Experimental Study on Iron Complexes Containing

N,N-Coordinated o-Diiminobenzosemiquinonate

Radicals

Dissertation for the degree of Doktor der Naturwissenschaften in the

Fakultt fr Chemie at the Ruhr-Universitt Bochum

Presented by

Krzysztof Chopek

Mlheim an der Ruhr, March 2005

Discovery is seeing what everybody else

has seen but thinking what nobody else has thought.

Sarah Ban Breathnach

This work was independently carried out between November 2001 and March 2005 at

the Max-Planck-Institut fr Bioanorganische Chemie, Mlheim an der Ruhr,

Germany.

Submitted on: 23 - 03 - 2005 Examination: 11 - 05 - 2005

Paper Published:

Molecular and Electronic Structure of Four- and Five- Coordinate Cobalt Complexes

Containing Two o-Phenylenediamine- or two o-Aminophenol-Type Ligands at

Various Oxidation Levels: An Experimental, Density Functional, and Correlated ab

initio Study. E. Bill, E. Bothe, P. Chaudhuri, K. Chlopek, D. Herebian, S. Kokatam,

K. Ray, T. Weyhermller, F. Neese, K. Wieghardt, Chem. Eur. J. 2005, 11, 204.

The Molecular and Electronic Structure of Five-Coordinate Complexes of Iron(II/III)

Containing o-Diiminobenzosemiquinonate(1-) Radical Ligands. K. Chopek, E.

Bill, T. Weyhermller, and K. Wieghardt, submitted.

Examination Committee:

Prof. Dr. K. Wieghardt (Referent)

Prof. Dr. W. S. Sheldrick (Koreferent)

Prof. Dr. W. Sander (Prfer)

Acknowledgements I would like to acknowledge all the members of Prof. Wieghardts laboratory for their

friendship and helpful support. I am especially indebted to:

Prof. Dr. Karl Wieghardt for the opportunity to work and to get scientific

experience under his excellent supervision. I am proud to say that I was guided by one

of the best experts in the world of paramagnetic chemistry and have worked in one of

the best equipped labs on the Earth for the investigation of paramagnetic species. I

will always keep in mind that only half of the world is diamagnetic.

Prof. Dr. Phalguni Chaudhuri for many helpful suggestions.

Dr. Eckhard Bill for teaching me the basics of the magnetochemistry (EPR, SQUID

and Mssbauer spectroscopies) which were of great help in the interpretation of my

results.

Dr. Dana S. Marlin and Dr. Sbastien Blanchard for many useful ideas and fruitful

discussions.

Dr. Thomas Weyhermller and Mrs. Heike Schucht for their X-ray crystal

structure analyses.

Dr. Eberhard Bothe and Mrs. Petra Hfer for their help in electrochemical

measurements.

Mr. Bernd Mienert, Mr. Frank Reikowski, Mr. Andreas Gbels, Mr. Jrg Bitter,

Mrs. Kerstin Sand and Mrs. Ursula Westhoff for their measurements of Mssbauer,

EPR, SQUID, NMR and GC.

I am very thankful to Dr. John F. Berry for the revision of the manuscript.

Dr. Nuria Aliaga-Alcade, Dr. Kathrin Merz, Dr. Yufei Song and Dr. Isabelle

Silvestre for many helpful ideas by writing the thesis.

Mrs. Jutta Theurich and Mrs. Rosemarie Schrader for ordered publications.

Dr. Kallol Ray, Dr. Taras Petrenko, Dr. Laurent Benisvy, Dr. Grettel Valle

Bourrouet, Ms. Ruta Kapre, Mr. Sumit Khanra, Mr. Chandan Mukherjee, Ms.

Swarnalatha Kokotam and Ms. Elham Safaei for lots of help and a friendly

atmosphere in the lab.

I am thankful to my parents for their faith in me.

I am highly indebted to my wife Patrycja who always supports me, for her faith,

patience, inspiration and love.

I am thankful to the Max-Planck-Gesellschaft (MPG) for financial support.

For Patrycja

On sengage et puis on voit .

(First start and then you will see.)

Napoleon Bonaparte

Contents

Contents

Chapter 1 1. Introduction 1

1.1. Formal and physical oxidation states 3

1.2. A special case intermediate spin iron(III) 5

1.2.1. Square pyramidal complexes 6

1.2.2. Six-coordinate complexes 15

1.2.3. Summary of studies on the intermediate spin state

Fe(III)

18

1.3. Objectives of the work 26

1.4. References 29

Chapter 2 Ligand syntheses 35

2. Ligand syntheses 37

2.1. N-[3,5-di(tert-butyl)phenyl]-1,2-benzenediamine 37

2.2. N,N-di[3,5-di(tert-butyl)phenyl]-1,2-benzenediamine 41

2.3. Rac-4-(1-phenylpentyl)pyridine 43

2.4. References 46

Chapter 3 Square pyramidal iron complexes containing o-

diiminobenzosemiquinonate radicals

47

3. Introduction 49

3.1. Synthesis and characterization of [Fe(1LISQ)(1LPDI)]2 and

[Fe(2LISQ)(2LPDI)]2

51

3.2. Synthesis and characterization of [Fe(1LISQ)2(PBu3)] 55

3.3. Synthesis and characterization of [Fe(1LISQ)2(PBu3)]PF6 61

3.4. Synthesis and characterization of [Fe(1LISQ)2I] 68

3.5. Synthesis and characterization of [Fe(1LISQ)2(CN-tBu)]

[Fe(1LISQ)2(CNCy)]

79

3.6. Synthesis and characterization of [Fe(1LISQ)2(1LH2)]PF6 87

3.7. Synthesis and characterization of [Fe(1LISQ)2(Ph2Im-H)] 92

3.8. Synthesis and characterization of

[Fe(1LISQ)(1LPDI)(tBu-py)] and

[Fe(1LISQ)(1LPDI)(DMAP)]

98

I

Contents

3.9. Synthesis and characterization of

[Fe(1LISQ)(1LPDI)(BuPhCH-py)] and

[Fe(2LISQ)(2LPDI)(tBu-py)]

118

3.10. Summary 128

3.11. References 132

Chapter 4 Square planar and square pyramidal cobalt complexes

containing two o-phenylenediamine type ligands at various

oxidation states

137

4.1. Introduction 139

4.2. Structural, spectroscopic characterization and discussion 143

4.3. References 159

Chapter 5 Iron, nickel, cobalt and palladium complexes containing bulky

N,N-disubstituted-1,2-benzenediamine derived ligands

161

5. Introduction 163

5.1. Synthesis and characterization of [Fe(3LISQ)3] 167

5.2. Synthesis and characterization of [Ni(3LISQ)2] 172

5.3. Synthesis and characterization of [Co(3L)2] 182

5.4. Synthesis and characterization of [Pd(3LISQ)2] 192

5.5. Summary 201

5.6. References 203

Chapter 6 Summary 207

Chapter 7 Methods and equipment 215

Chapter 8 Synthetic part 223

Chapter 9 Appendices 255

II

Abbreviations and symbols

List of Abbreviations and Symbols A hyperfine coupling constant

angstrom

B magnetic field

BINAP bis(diphenylphosphino)-1,1-binaphthyl

br broad

Bu butyl

cm centimeter

CGS Centimeter-Gram-Second unit system

CT charge transfer

CV cyclic voltammetry

Cy cyclohexyl

d doublet

D axial zero-field splitting parameter

dba dibenzylideneacetone

DFT density functional theory

DMAP 4-dimethylaminopyridine

dmf dimethylformamide

e electron

E electric potential

E/D rhombicity

eff effective

EFG electric field gradient

EI electron ionization

emu electromagnetic unit (= cm3 mol-1)

EPR electron paramagnetic resonance

Et ethyl

f Lamb-Mssbauer factor

Fc ferrocene

g electron Lande factor

gN nuclear Lande factor

G gauss

GC gas chromatography

III

Abbreviations and symbols

GHz gigahertz

H Hamiltonian operator

HOMO highest occupied molecular orbital

Hz hertz

I nuclear quantum number

IR infrared

ISO isotropic

IVCT intervalence charge transfer

J coupling constant

k = kB Boltzmann constant

K Kelvin

LLCT ligand-to-ligand charge transfer

LMCT ligand-to-metal charge transfer

LUMO lowest unoccupied molecular orbital

m meter (or multiplet in NMR)

mm millimeter

M molar = mol dm-3

MCD magnetic circular dichroism

Me methyl

MHz megahertz

Mmol molecular magnetization

MO molecular orbital

mV millivolt

n normal

nm nanometer

N Avogadro constant

NMR nuclear magnetic resonance

Oh octahedral

OTTLE optically transparent thin-layer electrochemical cell

Ph phenyl

ppm parts per million

py pyridine

q quartet

IV

Abbreviations and symbols

Q quadrupole moment

R gas constant

RT room temperature

s second (or singlet in NMR)

S local spin state (or spin quantum number)

sh shoulder

SOD second-order Doppler effect

SOMO singly occupied molecular orbital

SQUID superconductiv quantum interference device

ST total spin state

t triplet

T tesla (or absolute temperature)

Td tetrahedral

tert = t tertiary

THF tetrahydrofuran

TIP temperature independent paramagnetism

UV-Vis ultraviolet-visible

V volt

Vij EFG component

vs versus

v/v versus volume

XRD X-ray powder diffraction

N = N nuclear magneton

line-width

chemical shift in NMR (or isomer shift in Mssbauer spectroscopy)

EQ quadrupole splitting

H enthalpy change

S entropy change

extinction coefficient

asymmetry parameter

Weiss constant

D Debye temperature

V

Abbreviations and symbols

wavelength

dipole moment

A microamper

B Bohr magneton

N = N nuclear magneton

frequency

mol molar susceptibility

NH

NH2

NH

NH2

1LH2 2LH2

N-[3,5-di(tert-butyl)phenyl]-1,2- benzenediamine

N-phenyl-1,2-benzenediamine

NN

3LH2

H H

N,N-di[3,5-di(tert-butyl)phenyl]-1,2-benzenediamine

N

NR

R

(LPDI)2-

N

NR

R

. (LISQ)1-

o-diiminobenzosemiquinonate monoanion

VI

o-phenylenediamide dianion

N

NR

R

(LIBQ)0

o-diiminobenzoquinone

Chapter 1

CHAPTER 1

INTRODUCTION

1

Chapter 1

2

Chapter 1

1.1. Formal and physical oxidation states

The interaction of transition metal ions with organic radicals has captured

much attention in recent years.1,2 One of the driving forces for this research direction

is the realization that such systems exist in the active sites of metalloproteins.3 The

best understood example is galactose oxidase which features a single Cu(II) ion

coordinated to a modified tyrosyl radical.1 Due to this intricate bonding situation, the

enzyme can perform specific two-electron oxidation of alcohols to yield aldehydes.

More examples of such situations (organic radicals coordinated to metal ions) are

expected to be found in biochemistry, yet they may be difficult to detect, for example,

in the cases where there are two radical ligands which strongly couple through a

central metal ion. Thus, it is important to have a clear understanding of the bonding

and physical properties (especially oxidation state of the metal ions) of such species if

one wants to understand their reactivities.

It is important to recall the concept of formal and physical (or spectroscopic)

oxidation states of a metal ion in a given complex. The formal oxidation state of a

given metal ion in a mononuclear coordination compound is a nonmeasurable integer

described as the charge left on the metal after all ligands have been removed in their

normal, closed-shell configuration that is with their electron pair 4 (heterolytic

metal-ligand bond cleavage).

On the other hand, Jrgensen has suggested that the number derived from a known dn

configuration of the metal ion should be specified as physical or spectroscopic

oxidation number.5 This suggestion is based on the fact that a dn configuration is, in

principle, a measurable quantity and can be established by physical and spectroscopic

methods like magnetochemistry, EPR, UV-Vis, NMR and Mssbauer (e.g. for iron)

spectroscopies.

When innocent (redox inactive) ligands are present in a complex, the formal and

physical oxidation state are of course identical. The situation is different when

noninnocent (redox-active) ligands are present in a complex (a noninnocent ligand

may be bound to the central metal ion in an open-shell electronic configuration e.g.

as radical).

3

Chapter 1

A good example is an O-coordinated phenoxyl radical complex of a ferric ion

(d5 configuration), FeIIIOPh. According to the description of Hegedus the formal

oxidation number of the iron would have to be +IV because a closed shell phenolato

anion has to be removed. This is in contrast to the spectroscopic oxidation state (+III)

of the ferric ion which can be established by Mssbauer and resonance Raman

spectroscopy.6 Since Fe(IV)phenolato and Fe(III)phenoxyl complexes are distinctly

different species (with distinctly different electronic ground states), formal and

spectroscopic oxidations states are not synonymous.

Since 1960 there has been a debate concerning the oxidation states of metal

ions in complexes containing noninnocent ligands. In a potentially large number of

publications, wrong oxidation states of metal ions have been assigned because the

concept of formal oxidation number was used in cases where it should not have been

(noninnocent ligands were presumed to be innocent). Selected examples are the

supposed iron(IV)7,8 and iron(V)9 complexes synthesized by Sellmann et al. which in

fact contain iron(II) or iron(III) ions coordinated to radical(s).10-12

4

Chapter 1

1.2. A special case - intermediate spin iron(III)

In the course of this work we will be concerned mostly with intermediate spin

iron(III) and, hence, it is instructive to explain this unusual spin state. In this

subchapter the known examples of intermediate spin Fe(III) complexes and their

magnetic and spectroscopic features are reviewed. Interestingly, this spin state of the

ferric ion has been known since 196613 and many examples of ferric complexes with

this particular spin state have been found afterwards. However, they are still neglected

in academic textbooks.

The first theoretical model of the intermediate spin ferric ion was published by

Harris.14 In his model, wave functions of ferric species had been generated based on

the strong crystal field model (including spin-orbit coupling) assuming tetragonal

symmetry. It is suggested that the iron(III) ion, depending on the strength of the axial

ligand field (the equatorial field was assumed to be always stronger than axial one),

can exist generally in four qualitatively different ground states: a sextet, a quartet, a

doublet and a substantially spin-mixed state.

A very similar model was described by Maltempo15,16 to explain the observations

that some bacterial ferricytochromes (like ferricytochrome c from photosynthetic

Chromatium vinosum17) display anomalous magnetic and spectroscopic properties.

These were interpreted as a result of quantum-mechanical spin-mixing (through spin-

orbit coupling) between the 4A2 and 6A1 states in a tetragonal d5 system. It should be

noted that the ferricytochromes c under some conditions (pH ~ 7) contain a histidine

ligated five-coordinate ferric (heme) species. It was also postulated that the

ferricytochrome c isolated from other photosynthetic and denitrifying bacteria like

Rhodobacter capsulatus, Rhodopseudomonas palustris, Rhodospirillium

molischianum and Rhodspirillium rubrum contain spin admixed (S = 3/2, 5/2) ferric

species.18 Moreover, in Horseradish-Peroxidase the same spin admixture was

suggested.19

These models establish the criteria for obtaining a spin quartet ground state (S =

3/2) for a d5 system. The energy of the unoccupied antibonding dx2-y2 orbital must lie

well above that of the other d orbitals. The residual four d orbitals are filled with

electrons according to Hunds rule. This can be achieved when strong equatorial and

weak axial bonds are present so that the dz2 orbital remains close in energy to the t2g

5

Chapter 1

orbitals. The equatorial ligand field is an important factor leading to the formation of

intermediate spin iron(III).20 The splitting of the d orbitals under tetragonal distortion

allows the presence of the intermediate spin state as shown in Figure 1.1.

Ener

gy

eg

t2g

Tetragonal distortion

Oh D4h C4v

dx2-y2

dz2dxy

dyzdxz

Figure 1.1. Energy level diagram showing splitting under the influence of a tetragonaldistortion.

1.2.1. Square pyramidal complexes

The twenty-four iron-compounds discussed in this subchapter can be divided

into two groups: (1) complexes containing sulfur- or sulfur-nitrogen donor sets in

equatorial positions or (2) complexes with macrocyclic, nitrogen based ligands

occupying the equatorial positions. The second group contains porphyrin- and non-

heme compounds.

Since the appearance of the first report concerning intermediate-spin ferric

compound by White et al.,13 bis(N,N-dialkyl-dithiocarbamato)iron(III) complexes

have been extensively studied and the following work has included structural and

spectroscopic characterization.21,22-24 The investigated diethyl- and 4-morpholyl-

dithiocarbamato-complexes have the same geometrical features (Figure 1.2). They

contain two equatorial sulfur-ligands and a halogen ion at the apex. [Fe(S2CNEt2)2Cl]

6

Chapter 1

(1) and [Fe(S2CNC4H8O)2I] (2) or [Fe(S2CNC4H8O)2Br]1/4CH2Cl2 (3) display

similar features in their Mssbauer spectra. Their isomer shift values are large ( ~

0.50 mm s-1) and the quadrupole splitting (EQ ) is close to +3 mm s-1 (Table 1.1).

The room temperature effective magnetic moments ranging from 3.98 B to 4.01 B

are consistent with three unpaired electrons present in the complexes (see Table 1.2).

S

SFeIII

S

S

Cl

N N

0

Figure 1.2. Structure of complex 1.

Bis(dithiooxalato)ferric complexes were studied by Kostikas et al.25 Although

the investigated complexes with general formula [(O2C2S2)2FeX]2- (where X = I-, Br-,

Cl- for 4, 5, 6, respectively; Figure 1.3) were not structurally characterized, there is

enough spectroscopic evidence that they contain five-coordinate intermediate spin

ferric ions. Susceptibility and EPR experiments corroborated the quartet ground state

of these molecules (Table 1.2). Applied-field Mssbauer spectroscopy at 4.2 K

established that all complexes possess isomer shifts in the range 0.25 0.30 mm s-1

and large and positive quadrupole splittings +3.25 mm s-1 (Table 1.1).

S

SFeIII

S

S

X 2-O

O

O

O

X = Cl-, Br-, I- Figure 1.3. Structure of a dianion in complex 4, 5 and 6.

Kahn and coworkers26 have established that the five coordinate bis(cis-1,2-

dicyano-1,2-ethylenedithiolato)iron(III) [Fe(mnt)2(idzm)]2dmf (7, Figure 1.4)

undergoes a temperature-driven spin transition from S = 1/2 to S = 3/2 with eff between 40 and 300 K at 3.85 B. The transition occurs at extremely low temperature

(T1/2 ~ 3 K) and above 10 K only the intermediate spin component is observed. This

was shown also by Mssbauer spectroscopy ( = 0.36 mm s-1, |EQ| = 2.64 mm s-1, at

77 K).

7

Chapter 1

S

SFeIII

S

S

0NC

NC

CN

CN

N

HN NH+

Figure 1.4. Structure of complex 7.

A series of square pyramidal, intermediate-spin ferric complexes containing

equatorial sulfur donor ligands have been recently reinvestigated by Wieghardt and

coworkers12 (the complexes were originally synthesized by Sellmann et al.7). For two

complexes [Fe(SSLBu )(SSLBu)(PMe3)] (8, Figure 1.5) and [Fe(SSLBu )2(PMe3)]+ (9) it

has been established that the intermediate spin ferric species interact

antiferromagnetically with one or two radicals. In this case a triplet or doublet ground

state is observed for 8 and 9, respectively. On the other hand, a quartet ground state

has been found for molecules of [Fe(SSL)2(tBu-py)]- (10, Figure 1.5), which contain

no radicals.

S

S S

SFeIII

. PMe3 0S

S S

SFeIII

1-N

8 10

Figure 1.5. Structures of complexes 8 (left) and 10 (right).

All of these complexes exhibit a single doublet in their zero-filed Mssbauer spectra

with isomer shifts ranging from 0.12 mm s-1 to 0.34 mm s-1 (Table 1.1). An interesting

similarity between the hyperfine coupling tensor components of 8, 9 and 10 was

8

Chapter 1

detected by applied-field Mssbauer spectroscopy. For instance, complex 8 displays

two large negative and one small positive components: A/gNN(SFe = 3/2) = (-25.94,

-17.73, +0.28) T, and similar values were found for 9 and 10 (Table 1.1).

Sulfur-nitrogen donor ligands also form intermediate-spin ferric

complexes. Magnetic data of a Schiff-base-containing square-pyramidal compound

[(salenSS)FeCl] (11, Figure 1.6) revealed an effective magnetic moment 3.90 B

between 100 and 295 K.27 Below 100 K the magnetic moment decreases due to

intermolecular interactions. The Mssbauer spectrum recorded at 4.2 K exhibits one

doublet with the following data: = 0.38 mm s-1 and |EQ| = 3.13 mm s-1, which are

characteristic of intermediate spin state ferric ions.

S

NFeIII

S

N

Cl 0

Figure 1.6. Structure of complex 11.

Two other examples of iron compounds with sulfur-nitrogen donor ligands

contain radicals that are coupled antiferromagnetically to an intermediate spin ferric

species, resulting in a doublet ground state. Bis(thiosemicarbazide)iron(III)

compounds [Fe(LMe)2(SCH3)] (12, Figure 1.7) and [Fe(LMe)2Cl] (13) possess square

pyramidal geometry with two radicals in the plane and either methylthiolate or

chloride monoanion at the apex.28

HN

NFeIII

NS

NH

0

NN

S

S

. .

Figure 1.7. Structure of complex 12.

The low isomer shift value of = 0.06 mm s-1 observed in the Mssbauer spectrum of

12 at 80 K is explained by exceptionally strong covalent bond to the apical SCH3

9

Chapter 1

ligand. This is in contrast to this what is found in the chloride complex 13, for which

the isomer shift (measured at the same temperature) of = 0.20 mm s-1 suggests less

covalency in the Fe-Cl axial bond.

More detailed spectroscopic analysis was performed for the

bis(o-iminothiobenzosemiquinonate)iron(III) complex [Fe(NSLISQ)2I]11 (14, see Figure

1.8, left). In this case, magnetically perturbed Mssbauer spectroscopy (Table 1.1)

revealed two negative and one positive components of the intrinsic-field ferric value

(A/gNN(SFe = 3/2) = (-7.38, -16.05, +0.60) T), similar to complexes 8-10. The

isomer shift of = 0.12 mm s-1, and large positive quadrupole splitting (EQ = +3.18

mm s-1) corroborate the presence of an intermediate-spin ferric species in 14.

N

O N

OFeIII ..I

Ph

Ph

NH

SHN

SFeIII ..I 0 0

14 15

Figure 1.8. Structures of complexes 14 (left), and 15 (right).

We begin the discussion of intermediate spin iron complexes containing

nitrogen-based ligands with a unique example containing oxygen donor atoms. To the

best of our knowledge, no other example exists in which a spin quartet ground state

ferric ion is coordinated to equatorial oxygen-donor ligands.

The nitrogen-oxygen containing complex under consideration contains two

iminobenzosemiquinonate -radical mononanions coordinated to an intermediate spin

ferric species ([Fe(NOLISQ)2I], 15, Figure 1.8, right). By magnetically perturbed

Mssbauer spectroscopy it has been established that 15 possesses a larger intrinsic

field at the iron nucleus (A/gNN(SFe = 3/2) = (-12.40, -20.32, +1.30) T) and a larger

isomer shift value ( = 0.24 mm s-1) 11 comparing to 14. This is probably an effect of

reduced covalency of the N2O2I donor set with respect to the N2S2I set of 14.

The only intermediate spin ferric species described heretofore in the literature

containing four equatorial N donor atoms have been complexes of macrocyclic

ligands.

10

Chapter 1

A square pyramidal compound [Et4N]2[FeCl(4-MAC*)]CH2Cl2 (16, see

Figure 1.9) was investigated by Collins and coworkers.29 The intermediate spin state

of the ferric ion is here stabilized by a strong equatorial macrocyclic ligand and a

weak axial chloride ion. The ground state of the iron ion was determined by EPR-

(Table 1.2) and applied-field Mssbauer spectroscopy (Table 1.1). Again two large

negative and one small positive components are found for the iron hyperfine coupling

tensor (A/gNN = (-16.6, -22.0, +5.0) T).

N

NFeIII

N

N

Cl2-

O

O

O

O

O

Figure 1.9. Structure of a dianion in complex 16.

Very similar properties were recognized for complex [(L-N4)FeI] (17, Figure 1.10) of

Jger and coworkers30 (Table 1.1). The stabilization of a quartet ground state is

achieved via utilization of a strongly donating macrocyclic, equatorial ligand and

a weaker axial ligand, iodide.

N

NFeIII

N

N

I

O

O O

O

0

Figure 1.10. Structure of complex 17.

Porphyrin-ligated compounds are the next subclass of intermediate-spin ferric

complexes with N-donor ligands. In the majority a quantum-mechanic spin admixture

of 3/2, 5/2 is suggested20,31. The complexes discussed in this subchapter are those in

which the admixture of 3/2 spin exceeds 90 %.

11

Chapter 1

Reed at al. have extensively studied tetraphenylporphyrin iron complexes,

some of which containing a high degree of SFe = 3/2 in the ground state, as evidenced

by EPR, NMR, Mssbauer spectroscopy and magnetochemistry. The complexes

[Fe(TPP)(CB11H6X6)]32 (18 for X = Cl-, 19 for X = Br-, Fig. 1.11) and

[Fe(TPP)(CB11H12)]33 (20, Fig. 1.12) were synthesized by use of weakly donating

halogenocarborane or carborane monoanions, respectively. For 18, a pure

intermediate spin state for the ferric species was suggested on the basis of

susceptibility measurements (eff at 298 K is 4.0 B), whereas for 20, approximately

8 % of the ground state was determined to be high-spin (4.15 B at 300 K).

Figure 1.11. Crystal structure of complex 18, all hydrogen atoms omitted.

C(45)

C(1)

B(3)

B(7) Br(2)

Br(1)

Fe(1)

N(4)

N(3)

N(2) N(1)

Complexes [Fe(TPP)(H2O)]+ 32 (21) and [Fe(TPP)(FSbF5)]34,35 (22) contain a

negligible high-spin contribution to the ground state (< 2 %). All of these complexes

display a single doublet in zero-field Mssbauer spectra with isomer shifts ranging

from 0.33 to 0.39 mm s-1. Large quadrupole splitting values up to |EQ| = 4.29 mm s-1

(as for 22) are the most noticeable attribute of intermediate spin ferric ions.

Interestingly, in the 1H NMR spectra of complexes 18-22 (in benzene-d6, at 298 K)32

it has been found that chemical shifts of the -pyrrole protons appear at very negative

values (-43 ppm -62 ppm). It has been suggested that such a negative shift reflects

12

Chapter 1

depopulation of the dx2-y2 orbital,32,36 and supports the presence of intermediate spin

ferric species in the complexes.

Figure 1.12. Crystal structure of complex 20, all hydrogen atoms except H(12) are omitted.

C(12)

Fe(1)

B(8)

H(12)

C(45)

N(4)

N(3)N(1)

N(2)

Nakamura et al.37 have established a pure intermediate spin state of iron in the

porphyrin complex [Fe(OETPP)I] (23, Figure 1.13). For this compound the effective

magnetic moment is temperature independent between 10 and 300 K with eff = 3.90

B. The large isomer shift value ( = 0.42 mm s-1, at 77 K) and large quadrupole

splitting (|EQ| = 3.05 mm s-1) fit very well the features typically observed for an

intermediate spin ferric ion.

N

N

N

N

N

NFeIII

X

X = ClO4-

0

N

N N

N

FeIII

0

I

2423

Figure 1.13. Structures of complexes: 23 (left) and 24 (right).

13

Chapter 1

Of interest are octaethyltetraazaporphyrinate ferric complexes with weakly

coordinated anions (like Cl-, ClO4-, PF6-, SbF6- and CF3COO-) studied by Fitzgerald et

al.38,39 The perchlorate-compound [Fe(OETAP)(OClO3)] (24, see Figure 1.13)

displays one of the largest quadrupole splitting observed for SFe = 3/2 (|EQ| = 4.38(2)

mm s-1, = 0.35(1) mm s-1).39

Although highly admixed spin complexes (S = 3/2, 5/2) are not discussed here,

three of them should be mentioned, due to the properties of their Mssbauer spectra.

Square pyramidal [Fe(TPP)(OClO3)]40,41 (25) displays a room temperature

effective magnetic moment eff = 5.19 B and axial EPR signal with g = 4.75 and g||

= 2.03 (solid, 10 K). Hence, according to the Mapltempo model,16 the compound is

classified as a spin-admixed complex with a 35 % S = 5/2 and 65 % S = 3/2 ground

state. Interestingly, magnetically perturbed Mssbauer spectra at 4.2 K, with an

external magnetic field of 6.0 T, revealed the intrinsic magnetic field (A/gNN =

(-22.4, -22.8, +1.2) T)41 similar to those found for pure intermediate spin ferric

complexes. The isomer shift of 25 (at 4.2 K) at = 0.39 mm s-1, and the large,

positive quadrupole splitting (EQ = +3.50 mm s-1) also fit in the observed range for

intermediate spin iron(III) compounds.

Additionally, chromatium ferricytochrome c at pH = 7.8 investigated by Maltempo et

al.17 has been characterized by similar Mssbauer parameters: A/gNN = (-22.0, -22.0,

+1.1) T, = 0.29 mm s-1, EQ = +2.91 mm s-1, = 0.0 (at 4.2 K). In this case 30 %

of sextet is believed to be admixed with 70 % of quartet ground state was postulated.

More studies should be performed to elucidate the spectroscopic differences between

the spin admixed and intermediate spin Fe(III) complexes.

Attention must be drawn by an evaluation of spin-admixed complexes. A square

pyramidal porphyrin iron(III) complex investigated by Cheng et al.42 was originally

described, on the basis of its X-ray structure, magnetochemistry, X-band EPR- (in

2-methyltetrahydrofuran solution, at 77 K) and NMR-spewctroscopy, as containing

60 % of S = 5/2 admixed with 40 % of S = 3/2 (eff = 5.2 B at 300 K).

Reinvestigation of the complex by Weiss et al.43 has established, that the complex

possesses generally a high spin iron(III) with a small admixture of 3/2 state (< 10 %).

Zero-field Mssbauer data (recorded at 280 K) support the high spin configuration.

The observed isomer shift of = 0.35 mm s-1 with a small quadrupole splitting value

|EQ| = 0.95 mm s-1 match very well the features observed for high spin ferric

14

Chapter 1

complexes. The X-band EPR spectra recorded in 2-methyltetrahydrofuran at 10 K

revealed a well resolved rhombic signal with geff = 6.55, 5.28, 1.976 (g = 5.92) that is

in accord with the high spin configuration of ferric ion (with S = 5/2 ~ 96 %).

The X-band EPR spectrum recorded by Cheng et al has displayed a broad peak at g ~

6.00 and has been interpreted as g = 5.2. Using a Maltempo model,16 it has been

concluded that the complex under consideration contains 60 % of S = 5/2 and 40 % of

S = 3/2.

The Mssbauer data of Weiss et al. have confirmed on the other hand unambiguously

a high percentage of a sextet ground state ferric ion.

1.2.2. Six-coordinate complexes

Intermediate spin state ferric ions exist not only in five-coordinate but also in

six-coordinate complexes. As was already mentioned, a strong equatorial ligand-field

combined with weak axial ligands generate this spin state at the iron due to a

lowering of a symmetry from Oh to D4h.14 In this case, dz2 orbital comes close to t2g

orbitals while dx2-y2 is destabilized and becomes unoccupied (see Figure 1.1).

The few six-coordinate iron compounds with a spin quartet ground state that

are known in the literature were published by the groups of Krger, Latos-Grayski,

Nakamura and Wieghardt.

SS

FeIII

N

NN

N

1+

Figure 1.14. Structure of an anion in complex 26.

Krger et al.44 have studied a distorted octahedral complex [Fe(L-

N4Me2)(SSL)]ClO4H2O (26, Figure 1.14). From Mssbauer, EPR spectroscopies,

susceptibility measurements and X-ray crystallography it has been shown that 26

undergoes a temperature-driven, gradual spin transition from a doublet to a quartet

15

Chapter 1

state (SFe = ST). The intermediate spin ferric species are present at lower temperatures

and at 300 K essentially only a quartet state is observed (with eff = 3.7 B at 295 K).

The X-band EPR spectrum (at 10 K, MeCN/toluene solution, 1:3 v/v) displays a

rhombic signal with geff = 5.50, 1.86, 1.40, consistent with the presence of a quartet

spin state. The following Mssbauer data (zero-field, at 200 K) have been ascribed to

the intermediate spin ferric species: = 0.32 mm s-1, |EQ| = 2.22 mm s-1.

Latos-Grayski and coworkers36 have confirmed the presence of intermediate

spin ferric ion in a strongly distorted, octahedral porphyrin complex

[Fe(TMCP)(EtOH)2]ClO4 (27, Figure 1.15) in which the porphyrin core was strongly

ruffled (see Figure 1.16). A temperature independent effective magnetic moment eff

= 3.8 B between 50 and 300 K indicates the presence of three unpaired electrons in

27. The complex displays a single doublet in the zero-field Mssbauer spectra (at 77

K) with an isomer shift of = 0.35(1) mm s-1 and a large quadrupole splitting |EQ| =

3.79(1) mm s-1. The 1H NMR spectra in CDCl3-EtOH (at 295 K) exhibit upfield-

shifted resonances for the pyrrole protons (-30 ppm) that are consistent with the

depopulated iron dx2-y2 orbital.

N

N N

N

FeIII

1+

THF

THFN

N N

N

FeIII

1+

EtOH

EtOH

OO

O

O

O O

O

O

2827

Figure 1.15. Structures of cations in complexes: 27 (left) and 28 (right).

Nakamura et al. have studied a series of tetragonally distorted octahedral

complexes containing intermediate spin ferric species. A strongly ruffled

[Fe(TiPrP)(THF)2]ClO445 (28, see Figures 1.15 and 1.16) and highly saddle-like

[Fe(OETPP)(THF)2]ClO4 45-47 (29) complexes displays effective magnetic moments

16

Chapter 1

close to eff ~ 3.9 B (300 K). The following data have been recognized in zero-field

Mssbauer spectra: isomer shifts of 0.34 mm s-1 and 0.50 mm s-1 for 28 and 29,

respectively. The quadrupole splitting values are above 3.50 mm s-1 for both 28 and

29 (Table 1.1).

Additionally, the 1H NMR spectrum of 28 taken at 233 K exhibits -pyrrole signals at

~ -52 ppm indicating a depopulated iron dx2-y2 orbital.

A B Figure 1.16. Saddle-like (A) and ruffle-like (B) conformations for non-planar distortion in the porphyrin core; filled circles () correspond to atoms above the plane and open circles () represent atoms below the plane.

The distorted octahedral iron complex [Fe(SSLBu )(SSLBu)(PMe3)2] (30, Figure

1.17) has been studied recently by Wieghardt et al.12 To our knowledge this is the

only example of pure intermediate spin ferric complex with quasi-octahedral

geometry, which was investigated by applied-field Mssbauer spectroscopy. By an

array of spectroscopic methods combined with DFT calculations it has been shown

that 30 contains one thiolate radical coupled in an antiparallel fashion to an

intermediate spin ferric ion, yielding a triplet ground state (ST = 1). A large zero-field

splitting was established for this molecule (DT = +14.35 cm-1, E/D = 0.02, DFe =

+9.57 cm-1).48 Interestingly, solid sate Mssbauer spectroscopy (at 4.2 K) reveals two

large negative and one small positive components of iron hyperfine coupling tensor:

A/gNN(ST = 1) = (+1.56, -12.35, -38.22) T, with intrinsic ferric values A/gNN(SFe =

3/2) = (+1.25, -9.88, -30.58) T. The Mssbauer isomer shift of = 0.17 mm s-1, and

positive quadrupole splitting EQ = +1.46 mm s-1 (with asymmetry parameter =

0.10) corroborate the intermediate spin state of the ferric ion in 30, even though the

quadrupole splitting value is one of the lowest observed for the quartet ground state

ferric species (Table 1.1).

17

Chapter 1

SS S

SFeIII

P

P

0

.

Figure 1.17. Structure of complex 30.

1.2.3. Summary of studies on the intermediate spin state Fe(III)

Intermediate spin ferric complexes have been found for square pyramidal and

tetragonally distorted octahedral complexes containing strong equatorial and weak

axial ligands. To stabilize the unique spin state of iron(III) ions, strong sulfur- and (or)

nitrogen- based basal ligands are employed together with weakly coordinated apical

ligands. The following equatorial ligands have been used in the discussed complexes:

thiocarbamates, thiooxolates, thiosalen, thiosemikarbazides, benzenedithiolates,

iminothiobenzosemiquinonates, [N4]macrocycles, porphyrines and

tetraazaporphyrines. Among the axial ligands, which one can find, are: halogen-

anions, SbF61-, ClO41-, H2O, THF, Et2O, halogenocarborane CB11H6X61- and

carborane CB11H121- monoanions or in a few cases, phosphines, pyridines and

thiolates.

As was shown above, Mssbauer spectroscopy is a powerful tool in discerning

the spin and oxidation state of iron ions. The unique intermediate spin state iron(III)

complexes display isomer shifts between 0.06 mm s-1 and 0.50 mm s-1 (see Table 1.1).

Large quadrupole splittings up to 4.38 mm s-1 with a positive Vzz main electric field

gradient component (thus, a positive quadrupole splitting EQ) are additional features

of the quartet ground state of ferric ions.

Interestingly, according to a basic ligand-field picture, the intermediate spin

ferric configuration with an empty dx2-y2 orbital should possess a vanishing electric

field gradient with the quadrupole splitting value EQ ~ 0 (assuming that only ionic

interactions of the ligands to the iron ion exist).11 As early as in 1972 de Boer et al.49

used extended Hckel calculations to suggest that the large quadrupole splitting of

quartet ground state of ferric ion in 1 is caused by covalency effects. More

18

Chapter 1

sophisticated molecular orbital calculations by Grodzicki et al.30 have shown that the

large quadrupole-splitting phenomenon in 17 is indeed caused by a covalency effect

which populates the otherwise unpopulated dx2-y2 orbital.

An unambiguous assignment of the spin state of an iron ion is provided by

Mssbauer spectroscopy with applied external magnetic field. It has been established

that five coordinate square pyramidal complexes possess two large negative (Axx, Ayy)

and one small positive (Azz) components of the hyperfine coupling tensor A. On the

other hand, a tetragonally distorted octahedral complex 30 displays an opposite trend

small and positive x- and large and negative y- and z-components of A.

An X-band EPR signal of geff at ~ 4 and ~ 2 is an additional feature of the

quartet ground state of Fe(III) complexes, if SFe = ST = 3/2. In this case the observed

effective magnetic moment at room temperature is found close to the spin-only value

of eff = 3.87 B (Table 1.2) for S = 3/2.

In addition, porphyrin iron complexes with a quartet ground state exhibit upfield-

shifted resonances in the 1H NMR spectroscopy for the pyrrole protons (-30 ppm

-62 ppm) that are consistent with the highly depopulated iron dx2-y2 orbital.

Thus, spectroscopic methods especially magnetically perturbed Mssbauer

spectroscopy provide a convenient way of determining of the spin and oxidation

states of iron ions due to the unique properties which characterize every spin state,

including the intermediate spin state ferric species.

19

Chapter 1

Table 1.1. Mssbauer data of intermediate spin ferric complexes Complex

(geometry)a b

[mm s-1]

EQc

[mm s-1]

d A/gNN [T]e T [K] Ref.

1 (SP) 0.50 +2.70 0.16 |22.0|f, h 4.2 23, 24

2 (SP) 0.48 +2.80 0.12 |22.4|g, h 4.2 50

3 (SP) 0.47 +2.72 0.16 |26.8|g, h 4.2 50

4 (SP) 0.30 +3.33 0.1 |18.2|h 4.2 25

5 (SP) 0.29 +3.25 0.1 |21.0|h 4.2 25

6 (SP) 0.25 +3.60 0.2 |22.5|h 4.2 25

7 (SP) 0.36 |2.64| 77 26

8 (SP) 0.12 +3.05 0.74 -25.94, -17.73, 0.28;

(-14.46)h4.2 12

9 (SP) 0.29 +2.57 0.45 -18.66, -19.64, +1.01;

(-12.43)h4.2 12

10 (SP) 0.34 +2.94 0.02 -19.13, -32.24, +1.54;

(-16.61)h4.2 12

11 (SP) 0.38 |3.13| 4.2 27

12 (SP) 0.06 |2.32| 80 28

13 (SP) 0.20 |2.21| 80 28

14 (SP) 0.12 +3.18 0.53 -7.38, -16.05, +0.60;

(-7.61)h4.2 11

15 (SP) 0.24 +2.80 0.25 -12.40, -20.32, +1.30;

(-10.47)h4.2 11

16 (SP) 0.25 +3.60 0.15 -16.6, -22, +5.0;

(-11.2)h4.2 29

17 (SP) 0.19 +3.56 0.00 -12.7, -12.7, +0.5;

(-8.3)h4.2 30

18 (SP) 0.33 |3.68| 298 32

20 (SP) 0.33 |4.12| 4.2 33

21 (SP) 0.30 |3.24| 298 32

22 (SP) 0.39 |4.29| 4.2 35

23 (SP) 0.42 |3.05| 77 37

24 (SP) 0.35 |4.38| 298 39

25 (SP)i, j 0.39 +3.50 0.0 -22.4, -22.8, +1.2 4.2 41

20

Chapter 1

26 (Oh) 0.32 |2.22| 200 44

27 (Oh) 0.35 |3.79| 77 36

28 (Oh) 0.34 |3.71| 76 45

29 (Oh) 0.50 |3.50| 80 47

30 (Oh) 0.17 +1.46 0.10 +1.25, -9.88, -30.58;

(-13.07)h4.2 12

chromatium

ferricitochrome

c at pH=7.8

(SP) i

0.29

+2.91

0.0

-22.0, -22.0, +1.1

4.2

17

a) SP and Oh denote square pyramidal and octahedral, respectively. b) Isomer shift versus Fe at RT. c) Quadrupole splitting. d) Asymmetry parameter. e) Hyperfine coupling tensor components for intrinsic SFe = 3/2. f) Estimated at 1.2 K due to magnetic self-ordering. g) Estimated at 1.3 K due to magnetic self-ordering. h) Isotropic part. i) Spin admixed complex (S = 3/2, 5/2). j) Magnetic hyperfine coupling tensor estimated by external field of 6 T only. Table 1.2. Spectroscopic and magnetic properties of intermediate spin ferric complexes Complex geff values D a

[cm-1]

E/Db eff [B]c

(T)

ST d SFe e Ref.

1 4, 2 -2.0 3.98 (300 K) 3/2 3/2 13,23

2 +12.0 0.16 3.98 (300 K) 3/2 3/2 50

3 +8.0 0.05 4.01 (300 K) 3/2 3/2 50

4 4, 2 +4.2 0 4.00 (300 K) 3/2 3/2 25

5 4.5, 3, 2 +4.2 0.14 3/2 3/2 25

7 3.85 (300 K) 3/2 3/2 26

8 -18 0.01 3.80 (290 K)f 1 3/2 12

9 2.18, 2.07,

2.03

1/2 3/2 12

10 5.5, 2.0 -3.8 0.33 3.80 (290 K) 3/2 3/2 12

11 3.90 (295 K) 3/2 3/2 27

12 2.17, 2.05,

2.02

1.77 (290 K) 1/2 3/2 28

13 2.21, 2.06,

2.02

1.7 (290 K) 1/2 3/2 28

14 2.26, 2.12,

2.03

1.95 (290 K) 1/2 3/2 11

21

Chapter 1

15 2.34, 2.11,

2.05

1.9 (150 K) 1/2 3/2 51

16 6.19, 0,2,

0,2g;

(4.38, 3.74,

2.06)h

-3.7 0.05 3/2 3/2 29

17 5.38, 2.70,

1.74

+13 0.22 4.19 (300 K) 3/2 3/2 30

18 4.0 (298 K) 3/2 3/2 32

20 4.15 (300 K) 3/2i 3/2i 33

21 4.2 (298 K) 3/2 3/2 32

22 4.05, 2 4.14 (300 K) 3/2j 3/2j 34,35

23 4.14, 2.00 3.90 (300 K) 3/2 3/2 37

24 3.98, 1.99 3.87 (295 K) 3/2 3/2 38,39

26 5.50, 1.86,

1.40

3.7 (295 K) 3/2 3/2 44

27 4.3, 3.7, 2.0 |5.8| 0.05 3.8 (300 K) 3/2 3/2 36

28 3.99, 1.97 3.90 (300 K) 3/2 3/2 45

29 4.01, 2.00 3.85 (300 K) 3/2 3/2 45-47

30 +9.57 0.02 2.90 (290 K) 1 3/2 12

a) Axial zero-field splitting parameter for intrinsic DFe. b) Rhombicity. c) Effective magnetic moment. d) Total ground state, at the temperature given in the eff-column. e) Local spin state at the iron ion, at the temperature given in the eff-column. f) Simulated for a dimer with J = -2.0 cm-1. g) Kramers doublet Ms = 3/2. h) Kramers doublet Ms = . i) A quartet spin state admixed with 8 % of a sextet spin state. j) Quartet spin state admixed with 2 % of sextet spin state.

22

Chapter 1

List of compounds discussed in the subchapter 1.2.

1 [Fe(S2CNEt2)2Cl]

2 [Fe(S2CNC4H8O)2I]

3 [Fe(S2CNC4H8O)2Br]1/4 CH2Cl24 [(O2C2S2)2FeI]2-

5 [(O2C2S2)2FeBr]2-

6 [(O2C2S2)2FeCl]2-

7 [Fe(mnt)2(idzm)]2dmf

8 [Fe(SSLBu )(SSLBu)(PMe3)]

9 [Fe(SSLBu )2(PMe3)]+

10 [Fe(SSL)2(tBu-py)]-

11 [(salenSS)FeCl]

12 [Fe(LMe)2(SCH3)]

13 [Fe(LMe)2Cl]

14 [Fe(NSLISQ)2I]

15 [Fe(NOLISQ)2I]

16 [Et4N]2[FeCl(4-MAC*)]CH2Cl2

17 [(L-N4)FeI]

18 [Fe(TPP)(CB11H6Cl6)]

19 [Fe(TPP)(CB11H6Br6)]

20 [Fe(TPP)(CB11H12)]

21 [Fe(TPP)(H2O)]+

22 [Fe(TPP)(FSbF5)]

23 [Fe(OETPP)I]

24 [Fe(OETAP)(OClO3)]

25 [Fe(TPP)(OClO3)]

26 [Fe(L-N4Me2)(SSL)]ClO4H2O

27 [Fe(TMCP)(EtOH)2]ClO428 [Fe(TiPrP)(THF)2]ClO429 [Fe(OETPP)(THF)2]ClO430 [Fe(SSLBu )(SSLBu)(PMe3)2]

23

Chapter 1

List of abbreviations used in the subchapter 1.2.

(S2CNEt2)1- N,N-diethyldithiocarbamate anion

(S2CNC4H8O)1- 4-morpholyldithiocarbamate anion

(O2C2O2)2- dithiooxalate dianion

(mnt)2- cis-1,2-dicyano-1,2-ethylenedithiolate

dianion

(idzm)1+ 2-(p-pyridyl)- 4,4,5,5-tetramethyl-

imidazolinium cation

(SSLBu )1- 3,5-di-tert-butyl-1,2-

benzenedithiosemiquinonate anion

(SSLBu)2- 3,5-di-tert-butyl-1,2-benzenedithiolate

dianion

(SSL)2- 1,2-benzenedithiolate dianion

(salenSS)2- N,N-ethylene-bis-thiosalicylidenoiminate

dianion

(LMe)1- S-methyl-1-phenyl-isothiosemicarbazonate

radical anion

(SCH3)1- methylthiolate monoanion

(NSLISQ)1- 4,6-di-tert-butyl-o-

iminothiobenzosemiquinonate anion

(NOLISQ)1- N-phenyl-o-imino(4,6-di-tert-

butyl)benzosemiquinonate anion

(4-MAC*)4- tetraanion of 1,4,8,11-tetraaza-13,13-

diethyl-2,2,5,5,7,7,10,10-octamethyl-

3,6,9,12,14-pentaoxocyclotetra-decane

(L-N4)2- 6,13-bis(ethoxycarbonyl)-5,14-dimethyl-

1,4,8,11-tetraazacyclotetradeca-4,6,12,14-

tetraenate dianion

(TPP)2- tetraphenylporphyrinate dianion

(OETPP)2- dianion of 2,3,7,8,12,13,17,18-octaethyl-

5,10,15,20-tetraphenylporphyrin

(OEATP)2-

octaethyltetraazaporphyrinate dianion

24

Chapter 1

(L-N4Me2)0 N,N-dimethyl-2,11-

diaza[3.3](2,6)pyridinophane

(TMCP)2- tetramethylchiroporphyrin dianion

(TiPrP)2- dianion of 5,10,15,20-tetraisopropyl-

porphyrin

25

Chapter 1

1.3. Objectives of the thesis

A target of this work was to synthesize and characterize iron complexes with

ligands derived from o-phenylenediamine, such as commercially available N-phenyl-

1,2-benzenediamine (hereafter abbreviated to 1LH2), its new bulkier derivative

N-[3,5-di(tert-butyl)phenyl]-1,2-benzenediamine (2LH2) and new N,N-substituted

N,N-di[3,5-di(tert-butyl)phenyl]-1,2-benzenediamine (3LH2) see Figure 1.18.

NH

NH2

NH

NH2

NH

NH

1LH2 2LH2 3LH2 Figure 1.18. Structures of the ligands employed in the experimental work together with their

abbreviations.

o-Phenylenediamine belongs to the class of noninnocent ligands and may exist

after being doubly deprotonated, in three different oxidation states as

o-phenylenediamide dianion (LPDI

)2-

, o-diiminobenzosemiquinonate radical

monoanion (LISQ

)1-

and neutral o-diiminobenzoquinone (LIBQ

)0, as shown in Figure

1.19.

N

NH

HN

NH

HN

NH

H

- e - e+ e + e.

(LPDI)2- (LIBQ)0(LISQ)1- Figure 1.19. Redox scheme of the o-phenylenediamine ligand.

26

Chapter 1

The open-shell middle form of Figure 1.19 is only one of the many resonance

structures, which are displayed in Figure 1.20. This form of the ligand is also of

interest because no iron compound with a coordinated o-diiminobenzosemiquinonate

radical has been structurally characterized to date.

N

NH

H. -NNH

H. - NNH

H

.-N

NH

H

.-

Figure 1.20. Resonance structures of the o-diiminobenzosemiquinonato radical ligand.

High quality, low-temperature X-ray crystallography allows a convenient

determination of the oxidation states of the ligand presented in Figure 1.19, due to the

fact that the radical is most of the time delocalised over the phenyl ring and every

oxidation level of the ligand displays significantly different and characteristic bond

lengths. Studies of complexes of transition metals other than iron with

o-phenylenediamine reveal that the closed shell dianionic form contains an aromatic

phenyl ring with an average C-C distance of 1.40 and C-N bonds of 1.40 52 (see

Figure 1.21). The monoanionic radical form exhibits a quinoid distortion with two

alternating shorter C=C bonds at 1.37 and four longer ones at 1.41 and

additionally C-N distances of 1.35 .53 The closed-shell neutral structure of

o-diiminobenzoquinone displays a quinoid distortion with shorter C=C bonds of the

ring (1.34 ) in comparison to the previous form (Figure 1.4) and shorter C-N bond

distances at 1.30 .54

HN

NH

M

HN

NH

M

HN

NH

M .av. C-C 1.40

1.401.40

1.351.35

1.301.30

1.41

1.41

1.42

1.37

1.37

1.42

1.34

1.34

1.43

1.43

1.43

1.45

Figure 1.21. Average bond distances () in N,N-coordinated ligands (LPDI)2-, (LISQ)1-, and (LIBQ)0

as determined by X-ray crystallography (3 = 0.015 ).

27

Chapter 1

Iron chemistry of o-phenylenediamine-derived ligands was feebly explored in

the past probably due to the high air- and moisture-sensitivity of the complexes.

According to the Cambridge Crystallographic Data Centre only two iron compounds

with the deprotonated form of the discussed ligand have been structurally

characterized, both contain o-diiminobenzoquinones coordinated to low spin iron(II)

in octahedral geometry.55

Balch and Holm, who have synthesized cobalt, nickel, palladium and platinum

complexes with o-phenylenediamine-derived ligands, were unsuccessful in the

synthesis of their iron analogues.56

In 1977, L. F. Warren reported57 the synthesis of iron complexes with

o-diiminobenzoquinone derived ligands, namely [Fe{o-C6H4(NH)2}2I] and [Fe{o-

C6H4(NH)2}2(PPh3)]PF6. A doublet ground state has been assigned for the complexes.

At that time, the electronic structure of the iron and of the ligands had not been

explicitly addressed, nor had their molecular structures been determined.

Thus, so far the clear experimental evidence of the existence of iron ions with

N,N'-coordinated o-diiminobenzosemiquinonate(1-) radicals particularly with

iron(III) is lacking. These radicals may stabilize a quartet ground state at the iron ion

due to their significant acceptor character.58

It should be noted that square pyramidal intermediate spin iron(III) complexes with

non-macrocyclic nitrogen-donor set have never been investigated up to date (vide

supra). Therefore the aim of this work was to synthesize and fully characterize iron

complexes with o-phenylenediamine-derived ligands, especially with very unstable

o-diiminobenzosemiquinonate(1-) radicals.

28

Chapter 1

1.4. References

(1) Jadzewski, B. A.; Tolman, W. B. Coord. Chem. Rev. 2000, 200, 633.

(2) Chaudhuri, P.; Wieghardt, K. Prog. Inorg. Chem. 2001, 50,

151;Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331.

(3) Sigel, H. S. A. Metalloenzymes involving Amino Acid Residue and

Related Radicals; Marcel Dekker: New York, 1994; Stubbe, J.; Van der Donk, W. A.

Chem. Rev. 1998, 98, 705.

(4) Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic

Molecules; University Science Books: Mill Valley, CA, 1994.

(5) Jrgensen, C. K. Oxidation Numbers and Oxidation States; Springer:

Heidelberg, Germany, 1969.

(6) Snodin, M. D.; Ould-Moussa, L.; Wallmann, U.; Lecomte, S.; Bachler,

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29

Chapter 1

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J.; Safo, M. K.; Orosz, R. D.; Reed, C. A.; Debrunner, P. G.; Schulz, C. E. Inorg.

Chem. 1992, 31, 939; Safo, M. K.; Scheidt, W. R.; Gupta, G. P.; Orosz, R. D.; Reed,

30

Chapter 1

C. A. Inorg. Chim. Acta 1991, 184, 251; Scheidt, W. R.; Osvath, S. R.; Lee, Y. J.;

Reed, C. A.; Shaevitz, B.; Gupta, G. P. Inorg. Chem. 1989, 28, 1591; Masuda, H.;

Taga, T.; Osaki, K.; Sugimoto, H.; Yoshida, Z.; Ogoshi, H. Inorg. Chem. 1980, 19,

950; Einstein, F. W. B.; Willis, A. C. Inorg. Chem. 1978, 17, 3040; Summerville, D.

A.; Cohen, I. A.; Hatano, K.; Scheidt, W. R. Inorg. Chem. 1978, 17, 2906; Kennedy,

B. J.; Murray, K. S.; Zwack, P. R.; Homborg, H.; Kalz, W. Inorg. Chem. 1986, 25,

2539; Scheidt, W. R.; Geiger, D. K.; Hayes, R. G.; Lang, G. J. Am. Chem. Soc. 1983,

105, 2625; Kennedy, B. J.; Brain, G.; Murray, K. S. Inorg. Chim. Acta 1984, 81, L29;

Masuda, H.; Taga, T.; Osaki, K.; Sugimoto, H.; Yoshida, Z.; Ogoshi, H. Bull. Chem.

Soc. Jpn. 1982, 55, 3891.

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A. Inorg. Chem. 1987, 26, 3022.

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L.; Brewer, G. A. J. Chem. Soc.-Dalton Trans. 1996, 1249.

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Scheidt, W. R.; Mashiko, T.; Reed, C. A. J. Am. Chem. Soc. 1978, 100, 666.

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Chem. Soc. 1997, 119, 2563.

31

Chapter 1

(43) Schnemann, V.; Gerdan, M.; Trautwein, A. X.; Haoudi, N.; Mandon,

D.; Fischer, J.; Weiss, R.; Tabard, A.; Guilard, R. Angew. Chem., Int. Ed. 1999, 38,

3181.

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H. J. Chem.-Eur. J. 1998, 4, 686.

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Takahashi, M.; Takeda, M. Chem. Commun. 2000, 1989.

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M. J. Am. Chem. Soc. 2003, 125, 13028.

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Nakamura, M. Angew. Chem., Int. Ed. 2001, 40, 2617.

(48) The A tensor components of a doublet ground state system can be

converted to the intrinsic values for SFe = 3/2 by using the following relation Ai(SFe =

3/2) = 3/5Ai(ST = 1/2), derived from the spin projection techniques; for a triplet

ground sate the A components for SFe = 3/2 can be found by using the following

relation: Ai(SFe = 3/2) = 4/5Ai(ST = 1). The zero-field splitting parameter D(ST = 1) of

a triplet state may be converted to the intrinsic values DFe(SFe = 3/2) according to the

equation: DFe(SFe = 3/2) = 2/3 D(ST = 1).

(49) Vries, J.; Keijzers, C. P.; Boer, E. D. Inorg. Chem. 1972, 11, 1343.

(50) Grow, J. M.; Hopkins, T. E.; Robbins, G. L.; Silverthorn, W. E.;

Wickman, H. H. J. Chem. Phys. 1977, 67, 5275; Grow, J. M.; Robbins, G. L.;

Wickman, H. H. J. Chem. Phys. 1977, 67, 5282.

(51) Chun, H. P.; Bill, E.; Weyhermller, T.; Wieghardt, K. Inorg. Chem.

2003, 42, 5612.

(52) Gerber, T. I. A.; Kemp, H. J.; Dupreez, J. G. H.; Bandoli, G.; Dolmella,

A. Inorg. Chim. Acta 1992, 202, 191; Anillo, A.; Obesorosete, R.; Lanfranchi, M.;

Tiripicchio, A. J. Organomet. Chem. 1993, 453, 71.

(53) Herebian, D.; Bothe, E.; Neese, F.; Weyhermller, T.; Wieghardt, K. J.

Am. Chem. Soc. 2003, 125, 9116; Bill, E.; Bothe, E.; Chaudhuri, P.; Chlopek, K.;

Herebian, D.; Kokatam, S.; Ray, K.; Weyhermller, T.; Neese, F.; Wieghardt, K.

Chem.-Eur. J. 2005, 11, 204 .

(54) Ghosh, A. K.; Peng, S.-M.; Paul, R. L.; Ward, M. D.; Goswami, S. J.

Chem. Soc.-Dalton Trans. 2001, 336.

32

Chapter 1

(55) Peng, S. M.; Chen, C. T.; Liaw, D. S.; Chen, C. I.; Yu, W. Inorg. Chim.

Acta 1985, 101, L31; Christoph, G. G.; Goedken, V. L. J. Am. Chem. Soc. 1973, 95,

3869.

(56) Balch, A. L.; Holm, R. H. J. Am. Chem. Soc. 1966, 88, 5201.

(57) Warren, L. F. Inorg. Chem. 1977, 16, 2814.

(58) Herebian, D.; Wieghardt, K.; Neese, F. J. Am. Chem. Soc. 2003, 125,

10997.

33

Chapter 1

34

Chapter 2

CHAPTER 2

LIGANDS SYNTHESES

35

Chapter 2

36

Chapter 2

2. Ligands syntheses

For the purpose of the work described in this dissertation, three ligands have

been synthesized: N-[3,5-di(tert-butyl)phenyl]-1,2-benzenediamine, N,N-di[3,5-

di(tert-butyl)phenyl]-1,2-benzenediamine and rac-4-(1-phenylpentyl)pyridine. Their

syntheses and spectroscopic characteristics are discussed in the present chapter.

2.1. N-[3,5-di(tert-butyl)phenyl]-1,2-benzenediamine (2LH2)

N-[3,5-di(tert-butyl)phenyl]-1,2-benzenediamine (2LH2) was prepared in two

steps according to the Scheme 2.1.1:

NH

NO2

F

NO2

NH H

NaOH, in DMF

Step 1

NH

NH2

Step 2

HCOONH4, Pd/C

in MeOH

Scheme 2.1.1.

(2LHNO2)

(2LH2)

In the first step N-[3,5di(tert-butyl)phenyl]-N-(2-nitro)aniline* (2LHNO2) has

been synthesized: 3,5-di(tert-butyl)aniline2 was allowed to react with 2-

fluoronitrobenzene under basic conditions (NaOH) in DMF as solvent. A purification

* The compound was obtained in a similar fashion as the nitroanilines synthesized by Kirsch et al.1

37

Chapter 2

by column chromatography over silica (with ethyl acetate/n-hexane, gradient elution

0.1:10 up to 0.4:10 v/v, respectively) and subsequent evaporation of the solvent

yielded an orange solid consisting of 80% of 2LHNO2 and 20% of 2-nitrophenol. The

latter was removed by sublimation under reduced pressure yielding analytically pure

orange solid of 2LHNO2.

In the second step 2LHNO2 was reduced3 with anhydrous ammonium formate

HCOONH4 over a Pd/C catalyst in dry MeOH affording a yellowish solid of 2LH2.

In the IR spectrum (KBr pellet) 2LHNO2 displays one (N-H) stretching band

at 3334 cm-1, three characteristic bands of the tert-butyl CH bonds at 2952, 2904,

2866 cm-1 and additionally a stretching peak at 1258 cm-1 originating from (NO)

bond vibration. 2LH2 exhibits in the IR spectrum (KBr disc) three stretching bands associated with

(NH) vibrations at 3434, 3411, 3348 cm-1.

For both 2LHNO2 and 2LH2 1H (400 MHz) and 13C (100 MHz) NMR spectra have

been measured at room temperature. The spectra are shown in Figures 2.1.1 and 2.1.2.

Tables 2.1.1 and 2.1.2 summarize the spectral data. The assignment of peaks was

done on the basis of 2D correlation experiments (not discussed).

012345678910

6.66 .87.07.27.47 .67.88 .08 .2

[ppm ]

11 9

38

5

10

[ppm]

CD2Cl2

1

NHO

20406080100120140160

4

7

6

9

12

11

3

5

10

[ppm]

CD2Cl2

1

28

Figure 2.1.1. 1H (left) and 13C NMR (right) of 2LHNO2 in CD2Cl2.

The following 2D experiments were performed: 1H,1H-COSY (Correlation Spectroscopy), 1H,13C-

HMQC (Heteronuclear Multiple-Quantum Coherence experiment) and 1H,13C-HMBC (Heteronuclear

Multiple Bond Correlation experiment).

38

Chapter 2

123456789

6.76.86.97.07.17.2

[ppm]

11 9

3

8 10

5

NH2

[ppm]

CD2Cl2 NH

1

20406080100120140160

4

67

12

9

11

10

83

5

2

1

CD2Cl2

[ppm]

Figure 2.1.2. 1H (left) and 13C NMR (right) of 2LHNH2 in CD2Cl2.

As expected, 2LHNO2 displays only one broad NH peak at 9.50 ppm in the 1H NMR

spectrum. Such a downfield chemical shift may be caused by intramolecular hydrogen

bond between NO2 and NH groups. The integrated relative area under this peak

corresponds to one proton. On the other hand, 2LH2 shows two broad peaks at 3.84

and 5.32 ppm. The area under the former corresponds to two protons and is assigned

as the NH2 signal whereas the latter has a half intensity of the former and arises from

the NH proton. The positions of hydrogen atoms 9 and 11 (Scheme 2.1.2) of 2LHNO2 are more deshielded in comparison to that of 2LH2 which can be explained

by the presence of the strong electron withdrawing nitro group in the former

compound.

Table 2.1.1. 1H NMR chemical shifts (in ppm) of 2LHNO2 and 2LH2, in CD2Cl2, at 300 K Hydrogen

atom

1 NH2 NH 3 5 8 9 10 11

2LHNO2 s,

1.35

s,

9.50as,

7.34

s,

7.15

d,

7.22

(8.50)b

t,

7.38

(6.47)b

t,

6.76

(5.00)b

d,

8.19

(9.34)b

2LH2 s,

1.38

s,

3.84

s,

5.32

s,

7.02

s,

6.76

d,

6.88

(7.80)b

t,

7.04

(8.08)b

t,

6.82

(7.60)b

d,

7.20

(7.80)b

a) Hydrogen bonded NH group. b) Proton-proton coupling constant (in Hz).

39

Chapter 2

The peaks of carbon atoms 3-12 of both compounds are deshielded signals of

aromatic rings.

NH

R

121

14

3

56

712

8

11

9

10

2

4

5

11

1

R = NO2 for 2LNO2 NH2 for 2LNH2

Scheme 2.1.2. Numbering of the carbon and hydrogen atoms used in Tables 2.1.1 and 2.1.2.

Table 2.1.2. 13C NMR chemical shifts (in ppm) of 2LHNO2 and 2LH2, in CD2Cl2, at 300 K Carbon

atom

1 2 3 4 5 6 7 8

2LHNO2 31.52 35.28 120.25 152.98 119.33 138.37 144.08 116.51 2LH2 31.53 35.13 114.33 153.31 110.86 144.71 141.46 116.43

Carbon

atom

9 10 11 12

2LHNO2 136.01 117.37 126.84 133.34 2LH2 124.87 119.34 123.21 131.94

40

Chapter 2

2.2. N,N-di[3,5-di(tert-butyl)phenyl]-1,2-benzenediamine (3LH2)

The synthesis of N,N-di[3,5-di(tert-butyl)phenyl]-1,2-benzenediamine was

carried out according to the Buchwald-Hartwig coupling4 in an one-step reaction

from 1,2-dibromobenzene and 3,5-di(tert-butyl)aniline2 in dry toluene under basic

conditions (Scheme 2.2.1).

NHBr

Br

NH H

rac-BINAP, Pd2(dba)3, NaO-tBuin toluene

Scheme 2.2.1.

NH

This procedure afforded a dark brown oil upon evaporation of the solvent. The oil was

purified by flash chromatography on silica (gradient elution with n-heptane/CH2Cl2,

7 : 0 up to 7 : 2 v/v, respectively). The desired product 3LH2 was isolated as a white

solid (Rf = 0.16 in n-heptane/CH2Cl2 7:1 v/v). 3LH2 displays two stretching frequencies at 3374 and 3338 cm-1 in the IR

spectrum (KBr disc), which arise from NH vibrations, and a characteristic pattern of

three modes at 2961, 2901, 2864 cm-1 originating from CH vibrations of the tert-

butyl groups. 3LH2 exhibits well-resolved signals in the 1H (400 MHz) and 13C (100 MHz)

NMR spectra (at room temperature, recorded in CD2Cl2; Figure 2.2.1). On the basis of

2D (1H,1H-COSY, 1H,13C-HMQC and 1H,13C-HMBC) correlation experiments (not

discussed) it was possible to assign the signals to the appropriate hydrogen and carbon

atoms (Table 2.2.1, Scheme 2.2.2).

The Buchwald-Hartwig coupling is a crosscoupling of aryl halides and amines.

41

Chapter 2

The compound displays a broad signal at 5.67 ppm in the 1H NMR spectrum which is

assigned to the NH proton. The chemical shift of 1.29 ppm is assigned to the protons

of the tert-butyl groups. The high-field peaks at 31.51 and 34.74 ppm in the 13C NMR

spectrum arise also from the carbon atoms of this group. The positions of the other

signals in the 13C NMR spectrum correspond to the deshielded aromatic carbon atoms

(Table 2.2.1).

12345678

6.76.86.97.07.17.27.3

[ppm]

5.635.645.655.665.675.685.695.70

[ppm]

54

6

3

NH

[ppm]

1

H2O

CD2Cl2

406080100120140

9 8 7 6 5 4

3

2CD2Cl2

[ppm]

1

Figure 2.2.1. 1H (left) and 13C (right) NMR spectra of 3LH2 in CD2Cl2.

NNH

6

5

7

H

3

3

9

9

48

11

12

1

11

2

Scheme 2.2.2. Numbering of the carbon and hydrogen atoms used in Tables 2.2.1 and Figure 2.2.1.

42

Chapter 2

Table 2.2.1. 1H and 13C NMR chemical shifts (in ppm) of 3LH2, in CD2Cl2, at 300 K Atom 1 2 NH 3 4 5 6

1H s, 1.29 s, 5.67 d, 6.83

(1.67)at, 6.99

(1.67)add, 7.27

(3.56;2.31)add, 6.96

(3.49;2.47)a

13C 31.15 34.74 111.91 114.94 119.53 122.35

Atom 7 8 9 1H 13C 135.22 143.12 151.93

a) Proton-proton coupling constants (in Hz).

2.3. Rac-4-(1-phenylpentyl)pyridine (BuPhCH-py)

Rac-4-(1-phenylpentyl)pyridine (BuPhCH-py) was prepared in a similar

fashion to the 1955 patent of the Schering Corporation.5 No spectroscopic

characteristics of BuPhCH-py had been published in the patent and it was

characterized only by its boiling point.

4-Benzylpyridine was deprotonated with metallic Na in liquid NH3 and then it was

allowed to react with n-butyl bromide (Scheme 2.3.1).

NN

Na, NH3 (liquid)

n-C4H9Br

Scheme 2.3.1.

The compound BuPhCH-py was purified by fractional distillation under reduced

pressure. This procedure afforded yellowish oily BuPhCH-py.

43

Chapter 2

The compound displays five intense stretching modes in the range 3062 - 2860 cm-1 in

the IR spectrum. They originate from CH-bond vibrations of the alkyl chain.

BuPhCH-py exhibits nine sets of multiplets in the 1H NMR spectrum (400 MHz) and

twelve peaks in the 13C (100 MHz) NMR spectrum (both taken in CD2Cl2, see Figure

2.3.1). On the basis of 2D experiments (1H,1H-COSY, 1H,13C-HMQC and 1H,13C-

HMBC, not discussed) the signals were assigned to the appropriate hydrogen and

carbon atoms (Table 2.3.1, Scheme 2.3.2).

123456789

7.107.157.207.257.307.357.40

[ppm]1.01.21.41.61.82.02.2

[ppm]

98 + 10

6

7

CD2Cl2 5

43

2

[ppm]

1

20406080100120140160

11

12

9

8

10

7

6

CD2Cl25 4

32

[ppm]

1

Figure 2.3.1. 1H (left) and 13C (right) NMR spectra of BuPhCH-py in CD2Cl2.

N 6

7

11

5

4

3

2

1

12

8

9

10

Scheme 2.3.2. Numbering of the carbon and hydrogen atoms used in Tables 2.3.1 and Figure 2.3.1.

44

Chapter 2

Five multiplets have been observed between 0.5 - 4 ppm in the 1H NMR spectrum of

the compound: at 0.89 (triplet), 1.37 (multiplet), 1.25 (multiplet), 2.06 (quartet) and at

3.90 ppm (triplet). These signals have been assigned to the alkyl chain protons 1 - 5,

respectively. The downfield peaks are the signals of the aromatic protons 6 10

(Scheme 3.3.2).

Highly shielded signals between 14 and 52 ppm in the 13C NMR spectrum have been

assigned to the alkyl carbon atoms 1 5 (Table 5.3.1). The deshielded signals

between 120 and 160 ppm are due to the aromatic carbon atoms 6 - 12. Table 2.3.1. 1H and 13C NMR chemical shifts (in ppm) of 3LH2, in CD2Cl2, at 300 K

a) Proton-proton coupling constants (in Hz).

Atom 1 2 3 4 5 6 7 1H t, 0.89

(7.25)am, 1.37 m, 1.25 q, 2.06

(7.75)at, 3.90

(7.79)add, 8.47

(2.86;1.62)add, 7.18

(3.11;1.54)a

13C 14.10 22.98 30.41 35.00 51.17 150.19 123.54

Atom 8 10 9 11 12 1H m,

7.20-7.28

m, 7.31

13C 128.25 126.91 128.96 154.51 144.10

45

Chapter 2

2.4. References

(1) Kirsch, P.; Schnlebenjanas, A.; Schirmer, R. H. Liebigs Ann. 1995,

1275.

(2) Burgers, J.; Vanhartingsveldt, W.; Vankeulen, J.; Verkade, P. E.;

Visser, H.; Wepster, B. M. Recl. Trav. Chim. Pays-Bas-J. Roy. Neth. Chem. Soc.

1956, 75, 1327.

(3) Ram, S.; Ehrenkaufer, R. E. Tetrahedron Lett. 1984, 25, 3415.

(4) Alcazar-Roman, L. M.; Hartwig, J. F.; Rheingold, A. L.; Liable-Sands,

L. M.; Guzei, I. A. J. Am. Chem. Soc. 2000, 122, 4618; Hartwig, J. F. Angew. Chem.,

Int. Ed. 1998, 37, 2047; Rivas, F. M.; Riaz, U.; Diver, S. T. Tetrahedron: Asymmetry

2000, 11, 1703; Wolfe, J. P.; Buchwald, S. L. J. Org. Chem. 2000, 65, 1144; Yang, B.

H.; Buchwald, S. L. J. Organomet. Chem. 1999, 576, 125.

(5) Sperber, N.; Papa, D.; Bloomfield, N. J. Schering Corp.; US 2727895:

USA, 1955.

46

Chapter 3

CHAPTER 3

SQUARE PYRAMIDAL IRON COMPLEXES

CONTAINING o-DIIMINOBENZOSEMIQUINONATE

RADICALS

47

Chapter 3

48

Chapter 3

3. Introduction

Iron chemistry of o-phenylenediamine-derived ligands was feebly explored in

the past probably due to the high air- and moisture-sensitivity of the complexes.

According to the Cambridge Crystallographic Data Centre only two iron compounds

with the deprotonated form of the discussed ligand have been structurally

characterized, both contain o-diiminobenzoquinones coordinated to low spin iron(II)

in octahedral geometry.1 Balch and Holm, who have synthesized cobalt, nickel,

palladium and platinum complexes with o-phenylenediamine-derived ligands, were

unsuccessful in the synthesis of their iron analogues.2

In 1977, L. F. Warren reported3 the synthesis of iron complexes with

o-diiminobenzoquinone derived ligands, namely [Fe{o-C6H4(NH)2}2I] and [Fe{o-

C6H4(NH)2}2(PPh3)]PF6. A doublet ground state has been assigned for the complexes

on the basis of magnetic susceptibility measurements and the X-band EPR

spectroscopy. At that time, the electronic structure of the iron and of the ligands had

not been explicitly addressed, nor had their molecular structures been determined.

Thus, so far the clear experimental evidence of the existence of iron ions with

N,N'-coordinated o-diiminobenzosemiquinonate(1-) radicals particularly with

iron(III) is lacking.

As it will be shown below, these radicals stabilize a quartet ground state at the

iron ion due to their significant acceptor character.4 Note that square pyramidal

intermediate spin iron(III) complexes with non-macrocyclic nitrogen-donor set have

never been investigated up to date.

NH

NH2

NH

NH2

1LH2 2LH2

Figure 3.0. Structures of the ligands employed in the experimental work together with their abbreviations.

49

Chapter 3

Therefore the aim of this work was to synthesize and fully characterize iron

complexes with o-phenylenediamine-derived ligands, especially with very unstable

o-diiminobenzosemiquinonate(1-) radicals. Two types of the diamines were utilized

in the work described in the chapter to achieve this aim (Figure 3.0): a commercially

available N-phenyl-1,2-benzenediamine (1LH2) and its new bulky derivative N-[3,5-

di(tert-butyl)phenyl]-1,2-benzenediamine (2LH2).

50

Chapter 3

3.1. [Fe(1LISQ)(1LPDI)]2 (1) and [Fe(2LISQ)(2LPDI)]2 (2)

Synthesis

Complexes of iron with N-phenyl-1,2-benzenediamine (1LH2) or N1-[3,5-

di(tert-butyl)phenyl]-1,2-benzenediamine (2LH2) are obtained in the reaction of two

equivalents of 1LH2 or 2LH2 and one equivalent of [Fe(dmf)6](ClO4)35 in the presence

of triethylamine in dry acetonitrile under strictly anaerobic conditions. Dark violet

complexes [Fe(1LISQ)(1LPDI)]2 (1) and [Fe(2LISQ)(2LPDI)]2 (2) can be isolated in a pure

form as black precipitates in yields of 40 - 50%. It is noteworthy that an excess of the

ligand has no influence on the composition of the final product and octahedral iron

compounds containing three of the discussed ligands have not been prepared by this

method.

Other iron(III) salts like Fe(acac)3, FeCl3, Fe(ClO4)39H2O or iron(II) salts like

Fe(OOCCH3)2, FeCl24H2O, FeBr2, Fe(ClO4)26H2O do not react with 1LH2 or 2LH2 in

MeCN. Additionally, even [Fe(dmf)6](ClO4)3 does not react with 1LH2 if methanol is

used as the solvent. Thus, for complexation of iron by 1LH2 or 2LH2 under basic

conditions it appears that an easily displaced ligand must be employed in the starting

metal complex. Hence, the combination of [Fe(dmf)6](ClO4)3 and MeCN was found to

be ideal to obtain iron complexes with the deprotonated N-phenyl-1,2-

benzenediamine. Furthermore, the reaction of Fe3+ ion with 1LH2 or 2LH2 must be

carried out under strictly anaerobic and dry conditions.

The final dark violet products 1 and 2 are sensitive to oxygen and moisture in solution

as well as in solid state. The reaction of [Fe(dmf)6](ClO4)3 with 1LH2 in acetonitrile in

the presence of Et3N with traces of air affords immediately a condensed

benzoquinonate product (Figure 3.1.1).

N

N

NH2

NHPh Ph

ClO4-+

Figure 3.1.1. Product of the reaction of 1LH2 and

[Fe(dmf)6](ClO4)3 salt in the presence of air (Diels-Alderreaction); the structure is known from the single-crystal X-rayexperiment.

51

Chapter 3

Results and discussion

The elemental analysis of 1 and 2 confirmed the metal to ligand ratio as 1 : 2

and MS-EI and ESI (positive ion) yielded molecular ion peaks characteristic for

monomers (420 and 644 for 1 and 2, respectively). Both compounds are diamagnetic

and exhibit irreversible redox behavior in their cyclic voltammograms.

400 600 800 10000.0

0.4

0.8

1.2

1.6

1

[1

04 M

-1 c

m-1]

[nm]

2

Figure 3.1.2. Electronic spectra of 1 and 2 in toluene.

The electronic spectra of 1 and 2 were recorded in toluene solution at room

temperature (Figure 3.1.2.). Both compounds display very intense absorptions in the

visible range at 500 (1) and 504 (2) nm (Table 3.1.1).

Table 3.1.1. Electronic spectra of 1 and 2

Complex a max [nm], (b [104 M-1cm-1])

1 500 (1.02), 647 (0.68), 774 sh (0.44)

2 424 (0.77), 504 (1.30), 662 (0.67), 790 sh

(0.45)

a) Strictly under anaerobic conditions in toluene, at RT. b) Extinction coefficient calculated per metal ion.

The maxima at ~ 500 nm likely originate from ligand to metal charge transfer

transitions (LMCT),6 whereas the shoulders above 770 nm may arise from

intervalence-charge transfer transitions (IVCT).7 No bands above 1100 nm were

found.

52

Chapter 3

Note that the electronic spectra of compounds 1 and 2 closely resemble those

of similar complexes A and B characterized in reference 8. A and B were structurally

characterized by X-ray crystallography. Both are dimeric species (Figure 3.1.3). The

overall formula of compound A and B is [FeIII(LIP)(LISQ)]2. Each iron atom is in a

square pyramidal environment and the in-plane core contains one closed-shell N,S-

coordinated aminothiophenolate dianion (LIP)2- and one open-shell N,S-coordinated

radical monoanion (LISQ)1- (where (LISQ)1- represents here o-iminothiosemiquinonate

radical).

A B

S

NH

HN

S

FeIII

S

NH

HN

S

FeIII

.

.

S

N

S

N

FeIII.

H

S

N

S

N

.

H

FeIII

H H

Figure 3.1.3. Structures of compounds A and Bfrom Ref. 8.

Complexes A and B possess a diamagnetic ground state similar to the case of 1 and 2.

In UV-Vis spectra they display intense peaks at ~ 500 nm which were assigned as

LMCT transitions.8 Figure 3.1.4. Zero-fieldMssbauer spectra of 1 and 2.

-4 -3 -2 -1 0 1 2 3 4

0.88

0.92

0.96

1.00

0.94

0.96

0.98

1.00

Velocity [mm s-1]

1

Rel

ativ

e Tr

ansm

issi

on 2

Zero-field Mssbauer spectra of solid

samples of 1 and 2 were recorded at 80 K. These

results are listed in Table 3.1.2 and the spectra are

shown in Figure 3.1.4.

Both of the complexes show similar Mssbauer

spectra each consisting of one doublet. The

isomer shifts are the same for 1 and 2 (0.22 mm

s-1). Thus, the spin states of the iron in these

compounds are expected to be identical. Large

quadruple splitting Q values imply a large

53