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Molecular and electronic structure of square-planar nickel(II),nickel(III) and nickel(III) p-cation radical complexes with atetradentate o-phenylenedioxamidate redox-active ligand†‡

Rosa Carrasco,a Joan Cano,*a,b Xavier Ottenwaelder,c Ally Aukauloo,c Yves Journauxc andRafael Ruiz-Garcıa*d

a Departament de Quımica Inorganica, Centre de Recerca en Quımica Teorica (CERQT),Universitat de Barcelona, 08028, Barcelona, Spain. E-mail: joan.cano@qi.ub.es;Fax: 34 934907725; Tel: 34 934021270

b Institucio Catalana de Recerca i Estudis Avancats (ICREA), 08010, Barcelona, Spainc Laboratoire de Chimie Inorganique, UMR 8613, Universite Paris-Sud, 91405, Orsay, Franced Departament de Quımica Organica, Universitat de Valencia, 46100, Burjassot, Valencia,

Spain. E-mail: rafael.ruiz@uv.es; Fax: +34 963544328; Tel: +34 963544510

Received 17th February 2005, Accepted 6th June 2005First published as an Advance Article on the web 27th June 2005

The molecular and electronic structures of the electron transfer series of four-coordinate square-planar nickel complexeswith the ligand o-phenylenebis(N ′-methyloxamidate), [NiL]z (z = 2−, 1−, 0), have been evaluated by DFT and TDDFTcalculations, and most of their experimentally available structural and spectroscopic properties (X. Ottenwaelder et al.,Dalton Trans., 2005, DOI: 10.1039/b502478a) have been reasonably reproduced at the B3LYP level of theory. Theanionic species [NiL]2− and [NiL]− are genuine low-spin nickel(II) and nickel(III) complexes with diamagnetic singlet(S = 0) and paramagnetic doublet (S = 1/2) states, respectively. The nickel(III) complex presents shorter Ni–N(amidate)bond distances (1.85–1.90 A) than the parent nickel(II) complex (1.88–1.93 A) and characteristic LMCT bands in theNIR region (kmax = 794 and 829 nm) while the analogous MLCT bands for the nickel(II) complex are in the UV region(kmax = 346 and 349 nm). The neutral species [NiL] is a nickel(III) o-benzosemiquinonediimine p-cation radical complexwith a diamagnetic singlet (S = 0) and a paramagnetic triplet (S = 1) states fairly close in energy but fundamentallydifferent in orbital configuration. The singlet metal–radical ground state results from the antiferromagneticcoupling between the 3dyz orbital of the NiIII ion (SM = 1/2) and the pb orbital of the benzosemiquinone-typeradical ligand (SL = 1/2), which have a large overlap and thus strong covalent bonding. The triplet metal–radicalexcited state involves the ferromagnetic coupling between the NiIII 3dzx orbital and the benzosemiquinone-typepb orbital, which are orthogonal to each other. The singlet and triplet states of the nickel(III) p-cation radicalcomplex possess characteristic quinoid-type short–long–short alternating sequence of C–C bonds in the benzenering, as well as intense MLCT transitions in the VIS (kmax = 664 nm) and NIR (kmax = 884 nm) regions, respectively.

IntroductionThe chemistry of nickel complexes with redox-active ligandsrepresents one of the more promising developments of nickelcoordination chemistry with their potential applications in thearea of nanotechnology.1 Due to their peculiar physical prop-erties, there is a rapidly growing interest in ligand redox-activetransition metal complexes as building blocks for new magneticand conducting molecular materials.2,3 The development of thisclass of compounds relies on a firm understanding of the factorsthat influence the redox innocence or non-innocence of theligands in describing of their molecular and electronic structure,particularly the assignment of the oxidation state of the metalcenter in high-valent transition metal complexes.4 When thecoordinated ligand contains an extended p-conjugated bondsystem, the redox properties of the metal complex may involveboth the metal ion and the organic ligand. Nickel dioxolene andrelated nickel dithiolene chemistries offer well-known examplesof high-valent nickel complexes with redox-active ligands.5,6 The

† High-valent nickel oxamides. Part 2. Part 1 is ref. 11b.‡ Electronic supplementary information (ESI) available: Tables S1–S3: Selected MO energy and composition data for [NiL]− (2B), [NiL](1A) and [NiL] (3B). Tables S4–S6: Selected electronic spectroscopicdata for [NiL]− (2B), [NiL] (1A) and [NiL] (3B). Fig. S1: Optimizedbond lengths and angles for [NiL]2− (1A), [NiL]− (2B), [NiL] (1A)and [NiL] (3B). Fig. S2: Atomic charges and atomic spin densi-ties for [NiL]2− (1A), [NiL]− (2B) , [NiL] (1A) and [NiL] (3B). Seehttp://dx.doi.org/10.1039/b502481a

characteristic ligand-centered redox reactions among the threecatecholate-, semiquinonate- and quinone-type ligand oxidationstates combine with three possible metal-centered redox reac-tions involving divalent, trivalent and tetravalent metal oxida-tion states. Nickel complexes of o-phenylenediamine derivativesand their oxygen- and sulfur-monosubstituted analogues, o-aminophenolate and o-aminothiophenolate, respectively, alsoshare the same complex yet fantastic redox interplay betweenmetal and ligand.7–9

The redox non-innocence of the simple o-phenylenediamine,C6H4(NH2)2, when coordinating to the later 3d metal ionshas been a matter of debate since the discovery by Feigl andFurth of the square-planar neutral nickel complex with twobidentate N,N ′-coordinated o-phenylenediamine ligands.7a Thiscompound was originally formulated as a nickel(IV) complexof two o-benzenediamidate dianions, with a low-spin NiIV elec-tronic configuration consistent with the observed diamagnetism.Its molecular and electronic structure description has beenrevisited by Gray, who alternatively concluded on a diamagneticnickel(II) species with a singlet diradical character resultingfrom strong intramolecular antiferromagnetic coupling betweentwo o-benzosemiquinonediimine p-cation radical monoanionsthrough the low-spin NiII ion,7b as subsequently confirmedby Wieghardt and co-workers with the aid of theoreticalcalculations.7e,g In this regard, an investigation to assess the pos-sible use of non-innocent aromatic dioxalamide ligands derivedfrom o-phenylenediamine in the stabilization of unusual high-valent transition metal complexes of the late 3d metals, especiallyD

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T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5 D a l t o n T r a n s . , 2 0 0 5 , 2 5 2 7 – 2 5 3 8 2 5 2 7

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resistant to oxidative degradation, has been initiated by some ofus recently.10–12 Our goal is to obtain a further source of stablemetal compounds in high oxidation states, whereby not only themetal center but also the ligand can act as electron reservoir.

We have studied a new family of nickel complexes withredox-active ligands that were obtained through ligand designby aromatic functionalization of the parent tetradentate o-phenylenebis(oxamate) ligand.11a As the hydrogen atoms in the4 and 5 positions of the benzene ring are systematically replacedby electron-donating methyl, methoxy and dimethylaminogroups along this series, the redox properties of the square-planar diamagnetic nickel(II) complexes switch continuouslyfrom a metal- to a ligand-centered one-electron oxidation, asrevealed by EPR spectroscopy and DFT calculations.11a Onthe other hand, we have found that the related square-planardianionic nickel(II) complex with the tetradentate ligand o-phenylenebis(N ′-methyloxamidate) (L) undergoes not only onebut two successive one-electron oxidations to give the corre-sponding monoanionic and neutral oxidized complexes.11b Sincethis ligand also contains an o-phenylenediamidate fragment withpotential p-non-innocent character, these oxidation processesmay involve the metal ion as well as the ligand. Thus, theelectronic distribution in a given oxidized nickel species ofthis electron transfer series, [NiL]z (z = 2−, 1−, 0), can bedescribed by any one of the three redox isomers depicted inScheme 1. The formal oxidation state of the metal centerfor each redox isomer (valence isomer) is different as muchas the redox state of the ligand passes through the benzene,benzosemiquinone and benzoquinone forms. Our experimentalobservations indicate that the first oxidation process is clearlymetal-centered and the resulting monooxidized species [NiL]− isa genuine NiIII-o-benzenediamidate complex [Scheme 1(a)].11b

In contrast, the second oxidation process is mainly ligand-centered and the resulting dioxidized species [NiL] is likelya NiIII-o-benzosemiquinonediimine p-cation radical complex[Scheme 1(b)]. Yet, from the limited available experimen-tal data, the alternative NiIV-o-benzenediamidate and NiII-o-benzoquinonediimine descriptions [Scheme 1(a) and (c), respec-tively] can not be definitively ruled out.11b

Scheme 1 Redox isomers of the o-phenylenediamidate group in metalcomplexes.

We report here a computational study on the molecular andelectronic structure of this electron-transfer series of square-

planar nickel complexes with the tetradentate redox-active lig-and o-phenylenebis(N ′-methyloxamidate). The geometries andmolecular orbital descriptions of the lower valent nickel(II) com-plex and the corresponding high-valent nickel(III) and nickel(III)p-cation radical complexes have been addressed through DFTcalculations at the B3LYP level of theory. Most of theirexperimentally available electronic spectroscopic properties havebeen reasonably reproduced through use of the time-dependentformalism applied to DFT (TDDFT). We have paid particularattention to solving the ambiguity in the electronic structuredescription of the neutral metalloradical species. Both a singletand a triplet spin states, resulting from intramolecular magneticcoupling between the metal center and the radical ligand, arepossible descriptions of the ground electronic state. This workand the previous one in this series11b constitute a nice example ofcomplementarity between experimental and theoretical studies,and offer a unique opportunity to test the confidence in quantumchemical DFT methodology to determine the structure andbonding of transition metal complexes, both from a qualitativeand a quantitative points of view.13

Results and discussionEnergy calculations

DFT calculations on the energetics of the nickel complexes[NiL]z (z = 2−, 1−, 0) are summarized in Table 1. The geometriesof the dianionic complex [NiL]2−, with a closed-shell singlet (S =0) electronic state (1A), and the monoanionic complex [NiL]−,with an open-shell doublet (S = 1/2) electronic state (2B), wereoptimized with the C2 symmetry constraint by using the B3LYPfunctional and considering spin-restricted (RB3LYP) and spin-unrestricted (UB3LYP) wave functions, respectively. Two sets ofgeometry optimizations were undertaken on the neutral complex[NiL] for each one of the two low-lying, nearly degenerate singlet(S = 0) and triplet (S = 1) electronic states (1A and 3B) by usingRB3LYP and UB3LYP wave functions, respectively. In addition,a broken symmetry (BS) UB3LYP solution was obtained forthe singlet state of this neutral complex, [NiL] (1A), which is59.9 meV lower in energy than the RB3LYP solution. The com-putational instability of the spin-restricted solution favors anopen-shell description over the closed-shell one, according to awell-known criterion discussed elsewhere for singlet diradicals.7e

The final singlet–triplet UB3LYP energy difference (DEST) of8.2 meV (66.1 cm−1) is small but non-negligible, with the singletas the ground state. The potential energy minimum and thefirst vibrational level of the various optimized structures for theneutral complex [NiL] are depicted in Scheme 2.

DFT energy calculations clearly indicate a small but sig-nificant stabilization in terms of both standard enthalpy andfree energy on going from the dianionic complex [NiL]2− tothe monoanionic complex [NiL]− (Table 1). Instead, a largedestabilization is observed for both singlet and triplet statesof the neutral complex [NiL] compared to the monoanioniccomplex [NiL]− (Table 1). Thus, the corresponding gain in freeenergy upon the first oxidation (DG1

◦) is 6.8 kcal mol−1, while theloss in free energy for the second oxidation (DG2

◦) is as high as

Table 1 Energy data for [NiL]z (z = 2−, 1−, 0)

Complex Point group S Notation Method Ea/au Ezpb/au

H◦c/kcalmol−1

S◦d/calmol−1 K−1

G◦e/kcalmol−1

[NiL]2− C2 0 1A RB3LYP −2490.7400 −2490.5220 −315.7 138.0 −356.9[NiL]− C2 1/2 2B UB3LYP −2490.7536 −2490.5341 −324.0 133.3 −363.7[NiL] C2 0 1A RB3LYP −2490.6022 −2490.3820 −228.4 133.8 −268.3

UB3LYP −2490.6044 −2490.3855 −230.4 134.1 −270.4[NiL] C2 1 3B UB3LYP −2490.6041 −2490.3841 −229.9 132.7 −269.5

a E is the total energy of the potential energy minimum (1 au = 27.2113845 eV). b Ezp is the vibrational zero-point energy. c H◦ is the standard enthalpy.d S◦ is the standard entropy. e G◦ is the standard free energy.

2 5 2 8 D a l t o n T r a n s . , 2 0 0 5 , 2 5 2 7 – 2 5 3 8

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Scheme 2 Potential energy curves of the singlet (a) and triplet (b)electronic states for [NiL]. The dashed line corresponds to the RB3LYPsolution.

93.3 kcal mol−1 (Table 1). On the other hand, the correspondingsinglet–triplet standard free energy difference (DGST

◦) for theneutral complex [NiL] clearly identifies the singlet as the groundstate, with the triplet excited state being 0.9 kcal mol−1 higher(Table 1). The molar fraction of high-spin triplet molecules (x)and that of low-spin singlet molecules (1 − x) for an assembly ofmolecules in gas phase, with no intermolecular interaction, canbe easily obtained by assuming a simple Boltzmann distributionof molecules between the ground singlet and triplet excited states

according to x = [1 + exp(DGST/RT)]−1, where R is the gasconstant of 1.987 cal mol−1.14 At 298 K, the calculated molarfraction of molecules in the excited high-spin triplet state is 17%,while at 50 K practically all the molecules are in the ground low-spin singlet state.

Molecular geometry optimizations

DFT optimized geometries for the nickel complexes [NiL]z (z =2−, 1−, 0), with C2 point group symmetry, indicate more orless distorted square-planar metal coordination environments(Fig. S1, ESI‡). Selected calculated structural data for the titledcomplexes are summarized in Table 2.

Molecular structure of [NiL]2−. The optimized geometry forthe metal environment and the ligand skeleton in the gas phaseof the dianionic nickel(II) complex, [NiL]2− (1A), is in goodagreement with that experimentally determined in the solid stateby single-crystal X-ray diffraction.11b The geometry calculationsreproduce the two distinct metal–nitrogen amidate bond lengthscorresponding to the N-aryl- and N-alkylamidates (1.88 and1.93 A, respectively) (Table 2), yet they are slightly longerthan those observed experimentally (average values of 1.84and 1.87 A, respectively).11b The deviations observed betweencalculated and experimental Ni–N distances are less than 0.06 A,whereas the estimated error for the experimental bond distancesis 0.02 A (within the 3r criterion). The pattern of three fused five-membered chelate rings imposes a severe trapezoidal distortionof the ideal square-planar metal core: three of the calculatedbond angles at the metal are roughly equal and close to 90◦

(83.7–84.9◦), whereas the fourth one, being less constrained,

Table 2 Selected structural data (A,◦) of the optimized geometries for [NiL]z (z = 2−, 1−, 0)

[NiL]2− (1A) [NiL]− (2B) [NiL] (1A) [NiL] (3B)

Metal environment

Ni–N1 1.878 1.853 1.850 1.875Ni–N2 1.932 1.896 1.882 1.860N1–N2 2.543 2.534 2.531 2.532N1–N1′ 2.534 2.484 2.462 2.490N2–N2′ 3.120 3.022 3.000 2.974

N1–Ni–N2 83.7 85.1 85.4 85.3N1–Ni–N1′ 84.9 84.2 83.4 83.2N2–Ni–N2′ 107.7 105.7 105.7 106.1

sa 0.20 0.07 0.02 0.03

Ligand skeleton

C1–N1 1.360 1.380 1.413 1.385C1–O1 1.268 1.251 1.237 1.243C2–N2 1.363 1.377 1.388 1.400C2–O2 1.272 1.257 1.248 1.244C3–N1 1.402 1.393 1.356 1.378C3–C3′ 1.438 1.434 1.460 1.444C3–C4 1.411 1.415 1.431 1.420C4–C5 1.412 1.401 1.384 1.392C5–C5′ 1.406 1.414 1.438 1.426

a s is the dihedral angle between the N1NiN2 and N1′ NiN2′ planes.

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is appreciably larger (107.7◦), as experimentally observed forthe nickel(II) complex (bite angles in the range 83.3–84.7◦ and108.8◦, respectively).11b This is a common feature of the seriesof square-planar nickel complexes [NiL]z (z = 2−, 1−, 0) witha 5–5–5 chelating ring system of the open-chain tetradentateligand. The largest deviation from planarity is found precisely forcomplex [NiL]2−, which exhibits a slight tetrahedral distortionfrom the ideal square-planar coordination environment. Thecalculated value of the dihedral angle between the two oxamidatechelating donor sets (s = 0.2◦) is, however, somewhat smallerthan that experimentally observed for the nickel(II) complex(s = 1.7◦).11b

Molecular structure of [NiL]−. The optimized geometry forthe metal environment of the monoanionic nickel(III) complex,[NiL]− (2B), shows two Ni–N distances (1.85 and 1.90 A)consistently shorter by about 0.03 A than the correspondingdistances in the dianionic nickel(II) complex, [NiL]2− (1A), (1.88and 1.93 A) (Table 2). Concomitantly, the carbon–nitrogenamidate and carbon–oxygen carbonyl bond lengths in theoxamidate moiety undergo small but significant variations whenincreasing the metal oxidation state from 2+ to 3+ (Table 2).The two C–N distances lengthen by 0.02 A (from 1.36 to 1.38 A)indicating higher single bond character in the monooxidizedspecies, while the C–O distances shorten by 0.02 A (from 1.27to 1.25 A) suggesting higher double bond character instead.These calculated trends have been experimentally observedin earlier studies on the related copper(II) and copper(III)complexes.10a By contrast, there is no appreciable change in thecalculated carbon–carbon and carbon–nitrogen bond lengthsof the o-phenylenediamidate fragment for the pair of anioniccomplexes [NiL]2− and [NiL]− (Table 2). In both cases, theC–N distances (1.40 and 1.39 A, respectively) are those of asingle bond, while the six C–C distances are roughly identical(average values of 1.42 and 1.41 A, respectively), reflectingcomplete p delocalization within the aromatic ring. All thesefeatures are evidence against any ligand-centered oxidation and,hence, unambiguously comfort the NiIII formulation for themonooxidized species [Scheme 1(a)].

Molecular structure of [NiL]. The optimized geometries forthe metal environment of the two lowest singlet (1A) and triplet(3B) spin states of the neutral formally nickel(IV) complex, [NiL],are clearly different. The most noticeable structural difference isfound in the order of the metal–nitrogen amidate bond lengthscorresponding to the N-aryl- and N-alkylamides. They vary

according to Ni–N1 (1.85 A) < Ni–N2 (1.88 A) for 1A, andNi–N2 (1.86 A) < Ni–N1 (1.88 A) for 3B (Table 2). Yet, theaverage Ni–N distance is almost identical for both electronicstates (1.87 A), and comparable to that of the correspondingmonooxidized nickel(III) complex (1.88 A). The decrease byonly 0.01 A is consistent not with metal oxidation but ratherwith ligand oxidation, hence ruling out a NiIV formulationfor the dioxidized species [Scheme 1(a)]. For both singlet andtriplet state neutral molecules, [NiL] (1A) and [NiL] (3B), theC–N distances in the o-phenylenediamidate fragment (1.36and 1.38 A, respectively) are intermediate between those ofa single and a double bond (Table 2). In addition, the C–Cdistances in the benzene ring are asymmetric, with two of themremarkably shorter (1.38–1.39 A) than the other four (1.42–1.46 A) following a short–long–short alternating sequence.This situation contrasts with that found in the monoanionicnickel(III) complex, [NiL]− (2B), where the C–N distances aresignificantly longer (1.39 A) and the six C–C distances are almostequivalent (1.40–1.43 A) (Table 2). Overall, the shortening of thearomatic carbon–nitrogen bond lengths and the alternation ofthe aromatic carbon–carbon bond lengths for both the singletand the triplet state of the neutral complex [NiL] are typicalof quinoid species.4 The two limiting cases are representedby the NiIII-o-benzosemiquinonediimine p-cation radical andNiII-o-benzoquinonediimine valence isomers [Scheme 1(b) and(c), respectively]. In this regard, Collins has recently reportedthe preparation and the X-ray structures of stable, formallycobalt(IV) and iron(IV) complexes with a related class of o-phenylenediamidate ligands.15 The experimental structural datafor these complexes also display the aforementioned pattern ofbond length changes for the ligand skeleton, which clearly refutesa 4+ metal oxidation state.

Vibrational frequency analysis

DFT vibrational calculations have been performed on theoptimized structures of the square-planar nickel complexes[NiL]z (z = 2−, 1−, 0) to verify via frequency analysis thenature of the structural modifications associated with the changein oxidation state along this series. In particular, we focus onthe characteristic frequency shifts in carbonyl amide markerbands which should be informative about the structural and/orelectronic perturbations associated with metal and/or ligandoxidation. Selected calculated vibrational data for the titledcomplexes are summarized in Table 3.

Table 3 Vibrational spectroscopic data for [NiL]z (z = 2−, 1−, 0)a

Natureb

Complex Transition Energy/cm−1 Intensity/Km mol−1 m1 m2 m3 m4 Type

[NiL]2− (1A) 1 1649 36 62.6 0.0 37.4 0.0 Aromatic C=O (s)2 1646 622 0.0 97.8 0.0 2.2 Aromatic C=O (as)3 1632 938 31.3 0.0 67.1 0.0 Aliphatic C=O (s)4 1632 0.4 0.0 0.0 0.0 100.0 Aliphatic C=O (as)

[NiL]− (2B) 1 1679 2 60.4 0.0 39.6 0.0 Aromatic C=O (s)2 1671 1199 0.0 74.5 0.0 25.5 Aromatic C=O (as)3 1654 144 44.0 0.0 56.0 0.0 Aliphatic C=O (s)4 1640 296 0.0 19.4 0.0 80.6 Aliphatic C=O (as)

[NiL] (1A) 1 1702 127 53.9 34.2 7.3 4.6 Aromatic C=O (s)2 1697 391 33.4 51.2 5.9 9.5 Aromatic C=O (as)3 1662 156 14.5 0.0 81.2 4.3 Aliphatic C=O (s)4 1648 561 1.3 13.0 2.0 83.7 Aliphatic C=O (as)

[NiL] (3B) 1 1685 8 84.7 0.0 15.3 0.0 Aromatic C=O (s)2 1683 592 0.0 87.2 0.0 12.8 Aromatic C=O (as)3 1655 166 16.1 0.0 82.1 0.0 Aliphatic C=O (s)4 1655 78 0.0 11.4 0.0 88.6 Aliphatic C=O (as)

a m (cm−1) values of the carbonyl amide resonance frequencies. b Percentage composition (%) values of the four elemental vibrational modes.

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The calculations reveal a total of four vibrational transitionscorresponding to the carbonyl amide resonance frequencies,two of them being largely more intense than the other two.This agrees with the occurrence of a pair of C=O absorptionbands in the experimental IR spectrum of the dianionic nickel(II)complex, yet the calculations for [NiL]2− give higher energyvalues (1632 and 1646 cm−1) than the experimental ones (1608and 1620 cm−1) (Table 3).11b The four calculated transitions arecomposed by linear combinations of four elementary vibrationalmodes, namely m1 to m4. They correspond to the symmetric (m1)and antisymmetric (m2) combinations of the aromatic carbonylamide vibrations, and to the symmetric (m3) and antisymmetric(m4) combinations of the aliphatic carbonyl amide vibrations.With the only exception of the BS optimized structure of thesinglet-state neutral molecule, [NiL] (1A) (Table 3), the degreeof mixing is relatively small in such a way that it is possible toassign each main transition to a specific elemental vibrationalmode. For each complex, both symmetric (m1) and antisymmetric(m2) N(aryl)C=O frequencies are higher than the correspondingsymmetric (m3) and antisymmetric (m4) N(alkyl)C=O frequencies.This reflects the shorter C–O bond distances in the aromaticcarbonyl amides than in the aliphatic carbonyl amides (Table 2).

An overall shift to higher energy is calculated for all carbonylamide resonance frequencies along the series of nickel complexes[NiL]z (z = 2−, 1−, 0), as illustrated in Fig. 1. First, the fourC=O frequencies (m1–m4) shift uphill as the metal oxidationstate increases from 2+ to 3+ in the anionic complexes[NiL]2− (1632–1649 cm−1) and [NiL]− (1640–1679 cm−1) (Ta-ble 3), as experimentally observed for the related copper(II)and copper(III) complexes.10a This reflects the shorter C=Obonds in the oxamidate moiety for the monoanionic nickel(III)complex [NiL]− than for the dianionic nickel(II) complex [NiL]2−

(Table 2). The delocalization within the NCO amidate groupof the excess electron density residing on the deprotonatednitrogen becomes limited due to the stronger Ni–N bonds whenincreasing the metal oxidation state (Table 2).11b Secondly, thefour C=O frequencies (m1–m4) further shift uphill as a result ofligand oxidation for both singlet (1A) (1648–1702 cm−1) andtriplet (3B) (1655–1685 cm−1) spin states of the neutral complex[NiL] (Table 3). The shift is, however, most noticeable for thearomatic carbonyl amide resonance frequencies (m1 and m2). Thisamounts to the internal rearrangement of electrons in the o-phenylenediamidate fragment when increasing the contributionof the imine forms, which determines that the lone electronpair of nitrogen is less available for delocalization on the NCOamidate groups. In addition, the two C=O frequencies (m1 andm2) shift downhill from 1A (1697 and 1702 cm−1) to 3B (1683

Fig. 1 Plot of the carbonyl amide resonance frequencies correspondingto the vibrational modes m1 (�), m2 (�), m3 (�), and m4(�) for [NiL]2− (1A)(a), [NiL]− (2B) (b), [NiL] (1A) (c) and [NiL] (3B) (d). The solid lines areguides for the eye (data from Table 3).

and 1685 cm−1) (Table 3). This is likely due to lengthening ofthe Ni–N bonds on going from the singlet to the triplet state(Table 2), which facilitates delocalization of the excess electrondensity residing on the deprotonated nitrogen within the NCOamidate group.11b

Molecular orbital calculations

DFT MO calculations have been performed on the optimizedstructures of the nickel complexes [NiL]z (z = 2−, 1−, 0), withC2 point group symmetry, to correlate with the experimentalelectronic spectroscopic data.11b The energy spectra and the na-ture of the frontier MOs for the titled complexes are qualitativelydepicted in Fig. 2 (Tables S1–S3, ESI‡). The simulated electronicabsorption spectra are displayed in Fig. 3 (Tables S4–S6, ESI‡),and compared to the corresponding experimental spectra inacetonitrile.11b Selected calculated energy and composition dataof the frontier MOs and energy transition data for the dianionicnickel(II) complex [NiL]2−, as a representative example of thisseries of nickel complexes, are summarized in Tables 4 and 5,respectively.

Electronic structure of [NiL]2−. The MO diagram for thesinglet state dianionic complex, [NiL]2− (1A), is typical of alow-spin (S = 0) square-planar NiII species with a ligand oflarge r-field strength and distinctly p donating nature [Fig. 2(a)].The available out-of-plane p-type orbitals of the amidate donorgroups, made up of the 2pz nitrogen and oxygen lone pairs, areresponsible for the overall destabilization of the fully occupiedmetal 3dzx and 3dyz orbitals of p antibonding character [Fig. 2(a),middle left] relative to the fully occupied metal 3dx2−y2 and 3dz2

orbitals of mainly nonbonding character [Fig. 2(a), bottom left](Table 4). The interaction of the metal 3d orbitals with the in-plane r-type orbitals of the amidate donor groups, made upof the 2px and 2py nitrogen lone pairs, occurs primarily withthe metal 3dxy orbital (Table 4), which thus becomes strongly rantibonding and unoccupied [Fig. 2(a), top left]. Nonetheless,the metal 3dxy orbital is not the LUMO, but there are twonearly degenerate unfilled ligand p antibonding orbitals of lowerenergy, pa* and pb* [Fig. 2(a), top right], well separated from thefour lower lying fully occupied metal 3d orbitals. In addition,there exist two unusually high lying, nearly degenerate filledligand p bonding orbitals, pa and pb [Fig. 2(a), bottom right],with a relatively important contribution from the metal 3dzx

and 3dyz orbitals, respectively, compared with that in the relatedligand pa* and pb* orbitals (Table 4). The preferred 3dyz/pb

metal–ligand orbital mixing leads to a 3dyz HOMO with astronger p antibonding character and larger p-conjugation intothe aromatic ring relative to the lower lying metal 3dzx orbital[Fig. 2(a), middle left] (Table 4).

TDDFT calculations provide a reasonable MO interpreta-tion of the experimental electronic absorption spectrum ofthe nickel(II) complex.11b The calculated electronic absorptionspectrum for [NiL]2− reproduces fairly well the experimental onein terms of both transition energies and intensities [Fig. 3(a)] (seecomputational details). The most remarkable features of thecalculated spectrum are the MLCT transitions from the filledmetal 3dyz and 3dzx orbitals to the two lowest-energy unfilledligand pa* and pb* orbitals, respectively, located at 399 and346 nm (Table 5, transitions 5 and 6). The highest energy d–d transition from the lower-lying fully occupied 3dx2−y2 orbitalto the upper empty 3dxy orbital (1B ← 1A transition) at 424 nmis obscured by the above MLCT transitions [Fig. 3(a)] (Table 5,transition 4). The other three lower energy d–d transitions fromthe fully occupied 3dz2 , 3dzx and 3dyz orbitals to the upper empty3dxy orbital (1B ← 1A, 1B ← 1A and 1A ← 1A transitions,respectively) are less intense and appear as a band envelopecentered about 550 nm [inset of Fig. 3(a)] (Table 5, transitions1–3). In addition, there are two IL p–p* transitions from thefilled ligand pa and pb orbitals to the unfilled ligand pa* andpb* orbitals, and one LMCT transition from the filled ligand pa

D a l t o n T r a n s . , 2 0 0 5 , 2 5 2 7 – 2 5 3 8 2 5 3 1

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Tab

le4

Sele

cted

MO

ener

gyan

dco

mpo

siti

onda

tafo

r[N

iL]2−

(1A

)

Nat

urec

MO

aE

nerg

y/au

nb3d

x2−y

2

(Ni)

3dz2

(Ni)

3dzx

(Ni)

3dyz

(Ni)

3dxy

(Ni)

2px

(N1)

2py

(N1)

2pz

(N1)

2px

(N2)

2py

(N2)

2pz

(N2)

2pz

(O1)

2pz

(O2)

2pz

(C1)

2pz

(C2)

2pz

(C3)

2pz

(C4)

2pz

(C5)

Typ

e

75a

−0.0

2744

264

.70.

20.

00.

00.

00.

10.

70.

00.

20.

40.

00.

00.

00.

00.

00.

00.

00.

03d

x2−y

2

81a

0.01

525

20.

00.

08.

00.

00.

00.

00.

012

.40.

00.

09.

45.

84.

80.

20.

00.

18.

74.

0p

a

82b

0.01

538

20.

00.

00.

016

.50.

00.

00.

04.

30.

00.

09.

23.

28.

00.

20.

17.

01.

35.

5p

b

84a

0.03

874

20.

084

.70.

00.

00.

00.

20.

10.

00.

10.

20.

00.

00.

00.

00.

00.

00.

00.

03d

z2

85a

0.04

887

20.

00.

046

.60.

00.

00.

00.

01.

30.

00.

013

.01.

57.

60.

41.

10.

10.

20.

23d

zx

86b

0.06

292

20.

00.

00.

027

.90.

00.

00.

09.

80.

00.

06.

55.

21.

51.

40.

33.

12.

63.

33d

yz

87a

0.20

033

00.

00.

00.

60.

00.

00.

00.

00.

30.

00.

04.

56.

92.

69.

82.

99.

80.

09.

5p

a*

88b

0.20

298

00.

00.

00.

00.

00.

00.

00.

00.

90.

00.

02.

96.

83.

78.

74.

41.

511

.92.

9p

b*

89b

0.22

863

00.

00.

00.

00.

048

.52.

82.

30.

00.

62.

00.

00.

00.

00.

00.

00.

00.

00.

03d

xy

aT

hela

bels

(a)

and

(b)

refe

rto

the

orbi

tals

ymm

etry

inth

eC

2po

int

grou

p.b

nis

the

orbi

talo

ccup

ancy

.cP

erce

ntag

eco

mpo

siti

onva

lues

ofth

ere

leva

ntat

omic

orbi

tals

.

Table 5 Selected electronic spectroscopic data for [NiL]2− (1A)

Transition Energya/nm f b Naturec Type

1 579 2.1 × 10−9 86b → 89b (74.7) d–d2 547 2.6 × 10−6 85a → 89b (84.7) d–d3 533 3.6 × 10−6 84a → 89b (100.0) d–d4 424 3.1 × 10−5 75a → 89b (61.0) d–d5 399 2.8 × 10−4 86b → 87a (91.3) MLCT6 346 7.2 × 10−4 85a → 88b (94.3) MLCT7 280 3.8 × 10−4 82b → 87a (58.8) IL8 267 3.4 × 10−4 81a → 89b (67.4) LMCT9 264 4.3 × 10−4 81a → 87a (50.6) IL

82b → 88b (41.1) IL

a The transition energy is lower than 40000 cm−1 (250 nm). b f is theoscillator strength. c The transition contribution (%) values are given inparentheses.

orbital to the unfilled metal 3dxy orbital, which appear at higherenergies (264–280 nm) (Table 5, transitions 7–9).

Electronic structure of [NiL]−. The MO diagram for the dou-blet state monoanionic complex, [NiL]− (2B), is characteristicof a low-spin (S = 1/2) square-planar NiIII species [Fig. 2(b)].The spin-down (b) LUMO corresponds to the SOMO in a spin-restricted representation and indicates that the unpaired electronresides in the metal 3dyz orbital [Fig. 2(b), top right] (Table S1,ESI‡), which then behaves as the redox-active orbital for theNiII → NiIII process. The order of the metal 3d orbitals for themonoanionic nickel(III) complex, [NiL]−, is equivalent to thatin the parent dianionic nickel(II) complex, [NiL]2−, with a 3dyz

HOMO [Fig. 2(a), middle left] (Table 4). Yet, as the oxidationstate of the nickel increases from 2+ to 3+, the metal 3dzx and3dyz orbitals become deeper and the orbital mixing with thenearby ligand pa and pb orbitals also increases [boxed structurein Fig. 2(b)] (Table S1, ESI‡).

In terms of ligand field theory, this situation can be describedthrough the interaction between the fragment molecular or-bitals (FMOs) of the nickel center, 3dzx and 3dyz, and thoseof the o-phenylenediamidate unit of appropriate symmetry,pa and pb, respectively, as illustrated in Scheme 3. The o-phenylenediamidate pb orbital level is close to that of thedegenerate Ni 3dzx and 3dyz orbitals, with an energy separationgiven by DEM–L. The o-phenylenediamidate pa orbital level issituated at lower energy than the pb orbital, likely due to greaterantibonding contributions between the 2pz carbon and nitrogenlone pairs in the latter. DEL is the energy difference between thesetwo ligand orbitals before metal–ligand interaction. Becauseof the large orbital mixing between 3dyz and pb FMOs, theresulting antibonding combination (labeled 3dyz–pb) becomesthe highest energy orbital, well-separated from the 3dzx orbital,while the corresponding bonding combination (labeled pb +3dyz) approaches the lowest lying pa orbital. DE is the energysplitting between these two metal orbitals following 3dyz/pb

orbital mixing after metal–ligand interaction.In this simple four-orbital bonding model, DEa and DEb are

the CT energies for the 3dyz/pa and 3dyz/pb pairs, respectively.They are related to the two nearly superposed intense low-energy LMCT transitions in the calculated electronic absorptionspectrum of [NiL]−, located at 794 and 829 nm, respectively[Fig. 3(b)] (Table S4, ESI,‡ transitions 4 and 3). These twoLMCT bands are better resolved in the experimental electronicabsorption spectrum of the nickel(III) complex, appearing at760 and 910 nm. The relative intensities of both transitions areas expected from symmetry considerations, with the pb → 3dyz

LMCT transition being more intense than the pa → 3dyz LMCTtransition (Table S4, ESI‡). In addition, there are several lessintense high-energy MLCT transitions from the filled metal3dzx and half-filled metal 3dyz orbitals to the unfilled ligandpa* and pb* antibonding orbitals [Fig. 3(b)] (Table S4, ESI,‡transitions 8–11). Three weak individual transitions from the

2 5 3 2 D a l t o n T r a n s . , 2 0 0 5 , 2 5 2 7 – 2 5 3 8

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Fig. 2 Partial energy level diagram and selected surface contours of the frontier MOs for [NiL]2− (1A) (a), [NiL]− (2B) (b), [NiL] (1A) (c) and [NiL](3B) (d). The boxed structure outlines the four relevant MOs in the p bonding scheme description (see Scheme 3).

lower filled 3dzx, 3dz2 and 3dx2−y2 orbitals to the upper half-filled3dyz orbital (2A ← 2B transitions) are located at 3738, 1679 and456 nm, respectively (Table S4, ESI,‡ transitions 1, 2 and 7),and an individual transition from the half-filled 3dyz orbital tothe upper unfilled 3dxy orbital (2B ← 2B transition) appears at595 nm (Table S4, ESI,‡ transition 6). In general, it is expectedthat the energy separation between the 3d orbitals increases ongoing to a higher metal oxidation state because of a larger ligandfield strength. This explains why the calculated d–d transitionsare better resolved for the monoanionic nickel(III) complex[NiL]− than for the dianionic nickel(II) complex [NiL]2− [insetof Fig. 3(a)]. Experimentally, this situation had been previouslyobserved for the related copper(II) and copper(III) complexes:three well-resolved d–d transitions were noted for CuIII relativeto the unique d–d transition for CuII.10a

Electronic structure of [NiL]. The MO diagrams for bothsinglet (1A) and triplet (3B) spin states of the neutral complex[NiL] are consistent with the NiIII-o-benzosemiquinonediiminep-cation radical formulation as derived from molecular geom-etry calculations [Fig. 2(c) and (d), respectively]. The natureof the MOs for each spin state may be rationalized withinthe framework of the aforementioned four-orbital bondingmodel by assuming that the redox-active orbital in the NiIII →

NiIV (formal) process is the ligand pb orbital, as illustratedin Scheme 4. This situation would result from the inverteddisposition between the degenerate metal 3dzx and 3dyz orbitallevel and the ligand pb orbital level (with DEM–L < 0) for theneutral nickel(III) p-cation radical complex [NiL] [Scheme 4(a)and (b), left] relative to the anionic nickel(III) complex [NiL]−

(Scheme 3, bottom).For the singlet-state neutral molecule, [NiL] (1A), the HOMO

and LUMO of both spin-up (a) and spin-down (b) manifolds arethe 3dzx and pb − 3dyz orbitals, respectively [Fig. 2(c)] (Table S2,ESI‡). The HOMO is dominated by the Ni 3dzx FMO, whilethe LUMO has important contributions from both Ni 3dyz

and o-benzosemiquinonediimine pb FMOs (Table S2, ESI‡),which makes difficult the assignment of this LUMO as eithermetal or ligand in nature. The large HOMO–LUMO gap (DE =11870 cm−1) is attributed to the strong interaction between theNi 3dyz and o-benzosemiquinonediimine pb FMOs, thus raisingthe energy of the pb − 3dyz orbital well above the 3dzx orbital[Scheme 4(a), left]. This energy gap is large enough to overcomethe electron repulsion for spin pairing, hence resulting in arobust singlet (S = 0) spin state (strong interaction limit).16 Interms of an exchange model between the two unpaired electronslocated on the metal center (SM = 1/2) and the radical ligand(SL = 1/2), the large energy splitting (DEb) between the bonding(3dyz + pb) and antibonding (pb − 3dyz) combination of the Ni

D a l t o n T r a n s . , 2 0 0 5 , 2 5 2 7 – 2 5 3 8 2 5 3 3

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Fig. 3 Experimental and simulated (bold line) electronic absorption spectra for [NiL]2− (1A) (a), [NiL]− (2B) (b), [NiL] (1A) (c) and [NiL] (3B) (d).The bars correspond to the calculated transitions (data from Table 5 and Tables S4–S6, ESI‡).

3dyz and ligand pb orbitals is consistent with a strong metal–radical antiferromagnetic coupling that would be equivalent toa covalent bonding. This fact ultimately results from the largedirect overlap between these two FMOs of the same symmetryand which contain the two unpaired electrons (the so-called“magnetic orbitals”).2b

For the triplet state neutral molecule, [NiL] (3B), the spin-down (b) LUMO and LUMO + 1 are the 3dzx and pb − 3dyz

orbitals, respectively [Fig. 2(d)] (Table S3, ESI‡), which corre-spond to the two SOMOs in the spin-restricted representation[Scheme 4(b), left]. These two SOMOs are dominated by theNi 3dzx and o-benzosemiquinonediimine pb FMOs, respectively(Table S3, ESI‡). In this case, the higher SOMO has also animportant contribution from the Ni 3dyz FMO (Table S3, ESI‡),but smaller than for the related LUMO of the singlet state neutralmolecule, [NiL] (1A), (Table S2, ESI‡). The energy gap betweenthe two SOMOs is quite small (DE = 2040 cm−1) and accountsfor the resulting triplet (S = 1) spin state. This situation withone unpaired electron in the Ni 3dzx orbital and the other onein the ligand pb orbital formally correlates with the traditionalpicture of a metal–radical ferromagnetic coupling resulting fromthe strict orthogonality between “magnetic orbitals” of differentsymmetry with zero orbital overlap.2b

The difference in p bonding scheme descriptions for thesinglet and triplet states of the neutral nickel(III) p-cationradical complex [NiL] originates from the reduced 3dyz/pb

interaction (with lessened DEb) in the triplet state molecule[Scheme 4(a) and (b), left] as the result of longer metal–nitrogenamidate bonds with the N-arylamides (Table 2). This situationis consistent with the TDDFT calculations which predict asignificant bathochromic shift of the most intense CT bandcorresponding to the 3dyz/pb pair in the calculated electronicabsorption spectrum of [NiL] upon change of spin from thesinglet to the triplet state [Fig. 3(c) and (d)] (Tables S5 andS6, ESI,‡ transition 5). The calculated 3dyz → pb transition,formally a MLCT transition, is located at 664 nm in the VISregion for the singlet state neutral molecule, [NiL] (1A), whereasit appears at 844 nm in the NIR region for the triplet stateneutral molecule, [NiL] (3B) [Scheme 4(a) and (b), right]. Theexperimental electronic absorption spectrum of the nickel(III)p-cation radical species,11b which displays one intense VIS bandat 645 nm, matches best the calculated spectrum of the singletstate molecule [Fig. 3(c), transition 5]. This strongly suggeststhat the singlet state is both the experimentally observed groundstate and the most thermally populated state. According to theenergy calculations, which predict a small thermal populationof 17% for the triplet excited state at room temperature, it istempting to assign the weak NIR band observed experimentallyat 960 nm to that predicted theoretically for the triplet statemolecule [Fig. 3(d), transition 5].

These formally MLCT transitions for the singlet (1A) andtriplet (3B) electronic states of the neutral complex [NiL] are

2 5 3 4 D a l t o n T r a n s . , 2 0 0 5 , 2 5 2 7 – 2 5 3 8

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Scheme 3 MO diagram of the interaction between the metal dzx anddyz and ligand pa and pb FMOs for [NiL]−.

Scheme 4 Simplified p bonding scheme of the singlet (a) and triplet (b)electronic states for [NiL] with their lower-lying CT excited electronicstates and corresponding CT transitions (open arrows).

associated with the presence of additional, energeticallywell-separated CT singlet (1A*) and triplet (3B*) excitedstates, respectively [Scheme 4(a) and (b), right]. The low lyingCT singlet excited state molecule, [NiL] (1A*), is actually anickel(III)-benzosemiquinonediimine p-cation radical complexpossessing a (pa)2(pb)1(3dzx)2(3dyz)1 electronic configuration[Scheme 4(a), right]. It corresponds to an antiferromagneticallyexchange-coupled (S = 0) metalloradical species where the twoelectrons are not paired in the same orbital but are localizedon different metal and radical orbitals with opposite spins(weak interaction limit).16 By contrast, the low lying CT tripletexcited state molecule, [NiL] (3B*), is a true nickel(IV) speciespossessing a (pa)2(pb)2(3dzx)1(3dyz)1 electronic configuration[Scheme 4(b), right], which would be isoelectronic with therelated square-planar cobalt(III) complex with an intermediate(S = 1) spin state.12 The calculated energy differences betweenthe two lowest NiIII–L•+ singlet and triplet states, 1A and 3B, andthe corresponding low-lying NiIII–L•+ CT singlet and NiIV–LCT triplet excited states, 1A* and 3B*, are DEb = 15060 and11850 cm−1, respectively (Tables S5 and S6, ESI‡). In terms ofvalence bond theory, a larger interaction between the NiIII–L•+

and the NiIV–L CT states is predicted for the 3B/3B* pair whichare energetically closer than the NiIII–L•+ states of the 1A/1A*pair.

Charge and spin density distributions

DFT charge and spin density distributions of the nickel com-plexes [NiL]z (z = 2−, 1−, 0) (Fig. S2, ESI‡) have been analyzedto gain more insight into their electronic structure descriptionand, particularly, to confirm the nature of the lowest singlet andtriplet states of the neutral complex [NiL] as deduced from MOcalculations. Selected calculated charge and spin density datafor the metal and ligand fragments of the titled complexes aresummarized in Table 6, and the calculated electron populationsof the metal 3d shell for each complex are given in Table 7.

The relative changes in the charge distributions for the anionicnickel(II) and nickel(III) complexes, [NiL]2− (1A) and [NiL]− (2B),respectively, agree with the increase in metal oxidation state.In particular, the metal atomic charge (QM) increases by about0.175 from the divalent to the trivalent metal oxidation states(Table 6). A close inspection of the electron populations of the Ni3d shell allows to determine the electronic configuration for eachcomplex (Table 7). The total electron population of the metal 3dorbitals for the singlet state dianionic nickel(II) complex, [NiL]2−

(1A), equals 8.684 which is typical for a +2 metal oxidation state(low-spin, d8 electronic configuration). The nonzero yet smallelectron population of the 3dxy orbital is due to partial CTfrom the ligand to the Ni atom. The charge defect for [NiL]−

compared with [NiL]2− affects mainly the spin-down (b) 3dyz

orbital, which indicates the source of the electron loss uponoxidation. This situation with one unpaired electron from theNiIII center occupying the metal 3dyz orbital is also reflected in thespin density distribution for the monoanionic complex, [NiL]−

(2B) [Fig. 4(a)], which corresponds exactly with the square of this

Table 6 Selected charge and spin density data for [NiL]z (z = 2−,1−, 0)

Complex QMa/e QL

b/e qMc/e qMF

d/e qLFe/e

[NiL]2− (1A) 0.873 −2.873 0 0 0[NiL]− (2B) 1.048 −2.048 0.391 0.870 0.130[NiL] (1A) 1.032 −1.032 0.047 0.038 −0.038[NiL] (3B) 1.093 −1.093 0.479 1.660 0.340

a QM is the metal atomic charge. b QL is the ligand total charge. c qM isthe metal atomic spin density. d The metal fragment spin density qMF waschosen as the sum of the atomic spin densities in the metal and the twoN ′-methyloxamidate groups. e The ligand fragment spin density qLF waschosen as the sum of the atomic spin densities in the o-phenylene group.

D a l t o n T r a n s . , 2 0 0 5 , 2 5 2 7 – 2 5 3 8 2 5 3 5

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Tab

le7

Ni3

dsh

elle

lect

ron

popu

lati

onda

tafo

r[N

iL]z

(z=

2−,1

−,0)

Com

plex

3dx2

−y2

(a)

3dx2

−y2

(b)

3dz2

(a)

3dz2

(b)

3dzx

(a)

3dzx

(b)

3dyz

(a)

3dyz

(b)

3dxy

(a)

3dxy

(b)

3d(a

)3d

(b)

[NiL

]2−(1

A)

0.99

40.

994

0.95

60.

956

0.98

50.

985

0.99

40.

994

0.41

30.

413

4.34

24.

342

[NiL

]−(2

B)

0.99

40.

994

0.97

60.

973

0.99

20.

990

0.99

60.

630

0.50

90.

482

4.46

64.

069

[NiL

](1A

)0.

994

0.99

40.

977

0.97

70.

961

0.94

60.

880

0.85

90.

495

0.49

14.

307

4.26

0[N

iL](

3B

)0.

995

0.99

40.

981

0.97

80.

994

0.61

40.

996

0.91

70.

532

0.50

44.

498

4.00

8

Fig. 4 Projection views of the calculated spin density distribution for[NiL]− (2B) (a) and [NiL] (3B) (b) in the z direction perpendicular tothe ligand plane. Light and dark gray contours represent positive andnegative spin densities, respectively.

orbital [Fig. 2(b), top right]. The spin density is mainly centeredon the metal and the four nearest amidate nitrogen atoms but it isalso partially delocalized toward the peripheral oxygen atoms ofthe NCO arylamidate groups [Fig. 4(a)]. The same phenomenonhas been experimentally observed in square-planar copper(II)complexes with related oxamate and oxamidate ligands by po-larized neutron diffraction.17 The important spin delocalizationonto the oxamidate groups and, particularly, toward the N-arylamidate donor groups, is ultimately due to the p-conjugatedcharacter of the C–N and C=O bonds. The presence of somespin density at the aromatic ring carbon atoms is also worthnoting [Fig. 4(a)]. DFT calculations indicate that the bis(N ′-methyloxamidate)metal fragment carries together about 0.870 ofspin density, while the remaining 0.130 resides in the o-phenylenefragment (Table 6).

The calculated charge distributions for both the singlet (1A)and triplet (3B) states of the neutral complex [NiL] agree witha nickel(III) p-cation radical formulation. The metal atomiccharges (QM) are comparable and differ by no more than0.045 from that of the nickel(III) complex [NiL]−, as expectedfor a ligand-centered oxidation (Table 6). The total electronpopulations of the metal 3d shell are also similar for both thesinglet (1A) and triplet (3B) states of the neutral nickel(III) p-cation radical complex [NiL], yet a detailed inspection of theelectron population distributions among the Ni 3d orbitalsconfirms the distinct nature of the metal–radical coupling foreach spin state (Table 7). The charge defect is equally distributedbetween the spin-up (a) and the spin-down (b) 3dyz orbital forthe singlet state neutral complex, [NiL] (1A). This is character-istic of a strong metal–radical antiferromagnetic coupling thatwould be equivalent to a covalent bonding with complete spindelocalization.16 Moreover, a very small spin polarization witha negative spin density of −0.038 on the o-phenylene fragmentand a positive spin density of 0.047 on the metal are calculated(Table 6), thus indicating a highly covalent spin delocalized state.In contrast, for the triplet state neutral complex, [NiL] (3B), the

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charge defect is individualized on the spin-down (b) 3dzx orbital,as expected for metal–radical ferromagnetic coupling betweenorthogonal “magnetic orbitals”. This is also reflected in theshape of the spin density distribution for the triplet state neutralcomplex, [NiL] (3B) [Fig. 4(b)], which corresponds to the sum ofthe square of the metal 3dzx and the ligand pb orbitals [Fig. 2(d),top right]. The calculations indicate a spin density of 0.340 on theo-phenylene fragment and of 0.479 on the metal (Table 6). Thereis a large spin delocalization onto the oxamidate groups and,moreover, a weak but non-negligible spin polarization is presentin the NCO alkylamidate groups as evidenced by the negativespin density carried out by the central carbon atoms [Fig. 4(b)].This reflects the stronger delocalization of the metal spin densitytoward the N-alkylamidate donor groups compared to the N-arylamidate ones due to the larger metal interaction with theformer (shorter M–N distances) as deduced from molecularstructure optimizations (Table 2).

Overall, a picture emerges of the neutral complex [NiL] as amolecule possessing two nearly degenerate states of differentspin values, S = 0 and S = 1, both with predominantlymetalloradical character but fundamentally different in struc-ture and bonding interactions. The distinct spin multiplicityof these two states is associated with a switch of the metal-centered “magnetic orbital” that interacts with the ligand-centered “magnetic orbital” [Scheme 4(a) and (b), left]. Thisorbital reversal phenomenon is caused by a metal–ligand bondrearrangement of the square-planar low-spin NiIII ion uponchange of spin state. This situation differs from the traditionalpicture of an exchange-coupled metalloradical species for whichall the spin states are described with the same ligand- and metal-centered orbital configuration.

ConclusionsIn this work, we have theoretically elucidated the ligandcontributions to the structure and bonding of four-coordinatesquare-planar divalent, trivalent and formal tetravalent nickelcomplexes with a tetradentate aromatic dioxalamide ligandderived from o-phenylenediamine. The main results of thepresent study are summarized as follows.

(1) The ligand presents a very strong planar r-field to the NiII

ion through the four deprotonated amides of the two oxamidategroups. The r-donor interaction occurs primarily with just onemetal 3d orbital, namely 3dxy, which thus becomes strongly rantibonding and unoccupied to give a low-spin singlet (S = 0)nickel(II) complex.

(2) The ligand features an unusually high-lying fully occupiedp-type orbital, namely pb, which is mainly centered in the o-phenylenediamidate fragment. This ligand pb orbital partic-ipates actively in a p-donor interaction with the metal 3dyz

orbital, which thus becomes slightly p antibonding compared tothe mainly nonbonding 3dzx orbital, and constitutes the orbitalcontaining the unpaired electron in the low-spin doublet (S =1/2) nickel(III) complex.

(3) The ligand readily loses an electron from its pb orbitalgiving a o-benzosemiquinonediimine p-cation radical ligandwhich couples either antiferro- or ferromagnetically with theunpaired electron of the NiIII ion occupying the metal 3dyz or3dzx orbitals, respectively, then leading to a singlet (S = 0) anda triplet (S = 1) states close in energy for the nickel(III) p-cationradical complex.

Computational detailsDFT geometry optimization and MO calculations of complexes[NiL]z (z = 2−, 1−, 0) were realized under C2 point groupsymmetry with the hybrid B3LYP method,18 as implementedin the GAUSSIAN98 program.19 Double-f and triple-f qualitybasis sets proposed by Ahlrichs and co-workers have beenemployed for non-metal and metal atoms, respectively,20 with

two extra polarization p functions for the metal atoms. The BSformalism21 has been used to describe the URB3LYP solutionof the S = 0 state for the neutral complex [NiL].

The free energies have been evaluated for an ideal gasin a canonical ensemble taking into account the zero-pointcorrection, electronic, vibrational, rotational and translationalcontributions to enthalpy and entropy.22 The force constantsand vibrational frequencies have been determined analyticallyusing an harmonic approach from the optimized geometries.The theoretical electronic spectra have been simulated formthe energy of the excited states and transition strength forceconstants calculated by the TDDFT formalism as implementedin the GAUSSIAN98 program.23 These last have been deducedfrom transition electric dipole moments. A value equal to2000 cm−1 for the band-width at half-height has been used inthese simulations because this value often provide us with molarextinction coefficient values of the same order of magnitudethan the experimental ones (as it has been checked on severalsimple complexes). The atomic charges and spin densities wereobtained by a natural bond orbital (NBO) analysis.24

AcknowledgementsThis work was supported by the Ministerio de Ciencia yTecnologıa (MCYT) (Spain) through the “Ramon y Cajal”program.

References1 F. Meyer and H. Kozlowski, in Comprehensive Coordination Chem-

istry II: From Biology to Nanotechnology, ed. J. A. McCleverty andT. J. Meyer, Elsevier, Oxford, 2004, vol. 6, p. 247.

2 (a) A. Caneschi, D. Gatteschi, R. Sessoli and P. Rey, Acc. Chem. Res.,1989, 22, 392; (b) A. Caneschi, D. Gatteschi and P. Rey, Prog. Inorg.Chem., 1991, 39, 331; (c) J. S. Miller and A. J. Epstein, Angew. Chem.,Int. Ed. Engl., 1994, 33, 385; (d) P. Gutlich and A. Dei, Angew. Chem.,Int. Ed. Engl., 1997, 36, 2734; (e) C. G. Pierpont, Coord. Chem. Rev.,2001, 216, 99; (f) A. Dei, D. Gatteschi, C. Sangregorio and L. Sorace,Acc. Chem. Res., 2004, 37, 827.

3 (a) P. I. Clemenson, Coord. Chem. Rev., 1990, 106, 171; (b) P. Cassoux,L. Valade, H. Kobayashi, A. Kobayashi, R. A. Clark and A. E.Underhill, Coord. Chem. Rev., 1991, 110, 115; (c) R. M. Olk, B. Olk,W. Dietzsch, R. Kirmse and E. Hoyer, Coord. Chem. Rev., 1992, 117,99; (d) L. Ouahab, Coord. Chem. Rev., 1998, 178–180, 1501; (e) P.Cassoux, Coord. Chem. Rev., 1999, 185–186, 213; (f) A. E. Pullenand R. M. Olk, Coord. Chem. Rev., 1999, 188, 211; (g) N. Robertsonand L. Cronon, Coord. Chem. Rev., 2002, 227, 93.

4 (a) J. A. McCleverty, Prog. Inorg. Chem., 1968, 10, 49; (b) R. H. Holmand M. J. Olanov, Prog. Inorg. Chem., 1971, 14, 241; (c) C. G. Pierpontand C. W. Lange, Prog. Inorg. Chem., 1994, 41, 331; (d) T. J. Collins,Acc. Chem. Res., 1994, 27, 279; (e) A. Mederos, S. Domınguez, R.Hernandez-Molina, J. Sanchiz and F. Brito, Coord. Chem. Rev., 1999,193–195, 913.

5 (a) C. Benelli, A. Dei, D. Gatteschi and L. Pardi, Inorg. Chem., 1988,27, 2831; (b) C. Benelli, A. Dei, D. Gatteschi and L. Pardi, Inorg.Chem., 1989, 28, 1476; (c) C. W. Lange, B. J. M. Conklin and C. G.Pierpont, Inorg. Chem., 1994, 33, 1276; (d) C. W. Lange and C. G.Pierpont, Inorg. Chim. Acta, 1997, 263, 219; (e) D. A. Shultz, K. E.Vostrikova, S. H. Bodnar, H. J. Koo, M. H. Whangbo, M. L. Kirk,E. C. Depperman and J. W. Kampf, J. Am. Chem. Soc., 2003, 125,1607; (f) H. Ohtsu and K. Tanaka, Angew. Chem., Int. Ed., 2004, 43,6301.

6 (a) D. T. Sawyer, G. S. Srivatsa, M. E. Bodini, W. P. Schaefer andR. M. Wing, J. Am. Chem. Soc., 1986, 108, 936; (b) D. Sellmann, H.Binder, D. Haubinger, F. W. Heinemann and J. Sutter, Inorg. Chim.Acta, 2000, 300–302, 829; (c) B. S. Lim, D. V. Formitchev and R. H.Holm, Inorg. Chem., 2001, 40, 4257; (d) V. Bachler, G. Olbrich, F.Neese and K. Wieghardt, Inorg. Chem., 2002, 41, 4179.

7 (a) F. Feigl and M. Furth, Monatsh. Chem., 1927, 48, 445; (b) E. I.Stiefel, J. H. Waters, E. Billig and H. B. Gray, J. Am. Chem. Soc.,1965, 87, 3016; (c) A. L. Balch and R. H. Holm, J. Am. Chem. Soc.,1966, 88, 5201; (d) G. Swartz-Hall and R. H. Soderberg, Inorg. Chem.,1968, 7, 2300; (e) V. Bachler, G. Olbrich, F. Neese and K. Wieghardt,Inorg. Chem., 2002, 41, 4179; (f) D. Herebian, E. Bothe, F. Neese,T. Weyhermuller and K. Wieghardt, J. Am. Chem. Soc., 2003, 125,9116; (g) D. Herebian, K. Wieghardt and F. Neese, J. Am. Chem.Soc., 2003, 125, 10997.

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ishe

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d by

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8 (a) C. N. Verani, S. Gallert, E. Bill, E. Bothe, T. Weyhermuller, K.Wieghardt and P. Chaudhuri, Chem. Commun., 1999, 1747; (b) H.Chun, C. N. Verani, P. Chaudhuri, E. Bothe, T. Weyhermuller andK. Wieghardt, Inorg. Chem., 2001, 40, 4157; (c) P. Chaudhuri, C. N.Verani, E. Bill, E. Bothe, T. Weyhermuller and K. Wieghardt, J. Am.Chem. Soc., 2001, 123, 2213; (d) H. Chun, T. Weyhermuller, E. Billand K. Wieghardt, Angew. Chem., Int. Ed., 2001, 40, 2489; (e) K. S.Min, T. Weyhermuller and K. Wieghardt, Dalton Trans., 2003, 1126;(f) K. S. Min, T. Weyhermuller, E. Bothe and K. Wieghardt, Inorg.Chem., 2004, 43, 2922; (g) E. Bill, E. Bothe, P. Chaudhuri, K.Chlopek, D. Herebian, S. Kokatam, K. Ray, T. Weyhermuller, F.Neese and K. Wieghardt, Chem. Eur. J., 2004, 11, 204.

9 (a) W. Hieber and R. Bruck, Z. Anorg. Allg. Chem., 1952, 269, 13;(b) D. Herebian, E. Bothe, E. Bill, E. Bothe, T. Weyhermuller and K.Wieghardt, J. Am. Chem. Soc., 2001, 123, 10012.

10 (a) R. Ruiz, C. Surville-Barland, A. Aukauloo, E. Anxolabehere-Mallart, Y. Journaux, J. Cano and M. C. Munoz, J. Chem. Soc.,Dalton Trans., 1997, 745; (b) B. Cervera, J. L. Sanz, M. J. Ibanez, G.Vila, F. Lloret, M. Julve, R. Ruiz, X. Ottenwaelder, A. Aukauloo, S.Poussereau, Y. Journaux and M. C. Munoz, J. Chem. Soc., DaltonTrans., 1998, 781.

11 (a) X. Ottenwaelder, R. Ruiz-Garcıa, G. Blondin, R. Carrasco, J.Cano, D. Lexa, Y. Journaux and A. Aukauloo, Chem. Commun.,2004, 504; (b) X. Ottenwaelder, A. Aukauloo, Y. Journaux, R.Carrasco, J. Cano, B. Cervera, I. Castro, S. Curreli, M. C. Munoz,A. L. Rosello, B. Soto and R. Ruiz-Garcıa, Dalton Trans., 2005,DOI: 10.1039/b502478a (preceding paper).

12 (a) J. Estrada, I. Fernandez, J. R. Pedro, X. Ottenwaelder, R. Ruizand Y. Journaux, Tetrahedron Lett., 1997, 38, 2377; (b) I. Fernandez,J. R. Pedro, A. L. Rosello, R. Ruiz, I. Castro, X. Ottenwaelder andY. Journaux, Eur. J. Org. Chem., 2001, 1235.

13 (a) T. Ziegler, Chem. Rev., 1991, 91, 651; (b) B. Delley, in DensityFunctional Theory, A Tool for Chemistry, ed. J. M. Seminario andP. Politzer, Elsevier, Amsterdam, 1994; (c) W. Koch and M. C.Holthausen, A Chemist’s Guide to Density Functional Theory, VerlagChemie, Weinheim, 2000.

14 O. Kahn, Molecular Magnetism, VCH, New York, 1993, ch. 4, p. 53.15 (a) T. J. Collins, R. D. Powell, C. Slebodnick and E. S. Uffelman,

J. Am. Chem. Soc., 1991, 113, 8419; (b) M. J. Bartos, C. Kidwell,K. E. Kauffmann, S. W. Gordon-Wylie, T. J. Collins, G. R. Clark,E. Munck and S. Weintraub, Angew. Chem., Int. Ed. Engl., 1995, 34,1216.

16 P. Chen, D. E. Root, C. Campochiaro, K. Fujisawa and E. I.Solomon, J. Am. Chem. Soc., 2003, 125, 466.

17 (a) V. Baron, B. Gillon, O. Plantevin, A. Cousson, C. Mathoniere,O. Kahn, A. Grand, L. Ohrstrom and B. Delley, J. Am. Chem. Soc.,1996, 118, 11822; (b) V. Baron, B. Gillon, A. Cousson, C. Mathoniere,O. Kahn, A. Grand, L. Ohrstrom, B. Delley, M. Bonnet and J. X.Boucherle, J. Am. Chem. Soc., 1997, 119, 3500.

18 (a) A. D. Becke, Phys. Rev. A, 1988, 38, 3098; (b) C. Lee, W. Yangand R. G. Parr, Phys. Rev. B, 1988, 37, 785; (c) A. D. Becke, J. Chem.Phys., 1993, 98, 5648.

19 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr.,R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D.Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone,M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma,D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J.Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A.Liashenko, P. Piskorz, I. Komaromi , R. Gomperts, R. L. Martin,D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C.Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen,M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle and J. A.Pople, GAUSSIAN 98 (Revision A.6), Gaussian, Inc., Pittsburgh,PA, 1998.

20 (a) A. Schaefer, A. Horn and R. Ahlrichs, J. Chem. Phys., 1992, 97,2571; (b) A. Schaefer, C. Huber and R. Ahlrichs, J. Chem. Phys.,1994, 100, 5829.

21 (a) J. Cano, P. Alemany, S. Alvarez, M. Verdaguer and E. Ruiz, Chem.Eur. J., 1998, 4, 476; (b) E. Ruiz, J. Cano, S. Alvarez and P. Alemany,J. Am. Chem. Soc., 1998, 120, 11122; (c) E. Ruiz, J. Cano, S. Alvarezand P. Alemany, J. Comput. Chem., 1999, 20, 1391; (d) J. Cano, E.Ruiz, P. Alemany, F. Lloret and S. Alvarez, J. Chem. Soc., DaltonTrans., 1999, 1669.

22 D. A. McQaurrie, Statistical Thermodynamics, Harper and Row,New York, 1973.

23 M. E. Casida, C. Jamorski, K. C. Casida and D. R. Salahub, J. Chem.Phys., 1998, 108, 4439.

24 (a) J. E. Carpenter and F. Weinhold, J. Mol. Struct. (THEOCHEM),1988, 169, 41; (b) A. E. Reed, L. A. Curtis and F. Weinhold,Chem. Rev., 1988, 88, 899; (c) F. Weinhold and J. E. Carpen-ter, The Structure of Small Molecules and Ions, Plenum, 1988,p. 227.

2 5 3 8 D a l t o n T r a n s . , 2 0 0 5 , 2 5 2 7 – 2 5 3 8

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