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p-Conjugated N-heterocyclic compounds: correlation ofcomputational and electrochemical data†

M. Al-Anber,a S. Vatsadze,b R. Holze,c H. Langc and W. R. Thielda Department of Chemical Science, Faculty of Sciences, Mu’tah University, Al-Karak, Jordanb Chemistry Department, M.V. Lomonosov Moscow State University, Leninskie Gory, 119992,

Moscow, Russiac Technische Universitat Chemnitz, Fakultat fur Naturwissenschaften, Institut fur Chemie,

Straße der Nationen 62, D-09111, Chemnitz, Germanyd Fachbereich Chemie der Universitat Kaiserslautern, Gebaude 54, Erwin-Schrodinger Strasse,

D-67663, Kaiserslautern, Germany

Received 14th June 2005, Accepted 18th August 2005First published as an Advance Article on the web 8th September 2005

Electrochemical reduction potentials of a broad selection of nitrogen-containing molecules suitable as bridging(dipodal and tripodal) ligands in coordination and organometallic chemistry are reported and compared with resultsof semiempirical calculations. Trends of electrode potentials observed experimentally agree with respective calculateddata, deviations can be explained by invoking peculiarities of the involved molecular orbitals and ligand–electrodesurface interactions.

IntroductionN-Heterocycles are widely used as ligands in coordinationand organometallic chemistry1 and as building blocks forthe generation of supramolecular structures.2 The degree ofcomplexity of such compounds ranges from simple pyridine tothe quinconidine systems, which have found broad application inenantioselective cis-hydroxylation of olefins.3 Despite the broadusage of di- and polydentate aromatic heterocycles as linkersbetween redox-active transition metal sites,4 surprisingly littleis known about the electrochemical behavior of this class ofligand.5

In the present paper we describe the correlation betweenthe electrochemical and computational data of a series of p-conjugated nitrogen donor molecules.

Results and discussionBecause of their sp2-hybridized nitrogen atoms, all moleculesdiscussed below are excellent ligands for coordination to metal

† Paper presented in part at the 205th Meeting of the ElectrochemicalSociety, May 9–13, 2004, San Antonio, TX, USA.

Fig. 1 Pyridine-based ligands, where the six-membered heterocycles are either incorporated into a condensed system or directly linked to each otheror linked via heterocyclic units.

ions. They can be divided into three groups with differentchemical structures as shown in Fig. 1–3.

Compounds 1–25 were investigated by cyclic voltammetryand by semiempirical calculations (for further details see Exper-imental section). The electrochemical data are summarized inTable 1, the computational results are presented in Table 2.

Molecules 1–25 show one-electron reduction processes in thecyclic voltammograms. For a typical example see Fig. 4; furtherexamples are collected elsewhere5 (Table 1, eqn (1)).

L + e− → L−• (L = 1–25) (1)

In some cases, in addition to the first reduction a secondelectron transfer process can be observed. However, correlationof the electrochemical and the computational data is limitedto the reduction to the radical anion L−• as shown in eqn (1).In several cases the electrode peak potentials of the reductionand the corresponding reoxidation processes show considerablesplitting indicating a slow electron transfer. Nevertheless in allcases the formal potential E0 could be identified based on theexperimentally observed values of peak potential Eoxid and Ered

according to

E0 = Eoxid − (Eoxid − Ered)/2.

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3 6 3 2 D a l t o n T r a n s . , 2 0 0 5 , 3 6 3 2 – 3 6 3 7 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

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Fig. 2 Pyridine-based ligands, where the nitrogen heterocycles are spanned by ethylene or more complex p-conjugated bridges.

Fig. 3 Pyridine-based ligands, where the pyridines are spanned by 3-oxa-penta-1,4-diene-1,5-diyl fragments.

Fig. 4 Cyclic voltammogram of 1,2-di(4-pyridyl)ethylene (9) in THF,0.1 M [n-Bu4N][PF6], 25 ◦C, argon saturated, dE/dt = 200 mV s−1.

The value of this potential does not depend on the electrodepotential scan rate and the electron transfer kinetics, thus it isclosest to a thermodynamic value and thus most suitable for thesubsequent correlations. For the computational calculations weused the semiempirical method PM3 (see Experimental section)since it offers a rapid access to thermodynamic and electronicdata of large molecules. Ab-initio quantum chemical methods

(e.g. MP2 or higher) in combination with large basis sets wouldmost likely provide more accurate results, but would also requiremuch longer times for computation.

Solvent and surface effects will definitely play an importantrole in electrochemical processes proceeding in a condensedphase. They cannot be calculated with sufficient accuracy withany computational method. Therefore, semiempirical as well asab-initio methods assuming the molecules under study to be inthe gas phase are not expected to support the results of cyclicvoltammetry in every detail, but nevertheless a correlation ofcomputational and electrochemical data should be possible.

Molecular geometries of organic compounds obtained byPM3 are generally in good agreement with the results ofsolid state structures: as examples, the solid state structuresof 2,5-dibenzylidene cyclopentanone 176 and 2,6-dibenzylidenecyclohexanone 207 are compared with the results of the PM3calculations in Fig. 5.

From computational investigations heats of reaction‡(HR; L + e− → L−•), the energy of the LUMO and the HOMO(molecules L) and the energy of the SOMO (molecules L−•) havebeen obtained (see Table 2).

The negative heats of reaction are consistent with the exother-micity of the electron transfer in the gas phase. From this point of

‡ This is basically equivalent to electron affinity; but because the com-putational approach employed specifies results as heat of reac-tion/formation the former designation is not used in order to avoidconfusion with other computational approaches.

D a l t o n T r a n s . , 2 0 0 5 , 3 6 3 2 – 3 6 3 7 3 6 3 3

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Fig. 5 Comparison of the solid-state structures of 2,5-dibenzylidene cyclopentanone 17 (top, left) and 2,6-dibenzylidene cyclohexanone 20 (top,right) with the respective calculated structures (bottom).

Table 1 Electrochemical data for compounds 1–25 (data used incorrelations are in italics, all potentials vs. FeC (ferrocene/ferrocenium))

Compound E0/V Ered/V Eoxid/V DE/mV

1 −2.77 −2.90 −2.56 3402 −2.51 −2.78 −2.24 540

−3.08 −3.35 −2.81 5403 −2.71 −2.90 −2.51 3904 −2.15 −2.28 −2.01 270

−2.78 −2.91 −2.64 2705 −2.51 −2.69 −2.32 3706 −2.03 −2.19 −1.87 320

−2.65 −2.79 −2.51 280−2.82 −3.00 −2.64 360

7 −2.66 −2.80 −2.51 290−2.86 −3.05 −2.76 290

8 −2.17 −2.28 −2.05 230−2.52 −2.67 −2.37 300

9 −2.27 −2.42 −2.12 300−2.69 −2.82 −2.56 260

10 −2.21 −2.38 −2.03 350−2.40 −2.56 −2.23 330−2.60 −2.69 −2.51 180

11 −2.06 −2.16 −1.95 210−2.35 −2.45 −2.24 210−2.73 −2.85 −2.61 240

12 −2.00 −2.16 −1.85 310−2.23 −2.39 −2.08 310−2.48 −2.61 −2.34 270

13 −2.18 −2.28 −2.09 190−2.60 −3.71 −2.48 230

14 −1.92 −2.03 −1.80 230−2.30 −2.41 −2.18 230

15 −1.61 −1.70 −1.51 190−2.15 −2.25 −2.05 200

16 −1.78 −1.87 −1.69 180−2.37 −2.46 −2.28 180

17 −1.92 −2.05 −1.78 270−2.56 −2.69 −2.43 260

18 −1.75 −1.88 −1.62 260−2.25 −2.34 −2.16 180

19 −1.89 −2.00 −1.79 240−2.42 −2.54 −2.29 250

20 −2.07 −2.21 −1.93 280−2.60 −2.74 −2.46 280

21 −1.71 −1.81 −1.61 200−2.09 −2.18 −1.99 190

22 −1.83 −1.93 −1.73 200−2.33 −2.44 −2.23 210

23 −1.85 −1.93 −1.77 160−2.29 −2.38 −2.20 180

24 −1.93 −2.04 −1.82 220−2.42 −2.51 −2.33 180

25 −1.71 −1.79 −1.62 170−2.09 −2.20 −1.99 210

view, more negative calculated heats of reaction should correlatewith less negative electrochemically determined E0 values (easierreduction). However, the correlation of these data clearly shows

a pronounced difference between the conjugated dienones 15–25on one hand and compounds 1–14 on the other (see Fig. 6).

Fig. 6 Correlation of the calculated heats of reaction of 1–25 with theelectrochemically determined E0 values (� and —: compounds 1–8; �and ---: compounds 9–14; � and - - -: compounds 15–25).

With respect to the calculated heats of reaction, the dienones15–25 show reduction potentials which are about 0.4 to 0.6 Vmore positive than those of 1–14. In other words, the dienones15–25 are easier to reduce than 1–14, although they show similarheats of reaction in the gas phase. Since it is not probable thatthis feature is due to systematic errors in the computationalmethod we used, it should be related to specific differences inthe electrochemistry or interfacial chemistry of the two classesof compounds.

Correlation of the calculated LUMO energies (the orbital,where the electron is transferred to) of compounds 1–25 withthe electrochemically determined E0 values provides a similarpicture (Fig. 7).

Although the LUMO energies of the dienones 15–25 are inthe same range as those of 1–14, the two groups can again clearlybe distinguished by correlating these data with the experimentalE0 values. However, for generation of the trendlines in Fig. 7,one member (compound 6, for chemical structure see Fig. 1) ofgroup 1 was neglected. It is marked with an arrow in Fig. 7 andwill be discussed in more detail below. This compound almostbehaves like one of the dienones 15–25: its reduction potentialis shifted with respect to its LUMO energy by about 0.6 V to amore positive potential, making it much easier to reduce thanthe other compounds of group 1.

Correlation of the SOMO (semi occupied molecular orbital)energies of the radical anions L−• of 1–25 with the electro-chemical data (Fig. 8) again shows an analogous behavior tothe previous correlations. The dienones 15–25 can clearly bedistinguished from 1–14.

However, compound 6, which was excluded from the calcula-tion of the trendline for the group 1 ligands in Fig. 7 can now

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Table 2 Computational data and E0 of the first reduction process of compounds 1–25

CompoundHF of L/kcal mol−1

HF of L−•/kcal mol−1

HR L + e − →L−•/kcal mol−1 EHOMO of L/eV ELUMO of L/eV

EHOMO −ELUMO/eV ESOMO of L−•/eV E0,FeC/V

1 62.475 31.290 −31.185 −9.329 −0.710 8.620 −2.029 −2.772 62.061 24.806 −37.255 −10.062 −0.743 9.320 −2.251 −2.513 94.652 60.930 −33.723 −9.260 −0.672 8.587 −2.254 −2.714 102.875 56.211 −46.664 −9.265 −1.207 8.058 −2.841 −2.155 105.707 68.566 −37.141 −9.828 −0.572 9.256 −2.498 −2.516 146.556 100.003 −46.553 −10.110 −0.709 9.401 −2.963 −2.037 71.386 36.659 −34.726 −9.149 −0.843 8.306 −2.183 −2.668 83.956 28.869 −55.087 −9.818 −1.545 8.273 −3.283 −2.139 75.143 31.581 −43.562 −9.350 −1.128 8.221 −2.697 −2.2710 112.421 63.726 −48.695 −8.735 −1.265 7.469 −3.025 −2.2111 120.845 69.836 −51.009 −8.910 −1.329 7.581 −3.185 −2.0612 129.248 76.885 −52.363 −9.137 −1.396 7.741 −3.293 −2.0013 128.467 78.029 −50.438 −8.955 −1.299 7.657 −3.220 −2.1814 118.222 65.764 −52.458 −9.670 −1.307 8.363 −3.338 −1.9215 50.275 5.511 −44.764 −9.779 −1.178 8.602 −2.816 −1.6116 49.822 6.371 −43.451 −9.534 −1.122 8.412 −2.743 −1.7817 35.708 −1.325 −37.033 −9.217 −0.793 8.424 −2.447 −1.7118 48.703 6.813 −41.891 −9.850 −0.925 8.926 −2.779 −1.7519 48.320 7.778 −40.541 −9.583 −0.870 8.713 −2.702 −1.8920 34.317 0.312 −34.005 −9.271 −0.546 8.726 −2.425 −2.0721 60.905 17.360 −43.544 −9.484 −0.967 8.517 −2.849 −1.7122 60.589 18.344 −42.244 −9.404 −0.915 8.489 −2.773 −1.8323 50.978 7.810 −43.168 −9.447 −0.952 8.495 −2.833 −1.9324 50.708 8.798 −41.910 −9.376 −0.901 8.475 −2.758 −1.8525 89.205 45.148 −44.057 −9.471 −0.960 8.510 −2.896 −1.71

Fig. 7 Correlation of the calculated LUMO energies of compounds1–25 with the electrochemically determined E0 values (� and —:compounds 1–8; � and ---: compounds 9–14; � and - - -: compounds15–25).

Fig. 8 Correlation of the calculated SOMO energies of compounds1–25 with the electrochemically determined E0 values (� and —:compounds 1–8; � and ---: compounds 9–14; � and - - -: compounds15–25).

be included in the correlation without problems. This suggests acloser look at the molecular structure of this ligand (see below).

There are two conceivable ways for molecules 1–25 to interactwith the surface of an electrode: (i) coordination via thefree electron pair of the sp2-hybridized nitrogen atoms and(ii) coordination via the p-conjugated skeleton (including thecarbonyl group in compounds 15–25).

Both orientations have been observed with numerous exper-imental methods in a huge number of studies.8 Particularlyinteresting and helpful in understanding the metal–moleculeinteraction are in situ vibrational spectroscopies like infraredand Raman spectroscopy. Somewhat surprisingly the formermethod has been applied to pyridine adsorption on platinumonly infrequently. Surface enhanced Raman spectroscopy usingpyridine as a probe molecule is extremely popular. With coinagemetals the huge surface enhancement effect provides intense andwell defined spectra showing the metal–nitrogen stretching modeindicative of the former mode of coordination. In the case ofplatinum this metal-specific enhancement effect is not operative.Nevertheless the electromagnetic surface enhancement is stilleffective providing sufficiently intense Raman scattering. Inrecently published studies9–12 with platinum and other non-coinage metals there is evidence of a side-on coordination, thespectra nevertheless did not show in all cases bands assignedto metal–nitrogen stretching modes. In particular with platinumthis lack is most unfortunate. Further studies with ketones orother molecules similar to those studied here have not beenreported so far.

In the present environment the platinum (and accordingto our observations even the glassy carbon) electrode can beregarded as an arrangement of atoms in a low oxidation state(for simplicity atomic corrugation and details of the interac-tion are not considered below). From a series of structurallycharacterized compounds it is known, that a,b-unsaturatedketones are coordinating to low-valent late transition metalsvia their p-orbitals.13 A prominent example is Pd2(dba)3 (dba =dibenzylidene acetone), wherein each Pd atom is coordinated byone C=C double bond of each dba molecule.14 However, thereare structurally characterized complexes of a,b-unsaturatedketones known, where the C=O double bond is also involvedin binding the metal. This is mainly the case, when the metalcenters are becoming more Lewis acidic,15 e.g. in the ironcomplexes (Ph–CH=CH–C(=O)–CH3)Fe(CO)2(PR3).10a An ex-cellent model for a surface bound 3-oxa-penta-1,4-diene is

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the cluster Os3(CO)9(L) (L = 4-phenylcyclohexa-2,5-dienone),where the ligand is bound to three osmium atoms with both C=Cdouble bonds and the C=O double bond.16 All these findingsare corroborated by results of quantum chemical calculationsby different groups on the adsorption of enones on platinumsurfaces.17

From the Dewar–Chatt–Duncanson model we know thatboth the HOMO (p-donor) and the LUMO (p-acceptor) play arole in the description of the binding of olefins to (late) transitionmetals. Applying this model to explain the interaction betweenan a,b-unsaturated ketone and the atoms of an electrode surfaceimplies a close interaction between the surface and the electronaccepting orbital, which should give rise to a low barrier forelectron transfer (Fig. 9, left). This binding mode is supported bythe fact that compounds 17 and 20, which are dienones withouta pyridyl ring, show the same electrochemical behavior as thepyridine containing members of the dienone group.

Fig. 9 Interaction of characteristic frontier orbitals of dienones andpyridines with the surface of a platinum electrode.

In contrast, pyridines are known to interact with low-valentlate transition metal centers by r donation of the lone-pairof the nitrogen donor sites. For the adsorption of pyridineand related ligands, like cinchonidine and quinidine ligands,on metal surfaces the situation is slightly different: pyridineadsorbs on a platinum surface in a tilted mode, coordinatingto the surface with its lone-pair of electrons at the nitrogenatom.18 The mode of adsorption of larger molecules like quin-chonidine and quinidine depends on the molecular structure ofthe adsorbent, on its concentration, and the surface coverageand can therefore be mediated by the lone-pair or by thep system.19 These results can be extended to the binding ofpyridines to the platinum atoms of an electrode surface (Fig. 9,right), which presumably involves the lone-pair of electrons atthe nitrogen atom. However, the LUMO of a heteroaromaticbase like pyridine has p-character, which prevents a directcommunication between the drain (LUMO) and the source (Ptsurface) of the electron transfer. This is also consistent witha study by Thiel et al., who investigated the electrochemicalbehavior of the ferrocenyl site and the Mo(CO)4 fragment in(5-ferrocenyl-3-(2-pyridyl)pyrazole)Mo(CO)4 complexes.20 Noelectronic communication between the two redox active sitescould be found, since the p-conjugation is interrupted at thelinkage between the N,N ′-chelating ligand and the Mo(CO)4

moiety.In electrochemistry, an additional overpotential will be needed

to transfer an electron to a pyridine type of molecule comparedto an enone (described above). The frontier orbitals of alldienones are of p-character, the lone pair at the pyridyl ringsbelongs to the r-skeleton, while the electron has to be transferredto a p-type orbital. This is in our opinion the reason forthe different electrochemical behaviour of these two classes ofcompounds.

The different behavior of compound 6 in the correlation ofthe electrochemical data with the calculated HOMO energies(Fig. 7) can be explained by its special steric and electronicsituation. On one hand, the 1,3,5-tripyridyltriazine possesses

the most extended p-system of all class 1 molecules and onthe other hand, the triazine ring is electron deficient due tothree nitrogen atoms in the ring. These features may favor aninteraction between the p-system and the electrode surface overa r attack.

ExperimentalCompounds 1, 2, 3 and 5, were obtained from Aldrich/Sigma-Aldrich; 6 and 9 from Fluka Chemie; 7 from Acros organics andwere used without further purification. Compound 4 was syn-thesized according to literature procedures;21 8 was synthesizedand purified according to reference 22; 10–12 reference 23; 13reference 24; 14 reference 25; 15–18 reference 26; 19 reference27; 20–25 reference 26.

Cyclic voltammetry

Cyclic voltammetry (CV) was performed with a one-compartment glass cell with a platinum disc (1 mm2 apparentsurface area, 99%, Schiefer, Hamburg) or a glassy carbondisc (HTW, Thierhaupten) embedded into a PTFE cylinderas working electrode. A platinum wire counter electrode and asilver reference electrode (Ag/Ag+, 0.01 M AgNO3 in a solutionof 0.1 M tetra-n-butylammonium hexafluorophosphate, TBFP(Fluka, dried in an oil pump vacuum at 120 ◦C) in methylenechloride) were used. For ease of comparison, all electrodepotentials are converted using the ferrocene/ferrocenium redoxcouple as a reference point (EFeC = 0 mV).28 Peak potentialsEox and Ered were taken from the experimental curves; theformal (redox) potential E0 was calculated according to E0 =Eoxid − (Eoxid − Ered)/2. All CVs were run at a scan rate ofdE/dt = 200 mV s−1 using an electrolyte solution of 0.1 M tetra-n-butylammonium hexafluorophosphate in methylene chloridewith a concentration of the compounds of 5 mM. CVswere recorded using a Voltalab 3.1 potentiostat (Radiometer)equipped with a digital electrochemical analyser DEA 101 andan electrochemical interface IMT 102. All experiments were runat room temperature (25 ◦C) under nitrogen purified with a CuOcatalyst for dioxygen removal.

Semiempirical calculations

Semiempirical calculations were carried out on a personal com-puter using the program GAUSSIAN98W29 with the methodPM3.30 No imaginary frequencies (i.e. transition states) wereobserved; verticals of relaxed electron affinity (EA) were calcu-lated, they are synonymously called heat of reaction.

AcknowledgementsFinancial support from the Fonds der Chemischen Industrie,the Deutsche Forschungsgemeinschaft (Gaduiertenkolleg GRK829/1) is gratefully acknowledged, in addition a grant to one ofus (S.V.) from RFBR (# 03-03-32401) is appreciated.

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