Structural Reassessment of [W(CO)5(TCNE)]: N (σ) Coordination

Aug 14, 2012 - School of Chemistry, Faculty of Science & Agriculture, University of KwaZulu-Natal, Private Bag X01, Pietermaritzburg, 3209 South Afric...
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Structural Reassessment of [W(CO)5(TCNE)]: N (σ) Coordination Instead of an Olefin (π) Complex Martina Bubrin,† Michael J. Krafft,† Lisa Steudle,† Ralph Hübner,† John S. Field,‡ Stanislav Záliš,§ and Wolfgang Kaim*,† †

Institut für Anorganische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany School of Chemistry, Faculty of Science & Agriculture, University of KwaZulu-Natal, Private Bag X01, Pietermaritzburg, 3209 South Africa § J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-18223 Prague, Czech Republic ‡

S Supporting Information *

ABSTRACT: The blue title compound, long assumed to be an olefin complex on the basis of an apparently single unresolved CN stretching band in the IR spectrum, has been identified by experiment and through DFT analysis as a σ complex with the tungsten atom coordinated to one of the nitrile N centers. The previously reported data are reinterpreted in light of the new structural assignment, and spectroelectrochemical results (UV−vis, IR, EPR) are presented.

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INTRODUCTION The remarkable acceptor molecule TCNE (tetracyanoethene)1 can be coordinated by metal centers in complexes through π (side-on) bonding (A) or via σ (nitrile N, end-on) coordination (B).2,3 The former is a common binding mode with olefins devoid of other metal-coordinating functions;4 it can occur with π back-donation (A1) according to the Dewar−Chatt− Duncanson concept or with extensive two-electron transfer (A2) to form metallacyclopropanes (Scheme 1).5 While N coordination of partially oxidized metals to the nitrile functions of reduced TCNE ligands has been established through structural,6 spectroscopic,7 and theoretical analysis,8 the binding of M(CO)5 (M = W, Mo, Cr) to unreduced TCNE has been postulated to occur through the central CC bond. A major argument for this assignment originally made by Herberhold9 was the observation of one single unresolved CN stretching band around 2200 cm−1 in the IR spectra: “An unsymmetrical coordination of the ligands via the lone pair on the nitrogen atom or via the π system of a cyano group can therefore be ruled out”.9 Later Raman and other studies10 of the tungsten complex seemed to confirm this notion because of the similar Raman spectroscopic features of [W(CO)5(TCNE)] and [TCNE]0/−;10a an unsymmetrical sit© 2012 American Chemical Society

uation such as that in B was believed to result in the splitting of ν(CN) into several, easily detectable separate bands. Detailed studies of solvatochromism11 were perceived as supporting this notion of structure A for [W(CO)5(TCNE)], while electrochemical reduction clearly produced the EPR-detectable radical anion complex [W(CO)5(TCNE)]•− with the low-symmetry structure B.12 Poor crystallization behavior of the series [M(CO)5(TCNE)] with M = W as the most stable member has so far precluded a reliable structural assessment of the complexes. However, increasing confidence in DFT calculations to predict geometry and physical properties of coordination compounds has led us to reinvestigate the small H-free compound [W(CO)5(TCNE)], both by experimental and by computational methods. The outcome of these studies, the reassignment of the structure to the σ(N) coordinated form B, will be discussed in connection with new spectroelectrochemical experiments which take advantage of the reversible oneelectron reduction. Received: June 20, 2012 Published: August 14, 2012 6305

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Scheme 1

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RESULTS AND DISCUSSION Neutral State of [W(CO)5(TCNE)]. In the solid and in most solvents the complex [W(CO)5(TCNE)] is distinguished by a deep blue color, caused by a strong absorption with λmax ranging from 650 to 810 nm (Figure S1, Supporting Information).11 TD-DFT calculations of the two individually optimized structures, A and B (Figure 1), led to a good

Figure 2. Simulated UV−vis absorption spectra of isomers A (blue line, maximum at 309 nm) and B (red line, maxima at 660 and 329 nm) of [W(CO)5(TCNE)] from TD-DFT calculations (PBE0/PCMCH2Cl2).

Table 1. In Vacuo DFT Calculated Bond Lengthsa (Å) of [W(CO)5(TCNE)]n and TCNE N-bonded TCNE (B)

b

Figure 1. DFT (PBE0 in vacuo) calculated energy minimum structures of the isomeric forms A and B of [W(CO)5(TCNE)].

agreement in the case of B (TD-DFT: λmax 660 nm), while the π side-on coordination species A was calculated to result in an absorption at a much shorter wavelength of λmax 309 nm (Figure 2). The low-energy shoulder visible in electrolyte-free solutions (Figure S1) is attributed to forbidden transitions (triplet) enhanced by strong spin−orbit coupling; it is not observed for the chromium analogue [Cr(CO)5(TCNE)],9,10b,c (Figure S1) where the metal has a much smaller spin−orbit coupling constant.13 In accordance, the alternative B with largely planar TCNE and two eclipsed (W−)CO bonds is calculated at ΔE = 10.6 kcal/mol lower energy than the π sideon form A; solvent inclusion causes only a small change to ΔE = 8.5 kcal/mol. The most revealing1,2,8 central C−C distance of 1.376 Å for coordinated TCNE in B suggests some π backdonation (1.360 Å calculated for free TCNE) but no electron transfer in the ground state. The high-energy alternative A is calculated with a C−C bond length of 1.460 Å, indicating partial reduction of the acceptor ligand (Table 1). Numerous attempts to crystallize [W(CO)5(TCNE)] for Xray diffraction eventually produced a single-crystal material (Table 2) which was analyzed as [W(CO)5(TCNE)]·0.5TCNE, showing [W(CO)5(η1-TCNE)]

bond

n=0

W−N W−Cax W−Ceq Ce−C′e N−Cn Ce−Cn Cax−O Ceq−O

2.075 2.031 2.062 1.376 1.161 1.414 1.140 1.137

n = 1− n = 2− 2.168 1.987 2.049 1.430 1.159 1.402 1.153 1.143

2.159 1.978 2.052 1.489 1.167 1.392 1.162 1.146

free TCNE

n=0

1.360 1.150 1.422

n = 1−

1.429 1.158 1.405

π-bonded TCNE (A) n=0 2.325 (W−Ce) 2.046 2.076 1.460 1.151 1.433 1.130 1.128

a

Bond lengths of symmetry-linked bonds are averaged. bDefinitions: Ce, ethene-C of TCNE; Cn, nitrile C of TCNE.

molecules (Figure 3) separated by intracrystal TCNEcontaining regions (Figure S2, Supporting Information). Efforts to extract as much information as possible from the crystallographic analysis are described in detail in the Experimental Section. While the accuracy of this structure determination is not sufficient to discuss bond parameters in a meaningful way, the validity of the connectivity in B in contrast to that in A is without doubt. Close contacts (e.g., stacking) between molecules which may occur for TCNE-containing complexes1,6 were not observed (Figure S3, Supporting Information). Unfortunately, all attempts to record 13C NMR signals in CD2Cl2 or toluene-d8 solutions failed, even after experiments carried out for long times (>24 h). EPR signals 6306

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the operation of an MLCT (metal to ligand charge transfer) as the cause of the conspicuous long-wavelength transition;11 TDDFT calculations and the illustrated frontier orbitals in Figure 4 confirm this notion. With Mn(CO)2(C5H5), the resulting [Mn(CO)2(C5H5)(η1-TCNE)]7 was also established as a σ(N)-coordinated species,6a corresponding to B. DFT calculations (Table 3) shed light on the seemingly unresolved single ν(CN) bond of [W(CO)5(TCNE)] (Figure

Table 2. Crystal Data and Data Collection and Refinement Details for [W(CO)5(TCNE)]·0.5TCNE empirical formula Mr cryst size/mm T/K λ/Å cryst syst space group a/Å b/Å c/Å β/deg V/Å3 Z Dc/g cm−3 μ/mm−1 F(000) θ range/deg no. of collected (indep) rflns no. of obsd rflns (I > 2σ(I)) Rint no. of refined params (restraints) final R1 (I > 2σ(I)) final wR2 (all data) max, min Δρ/e Å−3

C14N6O5W·0.5C6N4 516.05 0.04 × 0.17 × 0.24 100 0.71073 monoclinic P21/c 23.694(2) 9.370(1) 7.389(1) 91.495(3) 1639.9(2) 4 2.090 7.084 960 2−26 20 785 (3336) 1968 0.0876 110 (0) 0.0703 0.1636 4.0 (C9), −8.9 (N5)

Table 3. DFT Calculated (PBE0/PCM-CH2Cl2) and Experimental CO (Top) and CN (Bottom) Stretching Frequenciesa ν/cm−1 of [W(CO)5(TCNE)]n n = −1

n=0 DFT 1963 1969 1971 2014 2057 2188 2248 2256 2274

(s) (s) (s) (vw) (s) (m)d (vw) (vw) (vw)

exptl 1950 1969 1969 2020 2044 2170 n.o. n.o. n.o.

(sh) (vs) (vs) (vw) (vs) (w)e

DFT 1901 1931 1932 1986 2073 2157 2165 2212 2219

(s) (s) (s) (vw) (w) (w) (w) (w) (w)

n = −2 exptl

1893 1938 1938 1980 2071 2152 2188 n.o. n.o.

b

(m) (vs) (vs) (sh) (w) (w) (w)

DFT 1868 1914 1918 1972 2063 2107 2113 2168 2183

(s) (s) (s) (vw) (w) (w) (w) (w) (w)

exptlb 1874 1928 1928 1975 2096 2146 2170 n.o. n.o.

(s) (vs) (vs) (sh) (m)c (m) (w)

a

Scaling factor 0.956. bFrom spectroelectrochemistry in CH2Cl2/0.1 M Bu4NBF4. cAdditional bands at 2080 and 2065 cm−1. dCoupled vibration ν(CN) + ν(COtrans). eAdditional shoulder in electrolytecontaining solutions at about 2188 cm−1.

S4, Supporting Information).9,10 As a consequence of very strong back-bonding between the W(CO)5 complex fragment and the unique π acceptor1 TCNE, the ν(CN) bands of η1TCNE exhibit very different intensities, with the vibration of the coordinated CN function dominating (Table 3 and Figure 5). This vibration is coupled to and enhanced by the stretching

Figure 3. Molecular structures of the constituents in the crystal of [W(CO)5(TCNE)]·0.5TCNE.

were not observed for the solid or solution of [W(CO)5(TCNE)] unless reduction was carried out (cf. below). Both DFT (Table 1 and Figure 4) and experimental data support the oxidation state distribution [W0(CO)5(TCNE0)] for the compound, in the observed form B. The absence of full metal to ligand electron transfer in the ground state allows for Figure 5. DFT calculated and scaled (scaling factor 0.956) absorption bands in the ν(CN) and ν(CO) stretching region for [W(CO)5(TCNE)]n: (solid line) n = 0; (broken line) n = 1−.

motion of the trans-positioned CO ligand on tungsten; the other three CN stretching vibrations were calculated with very low intensities and were experimentally undetected. This led to the appearance of only a single ν(CN) band in the IR spectrum,9,10 on which the assignment of the π(olefin) complex structure A1 was based. For compounds [Mn(CO)2(C5R5)(η1-TCNE)] the IR spectra showed two major ν(CN) absorption bands; however,

Figure 4. DFT-calculated frontier orbitals of A and B isomers of [W(CO)5(TCNE)]. 6307

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one further weak band was reported.1,6a,7 A total of four (expected) bands was observed for [V(C 5 H 5 ) 2 Br(η 1 TCNE)].1,6b The ν(CO) pattern observed and calculated for [W(CO)5(TCNE)] (Figures 5 and 8) is not unusual. Like ν(CN) the values for ν(CO) are well reproduced by DFT calculations after scaling (Table 3); when compared with related compounds,9b,14 they reflect the flow of electron density from the metal to the TCNE acceptor ligand via rather high values ν(CO). The unique solvatochromism11 of [W(CO)5(TCNE)] is independent of the coordination: in both cases, A and B, the unreduced acceptor ligand TCNE would be exposed to πdonating solvent molecules. Redox System [W(CO)5(TCNE)]0/−/2−. In light of the structural reassessment the redox system [W(CO)5(TCNE)]n was investigated spectroelectrochemically (EPR, IR, UV−vis). Cyclic voltammetry12a shows (Figure 6) that the title

process (square scheme).15 Spectroelectrochemistry (vide infra) confirms that the overall cycle is reversible. Couple [W(CO)5(TCNE)]0/−. The electrogenerated EPR spectrum of [W(CO) 5 (TCNE)] •− in CH 2 Cl 2 /0.1 M Bu4NPF6 could be simulated with a set of three different coupling constants (1N, 2.46 G; 2N, 1.62 G; 1N, 1.36 G; g = 2.017) for 14N nuclei of nonequivalent nitrile groups (Figure 7). This result clearly points to structure B, as established now also for the neutral form.

Figure 7. EPR spectrum of [W(CO)5(TCNE)]•− in 0.1 M Bu4NPF6/ CH2Cl2.

The hyperfine splitting is similar to the patterns observed and calculated for structurally characterized diruthenium(I) complexes of the η1-coordinated TCNE•−‑ radical ligands.12b The relatively large isotropic g factor of 2.017 in comparison to the free electron value of 2.0023 suggests some non-negligible participation of the third-row transition element tungsten with its high spin−orbit coupling constant13 at the singly occupied molecular orbital. IR spectroelectrochemistry in the CN and CO stretching regions reveals an absorption appearance of the neutral starting compound [W(CO)5(TCNE)] which is slightly different from that recorded in pure, electrolyte-free solution. Additional shoulders are observed for the main ν(CN) band in various electrolyte solutions (see Figure 11 and Figures S5−S9 (Supporting Information)). We infer that the polar, open structure B and the polar metal donor/TCNE acceptor interaction allow for an association with excess (0.1 M) electrolyte to influence the vibrational absorption features. The shifts of ν(CN) and ν(CO) on reduction [W(CO)5(TCNE)]0/− are well reproduced by DFT calculations (Figures 5 and 8, Table 3). More than one ν(CN) band is now visible, which is attributed to diminished coupling between the trans CO and coordinated CN function. The second reduction involving the couple [W(CO)5(TCNE)]•−/2− exhibits reversible spectroelectrochemical behavior, i.e. isosbestic points and near 100% spectral restoration on reoxidation, despite the distorted waves in the cyclic voltammogram (Figure 6). The shifts of the ν(CN) and ν(CO) stretching bands (Figure 9) are reproduced by the DFT calculations, confirming the computed twist of the TCNE2− ligand along the central C−C axis (single bond; see Figure 10).

Figure 6. Cyclic voltammograms for the reduction of [W(CO)5(TCNE)] in 0.1 M Bu4NPF6/CH2Cl2 at different scan rates.

compound can be reduced in two chemically reversible oneelectron steps (Epc1 = −0.17 V, Epc2 = −1.22 V vs Fc+/0 at a 100 mV/s scan rate), the second of which involves slow charge transfer due to extensive structural change (cf. Table 1 and Figure 10). DFT calculated structural changes (Table 1) on the first reduction [W(CO)5(TCNE)]0/− reflect the TCNE-based electron uptake. As a result, the central C−C bond lengthens (bond order change 2 → 1.5), and the N(TCNE)−W bond weakens because of greatly diminished back-bonding (halfoccupation of the π* MO). Further smaller changes include the weakening of the C−O bonds (cf. the ν(CO) shifts) and the strengthening of the W−C(O) bonds due to lack of competition from TCNE. The planarity of the conjugated π radical ligand TCNE•− is maintained, in agreement with a corresponding wave (Figure 6) for the couple [W(CO)5(TCNE)]0/−. In contrast, the acquisition of a second electron to produce [W(CO)5(TCNE)]2− is calculated to result in a twist of the ligand along the central C−C bond to orthogonality (see Figure 10), which reflects the calculated single-bond character (1.489 Å, Table 1).1,8 This significant rearrangement is accompanied by large Epc/Epa differences for the second reduction wave for [W(CO)5(TCNE)]•−/2− in the cyclic voltammogram (Figure 6), which corresponds to an EC/EC 6308

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Figure 8. IR spectral changes on the first reduction of [W(CO)5(TCNE)] in 0.1 M Bu4NBF4/CH2Cl2.

Figure 11. (top) UV−vis spectral changes on the first reduction of [W(CO)5(TCNE)] in 0.1 M Bu4NBF4/CH2Cl2. (bottom) Simulated UV−vis spectra of [W(CO) 5 (TCNE)] (red) and [W(CO)5(TCNE)]− (blue) from TD-DFT calculations.

Figure 9. IR spectral changes on the second reduction of [W(CO)5(TCNE)] in 0.1 M Bu4NBF4/CH2Cl2.

at 559 nm emerges. Figure 11 shows that TD-DFT calculations qualitatively reproduce the spectrum of [W(CO)5(TCNE)] and its change after reduction. The intense absorption of the neutral complex around 800 nm is assigned to a HOMO → LUMO (MLCT/IL) transition calculated at 660 nm. The second intense feature around 300 nm is assigned to an MLCT transition from metal-centered occupied orbitals to CO-based π* orbitals. The absorption at 559 nm of the reduction product is attributed to a βHOMO → βLUMO (MLCT/IL) transition. The shapes of the βHOMO and βLUMO are similar to those of the HOMO and LUMO of the neutral species, respectively. The second reduction producing [W(CO)5(TCNE)]2− with the twisted TCNE2− ligand exhibits absorptions (Figure 12) which are reproduced by TD-DFT calculations and can be attributed to preferably MLCT(CO) transitions. Summarizing, the reinvestigation of the title compound and its structural reassignment as an N-coordinated complex illustrate the value of DFT methodology, in addition to crystallographic studies (if possible), in the assessment of ambivalent structural situations. While an electronic description such as the MLCT formulation of the low-energy transition and the spectroelectrochemical response remain less affected, this study on [W(CO)5(TCNE)] obviously contains a clear caveat concerning relying on vibrational data alone for structural identification.

Figure 10. DFT calculated structure of [W(CO)5(TCNE)]2−.

In CH2Cl2/0.1 M Bu4NBF4 (Figure 11) the intense longwavelength absorption band of [W(CO)5(TCNE)] does not exhibit a pronounced shoulder as in the electrolyte-free solution (Figure S1, Supporting Information). On one-electron reduction that band disappears, and a less intense absorption 6309

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allowed for a solution of the Patterson function (SHELXS-97)18a to give the position of the tungsten atom; the light atoms were subsequently located in calculated difference electron density maps. In the first refinement the tungsten atom was assigned an anisotropic thermal factor and the light atoms were given isotropic thermal factorsthis refinement (SHELX-97)18b proceeded smoothly to full convergence. An attempt was then made to also refine the light atoms anisotropically, but this led to a nonpositive Uij tensor for four atoms, viz., two carbonyl carbon atoms (C1 and C5) and the two olefinic carbon atoms (C7 and C9) of the bonded TCNE ligand. This is not an unexpected result, since the refined values most affected by the systematic errors caused by absorption are the anisotropic displacement parameters. Atoms C1, C5, C7, and C9 were then restrained so that their Uij components approximated isotropic behavior: this does prevent these atoms from becoming “non-positive definite”, but only if unrealistically low effective standard deviations are used with the SHELX command ISOR. Moreover, anisotropic refinement of the light atoms does not improve the structural model in any way: the R factor hardly drops after an increase in the number of least-squares parameters from 110 to 235, and there is no associated reduction in the size of the peaks in the final difference Fourier. In the end it was decided to simply refine the light atoms isotropicallytheir contribution to the overall scattering is in any event relatively small. We then addressed the problem of the high residual electron density in the final difference Fourier map: in particular, the large positive peak corresponding to 4 electrons that is 0.88 Å from C9 and the very large negative peak of −9 electrons that is 1.48 Å from N5. Of most concern is whether this is a disorder problem involving a second orientation of the TCNE ligand. This does not appear to be the case, because plots of those residual peaks that are significantly above background do not fit with any kind of chemical speciesthey should, if the problem is one of disorder. Another aspect could be twinning, but a twin refinement was not successful. In fact, problems with high residual electron density are not necessarily caused by disorder or twinning but can be due to inadequately corrected absorptionwe believe that to be the case here. To conclude, this is not an accurate, well-refined structure, but it is fundamentally correct. Its value is that it solves a chemical problemthere can be no question that the TCNE ligand is σ-bonded through a nitrile nitrogen to the tungsten atom. Quantum Chemical Calculations. The electronic structures of [W(CO)5(TCNE)] and its reduced form were calculated by density functional theory (DFT) method using the Gaussian 0919 program package. G09/DFT calculations employed the Perdew−Burke−Ernzerhof20,21 (PBE0) hybrid functional. The geometry of the anionic form was calculated by the UKS approach. Low-lying excitation energies were calculated by time-dependent DFT (TD-DFT) at the optimized geometry of the corresponding oxidation state. For C, N, and O atoms, either polarized triple-ζ basis sets 6-311G(3df)22 for geometry optimization and vibrational analysis or cc-pvdz correlation consistent polarized valence double-ζ basis sets23 (TD-DFT) were used, together with quasi-relativistic effective core pseudopotentials and a corresponding optimized set of basis functions for W.24 The solvent was described by the polarizable conductor model (PCM).25

Figure 12. (top) UV−vis spectral changes on the second reduction of [W(CO)5(TCNE)] in 0.1 M Bu4NBF4/CH2Cl2. (bottom) Simulated UV−vis spectra of [W(CO) 5 (TCNE)] − (red) and [W(CO)5(TCNE)]2− (black) from TD-DFT calculations.

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EXPERIMENTAL SECTION

Instrumentation. EPR spectra in the X band were recorded with a Bruker System EMX instrument. IR spectra were obtained using a Nicolet 6700 FT-IR instrument; solid-state IR measurements were performed with an ATR unit (smart orbit with diamond crystal). UV− vis−near-IR absorption spectra were recorded on J&M TIDAS and Shimadzu UV 3101 PC spectrophotometers. Cyclic voltammetry was carried out in 0.1 M Bu4NPF6 solutions using a three-electrode configuration (glassy-carbon working electrode, Pt counter electrode, Ag reference) and a PAR 273 potentiostat and function generator. The ferrocene/ferrocenium (Fc/Fc+) couple served as internal reference. Spectroelectrochemistry was performed using an optically transparent thin-layer electrode (OTTLE) cell.16 A two-electrode capillary served to generate intermediates for X-band EPR studies.17 The synthesis of complexes [M(CO)5(TCNE)] (M = Cr, W) has been described.9−11 Crystal Structure Studies. Out of numerous crystallization attempts, the most suitable specimen for single-crystal X-ray diffraction was obtained via slow evaporation of a dichloromethane solution with exclusion of light. The measurements were carried out using a Bruker Kappa Apex 2 duo diffractometer at 100 K (Mo Kα radiation, λ = 0.710 73 Å). Crystallography of [W(CO)5(TCNE)]·0.5TCNE. The crystal used for the intensity data collection was a thin plate of variable thickness. As a result, it was difficult to apply accurate absorption corrections, a serious problem in view of the fact that the linear absorption coefficient is high and that the intensity data are therefore subject to systematic errors. Neither multiscan absorption correction nor numerical absorption correction gave a different result. The crystal also diffracts weakly, even at 100 K, the temperature used for the data collection. Nevertheless, it was possible to collect intensity data that

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ASSOCIATED CONTENT

S Supporting Information *

A CIF file giving crystal data for [W(CO)5(TCNE)], bond lengths for [W(CO)5(TCNE)]·0.5TCNE (Table S1), the absorption spectra of [W(CO) 5 (TCNE)] and [Cr(CO)5(TCNE)] in pure CH2Cl2 (Figure S1), the packing of molecules in the crystal of [W(CO)5(TCNE)]·0.5TCNE (Figure S2), intermolecular contacts in [W(CO)5(TCNE)]·0.5TCNE (Figure S3), the IR spectrum of [W(CO)5(TCNE)] in pure CH2Cl2 (Figure S4), the IR spectrum of [W(CO)5(TCNE)] in CH2Cl2/0.1 M Bu4NPF6 (Figure S5), the IR spectrum of [W(CO)5(TCNE)] in CH2Cl2/0.1 M Bu4NClO4 (Figure S6), the IR spectrum of 6310

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(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc., Wallingford, CT, 2009. (20) Perdew, J. P.; Burke, K.; Enzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (21) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158. (22) Curtiss, L. A.; McGrath, M. P.; Blaudeau, J.-P.; Davis, N. E.; Binning, R. C., Jr.; Radom, L. J. Chem. Phys. 1995, 103, 6104. (23) Woon, D. E.; Dunning, T. H., Jr. J. Chem. Phys. 1993, 98, 1358. (24) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (25) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669.

[W(CO)5(TCNE)] in CH2Cl2/0.1 M Bu4NOTf (Figure S7), the IR spectrum of [W(CO)5(TCNE)] in CH2Cl2/0.1 M Bu 4 NBF 4 (Figure S8), and the IR spectrum of [W(CO)5(TCNE)] in CH2Cl2/0.02 M Bu4NBPh4 (Figure S9). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work has been supported by the Deutsche Forschungsgemeinschaft, by the Fonds der Chemischen Industrie, and by the European Union (COST D35 and CM1002). S.Z. thanks the Ministry of Education of the Czech Republic (grant LD11086) for support. We thank Dr. W. Frey for crystal measurements.

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REFERENCES

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dx.doi.org/10.1021/om300565q | Organometallics 2012, 31, 6305−6311