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The first bismuth–NHC complexes† Cite this: Dalton Trans., 2014, 43, 764

Antonino Aprile, Robert Corbo, Kel Vin Tan, David J. D. Wilson and Jason L. Dutton*

Received 30th September 2013, Accepted 17th October 2013

The synthesis, isolation and crystallographic characterization of the first N-heterocyclic carbene adducts

DOI: 10.1039/c3dt52715h

NHC with BiCl3. This represents the last non-radioactive element from groups 13–17 for which an NHC– element fragment remained unreported.

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of bismuth is reported, by direct reaction of the Dipp2NHC (Dipp = 2,6-diisopropylphenyl) or iPr2(Me2)-

Introduction The use of N-heterocyclic carbenes (NHCs) as ligands has proven to be of great utility in both transition metal and main group chemistry. In transition metal chemistry the focus has been the incorporation of NHCs as spectator ligands in catalysts, which has often resulted in improved catalyst activities and lifetimes.1 In main group chemistry, using NHCs as ligands has allowed access to previously unobserved bonding about a variety of element centres. The field has been undergoing a renaissance in recent years, in particular with the use of N-heterocyclic and related carbenes to access low oxidation state centres such as compounds 1–8 comprising: B(0),2 B(I),3 C(0),4 Si(0),5 Si(II),6 Ge(0),7 Sn(0),8 P(0),9,10 P(I)11 and As(0)12 and also highly charged centres, such as Ge2+,13 P3+,14 Se2+,15 and Te2+.16

In nearly all of these cases the starting point was the formation of a simple coordination complex between a free NHC and a halide of the main group element of interest, followed by Department of Chemistry, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria, Australia. E-mail: [email protected] † Electronic supplementary information (ESI) available: Table of X-ray refinement details. Cartesian coordinates of optimized geometries. CCDC 942985 and 963493. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52715h

764 | Dalton Trans., 2014, 43, 764–768

either reduction (e.g. 1–3, 5) or halide abstraction/displacement (e.g. 6, 8). Most of the simple NHC–EXn complexes (E = element, X = halide), for example the NHC–SiCl4 precursor to NHC–SivSi–NHC 2E, had not been synthesized until recently, which is surprising given the explosion of NHC coordination chemistry in the past two decades. Despite the great deal of work in the field of main groupNHC coordination chemistry,17 for some elements such simple complexes are still scarce. In particular for the heavier p-block elements very few have been reported. For Tl there are two examples,18,19 three for Pb,8,20,21 one for Sb22 and none for Bi, the only element from groups 13–17 without a reported molecule containing an NHC–E fragment (radioactive Po and At excepted). The choice of halide is also important; attempts at isolating NHC–PbCl2 were unsuccessful, but NHC–PbBr2 could be crystallographically characterized for the Dipp2NHC (Dipp = 2,6-diisopropylphenyl).8 It is particularly surprising that there are no Bi–NHC complexes given the ease of handling Bi(III) halides as compared to Sb(III) or Sb(V) halides for example, and the substantial amount of known organobismuth chemistry.23 Previous theoretical work from our group has predicted the Ph2NHC–BiCl3 adduct to be stable by 118 kJ mol−1 with respect to dissociation to Ph2NHC and BiCl3.24 Of the NHC–ECl3 complexes with the heavier pnictogens (E = P, As, Bi, Sb), the NHC–BiCl3 complex was predicted to be the most stable; the stability increases monotonically going down the group from P to Bi. In this context we now report the isolation and crystallographic characterization of the first bismuth–NHC complexes from the direct reaction of BiCl3 with the NHCs Dipp2NHC or iPr2(Me2)NHC, and the attempts at their reduction to analogues of 3Pn.

Results and discussion The addition of a THF solution of Dipp2NHC to a THF solution of BiCl3 resulted in no visible change. After 10 minutes a white solid was precipitated by addition of n-hexane. After a short work-up a sample of the powder was dissolved in C6D6

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for proton NMR spectroscopy, which revealed a clean spectrum containing resonances consistent with one Dipp2NHC containing compound. The resonance associated with the diagnostic “backbone” protons was shifted slightly downfield to 6.49 ppm, nearly identical to the chemical shift reported for the AsCl3 adduct of the same NHC.12 Single crystals were grown by diffusion of Et2O into an MeCN solution of the bulk material. The crystals obtained were of average to poor quality, but sufficient for collection of a data set. Solution and refinement confirmed the crystals to be the Dipp2NHC–BiCl3 adduct 9, the first such NHC complex for bismuth (Scheme 1, Fig. 1). While the crystals were of low quality, the final refined data does allow for a limited discussion on the metrical parameters. The Bi–C bond distance is 2.389(8) Å, which can be compared to 2.24–2.27 Å in Ph3Bi.25 The Bi–Cl bonds are all different from each other, with bond distances of 2.611(2), 2.702(2) and 2.438(2) Å for Bi(1)–Cl(1), Bi(1)–Cl(2) and Bi(1)–Cl(3), respectively. DFT (M06-2X/def2-TZVP) optimized gas-phase geometries for Dipp2NHC–BiCl3 exhibit Cs molecular symmetry, yielding Bi–Cl bond distances of 2.635, 2.635 and 2.452 Å, respectively. The calculated Bi–C bond distance is 2.350 Å. Similar results were obtained for the analogous R2NHC–BiCl3 (R = Me, Ph) complexes. The elongation of Bi(1)–Cl(2) in the solid state is caused by a short contact (3.129(2) Å) with another unit of 9, donating into the vacant coordination site on Bi trans to C(1) forming a dimer, which is observed in other adducts of BiCl3.26

Scheme 1

Synthesis of compound 9.

Fig. 1 Solid state structure of 9, depicting the dimer formed between two asymmetric units. Hydrogen atoms, isopropyl groups and Et2O solvate omitted for clarity. Selected bond distances (Å) [M06-2X/def2TZVP calculated values in brackets]: Bi(1)–C(1) 2.389(8) [2.350], Bi(1)– Cl(1) 2.611(2) [2.642], Bi(1)–Cl(2) 2.702(2) [2.642], Bi(1)–Cl(3) 2.438(2) [2.488], Bi(1A)–Cl(2) 3.129(2).

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Scheme 2

Synthesis of compounds 10 and 11.

Mass spectral data on a THF solution of 9 gave fragments consistent with [NHC–Bi·3THF]+ (oxidation state for Bi +1) in positive ion mode and [NHC–Cl–Cl]− in negative ion mode. This indicates that the Bi centre may be susceptible to the same types of auto-REDOX processes observed for similar adducts of Se and P halides with NHCs.11,27 We investigated whether changing the NHC would influence the course of the reaction, as differing NHCs have been demonstrated to influence REDOX processes for PCl3; a PCl3 adduct was isolated for aryl substituted NHCs9 while bis-NHC P(I) cations 7 were isolated for alkyl substituted NHCs.11 In this context, the iPr2(Me2)NHC was reacted with BiCl3 in THF (Scheme 2). After a short workup a colourless powder was obtained. A sample of the solid dissolved in CD3CN for 1H NMR revealed a set of signals consistent with a single NHC containing compound, with the methine isopropyl protons shifted to 4.50 ppm from 4.25 ppm in the free NHC. Despite many attempts single crystals could not be obtained to conclusively identify the product as the iPr2(Me2)NHC–BiCl3 adduct 10. Derivatization was considered using a halide abstraction reaction; the solid product was mixed with one stoichiometric equivalent of TMS-OTf in THF. Addition of n-hexane after 4 hours resulted in the precipitation of a colourless solid. The 1H NMR of a solution of the solid in CD3CN showed two compounds, one with the resonance associated with the methine proton on the isopropyl R-groups were shifted downfield to 5.10 ppm and the other (integrating to approximately 20%) with identical chemical shifts to starting material 10. Single crystals were obtained by vapour diffusion of Et2O into a THF solution of the mixture at −30 °C; X-ray diffraction studies revealed the crystals to be 11, with the NHC, one triflate, two chlorides, and a THF bound to Bi (Fig. 2), consistent with product containing downfield 1H NMR chemical shifts. In the solid state a dimer is formed with another unit via the triflate giving an overall octahedral geometry about Bi. The geometry about the Bi centre and the charge on the complex is consistent with Bi(III), allowing for the inference that the first complex obtained in reaction with BiCl3 is the simple adduct 10. Attempts to drive the synthesis of 11 to completion with an excess of TMS-OTf or AgOTf and longer reaction times resulted in the appearance of decomposition products. The NHC–Bi bond distance in 11 is 2.37(1) Å and the Bi–Cl bonds are virtually identical at 2.508(3) and 2.509(3) Å. The Bi–O bonds with the triflate are slightly asymmetric at 2.708(7) and 2.785(7) Å. A THF bound to Bi with a Bi–O bond of 2.675(7) Å completes an octahedral geometry about the Bi centre. The 19F{1H} chemical shift of the [CF3–SO3]− group in

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Fig. 3 Fig. 2 Solid state structure of 11. Thermal ellipsoids are shown at 30% probability. Hydrogen atoms are omitted for clarity. Selected bond distances (Å): Bi(1)–C(1) 2.37(1), Bi(1)–Cl(1) 2.508(3), Bi(1)–Cl(2) 2.5096(3), Bi(1)–O(1) 2.708(7), Bi(1)–O(4) 2.785(7), Bi(1)–O(2) 2.675(7).

CD3CN was found to be −78.3 ppm, which suggests the triflate anion is not covalently bound to the Bi in CD3CN solution.15 Attempts were made to reduce compounds 9 and 11 to analogues of compound 3Pn (NHC–P–P–NHC and NHC–As–As– NHC). The reductions were attempted using 3 equivalents of KC8 or 1.5 equivalents of Mg0 (as Mg turnings) in C6D6. After stirring for 12 hours an aliquot was removed and filtered for 1 H NMR. In all instances one set of resonances was observed that corresponded to the free NHC. When Mg0 was used a black precipitate could also be observed that is presumably Bi metal given the generation of free NHC. This is similar to the results observed by Jones and co-workers in the attempted generation of the 2Pb and for 2Sn if held at room temperature.8 We surmise that the calculated stability with respect to R2NHC dissociation from Bi2 of −66.9 (R = Ph) and −41.7 (R = Me) kJ mol−1 is insufficient to offset the favourable formation of metallic bismuth.24 This appears to be a general trend and limiting factor for the isolation of the heavier analogues of these species for both group 14 and group 15. As these heavier main group element–NHC complexes are rare we performed some additional DFT calculations to probe the nature of the bonding in the NHC–BiCl3 adducts, using R2NHC (R = Me, Ph, Dipp). Energy decomposition analysis (EDA)28 calculations (Table 1) indicate that the interaction

Table 1 Results of the EDA calculations for NHCR–BiCl3 calculated with BP86/TZ2P (kcal mol−1)a

ΔEint ΔEPauli ΔEElstatb ΔEorbb

R = Me

R = Ph

R = Dipp

−49.2 189.0 −144.5 (60.7) −92.1 (39.3)

−44.9 181.0 −133.8 (59.2) −92.1 (40.8)

−46.3 185.3 −136.7 (59.0) −95.0 (41.0)

a

M06-2X/def2-TZVP optimized geometries. b Values in parentheses give the percentage of the total attractive interactions ΔEElstat + ΔEorb.

766 | Dalton Trans., 2014, 43, 764–768

Important MOs of R2NHC–BiCl3 (R = Me).

between the R2NHC and BiCl3 fragments is about 40% covalent (orbital) and 60% electrostatic, independent of R = Me, Ph or Dipp. That result is consistent with a donor–acceptor description of this complex.29,30 EDA-NOCV analysis shows the orbital contribution to be predominantly R2NHC→BiCl3 donation (80% of orbital interaction; Table 1). Important MOs of R2NHC–BiCl3 are illustrated in Fig. 3 (R = Me). The largest contribution to the Bi–C sigma bond is relatively stabilized, found in the HOMO−11. The corresponding antibonding orbital is found in the LUMO+1, which acts as the accepting orbital in the solid state for formation of the dimer. The HOMO contains contributions from chlorine atom lone pairs as well as the Bi s-lone pair, which is fairly high in energy for the lone pair of a heavy p-block atom.

Conclusions In conclusion, we have described the synthesis of the first NHC–Bi complexes. It appears the Bi(0) NHC–Bi–Bi–NHC compounds will be inaccessible, but the crystallographic characterization of 11 suggests that exploration of NHC–Bi cations will be possible, which we are now investigating further. From a wider perspective, Bi was the last non-radioactive group 13–17 element for which an NHC complex had not been reported, which is an indication of how entrenched NHC chemistry has become in the p-block.

Experimental section All manipulations were carried out under an N2 atmosphere in a glovebox. BiCl3 was anhydrous grade purchased from Alfa Aesar and used as received. TMS-OTf was purchased from Alfa Aesar and used as received. Solvents for both synthesis and NMR were dried by stirring for 2 days over CaH2, then distilled and stored over 3 Å molecular sieves in the glove box. The NHCs Dipp2NHC and iPr2(Me2)NHC were synthesized via literature procedures.31,32

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Synthesis and isolation of 9 A solution of BiCl3 (0.071 g, 0.226 mmol; THF 5 mL) was added to a solution of Dipp2NHC (0.088 g, 0.226 mmol; THF 3 mL) giving a colorless solution. After 10 minutes n-hexane (15 mL) was added, resulting in the precipitation of a colorless solid. The supernatant was decanted, the solids washed with n-hexane (3 × 5 mL) and then the volatiles removed in vacuo giving 9 as a colorless powder. Yield 0.090 g, 56%. Solid darkens >150 °C, melts with decomposition 190 °C; 1H NMR (C6D6) 7.21 (t, 2H, J = 6.0 Hz), 7.16 (overlap with CHD5), 6.49 (s, 2H), 3.17 (sept, 4H, J = 6.0 Hz), 1.58 (d, 12H, J = 6.0 Hz), 0.91 (d, 12H, J = 6.0 Hz); 13C NMR (C4H8O) 22.6, 25.4, 28.6, 123.6, 124.2, 127.1, 130.4, 146.4; ESI-MS (M+): 390 (Dipp2NHC– H+), 814 (Dipp2NHC–Bi·3THF+), (M−) 461 (Dipp2NHC–Cl·Cl−). Synthesis of 10 A solution of BiCl3 (0.107 g, 0.318 mmol; THF 3 mL) was added to a solution of iPr2(Me2)NHC (0.060 g, 0.318 mmol; THF 2 mL) giving a white precipitate. After 15 min the supernatant was decanted, the solid washed with THF (3 × 5 ml) and the volatiles removed in vacuo giving compound 10 as a colorless powder. Yield 0.112 g, 67%. Solid darkens at 160 °C decomposition 195 °C; 1H NMR (CD3CN) 4.50 (sept, 2H J = 8.0 Hz), 2.25 (s, 6H), 1.51 (d, 12H, J = 8.0 Hz); 13C NMR (CD3CN) 7.7, 10.0, 21.8, 50.3, 126.7. Synthesis of 11 To a suspension of compound 10 (0.101 g; 0.203 mmol) in THF (5 ml) was added TMSOTf (60 µl; 0.326 mmol). After four hours n-hexane (10 ml) was added resulting in a white solid. The supernatant was decanted and the solid washed with n-hexane (3 × 5 ml). Volatiles were removed in vacuo giving compound 11 as a white solid, contaminated with approximately 20% of 10. Yield 0.062 g, 51%; 1H NMR (CD3CN) 5.08 (sept, 2H J = 8.0 Hz), 2.37 (s, 6H), 1.61 (d, 12H J = 8.0 Hz); 19F NMR (CD3CN) −78.3. ESI-MS (M+): 539 (iPr2NHC–Bi·CF3SO3+), 181 (iPr2NHC–H+), (M−) 148.8 (CF3SO3−). DFT calculations of geometries were carried out within the Gaussian program33 with the M06-2X functional;34 Both def2SV(P) and def2-TZVP basis sets35 were considered, with minimal difference in calculated geometries. Stationary points were characterized as minima by calculating the Hessian matrix analytically at the same level of theory. B3LYP/TZVP results were equivalent. EDA and EDA-NOCV calculations were carried out within ADF36 at the BP86/TZ2P level of theory. We thank the La Trobe Institute for Molecular Sciences and La Trobe University for their generous funding of this work. Grants of computing resources from VPAC and NCI-NF are acknowledged. This work is also supported by an ARC DECRA fellowship (JLD, DE130100186).

Notes and references 1 C. S. J. Cazin, N-Heterocyclic Carbenes, in Transition Metal Catalysis and Organocatalysis, Springer, 2011, vol. 32.

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2 H. Braunschweig, R. D. Dewhurst, K. Hammond, J. Mies, K. Radacki and A. Vargas, Science, 2012, 336, 1420–1422. 3 (a) Y. Wang, B. Quillian, P. Wei, C. S. Wannere, Y. Xie, R. B. King, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2007, 129, 12412–12413; (b) Y. Wang, B. Quillian, P. Wei, Y. Xie, C. S. Wannere, R. B. King, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2008, 130, 3298–3299; (c) R. Kinjo, B. Donnadieu, M. A. Celik, G. Frenking and G. Bertrand, Science, 2011, 333, 610–613; (d) P. Bissiner, H. Braunschweig, A. Damme, T. Kupfer and A. Vargas, Angew. Chem., Int. Ed., 2012, 51, 9931–9934. 4 C. A. Dyker, V. Lavallo, B. Donnadieu and G. Bertrand, Angew. Chem., Int. Ed., 2008, 47, 3206–3209. 5 (a) Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. I. Schaefer, P. R. Schleyer and G. H. Robinson, Science, 2008, 321, 1069–1071; (b) K. C. Mondal, H. W. Roesky, M. C. Schwarzer, G. Frenking, B. Niepötter, H. Wolf, R. Herbst-Irmer and D. Stalke, Angew. Chem., Int. Ed., 2013, 52, 2963–2967; (c) Y. Xiong, S. Yao, S. Inoue, J. Epping and M. Driess, Angew. Chem., Int. Ed., 2013, 52, 7147. 6 (a) R. S. Ghadwal, H. W. Roesky, S. Merkel, J. Henn and D. Stalke, Angew. Chem., Int. Ed., 2009, 48, 5683–5686; (b) A. C. Fillippou, O. Chernov and G. Schnakenburg, Angew. Chem., Int. Ed., 2009, 48, 5687–5690. 7 (a) A. Sidiropoulos, C. Jones, A. Stasch, S. Klein and G. Frenking, Angew. Chem., Int. Ed., 2009, 48, 9701–9704; (b) Y. Xiong, S. Yao, G. Tan, S. Inoue and M. Driess, J. Am. Chem. Soc., 2013, 135, 5004–5007. 8 C. Jones, A. Sidiropoulos, N. Holzmann, G. Frenking and A. Stasch, Chem. Commun., 2012, 48, 9855–9857. 9 Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. I. Schaefer, P. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2008, 130, 14970–14971. 10 J. Masuda, W. W. Schoeller, B. Donnadieu and G. Bertrand, J. Am. Chem. Soc., 2007, 129, 14180–14181. 11 B. D. Ellis, C. A. Dyker, A. Decken and C. L. B. Macdonald, Chem. Commun., 2005, 1965–1967. 12 (a) M. Y. Abraham, Y. Wang, Y. Xie, W. Pingrong, H. F. Schaefer, III, P. v. R. Schleyer and G. H. Robinson, Chem.–Eur. J., 2009, 16, 432–435; (b) For use of 3As to generate an As radical and As dication see: M. Y. Abraham, Y. Wang, Y. Xie, R. J. Gillard Jr., P. Wei, B. J. Vaccaro, M. K. Johnson, H. F. Schaefer III, P. v. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2013, 135, 2486– 2488. 13 P. A. Rupar, V. N. Staroverov, P. J. Ragogna and K. M. Baines, J. Am. Chem. Soc., 2007, 129, 15138–15139. 14 J. Petukova, M. Patil, S. Holle, C. W. Lehmann, W. Thiel and M. Alcarazo, J. Am. Chem. Soc., 2011, 133, 20758–20760. 15 J. L. Dutton, T. L. Battista, M. J. Sgro and P. J. Ragogna, Chem. Commun., 2010, 46, 1041–1043. 16 J. L. Dutton, H. M. Tuononen and P. J. Ragogna, Angew. Chem., Int. Ed., 2009, 48, 4409–4413. 17 N. Kuhn and A. Al-Sheikh, Coord. Chem. Rev., 2005, 249, 829–857.

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18 H. Nakai, Y. Tang, P. Gantzel and K. Meyer, Chem. Commun., 2003, 24–25. 19 M. L. Cole, A. J. Davies and C. Jones, J. Chem. Soc., Dalton Trans., 2001, 2451. 20 F. Stabenow, W. Saak and M. Weidenbruch, Chem. Commun., 1999, 1131–1132. 21 B. Gerhus, P. B. Hitchcock and M. F. Lappert, J. Chem. Soc., Dalton Trans., 2000, 3094. 22 A. J. Arduengo III, F. Davidson, R. Krafczyk, W. J. Marshall and R. Schmutzler, Monatsh. Chem., 2000, 131, 251. 23 A search of the Cambridge Structural Database returns 627 hits for structures containing a Bi–C bond. 24 D. J. D. Wilson, S. Couchman and J. L. Dutton, Inorg. Chem., 2012, 51, 7657–7668. 25 P. G. Jones, A. Blaschette, D. Henschel and A. Weitze, Z. Kristallogr., 1995, 210, 377. 26 N. J. Hill, G. Reeske, J. A. Moore and A. H. Cowley, Dalton Trans., 2006, 4838. 27 J. L. Dutton, R. Tabeshi, M. C. Jennings, A. Lough and P. J. Ragogna, Inorg. Chem., 2007, 46, 8594–8602. 28 M. von Hopffgarten and G. Frenking, WIREs: Comput. Mol. Sci., 2012, 2, 43–62. 29 D. Nemcsok, K. Wichmann and G. Frenking, Organometallics, 2004, 23, 3640–3646. 30 G. Frenking, K. Wichmann, N. Fröhlich, J. Grobe, W. Golla, D. Le Van, B. Krebs and M. Läge, Organometallics, 2002, 21, 2921–2930. 31 L. Jararpour, E. D. Stevens; and S. P. Nolan, J. Organomet. Chem., 2000, 606, 49–54. 32 N. Kuhn and T. Kratz, Synthesis, 1993, 561–563. 33 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,

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H. Nakai, T. Vreven, J. J. A. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision A.1, Gaussian, Inc., Wallingford, CT, 2009. 34 Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215–241. 35 F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005, 7, 3297. 36 E. J. Baerends, T. Ziegler, J. Autschbach, D. Bashford, A. Bérces, F. M. Bickelhaupt, C. Bo, P. M. Boerrigter, L. Cavallo, D. P. Chong, L. Deng, R. M. Dickson, D. E. Ellis, M. van Faassen, L. Fan, T. H. Fischer, C. Fonseca Guerra, A. Ghysels, A. Giammona, S. J. A. van Gisbergen, A. W. Götz, J. A. Groeneveld, O. V. Gritsenko, M. Grüning, S. Gusarov, F. E. Harris, P. van den Hoek, C. R. Jacob, H. Jacobsen, L. Jensen, J. W. Kaminski, G. van Kessel, F. Kootstra, A. Kovalenko, M. V. Krykunov, E. van Lenthe, D. A. McCormack, A. Michalak, M. Mitoraj, J. Neugebauer, V. P. Nicu, L. Noodleman, V. P. Osinga, S. Patchkovskii, P. H. T. Philipsen, D. Post, C. C. Pye, W. Ravenek, J. I. Rodríguez, P. Ros, P. R. T. Schipper, G. Schreckenbach, J. S. Seldenthuis, M. Seth, J. G. Snijders, M. Solà, M. Swart, D. Swerhone, G. te Velde, P. Vernooijs, L. Versluis, L. Visscher, O. Visser, F. Wang, T. A. Wesolowski, E. M. van Wezenbeek, G. Wiesenekker, S. K. Wolff, T. K. Woo and A. L. Yakovlev, ADF2012, SCM, Theoretical Chemistry, Vrije Universiteit, Amsterdam, The Netherlands, http://www.scm. com

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