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Organometallics 2009, 28, 6458–6461 DOI: 10.1021/om900654g
Electronic and Steric Parameters of 76 N-Heterocyclic Carbenes in Ni(CO)3(NHC) Dmitry G. Gusev* Department of Chemistry, Wilfrid Laurier University, 75 University Avenue W., Waterloo, ON N2L 3C5, Canada. Received July 23, 2009
We report electron-donor and steric properties of a diverse group of representative N-heterocyclic carbene (NHC) ligands quantified with the help of DFT calculations. This study afforded the conventional TEP data (Tolman electronic parameter = νCO (A1) of Ni(CO)3(NHC)), which allowed ranking 76 NHC ligands in order of increasing donor power. The TEP data reveal several general trends concerning the influence of NHC ring size, substitution, and annulation. The calculations also provided reaction enthalpies for CO elimination from the Ni(CO)3(NHC) complexes and formation of the 16-electron Ni(CO)2(NHC) species. This reaction is largely under steric control, which allowed defining a new steric descriptor for NHC ligands, r (“repulsiveness”) = 10 (7.568 - 0.003172TEP - 0.0446ΔH), ranging from 0.0 for the smallest (ImNH2) to 8.0 for the most repulsive carbene (ImNAd2) of this work. Ni(CO)3L and Os(H2)Cl2(CO)L2 complexes eliminate CO and H2, respectively, more readily with L = NHC vs PR3 ligands. Apparently, even relatively small NHC ligands are very sterically demanding, and this property of N-heterocyclic carbenes may play a major role in coordination chemistry and catalysis.
Introduction The advance of N-heterocyclic carbene (NHC) ligands has become one of the most significant developments in modern *To whom correspondence should be addressed. E-mail: dgoussev@ wlu.ca. (1) (a) N-Heterocyclic Carbenes in Synthesis; Nolan, S. P., Ed.; WileyVCH: Weinheim, 2006. (b) N-Heterocyclic Carbenes in Transition Metal Catalysis; Glorius, F., Ed.; Springer-Verlag: Berlin, 2007. (c) Colacino, E.; Martinez, J.; Lamaty, F. Coord. Chem. Rev. 2007, 251, 726–764. (d) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776–1782. (e) Wuertz, S.; Glorius, F. Acc. Chem. Res. 2008, 41, 1523–1533. (f) Cavell, K. Dalton Trans. 2008, 6676–6685. (g) Marion, N.; Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440–1449. (h) Schuster, O.; Yang, L.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445–3478. (2) O’Brien, C. J.; Kantchev, E. A. B.; Valente, C.; Hadei, N.; Chass, G. A.; Lough, A.; Hopkinson, A. C.; Organ, M. G. Chem.;Eur. J. 2006, 12, 4743. (3) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1, 953. (4) (a) Altenhoff, G.; Goddard, R.; Lehmann, C. W.; Glorius, F. J. Am. Chem. Soc. 2004, 126, 15195–15201. (b) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo, L.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485–2495. (c) Herrmann, W. A.; Sch€utz, J.; Frey, G. D.; Herdtweck, E. Organometallics 2006, 25, 2437–2448. (d) T€urkmen, H.; Cetinkaya, B. J. Organomet. Chem. 2006, 691, 3749–3759. (e) Leuth€ausser, S.; Schwarz, D.; Plenio, H. Chem.;Eur. J. 2007, 13, 7195–7203. (f) Bazinet, P.; Ong, T.-G.; O'Brien, J. S.; Lavoie, N.; Bell, E.; Yap, G. P. A.; Korobkov, I.; Richeson, D. S. Organometallics 2007, 26, 2885–2895. (g) Bittermann, A.; H€arter, P.; Herdtweck, E.; Hoffmann, S. D.; Herrmann, W. A. J. Organomet. Chem. 2008, 693, 2079–2090. (h) Kelly, R. A.III; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P. Organometallics 2008, 27, 202–210. (i) Khramov, D. M.; Rosen, E. L.; Er, J. A. V.; Vu, P. D.; Lynch, V. M.; Bielawski, C. W. Tetrahedron 2008, 64, 6853–6862. (j) Song, G.; Zhang, Y.; Li, X. Organometallics 2008, 27, 1936–1943. (k) Hirano, K.; Urban, S.; Wang, C.; Glorius, F. Org. Lett. 2009, 11, 1019–1022. (l) Tonner, R.; Frenking, G. Organometallics 2009, 28, 3901–3905. pubs.acs.org/Organometallics
Published on Web 10/29/2009
organometallic chemistry and catalysis.1 This is best illustrated by the examples of efficient cross-coupling2 and second-generation olefin metathesis3 N-heterocyclic carbene metal catalysts, both types exhibiting higher catalytic activities than many of their non-NHC analogues. NHCs comprise a remarkably varied class of ligands possessing the structural and electronic diversity similar to, and in certain ways exceeding, that known for phosphines. Understanding ligand steric and electronic properties is essential for successful organometallic synthesis and catalyst design, and a growing amount of information about stereoelectronic properties of N-heterocyclic carbenes has been collected in the last 3-4 years.4 IR spectroscopy proved to be a particularly useful technique for the evaluation of ligand donor properties via CO stretching frequencies of metal carbonyls. A number of recent experimental and computational studies have employed Ni(CO)3(NHC), Rh(CO)2X(NHC), and Ir(CO)2X(NHC) (X = halide) complexes.4 Consequently, as noted by F€ urstner, the use of different metal carbonyl templates “requires careful consideration upon comparison of the different scales”.5 In order to place all representative NHC ligands on the same stereoelectronic scale, in a fashion similar to the seminal work of Chadwick Tolman,6 we have carried out a study of a large group of Ni(CO)3(NHC) (5) (a) F€ urstner, A.; Alcarazo, M.; Krause, H.; Lehmann, C. W. J. Am. Chem. Soc. 2007, 129, 12676–12677. (b) A particularly difficult aspect of this consideration involves answering the question of whether different carbonyl complexes all provide meaningful results. For example, the author noted in a recent paper8 the suspicious behavior of the IrCl(CO)2L carbonyl system, where PCy3 and PPh3 appear to be better donors than PtBu3 and PMePh2, respectively. (6) Tolman, C. A. Chem. Rev. 1977, 77, 313. r 2009 American Chemical Society
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complexes. The results presented below include νCO(A1) data (also known as the Tolman electronic parameter, or TEP) and a new steric descriptor, named r (“repulsiveness”), which correlates with the ease of CO loss from the Ni(CO)3(NHC) complexes.
Results and Discussion Good quality IR data can be obtained with the help of DFT calculations, and scaled calculated νCO frequencies match the experimental νCO values when solvent effects are systematic.4h,l,7,8 This relationship is observed for a representative series of 21 Ni(CO)3L complexes in Figure 1, where the standard error is 0.97 cm-1. The small residual differences between νexp and scaled νcalc values can be due to a combination of factors. The νexp values for Ni(CO)3(PR3) were obtained 40 years ago from the spectra plotted using a 5 cm-1 abscissa and calibrated against an external reference. This way, “band centers could be measured with a precision of ( 0.5 cm-l”.6 Additionally, experimental νCO values exhibit small variations due to specific solvent effects. For example, νCO(A1) values for IPr and sIMes, reported by Nolan, are the same in CH2Cl2 but differ by 0.4 cm-1 in hexane. ICy and IPr differ by ΔνCO(A1) = 1.9 cm-1 in CH2Cl2 and by 2.7 cm-1 in hexane.4b Calculated gas-phase νCO values are free from solvent effects, and the DFT method is a useful substitute for the traditional experimental approach, especially for compounds that could be challenging to make and/or handle, like many Ni(CO)3(NHC) complexes of this work. Seventy-six N-heterocyclic carbenes (Chart 1) have been included in this study. The majority of the NHC ligands of Chart 1 are found in structurally characterized compounds; additionally, this work includes carbenes that have not been made yet, possessing features that can be of interest for future research. Throughout this paper, the ligands are named following the following format: [parent heterocycle](substituents)nN(substituents)m. Parent heterocycles of carbenes of Chart 1 include imidazole (Im), benzimidazole (BIm), benzoxazole (BOx), imidazo[4,5]pyridines (Py[b]Im, Py[c]Im), imidazo[1,5-a]pyridine (Py[a]Im), dipyridoimidazole (DPyIm), imidazophenanthroline (ImPhen), isoxazole (isOx), oxazole (Ox), perimidine (Per), pyrazole (Pyraz), pyridine (PyC2, PyC3, PyC4), pyrimidine (Pm), pyrrole (Pyr), thiazole (Th), and triazole (Triaz). For example, ImNMe2 and ImNMeBut are derivatives of imidazole and have two NMe groups and one NMe and one NBut group, respectively. Saturated derivatives of the heterocycles are named accordingly, e.g., sImNMe2 or sImPh2NMe2. In the latter case, the heterocycle has Ph substituents at C-4,5. The “abnormal” (C-4 or C-5) binding of the ligands derived from imidazole is indicated by the prefix “a”, e.g., aImNMe2. Such abbreviations are not comprehensive, yet they offer a useful alternative to the less explicit names found in the literature, e.g., ICy, IPr, or SIPr. Three names are used in this paper as they appear in the literature: IMes, sIMes, and IBioxMe4. For practical reasons, two new ligands structurally related to IBioxMe4 have been nicknamed IBinitMe4 and IBicarMe4 (see Chart 1). (7) Perrin, L.; Clot, E.; Eisenstein, O.; Loch, J.; Crabtree, R. H. Inorg. Chem. 2001, 40, 5806. (8) Gusev, D. G. Organometallics 2009, 28, 763–770.
Figure 1. Plot of calculated vs experimental νCO(A1) values for a series of Ni(CO)3L complexes with L = PR3 or NHC ligands; νexp = 0.9541νcalc. Data for L = PR3 are from our recent work.8
Electronic Parameters. Calculated TEP (cm-1) values are collected in Chart 1 and Table S1. Figure 2 is a plot of TEP vs average C-O bond distances in the Ni(CO)3(NHC) complexes, with added points for typical phosphines. Figure 2 helps to visualize differences observed between the different ligands. Six known experimental νCO(A1) frequencies of Ni(CO)3(NHC) complexes agree well with the calculated TEP values. The experimental and calculated data are practically the same for ImNCy2, IMes, sIMes, and ImN(CHMePh)2. For ImNDip2 and sImNDip2, the calculated frequencies are slightly lower, by 1.0 and 0.7 cm-1, respectively. Solvent effects could play a role in these two cases. Calculated TEPs (cm-1) are 2050.5 for both IMes and ImNDip2, whereas their saturated analogues, sIMes and sImNDip2, exhibit slightly higher frequencies: 2051.2 and 2051.5, respectively. Several other saturated carbenes in Chart 1 show νCO(A1) values systematically greater than those of the corresponding unsaturated systems, e.g., sImNMe2 (2054.7) vs ImNMe2 (2054.1). Only two reversals of this trend are seen in Chart 1: sImNH2 (2057.5) vs ImNH2 (2058.1), and sDPyIm (2051.3) vs DPyIm (2055.9). Several trends are apparent in Chart 1: (a) in general, TEP (cm-1) decreases as the size of the NR groups increases, e.g., ImNMe2 (2054.1) > ImNEt2 (2052.8) > ImNPri2 (2051.5) > ImNCy2 (2049.7); (b) increasing NHC ring size from 5 to 6 atoms makes better donors, e.g., sImNMe2 (2054.7) > sPmNMe2 (2051.0), and sImNPri2 (2051.9) > sPmNPri2 (2048.3); (c) carbenes possessing extended π-systems (e.g., BImNR2, PerNR2, DPyIm, Py[a]ImNBut, Py[b]ImNMe2, Py[c]ImNMe2) form a group of relatively poor donors; (d) substitution at C-4,5 of the imidazole ring has a major effect on TEP, e.g., the donor power increases significantly in the order R = NO2 < CN < CF3 < F ≈ Cl < CO2Me < H < OMe < Me < NMe2 for ImR2NMe2. A combination of trends (a) and (d) makes sImBut2NPri2 a very good donor. Also NHC ligands with the NR substituents “backbiting” at C-4, 5 are very good donors, e.g., IBinitMe4, IBicarMe4, and IBioxMe4. Finally, moving one (as in PyrazC3NMe2 and aImNMe2) or both heteroatoms (as in Pyraz-3,5-Me2NMe2) away from the carbene carbon strongly decreases TEP values, as best seen in Figure 2. “Remote” carbenes (rNHC) derived from pyridine, particularly of the PyC4 and PyC3 type, are excellent donors. The pyridylidene
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Chart 1. Calculated TEP (cm-1) and r Values for NHC Ligands Arranged According to TEPa
a
ΔH values (kcal/mol) for CO elimination from Ni(CO)3 are in parentheses. Dip = 2,6-diisopropylphenyl; other abbreviations are explained in the text.
“pincer-type” ligand PyC3-2,4-(OPMe2)2-6-MeNH is an outstanding carbene donor. We are currently working with pincer complexes of this type.9 Steric Parameters. In this part, we first look at the reaction enthalpies for decarbonylation of Ni(CO)3(NHC) and formation of the corresponding 16-electron Ni(CO)2(NHC) species. The calculated ΔH values are given in Chart 1, and they range from 23.3 (NHC = ImNH2) to 6.2 kcal/mol (NHC = ImNAd2). This reaction appears to be largely under steric control since the most stable tricarbonyl complex possesses the smallest NHC ligand, ImNH2. A closer examination of 22 selected Ni(CO)3(NHC) complexes, all possessing two NMe groups, reveals a stereoelectronic relationship of the type ΔH = 86.35 - 0.06741TEP þ 21.63d(Ni-C) (SE 0.40 kcal/mol, Figure 3), where d(Ni-C) (9) For example: RuHCl[2,4-(OPBut2)2-6-CH3-(C5HNH)] and IrCl[2,4-(OPBut2)2-6-CH3-(C5HNMe)], work in progress.
is the distance from Ni to the carbon atoms of the NMe groups in Ni(CO)2(NHC).10 This distance, therefore, can be used as a formal steric parameter, which can be derived from ΔH and TEP for any NHC: d(Ni-C) = -4.075 þ 0.003172TEP þ 0.0446ΔH. For example, d(Ni-C) is 3.493 A˚ for ImNH2, meaning that this ligand has the repulsiveness of ImNMe2 with the methyl groups at 3.493 A˚ from the metal. Then, parameter r (repulsiveness) can be defined as r = 10 (3.493 - d(Ni-C)), ranging from 0.0 for the smallest (ImNH2) to 8.0 for the most repulsive NHC ligand (ImNAd2) of this work (see Figure 2). Parameter r is a measure of direct repulsive interactions between the NHC and carbonyl ligands of Ni(CO)3(NHC). (10) The NHC ligands of Ni(CO)2(NHC) are rotated out of the Ni(CO)2 plane. This geometry has little steric strain; therefore, the N-R groups are “relaxed” and the d(Ni-C) distances are those internally preferred in the Ni(NHC) fragment. The ΔH does not correlate well with the d(Ni-C) distances of Ni(CO)3(NHC).
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It is of interest to compare NHC and analogous phosphine complexes. In agreement with Nolan,4b we find that CO loss from Ni(CO)3L is more facile with L = NHC: ΔH e 23.3 (all NHC), 25.1 (PH3), 25.8 (P(OMe)3), 26.0 (PMe3), 26.1 (PEt3), and 24.7 kcal/mol (PPri3). This correlates with the finding8 that H2 elimination from Os(H2)Cl2(CO)L2 proceeds more readily when L = NHC: ΔH = 12.4 (sImNMe2), 13.7 (ImNMe2), 18.6 (PMe3), and 20.8 kcal/mol (PEt3). NHC ligands have gained the reputation of excellent donors, yet they also appear to be excessively sterically demanding. It is the latter property of N-heterocyclic carbenes that we should find playing a major role in coordination chemistry and catalysis.
Computational Details Figure 2. Plot of calculated TEP vs d(CO) values for a series of Ni(CO)3L complexes (based on the data from Chart 1 and Table S1).
The calculations were carried out using the mPW1PW91 functional implemented in Gaussian 03 (revisions C.02 and D.01),12 which included modified Perdew-Wang exchange and Perdew-Wang 91 correlation.13 The basis set employed in this work included 6-311þG(2d) for Ni and 6-311þG(d,p) for all other atoms.14,15 All geometries were obtained using tight optimizations and the ultrafine integration grid (a pruned (99 590) grid). All optimized geometries were verified to have all real harmonic frequencies by frequency calculations, which also provided the νCO frequencies and enthalpies.
Acknowledgment is made to Wilfrid Laurier University, Natural Sciences and Engineering Research Council of Canada, and the Donors of the American Chemical Society Petroleum Research Fund for support of this research, which was made possible by the facilities of the Shared Hierarchical Academic Research Computing Network (SHARCNET: www.sharcnet.ca).
Figure 3. Calculated CO elimination enthalpies, ΔH (kcal/mol), correlated with the TEP values of Ni(CO)3(NHC) and Ni-C distances (A˚) in Ni(CO)2(NHC) (based on the data from Chart 1 and Table S1).
This parameter is distinctly different from the other steric descriptor proposed for N-heterocyclic carbenes, %Vbur (buried volume), which is a measure of the overall NHC ligand bulk.4b For example, %Vbur = 35.7 and 35.5 for sImNDip2 and ImNBut2,11 yet their r values are 2.5 and 7.0, respectively. In fact, the nickel complex Ni(CO)3(sImNDip2) is known to be stable, whereas Ni(CO)3(ImNBut2) rapidly eliminates CO.4b However, the r values correlate with %Vbur and increase in the series ImNMe2 (1.1, 24.9) < BImNMe2 (1.6, 25.1) < sImNMe2 (1.8, 25.4). This can be explained by increasing C4-C5 distance and the NHC ring expansion in this series. As expected, lateral expansion of the NR groups has only a small effect on r (e.g., r = 1.1 (Me), 1.3 (Et), 1.3 (Bu), 1.6 (Pri), 1.6 (Cy) in ImNR2); also, flat NAr substituents have low repulsiveness when orthogonal to the Im plane (e.g., r = 1.5 for IMes). Most r data in Chart 1 can be rationalized on the basis of simple geometric arguments; for example, C-4,5 substitution in ImR2NR0 2, or NHC ring expansion in sPmNR2 and PerNR2, predictably results in greater r. (11) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759–1766.
Supporting Information Available: Calculated data (Table S1) and atomic coordinates of the Ni(CO)nL complexes. This material is available free of charge via the Internet at http:// pubs.acs.org. (12) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; 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.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian 03; Gaussian, Inc.: Wallingford, CT, 2004. (13) (a) Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664. (b) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B 1996, 54, 16533. (c) Burke K.; Perdew, J. P.; Wang, Y. In Electronic Density Functional Theory: Recent Progress and New Directions; Dobson, J. F., Vignale, G., Das, M. P., Eds.; Plenum: New York, 1998. (14) For more information about Gaussian 03 and additional references see: Frish, A.; Frish, M. J.; Trucks, G. W. Gaussian 03 User’s Reference; Gaussian, Inc.: Pittsburgh, PA, 2003. (15) (a) The basis sets are also available from the EMSL Basis Set Library (http://bse.pnl.gov).15b,c (b) Feller, D. J. Comput. Chem. 1996, 17, 1571. (c) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L. J. Chem. Inf. Model. 2007, 47, 1045.
