Organometallics 2009, 28, 3901–3905 DOI: 10.1021/om900206w
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Tolman’s Electronic Parameters for Divalent Carbon(0) Compounds Ralf Tonner*,† and Gernot Frenking*,‡ †
Centre for Theoretical Chemistry and Physics, New Zealand Institute for Advanced Study, Massey University Albany, Private Bag 102904, North Shore City, 0745 Auckland, New Zealand, and ‡ Fachbereich Chemie, Philipps-Universit€ at Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany Received March 18, 2009
The complexes [D-Ni(CO)3] and [D-RhCl(CO)2] have been calculated with DFT methods where D is a strong carbon or phosphorus donor ligand. The focus of the work is on the estimate of the donor strength of divalent carbon(0) compounds CL2 compared to common carbon(II) ligands (N-heterocyclic carbenes, NHC), phosphines, and several ligands recently introduced. Tolman’s electronic parameters are derived by a fit procedure to experimental data to enable better comparison of different donor strength scales. It turns out that carbon(0) ligands CL2 with L = PR3 (carbodiphosphoranes) or NHC (carbodicarbenes) are much stronger donors than carbene and phosphine ligands. Besides the calculation of known substitution patterns, several new ligands of this class are proposed that might be valuable in the search for new ligands with adjustable donor properties.
Introduction An important goal of coordination chemistry is the search for new ligand classes that exhibit desirable and adjustable donor properties that can be fine-tuned by electronic and steric modulation. Recently, we could show that divalent carbon(0) compounds with the general formula CL2 that possess two electron lone-pairs (Scheme 1) have a very high electron density at the central carbon atom, making them a promising ligand class.1 The characteristic electronic feature of the compounds CL2, for which the term carbones has been proposed,2 is two electron lone-pairs at the carbon atom. This distinguishes carbones from carbenes, which have only one electron lone-pair at carbon. Following earlier attempts to isolate transition metal complexes with carbone ligands,3 the chemistry of divalent C(0) has been revived by recent experimental studies in the groups of Petz, Bertrand, Bacereido, and F€ urstner.1,4,5 The best known examples for CL2 compounds are carbodiphosphoranes (CDP; L = PR3) and carbodicarbenes (CDC; L = N-heterocyclic carbenes, NHC), but other compounds have been proposed to show carbon(0) character as well6a (Scheme 2, 1-5). A characteristic feature of carbones is their high second proton affinity2 and their ability to bind to two *Corresponding authors. E-mail:
[email protected]; frenking@ chemie.uni-marburg.de. (1) (a) Tonner, R.; Oexler, F.; Neum€ uller, B.; Petz, W.; Frenking, G. Angew. Chem., Int. Ed. 2006, 45, 8038–8042. (b) Tonner, R.; Frenking, G. Angew. Chem., Int. Ed. 2007, 46, 8695–8698. (2) Tonner, R.; Heydenrych, G.; Frenking, G. ChemPhysChem 2008, 9, 1474–1481. (3) Schmidbaur, H. Nachr. Chem. Tech. Lab. 1979, 27, 620–622. (4) (a) Dyker, C. A.; Lavallo, V.; Donnadieu, B.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 3206–3209. (b) Marrot, S.; Kato, T.; Gornitzka, H.; Baceiredo, A. Angew. Chem., Int. Ed. 2006, 45, 2598–2601. (5) F€ urstner, A.; Alcarazo, M.; Goddard, R.; Lehmann, C. W. Angew. Chem., Int. Ed. 2008, 47, 3210–3214. r 2009 American Chemical Society
monodentate Lewis acids. Theoretical studies of reactivity and bonding of CL2 have been published,6 but a general evaluation of the particular donor properties in comparison to other common ligands in coordination chemistry is lacking. Single-parameter schemes have proven to be valuable in this respect for the ligand classes of N-heterocyclic carbenes7 (6) and phosphines8 (8). The vibrational frequencies of the CO ligands in transition metal complexes are often used as an indicator for the overall donor strength of ligands. The experimental or calculated frequencies may be scaled to represent Tolman’s electronic parameters (TEPs),8 which were originally developed for phosphines. For this purpose the A1 symmetrical CO stretching mode of [D-Ni(CO)3] complexes is often used for calibrating the donor strength of the ligand D. Experimental studies used also [D-RhCl(CO)2] complexes and their Ir analogues as reference systems,7,9,10 while theoretical studies preferred [D-Ni(CO)3] or [D-Cr(CO)5] complexes11-13 in the spirit of Tolman’s (6) (a) Tonner, R.; Frenking, G. Chem.;Eur. J. 2008, 14, 3260– 3272. (b) Tonner, R.; Frenking, G. Chem.;Eur. J. 2008, 14, 3273– 3289. (7) F€ urstner, A.; Alcarazo, M.; Krause, H.; Lehmann, C. W. J. Am. Chem. Soc. 2007, 129, 12676–12677. (8) Tolman, C. A. Chem. Rev. 1977, 77, 313–48. (9) (a) Canac, Y.; Lepetit, C.; Abdalilah, M.; Duhayon, C.; Chauvin, R. J. Am. Chem. Soc. 2008, 130, 8406–8413. (b) Mayr, M.; Wurst, K.; Ongania, K. H.; Buchmeiser, M. R. Chem.;Eur. J. 2004, 10, 1256– 1266. (10) (a) Chianese, A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.; Crabtree, R. H. Organometallics 2003, 22, 1663–1667. (b) Kelly, R. A.; 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. (11) Gusev, D. G. Organometallics 2009, 28, 763–770. (12) (a) Lai, C. L.; Guo, W. H.; Lee, M. T.; Hu, C. H. J. Organomet. Chem. 2005, 690, 5867–5875. (b) Lee, M. T.; Hu, C. H. Organometallics 2004, 23, 976–983. (13) Perrin, L.; Clot, E.; Eisenstein, O.; Loch, J.; Crabtree, R. H. Inorg. Chem. 2001, 40, 5806–5811.
Published on Web 06/08/2009
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Organometallics, Vol. 28, No. 13, 2009
Tonner and Frenking
Scheme 1. Schematic Representation of the Bonding Situation in Divalent Carbon(0) Compounds
been the subject of recent experimental and theoretical work.29 Our aim is the comparison of the σ-donor strength of divalent carbon(0) compounds with commonly used ligands and the investigation of new substitution patterns for fine-tuning of CL2.
Scheme 2. List of Investigated Ligandsa,b
Computational Details Geometry optimizations without symmetry constraints have been carried out using the Gaussian03 optimizer18 together with TurboMole519 energies and gradients at the BP8620/def-SVP21 level of theory. For Rh an additional f-function has been added (resulting in the def2-SVP basis set).22 Characterization of stationary points and derivation of unscaled vibrational frequencies have been achieved by calculating the Hessian matrix analytically.23 The resolution-of-identity method has been applied.24 For [D-Ni(CO)3] the A1 symmetrical CO stretching mode (νNi(CO)) has been taken for the evaluation of the donor parameters. For [D-RhCl(CO)2] the average of symmetrical and asymmetrical CO stretching mode was chosen (νRh(CO)), since the coupling of both modes disables the identification of a pure trans-CO mode. The separate values of these modes can be found in the Supporting Information.
Results and Discussion
a References indicate experimental studies introducing these ligands. Note that other representations are also possible that give a valid description of the bonding situation in the compounds.
b
original ansatz. In the work of Gusev11 it was pointed out that a single parameter such as the TEP may not always adequately describe the donor properties of ligands with very diverse bonding properties. As can be seen from the results presented here, the similar nature of carbon and phosphorus donors enables a good comparison of TEP values in this class of compounds and provides the basis for a classification scheme of donor strength. In this study we report the calculated vibrational frequencies of both [D-Ni(CO)3] and [D-RhCl(CO)2] complexes of divalent carbon(0) ligands (1-5). We compare the donor strength of CL2 with NHCs as examples for divalent carbon(II) ligands (6), a diaminocarbene (7), phosphines (8), and other recently developed ligands that are proposed as strong σ-donors (9-15).7,14-17 Compound 11 belongs to the class of donors that are designated as “abnormal” carbenes that have (14) Asay, M.; Kato, T.; Saffon-Merceron, N.; Cossio, F. P.; Baceiredo, A.; Bertrand, G. Angew. Chem., Int. Ed. 2008, 47, 7530–7533. (15) F€ urstner, A.; Alcarazo, M.; Radkowski, K.; Lehmann, C. W. Angew. Chem., Int. Ed. 2008, 47, 8302–8306. (16) Kobayashi, J.; Nakafuji, S.; Yatabe, A.; Kawashima, T. Chem. Commun. 2008, 6233–6235. (17) Mathew, P.; Neels, A.; Albrecht, M. J. Am. Chem. Soc. 2008, 130, 13534–13535.
Different scales for calibrating the donor strength of a ligand have been suggested in the literature, which complicates the comparison of results using different approaches. Theoretical work has the additional problem that different levels of theory yield electronic parameters with varying absolute numbers. An elegant solution for this problem, which has often been chosen in the past,10,11,13 is to scale the values that come from different models to a generally accepted scheme such as Tolman’s electronic parameters. A correlation of the frequencies calculated with our theoretical approach and experimentally derived TEPs is possible for selected compounds (Table 1). Although the absolute numbers of the calculated frequencies and the TEP values are different, the trend in (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J., J. A.; 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.; Pople, J. A. Gaussian 03, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (19) Ahlrichs, R.; Baer, M.; Haeser, M.; Horn, H.; Koelmel, C. Chem. Phys. Lett. 1989, 162, 165–169. (20) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (b) Perdew, J. P. Phys. Rev. B 1986, 33, 8822–8824. (21) Sch€afer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571–2577. (22) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. (23) Deglmann, P.; Furche, F. J. Chem. Phys. 2002, 117, 9535–9538. (24) (a) Eichkorn, K.; Treutler, O.; Ohm, H.; Haser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 240, 283–289. (b) Weigend, F. Phys. Chem. Chem. Phys. 2006, 8, 1057–1065.
Article
Organometallics, Vol. 28, No. 13, 2009
Table 1. Calculated and Experimental Vibrational Frequencies (in cm-1)
D
[D-Ni(CO)3]
[D-RhCl(CO)2]
νNi(CO)
νRh(CO)
exptl TEP
2024 2034 2046 2048 2049 2061 2081
2032a 2054b 2062c 2064c 2069c 2083c 2111c
1-Ph 2045 6-Mes 2062 8-NMe2 2076 8-PMe3 2079 8-PPh3 2080 8-PH3 2096 8-PF3 2118 a Ref 26. b Ref 27. c Ref 8.
Table 2. Calculated Vibrational Frequencies of [D-Ni(CO)3] (νNi(CO)) and [D-RhCl(CO)2] (νRh(CO)) and TEPs Derived by Equations 1 and 2a calculated frequencies D
νNi(CO)
νRh(CO)
TEPNi
TEPRh
(1) very strong donors
2-NMe2 1-Cy 1-THP 1-Mes 2-Mes 3-Me 2-Ad
2033 2034 2034 2035 2035 2036 2036
2003 2014 2010 2001 2014 2017 2011
2019 2020 2020 2021 2021 2022 2023
2008 2021 2017 2006 2022 2026 2017
(2) strong donors
2-Me 3-Et 1-Ph 1-Me 9 13 2-Ph 2-H 14 20 -Me
2045 2045 2045 2046 2047 2047 2048 2048 2049 2052
2022 2018 2024 2027 2027 2029 2019 2027 2032 2028
2031 2032 2032 2033 2034 2034 2034 2035 2036 2039
2032 2027 2035 2038 2040 2042 2028 2039 2045 2040
(3) intermediate donor strength
12 6-Ad 15 6-tBu 7 8-Ad 6-Mes 11 1-H 6-NMe2 10 6-Ph 6-Me 60 -Me 4 8-Mes 6-H
2054 2054 2055 2060 2061 2061 2062 2062 2062 2063 2064 2066 2066 2067 2067 2068 2071
2034 2037 2037 2039 2039 2031 2034 2037 2029 2041 2046 2041 2043 2043 2041 2044 2042
2041 2041 2042 2047 2048 2049 2049 2050 2050 2050 2051 2053 2054 2054 2055 2056 2059
2048 2052 2052 2054 2055 2044 2048 2053 2041 2057 2063 2058 2060 2060 2057 2062 2059
the donor strength is the same. A linear regression of theoretical and experimental data (Figure 1) gives the following values:
R2 ¼ 0:9871
ð1Þ
TEPRh ¼ 1:3080νRh ðCOÞ -612:39 R2 ¼ 0:9822 ð2Þ where TEPNi comes from correlating νNi(CO) with the TEP data, while TEPRh comes from the νRh(CO) values and the TEP data set. Both data sets correlate quite well with the experimental values. They span a broad range of TEPs and should thus enable a reliable interpolation of TEPs for ligands for which no experimental data are available. Table 2 gives the unscaled calculated frequencies for the ligands investigated in this work and the TEP values that have been obtained by applying eqs 1 and 2. In the following we will focus on the discussion of TEP values since they follow the same trend as the original data and are easier to compare with known literature values. We divided the ligands into four categories based on their TEPNi values: very strong donors (TEPNi
