Stable, three-coordinate Ni(CO)2(NHC) (NHC = N-heterocyclic carbene) complexes enabling the determination of Ni-NHC bond energies

Article abstract of DOI:10.1021/ja0362151

The synthesis and characterization of three- and four-coordinate Ni(CO)_n(NHC) (n = 2, 3; NHC = N-heterocyclic carbene) complexes are reported. Reactions with CO of the Ni(CO)₂(NHC) complexes lead to the quantitative formation of Ni(C

Full text of DOI:10.1021/ja0362151

Published on Web 08/08/2003
Stable, Three-Coordinate Ni(CO)2(NHC) (NHC ) N-Heterocyclic Carbene)
Complexes Enabling the Determination of Ni-NHC Bond Energies
Reto Dorta,† Edwin D. Stevens,† Carl D. Hoff,‡ and Steven P. Nolan*,†
Department of Chemistry, UniVersity of New Orleans, New Orleans, Louisiana 70148, and
Department of Chemistry, UniVersity of Miami, Coral Gables, Florida 33124
Received May 19, 2003; E-mail: snolan@uno.edu
Recent developments in the use of N-heterocyclic carbenes
(NHC) as ancillary ligands in organometallic chemistry and
homogeneous catalysis have drawn attention to this class of
compounds as an attractive alternative to the ubiquitous tertiary
phosphines.1 Several beneficial uses of NHC-modified organo-
metallic systems2 in ruthenium-mediated olefin metathesis,3 iridium-
catalyzed hydrogenation,4 and palladium cross-coupling5 chemistry
are now well recognized. The most frequently encountered NHCs
are presented in Figure 1.
To achieve a deeper understanding of the fundamental steric and
electronic factors characterizing these ligands and to afford a direct
comparison with a large array of tertiary phosphine data,6 we have
carried out a substitution reaction involving NHC ligands with Ni-
(CO)4.7 Treating a hexane solution containing IMes (1) or SIMes
(2) with a slight excess of Ni(CO)4 led to evolution of CO and to
isolation of the off-white complexes Ni(CO)3(IMes) (8) and Ni-
(CO)3(SIMes) (9) in high yield according to eq 1.8,9 The A1 carbonyl
stretching frequency (cm-1) of complexes 8 and 9 confirms the
electron richness of the NHC ligands.10
Figure 1. Common unsaturated and saturated NHCs.
Ni(CO)4 + NHC f Ni(CO)3(NHC) + CO
(1)
Surprisingly, when the sterically most demanding IAd (6) or
ItBu (7) was reacted with Ni(CO)4, we observed formation of
orange/red solutions. 1H and 13C NMR spectral data did not permit
unequivocal structural assignment. However, significant infrared
shifts to lower frequencies were observed, indicating a possible
different composition than Ni(CO)3(NHC) for these complexes.
Indeed, elemental analysis data for the orange microcrystalline
materials supported a Ni(CO)2(NHC) stoichiometry. To unambigu-
ously confirm the structure of these products, single crystals of both
complexes were grown by slow evaporation of saturated diethyl
ether (for 10) and pentane (for 11) solutions. ORTEP diagrams of
Ni(CO)2(IAd) (10) and Ni(CO)2(ItBu) (11) are presented in Figure
2. Both complexes show only slightly distorted trigonal structures
with Ni-C(NHC) bond distances of 1.9528(16) Å (10) and
1.9569(11) Å (11).11
Figure 2. ORTEP views of Ni(CO)2(IAd) (10, top) and Ni(CO)2(ItBu) (11,
bottom).
likely inherently unstable with respect to disproportionation.14 Both
complexes, 10 and 11, react with CO to produce Ni(CO)4, the
microscopic reverse of eq 2. We reasoned the equilibrium associated
with eq 2 could be investigated to afford thermodynamic parameters
and allow access to the Ni-NHC (NHC ) IAd, ItBu) bond
These two complexes, isolated in high yields (88% for 10 and
96% for 11) according to eq 2, are rare examples of three-coordinate
nickel carbonyl systems.12
dissociation energy (BDE). Under equilibrium conditions,15
a
Ni(CO)4 + NHC f Ni(CO)2(NHC) + 2CO
(2)
variation of the equilibrium constant can be monitored. Infrared
results for IAd are presented in Figure 3.
To obtain the sought after carbonyl stretching frequencies
associated with Ni(CO)3(IAd) and Ni(CO)3(ItBu), both 10 and 11
were reacted with CO in a high-pressure FTIR cell.13 Under 1 atm
of CO, no significant amount of Ni(CO)3(NHC) complex is
observed even at low temperature. The Ni(CO)3(NHC) is most
The van’t Hoff plot shows a linear relation (R2 ) 0.99) and
provides a ∆H of +10.2 ( 0.3 kcal/mol and a ∆S of 41( 3 cal/
(mol K) for the equilibrium involving IAd. Using the Ni-CO bond
energy values for the first and second CO dissociation from Ni-
(CO)4,16 a Ni-IAd BDE value of 43 ( 3 kcal/mol is calculated. A
similar treatment of the ItBu complex leads to a Ni-ItBu BDE value
of 39 ( 3 kcal/mol. The relative BDE values are in accord with
† University of New Orleans.
‡ University of Miami.
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J. AM. CHEM. SOC. 2003, 125, 10490-10491
10.1021/ja0362151 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Metal Phosphine Complexes; Plenum: New York, 1983. (c) Collman, J.
P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications
of Organotransition Metal Chemistry; University Science: Mill Valley,
CA, 1987.
(2) (a) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290-1309. (b)
Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000,
100, 39-91. (c) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am.
Chem. Soc. 1991, 113, 361-363.
(3) (a) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem.
Soc. 1999, 121, 2674-2678. (b) Jafarpour, L.; Nolan, S. P. AdV.
Organomet. Chem. 2001, 46, 181-222. (c) Jafarpour, L.; Nolan, S. P. J.
Organomet. Chem. 2001, 617, 17-27. (d) Trnka, T. M.; Grubbs, R. H.
Acc. Chem. Res. 2001, 34, 18-29.
(4) (a) Lee, H. M.; Jiang, T.; Stevens, E. D.; Nolan, S. P. Organometallics
2001, 20, 1255-1258 (b) Hillier, A. C.; Lee, H. M.; Stevens, E. D.; Nolan,
S. P. Organometallics 2001, 20, 4246-4252. (c) Vasquez-Serrano, L. D.;
Owens, B. T.; Buriak, J. M. Chem. Commun. 2002, 2518-2519.
(5) (a) Grasa, G. A.; Viciu, M. S.; Huang, J.; Nolan, S. P. J. Org. Chem.
2001, 66, 7729-7737. (b) Grasa, G. A.; Viciu, M. S.; Huang, J.; Zhang,
C.; Trudell, M. L.; Nolan, S. P. Organometallics 2002, 21, 2866-2873.
(6) Tolman, C. A. Chem. ReV. 1977, 77, 313-324.
Figure 3. Temperature variation of Keq monitored by FTIR.
(7) CAUTION should be used while manipulating Ni(CO)4 as it is
EXTREMELY toxic.
(8) For two examples of related Ni(CO)3(NHC) complexes, see: (a) Herrmann,
W. A.; Goossen, L. J.; Artus, G. R. J.; Ko¨cher, C. Organometallics 1997,
16, 2472-2477. (b) O¨ fele, K.; Herrmann, W. A.; Mihalios, D.; Elison,
M.; Herdtweck, E.; Scherer, W.; Mink, J. J. Organomet. Chem. 1993,
459, 177-184.
(9) Ligands 3-5 react in a similar manner. Complete characterization,
including X-ray diffraction studies of all of these saturated Ni(CO)3(NHC)
complexes, have been carried out and will be published elsewhere.
(10) For 8; νCO (A1, CH2Cl2) ) 2050.7 cm-1. For 9; νCO (A1, CH2Cl2) ) 2051.5
cm-1. For Ni(CO)3(PtBu3); νCO (A1, CH2Cl2) ) 2056.1 cm-1. For less
basic phosphines, see ref 6.
(11) (a) An unsaturated, homoleptic Ni(0)-(NHC) complex has been character-
ized by X-ray crystallography, Ni(IMes)2, showing shorter bond lengths
of 1.827 and 1.830 Å, respectively, see: Arduengo, A. J., III; Gamper, S.
F.; Calabrese, J. C.; Davidson, F. J. Am. Chem. Soc. 1994, 116, 4391-
4394. (b) 1:1 Ni(II)-NHC complexes with Ni-C(NHC) distances of
1.9025(16) and 1.8571(13) Å have been reported recently, see: Dible, B.
R.; Sigman, M. S. J. Am. Chem. Soc. 2003, 125, 872-873.
the infrared data that indicate that IAd (A1 νCO for Ni(CO)2(IAd)
) 2007.2 cm-1) is a better electron donor than ItBu (A1 νCO for
Ni(CO)2(ItBu) ) 2009.7 cm-1). On the basis of the Ni-CO BDE
values used in our calculations, the two Ni-NHC BDE values are
estimated to be accurate, in an absolute sense, to (5 kcal/mol.17
These two Ni-NHC BDEs represent the first experimentally
determined report of such values. Noteworthy is the fact that
equilibria are not established between the saturated Ni(CO)3(NHC)
complexes and CO, even at elevated pressures of CO.18 These
different reactivity profiles of Ni(CO)2(NHC) and Ni(CO)3(NHC)
complexes again highlight the very unique nature of complexes 10
and 11.19
Throughout the recent NHC literature, general statements are
made about the strength of M-NHC bonds, but none have so far
been measured on an absolute basis. Recent accounts of the
M-NHC bond being relatively labile have appeared.20 We suspect
that when sterically bulky NHCs are employed, a significant driving
force leading to dissociation is a relief of steric pressure around
the metal center. This tendency of four-coordinate NiL4 complexes
is well known from the pioneering work of Tolman with phosphine
ligands.6 In the case of the NHC ligands, the presumed Ni(CO)3-
(NHC) (NHC ) IAd, ItBu) complexes also display this tendency.
Loss of CO yields the unsaturated Ni(CO)2(NHC) compounds
shown in Figure 2. Dissociation of IAd (or ItBu) establishes the
reversible equilibrium shown in Figure 3. More detailed investiga-
tions on the reactivity and catalytic uses of complexes 10, 11, and
their Ni(CO)3(NHC) congeners are ongoing.
(12) (a) For the only other example found in the literature, see: Petz, W.;
Weller, F.; Uddin, J.; Frenking, G. Organometallics 1999, 18, 619-626.
(b) For recent examples of three-coordinate Ni(II) complexes, see:
Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte, R.
J. J. Am. Chem. Soc. 2002, 124, 14416-14424. (c) For Ni(0) complexes,
see: Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124, 9976-
9977. Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123,
4623-4624. Gosser, L. W.; Tolman, C. A. Inorg. Chem. 1970, 9, 2350-
2353.
(13) The experimental design of the FTIR cell has previously been described:
Bender, B. R.; Kubas, G. J.; Jones, L. H.; Swanson, B. I.; Eckert, J.;
Capps, K. B.; Hoff, C. D. J. Am. Chem. Soc. 1997, 119, 9179-9190.
(14) Disproportionation was also observed when reacting 10 and 11 with tert-
butylisoyanide and certain phosphines; unpublished results.
(15) For details, see the Supporting Information.
(16) Sunderlin, L. S.; Wang, D.; Squires, R. R. J. Am. Chem. Soc. 1992, 114,
2788-2796 and references cited.
(17) The present result can be compared to a Pd-NHC bond energy of 25.6
kcal/mol recently determined by Cloke and Caddick in a (NHC)2Pd(aryl)-
Cl complex: Caddick, S.; Cloke, F. G. N., private communication.
(18) A solution of 39 mg of Ni(CO)3(SiMes) in 10 mL of heptane (distilled
from Na benzophenone under Argon) was prepared in the glovebox and
loaded under argon in the high pressure FTIR cell. A spectrum was run
under argon, and then the cell was filled with 550 psi CO. A spectrum
run immediately after filling, 1 h later, and 1 day later showed no change;
in particular, no builidup of Ni(CO)4 at 2046 cm-1 was observed during
this period. Similar observations were made with Ni(CO)3(IMes) which
also failed to react with CO at 350 psi. See the Supporting Information
for spectra.
Acknowledgment. The National Science Foundation is grate-
fully acknowledged for financial support of this work. Dr. Kenneth
Moloy and DuPont CR&D are especially thanked for the gift of
Ni(CO)4, as are Dr. Emilio Bunel and Eli Lilly and Co. for a
generous donation of amines.
(19) Initial reactivity studies underline this: electron-poor olefins react with
complexes 10 and 11 giving unsaturated, mixed Ni(CO)(NHC)(olefin)
compounds. Allyl halides react instantaneously giving rise to carbonyl-
free, Ni(II)(NHC)(allyl)(X) complexes. These and related studies will be
discussed in detail elsewhere.
(20) (a) Simms, R. W.; Drewitt, M. J.; Baird, M. C. Organometallics 2002,
21, 2958-2963. (b) Jazzar, R. F. R.; Macgregor, S. A.; Mahon, M. F.;
Richards, S. P.; Whittlesey, M. K. J. Am. Chem. Soc. 2002, 124,
4944-4945.
Supporting Information Available: Experimental procedures,
compound characterization, crystallographic and thermochemical data
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) (a) Parshall, G. W.; Ittel, S. Homogeneous Catalysis; J. Wiley and Sons:
New York, 1992. (b) Pignolet, L. H., Ed. Homogeneous Catalysis with
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