Thermodynamics of Addition of H 2 , CO, N 2 , and C-H Bonds to M(P i Pr 3 ) 2 Cl (M ) Ir, Rh). An Unprecedented Metal-Carbonyl Bond Strength Glen P. Rosini, † Fuchen Liu, † Karsten Krogh-Jespersen,* ,† Alan S. Goldman,* ,† Chunbang Li, ‡ and Steven P. Nolan* ,‡ Contribution from the Departments of Chemistry, Rutgers, The State UniVersity of New Jersey, New Brunswick, New Jersey 08903, and UniVersity of New Orleans, New Orleans, Louisiana 70148 ReceiVed December 2, 1997. ReVised Manuscript ReceiVed June 3, 1998 Abstract: The thermodynamics of interconversion of various complexes containing the unit IrL* 2 Cl (L* ) P( i Pr) 3 ) have been investigated by calorimetry and equilibrium measurements. These complexes span a wide range of configurations including four- and five-coordinate d 8 (IrL* 2 ClL′, IrL* 2 Cl(CO) 2 ) and five- and six- coordinate d 6 (IrL* 2 ClRH and IrL* 2 ClRH(CO)). On the basis of kinetic experiments, a lower limit to the Ir-N 2 bond dissociation enthalpy (BDE) of IrL* 2 Cl(N 2 ) has been determined (36 kcal/mol). Using this value as an “anchor”, in conjunction with the relative addition enthalpies obtained calorimetrically, it is possible to derive lower limits for the absolute exothermicities of H 2 (48 kcal/mol) and CO (72 kcal/mol) addition to IrL* 2 Cl; estimates can also be made for the addition of benzene and acetylene C-H bonds. These values are unusually high; indeed, the magnitude of the Ir-CO BDE is unprecedented. In addition, kinetic methods have been used to determine a lower limit of 29 kcal/mol to the Rh-N 2 BDE of RhL* 2 Cl(N 2 ). Combined with previous calorimetric measurements on rhodium complexes, this value permits the calculation of lower limits to the absolute exothermicities of addition to RhL* 2 Cl for numerous small molecules including H 2 , CO, N 2 ,C 2 H 4 , and aldehydic C-H bonds. The results of electronic structure calculations (approximate DFT; PMe 3 used to model P i Pr 3 ) are in excellent agreement with the relative experimental enthalpies, while the absolute values calculated for addition to IrL 2 Cl are significantly greater than the experimentally determined lower limits. Addition of a methane C-H bond is calculated to be significantly less favorable than addition of benzene or acetylene C-H bonds, in accord with the fact that IrL* 2 Cl(alkyl)H complexes have not been reported. The significant differences in the enthalpies of addition for these three types of C-H bonds are briefly analyzed. Introduction The addition reactions of H 2 and CO to transition metal centers are certainly among the most important reactions in transition metal chemistry. These fundamental reactions (and their respective microscopic reverses) are ubiquitous in transi- tion-metal-catalyzed chemistry. Hydrogenations and carbon- ylations are of tremendous industrial importance and many important processes, such as hydroformylation and homoge- neous Fischer-Tropsch analogues, involve both CO and H 2 addition. 1 A related reaction, which has been the subject of intense study for about 15 years, is the addition of C-H bonds; development of systems incorporating this reaction into useful catalytic cycles is considered a “Holy Grail” of transition-metal- based catalysis. 2 It should also not be overlooked that the reverse reaction, C-H bond elimination, is involved for example in all of the above-mentioned catalytic reactions of CO and/or H 2 . 1 Yet despite the obvious importance of these fundamental reactions, our understanding of the factors that determine their thermodynamics is quite limited. Complexes of the group 9 metals play a particularly important role in catalyses of the type noted above. For example, probably the best known catalyst precursors for these respective processes are Rh(PPh 3 ) 3 Cl (olefin hydrogenation), Co 2 (CO) 8 (hydroformyl- ation), and RhI 3 (alcohol carbonylation). 1 In the field of homogeneous hydrocarbon C-H bond activation, Rh and Ir, particularly as phosphine complexes, have played a dominant role in the development of both stoichiometric and catalytic reactions. 2 We have recently elucidated the relative thermodynamics pertaining to a variety of reactions involving the fragment RhL* 2 Cl (L* ) P i Pr 3 ), including the addition of CO, N 2 ,H 2 , and aldehydic C-H bonds. 3 Herein we report our determination of the relative enthalpies for addition of CO, N 2 , and H 2 to the iridium congener, IrL* 2 Cl. In addition, the iridium manifold includes 18-electron complexes such as IrL* 2 Cl(CO)H 2 and IrL* 2 Cl(CO)PhH. This permits us to determine the absolute enthalpy, for example, of H 2 addition to IrL* 2 Cl(CO) and CO addition to IrL* 2 Cl(H) 2 . We also report on kinetic experiments which enable us to estimate upper limits to the absolute enthalpies of the additions to both the rhodium and iridium 14- † Rutgers, The State University of New Jersey. ‡ University of New Orleans. (1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; pp 523-576, 619-668. (2) For a recent review, see: Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154-162. (3) (a) Wang, K.; Rosini, G. P.; Nolan, S. P.; Goldman, A. S. J. Am. Chem. Soc. 1995, 117, 5082-5088. (b) Wang, K.; Goldman, A. S.; Li, C.; Nolan, S. P. Organometallics 1995, 14, 4010-4013. (c) Wang, K.; Emge, T. J.; Goldman, A. S.; Li, C.; Nolan, S. P. Organometallics 1995, 14, 4929- 4936. 9256 J. Am. Chem. Soc. 1998, 120, 9256-9266 S0002-7863(97)04100-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/26/1998