Organometallics 1993,12, zyxwvu 1823-1830 1823 Mechanism of the Hydrogenation of Phenylacetylene Catalyzed by [Ir( COD) (q2-I'Pr2PCH2CH20Me)]BF4 Miguel A. Esteruelas,t Ana M. L6pez,t Luis A. Oro,*?t Angel P6rez,t Michael Schulz,$ and Helmut Werner*?$ Departamento de Qulmica Inorg&nica,Instituto de Ciencia de Materiales de Aragbn, Universidad de Zaragoza, CSIC, 50009 Zaragoza, Spain, and Institut fur Anorganische Chemie, Universitat Wurzburg, Am Hubland, W-8700 Wiirzburg, Germany Received December 23, 1992 The synthesis of the mononuclear complexes of formulas IrCl(COD)(iPrzPCHzCH2OMe) (1) (COD zyxwvutsrq = 1,5-~yclooctadiene), IrCl(C0D)(iPrzPCH2CH2NMez) zyxw (21, [Ir(diolefin)(02-iPr2PCH2- CH20Me)lBF4(diolefin = COD (3), TFB (5) (TFB = tetrafluorobenzobarrelene)), [Ir(diolefin)- (+iPrzPCH&H2NMe2)]BF4 (diolefin = COD (4), TFB (6)), zyxwv [IrH(C2Ph)(COD)(s2-iPr,PCH,- CH20Me)]BF4 (7), and [I~Hz(COD)(~~-~P~ZPCH~CH~OM~)BF~ (8) are described. The X-ray crystal structure of 3 has been determined (monoclinic space group P21/n (No. 14) with a = 8.466(3) A, b = 9.004(3) A, c = 27.052(13) A, ,8 = 97.71(2)', and zyxw 2 = 4). 3 is found to be a very active and highly selective catalyst for the hydrogenation of phenylacetylene to styrene. Although the hydrido-alkynyl-diolefin compound 7 is the main species under catalytic conditions, the reaction proceeds via the dihydrido-diolefin intermediate 8 according to the following set of reactions: 8 + HZ * [I~H(CH=CHP~)(COD)(+~P~ZPCHZCHZOM~)IBF~ (9) and 9 + HZ - 8 + PhCH=CHZ where the reaction between 9 and molecular hydrogen is the rate-determining step. The catalytic activity of 3-6 in the hydrogenation of olefins is also reported. Introduction Shapley, Schrock, and Osbornl made the important discovery of the existence of the cationic rhodium and iridium complexes with the general formula [M(diene)- La]+ (a = 2 or 31, which are active catalysts for the hydrogenation of olefins,2 dienes,3 internal alkynes: and ketone^.^ In solvents such as acetone, ethanol or aceto- nitrile, these compounds react with molecular hydrogen to give the dihydrido-metal complexes [MHzS,L,I+ (M = Rh, Ir; S = The chemistry of the complex [Rh(NBD)(dppe)l+ (NBD = 2,5-norbornadiene,dppe = 1,2-bis(diphenylphosphino)- ethane) differs in some respects from that of complexes containing monodentate ligands L.6 In methanol solution, it reacts rapidly with hydrogen to give the solvated species [Rh(MeOH),(dppe)l+, which is an effective catalyst for the hydrogenation of alkene derivatives. Kinetic mea- surements of the hydrogenation of l-hexene were con- + Universidad de Zaragoza. i Universitiit Wurzburg. (1) Shapley, J. R.; Schrock, R. R.; Osborn, J. A. J. Am. Chem. SOC. 1969,91, 2816. (2) (a) Schrock, R. R.; Osborn, J. A. J. Am. zyxwvutsrqpo Chem. SOC. 1976,98,2134. (b) Crabtree, R. H.; Felkin, H.; Morris, G. E. J. Organomet. Chem. 1977, 141,205. (c) Usbn, R.; Oro, L. A.; Artigas, J.; Sariego, R. J. Organomet. Chem. 1979,179,65. (d) Crabtree, R. Acc. Chem. Res. 1979,12,331. (e) Usbn, R.; Oro, L. A.; Fernhdez, M. J.; Sariego, R. Reu. Acad. Ciencias Zaragoza, 1980,35,87. (0 Ush, R.; Oro, L. A.; Sariego, R.; Valderrama, M.; Rebullida, C. J. Organomet. Chem. 1980, 197,87. (g) Crabtree, R.; Demon, P. C.; Eden, D.; Michelcic, J. M.; Parnell, C. A.; Quirk, J. M.; Morris, G. E. J. Am. Chem. zyxwvutsrqp SOC. 1982, 104, 6994. (3) (a) Schrock, R. R.; Osborn, J. A. J. Am. Chem. SOC. 1976,98,4450. (b) Usbn,R.;Oro,L.A.;FernBndez,M. J. J. Organomet. Chem. 1980,193, 127. (c) Crabtree, R. H.; Felkin, H.; Filleben-Khan, T.; Morris, G. E. Zbid. 1979, 168, 183. (4) Schrock, R. R.; Osborn, J. A. J. Am. Chem. SOC. 1976, 98, 2143. (5) Schrock, R. R.; Osborn, J. A. J. Chem. Soc., Chem. Commun. 1970, (6) Halpern, J. Inorg. Chim. Acta. 1981, 50, 11 and references cited 567. therein. sistent with the following mechanism: [Rh(MeOH),(dppe)l+ + 1-hexene * MeOH [Rh( 7'- 1-hexene)(dppe) I (fast) (a) MeOH [Rh($-l-hexene)(dppe)l+ + H, - [Rh(MeOH),(dppe)l+ + hexane (slow) (b) The cation [Ir(COD)(py)zl+ (COD = 1,5-cyclooctadiene, py = pyridine),in contrast to [Rh(NBD)(dppe)l+, is totally inactive as a catalyst, apparently because it fails to add hydr~gen.~ However, the mixed-ligand complexes [Ir- (COD)L(PR3)2]+ (L = nitrogen donor ligand) are active catalysts for the reduction of a variety of substrates, particularly tetrasubstituted alkenes. Thus, in dichlo- romethane as solvent the complex [Ir(COD)(py)(PCy3)1+, known as Crabtree's catalyst, catalyzes the hydrogenation of 2,3-dimethyl-2-b~tene;~~ interestingly, better results are obtained for the benzonitrile derivative [Ir(COD)(NCPh)- (L)1+ (L = PCy3 or neomenthyldiphenylphosphine).8This benzonitrile catalyst precursor is able to hydrogenate tetrasubstituted alkene moieties of prochiral didehydro amino acid derivatives, although poor enantiomeric ex- cesses are obtained with chiral monodentate phosphine ligands. The reduction of terminal alkynes catalyzed by the cationic systems mentioned above has been scarcely observed,2fdue to the formation of nonactive alkynyl derivatives during the hydrogenation. In this article we describe the results obtained from a kinetic and spectro- (7) Crabtree, R. H.; Quirk, J. M. J. Organomet. Chem. 1980,199,99. (8) (a) Oro, L. A.; Cabeza, J. A.; Cativiela, C.; Dfaz de Villegas, M. D.; MelBndez, E. J. Chem. SOC., Chem. Commun. 1983,1383. (b) Cabeza, J. A.; Cativiela, C.; Dfaz de Villegas, M. D.; Oro, L. A. J. Chem. SOC., Perkin Trans. 1 1988, 1881. 0276-733319312312-1823$04.00/0 0 1993 American Chemical Society