ALKYNE-TO-VINYLIDENE TAUTOMERISM MEDIATED BY 2 ADJACENT METAL CENTERS - STRUCTURES OF [IR2I2(CO)2(MU-CC(H)R)(PH2PCH2PPH2)2] (R = H,PH)

The compound [Ir2I2(CO)(mu-CO)(dppm)2] (1) (dppm = Ph2PCH2PPh2) reacts with acetylene and phenylacetylene to give the vinylidene-bridged complexes [Ir2I2(CO)2(mu-CC(H)R)(dppm)2] (R = H (5), Ph (7)). With acetylene the first species observed is proposed to be the alkyne-bridged [Ir2I2(CO)2(mu-HC2H)(dppm)2] (3), in which the alkyne is bound parallel to the Ir-Ir vector. This compound transforms to [Ir2I(CO)2(mu-HC2B)(dppm)2][I] (4), in which it is proposed, on the basis of H-1, (H{P})-H-1-P-31, C-13 and C{P}-C-13-P-31 NMR experiments, that the alkyne is unsymmetrically bound, allowing it to pi bond with one metal while still remaining sigma bound to both. Compound 4 transforms to the vinylidene-bridged product 5 within a. 20 h at ambient temperature. When the reaction of 1 with phenylacetylene is carried out at -40-degrees-C, the only species detected is the acetylide-bridged hydrido complex [Ir2(H)(I)(CO)2(mu-C2Ph) (dppm)2][I] (6). However, at temperatures above 0-degrees-C, only compound 7 is observed. A mechanism is proposed for the acetylene-to-vinylidene transformation. Compound 5 reacts with additional acetylene to give the products [Ir2I(CO)2(mu-CCH2)(mu-HC2H)(dppm)2][1] (8) and [Ir2(CO)2(mu-CCH2)(mu-HC2H)-(dppm)2]-[I]2 (9a), in which the metals are bridged by a vinylidene unit and an alkyne group. These cationic complexes are also obtained in the reaction of [Ir2(CO)2(mu-I)(dppm)2]+ (2) with acetylene. The structures of 5 and 7 have been determined by X-ray crystallography. Compound 5 crystallizes with 0.75 equiv of CH2Cl2 in the orthorhombic space group Pbca with a = 16.474 (3) angstrom, b = 18.289 (3) angstrom, c = 37.642 (9) angstrom, V = 11 341 (6) angstrom3, and Z = 8, and is refined to R = 0.064 and R(w) = 0.087 based on 319 parameters varied and 3759 observations. Compound 7 crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 13.093 (3) angstrom, b = 18.177 (3) angstrom, c = 22.784 (4) angstrom, V = 5423 (1) angstrom, and Z = 4, and is refined to R = 0.046 and R(w) = 0.050 based on 331 parameters varied and 2985 unique observations. In both compounds the vinylidene moieties bridge the two Ir centers which are separated by 2.828 (1) angstrom (5) and 2.783 (1) angstrom (7). Both vinylidene C-C separations (1.25 (3) angstrom (5), 1.35 (3) angstrom (7)) are typical of double bonds. The phenyl substituent on the vinylidene unit of 7 causes significant skewing of the structure owing to repulsions involving the dppm phenyl groups.