MODELS FOR MULTICENTER CATALYSTS .4. REACTIONS OF BINUCLEAR HYDRIDES WITH ALKYNES AND THE STRUCTURE OF [IR2(H)2CL(CH3O2CC=C(H)CO2CH3)2(CO)2(PH2PCH2PPH2)2]-[BF4].3THF, A MODEL HYDROGENATION INTERMEDIATE CONTAINING MUTUALLY ADJACENT ALKENYL AND HYDRIDO LIGANDS

The reaction of [Ir2(H)2(CO)2(M-Cl)(DPM)2][BF4] (1) with 2 equiv of dimethyl acetylenedicarboxylate (DMA) in THF or acetone yields [Ir2(CH3O2CC=C(H)CO2CH3)2(CO)2(μ-Cl)(DPM)2][BF4]. This cationic species is the result of alkyne insertion into both of the Ir-H bonds of compound 1. Further reaction of this species with dihydrogen yields [Ir2(H)2Cl(CH3O2CC=C(H)CO2CH3)2(CO)2(pPM)2][BF4] (DPM = Ph2PCH2PPh2), which is a stable complex containing mutually adjacent alkenyi and hydrido hgands. This compound crystallizes in the space group P21/c with a = 18.675 (5) Å, b = 15.775 (3) Å, c = 26.371 (4) Å, β = 96.38 (2)°, and Z = 4. The structure has refined to R = 0.054 and Rv = 0.062 on the basis of 5775 unique observations and 590 parameters varied. Although the hydride ligands were not directly located in the X-ray study, their positions can be inferred from the positions of the heavy atoms and from the NMR studies. One is located in a terminal site on one Ir adjacent to an alkenyi group, while the other is bridging the two metals, resulting in an Ir-Ir separation of 3.2895 (7) Å. A mechanism is proposed for one pathway by which alkynes may be hydrogenated by binuclear species such as 1. It is proposed that hydrogenation occurs at each individual metal center in turn but that the coordinative unsaturation required at different stages occurs by movement of the chloride ligand back and forth from one metal to the other. The reactions of 1 with 2-butyne and ethylene yield 2-butenes and ethane, respectively, in the absence of H2 and suggest an alternate intermolecular mechanism for hydrogénation. The reaction of alkynes with the mixed-metal species, [RhIr(H)2(CO)2(M-Cl)(DPM)2]+, does not appear to result in insertion but rather displaces H2 from the complex, yielding alkyne-bridged species. © 1990, American Chemical Society. All rights reserved.