Mixed-metal cluster chemistry: 32. Synthesis, structure, and reactivity of a trimetallic molybdenum–iridium carbonyl cluster possessing a μ3-η2-benzyne ligand

Abstract

The reaction of Mo2Ir2(μ-CO)3(CO)7(η5-C5H5)2 and Ir(CCCH2OH)(CO)(PPh3)3 in refluxing toluene gives the molybdenum–iridium cluster MoIr2(μ3-η2-C6H4)(μ-PPh2)(μ-CO)2(CO)4(η5-C5H5) (1) in low yield. Mo2Ir2(μ-CO)3(CO)6(PPh3)(η5-C5H5)2 (2) is a possible intermediate en route to 1; reaction of Mo2Ir2(μ-CO)3(CO)7(η5-C5H5)2 and Ir(CCCH2OH)(CO)(PPh3)3 in refluxing dichloromethane affords low yields of 2, and thermolysis of the latter in refluxing toluene gives modest yields of 1. A structural study reveals that 1 consists of a triangular MoIr2 core with a molybdenum-bound cyclopentadienyl group, two terminal carbonyls at each of the iridium atoms, one carbonyl bridging each of the Mo–Ir bonds, and a PPh2 moiety spanning the Ir–Ir linkage. The cluster coordination sphere is completed by a face-capping benzyne ligand that ligates η1- to each iridium atom and η2- to the group 6 metal. A structural study of 2 confirms the tetranuclear Mo2Ir2 core with molybdenum-bound cyclopentadienyl groups, six terminal and three-edge-bridging carbonyls, the latter deployed about an MoIr2 face, and a triphenylphosphine ligand ligated axially with respect to the carbonyl-bridged plane. Reaction of 1 with PPh3 in refluxing toluene gives MoIr2(μ3-η2-C6H4)(μ-PPh2)(μ-CO)2(CO)3(PPh3)(η5-C5H5) (3) in low yield. While X-ray structural authentication of 3 has thus far proven elusive, theoretical studies indicate that an equatorially substituted structure is favored energetically over the axially-substituted isomer by around 20kJmol−1, but this energy difference is not sufficiently large so as to exclude formation of the axial isomer experimentally.