Bridge to terminal migration and ring closure in a dimolybdenum metallacyclopentadiene complex

Abstract

The dinuclear complex [Mo₂(O)(µ-O)(µ-C₄Ph₄)(η-C₅H₅)₂] 2a reacted with PhNCO in refluxing toluene to give [Mo₂(O)(µ-NPh)(µ-C₄Ph₄)(η-C₅H₅)₂] 3a. A crystal structure determination of the η-C₅H₄Me analogue 3b revealed that the bridging C₄Ph₄ ligand displays an unprecedented bonding mode in which one terminus bridges the two metal atoms whereas the other is bonded to only one molybdenum in the manner of a terminal alkylidene. Owing to the similarity of the ¹³C NMR spectra of 2a and 3a, we now propose that this bonding mode is also present in the former, and that the ligand rearrangement occurs during the air oxidation of the metallacyclopentadiene complex [Mo₂(CO)(µ-C₄Ph₄)(η-C₅H₅)₂] 1 to form 2a. The reaction of 2a with PrⁱSH afforded an analogous sulfido complex, [Mo₂(S)(µ-O)(µ-C₄Ph₄)(η-C₅H₅)₂] 4a. A second product of this reaction was identical to that formed by treatment of 2 with elemental sulfur and had the formula [Mo₂(O)(µ-O)(µ-S)(η-C₄Ph₄)(η-C₅H₅)₂] 5a. The crystal structure of the analogous complex [Mo₂(O)(µ-O)(µ-S)(η-C₄tol₄)(η-C₅H₄Me)₂] 5d was determined, and confirmed that the C₄R₄ ligand had closed up into an η⁴-cyclobutadiene ring. This sequence of reactions therefore represents the gradual migration of the bridging metallacyclopentadiene unit partially (in 2 or 3) or wholly (in 5) onto one molybdenum atom, a process believed to be sterically driven.