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.