BRIDGE TO TERMINAL MIGRATION AND RING-CLOSURE IN A DIMOLYBDENUM METALLACYCLOPENTADIENE COMPLEX

The dinuclear complex [Mo-2(O)(mu-O)(mu-C4Ph4)(eta-C5H5)(2)] 2a reacte d with PhNCO in refluxing toluene to give [Mo-2(O)(mu-NPh)(mu-C4Ph4)(e ta-C5H5)(2)] 3a. A crystal structure determination of the eta-C5H4Me a nalogue 3b revealed that the bridging C4Ph4 ligand displays an unprece dented 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 C-13 NMR spectra o f 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 ox idation of the metallacyclopentadiene complex [Mo-2(CO)(mu-C4Ph4)(eta- C5H5)(2)] 1 to form 2a. The reaction of 2a with (PrSH)-S-i afforded an analogous sulfido complex, [Mo-2(S)(mu-O)(mu-C4Ph4)(eta-C5H5)(2)] 4a. A second product of this reaction was identical to that formed by tre atment of 2 with elemental sulfur and had the formula [Mo-2(O)(mu-O)(m u-S)(eta-C4Ph4)(eta-C5H5)(2)] 5a. The crystal structure of the analogo us complex o-2(O)(mu-O)(mu-S)(eta-C(4)tol(4))(eta-C5H4Me)(2)] 5d was d etermined, and confirmed that the C4R4 ligand had closed up into an et a(4)-cyclobutadiene ring. This sequence of reactions therefore represe nts the gradual migration of the bridging metallacyclopentadiene unit partially (in 2 or 3) or wholly (in 5) onto one molybdenum atom, a pro cess believed to be sterically driven.