Trapping of Hemiquinone Radicals at Mo and P Sites by Phosphide-Bridged Dimolybdenum Species:  Chemistry of Complexes [Mo25-C5H5)2(OC6H4OH)(μ-PR2)(CO)4] and [Mo25-C5H5)2{μ-PR(OC6H4OH)}(CO)4]- (R = Cy, Ph)

The phosphide-bridged dimolybdenum complexes (H-DBU)[Mo2Cp2(μ-PR2)(CO)4] (R= Cy, Ph; DBU = 1,8-diazabicyclo[5.4.0.]undec-7-ene) react with p-benzoquinone to give the hemiquinone complexes [Mo2Cp2(OC6H4OH)(μ-PR2)(CO)4]. The latter experience facile homolytic cleavage of the corresponding Mo−O bonds and react readily at room temperature with HSPh or S2Ph2 to give the thiolate complexes [Mo2Cp2(μ-PCy2)(μ-SPh)(CO)4] or [Mo2Cp2(μ-PR2)(μ-SPh)(CO)2]. In contrast, PRH-bridged substrates experience overall insertion of quinone into the P−H bond to give the anionic compounds (H-DBU)[Mo2Cp2{μ-PR(OC6H4OH)}(CO)4], which upon acidification yield the corresponding neutral hydrides. The cyclohexyl anion experiences rapid nucleophilic displacement of the hemiquinone group by different anions ER- (ER = OH, OMe, OC4H5, OPh, SPh) to give novel anionic compounds (H-DBU)[Mo2Cp2{μ-PCy(ER)}(CO)4], which upon acidification yield the corresponding neutral hydrides. The structure of four of these hydride complexes [PPh(OC6H4OH), PCy(OH), PCy(OMe), and PCy(OPh) bridges] was determined by X-ray diffraction methods and confirmed the presence of cis and trans isomers in several of these complexes. In addition, it was found that the hydroxyphosphide anion [Mo2Cp2{μ-PCy(OH)}(CO)4]- displays in solution an unprecedented tautomeric equilibrium with its hydride-oxophosphinidene isomer [Mo2Cp2(μ-H){μ-PCy(O)}(CO)4]-.