Formation and cleavage of P-C, Mo-C, and C-H bonds involving arylphosphinidene and cyclopentadienyl ligands at dimolybdenum centers

The phosphinidene-bridged complex [Mo2Cp2(mu-PR*)(CO)(4)] (R = 2,4,6-(C6H2Bu3)-Bu-t) experiences an intramolecular C-H bond cleavage from a Bu-t group to give the phosphide-hydride derivative [Mo2Cp2(mu-H){mu-P(CH2CMe2)(C6H2Bu2)-Bu-t}(CO)(4)] in refluxing diglyme (ca. 438 K) or under exposure to near-UV-visible light. In contrast, its exposure to UV light yields two different dicarbonyl derivatives depending on the reaction conditions, either the triply bonded [Mo2Cp2(mu-PR*)(mu-CO)(2)] (Mo-Mo = 2.5322(3) angstrom) or its isomer [Mo2Cp2(mu-kappa(1): kappa(1), eta(6)-PR*)(CO)(2)], in which the phosphinidene ligand bridges asymmetrically the metal centers while binding its aryl group to one of the molybdenum atoms in a eta(6)-fashion. The latter complex experiences a proton-catalyzed tautomerization to yield the cyclopentadienylidene-phosphinidene derivative [Mo2Cp(mu-kappa(1): kappa(1), eta(5)-PC5H4)(eta(6)-R*H)(CO)(2)]. Carbonylation of the eta(6)-phosphinidene complex proceeds stepwise through the eta(4)-tricarbonyl complex [Mo2Cp2(mu-kappa(1): kappa(1), eta(4)-PR*)(CO)(3)] and then to the starting tetracarbonyl compound, whereas its reaction with (CNBu)-Bu-t yields only the eta(4)-complex [Mo2Cp2(mu-kappa(1): kappa(1), eta(4)-PR*)((CNBu)-Bu-t)(CO)(2)], which was characterized through an X-ray study. The eta(4)-tricarbonyl species reacts with (CNBu)-Bu-t in tetrahydrofuran to give the metal-metal bonded derivative [Mo2Cp2(mu-PR*)((CNBu)-Bu-t)(CO)(3)]. In petroleum ether, however, this reaction yields the bis(isocyanide) derivative [Mo2Cp2(mu-PR*)((CNBu)-Bu-t)(2)(CO)(3)], which has an asymmetric trigonal phosphinidene bridge and no metal-metal bond. All the above results can be explained by assuming the operation of two primary processes in the photolysis of [Mo2Cp2(mu-PR*)(CO)(4)], one of them involving a valence tautomerization of the phosphinidene ligand, from the trigonal (four-electron donor) to the pyramidal (two-electron donor) coordination mode. The carbonylation reaction of the eta(6)-complex is accelerated by the presence of CuCl, due to the formation of the trimetal species [CuMo2(Cl)Cp-2(mu-kappa(1): kappa(1): kappa(1), eta(6)-PR*)(CO)(2)] and [CuMo2(Cl)Cp-2(mu-kappa(1): kappa(1): kappa(1), eta(4)-PR*)(CO)(3)]. The latter complexes were also characterized by single-crystal X-ray studies.