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(μ-PR*)(CO)4] (R = 2,4,6- C6H2 tBu3) experiences an intramolecular C−H bond cleavage from a tBu group to give the phosphide-hydride derivative [Mo2Cp2(μ-H){μ-P(CH2CMe2)C6H2 tBu2}(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(μ-PR*)(μ-CO)2] (Mo−Mo = 2.5322(3) Å) or its isomer [Mo2Cp2(μ-κ 1:κ 1,η 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 η6-fashion. The latter complex experiences a proton-catalyzed tautomerization to yield the cyclopentadienylidene−phosphinidene derivative [Mo2Cp(μ-κ 1:κ 1,η 5-PC5H4)(η 6-R*H)(CO)2]. Carbonylation of the η 6-phosphinidene complex proceeds stepwise through the η 4-tricarbonyl complex [Mo2Cp2(μ-κ 1:κ 1,η 4-PR*)(CO)3] and then to the starting tetracarbonyl compound, whereas its reaction with CNtBu yields only the η 4-complex [Mo2Cp2(μ-κ 1:κ 1,η 4-PR*)(CNtBu)(CO)2], which was characterized through an X-ray study. The η 4-tricarbonyl species reacts with CNtBu in tetrahydrofuran to give the metal−metal bonded derivative [Mo2Cp2(μ-PR*)(CNtBu)(CO)3]. In petroleum ether, however, this reaction yields the bis(isocyanide) derivative [Mo2Cp2(μ-PR*)(CNtBu)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(μ-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 η 6-complex is accelerated by the presence of CuCl, due to the formation of the trimetal species [CuMo2(Cl)Cp2(μ-κ 1:κ 1:κ 1,η 6-PR*)(CO)2] and [CuMo2(Cl)Cp2(μ-κ 1:κ 1:κ 1,η 4-PR*)(CO)3]. The latter complexes were also characterized by single-crystal X-ray studies.