Chemistry of highly electrophilic binuclear cations. 6. Synthesis of the alkyne-bridged complexes [Mo2(η5-C5H5)2{μ-η2:η2-HC2(p-tol)}(CO)2(μ-L2)][B{3,5-C6H3(CF3)2}4]2 and their isocyanide derivatives (L2 = Ph2PCH2PPh2, Me2PCH2PMe2)

One-electron oxidation of the alkyne-bridged radicals [Mo2Cp2{μ-η2:η2-HC2(p-tol)}(CO)2(μ-L2)]View the MathML source(BAr4′) [Cp = η5 − C5H5; L2=Ph2PCH2PPh2 (dppm), Me2PCH2PMe2 (dmpm); Ar′ = 3,5-C6H3(CF3)2] with the ferricenium salts [FeCp2]X View the MathML source(X=BAr4′-, View the MathML sourceBF4-) gives the dipositive cationic complexes [Mo2Cp2{μ-η2:η2-HC2(p-tol)}(CO)2(μ-L2)]View the MathML source(BAr4′)(X) which display two isomeric geometries differing in the position (cis or trans ) of the alkyne ligand relative to the diphosphine bridge, with the cis isomer being dominant for the dmpm complex and the reverse for the dppm derivative. These 32-electron complexes turned out to be quite stable and do not experience deprotonation, dehydrogenation or any rearrangement in the bonded alkyne even under heating. However, they react rapidly with CNtBu at room temperature to give new isocyanide derivatives stepwise. Addition of one equivalent of CNtBu leads to the monocarbonyl complexes [Mo2Cp2{μ-η2:η2-HC2(p-tol)}(CO)(CNtBu)(μ-L2)]View the MathML source(BAr4′)2 (L2 = dppm, dmpm) resulting from substitution of one carbonyl ligand. Addition of a second equivalent of isocyanide on the dmpm cation replaces the remaining carbonyl to give the diisocyanide complex [Mo2Cp2{μ-η2:η2-HC2(p-tol)}(CNtBu)2(μ-dmpm)]View the MathML source(BAr4′)2, while a large excess of ligand forces the incorporation of a third isocyanide molecule to the unsaturated metal centre, to give the electron-precise triisocyanide derivative [Mo2Cp2{μ-η2:η2-HC2(p-tol)}(CNtBu)3(μ-dmpm)]View the MathML source(BAr4′)2. The latter is a labile cation which dissociates isocyanide in solution at room temperature. All new compounds were characterized on the basis of their spectroscopic IR and NMR (1H, 31P, 13C) data.