Voltammetry of carbocations stabilized by coordination with tetrahedral molybdenum- and tungsten-carbon clusters. [Cp2M2(CO)4-?-?2:?3-(HCCCR1R2)]+BF4 ?(M = Mo, W; R1, R2 = H, Me, and Pri) complexes

Abstract

The electrochemical reduction of the carbocationic complexes [Cp2M2(CO)4-μ-η2:η3(HCCCR1R2)]+BF4−, where M/R1, R2 = Mo/H, H (1+), Mo/H, Me (2+), Mo/Me, Me (3+), Mo/H, Pri (4+), has been studied by polarography and cyclic voltammetry on a Hg-electrode in THF solution. It has been suggested that carbocationic center-directed reversible two-electron reduction of1+−4+ takes place according to an ECE-mechanism and results in the carbanionic complexes [Cp2M2(CO)4-μ-η2:η3-(HCCCR1R2)] (1−−4−) as final productsvia carbon-centered radicals as intermediates. Anions1−-4− are capable of irreversible two-electron reduction at more negative potentials or protonation resulting in their transformation into the corresponding acetylene complexes [Cp2M2(CO)4(HCCCHR1R2)], which are also capable of irreversible two-electron reduction. Anions1−−4− and their protonated forms are reduced with cleavage of the Mo-Mo bond. The reduction pathways of complexes1+,2+, and4+ with C-H bonds at the carbon atom of the carbocationic center are different on a Pt-electrode. It is suggested that this difference is due to the abstraction of a H-atom from the intermediate radical species by platinum.