Amine–nitrile coupling in amino-monocarbollide complexes of molybdenum

Abstract

The Mo⁰ molybdenacarbaborane trianion [1-NH₂-2,2,2-(CO)₃-closo-2,1-MoCB₁₀H₁₀]³⁻ is formed in situ from the reaction between [7-NH₂-nido-7-CB₁₀H₁₀]³⁻ and [Mo(CO)₄{NH(CH₂)₅}₂] in THF solution (THF = tetrahydrofuran). In the absence of nitriles, oxidation of this trianion by CH₂CHCH₂Br gives the Mo^II complex [1,2-μ-NH₂-2,2,2-(CO)₃-closo-2,1-MoCB₁₀H₁₀]⁻, isolated as its [N(PPh₃)₂]⁺ salt. However, in the presence of nitriles NCR, the same oxidation ultimately affords the species [N(PPh₃)₂][1,2-μ-{NHC(R)NH}-2,2,2-(CO)₃-closo-2,1-MoCB₁₀H₁₀] in which amidine groups form a bridge between the carbon atom of the cage and the molybdenum, as a consequence of coupling of the nitrile with the cage-bound NH₂ group. The acetamidine product [N(PPh₃)₂][1,2-μ-{NHC(Me)NH}-2,2,2-(CO)₃-closo-2,1-MoCB₁₀H₁₀] is also formed by reaction of [7-NH₂-nido-7-CB₁₀H₁₂]⁻ with [Mo(CO)₆] in refluxing NCMe, and its structure was confirmed by an X-ray diffraction study. Further oxidation of this species by I₂ in the presence of CNBu^t gives the Mo^IV derivatives [1,2-μ-{NHC(Me)NH}-2,2-(CNBu^t)₂-2-L-2-I-closo-2,1-MoCB₁₀H₁₀] (L = CO, CNBu^t), of which the tris(isocyanide) complex has been studied by X-ray diffraction.