Chemistry of highly electrophilic binuclear cations. 6. Synthesis of the alkyne-bridged complexes [Mo 2(η 5-C 5H 5) 2{μ-η 2:η 2-HC 2(p-tol)}(CO) 2(μ-L 2)][B{3,5-C 6H 3(CF 3) 2} 4] 2 and their isocyanide derivatives (L 2 = Ph 2PCH 2PPh 2, Me 2PCH 2PMe 2)

Abstract

One-electron oxidation of the alkyne-bridged radicals [Mo 2Cp 2{μ-η 2:η 2-HC 2(p-tol)}(CO) 2(μ-L 2)] ( BAr 4 ′ ) [Cp = η 5 − C 5H 5; L 2=Ph 2PCH 2PPh 2 (dppm), Me 2PCH 2PMe 2 (dmpm); Ar′ = 3,5-C 6H 3(CF 3) 2] with the ferricenium salts [FeCp 2]X ( X = BAr 4 ′ - , BF 4 - ) gives the dipositive cationic complexes [Mo 2Cp 2{μ-η 2:η 2-HC 2(p-tol)}(CO) 2(μ-L 2)] ( BAr 4 ′ ) (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 CN t Bu at room temperature to give new isocyanide derivatives stepwise. Addition of one equivalent of CN t Bu leads to the monocarbonyl complexes [Mo 2Cp 2{μ-η 2:η 2-HC 2(p-tol)}(CO)(CN t Bu)(μ-L 2)] ( BAr 4 ′ ) 2 (L 2 = 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 [Mo 2Cp 2{μ-η 2:η 2-HC 2(p-tol)}(CN t Bu) 2(μ-dmpm)] ( BAr 4 ′ ) 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 [Mo 2Cp 2{μ-η 2:η 2-HC 2(p-tol)}(CN t Bu) 3(μ-dmpm)] ( BAr 4 ′ ) 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.