C−C and C−N Couplings Following Hydride Addition on Isocyanide Cyclopolyenyl Dimolybdenum Complexes to Give Tethered Aldimine and Aminocarbene Derivatives

Abstract

AbstractReaction of [Mo2Cp2(μ‐κ1:κ1,η6‐PMes*)(CO)2] with S or Se followed by protonation with [H(OEt2)2](BAr′4) gave the cationic derivatives [Mo2Cp2{μ‐κ2P,E:κ1P,η5‐EP(C6H3tBu3)}(CNR)(CO)2](BAr′4) (E=S; R=tBu, iPr, Ph, 4‐C6H4OMe, Xyl; or E=Se; R=tBu; Ar′=3,5‐C6H3(CF3)2). Reaction of the latter with K[BHsBu3] yielded the aldimine complexes [Mo2Cp2{μ‐κ2P,E:κ2P,N,η4‐SP(C6H3tBu3(CHNR))}(CO)2] and their aminocarbene isomers [Mo2Cp2{μ‐κ2P,E:κ2P,C,η4‐SP(C6H3tBu3(NRCH))}(CO)2] (R ≠ Xyl), following C−C and C−N couplings, respectively. Monitoring of these reactions revealed that the initial H− attack takes place at a Cp ligand to give cyclopentadiene intermediates [Mo2Cp{μ‐κ2P,S:κ1P,η5‐SP(C6H3tBu3)}(η4‐C5H6)(CNR)(CO)2], which then undergo C−H oxidative addition to give the hydride isomers [Mo2Cp2{μ‐κ2P,S:κ1P,η3‐SP(C6H3tBu3)}(H)(CNR)(CO)2]. In turn, the latter rearrange to give the aldimine and aminocarbene complexes. DFT calculations revealed that the hydride intermediates first undergo migratory insertion of the isocyanide ligand into the Mo−H bond to give unobservable formimidoyl intermediates, which then evolve either by nucleophilic attack of the N atom on the C6 ring (C−N coupling) or by migratory insertion of the formimidoyl ligand into the C6 ring (C−C coupling). Our data suggest that increasing the size of the substituent R at the isocyanide ligand destabilizes the aldimine isomer to a greater extent, thus favoring formation of the aminocarbene complex.