A new method for the preparation of N-stabilized allenylidene complexes of chromium and tungsten

Displacement of tetrahydrofuran in [(CO)5M(THF)] (M=Cr, W) by the anion [CCC(X)Y]− (X=O; NR; Y=NR′2, Ph) followed by alkylation of the resulting metalate with [R″3O]BF4 (R″=Me, Et) offers a convenient and versatile route to π-donor-substituted allenylidene complexes, [(CO)5MCCC(XR″)Y]. Allenylidene complexes in which the terminal carbon atom of the allenylidene ligand constitutes part of a heterocycle are likewise accessible by this reaction sequence. Reaction of [(CO)5M(THF)] with Li[CCC(NMe)Ph]− and subsequent protonation of the metalate afford [(CO)5MCCC(NMeH)Ph] in high yield. As indicated by the spectroscopic data of the compounds and the X-ray analyses of three representative examples, these allenylidene complexes are best described as hybrids of allenylidene and zwitterionic alkynyl complexes with delocalisation of the electron pair at nitrogen towards the metal center. Dimethylamine reacts with the amino(phenyl)allenylidene complex [(CO)5CrCCC(NMe2)Ph] (7a) by addition of the amine across the CαCβ bond to give selectively the E-alkenyl(amino)carbene complex [(CO)5CrC(NMe2)CHC(NMe2)Ph] (12). In contrast, the reaction of dimethylamine with the amino(methoxy)allenylidene complex [(CO)5CrCCC(NMe2)OMe] (1a) proceeds by addition of the amine to the Cγ atom and subsequent elimination of methanol to give the substitution product [(CO)5CrCCC(NMe2)2] (13). Triphenylphosphane neither adds to the Cα nor the Cγ atom of 7a but upon irradiation displaces a CO ligand to form a cis-allenylidene(tetracarbonyl)phosphane complex 15.