Cycloaddition and C–H Activation Reactions of a Tantalum Alkylidyne

The
alkylidyne complex (C5Me5)­Ta­(CPh)­(PMe3)2Cl (1) was first reported by Schrock
in 1978, but little if any follow-up work on 1 or other
group 5 metal alkylidynes has been reported. This work discloses two
avenues of reactivity of 1. Treatment of 1 with 3-hexyne resulted in the formation of a tantalacyclobutadiene.
Abstraction of chloride from 1 led to a mixture of products
that included isomers of [(C5Me5)­Ta­(CHPh)­(CH2PMe2)­(PMe3)]+ (4), in which a C–H bond of a PMe3 ligand was added
across the TaC bond. The C–H activation was found to
be reversible, and the equilibrium mixture functioned as an equivalent
of a cationic Ta alkylidyne in reaction with 3-hexyne, producing a
cationic tantalacyclobutadiene (6). Compounds 4 and 6 were structurally characterized in the solid
state by XRD methods, with 6 being the first structurally
characterized metallacyclobutadiene in group 5.