Carbon–Carbon Bond Forming Reactions with Tantalum Diamidophosphine Complexes That Incorporate Alkyne Ligands

The
incorporation of o-phenylene-linked diamidophosphine
ligands onto the readily available alkyne complexes Ta­(alkyne)­Cl3(DME) (where alkyne = hex-3-yne or 1,2-bis­(trimethylsilylacetylene);
DME = 1,2-dimethoxyethane) results in the formation of a versatile
set of starting materials of the general formula [PhNPN*]­Ta­(alkyne)­Cl
(where [PhNPN*] = PhP­(2-(N-mesityl)-5-Me-C6H3)2). Upon reaction with KBEt3H, the synthesis of the corresponding hydride complexes [PhNPN*]­Ta­(alkyne)H can be achieved; these complexes feature extremely
downfield (δ ∼21 ppm) doublet resonances (2JHP = ∼35 Hz) in the respective 1H NMR spectra that are assigned to the newly formed Ta–H
moieties. Subsequent reaction of these Ta hydrides with 2,6-dimethylphenylisocyanide
and phenylacetylene results in the insertion of these species into
the Ta–H bond and the formation of the corresponding iminoformyl
and phenylvinyl complexes, respectively. While the former intermediate
cannot be detected, the latter was characterized by NMR spectroscopy.
Both of these processes result in the further transformation to generate
C–C coupled products by a reductive elimination sequence with
the coordinated alkyne; in the case of the iminoformyl, an azadiene
results, whereas with the phenylvinyl derivative a butadienyl fragment
is generated. Single-crystal X-ray diffraction and a suite of NMR
spectroscopic techniques were used to characterize these species.
A discussion of the bonding of the products in the context of the
process by which they form is presented. The rate of formation of
the butadienyl moiety from the phenylvinyl intermediate results in
the activation parameters of ΔH⧧ = 22.2 ± 0.3 kcal/mol and ΔS⧧ = −8.7 ± 0.2 cal/(mol)­(K).