Mononuclear bis-aminocarbyne complexes of tungsten with Tp′ ligands; theory of metal-centred carbyne-carbyne coupling

An efficient procedure for the synthesis of mononuclear bis-aminocarbyne complexes of tungsten is reported starting from Tp′(CO)₂WCN (R)Et (1a: R Me; 1b: R Et) (Tp′= hydrido(3,5-dimethylpyrazol-1-yl)borate). In the first step oxidative decarbonylation of 1a and 1b with Br₂ occurs to afford the six-coordinate aminocarbyne complexes Tp′(Br)₂WCN(R)Et (2a, 2b) in high yield. Similarly, complexes 1a and 1b are converted by I₂ into the diiodo derivatives Tp′(I)₂WCN(R)Et (3a, 3b). Reductive dehalogenation of 3a and 3b by Na/Hg in the presence of EtNC gives the electron-rich mono-aminocarbyne complexes Tp′(EtNC)₂WCN(R)Et (4a, 4b), which are finally alkylated with [Et₃O]BF₄ at one isocyanide-nitrogen to yield the bis- aminocarbyne complexes {Tp′(EtNC)W[{A figure is presented}CN(R)Et][{A figure is presented}CNEt₂]}BF₄ (5a, 5b). Extended-Hückel calculations on the hypothetical 18e bis-carbyne complex [CpW(CO)(CH)₂]⁺ predict a spontaneous rearrangement to the energetically favoured 16e acetylene complex [CPW(CO)(HCCH)]⁺ and show, in full agreement with the experimental work, that introduction of amino-substituents on both carbyne-carbons stabilizes the bis-carbyne form relative to the alkyne isomer. The solid-state structure of 2b was determined by a single- crystal X-ray diffraction study revealing a distorted octahedral complex with a short WC_carbyne bond. The short C_carbyne N bond and the planar amino-nitrogen indicate a high degree of π-bonding between the carbyne-carbon and the amino-nitrogen.