MONONUCLEAR BIS-AMINOCARBYNE COMPLEXES OF TUNGSTEN WITH TP' LIGANDS -THEORY OF METAL-CENTERED CARBYNE-CARBYNE COUPLING

An efficient procedure for the synthesis of mononuclear bis-aminocarby ne complexes of tungsten is reported starting from Tp'(CO)2W=CN(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 oc curs to afford the six-coordinate aminocarbyne complexes Tp'(Br)2W=CN( R)Et (2a, 2b) in high yield. Similarly, complexes 1a and 1b. are conve rted by I2 into the diiodo derivatives Tp'(I)2W=CN(R)Et (3a, 3b). Redu ctive dehalogenation of 3a and 3b by Na/Hg in the presence of EtNC giv es the electron-rich mono-aminocarbyne complexes Tp'(EtNC)2W=CN(R)Et ( 4a, 4b), which are finally alkylated with [Et3O]BF4 at one isocyanide- nitrogen to yield the bis-aminocarbyne complexes {Tp'(EtNC)W[=CN(R)Et] [=CNEt2]}BF4 (5a, 5b). Extended-Huckel calculations on the hypothetica l 18e bis-carbyne complex [CpW(CO)(CH)2]+ predict a spontaneous rearra ngement to the energetically favoured 16e acetylene complex [CpW(CO)(H C=CH)]+ and show, in full agreement with the experimental work, that i ntroduction of amino-substituents on both carbyne-carbons stabilizes t he bis-carbyne form relative to the alkyne isomer. The solid-state str ucture of 2b was determined by a single-crystal X-ray diffraction stud y revealing a distorted octahedral complex with a short W-C(carbyne) b ond. The short C(carbyne)-N bond and the planar amino-nitrogen indicat e a high degree of pi-bonding between the carbyne-carbon and the amino -nitrogen.