Ligand elaboration mediated by a Cp*W(NO) template: Stepwise incorporation of small molecules into a tungsten vinyl fragment

Thermolysis of the alkyl vinyl complex Cp*W(NO)(CH₂SiMe₃)(CPh=CH₂) (1) in the presence of unsaturated, heteroatom-containing compounds such as esters and nitriles quantitatively affords metallacyclic products of reductive coupling. These are trapped as 18e complexes via either intramolecular rearrangement or intermolecular reaction with added trapping reagents. The nature of these metallacycles is consistent with the intermediacy of the acetylene complex Cp*W(NO)(η²-CPh≡CH) (A) derived in situ from the reductive elimination of SiMe₄ from 1. With esters ROAc (R = Me, Et), reductive coupling and C-O bond cleavage yields the alkoxide-containing oxametallacyclopentadiene complexes Cp*W-(NO)(η²-O=C(Me)CH=CPh)(OR) (2, R = Me; 3, R = Et). Thermolysis of 1 in RCN (R = Me, Et, ⁱPr) containing small excess amounts of R′OH yields the respective hydroxide or alkoxide compounds Cp*W(NO)(η²-NH=C(R)CH=CPh)(OR′) (4, R = Me, R′ = H; 5, R = Et, R′ = H; 6,R = ⁱPr, R′ = H; 7, R = Me, R′ = C₃H₅). Utilization of cyclopentadiene (CpH) as the trapping agent in MeCN affords the aminopentafulvene complex Cp*W(NO)(HNC(=C(C₄H₄))-(Me))(η²- NH=C(Me)CH=CPh) (8). VT ¹H NMR spectroscopy reveals the fluxional solution behavior of the fulvene ligand in 8. Thermolysis of 1 in RCN (R = Me, ⁱPr) containing trace amounts of acetone gives the bicyclic species Cp*W(NO)(η³-OC(Me)₂N=C(Me)CH=CPh) (9, R = Me; 10, R = ⁱPr). In the absence of added trapping reagent, thermolysis of 1 in RCN (R = Me, Et) yields the vinyl amidinate complexes Cp*W(NO)(η³-NHC(R)=NC(=C(R¹)(R ²))-CH=CPh) (11, R = Me, R¹ = R² = H; 12, R = Et, R¹ = H, R² = Me). The molecular structures proposed for 2, 4, 8, and 10 are confirmed by single-crystal X-ray analyses. Mechanistic proposals to account for the observed chemistry are corroborated by the results of labeling studies, while a kinetic study of the transformations yielding 3, 4, and 11 implicates the rate-limiting generation of acetylene intermediate A. A qualitative orbital overlap rationale is proposed to account for the observed chemistry.