Uranium Metalla-Allenes with Carbene Imido R2C=UIV=NR' Units (R = Ph2PNSiMe3; R' = CPh3): Alkali Metal-Mediated Push-Pull Effects with an Amido Auxiliary

We report uranium(IV)-carbene-imido-amide metalla-allene complexes [U(BIPMTMS)(NCPh3)(NHCPh3)(M)] (BIPMTMS = C(PPh2NSiMe3)2; M = Li or K) that can be described as R2C=U=NR' push-pull metalla-allene units, as organometallic counterparts of the well-known push-pull organic allenes. The solid state structures reveal that the R2C=U=NR' units adopt highly unusual cis-arrangements, which is also reproduced by gas-phase theoretical studies conducted without the alkali metals to remove their potential structure directing roles. Computational studies confirm the double-bond nature of the U=NR' and U=CR2 interactions, the latter increasingly attenuated by potassium then lithium when compared to the hypothetical alkali metal-free anion. Combined experimental and theoretical data show that the push-pull effect induced by the alkali metal cations and amide auxiliary gives a fundamental and tuneable structural influence over the C=UIV=N units.