Uranium metalla-allenes with carbene imido R₂C=U^IV=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(BIPM^TMS)(NCPh₃)(NHCPh₃)(M)] (BIPM^TMS=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 are 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 tunable structural influence over the C=U^IV=N units. (copyright 2016 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)