Rare-earth- and uranium-mesoionic carbenes. A new class of f-block carbene complex derived from an N-heterocyclic olefin

Neutral mesoionic carbenes (MICs) have emerged as an important class of carbene, however they are found in the free form or ligated to only a few d-block ions. Unprecedented f-block MIC complexes [M(N'')₃{CN(Me)C(Me)N(Me)CH}] (M=U, Y, La, Nd; N''=N(SiMe₃)₂) are reported. These complexes were prepared by a formal 1,4-proton migration reaction when the metal triamides [M(N'')₃] were treated with the N-heterocyclic olefin H₂C=C(NMeCH)₂, which constitutes a new, general way to prepare MIC complexes. Quantum chemical calculations on the 5f³ uranium(III) complex suggest the presence of a U=C donor-acceptor bond, composed of a MIC→U σ-component and a U(5f)→MIC(2p) π-back-bond, but for the d⁰f⁰ Y and La and 4f³ Nd congeners only MIC→M σ-bonding is found. Considering the generally negligible π-acidity of MICs, this is surprising and highlights that greater consideration should possibly be given to recognizing MICs as potential π-acid ligands when coordinated to strongly reducing metals. (copyright 2017 The Authors. Published by Wiley-VCH Verlag GmbH and Co. KGaA.)