An N-Heterocyclic Germylene with a Versatile Metal-Binding Pocket: Insights into Heterodinuclear Bonding and Reactivity

We report the synthesis, isolation, and characterisation of an N-heterocyclic germylene (NHGe) derived from a two-electron reduced Mg-synthon of the redox-active dippNBA ligand. This NHGe features a vacant binding pocket capable of coordinating various metals, which enables the formation of heterobimetallic Ge–Zn and Ge–Mg complexes. Electronic structure calculations reveal that the Ge–Zn interactions are weak, whilst the interactions between the metals found for the heterodinuclear Ge–Mg complex are stronger. These findings highlight how the nature of the Ge–M interactions adapts to the electron density requirements of the metal occupying the redox-active binding pocket flanking the Ge(II) centre. Notably, the Ge–Mg complex undergoes a ‘Metallo-Diels–Alder’ reaction with unsaturated C–C bonds, activating these bonds over the Ge centre and the ligand backbone – a transformation that does not proceed without Mg. This provides a compelling example of indirect cooperativity, where the Mg centre electronically stabilises a quadruply reduced ligand, upon which Ge engages in metal-ligand cooperative activation of C–C unsaturated bonds.