Ynamide and Azaalleneyl. Acid‐Base Promoted Chelotropic and Spin‐State Rearrangements in a Versatile Heterocumulene [(Ad)NCC(tBu)]

We introduce the heterocumulene ligand [(Ad)NCC(tBu)]− (Ad=1‐adamantyl (C10H15), tBu=tert‐butyl, (C4H9)), which can adopt two forms, the azaalleneyl and ynamide. This ligand platform can undergo a reversible chelotropic shift using Brønsted acid‐base chemistry, which promotes an unprecedented spin‐state change of the [VIII] ion. These unique scaffolds are prepared via addition of 1‐adamantyl isonitrile (C≡NAd) across the alkylidyne in complexes [(BDI)V≡CtBu(OTf)] (A) (BDI−=ArNC(CH3)CHC(CH3)NAr), Ar=2,6‐iPr2C6H3) and [(dBDI)V≡CtBu(OEt2)] (B) (dBDI2−=ArNC(CH3)CHC(CH2)NAr). Complex A reacts with C≡NAd, to generate the high‐spin [VIII] complex with a κ1‐N‐ynamide ligand, [(BDI)V{κ1‐N‐(Ad)NCC(tBu)}(OTf)] (1). Conversely, B reacts with C≡NAd to generate a low‐spin [VIII] diamagnetic complex having a chelated κ2‐C,N‐azaalleneyl ligand, [(dBDI)V{κ2‐N,C‐(Ad)NCC(tBu)}] (2). Theoretical studies have been applied to better understand the mechanism of formation of 2 and the electronic reconfiguration upon structural rearrangement by the alteration of ligand denticity between 1 and 2. In this communication, we highlight the chelotropic rearrangement of a relatively rare heterocumulene, [(Ad)NCC(tBu)]− on a [VIII] core from κ1‐N‐ynamide to a κ2‐C,N‐azaalleneyl and vice versa, with an accompanying change in the spin‐state of the metal ion. Our system demonstrates how Brønsted acid‐base chemistry can trigger, reversibly, chelotropic rearrangements along with significant spin‐state changes.