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

We introduce the heterocumulene ligand [(Ad)NCC( t Bu)] - (Ad=1-adamantyl (C 10 H 15 ), t Bu=tert-butyl, (C 4 H 9 )), 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 [V III ] ion. These unique scaffolds are prepared via addition of 1-adamantyl isonitrile (C≡NAd) across the alkylidyne in complexes [(BDI)V≡C t Bu(OTf)] (A) (BDI - =ArNC(CH 3 )CHC(CH 3 )NAr), Ar=2,6- i Pr 2 C 6 H 3 ) and [(dBDI)V≡C t Bu(OEt 2 )] (B) (dBDI 2- =ArNC(CH 3 )CHC(CH 2 )NAr). Complex A reacts with C≡NAd, to generate the high-spin [V III ] complex with a κ 1 -N-ynamide ligand, [(BDI)V{κ 1 -N-(Ad)NCC( t Bu)}(OTf)] (1). Conversely, B reacts with C≡NAd to generate a low-spin [V III ] diamagnetic complex having a chelated κ 2 -C,N-azaalleneyl ligand, [(dBDI)V{κ 2 -N,C-(Ad)NCC( t Bu)}] (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.