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

Abstract

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 [V^III] ion. These unique scaffolds are prepared via addition of 1-adamantyl isonitrile (C≡NAd) across the alkylidyne in complexes [(BDI)V≡C^tBu(OTf)] (A) (BDI^-=ArNC(CH₃)CHC(CH₃)NAr), Ar=2,6-ⁱPr₂C₆H₃) and [(dBDI)V≡C^tBu(OEt₂)] (B) (dBDI^2-=ArNC(CH₃)CHC(CH₂)NAr). Complex A reacts with C≡NAd, to generate the high-spin [V^III] 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 [V^III] 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.

Keywords: Alkylidynes; Azaalleneyl; Bent Allene; Metallacycles; Ynamide.