A dianionic allene bound to vanadium(iii) enables interconversion between vanadatetrahedranes, vanadacyclobutadienes...

Herein we disclose a novel route to redox-active vanadacyclobutadiene (VCBD) and vanadatetrahedrane (VT_d) complexes, which circumvents a vanadium alkylidyne precursor ([V^V][triple bond, length as m-dash]CR). [V^V] VCBD salts and VT_d's are prepared through an unusual, non-classical oxidative addition reaction via the addition of electrophiles to a low-spin [V^III] deprotiovanadacyclobutadiene (dVCBD). The microscopic reverse reaction, a reductive elimination, reverts the [V^V] VCBD and VT_d scaffolds back to the low-spin [V^III] dVCBD species via the addition of a Brønsted base. The interconversion of the [V^V] VCBD salts and VT_d's is mediated through anion exchange, which promotes a change in the spin-state and geometry of the organometallic species, highlighting the redox active nature of the allene ligand (C₃) bound to vanadium (V). Single electron reduction of the [V^V] VCBD or VT_d with cobaltocene (CoCp₂) led to the formation of neutral and radical-based [V^IV] VCBDs. Oxidation of the [V^IV] VCBDs with ferrocenium based salts ([FeCp₂][X], X = BArF₂₀ ^-, BArF₂₄ ^-, OTf^-) or chloride-delivering oxidants (i.e. triphenylmethyl chloride (Ph₃CCl) or lead(ii) chloride (PbCl₂)) regenerates the discrete [V^V] VCBD salts or VT_d's, respectively. Cyclic voltammetry studies reveal quasi-reversible one electron redox couples, while X-band electron paramagnetic resonance (EPR) spectroscopic studies confirms the presence of a paramagnetic [V^IV], d¹ system in the VCBD. Computational analysis of the dVCBD, VT_d, and VCBD complexes affords detailed insight into the structure and bonding of this unusual class of molecules and delineates the role of the anion in their interconversion. Further examination of the reaction of dVCBD with electrophiles corroborates the nonclassical character of the oxidative addition sequence involving metal-ligand cooperation, with ligand-centered redox activity, rather than a formal two-electron oxidation at the metal center. The present work demonstrates how these rare dVCBD, VCBD, and VT_d scaffolds can all be interconverted via reversible C-C bond formation and splitting pathways using the right combination of anion exchange, redox, and Brønsted acid-base chemistry.