New Chemistry of the Triply Bonded Divanadium (V₂⁴⁺) Unit and Reduction to an Unprecedented V₂³⁺ Core

We report here the synthesis and characterization of seven new divanadium compounds with the paddlewheel geometry bridged by nitrogen-donating ligands. Five of these contain the diamagnetic V24+ core with short V−V distances of less than 2.0 Å, consistent with a formal triple bond. The V−V distances vary with the basicity of the bridging ligands; more basic ligands such as those of the guanidinate type have the shorter metal−metal separations, and those with the less basic formamidinate groups have longer separations. One compound, V2(DPhF)4 (DPhF = the anion of N,N‘-diphenylformamidine), has been reduced by one electron, and two structures, [K(THF)3]V2(DPhF)4 and [K(18-crown-6)(THF)2]V2(DPhF)4, have been obtained. These have an unprecedented V23+ core where the formal oxidation state of each vanadium atom is +1.5. The decrease in V−V bond distance and the multiline EPR spectrum in the reduced species provide evidence that these two molecules contain a bond of order 3.5.