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 V-2(4+) core with short V-V distances of less than 2.0 Angstrom, 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, V-2(DPhF)(4) (DPhF = the anion of N,N'-diphenylformamidine), has been reduced by one electron, and two structures, [K(THF)(3)]V-2(DPhF)(4) and [K(18-crown-6)(THF)(2)]V-2(DPhF)(4), have been obtained. These have an unprecedented V-2(3+) 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.