Activation of White Phosphorus by Low-Valent Group 5 Complexes: Formation and Reactivity of cyclo-P4 Inverted Sandwich Compounds

We report the synthesis and comprehensive study of the electronic structure of a unique series of dinuclear group 5 cyclo-tetraphosphide inverted sandwich complexes. White phosphorus (P4) reacts with niobium­(III) and tantalum­(III) β-diketiminate (BDI) tert-butylimido complexes to produce the bridging cyclo-P4 phosphide species {[(BDI)­(NtBu)­M]2­(μ‑η3:η3P4)} (1, M = Nb; 2, M = Ta) in fair yields. 1 is alternatively synthesized upon hydrogenolysis of (BDI)­Nb­(NtBu)­Me2 in the presence of P4. The trinuclear side product {[(BDI)­NbNtBu]3­(μ‑P12)} (3) is also identified. Protonation of 1 with [HOEt2]­[B­(C6F5)4] does not occur at the phosphide ring but rather involves the BDI ligand to yield {[(BDI#)­Nb­(NtBu)]2­(μ‑η3:η3P4)}­[B­(C6F5)4]2 (4). The monocation and dication analogues {[(BDI)­(NtBu)­Nb]2­(μ‑η3:η3P4)}­{B­(ArF)4}n (5, n = 1; 6, n = 2) are both synthesized by oxidation of 1 with AgBArF. DFT calculations were used in combination with EPR and UV–visible spectroscopies to probe the nature of the metal–phosphorus bonding.