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

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