Selective P₄ Activation by a Highly Reduced Cobaltate: Synthesis of Dicobalt Tetraphosphido Complexes

Although the chemistry of transition metal polyphosphide anions has attracted significant attention, there are few reports of studies in which such species have been synthesized directly from white phosphorus. [K(OEt₂)₂{Co(BIAN)(cod)}] (1, BIAN=1,2-bis(2,6-diisopropylphenylimino)acenaphthene, cod=1,5-cyclooctadiene), which is readily prepared by ligand exchange from [K(thf)_x{Co(cod)₂}], reacts with P₄ to afford [{K(thf)}₂{(BIAN)Co}₂(μ-η⁴:η⁴-P₄)] (2 a) in 61 % yield (isolated product). [{K(OEt₂)}₂{(BIAN)Co}₂(μ-η⁴:η⁴-P₄)] (2 b) and [K([18]crown-6)(MeCN)]₂[{(BIAN)Co}₂(μ-η⁴:η⁴-P₄)] (2 c) were obtained by recrystallizing 2 a from diethyl ether and acetonitrile (and using [18]crown-6 in case of 2 c). Oxidation of 2 a with [Cp₂Fe]BAr^F₄ (one equivalent) and subsequent recrystallization of the product from different solvents gave [K(OEt₂){(BIAN)Co}₂(μ-η⁴:η⁴-P₄)] (3 a) and [K(dme)₄][{(BIAN)Co}₂(μ-η⁴:η⁴-P₄)] (3 b; dme=1,2-dimethoxyethane). Neutral [{(BIAN)Co}₂(μ-η⁴:η⁴-P₄)] (4) was obtained in moderate yield by oxidizing 2 a with two equivalents of [Cp₂Fe]BAr^F₄. The new complexes were characterized by NMR, EPR (in the case of 3 a), and UV/Vis spectroscopy, and elemental analysis. The molecular structures revealed by X-ray crystallography display planar cyclic or open-chain P₄⁴⁻ units sandwiched between {(BIAN)Co} fragments.