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 P-4 to afford [{K(thf)}(2){(BIAN)Co}(2)(-(4):(4)-P-4)] (2a) in 61% yield (isolated product). [{K(OEt2)}(2){(BIAN)Co}(2)(-(4):(4)-P-4)] (2b) and [K([18]crown-6)(MeCN)](2)[{(BIAN)Co}(2)(-(4):(4)-P-4)] (2c) were obtained by recrystallizing 2a from diethyl ether and acetonitrile (and using [18]crown-6 in case of 2c). Oxidation of 2a with [Cp2Fe]BAr4F (one equivalent) and subsequent recrystallization of the product from different solvents gave [K(OEt2){(BIAN)Co}(2)(-(4):(4)-P-4)] (3a) and [K(dme)(4)][{(BIAN)Co}(2)(-(4):(4)-P-4)] (3b; dme=1,2-dimethoxyethane). Neutral [{(BIAN)Co}(2)(-(4):(4)-P-4)] (4) was obtained in moderate yield by oxidizing 2a with two equivalents of [Cp2Fe]BAr4F. The new complexes were characterized by NMR, EPR (in the case of 3a), and UV/Vis spectroscopy, and elemental analysis. The molecular structures revealed by X-ray crystallography display planar cyclic or open-chain P-4(4-) units sandwiched between {(BIAN)Co} fragments.