Construction of alkyl-substituted pentaphosphido ligands in the coordination sphere of cobalt

Rare mono- and diorganopentaphosphido cobalt complexes are accessible by P–P condensation using the unprecedented, reactive cobalt-gallium tetraphosphido complex [K(dme)(2){((Mes)BIAN)Co(μ-η(4):η(2)-P(4))Ga(nacnac)}] (2). Compound 2 was prepared in good yield by reaction of [K(Et(2)O){((Mes)BIAN)Co(η(4)-1,5-cod)}] [1, BIAN = bis(mesitylimino)acenaphthene diimine, cod = 1,5-cyclooctadiene] with [Ga(nacnac)(η(2)-P(4))] (nacnac = CH[CMeN(2,6-iPr(2)C(6)H(3))](2)). Reactions with R(2)PCl (R = iPr, tBu, and Cy) selectively afford [((Mes)BIAN)Co(cyclo-P(5)R(2))] (3a–c), which feature η(4)-coordinated 1,1-diorganopentaphosphido ligands. The mechanism of formation of these species has been studied by (31)P{(1)H} NMR spectroscopy and DFT calculations. In the case of 3a (R = iPr), it was possible to identify the intermediate [((Mes)BIAN)Co(μ-η(4):η(2)-P(5)iPr(2))Ga(nacnac)] (4) by single-crystal X-ray diffraction. A related, monosubstituted organopentaphosphido cobalt complex [((Mes)BIAN)Co(μ-η(4):η(1)-P(5)tBu)GaCl(nacnac)] (5) was isolated by reacting dichloroalkylphosphane tBuPCl(2) with 2. Heterobimetallic complexes such as 2 thus may enable the targeted construction of a range of new metal-coordinated polyphosphorus frameworks by P–P condensation.