Anionic 1-Aza-3,4-diphospholides as redox active ligands

Abstract

Herein we present the redox active behaviour of the sodium salt of 1-aza-3,4-diphospholide Na[1]. Cyclic voltammetry in MeCN shows an irreversible reduction and an irreversible oxidation wave indicating EC mechanisms. This anionic heterocycle is chemically oxidized with hexachloroethane likely to a short-lived 1-aza-3,4-diphospholyl radical which rapidly dimerizes to give bis(1-aza-3,4-diphospholyl) (2) which contains a tetraphosphane unit, PPPP. In solution and in the solid state a rac- and meso-isomer have been identified. The oxidation reaction is chemically reversible, and using lithium metal, the central PP bond in 2 can be cleaved to afford Li[1]. Both the anion [1]− and the PP coupled dimer 2 have been applied as ligands in combination with iron carbonyl precursors to isolate the strawberry red complex [FeCp(η1-1)(CO)2] (3). This complex undergoes dimerization under the loss of carbon monoxide to form the complex [Fe2(​Cp)2(μ2-1)(CO)2] (4-syn and 4-anti). The reaction of bis(1-aza-3,4-diphospholyl) 2 with [Fe3(CO)12] in THF leads to cleavage of the PP bond, and a dinuclear complex [Fe2(μ2–1)2(CO)6] (5) with a direct Fe(I)Fe(I) bond [Å = 2.5866(4)] is obtained.