Solvent dependent reactivity: solvent activation vs. solvent coordination in alkylphosphane iron complexes

Abstract

The pyridine-derived tetrapodal tetraphosphane C₅H₃N[CMe(CH₂PMe₂)₂]₂ (1) is susceptible to selective protonolysis of a phosphorus–carbon bond in the presence of iron(II) salts. Water produces dimethylphosphinic acid, Me₂POH, and protonates the anionic remainder of the tetraphosphane. The resulting iron(II) complexes 2 and 3 (tetrafluoroborate and perchlorate salts, respectively) contain the residual chelate ligand in which a methyl group, derived from the ligand skeleton, is in agostic interaction with the metal centre, and in which Me₂POH, unavailable in the free state owing to rapid tautomerisation, is metal-coordinated and thus stabilised. Full NMR details are presented, including ³¹P simulations. The reactivity towards alcohols is similar (compounds 4, 5), and has been studied using deuterium labels (NMR). P–C bond cleavage may be suppressed only if all protic agents are rigorously excluded, as in the reaction of 1 with Fe(SO₃CF₃)₂·2CH₃CN in acetonitrile solution, which produces the complex [(1)Fe(NCMe)](SO₃CF₃)₂ (6). In it, the ligand acts as an NP₄ coordination cap but is severely distorted from square-pyramidal geometry. The reaction of 1 with anhydrous ferrous bromide, FeBr₂, in methanol again produces a dimethylphosphinic acid ester ligand, but the complex (7) now contains ferric iron coordinated by a carbanionic residual chelate ligand, implicating H⁺ as the oxidising agent under these conditions. Full spectroscopic and X-ray structural details are presented for all compounds.