Homoleptic iron(ii)–diphosphine and –diarsine complexes: syntheses, characterization and redox properties

Abstract

Treatment of methanol solutions of [Fe(H₂O)₆](BF₄)₂ with excess ligand gives low-spin [Fe(L–L)₃](BF₄)₂ where L–L = 1,2-C₆H₄(AsMe₂)₂, 1b, Me₂PCH₂PMe₂, 2, Me₂PCH₂CH₂PMe₂, 3 and Et₂PCH₂CH₂PEt₂, 4. These complexes have been characterized by ³¹P{¹H} and ¹H NMR spectroscopy, microanalyses, mass spectrometry, infrared and visible spectroscopy. The crystal structure of 1b·0.5H₂O has been determined. Although the Fe–As bond lengths are not significantly longer than the mean for the (very limited) range of other known Fe–1,2-C₆H₄(AsMe₂)₂ complexes, there is some evidence, from compressed Me–As–Me and extended Fe–As–Me angles, of steric crowding. Nevertheless, cyclic voltammetry reveals that the complexes have a quasi-reversible Fe(II)/Fe(III) redox process at potentials approximately consistent with the Lever electrochemical ligand parameter (E_L) scheme. This is in marked contrast to their Ru(II) analogues. Complex 4 reacts with CH₃CN to give trans-[Fe(depe)₂(CH₃CN)₂](BF₄)₂, 5, in near-quantitative yield.