Synthesis of the 17-electron cations [FeL(L′)(NO)2]+(L, L′= PPh3, OPPh3): structure and bonding in four-co-ordinate metal dinitrosyls, and implications for the identity of paramagnetic iron dinitrosyl complex catalysts

Abstract

The complex [FeL₂(NO)₂](L = PEt₃1a, L = PPh₃1b or L₂= dppe 1c) prepared from [{Fe(µ-I)(NO)₂}₂] and PPh₃ or Ph₂PCH₂CH₂PPh₂(dppe){in the presence and absence of [Co(cp)₂](cp =η⁵-C₅H₅) respectively} undergo one-electron oxidation at a platinum electrode in CH₂Cl₂. The complex [{Fe(µ-dppm)(NO)₂}₂]2, prepared from [{Fe(µ-I)(NO)₂}₂] and Ph₂PCH₂PPh₂(dppm) in the presence of [Co(cp)₂], undergoes two sequential one-electron oxidations. Complex 1b with [Fe(cp)₂]⁺ gave 1b⁺, X-ray studies of which show a distorted tetrahedral geometry with near C_2v symmetry. Oxidation of 1b leads to substantial lengthening of the Fe–P bonds and changes in the P–Fe–P and N–Fe–N angles. These changes are consistent with significant Fe–P π-bonding character in the singly occupied molecular orbital of 1b⁺. Cation 1b⁺ reacts with halide ions, giving [FeX(PPh₃)(NO)₂](X = Cl or I) and then [FeX₂(NO)₂]^–, and with OPPh₃ to give [Fe(OPPh₃)(PPh₃)(NO)₂]⁺3. X-Ray studies on the last, as its [PF₆]^– salt, show a distorted tetrahedral geometry; the co-ordination angles at iron approach trigonal bipyramidal with the PPh₃ ligand in one apical site and the other apical site vacant. The complex [Fe(OPPh₃)₂(NO)₂]⁺4⁺ resulted from the reaction between [{Fe(µ-I)(NO)₂}₂] and OPPh₃ in the presence of TlPF₆. An analysis of the ESR spectra of the paramagnetic cations 1b⁺, 3⁺ and 4⁺, together with extended-Hückel MO calculations on models of 1b⁺ and 3⁺, suggest that the complex catalysts formed from [{Fe(µ-Cl)(NO)₂}₂] and Ag⁺ or Tl⁺ are also four-co-ordinate 17-electron radicals. A crystallographic database study of four-co-ordinate dinitrosyl complexes of iron and other metals confirms that the N–Fe–N and O ⋯ Fe ⋯ O angles are linearly related. Consideration of these geometric effects, and those resulting from oxidation of 1b, in the light of a model proposed by Summerville and Hoffmann provides insight into the bonding in these and related species.