Synthesis and characterization of cationic iron complexes supported by the neutral ligands NPi-Pr3, NArPi-Pr3, and NSt-Bu3

Abstract

This paper compares the local geometries, spin states, and redox properties of a series of iron complexes supported by neutral, tetradentate NP3(tris(phosphine)amine) and NS3(tris(thioether)amine) ligands. Our consideration of an Fe-mediated N2fixation scheme similar to that proposed by Chatt for molybdenum motivates our interest in systems of these types. This report specifically describes the synthesis and characterization of cationic Fe(II) chloride complexes supported by the neutral ligands NPi-Pr3(NPi-Pr3 = [N(CH2CH2P-i-Pr2)3]), NArPi-Pr3(NArPi-Pr3 = [N(2-diisopropylphosphine-4-methylphenyl)3]), and NSt-Bu3(NSt-Bu3 = [N(CH2CH2S-t-Bu)3]). The solid-state structures, electrochemistry, and magnetic properties of these complexes are reported. Whereas the NPi-Pr3and NArPi-Pr3ligands provide pseudotetrahedral S = 2 ferrous cations [Fe(NPi-Pr3)Cl]PF6(1[PF6]) and [Fe(NArPi-Pr3)Cl]BPh4(2[BPh4]) featuring a long Fe—N bond distance, the NSt-Bu3ligand gives rise to a trigonal bipyramidal structure with a S = 1 ground state and a much shorter Fe–N interaction. The complexes 1[BPh4] and 2[BPh4] can be reduced under CO to give rise to the five-coordinate Fe(I) monocarbonyls [Fe(NPi-Pr3)CO]BPh4(4[BPh4]) and [Fe(NArPi-Pr3)CO]BPh4(5[BPh4]). The solid-state structures and electrochemistry of 4[BPh4] and 5[BPh4] are described, as is the EPR spectrum of 4[BPh4]. The synthesis and characterization of the hydride–dinitrogen complex [Fe(NPi-Pr3)(N2)(H)]PF6(6[PF6]) has also been accomplished and its properties are also reported.Key words: nitrogenase, iron, polydentate phosphines, thioether ligands, N2chemistry, nitrogen, Fe(I).