Pentanuclear complexes with unusual structural topologies from the initial use of two aliphatic amino-alcoholligands in Fe chemistry

Abstract

Five novel pentanuclear Fe³⁺ clusters with the aliphatic amino-alcohol ligands 3-amino-1-propanol (Hap) and 2-(hydroxymethyl)piperidine (Hhmpip) [Fe₅(μ₃-Ο)₂(L)₄(O₂CR)₇] [L = ap⁻, R = Ph (1); L = ap⁻, R = C(CH₃)₃ (2); L = hmpip⁻, R = Ph (3); L = hmpip⁻, R = C(CH₃)₃ (4)] and [Fe₅(μ₄-Ο)(μ₃-Ο)(O₂CC(CH₃)₃)₈(ap)₂Cl(HO₂CC(CH₃)₃)] (5) are reported. Compounds 1–4 were prepared from reactions of preformed trinuclear Fe³⁺ clusters with the ligands in a molar ratio 1 : 5 in MeCN (1, 3, 4) or DMF (2), whereas compound 5 was prepared from the reaction of FeCl₃ with Hap in the presence of HO₂CC(CH₃)₃ in a molar ratio 1 : 3 : 2 in MeCN. To the best of our knowledge, 1–5 are the first examples of Fe³⁺ complexes with the ligands Hap and Hhmpip. The structures of 1–4 are composed of a quasi-planar [Fe₅(μ₃-O)₂]¹¹⁺ core which consists of two vertex-sharing [Fe₃(μ₃-O)]⁷⁺ triangles. The structure of 5 is based on the [Fe₅(μ₄-O)(μ₃-O)]¹¹⁺ core, in which the five Fe³⁺ ions adopt a monocapped trigonal pyramidal topology. Variable-temperature magnetic susceptibility measurements on powdered microcrystalline samples of 1 and 5 revealed the existence of antiferromagnetic interactions which led to an S = 5/2 ground state. Mössbauer spectroscopy studies on powdered microcrystalline samples of 1 and 5 confirmed that all iron ions of both complexes are in the Fe³⁺ (S = 5/2) state. The variation of the ligand environment in the various iron sites was reflected in their different quadruple splitting parameters. At T < 50 K the Mössbauer spectra indicated the onset of spin relaxation effects in the time scale of the technique (10⁻⁷–10⁻⁸ s).