A Mössbauer spectroscopy investigation of iron chromophore equivalence in three tetrairon clusters

Abstract

The temperature dependence (1.6 to 300 K) of the zero field Mössbauer spectra of a series of tetrametal clusters demonstrates that the local iron environments are equivalent. Compounds investigated include the cubane-like Fe 4[NO] 4[μ 3-S] 4Fe 4[NO] 4((μ 3-S)) 2(μ 3NC(CH 3) 3 2) and the extensively studied black Roussin 's salt, NH 4 [Fe 4[NO] 7(μ-S) 3]. Two types of iron environments can be distinguished from the X-ray structures of the latter two compounds, leading to an expectation of four line Mössbuer spectra. However, least-squares analyses of the high resolution, zero-field spectra for all three compounds result tn a single quadrupole doublet with homer shifts ranging from 0.05 to 0.15 mm/sec and quadrupole splittings (at 78 K) of 1.46, 1,30 and 0.91 mm/sec, respectively. High field (0 to 60 kG) spectra of the three compounds are indicative of diamagnetic species The iron environments in the first two clusters exhibit axial symmetry (η = O) with V zz negative while for Roussin 's salt η ∼ 1.