Metastable Fe/S Clusters. The Synthesis, Electronic Structure, and Transformations of the (3-) Clusters (L=Cl(1-), Br(1-), I(1-), RS(1-), RO(1-)) and the Structure of 3

Article abstract of DOI:10.1021/ja00290a034

A series of new novel (3-) clusters (L=Cl, Br, I, SC6H4-p-CH3, OC6H4-p-CH3) have been synthesized and characterized as their Et4N(1+) salts.The crystal structure of (Et4N)3Fe6S6Cl6*CH3CN (I) is described in detail.The latter crystallizes in the C2/c space group with cell constants a=20.092(5) Angstroem, b=17.937(6) Angstroem, c=13.790(4) Angstroem, β=91.33(2) deg, Z=4, and V=4968 Angstroem³.The structure was solved by conventional methods from 2604 reflections and was refined by full-matrix least-squares techniques (206 parameters) to a final R value of 0.047.The anion in I contains the new (3+) distorted hexagonal prismatic core which consists of alternating tetrahedral Fe and triply bridging S atoms.Three of the Fe coordination sites are occupied by core sulfide atoms while the fourth coordination site is filled by the terminal chloride ligands.There are two sets of Fe...Fe distances and Fe-S-Fe angles in the Fe6S6 core with mean values of 2.765 (3) Angstroem, 3.790 (8) Angstroem and 74.7 (1) deg, 113.2 (3) deg, respectively.The (3+) core appears to be a metastable entity and is easily transformed, upon heating, to the thermodynamically more stable (2+) core.The chemical properties and electronic spectra of these clusters are reported.The clusters in CH2Cl2 solution display a reversible 3-/2-couple and an irreversible 3-/4-couple.Zero field and magnetically perturbed Moessbauer spectra are reported for all clusters.Their isomer shift and quadrupole splitting values are quite similar to those in the corresponding (Fe4S4(L)4)(2-) cubane clusters.The (Fe6S6(L)6)(3-) prismane clusters exhibit characteristic electron paramagnetic resonance (EPR) spectra (9 K) indicative of S=1/2 ground states.The Moessbauer and EPR results, in conjunction with magnetic susceptibility data in the 1.5-300 K temperature range for I, also are consistent with an S=1/2 ground state.The biological implications of the cores are discussed.