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A Continuous Symmetry Measure of [4Fe−4S]+ Core Distortions and Analysis of Supramolecular Synthons in Crystal Structures of (Et4N)3[Fe4S4Cl4]·Et4NCl at 100 and 295 K

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journal contribution
posted on 15.12.2008, 00:00 authored by Karl S. Hagen, Mohammad Uddin
Distortions of the [4Fe−4S]+ cores of synthetic models from Td symmetry are analyzed in terms of the continuous symmetry measures (CSM, S(Td)), and these are related to lattice effects in terms of the supramolecular synthon terminology common to crystal engineering of small molecule structures. The small tetragonal compression to D2d from idealized Td symmetry observed at low temperature is attributed to environmental factors. New members of the isomorphous series of compositional variations of double salts of the air-sensitive reduced cluster (Et4N)3[Fe4S4Cl4]·Et4NCl (1) are prepared in modest yield by treatment of FeCl2 with NaHS, or (Et4N)2[FeCl4] with Et4NSH and a base. The crystals are isomorphous with the corresponding HS ligated cluster. Crystal data: tetragonal, P4̅21c, Z = 2, a = b = 12.2550 (4), c = 16.278 (1) Å at 100 K, and a = b = 12.385 (1), c = 16.344 (2) Å at 295 K. The crystallographically imposed S4 symmetry obtained with sterically unincumbering ligands affords a better view of the intrinsic geometry of the core structure. The cocrystallization of the halide ion affords the opportunity to compare three types of weak C−H···X hydrogen bonds, or hydrogen bridges, between tetraalkylammonium cations and anions within the same crystal lattice. The C···Cl distances (3.590 and 3.634 at 100 K increase to 3.616 and 3.655 Å, respectively, at 295 K) are virtually temperature independent, indicative of hard hydrogen bridges, whereas the C···Cl−Fe distances are 3.702−3.718 Å at 100 K but are 3.753−3.764 Å at room temperature, suggesting a softer hydrogen bridge. A similar trend applies to the two sets of C···μ3-S distances (3.766−3.788 Å and 3.594−3.604 Å at 100 K and 3.821−3.848 Å and 3.614−3.676 Å at room temperature). The longer hydrogen bridges are more linear (170°) than the shorter ones (134°). The core distortions are correlated with spatial distribution of cations around the clusters.