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

Distortions of the [4Fe-4S](+) cores of synthetic models from T(d) symmetry are analyzed in terms of the continuous symmetry measures (CSM, S(T(d))), 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 D(2d) 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 (Et(4)N)(3)[Fe(4)S(4)Cl(4)] center dot Et(4)NCl (1) are prepared in modest yield by treatment of FeCl(2) with NaHS, or (Et(4)N)(2)[FeCl(4)] with Et(4)NSH and a base. The crystals are isomorphous with the corresponding HS(-) ligated cluster. Crystal data: tetragonal, P (4) over bar2(1)c, Z = 2, a = b = 12.2550 (4), c = 16.278 (1) angstrom at 100 K, and a = b = 12.385 (1), c = 16.344 (2) angstrom at 295 K. The crystallographically imposed S(4) 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 center dot center dot center dot Cl(-) distances (3.590 and 3.634 at 100 K increase to 3.616 and 3.655 angstrom, respectively, at 295 K) are virtually temperature independent, indicative of hard hydrogen bridges, whereas the C center dot center dot center dot Cl-Fe distances are 3.702-3.718 angstrom at 100 K but are 3.753-3.764 angstrom at room temperature, suggesting a softer hydrogen bridge. A similar trend applies to the two sets of C center dot center dot center dot mu(3)-S distances (3.766-3.788 angstrom and 3.594-3.604 angstrom at 100 K and 3.821-3.848 angstrom and 3.614-3.676 angstrom at room temperature). The longer hydrogen bridges are more linear (170 degrees) than the shorter ones (134 degrees). The core distortions are correlated with spatial distribution of cations around the clusters.