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 T_d 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.