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-45] + 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 (Et 4 N) 3 [Fe 4 S 4 Cl 4 ]. 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, P42 1 c, Z = 2, a = b = 12.2550 (4), c = 16.278 (1) A at 100 K, and a = b = 12.385 (1), c = 16.344 (2) A 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 . . . Cl- distances (3.590 and 3.634 at 100 K increase to 3.616 and 3.655 A, respectively, at 295 K) are virtually temperature independent, indicative of hard hydrogen bridges, whereas the C . . . Cl-Fe distances are 3.702-3.718 A at 100 K but are 3.753-3.764 A at room temperature, suggesting a softer hydrogen bridge. A similar trend applies to the two sets of C . . . μ 3 -S distances (3.766-3.788 A and 3.594-3.604 A at 100 K and 3.821-3.848 A and 3.614-3.676 A 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.