The reaction of colorless iron(II) formate or the mixed-valence cluster Fe3O(MeCOO)6(H2O)3 with formic acid in dimethylformamide exposed to air at 110 °C affords black crystals of the mixed-valence (Me2NH2)[FeIIFeIII(HCOO)6] three-dimensional (3D) structure in which the cations occupy half of the channels. The structure consists of alternating layers of FeIIO6 [Fe(1)−O(1), 2.119(1) Å] and FeIIIO6 [Fe(2)−O(2), 2.0049(9) Å] octahedra bridged by anti−anti-bonded formates to afford an open-framework 3D structure. The structure is very similar to those of (Me2NH2)[FeII(HCOO)3] and [FeIII(HCOO)3]·HCOOH, both of which are colorless. The black crystals appear dark-purple (λmax ≈ 520 nm) when powdered. The room-temperature Mössbauer spectrum confirms the 1:1 ratio of Fe(II) (δ = 1.03 mm/s, ΔEQ = 1.16 mm/s) and Fe(III) (δ = 0.62 mm/s, ΔE Q = 0.49 mm/s). Magnetic ordering that includes negative magnetization at low fields occurs at low temperature. The only molecular-based magnetic materials in which this phenomenon has been observed are the 2D polyiron(II,III) oxalates A[FeIIFeIII(C2O4)3] (A = R4N+ cation).

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.

A supramolecular aggregate of two Kemp's tri-acid (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid, KTA) and four acetic acid molecules is arranged in a centrosymmetric zero-dimensional (0D) motif (2). This unusual pairing of unlike carboxylic acids disrupts a 1D hydrogen-bonded rod motif that is thermodynamically preferred by KTA (1) under most conditions. The six-molecule 0D aggregate reported here behaves like a large molecule with excellent self-recognition properties. Efficient centrosymmetric packing leads to maximized van der Waals contact and multiple C–H⋯O interactions between aggregates. The synthesis and physical properties of the parent KTA rod structure (1) and the acetic acid co-crystal (2) are compared and discussed.