Noncentrosymmetric (NCS) structures are of particular interest owing to their symmetry-dependent physical properties, e.g., pyroelectricity, ferroelectricity, piezoelectricity, and nonlinear optical (NLO) behavior. Among them, chiral materials exhibit polarization rotation and host topological properties. Borates often contribute to NCS and chiral structures via their triangular [BO3] and tetrahedral [BO4] units and their numerous superstructure motifs. However, no chiral compound with the linear [BO2] unit has been reported to date. Herein, an NCS and chiral mixed-alkali-metal borate, NaRb6(B4O5(OH)4)3(BO2), with a linear BO2 – unit in the structure was synthesized and characterized. The structure features a combination of three types of basic building units (BBUs), [BO2], [BO3], and [BO4] with sp-, sp2-, and sp3-hybridization of boron atoms, respectively. It crystallizes in the trigonal space group R32 (No. 155), one of the 65 Sohncke space groups. Two enantiomers of NaRb6(B4O5(OH)4)3(BO2) were found, and their crystallographic relationships are discussed. These results not only expand the small family of NCS structures with the rare linear BO2 – unit but also prompt recognition to the fact that NLO materials have generally overlooked the existence of two enantiomers in achiral Sohncke space groups.

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We examine the role of molecular symmetry and hydrogen bonding in determining heterochiral intermolecular π–π stacking motifs in four racemic compounds with the formula [Cu­(phen)2(H2O)]­[MF6]·xH2O (M = Ti, Zr, Hf; phen = 1,10-phenanthroline) and two racemic compounds with the formula Cu­(phen)2MF6·H2O (M = Zr, Hf). In this work, equimolar combinations of C 2-symmetric Δ- and Λ-Cu­(phen)2(H2O)2+ complexes were found to organize via only face-to-face π–π stacking interactions to adopt a new horizontal packing motif in a series of compounds with the formula [Cu­(phen)2(H2O)]­[MF6]·xH2O (M = Ti, Zr, Hf). Previously, Δ- and Λ-Cu­(phen)2(H2O)2+ complexes had been observed to pack with only parallel displaced π–π stacking interactions in a diagonal packing motif or with both face-to-face and parallel displaced π–π stacking interactions in a zigzag packing motif. The horizontal arrangement reported here is associated with the formation of hydrogen-bonding networks that link cations, anions, and hydrating water molecules within these structures. Equimolar combinations of neutral Δ- and Λ-Cu­(phen)2MF6 (M = Zr, Hf) molecules organize in a zigzag stacking pattern that originates from the presence of both parallel displaced and face-to-face π–π stacking interactions. The symmetry of the Cu­(phen)2MF6 molecule is reduced to C 1 by tilting of the bound MF6 2– octahedron, which renders the two phen ligands symmetrically inequivalent.

The design of noncentrosymmetric (NCS) solid state materials, specifically how to break inversion symmetry between enantiomers, has intrigued chemists, physicists, and materials scientists for many years. Because the chemical complexity of molecular racemic building units is so varied, targeting these materials is poorly understood. Previously, three isostructural racemic compounds with a formula of [Cu(H2O)(bpy)2]2[MF6]2·2H2O (bpy = 2,2’=bipyridine; M = Ti, Zr, Hf) were shown to crystallize in the NCS space group Pna21, of polar, achiral crystal class mm2. In this work, we synthesized five new racemic compounds with the formula [Cu(H2O)(dmbpy)2]2[MF6]2·xH2O (dmbpy = 4,4′/5,5′-dimethyl-2,2′-bipyridine; M = Ti, Zr, Hf). Single crystal X-ray diffraction reveals that the five newly synthesized compounds feature equimolar combinations of Δ- and Λ-Cu(dmbpy)2(H2O)2+ complexes that are assembled into packing motifs similar to those found in the reported NCS structure but all crystallize in centrosymmetric (CS) space groups. Seven structural descriptors were created to analyze the intermolecular interactions on the assembly of Cu racemates in the CS and NCS structures. The structural analysis reveals that in the CS structures, the inversion center results from parallel heterochiral π–π stacking interactions between adjacent Cu racemates regardless of cation geometries, hydrogen bonding networks, or interlayer architectures, whereas in the NCS structure, nonparallel heterochiral π–π interactions between the adjacent Cu racemates preclude an inversion center. The parallel heterochiral π–π interactions in the CS structures can be rationalized by the restrained geometries of the methyl-substituted ligands. This work demonstrates that the introduction of nonparallel stacking can suppress the formation of an inversion center for an NCS racemate. A conceptual framework and practical approach linking the absence of inversion symmetry in racemates is presented for all NCS crystal classes.

Layered metal–organic frameworks are promising candidates for new two-dimensional (2D) magnets, as the synthetic programmability of these materials can provide a route to diverse structural and electronic properties. However, such framework materials typically lack the heavy elements that engender magnetocrystalline anisotropy in the monolayer ferromagnets reported to date. Alternative sources of magnetic anisotropy are therefore needed in these materials. Here, we report the synthesis of single crystals of the framework material (NMe4)2[Fe2L3] (H2L = 3,6-dichloro-2,5-dihydroxybenzoquinone) and evaluate the angular dependence of its magnetic properties. Oriented-crystal magnetization measurements reveal strong uniaxial anisotropy, where the easy axis is aligned with the crystallographic c axis. While the spin carriers of this structure are isotropic S = 5/2 FeIII metal centers and S = 1/2 organic linkers, the anisotropy energy of the framework material is comparable to that of reported 2D ferromagnets. Density functional theory calculations indicate that the observed magnetocrystalline anisotropy arises from ligand-to-metal charge transfer that enhances the magnetic anisotropy of the otherwise-isotropic Fe centers, suggesting that metal–ligand covalency can be utilized as a general additive for the development of 2D magnets. These results show the possibility for (NMe4)2[Fe2L3] to retain magnetic order down to the 2D monolayer limit. In addition, the combination of large magnetic anisotropy and semiconducting character in (NMe4)2[Fe2L3] highlights its potential as a new 2D magnetic semiconductor.