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.

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.

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.

The strength and sign of superexchange interactions are often predicted on the basis of the bond angles between magnetic ions, but complications may arise in situations with a nontrivial arrangement of the magnetic orbitals. We report on a novel molecular tetramer compound [Cu(H 2 O)dmbpy] 2 [V 2 O 2 F 8 ] (dmbpy = 4,4′-dimethyl-2,2′bipyridyl) that is composed of triangular "CuV 2 " fragments and displays a spin gap behavior. By combining first-principles calculations and electronic models, we reveal that superexchange Cu−V interactions carry drastically different coupling strengths along two Cu−F−V pathways with comparable bond angles in the triangular "CuV 2 " fragment. Counterintuitively, their strong disparity is found to originate from the restricted symmetry of the half-filled Cu d x 2 −y 2 orbital stabilized by the crystal field, leading to one dominating antiferromagnetic Cu−V coupling in each fragment. We revisit the magnetic properties of the reported spin-gapped chain compound [enH 2 ]Cu-(H 2 O) 2 [V 2 O 2 F 8 ] (enH 2 = ethylene diammonium) containing similar triangular "CuV 2 " fragments, and the magnetic behavior of the molecular tetramer and the chain compounds is rationalized as that of weakly coupled spin dimers and spin trimers, respectively. This work demonstrates that fundamentally different magnetic couplings can be observed between magnetic ions with similar bond angles in a single spin motif, thus providing a strategy to introduce various exchange interactions combined with low dimensionality in heterometallic Cu(II)−V(IV) compounds.

The syntheses and crystal structures of two bimetallic molecular compounds, namely, bis[bis(6,6′-dimethyl-2,2′-bipyridine)copper(I)] hexafluoridozirconate(IV) 1.134-hydrate, [Cu(dmbpy)2]2[ZrF6]·1.134H2O (dmbpy = 6,6′-dimethyl-2,2′-bipyridyl, C12H12N2), (I), and bis[bis(6,6′-dimethyl-2,2′-bipyridine)copper(I)] hexafluoridohafnate(IV) 0.671-hydrate, [Cu(dmbpy)2]2[HfF6]·0.671H2O, (II), are reported. Apart from a slight site occupany difference for the water molecule of crystallization, compounds (I) and (II) are isostructural, featuring isolated tetrahedral cations of copper(I) ions coordinated by two dmbpy ligands and centrosymmetric, octahedral anions of fluorinated early transition metals. The tetrahedral environments of the copper complexes are distorted owing to the steric effects of the dmbpy ligands. The extended structures are built up through Coulombic interactions between cations and anions and π–π stacking interactions between heterochiral Δ- and Λ-[Cu(dmbpy)2]+ complexes. A comparison between the title compounds and other [Cu(dmbpy)2]+ compounds with monovalent and bivalent anions reveals a significant influence of the cation-to-anion ratio on the resulting crystal packing architectures, providing insights for future crystal design of distorted tetrahedral copper compounds.

A bimetallic hydroxychalcogenide, BaZn 2 Se 2 (OH) 2 , was synthesized through hydrothermal pouch methods. The single crystal X-ray diffraction and electron diffraction indicates that the phase crystallizes in the orthorhombic space group Pnma and is composed of anionic layers [ZnSe 3/3 (OH) 1/1 ] À that are separated and charged balanced by Ba 2 + cations. The [ZnSe 3/3 (OH) 1/1 ]layer comprises two unique Zn sites, which form interpenetrating zigzag chains with an in-plane dipole moment and adopts a brownmillerite-type structural motif. The adjacent layers contain tetrahedrally coordinated Zn chains of opposite handedness related by an inversion center, which cancel the microscopic dipoles to minimize the macroscopic electric polarization. The adoption of a brownmillerite structural motif in BaZn 2 Se 2 (OH) 2 can be rationalized by the distinct charge difference between Se 2À and OH À anions, which creates a sufficient dipole moment in the ZnSe 3 (OH) tetrahedra to allow the occurrence of twisted chains. FTIR spectroscopy confirms the existence of OH À anions and DFT calculations indicate that BaZn 2 Se 2 (OH) 2 is a semiconductor with a direct band gap. This work expands the chemistry of the brownmillerite family from traditional homoanionic oxides to multianion hydroxychalcogenides, offering a new opportunity to explore tunable structural complexity for better design of functional materials.