Achiral Noncentrosymmetric Racemates

Nisbet, Matthew Lander

doi:https://doi.org/10.21985/n2-r3tg-sp26

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Achiral Noncentrosymmetric Racemates

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Chirality and polarity describe orthogonal mechanisms of inversion symmetry breaking, which is the origin of valuable properties in crystalline materials including nonlinear optical activity, ferroelectricity, and piezoelectricity. Noncentrosymmetric (NCS) materials have numerous applications yet opportunities remain for cooperative coupling between chiral and polar basic building units to realize high-performance materials. Enantiomerically pure samples of chiral molecules have been exploited as structure-directing agents based on the fact that a single enantiomer must crystallize without inversion symmetry. However, this strategy does not control the bulk polarity, which is required for ferroelectricity and associated with superior nonlinear optical properties, meaning that additional studies are needed tooptimize interactions between chiral and polar structural moieties. Racemic compounds, which contain both enantiomers of a chiral molecule, offer an underappreciated opportunity for achieving noncentrosymmetry in crystalline solids. In this work, we describe our efforts to arrange polar building units, namely d0 early transition metal fluorides and oxide-fluorides, and racemic combinations of chiral ∆- and Λ-Cu(bpy)2(H2O)2+ building units in achiral NCS structures via hydrothermal synthesis. Our investigation found that hydrogen bonding and nonparallel π − π stacking dictate inversion symmetry breaking in the [Cu(bpy)2(H2O)][MF6]·1.5H2O (M = Ti, Zr, Hf; bpy = 2,2’-bpiyridine; space group: Pna21) structural family. The first step of this study was finding the appropriate set of conditions to synthesize the Ti- and Zr- analogues of the compound [Cu(bpy)2(H2O)][HfF6]·1.5H2O. Our investigation of the relevant composition space revealed a phase competition between the NCS compounds and centrosymmetric compounds with the general formula [Cu(bpy)(H2O)2(MF6)]n (M = Ti, Zr, Hf) in each system that is strongly dependent on the identity of the early transition metal ion. Machine learning modeling was applied to generate an interpretable decision tree model which indicates that phase selection is driven primarily by the bpy:Cu molar ratio for reactions containing Zr or Hf and captures the additional requirement that the amount of HF present be decreased to raise the pH for reactions containing Ti. Ligand K-edge X-ray absorption spectroscopy allowed for the observation of strong ligand-to-metal π bonding that is unique to the TiF62− anion among the M = Ti, Zr, Hf series. Next, a series of compounds with the general formula [Cu(phen)2(H2O)][MF6 ]·xH2O (M = Ti, Zr, Hf ; phen = 1,10-phenanthroline) was synthesized to probe the role of π−π stacking interactions in the [Cu(bpy)2(H2O)][HfF6]·1.5H2O (M = Ti, Zr, Hf) system by comparing the intermolecular interactions of Cu(phen)2(H2O)2+ and Cu(bpy)2(H2O)2+ complexes. This study demonstrated that local inversion symmetry breaking by non-parallel heterochiral π−π stacking is a necessary but insufficient condition for inversion symmetry breaking. Finally, two additional centrosymmetric compounds, [Cu(bpy)2(H2O)][SiF6]·4H2O and [Cu(bpy)(H2O)2(SnF6)]n, were synthesized to compare the main group anions SiF62− and SnF62− to their group IV early transition metal counterparts. The SiF6 2− anion was found to direct the ∆- and Λ-Cu(bpy)2(H2O)2+ complexes to adopt distinct hydrogen bonding networks and homochiral π − π stacking owing to the increased amount of hydrating water relative to the polar [Cu(bpy)2(H2O)][MF6]·1.5H2O (M = Ti, Zr, Hf) phase. In contrast, [Cu(bpy)(H2O)2(SnF6)]n is isostructural to [Cu(bpy)H2O)2(HfF6)]n.

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