The emergence of intrinsic chirality in copper and palladium chloride materials

Abstract

In most chiral organic–inorganic hybrid metal halides, chirality is extrinsically introduced by chiral organic cations because the inorganic framework is otherwise achiral. However, there are a few hybrid metal halides where chirality emerges spontaneously, i.e. in the absence of chiral molecules. Yet, the mechanisms that cause this intrinsic chirality to emerge are unclear. In this work, we report the discovery and investigation of the intrinsically chiral material pyrrolidinium palladium chloride ((C₄H₁₀N)PdCl₃), which consists of bent edge-sharing square-planar palladium chloride dimers separated by [C₄H₁₀N]⁺ cations. Its crystal structure indicates that hydrogen bonding interactions between N-bound hydrogens and Cl⁻ anions induce chirality, and this hypothesis is supported by the achirality of the all-inorganic material CsPdCl₃ as well as our density functional theory calculations. The situation is reversed for octahedrally coordinated copper(II) chlorides, where previous reports show that (C₄H₁₀N)CuCl₃ is achiral while CsCuCl₃ exhibits intrinsic chirality. This reversal is caused by the electronic degeneracy in octahedral Cu²⁺. In CsCuCl₃, cooperative Jahn–Teller distortions cause Cu²⁺ ions to move off-center within their octahedra, inducing chirality, while hydrogen bonding interactions with (C₄H₁₀N)CuCl₃ break symmetry, so the cooperative chiral distortion no longer occurs. Although the source of intrinsic chirality is usually unclear, our findings show that hydrogen bonding interactions can cause intrinsic chirality to emerge in organic–inorganic hybrid materials. They also highlight the importance of the metal's electron configuration and metal-halide coordination geometry on the emergence of intrinsic chirality, thereby providing guidance for the targeted synthesis of intrinsically chiral organic–inorganic hybrid materials.

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Introduction

Halide perovskites and derivative metal halide materials have shown great promise for applications such as ionizing radiation detection, solar energy harvesting, circularly polarized light (CPL) detection, and spin-related optoelectronics.^1–4 For CPL and spin optoelectronic applications, a material is required to be chiral so that right- and left-handed polarized light can be differentiated and/or spin-polarized charge transport can be achieved.^2,4 Nearly all metal-halide frameworks are, by themselves, achiral.^5,6 In these instances, chirality must be extrinsically induced into the metal-halide framework, which is typically achieved by incorporating an enantiomerically pure chiral organic ligand into the metal halide.^7,8 Many extrinsically chiral organic–inorganic metal halide materials have been synthesized, including the copper-based (R/S-ClMBA)₂CuCl₄, (R/S-NA)₂CuBr₄, and (R-MBA)CuBr₂; and the lead-based (R-MBA)₂PbI₄, (R/S-MBnP)PbI₃, and (R/S-3AEP)Pb₂I₆ (where ClMBA = 4-(chloromethyl)benzylammonium, NA = nipecotic acid, MBA = a-methylbenzylammonium, 3AEP = 3-(1-aminoethyl)pyridin-1-ium, MBnP = methylbenzylpyridinium).^7–13

When an organic–inorganic hybrid metal halide material is synthesized with a racemic mixture of chiral cations, the resulting structure is typically achiral because it incorporates equal numbers of left- and right-handed molecules, so there is no net handed distortion.⁷ On some occasions, spontaneous resolution occurs, resulting in the formation of a mixture of left- and right-handed crystals that respectively contain exclusively left- or right-handed organic molecules.¹⁴

In the absence of chiral molecules, it is possible for intrinsic chirality to emerge in crystalline materials when the arrangement of atoms and/or molecules within a crystal is chiral, even though the atomic, ionic, or molecular building blocks are themselves achiral.¹⁵ In a single crystallization of an intrinsically chiral material, both left- and right-handed crystals can form as there are no chiral centers that force the formation of crystals of a single handedness.^15,16 Intrinsically chiral crystals occur in covalent inorganic materials such as α- and β-quartz (SiO₂),¹⁷ in ionic inorganic materials such as NaClO₃ and the metal halide CsCuCl₃,^15,18–21 in molecular solids such as o-terphenyl and C₆₀(SnI₄)₂,^22–24 and in organic–inorganic hybrid materials such as (TMA)CuCl₃ (TMA = tetramethylammonium).^25,26 However, it is not currently possible to predictively synthesize new intrinsically chiral materials.

Several phenomena have been hypothesized to be possible sources of intrinsic chirality. One such phenomenon is the cooperative Jahn–Teller effect,^27,28 which can be observed in the all-inorganic chiral hexagonal perovskite CsCuCl₃ as well as the isostructural organic–inorganic hybrid material (TMA)CuCl₃. Typical hexagonal perovskites consist of negatively charged one-dimensional (1D) chains of face-sharing octahedra separated from one another by cations, and nearly all hexagonal perovskites that do not contain chiral organic molecules are achiral. In contrast, CsCuCl₃ is chiral, crystallizing in the enantiomorphic hexagonal space groups P6₁22 and P6₅22. Chirality in CsCuCl₃ originates from cooperative Jahn–Teller distortions caused by the electronic degeneracy in and point symmetry of octahedrally-coordinated Cu²⁺ that cause each Cu²⁺ ion to move off-center in its octahedron, resulting in the formation of a helical structure within each one-dimensional chain.^29,30 The same phenomenon occurs in (TMA)CuCl₃,²⁶ which crystallizes in the P2₁ Sohncke space group, which supports chiral structures.

For achiral organic molecules, three factors are known to influence the formation of chiral crystal structures: hindered rotation about bonds, helical packing arrangements, and head-to-head stacking in columnar arrangements.²² Quasi-planar aromatic and/or floppy achiral molecules can form chiral crystal structures as these molecular shapes allow for many possible helical packing arrangements within crystals. However, high-symmetry achiral molecules can also crystallize into chiral crystal structures as demonstrated by the chiral crystallization of icosahedral C₆₀ and tetrahedral SnI₄,²³ indicating that much remains to be discovered about what induces the formation of intrinsically chiral materials.

In this work, we report that hydrogen bonding interactions cause chirality to emerge in the organic–inorganic hybrid material pyrrolidinium palladium chloride ([C₄H₁₀N]PdCl₃), which does not contain chiral molecules. In this structure, isolated Pd₂Cl₆ units consisting of bent dimers of edge-sharing Pd–Cl square planar motifs are separated from one another by [C₄H₁₀N]⁺ cations. Its crystal structure reveals that hydrogen bonding interactions between N-bound hydrogens on the [C₄H₁₀N]⁺ cation and Cl⁻ anions induce the formation of a chiral structure, and this conclusion is supported by density functional theory (DFT) calculations. In contrast, the all-inorganic material CsPdCl₃ is achiral, likely because symmetry-breaking hydrogen bonding interactions are not possible when [C₄H₁₀N]⁺ is replaced by Cs⁺. We also investigate fundamental differences in the emergence of intrinsic chirality in palladium(II) and copper(II) chlorides. Unlike the intrinsically chiral (C₄H₁₀N)PdCl₃, (C₄H₁₀N)CuCl₃ is an achiral one-dimensional hexagonal perovskite, in contrast to intrinsically chiral CsCuCl₃ and (TMA)CuCl₃, all of which are previously reported. Hydrogen bonding between N-bound hydrogens on the [C₄H₁₀N]⁺ cations and Cl⁻ anions breaks symmetry, thereby lowering the point symmetry of Cu²⁺ such that the cooperative Jahn–Teller distortions that induce chirality in CsCuCl₃ and TMACuCl₃ no longer occur. Our findings demonstrate that hydrogen bonding interactions can be used to induce the formation of intrinsically chiral organic–inorganic hybrid materials. They also highlight how the interplay of the metal's electronic configuration, point symmetry, and coordination geometry with the achiral cation's structure affect the emergence of intrinsic chirality, providing principles that can be used for the design and synthesis of new intrinsically chiral materials.

Results and discussion

Crystals of (C₄H₁₀N)PdCl₃ are grown in solution through the slow evaporation of concentrated hydrochloric acid and characterized using single-crystal X-ray diffraction (SCXRD) measurements (see ESI†). The crystal structure of intrinsically chiral (C₄H₁₀N)PdCl₃ is shown in Fig. 1 and Fig. S1 (ESI†), consisting of bent inorganic [Pd₂Cl₆]²⁻ dimers separated by [C₄H₁₀N]⁺ organic cations (Fig. 1a). At room temperature, (C₄H₁₀N)PdCl₃ crystallizes in the orthorhombic P2₁2₁2₁ (#19) Sohncke space group, which supports chiral structures, with unit cell parameters a = 11.4392(2), b = 19.7418(4), and c = 8.0453(2) Å (Table 1). (C₄H₁₀N)PdCl₃ is intrinsically chiral, and our structure has a Flack parameter of 0.020(7), indicating that we resolved the absolute configuration (i.e., handedness) of the studied crystal.¹⁵ In a single [Pd₂Cl₆]²⁻ dimer (Fig. 1b), the Cl–Cl–Cl angles are 139.96(7)° and 139.65(7)°. Within the [Pd₂Cl₆]²⁻ dimer, the Pd–Cl bond distance is measured to be ∼2.3 Å, which is slightly shorter than the sum of the Shannon ionic radii of 2.45 Å for four-coordinate square-planar Pd²⁺ and six-coordinate Cl⁻ ions;³¹ we use the six-coordinate Cl⁻ ionic radius because Shannon's table does not include ionic radii for one- or two-coordinate Cl⁻ as found in (C₄H₁₀N)PdCl₃. The measured Pd–Cl bond distance here in (C₄H₁₀N)PdCl₃ is similar to the Pd–Cl bond distance in other four-coordinate palladium chlorides such as (NH₄)₂PdCl₄ (2.345 Å, ref. 32), CsPdCl₃ (2.234 Å, ref. 33), K₂PdCl₄ (2.318 Å, ref. 34), and chiral (R/S-C₅H₁₄N₂)PdCl₄ (2.311 Å, ref. 35). The powder X-ray diffraction (PXRD) pattern of our bulk material (Fig. S2, ESI†) matches the simulated pattern obtained from our single-crystal structure, demonstrating that our material is phase-pure. (C₄H₁₀N)PdCl₃ is air-stable for at least eight weeks as the PXRD pattern does not change (Fig. S3, ESI†).

Fig. 1 (a) Single crystal structure of (C₄H₁₀N)PdCl₃ viewed down a. H atoms omitted for clarity. (b) A single [Pd₂Cl₆]²⁻ dimer to show the bond distance and bond angle within the dimer. (c) Helical packing arrangement of the crystal structure viewed down a, with (d) hydrogen bonding interactions shown in dashed lines.

Table 1 Crystallographic collection and structural refinement parameters of (C₄H₁₀N)PdCl₃

Formula	(C4H10N)PdCl3
Formula weight (g mol^−1)	284.88
Temperature (K)	300
Wavelength (Å)	1.54178
Crystal system	Orthorhombic
Space group	P212121 (#19)
Z	8
Unit cell parameters (Å)	a = 11.4392(2)
	b = 19.7418(4)
	c = 8.0453(2)
Volume (Å^3)	1816.87(7)
Density (g cm^−3)	2.083
Absorption coefficient (μ) (mm^−1)	23.979
θmin − θmax (°)	8.958 to 148.38
Reflections collected	23 053
Independent reflections	3671 [Rint = 0.0340, Rsigma = 0.0246]
Ra indices (I > 2σ(I))	R1 = 0.0219
	wR2 = 0.0563
Goodness-of-fit on F^2	1.051
Largest diff. peak/hole (e^− Å^−3)	0.42/−0.43
Flack parameter	0.020(7)

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The unexpected emergence of intrinsic chirality in (C₄H₁₀N)PdCl₃ encouraged us to further investigate the origin of its chirality, especially given the simplicity of the [C₄H₁₀N]⁺ organic cation. Based on our examination of the Cambridge Structural Database (CSD),³⁶ all organic–inorganic hybrid materials containing [Pd₂Cl₆]²⁻ moieties are achiral (excluding structures that contain chiral cations) including (C₈H₂₀N)₂[Pd₂Cl₆],³⁷ (C₁₆H₂₀N)₂[Pd₂Cl₆],³⁸ (C₁₉H₂₅N₂)₂[Pd₂Cl₆],³⁹ (C₃₀H₂₄N₆Ru)₂[Pd₂Cl₆]₂·H₂O,⁴⁰ (C₂₁H₂₈P)₂[Pd₂Cl₆],⁴¹ (C₃₅H₂₅ClN₅Pd)₂[Pd₂Cl₆],⁴² (H₃O)₂(C₅₀H₆₀N₂₀O₁₀)[Pd₂Cl₆]·3H₂O,⁴³ (C₄₅H₄₂ClOP₂PtS)₂[Pd₂Cl₆]·4(CHCl₃).⁴⁴ In addition, the all-inorganic material CsPdCl₃,⁴⁵ which also contains [Pd₂Cl₆]²⁻ dimers, is also achiral. A closer examination of the crystal structure of (C₄H₁₀N)PdCl₃ down the a axis (Fig. 1c) shows that the [Pd₂Cl₆]²⁻ dimers exhibit a helical packing arrangement. Hydrogen bonding interactions between the N bound hydrogens (which have refined coordinates in our structure) and Cl⁻ anions in [Pd₂Cl₆]²⁻ dimers (Fig. 1d) appear to be the driving force that induces the helical packing arrangement in (C₄H₁₀N)PdCl₃. In addition, hydrogen bonding interactions induce bending in [Pd₂Cl₆]²⁻ dimers; most structures in the CSD contain planar [Pd₂Cl₆]²⁻ dimers, and the structures containing bent [Pd₂Cl₆]²⁻ dimers exhibit strong hydrogen bonding interactions. Notably, both CsPdCl₃ and (C₈H₂₀N)₂[Pd₂Cl₆] ([C₈H₂₀N]⁺ = tetraethylammonium) are achiral, contain planar [Pd₂Cl₆]²⁻ dimers, and do not exhibit significant hydrogen bonding interactions.^37,45

To test our hypothesis that hydrogen bonding interactions between N-bound H atoms and nearby Cl atoms are the source of chirality in (C₄H₁₀N)PdCl₃, we use DFT calculations on our experimental structure as well as on model structures. Overall, we find that the DFT calculations agree well with the experimental results; for example, DFT accurately reproduces the lattice constants and the presence of chirality in (C₄H₁₀N)PdCl₃. We first perform DFT calculations on [Pd₂Cl₆]²⁻ dimers alone without any cations present to understand the role of the cation in determining whether the [Pd₂Cl₆]²⁻ dimers are planar or bent. We start by removing the [C₄H₁₀N]⁺ organic ligand from the obtained experimental crystal structure and add two electrons and an equivalent positive uniform background charge to compensate for the removal of the cation (Fig. 2a). The [Pd₂Cl₆]²⁻ dimers remain bent after relaxing the structure. Similarly, starting with a nearly planar [Pd₂Cl₆]²⁻ dimer configuration, [Pd₂Cl₆]²⁻ remains nearly planar with a bending angle of 174.02° (Fig. 2b). The two configurations are very similar in energy, with the bent configuration just 3 meV more stable. Because the energy difference between the two structures is so small, it is likely that interactions between the cation and [Pd₂Cl₆]²⁻ dimers are responsible for determining whether [Pd₂Cl₆]²⁻ dimers are planar or bent. We next introduce the [C₄H₁₀N]⁺ cation into a model structure with planar [Pd₂Cl₆]²⁻ dimers and find that the dimers spontaneously relax to the bent configuration during DFT optimization (Fig. 3). Thus, the [C₄H₁₀N]⁺ cation appears to cause the observed bending for the [Pd₂Cl₆]²⁻ dimers. To further test the role of hydrogen bonding, we next perform additional calculations to attempt to reduce or modify the hydrogen bonding interactions by rotating the organic cation in the presence of both bent and planar [Pd₂Cl₆]²⁻ dimers (Fig. S5, ESI†). These results confirm that hydrogen bonding interactions between N-bound H atoms and Cl⁻ cations are energetically favorable and support the conclusion that hydrogen bonding interactions are responsible for the arrangement of the organic [C₄H₁₀N]⁺ cation and bent [Pd₂Cl₆]²⁻ dimer.

Fig. 2 DFT optimization results for (a) bent [Pd₂Cl₆]²⁻ dimers and (b) linear [Pd₂Cl₆]²⁻ dimers with no organic cation and extra electrons added to maintain the charge.

Fig. 3 DFT geometry optimization of planar [Pd₂Cl₆]²⁻ dimers in the presence of [C₄H₁₀N]⁺ cations. H atoms omitted for clarity.

Lastly, we replace Cs⁺ with [C₄H₁₀N]⁺ in achiral CsPdCl₃⁴⁶ and relax the structure to see if a chiral structure forms (Fig. 4a and b). We note that the model input structure we created (Fig. 4b) is achiral with 100% certainty according to the NEWSYM test in PLATON.⁴⁷ However, the DFT-optimized structure (Fig. 4c) contains bent [Pd₂Cl₆]²⁻ dimers and is chiral with 100% probability according to the NEWSYM test. The emergence of chirality once Cs⁺ is replaced with [C₄H₁₀N]⁺ supports our hypothesis that hydrogen bonding interactions are responsible for inducing intrinsic chirality in (C₄H₁₀N)PdCl₃, and the presence of bent [Pd₂Cl₆]²⁻ dimers is again attributed to hydrogen bonding interactions between the organic [C₄H₁₀N]⁺ cation and Cl⁻ anions in the inorganic metal-halide [Pd₂Cl₆]²⁻ framework.

Fig. 4 (a) Crystal structure of achiral CsPdCl₃. (b) Crystal structure after replacing Cs⁺ with [C₄H₁₀N]⁺ in CsPdCl₃. (c) DFT optimized chiral structure after replacing Cs⁺ with [C₄H₁₀N]⁺ in CsPdCl₃. H atoms omitted for clarity in (b) and (c).

In agreement with previous work, we find that the presence of [C₄H₁₀N]⁺ does not guarantee the emergence of a chiral structure for metal cations other than Pd²⁺. In addition to chiral (C₄H₁₀N)PdCl₃, we also synthesized the known material (C₄H₁₀N)CuCl₃.⁴⁸ (C₄H₁₀N)CuCl₃ is an achiral hexagonal perovskite that consists of 1D chains of face-sharing Cu–Cl octahedra, separated from one another by [C₄H₁₀N]⁺ cations (Fig. 5a, b and Fig. S6, ESI†). At room temperature, (C₄H₁₀N)CuCl₃ crystallizes in the centrosymmetric monoclinic space group P2₁/n (#14) with unit cell parameters a = 9.3503(12) Å, b = 6.5136(8) Å, c = 14.1378(18) Å, and β = 102.931(3)° (see Table S1 for detailed refinement parameters, ESI†). In contrast, CsCuCl₃ (Fig. 5c and d) and (TMA)CuCl₃^26,49 form intrinsically chiral hexagonal perovskites because cooperative Jahn–Teller distortions cause the Cu²⁺ cations to move off-center within their octahedra (Fig. 5d), forming a helical structure in each 1D chain.³⁰ In (C₄H₁₀N)CuCl₃, hydrogen bonding interactions between N-bound hydrogens on the [C₄H₁₀N]⁺ cation and Cl⁻ anions change the point symmetry of the Cu²⁺ cation by causing some Cl⁻ anions to become symmetry inequivalent, so the cooperative helical Jahn–Teller distortions that cause CsCuCl₃ and (TMA)CuCl₃ to be intrinsically chiral no longer exist. Therefore, hydrogen bonding interactions serve to make intrinsically chiral Cu²⁺ hexagonal perovskites achiral, demonstrating the interplay between the metal's electron configuration, its coordination geometry, and hydrogen bonding interactions on the formation of intrinsically chiral materials. When cooperative Jahn–Teller distortions are not present, we hypothesize that hydrogen bonding interactions are likely to induce chirality; additional work is required to confirm this hypothesis. In materials where cooperative Jahn–Teller distortions are present, hydrogen bonding interactions will change the local symmetry and the nature of the cooperative distortions, causing intrinsically chiral materials to become achiral.

Fig. 5 (a) Crystal structure of achiral (C₄H₁₀N)CuCl₃, with hydrogen bonds shown as dashed lines, and (b) alternate view of its 1D Cu–Cl chain. (c) Crystal structure of chiral CsCuCl₃ and (d) alternate view of its 1D Cu–Cl chain.

Conclusions

In summary, we report the intrinsically chiral metal halide (C₄H₁₀N)PdCl₃ that contains no chiral molecules. Structural analysis and DFT calculations show that chirality emerges from a helical packing arrangement induced by hydrogen bonding interactions between the organic and inorganic components. In contrast, other palladium(II) chlorides such as CsPdCl₃ and (C₈H₂₀N)PdCl₃ that do not contain chiral molecules are achiral because they do not exhibit hydrogen bonding interactions that induce a similar helical packing arrangement. Copper(II) chlorides behave quite differently. CsCuCl₃ and (TMA)CuCl₃ are chiral because cooperative Jahn–Teller distortions cause Cu²⁺ to move off-center within its octahedron and form a helical structure in each 1D chain, whereas hydrogen bonding interactions in (C₄H₁₀N)CuCl₃ cause it to be achiral as these interactions stop the chirality-inducing cooperative Jahn–Teller distortions from occurring. Our findings demonstrate that organic–inorganic hydrogen-bonding interactions can induce chirality in achiral metal halide materials and reveal the interplay between hydrogen bonding interactions with the metal's electronic configuration, coordination geometry, and point symmetry in determining whether a material will be intrinsically chiral.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article, including experimental details and supplementary figures, have been included as part of the ESI.† Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre under deposition numbers 2434800 and 2434814.

Acknowledgements

Primary support is provided by the Louisiana Board of Regents under contract LEQSF(2024-27)-RD-A-28. Computational work is primarily supported by the National Science Foundation through grant CBET-2340356 and uses high performance computing resources and services provided by Technology Services at Tulane University. The authors acknowledge additional financial support from Tulane University and thank Dr Xiaodong Zhang for assistance with single crystal X-ray diffraction measurements.

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Footnote

† Electronic supplementary information (ESI) available: Experimental methods; additional figures and tables; crystal structures. CCDC 2434800 and 2434814. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5ma00682a

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