A competition between 2D and 3D magnetic orderings in novel mixed valent copper frameworks

Abstract

Low-dimensional hybrid inorganic–organic frameworks exhibit high structural flexibility and allow for the inclusion of various magnetic and optically-active species into their host structures. The emergence of copper-based hybrid structures for various optical applications provides a promising foundation for exploring the integration of magnetic sublattices, paving the way for advancements in magneto-optical coupling and multifunctional materials. Herein, we introduce a novel class of hybrid copper frameworks with covalently-connected alternating magnetic 2D copper(II) formate and non-magnetic copper(I) bromide layers. The anionic framework is stabilized by A⁺ cations to form ACu₅Br₄(COOH)₄ (A⁺ = Na⁺, K⁺, Rb⁺, NH₄⁺) semiconductors (bandgaps 2.1–2.2 eV) with optical transitions suitable for optoelectronic applications. Comprehensive magnetometry studies show that ACu₅Br₄(COOH)₄ compounds exhibit low-dimensional 2D short-range antiferromagnetic order within the formate layers, characterized by strong exchange coupling (J/k_B ∼ −100 K). Upon further temperature reduction, interactions between Cu(II) layers give rise to 3D long-range magnetic order at ∼40 K, despite the large (8.6–8.8 Å) spatial separation of the magnetic Cu(II) formate layers by nonmagnetic Cu(I)–Br bridging layers. This transition is further supported by electron paramagnetic resonance (EPR) spectroscopy. This study expands our understanding of low-dimensional hybrid frameworks and opens new avenues for the design of 2D multifunctional materials.

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1 Introduction

The study of low-dimensional inorganic magnetic materials has been an exciting area of research for physicists and chemists. These materials exhibit a wide range of phenomena driven by their electronic configuration and arrangement of magnetic ions.^1,2 While inorganic magnets hold great potential for various applications, most reported phases consist of all-inorganic components with limited chemical tunability and processability.^3–6 Additionally, their inherently dense structures result in high material weight, restricting their suitability for applications that require lightweight and portable designs, such as wearable electronics or compact energy-efficient devices.⁴ As a solution, researchers have been focusing on exploring hybrid inorganic–organic frameworks that can produce single molecules, 1D chains, 2D layered materials, and 3D networks with unique magnetic and optical properties.^2,7–14 These hybrid metal–organic materials have exhibited fascinating multifunctional properties, including long-range magnetic order, ferroelectricity, photo-absorption, and electronic conductivity, which are valuable in spintronics, multiferroics, and optical and quantum applications and devices.^15–22 Recently, there has been a lot of interest in the magnetic properties of materials with a layered copper(II) sublattice, mainly due to its relationship with quantum fluctuation and magnetic anisotropy.^23–25 The unique spin quantum number (S = ½) and Jahn–Teller distortion of Cu(II) ions leads to intriguing behaviors that have potential implications for developing new materials.^26–29 Moreover, these materials can yield geometrically frustrated systems, giving rise to exciting quantum phenomena at low temperatures.^28,30 Copper-based metal–organic materials offer an opportunity to implement strong magnetic coupling originating from magnetic exchange interactions between Cu²⁺ spin centers and organic linkers.

The length of linkers used in constructing magnetic coordination polymers affects the strength of magnetic coupling. Short ligands such as cyanide, azide, and formate are effective in promoting strong magnetic exchange coupling between metal centers, unlike their long-chain counterparts which can reduce coupling between moment carriers in the materials.^{23,29,31–34} As the smallest carboxylate, the formate ion acts as either a single-atom connector (similar to the azide and cyanide) or a three-atom linker – making it an excellent choice for facilitating strong exchange coupling between metal centers.^{7,8,33,35–40} The multiple bridging modes of the formate ion can facilitate ferro- (FM) or antiferromagnetic (AFM) coupling between metal centers, depending on the type of bridging and the identity of the metal atoms.^29,35 The coupling is due to the delocalization of electrons in the formate anion, which enables the transfer of π-electrons to neighboring metal ions when excited.²⁹ Apart from Ca- and Sr- copper formate salts, there are limited options for first-row transition metal formates without co-ligands.^7,8,41–44 The well-known families of homoleptic Cu-formates includes ferromagnetic 3D framework α-Cu(COOH)₂ and chain-like β-Cu(COOH)₂.⁴⁵ Some homoleptic Mn, Ni, Mg, Fe, and Co-formates, with the general formula M₃(COOH)₆ have recently been reported with varying magnetic properties.^10,46 While long-range antiferromagnetic ordering has been observed in some of these Fe, Co, and Ni-based formates their magnetic ordering is confined to the intralayer metal ions with weak magnetic features due to weak interlayer interactions in these compounds.^47,48 As a result, the search for materials with both intra (2D) and interlayer (3D) exchange interactions, resulting in intriguing magnetic properties, is in high demand.

Herein, we report a novel series of copper formate compounds that interconnect 2D copper(II) formate and copper(I) bromide layers to generate a 3D anionic framework. The anionic framework is stabilized by the intercalation of A⁺ cations (A⁺ = Na⁺, K⁺, Rb⁺, NH₄⁺) between Cu-formate/Cu–Br/Cu-formate trilayers. We detail the synthesis and structural characterization of ACu₅Br₄(COOH)₄ using various techniques including single-crystal and synchrotron powder X-ray diffraction, thermogravimetric analysis, infrared spectroscopy, and scanning electron microscopy. We also explore the optical and magnetic properties of the synthesized compounds via magnetometry, electron paramagnetic resonance spectroscopy (EPR), and linear optical property measurements. The outcomes of this study provide insight into the relationship between the structure and properties of these exciting copper formate/bromide materials and how the connectivity between alternating copper formate layers affects the magnetic and optical properties of the resulting compounds.

2 Experimental section

2.1 Safety warning

Solvothermal vessels may develop high autogenic pressures, which can result in the release of hot pressurized hazardous formic acid vapors during the reaction. Splashing of the ethanol/formic acid may occur upon opening of the reaction vessels, causing severe burns. It is highly recommended to wear proper personal protective equipment, such as face shields, long-sleeve gloves, and tight-cuff lab coats; to place the reaction vessels in secondary containment and allow them to cool to room temperature before opening; and to keep furnaces in well-ventilated spaces such as fume hoods.

2.2 Starting materials

Copper(II) bromide (Alfa Aesar, 99%), ammonium carbonate (Sigma-Aldrich, ≥30% NH₃ basis), sodium carbonate (Alfa Aesar, 99+%), potassium carbonate (Sigma-Aldrich, 99%), rubidium carbonate (Alfa Aesar, 99%), formic acid (Acros Organics, 99%), and 200-proof ethanol were used as received without further purification.

2.3 Synthesis of ACu₅Br₄(COOH)₄ (A⁺ = Na⁺, K⁺, Rb⁺, NH₄⁺)

To synthesize ACu₅Br₄(COOH)₄, a solvothermal method was utilized. 25 mL pyrex media bottles (Fig. S1†) were loaded with 2 mmol of CuBr₂ and 0.5 mmol of alkali metal/ammonium carbonate (A₂CO₃). The vessels were then taken into a well-ventilated fume hood, where 5 mL of formic acid (HCOOH) and 5 mL of ethanol (C₂H₅OH) were added to the vessels. Right after, the vessels were tightly sealed with a GL25 PTFE-lined silicone septum cap to prevent over-pressurization during the reaction (Fig. S1†). An instant release of CO₂ gas occurs due to the reaction between carbonates and formic acid. Therefore, it is essential to perform the solvent process in a well-ventilated fume hood to avoid direct exposure to CO₂ gas. The reactant vessels were then placed in a solvothermal furnace, rapidly heated up to 70 °C, and then slowly cooled to 50 °C at a rate of 1 °C h⁻¹, and further cooled to room temperature by turning off the furnace. The resulting products were filtered and washed with 200-proof ethanol to obtain dark yellow plates of varying sizes (ranging from 0.5 × 0.5 mm² to 3 × 3 mm²). The syntheses of lithium (Li) and cesium (Cs) analogs using the appropriate carbonate precursors were unsuccessful. All processes were performed under ambient conditions.

2.4 Characterization

The characterizations of solid samples, both single crystals and polycrystalline powders, were conducted using in-house and high-resolution synchrotron powder X-ray diffraction (PXRD), single-crystal X-ray diffraction (SCXRD), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS), solid-state diffuse reflectance spectroscopy, Fourier transform infrared (FTIR) spectroscopy, as well as thermogravimetric analysis, and differential scanning calorimetry (TGA-DSC). SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy were employed to investigate the magnetic properties of samples, while photoluminescence (PL) measurements elucidated optical behavior. Detailed descriptions of the experimental methodologies are available in the ESI.†

3 Results and discussion

3.1 Synthesis

Hybrid metal–organic frameworks are attractive for their intriguing physical properties, as well as their accessibility via low-temperature synthesis using earth-abundant reactants. By treating CuBr₂ in formic acid and ethanol with the addition of monovalent A⁺ carbonate salt (A⁺ = Na⁺, K⁺, NH₄⁺, Rb⁺) and slowly cooling the reaction mixture from 70 °C to 50 °C, large crystals of ACu₅Br₄(COOH)₄ were produced. Selected crystals were ground for in-house and synchrotron-based powder diffraction studies to confirm phase purity (Fig. 1a and S3†). The morphology and elemental composition of the products were studied using a SEM equipped with an EDS detector (Fig. 1b). Because EDS cannot reliably quantify low atomic weight elements such as C, O, N, and H, CHN/O organic component analysis was conducted to confirm the light element content (Table S1†).

Fig. 1 (a) Rietveld refinement of room-temperature synchrotron PXRD data (λ = 0.412642 Å) for KCu₅Br₄(COOH)₄. Experimental data: black open circles. Calculated pattern: red line. Difference profile: blue. Vertical maroon ticks indicate the positions for KCu₅Br₄(COOH)₄ reflections (space group: C2/m, no. 12). (b) SEM backscattered electron images of selected crystals of ACu₅Br₄(COOH)₄ with respective compositions. The heavy elements (Cu, Rb/K/Na, Br) were determined from EDS and normalized to 4 Br atoms, while lighter elements (C, H, N, O) are determined from CHN/O analysis and normalized to the total molecular weight of the respective compounds.

The size of the crystallites can be controlled by adjusting the cooling rate of the reaction. For instance, reactions at a cooling rate of 2 °C h⁻¹ produced smaller crystallites, while a cooling rate of 0.5 °C h⁻¹ formed larger crystallites. The resulting compounds contained Cu²⁺ and Cu¹⁺ cations, as determined from single-crystal diffraction analysis (discussed later), indicating a partial reduction of CuBr₂, likely facilitated by formic acid and/or alcohol. However, the exact reduction mechanism remains unknown. Reactions in the absence of alcohol (ethanol) do not result in the formation of the target phase, instead producing light blue crystallites of copper formate. The use of water as a substitute for alcohol yielded a dark-brown solution without any observable solid precipitate. The presence of absorbed moisture from the reaction atmosphere and the addition of a sacrificial amount of water (∼100 μL) did not impact the integrity of target compounds or the size of crystallites (Fig. S2a†), in sharp contrast with previously reported syntheses of formate-based compounds.^49,50 Switching from ethanol to 1-butanol did not significantly impact the formation of the target compound (Fig. S2b†). Therefore, the target hybrid compounds can be synthesized under ambient conditions as the presence of absorbed moisture, water of hydration from metal salts, and the addition of sacrificial water do not impede formation of the target phase, nor does the type of primary alcohol used.

3.2 Crystal structure description

Crystal structures of ACu₅Br₄(COOH)₄ compounds were solved from single crystal X-ray diffraction (SCXRD) data. The validity of the resulting structural models was confirmed by refining high-resolution powder X-ray diffraction data (PXRD) (Fig. 1a and S3†). All bond distances discussed herein correspond to those observed in the refined single crystal data. All ACu₅Br₄(COOH)₄, except A⁺ = Na⁺, crystallize in the monoclinic centrosymmetric space group C2/m (Fig. 2). In the layered crystal structure, copper(II) formate layers are linked to Cu(I) bromide layers via Cu(II)–Br–Cu(I) covalent bridges (Fig. 2c). Cu(II)formate–Cu(I)Br–Cu(II)formate slabs are separated by layers of monovalent A⁺ cations. The asymmetric unit has four distinct Cu sites: three Cu⁺ and one Cu²⁺ site. The oxidation state assignments of Cu⁺ and Cu²⁺ were determined based on bond valence sum (BVS) calculations derived from refined single-crystal data (Table S6†). The Cu⁺ atoms occupy the Wyckoff sites 4h, 4h, and 8j, with 75% occupancy each, resulting in three Cu⁺ ions per formula unit (Z = 4). These Cu⁺ ions are coordinated by four bromine atoms in a distorted tetrahedral environment (Fig. 2a) with Cu–Br distances ranging from 2.47 to 2.51 Å and Br–Cu–Br angles ranging from 104.38(1) to 115.47(1)°. Cu2/Cu4 (4h sites) double tetrahedra share edges to form 1D chains propagating along the [010] direction. These chains further share edges with chains of Cu3 (8j site) tetrahedra, resulting in infinite 2D [(0.75Cu)₄Br₄]¹⁻ layers propagating within the ab-plane (Fig. 2e).

Fig. 2 The structure of ACu₅Br₄(COOH)₄ featuring (a) [Cu(I)Br₄] tetrahedra and (b) square pyramidal [Cu(II)BrO₄] building units. (c) General view of the layered structure emphasizing the connectivity between the Cu-based layers. The unit cell is outlined in red. Two different Cu(II)–Cu(II) interlayer distances are labelled as D₁ (Cu(II)–Cu(II) distances across bridging Cu(I)Br layers) and D₂ (Cu(II)–Cu(II) distance across A⁺ cationic layer). (d) Rectangular antiprismatic geometry around A⁺ ions coordinated by four formate ions. (e) Striped arrangement of Cu(I) sites in the 2D [Cu₃Br₄]¹⁻ layer. (f) Connectivity of [CuBrO₄] units and formate ions in the Cu(II)-formate layer within the ab-plane, emphasizing interlayer voids filled by A⁺ cations. A⁺: purple, Cu(I): blue, Cu(II): yellow, C: cyan, O: black, H: grey, Br: red.

Based on bond valence sum calculations, the Cu atoms in the formate layer are in the +2 oxidation state. This single copper(II) site (Cu1, 8j) constitutes two Cu(II) ions per formula unit, resulting in an electron-precise semiconducting composition. The copper(II) ions exhibit square pyramidal geometry [CuO₄Br] (Fig. 2b and S9a†), with a base formed by four oxygen atoms from four different bridging formate ions in a μ-HCOO¹⁻ anti–anti arrangement and an apical Cu–Br bond. This bridging mode is typical for the formate ion in an environment without bulky ligands.^29,33

Formate layers are formed via corner-sharing, interconnecting linear chains of Cu₂(COOH)₄ propagating within the ab-plane through Cu(II)–O–C–O–Cu(II) bridges (Fig. 2f). Due to the anti–anti bridging mode of the formate anions (Fig. S9a†), alternating 2D copper formate layers are slightly tilted from one another in the 3D anionic framework. This results in the non-coplanar stacking of alternating copper formate layers (Fig. S9b†). This geometry facilitates the apical Cu(II)–Br bridging, which is crucial in the formation of the 3D anionic framework.

In all the target compounds, the Cu²⁺–O bond distances vary in the small range of 1.9514(1) to 1.9541(1) Å, and the O–Cu²⁺–O bond angles are almost 90°. These distances are consistent with those observed for Cu²⁺ oxides and formates.^51,52 The square planar Cu-formate layers are bridged via corner-sharing with the [Cu₃Br₄]¹⁻ anionic layer through Cu(II)–Br–Cu(I) bonds to form [Cu⁽²⁺⁾BrO₄] units with square pyramidal geometry, as shown in Fig. 2b and S9a.† The apical bridging Cu(II)–Br bond distance is 2.7001(2) Å, which is significantly longer than distances for tetrahedral Cu(II) in CuBr₂, 2.41 Å, but within the range of distances reported for octahedral Cu(II) in CsCuBr₃, 2.46–2.96 Å.⁵³ In Cu₃Mg(OH)₆Br₂ with heteroanionic [Cu(II)O₂Br₄] octahedral fragments, Cu–O distances are 1.972 Å and Cu–Br distances are 2.865 Å.⁵⁴

Cu²⁺ exhibits square pyramidal geometry featuring four short equatorial Cu–O bonds (1.95 Å) and one long apical Cu–Br bond (2.70 Å). This Jahn–Teller elongation is well known among various Cu²⁺-based compounds.^2,52,55,56 In the absence of the extended apical Cu–Br bonds, the structure would consist of 2D frameworks of Cu-formate and Cu–Br, which are upheld by van der Waals interactions. The elongated apical Cu–Br bond connects these two 2D layers, resulting in the 3D anionic [Cu₅Br₄(COOH)₄]⁻ framework.

The 3D anionic [Cu₅Br₄(COOH)₄]⁻ slabs are separated by A⁺ cationic layers. A⁺ cations adopt a rectangular antiprismatic coordination (Fig. 2d) by oxygen atoms from four formate anions from alternating Cu₂(COOH)₄ layers. The resulting [A(COOH)₄]³⁻ units connect to the [CuBrO₄] polyhedra, thus linking the anionic slabs together to yield the ACu₅Br₄(COOH)₄ structure (Fig. S8†). The A⁺ polyhedral volume and Cu²⁺⋯Cu²⁺ interatomic distance between alternating slabs are determined by the size of A⁺ ions. Attempts to synthesize the Cs and Li analogs were unsuccessful, either due to drastically different synthetic conditions required for Cs and Li, or due to the destabilization of the 3D framework caused by the extreme ionic radii, Cs⁺ being too large and Li⁺ too small. Additionally, the failure to synthesize these phases may be attributed to the inability of Cs⁺ and Li⁺ to adopt the required rectangular antiprismatic coordination, which is essential for the stabilization of the structure. The interlayer Cu²⁺⋯Cu²⁺ distances, D₂, for Na⁺, K⁺, NH₄⁺, and Rb⁺ analogues are 3.4982(9), 3.8379(3), 4.0871(8), and 4.1547(9) Å, respectively, with A⁺ polyhedral volumes ranging between 25.13–34.96 ų. The extrapolated Cu²⁺⋯Cu²⁺ distances expected for hypothetical Li⁺ and Cs⁺ analogs are 3.04 and 4.53 Å, respectively. Further details regarding the crystal structure refinement parameters may be obtained from Table S2† and the Cambridge Crystallographic Data Centre by quoting the depository numbers CCDC 2412816–2412819.

Attempts to solve NaCu₅Br₄(COOH)₄ within the monoclinic C2/m space group, similar to the other ACu₅Br₄(COOH)₄, were unsuccessful. Despite the high quality of synthesized crystals, refinements in C2/m yielded high R₁ values (10.4%) and revealed significant residual electron density around Cu(I) atomic sites in the structure. Based on these observations, we hypothesized that the Na-analogue may exhibit different ordering within the [Cu₃Br₄]⁻ layers due to the smaller size of Na⁺ ions compared to larger A⁺ ions. Re-evaluation of the single crystal diffraction data showed the presence of weak superstructural reflections suggesting the P2₁/c space group (no. 14) for NaCu₅Br₄(COOH)₄. The structure was successfully solved in P2₁/c, resulting in a significantly lower R₁ value of 3.0%. The crystal structure of NaCu₅Br₄(COOH)₄ is analogous to the other three compounds based on their structural motifs, connectivity, and bond distances and angles (Tables S3 and S4†), but there are some differences due to the change in symmetry.

In the C-centered structures of the K-, NH₄-, and Rb-analogs, there are one Cu(II) (8j) and three Cu(I) (8j, 4h, and 4h) sites. In turn, the asymmetric unit of the primitive P2₁/c structure contains two square pyramidal Cu(II) sites (4e) and four tetrahedral Cu(I) sites (4e) (Fig. S10 and Table S4†). Notably, the splitting of the 8j Cu(I) site in the C2/m model into two 4e sites in P2₁/c is accompanied by changes in the site occupancy and ordering of Cu(I) atoms and Cu vacancies within the Cu–Br layer (Fig. S10b†). NaCu₅Br₄(COOH)₄ exhibits near-complete ordering of Cu(I) vacancies such that two 4e Cu(I) sites are fully occupied while the other two exhibit partial occupancies of approximately 94.1(1)% and 5.9(1)%, respectively. In contrast, other ACu₅Br₄(COOH)₄ analogs exhibit 75% occupancy in all Cu(I) sites (Fig. S10a†). Despite the differences in ordering, in both structural models the composition of the Cu–Br layer refines to [Cu₃Br₄]¹⁻. Evidently, the Na-analogue closely approximates an idealized arrangement in which three of the 4e Cu(I) sites are fully occupied and there are tetrahedral voids in the Cu–Br layer (Fig. S10c†). Contrary, the structures with larger A⁺ cations exhibit a random distribution of 25% vacancies among all three Cu(I) crystallographic sites. The ACu₅Br₄(COOH)₄ structures are charge balanced, A⁺¹(Cu⁺²)₂(Cu⁺¹)₃(Br⁻¹)₄(HCOO⁻¹)₄, thus semiconducting behavior is expected together with potential magnetic and optical properties.

3.3 Thermal properties

The thermal stabilities of the synthesized compounds were studied using a combination of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Three out of the four ACu₅Br₄(COOH)₄ compounds (A⁺ = Na⁺, K⁺, and Rb⁺) showed one TGA weight loss event under an argon atmosphere (Fig. S4a, b and d†). In contrast, their DSC curves reveal multiple thermal events during heating to 400 °C (Fig. S5†). Among these, the prominent endothermic peaks observed above 180 °C can be attributed to the decomposition of residual inorganic [ACu₅Br₄] framework for A⁺ = Na⁺, K⁺, and Rb⁺. For A⁺ = NH₄⁺ the removal of NH₄/4COOH is completed at 230 °C. This assignment is supported by post-TGA PXRD analysis. The experimental percent weight loss for Na, K, and Rb analogs are close to the expected (calculated) loss of four formate ions per formula unit: 21.6 (21.4), 20.7 (21.0), and 20.5 (19.9), respectively. According to PXRD after these TG-DSC experiments, the loss of formate resulted in the formation of ternary (K₂CuBr₃ and RbCu₂Br₃), binary (NaBr and CuBr), and metallic Cu species which remain stable up to 400 °C (Fig. S4–S6†). From DSC, the thermal decomposition steps were mainly exothermic with an estimated energy release between 10–40 kJ mol⁻¹, which is lower than the values observed for other reported metal formate compounds.^33,51

During the TGA analysis of NH₄Cu₅Br₄(COOH)₄, two separate weight losses were observed, accounting for 16.98% and 6.28% of the total weight loss (23.26%), shown in Fig. S4c.† The total weight loss is likely due to removing four formate ions and one ammonium ion (ca. 23.7%). However, due to the potential formation of hydrogen bonds between these two species, a more complex mechanism for weight loss might be involved.

3.4 Fourier transform infrared (FTIR) spectroscopy

The FTIR spectra of the measured ACu₅Br₄(COOH)₄ (A = NH₄, K, and Rb) compounds are similar, which is expected for isostructural compounds. Characteristic IR bands of HCOO⁻ were observed (Fig. 3a) and their assignments agree with previous reports for copper formates.^2,33,49 The symmetric ν_s(C=O) and antisymmetric ν_as(C=O) stretching vibrations of the HCOO⁻ ions occur around 1360 cm⁻¹ and 1560 cm⁻¹, respectively, revealing the bridging modes of the carboxylate groups.² The absence of a split in the O–C–O symmetric deformation band, δ_s(O–C–O) around 800 cm⁻¹, implies all C–O bonds have the same conformation (equatorial). The band centered at ∼500 cm⁻¹ can be assigned to a Cu–O stretching mode, similar to what has previously been reported.^57,58

Fig. 3 (a) FTIR spectra showing the various vibration modes, (b) UV-vis absorption, and (c) photoluminescence (PL) spectra collected at 298 K of ACu₅Br₄(COOH)₄ compounds, A = K (black), NH₄ (blue), and Rb (red).

3.5 Linear optical properties

The studied compounds exhibit a dark-yellow color and two distinctive absorption bands in the UV-vis spectra (Fig. 3b). The absorption edge at approximately 30 000 cm⁻¹ is attributed to ligand-to-metal charge transfer (LMCT) between Cu²⁺ ions and negatively charged formate ligands.^59–61 Metal-to-ligand charge transfer (MLCT) between Cu¹⁺ with a full 3d¹⁰ configuration and Br with a lone pair of electrons in the Cu–Br layers is forbidden because of the π-donating nature of the Br⁻ ions. However, excitation from the filled 3d orbitals of Cu¹⁺ into the empty s/p shells cannot be ruled out given that this absorption band is expected around 32 000 cm⁻¹.⁶² Meanwhile, the band centered around 11 000 cm⁻¹ is due to the d–d transition of Cu²⁺, typical of Cu²⁺ species in a non-octahedral local environment.^51,60,62,63 The observed square pyramidal geometry (Fig. 2b) suggests that the d–d band of Cu²⁺ centers occurs due to p–d hybridization. The dark-yellow color of the crystals indicates a band gap of ∼2.2 eV, consistent with the observed band edge at 17 500 cm⁻¹. Furthermore, Tauc plots derived from diffuse reflectance spectra reveal direct band gaps between 2.20 and 2.22 eV for NH₄ and Rb analogues, which align well with the estimates from Kubelka–Munk analysis (Fig. S7†). Indirect band gaps were also determined from the Tauc plots, with slightly smaller values (2.13–2.16 eV) than the direct band gaps.

3.6 Photoluminescence properties

Room temperature PL spectroscopy of ACu₅Br₄(COOH)₄ reveals a very distinct emission peak (Fig. 3c). The PL emission maxima are at ∼600 nm which corresponds to a band-to-band transition of ∼2 eV. This is in good agreement with the band gap obtained from diffuse reflectance data and is evident of efficient radiative recombination across the band gap. Additionally, a secondary, broader emission peak is observed for these compounds. The coexistence of mixed-valence Cu⁺¹ and Cu⁺² oxidation states in these compounds suggest the contribution of intervalence charge-transfer (IVCT)^64,65 to the low-energy emission spectra observed in the PL spectra. IVCT transitions typically manifest as broad, lower-energy spectral bands, which could overlap significantly with the d–d transitions of Cu²⁺ (∼11 000 cm⁻¹) identified in the UV-vis spectra. In copper complexes with centrosymmetric octahedral environments, the d–d transitions,⁶² are generally weak in intensity due to their Laporte-forbidden nature. Such transitions become partially allowed in the non-octahedral non-centrosymmetric Cu local environments. Therefore, the broad and asymmetric nature of the secondary PL peaks likely reflect combined contributions from both IVCT and d–d transitions. Future studies involving temperature-dependent photoluminescence, transient absorption spectroscopy, or electro conductivity measurements will be valuable to clearly distinguish and elucidate the role of IVCT transitions in these compounds. The stability of the PL peaks across multiple measurements underscores the optical robustness of these crystals, suggesting their promise for device applications requiring consistent optical performance.

3.7 Magnetic properties

Exploration of magnetic properties of polycrystalline samples demonstrated a broad hump in the magnetic susceptibility in the 50–300 K range and a sharp transition below 40 K (Fig. 4A). Such a broad hump in susceptibility is typical for low-dimensional 1D or 2D magnetic ordering.^63,66,67 This indicates strong coupling of Cu²⁺ spins in the square formate layer but weak interlayer interactions due to separation by non-magnetic Cu–Br or A⁺ layers. Magnetic ordering occurs at slightly different temperatures depending on the nature of the A⁺ cation and corresponding Cu²⁺–Cu²⁺ intra- and interlayer separations (Fig. 4A, S11 and S13†). 3D magnetic ordering observed at low temperatures is of a ferro- or ferrimagnetic nature, which is corroborated by the observation of splitting between FC and ZFC susceptibilities of polycrystalline samples (Fig. S12†). For Rb⁺ and NH₄⁺, magnetic ordering is a single transition, while for Na⁺ and K⁺ a two-step transition occurs. At room temperature, the χT values ranged from 0.69–0.98 emu mol⁻¹ K⁻¹ (Fig. S11†), which is close to the expected value for two isolated Cu²⁺ ions with S = ½ spin states (assuming g-factor = 2) of 0.75 emu mol⁻¹ K⁻¹. The effective magnetic moments (μ_eff) calculated from these values range from 1.66–1.98μ_B per Cu²⁺, which differ slightly from the 1.73μ_B expected per Cu²⁺. However, the experimental g_average values (2.17 for KCu₅Br₄(COOH)₄) obtained from EPR studies (vide infra) result in μ_eff of ∼1.88μ_B per Cu²⁺, which is well within the range observed from the χT at room temperature. This consistency between the χT-derived magnetic moments and the EPR-derived g-values confirms that the magnetic nature of the Cu²⁺ ions is intrinsic to the target compounds.

Fig. 4 (A) Temperature dependence of molar susceptibility (M/H) under an applied dc magnetic field of 0.01 T for polycrystalline samples of ACu₅Br₄(COOH)₄. (Inset) Enlarged region of 45–275 K. (B) Isothermal field dependence at T = 2 K of magnetization for a polycrystalline sample of NaCu₅Br₄(COOH)₄. (Inset) Comparison of isothermal field dependences of polycrystalline and single crystal samples of RbCu₅Br₄(COOH)₄.

The isothermal magnetization data indicates weakly ferromagnetic or ferrimagnetic behavior characterized by rapid saturation at low applied field followed by a linear increase in magnetization. However, unlike the moments observed from the susceptibility plot, the M vs. H plot revealed moments less than 0.1μ_B per Cu²⁺. To elucidate the source of these disparities, we performed detailed studies of the magnetic properties on selected oriented single crystals (Fig. 4B). For the Rb-containing compound, magnetization appeared to be in line with polycrystalline data (Fig. 4B inset). EPR investigations confirmed that the observed magnetic transitions are intrinsic to the studied compounds (vide infra).

To ensure that the observed magnetic properties are intrinsic, we also performed studies on oriented single crystals of NH₄Cu₅Br₄(COOH)₄ (Fig. 5). Temperature dependence of magnetic susceptibility for both H‖c and H‖ab orientations at 0.1 T applied magnetic field revealed broad maxima in 50–200 K range, as depicted in the inset of Fig. 5A. This behavior is consistent with that observed in polycrystalline samples. The data, when fitted using a simple Ising model within the 50–200 K temperature range, yielded a coupling constant of J/k_B = −101 K, which indicates strong antiferromagnetic (AFM) coupling between Cu²⁺ spins in the square formate layers based on the negative value of J. Details for the fitting and data for other powdered and single crystal samples are provided in the ESI (Fig. S14, 15 and Table S5).† The Ising model is not the most applicable for describing a nearly square 2D lattice of magnetic ions. Due to the absence of an analytical expression for square lattices, polynomial expressions are used for the fitting susceptibility of square-lattice antiferromagnets. However, our attempts to fit magnetic data to a standard polynomial expression for square-lattice antiferromagnets^39,68 did not yield satisfactory results, probably due to a lack of higher temperature (T > 300 K) data. The weak interlayer magnetic interactions can be attributed to the presence of large, non-magnetic layers such as Cu–Br or A⁺ that spatially separate the magnetic layers, reducing the influence of neighboring layers. This suggests that while the in-plane interactions are strong, the system exhibits low-dimensional magnetic behavior with limited interplanar coupling, contributing to the observed magnetic anisotropy.

Fig. 5 (A) Temperature dependence of molar susceptibility (M/H) under an applied dc magnetic field of 0.1 T for oriented single crystals of NH₄Cu₅Br₄(COOH)₄. (Inset) Enlarged region of 50–200 K. (B) Double log plot of the 10–50 K region emphasizing the difference in directional behavior.

Higher values of susceptibility were observed with the applied magnetic field oriented along the c direction. Assuming the Cu²⁺ magnetic spins are antiferromagnetically coupled in the ab plane, applying a magnetic field perpendicular to the plane should result in spin canting and a larger susceptibility for the H‖c curve. Below 35 K, the susceptibility abruptly increases for the H‖ab direction while a smaller increase is observed for the H‖c direction (Fig. 5B). In the 3D ordered state, in-layer remains the easy magnetization direction. The easy magnetization direction along the ab-plane in the ordered magnetic state was confirmed for all studied compounds by temperature and field dependent measurements. The abrupt increase in susceptibility indicates the presence of spontaneous magnetization, suggesting long-range ferromagnetic or ferrimagnetic ordering. The observed ferromagnetic-like behavior may arise from spin canting, resulting in weak ferromagnetism. In the AFM Cu-formate layers, the spins of neighboring Cu²⁺ ions may not achieve perfect antiparallel alignment, leading to small uncompensated moments along the canting direction. This spin canting is attributed to weak anisotropic Dzyaloshinskii–Moriya (DM) interactions, which induce a slight tilting of spins and a weak deviation from collinearity – a characteristic feature commonly observed in quasi-2D 3d transition metal antiferromagnets.^{44,66,69–77} Further, the possibility of a long-range 3D ferrimagnetic state cannot be entirely ruled out. Neutron diffraction studies are necessary to figure out the exact nature of the ordered state. The observed magnetic moment was quite low, less than 0.1μ_B per Cu²⁺ at 7 T applied magnetic field. Such small moments support weak ferromagnetic or canted antiferromagnetic behavior. We hypothesize that while the 2D intralayer magnetic interactions are relatively strong, there is weak communication between formate layers due to large non-magnetic Cu–Br separating layers. The presence of Cu vacancies in Cu–Br layers may additionally distort the magnetic interactions along the [001] crystallographic direction. To confirm the intrinsic nature of magnetic ordering, we performed EPR investigations.

3.8 Electron paramagnetic resonance (EPR) spectroscopy

EPR spectra of powdered samples of ACu₅Br₄(COOH)₄ (A = K, NH₄, Rb) are shown in Fig. 6, S16 and S18.† Above 50 K, a simple spectrum characteristic of monomeric Cu(II) with g_x = g_y = 2.075 and g_z = 2.350 is observed for KCu₅Br₄(COOH)₄. The observed g-values can be attributed to large spin orbit coupling induced by Cu²⁺ ions and agrees with reported values for other Cu(II) complexes with equatorial oxygen coordination.^78,79 The anisotropic nature of the EPR signals indicates the elongated axial coordination environment around the Cu²⁺ centers, as evident from the square pyramidal geometry of Cu²⁺ with an elongated axial Cu(II)–Br bond and four equivalent equatorial Cu(II)–O bonds.

The plot of the resonance field dependence on the microwave frequency is depicted in Fig. 6b. Spectra were measured at different frequencies over the 25 GHz to 495 GHz range (Fig. S19†). Above the transition temperature, there are two resonances with g_x = g_y = 2.075 and g_z = 2.35 lying on straight lines (solid and dashed blue lines in Fig. 6b) passing through ν = 0, B = 0. As temperature decreases below 50 K, this EPR spectrum is replaced by a very different one, consisting mainly of two strong and sharp resonances which are significantly shifted downfield from their high-temperature positions. The magnitude of the shift increases with decreasing temperature (Fig. 6a, S15a and S17†). This shift is attributed to the development of internal magnetic fields due to 3D ferromagnetic ordering (vide supra). The low-temperature resonances lie on a straight line (red in Fig. 6b). The red line is parallel to the solid blue line which represents high-temperature “perpendicular” resonances.

Fig. 6 (a) Temperature-dependent EPR spectra of a powdered sample of KCu₅Br₄(COOH)₄ measured at 260 GHz and (b) the resonance field versus microwave frequency dependence in KCu₅Br₄(COOH)₄, read out from the spectra measured at various microwave frequencies over the range 25 GHz to 495 GHz. The small blue circles represent the resonance fields observed above the transition temperature. The blue solid and dashed lines were calculated using B = hν/(μ_B·g) with g_x = g_y = 2.075 and g_z = 2.350, respectively. Red open circles represent resonances seen at 3 K. The solid red line was calculated using B = hν/(μ_B·g) − B_internal, with g = 2.075 and B_internal equal to 1.25 T.

The shift of the red line from the blue line suggests the emergence of the internal magnetic field of ∼1.25 T at low temperatures. These values are estimated using the equation B = hν/(μ_B·g) − B_internal, using g_x = g_y = 2.075. However, none of the low-temperature experimental points align with a line parallel to the dashed blue line which represents the high-temperature “parallel” resonances, indicating that the low-temperature spectra lack the “parallel” spectral features. Similar to KCu₅Br₄(COOH)₄, a set of points below the transition temperature are observed in the NH₄Cu₅Br₄(COOH)₄ sample with the distinct internal magnetic fields of 1.30 T (Fig. S16b†).

These EPR studies confirm that the observed magnetic features in the studied compounds are intrinsic rather than resulting from admixtures. The detection of low magnetization, less than 0.1μ_B per Cu²⁺ ion, in combination with the high internal magnetic fields (∼1.3 T) detected by EPR, points to a complex magnetic ordering along the [001] direction below 40 K. This underscores the intricate magnetic character of the compounds, though the exact origin of this ordering is yet to be determined.

4 Conclusion

We have reported the synthesis of a series of novel hybrid copper formate compounds, showcasing an innovative approach toward a 3D anionic framework by interlinking 2D copper(II) formate and copper(I) bromide layers. Our comprehensive analysis has confirmed the structural integrity and stability of these compounds and illuminated their unique electronic and magnetic characteristics. Notably, strong antiferromagnetic coupling of Cu²⁺ spins in the formate 2D layer was observed while relatively weak interlayer interactions are moderated by large non-magnetic Cu–Br layers. Optical measurements on the dark yellow crystals revealed a bandgap of ∼2.2 eV and corroborated d–d and IVCT transitions characteristic of Cu²⁺ in a square pyramidal geometry. This work not only enriches the existing body of knowledge concerning copper formate frameworks but also opens new avenues for the design of materials by stacking layers of various functionality, such as optical and magnetic layers for application in fields including electronics, magnetism, and photonics.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre and can be retrieved by quoting the depository numbers CCDC 2412816–2412819.

Author contributions

Yao Abusa, Joshua Greenfield, Gayatri Viswanathan, Smitakshi Goswami, Emma Ross, Philip Yox, Richeal Oppong, Iyanu Ojo, Jifeng Liu, Andrew Ozarowski, Kirill Kovnir: investigation. Kirill Kovnir: supervision. All authors: writing – review & editing.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Division of Materials Science and Engineering, Physical Behavior of Materials program under award number DE-SC0023509. Use of the Advanced Photon Source was supported by U.S. Department of Energy, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement no. DMR-2128556 and the State of Florida. The authors would like to thank Colin Unger (UC Davis) for initial work on synthesis of the reported materials; Dr W. Straszheim (MARL-ISU) for the help with SEM/EDS data collection; Prof. J. V. Zaikina (ISU) for access to the UV-vis spectrometer; Dr Brett Boote (CIF-ISU) with help with TG-DSC; Dr S. Lapidus (ANL APS) for help with conducting high-resolution synchrotron PXRD at beamline 11-BM; and Prof. Warren Pickett (UC Davis) for the discussion regarding the electronic structure of the reported materials.

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Footnote

† Electronic supplementary information (ESI) available: Description of synthetic and characterization experimental procedures and additional figures and tables pertaining to powder X-ray diffraction, Rietveld refinement of synchrotron powder diffraction patterns, thermogravimetric (TG-DSC) analysis, BVS calculations, crystallographic files and data information for ACu₅Br₄(COOH)₄ (A⁺ = Na⁺, K⁺, NH₄⁺, Rb⁺), SEM/EDS, and CHNO elemental analysis, and magnetic and EPR studies. CCDC 2412816–2412819. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5sc00548e

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