A new tri-nuclear Cu-carbonate cluster utilizing CO₂ as a C1-building block – reactive intermediates, a probable mechanism, and EPR and magnetic studies

Abstract

This work demonstrates that simple copper-bipyridine compounds and atmospheric CO₂ react to produce useful/complex materials under appropriate conditions. Starting from a distorted square planar copper(II) complex, [(^tbubpy)CuCl₂](^tbubpy = 4-tert-butyl-2-(4-tert-butylpyridin-2-yl)pyridine), an air-sensitive, copper(I) complex, [(^tbubpy)₂Cu^I][BF₄], which exhibits a distorted tetrahedral geometry, was synthesized and characterized. Reactions of [(^tbubpy)₂Cu^I][BF₄] with CO₂ inside a sealed tube and with air were carried out over a week and three weeks, respectively. A new tricopper(II)-carbonato cluster, [{(^tbubpy)₂Cu}₃(μ-CO₃)][PF₆]₄, was isolated with three distorted octahedral copper(II) centres bound by a carbonate-bridge formed from atmospheric CO₂. NMR and UV-Vis spectroscopic analyses coupled with previous reports point to a multi-step process in the formation of a trinuclear Cu^II-carbonato cluster that includes the probable involvement of μ-hydroxo-bridged dicopper(II) type intermediates.

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Introduction

Novel chemical features of first row transition metal complexes have attracted enormous attention with the aim of developing cost-effective, non-toxic and essential catalysts/materials.^1–4 Copper is one of those metals which have shown significant promise.^5–10 Being able to accommodate oxidation states from 0 to +4, its complexes have been studied as catalysts for a variety of metal-assisted synthetic, oxidation (e.g. of water, C–H bond) and reduction (e.g. of protons and CO₂) reactions.^11–16 CO₂, a greenhouse gas, is being released from both natural processes (the respiration of living creatures) and anthropological processes (the burning of fossil fuels for industrial and household purposes of everyday life).^17,18 The increasing amount of CO₂ in the atmosphere is one of the major issues of the 21^st century. In the current fast growing society, the requirement for energy from carbon-based (fossil) fuel is increasing every day as renewable carbon-free energy sources are yet to fulfil the demand to a significant degree. Therefore, one of the best possible ways to mitigate the concentration of CO₂ in the atmosphere is to convert emitted CO₂ to useful materials. This is an active and important field of modern-day research and the major focus is on converting CO₂ to fuel and commodity chemicals. In this regard, direct air capture (DAC), more precisely direct extraction of CO₂ from air, a concept introduced by Lackner in 1999, is highly relevant.¹⁹ The direct capture of CO₂ from air and its conversion to useful materials (e.g. new chemicals, magnetic materials, metal organic frameworks etc) have been an active field of research in the last few decades.²⁰

Reduction and hydrogenation are the most common approaches for mitigating the amount of CO₂ in the atmosphere as well as for producing fuels and manufacturing useful organic and inorganic compounds.^21–23 In other words, the carbon atom that is trapped in waste CO₂ is utilized as a C1-building block. In 2015, Beller et al. reviewed extensively such uses of CO₂ as a building block in organic synthesis.²⁴ Formation of organic carbonate from CO₂ is one of the effective synthetic routes. CO₂ reacts rapidly in basic medium. In the presence of strong nucleophiles, it can produce the corresponding carboxyl and carbamoyl compounds. These compounds upon reaction with organic electrophiles lead to the formation of organic carbonates and carbamates, respectively.²⁴

In comparison, there are only a handful of examples of the formation of metal carbonates utilizing air.^25–27 The reasons can be the difficulties in finding a suitable ligand–metal system and appropriate conditions to interact with air, which contains a low concentration of the substrate (such as 410 ppm for CO₂).²⁸ Here we report, starting from a simple air-stable bipyridine-Cu^II(Cl)₂ complex {2: [(^tbubpy)CuCl₂] (^tbubpy = 4-tert-butyl-2-(4-tert-butylpyridin-2-yl)pyridine)}, in situ formation of an unstable bis(bipyridine)-Cu^I complex (3: [(^tbubpy)₂Cu^I][BF₄]) which on slow reaction with air (over a period of 3 weeks) forms a Cu^II₃-carbonate (4: [{(^tbubpy)₂Cu}₃(μ-CO₃)][PF₆]₄) cluster. Saturating the methanolic solution of 3 with CO₂ speeds up the formation of 4 (to less than a week). These types of clusters can be interesting candidates for use as magnetic materials (e.g. single-molecule magnets) comprising first row transition metals. The utilization of the atmospheric CO₂ as a C1-building block to form new materials and the understanding of the possible reactive intermediates are appealing topics.

Results and discussion

Freshly prepared 4-tert-butyl-2-(4-tert-butylpyridin-2-yl)pyridine (^tbubpy) (1) (1 mmol) was treated with one eq. of Cu(BF₄)₂ and two eq. of NaCl in methanol (Scheme S1†). The addition of 1.5 eq. of triethyl amine caused the pale blue solution to turn intense blue-green. A precipitate formed upon stirring for 90 min, which was successfully crystallized from acetonitrile solution to afford the complex-salt, [(^tbubpy)CuCl₂]·Et₃NH⁺BF₄⁻(2). Attempts made to crystallize [(^tbubpy)CuCl₂] using different combinations of Cu and Na salts, and without using Et₃N, were unsuccessful. In the single crystal X-ray structure of 2, Fig. 1a, the Cu^II ion binds to two nitrogen and two chloride donors in a distorted square planar coordination geometry. Cu^II being a d⁹ system prefers the square planar, square pyramidal or trigonal bi-pyramidal geometry depending on the ligands over the tetrahedral geometry as the latter readily goes through ligand rearrangement/attack from solvent molecules.²⁹ In 2, N₁CuN₂ has the smallest angle [80.14(9)°] out of the four angles around the Cu-center. The other angles are 93.90(7)°, 94.56(3)° and 95.63(6)° for N₁CuCl₁, Cl₂CuCl₁ and N₂CuCl₂, respectively.

Fig. 1 X-ray diffraction crystal structures of 2 (a), 3 (b) and 4 (c). A simplified depiction of the reaction pathway where (i) is NaBH₄/d-methanol under N₂ and (ii) is air. The details about the reaction procedure can be found in the Experimental section and Fig. S1.† The counter anions, methyl hydrogens of 4 and solvent molecules were removed for clarity.

In order to perform the direct and easy analysis of the reactive intermediate/s by NMR spectroscopy, d-methanol was used as a solvent for all of the follow-up reactions. Treatment of a d-methanol solution of 2 (0.5 mM) with 2.5 eq. of NaBH₄ under N₂ shows an immediate colour change to black-brown, followed by a black precipitation within thirty minutes of continuous stirring. The precipitate was insoluble in water, methanol and acetonitrile. The filtrate was collected under N₂. NMR spectra (¹H and ¹³C) of the colourless solution showed peaks for a single diamagnetic species (3) containing ^tbubpy ligand(s). Air sensitive crystals were obtained by the slow diffusion of diethyl ether to the d-methanol solution of 3 inside a glovebox. Careful crystal mounting (at low temperature and in the absence of air) followed by X-ray diffraction crystallography shows that 3 is [(^tbubpy)₂Cu^I][BF₄]. The Cu^I centre in 3 is surrounded by four N atoms from two ^tbubpy units in a distorted tetrahedral fashion (Fig. 1b). The instability of this complex can be explained by the soft nature of Cu^I and the structure. Being a d¹⁰ system, Cu^I prefers soft donors such as sulfur and iodide. With the intermediate donor capacity such as nitrogen (sp³ hybridized nitrogen is a harder donor, whereas sp² hybridized nitrogen is a relatively soft donor) in the current study and the additional electron density from the tertiary butyl groups, Cu^I always tends to form Cu^II given a source of oxidation. Moreover, with N-donors Cu^I tends to have proper tetrahedral geometry which provides thermodynamic stability (CFSE, crystal field stabilization energy).²⁹ Two ^tbubpy ligands make it impossible to achieve the tetrahedral geometry as N₁CuN₂ and N₃CuN₄ form an acute angle ∼80.4°. Two other angles of N₁CuN₃ and N₂CuN₄ are ∼107.9° and 122.3°, respectively.

NMR studies before (in a closed Young's NMR tube) and after careful purging of CO₂ (1 atm) into a d-methanol solution of 3 in a Young's NMR tube (in the absence of air) show no immediate fast reaction, but over a period of 48 h a slow reaction took place. It results in an appreciable amount of new species with singlets at δ 2.03 ppm and δ 161 ppm in the ¹H and ¹³C NMR spectra, respectively (Fig. S7 and S4†). The ¹³C chemical shift is characteristic of bicarbonate species (HCO₃)⁻.³⁰ Upon exposure to air, the solution starts changing colour slowly from yellow-orange to light brown to cyan over a period of four days. After a week, cyan coloured crystals of 4 (Fig. 1c) deposited from the reaction mixture. When another very similarly prepared d-methanol solution of 3 was directly exposed to air without prior exposure to CO₂, it took 3 weeks before the cyan colored crystalline material of 4 came out from the solution.

The immediate changes that happen on exposing a methanol solution of 3 to air were followed by UV-Vis spectroscopy (Fig. 2) over two hours. A colourless solution of 3 under N₂ turns light yellow-orange instantaneously upon the exposure to air. The formation of this new yellow-orange species was too fast to follow by room temperature UV-Vis spectroscopy. Previous reports by Stack et al. showed that a highly temperature sensitive bis(μ-oxo)dicopper(III) {[(TMED)₂Cu^III₂(μ₂-O)₂]²⁺} (TMED = N,N,N′,N′-tetramethylethylenediamine) type intermediate formed.³¹ This intermediate was only possible to isolate/characterize at low temperature (183 K) using acetone as the solvent.³¹ The mononuclear counterpart {(TMED)Cu^I}⁺ reacts with O₂ to form bis(μ-oxo)dicopper(III), which had a clear absorption maximum at around 390 nm with a shoulder at 470 nm. In the presence of an excess of Cu^I in the solution and with the limited steric demand from the ligand, {[(TMED)₂Cu^III₂(μ₂-O)₂]²⁺} species reacts further, and forms a compact trinuclear [Cu₃(μ₃-O)₂]³⁺ core.³¹

Fig. 2 Room temperature UV-Vis spectra recorded for the reaction of 3 (0.5 mM) with air over a period of 120 minutes. The first nine spectra were recorded at 5 min intervals and the last 5 spectra were recorded at 15 min intervals. The inset shows a magnification of the spectra from 450–825 nm.

In the current project, we carried out the reaction of the unstable Cu^I species (3) with air, instead of O₂ only, to get an idea of the possible intermediates that might be involved in the construction of 4 (bluish green) in the basic medium. The reaction of 3 with air is a multistep process. The first and very fast process, i.e. the immediate development of yellow-orange species from the colourless solution of 3, can be attributed to the formation and transient presence of a high-valent Cu-intermediate in the presence of air oxygen as discussed above.³¹ The decay or the reaction of this new yellow orange species with air, being a slower process, can be carried out at room temperature. A UV-Vis study for a period of 2 h shows a steady decrease of the absorption band at 435 and 535 nm and the appearance of a new broad absorption band at 570 nm with a shoulder at 700 nm. The process becomes much slower after 2 h. This slower process can be attributed to slow ligand reorganization under air during the formation of 4 (Fig. 1c). Our claim is further supported by the very slow reaction of 3 in the presence of only CO₂ in the absence of O₂ (Fig. S4–9†). ¹³C NMR and ¹H NMR studies over a period of two days reveal that 3 reacts very slowly with CO₂ in d-methanol solution in the absence of O₂. The clear formation of HCO₃⁻ was observed only after keeping 3 in the presence of 1 atm CO₂ over a period of 48 h (Fig. S4–9†). No further reaction was observed in the absence of O₂ (air). For the formation of 4, the presence of O₂ (air) is mandatory. All these observations point towards the formation of a semi-stable Cu^II–O(H)–Cu^II intermediate with oxygen that reacts further and rather slowly with atmospheric CO₂. The observations are consistent with the report from Mukherjee and co-workers, where the nucleophilic attack of the hydroxo group of an in situ generated, semi-stable di-μ-hydroxo-bridged dicopper(II) species, [L′Cu–OH–CuL′]²⁺ [L′ = methyl[2-(2-pyridyl)ethyl](2-pyridylmethy)amine], to the electrophilic carbon of the CO₂ unit was proposed.³² From the cumulative results of ours and the previous reports we propose a reaction pathway to form 4 starting from 2 (Scheme 1).

Scheme 1 Proposed reaction pathway for the formation of 4 from 2. # represents the probable reactive intermediate. L stands for ^tbubpy.

The X-ray diffraction structure reveals that 4 is a new trinuclear Cu^II-cluster {[(^tbubpy)₂Cu^II]₃(CO₃)(PF₆)₄} having a carbonato (CO₃²⁻) bridge as a central building block. Both O₂ and CO₂ from the air play important roles in forming the trinuclear Cu^II cluster (4). CO₃²⁻ uses all its possible coordination and occupies six coordination sites (two per Cu^II units). Each Cu^II center is attached to two oxygen atoms of CO₃²⁻ with O_(Carbonato)–Cu^II bond distances of 2.31–2.35 Å. The Cu^II centers have distorted octahedral geometry and are surrounded by four nitrogen donors from two ^tbubpy units and share two of the six possible binding sites of the central CO₃²⁻ (Fig. 1c). Although a few similar Cu-carbonate clusters are reported, to the best of our knowledge it is the first example where the formation of the cluster from the mononuclear counterpart in a reductive environment was followed up by NMR, UV-Vis spectroscopy and X-ray diffraction techniques.^27,33,34 The structure makes it an interesting candidate for magnetic studies.

The crystal packing diagram of 4 (Fig. 3a and b) shows that it consists of trimeric complex cations having the carbonate carbon at the centre of the three-fold axis. Out of the four PF₆⁻ anions (per trimeric unit), one has 1/3 occupancy. The PF₆⁻ anions play an important role in the formation of the crystal packing through weak C–H⋯F (with H⋯F distance >2.75 Å) interactions between the F and C–H of the tert-butyl units. The presence of tert-butyl groups in the ligand periphery and PF₆⁻ ions makes the crystal-packing unique and unprecedented. The UV-Vis spectrum of 4 in d-methanol solution shows distinct broad absorption spectra at 640 nm, very different from those of 2 and 3 under similar conditions (Fig. S10†).

Fig. 3 The packing diagram of 4·4PF₆ (a) viewed along the C-axis and (b) showing intermolecular weak C–H⋯F interactions. In (a) blue and red colors are assigned to the PF₆ anions where blue represents PF₆ anions with single occupancy, whereas red represents PF₆ anions with one third occupancy.

Important bond distances in complexes 2–4 are summarized in Table 1. Cu–N distances vary from 1.99 Å to 2.115 Å in these three complexes. Interestingly, the longest and shortest distances are present in 4. In the central Cu₃(CO₃)⁴⁻ unit, the Cu²⁺ centers are separated by 4.649 Å. Since both 2 and 4 are paramagnetic complexes, they were further characterized by EPR spectroscopy and magnetic measurements.

Table 1 Important bond distances in 2, 3 and 4

Cu–N bond distances	Å
2	Cu–N1 [2.018(2)], Cu–N2 [2.003(2)]
3	Cu–N1 [2.029], Cu–N2 [2.005], Cu–N3 [2.029], Cu–N4 [2.005]
4	Cu–N1 [2.115(9)], Cu–N2 [1.99(1)], Cu–N3 [2.00(1)], Cu–N4 [2.11(2)]
Cu–O bond distances (4)	2.21(1), 2.45(1)
Cu–Cu distance (4)	4.649(3)

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The EPR spectrum of a frozen solution of 2 (1 mM in acetonitrile, 5 K) shows a rhombic EPR signal, consistent with the distorted square planar coordination geometry of 2, where the hyperfine interactions with the Cu-nucleus are not very well resolved (Fig. 4a). The EPR spectrum could be simulated as a rhombic S = 1/2 signal using the g- and A-values given in Table 2. An anisotropic line broadening (g-strain) was needed to obtain a good simulation. The rhombic EPR signal from 2 in acetonitrile differs from the previously reported axial EPR signal for (^tbubpy)CuCl₂·0.5CH₃OH measured in methanol at 150 K where g_‖ = 2.269, g_⊥ = 2.01 and A_‖ = 490 MHz, A_⊥ ∼ 60 MHz.³⁵ Another related complex, (^tripbpy)CuCl₂ (^tripbpy = 6,6′-(2,4,6-triisopropylphenyl)-2,2′-bipyridine), is considerably more tetrahedral and has an EPR-signal (110 K) with g-values of ∼2.1 and ∼2.2 and unresolved hyperfine couplings.³⁶ Varying the temperature from 5–20 K changed the intensity of the EPR signal from 2 but did not alter the shape (Fig. S11†).

Fig. 4 (a) Experimental (black) and simulated (red) EPR spectra of 2 (1 mM in acetonitrile). (b) Experimental (black) and simulated (red) EPR spectra of 4 (1 mM in acetonitrile). EPR conditions: T: 5 K, microwave power: (a) 200 μW (b) 20 μW, microwave frequency 9.287 GHz. The simulation was performed using EasySpin.³⁷

Table 2 Parameters for the simulations of the EPR-signals from the frozen solutions of 2 and 4 at 5 K

	g 1	g 2	g 3	A 1, MHz	A 2, MHz	A 3, MHz	g 1-strain	g 2-strain	g 3-strain
2	2.26	2.16	2.06	510	200	10	0.11	0.06	0.15
4	2.256	2.10	2.075	500	225	10	0.09	0.15	0.03

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The EPR-spectrum of a frozen solution of 4 (1 mM in acetonitrile, 5 K) has more resolved hyperfine interactions. Since the magnetic measurements indicate that the exchange interaction between the three Cu²⁺ is very small (see below), the EPR spectrum was treated as originating from a single S = 1/2 species. A satisfactory simulation was obtained with the g- and A-values given in Table 2. The EPR signal from 4 is more axial than the signal from 2 but the hyperfine interactions are of the same magnitude in both signals. Also for this signal an anisotropic line broadening was used for the simulations, probably reflecting the distorted octahedral coordination around the Cu ions. Varying the temperature from 5–20 K had only a very minor influence on the shape of the EPR signal from 4, in agreement with a very small exchange interaction between the Cu²⁺ ions (Fig. S12†).

The temperature dependence of the molar magnetic susceptibility, χ_m, of 2 and 4 is shown in Fig. 5(a) and (b). The right-hand axes show fits of the respective ZFC data to the Curie–Weiss (C–W) law χ_m = C/(T − Θ), where Θ is the characteristic Weiss temperature and C is the Curie constant, from 2–150 K. For 2, least-squares fitting gives: μ_eff = 1.62(5)μ_B/Cu^II, which agrees well with the theoretical spin-only magnetic moment (1.73μ_B) of the Cu^II (d⁹, S = 1/2) ion; and Θ = −1.3(2) K, a nearly insignificant deviation from 0 K.

Fig. 5 DC molar magnetic susceptibility χ_m and inverse susceptibility 1/χ_m as functions of temperature for (a) the mononuclear Cu(II) complex 2 and (b) the trinuclear complex 4 (red lines show linear fits to the Curie–Weiss equation); and field-dependent magnetisation of (c) 2 and (d) 4 at 2 K (red), 20 K (green) and 50 K (blue).

No hysteresis is evident from the field-dependent magnetisation curves shown in Fig. 5. The mononuclear complex 2 is thus confirmed to contain the expected stoichiometric amount of Cu^II and to show purely paramagnetic behaviour, as expected. For 4, we observe a very small deviation from linear C–W behaviour. However, a fit over the 2–150 K range still yields μ_eff = 1.78(7)μ_B/Cu²⁺ and Θ = −0.8(3) K, and the field-dependent magnetisation behaviour is the same as for 2. These data are thus consistent with the purely paramagnetic behaviour of the expected stoichiometric amount of Cu²⁺ ions in the trinuclear complex, i.e., any exchange interactions among those ions through the central bridging carbonato unit must be very weak down to at least 2 K.

Conclusions

In this report, we have highlighted a new procedure that incorporates atmospheric CO₂ to form new magnetic materials comprising copper(II) units. The reactive/air sensitive Cu^I intermediate was isolated and characterized by X-ray crystallography and NMR techniques (inert atmosphere). A reaction pathway was proposed based on the experimental findings and previous reports. Detailed EPR and temperature dependent magnetic studies were performed on a trinuclear Cu^II cluster and a mononuclear Cu^II-bipyridine complex.

Experimental

General considerations

Commercially available reagents and deuterated solvents were used as received from Sigma Aldrich and Fisher chemicals. ¹H and ¹³C NMR spectra were recorded using Bruker DPX 400 spectrometers. The UV-Vis-NIR spectra were recorded with a Varian Cary 50 UV/Vis spectrometer.

Synthesis

Synthesis of 4-tert-butyl-2-(4-tert-butylpyridin-2-yl)pyridine (^tbubpy) (1). 1 was prepared following the synthetic procedure reported by Le Bozec et al. in 1993.³⁸

Synthesis of Cu^II(^tbubpy)(Cl)₂ (2). 0.268 g of freshly prepared, crystalline 4-tert-butyl-2-(4-tert-butylpyridin-2-yl)pyridine (^tbuBpy) (1) (1 mmol) was dissolved in 20 mL of methanol in a two necked 50 mL round bottomed flask and degassed with N₂ for 5 minutes. 0.236 g (1 mmol) of Cu(BF₄)₂ and 0.117 g of NaCl (2 mmol) were added to the methanol solution under continuous stirring. No immediate change in the color was observed. But the addition of 0.25 mL of triethyl amine changed the color of the solution immediately to intense blue-green. After rigorous stirring for one and half hours a precipitate was obtained. The precipitate was filtered and dissolved in a minimal amount of acetonitrile. Slow evaporation of the acetonitrile solution resulted in 0.429 g (72.5% yield) of green crystals of (^tbuBpy)Cu^IICl₂·Et₃NH⁺BF₄⁻ (2). Elemental analysis calculated for C₂₄H₄₂B₃Cl₂CuF₁₂N₃ [(^tbuBpy)Cu^IICl₂. Et₃NH.BF₄·2HBF₄]: C, 37.56; H, 5.52; N, 5.48%; found: C, 38.04; H, 5.31; N, 5.27%. [CCDC 1880852†].

Synthesis of [(^tbuBpy)Cu^I(BF₄)] (3). This synthesis was air sensitive. An appropriate air tight system and a continuous flow of inert gas (N₂ in our case) are important. 0.05 g (0.125 mmol) of 2 was dissolved in 5 mL of d-methanol in a two necked 50 mL round bottom flask. The solution was degassed by N₂ before adding 0.012 g (0.3125 mmol) of NaBH₄ and throughout the synthesis N₂ was purged through one of the necks of the round bottom flask. Upon the addition of NaBH₄ the color of the solution changes immediately to dark brown, followed by precipitation of a black material within half an hour of stirring. The solution was filtered under inert conditions and the NMR spectrum of the unprocessed light brown filtrate was recorded. It indicates the absence of any paramagnetic species. The filtrate was concentrated to its half volume under inert conditions and was left for crystallization inside a glovebox under slow diffusion of diethyl ether. It produced dark brown crystals of 3 and the structure was confirmed by X-ray diffraction crystallography (Fig. x) over a period of 5–6 days. Being highly unstable (hygroscopic), it was difficult to measure the yield with appropriate precision. Careful isolation of the crystals and drying under vacuum resulted in 0.028 g (32.5% yield) of semi-solid compound (3). 1H NMR (400 MHz, methanol-d4): δ 8.56 (d, 4H, J = 5.22 Hz), 8.35 (s, 4H), 7.50 (dd, 4H, J = 5.26 Hz), 1.41 (s, 36H) [CCDC 1880853†].

Synthesis of {[(^tbubpy)₂Cu^II]₃(CO₃)(PF₆)₄}(4). 2 mL of d-methanol solution of 3 (0.028 g, 0.041 mmol) was treated with 1 mL of degassed (by N₂ for 15 minutes) aqueous KPF₆ (0.0224 g, 0.122 mmol) and was stirred for half an hour under N₂. No immediate precipitation was observed. The solution was carefully purged with CO₂ for 5 minutes inside a Young's NMR tube. No immediate colour change was observed. The mixture was left under air for crystallization. After 5–6 days of slow evaporation, 0.019 g of cyan coloured crystals of 4 was found with 57.5% yield. 4 can also be prepared without purging CO₂, but the process is much longer and cyan crystals of 4 were isolated after 3 weeks. Elemental analysis calculated for C₁₀₉H₁₄₆Cu₃F₂₄N₁₂O₄P₄{[(^tbubpy)₂Cu^II]₃(CO₃)(PF₆)₄}·H₂O: C, 53.24; H, 5.98; N, 6.84%; found: C, 51.53; H, 5.49; N, 6.40%. [CCDC 1880854†].

EPR studies

EPR spectroscopic measurements were performed on 2 and 4 using a Bruker ESR-500 spectrometer, equipped with an ER 4122SHQ resonator, an ESR900 cryostat and an Oxford ITC503 temperature controller. The microwave frequency was 9.287 GHz and a modulation amplitude of 19.4 G and a modulation frequency of 100 kHz were used for all spectra. EPR signal simulations were performed with the EasySpin 5.2.21 software package.³⁷

Magnetic studies

DC magnetic susceptibility and magnetization data for the mononuclear Cu(II) complex 2 and the trinuclear complex 4 were collected on a Quantum Design Physical Property Measurement System (PPMS) using the vibrating sample magnetometer (VSM) method. Temperature-dependent susceptibility data were collected under zero field-cooled (ZFC) and field-cooled (FC) conditions in a coercive field of 1000 Oe between 2 and 300 K. Field-dependent magnetisation measurements were carried out at 2, 20 and 50 K up to 8 T. Molar magnetic parameters were calculated with respect to the magnetic Cu²⁺ ions [per Cu²⁺, i.e., 1/3 of a formula unit the trinuclear complex 4]. Diamagnetic corrections were applied using standard tabulated values of Pascal's constants.³⁹

X-ray crystallography

Crystals (2, 3 and 4) were mounted on a Bruker kappa-II CCD diffractometer at 150 K using an IμS Incoatec Microfocus Source with Mo-Kα radiation (λ = 0.710723 Å). Symmetry related absorption corrections using the program SADABS were applied and the data were corrected for Lorentz and polarisation effects using Bruker APEX3 software.⁴⁰ The structure was solved by program SHELXT⁴¹(with intrinsic phasing) and full-matrix least-squares refinements were carried out using SHELXL-2014⁴¹ through the Olex2⁴² suite of software.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

BD and SBC would like to thank the Australian Research Council (Grant No. DP 160104383) for providing the funding for this research.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1880852–1880854. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt04858d

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