Ruthenium polyhydrides supported by rigid PCP pincer ligands: dynamic behaviour and reactions with CO₂

Abstract

Two rigid β-elimination immune PC_carbeneP pincer ligands, differing in their electron donor properties by variation of the substitution pattern on the aromatic linker arms, were complexed to ruthenium to form the dichlorides L_RRuCl₂ (R = H or NMe₂). These compounds were converted to hydrido chlorides by treatment with dihydrogen (H₂) and a base. By converting to tert-butoxide derivatives in situ under an atmosphere of H₂, the poly hydride PC_alkylP complexes L_HRRu(H)₃ compounds were generated. In these complexes, H₂ has added across the Ru=C bond in the PC_carbeneP starting materials. The polyhydrides are dynamic in solution and extensive NMR studies helped to elucidate the speciation and fluxional processes operative in this dynamic system. The polyhydride complexes react rapidly with CO₂ to give the PC_carbeneP formato hydride complexes L_RRu(H)-κ²-O₂CH. For R = H, the 1,2-hydride shift from the anchoring alkyl of the PC_alkylP carbon to the metal is reversible, but for R = NMe₂ it is irreversible. The CO₂ incorporated into the formato ligand of these compounds exchanges with free CO₂via a bimolecular mechanism that is more rapid for R = NMe₂ than for R = H; plausible explanations for this observation are proffered. Experiments designed to evaluate the efficacy of the R = NMe₂ formato hydride complex as a catalyst precursor for CO₂ hydrogenation to formate salts reveal poor performance in comparison to state-of-the-art ruthenium-based catalysts. This is due primarily to the precipitation of a dimeric μ-κ²-κ¹-CO₃ carbonate complex that is not an active catalyst for the reaction.

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Introduction

Tridentate “pincer” ligands provide a thermally stable, highly tuneable ligand array for metals across the Periodic Table with a wide range of catalytic applications.¹ The ligands generally provide a fixed meridional geometry, leaving 2–3 other coordination sites for engagement of substrates and products in catalytic processes. For platinum group metals, particularly ruthenium and iridium-based complexes, dynamic polyhydride complexes² are effective catalysts for a variety of applications including CO₂ hydrogenation,^3–5 transfer hydrogenation,^6,7 alkane dehydrogenation,⁸ ammonia activation⁹ and H/D exchange reactions.^10–14 In many instances, the pincer ligand can engage in activating dihydrogen through metal–ligand cooperativity (MLC), acting as a reservoir for hydrogen equivalents in catalytic reactions involving delivery or evolution of multiple equivalents of dihydrogen.¹⁵

One specific family of PC_carbeneP ligands, which were first reported by Shaw¹⁶ (I, Chart 1) offer a unique form of MLC via reversible addition of H₂ across the anchoring C=M moiety of the pincer ligand framework. Because of the β-hydrogens present, the saturated hydrocarbon linker in these seminal ligands posed stability problems for polyhydride derivatives but in 2012,¹⁷ our group reported a “β-hydrogen elimination immune” variant that incorporated aryl groups as linkers (II) that allowed for generation of dynamic polyhydrides of rhodium¹⁸ and iridium.^13,19–23 Activation of other E–H bonds in small molecules across this linkage is also possible, especially in nickel complexes.^24,25 This family of PC_carbeneP ligands has also been studied by other research groups as summarized by a recent review article from Manzano and Young.²⁶

Chart 1 PC_carbeneP complexes.

Against this backdrop, we were interested in exploring the use of our PC_carbeneP ligands in ruthenium polyhydrides for use in catalytic small molecule conversions. Previously, Gusev had examined use of the saturated Shaw type ligand I for ruthenium complexes and found β-hydride elimination to be problematic in these systems as well.²⁷ Ozerov devised the PC_carbeneP ligand shown in III and used it to develop some ruthenium chemistry,²⁸ touting this ligand as being synthetically more accessible than the aryl-linked ligands II, which at that point had only been assembled within the coordination sphere of ruthenium, in early work by Roper et al.²⁹ Since we have available a small library of ligands II,³⁰ we report here the synthesis and characterization of some (PC_carbeneP)Ru(H)_n polyhydrides and their reactivity towards CO₂.

Results and discussion

Of the range of PC_carbeneP proligands we have available, we chose the rigidified ligands L_H and L_NMe2 in which the aryl linkers in the PC_carbeneP framework are bridged by a –C(CH₃)₂ group (L_R, Scheme 1). The groups in the para position relative to the anchoring pincer carbon allow for variance of the electron richness of this donor.³⁰ The ligands (R = H; NMe₂) were prepared using previously established methodologies.^23,31 When these proligands were heated in toluene with [Ru(p-cymene)Cl₂]₂, over the course of 24 hours a double C–H activation and loss of H₂ affords the dichloro ruthenium compounds L_RRuCl₂ in excellent to moderate yields. The dichlorides were fully characterized, including via X-ray crystallography (see the ESI† for details). These compounds can be viewed as structural analogs of Grubbs generation 1 olefin metathesis catalysts,³² with the caveat that the chelating nature of the pincer ligand dictates that the carbene orientation is perpendicular to that seen in the ground state of the Grubbs systems. The pincer framework also precludes easy phosphine dissociation (the initiation step in olefin metathesis catalysis) and so the dichlorides L_RRuCl₂ are, perhaps unsurprisingly, unreactive towards olefins.

Scheme 1 Synthesis of PC_carbenePRuX₂ and PC_alkylPRu(H)_n complexes.

When the dichlorides L_RRuCl₂ were treated with an atmosphere of H₂ in the presence of triethylamine at 70 °C (R = H) or room temperature (R = NMe₂), smooth conversion to the analogous monohydrido chlorides L_RRu(H)Cl was observed. These hydrido chlorides can also be prepared directly from the ligands and [Ru(p-cymene)Cl₂]₂ in one pot if triethylamine is included, albeit with some contamination (≈10%) by the dichlorides but, this can be driven forward by introducing additional dihydrogen. Conversion stops at the monohydrido derivatives, which are evidenced by upfield triplets at −11.75 ppm (R = H) and -14.57 ppm (R = NMe₂) in the ¹H NMR spectra. Both L_RRu(H)Cl derivatives were also characterized by X-ray crystallography (Fig. S1†).

Removal of the second chloride ligand for conversion to a polyhydride species required treatment of the L_RRu(H)Cl derivatives with NaO^tBu under an atmosphere of dihydrogen (Scheme 1), but this smoothly afforded solutions of the PC_alkylP ligated polyhydride species L_HRRu(H)_n. ³¹P{¹H} NMR spectra indicate relatively clean crude reaction mixtures, but upon work up by exposing to vacuum, a pale yellow to dark green color change was observed, and assay of the washed solid product by ³¹P NMR spectroscopy seemed to indicate the presence of several phosphorus containing products (Fig. S2 and S3†). Remarkably, simply reintroducing one atmosphere of H₂ reconstituted the polyhydride product L_HRRu(H)_n as the sole phosphorus containing species in solution.

The solution speciation of these compounds was probed by NMR spectroscopy; all spectra were obtained under 1–4 atmospheres of dihydrogen. In compounds L_HRRu(H)_n, hydrogenation of the C=Ru bond in the PC_carbeneP ligand has occurred, producing a PC_alkylP donor, and the dibenzylic hydrogen appears in the ¹H NMR spectra for L_HRRu(H)_n at 4.99 ppm (R = H) and 5.00 ppm (R = NMe₂), respectively. The ¹³C{¹H} spectra also support this conclusion as there are no carbene resonances but rather peaks for the HC–Ru groups at 39.7 ppm and 67.8 ppm, as confirmed by a ¹H–¹³C HSQC NMR experiment. In the hydride region of the spectra, broad resonances at −7.80 ppm and −7.52 ppm that integrate for a total of about three hydrogens are observed, suggesting that another two equivalents of H₂ have been added to the complex in the synthesis described above (one hydrogen atom is incorporated into the tert-butanol byproduct). The broadness of the signal in the hydride region of a room temperature acquired ¹H NMR spectrum is suggestive of complex dynamic behavior, and although the benzylic peaks do not coalesce fully with the hydride/dihydrogen resonances, exposure of these compounds to 1 atmosphere of D₂ results in rapid deuteration of the benzylic position on the ligand in addition to the hydrogens on the ruthenium. This indicates that the benzylic hydrogen is in exchange with the metal hydride/dihydrogen ligands on the chemical timescale, akin to what has been observed for iridium complexes with these types of ligands.^13,17

The variable temperature NMR spectra for the two compounds are similar in pattern, and here we focus here on the data for the L_HNMe2Ru(H)₃ derivative. Samples generated as described above and cooled from room temperature to 198 K leads to a spectrum with three broad signals in the hydride region, in addition to another minor species with a hydride multiplet at about −10.5 ppm (Fig. S4†). The relative integrations of the three major peaks are about 1 : 2 : 1 and their broadness and peak shape suggest there are further dynamic processes that cannot be frozen out at this limiting temperature. The benzylic region of the spectrum also shows multiple resonances, one of which is associated with the minor hydride resonance at −10.5 ppm, by EXSY spectroscopy (Fig. S5†). In the ³¹P NMR spectrum acquired at 198 K, two resonances at 97.1 and 90.0 ppm appear in a 6 : 1 ratio (Fig. S6†).

We interpret this (limited) spectral information as depicted in Scheme 2. The data suggest that the polyhydride compounds are best formulated as (PC_alkylP)Ru(II) hydrido/η²-dihydrogen complexes which may exist as two isomers (present in an approximately 6 : 1 ratio) delineated by the disposition of the terminal Ru hydride relative to the benzylic hydrogen across the Ru–C bond of the PC_alkylP ligand.¹⁸T₁ values at 298 K for the broad hydride resonances of L_HRRu(H)₃ were determined using inversion recovery experiments, giving values of 128 ms (R = H) and 61 ms (R = NMe₂). We were unable to determine T₁(min) values but the values obtained at room temperature are low for a purely classical hydride species (usually >350 ms) providing indirect evidence for some dihydrogen character in these systems.^33,34 The hydrogen atoms in these isomers are in dynamic exchange, likely involving intermediates L_H2RRu(H)₂, L_HRRu(H)₃, L_RRu(H₂)₂, or L_RRu(H)₂ as shown in Scheme 2. The nature of the minor species with a hydrido resonance at ≈−10.5 was elucidated by attempts to grow crystals of either trans-L_HRRu(H₂)H or cis-L_HRRu(H₂)H at low temperatures. Both isomers were highly soluble in aromatic and hydrocarbon solvents under H₂ and could not be coaxed out of solution under any conditions. However, exposure of these solutions to partial vacuum did lead to deposition of X-ray quality crystals, which turned out to be the dimeric species L_HHRu(H)(μ-H)₂(H)RuL_HH depicted in Scheme 2 and shown in Fig. 1. We propose that this compound forms upon loss of H₂ from cis/trans-L_HRRu(H₂)H and subsequent dimerization of the resulting unsaturated L_HRRuH species with (formally) intermediate L_HRRu(H)₃. Hydride locations were assigned based on residual electron density in open coordination sites suggested by the geometry of the ligand. For example, the P(1)–Ru(1)–P(2) angle of 119.12(6)° is consistent with a PC_alkylP ligand and accommodates a coordination site for a terminal hydrido ligand. The Ru1–Ru2 distance of 2.5714(7) Å is consistent with other hydride-bridged ruthenium dimers.^35,36 The ¹H NMR spectra of these crystals exhibited a doublet of triplets at −10.45 ppm and an H–C–Ru proton at 5.20 ppm, confirming the nature of this species (which appears in varying amounts from sample to sample) in the spectra of cis/trans-L_HRRu(H₂)H. Finally, it should be noted that exposure to 4 atm of H₂ gas results in loss of these signals as this dimer is converted back to cis/trans-L_HRRu(H₂)H.

Scheme 2 Speciation of L_HRRu(H)_n complexes under H₂, vacuum and N₂.

Fig. 1 Thermal ellipsoid diagram of L_HHRu(H)(μ-H)₂(H)RuL_HH with thermal ellipsoids drawn at 50% probability level. Ligand H atoms omitted for clarity; hydride ligands were unambiguously located from difference Fourier maps and isotropically refined. Selected bond lengths [Å] and bond angles [°]: Ru1–Ru2: 2.5714(7), Ru1–C13: 2.182(7); P1–Ru1–P2: 119.12(6), C13–Ru1–Ru2: 123.28(18).

Further support for the speciation map proposed in Scheme 2 arises from attempts to grow crystals of the green colored crude product mixture formed upon complete removal of H₂ atmosphere and solvent from cis/trans-L_HNMe2Ru(H₂)H. Here, long term recrystallizations lead to crystals that turned out to be the dinitrogen bridged dimer L_HNMe2Ru(H)(μ-N₂)(H)RuL_HNMe2 (see Fig. 2 for its molecular structure). This species formed in an argon filled box, picking up adventitious N₂ over the course of several days and underscoring the high affinity of the L_HNMe2RuH fragment for dinitrogen. This compound was confirmed by its intentional in situ synthesis by exposing cis/trans-L_HNMe2Ru(H₂)H to an atmosphere of dry N₂. This dinitrogen complex has a characteristic signal in the ³¹P NMR spectrum at 74.6 ppm. The rather short N(3)–N(3′) distance of 1.0892(95) Å indicates that the dinitrogen is relatively unactivated and essentially retains an N–N triple bond. Consistent with this, it is easily displaced by dihydrogen to return to clean samples of cis/trans-L_HNMe2Ru(H₂)H.

Fig. 2 Thermal ellipsoid diagram of L_HRRu(H)(μ-N₂)(H)RuL_HR with thermal ellipsoids drawn at 50% probability level. Ligand H atoms and disorder in the isopropyl groups omitted for clarity; hydride ligands were unambiguously located from difference Fourier maps and isotropically refined. Selected bond lengths [Å] and bond angles [°]: Ru1–C1: 2.0576(67), N3–N3′: 1.0892(95), Ru1–N3–N3′: 173.403(643).

Given that ruthenium polyhydride complexes are among the most active molecular catalysts for CO₂ hydrogenation,⁴ we evaluated the reactivity of PC_alkylP hydrides L_HRRuH with CO₂ (Scheme 3). In the products L_RRu(H)-κ²-O₂CH, the PC_carbeneP ligand is restored, as indicated by the absence of benzylic C–H groups in the ¹H NMR spectra, and the appearance of resonances at 275.6 (R = H) and 274.5 (R = NMe₂) ppm for the carbene carbons in the ¹³C NMR spectra. In addition, the formate carbons resonate at 170.9 and 170.4 ppm, and triplets for the terminal hydrido ligands are seen at −8.99 and −11.14 ppm. The nature of these products was further confirmed by their separate synthesis via treatment of the hydrido chlorides L_RRu(H)Cl with sodium formate as depicted in Scheme 3. The compound L_NMe2Ru(H)-κ²-O₂CH was also characterized by X-ray crystallography (Fig. 3) and the more planar tricyclic ligand backbone along with a short Ru(1)–C(13) distance of 1.890(9) Å confirms the presence of a PC_carbeneP ligand in these complexes.

Scheme 3 CO₂ insertion reactions and syntheses of L_RRu(H)-κ²-O₂CH.

Fig. 3 Structure of L_NMe2Ru(H)-κ²-O₂CH, with thermal ellipsoids drawn at 50% probability level. Unit cell contained 3 molecules of L_NMe2Ru(H)-κ²-O₂CH, only one shown for clarity. Ligand H atoms also omitted for clarity; hydride ligand was unambiguously located from difference Fourier maps and isotropically refined. Selected bond lengths [Å] and bond angles [°]: Ru1–C13: 1.890(9), Ru1–O1: 2.346(8), Ru1–O2: 2.265(9), O1–C33: 1.247(13), O2–C33: 1.56(12), C33–H33: 0.9504(92), O1–C33–O2: 127.8(11), C13–Ru1–O1: 173.3(4), C13–Ru1–O2: 117.7(4).

We presume that formation of these formato hydrides occurs via CO₂ insertion into the terminal hydride of the complexes L_HRRuH, followed by a 1,2-hydride shift from the anchoring alkyl group to the metal. Indeed, for R = H, the presumed PC_alkylP Ru(II) formato intermediate is observed spectroscopically (C–H benzylic proton at 5.55 ppm, ³¹P resonance at 65.8 ppm); the rate limiting 1,2-hydride shift to produce L_HRu(H)-κ²-O₂CH, occurs over the course of about 30 minutes. For R = NMe₂, however, the 1,2-shift is extremely rapid, and the PC_alkylP intermediate could not be detected. We hypothesize that for this more electron rich ligand, the step is driven by the restoration of conjugation of the π-electrons of the donating dimethyl amino groups with the carbene moiety that forms as the 1,2-shift proceeds. This is further evidenced by the observation that dissolution of these two complexes in acetonitrile indicates that, for R = H, the ruthenium hydride shifts back to the carbene carbon (5.70 ppm in the ¹H NMR spectrum, Fig. S7†) and forms an acetonitrile adduct (Scheme 3). For R = NMe₂, however, no reaction with acetonitrile is observed, indicating that for this species the 1,2-hydride shift is essentially irreversible. This is supported by the observation that when the headspace of NMR samples of these compounds is removed via freeze–pump–thaw routines, L_NMe2Ru(H)-κ²-O₂CH is indefinitely stable, while L_HRu(H)-κ²-O₂CH loses CO₂ over the course of about an hour, generating solutions of “L_HHRuH” that convert quantitatively to L_HHRu(H₂)H upon exposure to dihydrogen. Thus, the ability to open a coordination site via this transfer of a Ru–H to the carbene carbon renders the CO₂ insertion reversible in the less electron donating R = H ligand system, as is observed in other ruthenium-based CO₂ hydrogenation catalysts.³⁷

We also observed exchange of inserted CO₂ in the formato ligands of L_RRu(H)-κ²-O₂CH with free CO₂. Interestingly, the rate of this exchange of free CO₂ with the inserted carbon dioxide follows the opposite trend in that the R = NMe₂ system exchanges CO₂ faster than the parent R = H system. This was probed by preparing ¹³C labeled derivatives (using ¹³CO₂via the method shown in Scheme 3) and exposing solutions of the labelled L_RRu(H)-κ²-O₂¹³CH materials to various partial pressures of unlabelled ¹²CO₂. An observed pseudo first order rate constant (k_obs) could be obtained by monitoring the loss of ¹³C label in the formato ligand via¹H NMR spectroscopy. As shown in Fig. 4, the rate of label loss is about 3–4 times faster for the R = NMe₂ derivative and increases for both compounds as the partial pressure of ¹²CO₂ rises, tending towards saturation as the solution concentration of CO₂ in benzene plateaus. We interpret these results as being consistent with either an “inner sphere” or an “outer sphere” mechanism³⁸ as depicted in Scheme 4. The inner sphere process assumes that CO₂ insertion into the remaining hydride requires an open coordination site and slippage of the κ² formato to a κ¹ variant would be required to form L_RRu(H)-κ¹-O₂¹³CH. The equilibrium concentration of this intermediate should be higher for R = NMe₂ than for R = H, since the more electron donating former ligand should stabilize the unsaturated κ¹ species, increasing the rate of bimolecular CO₂ insertion to form the intermediate I (Scheme 4, bottom) from which exchange of label occurs. Loss of CO₂ from this species is rapid and can be either labeled or unlabeled, but due to the excess of ¹²CO₂ in the system, eventually the ¹³CO₂ label is washed out of the compounds. In addition to enabling higher concentrations of the key L_NMe2Ru(H)-κ¹-O₂¹³CH intermediate, the ruthenium hydride is likely more hydridic in the R = NMe₂ system possibly leading to further rate enhancements in the bimolecular step for this more electron rich ligand. An outer sphere process^39,40 (Scheme 4, top) wherein direct bimolecular hydride transfer from formato hydrides L_RRu(H)-κ²-O₂¹³CH to ¹²CO₂ occurs cannot be ruled out but would also favor the more hydridic R = NMe₂ derivative.

Fig. 4 The rate of exchange of ¹³CO₂ for ¹²CO₂ in compounds L_HRu(H)-κ²-O₂¹³CH plotted against the partial pressure of ¹²CO₂. Experiments were performed in triplicate to obtain the errors in the measurement of k_obs.

Scheme 4 CO₂ exchange in complexes L_RRu(H)-κ²-O₂CH.

Given its observed facile reactivity with CO₂, we evaluated the efficacy of polyhydride species L_HNMe2Ru(H)₃ as a carbon dioxide hydrogenation catalyst. Unsurprisingly, the formato hydride L_NMe2Ru(H)-κ²-O₂CH does not react with dihydrogen, and a basic additive was employed to generate a formate salt, providing a favorable thermodynamic impetus for reduction of CO₂.^4,5,37,41,42 Optimization of catalytic conditions led us to carry out catalytic reactions with 0.64% catalyst loading in dioxane/water mixtures (2 : 1 ratio), at 70 °C using NaOH as the basic additive (K₂CO₃ or NEt₃ did not lead to significant conversion of CO₂ to formate). Catalytic reactions were conducted with a 1 : 1 mixture of CO₂ and H₂ at pressures ranging from 1 to 40 atmospheres. Turnover numbers (TON) were determined via NMR integration of the C–H formate signal in NaO₂CH by diluting 0.20 mL aliquots of catalyst solution to 1.0 mL in D₂O and adding 5.0 mg (0.035 mmol) of sodium benzoate as an internal standard.³ In all cases it was found that turnovers were much lower (<10) than expected for the catalyst loading employed and in comparison to related catalyst systems (Table S1†). Furthermore, in most catalyst runs, a red precipitate was observed to form under catalytic conditions. Single crystals that were retrieved from these precipitates were subjected to X-ray analysis, which showed them to be the μ-κ²-κ¹ carbonato dinuclear compound [L_NMe2Ru(H)]₂(μ-κ²-κ¹-CO₃) (see Fig. 5). This species probably forms via insertion of CO₂ into ruthenium hydroxo complexes present in the catalytic reaction, and while this doesn't necessarily preclude catalytic turnover if this insertion is reversible,^37,43 here the apparent insolubility of this dinuclear species has the effect of suppressing catalytic turnover by removing catalyst from the system.

Fig. 5 Thermal ellipsoid diagram of [L_NMe2Ru(H)]₂(μ-κ²-κ¹-CO₃) with thermal ellipsoids drawn at 50% probability level. Ligand H atoms and disorder around bridging atoms omitted for clarity. Selected bond lengths of (Å) and bond angles (°): Ru1–C13: 1.9031(5), Ru1–O1: 2.18(1), Ru1–O2: 2.256(8), Ru1–O3: 2.0423(115), C33–O1: 1.51(2), C33–O2: 1.157(14), C33–O3: 1.17(2). C13–Ru1–O1: 171.3(3), C13–Ru1–O2: 112.2(3), C13–Ru1–O3: 150.1(3).

Conclusions

The coordination chemistry of two rigid, β-elimination immune PCP pincer ligands developed in our labs on ruthenium has been developed with the goal of making polyhydride complexes for activating and hydrogenating carbon dioxide. Herein we have fully characterized the L_RRu(H)Cl precursors and generated the highly dynamic L_HRRu(H)(H₂) polyhydrides, where the anchoring carbene ligand has accepted a hydrogen atom. For both ligands, these systems are highly dynamic, and we have mapped the speciation in some detail using variable temperature, multinuclear NMR spectroscopy and X-ray crystallography. While these hydrido complexes rapidly react with CO₂ to form the formato hydrides, the more electron rich example, L_NMe2Ru(H)-κ²-O₂CH, when tested under standard CO₂ hydrogenation conditions showed that catalyst deactivation occurs through the formation of insoluble bridging carbonato dinuclear compound after about 10 turnovers. Further catalyst development could involve adding steric bulk to the alkyl substituents on the P donors of the pincer ligand framework to prevent formation of dinuclear compounds. An alternate research direction would be to test the polyhydrides as catalysts for the dehydrogenation of formic acid; preliminary results indicate high activity for this process and these studies will be the focus of a future report.

Author contributions

The manuscript was prepared with contributions from LJD and WEP. CYC, JBL and BSG performed X-ray crystallographic analysis on the complexes reported.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was funded by the Natural Sciences and Engineering Research Council of Canada through a Discovery Grant to WEP. WEP also acknowledges and thanks the Canada Research Chairs program for a Tier I CRC (2020-2027). LJD thanks NSERD for a PGS-D award and the Province of Alberta for and Alberta Graduate Excellence Scholarship (AGES).

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Footnote

† Electronic supplementary information (ESI) available: General Experimental details, synthesis and characterization of all new compounds, dynamic NMR studies and crystallographic parameters. CCDC 2309264–2309271. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt04014c

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