Max
Mörtel
a,
Stephen J.
Goodner
a,
Johannes
Oschwald
b,
Andreas
Scheurer
a,
Thomas
Drewello
b and
Marat M.
Khusniyarov
*a
aDepartment of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Egerlandstraße 1, 91058, Germany. E-mail: marat.khusniyarov@fau.de
bDepartment of Chemistry and Pharmacy, Friedrich-Alexander University Erlangen-Nürnberg (FAU), Egerlandstraße 3, 91058, Germany
First published on 24th January 2024
Cobalt dioxolenes are a well-known class of switchable coordination compounds showing intramolecular electron transfer, which is always accompanied by a spin state change at the cobalt center. Here, we present the first example of thermally switchable cobalt bis-dioxolenes where intramolecular electron transfer seems to take place, but the spin state change is suppressed. This leads to the detection of thermal transition between a common ls-CoIII(SQ˙−)(Cat2−) and an extremely rare ls-CoII(SQ˙−)2 electronic state (hs – high-spin, ls – low-spin, SQ˙− – benzosemiquinonate(1−) radical and Cat2− – catecholate(2−)). Parallel to the present work, a similar work but on cobalt mono-dioxolenes has just appeared (Chem. Eur. J., 2023, 29, e202300091), suggesting thermal transition between ls-CoIII(Cat2−) and ls-CoII(SQ˙−) electronic states.
Scheme 1 Switchable cobalt bis-dioxolenes; note that mono-dioxolenes contain only one redox-active dioxolene ligand per molecule. |
Recently, we suggested the working principles of liquid-phase chemosensors based on VT cobalt dioxolenes and demonstrated the first prototype system.12 When weakly bound trans-4-styrylpyridine (stypy) ligands in the prototype sensor [Co(diox)2(stypy)2] 1 (Scheme 2) were replaced by different analytes – weakly and strongly coordinating ligands – VT equilibrium between hs-CoII(SQ˙−)2 and ls-CoIII(SQ˙−)(Cat2−) redox-isomers in 1 was shifted, resulting in a significant colorimetric and magnetic response at room temperature (hs – high-spin, ls – low-spin, diox – dioxolene ligand of this work at an unspecified oxidation level, SQ˙− – benzosemiquinonate(1−) radical and Cat2− – catecholate(2−)).12
By expanding the scope of analytes in this work, we studied the sensing of cyanide anions by 1, which has brought unexpected and highly interesting results. Upon reaction of 1 with 1 eq. of cyanide, a unique ls-CoII(SQ˙−)2 species 2 has been generated in situ (Scheme 2). Complex 2 shows an extremely rare thermally driven VT transition between two ls states: ls-CoII(SQ˙−)2 ⇄ ls-CoIII(SQ˙−)(Cat2−). To the best of our knowledge, the transition between two ls states in cobalt dioxolenes has been just reported by Krüger et al. for the very first time.13 Based on extensive analysis of electronic absorption spectra, they suggested detecting ls-CoII(SQ˙−) ⇄ ls-CoIII(Cat2−) transition in a series of VT mono-dioxolene complexes featuring one dioxolene ligand per complex. We became aware of this work only during the referential process. In our work, we deal with a bis-dioxolene system featuring two dioxolene ligands and rely on magnetic measurements, IR and EPR spectroscopy. Our original data deposited to ChemRxiv before the current submission can be found online.14
(1) |
(2) |
All solutions have been prepared under inert conditions.
Titration experiments monitored by UV-VIS-NIR spectroscopy, Evans NMR spectroscopy, and mass spectrometry were performed by keeping the initial concentration of the cobalt complex constant. A stock solution of the complex (c = 5 × 10−4 M) in DCM was prepared in a 50 mL volumetric flask. Another stock solution of the analyte salt (c = 3 × 10−2 M) in MeCN was prepared separately. For each titration point, the necessary equivalents of the analyte solution required for 3 mL of the complex solution were transferred to a vial. The solvent was carefully removed in vacuo, leaving behind the neat salt. Then, 3 mL of complex solution was added using an Eppendorf® pipette to achieve a solution with the desired amounts of analyte equivalents. For Evans titration experiments a DCM:DCM-d2:TMS = 10:2:1 mixture was used. For each titration point, an individual reference was prepared, which contained the same amount of analyte as the complex solution under investigation. Consequently, the diamagnetic contribution of the analyte and the solvent mixture has been automatically corrected.
For both IR and EPR spectroscopy, a stock solution of the complex (c = 7.5 × 10−3 M) in DCM was prepared in a 50 mL volumetric flask. Another solution of the analyte salt TBACN (c = 9.0 × 10−2 M) in DCM was prepared separately. The necessary aliquot of the analyte for each titration point was diluted with different aliquots of DCM, keeping both the concentration of the complex and the final volumes constant. The aliquot of the analyte was added dropwise to the solution of the complex and stirred slightly for 2–3 minutes. Samples for the reference complex (TBA)2[Co(tBu-dioxolene)(CN)4] were prepared by making a solution of the complex with the same concentration as the solutions for each data point (IR: c = 2.5 × 10−3 M; EPR: c = 1.0 × 10−3 M).
For time-dependent DFT, hybrid B3LYP26,27 was used. The same basis sets as for geometry optimization runs were used. The calculations were sped up by using RIJCOSX approximation with appropriate def2/J auxiliary basis sets.24 The first 100 excited states were calculated in each case.
To simulate the evolution of electronic absorption spectra upon titrations, we have calculated the electronic absorption spectra of complexes that should be present in solution at the starting and end points of titration and then calculated their weighted sum. Typical compositions correspond to ratios of 10:0, 8:2, 6:4, 4:6, 2:8, and 0:10 between two species. The calculated electronic spectra to simulate titrations were 3 = [Co(diox)(CN)4]2− dianion in the ls-CoIII(SQ˙−) state (S = 1/2) and 2five = [Co(diox)2(CN)]1− anion in the ls-CoII(SQ˙−)2 state (S = 3/2). Two cases were considered for parent 1. Since 1 presents as a mixture of states at RT, the electronic spectrum of 1 used for titration simulations was constructed as a weighted sum of a six-coordinate [Co(diox)2(stypy)2] in the ls-CoIII(SQ˙−)(Cat2−) state (S = 1/2, 37%) and a six-coordinate [Co(diox)2(stypy)2] in the hs-CoII(SQ˙−)2 state (S = 5/2, 63%). Since 1 is usually partially dissociated in solution at RT and the degree of dissociation in DCM at given concentrations is not known,17 we considered an alternative electronic absorption spectrum for 1 composed of the same six-coordinate [Co(diox)2(stypy)2] in the ls-CoIII(SQ˙−)(Cat2−) state (S = 1/2, 37%) and a five-coordinate [Co(diox)2(stypy)] in the hs-CoII(SQ˙−)2 state (S = 5/2, 63%). Both simulations provided similar results, whereas the simulation with five-coordinate species was slightly closer to the experimental data.
Molecular orbitals were visualized using Molekel.28 Calculated electronic absorption spectra were produced with orca_mapspc ORCA's tool by setting a constant linewidth of 1000 cm−1 for each transition.
Fig. 1 Electronic absorption spectrum of 1 in DCM (c = 5 × 10−4 M) titrated with TBACN (top: 0 to 1 eq., middle: 1 to 6 eq.); (*) is a spectrometer artefact, and (#) are due to solvent overtones. The evolution of absorption at given wavelengths (880 and 1330 nm) upon titration (bottom). See the ESI† for details. |
Since stypy ligands are weakly bound to the cobalt center in 1 (the first dissociation constant Kd,RT = Ka,RT−1 = 5.7(5) × 10−3 mol L−1) and therefore prone to substitution,12 we suggest the replacement of one stypy ligand by a strongly binding cyanide anion. Similar substitution reactions with other ligands have been observed previously.12 Distinct isosbestic points confirm a clear adduct-to-product conversion, suggesting the formation of a monocyanido species 2 in the first reaction. The absence of bands in the SWIR region points to its CoII(SQ˙−)2 electronic structure. Note that the fate of the second stypy ligand remains uncertain at this point.
As more TBACN is added (up to 4 eq.), the bands of 2 decrease in intensity, and the band at 535 nm becomes hypsochromically shifted to 505 nm, while the SWIR region remains featureless (Fig. 1). The gradual evolution of the spectrum provides no evidence for the formation of intermediate di/tricyanido species in any significant quantities. The spectra of solutions with >4 eq. are virtually identical, confirming that the second reaction is complete with 4 eq. of TBACN. Similarly, Wicholas et al. reported the exclusive formation of [ls-CoIII(SQ˙−)(CN)4]2− anions (accompanied by the loss of one dioxolene ligand) instead of a biscyanido complex, when the [hs-CoII(SQ˙−)2]4 tetramer was treated with 8 eq. of TBACN.16 We have independently synthesized this reference compound as the (TBA)2[ls-CoIII(SQ˙−)(CN)4] coordination salt 3. The electronic spectrum of 3 closely resembles the spectrum of 1 with 4 eq. of TBACN, which confirms the formation of 3 in the investigated solution (Fig. S8†). The apparent absence of particularly biscyanido species in this and previous16 work is surprising. With CN− showing both trans and cis effects in octahedral complexes, a single coordinated cyanide may activate the complex for subsequent substitution reactions.30,31
The magnetic properties of a solution of 1 upon titration with TBACN have been investigated by the Evans NMR method (Fig. 2). The χT product of 1 in DCM is 2.2(1) cm3 mol−1 K, which is in agreement with the co-presence of ls-CoIII(SQ˙−)(Cat2−) and hs-CoII(SQ˙−)2 redox-isomers: 37% and 63%, respectively. These numbers have been obtained using reference values of 0.357 and 3.25 cm3 mol−1 K for respective pure states, by fitting experimental data.17 Upon addition of TBACN, χT decreases and reaches a visible saddle point at 1.0(1) cm3 mol−1 K with 1 eq. of TBACN for in situ formed 2. More eq. of TBACN lead to a further drop of the χT value to 0.4(1) cm3 mol−1 K, which is in agreement with a spin-doublet state (S = 1/2) of tetracyanido species 3 (vide supra).
Fig. 2 Magnetic properties of 1 in DCM:DCM-d2:TMS (10:2:1) (c = 5 × 10−4 M) with different eq. of TBACN. |
The value of 1.0(1) cm3 mol−1 K measured for in situ formed 2 can, in principle, be explained as follows: Scenario A: species 2 is a classical VT complex, showing the co-presence of ls-CoIII(SQ˙−)(Cat2−) and hs-CoII(SQ˙−)2 redox-isomers at RT in solution (78 and 22%, respectively). Note that the major isomer would be ls-CoIII(SQ˙−)(Cat2−) in this case. Scenario B: complex 2 is a very rare ls-Co(II) species with an ls-CoII(SQ˙−)2 electronic structure. In the spin-only approximation of essentially uncoupled at RT magnetic centers, the χT value of 1.125 cm3 mol−1 K is expected for such species (3 × 0.375 = 1.125), which is close to the experimentally observed value of 1.0(1) cm3 mol−1 K. Scenario A is unlikely due to the absence of the LLIVCT band in SWIR/NIR, which is further supported by variable-temperature measurements (vide infra).
As our numerous attempts to isolate intriguing species 2 were unsuccessful (it seems that 2 is thermodynamically stable only in solutions, but becomes converted into thermodynamically stable solids 1 and 3), further challenging studies have been performed on 2 prepared in situ by adding 1.0 eq. of TBACN to 1.
Variable-temperature electronic absorption spectroscopy in DCM and MeCN solutions of 2 reveals similar temperature-dependent evolution of the spectra (Fig. 3 and S12,† for the color change see Fig. S21†). Starting at RT and upon lowering the temperature, all bands in the visible region decrease in intensity, which is typical of the thermal switching to the ls-CoIII(SQ˙−)(Cat2−) state at low temperatures.17,29,32,33 Unfortunately, our variable-temperature set-up does not allow us to probe the SWIR region at low temperatures. The thermal switching was fully reversible. Importantly, we were not able to observe any significant changes in the spectra upon heating above RT. Thus, scenario A, which implies that ls-CoIII(SQ˙−)(Cat2−) ⇄ hs-CoII(SQ˙−)2 VT equilibrium with the major ls-CoIII(SQ˙−)(Cat2−) component at RT, can be excluded, since large spectral changes due to thermal switching to hs-CoII(SQ˙−)2 would be expected in this case. Thus, 2 should be a unique ls-Co(II) species with the ls-CoII(SQ˙−)2 electronic structure.
Fig. 3 Temperature-dependent electronic absorption spectrum of in situ generated 2 in DCM (c = 5 × 10−4 M); (*) is a spectrometer artefact. |
The magnetic properties of 2 in solution have been investigated by the Evans NMR method at different temperatures. The χT product of 2 showed no significant changes upon heating from 298 to 313 K (Fig. S15†). This is in agreement with nearly unchanging UV-vis spectra at elevated temperatures (vide supra). Another sample of 2 was used for testing the low temperature region. The χT products of the two samples prepared independently are not identical at RT, which is, however, within the uncertainty of the method.
Upon lowering the temperature, the χT product of 1.25(10) cm3 mol−1 K at 298 K remains nearly constant up to 278 K, before falling sharply to 0.4(1) cm3 mol−1 K at 203 K (Fig. 4). The drop to 0.4(1) cm3 mol−1 K at low temperatures should be the thermal transition to a common ls-CoIII(SQ˙−)(Cat2−) state.34 Thus, 2 shows a unique VT transition between two different low-spin states, namely, ls-CoII(SQ˙−)2 ⇄ ls-CoIII(SQ˙−)(Cat2−). The thermodynamic parameters obtained by fitting available variable-temperature data are rather uncertain and slightly higher than those typically observed for classical VT cobalt-dioxolenes (see the caption to Fig. 4 and Table S1†).35 These deviations might be ascribed to the proposed unique electronic (and geometric) structure of 2 or might indicate some minor dissociation/association processes in solution.17,32 Thus, the determined parameters must be considered with care.
At RT, the parent 1 shows an 8-line signal at giso = 2.007, which is due to a ligand-based radical ls-CoIII(SQ˙−)(Cat2−) (Fig. S18†).17,32 The corresponding hs-CoII(SQ˙−)2 isomer, which is present at RT as well, is EPR inactive due to rapid relaxation at RT. Under similar conditions, 3 shows a 16-line signal of an organic-based radical at giso = 2.009, which is due to coupling to one 59Co nucleus (I = 7/2) and one aromatic proton of the SQ˙− ligand (Fig. S19†).16
The solution of in situ generated 2 is practically EPR-silent at RT, showing only very weak signals at g ∼ 2.0, which should be due to small amounts of unknown EPR-active species (by-product(s)) and traces of the unreacted 1 and the end-product 3. This is also expected for 2 with its unique electronic structure ls-CoII(SQ˙−)2, which implies very fast relaxation due to a cobalt-centered spin. This confirms that the electronic structure of 2 is very different from reference species 1 and 3.
We postulated and confirmed by UV-vis-NIR spectroscopy that tetracyanido species 3 is formed when 1 is reacted with 4 eq. of TBACN (vide supra). Now, we can further confirm this by EPR spectroscopy. The EPR spectra of 1 with 4 eq. of TBACN and reference 3 are very similar at RT, confirming the formation of 3 (Fig. S20†). However, a minor component might be hidden underneath the major 16-line signal of in situ formed 3, as can be judged from the slight disturbance in the hyperfine pattern. The presence of an unknown minor species in solution is also confirmed by IR spectroscopy (vide infra).
EPR-active impurities, detected in solutions of in situ formed 2 and 3, are actually to be expected. This is because substitution reactions in metal complexes with ligand radicals never proceed quantitatively according to our experience.
At low temperatures (∼100 K), frozen solutions of 1 and 3 show similar broad signals at g ∼ 2.0 due to the essentially ligand-based radicals ls-CoIII(SQ˙−)(Cat2−) and ls-CoIII(SQ˙−), respectively, the only thermally populated states at low temperatures (Fig. S22 and S23†).17 The hyperfine coupling pattern was not resolved under the given conditions in both cases.
The frozen solution of 2 shows a signal at gav = 2.001 with well-resolved (super) hyperfine coupling to one 59Co nucleus (Fig. 5). This 8-line spectrum is characteristic of the ls-CoIII(SQ˙−)(Cat2−) state with weak coupling to one 59Co nucleus,17 which, in turn, further supports our hypothesis on unique thermal VT transition from the ls-CoII(SQ˙−)2 to ls-CoIII(SQ˙−)(Cat2−) state in 2 upon cooling.
The EPR spectrum of 1 titrated with 4 eq. of TBACN shows a low-temperature spectrum with poorly but resolved hyperfine coupling to one 59Co nucleus, which generally resembles the spectrum of reference 3, for which the coupling was, however, not resolved in our hands (Fig. S24†). This discrepancy might be associated with different polarities of these two solutions. It is also interesting to note that the coupling is notably stronger for 2 than for in situ formed 3 (Fig. S25†).
Fig. 6 The evolution of the CN-stretching region of the IR spectrum of 1 titrated with TBACN (0…1 eq.) in DCM (c = 2.5 × 10−3 M). |
Further titration (1 to 4 eq. of TBACN) results in the complicated evolution of the spectrum (Fig. S28†). Although complete analysis is neither possible due to severe overlapping bands nor important for the goals of the present paper, we are able to make some conclusions. Firstly, the reaction 2 → 3 proceeds likely in several steps. The appearance of the band at 2110 cm−1, when 1.25 eq. of TBACN is added, suspiciously resembles the band of free TBACN (2111 cm−1), but is likely due to some other unknown species (vide infra). The spectrum with 4.0 eq. of TBACN resembles the spectrum of reference 3, however, with an additional band at 2112 cm−1. This indicates that unlike the clean 1 → 2 conversion, the subsequent 2 → 3 reaction probably competes with some other processes. All reactions are likely completed after 4 eq. of TBACN are added, since further addition of TBACN (5 and 6 eq.) results in the appearance of satellites of unreacted TBACN salt.
Fig. 7 Positive ion mode MS of 1 with (a) 0.0, (b) 0.5 and (d) 1.0 eq. of TBACN and the MS/MS of the ions with (c) m/z 1022 and (e) m/z 1109. |
The positive-ion mode spectra obtained for the reference species (TBA)2[Co(tBu-dioxolene)(CN)4] (3) is remarkably similar to the experimental results for 1 with 1.0 eq. of TBACN (Fig. S30†). This suggests the complete conversion of postulated labile species 2 into thermodynamically stable 3 during the ESI process. This is also in agreement with our numerous unsuccessful attempts to isolate 2 (see the Experimental section): all attempts resulted in a change of color and isolation of 3 only.
The analysis of the reaction mixture in the negative ion mode was, unfortunately, not particularly informative. The solutions of 1 with 0.5 eq. of TBACN did not yield any major signals that can be attributed to cobalt-containing ions (Fig. S31†). With 1 eq. of TBACN, a set of singly charged cobalt species with 1 to 4 cyanido ligands have been detected at relatively low intensities (Fig. S32†). As in the positive ion mode, no cobalt species containing stypy ligands have been detected, which might suggest complete displacement of stypy ligands in 1 upon reaction with 1 eq. of TBACN. Thus, in situ formed 2 is more likely a five-coordinate (TBA)[ls-CoII(SQ˙−)2(CN)] 2five and less likely a six-coordinate analog (TBA)[ls-CoII(SQ˙−)2(CN)(stypy)] 2six.
Note that certain biases and restrictions of ESI-MS should be considered: (1) only charged species can be detected, (2) the formation of charge carriers occurs through a dynamic electrospray process, which does not necessarily reflect equilibria in solution. Moreover, the solutions used for ESI-MS have to be much more dilute compared to those studied by other methods, which makes comparing ESI-MS results with other methods difficult.
Upon substitution of one stypy ligand in 1 with the CN− anion as in a hypothetical six-coordinate 2six, both S = 5/2 and S = 3/2 states become destabilized relative to the S = 1/2 state by 8.9 and 2.6 kcal mol−1, respectively (Fig. 8, see the ESI† for details). Since 1 shows a VT equilibrium between S = 1/2 and S = 5/2 states and no evidence for the population of the S = 3/2 intermediate spin state is available,32 the stabilization of the S = 1/2 state over all other states in 2six makes the thermal population of the S = 3/2 state, i.e. ls-CoII(SQ˙−)2, in the hypothetical 2six unlikely.
Fig. 8 Relative energy differences between the S = 1/2 state (set as zero for each species) and the states of higher multiplicity: OPBE-DFT + zero-point energy and thermal energy corrections, see the Experimental section and ESI† for further details. |
However, when the given substitution is accompanied by the loss of the second stypy ligand, thus giving a five-coordinate species 2five, the S = 5/2 state becomes similarly destabilized by 8.1 kcal mol−1, but the S = 3/2 state becomes stabilized by 0.1 kcal mol−1. Obviously, the stabilization of the S = 3/2 state is nearly negligible. However, the destabilization of the S = 5/2 state together with the nearly unchanged (small) relative energy of the S = 3/2 state might provide exactly the mechanism for accessing the unique S = 3/2 state observed experimentally for 2. Thus, our calculations suggest that 2 is likely a five-coordinate species (TBA)[ls-CoII(SQ˙−)2(CN)], i.e.2five. This can also be expected, since the already weak Co–stypy bond in 117 should be weakened further by the trans-effect of a newly introduced cyanido ligand in 2.31
For consistency, we calculated the spin energetics for [ls-CoIII(SQ˙−)(CN)4]2− species 3. Strong destabilization of both S = 5/2 and 3/2 states by 60.5 and 23.4 kcal mol−1, respectively, implies that only the S = 1/2 state should be thermally accessible here, which is also observed experimentally (Fig. S16†).
Further confirmations are obtained using time-dependent DFT calculations. Thus, we have calculated electronic absorption spectra for parent 1 (S = 1/2, 5/2), postulated 2five (S = 3/2 and 1/2), and reference species 3 (S = 1/2) (see the ESI†). Furthermore, we simulated the changes in the calculated absorption spectra upon reaction 1 → 2five and subsequently 2five → 3. The evolution of calculated absorption spectra (Fig. 9 and 10) seems to be surprisingly similar to the changes observed experimentally (Fig. 1).
Fig. 9 Simulated titration of 1 with TBACN (0…1 eq.): the evolution of the calculated electronic absorption spectrum upon the conversion of parent 1 (in green) to the postulated 2five (in purple). Method: TD-DFT, B3LYP, 1 = 37% ls-CoIII(SQ˙−)(Cat2−) + 63% hs-CoII(SQ˙−)2, and 2five = ls-CoII(SQ˙−)2, see the ESI† for further details. |
Fig. 10 Simulated titration of 1 with TBACN (1…4 eqs.): the evolution of the calculated electronic absorption spectrum upon the conversion of the postulated 2five (in purple) to tetracyanido species 3. Method: TD-DFT, B3LYP, 2five = ls-CoII(SQ˙−)2, and 3 = ls-CoIII(SQ˙−), see the ESI† for further details. |
Upon the first reaction (1 → 2five), the broad intense LLIVCT band of 1 at 1641 nm vanishes, whereas absorption in visible and NIR regions increases significantly, showing the development of three well-defined bands at 1030, 734, and 543 nm (Fig. 9). A similar pattern can be seen in the experimental data (Fig. 1). By taking into account the fact that 1 is also partially dissociated in solution at room temperature17 as well as the spectral signature of this dissociated species, the agreement between the theory and experiment becomes even better (Fig. S51†).
Our calculations confirm that the broad absorption of 1 in the SWIR region is indeed an LLIVCT band, which is trivial (see Fig. S42† for molecular orbitals). Furthermore, we can also probe the nature of the prominent absorption bands of 2five, which unsurprisingly show strong charge transfer character (see the ESI†).
Upon the second reaction (2five → 3), the absorption in the visible and NIR region decreases significantly at nearly all wavelengths except for the 400 nm region (Fig. 10). Exactly the same pattern is observed experimentally (Fig. 1). Thus, 2five is a much stronger chromophore than end-product 3, which is responsible for (may be suspicious at first sight, but now confirmed using calculations) the decrease of absorption at nearly all monitored wavelengths.
Finally, we would like to note that since all used methods confirm very strong binding of cyanide to 1 (at least the first eq.), thermally driven coordination/decoordination of cyanide in 2 can be safely excluded. However, the possibility of thermally driven coordination/decoordination of stypy ligands in the solution of 2 is difficult to exclude. Thus, the question whether the observed ls-CoII(SQ˙−) ⇌ ls-CoIII(Cat2−) VT transition is of pure molecular origin or (partially?) due to possibly coordinating stypy ligands remains open. In this context, we would like to mention that in a closely related recent publication by Krüger, a similar VT transition is of pure molecular origin.13
Footnote |
† Electronic supplementary information (ESI) available: General experimental methods, computational details and additional spectroscopy data. See DOI: https://doi.org/10.1039/d3dt03935h |
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