Probing Spin Crossover in a Solution by Paramagnetic NMR

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Cite This: Inorg. Chem. 2017, 56, 14759−14762

Probing Spin Crossover in a Solution by Paramagnetic NMR Spectroscopy Alexander A. Pavlov,† Gleb L. Denisov,† Mikhail A. Kiskin,‡ Yulia V. Nelyubina,† and Valentin V. Novikov*,† †

Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova str. 28, 119991 Moscow, Russia Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Leninskii prosp. 31, 117901 Moscow, Russia

‡

S Supporting Information *

Another option is to use the difference in the chemical shifts of the NMR signals for the LS and HS species.16 This is done by acquiring multiple NMR spectra at different temperatures and then fitting the observed dependence of the chemical shifts accounting for deviation from the Curie behavior17 caused by the LS complex. The SCO parameters thus obtained are at least as accurate as those from the Evans method;16 if those are already known, the chemical shifts may even be used to measure the temperature noninvasively in magnetic resonance imaging applications.18 While very convenient, this approach is only applicable to SCO compounds with a diamagnetic LS state, and its results should be treated with caution if deviation from the Curie behavior originates from the HS state, which is often the case of the compounds with large magnetic anisotropy.19 Here we propose an alternative NMR-based technique to probe the SCO behavior in solution. It is valid for compounds with both paramagnetic and diamagnetic LS states and avoids the need to isolate or purify them or to know their concentrations beforehand, with the only requirement being the possibility of recording their NMR spectra. The proposed approach is also immune to a possible presence of paramagnetic impurities in the sample because they influence the chemical shifts of all of the nuclei in the same way. If referenced to the residual solvent signal, which is a common practice in NMR spectroscopy, these chemical shifts are not affected by an admixture of any unwanted paramagnetic species. The presence of a paramagnetic ion [such as cobalt(II)] results in paramagnetic shifts of the signals in the NMR spectrum of the compound:

ABSTRACT: Spin transitions in spin-crossover compounds are now routinely studied in the solid state by magnetometry; however, only a few methods exist for studies in solution. The currently used Evans method, which relies on NMR spectroscopy to measure the magnetic susceptibility, requires the availability of a very pure sample of the paramagnetic compound and its exact concentration. To overcome these limitations, we propose an alternative NMR-based technique for evaluating spinstate populations by only using the chemical shifts of a spin-crossover compound; those can be routinely obtained for a solution that contains unknown impurities and paramagnetic admixtures or is contaminated otherwise. ince the first report in 1931,1 transition-metal complexes that undergo spin crossover (SCO) have become a focus of attention from the scientific community2,3 because of the possibility of using the energy difference between the low-spin (LS) and high-spin (HS) states of a transition-metal ion to create a molecular switch.4,5 Particularly suited for this application are complexes showing an abrupt SCO with thermal hysteresis, which is usually observed in a solid statewhere it relies on cooperative intermolecular interactions6and is easily followed by direct-current magnetometry. A temperature-induced SCO in solution is always gradual because of its statistical nature (however, there are other external stimuli for switching the spin state of isolated molecules7−10), and the methods that can be used to identify it are rather few. They either need high concentrations (such as solution magnetometry) or require low temperatures (such as electron paramagnetic resonance spectroscopy of cobalt(II) complexes11) and even specific ions (such as Mossbauer spectroscopy of iron compounds12). No such restrictions apply to nuclear magnetic resonance (NMR) spectroscopy, which is at the core of the Evans method13 proposed to measure the magnetic susceptibility of paramagnetic compounds in solution by comparing the chemical shift of a solvent (or another inert substance) relative to a pure solvent for which the NMR signal is simultaneously recorded. This popular approach only requires access to an NMR spectrometer, which is a tool generally available for chemists, but unfortunately suffers from limited precision because the concentration of the paramagnetic compound should be exactly known and the solution should be very pure.14,15

S

© 2017 American Chemical Society

δobs = δdia + δC + δ PC

(1)

δobs is the observed chemical shift (ppm), δdia is the diamagnetic chemical shift, δC is the contact contribution that arises from spin polarization conveyed through molecular orbitals, becoming negligible at a distance of 5−6 chemical bonds, and δPC is the pseudocontact contribution that arises from dipolar coupling between the magnetic moments of a nucleus and of an unpaired electron.20−23 Of the three contributions to the observed chemical shift, the value of the contact shift δC follows straightforwardly from the spin-density distribution (see the Supporting Information for computational details) readily accessible by density functional Received: October 16, 2017 Published: December 5, 2017 14759

DOI: 10.1021/acs.inorgchem.7b02649 Inorg. Chem. 2017, 56, 14759−14762

Communication

Inorganic Chemistry theory (DFT) calculations. The pseudocontact contribution δPC in the simplest axial case depends on the anisotropy of the magnetic susceptibility tensor Δχax, which is much harder to access computationally. It, however, can be reliably estimated21 by fitting the observed chemical shifts to the known structure of the compound by eq 2: i δ PC =

1 Δχax (3 cos2 θi − 1) 3 12πri

Nickel(II) complexes with the d8 configuration are paramagnetic, with the HS state (S = 1) alone being populated; because they are reported to have very low magnetic anisotropy,21 only diamagnetic and contact contributions to the paramagnetic shift are nonnegligible, thus providing an estimate of how well the DFT calculations reproduce the values δi,HS C . Finally, the cobalt(II) complexes are known for their SCO behavior28 and a significant (axial) magnetic anisotropy of their HS state,29 which implies a large pseudocontact contribution δiPC. After all other contributions are accounted for, this gives us an ultimate opportunity to test the performance of our approach (see the Supporting Information for its step-by-step procedure) for evaluating the populations of the two spin states in solution. Because these complexes are chemically stable, the implementation of the Evans method is also straightforward.30 Assignment of the signals in the NMR spectra to the diamagnetic iron(II) complexes was aided with 2D NMR spectroscopy (Figures S1−S6). To estimate the paramagnetic shifts (δiC + δiPC), assignment of the NMR signals may be assisted with the same 2D NMR pulse sequences21 or with analysis of their line widths.16 If, however, there is no pseudocontact contribution [as in our nickel(II) complexes], they are easier to access computationally by DFT calculations of the isotropic values of the hyperfine tensors.31,32 The latter provide the estimates of the contact shifts δiC. Summing them up with the diamagnetic contribution δidia, taken as the measured chemical shifts for the iron(II) complexes, results in chemical shifts that agree very well (Figures 2 and S19−S23) with those observed experimentally (Figures S7−S12).

(2)

θi and ri are the polar coordinates of the nuclei in the coordinate frame of the magnetic susceptibility tensor. For 3d metal complexes, the value of Δχax may be quite large owing to the large zero-field-splitting energy sometimes found in the HS complexes.24 For the LS state, the pseudocontact contribution is negligible in most cases.25 In contrast, the diamagnetic contribution to the chemical shifts δdia is virtually the same for the LS and HS states. If both of them are populated in a solution, the observed chemical shift δobs is a weighted average of those for the LS and HS species (ηLS and ηHS are their fractional populations) under an assumption of rapid exchange between the spin states in the case of a temperature-induced SCO:26,27 LS LS LS HS HS HS δobs = (δdia + δCLS + δ PC )η + (δdia + δCHS + δ PC )η

(3)

For the ith nuclei, it thus becomes ⎛ 1 i i δobs = δdia + δCi ,LS(1 − η HS) + ⎜δCi ,HS + Δχ 12πri 3 ax ⎝ ⎞ (3 cos2 θi − 1)⎟η HS ⎠

(4)

δidia

The values are directly measured by NMR spectroscopy of a suitable diamagnetic analogue, and the contact contributions i,HS δi,LS C and δC are accessible by DFT calculations, together with the geometrical parameters θi and ri. Therefore, there are only two variables, ηHS and Δχax, in eq 4 that need to be fitted simultaneously for all of the nuclei of the complex. In the case that the magnetic susceptibility tensor is rhombic, an additional variable Δχrh appears (and possibly other parameters describing the orientation of the tensor). This, however, does not limit the applicability of the proposed approach as long as the amount of the experimental data (i.e., the number of signals in the NMR spectra) exceeds the number of variables to fit. To put this approach to the test, we chose a series of 3d metal complexes with substituted terpyridine ligands (Figure 1). Among them, iron(II) complexes with the d6 configuration are diamagnetic and can be used as a source of the values δidia.

Figure 2. Experimental (δobs) versus theoretical (δdia + δC) 1H chemical shifts for Ni(pytpy)2.

For the cobalt(II) complexes, however, this approximation failed badly (Figure 3 and S13−S15), irrespective of whether the contact contribution was calculated for the LS state (S = 1/2) or for the HS state (S = 3/2). Taking into account the pseudocontact contribution that is nonnegligible for the HS state of the cobalt(II) ionby fitting multiple NMR spectra to eq 2also did not improve the correlation with the experimentally observed chemical shifts (Figures 3 and S24 and S25). Such behavior is strongly suggestive of an SCO. Indeed, both the Evans method (Figures 3 and S24 and S25) and the temperature dependence of the chemical shifts in the NMR spectra unambiguously show that all of the cobalt(II) complexes undergo an SCO in the temperature range studied (240−330 K). In the latter case (Figures S16−S18), it follows from a significant

Figure 1. Terpyridine-based iron(II), nickel(II), and cobalt(II) complexes. 14760

DOI: 10.1021/acs.inorgchem.7b02649 Inorg. Chem. 2017, 56, 14759−14762

Communication

Inorganic Chemistry

term, Δχrh, by accounting for exchange interactions in polynuclear compounds, or even by going beyond the pointdipole model.36 To apply it, one only needs a routine NMR spectrometer to perform 1D measurements (because high resolution and sensitivity are not required), a desktop computer to run a rather basic DFT calculation, and a few milligrams of the paramagnetic sample. Isolation of the SCO-active compound or of its diamagnetic analogue (which may be a free ligand) and even knowledge of their exact concentrations are optional. Because a temperature-induced SCO in solutions is governed by the Boltzmann statistics, its thermodynamic parameters (such as ΔH and ΔS) can be conveniently used for the simultaneous fitting of the NMR data at different temperatures, thus decreasing the number of variables even further. Finally, because the anisotropy of the magnetic susceptibility tensor depends on the electronic structure37 of the compound, namely, its g tensor and zero-field-splitting energy (accessible by computational or experimental methods24,38−40), NMR-based analysis of the SCO complexes may overcome the limits of a simple Bleaney model41 and evolve into a model-free approach.

Figure 3. Experimental versus theoretical 1H chemical shifts for Co(pytpy)2 at room temperature for the LS (black ■) and HS (blue ▲) states and after accounting for SCO (red ●).

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Curie-law-like20 increase in the paramagnetic shifts upon lowering of the temperature and nonuniform changes in the chemical shifts of nuclei depending on their position in the complexes. Therefore, the population of both spin states (LS and HS) has to be taken into account. The corresponding fit with eq 4, which resulted in the values of Δχ and ηHS at each temperature, led to a much better agreement between the experimental and calculated chemical shifts (Figures 3 and S24 and S25) for all of the cobalt(II) complexes. The obtained populations of the HS state (ηHS) at different temperatures closely mirror the SCO curve produced by the Evans method (Figures 4 and S26 and S27), thus confirming the

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02649. Supplementary methods, figures, and references (PDF) Accession Codes

CCDC 1565748−1565750 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yulia V. Nelyubina: 0000-0002-9121-0040 Valentin V. Novikov: 0000-0002-0225-0594 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the Russian Science Foundation (Project 17-13-01456). Synthesis of the complexes was supported by RFBR (16-03-00688) and CPRF (MK-6320.2016.3). X-ray diffraction data have been obtained at the Center for Molecular Composition Studies of INEOS RAS.

Figure 4. SCO curve as obtained from the Evans method (black ●) and from the proposed approach (red ●) for Co(pytpy)2.

validity of the proposed approach; SCO curves of previously studied cobalt(II) complexes behave in a similar way in the liquid NMR temperature range.33,34 Also note that, unlike the Evans method, our approach does not need prior knowledge of the magnetic moment for the HS state, which is not always possible to gain. The proposed approach rooted in the paramagnetic NMR spectroscopy thus emerges as a powerful and universal technique for probing an SCO behavior of 3d metal complexes in solutions; it can be easily adapted to other magnetic behaviors, e.g., by including the magnetic anisotropy of the LS state35 or a rhombic

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b02649 Inorg. Chem. 2017, 56, 14759−14762

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b02649 Inorg. Chem. 2017, 56, 14759−14762