Valence–Tautomeric Interconversion in a Bis(dioxolene)cobalt

Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Valence−Tautomeric Interconversion in a Bis(dioxolene)cobalt Complex with Iminopyridine Functionalized by a TEMPO Moiety. Phase Transition Coupled with Monocrystal Destruction Alexey A. Zolotukhin,† Michael P. Bubnov,*,† Alla V. Arapova,† Georgy K. Fukin,† Roman V. Rumyantcev,† Artem S. Bogomyakov,‡ Alexander V. Knyazev,§ and Vladimir K. Cherkasov† †

G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, 49 Tropinina Str., GSP-445, 603950 Nizhny Novgorod, Russia ‡ International Tomography Center, Siberian Branch of the Russian Academy of Sciences, Institutskaya Str. 3a, Novosibirsk 630090, Russia § Lobachevsky State University of Nizhny Novgorod, Gagarin Prospekt 23/2, 603950 Nizhny Novgorod, Russia S Supporting Information *

Scheme 1. Synthesis of Complex 1

ABSTRACT: Iminopyridine modified by TEMPO nitroxide was utilized for the synthesis of an octahedral bis(osemiquinonato)cobalt complex. Variable-temperature magnetic susceptibility measurements detect a valence tautomeric transformation in the temperature range 200−300 K. A reproducible hysteresis loop of about 40 K width is observed on the magnetic moment temperature dependence in the transition region. Differential scanning calorimetry measurements confirm different temperatures of phase transitions accompanying a valence−tautomeric transformation upon heating and cooling. Attempts to study the structural changes associated with the valence− tautomeric transformation by single-crystal X-ray diffraction failed because of the crystal destruction taking place upon cooling from 220 K. The powder X-ray diffraction pattern indicated an essential change of the unit cell upon cooling from 240 K.

A single-crystal X-ray diffraction (XRD) study at room temperature indicated that the Co(1) atom in 1a (1a means complex 1 at room temperature; CCDC 1572143 for 1a and 1572142 for 1b) has a distorted octahedral coordination environment (Figure 1 and Text S3 and Table S1). The Co(1)−O(1,2,3,4) distances vary in the range of 2.0150(15)− 2.0747(16) Å in 1a. Such distances are typical for high-spin cobalt(II) bonded with two semiquinonato anion radicals.3 The iminopyridine ligand is neutral. The Co−N distances are

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alence−tautomeric (redox−isomeric) transitions, like spin crossover, are typically accompanied by distinct and reversible changes in the structural, magnetic, and optical properties. It is the resulting effective switchability of these properties that may be exploited in future molecule-based materials for display devices, data storage, sensors, and molecular electronics or spintronics. In this context, of particular interest are bistable materials that display a hysteretic valence− tautomeric transition around room temperature, with a wide hysteresis loop.1 Bis(dioxolene)cobalt complexes with diaza ligands are some of the most promising and actively exploring sorts of such bistable compounds. In this work, the iminopyridine functionalized by a 2,2′,6,6′-tetramethylpiperidin-1-oxyl moiety was utilized as the diaza ligand. The ligand [2,2,6,6-tetramethyl-4-[[(E)-pyridin-2ylmethylidene]amino]cyclo-hexyl]oxidanyl was synthesized as described in ref 2. Implicating this ligand in a reaction with tris(3,6-di-tert-butyl-o-benzosemiquinonato)cobalt results in a six-coordinated bis(o-semiquinonato)cobalt complex with 4-NTEMPO-iminopyridine (complex 1 in Scheme 1 and Texts S1 and S2). © XXXX American Chemical Society

Figure 1. Molecular structure of 1a. Thermal ellipsoids are drawn at the 20% probability level. The H atoms and methyl groups of the tert-butyl substituents are omitted for clarity. Received: October 11, 2017

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DOI: 10.1021/acs.inorgchem.7b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

additional single-crystal XRD experiments at two temperatures: 240 and 200 K during separate cooling and heating. The experiment at 240 K was successful, but below this temperature, crystal destruction was observed (Table S3 and video files). The lattice strain caused by the temperature change during the redox−isomeric conversion is known.7 It is connected with changes of the complex geometry, especially the volume. In such cases, encapsulation of the crystal in the polymeric matrix and deceleration of cooling is useful to solving this problem. In the case of complex 1, the change of the lattice parameters appears to be so large that the crystal breaks the surface of the epoxy matrix (Table S3 and video files). To confirm the critical changes of the lattice parameters upon cooling, the powder XRD experiment was performed. The XRD pattern recorded from the powder of complex 1 (obtained by grinding in a mortar) at room temperature coincides with the theoretical spectrum calculated from the single-crystal XRD experiment fulfilled at room temperature and remains unchanged upon cooling to 240 K. Further cooling to 200 K results in considerable reorganization of the crystal structure, which is reflected in the XRD pattern (Figure S1). Such types of perturbations are typical for phase transitions of the first order (according to P. Ehrenfest).8 To evaluate the energies of phase transitions of complex 1, the differential scanning calorimetry (DSC) experiment was performed in repeated mode (Figure 3). When the crystalline

2.1226(18) and 2.1586(18) Å, which agrees with the analogous bond lengths in related cobalt complexes containing semiquinonato ligands [2.057(2)−2.518(16) Å].4 The piperidine cycle has a chair conformation. The N(3)−O(5) bond length is 1.266(3) Å. Such values of the distances are characteristic of related compounds with the 2,2′,6,6′-tetramethylpiperidin-1oxyl radical fragment.5 The variable-temperature magnetic susceptibility of complex 1 measured at 2.0 K detects a magnetic moment of 1.7 μB (red line in Figure 2). Heating causes a jump of the magnetic moment

Figure 2. Temperature dependence of the magnetic moment of complex 1.

value from 1.7 to 2.41 μB at 40 K. In the temperature range 40− 200 K, the magnetic moment grows very weakly (from 2.41 to 2.52 μB). The second intensive growth of the magnetic moment is observed in the temperature range 200−280 K (from 2.52 to 4.9 μB). The high-temperature limit is 4.93 μB at 295 K. The following sequence of cooling (down to 200 K) and second heating indicates a hysteresis loop of about 40 K width (Figure 2). The room temperature magnitude of the magnetic moment is close to the spin-only value 4.9 μB calculated for the system of spins S(Co2+) = 3/2, S(SQ) = 1/2(2SQ), and S(TEMPO) = 1/2. The magnetic moment magnitude at the plateau (40−200 K) corresponds to noninteracting spins S(Co3+) = 0, S(SQ) = 1 /2(SQ), and S(TEMPO) = 1/2 (2.45 μB). The fall of the magnetic moment at low temperature (down from 40 K) usually is caused by weak antiferromagnetic exchange interactions. So, it can be concluded that, between 200 and 280 K, the redox− isomeric interconversion takes place (Scheme 2). It should be mentioned that some redox−isomeric systems containing the radical substituent were already described.6 The following question appears: what is the reason for the hysteresis loop? To answer this question, we have tried to do the

Figure 3. DSC curves of complex 1 (rate of 5 K/min). First cooling (red line): Tmax = 211.4 K (197−221 K). First heating (violet line): Tmax = 268.6 K (241−277 K). Second cooling (black line): Tmax = 220.2 K (205−233 K). Second heating (aquamarine line): Tmax = 271.2 K (242− 278 K).

sample of the complex was cooled for the first time at the rate of 5 K/min, the exothermic peak was observed at 211.4 K (red line). Heating (5 K/min) detects the endothermic peak at about 268.6 K (violet line). A second cooling detects the exothermic peak at 220.2 K (black line). Following cyclic heating−cooling, measurements are reproducible and indicate the last temperatures of the anomalies to be the same: 220(↓) and 270(↑) K (Figure 3). The absolute enthalpy values of all processes (12.5 kJ/mol) are close to each other under measurement error. The first cooling differs from the following ones only by the temperature. The cooling of 1a from 296 to 240 K (complex 1b) leads to some systematic bond-length variations in 1b compared to 1a. Analysis of these variations allows one to conclude that the redox−isomeric equilibrium has already been slightly shifted toward the ls-Co(SQ)(Cat) isomer (Table S2). It is important to note that the distances Co−O and Co−N in 1b become slightly shorter, whereas C−O in the dioxolene ligands becomes slightly longer. The peculiarity of crystal packing is the 2D structure. It is formed because of short intermolecular contacts between the O atom of the N−O− group and the C(32)−H(32) atoms of the

Scheme 2. Redox−Isomeric Interconversion of Complex 1

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DOI: 10.1021/acs.inorgchem.7b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 4. Fragment of crystal packing (2D layer) of 1a and 1b. The figure shows only the H atoms involved in the intermolecular interactions. The methyl groups of the tert-butyl substituents are omitted for clarity.

[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

pyridine ring of an adjacent molecule, on the one hand, and the C atom of the O,O′-chelating ligand and the H(31) atom of the pyridine ring of an adjacent molecule, on the other hand (Figure 4). These intermolecular interactions form a layer. The 3D structure is formed because of intermolecular C···C short contacts between C atoms of the methyl groups of the tert-butyl substituents in dioxolenes and C atoms of the imino groups of the iminopyridine ligands of adjacent molecules [3.377(6) Å for 1a and 3.351(3) Å for 1b; Figure S2]. Cooling to 240 K leads to a slight shortening of the intermolecular contacts both within one layer and between layers. In order to determine the voids volume in the unit cell, we have carried out a study of complexes 1a and 1b by the multipurpose crystallographic tool PLATON.9 Calculations showed that cooling to 240 K led to an increase in the voids volume from 7.8% to 8.4% (Figure S3). We consider that a further decrease of the temperature can lead to substantial structural changes and can be the reason for destruction of the monocrystal. Now it is only assumption. Unambiguous interpretation requires additional investigations.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +7 831 4627682. Fax: +7 831 4627497. ORCID

Michael P. Bubnov: 0000-0002-0536-3711 Artem S. Bogomyakov: 0000-0002-6918-5459 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Russian Science Foundation (Grant 14-13-01296). Experiments were performed partly using the instruments of the Collective Use Analytic Center of G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences. Authors are thankful to A. V. Markin for help in the DSC experiment.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02597. Synthetic procedures, single-crystal and powder X-ray experimental descriptions, and evolution of the X-ray experiment (PDF) Video file (AVI) Video file (AVI) Video file (AVI)

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REFERENCES

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Accession Codes

CCDC 1572142−1572143 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 C

DOI: 10.1021/acs.inorgchem.7b02597 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

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DOI: 10.1021/acs.inorgchem.7b02597 Inorg. Chem. XXXX, XXX, XXX−XXX