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Cite This: Chem. Mater. 2018, 30, 1835−1838
A New S = 0 ⇄ S = 2 “Spin-Crossover” Scenario Found in a Nickel(II) Bis(nitroxide) System Yuta Homma and Takayuki Ishida* Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan S Supporting Information *
B
istable solid-sate materials without any change in chemical composition, spin-crossover complexes for instance, are of increasing interest for future applications to memory, display, and other devices.1−3 Transition between S = 0 and S = 2 states in discrete iron(II) 3d6 complexes is typical.3 Heterospin systems provide the wide diversity of the characters of paramagnetic centers including the symmetry of magnetic orbitals.4 Direct copper(II)-radical coordination compounds are the best documented among the 3d-2p heterospin molecules,5 and nickel(II)-radical compounds are the second but relatively scarce.6 Recently nickel(II)-iminosemquinonate chelates have been reported to exhibit ferromagnetic coupling, after the pioneering work on the copper(II)-semiquinonate compounds reported by Kahn and co-workers.7 The equatorial coordination in copper(II)-nitroxide radical complexes is well-known to favor strong antiferromagnetic coupling,5 but ferromagnetic interaction actually is available as well.5b,8 The magnitudes of the couplings often exceed the order of 300 K, whether ferro- or antiferromagnetic, being sensitive to the geometry. Ferromagnetic coupling can be realized when two magnetic orbitals (radical π* and metal dσ) are strictly orthogonal. On the other hand, severe twist around the Cu−O (or Ni−O) coordination bond would lead to an appreciable overlap between Cu (or Ni) 3dx2−y2 and 3dz2 and O 2pz magnetic orbitals, affording antiferromagnetic coupling.8 Spin-transition-like behavior in three- or four-centered spin systems has been reported owing to the chelate-ring deformation9,10 by using tert-butyl 2-pyridyl nitroxides as a paramagnetic ligand. We will report here the first example of nickel(II)-radical compounds showing spintransition-like behavior. An advantage of the present system resides in the feature where the drastic spin transition occurs between dia- and paramagnetic states. This report includes a mechanistic investigation of the spintransition-like behavior, and the ground high- and low-spin states are directly regulated with the switch of Ni2+-radical exchange couplings. The situation is completely different from those of the known materials having spin-crossover ions and additional exchange coupling.11 A 2p-3d-2p heterospin triad [Ni(phpyNO)2Cl2] (1) was prepared by complex formation of NiCl2·6H2O with a paramagnetic ligand, tert-butyl 5-phenyl-2-pyridyl nitroxide (phpyNO) ligand8c (Srad = 1/2). The spectroscopic and analytic characterizations were satisfactory, and finally the molecular structure was confirmed by X-ray crystallographic study (Figure 1). On cooling, chelate ring distortion very gradually took place in a single-crystal-to-single-crystal manner, and we determined the molecular structures at any temperature between 85 and 400 K. © 2018 American Chemical Society
Figure 1. (a) X-ray crystal structures of 1 measured at 400 K (light tone) and 85 K (dark tone). Thermal ellipsoids are drawn at the 50% probability levels. Structural formula of 1 is also shown (right). (b) Cell parameters and the torsion angles as a function of temperature (Figure S1, Supporting Information).
The space group retains monoclinic P21/c. The nickel(II) ion has an octahedral geometry, which guarantees the high spin (SNi = 1). The phpyNO ligands are arranged in a cis position; the O1−Ni−O2 angle is 80.44(11)° at 400 K. The molecule has a pseudo-C2 symmetry; namely the two coordination geometries are similar but independent. The five-membered chelate rings are clearly depicted in Figure 1a. The nitroxide oxygen atom is directly coordinate to the nickel(II) ion with the Ni−O1 and Ni−O2 distances of 2.083(3) and 2.103(3) Å, respectively, at 400 K. The unit cell smoothly shrinks on cooling (Figure 1b). The Ni−O1, Ni−O2, and other bond lengths hardly changed. On Received: December 27, 2017 Published: January 10, 2018 1835
DOI: 10.1021/acs.chemmater.7b05357 Chem. Mater. 2018, 30, 1835−1838
Communication
Chemistry of Materials
Repeated cycles exhibited complete reproducibility and no thermal hysteresis. The change to the molecular structure causes the change to the intramolecular exchange coupling. Simulation was unsuccessful to fit the data to any van Vleck equations derived from the spin-Hamiltonian on linear or triangular spin systems13 with S = 1/2, 1, and 1/2, and it can be rationalized by noting the temperature-dependent exchange coupling. Instead, we found that the data perfectly obeyed the van’t Hoff equation (eq 1), which is available for spin-crossover compounds.14 1 x(HS) = 1 + exp[(ΔH /R )(1/T − 1/Tc)] (1)
the other hand, the Ni−O−N−C2py torsion angles (ϕ) dramatically change from |ϕ1| = 13.0(4) and |ϕ2| = 25.8(3)° at 400 K to |ϕ1| = 29.2(2) and |ϕ2| = 38.9(2)° at 85 K. In short, the Ni−O bonds seem to rotate, as indicated with small curved arrows in Figure 1a. We have proposed8 that ϕ is a useful metric for orthogonal geometry between the nitroxide π* and Ni2+ 3dx2−y2 and 3dz2 orbitals (Scheme 1(i,iii)).5 At low temperatures, Scheme 1. Mutual Geometries between Ni 3dx2−y2 and O 2pz (i,ii) and between Ni 3dz2 and O 2pz (iii,iv)a
The molar fraction of the high-spin molecules (x(HS)) is proportional to the χmT value, and accordingly χmT = C0x(HS) + C1. The optimized parameters were: Tc (transition temperature) = 173.7(8) K and ΔH (enthalpy change of the phase transition) = 4.57(5) kJ mol−1 with the Curie constant for the high-spin state (C0) was 2.38(2) cm3 K mol−1 and impurity constant (C1) 0.093(4) cm3 K mol−1. The entropy change of the phase transition was estimated as ΔS = 26.3(1) J K−1 mol−1. At the low-temperature (LT) phase, the ground Stotal is 0. Both exchange couplings (J1 and J2) are antiferromagnetic, where the spin-Hamiltonian was defined as H = −J1SRad1·SNi − J2SRad2·SNi. Assuming the ground Stotal = 2 in the hightemperature (HT) phase, the entropy change from the spin multiplicity should be ΔSLH = R ln 5 = 13.4 J K−1 mol−1.15 The observed value has an excess entropy change, and the HT phase is assumed to involve other degrees of freedom such as vibration.14,16 The domain model16,17 indicates that the present compound is an almost uncooperative system. The crystal structure analysis clarifies that there are a few hydrogen-bond contacts with respect to the chlorine atoms (2.6−2.8 Å; Figure S2, Supporting Information), but they seem not so beneficial to cooperativity. The variable-temperature X-band EPR study on polycrystalline 1 was performed (Figure 3). In the LT phase, two resolved
a
Only radical chelate planes are shown. The oxygen atoms are located on the plane (i,iii) and dislocated out of the plane (ii,iv).
the breakdown of the d-π* orthogonality brings about antiferromagnetic coupling (Scheme 1(ii,iv)). Thus, the present geometrical deformation would enhance antiferromagnetic coupling. Magnetic susceptibility of 1 was measured on a SQUID magnetometer (Figure 2). The χmT value was 2.14 cm3 K mol−1
Figure 2. Temperature dependence of χmT for polycrystalline 1, measured in the applied magnetic field was 5000 Oe on heating, cooling, and heating again. For the solid line, see the main text.
at 400 K, which is larger than the calculated spin-only value of two radicals and one nickel(II) ion (1.96 cm3 K mol−1 from typical isotropic gNi is close to 2.212). The positive slope at 400 K indicates that the high-temperature limit would be ca. 2.38 cm3 K mol−1 (see below), leading to an unrealistic gNi = 2.55. It does not match the value from the EPR results (see below). This finding implies the presence of ferromagnetic interaction observable even around room temperature. On cooling, the χmT value was monotonically decreased, indicating the presence of considerably large antiferromagnetic coupling. Note that the temperature region of this behavior (around 170 K) corresponds to that of the structural change (Figure 1). Below 50 K, the specimen was practically diamagnetic (Stotal = 0).
Figure 3. Temperature dependence of EPR spectra of polycrystalline 1 (top) simulation and (bottom) observation.
signals appeared, which are assigned to the nickel(II) and radical spins. The simulation curve was drawn on a WIN-EPR SimFonia software18 with gx = 2.358, gy = 2.358, gz = 2.368, |D|/gμB = 18.5 mT, and |E|/gμB = 2.9 mT for the former, and gx = 1.986, gy = 2.005, gz = 2.018 for the latter. The resolved signals below 90 K are free from exchange coupling, suggesting the low concentration of the spin. The paramagnetic species in the LT phase are attributed to the lattice defect because it appeared only after the structural and spin transition. Namely, the majority of 1836
DOI: 10.1021/acs.chemmater.7b05357 Chem. Mater. 2018, 30, 1835−1838
Communication
Chemistry of Materials the molecules are EPR-silent below 90 K. This finding is consistent with the ground diamagnetic state for this morph. The density functional theory (DFT) MO calculation19 supports the antiferro- and ferromagnetic couplings are present in the LT and HT phases, respectively. For reducing calculation cost, the peripheral phenyl groups were replaced with hydrogen atoms, and the tert-butyl groups with methyl groups (Figure 4a).
also for Gd3+-nitroxide systems,23 and it has very recently been reported that the exchange coupling modulation in the Gd3+Cu2+-nitroxide compound was associated with the rotation around the Gd3+-O(nitroxide) bond.24 The present Ni2+bis(nitroxide) system may be advantageous to future application in the drastic dia-/paramagnetic spin-state change, like the iron(II) spin crossover. Moreover, the present system is close to the entropy-driven spin-crossover.25 The spin multiplicity is responsible for the number of microstates, and the TΔS term in ΔG = ΔH − TΔS would be substantial. The two Ni−O−N−C2py distortions were synchronized, and the ferromagnetic couplings become antiferromagnetic on both sides, because the degree of spin freedom is minimized (S total = 0). The results on [CuII(phpyNO)2(H2O)2](BF4)29 confirm this notion. The coupling switch occurred only on one side because of Stotal = 1/2, regardless of antiferromagnetic coupling on one side or both. No synchronized coupling switch is necessary in that case. One may still wonder: Why is the singlet-quintet equilibrium13 insufficient for the entropy loss with a constant antiferromagnetic exchange coupling? Why does the molecule have to struggle and twist? A plausible answer is that the HT form has ferromagnetic coupling and appreciable entropy can be discarded only when the coupling is switched to be antiferromagnetic. If the HT form had antiferromagnetic coupling or practically no coupling, there would never be a driving force for the molecular deformation. In fact, the molecule has to deform on both sides to accommodate the Stotal = 0 state and corresponding structure. This finding implies that the Stotal = 2 state cannot stand in the HT form on cooling. In summary, the spin triad 1 exhibited a thermally induced spin-transition (or crossover26) on the whole molecular basis. Thanks to the small geometrical change, i.e., inner angular torsion, 1 completely maintains the single-crystalline form throughout the transition. Such spin-state equilibrium has unprecedentedly been characterized in multicentered heterospin compounds, and is plausibly explained in terms of the entropydriven spin-crossover. Furthermore, this spin-transition drastically switches magnetic properties between dia- and paramagnetic states; actually, 1 is the first noniron(II) system to undergo an S = 0 ⇄ S = 2 spin-crossover. The present system will open a new class of spin-transition compounds and may be utilized as a potential sensor for external stimuli such as heat.
Figure 4. DFT Calculation results. (a) A model molecule. Substituents colored in red are introduced for reducing the calculation cost. (b) Spin density surfaces of the ground quintet state and (c) the excited singlet state at 400 K. (d) Spin density surfaces of the ground singlet state and (e) the excited quintet state at 85 K. Dark and light lobes stand for the positive and negative spin densities, respectively.
The geometries of other atoms were available from the X-ray crystallographic analysis. The SCF energies were calculated on the UB3LYP/6-311+G(d,p) level for the structure at 400 K and the quintet state is more stable by ca. 68 K.20 The spin density surfaces are drawn in Figure 4b,c, and the spin structure of the ground state was confirmed to be Rad1(↑)−Ni(↑↑)−Rad2(↑). In contrast, the calculation on the structure at 85 K indicated that the singlet state was ground with the spin-structure of Rad1(↓)−Ni(↑↑)−Rad2(↓) and that the energy gap was as large as 1600 K (Figure 4d,e).20 Such sensitive structural dependence has also been reported on a nickel(II) complex doubly chelated with 5-formylpyrrolyl nitronyl nitroxide.21 The theoretical treatment told us that a few degrees of the Ni−O bond rotation drastically changed the exchange coupling by 1 order of the magnitude as well as the sign of couplings. This work entirely supports our results. Ovcharenko and co-workers have reported several copper(II)-nitroxide complexes showing spin-transition-like behavior, and clarified that the Cu−O bond lengths change to convert the roles of axial and equatorial sites.22 It should be stressed that the present spin-transition phenomenon takes place in a completely different way from theirs. We have found an advantage in the nickel(II) system; the spin-transition occurs regardless of the coordination sites, axial or equatorial, and furthermore two spintransitions simultaneously occur in a molecule. The trigger of the transition is not elongation or shortening of coordination bonds. The angular torsion around the coordination bond is essential in the magneto-structural relationship. This logic holds
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b05357. Crystal structures at each temperature in Figure 1 (AVI) Molecular arrangement in the crystal, experimental details, and complete reference for Gaussian 03 (PDF) Crystallographic data of 1 at 85 K (CIF) Crystallographic data of 1 at 400 K (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Takayuki Ishida: 0000-0001-9088-2526 Notes
The authors declare no competing financial interest. 1837
DOI: 10.1021/acs.chemmater.7b05357 Chem. Mater. 2018, 30, 1835−1838
Communication
Chemistry of Materials
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and suggested from the theoretical calculation, the entropy change (ΔSLH = R ln(4 × 2) = 17.3 J K−1 mol−1) became closer to the experimental data. The moderately ferromagnetic coupling might be buried by the structural transition around 170 K, but the discussion on the transition mechanism indicates that the coupling is qualitatively ferromagnetic. (16) Boca, R. Theoretical Foundations of Molecular Magnetism: Current Methods in Inorganic Chemistry, Vol. 1; Elsevier: Amsterdam, 1999. (17) Sorai, M.; Seki, S. Phonon Coupled Cooperative Low-Spin 1A1 High-Spin 5 T 2 transition in [Fe(phen) 2 (NCS) 2 ] and [Fe(phen)2(NCSe)2] Crystals. J. Phys. Chem. Solids 1974, 35, 555−570. (18) Weber, R. T. WIN-EPR SimFonia, version 1.2; Bruker Instruments: Billerica, MA, 1995. (19) Frisch, M. J. et al. Gaussian 03, revision C.02; Gaussian Inc.: Wallingford, CT, 2004. (20) The S CF energies were − 3264.152 787 98 and −3264.152 572 87 au for the quintet and singlet states, respectively, at 400 K. Those of 85 K were −3264.146 307 16 and −3264.151 453 34 au, respectively. (21) Zueva, E. M.; Tretyakov, E. V.; Fokin, S. V.; Tkacheva, A. O.; Bogomyakov, A. S.; Petrova, O. V.; Trofimov, B. A.; Sagdeev, R. Z.; Ovcharenko, V. I.; Romanenko, G. V. Stereo Sensitivity of Exchange Interactions in NiII and CuII Heterospin Complexes with 5Formylpyrrolyl-Substituted Nitroxides. Russ. Chem. Bull. 2016, 65, 666−674. (22) (a) Ovcharenko, V.; Fokin, S.; Chubakova, E.; Romanenko, G.; Bogomyakov, A.; Dobrokhotova, Z.; Lukzen, N.; Morozov, V.; Petrova, M.; Petrova, M.; Zueva, E.; Rozentsveig, I.; Rudyakova, E.; Levkovskaya, G.; Sagdeev, R. A Copper−Nitroxide Adduct Exhibiting Separate Single Crystal-to-Single Crystal Polymerization−Depolymerization and Spin Crossover Transitions. Inorg. Chem. 2016, 55, 5853− 5861. (b) Fedin, M. V.; Veber, S. L.; Bagryanskaya, E.; Ovcharenko, V. I. Electron Paramagnetic Resonance of Switchable Copper-NitroxideBased Molecular Magnets: An Indispensable Tool for Intriguing Systems. Coord. Chem. Rev. 2015, 289−290, 341−356. (23) (a) Kanetomo, T.; Yoshitake, T.; Ishida, T. Strongest Ferromagnetic Coupling in Designed Gadolinium(III)−Nitroxide Coordination Compounds. Inorg. Chem. 2016, 55, 8140−8146. (b) Kanetomo, T.; Kihara, T.; Miyake, A.; Matsuo, A.; Tokunaga, M.; Kindo, K.; Nojiri, H.; Ishida, T. Giant Exchange Coupling Evidenced with a Magnetization Jump at 52 T for a GadoliniumNitroxide Chelate. Inorg. Chem. 2017, 56, 3310−3314. (24) Zhu, M.; Li, C.; Wang, X.; Li, L.; Sutter, J.-P. Thermal Magnetic Hysteresis in a Copper−Gadolinium−Radical Chain Compound. Inorg. Chem. 2016, 55, 2676−2678. (25) (a) Letard, J.-F.; Real, J. A.; Moliner, N.; Gaspar, A. B.; Capes, L.; Cador, O.; Kahn, O. Light Induced Excited Pair Spin State in an Iron(II) Binuclear Spin-Crossover Compound. J. Am. Chem. Soc. 1999, 121, 10630−10631. (b) Adams, D. M.; Dei, A.; Rheingold, A. L.; Hendrickson, D. N. Bistability in the [CoII(semiquinonate)2] to [CoIII(catecholate)(semiquinonate)] Valence-Tautomeric Conversion. J. Am. Chem. Soc. 1993, 115, 8221−8229. (26) The term “spin-crossover” can be expanded to multicentered systems.22a It may be acceptable from viewing a common basis; the spin-state is regulated from the balance of ferro- and antiferromagnetic contributions which originate in Hund’s rule and the Aufbau principle, respectively, in a molecular orbital picture.
ACKNOWLEDGMENTS This work was partly supported by KAKENHI (Grant Number JSPS/15H03793).
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