High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly

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

High-Spin and Incomplete Spin-Crossover Polymorphs in Doubly Chelated [Ni(L)2Br2] (L = tert-Butyl 5‑Phenyl-2-pyridyl Nitroxide) Yukiya Kyoden, Yuta Homma, and Takayuki Ishida* Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan

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S Supporting Information *

ABSTRACT: Complexation of nickel(II) bromide with tertbutyl 5-phenyl-2-pyridyl nitroxide (phpyNO) gave two morphs of doubly chelated [Ni(phpyNO)2Br2] as a 2p−3d−2p heterospin triad. The α phase crystallizes in the orthorhombic space group Pbcn. An asymmetric unit involves a half-molecule. The torsion angle around Ni−O−N−C2py is as small as 6.5(3)° at 100 K and 7.0(6)° at 400 K, guaranteeing an orthogonal arrangement between the magnetic radical π* and metal 3dx2−y2 and 3dz2 orbitals. Magnetic study revealed the high-spin ground state with the exchange coupling constant 2J/kB = +288(5) K, on the basis of a symmetrical spin Hamiltonian. The β phase crystallizes in the monoclinic space group P21/n. The whole molecule is an independent unit. The Ni−O−N−C2py torsion angles are 24.2(6) and 37.2(5)° at 100 K and 10.4(7) and 25.9(6)° at 400 K. A magnetic study revealed a very gradual and nonhysteretic spin transition. An analysis based on the van’t Hoff equation gave a successful fit with the spin-crossover temperature of 134(1) K, although the susceptibility did not reach the theoretical high-spin value at 400 K. Density functional theory calculation on the β phase showed ground Stotal = 0 in the low-temperature structure while Stotal = 2 in the high-temperature structure, supporting the synchronized exchange coupling switch on both sides. Consequently, the β phase can be recognized as an “incomplete spin crossover” material, as a result of conflicting thermal depopulation effects in a hightemperature region.

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been best described among 3d−2p heterospin compounds,2,5,6 since the magnetic orbitals are well-defined as 3dx2−y2 in a 3d9 copper(II) ion and π* in a paramagnetic ligand. The 3d−2p interaction in copper(II) complexes having a nitroxide radical coordinated at an equatorial site is often reported to be strongly antiferromagnetic,2 but in fact a strong ferromagnetic interaction is also available.5,6 The exchange interaction is sensitive to the coordination geometry. The couplings sometimes exceed the order of thermal energy of 300 K, even when they are ferromagnetic. A similar situation holds for both the axial and equatorial coordinations in octahedral 3d8 nickel(II) complexes, where the magnetic orbitals are 3dx2−y2 and 3dz2.6−8 Ferromagnetic coupling occurs when two magnetic orbitals (radical π* and metal dσ) are arranged in an orthogonal fashion. In contrast, an appreciable orbital overlap between them given by out-of-plane deformation around the Cu−O (or Ni−O) coordination bond leads to an antiferromagnetic interaction.8 Molecular design is required to realize ferromagnetic compounds. There have been a relatively small number of reports on tertbutyl 2-pyridyl nitroxides because of difficulty in their synthesis. Simple pyridyl nitroxides cannot be isolated under

INTRODUCTION Persistent organic radicals have been intensely studied in the various directions of molecule-based magnetic, conducting, chromic, and other related materials, and they are attracting considerable attention from materials chemistry researchers.1 Gatteschi and co-workers have proposed the versatile strategy “metal−radical approach” toward heterospin molecular magnets, where appreciable magnetic exchange interaction is operational through direct coordination bonds between paramagnetic metal ions and radical chromophores.2 Metal− radical compounds also afford rich chemistry and physics in the cutting-edge research of single-molecule and single-chain magnets and, in addition, spintronics.3 The exchange interactions have been investigated experimentally and theoretically on the basis of molecular geometries typically including bond lengths and angles,4 because they are regulated with symmetry and energy levels of the magnetic (singly occupied) orbitals as well as with their mutual spatial arrangement. Heterospin systems utilize a wide diversity of the nature of spin centers, including symmetry, energy levels, and localized/itinerant properties, in the context of frontier orbital engineering. Copper(II) radical complexes in which the paramagnetic center of the ligand is directly coordinated to the metal center have been the most studied, and the exchange mechanism has © XXXX American Chemical Society

Received: March 27, 2019

A

DOI: 10.1021/acs.inorgchem.9b00885 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry ambient conditions.9 Our group has explored a wide variety of ligands having a tert-butyl nitroxide group as a building block in 2p−3d6,8,10 and 2p−4f11 heterospin magnetic materials. The coordination formation improves persistency, thanks to bulkiness from the metal ion itself and ligands nearby. One of the most striking results in the 3d−2p systems may be the discovery of multicentered spin transitions in Stotal = 0 ⇄ 1,12 Stotal = 1/2 ⇄ 3/2,13 and Stotal = 0 ⇄ 2.14 Spin-crossover materials are of increasing interest because of possible applications to memory, display, sensor, and other devices in the future.15−18 Among them, the transition between S = 0 and S = 2 states has been the most investigated in detail and is found in iron(II) 3d6 complexes with suitable ligand field strength.19−21 Spin transition is observed between dia- and paramagnetic states, and the accompanying change of physical properties would be drastic. In the 2p−3d and 2p−3d−2p heterospin dyads and triads, there has so far been only one example reported on the spin transition between Stotal = 0 ⇄ 2, where two Srad = 1/2 ligands and an SNi2+ = 1 center are incorporated. The high-spin electron configuration in a conventional one-centered spin crossover is favored under the control of Hund’s rule, and this effect may also be called intrasite ferromagnetic interaction or potential exchange. Therefore, what “spin-crossover” means can be expanded to multicentered systems showing an essential spin-multiplicity switch. The spin state is regulated by ferroand antiferromagnetic contribution balance originating in the Hund and aufbau principles in a molecular orbital scheme,14 whether it be an intrasite or intersite interaction. Moreover, an entropy-driven spin-crossover mechanism22 has been discussed from comparison work of copper(II)− and nickel(II)−radical compounds.12−14 In a closely related system, Ovcharenko and co-workers reported “spin-crossover” materials in copper(II)−nitroxide complexes.23 This mechanism involves an interconversion of the role between axial and equatorial sites24 and works only for copper(II) systems. On the other hand, our logic can be applied also to octahedral nickel(II) compounds carrying two dσ spins. A prospective coupling switch occurs regardless of the sites in six-coordination. Another advantage of our system resides in the strength of 2p−3d ferromagnetic coupling. Rey and co-workers reported the spin-transition materials in copper(II)−nitroxide complexes, in which the copper coordination environment changed between a trigonal bipyramid and a square pyramid.25 Interestingly, the transition was accompanied by thermal hysteresis. They wrote that their compound could be regarded as a “pseudo-spin-transition” material because the exchange coupling switched from weakly ferromagnetic to strongly ferromagnetic. Along this classification, the development of genuine spin-transition materials involving a switch between antiferromagnetic and ferromagnetic is a target in the present study. We concentrated our attention on the preparation of complexes with nickel(II) ions and nitroxides with an M/L ratio of 2/1, where an orthogonal arrangement is expected between dσ and π*. Polymorphism has been found to occur in the doubly chelated complexes consisting of nickel(II) bromide and tert-butyl 5-phenyl-2-pyridyl nitroxide8a (Figure 1, abbreviated as phpyNO hereafter). One of the polymorphs has a ground high-spin state in all temperature ranges, while the other undergoes a spin transition. Comparing two temperature-independent/-dependent geometries in the poly-

Figure 1. Structural formulas of phpyNO and phpyCO.

morphs will provide decisive evidence for the novel mechanism of unconventional spin transition.

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RESULTS Preparation and Characterization. The complex [Ni(phpyNO)2Br2] (1) was prepared by simply mixing solutions of NiBr2·3H2O and the paramagnetic ligand phpyNO8a in an established manner.6,8,14 Polymorphism has been found; the major diamond-shaped platelets and minor rectangle-shaped prisms are named hereafter as the α and β phases, respectively, and could be easily recognized under a microscope. They originally crystallized by chance, though the initial crystallization more often gave α-1 rather than β-1. After separation of the crystals, “seeding” upon recrystallization is found to be a complementary method. Adding seeds of β-1 in an acetonitrile, acetone, 2-butanone, or 1-pentanol solution of 1 gave an increase in production of β-1. The spectroscopic and analytic characterizations were satisfactory on each phase. No solvent molecule was incorporated in any crystals. Crystal Structure Analysis. The two morphs α- and β-1 were characterized by single-crystal X-ray diffraction measured at 100 and 400 K, and the molecular structures at 100 K are shown in Figure 2. Selected crystallographic data and geometrical parameters at 100 and 400 K are summarized in Tables 1 and 2, respectively. Both phases consist of discrete [Ni(phpyNO)2Br2] molecules. The N−O bond lengths are typical of the aryl tert-butyl nitroxides.8a Five-membered chelate rings involve a nickel ion and nitroxide group for both phases. The nitroxide oxygen atoms are arranged in cis positions, and the two bromide anions occupy the remaining positions to also form a cis geometry. The geometry around the nickel(II) ion is an approximate octahedron, guaranteeing SNi2+ = 1. The molecular structures of α- and β-1 are apparently similar, but the space groups and molecular symmetries are different from each other. The α phase crystallizes in the orthorhombic space group Pbcn (Figure 2a,b). Half of the molecule corresponds to an asymmetric unit. When the temperature is varied, the coordination bond lengths around Ni1 hardly change, within ca. 0.003 Å on the average; only the natural thermal expansion was found. The five-membered chelate ring is very planar, as indicated by the torsion angles (|φ|) around Ni1−O1−N1−C1 of 6.5(3)° at 100 K and 7.0(6)° at 400 K. The φ values are practically unchanged from 100 to 400 K within experimental error (Figure 3a). Among various high-spin nickel(II)− bidentate nitroxide complexes,6,8 α-1 is the first example having a cis configuration with respect to radical positions. We have proposed that φ is a convenient metric to evaluate the atomic displacement out of the averaged plane defined with a chelate ring (Figure 3b,c).8 When a nickel ion is located just on the ligand π-conjugation plane defined by O−N−Csp2, the Ni−O−N−Csp2 torsion angle is ideally 0°. Thus resultant orthogonality between the magnetic radical π* and the metal dσ orbitals (i.e., 3dx2−y2 and 3dz2) would favor ferromagnetic coupling. According to this model, the small torsion in α-1 B


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Figure 2. Ortep drawings of a molecular structure of the α phase of [Ni(phpyNO)2Br2] (α-1) at 100 K (a) and molecular arrangement in an orthorhombic unit cell of α-1 viewed along the a axis (b). The symmetry operation code of * is 1 − x, y, 1/2 − z. Ortep drawings of a molecular structure of β-1 at 100 K (c) and molecular arrangement in a monoclinic unit cell of β-1 viewed along the a axis (d). Hydrogen atoms are omitted for clarity in all panels. Relatively short intermolecular contacts are indicated with broken hairlines. For details, see Figure S1 in the Supporting Information.

Table 1. Selected Crystallographic Parameters of α- and β-1 α phase C30H34Br2N4NiO2 Pbcn orthorhombic α phase

formula space group crystal system

a/Å b/Å c/Å β/deg V/Å3 Z dcalc/g cm−3 μ(Mo Kα)/ mm−1 R (I > 2σ(I))a Rw(F2) (all rflns)b goodness of fit param

β phase C30H34Br2N4NiO2 P21/n monoclinic β phase

100 K

400 K

100 K

400 K

14.279(2) 10.4169(15) 21.493(3) 90 3196.9(8) 4 1.457 3.146 0.0381 0.0728 1.375

14.383(4) 10.580(4) 21.779(7) 90 3314.0(17) 4 1.405 3.034 0.0496 0.1262 0.944

13.563(3) 16.700(4) 13.566(3) 95.953(10) 3056.1(12) 4 1.524 3.291 0.0677 0.1504 0.997

13.684(5) 16.995(4) 13.863(4) 96.895(13) 3200.7(16) 4 1.455 3.142 0.0692 0.1636 0.958

R = ∑[|Fo| − |Fc|]/∑|Fo|. bRw = [∑w(Fo2 − Fc2)/∑wFo4]1/2.

a

C


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Inorganic Chemistry Table 2. Selected Bond Lengths (d in Å), Bond Angles (θ in deg), and Torsion Angles (φ in deg) of α- and β-1a α phase d(N1−O1) d(N3−O2) d(Ni1−O1) d(Ni1−O2) d(Ni1−N2) d(Ni1−N4) d(Ni1−Br1) d(Ni1−Br2) θ(Br1−Ni1−Br2 (−Br1*)) θ(O1−Ni1−O2 (−O1*)) θ(N2−Ni1−N4 (−N2*)) |φ1(Ni1−O1−N1−C1)| |φ2(Ni1−O2−N3−C16)|

β phase

100 K

400 K

1.296(3)

1.278(5)

2.044(2)

2.047(4)

2.061(2)

2.066(4)

2.5213(5)

2.5048(10)

94.46(2) 89.92(13) 157.05(13) 6.5(3)

94.69(5) 89.2(3) 155.9(2) 7.0(6)

100 K

400 K

1.293(5) 1.284(5) 2.066(4) 2.082(4) 2.040(4) 2.038(4) 2.4896(11) 2.4797(10) 100.12(4) 81.07(16) 165.87(18) 24.2(6) 37.2(5)

1.271(6) 1.289(5) 2.060(4) 2.094(4) 2.042(4) 2.035(4) 2.4899(12) 2.4848(11) 98.31(4) 80.93(18) 163.36(19) 10.4(7) 25.9(6)

Symmetry operation code of * is 1 − x, y, 1/2 − z for α-1.

a

Figure 3. (a) Torsion angles (|φ(Ni−O−N−C2py)|) as a function of temperature for α- and β-1. (b, c) Orthogonal arrangements of Ni 3dx2−y2 and O 2pz and of Ni 3dz2 and O 2pz, respectively.

Waals radii.26 In the molecular arrangement of β-1, an intermolecular short distance is found between bromine and pyridine carbon atoms with 3.444(5) Å (Figure 2d and Figure S1b in the Supporting Information). The corresponding Br···H distance is ca. 2.65 Å, which is shorter than the sum of the van der Waals radii.26 Such an intermolecular contact may afford an interaction channel, but this could not be experimentally proven, because the β-1 molecule behaves as a diamagnetic species in a low-temperature region (see below). In contrast to the case of α-1, the planar chelate structure is lost in β-1 on cooling, and the degree of deformation was found to be temperature-dependent (Figure 3a). When a nickel ion is dislocated out of the ligand π-conjugation plane defined by O−N−Csp2, the Ni−O−N−Csp2 torsion angle becomes large. The structural change occurred in a singlecrystal to single-crystal manner, enabling us to trace it as a function of temperature: |φ1| = 10.4(7)° and |φ2| = 25.9(6)° at 400 K and |φ1| = 24.2(6)° and |φ2| = 37.2(5)° at 100 K (for the crystal structure of each temperature, see the Supporting Information). The low-temperature (LT) structure is more out-of-plane distorted in comparison to the high-temperature (HT) structure. On the other hand, the coordination bond lengths hardly changed within ca. 0.004 Å on the average. According to the magneto−structural relationship known for the copper(II)− and nickel(II)−radical systems,8 the breakdown of the d−π* orthogonal geometry would give rise to a

suggests considerably strong ferromagnetic coupling on both sides, giving a ground Stotal = 2 state in the entire temperature range studied here. A nonbonded O1···O1* distance is relatively short (2.888(2) Å at 100 K) but slightly larger than the sum of the van der Waals radii (2.8 Å for O/O).26 In the molecular arrangement of α-1 in a unit cell, two intermolecular short distances of 3.724(3) and 3.867(3) Å are found between bromine and pyridine carbon atoms (Figure 2b and Figure S1a in the Supporting Information). The corresponding Br···H distances are less than 3.0 Å, which is shorter than the sum of the van der Waals radii (3.15 Å for Br/H).26 These double contacts may serve as a channel of intermolecular exchange couplings. The space group of β-1 is monoclinic P21/n (Figure 2c,d), and it is maintained during the variable-temperature experiment from 100 to 400 K. The whole molecule is crystallographically independent, although a molecule has a pseudo-C2 symmetry. There are two coordination geometries and accordingly two kinds of 2p−3d interactions. In comparison with isomorphous [Ni(phpyNO)2Cl2],14 the cell parameters a and b in β-1 are longer than the corresponding values of the Cl analogue, being consistent with the atomic radius of Cl and Br, but the parameter c of β-1 is shorter. A nonbonded O1···O2 distance is relatively short (2.696(6) Å at 100 K) and slightly shorter than the sum of the van der D


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Inorganic Chemistry

Figure 4. Temperature dependence of χmT for α-1 (a) and β-1 (b), measured at 5000 Oe. The solid lines represent theoretical curves. For the equations and optimized parameters, see the text.

bonding contribution between the unpaired electrons. We will discuss this later after the magnetic results are combined. The motivation of the following study resides in a question about driving force for the torsion angle change by as large as Δφ1 = 13.8° and Δφ2 = 11.3°. It should be stressed that α-1 never changed its coordination structure while β-1 displayed a gradual transformation. No interconversion was found between the α and β phases, which can be reasonably understood by the quite different molecular packing motifs in the crystals (Figure 2b,d). Magnetic Analysis. The magnetic susceptibilities of αand β-1 were measured on a SQUID susceptometer, with the temperature varied from 1.8 to 400 K in a constant external magnetic field of 0.5 T. The χmT value at 400 K was 2.53 cm3 K mol−1 for α-1 (Figure 4a). This is larger than the spin-only value of 1.75 cm3 K mol−1 for noninteracting SNi2+ = 1 and two Srad = 1/2 paramagnetic centers and also larger than the corrected value 1.96 cm3 K mol−1 with a typical isotropic gNi2+ value of ca. 2.2.27 Upon cooling from 400 to 2 K, the χmT value of α-1 once increased and reached a broad maximum of 3.16 cm3 K mol−1 at around 75 K, indicating the presence of a ferromagnetic interaction. Since the material consists of discrete molecules (Figure 2a,b), the exchange coupling is reasonably assigned to an intramolecular origin. Thus, the magnetic study clearly indicates the ground high-spin nature (Stotal = 2) of α-1. This finding completely agrees with the prediction from the chelate structure; the small Ni1−O1− N1−C1 torsion angles in α-1 are related to ferromagnetic couplings on both sides. On further cooling below 75 K, the χmT value decreased. The chelate geometry is unique because of the molecular symmetry of α-1. The spin Hamiltonian is written as eq 1 with J1 = J2, where coupling between the terminal radicals was disregarded.28 The molecular structure was practically unchanged, so that J1 and J2 can be treated as temperatureindependent constants. Thus, we applied the van Vleck equation, to give eq 2.28,29 ̂ ̂ ) − 2J (S N3O2 ̂ ̂ ) ·S Ni ·S Ni Ĥ = −2J1(S N1O1 2

χm =

2 2NAgavg μB2

kBT exp(− 2J /kBT ) + 1 + 5 exp(2J /kBT ) exp(− 4J /kBT ) + 3 exp(− 2J /kBT ) + 3 + 5 exp(2J /kBT ) (2)

A final drop in the χmT value can be attributed to zero-field splitting (zfs) of Ni2+ in 1 and intermolecular coupling, but two contributions cannot be separated at this stage. The data in 75−400 K were fitted to eq 2, and the parameters are optimized as 2J/kB = +288(5) K and g = 2.068(3) for α-1. Figure S2 in the Supporting Information shows a possible analysis including an axial zfs parameter for the Ni2+ ion, after intermolecular interactions are disregarded. The simulation curves with 2J/kB = +288 K reproduced the experimental data well. The energy gap between singlet and quartet states is nominally given with 6J and accordingly ca. 864 K in this case. The χmT vs T plot of β-1 displayed an apparently contrasting profile, in comparison with that of α-1 (Figure 4b). The χmT value of β-1 was 1.96 cm3 K mol−1 at 400 K. The small positive slope at around 400 K indicates that the HT limit would be slightly higher. An equilibrium model gave ca. 2.18(2) cm3 K mol−1 as the HT limit (see below). In comparison with theoretical spin-only value of one nickel(II) ion and two radicals (1.75 cm3 K mol−1), a somewhat large gNi2+ = 2.39 would be deduced, or more likely there seems to be spin−spin interaction (see below). With a decrease in temperature, the χmT value was monotonically decreased, and the compound was practically diamagnetic below 60 K. The presence of a considerable antiferromagnetic interaction is indicated, and the coupling must be attributed to the intramolecular 2p−3d relations on both sides, from the discrete molecular form (Figure 2c,d). No thermal hysteresis was recorded in repeated measurements. The temperature range of the decrease is in good accord with that of the molecular structure deformation (Figure 3), and therefore a van Vleck analysis with exchange coupling constants does not work. There has been some reports describing magnetic analysis using temperature-dependent coupling parameters.30 In the present case, no suitable expression of function J(T) is known at present. Instead, the magnetic data of β-1 were assumed to obey the van’t Hoff equation (eq 3), and a satisfactory fit was found in the entire

(1) E


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Inorganic Chemistry

Figure 5. Schematic drawings of the relative energy levels of the quartet, triplet, and singlet states for (a) the low-temperature form and (b) hightemperature form of β-1. A DFT calculation was performed on the geometries from the crystallography at 100 and 400 K for (a) and (b), respectively, and the broken-symmetry method was applied. The spin density surfaces are drawn at the 0.002 e Å−3 level with dark and light lobes for the positive and negative spin densities, respectively. A molecular structure with selected atomic labels is also shown. (c) Temperature dependence of the energy gap between the singlet state (BS) and quintet state (HS). A solid line is drawn only for a guide to the eye.

system can be treated as a spin equilibrium. The optimized parameters were as follows: C0 = 2.18(2) cm3 K mol−1, C1 = 0.084(3) cm3 K mol−1, Tc = 134(1) K, and the enthalpy change ΔH = 3.22(6) kJ mol−1. The spin-crossover temperature T1/2 is defined as the temperature at which molar fractions of the high-spin and low-spin species are equal, and in this treatment Tc = T1/2. The entropy change was estimated as ΔS = 24.0(7) J K−1 mol−1 from the relation TcΔS = ΔH. The very gradual spin-transition profile suggests an almost uncooperative system, according to the domain model.32,33 The bond lengths or molecular bulkiness hardly changes regardless of temperature, probably bringing about negligible

temperature range. This equation has been often utilized to analyze spin equilibrium materials.31,32 γ(HS) =

1 1 + exp[(ΔH /R )(1/T − 1/Tc)

χm T = C0γ(HS) + C1

(3) (4)

The experimental χmT value is described as a function of the molar fraction of the high-spin molecules (γ(HS)) (eq 4), where C0 and C1 stand for the Curie constant for the high-spin state and the paramagnetic impurity, respectively. Figure S3 in the Supporting Information displays the van’t Hoff plot, clearly showing a linear fit. This finding suggests that the present F


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Inorganic Chemistry

Figure 6. Schematic drawings of the relative energy levels and the spin density surfaces drawn at the 0.002 e Å−3 level. A DFT calculation was performed on the geometries from the crystallography at 400 K for β-1, and the broken-symmetry method was applied. (a) Singlet and triplet states of a supramolecular model calculation on (phpyNO)2 after the removal of the NiBr2 portion. Doublet and quartet states of [Ni(phpyNO)(phpyCO)Br2] (b) to estimate J1 after J2 was masked and (c) to estimate J2 after J1 was masked. Molecular structures are also shown.

crystallographic analysis. In sharp contrast to the result on the 100 K structure, the ground state appeared as a quintet at 300−400 K (Figure 5b). This finding indicates that the exchange couplings in Ni−O1N1 and Ni−O2N3 are both ferromagnetic. The temperature dependence of the singlet and quintet energy levels is shown in Figure 5c, where the level crossing clearly appeared. The level crossing temperature of ca. 250 K seems to be higher than the experimental T1/2 (134(1) K). This finding is partially accounted for by the difference in the multiplicities of the high- and low-spin states. As Figure 5b shows, one of the triplet states is located higher than the singlet state and the other lower. Before evaluation of each exchange coupling parameter, we have to note that the calculated ⟨S2⟩ values were considerably large for any singlet state.36 Accordingly, direct quantitative evaluation of the exchange coupling constant J is assumed to be hardly reliable solely from the above calculation. The problem seems to reside in the accidentally short radical−radical distance. A possible through-space N1O1···O2N3 interaction is operative owing to the cis configuration, thus giving rise to a secondary effect on the energy level on each state and spin contamination. To check this, the DFT theoretical work was moved to a twocentered model calculation in place of a three-centered calculation. Eventually, these problems were completely solved by the simulation work, as described below. After the removal of the NiBr2 portion in the 400 K structure, a supramolecular model calculation on the virtual compound (phpyNO)2 clarified that the through-space 2p−2p interaction would be unexpectedly ferromagnetic with a singlet−triplet energy gap of 40.1 K (Figure 6a). The exchange coupling constant was deduced to be 2J/kB = +43.8 K, according to Yamaguchi’s equation (eq 5)37 on the basis of the

chemical pressure affecting spin states of neighboring molecules. The spin Hamiltonian for describing the magnetic properties of a β-1 molecule can be written as eq 1 with J1 ≠ J2. The most important point is that J1 and J2 are temperature-dependent. The ground Stotal is 0 in the LT form. Therefore, both exchange couplings must be antiferromagnetic. On the other hand, there are two possibilities for the ground Stotal of the HT form. When both wings are ferromagnetic in the HT form, the entropy change due to the spin multiplicity should be ΔS = R ln 5 = 13.4 J K−1 mol−1 from the LT form. An excess entropy change was observed, and the HT form is supposed to possess additional degrees of freedom such as phonon entropy.31−33 At this stage another possibility remains where one wing has an appreciable ferromagnetic coupling while the other has a relatively weak coupling, in comparison to thermal energy around 400 K. MO Calculation. To solve this issue, a density functional theory (DFT) MO calculation34 was performed for the structure of β-1. The self-consistent field (SCF) energies on the 100 K structure of β-1 were computed by a single-point calculation on the UB3LYP/6-311+G(2d,p) level35 (Figure 5a). The lowest state was a singlet, and the spin structure can be drawn as ↓−↑↑−↓ for N1O1−Ni−O2N3. The highest state was a quintet with ↑−↑↑−↑. Two triplet states, ↑−↑↑−↓ and ↓ −↑↑−↑, are between them. More quantitatively, the singlet state is more stable by ca. 660 and 668 K than the triplet states. Thus, the theoretical DFT analysis supports that both J1 and J2 are considerably antiferromagnetic at 100 K. This is consistent with the magnetic study on β-1 (Figure 4b). The same calculation protocol has been applied to each structure of β-1 (Figure 3a), determined by means of G


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Inorganic Chemistry

present finding is compatible with the magneto−structural relation proposed.

broken symmetry method.38 The ⟨S2⟩ values, 2.0003 and 0.1702 for the triplet and singlet states, respectively, were reasonably close to the ideal values. Another model calculation of [Zn(phpyNO)2Br2] after replacing the Ni2+ ion with a Zn2+ ion was performed (Figure S4 in the Supporting Information), although such a model may involve superexchange interaction between the radicals through a diamagnetic ion,39 which disturbs the analysis of a pure through-space interaction. Eventually the ferromagnetic coupling was confirmed with 2J/ kB = +16.2 K and the ⟨S2⟩ values are 2.004 and 0.2031 for the triplet and singlet states, respectively. The coupling parameter obtained from the Zn2+ model seems to be somewhat reduced from that of the (phpyNO)2 model. J=

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DISCUSSION As briefly stated in the Introduction, there have been an increasing number of compounds showing ferromagnetic copper(II)− and nickel(II)−radical couplings.6,7 A pioneering work should be cited here. Luneau et al. reported ferromagnetic behavior with 2J/kB = +407 K for a copper(II) and 2-pyridyl imino nitroxide complex, and the planar conformation around the imino nitrogen coordination bond was indicated by a Cu−N−C−C2py torsion (φ) of 1.0°.5a The five-membered chelate ring seems to possess an advantage for a planar conformation around the imino nitrogen coordination bond for steric reasons. In comparison with a six-membered chelate ring, a five-membered ring has a relatively open bite angle, reducing out-of-plane dislocation of the ligating atoms. Actually, the corresponding 2-pyridyl nitronyl nitroxide analogues having a six-membered chelate ring tend to display antiferromagnetic couplings.29a,44 On the basis of these experimental results, a plot of J versus |φ| for the octahedral copper(II) and nickel(II) complexes is proposed to describe the magneto−structural relationship (Figure 7), where φ is chosen as a useful metric. Note that the present data point on α-1 satisfactorily obeys this correlation.

E BS − E HS 2

S ⟩HS − S2⟩BS

(5)

The terminal 2p−2p ferromagnetic coupling can afford a reason for the stabilization of the singlet state having a parallel array of the two terminal 2p spins such as ↓−↑↑−↓. After the through-bond 3d−2p interaction decreases, the through-space 2p−2p interaction becomes comparably significant. A possible explanation for the ferromagnetic coupling is as follows: an orthogonal arrangement is realized by chance, where the lobe of N−O π* (for example O1 2pz in Figure 6a) is located near a node of the N−O π* in a neighboring radical (the center of O2 and N2). The model calculation has been expanded to separately evaluate J1 and J2, as follows. Replacing a diamagnetic functional group is an easily executable way to mask a paramagnetic group in a ligand.39a,40 In the case of tert-butyl nitroxide compounds, a tert-butylcarbonyl group would be fine as a diamagnetic analogue (Figure 1).41 After the replacement of the N3 atom with an sp2 carbon atom with other atomic geometries frozen, we extracted the doublet−quartet energy gap of [Ni(phpyNO)(phpyCO)Br2] as 47.0 K for the Ni− O1N1 interaction with the quartet state ground. The ⟨S2⟩ values, 3.7502 and 0.8084 for the quartet and doublet states, respectively, were sufficiently suppressed to the ideal values. The 2J1/kB value was calculated as +31.9 K (Figure 6b).42 Similarly, the replacement of the N1 atom with an sp2 carbon atom gave a doublet−quartet energy gap of 145.2 K (⟨S2⟩ = 3.7502 and 0.8121, respectively), thus leading to 2J2/kB = +98.9 K (Figure 6c).43 Furthermore, the molecular structure of α-1 (100 K) has also been subjected to a DFT single-point calculation at the same level. The quintet state is much more low-lying than the singlet state with an energy gap as large as 808 K (Figure S5 in the Supporting Information). The 3d−2p interactions are strong enough so that neglecting 2p−2p interactions is acceptable. Thus, ferromagnetic couplings are clearly indicated for both wings. In calculation on the model [Ni(phpyNO)(phpyCO)Br2] in a manner similar to that of β-1, the doublet− quartet energy gap of α-1 (425 K) is much larger than those of β-1 (47 and 145 K), after the ⟨S2⟩ values of the 100 K α-1 structure, 3.7503 and 0.8132 for the quartet and doublet states, respectively, were confirmed to be close to the ideal values. The exchange coupling was calculated as 2J1/kB = 2J2/kB = +290 K. The α-1 molecule has a more planar chelation structure than the β-1 molecule does, as the crystallographic study revealed (Figure 3a). The magnitude of the interaction in α-1 is much larger than those of the high-temperature β-1 structure (31.9 and 98.9 K) in this calculation. Again, the

Figure 7. Plot of the exchange coupling constant (2J) against the metal−radical (M−O−N−Csp2) torsion angle (φ) characterized in octahedral nickel(II) and copper(II) complexes with nitroxide and nitronyl nitroxide ligands. A filled mark stands for the data point on α1. Open marks denote the data on the complexes previously reported, and the sources of these data have appeared in refs 8 and 45. The broken line stands for a linear fit including the data of α-1.

Here the present data point from α-1 is now superimposed onto this plot, and the expression of the best linear fit is updated using all the data including that of α-1, though the change is slight: 2J/kB = a + b|φ| with a = 465(42) K and b = −35.9(2.7) K deg−1 (the broken line in Figure 7). The critical |φC|, at which the sign of the metal−radical exchange coupling changes between positive and negative, becomes 12.8(8)°. The observable exchange interaction consists of ferro- and antiferromagnetic terms: namely, J = Jferro + Jantiferro.46a The first term originates in the exchange integral (K), which is always positive, and the second term is regulated with orbital overlap (cS2), which is negative.46b Qualitatively, the intercept (a) stands for the first term and the slope (b)-related contribution the second term. The origin of antiferromagnetic H


Article

Inorganic Chemistry

profile exhibits a monotonic increase to the paramagnetic limit and not to the high-spin limit on heating. In short, the highspin nature is buried.51 Experimentally we observe that the spin crossover of β-1 is “incomplete” even at 400 K. The origin of “incomplete” behavior is different from that of typical iron(II) spin-crossover materials; incomplete spin crossover usually takes place in an LT phase.15,52 Finally, we have to mention an advantage of using tert-butyl 2-pyridyl nitroxides. Various 2-pyridyl derivatives and metal complexes have been developed so far, where a persistent radical is substituted, such as nitronyl nitroxide,44,53 verdazyl,54 and dithiadiazolyl55 groups (Figure 8). The spin density of the

coupling is rationalized by thinking of a covalent bond character. The magnitude of the antiferromagnetic contribution is sensitive to the angular geometry and is usually overwhelming, so that the ferromagnetic coupling would survive only when the magnetic orbital arrangement is strictly orthogonal. If we pursue ferromagnetic coupling, S and φ must be reduced. Compound α-1 is a successful example of a ground high-spin molecule with a wide gap, according to this molecular design. Now let us move to apply this relationship to the result on β1. As for the LT form, the |φ2| = 37.2(5)° value found in β-1 falls in an antiferromagnetic region. It is safely concluded that the magnetic coupling on the φ2 side should be antiferromagnetic. However, |φ1| = 24.2(6)° comes close to a borderline. Okazawa has recently revisited the magneto− structural relation after the data on the copper(II) and nickel(II) complexes are separated.47 The |φC| value for Ni2+ complexes was refined to be 21(1)° from the linear fit or 26(3)° from the cos2 φ fit. The magnetic study on β-1 (Figure 4b) clarified the ground diamagnetic state, and consequently the φ2 side also served antiferromagnetic coupling. At 100 K, the γ(HS) ratio remained 27% (Figure 4b) and the φ1(T) profiles still have a considerable slope at 100 K (Figure 3a). Thus, the chelate would be more distorted out of plane and at the same time antiferromagnetic coupling would be more enhanced at a 100% HS limit. A low-lying singlet state with a considerable gap below 100 K is reasonably acceptable. For the HT form of β-1, |φ1| = 10.4° comes in a ferromagnetic region, and it is likely that ferromagnetic coupling could be assigned to the φ1 side. On the other hand, |φ2| = 25.9° falls near the borderline, especially according to Okazawa’s formulation.47 Furthermore, there is another factor keeping us from evaluating the magnetic coupling. The measurements have been done up to 400 K, and under such conditions the thermal depopulation effect seems to be inevitable. To identify the character of the exchange coupling, a DFT calculation study is helpful (Figure 5). The hybrid unrestricted B3LYP method has been demonstrated to be highly reliable in the comprehension of magnetic exchange couplings,48,49 and the geometry dependence of the exchange coupling is one of its specialties.6,8a,12,50 In fact, in the calculation on α-1, the singlet−quintet energy gap was well reproduced: experimental 864 K versus computational 808 K with the quintet ground state (Figure S5 in the Supporting Information). On combination of all the DFT results (Figure 5), the ground spin multiplicity switches as a result of energy level crossing. Although a molecule of β-1 has no symmetry element, the couplings synchronously switch on both sides. This finding agrees well with the discussion of entropy-driven spin crossover,14 as suggested from comparison of a nonsynchronous coupling switch13 with a synchronous switch.12 The following is a possible explanation. The two Ni−O−N− C2py distortions in β-1 were synchronized, because the degree of spin freedom becomes minimal. The Stotal = 2 state cannot stand on cooling, the molecule must accommodate a target low-spin state, and the structure undergoes transformation to that suitable for Stotal = 0. As a result, the coordination bond twists by as large as Δφ1 = 13.8° and Δφ2 = 11.3°. In the HT form of β-1, the energy gaps (141 K etc.) are smaller than the order of the thermal energy of room temperature (Figure 5b). Under such conditions, the χmT(T)

Figure 8. Structural formulas of some persistent radicals.

ligating atom in a nitroxide group is assumed to be almost twice as large as that of the other spin-delocalized radicals; the delocalization area is almost half (N1O1 in nitroxides versus N2O2 in nitronyl nitroxides, N4 in verdazyls, and N2S2 in dithiadiazolyls), as quantified by means of electron spin resonance, polarized neutron diffraction studies, and so on.56,57 The exchange interaction is proportional to the spin densities at the contacting atoms.58 For the realization of spin transition, strongly exchange coupled systems in an intrasite or intersite manner seem to be needed for competition with the aufbau principle.

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CONCLUSION The five-membered chelate ring in α-1 is highly planar, and the Stotal = 2 high-spin state is the ground state with 2J/kB = +288(5) K. On the other hand, the β phase has considerably out of plane deformed chelate rings, and the distortion degree depends on temperature. Detailed structural, magnetic, and computational analyses showed that the exchange couplings changed from antiferro- to ferromagnetic on heating, and T1/2 was characterized to be 134(1) K. Comparing the two phases clarified the mechanism of the unconventional spin transition. The key is the internal angular rotation and not the bond lengths. Furthermore, attention must be paid to the exchange parameter J(T) as a variable, because the structural transformation is substantial. The X-ray crystal structure analysis at a single temperature may lead to overlooking a hidden structural transition, but a combination between magnetochemistry and structural chemistry is vigorous enough to tackle such issues. The spin density at the ligating atom plays a crucial role in realizing the multicentered spin-crossover material β-1. This advantage would also help to increase sensitivity in angular dependence on the exchange-coupling switch. A potential utility of these compounds is thought to be a sensor for external stimuli such as heat and light. Realizing thermal hysteresis is the next target by introduction of intermolecular interactions and cooperative effects. I


Article

Inorganic Chemistry

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Intermolecular contacts in the α- and β-1 crystals, simulated χmT(T) curves for α-1, the van’t Hoff plot of β-1, and DFT energy diagrams of α-1 and a model compound for β-1 (PDF) Crystal structures of β-1 viewed along the Npy−Ni−Npy direction measured at each temperature (AVI)

EXPERIMENTAL SECTION

Preparation and Characterization. After an acetone solution (0.3 mL) containing phpyNO8a (48.2 mg; 0.20 mmol) and a methanol solution (0.4 mL) containing NiBr2·3H2O (27.3 mg; 0.10 mmol) at room temperature were combined, additional ether (1.5 mL) was slowly added to the surface of the solution. The resultant mixture was allowed to stand at 0 °C for a few days. Dark red-purple diamond-shaped platelets of α phase [Ni(phpyNO)2Br2] (α-1) precipitated. They were collected on a filter. The yield was 24 mg (0.034 mmol; 34%). Mp: >200 °C dec. IR (neat, attenuated total reflection (ATR)): 554, 693, 856, 1005, 1181, 1253, 1315, 1343, 1374, 1401, 1449, 1578, 1578, 2937, 2993, 3055, 3095 cm−1. Anal. Calcd for C30H34Br2N4NiO2: C, 51.39; H, 4.89; N, 7.99. Found: C, 51.23; H, 4.77; N, 8.09. Another synthetic run gave black rectangular crystals of β phase [Ni(phpyNO)2Br2] (β-1) in 8% yield. The following seeding method for the isolation of β-1 is typical. After solutions of 48.3 mg of phpyNO (0.20 mmol) in 0.7 mL of acetone and 27.8 mg of NiBr2·3H2O (0.10 mmol) in 1.3 mL of 1-pentanol were combined, a tiny seed of β-1 was dropped. The resultant mixture was allowed to stand in a refrigerator for 2 days. Crystalline β-1 precipitated as rectangular prisms and was collected on a filter (44 mg, 63%). Mp: >200 °C dec. IR (neat, ATR): 554, 697, 854, 1010, 1178, 1257, 1306, 1342, 1374, 1401, 1448, 1583, 2933, 3106 cm−1. Anal. Calcd for C30H34Br2N4NiO2: C, 51.39; H, 4.89; N, 7.99. Found: C, 51.39; H, 4.91; N, 8.01. Crystallographic Analysis. X-ray diffraction data of single crystals of α- and β-1 were acquired on a Rigaku Saturn70 diffractometer with a CCD area detector and graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The structures were directly solved by a heavy-atom method and expanded using Fourier techniques in the CrystalStructure program package. 59 The parameters were refined in the Shelxl program.60 Numerical absorption correction was used. Hydrogen atoms were placed at calculated positions, with Uiso(H) set at 1.2 times larger than the Ueq value of the attached atom. The Csp2H and Csp3H bond lengths were fixed at 0.93 and 0.96 Å, and the hydrogen atom displacement parameters were treated in a riding model. The thermal displacement parameters of non-hydrogen atoms were anisotropically refined. Selected crystal data, data collection, and structure refinement details are summarized in Table 1, and selected bond distances and angles are given in Table 2. CCDC numbers 1899480, 1899481, 1899482, and 1899483 for α-1 measured at 100 K, α-1 at 400 K, β-1 at 100 K, and β-1 at 400 K, respectively, include the experimental details and full geometrical parameter tables. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html. Magnetic Analysis. Magnetic susceptibilities of the polycrystalline samples of α- and β-1 were acquired on an MPMS SQUID magnetometer (Quantum Design) at an applied magnetic field of 5000 Oe in the temperature range 1.8−400 K. The magnetic response was corrected with diamagnetic blank data of the sample holder measured separately. The diamagnetism of the sample itself was deduced from Pascal’s constant.61 DFT Calculation. Density functional theory (DFT) MO calculation investigations were performed with the Gaussian03 package revision C.0234 running on a Windows PC. The geometry was given from the X-ray crystallographic study. The self-consistentfield energy was converged to the 10−7 au level on the hybrid unrestricted B3LYP Hamiltonian62 with the 6-311+G(2d,p) basis set. The energy gap in au was determined with a single-point calculation for each spin multiplet state and converted to the conventional units with a factor of 3.156 × 105 K/au.

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Accession Codes

CCDC 1899480−1899483 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

*T.I.: e-mail, [email protected]; tel, +81-42-443-5490; fax, +81-42-443-5501. ORCID

Takayuki Ishida: 0000-0001-9088-2526 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by KAKENHI (JSPS/ 15H03793). REFERENCES

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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.inorgchem.9b00885. J


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Inorganic Chemistry

(36) The SCF energies and ⟨S⟩ 2 values respectively are −8189.80745528121 au and 6.0004 for the quintet state, −8189.80892450049 au and 2.0501 for the triplet state #1, −8189.80894949389 au and 2.0530 for the triplet state #2, and −8189.8110395769 au and 3.9336 for the singlet state of the 100 K structure of β-1; −8189.63569931043 au and 6.0004 for the quintet state, −8189.63538621572 au and 2.0579 for the triplet state #1, −8189.6350127868 au and 2.0591 for the triplet state #2, and −8189.63525244468 au and 4.1361 for the singlet state of the 400 K structure of β-1. (37) (a) Yamaguchi, K.; Takahara, Y.; Fueno, T.; Nasu, K. Ab initio MO calculations of effective exchange integrals between transitionmetal ions via oxygen dianions: nature of the copper-oxygen bonds and superconductivity. Jpn. J. Appl. Phys. 1987, 26, L1362−L1364. (b) Yamaguchi, K.; Kawakami, T.; Takano, Y.; Kitagawa, Y.; Yamashita, Y.; Fujita, H. Analytical and ab initio studies of effective exchange interactions, polyradical character, unpaired electron density, and information entropy in radical clusters (R)N: allyl radical cluster (N = 2 − 10) and hydrogen radical cluster (N = 50). Int. J. Quantum Chem. 2002, 90, 370−385. (38) (a) Noodleman, L.; Norman, J. G., Jr. The Xα valence bond theory of weak electronic coupling. Application to the low-lying states of Mo2Cl84‑. J. Chem. Phys. 1979, 70, 4903−4906. (b) Noodleman, L. Valence bond description of antiferromagnetic coupling in transition metal dimers. J. Chem. Phys. 1981, 74, 5737−5743. (c) O’Brien, T. A.; Davidson, E. R. Semiempirical local spin: Theory and implementation of the ZILSH method for predicting Heisenberg exchange constants of polynuclear transition metal complexes. Int. J. Quantum Chem. 2003, 92, 294−325. (d) Bencini, A.; Totti, F. DFT Description of the Magnetic Structure of Polynuclear Transition-Metal Clusters: The Complexes [{Cu(bpca)2(H2O)2}{Cu(NO3)2}2], (bpca = Bis(2pyridylcarbonyl)amine), and [Cu(DBSQ)(C2H5O)]2, (DBSQ = 3, 5-di-tert-butyl-semiquinonato). Int. J. Quantum Chem. 2005, 101, 819−825. (39) (a) Mochizuki, T.; Nogami, T.; Ishida, T. Ferromagnetic Superexchange Coupling through a Diamagnetic Iron(II) Ion in a Mixed-Valent Iron(III, II, III) meso-Helicate. Inorg. Chem. 2009, 48, 2254−2259. (b) Murakami, R.; Nakamura, T.; Ishida, T. Doubly TEMPO-coordinated gadolinium(III), lanthanum(III), and yttrium(III) complexes. Strong superexchange coupling across rare earth ions. Dalton Trans 2014, 43, 5893−5898. (40) Kahn, M. L.; Sutter, J. P.; Golhen, S.; Guionneau, P.; Ouahab, L.; Kahn, O.; Chasseau, D. Systematic investigation of the nature of the coupling between a Ln(III) ion (Ln= Ce(III) to Dy(III)) and its aminoxyl radical ligands. structural and magnetic characteristics of a series of {Ln(organic radical)2} compounds and the related {Ln(nitrone)2} derivatives. J. Am. Chem. Soc. 2000, 122, 3413−3421. (41) (a) Kanetomo, T.; Yoshii, S.; Nojiri, H.; Ishida, T. Singlemolecule magnet involving strong exchange coupling in terbium(III) complex with 2,2’-bipyridin-6-yl tert-butyl nitroxide. Inorg. Chem. Front. 2015, 2, 860−866. (b) Kanetomo, T.; Ishida, T. Luminescent single-ion magnets from Lanthanoid(III) complexes with monodentate ketone ligands. AIP Conf. Proc. 2015, 1709, 020015. (42) One of the reviewers pointed out that the present method was known as the doped cluster approach (a magnetic ion is substituted with a diamagnetic ion) and that there seemed sometimes to be a risk in determination of J. In the present study, the terminal spin, namely phpyNO, is masked, and the possible risk would be small, in comparison with calculation by masking the inner spin. See: Bencini, A.; Totti, F. A few comments on the application of density functional theory to the calculation of the magnetic structure of oligo-nuclear transition metal clusters. J. Chem. Theory Comput. 2009, 5, 144−154. (43) From a comparison of φ and J, Table 2 shows |φ1| < |φ2|, but Figure 6 demonstrates J1 < J2. We have to admit that the exchange coupling would be regulated not only by the angular torsion. Notwithstanding, both J1 and J2 qualitatively are ferromagnetic, being in good agreement with the experiment (Figure 4b). (44) (a) Richardson, P. F.; Kreilick, R. W. Copper Complexes with Free-Radical Ligands. J. Am. Chem. Soc. 1977, 99, 8183−8187.

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