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Article

Cobalt(II) Terpyridin-4′-yl Nitroxide Complex as an Exchange-Coupled Spin-Crossover Material

by
Akihiro Ondo
and
Takayuki Ishida
*
Department of Engineering Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
*
Author to whom correspondence should be addressed.
Crystals 2018, 8(4), 155; https://doi.org/10.3390/cryst8040155
Submission received: 2 March 2018 / Revised: 26 March 2018 / Accepted: 29 March 2018 / Published: 2 April 2018
(This article belongs to the Special Issue Synthesis and Applications of New Spin Crossover Compounds)
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Abstract

:
Spin-crossover (SCO) was studied in [Co(L)2](CF3SO3)2, where L stands for diamagnetic 2,2′:6′,2′′-terpyridine (tpy) and its paramagnetic derivative, 4′-{4-tert-butyl(N-oxy)aminophenyl}-substituted tpy (tpyphNO). The X-ray crystallographic analysis clarified the Co-N bond length change (Δd) in high- and low-temperature structures; Δdcentral = 0.12 and Δddistal = 0.05 Å between 90 and 400 K for L = tpy and Δdcentral = 0.11 and Δddistal = 0.06 Å between 90 and 300 K for L = tpyphNO. The low- and high-temperature structures can be assigned to approximate low- and high-spin states, respectively. The magnetic susceptibility measurements revealed that the χmT value of [Co(tpyphNO)2](CF3SO3)2 had a bias from that of [Co(tpy)2](CF3SO3)2 by the contribution of the two radical spins. The tpy compound showed a gradual SCO around 260 K and on cooling the χmT value displayed a plateau down to 2 K. On the other hand, the tpyphNO compound showed a relatively abrupt SCO at ca. 140 K together with a second decrease of the χmT value on further cooling below ca. 20 K. From the second decrease, Co-nitroxide exchange coupling was characterized as antiferromagnetic with 2JCo-rad/kB = −3.00(6) K in the spin-Hamiltonian H = −2JCo-rad(SCo·Srad1 + SCo·Srad2). The magnetic moment apparently switches double-stepwise as 1 μB ⇄ 3 μB ⇄ 5 μB by temperature stimulus.

Graphical Abstract

1. Introduction

Spin crossover (SCO) is a reversible transition between low-spin (LS) and high-spin (HS) states by external stimuli like heat, light, pressure, or magnetic field [1,2,3]. A number of materials are studied toward application in sensors [4]. Much attention has been paid to develop multifunctional SCO materials; for example, mesophase or liquid crystal properties [5,6,7] and magnetic exchange coupling [8,9]. Iron(II) (3d6) coordination compounds are the most developed materials among various SCO complexes [10,11,12,13], because SCO occurs between S = 0 dia- and S = 2 paramagnetic states [14], drastically exhibiting magnetic and chromic changes. However, if one develops SCO materials having exchange coupling from an adjacent paramagnetic center, iron(II) compounds are unsuitable. In this line, cobalt(II) (3d7) SCO behavior between S = 1/2 and S = 3/2 states are promising because the LS state is still paramagnetic (Scheme 1a). The entropy and geometry changes in cobalt(II) SCO compounds are less pronounced than those of iron(II) and iron(III) ones and it generally leads to a gradual SCO profile as a function of temperature [15,16,17,18]. A possible scenario of exchange-coupled SCO materials involves multi-step magnetic property jumps as a function of temperature. Namely, in the χmT versus T profile, a low-spin region has an additional spin-equilibrium regulated by magnetic exchange coupling. When a paramagnetic ligand (L) with Srad = 1/2 is available, a Co2+(LS)/L = 1/1 compound would show a singlet-triplet equilibrium, or a Co2+(LS)/L = 1/2 compound a doublet-quartet equilibrium.
The 2,2′:6′,2′′-terpyridine (tpy) ligands are popular to the cobalt(II) SCO materials [19,20,21,22]; for example, [Co(tpy)2](BF4)2 has been reported to exhibit SCO at 270 K [19] and [Co(tpy)2](ClO4)2·0.5H2O displayed SCO at 180 K [20]. There have been a number of reports on the valence tautomerism in cobalt(II)-radical coordination systems [23,24]. Various photomagnets have been developed from heterospin systems including prussian blue analogues [25,26]. Such charge transfer mechanism also works in cobalt(II) spin-crossover materials carrying a directly coordinated nitroxide ligand [8,9], often disturbing the analysis of exchange interaction. Thus, our hypothesis is as follows: the organic paramagnetic center should be remote from the SCO center. A ligand π-electron system is indispensable to maintain appreciable exchange coupling (Scheme 1a). As for a paramagnetic substituent, tert-butyl phenyl nitroxide radicals are sufficiently persistent under ambient conditions [27,28]. Therefore, we designed a novel paramagnetic ligand based on tpy, 4′-{4-tert-butyl(N-oxy)aminophenyl}-2,2′:6′,2′′-terpyridine (tpyphNO) (Scheme 1b). Its cobalt(II) complexes would be a target to realize the present project.

2. Materials and Methods

2.1. Materials

The ligands tpy and 4′-bromo-2,2′:6′,2′′-terpyridine (Br-tpy) are commercially available. The latter was subjected to the preparation of tpyphNO (Scheme 2), as follows. The counterpart 4-(N-tert-butyl-O-tert-butyldimethylsilylhydroxylamino)phenyl boronic acid (TBDMS-BA) was prepared according to the literature method [29].
A mixture of Br-tpy (730 mg; 2.34 mmol), TBDMS-BA (741 mg; 2.30 mmol), Pd(PPh3)4 (271 mg; 0.24 mmol), Na2CO3 (2.50 g; 14.3 mmol) in 50 mL of dioxane and 50 mL of water was heated at 100 °C for 72 h. The resultant mixture was extracted with dichloromethane, washed with brine and dried over anhydrous Na2SO4. The organic solution was filtered and concentrated under reduced pressure. The main product was separated through a basic alumina column eluted with dichloromethane. The concentration gave a yellow solid, which was characterized to be tpyphNOTBDMS (0.911 g; 1.79 mmol). Yield 85%. m.p: 212–223 °C. MS (ESI+) m/z: 511.29 (M + H+). 1H NMR (500 MHz CDCl3): δ −0.084 (6H, br), 0.93 (9H, s), 1.13 (9H, s), 7.36 (2H, J = 8.1 Hz, t), 7.36 (2H, J = 8.6 Hz, d), 7.77 (2H, J = 8.6 Hz, d), 7.88 (2H, J = 8.1 Hz, t) 8.68 (2H, J = 8.1 Hz, d), 8.73 (2H, s), 8.74 (2H, J = 8.1 Hz, d). 13C NMR (126 MHz, CDCl3): δ 156.34, 155.80, 152.14, 150.20, 149.12, 136.88, 134.56, 126.30, 125.46, 123.79, 121.34, 118.70, 61.44, 26.17, 26.12, 17.96, −4.64. IR (neat, attenuated total reflection (ATR)): 777, 857, 1466, 1061, 2853, 2928, 2957 cm−1.
To a dry tetrahedrofuran (THF) solution (10 mL) containing tpyphNOTBDMS (0.911 g; 1.79 mmol) 2.5 mL (2.5 mmol) of tetrabutylammonium fluoride in a THF solution (1 mol L−1) was added dropwise at 0 °C under nitrogen atmosphere. The mixture was stirred for further 1 h at room temperature. The resultant solution was extracted with dichloromethane after aqueous NaHCO3 was added. The organic layer was dried over anhydrous Na2SO4 and filtered. After addition of a small amount of hexane, the deprotected product (tpyphNOH) was precipitated as a colorless solid (0.570 g; 1.43 mmol). Yield 85%. m.p.: 211–212 °C. MS (ESI+) m/z: 397.20 (M + H+), 419.17 (M + Na+). 1H NMR (500 MHz CDCl3): δ 1.20 (9H, s), 7.34 (2H, J = 8.0 Hz, t), 7.38 (2H, J = 8.6 Hz, d), 7.83 (2H, J = 8.6 Hz, t) 7.88 (2H, J = 8.0 Hz, d), 8.66 (2H, J = 8.0 Hz, d), 8.72 (2H, s), 8.73 (2H, J = 8.0 Hz, d). 13C NMR (126 MHz, CDCl3): δ 156.36, 155.72, 150.84, 149.95, 149.16, 136.79, 135.09, 126.56, 124.56, 123.82, 121.22, 118.78, 60.97, 26.17. IR (neat, ATR): 730, 788, 1582, 2871, 2978, 3049, 3149, 3743 cm−1.
After the above product (87 mg; 0.22 mmol) was dissolved in dichloromethane (20 mL), freshly prepared Ag2O (510 mg; 2.2 mmol) was added and the resultant mixture was stirred at room temperature for 1 h. The solution portion was filtered and concentrated under reduced pressure. Crystallization from dichloromethane and hexane gave tpyphNO as a red solid (40 mg; 0.11 mmol). Yield 46%. M.p. 154–155 °C. MS (ESI+) m/z: 396.18 (M + H+), 418.16 (M + Na+). IR (neat, ATR): 660, 778, 1193, 1250, 148, 1651, 2980 cm−1. ESR (9.4 GHz, room temperature in toluene): aN = 1.165 mT, aH(ortho) = 0.212 mT (×2), aH(meta) = 0.094 mT (×2) at g = 2.0065.
The target complexes were prepared as follows. A methanol solution (7 mL) involving tpyphNO (40 mg; 0.10 mmol), CoCl2·6H2O (12 mg; 0.050 mmol) and LiCF3SO3 (16 mg; 0.10 mmol) was allowed to stand at 0 °C under nitrogen atmosphere, to give [Co(tpyphNO)2](CF3SO3)2 as a dark red polycrystalline precipitation (27 mg; 0.024 mmol). Yield: 24%. m.p.: 286 °C (decomp.). The product was subjected to elemental, crystallographic and magnetic analyses without further purification. All data satisfied the formula of the target compound. IR (neat, ATR): 633, 790, 1030, 1136, 1260, 1603, 2935, 2978, 3083 cm−1. Anal. Calcd. for C52H46Co1F6N8O8S2: C, 54.40; H, 4.04; N, 9.76; S, 5.59%. Found: C, 54.34; H, 3.85; N, 9.81; S, 5.69%.
A similar method using tpy in place of tpyphNO gave [Co(tpy)2](CF3SO3)2 as orange polycrystals in an 85% yield. m.p.: 321 °C (decomp.). The product was subjected to elemental, crystallographic and magnetic analyses without further purification. All data satisfied the formula of the target compound. IR (neat, ATR): 513, 570, 632, 763, 1028, 1126, 1256, 1452, 1600, 3080 cm−1. Anal. Calcd. for C32H22Co1N6O6S2: C, 46.67; H, 2.69; N, 10.20; S, 7.79%. Found: C, 46.29; H, 2.67; N, 10.08; S, 7.37%.

2.2. Crystallographic Analysis

X-ray diffraction data of [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy) were collected on a Rigaku Saturn70 CCD diffractometer with 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 [30]. Numerical absorption correction was used. Hydrogen atoms were located at calculated positions and their parameters were refined as “riding.” The thermal displacement parameters of non-hydrogen atoms were refined anisotropically. Selected crystallographic data are given in Table 1 and selected bond distances and angles are listed in Table 2 and Table 3. CCDC numbers 1826042, 1826043, 1826044 and 1826045 contain the crystallographic analysis details for [Co(tpyphNO)2](CF3SO3)2 at 90 and 300 K and [Co(tpy)2](CF3SO3)2 at 90 and 400 K, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

2.3. Magnetic Study

Magnetic susceptibilities of [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy) were measured on a Quantum Design MPMS-XL7 SQUID magnetometer with a static field of 0.5 T. The magnetic responses were corrected with diamagnetic blank data of the sample holder measured separately. The diamagnetic contribution of the sample itself was estimated from Pascal’s constants [31].

3. Results and Discussion

3.1. Preparation

A new ligand tpyphNO was prepared via the Suzuki coupling reaction [32] from commercially available 4′-bromoterpyridine (Br-tpy) and a protected hydroxylaminophenyl boronic acid (TBDMS-BA) [29] (Scheme 2a). Paramagnetic tpyphNO was prepared after the deprotection of the above product with tetrabutylammonium fluoride followed by the oxidation with Ag2O. The resultant nitroxide was isolated at room temperature under air and characterized as tpyphNO by means of spectroscopic methods including electron spin resonance (ESR) spectroscopy. A target complex [Co(tpyphNO)2](CF3SO3)2 was prepared by simply combining methanol solutions of the ligand and CoCl2 in the presence of the counter anion CF3SO3− (Scheme 2b). As a reference complex, [Co(tpy)2](CF3SO3)2 was also prepared in a similar manner, using tpy in place of tpyphNO. The nitroxide-carrying derivative is dark red and the reference is orange at room temperature.

3.2. Crystal Structures

The X-ray crystallographic analysis on [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy) was successful at 90 and 300 or 400 K (Table 1 and Figure 1). Though the crystal structure of [Co(tpy)2](CF3SO3)2 at 120 K has recently been reported [33], we measured them at 90 and 400 K to compare the LS and HS structures. The crystal structure of [Co(tpy)2](CF3SO3)2 possesses a relatively high symmetry orthorhombic Pbcn, which is kept between 90 and 400 K. A half molecule is crystallographically independent. Compound [Co(tpy)2](ClO4)2·0.5H2O is known to crystallize in a tetragonal cell [20] and the relatively low symmetry of CF3SO3– may cause the different crystal system. On the other hand, the crystal of [Co(tpyphNO)2](CF3SO3)2 belongs to monoclinic P21/c and the whole molecule corresponds to an independent unit. The linear spin triad structure is unequivocally characterized. There is no solvent molecule in any crystal.
The nitroxide group was characterized by the N–O bond lengths (1.300(4) Å N4–O1 and 1.293(4) Å for N8–O2 at 90 K) in a typical range of aryl tert-butyl nitroxides [34]. The two meridional chelate planes are arranged to be almost perpendicular with the dihedral angle of 94.99(6)°. The long molecular axis is somewhat bent at the metal center, as indicated with the N4...Co1...N8 angle of 159.98(3)° at 90 K, being considerably smaller than 180°. The 4-phenylpyridine core in each ligand is not coplanar. The dihedral angles between the pyridine and adjacent phenyl rings are 10.4(1) and 33.8(1)° at 90 K with respect to the N2- and N6-pyridine sides, respectively.
The cell volume expansion of the tpy derivative is 4.0% from 90 to 300 K and that of the tpyphNO derivative 5.5% from 90 to 400 K. The considerable volume changes originate in the distance changes between the metal and the coordinated donor atom (Δd) accompanying SCO. Usually Δd is not so large (0.07–0.11 Å) in cobalt(II) SCO complexes as those of the iron(II) complexes [15,19,20], because only one electron is transferred to the antibonding orbital upon SCO [16]. The present Co–N bond lengths are completely compatible with those of the previous SCO [Co(tpy)2]2+ compounds. For example, on the known SCO complex [Co(tpy)2](BF4)2, Kilner et al. [19] reported that d(Co–Ncentral) of the HS state is longer than that of the LS state by 0.12 Å on the average. In our case Δdcentral = 0.11 and 0.12 Å for the complexes with L = tpyphNO and tpy, respectively (Table 2). As for the Co–Ndistal bond lengths, the HS state possesses longer distances than the LS state by 0.06 Å in [Co(tpy)2](BF4)2 [19]. The present compounds showed Δddistal = 0.06 and 0.05 Å, respectively. These quite similar geometrical features strongly suggest that the low- and high-temperature structures can be assigned to approximate LS and HS states, respectively. This hypothesis is proven from the magnetic study (see below). The different sensitivity between Δdcentral and Δddistal is caused by the Jahn-Teller effect due to the LS eg1 state as well as the steric effect from the rigid ligand.
The HS states are known to favor distorted coordination geometry [35,36,37,38]. Among various geometrical parameters, Σ and CShM seem to be sensitive and convenient metrics [38]. The Σ values [39] were derived from the N–Co–N bond angles (Table 3), according to Equation (1). An ideal octahedron (Oh) possess Σ = 0°. By using the SHAPE software [40], the continuous shape measures (CShM) are calculated with respect to an Oh. An ideal Oh returns null. The HS states possess relatively distorted Oh, as expected (4.282 at 300 K versus 2.824 at 90 K for [Co(tpyphNO)2](CF3SO3)2 and so on). The bite angle of the five-membered chelate ring seems to be responsible to the difference of Σ; namely, ϕ in the HS state tends to be smaller than that of the LS state (79.37° at 90 K versus 76.47° at 300 K on the average). Furthermore, the ϕ reduction is related to the elongation of the five-membered ring. In short, the Co–N distance regulates these distortion parameters.
Σ = ∑ i = 1 12 | φ ( ∠ c i s   N - Fe - N ) i – 90 ° |
We have to make a comment on the intermolecular interaction in particular in the crystal of [Co(tpyphNO)2](CF3SO3)2. The shortest interatomic distances with respect to the N–O groups are 5.205(4) Å for O4...O2′ and 5.615(4) Å for N4...N8” at 90 K [the symmetry operation codes for ’ and ” are (1 + x, y, z) and (1 + x, 3/2 − y, 1/2 + z), respectively]. There hardly seems to be any exchange pathway. The tpy portions in the nearest neighboring molecules are arranged parallel with a separation of ca. 3.6 Å (Figure 2). The shortest Co...O(nitroxide) is found as 4.241(3) Å for Co1...O1* [the symmetry operation code for * is (1 − x, 1 − y, 1 − z)]. Two molecules are linked in a head-to-tail manner with two centrosymmetry-related Co1...O1* and Co1*...O1 distances. It is more likely that the intramolecular interaction through π-conjugation is dominant compared to the intermolecular through-space interaction but relatively short intermolecular distances cannot be neglected completely. In this case, the magnetic properties would be described as the sum of two Co...nitroxide pairs and two nitroxide doublets in every two molecules. This is another interpretation for exchange coupling in [Co(tpyphNO)2](CF3SO3)2. However, the motivation of this project never changes, because the cobalt(II) and nitroxide spins are exchange-coupled indeed, whether it works in an intra- or intermolecular fashion. By sharp contrast, such supramolecular contacts are absent from parent [Co(tpy)2](CF3SO3)2.

3.3. Magnetic Properties

The magnetic susceptibilities of polycrystalline specimens of [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy) were measured on a SQUID magnetometer in a temperature range of 1.8–300 K for the former and 1.8–400 K for the latter. As Figure 3 shows, the χmT values of [Co(tpy)2](CF3SO3)2 were 0.516 and 2.20 cm3 K mol−1 at 90 and 400 K, respectively. From the crystal structure analysis, the spin-state at 90 K is LS, namely, SCo2+ = 1/2 and accordingly the Landé factor gCo2+,LS = 2.35. The spin state at 400 K is HS, SCo2+ = 3/2, which leads to gCo2+,HS = 2.15. The latter involves a slight underestimation of gCo2+,HS, because the χmT value still has a small positive slope at 400 K. The SCO temperature T1/2 is defined as the temperature at which equimolar fractions of the HS and LS species are present. The gradual S-shaped curve in 150–400 K indicates T1/2 = ca. 260 K for [Co(tpy)2](CF3SO3)2. The χmT value is ideally flat below 100 K. Note that practically no exchange coupling took place, especially illustrated with the constant χmT in a lowest-temperature region.
Referring to the results of [Co(tpy)2](CF3SO3)2, we can analogously analyze the data of [Co(tpyphNO)2](CF3SO3)2. The χmT versus T profile of [Co(tpyphNO)2](CF3SO3)2 is apparently biased from that of [Co(tpy)2](CF3SO3)2, by the contribution of two radical spins (0.75 cm3 K mol−1). Another cause of this gap is the difference of the gCo2+ values between the two compounds. The high-temperature χmT value of [Co(tpyphNO)2](CF3SO3)2 was 3.47 cm3 K mol−1 at 300 K. On cooling the χmT value decreased to draw an S-shaped profile in 250–100 K and reached a plateau at ca. 1.45 cm3 K mol−1 around 80 K. On further cooling, the χmT value again decreased to the smallest value 0.607 cm3 K mol−1 at 1.8 K (the base temperature of the apparatus available). The first drop is ascribable to Co2+ SCO behavior with T1/2 = ca. 140 K. The second drop is accordingly assigned to exchange coupling behavior among the LS Co2+ spin and peripheral nitroxide spins.
The spin-Hamiltonian is defined as Equation (2), where JCo-rad stands for the exchange coupling constant. An approximation is introduced, where the spin centers are symmetrically arrayed in a linear manner and the interaction between the terminals is ignored. The fitting is performed only for analyzing the exchange behavior recorded in a low-spin region. The parameters were optimized according to the van Vleck equation, involving an averaged g value [41,42], giving gavg = 2.352(9) and 2JCo-rad /kB = −3.63(12) K. Alternatively, the grad and gCo2+,LS values can be separated with a more detailed van Vleck equation written as Equation (3) [43]. Assuming that the grad value is frozen to 2.006 (from the ESR spectrum of tpyphNO), the optimization gave gCo2+,LS = 2.98(2) together with 2JCo-rad/kB = −3.00(6) K. The calculation curve is superposed in Figure 3.
H = −2JCo-rad(SCo·Srad1 +SCo·Srad2)
χ m =   N A μ B 2 4 k B T   10 g 3 / 2 , 1 2 exp ( J Co - rad / k B T ) + g 1 / 2 , 0 2 + g 1 / 2 , 1 2 exp ( – 2 J Co - rad / k B T ) 2 exp ( J Co - rad / k B T ) + 1 + exp ( – 2 J Co - rad / k B T )
with
  • g1/2,1 = (4grad − gCo2+,LS)/3
  • g3/2,1 = (2grad + gCo2+,LS)/3
  • g1/2,0 = gCo2+,LS
At the ground state, Stotal should be 1/2; on the other hand, three paramagnetic spins are present in the almost constant χmT region in ca. 20–80 K. Thanks to the different temperature regions where spin-crossover and exchange coupling effects are operative, the exchange coupling parameter is well resolved to give a precise evaluation. Furthermore, the χmT plateau clearly appeared. In total, [Co(tpyphNO)2](CF3SO3)2 can be regarded as a doubly switchable material showing 1 μB ⇄ 3 μB ⇄ 5 μB by temperature stimulus.
The χmT versus T profile for [Co(tpy)2](CF3SO3)2 shows a very gradual SCO curve, whereas that of [Co(tpyphNO)2](CF3SO3)2 displays a relatively abrupt SCO curve (Figure 3). As described above (Figure 2), there are intermolecular interactions such as short Co...O(nitroxide) distances. The ligands in a neighboring molecule are centrosymmetry-related and planar portions are arranged in parallel with a separation of ca. 3.6 Å. Weak π-π stacking effects can be found in a dimeric structure as well as in interdimer relation. Owing to the spiro-type structure of the [Co(tpyphNO)2]2+ core, another parallel stacking motifs spread in the second direction, though the counter anion intervenes. The peripheral substituents like 4-tert-butyl(N-oxy)aminophenyl may serve additional intermolecular interaction, which may contribute cooperativity [44]. Such intermolecular interactions enhance an abrupt character of SCO [3,44,45].
Basically, the eg orbitals with σ-type symmetry possess no orbital overlap against π or π*-type orbitals of the ligand. This situation has been discussed when the nitroxide radical is directly coordinated to the metal ions [42,46,47] and in the present compound the ligating atom is a pyridine nitrogen atom. The 3d electron configuration of LS Co2+ is (t2g)6(eg)1 and the magnetic eg orbitals might lead to orthogonal geometry between the two magnetic orbitals (Figure 4a,b). However, the orthogonality is very sensitive to the coordination structure and out-of-plane deformation gives rise to loss of ferromagnetic coupling (Figure 4c,d) [14,34,42,48,49,50,51]. As the crystallographic analysis revealed, the long molecular axis is considerably bent (159.98(3)°) and the octahedral coordination sphere is largely distorted owing to the five-membered chelate ring. Therefore, the orthogonality is ready to breakdown.
The magnitude of the exchange coupling is limited to be small (2JCo-rad/kB = −3.00(6) K). It is comparable to several 3d-2p heterospin exchange coupling across a pyridine ring [52,53] and smaller than the 3d-3d exchange interaction found in the known dinuclear cobalt(II) SCO compound (2J/hc = 11.7 cm−1) [54]. There is an intervening organic portion between the 2p and 3d spins in [Co(tpyphNO)2](CF3SO3)2. The spin-polarization mechanism is well documented with respect to the 2-, 3- and 4-pyridyl-substituted isomers [52,53]. As shown in Scheme 3, the 1,p-position of the 4-phenylpyridine core plays a role of a magnetic coupler and the ligating nitrogen atom has a positive spin density. As stated above, ferromagnetic coupling would be expected with an orthogonally placed eg spin. However, the spin-polarization is not so effective across a long distance. Moreover, a non-planar biaryl conformation brings about a reduction of the exchange interaction [55,56,57]. The dihedral angles between the pyridine and adjacent benzene rings are 10.4(1) and 33.8(1)° in [Co(tpyphNO)2](CF3SO3)2. A shorter ligand without a para-phenylene spacer—namely, tert-butyl 2,2′:6′,2′’-terpyridin-4′-yl nitroxide—might be a promising exchange coupler to improve exchange interaction. Its cobalt(II) complexes will be a next target.

4. Conclusions

The SCO behavior was observed in [Co(tpy)2](CF3SO3)2 and [Co(tpyphNO)2](CF3SO3)2. The tpy compound showed a gradual SCO in 150–400 K. On the other hand, the tpyphNO derivative exhibited a relatively abrupt SCO in 100–250 K together with antiferromagnetic Co-nitroxide exchange coupling with 2J/kB = −3.00(6) K. The comparison work has proven the coexistence of SCO and exchange coupling in a complex ion [Co(tpyphNO)2]2+. The d-π magnetic exchange coupling is rationalized with the pyridine π-conjugation system. Thanks to the different temperature regions where they are operative, the magnetic moment apparently switches double-stepwise as 1 μB ⇄ 3 μB ⇄ 5 μB by temperature stimulus. The present work can be regarded as a successful example of development of multifunctional SCO materials including additional magnetic exchange coupling.

Acknowledgment

This work was financially supported from KAKENHI (JSPS/15H03793).

Author Contributions

Akihiro Ondo participated in the preparation, X-ray structural analysis and magnetic study. Takayuki Ishida designed the study and wrote the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 08 00155 sch001 550
Scheme 1. (a) Schematic drawing of proposal for exchange-coupled spin-crossover materials; (b) Structural formula of tpyphNO.
Scheme 1. (a) Schematic drawing of proposal for exchange-coupled spin-crossover materials; (b) Structural formula of tpyphNO.
Crystals 08 00155 sch001
Crystals 08 00155 sch002 550
Scheme 2. Synthetic routes of (a) tpyphNO and (b) [Co(tpyphNO)2](CF3SO3)2.
Scheme 2. Synthetic routes of (a) tpyphNO and (b) [Co(tpyphNO)2](CF3SO3)2.
Crystals 08 00155 sch002
Crystals 08 00155 g001 550
Figure 1. X-ray crystal structures of [Co(L)2](CF3SO3)2 (L = (a) tpyphNO; (b) tpy), measured at 90 K. The thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms and counter anions are omitted for clarity. The symmetry operation code for # is (1 − x, y, 1/2 − z) in (b).
Figure 1. X-ray crystal structures of [Co(L)2](CF3SO3)2 (L = (a) tpyphNO; (b) tpy), measured at 90 K. The thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms and counter anions are omitted for clarity. The symmetry operation code for # is (1 − x, y, 1/2 − z) in (b).
Crystals 08 00155 g001
Crystals 08 00155 g002 550
Figure 2. Molecular arrangement in the crystal of [Co(tpyphNO)2](CF3SO3)2. Only two molecules are shown. Hydrogen atoms and counter anions are omitted for clarity. Symmetry code for *: 1 − x, 1 − y, 1 − z.
Figure 2. Molecular arrangement in the crystal of [Co(tpyphNO)2](CF3SO3)2. Only two molecules are shown. Hydrogen atoms and counter anions are omitted for clarity. Symmetry code for *: 1 − x, 1 − y, 1 − z.
Crystals 08 00155 g002
Crystals 08 00155 g003 550
Figure 3. Temperature dependence of χmT for polycrystalline [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy), measured at 5000 Oe. A solid line represents the calculated curve for a low temperature region. See the text for the equation and optimized parameters.
Figure 3. Temperature dependence of χmT for polycrystalline [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy), measured at 5000 Oe. A solid line represents the calculated curve for a low temperature region. See the text for the equation and optimized parameters.
Crystals 08 00155 g003
Crystals 08 00155 g004 550
Figure 4. Schematic drawing of the absence or presence of orbital overlaps. (a,b) Geometries of (a) the Co 3dx2-y2 and N 2pz orbitals or (b) Co 3dz2 and N 2pz orbitals with a negligible angular torsion. (c,d) With an appreciable angular torsion.
Figure 4. Schematic drawing of the absence or presence of orbital overlaps. (a,b) Geometries of (a) the Co 3dx2-y2 and N 2pz orbitals or (b) Co 3dz2 and N 2pz orbitals with a negligible angular torsion. (c,d) With an appreciable angular torsion.
Crystals 08 00155 g004
Crystals 08 00155 sch003 550
Scheme 3. Canonical structures of tpyphNO.
Scheme 3. Canonical structures of tpyphNO.
Crystals 08 00155 sch003
Table
Table 1. Selected crystallographic parameters of [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy).
Table 1. Selected crystallographic parameters of [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy).
LtpyphNOtpyphNOtpytpy
T/K9030090400
Formula weight1148.031148.03823.61823.61
Crystal systemmonoclinicmonoclinicorthorhombicorthorhombic
Space groupP21/cP21/cPbcnPbcn
a/Å19.6868(14)20.0956(19)16.554(4)16.884(2)
b/Å16.2129(14)16.5156(18)21.145(5)21.384(3)
c/Å16.3614(10)16.4648(14)9.0805(19)9.2858(13)
β/°107.774(4)108.753(4)9090
V/Å34973.0(6)5174.4(9)3178.4(12)3352.7(8)
Z4444
dcalcd/g·cm−31.5331.4741.7211.632
μ (MoKα)/mm−10.5170.4970.7650.725
No. of unique reflections113731098935163569
R(F) (I > 2σ(I)) a0.06390.06980.05670.0700
wR (F2) (all reflections) b0.15970.18690.14260.1518
Goodness-of-fit parameter1.0361.0110.9511.031
a R = Σ[|Fo| − |Fc|]/Σ|Fo|; b wR = [Σw(Fo2 − Fc2)/ΣwFo4]1/2.
Table
Table 2. Co–N bond distances (d) in Å for [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy).
Table 2. Co–N bond distances (d) in Å for [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy).
LtpyphNO tpyphNOtpytpy
T/K9030090400
d(O1–N4)/Å1.300(4)1.282(4)--
d(O2–N8)/Å1.293(4)1.279(4)--
Ncentral
d(Co1–N2)/Å1.877(3)2.017(3)1.912(4)2.024(4)
d(Co1–N6/4)/Å1.941(3)2.025(3)1.886(4)2.023(4)
dcentral,avg/Å1.912.021.902.02
Δdcentral/Å 0.11 0.12
Ndistal
d(Co1–N1)/Å2.011(3)2.144(3)2.143(3)2.134(3)
d(Co1–N3)/Å2.010(3)2.130(3)2.017(3)2.120(3)
d(Co1–N5)/Å2.170(3)2.154(3)--
d(Co1–N7)/Å2.157(3)2.154(3)--
ddistal,avg/Å2.092.152.082.13
Δddistal/Å 0.06 0.05
Table
Table 3. N–Co–N bond angles (ϕ in °) and distortion geometrical parameters Σ (in °) and CShM for [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy).
Table 3. N–Co–N bond angles (ϕ in °) and distortion geometrical parameters Σ (in °) and CShM for [Co(L)2](CF3SO3)2 (L = tpyphNO, tpy).
LtpyphNOtpyphNOL atpytpy
T/K90300T/K90400
ϕ(N1–Co1–N2)/°80.99(11)76.99(11)ϕ(N1–Co1–N2)/°78.93(8)76.80(8)
ϕ(N1–Co1–N3)/°161.09(11)152.95(12)ϕ(N1–Co1–N1 #)/°157.87(17)153.59(17)
ϕ(N1–Co1–N5)/°97.53(10)100.25(11)ϕ(N1–Co1–N3)/°94.58(12)96.27(12)
ϕ(N1–Co1–N6)/°94.64(11)95.43(11)ϕ(N1–Co1–N3 #)/°89.07(11)89.74(12)
ϕ(N1–Co1–N7)/°89.03(10)89.93(11)ϕ(N1–Co1–N4)/°101.07(8)103.20(8)
ϕ(N2–Co1–N3)/°80.14(10)76.20(11)ϕ(N1 #–Co1–N4)/°101.06(8)103.21(8)
ϕ(N2–Co1–N5)/°101.31(11)103.23(12)ϕ(N1 #–Co1–N3)/°89.07(11)89.74(12)
ϕ(N2–Co1–N6)/°175.55(11)172.30(11)ϕ(N1 #–Co1–N3 #)/°94.58(12)96.27(12)
ϕ(N2–Co1–N7)/°102.63(11)104.88(12)ϕ(N2–Co1–N1 #)/°78.94(8)76.79(8)
ϕ(N3–Co1–N5)/°87.43(10)89.28(11)ϕ(N2–Co1–N3)/°99.53(9)103.24(9)
ϕ(N3–Co1–N6)/°104.24(11)111.44(11)ϕ(N2–Co1–N3 #)/°99.53(9)103.24(9)
ϕ(N3–Co1–N7)/°93.83(10)93.53(11)ϕ(N2–Co1–N4)/°180.0180.0
ϕ(N5–Co1–N6)/°78.32(11)76.55(12)ϕ(N3–Co1–N4)/°80.47(9)76.76(9)
ϕ(N5–Co1–N7)/°155.89(10)151.61(12)ϕ(N3–Co1–N3 #)/°160.94(18)153.52(18)
ϕ(N6–Co1–N7)/°78.04(11)76.14(12)ϕ(N4–Co1–N3 #)/°80.47(9)76.76(9)
Σ/°100.2123.7Σ/°93.4118.8
CShM (Oh)2.8244.282CShM (Oh)2.4843.672
a Symmetry code for #: 1 − x, +y, 1/2 − z.

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Ondo, A.; Ishida, T. Cobalt(II) Terpyridin-4′-yl Nitroxide Complex as an Exchange-Coupled Spin-Crossover Material. Crystals 2018, 8, 155. https://doi.org/10.3390/cryst8040155

AMA Style

Ondo A, Ishida T. Cobalt(II) Terpyridin-4′-yl Nitroxide Complex as an Exchange-Coupled Spin-Crossover Material. Crystals. 2018; 8(4):155. https://doi.org/10.3390/cryst8040155

Chicago/Turabian Style

Ondo, Akihiro, and Takayuki Ishida. 2018. "Cobalt(II) Terpyridin-4′-yl Nitroxide Complex as an Exchange-Coupled Spin-Crossover Material" Crystals 8, no. 4: 155. https://doi.org/10.3390/cryst8040155

APA Style

Ondo, A., & Ishida, T. (2018). Cobalt(II) Terpyridin-4′-yl Nitroxide Complex as an Exchange-Coupled Spin-Crossover Material. Crystals, 8(4), 155. https://doi.org/10.3390/cryst8040155

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