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

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 (∆d) in highand low-temperature structures; ∆dcentral = 0.12 and ∆ddistal = 0.05 Å between 90 and 400 K for L = tpy and ∆dcentral = 0.11 and ∆ddistal = 0.06 Å between 90 and 300 K for L = tpyphNO. The lowand high-temperature structures can be assigned to approximate lowand 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.

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 NaHCO 3 was added.The organic layer was dried over anhydrous Na 2 SO 4 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  The target complexes were prepared as follows.A methanol solution (7 mL) involving tpyphNO (40 mg; 0.10 mmol), CoCl 2 •6H 2 O (12 mg; 0.050 mmol) and LiCF 3 SO 3 (16 mg; 0.10 mmol) was allowed to stand at 0

Crystallographic Analysis
X-ray diffraction data of [Co(L) 2 ](CF 3 SO 3 ) 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 Tables 2 and 3. CCDC numbers 1826042, 1826043, 1826044 and 1826045 contain the crystallographic analysis details for [Co(tpyphNO) 2 ](CF 3 SO 3 ) 2 at 90 and 300 K and [Co(tpy) 2 ](CF 3 SO 3 ) 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.

Magnetic Study
Magnetic susceptibilities of [Co(L) 2 ](CF 3 SO 3 ) 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].

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 Ag 2 O.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 ](CF 3 SO 3 ) 2 was prepared by simply combining methanol solutions of the ligand and CoCl 2 in the presence of the counter anion CF 3 SO 3 − (Scheme 2b).As a reference complex, [Co(tpy) 2 ](CF 3 SO 3 ) 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.

Crystal Structures
The X-ray crystallographic analysis on [Co(L) 2 ](CF 3 SO 3 ) 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 ](CF 3 SO 3 ) 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 ](CF 3 SO 3 ) 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 ](ClO 4 ) 2 •0.5H 2 O is known to crystallize in a tetragonal cell [20] and the relatively low symmetry of CF 3 SO 3 -may cause the different crystal system.On the other hand, the crystal of [Co(tpyphNO) 2 ](CF 3 SO 3 ) 2 belongs to monoclinic P2 1 /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 (∆d) accompanying SCO.Usually ∆d is not so large (0.07-0.11Å) 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 ](BF 4 ) 2 , Kilner et al. [19] reported that d(Co-N central ) of the HS state is longer than that of the LS state by 0.12 Å on the average.In our case ∆d central = 0.11 and 0.12 Å for the complexes with L = tpyphNO and tpy, respectively (Table 2).As for the Co-N distal bond lengths, the HS state possesses longer distances than the LS state by 0.06 Å in [Co(tpy) 2 ](BF 4 ) 2 [19].The present compounds showed ∆d distal = 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 ∆d central and ∆d distal is caused by the Jahn-Teller effect due to the LS e g 1 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 ](CF 3 SO 3 ) 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.
We have to make a comment on the intermolecular interaction in particular in the crystal of [Co(tpyphNO) 2 ](CF 3 SO 3 ) 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 ](CF 3 SO 3 ) 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 ](CF 3 SO 3 ) 2 .

Magnetic Properties
The magnetic susceptibilities of polycrystalline specimens of [Co(L) 2 ](CF 3 SO 3 ) 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 χ m T values of [Co(tpy) 2 ](CF 3 SO 3 ) 2 were 0.516 and 2.20 cm 3 K mol −1 at 90 and 400 K, respectively.From the crystal structure analysis, the spin-state at 90 K is LS, namely, S Co2+ = 1/2 and accordingly the Landé factor g Co2+,LS = 2.35.The spin state at 400 K is HS, S Co2+ = 3/2, which leads to g Co2+,HS = 2.15.The latter involves a slight underestimation of g Co2+,HS , because the χ m T value still has a small positive slope at 400 K.The SCO temperature T 1/2 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 T 1/2 = ca.260 K for [Co(tpy) 2 ](CF 3 SO 3 ) 2 .The χ m T value is ideally flat below 100 K.Note that practically no exchange coupling took place, especially illustrated with the constant χ m T in a lowest-temperature region.cause of this gap is the difference of the g Co2+ values between the two compounds.The high-temperature χ m T value of [Co(tpyphNO) 2 ](CF 3 SO 3 ) 2 was 3.47 cm 3 K mol −1 at 300 K. On cooling the χ m T value decreased to draw an S-shaped profile in 250-100 K and reached a plateau at ca. 1.45 cm 3 K mol −1 around 80 K. On further cooling, the χ m T value again decreased to the smallest value 0.607 cm 3 K mol −1 at 1.8 K (the base temperature of the apparatus available).The first drop is ascribable to Co SCO behavior with T 1/2 = ca.140 K.The second drop is accordingly assigned to exchange coupling behavior among the LS Co 2+ spin and peripheral nitroxide spins.
The spin-Hamiltonian is defined as Equation ( 2), where J Co-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 g avg = 2.352( 9) and 2J Co-rad /k B = −3.63(12)K.Alternatively, the g rad and g Co2+,LS values can be separated with a more detailed van Vleck equation written as Equation ( 3) [43].Assuming that the g rad value is frozen to 2.006 (from the ESR spectrum of tpyphNO), the optimization gave g Co2+,LS = 2.98(2) together with 2J Co-rad /k B = −3.00(6)K.The calculation curve is superposed in Figure 3. with At the ground state, S total should be 1/2; on the other hand, three paramagnetic spins are present in the almost constant χ m T 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 χ m T plateau clearly appeared.In total, [Co(tpyphNO) 2 ](CF 3 SO 3 ) 2 can be regarded as a doubly switchable material showing 1 µ B 3 µ B 5 µ B by temperature stimulus.The χ m T versus T profile for [Co(tpy) 2 ](CF 3 SO 3 ) 2 shows a very gradual SCO curve, whereas that of [Co(tpyphNO) 2 ](CF 3 SO 3 ) 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 e g 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 Co 2+ is (t 2g ) 6 (e g ) 1 and the magnetic e g 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 (2J Co-rad /k B = −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 ](CF 3 SO 3 ) 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 e g 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 ](CF 3 SO 3 ) 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.

Conclusions
The SCO behavior was observed in [Co(tpy) 2 ](CF 3 SO 3 ) 2 and [Co(tpyphNO) 2 ](CF 3 SO 3 ) 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/k B = −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 µ B 3 µ B 5 µ B by temperature stimulus.The present work can be regarded as a successful example of development of multifunctional SCO materials including additional magnetic exchange coupling.

Figure 3 .
Figure 3. Temperature dependence of χ m T for polycrystalline [Co(L) 2 ](CF 3 SO 3 ) 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 4 .
Figure 4. Schematic drawing of the absence or presence of orbital overlaps.(a,b) Geometries of (a) the Co 3d x2-y2 and N 2p z orbitals or (b) Co 3d z2 and N 2p z orbitals with a negligible angular torsion.(c,d) With an appreciable angular torsion.