Redox Modulation of Spin Crossover within a Cobalt Metallogrid

Article pubs.acs.org/IC

Redox Modulation of Spin Crossover within a Cobalt Metallogrid Fuxing Shen,† Wei Huang,† Dayu Wu,*,† Zhe Zheng,† Xing-Cai Huang,† and Osamu Sato*,‡ †

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Collaborative Innovation Center of Advanced Catalysis & Green Manufacturing, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China ‡ Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan S Supporting Information *

ABSTRACT: A self-assembled cobalt molecular grid of a pyrazine-bridged bis-tridentate ligand (LR), where R = H (1), CH3 (2), and Br (3), was prepared and structurally characterized. Depending on the electronic effects of the substituents on the ligand, the redox of the metal center was systematically modulated, and the magnetic behavior from essentially high-spin CoII in 3 versus completely diamagnetic CoIII in 1 to CoII spin-crossover in 2 can be achieved.

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INTRODUCTION The preparation of molecular materials with possible nanotechnological applications is an area of intense interest.1,2 Spincrossover (SCO) materials based on molecules that exhibit crossover between the low-spin (LS) and high-spin (HS) states have a potential for data storage and visualization and for molecular electronics as well, in which the property of bistability is required.3−5 SCO compounds represent the typical bistability, and the transition from LS to HS is accompanied by a measurable change in the magnetism and often also in color.6 It is generally recognized that a SCO complex with a sharp and room-temperature transition, ideally with reproducible hysteresis loops, is most likely to fabricate as a memory device.7 The previous work in this area has been dominated by octahedral FeII compounds, which is primarily due to their bistability features showing abrupt spin transition with potential hysteresis loops.8−13 Among them, polynuclear SCO complexes stand out because of their designed magnetic multistability, and the past decade has witnessed progressive prosperity in the Fe4 grid complexes.14−22 In the CoII SCO field, although some important breakthroughs have been realized on valence tautomeric or Co−Fe complexes that exhibit charge-transferinduced spin transitions,23,24 less attention has been paid to the CoII center with a spin transition between S = 1/2 and 3/2 electronic states, perhaps due to more gradual transition character.25 Especially, the nuclearity reported to be higher than 2 is indeed rare to date. Brooker and co-workers reported the first pyridazine-bridged dinuclear CoII SCO complex with weak intramolecular antiferromagnetic exchange.26 Clérac et al. reported the CoII3 molecular wires exhibiting SCO behavior.27 Just recently, a pyrimidine-bridged tetranuclear Co SCO cluster was first prepared by Kou et al.28 Hence, CoII SCO complexes © XXXX American Chemical Society

with higher nuclearity remain challenging and should be developed. Following this point, herein, we are designing complexes of chelating ligands based on a pyrazine bridge and modifying the targeted ligand to realize a fine-tuning of the ligand field strengths, which might target the polynuclear Co SCO complexes. To date, no examples of pyrazine-bridged polynuclear CoII SCO complexes have been studied, and all of these contained HS CoII or LS CoIII ions throughout the temperature ranges studied.29,30 Here, we construct a series of gridlike tetranuclear complexes Co4LR4, where LR is a centrosymmetric Schiff-base ligand with two groups of tridentate chelating sites in trans position (Scheme 1). Through suitable tuning of the ligand field, we successfully modulated the redox property at the metal center by changing the Scheme 1. Ligands Used in This Work (Bottom: Fine Tuning of the Ligand Field Strength)

Received: October 21, 2015

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DOI: 10.1021/acs.inorgchem.5b02442 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Data Collection and Structure Refinement Parameters for Complexes 1−3 CCDC temperature (K) formula MW(g mol−1) cryst syst space group Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) dcalcd (g cm−3) F(000) R1a [I > 2σ(I)] wR2b [I > 2σ(I)] R1 (all data) wR2 (all data) GOF a

1

2

3

1432186 120 C90H64Cl4Co4N38O25 2455.31 monoclinic C2/c 4 21.314(4) 15.745(3) 33.445(6) 90 93.480(5) 90 11203(4) 1.456 4984 0.0929 0.2548 0.1765 0.2992 1.107

1432187 120 C80H88Cl8Co4N32O32 2529.14 triclinic P1̅ 2 15.857(3) 19.272(4) 23.286(4) 87.342(5) 74.832(5) 73.034(5) 6566(2) 1.279 2584 0.1069 0.2237 0.2371 0.254 1.051

1432188 120 C85H86Br8Cl8Co4N34O38 3350.47 monoclinic C2/c 4 21.917(5) 28.880(6) 22.603(5) 90 95.637(6) 90 14237(5) 1.563 6548 0.1113 0.2873 0.1914 0.3339 1.051

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2. bGOF = [∑[w(Fo2 − Fc2)2]/(Nobs − Nparams)]1/2, based on the data I > 2σ(I). Data Collection and Refinement. Single-crystal X-ray data for complexes 1−3 were collected on a Bruker APEX-2 CCD using Mo Kα radiation (λ = 0.71073 Å) at 120 K. Data collection, data reduction, and cell refinement were performed by using the Bruker Instrument Service, version 4.2.2, and SAINT, version 8.34A, software, respectively.33,34 The structure was solved by direct methods using the SHELXS program, and refinement was performed using SHELXL based on F2 through a full-matrix least-squares routine.35 Empirical multiscan absorption corrections using equivalent reflections were performed with the SADABS program.36 All non-H atoms were refined with anisotropic displacement parameters. H atoms were set in calculated positions and refined as riding models.37 The ClO4− group was disordered; therefore, large thermal displacement parameters were found for these atoms and refined with partial occupancy. For complex 2, unfortunately, we could not obtain high-quality crystals of solvated sample 2·S to collect single-crystal X-ray diffraction data, and attempts to define the highly disordered solvent molecules were unsuccessful. Therefore, electron density contributions from the highly disordered solvent molecules were handled using the SQUEEZE procedure from the PLATON software.38 Void volumes are calculated using the Void command from the Mercury software.39,40 Instead, the structure of 2·S was confirmed by TGA and elemental analysis studies. A summary of the crystallographic data and refinement parameters is shown in Table 1. CCDC 1432186−1432188 contain the supplementary crystallographic data, which are given in the Supporting Information and can be obtained free of charge from the Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/datarequest/cif. SQUEEZE details for 2·S at 120 K are as follows: Approximately 76.78% of the unit cell volume comprises a large region of disordered solvent that could not be modeled as discrete atomic sites. We employed PLATON SQUEEZE to calculate the contribution to the diffraction from the solvent region and thereby produced a set of solvent-free diffraction intensities. SQUEEZE estimated a total count of 826 electrons per unit cell. According to the final formula that was calculated from TGA combined with elemental analysis data, these electrons were assigned to be four MeCN and one H2O molecules per unit cell. The F(000) value with solvent was 2776, the μ (mm−1) value with solvent was 0.741, and the crystal density with solvent was 1.370. Magnetic Measurements. Solid-state, variable-temperature magnetic susceptibility measurements were performed using a Quantum Design MPMS XL-5 SQUID magnetometer. The magnetic suscept-

substituent groups from H to Br in a Co molecular grid. More interestingly, when the substituent group was replaced by CH3, the SCO phenomenon was observed in the corresponding metallogrid.

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EXPERIMENTAL SECTION

Materials and General Procedures. All of the reagents employed were commercially available and were used without further purification. Methanol (MeOH) and acetonitrile (MeCN) were dried using standard procedures. Perdeuterated dimethyl sulfoxide (DMSOd6) was purchased from Alfa Aesar Co. Ltd. 2-Chloropyridine, 2,6dibromopyridine, 2-hydrazinylpyridine (1a), and 1-(pyrazin-2-yl)ethanone were commercial sources. 2-(1-Methylhydrazinyl)pyridine (1b), 2-bromo-6-(1-methylhydrazinyl)pyridine (1c), and 1,1′-(pyrazine-2,5-diyl)diethanone (2a) were synthesized according to reported procedures.31,32 UV−visible studies were performed with a PerkinElmer Lambda 950 UV−visible instrument. Elemental analyses (C, H, and N) were conducted with a PerkinElmer 2400 analyzer. Micro-IR spectroscopy studies were performed on a Nicolet Magna-IR 750 spectrophotometer in the 4000−400 cm−1 region (w, weak; b, broad; m, medium; s, strong) by a KBr disk. 1H NMR spectra were obtained from a solution in deuterated DMSO using a Bruker-400 spectrometer (s, singlet; d, doublet; t, triplet; m, multiplet; dd, double doublet). Magnetic susceptibility measurements were carried out by using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL) under an applied magnetic field of 2500 Oe. Thermogravimetric analysis (TGA) was performed on a NETZSCH TG209F3 thermoanalyzer with filled alumina crucibles under a N2 atmosphere within the temperature range of 300−1000 K at a heating rate of 10 K min−1. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) experiments were undertaken in MeCN [0.1 M [Bu4N](PF6)] at 293 ± 2 K using a CHI620E computer-controlled electrochemical workstation and a standard three-electrode cell. A glassy carbon microelectrode was used as the working electrode, whereas a platinum mesh and a silver electrode were used as the counter and reference electrodes, respectively. All potentials given in this paper are referred to the ferrocene/ferrocenium ([FeCp2]0/+) reference couple under the same conditions. B


Article

Inorganic Chemistry Table 2. Selected Interatomic Distances (Å) and Distortion Parameters Σ (deg) for 1−3 complex 1

complex 3

Co1−N15 Co1−N6 Co1−N13 Co1−N8 Co1−N16 Co1−N5 Σ (deg)

1.873(5) 1.879(5) 1.902(5) 1.905(5) 1.923(5) 1.948(5) 65.70(2)



1.871(4) 1.922(4) 1.946(5) 1.975(4) 2.056(4) 2.080(5) 87.60(18)


1.873(5) 1.873(6) 1.906(6) 1.906(6) 1.938(5) 1.949(5) 66.30(2)

Co1−N6 Co1−N11 Co1−N9 Co1−N12 Co1−N8 Co1−N5 Σ (deg) complex 2

1.902(4) 1.941(4) 1.944(4) 1.983(4) 2.061(4) 2.097(4) 84.97(17)


2.090(5) 2.094(5) 2.117(6) 2.126(6) 2.144(5) 2.170(5) 127.83(6)

Co2−N1 Co2−N3 Co2−N4 Co2−N13 Co2−N15 Co2−16 Σ (deg)

2.133(5) 2.084(4) 2.145(5) 2.161(6) 2.066(5) 2.160(5) 134.86(14)

1.835(4) 1.927(4) 1.957(4) 1.973(4) 2.046(5) 2.124(4) 83.54(17)


1.863(5) 1.901(5) 1.970(4) 2.012(4) 2.072(5) 2.100(5) 82.41(17)

Σ is defined as the sum of deviation from 90° of 12 cis-N−Fe−N angles about the Fe atom. Preparation of Compound [Co4(LMe)4](ClO4)8·4MeCN·H2O (2). A solution of Co(ClO4)2·6H2O (18.30 mg, 0.05 mmol) in MeCN (3 mL) was added to a solution of LMe (18.71 mg, 0.05 mmol) in MeCN (5 mL). The resulting dark-red mixture was stirred under a N2 atmosphere for 30 min. The suspension was then filtered, and the filtrate was diffused with diethyl ether. Dark-red block-shaped single crystals suitable for X-ray diffraction analysis were obtained after several days. Yield: 48% (based on Co). IR (KBr pellet, cm−1): 625.8 (m), 763.6 (w), 940.6 (w), 1092.0 (s), 1121.2 (s), 1146.9 (m), 1313.5 (m), 1440.3 (m), 1470.9 (s), 1571.3 (w), 1606.5 (w), 3407.8 (m). Anal. Calcd for [Co4(C20H22N8)4](ClO4)8·4MeCN·H2O: H, 3.79; C, 38.98; N, 18.60. Found: H, 3.82; C, 38.91; N, 18.64. Preparation of Compound [Co4(LBr)4](ClO4)8·CH3OH·2MeCN· 5H2O (3). A solution of Co(ClO4)2·6H2O (18.30 mg, 0.05 mmol) in MeOH (3 mL) was added to a solution of LBr (26.50 mg, 0.05 mmol) in MeCN (10 mL). The resulting dark-red mixture was stirred under a N2 atmosphere for 30 min. The suspension was then filtered and left at room temperature. Dark-red block-shaped single crystals suitable for X-ray diffraction analysis were obtained after several days. Yield: 51% (based on Co). IR (KBr pellet, cm−1): 625.8 (m), 783.3 (w), 951.6 (w), 1033.8 (w), 1087.9 (s), 1108.3 (s), 1183.6 (w), 1260.8 (w), 1306.6 (w), 1333.2 (w), 1384.1 (w), 1413.2 (w), 1445.6 (m), 1575.9 (m), 1597.8 (m), 3418.5 (s). Anal. Calcd for [Co4(C20H20N8Br)4](ClO4)8·CH3OH·2MeCN·5H2O: H, 4.51; C, 43.18; N, 21.26. Found: H, 4.55; C, 43.12; N, 21.19.

ibility measurements were taken while the temperature was lowered from 400 to 5 K under an applied magnetic field of 2500 Oe. Directcurrent (dc) susceptibility measurements were taken on a freshly filtered crystal sample wrapped in a polyethylene membrane. Samples were prepared rapidly to avoid any loss of solvent for magnetic measurements. Corrections for diamagnetism were applied using Pascal’s constants.41 Synthetic Procedures. Synthesis of Ligand 2,5-Bis[1-[2-(pyridin2-yl)hydrazono]ethyl]pyrazine (LH). An ethanolic solution mixture of 1a (1.0906 g, 10.0 mmol) and 2a (0.8203 g, 5.0 mmol) was refluxed for 3 h under a N2 atmosphere; after cooling to room temperature, a pale-yellow solid was obtained by filtration. The crude product was washed with cold ethanol and dried in vacuo. Yield: 87%. 1H NMR (400 MHz, DMSO-d6): δ 10.19 (s, 2H), 9.28 (s, 2H), 8.20 (dd, J = 8 Hz, 2H), 7.71 (s, 2H), 7.46(d, J = 8 Hz, 2H), 6.88 (dd, J = 4 Hz, 2H), 2.40 (s, 6H). Synthesis of Ligand 2,5-Bis[1-[2-methyl-2-(pyridin-2-yl)hydrazono]ethyl]pyrazine (LMe). The pale-yellow solid ligand LMe was obtained by following the same procedure as that described for LH except that 1b was used instead of 1a. Yield: 90%. 1H NMR (400 MHz, DMSO-d6): δ 9.35 (s, 2H), 8.26 (dd, J = 4 Hz, 2H), 7.69 (t, J = 8 Hz, 2H), 7.25 (d, J = 8 Hz, 2H), 6.91 (m, 2H), 3.53 (s, 6H), 2.53 (m, 6H). Synthesis of Ligand 5-Bis[1-[2-(6-bromopyridin-2-yl)-2methylhydrazono]ethyl]pyrazine (LBr). The orange-red ligand LBr was obtained by following the similar procedure as that described for LH except that 1c was used instead of 1a. The ligand is insoluble in any organic solvent, including DMSO. Yield: 85%. IR (KBr pellet, cm−1): 765.6 (w), 778.6 (w), 809.8 (w), 933.3 (w), 998.1 (w), 1093.0 (m), 1124.9 (m), 1274.9 (w), 1325.9 (m), 1407.8 (m), 1451.8 (s), 1548.9 (m), 1582.3 (s), 1633.6 (w), 3440.1 (s). Caution ! Perchlorate salts are potentially explosive and should be treated with great caution. Only small amounts were used in the present work. Preparation of Compound [Co4(LH)4](ClO4)4·6DMF·3H2O (1). A solution of Co(ClO4)2·6H2O (36.60 mg, 0.10 mmol) in N,Ndimethylformamide (DMF; 3 mL) was added to a solution of LH (34.64 mg, 0.10 mmol) in DMF (5 mL). The resulting dark-green mixture was stirred for 20 min. The suspension was then filtered, and the filtrate was diffused with diethyl ether. Dark-green block-shaped single crystals suitable for X-ray diffraction analysis were obtained after several days. Yield: 62%. IR (KBr pellet, cm−1): 603.9 (w), 625.5 (w), 645.4 (w), 741.1 (w), 778.47 (w), 1020.4 (m), 1086.8 (s), 1120.7 (m), 1139.7 (s), 1238.2 (m), 1297.8 (s), 1413.9 (s), 1501.0 (w), 1546.7 (w), 1606.4 (w), 1654.7 (w), 3405.7 (m). Anal. Calcd for [Co4(C18H16N8)4](ClO4)4·6DMF·3H2O: H, 4.51; C, 43.18; N, 21.26. Found: H, 4.47; C, 43.24; N, 21.21.

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RESULTS AND DISCUSSION The starting point of our investigation was the green CoIII complex 1 containing the pyrazine derivative of the hydrazine ligand (LH, Scheme 1). The self-assembly between the LH ligand and Co(ClO4)2·6H2O gave rise to complex 1 in moderate yield. Single-crystal X-ray diffraction revealed that the coordination environment of each Co ion is formed by mercoordinated halves of different ligands, with each mer coordination involving a pyridine N atom, a pyrazine N atom, and an amine N atom. The axial metal−Nimine bonds are slightly shorter than the equatorial metal−Npyridyl and metal− Npyrazine bonds only by ca. 0.05 Å (Table 2). The angular distortion parameter, Σ (deg), is low, showing the more regular octahedral coordination geometries. The lengths of the Co−N bonds are quite characteristic of the respective spin and oxidation states of the coordinated metal ion and, therefore, can be used as diagnostic tools for determining the electronic state of the metal ion. Thus, the observation of rather short Co−N bond lengths of 1.873−1.949 Å as well as the small angular C


Article

Inorganic Chemistry

Figure 1. (a) Perspective view of the complex cation in 2 showing 50% thermal ellipsoids with the selected atom labels. The H atoms, anions, and solvent molecules are omitted for the sake of clarity. (b) Diffuse-reflection spectra of 1−3 at room temperature.

Figure 2. Temperature dependence of the product χMT for solid 2 (a) and 1 and 3 (b) under an applied magnetic field of 2500 Oe.

complexes containing the bis-tridendate ligand LMe, the apical Co−Npyrazine and Co−Npyridyl pairs are quite elongated compared to the equatorial bond distances, forming an elongated octahedral coordination sphere due to significant Jahn−Teller distortion of all LS CoII ions (Table 2). The angular distortion parameters, Σ (deg), are extended to the ones corresponding to LS CoII.25a These bond lengths within the equatorial plane and along the axial position contrast with those recently observed in the LS CoII4 grid [CoII4(HL3)4](ClO4) 4·8H 2O, where the compressed octahedron was observed.28 In the case of an elongated octahedron of a LS metal d7 ion, the Jahn−Teller effect places the unpaired electron in the σ-antibonding dz2 orbital, while in the case of compression, the unpaired electron will reside in the dx2−y2 orbital. In contrast to 1, inspection of the bond distances around the amine group in 2 reveals a less delocalized πbonding pattern, characteristic of a neutral ligand. In combination with the observation of eight perchlorate anions per molecule in the crystallographic analysis, we conclude that complex 2 can be solely described as a LS CoII complex at 120 K. The Br substituent is recognized to possess a weak electronwithdrawing effect, and it would decrease the negative electronic density of the N atom (Npy) if we position it on

distortion parameter around the Co ion is consistent with the presence of a LS CoIII ion. Further, the bond lengths of C5−N2 (1.395 Å) and N2−N3 (1.353 Å), together with other terminal bond lengths (1.375 Å for C14−N7 and 1.335 Å for N7−N6) within the ligand skeleton, are intermediate between a normal single bond and a double one, pointing out that the proton on the N2 and N7 sites is lost upon metal binding. Hence, because of the easy deprotonation character of the amine block, the coordinated ligand LH is monoanionic per tridentate binding site from the viewpoint of charge balance. To be able to reach the CoII electronic state and possible spin transition, the capability of the ligand to delocalize the charge of the metal ion has to be reduced. This objective can be accomplished because methyl substituents on the amine position will eliminate the possibility of deprotonation upon metal coordination, and, consequently, the neutral LMe provides the intermediate ligand field strength for possible SCO. An analogous synthetic procedure, but starting with LMe instead of LH in the reaction, afforded the pure, dark-red compound 2 in 48% yield. The structure of 2 was determined at 120 K (Figure 1a). As expected, the Co ion is coordinated to the two N-donor atoms of the pyridine moiety and the two pyrazine N-donor atoms in the equatorial plane and to the two amine N-donor atoms in the axial positions. Therefore, in CoII D


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Inorganic Chemistry

Figure 3. Cyclic (a) and differential pulse (b) voltammograms of complexes 1−3 referenced against Fc+/Fc at a scan rate of 100 mV s−1 recorded in CH3CN at a concentration of 0.1 mM. nBu4NPF6 (0.1 M) was used as the supporting electrolyte.

value at low temperatures most likely indicates the incomplete HS → LS conversion. The further decrease of χMT below 40 K should be ascribed to weak intramolecular antiferromagnetic coupling and/or to zero-field splitting of the remaining HS CoII in the Co4 molecule. However, the most remarkable finding is that, above 200 K, there is a steady increase of the χMT curve to ca. 3.78 cm3 K mol−1 at 400 K, revealing that a gradual spin transition from S = 1/2 to 3/2 takes place. This transition occurs typically at rather high temperature; however, the transition is not sharp but rather occurs over a wide temperature range reminiscent of CoII d7 crossover systems.25 The sequential heating/cooling cycles in the temperature range of 5−400 K were at least triply repeated, and no hysteresis occurs. The slight change of the magnetic data was related with the solvent loss upon heating based on the TGA data (Supporting Information, Figure S11). To further investigate the redox modulation by chemical modification, CV and DPV experiments in a MeCN solution were performed. All of the complexes undergo four quasireversible oxidation processes (vs Fc+/Fc; Figure 3), which are assigned to the stepwise one-electron CoII/CoIII couples that finally lead to CoIII4 species. Interestingly, the redox sequence consists of two pairs of relatively closely spaced processes (1/2 and 3/4 in Figure 3a) for complexes 1 and 2, with a much larger gap between the second and third redox couples. Hence, comproportionation equilibrium constants (Kc) of mixedvalence species could be calculated from the electrochemical data in the case of a stepwise oxidation process (Scheme 2). Frequently, Kc can be used as a parameter to determine the extent of metal−metal interaction in mixed-valence species and can be measured electrochemically from the following expression:42

the neighboring site of the pyridyl N atom and further weaken the ligand field around the metal centers. Along this line, the ligand LBr was prepared on purpose for the molecular assembly of 3 (Supporting Information, Figure S9). The selected bond distances and angles at 120 K are listed in Table 2 and Table S1. Compared to the data of 2 at 120 K, in addition to the large angular distortion parameters, Σ (deg), of 127.83 and 134.86 for Co1 and Co2, respectively, the equatorial Co−N bonding distances unequivocally extend to the expected range of HS CoII, confirming the feasibility of tuning the electron effect of the substituent on the spin state of the CoII center. The biggest difference between the UV−visible reflection spectra of 1−3 is the presence of an intense reflectance band with a maximum at 530 nm for 1 (Figure 1b). This observation and the paramagnetism of 3 (as evidenced by its 1H NMR in solution) at room temperature point toward a different electronic ground state between them. Magnetic susceptibility data further supported the data of Xray diffraction and reflection spectra, and complex 1 is diamagnetic over the measured temperature region, corresponding to the LS CoIII ions. χMT of compounds 3 is equal to 10.5 cm3 K mol−1 at 300 K, which is obviously higher than the expected values of 7.5 cm3 K mol−1 corresponding to four CoII ions in the HS state (Figure 2b), and the deviation stems from the substantial orbital contribution to the magnetic moment. As the temperature decreased, χMT did not significantly decrease until 40 K and then decreased more rapidly to reach a value of 5.1 cm3 K mol−1 at 2 K because of zero-field splitting of the local S = 3/2 ground-state spin. However, the dc magnetic susceptibility measurement of complex 2 was performed in the range of 5−400 K under a 2500 Oe external field (Figure 2a). The sample was first measured from room temperature to 5 K, followed by sequential heating/cooling cycles. The graph can be divided into three parts: Between 40 and 200 K, the curve reaches a slope plateau with a value for χMT of ca. 2.15 cm3 K mol−1. This value is higher than the expected value of 1.5 cm3 K mol−1 corresponding to four LS states of the S = 1/2 electronic ground state, with one unpaired electron residing in the σantibonding orbital. In such a case, the orbital contribution should be quenched by Jahn−Teller distortion, so the higher

ΔGm θ = −RT ln Kc θ = −neF(ΔE1/2)

where ΔGmθ is the free energy of the comproportionation reaction to form the mixed-valence species, R, T, ne, and F have their usual meanings, and ΔE1/2 is the potential difference between the E1/2 values of adjacent redox couples (vide supra). Comproportionation equilibrium constants Kc1θ, Kc2θ, and Kc3θ corresponding to the intermediate species Co II 3 Co III , E


Inorganic Chemistry

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Scheme 2. Redox Sequence within a Metallogrid and Derivation of the Comproportionation Equilibrium Constants of Three Intermediate Species

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02442. Spectral characterization, TGA curve, and additional electrochemical data (PDF) X-ray crystallographic data including CIF files for 1−3 (CIF)

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CoII2CoIII2, and CoIICoIII3, respectively, are summarized in Table 3, which reveals a pronounced thermodynamic stability of the dimixed-valence species CoII2CoIII2 in 1 and 2, assuming that the first two oxidations occur at opposite corners of the grid, where the next corners are still in the CoII ion, as in most of the mixed-valence systems.43 These huge Kc values of CoII2CoIII2 in 1 indicate significant metal−metal interaction (Kc ≥ 106 is typical of significant delocalization),44 which is possibly ascribed to the deprotonated form of the ligand, allowing higher spin delocalization onto the ligand’s π orbitals. It is also worth noting that the behavior of complex 1 is unusual because its highest oxidation state corresponding to four CoIII ions requires a quite large oxidation potential. In such a case, the complex is still isolated as CoIII4 species and does not undergo partial reduction when synthesized, as evidenced by magnetic characterization and X-ray diffraction analysis. Alternatively, the first oxidation of the CoII4 complex requires the most positive potential in complex 2 (the first wave from the left in Figure 3), suggesting that ligands in 1 and 3 provide a weaker ligand field than that in the case of 2. It is normal because both deprotonation and the presence of a π-donating Br substituent should render the delocalized aromatic system of the ligand as a weaker π acceptor, thus weakening the ligand strength in 1 and 3 versus that in 2. However, because complex 1 contains a CoIII ion, it remains in the LS state.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support by the Priority Academic Program Development of Jiangsu Higher Education Institutions. This experimental work is financially funded by the NSFC program (Grants 21371010 and 21471023), Jiangsu Provincial QingLan Project, and Jiangsu Province Key Laboratory of Fine Petrochemical Engineering (KF1302). D.W. also is thankful for positive comments and helpful suggestions from the anonymous reviewers concerning the quality of the manuscript.

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REFERENCES

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CONCLUSIONS In summary, the spin and valence states in [2 × 2] metallogrid complexes of the type [Co4L4R] depend directly on the nature of the substituent on the neighboring site of pyridyl N atoms. The compounds with a deprotonation ligand (R1 = R2 = H) gave rise to the LS CoIII state at all temperatures studied. Through anchoring of the Br atom on the 2 position of the pyridyl group (R1 = CH3; R2 = Br), the HS CoII state can be achieved for all of the metal centers. Only the complex bearing substituents that attenuate the ligand field by electronic effects (R1 = CH3; R2 = H) exhibits, although incomplete, a temperature-triggered spin transition. Gradual and incomplete transition without hysteresis seems to be typical for the investigated CoII-based SCO metallogrid. Further work is underway toward enhancing the intra- and/or intermolecular cooperativity by the introduction of hydrogen bonding between the grid units.

Table 3. Summary of Potential Difference and the Corresponding Comproportionation Constants complex

ΔE1θ

ΔE2θ

ΔE3θ

Kc1θ

Kc2θ

Kc3θ

1 2 3

0.1689 0.1344 0.2774

1.2711 0.6189 0.1186

0.1627 0.2950 0.1210

7.13 × 102 1.86 × 102 4.85 × 104

2.96 × 1021 2.85 × 1010 1.01 × 102

5.6 × 102 9.62 × 104 1.11 × 102

F


Article

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Redox Modulation of Spin Crossover within a Cobalt Metallogrid

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