Synthesis and Magnetic Properties of Mononuclear Cobalt（Ⅱ）Spin Crossover Complexes from Complementary Terpyridine Ligand Pairing

YANG RuiZHANG Shu-YaWANG Run-GuoMENG Yin-ShanLIU Tao＊,ZHU Yuan-Yuan＊,,

(1State Key Laboratory of Fine Chemicals,Dalian University of Technology,Dalian,Liaoning 116024,China)(2Anhui Key Laboratory of Advanced Catalytic Materials and Reaction Engineering,School of Chemistry and Chemical Engineering,Hefei University of Technology,Hefei 230009,China)

Abstract:The cobalt（Ⅱ） complexes containing terpyridine(terpy)and its derivatives compose a large family of Co（Ⅱ）SCO-active(SCO=spin-crossover)compounds and the reported cases are mainly built from homoleptic type terpy ligands.Herein we report the SCO properties in three mononuclear cobalt（Ⅱ）complexes constructed from complementary terpy ligand pairing.Their SCO behaviors are largely affected by the substituents of terpy at the 4-position.The archetypical complex 1 and its CF3-substituted one 3 showed a gradual and incomplete spin transition from the low spin state of S=1/2 to the high spin state of S=3/2.The fluorine-substituted complex 2 exhibited a solventdependent spin transition phenomenon.The solvated form which contains three lattice water molecules showed a similar gradually incomplete spin transition.Whereas the entire removal of water molecules resulted in a repeatable thermal hysteresis loop with a width of ca.50 K.Impressively,the adsorption and desorption of water molecules are reversible in structure and magnetism.In addition,absorption spectroscopy and cyclic voltammetry show that the substituent on the ligands can regulate the electronic structures of the central cobalt ion.CCDC:2162742,1(150 K);2162743,2(120 K);2162744,2(299 K);2162745,3(120 K).

Keywords:spin-crossover;cobalt（Ⅱ）complexes;terpyridine;self-assembly

Spin-crossover(SCO)complexes belong to a class of interesting compounds whose spin states can be switched through external perturbations including thermal,light,pressure,solvent,magnetic field,etc[1-2].The transition metal complexes with configurations ofd4-7electrons may be located in either the high-spin(HS)or low-spin(LS)state which is dependent on the ligand field strength.The interconversion of two spin states is essentially the consequence of the electron configuration rearrangement in thedorbital which is triggered by the relative energy magnitude of ligand field splitting between thet2gandegmolecular orbitals(Δ)and pairing energy of electrons(P)[3-4].WhenΔis larger thanP,the pairing of electrons is favored,resulting in an LS state.WhenΔis smaller thanP,electrons are inclined to occupy maximumdorbitals and thus prefer the HS state[5-6].Impressively,the transformations in electron configurations result in changes in several physical properties such as magnetic moment,color,dielectric constant,and electrical resistance[3,7].Since the first SCO compound,tris(dithiocarbamato)iron（Ⅲ）,was discovered by Cambi et al.in 1931[8],numerous mononuclear,polynuclear,and polymeric complexes have been found to show a variety of SCO properties[9-10].Among them,the vast majority of SCO-active cases are composed of Fe（Ⅱ） and Fe（Ⅲ） complexes[11-13].In contrast,the reported examples of Co（Ⅱ）complexes with SCO behavior are relatively limited[14-15].From the view of electron configuration,the transition in sixcoordinate Co（Ⅱ） complexes from LS to HS state involves a change fromwithS=1/2 to a4T1withS=3/2[16].Due to the larger ligand field splitting energy(Δ)of Co（Ⅱ） ion compared with that of Fe（Ⅱ） ion,organic ligands with relatively stronger ligand field are required to build Co（Ⅱ）SCO-active compounds[6].So that the cobalt（Ⅱ）complexes containing terpyridine(terpy)and its derivatives constitute SCO-active family compounds due to the moderate ligand field strength of these tri-imine type ligands[16-18].Through the suitable substitutions in archetypical terpy and variations of counter anions and lattice solvent[19],the occurrences of SCO behaviour have been revealed in a variety of cobalt（Ⅱ）compounds built from terpy and its derivatives[20-23].However,the majority of compounds are constructed from cobalt（Ⅱ）ion with two equivalent homoleptic terpy ligands so that the formed complexes have anS4improper axis[24].Due to the reversible characteristic of coordination reaction,it is difficult to control the complex that is constituted from two different terpy ligands[25].The exploration of heteroleptic terpy cobalt（Ⅱ）complexes has great significance in research on SCO.These novel structures not only possess the advantage to subtly tuning the ligand field through the introduction of substitution at different positions but also provide a feasible approach to directionally assembling complicated metallo-supramolecular architectures bearing multinuclear paramagnetic centres.In 2016,Chan and co-workers developed a smart strategy for the directional synthesis of transition metal complexes from heteroleptic terpy ligands[26-28].They designed and synthesized a complementary pair of terpy-based ligands,one is the normal terpy ligand and another one has 2,6-dimethoxyphenyl groups at 6,6″-positions of 4'-phenyl-terpy,to afford the directional heteroleptic complexation.By employing this structure as the connection unit,complicated metallo-supramolecular structures including macrocycles,cages,and multi-layered architectures have been accurately selfassembled from multi-components.To the best of our knowledge,there is no study on the magnetic properties of the cobalt（Ⅱ）complexes constructed from complementary terpy ligand pairing.The exploration of SCO behaviour on these novel terpy-based cobalt（Ⅱ）complexes will enrich the cobalt（Ⅱ）SCO-active compound family and find some unexpected properties.In addition,the substituents can influence the SCO behaviour via electronic effect and/or intermolecular weak interactions[29-31].In this contribution,we report the synthesis,characterization and magnetic properties of three mononuclear cobalt（Ⅱ）complexes built from complementary terpy ligand pairing(Scheme 1).

1 Experimental

1.1 Instruments and measurements

1.1.1 Structural characterization measurements

Scheme 1 Directional assembly of three mononuclear cobalt（Ⅱ）complexes based on complementary terpy ligand pairing

NMR spectra were recorded on a Bruker 400,500,or 600 MHz spectrometer.Elemental analysis of carbon,nitrogen,and hydrogen was performed using an Elementary Vario EL analyzer.Fourier transform infrared spectroscopy(FT-IR)data were collected on KBr pellet samples in a range of 4 000-400 cm-1using an IS-50 FT-IR spectrometer.

1.1.2 Magnetic properties measurements

Magnetic susceptibility data were collected using a Quantum Design MPMS XL-5 or PPMS-9T(EC-II)SQUID(superconducting quantum interference device)magnetometer.Measurements for all the samples were performed on microcrystalline powder restrained by a parafilm and loaded in a capsule.The magnetic susceptibility data were corrected for the diamagnetism of the samples using Pascal constants and the sample holder and parafilm by corrected measurement.

1.1.3 X-ray data collection and structure determinations

Crystals suitable for single-crystal X-ray diffraction were covered in a thin layer of hydrocarbon oil,mounted on a glass fiber attached to a copper pin,and placed under an N2cold stream.The data for three compounds were collected on a Bruker D8 Venture CMOS-based diffractometer(MoKαradiation,λ=0.071 073 nm)using the SMART and SAINT programs.Final unit cell parameters were based on all observed reflections from the integration of all frame data.The structures were solved with the ShelXT structure solution program using Intrinsic Phasing and refined with the ShelXL refinement package using Least Squares minimization that was implanted in Olex2.For all compounds,all non-hydrogen atoms were refined anisotropically and the hydrogen atoms of organic ligands were located geometrically and fixed under isotropic thermal parameters.For 1,the solvent molecule acetonitrile and substituted phenyl were disordered,and the refined sites occupancy factor(SOF)values for the major and minor components of this rotational disorder were 0.589 and 0.411 for N15,C9-C14,C67-C72,and C121.For 2,guest water molecules were disordered by symmetric elements with occupancy of 0.5 at 120 and 299 K,respectively.For 3,solvent molecule methanol and perchlorate were disordered,the refined values of this rotational disorder were 0.571 and 0.429 for O10-O12,and O14 and C60 were disordered by symmetric elements with occupancy of 0.5.

CCDC:2162742,1(150 K);2162743,2(120 K);2162744,2(299 K);2162745,3(120 K).

1.1.4 Powder X-ray diffraction(XRD),calorimetric analysis,and UV-Vis absorption measurements

XRD patterns were obtained on a D8 ADVANCE X-ray powder diffractometer with CuKαradiation(λ=0.154 18 nm)in a 2θrange of 5°-50°at room temperature.Testing voltage and current were 240 kV and 50 mA,respectively.Thermogravimetry analysis(TGA)was carried out with a TGA/DSC 1(Mettler Toledo)instrument from ambient temperature to 800℃at a warming rate of 5 K·min-1.Differential scanning calorimetry(DSC)was carried out with a DSC 823e(Mettler Toledo)at a cooling/warming rate of 3 K·min-1.The solution UV-Vis absorption spectra were recorded using a TU-1900 spectrophotometer with a sample concentration of 50 μmol·L-1in acetonitrile at ambient temperature.

1.1.5 Cyclic voltammetry(CV)measurements

The CV curves of the complexes and their corre-sponding ligands were measured in acetonitrile solution containing 1 mmol·L-1substrates and 0.1 mol·L-1tetrabutylammonium hexafluorophosphate electrolyte.The systems were run in a three-electrode cell(10 mL working volume),using a glassy carbon as a working electrode,Ag/AgCl in 3.5 mol·L-1KCl aqueous solution as a reference electrode,and a platinum plate as a counter electrode.The glassy carbon surface was polished by 0.05 mm alumina,then washed with deionized water before use every time.The solution was degassed by bubbling nitrogen for 15 min before measurements and maintaining inert nitrogen over the solution during the measurements.

1.2 Preparation of three Co（Ⅱ）complexes

General procedure.Four terpy-type ligands were prepared according to the literature method and the detailed synthetic procedure and structural characterization were depicted in the Supporting information.The mixed solution(H2O/MeOH,1∶1,V/V,10 mL)of Co(ClO4)2·6H2O(0.01 mmol·L-1)was mixed with a 20 mL acetonitrile solution of L1/L2/L3(0.1 mmol respectively)and L4(31 mg,0.1 mmol),and stirred for 5 h and then filtered.Dark red single crystals were obtained after slow evaporation of the solution at ambient temperature for several days.

[Co(L1)(L4)]2(ClO4)4·2H2O·3MeCN(1).Yield:75%.IR(KBr pellet,cm-1):3 425(br),3 068(w),2 940(w),2 838(w),1 616(m),1 604(m),1 568(m),1 548(w),1 474(m),1 439(m),1 421(w),1 380(w),1 303(w),1 283(w),1 252(m),1 107(s),1 096(s),1 022(m),999(w),882(w),820(w),785(m),770(m),754(w),734(w),695(w),649(w),624(m),602(w),502(w),440(w).Anal.Calcd.for C122H105Cl4Co2N15O26(%):C,59.59;H,4.27;N,8.55.Found(%):C,60.05;H,4.43;N,9.01.

[Co(L2)(L4)](ClO4)2·2.5H2O(2).Yield:70%.IR(KBr pellet,cm-1):3 425(br),3 070(w),2 839(w),1 616(w),1 602(m),1 568(m),1 549(m),1 516(w),1 474(m),1 438(m),1 400(w),1 380(w),1 283(w),1 252(w),1 162(m),1 144(s),1 108(s),1 022(m),1 000(w),896(w),850(w),836(m),821(m),785(w),773(w),754(w),735(w),693(m),650(w),625(w),602(w),518(w),483(w).Anal.Calcd.for C58H50Cl2CoFN6O14.5(%):C,57.48;H,4.16;N,6.93.Found(%):C,56.96;H,4.39.N,6.58.

[Co(L3)(L4)](ClO4)2·H2O·0.5MeOH(3).Yield:71%.IR(KBr pellet,cm-1):3 073(br),2 942(w),2 839(w),1 617(w),1 603(m),1 569(m),1 549(m),1 474(w),1 438(m),1 402(m),1 379(w),1 328(w),1 283(w),1 252(w),1 169(m),1 108(s),1 022(s),880(m),851(w),838(w),820(w),786(m),772(m),754(w),735(w),696(w),669(w),650(m),624(w),602(w),507(w),440(w).Anal.Calcd.for C59.5H49Cl2CoF3N6O13.5(%):C,57.13;H,3.95;N,6.72.Found(%):C,56.68;H,4.29;N,6.34.

CAUTION!Perchlorates are potentially explosive.Such compounds should be synthesized and used in small quantities,and treated with the utmost care at all times.

2 Results and discussion

2.1 Synthesis and structural characterization

In this work,four terpy-type ligands were synthesized according to the literature method,and the detailed synthetic procedure and NMR characterization are depicted in the Supporting information(Fig.S1-S7)[26,32-33].The preparation of complexes was performed in a mild and facile reaction condition.To acetonitrile solution of L1/L2/L3and L4was added Co(ClO4)2·6H2O mixed solution of methanol and water(the molar ratio of L1/L2/L3,L4,and Co2+was 1∶1∶1),and the selfassembly of heteroleptic complexes took place immediately at ambient temperature.Subsequently,dark red single crystals of complexes 1-3 were obtained by slow evaporation of the resulting solutions.Their structures were determined by single-crystal X-ray diffraction measurement and their crystal data and structure refinement is listed in Table 1.All the complexes crystallized in the triclinic space groupPat the test temperatures and their molecular structures and packing diagrams are illustrated in Fig.1 and 2.Different from that 1 contains two Co（Ⅱ）units,there is only one unique asymmetric unit in 2 and 3(Fig.1).In one unit cell,there are four,two,and two Co（Ⅱ）units in 1,2,and 3,respectively(Fig.2).The structure refinement revealed that there are lattice solvents including water and methanol in 1,water in 2,and water and acetonitrile in 3,respectively.The contents of the solvent were further confirmed by TGA(Fig.S8-S11)and elemental analysis.In addition,the imine-type ligand structure of complexes 1-3 was supported by FT-IR spectra(Fig.S12-S14).The purity of a large number of samples was confirmed by XRD(Fig.S15-S17).

For all complex cations in this series,the Co（Ⅱ）ion is coordinated by six N atoms from two different types of terpy ligands in a bis-meridional fashion,forming an axially compressed CoN6octahedron.Two 2,6-dimethoxyphenyl in L4and one pyridine in L1/L2/L3provide ancillary ion-dipole interactions during the coordination process.The formed heteroleptic complexes are stabilized byπ-πstacking between parallelly arranged 2,6-dimethoxyphenyl and pyridine by which considerable geometric distortion is generated due to this extra weak interaction(Fig.S18).The distances between 2,6-dimethoxyphenyl and pyridine planes are within 0.339-0.350 nm(Table S1),suggesting that one terpy ligand is tightly embraced by another substituted one throughπ-πstacking interactions.To quantitively analyze the structural distortions,some parameters including Co—N bond lengths,the continuous shape measure(CShM)values that reflect the deviation from idealOhsymmetry,the distortion parameters∑(the sum of the deviation from 90°of the 12cisN—Co—N angles),and the dihedral angles of two ligand planes are summarized in Table 2.The values of these parameters indicate that there exists considerably large geometrical distortion in the coordination sphere of this type of complex.In addition,the overlay of the crystal structures of 2 at 120 and 299 K shows a minor change in deformation,implying the spin state change in this temperature range is insignificant(Fig.S19).

Table 1 Crystal data and structure refinement for 1,2,and 3

Fig.1 Crystal structures of(a)1,(b)2,and(c)3

Fig.2 Projection of the unit cell of 1,2,and 3,where the hydrogen bondings are illustrated

The average Co—N bond lengths are in a range of 0.204 4-0.205 6 nm,suggesting that the cobalt（Ⅱ）ions in complexes 1-3 approach the LS state at the test temperature.The detailed bond length data of the CoN6coordination sphere in the structures are summarized in Table 3.For comparison of the structural discrimination between these asymmetric Co-terpy complexes and their symmetric analogues,the selective bond length data of four representative mononuclear cobalt（Ⅱ）complexes built from homoleptic ligands are listed in Table S2 as well.It shows that the existence ofπ-πstacking between two ligands causes the enhanced geometrical distortion in the part of cobalt（Ⅱ）ion with 2,6-dimethoxyphenyl substituted ligand L4,reflecting the substantially lengthening Co—N bond lengths.In general,the distances between cobalt（Ⅱ）ions and N atoms from bilateral pyridines in L4are more than 0.225 nm,0.02-0.03 nm longer than that in unsubstituted terpy.Additionally,the two bilateral pyridine rings in L4considerably deviate from the coplanarity than that of L1/L2/L3(Table S3).

Table 2 Summary of the structural parameters and spin state in the crystal structures of cobalt（Ⅱ）complexes in this work*

Table 3 Selected bond lengths(nm)for Co—N bonds of complexes 1-3 in the single-crystal structures

2.2 Magnetic properties

Variable-temperature magnetic susceptibilities of complexes 1-3 were measured in the solid-state using a SQUID magnetometer.The measurement was performed in sweep mode with a scan rate of 3 K·min-1at a field strength of 5 000 Oe.The solvated samples were sealed with parafilm to prevent solvent loss in the first cycle.At the end of the first heating semi-cycle,all samples were kept at 400 K for 2 h to ensure the complete removal of lattice solvents.Magnetic susceptibilities are displayed in the form ofχMTvsT,whereTis the absolute temperature andχMis the molar magnetic susceptibility.TheχMTvsTplots of complexes 1-3 under successive cooling/heating cycles are illustrated in Fig.3.Although they all displayed spin transition,their SCO behaviors had significant discriminations.For 1 and 3,both were observed to show gradual incomplete SCO behavior.At 2 K,theχMTvalues were 0.41 cm3·mol-1·K for 1 and 0.40 cm3·mol-1·K for 3,respectively.Upon heating to 400 K,it gradually increased to 1.00 cm3·mol-1·K for 1 and 1.19 cm3·mol-1·K for 3,indicating that they don't reach the complete HS state at the test upper limit temperature.After preservation at 400 K,the lattice solvent molecules were fully removed,and the subsequent cycles showed the spin transition behavior of desolvated samples.Their roughly coincident curves reveal that the influence of lattice solvent molecules in two compounds on SCO behavior is minor.

Fig.3 Plots of the temperature dependence of χMT under 5 kOe dc field for(a)1,(b)2,and(c)3 for successive cycles

Complex 2 showed an interesting solventdependent SCO behavior.At the beginning of the measurement,theχMTvalue at 300 K was 0.68 cm3·mol-1·K which was far from the saturated value of 2.5 cm3·mol-1·K for the spin-onlyd7cobalt（Ⅱ） ion withS=3/2.Upon cooling,it gradually decreased to 0.45 cm3·mol-1·K at 4 K,reaching a complete LS state withS=1/2.In the following heating semi-cycle,the curve entirely coincided with the cooling one below 300 K.Subsequently accompanied by the temperature rise,theχMTvalue increased quickly and an inflection point was observed at about 350 K.According to the TGA data of 2(Fig.S9),the dehydration occurred at the temperature range of 320-350 K.In combination of variable-temperature susceptibility and TGA data,theχMTvsTcurve above 300 K can be divided into two steps.The dehydration takes place at the first step(300-350 K)and the structural phase transition caused by dehydration possesses considerable contribution to the spin transition.When the temperature was above 350 K,the following measurements revealed the SCO behavior of the desolvated sample.It is found that the dehydration of 2 results in significantly distinct SCO behavior.Upon cooling in the second cycle,theχMTvalue gradually dropped from 1.59 cm3·mol-1·K at 400 K to 0.45 cm3·mol-1·K at 4 K,reflecting the typical SCO of cobalt（Ⅱ）ion from the incomplete HS state to the complete LS state.Unexpectedly,the second cooling and heating semi-cycles didn't overlap in the temperature range of 260-340 K and produced a large apparent thermal hysteresis loop of 50 K.This hysteresis loop was stable and repeatable which was reproduced in the following third cycle.Due to the diffraction data considerably getting worse during the heating process to remove the lattice water molecules,the attempt to get the accurate structure of dehydration crystals was unsuccessful.It is supposed that the removal of lattice solvents makes the stacking of molecules more tightly,leading to enhanced interaction between adjacent complexes which may be the main source of the occurrence of the thermal hysteresis loop.It was found that the adsorption and desorption of water in 2 were reversible[37-38].When the desolvated sample was exposed to air for a period of time,it could readsorb water to the originally solvated state,which was confirmed by the TGA trace of the re-adsorbed sample(Fig.S10).The re-adsorbed sample showed almost identical behaviour of variable-temperature magnetic susceptibilities to that of the pristine one(Fig.S20).DSC measurements of compound 2 were carried out at the temperature area of the thermal hysteresis loops.No noticeable thermal change was observed,suggesting that no apparent structural phase transition occurred in the SCO process(Fig.S21).

Fig.4 (a)UV-Vis spectra of complexes 1-3 in the 50 μmol·L-1acetonitrile solution at ambient temperature;(b)CV curves of complexes 1-3(1 mmol·L-1)in 0.1 mol·L-1Bu4NPF6/acetonitrile solution on a glassy carbon disk working electrode with a Pt counter electrode and Ag/AgCl reference with the potential sweep rate being 100 mV·s-1

2.3 Absorption spectra and electronic chemistry

The influence of substituents on the electronic structures of complexes was further investigated by using absorption spectroscopy and CV.According to the UV-Vis spectra recorded in acetonitrile at ambient temperature,complexes 1-3 showed a broad absorption peak at around 520 nm(Fig.4a).This wide absorption is ascribed to the metal to ligand charge transfer(MLCT)band.Their peak positions were at 515 nm for 1,and 511 nm for 2 and 3.Due to the strong electronwithdrawing effect,the MLCT band experienced a con-siderable blue shift.The strong electron-withdrawing groups of F and CF3benefit the back bonding of cobalt（Ⅱ）ion to ligands,resulting in the enhancement of ligand field strength.The electrochemical properties of complexes 1-3 and the ligands were investigated using CV in acetonitrile.The CV curve of each complex showed the reversible CoⅠ-CoⅡand CoⅡ-CoⅢcouples that centered at about-0.8 and 0.7 V,respectively(Fig.4b and S21)[39].It was found that their anode potential(Epa),cathode potential(Epc),and half-wave potential(E1/2)were all sensitive to the electronic effect of substituents(Table S4).In general,the presence of an electron drawing group pulls more electron density from the metal center,making its redox reaction easier.

3 Conclusions

In summary,three mononuclear SCO-active cobalt（Ⅱ）complexes have been prepared using the directional synthesis of complementary terpyridine ligand pairing.Complexes 1-3 were all located at complete LS state at low temperature and showed a gradually incomplete spin transition to HS state upon heating to 400 K.Impressively,the fluorine substituted complex 2 exhibited solvent-dependent SCO behavior.When three lattice water molecules were removed,a large thermal hysteresis loop with the width ofca.50 K emerged at the temperature range of 260-340 K.In addition,the influence of substituents was also confirmed using absorption spectroscopy and cyclic voltammetry.This work demonstrates that this strategy is effective in the construction of asymmetric SCO-active cobalt（Ⅱ）complexes with interesting magnetic properties.It also provides the possibility to introduce more functional groups into one SCO compound.The research on Co-terpy SCO compounds bearing multifunctions is in progress.

Acknowledgments:This work was financially supported by the National Natural Science Foundation of China(Grants No.21771049,21871039,91961114,22025101,22173015).ZHU Yuan-Yuan thanks the financial support from the Fundamental Research Funds for the Central Universities of China(Grants No.PA2021GDSK0063,PA2020GDJQ0028).

Supporting information is available at http://www.wjhxxb.cn