張翠娟 Sukhen Bala 倪兆平 童明良

(中山大學化學學院，生物無機與合成化學教育部重點實驗室，廣州 510275)

0 Introduction

Spin-crossover(SCO)complexes,as the bistable molecule-based materials, have attracted great attentions due to the promising applications in the field of molecular memory and switching.It is still challenging to design and synthesis a proper system which can show hysteretic SCO behavior near room temperature.The most commonly found SCO systems are based on the FeⅡ and FeⅢ ions[1].However,only few reports of cobaltⅡSCO systems were reported.The CoⅡion played an important role in magnetism among the all transition metals.The SCO cobaltⅡcomplexes can also show two states,high-spin S=3/2 and low-spin S=1/2.The commonly found SCO Co systems are six-coordinated complexes[2].Until now,five-coordinated CoⅡsystems are extremely rare[3].

Recently,Chen group reported the first example of SCO five-coordinate cobaltⅡcomplex [Co(TMC)(CH3CN)]X2(TMC=1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane)[4].However,it exhibited an incomplete SCO property.The χMT value at 400 K was still smaller than the values expected for the high-spin state of CoⅡions,suggesting that the ligand field strength should be reduced for the five-coordinated CoⅡcomplex.Considering the above mentioned facts,we chose the tetradentate ligand,TMC as the main ligand and adjusted the ligand field strength of the auxiliary ligand.

Herein,we report a pentacoordinate mononuclear cobaltⅡcomplex,[Co(TMC)Cl]ClO4,with four N atoms from the TMC ligand and one Cl-ion from decomposition of CHCl3.However,the ligand field of Cl-ion is weak,which only produces the high spin state of CoⅡions in a temperature range of 2.5～300 K.

1 Experimental

All the reagents were commercially available and used as received.The FT-IR spectra were recorded in KBr tablets in the range of 4000～400 cm-1on a PerkinElmer FT-IR spectrometer Frontier.The UVVis spectra were recorded in BaSO4in a range of 200～800 nm on UV-Vis-Nir Spectrophotometer UV-3600.The ESI-MS were recorded in MeOH in a range of m/z=50～800 on Liouid Chromatograph Mass Spectrometer LCMS-2010A.ThepowderXRD patternswere recorded on a Rigaku Smartlab X-Ray diffractometer with Cu Kα radiation(λ=0.154 178 nm,U=40 kV,I=26 mA)in a range of 5°～50° (2θ).The C,H and N microanalyses were performed for the dry crystals on an ElementarVario-ELCHNS elemental analyzer.Magnetic susceptibility measurementsforthe sample were performed on a Quantum Design PPMS instrument operating under a field of 1 000 Oe.Diamagnetic correction was performed based on Pascal′s coefficients.

1.1 Synthesis of[Co(TMC)Cl]ClO4

1,4,7,10-tetraazacyclododecane(3 g,17.4 mmol),HCOOH(98%～100%,20 mL)and HCHO(37%,5 mL)were mixed in 12 mL water.The solution was stirred and refluxed for 12 h at 105℃.The resulting solution was cooled in an ice bath and neutralized with KOH until pH=12.The TMC product was extracted with chloroform,and collected by rotary evaporation[5].Co(ClO4)2·6H2O(3.15 g,8.59 mmol)was dissolved in 15 mL MeCN.Then 15 mL acetonitrile solution of TMC containing 2 mL chloroform was added to this solution.The solution was stirred and refluxed for 12 h at 75℃.The violet crystalline product suitable for X-ray diffraction was collected by vacuum suction filtration,and finally washed with MeCN and MeOH.Yield:40%(based on Co(ClO4)2·6H2O).Anal.Calcd.for C12H28Cl2CoN4O4(%): C,34.14;H,6.68;N,13.27.Found(%):C,34.44;H,6.60;N,13.17.IR(KBr,cm-1):3 118(s),2 930(w),2 880(w),2 830(w),1 661(w),1 636(m),1 478(m),1 401(vs),1 384(s),1 298(m),1 151(w),1 101(s)and 1 084(vs),1 023(m),966(m),912(m),802(w),755(m),623(s),585(w),516(w),473(w).UV-Vis(BaSO4),λmax/nm:263,365,426,520,570,600,705.ESI-MS(CH3OH):m/z=322.10([Co(TMC)(Cl)]+).

1.2 Structure determination

Single-crystal diffraction data were recorded on a Bruker D8 QUEST diffractometer with Mo Kα (λ=0.071 073 nm)radiation at 120 K.The crystal structure was solved by direct methods,and all non-hydrogen atoms were refined anisotropically by least-squares on F2using the SHELXTL 2014/7 program[6].Hydrogen atoms on organic ligands were generated by the riding mode.The responses to the alerts from checkCIF are quoted within the validation response form.The details of single-crystal diffraction data and selected bond lengths and bond angles are listed in Table 1 and 2,respectively.

CCDC:1849513.

Table 1 Crystal structural data and refinement parameters for complex 1

Continued Table 1

Table 2 Selected bond lengths(nm)and bond angles(°)of complex 1

2 Results and discussion

Complex 1 is obtained by reaction of the TMC and Co(ClO4)2·6H2O in acetonitrile and chloroform medium.The coordinated Cl-ion maybe come from the decomposition of chloroform[7].Complex 1 is characterized by single-crystal X-ray diffraction analysis.As shown in Fig.1,1 is a mononuclear,five-coordinate CoⅡcomplex.It crystallizes in the monoclinic space group of P21,containing one CoⅡion,one TMC and Cl-ligand,and one ClO4-as counter anion in the asymmetric unit.The CoⅡion has a distorted square pyramidal coordination environment where four nitrogen donors from chelating TMC ligand form the basal plane,while the Cl-ion occupies the apical position.

Fig.1 Molecular structure of 1

The axial Co-Cl bond length is 0.226 2 nm,while the equatorial Co-N bond lengths are 0.213 5,0.214 2,0.216 8 and 0.217 8 nm.These bond lengths suggest a high-spin state of CoⅡion.The metal is deviated from the original N4 mean plane by ca.0.081nm.Thus,the Cl-Co-N bond angles are deviated from 90°and fall in a range of 109.05°～115.64°.To further evaluate the degree of distortion,the Addison parameter(τ=(α -β)/60)is used,where α and β are the two largest basal angles[8].Hence,the values of τ for the regular square pyramidal and trigonal bipyramidal geometries are 0 and 1,respectively.The calculated τ value is 0.006 for complex 1,suggesting a slightly distorted square pyramidalgeometry.Itis also confirmed by the SHAPE 2.1 program,which gives a value of 0.834 with respect to ideal square pyramidal geometry[9].

As shown in Fig.2,the[Co(TMC)(Cl)]+cations are linked with the ClO4-counter anions through very weak C-H…O interactions,in which the shortest C…O distance is 0.328 4 nm (C12…O4).The CoⅡ ions are effectively separated in 1 and the shortest Co…Co distance is 0.803 7 nm.

Fig.2 View of a fragment of weak C-H…O interactions(orange dash line)in 1

Fig.3 Experimental PXRD pattern of complex 1 at room temperature

The phase purity of solid-state sample was demonstrated by elemental analysis and powered X-ray diffraction determination(Fig.3).The magnetic property of 1 was studied at 1 000 Oe in a temperature range of 2.5～300 K.As shown in Fig.4a,the χMT value at 300 K was equal to 2.29 cm3·K·mol-1,which was larger than the spin-only value(1.87 cm3·K·mol-1)for a high-spin CoⅡion.It indicates the orbital contribution.When the temperature decreased to 100 K,the χMT value remained almost constant.Then,it gradually decreased to 1.44 cm3·K·mol-1at 2.5 K.Usually,the antiferromagnetic interaction and/or the thermal depopulation of the higher energy Kramers doublets of CoⅡ ion can contribute to the decrease of χMT at low temperature.Since the intermolecular Co…Co separations are larger,the possibility of any obvious magnetic interaction can be discarded.Therefore,the decline of the χMT value is mainly due to the intrinsic magnetic anisotropy of the CoⅡion.

Fig.4 (a)Temperature dependence of χMT and fielddependence of the magnetization(inset)for complex 1;(b)ac magnetic susceptibility measurements under the applied field of 1 000 Oe

The field dependence of the magnetization for 1 was carried out at 2.50,3.50 and 5.00 K in a field range of 0～7 T (Fig.4a inset).The magnetization at 7 T reached 1.83Nβ,which was lower than the theoretical saturation value(3.3Nβ)for CoⅡ ion with g=2.2.The lack of saturation and different isotherm magnetization plots suggest the presence of magnetic anisotropy in 1.

In order to qualitatively estimate the anisotropy parameters,the magnetic data of 1 were analyzed with PHI program[10].The spin Hamiltonian(μB:Bohr magneton;g:isotropic g-factor;B:magnetic induction;?:spin operator;D:axial zero-field splitting parameter;:Stevens operator;2:tensor rank;0:the component of tensor rank)was used.The best fit(Fig.4)gave g=2.226±0.003,D=(-33.066±1.346)cm-1.The magnitude of axial anisotropy|D|=33.066 cm-1was comparable with other anisotropic five-coordinate CoⅡcomplexes described in the literature[11].

To explore the dynamic magnetic behavior of 1,alternating current(ac)magnetic susceptibility data was collected (Fig.4b).Unfortunately,the lack of outof-phase signal(χM″)was observed under zero dc field.The appearance of frequency-dependence χM′and χM″signals could occur after the application of external dc field of 1 000 Oe.However,no peaks could be observed.Itsuggeststhatthe fastquantum tunneling of magnetization(QTM)can only be partially suppressed under non-zero dc field.

3 Conclusions

In conclusion,a pentacoordinate mononuclear cobaltⅡcomplex with field induced slow magnetic relaxation behavior is reported.The CoⅡion is coordinated to four N atoms from the TMC ligand and one Cl-ion,which can be described as a distorted square pyramid.The structural and magnetic data support the high-spin state for the CoⅡion.Therefore,to achieve a SCO pentacoordinate CoⅡcomplex,suitable ligands with the ligand field stronger than Clion will be required.