氰基桥联铁钴链状配合物的结构与磁性

类延瑞 朱海浪 黄 杰 周仁和 刘 涛

(大连理工大学精细化工国家重点实验室，智能材料化工前沿科学中心，大连 116024)

0 Introduction

Molecular magnetic bistability[1-2]refers to the phenomenon where a molecule can exist in two distinct spin states that can be interconverted by external stimuli such as light,heat,pressure,and magnetic or electric fields[3-9].This spin transition is generally mediated by metal-to-metal charge transfer (MMCT)between different metal sites or spin crossover (SCO) on a single metal site,which involves the redistribution ofd-electrons in response to appropriate crystal field environments[4,9].As the two spin states can represent the binary states of 0 and 1,the magnetic bistable molecular materials offer significant potential for the realization of high-density information devices at the molecular level[10].In addition,they also hold promise in molecular switching devices and sensors,which could lead to the development of spintronic devices[11]and quantum computing,offering improved performance and a wider range of functionalities[12].

As one of the well-known magnetic bistable systems,a considerable number of charge transfer coupled spin transition (CTCST) compounds have been documented.Among them,the building block strategy of cyanide-bridged metals has been demonstrated to be effective in fabricating CTCST compounds[4,13-16].One of the most typical systems of such compounds is the Prussian blue analogue (PBA)[17],which has a general formula of AnM1p[M2(CN)6]q·xH2O (A is a monovalent cation;M1and M2are redox-active metal ions with variable valence states).In 1996,Hashimoto and Sato et al.[1]initially reported the phenomenon of light - induced charge transfer in Co-Fe PBA systems,which stimulated extensive research on PBA analogues.In the early stages,the research focused on 3D grid structures,exploring the charge transfer behavior between metals by regulating vacancies and the number of alkali ions in the structures[18-20].Subsequently,researchers became interested in low-dimensional cyanide-bridged compounds[21-23]since these systems can precisely regulate the coordination environment of metal ions via ligands,thereby promoting complete charge transfer behaviors and increasing solubility,thus facilitating investigations of their structures and physicochemical properties.Notably,the tricyanoferrate （Ⅲ） building blocks can coordinate with metal ions to form lowdimensional compounds due to their unique conical structure.When appropriate ancillary ligands were applied,the redox potential of metal ions can be tuned to allow the occurrence of intermetallic charge transfer[18,24-25].The reported low-dimensional MMCT compounds using tricyanoferrate（Ⅲ）building blocks [Fe(Tp)(CN)3]-(Tp=tris(pyrazolyl)borate)have generated significant interest for MMCT-switched magnetism and other properties.For example,the 1D Fe—Co chain assembled by [Fe(Tp)(CN)3]-and a chiral ligand Pabn[26]showed the light-induced MMCT and conductivity conversion.The combination of asymmetric ancillary ligand and [Fe(Tp)(CN)3]-resulted in a light-induced single-chain magnet with large coercivity values[27-29].One can note that these functional cyanide-bridged MMCT systems usually contain solvent molecules and counterions.And the guest molecules and anions can influence charge transfer behavior[30-34]and the corresponding properties above-mentioned.For example,recently reported trinuclear {FeⅢ2FeⅡ} complexes[35]exhibit solvent-induced spin transition behavior and wide thermal hysteresis.The tetranuclear {FeⅢ2CoⅡ2}complexes[14]composed of [Fe(PzTp) (CN)3]-(PzTp=tetrakis(pyrazolyl)borate) revealed how different anions play a significant role in MMCT behavior by controlling intermolecular interactions.Therefore,understanding the roles of solvent and counterion effects on the MMCT behavior is crucial in manipulating the charge transfer more accurately and switching the physical properties more effectively.In light of this concern,we designed and synthesized two complexes (Scheme 1)composed of the asymmetric ligand Bzi and the tricyanoferrate （Ⅲ） building block [Fe(PzTp) (CN)3]-,namely [Fe(PzTp)(CN)3]2[Co(Bzi)4]2(ClO4)2·H2O (1) and[Fe(PzTp)(CN)3]2[Co(Bzi)2]·CH3OH (2).The effects of counterions and crystallizing solvents on the structures and properties of the complexes were studied.

Scheme 1 Self-assembly processes of complexes 1 and 2

1 Experimental

1.1 Materials

All chemical reagents were obtained from commercial suppliers and used without further purification.Tricyanoferrate （Ⅲ） building blocks (Bu4N) [Fe(PzTp)(CN)3],(Bu4N=tetrabutylammonium) and asymmetric ligand Bzi were synthesized according to the literature procedures[36-37].The physical measurements and detailed crystallographic data can be found in the Supporting information(Table S1-S6,Fig.S1-S17).

1.2 Synthesis of complex 1

Single crystals of 1 were synthesized by the liquid diffusion method.An aqueous solution containing Co(ClO4)2·xH2O (0.005 mmol,1.273 mg) was placed at the bottom of a clean test tube.Then a mixed solvent of methanol and water (1∶1,V/V,2.5 mL) was layered as the middle buffer.Finally,the methanol solution containing tricyanoferrate （Ⅲ） building blocks (Bu4N)[Fe(PzTp)(CN)3] (0.005 mmol,3.275 mg) and Bzi (0.02 mmol,3.4 mg) was placed on the top of the tube.The top of the tube was sealed and left for two months to obtain dark green block crystals.The yield based on Co(ClO4)2·xH2O was about 61%.Elemental analysis calculated for C118H106B2Cl2Co2Fe2N38O9(%)： C 56.19,H 4.23,N 21.10;Found(%)：C 56.05,H 4.26,N 20.17.

1.3 Synthesis of complex 2

The synthesis of 2 was similar to that of 1 by use of Co(NO3)2·xH2O instead of Co(ClO4)2·xH2O.Crimson long strips of crystals were obtained after about six weeks.The yield based on Co(NO3)2·xH2O was about 51%.Elemental analysis calculated for C53H48B2CoFe2N26O(%)： C 50.63,H 3.85,N 28.96; Found(%)： C 51.12,H 3.41,N 28.74.

CCDC： 2247684,1-120K; 2247683,2-120K;2247682,2-190K; 2247686,2-225K; 2247685,2-desolvated.

2 Results and discussion

2.1 Structure characterization

2.1.1 Crystal structure of 1

The coordination polymer 1 was synthesized by the reaction of (Bu4N) [Fe(PzTp) (CN)3],Bzi,and Co(ClO4)2·xH2O in a methanol-water mixture.The single crystals were obtained after staying solution in the dark for a few weeks.Due to the instability,attempts to collect X-ray diffraction data at higher temperatures were not performed.The dark green 1 crystallizes in a triclinic space groupat 120 K.The phase purity was confirmed by powder X-ray diffraction (PXRD)(Fig.S4) measurements.As shown in Fig.1,the cobalt center is located in a distorted octahedral environment.It can be verified by continuous symmetry measurements using the SHAPE program,which is similar to previously reported results.Cobalt ions are connected by tricyanoferrate building blocks,forming square -wave type chains along thec-axis.As shown in Fig.S1c,the stacking between chains is tight,but there are no obvious intra-chain or inter-chainπ…πinteractions.It should be noted that two free ClO4-ions are located in the lattice void,which plays a role in balancing the positive charges of the chain.In addition,hydrogen - bonding interactions between the solvent molecule and the free terminal cyanide groups are also observed,which may play a role in mediating the MMCT behavior.The asymmetric unit contains three crystallographically independent Co (Co1,Co2,and Co3)ions and two Fe(Fe1 and Fe2)ions(Fig.1a).Complex 1 comprises cyanide-bridged alternating Fe-Co square-wave type chains along thec-axis (Fig.S1b).Within the chain,two of the three cyanide groups in the [Fe(PzTp)(CN)3]-unit bridge two Co ions to form an alternating zigzag chain [Co1—NC—Fe1—CN—Co2—NC—Fe2—CN—Co3].In 1,the Co ions adopt a distorted octahedral coordination geometry with four nitrogen atoms from four Bzi and two nitrogen atoms from tricyanoferrate.At 120 K,the average Co1—N,Co2—N,and Co3—N bond lengths are 0.192 8,0.192 2,and 0.192 3 nm,respectively.All of them are typical of lowspin (LS) Co （Ⅲ）ions.The iron ion also locates in a distorted octahedral coordination environment that is composed of three nitrogen atoms from the PzTp ligand and three cyanide carbon atoms.Fe1—C and Fe1—N bond lengths are 0.187 6(4)and 0.200 9(3)nm,respectively,and the Fe2—C and Fe2—N bond lengths are 0.186 1(4) and 0.201 1(3) nm,respectively,which are in the range of character of the LS FeⅡspecies.

Fig.1 (a)Asymmetric unit of complex 1;(b)Square-wave type chain structure of 1

2.1.2 Crystal structure of 2

To investigate how solvent molecules and anions affect the crystal structure and magnetic properties.Complex 2 was synthesized in different solvents by changing the metal salts of Co(NO3)2·xH2O.Singlecrystal X-ray diffraction data for 2 was collected at 120 K.The phase purity was confirmed by PXRD (Fig.S5)measurement.2 crystallizes in the triclinic space groupP1.The cobalt center is located in an octahedral environment and is interconnected by tricyanoferrate to form a double zigzag chain skeleton [Co1—NC—Fe1—CN—Co2—NC—Fe2] along thea-axis (Fig.2b).2 shows a double zigzag chain structure.From thea-axis direction,all the cobalt ions fall in a straight line,and the iron ions in the building block form two planes located on both sides of the cobalt sites (Fig.2c),which is different from the structure of 1.In addition,lattice voids between the adjacent chains adopt a small amount of methanol solvent molecules.Thermogravimetric (TG) analysis shows that there exists one methanol molecule in the symmetric unit (Fig.S3).As shown in Fig.2a,the asymmetric unit contains two crystallographically independent Fe (Fe1 and Fe2)ions and two crystallographically independent Co (Co1 and Co2)ions.Each cobalt ion center is linked to four tricyanoferrate building blocks at the equatorial position,and the rest coordinating sites are coordinated with two Bzi ligands to form octahedral coordination.In contrast,each cobalt center in the asymmetric unit of complex 1 is linked to four Bzi ligands at the equatorial position and then connected to two building blocks.In complex 2,the average Co1—N and Co2—N bond lengths are 0.190 7 and 0.190 6 nm at 120 K,respectively.They are both typical for the LS Co（Ⅲ）ions.The Fe1—C and Fe1—N bond lengths are 0.189 7(4) and 0.198 7(3)nm,respectively,and the Fe2—C and Fe2—N bond lengths are 0.191 1(3) and 0.197 7(3) nm at 120 K,respectively,which are in the typical range for the LS FeⅡspecies.Single-crystal X-ray diffraction data for 2 at 190 and 225 K were also collected to investigate the effect of crystalline solvents on the structure and magnetic properties.The average Co1—N and Co2—N bond lengths are 0.211 7 and 0.211 2 nm at 190 K,0.212 0 and 0.211 6 nm at 225 K,respectively.Detailed crystallographic data are listed in Table S1.

Fig.2 (a)Asymmetric unit of complex 2;(b)Double zigzag chain structure of 2;(c)Packing diagram viewed along the a-axis of 2;(d)Packing diagram viewed along the c-axis of 2

2.2 Magnetic property

Temperature-variable magnetic susceptibility measurements were performed to probe the charge transfer behaviors in these complexes.As shown in Fig.3,for complex 1,theχMTvalue remained around 0.10 cm3·mol-1·K from 2 to 350 K,corresponding to diamagnetic FeⅡLS—CN—CoⅢLSlinkages.When further heated,theχMTvalue rapidly increased to 3.34 cm3·mol-1·K at 368 K and reached 6.29 cm3·mol-1·K at 371 K (T1/2↑=368 K).TheχMTvalue at 400 K was 6.27 cm3·mol-1·K,which was close to the value of 6.67 cm3·mol-1·K expected for magnetically isolated two FeⅢLS(S=1/2) and two CoⅡHS(S=3/2) ions.It suggests that about 94% of {FeⅡLS—CN—CoⅢLS} units underwent intermetallic charge transfer at this stage.Upon decreasing temperature,theχMTvalue decreased rapidly to 3.47 cm3·mol-1·K at 357 K.Then it reached a plateau value of about 0.11 cm3·mol-1·K with an 11 K-wide thermal hysteresis loop upon cooling.In addition,the isothermal field-dependent magnetization data for 1 was collected in a direct current (dc) field up to 5 T at 2 K (Fig.S9).The isothermal magnetization curve at 2 K increased slowly to 0.032Nβat 50 kOe,confirming the diamagnetic character of the {FeⅡLS—CN—CoⅢLS} unit.Differential scanning calorimetry (DSC)was performed under an N2atmosphere to verify the driving force of MMCT.As shown in Fig.S6,the DSC curves exhibited an endothermic peak in the heating mode,with the onset and maximum temperatures ofTon=362.5 K andTmax=368.0 K,respectively.TheTmaxwas consistent withT1/2↑=368 K,accompanied by enthalpy and entropy changes of ΔHm=90.33(3) kJ·mol-1and ΔSm=250.91(6) J·mol-1·K-1,respectively.In addition,an exothermic peak was recorded in the cooling mode,withTon=364.9 K,Tmax=360 K,ΔHm=91.23(3)kJ·mol-1and ΔSm=253.42(6)J·mol-1·K-1.Peak temperature was also close toT1/2↓=357 K.The distinct endothermic/exothermic peaks and the 8 K-width thermal hysteresis indicated a first-order phase transition and significant entropy changes suggested that MMCT is an entropy-driven process.

Fig.3 Temperature dependence observed for the χMT values of 1(a)and 2(b)under a dc field of 5 000 Oe(1)and 1 000 Oe(2)

Complex 2 exhibited an interesting two-step spin transition.The purple curve in Fig.3b showed that theχMTvalue below 100 K was about 1.39 cm3·mol-1·K,while the theoreticalχMTvalue of 2 in a low spin state was about 0.58 cm3·mol-1·K,suggesting the existence of high-spin(HS)Co（Ⅱ）ions.Upon heating,theχMTvalue increased to 1.91 cm3·mol-1·K at 180 K.The theoreticalχMTvalue of the {FeⅢLS—CN—CoⅡHS—NC—FeⅢLS}state of complex 2 was 4.32 cm3·mol-1·K.Based on the changes inχMTvalue,about 44% {FeⅡLS—CN—CoⅢLS—NC—FeⅢLS} units underwent the intermetallic charge transfer at this stage.When the temperature continued to increase to 210 K,theχMTvalue rapidly reached 4.12 cm3·mol-1·K,which was close to the theoretical value of 4.32 cm3·mol-1·K,indicating that about 95.4% {FeⅡLS—CN—CoⅢLS—NC—FeⅢLS} units underwent the intermetallic charge transfer and transformed into the {FeⅢLS—CN—CoⅡHS—NC—FeⅢLS}state.During the cooling process,theχMTvalue decreased rapidly to 2.06 cm3·mol-1·K at 190 K.A thermal hysteresis of 12 K was produced.Subsequently,the value ofχMTdecreased to 1.58 cm3·mol-1·K at 150 K.Further decreasing the temperature resulted in the decrease ofχMTvalue to 1.3 cm3·mol-1·K and produced a thermal hysteresis of about 30 K.2 exhibited a rare two-step charge transfer behavior.In the first step,the transition temperatures wereT1/2↑=183 K,T1/2↓=154 K; in the second step,the transition temperatures wereT1/2↑=204 K,T1/2↓=192 K.Remarkably,we observed the first step of spin transition behavior disappeared when 2 continued to be heated to 300 K (green curve in Fig.3b),possibly due to the loss of some solvents during the heating process.In addition,the TG curve of 2 (Fig.S3) showed a loss of solvents at 305 K,with a mass loss of 3.87%.Therefore,we collected the temperature-variable magnetic susceptibility for 2-desolvated.As shown in the blue curve in Fig.3b,theχMTvalues of 3.42 cm3·mol-1·K remained nearly constant from 30 to 150 K,then slowly increased to 4.12 cm3·mol-1·K at 198 K and reached a plateau value of 4.25 cm3·mol-1·K at 203 K.Corresponding to highspin {FeⅢLS—CN—CoⅡHS—NC—FeⅢLS} states.It indicates that the solvent molecules can greatly influence the charge transfer behavior of 2.It is noted that 2 exhibited higherχMTvalues below 10 K,which can be attributed to the intramolecular ferromagnetic coupling between the remaining FeⅢLSand CoⅡHSions.

Variable-temperature infrared spectra were collected to probe the charge transfer behavior.For complex 1,twoνCNabsorption bands at around 2 117 and 2 154 cm-1were observed at 400 K (Fig.4a).The band at 2 117 cm-1can be ascribed toνCNmodes for the nonbridging cyanide groups of [FeⅢ(PzTp)(CN)3]-,and the other is attributed toνCNmodes for the bridging cyanide groups of FeⅢLS—C≡N—CoⅡHSlinkages.As the temperature decreased,two new bands appeared,corresponding to the non-bridgingνCNmodes of [FeⅡ(PzTp)(CN)3]2-(2 064 cm-1) and the bridgingνCNmodes of FeⅡLS—C≡N—CoⅢLS(2 102 cm-1) linkages.Meanwhile,as the temperature was lowered,intensities of the non-bridgingνCNmodes of [FeⅢ(PzTp)(CN)3]-and the bridgingνCNmodes of the FeⅢLS—C≡N—CoⅡHSlinkages were reduced to disappear.These results established the MMCT behavior in 1.For complex 2,twoνCNabsorption bands at around 2 122 and 2 160 cm-1were also observed at 250 K (Fig.4b).It can be ascribed toνCNmodes for the non-bridging cyanide groups of [FeⅢ(PzTp)(CN)3]-and the bridging cyanide groups of FeⅢLS—C≡N—CoⅡHSlinkages,respectively.At the cooling process,threeνCNabsorption bands at around 2 064,2 101,and 2 199 cm-1were enhanced gradually,which can be ascribed toνCNmodes for the non-bridging cyanide groups of [FeⅡ(PzTp)(CN)3]2-and the bridging cyanide groups of FeⅢLS—C≡N—CoⅢLSand FeⅡLS—C≡N—CoⅢLSlinkages.These also probed the MMCT behavior in 2.

Fig.4 Variable-temperature solid-state infrared spectroscopy of 1(a)and 2(b)at the near transition temperature

Light-monitored magnetic susceptibility measurements were conducted to further explore the possible photo-responsive MMCT in complexes 1 and 2.Between 300 and 400 K,solid UV-Vis-NIR absorption spectra were recorded for 1.Spectral changes were observed at the bands approximately 500 and 800 nm(Fig.S12),which correspond to the {FeⅡLS—CN—CoⅢLS} and {FeⅢLS—CN—CoⅡHS} states,respectively.Based on the UV-Vis-NIR absorption spectra results,we chose 532 and 808 nm diode lasers to examine the photo-responsive characteristics of 1.Meanwhile,808 nm was selected based on the UV-Vis-NIR absorption spectra results of 2 (Fig.S13).As shown in Fig.5b,when 1 was irradiated with an 808 nm laser,theχMTvalue increased rapidly and reached a saturation value of 7.2 cm3·mol-1·K.The increase of magnetization demonstrated the occurrence of light-induced MMCT,corresponding to the transformation from low-spin{FeⅡLS—CN—CoⅢLS} to the metastable high-spin{FeⅢLS—CN—CoⅡHS} state.When the laser wavelength was changed to 532 nm,theχMTvalue of 1 experienced a gradual decrease from 7.2 to 1.3 cm3·mol-1·K after 175 min.The incomplete phase transition may be attributed to the partial overlap between the green light and the FeⅡ→CoⅢIVCT (intervalence charge transfer)band.It is noted that the interconversion between{FeⅢLS—CN—CoⅡHS} and {FeⅡLS—CN—CoⅢLS} spin states can be well repeated by alternating light irradiations of 808 and 532 nm,confirming the reversible light-induced MMCT.When 2 was irradiated with an 808 nm laser (Fig.S8),theχMTvalue increased slowly to 4.35 cm3·mol-1·K after 150 min.It also showed the light-induced MMCT behavior in 2.But it cannot be excited back to the initial state by other laser wavelengths for 2,which is consistent with its change in the UV-visible absorption spectrum.

Fig.5 (a)Plots of χMT vs temperature of 1 and 2 irradiated at 808 and 532 nm at 5 000 Oe(1)and 1 000 Oe(2);(b)Plots of χMT vs time during cycles of successive irradiation at 808 nm(orange)and 532 nm(green)at 10 K of 1

After the irradiation with 808 nm laser at 10 K for 2 h,complexes 1 and 2 were cooled down to the base temperature of 2 K.TheχMTvalues rapidly increased to 28.1 cm3·mol-1·K for 1 (orange curve in Fig.5a).After being irradiated by the 532 nm laser,only theχMTvalues of 1 increased to 1.8 cm3·mol-1·K at 2 K.During the heating process,theχMTvalues rapidly dropped to 5.8 cm3·mol-1·K at 17 K for 1,and finally returned to the thermodynamically stable {FeⅡLS—CN—CoⅢLS} state at 75 K.TheχMTvalues rapidly increased to 11.5 cm3·mol-1·K at 2 K and dropped to 5.3 cm3·mol-1·K for 2 at 11 K (blue curve in Fig.5a),then returned to {FeⅡLS—CN—CoⅢLS—NC—FeⅢLS}state at 150 K.The photomagnetic results confirmed that MMCT behavior could be excited by light irradiation and returned with thermal treatment.

The disparate magnetic properties of complexes 1 and 2 should lie in their structures,which may arise from their different intermolecular interactions and crystal packings.For 1,there are significant hydrogen bonding interactions between solvent molecules and the free terminal cyanide groups.No significant hydrogen bonding interactions are observed in 2.This could affect the crystal field experienced by the iron center,as well as the redox potential of the iron center.This explains why 1 and 2 have different transition temperatures.Furthermore,the different charge transfer behaviors of 1 and 2 prior to light irradiation stem primarily from distinct coordination environments and intra -chain structures for 1 and 2.

To further explore the influence of solvents on their charge transfer behavior,we collected X - ray diffraction data for 2 at temperatures ranging from 120 to 225 K.Detailed crystallographic data are presented in Table S1,and the structures of multiple temperatures for 2 were overlapped together to probe the trend of bond length and angle changes with temperature(Fig.6).The Co1—N and Co2—N bond lengths increase by 0.019 7 and 0.019 4 nm from 120 to 225 K,respectively.Meanwhile,the ∠N2—Co1—N3 decreases 1.21° and the ∠N5—Co2—N6 increases 1.31°.As for the Co—N≡C bond angles,the ∠C5—N2—Co1,∠C4—N5—Co2,and ∠C2—N6—Co2 decrease 2.9°,3.6°,and 3.5°,respectively.It indicates that the coordination environment of the cobalt center has changed.It is noted that the distortion degree of the octahedral field in the cobalt center is increased upon the heating process.The cobalt octahedron with a large distortion degree favors the high spin {FeⅢLS—CN—CoⅡHS—NC—FeⅢLS} state.This is consistent with the reported literature[14].In general,the parameter CShMMand∑M(M=Co,Fe) both can be used to evaluate the geometry deviation from the standard octahedron of the metal center (CShMMis analyzed by SHAPE software).For the Fe/Co charge-transfer systems,a smaller CShMMvalue favors the {FeⅡLS—CN—CoⅢLS—NC—FeⅢLS} state.From Table 1,the values of CShMCo1and CShMCo2increase by 0.029 and 0.018 from 120 to 225 K,respectively.And the values of∑Co1and∑Co2increase by 7.88 and 7.06,respectively.Based on these results,we think that the changes in the interaction between solvents and intra-chain molecules can modulate the degree of distortion of the[CoN6]octahedral and ligand fields around cobalt ions,resulting in different MMCT behaviors.

Table 1 Main structural parameters for 2 at different temperaturesa

Fig.6 Overlap diagrams of the structures of 2 at different temperatures centered on Co2

3 Conclusions

In this study,we report the synthesis of two chain complexes,denoted as 1 and 2,using anionic substitution in the methanol-water system.Detailed investigations of the structural and magnetic properties revealed that complex 1 exhibited thermal and light-induced charge transfer behavior,while complex 2 exhibited thermal and solvent-induced two-step spin transition behavior.Structural studies suggest that the different photomagnetic properties of 1 and 2 may be attributed to hydrogen bonding interactions between the solvent molecules and free terminal cyanide groups.Our findings demonstrate that the guest solvent molecules can also significantly modulate the metal-to-metal charge transfer (MMCT) through intermolecular interactions.Moreover,uncoordinated anions can modulate the molecular structure and MMCT by affecting the crystallographic environment of the complexes.These results provide valuable insights into the precise modulation of charge transfer.

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