含席夫碱配体的四方压缩高自旋Mnバ配合物的合成、晶体结构及磁性质

2013-09-15崔林芳李兴鳌汪联辉

无机化学学报 2013年8期

王石崔林芳李兴鳌汪联辉黄维

(南京邮电大学信息材料与纳米技术研究院，南京 210046)

Most of manganeseバcomplexes are known in high-spin (HS)elongated octahedral pattern,presenting interest for building magnets at molecular level[1]or in extended assemblies[2],since in elongated form the Mnバunits show favorable magnetic anisotropy,with D＜0 parameter for the Zero-Field-Splitting(ZFS)Hamiltonian．By contrary,the compressed Mnバcomplexeshaving D＞0 and anon-magnetic ground state with Sz=0,are apparently not suitable for molecular magnetism applications,but such species present a high interest being rare[3-6]．

Other rare varieties of Mnバion are related with its occurrence in low-spin (LS)and spin-crossover(SCO)systems．The pseudo-octahedral 3dnconfigurations able for SCO manifestation (4≤n≤7)are in their vast majority Fe（Ⅱ） and Feバ coordination compounds[7],to a lesser extent Co（Ⅱ）and only in a few cases of Cr（Ⅱ） or Mnバ[8-9]．

Hexadentate Schiff base ligands with a trans-(N4O2)set of donors usually stabilize manganeseバin its HSstate[10]．Challenging thecustomary status Morgan et al．reported the prototypic gradual SCO phenomenon of Mnバhexadentate Schiff base complexes[11-12]．Recently,we reported also a Mnバhexadentate Schiff base complex,[Mn(5-Br-sal-N-1,5,8,12)]ClO4(5-Brsal-N(1,5,8,12)is a hexadentate Schiff base ligand obtained from N,N′-bis(3-aminopropyl)ethylenediamine and 5-bromo-2-hydroxy-benzaldehyde), with SCO behavior and a detailed theoretical analysis[13]．The ligands of this class are determining,at relatively minor structural changes of their peripheral substituents,a versatile magnetic behavior of the complexes,since the exerted ligand field is at the critical limit in the balance of effects deciding HSor LSforms,or the SCO behavior．

Continuing our efforts in this line,we considered other manganeseバcomplexes based on a longer and more flexible hexadentate Schiff base ligand．Compared to the ligand 3-MeO-sal-N-1,5,8,12 reported before[11],which determined SCO transition behavior,the ligand 3-MeO-sal-N-1,5,9,13 (Scheme 1)stabilizes the HS form．It has one more CH2group (i．e．a central propylenediamine moiety instead of an ethylenediamine sequence),determining a specific balance of coordination effects and steric demands．Herein,we report the preparation,crystal structure and magnetic properties of complex,[Mn(3-MeO-sal-N-1,5,9,13)]ClO4·H2O(1)．

1 Experimental

1.1 Materials and physical measurements

N,N′-bis(3-aminopropyl)-1,3-propanediamine,2-hydroxy-3-methoxy benzaldehyde and manganese perchlorate hexahydrate were purchased from Aldrich．All solvents were of AR grade and used without further purification．Elemental analyses for C,H and N were performed on a Perkin-Elmer 240C analyzer．Variable-temperature magnetic susceptibility data were collected using a Quantum Design MPMS SQUID magnetometer．The experimental susceptibilities were corrected for the diamagnetism of the constituent atoms(Pascal′s constants)．

1.2 Synthesis of[MnⅢ(3-MeO-sal-N(1,5,9,13)]ClO4H 2O(1)

Complex 1 was synthesized by adding a methanol(6 mL)solution of 2-hydroxy-3-methoxy benzaldehyde(60．8 mg,0．4 mmol)into a methanol(6 mL)solution of N,N′-bis(3-aminopropyl)-1,3-propanediamine(37．7 mg,0．2 mmol),and the mixture was heated to reflux for 30 min．A methanol(6 mL)solution of Mn(ClO4)2·6H2O (72．4 mg,0．2 mmol)was added,and the mixture was stirred at room temperature for 30 min．The resulting solution was allowed to stand in air at room temperature for about several hours to yield wellformed dark-brown crystals．Yield:51%．Anal．Calcd．for C25H36ClMnN4O9(%):C,47．89;H,5．79;N,8．94．Found(%):C,47．65;H,6．01;N,8．70．(Caution!Although no problems have been encountered in the present work,perchlorates are potentially explosive and should be treated in small quantities with care．)

1.3 Structure determination

Crystal data was collected by a Bruker SMARTAPEX diffractometer employing graphite-monochromated Mo Kα radiation (λ=0．071 073 nm)at 296 K．The data integration and reduction were undertaken with SAINT[14]．The structure was solved by the direct method using SHELXS-97 and all the non-hydrogen atoms were refined anisotropically on F2by the fullmatrix least-squarestechniqueusingthe SHELXL-97[15]．All the hydrogen atoms (except for those bound to amine N atoms and water molecule)were placed in calculated positions with fixed isotropic thermal parameters．The hydrogen atoms of water molecule and amine N atoms were located from difference maps and constrained to ride on their parent O and N atoms．A summary of the crystallographic details of the X-ray analyses is given in Table 1 and selected bond lengths and angles are given in Table 2．

Table 1 Crystal data for complex 1

Table 2 Selected bond distances(nm)and bond angles(°)for compound 1

CCDC:926103．

2 Results and discussion

2.1 Description of the structure

At 296 K,complex 1 crystallizesin the monoclinic space group P21/c with discrete mononuclear[MnIII(3-MeO-sal-N(1,5,9,13))]+cation,uncoordinated perchlorate anion and one crystallizing water molecule．Manganeseバ ion is coordinated to 3-MeO-sal-N(1,5,9,13)ligand in a pseudo octahedral environment characterized by a {N4O2}donor set(Fig．1)．The two phenolic oxygen atoms are in a trans configuration and four nitrogen atoms placed in an approximate equatorial plane．

The two coordinating ONN ligand halves are facial．A pseudo-two-fold rotation axis through the Mn center bisects the C-C-C angle of the propanediamine moiety at the middle of the ligand．This generates pairs of cis amine(N2 and N3),cis imine(N1 and N4)and trans phenolate(O2 and O4)donors．The Mn-N and Mn-O distances(Mn1-O2:0．188 0(2),Mn1-O4:0．187 0(2);Mn1-N2:0．223 4(3),Mn1-N3:0．226 4(3);Mn1-N1:0．212 4(3),Mn1-N4:0．214 5(3)nm)are in good agreement with those observed in other HSmanganeseバ (S=2)hexadentate Schiff base complexes[10-13]．In complex 1,hydrogen bonds link the adjacent cation,anion,crystallzing water molecule forming a onedimensional(1D)zigzag chain along the a axis(Table 3 and Fig．2)．

Table 3 Hydrogen bond lengths and bond angles

Importantly,the presence of two short Mn-O and long Mn-N bonds are consistent with Jahn-Teller effects．Since in the chemistry of manganeseバ the common perspective is that the occurrence of the Jahn-Teller effect leads to axial elongation[16],the tetragonally compresed cases are regarded as intriguing and rare[17]．In compound 1 the compression occurs along the O-Mn-O axis．The primary reason is the relative strength of the coordinating donors,the phenoxo groups benefiting from the supplement of favorable electrostatic factors．At the same time,a role in the compressed pattern seems determined by steric demands of the ligands with this topology[13]．

2.2 Magnetic properties

The magnetic properties of compound 1 were investigated over a temperature range from 1．8 to 300 K with a SQUID magnetometer in an applied magnetic field of 2 000 Oe．The dependence of χmT vs T is depicted in Fig．3．The χmT value of 2．95 cm3·mol-1·K at room temperature is consistent with the spin-only value for HSMnバ,approximately(3．00 cm3·mol-1·K for g=2 and S=2)．As temperature was decreased,the χmT value remain almost constant between 300～50 K．The deep decrease ofχmT values below 25 K was observed and the χmT curve falls to a value of 1．08 cm3·mol-1·K．Despite cation-anion-crystallzing water hydrogen bonds being present,in the crystal packing the Mnバcations are well separated from each other and no effective intermolecular antiferromagetic interactions exist．Therefore,this behavior can be mainly attributed to manganeseバZFSwith positive D parameter．

To assess the ZFS,magnetization data were collected on compound 1 at a variety of fields in the temperature range 1．8～10 K (see Fig．4)．The seven isofield data sets were fitted using ANISOFIT[18]to give the following set of parameters:D=5．16 cm-1,E=0．12 cm-1and g=2．002．We note the positive value of the D parameter,associated with the compressed octahedral pattern and the quasi-tetragonal nature,with relatively small E parameter．