LI Jin-Hu WANG Xio-Zhong② WEI Hi-Wen HOU Qin LAI Xio-Yong LIU Xing-Yu YANG Qing-Feng

a (State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, National Demonstration Center for Experimental Chemistry Education, College of Chemistry and Chemical Engineering Ningxia Uniνersity Yinchuan750021, China)

b (College of Chemistry and Materials Science, Shandong Agricultural Uniνersity, Taian271018, China)

ABSTRACT Two new low-dimensional Mn(II) coordination compounds of Mn(H2pimdc)2(phen) (compound 1) and [Mn(Hpimdc)(phen)]n·nH2O (compound 2) were synthesized, where H3pimdc is a 2-propyl-imidazole- 4,5-dicarboxylic acid and phen is 1,10-phenanthroline.Both compounds were thoroughly analyzed by X-ray single-crystal diffraction, element analysis, IR spectra, PXRD and thermal analysis.X-ray single-crystal diffraction analyses indicate that the phen ligands terminate metal ions on two sides to prevent the linkage between the neighboring metal ions into a higher dimensional structure in compounds 1 and 2.Compound 1 is constructed from mononuclear molecules which form the 1D helical supramolecular chain by N-H···O hydrogen bonds.In compound 2, Hpimdc is a tetradentate bridging ligand, which binds Mn(II) ions to generate a single-stranded right helical chain parallel to the b axis.These 1D chains are additionally coupled by H-bonds as well as π··· π stacking interactions, which formed a 3D supramolecular structure.Additionally, compound 2 demonstrated temperature-dependent magnetic susceptibility and negative value of θ(equal to -9.09 K), which is indicative of weak antiferromagnetic interactions between Mn(II) ions.

Keywords: low-dimensional coordination compounds, helical structure, magnetic properties;

1 INTRODUCTION

Coordination compounds (CPs) attracted a lot of attention recently from scientists in the chemistry and material science disciplines[1,2].The CPs can be self-assembled from organic ligands[3]and metal ions into 0D, 1D, 2D or 3D networks[4-6], and formed entirely or partly by coordinate bond[7].And the CPs[1,2,8]have lots of potential application values (such as electricity, magnetism, photochemistry, catalyst adsorption and biological activity) in the field of materials[9-13]because of its novel skeleton structure[14], intriguing topologies and good performance[15-17], and combining the advantages of both inorganic materials and organic materials[18,19].Compared to traditional high dimensional CPs which can be synthesized relatively easy because of the high affinity of ligands to metals, low-dimensional CPs with 3D supramole- cular structures became popular in the crystal engineering field.In particular, as molecule-based magnetic materials, the low-dimensional CPs showed great potential for practical applications in the last years[20].As an example, single -molecule magnets (SMMs)[18,20,21]are extensively researched because they are great model substances, analysis of which might help scientists to understand magnetic exchange interactions at the molecular level.The metal ions in CPs (such as Fe(II)[12-25], Co(II)[26-28], Ni(II)[29,30]and Mn(II)[31]) significantly affect overall magnetic properties of these compounds.Additionally, low-dimensional CPs are also excellent materials to a broader topic related to the research of magnetism in molecular magnetic materials[[32].

However, crystal engineers and coordination chemists find it challenging to design and control targeted CPs structures because they are affected by variety of factors, including ligand and metal ion choice, chemical nature of the ligand (e.g.organic or inorganic) as well as their synthesis parameters, namely, choice of a template and a solvent, reaction pH and temperature, etc.In recent years, a lot of aromatic polycarboxylic acids and N/O-donor-bridging ligands were used in the construction of CPs because of their diverse coordination modes.Several coordination com-pounds based on H3pimdc (such as [Cd(H2pimdc)2(H2O)2]·4H2O[33], [Co3(pimdc)2(H2O)6]·5H2O[34], [Ni(H2pimdc)2(phen)][35]) and 4,5-dicarboxyimidazole([Mn(phen)-(Hdcbi)][36]) have been already reported in the literature.H3pimdc ligand offers several advantages for constructing low-dimensional CPs: (i) The N and O atoms in H3pimdc molecule offer several coordination sites, which allows H3pimdc to act as a multidentate ligand; (ii) Carboxylic groups orientation relative to the imidazole ring might allow H3pimdc to be used for the construction of CPs with helical structures; (iii) Partially or completely deprotonated H3pimdc ligands can form H2pimdc-, Hpimdc2-, and pimdc3-species under certain pH; thus, if pH is controlled, different acid-base coordination modes can be achieved; (iv) Propyl group of the imidazole ring in a position 2 can chelate metal ions and provide enough repulsion between the metal centers to keep molecules at a certain distance, which helps to obtain monodentate complexes.In this system, the phen ligands were selected as second ligand to construct new types of low-dimensional CPs due to its too concentrated coor- dination sites.Our group reported field-induced slow relaxation of magnetism obtained using two isomorphic magnetic compounds: [Co(H2pimdc)2(phen)]nand [Fe(H2pimdc)2(phen)]n[37].

In this article, we continuatively selected H3pimdc and phen ligands and have synthesized one mononuclear compound Mn(H2pimdc)2(phen) (1) and one 1D chain compound [Mn(H2pimdc)(phen)]n·nH2O (2) under hydrothermal conditions.Preparation procedures as well as structures and thermal stabilities of both compounds were thoroughly described.Magnetic properties were only reported for compound 2.

2 EXPERIMENTAL

2.1 Materials and physical measurements

All chemicals were purchased from well-established commercial vendors and used without any further treatment.Contents of C, H and N were determined using Perkin-Elmer 2400 LS II instrument.Infrared (IR) data were collected by a Perkin Elmer Spectrum II spectrophotometer in the 4000～400 cm−1range.Compounds were ground with preliminarily dried KBr and then pressed until clear pellets were obtained.The Powder X-ray diffraction (PXRD) data were obtained using Rigaku RU200 instrument operated using Mo-Kαradiationλ= 0.71073 Å.Thermogravimetric (TG) analysis was conducted in air by Setsys 16 instrument at a 10 °C/min heating rate.Magnetic analysis was performed by the SQUID-VSM instrument at 1000 Oe applied magnetic field.

2.2 Syntheses of compounds 1 and 2

The reactions were conducted under autogenous pressure using 20 mL Teflon-lined stainless-steel container.The single crystals were first filtered from the mother solution, then rinsed with distilled water and finally dried in air at ambient temperature.

Synthesis of Mn(H2pimdc)2(phen) (1).25 mg (0.1 mmol) of Mn(CH3COO)2·4H2O, 40 mg (0.2 mmol) of H3pimdc, 36 mg (0.2 mmol) of phen-ligand and 10 mL of 8:2 (by volume) mixture of water and acetonitrile were stirred for 3 h at ambient temperature.During the synthesis, pH was maintained at 3 by adding HCl.The resulting suspension was then placed into an autoclave for 96 h.The hydrothermal reaction was performed under autogenous pressure at 140 °C.At the end of the reaction, the autoclave was slowly cooled, after which the yellow block crystals of 1 were recovered from the mother solution.Yield: 51% based on Mn.Anal.Calcd.C28H26MnN6O8 (Mr = 629.49).Elemental analysis (%) calcd.for [Mn(H2pimdc)2(phen)]: C, 53.38; H, 4.13; N, 13.34.Found: C, 53.26; H, 4.24; N, 13.53.IR (KBr pellet, cm-1): 3430(m), 3199(s), 2961(s), 2026(w), 1695(s), 1527(s), 1476(s), 1119(m), 851(s), 773(s), 731(s), 618(s), 509(s).

Synthesis of [Mn(Hpimdc)(phen)]n·nH2O (2).Compound 2 was synthesized using a similar method but at 160 °C.Reaction yield was 56% (calculated based on initial Mn content).The resulting product was light yellow block crystals.Anal.Calcd.C20H18MnN4O5 (Mr = 449.31).Elemental analysis (%) calcd.for [Mn(Hpi- mdc)(phen)]n·nH2O: C, 52.22; H, 3.73; N, 13.10.Found: C, 52.06; H, 3.89; N, 13.25.IR (KBr pellet, cm-1): 3424(m), 3185(s), 2969(s), 2027(w), 1669(s), 1523(s), 1460(s), 1114(m), 859(s), 781(s), 732(s), 618(s), 513(s).

2.3 X-ray crystal structure determination

Crystallographic data were obtained using Mo-Kαradiation (λ= 0.71073 Å) and Rigaku R-AXIS RAPID IP instrument.The structures of compounds 1 and 2 were then processed by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least- squares methods with SHELXL[38,39].All atoms (with the exception of hydrogen) were refined using anisotropic temperature parameters.All H atoms of compounds 1 and 2 were processed by the program OLEX2[40].All relevant crystallographic data are given in Tables 1 and 2.

Table 1.Crystallographic Data for Compounds 1 and 2

Table 2.Selected Bond Lengths (Å) and Angles (°) for Compounds 1 and 2

O(1)-Mn(1)-N(2) 88.87(8) O(3)-Mn(1)-N(2) 86.92(4) O(1)-Mn(1)-N(3) 102.43(8) O(3)-Mn(1)-N(3) 87.58(4) O(1)-Mn(1)-N(5) 76.02(8) O(1)-Mn(1)-O(2) 90.61(4) O(1)-Mn(1)-N(1) 86.56(9) O(1)-Mn(1)-O(3) 88.27(4) N(2)-Mn(1)-N(3) 100.33(8) O(1)-Mn(1)-N(3) 160.98(4) N(2)-Mn(1)-N(5) 160.17(8) O(1)-Mn(1)-N(2) 96.95(4) N(2)-Mn(1)-N(1) 74.39(8) O(1)-Mn(1)-N(1) 97.70(4) N(3)-Mn(1)-N(1) 169.61(8) N(3)-Mn(1)-N(1) 92.64(4) N(5)-Mn(1)-N(3) 95.49(9) N(3)-Mn(1)-N(2) 101.35(4) N(5)-Mn(1)-N(1) 91.69(8) N(2)-Mn(1)-N(1) 72.84(4)

3 RESULTS AND DISCUSSION

3.1 Synthetic analysis

Both title compounds were synthesized by coordinating metal ions with aromatic polycarboxylic acids and phen-ligands under hydrothermal conditions.We found that pH, metal to ligand ratios, reaction temperature and solvent type affected the crystal phase formation as well as their structures.The specific influence can be described by the following aspects: (i) The reaction temperature affected the syntheses of both compounds.Yellow block crystals of compound 1 formed at 140 °C, while compound 2 was synthesized at 160 °C.(ii) Optimum pH values for both compounds preparation are 4 and 3, respectively.When the pH is higher than 5, no crystals are found.(iii) The optimum metal-to-ligand ratio was 1:2:2.AT other ratios, microcrystals or no crystal at all were observed.(iv) Higher yields and larger crystals were obtained when water- acetonitrile mixture was used as a solvent.Some microcrystals formed when absolute ethanol was used instead of water.When only absolute acetonitrile was used, no crystals were afforded at all.

Scheme 1.(a) Coordination fashions of H3pimda observed in compound 1; (b) Coordination fashions of H3pimdc observed in compound 2

3.2 Structural description of compounds 1 and 2

Structural analysis revealed that compound 1 is a mononuclear compound with monoclinic structure crystalli- zing in theP21/nspace group.Compound 2 also possesses a monoclinic structure and crystallizes inPbcaspace group.Fig.1a shows that the Mn(II) ion in 1 is coordinated by two N atoms (N(1) and N(2)) from a phen molecule, two N atoms (N(3) and N(5)) from two different H2pimdc molecules and two O atoms (O(1) and O(5)) from two different H2pimdc molecules.The resulting geometry can be presented as a distorted [MnN4O2] octahedron.The angles of N(2)-Mn(1)-N(5), N(1)-Mn(1)-N(3) and O(1)-Mn(1)-O(5), which are 160.2, 169.6 and 176.7°, respectively, deviate from an ideal angle (180°) of an octahedron.The bond distances of Mn-O (Mn(1)-O(1) = 2.185 Å and Mn(1)-O(5) = 2.171 Å) are comparable to that of Mn(1)-N(1) (2.286 Å), Mn(1)-N(2) (2.216 Å), Mn(1)-N(3) (2.246 Å) and Mn(1)-N(5) (2.236 Å), respectively.Such similarity led to the slight axial compression and rhombic deformation around the octahedrally coordinated Mn(II) ions.And through the intermolecular N-H···O hydrogen bonds (N(6)-H(6)···O(6)#1 = 2.811(3) Å, N(4)-H(4)···O(8)#2 = 2.805(3) Å, symmetry codes: #1: 1.5-x, 0.5+y, 1.5-z; #2: 1-x, 2-y, 1-z), the mononuclear units are alternately reversely arranged.Additionally, the phen ligands terminate the Mn(II) ions on two sides, resulting in a mononuclear structure, which, in turn, prevents the neighboring Mn(II) ions from forming a higher dimensional structure, such as 1D helical chain parallel to thebaxis with a 9.777 Å unit (Fig.1b).All helical chains possess the same components and are left-handed oriented.These 1D chains are further connected by additional N-H···O intermolecular hydrogen bonds (N(6)-H(6)···O(6)#1 = 2.811(3) Å, N(4)- H(4)···O(8)#2 = 2.805(3) Å, symmetry codes: #1: 1.5-x, 0.5+y, 1.5-z, #2: 1-x, 2-y, 1-z) to form an extended 2D supramolecular layer (Fig.2).

As shown in Fig.3a, the asymmetric units of compound 2 consist of one crystallographically independent Mn(II) ion, one Hpimdc, one phen-ligand, and one crystalline water molecule.The Mn(1) is six-coordinated by two N atoms (N(1) and N(2)) from the same phen molecule, one N atom (N3) from a Hpimdc molecule and three O atoms (O(1), O(2), and O(3)) from two different Hpimdc molecules, showing a distorted octahedral geometry with [MnN3O3] mode.Bond angles of O(1)-Mn(1)-N(3), O(3)-Mn(1)-N(1) and O(2)-Mn(1)-N(2) (161.0, 159.4 and 160.2°, respectively) deviate from an angle of an ideal octahedron (180°).The slight axial compression and rhombic deforma- tion occurring around octahedrally-coordinated metal can be explained by the fact that Mn(1)-N(2) and Mn(1)-O(3) bonds (2.260 and 2.147 Å) are shorter than Mn(1)-N(1) and Mn(1)-O(2) bonds (2.322 and 2.207 Å) and somewhat longer than the Mn(1)-N(3) and Mn(1)-O(1) bonds (2.210 and 2.096 Å).Additionally, the presence of phen-ligands containing N-donors is beneficial for Mn(II) ions terminated on two sides, which helps to form a 1D chain structure and to prevent bond formation between the neighboring Mn(II) ions and would otherwise lead to a structure with a higher-dimensional order.The Hpimdc ligand acts as a kind tetradentate bridging ligand adopting theµ2-kN,O:kO′,O″ mode (Scheme 1b) linking Mn(II) ions into a single-stranded right helical chain parallel to thebaxis (Fig.3b).These 1D chains are further connected by O(1W)νiahydrogen- bonding interactions (O(1W)-H(1WB)· O(2) = 2.910 Å, O(1W)-H(1WA)· O(3) = 2.832 Å, N(4)-H(4)· O(1W)#1 = 2.819 Å, symmetry code: #1: 0.5+x,y, 0.5-z), forming a 2D supramolecular structure (Fig.4).Furthermore, the adjacent layers are stacked into a 3D supramolecular architecture (Fig.5) byπ·πinteractions between the intermolecular phen-ligand planes, which are spaced 3.490 and 3.588 Å apart (Table 4).

Fig.1.(a) Structural unit of compound 1 showing a distorted MnN4O2 octahedron.

Fig.2.2D supramolecular layer of compound 1 parallel to the bc plane.

Fig.3.(a) Structure unit of compound 2.H atoms are omitted for clarity.(b) Single-stranded right helical chain of compound 2 parallel to the b axis

Fig.4.2D supramolecular layer of compound 2 parallel to the ab plane by intermolecular H-bonds (O(1W)-H(1WA)···O(2) = 2.910 Å, O(1W)-H(1WB)···O(3) = 2.832 Å, N(4)- H(4)·O(1W)#1 = 2.819, symmetry code: 0.5+x, y, 0.5-z)

Fig.5.(a) 3-D supramolecular architecture of compound 2; (b) π···π stacking interactions in the 3D supramolecular architecture (symmetry code: 1-x, 2-y, 1-z)

Table 3.Lengths and Angles of H-bonds Formed in Both Compounds 1 and 2

Table 4.Parameters of π-π Interactions Occurring in Compound 2 Centroid-to-plane Distance (d), Dihedral Angle (α) and Centroid-to-centroid Distance (c)

4 CHARACTERIZATION

4.1 Infrared (IR) spectroscopy and powder X-ray diffraction (PXRD)

The IR spectra of compounds 1 and 2 showed strong peaks at 1695 and 1669 cm−1, which can be attributed to C=O vibrations of H3pimdc molecules.Absorption bands observed in IR spectra at 3199 cm-1for 1 and 3185 cm−1for 2 are attributed to the C-H vibrations of phen-molecule.The bands around 3430 cm−1for 1 and 3424 cm−1for 2 are ascribed to the weakening ofν(-OH) in both compounds because of H+loss by the partial carboxyl group.PXRD data agreed with the spectra simulated using single-crystal XRD data (Fig.6), confirming that our as-synthesized coor- dination compounds did not contain any impurities or amorphous phases.

Fig.6.Experimental (red) and simulated (blue) powder XRD patterns for compounds 1 and 2

4.2 Thermal stability analysis

The thermal gravimetry behaviors of compounds 1 and 2 were investigated in the 30～800 °C range in the air at 10 °C/min heating rate (Fig.7).Weight loss of compound 1 consisted of two steps, implying that H2pimdc and phen decomposed accordingly.Experimentally determined residual content was 11.2%, consisting mostly of MnO.This value agrees very well with a theoretical residual content, which is equal to 11.27%.Decomposition of compound 1 started at ～296 °C and ended at ～430 °C.The two-step decomposition of compound 2 was less pronounced than that of 1 but the slope change at ～350 °C can be still clearly seen.Additionally, slow weight decline (～4%), attributed to the lattice water loss (theoretical content 4.01%), was observed for 2.Weight loss above 350 °C corresponded to the collapse of the whole structure.The residual weight for compound 2 was 16.1%, which agreed very well with the theoretical residual weight of 15.79%.Thus, the solid residue consisted mostly of MnO.Overall, both compounds showed accepta- ble thermal stability.

Fig.7.TGA curves of both compounds

4.2 Magnetic properties of compound 2

The form ofχMTagainst T plot (χMis the molar magnetic susceptibility per Mn(II) ion) was measured for compound 2 in the 2～300 K range at 1000 Oe.Fig.8 shows the χMT vs.T curve of compound 2.At 300 K,χMTvalue was equal to 6.06 cm3·mol-1·K, which is above the spin-only value of 4.38 cm3·mol-1·K for Mn(II) ions.Upon cooling,χMTgradually increased and equals to 5.51 cm3/(mol K) at 50 K.Upon further cooling to 2 K,χMTdecreased very quickly to 0.59 cm3/(mol·K).Such behavior indicates a classical antiferromagnetic order, showing that the Mn(II) ions interact with each other in an antiferromagnetic fashion.As shown in the insets of Fig.8, the χM-1 vs.Tcurve demonstrated almost linear behavior in the whole temperature range.The curve was fitted using the Curie-Weiss law.The resultingCandθvalues for compound 2 obtained from the fitting are equal to 6.33 cm3/mol and -9.09 K, respectively.The negativeθvalue points at the existence of weak antiferromagnetic interactions between the Mn(II) ions in compound 2.

Fig.8.Results of magnetic characterization of compound 2: χMT(main graph) and χM-1 (insert) plotted as a function of temperature.Linear χM-1 vs.T curve was fitted using the Curie-Weiss law

5 CONCLUSION

The preparation, structural characterizations, thermal stabilities as well as magnetic properties of the new compounds 1 and 2 which were hydrothermally synthesized of Mn(II) salts, 2-propyl-imidazole-4,5-dicarboxylic acid and 1,10-phenanthroline were reported.X-ray single-crystal diffraction analysis reveals that compound 1 is mononuclear, and its units are alternately and reversely arranged because of intermolecular N-H···O hydrogen bonds, thus forming the 1D left-hand helical supramolecular chain.The structure of compound 2 is a single-stranded right-hand helical chain, stabilized by hydrogen bonds andπ··· πstacking interactions.As a result, 2 forms a 3D supramolecular network.The phen ligands terminate metal ions on two sides, which prevents the linkage between the neighbouring metal ions into a higher-dimensional structure and H3pimdc ligands display two different coordination modes in both 1 and 2.Thermal gravimetric analysis demonstrated good thermal stability of 1 and 2.And Variable-temperature magnetic susceptibility measurements indicate the existence of weak antiferromag- netic interactions between the Mn(II) ions in 2.