Syntheses , Crystal Structures , Magnetic Behaviours , and Thermal Properties of Three Hydrogen-Bonding Networks Containing Dicyanamide and 4-Hydroxypyridine

Three new dicyanamide-bridged polymeric complexes of {[Mn(dca) 2 (L) 2 ] ⋅ 2H 2 O} n (1), {[Cd(dca) 2 (L) 2 ] ⋅ 2H 2 O} n (2), and {[Co(dca) 2 (L) 2 ]2(L)} n (3) (dca = dicyanamide, L = pyridinium-4-olate) have been synthesized and structurally characterized. In the three compounds, the protons of hydroxyl groups of 4-hydroxypyridine transfer to pyridyl nitrogen atoms. Compounds 1 and 2 are isomorphous forming one-dimensional [M(dca) 2 (L) 2 ] n chains where metals are connected by double dca anions. These one-dimensional chains are extended into two-dimensional layers through weak C–H⋅ ⋅ ⋅N hydrogen bonds. Further, these layers are assembled into a three-dimensional supramolecular network through N–H⋅ ⋅ ⋅O, O–H⋅ ⋅ ⋅O hydrogen bonds. Complex 3 is a coordination layer of (4, 4) topology with octahedral metal centers linked by four single μ 1,5 -bridges.These layers are interlocked by N–H⋅ ⋅ ⋅O, O–H⋅ ⋅ ⋅O hydrogen bonds from coordinated water molecules and free L molecules, which leads to a three-dimensional supramolecular architecture. The variable temperature magnetic susceptibilities measurement of compounds 1 and 3 shows the existence of weak antiferromagnetic interactions between the metal centers. The thermogravimetric analyses of the compounds 1–3 are also discussed.


Introduction
The coordination polymers [1] based on dicyanamide [dca, N(CN) 2 − ] have drawn great interest in the last two decades, due to their attractive structures and magnetic properties: three-dimensional rutile-like structure of [M(dca) 2 ] shows long-range magnetic ordering [2][3][4] and low-dimensional system (one-dimension or two-dimension) when auxiliary ligand is introduced to form [M(dca) 2 (L)  ] [5][6][7][8][9][10][11].Other ways have been developed to obtain three-dimensional system, using auxiliary bridging ligand, such as nicotinic acid [12], pyrazine-dioxide [13], and pyrimidine [14] through coordination interaction.In addition to coordination interaction, some weak interactions, such as intra-and/or intermolecular hydrogen-bonding interactions, and - stacking supramolecular interactions also greatly affect the ultimate structures, especially in the field of extending lowdimensional entities to higher-dimensional supramolecular frameworks [15,16].Compared with the bridging ligands, the terminal ligands seem to only occupy one position of the coordination sphere; in fact, they have important impacts on the ultimate networks.For example, when terminal ligands are 4-picoline [17] and cyanopyridine [18], although only one-dimensional chains are obtained, the intermolecular - stacking interactions extend these one-dimensional coordination chains into two-dimensional or three-dimensional networks, which indicates that intermolecular interactions play an important role in constructing supramolecular architectures.To date, the research focus on crystal engineering of dca through intramolecular interactions remains limited [19], especially using hydrogen-bonding interaction to construct higher-dimensional architectures.In our earlier research, we chose 4-(4H-imidazol-4-yl) benzenamine and isonicotinamide as the coligands, and five supramolecular architectures were obtained [20][21][22], in which the metal centers coordinates to the imidazole nitrogens or pyridyl nitrogen atoms, and the -NH 2 , -CONH 2 groups act as hydrogen bond donor to form strong intermolecular hydrogen bonds.We also found the in situ nucleophilic addition reactions between dca and some small molecules [23][24][25].To further explore the work of metal dca and discuss the influence of hydrogen bonding interactions on the ultimate architecture, we now focus on the context with other terminal ligands, in which 4-hydroxypyridine is employed as the auxiliary ligand, and three coordination polymers pyridinium-4-olate) via dca connectors are prepared and structurally determined by X-ray single crystal diffraction.To our knowledge, with dca and the 4-hydroxypyridine, no coordination compounds have been reported before.

Experimental
2.1.Materials and Methods.All the reagents and solvents employed were commercially available and used as received without further purification.The C, H, and N microanalyses were carried out with a Perkin-Elmer 240 elemental analyzer.The FT-IR spectra were recorded from KBr pellets in the range 4000-400 cm −1 with a Nicolet 5DX spectrometer.Variable-temperature magnetic susceptibility measurements were made using a SQUID magnetometer MPMS XL-7 (Quantum Design) at 0.5 T for 1 and 3.The diamagnetic correction for each sample was determined from Pascal's constants [26].Thermogravimetric analyses were carried out on a Netzsch TG-209 analyzer in atmosphere at a heating rate of 10 ∘ C min −1 .

Preparation of Compounds
): a solution (10 mL) of L (0.095 g, 0.5 mmol) in methanol (10 mL) was added dropwise to an aqueous solution (5 mL) of MnCl 2 ⋅4H 2 O (0.100 g, 0.5 mmol), under stirring to form a mixture.A solution (5 mL) of sodium dicyanamide (0.089 g, 1.0 mmol) in water (5 mL) was added to the mixture slowly.After 30 min, the clear solution was filtered and the filtrate was evaporated under room temperature.After a few days, yellow single crystals suitable for X-ray analysis were obtained.Yield: ca.54%.Anal

X-Ray Crystallography.
Crystallographic data for all the complexes were collected at 293 K on a Bruker Smart Apex CCD diffractometer with graphite-monochromated Mo-K radiation ( = 0.71073 Å).The frames were integrated with the Bruker SAINT + software package, and the data were corrected for absorption using the program SADABS [27].All structures were solved by direct methods using the SHELXS program of the SHELXTL package and refined by full-matrix least-squares methods with SHELXL [28,29].All nonhydrogen atoms were fined with anisotropic thermal parameters on  2 .The organic hydrogen atoms were generated geometrically (C-H 0.96 Å); the aqueous hydrogen atoms were located from different maps and refined with isotropic temperature factors.All crystallographic data and structural refinement details for the complexes are summarized in Table 1, selected bond distances and bond angles are listed in Table 2, and the hydrogen bonds are given in Table 3.

Result and Discussion
3.1.Syntheses and Characterization.Solution assemblies of dca and L with various metal salts led to complexes 1-3 at room temperature which were shown in Scheme 1. Complexes 1 and 2 have similar chains at ligand/dca/M ratios of 1 : 2 : 1, while 3 is synthesized at a 2 : 2 : 1 coordinative layer.In the specific self-assembled processes, the products are independent of the ligand-to-metal ratios (the results were confirmed by X-ray diffraction).In the three compounds, the protons of hydroxyl groups transfer to pyridyl nitrogen atoms to form pyridinium-4-olate, and similar cases have been reported before [30,31], which leads to rich hydrogen bonds.In compounds 1 and 2, the metal centers coordinate to oxygen atoms, while in 3, the L molecules behave as free molecules.However, L molecules act as both hydrogen bond donor and hydrogen bond acceptor in the three compounds.
The IR spectra of 1-3 are shown in experimental section, and the absorption peaks in 1400-1600 cm −1 can be ascribed to the vibrations of pyridyl groups of L [32].The characteristic peaks of dca are quite similar, and they all show strong absorptions in 2300-2100 cm −1 region, corresponding to the ]  + ]  (C ≡ N), ]  (C ≡ N), and ]  (C ≡ N) modes of the dca ligand, which shift to higher frequencies compared with those of free dca in its sodium salts (2286 cm −1 , 2232 cm −1 , and 2129 cm −1 ) and reveal the coordination of the ligand [33].Bands of the ]  (C−N) (1400-1300 cm −1 ) and ]  (C−N) (950-900 cm −1 ) vibration were also displayed in the spectra of 1-3.

The Crystal Structures of Compounds 1-3.
Single-crystal X-ray analysis shows that 1 and 2 are isostructural onedimensional linear complexes, so only 1 is described here.The The progress of syntheses.
metal coordination sphere with the atom-labeling scheme of 1 is given in Figure 1(a).Each unit consists of the neutral [Mn 0.5 (dca)(L)] and one water molecule.The octahedral manganese atom lies on a symmetric centre and is coordinated to four  1,5 -dca anions via nitrile nitrogen atoms and two trans-L ligands via the oxygen atoms.The Mn-N(nitrile) bond distances of 2.212(2) and 2.221(2) Å are slightly longer than that involving the L, of 2.210(2) Å, but all the values are comparable to those found in the pyridine derivatives [34].

(b)).
It is different from similar pyridine analogues that the onedimensional chains are cross-linked through - stacking interactions [17], and the hydrogen bonding interactions might make the distance of pyridyl rings slightly larger.
The existence of water molecules makes the hydrogenbonding interactions more complicated.These water molecules locate in the place of interchain, which is illustrated in Figure 1 The Co⋅ ⋅ ⋅ Co distance within the same layer is 7.894(1) Å, which is shorter than similar two-dimensional layer [36,37].It should be noted that only small number of compounds containing two-dimensional networks without bridging coligands have been reported [5][6][7][8].
Different to 1 and 2, the L ligands are used as free molecules and located in interlayer, which interact with the Thus the H-bonding acts here as a tool for controlling the whole coordinative architecture.The dca anion coordinates to metal centers in a  1,5 mode through two nitrile nitrogen atoms, the cases of amide nitrogen atom bind to metal centre are very few in dca chemistry [8,38].However, the amide nitrogen atom forms strong hydrogen bonds with other hydrogen donors.The hydrogen bonding interactions make the coordination polymer to higher dimensionality, which reveals the coordinative interaction of coordination and supramolecular interaction. 1 and 3. Solid state dc magnetic susceptibility studies were carried out on samples 1 and 3 in a field of 0.5 T. Plots of   versus  and    versus  plots are shown in Figure 3.For 1, taking into consideration the one-dimensional structure, magnetic data are taken for a Mn(II) ion.The    value at room temperature is 3.86 cm 3 mol −1 K, which is lower than a spin only value of 4.38 cm 3 mol −1 K, expected for an uncoupled high-spin  = 5/2 ions ( = 2.0).This value is almost constant up to 100 K, decreasing sharply to 2 K, owing to an increase of antiferromagnetic correlations.When the temperature is reduced, the   value increases as expected.This is due to the fact that there are more and more spins that are correlated in the lattice, showing that a peak maximum at ca. 3.5 K for 1D ordering occurs.This feature is indicative of weak antiferromagnetic coupling among Mn(II) ions.In order to estimate the intrachain exchange interaction between isotropic Mn II centers,   was also fit to the Heisenberg infinite chain model derived by the spin Hamiltonian [39],  = −2 Ŝ Ŝ+1 ,

Magnetic Properties of Compounds
where  is Avogadro number,   is the Boltzmann constant, and  is intrachain coupling constant.The best-fit parameters of the experimental data to theoretical model are  = −0.29 cm −1 ,  = 1.98, and  = 6.0 × 10 −6 ( = [∑ ( obs −  calc ) 2 /∑  obs 2 ] 1/2 ). and  values are typical for this type of Mn-dca complexes [40].The value of  is very small, indicating that the dicyanamide has a poor ability in mediating antiferromagnetic interactions when  1,5 bridging mode (metal-metal separation of ca.7.0 Å) is adopted.For 3, the    value is 2.86 cm 3 mol −1 K, which is significantly larger than that expected for an isolated spinonly Co II (   = 1.87 cm 3 mol −1 K for a  = 3/2 ion,  = 2.0), indicating that obvious orbital contribution is involved, typical of the 4 1 ground term for Co(II) in an octahedral field [41].No maximum in the corresponding susceptibilities is down to 2 K.The    value is in the range of typical range of a Co II of 2.75-3.40cm 3 mol −1 K [42].This temperaturedependent behavior is largely contributed to the combination of spin-orbit coupling and low symmetry ligand field effects that causes splitting of the 4 1 levels [43].The lack of an appropriate magnetic model to describe the complicated situation has precluded quantitative magnetic analyses for polynuclear or polymeric Co(II) complexes [44,45].

Thermogravimetric Analysis.
Thermogravimetric experiments for complexes 1-3 were performed to explore their thermal stabilities (Figure 4).The samples of 1-3 were heated to 425 ∘ C under atmosphere.With regard to 1, weight loss of 8.2% appeared between room temperature and 97.3 ∘ C corresponding to the loss of two crystallized water molecules (calcd.8.7%).The framework kept stable until ca.326 ∘ C for the appearance of a long platform.Then, the whole network started to decompose quickly.For 2, the TG profile is similar to compound 1.The two water molecules were released below ca.152 ∘ C, as revealed by a total weight loss of 7.5% (calcd.7.6%).Compared to 1, the water molecules were gotten off at higher temperature, which is ascribed to the more strong hydrogen bonding interactions in 2 (O1W⋅ ⋅ ⋅ O1 2.849, 3.100 Å; N1⋅ ⋅ ⋅ O1, 2.789 Å).Then, the skeleton kept stable until ca.300 ∘ C. When further heated, it began to decompose.Complex 3 was thermally stable up to 100 ∘ C, and then it began to dehydrate.The weight loss of 8.1% appeared between 100 ∘ C and 160 ∘ C, corresponding to the loss of two coordinated water molecules (calcd.8.6%) in one unit cell, and when further heated it started to decompose.The coordinate water molecules were released in higher temperature than crystallization water molecules.

Conclusions
The new three-dimensional supramolecular networks have been synthesized by using dca anions together with 4hydroxypyridine as coligands.The results demonstrate that the dca anions link metal ions through  1,5 -bridge, which is most common in dca chemistry.However, the introduction of coligand plays an important role in the crystal packing of polymers.The hydrogen bonding interactions between coligand and polymer extend the polymers into higher dimensionality.Finally, magnetic studies of 1 and 3 display only very weak antiferromagnetic interactions between metal centers through  1,5 -dca anions and the long distance of metal centers.

Figure 1 :
Figure 1: Views of (a) the coordination environment of Mn II atoms, (b) the 2D network formed by the interchain C-H⋅ ⋅ ⋅ N hydrogenbonding interactions, (c) the water molecules in the architecture, and (d) the hydrogen-bonding interactions between water molecules and L in 1.

Figure 2 :
Figure 2: Views of (a) the coordination environment of Co II atoms, (b) the 2D (4, 4) layer, and (c) the hydrogen-bonding interactions between water molecules and free pyridinium-4-olate molecules in 3.

Figure 3 :
Figure 3: (a) Plots of the    versus  and   versus .Solid lines represent the best fit with the parameters given in the text in 1.(b) Plots of the    versus  and   versus  in 3.

Table 1 :
Crystal data and structure refinements for compounds 1-3.