Synthesis, structure, and properties of a new Co(II) diphosphonate based on auxiliary ligand 2,2'-bipyridine

ABSTRACT The example of Co(II)-N-heterocyclic complex based on 1-hydroxyethylidenediphosphonic acid (H5L = CH3C(OH)(PO3H2)2), namely [Co2(H3L)2(2,2′-bipy)2] 1, has been solvothermally isolated using the second ligand 2,2'-bipyridine (2,2'-bipy) and characterized by powder X-ray diffraction (PXRD), elemental analysis, IR, and thermal gravimetric analyses. The single-crystal X-ray diffractions show that complex 1 possesses a 0-D structure built from binuclear unit [Co2(O–P–O)2] by μ2-(O–P–O) bridge. Then, H-bonding and π–π stacking interactions further expand this 0-D structure into a 3-D supramolecular framework. Fluorescent measurements reveal that the maximum emission peak is centered at 421.5 nm, mainly deriving from intraligand π*–π transition state of the second ligand 2,2'-bipy (λem = 419.5 nm, λex = 235 nm). Magnetism data indicate that 1 exhibits ferromagnetic behavior within binuclear Co(II) unit via μ2-(O–P–O) bridge in syn-anti mode.


Introduction
As a new type of functional molecular materials, coordination compounds have not only abundant space topology but also huge potential applications in gas storage, separation, optics, electron, magnetism, chiral separation and catalysis, and other fields. [1][2][3][4] The assembly of the coordination compounds based on phosphonate is currently of significant interest. [5][6][7][8][9][10] Any material properties mainly depend on their structures. So, both the structure of organic ligand and the coordination behavior of metal ion are generally dominant for the self-assembly of phosphonates. In addition, we do not know enough about the synthetical rules and actual compound structures. It means that the rational synthesis of compounds with the target structure is an extremely creative and intellectually challenging task. In our work, first, 1-hydroxyethylidenediphosphonic acid (H 5 L) with a flexible hydroxyl group is often used as building block in the construction of the phosphonates. The additional -OH group attached to the organic tether not only provides a possible hydrophobic/hydrophilic environment but also increases solubility of the resulting metal phosphonate. Importantly, it is more likely to behave as a bis(bidentate) chelating ligand using its four phosphonate oxygen atoms, which would be good for constructing low-dimensional phosphonates. [11,12] Second, many auxiliary ligands with N-heterocycle as one assistant method have been applied widely during the self-assembly of the phosphonates. Polypyridyl ligands have been intensively used as ancillary building blocks in the phosphonate coordination chemistry, [13] due to their interesting electronic, photonic, and magnetic properties, as well as p-stacking ability and directional H-bonding when coordinating to transition metals. Although many chemists devoted to the study of self-assembly of the phosphonates by means of the mixed ligands, [14][15][16][17][18] the low-dimensional phosphonates have not been well developed based on auxiliary N-heterocyclic ligands under the solvothermal condition. So, 2,2'-bipy has been selected as the auxiliary ligand in our work. Finally, we focused our attention on Co(II) ion because of its unique high-spin d 7 electron configuration and larger spin-orbital coupling effect for a octahedral Co(II) complex. A great many compounds of Co (II)-H 5 L have been resolved, but a few Co(II)-diphosphonate compounds with 2,2'-bipy have been structurally determined. [19] By introducing the auxiliary ligand 2,2'-bipy, a new diphosphonate [Co 2 (H 3 L) 2 (2,2 0 -bipy) 2 ] 1 is successfully obtained. In this article, its crystal structure, fluorescent, and magnetic properties are studied.

Materials and general methods
All reagents were purchased from commercial sources and used without further purification. Element analyses were performed on a Perkin-Elmer 2400 LS elemental analyzer. IR spectra were recorded from 4000 to 400 cm ¡1 with a Nicolet AVATAR360 instrument. The thermal gravimetric analyses (TG-DSC) under N 2 were carried out with a NETZSCH STA 449 F3 thermal analyzer with a heating rate of 10 K min ¡1 . Powder X-ray diffraction (PXRD) patterns were performed on an ARL X'TRA diffractometer using Cu-Ka radiation. Emission spectra in the solid state at room temperature were taken on a Perkin-Elmer LS-55 fluorescence spectrophotometer. Magnetic susceptibility data were collected on 0.0221 g of sample using a Quantum Design MPMS-7 SQUID magnetometer.

Crystallography
Intensity data were collected on a Bruker SMART CCD diffractometer equipped with a graphite-monochromated Mo Ka radiation (λ D 0.71073 A ) at 296 K for 1 using the v-2u scan technique. The structure was solved by direct methods and refined by full-matrix least-squares fitting on F 2 by SHELXL-97. A total of 11086 for 1 reflections were collected, of which 2883 (R int D 0.0473) were unique. All non-hydrogen atoms were located from the initial solution and refined with anisotropic thermal parameters. The position of hydrogen atoms was either located by difference Fourier maps or calculated geometrically, and their contributions in structural factor calculations were included. Crystallographic data and structural refinements are summarized in Table 1

Results and discussion
Description of the structure Single-crystal X-ray diffraction anlaysis reveals that the asymmetric unit of 1 contains a distinct Co(II) atom, a H 3 L 2¡ anion and a 2,2 0 -bipy ligand ( Figure 1a). As shown in Figure 1a, the Co(II) ion is octahedrally coordinated by four oxygen atoms (O1, O2, O3, and O4) from two different H 3 L 2¡ anions, and two nitrogen atoms (N1, N2) from a 2,2 0 -bipy ligand. The Co(II)¡O/N distances fall into the range of 1.980(4)-2.240(4) A /2.082(6)-2.091 (6) A . These values are in agreement with those reported for other Co(II)-2,2 0 -bipy phosphonate compounds. [19] Each H 3 L 2¡ anion in compound 1 serves as a tetradentate chelating-bridging ligand in m 2 :(h 1 h 0 h 0 )(h 1 h 1 h 0 ) fashion, in which it chelates a Co(II) ion with two of its six diphosphonate oxygens and a hydroxyl oxygen and bridges another Co(II) ion with one of its six diphosphonate oxygens. This bonding mode is different from the reported article, [19] in which each hedpH 3 ¡ group in compound serves as a bis-chelating ligand and connects one Co(II) ion in m 1 :    86.53 (7) Symmetry transformations used to generate equivalent atoms: #1: ¡x, ¡y C 1, ¡z C 1. , 165.7 ), to result in a 3-D supramolecular network. These hydrogen-bonding interactions play an important role in controlling the packing of the title compound. In this work, the compound 1 acts as a 3-D supramolecular role based on binuclear unit [Co 2 (OPO) 2 ]; however, the reported one [19] displays a 2-D supramolecular network based on mononuclear unit [Co(2,2'-bipy)(hedpH 3 ) 2 ] by H-bonding and p-p stacking interactions.

Spectral studies
The PXRD pattern of 1 indicates that the as-synthesized product is a new material, and the pattern is entirely consistent with Symmetry transformations used to generate equivalent atoms: #1: ¡x, ¡y C 1, ¡z C 1; #2: x, ¡y C 3/2, z C 1/2; #3: ¡x, ¡y C 1, ¡z C 2; #4: ¡xC1, ¡y C 1, ¡z C 2. the simulated one from the single-crystal X-ray diffraction (see Supplement Figure A2). The fluorescent spectra of compound 1 and the second ligand 2,2'-bipy are shown in Figure 2. The free H 5 L ligand does not emit fluorescence at room temperature in the visible region. However, the second ligand 2,2'-bipy shows a broad peak at 375.5 nm and a sharp peak at 419.5 nm (λ ex D 235 nm). The fluorescent emissions of 2,2'-bipy have a few minor variations when it coordinates with Co(II) ion, including slight redshifts of the maximum peak band (419.5 nm!421.5 nm) and weakening of fluorescent intensity at 375.5 nm for 1 (λ ex D 235 nm). The emission bands of 1 are mainly attributed to intraligand p Ã -p transition states of 2,2'-bipy. With the increase in molecular rigidity, the energy loss of the non-radiative transition was decreased, and the energy gap from the excited state to the ground state was reduced. Thus, the reason of the fluorescent intensity increase of the maximum peak at 421.5 nm may be that polycyclic conjugated structure not only effectively increases the rigidity but also reduces the loss of energy via radiation-less decay of the intraligand emission excited state.

Thermal characteristics
TG-DSC measurement was conducted to examine the stability of compound 1 in the temperature range of 25-800 C. The combined TG-DSC analyses for 1 are shown in Figure 3. TGA curve shows three main weight losses. The first is from 310 C to 345 C, attributing to the departure of hydroxyl groups [P/C-OH] with the loss of 12.14% (calc. 12.17%), coming with a weak endothermic peak centered at 338 C. The last two steps are mainly the losses of organic moieties of diphosphonate ligand and 2,2'-bipy ligand with a weak endothermic peak centered at 373 C and two exothermic peaks centered at 481 C and 564 C. The two-step losses are in the range of 345-800 C. The total weight loss of 46.54% is slightly less than the calculated value (48.24%), which means the decomposing process was not complete at 800 C. The final thermal decomposition residue is mainly Co(PO 3 ) 2 based on the PXRD patterns.

Magnetism properties
The temperature dependence of x m of 1 has been measured in 2 KOe (Figure 4) from 2.7 to 300 K. At room temperature, the x m T value of 5.38 cm 3 ¢mol ¡1 ¢K is far greater than that expected of 3.74 cm 3 ¢mol ¡1 ¢K for two free Co II ions (g D 2.0, S D 3/2), suggesting the orbital contribution of the octahedral Co(II) ions. This is a ubiquitous phenomenon for the high-spin Co(II) compounds. [20] The value of x m T increases gradually with the decreasing temperature until the maximum of 5.79 cm 3 mol ¡1 K at 72 K and then starts to decrease quickly until it reaches a value of 4.26 cm 3 mol ¡1 K at 2.7 K. In the high temperature region between 72 and 300 K, the product of x m T suggests the typical ferromagnetic behavior. Fit of the magnetic data was done above 70 K using the Curie-Weiss law x m D C m /(T-u): C D 5.27 emu K mol ¡1 and u D 7.67 K. The positive u further confirmed ferromagnetic interactions between Co(II) ions.
The structure of 1 is built from the binuclear unit,, which is m 2 -bridged by  groups. So, the magnetic behavior of 1 depends on the superexchange pathway of double O-P-O bridges in syn-anti mode within a binuclear unit, and the distances between Co(II) cent ers is 4.758 A . _ Zurowska [21] has demonstrated the fact that when a single-or double-carboxylato bridge link with two copper(II) ions is in syn-anti mode, a   weak ferromagnetic coupling behavior will be observed within binuclear copper(II) complexes based on experimental and calculated results. Based on his discussions, the article further investigates the impact of different types of O-P-O bridge on magnetic behavior in binuclear copper(II) system by _ Zurowska [22] in detail. It is because of syn-anti conformation of O-P-O bridges between Co(II) ions that the advent of weak ferromagnetic behavior is just about rational. The magnetic data of 1 were analyzed by the expression (1) [23] for isotropically coupled binuclear S D 3/2 Co(II) ions based on spin Hamiltonian (H D -JŜ 1Ŝ2 ) as follows: where x D 2J/kT. N, g, b, k, and T have their usual meanings; J is the exchange integral. x m is the measured magnetic susceptibility. The best fitted parameters above 70 K are: J D 2.14 cm ¡1 , g D 2.38. As shown in the inset of Figure 4 for 1, the calculated curve matches very well with the magnetic data from room temperature to 70 K. No analytical expression is available in the literature describing the temperature dependence of x m T for binuclear unit of Co II ions with spin-orbit coupling at low temperature. The reason may be that at higher temperatures (greater than 70 K), the ion can be thought of as a nearly isotropic S D 3/2 entity; however, at lower temperatures, the Co II ions serve as anisotropic pseudospins S D 1/2 below 30 K. [24,25]

Conclusion
We have successfully obtained a binuclear Co(II)-N-heterocyclic-diphosphonate under solvothermal condition. Complex 1 displays the 3-D supramolecular structure via H-bonding and p-p stacking interactions. Luminescent studies at room temperature show fluorescent emission bands, which are mainly derived from the ancillary ligands. The correlation of the structure and magnetic property of complex is less focused on lowdimensional Co(II)-N-heterocyclic-diphosphonate. Especially, the multinuclear Co II complexes linked through single or double O-P-O bridges may lead to an unexpected magnetic behavior. In future, we will continue to make efforts to explore the related magnetostructural relationship of low-dimensional multinuclear M(II)-diphosphonates.