Hg(II) Coordination Polymers Based on N,N’-bis(pyridine-4-yl)formamidine

Reactions of N,N’-bis(pyridine-4-yl)formamidine (4-Hpyf) with HgX2 (X = Cl, Br, and I) afforded the formamidinate complex {[Hg(4-pyf)2]·(THF)}n, 1, and the formamidine complexes {[HgX2(4-Hpyf)]·(MeCN)}n (X = Br, 2; I, 3), which have been structurally characterized by X-ray crystallography. Complex 1 is a 2D layer with the {44·62}-sql topology and complexes 2 and 3 are helical chains. While the helical chains of 2 are linked through N–H···Br hydrogen bonds, those of 3 are linked through self-complementary double N–H···N hydrogen bonds, resulting in 2D supramolecular structures. The 4-pyf- ligands of 1 coordinate to the Hg(II) ions through one pyridyl and one adjacent amine nitrogen atoms and the 4-Hpyf ligands of 2 and 3 coordinate to the Hg(II) ions through two pyridyl nitrogen atoms, resulting in new bidentate binding modes. Complexes 1–3 provide a unique opportunity to envisage the effect of the halide anions of the starting Hg(II) salts on folding and unfolding the Hg(II) coordination polymers. Density function theory (DFT) calculation indicates that the emission of 1 is due to intraligand π→π * charge transfer between two different 4-pyf- ligands, whereas those of 2 and 3 can be ascribed to the charge transfer from non-bonding p-type orbitals of the halide anions to π * orbitals of the 4-pyf- ligands (n→π *). The gas sorption properties of the desolvated product of 1 are compared with the Cu analogues to show that the nature of the counteranion and the solvent-accessible volume are important in determining their adsorption capability.


Introduction
Functional coordination polymers have been a research focus in recent years due to their potential applications in separation, ion exchange, catalysis, and adsorption [1][2][3][4][5]. Of these coordination polymers, the 1D and 2D ones that show the simple topological types of coordination arrays are found to be dominating the literature [6,7]. The relative simplicity of the 1D and 2D coordination polymers and their ease of formation through self-assembly facilitate the incorporation of functional properties at the metal centers and the backbone of the organic linkers. Although metal cations are essential common constituents of coordination polymers, the contribution of organic ligands in the design and construction of desired networks is highly appreciable due to their possible changes in flexibility, length, and symmetry [8,9].
As our continuing efforts to investigate the correlation between the binding modes of formamidinate ligand and the structural diversity of novel coordination polymers, we sought to investigate the halide anion effect of metal salts on the construction of formamidinate ligand-based Hg(II) coordination polymers. The synthesis, structures, and luminescence and adsorption properties of {[Hg(4-pyf) 2  which was reacted with Hg(II) salts to afford the first 2D and 3D heteronuclear coordination networks based on quadruple-bonded dimolybdenum units [14]. By one-pot solvothermal reactions of 4aminopyridine and triethylorthoformate with divalent copper salts, several 2D coordination networks of the types anti-{[Cu (4-pyf)]·solvent}n and syn-{[Cu4(4-pyf)4]·2solvent}n (solvent = MeOH and EtOH) that show crystal-to-crystal transformations and photoluminescence changes have also been reported [15]. In these complexes, the anionic 4-pyf-ligands show bidentate binding mode through two inner amine nitrogen atoms, Figure 1b, tetradentate binding mode through all the four nitrogen atoms, Figure 1c, or tridentate binding mode with one dangling pyridyl nitrogen atom, Figure 1d. As our continuing efforts to investigate the correlation between the binding modes of formamidinate ligand and the structural diversity of novel coordination polymers, we sought to investigate the halide anion effect of metal salts on the construction of formamidinate ligand-based Hg(II) coordination polymers. The synthesis, structures, and luminescence and adsorption properties of {[Hg(4-pyf)2]·(THF)}n, 1, {[HgBr2(4-Hpyf)]·(ACN)}n, 2, and {[HgI2(4-Hpyf)]·(ACN)}n, 3, form the subject of this report. New types of bidentate binding modes, Figure 1e and Figure 1f, are found for the anionic and neutral 4-Hpyf ligands. The halide anions show significant effect on the structural diversity, and complexes 1-3 represent a unique example in folding and unfolding the Hg(II) coordination polymers. Moreover, the formation of complex 1, in which the 4-pyf-ligand adopts the binding mode of Figure 1e, provides an opportunity to investigate the effect of the dangling pyridyl nitrogen atoms on the pore structure and gas storage capability.

General Procedures
Elemental analyses were carried out using an Elementar Vario EL cube analyzer (Elementar Analysensysteme GmbH, Hanau, Germany). IR spectra (KBr disk) were recorded on a Jasco FT/IR-460 plus spectrometer (Jasco, Easton, PA, USA). Emission spectra were recorded on a Hitachi F-4500 spectrometer (Hitachi, Tokyo, Japan). Powder X-ray diffraction measurements were measured using

General Procedures
Elemental analyses were carried out using an Elementar Vario EL cube analyzer (Elementar Analysensysteme GmbH, Hanau, Germany). IR spectra (KBr disk) were recorded on a Jasco FT/IR-460 plus spectrometer (Jasco, Easton, PA, USA). Emission spectra were recorded on a Hitachi F-4500 spectrometer (Hitachi, Tokyo, Japan). Powder X-ray diffraction measurements were measured using a PANalytical PW3040/60 X'Pert Pro diffractometer (PANalytical, EA Almelo, Netherlands) or a Bruker D2 PHASER X-ray diffractometer (Bruker Corporation, Karlsruhe, Germany). 1 H NMR spectra were recorded on a Bruker Avance II 400 MHz FT-NMR spectrometer (Bruker Corporation, Rheinstetten, Germany) by using DMSO-d 6 as an internal standard.

Synthesis of 4-Hpyf
Triethyl orthoformate (12 mL, 0.07 mol) was added to a round-bottom flask containing 4-aminopyridine (9.50 g, 0.10 mol) and equipped with a condenser. The mixture was then stirred at 140˝C for 24 h to yield a brown solution and khaki precipitation. The precipitate was filtered, washed with 100 mL of diethylether, and dried under vacuum. Yield: 8.82 g (89%). 1  4-Hpyf (0.40 g, 2.02 mmol) and HgCl 2 (0.27 g, 0.99 mmol) were placed in a flask containing 20 mL of THF. The mixture was then stirred at room temperature for 24 h to produce a colorless solution and a colorless solid. The solution was filtered and diethylether was added to introduce a precipitate. The precipitate was filtered and washed with diethyl ether (2ˆ10 mL) and THF (2ˆ10 mL) and then dried under vacuum to produce a white product. Yield: 0.38 g (51%). Anal. Calcd. for C 30 H 34 HgN 8  4-Hpyf (0.20 g, 1.01 mmol) was placed in a flask containing 20 mL of acetonitrile and then HgBr 2 (0.36 g, 1.00 mmol) or HgI 2 (0.45 g, 0.99 mmol) was added. The mixture was then stirred at room temperature for 24 h to produce a colorless solution and a colorless solid. The solution was filtered and diethylether was added to introduce precipitate. The precipitate was filtered and washed with diethylether (2ˆ10 mL) and THF (2ˆ10 mL), and then dried under vacuum to produce a white product. Yield: 0.44 g (73%) for 2. Anal. Calcd. for C 13

X-ray Crystallography
The diffraction data for 4-Hpyf and complexes 1-3 were collected on a Bruker AXS SMART APEX II CCD diffractometer at 22˝C, which was equipped with a graphite-monochromated MoK α (λ α = 0.71073 Å) radiation. The structure factors were obtained after Lorentz and polarization. An empirical absorption correction based on "multi-scan" was applied to the data [16]. The positions of some of the heavier atoms, including the Hg atom, were located by the direct method or Patterson method of the SHELXS program, and the remaining atoms were found in a series of alternating difference Fourier maps and least-square refinements, while the hydrogen atoms were added by using the HADD command and refined as riding atoms [17]. The refined model for 4-Hpyf only represents one of several possible orientations. Basic information pertaining to crystal parameters and structure refinement is summarized in Table 1. Selected bond distances and angles are listed in Table 2.    2) 148.40 (3) A:´x + 2,´y + 1,´z; B:´x + 2, y´1/2,´z + 1/2; C: x,´y + 3/2, z´1/2 for 1. A:´x, y + 1/2,´z + 3/2 for 2. A:´x + 2, y´1/2,´z + 3/2 for 3.

Computational Methods
The density functional theory (DFT) calculations were performed for complexes 1-3 at the rb3lyp/lanl2dz level [18], which was implemented using the Gaussain 09 software package [19]. Coordinates of these complexes were obtained directly from X-ray crystallography.

Gas Adsorption Measurements
The adsorption isotherms for N 2 , H 2 , and CO 2 were carried out on a Micrometrics ASAP 2020 Series analyzer by using gases of the highest quality at 77 and 293 K in a liquid nitrogen bath and an ice-water bath, respectively. Before the measurement, the sample was degassed (10´3 torr) at 423 K overnight to remove the co-crystallized solvent molecules.

Structure of 4-Hpyf¨0.16THF
The asymmetric unit of 4-Hpyf¨0.16THF contains six 4-Hpyf molecules and one THF molecule. 2˝] to form a ring, with the THF molecule occupying the center. It is noted that the structure of 4-Hpyf without co-crystallization of solvent has been reported, in which the 4-Hpyf molecules are linked by the similar N-H¨¨¨N hydrogen bonds that results in a 1D linear chain [20]. The difference in the supramolecular structures indicates the structure-directing role of the THF solvent molecule.  (6) Å is significantly shorter than the sum of the van der Waals radius of Hg and N atoms, which is 3.65 Å, indicating a strong interaction. Noticeably, the 4-pyfligand coordinates to the Hg(II) ions through one amine and one adjacent pyridyl nitrogen atoms, resulting in a unique bidentate binding mode, Figure 1e. Subsequently, the Hg(II) ions are linked by a series of Hg-N bonds to form a 2D network, Figure 3b, which can be simplified to a 4-connected 2D net {4 4 ·6 2 }-sql topology determined using TOPOS [21]. The area of each of the window is 8.26 × 8.26 Å 2 . Figure 3c shows that the THF solvent molecules are located in the space between the 2D layers, and the solvent-accessible volume calculated by PLATON program [22] is 605.7 Å 3 , which is 39.8 % of the unit cell volume.   (6) Å is significantly shorter than the sum of the van der Waals radius of Hg and N atoms, which is 3.65 Å, indicating a strong interaction. Noticeably, the 4-pyfligand coordinates to the Hg(II) ions through one amine and one adjacent pyridyl nitrogen atoms, resulting in a unique bidentate binding mode, Figure 1e. Subsequently, the Hg(II) ions are linked by a series of Hg-N bonds to form a 2D network, Figure 3b, which can be simplified to a 4-connected 2D net {4 4¨62 }-sql topology determined using TOPOS [21]. The area of each of the window is 8.26ˆ8.26 Å 2 . Figure 3c shows that the THF solvent molecules are located in the space between the 2D layers, and the solvent-accessible volume calculated by PLATON program [22] is 605.7 Å 3 , which is 39.8 % of the unit cell volume.  (6) Å is significantly shorter than the sum of the van der Waals radius of Hg and N atoms, which is 3.65 Å, indicating a strong interaction. Noticeably, the 4-pyfligand coordinates to the Hg(II) ions through one amine and one adjacent pyridyl nitrogen atoms, resulting in a unique bidentate binding mode, Figure 1e. Subsequently, the Hg(II) ions are linked by a series of Hg-N bonds to form a 2D network, Figure 3b, which can be simplified to a 4-connected 2D net {4 4 ·6 2 }-sql topology determined using TOPOS [21]. The area of each of the window is 8.26 × 8.26 Å 2 . Figure 3c shows that the THF solvent molecules are located in the space between the 2D layers, and the solvent-accessible volume calculated by PLATON program [22] is 605.7 Å 3 , which is 39.8 % of the unit cell volume.     The thermogravimetric analyses (TGA) were performed for (4-Hpyf)6·THF and 1 to examine their thermal stabilities, which were carried out in nitrogen atmosphere from 30 to 900 °C, Figure S5. The TGA curve of 4-Hpyf·1/6THF shows the gradual weight loss of THF of solvent molecules (calculated 5.7%; observed 5.5%) in 140-200 °C. The weight loss in 230-350 °C corresponds to the decomposition of 4-pyfligand (calculated 94.3%; observed 94.1%). The TGA curve of 1 shows the gradual weight loss of THF of solvent molecules (calculated 19.5%; observed 19.1%) in 140-215 °C, and the weight loss of 53.6% in 270-420 °C corresponds to the decomposition of 4-pyfligand (calculated 53.4%). The higher decomposition temperature range for the 4-pyfligand in 1 than for the 4-Hpyf ligand in 4-Hpyf·1/6THF indicates the Hg-N bond is much stronger than the N-H···N hydrogen bond.

Luminescent Properties
Luminescent metal complexes are able to enhance, shift, and quench the luminescent emission of organic ligands through metal coordination. The emission and excitation spectra of 1-3 were measured in the solid-state at room temperature, Figure 5. The emission spectrum of 1 exhibits a broad band at 448 nm upon excitation at 381 nm, and those of 2 and 3 show emissions at 432 and 445 nm upon excitation at 344 and 348 nm, respectively. The difference in the emission wavelengths between 2 and 3 can be ascribed to the different electronegativity of the halide anions.
The results of the density functional theory (DFT) calculations show that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy gaps of 1-3 are 3.62, 3.84, and 3.80 eV, respectively, which match quite well with the excitation spectra. The  The thermogravimetric analyses (TGA) were performed for (4-Hpyf) 6¨T HF and 1 to examine their thermal stabilities, which were carried out in nitrogen atmosphere from 30 to 900˝C, Figure S5. The TGA curve of 4-Hpyf¨1/6THF shows the gradual weight loss of THF of solvent molecules (calculated 5.7%; observed 5.5%) in 140-200˝C. The weight loss in 230-350˝C corresponds to the decomposition of 4-pyfligand (calculated 94.3%; observed 94.1%). The TGA curve of 1 shows the gradual weight loss of THF of solvent molecules (calculated 19.5%; observed 19.1%) in 140-215˝C, and the weight loss of 53.6% in 270-420˝C corresponds to the decomposition of 4-pyfligand (calculated 53.4%). The higher decomposition temperature range for the 4-pyfligand in 1 than for the 4-Hpyf ligand in 4-Hpyf¨1/6THF indicates the Hg-N bond is much stronger than the N-H¨¨¨N hydrogen bond.

Luminescent Properties
Luminescent metal complexes are able to enhance, shift, and quench the luminescent emission of organic ligands through metal coordination. The emission and excitation spectra of 1-3 were measured in the solid-state at room temperature, Figure 5. The emission spectrum of 1 exhibits a broad band at 448 nm upon excitation at 381 nm, and those of 2 and 3 show emissions at 432 and 445 nm upon excitation at 344 and 348 nm, respectively. The difference in the emission wavelengths between 2 and 3 can be ascribed to the different electronegativity of the halide anions.
The results of the density functional theory (DFT) calculations show that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy gaps of 1-3 are 3.62, 3.84, and 3.80 eV, respectively, which match quite well with the excitation spectra. The results show that the HOMO and LUMO of 1 are derived from the π and π * orbitals of two different 4-pyfligands, whereas those of 2 and 3 are primarily composed of non-bonding p-type orbitals from halide anions and the π * orbitals of the 4-pyfligands, Figure 6 and Table 3. The emission of 1 is thus due to intraligand πÑπ * charge transfer between two different 4-pyfligands, and those of 2 and 3 can be ascribed to the charge transfer from non-bonding p-type orbitals of the halide anions to π * orbitals of the 4-pyfligands (nÑπ *). results show that the HOMO and LUMO of 1 are derived from the π and π * orbitals of two different 4-pyfligands, whereas those of 2 and 3 are primarily composed of non-bonding p-type orbitals from halide anions and the π * orbitals of the 4-pyfligands, Figure 6 and Table 3. The emission of 1 is thus due to intraligand π→π * charge transfer between two different 4-pyfligands, and those of 2 and 3 can be ascribed to the charge transfer from non-bonding p-type orbitals of the halide anions to π * orbitals of the 4-pyfligands (n→π *).  results show that the HOMO and LUMO of 1 are derived from the π and π * orbitals of two different 4-pyfligands, whereas those of 2 and 3 are primarily composed of non-bonding p-type orbitals from halide anions and the π * orbitals of the 4-pyfligands, Figure 6 and Table 3. The emission of 1 is thus due to intraligand π→π * charge transfer between two different 4-pyfligands, and those of 2 and 3 can be ascribed to the charge transfer from non-bonding p-type orbitals of the halide anions to π * orbitals of the 4-pyfligands (n→π *).
The permanent macropore features are established by N 2 adsorption isotherms at 77 K, which show the H3 type hysteresis loop for 1' and 4', and typical type-II sorption behavior for 5a' and 5b' (Figure 7), and the maximum adsorption volumes were 64.14, 91.19, 22.31, and 70.68 cm 3 /g at P/P 0 = 0.99, respectively. The Langmuir surface areas are determined by the linear fitting of N 2 adsorption branch data and the result is 77, 154, 99, and 61 m 2 /g (BET surface areas are 45, 146, 94, and 59 m 2 /g) at 77 K. The amounts of hydrogen adsorption increase gradually with increasing hydrogen pressure and reveal maximum sorption amounts of 0.27, 0.25, 2.14, and 0.90 mmol/g for 1', 4', 5a', and 5b' at 760 mmHg, respectively ( Figure 8). The CO 2 uptakes of 1', 4', 5a', and 5b' at 273 and 760 mmHg are 0.71, 0.44, 1.15, and 0.74 mmol/g, respectively, which are lower than N 2 adsorption (Figure 9a). The CO 2 uptake capacity at 298 K is shown in Figure 9b. The different uptake amounts in the gas adsorption for 5a' and 5b' indicate that the nature of the BF 4 and ClO 4 anions are important in determining the adsorption capability. The Clausius-Clapeyron equation implemented in the software of the Micrometrics ASAP 2020 sorptometer was employed to calculate the isosteric heats of CO 2 adsorption (Q st ) ( Figure 10). The Q st against loading amount was found to be 7.75 KJ/mol for 1' at 0.0013 mmol/g, which displays the weak adsorption for CO 2 between the layers of 1'. 2D coordination polymers are often less stable with temperature and pressure variation than 3D ones during adsorption investigation because 2D coordination polymers usually present the supramolecular structure linked by hydrogen bonds. Presumably, the porosity of 1' may have changed at higher gas pressure. The poorer data for 1' may indicate the overloading of CO 2 in the range of 600-760 mmHg, where saturation is reached. The Q st of 4', 5a', and 5b' exhibits maximum values of 39.31, 32.41, and 30.50 KJ/mol at near zero CO 2 coverage and decreases with increasing CO 2 loading amounts. Note that these are higher than the enthalpy of liquefaction of CO 2 (17 kJ/mol) but lower than the values observed for zeolites such as NaX and Na-ZSM-5 at zero coverage (ca. 50 kJ/mol) [23][24][25], indicating relatively strong interactions between CO 2 and the pore surfaces of 4', 5a', and 5b'. The adsorption enthalpy leveled off rapidly when the loading amounts were over 0.039 mmol/g for 1' and 0.051 mmol/g for 4'. When the CO 2 coverage increases, the Q st remains steadily above 4.35 KJ/mol for 1' and 8.46 KJ/mol for 4', respectively.
To compare the Q st values for the 2D coordination networks of 1' and 4', we propose that the outward dangling pyridyl nitrogen atoms may present the weak interactions with CO 2 at zero coverage. From the Q st values, it is found that 4', which has a smaller solvent-accessible volume, presents the larger Q st value in CO 2 adsorption. By DFT calculations, it can be shown that the Mulliken charges of the outward-dangling pyridyl nitrogen atoms ( Figures S9 and S10, Supplementary Material) are 0.028 and 0.0033 for 1' and -0.135 and -0.169 for 4', respectively. The smaller Mulliken charges of 1' probably cannot invoke the interactions for any gas molecules and displays the physical adsorption for CO 2 . The larger CO 2 uptake amounts for 1' as compared with 4' is thus probably due to the larger solvent-accessible volume with a more attractive surface to the CO 2 molecules.    coverage. From the Qst values, it is found that 4', which has a smaller solvent-accessible volume, presents the larger Qst value in CO2 adsorption. By DFT calculations, it can be shown that the Mulliken charges of the outward-dangling pyridyl nitrogen atoms ( Figures S9 and S10, Supplementary Material) are 0.028 and 0.0033 for 1' and -0.135 and -0.169 for 4', respectively. The smaller Mulliken charges of 1' probably cannot invoke the interactions for any gas molecules and displays the physical adsorption for CO2. The larger CO2 uptake amounts for 1' as compared with 4' is thus probably due to the larger solvent-accessible volume with a more attractive surface to the CO2 molecules.   coverage. From the Qst values, it is found that 4', which has a smaller solvent-accessible volume, presents the larger Qst value in CO2 adsorption. By DFT calculations, it can be shown that the Mulliken charges of the outward-dangling pyridyl nitrogen atoms ( Figures S9 and S10, Supplementary Material) are 0.028 and 0.0033 for 1' and -0.135 and -0.169 for 4', respectively. The smaller Mulliken charges of 1' probably cannot invoke the interactions for any gas molecules and displays the physical adsorption for CO2. The larger CO2 uptake amounts for 1' as compared with 4' is thus probably due to the larger solvent-accessible volume with a more attractive surface to the CO2 molecules.

Conclusions
The reactions of 4-Hpyf with Hg(II) halide salts afforded three coordination polymers showing 2D net and 1D chains, in which the 4-pyf − anions and the neutral 4-Hpyf ligands adopt the new bidentate binding modes. The formation of 1-3 indicates that the Clanion is more readily removed by the amine hydrogen atom of the 4-Hpyf ligand than the Brand Ianions. The halide anions also show significant effect on the supramolecular structures of 2 and 3. While the helical chains of 2 are linked through N-H···Br hydrogen bonds, those of 3 are linked through self-complementary double N-H···N hydrogen bonds. Structural comparisons of 1-3 show that deliberate choice of the starting Hg(II) halide salts is important in controlling the folding and unfolding of the Hg(II) coordination polymers in terms of topology. Density function theory (DFT) calculation indicates that the emission of 1 is due to intraligand π→π * charge transfer, and those of 2 and 3 are due to the n→π * charge transfer. It is also shown that the nature of the BF4 − and ClO4 − anions in 5a and 5b and the larger solvent-accessible volume of 1 are important in determining the adsorption capability. However, the effect of the electron densities on the dangling nitrogen atoms cannot be overlooked.