Inorganica Chimica Acta 453 (2016) 263–267 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica Research paper Synthesis, characterization and molecular structure of a zinc(II) formate-2,20 -bipyridine mono-dimensional coordination polymer. Comparison with other 2,2-bipyridine coordination compounds Jean Ngoune a,⇑, Jean Jacques Anguile b, Justin Nenwa c, Golngar Djimassinga d, Claudio Pettinari e,⇑, Eleuterio Álvarez f, Luciano Pandolfo g a Department of Chemistry, The University of Dschang, PO Box 67, Dschang, Cameroon Department of Chemistry, The University of Sciences & Techniques of Masuku, PO Box 238, Franceville, Gabon c Department of Inorganic Chemistry, The University of Yaoundé 1, PO Box 812, Yaoundé, Cameroon d Department of Chemistry, Mongo Polytechnique University Institute (IUPM), Chad e Scuola di Farmacia, Università di Camerino, via S. Agostino 1, 62032 Camerino (MC), Italy f Instituto de Investigaciones Quimicas (IIQ), Consejo Superior de Investigaciones Cientificas (CSIC), Universidad de Sevilla, Avda. Américo Vespucio 49, Isla de La Cartuja, 41092 Sevilla, Spain g Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, 35131 Padova, Italy b a r t i c l e i n f o Article history: Received 26 June 2016 Received in revised form 3 August 2016 Accepted 4 August 2016 Available online 4 August 2016 Keywords: Zinc(II) coordination polymer Formate complexes 2,20 -Bipyridine X-ray single crystal structure Thermal analysis a b s t r a c t Hydrated zinc(II) formate reacts with 2,20 -bipyridine (2,20 -bpy) to form a novel zig-zag mono-dimensional coordination polymer (1D CP) [{Zn(l-OCHO)(OCHO)(2,20 -bpy)(H2O)}H2O]n, 1, which was structurally characterized by a single-crystal X-ray diffraction study. Title compound crystallizes in the orthorhombic system, Pbca space group, [a = 6.7651(8), b = 17.4876(19), c = 23.180(3)]. Each zinc ion displays a distorted octahedral geometry due to the coordination of a monotopic formate, one molecule of water and the two nitrogen atoms of 2,20 -bpy, while the octahedral scheme is completed by the coordination of a ditopic formate connecting two zinc ions according to a syn-anti configuration, thus generating the polymeric structure. Moreover, a crystallization water molecule acts as a supramolecular bridge connecting formate ions belonging to neighboring, symmetry equivalent, 1D CPs through quite strong H-bonds, thus leading to an overall waved 2D open framework. Thermal analyses show significant weight losses corresponding to the elimination of water molecules followed by the decomposition of both formate groups and 2,20 -bipyridine. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction The design and synthesis of coordination polymers (CPs) has undergone explosive growth because of their intriguing structures and their possible applications as functional materials in various fields such as gas sorption, storage, separation and purification, catalysis, ion exchange or sensor devices [1–15]. A large variety of metal ions or oligonuclear metal fragments have been employed as nodes to obtain CPs, through the (self)-assembly with polytopic linkers. Particularly, Zn ions, having versatile coordination properties [16–19], have been employed to obtain a large number of complexes showing interesting characteristics, as photoluminescence [20–22] and have generated a large number of interesting CPs, ⇑ Corresponding authors. E-mail addresses: [email protected], [email protected] (J. Ngoune), [email protected] (C. Pettinari). http://dx.doi.org/10.1016/j.ica.2016.08.005 0020-1693/Ó 2016 Elsevier B.V. All rights reserved. being MOF-5, having the Zn4(l4-O) oxocluster as node, the most famous example [1,2,5,6,20]. Beside suitable dinitrogen ligands (bipyridine, imidazolate, pyrazolate, triazolate, etc.) the most employed linkers to generate CPs are bicarboxylates, but some of us have evidenced that also monocarboxylates ions, endowed with a large variety of bridging coordination modes may act as good linkers, connecting two or three metal ions and generating 1- and 2-D CPs [17,23–31]. Very recently a chiral flexible MOF [Zn3(bpydc)2(HCOO)2] H2ODMF containing the N,O-donor 2,20 -bipyridyl-5,50 -dicarboxylate (bpydc) has been reported which shows unexpected and uncommon stepwise N2 and CO2 adsorption [32]. Here we report the synthesis, the thermal behavior and the single-crystal (SC) XRD molecular structure determination of a novel 1D coordination polymer, based on the mononuclear Secondary Building Unit (SBU) [Zn(l-OCHO)(OCHO)(2,20 -bpy)(H2O)]H2O, obtained by exploiting 264 J. Ngoune et al. / Inorganica Chimica Acta 453 (2016) 263–267 the bridging ability of the formate ion and the chelating behavior of 2,20 -bipyridine (2,20 -bpy). 2. Experimental section 2.1. Materials and methods All the reactions and manipulations were carried out in air by using laboratory grade toluene as solvent. All the commercially available reagents were used as purchased from Aldrich and Alfa Aesar. The IR spectrum was recorded with a Perkin-Elmer Spectrum 100 FT-IR spectrometer in the range 4000–650 cm1. The 1 H NMR spectrum was recorded on a Mercury Plus Varian 400 NMR spectrometer (400 MHz) using Me4Si as external standard. Elemental analysis (C, H, N) were carried out on a LECO TruSpec CHN instrument. Thermal analyses were performed with a Perkin-Elmer Simultaneous Thermal Analyzer (STA) 6000 instrument. 2.2. Crystallographic data collection and structure determination Compound 1 crystallizes as nicely shaped colorless crystals, one of which, coated with dry perfluoropolyether, was glued on the tip of a glass fiber and, in a cold nitrogen stream [T = 173(2) K] mounted on top of a goniometer head. Data collection was performed on Bruker-Nonius X8APEX-II CCD diffractometer, using monochromatic radiation k(Mo, Ka) = 0.71073 Å. A total of 33,674 intensity data were collected in the x-scan mode for one hemisphere in the 1.76° < h < 26.40° range. The data collected were reduced using the program SAINT [33] and corrected for Lorentzpolarization and for absorption effects by multi-scan method using the program SADABS [34]. The structure was solved by direct methods (SIR-2004) [35] and refined against all F2 data by fullmatrix least-squares techniques (SHELXTL-6.12) [36]. All the non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms attached to carbon atoms were included from calculated positions and refined riding on their respective carbon atoms with isotropic displacement parameters, while the water hydrogen atoms were located on a Fourier difference map and refined isotropically with a shelxl DFIX restraint of 0.90 Å for the OAH distances. X-ray crystallographic data collection and structure refinements are reported in the Table 1 while the supplementary crystallographic data have been deposited at the Cambridge Crystallographic Data Centre with deposition number CCDC 970035. Moreover, relevant selected bond lengths and bond angles are reported in Table S1 (Supplementary Information) while the atomic coordinates and isotropic displacement parameters are listed in Table S2 and relevant H-bonds parameters are reported in Table S3. Molecular graphics were generated by using Mercury 3.8 program [37,38]. Color codes for all molecular graphics: light gray (Zn), blue (N), red (O), gray (C), white (H). 2.3. Synthesis of [{Zn(l-OCHO)(OCHO)(2,20 -bpy)(H2O)}H2O]n, 1 Dihydrate zinc formate (0.384 g, 2.0 mmol) was suspended in 15 mL of toluene and 2,20 -bpy (0.32 g, 2.1 mmol) was added under stirring. The mixture was vigorously stirred at room temperature for 24 h obtaining a colorless solid that was filtered, washed with 5 mL of toluene and dried in the air at room temperature. Colorless crystals suitable for a SCXRD determination were obtained by slow evaporation of a dimethylformamide solution of the dried precipitate. 1. Yield: 0.555 g, 80%. Elem. Anal.: Calcd for C12H14N2O6Zn: C, 41.46; H, 4.06; N, 8.06, Found: C, 41.78; H, 3.79 N, 8.23. IR absorption: 3274(br), 3162(br), 2843(w), 1605(s), 1564(s), 1475(w), 1439 (s), 1397(w), 1373(m), 1340(br), 1323(br), 1248(w), 1156(w), 1062 Table 1 Crystal data and structure refinement for 1. Empirical formula Formula weight Temperature Wavelength Crystal system Space group Unit cell dimensions Volume Z Density (calculated) Absorption coefficient F(0 0 0) Crystal size Theta range for data collection Index ranges Reflections collected Independent reflections Completeness to theta = 26.40° Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Largest diff. peak and hole C12H14N2O6Zn 347.62 173(2) K 0.71073 Å Orthorhombic Pbca a = 6.7651(8) Å a = 90° b = 17.4876(19) Å b = 90° c = 23.180(3) Å c = 90° 2742.3(6) Å3 8 1.684 Mg/m3 1.821 mm1 1424 0.34 0.14 0.10 mm3 1.76–26.40° 5 6 h 6 8, 21 6 k 6 12, 28 6 l 6 23 33,674 2810 [R(int) = 0.0886] 99.6% Semi-empirical from equivalents 0.8389 and 0.5765 Full-matrix least-squares on F2 2810/4/202 1.086 R1 = 0.0394, wR2 = 0.0801 R1 = 0.0643, wR2 = 0.0916 0.801 and 0.444 e.Å3 R1 = R||Fo| |Fc||/R|Fo|, wR2 = [R(w(F2o F2c )2)/R(w(F2o)2)]1/2. (vw), 1021(w), 766(br), 735(br). 1H NMR (400 MHz, Acetone d6): d: 7.51 [m, 2H, 2,20 -bpy], 7.53 [m, 2H, 2,20 -bpy], 7.81 [m, 2H, 2,20 -bpy], 8.21 [d, 2H, 2,20 -bpy], and 9.06 [s, 1H, HCOO]. 3. Results and discussion The reaction of dihydrate zinc formate with 2,20 -bpy in toluene at room temperature leads to [{Zn(l-OCHO)(OCHO)(2,20 -bpy) (H2O)}H2O]n, 1, in high yield. Compound 1 is an air-stable colorless solid, slightly soluble in some common organic solvents such as acetone, chloroform, dimethylsulfoxide, dimethylformamide. The FT-IR spectrum shows medium intensity broad bands, centered at 3274 and 3162 cm1 due to coordinated and crystallization water, while strong signals at 1605 and 1564 cm1 are assigned to two different mCO stretchings [39]. 1H NMR spectrum exhibits three multiplets at d 7.51, 7.53, 7.81 and a doublet at 8.21 ppm assigned to coordinated 2,20 -bpy, while one singlet at d 9.06 ppm is assigned to the formate H. The results of the thermal analyses carried out by gradually heating compound 1 until 450 °C under a N2 flux, are shown in Fig. 1. The TGA curve evidences three distinct weight losses in the approximate range 60–250 °C, with three corresponding endothermic process (DTA diagram). The two losses at ca. 60 °C and ca. 80 °C correspond to the release of crystallization and coordinated water molecules (calc. 4.52%, exp. 5.15%, each). The resulting uncharacterized compound is stable until about 180 °C where it starts the third weight loss (ca. 40%) due to the decomposition of the formate and 2,20 -bpy ligands. The molecular structure of compound 1 (Fig. 2) has been determined by a SCXRD study showing 1 crystallizing in the orthorhombic crystal system, Pbca space group. The structure analysis reveals that each zinc ion is octahedrally coordinated by a water molecule [Zn1AO2 2.058(3) Å], one bipyridine molecule, almost symmetrically chelating in the common N, N0 -bidentate mode [Zn1AN1 2.131(3), Zn1AN2 2.161(3) Å], (being the two pyridyl rings slightly twisted around the C7AC8 bond with a dihedral angle of ca. 4°), and three oxygen atoms pertaining to J. Ngoune et al. / Inorganica Chimica Acta 453 (2016) 263–267 265 Fig. 1. TGA (blue) and DTA (red) diagrams for compound 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 3. View down the crystallographic b axis of one CP. 2,20 -bpy H atoms are omitted for clarity. Fig. 2. Molecular structure of 1 with a partial atom labeling scheme. three distinct formate ions. Actually, while O5AC2AO4 carboxylate coordinates to Zn1 in a monotopic fashion, [Zn1AO4 2.081(2) Å], O1AC1AO3 behaves as a ditopic ligand, joining Zn1 to the symmetry equivalent Zn1i according to a syn-anti bridging geometry [Zn1AO1 2.083(2), Zn1iAO3 2.228(2) Å], thus generating a bent ZnAOCHOAZn conformation [20,39,40]. The coordination scheme is completed by the interaction Zn1AO3iii. Thus, the overall coordination geometry resembles that found in the chiral 3D coordination polymer [Zn(OCHO)2(N2H8C10)]n (N2H8C10 = 4,40 -bipyridine) [40]. Due to the ditopic behavior of O1AC1AO3, [Zn(l-OCHO) (OCHO)(2,20 -bpy)(H2O)H2O] self-assemble in a series of parallel zig-zag CPs, [where the closest Zn Zn separation is 5.4745(7) Å] one of which is shown in Fig. 3. Interestingly, coordinated and crystallization water molecules, coming from hydrated zinc formate and, likely, from the atmospheric moisture, play a relevant role in the crystal packing of 1, being involved in some strong H-bonds [41]. Actually, as shown in Fig. 4, coordinated water acts as a strong H-donor toward formate O1 [O2 O1i 2.653(3) Å, O2AH2O2 O1i 162(4)°] and O6 pertaining to the crystallization water molecule [O2 O6iii 2.612 Fig. 4. Strong H-bonds (light-blue dashed lines) connecting water molecules and oxygens from formates in one 1D CP of compound 1. 2,20 -bpy H atoms are omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) (4) Å, O2AH1O2 O6iii 174(4)°], that, in turn acts a strong H-donor toward O4 pertaining to the monotopic formate ion [O6 O4 2.755 266 J. Ngoune et al. / Inorganica Chimica Acta 453 (2016) 263–267 (4) Å, O6AH2O6 O4 177(4)°], likely reinforcing the 1D CP assembly. Moreover, the crystallization water molecules cover another relevant role, having strong H-bond interactions with O5 formate pertaining to neighboring CPs, [O6 O5iv 2.699(4) Å, O6AH1O6 O5iv 170(4)°]. Thus, the overall network of this compound results as waved 2D open frameworks, perpendicular to the crystallographic bc plane, one of which is shown in Fig. 5. By examining the CAO distances of carboxylate ions, the relevant role played by H-bonds in compound 1 is further highlighted. As matter of fact, while it is expected that, for the ditopic O1AC1AO3 formate, the C1AO1 and C1AO3 bond distances are very similar, according to a good delocalization of the negative charge of this ion, for monotopic O4AC2AO5 you would expect that C2AO5 was shorter than C2AO4. The measured bond lengths [C2AO4 1.255(4) and C2AO5 1.253(5)] are instead very similar and, incidentally, very similar to C1AO1 and C1AO3 distances [1.260(4) and 1.242(4) Å, respectively], and this feature is very likely related to the involvement of O5 into the above discussed very strong H-bond [41]. This supramolecular interaction has likely the effect to make the C2AO5 bond overall comparable to the C2AO4, with a good electron delocalization also in this carboxylate fragment. Finally, parallel 2D supramolecular open networks generate the crystal packing shown in Fig. 6. A further point worth to be considered concerns the use of 2,20 -bpy and formate ion to generate Zn complexes and related CPs. 2,20 -bpy is a well-known, largely used ligand, normally acting in the symmetrically chelating N,N0 -bidentate mode [42,43]. The searching and retrieving information from the Cambridge Structural Database (CSD version 5.37 update Feb 2016) reveals that in literature are present 568 structurally validated Zn complexes containing at least one molecule of 2,20 -bpy, while only 158 contain also two carboxylate moieties. Moreover, only 32 of them correspond to CPs, all having di-, tri- or tetracarboxylates as linkers. The formate anion has been scarcely employed to build CPs likely due to the fact that it may easily decomposed in drastic solvothermal conditions: 70 derivatives containing the Zn(HCOO)2 fragment are reported in the CSD, 57 correspond to CPs and the OACHAO moiety acts as linker in 48 species only. Moreover, a large number of these species corresponds to Zn/ammonium oxalate mixed salts or Zn oxalate containing clathrate molecules. By considering both ligands (formate and 2,20 -bpy) the CSD returns Fig. 5. View down the crystallographic b axis of compound 1 evidencing the H-bond interactions connecting neighboring parallel CPs (magenta dashed lines). 2,20 -bpy H atoms are omitted for clarity. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Fig. 6. View down the crystallographic a axis of the crystal packing of compound 1. Different colors indicate different 1D CPs running perpendicular the bc plane. only two compounds (indicated as A [44] and B [45] in Fig. 7) containing Zn coordinating one formate anion and one 2,20 -bpy molecule, beside another carboxylate species and, in both cases, the formate ion was serendipitously obtained by in situ solvothermal decomposition of DMF employed as reaction solvent. On the other hand, the CSD returns only six distinct metal complexes where at least one 2,20 -bpy molecule and two formate ligands are present, namely Mn [46], Cu [47,48] and Rh [49–51]. Only a Mn [Mn (HCOO)2(2,20 -bpy)] and a Cu [47] {[Cu(HCOO)2(2,20 -bpy)HCOOH]} derivatives have a structure someway resembling that of 1. More specifically, in [Mn(HCOO)2(2,20 -bpy)] both formate ions display a ditopic behavior, connecting MnII ions according to an anti-anti geometry, thus generating a 3D CP. The structure of [Cu(HCOO)2(2,20 -bpy)]HCOOH is much more similar to that of 1, Fig. 7. The sole two structurally validated Zn complexes containing both formate and 2,20 -bibyridine ligands (evidenced in red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) J. Ngoune et al. / Inorganica Chimica Acta 453 (2016) 263–267 267 References [1] [2] [3] [4] [5] [6] [7] 0 Fig. 8. Comparison of the schematic structures of 1 (left) and [Cu(HCOO)2(2,2 bpy)]HCOOH (right). Crystallization water (in 1) and formic acid (in [Cu (HCOO)2(2,20 -bpy)]HCOOH) are omitted for clarity. [8] [9] [10] [11] one formate anion acting as a monotopic ligand, while the second one connects two CuII ions in an anti-anti geometry. By this way, a 1D CP is formed and in Fig. 8 the schematic structures of 1 and [Cu(HCOO)2(2,20 -bpy)]HCOOH are shown, evidencing the resemblance between the two structures. In both cases 2,20 -bpy chelates the metal ion and one formate is bonded in a monodentate way, while the second formate is responsible of the polymeric assembly. The differences consist in one water molecule coordinated to Zn, which thus reaches its usual hexacoordination, while in the case of copper the pentacoordination is achieved, and in the syn-anti geometry of ditopic formate in the case of 1 against the anti-anti geometry present in the copper derivative. 4. Conclusions We have here reported the synthesis, the thermal and spectroscopic characterization and the structural analysis of a 2,20 -bipyridine-containing zinc(II) CP, [{Zn(l-OCHO)(OCHO)(2,20 -bpy)(H2O)} H2O]n, 1, by simply mixing hydrated zinc bis-formate with 2,20 -bpy in toluene. In this zig-zag 1D CP Zn ions are connected each-other by one formate anion in syn-anti geometry, while the second formate ion is coordinated to Zn in a monodentate way and is involved into some strong H-bonds connecting parallele 1D CPs forming waved 2D supramolecular networks. Noteworthy, despite polycarboxylates and even monocarboxylates have largely employed in the synthesis on a plethora of Zn CPs, formate anion has been rarely employed for this purpose. On the other hand, 2,20 -bpy has been largely employed as zinc chelating ligand [42,43], but at the present Zn bis(formate)(2,20 -bpy) derivatives are not present in the CSD. Thus, compound 1 is not only the first ‘‘Zn(HCOO)2(2,20 -bpy)” coordination polymer, but also the first species containing both these ligands. Acknowledgments We would like to thank Dr. Corrado Di Nicola of the School of Pharmacy, the University of Camerino, Italy, for assistance with some analysis and their interpretations. 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