Multidimensional Transition Metal Complexes Based on 3-Amino-1H-1,2,4-triazole-5-carboxylic Acid: From Discrete Mononuclear Complexes to Layered Materials

The synthesis and structural characterization of five transition metal complexes with different dimensionality and incorporating residues of 3-amino-1H-1,2,4-triazole-5-carboxylic acid (H2atrc) is reported: [Zn(Hatrc)2(H2O)] (1), [Mn(Hatrc)2(H2O)2]·2H2O (2), [Fe2(Hatrc)4(OH)2]·6H2O (3), [Cd(Hatrc)2(H2O)]n (4), and [Mn(atrc)(H2O)]n·nH2O (5). These materials could be prepared from solution (1–3), diffusion (4), or hydrothermal reactions (5) with various anions and L:M ratios. Structural details were revealed by single crystal X-ray diffraction. The discrete units composing compounds 1–3, the polymeric 1D chain of 4 and the 2D layer of 5 are further extended into 3D supramolecular architectures through the formation of hydrogen bonds.


Introduction
The rational strategy to design new coordination complexes, especially Metal-Organic Frameworks (MOFs) or coordination networks, by self-assembly has received remarkable interest due to their fascinating structural features and potential to be applied as novel functional materials [1][2][3][4][5][6][7]. The final assembly can be influenced by numerous factors, such as geometric requirements of metal centers, shape and nature of the ligands, reaction routes, solvents, templates, pH of the reactive medium and counterions [8][9][10][11][12]. At the moment, immense research activity is focused on integrating the advantages of metal centers and organic spacers in a complementary way. The inorganic component imparts magnetism, mechanical strength and thermal stability, while the organic part offers a way to improved luminescence, structural diversifications, and processability [13][14][15][16]. The effort exerted in this field promotes the understanding of the structure-property relationship, being attributed to the purposeful design and controlled synthesis of the aimed complexes [17][18][19][20][21].

Synthesis
All compounds were directly isolated from the solutions or contents of autoclaves mostly as good quality single crystals. The influence of several parameters, such as the type of metal centers, anions, inorganic/organic bases, ligand:metal ratio, volume of water, temperature and stirring in the formation and crystallization of the compounds was investigated. A number of drawbacks in the crystallization process were found: (i) the use NaOH or triethylamine (Et3N) to deprotonate H2atrc in reactions with transition metal salts (such as Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ and Cd 2+ ) led to poor crystalline powders; (ii) the use of temperatures above 60 °C in the reactions with the aforementioned metals also led to the formation of microcrystalline powders; (iii) with the use of less than ca. 6 mL of water in the systems mixed crystals of the new materials and the parent H2atrc ligand were easily produced; (iv) with 30 min of stirring, the clear mixed solutions become cloudy and no pure single crystals were produced.
As a result of this study and optimization, the most adequate reaction conditions for each compound were ultimately optimized for a mixture of an aqueous solution of H2atrc and the metal, without stirring and standing at ambient temperature. The optimized conditions (see Experimental Section for details) were used in the reactions of H2atrc with several metal salts. The use of distinct Zn 2+ salts [ZnCl2, Zn(NO3)2·6H2O, ZnSO4·7H2O and Zn(ClO4)·6H2O] with H2atrc at a ratio of 0.1:0.1-0.6 mmol (with a step of 0.1 mmol) resulted always in the isolation of the same complex 1, indicating that this is a governing product under the fixed conditions. A similar phenomena was observed for the Mn 2+ complexes, but the optimized reaction conditions are not suitable for the Cu 2+ , Ni 2+ and Co 2+ , whose products appeared as powders [Cu 2+ and Ni 2+ series] or solutions [Co 2+ series; slow evaporation of the solution did not result in suitable crystals], ultimately indicating that the metal centers play a decisive role in the formation of the final compounds.

Mononuclear Complexes
Complex 1, [Zn(Hatrc)2(H2O)], crystallizes in the orthorhombic space group Pbcn with a crystallographic C2 rotation axis located along the Zn1-O1W bond. The metal center coordinates to one water molecule to two symmetry related Hatrc − moieties, having a distorted trigonal bipyramidal coordination geometry whose triangular equatorial plane is formed by O1W, N14 and N14 a (symmetry operation: a = 1−x, y, 1.5−z), while the axial positions are occupied by O12 and O12 a atoms ( Figure 1). The metal center is ideally located in the triangular plane with no deviation from the best least-squares plane fitted by the three coordinated atoms. The {ZnN2O3} trigonal bipyramid displays τ = (β − α)/60 = 0.6255, where α and β are the two largest angles found in the metal coordination center (Please note: τ = 0 for an ideal square pyramid, and τ = 1 for an ideal trigonal bipyramid) [48]. The Hatrc − residue adopts a typical N,O-bidentate chelating mode (Scheme 1, type I) with the Zn-O and Zn-N found within the expected ranges (Table 1). Considering compounds 1, 2 and the isotypical [Cd(Hatrc)2(H2O)2]·2H2O [34], the distances between the metal centers and the atoms in the first coordination sphere follow the tendency of the ionic radius Zn 2+ < Mn 2+ < Cd 2+ . Conversely, the O-M-N bite angles subtended by the Hatrc − residue follow, as expected, the opposite tendency (Table 1).

Scheme 1.
Coordination modes of Hatrc − (type I and II) and atrc 2− (type III) anionic residues found in compounds 1-5. Non-H atoms of the asu are drawn as thermal ellipsoids at the 30% probability level and the remaining atoms as spheres with arbitrary radius. For selected bond lengths and angles see Table 1. Symmetry transformation: a = 1−x, y, 1.5−z.
Adjacent [Zn(Hatrc)2(H2O)] complexes interact via strong hydrogen bonds leading to the formation of a 2D supramolecular structure (layers) extended in the ab plane of the unit cell ( Figure 2a). The intermolecular hydrogen bonds involved in the formation of these layers are of the type O1W-H1WA···O11 and O1W-H1WA···O12 (yellow dashed lines in Figure 2a; Table 2). Neighboring layers pack close along the [001] direction and are interconnected into a 3D supramolecular architecture network via the N11-H11A···O11 interaction (orange dashed lines in Figure 2b; Table 2). Furthermore, the distance between the centroid of triazole ring and N11 is 3.510(3) Ǻ, pointing to the existence of a weak N11-H11A···π stacking interaction in the crystal structure (not shown), which also contributes for the stabilization of the 3D supramolecular architecture.     Table 2.
Compound 2, [Mn(Hatrc)2(H2O)2]·2H2O features a mononuclear complex ( Figure 3) with the asu being composed of one metal center (Mn1), one Hatrc − anionic ligand and one coordinated water molecule. An additional uncoordinated water molecule of crystallization is, however, present. The metal center has a distorted octahedron geometry, with the equatorial plane formed by O1, N14 and their symmetrical counterparts (O1 b and N14 b , symmetry transformation: b: 1−x, 1−y, 1−z); O1W and O1W b occupy the axial positions ( Figure 3 and Table 1). The coordination fashion of the ligand is of type I, as that observed in compound 1. Bond-valence-sum (BVS) calculations gives a value of 2.02 for Mn1, indicating that Mn1 has the +2 oxidation state (command Calc Coord in PLATON [49]). This compound is isotypical to the already described compound [Cd(Hatrc)2(H2O)2]·2H2O [34]. Crystallographically independent non-H atoms are represented as thermal ellipsoids drawn at the 50% probability level. Symmetry-related atoms are represented as spheres with arbitrary radius. For selected bond lengths and angles see Table 1.
Individual [Mn(Hatrc)2(H2O)2] complexes present in compound 2 are interconnected through strong hydrogen bonds of type N-H···O1W involving the coordinated water molecules and adjacent Hatrc − residues into 1D chains running parallel to the a-direction of the unit cell (Figure 4a), forming a graph set motif of the type R2 2 (12) [50]. Chains further grow into supramolecular layers by way of the lattice water molecules O2W, which act as bridges between adjacent chains: O2W-H2WB···O2, O2W-H2WA···N12 and N11-H11A···O2W ( Figure 4b and Table 3). Layers pack along the b-axis of the unit cell mediated by hydrogen bonds, ultimately originating the extended 3D supramolecular structure in 2 ( Figure 4c).  Table 2.   (15) Symmetry transformations used to generate equivalent atoms:

Binuclear Complex
Compound 3, [Fe2(Hatrc)4(OH)2]·6H2O, is composed by a discrete neutral binuclear unit ( Figure 5). The asu is composed of one independent Fe 3+ center, two anionic Hatrc − ligands, one μ2-bridging OH − anion and three lattice water molecules. Besides the presence of the aforementioned anions, the charge of the metallic center is supported by Bond Valence Sum (BVS) calculations, which produces a value for the ferric atom of ca. 3.01, thereby indicating that iron center is indeed in the +3 oxidation state [49]. The six-coordinated Fe1 center displays a distorted octahedral coordination geometry with the apical positions being occupied by N14 and N24 from two coordinated triazole moieties. The carboxylic oxygen atoms O2 and O4 from two distinct carboxylate groups plus O5 and O5 l from two crystallographic equivalent hydroxyl moieties shape the equatorial plane of the octahedron (symmetry transformation to generate equivalent atoms: l = x, 0.5−y, 0.5−z). The resulting binuclear [Fe2(Hatrc)4(OH)2] complex is thus formed due two μ2-OH − bridging anions, with the Fe1-O5 distances being 1.954 (2) Table 3 for geometric details on selected bond lengths and angles). It is noteworthy that the coordination mode of Hatrc − found in 3 is identical to that described for compounds 1 and 2.  Table 4 for geometrical details on the hydrogen bonds). Adjacent layers are fused into a 3D supramolecular architecture by ways of several strong N-H···O, O-H···N and O-H···O interactions (Figure 6c and Table 4).    Table 4.

One-Dimensional Chain (1D)
Compound 4, [Cd(Hatrc)2(H2O)]n, crystallizes in monoclinic space group P21/n with one Cd 2+ cation, two Hatrc − ligands and one coordinated water composing the asu, which features the building unit of a 1D coordination chain (Figure 7). The Cd1 center coordinates to three Hatrc − anionic ligands and one water molecule (Figure 7a), displaying a coordination geometry which resembles a relatively rare seven-coordinated capped trigonal prism: the atom groups N14, O2, O4 and N24, O3 and O1W build the two opposite triangular facets, and another symmetrical O3 is the capping atom located at the rectangular facet fitted by O2, O4, N24 and O1W (selected bond distances and angles are in given in Table 3). Although seven-coordinated Cd 2+ centers are not so numerous as those of four or six-coordinated, a search in the CCDC [33] gives up to 700 related hits. Limiting the search to 1,2,4-triazole residues only, 33 examples of seven-coordinated cadmium(II) are known: three of them are single-capped octahedra [48,[51][52][53] while the remaining ones are pentagonal bipyramids. The two crystallographically distinct Hatrc − ligands in compound 4 adopt two different coordination fashions: one is a η 2 -type (type I in Scheme 1) and the other is η 4 -type (type II in Scheme 1). The first coordination mode is the same as those observed in    Table 3. Symmetry operation: m = 1.5−x, y+0.5, 0.5−z.
Neighboring chains interact trough cooperative N-H···O hydrogen bonds, in particular interactions of the type N11-H11A···O4, N2-H2A···O1W and N21-H21A···O1, leading to the formation of 2D supramolecular layers as depicted in Figure 8a (see Table 5 for geometrical details on the existent hydrogen bonds). Additionally, these supramolecular layers are interconnected to other adjacent ones via strong N-H···O and O-H···O interactions forming a 3D supramolecular architecture (Figure 8b and Table 5).    Table 5.

Coordination Layer (2D)
The structure of compound 5, as unveiled from single-crystal XRD analysis, was formulated as [Mn(atrc)(H2O)]·H2O, featuring a 2D coordination layered structure. The asu comprises one Mn 2+ center, one fully deprotonated atrc 2− anionic ligand, one coordinated and one lattice water molecule. The Mn1 metal center is coordinated by three crystallographic equivalent atrc 2− and one water molecule, leading to a six-coordinated coordination environment with a geometry that resembles a distorted octahedron: N14, N12 n , O2 o and O1W atoms form the equatorial plane, and O1 and N11 o occupy the two axial positions (Figure 9; for details concerning bond lengths and angles see Table 3; symmetry operations: n = −x, 0.5+y, 0.5−z and o = x, 0.5−y, −0.5+z). BVS calculations gave a charge of +2.02 for Mn center suggesting that the oxidation state of Mn1 should be +2, in good agreement with the charge assignment from the crystal structure determination. Each atrc 2− residue in 5 connects to three Mn 2+ metal centers through two N,O-bidentate interactions (chelating two metal cations through N11/O2 and N14/O1) and one monodentate-N mode to bind the fourth Mn 2+ by N12 (Scheme 1, type III). As a consequence of these coordination modes, a 2D coordination framework (layer) based on the interconnection of [Mn8(atrc)8] macro rings is ultimately (Figure 10a). The 2D layers pack along the [100] direction of the unit cell being mediated by numerous strong O-H···O, O-H···N and N-H···O hydrogen-bonding interaction leading to the formation of a 3D supramolecular architecture (Figure 10b).
The coordination fashions of the organic ligand (Scheme 1) can be related with the structural features and dimensionality of the reported complexes. In mild conditions, H2atrc tends to firstly deprotonate the hydrogen atom of the carboxylic acid group. The resulting anionic Hatrc − ligand chelates one metal center using the 4-positioned N-atom and one O-atom of the carboxylate group to form a rather stable five-membered chelate ring (type I in Scheme 1), as observed in compounds 1-3. The coordination fashion of Hatrc − in compound 4 (type II in Scheme 1) has one more chelating carboxylate group than type I. Usually, a more complex coordination fashion tends to promote more complicated structures. With the use of a higher synthetic temperature and the presence of a strong base (NaN3 or NaOH), the hydrogen atom on the 1-positioned N is liberated, thereby giving rise to a bidentate-chelating-N,O and one monodentate-N coordination donor (type III) contributing to the construction of a higher dimensional network.  Table 5.

Synthesis
[Zn (Hatrc)2(H2O)] (1). H2atrc (4.992 mmol, 0.6394 g) was dissolved in 500 mL of hot water, cooled to ambient temperature and then mixed with a 5 mL ZnSO4·7H2O (5.019 mmol, 1.4404 g) aqueous solution without stirring. Single crystals suitable for X-ray diffraction were obtained from the standing solution over a period of two weeks.

Single Crystal X-ray Diffraction
Single crystals of compounds 1-5 were manually harvested from the crystallization vials and mounted on Hampton Research CryoLoops using FOMBLIN Y perfluoropolyether vacuum oil (LVAC 25/6, purchased from Aldrich) [54] with the help of a Stemi 2000 stereomicroscope equipped with Carl Zeiss lenses. Data were collected with a Bruker X8 Kappa APEX II charge-coupled device (CCD) area-detector diffractometer (Mo-Kα graphite-monochromated radiation, λ = 0.71073 Å) controlled by the APEX2 software package [55], and equipped with an Oxford Cryosystems Series 700 cryostream monitored remotely using Cryopad [56]. Images were processed using SAINT+ [57], and data were corrected for absorption by the multi-scan semi-empirical method implemented in SADABS [58]. The structures of compounds 1-5 were solved by direct methods using SHELXS [59] and refined using SHELXL [60] by full-matrix least-squares technique on F 2 . All non-hydrogen atoms were refined anisotropically. Hydrogen atoms attached to C atoms were placed at geometrically calculated positions to their carrier atoms and refined with isotropic thermal parameters included in the final stage of the refinement, with Uiso = 1.2 × Ueq of the atoms which they are attached. Hydrogen atoms associated with the water molecules and nitrogen atoms were located in difference Fourier maps. The N-H, O-H and H···H distances were fixed using with Uiso = 1.5 × Ueq of the atoms to which they are attached, plus appropriated DFIX distances (N-H 0.88 Å, O-H 0.95 Å and H···H 1.54 Å). A summary of the structural determination and refinement details for compounds 1-5 is listed in Table 6. The highest peaks and deepest holes found in the structures refinement is listed in Table 7. CCDC 1403898-1403902 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.   In this series, (H)atrc ligands adopt three types of connection fashions: in 1-3, they chelate preferentially with the metal centers via the carboxylic O atom and the 4-position N atom to form a five-membered ring; in 4, with one more metal center chelated by two carboxylic O atoms, the discrete units can shape a polymeric 1D chain; in 5, based on the fashion in 4, two more metal centers are bound through, on the one hand, a similar five-membered ring being formed by the 2-positioned N atom and the second carboxylic O atom and, on the other, the fourth metal being connected to the 1-positioned N atom. The various coordination fashions play, in this respect, a significant role in the structural fabrication of the various compounds. The residual uncoordinated donor sites in the anionic ligands in compounds 1-4 open the possibility to use compounds 1-4 as precursors of building blocks to further isolate multidimensional frameworks. With the rich existence of donors and acceptors in the present structures, the hydrogen-bonding interactions with the various water molecules incorporated into the materials play key roles in the supramolecular organization leading to the formation of 3D supramolecular architectures.