Three Azamacrocycle-Based Coordination Complexes Bearing a New Triazine Derived Carboxylic Ligand Via In Situ Ligand Hydrolysis: The Trap of Resonance Structure

The in situ ligand hydrolysis reaction of 2,4,6-tris-(4-carboxyphenoxy)-1,3,5-triazine and macrocyclic complexes yields three coordination complexes bearing new triazine derived carboxylic ligands, namely, {(NiL1)(L0)·DMF}n (1), {(NiL2)(L0)·DMF·H2O}n (2), and [NiL3][(NiL3)(L0)2]·2H2O (3) (Lo = 4-(6-Hydroxy-4-oxo-4,5-dihydro-[1,3,5]triazine-2-yloxy)-benzoic acid, L1 = 1,4,8,11-tetraazacyclotetradecane, L2 = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradeca and L3 = 1,3,6,9,11,14-hexaazatricyclooctadecane). Single-crystal X-ray diffraction analyses reveal that 1 and 2 exhibit a one-dimensional (1D) chain structure, which is further connected into a three-dimensional (3D) supramolecular structure by hydrogen bonds. In complex 2, π···π stacking interaction is observed. In contrast, 3 shows a ion-pair structure, which is connected into a 2D 44 hydrogen bonded supramolecular structure. The results indicate the subtle difference of azamacrocycle may lead to diverse structures. Importantly, in complexes 1–3, the resonance structure is trapped in crystals 1–3.


Single-crystal X-ray crystallography
Single-crystal X-ray diffraction data for complexes were collected on a Bruker Apex CCD diffractometer with graphitemonochromated Mo-Ka radiation (λ D 0.71073 A ). Absorption corrections were applied to the data using the SADABS program. [21] The structures were solved using SHELXL-97 and refined by full-matrix least-squares on F 2 . [22,23] All nonhydrogen atoms of the networks were refined with anisotropic temperature parameters and hydrogen atoms were placed in calculated positions and refined with a riding model. The hydrogen atoms on protonated triazine ring were found from difference Fourier maps. Crystallographic data and structure determination summaries are listed in Table 1, and the selected bond lengths for complexes 1-3 are listed in Table S1.
triazine derived carboxylic ligand was isolated after the hydrolysis of the TCPT. Based on Park and Suh [24] and Zhu et al.'s [25] work, the TCPT was stable under high temperature and in the water solution. Combined with our experiments, the only difference is the addition of macrocyclic precursors, indicating there may be a macrocyclic complex catalysis process in existence. Therefore, parallel experiments were designed to study the mechanism of the hydrolysis. Without the macrocyclic precursors, no L o ligand was detected after several days. Hence, the hydrolysis of TCPT was ascribed to the macrocyclic complex catalysis.
The hydrolyzed product may have four different resonance structures (a, b, g, d), including full and partial resonance conformations (Scheme 2 is the simplified diagram). The precise structure of L o is confirmed by the bond distance of triazine ring, C-O/C D O distance, charge balance and the rationality of hydrogen bond. In 1-3, two C-N bond distances in the triazine ring are larger than the unprotonated tri-    respectively. Each L 0 ligand binds two Ni(II) ions in a bis-monodentate mode, which results in a zigzag 1D coordination polymer chain. Hydrogen bonds interactions play crucial roles in the formation of supramolecular structures. Within a chain, two kinds of hydrogen bonds were formed, (a) hydrogen-bonding interaction between the -COO ¡ group and the -NH-group on the macrocyclic ligand and (b) hydrogen-bonding interaction between N atom on triazine and the -NH-group on the macrocyclic ligand ( Figure 2). The former one is usually observed in azamacrocycle based coordination complexes. To the best of our knowledge, the later one is never been observed before. Furthermore, zigzag chains are held together through interchains hydrogen bonds (Figure 3), giving rise to a 2D supramolecular layer.
As shown in Figure 4, single-crystal X-ray diffraction analysis reveals that the complex 2 consists of one L 0 anion,  , respectively, which range in the normal distance. Each L 0 binds two Ni(II) ions in a monodentate mode, which results in a zigzag 1D coordination polymer chain. Similarly, two kinds of hydrogen bonds are observed within a chain ( Figure S1). Those zigzag chains are further held together through inter-chains hydro-   Xia et al.
gen bonds and p¢¢¢p stacking ( Figure 5), giving rise to a 2D supramolecular structure. The molecular structure of complex 3 with atom labeling is shown in Figure 6. The complex 3 consists of two discrete units: [NiL 3 ] 2C and [(NiL 3 )(L 0 ) 2 ] 2-. Compared with L 1 and L 2 macrocyclic ligands, the discrete [Ni(1)L 3 ] 2C fragment locates in the crystal maybe due to the steric hindrance of L 3 macrocycle. Similar to complexes 1 and 2, the Ni(2) atoms sit on an coordination center, which displays distorted [NiN 4 O 2 ] octahedral geometry, with four secondary nitrogens from the macrocycle and two oxygens from carboxylates. The average Ni(1)-N and Ni(1)-O bond distances are 2.0719 A and 2.1477 A , respectively, which range in the normal distance. In the [(NiL 3 )(L 0 ) 2 ] 2anion, the hydrogen bond between the -COO ¡ group and the -NH-group on the macrocyclic ligand is also observed ( Figure S2). Two different ion pairs are connected by the hydrogen bonds between tertiary amine and protonated triazine, secondary amine (-NH-) and triazine ( Figure 7). It is deserved to note that the hydrogen bond between tertiary amine and protonated triazine is never been observed before. When we treat as [NiL 3 ] 2C a 4-connected node and [(NiL 3 )(L 0 ) 2 ] 2as linker, the hydrogen bonded framework can be simplified to a 2D 4 4 network ( Figure S3). Furthermore, the 4 4 layer are linked by hydrogen bond between water and L 0 , which form a twelve membered ring ( Figure 8).
It had been clearly shown that the crystallization of complexes was significantly influenced by pH, the reaction temperature, solvent, metal cation, the conformation of ligand and so on. As described previously, complexes 1-3 were synthesized under the same external experimental environment except for the macrocyclic ligand, however, they exhibit huge structural changes. Very recently, such peripheral macrocycles regulated drastic different structures had been observed by our group. [26,27] Complexes are air-stable and insoluble in water and common organic solvents. The simulated and experimental XRD patterns of complexes obtained are shown in Figure 9. Their peak positions are in good consistency with each other, indicating the phase purity of the synthesized sample.

Conclusion
In summary, three entire different nickel(II) coordination complexes based on various azamacrocyclic precursors and a new triazine derived ligand were obtained and characterized.
The results indicate the subtle difference of azamacrocycle may lead to diverse structures. In additional, our research shows the resonance structure can be trapped in coordination complex. This phenomenon may provide a straightforward approach to characterize the unstable resonance structure.

Funding
The work was supported by the Fundamental Research Funds for National University, China University of Geosciences (Wuhan) (No. 1210491B03), and the College Students' Innovative Experiment Project of China (No. 091049148).

Supplementary Material
Supplemental data for this article can be accessed at the publisher's website. CCDC-1018670 (for 1), 1018671 (for 2), and 1018672 (for 3) contain the supplementary crystallographic data for this paper. These date can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.