b Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, China;
c Department of Chemistry, Guangdong University of Education, Guangzhou 510303, China;
d Department of Chemistry, Michigan State University, E. Lansing, MI 48824, USA
Porphyrin is an 18-π electron conjugated tetrapyrrolic macrocycle and regularly used for synthesize coordination polymers [1-3]. It can form complexes with most metals of the periodic table . The IB group metal copper, silver and gold may form corresponding metal porphyrin smoothly and these metal porphyrins are good catalysts for many reactions [5, 6]. When forming metal porphyrins, the common oxidation states of copper, silver and gold are +2, +2 and +3, respectively. Generally, copper porphyrin has a four-coordinate structure . When existing axial ligands such as pyridine , N, N-dimethylformamide (DMF)  and H2O , five coordinate pyramid geometry copper porphyrins may be formed. Six-coordinate structure of copper porphyrin could be found when the axial ligand was 1, 4-dioxane  or tetrahydrofuran . Also, when porphyrin bearing o-nicotinoylamidophenyl , ethoxyphosphoryl  or 4-bromophenyl  substituents, the hetero atom of these substituents would coordinate with copper atom to form a six coordinated packing structure. As for the gold porphyrin, the oxidation state of central gold metal is +3 and generally has a five-coordinated structure with the fifth axial ligand chloride anion  or four coordinated structure [16-18].
The common oxidation state of silver is +1, which tends to form double coordination linear structure. In silver porphyrin cases, all of +1, +2 and +3 oxidant state of silver have been well documented . Ag(Ⅱ) porphyrin is the most common complex, which is generated from the reaction of free-base porphyrin and Ag(Ⅰ) salt. Silver(Ⅱ) porphyrins through photolysis are able to function as sensitizers in photodynamic therapy. The released Ag+ ions could supplement or enhance the therapeutic effect . Ag(Ⅰ) porphyrin was observed in the reductive demetallation of Ag(Ⅱ) porphyrin, while Ag(Ⅲ) porphyrin could be obtained from the oxidation of Ag(Ⅱ) porphyrin. Although four, five and six coordinated copper(Ⅱ) porphyrins are known very well, to the best of our knowledge, only four coordinated silver(Ⅱ) porphyrins have been reported so far [5, 21-28]. This may be due to the fact that the steric hindrance of meso-aryl-substituted porphyrins does not permit it to form well packed stacking structure. It is noteworthy, for those silver(Ⅱ) complexes of non-porphyrin ligand, six-coordinated silver complexes had been reported quite a long time ago. [Ag2(μ4-hmt)(NO2)2]n (hmt = hexamethylenetetramine) exampled the first report of six-coordinated silver(Ⅰ) complex which was of a triangular prism structure . While bis(pyridine-2, 6-dicarboxylate)silver(Ⅱ) monohydrate is the six-coordinated complex of a distorted octahedron structure . Six-coordinated Ag(Ⅱ) complexes such as bis(pyridine-2, 3-dicarboxylate)silver(Ⅱ) dehydrate , dinitratodipyridyl silver(Ⅱ)  and meso-5, 5, 7, 12, 12, 14- hexamethyl-1, 4, 8, 11-tetraazacyclotetradecane silver(Ⅱ) nitrate  generally have a tetragonally elongated distorted octahedral structure. The maximum coordination number of Ag(Ⅱ) atom may reach eight .
5, 10, 15, 20-Tetra(ethoxycarbonyl)porphyrin (Scheme 1, TECP)  is an electron deficient flat tetrapyrrolic macrocycle having interesting property. It is also a good precursor for the preparation of porphine . Recently, We found manganese(Ⅲ) TECP was even a recyclable catalyst for styrene oxidation in homogenous system . X-Ray structure determination showed zinc(Ⅱ)TECP , Cu(Ⅱ)TECP  and Mn(Ⅲ)TECP  displayed an interesting six-coordinated structure. Whether Ag(Ⅱ)TECP (Scheme 1) would has a similar coordination structure? Here we wish to report the crystal structure of Ag(Ⅱ)TECP and find it do displays a sixcoordinated linear assembled structure, which stands for the first example of six-coordinated Ag(Ⅱ) porphyrin.
Ag(Ⅱ)TECP was prepared by reaction of AgNO3 and TECP in DMF solution and was well characterized. In dichloromethane, the Soret band of Ag(Ⅱ)TECP showed a significant red-shift (9 nm) as compared to free base TECP (Fig. 1), this indicated the central Ag atom might be coordinated by oxygen of the meso-ester group. When the concentration of Ag(Ⅱ)TECP is less than 14 μmol/L, the absorbance and concentration is of a linear relationship (Fig. S1 in Supporting information). A further increase in concentration of Ag (Ⅱ)TECP would lead to aggregation, and the obvious blue-shift of the Soret band indicated it formed a H-aggregate . Fig. 2 shows the X-ray photoemission spectra (XPS) of Ag(Ⅱ)TECP. The XPS-3d signal of Ag exhibited two peaks at 369.1 eV (3d5/2) and 375.1 eV (3d3/2), with an intensity ratio larger than 1:1 and a gap of 6.0 eV. This indicated the central Ag atom of Ag(Ⅱ)TECP is +2 oxidation state . Fig. 3 shows the electron paramagnetic resonance (EPR) spectra of Ag(Ⅱ)TECP, which consists eleven lines with an equal separation of approximately 22 Gauss. This splitting, together with the observed peak ration, can be explained by assuming an overlap of the expected two sets of nine nitrogen hyperfine lines, where each set corresponds to hyperfine interactions between the unpaired electron and four equivalent nitrogen atoms with a unity nuclear spin. The splitting into two sets is due to the spin of one half of the silver nucleus. From the spectra, we obtain an isotropic g-value of 2.06. This value is in close agreement with those reported Ag(Ⅱ) porphyrin earlier [39, 40]. Thus, the appearance of the EPR signal from Ag(Ⅱ)TECP further demonstrated a Ag (Ⅱ) central atom.
|Fig. 3. EPR spectrum of Ag(Ⅱ)TECP in chloroform at room temperature and a microwave frequency of 9.8749 GHz.|
The electrochemical characteristics of TECP and Ag(Ⅱ)TECP were investigated by cyclic voltammetry in DMF solutions. TECP have one oxidation potential peak at 0.756 V and two reductions potential peaks at -0.186 V and -0.677 V. Ag(Ⅱ)TECP have three oxidation potential peaks at 0.155 V, 0.807 V and 1.531 V and three reduction potential peaks at -0.454 V, -1.089 V and -1.332 V.
The crystals of TECP and Ag(Ⅱ)TECP were obtained by evaporation of solvent from a solution of methylene chloride and n-hexane. The monoclinic crystal form of TECP was found in the centrosymmetric space group P21/c with a required 2-fold symmetry. The asymmetric unit contained eight TECP molecules with no solvent molecule (Fig. 4).
|Fig. 4. Thermal ellipsoid diagram of TECP displaying a partial atom labelling scheme. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity.|
The average C-N bond in TECP is about 1.37Å. The 24- membered macrocyclic core of the porphyrin is coplanar, and the displacement of each atom in the equatorial mean plane is within ±0.069Å. In the crystal, two TECP molecules are nearly parallel and separated by 3.3Å (Fig. 5A). TECP molecules were packed by π-π stacking interactions and Van der Waals forces to construct three-dimensional structure (Fig. 5B). In contrast to TECP, the X-ray diffraction analysis revealed that Ag(Ⅱ)TECP crystallized with P-1 space group of a triclinic structure with one formula unit in a cell without solvent molecule. All crystallographically independent atoms are in general positions with exception of the Ag atom, which is at the crystallographic center of inversion. The 24-membered macrocyclic core of the porphyrin is coplanar, and the displacement of each atom in the equatorial mean plane is within ±0.039Å. The silver atom is located perfectly at the center of planar porphyrin macrocycle and bonded to four nitrogen and two oxygen atoms to form an octahedron structure (Fig. 6). The average Ag–N bond length is 2.0997Å, which is similar to those Ag porphyrins reported in the literature [22, 26]. The distance between carbonyl oxygen atom and the Ag centre is 2.8594Å, which is comparable to those found in Ag complexes with weak Ag–O coordination bond . Thus, the crystal structure of Ag(Ⅱ)TECP indicates it has a six coordinated Ag central atom. Two axial oxygen atoms (O1A and O1B) sit at the opposite position of silver atom Ag1, the O1A–Ag1–O1B angle is 180°. This examples the first observation of six coordinated Ag(Ⅱ) porphyrin complex. As pointed out in the introduction, all reported Ag(Ⅱ) porphyrins are of four coordinated structure [5, 21-28].
|Fig. 5. (A) π-π interactions between two TECP molecules, (B) Wire frame partial packing diagram of TECP.|
|Fig. 6. Thermal ellipsoid diagram of Ag(Ⅱ)TECP displaying a partial atom labelling scheme. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity.|
X-Ray structure of Ag(Ⅱ)TECP showed it formed an onedimensional long-chain through weak intermolecular Ag–O coordination bonds and π-π stacking interactions (Fig. 7). These observations were similar to our previously reported structure of ZnTECP  and CuTECP . The distance between two neighbouring Ag(Ⅱ)TECP planes is 3.415Å with a plane dihedral angle of 0°. And there is a partial overlap between the two Ag(Ⅱ) TECP ring planes. One-dimensional Ag(Ⅱ)TECP constructed 3D supramolecular network via hydrogen bonding (Fig. 8). Viewing from side b, two types of hydrogen-bonding interactions could be found (Fig. 8A). One hydrogen bond formed by methylene hydrogen and carbonyl oxygen atom between two adjacent silver porphyrin moieties of two coordination polymer chains, and the other one formed between pyrrole hydrogen atom of one polymer chain and coordinated carbonyl oxygen atom of other polymer chain. When viewing from side c view, only one type of hydrogen bond could be observed, which formed by carbonyl oxygen atom (coordinate with silver) and methylene hydrogen atom (Fig. 8B).
|Fig. 8. Packing diagram of Ag(Ⅱ)TECP with the dashed lines representing hydrogen-bonding, (A) side b view, (B) side c view.|
In summary, we have successfully resolved the crystal structure of Ag(Ⅱ) 5, 10, 15, 20-tetra(ethoxycarbonyl) porphyrin, which revealed its central Ag(Ⅱ) atom was axially coordinated by two oxygen atoms of the carbonyl group from neighbouring porphyrin to form a well packed six-coordinated array. Also, free base TECP exhibited quite different packing and hydrogen bonding mode from Ag(Ⅱ)TECP. It seems TECP prefers to form six-coordinated metal complexes with a linear assembled structure. Systematic investigations extending to other metal complexes are going on in our lab.Acknowledgment
We gratefully acknowledge financial support from the National Natural Science Foundation of China (Nos. 21371059, 21376099, 21671068).Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2017.12.027.
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