An Unsymmetrical Trinickel Metal Complex with 4-sulfobenzoate Axial Ligand: [ni 3 (dpa) 4 (4-sb)(h 2 O)]⋅ ⋅ ⋅ ⋅3h 2 O

An unsymmetrical trinickel metal complex, [Ni 3 (dpa) 4 (4-sb)(H 2 O)]⋅3H 2 O (1), where dpa is a 2,2'-dipyridylamine anion and 4-sb is 4-sulfobenzoate dianion, was synthesized and characterized by X-ray crystallography, IR, elemental analysis, fluorescence, and cyclic voltammetry. The molecular structure of 1 is a cluster with a linear trimer. The extended structure is a 3D architecture assembled by hydrogen bonds. The 4-sulfobenzoate axial replacement leads to the change of Ni-Ni distances, the shift of the fluorescence emission, and the reversible one-electron reduction process without oxidation process.


Results and Discussions
The unusual character of complex 1 is an unsymmetrical trinickel complex.The molecular structure of 1 is shown in Figure 1 and selected bond lengths and angles are listed in Table 1 The two axial ligands of 4-sb and water have potential hydrogen bonding ability, therefore the extended hydrogen bonding structure is a 3D architecture (Figure 2).The fluorescent spectrum of 1 shows emission bands at 413 nm, 438 nm, 453 nm and 470 nm (λ ex = 220 nm) in the solid state at room temperature (Figure 3).The maximal peak is at 413 nm.Comparison of emission bands between 1 and ligands suggests that the origin of these emissions is mainly caused by dpa -and 4-sb 2-ligands and their emission strength is stronger than those of free ligands.The strongest peaks in 1, [Ni 3 (dpa) 4 Cl 2 ] (421 nm, 452 nm, 470 nm) [11], and [Ni 3 (dpa) 4 (Cl)(ClO 4 )](CH 2 Cl 2 ) (417 nm, 453 nm, and 470 nm) are somewhat different and shifted, illustrating that different axial ligands truly affect the fluorescent properties.The electrochemical spectrum of 1 in DMSO solution with 0.1 M TBAP as the supporting electrolyte is shown in Figure 4. We noted that there is no oxidation process for 1 in the measurement range of 1.2~-1.2V and a reversible one-electron reduction process was found according to the explanation of Cotton and his co-workers [12].The reduction site is at -0.85 V and the E 1/2 is -0.726V.

Materials and synthesis
2,2'-Dipyridylamine (Hdpa), [Ni 3 (dpa) 4 Cl 2 ], and [Ni 3 (dpa) 4 (ClO 4 ) 2 ] were prepared according to previous reports [1,3,4].Other chemicals used in the experiments were obtained from commercial sources.IR spectrum was recorded by a Nicolet Nexus 470 infrared spectrophotometer in KBr pellet.Elemental analyses for C, H, and N were done on a Flash EA 1112 analyzer.The fluorescent spectrum was determined using a Shimadzu RF-50 spectrometer on a powdered sample in the solid state at room temperature.Cyclic voltammogram was carried out on a CHI 600C potentiostat and the detailed operation and experimental conditions were described in our previous reports [1,3,4].
So far, there is no common strategy to prepare unsymmetrical trinickel complexes.In the formation of 1, AgNO 3 is necessary because its absence does not lead to complex 1, instead a symmetric complex with two same axial ligands.The addition of other salts such as copper salt also could not precipitate the complex 1.

Crystallographic determination
Data collection for a suitable crystal of complex 1 were carried out using a Bruker SMART APEX CCD detector with graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å).The data frames were integrated by the SAINT program [14].Empirical absorption correction was done by the SADABS program [15].The structure was solved by the Patterson method and successive Fourier synthesis, and refined by the full-matrix least squares method on F 2 .All non-hydrogen atoms were anisotropically refined.Hydrogen atoms on carbon atoms were placed in calculated positions and refined as riding, with C-H = 0.93 Å and U iso (H) = 1.2 U eq (C).All of the programs used are included in the WinGX Suite version 1.70 [16].The drawings of the molecule were realized with the help of ORTEP-3 for Windows [17].
Further details of the crystal structure investigation may be obtained free of charge from http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or E-Mail: deposit@ccdc.cam.ac.uk.upon quoting the 796463 CCDC number.Detailed crystallographic data and structure refinements of complex 1 are listed in Table 2.

Figure 1 .
Figure 1.View of molecular structure of 1. H atoms and lattice water molecules are omitted for clarity.

Table 2 .
Crystallographic data and refinement parameters for complex 1.