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Crystal Structures and Spectroscopic and Theoretical Properties of Pentacoordinate Nickel(II) Complexes Containing Tris(pyrazolyl)borate and Quinolinate Ligands



A series of quinolinate nickel–tris(pyrazolyl)borate complexes [Ni(Tp*)(N,O-2,5,7-R1,R2,R3-8-hq)] {Tp* = hydrotris(3,5-dimethylpyrazolyl)borate, 8-hq = 8-hydroxyquinoline} were synthesized and characterized by spectroscopic methods. Single-crystal X-ray studies indicate that these complexes exhibit a pentacoordinate arrangement with chelating quinolinate and hydrotris(3,5-dimethylpyrazolyl)borate ligands. The substituents on the quinolinate generate variations in the absorption and emission wavelengths. Substituents in the 5,7-positions and a 2-CN substituent in the quinolinate induce a bathochromic shift with respect to the unsubstituted derivative; however, 2-Me and 2-NH2 substituents cause a blueshift. The UV/Vis spectra were investigated by time-dependent (TD)-DFT on the basis of the optimized geometries; further calculations were performed to identify the main contributions to the corresponding molecular orbitals. These nickel(II) complexes show emissions between 458 and 561 nm in the solid state. The experimental results in combination with the theoretical approaches led us to propose that the lowest-energy absorption can be assigned to both ligand-to-metal charge-transfer and intraligand charge-transfer (ILCT) transitions for the 2-Me- and 2-NH2-substituted derivatives. Nevertheless, for the 5,7-substituted and 2-CN derivatives the transitions are mainly assigned to ILCT transitions with an admixture of metal orbitals. TD-DFT calculations indicate that the contribution of the metal to the transitions is one of the essential factors.