Hemilability of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands: Imidazole and oxazole ring Lewis basicity, Ni(II)/Pd(II) complex structures and spectra

Fourteen new organic molecules A1-A4, B1-B5, C1-C4 and D and a series of transition metal(II) complexes (Ni1-Ni9 and Pd1-Pd2b) were synthesized and studied in order to characterize the hemilability of 2-(1H-imidazol-2-yl)pyridine and 2-(oxazol-2-yl)pyridine ligands (A1-A4 = 2-R-2-6-(4,5-diphenyl-1R(1)-imidazol-2-yl)pyridines, R-1 = H or CH3, R-2 = H or CH3; B1-B5 = 1-R-2-2-(pyridin-2-yl)-1R(1)-phenanthro[9,10-d]imidazoles/oxazoles, R-1 = H or CH3, R-2 = H or CH3; C1-C4 = 2-(6-R-2-pyridin-2-yl)-1H-imidazo/oxazo[4,5-f][1,10]phenanthrolines, R-2 = H or CH3; D = 2-mesityl-1H-imidazo[4,5-f][1,10]phenanthroline). They were also used to study the substituent effects on the donor strengths as well as the coordination chemistries of the imidazole/oxazole fragments of the hemilabile ligands. All the observed protonationdeprotonation processes found within pH 114 media pertain to the imidazole or oxazole rings rather than the pyridyl Lewis bases. The donor characteristics of the imidazole/oxazole ring can be estimated by spectroscopic methods regardless of the presence of other strong N donor fragments. The oxazoles possessed notably lower donor strengths than the imidazoles. The electron-withdrawing influence and capacity to hinder the azole base donor strength of 4,5-azole substituents were found to be in the order phenanthrenyl (B series) > 4,5-diphenyl (A series) > phenanthrolinyl (C series). An X-ray structure of Ni5b gave evidence for solvent induced ligand reconstitution while the structure of Pd2b provided evidence for solvent induced metalligand bond disconnection. Interestingly, alkylation of 1H-imidazoles did not necessarily produce the anticipated push of electron density to the donor nitrogen. Furthermore, substituents on the 4,5-carbons of the azole ring were more important for tuning donor strength of the azole base. DFT calculations were employed to investigate the observed trends. It is believed that the information provided on substituent effects and trends in this family of ligands will be useful in the rational design and synthesis of desired azole-containing chelate ligands, tuning of donor properties and application of this family of ligands in inorganic architectural designs, template-directed coordination polymer preparations, mixed-ligand inorganic self-assemblies, etc. (c) 2010 Elsevier Ltd. All rights reserved.