Insights into Different Donor Abilities in Bis(pyrazolyl)pyridinylmethane Transition Metal Complexes.

We synthesised and characterised eleven complexes of the biomimetic bis(pyrazolyl)methanes (2-pyridinyl)bis(pyrazolyl)methane [HC(Pz)2(Py)] and (2-quinolinyl)bis(pyrazolyl)methane [HC(Pz)2(Qu)] with the transition metals cobalt, iron, copper and zinc. With the pyridinyl ligand HC(Pz)2(Py), we obtained the bifacial complexes [Co{HC(Pz)2(Py)}2]Cl2 ·2CH3CN ( C1), [Co{HC(Pz)2(Py)}2][CF3SO3]2 ( C2), [Fe{HC(Pz)2(Py)}2]Cl2 ·2CH3OH [ C3], [Cu{HC(Pz)2(Py)}2]Cl2 ·2CH3OH ( C4a) and [Cu{HC(Pz)2(Py)}2][CuCl3H2O] ( C4b) and the monofacial complex [CuCl2{HC(Pz)2(Py)}] ·CH3OH ( C5); with the quinolinyl ligand HC(Pz)2(Qu), we obtained the bifacial complexes [Co{HC(Pz)2(Qu)}2][CoCl4] ·4CH3CN ·0.88CH2Cl2 ( C6) and [Fe{HC(Pz)2(Qu)}2][FeCl4] ·2CH3CN ( C7), the monofacial complexes [FeCl2{HC(Pz)2(Qu)}] ( C8) and [CuBr2{HC(Pz)2(Qu)}] ( C9) and the bifacial zinc complex [Zn{HC(Pz)2(Qu)}2][ZnCl4] ( C10). We explored the donor properties of the free ligands and the ligands in these complexes and found that the delicate donor competition is an intriguing field. By second-order perturbation theory, pyrazolyl units are the stronger donors, although pyridinyl units are more basic. Isodesmic reactions indicate that the pyridinyl donors give a stronger complex stabilisation. This small bias can be disturbed by subtle variations of the coordination geometry, which is important in the bioinorganic context. [ABSTRACT FROM AUTHOR]