New Pyrazole-Based Ligands with Two Tripodal Binding Pockets: Potential Scaffolds for Metallobiosite Modeling

The synthesis of a new set of bioinspired dinucleating ligand scaffolds HL1−HL3 based on a bridging pyrazolate with appended chelate arms is reported. The ligands provide two binding compartments akin to the tris(imidazolyl)methane motif, predisposed to act as facially tridentate coordination caps. Potentiometric titrations of HL1 in the presence of Ni2+ and Zn2+ reveal formation of species with a metal:ligand ratio 1:1 in aqueous solution, and UV−vis data for the NiII system suggest that the complex [L12Ni2]2+ with {NiN6} chromophore is formed under appropriate pH conditions. In contrast, trinickel(II) complexes [L22Ni3(NO3)4(MeOH)2] (4) and [L22Ni3Cl2(MeOH)4]Cl2 (5) could be obtained from MeOH solutions and characterized crystallographically. The anticipated tripodal {N3} binding mode of the ligand is indeed realized for the central NiII ion, but the counteranions or MeOH solvent molecules lead to dissociation of one of the N donor legs for the outer NiII ions with formation of intramolecular H-bonds between a Ni-bound MeOH and the pyrazolate-N. X-ray crystals structures were also obtained for three CuI complexes [L32Cu4X2](PF6)2 with X = PMe3 (6), CNnBu (7), CNC6H3Me2-2,6 (8), where all CuI ions are three-coordinate in a distorted trigonal-planar arrangement. The two inner metals are bound to two imidazole-N from one ligand sidearm and a pyrazolate-N from the other ligand while the outer CuI ions are hosted by the pyrazolate-N and one imidazole-N from the nearby sidearm with the third coordination site filled by the coligand X. Spectroscopic and ESI-MS data suggest that the trinickel complexes stay intact even in coordinating solvents while the CuI complexes in solution are partly dissociated into their bimetallic constituents. The solid state structures observed for the oligonuclear complexes 4−8 are reminiscent of the coordination motifs previously found for related mononuclear complexes based on tripodal tris(imidazolyl)methane, which corroborates the description of HL1−HL3 as novel binucleating versions of such tris(imidazolyl)methane ligands.