New Zinc Complexes bearing κ2-Heteroscorpionate Ligands: Influence of Second-Sphere Bonding Interactions on Reactivity and Properties

New zinc complexes (LOMe)ZnCl2 (1) and (LOH)ZnCl2(2) of the heteroscorpionate ligands 1-[(3,5-di-terf-butyl-2-methoxyphenyl)(3,5-dimethyl-pyrazol-1-yl)methyl)]-3,5-dim ethyl-pyrazole (LOMe) and 2,4-di-ter1-butyl-6-[bis(3,5dimethyl-pyrazol-1-yl)methyl]phenol (LOH) have been synthesized. The X-ray molecular structure of 2 was reported and compared with the one of the iron(II) complex (LOH)Fel(2) (3). The complexes 2-3 adopt a tetrahedral structure in the solid state in which the LOH ligand is kappa(2)-coordinated to the metal via the imino nitrogens of the two pyrazolyl rings. The hydroxyl phenyl group is not coordinated to the metal but found to be involved in an intermolecular hydrogen bond. The solution structures of 1 and 2 are consistent with this tetrahedral C-S symmetric geometry. Dilution and H-1-H-1 Nuclear Overhauser Effect Spectroscopy (NOESY) experiments revealed that the free ligands LOMe and LOH are involved in intra- and intermolecular hydrogen bonding interactions. Coordination of LOMe and LOH to ZnCl2 was investigated by NMR titration methods. Association constants (K-a) of (8.6 +/- 0.4) x 10(2) M-1 and (7.8 +/- 0.3) x 10(2) M-1 were obtained in methanol/water solutions (95:5) for LOMe and LOH, respectively. Coordination of bis(3,5-dimethylpyrazol-1-yl)methane (bpm) ligand to ZnCl2 is weaker, as evidenced by the lower value of the association constant (5.3 +/- 0.3) x 10(2) M-1. When bpm was added to solutions of 1 or 2, an equilibrium shifted toward the (bmp)ZnCl2 species was observed. The thermodynamic parameters for this reaction were determined by VT NMR analysis. The optical properties of the ligands (LOMe, LOH) and of the corresponding zinc complexes I and 2 were also investigated by means of UV-vis and fluorescence spectroscopy to assess the potential use of these ligands as fluorescent sensors for Zn2+ detection.