Heteroscorpionate-Based Co2+, Zn2+, and Cu2+ Complexes: Coordination Behavior, Aerobic Oxidation, and Hydrogen Sulfide Detection

The coordination behavior and reactivity of the phenol-substituted bis(pyrazolyl)methane ligands, (3,5-tBu2-2-phenol)bis(3,5-Me2-pyrazol-1-yl)methane (L1-H) and 2-phenol-bis(3,5-Me2-pyrazol-1-yl)methane (L2-H) have been investigated in the metal complexes (L1-H)CoCl2 (1), (L1-H)ZnCl2 (2), (L3)CuCl2 (3), (L2)2Co2Cl2 (4) (L2-H)ZnCl2 (5), and (L2-H)CuCl2·H2O (6). The mononuclear tetrahedral cobalt complex 1 was isolated and fully characterized by X-ray single crystal diffraction and 1H NMR spectroscopy and relaxometry. The neutral L1-H is κ2-coordinated to the metal center whereas the not coordinated hydroxy-phenyl group is involved in extended intermolecular hydrogen bonds. Aerobic oxidation of L1-H was observed in the reaction of this ligand with CuCl2 to yield the para-quinone derivative L3 (L3 = 2-tBu-6-(bis(3,5-Me2-pyrazol-1-yl)methyl)cyclohexa-2,5-diene-1,4-dione). Upon oxidation L3 resulted κ2-coordinated to the tetrahedral Cu(II) metal center, affording 3. The reaction of L2-H with CoCl2·6H2O produced the elimination of 1 equiv of hydrochloric acid and the formation of the binuclear complex 4 in which one cobalt is in an octahedral environment featuring two κ3-coordinated deprotonated ligands whereas the second cobalt center is detected in tetrahedral coordination geometry, bound to the octahedral cobalt via two phenoxo bridging moieties. Interestingly L2-H, (3-tBu-2-phenol)bis(3,5-Me2-pyrazol-1-yl)methane (L4-H), or (5-tBu-2-phenol)bis(3,5-Me2-pyrazol-1-yl)methane (L5-H) were not oxidized in the reaction with CuCl2. The reaction of the ligand L2-H with ZnCl2 and CuCl2·2H2O yielded the κ2-coordinated tetrahedral complex 5 and the square planar complex 6, respectively. The application of the cobalt complex 1 as molecular dosimeter for H2S was explored and compared to that of the zinc analogue 2. Density functional theory (DFT) calculations and NMR experiments to assess the possible mechanisms of H2S detection by both 1 and 2 are also described.