Pyridazine- versus Pyridine-Based Tridentate Ligands in First-Row Transition Metal Complexes

A series of first-row transition metal complexes with the unsymmetrically disubstituted pyridazine ligand picolinaldehyde (6-chloro-3-pyridazinyl)hydrazone (PIPYH), featuring an easily abstractable proton in the backbone, was prepared. Ligand design was inspired by literature-known picolinaldehyde 2-pyridylhydrazone (PAPYH). Reaction of PIPYH with divalent nickel, copper, and zinc nitrates in ethanol led to complexes of the type [Cu-II(PIPYH)(NO3)(2)] (1) or [M(PIPYH)(2)](NO3)(2) [M = Ni-II (2) or Zn-II (3)]. Complex synthesis in the presence of triethylamine yielded fully- or semideprotonated complexes [Cu-II(PIPY)(NO3)] (4), [Ni-II(PIPYH)(PIPY)] (NO3) (5), and [Zn-II(PIPY)(2)] (6), respectively. Cobalt(II) nitrate is quantitatively oxidized under the reaction conditions to [Co-III(PIPY)(2)](NO3) (7) in both neutral and basic media. X-ray diffraction analyses reveal a penta- (1) or hexa-coordinated (2, 3, and 7) metal center surrounded by one or two tridentate ligands and, eventually, kappa-O,O' nitrate ions. The solid-state stoichiometry was confirmed by electron impact (EI) and electrospray ionization (ESI) mass spectrometry. The diamagnetic complexes 5 and 6 were subjected to H-1 NMR spectroscopy, suggesting that the ligand to metal ratio remains constant in solution. Electronic properties were analyzed by means of cyclic voltammetry and, in case of copper complexes 1 and 4, also by electron paramagnetic resonance (EPR) spectroscopy, showing increased symmetry upon deprotonation for the latter, which is in accordance with the proposed stoichiometry [Cu-II(PIPY)(NO3)]. Protic behavior of the nickel complexes 2 and 5 was investigated by UV/vis spectroscopy, revealing high pi-backbonding ability of the PIPYH ligand resulting in an unexpected low acidity of the hydrazone proton in nickel complex 2.