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

Abstract

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)(NO(3))(2)] (1) or [M(PIPYH)(2)](NO(3))(2) [M = Ni(II) (2) or Zn(II) (3)]. Complex synthesis in the presence of triethylamine yielded fully- or semideprotonated complexes [Cu(II)(PIPY)(NO(3))] (4), [Ni(II)(PIPYH)(PIPY)](NO(3)) (5), and [Zn(II)(PIPY)(2)] (6), respectively. Cobalt(II) nitrate is quantitatively oxidized under the reaction conditions to [Co(III)(PIPY)(2)](NO(3)) (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, κ-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 (1)H 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)(NO(3))]. Protic behavior of the nickel complexes 2 and 5 was investigated by UV/vis spectroscopy, revealing high π-backbonding ability of the PIPYH ligand resulting in an unexpected low acidity of the hydrazone proton in nickel complex 2.

Synthesis and characterization of diazine-ring containing hydrazones and their Zn(II) complexes

“…Intra ligand bond lengths have typical values for sp2 hybridized atoms and are in accordance with the literature data[24][25][26]. The N3-C6 and N6-C15 bonds have lengths that correspond to localized double bonds.…”

“…As the single crystal X-ray diffraction measurement confirmed that Hz•HCl exists in the diazine ring in -NH tautomeric form [20] it was expected to keep it in its Schiff base HzDPK, too. furthermore, the selected structural parameters are given in The CSD contains the structural data for three previously reported octahedral Zn(II) complexes with structurally related 1-hydrazinophthalazine and 3-chloro-6-hydrazinopyridazine based Schiff bases, refcodes: DITQOO [24] FARCEI [25] and CAJJAB [26]. In DITQOO and FARCEI structures, coordination mode of the tridentate ligands is analogous to that of HpDPK, while in CAJJAB both nitrogen atoms of the pyridazine ring are involved in coordination, thus forming a bridge between two metal atoms.…”

X‐ray structures, spectroscopic, electrochemical, thermal, antibacterial, and DFT studies of two nickel(II) and cobalt(III) complexes constructed from a new quinazoline‐type ligand

“…As shown in Figure (b), the cyclic voltammogram of 2 displays two reversible reduction waves, which are assigned to the consecutive of Co(III)/Co(II) and Co(II)/Co(I) redox process . The first pair of oxidation–reduction peaks corresponding to the oxidation–reduction couples Co(III)/Co(II), Epa 1 = −0.814 V, Epc 1 = −1.212 V, the average formal potential [E 1/2 = (Epa 1 + Epc 1 )/2] is −1.013 V, the peak‐to‐peak separation between the anodic and cathodic (ΔEp 1 ) is 0.916 V and the proportional of the peak current (ipc 1 = −2.96 μA) /(ipa 1 = 1.76 μA) is 1.42.…”

Two 1-D and 2-D cobalt(II) complexes: synthesis, crystal structures, spectroscopic and electrochemical properties

“…As shown in figure 8, the Co(II) complexes display two reversible redox waves, which are assigned to consecutive Co(II)/Co(I) and Co(III)/Co(II) redox processes [46]. The first pair of oxidationreduction peaks correspond to oxidation-reduction couples Co(III)/Co(II); Epa 1 = 0.107 V, Epc 1 = −0.829 V, with average formal potential [E 1/2 = (Epa 1 +Epc 1 )/2] is −0.358 V. The peak-to-peak separation between the anodic and cathodic processes (ΔEp 1 ) is 0.936 V and the proportion of the peak current (ipa 1 /ipc 1 ) is 0.621.…”