Targeted syntheses of homo- and heterotrinuclear complexes involving M~(II)-Ni~(II)-M~(II) (M = Ni, Cu, and Pd) nonlinear core: Structure, spectroscopy, magnetic and redox studies

摘要

Homo- and heterotrinuclear complexes [LNi{M(L~n)}_2](ClO_4)_2·H_2O involving Ni~(II)M~(II) _2 nonlinear cores (M = Ni, Cu, and Pd) (1-6) have been synthesized by a single-pot reaction when the oximato metal complexes [ML~n(H_2O)]ClO_4 (HL~n are tridentate oxime ligands), prepared in situ in methanol are allowed to react with the precursor nickel(II) complex [LNi(H_2O)_2] (H_2L = N,N'-dimethyl-N,N'-bis(2-hydroxy-3,5- dimethylbenzyl)ethylenediamine). Single crystal X-ray diffraction analysis, and ESI-MS spectroscopy have been used to establish their identities which involve an octahedral Ni(II) site flunked by two metal-oximate moieties, each in a square planar environment. The electronic and molecular structures of these compounds are interesting due to a synergistic bonding mechanism operative through the deprotonated oxime and the phenolate oxygen atoms via the metal centers. Thus the weak spin-forbid den~3A_2→~1E transition (ε, 4-8 mol~(-1) cm~2), characteristic of the octahedral Ni(II) center in Ni(II)-Ni(II)_2 (1, 2) and Ni(II)-Pd(II)_2 complexes (5, 6), gains intensity by ca. 25-fold in the spin-coupled Ni(II)-Cu(II)_2 complexes (3, 4) in which the metal centers are connected antiferromagnetically through a moderately strong coupling (JCu-Ni/kB = -119.6(6) K). In the presence of excess pyridine (5 ×10~(-2) M), the trinickel compound (1) in acetonitrile displays a pair of quasi-reversible oxidations at E_(1/2) = 0.48 and 0.69 V (vs. Ag/AgCl reference at a high scan speed, 1000-3000 mVs~(-1)) due to Ni(II/III) oxidations at the square planar nickel sites. Coordinated oxime helps in stabilizing these Ni centers in +3 oxidation state in the time-scale of cyclic voltammetry possibly through the formation of octahedral geometry with the added pyridine as axial ligands. In the cathodic range, all the three Ni centers undergo Ni(II/I) reductions at E_(1/2) = -1.17, -1.35 and -1.62 V, the most cathodic one being due to the octahedral site.