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

Homo- and heterotrinuclear complexes [LNi{M(Ln)}2](ClO4)2·H2O involving NiIIMII2 nonlinear cores (M = Ni, Cu, and Pd) (1–6) have been synthesized by a single-pot reaction when the oximato metal complexes [MLn(H2O)]ClO4 (HLn are tridentate oxime ligands), prepared in situ in methanol are allowed to react with the precursor nickel(II) complex [LNi(H2O)2] (H2L = 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-forbidden 3A2 → 1E transition (ε, 4–8 mol−1 cm2), 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 E1/2 = 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 E1/2 = −1.17, −1.35 and −1.62 V, the most cathodic one being due to the octahedral site.

A methodology has been developed for the synthesis of both homo- and heterotrinuclear complexes [LNi{M(Ln)}2](ClO4)2·H2O (1–6) involving Ni(II)M(II)2 (M = Ni, Cu, and Pd) nonlinear core. In the exchange-coupled Ni(II)Cu(II)2 complex 4, the metal centers are antiferromagnetically connected (JCu–Ni/kB = −119.6 (6) K). The weak spin-forbidden 3A2 → 1E transition in the Ni(II)M(II)2 (M = Ni(II), Pd(II)) complexes (1, 2 and 5, 6) undergoes ca. 25-fold intensity enhancement in the Ni(II)Cu(II)2 complexes (3 and 4) due to this exchange-coupling.Figure optionsDownload as PowerPoint slide