Six-coordinate Co^III and four-coordinate M^II (M = Co, Zn) mixed-valence dimers supported by a deprotonated pyridine amide ligand: magnetism of a Co^IIICo^II complex and C-H···O/Cl/Br interactions

Abstract

Using a pyridine amide ligand 4-methyl-2-{N-(2-pyridyl)carbamoyl}pyridine (HL), in its deprotonated form, binuclear mixed-valence homonuclear bimetallic complexes [Co^III,II₂(L)₃(X)]X·S [X = Cl (1); X = Br, S = CH₃OH (2)] and heteronuclear bimetallic complex [Co^IIIZn^II(L)₃(Cl)]Cl·CH₃OH·5H₂O (3) have been synthesized. Structural analysis revealed that trivalent cobalt is in distorted octahedral and bivalent cobalt/zinc is in distorted tetrahedral environment. Three L(-) ligands provide six-coordination by utilizing three pyridine amide units (pyridine N and amide N donor set) in a facial mode, which in turn places three 4-methylpyridine nitrogens to coordinate to another metal center, which completes four-coordination by a chloride/bromide ion. Temperature-dependent magnetic susceptibility measurements in the solid state of a representative complex 1 revealed the spin-states as Co^III (S = 0) and Co^II (S = 3/2), with a zero-field splitting parameter for Co^II (g = 2.20 and D = 3.9 cm^-1; 2D is the energy gap between the two Kramers doublets M_s = |±3/2> and |±½>). To the best of our knowledge, this is for the first time that a pyridine amide ligand has been utilized to accommodate two cobalt ions or a cobalt and a zinc ion in different geometry, oxidation state, and spin-state (class I mixed-valence dimer). The crystal packing diagram reveals interesting non-covalent interactions involving C-H···O/Cl/Br, leading to the generation of 1D, 2D and 3D supramolecular architectures. Absorption spectral and redox properties of the complexes have also been investigated.