Synthesis and Characterization of Axially Linked Heteroleptic Polymeric and Monomeric Lewis Base Adducts of bis-(2-mercaptobenzothiazolato) Nickel(II) and Its Extraordinary Thermal Decomposition

Abstract

Six coordinated axially bridged heteroleptic polymeric and monomeric Lewis base adducts from coordinatively unsaturated bis-(2-mercaptobenzothiazolato) nickel(II) {Ni(mbt)₂, [NiC₁₄H₈N₂S₄]} have been synthesized and charecterized by elemental analysis, spectral techniques, magnetic susceptibility measurements and thermal analysis using coordination sphere expansion method. These adducts have general molecular formula [Ni(mbt)₂L]n for polymeric adducts and [Ni(mbt)₂L*₂] for monomeric adducts, where L = pyrazine, 4,4'-bipyridine and 4,4'-azopyridine, L* = pyridine and 4-amino pyridine. The infrared spectral studies suggested that 2-mercaptobenzothiazole is bidentate coordinating through its nitrogen and sulphur atoms. The bidentate Lewis bases used were capable to act as bridging ligand coordinated to two metal atoms which resulted into octahedral species with MN₄S₂ chromophore but 4-aminopyridine is coordinating through its pyridyl nitrogen eventhough it was bidentate. The perceptible changes in the spectroscopic parameters like Dq, Nephelauxetic parameter (b) and Racah parameter (B) of all adducts were evaluated and explained. The variation of magnetic behaviour on adduct formation was also studied. Ni(mbt)₂ decomposes to yield NiS, a stable residue with Ni(SCN)₂ as one of the major intermediate but the adducts show an additional step due to the loss of Lewis bases leaving back the parent compound. The kinetic and thermodynamic parameters were also evaluated using Coats-Redfern equation. In most of the cases, it follows random nucleation, one nucleus on each particle obeying Mampel equation.