A novel bis-1,2,4-benzothiadiazine pincer ligand: synthesis, characterization and first row transition metal complexes

Abstract

Reaction of 2,6-dicyanopyridine with 2 equiv. of 2-(propylthio)benzenamine in the presence of lithium bis(trimethylsilyl)amide, followed by ring-closing oxidation with N-chlorosuccinimide affords the novel tridentate ligand, 2,6-bis-(1′,2′,4′-benzothiadiazinyl)pyridine (LH₂). Electrochemical studies on the free ligand LH₂ reveal a single well-defined 2e⁻ oxidation process with E_1/2 = +0.90 V. EPR studies of the in situ chemical oxidation of LH₂ reveal the generation of a benzotriazinyl radical. Reactions of ligand LH₂ with a range of divalent transition metal salts in either MeOH or MeCN in a 2 : 1 ratio at ambient temperatures afforded mononuclear complexes with general formula [M(LH₂)₂][X]₂ (M = Mn, X = CF₃SO₃ (1); Fe, X = CF₃SO₃ (2); Fe, X = BF₄ (3); Co, X = Cl (4); Ni, X = Cl (5); Zn, X = CF₃SO₃ (6)) and the 1 : 1 complex [Cu(LH₂)(NO₃)₂] (7). In all cases the LH₂ ligand binds in a tridentate N,N′,N′′ chelate fashion via benzothiadiazinyl N_BTDA and pyridyl N_py atoms. The low spin Fe^II complexes (2 and 3) were implemented for NMR and UV-Vis solution studies of ligand reactivity as well as cyclic voltammetry which reveal two 1e-oxidation waves. The metal complexes 1–6 are discussed and reveal a range of geometries between octahedral and trigonal prismatic with the greatest deviation from octahedral symmetry apparent for ions with no crystal field stabilisation energy, i.e. d¹⁰ Zn^II and high spin d⁵ Mn^II ions.