Synthesis and coordination chemistry of chelating verdazyl radicals

Date

2018-11-08

Authors

Lemaire, Martin Trent

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Abstract

A series of new 1.5-dimethyI-6-oxo-verdazyl radicals was prepared bearing substituents such as 2-pyridyl, 2.2'-bipyridyl, and 2.2'-bipyrimidine, among others, in the 3 position of the verdazyl ring, generating a viable family of new open-shell chelating ligands. All verdazyl radical precursors (including aldehydes and tetrazanes) were characterized using elemental analysis, MS, 1H/13C NMR and FT-IR spectroscopies. The radicals were obtained by oxidation of the tetrazane precursors with benzoquinone. NalO4, or K3Fe(CN)6, and were isolated in pure crystalline form (in two cases as 1:1 molecular complexes with the hydroquinone reaction by-product) or as powders. Crystalline radicals exhibited indefinite stability to ambient conditions, but the radicals obtained as powders were immediately characterized and used in subsequent coordination reactions as a result of decomposition. All verdazyl radicals were characterized by EPR, UV-visible, FT-IR spectroscopies, and high-resolution MS. In selected cases, variable temperature magnetic susceptibility data were obtained and fairly strong intermolecular antiferromagnetic interactions were observed. Based on structural and magnetic data, as well as DFT calculations, we postulate a new intermolecular exchange mechanism for organic solids analogous to metal-oxide-based superexchange. A wide-variety of homoleptic and heteroleptic mono- and bimetallic coordination complexes with diamagnetic and paramagnetic transition metal salts were prepared with the verdazyl ligands described above. Metal-verdazyl complexes were structurally characterized, in many cases using X-ray crystallography, and in all cases by elemental analysis, and by MS, FT-IR, and UV-visible spectroscopies. Variable temperature magnetic susceptibility data indicated very strong ferromagnetic intramolecular nickel(II)-verdazyl exchange coupling in a wide-range of complexes (JNi-vd ranging from +35 to ≥ +240 cm-1). Antiferromagnetic manganese(11)-verdazyl exchange was observed in a number of complexes, generally around -50 cm-1 in magnitude. Exchange couplings between verdazyls and nickel(II) or manganese(II) centres was rationalized with established molecular orbital symmetry arguments. Cobalt(II)-verdazyl exchange coupling was tentatively assigned as ferromagnetic, however, no quantitative conclusions were drawn owing to other complications associated with cobalt(II) magnetochemistry. coupling through diamagnetic zinc(II) and copper(I) metals between coordinated verdazyls was weakly antiferromagnetic in all cases. Ruthenium(II)-verdazyl complexes were prepared as mimics of well known Ru(bipy)3 2+ complexes and the electronic properties (including absorption and emission spectroscopy, EPR, and CV) of these complexes were investigated. Interesting absorption features, including CT transitions between ruthenium(Il) centres and verdazyl ligands, and weak emission upon excitation into the ruthenium(II)-bipy CT band, were observed from these complexes at room temperature. Complicated EPR spectra were obtained from selected ruthenium(II)-verdazyl complexes, suggesting some interaction with the ruthenium(ll) centre, but no quantitative conclusions were drawn. CV data suggest facile verdazyl reductions and oxidative stability of the verdazyl ligand in selected complexes.

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Keywords

Radicals (Chemistry), Verdazyl radicals, Coordination chemistry, Pure sciences

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