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.
Description
Keywords
Radicals (Chemistry), Verdazyl radicals, Coordination chemistry, Pure sciences