Synthesis and Electronic Structure Determination of Uranium(VI) Ligand Radical Complexes

   Pentagonal bipyramidal uranyl complexes of salen ligands, N,N’-bis(3-tert-butyl-(5R)-salicylidene)-1,2-phenylenediamine, in which R = tBu (1a), OMe (1b), and NMe2 (1c), were prepared and the electronic structure of the one-electron oxidized species [1a-c]+ were investigated in solution. The solid-state structures of 1a and 1b were solved by X-ray crystallography, and in the case of 1b an asymmetric UO22+ unit was found due to an intermolecular hydrogen bonding interaction. Electrochemical investigation of 1a-c by cyclic voltammetry showed that each complex exhibited at least one quasi-reversible redox process assigned to the oxidation of the phenolate moieties to phenoxyl radicals. The trend in redox potentials matches the electron-donating ability of the para-phenolate substituents. The electron paramagnetic resonance spectra of cations [1a-c]+ exhibited gav values of 1.997, 1.999, and 1.995, respectively, reflecting the ligand radical character of the oxidized forms, and in addition, spin-orbit coupling to the uranium centre. Chemical oxidation as monitored by ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy afforded the one-electron oxidized species. Weak low energy intra-ligand charge transfer (CT) transitions were observed for [1a-c]+ indicating localization of the ligand radical to form a phenolate / phenoxyl radical species. Further analysis using density functional theory (DFT) calculations predicted a localized phenoxyl radical for [1a-c]+ with a small but significant contribution of the phenylenediamine unit to the spin density. Time-dependent DFT (TD-DFT) calculations provided further insight into the nature of the low energy transitions, predicting both phenolate to phenoxyl intervalence charge transfer (IVCT) and phenylenediamine to phenoxyl CT character. Overall, [1a-c]+ are determined to be relatively localized ligand radical complexes, in which localization is enhanced as the electron donating ability of the para-phenolate substituents is increased (NMe2 > OMe > tBu)

Vanadium(iii) phenoxyimine complexes for ethylene or ε-caprolactone polymerization: mononuclear versus binuclear pre-catalysts

The mononuclear {[C6H4NCH(ArO)]2VCl(THF)} (Ar = 2,4-t-Bu2C6H2 (1), Ar = C 6H4 (2)), {O[C6H4NCH(ArO)] 2}VCl(THF) (Ar = 2,4-t-Bu2C6H2 (3), Ar = C6H4 (4)) and the binuclear vanadium(iii) complexes {[C6H4NCH(ArO)]VCl2(THF)2} 2(μ-CH2CH2) (Ar = 2,4-t-Bu2C 6H2 (5), Ar = C6H4 (6)), have been synthesized and fully characterized. The compounds [C6H 5NCH(ArO)]VCl2(THF)2 (Ar = 2,4-t-Bu 2C6H2 (7), Ar = C6H4 (8)), [2,4,6-Me3-C6H2NCH(ArO)]VCl2 (Ar = 2,4-t-Bu2C6H2 (9), Ar = C 6H4 (10)) and [2,6-i-Pr2-C6H 3NCH(ArO)]VCl2(THF)2 (Ar = 2,4-t-Bu 2C6H2 (11), Ar = C6H4 (12)), {μ-CH2CH2[NCH(C6H4O)] 2VCl(THF)} (14) and {C6H4[NCH(C 6H4O)]2VCl(THF)} (15) were synthesized for comparative polymerization studies. The dizwitterionic compound [2,6-i-Pr 2-C6H3N+(H)CH(C6H 4O)]2VCl2O (13) was also isolated, and presumably formed via a fortuitous hydrolysis reaction. The complexes 2, 5 and 13 have been structurally characterized; the molecular structure of the parent ligand (L5) in 5 is also reported. All complexes have been screened for ethylene as well as ε-caprolactone polymerization, and results are compared against those for known related mono- and bi-nuclear counterparts to evaluate for possible cooperative effects. The compounds 10 and 12 have been supported on modified SiO2, analysed by XPS and subjected to homo-polymerization (ethylene) and co-polymerization (1-hexene and ethylene) studies. © The Royal Society of Chemistry 2013