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)

Stable anilinyl radicals coordinated to nickel: X-ray crystal structure and characterization

Two anilinosalen and a mixed phenol-anilinosalen ligands involving sterically hindered anilines moieties were synthesized. Their nickel(II) complexes 1, 2, and 3 were prepared and characterized. They could be readily one-electron oxidized (E1/2=-0.30, -0.26 and 0.10V vs. Fc+/Fc, respectively) into anilinyl radicals species [1]+, [2]+, and [3]+, respectively. The radical complexes are extremely stable and were isolated as single crystals. X-ray crystallographic structures reveal that the changes in bond length resulting from oxidation do not exceed 0.02Å within the ligand framework in the symmetrical [1]+ and [2]+. No quinoid bond pattern was present. In contrast, larger structural rearrangements were evidenced for the unsymmetrical [3]+, with shortening of one Cortho-Cmeta bond. Radical species [1]+ and [2]+ exhibit a strong absorption band at around 6000cm-1 (class III mixed valence compounds). This band is significantly less intense than [3]+, consistent with a rather localized anilinyl radical character, and thus a classification of this species as class II mixed-valence compound. Magnetic and electronic properties, as well as structural parameters, have been computed by DFT methods

Structural and Spectroscopic Investigation of an Anilinosalen Cobalt Complex with Relevance to Hydrogen Production

A Co(II) anilinosalen catalyst containing proton relays in the first coordination sphere has been synthesized that catalyzes the electrochemical production of hydrogen from acid in dichloromethane and acetonitrile solutions. The complex has been spectroscopically and theoretically characterized in different protonation and redox states. We show that both coordinated anilido groups of the neutral Co(II) complex can be protonated into aniline form. Protonation induces an anodic shift of more than 1 V of the reduction wave, which concomitantly becomes irreversible. Hydrogen evolution that originates from the aniline protons located in the first coordination sphere is observed upon bulk electrolysis at −1.5 V of the protonated complex in absence of external acid. Structures for intermediates in the catalytic reaction have been identified based on this data

Ligand contributions to the electronic structures of the oxidized cobalt(II) salen complexes

Square planar cobalt(II) complexes of salen ligands N,N′-bis(3-tert-butyl-5R-salicylidene)-1,2-cyclohexanediamine), where R = OMe (1) and tert-butyl (2), were prepared. 1 and 2 were electrochemically reversibly oxidized into cations [1-H2O]+ and [2-H2O]+ in CH2Cl2. The chemically generated [1-H2O](SbF6)*0.68 H2O*0.82CH2Cl2 and [2-H2O](SbF6)*0.3H2O*0.85CH2Cl2 were characterized by X-ray diffraction and NIR spectroscopy. Both complexes are paramagnetic species containing a square pyramidal cobalt ion coordinated at the apical position by an exogenous water molecule. They exhibit remarkable NIR bands at 1220 (7370 M-1 cm-1) and 1060 nm (5560 M-1 cm-1), respectively, assigned to a CT transition. DFT calculations and magnetic measurements confirm the paramagnetic (S = 1) ground spin state of the cations. They show that more than 70% of the total spin density in [1-H2O]+ and [2-H2O]+ is localized on the metal, the remaining spin density being distributed over the aromatic rings (30% phenoxyl character). In the presence of N-methylimidazole 1 and 2 are irreversibly oxidized by air into the genuine octahedral cobalt(III) bis(phenolate) complexes [1-im2]+ and [2-im2]+, the former being structurally characterized. Neither [1-im2]+ nor [2-im2]+ exhibits a NIR feature in its electronic spectrum. 1 and 2 were electrochemically two-electron oxidized into [1]2+ and [2]2+. The cations were identified as Co(III)-phenoxyl species by their characteristic absorption band at ca. 400 nm in the UV-vis spectrum. Coordination of the phenoxyl radical to the cobalt(III) metal ion is evidenced by the EPR signal centered at g = 2.00

One-electron oxidized copper(II) salophen complexes: phenoxyl versus diiminobenzene radical species

Where is the radical? The π-system of the diiminobenzene bridge adds a putative oxidation site to prophenoxyl copper(II) salophen complexes. It is shown that the bridge is effectively redox active and that the radical site (phenoxyl vs. diiminobenzene bridge) in the above cations is tuned by selective methoxy substitution (see figure)

Radical localization in a series of symmetric Ni^II complexes with oxidized salen ligands

Square-planar nickel(II) complexes of salen ligands, N,N′-bis(3-tert-butyl-(5R)-salicylidene)-1,2-cyclohexanediamine), in which R=tert-butyl (1), OMe (2), and NMe2 (3), were prepared and the electronic structure of the one-electron-oxidized species [1-3]+. was investigated in solution. Cyclic voltammograms of [1-3] showed two quasi-reversible redox waves that were assigned to the oxidation of the phenolate moieties to phenoxyl radicals. From the difference between the first and second redox potentials, the trend of electronic delocalization 1+.>2+.>3+. was obtained. The cations [1-3]+. exhibited isotropic g tensors of 2.045, 2.023, and 2.005, respectively, reflecting a lower metal character of the singly occupied molecular orbital (SOMO) for systems that involve strongly electron-donating substituents. Pulsed-EPR spectroscopy showed a single population of equivalent imino nitrogen atoms for 1+., whereas two distinct populations were observed for 2+.. The resonance Raman spectra of 2+. and 3+. displayed the ν8a band of the phenoxyl radicals at 1612 cm−1, as well as the ν8a bands of the phenolates. In contrast, the Raman spectrum of 1+. exhibited the ν8a band at 1602 cm−1, without any evidence of the phenolate peak. Previous work showed an intense near-infrared (NIR) electronic transition for 1+. (Δν1/2=660 cm−1, ε=21 700 M−1 cm−1), indicating that the electron hole is fully delocalized over the ligand. The broader and moderately intense NIR transition of 2+. (Δν1/2=1250 cm−1, ε=12 800 M−1 cm−1) suggests a certain degree of ligand-radical localization, whereas the very broad NIR transition of 3+. (Δν1/2=8630 cm−1, ε=2550 M−1 cm−1) indicates significant localization of the ligand radical on a single ring. Therefore, 1+. is a Class III mixed-valence complex, 2+. is Class II/III borderline complex, and 3+. is a Class II complex according to the Robin-Day classification method. By employing the Coulomb-attenuated method (CAM-B3LYP) we were able to predict the electron-hole localization and NIR transitions in the series, and show that the energy match between the redox-active ligand and the metal d orbitals is crucial for delocalization of the radical SOMO