Chandan Mukherjee

Biomimetic metal-radical reactivity: aerial oxidation of alcohols, amines, aminophenols and catechols catalyzed by transition metal complexes.

Abstract The contributions of the authors to the research program ‘Radicals in Enzymatic Catalysis’ over the last ca. 5 years are summarized. Significant efforts were directed towards the design and testing of phenol-containing ligands for synthesizing radical-containing transition metal complexes as potential candidates for catalysis of organic substrates like alcohols, amines, aminophenols and catechols. Functional models for different copper oxidases, such as galactose oxidase, amine oxidases, phenoxazinone synthase and catechol oxidase, are reported. The copper complexes synthesized can mimic the function of the metalloenzymes galactose oxidase and amine oxidases by catalyzing the aerial oxidation of alcohols and amines. Even methanol could be oxidized, albeit with a low conversion, by a biradical-copper(II) compound. The presence of a primary kinetic isotope effect, similar to that for galactose oxidase, provides compelling evidence that H-atom abstraction from the α-C-atom of the substrates is the rate-limiting step. Although catechol oxidase and phenoxazinone synthase contain copper, manganese(IV) complexes containing radicals have been found to be useful to study synthetic systems and to understand the naturally occurring processes. An ‘on-off’ mechanism of the radicals without redox participation from the metal centers seems to be operative in the catalysis involving such metal-radical complexes.

Oxidation of an o-Iminobenzosemiquinone Radical Ligand by Molecular Bromine: Structural, Spectroscopic, and Reactivity Studies of a Copper(II) o-Iminobenzoquinone Complex

The bis(o-iminobenzosemiquinonato)copper(II) complex 1, containing the radical form [L(*)SQ](1-) arising from the aerial oxidation of the noninnocent ligand 2-anilino-4,6-di- tert-butylphenol, H2L, is readily oxidized by molecular bromine to a bis(o-iminobenzoquinone)copper(II) complex, 2. Thus, a ligand-based oxidative addition is reported for complex 1 containing an electron-rich Cu(II) d(9) metal ion. The crystal structure of the synthesized hexacoordinated complex [Cu(II)(LBQ)2Br2] (2) has been determined by X-ray crystallography at 100 K. Variable-temperature (2-290 K) magnetic susceptibility measurements and an X-band electron paramagnetic resonance spectrum establish the spin state to be St = 1/2 because of localized spin moments mainly in the (d(x(2)-y(2)))(1) orbital of a Cu(II) d(9) ion, indicating clearly the presence of a neutral iminobenzoquinone form, [LBQ](0), of the ligand in 2, as is found also in the X-ray structure. Electrochemical measurements (cyclic voltammograms and coulometry) indicate two successive one-electron reductions of the ligand. The reactivity of complex 2 as an oxidizing agent toward ethanol and triethylamine has been investigated.

Trinuclear C3-Symmetric Extension of Jacobsen’s Catalyst: Synthesis, Characterization, and Catalytic Properties of a Chiral Trinuclear MnIII Triplesalen Complex

The synthesis of a chiral version of a triplesalen ligand has been performed in two steps starting from 2,4,6-triacetyl-1,3,5-trihydroxybenzene (1). Reaction with excess trans-(1R,2R)-1,2-cyclohexanediamine and trans-(1S,2S)-1,2-cyclohexanediamine provided the chiral triplesalen half units 2,4,6-tris[1-((1R,2R)-2-aminocyclohexylimino)ethyl]-1,3,5-trihydroxybenzene (2(RR)) and 2,4,6-tris[1-((1S,2S)-2-aminocyclohexylimino)ethyl]-1,3,5-trihydroxybenzene (2(SS)), respectively. The two enantiomeric pure triplesalen ligands H(6)chand(RR) and H(6)chand(SS) were obtained by reaction of the triplesalen half units with 3,5-di-tert-butylsalicylaldehyde. Reaction with MnCl(2) x 2 H(2)O under basic aerobic conditions afforded the chiral trinuclear triplesalen complexes 3(RR) and 3(SS). Single-crystal X-ray diffraction studies on both enantiomers showed the presence of the two ionization isomers [(chand){Mn(III)Cl(MeOH)}(3)] and [(chand){Mn(III)(MeOH)(2)}(3)]Cl(3) in the solid state, resulting in the formulation of 3 (either 3(RR) or 3(SS)) as [(chand){Mn(III)Cl(MeOH)}(3)][(chand){Mn(III)(MeOH)(2)}(3)]Cl(3). The crystal structures exhibit chiral hydrophobic channels of approximately 8 A diameter decorated with tert-butyl groups. These form left-handed helices in the 3(RR) enantiomer and right-handed helices in the 3(SS) enantiomer. Magnetic measurements are in accord with weak exchange interactions between the Mn(III) S(i) = 2 ions and strong local magnetic anisotropy as has been found in other trinuclear Mn(III)(3) triplesalen complexes. As a proof-of principal, we have investigated the catalytic ability of the two enantiomers in the enantioselective epoxidation of unfunctionalized olefins. The chiral trinuclear Mn(III)(3) triplesalen acts under non-optimized conditions as a catalyst with relatively good yields and moderate enantiomeric excess.

Ortho-substituent induced triradical-containing tetranuclear oxo-vanadium(IV) cluster formation via ligand C–N bond breaking and C–O bond making

Substituent having weak-coordination character, and attached at the ortho-carbon atom to the aniline moiety of 2-anilino-4,6-di-tert-butylphenol, provided a triradical-containing tetranuclear vanadium(IV) complex via ligand C-N bond breaking and C-O bond making.

Effect of Geometrical Distortion on the Electronic Structure: Synthesis and Characterization of Monoradical-Coordinated Mononuclear Cu(II) Complexes

Ligand H3Sami(Mixed(tBu)) was composed of two different compartments, a redox-active 2-aminophenol and a salen salicylidene. Both compartments were linked via a benzyl linker. The ligand reacted with CuCl2·2H2O under air in the presence of Et3N and provided the corresponding monoradical-coordinated mononuclear Cu(II) complex (1). Complex 1, in solution, reacted with air and provided complex 2 via ligand-centered oxygenation at the benzyl-CH2 position. Both complexes were characterized via IR, mass spectrometry, X-ray single-crystal diffraction, variable-temperature magnetic susceptibility, cyclic voltammograms (CVs), and UV-vis/NIR spectroscopic techniques. X-ray crystallographic analyses clearly showed almost equally distorted square planar geometry around the Cu(II) atom in both complexes. However, the bending of the radical-containing C6 ring compared to the N1-Cu1-O1 plane was different in both complexes. While complex 1 was paramagnetic and showed a ferromagnetic coupling between the d(x(2)-y(2)) magnetic orbital of Cu(II) ion and the p(z) orbital of coordinated π-radical, complex 2 was diamagnetic by experiencing a strong antiferromagnetic coupling between the two magnetic orbitals. UV-vis/NIR spectra of the complexes were dominated by charge-transfer transitions. CVs of the complexes showed two reversible one-electron oxidations and one reversible one-electron reduction. E(1/2)(ox2) and E(1/2)(red1) potentials were different in both complexes, while E(1/2)(ox1) values were almost the same and the process corresponded to the formation of phenoxyl radical. Theoretical studies were also performed to understand the magnetic coupling phenomena, and TD-DFT calculations were employed for the assignment of charge-transfer absorption bands.

Copper(II)‐Mediated Transformation of a Tridentate Non‐Innocent Ligand into a Tetradentate Salen‐Type Innocent Ligand

A non-innocent ligand, H4L(CH2NH2), was synthesized by introducing a -CH2NH2 group at the ortho carbon atom to the aniline moiety of 2-anilino-4,6-di-tert-butylphenol. The new ligand was characterized by IR and NMR spectroscopy and mass spectrometry techniques. Upon treatment with CuCl2⋅2H2O, this non-innocent ligand provided a mononuclear four-coordinate salen-type Cu(II) complex by complete modification of the ligand backbone. The complex was characterized by IR spectroscopy, mass spectrometry, X-ray single-crystal diffraction, electron paramagnetic resonance (EPR) spectroscopy, and UV/Vis/near-IR spectroscopy techniques. X-ray crystallographic analysis showed an asymmetric environment around the Cu(II) center with a small (≈12°) twist between the two biting planes. Analysis of the X-band EPR spectrum also supported the asymmetric environment and also indicated the presence of an unpaired electron on the dx2-y2 orbital. The UV/Vis/near-IR spectrum showed strong absorption bands for metal-to-ligand charge transfer and ligand-to-metal charge transfer along with a Cu(II) -centered d-d transition. Mechanistic investigation of the formation of complex 1 indicated that modification of the ligand backbone proceeded through ligand-centered amine to imine oxidation as well as through C-N bond-breaking processes. During these processes, 3,5-di-tert-butyl-1,2-benzoquinone and 2-aminobenzylidene were produced. Ammonia, generated in situ through hydrolysis of the imine to the aldehyde, reacted with 3,5-di-tert-butyl-1,2-benzoquinone to form the corresponding 3,5-di-tert-butyl-1,2-iminobenzoquinone moiety, which upon two-electron reduction in the reaction medium formed 3,5-di-tert-butyl-1,2-aminophenol. This aminophenol underwent condensation with the H2L5 ligand that was formed by self-condensation of two molecules of 2-aminobenzaldehyde and provided the modified ligand backbone.

Solvatochromic fluorescent cyanophenoxazine: design, synthesis, photophysical properties and fluorescence light-up sensing of ct-DNA

We report our new methodology for the Mn2+-catalysed synthesis of donor–acceptor substituted classical fluorescent phenoxazine which showed high solvatochromic emission properties and ct-DNA sensing efficiency via a light-up fluorescence response.

Solid-state valence tautomeric octahedral {CoII[(BQ-N-Cat)]2}0 complex formation via ligand-centered phenolic C–O bond breaking and Co–O bond making

Ligand H4LO(AP/AP) underwent ligand-centered C-O bond cleavage during a complexation reaction with Co(ii)-salt. The thus formed octahedral {CoII[(BQ-N-Cat)]2}0 complex showed valence tautomerization in the solid state. While the process was triggered by the presence of lattice solvent, the nature of the solvent molecule has less effect on the process.

Development of catalytic activity protocol for electrochemical reduction of carbon dioxide to value added products

Utilization of carbon dioxide for the production of value added products is a challenging task. Electrochemical reduction of carbon dioxide is one of the most promising techniques to convert the carbon dioxide into value added products. However, the development and selection of a suitable electrocatalyst is not so straightforward. The main problems are non-selectivity of the electrocatalyst toward CO2 reduction, simultaneous hydrogen evolution reaction, and low efficiency of the process. To overcome these problems, many electrocatalysts have been studied and reported in the literature. However, there is no effective guideline to screen the electrocatalysts in a quick and simple way. Moreover, it is found that the method used for the screening of electrocatalyst is not accurate and has anomalies. Therefore, in this paper, a simple and quick protocol based on half-cell tests is proposed. The method provides a first-hand prediction about the electrochemical activity of an electrocatalyst toward electrochemical reduction of carbon dioxide. The protocol was validated and compared along with confirming the results by product analysis of the electrochemical reduction of CO2 using full electrochemical reactor. The method was found satisfactory for the preliminary screening of electrocatalyst to reduce carbon dioxide electrochemically.

Synthesis and characterization of a tetraradical-containing octanuclear vanadium cluster formed via ligand C–N bond breaking and C–O bond making

A tetraradical-containing octanuclear vanadium cluster (V8, complex 2) was synthesized by reacting N(2-methoxyanilino)-3,5-di-tert-butylphenol and VOSO4·5H2O in the presence of Et3N in MeOH. The reaction proceeded via a ligand centre C–N bond cleavage and C–O bond formation. The V8 cluster was comprised of two identical V4 clusters where one V4 cluster was replicated by the other by an inversion centre. A strong antiferromagnetic coupling between the two V4 clusters, having S = 1/2 spin state each, provided a diamagnetic ground state.