Synthesis and Electrochemistry of Schiff Base Cobalt(III) Complexes and Their Catalytic Activity for Copolymerization of Epoxide and Carbon Dioxide

Solvent Effects on the Photophysical Properties of a Donor-acceptor Based Schiff Base

In this work, a donor-acceptor substituted aromatic system ((E)-N-((E)-3-(4 (dimethylamino)phenyl) allylidene)-4-(trifluoromethyl) benzenamine (DPATB) has been synthesized and its detailed photophysics of intramolecular charge transfer process have been explored on the basis of steady state absorption, fluorescence and time resolved spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring from the donor group to the acceptor moiety in the excited state. Effect on addition of acid and pH on steady state spectral properties further reveals excited state charge transfer character. Quantum chemical calculations were performed in order to study the conformation and polarity of DPATB at their ground as well as excited electronic states. The HOMO and LUMO molecular orbital pictures are obtained at DFT level using B3LYP functional and 6-311 + g(d,p) basis set which clearly support excited state intramolecular charge transfer process. The molecular electrostatic potential maps for the optimized ground state, donor twisted and acceptor twisted geometry shed insight on the electrostatic potential and charge distribution in a system which gives information about the reacting site of the probe and nature of the reaction. In this work, detailed photophysics of excited state intramolecular charge transfer process in donor-acceptor system (DPATB) was evaluated using steady state and time-resolved fluorescence spectroscopy in combination with density functional theory calculations. Large solvent dependency fluorescence spectral shift and the calculated large excited state dipole moment clearly indicate an efficient charge transfer occurring in DPATB. Molecular orbital pictures as obtained from DFT based computational analysis reveals a significant change in the distribution of electron density upon transition from HOMO to LUMO which confirms an ICT process occurring from the donor group to the acceptor moiety in the excited state.

Chiral salenCo(iii) complexes with bulky substituents as catalysts for stereoselective alternating copolymerization of racemic propylene oxide with carbon dioxide and succinic anhydride

Stereoregular poly(propylene carbonate)s and poly(propylene succinate-block-carbonate)s were synthesized with new chiral salenCo(iii) catalysts carrying bulky substituents.

Cobalt amino-bis(phenolate) complexes for coupling and copolymerization of epoxides with carbon dioxide

Electron-withdrawing groups on phenolate donors enhance polycarbonate production from CO₂ and cyclohexene oxide by Co(ii) amino-bis(phenolate) complexes.

Development of Highly Active and Regioselective Catalysts for the Copolymerization of Epoxides with Cyclic Anhydrides: An Unanticipated Effect of Electronic Variation

Recent developments in polyester synthesis have established several systems based on zinc, chromium, cobalt, and aluminum catalysts for the ring-opening alternating copolymerization of epoxides with cyclic anhydrides. However, to date, regioselective processes for this copolymerization have remained relatively unexplored. Herein we report the development of a highly active, regioselective system for the copolymerization of a variety of terminal epoxides and cyclic anhydrides. Unexpectedly, electron withdrawing substituents on the salen framework resulted in a more redox stable Co(III) species and longer catalyst lifetime. Using enantiopure propylene oxide, we synthesized semicrystalline polyesters via the copolymerization of a range of epoxide/anhydride monomer pairs.

Aluminium salabza complexes for fixation of CO2 to organic carbonates

A highly stable and easy to synthesize aluminium complex bearing a flexible N₂O₂-donor salabza ligand (N,N′-bis(salicylene)-2-aminobenzylamine) in combination with tetrabutylammonium bromide forms an active binary catalytic system for the cycloaddition of CO₂ to epoxides (TOFs 120–3434 h⁻¹) under mild conditions (10 bar, 80 °C) and low catalyst loadings (0.05–0.2 mol%).

Surprisingly facile CO2 insertion into cobalt alkoxide bonds: A theoretical investigation

Exploiting carbon dioxide as co-monomer with epoxides in the production of polycarbonates is economically highly attractive. More effective catalysts for this reaction are intensively being sought. To promote better understanding of the catalytic pathways, this study uses density functional theory calculations to elucidate the reaction step of CO2 insertion into cobalt(III)-alkoxide bonds, which is also the central step of metal catalysed carboxylation reactions. It was found that CO2 insertion into the cobalt(III)-alkoxide bond of [(2-hydroxyethoxy)Co(III)(salen)(L)] complexes (salen = N,N"-bis(salicyliden-1,6-diaminophenyl)) is exothermic, whereby the exothermicity depends on the trans-ligand L. The more electron-donating this ligand is, the more exothermic the insertion step is. Interestingly, we found that the activation barrier decreases with increasing exothermicity of the CO2 insertion. Hereby, a linear Brønsted-Evans-Polanyi relationship was found between the activation energy and the reaction energy.

Bis[4-chloro-2-(quinolin-8-yl-imino-meth-yl)phenolato-κ(3) N,N',O]cobalt(III) trichlorido-methano-lcobaltate(II)

The reaction of 4-chloro-2-(quinolin-8-yl­imino­meth­yl)phenol (HClQP) with cobalt(II) dichloride hexa­hydrate in methanol/chloro­form under solvothermal conditions yielded the title compound, [Co(C₁₆H₁₀ClN₂O)₂][CoCl₃(CH₃OH)]. The Co^III atom is six-coordinated in a slightly distorted octa­hedral geometry by four N atoms and two O atoms of two tridentate HClQP ligands, which are nearly perpendicular to each other, making a dihedral angle of 86.95°. The Co^II atom is four-coordinated by three Cl atoms and one O atom from a methanol ligand in a distorted tetra­hedral geometry. The crystal packing is consolidated by inter­molecular O—H⋯Cl, C—H⋯Cl and C—H⋯O hydrogen bonds, forming a three-dimensional supra­molecular structure, in which [Co^IICl₃(CH₃OH)] anions are connected via O—H⋯Cl and C—H⋯Cl hydrogen bonds into centrosymmetric dimers. Neighboring cobalt(III) complexes form dimers through C—H⋯O hydrogen bonds, as well as π–π stacking [centroid–centroid distances = 3.30 (2) Å] between the planar quinoline systems of one HClQP ligand and the phenolate ring of another.

Alternating copolymerization of carbon dioxide and cyclohexene oxide catalyzed by salen CoIII(acetate) complexes

A series of Co^III carboxylate based upon N,N,O,O-tetradentate Schiff base ligand framework have been prepared. X-ray diffraction analysis confirms that these Schiff base Co^III carboxylate are all monomeric species with a six-coordinated central Co in their solid structures. The activities and polycarbonate selectivity of these complexes toward the copolymerization of epoxide (cyclohexene oxide and propylene oxide) and carbon dioxide have been investigated in the presence of bis(triphenylphosphine)iminium chloride. Copolymerization experiments indicate that [bis(α-methyl-3,5-di-tertbutyl-salicylaldehyde) ethylenediiminato] Co^IIIOOCH₃ exhibits the highest activity and polycarbonate selectivity among these Co^III carboxylate. The resultant copolymer contained almost 100 % carbonate linkages with the molecular weight up to 71.8 kg mol⁻¹ as well as narrow polymer dispersity index (polymer dispersity index = 1.5). The substituents and the mode of the bridging part between the two nitrogen atoms both exert significant influences upon the progress of the copolymerizations, influencing both the polycarbonate selectivity and the rate of copolymerization.

Proton transfer assisted charge transfer phenomena in photochromic Schiff bases and effect of -NEt2 groups to the anil Schiff bases

Photochromic Schiff bases 5-diethylamino-2-[(4-diethylamino-benzylidene)-hydrazonomethyl]-phenol (DDBHP) and N,N'-bis(4-N,N-diethylaminosalisalidene) hydrazine (DEASH) with both the proton and charge transfer moieties have been synthesized, and their photophysical properties such as excited state intramolecular charge transfer (ICT) and proton transfer (ESIPT) processes have been reported on the basis of steady-state and time-resolved spectral measurement in various solvents. The ground-state six-membered intramolecular hydrogen bonding network at the proton transfer site accelerates the ESIPT process for these compounds. Both the compounds show large Stokes-shifted emission bands for proton transfer and charge transfer processes. The hydrogen bonding solvents play a crucial role in these photophysical processes. Excited-state dipole moment of DDBHP and DEASH calculated by the solvatochromic method supports the polar character of the charge transfer excited state. Introduction of -NEt(2) groups to the reported salicylaldehyde azine (SAA) Schiff base results an increase in fluorescence lifetime from femtosecond to picosecond time scale for the proton transfer process.