Bimetallic Cobalt Complexes Research Articles

Bimetallic Cobalt Complex-Mediated Enantioselective Terpolymerizations of Carbon Dioxide, Cyclohexene Oxide, and β-Butyrolactone

Enantioselective terpolymerization of carbon dioxide (CO2), cyclohexene oxide (CHO), and racemic β-butyrolactone (BBL) was first achieved in a one-pot reaction utilizing the catalyst system based on enantiopure bimetallic cobalt complex. The resultant terpolymers possess one high melting temperature (Tm) of more than 200 °C and a mixed glass transition temperature (Tg), which could be adjusted by CO2 pressure used in the terpolymerization. 1H NMR analysis indicated that the terpolymers had the carbonate-ester junction unit content in the range of 10.9–28.5%, dependent on CO2 pressure and CHO/BBL feed ratio. Combined with matrix-assisted laser desorption ionization time-of-flight mass spectrometry, two-dimensional NMR spectroscopy, 13C NMR spectroscopy, and polarimetry, an intramolecular bimetallic cooperation mechanism was proposed to be responsible for the terpolymerization process.

Copolymerization of propylene oxide and CO2 catalyzed by dinuclear salcy–CoCl complex

The development of catalytic systems is always a research focus on the field of epoxides/carbon dioxide (CO2) copolymerization. A novel bimetallic Cobalt complex (1) bearing a piperazine-bridged salcy ligand and ancillary chloride group (salcyCoCl) has been exploited as catalyst for the ring-opening copolymerization (ROCOP) of propylene oxide (PO) and CO2 to afford poly(propylene carbonate) (PPC). The catalyst loading, CO2 pressure, polymerization temperature, reaction time, type of cocatalyst were employed in order to explore the influences on the polymeration activity of 1. It was found that after adding the cocatalyst PPNCl, the bimetallic salcy–CoCl complex can exhibit relatively good activity under mild conditions in the catalytic copolymerization of PO and CO2. A reasonable bimetallic polymerization mechanism has been proposed according to the polymer structure and the interaction between metal complex catalyst system and monomer.

Electronic Structure Description of a Doubly Oxidized Bimetallic Cobalt Complex with Proradical Ligands.

The geometric and electronic structure of a doubly oxidized bimetallic Co complex containing two redox-active salen moieties connected via a 1,2-phenylene linker was investigated and compared to an oxidized monomeric analogue. Both complexes, namely, CoL(1) and Co2L(2), are oxidized to the mono- and dications, respectively, with AgSbF6 and characterized by X-ray crystallography for the monomer and by vis-NIR (NIR = near-infrared) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) magnetometry, and density functional theory (DFT) calculations for both the monomer and dimer. Both complexes exhibit a water molecule coordinated in the apical position upon oxidation. [CoL(1)-H2O](+) displays a broad NIR band at 8500 cm(-1) (8400 M(-1) cm(-1)), which is consistent with recent reports on oxidized Co salen complexes (Kochem, A. et al., Inorg. Chem., 2012, 51, 10557-10571 and Kurahashi, T. et al., Inorg. Chem., 2013, 52, 3908-3919). DFT calculations predict a triplet ground state with significant ligand and metal contributions to the singularly occupied molecular orbitals. The majority (∼75%) of the total spin density is localized on the metal, highlighting both high-spin Co(III) and Co(II)L(•) character in the electronic ground state. Further oxidation of CoL(1) to the dication affords a low-spin Co(III) phenoxyl radical species. The NIR features for [Co2L(2)-2H2O](2+) at 8600 cm(-1) (17 800 M(-1) cm(-1)) are doubly intense in comparison to [CoL(1)-H2O](+) owing to the description of [Co2L(2)-2H2O](2+) as two non-interacting oxidized Co salen complexes bound via the central phenylene linker. Interestingly, TD-DFT calculations predict two electronic transitions that are 353 cm(-1) apart. The NIR spectrum of the analogous Ni complex, [Ni2L(2)](2+), exhibits two intense transitions (4890 cm(-1)/26 500 M(-1) cm(-1) and 4200 cm(-1)/21 200 M(-1) cm(-1)) due to exciton coupling in the excited state. Only one broad band is observed in the NIR spectrum for [Co2L(2)-2H2O](2+) as a result of the contracted donor and acceptor orbitals and overall CT character.

Exploration of Cocatalyst Effects on a Bimetallic Cobalt Catalyst System: Enhanced Activity and Enantioselectivity in Epoxide Polymerization

Organic ionic compounds were synthesized and investigated as cocatalysts with a bimetallic cobalt complex for enantioselective epoxide polymerization. The identities of both the cation and the anion were systematically varied, and the subsequent reactivity was studied. The nature of the ionic cocatalyst dramatically impacted the rate and enantioselectivity of the catalyst system. The ionic cocatalyst [P(N═P(N(CH2)4)3)4+][tBuCO2–] in combination with a bimetallic cobalt complex produced a catalyst system that exhibited the greatest activity and selectivity for a variety of monosubstituted epoxides.

Enantioselective Epoxide Polymerization Using a Bimetallic Cobalt Catalyst

A highly active enantiopure bimetallic cobalt complex was explored for the enantioselective polymerization of a variety of monosubstituted epoxides. The polymerizations were optimized for high rates and stereoselectivity, with s-factors (k(fast)/k(slow)) for most epoxides exceeding 50 and some exceeding 300, well above the threshold for preparative utility of enantiopure epoxides and isotactic polyethers. Values for mm triads of the resulting polymers are typically greater than 95%, with some even surpassing 98%. In addition, the use of a racemic catalyst allowed the preparation of isotactic polyethers in quantitative yields. The thermal properties of these isotactic polyethers are presented, with many polymers exhibiting high T(m) values. This is the first report of the rapid synthesis of a broad range of highly isotactic polyethers via the enantioselective polymerization of racemic epoxides.

Solvent dependent assembly and disassembly of a hydrogen bonded helical structure in a Co–Mo bimetallic complex

The effect of solvent on the assembly and disassembly of a hydrogen bonded helical structure in a bimetallic cobalt complex of a molybdenum-containing metalloligand and their spontaneous resolution has been demonstrated.

Synthesis and Characterization of a Bimetallic Boratabenzene Cobalt Complex

Reaction of 1,4-(BBr2)2C6H4 with cobaltocene gives a bimetallic cobalt complex with cyclopentadienyl and boratabenzene ligands. Oxidation allows the dicationic dicobalt(III) complex to be isolated; the bis(hexafluorophosphate) salt has been structurally characterized. Variable-temperature magnetic susceptibility measurements on the neutral dicobalt(II) complex showed substantial deviation from the Curie−Weiss law; a fit to the Bleaney−Bowers equation for a singlet ground state and a triplet excited state gave an exchange interaction of J = ca. −28 cm-1. Electrochemical studies, as well as near-infrared data on the mixed-valence species, suggest that the interaction between the metal centers is weak.

Bimetallic Reactivity: Factors Controlling One‐Site Addition Two‐Metal Oxidation Reactions in Bimetallic Complexes

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Bimetallic reactivity. Factors controlling one-site addition two-metal oxidation reactions in bimetallic complexes

A number of bimetallic complexes bearing five- and six-coordinate sites are described for the purpose of achieving one-site addition two-metal oxidation reactions where a substrate adds to the metal at the five-coordinate site and is reduced by both metals. One set of such bimetallic cobalt complexes is found to be either oxidatively unreactive or in some cases when one or the other of the metals is oxidized, the other becomes unreactive to oxidation. This oxidative deactivation is evident even when it is demonstrated that the monometallic constituent parts of the bimetallic complex allow for ready oxidation. It is concluded that the major impediment to bimetallic oxidation is mechanical coupling where the ligand conformation cannot adjust to the structural demands of the higher oxidation state metal. When mechanical coupling is minimized, it is shown that one-site addition two-metal oxidation reactions are observed. With such bimetallic Co 2+ complexes NO 2 +, Br 2 and O 2 are shown to undergo the expected reactions.