Indium Catalysts for Low-Pressure CO2/Epoxide Ring-Opening Copolymerization: Evidence for a Mononuclear Mechanism?

The alternating copolymerization of CO2/epoxides is a useful means to incorporate high levels of carbon dioxide into polymers. The reaction is generally proposed to occur by bimetallic or bicomponent pathways. Here, the first indium catalysts are presented, which are proposed to operate by a distinct mononuclear pathway. The most active and selective catalysts are phosphasalen complexes, which feature ligands comprising two iminophosphoranes linked to sterically hindered ortho-phenolates. The catalysts are active at 1 bar pressure of carbon dioxide and are most effective without any cocatalyst. They show low-pressure activity (1 bar pressure) and yield polymer with high carbonate linkage selectivity (>99%) and isoselectivity ( Pm > 70%). Using these complexes, it is also possible to isolate and characterize key catalytic intermediates, including the propagating indium alkoxide and carbonate complexes that are rarely studied. The catalysts are mononuclear under polymerization conditions, and the key intermediates show different coordination geometries: the alkoxide complex is pentacoordinate, while the carbonate is hexacoordinate. Kinetic analyses reveal a first-order dependence on catalyst concentration and are zero-order in carbon dioxide pressure; these findings together with in situ spectroscopic studies underpin the mononuclear pathway. More generally, this research highlights the future opportunity for other homogeneous catalysts, featuring larger ionic radius metals and new ligands, to operate by mononuclear mechanisms.

Unusual coordination mode of tetradentate Schiff base cobalt(III) complexes.

Contrary to the stereotype, Jacobsen's catalyst, chiral (salcy)Co(III)OAc adopts an unusual binding mode. The tetradentate {ONNO} ligand does not form a square plane but wraps cobalt in a cis-ß fashion while acetate is chelating.

Preparation of half-metallocenes of thiophene-fused and tetrahydroquinoline-linked cyclopentadienyl ligands for ethylene/α-olefin copolymerization.

Directed ortho-lithiation of the lithium carbamates generated from tetrahydroquinoline or tetrahydroquinaldine enables one-step preparation of thiophene-fused and tetrahydroquinoline-linked cyclopentadienes [2-R(1)-3-R(2)-4,5-dimethyl-6-(2-R(3)-2,3,4,5-tetrahydroquinolin-8-yl)-4H-cyclopenta[b]thiophene (R(1), R(2), R(3) = H or methyl)], from which titanium(iv) and zirconium(iv) complexes are prepared. The molecular structures of Me(2)Ti-complexes (12, R(1) = R(2) = Me, R(3) = H; 14, R(1) = R(2) = R(3) = Me) and Cl(2)Zr-complex (17, R(1) = R(2) = Me, R(3) = H) are determined by X-ray crystallography. The Me(2)Ti-complexes, 14 and 15 (R(1) = R(3) = Me, R(2) = H) show excellent activities (62 and 54 × 10(6) g/molTi·h) in ethylene/1-octene copolymerization, even when activated with small amount of MAO (Al/Ti = 1000).

Anion variation on a cobalt(III) complex of salen-type ligand tethered by four quaternary ammonium salts for CO2/epoxide copolymerization.

Anion exchange of BF(4)(-) occurs by stirring a cobalt(III) complex of salen-type ligand tethered by four quaternary ammonium BF(4)(-) salts over a slurry of NaX in CH(2)Cl(2), affording a complex containing four X's per cobalt (X = 2,4,5-trichlorophenolate, 6; X = 4-nitrophenolate, 10; X = 2,4-dichlorophenolate, 12). The (1)H and (13)C NMR spectra are in agreement with an unusual imine uncoordinated structure. The two salen-phenoxys and the two X's persistently coordinate with cobalt(III) to form a square planar cobaltate complex while the other two X's scramble through coordination and decoordination to the axial sites of the square plane. Another form of the complex (X = 2,4,5-trichlorophenolate, 14; X = 4-nitrophenolate, 15; X = 2,4-dichlorophenolate, 16) is also prepared, in which the scrambling two X's in 6, 10, or 12 are replaced with the corresponding [X...H...X](-) homoconjugate. These complexes, which adopt an unusual imine uncoordinated structure, are excellent catalysts for CO(2)/propylene oxide copolymerization (turnover frequency (TOF), 8300-16,000 h(-1)). In all cases, the complex containing the homoconjugate [X...H...X](-) shows higher activity than the corresponding phenol-free complex. Among the prepared complexes, 4-nitrophenol-4-nitrophenolate homoconjugate complex 15 showed the best performance (TOF, 16,000 h(-1); selectivity, 98%; M(n), 273,000), allowing for replacement of the explosive 2,4-dinitrophenolate complex.

Elucidation of the structure of a highly active catalytic system for CO2/epoxide copolymerization: a salen-cobaltate complex of an unusual binding mode.

Salen-type ligands comprised of ethylenediamine or 1,2-cyclohexenediamine, along with an salicylaldehyde bearing a methyl substituent on its 3-position and a -[CR(CH(2)CH(2)CH(2)N(+)Bu(3))(2)] (R = H or Me) on its 5-position, unexpectedly afford cobalt(III) complexes with uncoordinated imines. In these complexes, two salen-phenoxys and two 2,4-dinitrophenolates (DNPs), which counter the quaternary ammonium cations, coordinate persistently with cobalt, while two other DNPs are fluxional between a coordinated and an uncoordinated state in THF at room temperature. The complexes of this binding mode show excellent activities in carbon dioxide/propylene oxide copolymerization (TOF, 8300-13,000 h(-1)) but with some fluctuation in induction times (1-10 h), depending on how dry the system is. The induction time is shortened (<1.0 h) and activity is increased approximately 1.5 times upon the replacement of the two fluxional DNPs with 2,4-dinitrophenol-2,4-dinitrophenolate homoconjugation ([DNP...H...DNP](-)). Imposing steric congestion either by replacing the methyl substituent on the salicylaldehyde with tert-butyl or by employing H(2)NCMe(2)CMe(2)NH(2) instead of ethylenediamine or 1,2-cyclohexenediamine results in conventional imine-coordinating complexes, which show lower activities than uncoordinated imine complexes.