Copolymerization of Phthalic Anhydride with Epoxides Catalyzed by Amine-Bis(Phenolate) Chromium(III) Complexes

The effect of ligand structure on the catalytic activity of amine-bis(phenolate) chromium(III) complexes in the ring-opening copolymerization of phthalic anhydride and a series epoxides was studied. Eight complexes differing in the donor-pendant group (R₁) and substituents (R₂) in phenolate units were examined as catalysts of the model reaction between phthalic anhydride and cyclohexane oxide in toluene. They were used individually or as a part of the binary catalytic systems with nucleophilic co-catalysts. The co-catalyst was selected from the following organic bases: PPh₃, DMAP, 1-butylimidazole, or DBU. The binary catalytic systems turned out to be more active than the complexes used individually, and DMAP proved to be the best choice as a co-catalyst. When the molar ratio of [PA]:[epoxide]:[Cr]:[DMAP] = 250:250:1:1 was applied, the most active complex (R₁-X = CH₂NMe₂, R₂ = F) allowed to copolymerize phthalic anhydride with differently substituted epoxides (cyclohexene oxide, 4-vinylcyclohexene oxide, styrene oxide, phenyl glycidyl ether, propylene oxide, butylene oxide, and epichlorohydrin) within 240 min at 110 °C. The resulting polyesters were characterized by M_n up to 20.6 kg mol^-1 and narrow dispersity, and they did not contain polyether units.

Lithium, sodium, potassium and calcium amine-bis(phenolate) complexes in the ring-opening polymerization of rac-lactide

Compounds of Li, Na, K and Ca of a tetradentate amino-bis(phenolato) ligand were prepared. Bimetallic compounds formulated as M₂[L](THF)_n (where M = Na, n = 1 (1·THF) or Li, n = 1 (2·THF)) were synthesized via the reaction of H₂[L] (where [L] = 2-pyridylmethylamino-N,N-bis(2-methylene-4-methoxy-6-tert-butylphenolato) with sodium hydride or n-butyllithium, respectively, in THF. Monometallic complexes MH[L](THF)_n (where M = Na, n = 1 (3·THF), Li, n = 0 (4) and K, n = 0 (5)) were obtained by reaction of H₂[L] with MN(SiMe₃)₂ where M = Na, Li, or K. Calcium complex Ca[L](THF) (6·THF) was synthesized in two ways; reaction of Na₂[L] with calcium iodide in THF, and reaction of Ca[N(SiMe₃)₂]₂ with H₂[L] in toluene. Compounds 1-6 exhibit activity for rac-lactide polymerization under melt and solution conditions. Moderate control of polymer molecular weights was achieved in toluene, whereas polydisperse polymer was obtained under solvent free conditions. MALDI-TOF MS analysis of the polymer end groups revealed a predominantly cyclic nature for the polylactides.

cis-Thioindigo (TI) - a new ligand with accessible radical anion and dianion states. Strong magnetic coupling in the {[TI-(μ2-O),(μ-O)]Cp*Cr}2 dimers

Reaction of decamethylchromocene (Cp*₂Cr) with thioindigo (TI) yields a coordination complex {[TI-(μ₂-O), (μ-O)]Cp*Cr}₂·C₆H₁₄ (1) in which one Cp* ligand in Cp*₂Cr is substituted by TI. TI adopts cis-conformation in 1 allowing the coordination of both carbonyl groups to chromium. Additionally, one oxygen atom of TI becomes a μ₂-bridge for two chromium atoms to form {[TI-(μ₂-O), (μ-O)]Cp*Cr}₂ dimers with a CrCr distance of 3.12 Å. According to magnetic data, diamagnetic TI^2- dianions and two Cr³⁺ atoms with a high S = 3/2 spin state are present in a dimer allowing strong antiferromagnetic coupling between two Cr³⁺ spins with an exchange interaction of -35.4 K and the decrease of molar magnetic susceptibility below 140 K. Paramagnetic TI˙^- radical anions with the S = 1/2 spin state have also been obtained and studied in crystalline {cryptand[2,2,2](Na⁺)}(TI˙^-) (2) salt showing that both radical anion and dianion states are accessible for TI.

Chromium(III) complexes with 1,2,4-diazaphospholide and 2,6-bis(N-1,2,4-diazaphosphol-1-yl) pyridine ligands: synthesis, X-ray structural characterization, EPR spectroscopy analysis, and magnetic susceptibility studies

Three chromium(III) 1,2,4-diazaphospholide complexes were prepared: 3,5-di-tert-butyl-1,2,4-diazaphospholide chromium(III) dichloride [(η 2-3,5-tBu2dp)Cr(THF)2Cl2] (1), 3,5-di-phenyl-1,2,4-diazaphospholide chromium(III) dichloride [(η 2-3,5-Ph2dp)Cr(THF)2Cl2] (2), and 2,6-bis(N-1,2,4-diazaphosphol-1-yl)pyridine chromium(III) trichloride {[2,6-bis(N-1,2,4-diazaphosphol-1-yl)pyridine]CrCl3} (3). X-ray diffraction analysis has shown that these six-coordinate complexes each have a pseudo-octahedral configuration. Electron paramagnetic resonance (EPR) spectroscopy data for complex 1 for the paramagnetic S = 3/2 system (d 3 electron configuration) confirm a Cr(III) center in the octahedral coordination environment. The magnetic susceptibility of complex 1 followed the Curie–Weiss law well between 25 and 300 K. The magnetic moment of 1 was found to be close to the spin-only magnetic moment expected for three unpaired electrons (3.87 μB).

Cyclohexene oxide/carbon dioxide copolymerization by chromium(iii) amino-bis(phenolato) complexes and MALDI-TOF MS analysis of the polycarbonates

Single-component Cr(iii) catalysts for copolymerization of CO₂ and cyclohexene oxide have been prepared. MALDI-TOF MS studies provide mechanistic information.

A MALDI-TOF MS analysis study of the binding of 4-(N,N-dimethylamino)pyridine to amine-bis(phenolate) chromium(iii) chloride complexes: mechanistic insight into differences in catalytic activity for CO2/epoxide copolymerization

Amine-bis(phenolato)chromium(iii) chloride complexes, [LCrCl], are capable of catalyzing the copolymerization of cyclohexene oxide with carbon dioxide to give poly(cyclohexane) carbonate. When combined with 4-(N,N-dimethylamino)pyridine (DMAP) these catalyst systems yield low molecular weight polymers with moderately narrow polydispersities. The coordination chemistry of DMAP with five amine-bis(phenolato)chromium(iii) chloride complexes was studied by matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The amine-bis(phenolato) ligands were varied in the nature of their neutral pendant donor-group and include oxygen-containing tetrahydrofurfuryl and methoxyethyl moieties, or nitrogen-containing N,N-dimethylaminoethyl or 2-pyridyl moieties. The relative abundance of mono and bis(DMAP) adducts, as well as DMAP-free ions is compared under various DMAP : Cr complex ratios. The [LCr]⁺ cations show the ability to bind two DMAP molecules to form six-coordinate complex ions in all cases, except when the pendant group is N,N-dimethylaminoethyl (compound 3). Even in the presence of a 4 : 1 ratio of DMAP to Cr, no ions corresponding to [L3Cr(DMAP)₂]⁺ were observed for the complex containing the tertiary sp³-hybridized amino donor in the pendant arm. The difference in DMAP-binding ability of these compounds results in differences in catalytic activity for alternating copolymerization of CO₂ and cyclohexene oxide. Kinetic investigations by infrared spectroscopy of compounds 2 and 3 show that polycarbonate formation by 3 is twice as fast as that of compound 2 and that no initiation time is observed.

Chromium(iii) amine-bis(phenolate) complexes as catalysts for copolymerization of cyclohexene oxide and CO2

Chromium complexes of tri- and tetradentate amine-bis(phenolate) ligands in combination with chloride, azide or DMAP nucleophiles are effective catalysts for the copolymerization of CO₂ with cyclohexene oxide to give polycarbonates.

Ring-opening polymerization of cyclic esters with lithium amine-bis(phenolate) complexes

Lithium compounds of tetradentate amino-bis(phenolato)-tetrahydrofuranyl ligands, Li(2)[L1] (1) and Li(2)[L2] (2) (where [L1] = 2-tetrahydrofuranyl-N,N-bis(2-methylene-4-methyl-6-tert-butylphenolate), and [L2] = 2-tetrahydrofuranyl-N,N-bis(2-methylene-4,6-tert-butylphenolate)) were characterized by multinuclear solution NMR and solid-state (6)Li and (7)Li NMR spectroscopy. The proligands, n-propylamino-N,N-bis(2-methylene-4-methyl-6-tert-butylphenol), (H(2)[L3]) and benzylamino-N,N-bis(2-methylene-4,6-di-tert-amylphenol), H(2)[L4] were reacted with n-butyllithium in THF to give the related dilithium compounds Li(2)[L3] (4) and Li(2)[L4] (5), respectively. The pyridine adduct of 1, (py)(2)Li(2)[L1] (3) and complexes 4 and 5 have been structurally characterized by single-crystal X-ray diffraction and NMR spectroscopy. The reactivity of these complexes for the ring-opening polymerization of rac-lactide, as well as the influences of monomer concentration, monomer/Li molar ratio, polymerization temperature and time, were studied.

Supramolecular architectures of novel chromium(III) oxalate complexes: steric effects of the ligand size and building-blocks approach

Five new oxalate complexes of chromium(III), [Hphen][Cr(phen)(C(2)O(4))(2)]·2H(2)O (1), [Cr(phen)(2)(C(2)O(4))][Cr(phen)(C(2)O(4))(2)]·3H(2)O (2), [Cr(phen)(2)(C(2)O(4))]NO(3)·H(2)C(2)O(4)·H(2)O (3), [Cr(bpy)(2)(C(2)O(4))][Cr(bpy)(C(2)O(4))(2)]·3H(2)O (4) and [Cr(bpy)(2)(C(2)O(4))]NO(3)·1/2H(2)C(2)O(4)·4H(2)O (5) (phen = 1,10-phenanthroline, bpy = 2,2'-bipyridine), were prepared by using an (oxalato)tantalate(V) solution as a source of oxalate ligands. The compounds contain either the discrete mononuclear [Cr(L)(2)(C(2)O(4))](+) cation [L = phen (3); L = bpy (5)] or the discrete mononuclear [Cr(L)(C(2)O(4))(2)](-) anion [L = phen (1)], or both types of mononuclear ions [L = phen (2); L = bpy (4)]. The crystal structures are dominated by the hydrogen-bonding and π···π-stacking interactions that give rise to the overall two- (compounds 1, 2, 4, 5) or three-dimensional (compound 3) architectures. Compounds 2 and 4 represent a borderline case between isostructurality and non-isostructurality; they exhibit an analogous packing of the cation and the anion units, but the crystallization water molecules occupy different positions - due to a difference in size between the phen and bpy ligands. The influence of steric factors is evident also in the case of 3 and 5, which, despite very similar chemical formulae, exert a completely different packing of the constituents. By the self-assembling of 1 and 4, used as building blocks in the reaction with calcium(II) cations, the heterobimetallic polymeric compounds {[CaCr(2)(phen)(2)(C(2)O(4))(4)]·5H(2)O}(n) (6) and {[CaCr(2)(bpy)(2)(C(2)O(4))(4)]·0.83H(2)O}(n) (7) were obtained. The crystal structure of 7 is reported: the [Cr(bpy)(C(2)O(4))(2)](-) unit, through the two oxalate groups, acts as a chelating ligand towards Ca cations, resulting in heterometallic one-dimensional double zigzag chains, formed of diamond-shaped units. The characterization of the compounds obtained was accomplished by the spectroscopy and thermal analysis methods.

Ring-opening polymerization of ε-caprolactone by lithium piperazinyl-aminephenolate complexes: synthesis, characterization and kinetic studies

A series of lithium complexes were prepared from 2(N-piperazinyl-N'-methyl)-2-methylene-4-R'-6-R-phenols ([ONN](RR')) and characterized through elemental analysis, (1)H and (13)C{(1)H} NMR spectroscopy, and X-ray crystallography. Treatment of the ligands with n-butyllithium afforded {Li[ONN](RR')}(3) [R = Me, R' = (t)Bu, (1); R = R' = (t)Bu (2); R = R' = (t)Am, (3), (t)Am = C(CH(3))(2)CH(2)CH(3)], with trimetallic structures in the solid-state as shown by single-crystal X-ray diffraction. The reactivity of these complexes in the ring-opening polymerization of ε-caprolactone (ε-CL), as well as the influences of monomer concentration, monomer/Li molar ratio, polymerization temperature and time, was studied. Rates of polymerization were first order with respect to both monomer and lithium concentrations, and activation energies for the reactions were determined. MALDI-TOF MS analysis revealed that transesterification had occurred during the polymerization.