Aluminum Methyl Complexes Research Articles

Synthesis of Mono- and Dinuclear Aluminum Complexes Bearing Aromatic Amino-Phenolato Ligands: A Comparative Study in the Ring-Opening Polymerization of Cyclohexene Oxide.

Dinuclear aluminum methyl complexes bearing aromatic diamine-bridged tetra(phenolato) ligands and the mononuclear aluminum methyl complex with the phenylamine-bridged bis(phenolato) ligand have been synthesized and characterized. Structure determination revealed that the Al-Al distances in these dinuclear aluminum complexes are tunable by the choice of the suitable aromatic backbone of the diamine-bridged tetra(phenolato) ligands. The catalytic behaviors of these mono- and dinuclear aluminum complexes for cyclohexene oxide (CHO) polymerization were investigated. The activities of these dinuclear Al complexes were observed to increase with the decrease of Al-Al distances, and the dinuclear Al complexes appeared to have better catalytic activity than the mononuclear Al complex, even if the Al-Al distance is as long as 9.401 Å. Dinuclear aluminum complex 2, with the shortest Al-Al distance (7.236 Å), showed the highest activity toward CHO polymerization with TOFs up to 6460 h-1 in neat CHO at 30 °C. Furthermore, comparative kinetic studies revealed that the polymerization is first-order for CHO concentration, and the reaction orders for initiator concentration are different for the mono- and dinuclear Al complexes. The polymerization mechanism study revealed that both the methyl and phenolate groups were involved in the initiation process.

Highly Isoselective Ring‐Opening Polymerization of rac‐Lactide Using Chiral Binuclear Aluminum Catalyst

The microstructure can significantly affect the physical and mechanical properties of polymers. Five chiral binuclear aluminum methyl complexes (rac1 to rac5) are synthesized by using chiral binaphthalene diamine ligand and characterized by both 1 H and 13 C NMR spectroscopy. The crystal structures of rac3 and rac5 are also identified by single crystal X-ray diffraction. In the presence of i PrOH initiator, these aluminum complexes catalyze the ring-opening polymerization (ROP) of rac-lactide (rac-LA) in a controlled manner and produce polymers with high to excellent isoselectivity (Pm up to 0.93). Even with a catalyst loading as low as 0.1%, polylactide with a Pm of 0.83 can still be obtained. Kinetic studies reveal the first-order dependence on monomer concentration whereas kinetic resolution polymerization provides the evidence to support that these binuclear aluminum catalysts catalyzed ROP of rac-LA adopts enantiomorphic site control mechanism. Furthermore, this strategy can also be applied to the ROP of both ε-caprolactone and δ-valeroactone.

A Comparative Study on Dinuclear and Mononuclear Aluminum Methyl Complexes Bearing Piperidyl-Phenolato Ligands in ROP of Epoxides.

Dinuclear aluminum methyl complexes stabilized by piperidyl-phenolato ligands were prepared and characterized. The ring-opening polymerizations of cyclohexene oxide (CHO) and propylene oxide (PO) initiated by dinuclear complexes and mononuclear analogues were investigated and compared. Enhanced activity of dinuclear complexes compared to that of mononuclear analogues in both the ring-opening polymerization of CHO and PO proves the synergistic interaction of two Al centers in the former. End-group analysis of oligomers by MALDI-TOF mass spectrometry confirms the role of methyl groups as initiating groups. A bimetallic mechanism is proposed, in which the cooperation of two Al centers are involved in polymerization processes.

Aluminum methyl, alkoxide and α-alkoxy ester complexes supported by 6,6'-dimethylbiphenyl-bridged salen ligands: synthesis, characterization and catalysis for rac-lactide polymerization.

The synthesis and characterization of aluminum alkyl and alkoxide complexes bearing racemic 6,6'-dimethylbiphenyl-bridged salen-type ligands, and their catalysis in the ring-opening polymerization (ROP) of rac-lactide are reported. Reactions of AlMe3 with various amounts of the proligands (6,6'-[(6,6'-dimethyl-[1,1'-biphenyl]-2,2'-diyl)bis(nitrylomethilidyne)]-bis(2-R(1)-4-R(2)-phenol): , R(1) = R(2) = Me; , R(1) = (t)Bu, R(2) = Me; , R(1) = R(2) = cumyl; , R(1) = Br, R(2) = (t)Bu) afforded the corresponding mono- and dinuclear aluminum methyl complexes [AlMe (), Al2Me4 ()]. Aluminum alkoxide complexes AlO(i)Pr (), AlOBn (), and α-alkoxy ester complexes Al(OCMe2CO2Me) (), Al[(S)-OCHMeCO2Me] () were prepared via in situ alcoholysis of the parent aluminum methyl complex with the corresponding alcohols. The molecular structures of mononuclear complexes , dinuclear complex , alkoxide complexes and α-alkoxy ester complexes were established by single-crystal X-ray diffraction studies. Two broad resonances at about 69-70 ppm and 25-41 ppm were observed in the (27)Al NMR spectra of complexes and , indicating the existence of both four- and five-coordinate aluminum centers in solution, which results from the dissociation of one N donor of the salen ligand, accompanied by an association and dissociation equilibrium of the carbonyl group of the α-alkoxy ester ligand to the aluminum center. Complex is also a rare example of an O-lactate model complex that mimics the first insertion of l-LA. All complexes were investigated for the ROP of rac-LA at 110 °C in toluene. The polymerization initiated by complexes in the presence of (i)PrOH showed living features, affording PLAs with narrow molecular weight distributions (PDIs = 1.03-1.05) and 65-73% isotacticities. Particularly, complex showed an "immortal" behavior for the polymerization of rac-LA in the presence of excess alcohol. Compared with the mononuclear counterparts, the tetra-coordinate dinuclear aluminum complexes enabled a few fold boosts in activity, but gave atactic PLAs with broadened PDIs.

Aluminum methyl and isopropoxide complexes with ketiminate ligands: Synthesis, structural characterization and ring-opening polymerization of cyclic esters

Mono(ketiminate) aluminum dimethyl complexes 1a–5a and bis(ketiminate) aluminum methyl complex 2b were synthesized via the reactions of AlMe3 with various N-aryl substituted ketimine compounds in a 1:1 or 1:2 molar ratio, respectively. The reaction of Al(OiPr)3 with the ketimine compound L2H resulted in a bis(ketiminate) aluminum isopropoxide complex 2c. All the complexes were characterized by 1H and 13C{1H} NMR spectroscopy, either EA or HRMS analysis. X-ray diffraction measurement of complexes 2a and 3a confirmed that in the solid state both 2a and 3a exist as a tetrahedron structure with the aluminum atom center surrounded by the oxygen and nitrogen donors of the bis-chelating ketiminate ligand as well as two methyl groups. Besides, a triligated aluminum complex 3 (Al(L3)3) with a six-coordinated metal core and a dinuclear aluminum isopropoxide complex 6d [(L6)2Al(μ-OiPr)2Al(OiPr)2] with C2-symmetry were isolated and characterized by X-ray diffraction studies. All aluminum methyl complexes behaved as active initiators in the ring-opening polymerization (ROP) of rac-lactide (rac-LA), affording isotactic-enriched polylactides (PLAs) with high molecular weights and broad molecular weight distributions at 70°C. The steric and electronic characters of the ancillary ligands show significant effects both on the activity and the stereoselectivity. The introduction of an electron-withdrawing group on the para-position of N-aryl ring of the ligand resulted in an increase of the catalytic activity. However, the electronic character of ancillary ligands showed a neglectable effect on the ROP of ε-CL initiated by these aluminum complexes. For both monomers, bis(ketiminate) aluminum complex 2b was found to be more active than the corresponding mono(ketiminate) complex 2a of the same ligand. Aluminum isopropoxide complex 2c exerted better control for the polymerizations of both monomers, providing polymers with narrower molecular weight distributions and smaller molecular weights which are comparable to the theoretical values

Aluminum complexes of bidentate phenoxy-amine ligands: Synthesis, characterization and catalysis in ring-opening polymerization of cyclic esters

A series of aluminum complexes 1a–5a bearing bidentate phenoxy-amine ligands were synthesized and characterized by NMR spectroscopy and elemental analyzes. The variation of the ortho-substituent of phenoxy moiety and the amino group of the ancillary ligands has crucial influence on the stoichiometric structure of the obtained aluminum complexes. Reaction of 2-(N-benzyl-N-R3-aminomethyl)-4-R2-6-R1-phenols (R1 = tBu, R2 = Me, R3 = ethyl, 1; R1 = R2 = cumyl, R3 = ethyl, 2; R1 = R2 = cumyl, R3 = tBu, 3) with either one or half equivalent of AlMe3 exclusively afforded monoligated complexes LBu,EtAlMe2 (1a), LCumyl,EtAlMe2 (2a) and LCumyl,BuAlMe2 (3a), whereas the treatment of 2-(N-benzyl-N-R3-aminomethyl)-4,6-dichlorophenols (R3 = ethyl, 4; R3 = tBu, 5) with 0.5 equivalent of AlMe3 resulted in the formation of (LCl,Et)2AlMe (4a) or (LCl,Bu)2AlMe(HLCl,Bu) (5a) respectively. All of these aluminum methyl complexes can efficiently polymerize rac-lactide to atactic polymers and actively initiate the ring-opening polymerization of ɛ-caprolactone.

Syntheses of bis(pyrrolylaldiminato)aluminum complexes for the polymerisation of lactide

Seven bis(pyrrolylaldiminato)aluminum methyl complexes were synthesized from the reactions of AlMe3 and two equiv. of the corresponding pyrrolylaldimine ligands. The ligands were modified to have different steric hindrances (C6H5 (1), 2,6-Me2C6H3 (2), 2,4,6-Me3C6H2 (3), 2,6-Et2C6H3 (4) and 2,6-(i)Pr2C6H3 (5)) and electronic contributions (4-CF3C6H4 (6) and 4-OMeC6H4 (7)). Crystal structures of complexes 3-7 were determined and shown to have distorted trigonal bipyramidal geometry (4, 6, 7) and intermediates between trigonal bipyramidal and square pyramidal geometries (3 and 5). The rotation around the N-C(aryl) bond was fast for ligands having small ortho substituents and became slower as the size of the substituents increased. Polymerisations of l-lactide using complexes 1-7 and benzyl alcohol as an initiator were carried out giving the rate dependence on steric hindrance (5 < 4 < 3 < 2 < 1) and electronic contribution (6 < 7 < 1). Larger substituents and electron withdrawing groups were found to suppress the polymerisation rates. Despite having C2 symmetry in the crystal structures of compounds 3-7, only slight enhancement for isotactic enchainment was found in the polymerisation of rac-lactide.

Aluminum Methyl and Chloro Complexes Bearing Monoanionic Aminephenolate Ligands: Synthesis, Characterization, and Use in Polymerizations

A series of aluminum methyl and chloride complexes bearing 2(N-piperazinyl-N′-methyl)-2-methylene-4-R′-6-R-phenolate or 2(N-morpholinyl)-2-methylene-4-R′-6-R-phenolate ([ONER1,R2]-) {[R1 = tBu, R2 = Me, E = NMe (L1); R1= R2 = tBu, E = NMe (L2); R1 = R2 = tBu, E = O (L3)} ligands were synthesized and characterized through elemental analysis, 1H, 13C{1H}, and 27Al NMR spectroscopy, and X-ray crystallography. Reactions of AlMe3 with two equivalents of L1H-L3H gave {[ONER1,R2]2AlMe} (1–3), while reaction of Et2AlCl with two equivalents of L1H and L3H afforded {[ONER1,R2]2AlCl} (4 and 5) as monometallic complexes. The catalytic activity of complexes 1–3 toward ring-opening polymerization (ROP) of ε-caprolactone was assessed. These complexes are more active than analogous Zn complexes for this reaction but less active than the Zn analogues for ROP of rac-lactide. Characteristics of the polymer as well as polymerization kinetics and mechanism were studied. Polymer end-group analyses were achieved using 1H NMR spectroscopy and MALDI-TOF MS. Eyring analyses were performed, and the activation energies for the reactions were determined, which were significantly lower for 1 and 2 compared with 3. This could be for several reasons: (1) the methylamine (NMe) group of 1 and 2, which is a stronger base than the ether (O) group of 3, might activate the incoming monomer via noncovalent interactions, and/or (2) the ether group is able to temporarily coordinate to the metal center and blocks the vacant coordination site toward incoming monomer, while the amine cannot do this. Preliminary studies using 4 and 5 toward copolymerization of cyclohexene oxide with carbon dioxide have been performed. 4 was inactive and 5 afforded polyether carbonate (66.7% epoxide conversion, polymer contains 54.0% carbonate linkages).

Phenoxy-Thioether Aluminum Complexes as ε-Caprolactone and Lactide Polymerization Catalysts

A series of new phenoxy-thioether (OS) proligands have been synthesized. They were found to readily react with 1 equiv of AlMe3 to afford the corresponding Al chelate complexes {4,6-tBu2-OC6H2-2-CH2S(2-R-C6H4)}AlMe2 (R = H (1), Br (2), CH3 (3), CF3 (4)) in quantitative yields. All the aluminum methyl complexes are stable monomeric species. In the solid state, as determined from X-ray crystallographic studies, complex 2 consists of a four-coordinate aluminum species in which the metal center is chelated by the sulfur and oxygen atoms of the bidentate ligand. All complexes promote the ring-opening polymerization of ε-caprolactone and l- and rac-lactide. Upon addition of methanol, efficient binary catalytic systems for the immortal ring-opening polymerization of the cyclic esters are produced (in detail, 300 equiv of ε-CL were converted in 20 min at 50 °C and 100 equiv of rac-LA were converted in 1 day at 80 °C). Kinetic studies show that polymerizations promoted by 1–4 are first order with respect to monomer concentration. The steric and electronic characteristics of the ancillary ligands have moderate influence on the polymerization performance of the corresponding aluminum complexes. However, the introduction of a substituent at the ortho position of the thiophenol aryl ring showed an opposite effect on the catalytic activities of the two different cyclic esters, increasing the activity in the ε-caprolactone polymerization and decreasing it in the polymerization of lactide.

Bimetallic amidinate aluminum methyl complexes: Synthesis, crystal structure and activity for ε-caprolactone polymerization

Four novel bridged-amidines H2L (1,4-R1[C(NR2)(NHR2)]2 (R1=C6H4, R2=2,6-iPr2C6H3 (H2L1), R1=C6H4, R2=2,6-Me2C6H3 (H2L2), R1=C6H10, R2=2,6-iPr2C6H3 (H2L3), R1=C6H10, R2=2,6-Me2C6H3 (H2L4)) were prepared in 65–78% isolated yields by the condensation reaction of dicarboxylic acid with four equimolar amounts of amines in the presence of PPSE (polyphosphoric acid trimethylsilyl ester) at 180°C. Alkane elimination reaction of AlMe3 with 0.5 equivalent of the bridged-amidines (H2L) in THF at room temperature afforded the corresponding bimetallic aluminum methyl complexes Me2AlLAlMe2 (L1 (1), L2 (2), L3 (3), L4 (4)) in 83–93% isolated yields. In order to understand whether the bimetallic complexes could show cooperative effect in the polymerization, their related mononuclear analogues LAlMe2 (L5 (5), L6 (6), L7 (7), L8 (8)) had also been synthesized in 85–91% isolated yields by treatment of AlMe3 with 1 equivalent of amidines HL ([PhC(N-2,6-iPr2C6H3)2]H (HL5), [PhC(N-2,6-Me2C6H3)2]H (HL6), [CyC(N-2,6-iPr2C6H3)2]H (HL7), [CyC(N-2,6-Me2C6H3)2]H (HL8)) in THF at room temperature. All these complexes were characterized by elemental analysis, FT-IR, and NMR spectroscopy. The crystal structures of H2L3, 1, 3, 5 and 7 were determined by X-ray diffraction. In all these amidinate aluminum methyl complexes, Al is four-coordinated by two nitrogen atoms from the dihapto-amidinate ligand and two methyl groups to adopt a distorted tetrahedral geometry. These complexes showed activity towards the ring-opening polymerization of ε-caprolactone at room temperature to give unimodal molecular weight distribution polyesters.

One-Pot Synthesis of α,ω-Chain End Functional, Stereoregular, Star-Shaped Poly(lactide)

The synthesis of stereoregular α,ω-chain end functional linear, telechelic, and star-shaped polymers and copolymers is reported using a one-pot chain end functionalization methodology. Aluminum methyl complexes are applied in combination with functional and multifunctional initiating species to synthesize stereoregular poly(lactide)s by the stereospecific ring-opening polymerization of rac-lactide. The quenching of the reactions with an excess of acid chloride functional molecules has enabled the in situ quantitative modification of the ω-chain ends of the polymers such that primary and secondary alkyl and aryl groups can be incorporated. This methodology has been extended to the synthesis of linear, telechelic, and star-shaped polymers with “click” functional handles for both copper-catalyzed Huisgen 1,3-dipolar cycloaddition and thiol−ene Michael additions and block copolymers through the application of a trithiocarbonate, suitable for mediating RAFT polymerization. This tolerant and mild method is used...

Rac-Lactide polymerization using aluminum complexes bearing tetradentate phenoxy-amine ligands

A family of aluminum-methyl complexes supported by tetradentate phenoxy-amine ligands has been prepared and employed in the ring-opening polymerization of rac-lactide; the ligands include N,N-bis(3,5-dimethyl-2-hydroxybenyl)- N′, N′-dimethyl-1,2-diaminoethane (L 1), N, N-bis(3,5-diisopropyl-2-hydroxybenyl)- N′, N′-dimethyl-1,2-diaminoethane (L 2) and N, N-bis(3,5-dichloro-2-hydroxybenyl)- N′, N′-dimethyl-1,2-diaminoethane (L 3). Polymerizations of rac-lactide were carried out by treatment of the aluminum-methyl complexes with PhCH 2OH and rac-lactide at 70 °C, affording well-controlled formation of polylactide (PLA) and a moderate isotactic bias for initiators bearing L 1 and L 2; the chloro-substituted ligand L 3 afforded largely atactic PLA.

Aluminum Complexes Supported by Tridentate Aminophenoxide Ligand as Efficient Catalysts for Ring-Opening Polymerization of ε-Caprolactone

Aluminum complexes bearing dianionic amine bis(phenolate) ligand are described. Reactions of ligand precursors H2O2NMe or H2O2NPr [H2O2NMe = (CH3)N-(CH2-2-HO-3,5-C6H2(tBu)2)2; H2O2NPr = (CH3CH2CH2)N-(CH2-2-HO-3,5-C6H2(tBu)2)2] with 1.1 mol equiv of AlMe3 in toluene afford MeAl(O2NMe) (1) and MeAl(O2NPr) (2) as four-coordinate aluminum methyl complexes. The molecular structures are reported for compounds 1 and 2. Both compounds show excellent catalytic activity toward the ring-opening polymerization of ε-caprolactone in the presence of benzyl alcohol.