Aluminum Alkyl Complexes Research Articles

Alkyl initiated ring opening polymerization of lactide by indium complexes supported by NCN pincer ligands.

Indium-alkyl NCN pincer complexes supported by amine and imine donors are active catalysts for the ring opening polymerization of lactide. The amine indium complexes are significantly more active for polymerization due to the decoordination of the amine donors at higher temperatures. Solid state structures of the imine and amine indium complexes indicate that the imine-indium bonds are stronger than the analogous amine-indium bonds. Higher molecular weight polymers that were not observable by MALDI-TOF were shown to be linear by comparing their intrinsic viscocities with those of known linear polymers. The alkyl indium complexes are more active than the analogous alkyl aluminum complexes but are prone to transesterification reactions.

Alkylaluminum Complexes Featuring Bridged Bis-Formylfluorenimide Ligands for Hydroboration of Aldehyde, Ketone, and Imines.

Three bis-formylfluorenimide ligands with different bridging groups were designed and synthesized, leading to the successful preparation of six novel alkylaluminum complexes through their reaction with alkylaluminum reagents (AlMe3 or AlEt3). Complexes 1 and 2 were obtained by the reaction of 1,2-propylene-bridged diamine (L1) with AlMe3 or AlEt3. By reacting 1,2-cyclohexylene-bridged diamine (L2) with AlMe3 or AlEt3 to obtain complexes 3 and 4. The above ligands formed a bidentate four-coordinate structure with alkylaluminum, which involved the elimination of one alkyl group as the ligand reacted with alkylaluminum. The complexes 5 and 6 were synthesized through the reaction of 1,2-phenylene-bridged diamine (L3) with AlEt3 in toluene or tetrahydrofuran. Notably, L3 exhibited unique reactivity compared with the other ligands, which formed a tridentate four-coordinated structure when reacting with alkylaluminum. The formation of the tridentate complex resulted from the introduction of a benzimidazole derivative or tetrahydrofuran (THF) molecule along with the elimination of two alkyl groups during its coordination with alkylaluminum. All complexes were characterized via 1H NMR, 13C NMR, and elemental analysis, with structural determination confirmed through X-ray. Furthermore, the catalytic activity in the hydroboration reaction of aldehyde, ketone, and imines was investigated with these complexes as catalysts. Among them, complex 1 demonstrated excellent catalytic performance (up to 99% yield) and broad substrate compatibility (more than 30 substrates) at low catalyst loading (1 mol %) under mild reaction conditions.

Aluminum Tris(2-pyridyl)borates: Structure, reactivity and catalysis

Tris(2-pyridyl)borates are emerging scorpionate ligands distinguished by their robustness and modularity, as well as their strong electron-donicity. Metal tris(2-pyridyl)borates show a strong tendency toward six-coordinate, pseudooctahedral metal geometries with static κ3-ligation. Most exceptions involve sterically hindered tris(2-pyridyl)borates. We report four-coordinate alkylaluminum complexes with the sterically unhindered ligand phenyl tris(2-pyridyl)borate (Tpy–), using single-crystal XRD, variable-temperature NMR, and computational modelling to characterize their structure and dynamics. Thus proton exchange between TpyH and trialkylaluminum gave four-coordinate complexes TpyAlR2 (–R = methyl, ethyl, and isobutyl), the first κ2-complexes of sterically unhindered tris(2-pyridyl)borates. Moreover, pyridine coordination in TpyAlR2 complexes is fluxional, showing NMR symmetry down to −80 °C. Computational modelling suggests that pyridine exchange occurs through a higher energy five-coordinate κ3-conformation. By contrast, the analogous gallium complex TpyGaMe2 is also κ2, but NMR does not show pyridine interconversion even up to 70 °C. Alternatively, proton exchange involving TpyH, trialkylaluminums, and acids gives κ3, four-coordinate cations with the composition [TpyAlR]+ (–R = methyl, ethyl, and isobutyl), in addition to an unusual protonated tris(2-pyridyl)borate complex [TpyHAliBu2]+. Furthermore, reaction of TpyH with two equivalents of AlMe3 gave the first bimetallic tris(2-pyridyl)borate complex TpyAl2Me5, with distinct κ2- and κ1-bound aluminum sites. Finally, we characterize controlled polymerization of ε-caprolactone by these complexes at high temperatures.

Novel bidentate N-coordinated alkylaluminum complexes: synthesis, characterization, and efficient catalysis for hydrophosphonylation.

Five new alkylaluminum complexes with different pyridinyl-substituted imines or cyclohexyl-substituted imines were synthesized and characterized successfully. The aluminum complex [FlCHNCH(CH3)Py]AlMe2(Py = 2-pyridyl) (1) was obtained by reacting 9-[2-pyridyl-CH(CH3)-NCH]Fl (Fl = fluorenyl) (L1) and equimolar AlMe3. The reactions of 9-(2-pyridyl-NCH)Fl (L2) and 9-[2-N(CH3)2-cyclohexyl-NCH]Fl (L3) with equimolar AlMe3 or AlEt3 afforded other alkylaluminum complexes [FlCHNPy]AlMe2(Py = 2-pyridyl) (2), [FlCHNPy]AlEt2 (Py = 2-pyridyl) (3), [FlCHNCyN(CH3)2]AlMe2 (Cy = 2-cyclohexyl) (4) and [FlCHNCyN(CH3)2]AlEt2 (Cy = 2-cyclohexyl) (5). All these complexes (1-5) were characterized using NMR spectroscopy, elemental analysis, and X-ray crystal structure analysis. The catalytic properties of these new alkylaluminum complexes for the hydrophosphonylation of aldimines were examined. Complex 5 showed the best catalytic performance under mild reaction conditions with a low catalyst loading (1 mol%), and 20 different substituents of aldimines were isolated with more than 90% yields.

A binuclear aluminium complex as a single competent catalyst for efficient synthesis of urea, biuret, isourea, isothiourea, phosphorylguanidine, and quinazolinones.

The synthesis and characterisation of two mononuclear aluminium alkyl complexes with the general composition [Al(Me)2{Ph2P(E)N(CH2)2N(CH2CH2)2O}] (E = Se (2a); S (2b)), and two binuclear aluminium complexes, [Al(Me)2{Ph2P-(E)N(CH2)2N(CH2CH2)2O}(AlMe3)] (E = Se (3a) and S (3b)), are described. The binuclear aluminium alkyl complex 3a proved to be a proficient catalyst for the addition of simple nucleophiles to heterocumulenes, leading to the synthesis of a variety of products such as urea, biuret, isourea, isothiourea, phosphorylguanidine, and quinazolinone derivatives, in contrast to its mononuclear analogues. Complex 3a is the first example of a single competent catalyst, which is also low-cost and eco-friendly and derived from a main-group metal, under solvent-free conditions either at room temperature or mild temperatures. Complex 3a possessed a wide functional group tolerance including heteroatoms, yielding the corresponding insertion products in good quantities and with high selectivity.

Improvement of catalytic activity for α‐diimine nickel complex with active sites stabilized by bulky boron counterions at elevated temperature

Three α‐diimine Ni (II) catalysts (Cat.1, Cat.2 and Cat.3) were synthesized and investigated for ethylene polymerization using alkyl aluminum and organoboron compounds as binary cocatalyst. The effects of different alkyl aluminum and boron complexes on ethylene polymerization catalyzed by Cat.1, Cat.2, and Cat.3 were studied. The sodium tetrakis[3,5‐bis (trifluoromethyl)phenyl]borate (NaBArF) was found to have best promotion for the catalytic activity of Cat.1, compared to negative effect of dimethylanilinium tetrakis (pentafluorophenyl)borate (TTB) and trityl tetrakis (pentafluorophenyl)borate (AB). All the three catalysts showed higher catalytic activity when NaBArF was introduced into the catalytic system, especially for Cat.3 (increased by 41.3% at 70°C). There was still minor increase of the catalytic activity for Cat.2 which have much better thermostability than Cat.3. The charging sequence of NaBArF and the [B]/[Ni] ratio have significant effects on the catalytic activity. The better catalytic performance of Cat.1/AlEt2Cl/NaBArF could be explained by its ability to raise the number of active centers. Namely, the molar percentage of active centers of Cat.1/AlEt2Cl/NaBArF was still above 30%, which was 1.7 times that of Cat.1/AlEt2Cl even if polymerization ran up to 30 min based on 2‐thiophenecarbonyl chloride (TPCC) quench‐labeling method. Lower [Al]/[Ni] ratio but more active centers and longer active centers lifetime of this binary cocatalyst is promising for improving the activity of ethylene polymerization industrially at elevated temperature.

Aluminium alkyl complexes supported by imino-phosphanamide ligand as precursors for catalytic guanylation reactions of carbodiimides.

Herein, we report the synthesis, characterisation, and application of three aluminium alkyl complexes, [κ2-{NHIRP(Ph)NDipp}AlMe2] (R = Dipp (2a), Mes (2b); tBu (2c), Dipp = 2,6-diisopropylphenyl, Mes = mesityl, and tBu = tert-butyl), supported by unsymmetrical imino-phosphanamide [NHIRP(Ph)NDipp]− [R = Dipp (1a), Mes (1b), tBu (1c)] ligands as molecular precursors for the catalytic synthesis of guanidines using carbodiimides and primary amines. All the imino-phosphanamide ligands 1a, 1b and 1c were prepared in good yield from the corresponding N-heterocyclic imine (NHI) with phenylchloro-2,6-diisopropylphenylphosphanamine, PhP(Cl)NHDipp. The aluminium alkyl complexes 2a, 2b and 2c were obtained in good yield upon completion of the reaction between trimethyl aluminium and the protic ligands 1a, 1b and 1c in a 1 : 1 molar ratio in toluene via the elimination of methane, respectively. The molecular structures of the protic ligands 1b and 1c and the aluminium complexes 2a, 2b and 2c were established via single-crystal X-ray diffraction analysis. Complexes 2a, 2b and 2c were tested as pre-catalysts for the hydroamination/guanylation reaction of carbodiimides with aryl amines to afford guanidines at ambient temperature. All the aluminium complexes exhibited a high conversion with 1.5 mol% catalyst loading and broad substrate scope with a wide functional group tolerance during the guanylation reaction. We also proposed the most plausible mechanism, involving the formation of catalytically active three-coordinate Al species as the active pre-catalyst.

Aluminium complexes supported by a thioether-bridged salen ligand: synthesis, characterization and application in ε-caprolactone homopolymerization and copolymerization with L-lactide

Several aluminium alkyl and alkyloxide complexes supported by a thioether-bridged salen ligand were synthesized by the protonolysis reaction. Reactions of N,N’-bis(3,5-di-tert-butylsalicylidene)-2,2’-diaminophenylsulfide (H2L) with one or two equivalents of AlMe3 in toluene at 100 °C afforded the corresponding mono- and binuclear aluminium methyl complexes LAlMe (1) and L(AlMe2)2 (2). Treatment of the in situ generated complex 1 with one equivalent of ROH (R = PhCH2, iPr, CH3OCH2CH2) produced the desired aluminium alkyloxide complexes LAlOCH2Ph (3), LAl(OiPr) (4) and LAlO(CH2)2OCH3 (5). All the molecular structures were established by single crystal X-ray diffraction studies. It was found that aluminium alkyloxide complexes 3-5 showed higher catalytic activity for the homopolymerization of ε-caprolactone (ε-CL) than corresponding aluminium methyl complexes 1 and 2. Furthermore, these alkyloxide complexes were also efficient initiators for the copolymerization of L-lactide and ε-CL to give practically random copolymers.

Aluminum Amidinates: Insights into Alkyne Hydroboration.

The mechanism of the aluminum-mediated hydroboration of terminal alkynes was investigated using a series of novel aluminum amidinate hydride and alkyl complexes bearing symmetric and asymmetric ligands. The new aluminum complexes were fully characterized and found to facilitate the formation of the (E)-vinylboronate hydroboration product, with rates and orders of reaction linked to complex size and stability. Kinetic analysis and stoichiometric reactions were used to elucidate the mechanism, which we propose to proceed via the initial formation of an Al-borane adduct. Additionally, the most unstable complex was found to promote decomposition of the pinacolborane substrate to borane (BH3), which can then proceed to catalyze the reaction. This mechanism is in contrast to previously reported aluminum hydride-catalyzed hydroboration reactions, which are proposed to proceed via the initial formation of an aluminum acetylide, or by hydroalumination to form a vinylboronate ester as the first step in the catalytic cycle.

Electron rich (salen)AlCl catalysts for lactide polymerisation: Investigation of the influence of regioisomers on the rate and initiation efficiency

Aluminium-alkyl complexes are well known as initiators for lactide ring-opening polymerisation, yet aluminium-chloride complexes remain underexplored despite benefits such as ease of synthesis and improved air-stability. While aluminium-chloride complexes are typically poor initiators, recent studies have shown that electron rich amino-substituted (salen)AlCl complexes can efficiently initiate lactide polymerisation in the presence of an epoxide. Herein, we report eight ether-substituted complexes as efficient initiators for lactide polymerisation, where exchanging strongly electron-donating amino groups for weaker electron-donating methoxy substituents maintains efficient initiation and also improves the propagation rate by a factor of four. Investigation of ortho-, meta-, para- and meta’-methoxy-substituted regioisomers established that the ortho-substituted complex was twice as active as the other regioisomers. Kinetic and spectroscopic studies suggest that the initiation efficiency is influenced by the electronics (ortho and para > meta and meta’), with substituents closer to Al giving improved initiation (ortho > para and meta’ >meta). While electron-donating ortho-substituents often decrease catalyst activity in lactone polymerisation, here we show that ether groups can act as σ-electron-withdrawing groups and π-electron donors, to deliver improved propagation rates, initiation and tacticity control.

CO2 Capture by 2-(Methylamino)pyridine Ligated Aluminum Alkyl Complexes.

A set of novel, easily synthesized aluminum complexes, Al[κ2-N,N-2-(methylamino)pyridine]2R (R = Et, iBu) are reported. When subjected to 1 atm of CO2 pressure, each hemilabile pyridine arm dissociates and facilitates cooperative activation of the CO2 substrate reminiscent of a Frustrated Lewis Pair. This reaction has limited precedent for Al/N based Lewis Pair systems, and this is the first system readily shown to sequester multiple equivalents of CO2 per aluminum center. The ethyl variant then reacts further, inserting a third equivalent of CO2 into the aluminum alkyl to generate an aluminum carboxylate. Examples of this type of reactivity are rare under thermal conditions. A joint experimental/computational study supports the proposed reaction mechanism.

Mono- and Bimetallic Aluminum Alkyl, Alkoxide, Halide and Hydride Complexes of a Bulky Conjugated Bis-Guanidinate(CBG) Ligand and Aluminum Alkyls as Precatalysts for Carbonyl Hydroboration.

Tetra-aryl-substituted symmetrical conjugated bis-guanidine (CBG) ligands such as L1-3 (3H) [L(3H) = {(ArHN)(ArHN)C═N-C═NAr(NHAr)}; Ar = 2,6-Me2-C6H3 (L1(3H)), 2,6-Et2-C6H3 (L2(3H)), and 2,6-iPr2-C6H3 (L3(3H))] have been employed to synthesize a series of four- and six-membered aluminum heterocycles (1-8) for the first time. Generally, aluminum complexes bearing N,N'- chelated guanidinate and β-diketiminate/dipyrromethene ligand systems form four- and six-membered heterocycles, respectively. However, the conjugated bis-guanidine ligand has the capability of forming both four- and six-membered heterocycles possessing multimetal centers within the same molecule; this is due to the presence of three acidic protons, which can be easily deprotonated (at least two protons) upon treatment with metal reagents. Both mono- and dinuclear aluminum alkyls and mononuclear aluminum alkoxide, halide, and hydride complexes have been structurally characterized. Further, we have demonstrated the potential of mononuclear, six-membered CBG aluminum dialkyls in catalytic hydroboration of a broad range of aldehydes and ketones with pinacolborane (HBpin).

Correction to “Alkylaluminum Complexes as Precatalysts in Hydroboration of Nitriles and Carbodiimides”

Correction to “Alkylaluminum Complexes as Precatalysts in Hydroboration of Nitriles and Carbodiimides”

Aluminum complexes with new non-symmetric ferrocenyl amidine ligands and their application in CO2 transformation into cyclic carbonates.

A set of alkyl aluminum complexes supported by non-symmetric ferrocenyl amidine ligands were used as catalysts for the preparation of cyclic carbonates from epoxides and carbon dioxide using Bu4NI as a co-catalyst. A modified method for the synthesis of aminoferrocene allowed us to obtain this precursor in quantitative yield. Treatment of aminoferrocene with the corresponding acetimidoyl chloride afforded the desired ferrocenyl amidine ligands L1H, (E)-N-(2,6-diisopropylphenyl)-N'-(ferrocenyl)acetimidamide, and L2H, (E)-N-(2,6-dimethylphenyl)-N'-(ferrocenyl)acetimidamide. The reaction of these ligands with 1.0 or 0.5 equiv. of AlMe3 led to the synthesis of aminoferrocene based aluminum complexes ((L1)AlMe2 (1), (L2)AlMe2 (2), (L1)2AlMe (3), and (L2)2AlMe (4)) in excellent yields, which were characterized by spectroscopic and X-ray diffraction methods. In addition, we have studied their electrochemical properties and complex 1 was found to be the most active catalyst for the formation of cyclic carbonates 6a-j from their corresponding epoxides 5a-j and CO2.

Synthesis and Structures of Dinuclear Aluminum Complexes Based on Bis(β‐diketiminate) Ligands

A series of ten dinuclear aluminum alkyl complexes based on rigid, semirigid, and flexible bis(β‐diketiminate) ligands (NacNac) has been obtained from the reaction of trimethylaluminum and the corresponding bis(β‐diketimine)s. All compounds were fully characterized using NMR and IR spectroscopy and elemental analysis. The molecular structures of five compounds have been investigated by means of single‐crystal X‐ray diffraction analysis.

Correction to “Alkylaluminum Complexes as Precatalysts in Hydroboration of Nitriles and Carbodiimides”

Correction to “Alkylaluminum Complexes as Precatalysts in Hydroboration of Nitriles and Carbodiimides”

Alkylaluminum Complexes as Precatalysts in Hydroboration of Nitriles and Carbodiimides

The metathesis reaction of aluminum(III) dihydride LAlH2 (1) (L = (ArNCMe)2CH, Ar = 2,6-i-Pr2C6H4) with n-BuLi and n-Bu2Mg, respectively, resulted in di-n-butyl aluminum complex LAl(n-Bu)2 (2). Mea...

Syntheses of biodegradable and biorenewable polylactides initiated by aluminum complexes bearing porphyrin derivatives by the ring-opening polymerization of lactides

Three aluminum alkyl complexes bearing porphyrin derivatives were synthesized by pyrrole derivatives and benzaldehyde derivatives. These complexes were characterized by NMR and elemental analysis. These complexes are used for racemic-lactide polymerization. In the ring-opening polymerization of racemic lactide, complex 3(2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphyrin aluminum methyl, R1=H, R2=Me in Scheme 2) with isopropanol showed the highest stereoselectivity, and an isotactic polylactide with a Pm of 0.68 was obtained. The polymerization kinetics with complex 3 as catalyst was studied in detail. The kinetics of the polymerization data indicated that the rate of polymerization initiated by complex 3 had first-order dependency on monomer and catalyst. There is a linear relationship between lactide conversion and the number average molecular weight of PLA.

Aluminum complexes with Schiff base bridged bis(indolyl) ligands: synthesis, structure, and catalytic activity for polymerization of rac-lactide.

A series of Schiff base bridged bis(indolyl) ligands were developed for aluminum chemistry. The reactions of AlEt3 or AlMe3 with the Schiff base bridged bis(indolyl) proligands R1(-N[double bond, length as m-dash]CHC8H5NH)2 (R1 = -CH2CH2- (H2L1); -CH2CH2CH2- (H2L2); -CH2CMe2CH2- (H2L3); rac-Cy (H2L4); and R,R-Cy (H2L5)) were studied leading to the synthesis of a series of aluminum alkyl complexes L1AlEt (1)-L5AlEt (5) and L3AlMe (3b) in good yields, while the reaction of H2L3 with Al(OiPr)3 gave the aluminum alkoxide complex L3AlOiPr (3a). These aluminum complexes were characterized by spectroscopic methods and elemental analyses. The solid state structures of the aluminum complexes 1-5 and 3a were confirmed by the X-ray diffraction study. X-ray analyses revealed that the aluminum centre in these complexes is five-coordinated. The coordination geometry is between square pyramidal and trigonal bipyramidal. In the presence of 1 equiv. of isopropanol, the aluminum alkyl complexes exhibited notable activity towards the ring-opening polymerization of rac-lactide at 70 °C in toluene, with good control over molecular weights and dispersities. The substituents and the length of the bridging part between the two Schiff base nitrogen atoms have an influence on either the tacticity of isolated polymers or the rate of polymerization. The kinetics of complex L3AlOiPr (3a) in C6D6 was also investigated, and the experimental results revealed that the rate of polymerization was first-order with respect to rac-lactide.

Four- and five-coordinate aluminum complexes supported by N,O-bidentate β-pyrazylenolate ligands: synthesis, structure and application in ROP of ε-caprolactone and lactide

In this paper we report a series of Al(iii) complexes supported by N,O-bidentate β-pyrazyl functionalized enolate ligands HL1-HL5 (L = (6-Me-2,5-C4H2N2)-CH[double bond, length as m-dash]C(R)-O-), (R = tBu, Ph, p-tolyl, p-OMePh, o-tolyl) and their exploitation for the ring-opening polymerization of ε-caprolactone (ε-CL) and rac-lactide (rac-LA). The structures of the form LAlMe2 (1a-5a) or L2AlMe (1b-4b) were isolated depending on the ligand or stoichiometry of complexation. The investigation of these complexes toward the ROP of both ε-CL and rac-LA under the same conditions showed that dimethyl-aluminum complexes LAlMe2 (1a-5a) gave a higher activity than monomethyl-aluminum complexes L2AlMe (1b-4b). Meanwhile, monomethyl-aluminum complexes 1b-4b promoted the ring-opening polymerization with a better control over molecular weights and polydispersities than 1a-5a. Moreover, all of the LAlMe2 and L2AlMe produced PLA with strong isotactic bias (Pm up to 0.77) and narrow distributions. Generally, a β-pyrazyl enolate system exhibited significantly higher catalytic activity for the ROP of ε-CL and rac-LA than the analogous β-quinolyl enolate systems reported recently. The results are discussed in terms of electronic and steric properties affected by the substituents on the ligands.