Four-coordinate Aluminum Research Articles

Preparation and Characterization of Aluminum Alkoxides and their Application to Ring-Opening Polymerization of ϵ-Caprolactones

The reaction of 2-methoxybenzyl alcohol with one molar equiv of R2AIX in diethyl ether at 0°C gives [(2-MeOC6H4CH2-μ-O)AlRX]2 (1: R = Et, X = Cl, 2: R = X = Et). In addition, 2,4-di-tert-butylphenol reacts with iBu3Al affording a four-coordinated aluminum compound [(μ-2,4-tBu2-C6H4O)Al(iBu)2]2 (4). Single crystal X-ray structure analysis of 4 shows a C2h-symmetry with a planar Al2O2 core. Ring-opening polymerization (ROP) of caprolactones initiated by 1, 4 and [(μ-OCH2C6H4OMe)Al(iBu)2]2 (3) is performed and polyesters with narrow molecular weight distributions were obtained from the “living” ROP of caprolactones. 1H NMR spectroscopic studies of PCL reveal that the initiator of 1 and 3 is through the Al-OAr function, but the initiator of 4 is through the Al- iBu group.

Reactivity of Various Four-Coordinate Aluminum Alkyls towards Dioxygen: Evidence for Spatial Requirements in the Insertion of an Oxygen Molecule into the Al−C Bond

Reactivity of Various Four-Coordinate Aluminum Alkyls towards Dioxygen: Evidence for Spatial Requirements in the Insertion of an Oxygen Molecule into the Al−C Bond

Reactivity of Various Four-Coordinate Aluminum Alkyls towards Dioxygen: Evidence for Spatial Requirements in the Insertion of an Oxygen Molecule into the Al−C Bond

Reactivity of Various Four-Coordinate Aluminum Alkyls towards Dioxygen: Evidence for Spatial Requirements in the Insertion of an Oxygen Molecule into the Al−C Bond

Reactivity of various four-coordinate aluminum alkyls towards dioxygen: evidence for spatial requirements in the insertion of an oxygen molecule into the AI-C bond

The interaction of dioxygen with various tetrahedral aluminum alkyls, (tBu)3Al.OEt2 (1), tBu2Al(mu-OtBu)2AltBu2 (6), (tBu)2Al(mesal) (2) [mesal=methyl salicylate anion], R2Al(mu-pz)2AlR2 [pz=deprotonated pyrazole, R= Me (3a), Et (3b), and tBu (3c)], R2Al(mu-3,5-Me2pz)2AIR2[3,5-Me2pz = deprotonated 3,5-dimethylpyrazole, R= Me (4a), and Et (4b)], and Et2B(mu-pz)2AlEt2 (5), has been investigated. We were particularly interested in the effect of steric hindrances both caused by the metal-bonded substituents and those that result from the nature of the bifunctional ligand used in the oxygenation reaction. In the reaction of 1 with O2, only the formation of the monoalkoxide compound6 was observed. The latter di-tert-butyl compound as well as all planar aluminapyrazoles, that is, the tert-butyl derivative 3c and lower alkylaluminum derivatives with the more demanding 3,5-dimethylpyrazoyl ligands 4a and 4b, are stable under an atmosphere of dry oxygen and ambient conditions. Inspection of the space-filling representation of these compounds has undoubtedly shown that the bulky tert-butyl groups or pyrazolyles ligands, respectively, provide steric protection for the metal center from the dioxygen attack. In contrast, the dialkylaluminum derivatives of pyrazole, 3a and 3b, and the diethylaluminum bis(1-pyrazolyl)borate complex 5, all with the metal center eclipsed with respect to the plane defined by the four nitrogen atoms, react smoothly with O2 to form the alkyl(alkoxy)aluminum complexes. In the reaction of 5 with O2 for example, the Et-B bonds remained intact, and the dimeric five-coordinate compound [Et2B(mu-pz)2 Al(mu-OEt)Et]2 (9) was isolated in good yield. The interaction of mononuclear di-tert-butyl chelate complex 2 with O2 at -15 degrees C gives (tBuOO)(tBuO)Al(mu-OtBu)2Al(mesal)2 (7) in high yield, and the presence of the alkylperoxo moiety is a particularly significant point in the resulting product. All the compounds have been characterized spectroscopically, and the structures of 3c, 4a, 6, 7, and 9 have been confirmed by X-ray crystallography. Structural features of 1-6 are discussed and are considered in relation to the possible approach pathways of the O2 molecule to the four-coordinate metal center. This analysis and the observed apparent dissimilarity in the reactions of model four-coordinate aluminum alkyls with O2 clearly show that the stereoelectronic prerequisites are responsible for the fundamentally different reactivity.

Short and Medium Range Order in Sodium Aluminoborate Glasses. 2. Site Connectivities and Cation Distributions Studied by Rotational Echo Double Resonance NMR Spectroscopy

A quantitative NMR strategy is developed for the study of the network connectivity (intermediate range structure) and the cation distribution in the sodium aluminoborate glass system. The strategy is based on the analysis of rotational echo double resonance (REDOR) spectroscopy applied to the glasses and the crystalline model compounds Li6Al2(BO3)4, NaBO2, and Na2B4O7. The heterodipolar multispin interaction between the quadrupolar spin systems 27Al ↔ 11B and 23Na ↔ 11B is analyzed in terms of approximate second moments, which are extracted from REDOR data measured at short dipolar evolution times. On the basis of 27Al{11B} and 11B{27Al}-REDOR results, the framework connectivity distribution is extracted and compared to statistical and preferential bonding distribution scenarios, respectively. The sodium aluminoborate system is characterized by a large degree of bonding regularity in the framework. Four-coordinated aluminum is preferentially linked to BO3/2 and BO2/2O- units and vice versa, while linking ...

Syntheses of cationic four-coordinated aluminum complexes: counter-ion induced aggregation through the NH⋯I hydrogen bonds

Synthesis of cationic aluminum complexes from the reactions of Lewis bases with Me 2AlI is reported. The reactions of two molar equivalents of ( S)-(−)-1-phenylethylamine, ( R)-(−)-pantolactone (PLH), or ( S)-(−)-ethyl lactate (ELH) with Me 2AlI ( 1), generated in situ from the reaction of Me 3Al with one molar equivalent of I 2 in toluene at room temperature, afford white crystalline solids [Me 2Al(( S)-NH 2(Ph)C(H)CH 3) 2] +I − ( 2), [(PL) 2Al] +I − ( 3) or [(EL) 2Al] +I − ( 4), respectively. However, Me 3Al reacts with one molar equivalent of PLH producing [(Me 2Al(μ-PL)) 2 ( 5). The reaction of Me 2AlI with 2,2′-ethylidene-bis-(4,6-di- tert-butylphenol) (EDBPH 2) yields [Al(EDBP)I(Et 2O)] ( 6) which further reacts with one molar equivalent or two molar equivalents of OPPh 3 giving a neutral complex [Al(EDBP)(OPPh 3)I] ( 7) or a cationic complex [Al(EDBP)(OPPh 3) 2] +I − ( 8). However, the cationic aluminum derivative [Al(EDBP)(HMPA) 2] +I − ( 9) is prepared from the reaction of 6 with excess of HMPA. Crystal structures of 2, 5, and 8 determined by X-ray diffraction method are also presented.

On the mechanism of four-coordinate aluminum alkyls interaction with dioxygen: evidence for spatial demands in the autoxidation reaction

The reactivity of four-coordinate dialkylaluminum derivatives of pyrazole towards dioxygen, particularly with regard to the effect of steric hindrances caused by the metal center bonded substituents and those resulting from the ligand, has been investigated. Our preliminary results show that spatial demands play a vital role in the autoxidation of four-coordinate aluminum alkyls.

Electronic and steric factors affecting the formation of four or five-coordinated aluminum complexes: syntheses and crystal structures of some aluminum alkoxides

The reaction of 2-phenoxyethanol with AlX 3 affords a five-coordinated dimeric product [( μ-OCH 2CH 2OPh)AlX 2] 2 ( 1, X=Cl; 2, X=Br) in high yields. However, the reaction of 2-phenoxyethanol with Al(Bu i ) 3 yields a four-coordinated dimeric product [( μ-O(CH 2) 2OPh)Al(Bu i ) 2] 2 ( 3). A mixture, [( μ-O(CH 2) 2OPh)Al(Et) 0.75(Cl) 1.25] 2 ( 4) is obtained from the reaction of 2-phenoxyethanol with one equivalent of Et 2AlCl. The reaction of 2-methoxybenzyl alcohol with AlR 3 gives a semi-pentacoordinated dimeric product [( μ-OCH 2C 6H 4OMe)AlR 2] 2 ( 5, R=Me; 6, X=Bu i ) in high yields. Crystal structure studies of these compounds reveal that the distance between Al and the phenoxy oxygen or the methoxy oxygen is 3> 6> 5⪢ 4> 1∼ 2. Although both the electronic and steric effect influence the nature of the O,O′-bifunctional ligand chelating on the aluminum center to generate a five- or four-coordinated aluminum, the electronic effect is considered to play a major role. The interaction of aluminum and ethereal oxygen can be monitored by 13C-NMR spectroscopic studies.

Synthesis and characterization of cobalt-containing hydrated aluminophosphate molecular sieves CoAPO4-H3

Pure, highly crystalline AlPO4-H3 (structure type APC) and cobalt-substituted materials CoAPO4-H3 have been synthesized hydrothermally in the presence of dipropylamine. As-made materials are free from organic molecules, which indicates that the amine does not act as a templating molecule but rather as a pH moderator. Under the synthesis conditions, cobalt could substitute for framework aluminum up to an extent of approximately two cobalt atoms per unit cell. Spectroscopic characterization of the samples showed that cobalt substitutes predominantly for four-coordinated aluminum species; i.e., in the 4.82 layers of the structure. The framework charge defect associated with the presence of Co2+ is compensated by protons, and the materials contain essentially Brönsted acid sites. As for pure AlPO4-H3, CoAPO4-H3 materials undergo structure modifications at elevated temperatures, thus leading to CoAPO4-C, CoAPO4-D and CoAPO4-H6. These transformations have been followed by various spectroscopic techniques, including 27Al two-dimensional–five-quantum magic-angle spinning nuclear magnetic resonance spectroscopy, and occur without any modification of the cobalt coordination and the oxidation state.

Effect of composition on the glass-forming tendency of barium aluminosilicate gels derived from metal alkoxides

The effect of composition on the glass-forming tendency of barium aluminosilicate gels derived from metal alkoxides with compositions near the theoretical composition of barium feldspar was studied. Results indicated that, in the experimental range, larger SiO 2 content and larger BaO/Al 2O 3 ratio were advantageous to glass formation. A criterion for glass formation in the studied system was established as ( b+3−4 y)/(2 y+ s(1+ b)/(100− s))< J c, where b is the BaO/Al 2O 3 ratio, s the mol percentage of SiO 2, y the fraction of four-coordinated aluminum over the total amount of aluminum, and J c is a constant for a definite experimental condition. The criterion showed good fitness with the experimental results.

Neutral and Cationic Tetracoordinated Aluminum Complexes Featuring Tridentate Nitrogen Donors: Synthesis, Structure, and Catalytic Activity for the Ring-Opening Polymerization of Propylene Oxide and (d,l)-Lactide

The dilithium salts of N,N‘,N‘‘-tris(trimethylsilyl)diethylenetriamine (1), N-methyl-N‘,N‘‘-bis(trimethylsilyl)diethylenetriamine (2), and N-methyl-N‘,N‘‘-bis(diisopropyl)diethylenetriamine (3) reacted in THF, at −40 °C, with AlCl3 affording monomeric aluminum chloride derivatives 4 (55% yield), 5 (76% yield), and 6 (71% yield), respectively. Addition of 1 equiv of AlMe3 to the amine 2 gave rise to the five-coordinate dimethylaluminum derivative 9 (67% yield); heating a toluene solution of 9 overnight at 80 °C induces a loss of methane giving rise to the four-coordinate methylaluminum compound 10 (50% yield). The amine 2 also reacted at low temperature with LiAlH4 affording derivative 11 (38% yield). Compounds 4−6 reacted subsequently in toluene with 1 equiv of HCl and 1 equiv of AlCl3 to afford the first chiral tetracoordinated aluminum cations 12 (94% yield), 13 (93% yield), and 14 (90% yield), respectively. Single-crystal X-ray diffraction studies of derivatives 4, 5, 8, 10, 11, 12, and 15 have been carried out. They revealed that the tridentate nitrogen donors enforce an approximately trigonal-monopyramidal coordination geometry for neutral and cationic four-coordinate aluminum complexes. All cationic aluminum derivatives 12−14 and the neutral aluminum chloride 4 brought about the oligomerization of propylene oxide (PO). Low molecular weight polymers having narrow molecular weight distributions were obtained. The 13C NMR spectra of these polymers indicate that they consist exclusively of head-to-tail linkages and that these macromolecules are rich in meso diad and isotactic triad sequences. Due to the presence of a free axial site, a new mechanism for PO polymerization is proposed. Methyl- and hydridoaluminum derivatives 10 and 11, as well as aluminum alkoxide, prepared in situ by reacting derivative 13 with PO, initiated the polymerization of (d,l)-lactide in benzene at 80 °C. High molecular weight polymers of broad molecular weight distributions were obtained in good yields.

Reactions of trialkylaluminium with sterically hindered diols

The reactions of trialkylaluminium R 3Al (R=Me, Et) with 2,4-pentanediols [(CH 3) 2(OH)CCH 2C(CH 3) 2OH ( A̱)], [(CH 3) 2(OH)CCH 2CH(OH)CH 3 ( Ḇ)] and 2,5-dimethyl-2,5-hexanediol ( C̱) were studied. Sterically hindered 2,4-pentanediols ( A̱, Ḇ) react with alkylaluminium compounds to form mainly cyclic compounds 1̱– 3̱ of a formula R 5Al 3[diol(−2H)] 2 possessing one central five-coordinated aluminium atom and two terminal four-coordinated aluminium atoms. All the compounds were characterised by means of NMR spectra, elemental analysis and molecular weight determination. The molecular structure of the solid Me 5Al 3[O(CH 3) 2CCH 2C(CH 3) 2O] 2 1̱ was determined by X-ray diffraction analysis. Substituted at terminal carbon atoms 2,5-dimethyl-2,5-hexanediol ( C̱) forms organoaluminium oligomers exclusively. The influence of the diol carbonchain lengths and its substituents on the yield of the cyclic complexes and organoaluminium oligomers are discussed.

Structure, Spectra, and Reaction Energies of the Aluminum−Nitrogen (HAl−NH)2 and (H2Al−NH2)2 Rings and the (HAl−NH)4 Cluster

Rings of four-coordinate aluminum and nitrogen are easily synthesized and well studied, as are clusters of four-coordinate Al and N. Only recently, however, have rings that are derivatives of the model compounds (HAl-NH)(n)() (n = 2, 3) with three-coordinate Al and N been synthesized. Ab initio investigations of the structure, bonding, vibrational spectra, and reaction energies for the three-coordinate ring (HAl-NH)(2), the four-coordinate ring (H(2)Al-NH(2))(2), and the four-coordinate cluster (HAl-NH)(4) are presented. Even in the absence of differences in steric factors, the four-membered ring has longer Al-N bonds than either the six-membered ring or the unassociated aluminum amide, H(2)Al-NH(2). This is due to both rehybridization and pi interactions. Theoretical reaction energies for formation of the (HAl-NH)(4) cluster from the (H(2)Al-NH(2))(2) ring are consistent with intermolecular loss of hydrogen, or the necessity of surface catalysis.

Substitution Reactions of Aluminum Organoamide Complexes [Al(&mgr;-NR(2))(n)Cl(3-n)](2) (R = Et, (i)Pr; n

As part of our ongoing study of different substituents of Al(2)N(2) ring systems, the synthesis, structural characterization, and spectroscopic studies on several amido derivatives of four-coordinated aluminum are described. The compounds Al(2)(NEt(2))(2)(N(i)Pr(2))(2)Cl(2), 1, Al(2)((N(i)Pr(2))(4)Cl(2), 2, Al((N(i)Pr(2))(3), 3, Al(NEt(2))(3), 4, {Al(N(i)PrH)(3)}(3), 5, Al(2)(NEt(2))(2)(N(i)Pr(2))(2)H(2), 6, and AlCl(3)(N(i)PrH(2))(2){Al(NH(3))(NH(2))[Al(N(i)PrH)(N(i)Pr)Cl](2)}(2), 7, were synthesized by substitution reactions and characterized by mass spectra, IR spectra, elemental analysis, and (1)H, (13)C, and (27)Al NMR data. The structures of three compounds, viz., 1, 6, and 7, have been determined by single-crystal X-ray diffraction analysis. Crystal data with Mo Kalpha (1 and 7, lambda = 0.710 73 Å) or Cu Kalpha (6, lambda = 1.541 78 Å) radiation: (1) Al(2)N(4)C(20)H(48)Cl(2), a = 7.747(2) Å, b = 9.648(2) Å, c = 10.110(3) Å, alpha = 102.91(2) degrees, beta = 83.54(2) degrees, gamma = 110.19(2) degrees, triclinic, space group P&onemacr;, Z = 1, R = 0.053 for 1789 (I > 3sigma(I)) reflections; (6) Al(2)N(4)C(20)H(50), a = 15.088(2) Å, b = 14.506(3) Å, c = 12.439(3) Å, orthorhombic, space group Cmca (No. 64), Z = 4, R = 0.045 for 701 (I > 3sigma(I)) reflections; (7) Al(7)N(14)C(30)H(86)Cl(7), a = 29.914(9) Å, b = 10.197(3) Å, c = 20.525(5) Å, beta = 90.99(4) degrees, monoclinic, space group C2/c, Z = 4, R = 0.042 for 2472 (I > 2sigma(I)) reflections.

A Synthetic and Theoretical Study of the Aggregation of Amidoaluminium Hydrides: Solid‐State Structure of the Trimethylamine Adduct [(Me3Si)2NA1(Cl)(H) · NMe3

AbstractThe amidoaluminium hydride complexes [(Me3Si)2NA1(X)(H) · NMe3], X = H (1), Cl (2), were prepared by the metallation of bis(trimethylsilyl)amine by Al(X)(H)2 · NMe3 (X = H and Cl). The molecular structure of 2 as a monomeric Lewis base adduct with four‐coordinate aluminium centres and terminal amido groups was confirmed by X‐ray crystal structure determination. We also find that bis(trimethylsilyl)amine forms a thermally stable adduct of alane, (Me3Si)2N(H) · AlH3 (3). Ab initio molecular orbital calculations on the possible products arising from these reactions yielding 1 and 2 revealed that the amido‐bridged species, {(μ‐H2N)Al(X)H}2 (X = H and Cl), are favoured over nitrogen donor Lewis base adduct formation, H2NAl(X)(H) · NH3 (X = H and Cl), and then chloro‐bridged, {H2NAl(μ‐X)(H)}2, (X = Cl only), and hydrido‐bridged species, {H2NAl(X)(μ‐H)}2 (X = H and Cl).

Reactions of trimethylaluminium with aliphatic diols

The reactions of trimethylaluminium with aliphatic diols, derivatives of 1,4-butandiol and 1,3-propadiol were studied. A mixtures of linear and cyclic organoaluminium compounds were the products of these reactions carried out for AIME 3: diol molar ratios of 3:2 or greater. One of the products isolated in all the studied reactions was a complex of a formula Me 5Al 3[diol(-2H)] 2 possessing one central five-coordinated aluminium atom and two terminal four-coordinated aluminium atoms. All the complexes were characterized by means of 1H, 13C and 27Al NMR spectra, elemental analysis and molecular weight determination.

IR spectroscopic studies of the acidic properties of the mesoporous molecular sieve MCM-41

The concentration, acid strength and thermal stability of the Brønsted- and Lewis-acid sites in the proton form of the mesoporous molecular sieve MCM-41 were monitored by IR spectroscopy as function of the Al content. The results were compared with those for zeolite H+-ZSM-5 with the same elemental composition. The spectra of hydroxy groups in purely siliceous [Si]-MCM-41 and H+-[Si, Al]-MCM-41 contain a single band at 3740 cm–1, assigned to the Si—OH groups, and are independent of the Al content. Sorption of ammonia or pyridine generates bands from the ammonium and pyridinium ions. The concentrations of both Brønsted- and Lewis-acid sites in H+-[Si, Al]-MCM-41 are lower than the concentration of four-coordinate aluminium. The Brønsted sites in H+-MCM-41 are more prone to dehydroxylation than those in zeolite H+-ZSM-5, and the effect increases with the Al content. The acid strength of the Brønsted sites in H+-MCM-41 is independent of the Al content and much lower than in zeolite H+-ZSM-5.

The AlO bond interaction in four-coordinate aluminum aryloxide compounds

What is a normal AlO bond? The nature of this common chemical bond is discussed with respect to the observed structural and spectroscopic parameters for monomeric aluminum aryloxide compounds, e.g. AlR x (OAr) 3− x (L) n ( x = 0, 1, 2 and n = 0, 1). Experimental evidence for the presence of short AlO bond distances, the low reactivity of residual aluminum-alkyl groups, the electron rich nature of the aluminum center, and the weakening of AlX bonding by the aryloxide ligand are presented. Based upon the experimental observations and theoretical studies, models for the AlO bond are presented and discussed: steric effects, highly ionic bonding, dπ— pπ interactions and π-bonding via donation into AlX σ* orbitals.

Observation of four-coordinate aluminum oxynitride (AlO4-xNx) environments in AlON solids by MAS 27Al NMR at 14 T

The synthesis and magic angle spinning (MAS) [sup 27]Al NMR characterization of a series of aluminum oxynitride powders prepared at various AlN/Al[sub 2]O[sub 3] reaction ratios from 16.7 to 88.0 mole percent AlN are reported. The MAS NMR at 14 T reveal the presence of both six-(AlO[sub 6]) and four-coordinate (AlO[sub 4]) sites in lamda-AlON. Also observed were resonances dur to four-coordinate AlNO[sub 3], AlN[sub 2]O[sub 2], and AlN[sub 3]O sites in the various AlON reaction products, as well as NMR peaks due to the unreacted AlN and Al[sub 2]O[sub 3]. The results of these investigations indicate that sufficient spectral resolution can be obtained via MAS at this field to distinguish and assign four-coordinate aluminum oxynitride moieties in these unique AlON ceramic materials.

27Al and 23Na MAS NMR and Powder X-ray Diffraction Studies of Sodium Aluminate Speciation and the Mechanistics of Aluminum Hydroxide Precipitation upon Acid Hydrolysis

{sup 27}Al and {sup 23}Na MAS NMR, powder X-ray diffraction, and infrared spectroscopic investigations of freeze-dried sodium aluminates and aluminum hydroxides formed through acid hydrolysis have been undertaken, with OH/Al hydrolysis ratios between 5.3 and 2.8 being analyzed. Numerous {sup 27}AlNMR resonances were observed, the intensities of which vary as a function of OH/Al ratio, and these have been assigned to four-, five-, and six-coordinate aluminum species constituting a variety of structural moieties. The dominant species at an OH/Al ratio above 4.4 appears to be a Q{sup o}Na[Al(OH);{sub 4}] salt, as indicated by a {sup 27}Al resonance at 86.6 ppm. In addition, a second, broader resonance at 71.3 ppm demonstrates the simultaneous existence of further four-coordinate aluminum species linked thorough oxo bonds to other four-coordinate aluminums (e.g., Q{sup 2} [Al(OH);{sub 2}(OAl){sub 2}];{sup x-}). At an OH/Al ratio between 4.4 and 4.1, a water-soluble phase forms that contains both four- and six-coordinate aluminum. At OH/Al ratios fo 4.0 and below, a water-soluble phase forms that contains both four-and six-coordinate aluminum. AT OH/Al ratios of 4.0 and below, a water-insoluble phase exists possessing four-, five-, and six-coordinate aluminum. At OH/Al{le}3.9 range exhibits {sup 27}Al chemical shifts similar to those reported for transitionalmore » aluminas such as {gamma}-, {eta}-, and 0-Al{sub 2}O{sub 3} and an infrared spectrum similar to pseudo-spinel gels, suggesting that a pseudo-spinel intermediate is the first phase involved in the crystallization of gibbsite. The resonance assigned to five-coordinate aluminum probably results from species involved in the transformation of the pseudo-spinal phase to pseudo-boehmite. The formation of gibbssite on the acid hydrolysis of alkaline sodium aluminate solutions thus appears to follow the pathway pseudo-spinel {r_arrow} pseudo-boehmite {r_arrow} bayerite {r_arrow} gibbsite. 82 refs., 7 figs., 3 tabs.« less