On the difference of reactivities of various aggregated forms of aluminium triisopropoxide in initiating ring-opening polymerizations

The polymerization of ε-caprolactone (εCL) initiated with aluminium triisopropoxide (Al(OⁱPr)₃) trimer ( A ₃) and/or tetramer ( A ₄) was studied. The rate of A ₃ ⇔︁ A ₄ interconversions in the diluted (≤0,1 mol · L^-1) C₆D₆, C₆D₆/εCL, and THF/εCL solutions were found to be slow, when compared with the rate of propagation. Comparison of the ¹H NMR spectra of the initiators with those of the polymerization mixtures revealed that A ₃ is much more reactive than A ₄ in their reactions with εCL. From the initiator reacted with εCL all threeOⁱPr groups from Al(OⁱPr)₃ are transferred into the poly(εCL) as end groups. Kinetic studies of polymerization confirmed the large reactivity difference between A ₃ and A ₄.     

Ring-opening copolymerization of α-chloromethyl-α-methyl-β-propionolactone with ε-caprolactone

α-Chloromethyl-α-methyl-β-propionolactone (CMMPL) has been copolymerized with ε-caprolactone (CL) using a wide range of feed compositions and aluminium triisopropoxide [Al(OⁱPr)₃] as an initiator. Random copolymers of CMMPL with CL were obtained. The pendant chloromethyl groups of the copolymer were converted to quaternary ammonium salts by reaction with pyridine, resulting in an increased hydrophilicity of the copolymers.     

“Coordination‐insertion” ring‐opening polymerization of 1,4‐dioxan‐2‐one and controlled synthesis of diblock copolymers with ε‐caprolactone

This communication deals with the coordination‐insertion ring‐opening polymerization of 1,4‐dioxan‐2‐one (DX) as initiated by aluminium triisopropoxide (Al(OⁱPr)₃) either in bulk or in solution. First, polymerization of DX has been carried out in bulk at 100°C and compared to the ring‐opening polymerization promoted by tin(II)octoate. Block copolymers of ε‐caprolactone (CL) and DX have been then selectively obtained by first initiating CL polymerization with Al(OⁱPr)₃ in toluene and then adding DX to the living PCL macroinitiator solution at room temperature. In spite of the inherent poor solubility of poly(1,4‐dioxan‐2‐one) in most organic solvents, DX polymerization has proven to proceed through a “living” mechanism. Interestingly enough, the semi‐crystalline P[CL‐b‐DX] block copolymers displayed two well separated melting endotherms at ca. 55 and 102°C for PCL and PDX sequences, respectively.     

3‐[4′‐(Diethylboryl)phenyl]pyridine: Exclusive Crystallization of the Cyclic Tetramer

The crystallization of 3‐[4′‐(diethylboryl)phenyl]pyridine ( 1 ), which formed a mixture of oligomers in solution with the cyclic trimer as a major component, in acetone at 0 °C afforded a cyclic tetramer that co‐crystallized with solvent molecules. Similarly, solutions of compound 1 in toluene at 10 °C and in benzene at 8 °C furnished the cyclic tetramer with the incorporation of toluene and benzene molecules, respectively, thus suggesting that the cyclic tetramer was the minor component. ¹³C CP/MAS NMR spectroscopy of precipitates of compound 1 suggested that precipitation from acetone and toluene each afforded mixtures of the cyclic trimer and the cyclic tetramer, whereas precipitation from benzene exclusively furnished the cyclic tetramer. Therefore, it appeared that crystallization readily shifted the equilibrium towards the cyclic tetramer in benzene. The thermodynamic parameters for the equilibrium between these two oligomers in [D₆]benzene, as determined from a van′t Hoff plot, were ΔH°=?8.8 kcal mol^?1 and ΔS°=?23.7 cal mol^?1 K^?1, which were coincident with previously reported calculations and observations.     

Micellization of cetyltrimethylammonium bromide in the presence of 9-anthryl alkanols

 Surface tension measure-ments in aqueous cetyltrimethyl ammonium bromide were performed in presence of various amounts of 9-(hydroxymethyl)anthracene (AM), 9-[1-(1-hydroxy)ethyl]anthracene (THAE), and 9-[1-(1-hydroxy-2,2,2-trifluoro)ethyl]anthracene (TFAE). Free energies ΔG ^⊖ _m and ΔG ^⊖ _i of micellization and of adsorption to the air–water interface, respectively, were determined as well as the corresponding enthalpies and entropies. ΔG ^o− _m of micellization increased in the presence of AM and THAE, but became more negative when TFAE was added. In contrast to AM and THAE, TFAE addition decreases ΔS ^⊖ _i. For this peculiarity of TFAE, its location and orientation in micellar solution was investigated by means of UV and ¹⁹F-NMR spectroscopy. Received: 26 March 1997 Accepted: 16 May 1997     

Structural peculiarities and the possibility of the existence of small water cluster anions (H2O) n − withn≤4

Structures of (H₂O) _n ⁻ anions withn≤4 were optimized at the UHF/4-31++G^** level and their stability was estimated at the MP2/4-31++G^** level. The trimer anion has a chain-like structure while the tetramer anion can exist either in a chain-like or a cyclic configuration. In the dimer anion and in the chain-like anions, the excess electron density is localized on the terminal water molecule, an acceptor of the H-bond proton. In the cyclic anion, it is uniformly distributed over the free hydrogen atoms. All considered anions have energy values higher than those of the corresponding neutral oligomers. The detachment of an electron from the anions should proceed with the liberation of energy. However, trimer and larger anions are stable against dissociation into individual water molecules and a free electron. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 41–46, January, 1997.     

Isolation of a Cationic Platinum(II) σ‐Silane Complex

The platinum complex [Pt(I^tBuⁱPr′)(I^tBuⁱPr)][BAr^F] interacts with tertiary silanes to form stable (<0 °C) mononuclear Pt^II σ‐SiH complexes [Pt(I^tBuⁱPr′)(I^tBuⁱPr)(η¹‐HSiR₃)][BAr^F]. These compounds have been fully characterized, including X‐ray diffraction methods, as the first examples for platinum. DFT calculations (including electronic topological analysis) support the interpretation of the coordination as an unusual η¹‐SiH. However, the energies required for achieving a η²‐SiH mode are rather low, and is consistent with the propensity of these derivatives to undergo Si?H cleavage leading to the more stable silyl species [Pt(SiR₃)(I^tBuⁱPr)₂][BAr^F] at room temperature.     

Quantum chemical exploration of formaldehyde clusters (H2CO)n (n = 2–4)

Global exploration of equilibrium structures and interconversion pathways on the quantum chemical potential energy surface (PES) is performed for (H₂CO)n (n = 2–4) by using the Scaled Hypersphere Search‐Anharmonic Downward Distortion Following (SHS‐ADDF) method. Density functional theoretical (DFT) calculations with empirical dispersion corrections (D3) yielded comparable results for formaldehyde dimer in comparison with recent detailed studies at CCSD(T) levels. Based on DFT‐D3 calculations, trimer and tetramer structures and their stabilities were studied. For tetramer, a highly symmetrical S₄ structure was found as the most stable form in good accordance with experimentally determined tetramer unit in the formaldehyde crystal. © 2018 Wiley Periodicals, Inc.     

Synthetic,spectral and structural studies of mononuclear tris(κ-amidate) aluminium complexes supported by tripodal ligands

The synthesis and characterization of mononuclear tris(κ²-amidate) aluminium complexes supported by the tripodal ligands, [N(o-PhNC(O)R)₃]³⁻ (R = ⁱPr and ^tBu), are described. The molecular structures of [Al(N(o-PhNC(O)ⁱPr)₃)] and [Al(N(o-PhNC(O)^tBu)₃)] have been determined by X-ray diffraction studies. Both neutral six-coordinate aluminium complexes display coordination geometries that are intermediate between octahedral and trigonal prismatic. Solution-state NMR studies (¹H, ¹³C and ²⁷Al) indicate that these structures are non-fluxional in solution. Detailed analysis of the solid-state structures shows that slight changes in the relative size of the amidate acyl substituents do not significantly impact the solid-state structures. However, large substituents may be required to prevent the formation of multinuclear species.     

Intermolecular chain transfer to polymer with chain scission: general treatment and determination of kp/ktr in L,L-lactide polymerization

A general kinetic treatment of the system with intermolecular chain transfer followed by fast reinitiation is given. It leads to the broadening of the molecular weight distribution (MWD), the number of growing chains being invariable. Thus, this system can be considered as a special case of living polymerization. A general method has been elaborated allowing the determination of the ratio of the rate constant of propagation (k_p) to the rate constant of the bimolecular transfer (k⁽²⁾_tr) from the dependence of the MWD on monomer conversion. Numerical values of k_p/k⁽²⁾_tr equal to ≈ 10² and 25 were thus determined for the polymerization of L , L -lactide (L , L -dilactide) initiated with aluminium tris(isopropoxide) trimer ({Al(OⁱPr)₃}₃) and tributyltin ethoxide (ⁿBu₃SnOEt), respectively.     

Synthesis,structure, and catalytic activity of lanthanide Gd(III) and La(III) complexes with N-(cyclohexyl)isopropyl amidate ligand

Two new lanthanide amidate complexes, {Gd₂[Cy(NCO)ⁱPr]₆} (1) and {La₂[Cy(NCO)ⁱPr]₆[Cy(HNCO)ⁱPr]} (2) (ⁱPr = isopropyl, Cy = cyclohexyl), have been synthesized in good yields by silylamine elimination reaction between Gd[N(SiMe₃)₂]₃ or La[N(SiMe₃)₂]₃ and N-(cyclohexyl)isopropyl amide. Complexes 1 and 2 have been characterized by NMR, elemental analyzes, and X-ray diffraction. The molecular structures of {[Cy(NCO)ⁱPr]Gd[μ₂-Cy(NCO)ⁱPr]₃Gd[Cy(NCO)ⁱPr]₂} (1) and {[Cy(NCO)ⁱPr]La[μ₂-Cy(NCO)ⁱPr]₃La[Cy(NCO)ⁱPr]₂[Cy(HNCO)ⁱPr]} (2) exhibit a dimer structure with three μ₂-O bridging bonds that look like a windmill. Additionally, 2 formed an intramolecular N–H···O hydrogen bond via a neutral amide. The catalytic properties of 1 and 2 for ring-opening polymerization (ROP) of ε-caprolactone have been studied. The results show that 1 and 2 are efficient catalysts for the ROP of ε-caprolactone.     

Isolation and single crystal study of [Nb2(μ-OMe)2(OiPr)8]. Can alcohol interchange provide the homoleptic niobium isopropoxide?

Niobium isopropoxide, Nb(OⁱPr)₅, is an attractive precursor of simple and complex niobium oxides in sol-gel technology. This compound cannot, unfortunately, be obtained by alcohol interchange starting from linear chain homologues such as Nb(OMe)₅ or Nb(OEt)₅. The equilibrium in the latter reaction favours formation of mixed-ligand complexes, [Nb₂(OR)₂(OⁱPr)₈], R = Me, Et. In particular, [Nb₂(OMe)₂(OPrⁱ)₈] (1) has been isolated in high yield from repeated treatment of Nb₂(OMe)₁₀ with excess of isopropanol. The X-ray single crystal study reveals a dinuclear structure containing a pair of edge-sharing octahedra with methoxide ligands in the bridging position. Infrared (IR) and mass spectroscopy (MS) studies confirmed the incomplete ligand substitution. The ¹H-NMR spectra suggest equilibrium between different molecular forms in solution. Solvothermal interaction of 1 with La chips in toluene/isopropanol media results in formation of a mixture of LaNb₂(OⁱPr)₁₃ and La₂Nb₄(μ₄−O)₄(OH)₂(μ−OⁱPr)₈(OⁱPr)₈ (2). Electronic Supplementary Material The online version of this article (doi: ) contains supplementary material, which is available to authorised users.     

The Mechanism of CO2 Insertion into Iridium(I) Hydroxide and Alkoxide Bonds: A Kinetics and Computational Study

The facile insertion of CO₂ into iridium(I) hydroxide, alkoxide, and amide bonds was recently reported. In particular, [Ir(cod)(IiPr)(OH)] (IiPr=1,3‐bis(isopropyl)imidazol‐2‐ylidene) reacted with CO₂ in solution and in the solid state in a matter of minutes to give the novel [{Ir(cod)(IiPr)}₂(μ‐κ¹O:κ²O,O‐CO₃)] complex. In the present study, this reaction is probed using kinetics and theoretical studies, which enabled us to analyse its facile nature and to fully elucidate the reaction mechanism with excellent correlation between the two methods.     

二（胍基）锆配合物的合成和表征

Addition of one equivalent of LiN（i-Pr）2 or LiN（CH2）5 to carbodiimides, RN=C=NR [R=cyclohexyl （Cy）, isopropyl （i-Pr）], generated the corresponding lithium of tetrasubstituted guanidinates {Li[RNC（N R^′2）NR]（THF）}2 [R=i-Pr, N R^′2=N（i-Pr）2 （1）, N（CH2）5 （2）; R=Cy, N R^′2=N（i-Pr）2 （3）, N（CH2）5 （4）]. Treatment of ZrCl4 with freshly prepared solutions of their lithium guanidinates provided a series of bis（guanidinate） complexes of Zr with the general formula Zr[RNC（N R^′2）NR]2Cl2 [R=i-Pr, N R^′2=N（i-Pr）2 （5）, N（CH2）5 （6）; R=Cy, N R^′2=N（i-Pr）2 （7）, N（CH2）5 （8）]. Complexes 1, 2, 5-8 were characterized by elemental analysis, IR and ^1H NMR spectra. The molecular structures of complexes 1, 7 and 8 were further determined by X-ray diffraction studies.     

Polymerization of ε-Caprolactone and its Copolymerization with γ-Butyrolactone using Metal Complexes

Solution polymerization of ε-caprolactone (ε-CL) was performed using four different initiators namely: tin(II) octanoate (Sn(Oct)₂)/ethanolamine, aluminium Schiff's base complex-HAPENAlOⁱPr, lithium diisopropyl amide (LDA) and aluminium isopropoxide. The reaction conditions varied with the initiator used. LDA gave rise to the most rapid polymerization with the highest amount of cyclic species as detected by ¹³C NMR. However, no cyclic species were detected when HAPENAlOⁱPr was used as initiator. The tin(II) octanoate/ethanolamine system lead to an α,ω-dihydroxy-polycaprolactone (PCL). The copolymerization of ε-CL was then performed with the hard to oligomerize γ-butyrolactone using the four initiators. GPC (Gel Permeation Chromatography) analyses showed the formation of copolymers. The highest incorporation of polybutyrolactone (PBL) in the copolymer was obtained using HAPENAlOⁱPr as evidenced by ¹H NMR. ¹³C NMR indicated the presence of pseudoperiodic random copolymers with short blocks of PCL whose block length varied with initiator used. The longest and shortest block length were obtained using Sn(Oct)₂ and HAPENAlOⁱPr respectively as calculated from ¹³C NMR. The reactivity ratios were determined using the Finemann-Ross method at low conversion with HAPENAlOⁱPr as initiator. The values obtained, r_CL = 19.4 and r_BL = 0.11, confirmed the presence of long blocks of CL units in the copolymer.     

Tuning the Stability and Reactivity of Metal‐bound Alkylperoxide by Remote Site Substitution of the Ligand

An alkylperoxonickel(II) complex with hydrotris(3,5‐diisopropyl‐4‐bromo‐1‐pyrazolyl)borate, [Ni^II(OOtBu)(Tp^iPr2,Br)] ( 3a ), is synthesized, and its chemical properties are compared with those of the prototype non‐brominated ligand derivative [Ni^II(OOtBu)(Tp^iPr2)] ( 3b ; Tp^iPr2=hydrotris(3,5‐diisopropyl‐1‐pyrazolyl)borate). Same synthetic procedures for the prototype 3b and its precursors can be employed to the synthesis of the Tp^iPr2,Br analogues. The dimeric nickel(II)‐hydroxo complex, [(Ni^IITp^iPr2,Br)₂(μ‐OH)₂] ( 2a ), can be synthesized by the base hydrolysis of the labile complexes [Ni^II(Y)(Tp^iPr2,Br)] (Y=NO₃ ( 1a ), OAc ( 1a′ )), which are obtained by the metathesis of NaTp^iPr2,Br with the corresponding nickel(II) salts, and the following dehydrative condensation of 2a with the stoichiometric amount of tert‐butylhydroperoxide yields 3a . The unique structural characteristics of the prototype 3b , that is, highly distorted geometry of the nickel center and intermediate coordination mode of the O O moiety between η¹ and η², are kept in the brominated ligand analogue 3a . The introduction of the electron‐withdrawing substitutents on the distal site of Tp^R affects the thermal stability and reactivity of the nickel(II)‐alkylperoxo species.     

Modification of aluminium alkoxides with dialkylmalonates

Abstract  

Depending on the reaction conditions, different aluminium dialkylmalonate derivatives were obtained by reaction of aluminium alkoxides Al(OR)₃ (R = Et, iPr, tBu) with dialkylmalonates, viz. Al(malonate)₃ (malonate = dimethyl, diethyl, di-iso-propyl and di-tert-butyl malonate), Al₂(OiPr)₄(malonate)₂ (malonate = di-iso-propyl and di-tert-butyl malonate), Al₂(OiPr)₂(di-iso-propylmalonate)₄ and Al₃(OH)(OEt)₃(diethylmalonate)₅. All compounds were characterized by NMR spectroscopy, and single crystal structure analyses are reported for each type of compound. An Al₂(OiPr)₂(dialkylmalonate)₄ derivative was only obtained by disproportionation of Al₂(OiPr)₄(di-iso-propylmalonate)₂, but not by reaction of Al(OiPr)₃ with dialkylmalonates in the corresponding molar ratio. Reactions of Al(OtBu)₃ only resulted in Al(malonate)₃ derivatives, but no transesterification was observed, contrary to the reaction of Al(OiPr)₃ with dimethyl or diethyl malonate.     

胍基锆酰胺配合物与四氯化碳的反应生成氯化锆胍基衍生物

四氯化碳与Zr（NMe₂）₂[ⁱPrNC（NMe₂）NⁱPr]₂（1）反应先生成中间体ZrCl（NMe₂）₂[ⁱPrNC（NMe₂）NⁱPr]₂（2）,然后生成ZrCl₂[ⁱPrNC（NMe₂）NⁱPr]₂（3）。该反应可能是自由基反应。另外,配合物2可由ZrCl（NMe₂）₃与二异丙基碳二亚胺ⁱPr-N=C=N-ⁱPr反应制备。对配合物2进行了核磁共振（NMR）和元素分析。     

Mechanism of Copper(I)‐Catalyzed Allylic Alkylation of Phosphorothioate Esters: Influence of the Leaving Group on α Regioselectivity

The mechanism of Cu^I‐catalyzed allylic alkylation and the influence of the leaving groups (OPiv, SPiv, Cl, SPO(OiPr)₂; Piv: pivavloyl) on the regioselectivity of the reaction have been explored by using density functional theory (DFT). A comprehensive comparison of many possible reaction pathways shows that [(iPr)₂Cu]^? prefers to bind first oxidatively to the double bond of the allylic substrate at the anti position with respect to the leaving group, and this is followed by dissociation of the leaving group. If the leaving group is not taken into account, the reaction then undergoes an isomerization and a reductive elimination process to give the α‐ or γ‐selective product. If OPiv, SPiv, Cl, or SPO(OiPr)₂ groups are present, the optimal route for the formation of both α‐ and γ‐substituted products changes from the stepwise elimination to the direct process, in which the leaving group plays a stabilizing role for the reactant and destabilizes the transition state. The differences to the energy barrier for the α‐ and γ‐substituted products are 2.75 kcal mol^?1 with SPO(OiPr)₂, 2.44 kcal mol^?1 with SPiv, 2.33 kcal mol^?1 with OPiv, and 1.98 kcal mol^?1 with Cl, respectively; these values show that α regioselectivity in the allylic alkylation follows a SPO(OiPr)₂>SPiv>OPiv>Cl trend, which is in satisfactory agreement with the experimental findings. This trend mainly originates in the differences between the attractive electrostatic forces and the repelling steric interactions of the SPO(OiPr)₂, SPiv, OPiv, and Cl groups on the Cu group.     

Cationic Five‐Coordinate Bis(guanidinato)silicon(IV) Complexes with SiN4El Skeletons (El=S,Se): “Heterolytic Activation” of S−S and Se−Se Bonds

The donor‐stabilized silylene [iPrNC(NiPr₂)NiPr]₂Si ( 2 ) reacts with PhEl?ElPh (El=S, Se) to form the respective cationic five‐coordinate bis(guanidinato)silicon(IV) complexes {[iPrNC(NiPr₂)NiPr]₂SiSPh}⁺PhS^? ( 4 ) and {[iPrNC (NiPr₂)NiPr]₂SiSePh}⁺PhSe^? ( 5 ). Compounds 4 and 5 were characterized by crystal structure analyses and NMR spectroscopic studies in the solid state.