Comparison of three elements (In, Sn, and Sb) in the same period as catalysts in the ring-opening polymerization of l-lactide: from amorphous to semicrystalline polyesters

The ring-opening polymerization (ROP) of l-lactide (l-LA) is the main method for synthesizing poly(l-lactide) (PLLA), in which choosing the catalyst is one of the most important parameters. In this work, we focused on the systematic study of catalysts based on p-block elements from period 5, such as indium(iii), tin(ii), tin(iv) and antimony(iii) acetates, which displayed contrasting performances influenced by the oxidation state of the metal center. Analysis of the obtained oligomers by different techniques, including nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), polarized optical microscopy (POM) and matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF), revealed the selectivity of each catalyst toward the ROP of l-LA. Tin(ii) acetate showed the best performance, making it the best catalyst of this series for synthesizing PLLA. Indium(iii) and tin(ii) acetates induced an amorphous and semicrystalline polyester, respectively. The kinetic study evidenced the excellent performance of tin(ii) acetate in the ROP of l-LA. This catalyst reached high conversions in a quarter of the total reaction time, positioning it as the most catalytically active of the selected p-block acetate catalysts. Finally, the coordination-insertion mechanism by the catalyst in the initiation step was corroborated through the development of a mechanistic study applying the density functional theory (DFT).

Luminescent Bis(amidinate) Indium Complexes

Bis-amidinate indium(III) monochlorides [(tBuN)2C(Ph)]2InCl (1), [(tBuN)2C(2-naphthyl)]2InCl (2), [(tBuN)2C(2-anthryl)]2InCl (3), [(tBuN)2C(9-anthryl)]2InCl (4), [(tBuN)2C(9-phenanthryl)]2InCl (5), and [(tBuN)2C(1-pyrene)]2InCl (6) were prepared by the reaction of the corresponding Li-amidinate ligand with InCl3. Single crystal X-ray analysis of compounds 1, 2, 4, and 5 reveals that the In(III) center is bound with two chelating amidinate ligands. The overall coordination geometry around In(III) is distorted trigonal bipyramidal with the chloride occupying one of the equatorial positions. The photophysical properties of these compounds have been analyzed. Compounds 2-6 are emissive in the solution state. The 9-anthryl substituted compound 4 was found to exhibit a maximum quantum yield of 45.5% in dichloromethane. Compound 3 has a maximum lifetime of 11 ns in solution. Theoretical studies were performed to validate the photophysical properties observed in these compounds.

Bis(diiminate)-based boron difluoro complexes: effective synthon for bis(borenium) cations

A series of boron difluoro bis(diiminate) complexes have been prepared and used to obtain triflate substituted fluoroborane complexes. The corresponding well-defined bis(borenium) cations were subsequently synthesized and structurally authenticated. We are also presenting the first experimental and theoretical study of bis(borenium) cations that are derivative of cationic borinic acid.

Expanding the Scope of Aluminum Chemistry with Noninnocent Ligands

ConspectusAluminum is the most abundant metal in the earth's crust at 8%, and it is also widely available domestically in many countries worldwide, which ensures a stable supply chain. To further the applications of aluminum (Al), such as in catalysis and electronic and energy storage materials, there has been significant interest in the synthesis and characterization of new Al coordination compounds that can support electron transfer (ET) and proton transfer (PT) chemistry. This has been achieved using redox and chemically noninnocent ligands (NILs) combined with the highly stable M(III) oxidation state of Al and in some cases the heavier group 13 ions, Ga and In.When ligands participate in redox chemistry or facilitate the breaking or making of new bonds, they are often termed redox or chemically noninnocent, respectively. Al(III) in particular supports rich ligand-based redox chemistry because it is so redox inert and will support the ligand across many charge and protonation states without entering into the reaction chemistry. To a lesser extent, we have reported on the heavier group 13 elements Ga and In, and this chemistry will also be included in this Account, where available.This Account is arranged into two technical sections, which are (1) Structures of Al-NIL complexes and (2) Reactivity of Al-NIL complexes. Highlights of the research work include reversible redox chemistry that has been enabled by ligand design to shut down radical coupling pathways and to prevent loss of H2 from unsaturated ligand sites. These reversible redox properties have in turn enabled the characterization of Class III electron delocalization through Al when two NIL are bound to the Al(III) in different charge states. Characterization of the metalloaromatic character of square planar Al and Ga complexes has been achieved, and characterization of the delocalized electronic structures has provided a model within which to understand and predict the ET and PT chemistry of the NIL group 13 compounds. The capacity of Al-NIL complexes to perform ET and PT has been employed in reactions that use ET or PT reactivity only or in reactions where coupled ET/PT affords hydride transfer chemistry. As an example, ligand-based PT reactions initiate metal-ligand cooperative bond activation pathways for catalysis: this includes acceptorless dehydrogenation of formic acid and anilines and transfer hydrogenation chemistry. In a complementary approach, ligand based ET/PT chemistry has been used in the study of dihydropyridinate (DHP-) chemistry where it was shown that N-coordination of group 13 ions lowers kinetic barriers to DHP- formation. Taken together, the discussion presented herein illustrates that the NIL chemistry of Al(III), and also of Ga(III) and In(III) holds promise for further developments in catalysis and energy storage.

Diversity of transformation of heteroallenes on acenaphthene-1,2-diimine aluminum oxide

The reactions of oxide [(dpp-bian)Al(μ2-O)2Al(dpp-bian)] (1) (dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with phenyl- or cyclohexylisocyanates result in the formation of carbonimidate derivatives [(dpp-bian)Al(μ-O)(μ-RNCO2)Al(dpp-bian)] (R = Ph, 2; Cy, 3). Addition of N,N'-dicyclohexylcarbodiimide to compound 1 leads to the formation of ureate complex [(dpp-bian)Al(μ-O)(μ-(CyN)2CO)Al(dpp-bian)] (4). The reactions of the oxide 1 with pinacolborane and catecholborane afford oxo-bridged hydride [{(dpp-bian)Al(H)}(μ-O){Al(OBpin)(dpp-bian)}] (5) and compound [{(dpp-bian)Al(OBCat)}2(μ-O)] (7), respectively. Insertion of cyclohexylisocyanate into the Al-H bond of compound 5 gives CO insertion product [{(dpp-bian)Al(OC(H)NCy)}(μ-O){Al(OBpin)(dpp-bian)}] (6). New compounds have been characterized by ESR and IR spectroscopy; their molecular structures have been established by single-crystal X-ray analysis. The oxide 1 serves as a catalyst for the hydroboration of heteroallenes (isocyanates, carbodiimides) with pinacolborane.

Synthesis of High-Molecular-Weight Poly(ether-alt-ester) by Selective Double Ring-Opening Polymerization of Spiroorthoesters

We report the selective double ring-opening polymerization of presequenced spiroorthoester monomers to form high-molecular-weight (≈90 kDa) poly(ether-alt-ester)s with a simple cationic alkyl gallium catalyst. The selective formation of double ring-opened polymer units was confirmed by NMR and IR spectroscopies. Thermal and rheological properties of homo- and copolymers were further characterized by differential scanning calorimetry, thermogravimetric analysis, and stress-controlled rotational rheometry. Linear viscoelastic moduli show that these systems are well entangled (plateau modulus), thereby possessing nearly terminal relaxation at long time scales (low frequencies) and Rouse segmental dynamics at short time scales (high frequencies) with characteristic slopes. These are the highest-molecular-weight poly(ether-alt-ester)s reported to date.

Flexible NHC-aryloxido aluminum complex and its zwitterionic imidazolium aluminate precursor in ring-opening polymerization of ε-caprolactone

Anionic donor-functionalized NHC (N-heterocyclic carbene) complexes of Al are rare. We report one such case here, an NHC-aryloxido AlMe2 complex [Al(L)Me2] (2), following a stepwise synthesis from the proligand [HO-4,6-tBu2-C6H2-2-CH2{CH(NCHCHNAr)}]Br [LH2Br; Ar = 2,6-iPr2-C6H3 (Dipp)] and AlMe3via the zwitterionic intermediate [Al(LH)Me2Br] (1). The ligand's flexibility in 2 is evident from the conformational fluxionality revealed by VT-1H NMR spectroscopic analysis. The ∠O-Al-C (ca. 100.5°) bite angle is also wider than the ∠O-Ti-C (ca. 80.6°) as seen in our recently reported Ti complex [Ti(L)(NMe2)2Br]. DFT analysis showed that the CNHC-Al bond is significantly ionic, as is the CNHC-Ti bond. Both 1 and 2 are active in the ring-opening polymerization (ROP) of ε-caprolactone (CL). 2, similar to [Ti(L)(NMe2)2Br], exhibits bifunctional MLC-type monomer activation, but only at an elevated temperature. However, the 2/BnOH combination is catalytically active at room temperature, likely through a zwitterionic [Al(LH)Me2(OBn)]. The 1/BnOH combination follows a similar mechanism but surprisingly at a faster rate.

Highly Electrophilic Mononuclear Cationic Aluminium Alkoxide Complexes: Syntheses, Reactivity and Catalytic Applications

Herein, we report the synthesis of β-diketiminate-supported aluminium complexes bearing terminal alkoxide and mono-thiol functional groups: LAlOMe(Et) (2), LAlOtBu(Et) (3), and LAlSH(Et) (4), (L=[HC{C(Me)N-(2,6-iPr2 C6 H3 )}2 ]). Complexes 2 and 3 are further used as synthons to generate the fascinating cationic aluminium alkoxide complexes, [LAlOMe(μ-OMe)-Al(Et)L][EtB(C6 F5 )3 ] (5), [LAlOMe(OEt2 )][EtB(C6 F5 )3 ] (6), and [LAlOtBu(OEt2 )][EtB(C6 F5 )3 ] (8). These electrophilic cationic species are well characterized by spectroscopic and crystallographic techniques. The assessment of Lewis acidity by the Gutmann-Beckett method revealed superior Lewis acidity of the cations substituted with electron-demanding alkoxy groups in comparison to the known methyl analogue [LAlMe][B(C6 F5 )4 ]. This has been further endorsed by computational calculations to determine the NBO charges and hydride ion affinity for complexes 6 and 8. These complexes are also capable of activating triethylsilane in stoichiometric reactions. The applicability of these complexes has been realized in the hydrosilylation of ethers, carbonyls, and olefines. Additionally, the solid-state structure of a new THF stabilized aluminium halide cation [LAlCl(THF)][B(C6 F5 )4 ] (11) has also been reported.

Uncatalyzed Carbometallation Involving Group 13 Elements: Carboboration and Carboalumination of Alkenes and Alkynes

Carbometallation of alkenes and alkynes are powerful carbon-carbon bond-forming reactions. The use of compounds containing bonds between carbon and group 13 elements, particularly boron and aluminum, are particularly attractive because of the versatility of subsequent transformations. Uncatalyzed carboboration and carboalumination represent less common classes of reactions. This Short Review discusses uncatalyzed carboboration and carboalumination reactions of alkenes and alkynes, including the reaction design and mechanism.

Gallium(III)- and Indium(III)-Containing Ionic Liquids as Highly Active Catalysts in Organic Synthesis

The chemical industry still requires development of environmentally friendly processes. Acid-catalysed chemical processes may cause environmental problems. Urgent need to replace conventional acids has forced the search for sustainable alternatives. Metal-containing ionic liquids have drawn considerable attention from scientists for many years. These compounds may exhibit very high Lewis acidity, which is usually dependent on the composition of the ionic liquid with the particular content of metal salt. Therefore, metal-containing ionic liquids have found a lot of applications and are successfully employed as catalysts, co-catalysts or reaction media in various fields of chemistry, especially in organic chemistry. Gallium(III)- and indium(III)-containing ionic liquids help to transfer the remarkable activity of metal salts into even more active and easier-to-handle forms of ionic liquids. This review highlights the wide range of possible applications and the high potential of metal-containing ionic liquids with special focus on Ga(III) and In(III), which may help to outline the framework for further development of the presented research topic and synthesis of new representatives of this group of compounds.

Direct Comparison of Subvalent, Polycationic Group 13 Cluster Compounds: Lessons learned on Isoelectronic DMPE Substituted Gallium and Indium Tetracation Salts

The tetracationic, univalent cluster compounds [{M(dmpe)}4 ]4+ (M=Ga, In; dmpe=bis(dimethylphosphino)ethane) were synthesized as their pf salts ([pf]- =[Al(ORF )4 ]- ; RF =C(CF3 )3 ). The four-membered ring in [{M(dmpe)}4 ]4+ is slightly puckered for M=Ga and almost square planar for M=In. Yet, although structurally similar, only the gallium cluster is prevalent in solution, while the indium cluster forms temperature dependent equilibria that include even the monomeric cation [In(dmpe)]+ . This system is the first report of one and the same ligand inducing formation of isoelectronic and isostructural gallium/indium cluster cations. The system allows to study systematically analogies and differences with thermodynamic considerations and bonding analyses, but also to outline perspectives for bond activation using cationic, subvalent group 13 clusters.

Comparison of Imine- and Phosphinimine-Supported Indium Complexes: Tuning the Reactivity for the Sequential and Simultaneous Copolymerization of Lactide and ε-Caprolactone

Imine- and phosphinimine-supported indium complexes were used as catalysts in the polymerization of racemic lactide and ε-caprolactone as well as their copolymerization by the sequential and simultaneous addition of monomers. Tuning the electronics and sterics of the indium centers by either (i) changing the nature of the nitrogen donors and (ii) coordinating a hemilabile side group had a significant effect on the reactivity of the complexes, their stability, and their control in the synthesis of block copolymers. Specifically, the imine-supported complex (5) showed the highest activity in the homo- and copolymerization of the cyclic esters, in contrast to the phosphinimine-supported complex (7), which was significantly slower and less stable. The presence of morpholine and thiomorpholine hemilabile side groups either reduced the activity or prevented the formation of alkoxide complexes.

Ga+-catalyzed hydrosilylation? About the surprising system Ga+/HSiR3/olefin, proof of oxidation with subvalent Ga+ and silylium catalysis with perfluoroalkoxyaluminate anions

Already 1 mol% of subvalent [Ga(PhF)2]+[pf]- ([pf]- = [Al(ORF)4]-, RF = C(CF3)3) initiates the hydrosilylation of olefinic double bonds under mild conditions. Reactions with HSiMe3 and HSiEt3 as substrates efficiently yield anti-Markovnikov and anti-addition products, while bulkier substrates such as HSiiPr3 are less reactive. Investigating the underlying mechanism by gas chromatography and STEM analysis, we unexpectedly found that H2 and metallic Ga0 formed. Without the addition of olefins, the formation of R3Si-F-Al(ORF)3 (R = alkyl), a typical degradation product of the [pf]- anion in the presence of a small silylium ion, was observed. Electrochemical analysis revealed a surprisingly high oxidation potential of univalent [Ga(PhF)2]+[pf]- in weakly coordinating, but polar ortho-difluorobenzene of E 1/2(Ga+/Ga0; oDFB) = +0.26-0.37 V vs. Fc+/Fc (depending on the scan rate). Apparently, subvalent Ga+, mainly known as a reductant, initially oxidizes the silane and generates a highly electrophilic, silane-supported, silylium ion representing the actual catalyst. Consequently, the [Ga(PhF)2]+[pf]-/HSiEt3 system also hydrodefluorinates C(sp3)-F bonds in 1-fluoroadamantane, 1-fluorobutane and PhCF3 at room temperature. In addition, both catalytic reactions may be initiated using only 0.2 mol% of [Ph3C]+[pf]- as a silylium ion-generating initiator. These results indicate that silylium ion catalysis is possible with the straightforward accessible weakly coordinating [pf]- anion. Apparently, the kinetics of hydrosilylation and hydrodefluorination are faster than that of anion degradation under ambient conditions. These findings open up new windows for main group catalysis.

Temperature triggered alternating copolymerization of epoxides and lactones via pre-sequenced spiroorthoester intermediates

We report the alternating copolymerization of caprolactone and epoxide through the in situ formation of pre-sequenced spiroorthoester monomer. The reaction is catalyzed by the temperature triggered, bifunctional cationic indium complex (±)-[(NN_iO_tBu)In(CH₂SiMe₃)][B(C₆F₅)₄] (1). 1 can catalyze the coupling of epoxide and lactone to form spiroorthoester at 60 °C and its double ring-opening polymerization at 110 °C to form poly(ether-alt-ester). The post-polymerization modification and degradation of the poly(ether-alt-ester) are further investigated.

We report the alternating copolymerization of caprolactone and epoxide through the in situ formation of pre-sequenced spiroorthoester monomer.

Use of group 13 aryloxides for the synthesis of green chemicals and oxide materials

In this work, group 13 metal aryloxides [Al(MesalO)3] (1), [Me2Ga(MesalO)]2 (2), [AlLi3(MesalO)6] (3) and [Me2GaLi(MesalO)2(THF)] (4), were obtained by reaction of methyl salicylate (MesalOH) with group-13 alkyls MMe3 (for M...

Indium-catalyzed synthesis of benzannulated spiroketals by intramolecular double hydroalkoxylation of ortho-(hydroxyalkynyl)benzyl alcohols

The novel indium-catalyzed synthesis of benzannulated spiroketals by a double intramolecular hydroalkoxylation reaction of o-(hydroxyalkynyl)benzyl alcohols is reported.