Bor-Hunn Huang

Aluminium metal complexes supported by amine bis-phenolate ligands as catalysts for ring-opening polymerization of ε-caprolactone

A family of aluminium complexes supported by dianionic amine bis(phenolate) ligands are described. Treatment of ligand precursors H2O2NNPy or H2O2NNMe [H2O2NNPy = (2-C5H4N)CH2N(CH2-2-HO-3,5-C6H2(tBu)2)2; H2O2NNMe = Me2NCH2CH2N(CH2-2-HO-3,5-C6H2(tBu)2)2] with 1.1 molar equivalent of AlMe3 in toluene affords MeAl(O2NNPy) (1) and MeAl(O2NNMe) (2) as five-coordinate aluminium methyl complexes, which further react with one equivalent of benzyl alcohol affording (C6H5CH2O)Al(O2NNPy) (3) and (C6H5CH2O)Al(O2NNMe) (4) as five-coordinate aluminium benzyloxy complexes. The molecular structures are reported for compounds 1 and 4. Compounds 3 and 4 show excellent catalytic activity toward the ring-opening polymerization of e-caprolactone.

Recent Developments in Metal-Catalyzed Ring-Opening Polymerization of Lactides and Glycolides: Preparation of Polylactides, Polyglycolide, and Poly(lactide-co-glycolide)

A good synergy of a catalytic system’s components, such as ancillary ligands and leaving groups at the active site of a catalyst, is of fundamental importance in the ring-opening polymerization (ROP) of lactides and lactones. This article surveys recent advances in the metal-promoted ROP of lactide and glycolide for the preparation of poly(lactide) (PLA), poly(glycolide) (PGA), and their copolymer poly(lactide-co-glycolide) (PLGA). First, there is a general discussion on mechanisms, the undesirable effects of side reactions on the rate of polymerization, and how the first generation of ROP catalysts such as SnOct2 [tin(II) 2-ethylhexanoate, also known as tin(II) octoate] work as efficient initiators. Then, the study focuses on the ROP capability of monomeric and multinuclear complexes of Li, Ca, Mg, Zn, Al, and Ti metals for the efficient preparation of PLA. Special emphasis is given to the factors controlling polymer molecular weight, molecular weight distribution, and the suppression of transesterification side reactions and epimerization of monomers. Surpassing the pure model nature of many structurally well-defined catalytic systems such as bis(phenoxide)-Li, bis(phenoxide)-Mg, trispyrazolylborate-Mg/Zn, and β-diketiiminate-Mg/Zn, the applicability and performance of N,O-donor Schiff base Mg/Zn systems in the production of PLAs are highlighted. The emerging “structure-polymerization” activity is also addressed. Special attention is given to Ca, Mg, and Zn initiators which, due to their biocompatibility, are considered the safest to be used in the preparation of PLAs for biomedical purposes. Likewise, the polymerization activity of the metal initiators is evaluated on the basis of the Lewis acidic properties of the central metal. Alternatives like trivalent lanthanide systems with ancillary ligands such as bis(amidinate), β-ketoiminate, bis(phenolate), and Schiff bases are considered. Some recently investigated coordination complexes of Cu, Ni, Ag, and Au metals used in the solvent-free melt polymerization of lactide are discussed in terms of structure–activity relationships. The substantial role of the ligand geometry on the stereocontrol of the rac-lactide polymerization is addressed to finally summarize the key components essential for obtaining PLAs of desired microstructure from rac- and meso-lactides. The development of glycolide ROP catalysts based on Sn, Zn, Sm, and Bi is addressed and their effectiveness in producing copolyesters of lactide and glycolide such as PLGA assessed.

Structurally Diverse Copper Complexes Bearing NNO-Tridentate Schiff-Base Derivatives as Efficient Catalysts for Copolymerization of Carbon Dioxide and Cyclohexene Oxide

Structurally diverse copper acetate complexes based on NNO-tridentate Schiff-base ligands were synthesized and characterized as mono-, di-, and trinuclear complexes with respect to varied ancillary ligands. Treatment of the ligand precursors (L(1)-H = 2-(1-((2-(dimethylamino)ethyl)imino)ethyl)-4-methylphenol, L(2)-H = 4-chloro-2-(1-((2-(dimethylamino)ethyl)imino)ethyl)phenol, and L(3)-H = 2-(1-((2-(dimethylamino)ethyl)imino)ethyl)-5-methylphenol) with Cu(OAc)2·H2O (1 equiv) in refluxing ethanol afforded five-coordinate mono- or bimetallic copper complexes ([(L(1))Cu(OAc)(H2O)] (1); [(L(2))Cu(OAc)(H2O)] (2); [(L(3))2Cu2(OAc)2] (3)) in high yields. Dinuclear copper acetate analogue [(L(1))2Cu2(OAc)2] (4) resulted from treatment of L(1)-H as the ligand precursor in refluxing anhydrous MeOH with equimolar proportions of metal acetate salt under a dry nitrogen atmosphere. However, a trinuclear complex, [(L(4))2Cu3(OAc)4] (5), was obtained on utilizing 2-(1-((2-(dimethylamino)ethyl)imino)ethyl)-5-methoxyphenol (L(4)-H) as the proligand under the same synthetic route of 1-3; this complex was also synthesized in the reaction of L(4)-H and copper(II) acetate monohydrate in the ratio of 2:3, giving a quantitative yield. All complexes are active catalysts for copolymerization of cyclohexene oxide (CHO) and CO2 without cocatalysts. In particular, dinuclear Cu complex 3 performed satisfactorily to produce polycarbonates with controllable molecular weights and high carbonate linkages. These copper complexes are the first examples that are effective for both CO2/CHO copolymerization and formation of polymers in a controlled fashion.

Synthesis of Sodium Complexes Supported with NNO-Tridentate Schiff Base Ligands and Their Applications in the Ring-Opening Polymerization of l-Lactide

A series of sodium complexes bearing NNO-tridentate Schiff base ligands with an N-pendant arm were synthesized and used as catalysts for the ring-opening polymerization of L-lactide (L-LA). Electronic effects of ancillary ligands coordinated by sodium complexes substantially influence the catalysis, and ligands with electron-donating groups increase the catalytic activity of the sodium complexes for catalyzing L-LA polymerization. In particular, a sodium complex bearing a 4-methoxy group has the highest activity with conversion up to 95% within 30 s at 0 °C and a low polydispersity index of 1.13, whereas the 4-bromo group showed the poorest performance with regard to the catalytic rate of L-LA polymerization in the presence of benzyl alcohol (BnOH). (1)H NMR pulsed-gradient spin-echo diffusion experiments and single-crystal X-ray analyses showed that sodium complexes [L(H)Na(THF)]2 and [L(4-Cl)Na(THF)]2 were dinuclear species in both solution and the solid state. The kinetic results indicated a first-order dependence on each of [[L(4-Cl)Na]2], [l-LA], and [BnOH].

Metal complexes containing nitrogen-heterocycle based aryloxide or arylamido derivatives as discrete catalysts for ring-opening polymerization of cyclic esters

The development of well-defined homogeneous catalysts for the ring-opening polymerization (ROP) of cyclic esters has made enormous progress over the past decade. This perspective focuses on some recent advances in the field of discrete metal complexes modified by various aryloxide or arylamido ligands bearing the nitrogen-containing heterocycle moiety, and their catalytic applications in ROP of lactones. It mainly highlights aryloxide/arylamido ligands that are directly installed by the N-heterocyclic group. The complex structure-ROP performance relationships and the observed trends with respect to their catalytic efficiency affected by ligand modifications are also discussed.

Bis{1-[(E)-(2-methyl-phen-yl)diazen-yl]-2-naphtho-lato}palladium(II).

In the title compound, [Pd(C17H13N2O)2], the PdII atom is tetracoordinated by two N atoms and two O atoms from two bidentate methylphenyldiazenylnaphtolate ligands, forming a square-planar complex. The two N atoms and two O atoms around the PdII atom are trans to each other (as the PdII atom lies on a crystallographic inversion centre) with O—Pd—N bond angles of 89.60 (11) and 90.40 (11)°. The distances between the PdII atom and the coordinated O and N atoms are 1.966 (3) and 2.009 (3) Å, respectively.

Dinuclear and Trinuclear Nickel Complexes as Effective Catalysts for Alternating Copolymerization on Carbon Dioxide and Cyclohexene Oxide

A series of novel nickel complexes 1-9 supported by NNO-tridentate Schiff-base derivatives have been synthesized and characterized. Treatment of the pro-ligands [L(1)-H = 2,4-di-tert-butyl-6-(((2-(dimethylamino)ethyl)imino)methyl)phenol, L(2)-H = 2-(((2-(dimethylamino)ethyl)imino)methyl)-4,6-bis(2-phenylpropan-2-yl)phenol, L(3)-H = 2-(((2-(dimethylamino)ethyl)imino)methyl)phenol] with Ni(OAc)2·4H2O in refluxing ethanol afforded mono- or bimetallic nickel complexes {[(L(1))Ni(OAc)] (1); (L(2))Ni(OAc)] (2); (L(3))2Ni2(OAc)2(H2O)] (3)}. Alcohol-solvated trimetallic nickel acetate complexes {[(L(3))2Ni3(OAc)4(MeOH)2] (4); (L(3))2Ni3(OAc)4(EtOH)2] (5)} could be generated from the reaction of L(3)-H and anhydrous nickel(II) acetate with a ratio of 2:3 in refluxing anhydrous MeOH or EtOH. The reaction of nickel acetate tetrahydrate and L(4)-H to L(6)-H [L(4)-H = 2-(((2-(dimethylamino)ethyl)imino)methyl)-5-methoxyphenol, L(5)-H = 2-(((2-(dimethylamino)ethyl)imino)methyl)-4-methoxy-phenol, L(6)-H = 2-(((2-(dimethylamino)ethyl)imino)(phenyl)methyl)phenol] produced, respectively, the alcohol-free trinuclear nickel complexes {[(L(4))2Ni3(OAc)4] (7); [(L(5))2Ni3(OAc)4] (8); [(L(6))2Ni3(OAc)4] (9)} with the same ratio in refluxing EtOH under the atmospheric environment. Interestingly, recrystallization of [(L(3))2Ni3(OAc)4(MeOH)] (4) or [(L(3))2Ni3(OAc)4(EtOH)] (5) in the mixed solvent of CH2Cl2/hexane gives [(L(3))2Ni3(OAc)4] (6), which is isostructural with analogues 7-9. All bi- and trimetallic nickel complexes exhibit efficient activity and good selectivity for copolymerization of CO2 with cyclohexene oxide, resulting in copolymers with a high alternating microstructure possessing ≥99% carbonate-linkage content. This is the first example to apply well-defined trinuclear nickel complexes as efficient catalysts for the production of perfectly alternating poly(cyclohexene carbonate).

Main-Group Catalysts for Lactide Polymerization

This chapter surveys the ring-opening polymerization (ROP) of lactide (LA) initiated/catalyzed by well-defined main-group metal complexes especially papers published from 2000 to 2012. Special emphasis is placed on factors such as ligand, metal, and solvent, which affect the molecular weight, molecular-weight distribution, reaction rate, and stereoselectivity of the polymer. The mechanistic studies of ROP of LA by the catalysts/initiators are addressed.

Bis[(1Z,3Z)-1,3-bis­(4-fluoro­phen­yl)-N,N′-diphenyl­propane­diiminato]magnesium(II)

In the title complex, [Mg(C27H19F2N2)2], the MgII atom, lying on a crystallographic twofold rotation axis, is tetrahedrally coordinated by four N atoms from two diiminate ligands.

Synthesis and characterisation of aluminium and magnesium complexes supported by pendant oxalic amidinate ligands

Two pendant oxalic amidine compounds [C6H5NC{NH(CH2)2OMe}–C{NH(CH2)2OMe}NC6H5] (1) (oxam(OMe)2H2) and [C6H5NC{NH(CH2)2NMe2}–C{NH(CH2)2NMe2}NC6H5] (2) (oxam(NMe2)2H2) are described. Reactions of 1 or 2 with two molar equivalents of AlMe3 in toluene give the bimetallic complexes [(Me2)Al(oxam(OMe)2)Al(Me2)] (3) and [(Me2)Al(oxam(NMe2)2)Al(Me2)] (4), respectively. Treatment of 1 or 2 with two molar equivalents of MeMgBr in THF affords the bimetallic complexes [(Br)(THF)Mg(oxam(OMe)2)Mg(THF)(Br)] (5) and [(Br)(THF)Mg(oxam(NMe2)2)Mg(THF)(Br)] (6) respectively. The crystal and molecular structures are reported for compounds 1, 2, 3, 5 and 6.