Titanium Complexes of Tridentate Aminebiphenolate Ligands Containing Distinct N-Alkyls: Profound N-Substituent Effect on Ring-Opening Polymerization Catalysis

Niobium Complexes Supported by Chalcogen-Bridged [OEO]-Type Bis(phenolate) Ligands (E = S, Se): Synthesis, Characterization, and Phenylacetylene Polymerization

Trichloro niobium(V) complexes 3 and 4 with the sulfur- or selenium-bridged [OEO]-type bis(phenolate) ligands (E = S, Se) were synthesized and fully characterized on the basis of their NMR spectroscopic data and X-ray crystallographic analysis. In the crystalline state of 4, the [OSeO]-core of the ligand was coordinated to the niobium center in a fac-fashion. The corresponding tribenzyl niobium(V) complexes 5 and 6 were also prepared by the reactions of 3 and 4 with 3 equivalents of PhCH2MgCl in toluene. The X-ray diffraction analysis of 6 revealed that the distorted six-coordinated niobium center incorporated in the [OSeO]-type ligand took a mer-fashion, and one benzyl ligand was coordinated to the niobium center by η2-fashion. Complexes 5 and 6 were tested for the phenylacetylene polymerization that produced poly(phenylacetylene)s (PPAs), oligomers, and triphenylbenzenes (TPBs) depending on the chalcogen atom in the [OEO]-type ligand.

Titanium Complexes of Salicylbenzoxazole and Salicylbenzothiazole Ligands for the Ring-Opening Polymerization of ε-Caprolactone and Substituted ε-Caprolactones and Their Copolymerizations

Two series of titanium complexes, including salicylbenzoxazole titanium complexes (1-4) and salicylbenzothiazole titanium complexes (5-8), were successfully synthesized and characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction crystallography (for 2 and 5). The ¹H NMR spectra of complexes 7 and 8 reveal fluxional behavior in solution at room temperature, and the activation parameters were determined by lineshape analysis of variable-temperature (VT) NMR spectra in toluene-d₈: for 7, ΔH^⧧ = 73.0 ± 1.8 kJ mol^-1, ΔS^⧧ = 22.1 ± 5.5 J mol^-1 K^-1; for 8, ΔH^⧧ = 73.7 ± 1.2 kJ mol^-1, ΔS^⧧ = 20.3 ± 3.8 J mol^-1 K^-1. The positive values of ΔS^⧧ suggested that the isomerization occurred via a dissociative mechanism. All complexes were active initiators for the ring-opening polymerization of ε-caprolactone (ε-CL) and three substituted ε-CLs: γ-methyl-ε-caprolactone (γMeCL), γ-ethyl-ε-caprolactone (γEtCL), and γ-phenyl-ε-caprolactone (γPhCL). Of all complexes, complex 5 was found to be the most active initiator in this study. The copolymerizations between ε-CL and three substituted ε-CLs produced completely random copolymers. The polymerization was proposed to proceed via a dissociative coordination-insertion mechanism. The catalytic activity of the salicylbenzoxazole titanium complex was lower than that of its closely related salicylbenzothiazole titanium congener. Additionally, DFT calculations unveiled that the ligand decoordination step and the less steric congestion at the titanium center in the salicylbenzothiazole titanium complexes were the key factors in enhancing the catalytic rate.

Use of pyrazoles as ligands greatly enhances the catalytic activity of titanium iso-propoxide for the ring-opening polymerization of l-lactide: a cooperation effect

Using TiOⁱPr₄ with a pyrazole ligand for one-pot LA polymerization improved catalytic activity compared with using TiOⁱPr₄ only. At 60 °C, TiOⁱPr₄ with ^furPz exhibited a higher catalytic activity (approximately 3-fold) than TiOⁱPr₄. At room temperature, TiOⁱPr₄ with ^BuPz exhibited a higher catalytic activity (approximately 17-fold) than TiOⁱPr₄. High molecular mass PLA (M _nGPC = 51 100, and Đ = 1.10) could be produced by using TiOⁱPr₄ with ^furPz in melt polymerization ([TiOⁱPr₄] : [^furPz] = 1000 : 1 : 1 at 100 °C, 240 min). The crystal structure of ^MePz₂Ti₂OⁱPr₇ revealed the cooperative activation between two Ti atoms during LA polymerization.

Enhanced Reactivity of Aluminum Complexes Containing P-Bridged Biphenolate Ligands in Ring-Opening Polymerization Catalysis

Aluminum complexes containing [RP(O)(2-O-3,5-tBu₂C₆H₂)₂]²⁻ [R = tBu (3a), Ph (3b)] have been synthesized, structurally characterized, and their reactivity studied in comparison with those of their [RP(2-O-3,5-tBu₂C₆H₂)₂]²⁻ [R = tBu (2a), Ph (2b)] analogs. Treating AlMe₃ with one equiv of H₂[3a-b] in THF at 0°C affords quantitatively [3a-b]AlMe, subsequent reactions of which with benzyl alcohol in THF at 25°C generate {[3a-b]Al(μ₂-OCH₂Ph)}₂. The methyl [3a-b]AlMe and the benzyloxide {[3a-b]Al(μ₂-OCH₂Ph)}₂ are all active for catalytic ring-opening polymerization (ROP) of ε-caprolactone and rac-lactide (rac-LA). Controlled experiments reveal that {[3a]Al(μ₂-OCH₂Ph)}₂ is competent in living polymerization. Kinetic studies indicate that [3a]AlMe, in the presence of benzyl alcohol, catalyzes ROP of rac-LA at a rate faster than [3b]AlMe and [2a]AlMe(THF) by a factor of 1.8 and 23.6, respectively, highlighting the profound reactivity enhancement in ROP catalysis by varying the P-substituents of these biphenolate complexes of aluminum.

Polymerization of ε-Caprolactone Using Bis(phenoxy)-amine Aluminum Complex: Deactivation by Lactide

Polymerizations of biodegradable lactide and lactones have been the subjects of intense research during the past decade. They can be polymerized/copolymerized effectively by several catalyst systems. With bis(phenolate)-amine aluminum complex, we have shown for the first time that lactide monomer can deactivate the aluminum complex during the ongoing polymerization of ε-caprolactone to a complete stop. After hours of dormant state, the aluminum complex can be reactivated again by heating at 100 °C without the addition of any external chemicals still giving polymer with narrow dispersity. Studies using NMR, in situ FTIR, and single-crystal X-ray crystallography indicated that the coordination of the carbonyl group in lactyl unit was responsible for the unusual behavior of lactide. In addition, the unusual methyl-migration from methyl lactate ligand to the amine side chain of the aluminum complex was observed through intermolecular nucleophilic-attack mechanism.

Heterodinuclear titanium/zinc catalysis: synthesis, characterization and activity for CO2/epoxide copolymerization and cyclic ester polymerization

The preparation of heterodinuclear complexes, especially those comprising early-late transition metals coordinated by a simple or symmetrical ancillary ligand, represents a fundamental challenge and an opportunity to prepare catalysts benefitting from synergic properties. Here, two new mixed titanium(iv)-zinc(ii) complexes, [LTi(OⁱPr)₂ZnEt] and [LTi(OⁱPr)₂ZnPh], both coordinated by a diphenolate tetra(amine) macrocyclic ligand (L), are prepared. The synthesis benefits from the discovery that reaction of the ligand with a single equivalent of titanium tetrakis(iso-propoxide) allows the efficient formation of a mono-Ti(iv) complex, [LTi(OⁱPr)₂]. All new complexes are characterized by a combination of single crystal X-ray diffraction, multinuclear NMR spectroscopy and mass spectrometry techniques. The two heterobimetallic complexes, [LTi(OⁱPr)₂ZnEt] and [LTi(OⁱPr)₂ZnPh], feature trianionic coordination by the macrocyclic ligand and bridging alkoxide groups coordinate to both the different metal centres. The heterodinuclear catalysts are compared to the mono-titanium analogue, [LTi(OⁱPr)₂], in various polymerization reactions. In the alternating copolymerizations of carbon dioxide and cyclohexene oxide, the mono-titanium complex is totally inactive whilst the heterodinuclear complexes show moderate activity (TOF = 3 h^-1); it should be noted the activity is measured using just 1 bar pressure of carbon dioxide. In the ring opening polymerization of lactide and ε-caprolactone, the mono-Ti(iv) complex is totally inactive whilst the heterodinuclear complexes show moderate-high activities, qualified by comparison to other known titanium polymerization catalysts (l-lactide, k_obs = 11 × 10^-4 s^-1 at 70 °C, 1 M in [lactide]) and ε-caprolactone (k_obs = 5 × 10^-4 s^-1 at 70 °C, 0.9 M in [ε-caprolactone]).

Aluminum complexes containing biphenolate phosphine ligands: synthesis and living ring-opening polymerization catalysis

This report describes the synthesis, structure, and reactivity of aluminum complexes containing tridentate biphenolate phosphine ligands of the type [RP(2-O-3,5-C₆H₂tBu₂)₂]^2- (R = tBu (2a), Ph (2b)). Alkane elimination of AlMe₃ with one equiv. of H₂[2a] or H₂[2b] in THF at 0 °C cleanly affords colorless crystalline [2a]AlMe(THF) (3a) and [2b]AlMe(THF) (3b), respectively. An X-ray diffraction study of 3a showed it to be a five-coordinate THF-bound species, whose coordination geometry is best described as trigonal bipyramidal, having the phosphorus donor and THF at axial positions. Treatment of either in situ prepared or isolated methyl complexes 3a,b with one equiv. of benzyl alcohol in toluene or THF generated their corresponding benzyloxides {[2a,b]Al(μ₂-OCH₂Ph)}₂ (4a,b). An X-ray diffraction study of 4a revealed a dimeric structure, in which the coordination geometry of aluminum is also distorted trigonal bipyramidal with the tridentate 2a ligand being facially bound. In the presence of one equiv. of benzyl alcohol, complex 3a is a competent catalyst precursor for the living ring-opening polymerization of ε-caprolactone (ε-CL) and rac-lactide (rac-LA), producing poly(ε-caprolactone) and poly(rac-lactide), respectively, in a controlled manner. As such, well-defined block copolymers of ε-CL with rac-LA can also be prepared by catalytic 3a. Kinetic studies revealed that 3a catalyzes the polymerization of rac-lactide at a rate 2-fold faster than that of 3b, indicating the significance of the P-substituent effect on this catalysis. Interestingly, the polymerization rate of rac-lactide by 3a is 16.5 times faster than that of l-lactide under otherwise identical conditions.

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].

Homo- and Heteropolynuclear Clusters of Phosphine Triphenolates

The synthesis and structural characterization of a series of homo- and heteropolynuclear clusters constructed with a potentially tetradentate phosphine triphenolate ligand are presented. Treatment of tris(3,5-di-tert-butyl-2-hydroxyphenyl)phosphine (H3[O3P]) with 3 equiv of nBuLi in diethyl ether at -35 °C affords hexanuclear Li6[O3P]2(OEt2)2 (1) as colorless crystals. In situ lithiation of H3[O3P] with 3 equiv of nBuLi in THF at -35 °C followed by metathetical reactions with MnCl2 or NiCl2(DME) gives crystals of forest green pentanuclear MnLi4[O3P]2(THF)3 (2) or dark brown tetranuclear Ni2Li2[O3P]2(THF)2 (3), respectively. Alkane elimination of ZnR2 (R = Me, Et) with H3[O3P] in THF at 25 °C generates high yields of colorless crystalline trinuclear Zn3[O3P]2(THF)2 (4). The cluster structures of 1-4 were all determined by single crystal X-ray diffraction studies. These molecules represent the first examples of metal complexes supported by phosphine triphenolate derivatives. The cluster 2 contains a paramagnetic core of high spin Mn(II) (S = 5/2) as indicated by solution and solid state magnetic susceptibility measurements.

Titanium pyridonates for the homo- and copolymerization of rac-lactide and ε-caprolactone

A series of titanium pyridonate complexes have been synthesized under very mild reaction conditions from a common precursor, Ti(NMe₂)₄. These complexes have been explored as initiators for the ring-opening polymerization of rac-lactide and ε-caprolactone and have proven to be competitive with leading titanium initiators.