Aggregated initiators: defining their role in the ROP of rac-lactide

Switchable, chiral aluminium catalysts for ring opening polymerisations

A switchable, solvent-free catalytic system was developed in which Al methyl aminebis(phenolate) catalysts selectively initiate the formation of a polyether from cyclohexene oxide under CO2 atmosphere or the ring opening copolymerisation (ROCoP) of cyclohexene oxide and CO2 through the addition of a PPNCl (bis(triphenylphosphine)iminium chloride) cocatalyst to form poly(cyclohexene carbonate).

Exploiting Multimetallic Cooperativity in the Ring-Opening Polymerization of Cyclic Esters and Ethers

The use of multimetallic complexes is a rapidly advancing route to enhance catalyst performance in the ring-opening polymerization of cyclic esters and ethers. Multimetallic catalysts often outperform their monometallic analogues in terms of reactivity and/or polymerization control, and these improvements are typically attributed to "multimetallic cooperativity". Yet the origins of multimetallic cooperativity often remain unclear. This review explores the key factors underpinning multimetallic cooperativity, including metal-metal distances, the flexibility, electronics and conformation of the ligand framework, and the coordination environment of the metal centers. Emerging trends are discussed to provide insights into why cooperativity occurs and how to harness cooperativity for the development of highly efficient multimetallic catalysts.

Ring Opening Polymerization of Lactides and Lactones by Multimetallic Titanium Complexes Derived from the Acids Ph2C(X)CO2H (X = OH, NH2)

The reactions of the titanium alkoxide [Ti(OR)4] (R = Me, nPr, iPr, tBu) with the acids 2,2’-Ph2C(X)(CO2H), where X = OH and NH2, i.e., benzilic acid (2,2’-diphenylglycolic acid, L1H2), and 2,2’-diphenylglycine (L2H3), have been investigated. The variation of the reaction stoichiometry allows for the isolation of mono-, bi-, tri or tetra-metallic products, the structures of which have been determined by X-ray crystallography. The ability of the resulting complexes to act as catalysts for the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) and r-lactide (r-LA) has been investigated. In the case of ε-CL, all catalysts except that derived from [Ti(OnPr)4] and L2H3, i.e., 7, exhibited an induction period of between 60 and 285 min, with 7 exhibiting the best performance (>99% conversion within 6 min). The PCL products are moderate- to high-molecular weight polymers. For r-LA, systems 1, 3, 4 and 7 afforded conversions of ca. 90% or more, with 4 exhibiting the fastest kinetics. The molecular weights for the PLA are somewhat higher than those of the PCL, with both cyclic and linear PLA products (end groups of OR/OH) identified. Comparative studies versus the [Ti(OR)4] starting materials were conducted, and although high conversions were achieved, the control was poor.

Amino acid-derived bisphenolate palladium complexes as C-C coupling catalysts

A series of amine bisphenol (ABP) pro-ligands featuring amino acid ester pendant arms were prepared. Optimisation of the synthetic method allowed the facile incorporation of naturally occurring, chiral amino acids into the ABP scaffold with minimal racemisation. Reaction of the pro-ligands (LH₂) with Pd(OAc)₂, in the presence of amines, led to the formation of complexes with an unprecedented pincer-like O,N,O coordination mode around the Pd^II centre. The complexations in the presence of trialkylamines (NR₃) afforded a mixture of LPdNR₃ and LPdNHR₂ species. The latter was shown to form via an ambient-temperature C-N cleavage involving unstable Pd(OAc)₂(NHR₂)₂ intermediates. Using pyridine as base eliminated this dealkylation and resulted in the exclusive formation of LPd(py) complexes in high yields. In total, seven novel Pd^II ABP complexes were prepared, exhibiting distorted square-planar geometries with the asymmetric ligand moieties orientated towards the metal centre. The air- and moisture-stable LPd(py) complexes were successfully employed as catalysts in two types of C-C coupling reactions. The Suzuki-Miyaura coupling of 4'-bromoacetophenone and phenylboronic acid reached high yields (up to 81%), while a scope of further alkyl bromides was also efficiently converted using low catalyst loadings (1 mol%) and mild temperatures (40 °C). Furthermore, a Pd-pyridine complex achieved high activity in the Mizoroki-Heck coupling of styrene and 4'-bromoacetophenone.

Polymetallic Group 4 Complexes: Catalysts for the Ring Opening Polymerisation of rac-Lactide

Five novel air- and moisture-stable polymetallic Ti and Zr amino acid-derived amine bis(phenolate) (ABP) complexes were synthesised and fully characterised, including X-ray crystallographic studies. The reaction of the ABP proligands with Ti or Zr alkoxides has resulted in the formation of polymetallic aggregates of different nuclearity. The steric bulk on the pendant arm of the ligand was found to play a critical role in establishing the nuclearity of the aggregated complex. Sterically, less-demanding groups, such as H or Me, facilitated the formation of tetrametallic Ti clusters, bridged by carboxylate groups, while increased steric bulk (tBu) led to the formation of binuclear μ-oxo-bridged species. The isolated complexes were employed as catalysts for the ring opening polymerisation (ROP) of rac-lactide. Overall, the Ti catalysts were all active with the smaller, bimetallic Ti aggregates exhibiting relatively faster rates. A monometallic, bis(ABP) Zr complex was found to exert remarkable ROP activity, albeit with limited control over the tacticity and molecular weight distribution of the polymer. A further oxo-bridged Zr cluster was shown to display a previously unprecedented trimetallic structure and achieved a moderate rate in the ROP of rac-lactide.

Discrete Ti-O-Ti Complexes: Visible-Light-Activated, Homogeneous Alternative to TiO2 Photosensitisers

A series of novel bimetallic TiIV amine bis(phenolate) complexes was synthesised and fully characterised. X-ray crystallography studies revealed distorted octahedral geometries around the Ti centres with single or double oxo-bridges connecting the two metals. These robust, air- and moisture-stable complexes were employed as photosensitisers generating singlet oxygen following irradiation with visible light (420 nm) LED module in a commercial flow reactor. All five complexes showed high activity in the photo-oxygenation of α-terpinene and achieved complete conversion to ascaridole in four hours at ambient temperature. The excellent selectivity of these photosensitisers towards ascaridole (vs. transformation to p-cymene) was demonstrated with control experiments using a traditional TiO2 catalyst. Further comparative studies employing the free pro-ligands as well as a monometallic analogue highlighted the importance of the 'TiO2 -like' moiety in the polymetallic catalysts. Computational studies were used to determine the nature of the ligand to metal charge transfer (LMCT) states and singlet-triplet gaps for each complex, the calculated trends in the UV-vis absorption spectra across the series agreed well with the experimental results.

Isoselective Ring-Opening Polymerization of rac-Lactide Catalyzed by Sodium/potassium Tetradentate Aminobisphenolate Ion-paired Complexes

Two sodium/potassium tetradentate aminobisphenolate ion-paired complexes were synthesized and structurally characterized. These ion-paired complexes are efficient catalysts for the ring-opening polymerization of rac-lactide (rac-LA) in the presence of 5 equivalents BnOH as an initiator and the side reaction of epimerization can be suppressed well at low temperatures. The polymerizations are controllable, affording polylactides with desirable molecular weights and narrow molecular weight distributions; the highest molecular weight can reach 50.1 kg mol^-1 in this system, and a best isoselectivity of P_m =0.82 was achieved. Such polymerizations have rarely been reported for isoselective sodium/potassium complexes without crown ether as an auxiliary ligand. The solid structures suggest that BnOH can be activated by an interaction with the anion of sodium/potassium complex via a hydrogen bond and that the monomer is activated by coordination to sodium/potassium ion.