Cooperative effects of Schiff base binuclear zinc complexes on the synthesis of aliphatic and semi-aromatic polyesters

In this paper, we use mono- and bimetallic complexes based on Earth-abundant, cheap and benign zinc for the synthesis of sustainable aliphatic and semi-aromatic polyesters. Tridentate and hexadentate aldimine-thioetherphenolate ligands were used to obtain the desired zinc complexes by the reaction of proligands with opportune equivalents of zinc bis[bis(trimethylsilyl)amide]. The obtained bimetallic complexes 1 and 2 and the monometallic complex 3 were used as catalysts in the Ring-Opening Polymerization (ROP) of landmark cyclic esters, such as ε-caprolactone and lactide, and in the Ring-Opening COPolymerization (ROCOP) of cyclohexene oxide and phthalic anhydride under different reaction conditions. All catalysts were active in these two classes of reactions, showing good control of the polymerization processes. Interestingly, the bimetallic complexes have higher activity compared to their monometallic counterparts, highlighting the cooperation between the two zinc centers.

Metal Complexes in the Synthesis of Biodegradable Polymers: Achievements and Prospects

This review describes recent advances in the synthesis of homopolymers of lactide and related cyclic esters via ring-opening polymerization (ROP) in the presence of metal complexes based on group 1, 2, 4, 12, 13 and 14 metals. Particular attention is paid to the influence of the initiator structure on the properties of the obtaining homo- and copolymers. Also, a separate chapter is devoted to the study of metal complexes in the synthesis of copolymers of lactide and lactones. This review highlights the efforts made over the last ten years or so, and shows how main-group metals have received increasing attention in the field of the polymerization of lactide and related cyclic esters.

Synthesis, characterization, and alkoxide transfer reactivity of dimeric Tl2(OR)2 complexes

Reaction of LiOC^tBu₂Ph with TlPF₆ forms the dimeric Tl₂(OC^tBu₂Ph)₂ complex, a rare example of a homoleptic thallium alkoxide complex demonstrating formally two-coordinate metal centers. Characterization of Tl₂(OC^tBu₂Ph)₂ by ¹H and ¹³C NMR spectroscopy and X-ray crystallography reveals the presence of two isomers differing by the mutual conformation of the alkoxide ligands, and by the planarity of the central Tl-O-Tl-O plane. Tl₂(OC^tBu₂Ph)₂ serves as a convenient precursor to the formation of old and new [M(OC^tBu₂Ph)_n] complexes (M = Cr, Fe, Cu, Zn), including a rare example of T-shaped Zn(OC^tBu₂Ph)₂(THF) complex, which could not be previously synthesized using more conventional LiOR/HOR precursors. The reaction of [Ru(cymene)Cl₂]₂ with Tl₂(OC^tBu₂Ph)₂ results in the formation of a ruthenium(ii) alkoxide complex. For ruthenium, the initial coordination of the alkoxide triggers C-H activation at the ortho-H of [OC^tBu₂Ph] which results in its bidentate coordination. In addition to Tl₂(OC^tBu₂Ph)₂, related Tl₂(OC^tBu₂(3,5-Me₂C₆H₃))₂ was also synthesized, characterized, and shown to exhibit similar reactivity with iron and ruthenium precursors. Synthetic, structural, and spectroscopic characterizations are presented.

Pyridyldiimine macrocyclic ligands: Influences of template ion, linker length and imine substitution on ligand synthesis, structure and redox properties

A series of 2,6-diiminopyridine-derived macrocyclic ligands have been synthesized via [2+2] condensation around alkaline earth metal triflate salts. The inclusion of a tert-butyl group at the 4-position of the pyridine ring of the macrocyclic synthons results in macrocyclic complexes that are soluble in common organic solvents, thereby enabling a systematic comparison of the physical properties of the complexes by NMR spectroscopy, mass spectrometry, solution-phase UV-Vis spectroscopy, cyclic voltammetry and single-crystal X-ray crystallography. Solid-state structures determined crystallographically demonstrate increased twisting in the ligand, concurrent with either a decrease in ion size or an increase in macrocycle ring size (18, 20, or 22 membered rings). The degree of folding and twisting within the macrocycle can be quantified using parameters derived from the N_pyr-M-N_pyr bond angle and the relative orientation of the pyridinediimine (PDI) and pyridinedialdimine (PDAI) fragments to each other within the solid state structures. Cyclic voltammetry and UV-Vis spectroscopy were used to compare the relative energies of the imine π* orbital of the redox active PDI and PDAI components in the macrocycle when coordinated to redox inactive metals. Both methods indicate the change from a methyl to hydrogen substitution on the imine carbon lowers the energy of the ligand π* system.

Dinucleating Amino-Phenolate Platform for Zinc Catalysts: Impact on Lactide Polymerization

We report imine- and amine-based dinucleating ligands bearing a bisphenol backbone and explore their coordination chemistry with zinc to form zinc alkyl, alkoxide, acetate, and amide complexes. Full characterization of the complexes shows that this ligand framework can support dinuclear and trinuclear complexes. We explore the reactivity of the zinc alkyl and alkoxide complexes as catalysts for the ring opening polymerization of lactide and compared this reactivity to analogous mononuclear complexes. We show that 1) The amine-based complexes are more reactive than the imine-based analogues; 2) The trinuclear zinc alkyl species show unusual control and reproducibility for lactide polymerization; and 3) The extent of bimetallic cooperation is hampered by the ability of the ligand framework to form trinuclear clusters.

Synthesis of a mononuclear magnesium bis(alkoxide) complex and its reactivity in the ring-opening copolymerization of cyclic anhydrides with epoxides

Synthesis of a new mononuclear magnesium complex with a bulky bis(alkoxide) ligand environment and its reactivity in ring-opening polymerization (ROP) and ring-opening copolymerization (ROCOP) are reported. Reaction of n-butyl-sec-butylmagnesium with two equivalents of HOR (HOR = di-tert-butylphenylmethanol, HOCtBu2Ph) formed Mg(OR)2(THF)2. The reaction proceeded via the Mg(OR)(sec-Bu)(THF)2 intermediate that was independently synthesized by treating n-butyl-sec-butylmagnesium with one equivalent of HOR. Mg(OR)2(THF)2 led to active albeit not well-controlled ROP of rac-lactide. In contrast, well-controlled ROCOP of epoxides with cyclic anhydrides was observed, including efficient and alternating copolymerization of phthalic anhydride with cyclohexene oxide as well as rare copolymerization of phthalic anhydride with limonene oxide and terpolymerization of phthalic anhydride with both cyclohexene oxide and limonene oxide. In addition, novel copolymerization of dihydrocoumarin with limonene oxide is described.

Mono- and bimetallic pentacoordinate silicon complexes of a chelating bis(catecholimine) ligand

Schiff base condensation of 4,5-diamino-9,9-dimethylxanthene with 4,6-di-tert-butylcatechol-3-carboxaldehyde affords the bis(catecholimine) ligand XbicH₄, which can bind metals in both a square bis(catecholate) upper pocket and a pentagonal N₂O₃ lower pocket. Metalation with PhSiCl₃ results in [(XbicH₂)SiPh][HCl₂], where the silicon adopts a five-coordinate, square pyramidal geometry in the upper pocket and the lower pocket binds to two protons on the imine nitrogens. Deprotonation of the imines with LiO^tBu, NaN[SiMe₃]₂, or AgOAc results in binding of the univalent metal ion in the lower pocket, where it adopts an unusual pentagonal monopyramidal geometry in the solid state. The complexes show irreversible electrochemistry, with oxidations taking place at relatively high potentials.

Reactions of dicobalt octacarbonyl with dinucleating and mononucleating bis(imino)pyridine ligands

This work focuses on the application of dicobalt octacarbonyl (Co₂(CO)₈) as a metal precursor in the chemistry of formally low-valent cobalt with redox-active bis(imino)pyridine [NNN] ligands. The reactions of both mononucleating mesityl-substituted bis(aldimino)pyridine (L¹) and dinucleating macrocyclic xanthene-bridged di(bis(aldimino)pyridine) (L²) with Co₂(CO)₈ were investigated. Independent of the metal-to-ligand ratio (1 : 1 or 1 : 2 ligand to Co₂(CO)₈), the reaction of the dinucleating ligand L² with Co₂(CO)₈ produces a tetranuclear complex [Co₄(L²)(CO)₁₀] featuring two discrete [Co₂[NNN](CO)₅] units. In contrast, a related mononucleating bis(aldimino)pyridine ligand, L¹, produces different species at different ligand to Co₂(CO)₈ ratios, including dinuclear [Co₂(CO)₅(L¹)] and zwitterionic [Co(L¹)₂][Co(CO)₄]. Interestingly, [Co₄(L²)(CO)₁₀] features metal-metal bonds, and no bridging carbonyls, whereas [Co₂(CO)₅(L¹)] contains cobalt centers bridged by one or two carbonyl ligands. In either case, treatment with excess acetonitrile leads to disproportionation to the zwitterionic [Co[NNN](NCMe)₂][Co(CO)₄] units. The electronic structures of the complexes described above were studied with density functional theory. All the obtained bis(imino)pyridine complexes serve as catalysts for cyclotrimerization of methyl propiolate, albeit their reactivity is inferior compared with Co₂(CO)₈.

Cooperative bimetallic reactivity of a heterodinuclear molybdenum–copper model of Mo–Cu CODH

Modeling the reactivity of Mo–Cu CODH: Cu(i) brings the substrate close to Mo–oxo and develops electrophilic character in CO carbon.