CO2-Based Polycarbonates through Ring-Opening Polymerization of Cyclic Carbonates Promoted by a NHC-Based Zinc Complex.

A backbone-substituted N-heterocyclic carbene (NHC) zinc complex, in combination with alcohol initiators, has been shown to be an effective catalyst for the ring-opening polymerization (ROP) of trimethylene carbonate (TMC) to poly(trimethylene carbonate) (PTMC) devoid of oxetane linkages. The ROP of TMC proceeded in solution to give PTMC, possessing controlled molecular mass (2500 < Mn < 10000) and low dispersity (D ∼ 1.2). Changing the alcohol initiators, PTMCs with different end-groups were obtained, included a telechelic polymer. The results of MALDI-ToF and NMR analysis confirmed the controlled/living nature of the present ROP catalytic system, where side reactions, such as inter- and intramolecular transesterifications, were minimized during the polymerization. Solution studies in different solvents demonstrated the polymerization reaction to proceed via a mechanism first order in monomer and in catalyst. The zinc complex was also able to convert substituted cyclic carbonates, which were purposely synthesized from renewable feedstocks such as CO2 and 1,3-diols. For the asymmetric 2-Me TMC monomer, good regioselectivity was observed (Xreg up to 0.92). The excellent control of the polymerization process was finally brought to light through the preparation of polycarbonate/polyether triblock copolymers by using polyethylene glycol (PEG) as a macroinitiator and of well-defined di- and triblock polycarbonate/polylactide copolymers by sequential ROP of TMC and L-LA.

Sol-Gel Dipping Devices for H2S Visualization.

In this contribution we report the synthesis and full characterization, via a combination of different spectroscopies (e.g., 1H NMR, UV-vis, fluorescence, MALDI), of a new family of fluorescent zinc complexes with extended π-conjugated systems, with the final aim of setting up higher performance H2S sensing devices. Immobilization of the systems into a polymeric matrix for use in a solid-state portable device was also explored. The results provided proof-of-principle that the title complexes could be successfully implemented in a fast, simple and cost-effective H2S sensing device.

Chromium Complexes Supported by Salen-Type Ligands for the Synthesis of Polyesters, Polycarbonates, and Their Copolymers through Chemoselective Catalysis.

Salen, Salan, and Salalen chromium (III) chloride complexes have been investigated as catalysts for the ring-opening copolymerization reactions of cyclohexene oxide (CHO) with CO2 and of phthalic anhydride (PA) with limonene oxide (LO) or cyclohexene oxide (CHO). In the production of polycarbonates, the more flexible skeleton of salalen and salan ancillary ligands favors high activity. Differently, in the copolymerization of phthalic anhydride with the epoxides, the salen complex showed the best performance. Diblock polycarbonate-polyester copolymers were selectively obtained by one-pot procedures from mixtures of CO2, cyclohexene oxide, and phthalic anhydride with all complexes. In addition, all chromium complexes were revealed to be very active in the chemical depolymerization of polycyclohexene carbonate producing cyclohexene oxide with high selectivity, thus offering the opportunity to close the loop on the life of these materials.

Salen-like Chromium and Aluminum Complexes as Catalysts in the Copolymerization of Epoxides with Cyclic Anhydrides for the Synthesis of Polyesters.

Chromium and aluminum complexes bearing salalen ligands were explored as catalysts for the ring-opening copolymerization (ROCOP) of succinic (SA), maleic (MA), and phthalic (PA) anhydrides with several epoxides: cyclohexene oxide (CHO), propylene oxide (PO), and limonene oxide (LO). Their behavior was compared with that of traditional salen chromium complexes. A completely alternating enchainment of monomers to provide pure polyesters was achieved with all the catalysts when used in combination with 4-(dimethylamino)pyridine (DMAP) as the cocatalyst. Poly(propylene maleate-block-polyglycolide), a diblock polyester with a precise composition, was obtained by switch catalysis, in which the same catalyst was able to combine the ROCOP of propylene oxide and maleic anhydride with the ring-opening polymerization (ROP) of glycolide (GA) through a one-pot procedure, starting from an initial mixture of the three different monomers.

Paper-Strip-Based Sensors for H2S Detection: A Proof-of-Principle Study.

In this work, the authors explored the interaction of a suite of fluorescent zinc complexes with H2S. The authors provide evidence that HS- binds the zinc center of all the complexes under investigation, allowing them to possibly function as sensors by a 'coordinative-based' approach. Naked-eye color changes occur when treating the systems with HS-, so the fluorescence responses are modulated by the presence of HS-, which has been related to a change in the energy level and coupling of excited states through a computational study. The results show the potential of the systems to function as HS-/H2S colorimetric and fluorescent sensors. Paper-strip-based sensing experiments foresee the potential of using this family of complexes as chemosensors of HS- in more complex biological fluids.

Fluorescent salen-type Zn(II) Complexes As Probes for Detecting Hydrogen Sulfide and Its Anion: Bioimaging Applications.

In this work, we investigate the mode of interaction of a family of fluorescent zinc complexes with HS- and H2S. Different experiments, performed by diverse spectroscopic techniques, provide evidence that HS- binds the zinc center of all the complexes under investigation. Treatment with neutral H2S exhibits a markedly different reactivity which indicates selectivity for HS- over H2S of the systems under investigation. Striking color changes, visible to the naked eye, occur when treating the systems with HS- or by an H2S flow. Accordingly, also the fluorescence is modulated by the presence of HS-, with the possible formation of multiple adducts. The results highlight the potential of the devised systems to be implemented as HS-/H2S colorimetric and fluorescent sensors. Bioimaging experiments indicate the potential of using this class of compounds as probes for the detection of H2S in living cells.

Gradient isotactic multiblock polylactides from aluminum complexes of chiral salalen ligands.

Aluminum complexes of enantiomerically pure aminomethylpyrrolidine-based salalen ligands and their application in the stereoselective polymerization of lactide are described. Poly(lactic acid) featuring the new gradient isotactic multiblock microstructure was synthesized by isoselective catalysts, which operate by a combination of enantiomorphic-site and chain-end control mechanisms.

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.

Salen, salan and salalen zinc(II) complexes in the interaction with HS-: time-resolved fluorescence applications.

In the current work we investigate the route of interaction of a newly synthesized family of zinc complexes with HS- by a plethora of different spectroscopic techniques. A computational analysis on the time dependent density functional theory (TD-DFT) level explored the overall fluorescence properties of the title complexes and their different fluorescence responses to HS-. Time-resolved fluorescence experiments were also performed and highlight the great potential of the current systems to be implemented as HS- fluorescent sensors.

Coordination chemistry and reactivity of zinc complexes supported by a phosphido pincer ligand.

The preparation and characterization of new Zn(II) complexes of the type [(PPP)ZnR] in which R = Et (1) or N(SiMe(3))(2) (2) and PPP is a tridentate monoanionic phosphido ligand (PPP-H = bis(2-diphenylphosphinophenyl)phosphine) are reported. Reaction of ZnEt(2) and Zn[N(SiMe(3))(2)](2) with one equivalent of proligand PPP-H produced the corresponding tetrahedral zinc ethyl (1) and zinc amido (2) complexes in high yield. Homoleptic (PPP)(2) Zn complex 3 was obtained by reaction of the precursors with two equivalents of the proligand. Structural characterization of 1-3 was achieved by multinuclear NMR spectroscopy ((1)H, (13)C, and (31)P) and X-ray crystallography (3). Variable-temperature (1)H and (31)P NMR studies highlighted marked flexibility of the phosphido pincer ligand in coordination at the metal center. A DFT calculation on the compounds provided theoretical support for this behavior. The activities of 1 and 2 toward the ring-opening polymerization of ε-caprolactone and of L- and rac-lactide were investigated, also in combination with an alcohol as external chain-transfer agent. Polyesters with controlled molecular parameters (M(n), end groups) and low polydispersities were obtained. A DFT study on ring-opening polymerization promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive zinc center. Preliminary investigations showed the ability of these complexes to promote copolymerization of L-lactide and ε-caprolactone to achieve random copolymers whose microstructure reproduces the composition of the monomer feed.

Bis{bis-[2-(diisopropyl-phosphan-yl)phen-yl]phosphanido-κ(3) P,P',P''}chloridonickel(II).

In the title compound, [Ni(C24H36P3)Cl], the Ni(II) atom adopts a distorted square-planar geometry with the two neutral P atoms of the tridentate ligand trans to one another. Bond lengths and angles of the phosphide P atom feature a pyramidal geometry of the donor atom, which forms a single bond with the Ni(II) atom, retaining a stereochemically active lone pair.

A Single Catalyst for Promoting Reverse Processes: Synthesis and Chemical Degradation of Polylactide.

A simple zinc catalyst showing high activity for both the synthesis of polylactide, a biodegradable polymer produced from renewable feedstock, and its degradation was described. In the ring-opening polymerization of lactides, the zinc catalyst showed one of the highest activities reported in the literature for reactions carried out in solution at room temperature. This excellent performance was preserved even when the process was performed under industrial conditions: at high temperature, in the absence of solvent, and by using a low catalyst loading with unpurified monomers. The same complex revealed high efficiency also in depolymerization of polylactide by alcoholysis, a process that occurred efficiently at room temperature and in the absence of solvent, conditions that reduce costs and guarantee low environmental impact.

The contribution of metalloporphyrin complexes in molecular sensing and in sustainable polymerization processes: a new and unique perspective.

This review highlights the recent developments in the field of metalloporphyrins as optical probes for biologically relevant molecules, such as nitric oxide (NO) and hydrogen sulfide (H2S), and as catalysts for the preparation of sustainable polymers such as polyesters, by the ring-opening polymerization (ROP) of cyclic esters and the ring-opening co-polymerization (ROCOP) of epoxides and anhydrides, and polycarbonates by the chemical fixation of carbon dioxide (CO2). The great potential of porphyrins is mainly due to the possibility of making various synthetic modifications to the porphyrin ring, such as modifying the coordinated metal, peripheral substituents, or even the molecular skeleton. Due to the strict structure-property relationships, one can use porphyrinoids in several different applications such as, for instance, activation of molecular oxygen or catalysis of photosynthetic processes. These possibilities broaden the application of porphyrins in several different fields of research, further mimicking what nature does. In this context, here, we want to provide evidence for the great flexibility of metalloporphyrins by presenting an overview of results obtained by us and others in the research fields we are currently involved in. More specifically, we report a survey of our most significant achievements regarding their use as optical probes in the context of the results reported in the literature from other research groups, and of the use of porphyrin metal(iii) complexes as catalysts for sustainable polymerization processes. As for the optical probe section, in addition to the metalloporphyrins synthesized ad hoc in the laboratory, the present work also covers the natural proteins containing a porphyrin core.

Copolymerization of L-Lactide and ε-Caprolactone promoted by zinc complexes with phosphorus based ligands.

The zinc complexes 1 and 2 bearing chelating phosphorous based pincer ligands were found to catalyze the ring-opening copolymerization of L-lactide and ε-caprolactone in the melt at 110 °C to obtain a series of random copolymers in which the monomer distributions were coherent with the monomer ratio in the feed. All the obtained copolymers showed high molecular masses with monomodal and moderately narrow distributions. The thermal properties of the achieved copolymers (Tg, glass transition temperature, and Tm, melting temperature) were strongly dependent on their composition. A linear dependence of Tg with molar percentage of lactide in the copolymers was observed over a temperature range from -59 °C (pure polycaprolactone) to 55 °C (pure polylactide).

Imidazo-pyridine-based zinc(II) complexes as fluorescent hydrogen sulfide probes.

In this work we explore the interaction of HS- with a family of fluorescent zinc complexes. In particular we selected a family of complexes with N,O-bidentate ligands aiming at assessing whether the zinc-chelating ligand plays a role in influencing the reactivity of HS- with the complexes under investigation. Different experiments, performed by diverse spectroscopic techniques, provide evidence that HS- binds the zinc center of all the complexes included in this study. The results highlight the potential of the devised systems to be used as HS-/H2S fluorescent sensors via a coordinative-based approach. To shed light on the species formed in solution when HS-/H2S interacts with the title complexes and aiming to rationalize the photophysical properties of the sensing constructs, we performed a computational analysis based on the time dependent density functional theory (TD-DFT). Preliminary bio-imaging experiments were also performed and the results indicate the potential of this class of compounds as probes for the detection of H2S in living cells.

Salen-type aluminum and zinc complexes as two-faced Janus compounds: contribution to molecular sensing and polymerization catalysis.

The aim of the present review is to highlight the most recent achievements in different fields of application of salen-based zinc and aluminum complexes. More specifically this article focuses on the use of aluminum and zinc salen-type complexes as optical probes for biologically relevant molecules, as catalysts for the ring opening polymerization (ROP) of cyclic esters and co-polymerization of epoxides and anhydrides (ROCOP) and in the chemical fixation of carbon dioxide (CO2). The intention is to provide an overview of the most recent results from our group within the framework of the state-of-art-results in the literature.

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.

Interaction of monohydrogensulfide with a family of fluorescent pyridoxal-based Zn(ii) receptors.

H2S and its conjugate base HS- have recently gained increasing attention for their reactivity with bioinorganic targets. However, so far, stable adducts of bioinorganic compounds with H2S/HS- are still scarce due to the propensity of sulfide to form insoluble metal sulfides. In this work, we studied the reactivity of HS- with a family of fluorescent zinc complexes via a variety of spectroscopic techniques. The complexes selected for this study feature a pyridoxal moiety with different substituent groups on the ligand framework. Interaction of the complexes under investigation with HS- results in the displacement of the coordinated ligand from the Zn center with the concomitant precipitation of ZnS in the case of complexes 1 and 3, whereas for complex 2 our data points to the coordination of HS- to the metal center likely assisted by hydrogen bonding with the OH of the pyridoxal moiety. In the presence of HS-, the fluorescence emission of complex 2 is enhanced, whereas the fluorescence emission of complexes 1 and 3 is quenched. The results highlight the potential of complex 2 to be implemented as a HS- fluorescent sensor via a coordinative-based approach.

Salen, salan and salalen iron(iii) complexes as catalysts for CO2/epoxide reactions and ROP of cyclic esters.

A salen ligand and its derivatives salan and salalen, bearing the same substituents on the phenolate rings, have been synthesized and then used for the preparation of the corresponding Fe(iii) complexes. All complexes were characterized by MALDI ToF mass spectroscopy, Evans method, and UV-Vis and IR spectroscopy. Moreover, for the salalen-based iron complex, the X-ray structure was studied. A comparison of their behaviour in the catalysis of CO2 with benchmark epoxide substrates allowed us to individuate an order of reactivity of the different classes of complexes. A salen Fe(iii) complex was prepared with a longer bridge between the two nitrogen atoms of the ligand skeleton in order to evaluate the influence of the flexibility on the activity of the catalyst. Finally, the same complexes were tested as catalysts in the ring-opening polymerization of l-lactide and ε-caprolactone.

Chemically reversible binding of H2S to a zinc porphyrin complex: towards implementation of a reversible sensor via a "coordinative-based approach".

Binding of hydrogen sulfide (H2S) to a zinc porphyrin complex and the stabilization of the related zinc hydrosulfido adduct are explored. High-resolution MALDI Fourier transform ion cyclotron resonance mass spectrometry (HR MALDI-FT-ICR) and 1H NMR experiments provide evidence that HS- coordination occurs at the zinc centre. The coordination of HS- occurs in a reversible manner and modulates fluorescence emission of a tetra(N-methylpyridyl)porphine zinc complex (TMPyPZn). The results highlight the potential of TMPyPZn and related systems for the implementation of fast and simple H2S sensors via a coordinative-based approach.