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Paolo ZARDI

Ricercatore t.d. art. 24 c. 3 lett. A
Dipartimento di Scienze Chimiche e Geologiche - Sede Dipartimento di Scienze Chimiche e Geologiche

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Pubblicazioni

2024 - Functionalized graphene-based materials for cementitious applications [Articolo su rivista]
Cacciatore, Andrea; Zardi, Paolo; Capone, Laura; Maggini, Michele
abstract

Graphene-based materials (GBM) are promising cementitious composite additives that can significantly improve the mechanical characteristics and durability of concrete due to their unique properties, such as high surface area and aspect ratio and excellent tensile strength, to name a few. To display their full potential, GBM have to be homogeneously dispersed into the aqueous environment of cement-based matrices. The present study addresses the issue of limited dispersibility in the aqueous media of GBM through the chemical functionalization of mono- and few-layer graphene structures with hydrophilic aryl sulfonate groups and shows that a series of mortar samples containing modified GBM exhibit increased flexural and compressive strength by up to 17% and 30%, respectively, compared to mortar references without additives.Few-layer graphene, functionalized with covalently attached arylsulfonate groups, is proposed as an additive to improve the compressive and flexural strength of concrete towards smart and sustainable building materials.

2023 - Direct Arylation of Thiophenes in Continuous Flow [Articolo su rivista]
Petronilli, A; Carofiglio, T; Zardi, P
abstract

Synthetic methodologies involving direct C-H functionalization are promising to improve sustainability in organic synthesis. However, these newly developed strategies may have a scarce appeal for larger scale applications due to the high catalyst loading, harsh conditions or their typically long reaction times that affect severely the process productivity. Flow chemistry technology is a recognized tool to improve both the efficiency and scalability in organic synthesis that can overcome these issues. In the present paper we studied an "in flow" method for the direct arylation of thiophene derivatives with aromatic bromides to promptly afford heteroaromatic biaryls, which are recurrent motifs both in biologically active molecules and in functional materials. By using a packed-bed reactor containing potassium carbonate as the solid base and an automated system, we could develop a reliable methodology for thiophene arylation in flow with yields up to 90 % within a residence time of 30-60 minutes. This strategy is suitable for a wide variety of substrates and allowed the reaction to be carried out at gram-scale reaching a productivity value of 1.1 g h(-1).

2022 - Evaluation of Heteroleptic Pt (II) β-Diketonate Complexes as Precatalysts for the Photoactivated Curing of Silicone Resins [Articolo su rivista]
Moro, M.; Zardi, P.; Rossi, M.; Biffis, A.
abstract

A library of platinum (II) complexes of general formula (O,O′-β-diketonate)PtLX (L = dimethylsulfoxide, pyridine, triphenylphosphine; X = chloride, γ-acetylacetonate) has been prepared, using synthetic methodologies available in the literature. The library has been supplemented with a novel platinum (II) complex bearing a triazenido N-oxide ligand. The complexes have been characterized and tested as precatalysts for the photoactivated curing of silicone resins. Correlations have been established between the nature of the employed ligands, the ultraviolet-visible (UV-Vis) absorption spectrum of the complexes and their catalytic performance, which enable the tailored preparation of complexes with improved performance in view of possible technological applications.

2022 - Mild Microfluidic Approaches to Oxide Nanoparticles Synthesis [Articolo su rivista]
Zardi, P.; Carofiglio, T.; Maggini, M.
abstract

Oxide nanoparticles (oxide NPs) are advanced materials with a wide variety of applications in different fields. The use of continuous flow methods is particularly appealing for their synthesis due to the high control achieved over the reaction conditions and the easy process scalability. The present review focuses on the preparation of oxide NPs using microfluidic setups at low temperature (≤80 °C), since the employment of mild reaction conditions is crucial for developing sustainable and cost-effective processes. A particular emphasis will be put on the improvement over the final product features (e. g., size, shape, and size distribution) given by flow methods with respect to conventional batch procedures. The main issues that arise by treating NPs suspensions in microfluidic systems are product deposition or channel clogging; mitigation strategies to overcome these drawbacks will also be presented and discussed.

2021 - Achieving selectivity in porphyrin bromination through a DoE-driven optimization under continuous flow conditions [Articolo su rivista]
Zardi, P.; Maggini, M.; Carofiglio, T.
abstract

The post-functionalization of porphyrins through the bromination in β position of the pyrrolic rings is a relevant transformation because the resulting bromoderivatives are useful synthons to covalently link a variety of chemical architectures to a porphyrin ring. However, single bromination of porphyrins is a challenging reaction for the abundancy of reactive β-pyrrolic positions in the aromatic macrocycle. We herein report a synthetic procedure for the efficient preparation of 2-bromo-5,10,15,20-tetraphenylporphyrin (1) under continuous flow conditions. The use of flow technology allows to reach an accurate control over critical reaction parameters such as temperature and reaction time. Furthermore, by performing the optimization process through a statistical DoE (Design of Experiment) approach, these parameters could be properly adjusted with a limited number of experiments. This process led us to a better understanding of the relevant factors that govern porphyrins monobromination and to obtain compound 1 with an unprecedent 80% yield. [Figure not available: see fulltext.]

2021 - Enzyme-like Supramolecular Iridium Catalysis Enabling C−H Bond Borylation of Pyridines with meta-Selectivity [Articolo su rivista]
Trouve, J.; Zardi, P.; Al-Shehimy, S.; Roisnel, T.; Gramage-Doria, R.
abstract

The use of secondary interactions between substrates and catalysts is a promising strategy to discover selective transition metal catalysts for atom-economy C−H bond functionalization. The most powerful catalysts are found via trial-and-error screening due to the low association constants between the substrate and the catalyst in which small stereo-electronic modifications within them can lead to very different reactivities. To circumvent these limitations and to increase the level of reactivity prediction in these important reactions, we report herein a supramolecular catalyst harnessing Zn⋅⋅⋅N interactions that binds to pyridine-like substrates as tight as it can be found in some enzymes. The distance and spatial geometry between the active site and the substrate binding site is ideal to target unprecedented meta-selective iridium-catalyzed C−H bond borylations with enzymatic Michaelis–Menten kinetics, besides unique substrate selectivity and dormant reactivity patterns.

2020 - Selective formation, reactivity, redox and magnetic properties of MnIII and FeIII dinuclear complexes with shortened salen-type schiff base ligands [Articolo su rivista]
Rigamonti, L.; Zardi, P.; Carlino, S.; Demartin, F.; Castellano, C.; Pigani, L.; Ponti, A.; Ferretti, A. M.; Pasini, A.
abstract

The reactivity of the shortened salen-type ligands H3salmp, H2salmen and H2sal(p-X)ben with variable para-substituent on the central aromatic ring (X = tBu, Me, H, F, Cl, CF3, NO2) towards the trivalent metal ions manganese(III) and iron(III) is presented. The selective formation of the dinuclear complexes [M2(m-salmp)2], M = Mn (1a), Fe (2a), [M2(m-salmen)2(m-OR)2)], R = Et, Me, H and M = Mn (3a–c) or Fe (4a–c), and (M2(m-sal[p-X]ben)2(m-OMe)2), X = tBu, Me, H, F, Cl, CF3, NO2 and M = Mn (5a–g) or Fe (6a–g), could be identified by reaction of the Schiff bases with metal salts and the base NEt3, and their characterization through elemental analysis, infrared spectroscopy, mass spectrometry and single-crystal X-ray diffraction of 2a.2AcOEt, 2a.2CH3CN and 3c.2DMF was performed. In the case of iron(III) and H3salmp, when using NaOH as a base instead of NEt3, the dinuclear complexes [Fe2(m-salmp)(m-OR)(salim)2], R = Me, H (2b–c) could be isolated and spectroscopically characterized, including the crystal structure of 2b.1.5H2O, which showed that rupture of one salmp3– to two coordinated salim– ligands and release of one salH molecule occurred. The same hydrolytic tendency could be identified with sal(p-X)ben ligands in the case of iron(III) also by using NEt3 or upon standing in solution, while manganese(III) did not promote such a C–N bond breakage. Cyclic voltammetry studies were performed for 3b, 4b, 5a and 6a, revealing that the iron(III) complexes can be irreversibly reduced to the mixed-valence FeIIFeIII and FeII2 dinuclear species, while the manganese(III) derivatives can be reversibly oxidized to either the mixed-valence MnIIIMnIV or to the MnIV2 dinuclear species. The super-exchange interaction between the metal centers, mediated by the bridging ligands, resulted in being antiferromagnetic (AFM) for the selected dinuclear compounds 3b, 4b, 5a, 5e, 5f, 6a and 6e. The coupling constants J (–2J Ŝ1·Ŝ2 formalism) had values around –13 cm–1 for manganese(III) compounds, among the largest AFM coupling constants reported so far for dinuclear MnIII2 derivatives, while values between –3 and –10 cm–1 were obtained for iron(III) compounds.

2019 - A Supramolecular Palladium Catalyst Displaying Substrate Selectivity by Remote Control [Articolo su rivista]
Zardi, P.; Roisnel, T.; Gramage-Doria, R.
abstract

Inspired by enzymes such as cytochrome P-450, the study of the reactivity of metalloporphyrins continues to attract major interest in the field of homogeneous catalysis. However, little is known about benefitting from the substrate-recognition properties of porphyrins containing additional, catalytically relevant active sites. Herein, such an approach is introduced by using supramolecular ligands derived from metalloporphyrins customized with rigid, palladium-coordinating nitrile groups. According to different studies (NMR and UV/Vis spectroscopy, XRD, control experiments), the supramolecular ligands are able to accommodate pyridine derivatives as substrates inside the porphyrin pocket while the reactivity occurs at the peripheral side. By simply tuning a remote metal center, different binding events result in different catalyst reactivity, and this enzyme-like feature leads to high degrees of substrate selectivity in representative palladium-catalyzed Suzuki–Miyaura reactions.

2019 - Indoles from Alkynes and Aryl Azides: Scope and Theoretical Assessment of Ruthenium Porphyrin-Catalyzed Reactions [Articolo su rivista]
Intrieri, D.; Carminati, D. M.; Zardi, P.; Damiano, C.; Manca, G.; Gallo, E.; Mealli, C.
abstract

A symbiotic experimental/computational study analyzed the Ru(TPP)(NAr)2-catalyzed one-pot formation of indoles from alkynes and aryl azides. Thirty different C3-substituted indoles were synthesized and the best performance, in term of yields and regioselectivities, was observed when reacting ArC≡CH alkynes with 3,5-(EWG)2C6H3N3 azides, whereas the reaction was less efficient when using electron-rich aryl azides. A DFT analysis describes the reaction mechanism in terms of the energy costs and orbital/electronic evolutions; the limited reactivity of electron-rich azides was also justified. In summary, PhC≡CH alkyne interacts with one NAr imido ligand of Ru(TPP)(NAr)2 to give a residually dangling C(Ph) group, which, by coupling with a C(H) unit of the N-aryl substituent, forms a 5+6 bicyclic molecule. In the process, two subsequent spin changes allow inverting the conformation of the sp2 C(Ph) atom and its consequent electrophilic-like attack to the aromatic ring. The bicycle isomerizes to indole via a two-step outer sphere H-migration. Eventually, a ′Ru(TPP)(NAr)′ mono-imido active catalyst is reformed after each azide/alkyne reaction.

2017 - Concentration-Independent Stereodynamic g-Probe for Chiroptical Enantiomeric Excess Determination [Articolo su rivista]
Zardi, P.; Wurst, K.; Licini, G.; Zonta, C.
abstract

Enantiomeric excess (ee) determination is crucial in many aspects of science, from synthesis to materials. Within this subject, coupling molecular sensors with chiroptical techniques is a straightforward approach to the stereochemical analysis of chiral molecules, especially in terms of process immediacy and labor. Stereodynamic probes typically consist of racemic mixtures of rapidly interconverting enantiomeric conformers able to recognize a chiral analyte and greatly amplify its chiroptical readout. A great number of sensors have been developed, but their activity is generally restricted to one or a few classes of chemicals, and the analysis outcome relies on precise knowledge of the probe and analyte concentrations. This aspect in particular limits the potential practical applications. Here we report an oxo-vanadium(V) aminotriphenolate complex that was found to act as a concentration-independent stereodynamic sensor for a wide range of compounds. The bare complex is CD-silent, but coordination of an enantioenriched substrate immediately gives rise to intense Cotton effects in the visible region. Furthermore, a geometry change during the substrate complex interaction leads to a marked optical response, as witnessed by a strong red-shift of the probe absorption bands, thus allowing the generation of dichroic signals in an "interference-free" area of the spectrum. This peculiarity allows for a linear correlation at high wavelengths between the ee of the analyte and anisotropy g factor. This parameter derives from the differential circularly polarized light absorption of the sample but is independent of concentration. The newly developed sensor based on a simple coordination process has an unprecedented general character in terms of substrate scope and employment.

2016 - Resonance Raman spectroscopy as an: In situ probe for monitoring catalytic events in a Ru-porphyrin mediated amination reaction [Articolo su rivista]
Zardi, P.; Gallo, E.; Solan, G. A.; Hudson, A. J.
abstract

Resonance Raman microspectroscopy has been widely used to study the structure and dynamics of porphyrins and metal complexes containing the porphyrin ligand. Here, we have demonstrated that the same technique can be adapted to examine the mechanism of a homogeneously-catalysed reaction mediated by a transition-metal-porphyrin complex. Previously it has been challenging to study this type of reaction using in situ spectroscopic monitoring due to the low stability of the reaction intermediates and elevated-temperature conditions. We have made a straightforward modification to the sample stage on a microscope for time-lapsed Raman microspectroscopy from reaction mixtures in these media. The allylic amination of unsaturated hydrocarbons by aryl azides, which can be catalysed by a ruthenium-porphyrin complex, has been used as an illustrative example of the methodology. The mechanism of this particular reaction has been studied previously using density-functional theory and kinetic approaches. The Raman measurements support the mechanism proposed in the earlier publications by providing the first experimental verification of a precursor reaction complex between the aryl azide and the ruthenium metal ion, and evidence for the formation of a mono-imido intermediate complex under conditions of high concentration of the reactant olefin.

2016 - Synthesis and catalytic activity of μ-oxo ruthenium(IV) porphyrin species to promote amination reactions [Articolo su rivista]
Zardi, P.; Intrieri, D.; Carminati, D. M.; Ferretti, F.; Macchi, P.; Gallo, E.
abstract

This work describes the synthesis of ruthenium(IV) m-oxo porphyrin complexes of general formula [RuIV(TPP)(X)]2O which have been applied as catalysts in nitrene transfer reactions using aryl azides (ArN3) as nitrene sources. Collected data indicated that the catalytic efficiency of [RuIV(TPP)(OCH3)]2O was comparable to that of RuII(TPP)CO because of their analogous reactivity towards aryl azides to give the same catalytically active bis-imido species RuVI(TPP)(ArN)2. The reaction of [RuIV(TPP)(OCH3)]2O with Ph3CN3 or (CH3)3SiN3 afforded [RuIV(TPP)(N3)]2O which was fully characterised, its molecular structure was also determined by single crystal X-ray analysis.

2015 - A mechanistic investigation of the ruthenium porphyrin catalysed aziridination of olefins by aryl azides [Articolo su rivista]
Zardi, P.; Pozzoli, A.; Ferretti, F.; Manca, G.; Mealli, C.; Gallo, E.
abstract

A mechanism for the aziridination of olefins by aryl azides (ArN3), promoted by ruthenium(ii) porphyrin complexes, is proposed on the basis of kinetic and theoretical studies. All the recorded data support the involvement of a mono-imido ruthenium complex as the active intermediate in the transfer of the nitrene moiety "ArN" to the olefin. The selectivity of the aziridination vs. the uncatalysed triazoline formation can be enhanced by fine-tuning the electronic features of the porphyrin ligand and the olefin/azide catalytic ratio. The DFT study highlights the importance of an accessible triplet ground state of the intermediate ruthenium mono-imido complex to allow the evolution of the aziridination process.

2015 - Glycoporphyrin Catalysts for Efficient C-H Bond Aminations by Organic Azides [Articolo su rivista]
Tseberlidis, G.; Zardi, P.; Caselli, A.; Cancogni, D.; Fusari, M.; Lay, L.; Gallo, E.
abstract

We report herein the synthesis of new glycoporphyrin ligands which bear a glucopyranoside derivative on each meso-aryl moiety of the porphyrin skeleton. The saccharide unit is directly conjugated to the porphyrin or a triazole spacer is placed between the carbohydrate and porphyrin ring. The obtained glycoporphyrin ligands were employed to synthesize cobalt(II), ruthenium(II), and iron(III) complexes which were tested as catalysts of C-H bond aminations by organic azides. Two of the synthesized complexes were very efficient in promoting catalytic reactions, and the results achieved indicated that ruthenium and iron complexes show an interesting complementary catalytic activity in several amination reactions. The eco-friendly iron catalyst displayed very good chemical stability in catalyzing the amination reaction for three consecutive runs without losing catalytic activity. (Chemical Equation Presented).

2014 - Correction: Organic azides: ‘energetic reagents’ for the intermolecular amination of C–H bonds [Articolo su rivista]
Intrieri, D.; Zardi, P.; Caselli, A.; Gallo, E.
abstract

Correction for ‘Organic azides: “energetic reagents’ for the intermolecular amination of C–H bonds’ by Daniela Intrieri et al., Chem. Commun., 2014, DOI: 10.1039/c4cc03016h. © 2014 The Partner Organisations.

2014 - Indoles rather than triazoles from the ruthenium porphyrin-catalyzed reaction of alkynes with aryl azides [Articolo su rivista]
Zardi, P.; Savoldelli, A.; Carminati, D. M.; Caselli, A.; Ragaini, F.; Gallo, E.
abstract

An unprecedented reactivity of aryl azides toward alkynes is presented herein. The reaction performed well in the presence of 2 mol % of ruthenium porphyrin catalysts and afforded substituted indoles instead of triazoles. The procedure is particularly appealing for the synthesis of C3-functionalized indoles which bear EWG on the fragment coming from the azide. The method allowed the synthesis of 15 derivatives with yields up to 95%, high regioselectivity, and without requiring the time-consuming prefunctionalization of reagents and the addition of oxidants and/or additives.

2014 - Organic azides: “energetic reagents” for the intermolecular amination of C–H bonds [Articolo su rivista]
Intrieri, Daniela; Zardi, Paolo; Caselli, Alessandro; Gallo, Emma
abstract

This feature article provides an overview of the application of organic azides for the intermolecular amination of sp3 and sp2 C-H bonds. The catalytic activity of several metal complexes was reviewed underlining both synthetic and mechanistic aspects of the C-H amination. The majority of the aminated compounds reported in literature have been collected in this paper to provide a compendium of published procedures. In addition, the discussion of involved mechanisms has been included to assist the reader to envisage the future potential of organic azides in the synthesis of aza-derivatives. © Partner Organisations 2014.

2014 - Synthesis of biologically relevant compounds by ruthenium porphyrin catalyzed amination of benzylic C-H bonds [Articolo su rivista]
Zardi, P.; Caselli, A.; Macchi, P.; Ferretti, F.; Gallo, E.
abstract

Herein we report the catalytic activity of ruthenium porphyrin complexes to promote the amination of benzylic C-H bonds by aryl azides, yielding α- and β-amino esters. The catalytic methodology is also effective to synthesize two derivatives of methyl l-3-phenyllactate in order to convert one of them into the corresponding β-lactam. The catalytic experimental conditions have been optimized on the basis of a preliminary mechanistic investigation which underlines the pivotal role of the substrate concentration to maximize the reaction productivity. © 2014 American Chemical Society.

2013 - Silica "sHB" chiral Pc-L* copper complexes for halogen-free solvent cyclopropanation reactions [Articolo su rivista]
Castano, B.; Zardi, P.; Honemann, Y. C.; Galarneau, A.; Gallo, E.; Psaro, R.; Caselli, A.; Santo, V. D.
abstract

The grafting of the preformed Pc-L* (pyridine containing macrocyclic ligands) copper(i) complexes on different ordered and non-ordered silicas, and their use, under heterogeneous batch conditions, as catalysts for the olefin cyclopropanation are reported. High yields (up to 99%) and good recyclability in halogen-free solvent reactions were obtained, together with negligible copper leaching (0.1% of total copper). © The Royal Society of Chemistry 2013.

2012 - Co(porphyrin)-catalysed amination of 1,2-dihydronaphthalene derivatives by aryl azides [Articolo su rivista]
Zardi, P.; Intrieri, D.; Caselli, A.; Gallo, E.
abstract

Co(porphyrin) complexes promote an unusual reactivity of dihydronaphthalene towards several aryl azides. The reaction affords the benzylic amine of tetrahydronaphthalene instead yielding the amine of dihydronaphthalene as it normally happens in the presence of Ru(porphyrin)CO catalysts. The amination process occurs with the concomitant reduction of the dihydronaphthalene double bond probably due to the high reactivity of the endocyclic CC bond coupled with the good hydrogen donor capability of dihydronaphthalene. Two mechanisms for this reaction are proposed. © 2012 Elsevier B.V. All rights reserved.