The paradigm shift from a linear to a circular economy is a very important goal, which involves several fields of chemistry, including the recycling of plastic materials and of critical raw materials, including platinum-group metal catalysts. The introduction of polar functional groups into the polyolefin skeleton, to yield functionalized polyolefins (FPOs), might open new and easier pathways for plastic depolymerization processes compared to simple polyolefins. However, the controlled copolymerization of ethylene with polar vinyl monomers via homogeneous catalysis to produce FPOs is a highly challenging reaction; at present, Pd-alpha-diimine complexes are the most promising catalysts for this reaction. In this contribution, we have revisited the catalytic behaviors of three known palladium complexes with benchmark alpha-diimines (N-N), with the general formula [Pd(N-N)(Me)(NCMe)][PF6], by carrying out, for the first time, the copolymerization of ethylene with methyl acrylate in trifluoroethanol/dichloromethane mixtures of different compositions. We found that the solvent composition had an unprecedented effect on catalyst productivity, the content of inserted polar monomers and the mode of incorporation. Detailed NMR investigations in CD2Cl2/TFE-d3 mixtures of the reaction of one of the Pd-complexes with methyl acrylate allowed us to correlate the observed catalytic behavior with the organometallic intermediates present in solution. Moreover, at specific solvent compositions, the spontaneous formation of two phases occurred at the end of the catalytic runs. We exploited this phenomenon to successfully perform, for the first time in this field, the recovery and recycling of the catalyst.

Ethylene polymerization with late transition metals offers the possibility of including polar monomers for the generation of functionalized polymers. However, several palladium complexes, including those with pyridyl-functionalized pyridinium amidate (PYA) ligands [Pd(Me)(MeCN)(N,N ')]+ (with N = PYA, N ' = pyridyl), undergo rapid beta-hydrogen elimination and form predominantly butene derivatives. Here, we have modified a range of elements in the catalyst design, including (i) the PYA substituents (Me, Bu, CH2OCH3), (ii) the chelating imine donor, (iii) the labile neutral ligand L, and (iv) the noncoordinating anion. These variations indicated factors that prevent (L = lutidine) or slow down ethylene conversion (imine = oxalyl, triazolyl, and pyrazolyl) and factors that accelerate it. In particular, the absence of MeCN as the coordinating ligand and the introduction of BArF as the counterion are highly beneficial and lead to efficient ethylene conversion and formation of oligomers with C20-C30 chain length. Time-dependent reaction monitoring suggests a step-growth mechanism rather than the more common chain-growth mechanism with the initial formation of butene and the subsequent conversion of butene and higher olefins. Indeed, also higher alpha-olefins such as 1-hexene were oligomerized with this in situ-prepared catalytic PYA palladium system.

The migratory insertion reaction of a polar vinyl monomer into the Pd-alkyl bond and the chain walking process are two of the key steps in the catalytic cycle for the synthesis of functionalized polyolefins through coordination/insertion polymerization. Here, we present a detailed NMR investigation to gain insight into these fundamental steps and demonstrate the critical role of traces of MeCN in this process. We used Pd(II) complexes containing a N-N' bidentate pyridyl-pyridylidene amide (py-PYA) ligand, which are known to cooligomerize ethylene and methyl acrylate (MA). The reaction of three related Pd-(py-PYA) complexes, viz. neutral [Pd(CH3)Cl(py-PYA)], 1a, and cationic derivatives [Pd(CH3)(NCCH3)(py-PYA)][X], X = BArF 1b and PF61c, with either MA or N,N-dimethylacrylamide (DMA), showed distinct reactivity with the two polar monomers. While 4-, 5-, and 6-membered palladacycles, resulting from the migratory insertion reaction of the polar monomer into the Pd-CH3 bond and subsequent chain walking, were detected with both monomers, their amounts varied considerably with the type of polar monomer, the anion, and the amount of MeCN. Specifically, we found that the coordinating ability of MeCN plays a critical and ambivalent role: on one hand, it hampers the coordination and insertion of the polar olefin, and on the other hand, it markedly suppresses the chain walking process. Moreover, we report here the first solid state structure of a 5-membered metallacyclic species derived from DMA insertion into the Pd-CH3 bond. The palladacyclic complexes are remarkably robust towards ethylene, though they react with carbon monoxide to form the palladium acyl species, opening perspectives for these complexes to catalyze CO/DMA copolymerization.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

1,8-Naphthalimides (NIs) represent a class of organic dyes with interesting optical properties that has been extensively explored in the last decades in lighting devices, chemosensors, optical probes or medicinal chemistry. However, despite their remarkable potential, reports on organometallic dyes bearing NIs are scarce and virtually inexistent regarding palladium(II) complexes. Herein, we report the synthesis of NIs bearing phosphine and amine chelating moieties and the characterization of their optical properties both as single molecules and when complexed on Pd(II) ions. It is shown that the introduction of phosphine moieties in the naphthalimide core results in a marked increase in non-radiative processes, leading to a significant reduction of the emission efficiency and lifetime of these dyes, compared to amine-bearing counterparts. The complexation to Pd(II) sequesters the electronic contribution of chelating moieties, with complexes assuming an optical behavior similar to that of unsubstituted 1,8-naphthalimide. The complexation significantly increases the acidity of chelating secondary amines, giving rise to an unexpected intramolecular reaction that results in the formation of a novel 1,8-naphthalimide dye bearing a cyclic phosphorylamide moiety. The new dye exhibits good emission quantum yield, long fluorescence lifetime and sensitivity to basic media, evidencing potential for application in optical imaging and sensing scenarios.

Metal catalyzed polymerizations are among the most important chemical reactions, accounting for the production of about 400 million tons per year of polymeric materials, 50 % of which are polyolefins. The CIRCC research units at the University of Salerno, founded by the late Professor Adolfo Zambelli, a coworker of Giulio Natta and a pioneer in the studies of stereospecific polymerization catalysts, has a consolidated expertise in this field. Although often considered a "mature" area of research, olefin polymerization catalysis continues to drive great interest of both industrial and academic scientists. On the other hand, strong political and economic pressure toward the development of "green" and possibly biodegradable alternatives to olefin-based polymers stimulated our group to direct increasing research efforts in the area of sustainable polymers. In this perspective, we focus on the most recent work from the CIRCC research units involved in homogeneous catalysis for polymerization of a variety of monomers, with the aim to address how the concepts and the expertise developed for olefin polymerization can be applied to the development of different metal-catalyzed polymerizations and copolymerizations. Of course, although the results are discussed in the frame of the most important literature contributions, a comprehensive review of such a wide and diversified topic is out of the scope of the paper. References to reviews covering the different types of metal catalyzed polymerizations are provided in each chapter.

The last two decades have witnessed the development of homogeneous catalysts for ethylene homo- and co-polymerization reactions based on late transition metals. When Pd(II) is the metal of choice, the best ligand-metal combination deals with either bidentate nitrogen-donor molecules or phosphinobenzene sulfonate derivatives. In this contribution we have investigated the coordination chemistry to Pd(II) of a bidentate phosphorus ligand, namely 4,5-bis(diphenylphosphino)acenaphthene (1). Starting from the neutral complex, [Pd(1)(CH3)Cl], we obtained the cationic derivatives [Pd(1)(CH3)(L)][SbF6], with L being either CH3CN or 3,5-lutidine. Using in situ NMR spectroscopy we investigated the reaction of [Pd(1)(CH3)(NCCH3)][SbF6] with ethylene, at room temperature, and ambient ethylene pressure. We discovered that [Pd(1)(CH3)(NCCH3)][SbF6] acts as a catalyst for butenes and hexenes synthesis with the relevant Pd-ethyl intermediate as the catalyst resting state. At the same time the color of the solution turned from pale yellow to light red due to the formation of the dinuclear species [Pd(mu-eta(2)-C6H5)PPh)-PPh2](2)[SbF6](2). Both the neutral Pd(II) complex, activated in situ by NaSbF6, and the monocationic acetonitrile derivative were tested in the ethylene homopolymerization reaction at high pressure, leading to low molecular weight, branched, polyethylene.

The synthesis of functionalized polyolefins through coordination-insertion polymerization is a highly challenging reaction. The ideal catalyst, in addition to showing a high productivity, has to be able to control the copolymer microstructure and, in particular, the way of the polar vinyl monomer incorporation. In this contribution, we modified the typical Brookhart's catalyst by introducing in the fourth coordination site of palladium a hemilabile, potentially bidentate ligand, such as a thiophenimine (N-S). The obtained cationic Pd(II) complexes, [Pd(Me)(N-N)(N-S)][PF6], generated active catalysts for the ethylene/methyl acrylate (MA) copolymerization leading to the desired copolymer with a different incorporation of the polar monomer depending on both the reaction medium and the N-S ligand. Surprisingly enough, the produced copolymers have the inserted acrylate both at the end of the branches (T(MA)) and in the main chain (M(MA)) in a ratio M(MA)/T(MA) that goes from 9:91 to 45:55 moving from dichloromethane to trifluoroethanol (TFE) as a solvent for the catalysis and varying the N-S ligand. The catalytic behavior of the new complexes was compared to that of the parent compound [Pd(Me)(N-N)(MeCN)][PF6], highlighting the fact that when the copolymerization is carried out in trifluoroethanol, this complex is also able to produce the E/MA copolymer with MA inserted both in the main chain and at the end of the branches. Accurate NMR studies on the reactivity of the precatalyst [Pd(Me)(N-N)(MeCN)][PF6] with the two comonomers allowed us to discover that in the fluorinated solvent, the catalyst resting state is an open-chain intermediate having both the organic fragment, originated from the migratory insertion of MA into the Pd-Me bond, and the acetonitrile coordinated to palladium and not the six-membered palladacycle typically observed for the Pd-alpha-diimine catalysts. This discovery is also supported by both DFT calculations and in situ NMR studies carried out on [Pd(Me)(N-N)(N-S)][PF6] complexes that point out that N-S remains in the palladium coordination sphere during catalysis. The open-chain intermediate is responsible for the growth of the copolymer chain with the polar monomer inserted into the main chain.

Treatment of the Ru(II) precursor cis,cis,trans-[RuCl2(dmso-S)(2)(PTA)(2)] (1, PTA=1,3,5-triaza-7-phosphaadamantane) with 2,2 '-bipyridine (bpy) in refluxing ethanol selectively affords cis,cis-[Ru(bpy)Cl-2(PTA)(2)] (2), whereas with pyridine (py), under the same conditions, it gives trans,cis,cis-[RuCl2(PTA)(2)(py)(2)] (6). The slightly less stable stereoisomer of 2, cis,trans-[Ru(bpy)Cl-2(PTA)(2)] (3), is obtained selectively through a different synthetic route. Isomers 2 and 3 are thermally stable, but cleanly equilibrate upon irradiation of an aqueous solution of either one with blue light. Intrigued by the stereoisomeric outcome in the preparations of this homogeneous set of complexes, we also investigated 2, 3, and 6 (and the mono-pyridine complex trans,mer-[RuCl2(py)(PTA)(3)] (7)) through a topological analysis of the electron density map using the quantum theory of atoms in molecules (QTAIM). The wealth of acquired experimental and calculated data allow us to discuss the stereochemical preferences of the [RuCl2(PTA)(2)(2 L)] complexes (2 L=bpy or 2py) in terms of electronic and steric contributions. The results of this speculative study on model complexes are transferable to similar systems. As an example, our findings from the reactivity of 1 towards pyridine allowed us to prepare the 2+2 pyridylporphyrin metallacycle trans,cis,cis-[RuCl2(PTA)(2)(4 '-cisDPyP)](2) (10, 4 '-cisDPyP=5,10-(4 '-pyridyl)-15,20-(phenyl)-porphyrin), whose X-ray molecular structure is also reported.

The efficient copolymerisation of functionalised olefins with alkenes continues to offer considerable challenges to catalyst design. Based on recent work using palladium complexes containing a dissymmetric N^N'-bidentate pyridyl-PYA ligand (PYA = pyridylidene amide), which showed a high propensity to insert methyl acrylate, we have here modified this catalyst structure by inserting shielding groups either into the pyridyl fragment, or the PYA unit, or both to avoid fast β-hydrogen elimination. While a phenyl substituent at the pyridyl side impedes catalytic activity completely and leads to an off-cycle cyclometallation, the introduction of an ortho-methyl group on the PYA side of the N^N'-ligand was more prolific and doubled the catalytic productivity. Mechanistic investigations with this ligand system indicated the stabilisation of a 4-membered metallacycle intermediate at room temperature, which has previously been postulated and detected only at 173 K, but never observed at ambient temperature so far. This intermediate was characterised by solution NMR spectroscopy and rationalises, in part, the formation of α,β-unsaturated esters under catalytic conditions, thus providing useful principles for optimised catalyst design.

The introduction of polar functional groups into the polyolefin skeleton is a challenging goal of high interest, and coordination-insertion polymerization represents the most powerful and environmentally friend approach to achieve it. Until now the most considerable catalysts are based on Pd(II) complexes and only a few examples on Ni(II) derivatives have been reported. We have now investigated a series of Ni(II) complexes with four pyridylimino ligands, both aldimines and ketimines, differing for the substituent present in position 6 on the pyridine ring (either a methyl group or a 2,6-dimethyl-substituted phenyl ring). These complexes generated active catalysts for the copolymerization of ethylene with methyl acrylate, yielding low-molecular weight, hyperbranched copolymers with the polar monomer content ranging between 0.2 and 35 mol % and inserted in a variety of modes, some of which were never observed before. The way of incorporation of the polar monomer goes from "in-chain only" to "everywhere but in-chain", and it is dictated by both the activation mode and the solvent used to dissolve the nickel precatalyst.

The control of the stereochemistry of macromolecules is a very important goal, and coordination-insertion polymerization is superior with respect to the other polymerization methods for its achievement. In this contribution, we focus on Pd(II) homogeneous catalysts for the stereocontrolled synthesis of CO/vinyl arene polyketones. We developed a library of aldo- and keto-iminopyridine ligands N-N' that feature an alpha- or beta-naphthyl or anthracenyl moiety on the imino nitrogen atom (N-imm). With such ligands, the Pd(II) monocationic complexes [Pd(CH3)(CH3CN)(N-N')][PF6] were synthesized. NMR spectroscopy shows that in solution, each complex exists as an equilibrium mixture of cis and trans stereoisomers, the latter having the CH3 ligand opposite to the Pd-N-imm bond. The isomeric population depends on the N-N' ligand: an almost 1:1 ratio is found for the ketimine complexes, whereas those with the aldimines show a preference for the trans geometry. These complexes generate very efficient catalysts for the CO/vinyl arene copolymerization. Catalyst performances depend both on the nature of N-N' and of the vinyl arene comonomer. The ketimine-based catalysts are more stable and more productive than the aldimine counterpart, leading to prevailingly syndiotactic macromolecules of high M-w (up to 280 kDa). The aldimine derivatives produce copolymers with isotactic and syndiotactic stereoblocks of different lengths depending on the vinyl arene. The effect of the prochiral monomer on the copolymer tacticity is steric in nature as demonstrated by the stereochemistry of the obtained CO/4-fluorostyrene polyketone, whose synthesis is reported here for the first time. As a conclusion, we have now demonstrated that when catalysts with nonsymmetric ancillary ligands are used, and stereoisomers are present, the stereochemistry of the copolymerization is driven by both the catalyst isomeric distribution and the prochiral comonomer.

This contribution, that readdresses the insertion of the Ru II –CO fragment into model porphyrins (i.e. ruthenation), has a Janus character, with one speculative and one practical side. As a proof of concept we demonstrate that ruthenation of a porphyrin can be performed under relatively mild conditions using the Ru II monocarbonyl complex [Ru(CO)(dmso) 5 ][PF 6 ] 2 that – besides CO – features exclusively labile dmso ligands. Even though this finding might seem trivial, it is only the second example that uses a Ru II carbonyl for porphyrin ruthenation, the first one having been reported almost 50 years ago and then neglected. From a practical point of view, we show the spectacular effect of propionic acid as solvent for performing the ruthenation of neutral and anionic model porphyrins with Ru 3 (CO) 12 ( 1 ). This process turned out to be extremely efficient and advantageous in terms of both reaction rates and yields (e.g. 100 % ruthenation of TPP in 30 min at 140 °C) compared to the procedures described in the literature.

The development of efficient homogeneous catalysts for the synthesis of functionalized polyolefins is a challenging topic. Palladium(II) complexes with alpha-diimine ligands having a phenanthrene skeleton and 2,6-disubstituted aryl rings (Ar-BIP) were synthesized, characterized, and tested as precatalysts in the copolymerization of ethylene with methyl acrylate. The direct comparison with analogous complexes having the corresponding alpha-diimines with an acenaphthene skeleton (Ar-BIAN) was performed. X-ray characterization in the solid state and NMR analysis in solution of both neutral [Pd(Ar-BIP)(CH3)Cl] and monocationic [Pd(Ar-BIP)(CH3)(NCCH3)][PF6] complexes indicate that the Ar-BIP ligands have a higher Lewis basicity and are more strongly coordinated to the metal center in comparison to the Ar-BIAN counterparts. Therefore, the Pd(Ar-BIP) cationic complexes can be regarded as electron-rich metal cations. In addition, they create a higher steric congestion around palladium in comparison to Ar-BIAN, regardless of the substituents on the aryl rings. The monocationic species generate active catalysts for the ethylene/methyl acrylate copolymerization leading to copolymers with M-n values up to 37000 and a content of polar monomer of 5.3 mol %. A detailed study of the catalytic behavior points out that Pd(Ar-BIP) catalysts show a good affinity for the polar monomer, have a good thermal stability, and favor the cleavage of the catalyst resting state, leading to copolymers with M-w values higher than those of the macromolecules produced with the corresponding Pd(Ar-BIAN) under the same reaction conditions. NMR characterization of the produced copolymers points out that the polar monomer is inserted both at the end of the branches and into the main chain, with an enchainment more selective than that achieved when the copolymerization is carried out in dichloromethane. In situ NMR investigations allowed us to detect relevant intermediates of the catalytic cycle and shed light on the nature of possible deactivated species.

Palladium(ii) complexes with a bidentate, anionic formazanate ligand are described. Attempts to prepare mono(formazanate) palladium alkyl complexes often leads to the homoleptic bis(formazanate) complex, which shows rich electrochemistry due to the redox-active nature of the ligands. Performing salt metathesis between the precursor [Pd(COD)(CH3)Cl] and the potassium salt of the ligand in the presence of tetrabutylammonium chloride yields a square planar mono(formazanate) palladate complex through coordination of chloride anion. Ligand exchange allows binding of unsaturated molecules and evaluation of the reactivity of the Pd-CH3 fragment. Using this approach, insertion reactions of CO, isocyanide and methyl acrylate into the Pd-CH3 bond are demonstrated.

Two new nonsymmetric bis(aryl-imino)acenaphthene ligands (Ar,Ar'-BIAN) and one symmetric Ar2-BIAN were studied. The three ligands share the presence of at least one methoxy group on one of the two aryl rings. These ligands were used for the synthesis of neutral and monocationic palladium(ii) complexes of general formula [Pd(CH3)Cl(N-N)] and [Pd(CH3)(L)(N-N)][PF6] (N-N = Ar,Ar'-BIAN, Ar2-BIAN; L = CH3CN, dmso). Due to the nonsymmetric nature of the ligands and their coordination to palladium in a nonsymmetric chemical environment, cis and trans isomers are possible for the three series of complexes with Ar,Ar'-BIANs. Both a detailed NMR investigation in solution and the X-ray characterization in the solid state point out that the trans isomer is the preferred species for the neutral derivatives, whereas for the cationic compounds a decrease in the stereoselectivity of the coordination is observed. One of the new Ar,Ar'-BIANs differs from an already reported nonsymmetric α-diimine for the replacement, on one aryl ring, of a methyl group with a methoxy substituent, thus allowing a comparison of the structural features of the relevant complexes. The monocationic complexes were tested as precatalysts for the ethylene/methyl acrylate copolymerization under mild reaction conditions. Despite the structural similarities observed in solution with the already known precatalysts, the present compounds demonstrated a remarkable decrease in the productivity values associated with a higher affinity for the polar monomer.

As a continuation of our strategy for preparing new Ru(II) precursors with improved water solubility through the introduction of highly water-soluble 1,3,5-triaza-7-phosphoadamantane (PTA) supporting ligands in the coordination sphere, in this work, we address the largely unexplored preparation of Ru(II)-PTA carbonyls. Two complementary synthetic approaches were used: (1) the treatment of a series of neutral Ru(II)-CO-dmso compounds of general formula RuCl2(CO) n(dmso)4- n ( n = 1-3, 1-5) with PTA; (2) the reaction of Ru(II)-PTA complexes with CO. Through the first approach, we obtained and fully characterized seven novel neutral compounds bearing from one to three PTA ligands per Ru atom, namely, the four monocarbonyls, cis, cis, trans-RuCl2(CO)(dmso-S)(PTA)2 (6), trans-RuCl2(CO)(PTA)3 (7), cis, mer-RuCl2(CO)(PTA)3 (8), and trans, trans, trans-RuCl2(CO)(OH2)(PTA)2 (10), and the three dicarbonyls, trans, trans, trans-RuCl2(CO)2(PTA)2 (11), [RuCl2(CO)2(PTA)]2 (12), and cis, cis, trans-RuCl2(CO)2(PTA)2 (13). The less stable, and thus more elusive, species fac-RuCl2(CO)(PTA)3 (9) and cis, cis, cis-RuCl2(CO)2(PTA)2 (14) were also unambiguously identified but could not be obtained in pure form and fully characterized. The complementary synthetic approach, that involved the treatment of the trans- and cis-RuCl2(PTA)4 (15, 16) isomers with CO, afforded only one new Ru(II)-PTA carbonyl, the cationic species cis-[RuCl(CO)(PTA)4]Cl (17). In general, the choice of the solvent was very relevant for obtaining the products with high yield and purity. We were unable to isolate Ru(II)-PTA compounds with more than two carbonyls. The thermodynamically preferred species have CO trans to Cl and two mutually trans PTAs, and only in the dinuclear compound 12 there is a single PTA per Ru atom. Compounds 7 and 17 feature the unprecedented trans-{Ru(CO)(PTA)} fragment. The X-ray structures of cis, cis, cis-RuCl2(CO)2(dmso)2 (3), 6-8, 10, 11, 13, and 17 are also reported. All compounds are new, are air-stable, and show a good solubility in water ( S from 10 to 165 g·L-1) and, most often, also in chloroform.

A series of cationic palladium complexes [Pd-((NN)-N-boolean AND')Me(NCMe)](+) was synthesized, comprising three different N<^>N'-bidentate coordinating pyridyl-pyridylidene amide (PYA) ligands with different electronic and structural properties depending on the PYA position (o-, m-, and p-PYA). Structural investigation in solution revealed cis/trans isomeric ratios that correlate with the donor properties of the PYA ligand, with the highest cis ratios for the complex having the most donating o-PYA ligand and lowest ratios for that with the weakest donor p-PYA system. The catalytic activity of the cationic complexes [Pd((NM)-M-boolean AND)Me(NCMe)](+) in alkene insertion and dimerization showed a strong correlation with the ligand setting. While complexes bearing more electron donating m- and o-PYA ligands produced butenes within 60 and 30 min, respectively, the p-PYA complex was much slower and only reached 50% conversion of ethylene within 2 h. Likewise, insertion of methyl acrylate as a polar monomer was more efficient with stronger donor PYA units, reaching a 32% ratio of methyl acrylate vs ethylene insertion. Mechanistic investigations about the ethylene insertion allowed detection, for the first time, by NMR spectroscopy both cis- and trans-Pd-ethyl intermediates and, furthermore, revealed a trans to cis isomerization of the Pd-ethyl resting state as the rate-limiting step for inducing ethylene conversion. These PYA palladium complexes induce rapid double-bond isomerization of terminal to internal alkenes through a chain-walking process, which prevents both polymerization and also the conversion of higher olefins, leading selectively to ethylene dimerization.