Modulation of N^N′-bidentate chelating pyridyl–pyridylidene amide ligands offers mechanistic insights into Pd-catalysed ethylene/methyl acrylate copolymerisation

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 NN '-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 beta-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 NN '-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 alpha,beta-unsaturated esters under catalytic conditions, thus providing useful principles for optimised catalyst design

Palladium catalyzed copolymerizations: from ligand architecture to macromolecule microstructure

Polyolefins represent one of the major products of the plastic industry, that satisfy 55 % of the worldwide plastic demand. However, due to their chemical composition, they suffer of scarce surface properties, such as dyeability, compatibility with other materials, printability and adhesion. In this regard, the introduction of polar functional groups into the polyolefin skeleton would overcome this problem. Up to now this target is industrially achieved by radical polymerization reactions, that require harsh conditions of temperature and pressure and do not allow the control on the microstructure of the produced functionalized polyolefin. The direct, homogeneously catalyzed copolymerization reaction of ethylene with polar vinyl monomers would overcome all of these limits, allowing to use milder reaction conditions but, most importantly, to have a precise control on the copolymer microstructure. Much efforts have been done in this direction, and in the last two decades a wide number of systems based on palladium catalysts with a great variety of ligands has been reported. However, the catalytic performances of these systems, in terms of activity/productivity values, incorporation of the polar monomers, molecular weight and molecular weight distribution and copolymer microstructure, do not satisfy the requirements for a potential industrial application, thus better performing catalysts are needed. This PhD thesis aims to develop new homogeneous catalysts for the target reaction based on palladium(II) complexes with bidentate nitrogen-donor ligands (N-N). The strategy of this research project is based on expanding the library of N-N donor ligands reported in literature, and to study the influence of the ligand design on the catalytic outcome. In each Chapter the synthesis and characterization of different N-N ligands will be reported, along with the synthesis and characterization of the relevant Pd(II) complexes, and the study of their catalytic behaviour including the characterization of the catalytic products

Catalysts and process for olefins and polar vinyl monomers copolymerization and olefins homopolymerization

The invention relates to a family of Pd(II)-based catalysts of general formula (I) reproduced below for use in the production of olefin/polar vinyl monomer copolymers and olefin homopolymers. The invention also refers to ligands that are intermediates in the synthesis of the catalysts, to the process for the synthesis of homo- and copolymers using these catalysts, and to the homo- and copolymers thus obtained

Copolymerization of Ethylene and Methyl Acrylate by Pyridylimino Ni(II) Catalysts Affording Hyperbranched Poly(ethylene- co -methyl acrylate)s with Tunable Structures of the Ester Groups

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. Till 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 \u201cin-chain only\u201d to \u201ceverywhere but in-chain\u201d, and it is dictated by both the activation mode and the solvent used to dissolve the nickel precatalyst

Additive-free cobalt-catalysed hydrogenation of carbonates to methanol and alcohols

Reduction of various organic carbonates to methanol and alcohols can be achieved in the presence of a molecularly-defined homogeneous cobalt catalyst. Specifically, the use of Co(BF4)2 in combination with either commercial or tailor-made tridentate phosphine ligands allows for additive-free hydrogenations of carbonates. Optimal results are obtained at relatively mild conditions (120 \ub0C, 50 bar hydrogen pressure) in the presence of xylyl-Triphos L4

Palladium alkyl complexes with a formazanate ligand: synthesis, structure and reactivity

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

A square planar gold(III) bis-(1,1'-dimethyl-3,3'-methylene-diimidazol-2,2'-diylidene) trication as an efficient and selective receptor towards halogen anions: The cooperative effect of Au---X and X---HC interactions

Treatment of the tricationic gold(III) [Au(MeImCH(2)]mMe)(2)](PF6)(3) complex 1-3PF(6) (Im = imidazol-2-ylidene) with excess halides affords complexes 1-3X (X = Cl, Br, and I), resulting from counter anion PF6 (-)/ X- exchange. The H-1 chemical shift of the CH3 groups and particularly that of the CH2 linker in DMSO-d6 are different in the three complexes, thus suggesting selective X center dot center dot center dot HC interactions. Complex 1(3+) can therefore be used as a halide sensor in DMSO and water. The host-guest interaction between the tricationic gold(III) complex and the halides C-l-, Br- and I- in solution and in the solid state has been investigated by means of NMR titration experiments, DFT calculations and X-ray structure analysis. The electrostatic interaction between the halides and the triple formal positive charge on the metal centre, together with the CH center dot center dot center dot X hydrogen bonding between the NHC ligand and halides, contributes to the formation of stable supramolecular aggregates in solution and in the solid state. The complexing properties of 1(3+) are strongly influenced by the nature of the solvent. Formation of the 1 : 1 and 1 : 2 species (1X(2+) and 1X(2+)) is observed in DMSO-d(6), while that of only the 1 : 1 aggregates (1X(2+)) is observed in D2O (X = Cl, Br, and I). Moreover, the selectivity towards the various halides is reversed in the two solvents, being in the order Cl-> Br-> Iin DMSO-d(6) and I-> Br-> Cl- in D2O. The formation constants of the species 1X2+ and 1X2 + in DMSO and 1X(2+) in water have been determined by fitting the NMR titration curves

New Homoleptic Gold Carbene Complexes via Ag-Au Transmetalation: Synthesis and Application of the [Au(diNHC)2]3+ Cations as 1H-NMR and UV-vis Halides Sensors

Three novel gold(III) complexes with bidentate di(N-heterocyclic carbene) (diNHC) ligands of general formula [AuL2](PF6)3 (L = diNHC) were isolated by transmetallation of the carbene ligand from the corresponding silver(I) complexes [Ag2L2](PF6)2 to KAuBr4 in the presence of AgPF6. In this regard, the scope of ligand stereo-electronic properties has been investigated. Over a number of seven tested ligands, three ligands leaded to the formation of the desired gold(III) complex, whereas in the other cases a gold(I) complex of formula [Au2L2](PF6)2 was isolated as main product. The anion sensing ability of the prepared gold(III) complexes towards the halides Cl-, Br- and I- has been studied in DMSO-d6 and D2O by means of 1H-NMR titration experiments and in one case also by UV-vis. Replacement of a methyl with a n-butyl wingtip substituent on the imidazole ring strongly favours the interaction with the larger iodide anion with respect to chloride. Introduction of an OH group in the bridge gives raise to metal- and ligand-centred interactions, which allow to discriminate on the nature of the involved anion. The formation of the gold(III) complex-halide aggregates has been also analysed with X-ray crystallography studies

CCDC 1835847: Experimental Crystal Structure Determination

Related Article: Francesca Milocco, Folkert de Vries, Anna Dall'Anese, Vera Rosar, Ennio Zangrando, Edwin Otten, Barbara Milani|2018|Dalton Trans.|47|14445|doi:10.1039/C8DT03130D,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.

CCDC 1835846: Experimental Crystal Structure Determination

Related Article: Francesca Milocco, Folkert de Vries, Anna Dall'Anese, Vera Rosar, Ennio Zangrando, Edwin Otten, Barbara Milani|2018|Dalton Trans.|47|14445|doi:10.1039/C8DT03130D,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.