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Two pairs of enantiomerically pure chiral bidentate nitrogen-donor ligands (N*-N) featuring one sp(3) and one sp(2) nitrogen atom have been applied to the CO/vinyl arene copolymerization reaction catalyzed by the corresponding monocationic palladium(11) complexes [Pd(CH3)(CH3CN)-(N*-N)][PF6]. The ligand frame containing the sp(2) nitrogen atom is a 2-pyridinyl or an 8-quinolinyl building block, while the chiral framework of the ligand derives from (S)-(+)-2.2'-(2-azapropane-1,3-diyl)-1,1'-binaphthalene or from trans-2.5-dimethylpyrrolidinyl. Ligands with the binaphthyl moiety generate catalysts showing a moderate activity, while ligands having the pyrrolidinyl fragment results in very low activity species. The stereochemistry of the synthesized polyketones depends of the sp(2) nitrogen-containing fragment: ligands with the 2-pyridinyl group lead to atactic copolymers, whereas those with the quinolinyl moiety give an isotactic polyketone. The effect of the nature of the quinone, added to the reaction mixture as oxidant, has been investigated MALDI-TOF analysis reveals the formation of several polymeric chains, differing in the presence of various end-groups.

The catalytic CO/styrene and ethylene/methyl acrylate copolymerizations are compared for the first time by applying the same precatalysts. With this aim, Pd-II neutral, [Pd(CH3) Cl(N-N)], and monocationic, [Pd(CH3)(L)(N-N)][PF6] (L= CH3CN, DMSO), complexes with 1-naphthyl- and 2-naphthyl-substitued alpha-diimines with both an acenaphthene and a diazabutadiene skeleton as chelating nitrogen-donor ligands (N-N) have been studied. In the case of the complexes with the 1-naphthyl-substituted ligands, syn and anti isomers are found both in solid state and in solution. All the monocationic complexes generate active catalysts for the CO/styrene copolymerization leading to atactic or isotactic/atactic stereoblock copolymers depending on the ligand bonded to palladium. Instead, in the case of the ethylene/methyl acrylate copolymerization only the complexes with the 1-naphthyl-substituted ligands generate catalysts for this reaction.

A series of Pd-complexes containing nonsymmetrical bis(aryl-imino)acenaphthene (Ar-BIAN) ligands, characterized by substituents on the meta positions of the aryl rings, have been synthesized, characterized and applied in CO/vinyl arene copolymerization reactions. Crystal structures of two neutral Pd-complexes have been solved allowing comparison of the bonding properties of the ligand. Kinetic and mechanistic investigations on these complexes have been performed. The kinetic investigations indicate that in general ligands with electron-withdrawing substituents give more active, but less stable, catalytic systems, although steric effects also play a role. The good performance observed with nonsymmetrical ligands is at least in part due to a compromise between catalyst activity and lifetime, leading to a higher overall productivity with respect to catalysts based on their symmetrical counterparts. Additionally, careful analysis of the reaction profiles provided information on the catalyst deactivation pathway. The latter begins with the reduction of a Pd(II) Ar-BIAN complex to the corresponding Pd(0) species, a reaction that can be reverted by the action of benzoquinone. Then the ligand is lost, a process that appears to be facilitated by the contemporary coordination of an olefin or a CO molecule. The so formed Pd(0) complex immediately reacts with another molecule of the initial Pd(II) complex to give a Pd(I) dimeric species that irreversibly evolves to metallic palladium. Mechanistic investigations performed on the complex with a nonsymmetrical Ar-BIAN probe evidence that the detected intermediates are characterized by the Pd-C bond trans to the Pd-N bond of the aryl ring bearing electron-withdrawing substituents. In addition, the intermediate resulting from the insertion of 4-methylstyrene into the Pd-acyl bond is a five-member palladacycle and not the open-chain η(3)-allylic species observed for complexes with Ar-BIANs substituted in ortho position.

Pd(II) complexes bearing -diimines as ancillary ligands have been widely applied as catalysts to CO/alkene1 and ethylene/methyl acrylate (MA)2 copolymerization reactions. However, up to now there is a lack of a homogeneous comparison between these two reactions, hampering to highlight their analogies and differences. For this purpose, we have studied two -diimines with an acenaphthene (BIAN) and two with a diazabutadiene (DAB) skeleton, bearing 1-naphthyl (1-napht) or 2-naphthyl (2-napht) groups on the N-donors. The corresponding Pd(II) complexes [Pd(N-N)(Me)(L)]0/+ (N-N = 1-naphtBIAN, 2-naphtBIAN, 1-naphtDAB, 2-naphtDAB; L = Cl-, MeCN) were characterized both in solid state and in solution. For [Pd(Me)Cl(1-naphtBIAN)], syn and anti isomers (Figure) were found in the crystal structure, in agreement with the characterization in solution, that shows the equilibrium between the two conformers. On the other hand, for the complexes bearing 2-naphthyl-substituted ligands a single species was observed in solution. The cationic derivatives were tested as precatalysts both in CO/styrene and ethylene/MA copolymerizations under mild conditions, highlighting that: i. in CO/styrene copolymerization BIAN ligands generate more productive and more stable catalysts than DAB derivatives (2.73 kg CP/g Pd vs 0.79 kg CP/g Pd); ii. in ethylene/MA copolymerization opposite trend is found: DAB-containing catalysts are better performing than BIAN-analogues (201.3 g P/g Pd vs 27.0 g P/g Pd); iii. for both reactions, regardless to the ligand skeleton, catalysts with 2-naphthyl-substituted ligands show a catalytic behaviour analogous to that of catalysts with meta-substituted aryl-BIANs, while catalysts with 1-naphthyl-substituted ligands show a catalytic behaviour analogous to that of catalysts with ortho-substituted aryl-BIANs. Catalysts with 1-naphtBIAN or 1-naphtDAB, detected in solution as syn/anti isomers, lead to CO/styrene polyketones with isotactic/atactic stereoblocks, while atactic CO/styrene copolymers are produced with catalysts having the 2-napht-ligands. Ethylene/MA cooligomers, obtained with 1-naphDAB-containing catalyst, are highly branched with 29 % of inserted polar monomer. 1 a) A. Scarel, M.R. Axet, F. Amoroso, F. Ragaini, J.C. Elsevier, A. Holuigue, C. Carfagna, L. Mosca, B. Milani Organometallics 2008, 27, 1486; b) B. Binotti, C. Carfagna, C. Zuccaccia, A. Macchioni Chem. Commun. 2005, 92; c) J.H. Groen, J.G.P. Delis, P.W.N.M. van Leeuwen, K. Vrieze, Organometallics 1997, 16, 68. 2 L. K. Johnson, S. Mecking, M. Brookhart J. Am. Chem. Soc. 1996, 118, 267.

New atropisomeric bidentate bipyridine-based ligands (3,3’-(ethylenedioxy)-2,2’-bipyridine 3;3,3’-(propylenedioxy)-2,2’-bipyridine 4; 3,3’-(butylenedioxy)-2,2’-bipyridine 5) containing a bridge between the 3,3’ positions of the aromatic rings have been prepared. Together with the previously reported analogous ligands ((R)-3,3’-(1-methylethylenedioxy)-2,2’-bipyridine) 1 and ((S,S)-3,3’-(1,2-dimethylethylenedioxy)-2,2’-bipyridine) 2, they were used to synthesize the corresponding bis-chelated dicationic complexes [Pd(N–N)2][PF6] 2. Crystal structures and comparison of the data obtained by X-ray analysis on four of these complexes is reported. These palladium compounds were used as precatalysts in the CO/styrene and CO/4-Me-styrene copolymerization reactions, where they showed that small variations in the ligand backbone remarkably affects the productivity of the catalytic system.

Palladium−azabis(oxazoline) complexes proved to be highly selective and efficient in the asymmetric copolymerization of styrene and CO at ambient pressure.