Comparing Pd-catalyzed CO/vinyl arene and ethylene/acrylate copolymerization

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.