Synthesis of mononuclear and dinuclear palladium (II) complexes containing oxadithioether ligands and their catalytic activities in norbornene polymerization

Abstract

The Pd (II) complexes trans-[PdCl 2 (μ-L)] 2 and [Pd(acac)(L)][BF 4 ] (L = RS (CH 2 ) 2 O(CH 2 ) 2 SR, R = Me, Et, n Pr, i Pr, n Bu, i Bu, n-hexyl, benzyl) were obtained through the reaction of Pd(cod)Cl 2 or [Pd(acac)(MeCN) 2 ][BF 4 ] with one equivalent of oxadithioether (L). The structural features of these complexes were analyzed by nuclear magnetic resonance (NMR) and Fouriertransform infrared spectroscopy (FTIR), as well as electrospray ionisation mass spectrometry and density functional theory calculations. The complexes diμ-(2,10-dimethyl-6-oxa-3,9-dithiaundecane-κ 2 S,S 0 )-bis[trans-dichloropalladium (II)] and (acetylacetonate-κ 2 O,O 0 )(2,10-dimethyl-6-oxa-3,9-dithiaundecane-κ 2 S, S 0 )palladium (II) tetrafluoroborate were characterized by X-ray diffractometry. The X-ray structures of each complex indicate an axial M-O interaction formed by the endodentate conformation of the oxadithioether ligand. Palladium (II) dichloride complexes with oxadithioethers were demonstrated to have a 16-membered dinuclear structure with a trans-configured S 2 PdCl 2 fragment. In the case of cationic palladium acetylacetonate complexes, only mononuclear complexes with a cis configuration of the oxadithioether fragment were observed. Variable temperature 1 H and 13 C NMR studies of the complexes demonstrate dynamic bonding of the oxadithioether ligands consistent with the presence of diastereoisomers that differ in the orientation of the S-R groups along with both endodentate and exodentate bonding modes in solution. FTIR studies of the complexes indicate the presence of isomers in the solid state. The palladium catalyst precursors trans-[PdCl 2 (μ-L)] 2 and [Pd (acac)(L)][BF 4 ] were found to be active in the addition polymerization of norbornene. In the presence of cocatalysts, such as diisobutylaluminum chloride and boron trifluoride etherate, Pd (II) complexes exhibited activities in the range of 10 5 to 10 7 g of polymer (mol of Pd) À1 h À1 . The catalyst systems