www.elsevier.nl/locate/jorganchem Journal of Organometallic Chemistry 587 (1999) 58 – 66 Chiral palladium(II) complexes bearing tetradentate nitrogen ligands: synthesis, crystal structure and reactivity towards the polymerization of norbornene Adnan S. Abu-Surrah a , Ulf Thewalt b , Bernhard Rieger a, * a Abteilung Anorganische Chemie II, Uniersita ¨t Ulm, D-89069 Ulm, Germany b Sektion Ro ¨ntgen - und Elektronenbeugung, Uniersita ¨t Ulm, D-89069 Ulm, Germany Received 24 March 1999; received in revised form 15 April 1999 Abstract A synthesis for a series of tetrapodal nitrogen ligands (N,N-dibenzyl-N,N-di(quinoline-2-methyl)-1,2-ethylene diamine (DBQED, 1), N,N-dimethyl-N,N-di(quinoline-2-methyl)-1,2-ethylene diamine (DMQED, 2), (1R,2R )-( -)-N,N-di(quinoline-2- methylene) diiminocyclohexane (DQEDC, 3), N,N-di(quinoline-2-methylene)-1,2-phenylene diimine (DQPD, 4) and for their corresponding neutral and cationic palladium(II) complexes of the types [(1,2,4)PdCl 2 ](5a, 6a, 7) and [(1,2,3,4)Pd(NCCH 3 ) x ](Y) 2 (5b, 5c, 6b, 6c, 8, 9), x =0, 2; Y =BF 4 - , NO 3 - is reported. The dicationic palladium(II) complex [Pd(DBQED)(NO 3 ) 2 ·H 2 O] (5c) crystallizes in the monoclinic space groups P 2 1 /c (no. 14) with a =11.458(1), b =15.302(1) A  , c =20.644(2) A  ,  =99.23(1)° and V =3572.7 A  3 , Z =4. All four nitrogen donors are attached to the Pd(II)-center in the chiral and C 2 -symmetric complex 5c. The two quinoline nitrogen atoms of the more rigid Schiff bases (DQEDC, 3; DQPD, 4) do not coordinate coincidentally to Pd(II). A fluxional coordination behavior is suggested for these quinoline donors, which enables this new complex family to act as catalysts for norbornene polymerization, even when chloride is present as a counter ion. Introduction of an enantiomerically pure trans -1,2-cyclohexyl bridge into the Schiff base ligand systems afforded the formation of partially stereoregular polynorbornene. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Palladium(II) polymerization catalysts; Tetradentate nitrogen ligands; Schiff bases; Chirality; Crystal structure; Norbornene polymer- ization 1. Introduction The coordination chemistry of late transition metal complexes bearing multidentate ligands which contain terminal pyridyl donor groups has been intensively studied [1]. The palladium(II) or platinum(II) com- plexes of those ligands show a dynamic behavior by exchange of the terminal pyridyl donor groups. This fluctuation phenomenon, which is common in the coor- dination chemistry of kinetically labile palladium(II) complexes, is explained as a result of preliminary ligand dissociation enabling a variety of processes to occur [2]. In model studies of the separate steps of the alternating copolymerization of carbon monoxide with alkenes [3,4], it has been shown that palladium compounds with nitrogen (imine) ligands can be more reactive than analogous palladium phosphine complexes. However, they are less stable under catalytic conditions, particu- larly due to their tendency to dissociate from the metal center. Recently, we reported on new tetradentate, stereo- rigid amine ligand systems containing terminal 2- quinaldinyl-N -donor groups. The coordination of these ligands with late transition metals, like iron(II) and cobalt(II), has been investigated [5]. The results of the X-ray structure analysis of an octahedral complex prove the desired chiral and C 2 -symmetric coordination of the ligands, where the quinoline units open a chiral cage around the metal center, which resembles that of the two indenyl fragments in C 2 -symmetric ansa-metal- locene dichlorides [6]. Herein, we report on the synthesis and characteriza- tion of some neutral and cationic palladium(II) com- * Corresponding author. Tel.: +49-731-5023038; fax: +49-731- 5023039. E-mail address: bernhard.rieger@chemie.uni-ulm.de (B. Rieger) 0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII:S0022-328X(99)00273-9