Substitution reactions of tetrahydrofuran in [Cr(thf) 3 Cl 3 ] with mono and bidentate N-donor ligands: X-ray crystal structures of [Cr(bipy)(OH 2 )Cl 3 ] and [HpyNH 2 ][Cr(bipy)Cl 4 ] Nicholas F. Brennan a , Burgert Blom a,1 , Simon Lotz a , Petrus H. van Rooyen a, * , Marile ´ Landman a , David C. Liles a , Mike J. Green b a Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa b Sasol Technology, 1 Klasie Havenga Road, Sasolburg 1947, South Africa Received 29 November 2007; received in revised form 24 January 2008; accepted 26 January 2008 Available online 6 February 2008 Dedicated to Professor Robert J. Angelici. Abstract Substitution of thf ligands in [Cr(thf) 3 Cl 3 ] and [Cr(thf) 2 (OH 2 )Cl 3 ] was investigated. 2,2 0 -Bipyridine (bipy) was reacted with [Cr(thf) 3 Cl 3 ] to form [Cr(bipy)(thf)Cl 3 ](1), which was subsequently reacted with water to give [Cr(bipy)(OH 2 )Cl 3 ](2). Reaction of 1 with acetonitrile (CH 3 CN), pyridine (py) and pyridine derivatives to form [Cr(bipy)(L)Cl 3 ] (L = CH 3 CN 3, py 4 and 4-pyR with R = NH 2 5, Bu t 6 and Ph 7). In addition, the substitution of bipy in [Cr(thf) 3 Cl 3 ] was followed by 1 H NMR spectroscopy at room temperature, which showed completion of the reaction in ca. 100 min. Complex 2 was characterised by single crystal X-ray diffraction. The theoretical pow- der diffraction pattern of 2 was compared to the experimentally obtained powder X-ray diffraction pattern, and shows excellent agree- ment. The dimer [Cr 2 (bipy) 2 Cl 4 (l-Cl) 2 ] was cleaved asymmetrically to give the anionic complex [Cr(bipy)Cl 4 ]  (8) and [Cr(bipy) 2 Cl 2 ] + (9). Complexes 8 and 9 were characterised by single crystal X-ray diffraction. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Chromium(III), 2,2 0 -Bipyridine; Substitution reactions; Asymmetrically cleaved dimers; Catalysis; a-Olefins 1. Introduction In recent years, interest in the selective trimerization and tetramerization of ethylene to afford linear a-olefins has escalated because of their industrial importance [1,2]. Among the many catalysts known, chromium(III) precur- sors are preferred for their better activity and selectivity. In particular, [CrCl 3 (thf) 3 ] has been used as a precursor with bidentate PNP- [3], bidentate PNP with pendant side-arms containing heteroatoms [4] and tridentate XYX (X = PR 2 or SR and Y = PR or NH; XYX = N,N,N- donor ligands) [5] ligands and catalyst activators such as MAO or aluminium alkyl compounds [1,6], have been investigated. Some outstanding issues to be resolved are the reaction mechanism (especially for the tetramerization reaction) as well as the identification of the catalytically active species [2,7]. The latter deals with complex composi- tion, mode of activation and the metal redox couple responsible for the olefin coordination and carbon–carbon coupling reactions during the chain growth [1,8,9]. It is generally believed that the co-catalyst acts upon the chro- mium precursor to give a cationic catalytically active inter- mediate [4a,6,8,9]. Also generally accepted is the role of dimeric chloro bridged intermediates of which the existence has been postulated or indicated by solid state structures 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.01.033 * Corresponding author. Tel.: +27 12 4202519; fax: +27 12 4204687. E-mail address: phvr@up.ac.za (P.H. van Rooyen). 1 Present address: Institut fu ¨ r Anorganische Chemie, Universita ¨t Bonn, Bonn 53115, Germany. www.elsevier.com/locate/ica Available online at www.sciencedirect.com Inorganica Chimica Acta 361 (2008) 3042–3052