Cationic rhodium(I) complexes containing 4,4 0 -disubstituted 2,2 0 -bipyridines: A systematic variation on electron density over the metal centre Paulo E.A. Ribeiro 1 , Claudio L. Donnici * , Eduardo N. dos Santos * Departamento de Quı ´mica, Universidade Federal Minas Gerais, Av. Antonio Carlos, 6627, 31270-901 Belo Horizonte, MG, Brazil Received 26 October 2005; received in revised form 21 December 2005; accepted 22 December 2005 Available online 28 February 2006 Abstract A series of [Rh(COD)(X 2 -bipy)]BF 4 complexes (COD = 1,5-cyclooctadiene; X 2 -bipy = 4,4 0 -disubstituted 2,2 0 -bipyridines; X = OCH 3 , CH 3 , H, Cl or NO 2 ) has been prepared from [Rh(COD)Cl] 2 . The complexes for X = OCH 3 , Cl and NO 2 have not been described previously in the literature. All complexes have been characterised by elemental analysis, IR, 1 H NMR and UV–Vis spectrometry. This series of complexes presents a wide variation on electron density over the metal centre with virtually no variation on its steric environ- ment which discloses interesting possibilities for catalytic and electro-catalytic studies. A preliminary evaluation of these complexes on the hydroformylation of camphene and b-pinene showed that under the rather drastic conditions employed the complexes acted as a precursor for [Rh(CO) 3 H], which accounts for most of the catalytic activity. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Cationic rhodium(I) complexes; Substituted bipyridine; Hydroformylation; Rhodium catalysts 1. Introduction Cationic rhodium(I) complexes have long been acknowl- edged as catalysts precursors for many reactions such as hydrogenation [1] or transfer hydrogenation [2] of dou- ble-bonds, water gas shift reaction [3], amination [4], hydroamination [5], hydroformylation [6–8], among others. Although phosphorus ligands are more widely used for these reactions, in many instances nitrogen-donors such as 2,2 0 -bipyridines have been successfully employed [9–20]. We have been studying the diastereoselective hydro- formylation of monoterpenes as a way of better understand- ing the diastereoselectivity control in hydroformylation employing rhodium complexes [21] or platinum/tin systems [22,23] as catalysts. In a previous work [21] we described the rhodium-catalysed hydroformylation of b-pinene and camphene in the presence of various phosphines and phos- phites ancillaries. It has been demonstrated that the diastero- isomeric excess for the cis aldehyde is directly related to the electron-donating ability of the ancillary measured as described by Tolman [24]. Nevertheless, no clear correlation was found for its steric parameter (cone angle). In order to investigate whether or not this influence is purely electronic we decided to synthesise rhodium(I) complexes containing various 2,2 0 -bipyridines disubstituted in the 4,4 0 positions by the methoxy, methyl, chloro, or nitro groups. In this series there is a considerable variation on the electron-donating ability of the ligands with virtually no variation on their ste- ric parameters. In this work, we describe the synthesis and the spectro- scopic characterisation of rhodium complexes of the type [Rh(COD)(X 2 -bipy)]BF 4 (COD = 1,5-cyclooctadiene; X 2 -bipy = 2,2 0 bipirydine or 2,2 0 bipirydine 4,4 0 -disubsti- tuted by Cl, OCH 3 , NO 2 , CH 3 ), as well as their use as a cat- alyst for the hydroformylation camphene and b-pinene. 0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2005.12.049 * Corresponding authors. Tel.: +553134995743; fax: +553134995700. E-mail addresses: cdonnici@ufmg.br (C.L. Donnici), nicolau@ufmg.br (E.N. dos Santos). 1 Present address: Empresa Brasileira de Agropecua ´ria – EMBRAPA. www.elsevier.com/locate/jorganchem Journal of Organometallic Chemistry 691 (2006) 2037–2043