Synthesis of a bis(phenoxyketimine) palladium(II) complex and its activity in the Suzuki–Miyaura reaction Daniel F. Brayton, Timothy M. Larkin, David A. Vicic, Oscar Navarro * Department of Chemistry, University of Hawai‘i at Ma¯ noa, 2545 McCarthy Mall, Honolulu, HI 96822, United States article info Article history: Received 13 March 2009 Received in revised form 29 April 2009 Accepted 30 April 2009 Available online 8 May 2009 Keywords: Palladium catalyst Suzuki–Miyaura reaction Unactivated aryl bromides Phenoxyketimine abstract The synthesis of a new charge-neutral, air- and moisture-stable fluorinated bis(phenoxyketimine) Pd(II) complex is presented. Its activity as a precatalyst in the Suzuki–Miyaura cross-coupling reaction of acti- vated and unactivated bromides has been explored. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Schiff bases are attractive ligands due to their facile preparation and simple synthetic modification, both electronic and steric, and have been used to prepare a broad range of organometallic com- pounds with a wide variety of applications [1–4]. For instance, bis-phenoxyketimine (PHI) titanium complexes were originally re- ported for the polymerization of ethylene [5]. The improved activ- ity over known metallocenes spurred further interest in these early transition metal complexes, with general formula [M(PHI) 2 X 2 ] (M = Ti, Zr; X = Cl, Br). It was later discovered that activity increases dramatically when two or more F or CF 3 groups are introduced to the imido phenyl ring. Both synthetic and computational methods confirmed that in particular an ortho fluorine (or other lone pair donating substituent) can inhibit b-hydride elimination and pro- mote living catalyst conditions [6]. It is this feature that is believed to promote high speed living polymerizations and lead to monodis- perse high molecular weight polymers. A new derivative of PHI li- gands with a phenyl ring on the N–O backbone, PKI-F 5 (1), was recently employed by Coates and co-workers, for the living, isose- lective polymerization of propylene [7]. Recently, the growing interest on the development of systems for the polymerization of polar vinyl monomers, such as acrylates, has led to the synthesis of a variety of complexes of palladium and nickel for this purpose [8], because of the tolerance to functional- ities that late transition metals display [9]. Many of those com- plexes include [N,O] chelating ligands that resemble [P,O] ligands used for SHOP-type (SHOP = Shell Higher Olefin Process) [10].A search of the literature uncovered several related [Pd(PHI) 2 ] and [Pd(PHI)(Me)(X)] compounds, not only for that use but for many other applications [11]. However, only one other monofluorinated example is known in the literature [12]. When attempting the syn- thesis of a 1:1 PdCl 2 :1 complex, we obtained as a byproduct the new, neutral, air- and moisture-stable bis(phenoxyketimine) Pd(II) complex 2 (Fig. 1), whose synthesis we optimized and report in here. The activity of the complex in the Suzuki–Miyaura cross-cou- pling [13] of unactivated aryl bromides and boronic acids is also discussed. 2. Results and discussion 2.1. Complex synthesis The ligand was prepared following the reported procedure by Mason and Coates [14], then deprotonated with n-butyllithium at 78 °C and allowed to react with PdCl 2 at room temperature over- night. Elution of the unreacted ligand through a plug of silica with pentane, followed by elution with methylene chloride, yielded the desired complex 2 (81%) with no observable complex decomposi- tion. X-ray quality crystals were obtained by slow diffusion of pen- tane into a concentrated solution of 2 in methylene chloride at room temperature, and the structure of the complex was deter- mined by X-ray crystallography. An ORTEP plot of 2 is shown in Fig. 2, and selected crystallographic parameters are listed in Table 1. The complex displays a nearly ideal square planar geometry, with both ligands trans to each other. On each PKI-F 5 ligand the pentaflu- oro aryl ring and ketimine aryl ring have a centroid distance of 0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2009.04.044 * Corresponding author. Fax: +1 808 956 5908. E-mail address: oscarnf@hawaii.edu (O. Navarro). Journal of Organometallic Chemistry 694 (2009) 3008–3011 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem