Synthesis of aluminium complexes bearing a piperazine-based ligand system Nick C. Johnstone, Elham S. Aazam, Peter B. Hitchcock, J. Robin Fulton * Department of Chemistry, University of Sussex, Brighton BN1 9QJ, UK article info Article history: Received 17 September 2009 Received in revised form 9 October 2009 Accepted 14 October 2009 Available online 20 October 2009 Keywords: Aluminium N,O-ligands Piperazine-ligands Polymerisation Caprolactone abstract Aluminium complexes bearing the N,N-chelating ligand 1,4-bis(2-hydroxy-3,5-di-tert-butyl)piperazine (1) have been synthesised. Both monometallic and bimetallic aluminium methyl complexes (2 and 3, respectively) were prepared by treatment of 1 with the appropriate amount of AlMe 3 . Complex 2 can be converted to 3 by addition of excess AlMe 3 . Bimetallic aluminium-ethyl complex 4 was also prepared. Treatment of 1 with AlEt 2 Cl afforded the monometallic chloride complex 5. Treatment of this latter com- plex with potassium alkoxides (KOR, R = Me, Et, i Pr, t Bu) or AgOTf afforded the corresponding aluminium alkoxide complexes (6, R = Et; 7, R = Me; 8,R= i Pr; 9,R= t Bu; 10, R = OTf) in good yields. Aluminium eth- oxide complex 6 was also synthesised by treatment of 1 with AlEt 2 OEt. All of these complexes were tested as potential catalysts in the ring-opening polymerisation of rac-lactide and caprolactone with lim- ited success. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Aluminium complexes have been shown to be active on the ring-opening polymerisation (ROP) of cyclic esters such as lactides and lactones [1–3]. As such, a variety of aluminium alkoxides as well as aluminium alkyl complexes have been synthesised and have shown to be active catalytic precursors [4–10]. Many of these species employ phenoxy-based ancillary ligands, including biden- tate and tetradentate Schiff-based systems [5–7,10,11] as well as phenoxymethyl-bisamine-based ligand systems [8]. N-substituted piperazines are versatile ligands as they can potentially be used to bind either one- or two-metal centres, the for- mer of which results in a conformational strain around the pipera- zine ring as the 6-membered piperazine ring forms the less stable boat conformation [12]. Generally, the nitrogen atoms of the piper- azine ring are further substituted in order in increase the tacticity of the ligand, resulting in a wide variety of binding possibilities to create monometallic and bimetallic species (Fig. 1). For instance, Limberg has generated the neutral ligand 1,4-bis(2-pyridyl- methyl)piperazine ligand and has shown that it can coordinate to iron(II) as either a tetradentate or bidentate ligand, the latter of which forms a bimetallic species in which the piperazine bridges two iron metal centres [13]. Other uses of similar piperazine ligands include Wieghardt’s 1,4-bis(2-amino-benzyl)piperazine [14], which was used as a neutral ligand on late transition metals, and Mountford’s 1,4-bis(2-amino-4-tert-butylbenzyl)piperazine for use as a dianionic ligand to stabilise titanium imido complexes [15]. Recently, the groups of Lappert as well as Balakrishna have uti- lised 1,4-bis(2-hydroxy-3,5-di-tert-butyl)piperazine (1) as a ligand to stabilise zinc and palladium complexes [16,17]. In most of the above cases, the piperazine-based ligands were not used inter- changeably between the monometallic and bimetallic binding modes; only Limberg has been able to take advantage of the diver- sity of the piperazine-based ligand to ligate both one or two iron centres [13]. We have utilised ligand 1 to generate a series of new aluminium complexes, including aluminium alkyl and aluminium alkoxide species. With regard to the alkyl complexes, we have shown that this ligand can bind either one or two aluminium metal centres; however, the bimetallic species is significantly more stable than the corresponding monometallic complex. All of the aluminium alkoxide complexes were obtained as monometallic complexes; at- tempts to obtain their corresponding bimetallic species were unsuccessful. 2. Results and discussion 2.1. Aluminium alkyl complexes Monometallic aluminium methyl complex 2 was obtained in 86% yield through treatment of a toluene solution of ligand 1 with one equivalent of AlMe 3 at 50 °C, followed by a five-hour reflux (Eq. (1)). A white solid was isolated which did not contain any measurable amount of impurities. The 1 H NMR spectrum of com- plex 2 revealed a singlet integrating to three hydrogens at d 0.79 ppm, consistent with an Al–Me resonance. Three sets of piperazine-based resonances with a ratio of 1:1:2, respectively, 0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2009.10.016 * Corresponding author. Tel.: +44 (0) 1273 873170; fax: +44 (0) 1273 876687. E-mail address: j.r.fulton@sussex.ac.uk (J.R. Fulton). Journal of Organometallic Chemistry 695 (2010) 170–176 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem