Structural characterisation of difluoro-boron chelates of quino[7,8-h]quinoline Karl J. Shaffer, Tracey M. McLean, Mark R. Waterland, Marco Wenzel, Paul G. Plieger ⇑ Institute of Fundamental Sciences, Massey University, Private Bag 11 222, Palmerston North, New Zealand article info Article history: Available online 29 September 2011 Young Investigator Award Special Issue Keywords: Quino[7,8-h]quinoline Synthesis X-ray structure Boron difluoride Metalloid binding Proton sponge abstract We report a milder synthetic route for the compounds 4,9-dichloroquino[7,8-h]quinoline L 1 and quino[7,8-h]quinoline L 2 and we utilise these ligands in the synthesis of boron difluoride chelates. X-ray crystallographic analysis of L 1 , [HL 1 ]BF 4 and the coordinated structures of [BF 2 L 1 ]BF 4 and [BF 2 L 2 ]BF 4 reveal the flexible nature of the ligand. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction The main group coordination chemistry of boron continues to receive considerable attention in the areas of catalysis [1], soft materials research [2] and fluorophores, the latter due to the boron difluorides utilisation in BODIPY dyes [3]. Despite the continued interest there are still relatively few examples of BF 2 chelates with neutral ligands and to our knowledge there exists only one structurally characterised chelate of boron with a neutral aromatic ligand [4]. Our group has an interest in chelating ligands designed to selectively bind small cations and to this end we have turned our attention to the quinoquinoline class of ligands. Quino[7,8- h]quinoline (L 2 , Scheme 2) has had a controversial history regarding its synthesis, with several reports stating the successful synthesis dating back to 1950 [5–7]. These later proved to be incorrect [8] and it was not until 1987 that a viable synthetic pathway was identified [9], with subsequent X-ray structure confirmation [10]. Twenty-three years later and little attention has been placed on this structurally intriguing molecule. Not-withstanding several theoretical papers [11–13], there has been just one paper published which reports on several coordination complexes [14]. Given that related ligands such as 2,2 0 -bipyridine and 1,10-phenanthroline are routinely used by both coordination and supramolecular chemists for a wide variety of applications we found the lack of complexes for the quinoquinoline class of ligands surprising. The reason may well lie in a problematic two step de-esterification sequence, which utilises a high temper- ature/low pressure protocol (370 °C and 10 5 torr). We now report a milder synthetic route for the quino-quinolines L 1 and L 2 (Scheme 1) which avoids these conditions and utilise these ligands in the synthesis of boron difluoride chelates. 2. Experimental 2.1. Materials and methods Unless specified, commercial reagents and solvents were used without purification. Dichloromethane was dried by passing the solvent through activated alumina. All reactions were carried out in oven-dried glassware, under an atmosphere of nitrogen or ar- gon. NMR spectra were collected on Bruker Avance 400 and 500 MHz spectrometers; the particular instrument is specified for each compound. In CDCl 3 , all chemical shifts are reported relative to TMS ( 1 H) and residual solvent ( 13 C). In all other deuterated sol- vents, the chemical shifts are reported relative to residual solvent ( 1 H, 13 C) or reference compounds ( 11 B, 19 F). The full assignment of 1 H and 13 C spectra were aided using COSY and HMQC experi- ments. Electrospray mass spectra were recorded on a Micromass ZMD spectrometer run in positive ion mode. Elemental analyses were determined by the Campbell Microanalytical Laboratory at the University of Otago. UV–Vis spectra were recorded on a Shimadzu UV-3101PC spectrophotometer using UV Probe v1.1. Fluorescence measurements were made on a Perkin Elmer LS50B luminescence spectrometer using FL Winlab v4.00.02. The slit width was 2.5 nm for both excitation and emission. Relative quan- tum efficiencies were obtained by comparing the areas under the 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.09.046 ⇑ Corresponding author. Tel.: +64 6 356 9099x7825; fax: +64 6 350 5682. E-mail address: p.g.plieger@massey.ac.nz (P.G. Plieger). Inorganica Chimica Acta 380 (2012) 278–283 Contents lists available at SciVerse ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica