Homo- and Heteroleptic 8-Quinaldinolate Complexes From Elevated-temperature Rearrangements Glen B. Deacon, A Peter C. Junk, B,C David R. Turner, A,C and Julia A. Walker A A School of Chemistry, Monash University, Clayton, Vic. 3800, Australia. B School of Pharmacy and Molecular Sciences, James Cook University, Townsville, Qld 4811, Australia. C Corresponding authors. Email: david.turner@monash.edu; peter.junk@jcu.edu.au Three new zinc 8-quinaldinolate complexes have been obtained from rearrangement reactions at elevated temperatures including the first homoleptic zinc 8-quinaldinolate complex. The homoleptic, trinuclear complex [Zn 3 (MQ) 6 ](1) (MQ ¼ 8-quinaldinolate) was obtained by the recrystallisation of amorphous Zn(MQ) 2 from a 1,2,4,5-tetramethylbenzene flux at 2708C. The heteroleptic complexes [Zn 4 Cl 4 (MQ) 4 ](2) and [Zn 4 Cl 2 (MQ) 6 ](3) were simultaneously obtained by the reaction between Zn(MQ) 2 and anhydrous ZnCl 2 under the same conditions. All complexes contain quinaldinolate ligands in a mixed chelating–bridging coordination mode. The homoleptic complex adopts a V-shaped geometry whereas the heteroleptic complexes adopt closely related cyclic structures. Manuscript received: 18 April 2013. Manuscript accepted: 2 May 2013. Published online: 12 June 2013. Introduction The coordination chemistry of 8-hydroxyquinoline (HOQ) and quinaldine (2-methyl-8-hydroxyquinoline, HMQ) largely stems from the ability of their deprotonated forms to act as strong chelating agents, typically to yield quite insoluble compounds, hence their early use as agents in gravimetric analysis. [1] For example, [Al(OQ) 3 ] is a common component of organic light- emitting diode (with ring substituents altering the luminescence of the compound). [2] There are also biological applications, with radiolabelled clioquinol (5-chloro-7-iodo-8-hydroxyquinoline) having been investigated as a biomarker for b-amyloid with the ligand targeting zinc and copper ions that aid the formation of amyloid aggregates. [3] Metal quinolinolate complexes have also been investigated as active ingredients in antifungal treatments. [4] The relatively poor solubility of OQ and MQ complexes leads to great difficulty in being able to structurally characterise complexes containing these ligands despite their growing rele- vance in medicinal applications. We recently turned our atten- tion to monometallic zinc 8-quinaldinolate complexes, for which a homoleptic species has never been isolated and struc- turally characterised despite the quinolinolate complex [Zn 4 (OQ) 8 ] having been isolated by sublimation and structurally characterised in the mid-1980s. [5] Heteroleptic zinc quinaldino- late complexes have been obtained by several different routes. The mononuclear complex [Zn(MQ) 2 (H 2 O)] has been prepared by solution methods using methanol and aqueous ammonia. [6] The clioquinolate analogue has also been reported to have been prepared from H 2 O/THF. [7] These two observations suggest that the biologically important forms of zinc 8-quinolinolate complexes are of the form ML 2 (H 2 O), although the quinolinolate anion has been reported to give the bis-aqua complex [Zn(OQ) 2 (H 2 O) 2 ]. [8] The more soluble, and sterically demanding, 2-methyl-7-nonyl derivative forms an unsolvated, tetrahedral ML 2 complex with zinc whereas its 2-nonyl-7- methyl isomer forms an M 2 L 4 species to extract metal ions into organic solvents. [9] A tetrahedral ML 2 complex is also observed for a more heavily functionalised ligand that contains chelating quinolinolate groups. [10] Several complexes with anionic co-ligands have also been reported. The charge-separated compound (H 2 MQ)[Zn(MQ)Cl 2 ], with only one MQ ligand chelating to the distorted tetrahedral metal centre, has been prepared from both methanol and from acetonitrile in convection tubes at 608C. [11] The dimeric com- plex [Zn(MQ)(OAc)(MeOH)] 2 was prepared using the same procedure with a different zinc reagent. [12] With the limited solubility of OQ and MQ complexes, we have utilised, to great effect, pseudo-solid-state synthetic methods (using reactions that are mediated by an inert flux) to synthesise alkali metal, rare earth and transition metal complexes, in addition to heterometallic complexes. [13] Herein, we apply this synthetic methodology to synthesise a homoleptic zinc quinal- dinolate complex, [Zn 3 (MQ) 6 ](1). We also report the synthesis and structures of two mixed MQ/Cl complexes, [Zn 4 Cl 4 (MQ) 4 ] (2) and [Zn 4 Cl 2 (MQ) 6 ](3)(Scheme 1). Experimental General Details Zn(MQ) 2 was prepared by mixing aqueous solutions of zinc chloride with a stoichiometric amount of Na(MQ), with the precipitated product recovered by filtration. Infrared spectra CSIRO PUBLISHING Aust. J. Chem. http://dx.doi.org/10.1071/CH13185 Journal compilation Ó CSIRO 2013 www.publish.csiro.au/journals/ajc Full Paper RESEARCH FRONT