Synthesis of Substituted 2-Pyridyl-4-phenylquinolines

The acid-catalyzed condensation of o-aminobenzophenones with aromatic acetyl derivatives, in a basic methanol/tetrahydrofuran medium, has been used to prepare a series of substituted 2-pyridyl-4-phenylquinolines. Derivatives having two aza binding sites can act as asymmetric bidendate ligands to complex transition metals such as ruthenium, osmium or iridium. All the compounds were characterized by elemental analysis, Ei or FAB (+) MS, 1H- and 13C-NMR spectroscopies. Complete assignments of the 1H spectra were accomplished by using a combination of one- and two-dimensional NMR techniques.


Introduction
In the past few decades luminescent transition metal complexes based on polypyridine ligands, owing to their long-lived metal-to-ligand charge-transfer (MLCT) excited states, have already been used in various fields such as solar energy conversion [1], information storage [2], photocleavage of DNA [3], and oxygen sensors [4]. Although the photophysics and photochemistry of [Ru(bpy) 3 ] 2+ (bpy = 2,2' bipiridine) have been the subject of extensive research [1][2][3][4], few other bidentate ligands, i.e. having two aza binding sites, have been prepared and the photophysical and/or photochemical properties of their complexes with transition metals studied [5]. As a continuation of previous studies in this field [6], we now report the synthesis and characterization of the ligands shown in Scheme 1, with the aim of studing the photochemical properties of their complexes with transition metals such as ruthenium, osmium or iridium. Three of these asymmetric bidendate ligands (L 2 -L 4 ) are new. All the compounds were characterized by elemental analysis, EI or FAB mass, 1 H and 13 C NMR spectroscopies. Complete assignments of the 1 H spectra of the various compounds were accomplished by using a combination of one-and two-dimensional NMR techniques.
The resonances for H 5 and H 6 could be assigned to the signals at 7.96 and 7.57-7.50 ppm, respectively. The 1 H double triplet at 7.90 ppm, diagnostic for a γ-pyridine [19], and involved in another four spin system connecting the 1 H signals at 8.74, 8.71, 7.90, and 7.37 ppm, was assigned to the pyridine proton H 4' . As a consequence of the meta and ortho couplings showed by H 4' , the doublets at 8.74, 8.71, and the broad triplet at 7.37 ppm, that in turn are correlated themselves, were easily assigned at H 6' , H 3' , and H 5' , respectively. It is worth noting that ortho, meta, and para crosspeaks are observable in the COSY-45 spectrum and can be distinguished from the number and/or the intensity of the spots. The highest downfield shift experienced by the H 3' protons, due to deshielding by the nonbonding electrons of the nitrogen on the pyridine ring, is indicative of an anti conformation for the ligands, in agreement with the conformation considered the most probable for bipyridine [5]. According to literature data [19], confirmed by our 1 H-NMR analyses, these uncomplexed molecules show an anti conformation (as depicted in Schemes 1 -3) that changes to a syn one when they act as ligands by using the nitrogen of the pyridine and quinoline rings as binding sites. According to the inductive and/or mesomeric effects of the substituents, their introduction onto the skeleton of the N-N bidendate ligand L 1 influence the upfield and/or downfield chemical shift of the nearest protons, and the reactivity of these molecules as well. The structures of ligands L 1 -L 4 was further confirmed by their 13 C-NMR spectra (see Table II), which displayed the expected patterns.

Conclusions
We report the synthesis of a series of bidentate aza chelating molecules, based on a substituted 2-pyridyl-4-phenylquinoline skeleton, that may be useful for the complexation of metal cations such as Ru, Os, and Ir. These complexes, owing to their asymmetry, may display new and interesting photophysical properties.

Acknowledgements
The research was financially supported by the University of Catania (PRA funds).

General
The starting materials 2-acetylpyridine, 2-aminobenzophenone, 2-amino-4-methylbenzophenone, pnitrobromobenzene, and phenylacetonitrile were purchased from Aldrich. All other chemicals were reagent grade. 6-Methyl-2-acetylpyridine [20] and the ligand 4-phenyl-2-(2'-pyridyl)pyridine (L 1 ) [16], were prepared as described in the literature. All reactions were performed under an inert atmosphere of nitrogen except when otherwise stated and the solvents were dried and stored under nitrogen and over 4Å molecular sieves. Melting points are uncorrected. Elemental analyses were determined by a commercial laboratory. 1 H-and 13 C-NMR spectra were performed in deuterated chloroform (CDCl 3 ) with a Varian INOVA 500 instrument. Chemical shifts were calibrated relative to the solvent resonance considered at 7.26 ppm for residual CHCl 3 and at 77.0 ppm for CDCl 3 . The analysis of the proton spectra was carried out according to the rules for the first-order splitting with the help of integral intensities. The 13 C-NMR spectra were measured with full decoupling from the protons, and the signals were assigned with the help of SCS. The quaternary carbon atoms and CH groups were differentiated by means of the APT pulse sequence. Positive ion FAB mass spectra were obtained on a Kratos MS 50 S double-focusing mass spectrometer equipped with a standard FAB source, using 3-nitrobenzyl alcohol as a matrix. The yields, melting points and elemental analyses of the ligands synthetized are presented in Table III. The 1 H-and 13 C-NMR spectra with signal assignments are given in Tables I and II, respectively. 3-phenyl-5-bromo-2,1-benzisoxazole (1): Phenylacetonitrile (1.75 g, 15 mmol) was slowly added to a vigorously stirred solution of potassium hydroxide (17.76 g, 310 mmol) in methanol (35 mL) at room temperature. After dissolution was complete, 36 mL of a methanol/tetrahydrofuran (2 : 1 v/v) solution containing p-nitrobromobenzene (3.0 g, 15 mmol) was added dropwise at 0 °C. The resulting dark mixture was stirred at 0 °C for 3 hours, at room temperature for 4 hours, refluxed overnight, and then poured into ice-water (300 mL), filtered, washed successively with cold water and methanol and recrystallized from methanol to afford compound 1 as yellow crystals; 2. 2 Amino-5-bromo-benzophenone (2): Following the procedure of Simpson and Stephenson [21], a solution, containing 0.44 g (1.6 mmol) of 1 in acetic acid (70 mL), was heated on a water-bath, and 1.0 g (18 mmol) of iron powder was added over 2.5 hours, during which time, 12 ml of water was also added. The mixture was filtered while hot and then 100 ml of water was added. The yellow precipitate was collected by filtration, washed with cold water until the water washings were clear and dried. The product was purified by column chromatography (silica; cyclohexane / ethyl acetate 9:1) followed by recrystallization from ethanol-water to afford 2 as a yellow powder; 0.31 g (70 %); m.p. The synthesis of L 4 is given below as a general procedure for the synthesis of ligands.