New N^C^N ligands obtained through Pd-catalyzed direct arylation of difluorinated substrates

Introduction Phosphorescent chromophores are in continual demand for applications ranging from chemosensing to bioimaging, light emitting diodes and artificial photosynthesis. Squareplanar platinum(II) complexes are particularly useful in such contexts because of the great emission efficiencies that they may give thanks to the intersystem crossing caused by the heavy metal, which allows emission from the otherwise lost triplet states that are up to three quarters of the excited states obtained upon charge-recombination in electroluminescent devices. Among these, platinum(II) chloride complexes bearing a cyclometalated 1,3-di(2-pyridyl)benzene ligand, which allows rigid N^C^N coordination surroundings, are really fascinating for their luminescence properties, being among the brightest emitters based on platinum, and they have found successful applications in OLEDs1 and bio-imaging2. The use of catalysis is of paramount importance to prepare in a convenient manner difluorinated ligands functionalized at the para position of the central phenyl ring: the presence of two fluorine atoms on the central phenyl ring makes the phenyl functionalization particularly attractive because of the higher acidity and reactivity of the hydrogen atom between them rather than that of a common hydrogen atom on an aromatic ring3. Direct arylation of difluorinated aryls has already been described in literature4,5, and tested on difluorinated phenylpiridine ligands for Ir(III) complexes6. Despite the similarity with other cross-coupling reactions, direct arylation of difluorinated substrates does not involve the need of reactants such as boronic acids or stannanes, thus representing an interesting alternative to them because of the reduction in the number of reaction steps and the avoided generation of wastes containing boron or tin. Materials and Methods All reactions were carried out in a 10 mL Schlenk tube, in DMF, under argon atmosphere at 150°C. All reactants All reactants were bought from Fluorochem and all solvents from Fisher, Sigma-Aldrich, VWR, and Carlo Erba, without any further purification. All products were purified by flash chromatography using Macherey-Nagel silica gel (pore size 60 Å, mesh size 0.040-0.063 mm), and characterized by NMR using a Bruker Ascend 400 instrument and GC-MS using a Shimadzu GCMS-QP2010 SE instrument. Results and Discussion Two classes of phenyl-functionalized N^C^N ligands were successfully synthesized through Pd-catalyzed direct arylation of difluorinated precursors: the first, without aryl groups on the pyridine rings, the second, with different aryl groups on the pyridines. The reactions were successful with non-hindered (para- and meta-substituted) and electron-poor aryl bromides (bearing strong electron-attracting groups like -CF3, -CN, and -COOEt), and with reasonable yields, ranging from 20 to 70 %. However, they failed when electron-rich bromides (with electron-donating groups as -tBu) or hindered (ortho-substituted) bromides were employed. Another important aspect of these reaction is the high selectivity towards the position between the two fluorine atoms, that lead to the desired product only, without the generation of isomers. The resulting ligands were coordinated to a Pt center to synthesize the corresponding complexes, by reacting them with K2PtCl4. Significance This work is of great scientific interest because it represents a new convenient way to obtain N^C^N ligands, hence, the corresponding Pt(II) complexes. The main aspect is the new synthetic way, involving a Pd-catalyzed direct arylation step. Last, but not least, the synthesized complexes may be employed in the fields of OLEDs and bioimaging.