Linear ditopic acetylide gold or mercury complexes: synthesis and photophysic studies: X-ray crystal structure of PPh4[Au(CCC5H4N)2]

The AlkH alkyne compounds 4-ethynylpyridine (1), 4-ethynylbenzonitrile (2) and (4-ethynylphenyl)(4-pyridyl)acetylene (3) react with [PPh4][Au(acac)2] to yield three new linear ditopic bis-alkyne gold(I) compounds [PPh4][Au(Alk)2] (for AlkH=1, 1a; for AlkH=2, 2a and for AlkH=3, 3a). The reaction of 1a with an excess of methyl iodide results in the methylation of both N-pyridine atoms. Analogous mercury(II) [Hg(Alk)2] compounds are obtained by the reaction of the lithium derivatives of 1, 3 and (4-bromophenyl)(4-pyridyl)acetylene (4) with HgCl2. All these compounds have been characterised by IR and 1H-NMR spectroscopies and mass spectrometry. The X-ray crystal structure of 1a shows that the gold atom is surrounded by two alkyne ligands in a linear fashion. The reported compounds have been tested as building blocks for the synthesis of molecular polygons by self-assembly. Emission of the compounds was studied and the measured quantum yields were in the range 0.01<Φ<0.04. Room-temperature emission of compounds 1aH and 1a′ exhibits a large Stokes shift, and is assigned to the lower triplet state. Decay times were measured at room temperature in CH2Cl2 for 1aH (1.5 μs) and in ethanol at 77 K (149 and 500 μs for compounds 1aH and 1a′, respectively). Room-temperature emission of compounds 3a and 3aH arises from the lower singlet state and phosphorescence is only observed at 77 K. Theoretical calculations on the nature and energy of singlet and triplet states corroborate the assignments.