The absorption spectra, luminescence spectra, and luminescence lifetimes of the isomeric [M(bph)(bpy)] and [M(phpy)2] complexes M = Pt(II) or Pd(II), bph2− = 1,1′‐biphenyl‐2,2′‐diyl dianion, phpy− = 2‐phenylpyridine‐2′‐yl anion, and bpy = 2,2′‐bipyridine have been investigated and compared with those of [M(bpy)2]2+ complexes and of the free protonated ligands H2bph, Hbpy+, and Hphpy. In the absorption spectra, the region below 320 mm is dominated by ligand‐centered (LC) transitions, whereas metal‐to‐ligand charge transfer (MLCT) transitions are responsible for the bands present in the near UV/VIS region. The MLCT bands move to higher energies on replacing Pt with Pd and in going from [M(bph)(bpy)] to the [M(phpy)2] isomer. For the mixed‐ligand complexes, evidence for both M → bph2− (at higher energies) and M → bpy bands is found. The structured luminescence observed at 77 K shows lifetimes of 4.0 and 1.1 μs for [Pt(phpy)2] and [Pt(bph)(bpy)], respectively, and 480 and 250 μs for the analogous Pd complexes. On the basis of the energy and lifetime data, the luminescence of the Pt(II) complexes is assigned to the lowest triplet MLCT excited state, whereas for the Pd complexes the luminescent state is thought to result from a mixture of MLCT and LC triplet levels.
The X-ray structure of cis-bis(2,6-diphenylpyridinato-N,O')platinum(11) shows the first example of helical chirality for a 'square planar' co-ordination compound with nonchiral ligands; 1H NMR investigations indicate no racemization of the compound in solution on the NMR time scale.
A synthetic route leading to bis‐heteroleptic cyclometalated complexes
is described. The complexes [2‐(2′‐thienyl)pyridinato‐N, C‐3′]{2‐[3′‐(trimethylsilyl)2′‐thienyl]pyridinato‐N, C3′}platinum(II) ([Pt(thpy) (TMS‐thpy)]; I) and (l‐phenylpyrazolato‐N2, C2′)[2‐(2′‐thienyl)pyridinato‐N, C3′]platinum ([Pt(Phpz)(thpy)]; II) are characterized by UV/VIS, NMR, and mass spectroscopy. Thermal and photochemical oxidative addition reactions yield two out of the 10 possible pairs of enantiomers of octahedral Pt(IV) compounds.
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