Synthesis and Spectroscopic and Electronic Characterization of New Cis-Configured Di- to Multiplatinum Alkynyls

Utilization of Sonogashira coupling reactions with cis-[Pt(dppe)Cl2] and varying amounts of the alkynyl
ligands HC⋮CRC⋮CH (where R = p-C6H4, p-C6H4-p-C6H4) afford the monoplatinum complexes cis-[Pt(dppe)(C⋮CRC⋮CH)2] (R = p-C6H4 (1a), p-C6H4-p-C6H4 (1b)), the diplatinum complexes cis-[(HC⋮CRC⋮C)Pt(dppe)(C⋮CRC⋮C)Pt(dppe)(C⋮CRC⋮CH)] (R = p-C6H4 (2a), p-C6H4-p-C6H4 (2b)),
the triplatinum complexes cis-[(HC⋮CRC⋮C)Pt(dppe)(C⋮CRC⋮C)Pt(dppe)(C⋮CRC⋮C)Pt(dppe)(C⋮CRC⋮CH)] (R = p-C6H4 (3a), p-C6H4-p-C6H4 (3b)), and the respective oligomeric species 4a,b, all
in reasonable yields. X-ray diffraction studies of 2a reveal the presence of two independent diplatinum
molecules in the asymmetric unit, each with Ci symmetry. All the complexes have been fully characterized
by NMR and mass spectrometry, and GPC analysis of 4a,b indicate their oligomeric nature, with a degree
of polymerization of ca. 4. Electronic spectral studies indicate that the optical band gap decreases with
increasing chain length but that the intensity of the triplet state emission increases with the number of
repeating units. As expected, there is less effective π-conjugation along the chain in the cis configuration
as compared to the analogous trans-oriented species.