Photoinduced Reductive C-C and C-Heteroatom Couplings from Bis-cyclometalated Pt(IV) Alkynyl Complexes

Unsymmetrical dicarboxylato complexes [Pt-(tpy)(2)(O2CR)(2)] [tpy = cyclometalated 2-(p-tolyl)pyridine, R = Me, CF3] react with the terminal alkynes 4-methoxyphenylacetylene, phenylacetylene, 4-(trifluoromethyl)phenylacetylene or 3,5-difluoro-phenylacetylene in the presence of a base to produce complexes mer-[Pt(tpy)(2)(O2CR)(CCAr)], in which the metalated carbon atoms are in a meridional arrangement. Irradiation of the trifluoroacetato derivatives with a 365 nm LED source leads to isomerization to the facial complexes, which can be converted to chlorido derivatives upon reaction with NH4Cl. In contrast, irradiation of the acetato derivatives leads to four different processes, namely, reduction to cis-[Pt(tpy)(2)], annulations involving one of the tpy ligands and the C-alpha and C-beta atoms of the alkynyl to give benzoquinolizinium derivatives, isomerization to the facial geometry, or C-O couplings between the acetato ligand and one tpy. The first two processes are favored by the presence of electron-donating groups on the alkynyl, whereas electron-withdrawing groups favor the last two. Irradiation of complexes fac-[Pt(tpy)(2)(O2CCF3)(CCAr)] with a medium-pressure Hg UV lamp leads to a reductive C-C coupling involving the alkynyl C-alpha atom and one of the tpy ligands to give pyridoisoindolium derivatives, except for the methoxyphenylacetylide derivative, which is photostable. On the basis of TDDFT calculations, the photoreactivity of the mer complexes is attributed to (LLCT)-L-3 [pi(alkynyl) -> pi*(tpy)] excited states for annulations or (LMCT)-L-3 [pi(alkynyl) -> d sigma*] excited states for the rest of the processes, which are accessible through thermal population from (LC)-L-3(tpy) states. The C-C couplings from the fac complexes are attributed to photoreactive pentacoordinate intermediates.