Light- induced cis/trans isomerization of cis-[ Pd( L- S, O) 2] and cis-[ Pt( L- S, O) 2] complexes of chelating N, N- dialkyl- N'- acylthioureas: key to the formation and isolation of trans isomers

Irradiation cis-[M(L-n-S,O)(2)] complexes (M=Pt-II, Pd-II) derived from N,N-dialkyl-N-benzoylthioureas (HLn) with various sources of intense visible polychromatic or monochromatic light with < 500nm leads to light-induced cistrans isomerization in organic solvents. In all cases, white light derived from several sources or monochromatic blue-violet laser 405nm light, efficiently results in substantial amounts of the trans isomer appearing in solution, as shown by H-1 NMR and/or reversed-phase HPLC separation in dilute solutions at room temperature. The extent and relative rates of cis/trans isomerization induced by in situ laser light ( = 405nm) of cis-[Pd(L-2-S,O)(2)] was directly monitored by H-1 NMR and Pt-195 NMR spectroscopy of selected cis-[Pt(L-S,O)(2)] compounds in chloroform-d; both with and without light irradiation allows the (Pt-195) chemical shifts cis/trans isomer pairs to be recorded. The cis/trans isomers appear to be in a photo-thermal equilibrium between the thermodynamically favored cis isomer and its trans counterpart. In the dark, the trans isomer reverts back to the cis complex in what is probably a thermal process. The light-induced cis/trans process is the key to preparing and isolating the rare trans complexes which cannot be prepared by conventional synthesis as confirmed by the first example of trans-[Pd(L-S,O)(2)] characterized by single-crystal X-ray diffraction, deliberately prepared after photo-induced isomerization in acetonitrile solution.