Configurational E/Z and photo-induced cis-trans isomerism in the Pd(II) complex of asymmetrical N,N-methyl-ethyl-N′-benzoylthiourea

Abstract

The palladium complex prepared from an asymmetrically di-substituted N,N-methyl-ethyl-N′-benzoylthiourea ligand exists in chloroform as a pure mixture of the configurational isomers cis-ZZ-[Pd(L-κS,O)2], cis-EE-[Pd(L-κS,O)2] and cis-ZE-[Pd(L-κS,O)2]. These have been have been characterized by 1H, 13C, HMBC and 1D NOE difference NMR spectroscopy in organic solvents. The single-crystal X-ray diffraction structures of crystals of the cis-EE-[Pd(L-κS,O)2] isomer isolated from either acetonitrile or dichloromethane solutions in the dark, confirmed its molecular structure. In acetonitrile, the analytically pure mixture of the cis-ZZ/ZE/EE-[Pd(L-κS,O)2] complexes undergo reversible photo-induced cis → trans isomerization when irradiated with polychromatic light to form all the configurational isomers trans-ZZ/ZE/EE-[Pd(L-κS,O)2]. In the absence of light, the latter trans-complexes slowly revert back to trans-ZZ/ZE/EE-[Pd(L-κS,O)2] in the dark in both acetonitrile and chloroform-d solutions, as monitored by RP-HPLC and 1H NMR spectroscopy as a function of time at 25 °C. Estimates of the relative rate of the trans → cis reversion reaction in the dark, is 2.8 × 10−2 min−1 in acetonitrile, compared to that in chloroform-d of ca 0.38 × 10−2 min−1.