Structural and conformational changes in [M(1,10-diaza-18-crown-6)Cl2] (M = Pd, Pb) complexes: a crystallographic and theoretical study

The 1 : 1 adduct formation between crown ether (1,10-diaza-18-crown-6) and PdCl2 was observed as an inclusion compound and the molecular structure of complex PdLCl2 (L = 1,10-diaza-18-crown-6) was analyzed using a single crystal X-ray diffraction technique. Two different isomers were isolated upon changing the crystallization solvent system, i.e. CH2Cl2 afforded isomer PdLCl2(a) and a 1 : 1 CH2Cl2 : MeOH solvent system produced isomer PdLCl2(b); both structures were compared with a previously reported isomer PdLCl2(c) that exhibited a different structural arrangement. The three crystalline structures showed trans/syn, trans/anti and cis/syn relationships in relation to the Cl–Pd–Cl angle and the hydrogen atoms at the HN–Pd–NH moieties, respectively. In addition, DFT computational studies revealed the trans/syn isomer to be the most stable conformation by 5.8 kcal mol−1 and allowed the proposal of a potential mechanism for the conversion of this isomer into the trans/anti and cis/syn isomers. Along the same line, the related lead complex PbLCl2(d) was recrystallized from CH2Cl2 and analyzed using a single crystal X-ray diffraction technique, which showed to be structurally different to another previously reported structure of this complex PbLCl2(e), due to the inert pair effect. The analysis was expanded by comparing both series of structures with other relevant analogous complexes of different transition metals.