Diastereoselectively self-sorted low-symmetry binuclear metallomacrocycle and trinuclear metallocage

A pyridine/aniline appended unsymmetrical bis-monodentate ligand N-(3-aminophenyl)nicotinamide, Lun is synthesized via condensation of nicotinic acid with excess m-phenylene diamine. A low-symmetry binuclear complex of the Pd2L′2Lun2 type and an extremely rare trinuclear complex of the Pd3Lun6 type are produced by self-assembly of the ligand Lun with cis-protected palladium(ii) (i.e., PdL′) and palladium(ii), respectively. Two isomers (i.e. [(2,0), (2,0)] and [(1,1), (1,1)]-forms) are theoretically possible for the Pd2L′2Lun2-type complex whereas nine isomers can be envisaged in the case of the Pd3Lun6-type arrangement. However, one of the isomers of the Pd2L′2Lun2-type complex as well as the one for the Pd3Lun6-type complex are experimentally obtained. The exclusive formation of specific isomers could be predicted from the 1D/2D NMR study in the solution state and the DFT calculations in the gas phase/implicit solvent media. The formation of the predicted all-(1,1)-[Pd2(en)2Lun2](NO3)4 has been confirmed by a single-crystal XRD study. DFT calculations for the isomers of the Pd3Lun6-type arrangement show that a [cis(2,2), cis(2,2), cis(2,2)] isomer is energetically favourable than the alternatively predicted [trans(2,2), trans(2,2), trans(2,2)] isomer. Conformational changes within the build of the exclusively formed isomers are proposed on the basis of NMR study.