Coordination chemistry of tripyridinedimethane

The ligand tripyridinedimethane (tpdm), consisting of three pyridine residues linked at their ortho carbons by two CH2 groups, is shown to be a sterically flexible ligand capable of binding in a meridional arrangement in trigonal bipyramidal (tpdm) (CUCl2)-Cl-II but binding in a facial arrangement in tetrahedral (tpdm) (CuCl)-Cl-I. Nucleophilic substitution of chloride by (BuO-)-Bu-t and PhCdropC(-) is possible, and deprotonation of the acidic benzylic protons does not take place because the resulting carbanion cannot achieve coplanarity with the aryl rings. RhCl3 forms, with tpdm in boiling methanol, a 1:1 kinetic mixture of fac(-) and mer-isomers RhCl3(tpdm). The former isomerizes slowly at RT (room temperature) in DMSO solution into the latter with Rh-N bond dissociation as the rate-determining step.