Copper(II) and palladium(II) complexes of a terdentate pyrrolidine diester ligand

The synthesis and characterisation of three copper(II) complexes, [CuLCl] (4), [CuLBr] (5) and [CuL(NCS)] (6), and a palladium(II) complex, [PdLCl] (7), of the monoanionic terdentate ligand L− [L− is the deprotonated form of diethyl 2,2′-(pyrrolidine-2,5-diylidene)diacetate (HL) (3)] are reported. The X-ray crystal structure determinations show that both 4 and 6 feature square planar copper(II) centres. The cis bond angles are close to 90° (85–93°) thanks in large part to the introduction of an additional carbon atom between the pyrrole ring and each of the ester moieties. These results indicate that the failure of the 2,5-diiminopyrrole head unit to coordinate in a terdentate manner to first row transition metal ions can, in principle, be resolved by introducing an extra carbon atom between the pyrrole ring and each of the imine moieties. A comparison of structural parameters between the two structurally characterised complexes (4 and 6) and the non-deprotonated free ligand HL (3) indicates that there is an increase in delocalisation of the electron density throughout the π system of the deprotonated ligand on coordination, as the CC and CO double bonds are longer and C–C and C–O single bonds are shorter than the corresponding bond distances in 3.

A new “expanded” head unit, featuring an “extra” carbon atom between the pyrrolidine ring nitrogen and each of the two flanking donor atoms, is shown to bind in a terdentate manner (shown) in contrast to the bidentate binding mode adopted by 2,5-diformylpyrrole-based ligands.Figure optionsDownload as PowerPoint slide