Planar-Chiral Metal Complexes Comprised of Square-Planar Metal and Achiral Tetradentate Ligands: Design, Optical Resolution, and Thermodynamics

Planar-chiral palladium complexes {[[N,N′-[1,4-butanediylbis­(oxy-7,1-naphthalenediyl)]­bis­(2-pyridinecarboxamidato)]­(2−)-κN1,κN1′,κN2,κN2′]­palladium
(PdL4) and [[2,2′-[1,4-butanediylbis­[[(oxy-7,1-naphthalenediyl)­imino]­methyl]]­dipyrrolato](2−)-κN1,κN1′,κN2,κN2′]­palladium
(PdL5)} were synthesized from achiral tetradentate ligands N,N′-[1,4-butanediylbis­(oxy-7,1-naphthalenediyl)]­bis­(2-pyridinecarboxamide)
(H2L4) and N,N′-bis­[(1H-pyrrol-2-yl)­methylidene]-7,7′-(1,4-butanediyldioxy)­bis­(1-naphthalenamine)
(H2L5) bearing two dissymmetric bidentate units at both ends and
a PdII ion, respectively. The palladium complexes were
crystallized in the monoclinic space group P21/n with the unit cell parameters a = 16.5464(6) Å, b = 11.3534(4) Å, c = 17.6697(7) Å, β = 115.5300(10)°, and Z = 4 for PdL4 and a = 17.2271(8) Å, b = 10.1016(5) Å, c = 17.9361(9) Å, β = 105.6310(10)°, and Z = 4 for PdL5. The
planar-chiral structures of PdL4 and PdL5 were confirmed by
single-crystal X-ray analyses, resulting in the fact that the crystals
were racemic mixtures. The racemic mixtures were successfully resolved
by using chiral high-performance liquid-chromatography techniques.
Racemizations of the complexes were found to be drastically dependent
on the arrangement of the charged or uncharged metal-binding N atoms
of the ligands.