Effects of counter anions on stereospecific interactions through π-conjugated systems of sulfur-bridged dinuclear cobalt(III)–platinum(II) complexes with optically active penicillaminates: Synthetic, structural, and spectroscopic studies

The optically active S-bridged dinuclear complex, [Pt(bpy){Co(d-pen)2}]X (1X: bpy = 2,2′-bipyridine; pen = penicillaminate; X=NO3-, ClO4-, Br−, or I−), and its enantiomer, [Pt(bpy){Co(l-pen)2}]X (2X), were prepared by the reactions of [PtCl2(bpy)] with K[Co(d-pen)2] and K[Co(l-pen)2], respectively. The racemates, [Pt(bpy){Co(d-pen)2}]0.5[Pt(bpy){Co(l-pen)2}]0.5X (3X), were also obtained by the slow evaporation of aqueous solutions containing equimolar amounts of 1X and 2X. The crystal structures of 3NO3 · 3H2O, 3ClO4 · 2H2O, 3Br · 2H2O and 3I · 5.5H2O were determined by X-ray crystallography, and compared with those of 1ClO4 · H2O, 1Br · 2H2O, 1I · 2H2O, 2ClO4 · H2O, 2Br · 2H2O and 2I · 2H2O. The optically active complexes 1X and 2X exist as monomers, accompanied by no intermolecular interactions in the π-electronic systems of the bpy moieties. In the crystals of 3Br · 2H2O, on the other hand, the enantiomeric complex cations, [Pt(bpy){Co(d-pen)2}]+ (D) and [Pt(bpy){Co(l-pen)2}]+ (L), are arranged alternately with overlapping bpy planes along the c axis, and the π electronic system of the bpy framework in D interacts with those in L. The remaining three racemates, 3NO3 · 3H2O, 3ClO4 · 2H2O and 3I · 5.5H2O, afford dimeric structures with π–π stacking contacts between the enantiomeric D and L complex cations. Differences in the crystal structures between the optically active complexes and the racemates significantly mirror the diffuse reflectance spectral behaviors.

The racemic crystals containing two enantiomers, [Pt(bpy)Co(d-pen)2]+ (D) and [Pt(bpy)Co(l-pen)2]+ (L), can be obtained when their aqueous solutions are mixed in equimolar amounts. Depending on the nature of the counter anion in these crystals, dimers or linear-chains are established by the stereospecific interactions of the electronic systems between a pair of enantiomers, D and L.Figure optionsDownload as PowerPoint slide