Crystal Structures of and Polarized Absorption-Spectroscopy in Racemic and Resolved [Ru(bpy)₃] (ClO₄)₂ and [Zn(bpy)₃] (ClO₄)₂Bpy=2,2'-Bipyridine

Abstract

[Zn( bpy )₃] (ClO₄)₂ and [ Ru ( bpy )₃] (ClO₄)₂ are isomorphous in both their racemic and resolved crystal forms. The resolved materials are monohydrates and have a C 2, Z = 8, structure with two independent formula units on general sites in the asymmetric unit. The cations have the same chirality. The inherent threefold axis of each cation lies approximately parallel to the c axis. The unrelated racemic form has a C2/c, Z = 4, structure which is a commensurate modulation of a P3c1, Z = 2, parent structure, typified by the room-temperature structure of [ Ru ( bpy )₃] (PF₆)₂. A primary, secondary and tertiary axis of P3c1 become the c, b and a axes respectively of C2/c, retaining a third of the symmetry elements of P3c1. The crystals grow as multiply contacted twins. This structure bas just one spectroscopic site with the cation lying on a twofold axis that passes through the metal and one of the bidendate ligands and relates the other two ligands to each other. This feature is particularly useful in the study of the optical spectroscopy of the metal-to- ligand charge transfer excitations of [ Ru ( bpy )₃]²⁺ and related systems. A comparison of structural and spectral data indicates that the positions of the anions have a dominant influence on the relative energies of the metal-to- ligand excitations. An energy difference between excitations involving the two (lower-energy) equivalent ligands and the third ligand of the order of 800 cm-¹ is indicated in both singlet and triplet regions for the racemic perchlorate. The absorption spectra of [ Ru ( bpy )₃]²⁺and [Os( bpy )₃]²⁺ in a number of crystalline hosts are compared and discussed.