Energy-Transfer Rate in Crystals of Double-Complex Salts Composed of [Ru(N-N)₃]²⁺ (N-N = 2,2‘-Bipyridine or 1,10-Phenanthroline) and [Cr(CN)₆]^3-: Effect of Relative Orientation between Donor and Acceptor

A block single-crystal was obtained using a diffusion method with a concentrated acetone−water (vol. 1/1) solution of [Ru(phen)3]Cl2·6H2O (phen = 1,10-phenanthroline) and a concentrated aqueous solution of K3[Cr(CN)6], without evaporating solvents. The crystal was identified as a double-complex salt including two acetone and fourteen solvent water molecules, [Ru(phen)3]2[Cr(CN)6]Cl·2(CH3)2CO·14H2O (1). Measurement of the X-ray diffraction pattern of the double-complex salt was performed using an X-ray diffractometer with an Imaging-Plate (IP) Weissenberg camera. 1 crystallizes in the triclinic space group P1̄, with a = 13.930(5) Å, b = 14.783(5) Å, c = 11.137(6) Å, α = 89.87(4)°, β = 107.47(3)°, γ = 96.68(3)°, and Z = 2. The crystal structure is very different from that of [Ru(bpy)3]2[Cr(CN)6]Cl·8H2O (2) (bpy = 2,2‘-bipyridine), which could be obtained using the same procedure and crystallizes in the monoclinic space group C2, with a = 22.414(2) Å, b = 13.7686(15) Å, c = 22.207(2) Å, β = 90.713(8)°, and Z = 4. The distance between the central-metal ions of ruthenium(II) and chromium(III) complexes in [Ru(phen)3]2[Cr(CN)6]Cl·2(CH3)2CO·14H2O (7.170 Å) is shorter than that in [Ru(bpy)3]2[Cr(CN)6]Cl·8H2O (9.173 Å) by about 2 Å, while the rate of energy transfer from the 3MLCT state of [Ru(N-N)3]2+ to the 2Eg state of [Cr(CN)6]3- in the former salt (9.5 × 105 s-1) is far slower than that in the latter one (6.0 × 106 s-1) at 77 K. These results indicate that the energy-transfer rate strongly depends, not upon the distance between central metal ions, rather, upon the mutual relative orientation between the donor and the acceptor complexes in double-complex salts.