Steric Ligand Effects of Six Bidentate Bipyridyl Ligands

The synthesis and characterization of a series of [Ru(H₂O)(N-N)(trpy)]²⁺ complexes (where N-N = 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), 4,7-dimethyl-1,10-phenanthroline (4,7-Me₂phen), 2,9-dimethyl-1,10-phenanthroline (2,9-Me₂phen), 2,2′-biquinoline (biq), and 6,6′-dichloro-2,2′-bipyridine (6,6′-Cl₂bpy), and trpy = 2,2′: 6′,2″-terpyridine) are described. The rate constants for ligand substitution of the aqua ligand for acetonitrile were determined for the [Ru(H₂O) (N-N) (trpy)]²⁺ complexes as well as for a series of [Ru(H₂O)(bpy)₂(PR₃)]²⁺ complexes in pH = 2.2 HNO₃/NaNO₃ (μ = 0.1) solution at 25 °C. The electronic (E) and steric (S) properties of the phopshine ligands in the [Ru(H2O)(bpy)₂(PR₃)]²⁺ complexes correlated well with the rate constants for ligand substitution (k_ls) using the relationship In k_ls = aE + bS + c (where a–c are constants which were determined by linear regression analyses). The equation for the [Ru(H₂O)(bpy)₂(PR₃)]²⁺ complexes was used to calculate the cone angles of the six bidentate bipyridyl ligands (N-N). These cone angles represent the first quantitative estimates of steric ligand effects for bidentate bipyridyl ligands.