Synthesis of Pd Complexes Combined with Photosensitizing of a Ruthenium(II) Polypyridyl Moiety through a Series of Substituted Bipyrimidine Bridges. Substituent Effect of the Bridging Ligand on the Photocatalytic Dimerization of α-Methylstyrene

Mononuclear ruthenium complexes and dinuclear Ru···Pd complexes having a series of 2,2‘-bipyrimidine ligands,
[(bpy)2Ru(Ln)]2+ [Ln = 2,2‘-bipyrimidine (L1), 5,5‘-dimethyl-2,2‘-bipyrimidine (L2), 5,5‘-dibromo-2,2‘-bipyrimidine (L3),
4,4‘-dimethyl-2,2‘-bipyrimidine (L4), and 4,4‘,6,6‘-tetramethyl- 2,2‘-bipyrimidine (L5)] and [(bpy)2Ru(Ln)PdL]m+ [Ln
= L1−L3; PdL = PdMeCl (m = 2) and PdMe(solvent) (m = 3)], are prepared, and the obtained complexes are
characterized by means of spectroscopic and crystallographic methods. Introduction of the substituents on the
bipyrimidine ligands led to the substantial differences in their electrochemical and photophysical properties. Density
functional theory calculations have been performed to understand the substituent effect on the ground-state molecular
orbital energy level. Reactivity studies on the catalytic dimerization of α-methylstyrene revealed that the Pd complex
having a Br-substituted bipyrimidine ligand were much more active than those of the corresponding Pd complexes
having methyl-substituted or nonsubstituted bipyrimidine ligands.