Synthesis of Pd complexes directly linked to the light-absorbing [(bpy)3Ru]2+ unit and their photochemical reactions toward styrenes

Abstract

A series of bipyridyl–Pd and Rh complexes containing a [(bpy)₃Ru]²⁺ or [(bpy)₂Ru(phen)]²⁺ (bpy = 2,2′-bipyridyl, phen = 1,10-phenanthroline) moiety as visible-light absorbing unit was synthesized. The complexes were synthesized via a Suzuki–Miyaura coupling reaction between the Ru complexes having a 4-bromo-2,2′-bipyridyl ligand and a 2,2′-bipyridyl-4-boronic acid and a subsequent reaction with various mononuclear Pd and Rh precursors. There was a noticeable structural difference between the QP (2,2′:4′,4″:2″,2‴-quaterpyridyne) and PB (5-(2,2′-bipyridyl)-yl-1,10-phenanthroline) complexes, which involved the dihedral angles within the bridging ligand; the PB complexes possessed large dihedral angles but the QP complexes showed small values. This structural difference clearly indicated a strong π-conjugation through the QP ligand. The electrochemical and photophysical properties of the QP and PB complexes were compared with the parent mononuclear Ru complexes, such as [(bpy)₃Ru]²⁺, [(bpy)₂Ru(phen)]²⁺, and [(bpy)₂Ru(bpm)]²⁺ (bpm = 2,2′-bipyrimidine). The QP and PB complexes showed a ³MLCT life time that was similar to [(bpy)₃Ru]²⁺ and [(bpy)₂Ru(phen)]²⁺, which was about 10 times longer (ca. 1 μs) than the corresponding bpm complexes. Reactivity studies with Pd complexes toward styrene dimerizations were examined. The reaction proceeded under visible-light irradiated conditions and the reactivity of the QP complexes was much higher than the corresponding PB complexes. Substantial acceleration of the reaction was observed with the introduction of an Me substituent on the bipyridyl ligand.