Intramolecular Hydrogen Bonding: A Key Factor Controlling the Photosubstitution of Ruthenium Complexes

Abstract

Photosubstitution reactions of ruthenium complexes with pyrazole ligands, cis-[Ru­(bpy)2(pzH)2]2+ (1a), cis-[Ru­(bpy)2(pz)­(pzH)]+ (1b), and cis-[Ru­(bpy)2(pz)2]0 (1c) (pzH = pyrazole, bpy = 2,2′-bipyridine), were investigated. Dicationic complex 1a was deprotonated to 1b using moderate base (pK a = 15.2, MeCN), while the second deprotonation to give 1c required more severe conditions (pK a = 26.9). Monocationic complex 1b possessed an N–H···N-type intramolecular hydrogen bond between the pyrazole and pyrazolate ligands, as corroborated by the solid-state crystal structure. The photosubstitution quantum yield of 1a (Φ = 0.26) was comparable to that of cis-[Ru­(bpy)2(pyridine)2]2+ (Φ = 0.24) in acetonitrile solution. In contrast, the photodissociation of a pzH ligand was strongly suppressed by the deprotonation of a pyrazole ligand N–H group. In the presence of 10 000 equiv of 4,4′-dimethylaminopyridine, the quantum yield dropped to ∼2 × 10–6 in acetonitrile. The photosubstitution quantum yield of 1b was even smaller than that of neutral complex 1c, although 1c had a smaller HOMO–LUMO energy gap than monocationic complex 1b. The small quantum yield of 1b was attributed to intramolecular hydrogen bonding between pyrazole and pyrazolate ligands. The apparent rate constants for the photosubstitution of 1b were highly solvent-dependent. The photosubstitution of 1b was suppressed in aprotic solvents, while the reaction was accelerated by 2 orders of magnitude in protic solvents with strong proton donor abilities.

A Synthetic Route to a Ruthenium Complex via Successive Photosubstitution Reactions

“…The decrease of the yield is attributed to the enhancement of the photodissociation of a pyrazole ligand in 1b . We earlier reported that the photosubstitution reaction of 1b was highly dependent on the solvent, being highly accelerated in a protic solvent with a strong proton donor ability, such as water or 2,2,2-trifluoroethanol . Additionally, the relatively larger yield of 1b in D 2 O rather than in H 2 O is consistent with the deuterium effect on the photosubstitution of 1b .…”

“…cis -Ru­(bpy) 2 Cl 2 , cis -[Ru­(bpy) 2 (MeCN) 2 ]­(PF 6 ) 2 , cis -[Ru­(bpy) 2 (pzH) 2 ]­(PF 6 ) 2 , and 1b were prepared as previously described. The extinction coefficients and absorption maxima agreed with the reported values .…”

“…For UV–vis measurements under light irradiation, one edge of an optical fiber (diameter 400 μm) was immersed to the top of the sample solution in the cuvette. The sample solution was irradiated with a fiber-coupled blue LED (M470F3, Thorlab, diameter = 470 nm, 14 mW cm –2 ) connected to an LED controller (DC2200, Thorlab), as described in our previous study …”

“…We have recently reported the effect of intramolecular hydrogen bonding in photosubstitution reactions using a ruthenium complex with two pyrazole ligands, cis -[Ru­(bpy) 2 (pzH) 2 ] 2+ ( 1a ) and cis -[Ru­(bpy) 2 (pz)­(pzH)] + ( 1b ) . The complex 1a was first synthesized by Meyer et al .…”

Recent advances in photorelease complexes for therapeutic applications

“…There is a clear chelate effect, explained that even if one pyridine became detached the other anchoring ring would still hold the ligand in place. 29,30 A bond elongation process, by population of the anti-bonding metalcentred d-orbital, would appear to be more significant in a photorelease mechanism for mono-dentate ligands. The full ejection of a chelate after excitation may require a secondary agent, utilising the excited state redox properties of a complex.…”