Synthesis, Characterization, and Electrochemistry of Biorelevant Photosensitive Low-Potential Orthometalated Ruthenium Complexes

Redox potentials of photosensitive cyclometalated RuII derivatives of 2-phenylpyridine or 2-(4-tolyl)pyridine are
controllably decreased by up to 0.8 V within several minutes. This is achieved by irradiation of the ruthena(II)cycles
cis-[Ru(o-X-2-py)(LL)(MeCN)2]PF6 (2, X = C6H4 (a) or 4-MeC6H3 (b), LL = 1,10-phenanthroline or 2,2‘-bipyridine).
The cis geometry of the MeCN ligands has been confirmed by the X-ray structural studies. The σ-bound sp2
carbon of the metalated ring is trans to LL nitrogen. Complexes 2 are made from [Ru(o-X-2-py)(MeCN)4]PF6 (1)
and LL. This “trivial” ligand substitution is unusual because 1a reacts readily with phen in MeCN as solvent to give
cis-[Ru(o-C6H4-2-py)(phen)(MeCN)2]PF6 (2c) in a 83% yield, but bpy does not afford the bpy-containing 2 under
the same conditions. cis-[Ru(o-C6H4-2-py)(bpy)(MeCN)2]PF6 (2e) has been prepared in CH2Cl2 (74%). Studies of
complexes 2c,e by cyclic voltammetry in MeOH in the dark reveal RuII/III quasy-reversible redox features at 573
and 578 mV (vs Ag/AgCl), respectively. A minute irradiation 2c and 2e converts them into new species with redox
potentials of −230 and 270 mV, respectively. An exceptional potential drop for 2c is accounted for in terms of a
photosubstitution of both MeCN ligands by methanol. ESR, 1H NMR, and UV−vis data indicate that the primary
product of photolysis of 2c is an octahedral monomeric low-spin (S = 1/2) RuIII species, presumably cis-[RuIII(o-C6H4-2-py)(phen)(MeOH)2]2+. The primary photoproduct of bpy complex 2e is cis-[RuII(o-C6H4-2-py)(bpy)(MeCN)(MeOH)]+, and this accounts for a lower decrease in the redox potential. Irradiation of 2c in the presence of added
chloride affords [(phen)(o-C6H4-2-py)ClRuIIIORuIVCl(o-C6H4-2-py)(phen)]PF6, a first μ-oxo-bridged mixed valent dimer
with a cyclometalated unit. The structure of the dimer has been established by X-ray crystallography.