Spontaneous Formation in the Dark, and Visible Light-Induced Cleavage, of a Ru–S Bond in Water: A Thermodynamic and Kinetic Study

In
this work the thermal and photochemical reactivity of a series of
ruthenium complexes [Ru­(terpy)­(N–N)­(L)]­(X)2 (terpy
= 2,2′;6′,2″-terpyridine, L = 2-(methylthio)­ethanol
(Hmte) or water, and X is Cl– or PF6–) with four different bidentate chelates N–N
= bpy (2,2′-bipyridine), biq (2,2′-biquinoline), dcbpy
(6,6′-dichloro-2,2′-bipyridine), or dmbpy (6,6′-dimethyl-2,2′-bipyridine),
is described. For each chelate N–N the thermodynamic constant
of the dark equilibrium between the aqua- and Hmte- complexes, the
Hmte photosubstitution quantum yield, and the rate constants of the
thermal interconversion between the aqua and Hmte complexes were measured
at room temperature. By changing the steric hindrance and electronic
properties of the spectator N–N ligand along the series bpy,
biq, dcbpy, dmbpy the dark reactivity clearly shifts from a nonlabile
equilibrium with N–N = bpy to a very labile thermal equilibrium
with N–N = dmbpy. According to variable-temperature rate constant
measurements in the dark near pH = 7 the activation enthalpies for
the thermal substitution of H2O by Hmte are comparable
for all ruthenium complexes, whereas the activation entropies are
negative for bpy and biq, and positive for dcbpy and dmbpy complexes.
These data are indicative of a change in the substitution mechanism,
being interchange associative with nonhindered or poorly hindered
chelates (bpy, biq), and interchange dissociative for more bulky ligands
(dcbpy, dmbpy). For the most labile dmbpy system, the thermal equilibrium
is too fast to allow significant modification of the composition of
the mixture using light, and for the nonhindered bpy complex the photosubstitution
of Hmte by H2O is possible but thermal binding of Hmte
to the aqua complex does not occur at room temperature. By contrast,
with N–N = biq or dcbpy the thermodynamic and kinetic parameters
describing the formation and breakage of the Ru–S bond lie
in a range where the bond forms spontaneously in the dark, but is
efficiently cleaved under light irradiation. Thus, the ratio between
the aqua and Hmte complex in solution can be efficiently controlled
at room temperature using visible light irradiation.