Characterization of Low Energy Charge Transfer Transitions in (terpyridine)(bipyridine)Ruthenium(II) Complexes and their Cyanide-Bridged Bi- and Tri-Metallic Analogues

The lowest energy metal-to-ligand charge transfer (MLCT) absorption
bands found in ambient solutions of a series of [Ru(tpy)(bpy)X]m+ complexes (tpy = 2,2′:3′,2″-terpyridine;
bpy = 2,2′-bipyridine; and X = a monodentate ancillary ligand)
feature one or two partly resolved weak absorptions (bands I and/or
II) on the low energy side of their absorption envelopes. Similar
features are found for the related cyanide-bridged bi- and trimetallic
complexes. However, the weak absorption band I of [(bpy)2Ru{CNRu(tpy)(bpy)}2]4+ is missing in its [(bpy)2Ru{NCRu(tpy)(bpy)}2]4+ linkage isomer
demonstrating that this feature arises from a RuII/tpy
MLCT absorption. The energies of the MLCT band I components of the
[Ru(tpy)(bpy)X]m+ complexes are proportional
to the differences between the potentials for the first oxidation
and the first reduction waves of the complexes. Time-dependent density
functional theory (TD-DFT) computational modeling indicates that these
band I components correspond to the highest occupied molecular orbital
(HOMO) to lowest unoccupied molecular orbital (LUMO) transition, with
the HOMO being largely ruthenium-centered and the LUMO largely tpy-centered.
The most intense contribution to a lowest energy MLCT absorption envelope
(band III) of these complexes corresponds to the convolution of several
orbitally different components, and its absorption maximum has an
energy that is about 5000 cm–1 higher than that
of band I. The multimetallic complexes that contain RuII centers linked by cyanide have mixed valence excited states in which
more than 10% of electronic density is delocalized between the nearest
neighbor ruthenium centers, and the corresponding stabilization energy
contributions in the excited states are indistinguishable from those
of the corresponding ground states. Single crystal X-ray structures
and computational modeling indicate that the Ru-(CN)-Ru linkage
is quite flexible and that there is not an appreciable variation in
electronic structure or energy among the conformational isomers.