Thiocyanate Linkage Isomerism in a Ruthenium Polypyridyl Complex

Ruthenium polypyridyl complexes have seen extensive use
in solar energy applications.
One of the most efficient dye-sensitized solar cells produced to date
employs the dye-sensitizer N719, a ruthenium polypyridyl thiocyanate
complex. Thiocyanate complexes are typically present as an inseparable
mixture of N-bound and S-bound linkage isomers. Here we report the
synthesis of a new complex, [Ru­(terpy)­(tbbpy)­SCN]­[SbF6]
(terpy = 2,2′;6′,2″-terpyridine, tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine), as a mixture of N-bound
and S-bound thiocyanate linkage isomers that can be separated based
on their relative solubility in ethanol. Both isomers have been characterized
spectroscopically and by X-ray crystallography. At elevated temperatures
the isomers equilibrate, the product being significantly enriched
in the more thermodynamically stable N-bound form. Density functional
theory analysis supports our experimental observation that the N-bound
isomer is thermodynamically preferred, and provides insight into the
isomerization mechanism.