Thiocyanate linkage isomerism in the isobutyl ester form of the ruthenium dye known as N3

Using a modification of a recently reported convenient synthesis of the isobutyl ester form of the bis(isothiocyanato) ruthenium dye (N3), we report the isolation and spectroscopic characterization of the minor isomer in which one NCS^- is N-bound and the other is S-bound. The synthesis involves the reaction of Ru(ⁱBu₂dcbpy)₂Cl₂, where ⁱBu₂dcbpy is the diisobutyl ester of 4,4′- dicarboxy-2,2′-bipyridine, with a source of thiocyanate. The impact of the isocyanate salt, solvent, and temperature on the yield of the linkage isomers is presented. In addition to the two linkage isomers 1 and 2, the partially substituted chloroisothiocyanato complex 3, Ru(ⁱBu ₂dcbpy)₂Cl(NCS), was also isolated. Selective removal of the chloride using silver triflate provided a path to [Ru(ⁱBu ₂dcbpy)₂(pyridine)(NCS)]OTf, 4, in high yield. The complexes were characterized by ¹H and ¹³C NMR, IR, and electronic absorption spectroscopies and mass spectrometry. At 80 °C in DMSO-d₆, the isomerization of 2 to 1 is complete and exhibits first-order kinetics with a rate constant of 0.00014 s^-1. Room temperature hydrolysis of the isobutyl ester groups of 2 using [ ⁿBu₄ N]OH in acetonitrile produced a mixture of the two linkage isomers.