Unexpected reversible pyrazine based methylation in a Ru(ii) complex bearing a pyrazin-2′-yl-1,2,4-triazolato ligand and its effect on acid/base and photophysical properties

The regioselective methylation of a ruthenium polypyridyl complex bearing both a 1,2,4-triazolato and a pyrazine moiety is reported. In contrast to previous studies in which methylation of the 1,2,4-triazolato ring was observed, in the present system methylation takes place exclusively at the non-coordinated nitrogen of the pyrazine ring. The monomethylation is confirmed by ¹H NMR spectroscopy and ESI-MS and the electronic properties of the methylated complexes are studied by UV/vis absorption, emission, surface enhanced, resonance and transient resonance Raman spectroscopy. Ligand deuteriation is used to simplify the ¹H NMR spectra and to assign definitively the Raman spectra. Acid-base studies show that the triazolato ring of the N-methylated complexes can be protonated at low pH and that at high pH the N-methyl group can be deprotonated reversibly. Furthermore it is shown that under conditions where the methyl group is deprotonated, demethylation occurs to recover the initial complex.