Effects of Methyl Substitution in Ruthenium Tris(2-pyridylmethyl)amine Photocaging Groups for Nitriles

Four complexes of the general formula
[Ru­(L)­(CH3CN)2]­(PF6)2,
[L = TPA (5), MeTPA (6), Me2TPA
(7), and Me3TPA (8)] [TPA = tris­[(pyridin-2-yl)­methyl]­amine,
where methyl groups were introduced consecutively onto the 6-position
of py donors of TPA, were prepared and characterized by various spectroscopic
techniques and mass spectrometry. While 5 and 8 were isolated as single stereoisomers, 6 and 7 were isolated as mixtures of stereoisomers in 2:1 and 1.5:1
ratios, respectively. Steric effects on ground state stability and
thermal and photochemical reactivities were studied for all four complexes
using 1H NMR and electronic absorption spectroscopies and
computational studies. These studies confirmed that the addition of
steric bulk accelerates photochemical and thermal nitrile release.