Photochemical Expulsion of the Neutral Monodentate Ligand L in Ru(Terpy*)(Diimine)(L)²⁺:  A Dramatic Effect of the Steric Properties of the Spectator Diimine Ligand

A series of photoreactive complexes of the type Ru(terpy*)(N−N)(L)²⁺, where terpy* is 4‘-(3,5-ditertiobutylphenyl)-2,2‘:6‘,2‘ ‘-terpyridine, N−N is the bidentate chelate phen or dmp (phen = 1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline), and L is the monodentate ligand dms, MeBN, or MeOBN (dms = dimethyl sulfide, MeBN = 2,6-dimethyl benzonitrile, MeOBN = 2,6-dimethoxybenzonitrile), has been synthesized and fully characterized by proton NMR spectroscopy, electrospray mass spectrometry, and UV−vis spectroscopy. The X-ray structures of four complexes were also obtained. In neat pyridine, the quantum yields for the photosubsitution of L by pyridine were measured and showed dramatic variations depending on the steric interactions between the spectator bidentate ligand and the leaving monodentate ligand L. The use of dmp instead of phen multiplied the photosubstitution efficiency by a factor of 20−50, depending on L. This effect could be qualitatively correlated to the distortions observed in the X-ray structures of the corresponding complexes. The highly distorted structure of Ru(terpy*)(dmp)(dms)(PF₆)₂ showed a very high photosubsitution quantum yield φ = 0.36 in neat pyridine. The high photoreactivity of some of the compounds makes them particularly promising as components of future light-driven molecular machines.