Follow-Up Study of Trans-C to Cis-C Thermally or Photochemically Induced Isomerization of Terpyridine Adducts of Cycloruthenated 2-Aryl-2′-pyridine Compounds

The mechanism of isomerization of the known 2-phenyl,pyridine (phpy) derivatives [Ru(phpy-κC,N) (MeCN-trans-N)(terpy)]PF6, 2, to [Ru(phpy-κC,N)(MeCN-trans-C)(terpy)]PF6 (terpy = 2,2′;6′,2″-terpyridine), 3, at temperatures >50 °C has been investigated both by 1H NMR spectroscopy and by DFT calculations. The photoisomerization of 2 to 3 by UV light occurred also quantitatively in MeCN after 20 h at room temperature. A similar behavior to that of 2 could be established for the related compound [Ru(3-acridine-2′-C5H4N-κC,N)(MeCN-trans-N)(2,2′;6′,2″-terpyridine)]PF6, 6 (acridine = dibenzo[b,e]pyridine or 2,3-benzoquinoline), that was obtained from the reaction between [Ru(3-acridine-2′-C5H4N-κC,N) (MeCN)4]PF6, 4, and terpy in MeOH/MeCN at 60 °C for 24 h. Similar to 2, the isomerization of 6 to [Ru(3-acridine-2′-C5H4N-κC,N)(MeCN-trans-C) (terpy)]PF6, 7, could be induced thermally (48 h at 60 °C in pure MeOH) or photochemically under UV radiation in MeCN at room temperature. A compound closely related to 7 but in which MeCN was replaced by H2O was described earlier (Tanaka et al. Inorg. Chem. 2012, 51, 5386–539). The presence of water on this compound had a dramatic effect as far as the coordination of terpy was concerned as its isomerization to a compound related to 6 (in which H2O instead of MeCN is coordinated to Ru) occurred indeed photochemically via irradiation with visible light.