Synthesis and characterization of cycloruthenated 2-(phenylimino)phenyls: A useful probe for the elucidation of the tautomeric process in 2-hydroxyphenyl-schiff bases

Treatment of [RuHCl(CO)(PPh3)3] with a 1:1 molar ratio of Hg{o-C6H4C(H)=NC6H4-4-R}2 in refluxing toluene afforded the new compounds [RuCl(CO){η2-C,N-C6H4C(H)=NC6H 4-4-R}(PPh3)2] (R = NMe2, 1a; Me, 1b; I, 1c; NO2, 1d) in good yield. The new compounds have been fully characterized by elemental analysis, IR spectroscopy, and 1H, 13C{1H}, and 31P-{1H} NMR spectroscopy. A linear correlation in a Hammett plot of the metalated carbon resonance of the imine ligand versus σ+-values is observed, and this is used to comment on, along with the reported ν(CO) data and data previously reported for analogous azo-containing complexes, the keto/enol tautomerization undergone by 2-hydroxyphenyl-Schiff bases. These data suggest that the position of the tautomeric equilibrium is affected by electron-withdrawing or -releasing substituents in the same way as for the analogous 2-phenylazophenols. During the purification of 1a and 1b small amounts of the bis-cyclometalated imine-containing complexes [Ru(CO){η2-C,N-C6H4C(H)=NC6H 4-4-R}2(PPh3)] (R = NMe2, 2a; Me, 2b) were isolated. These compounds have been subsequently synthesized and fully spectroscopically characterized. The compounds 1a, 1b, 2a, and 2b have also been further characterized by single-crystal X-ray diffraction studies.