Synthesis, Structure, and Substitution Mechanism of New Ru(II) Complexes Containing 1,4,7-Trithiacyclononane and 1,10-Phenanthroline Ligands

Two new Ru complexes containing the 1,10-phenanthroline (phen) and 1,4,7-trithiacyclononane ([9]aneS3, ligands of general formula [Ru(phen)(L)([9]aneS3)]2+ (L = MeCN, 3; L = pyridine (py), 4)
have been prepared and thoroughly characterized. Structural characterization in the solid state has been performed
by means of X-ray diffraction analyses, which show a distorted octahedral environment for a diamagnetic d6 Ru(II),
as expected. 1H NMR spectroscopy provides evidence that the same structural arrangement is maintained in solution.
Further spectroscopic characterization has been carried out by UV−vis spectroscopy where the higher π acceptor
capability of MeCN versus the py ligand is manifested in a 9−15-nm blue shift in its MLCT bands. The E1/2 redox
potential of the Ru(III)/Ru(II) couple for 3 is anodically shifted with respect to its Ru−py analogue, 4, by 60 mV,
which is also in agreement with a higher electron-withdrawing capacity of the former. The mechanism for the
reaction Ru−py + MeCN → Ru−MeCN + py has also been investigated at different temperatures with and without
irradiation. In the absence of irradiation at 326 K, the thermal process gives kinetic constants of k2 = 1.4 × 10-5
s-1 (ΔH⧧ = 108 ± 3 kJ mol-1, ΔS⧧ = −8 ± 9 J K-1 mol-1) and k-2 = 2.9 × 10-6 s-1 (ΔH⧧ = 121 ± 1 kJ mol-1,
ΔS⧧ = 18 ± 3 J K-1 mol-1). The phototriggered process is faster and consists of preequilibrium formation of an
intermediate that thermally decays to the final Ru−MeCN complex with an apparent rate constant of (k1Khν)app =
1.8 × 10-4 s-1 at 304 K, under the continuous irradiation experimental conditions used.