We have applied Density Functional Theory (DFT) at the generalized gradient approximation (GGA) level
to investigate the C−S bond cleavage in hexathioether complexes of the form [M(9S3)2]
n
+ (with 9S3 =
1,4,7-trithiacyclononane and M = Re, Tc; n = 1, 2; as well as M = Ru; n = 2, 3). The experimental trends
in C−S bond lengths of the different compounds are reproduced faithfully. Reduction leads to a lowering of
the calculated reaction energies by ≈20 kcal/mol to values of 4, 10, and 44 kcal/mol for M = Re, Tc, Ru,
respectively. The corresponding values for the activation energy are 10, 15, and 44 kcal/mol, which is in
agreement with the experimental observation that the rhenium and technetium compounds lose an ethene
molecule immediately after reduction, while the ruthenium compound is stable toward such a loss. Our
calculations suggest that the unique reactivity of the reduced rhenium and technetium complexes is a result
of the higher energies of metal t2g-orbitals, resulting from the lower overall charge of the complex. π-Back-donation from t2g-orbitals into C−S σ*-orbitals is another important effect, leading to low activation barriers,
as only little electronic rearrangement is necessary upon cleavage of the C−S bonds.
The binding capability of three ruthenium polypyridyl compounds of structural formula [Ru(apy)(tpy)Ln-](ClO4)(2-n) [1a-c; apy = 2,2'-azobis(pyridine), tpy = 2,2':6',2''-terpyridine, L = Cl, H2O, CH3CN] to a fragment of DNA was studied. The interaction between each of these complexes and the DNA model base 9-ethylguanine (9-EtGua) was followed by means of 1H NMR studies. Density functional theory calculations were carried out to explore the preferential ways of coordination between the ruthenium complexes and guanine. The ruthenium-9-EtGua adduct formed was isolated and fully characterized using different techniques. A variable-temperature 1H NMR experiment was carried out that showed that while the 9-EtGua fragment was rotating fast at high temperature, a loss of symmetry was suffered by the model base adduct as the temperature was lowered, indicating restricted rotation of the guanine residue.
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