Diruthenium Dithiolato Cyanides: Basic Reactivity Studies and a Post Hoc Examination of Nature's Choice of Fe versus Ru for Hydrogenogenesis

The reaction of Ru2(S2C3H6)(CO)6 (1) with 2 equiv of Et4NCN yielded (Et4N)2[Ru2(S2C3H6)(CN)2(CO)4], (Et4N)2[3],
which was shown crystallographically to consist of a face-sharing bioctahedron with the cyanide ligands in the axial
positions, trans to the Ru−Ru bond. Competition experiments showed that 1 underwent cyanation >100× more
rapidly than the analogous Fe2(S2C3H6)(CO)6. Furthermore, Ru2(S2C3H6)(CO)6 underwent dicyanation faster than
[Ru2(S2C3H6)(CN)(CO)5]-, implicating a highly electrophilic intermediate [Ru2(S2C3H6)(μ-CO)(CN)(CO)5]-. Ru2(S2C3H6)(CO)6 (1) is noticeably more basic than the diiron compound, as demonstrated by the generation of [Ru2(S2C3H6)(μ-H)(CO)6]+, [1H]+. In contrast to 1, the complex [1H]+ is unstable in MeCN solution and converts to [Ru2(S2C3H6)(μ-H)(CO)5(MeCN)]+. (Et4N)2[3] was shown to protonate with HOAc (pKa = 22.3, MeCN) and, slowly, with MeOH and
H2O. Dicyanide [3]2- is stable toward excess acid, unlike the diiron complex; it slowly forms the coordination
polymer [Ru2(S2C3H6)(μ-H)(CN)(CNH)(CO)4]n, which can be deprotonated with Et3N to regenerate [H3]-.
Electrochemical experiments demonstrate that [3H]- catalyzes proton reduction at −1.8 V vs Ag/AgCl. In contrast
to [3]2-, the CO ligands in [3H]- undergo displacement. For example, PMe3 and [3H]- react to produce [Ru2(S2C3H6)(μ-H)(CN)2(CO)3(PMe3)]-. Oxidation of (Et4N)2[3] with 1 equiv of Cp2Fe+ gave a mixture of [Ru2(S2C3H6)(μ-CO)(CN)3(CO)3]- and [Ru2(S2C3H6)(CN)(CO)5]-, via a proposed [Ru2]2(μ-CN) intermediate. Overall, the ruthenium
analogues of the diiron dithiolates exhibit reactivity highly reminiscent of the diiron species, but the products are
more robust and the catalytic properties appear to be less promising.