Mechanistic and Structural Studies of Electron-Deficient Quinoline Triosmium Clusters

Abstract

Deuterium-labeling experiments on the sequential reactions of the previously reported electron-deficient complexes Os3(CO)9(μ3-η2-C9H4NRR‘)(μ-H)(R = R‘ = H, 1a; R = 4-Me, R‘ = H, 1b; R = H, R‘ = 6-CH3, 1c) with X-/X+ (X = H or D) reveal that initial attack of H- is at the 5-position of the quinoline ring and that the reduction of the C(5)−C(6) double bond to yield Os3(CO)9(μ3-η3-C9H6RR‘N)(μ-H) (2a−c) is not stereoselective. Related experiments with 2a−c reveal that hydride attack at the 7-position is followed by protonation at the metal core to yield Os3(CO)9(μ3-η2-C9H7RR‘N)(μ-H)2 (3a−c). The conversion of 2a to 3a is also achieved by reaction with H2 at 75 °C and 100 psi. When this reaction is carried out with excess D2, deuterium incorporation is observed at C(7) and at the metal core, suggesting a concerted, irreversible hydrogen addition or a radical chain reaction. The related 46-electron cluster Os3(CO)9(μ3-η2-C9H8N)(μ-H) (5) containing a CN bond in a partially reduced heterocyclic ring, as well as the three-center two-electron bond at C(8), undergoes H- attack at C(2) and not at C(5), as for 1a−c, followed by protonation at the metal core to yield Os3(CO)9(μ3-η2-C9H9N)(μ-H)2 (4). Photolysis or thermolysis of the previously reported Os3(CO)9(μ-η2-(4-Me)C9H5N)(μ-H)(P(OEt)3) (6b) does not yield the phosphite-substituted 46-electron clusters related to 1a−c but leads only to nonspecific decomposition. Partially selective incorporation of 13CO into 1a−c is observed to yield the corresponding decacarbonyl derivatives, and the pattern of 13CO incorporation helps to elucidate the interconversion of the nona- and decacarbonyl derivatives. The electrochemical behavior of 1a, the dynamical behavior of 2b, and the solid-state structures of 2b, 3a, 5, and 6b are reported.