Mechanistic and structural studies of electron-deficient quinoline triosmium clusters

Deuterium-labeling experiments on the sequential reactions of the previously reported electron-deficient complexes Os₃(CO)₉(μ₃-η₂-C ₉H₄NRR′)(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-C ₉H₆RR′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-C ₉H₇RR′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-C ₉H₈N)(μ-H) (5) containing a C=N 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-C ₉H₉N)(μ-H)₂ (4). Photolysis or thermolysis of the previously reported Os₃(CO)₉-(μ-η²-(4-Me)C₉H ₅N)(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.