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 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-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.

Spectroscopic and Computational Investigations of Stable Radical Anions of Triosmium Benzoheterocycle Clusters

“…[13] It is worth noting that in this case the line width is independent of the hyperfine coupling constant, and uniform broadening of the resonances is expected, as observed. This is also the case for 1 after exhaustive electrolysis for which we observe a uniform sharpening of the 1 H NMR resonances for the small amount of unreduced cluster as the temperature is decreased Density functional theory calculations and spectral simulations: The density functional theory (DFT) approach has proven to be a very useful method for understanding chemical bonding in transition-metal and polymetallic complexes. [17,18] DFT methods have been previously successfully applied to calculations of triosmium clusters: Morokuma and co-workers studied the bonding in [Os 3 (CO) 9 (C 6 H 6 )], [19] and Calhorda et al studied in detail structures, frontier orbitals, and bonding of [Os 3 (CO) 10 (a-diimine)] clusters.…”

“…The radical anions of 1 and 2 prepared by exhaustive electrolysis show an EPR signal at room temperature, while the NMR signals are uniformly broadened. Compound 3 appears to be partially reduced by sodium at room temperature and shows uniformly broadened 1 H NMR resonances at room temperature that sharpen significantly at À80 8C. The temperature dependence of the spectra are discussed in terms of the effects of relative electron nuclear relaxation processes, chemical exchange, and the results of the DFT calculations.…”

“…That 3 reacts with sodium but not cobaltocene is not surprising, because its reduction potential is significantly more negative than that of 1 (À1.63 V). [6] Exhaustive electrolysis of a solution of 1 in methylene chloride (5 mm) led to a completely broadened 1 H NMR spectrum and a well-defined ESR signal at room temperature with a g value of 1.99707, typical for aromatic hydrocarbon radicals. [15] This is in sharp contrast to solutions of 1 with cobaltocene, for which a broad, ill-defined ESR signal is observed at all temperatures ( Figure 7).…”

Water-soluble benzoheterocycle triosmium clusters as potential inhibitors of telomerase enzyme

“…We have recently developed synthetic procedures for a class of benzoheterocycle triosmium clusters of general formula [(l-H)Os 3 (CO) 9 (L)(l 3 -g 2 -(Q-H))], where L = [P(C 6 H 4 SO 3 Na) 3 ] or [P(OCH 2 CH 2 NMe 3 I) 3 ] and Q = quinoline, 3-aminoquinoline, quinoxaline, phenanthridine [23][24][25][26][27][28][29][30] (Fig. 2).…”

Competition studies on the activation of the C_H bond of diazines by the unsaturated triangular cluster anion [Re3(μ-H)4(CO)10]−

“…It is known in fact that stable organometallic compounds can provide good models of the reactive intermediates in both homogeneous and heterogeneous catalytic processes. A particular interest is devoted to systems able to activate C-H bonds of organic ligands co-ordinated on metal clusters and several examples of these complexes have been described [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. 2 …”