Mechanically Induced Phase Change in Barbituric Acid

By grinding a commercial sample of barbituric acid in its trioxo form (polymorph II, 99%) for 24 h, a new compound has been isolated. The new phase has been identified as the trihydroxyl isomer. The characterization of the new isomer has been carried out by means of X-ray powder diffraction, solid-state NMR (1H MAS, 13C and 15N CPMAS, 2D PASS, and FSLG-HETCOR), IR and Raman spectroscopies. The conversion results from the complete tautomeric shift of a methylene and of two N−H hydrogen atoms. 1H MAS spectra allow the characterization of the hydrogen bond interactions on the basis of their strength in the starting compound and in the new isomer. In solution, the trihydroxyl isomer immediately converts to the trioxo form as demonstrated by 1H NMR experiments in protic, aprotic, and amphiprotic solvents (D2O, MeOH-d4, DMSO-d6, acetone-d6)

Comparative Reactivity of Triruthenium and Triosmium μ₃-η²-Imidoyls. 1. Dynamics and Reactions with Carbon Monoxide, Phosphine, and Isocyanide

The reactivity and ligand dynamics of the μ3-η2-imidoyl clusters Ru3(CO)9(μ3-η2-RCNR‘)(μ-H) (R = CH3, R‘ = Et 2; RR‘ = (CH2)3, 3; RR‘ = (CH2)2C(H)CH2OCH3 4, R = R‘ = CH3, 5) are compared with the previously reported osmium analogs. The lowest energy dynamical process in these clusters is the “windshield wiper” motion over the face of the cluster whereas tripodal rotation of the carbonyl groups on the unbridged metal atom is the fastest process in the analogous osmium compounds. Although the structures of the phosphine and isocyanide substitution products reported, Ru3(CO)8(μ3-η2-RCNR‘)(μ-H)L (R = CH3, R‘ = CH2CH3, L = PPh3 (8a), L = CNMe (12); R = R‘ = (CH2)3, L = PPh3 (9), L = CNMe (13); R = R‘ = CH3, L = PPh3 (10), are identical to their osmium analogs, the pathway to their formation reflects the lower CO dissociation energies for ruthenium clusters and a greater sensitivity to the substituents on the imidoyl group. The solid state structures of 9 and 12 are reported as well as that of Os3(CO)8(μ-η2-CN(CH2)3(μ-H)(PPh3)(MeNC) (15). The latter illustrates the hemilabile nature of the μ3-imidoyl ligand even in phosphine-substituted derivatives and the structural preferences of phosphine and isocyanide ligands in this class of clusters

Comparative Reactivity of Triruthenium and Triosmium μ₃-η²-Imidoyls. 2. Reactions with Alkynes

The reactions of Ru3(CO)9(μ3-η2-CH3CNCH2CH3)(μ-H) (1), M3(CO)9(μ3-η2-CN(CH2)3)(μ-H) (M = Ru (2), M = Os (3)) with the alkynes RC⋮CR (R = CH3, C6H5, CO2Me) have been studied. The ruthenium complexes 1 and 2 react with 2-butyne at 70 °C to give two very different trimetallic alkyne derivatives:  Ru3(CO)7(μ-η2:η4-C4(CH3)4)(μ-η2-CH3CNCH2CH3)(η1-COC(CH3)C(H)CH3) (5) and Ru3(CO)8(μ3-η2-CN(CH2)3)(μ-η2-CH3C(H)CCH3) (6). The osmium imidoyl 3 does not react with 2-butyne even at elevated temperatures. However, the reaction of Os3(CO)9(μ-η2-CN(CH2)3)(μ-H)(CH3CN) (7b), synthesized from Os3(CO)10(μ-η2-CN(CH2)3)(μ-H) (7a), with 2-butyne yields the analog of 6, Os3(CO)8(μ3-η2-CN(CH2)3)(μ-η2-RC(H)CR) (R = CH3 (10), R = C6H5 (11)) on thermolysis of the initially formed nonacarbonyl precursors (8 and 9 for R = CH3), which are a mixture of isomers. Direct reaction of 7a with diphenylacetylene at 100 °C gives somewhat lower yields of 11. The reaction of 7b with dimethylacetylenedicarboxylate or the direct reaction of 3 with this alkyne yields the nonacarbonyl derivative Os3(CO)9(μ-η2-CN(CH2)3)(μ3-η3-CH3O2CCC(H)CO2CH3) (12). Direct reaction of 7a with a 2.5 molar excess trimethylamine N-oxide at room temperature yields the alkyne−imidoyl-coupled product Os3(CO)8(μ-η6-CH3C(H)C(CH3)C(CH3)C(CH3)CN(CH2)3) (13). The solid state structures of 5, 11, 12, and 13 are reported. A comparative study of the electrochemical properties of 5 and 1 is also reported

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-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