HYDROGENATION OF THE 2 DIASTEREOMERS OF THE 66-ELECTRON LINEAR CLUSTER RU4(CO)10[R1C=C(H)C(H)=NR2]2 - HYDROGEN-TRANSFER REACTIONS AND THE MOLECULAR-STRUCTURE OF THE ONLY ISOLABLE DIASTEREOMER OF THE 64-ELECTRON BUTTERFLY CLUSTER (MU-H)2RU4(CO)8[CH3C=C(H)C(H)=N-I-PR]2

The linear tetranuclear 66-electron clusters Ru4(CO)10[R1C=C(H)C(H)=NR2]2(R1,R2 = CH3, i-Pr (3a), CH3, c-Hex (3b), C6H5, i-Pr (3d)), which consist of mixtures of diastereomers (CC/AA)-3a,b,d and (CA/AC)-3a,b,d, react in heptane solution at 90-degrees-C with dihydrogen to yield only one of the two possible diastereomers of the tetranuclear 64-electron butterfly clusters (mu-H)2RU4(CO)8[R1C=C(H)C(H)=NR2]2 ((CC/AA)-5a,b,d; ca. 45%), together with H4Ru4(CO)12 (ca. 25%) and R1CH2CH2CH2N(H)R2. Reaction of 3c (R1, R2 = CH3, t-Bu) with dihydrogen gave H4RU4(CO)12 and CH3CH2CH2CH2N(H)(t-Bu). These conversions proceed via the intermediacy of the dinuclear species HRu2(CO)5[R1C=C(H)C(H)=NR2] (2). The reactivity of the two diastereomers of 3a toward dihydrogen differ. (CA/AC)-3a reacts with dihydrogen at 40-degrees-C to give first two molecules of the dinuclear complex HRU2(CO)5[CH3C=C(H)C(H)=N-i-Pr] (2a), which are subsequently converted into (CC/AA)-5a, H4RU4(CO)12, and CH3CH2CH2CH2N(H)(i-Pr), whereas (CC/AA)-3a has to be heated at 70-degrees-C before conversion into 2a and subsequent conversions take place. The compounds 5a,b,d have been characterized by H-1 NMR, C-13 NMR, and IR spectroscopy, FI/FD mass spectrometry, and elemental analysis, and the X-ray crystal structure of (CC/AA)-5a has been determined. Crystals of (CC/AA)-5a are monoclinic, space group P2(1)/n, with a = 10.852 (1) angstrom, b = 16.878 (2) angstrom, c = 15.476 (2) angstrom, beta = 95.64 (1)-degrees, Z = 4, and R = 0.029 (R(W) = 0.031). Compound (CC/AA)-5a is an aggregate of two Ru2(CO)4[CH3C=C(H)C(H)=N-i-Pr] units and two bridging hydrides, the presence of which was established chemically. The four ruthenium atoms in (CC/AA)-5a occupy a butterfly arrangement (phi = 137.05-degrees) with two metal-metal bonds of normal length (ca. 2.82 angstrom) and three long bonds (3.12-3.14 angstrom). The monoazadienyl ligands in (CC/AA)-5a adopt, as in 2 and 3, the bridging seven-electron-donating sigma-(N)-sigma-(C)-eta-2-(C=C):eta-2(C=N) coordination mode, and the eight CO ligands are all terminally bonded. On the basis of the structural data for (CC/AA)-5a, the instability of the other diastereomers (CA/AC)-5a-d and of (CC/AA)-5c(R2 = t-Bu) is discussed. Several hydrogenation and H-transfer reactions involving the monoazadienyl ligand have been observed. For instance, reaction of 3a-d at 90-degrees-C under an atmosphere of H2/CO (9:1) gives RU2(CO)6[R1C=C(H)CH2NR2] (1a-d; 60-80%). Reaction of 5a,b,d with CO at 90-degrees-C also gives 1a,b,d in nearly quantitative yield. Both reactions probably proceed via HRu2(CO)5[R1C=C(H)C(H)=NR2] (2) and HRU2(CO)6[R1C=C(H)C(H)=NR2] (6), since at ambient temperature 2c rapidly takes up one CO ligand to give 6c, which contains a five-electron-donating sigma(N)-sigma-(C)-eta-2(C=C)-bonded monoazadienyl ligand, which can be thermally isomerized to give 1c. During this conversion, the hydride in 6 is transferred to the imine C atom of the monoazadienyl ligand. Conversion of 6c into 1c, however, only takes place at a reasonable rate at 80-degrees-C under an atmosphere of CO; otherwise conversion into 3c dominates. On silica, 6 isomerizes into RU2(CO)6[R1CC(H)C(H)N(H)R2] (7). This isomerization takes place stereospecifically, as only one diastereomer of 7 is formed. Thermal reaction of Ru2(CO)6[CH2CC(H)C(H)=NR2] (4a,C) with dihydrogen at 100-degrees-C gives la and 3c, respectively. These conversions probably take place via 6a,c. The deviating reactivity of 6 with R2 = t-Bu (6c) compared to that with R2 = i-Pr or c-Hex (6a,b) and the intermediacy of this crucial compound during thermal reactions of RU3(CO)12 with monoazadienes R1C(H)=C(H)C(H)=NR2 are discussed.