SYNTHESIS, STRUCTURAL CHARACTERIZATION AND THERMAL REACTIVITIES OF OSMIUM CARBONYL CLUSTERS CONTAINING 4,6-DIMETHYLPYRIMIDINE-2-THIONE

The reaction of the labilized cluster [Os-3(CO)(10)(MeCN)(2)] and bis(4,6-dimethylpyrimidin-2-yl) disulfide affords two major products [Os-3(CO)(10)(mu-dmpymt)(2)] 1 (36%) and [Os-3(CO)(10)(mu-SH) (mu-dmpymt)] 2 (19%) (dmpymt = 4,6-dimethylpyrimidine-2-thione). Cluster 2 is formed from C-S bond cleavage of one of the dmpymt ligands at ambient conditions. Thermolysis of cluster 1 in n-heptane (98 degrees C) for 1 h produces an isomeric cluster of 1, [OS3(CO)(10)(mu-dmpymt)(2)] 3 (62%), which contains two dmpymt ligands co-ordinated across the same non-bonding Os Os edge. Thermolysis of cluster 3 in n-heptane for 2 h affords the cluster [Os-3(CO)(9)(mu-dmpymt)(mu(3)-eta(2)-dmpymt)] 4 as the major product (42%) in addition to a minor product [Os-3(CO)(8)(mu-eta(2)-dmpymt)(mu(3)-eta(2)-dmpymt)] 5 (9%). Cluster 4 contains two dmpymt moieties co-ordinating in mu and mu(3)-eta(2) modes respectively. Cluster 5 has two dmpymt moieties bonding in the mu(3)-eta(2) and mu-eta(2) modes respectively. Further thermolysis of cluster 4 in n-octane (125 degrees C) leads to a much higher yield of cluster 5 (60%) at the expense of cluster 4. Chemical activation of cluster 1 using Me(3)NO in CH2Cl2 at -78 degrees C also produces clusters 4 and 5 in 20 and 15% yield respectively. A new, orange product (31%) is also formed but remains uncharacterized. When cluster 4 is allowed to stand under ambient conditions over a period of ten days, it isomerizes to the cluster [Os-3(CO)(9)(mu-dmpymt)(mu-eta(2)-dmpymt)] 6 in almost quantitative yield (87%). Cluster 6 contains one dmpymt ligand co-ordinating in the mu-eta(2) five-electron donating mode. Cluster 5 undergoes carbonylation to form cluster 4 (62%) under a CO atmosphere in CH2Cl2 while cluster 4 also changes back to cluster 3 (47%) in refluxing n-hexane (69 degrees C) under a CO atmosphere. Formation of clusters 3-5 is thus reversible. Mechanisms for the formation of 3, 4 and 6 have also been proposed. All the clusters 1-6 isolated have been fully characterized by conventional spectroscopic methods as well as single-crystal X-ray analyses.