RING-OPENING VERSUS PHENYL PHOSPHORUS BOND-CLEAVAGE IN INCORPORATING A PHOSPHOLE INTO TRIOSMIUM CLUSTERS

The five-membered heterocyclic compound 3,4-dimethyl-1-phenylphosphole (Ph activated PCH = CMeCMe = CH) reacts with [Os3(CO)11(MeCN)] or [Os3(CO)10(MeCN)2] to give the simple substitution products [Os3(CO)12-x(PhPC4H2Me2)x] (x = 1 or 2) in which the phospholes are co-ordinated as tertiary phosphines through the phosphorus atoms. Thermolysis of these compounds gives decarbonylation compounds containing modified phosphole ligands. Phosphorus-carbon bonds either to the phenyl group or within the five-membered ring have been cleaved. The X-ray structure of the main product [Os3(CO)9(mu-3-PhPCHCMeCMeCH)] 1 shows that the organic mu-3 ligand is a six-electron donor which is co-ordinated differently from the corresponding eight-electron donating ligand derived from 1-phenylphosphole in the cluster [Os3(CO)9(mu-3-PhPCHCHCHCH)]. There is evidence for the reversible generation of this alternative form, 3, of the methylated compound 1 by photolysis. A minor product of the thermolysis is the hydrido cluster [Os3(mu-H)(mu-activated PCH = CMeCMe = CH)(mu-3-C6H4)(CO)9] 2 which contains a five-membered phospholyl ring and an omicron-phenylene (benzyne) ligand which originated from the phenyl group. The X-ray structure shows that the phospholyl ligand bridges two osmium atoms as a phosphido ligand through the phosphorus atom. The major and minor products, 1 and 2, are both derived by C-P bond cleavage, either in the phosphole ring or to the phenyl substituent respectively.