Synthesis, structural characterisation and reactivity of molybdenum half-sandwich complexes containing keto- and amido-phosphines

The keto-functionalised N-pyrrolyl phosphine ligand PPh2NC4H3{C(O)CH3-2} L1 reacts with [MoCl(CO)3(η5-C5R5)] (R=H, Me) to give [MoCl(CO)2(L1-κ1P)(η5-C5R5)] (R=H 1a; Me 1b). The phosphine ligands PPh2CH2C(O)Ph (L2) and PPh2CH2C(O)NPh2 (L3) react with [MoCl(CO)3(η5-C5R5)] in an analogous manner to give the compounds [MoCl(CO)2(L-κ1P)(η5-C5R5)] (L=L2, R=H 2a, Me 2b; L=L3, R=H 3a, Me 3b). Compounds 1–3 react with AgBF4 to give [Mo(CO)2(L-κ2P,O)(η5-C5R5)]BF4 (L=L1, R=H 4a, Me 4b; L=L2, R=H 5a, Me 5b; L=L3, R=H 6a, Me 6b) following displacement of chloride. The X-ray crystal structure of 4a revealed a lengthening of both Mo–P and CO bonds on co-ordination of the keto group. The lability of the co-ordinated keto or amido group has been assessed by addition of a range of phosphines to compounds 4–6. Compound 4a reacts with PMe3, PMe2Ph and PMePh2 to give [Mo(CO)2(L1-κ1P)(L)(η5-C5H5)]BF4 (L=PMe37a; PMe2Ph 7b; PMePh27c) but does not react with PPh3, 5a reacts with PMe2Ph, PMePh2 and PPh3 to give [Mo(CO)2(L2-κ1P)(L)(η5-C5H5)]BF4 (L=PMe2Ph 8b; PMePh28c; PPh38d), and 6a reacts with PMe3, PMe2Ph, PMePh2 and PPh3 to give [Mo(CO)2(L3-κ1P)(L)(η5-C5H5)]BF4 (L=PMe310a; PMe2Ph 10b; PMePh210c; PPh310d). No reaction was observed for the pentamethylcyclopentadienyl compounds 4b–6b with PMe3, PMe2Ph, PMePh2 or PPh3. These results are consistent with the displacement of the co-ordinated oxygen atom being influenced by the steric properties of the P,O-ligand, with PPh3 displacing the keto group from L2 but not from the bulkier L1. In the reaction of [Mo(CO)2(L2-κ2P,O)(η5-C5H5)]BF4 (5a) with PMe3 the phosphine does not displace the keto group, instead it acts as a base, with the only observed molybdenum-containing product being the enolate compound [Mo(CO)2{PPh2CHC(O)Ph-κ2P,O}(η5-C5H5)] 9. Compound 9 can also be formed from the reaction of 2a with BuLi or NEt3, and a single crystal X-ray analysis has confirmed the enolate structure.