ETA-5[--]ETA-3 HAPTICITY INTERCONVERSION IN CYCLOHEPTADIENYL MOLYBDENUM COMPLEXES

A series of reactions of the general type [Mo(CO)2L2(eta5-C7H9)]+ + L' --> [Mo(CO)2(L')L2(eta3-C7H9)]+, involving eta5 --> eta3 hapticity conversion at a molybdenum co-ordinated cycloheptadienyl ring, have been examined. The specific examples investigated comprise reactions of [Mo(CO)2L2(eta5-C7H9)]+ [L2 = 2,2'-bipyridyl (bipy) 1, Ph2PCH2PPh2 (dppm) 6, Ph2PCH2CH2PPh2 (dppe) 7; L = CNBu(t) 17 or PPh3 18] with NCMe or CNBu(t). Complex 1 reacts reversibly with NCMe to give [Mo(CO)2-(NCMe)(bipy)(eta3-C7H9)]+ 2 and, in NCMe, 7 exists as an equilibrium mixture with [Mo(CO)2(NCMe) (dppe) (eta3-C7H9)]+ 8. The complexes 6, 17 and 18 do not react with NCMe at ambient temperature. Treatment of 1, 6 and 7 with CNBu(t) affords [Mo(CO)2(CNBu(t))L2(eta3-C7H9)]+ (L2 = bipy 4, dppm 9 or dppe 11) although 4 is better prepared by reaction of [Mo(CO)2-(CNBu(t))(NCMe)2(eta-C7H9)]+ 5 with bipy. Complex 4 is resistant to carbonyl elimination but 9 and 11 undergo facile loss of CO to give [Mo(CO)(CNBu(t))L2(eta5-C7H9)]+ (L2 = dppm 10 or dppe 12) so establishing examples of associative ligand-substitution processes which proceed with eta5 --> eta3 --> eta5 hapticity interconversion at the C7H9 dienyl ligand. The reaction between 17 and CNBu(t) affords [Mo(CO)(CNBu(t))3(eta-C7H9)]+ 19 with no observable eta3-C7H9 intermediate but 19 reacts with further CNBu(t) to yield [Mo(CO)(CNBu(t))4(eta3-C7H9)]+ 20. The crystal structures of complexes 1 and 4 have been determined. Complex 1 adopts an asymmetric ligand arrangement similar to that previously described for 6. Complex 4 has a pseudo-octahedral molecular geometry in which CNBu(t) is located trans to the eta3-bonded C7H9 ligand.