η5↔η3 Hapticity interconversion in cycloheptadienyl molybdenum complexes

Abstract

A series of reactions of the general type [Mo(CO)₂L₂(η⁵-C₇H₉)]⁺+ L′→[Mo(CO)₂(L′)L₂(η³-C₇H₉)]⁺, involving η⁵→η³ hapticity conversion at a molybdenum co-ordinated Cycloheptadienyl ring, have been examined. The specific examples investigated comprise reactions of [Mo(CO)₂L₂(η⁵-C₇H₉)]⁺[L₂= 2,2′-bipyridyl (bipy)1, Ph₂PCH₂PPh₂(dppm)6, Ph₂PCH₂CH₂PPh₂(dppe)7; L = CNBu^t17 or PPh₃18] with NCMe or CNBu^t. Complex 1 reacts reversibly with NCMe to give [Mo(CO)₂(NCMe)(bipy)(η³-C₇H₉)]⁺2 and, in NCMe, 7 exists as an equilibrium mixture with [Mo(CO)₂(NCMe)(dppe)(η³-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(η³-C₇H₉)]⁺(L₂= bipy 4, dppm 9 or dppe 11) although 4 is better prepared by reaction of [Mo(CO)₂(CNBu^t)(NCMe)₂(η³-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(η⁵-C₇H₉)]⁺(L₂= dppm 10 or dppe 12) so establishing examples of associative ligand-substitution processes which proceed with η⁵→η³→η⁵ hapticity interconversion at the C₇H₉ dienyl ligand. The reaction between 17 and CNBu^t affords [Mo(CO)(CNBu^t)₃(η⁵-C₇H₉)]⁺19 with no observable η³-C₇H₉ intermediate but 19 reacts with further CNBu^t to yield [Mo(CO)(CNBu^t)₄(η³-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 η³-bonded C₇H₉ ligand.