Structure, bonding, and reactivity of molybdenum η3-cyclohexenone complexes in comparison with their cyclopentenone analogues: η3-allyl/η4-diene conversion‡

Abstract

The neutral η³-cyclohexenone complexes [Mo(η³-C₆H₇O)(CO)₂(MeCN)₂Br] 1, [Mo(η³-C₆H₇O)(CO)₂{HB(pz)₃}] 2, [Mo(η³-C₆H₇O)(CO)₂(bipy)Br] 3 (bipy =  2,2′-bipyridine) and [Mo(η³-C₆H₇O)(CO)₂(dppm)Br] 4 (dppm = Ph₂PCH₂PPh₂) have been synthesized. The structure of 2 has been determined by X-ray crystallography. All these complexes resist hydride abstraction using Ph₃C⁺PF₆^–, in sharp contrast to the η³-cyclopentenone analogues where η³-allyl/η⁴-diene conversion is a facile process. A rationale for this different behaviour is provided by extended-Hückel calculations combined with a Walsh analysis of hydrogen abstraction. Thus, while in the η³-C₅H₅O → η⁴-C₅H₄O conversion a Möbius system is formed upon release of hydride via electrophilic attack, this is not possible in the hypothetical η³-C₆H₇O → η⁴-C₆H₆O process. Therefore, η³-C₆H₇O is a C–H acid. Also, the occurrence of different conformations, exo for allyl and endo for diene complexes, is rationalized.