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.