Transition metal–diene complexes. Part 4. The crystal and molecular structure and isomerization of η-cyclopentadienyl(η-3-methoxycarbonylcyclohexa-1,4-diene)rhodium(I)

Abstract

Crystals of the title compound, [Rh(C₅H₆)(C₅H₇CO₂Me)], are monoclinic, space group I2/a with cell dimensions a= 28.124(14), b= 6.141(8), c= 14.569(11)Å, β= 112.43(9)°, and Z= 8. The intensities of 1 325 reflections above background were collected by counter methods and refined to R 0.052. The rhodium atom is bonded to the five carbon atoms of the planar cyclopentadienyl ring [Rh–C 2.197(10)–2.262(13)Å] and to the four olefinic carbon atoms [Rh–C 2.125(13)–2.138(12)Å] of the cyclohexa-1,4-diene. The cyclohexa-1,4-diene ring is in the boat conformation with the two saturated carbon atoms lying 0.66, 0.70 Å above the plane of the four olefinic carbons in a direction away from the metal. The carbomethoxy-subtituent is planar and occupies an axial position with respect to the cyclohexadiene boat. The title complex undergoes a kinetically controlled isomerization when heated in toluene or acid-free o-dichlorobenzene to a mixture of complexes containing co-ordinated 5-methoxycarbonyl-(major product) and 2-methoxycarbonyl-cyclohexa-1,3-diene (minor product). In the presence of acid in o-dichlorobenzene the major product undergoes a secondary isomerization to the complex containing co-ordinated 1-methoxycarbonylcyclohexa-1,3-diene. For the primary isomerization a mechanism involving the formation of a hydrido-η³-allylic intermediate is proposed in which the rate-determining step is a carbon-to-metal hydride-ion transfer. For the secondary isomerization, in the presence of acid, a reverse metal-to-carbon hydrogen transfer is believed to operate.