Synthesis and reactions of acyl(cyclohexadienyl)manganates

Abstract

The cyclohexadienyl complexes [Mn(η⁵-C₆H₆R¹)(CO)₃]1(R¹=exo-H, Me or C₆H₄Me-4) react with LiR²(R²= Me or Ph) to give acylmetalates [Li(OEt₂)][Mn(η⁵-C₆H₆R¹)(CO)₂{C(O)R²}]2. Complex 2(R¹= Me, R²= Ph) has been characterised by an X-ray crystallography study: orthorhombic, space group P2₁2₁2₁, a= 9.113(2), b= 14.491(8), c= 32.803(9)Å, R= 0.064 for 1458 independent reflections. Protonation of 2 with HBF₄·Et₂O induces an aryl- or alkyl-group transfer to the endo face of the cyclohexadienyl ring yielding the new agostic cyclohexenyl complexes [Mn(η^3:CHC₆H₇R¹R²)(CO)₃]3. The cyclohexenyl ligands can be decomplexed from the metal via treatment of 3 with 1,2-bis-(diphenylphosphino)ethane (dppe) in tetrahydrofuran to afford mixtures of substituted cyclohexa-1,3-dienes and [MnH(CO)₃(dppe)]. The acylmetalates 2 react with [NO][BF₄] at –78 °C to give [Mn(η⁵-C₆H₆R¹)(CO)(NO){C(O)R²}] which decompose at room temperature to form the trans-disubstituted acylcyclohexadienes C₆H₆R¹[C(O)R²]. Reaction of 2 with electrophiles SiMe₃Cl, [Me₃O][BF₄] or [Et₃O][BF₄] results in O alkylation and formation of stable carbene complexes [Mn(η⁵-C₆H₆R¹)(CO)₂{C(OE)R²}](E = SiMe₃, Me or Et).