Addition of Alkynes to the Metal-Metal Double Bond in (η-C₅Me,₅)₂Rh₂(µ-CO)₂; Crystal and Molecular Structure of the Complex (η-C₅Me₅)₂Rh₂(µ-CO){µ-C(O)C₂(CF₃)₂}.

Abstract

Various alkynes (EtC₂Et, MeO₂CC₂CO₂Me, CF₃C₂CF₃, PhC₂Ph, C₆F₅C₂C₆F₅, PhC₂Me, PhC₂C₆-F₅, PhC₂CO₂Me) react with (η-C₅Me₅)₂Rh₂(μ-CO)₂ in acetone at room temperature; no reaction was observed with Bu^tC₂Bu^t. The σ-bridging alkyne complex, trans-(η-C₅Me₅)₂Rh₂(CO)₂(μ-η¹:η¹C₆F₅C₂C₆F₅), was obtained from the reaction with C₆F₅C₂C₆F₅. With the other alkynes, dimetallaeneone complexes, (η-C₅Me₅)₂Rh₂(μ-CO){μ-η²:η²-C(O)C₂RR′}, are formed. Some of these 'eneone' complexes co-exist with (η- C₅Me₅)₂Rh₂(CO)₂(μ-η¹:η¹-RC₂R¿) in the solid state (RC₂R¿ = PhC₂C₆F₅) and/or in solution (EtC₂Et, CF₃C₂CF₃, PhC₂C₆F₅); others (MeO₂CC₂CO₂Me, PhC₂Ph, PhC₂Me, PhC₂CO₂Me) exist exclusively as (η-C₅Me₅)₂Rh₂(μ-CO){μ-η²:η²- C(O)C₂RR′} in both the solid and solution states. The geometry of the bridging group in (η-C₅Me₅)₂Rh₂(μ-CO){μ-η²:η²-C(O)C₂(CF₃)₂} has been determined from an X-ray diffraction study. The compound crystallizes with four molecules in the monoclinic space group P2₁/n in a unit cell of dimensions a 9.451(4), b 15.287(5), c 18.821(8)Ǻ, β 98.66(5)°. The structure was solved by conventional heavy atom methods and refined to R 0.066 based on 4356 observed reflections above background. The structure contains a metalla-eneone ring, Rh -C(=O)-C(CF₃)=C(CF₃), with the alkene bond η²-attached to the second rhodium atom. Variable temperature N.M.R . measurements establish that the complexes (η- C₅Me₅)₂Rh₂(μ-CO){μ-η²:η²-C(O)C₂R₂}(R = CF₃ or CO₂Me) are fluxional in solution. Facile cleavage of a C(O)-C(R) bond enables the metalla-eneone ring to shift rapidly from one rhodium atom to the other.

Other products formed in the reactions between (η-C₅Me₅)₂Rh₂(μ-CO)₂ and RC₂R′ include. (η-C₅Me₅) Rh {C₄(CF₃)₄CO}, (η-C₅Me₅) Rh (η⁴-C₆R₆) (R = CF₃ or CO₂Me), (η-C₅Me₅)₂Rh_2-(C₄R₂R′₂) (R = R′ = CO₂Me; or R, R′ = Ph, C₆F₅), (η- C₅Me₅)₂Rh₂(CO₂C₂Ph₂), (η-C₅Me₅)- Rh (C₄Ph₄CO₂), (η-C₅Me₅)₂Rh₂(CO)₂{C₄(CF₃)₄}, (η-C₅Me₅)₂Rh₂(CO)(μ-CO){COC₄(C₆F₅)₄} and C₆R₃R′₃ (R = R′ = Ph or CO₂Me; R = Ph and R′ = Me). Reactions between (η-C₅Me₅)_2-Co₂(μ-CO)₂ and alkynes at room temperature or above yield mononuclear cyclopentadienone complexes (η-C₅Me₅)Co{C₄R₄CO}(R = Me, CF₃ or C₆F₅), and the mononuclear arene complex (η-C₅Me₅)Co{C₆(CF₃)₆}.