Synthesis and Characterization of Fluorinated Tris(pyrazolyl)borate Complexes. Observation of an (η⁵-Pyrazole)−K⁺ Interaction in the Solid State

Cyclopentadienyl (Cp) ligands have received considerable attention mainly because of their π six-electron donation capability. Tris(pyrazolyl)borate ligands (Tp) are often compared with Cp because of their identical charge, number of donated electrons, and similar facial coordinating geometry. Their six-electron donation, however, is formally σ-type. The X-ray structure of the [TpCF3,CH3CuK(μ4-CO3)KTpCF3,CH3]2 aggregate, 1, reveals for the first time an unprecedented η5-TpCF3,CH3 potassium bonding interaction. 1 crystallizes in the triclinic space group P1̄ with a = 12.0411(2) Å, b = 14.9791(2) Å, c = 16.0567(3) Å, α = 71.301(1)°, β = 69.785(1)°, γ = 66.539(1)°, and Z = 2. Both K−F and K−μ4-CO32- interactions stabilize the aggregate, as suggested by the lack of hexanuclear aggregation and K incorporation in the absence of fluorine groups or when OCO22- is replaced by CH3−CO2-. In the latter case we have isolated the complex [CuTpCF3,CH3(CH3CO2)], 2, which retains a Cu coordination sphere similar to that encountered in the TpCF3,CH3Cu subset of 1. The mononuclear complex 2 crystallizes in the monoclinic space group P2(1)/c with a = 14.1474(2) Å, b = 14.1474(2) Å, c = 19.0456(6) Å, β = 99.012(2)°, and Z = 4. The novel η5-coordination mode revealed in 1 suggests that Tp ligands might function not only as σ donors but also as Cp-like π donors. The η5-coordination mode might therefore constitute a new potential common denominator of these two important classes of ligands.