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 pi 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 sigma-type. The X-ray structure of the [TpCF3,CH3CuK(mu 4-CO3)KTpCF3,CH3]2 aggregate, 1, reveals for the first time an unprecedented eta 5-TpCF3,CH3 potassium bonding interaction. 1 crystallizes in the triclinic space group P1 with a = 12.0411(2) A, b = 14.9791(2) A, c = 16.0567(3) A, alpha = 71.301(1) degrees, beta = 69.785(1) degrees, gamma = 66.539(1) degrees, and Z = 2. Both K-F and K-mu 4-CO3(2-) interactions stabilize the aggregate, as suggested by the lack of hexanuclear aggregation and K incorporation in the absence of fluorine groups or when O=CO2(2-) 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) A, b = 14.1474(2) A, c = 19.0456(6) A, beta = 99.012(2) degrees, and Z = 4. The novel eta 5-coordination mode revealed in 1 suggests that Tp ligands might function not only as sigma donors but also as Cp-like pi donors. The eta 5-coordination mode might therefore constitute a new potential common denominator of these two important classes of ligands.