For this research, the title compound has been prepared by the reaction of tris(cyclopentadienyl) uranium chloride (UCp3Cl) and sodium pyrazolate (NaC3H3N2) in tetrahydrofuran (THF). The compound has been characterized by its infrared, visible-near IR, pmr, and mass spectra; and by single crystal X-ray diffraction. The molecular structure consists of discrete UCp3(C3H3N2) molecules in which the uranium(IV) ion is coordinated by three η5-Cp rings in a nearly trigonal array similar to that of other lanthanide and actinide MCp3X structures. However, unlike these other structures, an eleven-coordinate geometry is achieved by having both the adjacent nitrogen atoms of the pyrazolate ring coordinate to the metal ion, such that the local twofold axis of the pyrazolate ring and the local threefold axis of the UCp3 fragment coincide. This is the first example of this type of endo-bidentate η2 coordination for the pyrazolate anion. Red-brown crystals from toluene conform to space group P21/a (an alternate setting of P21/c) with a = 14.295(1)Å, b = 8.383(1)Å, c = 14.282(1)Å, β = 112.80(1)⁰, and there are four molecules per unit cell. For the 3631 independent reflections collected by counter methods with F2 > 3σ(F2). the final weighted and unweighted R factors are both 3.14 percent. The U-N distances are 2.40(1) and 2.36(1)Å. The average U-C distance of 2.76 Å is consistent with that predicted for an eleven-coordinate uranium(IV) cyclopentadienyl complex. The 90 MHz proton resonances of the Cp rings occur at -9.51 (35°C), -11.59 (-10°C), -14.22 (-50°C), and -15.84 (-75°C) ppm. Finally, susceptibility studies from 5 to 80 K show a temperature-independent paramagnetism from 7.5 to 37.1 K; above this temperature μeff = 2.67 μB·
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Eigenbrot et al. (1981) studied this question.