ENCAPSULATED ALKALINE-EARTH METALLOCENES - SYNTHESIS, SOLUTION BEHAVIOR, AND SOLID-STATE STRUCTURES OF BIS(TETRAISOPROPYLCYCLOPENTADIENYL)CALCIUM AND BIS(TETRAISOPROPYLCYCLOPENTADIENYL)BARIUM, [(C3H7)4C5H]2CA AND [(C3H7)4C5H]2BA

Reaction of 2 equiv of KCp4i (Cp4i = (C3H7)4C5H) with CaI2 in diethyl ether or BaI2 in THF produces the metallocenes (Cp4i)2Ca and (CP4i)2Ba in 77 and 83% yields, respectively. The calcium metallocene (Cp4i)2Ca displays a remarkable degree of air stability and can be handled in dry air for several minutes without decomposition. It displays virtually no tendency to form adducts in solution with ethers or aromatic amines. The barium metallocene is the most volatile known organobarium compound, subliming at 90-degrees-C and 0.01 Torr. These physical properties of the metallocenes are most likely related to the ability of the isopropyl groups of the Cp4i rings to interlock, forming ''cages'' around the metal centers. DNMR experiments have provided evidence for hindered rotation in the complexes, either around the ring-C-CHMe2 bonds or around the metal-ring centroid axis, with energies of activation of DELTA-G less-than-or-equal-to 11.1 kcal mol-1 in (Cp4i)2Ca and 11.1 (+/- 0.5) kcal mol-1 in (Cp4i)2Ba. Conformations of the Cp4i anions were probed with semiempirical MO (AM1) calculations and found to be consistent with the solid-state structures. Crystals of (Cp4i)2Ca grown from hexane are monoclinic, space group C2/c, with a = 34.033 (6) angstrom, b = 12.432 (6) angstrom, c = 16.495 (5) angstrom, beta = 111.29 (2) degrees, and D(c) = 1.036 g cm-3 for Z = 8. Least-squares refinement on the basis of 2102 observed reflections measured at 23-degrees-C led to a final R value of 0.044. The average Ca-C distance is 2.64 (1) angstrom, and the ring centroid-Ca-ring centroid angle is 162-degrees. Crystals of (Cp4i)2Ba grown from hexane are triclinic, space group P1BAR, with a = 16.682 (4) angstrom, b = 17.862 (2) angstrom, c = 12.396 (4) angstrom, a = 108.02 (3) degrees, beta = 90.56 (3) degrees, gamma = 111.12 (2)degrees, and D(c) = 1.236 g cm-3 for Z = 4. Least-squares refinement on the basis of 6021 observed reflections measured at -120-degrees-C led to a final R value of 0.039. Two crystallographically independent but nearly identical molecules occur in the unit cell. The average Ba-C distance is 2.94 (2) angstrom, and the ring centroid-Ba-ring centroid angle is 154-degrees.