BIS(CYCLOPENTADIENYL)ZIRCONIUM AND HAFNIUM MACROCYLES CONTAINING SILOXANE BRIDGES - SYNTHESIS OF [(ETA-5-C5H5)2M(ETA-OSI(C6H5)2O)]2 (M=ZR, HF) AND CRYSTAL-STRUCTURE AND EHMO STUDY FOR M=ZR

The reaction of Cp2MMe2 (M = Zr, Hf) with Ph2Si(OH)2 yields the eight-membered ring [Cp2M(mu-OSiPh2O)]2 (M = Zr (1), Hf (2)). 1 crystallizes in the space group P1BAR with a = 10.410(9) angstrom, b = 12.140(6) angstrom, c = 16.318(5) angstrom, alpha = 94.26(3)degrees, beta = 103.42(5)degrees, gamma = 93.16(6)degrees, and Z = 2. The crystal structure reveals that the ring is not planar. The Cp2Zr units are bent 20-degrees out of the Si2O4 Plane to give a ''chaise longue conformation; however, in solution the molecule is fluxional on the NMR timescale. The zirconium-oxygen distances of 1.974(2) and 1.983(2) angstrom are significantly longer than those reported in other Zr-O-Si compounds. EHMO calculations suggest that the Zr-O bond order may be rationalized in terms of an in-plane pi-system.