Ring-Opening Polymerization Behavior of <i>ansa</i>- and Spirocyclic <i>ansa</i>-Zirconocene Complexes

Attempts to develop a ring-opening polymerization (ROP) approach to polymeric zirconocene complexes are reported. A new silicon-bridged [1][1]zirconocenophane, (SiMe2)2(η5-C5H3)2Zr(NEt2)2 (3a), was synthesized by reaction of Zr(NEt2)4 with (C5H4)2(SiMe2)2 (4). Single-crystal X-ray diffraction revealed a highly tilted structure with an angle of 73.1(4)°, a significant increase over the untethered compound (η5-MeC5H4)2ZrCl2 (2), which possesses a tilt angle of 54.2°. This and the related compounds (SiMe2)(η5-C5H4)2MCl2 (1a, M = Ti; 1b, M = Zr) and (SiMe2)2(η5-C5H3)2ZrCl2 (3b) were tested for transition-metal catalyzed ROP behavior in the presence of Pt(0) catalysts but were found to be inactive. The novel spirocyclic silacyclobutane-bridged monomer (CH2)3Si(η5-C5H4)2ZrCl2 (7b) was prepared via a “fly trap” amine elimination reaction between cyclotrimethylenedicyclopentadienylsilane (8) and Zr(NMe2)4; this initially yielded (CH2)3Si(η5-C5H4)2Zr(NMe2)2 (7a), which was converted to 7b by reaction with a slight excess of Me3SiCl. 7a and 7b were characterized by 29Si, 13C, and 1H NMR spectroscopy and mass spectrometry as well as single-crystal X-ray diffraction. Both showed moderate tilt angles (61.7(2)° and 59.69(7)°, respectively) as well as highly strained silacyclobutane rings with C−Si−C bond angles of 79.7(2)° and 80.64(10)°, respectively. Transition-metal-catalyzed ROP of 7b led to the formation of the polycarbosilane [(CH2)3Si(η5-C5H4)2ZrCl2]n (10). Both the soluble and insoluble fractions of 10 were characterized by CP-MAS 29Si and 13C NMR, and the soluble, oligomeric fraction was further characterized by solution 29Si, 13C, and 1H NMR. Complexes 7a, 7b, and polyzirconocene 10 were found to exhibit moderate activity as ethylene polymerization catalysts.