The reaction of the oreanolanthanide complexes Cp'2MCl2 -(ether)2 +, Cp'= η5-(CH3)5C5, M = La. Nd. Sm. and Lu, with LiCH[Si(CH3)3]2provides a straightforward route to the broad class of ether- and halide-free bis(pentamethylcyclopentadienyl) lanthanide alkyls, Cp'2MCH[Si(CH3)3]2. These complexes were characterized by standard analytical methodology. The M = Nd complex crystallizes in the monoclinic space group P21/n with four molecules in a cell of dimensions (-100 °C) a = 11.045 (9) A, b = 23.554 (7) A, c = 11.648 (9) A, and β= 103.10 (6)°. Least-squares refinement led to a value for the conventional R index (on F) of 0.078 for 4287 independent, absorption-corrected reflections having I > 3σ(I). The molecular structure consists of monomeric Cp'2NdCH[Si(CH3)3]2 units with a “bent-sandwich” Cp'2Nd configuration: (ring centroid)-Nd-(ring centroid) = 134.4°. The CH[Si(CH3)3]2 fragment is coordinated to the Cp'2Nd group in a highly unsymmetrical fashion with a Nd-C σ-bond distance of 2.517 (7) A and a rather short secondary contact to a single methyl carbon atom of 2.895 (7) A. Spectroscopic data suggest that this interaction is higly metal-to-carbon in character. The Cp'2NdCH[Si(CH3)3]2 complexes react with H2 under mild conditions to yield the corresponding hydrides (Cp'2MH)2. These complexes are extremely active homogeneous ethylene polymerization catalysts with turnover frequencies exceeding 1800 s-1(25 °C, 1 atm of ethylene pressure) for M = La. Activities are in the order La ≥ Nd » Lu. The kinetics of this process and the molecular weight characteristics of the resulting polymer are severely limited by mass transport effects; evidence is presented that these are “living” polymers and that chain transfer by β-hydride elimination is not important. For La and Nd, propylene and 1-hexene react rapidly at -10 °C to yield equal quantities of the disproportionation products alkane + metal η3-alkenyl. Butadiene undergoes reaction with these hydrides to yield η3-crotyl complexes, and significant polymerization does not occur. For 1-hexene under low H2 (or D2) pressure at room temperature, some formation of 2-methylundecane is also observed. The analogous reaction of cyclohexene with these hydrides is sluggish and appears to produce an ηl-cyclohexenyl complex. There is no evidence for cyclohexene oligomers. Although unreactive toward olefins, Cp'2NdCH[Si(CH3)3]2reacts with CO to yield a dimeric dionediolate.
ASJC Scopus subject areas
- Colloid and Surface Chemistry