Synthesis, characterization, and reactivity of organometallic complexes of early and late metals and the functionalization of polydienes

Abstract

<p>A series of rhodium and iridium organometallic complexes supported by 1-cyclopentadienyl-1,1-bis(4,4-dimethyl-2-oxazolinyl)ethane (MeC(OxMe2)2(C5H4); BoMCp) and 1-fluorenyl-1,1-bis(4,4-dimethyl-2-oxazolinyl)ethane (MeC(OxMe2)2(C13H8); BoMFlu) are described. Metalation of BoMCp readily occurred through salt metathesis from a thallium intermediate or protonolysis with appropriate metal precursors. In contrast, metalation of the more basic BoMFlu ligand required in situ generation of a potassium carbanion with potassium benzyl and salt metathesis with appropriate metal precursors. The piano-stool complexes of BoMCpM (M = Rh or Ir) were unreactive to substitution chemistry and forcing condition required for C–H activation reactions resulted in decomposition of catalysts prior to successful reactions. However, the two-electron oxidation of BoMCpRh(C2H4)2 with Br2 results in BoMCpRhBr2, a new RhIII species. BoMFluM (M = Rh or Ir) complexes readily underwent substitution chemistry. In addition, BoMFluRhL2 (L2 = C8H12 or C16H12) displayed unique electrochemistry of two reversible 1-electron oxidations. The RhII species could be generated in solution with life time greater than 24 hours for L2 = C16H12.</p> <p>A series of lanthanide organometallic complexes, Ln{C(SiHMe2)3}3 (Ln = La, Ce, Pr, Nd) were activated by the abstraction of Si–H with 1 and 2 equivalents B(C6F5)3 to generate Ln{C(SiHMe2)3}2HB(C6F5)3 and Ln{C(SiHMe2)3{HB(C6F5)3}2 respectively. The addition of AlR3 (R = Me or iBu) to Ln{C(SiHMe2)3}3 resulted in the complexation of the weaker lewis acid rather than Si–H abstraction. The hydridoborate alkyl complexes, with AliBu3 co-catalysts, are active in butadiene polymerization. Neodymium and cerium were shown to be the most active and all lanthanides showed a ~50:50 selectivity for cis-1,4:trans-1,4 insertions, with exception of lanthanum which showed a slightly higher selectivity for trans-1,4. Further studies with precatalysts Nd{C(SiHMe2)3}3 indicate a polymerization with living character with respect to reaction time but also showed a dependence of molecular weight on Nd:AliBu3 ratio supporting the proposed chain transfer mechanism. Polymerization in saturated hydrocarbon solvent (heptane) improved cis-1,4 selectivity to nearly 90% with Nd{C(SiHMe2)3}3 pre-catalyst.</p> <p>Nd{C(SiHMe2)3{HB(C6F5)3}2 with 10 equivalents AliBu3 was >95% selective in the polymerization of isoprene to cis-1,4 polyisoprene with good activity in toluene. In contrast, Nd{C(SiHMe2)3}2HB(C6F5)3 was less active in the polymerization of isoprene and displayed a lower selectivity yielding ~40% trans-1,4 polyisoprene. The activation of Nd{C(SiHMe2)3}3 in toluene with different protocols containing the organochloride Ph3CCl were also successful for cis-1,4 selective polymerizations of isoprene. In addition, activation protocols with Ph3CCl improved the cis-1,4 selectivity of polybutadiene to 90% and above. Attempts to utilize Ph3CCl based activations in heptane resulted in highly reactive polymerization ultimately isolating gelled polybutadiene.</p> <p>Functionalizing agents for the functionalization of polydienes were synthesized from modified literature procedures. The modifications allowed for higher yields for compounds such as (EtO)3Si-CC-Si(OEt)3. The application of (EtO)3Si-CC-Si(OEt)3 as a quenching agent for neodymium-based diene polymerizations resulted in the incorporation of silyl functionalized polydiene and improved physical properties of the resulting rubber composites. 2-Me2XSi-1,3-C4H5 (X = OiPr, OtBu, and NiPr2) monomers were also synthesized and studied for the functionalization of polydienes. No functional group incorporation during neodymium-based polymerizations was observed for X = OiPr or OtBu. However, when X = NiPr2 SiMe groups were observed to be incorporated into polydienes indicating successful functionalization.</p>