This contribution describes lutetium and yttrium hydrocarbyl and hydride chemistry based upon the chelating R2Si(η5-C5H4)(η 5-Me4C5)2- ligand (R = Me, Et; abbreviated R2SiCpCp″). The ligand is prepared by reaction of the corresponding R2Si(Cp″)Cl derivative with NaC5H5. Subsequent metalation and reaction with MCl3·3THF (M = Y, Lu) yields R2SiCpCp″MCl2-Li(OEt2)2 + complexes, which in turn can be alkylated to yield R2SiCpCp″MCHTMS2 derivatives (TMS = SiMe3). Pertinent crystallographic data for Me2SiCpCp″LuCHTMS2 at -120 °C: P1 (no. 2), z = 4, a = 16.049 (3) Å, b = 17.945 (4) Å, c = 8.993 (3) Å, α = 93.36 (2)°, β= 90.92 (2)°, and γ = 82.54 (2)°; A(F) = 0.030 for 6085 independent reflections with I > 3σ(I). The structure is of a "bent-sandwich" Cp′2MX-type (Cp′ = η5-Me5C5) with relaxed interligand nonbonded interactions vis-â-vis the Cp′2M and Me2SiCp″2M analogues (Lu-CHTMS2 = 2.365 (7) Å) and having one close Lu⋯MeSi (Lu-C = 2.820 (8) Å) secondary interaction. These alkyls initiate the polymerization of ethylene and undergo relatively slow hydrogenolysis to yield dihydrides of stoichiometry (R2SiCpCp″MH)2 via detectable intermediates of stoichiometry (R2SiCpCp″)2M2(H)(CHTMS2). Pertinent crystallographic data for (Et2SiCpCp″LuH)2 at -120 °C: p21/n (no. 14), z = 2, a = 11.558 (3) Å, b = 8.590 (2) Å, c = 18.029 (3) Å, β= 100.10 (2) Å, R(F) = 0.022 for 2656 independent reflections with I > 3σ(I). The structure has an idealized C2h,Lu(μ-Et2SiCpCp″)2(μ-H) 2Lu geometry with both bridging Et2SiCpCp″ and hydride ligands (Lu-H = 2.16 (4), 2.13 (4) Å). These complexes react slowly (compared to monomeric Cp′2MH and Me2SiCp″2MH), reversibly, and regiospecifically with α-olefins to form bridging alkyls of structure M(μ-R2SiCpCp″)2(μ-H)(μ-R′)M, R′ = ethyl, n-propyl, n-hexyl. Pertinent crystallographic data for Lu(μ-Et2SiCpCp″)2(μ-H)(μ-CH 2CH3)Lu at -120 °C: P21/c (no. 14), z = 6, a= 11.679 (4) Å, b = 25.755 (5) Å, c = 18.074 (2) Å, β= 99.41 (2)°; A(F) = 0.058 for 4643 independent reflections with I > 3σ(I). The Lu(μ-Et2SiCpCp″)2(μ-H)Lu framework is nearly identical to that in the dihydride above. The μ-ethyl fragment is bound very unsymmetrically with Lu-C(α) = 2.46 (2) and 2.58 (2) Å, 〈 Lu-C(α)-C(β) = 148 (1)° and 84.7 (5)°. In addition, Lu-C(β) = 2.78 (2) Å suggests a strong secondary bonding interaction. Hydrogenolysis of the μ-alkyl linkages is considerably slower than for terminal alkyl bonds in Cp′2M(alkyl) and Me2SiCp″2M(alkyl) complexes. NMR studies of the μ-alkyls reveal rapid rotation of the μ-alkyl ligands about the μ-H-μ-C(α) vectors down to -85 °C and rapid inversion at C(α) occurring with ΔG‡ = 12.5-13.5 kcal/mol (Tc = +11- +39 °C). Kinetic (rate law: ν = k[dihydride][olefin]) and equilibration measurements reveal that the hydride addition process to 1-hexene (Et2SiCpCp″LuH)2 + 1-hexene ⇄ Lu(μEt2SiCpCp)2(μ-H)(μ-n-hexyl)Lu is described by ΔH = -10.7 (6) kcal/mol, ΔS = -25 (2) eu, ΔH‡ = 12.0 (4) kcal/mol, and ΔS‡ = -38.6 (7) eu. These results indicate that, in comparison to terminal bonding modes with similar metal ancillary ligation, lanthanide μ-H ligands are kinetically deactivated with respect to olefin insertion (a rate depression of ∼ 108-1010), and μ-alkyl ligands are kinetically deactivated with respect to hydrogenolysis (a rate depression of ∼108-109). Moreover, relative to a bridging hydride ligand, lanthanide μ-alkyl coordination is found to be no more and probably less thermodynamically stable than terminal alkyl coordination.
|Number of pages||18|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - Dec 19 1990|
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