Manipulation of Organolanthanide Coordinative Unsaturation. Synthesis, Structures, Structural Dynamics, Comparative Reactivity, and Comparative Thermochemistry of Dinuclear μ-Hydrides and μ-Alkyls with [μ-R₂SiC(Me₄C₅)(C₅H₄)]₂ Supporting Ligation

This contribution describes lutetium and yttrium hydrocarbyl and hydride chemistry based upon the chelating R₂Si(η⁵C₅H₄)(η⁵Me₄C₅)^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 MCI₃-3THF (M = Y, Lu) yields R₂SiCpCp^”MCI₂-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: PI (no. 2), z = 4, a = 16.049 (3) A, b = 17.945 (4) A, c = 8.993 (3) A, a = 93.36 (2)°, s = 90.92 (2)°, and y = 82.54 (2)°; R(F) = 0.030 for 6085 independent reflections with I > 3σ(I). The structure is of a gbent-sandwich Cp^-₂MX-type (Cp^- = η⁵-Me₅C₅) with relaxed interligand nonbonded interactions vis-a-vis the Cp^'₂M and Me₂SiCp^”₂M analogues (Lu-CHTMS₂ = 2.365 (7) A) and having one close Lm-MeSi (Lu-C = 2.820 (8) A) 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: P₂fn (no. 14), z = 2, a = 11.558 (3) A, b = 8.590 (2) A, c = 18.029 (3) A, s = 100.10 (2) A, R(F) = 0.022 for 2656 independent reflections with I > 3 (I). The structure has an idealized C₂i, Lu(M-Et₂SiCpCp^”)₂-H)₂Lu geometry with both bridging Et₂SiCpCp^” and hydride ligands (Lu-H = 2.16 (4), 2.13 (4) A). 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-Et₂SiCpCp^”)₂(μ-H)(μ-CH₃CH₂)Lu at-120°C: P2Jc (no. 14), z = 6, a= 11.679 (4) A, 6 = 25.755 (5) A, c = 18.074 (2) A, s = 99.41 (2)°; R(F) = 0.058 for 4643 independent reflections with 1 > 3 (7). The Lu-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(cr) = 2.46 (2) and 2.58 (2) A,. Lu-C(a)-C(/3) = 148 (1)° and 84.7 (5)°. In addition, Lu-C(η) = 2.78 (2) A 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-μ-(H) vectors down to-85°C and rapid inversion at C(a) occurring with AG^* = 12.5-13.5 kcal/mol (7C = +11-+39°C). Kinetic (rate law: v = A:[dihydride] [olefin]) and equilibration measurements reveal that the hydride addition process to 1-hexene (Et₂SiCpCp^“LuH)₂ + 1-hexene Lu(μ-Et₂SiCpCp)₂(μ-H)(μ-7-hexyl)Lu is described by AH =-10.7 (6) kcal/mol, AS =-25 (2) eu, AH^* = 12.0 (4) kcal/mol, and AS^* =-38.6 (7) eu. These results indicate that, in comparison to terminal bonding modes with similar metal ancillary ligation, lanthanide μ-ligands are kinetically deactivated with respect to olefin insertion (a rate depression of 10s. 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.