Hydrocarbon Activation via Reversible 1,2-RH-Elimination from (tBu3SiNH)3ZrR:  Synthetic, Structural, and Mechanistic Investigations

• Published in: Journal of the American Chemical Society Vol. 118; no. 3; pp. 591 - 611
• Main Authors: Schaller, Christopher P, Cummins, Christopher C, Wolczanski, Peter T
• Format: Journal Article
• Language: English
• Published: American Chemical Society 24-01-1996
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja950745i

Hydrocarbyl complexes, (tBu3SiNH)3ZrR (1-R), were prepared via metatheses of (tBu3SiNH)3ZrCl (1-Cl) with RMgX or RLi (R = Me, Et, Cy, CH2Ph, allyl, CHCH2, Ph, CH2 tBu, C⋮CPh, C⋮CtBu), through addition of isobutylene, H2CCCMe2, and acetylene to 1-H (R = iBu, dma, or CHCH2), and by CH-bond activation; thermal 1,2-RH-elimination from 1-R produced putative (tBu3SiNH)2ZrNSitBu3 (2), which was subsequently trapped by R‘H. Thermolysis of 1-R (∼100 °C, R = Me or Cy) in the presence of H2, c-C3H6, and CH4 in cyclohexane or neat C6H6, mesitylene, and toluene afforded 1-R (R = H, cPr, Me, Ph, CH2-3,5-Me2C6H3) and a mixture of 1-CH2Ph and 1-C6H4Me, respectively. Exposure of 1-Cy to C2H4 or C6H6 in cyclohexane provided 1-CHCH2 or 1-Ph, respectively, but further reaction produced 1 2-(trans-HCCH) and 1 2-(p-C6H4) through double CH-bond activation. Thermolysis of (tBu3SiND)3ZrCH3 (1-(ND)3-CH3) in C6H6 or C6D6 yielded CH3D, and 1C6H5 or 1-(ND)3C6D5, through reversible benzene activation. Thermolysis of 1-Cy in neat cyclohexane, and with C2H6 or CMe4 present, gave cyclometalation product (tBu3SiNH)2ZrNHSitBu2CMe2CH2 (3) and 1-NHSitBu3. In THF, thermolysis of 1-CH3 afforded (tBu3SiNH)2(THF)ZrNSitBu3 (2-THF); at 25 °C, 1-H lost H2 in the presence of L (L = THF, Et2O, NMe3, PMe3) generating 2-L; 2-L (L = Et2O, py) was also prepared via ligand exchange with 2-THF. Single crystal X-ray diffraction studies of 2-THF revealed a pseudotetrahedral core, with a long ZrN bond distance (1.978(8) Å), normal Zr−N(H) bond lengths (2.028(8), 2.031(8) Å), similar amide (154.7(5), 158.1(5)°) and imide (156.9(5)°) bond angles, and little O(pπ) → Zr(dπ) bonding. Crystal data:  monoclinic, P2 1 /n, a = 13.312(5) Å, b = 18.268(6) Å, c = 20.551(7) Å, β = 92.30(3)°, Z = 4, T = 25 °C. 2-Et2O thermally eliminated C2H4 to give 1-OEt through γ-CH activation. Kinetic isotope effects (KIE) on 1,2-RD-elimination from 1-(ND)3-R (96.7 °C, R = CH3, z Me = 6.3(1); CH2Ph, z Bz = 7.1(6); Ph, z Ph = 4.6(4)) and CD3H loss from 1-CD3 (k(CH3)/k(CD3) = (z‘Me)3 = 1.32) revealed a symmetric H-transfer in a loose transition state. 1,2-RH-elimination rates follow:  (96.7 °C, k R (×104 s-1) = 22.6(2), Ph; 15.5(2), cPr; 13.2(4), CHCH2; 10.4(2), Cy; 3.21(6), Et; 3.2(1), iBu; 1.3(1), dma; 1.51(6), H; 1.42(4), CH2 tBu; 1.06(2), Me; 0.34(2), CH2-3,5-Me2C6H3; 0.169(3), CH2Ph). Competition for (tBu3SiNH)2ZrNSitBu3 (2) by RH/R‘H and equilibria provided information about the stabilities of 1-R relative to 1-cPr (R = cPr (0.0 kcal/mol) < Ph (0.3) < CH2Ph (0.7) < Me (1.2) < CH2 tBu (≥7.6) < Et (≥7.8) < Cy (≥10.9)). Transition state energies afforded relative C−H bond activation selectivities (ΔΔG ⧧ relative to cPr-H):  cPrH ≈ ArH (0.0 kcal/mol) > MeH (3.4) > PhCH2H (4.0) > cyclometalation (≥8.5) > EtH (≥8.9) > tBuCH2H (≥9.3) > CyH (≥11.2). A correlation of ΔG ⧧(1,2-RH-elimination) with D(R−H) indicated generally late transition states but suggested an earlier composition for the alkyls, as rationalized through a Hammond analysis. Correlation of ΔG ⧧(1,2-RH-elimination) with RH proton affinity implicated tight binding of RH in the transition state and possible RH-binding intermediates (2-RH). 1,2-HC⋮CR-elimination from 1-C⋮CR was not observed, but second-order exchanges of 1-C⋮CPh with tBuC⋮CH, and 1-C⋮CtBu with HC⋮CPh were indicative of an associative pathway. All data can be accommodated by the following mechanism:  1-R + R‘H ⇌ 2-RH + R‘H ⇌ 2-R‘H + RH ⇌ 1-R‘ + RH; a variant where 2 mediates reversible 2-RH + R‘H exchange is less likely.