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

Hydrocarbyl complexes, ( t Bu 3 SiNH) 3 ZrR ( 1 -R), were prepared via metatheses of ( t Bu 3 SiNH) 3 ZrCl ( 1 -Cl) with RMgX or RLi (R = Me, Et, Cy, CH 2 Ph, allyl, CH CH 2, Ph, CH 2 t Bu, C⋮CPh, C⋮C t Bu), through addition of isobutylene, H 2 C C CMe 2, and acetylene to 1 -H (R = i Bu, dma, or CH CH 2 ), and by CH-bond activation; thermal 1,2-RH-elimination from 1 -R produced putative ( t Bu 3 SiNH) 2 Zr NSi t Bu 3 ( 2 ), which was subsequently trapped by R‘H. Thermolysis of 1 -R (∼100 °C, R = Me or Cy) in the presence of H 2, c-C 3 H 6, and CH 4 in cyclohexane or neat C 6 H 6, mesitylene, and toluene afforded 1 -R (R = H, c Pr, Me, Ph, CH 2 -3,5-Me 2 C 6 H 3 ) and a mixture of 1 -CH 2 Ph and 1 -C 6 H 4 Me, respectively. Exposure of 1 -Cy to C 2 H 4 or C 6 H 6 in cyclohexane provided 1 -CH CH 2 or 1 -Ph, respectively, but further reaction produced 1 2 -( trans- HC CH) and 1 2 -( p -C 6 H 4 ) through double CH-bond activation. Thermolysis of ( t Bu 3 SiND) 3 ZrCH 3 ( 1 -(ND) 3 -CH 3 ) in C 6 H 6 or C 6 D 6 yielded CH 3 D, and 1 C 6 H 5 or 1 -(ND) 3 C 6 D 5, through reversible benzene activation. Thermolysis of 1 -Cy in neat cyclohexane, and with C 2 H 6 or CMe 4 present, gave cyclometalation product ( t Bu 3 SiNH) 2 ZrNHSi t Bu 2 CMe 2 CH 2 ( 3 ) and 1 -NHSi t Bu 3 . In THF, thermolysis of 1 -CH 3 afforded ( t Bu 3 SiNH) 2 (THF)Zr NSi t Bu 3 ( 2 -THF); at 25 °C, 1 -H lost H 2 in the presence of L (L = THF, Et 2 O, NMe 3, PMe 3 ) generating 2 -L; 2 -L (L = Et 2 O, 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 -Et 2 O thermally eliminated C 2 H 4 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 = CH 3, z Me = 6.3(1); CH 2 Ph, z Bz = 7.1(6); Ph, z Ph = 4.6(4)) and CD 3 H loss from 1 -CD 3 ( k (CH 3 )/ k (CD 3 ) = ( 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 (×10 4 s - 1 ) = 22.6(2), Ph; 15.5(2), c Pr; 13.2(4), CH CH 2; 10.4(2), Cy; 3.21(6), Et; 3.2(1), i Bu; 1.3(1), dma; 1.51(6), H; 1.42(4), CH 2 t Bu; 1.06(2), Me; 0.34(2), CH 2 -3,5-Me 2 C 6 H 3; 0.169(3), CH 2 Ph). Competition for ( t Bu 3 SiNH) 2 Zr NSi t Bu 3 ( 2 ) by RH/R‘H and equilibria provided information about the stabilities of 1 -R relative to 1 - c Pr (R = c Pr (0.0 kcal/mol) < Ph (0.3) < CH 2 Ph (0.7) < Me (1.2) < CH 2 t Bu (≥7.6) < Et (≥7.8) < Cy (≥10.9)). Transition state energies afforded relative C−H bond activation selectivities (ΔΔ G ⧧ relative to c Pr-H): c PrH ≈ ArH (0.0 kcal/mol) > MeH (3.4) > PhCH 2 H (4.0) > cyclometalation (≥8.5) > EtH (≥8.9) > t BuCH 2 H (≥9.3) > CyH (≥11.2). A correlation of Δ G ⧧ (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 Δ G ⧧ (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 t BuC⋮CH, and 1 -C⋮C t Bu 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.