HYDROCARBON ACTIVATION VIA REVERSIBLE 1,2-RH-ELIMINATION FROM ((T)BN(3)SINH)(3)ZRR - SYNTHETIC, STRUCTURAL, AND MECHANISTIC INVESTIGATIONS

Hydrocarbyl complexes, ((t)Bu(3)SiNH)(3)ZrR (1-R), were prepared via m etatheses of ((t)Bu(3)SiNH)(3)ZrCl (1-Cl) with RMgX or RLi (R = Me, Et , Cy, CH(2)Ph, allyl, CH=CH2, Ph, CH(2)(t)Bu, C=CPh, C=C(t)Bu), throug h addition of isobutylene, H2C=C=CMe(2), and acetylene to 1-H (R = (i) Bu, dma, or CH=CH2), and by CH-bond activation; thermal 1,2-RH-elimina tion from 1-R produced putative ((t)Bu(3)SiNH)(2)Zr=NSi(t)Bu(3) (2), w hich was subsequently trapped by R'H. Thermolysis of 1-R (similar to 1 00 degrees C, R = Me or Cy) in the presence of H-2, c-C3H6, and CH4 in cyclohexane or neat C6H6, mesitylene, and toluene afforded 1-R (R = i i, Pr-c, Me, Ph, CH2-3,5-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 C2H4 or C6H6 in cyclo hexane provided 1-CH=CH2 or 1-Ph, respectively, but further reaction p roduced 1(2)-(trans-HC=CH) and 1(2)-(p-C6H4) through double CH-bond ac tivation. Thermolysis of ((t)Bu(3)SiND)(3)ZrCH3 (1-(ND)(3)-CH3) in C6H 6 or C6D6 yielded CH3D, and 1C(6)H(5) or 1-(ND)(3)C6D5, through revers ible benzene activation. Thermolysis of l-Cy in neat cyclohexane, and with C2H6 Or CMe(4) present, gave cyclometalation product ((t)Bu(3)SiN H)(2)ZrNHSi(t)Bu(2)CMe(2)CH(2) (3) and 1-NHSi(t)Bu(3). In THF, thermol ysis of 1-CH3 afforded ((t)Bu(3)SiNH)(2)-(THF)Zr=NSi(t)Bu(3) (2-THF); at 25 degrees C, 1-H lost H-2 in the presence of L (L = THF, Et(2)O, N Me(3), PMe(3)) generating 2-L; 2-L (L = Et(2)O py) was also prepared v ia ligand exchange with 2-THF. Single crystal X-ray diffraction studie s of 2-THF revealed a pseudotetrahedral core, with a long Zr=N bond di stance (1.978(8) Angstrom), normal Zr-N(H) bond lengths (2.028(8), 2.0 31(8) Angstrom, similar amide (154.7(5), 158.1(5)degrees) and imide (1 56.9(5)degrees) bond angles, and little O(p pi) --> Zr(d pi) bonding. Crystal data: monoclinic, P2(1)/n, a = 13.312(5) Angstrom, b = 18.268( 6) Angstrom, c = 20.551(7) Angstrom, beta = 92.30(3)degrees, Z = 4, T = 25 degrees C. 2-Et(2)O thermally eliminated C2H4 to give 1-OEt throu gh gamma-CH activation. Kinetic isotope effects (KIE) on 1,2-RD-elimin ation from 1-(ND)(3)-R (95.7 degrees C, R = CH3, z(Me) = 6.3(1); CH(2) Ph, z(Bz) = 7.1(6); Ph, z(Ph) = 4.6(4)) and CD3H loss from 1-CD3((CH3) /k(CD3) = (z'(Me))(3) = 1.32) revealed a symmetric H-transfer in a loo se transition state. 1,2-RH-elimination rates follow: (96.7 degrees C, k(R) (x10(4) s(-1)) = 22.6(2), Ph; 15.5(2), Pr-c; 13.2(4), CH=CH2; 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), CH2-3,5-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 rel ative to 1-Pr-c (R = CPr (0.0 kcal/mol) < Ph (0.3) < CH(2)Ph (0.7) < M e (1.2) < CH(2)(t)Bu (greater than or equal to 7.6) < Et (greater than or equal to 7.8) < Cy (greater than or equal to 10.9)). Transition st ate energies afforded relative C-H bond activation selectivities (Delt a Delta G double dagger relative to Pr-c-H): (PrH)-Pr-c approximate to ArH (0.0 kcal/mol) > MeH (3.4) > PhCH(2)H (4.0) > cyclometalation (gr eater than or equal to 8.5) > EtH(greater than or equal to 8.9) > (t)B uCH(2)H (greater than or equal to 9.3) > CyH (greater than or equal to 11.2). A correlation of Delta G double dagger(1,2-RH-elimination) wit h D(R-H) indicated generally late transition states but suggested an e arlier composition for the alkyls, as rationalized through a Hammond a nalysis. Correlation of Delta G double dagger(1,2-RH-elimination) with RH proton affinity implicated tight binding of RH in the transition s tate and possible RH-binding intermediates (2-RH). 1,2-HC=CR-eliminati on 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 asso ciative pathway. All data can be accommodated by the following mechani sm: 1-R + R'H reversible arrow 2-RH + R'H reversible arrow 2-R'H + RH reversible arrow 1-R' + RH; a variant where 2 mediates reversible 2-RH + R'H exchange is less likely.