METHANE VS BENZENE ACTIVATION VIA TRANSIENT TBU3SINHTA(=NSITBU3)2 - STRUCTURE OF (PY)2META(=NSITBU3)2

Addition of Me3TaCl2 to 2.0 equiv of LiNHSitBu3 in hexanes resulted in CH4 and (tBu3SiNH)Me2Ta=NSitBu3 (1, 63%). Thermolysis of 1 in benzene resulted in disproportionation, but in pyridine and THF, L2MeTa(=NSitBu3)2 (L = py, 2(py)2, 68%; THF, 2(THF)2, 13%) and MeH were produced. The bis adduct 2(py)2 is considered to form via 1,2-MeH-elimination from (tBu3SiNH)Me2(py)Ta=NSitBu3 (1-py), which is obtained from 1 and pyridine at 250-degrees-C. H-1 and C-13{H-1} NMR spectra of 2(py)2 manifested equivalent pyridines, but an X-ray structure determination revealed a trigonal bipyramidal, pseudo-C(s) stereoisomer with an axial methyl group and equatorial imides that reflects the steric requirements of the bulky tBu3SiN units: orthorhombic, P2(1)2(1)2(1), a = 12.134 (2) angstrom, b = 13.421 (2) angstrom, c = 24.865 (4) angstrom, Z = 4, V = 4049.3 (11) angstrom3, T = 295 K, R = 6.20%, R(w) = 6.64%, and GOF = 1.35 for 3257 (88.9%) reflections with (\F(o)\ > 3sigma\F(o)\). Rather long Ta=N bond distances (1.810 (13) and 1.819 (13) angstrom) support electronic arguments suggesting the imides donate a maximum of 6 electrons to the metal center. Addition of TaCl5 to 4.0 equiv of LiNHSitBu3 in Et2O at -78-degrees-C afforded (tBu3SiNH)2ClTa-NSitBu3 (3-Cl) and tBu3SiNH2. Alkylation of 3-Cl with AlMe3 (hexanes), PhLi (Et2O/hexanes), PhCH2K (toluene), and tBuCH2Li (Et2O) provided (tBu3SiNH)2RTa-NSitBu3 (R = Me, 3-Me, 78%; Ph, 3-Ph, 64%; CH2Ph, 3-CH2Ph, 51%; CH2tBu, 3-CH2tBu, 39%). Addition of tBu3SiNH2 or tBu3SiOH to 1 yielded 3-Me or (tBu3SiNH)(tBu3SiO)MeTa=NSitBu3 (5-Me, 52%) and CH4. Thermolysis of 3-R effected 1,2-RH-elimination to form transient (tBu3SiNH)Ta(=NSitBu3)2 (4), a species capable of adding C-H bonds across one imido linkage. Moderate rates of elimination from 3-R could be obtained only at 182.8 (4) degrees-C: k(MeH) = 8.0 (1) X 10-6 s-1, DELTAG(double dagger) = 37.7 kcal/mol, k(MeH)/k(MeD) greater-than-or-equal-to 3.4; k(PhH) = 1.67 (4) X 10(-4) s-1, DELTAG(double dagger) = 35.0 kcal/mol; k(BzH) = 1.71 (5) X 10(-6) s-1, DELTAG(double dagger) = 39.1 kcal/mol. Ground-state information was obtained via the approach to equilibrium of 3-Ph and CH4, but observation of a para-ditantalum phenyl derivative, [(tBu3SiNH)2Ta=NSitBu3]2(mu2:eta1,eta1-1,4-C6H4) ((3)2C6H4) complicated the measurement. Simulation of the approach to equilibrium yielded rate constants consistent with the previously measured 1,2-RH-elimination rates and showed that 3-Me, 3-Ph, and (3)2C6H4 possess relatively similar ground-state free energies. Equilibration of 3-CH2Ph to aryl complexes (tBu3SiNH)2(C6H4Me)Ta=NSitBu3 (3-C6H4Me) in toluene at 182.8 (4) degrees-C gave similar results. The data portray differing 1,2-RH-elimination rates that result from significant transition state energy differences, ruling out a late transition state despite a rough correlation of rate with the C-H bond strength of the eliminated alkane/arene. The implications of these measurements, including the possibility of d0 alkane or arene complexes as intermediates and differences in tantalum-carbon bond strengths, are discussed in detail.