MECHANISTIC ASPECTS OF CARBON-NITROGEN BOND-CLEAVAGE IN AN ETA(2)(N,C)-PYRIDINE COMPLEX - INTIMATE DETAILS OF METAL TO LIGAND ARYL MIGRATIONS AND THEIR RELEVANCE TO HYDRODENITROGENATION CATALYSIS

The reaction of the eta(2)(N,C)-pyridine complex [eta(2)(N,C)-2,4,6-NC (5)(t)Bu(3)H(2)]Ta(OAr)(2)Cl (1, Ar = 2,6-(C6H3Pr2)-Pr-i) with PhLi af fords the phenyl complex [eta(2)(N,C)-2,4,6-NC(5)(t)Bu(3)H(2)]Ta(OAr)( 2)-Ph (2) in moderate yield. The para-substituted phenyl derivatives - 2,4,6-NC(5)(t)Bu(3)H(2)]-Ta(OAr)(2)(4-C(6)H(4)X), where X = OMe (3), M e (4), Cl(5), and CF3 (6), are prepared similarly from 1 and the appro priate aryl Grignard or lithium reagent. Complexes 2-6 represent the k inetic products of this reaction since, upon their thermolysis, aryl m igration from metal to ligand occurs and the ring-opened compounds Ta[ =NC(t)Bu=CHC(t)Bu=CHC(t)Bu(4-C(6)H(4)X)]- (OAr)(2), where X = H (7), O Me (8), Me (9), Cl (10), and CF3 (11), are formed by C-N bond scission . Kinetic and mechanistic studies of the C)-2,4,6-NC(5)(t)Bu(3)H(2)]Ta (OAr)(2)(4-C(6)H(4)X) --> a[=NC(t)Bu=CHC(t)Bu=CHC(t)Bu(4-C(6)H(4)X)](O Ar)(2) rearrangement reveal that aryl migration follows clean first-or der kinetics, implying an intramolecular, endo attack of the migrating aryl group on the eta(2)(N,C)-pyridine ligand. An Eyring plot provide s activation parameters for the -2,4,6-NC(5)(t)Bu(3)H(2)]Ta(OAr)(2)(4- C(6)H(4)OMe) (3) --> NC(t)Bu=CHC-(t)Bu=CHC(t)Bu(4-C(6)H(4)OMe)](OAr)(2 ) (8) rearrangement of Delta H double dagger=+19.2+/-1.3 kcal mol(-1) and Delta S-double dagger=127+/-4 cal K-1 mol(-1), for a Delta G(doubl e dagger)=+27l3+/-1.3 kcal mol(-1) at 25 degrees C, suggesting a highl y ordered transition state leading to aryl transfer. Hammett considera tions for the transformation of complexes 2-6 into 7-11 provide rho=-0 .58+/-0.10 for the aryl migration reaction. While the sign of rho is i n agreement with an electrophilic aromatic substitution at the migrati ng phenyl ligand, the small value of rho argues against this pathway a nd against the involvement of the aryl pi framework during migration. These data suggport the direct attack of the formal [C(6)H(4)X](-) sig ma bond on C-alpha of the eta(2)(N,C)-pyridine ligand and C-N bond sci ssion resulting from migration of the aryl group as a sigma-nucleophil e. Unlike their alkyl congeners, these ring-opened aryl derivatives [= NC(t)Bu-CHC(t)Bu=CHC(t)Bu(4-C(6)H(4)X)]-(OAr)(2) (7-11) are stable to subsequent rearrangement. The structure of Ta[=NC(t)Bu=CHC-(t)Bu=CHC(t )BuPh](OAr)(2) (7) indicates a highly localized metallacycle with an i mido nitrogen linkage characterized by a Ta-N-C angle of 145.3(3)degre es and Ta-N bond distance of 1.771(3) Angstrom. Although an endo attac k of the phenyl group is evident from mechanistic studies, the solid-s tate structure intimates an exo addition, and a simple pathway that in terconverts endo and exo isomers is described to account for this obse rvation. The reactions and structures of this model system are discuss ed in terms of a fundamental mechanistic knowledge of C-N bond scissio n that is relevant to hydrodenitrogenation catalysis.