CARBON-NITROGEN BOND-CLEAVAGE IN AN ETA(2)(N,C)-PYRIDINE COMPLEX-INDUCED BY INTRAMOLECULAR METAL-TO-LIGAND ALKYL MIGRATION - MODELS FOR 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 LiBEt(3)H affords the C-N bond scission product Ta(=NC(t)Bu=CHC(t)Bu=CHCH(t)Bu) OAr)(2) (2). The reactions of [eta(2)(N,C)-2,4,6-NC(5)(t)Bu(3)H(2)]Ta( OAr)(2)Cl (1) with carbon nucleophiles RLi or RMgX provide the alkyl d erivatives [eta(2)(N,C)-2,4,6-NC(5)(t)Bu(3)H(2)]Ta(OAr)(2)R [R = Me (3 ), Et (4), Pr-n (5), (n)Bu (6), and CH(2)SiMe(3) (7)]. Complexes 3-6 r epresent the kinetic products of the reaction since upon their thermol ysis, alkyl migration from metal to ligand occurs and the C-N bond cle avage compounds Ta(=NC(t)Bu=CHC(t)Bu=CHC(t)BuR)(OAr)(2) [R = Me (8), E t (9), Pr-n (10), (n)Bu (11)] are formed. Kinetic and mechanistic stud ies of the 3 --> 8 rearrangement reveal that methyl migration is stric tly intramolecular. Further studies of Ta(=NC(t)Bu=CHC(t)Bu=CHC(t)BuMe )(OAr)(2) (8) reveal that this complex subsequently rearranges to affo rd the eight-membered metallacycle Ta(=NC(t)Bu=CHC(t)Bu=CHC(t)BuHCH(2) )(OAr)(2) (12), which further decomposes to give the metallapyridine d imer [Ta(mu-NC(t)Bu=CHC(t)Bu=CH)(OAr)(2)](2) (13) and (t)BuCH=CH2. Syn thetic and mechanistic studies on the 8 --> 12 --> 13 rearrangement re veal the source of the (t)BuCH=CH2 through labeling experiments, allow the isolation of an adduct of 12, viz. Ta(=NC(t)Bu=CHC(t)Bu=CHC(t)BuH CH(2))(OAr)(2) .(2)NCMe (12-NCMe, and suggest a mechanistic scheme to account for these arrangements. Complexes 2,4 and 13 have been structu rally characterized. Ta(NC(t)Bu=CHC(t)Bu=CHCH(t)Bu)(OAr)(2) (2) crysta llizes in the monoclinic space group P2(1)/n (No. 14) and displays a h ighly localized metallacyclic structure with an imido nitrogen linkage characterized by a Ta-N-C angle of 145.7(6)degrees. [eta(2)(N,C)-2,4, 6-NC(5)(t)Bu(3)H(2)]Ta(OAr)(2)Et (4) crystallizes in the monoclinic gr oup P2(1)/n (No. 14) and is characterized by an interruption of aromat icity to the heterocyclic ring through a 1,3-diene-like pi electron lo calization. Metallapyridine [Ta(mu-NC(t)Bu=CHC(t)Bu=CH)(OAr)(2)](2) (1 3) crystallizes in the triclinic space group P (1) over bar (No. 2) an d reveals an extremely crowded structure with a pi localized, formal [ mu-NC(t)Bu=CHC(t)Bu=CH](3-) mu-imido ligand. The reactions of this mod el system delineate one process by which heterocyclic C-N bonds are cl eaved and offer new insight as to how nitrogen heterocycles may be fur ther degraded after C-N bond cleavage in hydrodenitrogenation catalysi s.