STRUCTURAL MODELS FOR THE SUBSTRATE-CATALYST ADDUCT IN HYDRODENITROGENATION CATALYSIS - OXYGEN VS SULFUR LIGATION

A comparison between models for the substrate-catalyst adduct in hydro denitrogenation (HDN) catalysis is made with respect to oxygen vs sulf ur ancillary ligands. Reacting [eta(2)(N,C)(NC5Bu3H2]Ta)-Bu-t(OAr)(2)C l (1, Ar = 2,6-(C6H3Pr2)-Pr-i) With (KOBu)-Bu-t affords orange crystal s of the alkoxide [eta(2)(N, C)-(NC5Bu3H2)-Bu-t]Ta(OAr)(2)((OBu)-Bu-t) (2), while 1 and (LiSBu)-Bu-t react to form the red thiolate analogue [eta(2)(N, C)-(NC5Bu3H2)-Bu-t]Ta(OAr)(2)((SBu)-Bu-t) (3). Structural studies of both complexes 2 and 3 are reported and compared with other eta 62(N,C)-(NC5Bu3H2)-Bu-t derivatives. A trace of the bromide compl ex [eta(2)(N, C)-(NC5Bu3H2)-Bu-t]Ta(OAr)(2)Br (4) is isolated from rea cting [eta(2)(N, C)-(NC5Bu3H2)-Bu-t]Ta(OAr)(2)Cl (1) with EtMgBr in TH F/Et2O solution and is also structurally characterized for comparison. Complexes 2-4 reveal a severe interruption of aromaticity within the heterocycle, different rotational preferences of the pyridine NC5 plan e with respect to the Ta(OAr)(2)X moiety, and various aryloxide ligand structural differences. From this comparison, arguments will be prese nted that support the ancillary ligand cc-donor ability decreasing as (OBu)-Bu-t > OAr > (SBu)-Bu-t > Cl approximate to Br > Et, although ev idence suggests that the (SBu)-Bu-t ligand is a better sigma + pi dono r overall than OAr or (OBu)-Bu-t.