Structural Models for the Substrate−Catalyst Adduct in Hydrodenitrogenation Catalysis:  Oxygen vs Sulfur Ligation

• Published in: Organometallics Vol. 17; no. 13; pp. 2720 - 2729
• Main Authors: Fox, Peter A, Bruck, Michael A, Gray, Steven D, Gruhn, Nadine E, Grittini, Carina, Wigley, David E
• Format: Journal Article
• Language: English
• Published: American Chemical Society 22-06-1998
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om970559g

A comparison between models for the substrate−catalyst adduct in hydrodenitrogenation (HDN) catalysis is made with respect to oxygen vs sulfur ancillary ligands. Reacting [η2(N,C)-NC5 tBu3H2]Ta(OAr)2Cl (1, Ar = 2,6-C6H3 iPr2) with KOtBu affords orange crystals of the alkoxide [η2(N,C)-NC5 tBu3H2]Ta(OAr)2(OtBu) (2), while 1 and LiStBu react to form the red thiolate analogue [η2(N,C)-NC5 tBu3H2]Ta(OAr)2(StBu) (3). Structural studies of both complexes 2 and 3 are reported and compared with other η2(N,C)-NC5 tBu3H2 derivatives. A trace of the bromide complex [η2(N,C)-NC5 tBu3H2]Ta(OAr)2Br (4) is isolated from reacting [η2(N,C)-NC5 tBu3H2]Ta(OAr)2Cl (1) with EtMgBr in THF/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 plane with respect to the Ta(OAr)2X moiety, and various aryloxide ligand structural differences. From this comparison, arguments will be presented that support the ancillary ligand π-donor ability decreasing as OtBu > OAr > StBu > Cl ≈ Br > Et, although evidence suggests that the StBu ligand is a better σ + π donor overall than OAr or OtBu.