Steven D. Gray

Homogeneous models for hydrodenitrogenation catalysis

Abstract Hydrodenitrogenation (HDN) catalysis is the process of removing nitrogen from petroleum feed-stocks in the form of NH3 to provide more processable and environmentally compatible liquid fuels. In practice, HDN is carried out simultaneously with other catalytic hydrotreating reactions such as hydrodesulfurization (HDS), yet HDN is significantly less well-studied than HDS. This contribution provides an overview of the heterogeneous HDN process, then outlines various homogeneous models for hydrodenitrogenation catalysis including binding modes of HDN substrates, catalytic hydrogenation processes and recent CN bond cleavage reactions of nitrogen heterocycles. Emphasis is placed on aryloxide and alkoxide complexes of the early metals that afford some of the best homogeneous models for hydrodenitrogenation catalysis to date.

QUINOLINE BINDING MODE AS A FUNCTION OF OXIDATION STATE IN ARYLOXIDE-SUPPORTED TANTALUM COMPLEXES: MODELS FOR HYDRODENITROGENATION CATALYSIS

Abstract The heterocyclic complexes [η 1 ( N )-QUIN]Ta(OAr) 3 Cl 2 ( 1 ) and [η 1 ( N )-6MQ] Ta(OAr) 3 Cl 2 ( 2 ) (where Ar = 2,6-diisopropylphenyl, QUIN = quinoline, and 6MQ = 6-methylquinoline) are prepared from Ta(OAr) 3 Cl 2 (OEt 2 ) and QUIN or 6MQ in pentane. [η 1 ( N )-6MQ]Ta(OAr) 2 Cl 3 ( 4 ) is prepared similarly from Ta(OAr) 2 Cl 3 (OEt 2 ). Upon rapid, two-electron reduction of these complexes, an η 1 ( N ) → η 2 ( N,C ) bonding rearrangement is effected and the thermally sensitive, d 2 species [η 2 ( N,C )-QUIN]Ta(OAr) 3 ( 5 ), [η 2 ( N,C )-6MQ]Ta(OAr) 3 ( 6 ), and [η 2 ( N,C )-6MQ]Ta(OAr) 2 Cl(OEt 2 ) ( 9 ) can be isolated. Alternatively, [η 2 ( N,C )-6MQ]Ta(OAr) 2 Cl(OEt 2 ) ( 9 ) can be prepared in higher yield from (η 6 -C 6 Me 6 )Ta(OAr) 2 Cl and 6MQ. The trimethylphosphine adducts [η 2 ( N,C )-QUIN] Ta(OAr) 3 (PMe 3 ) ( 7 ) and [η 2 ( N,C )-6MQ]Ta(OAr) 3 (PMe 3 ) ( 8 ) can be prepared by simple coordination of PMe 3 to the base-free compounds 5 and 6 . When Ta(OAr) 2 Cl 3 (OEt 2 ) is reduced by one electron in the presence of QUIN, 6MQ, or pyridine, the d 1 bis(ligand) complexes [η 1 ( N )-QUIN] 2 Ta(OAr) 2 Cl 2 ( 10 ), [η 1 ( N )-6MQ] 2 Ta(OAr) 2 Cl 2 ( 11 ), and [η 1 ( N )-py] 2 Ta(OAr) 2 Cl 2 ( 12 ) can be isolated. Complexes 10 and 11 are not readily converted to the η 2 ( N,C ) analogues 5 and 6 by further reduction. Under mild hydrogenation conditions, the only heterocyclic ligands which are hydrogenated are those bound in the η 2 ( N,C ) mode to a d 2 metal. Structural studies on [η 2 ( N,C )-6MQ]Ta(OAr) 3 (PMe 3 ) ( 8 ) and [η 2 ( N,C )-6MQ]Ta(OAr) 2 Cl(OEt 2 ) ( 9 ) have been undertaken. [η 2 ( N,C )-6MQ]Ta(OAr) 3 (PMe 3 ) ( 8 ) crystallizes in the monoclinic space group C 2 1 / c (No. 15), with a = 32.849 (3) A, b = 19.579 (2) A, c = 23.822 (2) A, β = 135.69 (49)°, and V = 10702 (2) A 3 with Z = 8 and ρ caled = 1.16 g cm −3 . [η 2 ( N,C )-6MQ]Ta(OAr) 2 Cl(OEt 2 ) ( 9 ) crystallizes in the monoclinic space group P 2 1 / n (No. 14) with a = 12.059 (9) A, b = 17.975 (14) A, c = 17.949 (13) A, β = 100.29 (3)°, and V = 3828 (9) A 3 with Z = 4 and ρ caled = 1.37 g cm −3 . Both structures indicate an interruption of aromaticity to the heterocyclic ring only when bound in this fashion, consistent with the observation of 1,2,3,4-tetrahydroquinoline as the principal hydrogenation product of [η 2 ( N,C )-QUIN]Ta(OAr) 3 ( 5 ) with no decahydroquinoline being observed.