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

The heterocyclic complexes [eta(1)(N)-QUIN]Ta(OAr)(3)Cl-2 (1) and [eta(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. [eta(1)(N)-6MQ]Ta(OAr)(2)Cl-3 (4) is prepared similarly from Ta(OAr)(3)Cl-2(OEt(2)). Upon rapid, two-electron reduction of these complexes, an eta(1)(N) --> eta(2)(N,C) bonding rearrangement is effected and the thermally sensitive, d(2) species [eta(2)(N,C)-QUIN]Ta(OAr)(3) (5), [eta(2)(N,C)-6MQ]Ta(OAr)(3) (6), and [eta(2)(N,C)-6MQ]Ta(OAr)(2)Cl(OEt(2)) (9) can be isolated. Alternatively, [eta(2)(N,C)-6MQ]Ta(OAr)(2)Cl(Oet(2)) (9) can be prepared in higher yield from (eta(6)-C(6)Me(6))Ta(OAr)(2)Cl and 6MQ. The trimethylphosphine adducts [eta(2)(N,C)-QUIN] Ta(OAr)(3)(PMe(3)) (7) and [eta(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 [eta(1)(N)-QUIN]Ta-2(OAr)(2)Cl-2 (10), [eta(1)(N)-6MQ]Ta-2(OAr)(2)Cl-2 (11), and [eta(1)(N)-py]Ta-2(OAr)(2)Cl-2 (12) can be isolated. Complexes 10 and 11 are not readily converted to the eta(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 eta(2)(N,C) mode to a d(2) metal. Structural studies on [eta(2)(N,C)-6MQ]Ta(OAr)(3)(PMe(3)) (8) and [eta(2)(N,C)-6MQ]Ta(OAr)(2)Cl (OEt(2)) (9) have been undertaken. [eta(2)(N,C) -6MQ]Ta(OAr)(3)(PMe(3)) (8) crystallizes in the monoclinic space group C2(1)/c (No. 15) with a = 32.849 (3) Angstrom, b = 19.579 (2) Angstrom, c = 23.822 (2) Angstrom, beta = 135.69 (49)degrees, and V = 10702 (2) Angstrom(3) with Z = 8 and rho(calcd) = 1.16 g cm(-3). [eta(2)(N,C)-6MQ]Ta(OAr)(2)Cl(OEt(2)) (9) crystallizes in the monoclinic space group P2(1)/n (No. 14) with a = 12.059 (9) Angstrom, b = 17.975 (14) Angstrom, c = 17.949 (13) Angstrom, beta = 100.29 (3)degrees, and V = 3828 (9) Angstrom(3) with Z = 4 and rho(calcd) = 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 [eta(2)(N,C)-QUIN]Ta(OAr)(3) (5) with no decahydroquinoline being observed.