Quinoline binding mode as a function of oxidation state in aryloxide-supported tantalum complexes: models for hydrodenitrogenation catalysis

Article abstract of DOI:10.1016/0277-5387(95)85013-9

The heterocyclic complexes [η1(N)-QUIN]Ta(OAr)3Cl2 (1) and [η1(N)-6MQ]Ta(OAr)3Cl2 (2) (where Ar=2,6-diisopropylphenyl, QUIN=quinoline, and 6MQ=6-methylqunoline) are prepared from Ta(OAr)3Cl2(OEt2) and QUIN or6-MQ in pentane. [η1(N)-6MQ]Ta(OAr)2Cl3 (4) is prepared similarly f rom Ta(OAr)2Cl3(OEt2). Upon rapid, two-electron reduction of these complexes, an η1(N) η2(N,C) bonding rearrangement is effected and the thermally sensitive, d(2) species [η2(N,C)-QUIN]Ta(OAr)3 (5), [η2(N,C)-6MQ]Ta(OAr)3 (6), and [η2(N,C)-6MQ]Ta(OAr)2Cl(OEt2)(9) can be isolated. Alternatively, [η2(N,C)-6MQ]Ta(OAr)2Cl(OEt2) ( 9) can be prepared in higher yield from (η6-C6Me6)Ta(OAr)2Cl and 6MQ. The trimethylphosphine adducts [η2(N,C)-QUIN]Ta(OAr)3(PMe3) (7) and [η2(N,C)-6MQ]Ta(OAr)3(PMe3) (8) can be prepared by simple coordination of PMe3 to the base-free compounds 5 and 6. When Ta(OAr)2Cl3(OEt2) is reduced by one electron in the presence of QUIN, 6MQ, or pyridine, the d(1) bis(ligand) complexes [η1(N)-QUIN]2Ta(OAr)2Cl2 (10), [η1(N)-6MQ]2Ta(OAr)2Cl2 (11), and [η1(N)-py]2Ta(OAr)2Cl2 (12) can be isolated. Complexes 10 and 11 are not readily converted to the η2(N,C) analogues 5 and 6 by further reduction. Under mild hydrogenation conditions, the only heterocyclic ligands which are hydrogenated are those in the η2(N,C) mode to a d(2) metal. Structural studies on [η2(N,C)-6MQ]Ta(OAr)3(PMe3) (8) and [η2(N,C)-6MQ]Ta(OAr)2Cl(OEt2) (9) have been undertaken. [η2(N,C)-6MQ]Ta(OAr)3(PMe3) (8) crystallizes in the monoclinic space group C21/C (No. 15) with a=32.849 (3) ?, b=19.579 (2) ?, c=23.822 (2) ?, β=135.69 (49)°, and V=10702 (2) ?**3 with Z=8 and ρ(calcd)=1.16 g cm**-3. [η2(N,C)-6MQ]Ta(OAr)2Cl(OEt2) (9) crystallizes in the monoclinic space group P21/N (No. 14) with a=12.059 (9) ?, b=17.975 (14) ?, c=17.949 (13) ?, β=100.29 (3)°, and V=3828 (9) ?**3 with Z=4 and ρ(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 [η2(N,C)-QUIN]Ta(OAr)3 (5) with no decahydroquinoline being observed.