A sterically hindered N,N,O tripod ligand and its zinc complex chemistry

The new ligand bis(2-picolyl)(2-hydroxy-3,5-di-tert-butylbenzyl)amine (HL) was prepared from bis(2-picolyl)amine and 2,4-di-tert-butyl-6-(chloromethyl)phenol. It acts as a tetradentate N,N,O tripod ligand ensuring 5-fold coordination in all its zinc complexes L.Zn-X. The central complex of the series was [L.Zn(OH(2))]ClO(4) (1) obtained from zinc perchlorate. Together with the more labile complex L.Zn-C(2)H(5) (2), obtained from diethyl zinc, it was used as a starting material for ligand substitutions. In the presence of bases, 1 was converted to L.Zn-OH (3), [L.Zn(py)]ClO(4) (4), and [(L.Zn)(3)(mu(3)-CO(3))]ClO(4) (5). Metathetical reactions produced the neutral complexes L.Zn-X with X = Br (6), OAc (7), OC(6)H(5) (8), SC(6)H(5) (9), OP(O)(OPh)(2) (10), p-nitrophenolate (11), 1-methyluracilate (12), o-formylphenolate (13), and o-hydroxymethylphenolate (14). Structure determinations of 1, 5, 7, 10, 11, 13, and 14 confirmed the strictly monodentate attachment of all units X in L.Zn-X. The hydrolytic cleavage of tris(p-nitrophenyl) phosphate by 1 was investigated preparatively and kinetically. L.Zn-OH was found to be the hydrolytically active nucleophile. The second-order rate constant for the cleavage reaction was found to be slightly lower than the values for related systems, reflecting the steric hindrance in the tert-butyl-substituted ligand L.