A Sterically Hindered N,N,O Tripod Ligand and Its Zinc Complex Chemistry

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Abstract
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(OH2)]ClO4 (1) obtained from zinc perchlorate. Together with the more labile complex L·Zn−C2H5 (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)]ClO4 (4), and [(L·Zn)3(μ3-CO3)]ClO4 (5). Metathetical reactions produced the neutral complexes L·Zn−X with X = Br (6), OAc (7), OC6H5 (8), SC6H5 (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.