Phosphate diester hydrolysis promoted by Zinc(II) complexes with alkoxide pendants

Two new Zn(II) complexes of tripodal pyridyl-amine ligands with alcohol pendants, 2-[bis-(2-pyridin-2-yl-ethyl)-amino]-ethanol (L1) and 3-[bis-(2-pyridin-2-yl-ethyl)-amino]-propan-1-ol (L2), have been synthesized and structurally characterized. The reactivities of ZnL1 (1) and ZnL2 (2) for promoting the hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) have been investigated, and both complexes are highly active in catalyzing the hydrolysis of BNPP, with 2 (k = 1.8 × 10−1 M−1 s−1) being about two orders of magnitude more active than 1 (k = 2.7 × 10−3 M−1 s−1) in promoting the hydrolysis of BNPP. The differences in reactivity can be attributed to the disparate abilities of the alkoxide nucleophiles in the pendant groups in ligands L1 and L2 to attack the substrate phosphate diester BNPP due to differences in their tether lengths. X-ray crystal structures of 1 and 2 are also reported, which show that the complexes form dimers with bridging alkoxide ligands in the solid state.