Metallohydrolase biomimetics with catalytic and structural flexibility
The structural and functional properties of zinc(II) complexes of two nitrogen rich polydentate ligands, HTPDP = 1,3-bis(bis-pyridin-2-ylmethylamino)propan-2-ol and HTPPNOL = N,N,N′-tris-(2-pyridylmethyl)-1,3-diaminopropan-2-ol, are compared. HTPDP is a hepta-dentate ligand with four pyridyl groups attached to a 1,3-diaminopropan-2-ol backbone while HTPPNOL contains only three pyridyl groups. In reactions with Zn(ClO4)2, HTPDP forms a dinuclear zinc compound [Zn2(TPDP)(OAc)](ClO4)2, 1. On the other hand, mononuclear [Zn(HTPPNOL)](ClO4)2, 2, and tetranuclear [Zn4(TPPNOL)2(OAc)3](ClO4)3, 3, complexes were isolated with the ligand HTPPNOL. Kinetic measurements with the substrate bis(2,4-dinitrophenyl)phosphate (BDNPP) revealed that compound 1 (kcat = 31.4 × 10−3 min−1) is more reactive than 3 (kcat = 7.7 × 10−3 min−1) at pH = 8.5, whilst the mononuclear compound 2 is inactive. Compound 1 displays a typical steady-state kinetic behaviour, while compound 3 exhibits steady-state behaviour only ∼120 s after starting the reaction, preceded by a burst-phase. 31P NMR studies confirm that 1 can promote the hydrolysis of both ester bonds in BDNPP, generating the monoester DNPP and inorganic phosphate in the process. In contrast, DNPP is not a substrate for 3. The crystal structure of the complex formed by 3 and DNPP reveals the formation of a tetranuclear zinc complex [Zn4(TPPNOL)2(DNPP)2](ClO4)2, 4, in which the phosphate moiety of DNPP adopts an unusual tridentate μ–η1:η1:η1 coordination mode.