Quantifying the relative contribution of hydrogen bonding and hydrophobic environments, and coordinating groups, in the zinc(II)-water acidity by synthetic modelling chemistry

Ligands derived from the tripodal N-4 ligand tris(pyridylmethyl) amine ((pyCH(2))(3)N, tpa) of general formula (6-RNHpyCH(2))(n)N(CH(2)py)(3-n) (R = H, n = 1 - 3 L1-3; R = neopentyl, n = 1 - 3 L'(1-3)) were used to elucidate and quantify the magnitude of the effects exerted by hydrogen bonding and hydrophobic environments in the zinc - water acidity of their complexes. The pK(a) of the zinc-bound water molecule of [(L1-3) Zn(OH2)](2+) 1 - 3 and [(L'1-3) Zn(OH2)](2+) 1'- 3' was determined by potentiometric pH titrations in water (1 - 3) or water-ethanol (1 : 1) (1'-3'). The zinc(II) water acidity gradually increases as the number of -NH2 hydrogen bonding groups adjacent to the water molecule increases. Thus, the zinc-bound water of [(L-3) Zn(OH2)](2+) and [(tpa) Zn(OH2)](2+) deprotonate with pK(a) values of 6.0 and 8.0, respectively. The pK(a) of the water molecule, however, is only raised from 8.0 in [(tpa) Zn(OH2)](2+) to 9.1 in [(bpg) Zn(OH2)](+) (bpa = (pyCH(2))(2)N(CH2COO-)). Moreover, the acidity of the zinc-bound water of several of the five-coordinate zinc(II) complexes with the hydrogen bonding groups is greater than that of four-coordinate [((12) aneN(3)) Zn(OH2)](2+)(pK(a) = 7.0). This result shows that the magnitude of the effect exerted by the hydrogen bonding groups can be larger than that induced by changing one neutral by one anionic ligand, and/or even by changing the coordination number of the zinc(II) centre. The X-ray structure of [(L-'2) Zn(OH)] ClO4 2' and [(L-'3) Zn(OH)] ClO4 . CH3CN 3'.CH3CN is reported, and show the neopentylamino groups forming N - H...O hydrogen bonds with the zinc-bound hydroxide. Although 1'-3', which have hydrogen bonding and hydrophobic groups, have a zinc-bound water more acidic than [(tpa) Zn(OH2)](2+), their pK(a) is not always lower than that of 1 - 3. This result suggests that a hydrogen bonding microenvironment may be more effective than a hydrophobic one to increase the zinc - water acidity.