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

Abstract

Ligands derived from the tripodal N₄ ligand tris(pyridylmethyl)amine ((pyCH₂)₃N, tpa) of general formula (6-RNHpyCH₂)_nN(CH₂py)_3−n (R = H, n = 1–3 L^1–3; 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 [(L^1–3)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 [(L³)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 = (pyCH₂)₂N(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)aneN₃)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)]ClO₄2′ 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.