Attaching tweezers like ionophore to a proton crane: theoretical design of new tautomeric sensors

ABSTRACT

The tautomeric optical sensors based on 4-(phenyldiazenyl)naphthalen-1-ol exist in their pure enol tautomeric form as free ligands, while the addition of metal ion fully shifts the equilibrium towards the keto tautomer allowing a red shift in the measured absorbance. This effect is achieved when a side ionophore group is connected to a tautomeric backbone by a spacer in a way that stabilizes the enol form via hydrogen boding. When the ionophore captures the metal ion the keto form is stabilized due to C─O tautomeric group participation in the complex. In the current study, we model theoretically the effect of symmetric tweezer like ionophores (RCOXCOR, where X, being CH or N, is the linker to the tautomeric backbone) on the tautomeric state and complexation ability of 4-(phenyldiazenyl)naphthalen-1-ol containing ligands. It was found that enol form stabilisation is achieved when R = NMe₂, independing on the linker. Both ligands are unsuitable for capturing alkali metal ions. The calculations predict that the complexation with alkali earth metal ions could lead to a full shift of the tautomeric equilibrium towards keto tautomer.

GRAPHICAL ABSTRACT

KEYWORDS:

• DFT
• tautomerism
• ion sensing
• alkali and alkaline-earth metal ions

Acknowledgement

The authors acknowledge the COST Action CM1405 (Molecules in motion, MOLIM) for the ideas and knowledge exchange.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Liudmil Antonov http://orcid.org/0000-0003-0520-1517

Notes

1 Negative values correspond to more stable K tautomer and vice versa.

2 In the discussion below ΔE is electronic energy.

Additional information

Funding

This work was supported by Bulgarian National Science Fund [Grant Number DFNI DM09/7].