Aldimines generated from aza-Wittig reaction between bis(iminophosphoranes) derived from 1,1′-diazidoferrocene and aromatic or heteroaromatic aldehydes: electrochemical and optical behaviour towards metal cations

Abstract

Aldimine 4 bearing a 2-quinolyl group was prepared by aza-Wittig reaction between the triphenyliminophosphorane derived from the 1,1′-diazidoferrocene and 2-formylquinoline. However, aldimine 5, bearing a pyrene ring, was prepared using the most reactive tributyliminophosphorane derivative and the corresponding 1-formylpyrene. On the other hand, formation of aldimine 8 involves a tandem process, Staudinger reaction/intramolecular aza-Wittig reaction, by using directly 1,1′-diazidoferrocene and 2-(diphenylphosphonyl)benzaldehyde. Aldimine 4 behaves as chemosensor molecule for Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺ cations through two different channels: electrochemical (ΔE_1/2 = 222–361 mV) and chromogenic (Δλ = 122–153 nm), which can be used for the “naked eye” detection of these metal cations. Aldimine 5 behaves as a highly selective redox (in CH₃CN) and fluorescent (in CH₃Cl–DMF) probe for Hg²⁺ metal cations even in the presence of a large excess of the other metal cations tested. Aldimine 8 displays electrochemical affinity (ΔE_1/2 = 60–288 mV) to Li⁺, Ca²⁺, Mg²⁺, Zn²⁺ and Pb²⁺ metal cations, with the phosphorus oxide functionality as a binding site. From the ¹H NMR titration data as well as DFT calculations, different tentative binding modes have been established, for these structurally related ferrocenyl derivatives.