Nucleophilic addition of phosphorus(iii) derivatives to squaraines: colorimetric detection of transition metal-mediated or thermal reversion

Abstract

Nucleophilic addition of phosphorus(iii) agents to the electrophilic core of intensely colored squaraine dyes gives a bleached zwitterionic adduct in good to excellent yields (up to 99%) at room temperature. The process can be reversed by adding specific transition metal complexes with high phosphorous(iii) affinity.

Figure 1.

Additions to the central ring of squaraine dyes. (a) Established nucleophilic additions to squaraines. (b) Colorimetric sensors for trace metal impurities. (c) Reversible formation of bleached squaraine/phosphine adducts.

Figure 2.

A) Titration plots showing partial regeneration of squaraine 2b absorbance at 656 nm when separate solutions of 3b (5 × 10⁻⁵ M in DMSO) were titrated with Rh(PPh₃)₃Cl, Pd(OAc)₂, [Ir(cod)Cl]₂, or Au(PPh₃)Cl. B) Appearance of squaraine 2b absorption maxima band upon titration of adduct 3b (3 × 10⁻⁵ M in DMSO) with increasing amounts of Pd(OAc)₂.

Figure 3.

Photographs of a vial containing a binary mixture of P(p-MeOC₆H₄)₃ and 2a in CHCl₃ at different temperatures. At 40 °C the solution exhibits a dark blue color indicating free squaraine, whereas cooling the mixture to –40 °C results in appearance of pale yellow adduct 3g.

Figure 4.

Partial ¹H NMR spectra (500 MHz, CDCl₃) for a single sample, showing how the equilibrium for reversible addition of P(p-MeOC₆H₄)₃ to squaraine dye 2b to produce adduct 3h changes with temperature. The sample temperature was cycled from 20 °C (top spectrum) through seven different temperatures to end up at –40 °C (bottom). The table provides the 3h:2b ratio at each temperature, as determined by ¹H NMR peak integration.

Scheme 1.

Observed product 3a for addition of PⁿBu₃ to squaraine 2a.