(Me,Me)bimane as a metal-binding ligand: Complexation and photophysical effects

syn-Bimanes are fluorescent dyes used in varied applications, mainly as labeling agents for biomolecules.¹ Although extensive research on these compounds has been done, their coordination chemistry was unexplored until recently, when the first bimane-metal complex was reported by our group.² The latter contained Pd(II) that is coordinated to syn-(Me,Me)bimane through the two carbonyl groups of the bimane. Recently, we have also shown that syn-(Me,Me)bimane coordinates Na⁺, and that the fluorescence of the bimane is quenched by this alkali cation in organic solvents and water.³ These results may have consequences for the use of bimane dyes, since they are used in biological media, which normally contain an abundance of metal ions that may change the chemical and photophysical properties of the dye. Therefore, data generated with these fluorophores in biological media should be carefully considered.

The above results prompted us to study the effect of different biologically-relevant alkali and alkaline earth metal cations, e.g., Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺, on the chemical and photophysical properties of syn-(Me,Me)bimane. Preliminary spectroscopic studies of different bimane-metal systems, mainly containing the metal cations Li⁺, Na⁺ and K⁺, revealed that quenching could occur selectively at one of the two bimane fluorescence bands, or at both fluorescence bands together. The metal cation was found to be the primary quencher in salts that do not contain the known anion quenchers I^‒ and Br^‒. Furthermore, our structural studies of bimane-metal complexes revealed that syn-(Me,Me)bimane binds the metal cations via its carbonyl oxygen atoms as a bidentate chelating ligand. Some of the bimane-metal complexes displayed polymeric structures, while others exhibited discrete structures.

_{(1) Goldberg, I.; Kosower, E. M. J. Phys. Chem 1982, 86, 332–335.}

_{(2) Das, P. J.; Diskin-Posner, Y.; Firer, M.; Montag, M.; Grynszpan, F. Dalton Trans. 2016, 45 (43), 17123–17131.}

_{(3) Roy, A.; Das, P. J.; Diskin-Posner, Y.; Firer, M.; Grynszpan, F.; Montag, M. New J. Chem. 2018, 2–6.}