syn-Bimanes: established as Fluorescent Labels, Overlooked as Ligands

Bimanes, which are a family of fluorescent molecules based on the 1,5-diazabicyclo[3.3.0]octadienedione framework (Scheme 1), were introduced nearly four decades ago by Kosower and coworkers.^[1] These compounds exist in two diastereomeric forms, namely, syn- and anti-bimanes. The former are strongly fluorescent and are utilized as fluorescent probes, primarily for biological labeling, whereas the latter fluoresce only weakly. All bimanes bear the typical hallmarks of a metal-binding ligand, i.e., both O- and N-donors, as well as a sterically-accessible p system. However, despite being known and used for almost 40 years, often in metal-containing media (e.g., living cells), the coordination chemistry of bimanes and its spectroscopic implications have not been studied. Recently, we reported the first example of a metal-coordinated bimane core, a cationic Pd(II) complex containing syn-(Me,Me)bimane (Scheme 1; R^1-4 = Me) as an O-donor chelating ligand.^[2] We have since expanded our investigation to include complexes of syn-(Me,Me)bimane with biologically-relevant metal ions, among others. Herein, we shall explore the properties of syn-bimane as a ligand, and the coordination-induced changes in its absorbance and fluorescence spectra - effects that are relevant to its use as a fluorescent label in biological systems.

^[1] Kosower, E. M.; Pazhenchevsky, B.; Hershkowitz, E., J. Am. Chem. Soc. 1978, 100, 6516-6518.

^[2] Das, P. J.; Diskin-Posner, Y.; Firer, M.; Montag, M.; Grynszpan, F., Dalton Trans. 2016, 45, 17123-17131.