A ruthenium nitrosyl cyclam complex with appended anthracenyl fluorophore

What is it about?

The complex trans-[Ru(NO)(H2 O)(L)](ClO 4)3 (L = (1-anthracen-9-ylmethyl)-1,4,8,11-tetraazacyclotetradecane) (RuNOL) was synthesized and characterized, and its photophysics and reactivity were investigated. The FTIR spectrum displays an mNO band at 1840 cm-1, indicating a nitrosonium character. 1H and 13C signals in 1D and 2D NMR spectra were assigned and are consistent with the trans configuration. The UV–Vis spectrum displays maximal absorption bands at 341 nm (log e 3.74), 363 nm (log e 3.91), 378 nm (log e 3.97), and 397 nm (log e 3.91). Coordination of the [RuNO] moiety to the L ligand strongly quenches the anthracenyl fluorescence, and thus RuNOL exhibits only very weak emission at 390, 418, 440 and 472 nm. Electrochemical studies indicate that cathodic peak potentials at +1.10 V(Ic), -0.1 V(IIc), and -0.4 V(IIIc) (vs Ag/AgCl) are related to An+/An, {RuNO}6/7, and {RuNO}7/8 reduction processes, respectively. The pKa of coordinated water was estimated at 2.8 ± 0.2 by DPV. The emission of the pendant fluorophore increases upon electrochemical or chemical reduction of RuNOL, and the resulting switch-ON fluorescence is possibly due to NO release from the target complex. Spectroscopic titrations with DNA resulted in hypochromic and redshifts and allowed estimation of binding constants of 1.9 x103 (L) and 1.8 x103 (RuNOL). Molecular docking showed that the RuNOL complex has a great affinity with DNA. The RuNOL complex showed low cytotoxicity toward MCF-7 and NIH-3T3 and was ineffective in healthy HUVEC and A7r5 cell lines in the range of concentration of 1x10-7 – 1x10-4 mol /L.

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Photo by Shahadat Rahman on Unsplash

Photo by Shahadat Rahman on Unsplash

Why is it important?

The photophysical properties of cyclam metal complexes bearing anthracenyl or dansyl fluorophores N-appended to the macrocyclic ring are sensitive to the metal center and its oxidation state as well as to axial ligands such as NO. Conceivably, changes to the {RuNO}6 center in a ruthenium nitrosyl tetraazamacrocyclic complex with an appended fluorophore could result in a fluorescence response.

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