Dirhodium Tetracarboxylate Scaffolds as Reversible Fluorescence-Based Nitric Oxide Sensors

We report the synthesis and characterization of dirhodium tetracarboxylate complexes [Rh2(μ-O2CR)4(L)2], with R = Me and L = dansyl-imidazole (Ds-im) or dansyl-piperazine (Ds-pip). The fluorophores
coordinate to the axial sites of the dirhodium core through the imidazole or piperazine N-atom and emit
only weakly when excited at 365 or 345 nm for the Ds-im and Ds-pip complexes, respectively. These
fluorophore-containing complexes were investigated for their ability to elicit a fluorescence response in the
presence of NO. An immediate increase in fluorescence emission of greater than 15-fold occurs when NO
is admitted to solutions containing [Rh2(μ-O2CMe)4] and Ds-pip or Ds-im. In both systems, the fluorescence
response, which arises by NO-induced displacement of the axially coordinated fluorophore, is reversible
with a sensitivity of ∼4 μM. The related dinitrosyl complexes [Rh2(μ-O2CR)4(NO)2], where R = Me, Et, or
n-Pr, were prepared, structurally characterized, and found to be air-stable, losing NO upon standing in
solution. Sequestration of a methylene chloride solution of the Ds-pip complex from aqueous media by a
NO-permeable membrane allows for fluorescence detection of NO for potential applications in biological
fluids.