Excitation Wavelength λ / nm Emission Intensity (AU) Figure 6. (A) Effects of ligand structure on iridium catalyst selectivity for formate vs NADH as hydride donors for transfer hydrogenation catalysis. Systematic screening efforts used aldehyde and Ir catalyst ([10 µM] each) in 20 mM PBS (pH 7.4) at 37 ºC. Fluorogenic reactions were monitored with λex = 500 nm and λem = 510-600 nm. Initial rates were estimated based on the increase in fluorescence emission intensity between t = 0 and t = 30 min. (B) Proposed model for observation of higher selectivity for formate over NADH catalyzed by Catalyst 19, due to a disfavorable steric interaction with NADH. ion-paired, or 6-membered hydrogen bonded (via a pendant ligand N-H moiety) transition state. Indeed, selectivity with a sulfonamide amine complex could be significantly improved to 2.7:1 in favor of formate by using a steric interaction at R 1 in the case of complex 19, which may be due to a disfavorable 1,3diaxial interaction between the benzyl group and NADH, which is not present with formate (Figure ). Next, we conducted analyte selectivity assays to evaluate reduction of F-1 using sulfonamide amine complex 19 and pyridine amide complex 9 with several biologically relevant endogenous hydride sources, reactive oxygen species (ROS), and reactive carbon species (RCS), including NAD(P)H, FADH, 5,10-Me-THF, formaldehyde, methanol, pyruvate, acetate, acrolein, glucose, hydrogen peroxide and hypochlorite (Figure and).