Pd(0)-Mediated Deallylation Chemistry: A Reassessment of Its Application in Sensing CO

Pd(0)-mediated deallylation has been employed for developing fluorescent probes for carbon monoxide (CO). The key idea relied on the ability of CO to reduce Pd(II) to Pd(0). However, most studies used Ru-based CORM-2 and/or CORM-3 as CO sources, despite their known chemical reactivity and idiosyncratic CO release. Herein, we conducted studies using one of the most widely used probes (FL-CO-1), evaluating its response to various CO sources and to Pd(0). We found that (1) the activation of FL-CO-1 by CORM-2/-3 has CO-independent component(s); (2) vitamin C and cysteine were found to interfere with the probe's performance; and (3) Pd(0) only led to moderate fluorescence turn-on, while a combination of Pd(0) and CO resulted in a pronounced fluorescence turn-on response. Such findings indicate that the role(s) of CO goes beyond Pd(II) reduction, and accurate in vivo detection of CO using this approach is unlikely because of the presence of vitamin C and thiols in living systems. These new insights suggest the need to reinterpret some results, particularly when chemically reactive CORM-2 and CORM-3 were employed as CO surrogates. We recommend that future studies avoid using reactive CORMs to ensure experimental rigor.