11B NMR Probes of Copper(II): Finding and Implications of the Cu2+‐Promoted Decomposition of ortho‐Carborane Derivatives

Abstract

AbstractThe development of noninvasive methodologies for the detection of d‐block metal ions such as copper (Cu2+), zinc (Zn2+), and manganese (Mn2+) is important for understanding their biological roles and relationship with diseases. We have been interested in the use of 11B NMR probes for the detection of d‐block metal ions, because 11B is an ultratrace element in living systems. o‐Carboranes, which consist of ten boron and two carbon atoms, have been applied to numerous drugs and biological active agents. In this work, we found that the o‐carborane‐pendant cyclens (L3–L5) (cyclen = 1,4,7,10‐tetraazacyclododecane) are decomposed in the presence of Mn2+ or Cu2+ in aqueous solution at neutral pH, accompanied by the release of 4–9 equiv. of B(OH)3. Furthermore, it was found that o‐carborane derivatives that contain hydroxyl groups instead of a cyclen unit also undergo decomposition in the presence of Cu2+ and the corresponding complexes such as Cu(bpy) to afford 10 equiv. of B(OH)3, as confirmed by 11B NMR spectroscopic analysis and an azomethine‐H assay. These reactions are applied to 11B MRI (magnetic resonance imaging) probes for Cu2+.