How does Cu(II) convert into Cu(I)? An unexpected ring-mediated single-electron reduction

CuxO (x=1,2) nanomaterials with tailored composition and properties-a hot topic in sustainable technologies-may be fabricated from molecular sources through bottom-up processes that involve unexpected changes in the metal oxidation state and open intriguing challenges on the copper redox chemistry. How copper(II) sources may lead to copper(I) species in spite of the absence of any explicit reducing agent, and even in the presence of oxygen, is one such question-to date unanswered. Herein, we study copper "reduction without reductants" within one molecule and reveal that the actual reducing agent is abstracted atomic hydrogen. By investigating the fragmentation of a copper(II) precursor for copper oxide nanostructures by combined ESI-MS with multiple collisional experiments (ESI/MSn) and theoretical calculations, we highlight a copper-promoted C-H bond activation, leading to reduction of the metal center and formation of a CuI-C-NCCN six-membered ring. Such a novel ring system is the structural motif for a new family of cyclic copper(I) adducts, which show a bonding scheme, herein reported for the first time, that may shed unprecedented light on copper chemistry. Beyond the relevance for the preparation of copper oxide nanostructures, the hydrogen-abstraction/proton-delivery/electron-gain mechanism of copper(II) reduction disclosed herein appears to be a general property of copper and might help to understand its redox reactivity.