Copper is one of the most abundant and less toxic transition metals, which plays a key role in many natural processes. The ability of Cu to access four oxidation states (Cu0, CuI, CuII and CuIII) using O2 as oxidant endows it with rich redox reactivity to carry out multifarious oxidation reactions in biological and chemical transformations. In these Cu/O2 systems, Cu-O species are commonly regarded as the active centers and a number of important Cu-O complexes have been identified. However, the detailed mechanisms are not well understood and the study on the structure of intermediate species involving Cu-O and organic substrates are less explored. It is expected that the isolation and characterization of organometallic Cu/O2 mixed-cluster intermediates would promote our understanding of catalytic mechanisms of the Cu/O2-based organic transformations.
Our group has been working on controllable synthesis and reactivity studies of polynuclear metal clusters based on size-tunable macrocyclic ligands (Py[n]s, azacalixpyridines). Taking advantage of the known macrocyclic effect and highly fluxional conformation of the size-tunable Py[n]s, our investigations have revealed that Py[n]s show a significant cooperative coordination effect in the metal ion binding process that facilitated the easy formation of a polymetallic assembled structure. It is envisioned that the employment of these polydentate macrocyclic ligands Py[n]s may provide a convenient tool to access multi-component organometallic Cu/O2 mixed-cluster intermediates in the Cu/O2-based organic transformations.
Glaser coupling reaction of terminal alkynes is one of the earliest known metal-catalyzed coupling reactions. Chemists have paid considerable attention to the mechanistic studies, however, detailed studies on the intermediate structures involving Cu-O species and copper-acetylide cluster moieties remains unclear up to now. Therefore, we attempted to capture the possible intermediate species in Glaser coupling reactions by using differently sized macrocyclic ligand. Intriguingly, two different copper-oxygen species [(tBuC≡CCuI3)-(μ2-OH)-CuII]@Py (1) and [CuII-(μ2-OH)-CuII]@Py (2) are in situ isolated from the same Glaser coupling reaction system. It is noteworthy that these two copper-oxygen clusters show distinct oxidative capacity and reactivity despite of the similar coordination environments of Py and Py. The [(tBuC≡CCuI3)-(μ2-OH)-CuII] cluster in complex 1, which can be regarded as merged from a [tBuC≡CCu3] and a [Cu-(μ2-OH)-Cu] unit in complex 2, features a remarkably strong oxidation capacity, comparative with some copper(III)-oxygen species and thus showing high reactivity in single-electron transfer (SET) and hydrogen atom transfer (HAT) reactions. In contrast, the simple copper-oxygen species [CuII-(μ2-OH)-CuII] in 2 encapsulated in a small macrocycle Py exhibits no HAT reactivity due to the poor oxidative ability of Cu(II). Detailed characterizations and theoretical calculations indicate that the oxidation ability difference between [(tBuC≡CCuI3)-(μ2-OH)-CuII]@Py and [CuII-(μ2-OH)-CuII]@Py is mainly ascribed to the uneven positive charge distribution of Cu(I) in the tBuC≡CCuI3 unit due to significant [dCu(I)→π*(C≡C)] back donation. The obtaining of the highly reactive bi-cluster merged structure expands the library of copper-oxygen species and sheds light on the mechanistic study of Glaser coupling. Further mechanistic studies on the Cu/O2-based organic transformations are ongoing.