Photophysical Characteristics and Reactivity of Bis(2,9-di-<i>tert</i>-butyl-1,10-phenanthroline)copper(I)

The recently synthesized sterically constrained copper(I) complex [Cu(dtbp)2]+ (1), where dtbp is 2,9-di-tert-butyl-1,10-phenanthroline, exhibits unique photophysical and reactivity properties. Complex 1 (λabs, 425 nm; ε, 3100 L M−1 cm−1; λemission, 599 nm) has the longest metal-to-ligand charge-transfer (MLCT) emission lifetime (τ, 3260 ns) and largest quantum yield (ϕ, 5.6%) of all [Cu(R2phen)2]+ complexes. Complex 1 also exhibits a large positive reduction potential for the [Cu2+(dtbp)2]|[Cu+(dtbp)2] couple (E1/2 = 0.70 V vs Fc+/0) and a large negative excited-state reduction potential for the [Cu2+(dtbp)(dtbp−•)]|[Cu2+(dtbp)2] couple (E1/2 = −1.66 V vs Fc+/0), indicating that this complex is a potent photoreductant in the excited state. The steric constraint imposed by the t-butyl substituents in 1 enables unusual ligand replacement reactivity. Either CH3CN or CO replaces one of the dtbp ligands, a reaction that is readily followed by loss of the unique emission signature of 1. Monodentate CH3CN binds to the copper(I) center with an affinity 2 orders of magnitude greater than that of the displaced dtbp, despite the fact that the displaced ligand is bidentate. CO-induced displacement of dtbp from 1 is reversible, but only in the presence of 1 equiv of unbound dtbp. The exceptionally strong donor ligand CH3NC displaces both dtbp ligands from 1. In contrast to the facile ligand displacement reactivity with good donor ligands, 1 does not react readily with O2, by either a ligand displacement or an oxidative pathway. Rather, O2 induces partial quenching of emission via an outer-sphere interaction with 1.