Well-Defined Dinuclear Copper(II) Complexes with Distorted Coordination Environments Promote C−N Coupling Reactions via Tandem CuII−CuIII Redox Catalysis

Dinuclear copper(II) complexes of the general formula [Cu2(μ-Ln)2] (1−6) with shortened salen-type N2O2 tetradentate Schiff base ligands named H2sal(X)ben (H2Ln, n = 1−7, X = H, 2-OMe, 4-OMe, 2-Cl, 4-Cl, 2,6-diOMe, 2,4,6-triOMe, respectively) revealed to be efficient and selective catalysts for Ullmann-type C−N coupling reactions under relatively mild conditions. These compounds display a distorted metal coordination environment, intermediate between tetrahedral and square planar, as shown by single-crystal X-ray diffraction, caused by the one-carbon bridge linking the iminic nitrogen atoms. This peculiar feature suggests that the metal ions can easily modulate and adapt to different redox states, evoking their potential as efficient catalysts. In fact, they revealed excellent performance in the Ullmann-type C−N coupling reaction, being reactive toward a wide range of N-nucleophiles and successfully coupling sterically hindered and electronically deactivated aryl iodides. Mechanistic studies support a 2-electron oxidative addition/reductive elimination event taking place on one copper(II) center, while the other acts as an electron reservoir to assist the redox process. Noteworthy, catalysts 1−6 are easily prepared from nonexpensive starting materials and are stable to air and moisture, characteristics that render them a very attractive alternative to classic in situ-generated catalytic systems, notably in the context of industrial applications.