Flexible Silylamine Ligands to Stabilize Copper Nanoclusters for Catalyzed Ullmann N-Arylation

Nanoclusters (NCs) with accessible sites have gradually become valued because of their outstanding performance. Herein, we propose a steric hindrance effect to construct CuNCs with electrostatic forces for Ullmann N-arylation reaction. Two CuNCs, Cu12(L1H)2(L1H2)2(PhNH)2 (Cu12) and Cu16(L2)4(Mes)4 (Cu16) (where L1H4 = N,N',N″,N‴-tetraphenylsilanetetraamine, L2H3 = 1-methyl-N,N',N″-triphenylsilanetriamine, Mes = mesityl, respectively), were synthesized by a similar metathesis reaction of an organocopper (CuMes)4 precursor and silylamine ligands under solvothermal conditions. The steric hindrance effect of L1H4 results in S-shaped 12-coordinated cores in Cu12, compared to beading-shaped 16-coordinated ones in Cu16. The catalytic efficiency for Cu12/CeO2 reaches a high yield of over 90% for a broad scope of substrates, indicating a rather broad versatility and practicability. Theoretical calculations reveal that the more accessible sites in Cu12, endowed with flexible ligands, exert stronger electrostatic attraction toward the substrates, thereby promoting Ullmann N-arylation.