Structure–function correlation of mononuclear nonheme copper(ii) compounds based on a ligand backbone effect and phenoxazinone synthase activity

Abstract

Mononuclear nonheme copper(II) compounds [Cu(BQEN)(CH₃CN)]²⁺1 and [Cu(BQPN)(CH₃CN)]²⁺2 (BQEN = N,N′-dimethyl-N,N′-di(quinolin-8-yl)ethaneane-1,2-diamine; BQPN = N,N′-dimethyl-N,N′-di(quinolin-8-yl)propane-1,2-diamine) were synthesized and characterized by elemental analysis, ESI-MS, EPR, and single-crystal X-ray diffratrometry. Compound 1 crystalized in the P space group, while 2 crystallized in the P2₁/n group. Compounds 1 and 2 exhibited distorted square pyramidal geometry, which deviated interestingly by 4.8% and 46.0% from a regular square pyramidal structure in 1 and 2, respectively. This was attributed to the marginal increase in the carbon chain backbone of the ligand in 2. Compounds 1 and 2 were used in the aerial oxidation of 2-aminophenol (H₂AP) to 2-amino-phenoxazine-3-one (APX), which revealed phenoxazinone synthase activity. DFT steric maps were utilized to understand the catalytic activity of 1 (% Vbur, 71.3%) and 2 (% Vbur, 75.6%). The current results clearly demonstrate an interesting structure–function relationship based on the effect of the ligand backbone on the geometry.