Copper(II)‐Bioinspired Models for Copper Amine Oxidases: Oxidative Half‐Reaction in Water

The quinone cofactor is generated at the oxidative half‐reaction of copper amine oxidases (CAO) activity, which is key biogenesis step in biology. The copper(II) complexes [Cu(L1)SO 3 CF 3 ]SO 3 CF 3 , 1 ; [Cu(L1)ClO 4 ]ClO 4 , 2 [L1=1,4‐bis[(pyridin‐2‐yl‐methyl)]‐1,4‐diazepane]; [Cu(L2)SO 3 CF 3 ]SO 3 CF 3 , 3 and [Cu(L2)ClO 4 ]ClO 4 , 4 [L2= 1 , 4‐bis[2‐(pyridin‐2‐yl)ethyl]‐1,4‐diazepane] were synthesized as models for copper amine oxidases. The molecular structure of complexes was determined by single crystal X‐ray studies and shows distorted square pyramidal geometry (τ, 0.064 ‐ 0.285). The average Cu‐N(2.0 Å) bond distances of model complexes are similar to the Cu‐N His bond distances in native CAOs enzyme (Cu‐N His‐456 , 2.0‐2.1 Å, Cu‐N His‐458, 1.9‐2.2 Å and Cu‐N His‐624 ,1.9 −2.1 Å).Only one Cu(II)/Cu(I) redox couple (‐0.296 to −0.343 V) was noted in acetonitrile for 1–4 but a well‐defined redox couple around −0.349 to −0.404 V with an additional anodic peak around −0.056 to −0.128 V appeared in the water. The electronic spectra of 1–4 at pH ∼ 6.0 in water, shows ligand‐based absorption band around 260 −264 nm with a shoulder around 290 nm and the d‐d transition appeared around 593–640 nm. Also, an unusual low‐energy transition is centered at 965 nm due to axial field splitting of the e g orbital sets. Interestingly, on raising of the pH solution to ∼7 ‐ 10 instigates a transition of square pyramidal coordination geometry into trigonal bipyramidal for 1–4 . This geometrical interconversion is further supported by appearance of two Cu(II)/Cu(I) redox couples around −0.313 to −0.390 V and −0.056 to −0.309 V at pH ranges of ∼7 ‐ 10. All the complexes exhibits almost similar solution EPR parameter (g || , 2.21 ‐ 2.45; A ||, 174 ‐ 193 × 10 −4 cm −1 ) to CAO (g || , 2.32; A ||, 153×10 −4 cm −1 ) 5 at 70 K. The model complexes convert substrate 2‐aminophenol into o ‐quinone simultaneously using molecular oxygen via oxidative deamination pathway and this formation accomplished by new absorption band at 430 nm with rate of 0.53 ‐ 11.2 ×10 −3 s −1 in water. The asymmetric Cu‐N py bonds and higher distortion in the square pyramidal geometry of 1 and 2 presumed to facilitate geometrical change via decoordination of one the pyridine arms and leads faster dioxygenation reaction than 3 and 4 . This decoordinated pyridine arm act as like an acid‐base catalyst to accept and donate protons as in CAO catalysis by amino acid residues at secondary coordination sphere.