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

Abstract

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)SO3CF3]SO3CF3, 1; [Cu(L1)ClO4]ClO4, 2 [L1=1,4‐bis[(pyridin‐2‐yl‐methyl)]‐1,4‐diazepane]; [Cu(L2)SO3CF3]SO3CF3, 3 and [Cu(L2)ClO4]ClO4, 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‐NHis bond distances in native CAOs enzyme (Cu‐NHis‐456, 2.0‐2.1 Å, Cu‐NHis‐458, 1.9‐2.2 Å and Cu‐NHis‐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 eg 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||, 153×10−4cm−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‐Npy 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.

Benzene Hydroxylation by Bioinspired Copper(II) Complexes: Coordination Geometry versus Reactivity

“…below). Complexes 4 and 6 were crystallized, and their molecular structures were reported as distorted SP geometry . Suitable single crystals of 2a and 4a were isolated by diethyl ether diffusion into complexes in acetonitrile (CH 3 CN) and used for X-ray diffraction studies.…”

“…The ligands N , N -bis­(2-pyridylmethyl)- N ′, N ′-dimethylethane-1,2-diamine (L1), N , N -bis­(2-pyridylmethyl)- N ′, N ′-dimethylpropane-1,3-diamine (L2), N , N -bis­(4-methoxy-3,5-dimethylpyridin-2-ylmethyl)- N ′, N ′-dimethylpropane-1,3-diamine (L3), 1,4-bis­(2-pyridinylmethyl)-1,4-diazepane (L4), 1,4-bis­(2-quinolylmethyl)-1,4-diazepane (L5), and 1,4-bis­(2-pyridinylethyl)-1,4-diazepane (L6) were synthesized as reported previously by us. − The copper­(II) complexes [Cu­(L)­(ClO 4 )]­ClO 4 ( 1 – 6 ) have been isolated by the reactions of Cu­(ClO 4 ) 2 ·6H 2 O and the respective tetradentate N 4 ligands in methanol (Scheme ). Additionally, SO 3 CF 3 – -ion-containing complexes [Cu­(L)­(SO 3 CF 3 )]­SO 3 CF 3 ( 1a – 6a ) were also synthesized to understand the influence of counterions in catalysis. The complexes have been characterized by various analytical methods (cf.…”

“…The distortion in the TBP geometry is revealed by variation of the N(1)–Cu–N(3) [125.2(2)°], N(3)–Cu–N(4) [119.3(2)°], and N(1)–Cu–N(4) [113.74(19)°] bond angles at the equatorial site. The axial bond angle N(2)–Cu–O(1) deviated from 180°, which additionally confirmed the existence of significant distortion in the coordination geometry . The complexes of the other tripodal ligands L1 and L3 are expected to display similar TBP-based coordination geometries.…”

Chlorocobalt complexes with pyridylethyl-derived diazacycloalkanes

“…For Ppz, both copper(II) (Mautner et al, 2008(Mautner et al, , 2009O'Connor et al, 2012) and nickel(II) (O'Connor et al, 2012) complexes have been described. In the case of Phpz, there are reports of copper(II) complexes (O'Connor et al, 2012), including their application as oxidation catalysts (Muthuramalingam et al, 2017(Muthuramalingam et al, , 2020. In addition, nickel(II) complexes of Phpz have been studied as catalysts (Muthuramalingam et al, 2019a,b) as has a recent cobalt(II) complex (Anandababu et al, 2020).…”

“…As the same ligand behaves as a straightforward mononucleating quadridentate in the copper and nickel complexes (O'Connor et al, 2012;Muthuramalingam et al, 2017Muthuramalingam et al, , 2019a, this led to the question as to whether the coordination is governed by the ligand bite vs the larger ionic radius of Co 2+ vs Cu 2+ /Ni 2+ . This proposal was approached by synthesising the homopiperazine analogue, Phpz, whose ligand has a larger (N2-N2A) bite.…”