A series of bioinspired copper(II) complexes of N 4 -tripodal and sterically crowded diazepane-based ligands have been investigated as catalysts for functionalization of the aromatic C−H bond. The tripodalligand-based complexes exhibited distorted trigonal-bipyramidal (TBP) geometry (τ, 0.70) around the copper(II) center; however, diazepaneligand-based complexes adopted square-pyramidal (SP) geometry (τ, 0.037). The Cu−N Py bonds (2.003−2.096 Å) are almost identical and shorter than Cu−N amine bonds (2.01−2.148 Å). Also, their Cu−O (Cu− O water , 1.988 Å; Cu−O triflate , 2.33 Å) bond distances are slightly varied. All of the complexes exhibited Cu 2+ → Cu + redox couples in acetonitrile, where the redox potentials of TBP-based complexes (−0.251 to −0.383 V) are higher than those of SP-based complexes (−0.450 to −0.527 V). The d−d bands around 582−757 nm and axial patterns of electron paramagnetic resonance spectra [g ∥ , 2.200−2.251; A ∥ , (146−166) × 10 −4 cm −1 ] of the complexes suggest the existence of five-coordination geometry. The bonding parameters showed K ∥ > K ⊥ for all complexes, corresponding to out-ofplane π bonding. The complexes catalyzed direct hydroxylation of benzene using 30% H 2 O 2 and afforded phenol exclusively. The complexes with TBP geometry exhibited the highest amount of phenol formation (37%) with selectivity (98%) superior to that of diazepane-based complexes (29%), which preferred to adopt SP-based geometry. Hydroxylation of benzene likely proceeded via a Cu II -OOH key intermediate, and its formation has been established by electrospray ionization mass spectrometry, vibrational, and electronic spectra. Their formation constants have been calculated as (2.54−11.85) × 10 −2 s −1 from the appearance of an O (π* σ ) → Cu ligand-to-metal charge-transfer transition around 370−390 nm. The kinetic isotope effect (KIE) experiments showed values of 0.97−1.12 for all complexes, which further supports the crucial role of Cu-OOH in catalysis. The 18 O-labeling studies using H 2 18 O 2 showed a 92% incorporation of 18 O into phenol, which confirms H 2 O 2 as the key oxygen supplier. Overall, the coordination geometry of the complexes strongly influenced the catalytic efficiencies. The geometry of one of the Cu II -OOH intermediates has been optimized by the density functional theory method, and its calculated electronic and vibrational spectra are almost similar to the experimentally observed values.

Nickel(ii)complexes of N4-ligands are reported as efficient catalysts for direct benzene hydroxylation via bis(μ-oxo)dinickel(iii) intermediate species. The exclusive phenol formation is achieved with a yield of 41%.

Cobalt(ii) complexes reported as efficient and selective catalysts for single-step phenol formation from benzene using H2O2. The catalysis proceeds likely via cobalt(iii)-hydroperoxo species.

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