Models for the trinuclear copper(II) cluster in the particulate methane monooxygenase from methanotrophic bacteria: Synthesis, spectroscopic and theoretical characterization of trinuclear copper(II) complexes

Three trinuclear Cu(II) complexes [Cu3(tacp)(μ3-Cl)2](Cl)4 (1), [Cu3(tacp)(μ3-Br)2](Br)4 (2) and [Cu3(tacp)(μ3-OH)2](Cl)4 (3) (tacp, 1,10,19-trioxa-4,7,13,16,22,25-hexaazacycloheptaeicosane) are synthesized to model the oxidized tricopper cluster implicated in the particulate methane monooxygenase from Methylococcus capsulatus (Bath). In the enzyme, the three Cu(II) ions are coupled by weak ferromagnetic interactions. The Cu(II) ions in 1 and 2 are shown to be ferromagnetically coupled from magnetic susceptibility and electron paramagnetic resonance (EPR) measurements. EPR suggests anti-ferromagnetic interactions among the Cu(II) ions in 3. Density functional theory calculations reproduce well the geometric, electronic and magnetic properties observed in these complexes and provide insights into the spin-coupling interactions mediated by the bridging ligands.