Triangular, Ferromagnetically-Coupled Cu^II₃−Pyrazolato Complexes as Possible Models of Particulate Methane Monooxygenase (pMMO)

Magnetic susceptibility and EPR studies show that trinuclear Cu^II−pyrazolato complexes with a Cu₃(μ₃-X)₂ core (X = Cl, Br) are ferromagnetically coupled:  J_Cu-Cu = +28.6 cm^-1 (X = Cl), +3.1 cm^-1 (X = Br). The orderly transition from an antiferromagnetic to a ferromagnetic exchange among the Cu centers of Cu₃(μ₃-X) complexes, X = O, OH, Cl, Br, follows the change of the Cu−X−Cu angle from 120° to ∼80°. The crystal structures of [Bu₄N]₂[Cu₃(μ₃-Br)₂(μ-pz*)₃Br₃] (pz* = pz (1a) or 4-O₂N-pz (1b), pz = pyrazolato anion, C₃H₃N₂^1-) are presented.