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

Magnetic susceptibility and EPR studies show that trinuclear CuII−pyrazolato complexes with a Cu3(μ3-X)2 core (X = Cl, Br) are ferromagnetically coupled: JCu-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, C3H3N21-) are presented.