Ground-State Electronic and Magnetic Properties of a μ₃-Oxo-Bridged Trinuclear Cu(II) Complex:  Correlation to the Native Intermediate of the Multicopper Oxidases

Abstract

The ground state electronic and magnetic properties of one of the possible structures of the trinuclear Cu II site in the native intermediate (NI) of the multicopper oxidases, the μ 3 -oxo bridged structure, are evaluated using the C 3 -symmetric Cu 3 II complex, μ 3 O. μ 3 O is unique in that no ligand, other than the oxo, contributes to the exchange coupling. However, μ 3 O has a ferromagnetic ground state, inconsistent with that of NI. Therefore, two perturbations have been considered: protonation of the μ 3 -oxo ligand and relaxation of the μ 3 -oxo ligand into the Cu 3 plane. Notably, when the oxo-ligand is sufficiently close to the Cu 3 plane (< 0.3 Å), the ground state of μ 3 O becomes antiferromagnetic and can be correlated to that of NI. In addition, the ferromagnetic 4 A ground state of μ 3 O is found from variable-temperature EPR to undergo a zero-field splitting (ZFS) of 2D = -5.0 cm -1 , which derives from the second-order anisotropic exchange. This allows evaluation of the σ-to-π excited state exchange pathways and provides experimental evidence that the orbitally-degenerate 2 E ground state of the antiferromagnetic μ 3 O would also undergo a ZFS by the first-order antisymmetric exchange that has the same physical origin as the anisotropic exchange. The important contribution of the μ 3 -oxo bridge to the ground-to-ground and ground-to-excited state superexchange pathways that are responsible for the isotropic, antisymmetric and anisotropic exchange are discussed.

Copper Active Sites in Biology

“…Both structures have the appropriate orbital arrangements for ground to ground and ground to excited state exchange coupling to have significant antisymmetric exchange (Figure 157C) and indeed we have observed this experimentally. 66,916,917 Single crystal EPR experiments on TrisOH, Figure 158, show that very low g-values are in fact observed for this structure, when the field is perpendicular to its z-axis. 918 From Figure 157C, for TrisOH two d x2-y2 ground state orbitals on two Cu(II)’s strongly antiferromagnetically couple, and SOC on one Cu gives a d xy excited state on that Cu, that can strongly couple to the d x2-y2 ground state of the adjacent Cu, thus fulfilling the requirements for efficient antisymmetric exchange.…”

“…A similar mechanism is realized for µ 3 -oxo when the µ 3 -bridging oxo group resides in or close to the plane of the three Cu(II)’s. 917 In this case, each d z2 ground state can SOC to a d xz excited state on an adjacent Cu(II) via the L z orbital angular momentum operator. Thus both TrisOH and µ 3 -oxo are possible structures for NI.…”

Electron transfer and reaction mechanism of laccases

“…The resulting analysis yields exchange coupling constants (−2J) of 430 and 470 cm −1 for the other two bridges [100]. This spin frustration also gives rise to the g values below 2.0 due to a phenomenon known as antisymmetric exchange [101]. This requires good ground state to ground state exchange coupling between two adjacent Cu(II)s, spin orbit coupling between the ground and excited states on a single Cu(II), and exchange coupling between this excited state and the ground state on the adjacent Cu(II).…”

Diastereomerism in tetranuclear copper(II) complexes of a phenol based “end-off” compartmental ligand

“…Since the foremost reports of tetranuclear copper(II) complexes with a Cu 4 (μ 4 -O) core by Bertrand et al [2] and by Bock et al [3] at the end of the sixties, there has been a continuous interest towards such complexes [4]. This attention is still prevailing in the present decade due to the possible applications of these species in catalysis, biochemistry, theoretical chemistry and material science [5][6][7][8][9][10][11][12]. In fact several tetranuclear copper(II) complexes with N 2 , O-donor 4-methyl-2,6-bis(R-iminomethyl)-phenol ligands have been reported.…”