The phenoxy/phenol/copper cation: A minimalistic model of bonding relations in active centers of mononuclear copper enzymes

The bare complex [Cu(PhOH)(PhO)](+) with a phenol (PhOH) and a phenoxy (PhO) ligand bound to copper is studied both experimentally and computationally. The binding energies and structure of this complex are probed by mass spectrometry, infrared multi-photon dissociation, and DFT calculations. Further, the monoligated complexes [Cu(PhO)](+) and [Cu(PhOH)](+) are investigated for comparison. DFT calculations on the [Cu(PhOH)(PhO)](+) complex predict that a phenolate anion interacts with copper(II) preferentially through the oxygen atom, and the bonding is associated with electron transfer to the metal center resulting in location of the unpaired electron at the aromatic moiety. Neutral phenol, on the other hand, interacts with copper preferentially through the aromatic ring. The same arrangements are also found in the monoligated complexes [Cu(PhO)](+) and [Cu(PhOH)](+). The calculations further indicate that the bond strength between the copper atom and the oxygen atom of the phenoxy radical is weakened by the presence of neutral phenol from 2.6 eV in bare [Cu(PhO)](+) to 2.1 eV in [Cu(PhOH)(PhO)](+).