Can an ancillary ligand lead to a thermodynamically stable end-on 1: 1 Cu-O₂ adduct supported by a β-diketiminate ligand?

The finding that dioxygen binds end-on to the Cu_B site in the crystal structure of a precatalytic complex of peptidylglycine α-hydroxylating monooxygenase has spurred the search for biomimetic model complexes exhibiting the same dioxygen coordination. Recent work has not only indicated that sterically hindered β-diketiminate ligands (L¹) could support side-on 1: 1 Cu-O₂ adducts, but also that an end-on L¹Cu(THF)O₂ structure occurs as an unstable intermediate in the oxygenation mechanism of the Cu(i) complex. In this work, density functional theory and multireference methods are used to determine the potential of ancillary ligands, X, other than THF to yield thermodynamically stable end-on L¹CuXO₂ species. A diverse set of ligands X, comprising phosphines, thiophene, cyclic ethers, acetonitrile, para-substituted pyridines, N-heterocyclic carbenes, and ligands bearing hydrogen bond donors, has been considered in order to identify ligand characteristics which energetically favor end-on L¹CuXO₂ over: a) reversion to the Cu(i) complex and dioxygen, b) isomerization to side-on L ¹CuXO₂, and c) decay to L¹CuO₂ and X. Ancillary ligands with judiciously chosen degrees and orientation of steric bulk and which bear potential hydrogen bond donors to an end-on bound dioxygen moiety most favor oxygenation of L¹CuX to yield end-on L ¹CuXO₂. Conversion to the side-on isomer can be deterred through the use of a sufficiently bulky ligand X, such as one that is at least the size of a 5-membered ring. Loss of X to give L¹CuO₂ can be made prohibitively endergonic by employing ligands X which are highly electron donating and which backbond strongly with and σ-donate significantly to copper.