Heterogeneous Binding of Dioxygen: EPR and DFT Evidence for Side-On Nickel(II)-Superoxo Adduct with Unprecedented Magnetic Structure Hosted in MFI Zeolite

This article reports on the activation of dioxygen on nickel(I) dispersed inside the nanopores of the ZSM-5 zeolite, which can be regarded as a heterogeneous mimetic system (zeozyme) for Ni-bearing enzymes. The side-on eta 2-coordination of the resulting nickel-bound superoxo adduct was ascer16O2 species supported by computer simulations of the spectra tamed by detailed analysis of the EPR spectra of both 16O2 and 17O2 species supported by computer simulations of the spectra and relativistic DFT calculations of the EPR signatures. Molecular analysis of the g and A(17O) tensors (gxx = 2.0635, gyy = 20.884, gzz = 2.1675; vertical bar Axx vertical bar approximate to 1.0 mT, vertical bar Ayy vertical bar = 5.67 mT, vertical bar A(zz)vertical bar approximate to 1.3 mT 1.3 mT) and quantum chemical modeling revealed an unusual electronic and magnetic structure of the observed adduct (with g(zz)(g(max)) > g(yy)(g(mid)) > g(xx)(g(min)) and the largest O-17 hyperfine splitting along the g(mid) direction) in comparison to the known homogeneous and enzymatic nickel-superoxo systems. It is best described as a mixed metalloradical with two supporting oxygen donor-ligands and even triangular spin-density redistribution within the)eta 2(-){NiO2}(11) magnetophore. The semioccupied molecular orbital (SOMO) is constituted by highly covalent 6 overlap between the out-of-plane 2p(pi(g)*) MO of dioxygen and the 3d(x2-y2) MO nickel. By means of the extended transition state-natural orbitals for the chemical valence approach (ETS-NOCV), three distinct orbital channels (associated with a, pi, and delta overlap) of congruent and incongruent charge and spin density flows within the eta(2)-{NiO2}(11) unit, contributing jointly to activation of the attached dioxygen, were identified. Their individual energetic relevance was quantified, which allowed for explaining the oxygen binding mechanism with unprecedented accuracy. The nature and structure sensitivity of the g tensor was rationalized in terms of the contributions due to the magnetic field-induced couplings of the relevant molecular orbitals that control the g-tensor anisotropy. The calculated O-17 hyperfine coupling constants correspond well with the experimental parameters, supporting assignment of the adduct. To the best of our knowledge, the eta(2)-{NiO2}(11) superoxo adducts have not been observed yet for digonal mononuclear nickel(I) centers supported by oxygen donor ligands.