Stabilising High-Spin, High-Valent Transition-Metal-Oxo Species in Cucurbit[5]uril: Correlating Structure, Spin State, and Reactivity

Abstract

Stabilising highly reactive high-spin (HS) transition-metal-oxo intermediates outside enzymatic environments remains a fundamental challenge in oxidation catalysis, as such species are intrinsically prone to rapid decomposition and loss of selectivity. Synthetic ligand frameworks capable of enforcing enzyme-like electronic structures while retaining high reactivity under catalytic conditions are therefore exceedingly rare. In this work, we demonstrate that cucurbit[5]uril (CB[5]) functions as a rigid, weak-field, biomimetic hostligand scaffold that stabilises enzymatically relevant HS metal-oxo species without sacrificing oxidative power. Using a combination of calibrated density functional theory, benchmark DLPNO-CCSD(T) calculations, and explicit reaction-pathway analysis, we investigate a series of CB[5]-encapsulated Mn, Fe, and Co oxo complexes, [(CB [5])M IV/V =O(H₂O)]²⁺ / ³⁺. Coupledcluster benchmarks unequivocally confirm HS ground states for all complexes, with substantial energetic separation from competing low-spin manifolds, establishing that HS stabilisation is an intrinsic consequence of the weak equatorial ligand field imposed by the carbonyl portals of CB [5]. DFT benchmarking shows that functionals with moderate exact exchange (≈15-20%), particularly B3LYP, most reliably reproduce spin-state energetics across d³-d⁵ metal-oxo systems. Electronic-structure analyses reveal a systematic evolution from strongly covalent ferryl character in Mn IV =O and Fe IV =O-to pronounced oxygen-centred oxylradical character in Co IV =O. These trends directly govern reactivity: computed methane C-H activation barriers decrease along the Mn < Fe < Co series, with all systems operating via a concerted proton-coupled electron transfer mechanism. The predicted reactivity trend and the exceptional activity of cobalt are strongly supported by experimental reports of CB[5]stabilised Co IV =O/Co III -O • intermediates active even under aqueous conditions.