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 host–ligand scaffold that stabilises enzymatically relevant HS metal–oxo species without sacrificing oxidative power. Using a combination of calibrated density functional theory (DFT), 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/VO(H₂O)]^2+/3+. Coupled-cluster 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 high valent metal–oxo systems. Electronic-structure analyses reveal a systematic evolution from strongly covalent ferryl character in Mn^IVO and Fe^IVO to pronounced oxygen-centred oxyl-radical character in Co^IVO. These trends directly govern reactivity: the computed methane C–H activation barriers decrease along the Mn < Fe < Co series, with all systems operating via a concerted proton-coupled electron transfer (PCET) mechanism. The predicted reactivity trend and the exceptional activity of cobalt are strongly supported by experimental reports of CB[5]-stabilised Co^IVO/Co^III–O˙ intermediates active even under aqueous conditions.