Host-guest complexes of cucurbit[8]uril and [(η5-C5H4R)Mo(CO)3CH3] (R = H, CO2CH3) for controlled release of carbon monoxide

The monosubstituted cyclopentadienyl half-sandwich molybdenum(II) tricarbonyl complex [(η⁵-C₅H₄CO₂CH₃)Mo(CO)₃CH₃] (1) and its 1:1 inclusion compound with cucurbit[8]uril (1@CB8) have been prepared and characterised by elemental and thermogravimetric analyses, powder X-ray diffraction, diffuse reflectance UV–vis, solid-state ¹³C{¹H} MAS NMR, FT-IR, and Raman spectroscopies. The CO-releasing behaviours of 1, 1@CB8 and, for comparison, [(η⁵-C₅H₅)Mo(CO)₃CH₃] (2) and its 1:1 inclusion complex with CB8, were assessed using a deoxymyoglobin-carbonmonoxymyglobin assay. For assays performed in the dark at 37 °C, complex 1 undergoes thermally assisted spontaneous CO release, with ca. 0.5 equivalents of CO being released after 6 h. The half-life (t_1/2) of 325 min identifies 1 as a slow releaser when compared to complex 2 bearing the unsubstituted cyclopentadienyl ligand (t_1/2 = 25 min). CO release from 1 was promoted by exposure to UV light (t_1/2 = 85 min), establishing the complex as a photochemically activated CO-releasing molecule (photoCORM). For 1@CB8, t_1/2 for photo-assisted CO release increased to ca. 7 h, and for 2@CB8 the dark-release t_1/2 increased to 165 min, showing that molecular acceptors like cucurbiturils can be used effectively as second-sphere ligands to modulate the CO release profile of CORMs.