Abstract
Biradicals are important intermediates in the formation and breaking of a chemical bond. Their use as molecular switches is of particular interest. Much less is known about tetraradicals, which can, for example, consist of two biradical(oid) units. Here we report the synthesis of the first persistent phosphorus-centred tetraradical bound to a transition metal fragment. Starting from a zirconocene complex, rac-(ebthi)ZrCl2 (rac-(ebthi) = 1,2-ethylene-1,10-bis(h5-tetrahydroin-denyl), two cyclo-1,3-diphospha-pentane-1,3-diyls were successfully introduced, which finally led to the isolation of a deep green zircon-cene-bridged bis(biradicaloid) complex (5) that can act as a double molecular switch. Under the influence of light (570nm), this tetra-radical forms a transannular bond in each of the two five-membered biradical units, leading to the formation of housane 5h. Upon irradiation at 415nm, the reverse reaction is observed, fully reco-vering tetraradical 5. Through single-crystal-to-single-crystal trans-formation, both stable species of the molecular switch could be structurally characterised using SCXRD. The switching under the influence of light and the activation of molecular hydrogen were analysed in solution using NMR and UV spectroscopy. It was found that the addition of one or two equivalents of molecular hydrogen can be switched on and off by light.
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