A novel metal-wingtip pentalene system: the synthesis, structure, and reactivity of non-aromatic wingtip osmapentalenes†

Since the concept of aromaticity was first introduced in transition metal complexes, metals have become a crucial component for modulating aromaticity, leading to a variety of structural frameworks. Initial studies successfully achieved aromatic pentalene dianions through metal-ion coordination, and recent advancements in bridgehead metallapentalenes have demonstrated the transformation of antiaromaticity into aromaticity. This research introduces an efficient [2 + 2 + 1] cycloaddition strategy with a dual-path cooperative mechanism for constructing the first structurally defined non-aromatic wingtip metallapentalene, including wingtip osmapentalene and its methoxy and hydroxy derivatives. Osmapentalenes display versatile reactivity, allowing for multi-site functionalization via nucleophilic and electrophilic substitutions. Furthermore, the hydroxy derivative easily undergoes a ring-opening reaction with methanol to form osmafuran. The lowest-energy absorption of wingtip osmapentalene falls within the visible range, with adjustable optical and electrochemical properties.