Synthesis of Cobalt-Centered Inorganic Sandwich Complexes

Complete inorganic sandwich complexes with group 15 element rings at a single metal center have remained elusive for more than 30 years. Herein, we report that the readily accessible Zintl ion E₇^3- (E = P, As) reacts with Co(II) sources to afford the first single-centered cobalt inorganic sandwiches, [(η³-E₃)Co(η⁴-E₄)]^2-. This work also represents the first extension of inorganic sandwiching from phosphorus to arsenic analogues. Mechanistic studies on the Co-P system reveal a two-step pathway via a [Co(P₇)₂]^4- intermediate that undergoes thermally induced P₇^3- fragmentation and rearrangement. While Ti and Fe analogues can be accessed efficiently using the same P₇^3- precursor, their transformation pathways differ, reflecting metal-dependent reactivity. The P₇^3--based route also provides superior access to [Ti(η⁵-P₅)₂]^2- under mild conditions, and [Fe(η⁴-P₄)₂]^2- with higher yield and purity than the previous approach. These results suggest that P₇^3- acts as a versatile phosphorus source for inorganic sandwich synthesis, wherein cobalt systems allow product distribution to be modulated by the solvent choice and reaction conditions.