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 E 7 3– (E = P, As) reacts with Co(II) sources to afford the first single-centered cobalt inorganic sandwiches, [(η 3 -E 3 )Co(η 4 -E 4 )] 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(P 7 ) 2 ] 4– intermediate that undergoes thermally induced P 7 3– fragmentation and rearrangement. While Ti and Fe analogues can be accessed efficiently using the same P 7 3– precursor, their transformation pathways differ, reflecting metal-dependent reactivity. The P 7 3– -based route also provides superior access to [Ti(η 5 -P 5 ) 2 ] 2– under mild conditions, and [Fe(η 4 -P 4 ) 2 ] 2– with higher yield and purity than the previous approach. These results suggest that P 7 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.