A Homologous Series of Regioselectively Tetradeprotonated Group 8 Metallocenes:  New Inverse Crown Ring Compounds Synthesized via a Mixed Sodium−Magnesium Tris(diisopropylamide) Synergic Base

Subjecting ferrocene, ruthenocene, or osmocene to the synergic amide base sodium−magnesium tris(diisopropylamido) affords a unique homologous series of metallocene derivatives of general formula [{M(C₅H₃)₂}Na₄Mg₄(i-Pr₂N)₈] (where M = Fe (1), Ru (2), or Os (3)). X-ray crystallographic studies of 1−3 reveal a common molecular “inverse crown” structure comprising a 16-membered [(NaNMgN)₄]⁴⁺ “host” ring and a metallocenetetraide [M(C₅H₃)₂]^4- “guest” core, the cleaved protons of which are lost selectively from the 1, 1‘, 3, and 3‘-positions. Variable-temperature NMR spectroscopic studies indicate that 1, 2, and 3 each exist as two distinct interconverting conformers in arene solution, the rates of exchange of which have been calculated using coalescence and EXSY NMR measurements.