The unusual structure of potassium complexes with amidinylcyclopentadienyl ligands

Abstract

Cyclopentadienyl (Cp) complexes of alkali metals, with a sandwich structure and π interaction between the metal cation and the cyclopentadienyl ring anion, are widely used in the synthesis of organometallic compounds [1–5]. In pentakis(methoxycarbonyl)cyclopentadiene complexes, hard alkali-metal and alkaline-earth cations are coordinated to the hard carbonyl oxygen atoms [6]. We synthesized potassium complexes with tetrakis(methoxycarbonyl)cyclopentadienyl ligands containing an amidine substituent in the side chain in which the potassium atom is coordinated to a nitrogen atom of the side chain and to the Cp ring ( η 5 bonding). This coordination mode is unusual for the series of polykis(methoxycarbonyl)cyclopentadienyl ligands. Such a coordination mode is determined by the delocalization of the positive charge in the amidinyl substituents, which favors the stabilization of the ylide structure of complexes. The structure of these complexes was studied by X-ray crystallography, 1 c and 13 C NMR, 13 C CP/MAS NMR, and ESI mass spectrometry.