Pentakis(methoxycarbonyl)cyclopentadiene chemistry. Part 1. Preparation and properties of the diene, and of derivatives containing the alkali metals or thallium(I): crystal and molecular structures of HC5(CO2Me)5, Li[C5(CO2Me)5]·H2O, K[C5(CO2Me)5]·MeOH, and Tl[C5CO2Me)5]

Abstract

The alkali metal (Li, Na, K, Rb, and Cs) and thallium(I) derivatives of HC₅(CO₂Me)₅ have been prepared and characterised; their i.r., u.v., ¹H, and ^l3C n.m.r. spectra are reported and discussed. Single-crystal X-ray structure determinations have been carried out on the diene and its Li, K, and Tl salts at 295 K. In the solid, as in non-polar solvents, the diene has the fulvenoid configuration, with the acidic proton bridging two adjacent carbonyl groups. This configuration is preserved in univalent metal cation salts. Lithium is four-co-ordinate (from two carbonyl oxygens, the water molecule, and a carbonyl oxygen from a second anion). In the potassium salt, the metal is co-ordinated by methanol, and five carbonyl oxygens from three different anions; all carbonyl oxygens of each anion are likewise involved in co-ordination to three different metal atoms. In the thallium salt the metal has irregular five-co-ordination, with two chelating carbonyl groups from one anion and three from separate anions; all carbonyl oxygens of each anion co-ordinate to four different metal atoms.