Metal Vapors in Synthesis: The Chemistry of Chromium Sandwich Complexes

Abstract

<p>A series of substituted bis(fluorobenzene)chromium complexes, (C₆H₄FX)₂Cr, where X is ortho, meta and para H, F, Cl, CH₃ and CF₃, have been synthesized by cocondensing chromium atoms and fluoroarenes. The ¹⁹F n.m.r. chemical shifts of these compounds correlate well with those of the corresponding C₆F₅X system suggesting that the overall electron-withdrawing effect of a π-bonded chromium atom is similar to the effect of four ring fluorine substituents.</p> <p>This correlation suggested that nucleophilic substitution reactions might be possible for fluorosubstituted bis(arene)chromium complexes; indeed bis(anisole) chromium was obtained in low yield from the reaction of bis{η-fluorobenzene)- chromium and sodium methoxide. Furthermore, a series of alkyl-and aryl-lithium reagents reacted readily with (C₆H₆)Cr(C₆F₆) at low temperature to yield polysubstituted products. Proton abstraction from the π-complexed C₆H₆ ring was a competing process and led to telomeric sandwich complexes of which several were identified spectroscopically.</p> <p>The behaviour of these complexes was consistent with the view that the π-{C₆H₆)Cr moiety donated electron density to the C₆F₆ ring and, to investigate this further, a series of complexes (C₆H₆)Cr(C₆F₅X) was prepared via the lithium reagent (C₆H₆)Cr(C₆F₅Li). These data supported the view of the π-complexed C₆F₅X system being more electron rich than the free C₆F₅X rings.</p> <p>These new sandwich complexes pose severe nomenclatural problems, and the CHROMARENE nomenclature has been proposed in an attempt to overcome these difficulties.</p> <p>Preliminary studies with other metals, viz., iron and germanium have provided direct routes to ferrocenophanes and organogermenes respectively.</p>