Isolable Cuprocenes: Bis(tri- <i>tert</i> -butylcyclopentadienyl) Complexes of Copper

Seventy years after the discovery of ferrocene, the first neutral bis(cyclopentadienyl)metal complex of copper has been synthesized: Cp^ttt₂Cu, <b>1</b> (Cp^ttt = C₅H₂^tBu₃). Comparison of this compound and the related anion, (Cp^ttt₂Cu)^1-, and cation, (Cp^ttt₂Cu)¹⁺, shows how the (Cp^ttt)^1- ligand can provide stability to copper compounds in three different levels of charge. The reaction between KCp^ttt and copper(I) triflate in Et₂O results in disproportionation to Cu metal and the blue-green neutral bis(cyclopentadienyl) copper metallocene Cp^ttt₂Cu, <b>1</b>, which has a slipped η⁵-Cp^ttt:η²-Cp^ttt structure. This reaction was found to proceed through the red intermediate [Cp^tttCu(OTf)]₂, <b>2</b> which has one η²-bound (Cp^ttt)^1- ligand. Complexes <b>1</b> and <b>2</b> could also be prepared from the reaction of KCp^ttt with copper(II) triflate. Reduction of Cp^ttt₂Cu with KC₈ in the presence of 2.2.2-cryptand (crypt) affords the colorless metallocene anion complex [K(crypt)][Cp^ttt₂Cu], <b>3</b>, in which both (Cp^ttt)^1- ligands are η²-bound. Oxidation of Cp^ttt₂Cu with the ammoniumyl radical salt [N(C₆H₄Br-4)₃][B(C₆F₅)₄] gives the purple cationic cuprocenium complex [Cp^ttt₂Cu][B(C₆F₅)₄], <b>4</b>, which has η³:η²-coordination of the (Cp^ttt)^1- ligands. Although <b>1</b>, <b>3</b>, and <b>4</b> have copper in the formal oxidation states of +2, +1, and +3, respectively, density functional theory calculations indicate that the charge on copper is similar in all three complexes, which attests to the ability of the (Cp^ttt)^1- ligand to delocalize electron density by varying the coordination mode.