Isolable Cuprocenes: Bis(tri-tert-butylcyclopentadienyl) Complexes of Copper.

Seventy years after the discovery of ferrocene, the first neutral bis(cyclopentadienyl)metal complex of copper has been synthesized: Cpttt2Cu, 1 (Cpttt = C5H2tBu3). Comparison of this compound and the related anion, (Cpttt2Cu)1-, and cation, (Cpttt2Cu)1+, shows how the (Cpttt)1- ligand can provide stability to copper compounds in three different levels of charge. The reaction between KCpttt and copper(I) triflate in Et2O results in disproportionation to Cu metal and the blue-green neutral bis(cyclopentadienyl) copper metallocene Cpttt2Cu, 1, which has a slipped η5-Cpttt:η2-Cpttt structure. This reaction was found to proceed through the red intermediate [CptttCu(OTf)]2, 2 which has one η2-bound (Cpttt)1- ligand. Complexes 1 and 2 could also be prepared from the reaction of KCpttt with copper(II) triflate. Reduction of Cpttt2Cu with KC8 in the presence of 2.2.2-cryptand (crypt) affords the colorless metallocene anion complex [K(crypt)][Cpttt2Cu], 3, in which both (Cpttt)1- ligands are η2-bound. Oxidation of Cpttt2Cu with the ammoniumyl radical salt [N(C6H4Br-4)3][B(C6F5)4] gives the purple cationic cuprocenium complex [Cpttt2Cu][B(C6F5)4], 4, which has η3:η2-coordination of the (Cpttt)1- ligands. Although 1, 3, and 4 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 (Cpttt)1- ligand to delocalize electron density by varying the coordination mode.