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 ttt 2 Cu, 1 (Cp ttt = C 5 H 2 t Bu 3 ). Comparison of this compound and the related anion, (Cp ttt 2 Cu) 1–, and cation, (Cp ttt 2 Cu) 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 Et 2 O results in disproportionation to Cu metal and the blue-green neutral bis(cyclopentadienyl) copper metallocene Cp ttt 2 Cu, 1, which has a slipped η 5 -Cp ttt:η 2 -Cp ttt structure. This reaction was found to proceed through the red intermediate [Cp ttt Cu(OTf)] 2, 2 which has one η 2 -bound (Cp ttt ) 1– ligand. Complexes 1 and 2 could also be prepared from the reaction of KCp ttt with copper(II) triflate. Reduction of Cp ttt 2 Cu with KC 8 in the presence of 2.2.2-cryptand (crypt) affords the colorless metallocene anion complex [K(crypt)][Cp ttt 2 Cu], 3, in which both (Cp ttt ) 1– ligands are η 2 -bound. Oxidation of Cp ttt 2 Cu with the ammoniumyl radical salt [N(C 6 H 4 Br-4) 3 ][B(C 6 F 5 ) 4 ] gives the purple cationic cuprocenium complex [Cp ttt 2 Cu][B(C 6 F 5 ) 4 ], 4, which has η 3:η 2 -coordination of the (Cp ttt ) 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 (Cp ttt ) 1– ligand to delocalize electron density by varying the coordination mode.