Lanthanide−Transition-Metal Carbonyl Complexes: New [Co₄(CO)₁₁]^2- Clusters and Lanthanide(II) Isocarbonyl Polymeric Arrays

Two types of Ln(II)−Co4 isocarbonyl polymeric arrays, {(Et2O)3-x(THF)xLn[Co4(CO)11]}∞ (1−3; x = 0, 1) and {(THF)5Eu[Co4(CO)11]}∞ (4), were prepared and structurally characterized. Transmetalation involving Ln(0) and Hg[Co(CO)4]2 in Et2O yields {(Et2O)3Ln[Co4(CO)11]}∞ (1, Ln = Yb; 2, Ln = Eu). Dissolution of the solvent-separated ion pairs [Ln(THF)x][Co(CO)4]2 (Ln = Yb, x = 6; Ln = Eu) in Et2O affords {(Et2O)2(THF)Yb[Co4(CO)11]}∞ (3) and {(THF)5Eu[Co4(CO)11]}∞ (4). In these reactions, oxidation and condensation of the [Co(CO)4]- anions result in formation of the new tetrahedral cluster [Co4(CO)11]2-. The two types of Ln(II)−Co4 compounds contain different isomers of [Co4(CO)11]2-, and, consequently, the structures of the infinite isocarbonyl networks are distinct. The cluster in {(Et2O)3-x(THF)xLn[Co4(CO)11]}∞ (1−3) possesses pseudo C3v symmetry (an apical Co, three basal Co atoms; one face-bridging, three edge-bridging, seven terminal carbonyls) and connects to Ln(II) centers through η2,μ4- and η2,μ3-carbonyls to generate a 2-D puckered sheet. In contrast, {(THF)5Eu[Co4(CO)11]}∞ (4) incorporates a C2v symmetric cluster (two unique Co environments; two face-bridging, one edge-bridging, eight terminal carbonyls), and isocarbonyl linkages (η2,μ4-carbonyls) to Eu(II) atoms create a 1-D zigzag chain. Complexes 1−4 contain the first reported η2,μ4-CO bridges between a Ln and a transition-metal carbonyl cluster. Infrared spectroscopic studies revealed that the isocarbonyl associations to Ln(II) persist in solution. The solution structure and dynamic behavior of the [Co4(CO)11]2- cluster in 1 was investigated by variable-temperature 59Co and 13C NMR spectroscopies.