Synthetic, electrochemical, and theoretical studies of tetrairidium clusters bearing mono- and bis[60]fullerene ligands

Heating a mixture of lr₄(CO)₉(PPh₃) ₃ (1) and 2 equiv of C₆₀ in refluxing chlorobenzene (CB) affords a "butterfly" tetrairidium-C₆₀ complex lr ₄(CO)₆{μ₃-κ³-PPh ₂(o-C₆H₄)P(o-C₆H₄) PPh(η¹-o-C⁶H⁴}}-(μ₃- η²:η²-C₆₀) (3, 36%). Brief thermolysis of 1 in refluxing chlorobenzene (CB) gives a "butterfly" complex lr₄(CO)₈{μ-k²-PPh₂(o-C ₆H₄)PPh}{μ₃-PPh₂(η ¹:η²-o-C₆H₄)} (2, 64%) that is both ortho-phosphorylated and ortho-metalated. Interestingly, reaction of 2 with 2 equiv of C₆₀ in refluxing CB produces 3 (41%) by C ₆₀-assisted ortho-phosphorylation, indicating that 2 is the reaction intermediate for the final product 3. On the other hand, reaction of lr ₄(CO)₈(PMe₃)4 (4) with excess (4 equiv) C ₆₀ in refluxing 1,2-dichlorobenzene, followed by treatment with CNCH₂Ph at 70 °C, affords a square-planar complex with two C ₆₀ ligands and a face-capping methylidyne ligand, lr ₄(CO)₃(μ₄-CH)(PMe₃) ₂(μ-PMe₂)(CNCH₂Ph)(μ-η²: η²-C₆₀)(μ₄-η¹: η¹:η²:η²-C₆₀) (5, 13%) as the major product. Compounds 2, 3, and 5 have been characterized by spectroscopic and microanalytical methods, as well as by single-crystal X-ray diffraction studies. Cyclic voltammetry has been used to examine the electrochemical properties of 2, 3, 5, and a related known "butterfly" complex lr₄(CO)₆(μ-CO){μ₃-k ²-PPh₂(o-C₆H₄)P(η¹-o- C₆H₄)}(η₃-η²: η²:η²-C₆₀) (6). These cyclic voltammetry data suggest that a C₆₀-mediated electron transfer to the iridium cluster center takes place for the species 3^3- and 6 ^2- in compounds 3 and 6. The cyclic voltammogram of 5 exhibits six well-separated reversible, one-electron redox waves due to the strong electronic communication between two C₆₀ cages through a tetrairidium metal cluster spacer. The electrochemical properties of 3, 5, and 6 have been rationalized by molecular orbital calculations using density functional theory and by charge distribution studies employing the Mulliken and Hirshfeld population analyses.