Synthetic, Electrochemical, and Theoretical Studies of Tetrairidium Clusters Bearing Mono- and Bis[60]fullerene Ligands

Heating a mixture of Ir4(CO)9(PPh3)3 (1) and 2 equiv of C60 in refluxing chlorobenzene (CB)
affords a “butterfly” tetrairidium−C60 complex Ir4(CO)6{μ3-κ3-PPh2(o-C6H4)P(o-C6H4)PPh(η1-o-C6H4)}(μ3-η2:η2:η2-C60) (3, 36%). Brief thermolysis of 1 in refluxing chlorobenzene (CB) gives a “butterfly” complex
Ir4(CO)8{μ-k2-PPh2(o-C6H4)PPh}{μ3-PPh2(η1: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 C60-assisted ortho-phosphorylation, indicating that 2 is the reaction intermediate for the final product 3. On the
other hand, reaction of Ir4(CO)8(PMe3)4 (4) with excess (4 equiv) C60 in refluxing 1,2-dichlorobenzene,
followed by treatment with CNCH2Ph at 70 °C, affords a square-planar complex with two C60 ligands and
a face-capping methylidyne ligand, Ir4(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
Ir4(CO)6(μ-CO){μ3-k2-PPh2(o-C6H4)P(η1-o-C6H4)}(μ3-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 33- and 62-
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.