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

Abstract

Heating a mixture of Ir-4(CO)(9)(PPh3)(3) (1) and 2 equiv of C-60 in refluxing chlorobenzene (CB) affords a "butterfly" tetrairidium-C-60 complex Ir-4(CO)(6){mu(3)-k(3)-PPh2(o-C6H4)P(o-C6H4)PPh(eta(1)-o-C6H4)}-( mu(3)-eta(2):eta(2):eta(2)-C-60) (3, 36%). Brief thermolysis of 1 in refluxing chlorobenzene (CB) gives a " butterfly" complex Ir-4(CO)(8){mu-k(2)-PPh2(o-C6H4)PPh}{mu(3)-PPh2(eta(1):eta(2)-o-C6H4)} (2, 64%) that is both ortho-phosphorylated and ortho-metalated. Interestingly, reaction of 2 with 2 equiv of C-60 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 Ir-4(CO)(8)(PMe3)(4) (4) with excess (4 equiv) C-60 in refluxing 1,2-dichlorobenzene, followed by treatment with CNCH2Ph at 70 degrees C, affords a square-planar complex with two C60 ligands and a face-capping methylidyne ligand, Ir-4(CO)(3)(mu(4)-CH)(PMe3)(2)(mu-PMe2)(CNCH2Ph)(mu-eta(2):eta(2)-C-60)(mu(4)-eta(1):eta(1):eta(2):eta(2)-C-60) (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 Ir-4(CO)(6)(mu-CO){mu(3)-k(2)-PPh2(o-C6H4)P(eta(1)-o-C6H4)}(mu(3)-eta(2):eta(2):eta(2)-C-60) (6). These cyclic voltammetry data suggest that a C-60-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 C-60 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.