Mixed-metal cluster chemistry. 19. Crystallographic, spectroscopic, electrochemical, spectroelectrochemical, and theoretical studies of systematically varied tetrahedral group 6-iridium clusters

Abstract

A systematically varied series of tetrahedral clusters involving ligand and core metal variation has been examined using crystallography, Raman spectroscopy, cyclic voltammetry, UV - vis - NIR and IR spectroelectrochemistry, and approximate density functional theory, to assess cluster rearrangement to accommodate steric crowding, the utility of metal-metal stretching vibrations in mixed-metal cluster characterization, and the possibility of tuning cluster electronic structure by systematic modification of composition, and to identify cluster species resultant upon electrochemical oxidation or reduction. The 60-electron tetrahedral clusters MIr3(CO)11-x(PMe3)x (η5-Cp) [M = Mo, x = 0, Cp = C5H4Me (5), C5HMe4 (6), C5Me5 (7); M = W, Cp = C5H4Me x = 1 (13), x = 2 (14)] and M2Ir2(CO)10-x (PMe3)x(η5-Cp) [M = Mo, x = 0, Cp = C5H4Me (8), C5HMe4 (9), C5Me5 (10); M = W, Cp = C5H4Me, x = 1 (15), x = 2 (16)] have been prepared. Structural studies of 7, 10, and 13 have been undertaken; these clusters are among the most sterically encumbered, compensating by core bond lengthening and unsymmetrical carbonyl dispositions (semi-bridging, semi-face-capping). Raman spectra for 5, 8, WIr3(CO)11(η5- C5H4Me) (11), and W2Ir2(CO)10- (η5-C5H4Me)2 (12), together with the spectrum of Ir4(CO)12, have been obtained, the first Raman spectra for mixed-metal clusters. Minimal mode-mixing permits correlation between A1 frequencies and cluster core bond strength, frequencies for the A1 breathing mode decreasing on progressive group 6 metal incorporation, and consistent with the trend in metal-metal distances [Ir-Ir < M-Ir < M-M]. Cyclic voltammetric scans for 5-15, MoIr3(CO)11(η5- C5H5) (1), and Mo2Ir2(CO)10(η5- C5H5)2 (3) have been collected. The [MIr3] clusters show irreversible one-electron reduction at potentials which become negative on cyclopentadienyl alkyl introduction, replacement of molybdenum by tungsten, and replacement of carbonyl by phosphine. These clusters show two irreversible one-electron oxidation processes, the easier of which tracks with the above structural modifications; a third irreversible oxidation process is accessible for the bis-phosphine cluster 14. The [M2Ir2] clusters show irreversible two-electron reduction processes; the tungsten-containing clusters and phosphine-containing clusters are again more difficult to reduce than their molybdenum-containing or carbonyl-containing analogues. These clusters show two one-electron oxidation processes, the easier of which is reversible/quasi-reversible, and the more difficult of which is irreversible; the former occur at potentials which increase on cyclopentadienyl alkyl removal, replacement of tungsten by molybdenum, and replacement of phosphine by carbonyl. The reversible one-electron oxidation of 12 has been probed by UV-vis-NIR and IR spectroelectrochemistry. The former reveals that 12+ has a low-energy band at 8000 cm-1, a spectrally transparent region for 12, and the latter reveals that 12+ exists in solution with an all-terminal carbonyl geometry, in contrast to 12 for which an isomer with bridging carbonyls is apparent in solution. Approximate density functional calculations (including ZORA scalar relativistic corrections) have been undertaken on the various charge states of W2Ir2(CO)10(η5- C5H5)2 (4). The calculations suggest that two-electron reduction is accompanied by W-W cleavage, whereas one-electron oxidation proceeds with retention of the tetrahedral core geometry. The calculations also suggest that the low-energy NIR band of 12+ arises from a σ(W-W) → σ*(W-W) transition.