Syntheses and Characterization of a Series of Binuclear Iridium Complexes

Citation

Smith, Terrance Patrick (1982) Syntheses and Characterization of a Series of Binuclear Iridium Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/xc18-n193. https://resolver.caltech.edu/CaltechETD:etd-02252009-153626

Abstract

A binuclear iridium(I) isocyanide complex has been synthesized and its photophysics and chemistry have been synthesized and its photophysics and chemistry have been investigated. The complex Ir₂(TMB)²⁺₄ (TMB = 2.5-dimethyl 2,5 diisocyanohexane), possesses an intense absorption band maximizing at 625 nm in the visible region. Excitation into this feature results in two long wavelength luminescence bands at 735 nm and 1080 nm. The longer wavelength feature has been assigned as phosphorescence (E(³A₂) → A₁(¹A₁)) from the lowest electronic excited state; the lifetime of this state is 200 ns and is independent of temperature between 77 K and 298 K. In contrast, the E(³A₂) excited state of the analogous rhodium complex exhibits a dramatic temperature dependence. This difference is ascribed to the proximity of ligand field states to the lowest excited state; in the rhodium case the ligand field states are thermally accessible whereas for the iridium case they lie at much higher energy.

The d⁸ - d⁸ iridium complex oxidatively adds a variety of small molecules (e.g. I₂, Br₂, Cl₂, CH₃I, HCl). The reactivity is quite similar to that of the analogous rhodium system (Rh₂(TMB)²⁺₄). One interesting difference is the fact that the HC1 adduct of Rh₂(TMB)²⁺₄ is directly observable only at low temperature. Nonetheless, a similar hydride species is postulated as an important intermediate in the production of hydrogen in another binuclear rhodium system.

A number of spectroscopic methods have been employed to explore the bonding in symmetric oxidative addition adducts. A linear correlation between metal-metal-force constant and intermetallic distance has been uncovered. The structure of the iodine adduct, Ir₂(TMB)₄I₂(BPh₄)₂, has been determined; the intermetallic distance 2.803(4) Å. Also dynamic ¹H NMR reveals that binuclear complexes containing the TMB ligand are rapidly undergoing conformation changes at ambient temperatures; the activation barriers for this process range from 12 to 15 kcal/mole.

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