Structural properties of pyrazolyl-bridged diiridium complexes




Brost, Ron D.

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The x-ray structures of several alkyl halide, alkyl dihalide, and hydrogen adducts to pyrazolyl-bridged diiridium complexes [special characters omitted] are determined. The diiridium (bis-pyrazolyl) core of these complexes enables contact between the two centers so that metal-metal bond formation may occur, exemplified by a short iridium-iridium distance of 2.78(I) A in the diiridium(II) complex [special characters omitted]. Oxidation mechanisms are postulated based on reaction kinetics. The oxidative addition of methyl iodide to [special characters omitted] is observed to occur by a two-step mechanism, where a high positive ΔS‡ term may be due to a highly ordered intermediate. This is proposed as evidence for an [special characters omitted] addition, where coordination of the alkyl halide is followed by halide dissociation and migration to a trans diaxial coordination site. Different kinetics of the reaction are observed in THF and benzene, which is also attributed to a polar [special characters omitted] intermediate. Occupation of the 3,3’ and 5,5’ positions of the pyrazolyl ligand decreases the reaction rate by an order of magnitude or greater, which indicates steric inhibition of the reaction by the bridging ligands. Experimental evidence for a competing light-induced reaction that corresponds to a radical-chain mechanism rather than the dark [special characters omitted] reaction is also presented. Oxidative isomerization of an iodo (iodomethylene) complex to the methylene-bridged isomer is determined to be an intramolecular process based on isotope labelling experiments and kinetics. Negligible isomerization to the bridging methylene complex under ambient conditions is attributed to coordinative saturation; the stability of [special characters omitted] is likewise due to coordinative saturation of the metal centers. The addition of hydrogen or hydride to [special characters omitted] is possible through a number of synthetic routes, but the stereochemistry of the iridium(II) hydrido complexes is such that the metal-hydride and iridium-iridium bonds do not occupy coordination sites trans to each other: it is proposed that the strong σ-trans effect of the hydride induces structural rearrangements in substitution reactions so that stereochemistry of parent complexes is not conserved. This is demonstrated by the x-ray structures of [special characters omitted]. The hydride ligand promotes nucleophilic attack on an electron-rich iridium center; thus water and other Lewis bases are found to react with the cationic diiridium hydride complex [special characters omitted].



Pyrazoles, Iridium, Transition metal compounds