An Investigation of the Photochemistry and Structures of Selected Second and Third Row Transition Metal Complexes

Citation

Eidem, Penny Kristen (1981) An Investigation of the Photochemistry and Structures of Selected Second and Third Row Transition Metal Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1s9g-cn19. https://resolver.caltech.edu/CaltechTHESIS:05072018-093217287

Abstract

Examination of the electronic spectrum of IrCl^3-₆ has led to a reassignment of the electronic transitions involved. The band at 206 nm, formerly assigned as the spin allowed π_L to metal e_g transition, is shown to be 1_t1u (σ) → 2e_g(z², x² - y²) [¹A_1g → ¹T_1u]. Low temperature spectra revealed the presence of additional features at 250 and 278 nm. These are ascribed to the 1_t2u (π) → 2e_g(z², x² - y²) [¹A_1g → ¹T_1u] and 1_t2u (π) → 2e_g(z², x² - y²) [¹A_1g → ³T_1u] transitions, respectively. Irradiation (λ = 254 nm) of 1-12 M HCl solutions of IrCl^3-₆ yields IrCl^2-₆ and H₂. Since the excited state populated at this wavelength has been shown to be ligand to metal charge transfer in nature, the reactive intermediate is proposed to he an Ir(II) species with a chlorine atom still formally bound. Photolysis of the reaction product, IrCl^2-₆ in HCl results in the formation of IrCl^3-₆ and Cl₂. This reaction prevails regardless of wavelength of excitation. The reactive state is again LMCT in nature. Coupling of these reactions effects a reversible photochemical hydrohalic acid splitting catalyst.

The photochemistry of Mo(III), Mo(IV), Mo(V) in aqueous solution was investigated, and these ions were shown to be photochemically inert. Structural characterizations via Raman spectroscopy and X-ray ahsorption edge and EXAFS were undertaken. The Mo(II) structure is shown to be q quadruply bound dinuclear species. The Mo(III) is singly bonded with hydroxy bridges. Data for the Mo(V) ion are typical for oxobridged dinuclear compounds. In strong acid, Mo(IV) is shown to exist as a trinuclear species. As the pH of the medium is increased, the Mo-Mo amplitude decreased, indicating possible cluster fragmentation. In basic solution, a major structural change occurs. The presence of halide ions had no effect on the spectra.

Ru₃(CO)₁₂ reacts photochemically in the presence of olefins, CO, and H₂ to catalyze the hydroformylation reaction. Typical yields are 1.5 x 10^-3 moles of aldehydes in a 2:1 linear to branched chain ratio. A heterogeneous catalyst can also be effected by photoinduced fragmentation of the cluster in the presence of PV4P. Attachment of a Ru-CO moiety was confirmed by IR and elemental analysis. The first step in catalyst activation was shown to be formation of Ru(CO)₄ olefin with a quantum yield of 0.03 for 1-pentene. Subsequent steps involved formation of a hydride-olefin complex, rearrangement to a hydride alkyl, "CO insertion," and reductive elimination of aldehyde. Olefin isomerization and alkane production are also seen under reaction conditions. Formation of larger ruthenium carbonyl clusters led to catalyst deactivation. Photolysis of Ru₃(CO)₁₂ in the presence of H₂ led to the formation of αH₄-Ru₄(CO)₁₂. This molecule can also effect catalysis of the hydroformylation reaction, although yields are an order of magnitude less than for the parent cluster.

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