Abstract
We have found1 that 546 nm irradiation of solutions of the binuclear Rh(I) complex Rh2(bridge)4 2+ (bridge = 1, 3-diisocyano- propane), or Rh2 2+, in 12 M HCl(aq) results in clean conversion to H2 and a Rh(II) complex. The Rh(II) complex can also be prepared by Cl2 oxidation and has been structurally characterized.2 Recent work in our laboratory has shown that a thermal reaction generates the blue photoactive species, Rh4C12 4+, as follows:
Notably, the photochemical step is uphill, as H2 reacts slowly with Rh2C12 2+ to regenerate Rh4C12 4+.
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References and Notes
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Higher sulfate concentrations result in spectral shifts consistent with sulfate binding; thus, λmax = 590 nm in 32 N H2SO4. We distinguish ligation from ion-pairing, which our data suggest to be extensive for N (H2SO4) > 10-2.
Flash photolysis experiments were performed at the University of California at Santa Cruz. Dilute (10-6-10-5 M) degassed samples were excited by a coaxial filtered xenon flash lamp (output ≤ 90 joules, τ1/2 ~ 20μs).
V. M. Miskowski, K. R. Mann, H. B. Gray, S. J. Milder, G. S. Hammond, and P. R. Ryason, J. Am. Chem. Soc., submitted for publication.
Laser flash photolysis measurements were made at the University of Southern California.
Decay was second-order for over 5 half-lives, and the rate constant was independent of total rhodium concentration and flash pulse intensity.
The rate is nearly independent of ionic strength for > 1 N H2SO4.
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Fe(II) product was determined as the tris(o-phenanthroline) complex. The quantitative formation of the Rh(II) product, which has been independently characterized as the product of Ce (IV) thermal oxidation of the H2SO4 (aq) solutions and has an absorption maximum at 311 nm (ℇ = 33,600), was determined by the spectral changes that occur during the photoreaction, and by the clean reduction back to starting material that occurs upon addition of a large excess of Fe2+.
Assuming our kinetic scheme (eqs. 4, 5, 7), the limiting product quantum yield for (6) should be twice the primary quantum yield ∅0 for (4). Therefore, 0 ≃ 0.01. Measurements of ∅0 from the magnitude of transient signal in the flash photolysis experiments were in reasonable agreement. The transient quantum yield increases at lower ionic strength, and is much larger in CH3CN, suggesting that cage recombination is important.
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Gray, H.B. et al. (1979). Mechanistic Aspects of Solar Energy Storage Reactions Involving Polynuclear Rhodium Isocyanide Complexes. Preparation of New Binuclear Isocyanide Complexes of Iridium, Cobalt, Nickel, and Ruthenium. In: Tsutsui, M. (eds) Fundamental Research in Homogeneous Catalysis. Springer, Boston, MA. https://doi.org/10.1007/978-1-4613-2958-9_54
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DOI: https://doi.org/10.1007/978-1-4613-2958-9_54
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