The cobalt cubium Co4O4(OAc)(4)(py)(4)(ClO4) (1A(+)) containing the mixed valence [Co4O4](5+) core is shown by multiple spectroscopic methods to react with hydroxide (OH-) but not with water molecules to produce O-2. The yield of reaction products is stoichiometric (>99.5%): 41A(+) + 40H(-) -> O-2 + 2H(2)O + 41A. By contrast, the structurally homologous cubium Co4O4(trans-OAc)(2)(bpy)(4)(ClO4)(3), 1B(ClO4)(3), produces no O-2. EPR/NMR spectroscopies show clean conversion to cubane 1A during O-2 evolution with no Co2+ or Co3O4 side products. Mass spectrometry of the reaction between isotopically labeled mu-O-16(bridging-oxo) 1A(+) and O-18-bicarbonate/water shows (1) no exchange of O-18 into the bridging oxos of 1A(+), and (2) O-36(2) is the major product, thus requiring two OH- in the reactive intermediate. DFT calculations of solvated intermediates suggest that addition of two OH- to 1A(+) via OH- insertion into Co-OAc bonds is energetically favored, followed by outer-sphere oxidation to intermediate [1A(OH)(2)](0). The absence of O-2 production by cubium IB3+ indicates the reactive intermediate derived from 1A(+) requires gem-1,1-dihydoxo stereochemistry to perform O-O bond formation. Outer-sphere oxidation of this intermediate by 2 equiv of 1A(+) accounts for the final stoichiometry. Collectively, these results and recent literature (Faraday Discuss., doi:10.1039/C5FD00076A and J. Am. Chem. Soc. 2015, 137, 12865-12872) validate the [Co4O4](4+/5+) cubane core as an intrinsic catalyst for oxidation of hydroxide by an inner-sphere mechanism.
Water Oxidation by the [Co4O4(OAc)4(py)4]+ Cubium is Initiated by OH- Addition
Marotta, Gabriele;Mosconi, Edoardo;De Angelis, Filippo;
2015
Abstract
The cobalt cubium Co4O4(OAc)(4)(py)(4)(ClO4) (1A(+)) containing the mixed valence [Co4O4](5+) core is shown by multiple spectroscopic methods to react with hydroxide (OH-) but not with water molecules to produce O-2. The yield of reaction products is stoichiometric (>99.5%): 41A(+) + 40H(-) -> O-2 + 2H(2)O + 41A. By contrast, the structurally homologous cubium Co4O4(trans-OAc)(2)(bpy)(4)(ClO4)(3), 1B(ClO4)(3), produces no O-2. EPR/NMR spectroscopies show clean conversion to cubane 1A during O-2 evolution with no Co2+ or Co3O4 side products. Mass spectrometry of the reaction between isotopically labeled mu-O-16(bridging-oxo) 1A(+) and O-18-bicarbonate/water shows (1) no exchange of O-18 into the bridging oxos of 1A(+), and (2) O-36(2) is the major product, thus requiring two OH- in the reactive intermediate. DFT calculations of solvated intermediates suggest that addition of two OH- to 1A(+) via OH- insertion into Co-OAc bonds is energetically favored, followed by outer-sphere oxidation to intermediate [1A(OH)(2)](0). The absence of O-2 production by cubium IB3+ indicates the reactive intermediate derived from 1A(+) requires gem-1,1-dihydoxo stereochemistry to perform O-O bond formation. Outer-sphere oxidation of this intermediate by 2 equiv of 1A(+) accounts for the final stoichiometry. Collectively, these results and recent literature (Faraday Discuss., doi:10.1039/C5FD00076A and J. Am. Chem. Soc. 2015, 137, 12865-12872) validate the [Co4O4](4+/5+) cubane core as an intrinsic catalyst for oxidation of hydroxide by an inner-sphere mechanism.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
