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The silicon clathrates--materials composed of metal-doped Si(20) dodecahedra--were identified as the first superconductors based on pure silicon networks. The mechanism of superconductivity in these materials can be obtained by studying their phonon modes, as modified by isotope substitution, and specific-heat measurements. Here, we present experimental studies that provide strong evidence that superconductivity in Ba(8)Si(46) is explained in the framework of phonon-mediated Bardeen-Cooper-Schriefer theory. Analyses using the McMillan approximation of the Eliashberg equation indicate that the superconducting mechanism is in the medium coupling regime, but at the high-end limit. The large density of states at the Fermi level, which arises from hybridization of the Si(20) cluster and Ba orbitals, is responsible for the unexpectedly high superconducting temperature. The temperature evolution of the specific heat unambiguously shows that this is an s-wave symmetry superconductor.

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The reactions of copper(II) complexes and hydrogen peroxide (H 2 O 2 ) have been studied extensively in order to gain insight into reactive intermediates involved in copper monooxygenases and copper oxidases as well as copper-catalyzed oxidation reactions. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Several types of mononuclear and dinuclear copper/active-oxygen complexes have been reported, and their structures and physicochemical properties have been explored in detail. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] However, less is known about the intrinsic reactivity of the generated copper/ active-oxygen complexes.We herein report a new copper(II)-alkylperoxo species 2 X [2-hydroxy-2-hydroperoxypropane (HHPP) adduct], which is generated by the reaction of H 2 O 2 and copper(II) complex 1 X supported by the bis(pyridylmethyl)amine tridentate ligand containing msubstituted phenyl groups at the 6-positions of the pyridine rings (L X ) in acetone in the presence of triethylamine (NEt 3 ) (Scheme 1). The alkylperoxo intermediate 2 X undergoes an efficient aromatic ligand hydroxylation reaction, producing phenolate complex 4 X via another intermediate 3 X . Kinetic studies on the aromatic hydroxylation process are reported here together with spectral characterization of 2 X .Starting mononuclear copper(II) complexes 1 X supported by ligand L X (X ) NO 2 , Cl, H, Me, or OMe; Y ) ClO 4 -or H 2 O; S ) CH 3 CN or H 2 O) were prepared by mixing the ligands and Cu-(ClO 4 ) 2 ‚6H 2 O in acetone or acetonitrile (Figures S1-S4). 20 The reaction of 1 X and H 2 O 2 (1 equiv) was then examined in acetone at -70°C in the presence of triethylamine (1 equiv). Figure 1A shows a spectral change for the reaction of 1 NO 2 as a typical example, where intermediate 2 NO 2 exhibiting a characteristic absorption band at 420 nm ( ) 1350 M -1 cm -1 ) together with a weak d-d band at 630 nm ( ) 200 M -1 cm -1 ) becomes apparent. Spectroscopic titration for the generation of 2 NO 2 established that the stoichiometry of 1 NO 2 to H 2 O 2 was 1:1 ( Figure S5). Similar spectral changes were obtained with other ligand systems ( Figures S6-S9). A more detailed characterization of intermediate 2 X was carried out for 2 NO 2 since it showed higher stability than any other (as discussed further below).Intermediate 2 NO 2 generated with H 2 16 O 2 showed isotope sensitive Raman bands at 855, 823, 792, and 545 cm -1 when an acetone-d 6 (CD 3 COCD 3 ) solution of 2 NO 2 was excited with a 441.6 nm laser light ( Figure S10). These Raman bands shifted to 825, 803, 785, and 525 cm -1 , respectively, when H 2 18 O 2 was used ( Figure S10). The appearance of multiple Raman bands in the 800 cm -1 region and their associated isotope shifts (∆ν ) 30, 20, and 7 cm -1 ) as well as their intensity patterns are similar to those reported from resonance Raman studies of copper(II)-alkylperoxo (Cu II -OOR) and iron(III)-alkylperoxo (Fe III -OOR) complexes (R ) tert-butyl and cumyl), where such bands have been assigned as...