High-valent cobalt-oxo intermediates are proposed as reactive intermediates in a number of cobalt complex-mediated oxidation reactions. Herein we report the spectroscopic capture of low-spin (S = 1/2) Co(IV)-oxo species in the presence of redox-inactive metal ions, such as Sc3+, Ce3+, Y3+, and Zn2+ and investigation of their reactivity in C-H bond activation and sulfoxidation reactions. Theoretical calculations predict that the binding of Lewis-acidic metal ions to the cobalt-oxo core increases the electrophilicity of the oxygen atom, resulting in the redox tautomerism of a highly unstable [(TAML)CoIII-(O•)]2− species to a more stable [(TAML)CoIV-(O)(Mn+)] core. The present report supports the proposed role of the redox-inactive metal ions in facilitating formation of high-valent metal-oxo cores as a necessary step for oxygen evolution in chemistry and biology.

High-valent cobalt-oxo intermediates are proposed as reactive intermediates in a number of cobalt complex-mediated oxidation reactions. Herein we report the spectroscopic capture of low-spin (S = 1/2) Co(IV)-oxo species in the presence of redox-inactive metal ions, such as Sc 3+ , Ce 3+ , Y 3+ , and Zn 2+ and investigation of their reactivity in C-H bond activation and sulfoxidation reactions. Theoretical calculations predict that the binding of Lewis-acidic metal ions to the cobalt-oxo core increases the electrophilicity of the oxygen atom, resulting in the redox tautomerism of a highly unstable [(TAML)Co III -(O • )] 2− species to a more stable [(TAML)Co IV -(O)(M n+ )] core. The present report supports the proposed role of the redox-inactive metal ions in facilitating formation of high-valent metal-oxo cores as a necessary step for oxygen evolution in chemistry and biology. KeywordsCobalt; Oxo ligand; Lewis acid stabilization; Redox tautomerization; Oxygenation High-valent metal-oxo complexes have been implicated as active oxidants in oxygenation and water oxidation reactions by a number of biological and chemical catalysts. 1 Although high-valent terminal metal-oxo complexes of groups 3-8 are ubiquitous, isolation of metaloxo species to the right of group 8 (also known as the "oxo wall" for C4v symmetry) has only been achieved for heavy transition metals like iridium 2 and platinum. 3 For example, high-valent terminal metal-oxo complexes of the lighter analogues like cobalt, nickel, and copper still remain elusive, although they are often invoked as highly reactive transient * Prof. Dr. Wonwoo Nam, Dr. Kallol Ray wwnam@ewha.ac.kr, kallol.ray@chemie.hu-berlin.de.Supporting information for this article, including Experimental section, Tables S1 -S8 and Figures S1 -S17, is available on the WWW under http://dx.doi.org/10.1002/anie.201xxxxxx. Tables S1-S2 and Figures S1-S2). Complex 1 is paramagnetic with S = 1 (μ eff = 3.1 Bohr magnetons) ground state, as expected for a Co(III) ion in the observed square planar geometry. Cyclic voltammetry (CV) of 1 at 25 °C in CH 3 CN reveals a reversible oxidation wave centered at 1.00 V vs saturated calomel electrode (SI, Figure S3); coulometric measurements show that the oxidation corresponds to a 1e -process. The reversibility of this oxidation wave at room temperature suggests that a formal Co(IV) state is thermally and kinetically accessible. HHS Public AccessIn agreement with the electrochemical data, 1 is readily oxidized in the presence of cerium ammonium nitrate (CAN) in acetone at 5 °C to form a metastable blue species (2-Ce) with a half-life (t 1/2 ) of 20 min (SI, Figure S4, inset). The blue color is associated with a band at 600 nm (ε = 7200 M −1 cm −1 ) with a shoulder near 730 nm (SI, Figure S4). We tentatively assign these bands to be ligand to metal charge transfer (LMCT) in origin, which presumably arise from transitions from the amide nitrogens of TAML to the Co(IV) center in 2-Ce; as expected, these bands are significantly blue-shifted in the corre...

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The reaction of Pd black with [Mo 3 Se 4 (H 2 O) 9 ] 4ϩ in 2 M HCl gives the single cube [Mo 3 (PdCl)Se 4 (H 2 O) 9 ] 3ϩ , which on removal of Cl Ϫ forms the edge-linked double cube [{Mo 3 PdSe 4 (H 2 O) 9 } 2 ] 8ϩ . No similar reactions of [W 3 S 4 (H 2 O) 9 ] 4ϩ and [W 3 Se 4 (H 2 O) 9 ] 4ϩ are observed, and in no case is Pt black incorporated into the trinuclear species. The crystal structure of [{Mo 3 PdSe 4 (H 2 O) 9 } 2 ](pts) 8 ؒ18H 2 O has been determined (pts Ϫ = p-toluenesulfonate), and is consistent with Mo-Mo and Mo-Pd bonding. Properties of the Pd derivatives of both [Mo 3 Q 4 (H 2 O) 9 ] 4ϩ (Q = S, Se) are considered. No heteroatom transfer is observed on mixing [Mo 3 (PdCl)Se 4 (H 2 O) 9 ] 3ϩ with [Mo 3 S 4 (H 2 O) 9 ] 4ϩ as is the case of [Mo 3 SnSe 4 (H 2 O) 12 ] 6ϩ with [Mo 3 S 4 (H 2 O) 9 ] 4ϩ . The single cubes [Mo 3 (PdCl)Q 4 (H 2 O) 9 ] 3ϩ (Q = S, Se), react 1 : 1 with SnCl 3 Ϫ to give [Mo 3 (PdSnCl 3 )Q 4 (H 2 O) 9 ] 3ϩ with Pd-Sn bonding. Formation constants K (25 ЊC) are 1.15 × 10 3 M Ϫ1 (Q = S) and 9.5 × 10 3 M Ϫ1 (Q = Se). On mixing the heterometal cubes [Mo 3 PdS 4 (H 2 O) 10 ] 4ϩ and [Mo 3 SnS 4 (H 2 O) 12 ] 6ϩ in 2 M HCl no Pd-Sn bonding occurs. With [Pd(H 2 O) 4 ] 2ϩ and [Mo 3 SnS 4 (H 2 O) 12 ] 6ϩ in 2.0 M HClO 4 , reaction steps Mo 3 SnS 4 6ϩ Mo 3 S 4 4ϩ ϩ Sn II , followed by Sn II ϩ Pd II Sn IV ϩ Pd 0 are observed, and Pd 0 is precipitated. Rate constants for the oxidation of [Mo 3 (PdCl)S 4 (H 2 O) 9 ] 4ϩ and [{Mo 3 PdS 4 (H 2 O) 9 } 2 ] 8ϩ with [Co(dipic) 2 ] Ϫ (dipic = 2,6-pyridinedicarboxylate) are within a factor of 2, indicating no significant change in redox properties. This contrasts with the behaviour of single and corner-shared double cubes e.g. MЈ = Sn.

Preparation, Structure, and Properties of the Corner-Shared Double Cubes [Mo 6 HgO 8 (H 2 O) 18 ] 8+ (Q: S, Se) and Tungsten Analogues. -Corner-shared double cubes [Mo 6 HgQ 8 (H 2 O) 18 ] 8+ (Q: S, Se) are obtained by reacting Hg metal or Hg 1 2 with [Mo 3 Q 4 (H 2 O) 9 ] 4+ under air-free conditions (2 M HCl or 4 M TosOH). The reactions are monitored by UV/VIS spectrophotometry. As revealed by XRD the compounds [M 6 HgQ 8 Cl 4 (H 2 O) 14 ] (C 36 H 36 N 24 O 12 )Cl 4 ·14H 2 O (M: Mo, W) are isostructural. They have six carbonyl O atoms above and six below the ring of the cucurbituril molecule, which form hydrogen bonds to H 2 O ligands of the double cube. Four chlorides are coordinated to M, and four chlorides act as counterions. Redox properties of the title cations are reported. -(FEDIN, VLADIMIR P.; SOKOLOV, MAXIM; LAMPRECHT, GERT J.; HERNANDEZ-MOLINA, RITA; SEO, MI-SOOK; VIROVETS, ALEXANDER V.; CLEGG, WILLIAM; SYKES, A.