Spectroscopic Capture and Reactivity of a Low‐Spin Cobalt(IV)‐Oxo Complex Stabilized by Binding Redox‐Inactive Metal Ions

Abstract

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...

ChemInform Abstract: Lewis Acid Coupled Electron Transfer of Metal—Oxygen Intermediates

“…Althoughh igh-valent cobalt-oxo complexes remain elusive, bindingo fr edox-inactivem etal ions has been shownt os tabilize ac obalt(IV)-oxo complex (Scheme 3). [71] The cobalt(III) complex, [(TAML)Co III ] À (TAML = tetraamidom acrocyclic ligand), is readily oxidized by cerium ammonium nitrate (CAN) in acetone to afford the [(TAML)Co IV (O)-Ce] 2 + complex. [71] The reaction of [(TAML)Co III ] À with iodosylbenzene (PhIO) in the presence of Sc(OTf) 3 also generatedt he [(TAML)Co IV (O)ÀSc] 2 + complex, whichi ss tabilized by binding of Sc 3 + ion to the metaloxo moiety.…”

“…[71] The cobalt(III) complex, [(TAML)Co III ] À (TAML = tetraamidom acrocyclic ligand), is readily oxidized by cerium ammonium nitrate (CAN) in acetone to afford the [(TAML)Co IV (O)-Ce] 2 + complex. [71] The reaction of [(TAML)Co III ] À with iodosylbenzene (PhIO) in the presence of Sc(OTf) 3 also generatedt he [(TAML)Co IV (O)ÀSc] 2 + complex, whichi ss tabilized by binding of Sc 3 + ion to the metaloxo moiety. [71] Theoretical calculations predict that the binding of Lewis acidic metal ions (M n + )t ot he cobalt-oxo core increases the electrophilicity of the oxygen atom, resultingi n the redox tautomerism of ah ighly unstable[ (TAML)Co III (OC)] 2À speciest oam ore stable [(TAML)Co IV (O)(M n + )] core.…”

“…[71] The reaction of [(TAML)Co III ] À with iodosylbenzene (PhIO) in the presence of Sc(OTf) 3 also generatedt he [(TAML)Co IV (O)ÀSc] 2 + complex, whichi ss tabilized by binding of Sc 3 + ion to the metaloxo moiety. [71] Theoretical calculations predict that the binding of Lewis acidic metal ions (M n + )t ot he cobalt-oxo core increases the electrophilicity of the oxygen atom, resultingi n the redox tautomerism of ah ighly unstable[ (TAML)Co III (OC)] 2À speciest oam ore stable [(TAML)Co IV (O)(M n + )] core. [71] The Co IV oxidation state of [(TAML)Co IV (O)ÀSc] 2 + was confirmed by EPR and XAS measurements.…”

“…[71] Theoretical calculations predict that the binding of Lewis acidic metal ions (M n + )t ot he cobalt-oxo core increases the electrophilicity of the oxygen atom, resultingi n the redox tautomerism of ah ighly unstable[ (TAML)Co III (OC)] 2À speciest oam ore stable [(TAML)Co IV (O)(M n + )] core. [71] The Co IV oxidation state of [(TAML)Co IV (O)ÀSc] 2 + was confirmed by EPR and XAS measurements. It shouldb en oted that the CoÀOd istance of 1.67 f or [(TAML)Co IV (O)ÀSc] 2 + determined by EXAFS is significantly shorter than the CoÀOd istances of 1.79 a nd 1.85 d etermined for [(a-P 2 W 17 O 61 Co IV ) 2 O] 14À by X-ray diffraction [72] and [(TMG 3 tren)Co IV (O)(Sc)(OTf) 3 ]] 2 + (TMG 3 tren = tris[2-(N-tetramethylguanidyl)ethyl]amine) by EXAFS.…”

“…Scheme3.Oxidationofac obalt(III) complex to ac obalt(IV)-oxo-metalion complexesb yCAN/H 2 Oo rP hIO/M n + . Scheme3was taken with permission from reference [71].…”