Correlation Between Structural, Spectroscopic, and Reactivity Properties Within a Series of Structurally Analogous Metastable Manganese(III)–Alkylperoxo Complexes

Manganese–peroxos are proposed as key intermediates in a number of important biochemical and synthetic transformations. Our understanding of the structural, spectroscopic, and reactivity properties of these metastable species is limited, however, and correlations between these properties have yet to be established experimentally. Herein we report the crystallographic structures of a series of structurally related metastable Mn­(III)–OOR compounds, and examine their spectroscopic and reactivity properties. The four reported Mn­(III)–OOR compounds extend the number of known end-on Mn­(III)–(η¹-peroxos) to six. The ligand backbone is shown to alter the metal–ligand distances and modulate the electronic properties key to bonding and activation of the peroxo. The mechanism of thermal decay of these metastable species is examined via variable-temperature kinetics. Strong correlations between structural (O–O and Mn···N^py,quin distances), spectroscopic (E­(π_v*­(O–O) → Mn CT band), ν_O–O), and kinetic (Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧) parameters for these complexes provide compelling evidence for rate-limiting O–O bond cleavage. Products identified in the final reaction mixtures of Mn­(III)–OOR decay are consistent with homolytic O–O bond scission. The N-heterocyclic amines and ligand backbone (Et vs Pr) are found to modulate structural and reactivity properties, and O–O bond activation is shown, both experimentally and theoretically, to track with metal ion Lewis acidity. The peroxo O–O bond is shown to gradually become more activated as the N-heterocyclic amines move closer to the metal ion causing a decrease in π-donation from the peroxo π_v*­(O–O) orbital. The reported work represents one of very few examples of experimentally verified relationships between structure and function

Correlation Between Structural, Spectroscopic, and Reactivity Properties Within a Series of Structurally Analogous Metastable Manganese(III)–Alkylperoxo Complexes

Manganese–peroxos are proposed as key intermediates in a number of important biochemical and synthetic transformations. Our understanding of the structural, spectroscopic, and reactivity properties of these metastable species is limited, however, and correlations between these properties have yet to be established experimentally. Herein we report the crystallographic structures of a series of structurally related metastable Mn­(III)–OOR compounds, and examine their spectroscopic and reactivity properties. The four reported Mn­(III)–OOR compounds extend the number of known end-on Mn­(III)–(η¹-peroxos) to six. The ligand backbone is shown to alter the metal–ligand distances and modulate the electronic properties key to bonding and activation of the peroxo. The mechanism of thermal decay of these metastable species is examined via variable-temperature kinetics. Strong correlations between structural (O–O and Mn···N^py,quin distances), spectroscopic (E­(π_v*­(O–O) → Mn CT band), ν_O–O), and kinetic (Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧) parameters for these complexes provide compelling evidence for rate-limiting O–O bond cleavage. Products identified in the final reaction mixtures of Mn­(III)–OOR decay are consistent with homolytic O–O bond scission. The N-heterocyclic amines and ligand backbone (Et vs Pr) are found to modulate structural and reactivity properties, and O–O bond activation is shown, both experimentally and theoretically, to track with metal ion Lewis acidity. The peroxo O–O bond is shown to gradually become more activated as the N-heterocyclic amines move closer to the metal ion causing a decrease in π-donation from the peroxo π_v*­(O–O) orbital. The reported work represents one of very few examples of experimentally verified relationships between structure and function

Correlation Between Structural, Spectroscopic, and Reactivity Properties Within a Series of Structurally Analogous Metastable Manganese(III)–Alkylperoxo Complexes

Manganese–peroxos are proposed as key intermediates in a number of important biochemical and synthetic transformations. Our understanding of the structural, spectroscopic, and reactivity properties of these metastable species is limited, however, and correlations between these properties have yet to be established experimentally. Herein we report the crystallographic structures of a series of structurally related metastable Mn­(III)–OOR compounds, and examine their spectroscopic and reactivity properties. The four reported Mn­(III)–OOR compounds extend the number of known end-on Mn­(III)–(η¹-peroxos) to six. The ligand backbone is shown to alter the metal–ligand distances and modulate the electronic properties key to bonding and activation of the peroxo. The mechanism of thermal decay of these metastable species is examined via variable-temperature kinetics. Strong correlations between structural (O–O and Mn···N^py,quin distances), spectroscopic (E­(π_v*­(O–O) → Mn CT band), ν_O–O), and kinetic (Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧) parameters for these complexes provide compelling evidence for rate-limiting O–O bond cleavage. Products identified in the final reaction mixtures of Mn­(III)–OOR decay are consistent with homolytic O–O bond scission. The N-heterocyclic amines and ligand backbone (Et vs Pr) are found to modulate structural and reactivity properties, and O–O bond activation is shown, both experimentally and theoretically, to track with metal ion Lewis acidity. The peroxo O–O bond is shown to gradually become more activated as the N-heterocyclic amines move closer to the metal ion causing a decrease in π-donation from the peroxo π_v*­(O–O) orbital. The reported work represents one of very few examples of experimentally verified relationships between structure and function

Correlation Between Structural, Spectroscopic, and Reactivity Properties Within a Series of Structurally Analogous Metastable Manganese(III)–Alkylperoxo Complexes

Manganese–peroxos are proposed as key intermediates in a number of important biochemical and synthetic transformations. Our understanding of the structural, spectroscopic, and reactivity properties of these metastable species is limited, however, and correlations between these properties have yet to be established experimentally. Herein we report the crystallographic structures of a series of structurally related metastable Mn­(III)–OOR compounds, and examine their spectroscopic and reactivity properties. The four reported Mn­(III)–OOR compounds extend the number of known end-on Mn­(III)–(η¹-peroxos) to six. The ligand backbone is shown to alter the metal–ligand distances and modulate the electronic properties key to bonding and activation of the peroxo. The mechanism of thermal decay of these metastable species is examined via variable-temperature kinetics. Strong correlations between structural (O–O and Mn···N^py,quin distances), spectroscopic (E­(π_v*­(O–O) → Mn CT band), ν_O–O), and kinetic (Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧) parameters for these complexes provide compelling evidence for rate-limiting O–O bond cleavage. Products identified in the final reaction mixtures of Mn­(III)–OOR decay are consistent with homolytic O–O bond scission. The N-heterocyclic amines and ligand backbone (Et vs Pr) are found to modulate structural and reactivity properties, and O–O bond activation is shown, both experimentally and theoretically, to track with metal ion Lewis acidity. The peroxo O–O bond is shown to gradually become more activated as the N-heterocyclic amines move closer to the metal ion causing a decrease in π-donation from the peroxo π_v*­(O–O) orbital. The reported work represents one of very few examples of experimentally verified relationships between structure and function

Correlation Between Structural, Spectroscopic, and Reactivity Properties Within a Series of Structurally Analogous Metastable Manganese(III)–Alkylperoxo Complexes

Manganese–peroxos are proposed as key intermediates in a number of important biochemical and synthetic transformations. Our understanding of the structural, spectroscopic, and reactivity properties of these metastable species is limited, however, and correlations between these properties have yet to be established experimentally. Herein we report the crystallographic structures of a series of structurally related metastable Mn­(III)–OOR compounds, and examine their spectroscopic and reactivity properties. The four reported Mn­(III)–OOR compounds extend the number of known end-on Mn­(III)–(η¹-peroxos) to six. The ligand backbone is shown to alter the metal–ligand distances and modulate the electronic properties key to bonding and activation of the peroxo. The mechanism of thermal decay of these metastable species is examined via variable-temperature kinetics. Strong correlations between structural (O–O and Mn···N^py,quin distances), spectroscopic (E­(π_v*­(O–O) → Mn CT band), ν_O–O), and kinetic (Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧) parameters for these complexes provide compelling evidence for rate-limiting O–O bond cleavage. Products identified in the final reaction mixtures of Mn­(III)–OOR decay are consistent with homolytic O–O bond scission. The N-heterocyclic amines and ligand backbone (Et vs Pr) are found to modulate structural and reactivity properties, and O–O bond activation is shown, both experimentally and theoretically, to track with metal ion Lewis acidity. The peroxo O–O bond is shown to gradually become more activated as the N-heterocyclic amines move closer to the metal ion causing a decrease in π-donation from the peroxo π_v*­(O–O) orbital. The reported work represents one of very few examples of experimentally verified relationships between structure and function

X‑ray Absorption and Emission Study of Dioxygen Activation by a Small-Molecule Manganese Complex

Manganese K-edge X-ray absorption (XAS) and Kβ emission (XES) spectroscopies were used to investigate the factors contributing to O–O bond activation in a small-molecule system. The recent structural characterization of a metastable peroxo-bridged dimeric Mn­(III)₂ complex derived from dioxygen has provided the first opportunity to obtain X-ray spectroscopic data on this type of species. Ground state and time-dependent density functional theory calculations have provided further insight into the nature of the transitions in XAS pre-edge and valence-to-core (VtC) XES spectral regions. An experimentally validated electronic structure description has also enabled the determination of structural and electronic factors that govern peroxo bond activation, and have allowed us to propose both a rationale for the metastability of this unique compound, as well as potential future ligand designs which may further promote or inhibit O–O bond scission. Finally, we have explored the potential of VtC XES as an element-selective probe of both the coordination mode and degree of activation of peroxomanganese adducts. The comparison of these results to a recent VtC XES study of iron-mediated dintrogen activation helps to illustrate the factors that may determine the success of this spectroscopic method for future studies of small-molecule activation at transition metal sites

Selective Synthesis and Redox Sequence of a Heterobimetallic Nickel/Copper Complex of the Noninnocent Siamese-Twin Porphyrin

The Siamese-twin porphyrin (<b>1H</b>_<b>4</b>) is a redox noninnocent pyrazole-expanded porphyrin with two equivalent dibasic {N₄} binding sites. It is now shown that its selective monometalation can be achieved to give the nickel­(II) complex <b>1H</b>_<b>2</b><b>Ni</b> with the second {N₄} site devoid of a metal ion. This intermediate is then cleanly converted to <b>1Ni</b>_<b>2</b> and to the first heterobimetallic Siamese-twin porphyrin <b>1CuNi</b>. Structural characterization of <b>1H</b>_<b>2</b><b>Ni</b> shows that it has the same helical structure previously seen for <b>1Cu</b>_<b>2</b>, <b>1Ni</b>_<b>2</b>, and free base <b>1H</b>_<b>6</b>^<b>2+</b>. Titration experiments suggest that the metal-devoid pocket of <b>1H</b>_<b>2</b><b>Ni</b> can accommodate two additional protons, giving <b>[1H</b>_<b>4</b><b>Ni]</b>^<b>2+</b>. Both bimetallic complexes <b>1Ni</b>_<b>2</b> and <b>1CuNi</b> feature rich redox chemistry, similar to the recently reported <b>1Cu</b>_<b>2</b>, including two chemically reversible oxidations at moderate potentials between −0.3 and +0.5 V (vs Cp₂Fe/Cp₂Fe⁺). The locus of these oxidations, in singly oxidized [<b>1Ni</b>_<b>2</b>]⁺ and [<b>1CuNi</b>]⁺ as well as twice oxidized [<b>1CuNi</b>]²⁺, has been experimentally derived from comparison of the electrochemical properties of the complete series of complexes <b>1Cu</b>_<b>2</b>, <b>1Ni</b>_<b>2</b>, and <b>1CuNi</b>, and from electron paramagnetic resonance (EPR) spectroscopy and X-ray absorption spectroscopy (XAS) (Ni and Cu K edges). All redox events are largely ligand-based, and in heterobimetallic <b>1CuNi</b>, the first oxidation takes place within its Cu-subunit, while the second oxidation then occurs in its Ni-subunit. Adding pyridine to solutions of [<b>1Ni</b>_<b>2</b>]⁺ and [<b>1CuNi</b>]²⁺ cleanly converts them to metal-oxidized redox isomers with axial EPR spectra typical for Ni^III having significant d_<i>z</i>²¹ character, reflecting close similarity with nickel complexes of common porphyrins. The possibility of selectively synthesizing heterobimetallic complexes <b>1MNi</b> from a symmetric binucleating ligand scaffold, with the unusual situation of three distinct contiguous redox sites (M, Ni, and the porphyrin-like ligand), further expands the Siamese-twin porphyrin’s potential to serve as an adjustable platform for multielectron redox processes in chemical catalysis and in electronic applications

Carboxylate-Assisted Formation of Aryl-Co(III) Masked-Carbenes in Cobalt-Catalyzed C–H Functionalization with Diazo Esters

Herein we describe the synthesis of a family of aryl-Co­(III)-carboxylate complexes and their reactivity with ethyl diazoacetate. Crystallographic, full spectroscopic characterization, and theoretical evidence of unique C-metalated aryl-Co­(III) enolate intermediates is provided, unraveling a carboxylate-assisted formation of aryl-Co­(III) <i>masked-carbenes</i>. Moreover, additional evidence for an unprecedented Co­(III)-mediated intramolecular S_N2-type C–C bond formation in which the carboxylate moiety acts as a relay is disclosed. This novel strategy is key to tame the hot reactivity of a metastable Co­(III)-carbene and elicit C–C coupling products in a productive manner

Carboxylate-Assisted Formation of Aryl-Co(III) Masked-Carbenes in Cobalt-Catalyzed C–H Functionalization with Diazo Esters

Herein we describe the synthesis of a family of aryl-Co­(III)-carboxylate complexes and their reactivity with ethyl diazoacetate. Crystallographic, full spectroscopic characterization, and theoretical evidence of unique C-metalated aryl-Co­(III) enolate intermediates is provided, unraveling a carboxylate-assisted formation of aryl-Co­(III) <i>masked-carbenes</i>. Moreover, additional evidence for an unprecedented Co­(III)-mediated intramolecular S_N2-type C–C bond formation in which the carboxylate moiety acts as a relay is disclosed. This novel strategy is key to tame the hot reactivity of a metastable Co­(III)-carbene and elicit C–C coupling products in a productive manner

Carboxylate-Assisted Formation of Aryl-Co(III) Masked-Carbenes in Cobalt-Catalyzed C–H Functionalization with Diazo Esters

Herein we describe the synthesis of a family of aryl-Co­(III)-carboxylate complexes and their reactivity with ethyl diazoacetate. Crystallographic, full spectroscopic characterization, and theoretical evidence of unique C-metalated aryl-Co­(III) enolate intermediates is provided, unraveling a carboxylate-assisted formation of aryl-Co­(III) <i>masked-carbenes</i>. Moreover, additional evidence for an unprecedented Co­(III)-mediated intramolecular S_N2-type C–C bond formation in which the carboxylate moiety acts as a relay is disclosed. This novel strategy is key to tame the hot reactivity of a metastable Co­(III)-carbene and elicit C–C coupling products in a productive manner