21 research outputs found
Correlation Between Structural, Spectroscopic, and Reactivity Properties Within a Series of Structurally Analogous Metastable Manganese(III)âAlkylperoxo Complexes
Manganeseâperoxos
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 MnÂ(III)âOOR compounds, and examine their
spectroscopic and reactivity properties. The four reported MnÂ(III)âOOR
compounds extend the number of known end-on MnÂ(III)â(Ρ<sup>1</sup>-peroxos) to six. The ligand backbone is shown to alter the
metalâligand 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 (OâO and Mn¡¡¡N<sup>py,quin</sup> distances), spectroscopic (EÂ(Ď<sub>v</sub>*Â(OâO)
â Mn CT band), ν<sub>OâO</sub>), and kinetic (Î<i>H</i><sup>⧧</sup> and Î<i>S</i><sup>⧧</sup>) parameters for these complexes provide compelling evidence for
rate-limiting OâO bond cleavage. Products identified in the
final reaction mixtures of MnÂ(III)âOOR decay are consistent
with homolytic OâO bond scission. The N-heterocyclic amines
and ligand backbone (Et vs Pr) are found to modulate structural and
reactivity properties, and OâO bond activation is shown, both
experimentally and theoretically, to track with metal ion Lewis acidity.
The peroxo OâO bond is shown to gradually become more activated
as the N-heterocyclic amines move closer to the metal ion causing
a decrease in Ď-donation from the peroxo Ď<sub>v</sub>*Â(OâO) 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)âAlkylperoxo Complexes
Manganeseâperoxos
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 MnÂ(III)âOOR compounds, and examine their
spectroscopic and reactivity properties. The four reported MnÂ(III)âOOR
compounds extend the number of known end-on MnÂ(III)â(Ρ<sup>1</sup>-peroxos) to six. The ligand backbone is shown to alter the
metalâligand 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 (OâO and Mn¡¡¡N<sup>py,quin</sup> distances), spectroscopic (EÂ(Ď<sub>v</sub>*Â(OâO)
â Mn CT band), ν<sub>OâO</sub>), and kinetic (Î<i>H</i><sup>⧧</sup> and Î<i>S</i><sup>⧧</sup>) parameters for these complexes provide compelling evidence for
rate-limiting OâO bond cleavage. Products identified in the
final reaction mixtures of MnÂ(III)âOOR decay are consistent
with homolytic OâO bond scission. The N-heterocyclic amines
and ligand backbone (Et vs Pr) are found to modulate structural and
reactivity properties, and OâO bond activation is shown, both
experimentally and theoretically, to track with metal ion Lewis acidity.
The peroxo OâO bond is shown to gradually become more activated
as the N-heterocyclic amines move closer to the metal ion causing
a decrease in Ď-donation from the peroxo Ď<sub>v</sub>*Â(OâO) 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)âAlkylperoxo Complexes
Manganeseâperoxos
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 MnÂ(III)âOOR compounds, and examine their
spectroscopic and reactivity properties. The four reported MnÂ(III)âOOR
compounds extend the number of known end-on MnÂ(III)â(Ρ<sup>1</sup>-peroxos) to six. The ligand backbone is shown to alter the
metalâligand 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 (OâO and Mn¡¡¡N<sup>py,quin</sup> distances), spectroscopic (EÂ(Ď<sub>v</sub>*Â(OâO)
â Mn CT band), ν<sub>OâO</sub>), and kinetic (Î<i>H</i><sup>⧧</sup> and Î<i>S</i><sup>⧧</sup>) parameters for these complexes provide compelling evidence for
rate-limiting OâO bond cleavage. Products identified in the
final reaction mixtures of MnÂ(III)âOOR decay are consistent
with homolytic OâO bond scission. The N-heterocyclic amines
and ligand backbone (Et vs Pr) are found to modulate structural and
reactivity properties, and OâO bond activation is shown, both
experimentally and theoretically, to track with metal ion Lewis acidity.
The peroxo OâO bond is shown to gradually become more activated
as the N-heterocyclic amines move closer to the metal ion causing
a decrease in Ď-donation from the peroxo Ď<sub>v</sub>*Â(OâO) 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)âAlkylperoxo Complexes
Manganeseâperoxos
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 MnÂ(III)âOOR compounds, and examine their
spectroscopic and reactivity properties. The four reported MnÂ(III)âOOR
compounds extend the number of known end-on MnÂ(III)â(Ρ<sup>1</sup>-peroxos) to six. The ligand backbone is shown to alter the
metalâligand 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 (OâO and Mn¡¡¡N<sup>py,quin</sup> distances), spectroscopic (EÂ(Ď<sub>v</sub>*Â(OâO)
â Mn CT band), ν<sub>OâO</sub>), and kinetic (Î<i>H</i><sup>⧧</sup> and Î<i>S</i><sup>⧧</sup>) parameters for these complexes provide compelling evidence for
rate-limiting OâO bond cleavage. Products identified in the
final reaction mixtures of MnÂ(III)âOOR decay are consistent
with homolytic OâO bond scission. The N-heterocyclic amines
and ligand backbone (Et vs Pr) are found to modulate structural and
reactivity properties, and OâO bond activation is shown, both
experimentally and theoretically, to track with metal ion Lewis acidity.
The peroxo OâO bond is shown to gradually become more activated
as the N-heterocyclic amines move closer to the metal ion causing
a decrease in Ď-donation from the peroxo Ď<sub>v</sub>*Â(OâO) 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)âAlkylperoxo Complexes
Manganeseâperoxos
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 MnÂ(III)âOOR compounds, and examine their
spectroscopic and reactivity properties. The four reported MnÂ(III)âOOR
compounds extend the number of known end-on MnÂ(III)â(Ρ<sup>1</sup>-peroxos) to six. The ligand backbone is shown to alter the
metalâligand 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 (OâO and Mn¡¡¡N<sup>py,quin</sup> distances), spectroscopic (EÂ(Ď<sub>v</sub>*Â(OâO)
â Mn CT band), ν<sub>OâO</sub>), and kinetic (Î<i>H</i><sup>⧧</sup> and Î<i>S</i><sup>⧧</sup>) parameters for these complexes provide compelling evidence for
rate-limiting OâO bond cleavage. Products identified in the
final reaction mixtures of MnÂ(III)âOOR decay are consistent
with homolytic OâO bond scission. The N-heterocyclic amines
and ligand backbone (Et vs Pr) are found to modulate structural and
reactivity properties, and OâO bond activation is shown, both
experimentally and theoretically, to track with metal ion Lewis acidity.
The peroxo OâO bond is shown to gradually become more activated
as the N-heterocyclic amines move closer to the metal ion causing
a decrease in Ď-donation from the peroxo Ď<sub>v</sub>*Â(OâO) orbital. The reported work represents one of very few
examples of experimentally verified relationships between structure
and function
Xâray 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 OâO bond activation in a small-molecule
system. The recent structural characterization of a metastable peroxo-bridged
dimeric MnÂ(III)<sub>2</sub> 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 OâO 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><sub><b>4</b></sub>) is a redox noninnocent pyrazole-expanded
porphyrin with two equivalent dibasic {N<sub>4</sub>} binding sites.
It is now shown that its selective monometalation can be achieved
to give the nickelÂ(II) complex <b>1H</b><sub><b>2</b></sub><b>Ni</b> with the second {N<sub>4</sub>} site devoid of a
metal ion. This intermediate is then cleanly converted to <b>1Ni</b><sub><b>2</b></sub> and to the first heterobimetallic Siamese-twin
porphyrin <b>1CuNi</b>. Structural characterization of <b>1H</b><sub><b>2</b></sub><b>Ni</b> shows that it has
the same helical structure previously seen for <b>1Cu</b><sub><b>2</b></sub>, <b>1Ni</b><sub><b>2</b></sub>,
and free base <b>1H</b><sub><b>6</b></sub><sup><b>2+</b></sup>. Titration experiments suggest that the metal-devoid pocket
of <b>1H</b><sub><b>2</b></sub><b>Ni</b> can accommodate
two additional protons, giving <b>[1H</b><sub><b>4</b></sub><b>Ni]</b><sup><b>2+</b></sup>. Both bimetallic
complexes <b>1Ni</b><sub><b>2</b></sub> and <b>1CuNi</b> feature rich redox chemistry, similar to the recently reported <b>1Cu</b><sub><b>2</b></sub>, including two chemically reversible
oxidations at moderate potentials between â0.3 and +0.5 V (vs
Cp<sub>2</sub>Fe/Cp<sub>2</sub>Fe<sup>+</sup>). The locus of these
oxidations, in singly oxidized [<b>1Ni</b><sub><b>2</b></sub>]<sup>+</sup> and [<b>1CuNi</b>]<sup>+</sup> as well
as twice oxidized [<b>1CuNi</b>]<sup>2+</sup>, has been experimentally
derived from comparison of the electrochemical properties of the complete
series of complexes <b>1Cu</b><sub><b>2</b></sub>, <b>1Ni</b><sub><b>2</b></sub>, 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><sub><b>2</b></sub>]<sup>+</sup> and [<b>1CuNi</b>]<sup>2+</sup> cleanly converts them to metal-oxidized
redox isomers with axial EPR spectra typical for Ni<sup>III</sup> having
significant d<sub><i>z</i><sup>2</sup></sub><sup>1</sup> 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
porphyrinâs 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 CâH Functionalization with Diazo Esters
Herein
we describe the synthesis of a family of aryl-CoÂ(III)-carboxylate
complexes and their reactivity with ethyl diazoacetate. Crystallographic,
full spectroscopic characterization, and theoretical evidence of unique
C-metalated aryl-CoÂ(III) enolate intermediates is provided, unraveling
a carboxylate-assisted formation of aryl-CoÂ(III) <i>masked-carbenes</i>. Moreover, additional evidence for an unprecedented CoÂ(III)-mediated
intramolecular S<sub>N</sub>2-type CâC 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 CoÂ(III)-carbene
and elicit CâC coupling products in a productive manner
Carboxylate-Assisted Formation of Aryl-Co(III) Masked-Carbenes in Cobalt-Catalyzed CâH Functionalization with Diazo Esters
Herein
we describe the synthesis of a family of aryl-CoÂ(III)-carboxylate
complexes and their reactivity with ethyl diazoacetate. Crystallographic,
full spectroscopic characterization, and theoretical evidence of unique
C-metalated aryl-CoÂ(III) enolate intermediates is provided, unraveling
a carboxylate-assisted formation of aryl-CoÂ(III) <i>masked-carbenes</i>. Moreover, additional evidence for an unprecedented CoÂ(III)-mediated
intramolecular S<sub>N</sub>2-type CâC 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 CoÂ(III)-carbene
and elicit CâC coupling products in a productive manner
Carboxylate-Assisted Formation of Aryl-Co(III) Masked-Carbenes in Cobalt-Catalyzed CâH Functionalization with Diazo Esters
Herein
we describe the synthesis of a family of aryl-CoÂ(III)-carboxylate
complexes and their reactivity with ethyl diazoacetate. Crystallographic,
full spectroscopic characterization, and theoretical evidence of unique
C-metalated aryl-CoÂ(III) enolate intermediates is provided, unraveling
a carboxylate-assisted formation of aryl-CoÂ(III) <i>masked-carbenes</i>. Moreover, additional evidence for an unprecedented CoÂ(III)-mediated
intramolecular S<sub>N</sub>2-type CâC 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 CoÂ(III)-carbene
and elicit CâC coupling products in a productive manner