33 research outputs found
Magnetic Properties and Electronic Structure of Manganese-Based Blue Pigments: A High-Frequency and -Field EPR Study
A variety of new oxide-based materials
based on hexagonal phase
of YInO<sub>3</sub> have been recently described. In some of these
materials, the InĀ(III) ions are substituted by MnĀ(III), which finds
itself in a trigonal-bipyramidal (TBP) coordination environment. While
YInO<sub>3</sub> is colorless and YMnO<sub>3</sub> is black, mixed
systems YIn<sub>1ā<i>x</i></sub>Mn<sub><i>x</i></sub>O<sub>3</sub> (0.02 < <i>x</i> < 0.25) display
intense blue color and have been proposed as novel blue pigments.
Since the MnĀ(III) ion is paramagnetic, its presence imparts distinct
magnetic properties to the whole class of materials. These properties
were investigated by electron paramagnetic resonance (EPR) in its
high-frequency and -field version (HFEPR), a technique ideally suited
for transition metal ions such as MnĀ(III) that, in contrast to, for
example, MnĀ(II), are difficult to study by EPR at (conventional) low
frequency and field. YIn<sub>1ā<i>x</i></sub>Mn<sub><i>x</i></sub>O<sub>3</sub> with 0.02 < <i>x</i> < 0.2 exhibited high-quality HFEPR spectra up to room temperature
that could be interpreted as arising from isolated <i>S</i> = 2 paramagnets. A simple ligand-field model, based on the structure
and optical spectra, explains the spin Hamiltonian parameters provided
by HFEPR, which were <i>D</i> = +3.0 cm<sup>ā1</sup>, <i>E</i> = 0; <i>g</i><sub>ā„</sub> =
1.99, <i>g</i><sub>ā„</sub> = 2.0. This study demonstrates
the general applicability of a combined spectroscopic and classical
theoretical approach to understanding the electronic structure of
novel materials containing paramagnetic dopants. Moreover, HFEPR complements
optical and other experimental methods as being a sensitive probe
of dopant level
Observation of a Photogenerated Rh<sub>2</sub> Nitrenoid Intermediate in CāH Amination
Rh<sub>2</sub>-catalyzed CāH amination is a powerful method
for nitrogenating organic molecules. While Rh<sub>2</sub> nitrenoids
are often invoked as reactive intermediates in these reactions, the
exquisite reactivity and fleeting lifetime of these species has precluded
their observation. Here, we report the photogeneration of a transient
Rh<sub>2</sub> nitrenoid that participates in CāH amination.
The developed approach to Rh<sub>2</sub> nitrenoids, based on photochemical
cleavage of NāCl bonds in <i>N</i>-chloroamido ligands,
has enabled characterization of a reactive Rh<sub>2</sub> nitrenoid
by mass spectrometry and transient absorption spectroscopy. We anticipate
that photogeneration of metal nitrenoids will contribute to the development
of CāH amination catalysis by providing tools to directly study
the structures of these critical intermediates
High Spin Co(I): High-Frequency and -Field EPR Spectroscopy of CoX(PPh<sub>3</sub>)<sub>3</sub> (X = Cl, Br)
The previously reported pseudotetrahedral CoĀ(I) complexes,
CoXĀ(PR<sub>3</sub>)<sub>3</sub>, where R = Me, Ph, and chelating analogues,
and X = Cl, Br, I exhibit a spin triplet ground state, which is uncommon
for CoĀ(I), although expected for this geometry. Described here are
studies using electronic absorption and high-frequency and -field
electron paramagnetic resonance (HFEPR) spectroscopy on two members
of this class of complexes: CoXĀ(PR<sub>3</sub>)<sub>3</sub>, where
R = Ph and X = Cl and Br. In both cases, well-defined spectra corresponding
to axial spin triplets were observed, with signals assignable to three
distinct triplet species, and with perfectly axial zero-field splitting
(zfs) given by the parameter <i>D</i> = +4.46, +5.52, +8.04
cm<sup>ā1</sup>, respectively, for CoClĀ(PPh<sub>3</sub>)<sub>3</sub>. The crystal structure reported for CoClĀ(PPh<sub>3</sub>)<sub>3</sub> shows crystallographic 3-fold symmetry, but with three structurally
distinct molecules per unit cell. Both of these facts thus correlate
with the HFEPR data. The investigated complexes, along with a number
of structurally characterized CoĀ(I) trisphosphine analogues, were
analyzed by quantum chemistry calculations (both density functional
theory (DFT) and unrestricted HartreeāFock (UHF) methods).
These methods, along with ligand-field theory (LFT) analysis of CoClĀ(PPh<sub>3</sub>)<sub>3</sub>, give reasonable agreement with the salient
features of the electronic structure of these complexes. A spin triplet
ground state is strongly favored over a singlet state and a positive,
axial <i>D</i> value is predicted, in agreement with experiment.
Quantitative agreement between theory and experiment is less than
ideal with LFT overestimating the zfs, while DFT underestimates these
effects. Despite these shortcomings, this study demonstrates the ability
of advanced paramagnetic resonance techniques, in combination with
other experimental techniques, and with theory, to shed light on the
electronic structure of an unusual transition metal ion, paramagnetic
CoĀ(I)
Synthesis, Characterization, and Electrochemical Analyses of Vanadocene Tetrametaphosphate and Phosphinate Derivatives
The synthesis and
characterization of a series of new vanadocene-derived
tetrametaphosphate or diphenylphosphinate
complexes, [PPN]<sub>2</sub>[CpVĀ(P<sub>4</sub>O<sub>12</sub>)] (<b>1</b>), [Cp<sub>2</sub>VOPĀ(O)ĀPh<sub>2</sub>] (<b>2</b>),
[CpVĀ(Ī¼<sup>2</sup>-O<sub>2</sub>PPh<sub>2</sub>)<sub>4</sub>VCp] (<b>3</b>), and [Cp<sub>2</sub>VĀ(Ī¼<sup>2</sup>-O<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub>VCp<sub>2</sub>]Ā[PF<sub>6</sub>]<sub>2</sub> (<b>4</b>), is reported ([PPN]<sup>+</sup> =
bisĀ(triphenylphosphoranylidene)Āammonium; Cp = Ī·<sup>5</sup>-cyclopentadienyl).
The complexes were synthesized from the methyl vanadocene (Cp<sub>2</sub>VMe) precursor, through protonation of the Me or Cp linkages
using the tetrametaphosphate or phosphinate acid precursors [PPN]<sub>2</sub>[P<sub>4</sub>O<sub>12</sub>H<sub>2</sub>] (for <b>1</b>) and Ph<sub>2</sub>PĀ(O)ĀOH (for <b>2</b> and <b>3</b>). Oxidation of <b>2</b> with [Fc]Ā[PF<sub>6</sub>] resulted
in dimerization, yielding the bimetallic <b>4</b> ([Fc]<sup>+</sup> = ferrocenium). Electrochemical analysis of this reaction
revealed a possible ECE mechanism that includes prior and subsequent
electron transfer to this dimerization. The electronic structure of
the dimer <b>4</b> was probed by SQUID magnetommetry and X-band
EPR spectroscopy (100 and 4 K). The dimer was found to contain two
antiferromagnetically coupled V<sup>IV</sup> centers, as well as a
small portion of monomeric V<sup>IV</sup> species, <b>2</b><sup><b>+</b></sup>. In contrast to <b>2</b>, oxidation of <b>1</b> resulted in an EC mechanism, the product of which remains
unknown. Preliminary reactions with compounds <b>1</b> and <b>2</b> bearing free Pī»O groups were performed using H atom
donors to test their ability to undergo H atom transfer in the context
of the proposed āreduction-coupled oxo activationā mechanism;
however, no clear reaction pathway supporting this mechanism has yet
been observed
Secondary Coordination Sphere Effects in Ruthenium(III) Tetraammine Complexes: Role of the Coordinated Water Molecule
The
complexes <i>trans</i>-[Ru<sup>III</sup>(NH<sub>3</sub>)<sub>4</sub>(4-pic)Ā(H<sub>2</sub>O)]Ā(CF<sub>3</sub>SO<sub>3</sub>)<sub>3</sub> (<b>1</b>) and [Ru<sup>III</sup>(NH<sub>3</sub>)<sub>5</sub>(4-pic)]Ā(CF<sub>3</sub>SO<sub>3</sub>)<sub>3</sub> (<b>2</b>) were isolated and studied experimentally by electron paramagnetic
resonance (EPR) and UVāvis spectroscopies, cyclic voltammetry,
and X-ray crystallography and theoretically by ligand-field theory
(LFT) and density functional theory (DFT) calculations. Complex <b>1</b> is reported in two different crystal forms, <b>1a</b> (100 K) and <b>1b</b> (room temperature). EPR and UVāvis
spectroscopies suggest that aqua ligand interaction in this low-spin
rutheniumĀ(III) complex changes as a function of hydrogen bonding with
solvent molecules. This explicit water solvent effect was explained
theoretically by DFT calculations, which demonstrated the effect of
rotation of the aqua ligand about the N<sub>pic</sub>āRuāO<sub>aq</sub> axis. The UVāvis spectrum of <b>1</b> shows
in an aqueous acid solution a broad- and low-intensity absorption
band around 28āÆ500 cm<sup>ā1</sup> (Īµ ā
500 M<sup>ā1</sup> cm<sup>ā1</sup>) that is assigned
mainly to a charge-transfer (CT) transition from the equatorial ligands
to the Ru Ī²-4d<sub><i>xy</i></sub> orbital (Ī²-LUMO)
using DFT calculations. The electronic reflectance spectrum of <b>1</b> shows a broad and intense absorption band around 25āÆ500
cm<sup>ā1</sup> that is assigned to a CT transition from 4-picoline
to the Ru Ī²-4d<sub><i>xz</i></sub> orbital (Ī²-LUMO)
using DFT calculations. The t<sub>2g</sub><sup>5</sup> set of orbitals
had its energy splitting investigated by LFT. LFT analysis shows that
a rhombic component arises from <i>C</i><sub>2<i>v</i></sub> symmetry by a simple Ļ-bonding ligand (H<sub>2</sub>O in our case) twisting about the trans (<i>C</i><sub>2</sub>) axis. This twist was manifested in the EPR spectra, which were
recorded for <b>1</b> as a function of the solvent in comparison
with [RuĀ(NH<sub>3</sub>)<sub>5</sub>(4-pic)]<sup>3+</sup> and [RuĀ(NH<sub>3</sub>)<sub>5</sub>(H<sub>2</sub>O)]<sup>3+</sup>. Only <b>1</b> shows an evident change in the <b>g</b>-tensor values, wherein
an increased rhombic component is correlated with a higher nucleophilicity
(donor) solvent feature, as parametrized by the Abraham system
Secondary Coordination Sphere Effects in Ruthenium(III) Tetraammine Complexes: Role of the Coordinated Water Molecule
The
complexes <i>trans</i>-[Ru<sup>III</sup>(NH<sub>3</sub>)<sub>4</sub>(4-pic)Ā(H<sub>2</sub>O)]Ā(CF<sub>3</sub>SO<sub>3</sub>)<sub>3</sub> (<b>1</b>) and [Ru<sup>III</sup>(NH<sub>3</sub>)<sub>5</sub>(4-pic)]Ā(CF<sub>3</sub>SO<sub>3</sub>)<sub>3</sub> (<b>2</b>) were isolated and studied experimentally by electron paramagnetic
resonance (EPR) and UVāvis spectroscopies, cyclic voltammetry,
and X-ray crystallography and theoretically by ligand-field theory
(LFT) and density functional theory (DFT) calculations. Complex <b>1</b> is reported in two different crystal forms, <b>1a</b> (100 K) and <b>1b</b> (room temperature). EPR and UVāvis
spectroscopies suggest that aqua ligand interaction in this low-spin
rutheniumĀ(III) complex changes as a function of hydrogen bonding with
solvent molecules. This explicit water solvent effect was explained
theoretically by DFT calculations, which demonstrated the effect of
rotation of the aqua ligand about the N<sub>pic</sub>āRuāO<sub>aq</sub> axis. The UVāvis spectrum of <b>1</b> shows
in an aqueous acid solution a broad- and low-intensity absorption
band around 28āÆ500 cm<sup>ā1</sup> (Īµ ā
500 M<sup>ā1</sup> cm<sup>ā1</sup>) that is assigned
mainly to a charge-transfer (CT) transition from the equatorial ligands
to the Ru Ī²-4d<sub><i>xy</i></sub> orbital (Ī²-LUMO)
using DFT calculations. The electronic reflectance spectrum of <b>1</b> shows a broad and intense absorption band around 25āÆ500
cm<sup>ā1</sup> that is assigned to a CT transition from 4-picoline
to the Ru Ī²-4d<sub><i>xz</i></sub> orbital (Ī²-LUMO)
using DFT calculations. The t<sub>2g</sub><sup>5</sup> set of orbitals
had its energy splitting investigated by LFT. LFT analysis shows that
a rhombic component arises from <i>C</i><sub>2<i>v</i></sub> symmetry by a simple Ļ-bonding ligand (H<sub>2</sub>O in our case) twisting about the trans (<i>C</i><sub>2</sub>) axis. This twist was manifested in the EPR spectra, which were
recorded for <b>1</b> as a function of the solvent in comparison
with [RuĀ(NH<sub>3</sub>)<sub>5</sub>(4-pic)]<sup>3+</sup> and [RuĀ(NH<sub>3</sub>)<sub>5</sub>(H<sub>2</sub>O)]<sup>3+</sup>. Only <b>1</b> shows an evident change in the <b>g</b>-tensor values, wherein
an increased rhombic component is correlated with a higher nucleophilicity
(donor) solvent feature, as parametrized by the Abraham system
Mechanistic Studies of the Spore Photoproduct Lyase via a Single Cysteine Mutation
5-Thyminyl-5,6-dihydrothymine (also called spore photoproduct
or
SP) is the exclusive DNA photodamage product in bacterial endospores.
It is repaired by a radical SAM (<i>S</i>-adenosylmethionine)
enzyme, the spore photoproduct lyase (SPL), at the bacterial early
germination phase. Our previous studies proved that SPL utilizes the
5ā²-dAā¢ generated by the SAM cleavage reaction to abstract
the H<sub>6pro<i>R</i></sub> atom to initiate the SP repair
process. The resulting thymine allylic radical was suggested to take
an H atom from an unknown protein source, most likely cysteine 141.
Here we show that C141 can be readily alkylated in the native SPL
by an iodoacetamide treatment, suggesting that it is accessible to
the TpT radical. SP repair by the SPL C141A mutant yields TpTSO<sub>2</sub><sup>ā</sup> and TpT simultaneously from the very beginning
of the reaction; no lag phase is observed for TpTSO<sub>2</sub><sup>ā</sup> formation. Should any other protein residue serve
as the H donor, its presence would result in TpT being the major product
at least for the first enzyme turnover. These observations provide
strong evidence to support C141 as the direct H atom donor. Moreover,
because of the lack of this intrinsic H donor, the C141A mutant produces
TpT via an unprecedented thymine cation radical reduction (proton-coupled
electron transfer) process, contrasting to the H atom transfer mechanism
in the wild-type (WT) SPL reaction. The C141A mutant repairs SP at
a rate that is ā¼3-fold slower than that of the WT enzyme. Formation
of TpTSO<sub>2</sub><sup>ā</sup> and TpT exhibits a <i>V</i><sub>max</sub> deuterium kinetic isotope effect (KIE) of
1.7 Ā± 0.2, which is smaller than the <sup>D</sup><i>V</i><sub>max</sub> KIE of 2.8 Ā± 0.3 determined for the WT SPL reaction.
These findings suggest that removing the intrinsic H atom donor disturbs
the rate-limiting process during enzyme catalysis. As expected, the
prereduced C141A mutant supports only ā¼0.4 turnover, which
is in sharp contrast to the >5 turnovers exhibited by the WT SPL
reaction,
suggesting that the enzyme catalytic cycle (SAM regeneration) is disrupted
by this single mutation
HFEPR and Computational Studies on the Electronic Structure of a High-Spin Oxidoiron(IV) Complex in Solution
Nonheme
iron enzymes perform diverse and important functions in biochemistry.
The active form of these enzymes comprises the ferryl, oxidoironĀ(IV),
[FeO]<sup>2+</sup> unit. In enzymes, this unit is in the high-spin,
quintet, <i>S</i> = 2, ground state, while many synthetic
model compounds exist in the spin triplet, <i>S</i> = 1,
ground state. Recently, however, Que and co-workers reported an oxidoironĀ(IV)
complex with a quintet ground state, [FeOĀ(TMG<sub>3</sub>tren)]Ā(OTf)<sub>2</sub>, where TMG<sub>3</sub>tren = 1,1,1-trisĀ{2-[<i>N</i>2-(1,1,3,3-tetramethylguanidino)]Āethyl}Āamine and OTf = CF<sub>3</sub>SO<sub>3</sub><sup>ā</sup>. The trigonal geometry imposed
by this ligand, as opposed to the tetragonal geometry of earlier model
complexes, favors the high-spin ground state. Although [FeOĀ(TMG<sub>3</sub>tren)]<sup>2+</sup> has been earlier probed by magnetic circular
dichroism (MCD) and MoĢssbauer spectroscopies, the technique
of high-frequency and -field electron paramagnetic resonance (HFEPR)
is superior for describing the electronic structure of the ironĀ(IV)
center because of its ability to establish directly the spin-Hamiltonian
parameters of high-spin metal centers with high precision. Herein
we describe HFEPR studies on [FeOĀ(TMG<sub>3</sub>tren)]Ā(OTf)<sub>2</sub> generated in situ and confirm the <i>S</i> = 2 ground
state with the following parameters: <i>D</i> = +4.940(5)
cm<sup>ā1</sup>, <i>E</i> = 0.000(5), <i>B</i><sub>4</sub><sup>0</sup> = ā14(1) Ć 10<sup>ā4</sup> cm<sup>ā1</sup>, <i>g</i><sub>ā„</sub> =
2.006(2), and <i>g</i><sub>ā„</sub> = 2.03(2). Extraction
of a fourth-order spin-Hamiltonian parameter is unusual for HFEPR
and impossible by other techniques. These experimental results are
combined with state-of-the-art computational studies along with previous
structural and spectroscopic results to provide a complete picture
of the electronic structure of this biomimetic complex. Specifically,
the calculations reproduce well the spin-Hamiltonian parameters of
the complex, provide a satisfying geometrical picture of the <i>S</i> = 2 oxidoironĀ(IV) moiety, and demonstrate that the TMG<sub>3</sub>tren is an āinnocentā ligand
A Neutrally Charged Trimethylmanganese(III) Complex: Synthesis, Characterization, and Disproportionation Chemistry
The
synthesis and properties of an unusual, neutrally charged and
volatile <i>N</i>,<i>N</i>,<i>N</i>ā²,<i>N</i>ā²-tetramethylethylenediamine trimethyl manganeseĀ(III)
complex, (TMEDA)ĀMnMe<sub>3</sub>, are described, along with its facile
disproportionation to the corresponding MnĀ(II) and MnĀ(IV) complexes.
Characterization by single-crystal XRD, UVāvis spectroscopy,
high-frequency and -field EPR (HFEPR), magnetic susceptibility, and
density functional theory (DFT) computations indicate that the (TMEDA)ĀMnMe<sub>3</sub> electronic structure can be described as largely square pyramidal
MnĀ(III) centered. The paucity of manganeseĀ(III) polyalkyls and the
simplicity and reactivity of this compound implicate it as a potentially
useful synthetic building block
Ligand Substituent Effects in Manganese Pyridinophane Complexes: Implications for Oxygen-Evolving Catalysis
A series
of MnĀ(II) complexes of differently substituted pyridinophane ligands,
(Py<sub>2</sub>NR<sub>2</sub>)ĀMnCl<sub>2</sub> (R = <i><sup>i</sup></i>Pr, Cy) and [(Py<sub>2</sub>NR<sub>2</sub>)ĀMnF<sub>2</sub>]Ā(PF<sub>6</sub>) (R = <i><sup>i</sup></i>Pr, Cy, <i><sup>t</sup></i>Bu) are synthesized and characterized. The electrochemical
properties of these complexes are investigated by cyclic voltammetry,
along with those of previously reported (Py<sub>2</sub>NMe<sub>2</sub>)ĀMnCl<sub>2</sub> and the MnĀ(III) complex [(Py<sub>2</sub>NMe<sub>2</sub>)ĀMnF<sub>2</sub>]Ā(PF<sub>6</sub>). The electronic structure
of this and other MnĀ(III) complexes is probed experimentally and theoretically,
via high-frequency and -field electron paramagnetic resonance (HFEPR)
spectroscopy ab initio quantum chemical theory (QCT), respectively.
These studies show that the complexes contain relatively typical six-coordinate
MnĀ(III). The catalytic activity of these complexes toward both H<sub>2</sub>O<sub>2</sub> disproportionation and H<sub>2</sub>O oxidation
has also been investigated. The rate of H<sub>2</sub>O<sub>2</sub> disproportionation decreases with increasing substituent size. Some
of these complexes are active for electrocatalytic H<sub>2</sub>O
oxidation; however this activity cannot be rationalized in terms of
simple electronic or steric effects