8 research outputs found
Copper Complexes with NH-Imidazolyl and NH-Pyrazolyl Units and Determination of Their Bond Dissociation Gibbs Energies
We synthesized two dinuclear copper
complexes, which have ionizable <i>N</i> imidazole and <i>N</i> pyrazole protons in the ligand, respectively, and determined
the BDFE of the hypothetical H atom transfer reactions Cu<sup>II</sup>(LH<sub>ā1</sub>) + H<sup>ā¢</sup> ā Cu<sup>I</sup>(L) in MeOH/H<sub>2</sub>O (BDFE: bond dissociation Gibbs (free)
energy). The ligands have two adjacent <i>N</i>,<i>N</i>ā²,<i>O</i>-binding pockets, which differ
in one <i>N-</i>heterocycle: L<sup>a</sup> has an imidazole
unit and L<sup>c</sup>, a pyrazole unit. The copperĀ(II) complexes
of L<sup>a</sup> and L<sup>c</sup> have been characterized, and the
substitution pattern has only little influence on the structural properties.
The BDFEs of the hypothetical PCET reactions have been determined
by means of the species distribution and the redox potentials of the
involved species in MeOH/H<sub>2</sub>O (80/20 by weight). The pyrazole
copper complex <b>3</b> exhibits a lower BDFE than the isoelectronic
imidazole copper complex <b>1</b> (<b>1</b>, 292(3) kJ
mol<sup>ā1</sup>; <b>3</b>, 279(1) kJ mol<sup>ā1</sup>). The difference is mainly caused by the higher acidity of the <i>N</i> pyrazole proton of <b>3</b> compared to the <i>N</i> imidazole proton of <b>1</b>. The redox potentials
of <b>1</b> and <b>3</b> are very similar
Copper Complexes with NH-Imidazolyl and NH-Pyrazolyl Units and Determination of Their Bond Dissociation Gibbs Energies
We synthesized two dinuclear copper
complexes, which have ionizable <i>N</i> imidazole and <i>N</i> pyrazole protons in the ligand, respectively, and determined
the BDFE of the hypothetical H atom transfer reactions Cu<sup>II</sup>(LH<sub>ā1</sub>) + H<sup>ā¢</sup> ā Cu<sup>I</sup>(L) in MeOH/H<sub>2</sub>O (BDFE: bond dissociation Gibbs (free)
energy). The ligands have two adjacent <i>N</i>,<i>N</i>ā²,<i>O</i>-binding pockets, which differ
in one <i>N-</i>heterocycle: L<sup>a</sup> has an imidazole
unit and L<sup>c</sup>, a pyrazole unit. The copperĀ(II) complexes
of L<sup>a</sup> and L<sup>c</sup> have been characterized, and the
substitution pattern has only little influence on the structural properties.
The BDFEs of the hypothetical PCET reactions have been determined
by means of the species distribution and the redox potentials of the
involved species in MeOH/H<sub>2</sub>O (80/20 by weight). The pyrazole
copper complex <b>3</b> exhibits a lower BDFE than the isoelectronic
imidazole copper complex <b>1</b> (<b>1</b>, 292(3) kJ
mol<sup>ā1</sup>; <b>3</b>, 279(1) kJ mol<sup>ā1</sup>). The difference is mainly caused by the higher acidity of the <i>N</i> pyrazole proton of <b>3</b> compared to the <i>N</i> imidazole proton of <b>1</b>. The redox potentials
of <b>1</b> and <b>3</b> are very similar
(Electro)chemical N<sub>2</sub> Splitting by a Molybdenum Complex with an Anionic PNP Pincer-Type Ligand
Molybdenum(III) complexes
bearing pincer-type ligands
are well-known
catalysts for N2-to-NH3 reduction. We investigated
herein the impact of an anionic PNP pincer-type ligand in a Mo(III)
complex on the (electro)chemical N2 splitting ([LMoCl3]ā, 1ā, LH = 2,6-bis((di-tert-butylphosphaneyl)methyl)-pyridin-4-one). The increased electron-donating
properties of the anionic ligand should lead to a stronger degree
of N2 activation. The catalyst is indeed active in N2-to-NH3 conversion utilizing the proton-coupled
electron transfer reagent SmI2/ethylene glycol. The corresponding
Mo(V) nitrido complex 2H exhibits similar catalytic activity
as 1H and thus could represent a viable intermediate.
The Mo(IV) nitrido complex 3ā is also accessible by electrochemical reduction of 1ā under a N2 atmosphere. IR- and UV/vis-SEC
measurements suggest that N2 splitting occurs via formation
of an āoverreducedā but more stable [(L(N2)2Mo0)2Ī¼-N2]2ā dimer. In line with this, the yield
in the nitrido complex increases with lower applied potentials
Dinuclear Rhenium Complex with a Proton Responsive Ligand as a Redox Catalyst for the Electrochemical CO<sub>2</sub> Reduction
Herein,
we present the reduction chemistry of a dinuclear Ī±-diimine
rhenium complex, <b>1</b>, [Re<sub>2</sub>(L)Ā(CO)<sub>6</sub>Cl<sub>2</sub>], with a proton responsive ligand and its application
as a catalyst in the electrochemical CO<sub>2</sub> reduction reaction
(L = 4-<i>tert</i>-butyl-2,6-bisĀ(6-(1<i>H</i>-imidazol-2-yl)-pyridin-2-yl)Āphenol).
The complex has a phenol group in close proximity to the active center,
which may act as a proton relay during catalysis, and pyridine-NH-imidazole
units as Ī±-diimine donors. The complex is an active catalyst
for the electrochemical CO<sub>2</sub> reduction reaction. CO is the
main product after catalysis, and only small amounts of H<sub>2</sub> were observed, which can be related to the ligand reactivity. The <i>i</i><sub>c</sub>/<i>i</i><sub>p</sub> ratio of 20
in dimethylformamide (DMF) + 10% water for <b>1</b> points to
a higher activity with regard to [ReĀ(bpy)Ā(CO)<sub>3</sub>Cl]
in MeCN/H<sub>2</sub>O, albeit <b>1</b> requires a slightly
larger overpotential (bpy = 2,2ā²-bipyridine). Spectroscopic
and theoretical investigations revealed detailed information about
the reduction chemistry of <b>1</b>. The complex exhibits two
reduction processes in DMF, and each process was identified as a two-electron
reduction in the absence of CO<sub>2</sub>. The first 2e<sup>ā</sup> reduction is ligand based and leads to homolytic NāH bond
cleavage reactions at the imidazole units of <b>1</b>, which
is equal to a net double proton removal from <b>1</b> forming
[Re<sub>2</sub>(LH<sub>ā2</sub>)Ā(CO)<sub>6</sub>Cl<sub>2</sub>]<sup>2ā</sup>. The second 2e<sup>ā</sup> reduction
process has been identified as an OāH bond cleavage reaction
at the phenol group, removal of chloride ions from the coordination
spheres of the metal ions, and a ligand-centered one-electron reduction
of [Re<sub>2</sub>(LH<sub>ā3</sub>)Ā(CO)<sub>6</sub>Cl]<sup>2ā</sup>. In the presence of CO<sub>2</sub>, the second reduction
process initiates catalysis. The reduced species is highly nucleophilic
and likely favors the reaction with CO<sub>2</sub> instead of OāH
bond cleavage
Syntheses and Anion Binding Capabilities of Bis(diarylboryl) Ferrocenes and Related Systems
Isomeric diborylated ferrocenes featuring
1,1ā²-, 1,2-, and
1,3-substitution patterns have been targeted via a combination of
electrophilic
aromatic substitution
and directed ortho-lithiation protocols. While none of these systems
are competent for the Lewis acid chelation of fluoride, related systems
featuring a mixed B/Si acceptor set capture 1 equiv of fluoride via
a SiāFāB bridging motif
Syntheses and Anion Binding Capabilities of Bis(diarylboryl) Ferrocenes and Related Systems
Isomeric diborylated ferrocenes featuring
1,1ā²-, 1,2-, and
1,3-substitution patterns have been targeted via a combination of
electrophilic
aromatic substitution
and directed ortho-lithiation protocols. While none of these systems
are competent for the Lewis acid chelation of fluoride, related systems
featuring a mixed B/Si acceptor set capture 1 equiv of fluoride via
a SiāFāB bridging motif
Mechanism of Chemical and Electrochemical N<sub>2</sub> Splitting by a Rhenium Pincer Complex
A comprehensive
mechanistic study of N<sub>2</sub> activation and
splitting into terminal nitride ligands upon reduction of the rhenium
dichloride complex [ReCl<sub>2</sub>(PNP)] is presented (PNP<sup>ā</sup> = NĀ(CH<sub>2</sub>CH<sub>2</sub>P<i>t</i>Bu<sub>2</sub>)<sub>2</sub><sup>ā</sup>). Low-temperature studies using
chemical reductants enabled full characterization of the N<sub>2</sub>-bridged intermediate [{(PNP)ĀClRe}<sub>2</sub>(N<sub>2</sub>)] and
kinetic analysis of the NāN bond scission process. Controlled
potential electrolysis at room temperature also resulted in formation
of the nitride product [ReĀ(N)ĀClĀ(PNP)]. This first example of molecular
electrochemical N<sub>2</sub> splitting into nitride complexes enabled
the use of cyclic voltammetry (CV) methods to establish the mechanism
of reductive N<sub>2</sub> activation to form the N<sub>2</sub>-bridged
intermediate. CV data was acquired under Ar and N<sub>2</sub>, and
with varying chloride concentration, rhenium concentration, and N<sub>2</sub> pressure. A series of kinetic models was vetted against the
CV data using digital simulations, leading to the assignment of an
ECCEC mechanism (where āEā is an electrochemical step
and āCā is a chemical step) for N<sub>2</sub> activation
that proceeds via initial reduction to Re<sup>II</sup>, N<sub>2</sub> binding, chloride dissociation, and further reduction to Re<sup>I</sup> before formation of the N<sub>2</sub>-bridged, dinuclear
intermediate by comproportionation with the Re<sup>III</sup> precursor.
Experimental kinetic data for all individual steps could be obtained.
The mechanism is supported by density functional theory computations,
which provide further insight into the electronic structure requirements
for N<sub>2</sub> splitting in the tetragonal frameworks enforced
by rigid pincer ligands
Mechanism of Chemical and Electrochemical N<sub>2</sub> Splitting by a Rhenium Pincer Complex
A comprehensive
mechanistic study of N<sub>2</sub> activation and
splitting into terminal nitride ligands upon reduction of the rhenium
dichloride complex [ReCl<sub>2</sub>(PNP)] is presented (PNP<sup>ā</sup> = NĀ(CH<sub>2</sub>CH<sub>2</sub>P<i>t</i>Bu<sub>2</sub>)<sub>2</sub><sup>ā</sup>). Low-temperature studies using
chemical reductants enabled full characterization of the N<sub>2</sub>-bridged intermediate [{(PNP)ĀClRe}<sub>2</sub>(N<sub>2</sub>)] and
kinetic analysis of the NāN bond scission process. Controlled
potential electrolysis at room temperature also resulted in formation
of the nitride product [ReĀ(N)ĀClĀ(PNP)]. This first example of molecular
electrochemical N<sub>2</sub> splitting into nitride complexes enabled
the use of cyclic voltammetry (CV) methods to establish the mechanism
of reductive N<sub>2</sub> activation to form the N<sub>2</sub>-bridged
intermediate. CV data was acquired under Ar and N<sub>2</sub>, and
with varying chloride concentration, rhenium concentration, and N<sub>2</sub> pressure. A series of kinetic models was vetted against the
CV data using digital simulations, leading to the assignment of an
ECCEC mechanism (where āEā is an electrochemical step
and āCā is a chemical step) for N<sub>2</sub> activation
that proceeds via initial reduction to Re<sup>II</sup>, N<sub>2</sub> binding, chloride dissociation, and further reduction to Re<sup>I</sup> before formation of the N<sub>2</sub>-bridged, dinuclear
intermediate by comproportionation with the Re<sup>III</sup> precursor.
Experimental kinetic data for all individual steps could be obtained.
The mechanism is supported by density functional theory computations,
which provide further insight into the electronic structure requirements
for N<sub>2</sub> splitting in the tetragonal frameworks enforced
by rigid pincer ligands