24 research outputs found
Comparison of [Ni(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)]<sup>2+</sup> and [Pd(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> as Electrocatalysts for H<sub>2</sub> Production
The
complexes [NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)]<sup>2+</sup> and [PdÂ(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup>, where P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub> is 1,5-diphenyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane,
are compared as electrocatalysts for H<sub>2</sub> production under
identical experimental conditions. With [(DMF)ÂH]<sup>+</sup> as the
acid in acetonitrile solution, [PdÂ(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> afforded a turnover
frequency (TOF) of 230 s<sup>–1</sup> for formation of H<sub>2</sub> under dry conditions and a TOF of 640 s<sup>–1</sup> when H<sub>2</sub>O was added. These rates are similar to the TOFs
of 590 s<sup>–1</sup> (dry) and 720 s<sup>–1</sup> (wet)
that were previously measured for [NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)]<sup>2+</sup> using [(DMF)ÂH]<sup>+</sup>. The [NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)]<sup>2+</sup> and [PdÂ(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> complexes both exhibited large current enhancements
when treated with trifluoroacetic acid (TFA). At a TFA concentration
of 1.8 M, TOF values of 5670 and 2060 s<sup>–1</sup> were measured
for [NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)]<sup>2+</sup> and [PdÂ(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup>, respectively.
The fast rates observed using TFA are, in part, attributed to homoconjugation
of TFA in acetonitrile solutions, which decreases the effective p<i>K</i><sub>a</sub><sup>MeCN</sup> of the acid. In support of
this hypothesis, dramatically lower rates of H<sub>2</sub> production
were observed using <i>p</i>-anisidinium, which has a p<i>K</i><sub>a</sub><sup>MeCN</sup> value comparable to that of
TFA but does not homoconjugate significantly in acetonitrile solutions
Kinetic Analysis of Competitive Electrocatalytic Pathways: New Insights into Hydrogen Production with Nickel Electrocatalysts
The
hydrogen production electrocatalyst NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub><sup>2+</sup> (<b>1</b>) is
capable of traversing multiple electrocatalytic pathways. When
using dimethylformamidium, DMFÂ(H)<sup>+</sup>, the mechanism of H<sub>2</sub> formation by <b>1</b> changes from an ECEC to an EECC
mechanism as the potential approaches the NiÂ(I/0) couple. Two electrochemical
methods, current–potential analysis and foot-of-the-wave analysis
(FOWA), were performed on <b>1</b> to measure detailed kinetics
of the competing ECEC and EECC pathways. A sensitivity analysis was
performed on the methods using digital simulations to understand their
strengths and limitations. Chemical rate constants were significantly
underestimated when not accounting for electron-transfer kinetics,
even when electron transfer was fast enough to afford a reversible
noncatalytic wave. The EECC pathway of <b>1</b> was faster than
the ECEC pathway under all conditions studied. Buffered DMF:DMFÂ(H)<sup>+</sup> mixtures afforded an increase in the catalytic rate constant
(<i>k</i><sub>obs</sub>) of the EECC pathway, but <i>k</i><sub>obs</sub> for the ECEC pathway did not change when
using buffered acid. Further kinetic analysis of the ECEC path revealed
that base increases the rate of isomerization from exo-protonated Ni(0)
isomers to the catalytically active endo-isomers, but decreases the
rate of protonation of NiÂ(I). FOWA did not provide accurate rate constants,
but FOWA was used to estimate the reduction potential of the previously
undetected exo-protonated NiÂ(I) intermediate. Comparison of catalytic
Tafel plots for <b>1</b> under different conditions reveals
substantial inaccuracies in the turnover frequency at zero overpotential
when the kinetic and thermodynamic effects of the conjugate base are
not accounted for properly
Nickel Bis-Diphosphine Complexes: Controlling the Binding and Heterolysis of H<sub>2</sub>
Two new NiÂ(II) complexes, the homoleptic [NiÂ(8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> complex (8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup> = 3,7-cyclohexyl-1-amino-3,7-diphosphacyclooctane)
containing two
pendant amines on adjacent ligands, and the heteroleptic [NiÂ(8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup>)Â(dppe)]<sup>2+</sup> complex (dppe
= bisÂ(diphenylphosphino)Âethane) containing only a single pendant amine,
have been synthesized, and their electrochemical properties are reported.
The [NiÂ(8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup>)Â(dppe)]<sup>2+</sup> complex is capable of heterolytically cleaving hydrogen, allowing
for the first observation of an endo-protonated nickel hydride related
to the [NiÂ(P<sup>R</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> family of complexes. The
[NiÂ(8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup>)Â(dppe)]<sup>2+</sup> complex
did not exhibit electrocatalytic H<sub>2</sub> oxidation activity;
however, [NiÂ(8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> is an active electrocatalyst for H<sub>2</sub> oxidation
with a maximum turnover frequency (<i>k</i><sub>obs</sub>) of 18 s<sup>–1</sup> under 1 atm H<sub>2</sub> at <i>E</i><sub>cat/2</sub> = −0.71 V versus the ferrocenium/ferrocene
(Cp<sub>2</sub>Fe<sup>+/0</sup>) couple. In addition to an analysis
of the effect of the number of pendant amines on the rates of electrocatalytic
H<sub>2</sub> oxidation of compounds related to the [NiÂ(P<sup>R</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> family, the effect of the secondary pendant amine
on intermolecular deprotonation is discussed
Nickel Bis-Diphosphine Complexes: Controlling the Binding and Heterolysis of H<sub>2</sub>
Two new NiÂ(II) complexes, the homoleptic [NiÂ(8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> complex (8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup> = 3,7-cyclohexyl-1-amino-3,7-diphosphacyclooctane)
containing two
pendant amines on adjacent ligands, and the heteroleptic [NiÂ(8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup>)Â(dppe)]<sup>2+</sup> complex (dppe
= bisÂ(diphenylphosphino)Âethane) containing only a single pendant amine,
have been synthesized, and their electrochemical properties are reported.
The [NiÂ(8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup>)Â(dppe)]<sup>2+</sup> complex is capable of heterolytically cleaving hydrogen, allowing
for the first observation of an endo-protonated nickel hydride related
to the [NiÂ(P<sup>R</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> family of complexes. The
[NiÂ(8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup>)Â(dppe)]<sup>2+</sup> complex
did not exhibit electrocatalytic H<sub>2</sub> oxidation activity;
however, [NiÂ(8P<sup>Cy</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> is an active electrocatalyst for H<sub>2</sub> oxidation
with a maximum turnover frequency (<i>k</i><sub>obs</sub>) of 18 s<sup>–1</sup> under 1 atm H<sub>2</sub> at <i>E</i><sub>cat/2</sub> = −0.71 V versus the ferrocenium/ferrocene
(Cp<sub>2</sub>Fe<sup>+/0</sup>) couple. In addition to an analysis
of the effect of the number of pendant amines on the rates of electrocatalytic
H<sub>2</sub> oxidation of compounds related to the [NiÂ(P<sup>R</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> family, the effect of the secondary pendant amine
on intermolecular deprotonation is discussed
Catalytic Oxidation of Alcohol via Nickel Phosphine Complexes with Pendant Amines
Nickel complexes were prepared with
diphosphine ligands that contain
pendant amines, and these complexes catalytically oxidize primary
and secondary alcohols to their respective aldehydes and ketones.
Kinetic and mechanistic studies of these prospective electrocatalysts
were performed to understand what influences the catalytic activity.
For the oxidation of diphenylmethanol, the catalytic rates were determined
to be dependent on the concentration of both the catalyst and the
alcohol and independent of the concentration of base and oxidant.
The incorporation of pendant amines to the phosphine ligand results
in substantial increases in the rate of alcohol oxidation with more
electron-donating substituents on the pendant amine exhibiting the
fastest rates
Catalytic Oxidation of Alcohol via Nickel Phosphine Complexes with Pendant Amines
Nickel complexes were prepared with
diphosphine ligands that contain
pendant amines, and these complexes catalytically oxidize primary
and secondary alcohols to their respective aldehydes and ketones.
Kinetic and mechanistic studies of these prospective electrocatalysts
were performed to understand what influences the catalytic activity.
For the oxidation of diphenylmethanol, the catalytic rates were determined
to be dependent on the concentration of both the catalyst and the
alcohol and independent of the concentration of base and oxidant.
The incorporation of pendant amines to the phosphine ligand results
in substantial increases in the rate of alcohol oxidation with more
electron-donating substituents on the pendant amine exhibiting the
fastest rates
Structural and Spectroscopic Characterization of 17- and 18-Electron Piano-Stool Complexes of Chromium. Thermochemical Analyses of Weak Cr–H Bonds
The 17-electron radical CpCrÂ(CO)<sub>2</sub>(IMe)<sup>•</sup> (IMe = 1,3-dimethylimidazol-2-ylidene) was synthesized
by the reaction of IMe with [CpCrÂ(CO)<sub>3</sub>]<sub>2</sub>, and
characterized by single crystal X-ray diffraction and by electron
paramagnetic resonance (EPR), IR, and variable temperature <sup>1</sup>H NMR spectroscopy. The metal-centered radical is monomeric under
all conditions and exhibits Curie paramagnetic behavior in solution.
An electrochemically reversible reduction to 18-electron CpCrÂ(CO)<sub>2</sub>(IMe)<sup>−</sup> takes place at <i>E</i><sub>1/2</sub> = −1.89(1) V vs Cp<sub>2</sub>Fe<sup>+•/0</sup> in MeCN, and was accomplished chemically with KC<sub>8</sub> in
tetrahydrofuran (THF). The salts K<sup>+</sup>(18-crown-6)Â[CpCrÂ(CO)<sub>2</sub>(IMe)]<sup>−</sup>·<sup>1</sup>/<sub>2</sub>THF
and K<sup>+</sup>[CpCrÂ(CO)<sub>2</sub>(IMe)]<sup>−</sup>·<sup>3</sup>/<sub>4</sub>THF were crystallographically characterized.
Monomeric ion pairs are found in the former, whereas the latter has
a polymeric structure because of a network of K···O<sub>(CO)</sub> interactions. Protonation of K<sup>+</sup>(18-crown-6)Â[CpCrÂ(CO)<sub>2</sub>(IMe)]<sup>−</sup>·<sup>1</sup>/<sub>2</sub>THF
gives the hydride CpCrÂ(CO)<sub>2</sub>(IMe)ÂH, which could not be isolated,
but was characterized in solution; a p<i>K</i><sub>a</sub> of 27.2(4) was determined in MeCN. A thermochemical analysis provides
the Cr–H bond dissociation free energy (BDFE) for CpCrÂ(CO)<sub>2</sub>(IMe)H in MeCN solution as 47.3(6) kcal mol<sup>–1</sup>. This value is exceptionally low for a transition metal hydride,
and implies that the reaction 2 [Cr–H] → 2 [Cr<sup>•</sup>] + H<sub>2</sub> is exergonic (Δ<i>G</i> = −9.0(8)
kcal mol<sup>–1</sup>). This analysis explains the experimental
observation that generated solutions of the hydride produce CpCrÂ(CO)<sub>2</sub>(IMe)<sup>•</sup> (typically on the time scale of days).
By contrast, CpCrÂ(CO)<sub>2</sub>(PCy<sub>3</sub>)H has a higher Cr–H
BDFE (52.9(4) kcal mol<sup>–1</sup>), is more stable with respect
to H<sub>2</sub> loss, and is isolable
Electrocatalytic Hydrogen Production by [Ni(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup>: Removing the Distinction Between Endo- and Exo-Protonation Sites
A new NiÂ(II) complex, [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>H]<sup>3+</sup> (7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup> = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane),
has been synthesized, and its electrochemical properties have been
reported. The 7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup> ligand features
an NH, ensuring properly positioned protonated amine groups (N–H<sup>+</sup>) for electrocatalysis, regardless of whether protonation
occurs exo or endo to the metal center. The compound is an electrocatalyst
for H<sub>2</sub> production in the presence of organic acids (p<i>K</i><sub>a</sub> range 10–13 in CH<sub>3</sub>CN), with
turnover frequencies ranging from 160 to 780 s<sup>–1</sup> at overpotentials between 320 and 470 mV, as measured at the potential
of the catalytic wave. In stark contrast to [NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> (P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub> = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane)
and other [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup>)<sub>2</sub>]<sup>2+</sup> complexes, catalytic turnover frequencies for H<sub>2</sub> production by [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> do not show catalytic rate enhancement upon
the addition of H<sub>2</sub>O. This finding supports the assertion
that [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> eliminates the distinction between the endo- and exo-protonation
isomers
Effects of Phosphine–Carbene Substitutions on the Electrochemical and Thermodynamic Properties of Nickel Complexes
NickelÂ(II) complexes containing chelating
N-heterocyclic carbene–phosphine
ligands ([NiL<sub>2</sub>]Â(BPh<sub>4</sub>)<sub>2</sub>, for which
L = [MeImÂ(CH<sub>2</sub>)<sub>2</sub>PR<sub>2</sub>], MeIm = 1-methylimidizolylidene,
and R = Ph or Et) have been synthesized for the purpose of studying
how this class of ligand affects the electrochemical and thermodynamic
properties compared to the nickel bis-diphosphine analogues. The nickel
complexes were synthesized and then characterized by X-ray crystallography,
electrochemical methods, and thermodynamic studies, including DFT
calculations. On the basis of the reduction potentials (<i>E°</i>), substitution of an NHC for one of the phosphines in a diphoshine
ligand resulted in negative shifts in potential by 0.6 to 1.2 V relative
to the corresponding nickel bis-diphosphine complexes. From computational
studies of the nickel hydride complex of the phenyl-substituted phosphine–carbene
ligand, the hydride donor ability was determined to improve by 32
kcal/mol relative to the estimated hydride donor ability for the analogous
nickel complex of the chelating diphosphine ligand 1,3-bisÂ(diphenylphosphino)Âpropane.
The free energy for addition of H<sub>2</sub> is presented, and the
implications for catalysis are discussed. These quantitative results
highlight the substantial effect that NHC ligands can have on the
electronic properties of the metal complexes
Electrocatalytic Hydrogen Production by [Ni(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup>: Removing the Distinction Between Endo- and Exo-Protonation Sites
A new NiÂ(II) complex, [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>H]<sup>3+</sup> (7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup> = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane),
has been synthesized, and its electrochemical properties have been
reported. The 7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup> ligand features
an NH, ensuring properly positioned protonated amine groups (N–H<sup>+</sup>) for electrocatalysis, regardless of whether protonation
occurs exo or endo to the metal center. The compound is an electrocatalyst
for H<sub>2</sub> production in the presence of organic acids (p<i>K</i><sub>a</sub> range 10–13 in CH<sub>3</sub>CN), with
turnover frequencies ranging from 160 to 780 s<sup>–1</sup> at overpotentials between 320 and 470 mV, as measured at the potential
of the catalytic wave. In stark contrast to [NiÂ(P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub>)<sub>2</sub>]<sup>2+</sup> (P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup><sub>2</sub> = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane)
and other [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>R<sup>′</sup></sup>)<sub>2</sub>]<sup>2+</sup> complexes, catalytic turnover frequencies for H<sub>2</sub> production by [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> do not show catalytic rate enhancement upon
the addition of H<sub>2</sub>O. This finding supports the assertion
that [NiÂ(7P<sup>Ph</sup><sub>2</sub>N<sup>H</sup>)<sub>2</sub>]<sup>2+</sup> eliminates the distinction between the endo- and exo-protonation
isomers