10 research outputs found
Mechanistic Insights into the Formation of N<sub>2</sub>O by a Nickel Nitrosyl Complex
Reaction
of [NiĀ(NO)Ā(bipy)Ā(Me<sub>2</sub>phen)]Ā[PF<sub>6</sub>] with 1 equiv
of nitric oxide (NO) in CH<sub>2</sub>Cl<sub>2</sub> results in the
formation of N<sub>2</sub>O and [{(Me<sub>2</sub>phen)ĀNiĀ(NO)}<sub>2</sub>(Ī¼āĪ·<sup>1</sup>-<i>N</i>:Ī·<sup>1</sup>-<i>O</i>)-NO<sub>2</sub>)]Ā[PF<sub>6</sub>] (<b>3</b>), along with the known complex, [NiĀ(bipy)<sub>3</sub>]Ā[PF<sub>6</sub>]<sub>2</sub> (<b>4</b>). The isolation of complex <b>3</b>, which contains a nitrite ligand, demonstrates that the
reaction of [NiĀ(NO)Ā(bipy)Ā(Me<sub>2</sub>phen)]Ā[PF<sub>6</sub>] with
exogenous NO results in NO disproportionation and not NO reduction.
Complex <b>3</b> could also be accessed by reaction of [NiĀ(NO)Ā(Me<sub>2</sub>phen)]Ā[PF<sub>6</sub>] (<b>5</b>) with (Me<sub>2</sub>phen)ĀNiĀ(NO)Ā(NO<sub>2</sub>) (<b>7</b>) in good yield. Complexes <b>3</b>, <b>5</b>, and <b>7</b> were fully characterized,
including analysis by X-ray crystallography in the case of <b>3</b> and <b>7</b>. Furthermore, addition of 0.5 equiv of bipy to
[NiĀ(NO)Ā(bipy)]Ā[PF<sub>6</sub>] results in formation of the hyponitrite
complex, [{(bipy)ĀNiĀ(Īŗ<sup>2</sup>-O<sub>2</sub>N<sub>2</sub>)}ĀĪ·<sup>1</sup>:Ī·<sup>1</sup>-<i>N</i>,<i>N</i>-{NiĀ(NO)Ā(bipy)}<sub>2</sub>]Ā[PF<sub>6</sub>]<sub>2</sub> (<b>9</b>), in modest yield. Importantly, the hyponitrite
ligand in <b>9</b> is thought to form via coupling of two NO<sup>ā</sup> ligands and not by coupling of a nucleophilic nitrosyl
ligand (NO<sup>ā</sup>) with an electrophilic nitrosyl ligand
(NO<sup>+</sup>). X-ray crystallography reveals that complex <b>9</b> features a new binding mode of the <i>cis</i>-hyponitrite
ligand
Nitric Oxide Release from a Nickel Nitrosyl Complex Induced by One-Electron Oxidation
Reaction of [NiĀ(NO)Ā(bipy)]Ā[PF<sub>6</sub>] (<b>2</b>) with AgPF<sub>6</sub> or [NO]Ā[PF<sub>6</sub>] in MeCN results in formation of [NiĀ(bipy)<sub><i>x</i></sub>(MeCN)<sub><i>y</i></sub>]<sup>2+</sup> and release
of NO gas in moderate yields. In contrast, the addition of the inner
sphere oxidant Ph<sub>2</sub>S<sub>2</sub> to <b>2</b> does
not result in denitrosylation. Instead, the diphenyldisulfide adduct
[{(bipy)Ā(NO)ĀNi}<sub>2</sub>(Ī¼-S<sub>2</sub>Ph<sub>2</sub>)]Ā[PF<sub>6</sub>]<sub>2</sub> (<b>3</b>) is formed in good yield. However,
oxidation of <b>2</b> with 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO) does results in cleavage of the NiāNO bond and generation
of NO. The metal-containing product, [(bipy)ĀNiĀ(Ī·<sup>2</sup>-TEMPO)]Ā[PF<sub>6</sub>] (<b>4</b>), can be isolated as an
orange-brown solid in excellent yields. In the solid state, complex <b>4</b> contains a side-on bound TEMPO<sup>ā</sup> ligand,
which is characterized by a long NāO bond length [1.383(2)
Ć
]. The contrasting reactivity of Ph<sub>2</sub>S<sub>2</sub> and TEMPO likely relates to their different redox potentials, as
Ph<sub>2</sub>S<sub>2</sub> is a relatively weak oxidant. Finally,
the addition of pyridine-<i>N</i>-oxide to <b>2</b> results in the formation of the adduct, [(bipy)ĀNiĀ(NO)Ā(ONC<sub>5</sub>H<sub>5</sub>)]Ā[PF<sub>6</sub>] (<b>5</b>). No evidence of
NO release is observed in this reaction, probably because of the low
one-electron (1e<sup>ā</sup>) reduction potential of pyridine-<i>N</i>-oxide
Mechanistic Insights into the Formation of N<sub>2</sub>O by a Nickel Nitrosyl Complex
Reaction
of [NiĀ(NO)Ā(bipy)Ā(Me<sub>2</sub>phen)]Ā[PF<sub>6</sub>] with 1 equiv
of nitric oxide (NO) in CH<sub>2</sub>Cl<sub>2</sub> results in the
formation of N<sub>2</sub>O and [{(Me<sub>2</sub>phen)ĀNiĀ(NO)}<sub>2</sub>(Ī¼āĪ·<sup>1</sup>-<i>N</i>:Ī·<sup>1</sup>-<i>O</i>)-NO<sub>2</sub>)]Ā[PF<sub>6</sub>] (<b>3</b>), along with the known complex, [NiĀ(bipy)<sub>3</sub>]Ā[PF<sub>6</sub>]<sub>2</sub> (<b>4</b>). The isolation of complex <b>3</b>, which contains a nitrite ligand, demonstrates that the
reaction of [NiĀ(NO)Ā(bipy)Ā(Me<sub>2</sub>phen)]Ā[PF<sub>6</sub>] with
exogenous NO results in NO disproportionation and not NO reduction.
Complex <b>3</b> could also be accessed by reaction of [NiĀ(NO)Ā(Me<sub>2</sub>phen)]Ā[PF<sub>6</sub>] (<b>5</b>) with (Me<sub>2</sub>phen)ĀNiĀ(NO)Ā(NO<sub>2</sub>) (<b>7</b>) in good yield. Complexes <b>3</b>, <b>5</b>, and <b>7</b> were fully characterized,
including analysis by X-ray crystallography in the case of <b>3</b> and <b>7</b>. Furthermore, addition of 0.5 equiv of bipy to
[NiĀ(NO)Ā(bipy)]Ā[PF<sub>6</sub>] results in formation of the hyponitrite
complex, [{(bipy)ĀNiĀ(Īŗ<sup>2</sup>-O<sub>2</sub>N<sub>2</sub>)}ĀĪ·<sup>1</sup>:Ī·<sup>1</sup>-<i>N</i>,<i>N</i>-{NiĀ(NO)Ā(bipy)}<sub>2</sub>]Ā[PF<sub>6</sub>]<sub>2</sub> (<b>9</b>), in modest yield. Importantly, the hyponitrite
ligand in <b>9</b> is thought to form via coupling of two NO<sup>ā</sup> ligands and not by coupling of a nucleophilic nitrosyl
ligand (NO<sup>ā</sup>) with an electrophilic nitrosyl ligand
(NO<sup>+</sup>). X-ray crystallography reveals that complex <b>9</b> features a new binding mode of the <i>cis</i>-hyponitrite
ligand
Nitric Oxide Release from a Nickel Nitrosyl Complex Induced by One-Electron Oxidation
Reaction of [NiĀ(NO)Ā(bipy)]Ā[PF<sub>6</sub>] (<b>2</b>) with AgPF<sub>6</sub> or [NO]Ā[PF<sub>6</sub>] in MeCN results in formation of [NiĀ(bipy)<sub><i>x</i></sub>(MeCN)<sub><i>y</i></sub>]<sup>2+</sup> and release
of NO gas in moderate yields. In contrast, the addition of the inner
sphere oxidant Ph<sub>2</sub>S<sub>2</sub> to <b>2</b> does
not result in denitrosylation. Instead, the diphenyldisulfide adduct
[{(bipy)Ā(NO)ĀNi}<sub>2</sub>(Ī¼-S<sub>2</sub>Ph<sub>2</sub>)]Ā[PF<sub>6</sub>]<sub>2</sub> (<b>3</b>) is formed in good yield. However,
oxidation of <b>2</b> with 2,2,6,6-tetramethylpiperidine-1-oxyl
(TEMPO) does results in cleavage of the NiāNO bond and generation
of NO. The metal-containing product, [(bipy)ĀNiĀ(Ī·<sup>2</sup>-TEMPO)]Ā[PF<sub>6</sub>] (<b>4</b>), can be isolated as an
orange-brown solid in excellent yields. In the solid state, complex <b>4</b> contains a side-on bound TEMPO<sup>ā</sup> ligand,
which is characterized by a long NāO bond length [1.383(2)
Ć
]. The contrasting reactivity of Ph<sub>2</sub>S<sub>2</sub> and TEMPO likely relates to their different redox potentials, as
Ph<sub>2</sub>S<sub>2</sub> is a relatively weak oxidant. Finally,
the addition of pyridine-<i>N</i>-oxide to <b>2</b> results in the formation of the adduct, [(bipy)ĀNiĀ(NO)Ā(ONC<sub>5</sub>H<sub>5</sub>)]Ā[PF<sub>6</sub>] (<b>5</b>). No evidence of
NO release is observed in this reaction, probably because of the low
one-electron (1e<sup>ā</sup>) reduction potential of pyridine-<i>N</i>-oxide
Tuning the Reactivity of TEMPO by Coordination to a Lewis Acid: Isolation and Reactivity of MCl<sub>3</sub>(Ī·<sup>1</sup>āTEMPO) (M = Fe, Al)
Addition of 2,2,6,6-tetramethylpiperidine-<i>N</i>-oxyl
(TEMPO) to MCl<sub>3</sub> (M = Fe, Al) results in the formation of
MCl<sub>3</sub>(Ī·<sup>1</sup>-TEMPO) [M = Fe (<b>1</b>), Al (<b>2</b>)]. Both <b>1</b> and <b>2</b> oxidize
alcohols to generate ketones or aldehydes along with the reduced complexes
MCl<sub>3</sub>(Ī·<sup>1</sup>-TEMPOH) [M = Fe (<b>3</b>), Al (<b>4</b>)]. Complexes <b>1</b>ā<b>4</b> were fully characterized, including analysis by X-ray crystallography.
Additionally, control experiments indicated that neither MCl<sub>3</sub> (M = Al, Fe) nor TEMPO are capable of effecting the oxidation of
alcohols independently
Oxidation of Alcohols and Activated Alkanes with Lewis Acid-Activated TEMPO
The
reactivity of MCl<sub>3</sub>(Ī·<sup>1</sup>-TEMPO) (M
= Fe, <b>1</b>; Al, <b>2</b>; TEMPO = 2,2,6,6-tetramethylpiperidine-<i>N</i>-oxyl) with a variety of alcohols, including 3,4-dimethoxybenzyl
alcohol, 1-phenyl-2-phenoxyethanol, and 1,2-diphenyl-2-methoxyethanol,
was investigated using NMR spectroscopy and mass spectrometry. Complex <b>1</b> was effective in cleanly converting these substrates to
the corresponding aldehyde or ketone. Complex <b>2</b> was also
able to oxidize these substrates; however, in a few instances the
products of overoxidation were also observed. Oxidation of activated
alkanes, such as xanthene, by <b>1</b> or <b>2</b> suggests
that the reactions proceed via an initial 1-electron concerted protonāelectron
transfer (CPET) event. Finally, reaction of TEMPO with FeBr<sub>3</sub> in Et<sub>2</sub>O results in the formation of a mixture of FeBr<sub>3</sub>(Ī·<sup>1</sup>-TEMPOH) (<b>23</b>) and [FeBr<sub>2</sub>(Ī·<sup>1</sup>-TEMPOH)]<sub>2</sub>(Ī¼-O) (<b>24</b>), via oxidation of the solvent, Et<sub>2</sub>O
Charge Transfer and Blue Shifting of Vibrational Frequencies in a Hydrogen Bond Acceptor
A comprehensive
Raman spectroscopic/electronic structure study of hydrogen bonding
by pyrimidine with eight different polar solvents is presented. Raman
spectra of binary mixtures of pyrimidine with methanol and ethylene
glycol are reported, and shifts in Ī½<sub>1</sub>, Ī½<sub>3</sub>, Ī½<sub>6a</sub>, Ī½<sub>6b</sub>, Ī½<sub>8a</sub>, Ī½<sub>8b</sub>, Ī½<sub>9a</sub>, Ī½<sub>15</sub>, Ī½<sub>16a</sub>, and Ī½<sub>16b</sub> are compared
to earlier results obtained for water. Large shifts to higher vibrational
energy, often referred to as blue shifts, are observed for Ī½<sub>1</sub>, Ī½<sub>6b</sub>, and Ī½<sub>8b</sub> (by as much
as 14 cm<sup>ā1</sup>). While gradual blue shifts with increasing
hydrogen bond donor concentration are observed for Ī½<sub>6b</sub> and Ī½<sub>8b</sub>, Ī½<sub>1</sub> exhibits three distinct
spectral components whose relative intensities vary with concentration.
The blue shift of Ī½<sub>1</sub> is further examined in binary
mixtures of pyrimidine with acetic acid, thioglycol, phenylmethanol,
hexylamine, and acetonitrile. Electronic structure computations for
more than 100 microsolvated structures reveal a significant dependence
of the magnitude of the Ī½<sub>1</sub> blue shift on the local
microsolvation geometry. Results from natural bond orbital (NBO) calculations
also reveal a strong correlation between charge transfer and blue
shifting of pyrimidineās normal modes. Although charge transfer
has previously been linked to blue shifting of the XāH stretching
frequency in hydrogen bond donors, here, a similar trend in a hydrogen
bond acceptor is demonstrated
Probing Dative and Dihydrogen Bonding in Ammonia Borane with Electronic Structure Computations and Raman under Nitrogen Spectroscopy
Although
ammonia borane is isoelectronic with ethane and they have
similar structures, BH<sub>3</sub>NH<sub>3</sub> exhibits rather atypical
bonding compared to that in CH<sub>3</sub>CH<sub>3</sub>. The central
bond in ammonia borane is actually a coordinate covalent or dative
bond rather than the conventional covalent CāC bond in ethane
where each atom donates one electron. In addition, strong intermolecular
dihydrogen bonds can form between two or more ammonia borane molecules
compared to the relatively weak dispersion forces between ethane molecules.
As a result, ammonia boraneās physical properties are very
sensitive to the environment. For example, gas-phase and solid-state
ammonia borane have very different BN bond lengths and BN stretching
frequencies, which led to much debate in the literature. It has been
demonstrated that the use of cluster models based on experimental
crystal structures led to better agreement between theory and experiment.
Here, we employ a variety of cluster models to track how the interaction
energies, bond lengths, and vibrational normal modes evolve with the
size and structural characteristics of the clusters. The M06-2X/6-311++GĀ(2df,2pd)
level of theory was selected for this analysis on the basis of favorable
comparison with CCSDĀ(T)/aug-cc-pVTZ data for the ammonia borane monomer
and dimer. Fourteen unique fully optimized molecular cluster geometries,
(BH<sub>3</sub>NH<sub>3</sub>)<sub><i>n</i>ā¤12</sub>, and nine crystal models, (BH<sub>3</sub>NH<sub>3</sub>)<sub><i>n</i>ā¤19</sub>, were used to elucidate how the local
environment impacts ammonia boraneās physical properties. Computational
results for the BN stretching frequencies are also compared directly
to the Raman spectrum of solid ammonia borane at 77 K using Raman
under liquid nitrogen spectroscopy (RUNS). A strong linear correlation
was found to exist between the BN bond length and stretching frequency,
from an isolated monomer to the most distorted BH<sub>3</sub>NH<sub>3</sub> unit in a cluster or crystal structure model. Excellent agreement
was seen between the frequencies computed for the largest crystal
model and the RUNS experimental spectra (typically within a few wavenumbers)
Reaction of a Bridged Frustrated Lewis Pair with Nitric Oxide: A Kinetics Study
Described
is a kinetics and computational study of the reaction
of NO with the intramolecular bridged P/B frustrated Lewis pair (FLP) <i>endo</i>-2-(dimesitylphosphino)-<i>exo</i>-3-bisĀ(pentafluorophenyl)Āboryl-norbornane
to give a persistent FLP-NO aminoxyl radical. This reaction follows
a second-order rate law, first-order in [FLP] and first-order in [NO],
and is markedly faster in toluene than in dichloromethane. By contrast,
the NO oxidation of the phosphine base 2-(dimesitylphosphino)Ānorbornene
to the corresponding phosphine oxide follows a third-order rate law,
first-order in [phosphine] and second-order in [NO]. Formation of
the FLP-NO radical in toluene occurs with a Ī<i>H</i><sup>ā§§</sup> of 13 kcal mol<sup>ā1</sup>, a feature
that conflicts with the computation-based conclusion that NO addition
to a properly oriented B/P pair should be nearly barrierless. Since
the calculations show the B/P pair in the most stable solution structure
of this FLP to have an unfavorable orientation for concerted reaction,
the observed barrier is rationalized in terms of the reversible formation
of a [B]-NO complex intermediate followed by a slower isomerizationāring
closure step to the cyclic aminoxyl radical. This combined kinetics/theoretical
study for the first time provides insight into mechanistic details
for the activation of a diatomic molecule by a prototypical FLP
Ī²āHydride Elimination and CāH Activation by an Iridium Acetate Complex, Catalyzed by Lewis Acids. Alkane Dehydrogenation Cocatalyzed by Lewis Acids and [2,6-Bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl]iridium
NaBAr<sup>F</sup><sub>4</sub> (sodium
tetrakisĀ[(3,5-trifluoromethyl)Āphenyl]Āborate)
was found to catalyze reactions of (Phebox)ĀIr<sup>III</sup>Ā(acetate)
(Phebox = 2,6-bisĀ(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complexes,
including (i) Ī²-H elimination of (Phebox)ĀIrĀ(OAc)Ā(<i>n</i>-alkyl) to give (Phebox)ĀIrĀ(OAc)Ā(H) and
the microscopic reverse, alkene insertion into the IrāH bond
of (Phebox)ĀIrĀ(OAc)Ā(H), and (ii) hydrogenolysis of
the Irāalkyl bond of (Phebox)ĀIrĀ(OAc)Ā(<i>n</i>-alkyl)
and the microscopic reverse, CāH activation by (Phebox)ĀIrĀ(OAc)Ā(H),
as indicated by H/D exchange experiments. For example, Ī²-H elimination
of (Phebox)ĀIrĀ(OAc)Ā(<i>n</i>-octyl) (<b>2-Oc</b>) proceeded on a time scale of minutes at ā15 Ā°C in the
presence of (0.4 mM) NaBAr<sup>F</sup><sub>4</sub> as compared with
a very slow reaction at 125 Ā°C in the absence of NaBAr<sup>F</sup><sub>4</sub>. In addition to NaBAr<sup>F</sup><sub>4</sub>, other
Lewis acids are also effective. Density functional theory calculations
capture the effect of the Na<sup>+</sup> cation and indicate that
it operates primarily by promoting Īŗ<sup>2</sup>āĪŗ<sup>1</sup> dechelation of the acetate anion, which opens the coordination
site needed to allow the observed reaction to proceed. In accord with
the effect on these individual stoichiometric reactions, NaBAr<sup>F</sup><sub>4</sub> was also found to cocatalyze, with (Phebox)ĀIrĀ(OAc)Ā(H),
the acceptorless dehydrogenation of <i>n</i>-dodecane