Mechanistic Insights into the Formation of N₂O by a Nickel Nitrosyl Complex

Reaction of [Ni­(NO)­(bipy)­(Me₂phen)]­[PF₆] with 1 equiv of nitric oxide (NO) in CH₂Cl₂ results in the formation of N₂O and [{(Me₂phen)­Ni­(NO)}₂(μ–η¹-<i>N</i>:η¹-<i>O</i>)-NO₂)]­[PF₆] (<b>3</b>), along with the known complex, [Ni­(bipy)₃]­[PF₆]₂ (<b>4</b>). The isolation of complex <b>3</b>, which contains a nitrite ligand, demonstrates that the reaction of [Ni­(NO)­(bipy)­(Me₂phen)]­[PF₆] with exogenous NO results in NO disproportionation and not NO reduction. Complex <b>3</b> could also be accessed by reaction of [Ni­(NO)­(Me₂phen)]­[PF₆] (<b>5</b>) with (Me₂phen)­Ni­(NO)­(NO₂) (<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 [Ni­(NO)­(bipy)]­[PF₆] results in formation of the hyponitrite complex, [{(bipy)­Ni­(κ²-O₂N₂)}­η¹:η¹-<i>N</i>,<i>N</i>-{Ni­(NO)­(bipy)}₂]­[PF₆]₂ (<b>9</b>), in modest yield. Importantly, the hyponitrite ligand in <b>9</b> is thought to form via coupling of two NO^– ligands and not by coupling of a nucleophilic nitrosyl ligand (NO^–) with an electrophilic nitrosyl ligand (NO⁺). 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 [Ni­(NO)­(bipy)]­[PF₆] (<b>2</b>) with AgPF₆ or [NO]­[PF₆] in MeCN results in formation of [Ni­(bipy)_<i>x</i>(MeCN)_<i>y</i>]²⁺ and release of NO gas in moderate yields. In contrast, the addition of the inner sphere oxidant Ph₂S₂ to <b>2</b> does not result in denitrosylation. Instead, the diphenyldisulfide adduct [{(bipy)­(NO)­Ni}₂(μ-S₂Ph₂)]­[PF₆]₂ (<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 Ni–NO bond and generation of NO. The metal-containing product, [(bipy)­Ni­(η²-TEMPO)]­[PF₆] (<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^– ligand, which is characterized by a long N–O bond length [1.383(2) Å]. The contrasting reactivity of Ph₂S₂ and TEMPO likely relates to their different redox potentials, as Ph₂S₂ 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)­Ni­(NO)­(ONC₅H₅)]­[PF₆] (<b>5</b>). No evidence of NO release is observed in this reaction, probably because of the low one-electron (1e^–) reduction potential of pyridine-<i>N</i>-oxide

Mechanistic Insights into the Formation of N₂O by a Nickel Nitrosyl Complex

Reaction of [Ni­(NO)­(bipy)­(Me₂phen)]­[PF₆] with 1 equiv of nitric oxide (NO) in CH₂Cl₂ results in the formation of N₂O and [{(Me₂phen)­Ni­(NO)}₂(μ–η¹-<i>N</i>:η¹-<i>O</i>)-NO₂)]­[PF₆] (<b>3</b>), along with the known complex, [Ni­(bipy)₃]­[PF₆]₂ (<b>4</b>). The isolation of complex <b>3</b>, which contains a nitrite ligand, demonstrates that the reaction of [Ni­(NO)­(bipy)­(Me₂phen)]­[PF₆] with exogenous NO results in NO disproportionation and not NO reduction. Complex <b>3</b> could also be accessed by reaction of [Ni­(NO)­(Me₂phen)]­[PF₆] (<b>5</b>) with (Me₂phen)­Ni­(NO)­(NO₂) (<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 [Ni­(NO)­(bipy)]­[PF₆] results in formation of the hyponitrite complex, [{(bipy)­Ni­(κ²-O₂N₂)}­η¹:η¹-<i>N</i>,<i>N</i>-{Ni­(NO)­(bipy)}₂]­[PF₆]₂ (<b>9</b>), in modest yield. Importantly, the hyponitrite ligand in <b>9</b> is thought to form via coupling of two NO^– ligands and not by coupling of a nucleophilic nitrosyl ligand (NO^–) with an electrophilic nitrosyl ligand (NO⁺). 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 [Ni­(NO)­(bipy)]­[PF₆] (<b>2</b>) with AgPF₆ or [NO]­[PF₆] in MeCN results in formation of [Ni­(bipy)_<i>x</i>(MeCN)_<i>y</i>]²⁺ and release of NO gas in moderate yields. In contrast, the addition of the inner sphere oxidant Ph₂S₂ to <b>2</b> does not result in denitrosylation. Instead, the diphenyldisulfide adduct [{(bipy)­(NO)­Ni}₂(μ-S₂Ph₂)]­[PF₆]₂ (<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 Ni–NO bond and generation of NO. The metal-containing product, [(bipy)­Ni­(η²-TEMPO)]­[PF₆] (<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^– ligand, which is characterized by a long N–O bond length [1.383(2) Å]. The contrasting reactivity of Ph₂S₂ and TEMPO likely relates to their different redox potentials, as Ph₂S₂ 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)­Ni­(NO)­(ONC₅H₅)]­[PF₆] (<b>5</b>). No evidence of NO release is observed in this reaction, probably because of the low one-electron (1e^–) reduction potential of pyridine-<i>N</i>-oxide

Tuning the Reactivity of TEMPO by Coordination to a Lewis Acid: Isolation and Reactivity of MCl₃(η¹‑TEMPO) (M = Fe, Al)

Addition of 2,2,6,6-tetramethylpiperidine-<i>N</i>-oxyl (TEMPO) to MCl₃ (M = Fe, Al) results in the formation of MCl₃(η¹-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 MCl₃(η¹-TEMPOH) [M = Fe (<b>3</b>), Al (<b>4</b>)]. Complexes <b>1</b>–<b>4</b> were fully characterized, including analysis by X-ray crystallography. Additionally, control experiments indicated that neither MCl₃ (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 MCl₃(η¹-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 proton–electron transfer (CPET) event. Finally, reaction of TEMPO with FeBr₃ in Et₂O results in the formation of a mixture of FeBr₃(η¹-TEMPOH) (<b>23</b>) and [FeBr₂(η¹-TEMPOH)]₂(μ-O) (<b>24</b>), via oxidation of the solvent, Et₂O

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 ν₁, ν₃, ν_6a, ν_6b, ν_8a, ν_8b, ν_9a, ν₁₅, ν_16a, and ν_16b are compared to earlier results obtained for water. Large shifts to higher vibrational energy, often referred to as blue shifts, are observed for ν₁, ν_6b, and ν_8b (by as much as 14 cm^–1). While gradual blue shifts with increasing hydrogen bond donor concentration are observed for ν_6b and ν_8b, ν₁ exhibits three distinct spectral components whose relative intensities vary with concentration. The blue shift of ν₁ 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 ν₁ 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 pyrimidine’s normal modes. Although charge transfer has previously been linked to blue shifting of the X–H 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, BH₃NH₃ exhibits rather atypical bonding compared to that in CH₃CH₃. The central bond in ammonia borane is actually a coordinate covalent or dative bond rather than the conventional covalent C–C 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 borane’s 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++G­(2df,2pd) level of theory was selected for this analysis on the basis of favorable comparison with CCSD­(T)/aug-cc-pVTZ data for the ammonia borane monomer and dimer. Fourteen unique fully optimized molecular cluster geometries, (BH₃NH₃)_{<i>n</i>≤12}, and nine crystal models, (BH₃NH₃)_{<i>n</i>≤19}, were used to elucidate how the local environment impacts ammonia borane’s 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 BH₃NH₃ 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-bis­(pentafluorophenyl)­boryl-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)­norbornene 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 Δ<i>H</i>^⧧ of 13 kcal mol^–1, 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 isomerization–ring 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

β‑Hydride Elimination and C–H 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^F₄ (sodium tetrakis­[(3,5-trifluoromethyl)­phenyl]­borate) was found to catalyze reactions of (Phebox)­Ir^III­(acetate) (Phebox = 2,6-bis­(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) complexes, including (i) β-H elimination of (Phebox)­Ir­(OAc)­(<i>n</i>-alkyl) to give (Phebox)­Ir­(OAc)­(H) and the microscopic reverse, alkene insertion into the Ir–H bond of (Phebox)­Ir­(OAc)­(H), and (ii) hydrogenolysis of the Ir–alkyl bond of (Phebox)­Ir­(OAc)­(<i>n</i>-alkyl) and the microscopic reverse, C–H activation by (Phebox)­Ir­(OAc)­(H), as indicated by H/D exchange experiments. For example, β-H elimination of (Phebox)­Ir­(OAc)­(<i>n</i>-octyl) (<b>2-Oc</b>) proceeded on a time scale of minutes at −15 °C in the presence of (0.4 mM) NaBAr^F₄ as compared with a very slow reaction at 125 °C in the absence of NaBAr^F₄. In addition to NaBAr^F₄, other Lewis acids are also effective. Density functional theory calculations capture the effect of the Na⁺ cation and indicate that it operates primarily by promoting κ²–κ¹ 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^F₄ was also found to cocatalyze, with (Phebox)­Ir­(OAc)­(H), the acceptorless dehydrogenation of <i>n</i>-dodecane