Nilay Hazari

Lewis Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-Supported Iron Catalyst

Formic acid (FA) is an attractive compound for H2 storage. Currently, the most active catalysts for FA dehydrogenation use precious metals. Here, we report a homogeneous iron catalyst that, when used with a Lewis acid (LA) co-catalyst, gives approximately 1,000,000 turnovers for FA dehydrogenation. To date, this is the highest turnover number reported for a first-row transition metal catalyst. Preliminary studies suggest that the LA assists in the decarboxylation of a key iron formate intermediate and can also be used to enhance the reverse process of CO2 hydrogenation.

Synthesis and Structure of Six-Coordinate Iron Borohydride Complexes Supported by PNP Ligands

The preparation of a number of iron complexes supported by ligands of the type HN{CH2CH2(PR2)}2 [R = isopropyl (((i)Pr)PNP) or cyclohexyl ((Cy)PNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron(II) complexes (((i)Pr)PNP)FeCl2(CO) (1a) and ((Cy)PNP)FeCl2(CO) (1b) were synthesized through the reaction of the appropriate free ligand and FeCl2 in the presence of CO. The iron(0) complex (((i)Pr)PNP)Fe(CO)2 (2a) was prepared through the reaction of Fe(CO)5 with ((i)Pr)PNP, while irradiating with UV light. Compound 2a is unstable in CH2Cl2 and is oxidized to 1a via the intermediate iron(II) complex [(((i)Pr)PNP)FeCl(CO)2]Cl (3a). The reaction of 2a with HCl generated the related complex [(((i)Pr)PNP)FeH(CO)2]Cl (4a), while the neutral iron hydrides (((i)Pr)PNP)FeHCl(CO) (5a) and ((Cy)PNP)FeHCl(CO) (5b) were synthesized through the reaction of 1a or 1b with 1 equiv of (n)Bu4NBH4. The related reaction between 1a and excess NaBH4 generated the unusual η(1)-HBH3 complex (((i)Pr)PNP)FeH(η(1)-HBH3)(CO) (6a). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η(1)-HBH3 ligand and the N-H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of 6a with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(((i)Pr)PNP)FeH(CO)}2(μ2,η(1):η(1)-H2BH2)][BPh4] (7a), which features a rare example of a μ2,η(1):η(1)-H2BH2 ligand. Unlike all previous examples of complexes with a μ2,η(1):η(1)-H2BH2 ligand, there is no metal-metal bond and additional bridging ligand supporting the borohydride ligand in 7a; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes 1a, 2a, 3a, 4a, 5a, 6a, and 7a were characterized by X-ray crystallography.

A mechanistic study of allene carboxylation with CO2 resulting in the development of a Pd(II) pincer complex for the catalytic hydroboration of CO2

The carboxylation of allenes with CO2 represents a potentially important method for the synthesis of unsaturated carboxylic acids. Here, we describe a detailed mechanistic study of the catalytic carboxylation of allenes using CyPSiP (CyPSiP = Si(Me)(2-PCy2-C6H4)2) supported Pd complexes. As part of this work we have identified, characterized and isolated all of the proposed intermediates in the catalytic cycle and shown that they are kinetically competent catalysts. In addition, we have isolated several off-cycle species, which are in equilibrium with complexes in the catalytic cycle, and established that they are also active catalysts. Several of these off-cycle species are formed through an unusual ligand rearrangement of the CyPSiP pincer ligand, in which a Si–C bond is reversibly cleaved. The major catalyst deactivation pathway has been identified. Furthermore, our mechanistic study has allowed us to develop a new catalyst for the hydroboration of carbon dioxide, which gives a maximum turnover number (TON) greater than 60u2006000; the highest reported to date.

Nickel(I) Monomers and Dimers with Cyclopentadienyl and Indenyl Ligands

The reaction of (μ-Cl)2Ni2(NHC)2 (NHC = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IPr) or 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr)) with either one equivalent of sodium cyclopentadienyl (NaCp) or lithium indenyl (LiInd) results in the formation of diamagnetic NHC supported Ni(I) dimers of the form (μ-Cp)(μ-Cl)Ni2(NHC)2 (NHC = IPr (1u2009a) or SIPr (1u2009b); Cp = C5H5) or (μ-Ind)(μ-Cl)Ni2(NHC)2 (NHC = IPr (2u2009a) or SIPr (2u2009b); Ind = C7H9), which contain bridging Cp and indenyl ligands. The corresponding reaction between two equivalents of NaCp or LiInd and (μ-Cl)2Ni2(NHC)2 (NHC = IPr or SIPr) generates unusual 17 valence electron Ni(I) monomers of the form (η(5)-Cp)Ni(NHC) (NHC = IPr (3u2009a) or SIPr (3u2009b)) or (η(5)-Ind)Ni(NHC) (NHC = IPr (4u2009a) or SIPr (4u2009b)), which have nonlinear geometries. A combination of DFT calculations and NBO analysis suggests that the Ni(I) monomers are more strongly stabilized by the Cp ligand than by the indenyl ligand, which is consistent with experimental results. These calculations also show that the monomers have a lone unpaired-single-electron in their valence shell, which is the reason for the nonlinear structures. At room temperature the Cp bridged dimer (μ-Cp)(μ-Cl)Ni2(NHC)2 undergoes homolytic cleavage of the Ni-Ni bond and is in equilibrium with (η(5)-Cp)Ni(NHC) and (μ-Cl)2Ni2(NHC)2. There is no evidence that this equilibrium occurs for (μ-Ind)(μ-Cl)Ni2(NHC)2. DFT calculations suggest that a thermally accessible triplet state facilitates the homolytic dissociation of the Cp bridged dimers, whereas for bridging indenyl species this excited triplet state is significantly higher in energy. In stoichiometric reactions, the Ni(I) monomers (η(5)-Cp)Ni(NHC) or (η(5)-Ind)Ni(NHC) undergo both oxidative and reductive processes with mild reagents. Furthermore, they are rare examples of active Ni(I) precatalysts for the Suzuki-Miyaura reaction. Complexes 1u2009a, 2u2009b, 3u2009a, 4u2009a and 4u2009b have been characterized by X-ray crystallography.

Comparison of dppf‐Supported Nickel Precatalysts for the Suzuki–Miyaura Reaction: The Observation and Activity of Nickel(I)

Ni-based precatalysts for the Suzuki-Miyaura reaction have potential chemical and economic advantages compared to commonly used Pd systems. Here, we compare Ni precatalysts for the Suzuki-Miyaura reaction supported by the dppf ligand in 3 oxidation states, 0, I and II. Surprisingly, at 80u2009°C they give similar catalytic activity, with all systems generating significant amounts of Ni(I) during the reaction. At room temperature a readily accessible bench-stable Ni(II) precatalyst is highly active and can couple synthetically important heterocyclic substrates. Our work conclusively establishes that Ni(I) species are relevant in reactions typically proposed to involve exclusively Ni(0) and Ni(II) complexes.

Effect of Sodium Cation on Metallacycle β-Hydride Elimination in CO2–Ethylene Coupling to Acrylates

The catalytic conversion of carbon dioxide and olefins into acrylates has been a long standing target, because society attempts to synthesize commodity chemicals in a more economical and sustainable fashion. Although nickel complexes have been known to successfully couple CO2 and ethylene for decades, a key β-hydride elimination step has proven a major obstacle to the development of a catalytic process. Recent studies have shown that Lewis acid additives can be used to create a lower-energy pathway for β-hydride elimination and facilitate a low number of catalytic turnovers. However, the exact manner, in which the Lewis acid promotes β-hydride elimination remains to be elucidated. Herein, we describe the kinetic and thermodynamic role that commercially relevant and weakly Lewis acidic sodium salts play in promoting β-hydride elimination from nickelalactones synthesized from CO2 and ethylene. This process is compared to a non-Lewis acid promoted pathway, and DFT calculations were used to identify differences between the two systems. The sodium-free isomerization reaction gave a rare CO2 -derived β-nickelalactone complex, which was structurally characterized.

Controlled doping of carbon nanotubes with metallocenes for application in hybrid carbon nanotube/Si solar cells.

There is considerable interest in the controlled p-type and n-type doping of carbon nanotubes (CNT) for use in a range of important electronics applications, including the development of hybrid CNT/silicon (Si) photovoltaic devices. Here, we demonstrate that easy to handle metallocenes and related complexes can be used to both p-type and n-type dope single-walled carbon nanotube (SWNT) thin films, using a simple spin coating process. We report n-SWNT/p-Si photovoltaic devices that are >450 times more efficient than the best solar cells of this type currently reported and show that the performance of both our n-SWNT/p-Si and p-SWNT/n-Si devices is related to the doping level of the SWNT. Furthermore, we establish that the electronic structure of the metallocene or related molecule can be correlated to the doping level of the SWNT, which may provide the foundation for controlled doping of SWNT thin films in the future.

Role of HF in oxygen removal from carbon nanotubes: implications for high performance carbon electronics.

Oxygen removal from SWNTs is crucial for many carbon electronic devices. This work shows that HF treatment followed by current stimulation is a very effective method for oxygen removal. Using a procedure involving HF treatment, current stimulation and spin-casting AgNWs onto a SWNT thin film, record high efficiency SWNT/p-Si solar cells have been developed.

Flexible Binding of PNP Pincer Ligands to Monomeric Iron Complexes

Transition metal complexes supported by pincer ligands have many important applications. Here, the syntheses of five-coordinate PNP pincer-supported Fe complexes of the type (PNP)FeCl2 (PNP = HN{CH2CH2(PR2)}2, R = iPr ((iPr)PNP), tBu ((tBu)PNP), or cyclohexyl ((Cy)PNP)) are reported. In the solid state, ((iPr)PNP)FeCl2 was characterized in two different geometries by X-ray crystallography. In one form, the (iPr)PNP ligand binds to the Fe center in the typical meridional geometry for a pincer ligand, whereas in the other form, the (iPr)PNP ligand binds in a facial geometry. The electronic structures and geometries of all of the (PNP)FeCl2 complexes were further explored using (57)Fe Mössbauer and magnetic circular dichroism spectroscopy. These measurements show that in some cases two isomers of the (PNP)FeCl2 complexes are present in solution and conclusively demonstrate that binding of the PNP ligand is flexible, which may have implications for the reactivity of this important class of compounds.

An Unusual Example of Hypervalent Silicon: A Five‐Coordinate Silyl Group Bridging Two Palladium or Nickel Centers through a Nonsymmetrical Four‐Center Two‐Electron Bond

Pd and Ni dimers supported by PSiP ligands in which two hypervalent five-coordinate Si atoms bridge the two metal centers are reported. Crystallographic characterization revealed a rare square-pyramidal geometry at Si and an unusual asymmetric M2 Si2 core (M=Pd or Ni). DFT calculations showed that the unusual structure of the core is also found in a model in which the phosphine and Si centers are not part of a pincer group, thus indicating that the observed geometry is not imposed by the PSiP ligand. NBO analysis showed that an asymmetric four-center two-electron (4c-2e) bond stabilizes the hypervalent Si atoms in the M2 Si2 core.