Trends in the Usage of Bidentate Phosphines as Ligands in Nickel Catalysis.

A critically important process in catalysis is the formation of an active catalyst from the combination of a metal precursor and a ligand, as the efficacy of this reaction governs the amount of active catalyst. This Review is a comprehensive overview of reactions catalyzed by nickel and an added bidentate phosphine, focusing on the steps transforming the combination of precatalyst and ligand into an active catalyst and the potential effects of this transformation on nickel catalysis. Reactions covered include common cross-coupling reactions, such as Suzuki, Heck, Kumada, and Negishi couplings, addition reactions, cycloadditions, C-H functionalizations, polymerizations, hydrogenations, and reductive couplings, among others. Overall, the most widely used nickel precatalyst with free bidentate phosphines is Ni(cod)2, which accounts for ∼50% of the reports surveyed, distantly followed by Ni(acac)2 and Ni(OAc)2, which account for ∼10% each. By compiling the reports of these reactions, we have calculated statistics of the usage and efficacy of each ligand with Ni(cod)2 and other nickel sources. The most common bidentate phosphines are simple, relatively inexpensive ligands, such as DPPE, DCPE, DPPP, and DPPB, along with others with more complex backbones, such as DPPF and Xantphos. The use of expensive chiral phosphines is more scattered, but the most common ligands include BINAP, Me-Duphos, Josiphos, and related analogs.

Removal of uranium from contaminated groundwater using monorhamnolipids and ion flotation.

Mining of uranium for defense-related purposes has left a substantial legacy of pollution that threatens human and environmental health. Contaminated waters in the arid southwest are of particular concern, as water resource demand and water scarcity issues become more pronounced. The development of remediation strategies to treat uranium impacted waters will become increasingly vital to meet future water needs. Ion flotation is one technology with the potential to address legacy uranium contamination. The green biosurfactant rhamnolipid has been shown to bind uranium and act as an effective collector in ion flotation. In this study, uranium contaminated groundwater (∼440 µg L-1 U) from the Monument Valley processing site in northeast Arizona was used as a model solution to test the uranium removal efficacy of ion flotation with biosynthetic (bio-mRL) and three synthetic monorhamnolipids with varying hydrophobic chain lengths: Rha-C10-C10, Rha-C12-C12, and Rha-C14-C14. At the groundwater's native pH 8, and at an adjusted pH 7, no uranium was removed from solution by any collector. However, at pH 6.5 bio-mRL and Rha-C10-C10 removed 239.2 µg L-1 and 242.4 µg L-1 of uranium, respectively. By further decreasing the pH to 5.5, bio-mRL was able to reduce the uranium concentration to near or below the Environmental Protection Agency maximum contaminant level of 30 µg L-1. For the Rha-C12-C12 and Rha-C14-C14 collector ligands, decreasing the pH to 7 or below reduced the foam stability and quantity, such that these collectors were not suitable for treating this groundwater. To contextualize the results, a geochemical analysis of the groundwater was conducted, and a consideration of uranium speciation is described. Based on this study, the efficacy of monorhamnolipid-based ion flotation in real world groundwater has been demonstrated with suitable solution conditions and collectors identified.

Highly active electrolytes for rechargeable Mg batteries based on a [Mg2(µ-Cl)2](2+) cation complex in dimethoxyethane.

A novel [Mg2(µ-Cl)2](2+) cation complex, which is highly active for reversible Mg electrodeposition, was identified for the first time in this work. This complex was found to be present in electrolytes formulated in dimethoxyethane (DME) through dehalodimerization of non-nucleophilic MgCl2 by reacting with either Mg salts (such as Mg(TFSI)2, TFSI = bis(trifluoromethane)sulfonylimide) or Lewis acid salts (such as AlEtCl2 or AlCl3). The molecular structure of the cation complex was characterized by single crystal X-ray diffraction, Raman spectroscopy and NMR. The electrolyte synthesis process was studied and rational approaches for formulating highly active electrolytes were proposed. Through control of the anions, electrolytes with an efficiency close to 100%, a wide electrochemical window (up to 3.5 V) and a high ionic conductivity (>6 mS cm(-1)) were obtained. The understanding of electrolyte synthesis in DME developed in this work could bring significant opportunities for the rational formulation of electrolytes of the general formula [Mg2(µ-Cl)2][anion]x for practical Mg batteries.

The discovery of [Ni(NHC)RCN]2 species and their role as cycloaddition catalysts for the formation of pyridines.

The reaction of Ni(COD)(2), IPr, and nitrile affords dimeric [Ni(IPr)RCN](2) in high yields. X-ray analysis revealed these species display simultaneous η(1)- and η(2)-nitrile binding modes. These dimers are catalytically competent in the formation of pyridines from the cycloaddition of diynes and nitriles. Kinetic analysis showed the reaction to be first order in [Ni(IPr)RCN](2), zeroth order in added IPr, zeroth order in nitrile, and zeroth order in diyne. Extensive stoichiometric competition studies were performed, and selective incorporation of the exogenous, not dimer bound, nitrile was observed. Post cycloaddition, the dimeric state was found to be largely preserved. Nitrile and ligand exchange experiments were performed and found to be inoperative in the catalytic cycle. These observations suggest a mechanism whereby the catalyst is activated by partial dimer-opening followed by binding of exogenous nitrile and subsequent oxidative heterocoupling.

Nickel-Catalyzed [2+2+2] Cycloaddition of Diynes and Cyanamides.

A variety of bicyclic N,N-disubstituted 2-aminopyridines have been prepared from diynes and cyanamides by nickel-catalyzed [2+2+2] cycloaddition reactions. The reactions proceeded at room temperature with low catalyst loading to afford 2-aminopyridines in good to excellent yields. The method is amenable to both internal and terminal diynes and proceeds in a regioselective manner. A number of cyanamides with diverse functional group tolerance were used. The intermolecular version employing 3-hexyne and N-cyanopyrrolidine also afforded the desired N,N-disubstituted 2-aminopyridine in good yield.

Spin Wave Excitation, Detection, and Utilization in the Organic-Based Magnet, V(TCNE)x (TCNE = Tetracyanoethylene).

Spin waves, quantized as magnons, have low energy loss and magnetic damping, which are critical for devices based on spin-wave propagation needed for information processing devices. The organic-based magnet [V(TCNE)x ; TCNE = tetracyanoethylene; x ≈ 2] has shown an extremely low magnetic damping comparable to, for example, yttrium iron garnet (YIG). The excitation, detection, and utilization of coherent and non-coherent spin waves on various modes in V(TCNE)x is demonstrated and show that the angular momentum carried by microwave-excited coherent spin waves in a V(TCNE)x film can be transferred into an adjacent Pt layer via spin pumping and detected using the inverse spin Hall effect. The spin pumping efficiency can be tuned by choosing different excited spin wave modes in the V(TCNE)x film. In addition, it is shown that non-coherent spin waves in a V(TCNE)x film, excited thermally via the spin Seebeck effect, can also be used as spin pumping source that generates an electrical signal in Pt with a sign change in accordance with the magnetization switching of the V(TCNE)x . Combining coherent and non-coherent spin wave detection, the spin pumping efficiency can be thermally controlled, and new insight is gained for the spintronic applications of spin wave modes in organic-based magnets.

Total Synthesis of Indolizidine Alkaloids via Nickel-Catalyzed (4 + 2) Cyclization.

A Ni-catalyzed (4 + 2) cycloaddition of alkynes and azetidinones toward piperidinones was used as key reaction in the enantioselective synthesis of naturally occurring indolizidine alkaloids. The reaction benefits from the use of an easily accessible azetidinone as an advanced and divergent intermediate to build the indolizidine core. This methodology has been applied in the total syntheses of (+)-septicine, (+)-ipalbidine, and (+)-seco-antofine to illustrate the applicability of the general approach.

Synthesis, mechanism of formation, and catalytic activity of Xantphos nickel π-complexes.

A general synthetic route to the first Xantphos nickel alkyne and alkene complexes has been discovered. Various Ni π-complexes were prepared and characterized. NMR experiments indicate benzonitrile undergoes ligand exchange with a Xantphos ligand of (Xant)2Ni, a compound that was previously believed to be unreactive. The Ni π-complexes were also shown to be catalytically competent in cross coupling and cycloaddition reactions. (Xant)2Ni is also catalytically active for these reactions when activated by a nitrile or coordinating solvent.

Solvent and electrolyte effects on Ni(P(R)(2)N(R')2)(2)-catalyzed electrochemical oxidation of hydrogen.

We report solvent and electrolyte effects on the electrocatalytic oxidation of H2 using Ni(P(Cy)2N(R')2)2 (R = Bn, (t)Bu) complexes. A turnover frequency of 46 s(-1) for Ni(P(Cy)2N(Bn)2)2 was obtained using 0.2 M [(n)Bu4N][BF4] in THF. A turnover frequency of 51 s(-1) was observed for Ni(P(Cy)2N(tBu)2)2 using 0.2 M [(n)Bu4N][B(C6F5)4] in fluorobenzene. These observations, in conjunction with previous studies, indicate nitrile binding inhibits catalysis supported by Ni(P(Cy)2N(Bn)2)2.

Mechanistic Evaluation of the Ni(IPr)2-Catalyzed Cycloaddition of Alkynes and Nitriles to Afford Pyridines: Evidence for the Formation of a Key η1-Ni(IPr)2(RCN) Intermediate.

A detailed mechanistic evaluation of the Ni(IPr)2-catalyzed [2+2+2]-cycloaddition of diynes and nitriles was 2 conducted. Through kinetic analysis of these reactions, observed regioselectivities of the products, and stoichiometric reactions, Ni(IPr)2-catalyzed cycloadditions of diynes and nitriles appear to proceed by a heterooxidative coupling mechanism, contrary to other common cycloaddition catalysts. Reaction profiles demonstrated strong dependence in nitrile, resulting in variable nitrile-dependent resting states. Strong coordination and considerable steric bulk of the carbene ligands facilitate selective initial binding of nitrile thereby forcing a heterocoupling pathway. In situ IR data suggests the initial binding of the nitrile resides in a rare, η1-bound conformation. Following nitrile coordination are a rate-determining hapticity shift of the nitrile and subsequent loss of carbene. Alkyne coordination then leads to heterooxidative coupling, insertion of the pendant alkyne, and reductive elimination to afford pyridine products.

Palladium-catalyzed arylation of cyanamides.

The cross-coupling of alkyl cyanamides with a number of aryl, heteroaryl, and vinyl halide and pseudohalide coupling partners has been developed via a modification of Pd-catalyzed amidation methods. The reactions proceed selectively under mild conditions with reasonable reaction times in moderate to excellent yields.