A nickel bis(diphosphine) complex containing pendant amines in the second coordination sphere, [Ni(P(Cy)2N(t-Bu)2)2](BF4)2 (P(Cy)2N(t-Bu)2 = 1,5-di(tert-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the Ni(II) complex gives three isomers of the doubly protonated Ni(0) complex [Ni(P(Cy)2N(t-Bu)2H)2](BF4)2. Using the pKa values and Ni(II/I) and Ni(I/0) redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni(P(Cy)2N(t-Bu)2)2](2+) was determined to be -7.9 kcal mol(-1). The catalytic rate observed in dry acetonitrile for the oxidation of H2 depends on base size, with larger bases (NEt3, t-BuNH2) resulting in much slower catalysis than n-BuNH2. The addition of water accelerates the rate of catalysis by facilitating deprotonation of the hydrogen addition product before oxidation, especially for the larger bases NEt3 and t-BuNH2. This catalytic pathway, where deprotonation occurs prior to oxidation, leads to an overpotential that is 0.38 V lower compared to the pathway where oxidation precedes proton movement. Under the optimal conditions of 1.0 atm H2 using n-BuNH2 as a base and with added water, a turnover frequency of 58 s(-1) is observed at 23 °C.
An efficient ligand combination: A new bis(diphosphine) nickel(II) complex (see picture) is described. A ΔG° value of 0.84 kcal mol−1 for hydrogen addition for this complex was calculated from the experimentally determined equilibrium constant. This complex displayed reversible electrocatalytic activity for hydrogen production and oxidation at low overpotentials, which are characteristic for hydrogenase enzymes.
Carbon
nanothreads, which are one-dimensional sp3-rich
polymers, combine high tensile strength with flexibility owing to
subnanometer widths and diamond-like cores. These extended carbon
solids are constructed through pressure-induced polymerization of
sp2 molecules such as benzene. Whereas a few examples of
carbon nanothreads have been reported, the need for high onset pressures
(≥17 GPa) to synthesize them precludes scalability and limits
scope. Herein, we report the scalable synthesis of carbon nanothreads
based on molecular furan, which can be achieved through ambient temperature
pressure-induced polymerization with an onset reaction pressure of
only 10 GPa due to its lessened aromaticity relative to other molecular
precursors. When slowly compressed to 15 GPa and gradually decompressed
to 1.5 GPa, a sharp 6-fold diffraction pattern is observed in situ, indicating a well-ordered crystalline material
formed from liquid furan. Single-crystal X-ray diffraction (XRD) of
the reaction product exhibits three distinct d-spacings
from 4.75 to 4.9 Å, whose size, angular spacing, and degree of
anisotropy are consistent with our atomistic simulations for crystals
of furan nanothreads. Further evidence for polymerization was obtained
by powder XRD, Raman/IR spectroscopy, and mass spectrometry. Comparison
of the IR spectra with computed vibrational modes provides provisional
identification of spectral features characteristic of specific nanothread
structures, namely syn, anti, and syn/anti configurations. Mass spectrometry suggests that
molecular weights of at least 6 kDa are possible. Furan therefore
presents a strategic entry toward scalable carbon nanothreads.
The effects of the ligand to metal ratio, temperature, syngas pressure, partial pressures of H2 and CO, and new ligand structures have been examined on 12 of the most reasonable products resulting from the rhodium-catalyzed low-pressure hydroformylation of 1,3-butadiene. The selectivity for the desired linear dihydroformylation product, 1,6-hexanedial (adipic aldehyde), is essentially independent of all of these reaction parameters, except for ligand structure. However, the reaction parameters do have a substantial effect on the selectivity for the products, resulting from the branched addition of the rhodium hydride to the carbon–carbon double bond. The optimum reaction parameters and ligand have resulted in a so far unprecedented maximum selectivity of 50% for adipic aldehyde.
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