3 research outputs found
Computational Transition-State Design Provides Experimentally Verified Cr(P,N) Catalysts for Control of Ethylene Trimerization and Tetramerization
Computational design
of molecular homogeneous organometallic catalysts
followed by experimental realization remains a significant challenge.
Here, we report the development and use of a density functional theory
transition-state model that provided quantitative prediction of molecular
Cr catalysts for controllable selective ethylene trimerization and
tetramerization. This computational model identified a general class
of phosphine monocyclic imine (P,N)-ligand Cr catalysts where changes
in the ligand structure control 1-hexene versus 1-octene selectivity.
Experimental ligand and catalyst synthesis as well as reaction testing
quantitatively confirmed predictions
Hydrogen Production Using Nickel Electrocatalysts with Pendant Amines: Ligand Effects on Rates and Overpotentials
A Ni-based electrocatalyst for H<sub>2</sub> production, [NiÂ(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]Â(BF<sub>4</sub>)<sub>2</sub>,
featuring eight-membered cyclic
diphosphine ligands incorporating a single amine base, 1-<i>para</i>-bromophenyl-3,7-triphenyl-1-aza-3,7-diphosphacycloheptane (8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>) has
been synthesized and characterized. X-ray diffraction studies reveal
that the cation of [NiÂ(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>(CH<sub>3</sub>CN)]Â(BF<sub>4</sub>)<sub>2</sub> has a distorted trigonal bipyramidal geometry. In CH<sub>3</sub>CN, [NiÂ(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]<sup>2+</sup> is an electrocatalyst
for reduction of protons, and it has a maximum turnover frequency
for H<sub>2</sub> production of 800 s<sup>–1</sup> with a 700
mV overpotential (at <i>E</i><sub>cat/2</sub>) when using
[(DMF)ÂH]ÂOTf as the acid. Addition of H<sub>2</sub>O to acidic CH<sub>3</sub>CN solutions of [NiÂ(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]<sup>2+</sup> results in
an increase in the turnover frequency for H<sub>2</sub> production
to a maximum of 3300 s<sup>–1</sup> with an overpotential of
760 mV at <i>E</i><sub>cat/2</sub>. Computational studies
carried out on [NiÂ(8P<sup>Ph</sup><sub>2</sub>N<sup>C<sub>6</sub>H<sub>4</sub>Br</sup>)<sub>2</sub>]<sup>2+</sup> indicate the observed
catalytic rate is limited by formation of nonproductive protonated
isomers, diverting active catalyst from the catalytic cycle. The results
of this research show that proton delivery from the exogenous acid
to the correct position on the proton relay of the metal complex is
essential for fast H<sub>2</sub> production
Studies of a Series of [Ni(P<sup>R</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)]<sup>2+</sup> Complexes as Electrocatalysts for H<sub>2</sub> Production: Substituent Variation at the Phosphorus Atom of the P<sub>2</sub>N<sub>2</sub> Ligand
A series of [Ni(P<sup>R</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)](BF<sub>4</sub>)<sub>2</sub> complexes containing the cyclic diphosphine ligands [P<sup>R</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub> = 1,5-diaza-3,7-diphosphacyclooctane; R = benzyl (Bn), <i>n</i>-butyl (<i>n-</i>Bu), 2-phenylethyl (PE), 2,4,4-trimethylpentyl (TP), and cyclohexyl (Cy)] have been synthesized and characterized. X-ray diffraction studies reveal that the cations of [Ni(P<sup>Bn</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)](BF<sub>4</sub>)<sub>2</sub> and [Ni(P<sup><i>n</i>‑Bu</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)](BF<sub>4</sub>)<sub>2</sub> have distorted trigonal bipyramidal geometries. The Ni(0) complex [Ni(P<sup>Bn</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>] was also synthesized and characterized by X-ray diffraction studies and shown to have a distorted tetrahedral structure. These complexes, with the exception of [Ni(P<sup>Cy</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)](BF<sub>4</sub>)<sub>2</sub>, all exhibit reversible electron transfer processes for both the Ni(II/I) and Ni(I/0) couples and are electrocatalysts for the production of H<sub>2</sub> in acidic acetonitrile solutions. The heterolytic cleavage of H<sub>2</sub> by [Ni(P<sup>R</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>(CH<sub>3</sub>CN)](BF<sub>4</sub>)<sub>2</sub> complexes in the presence of <i>p</i>-anisidine or <i>p</i>-bromoaniline was used to determine the hydride donor abilities of the corresponding [HNi(P<sup>R</sup><sub>2</sub>N<sup>Ph</sup><sub>2</sub>)<sub>2</sub>](BF<sub>4</sub>) complexes. However, for the catalysts with the most bulky R groups, the turnover frequencies do not parallel the driving force for elimination of H<sub>2</sub>, suggesting that steric interactions between the alkyl substituents on phosphorus and the nitrogen atom of the pendant amines play an important role in determining the overall catalytic rate