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 H₂ production, [Ni­(8P^Ph₂N^C₆H₄Br)₂]­(BF₄)₂, 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^Ph₂N^C₆H₄Br) has been synthesized and characterized. X-ray diffraction studies reveal that the cation of [Ni­(8P^Ph₂N^C₆H₄Br)₂(CH₃CN)]­(BF₄)₂ has a distorted trigonal bipyramidal geometry. In CH₃CN, [Ni­(8P^Ph₂N^C₆H₄Br)₂]²⁺ is an electrocatalyst for reduction of protons, and it has a maximum turnover frequency for H₂ production of 800 s^–1 with a 700 mV overpotential (at <i>E</i>_cat/2) when using [(DMF)­H]­OTf as the acid. Addition of H₂O to acidic CH₃CN solutions of [Ni­(8P^Ph₂N^C₆H₄Br)₂]²⁺ results in an increase in the turnover frequency for H₂ production to a maximum of 3300 s^–1 with an overpotential of 760 mV at <i>E</i>_cat/2. Computational studies carried out on [Ni­(8P^Ph₂N^C₆H₄Br)₂]²⁺ 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 H₂ production

Studies of a Series of [Ni(P^R₂N^Ph₂)₂(CH₃CN)]²⁺ Complexes as Electrocatalysts for H₂ Production: Substituent Variation at the Phosphorus Atom of the P₂N₂ Ligand

A series of [Ni(P^R₂N^Ph₂)₂(CH₃CN)](BF₄)₂ complexes containing the cyclic diphosphine ligands [P^R₂N^Ph₂ = 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^Bn₂N^Ph₂)₂(CH₃CN)](BF₄)₂ and [Ni(P^{<i>n</i>‑Bu}₂N^Ph₂)₂(CH₃CN)](BF₄)₂ have distorted trigonal bipyramidal geometries. The Ni(0) complex [Ni(P^Bn₂N^Ph₂)₂] 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^Cy₂N^Ph₂)₂(CH₃CN)](BF₄)₂, all exhibit reversible electron transfer processes for both the Ni(II/I) and Ni(I/0) couples and are electrocatalysts for the production of H₂ in acidic acetonitrile solutions. The heterolytic cleavage of H₂ by [Ni(P^R₂N^Ph₂)₂(CH₃CN)](BF₄)₂ 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^R₂N^Ph₂)₂](BF₄) complexes. However, for the catalysts with the most bulky R groups, the turnover frequencies do not parallel the driving force for elimination of H₂, 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