Two Pathways for Electrocatalytic Oxidation of Hydrogen by a Nickel Bis(diphosphine) Complex with Pendant Amines in the Second Coordination Sphere

Abstract

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.

Molecular Electrocatalysts for Oxidation of Hydrogen Using Earth-Abundant Metals: Shoving Protons Around with Proton Relays

“…Scheme 2 shows the synthesis of and abbreviations used for P 2 N 2 ligands. 25 Instead, the Ni II complex is reduced by H 2 to give a Ni 0 complex, and …”

Role of pendant proton relays and proton-coupled electron transfer on the hydrogen evolution reaction by nickel hangman porphyrins

“…Studies that explore the interplay between these factors are crucial to designing catalytic reactions of high efficiency. Along these lines, the incorporation of proton relays in the second coordination sphere of molecular catalysts has emerged as a useful tool in optimizing PCET transformations (23)(24)(25)(26)(27)(28)(29). We have focused on the synthesis and mechanistic investigation of a class of metalloporphyrin molecules attached to a xanthene backbone bearing a carboxylic acid functional group-"hangman" porphyrins (1-M, Scheme 1) (23,30,31 (35).…”

X-ray Crystallographic, Multifrequency Electron Paramagnetic Resonance, and Density Functional Theory Characterization of the Ni(PCy2NtBu2)2n+Hydrogen Oxidation Catalyst in the Ni(I) Oxidation State

“…29 Butyronitrile and CD 3 CN were stored under nitrogen over activated 4 Å molecular sieves. The compounds 1 18 and 1 [BF 4 ] 2 27 were prepared by standard procedures. 1 H- and 31 P{ 1 H} NMR spectra were recorded at room temperature with a Bruker DRX 400 NMR spectrometer.…”

“…These details are particularly rich for catalyst intermediates in the Ni(II) and Ni(0) oxidation states, which are amenable to study by multinuclear NMR spectroscopic methods. 18 …”

“…15,18,19 In the catalytic oxidation of hydrogen, the one-electron oxidation of [Ni(P R 2 N R′ 2 ) 2 ] + compounds produces the [Ni(P R 2 N R′ 2 ) 2 ] 2+ species that coordinate dihydrogen, while in catalytic proton reduction, [Ni(P R 2 N R′ 2 ) 2 ] + species initiate the cycle by binding the first proton. Because [Ni(P R 2 N R′ 2 ) 2 ] + compounds are paramagnetic (d 9 electron configuration), their structures and properties are not fruitfully probed by NMR spectroscopic techniques.…”