Electrocatalytic Hydrogen Production by [Ni(7P^Ph₂N^H)₂]²⁺: Removing the Distinction Between Endo- and Exo-Protonation Sites

Abstract

2 N H = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane), has been synthesized, and its electrochemical properties have been reported. The 7P Ph 2 N H ligand features an NH, ensuring properly positioned protonated amine groups (N−H + ) for electrocatalysis, regardless of whether protonation occurs exo or endo to the metal center. The compound is an electrocatalyst for H 2 production in the presence of organic acids (pK a range 10−13 in CH 3 CN), with turnover frequencies ranging from 160 to 780 s −1 at overpotentials between 320 and 470 mV, as measured at the potential of the catalytic wave. In stark contrast to [Ni(P Ph 2 N R′ 2 ) 2 ] 2+ (P Ph 2 N R′ 2 = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and other [Ni(7P Ph 2 N R′ ) 2 ] 2+ complexes, catalytic turnover frequencies for H 2 production by [Ni(7P Ph 2 N H ) 2 ] 2+ do not show catalytic rate enhancement upon the addition of H 2 O. This finding supports the assertion that [Ni(7P Ph 2 N H ) 2 ] 2+ eliminates the distinction between the endo-and exo-protonation isomers.

Combining theory and experiment in electrocatalysis: Insights into materials design

“…Homogeneous catalysts with high TOF avg have also been developed, though they typically require large overpotentials to reach appreciable current densities (Fig. 3B) (67)(68)(69).…”

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

“…283,294 In support of this proposal, Ni44 showed no change in activity upon H 2 O addition. 299 Conversely to the above examples, which all involve tertiary amines, Ni44 has secondary N-atoms. As such, it features both an endo - and exo -proton in the corresponding catalytic intermediate, and thus shows activity that is not influenced by external bases.…”

“…Avoiding inactive pinched- exo isomers by removing a pendant amine from the chelate rings of DuBois-type HECs has also been thoroughly explored. 283,293,294,299−301 Ni38 - Ni43 are coordinated by two 7-membered cyclic diphosphines featuring a single N-atom, with Ni38 and Ni39 showing CV-derived H 2 evolution rates of 100 000 s –1 in MeCN containing H 2 O and [(DMF)H] + . 283,294 These TOF H2 values are ∼100-fold greater than those achieved under comparable conditions with 8-membered analogues having two pendant-amines.…”

Electrochemical and Computational Study of the Reactivity of a Diiron Azadithiolate Complex towards Protons in the Presence of Coordinating Anions

“…endo vs. exo isomers). [40,41] Finally, as discussed earlier in the text, the voltammetric behavior of 1 in the presence of HCl (pK a ≈ 9.5) is very similar to that of the adt R derivatives in the presence of HBF 4 ·Et 2 O (pK a ≈ 3), suggesting rather an effect of coordinating anion (i.e. Cl -or BF 4 -) or added Et 2 O that comes with both HCl and HBF 4 than an effect of acid strength on the catalytic pathway.…”