Nickel Complexes of Pyridine‐Functionalized N‐Heterocyclic Carbenes: Syntheses, Structures, and Activity in Electrocatalytic Hydrogen Production

Siyuan Luo,Maxime A Siegler,Elisabeth Bouwman

doi:10.1002/ejic.201600917

Siyuan Luo, Maxime A Siegler + Show 1 more

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https://doi.org/10.1002/ejic.201600917

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Abstract

Two bis(benz)imidazolium salts alkylated with pyridyl side arms, H2L1Br2 (1,2‐bis{[1‐(2‐pyridylmethyl)benzimidazolium‐3‐yl]methyl}benzene bromide) and H2L2Br2 (1,2‐bis{[1‐(2‐pyridylmethyl)imidazolium‐3‐yl]methyl}benzene bromide) have been prepared and were used as precursors in the synthesis of novel nickel compounds of N‐heterocyclic carbenes (Ni‐NHCs). The four Ni‐NHC complexes [Ni(L1)Br]Br (1a), [Ni(L1)](PF6)2 (1b), [Ni(L2)]Br2 (2a) and [Ni(L2)](PF6)2 (2b) were isolated and characterized by various methods, and the X‐ray crystal structures of 1a, 2a and 2b are reported. The nickel ion in 1a is in a square‐pyramidal geometry with one of the bromide ions in the apical position, the nickel ions in 2a and 2b are in square‐planar geometries. The compounds of the ligand with an imidazole‐based carbene revealed much higher activity in electrocatalytic proton reduction and better acid tolerance, although their overpotentials are higher than those of the benzimidazole‐based compounds. The presence of bromide ions has an adverse effect on the redox potentials as well as the overpotentials for proton reduction. Complex 2b, having the most planar coordination geometry, appeared to have the highest catalytic efficiency for proton reduction in DMF (ic/ip = 50, kobs = 490 s–1 at 0.1 V/s) when using acetic acid as the proton source. To the best of our knowledge, this is the first report of Ni‐NHC complexes that are active in electrocatalytic proton reduction.

Highlights

We report four novel nickel compounds of N-heterocyclic carbenes (Ni-N-heterocyclic carbene (NHC)) complexes obtained with the ligand precursors H2L1Br2 and H2L2Br2, which are based on bis(benz)imidazolium salts bridged by a xylyl linker (Scheme 1)
The low-spin character of the NiII ion in all compounds is evidenced by their diamagnetic 1H NMR spectra, indicating that the square-planar geometry of the Ni centers is retained in solution
In this paper we report four novel pyridine-functionalized NiNHC complexes and their redox and electrocatalytic properties

Summary

Introduction

Dihydrogen is an environmentally friendly energy carrier as upon combustion it only produces H2O.[1,2] In order to enable the establishment of a society based on dihydogen as an energy source, for many years researchers have focused on the search for efficient proton-reduction catalysts, notably for catalysts that are not based on noble metals.[3,4,5,6,7] In the field of bioinorganic chemistry the synthesis of structural models of hydrogenases is a common strategy to devise molecular catalysts for proton reduction.[8,9] the so-called “functional models” of hydrogenases, which do not exhibit exactly similar structures, appear to result in more active and more stable catalytic systems. The N-heterocyclic carbene (NHC) ligand is a two-electron donor, with strong binding properties comparable to that of phosphane ligands.[14] Ni-NHC complexes have found various applications in organometallic chemistry,[15,16,17,18,19,20,21] the electrocatalytic properties of this kind of compounds so far mostly have been neglected. Complexes reported in this paper (linker = xylyl) and related compounds previously reported (with linker = propyl[26] or methylquinoxaline[27]). We have studied their redox properties and electrocatalytic ac- diethyl ether).

Results and Discussion

Conclusions

Synthetic Procedures