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

Abstract

PF 6 ) 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

Transition Metal Compounds of Pyridine‐Amide‐Functionalized Carbene Ligands: Synthesis, Structure, and Electrocatalytic Properties in Proton Reduction

“…Combined with earlier results, we believe that the presence of pendant pyridine groups indeed facilitates proton transfer in solution. Nevertheless, the large overpotentials and rather poor catalytic activity of the compounds described in this paper lead us to conclude that the carbene ligands are too electron‐donating to be applied as auxiliary ligands for HER catalysts.…”

“…Such NHC ligands functionalized with pyridine groups have been used to synthesize nickel and palladium complexes that were found to be efficient catalysts for the Kumada cross‐coupling and Heck‐type coupling reactions . Nevertheless, so far metal‐NHC complexes have received little attention regarding their potential electrochemical properties and activity in electrocatalysis …”

Rational Design of Penta-Coordinated Nickel(II) Dicarbene Complexes

“…11,12 In general, most Pd II NHC complexes are square-planar, but its nickel counterparts were also found to form penta-and hexa-coordinated compounds in addition to the common square-planar ones. 13,14 Among these, pentacoordinated Ni II −NHC complexes occupy a special position, because they are the rarest and can adopt square-pyramidal (SPy) or trigonal-bipyramidal (TPB) structures. Despite a few reports on such rare structures, it is still unclear how the design of the NHC ligand will influence the structure of the respective complex.…”

“…Notably, a similar nickel complex with more flexible pyridyl arms showed a SPy geometry with more regular bond parameters. 14 Conductivity Study. Dissolution of all the monocationic nickel(II) complexes should give rise to 1:1 electrolytes.…”

“…For example, Pd II –NHC complexes are easily prepared using an azolium salt and Pd­(OAc) 2 , while the use of Ni­(OAc) 2 requires significantly harsher and/or solvent-free conditions or ionic liquids . As for their structures, dihalido-bis­(NHC) complexes of palladium­(II) often adopt both cis and trans geometries, , while those of nickel are almost exclusively trans configured. , The use of cis -chelating methylene-bridged diNHCs also does not lead to the neutral dihalido-diNHC complexes, but formation of cationic bis­(chelate) nickel­(II) complexes was observed instead. , In general, most Pd II NHC complexes are square-planar, but its nickel counterparts were also found to form penta- and hexa-coordinated compounds in addition to the common square-planar ones. , Among these, penta-coordinated Ni II –NHC complexes occupy a special position, because they are the rarest and can adopt square-pyramidal (SPy) or trigonal-bipyramidal (TPB) structures. Despite a few reports on such rare structures, it is still unclear how the design of the NHC ligand will influence the structure of the respective complex.…”