Ligand Protonation Triggers H₂ Release from a Dinickel Dihydride Complex to Give a Doubly “T”‐Shaped Dinickel(I) Metallodiradical

Abstract

The dinickel(II) dihydride complex (1 K)o fap yrazolate-based compartmental ligand with b-diketiminato (nacnac) chelate arms (L À), providing two pincer-type {N 3 }binding pockets,has been reported to readily eliminate H 2 and to serve as amasked dinickel(I) species.Discrete dinickel(I) complexes (2 Na , 2 K)ofL À are now synthesized via adirect reduction route. They feature two adjacent T-shaped metalloradicals that are antiferromagnetically coupled, giving an S = 0g round state. The two singly occupied local d x 2 Ày 2 type magnetic orbitals are oriented into the bimetallic cleft, enabling metal-metal cooperative 2e À substrate reductions as shown by the rapid reaction with H 2 or O 2 .X-ray crystallography reveals distinctly different positions of the K + in 1 K and 2 K ,s uggesting as tabilizing interaction of K + with the dihydride unit in 1 K .H 2 release from 1 K is triggered by peripheral g-C protonation at the nacnac subunits,w hich DFT calculations showl owers the barrier for reductive H 2 elimination from the bimetallic cleft.

Bimetallic Nickel Complexes Supported by a Planar Macrocyclic Diphosphoranide Ligand

“…Thus the spatial and electronic relationship between the two metals is extremely important for MMC, and is dictated by the scaffolding ligands. Significant success has been attained with ligands such as expanded pincers, [15,19–24] fused mononucleating multidentate systems, [25–29] and macrocycles, [17,30–32] which can be either flexible or rigid (Figure 1). Rigid macrocyclic ligands typically offer trans reactive sites on either face of the macrocycle, such as in quintessential monometallic porphyrins and phthalocyanines.…”

Interplay between β-Diimino and β-Diketiminato Ligands in Nickel Complexes Active in the Proton Reduction Reaction

“…In this context, a recent report has demonstrated both metal–metal and metal−β-diketiminate ligand cooperative stoichiometric hydrogen evolution by a bimolecular dihydrido nickel complex in the presence of a proton source (Figure ). , Further, the dihydride derivative was stable enough to be isolated and characterized but only in the presence of an acid source, and thanks to the cooperative protonation of the BDK/BDI ligand, the complex was able to evolve H 2 gas. In contrast, in the case of complexes 1 2+ and 2 2+ reported here, the putative hydrido species 1 ′ + and 2 ′ + are too reactive to be isolated, and they rapidly transform into ( 1-H ) + and ( 2-H ) + , respectively.…”

“…First-row transition metal β-diketiminato complexes have been used for a wide variety of applications including stoichiometric and catalytic transformations such as functionalization of alkenes, cross-coupling reactions, and energy-related applications. − In addition, late transition metal complexes have been used for synthesizing low coordination metal centers mimicking the active site of metalloproteins for the conversion of organic substrates but also the activation of small molecules such as carbon dioxide or dinitrogen. , More recently, Meyer and coworkers have published a dinuclear bis­(β-diketiminato) complex bridged by a pyrazolate ring, which has been shown to be involved in the stoichiometric proton reduction reaction to produce hydrogen through a proposed metal–metal and metal–ligand cooperative two-electron reductive process. , In their work, they fully characterized the dihydride species Ni 2 H 2 (BDK) 2 in Figure (left) with two Ni­(II) centers, prepared from the corresponding bromido-bridged derivative in the presence of KHBEt 3 . The species Ni 2 H 2 (BDK) 2 converts to the corresponding bis­(β-diimino) derivative Ni 2 (BDI) 2 containing two three-coordinated Ni­(I) centers in the presence of an acid source, as demonstrated by a full range of spectroscopic and magnetic techniques.…”

“…(Left) Stoichiometric H 2 evolution from a dihydrido dinickel bis­(β-diketiminato) complex reported in the literature. , (Right) β-Diimino ligands containing two phosphine arms designed in this work as precursors to tetradentate β-diketiminato complexes.…”

“… 14 , 21 More recently, Meyer and coworkers have published a dinuclear bis(β-diketiminato) complex bridged by a pyrazolate ring, which has been shown to be involved in the stoichiometric proton reduction reaction to produce hydrogen through a proposed metal–metal and metal–ligand cooperative two-electron reductive process. 22 , 23 In their work, they fully characterized the dihydride species Ni 2 H 2 (BDK) 2 in Figure 1 (left) with two Ni(II) centers, prepared from the corresponding bromido-bridged derivative in the presence of KHBEt 3 . The species Ni 2 H 2 (BDK) 2 converts to the corresponding bis(β-diimino) derivative Ni 2 (BDI) 2 containing two three-coordinated Ni(I) centers in the presence of an acid source, as demonstrated by a full range of spectroscopic and magnetic techniques.…”