Metal‐Metal Interactions in Bi‐, Tri‐ and Multinuclear Fe, Ru and Os N‐Heterocyclic Carbene Complexes and their Catalytic Applications

Abstract

N‐heterocyclic carbenes (NHCs) have been the focal point of advanced research on catalysis and bioorganometallic chemistry in the last decade. The field of NHC‐metal mediated catalysis has steadily grown over past years, commencing from the seminal discoveries on various C−C, C−N coupling and acceleration of C−H bond activation reactions. This review presents an overview of the work about the effects of NHC ligand field on metal‐metal contacts in Fe, Ru and Os metal NHC complexes. The influence of these contacts in making the supramolecular assemblies and their consequences on the reactivity patterns is reviewed. Correspondingly, the synergic effects between two metals leading to a unique reactivity pattern are correlated. The suitability and efficiency of various synthetic protocols used to prepare bi‐, tri‐ and multinuclear transition metal NHC complexes where metal‐metal contacts are possible, is discussed. Alternatively, this study on metal‐metal contacts in Fe, Ru and Os metal NHC complexes has great potential in the fields of homogeneous and heterogonous catalysis including the new designs for efficient hydrogen evolution electrocatalysts and oxidation and acylation catalysts. Rational ligand design leading to metal‐metal contacts by the appropriate functionalization of NHCs to investigate their applications in mentioned fields are thus fundamental to our efforts. Apart from their applications, the nature of the metal‐carbene carbon bond, nature of auxiliary ligand(s) and the influence of suitably functionalized NHC ligands led to generate advanced structural motifs have also been emphasized.

Enhancing Electrochemical Hydrogen Evolution Performance of N-Heterocyclic Carbene-Coordinated Palladium(II) Complexes with Conductive Carbon: Insights from Hydrogen Oxidation Reactions

“…N-Heterocyclic carbenes (NHCs) are particularly exceptional as ligands in stabilizing transition metal centers because of their binding characteristics. − The strong σ–donor and poor π–acceptor attributes of the NHCs render the Pd metal center highly stable ensuring longer catalyst lifetime and efficiency in Pd–NHC complexes . There are several benefits that NHCs offer as ligands through their various substitutions and structure tunability in designing efficient catalysts for specific applications. − These qualities of NHCs as ligands have enabled Pd–NHCs to be employed as alternatives to traditional Pd-based complexes for catalysis catered toward various cross-coupling reactions . In the field of electrochemistry, there is limited but notable amount of literature that explores Pd–NHCs as molecular electrocatalysts for CO 2 reduction applications with some of these reports showing selectivity for hydrogen generation. − It is worth mentioning that the reported Pd–NHC complexes have been used as homogeneous electrocatalysts, a method commonly used when evaluating molecular electrocatalysts, while there are several advantages offered by homogeneous electrocatalysis such as specificity and more detailed insights into the reaction mechanism.…”

Interfacial Heterojunction Electronic Modulation on RuSe2@NiFeLDH Heterostructures for Water Splitting

“…Due to this, the LDH volume reduces that decreases electrochemical activity . Fabricating LDHs with highly conducting transition-metal chalcogenides helps to a better catalyst for electrochemical water splitting. − Ruthenium belongs to platinum-related metals with high d-orbital electrons that provide ideal adsorption energy for oxygen and hydrogen. , Ruthenium costs 1/5th of Pt metal and exhibits a similar range of metal hydrogen bond strengths, resulting in a better catalyst for electrochemical water splitting. , When compared to Pt and Ir, Ru-based catalysts possess a lower cost and higher activity than Ir. , Se-based catalysts possess flexible coordination modes, better conductivity, and less ionization energy. Transition-metal-based selenides exhibit less resistivity and also help in accelerating the electrons in the conduction band that makes it a better catalyst for electrochemical water splitting .…”

“…10−15 Ruthenium belongs to platinum-related metals with high dorbital electrons that provide ideal adsorption energy for oxygen and hydrogen. 16,17 Ruthenium costs 1/5th of Pt metal and exhibits a similar range of metal hydrogen bond strengths, resulting in a better catalyst for electrochemical water splitting. 17,18 When compared to Pt and Ir, Ru-based catalysts possess a lower cost and higher activity than Ir.…”