Structure and Reactivity of NO/NO⁺/NO⁻Pincer and Porphyrin Complexes

Abstract

This review covers recent progress in the field of nitrosyl pincer and porphyrinate complexes, and how their reactivity depends on the redox state of the NO ligand. Synthetic methods, and the most significant structural information, spectroelectrochemical studies, and different reactivities of nitrosyl pincer complexes are presented. These include selective activation and functionalization of carbon-halogen bonds, catalysis for a variety of specific organic reactions, mediation of NO dispro-portionation and reversible coordination of solvents and weakly-coordinating anions. Regarding the porphyrinate complexes, the focus is on biorelevant nitroxyl (NO À /HNO) complexes, discussing pioneering studies along with preparation and common characterization methods. Their relative stability in different conditions is compared, and possible decomposition mechanisms are discussed in terms of their ability to act as biomimetic models for enzymatic systems.

Phosphine-NHC-Phosphonium Ylide Pincer Ligand: Complexation with Pd(II) and Unconventional P-Coordination of the Ylide Moiety

“…The growing interest in pincer complexes can be mainly attributed to their stability and structural variability, thus making these metal species essential today in various fields such as homogeneous catalysis, biological systems, and functional materials . Historically, the first pincers were prepared in the late 1970s based on an anionic central phenyl ring bearing two neutral pendant phosphine or amine donors.…”

Nitric Oxide Disproportionation Mediated by a Rh Complex with an Asymmetric Pincer Ligand: Spectroscopic Detection of a Dinitrosyl Intermediate

“…Dinitrosyl species are oftentimes proposed as reaction intermediates in NO⋅ disproportionation mechanisms, but they are seldom detected [1] . In fact, for pincer complexes, we only have knowledge of two structures reported to date, one with two equivalent NO⋅ ligands, [36] and another one with inequivalent NO + and NO − [26] .…”

“…Dinitrosyl species are oftentimes proposed as reaction intermediates in NO * disproportionation mechanisms, but they are seldom detected. [1] In fact, for pincer complexes, we only have knowledge of two structures reported to date, one with two equivalent NO * ligands, [36] and another one with inequivalent NO + and NO À . [26] They are also biologically relevant, since they have been proposed to function both as reservoirs and as carriers of nitric oxide in biological systems, particularly as iron complexes.…”

“…The combined use of both nitric oxide and pincer ligands is an incipient field of growing interest in coordination chemistry [1] . On the one hand, nitric oxide's great appeal as a ligand lies on its redox versatility, given its bond to the metallic center can be understood in three limiting descriptions: M z−1 NO + , M z NO⋅, M z+1 NO − .…”

“…The combined use of both nitric oxide and pincer ligands is an incipient field of growing interest in coordination chemistry. [1] On the one hand, nitric oxide's great appeal as a ligand lies on its redox versatility, given its bond to the metallic center can be understood in three limiting descriptions: M zÀ 1 NO + , M z NO * , M z + 1 NO À . In truth, the metal-NO bond's complex character is best described by the notation proposed by Enemark and Feltham: {MNO} n , where n accounts for the joint electrons in the d orbitals of the metal and in the π* orbitals of NO * , thus acknowledging the electronic delocalization and the possibility of intermediate configurations between the three limiting ones.…”