Designing Catalysts for Nitrene Transfer Using Early Transition Metals and Redox-Active Ligands

Abstract

In this Forum Article, we discuss the use of redox-active pincer-type ligands to enable multielectron reactivity, specifically nitrene group transfer, at the electron-poor metals tantalum and zirconium. Two analogous ligand platforms, [ONO] and [NNN], are discussed with a detailed examination of their similarities and differences and the structural and electronic constraints they impose upon coordination to early transition metals. The two-electron redox capabilities of these ligands enable the transfer of organic nitrenes to tantalum(V) and zirconium(IV) metal centers despite formal d(0) electron counts. Under the correct conditions, the resulting metal imido complexes can participate in further multielectron reactions such as imide reduction, nitrene coupling, or formal nitrene transfer to an isocyanide.

Redox non-innocence permits catalytic nitrene carbonylation by (dadi)TiNAd (Ad = adamantyl)

“…The RNI of (dadi) n is implicit in the failure of CO to compete with THF in binding to titanium, 11–13 rendering it capable of catalyzing carbonylation reactions. Scheme 4 illustrates a plausible mechanism for the carbonylation of the nitrene derived from adamantyl azide (>20 turnovers) as catalyzed by (dadi)TiNAd ( 2 NAd).…”

“…Redox non-innocent (RNI) ligands 1–20 like dadi m can support catalytic activity 11–17 by expanding the redox capability of a system, essentially electronically buffering the transition metal center. As Fig.…”

Combination of Scanning Probe Microscopy and Coordination Chemistry: Structural and Electronic Study of Bis(methylbenzimidazolyl)ketone and Its Iron Complex

“…3,7−9 This expands the scope of reactions that can be catalyzed using affordable and nontoxic first-row transition metals. 10−13 Some examples of NILs with great potential are bis(imino)pyridines, 8,14 bis(amidophenolates), 15 bis(amido-phenyl)amides, 16,17 formazanates, 18 and catecholates. 19 Analytical and theoretical studies by Chirik et al have allowed the assignment of the oxidation and spin state of the metal centers in complexes bearing a bis(imino)pyridine ligand and have led to a proposed mechanism for catalytic olefin hydrogenation using such Fe complexes as catalysts.…”

Metal effects on ligand non-innocence in Group 5 complexes of the redox-active [ONO] pincer ligand

“…1. 2,11 Notably the electronic structure of the S = 1/2 complex (ONO) TaCl 3 could be interrogated by structural, spectroscopic, and electrochemical techniques, providing a concise view of the metal-ligand interaction. This discovery prompted us to pursue analogous niobium and vanadium complexes of the (ONO) ligand platform in order to evaluate how these changes to the central metal ion would influence the overall properties of the redox-active ligand complex.…”