The Coordination Chemistry of Multidentate Isocyanide Ligands

Abstract

In spite of the excellent ligation properties of isocyanides, until a few years ago there was only a small number of known multidentate ligands of this type. One of the reasons for this lack of interest, when compared to monodentate isocyanides, was the linear arrangement of the M-C=N-R group, which usually inhibits the formation of mononuclear chelate complexes and leads to the formation of multinuclear or polymeric metal complexes. In these, the multidentate ligand acts in a monodentate Fdshion towards each metal atom. Only recently has a series of polyisocyanides with large ligand backbones been synthesized successfully. Bidentate isocyanides can bridge two metal atoms or react to give chelates with only one metal center. Tripodal ligands form mono-or binuclear complexes, in which the largest organometallic rings observed to date occur (up to 36 atoms). This class of ligands promises to be interesting for the synthesis of stable, diagnostically important technetium complexes of the type [Tc(CNR),]'. There also appear to be applications for tripodal isocyanides in catalysis. A facial, chiral Cr(CNR*), unit might be able to catalyze the hydrogenation or isomerization of prochiral double bonds. It is even possible to bind triisocyanides with suitable backbones to carbonyl trimetal clusters, thereby stabilizing them, or making selective cluster formation possible. Coordinated isocyanides can be transformed readily into carbene ligands, which, in the future, could lead to complexes with polycarbene ligation.

A Molybdenum(0) Isocyanide Analogue of Ru(2,2′‐Bipyridine)32+: A Strong Reductant for Photoredox Catalysis

“…Compared to a-diimine ligands such as 2,2'-bipyridine, there has been surprisingly little prior work on chelating isocyanide ligands, [18] particularly with regard to luminescent metal complexes. [19] We prepared the new 2,2''-diisocyano-3,5,3'',5''-tetramethyl-1,1':3',1''-terphenyl (CNAr 3 NC) ligand and reacted it with Mo(THF) 2 Cl 4 in the presence of Na/Hg to obtain the Mo(CNAr 3 NC) 3 complex shown in Figure 1 (pages S2-S5 in the Supporting Information).…”

Excited-State Relaxation in Luminescent Molybdenum(0) Complexes with Isocyanide Chelate Ligands

“…complexes with monodentate arylisocyanide ligands are very strong photoreductants, capable, for example, of reducing anthracene to its radical anion form. Many different kinds of metal complexes with isocyanide ligands have been explored over the past few decades [11][12][13][14][15], but metals with the d 6 or d 10 electron configurations are unique in their ability to show luminescence from a 3 MLCT excited state.Whilst structurally more flexible, multidentate isocyanide chelate ligands had been known for some time [16], we found that chelating diisocyanide ligands based on a m-terphenyl backbone permit the synthesis of homoleptic tris(diisocyanide) complexes of Cr 0 and Mo 0 that luminesce from 3 MLCT excited states (Figure 1) [17]. The molecular and the electronic structures of these compounds are reminiscent of Fe II and Ru II polypyridine complexes, which have been investigated extensively in the past.…”

“…Whilst structurally more flexible, multidentate isocyanide chelate ligands had been known for some time [16], we found that chelating diisocyanide ligands based on a m-terphenyl backbone permit the synthesis of homoleptic tris(diisocyanide) complexes of Cr 0 and Mo 0 that luminesce from 3 MLCT excited states (Figure 1) [17]. The molecular and the electronic structures of these compounds are reminiscent of Fe II and Ru II polypyridine complexes, which have been investigated extensively in the past.…”

An Alternative Approach to PEPPSI Catalysts: From Palladium Isonitriles to Highly Active Unsymmetrically Substituted PEPPSI Catalysts

“…The pioneering contributions from the groups of Minghetti and Bonati,14 as well as Hahn,15 on the synthesis of metal carbenes starting from metal‐coordinated isonitriles, inspired us to develop an alternative strategy for the synthesis of Pd–PEPPSI complexes. Preparing NHC ligands through an isonitrile strategy not only offers a route to unsymmetrically substituted NHC ligands, but also avoids a stepwise pre‐synthesis of the corresponding imidazolium salts.…”