Alkyl isocyanides, binding

Any adequate theoretical treatment must also explain how iron-porphyrin systems can bind not only O2, but also CO, NO, alkyl isocyanides, and alkyl-nitroso moieties. A simple qualitative model presented by Wayland and coworkersconveniently summarizes ligand-binding geometries of cobalt and iron porphyrins. Although a reasonable quantitative theoretical consensus exists for 1 1 cobalt-dioxygen species, the same cannot be said yet for iron-dioxygen systems. 

Alkylidenes have been prepared by reduction of alkyli-dynes, by C H oxidative addition from alkyls, and by treatment of unsaturated metal clusters with diazoalkanes. In most instances, the alkylidene adopts a /r2-h coordination mode. However, alkylidenes with heteroatom substituents may also be found in terminal coordination modes. The latter are typically prepared by the Fischer-type carbene route (see Fischer-type Carbene Complexes) (sequential addition of nucleophilic and electrophilic alkylating agents to carbonyl or isocyanide ligands), by condensation of metal fragments with mono- or dimetallic carbene complexes, or by C-H activation of alkylamines. These heteroatom substituted carbenes may also bind in a p3-ri mode, as in (12). 

Despite being three-coordinate in the solid, (18) (20), (30), (31) or other compounds closely related to (g) (11), " " (19) and (30) will readily add a monodentate ligand such as Cl , CO, MeCN, or PPhs, or an endogenous alkene, imine, phenoxide, or thioether donor, to afford four-coordinate products. This affinity for a fourth donor mimics the ability of CO and isocyanides to bind to the Cuh site. " However, given the spectroscopic inertness of Cu(I), this affinity for exogenous ligands means that (8)-(31) cannot be assumed to be three-coordinate in solution, particularly in donor solvents such as MeCN. Compounds (8)-(ll) and copper(I) complexes of several other A-alkyl- A(2- pyrid-2-yl ethyl)amines, (13), ... 

The ligating isocyanide is activated toward electrophilic attack and aminocarbyne-type species are formed by reactions with acids or alkylating agents. The nucleophilicity of the isocyanide ligand in those electron-rich complexes is in contrast to its usual electrophilic character " reported for this ligand when it binds a metal ion in its normal or higher oxidation states. 

An alternative method for the synthesis of [(NHC)M] complexes is to bind ligands to a transition metal that can subsequently be converted into an NHC ligand. Using 2-azidophenyl isocyanide" " or 2-nitrophenyl isocyanide" as synthons for the unstable 2-amino isocyanide, diprotic benzannulated NHC-metal complexes were obtained, and easily alkylated to give A,A -disubstituted benzimidazolylidenes. This methodology was also applied to ruthenium-based catalysts (Scheme 3.6)." ... 

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