Isocyanide group

Given that the presence of three alkoxy chains in the phenyl group produces such a dramatic change in the properties of the material to the point that columnar mesophases are formed at room temperature, the structure of the aryl isocyanide ligand has been further modified to introduce more paraffinic chains, and examples of metallodendrimers containing monodendrons with an isocyanide group in the focal point, and its gold compound 9, have been reported [26]. 

A novel polysiloxane, containing the isocyanide group pendent to the backbone, has been synthesized. It is observed to react with the metal vapors of chromium, iron and nickel to afford binary metal complexes of the type M(CN-[P])n, where n = 6, 5, 4 respectively, in which the polymer-attached isocyanide group provides the stabilization for the metal center. The product obtained from the reaction with Fe was found to be photosensitive yielding the Fe2(CN-[P])q species and extensive cross-linking of the polymer. The Cr and Ni products were able to be oxidized on exposure of thin films to the air, or electrochemically in the presence of an electron relay. The availability of different oxidation states for the metals in these new materials gives hope that novel redox-active polymers may be accessible. 

We shall focus here on the synthesis of the isocyanide-containing polymer. Several reactions of the polymer with the metal vapors of Cr, Fe and Ni using a matrix-scale modeling technique, as well as synthetic-scale metal vapor methods, are then presented in order to demonstrate the reactivity of the isocyanide groups on the polymer. Finally, preliminary studies of the reactivity of the polymer-based metal complexes are described. 

Focusing on reactions using the Fluid Matrix Technique, we have studied the interaction of chromium vapor with 2 at 200 K (13). The resulting film was found to contain metal complexes encapsulated within the polymer in which the isocyanide group adopts a well-defined octahedral arrangement around the chromium center, i.e. a species of type Cr(CN-[P])g. Since characterization of this metal complex within the polymer is not trivial we shall develop the analysis in a little detail. 

This transformation proceeds through coordination of the isocyanide group to the ruthenium complex (structure 172), followed by insertion of the C-bound ruthenium into the benzylic C-H bond (intermediate 173). After ruthenium-mediated addition of the benzylic carbon to the isonitrile carbon and tautomerization, the desired product was obtained via elimination of the ruthenium complex. 

ATR-IR spectra of PDI, CN-QH4-NC, adsorbed on a Pd film deposited on one face of a ZnSe crystal exhibit three v(N=C) peaks, at 2170, 2120 and 1960 cm" [26]. The 2120 cm" band is shifted only 8 cm" as compared with that for the free molecule and is assigned to the uncoordinated isocyanide group. The 2170cm" band is close to that observed for PDI adsorbed on Au film and was assigned an T) adsorphon mode. The 1960 cm" band is characteristic of an adsorption mode in which the N=C bond is weakened. The authors indicate two possible modes a bent T mode (B in Figure 13.1) or a mode (C in Figure 13.1) in which... 

The isocyanide monolayer was formed by immersing the Cr slide in a 1 mM isocyanide solution under argon for 30 min. The redox wave, which is observed in cyclic voltammograms of the modified Cr electrode, proves that the 4-ferrocenyl-phenyl isocyanide is adsorbed on the metal surface. When a Cr electrode is exposed for 2 h to a solution of ferrocene, no redox wave was observed in the CV, which demonstrates that the isocyanide group is necessary for adsorption. No spectroscopic studies were performed on the adsorbed isocyanide. 

On Ni there is no evidence for T) adsorption proposed modes of adsorphon involve bonding of both the C and N atoms of the isocyanide group. 

VC13 treated with Bu NC produces mer-[VCl3(CNBu,)3]. The structure was determined by X-ray diffraction.182 The HNMR spectra in CDC13 showed that the free ligand exchanges more rapidly with the isocyanide tram to chloride than with the trans isocyanide groups. Therefore, the kinetic trans effect of chloride ion is larger than that of isocyanide. 

The generally accepted valence bond and molecular orbital (MO) approach to the bonding of metal isocyanides has been well described in Treichel s review (6), and has been used to rationalize (i) variations in IR stretching frequencies between bonded and nonbonded isocyanides, and (ii) the better Tt-acceptor qualities of aryl versus alkyl isocyanide groups (53,54). In valence bond theory the canonical forms involved in metal isocyanide bonds are... 

A 3C NMR study of the compounds Os3(CO), 2 CNR) (x= 1, 2, R = Me, C6H4OMe-p, Bu" jc = 1-4, R = Bu", Bu ) has found axial and equatorial isomers at low temperatures in solution, the ratio of which depends upon the size of the isocyanide group. Thus, for the monosubsti-tuted derivative, the axial isomer is present in significant concentrations in solution at low temperatures except for CNBu, for which an equilibrium mixture of the axial and equatorial isomer is obtained at — 60°C. As substitution increases the proportion of the equatorial isomer increases except for CNBu, for which in the tetrasubstituted derivative an all-axial isomer is found. Above 0°C these isomers rapidly interconvert (174). 

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