Metal-coordinated isocyanide ligands

Carbene gold(III) with diamine ligand can coordinate a second metallic center, since the N -ligand contains a free N H 2 group and can react with a new gold isocyanide under the formation of double dinuclear carbenes [46] (32). 

Figure 1.12 suggests that for carbonyl complexes the HOMO is localized primarily on the metal centre, with only a modest contribution from oxygen orbitals. Thus by far the majority of reactions of metal carbonyls with electrophiles involve direct attack at the metal, with the carbonyl serving as a spectator ligand. If, however, the metal centre is (i) particularly electron rich and (ii) sterically shielded and the electrophile is hard (in the HSAB sense) and also sterically encumbered, then attack may occur at the oxygen. Thiocarbonyls (LM-CS) are stronger 71-acids than CO and the sulfur is both softer and more nucleophilic. Thus electrophilic attack at the sulfur of thiocarbonyls is more common if the metal centre is electron rich (vcs < 1200 cm-1). Similarly, coordinated isocyanides (CNR) are more prone to attack by electrophiles at nitrogen. This is noteworthy in the sense that free isocyanides are attacked by electrophiles at carbon (Figure 3.19). The resulting carbyne ligands will be discussed in Chapter 5.

The 1,3-dipolar addition of phenylazide to metal coordinated ligands affords carbodi-imide ligands. An example is the reaction of the bis-azidopalladium complex 38 with two equivalents of 2,6-dimethylphenyl isocyanide to give the bis-carbodiimide complex 39. ... 

The aminocarbenes derived from reactions of coordinated isocyanides are treated in Section 14 and bridging alkyhdenes arising from C-H activation by Re atoms are presented in Section 9. The recent boom of metal complexes derived from simple ligand substitution with the stabilized N-heterocychc carbenes has produced only a very limited number of Re complexes like the luminescent (48). ... 

Isocyanides, when bound to weak tt-donor metal centers, can also undergo addition of protic nucleophiles (HNu) to form aminocarbene M=C(Nu)NHR species. Hence, a parallelism of behavior is observed for nitriles and isocyanides. In both ligands, either the electrophilic or the nucleophilic addition occurs at the unsaturated atom at the jS-position that exhibits a higher reactivity than the metal coordinated terminal atom. 

Noncarbonyl ligands can participate more readily in the hydrogenation of the metal carbonyl as for RUjfCOljjfCNBu-t). Bubbling through the cluster in cyclohexane at reflux for 1 h yields HRu3(ju.3-HCNBu-t)(CO)g (52%) by addition of Hj to the coordinated isocyanide. These reactions require milder conditions than those used for the unfunctionalized RujfCOljj with Hj. 

Both 2-azido [184] and 2-nitrophenyl isocyanides [185] are suitable synthons for the generation of the freely unstable 2-aminophenyl isocyanide and they have been used (Fig. 23) in the template-controlled preparation of NH,NH-stabilized benzim-idazolin-2-ylidene ligands. Both phenyl isocyanides coordinate readily to transition metal centers. The isocyanide ligand in complex 65 reacts with PPhs and the... 

Just like the isoelectronic carbon monoxide, an isocyanide is an excellent ligand to metal ions. The chemistry of metal isocyanide complexes has been reviewed by Singleton and Oosthuizen. Only a few examples will be given here. Insertion of an isocyanide into a metal-carbon bond frequently occurs. It is not always clear whether the key step is electrophilic or nucleophilic attack on the coordinated isocyanide or whether the reaction is concerted. Insertion into metal-carbene and metal-carbyne complexes have been reviewed by Aumann. Coordination to the metal considerably affects the chemistry of the isocyanide. If the metal is electron-donating, as in nitrogenase-like centres, the coordinated isocyanide is apt to electrophilic attack at nitrogen cf. Section III. 

Table 10. IR-, TG/DTA- and room temperature conductivity (powder, 1 kbar) data of mononuclear and polynuclear metal complexes with macrocyclic ligands and bisaxially coordinated isocyanides...

The IR spectra of all isocyanide complexes [MacML] and MacML2 show the intense stretching frequency of the isocyanide group at about 2080-2150 cm (see Tables 10 and 11). The shift of this absorption by changing from free to metal coordinated ligand is attributed to the or-donor and n-acceptor abilities of the metal ligand bond. The strength of the jt-acceptor bond... 

The reaction of alkoxyarylcarbene complexes with alkynes mainly affords Dotz benzannulated [3C+2S+1C0] cycloadducts. However, uncommon reaction pathways of some alkoxyarylcarbene complexes in their reaction with alkynes leading to indene derivatives in a formal [3C+2S] cycloaddition process have been reported. For example, the reaction of methoxy(2,6-dimethylphenyl)chromium carbene complex with 1,2-diphenylacetylene at 100 °C gives rise to an unusual indene derivative where a sigmatropic 1,5-methyl shift is observed [60]. Moreover, a related (4-hydroxy-2,6-dimethylphenyl)carbene complex reacts in benzene at 100 °C with 3-hexyne to produce an indene derivative. However, the expected Dotz cycloadduct is obtained when the solvent is changed to acetonitrile [61] (Scheme 19). Also, Dotz et al. have shown that the introduction of an isocyanide ligand into the coordination sphere of the metal induces the preferential formation of indene derivatives [62]. 

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