Silver complexes pseudohalides

Hydrogen cyanide (Table 15.1) is a colorless, flammable liquid or gas that boils at 25.7°C and freezes at minus 13.2°C. The gas rarely occurs in nature, is lighter than air, and diffuses rapidly. It is usually prepared commercially from ammonia and methane at elevated temperatures with a platinum catalyst. It is miscible with water and alcohol, but is only slightly soluble in ether. In water, HCN is a weak acid with the ratio of HCN to CN about 100 at pH 7.2, 10 at pH 8.2, and 1 at pH 9.2. HCN can dissociate into H+ and CN. Cyanide ion, or free cyanide ion, refers to the anion CN derived from hydrocyanic acid in solution, in equilibrium with simple or complexed cyanide molecules. Cyanide ions resemble halide ions in several ways and are sometimes referred to as pseudohalide ions. For example, silver cyanide is almost insoluble in water, as are silver halides. Cyanide ions also form stable complexes with many metals. 

The cyanide ion is called a pseudohalide ion because it behaves like Cl- in forming an insoluble, white silver salt, AgCN. In complex ions such as Fe(CN)63-, CN - acts as a Lewis base (Section 15.16), bonding to transition metals through the lone pair of electrons on carbon. In fact, the toxicity of HCN and other cyanides is due to the strong bonding of CN- to iron(III) in cytochrome oxidase, an important enzyme involved in the oxidation of food molecules. With CN attached to the iron, the enzyme is unable to function. Cellular energy production thus comes to a halt, and rapid death follows. 

Other coordination modes in pseudohalide complexes are comparatively rare. Amongst them, we note the structure of complex [AgLSCN] 0.25L (L = bipy), in which two silver ions are simultaneously bound with N atoms from NCS groups [166], The pseudohalide complexes with simultaneous different kinds of coordination of the NCS group are also rare. In particular, the complex compound [CuL(HL)2][Cu(L)(SCN)(p-NCS)], where LH is 2-dimethylaminoethanol, contains within its coordination sphere both kinds S-terminal (108) and -bridge (109) thiocyanate groups [178],... 

The azide ion is a good ligand, and it forms numerous complexes with metal ions. Chlorazide (C1N3) is an explosive compound prepared by the reaction of OCT and N3. As in the case of CN, the azide ion is a pseudohalide ion. Pseudohalogens are characterized by the formation of an insoluble silver salt, the acid H-X exists, X-X is volatile, and they combine with other... 

Dichlorosilicon phthalocyanine (XIX) is prepared from silicon tetrachloride and phthalonitrile in quinoline at 200°C 168,170). The blue-green crystals, which sublime readily at 430°C in vacuo, hydrolyze forming dihydroxysilicon phthalocyanine (XX) when refluxed with equal volumes of pyridine and aqueous ammonia (200). The corresponding difluorosilicon phthalocyanine is resistant to hydrolysis. Conversion of the chloride to the corresponding dicyanate, dithiocyanate, and diselenocyanate occurs upon reaction with the appropriate silver pseudohalide (178). The complexes are believed to involve nitrogen to silicon bonding in the case of the thiocyanate and selenocyanate. 

We reasoned that such a decarboxylation step could also be employed in a redox-neutral cross-coupling reaction with carbon electrophiles. On this basis, we drew up a catalytic cycle that starts with an oxidative addition of aryl halides or pseudohalides to a coordinatively unsaturated palladium(O) species f (Scheme 5). The more weakly coordinating the leaving group X, the easier should be its subsequent replacement by a carboxylate. At least for X = OTf, the palladium(ll) carboxylate h should form quantitatively, whereas for X = halide, it should be possible to enforce this step by employing silver or thallium salts as species g. The ensuing thermal decarboxylation of the palladium(ll) intermediate i represents the most critical step. Myers results indicated that certain palladium(ll) carboxylates liberate carbon dioxide on heating. However, it remained unclear whether arylpalladium (II) carboxylate complexes such as i would display a similar reactivity. If this were to be the case, they would form Ar-Pd-Ar intermediates k, which in turn are... 

A series of anionic monodentate A-donor ligands such as azido, isocyanato, isothiocyanato, and nitro groups have been used to form neutral pseudohalide complexes. They are usually obtained by exchange reactions between a chloride precursor and the appropriate sodium or silver salt of the ligand. Representative examples include the following ... 

One successful strategy for ligand substitution by ion exchange involves the use of halide or pseudohalide metal complexes as starting materials in reactions with silver salts that contain the anion that is to be introduced as a new ligand. Since the solubility of silver halides in nonpolar, aprotic solvents is low (see Table 3.1), ion exchange in solvents of poor donor ability typically occurs according to Eq. 5.1 ... 

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