Acetonitrile, complexes with metal

Displacement of weakly coordinating ligands is also very effective. Common ligands that form weakly stabilized complexes with metals include acetonitrile, ethylene, and other mono- and diolefins. The metal nucleophile may be a metal carbonyl anion, a metal hydride, or a neutral, low-valent metal complex ... 

In order to prepare a homoleptic solvento-complex by metal oxidation in a non-aqueous medium, the reduction products must not compete with the solvent molecules as ligands. Nitrosonium perchlorate, N(DC104, is a suitable oxidant. 4,15 Reactions have been performed in acetonitrile, nitromethane and ethylacetate. In acetonitrile this oxidant converts copper powder into the copper(I) and copper(II) homoleptic solvento-complexes, [Cu(MeCN)4][C104] and [Cu(MeCN)4][C104]2. On boiling an acetonitrile solution of these complexes with metallic copper, reduction of the copper(II)... 

The work of Hyatt on cyclotriveratrylene—derived octopus molecules contrasts with this. Of course, these species have the advantage of ligand directionality absent in the benzene-derived octopus molecules. Except for the shortest-armed of the species (i.e., n = 1), all of the complexing agents (i.e., n = 2—4) were capable of complexing alkali metal cations. Synthesis of these species was accomplished as indicated below in Eq. (7.7). These variations of the original octopus molecules were also shown to catalyze the reaction between benzyl chloride and potassium acetate in acetonitrile solution and to effect the Wittig reaction between benzaldehyde and benzyltriphenylphos-phonium chloride. 

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]. 

It is expected, therefore, that the cyclic hexamer also exhibits a characteristic tendency to complex with cations. In fact, the addition of an acetonitrile solution of metal thiocyanates to a solution of the cyclic hexamer in the same solvent shifted the carbonyl absorption to a lower wave number46,52 The shift values depended upon the kind of metal ions present, and the largest shift value of 40 cm-1 was observed for barium thiocyanate (molar ratio of Ba2+/hexamer = 10). In addition to the shift of the carbonyl absorption, the intensities of the C—O-C stretching vibrations around 1200 cm-1 varied appreciably. 

The donor properties of N3P3CI6 appear to be too weak to allow complex formation with metal halides, but it has been reported that complex formation between N3P3Cl5 NHBu" and Cu" or Co" chlorides in acetonitrile solutions can be detected by u.v. spectroscopy. Attempts to isolate the complexes were unsuccessful. 

The short Au Ag contacts may be due to some degree of metal-metal bonding which means that these were the first reported Au—Ag bonds. The complexes with acetone and acetonitrile shown are also the first reported examples in gold chemistry in which the pentafiuorophenyl ligands act as a bridge between Au and Ag centers. This type of behavior is generally more common in Pt chemistry. 

The l-(7V-methyl)benzimidazolyl-3-benzimidazolyl-2-oxapropane ligand (107) shows coordination through the N20 donor set of the ligand, a water molecule and an acetonitrile to give a five-coordinate distorted trigonal-bipyramidal metal center.123 This contrasts with the behavior of the related thiapropane ligand that forms bidentate complexes with zinc. 

An enormous variety of solvates associated with many different kinds of compounds is reported in the literature. In most cases this aspect of the structure deserved little attention as it had no effect on other properties of the compound under investigation. Suitable examples include a dihydrate of a diphosphabieyclo[3.3.1]nonane derivative 29), benzene and chloroform solvates of crown ether complexes with alkyl-ammonium ions 30 54>, and acetonitrile (Fig. 4) and toluene (Fig. 5) solvates of organo-metallic derivatives of cyclotetraphosphazene 31. In most of these structures the solvent entities are rather loosely held in the lattice (as is reflected in relatively high thermal parameters of the corresponding atoms), and are classified as solvent of crystallization or a space filler 31a). However, if the geometric definition set at the outset is used to describe clathrates as crystalline solids in which guest molecules... 

Metal dithiophosphinato complexes are usually prepared by metathesis of metal halides with alkali metal or ammonium dithiophoshinates, but can also be conveniently prepared by reactions of /i .v(thiophosphinyl)disul fanes, R2(S)PSSP(S)R2, with metal species.87 The electrochemical oxidation of metals in acetonitrile solution, in the presence of diphenylphosphine and sulfur affords M(S2PPh2)2 (M = Co, Zn, Cd),88 but this is not a preparative method. 

In many instances it is not necessary to isolate the acetonitrile complex or to carry out the reaction in acetonitrile. The use of amine oxide as a means of displacing carbonyl groups in metal carbonyls is well documented, and reaction proceeds smoothly with the carbonyl in the presence of a variety of ligands—e.g., ethylene or pyridine—to yield the monosubstituted derivatives. The advantage of the acetonitrile adducts is the stability of the compounds and the reactivity of the amine oxide toward acidic ligands. 

The non-cyclic ethers E-2 (Figure 10.26), with two pyrenes linked at both ends of the chain, show strong intramolecular excimer formation. Addition of alkaline earth metal ions leads to an increase in monomer emission at the expense of the excimer band. The helical structure of the 1 1 complexes is supported by NMR spectra. Thanks to the pseudocyclic structure, the stability constants of the complexes with Ca2+, Sr2+ and Ba2+ in acetonitrile are quite high (106-107 for n — 5), but the selectivity is poor as a consequence of the flexibility of the oxyethylene chain. 

Fig. 10. NMRD curves of free Gd (O) and its complexes with calix[4]arenes2(0) (at the maximum of the relaxivity titration curve), 3 ( ) (at the maximum of the relaxivity titration curve), 4 (A) (ligand to metal ratio = 2) in anhydrous acetonitrile at 25° C (63,66,67).

Owing to the high Lewis acidity the group 14 organometallic cations are polymerization catalysts par excellence. so Silanorbonyl cations and triethylsilyl arenium have been shown to be efficient catalysts for metal-free hydrosilylation reactions. Chiral silyl cation complexes with acetonitrile have been applied as cata -lysts in Diels Alder-type cyclization reactions °792 intramolecularly stabilized tetracoordinated silyl cations have been successfully used as efficient catalysts in Mukaiyama-type aldol reactions. 

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