Acetonitrile solution stability study

Interaction of the nitrate ion with lanthanide(III) in acetonitrile solution was studied by conductivity, vibrational spectroscopy and luminescence spectroscopy. Bidentate nitrate with approximate C2V local symmetry was detected. FT-IR spectral evidence for the formation of [La(N03)5]2, where La = Nd, Eu, Tb and Er with coordination number 9.9 has been obtained [128]. Two inequivalent nitrate ions bound to lanthanides were detected by vibrational spectroscopy. The inequivalent nature varied with different lanthanides. For example three equivalent nitrate groups for La and Yb, one nitrate different from the other two for Eu ion were detected. Vibrational spectral data point towards strong La-NC>3 interaction in acetonitrile [129]. Stability constants for lanthanide nitrate complexes are given in Table 4.10. 

Polarographic studies gave no evidence for the existence of the bivalent oxidation states of selected actinides in acetonitrile solution. Only one wave corresponding to reduction of americium(iii) or curium(iii) to the zero-valent state was observed and experiments with berkelium(iii) and einsteinium(iii) failed to give conclusive results because of rapid radiolysis of the acetonitrile solution. A study of the electrochemical reduction of americium, thulium, erbium, samarium, and europium showed that the elements did assume the bivalent state with the actinide bivalent cations having a smaller stability than the lanthanides. The half-wave potential of nobelium was found to be —1.6 V versus the standard hydrogen electrode for the reaction... 

The data obtained for [2.2.2]cryptand in acetonitrile solutions were further investigated.479,480 Silver ions are strongly solvated by acetonitrile and a competition was found to exist between complexation of the ligand and the solvent. This was claimed to be predominantly responsible for the lower stability of Ag[2.2.2]+ in acetonitrile than in water and for the rapid decrease in the stability constant at low mole fraction of acetonitrile (xMccn)> This phenomenon was then studied by determining the rate of formation and dissociation of Ag[2.2.2]+ in acetonitrile-water mixtures.481... 

Detailed studies on the UV absorption spectra of alkenyl-A3-iodane 133 (X = Cl,Br,I) in acetonitrile solutions in the presence of tetrabutylammonium halides show the equilibrium formation of the iodate 134 (Sect. 2.1), in addition to the iodonium ion 132 coordinated by acetonitrile via hypervalent bonding. [Eq. (116)] [213]. The equilibrium constants are summarized in Table 5. Determinations of the equilibrium constants by UV spectra were carried out at a low concentration of the A3-iodane (<10 4 M), without considering the formation of the corresponding dimers. The magnitudes of the equilibrium constant K1 clearly decrease in the order Cl >Br >1, which reflects the differences in the stability of alkenyl-A3-iodane 133. 

The Ni and Pt complexes can also be incorporated into polymer films of quaternized poly(vinylpyridine) (PVP) and deposited onto the transparent electrode (84). Photocurrents are enhanced to microamps (pA), an increase that may be attributed to either the effect of immobilization of the complexes near the electrode surface or an increase of the excited-state lifetimes in the polymer matrix. However, the effective concentrations of the complexes in this study were much greater than for the acetonitrile solutions in their earlier work. The polymer films are not stable to continuous photolysis, and voltammograms of the films are quite sensitive to anions used in the supporting electrolyte. The system can be stabilized by using a polymer blend of PVP and a copolymer containing quaternary ammonium ion and including [Fe(CN)6]4- in the electrolyte solution (85). Upon irradiation of the visible MLCT bands of [M(mnt)2]2 (M = Ni, Pt), photocurrents are produced. The mechanism (Scheme 4) is believed to involve photooxidation of the metal bis(dithiolene) triplet state by the Sn02 electrode, followed by [Fe(CN)6]4 reduction of the monoanion, with completion of the ET cycle as ferricyanide, Fe(CN)6 3, diffuses to the other electrode and is reduced. 

Stable in methanol than in acetonitrile however, in both solvents, the sample solutions are stable in the refrigerator (4°C) for up to 50 hours. Solution light stability studies (dark and light chamber) in both solvents at ambient temperature were also performed in order to further determine whether the formation of the degradation products is photocatalyzed or generated at ambient temperature ( 25°C). 

Electron Transfer Processes - A study of the benzophenone/acetonitrile/t-butylamine system has shown that electron transfer occurs from the amine to the excited state benzophenone. Hydrogen abstraction processes within this system ultimately yield the CH2CN radical. When this species is generated in the presence of a diarylethene such as that shown in Scheme 1 then addition affords reasonable yields of the adducts (16). The mode of addition is dictated by the stability of the radical formed on addition to the ethene bond. Addition can also occur to alkenes such as (17) when the adduct (18) is obtained. SET processes also provide a novel synthetic path to 5,6-dihydro-4/f-l,2-oxazines. These reactions involve the DCA-sensitized transformations of the y,5-unsaturated oximes (19). The reactions are carried out in acetonitrile solution with irradiation through Pyrex for no longer than 30 min. This treatment yields the products (20) in reasonable yields. The likely mechanism for the process involves the formation of the intermediate (21) which cyclizes to yield the final products. The reaction is to some extent substituent dependent and when the oxime (19, R = H, R = R = R = Me) is irradiated under the same conditions for 3 h no reaction is... 

Therefore, much effort has been directed toward the synthesis of compounds capable of the cotransport of HCl across lipid bilayer membranes. For example, Sessler and coworkers have studied the anion-binding and transport properties of prodigiosin model compounds 16-20 (Figure 8). " Binding studies conducted in acetonitrile solution by using ITC showed that the monoprotonated iodide salts of compounds 16-20 bound chloride with stability constants between 1.1 x 10 and 8.8 x 10 M at 30 °C. The crystal structure of the HCl complex of compound 20 was elucidated, showing chloride bound to the protonated receptor by three hydrogen bonds in... 

The effect of acetonitrile on the kinetics of oxidation of ascorbic acid by Cu(II) and the ferrocenium ion in aqueous acetonitrile has been studied, as has the similar oxidation using the Cu(II)-tyrosine complex, for which a mechanism has been proposed. A reinvestigation of the oxidation of ascorbic acid by Fe(III) has shown catalysis by even small amounts of added chloride, 7 and the kinetics and mechanism of the oxidation of ascorbate by manganese pyrophosphate have been studied. The heat stability of aqueous solutions of ascorbic acid is increased witli reduced o gen concentrations, with Cu(II) and Fe(II) accelerating decomposition. ... 

---