Transition metal complexes with acetonitrile

In the case of transition metal complexes with large g anisotropy in disordered matrices, mw frequencies <9.4 GHz are sometimes preferable, because local heterogeneities (strain) of the matrix lead to a distribution of the principal values of the g- and A-tensors g- and A-strain) and thus to field-dependent line broadening. Such a situation is illustrated in Fig. 11 for Cu(II) in Nation perfluorinated ionomers swollen by acetonitrile the line width of the parallel components was measured at four mw frequencies in the range 1.2-9.4 GHz, and the narrowest line widths were detected for the two low-field lines of the parallel quartet at C band (4.7 GHz) and L band (1.2 GHz). In this way, clear superhyperfine splittings from nuclei were resolved, in addition of course to the hyperfine splittings from Cu(ll). 

The free electron pair(s) in the concave pyridines 3 (Table 1), 13 (s. Scheme 3) and 29 (s. Scheme 5) and especially in the concave 1,10-phenanthrolines 11 (s. Scheme 2) and 21 (Structures 3) are not only able to bind a proton, they may also be used to coordinate a metal ion. For concave 1,10-phenanthrolines 11 and 21, transition metal complexes 87 (Structure 11) have already been generated [18, 55]. They form readily in acetonitrile solution with binding constants of 10 10 and larger. Of great importance is the nature of the chains X in the concave 1,10-phenanthrolines 21 (Structures 3). Pure aliphatic chains lead to smaller association constants than polyether chains. 

Many transition-metal complexes have been widely studied in their application as catalysts in alkene epoxidation. Nickel is unique in the respect that its simple soluble salts such as Ni(N03)2 6H20 are completely ineffective in the catalytic epoxidation of alkenes, whereas soluble manganese, iron, cobalt, or copper salts in acetonitrile catalyze the epoxidation of stilbene or substituted alkenes with iodosylbenzene as oxidant. However, the Ni(II) complexes of tetraaza macrocycles as well as other chelating ligands dramatically enhance the reactivity of epoxidation of olefins (90, 91). 

A phosphenium cation reacts with a transition metal complex having an appropriate leaving ligand such as carbonyl, phosphine, and acetonitrile to generate a cationic phosphenium complex [Eqs. (10)-( 14)].2,26-29 This reaction seems to be the simplest method for the preparation of a cationic phosphenium complex. The drawback of this method is that known stable phosphenium cations are limited in number. 

Photolysis (see Photochemistry of Transition Metal Complexes) of a mixture of CpRh(C2H4)2 and hexamethylbenzene in hexane solution forms a mixture of (183) and (184). Analogs are similarly formed with biphenyl under the same conditions. The tetrameric borole complex [Rh(/u-3-I)( -C4H4BPh)]4 is converted to [( ] -C4H4BPh)Rh(MeCN)3]BF4 by halide abstraction with AgBF4. The acetonitrile can be... 

Kinetic data for acetonitrile exchange with transition metal complexes... 

Acetonitrile is a convenient solvent in which to study, by pulse radiolysis, the one-electron reduction of transition metal complexes that are not stable in water or hy-droxylic media. For example, the tantalum compound [Ta2Cl6(4-methylpyridine)4] has been shown to be reduced by CH3CN with A = 1.2 x 10 dm mol s [24], The absorption spectrum of the product shows the characteristic features of d-d transitions and it was suggested that the added electron is delocalized over the double-bonded Ta=Ta moiety. Another example is the radiolytic reduction of the vanadium(III) complex [VCl3(y-pic)3], where y-pic is 4-methylpyridine, to the derivative via the V complex [25]. It was shown by pulse radiolysis that the electron adduct of the complex decayed in a first-order process with k= 1.3 x 10 s which is thought to involve loss of Cl". The intermediate V complex had a... 

Due to the low nucleophilicity of the lone pair, tertiary bismuthines have found little use in the construction of onium salts and transition metal complexes. Trimethylbismuthine reacts with methyl triflate in acetonitrile to yield tetramethylbismuthonium triflate, which has so far been the only known example of tetraalkylbismuthonium salts [94AG(E)976]. Some transition metal complexes coordinated by tertiary bismuthine have been reported. They are described in Section 2.4. 

A transformation showing enhancement of the reactivity of phenol through transition metal complexation occurs in the reaction of [Os(NH3)s(fi -phenol]-(OTO2 with maleic anhydride in acetonitrile over 20 hours at ambient temperature followed by recovery of the product, dimethyl (4-hydroxyphenyl)succinate in 85% yield by simple ethereal precipitation and removal of the osmium by refluxing in acidic methanol (ref.39). These last two examples illustrate the versatility of the appropriately modified phenolic structure to function either in a nucleophilic or in an electrophilic manner. 

The oxidation of OH by [Fe(CN)6] in solution has been examined. Application of an electrical potential drives the reaction electrochemically, rather than merely generating a local concentration of OH at the anode, as has been suggested previously, to produce both O and [Fe(CN)6] in the vicinity of the same electrode. With high [OH ] or [Fe(CN)6] /[Fe(CN)6] ratio, the reaction proceeds spontaneously with a second-order rate constant of 2.2 x 10 M s at 25 °C. Under anaerobic conditions, iron(III) porphyrin complexes in dimethyl sulfoxide solution are reduced to the iron(II) state by addition of hydroxide ion or alkoxide ions. Excess hydroxide ion serves to generate the hydroxoiron(II) complex. The oxidation of hydroxide and phenoxide ions in acetonitrile has been characterized electrochemically " in the presence of transition metal complexes [Mn(II)L] [M = Fe,Mn,Co,Ni L = (OPPh2)4,(bipy)3] and metalloporphyrins, M(por) [M = Mn(III), Fe(III), Co(II) por = 5,10,15,20-tetraphenylpor-phinato(2-), 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphinato(2-)]. Shifts to less positive potentials for OH and PhO are suggested to be due to the stabilization of the oxy radical products (OH and PhO ) via a covalent bond. Oxidation is facilitated by an ECE mechanism when OH is in excess. 

However, donor-acceptor interactions are affected not only by the Lewis acid and base strengths, but also by other, steric and electron structural, factors. Thus, even in systems where either solely the donor or the acceptor property of the solvent is manifested, solvents with different space requirements may interact to different extents because of the steric properties of the reference solute and a reference acceptor with a tendency for dative 7c-bonding (back-coordination) will interact more strongly with jr-acceptor solvent molecules (e.g., acetonitrile) than would be expected from their basicity. The solvent donicity investigations by Burger et al [Bu 71, 74] with transition metal complex reference acceptor model systems have clearly shown the great extent to which such secondary effects may distort the solvent scale. 

The penultimate unit effect may play a very important role in ATRR The rate constants of activation of monomeric and dimeric alkyl bromides with a CuBr-bpy (bpy=2,2 -bipyridine) complex as activator were determined. The ATRP relies on the reversible activation of a dormant alkyl halide through halogen abstraction by a transition metal complex to form a radical that participates in the classical free-radical polymerization figure (Fig. 2) prior to deactivation. In this equiUbrium, the alkyl radical (Pm ) is formed in an activated process, with a rate constant kact> by the homolytic cleavage of an alkyl halogen bond (Pm-Z) catalyzed by a transition metal complex in its lower oxidation state (Cu ). The relative values of fcact of the alkyl bromides were determined for CuBr/bpy catalyst systems in acetonitrile at 35°C. These systems followed the order EBriB (30) MBrP (3)>iBBrP (1) for monomeric initia-tors and MMA-MMA-Br (100) MA-MMA-Br (20) > MMA-MA-Br (5) > MA-MA-Br (1) for dimeric initiators. ... 

Example Transition metal complexes are readily analyzed by LIFDI-MS. A dichloro nickel carbeaie complex dissolved in acetonitrile not only forms the expected [M-Cl] ion, m/z 375.3, but also a molecular ion, m/z 410.3, of somewhat lower abundance. The isotopic patterns of the signals are in very good agreement with the calculated isotopic distributions (Fig. 8.23). Peaks due to the free ligand and its oxide are also observed. The corresponding TIC is typical for FD experiments and clearly reveals the onset of desorption/ionization at medium EHC and the completion of the measurement when the sample is consumed [123]. 

Given the approximate nature of these calculations, the frequency factor for bimolecu-lar electron transfers in transition-metal complexes can be taken as v = 10 M sec With this pre-exponential factor and the barrier calculated above for the Fe(Cp)2/Fe(Cp)2 exchange, AG = 26.5 kJ mol with n = 1.25, we obtain kjjM = 2.1X10 M sec . The experimental rate in acetonitrile, extrapolated to zero ionic strength, is 9.3x10 M sec [21]. It could be anticipated that the calculated rate would be an upper limit for the experimental values in solution because the bonding character of the t2 a/ orbitals was overemphasised. With = 1, a lower limit for this parameter, the calculated rate is = 5.0X10 M sec . ... 

---