Paramagnetic counterions

The aromatic shifts that are induced by 5.1c, 5.If and S.lg on the H-NMR spectrum of SDS, CTAB and Zn(DS)2 have been determined. Zn(DS)2 is used as a model system for Cu(DS)2, which is paramagnetic. The cjkcs and counterion binding for Cu(DS)2 and Zn(DS)2 are similar and it has been demonstrated in Chapter 2 that Zn(II) ions are also capable of coordinating to 5.1, albeit somewhat less efficiently than copper ions. Figure 5.7 shows the results of the shift measurements. For comparison purposes also the data for chalcone (5.4) have been added. This compound has almost no tendency to coordinate to transition-metal ions in aqueous solutions. From Figure 5.7 a number of conclusions can be drawn. (1) The shifts induced by 5.1c on the NMR signals of SDS and CTAB... 

Figure 7.7 Schematic representation of the temperature-induced change in the MSP composed of terpy end-capped ditopic monomers and Fe dihexadecyl phosphate. The temperature-induced transition resuits in a distortion of the metal ion coordination geometry, which occurs because of the melting of the amphiphilic counterions, giving rise to a reversible transition from a diamagnetic low spin state to a paramagnetic high spin state (Bodenthin et al. 2005).

The paramagnetic purple trimer [Nb3(jti-Cl)6(C6Me6)3]2+ could be stabilized when using the dimeric (TCNQ)2 as counterion.655 It has m symmetry with two Nb—Nb distances of 3.327(2)... 

Complexation of macrocyclic ligands to lanthanide cations has been studied extensively [207,208], One main reason for the current interest in those macro-cyclic complexes are their intrinsic paramagnetic and luminescent properties. There is also the steadily increasing number of tailor-made macrocyclic ligands [209], This section will focus on complexes which contain macrocycles as discrete counterions and in particular on the coordination chemistry of phthalocyanine (Pc) and porphyrin (Por) ligands. Schiff base ligands which display another source of amine functionalities are usually not deprotonated under the prevailing reaction conditions [210]. 

The a- and /3-isomers of [Ru2(S2CNR2)5]f can be reduced to the paramagnetic a- and /J-[Ru2(S2CNR2)5] respectively by reaction with Na[BH4] in ethanol. For R = Et, the green a-isomer converts in toluene to the purple /S-isomer with a first-order rate constant of 8 x 10-4s-1 at 30°C. Photolysis in CHC13 yields the oxidized cations with chloride counterions and retained a and... 

These light yellow, air- and water-stable cations can be isolated with a variety of counterions, for example, PFe and BP1)4 but, probably due to the very low yields, the chemistry of the complexes has been relatively unexplored. Reduction by Na in liquid NH3 or H addition to (140) (R = H, Me) gives [Re( -C6R6) ( -C6R<5H-6-exo)]. A very unstable, paramagnetic complex, [Re(jj -C6Me6)2], has been isolated and identified by electron spin resonance (ESR) upon reduction of (135) (R = Me) with Li. 

A novel class of dithiolene complexes based on thiophenedithiolate ligands with interesting electrical and magnetic properties has been recently reported by Almeida and coworkers (Section IV.B.2). In particular, the paramagnetic [Ni(a-tpdt)2] (56, M = Ni) anionic dithiolene complex in combination with a suitable counterion has proved to be... 

Its structure and optical properties indicate both inter- and intramolecular CT bands. The third product is a dark green solid whose paramagnetism points to 142 cation radicals and whose stoichiometry is consistent with partlyhydrolized TCNE counterions. 

An analogous explanation can be invoked to account for the counterion effects observed in the base-promoted reduction of nitroarenes described earlier. Indications in this sense have been obtained by means of in situ electron paramagnetic resonance (EPR) analysis of reacting solutions (32). In Figure 4, for example, plots are shown of the concentration of 4-ClC6H4N02 and of the intensity of the EPR signal due to 4-ClC6H4N02, as a function of time. Experiments of this sort provide evidence that these alkoxy-promoted reductions involve nitroarene radical anion intermediates. 

On the other hand, in the case of anionic-coordination polymerization on paramagnetic catalytic centers, the existence of free spins in the active center can lead to the stabilization of the triplet intermediate by involving unpaired electrons of the transition metal in the exchange interaction with those of the intermediate. This should increase the effect of spin exclusion. However, the final result will depend on the competition of two groups of factors stabilizing (pseudosymmetry and exchange interaction) and destabilizing (electric and quantum-chemical fields of the counterion) a locally symmetric triplet intermediate. 

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