Porphyrins anions

Chemical transformations at the macroeyclic chromophorc of expanded porphyrins are still not known. The complexation behavior of expanded porphyrins is very different from that of nonexpanded porphinoid macrocycles. The coordination hole of the expanded porphyrins is often too big for the complexation of a single metal ion, so in fact two metal ions can be chelated. With some expanded porphyrins, anion binding is observable, a striking difference to the nonexpanded porphyrins. The complexation behavior and the host-guest chemistry of expanded porphyrins is a rapidly growing field of research. The work in this field has been reviewed. Ie f... 

The radiolytic technique has also been applied to the reaction of alkyl radicals R with Ni1 porphyrins anions.279 In analogy with the postulated reaction of NiIF43o to form CH3NiinF430, short lived R-Ni111 products have been detected. 

The catalytic activity of electrogenerated nickel-porphyrin anions toward Mel has been investigated for a series of nickel(II) porphyrins.290 As previously suggested,291 results are in agreement with the presence of Ni1 in the singly reduced [nickel-porphyrin]- anions. 

The adsorption of transition metal complexes by minerals is often followed by reactions which change the coordination environment around the metal ion. Thus in the adsorption of hexaamminechromium(III) and tris(ethylenediamine) chromium(III) by chlorite, illite and kaolinite, XPS showed that hydrolysis reactions occurred, leading to the formation of aqua complexes (67). In a similar manner, dehydration of hexaaraminecobalt(III) and chloropentaamminecobalt(III) adsorbed on montmorillonite led to the formation of cobalt(II) hydroxide and ammonium ions (68), the reaction being conveniently followed by the IR absorbance of the ammonium ions. Demetallation of complexes can also occur, as in the case of dehydration of tin tetra(4-pyridyl) porphyrin adsorbed on Na hectorite (69). The reaction, which was observed using UV-visible and luminescence spectroscopy, was reversible indicating that the Sn(IV) cation and porphyrin anion remained close to one another after destruction of the complex. 

Association constant values for the 1 1 porphyrin-anion complex on the nanoparticle surface. ... 

Only the (F20TPPT)Fem/ alkyl-metal porphyrin anion exhibits another reversible couple ( 1/2, —1.63 V) to yield a dianion [the four electron-withdrawing pentafluorophenyl groups on the porphyrin ring make possible the addition of an election to give the (F2oTPP2-)Fe dianion within the voltage limit of the solvent]. 

The alkyl-metal porphyrin anion [(porT)Fem/ ] is oxidized reversibly on reversal of a cathodic scan (—1.36 to —0.54 V, Table 13.8). The further oxidation of (por)Fem does not occur when is a tertiaiy-butyl group [apparently steric effects weaken the (por)Fein—R bond]. The oxidation potentials for the iron porphyrins (with the same porphyrin ring) are in the order (por)Fem > (por)Feu > (porT)Fem > (por) Fe- > (porT)nFe- (most negative). 

In summary, it has been demonstrated that ISEs can be designed by employing molecular recognition principles. In particular, the feasibility of using hydrophobic vitamin B12 derivatives and electropolymerized porphyrin films in the development of polymer membrane anion-selective electrodes has been demonstrated. The studies indicated that the changes in the selectivity of these ISEs can be explained by the difference in structure of the ionophores. In addition, it was shown that by electropolymerization of a cobalt porphyrin, anion-selective electrodes can be prepared that have extended lifetimes compared with PVC-based ISEs, which use a similar compound as the ionophore. 

As expected from the extremely low fluorescence of fibres made of alkyl-substituted porphyrin amphiphiles, flash photolysis is ineffective. Nevertheless, the formation of porphyrin anion radicals was detected on a millisecond time scale and was traced back to a charge separation within the porphyrin fibre. ... 

The alkylzirconium(m) octaethylporphyrin complex, (OEP)ZrCH2SiMe3 1, was prepared from the dialkylzirconium(rv) complex by reduction with H2 (1 atm) in toluene at 20 °C (Scheme 1). This reaction therefore appears to be a rather rare example of the chemical reduction of Zr(rv) to Zr(m) by H2. The structure of 1 was elucidated by single crystal X-ray diffraction and has a Zr-C bond length of 2.216(8) A. Although this complex formally contains zirconium in oxidation state hi, careful consideration of the structural and spectroscopic data led the authors to conclude that this was an overly simplistic view. At 77 K, an EPR signal typical of a metal-centered radical was observed, while no signal was detected at 293 K. The UV/Vis spectrum of 1 contains bands typical of a porphyrin anion. The electronic structure of 1 is therefore better described as a combination of two resonance forms a Zr(m) metal-based radical, and a zwitterionic form with a positively charged Zr(iv) center and a porphyrin radical anion. 

Treatment of tetraphenylporphyrinato (tpp) zirconium dichloride with TICp in the presence of sodium amalgam afforded (tpp)CpZr 11 (Scheme 5). The structure of 11 was elucidated by X-ray crystallography (Zr-Cp(centTOid) = 2.206(6) A). Compound 11 was also obtained from the reaction between [Li(TH F)4] [(tpp) Zr(C CPh)3] and O p 2T i (M e3 S i C=C S iMc3). Compound 11 has a number of similarities to ((OEP)ZrCH2SiMe3) 1 both are paramagnetic with ESR spectra consistent with metal-centered radicals at low temperature and radical anions at room temperature. As was seen for 1, the I JV/Vis spectrum of 11 also has a band (at 638 nm) characteristic of a porphyrin radical anion. So while 11 is formally an organozirconium(lll) compound, it is better described as a resonance hybrid between a metal-centered radical and a zwitterion with a cationic Zr(iv) and radical porphyrin anion. 

Figure 6.25 Mixed liquid and solid state Arrhenius diagram of the tautomerism of the porphyrin anion. Adapted from Ref [23b].

Finally, for TPiBC the rate constants of the processes AD o DB and AD BD (Fig. 6.21(c)) could be measured [19b]. The results are included in the Arrhenius curves of Fig. 6.28(b). The AD o BD reaction is slower than in TPP which is not surprising, as the molecule is not aromatic. By contrast, the AD o DB reaction is substantially faster than in TPP, an effect which has been associated with the formation of the aromatic cis-intermediate. The reaction rates are similar to those of the porphyrin anion. Although only a few rate constants were measured, one can anticipate with the accepted pre-exponential factor of lO s s a substantial concave curvature of the Arrhenius curves, i.e. a tunneling process occurring at much lower energies as compared to TPP. This is again the consequence of a more symmetric reaction profile as compared to TPP because the energy gap between the non-aromatic initial state AC and the aromatic cis-intermediate DC is substantially reduced. 

For iron(IlI)-porphyrinato complexes, strong-held ligands lead to low-spin (5 = 2) complexes. A pair of identical weak-held ligands, such as tetrahydro-furan, leads to intermediate-spin (5 = ) species. Five-coordinate species are, with few exceptions, high-spin (5 = f), with all hve 3d electrons in separate orbitals. Spin equilibria 5 = i 5 = f and 5 = 15 = i are not unusual. Specihc examples of these spin systems are given in Table 4.4. Higher oxidation states are found in some other hemoproteins. Fe(V)-porphyrin systems actually occur as Fe(IV)-porphyrin cation radical species, and Fe(I)-porphyrin systems exist as Fe(II)-porphyrin anion radical species. 

Highly electropositive ions deactivate the porphyrin towards oxidants. The Sn(IV)-porphyrinates present an extreme here. They are totally stable against molecular bromine but can easily be photoreduced to form porphyrin anion radicals and, in proton-containing solvents, phlorins (Fuhrhop and Lumbantobing, 1970). Both porphyrin Ji-anion radicals and the protonated neutral phlorin radicals usually produce absorption bands around 850 nm (see Sec. 6.5). The lifetimes of a-anion radicals of metalloporphyrins as obtained by pulse radiolysis are also dependent on the central metal ions. Sb(V) and Sn(IV) produce stable... 

Only the lateral assembly produces an 850 nm signal upon flashing, indicating formation of a porphyrin anion radical presumably resulting from light-induced charge separation. 

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