Porphyrins deformations

Metal-ligand modes were identified by resonance Raman spectroscopy, and assigned after a normal coordinate analysis, for Mn2(0)(0Ac)2(bipy)2 (H20)2.24° Resonance Raman spectra of MnN(P), where P = a range of porphyrins, included vMn=N near 1050 cm-1.241 The IR spectra of Min(P)Cl, Mn(P), where M111 = Mn, Fe Mn = Co, Ni, Cu, Zn, P = meso-tetra-(4-myristyloxy-phenyl)porphyrin, included vMnN (coupled with a porphyrin deformation mode) at 250 cm 1, and vMninCl 320 cm 1.242... 

One of the more extensively studied and discussed structural variables has been equilibrium out-of-plane distortion of the porphyrin. Out-of-plane porphyrin deformation typically results in a bathochromic (red) shift of the optical absorption spectrum and a shift to more negative reduction potentials (easier to oxidize). These tantalizing correlations, based upon studies of model complexes comprising porphyrin ligands such as octaethyltetraphenylporphyrinate (OETPP ), having sterically crowded peripheries that force large out-of-plane deformations, have driven... 

Regardless of the driving force(s), out-of-plane porphyrin deformations invariably occur along low-frequency, normal coordinates. A recent computational method, normal-coordinate structural decomposition (NSD), yields linear combinations of equilibrium distortions along the lowest-... 

Miscellaneous Metal Ions.—Mention has been made already of a relatively slow formation reaction for the normally labile manganese(ii) ion. Incorporations of the labile bivalent metal ions zinc(ii), copper(ii), manganese(n), cobalt(ii), and nickel(n) into water-soluble porphyrin molecules such as tetrakis-(4-N-methyI-pyridyl)porphine, tetrasulphonated tetraphenylporphine, and uroporphine are also relatively slow reactions. However, by taking into account the porphyrin deformation which is necessary, an Id mechanism can be fitted to these reactions. The rates of formation and dissociation of nickel(ii), copper(n), cobalt(ii), and zinc(ii) complexes of the sterically hindered ligand 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetra-decane (Meicyclam) are also 10 —10 smaller than is normal for these metal ions. ... 

Bain-Ackerman and Lavallee " have measured the rate parameters for the entry of five bivalent metal ions into A-methyltetraphenylporphyrin in dmf solution. All reactions follow a second-order rate law and with the exception of Mn(II), the order of the porphyrin metalation rate constants (Cu(II) > Zn(II) > Co(II) > Ni(II)) coincides with that of solvent exchange at the metal ions. The values of k for this deformed porphyrin (Table 6.4) are all larger than for the corresponding metal ion reacting with planar porphyrins. The authors favor a mechanism which involves solvent dissociation from the metal ion as an important rate-determining factor but they also point out that porphyrin deformation... 

A general deformation mechanism has been proposed which rationalizes the kinetic data that have been obtained for the incorporation of metals into porphyrins and the acid-catalysed solvolysis of metal porphyrins. In addition, the idea of porphyrin deformation by metal ions, protons, or JV-alkyl groups, leading to subsequent reactions on the opposite side of the porphyrin plane,... 

The cucurbit [n]uril family (CB[n]) of molecular containers possess remarkable binding affinities and selectivities (Ka values up to 1012M-1, Krei values up to 106) which renders them useful as a component of molecular machines, sensors, and biomimetic systems (123-125). Recently, Wagner and coworkers have reported (126) that CB[10] - with its spacious 870A3 cavity - is capable of acting as a host for free base and metalated tetra (Af-methylpyridinium)porphyrins 19a-d (Fig. 17). Despite the large ellipsoidal deformation of CB[10] upon complexation, the complexed porphyrins retain their fundamental UV/VIS, fluorescence, and electrochemical properties. The CB[ 10] porphyrin... 

Detailed kinetic studies of the reaction of Fe(II) in cyclophane hemes with 02 and with CO probed polarity and steric effects the effects of deformation of the porphyrin skeleton from planarity were assessed for one compound (121). Volume profiles have been established for reactions of a lacunar Fe(II) complex with CO (122 for myoglobin with 02 and with CO and for hemerythrin with 02 (123). 

The importance of this deformation in easing metal entry is illustrated by the fact that the porphyrin N-Metetraphenylporphyrin is deformed and reacts rapidly and simply with many metal ions. 

One of the two most common nonplanar deformations of the porphyrin ligand is the saddle conformation in which the pyrrole Cp—C bonds are displaced alternately above and below the mean 24-atom plane. The other is the ruffled conformation in which the Cmeso carbon atoms are displaced alternately above and below the mean 24-atom plane with concomitant twisting of the pyrrole rings. There are various measures for the extent of mffling for example, the C eso cross-ring distance decreases and the mean displacement of Cmeso from the mean 24-atom plane increases as the structures become more ruffled. The average M—N... 

This reinforces a view that the thermal isomerization, while perhaps assisted by deformations originating near the pyrrole nitrogens, does not require severe ruffling of the porphyrin macrocycle. 

The above mentioned deformation in the porphyrin diacid results in decreases in the free energy of activation for thermal atrop-isomerization, AG, of 2-3 kcal/mole compared to the corresponding free bases ( l) Even in the thermal isomerization the greater ground state stability of the trans-2,2 compared to the other isomers is apparent, although not nearly so much so as in the corresponding excited state. 

The distinction between the two states is readily illustrated in a deformation density section perpendicular to the porphyrin plane through the iron atom. Because of the transfer of an electron into the d2z orbital in going from 3EgA, and to 3A2g, the former configuration shows a deficiency and the latter an excess of density above and below the nitrogen atom. This is confirmed by theoretical deformation density maps (Fig. 10.10) (Rohmer 1985). The experimental map for FePc shows the deficiency along the z axis, as expected from the 4-orbital populations listed in Table 10.10. 

Irngartinger et al. 1977), and the hydro-bis(squarate) anion (Lin et al. 1994). Even in the deformation density maps of the five-membered pyrrole rings in transition metal tetraphenyl porphyrins (chapter 10), bond bending is visible. 

Several metal insertion mechanisms have been proposed, but none of them is conclusive.18 The rate of metallation varies from square root to second order in metal salt from one system to another, and apparently there exists more than one pathway. Where the rate law is second order in metal salt, a so called sitting-atop metal ion-porphyrin complex intermediate or metal ion-deformed porphyrin intermediate, which then incorporates another metal ion into the porphyrin centre, has been postulated (Figure 3).19 For the reactions with the square root dependence on the metal salt concentration, the aggregation of metal salts is suggested.18 Of course, there are many examples which follow simple kinetics, i.e. d[M(Por)]/df = k[M salt][H2Porj. 

Figure 3 Sitting-atop and deformed intermediates in porphyrins metallation...

The lack of ring deformation on the first N protonation in the predeformed TV-alkylporphyrins renders them more basic [pX3 as 5.64 for (TV-Me)H(TPP)] than the planar porphyrins [pX3 a 4.38 for H2(TPP)], whereas their pX4 values (3.85) are identical because their monoprotonated forms have already been distorted.149... 

An area of increasing interest is the selective complexation of Sn2+ and more particularly Pb2+ for the treatment of heavy metal poisoning. Molecular mechanics has been extensively applied to the problem of metal ion selectivity (see Chapter 8) but there have been few studies of lead or tin complexes. The fit of Sn2+ to 18-crown-6 has been considered12811, as has the size selectivity of tetraazamacrocycles with respect to Pb2+ binding131. The binding of Pb2+ to porphyrin-1 has been modeled, though in this case the point of interest was the structural deformations caused by the metal cation11901. 

The deformation of the n-system of the porphyrin ring affects its chemical reactivity and various physical properties. Highly substituted systems are more susceptible... 

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