Electronic structure, cobalt porphyrins

Interpreting these results on a detailed molecular basis is difficult because we have at present no direct structural data proving the nature of the split Co(IIl/lI) voltammetry (which seems critical to the electrocatalytic efficacy). Experiments on the dissolved monomeric porphyrin, in CH-C solvent, reveal a strong tendency for association, especially for the tetra(o-aminophenyl)porphyrin. From this observation, we have speculated (3) that the split Co(III/II) wave may represent reactivity of non-associated (dimer ) and associated forms of the cobalt tetra(o-aminophenyl)porphyrins, and that these states play different roles in the dioxygen reduction chemistry. That dimeric cobalt porphyrins in particular can yield more efficient four electron dioxygen reduction pathways is well known (24). Our results suggest that efforts to incorporate more structurally well defined dimeric porphyrins into polymer films may be a worthwhile line of future research. 

It has been recently demonstrated that the simplest of the cobalt porphyrins (Co porphine) adsorbed on a pyrolytic graphite electrode is also an efficient electrocatalyst for reduction of 02 into 1120.376 The catalytic activity was attributed to the spontaneous aggregation of the complex on the electrode surface to produce a structure in which the cobalt-cobalt separation is small enough to bridge and activate 02 molecules. The stability of the catalyst is quite poor and largely improved by using porphyrin rings with mew-substitu-tion.377-380 Flowever, as the size of the mew-substituents increases the four-electron reduction efficiency decreases. 

ESR studies also show the similarity between oxycoboglobin and oxygenated cobalt porphyrin, and suggest that the protein does not measurably influence the electronic structure of the heme group, although the protein does control the extent to which dioxygen is bound. 

For molecular electrocatalysts otherwise, and especially transition metal macrocycles, the electrocatalytic activity is often modified by subtle structural and electronic factors spanning the entire mechanistic spectrum, that is, from strict four-electron reduction, as for the much publicized cofacial di-cobalt porphyrin, in which the distance between the Co centers was set at about 4 A [12], to strict two-electron reduction, as in the monomeric (single ring) Co(II) 4,4, 4",4" -tetrasulfophthalo-cyanine (CoTsPc) [20] and Co(II) 5,10,15,20-tetraphenyl porphyrin (CoTPP) [21]. Not surprisingly, nature has evolved highly specific enzymes for oxygen transport, oxygen reduction to water, superoxide dismutation and peroxide decomposition. 

Abstract The transition metal complexes of the non-innocent, electron-rich corrole macrocycle are discussed. A detailed summary of the investigations to determine the physical oxidation states of formally iron(IV) and cobalt(IV) corroles as well as formally copper(III) corroles is presented. Electronic structures and reactivity of other metallocorroles are also discussed, and comparisons between corrole and porphyrin complexes are made where data are available. The growing assortment of second-row corrole complexes is discussed and compared to first-row analogs, and work describing the synthesis and characterization of third-row corroles is summarized. Emphasis is placed on the role of spectroscopic and computational studies in elucidating oxidation states and electronic configurations. 

Recently we have seen identical chemistry in the carbahemiporphyrazines. As can be seen in Figure 4, we have observed this chemistry with the metals manganese, iron, cobalt, copper and silver with the dicarbahemiporphyrazine macrocycle (30, 33). With the exception of copper, which will be described below, both internal C-H bonds are retained upon metalation. The lack of C-H activation in these complexes distorts the macrocycle in these compounds from planarity. Since there are two intact C-H bonds at the core, the planar distortion is much greater than that seen in A -confiised porphyrin. The C-M distances are longer than those seen in iV-confused porphyrins (> 2.4 A), and, based on our H NMR measurement of the Ag(I) complex, the electronic structure of the C-H bond is not greatly perturbed when compared to the free base macrocycle (6.77 ppm in the silver adduct versus 6.91 ppm). 

Using a novel NMR shift reagent, for example cobalt(III)meso-tetraphenyl porphyrin (CoTPP), the conformation of (3) is found to be a mixture of many conformations. Theoretical and experimental results revealed ring current shift for (3) <90MRC343>. Electronic structure and conformational properties of the amide linkage were studied in lactams based on MNDO calculations and photoelectron spectroscopy <85Mi 9l8-oi>. 

Another insight provided by their study is that the side-on oxygen adsorption requires more space than the end-on adsorption configuration. This may be one of the reasons why cobalt porphyrin and phthalocyanine systems cannot form stable side-on adducts which generally lead to 4e transfer products. The destruction of the ordered structure of such a macrocyclic catalyst during the heat treatment is likely to increase the number of sites that facilitate the side-on oxygen adsorption, and this leads to an increase in the number of electrons transferred in the ORR. 

Liao M-S, Watts JD, Huang M-.T (2005) Effects of peripheral substituents and axial Ugands on the electronic structure and properties of cobalt porphyrins. J PhysChem A 109(51) 11996-12005... 

One-electron oxidation of the vinylidene complex transforms it from an Fe=C axially symmetric Fe(ll) carbene to an Fe(lll) complex where the vinylidene carbon bridges between iron and a pyrrole nitrogen. Cobalt and nickel porphyrin carbene complexes adopt this latter structure, with the carbene fragment formally inserted into the metal-nitrogen bond. The difference between the two types of metalloporphyrin carbene, and the conversion of one type to the other by oxidation in the case of iron, has been considered in a theoretical study. The comparison is especially interesting for the iron(ll) and cobalt(lll) carbene complexes Fe(Por)CR2 and Co(Por)(CR2) which both contain metal centers yet adopt... 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

Yb(5d)(H20)3]Cl, and [Yb(5d)(CoP] (fig. 12) where CoP is the cyclopentadienyl-tris(dieth-ylphosphito)cobaltate(I) anion (see fig. 13) for which the luminescence intensity increases in the proportions 1 22 36 62 271, upon excitation at 512 nm (Meng et al., 2000). The effect of coordinated water molecules on the metal-centered fluorescence intensity is clearly seen in the more than four-fold enhancement obtained by replacing the three water molecules in [Yb(5d)(H20)3 ]C1 by CoP the 2F5/2 lifetime of the latter complex (40 ps) is also much longer than lifetimes reported for other Ybm porphyrinates. In addition, the cobaltate anion rigidities the molecule, which results in a much finer structure of the ligand-field split electronic levels. 

Cobalt Complexes. A fair number of complexes of cobalt have been studied theoretically. Veillard and co-workers have studied the Schiff base adduct Co(acacen)L02 (L = none, H2O, CO, CN", imidazole) and the porphyrin complex Co(porph)(NH3)02 using ab initio LCAO SCF methods. The calculations show the structure to be more stable than the if a linear structure is also found to be unstable. The most important interaction is that between the cobalt d a orbital and the in- plane rg orbital. The interactions of the in-plane jig orbital with the dyz orbital and TTg (i) with dxz, although present, are much less than in the analogous iron complexes as a result of the tighter binding of the d-orbitals in cobalt. The unpaired electron is localised essentially in the Tig (1) orbital of dioxygen. 

---