Porphyrinogen metal complexes

The porphyrinogen-metal complexes 1-4, which are isostructuraH (see the structure in Figure 3), and complex 5 have some remarkable structural characteristics, particularly helpful for understanding their chemistry. 

Metal complexes of porphyrinogens (XI), which are easily accessible by various routes (78MI15), are unstable. Rearrangements were studied carefully, mainly by the Eschenmoser group. Some examples may illustrate this. Tautomerization of octaethylporphyrinogen (45) on complexation with the Mg salt of 1,5,7-triazabicyclo [4.4.0] dec-5-ene, followed by decomplexation... 

Compared with metal enolates, there have been very few reports on the direct structural analysis and theoretical studies of ynolates. An X-ray crystal structure of a vanadium complex of lithium ynolate with a porphyrinogen ligand (56) is reported. This metal complex was incidentally formed from VCl3(THF)3 with tetralithium salt of the octaethyl-porphyrinogen ligand. In this complex, the lithium cation seems to interact with the 7T-electrons of the ynolate. The four atoms of the ynolate group in 56 are not collinear due to a partial sp character of the group in this complex. 

The w 5 o-octaalkylporphyrinogen does not undergo the fast and easy oxidation observed for the w o-tetrahydrotetraalkylporphyrinogen, although we observed a remarkably slow absorption of oxygen by its solutions in different solvents. Such a process never led as far as well defined oxidized forms of the porphyrinogen skeleton. This prompted us to undertake a systematic synthetic and mechanistic study of the redox chemistry of the /w. yo-octaethylporphyrinogen transition metal complexes." The ethyl substituted form was mainly employed as dictated by its appropriate solubility in hydrocarbon solvents. 

Porphyrinogens make excellent frameworks for synthetic elaboration toward a variety of applications because they can be prepared in high yields from commonly available precursors (substituted pyrroles and ketones). They have been exploited from a variety of viewpoints, starting with the work of Horiani and coworkers who developed several porphyrinogens as scaffolds for uansition metal complexes. Of particular note are the dimeric and organometallic species generated through their work. 

The tautomerization is induced by cobalt(II) which forms the thermodynamically more stable metalatcd hydroporphyrins from which the cobalt can be removed using trifluoroacctic acid under kinetic control. Experiments with porphyrinogen and hexahydroporphyrin show that the porphyrinogen-hexahydroporphyrin equilibrium can be shifted by complexation of porphyrinogen with metal ions to the more stable metal hexahydroporphyrins and that metal-free hexahydroporphyrins tautomerize back to the more stable metal-free porphyrinogens.29... 

Systematic investigations have deary demonstrated that in the presence of metal ions the conjugated hexahydroporphyrin forms are thermodynamically favored by complexation whereas in the absence of metal ions the porphyrinogen form with isolated aromatic pyrrole rings is the thermodynamically stable tautomer. 

The precursors and the synthetic route leading to rotaxane 102 are represented in Figure 2.37.57b First macrocycle 58 was threaded onto the presynthesized Au(III) porphyrin-substituted phenanthroline 103, a semidumbbell molecule, in the presence of Cu(I), to afford prerotaxane 104 quantitatively. The second stopper (and functional end cap) was installed by the meso-porphyrin construction method. Thus reaction of 104 with 4,4/-dimethyl-3,3/-diethyl-2,2/-dipyrrylmethane 105 and 3,5-di-/m-bulyI benzaldehyde 84, followed by oxidation of the intermediate porphyrinogen with chloranil 85, gave the free-base Cu(I)-complexed [2]-rotaxane 106 in 25% yield. After metal-lation with Zn(0Ac)2 2H20 and exhaustive purification, Cu(I) complex 102 was obtained. It was subsequently demetallated with KCN, to afford the free [2]-rotaxane 107 in quantitative yield. 

The complexation of transition metals " has been achieved in non-aqueous solvents using the lithiated form [EtgN4Li4(thf)4] of the m o-octaethylporphyrinogen. The metal helps fix one of the many possible porphyrinogen conformations, as observed both in solution and in the solid state. Reaction 1 exemplifies the complexation of several bivalent metals. ... 

In complexes 1-4 the lithium cation is ri -bonded to the pyrrolyl anions at the porphyrinogen periphery. The binding ability of the porphyrinogen periphery thus allows such complexes to display bifijiictional properties, the acidic center being the transition metal ion. Such a bifunctional peculiarity will be relevant in porphyrinogen based organometallic chemistry. ... 

The oxidation of porphyrinogen displayed in Scheme 1 is an overall process which can be formally viewed as the result of two distinct molecular actions the removal of six electrons, followed by the removal of six protons. A picture of this view applied to the porphyrinogen tetraanion, namely in the form complexed to transition metals, is shown in Scheme 2. 

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