Porphyrins square

Very available and promising square panels are tetrapyridyl porphyrins 15 and 16. In fact, 4-pyridyl-substituted porphyrin 15 has been frequently employed for infinite assemblies. In contrast, there seems to be difficulty in constructing discrete 3D assemblies from tetrapyridyl porphyrins, though there are some examples of 2D assemblies such as porphyrin squares 17 [27] and 18 [28] from dipyridyl porphyrins. 

Similar measurements using either walljet electrochemistry or steady-state microelectrode voltammetry have been reported for layer-by-layer assembled and interfadally polymerized materials, respectively [24,28]. Additional measmements were made spectrophotometrically with polymerized porphyrin squares by using a U-tube. Results summarized in Fig. 5 revealed the following (a) After normalizing for differences in film thickness, transport through polymeric membranes is two to three orders of magnitude faster... 

A brief report showed that molecular square IZn could be used to detect binding of sodium and potassium cations by a crown ether featuring a pendant pyridine [32], The crown ether ligand was observed to bind via the pendant pyridine to the available Zn(II) sites of the porphyrin square. Accompanying the binding was a decrease in fluorescence intensity that was reversed upon capture of an alkali metal cation by the crown. 

In contrast to the vast assortment of molecular squares now in existence, relatively few examples of molecular boxes or cubes were synthesized, unless one views squares having tall edges (for example, porphyrinic squares) as open-ended flexible boxes. As defined here molecular boxes are right-angle-containing complexes, where the metal coordination units are arranged in three dimensions, instead of two. 

Another type of catalytic system being developed involves the tethering of a catalytically active metal center to the cavity of a supramolecular porphyrin square. In this example, the square cavity serves as a protective barrier that significantly lengthens catalyst lifetime. 

While only a few have been examined with respect to their effects upon molecular transport through thin films, binding of a fairly large number of ligands has been demonstrated for porphyrinic squares ... 

In the catalase, manganese is held to the protein with complicated nitrogen-ring amino acids that look a little like the nitrogens on the inside of porphyrin squares. This amino acid is the second-most expensive amino acid and would have been difficult to make. The Caetano-Anolles molecular clock agrees and says this particular... 

Fig. 3 Hydrogen-bonded tetramer (1) formed by monopyrazolylporphyrins and the self-assembled porphyrin square (2) mediated hy self-complementary quadruple hydrogen...

The macrocyclic effect can also be employed. MOF [Zrg04(0FI)4(tcpp-Fl2)3] (tcpp-H2 = tetra(carboxyphenyl) porphyrin. Chart 2) is constructed using unmetallated porphyrin square planar tectons. The MOF can be quantitatively metalled in an SCSC manner to form [Zrg04(0FI)4(tcpp-FeCl)3] by immersion in a DMF solution of FeCl3 at 100 °C. Interestingly, this MOF could not be formed directly using a premetallated H4tcppFeCl precursor. 

Current synthesis techniques provide an unprecedented level of control over nanoparticle shape and composition, resulting in an almost limitless catalog of nanoparticles with varying geometry and interactions.Examples of different shapes include spheres, rods, cubes and other polyhedra, plates, ° and multipods, along with molecular nanoparticles such as carbon fullerenes, porphyrin squares, polyhedral oligomeric silsesquioxane (POSS)... 

In the genuine low-temperature chemical conversion, which implies the incoherent tunneling regime, the time dependence of the reactant and product concentrations is detected in one way or another. From these kinetic data the rate constant is inferred. An example of such a case is the important in biology tautomerization of free-base porphyrines (H2P) and phtalocyanins (H2PC), involving transfer of two hydrogen atoms between equivalent positions in the square formed by four N atoms inside a planar 16-member heterocycle (fig. 42). 

Green-yellow salts of the tetrahedral [MX4] (X = Cl, Br, I) ions can be obtained from ethanolic solutions and are well characterized. Furthermore, a whole series of adducts [MnX2L2] (X = Cl, Br, I) are known where L is an N-, P- or A -donor ligand, and both octahedral and tetrahedral stereochemistries are found. Of interest because of the possible role of manganese porphyrins in photosynthesis is [Mn (phthalocyanine)] which is square planar. The reaction of aqueous edta with MnC03 yields... 

With their preference for square planar coordination, palladium(II) and platinum(II) are well suited to binding to porphyrins and related N4 donor macrocycles. Therefore, Pd(octaethylporphyrin) is readily synthesized starting from the labile PhCN complex (like the platinum analogue) [92]... 

With a tridentate ligand Au(terpy)Cl3.H20 has, in fact, AuCl(terpy)2"1" with weakly coordinated chloride and water while Au(terpy)Br(CN)2 has square pyramidal gold(III) the terpyridyl ligand is bidentate, occupying the axial and one basal position [124]. Macrocyclic complexes include the porphyrin complex Au(TPP)Cl (section 4.12.5) cyclam-type macrocyclic ligands have a very high affinity for gold(III) [125],... 

Enikolopyan et al.til found that certain Co11 porphyrin complexes (eg. 87) function as catalytic chain transfer agents. Later work has established that various square planar cobalt complexes (e.g. the cobaloximes 88-92) are effective transfer agents.Ij2 m The scope and utility of the process has been reviewed several times,1 lt>JM ns most recently by Hcuts et al,137 Gridnev,1 3X and Gridnev and Ittel."0 The latter two references1provide a historical perspective of the development of the technique. 

Like their Fe analogs, Co porphyrins are aerobically stable. Co , being a if ion, favors a square pyramidal coordination sphere, and simple Co porph5Tins t)q)ically have fairly low affinity to axial hgands. As a result, many simple Co porphyrins are aerobically quite stable in the sohd state, but oxidize slowly if dissolved in a coordinating solvent. Simple Co porphyrins are typically stronger oxidants than the Fe ... 

Macrocyclic receptors made up of two, four or six zinc porphyrins covalently connected have been used as hosts for di- and tetrapyridyl porphyrins, and the association constants are in the range 105-106 M-1, reflecting the cooperative multipoint interactions (84-86). These host-guest complexes have well-defined structures, like Lindsey s wheel and spoke architecture (70, Fig. 27a), and have been used to study energy and electron transfer between the chromophores. A similar host-guest complex (71, Fig. 27b) was reported by Slone and Hupp (87), but in this case the host was itself a supramolecular structure. Four 5,15-dipyridyl zinc porphyrins coordinated to four rhenium complexes form the walls of a macrocyclic molecular square. This host binds meso-tetrapyridyl and 5,15-dipyridyl porphyrins with association constants of 4 x 107 M-1 and 3 x 106 M-1 respectively. 

There are few well-established examples of Co111—Sn bonds. Stannanes R3SnH react with OEP complexes (124) or the divalent Co(OEP) to form Co(OEP)SnR3.550 This is also accessible from R3SnCl and [CoT(OEP)]. The structure shows a square-pyramidal Co displaced 0.777 A out of the porphyrin plane and towards the Sn, which is 2.510(2) A distant from the Co111 center. 

Reduction of both nickel porphyrins and thiaporphyrins to Ni1 species has been studied by EPR and 2H NMR spectroscopy.179, 2 58 The Ni1 complex of 5,10,15,20-tetraphenyl-21-thiaporphyrin has been isolated and characterized. Reaction of this complex with sulfur dioxide produced a paramagnetic five-coordinated Ni1 S02 adduct, while reaction with nitrogenous base ligands (amines, pyridines, imidazoles) yielded five- and six-coordinate complexes. In addition, the crystal structure of Ni1 diphenyldi-p-tolyl-21-thiaporphyrin has been determined. The coordination geometry about the nickel center is essentially square planar with extremely short Ni—N and Ni—S bonds (Ni—N = 2.015(2) A, 2.014(12) A, and 1.910(14) A and Ni—S = 2.143(6) A).2359... 

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