Ligands porphyrin assemblies

Ligand self-assembly through coordinative bonding has been used to increase the bulkiness of a monodentate tris-3-pyridyl phosphine ligand employing the zinc porphyrin/pyridine interaction (Scheme 33) [95-97]. The corresponding rhodium catalyst allowed for regioselective hydroformylation of2-octene [95]. 

Kleij, A.W., Kuil, M., Tooke, D.M., Spek, A.L. and Reek, J.N.H. (2005) Template-assisted ligand encapsulation the impact of an unusual coordination geometry on a supramolecular pyridylphosphine-Zn(ii) porphyrin assembly. Inorg. Chem., 44, 7696-7698. 

Slagt. V.F., van Leeuwen, P.W.N.M. and Reek, f.N.H. (2003) Midticomponent porphyrin assemblies as functional bidentate phosphite ligands for regioselective rhodium-catalyzed hydroformylation. Angew. Chem., Int. Ed., 42, 5619-5623. 

Two of the classical bis-nitrogen ligands to assemble porphyrins (predominantly zinc metallated) are the bidentate l,4-diazabicyclo[2.2.2]octane (DABCO) and 4,4 -bipyridine (bipy). Other di-topic nitrogen-ligands studied include hydrazine, 4,4 -bipyrimidine, diaza-pyrene, 5,5 -dicyano-2,2 -bipyri-dine, perylene-bisimide, extended bipyridine units and non-covalently linked di-pyridyl ligands via bipy-metal interactions. The formation of various bis-porphyrin and multiporphyrin assemblies using these versatile ligands were studied in detail by Sanders, Anderson, Hunter, Branda and many others. The structures of the supramolecular multi-porphyrin architectures are schematically represented in Fig. 9. 

Fig. 7 Schematic representation of the multivalent ligands 1-3 capable of forming non-americ and heptadecameric porphyrin assemblies upon binding of the dimer 4...

Fig. 5 Porphyrin assemblies constructed from 5 and 6 using hydrogen bonding and metal-ligand interactions...

Fig. 2. Trivial closed trimers (a,b), tetramers (c), and pentamers (d) assembled on multi-dentate 4-pyridyl porphyrin ligands.

Zinc porphyrins also interact with diaza ligands to form dimeric assemblies, but these systems have significantly lower kinetic and thermodynamic stability. The zinc porphyrin-DABCO system has been widely studied by Sanders (35) and Anderson (36,37), while Ballester... 

A four-component self-assembling system was described by Kuroda (46). Two rhodium porphyrins are coordinated by the terminal pyridine groups of an extended ligand constructed from a tartrate derivative. 

The axial coordination of metalloporphyrins to a pyridyl ligand was successfully exploited by two groups to produce porphyrin-stoppered rotaxanes. Sanders (48) assembled a rotaxane by simply mixing the constituent parts. Zn(II), Ru(II)CO, and Rh(II)Cl porphyrins were used as stoppers. Branda (49) reported the stoppering of a pseudorotaxane by adding two equivalents of a Ru(II)CO porphyrin that coordinated to... 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

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