2. Supramolecular chemistry of Rh(III) porphyrin

The coordination chemistry of Rh(III) porphyrin was outlined by Sadasivan and Fleischer in 1967.199,200 Their Rh(III) porphyrin, obtained by a metallation reaction using [Rh(CO)2Cl]2 and aerial oxidation, was believed to coordinate a labile water molecule which could be displaced by pyridine. A coordinated chloride counterion could be exchanged for azide, cyanide or hydroxide. Work by other groups has demonstrated that six coordinate Rh(III) porphyrins with two identical axial ligands may be generated by reaction of Rh(III)Cl porphyrins (OEP, TPP) with Ag+ to remove the chloride anion, followed by treatment with ligands such as BuNC, PPh2Me and POMe3.201,290 Reaction of Rh(III)Cl porphyrins with organolithium reagents232 or metal carbonylate anions202 afforded organometallic and metal-metal bonded derivatives respectively. The [(OEP)Rh]+ intermediate formed by treatment of (OEP)RhCl with AgBF4 was found to be active in electrophilic aromatic substitution reactions to afford s-bonded Rh aryl porphyrins.203,204 Reduction of (OEP)RhCl with NaBH4, followed by protonation yields air sensitive (OEP)RhH which on oxidation is converted to a metal-metal bonded Rh(II) dimer.107 These reduced derivatives have been shown to undergo unusual reactions such as CH activation,205,206 but this is outside the scope of this introduction. A summary of the reactions of Rh(III) porphyrins is given in scheme 2.1.

Reactions of Rh(III) porphyrin

A variety of other synthetically useful transformations have been found to be catalysed by Rh(III) porphyrins. These include cyclopropanation,207-213 enolization,214 olefin oxygenation,215 and borohydride reduction.216,217 The reader is referred to the literature for a discussion of the mechanistic aspects of catalysis, as catalytic applications are not the focus of this thesis.

Reports of structures containing multiple Rh porphyrins are comparatively scarce and are confined to several covalently linked dimers218,219 and the structures already described in chapter 1.101,107,122, 220

Supramolecular applications of Rh(III) porphyrins have been explored by Aoyama and Ogoshi. These workers have reported bifunctional receptors for amino acids and esters which combine coordination of the amine moiety to the Rh centre with hydrogen bonding between hydroxyl groups on the porphyrin periphery and the ester or acid group.221-223 Binding of amino acids to Rh(III)Cl porphyrins was sufficiently strong to be effectively irreversible, however organometallic derivatives (figure 2.1) bound amino acids reversibly. Binding of nucleobases was also studied.224,225

Rh(III) porphyrin receptor for amino acids

The use of Rh(III) porphyrin as a coating on quartz crystal microbalances for sensing the volatile products of food spoilage has been investigated.226,227 Such microbalances were found to be sensitive to amines and sulfur compounds, although there were some inconsistencies in the results. From this evidence alone it is not possible to determine whether axial coordination of the analyte to the porphyrin was responsible for the sensor response.

Rh(III) porphyrins seemed attractive for preparation of self assembled structures due to the facile replacement of a weakly coordinated solvent molecule with a tightly bound nitrogen donor ligand. The diamagnetic nature and slow exchange kinetics of the resulting complexes facilitates solution characterization by NMR spectroscopy. The work of Aoyama and Ogoshi221 indicated that carboxylic acids would not interfere with amine binding to Rh(III), thus offering the potential for binding of another metal to a free acid group. In this chapter work is presented on the assembly and characterization of structures in the solution and solid state using Rh(III) porphyrin axial coordination chemistry.

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