2.9 Conclusions and outlook

Rh(III) diaryl porphyrin was shown to bind to a range of mono- and bidentate nitrogen ligands. The aromatic heterocycles pyridine, 4,4'-bipyridine, 4,4'-bipyrimidine and diazapyrene bound to the Rh porphyrin with similar geometries in the solid state, and solution. However tilting of the ligand with respect to the normal to the porphyrin was observed in several cases in the solid state and this is attributed to a crystal packing effect. Pyridine ligands bound with an affinity constant > 107 and were always observed to be in slow exchange with free ligand on the 400 MHz 1H NMR chemical shift time-scale. However the lability of the complexes is such that equilibration occurs in seconds or less at room temperature. For the bidentate ligands the equilibrium between 1:1, 2:1 complexes and free ligand is easily displaced to increase the proportion of 2:1 complex by removal of the free ligand by crystallization from a polar solvent, or filtration through silica gel.

The complementary binding properties of Rh(III) and Sn(IV) porphyrins for pyridyl and carboxylate ligands enabled assembly of large porphyrin based structures. A limitation appeared to be side reactions of the Sn(IV) porphyrin, and the slow kinetics of complex formation were a disadvantage. Both types of porphyrin possess excellent NMR properties which facilitated confirmation of the solution structures of these compounds. Ligand exchange is always slow on the chemical shift time-scale, so direct measurement of the chemical shifts of each species present is possible. Several of the characteristic chemical shifts of bound ligands and the meso protons lie within uncluttered regions of the NMR spectrum thus permitting location of all the important peaks in the spectrum by 2D COSY and NOESY techniques.

The Rh(III) porphyrin was also found to coordinate to hydrazine and methyl substituted hydrazines. Addition of less than 0.5 eq of the hydrazine afforded a complex with a bridging ligand, although addition of further equivalents of the hydrazine afforded 1:1 complexes. In the 1:1 complexes the substituted hydrazines bound through the substituted nitrogen atom. Paralleling the behaviour observed with bipyridine, the 1:1 complexes could be converted to the bridging 2:1 complex by passage through silica gel. Crystal structures of the complexes with bridging hydrazines demonstrated the steric influence of the methyl substituents.

Nitriles coordinated only weakly, but in the absence of competitive ligands Rh porphyrin could still be organized into a bridging complex with 5,5'-dicyano-2,2'-bipyridine in the solid state. There is potential for introduction of further metals by coordination to the chelating nitrogens of the bipyridine which are sterically unavailable for porphyrin coordination.

The binding of sulfur ligands to Rh(III) porphyrin was investigated. A wide range of sulfur compounds, in which the sulfur is in the +2 or +4 oxidation state were found to bind. Me2S was the strongest binding ligand examined, although this bound less strongly than pyridine. In many cases broad NMR spectra were observed due to exchange processes. Dimethyl disulfide and dimethyl diselenide were found to act as bridging ligands in a manner analogous to N,N'-dimethyl hydrazine. Sulfoxides bind preferentially through the softer sulfur atom, rather than through oxygen. Due to the weaker binding and faster exchange the Rh(III) - S coordination chemistry appears less well suited to the assembly of porphyrin based nanostructures than Rh - pyridyl coordination. However these features could be advantageous in applications such as templating and catalysis, in which ultimate decomplexation of the ligand is required.


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