Trimerization, metalloporphyrin

Fig. 3. A trivial closed trimer assembled on a six-coordinate metalloporphyrin.

Sanders (14) has exploited the strong and selective coordination of phosphine donor groups to Ru(II) to construct hetero-dimetallic porphyrin dimers (17, Fig. 5). An alkyne-phosphine moiety introduced on the periphery of a free base or metalloporphyrin (M = Zn or Ni) spontaneously coordinates to a Ru(II)(CO) porphyrin when the two porphyrins are mixed in a 1 1 ratio. Coordination is characterized by a downfield shift of the 31P resonance (A<531P = 19 ppm). There is no evidence of self-coordination of the zinc porphyrin at 10 6 m in toluene, there is no shift in the Soret band in the UV-Vis absorption spectrum. The Ni-Ru dimer was observed by MALDI-TOF mass spectrometry. Heating the Ru(II)CO porphyrin with 2 equivalents of the phosphine porphyrins led to quantitative formation of trimeric assemblies. 

Sanders and coworkers96 97 catalyzed and directed the Diels-Alder reaction between 4-(maleimidomethyl)pyridine and 4-(3-furyl)pyridine using metalloporphyrin oligomers. When trimer 115a having three butadiyne linkers was used as the catalyst, the exo adduct was the exclusive product isolated at both 30 °C and 60 °C, whereas the uncatalyzed reaction provided an endo/exo ratio of 2/1 at 30 °C and a transient trace of endo adduct... 

Marty, M., Clyde-Watson, Z.,Twyman, L.J., Nakash, M. and Sanders, J.K.M. (1998) Acceleration of a hetero-Diels-Alder reaction by cyclic metalloporphyrin trimers. Chem. Commun., 2265-2266. 

The principle for construction of aggregates ba.sed on these features is. schematically represented in Figure 11. Here tme oligomerization processes such as dimerization, trimerization and so on are considered, because the metalloporphyrins assemble with themselves contrary to what will be described in other sections of this chapter, no additional ligand or metal is required for the assembly... 

Figure 11. Porphyrin dimer (a), trimer (b) and tetramer (c) made from metalloporphyrins (thick line) bearing a pendant-coordinating arm symbolized ty an arrow. The relative orientation of the latter with respect to the porphyrin plane determines the structure of the assembly.

Figure 31. Multiporphyrin assemblies based on porphyrins bearing one, two, three or four coordinating subunits, (a) dimer, (b)-(d) trimers, (e) tetramer (f) pentamer. The metalloporphyrins are represented schematically by a thick line and the free-base porphyrins bearing coordinating subunits (arrows) are represented by empty squares.

The influence of the central metal ion on the TPA properties—as well as on the second-order NLO response—of push-pull porphyrins has been investigated by Ray et alP by employing the ZINDO/Correction Vector (ZINDO/CV) method combined with the Onsager Self-Consistent Reaction Field (SCRF) method to account for solvent effects. Metalloporphyrins with strong CT bands are shown to exhibit an increase of both NLO responses by one order of magnitude with respect to the porphyrin without metal. These NLO responses further increase by at least one or two orders of magnitude when going from the monomer to the dimer and trimer. 

The most used approaches for the construction of multiporphyrin systems are the self-assembly methods. For example, electrostatic assembly is an easy and widely used method based on the interaction of oppositely charged porphyrins for preparation of more complex systems [1-12]. Self-assembly of porphyrins are also possible by coordination of nucleophihc groups to the central metal ion or using compounds having two nucleophihc sites to coordinate two metalloporphyrins [13-22]. These methods have been used not only for the preparation of porphyrin dimers and trimers, but also for the formation of larger supramolecular networks. Despite the apparent simphcity of the preparation methods of multiporphyrin materials by self-assembly, they generally are less stable as compared to covalently bond... 

---