Design, self-assembly and structural characterization of novel supramolecular systems containing porphyrins and coordination compounds.

There is substantial recent interest worldwide in the construction of multiporphyrin assemblies which can either mimic naturally occurring multichromophore aggregates, such as the photosynthetic reaction center and the light harvesting complex of purple bacteria, or which can be used as electron- and/or energy-transfer molecular devices for advanced technological tasks.

The metal-mediated self-assembly approach, which exploits the formation of coordination bonds between peripheral basic site(s) on the porphyrins and metal centers, has recently allowed the design and preparation of sophisticated supramolecular architectures whose complexity and function begin to approach the properties of naturally occurring systems. Within this framework, meso-pyridyl/phenyl porphyrins (PyPs, Figure 1), or strictly related chromophores, can provide geometrically well-defined connections to as many as four metal centers by coordination of the pyridyl groups.

Several discrete assemblies of various nuclearities, in which the pyridylporphyrins are linker binding metallo-porphyrins and/or coordination compounds, have been constructed in recent years by our group.12 All the adducts were thoroughly characterized in solution, mainly by NMR techniques; in several cases, also the solid state structure was determined by X-ray crystallography.

Molecular squares of porphyrins

We found that treatment of the octahedral Ru(II)-dimethylsulfoxide complexes trans-RuCl2(dmso-S)4, trans-RuCl2(dmso-O)2(CO)2, and trans-RuCl2(dmso)3(CO) with an excess of 5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin (4'-cisDPyP) yields the bisporphyrin adducts trans,cis,cis-RuCl2(X)(Y)(4'-cisDPyP)2 (X = Y = CO, 1; X = Y = dmso-S, 2; and X = dmso-S, Y = CO, 3; trans,cis-RuCl2(X)(Y) = tRu) (Figure 2). Treatment of compounds 1 - 3 with the square planar complex of Pd(II), Pd(dppp)(OTf)2 (dppp = 1,3-bis(diphenylphosphanyl)-propane, OTf = trifluoromethanesulfonate = triflate), leads efficiently to unprecedented heterobimetallic molecular squares of porphyrins, [tRu(4'-cisDPyP)2Pd(dppp)](OTf)2, featuring different diagonally opposed metal ions (Figure 2).8 The solid state structure of [Pd(dppp)trans,cis,cis-RuCl2(CO)2(4'-cisDPyP)2](OTf)2 was also determined by X-ray single crystal analysis (Figure 3). The metal-to-porphyrin center distances are about 9.83 Å long, and the diagonal Pd···Ru distance is 14.015(3) Å.

On the other hand, treatment of 4'-cisDPyP with a stoichiometric amount of the above Ru(II) complexes leads to the formation of the corresponding 2+2 homonuclear neutral squares of porphyrins of formula [trans,cis,cis-RuCl2(X)(Y)(4'-cisDPyP)]2 (X = Y = dmso-S, 4; X = Y = CO, 5; and X = dmso-S, Y = CO, 6) (Figure 2).14 Compared to most previous examples of molecular squares of porphyrins, which used square planar charged PtII and PdII complexes as structural units, the metallacyclic species 4 - 6 are neutral, thus providing greater solubility in organic solvents. Moreover, the two additional coordinated "spectator" ligands in the octahedral complexes at each corner, besides being, in principle, useful extra spectroscopic handles for characterization, might prove to be advantageous for a better fine-tuning of the properties of the metal and for further functionalization of the assemblies as well.

Molecular squares of porphyrins as building blocks for the construction of supramolecular arrays of higher order.14

We also showed how these discrete units, after metallation of the porphyrins, are suitable building blocks for the construction of supramolecular arrays of higher order by axial coordination of bridging ligands.14 In particular, introduction of zinc centers inside the porphyrin cores of the molecolar square 5, followed by treatment with one equivalent of trans ditopic N-donor ligands L (L = 4,4'-bipy, 4'-transDPyP), leads selectively to the quantitative assemblying of sandwich-like supramolecular adducts of stacked metallacycles of porphyrins of formula [(5Zn)2(µ-L)2] (Figure 4). Single crystal X-ray investigations showed that, depending on the nature of the bridging ligand, in the solid state these sandwich structures can be either maintained or originate polymeric chains, [(5Zn)(µ-L)]n.

When L = 4'-transDPyP, both solution and solid state data indicate that [(5Zn)2(µ-4'-transDPyP)2] can be regarded as a molecular box featuring two coplanar bridging porphyrins at a distance of about 11.4 Å (Figure 5)

When L = 4,4'-bipy, the corresponding adduct has the anticipated sandwich-like discrete architecture [(5Zn)2(µ-4,4'-bipy)2] in solution, but it assumes a stair-like polymeric wire structure [(5Zn)(µ-4,4'-bipy)]n in the solid state (Figure 6). The polymer [(5Zn)(µ-4,4'-bipy)]n is made by 5Zn squares bridged by 4,4'-bipy ligands which are axially coordinated alternatively on the two opposite sides of each square.

Two-Point Self-Coordination of the Dizinc(II) Bis-Pyridylporphyrin Ruthenium Complex (1Zn).16

The dizinc(II) bis-pyridylporphyrin ruthenium complex trans,cis,cis-RuCl2(CO)2(Zn·4'-cisDPyP)2 (1Zn) features two donor (the uncoordinated 4'-N(py) atoms) and two acceptor (the Zn atoms) sites and is thus a building block suited for two-point self-coordination. 1H NMR spectroscopy indicates that 1Zn self-assembles in solution through 4'-N(py)–Zn interactions to yield selectively a highly symmetrical discrete species, in which all donor and all acceptor sites of 1Zn are mutually saturated (Figure 7). Single crystal X-ray analysis established that this adduct is a dimeric species, (1Zn)2, with a global S4 symmetry, in which the four porphyrins have a propeller-like arrangement (Figure 8 and Figure 9). (1Zn)2 is a meso form derived from the combination of two 1Zn units with opposite helical chirality. The geometry of this highly symmetrical tetraporphyrin assembly in solution, determined by NMR spectroscopy, is essentially the same as that found in the solid state. Thus 1Zn is an unprecedented example of metal-containing self-complementary building block that selectively recognizes itself through four Zn-N(py) interactions yielding a very stable and symmetrical dimeric species, (1Zn)2, that features four porphyrins and six metal atoms (two Ru and four Zn).

Side-to-face assemblies of porphyrins

In the past, we prepared a series of oligomers of perpendicularly linked porphyrins by axial coordination of the 4'-pyridyl groups of 4'PyPs to [Ru(TPP)(CO)(EtOH)], namely the dimer, [Ru(TPP)(CO)(4'-MPyP)] (6), two trimers with different geometries, (4'-cisDPyP)[Ru(TPP)(CO)]2 (7) and (4'-transDPyP)[Ru(TPP)(CO)]2 (8), and the pentamer, (4'-TPyP)[Ru(TPP)(CO)]4 (9) (Figure 10).1 Later, by using meso 3'-pyridylporphyrins (3'PyPs) as building blocks, instead of 4'PyPs, we synthesized the canted analogs of selected perpendicular arrays.3, 4 In particular, we described the solution and solid state structure of the canted dimeric adduct, [Ru(TPP)(CO)(3'MPyP)] (10),3 and a thorough NMR characterization of the corresponding pentameric assembly, (3'TPyP)[Ru(TPP)(CO)]4 (11), together with a well-defined X-ray structure of the corresponding zinc derivative, (Zn·3'TPyP)[Ru(TPP)(CO)]4 (11Zn) (Figure 11).4

We also demonstrated that unsymmetrical architectures containing porphyrins and coordination compounds may be assembled by stepwise coordination of 4'-PyP’s to different metal centers.7

The detailed photophysical investigation of the assemblies of porphyrins prepared in Trieste have been performed in collaboration with the group of Professor F. Scandola at the University of Ferrara.5, 11, 15

Recent Publications

1) E. Alessio, M. Macchi, S. Heath, L. G. Marzilli «A novel open-box shaped pentamer of vertically linked porphyrins that selectively recognizes S-bonded Me2SO complexes.» Chem. Commun. 1996, 1411-1412.

2) E. Alessio, M. Macchi, S. L. Heath, L. G. Marzilli «Ordered supramolecular porphyrin arrays from a building block approach utilizing pyridylporphyrins and peripheral ruthenium complexes and identification of a new type of mixed-metal building block.» Inorg. Chem. 1997, 36, 5614-5623.

3) E. Alessio, S. Geremia, S. Mestroni, E. Iengo, I. Srnova, M. Slouf «Solution and solid state structure of a canted, side-to-face, bis-porphyrin adduct.» Inorg. Chem. 1999, 38, 869-875.

4) E. Alessio, S. Geremia, S. Mestroni, I. Srnova, M. Slouf, T. Gianferrara, A. Prodi « Porphyrin «flying saucers»: solid state and solution structure of a novel pentameric array of axially-ligated canted porphyrins.» Inorg. Chem., 1999, 38, 2527-2529.

5) A. Prodi, M. T. Indelli, C. J. Kleverlaan, F. Scandola, E. Alessio, T. Gianferrara, L. G. Marzilli «Side-to-face ruthenium porphyrin arrays. Photophysical behavior of dimeric and pentameric systems.» Chem. Eur. J. 1999, 5, 2668-2679.

6) T. Da Ros, M. Prato, D. Guldi, E. Alessio, M. Ruzzi, L. Pasimeni «A noncovalently linked, dynamic fullerene porphyrin dyad. Efficient formation of long-lived charge separated states through complex dissociation.» Chem. Commun. 1999, 635-636.

7) E. Alessio, E. Ciani, E. Iengo, V.Yu. Kukushkin, L. G. Marzilli «Stepwise Assembly of Unsymmetrical Supramolecular Arrays Containing Porphyrins and Coordination Compounds.» Inorg. Chem. 2000, 39, 1434-1443.

8) E. Iengo, B. Milani, E. Zangrando, S. Geremia, E. Alessio « Novel Ruthenium Building Blocks for the Efficient Modular Construction of Heterobimetallic Molecular Squares of Porphyrins.» Angew. Chem. Int. Ed. 2000, 39, 1096-1099.

9) E. Iengo, R. Minatel, B. Milani, L. G. Marzilli, E. Alessio «Metal-mediated self-assembly of molecular squares of porphyrins rimmed with coordination compounds.» Eur. J. Inorg. Chem. 2001, 609-612.

10) E. Iengo, E. Zangrando, S. Mestroni, G. Fronzoni, M. Stener, E Alessio «Complexed bridging ligands: oxorhenium(V) compounds with mono-coordinated pyrazine or pyrimidine as possible building-blocks for the construction of polynuclear architectures.» J. Chem. Soc., Dalton Trans., 2001, 1338-1346.

11) A. Prodi, C. J. Kleverlaan, M. T. Indelli, F. Scandola, E. Alessio, E. Iengo «Photophysics of Pyridylporphyrin Ru(II) Adducts. Heavy Atom Effects and Intramolecular Decay Pathways.» Inorg. Chem., 2001, 40, 3498-3504.

12) E. Alessio, E. Iengo, L. G. Marzilli «Metal-mediated discrete supramolecular assemblies of porphyrins.» Supramol. Chem., 2002, 14, 103-120.

13) D. M. Guldi, T. Da Ros, P. Braiuca, M. Prato, E. Alessio «C60 in the box. A supramolecular C60 – porphyrin assembly.» J. Mat. Chem., 2002, 12, 2001-2008.

14) E. Iengo, E. Zangrando, R. Minatel, E. Alessio «Metallacycles of porphyrins as building blocks in the construction of higher order assemblies through axial coordination of bridging ligands: solution and solid state characterization of molecular sandwiches and molecular wires.» J. Am. Chem. Soc. 2002, 124, 1003-1013.

15) A. Prodi, M. T. Indelli, C. J. Kleverlaan, E. Alessio, F. Scandola «Energy transfer pathways in pyridylporpyrin metal adducts and side-to-face arrays.» Coord. Chem. Rev., 2002, 229, 51-58.

16) E. Iengo, E. Zangrando, S. Geremia, R. Graff, B. Kieffer, E. Alessio «Two-point self-coordination of a dizinc(II) bis-pyridylporphyrin ruthenium complex leading selectively to a discrete molecular assembly: solution and solid state characterization.» Chem. Eur. J., 2002, 8, 4670-4674.

17) E. Iengo, E. Zangrando, E. Alessio, J.-C. Chambron, V. Heitz, L. Flamigni, J.-P. Sauvageo «A functionalized non-covalent macrocyclic multiporphyrin assembly from a dizinc(II) bis-porphyrin receptor and a free base bis-pyridyl porphyri.» Chem. Eur. J., 2003, 9, 5879-5887.

18) E. Iengo, E. Zangrando, E. Alessio «Discrete Supramolecular Assemblies of Porphyrins Mediated by Coordination Compounds.»  Eur. J. Inorg. Chem., 2003, 2371-2384.

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