Solvent-free supramolecular complexation

Besides reactions with alkali salts, there are several groups investigating metal-ligand bond formation by mechanochemical methods. For example Orita et al. have shown that the supramolecular self-assembly of a number of complexes can take place under solvent-free conditions, leading to higher-order fabrications of two- or three-dimensional topology and even double... 

Therefore, thermodynamics plays a fundamental role in supramolecular chemistry. However, thermodynamics is rigorous and as such, a great deal of ancillary information is required prior to the formulation of an equation representative of the process taking place in solution, such as, the composition of the complex and the nature of the speciation in solution. For the latter and when electrolytes are involved, knowledge of the ion-pair formation of the free and complex salts in the appropriate solvent is required particularly in non-aqueous solvents. This information would allow the establishment of the concentrations at which particular ions are the predominant species in solution. Similar considerations must be taken into account when neutral receptors are involved, given that in dipolar aprotic or inert solvents, monomeric species are not always predominant in solution. In addition, awareness of the scope and limitations of the methodology used for the derivation of thermodynamic data for the complexation process is needed and this aspect has been addressed elsewhere [18]. 

Solvent free methods have been used extensively in supramolecular chemistry, coordination chemistry and the formation of transition metal clusters and polymers. Reactions range from very simple ligand substitution reactions for salts of labile metal ions to more complex procedures, some of which are outlined below. 

Type of reaction supramolecular complexation Reaction condition solvent-free... 

Type of reaction Supramolecular Complexation Reaction condition solvent-free Keywords [2]rotaxan... 

A solvent-free self-assembly offullerene Cjq using high-speed vibration milling has been described [19]. This simple, efficient, and green pathway to the synthesis of a supramolecular complex in the solid state opened a new avenue for the application of solventless reactions. 

Kim et al. reported the formation of supramolecular complex 22 of the benzo-annulated TTF calix [4] pyrrole (TTF-C4P) as an electron donor with porphyrin as an electron acceptor in benzonitrile [78]. The TTF-C4P binds to the carboxylate moiety of the porphyrin through H-bonding with a 1 1 stoichiometiy and a binding constant of 6.3 x 10" IVT in this solvent at 298 K. Photoexcitation of complex 22 formed between these two components in PhCN at 298 K afforded the CS state, which was characterized by forward and backward intramolecular ET rate constants of 2.1 X 10" and 3.6 x 10 s, respectively. The triplet CS state produced upon photoirradiation of porphyrin was found to be 2.8 ms, one of the longest known lifetimes for a CS generated via a photoinduced electron transfer process within a noncovalentiy bound complex. It is interesting to see that free metal porphyrin acts as electron acceptor instead of donor in this case. 

In a supramolecular approach to fullerene-porphyrin hybrids, the assembly of a rigidly connected dyad, in which a zinc tetraphenylporphyrin, Zn(TPP), is noncovalently linked to a C60 derivative via axial pyridine coordination to the metal, was reported [219-222]. Photo excitation of the dyad Zn-complex led to electron transfer with very long lifetimes of the charge-separated pairs, as revealed by optical spectroscopy and confirmed by time-resolved electron paramagnetic resonance spectroscopy. Accordingly, two different solvent-dependent pathways can be considered for the electron-transfer processes. Either the excitation of the porphyrin chromophore is followed by fast intramolecular electron transfer inside the complex, or alternatively the free porphyrin is excited undergoing intermolecular electron transfer when the acceptor molecules ap-... 

The reactant state is converted to the product state by the transfer of one electron. The participants in the reactant state may be individual molecules held transiently in proximity by a solvent cage or they can be distinct parts of a supramolecular unit. Several types of chemical species can make up the reactant state it may contain only ground-state, spin-paired entities, or electronically excited entities (singlet or other multiplicity), or reactive entities (free radicals, metal complexes in unusual oxidation states, etc.) Many combinations are possible, and a large variety of reactant states can be prepared from some precursor state by photon absorption. The chapters in this series of volumes contain an abundance of examples. In every case, however, no matter what the identity of the entities participating in the process, the... 

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