Amidinium-carboxylate salt bridges

As previously shown, /V, A-dimetylan i I i ne acts as an electron donor toward the electronically excited Ru(II) tris(dipirydyl) complex (Bock et al. 1979). Recently, this complex was juxtaposed to N,N -dimethy laniline via a salt bridge (Deng et al. 1997 Kirby et al. 1997 Roberts et al. 1997). Supramolecular assemblies have been prepared in which an electron may transfer from A.A-dimethylaniline to the electronically excited Ru(II) tris(dipyridyl) acceptor through the intervening amidinium-carboxylate salt bridge interface (Scheme 5-15). 

A more pronounced role of the proton in the photoinduced PCET event can be imposed in model systems that contain a hydrogen bond interface composed of Dp and Ap pairs possessing pK s that can support the transfer of a proton. We have designed PCET networks assembled from asymmetric amidinium-carboxyl-ate salt bridges. In the solid state, the amidinium-carboxylate interface is ionic in nature and combines the dipole of an electrostatic ion-pair interaction within a hydrogen bonding network [108, 109]. The amidinium-carboxylate salt bridge in-... 

Figure 17.13 Two-point binding modes for guandinium-carboxylate and amidinium-carboxylate salt-bridges.

Yashima etal. [92] have designed and synthesized novel artificial double helixes, consisting of two complementary m-terphenyl-based strands intertwined through chiral amidinium-carboxylate salt bridges. Due to the chiral substituents on the amidine groups, the double... 

Otsuki et al. [27] have demonstrated that amidinium-carboxylate salt bridges, which have been used earlier to construct electron donor-acceptor dyads or a donor-spacer-acceptor triad, can also be used to assemble energy donor-acceptor dyad 13 and pentad 14. The salt bridge consists of complementary double hyi-ogen bonds and electrostatic interactions and, therefore, offers... 

An intriguing example of a structurally very simple self-replicating systems utilizes amidinium carboxylate salt bridges to enhance the catalytic condensation of an aniline structure with a benzaldehyde derivative (Figure IS). " Several structures with various substituents in para-position to the recognition sites have been investigated. For instance, autocatalysis was established in the formation of imine 14 from aniline 12 and aldehyde 13. Adding preformed template to the reaction mixture increased the initial rate of product formation, and the obtained experimental data was in accordance with the square root model. 

SCHEME 10.4 Amidinium-carboxylate salt bridge templated catenane 6. 

The effect of the salt bridge on electron transfer can be determined directly by a comparative kinetics study of a D—[amidinium-carboxylate]—A complex and its switched interface D—[carboxylate-amidinium]—A congener. We have reported such a study for a supramolecular series of complexes where the donor is a ruthenium(II) polypyridyl with one bipyridine (bpy) ligand modified by either amidinium or carboxylate and the acceptor is the complementarity modified 3,5-dinitrobenzene (DNB) [162, 163]. The same donor-acceptor pair bridged by a symmetrical dicarboxylic acid interface has also been examined. 

Nocera and co-workers (259) reported an amidinium functionalized ruthen-ium(II) bipyridyl system (138) binding 3,5-dinitrobenzoate (259). On binding the carboxylate anion, the chemical shift of the amidinium protons changes by >2.0 ppm (in DMSO). Electron-transfer kinetics in dichloromethane were investigated in order to elucidate the role of proton motion within the salt bridge... 

Electrostatic Interaction. Electron transfers in supramolecular systems with electrostatic interaction have also been reported. A rapid electron transfer within a cofacial heteroporphyrin dim was reported [621]. A cofacial ionic interaction between porphyrin and anthraquinone sulfonate was observed to be controlled by sugar hydroxylate [620]. As a model system of cytochrome c-peroxidase interaction [608,609], a reconstituted myoglobin with Zn porphyrin was associated with methylviologen through electrostatic interaction and the forward electron transfer was observed to be more favored than the back electron transfer [610] (Fig. 56). In a salt-bridged supramolecules having Zn amidinium porphyrin and aromatic carboxylate as a model of arginine-aspartate interaction [606], the electron transfer was considerably more decreased than expected [607]. 

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