Four bis(guanidinium) receptors have been synthesized in which the guanidinium groups are spatially preorganized by an octahydroacridine (meso-3 and d,l-3) or hexahydrodicyclopenta [b,e\pyridine (meso-4 and d,l-4) spacer to complement a phosphodiester. These structures are designed to mimic the active site of staphylococcal nuclease and, thereby, form four hydrogen bonds to a bound phosphodiester with little reorganization of the host structures. The syntheses involve two parts: construction of the spacer and formation of the aminoimidazoline groups via an intramolecular cyclization between an amine and a thiouronium salt. Binding constants between the receptors and dibenzyl phosphate range from 4.0 X 10* 123 to 10 M~' in highly competitive solvent systems such as aqueous DMSO. Each receptor forms both a 1:1 and 2:1 phosphate to host complex. The methods for determining K\ and K2 are discussed in detail and involve both 31P and *H NMR titration experiments followed by a linear treatment of the data. Binding in pure DMSO is worth 3-4 kcal/mol, but the addition of water significantly decreases the degree of complexation. When the guanidinium counterions are tetraphenylboron, the meso forms of the hosts are the best receptors due to preorganization of the guanidinium groups on the same face of the spacer. When the counterions are chloride, the d,l forms can be the best receptors due to a specific ion effect where a chloride is involved in the host-guest complex. Addition of chloride salts increases binding, possibly due to a chaotropic "salting-out" phenomenon. The structures of the host-guest complexes of meso-4 with dibenzyl phosphate and phenyl phosphate have been determined by X-ray analysis. The structures demonstrate the chloride-counter ion assistance and confirm the four hydrogen bonds between the host and the guest. Near-identical structures to the crystal structures are calculated by molecular mechanics for the complex formed between dimethyl phosphate and meso-3 and meso-4. meso-3 has been found to act as an RNA hydrolysis catalyst and is the first step toward the optimization of a functional RNA-cleaving artificial enzyme.

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In the presence of moderate to high concentrations of electrolytes, the emission of *[Pt2(pop)4]4- (where pop = μ-pyrophosphite-P,P‘) is quenched by the complexes [Co(CN)5X]3- (where X = N3 -, I-, Br-, Cl-, but not CN-). The salt effects on the emission decay lifetime quenching rate constants between these anionic species have been studied in the presence of MCl, M‘Cl2, or R n NH4 - n Cl (where M, M‘, and R represent alkali, alkaline earth metals, and alkyl respectively, n = 0−3) and K n X (X = Cl-, Br-, NO3 -, SO4 2-, [Co(CN)6]3-, n = 1−3). At 0.5 M cation concentration, second-order quenching rate constants, k q, are in the “nearly diffusion-controlled” range, 107−109 L mol-1 s-1, and k q decreases by an order of magnitude across the series of quenchers [Co(CN)5I]3- > [Co(CN)5N3]3- > [Co(CN)5Br]3- > [Co(CN)5Cl]3-. On the basis of a detailed study of [Co(CN)5I]3-, the quenching efficiency increases with background electrolyte concentration and the measured rate constants are in good agreement with predictions based on the Debye−Smoluchowski and Debye−Eigen equations for diffusion-controlled formation and dissociation in ionic solution of an encounter pair, together with a rate constant of 1.2 × 109 s-1 for the quenching step. However, the analysis provides further evidence for the Olson−Simonson effect; that is, in the presence of multivalent electrolyte ions, the salt effects are determined by the counterion concentration, here the cation, rather than by the ionic strength. Specific cation effects are observed such that the quenching rate constants increase in the following sequences:  Li+ < Na+ < K+ < Cs+; Mg2+ < Ca2+ < Sr2+ < Ba2+; NH4 + < MeNH3 + < Me2NH2 + < Me3NH+; Et3NH+ < Et2NH2 + < EtNH3 +; n-PrNH3 + < EtNH3 + < MeNH3 +. For the alkali or alkaline-earth cations the large effects seen require participation of the cation in the transition state for the quenching step; the alkylammonium cations are also effective in this role, but the small differences in their efficiencies can be rationalized in terms of their effects on water structure.

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The thermal anation of Co(CN)sBr3by 2 M thiocyanate ion at 22 °C in 0.1 M NaOH proceeds by two pathways. Direct anation occurs with a pseudo-first-order rate constant of about 8 x 10-6 s-1, while anation of Co(CN)5(H20)2-in equilibrium with Co(CN)j(C>H)3-formed by the concurrent base hydrolysis of Co(CN)5Br3_ occurs with a pseudo-first-order rate constant of 2.15 x 10-6 s-1. The thiocyanate anation product obtained shows an overall S:N-bonded linkage isomer ratio of 4.5 ± 0.2. Measurements of the product S:N linkage isomer ratio and of the ratio of aquation to anation were obtained for photosubstitution of CoCCNjsNa3-at 5 °C and pH 13 in 2 M aqueous thiocyanate with various counterions. For the group of singly charged ions Li+, Na+, K+, and NH4"* 1", the aquation:anation ratio ranged from 9.9 ± 1.4 for Li+ to 2.5 ± 0.5 for NH4"1" while the range of S:N product ratios was 12.7 ± 1.1 to 10.6 ± 2.6 over the same group. These results reveal a linear relationship between ln(aquation/anation) and 1/hydrated radius, while the S:N ratio is essentially constant. Compared with Li+, Ca2+ showed the increase in anation efficiency expected on the basis of its higher charge. A constant total quantum yield of 0.073 ± 0.005 was found for photosubstitution of Co(CN)5l3-45in NaClOVNaSCN mixtures of total concentration 4 M, showing that the anation and aquation are competitive processes. The cation apparently assists anation by helping overcome the Coulomb repulsion between the entering thiocyanate and a coordinatively unsaturated intermediate of like charge. The higher S:N ratio for the photoprocess is not a result of the cation assistance but is an intrinsic property of the intermediate, consistent with involvement of the triplet state fiveindependent aquation to anation product ratio. These results were attributed to the participation in the photoreaction of a