Cooperative interactions play a very important role in both natural and synthetic supramolecular systems. We report here on the cooperative binding properties of a tetratopic ion-pair host 1. This host combines two isophthalamide anion recognition sites with two unusual "half-crown/two carbonyl" cation recognition sites as revealed by the combination of single-crystal X-ray analysis of the free host and the 1:2 host:calcium cation complex, together with two-dimensional NMR and computational studies. By systematically comparing all of the binding data to several possible binding models and focusing on four different variants of the 1:2 binding model, it was in most cases possible to quantify these complex cooperative interactions. The data showed strong negative cooperativity (α = 0.01-0.05) of 1 toward chloride and acetate anions, while for cations the results were more variable. Interestingly, in the competitive (CDCl3/CD3OD (9:1, v/v)) solvent, the addition of calcium cations to the tetratopic ion-pair host 1 allosterically switched "on" chloride binding that is otherwise not present in this solvent system. The insight into the complexity of cooperative interactions revealed in this study of the tetratopic ion-pair host 1 can be used to design better cooperative supramolecular systems for information transfer and catalysis.

The reaction of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) with RhCl 3 ‚3H 2 O has been studied under different experimental conditions. This reaction in ethanol resulted in the formation of [Rh(tptz)Cl 3 ]‚2H 2 O (1), whereas the bis-chelate complex [Rh(tptz) 2 ][ClO 4 ] 3 ‚2H 2 O (2) was obtained in a two-step reaction in acetone; the chlorides from RhCl 3 were removed in the first step using AgClO 4 , and the ligand tptz was added in the second step. Complexes 1 and 2, when refluxed in ethanol-water (1:1), resulted in metal-promoted hydrolysis of tptz to bis-(2-pyridylcarbonyl)amide anion (bpca) and 2-picolinamide (pa), yielding the complexes [Rh(bpca)(pa)Cl][PF 6 ]‚ H 2 O (3) and [Rh(bpca) 2 ][ClO 4 ] (6), respectively. A mixed-ligand complex, [Rh(bpca)(tpy)][PF 6 ] 2 ‚CH 3 CN (4), was obtained by the reaction of either 1 with tpy or [Rh(tpy)Cl 3 ] (5) with tptz in ethanol-water medium. The crystal structures of complexes 1 and 4 have been determined. Crystal data: complex 1, monoclinic, P2 1 /c, a ) 11.642(2) Å, b ) 7.302(2) Å, c ) 24.332(3) Å, β ) 96.420(10)°, Z ) 4, R ) 0.040, and wR2 ) 0.117; complex 4, triclinic, P1 h, a ) 9.581(1) Å, b ) 12.933(2) Å, c ) 14.493(2) Å, R ) 82.480(10)°, β ) 71.810(10)°, γ ) 75.100(10)°, Z ) 2, R ) 0.030, and wR2 ) 0.082. The two water molecules of complex 1 make short contacts with the carbon atoms adjacent to the metal-bound nitrogen atom of the triazine ring; this observation provides some insight about the "intermediate" of the hydrolysis. X-ray and NMR data suggest that the electron-withdrawing effect of the metal ion is the major responsible factor for the hydrolysis of tptz. The cyclic voltammograms of the complexes exhibit a metal-based 2e reduction (Rh(III) f Rh(I)) at the potential range -0.42 to -0.98 V vs SCE, followed by ligand-based redox couple(s). These novel complexes show effective catalytic properties for the electrocatalytic reduction of carbon dioxide in the potential range -1.26 to -1.44 V.

Three copper(II) complexes, Cu(S-X-salen), where salen2-= N,N-ethylenebis(salicylidenaminato) and X =(1), CH3O (2), or Cl (3), have been synthesized and characterized by various physicochemical techniques. Single crystal X-ray data of the complexes are as follows: 1, Ci6Hi4N202Cu, monoclinic C2/c, a = 26.

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