| Abstract
| - The seven-coordinate [Fe(dapsox)(CH3CN)2]+ complex, in its Fe(III), Fe(II), and anion-substituted forms, having a shape of “charged tweezers”, is able to intimately associate with the lithium salts by means of shape recognition, hydrogen-bond complementarity, and charge assistance.
- Interactions between the seven-coordinate tweezerlike [Fe(dapsox)(H2O)2]ClO4 complex (H2dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with different lithium salts (LiOTf, LiClO4, LiBF4, and LiPF6) in CH3CN havebeen investigated by electrochemical, spectrophotometric, 7Li and 19F NMR, kinetic, and DFT methods. It has beendemonstrated that this complex acts as ditopic receptor, showing spectral and electrochemical ion-pair-sensingcapability for different lithium salts. In general, the apparent binding constants for lithium salts increase in the orderLiOTf < LiClO4< LiBF4. From the electrochemical measurements, the apparent lithium salt binding constants forthe Fe(III) and Fe(II) forms of the complex have been obtained, suggesting a stronger host−guest interaction withthe reduced form of the complex. In the presence of LiPF6, the solution chemistry is more complex because of thehydrolysis of PF6-. The kinetics of the complexation of [Fe(dapsox)(CH3CN)2]+ by thiocyanate at −15 °C in acetonitrilein the presence of 0.2 M NBu4OTf shows two steps with the following rate constants and activation parameters: k1 = 411 ± 14 M-1 s-1; ΔH1≠ = 9 ± 2 kJ mol-1; ΔS1≠ = −159 ± 6 J K-1 mol-1; k2 = 52 ± 1 M-1 s-1; ΔH2≠= 4 ± 1 kJ mol-1; ΔS2≠ = −195 ± 3 J K-1 mol-1. The very negative ΔS≠ values are consistent with an associative(A) mechanism. Under the same conditions but with 0.2 M LiOTf, k1Li and k2Li are 1605 ± 51 and 106 ± 2 M-1 s-1,respectively. The increased rate constants for the {[Fe(dapsox)(CH3CN)2]·LiOTf}+ adduct are in agreement withan associative mechanism. Kinetic and spectrophotometric titration measurements show stronger interaction betweenthe lithium salt and the anion-substituted forms, [Fe(dapsox)(CH3CN)(NCS)] and [Fe(dapsox)(NCS)2]-, of the complex.These experiments demonstrate that in nonaqueous media lithium salts cannot be simply used as supportingelectrolytes, since they can affect the kinetic behavior of the studied complex. DFT calculations revealed that thenegatively charged α-oxyazine oxygen atoms are responsible for cation binding (electrostatic interactions), whereasthe two terminal amide groups bind the anion via hydrogen bonding.
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