Seven-Coordinate Iron Complex as a Ditopic Receptor for Lithium Salts:  Study of Host−Guest Interactions and Substitution Behavior

Abstract

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 have been investigated by electrochemical, spectrophotometric, 7Li and 19F NMR, kinetic, and DFT methods. It has been demonstrated that this complex acts as ditopic receptor, showing spectral and electrochemical ion-pair-sensing capability for different lithium salts. In general, the apparent binding constants for lithium salts increase in the order LiOTf < LiClO4 < LiBF4. From the electrochemical measurements, the apparent lithium salt binding constants for the Fe(III) and Fe(II) forms of the complex have been obtained, suggesting a stronger host-guest interaction with the reduced form of the complex. In the presence of LiPF6, the solution chemistry is more complex because of the hydrolysis of PF6-. The kinetics of the complexation of [Fe(dapsox)(CH3CN)2]+ by thiocyanate at -15 degrees C in acetonitrile in the presence of 0.2 M NBu4OTf shows two steps with the following rate constants and activation parameters: k(1) = 411 +/- 14 M(-1) s(-1); DeltaH(1) not equal = 9 +/- 2 kJ mol(-1); DeltaS1 not equal = -159 +/- 6 J K(-1) mol(-1); k(2) = 52 +/- 1 M(-1) s(-1); DeltaH(2) not equal = 4 +/- 1 kJ mol(-1); DeltaS(2) not equal = -195 +/- 3 J K(-1) mol(-1). The very negative DeltaS not equal 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] x LiOTf}+ adduct are in agreement with an associative mechanism. Kinetic and spectrophotometric titration measurements show stronger interaction between the 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 supporting electrolytes, since they can affect the kinetic behavior of the studied complex. DFT calculations revealed that the negatively charged alpha-oxyazine oxygen atoms are responsible for cation binding (electrostatic interactions), whereas the two terminal amide groups bind the anion via hydrogen bonding.

Beyond H2S and NO Interplay: Hydrogen Sulfide and Nitroprusside React Directly to Give Nitroxyl (HNO). A New Pharmacological Source of HNO

“…Similar behavior has been reported for the binding of thiocyanate to another type of iron center. 32 The 720 nm band for the above-mentioned transient species is characteristic of a mixed-valent cyanobridged complex (a Prussian blue type compound) that could be independently o b t a i n e d e i t h e r f r o m t h e r e a c t i o n o f a q u a [Fe II (CN) 5−x (SCN) x (H 2 O)] 3− complexes in the presence of oxygen or upon addition of the corresponding Fe(III) complexes. 30 The UV/vis band at approximately 440 nm can be attributed to transient [(CN) 5 Fe(HNO)] 3− (inset in Figure 4A).…”

Role of Spin State and Ligand Charge in Coordination Patterns in Complexes of 2,6-Diacetylpyridinebis(semioxamazide) with 3d-Block Metal Ions: A Density Functional Theory Study

“…22,23,25,60 Because these complexes represent mimics of superoxide dismutase (SOD) and ditopic receptors for lithium salts, they are of the utmost importance, and hence detailed kinetic and mechanistic experimental studies were reported previously. 29 2 ] n+ , explaining the nature of correctly tuned redox potentials in catalytic superoxide disproportionation.…”

An Ionophore for High Lithium Loading and Selective Capture from Brine

“…Although ionophores have been considered early on, the synthetic ionophores with high affinity and selectivity for lithium ion is a challenging task. In this regard, considerable efforts have been given on crown ether derivatives, − spherands, hemispherands, , metallamacrocycles, − and various lithium ionophores/​receptors − which often require substantial synthetic efforts.…”