Outer-Sphere Coordination Chemistry: Amido-Ammonium Ligands as Highly Selective Tetrachloridozinc(II)ate Extractants

Abstract

Eight new amido functionalized reagents, L 1 −L 8 , have been synthesized containing the sequence of atoms R 2 N−CH 2 −NR′−CO−R″, which upon protonation forms a six-membered chelate with a hydrogen bond between the tertiary ammonium N−H + group and the amido oxygen atom. The monocationic ligands, LH + , extract tetrachloridometal(II)ates from acidic solutions containing high concentrations of chloride ions via a mechanism in which two ligands address the "outer sphere" of the [MCl 4 ] 2unit using both N−H and C−H hydrogen bond donors to form the neutral complex as in 2L + 2HCl + MCl 2 ⇌ [(LH) 2 MCl 4 ]. The strengths of L 1 −L 8 as zinc extractants in these pH-dependent equilibria have been shown to be very dependent on the number of amide groups in the R 3-n N(CH 2 NR′COR″) n molecules, anti-intuitively decreasing with the number of strong hydrogen bond donors present and following the order monoamides > diamides > triamides. Studies of the effects of chloride concentration on extraction have demonstrated that the monoamides in particular show an unusually high selectivity for [ZnCl 4 ] 2over [FeCl 4 ] − and Cl − . Hybrid-DFT calculations on the tri-, di-, and monoamides, L 2 , L 3 , and L 4 , help to rationalize these orders of strength and selectivity. The monoamide L 4 has the most favorable protonation energy because formation of the LH + cation generates a "chelated proton" structure as described above without having to sacrifice an existing intramolecular amide−amide hydrogen bond. The selectivity of extraction of [ZnCl 4 ] 2over Cl − , represented by the process 2[(LH)Cl] + ZnCl 4 2-⇌ [(LH) 2 ZnCl 4 ] + 2Cl − , is most favorable for L 4 because it is less effective at binding chloride as it has fewer highly polar N−H hydrogen bond donor groups to interact with this "hard" anion.

Catalytic epoxidation by perrhenate through the formation of organic-phase supramolecular ion pairs

“…7 Furthermore, the activity of molecular catalysts such as methyltrioxorhenium (MTO) in the biphasic epoxidation of alkenes strongly depends on the nature of the reaction medium, being highest in hydrophobic ionic liquids. [8][9][10] Given that the transfer of [ReO 4 ] À into an organic phase has been established, 11 we considered it likely that the amido-ammonium and -pyridinium receptors L 1 -L 3 (Scheme 1), used recently in the solvent extraction of halidometalates, 12,13 could be exploited to transfer [ReO 4 ] À into a hydrophobic environment; this would potentially enhance its activity in oxidation catalysis by favouring supramolecular interactions with substrates such as H 2 O 2 . Herein, we report the synthesis and characterization of supramolecular ion pairs (SIPs) constructed from perrhenate and the organic receptor cations [HL 1 ] + -[HL 3 ] + , and their application in biphasic epoxidation catalysis.…”

New Insights into the Recovery of Strategic and Critical Metals by Solvent Extraction

“…shown above have been reported(Figure 11, right)[43]. Intrinsic to this design is the feature that, on protonation, a stable, six-membered pseudo chelate is formed.…”

Mutual stabilisation between MII4L6 tetrahedra and MIIX42− metallate guests

“…46–48 Secondly, Br – binds within the centre of Co II 10 L 15 pentagonal prism, Co- 2 , 40 which self-assembles from p -toluidine, 6,6′-diformyl-3,3′-bipyridine and Co II (ClO 4 ) 2 in acetonitrile. 40 Similarly, addition of KBr (0.6 equiv.)…”