Cavitands Incorporating a Lewis Acid Dinickel Chelate Function as Receptors for Halide Anionsby Alexander Jeremies, Ulrike Lehmann, Sina Gruschinski, Frederik Schleife, Michel Meyer, Vitaly Matulis, Oleg A. Ivashkevich, Marcel Handke, Karolin Stein, Berthold Kersting

Inorganic Chemistry

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Year
2015
DOI
10.1021/acs.inorgchem.5b00123
Subject
Inorganic Chemistry / Physical and Theoretical Chemistry

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Text

Cavitands Incorporating a Lewis Acid Dinickel Chelate Function as

Receptors for Halide Anions

Alexander Jeremies,† Ulrike Lehmann,† Sina Gruschinski,† Frederik Schleife,† Michel Meyer,*,‡

Vitaly Matulis,§ Oleg A. Ivashkevich,∥ Marcel Handke,† Karolin Stein,† and Berthold Kersting*,† †Institut für Anorganische Chemie, Universitaẗ Leipzig, Johannisallee 29, 04103 Leipzig, Germany ‡Institut de Chimie Molećulaire de l’Universite ́ de Bourgogne (ICMUB), UMR CNRS 6302, 9, avenue Alain Savary, BP 47870, 21078

Dijon Cedex, France §Research Institute for Physical Chemical Problems, Belarusian State University, Leningradskaya 14, 220030 Minsk, Belarus ∥Belarusian State University, 4 Nezavisimisti avenue, 220050 Minsk, Belarus *S Supporting Information

ABSTRACT: The halide binding properties of the cavitand [Ni2(L

Me2H4)]2+ (4) are reported. Cavitand 4 exhibits a chelating

N3Ni(μ-S)2NiN3 moiety with two square-pyramidal Ni

IIN3S2 units situated in an anion binding pocket of ∼4 Å diameter formed by the organic backbone of the (LMe2H4)2− macrocycle. The receptor reacts with fluoride, chloride (in MeCN/MeOH), and bromide (in MeCN) ions to afford an isostructural series of halogenidobridged complexes [Ni2(L

Me2H4)(μ-Hal)]+ (Hal = F− (5), Cl− (6), and Br− (7)) featuring a N3Ni(μ-S)2(μ-Hal)NiN3 core structure.

No reaction occurs with iodide or other polyatomic anions (ClO4 −, NO3 −, HCO3 −, H2PO4 −, HSO4 −, SO4 2−). The binding events are accompanied by discrete UV−vis spectral changes, due to a switch of the coordination geometry from square-pyramidal (N3S2 donor set in 4) to octahedral in the halogenido-bridged complexes (N3S2Hal donor environment in 5−7). In MeCN/MeOH (1/1 v/v) the log K11 values for the 1:1 complexes are 7.77(9) (F−), 4.06(7) (Cl−), and 2.0(1) (Br−). X-ray crystallographic analyses for 4(ClO4)2, 4(I)2, 5(F), 6(ClO4), and 7(Br) and computational studies reveal a significant increase of the intramolecular distance between two propylene groups at the cavity entrance upon going from F− to I− (for the DFT computed structure). In case of the receptor 4 and fluorido-bridged complex 5, the corresponding distances are nearly identical. This indicates a high degree of preorganization of the [Ni2(L

Me2H4)]2+ receptor and a size fit mismatch of the receptor binding cavity for anions larger than F−. ■ INTRODUCTION

Since the seminal work of Simmons and Park,1 the host−guest chemistry of macrocyclic anion receptors has been extensively investigated, and more sophisticated examples with better sizecomplementarity and higher degrees of preorganization2 have been reported.3,4 A particularly large amount of work has been devoted to the design of artificial halide receptors with the goal to design molecular-based sensors, receptors, and transporters.5,6 Designing a selective halide host is very challenging, given that the halide ions behave like spherical charges adopting various coordination geometries without specific binding sites.7

As with cation hosts selectivity is generally enhanced for cyclic or polycyclic structures, although acyclic structures exhibit interesting halide-binding properties and selectivity as well.8

One very successful approach to bind halide anions is chelation by Lewis acids,9 and several examples of multidentate

Lewis-acids incorporating d-block and p-block metals have now been reported.10 Of these, the cascade complexes with closedshell structures have received much attention.11−13 The analogous chemistry of metallocavitands featuring an open binding site has been investigated far less frequently.14−16

Recently, we described the binuclear complex [Ni2(L

H6)]2+ (1) supported by the macrocycle H2L

H6 (Figure 1).17 Although being coordinatively unsaturated, the [Ni2(L

H6)]2+ dication has no affinity for halide ions, in striking contrast to nickel complexes supported by smaller N6S2 macrocycles with lateral diethylene triamine linkers18,19 and other chelating ligands.20−22 We reasoned that partial N-alkylation would decrease the ligand-field strength of the macrocycle to access coordination numbers higher than 5, thereby generating an active receptor. In this Paper we show that the corresponding [Ni2(L

Me2H4)]2+ complex (4)23 strongly binds the small spherical fluoride and chloride ions in acetonitrile/methanol solution, has little affinity for the larger bromide ion, and has essentially no affinity for iodide or polyatomic ions of trigonalplanar or tetrahedral geometry. This behavior is discussed in

Received: January 19, 2015

Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.5b00123

Inorg. Chem. XXXX, XXX, XXX−XXX the light of the confined binding cavity and preorganization of 4. ■ RESULTS AND DISCUSSION

The targeted supporting ligand H2L

Me2H4 was prepared in two steps starting from known tetraaldehyde 2 (Scheme 1). A [2 + 1] condensation reaction between 2 and N1-(3-aminopropyl)N1-methylpropane-1,3-diamine, followed by NaBH4 reduction provided bicyclic macrocycle 3 whose thioether linkage was subsequently cleaved by Na/NH3. The free macrocycle was isolated as an air-stable hexahydrobromide salt in 62% overall yield (based on 2). 1H NMR and 13C NMR spectroscopies, mass spectrometry, and elemental analysis are consistent with the proposed formulation.

The synthesized complexes and their labels are collected in

Scheme 2. Reaction of H2L

Me2H4·6HBr with NiBr2·6H2O and

NEt3 in MeOH gave a dark green solution, from which the receptor [Ni2(L

Me2H4)]2+ (4) could be reproducibly isolated as a dark green perchlorate salt in 70% yield. Although isostructural with the parent complex 1,17 the host−guest properties of 4 were found to be strikingly different. In contrast to inactive 1,24 the dark green receptor 4 reacts readily with fluoride or chloride to form the pale green 1:1 complexes [Ni2(L

Me2H4)(μ-Hal)]+ (Hal = F− (5), Cl− (6)).

The receptor 4 has only little affinity for anions larger than