Complementary, Cooperative Ditopic Halogen Bonding and Electron Donor-Acceptor π-π Complexation in the Formation of Cocrystals

This study expands and combines concepts from two of our earlier studies. One study reported the complementary halogen bonding and π-π charge transfer complexation observed between isomeric electron rich 4-N,N-dimethylaminophenylethynylpyridines and the electron poor halogen bond donor, 1-(3,5-dinitrophenylethynyl)-2,3,5,6-tetrafluoro-4-iodobenzene while the second study elaborated the ditopic halogen bonding of activated pyrimidines. Leveraging our understanding on the combination of these non-covalent interactions, we describe cocrystallization featuring ditopic halogen bonding and π-stacking. Specifically, red cocrystals are formed between the ditopic electron poor halogen bond donor 1-(3,5-dinitrophenylethynyl)-2,4,6-triflouro-3,5-diiodobenzene and each of electron rich pyrimidines 2- and 5-(4-N,N-dimethyl-aminophenylethynyl)pyrimidine. The X-ray single crystal structures of these cocrystals are described in terms of halogen bonding and electron donor-acceptor π-complexation. Computations confirm that the donor-acceptor π-stacking interactions are consistently stronger than the halogen bonding interactions and that there is cooperativity between π-stacking and halogen bonding in the crystals.

Selective binding of anions by rigidified nanojars: sulfate vs. carbonate

Selective binding and transport of highly hydrophilic anions is ubiquitous in nature, as anion binding proteins can differentiate between similar anions with over a million-fold efficiency. While comparable selectivity has occasionally been achieved for certain anions using small, artificial receptors, the selective binding of certain anions, such as sulfate in the presence of carbonate, remains a very challenging task. Nanojars of the formula [anion⊂{Cu(OH)(pz)}_n]^2- (pz = pyrazolate; n = 27-33) are totally selective for either CO₃^2- or SO₄^2- over anions such as NO₃^-, ClO₄^-, BF₄^-, Cl^-, Br^- and I^-, but cannot differentiate between the two. We hypothesized that rigidification of the nanojar outer shell by tethering pairs of pyrazole moieties together will restrict the possible orientations of the OH hydrogen-bond donor groups in the anion-binding cavity of nanojars, similarly to anion-binding proteins, and will lead to selectivity. Indeed, by using either homoleptic or heteroleptic nanojars of the general formula [anion⊂Cu_n(OH)_n(L2-L6)_y(pz)_n-2y]^2- (n = 26-31) based on a series of homologous ligands HpzCH₂(CH₂)_xCH₂pzH (x = 0-4; H₂L2-H₂L6), selectivity for carbonate (with L2 and with L4-L6/pz mixtures) or for sulfate (with L3) has been achieved. The synthesis of new ligands H₂L3, H₂L4 and H₂L5, X-ray crystal structures of H₂L4 and the tetrahydropyranyl-protected derivatives (THP)₂L4 and (THP)₂L5, synthesis and characterization by electrospray-ionization mass spectrometry (ESI-MS) of carbonate- and sulfate-nanojars derived from ligands H₂L2-H₂L6, as well as detailed selectivity studies for CO₃^2-vs. SO₄^2- using these novel nanojars are presented.

Effect of substitution on halide/hydrated halide binding: a case study of neutral bis-urea receptors

The impact of the substituted moiety on the binding nature of the anion with a neutral receptor through non-cooperative interaction.

The contribution of polar C–H hydrogen bonds to anion binding

Binding abilities depend on the magnitude of C–H polarization.

Anion complexation with cyanobenzoyl substituted first and second generation tripodal amide receptors: crystal structure and solution studies

Solid and solution state anion binding via a side cleft and pseudocapsular fashion by two cyanophenyl substituted first (L1) and second (L2) generation tripodal amides has been demonstrated along with a selective anion induced conformational change of L2 from open C_2v to folded C_3v symmetry.

Hydrated anion glued capsular and non-capsular assembly of a tripodal host: Solid state recognition of bromide–water [Br5–(H2O)6]5− and iodide–water [I2–(H2O)4]2− clusters in cationic tripodal receptor

In this report we describe capsular and non-capsular assembly of polyammonium tripodal receptor into supramolecular network driven by anion or anion–water cluster and solid state recognition of unique bromide–water [Br₅–(H₂O)₆]⁵⁻ and iodide–water [I₂–(H₂O)₄]²⁻ clusters.

Oxyanion-encapsulated caged supramolecular frameworks of a tris(urea) receptor: evidence of hydroxide- and fluoride-ion-induced fixation of atmospheric CO2 as a trapped CO3(2-) anion

A tris(2-aminoethyl)amine-based tris(urea) receptor, L, with electron-withdrawing m-nitrophenyl terminals has been established as a potential system that can efficiently capture and fix atmospheric CO(2) as air-stable crystals of a CO(3)(2-)-encapsulated molecular capsule (complex 1), triggered by the presence of n-tetrabutylammonium hydroxide/fluoride in a dimethyl sulfoxide solution of L. Additionally, L in the presence of excess HSO(4)(-) has been found to encapsulate a divalent sulfate anion (SO(4)(2-)) within a dimeric capsular assembly of the receptor (complex 2) via hydrogen-bonding-activated proton transfer between the free and bound HSO(4)(-) anions. Crystallographic results show proof of oxyanion encapsulation within the centrosymmetric cage of L via multiple N-H···O hydrogen bonds to the six urea functions of two inversion-symmetric molecules. The solution-state binding and encapsulation of oxyanions by N-H···O hydrogen bonding has also been confirmed by quantitative (1)H NMR titration experiments, 2D NOESY NMR experiments, and Fourier transform IR analyses of the isolated crystals of the complexes that show huge spectral changes relative to the free receptor.

Anion-templated supramolecular C3 assembly for efficient inclusion of charge-dispersed anions into hydrogen-bonded networks

The binding properties and conformational adaptability of a known nitrate/sulfate receptor N,N'-3-azapentane-1,5-bis[3-(1-aminoethylidene)-6-methyl-3H-pyran-2,4-dione] (L) toward various charge-dispersed monoanions (HSO(3)(-), ClO(4)(-), IO(4)(-), PF(6)(-), and SbF(6)(-)) are considered. These anions template the folding of three HL(+) species through a self-assembly process into a new hollow supramolecular trication. During the self-assembly, all strong hydrogen-bond donors of the podand become coordinatively saturated by interactions with the oxo functionalities from other HL(+) molecules. In that way, only the weak hydrogen-bond-donating groups in the exterior part of the receptor are accessible for anion binding. The investigated anions are accommodated in the hydrophobic pockets of the isomorphous hydrogen-bonded frameworks, which serve as a basis for selective crystallization from the highly competitive anion/solvent systems. This behavior is discussed in terms of size and geometry of the anions as well as the receptor's coordination capabilities to provide the most favorable surroundings for guest inclusion both in solution and in the solid state.