Anion Binding Hosts & Supramolecular Coordination Assemblies

While supramolecular chemistry initially flourished around cation coordination, designing synthetic receptors for anions presented substantially greater chemical hurdles. Anion recognition chemistry developed several decades after cation coordination due to four fundamental physical differences between anions and cations.

The Four Physical Challenges of Anion Binding

1. Larger Size and Diffuse Charge:

Anions possess significantly larger ionic radii than isoelectronic cations (e.g., fluoride $\text{F}^-$ has radius $1.33\text{ Å}$, whereas isoelectronic sodium $\text{Na}^+$ has radius $1.02\text{ Å}$; chloride $\text{Cl}^-$ has $r = 1.81\text{ Å}$ vs $\text{K}^+$ with $1.38\text{ Å}$). Because charge is distributed over a larger volume, anions possess lower charge-to-radius ratios ($q/r$), resulting in weaker purely electrostatic attractions per contact.

2. High Solvation Energies:

Anions have high hydration enthalpies because protic solvents (especially water) form directional hydrogen bonds to anion lone pairs:

To bind an anion, a synthetic receptor must pay an immense thermodynamic penalty to strip away these tightly bound water molecules.

3. Geometric and Dimensional Anisotropy:

Unlike spherically symmetric monoatomic cations ($\text{Na}^+, \text{K}^+$), anions exhibit diverse, non-spherical spatial geometries:

• Spherical ($K_h$ symmetry): halides ($\text{F}^-, \text{Cl}^-, \text{Br}^-, \text{I}^-$)
• Linear ($D_{\infty h}$): azide ($\text{N}_3^-$), thiocyanate ($\text{SCN}^-$)
• Trigonal Planar ($D_{3h}$): nitrate ($\text{NO}_3^-$), carbonate ($\text{CO}_3^{2-}$)
• Tetrahedral ($T_d$): sulfate ($\text{SO}_4^{2-}$), phosphate ($\text{PO}_4^{3-}$), perchlorate ($\text{ClO}_4^-$)
• Octahedral ($O_h$): hexafluorophosphate ($\text{PF}_6^-$)

A host must be engineered with geometric complementarity tailored to these specific point group symmetries.

4. Pronounced pH Sensitivity:

Many biologically and environmentally vital anions are conjugate bases of weak acids (e.g., carboxylates, hydrogen phosphate $\text{HPO}_4^{2-}$, bicarbonate $\text{HCO}_3^-$). Altering solution pH by $1 - 2$ units drastically shifts protonation equilibria, altering the net charge and hydrogen-bonding donor/acceptor profile of the guest.

The development of neutral, uncharged anion receptors by Kelly, Hamilton, Reinhoudt, and Gale revolutionized anion supramolecular chemistry. Neutral receptors bind anions through directional hydrogen-bonding arrays without requiring positively charged counterions.

Urea and Thiourea Receptors

Ureas and thioureas possess two convergent, coplanar $\text{N-H}$ hydrogen-bond donor groups oriented with an angle of $\approx 60^\circ$ between their donor axes:

• In the anti,anti (syn,syn) binding conformation, the two $\text{N-H}$ protons point directly toward the anion, forming a six-membered chelate-like hydrogen-bonded ring with halides or bidentate carboxylate/phosphate oxygens:

• Thiourea vs Urea: Thioureas are superior anion receptors compared to ureas:

1. The $\text{N-H}$ protons of thioureas are substantially more acidic ($ ext{p}K_a \approx 21$ in DMSO for diphenylthiourea vs $\text{p}K_a \approx 27$ for diphenylurea) due to the greater polarizability and lower basicity of sulfur.
2. The thiocarbonyl sulfur is a weaker hydrogen-bond acceptor than oxygen, preventing self-association (dimerization) of the host that would otherwise compete with anion binding.

Squaramides: Four-Membered Aromatic Hydrogen-Bonding Clefts

Squaramides (diamides of squaric acid) have emerged as powerful hydrogen-bonding scaffolds:

• The cyclobutenedione ring enforces a planar geometry with an increased spacing of $d(\text{N}\cdots\text{N}) \approx 2.72\text{ Å}$ compared to ureas ($2.2\text{ Å}$), providing an ideal bite angle for larger oxoanions ($\text{SO}_4^{2-}, \text{HPO}_4^{2-}$).
• Aromaticity Gain: Upon deprotonation or hydrogen bonding to an anion, polarization of the two carbonyl groups contributes to a $2\pi$-electron Hückel aromatic resonance structure across the four-membered ring, thermodynamically augmenting the hydrogen-bond donor strength via resonance-assisted hydrogen bonding (RAHB).

Heterocyclic compounds containing acidic $\text{N-H}$ bonds—most prominently pyrrole, dipyrromethanes, and calixpyrroles—constitute a fundamental class of neutral anion receptors pioneered by Jonathan L. Sessler.

Calix[4]pyrrole: Structure and Synthesis

Calix[4]pyrroles are macrocycles consisting of four pyrrole rings linked through their 2- and 5-positions by $sp^3$-hybridized quaternary carbon meso-bridges.

• First prepared by Baeyer in 1886 from the acid-catalyzed condensation of pyrrole with acetone:

• The four $sp^3$ meso-carbons prevent the macrocycle from becoming planar and aromatic, leaving it flexible with four distinct conformational states: cone, partial cone, 1,2-alternate, and 1,3-alternate.

Induced-Fit Conformational Switching

In the uncomplexed state in solution and solid state, calix[4]pyrrole adopts the 1,3-alternate conformation:

• Two opposing pyrrole rings point upward, and the other two point downward, minimizing electrostatic repulsion between nitrogen lone pairs and dispersing the $\text{N-H}$ dipole vectors.
• Upon addition of a halide (especially fluoride $\text{F}^-$ or chloride $\text{Cl}^-$), the macrocycle undergoes an allosteric induced-fit conformational reorganization into the cone conformation:

In the cone conformation, all four pyrrole $\text{N-H}$ bonds point in the same direction, converging onto the central halide ion in a square-pyramidal coordination geometry with four simultaneous $\text{N-H}\cdots\text{Cl}^-$ hydrogen bonds ($d(\text{N}\cdots\text{Cl}) \approx 3.25\text{ Å}$).

Calix[4]pyrrole exhibits remarkable selectivity for fluoride ($K_a > 10^5\text{ M}^{-1}$ in acetonitrile) and chloride ($K_a \approx 3.5 \times 10^3\text{ M}^{-1}$), while completely excluding larger iodide ($\text{I}^-$) due to steric clash within the narrow cone cavity.

The earliest synthetic anion hosts relied on electrostatic Coulomb attractions generated by protonated polyammonium or permanently charged quaternary ammonium/guanidinium centers.

Katapinands and Polyazacryptands

In 1968, C. H. Park and H. E. Simmons synthesized katapinands (from Greek katapino, to swallow), macrobicyclic diamines capable of encapsulating chloride inside their cavity:

• Upon protonation of both bridgehead tertiary amines, the host adopts an $in,in$-diprotium state ($[\text{H}_2\text{L}]^{2+}$) where both $\text{N}^+-\text{H}$ protons point inward.
• The chloride ion is coordinated between the two internal ammonium protons:

• Jean-Marie Lehn expanded this concept to spherical polyazacryptands containing six or eight secondary/tertiary amino groups. In acidic aqueous solution, hexaprotonation produces a hexacationic cage $[\text{L}\cdot 6\text{H}]^{6+}$:
• Encloses a spherical cavity lined with six inward-directed ammonium groups.
• Displays astronomical affinity for chloride ($\log K_a > 4.0$ in water!), but rejects smaller fluoride because $\text{F}^-$ cannot simultaneously span the six divergent $\text{N}^+-\text{H}$ bonds.

The Guanidinium Motif: Biological Phosphate Recognition

The guanidinium cation ($\text{C}(\text{NH}_2)_3^+$, side chain of arginine) is uniquely suited for anion recognition:

1. High Basicity: Possesses a $\text{p}K_a \approx 13.5$, ensuring it remains $100\%$ positively charged across the entire physiological pH range ($1 - 12$).

2. Resonance Planarity and Dual Donors: Resonance delocalizes the $+1$ charge symmetrically across three nitrogen atoms. The planar guanidinium ring presents two parallel $\text{N-H}$ hydrogen-bond donors with an ideal distance of $2.23\text{ Å}$ to form a planar, bidentate salt bridge with oxoanions:

This structural motif governs substrate binding in staphylococcal nuclease, ATP-dependent kinases, and synthetic DNA-binding clefts.

Beyond classical hydrogen bonding and Coulombic interactions, modern supramolecular chemistry exploits non-covalent interactions driven by anisotropic electrostatic potential distributions: halogen bonding (XB) and chalcogen bonding (ChB).

The sigma-Hole Concept

When a halogen atom $\text{X}$ (such as $\text{I}$ or $\text{Br}$) is covalently bonded to an electron-withdrawing group $\text{R}$ (such as a perfluoroalkyl or perfluoroaryl moiety, e.g., $-\text{C}_6\text{F}_5$ or $-\text{CF}_3$), the covalent $\sigma$-bond draws electron density along the $\text{R-X}$ bonding axis:

• The halogen's valence shell forms an anisotropic electron distribution: a torus (belt) of negative electrostatic potential around its equator, and a region of positive electrostatic potential directly on the outermost tip along the extension of the $\text{R-X}$ bond axis.
• This electropositive region is termed the $\sigma$-hole (Politzer and Murray, 2007).

Characteristics of Halogen-Bonded Anion Receptors

An anion $\text{A}^-$ (Lewis base) is attracted to the electropositive $\sigma$-hole:

1. Strict Linearity: Because the $\sigma$-hole is situated precisely along the extension of the covalent $\sigma$-bond, halogen bonds exhibit extreme directional rigidity ($ngle \text{R-X}\cdots\text{A}^- \approx 175 - 180^\circ$), far more directional than hydrogen bonds which tolerate bending down to $140^\circ$.

2. Tunable Strength: The magnitude of the $\sigma$-hole potential scales directly with the polarizability of the halogen atom:

Iodine hosts form the strongest halogen bonds, with binding energies reaching $20 - 150\text{ kJ/mol}$ for halides.

3. Hydrophobic Resilience: Unlike hydrogen bonds, which are severely weakened by competitive hydrogen-bond accepting solvents (acetone, DMSO, water), halogen-bond donors possess no protic character and operate effectively in competitive polar and aqueous-organic media.

Chalcogen and Pnictogen Bonding

Extending to Group 16 elements ($\text{S, Se, Te}$), atoms bearing two covalent bonds develop two distinct $\sigma$-holes. Bidentate chalcogen-bonding receptors containing dual tellurophene or 1,2,5-chalcogenadiazole rings bind halides through collinear $\text{C-Te}\cdots\text{X}^-$ interactions.

Complementing organic hydrogen-bonding receptors, inorganic and organometallic complexes provide potent anion hosts utilizing coordination to Lewis acidic metal centers or redox-active metallocenes.

Metallocene-Based Electrochemical Anion Sensors

Pioneered by Paul Beer, incorporating redox-active organometallic reporter groups (such as ferrocene or cobaltocenium) into an anion-binding cleft enables electrical sensing:

1. Cobaltocenium Receptors: The cobaltocenium cation ($[\text{Co}(\text{Cp})_2]^+$) carries a permanent $+1$ charge that enhances anion binding via Coulombic attraction while resisting protonation changes over a wide pH range.

2. Ferrocenyl Amide / Urea Receptors: A neutral ferrocene unit is functionalized with amide or urea arms:

• In the initial $\text{Fe(II)}$ state, the receptor binds an anion (such as $\text{H}_2\text{PO}_4^-$) via $\text{N-H}\cdots\text{O}$ hydrogen bonds.
• The close proximity of the bound negative charge stabilizes the oxidized ferrocenium cation $\text{Fe(III)}$.
• Consequently, the reversible redox potential $E_{1/2}$ of the $\text{Fe(II)/Fe(III)}$ couple shifts cathodic (to more negative potentials) by $\Delta E_{1/2} = 50 - 250\text{ mV}$ upon anion binding:

Measuring $\Delta E_{1/2}$ via cyclic voltammetry directly quantifies anion concentration.

Lewis Acidic Receptors: Organoboranes and Mercuracarborands ("Anti-Crowns")

Direct coordinate covalent bonding between an electron-rich anion and a Lewis acidic center:

1. Bidentate Organoboranes: Molecules containing two triarylboron moieties ($\text{Ar}_2\text{B-R-BAr}_2$) with empty $p_z$ orbitals directed toward a central cavity chelate small Lewis basic anions such as fluoride ($\text{F}^-$) or cyanide ($\text{CN}^-$) via dual $\text{B-F}$ coordinate bonds, producing binding constants $K_a > 10^8\text{ M}^{-1}$.

2. Mercuracarborands (Hawthorne's "Anti-Crown Ethers"): Cyclic oligomers consisting of repeating electrophilic mercury(II) atoms bridged by *o*-carborane clusters, such as cyclo-$[\text{Hg}(\text{C}_2\text{B}_{10}\text{H}_{10})]_4$:

• Possess a planar, macrocyclic ring containing four or five electrophilic $\text{Hg(II)}$ centers pointing into the central cavity.
• Form exceptionally stable complexes with spherical halides ($[\text{Cl}^- \subset \text{anti-crown}]$), with the chloride held in the ring plane by four or five convergent $\text{Hg}\cdots\text{Cl}$ interactions.

A pinnacle of modern supramolecular design is the creation of switchable molecular receptors whose binding affinity and selectivity can be inverted reversibly by an external chemical or physical trigger (pH, light, or redox potential).

pH-Gated Cation-to-Anion Receptor Inversion

Polyamino-polyether macrocycles and cryptands can be switched between cation-selective and anion-selective states by modulating pH:

1. Neutral/Basic Regime ($ ext{pH} > 9$):

• The amino nitrogens are unprotonated, possessing free Lewis basic lone pairs directed into the macrocyclic cavity.
• In this state, the host acts as a cation receptor, binding alkali or transition metal cations ($\text{Na}^+, \text{Cu}^{2+}$) via cooperative lone-pair coordination.

2. Acidic Regime ($ ext{pH} < 4$):

• The amino nitrogens undergo protonation to form positive ammonium groups ($-\text{NH}_2^+-$).
• Inward-pointing positive charges create electrostatic repulsion that ejects any bound metal cation.
• Concurrently, the inward-directed $\text{N}^+-\text{H}$ protons and positive electrostatic field transform the cavity into an anion receptor, binding halides ($\text{Cl}^-$) or oxoanions ($\text{SO}_4^{2-}$) with high affinity.

Redox-Switchable Receptors

Receptors incorporating viologen ($N,N'$-dialkyl-4,4'-bipyridinium, $\text{V}^{2+}$) units:

• In the dicationic oxidized state $\text{V}^{2+}$, the host binds electron-rich planar anions (such as iodide or 2-naphthalenesulfonate) through coupled electrostatic attraction and charge-transfer complexation.
• Upon single-electron reduction to the radical cation $\text{V}^{\bullet+}$, electrostatic attraction diminishes, triggering quantitative anion release.

Light-Responsive Azobenzene Receptors

Functionalizing an anion-binding bis-urea host with a central azobenzene linker allows photochemical gating:

• trans-Isomer (visible light, $\lambda = 450\text{ nm}$): The two urea arms are separated by $9\text{ Å}$, incapable of simultaneously binding an oxoanion.
• cis-Isomer (UV light, $\lambda = 365\text{ nm}$): Photoisomerization bends the azobenzene backbone into a U-shaped geometry, bringing the two urea clefts within $4.5\text{ Å}$ to form a synergistic bidentate binding pocket for sulfate.

Achieving high selectivity among competing anions in complex biological or environmental fluids requires receptors designed to discriminate on the basis of charge density, shape, and basicity.

Spherical Halide Discrimination: $\text{F}^-$ vs $\text{Cl}^-$ vs $\text{Br}^-$ vs $\text{I}^-$

The halides present identical spherical symmetry ($K_h$) but vary across a wide range of ionic radii, polarizabilities, and hydration energies:

| Halide | Ionic Radius (Å) | $\Delta G_{\text{hyd}}^\circ$ (kJ/mol) | Polarizability ($\text{Å}^3$) | Optimal Recognition Motif | | :--- | :--- | :--- | :--- | :--- | | $\text{F}^-$ | $1.33$ | $-465$ | $0.64$ | Tight, hard hydrogen-bond donors (ureas, calixpyrrole) | | $\text{Cl}^-$ | $1.81$ | $-340$ | $2.98$ | Macrobicyclic azacryptands, katapinands, cone calix[4]pyrrole | | $\text{Br}^-$ | $1.96$ | $-315$ | $4.24$ | Expanded macrocycles, bis-thioureas | | $\text{I}^-$ | $2.20$ | $-275$ | $6.45$ | Halogen bonding ($\sigma$-hole donors), large cavitands |

Because fluoride has by far the highest hydration penalty ($-465\text{ kJ/mol}$), a receptor that binds fluoride in water must provide immense electrostatic stabilization to overcome this desolvation threshold.

Tetrahedral Oxoanion Recognition: Sulfate vs Phosphate vs Perchlorate

Tetrahedral oxoanions possess four oxygen atoms directed toward the vertices of a tetrahedron, but exhibit drastically different formal charges and basicities:

1. Sulfate ($\text{SO}_4^{2-}$):

• High charge density, strong Lewis base, high hydration penalty ($\Delta G_{\text{hyd}}^\circ = -1080\text{ kJ/mol}$).
• Bound with extreme selectivity by rigid bicyclic or tricyclic cages presenting six or eight convergent hydrogen-bond donors oriented along tetrahedral axes, mimicking the biological Sulfate-Binding Protein (SBP) which encapsulates $\text{SO}_4^{2-}$ with 16 hydrogen bonds without a single direct water contact.

2. Hydrogen Phosphate ($\text{HPO}_4^{2-}$ / $\text{H}_2\text{PO}_4^-$):

• Protic oxoanion capable of acting simultaneously as hydrogen-bond acceptor and donor. Receptors bearing complementary donor-acceptor arrays (such as guanidiniums or zinc(II)-dipicolylamine complexes) bind phosphates with sub-micromolar affinity.

3. Perchlorate ($\text{ClO}_4^-$):

• Mononegative charge dispersed over four oxygens; exceptionally low hydration penalty ($-205\text{ kJ/mol}$). Follows the classical Hofmeister series:

Non-preorganized, hydrophobic receptors in water naturally bind $\text{ClO}_4^-$ most strongly simply because it is the easiest anion to dehydrate. Overcoming this Hofmeister bias to selectively bind more hydrophilic anions ($ ext{SO}_4^{2-}$ or $ ext{Cl}^-$) requires rigorous structural preorganization and high donor density.