Stabilizing Anion–Anion Aggregates via Dihydrogen Bonds in Non-Classical Inorganic Molecules

Abstract

Coulomb's law predicts that like-charge ions repel and avoid dimerization. However, a class of dimers between like-charge ions is characterized. The [3,3’-Fe(1,2-C₂B₉H₁₁)₂]⁻ (abbreviated as [o-FESAN]⁻) represents an innovative non-classical inorganic anion apart from hydroxyanions that exhibits anion-anion stabilization via dihydrogen bonding. Experimental methods (nuclear magnetic resonance [NMR], dynamic light scattering [DLS], and X-ray diffraction) and theoretical approaches (density functional theory) reveal that [o-FESAN]⁻ clusters aggregate by overcoming long-range electrostatic repulsion. The synthesis of [H₃O][o-FESAN]•3H₂O and its crystal structure confirm the formation of stabilized anion-anion aggregates, with [H₃O]⁺ counterions residing freely in the channels rather than between the anionic clusters. The structure exhibits the cisoid rotamer, which facilitates the ability of the anionic [o-FESAN]⁻ cluster to form interactions stabilized by dihydrogen bonds (head-to-middle cluster) shorter than the sum of the Van der Waals radii. These shorter bonds are crucial for the formation of anion-anion interactions mediated by dihydrogen bonds. X-ray structures show that anions aggregate in the solid state, overcoming long-range electrostatic repulsion through dihydrogen bonds, which are distinct from the hydrogen bonds commonly observed in anion systems involving highly electronegative elements. Consistent with crystal structure evidence, ¹H NMR, transmission electron microscopy, and DLS confirm [o-FESAN]⁻ anion-anion aggregates in solution. Theoretical calculations support the formation of these anion-anion aggregates, primarily via C_cluster-H···H-B bonds. While individual B-H···H-B interactions are weakly attractive, their cumulative effect significantly enhances aggregate stability. Additionally, the crystal structure of Na(H₂O)₃[o-FESAN] is reported and analyzed, providing further evidence of unconventional interactions stabilized by dihydrogen bonds.

Keywords

anion-anion / boron clusters / dihydrogen bond / ferrabis(dicarbollide) / metallacarboranes

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