Metal Inorganic-Organic Complex Crystals

• MIOC crystals are hybrid materials combining metal-based inorganic components and organic moieties, offering tunable magnetic, optical, and catalytic properties.
• Controlled ligand-tuning via substitution and hydrogen-bond manipulation allows systematic modulation of thermal and photophysical behavior.
• Their vitrification dynamics enable conversion from crystalline to amorphous glass states while preserving key tetrahedral coordination, enhancing multifunctional applications.

Metal inorganic-organic complex (MIOC) crystals are hybrid solids comprising both metal-based inorganic components and organic structural moieties, organized into molecular or extended architectures. These materials can be engineered for diverse functionalities—magnetic, optical, catalytic, or glassy—by modulating their composition, structural topology, and supramolecular interactions. Recent research has illuminated new design principles for producing MIOC crystals that are competent glass-formers, exploiting ligand-exchange strategies within frameworks akin to zeolitic imidazolate frameworks (ZIFs), and demonstrating systematic control of network topology and property modulation through ligand and anion selection (Xu et al., 29 Sep 2025).

1. Ligand-Tuning Synthesis Strategies

A central approach for generating glass-forming MIOC crystals relies on controlled ligand substitution within conventional tetrahedral coordination environments. Starting from prototypical ZIFs, in which metal centers (e.g., Zn^2+) are tetrahedrally coordinated by imidazolate-type linkers, a portion of the imidazole or benzimidazole ligands is replaced by acid anions such as chloride or thiocyanate. This substitution scheme introduces directional hydrogen-bonding interactions (notably N–H···Cl and C–H···Cl) in place of a fraction of strong metal–N bonds.

The representative synthesis is executed via slow evaporation, yielding crystals such as ZnCl₂·HIm₂ (“MIOC-4”) and ZnCl₂·HbIm₂ (“MIOC-7”), where HIm = imidazole and HbIm = benzimidazole. The weaker hydrogen-bonding interactions reduce the energetic barrier to melting and produce frameworks that readily vitrify upon melt-quenching. This method systematically expands the glass-forming compositional space, facilitating the rational design of MIOC glasses across a broader chemical landscape.

2. Structural Architecture and Supramolecular Design

The construction of MIOC crystal lattices by the ligand-tuning strategy involves three principal components: the metal center, the acid anion, and the organic ligand. Zn^2+ typically forms tetrahedral units, with coordination spheres completed by imidazole- or benzimidazole-type ligands and bridging anions.

In MIOC-4 (ZnCl₂·HIm₂), N–H···Cl hydrogen bonds link the tetrahedral units into zigzag chains, which aggregate into two-dimensional layers and further into three-dimensional networks via van der Waals forces. In MIOC-7 (ZnCl₂·HbIm₂), the larger benzimidazole ligands sustain both hydrogen bonding and π–π (T–T) stacking, resulting in an interdigitated supramolecular architecture. The choice of ligand enables control over the dimensionality and robustness of these supramolecular units, as larger or more polarizable ligands can augment chain persistence or introduce stacking motifs.

3. Thermal and Glass-Forming Properties

The thermal behavior of MIOC crystals, in particular their ability to form glasses, is closely tied to the precise stoichiometry of the constituent ligands and anions. Thermal analysis reveals that the glass-transition temperature (T_g) is a tunable function of the ligand composition. In a continuous solid-solution between HIm and HbIm ligands, T_g rises linearly with increased fractional benzimidazole content:

$T_g = T_{g,0} + k \cdot R$

Here, $T_{g,0}$ is the T_g for the imidazole-only case, $R$ is the molar ratio HbIm/(HIm + HbIm), and $k$ quantifies the incremental increase per unit substitution. Experimentally, T_g is modulated from 282 K (MIOC-4) to 360 K (MIOC-7), reflecting the higher steric hindrance and glass-forming propensity imparted by the benzimidazole ligand. This linear relationship demonstrates the effectiveness of the ligand-tuning strategy for thermal property engineering.

4. Vitrification Dynamics and Network Reorganization

Upon heating, the hydrogen-bonded MIOC crystal network undergoes stepwise disordering, starting with the disruption of more labile hydrogen bonds (e.g., C–H···Cl) and culminating in loss of long-range supramolecular order. Notably, the primary tetrahedral coordination environment of the metal center is preserved even as the framework transitions from a crystalline to an amorphous glass state.

Rapid melt-quenching results in the “freezing-in” of a disordered network, typified by a short-range hydrogen-bonded skeleton with retained tetrahedral units but medium-range structural disorder. Spectroscopic analyses (Fourier transform IR, Raman, solid-state NMR) confirm that while Zn–N and Zn–Cl linkages endure, structural features associated with long-range order and π–π stacking are lost, giving rise to a glass characterized by broad and asymmetric spectral signatures indicative of conformational multiplicity and static disorder.

5. Functional Property Modulation

The exploited ligand-tuning protocol not only enables glass formation but also permits systematic modulation of physical properties:

• Thermal Tuning: The ability to linearly increase T_g through increasing benzimidazole content facilitates the tailoring of the glass-forming range for processing and operational requirements.
• Photophysical Behavior: MIOC glasses, especially those derived from benzimidazole (e.g., MIOC-7), exhibit enhanced photoluminescence, including excitation-dependent emission, red-edge effects, and high photoluminescence quantum yields compared to their crystalline antecedents. These properties stem from the restrained and yet disordered ligand environment that enhances radiative transitions and diminishes non-radiative relaxation pathways.
• Structural Versatility: The design flexibility extends to the incorporation of other coordinating metals (such as Co or Cu) and a variety of hydrogen-bond acceptors, allowing further expansion of glass-forming compositions.

6. Applications and Outlook

MIOC glasses, by virtue of their processability, tunable thermal properties, and photophysical performance, present a spectrum of potential applications:

• Optoelectronics and Anti-counterfeiting: Enhanced, excitation-dependent emission characteristics combined with glass processability point towards utility in photonic tags, luminescent hosts, and security features.
• Gas Separation and Membranes: The tailored network topology and mixed short-range/medium-range order may enable customized transport properties, relevant for separations or catalysis.
• Energy Storage: The possibility of compositional adjustment and thermal stability positions MIOC glasses as candidate matrices for lithium-ion (or other ion) transport and storage.
• Process Engineering: The ligand-tuning approach increases the feasibility of producing glasses with exact processing temperatures and stability profiles, essential for deployment in device systems.

This approach to constructing and tuning MIOC structures via ligand selection and hydrogen-bond network engineering provides a framework for developing a new generation of hybrid glasses with properties rivaling both inorganic glasses and organic polymers, while offering the functional diversity inherent to supramolecular design (Xu et al., 29 Sep 2025).