Template Role of Long Alkyl-Chain Amides in the Synthesis of Zeolitic Imidazolate Frameworks

Organic amides as solvents and structure directing agents
(SDAs)
are crucial for synthesizing zeolitic imidazolate frameworks (ZIFs).
However, current research focuses only on the use of short alkyl-chain
amides as solvents/SDAs. Here, we investigate the role of amides with
varying alkyl-chain lengths on the structures and topologies of Zn(Im)2 polymorphs. Using short alkyl-chain amides as solvents, the
Zn(Im)2 topological structures are affected by the synthesis
conditions, leading to “one SDA/multiple topological structures”.
In contrast, when long alkyl-chain amides are used as solvents, the
Zn(Im)2 topological structures are essentially unaffected
by other synthesis conditions. Thus, long alkyl-chain amides are shown
for the first time to exhibit a significant template role, leading
to “one template/one topological structure”. Specifically,
the use of long alkyl-chain N,N-dimethyl-Cn amides (abbreviated as DM-Cn amides, n = 3, 4, 6, 8, and 10) can lead to only DTF-type Zn(Im)2 frameworks under broad crystallization conditions. Single-crystal
X-ray diffraction confirmed that the exquisite structural compatibility
between long alkyl-chain DM-Cn amides and the DFT-type
Zn(Im)2 framework results in a highly regular head-to-tail
arrangement of amides along the (kaa-lov)n chain of the DFT framework. The template role for
long alkyl-chain amides was further identified to be multiple C–H···π
interactions between DM-Cn amides and Zn(Im)2 frameworks thanks to molecular simulations.