Crystallographic investigation into the self-assembly, guest binding, and flexibility of urea functionalised metal-organic frameworks

Ross J Marshall,Jake Mcguire,Claire Wilson,Ross S Forgan

doi:10.1080/10610278.2017.1370095

Abstract

Introduction of hydrogen bond functionality into metal-organic frameworks can enhance guest binding and activation, but a combination of linker flexibility and interligand hydrogen bonding often results in the generation of unwanted structures where the functionality is masked. Herein, we describe the self-assembly of three materials, where Cd2+, Ca2+, and Zn2+ are linked by N,Nʹ-bis(4-carboxyphenyl)urea, and examine the effect of the urea units on structure formation, the generation of unusual secondary building units, structural flexibility, and guest binding. The flexibility of the Zn MOF is probed through single-crystal to single-crystal transformations upon exchange of DMF guests for CS2, showing that the lability of the [Zn4O(RCO2)6] cluster towards solvation enables the urea linkers to adopt distorted conformations as the MOF breathes, even facilitating rotation from the trans/trans to the trans/cis conformation without compromising the overall topology. The results have significant implications in the mechanistic understanding of the hydrolytic stability of MOFs, and in preparing heterogeneous organocatalysts.

Highlights

Metal-organic frameworks (MOFs) are network materials comprising metal ion or cluster secondary building units (SBUs) linked by organic linkers into multidimensional structures that often exhibit considerable porosity [1,2,3]
Attempts were subsequently made to prepare MOFs containing the ligand with free urea units to examine guest binding
We have exploited the flexibility of the urea-based linker, N,N-bis(4-carboxyphenyl)urea, to prepare three new framework materials with unusual SBUs: [Cd(L)(DMF)3]n, a one-dimensional coordination polymer; [Ca5(L)5(DMF)3(H2O)2]n a three-dimensional MOF linked by infinite calcium chains; and two solvates of [Zn4O(L)3(DMF)2]n, which has the prototypical IRMOF topology

Summary

Introduction

Metal-organic frameworks (MOFs) are network materials comprising metal ion or cluster secondary building units (SBUs) linked by organic linkers into multidimensional structures that often exhibit considerable porosity [1,2,3]. Small triangular pores run down the crystallographic a axis (Figure 3(c)) and these spaces are filled in the crystal structure with a large number of water and DMF solvent molecules, with two of the five independent urea units binding water guests through a bifurcated

Results

Conclusion