Building MOFs: nodes, linkers and crystal engineering

MOFs are built the way you build with LEGO: pick the pieces and let them click

Intuition

Imagine a box of LEGO with two kinds of brick: a hub with six studs and a straight rod. Shake the box slowly and the hubs and rods click together into a big open cube lattice. A metal-organic framework (MOF) is that, made with metal ions as hubs and molecules as rods. The structure is built in, because the pieces only click in certain ways.

Lecture 37 opens the MOF series: L37 covers structures and structural properties, L38 surfaces and host-guest chemistry, and L39 magnetic, electronic, ion-transport and spin-switching properties. The slide notes that the Nobel Prize in Chemistry 2025 was awarded for the development of MOFs.

Self-assembly lets parts find their own stable arrangement

Self-assembly is when pieces explore many arrangements and settle into a stable one without anyone placing them. The lecture’s macroscale picture is coloured triangles floating on water. As the system cools, they explore many configurations and find a thermodynamically stable one. On molecules, trimesic acid (1,3,5-benzenetricarboxylate) on a surface assembles through hydrogen bonds, with two motifs: a dimer and a trimer.

MOFs use a stronger glue, the coordinate bond. This is a bond from a ligand $L$ to a metal ion $M^{n+}$. The lecture shows it next to an ordinary covalent bond for comparison.

Nodes plus linkers give a framework, and Prussian blue was the first

A node with six connections and a linker with two connections give a cubic, three-dimensional framework. That is a metal-organic framework. The historic example is Prussian blue (1710): an octahedral metal node with a linear $CN^-$ linker, with Fe(II) and Fe(III) centres. The slide writes the bridging unit as $4[Fe^{II}(CN)_6]^{3-}$, as written on the slide.

Compare

Hydrogen-bonded assembly

Molecules such as trimesic acid link through hydrogen bonds on a surface, in a dimer or trimer motif.

Coordinate-bonded framework

Metal nodes and linkers link through coordinate bonds from ligand L to metal ion M(n+). The result is a 3D framework.

Crystal engineering designs the crystal before it exists

The lecture calls this “molecular LEGO” or crystal engineering: design and construction of crystals from chosen pieces. The diamond structure is built from tetrahedral centres. Robson’s 1990 example uses CuI, $[Zn^{II}(CN)_4]^{2-}$ and $N(CH_3)_4^+$. It was the first family of compounds where the structure could be reliably predicted before the crystals were made.

Larger tetrahedral building units give expanded diamond-type frameworks. A central carbon with benzonitrile “arms” is one such ligand. The voids in these frameworks hold anions and solvent of crystallisation, which is an opportunity for host-guest chemistry. L38 picks this up for gas storage.

The symmetry example links back to the space-group lectures

The slide shows a crystal with space group $I4_1/amd$ from the earlier lectures on symmetry and space groups. It is a body-centred tetragonal cell with 32 symmetry elements. The panels mark the $4_1$ screw axes, 2-fold rotation axes, mirror planes and two kinds of glide planes. Rotary-inversion axes ($\bar{n}$) also appear. The slide says these materials are covered in lectures 37 to 39 in week 13.

Optional aside

The slide on page 6 to 7 identifies the framework only as a “Metal-Organic Framework” with octahedral nodes and cyanide-type linkers. It does not name the compound, so this page does not either.

Exam detail

Be able to describe in a sentence what nodes, linkers and a coordinate bond are, and name Prussian blue and Robson’s diamond-type framework as the examples.

Recall

Why does using nodes and linkers make a MOF's structure predictable?

Each node connects in a fixed number of directions and each linker has fixed ends. The pieces can only click together in a limited number of ways, so the finished framework follows from the choice of pieces, as in Robson’s diamond-type frameworks.