1. The actors
Valentina, an Inorganic structural chemist and Sergio, an organic chemist... two different worlds that speak to each others!
1. The idea
The IDEA came from a collaboration between Valentina (@VaColomb) and Sergio (@_sergiorossi_) during the development of novel activities focused in the engagement of high school students in a PLS educational program of the Department of Chemistry at the University of Milan. Here is how things began:
V: "Sergio, do you think we can use a 3D printer to print molecules?"
S: "Of course! There are a lot of examples in the literature!"
V: "Can we use it with students?"
S: "Probably... but we have to print small molecules"
V: "And... Could you print a crystal structure for me...?"
S: "Well, this is more complicated, but it should be feasible using a SLA 3D printer"
V: "SLA stay for?"
S: "Stereolitography... the printer that cures a photopolymer resin by a laser to create objects layer by layer. We have the SLA printer here in the department..."
V: "Could you 3D-print a crystal for me ...?"
S: "Uhm... "
V: "Pleeeeeeeeease! Do you know that having a 3D-printed crystal is the dream of every crystallographer?!?"
S: "The problem is not the printer but the crystal structure! We need a .stl file which contains the structure of the entire crystal: this is difficult to create. Generally, virtual molecular softwares which convert your structure into a 3D printable file only generates the structure of molecule itself and not the way in which molecules are packed in space, like a crystal..."
V: "Oh, this should not be a problem... I know how to do it!"
V: "Yes, Mercury software of Cambridge Crystallographic Data Centre has a specific function to do that! We should try it!"
S: "But... do you have a crystal structure of a nice 3D porous network to be printed out?"
V: "Oh, I have also a perfect crystal!"
V: "C'mon... stop to say "but", and let’s try it!"
2. The story
Valentina shows to Sergio one of her favourite crystal structures: [Ni8(OH)4(H2O)2(L)6] in which L = bis-pyrazolate based linker (alias = Ni-fcu)... and she tells to Sergio that Ni-fcu comes from a long story. Ni-fcu was discovered during her Ph.D. studies, which, again, comes back to 2010... (see more info here)This metal-organic framework is constituted by octanuclear Ni(II) hydroxo clusters, linked by tetradentate (μ4-) linear bis-pyrazolate based ligands in a complex Ni8(μ4-X)6(μ4-L)6 polyhedron of rigorous cubic symmetry (X = OH- or H2O; L = ligand). The inset of the figure shows the local stereochemistry of the octa-metallic node, where each Ni(II) ion is hexacoordinated in a fac-NiN3O3 fashion. The presence of rigid bis(exobidentate) spacers, possessing pyrazolato moieties at both ends, induces the formation of a fcc packing of the [Ni8(μ4-X)6]12+ clusters, each linked to twelve symmetry-related nodes by the μ4-L2- ligands.
Intriguingly, many pyrazolate-based MOFs quickly precipitates in the form of insoluble polycrystalline powders, which hamper their structural characterization by single crystal X-ray diffraction. However, the "art" of ab-initio structural determination from powder diffraction data can be nicely applied on powders of crystalline metal-organic frameworks for the description of their crystallographic features (see here). For Ni-fcu, these techniques were coupled with X-ray absorption techniques and spectroscopic measurements, in a beautiful collaboration between the structural chemistry group of the University of Insubria (Como, PI Prof. Masciocchi) and the group of physical chemistry of surfaces and interfaces of the University of Turin (Italy, PI Prof. Silvia Bordiga). This collaboration allowed the detection and confirmation of the relevant stereochemical features and the correct stoichiometry of this class of metal-organic frameworks with formula [Ni8(OH)4(OH2)8(μ4-L)6].
The same MOF can be prepared with pyrazolate-based ligands possessing different length of the inner spacer, resulting in an isoreticular series of MOFs with tunable porosity (next figure): indeed, upon elimination of the residual solvent from the as-prepared samples, they possess accessible octahedral and tetrahedral cavities with dimensions dependent from the length of the spacer (see here).
And this is not enough to make Ni-fcu a fabulous structure!
As known for UiO66/UiO67 MOF systems containing Zr6O4(OH)4 nodes and linear dicarboxylate linkers, the treatment with modulating agents leads to porosity improvement as a consequence of the creation of missing linker defects. This is true and feasible also for our Ni-fcu MOF! Valentina’s friend and collaborator, Prof. Jorge A.R. Navarro (Universidad de Granada) has succeeded in the introduction of structural defects by post-synthetic treatment with KOH, showing that the properties of this type of material can be tuned to improve CO2 capture from flue gas (here); sulfur dioxide adsorption (here) and ion conductivity (here)
So... now you know why Ni-fcu is one of Valentina’s favourite crystal structure?!?!
This is the 3D-printed model of the cubic octanuclear cluster of the Ni-fcu MOF. In the back, the Powder X-ray Diffractometer from which its crystal structure has been determined.
3. The .stl file
We know how to 3D-print molecules (here). In addition, according to Mercury guidelines we were able to generate
the .stl file in just few clicks with all the parameters ready for the printing process. Here you can see a screenshot of the software with the structure of Ni-fcu.
With this file in our hand, we are ready for the next level!
4. (SLA) 3D-printing process
3D printing technologies create models by an additive manufacturing process. In a stereolithography (SLA) approach, the object is
created by selectively curing a liquid polymer resin layer-by-layer using an ultraviolet (UV) laser beam.
The polymer is solidified through a photopolymerization process in which monomers are activated by the light of the UV laser
and creates new unbreakable bonds between a monomer and another one. This photopolymerization process is irreversible.
At the beginning of the process, the build platform (the black part on the top of the right picture) is immersed in the resin vat (the orange part on the bottom of the right picture) at the distance of one layer height from the transparent bottom of the vat. Then a UV laser creates the first layer by selectively curing and solidifying the photopolymer resin according to the virtual design. When this layer is finished, the platform moves to the top, the resin is mixed and then the build platform is re-immersed in the resin vat the distance of a two layer height and the UV laser create the second layer. This process is repeated until the part is complete.
All 3D printing processes start with a .stl file (in our case, it was generated with Mercury software). Then, using a print preparation software (different from each 3D-printer) it is possible
to setup all the parameters necessary to realize a succesfull 3D-printing process. These parameters are related to the choice of the object orientation;
in the creation of support structures (which are always required to in SLA process) as well as in the selection of the parameters typical for each
printable polymer and in the printing resolution. When all these parameters are fixed, the file can be sent to the printer.
In our case, we use Preform software using a resolution of 50 microns and the default parameters for the Clear V4 resin. The crystal (left picture, blue color) with supports (left picture, grey color) were printed using a Formalbs 2 printer.
After the printing process (consisting in 2596 layers!!) the object was cleaned with iPrOH in order to remove the excess of the unpolymerized resin then supports were manually removed using a nail clipper. (left picture: the crystal during the removal of the supports, right picture: the crystal after the complete removal of the supports).