Platinum-Group Based Pentazolatocarboxylate Metal Inorganically Linked Frameworks for Direct CO2 Capture and Solar Fuel Applications

By C. Klein, J. Kimmel, T. Kaputtke, Omar Yogi and R. A. Freud :: The Institute of Exploives and Corrosives, Department of Alchemy, Brewery and Conspiracy Springfield Idunno

Abstract: We report the preparation of a PtO-pentazole Metal Inorganically Linked Framework (MILF). This MILF was characterised by PXRD and elemental analysis, and exhibits pronounced host-guest interactions, stimulus-response behaviour, and high internal surface area. The material also shows promise as a catalyst for the conversion of CO₂ to octane.

Size matters…   Metal-organic frameworks (MOFs) are 3D networks of organically linked metal nodes which have gained considerable attention in the scientific community as potential (photo)electrodes, shape-selective and shape-shifting catalysts, e.g. in the production of solar fuels, and also as gas-adsorbents due to their remarkable inner surface area.¹ Their importance for a sustainable future is as obvious as their widely standardized and intuitive nomenclature system. However recently, another promising class of porous 3D materials has been rediscovered, namely metal-inorganically linked frameworks (MILFs). The intriguing properties of these compounds have already been established by Finch et al. in their ground-breaking work from 1999.² Excellent mechanical properties, triboluminescence and high shear moduli are among their many attributes. The most important feature is their flexible pore structure for equally flexible catalysis. Thus, MILFs are the crown of creation and superior to all other porous materials. In the following article, our MILF-experienced group will present a new and novel, tuneable and switchable, as well as solar fuel-producing, undoubtedly world-saving catalyst.³

Based on previous literature reports by Kaputtke et al. on the synthesis of stimuli-responsive MILFs,^[4] we envisioned even more responsive pentazolato-MILFs with abundant metals (in this case platinum), which is ubiquitous (at least in catalytic converters) and primed for the formation of stable oxo-clusters.⁵ We developed a straightforward procedure for the synthesis of methylpentazole 1 by bubbling methylamine through an acidic concentrations. Subsequent reaction of 1 with CO₂ in DMF under mild conditions (5 GPa) led to 2-pentazole malonic acid 2 in nearly quantitative yield (103%) after centrifugation and thorough DMF washing. 

Following a well-known Kawasaki-Miau-Wau coupling, moronic acid 3 was obtained as previously described.⁶ Afterwards, the easily and cheaply accessible palladium(II) precatalyst Pd/OwlPHOS was employed in a Hirohito coupling⁷ between 2 and 3 in DMF-d₇ to yield ^HypaHypasilyl-protected⁸ linker 4. Linker 4 was purified by washing with more deutero-DMF and drying in high vacuum at the rotary evaporator (300 RPM, ISO 9 cleanroom). OwlPHOS® is likened to an owl, being the predator of the PHOrest, in analogy to Takasago’s naming scheme: “BINAP is compared to a butterfly for its high ability and beautiful structure. SEGPHOS is named after a seagull, which flies higher and faster than a butterfly”.^9 ¾ Final deprotection in acetone/H₂O₂ under ground-shaking conditions with catalytic amounts of concentrated sulfuric acid (D5-potentized from the same solvent mixture) gave linker 5.

Platinum group metals were obtained from a catalytic converter and hexachloroplatinic acid was then isolated by coarsely crushing the supported catalyst with a NicerDicer® and extracting the residue with aqua regia. Seventeen coffee filters were used to remove the undissolved material, which was washed with more DMF.

Linker 5, platinum precursor and acetic acid as a modulator were incubated in a drying oven in DMF over the weekend. The green crude metal inorganically linked framework (MILF) was then purified by washing with dry DMF twice and dry µ-oxido-dihydrogen(I) (DHMO) once to afford the colourless, pristine MILF (see Scheme 2, bottom). Calcd. = C: 31.5783, H: 3.14159, N: 6.9420. Found = C: 31.5783, H: 3.14159, N: 6.9420. We shall call it UiUiUi-69 in order to keep the nomenclature consistent and intuitive with similarly spin-polarized carbon oxide-capturing MOFs (COCMOFs)⁹ like Chanel’s MOF N^o5,¹⁰ HKUST-420,¹¹ or IRMOF-θ.¹²

Single crystals were unfortunately not good enough for structure solution. However, synchrotron PXRD at the Quartz light source (Worcestershire, UK) allowed for Rightfelt refinement of the network that forms the MILF. Structure fine solution was then achieved by transformation to the pair distribution function (PDF, see Figure 1). The crystallographic information file is available free of charge from the CCCCCC.

Analysis revealed an intimately interpenetrating network with pronounced host-guest interaction and remarkable pore structure (see Figure 2). Because of these findings, variable pressure powder X-ray diffraction (PP-XRD) experiments were added, indicating stimulus responsive behaviour. Phase transition was observed on infrasonication with a St. John Smythe apparatus (Hitachi, JP) during CO₂ pressurization and can be reversed by removing adsorbed gas molecules from the engorged pores by turbomolecular pump suction. Pore elongation was accompanied by a significant increase of the material’s shear modulus. The material was then subjected to BED isotherms (up to 50 oz. sq.in.^-1) in order to measure the manipulated pore length and circumference (6 by 3 attoparsec).

Owing to these remarkable analytic results, we attempted to produce solar fuels by supplying hydrogen to the CO₂-saturated MILF and irradiating in a solarium. Afterwards, the product was desorbed from the pores by washing with a minimal amount of n-octane to achieve a staggering yield of 181.7%. Product distribution was analysed by molecular autoionization supercritical liquid-time of flight-mass spectrometry (MASL-TOF-MS)¹³ and olfacto-gravimetry¹⁴ to reveal perfect selectivity for C₈H₁₈. Between different MILF batches we recognized significant yield dependence on the pore volume. Size matters!

Conclusion

In this work, we have once again highlighted the need for MILFs to replace fossil fuels and make our planet Greta-friendly. Based on these findings, the straightforward and safe synthesis as well as cost-efficiency, a pilot scale plant for CO₂ direct air capture in the kiloton scale is currently under construction.

Chem3D molecular modelling optimization has not only suggested superior performance of UiUiUi-69’s Darmstadtium congener due to the metal’s higher oxophilicity, but it also predicted the MILF’s fac isomer to have even higher catalytic abilities, encouraging its synthesis in future endeavours. Likewise, sildenofilic acid might replace linker 5 for increased catalytic potency and greater turnover numbers. Further research is thus highly recommended.

Experimental data

Chemicals, unless otherwise noted, were purchased from suppliers such as FCK Chemicals, Smegma Aldrich, or Merck Macropore and used without further purification. All reaction steps were performed in a glovebox with oxygen concentrations usually below 1000 ppm. D5 potentizing was achieved by slapping the flask containing H₂O₂/H₂SO₄/CO(CH₃)₂ 10 against the fume hood window at full moon, and repeating with 1:10 dilution 5. µ-Oxido-dihydrogen(I) was dried over molar sieves 3 Å and alcohol content was checked to be lower than 1.5‰ (Karl Emil Otto Ernst Hans Fischer titration). Carbon dioxide was evolved by treating CaCO₃ (harvested from our coffee machine) with acetic acid in a Schütt-Kipp apparatus. Supporting Information with detailed procedures and analytic data is available free of charge under the following link: http://bitly.com/98K8eH.

Conflicts of Interest

No consensus has been reached between the authors on the pressing subject of whether residual DMF can be washed away with more DMF.^[15] TK proposes that any rusty nail possesses higher catalytic performances than UiUiUi-69. 

Acknowledgements 

To the guy who wild parked his Çïτяøëй C4 on campus: Thank you for the catalytic converter! We greatly acknowledge Cooper I. Odide’s sacrifice to the glovebox gods. T. Piquèl is acknowledged for crude MILF synthesis. 2D ¹⁹⁵Pt–¹⁴N {¹H} SHMOSY NMR for fac/mer stereochemical assignment was performed by F. Bloch.

Notes and references

1. L. R. Hubbard, J. Unint. Sci. 1995, 11, 123-123.
2. P. Finch, S. Stifler, K. Myers, C. Ostreicher, J. Levenstein, JACoffs 1999, 22, 149-155.
3. C. B, D. Cat, K. West, Tetrahedron Mails 2016, 21, 69-76.
4. T. Kaputtke, Expl. Today 2021, 61, 42-71.
5. T. M. Anderson, W. A. Neiwert, M. L. Kirk, P. M. B. Piccoli, A. J. Schultz, T. F. Koetzle, D. G. Musaev, K. Morokuma, R. Cao, C. L. Hill, Science 2004, 306, 2074–2077. First proof of platinum-oxo species. Retracted for reasons we don’t fully understand.
6. M. Kawasaki, L. Miau-Wau, Org. Bullsh. 1985, 9, 1-32. 
7. L. Gordon, A. Stanheight, Molecular Pain 2004, 7. 33-39.
8. H. P. Baxxta, Inorganometallics, ASAP. 
9. https://www.takasago.com/en/news/2002/0607_0528.html accessed 08/11/2021
   1. C. Bandersnatch, ChemSucksChem 2012, 75, 33-50. (9¾ See pdf haha)
10. C. Chanel, Sci. Hub. 1921, 33, 149-151.
11. G. Schlonk, Pentagondodecahedron 2012, 5, 5-12.
12. P. Desolate, G. Schlonk, Chim. ad Acta C 1995, 1, 122.
13. D. Duck, JACS’o’lantern 1996, 12, 666-777.
14. O. McDonald, Old J. Chem. 1866, 1, 1-6812.
15. Socrates, J. Philosoph. Chem. 400 BC, 1, 1.

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