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. 2025 May 4;44(10):1018–1021. doi: 10.1021/acs.organomet.5c00112

[B(O2C2(CF3)4)2]− ([FPB]−): Repurposing This Weakly Coordinating Anion for Solid-State Molecular Organometallic (SMOM) Chemistry

Kristof M Altus †, M Arif Sajjad ‡, Stuart A Macgregor ‡,*, Andrew S Weller †,*
PMCID: PMC12117562  PMID: 40444032

Abstract

The perfluoropinacol borate-based anion [B­(O2C2(CF3)4)2]−, [FPB] –, is developed as a weakly coordinating anion for single-crystal to single-crystal organometallic solid/gas reactivity, resulting in the isolation and characterization (including periodic DFT and IGMH analysis) of the σ-alkane complex [Rh­(Cy2PCH2CH2PCy2)­(exo-η2η2-norbornane)]­[FPB]. The synthetically useful solvent-free Na+ salt, Na­[FPB], and oxonium acid [H­(OEt2)2]­[FPB] are also reported.

graphic file with name om5c00112_0006.jpg

graphic file with name om5c00112_0005.jpg

Weakly coordinating anions (WCAs) have been instrumental in the development of the synthetic and catalytic organometallic and main-group chemistry of reactive cationic species. − The ideal WCA should be chemically robust and of low polarizability, with the negative charge delocalized over a large surface area. While there are many different WCAs, the most popular are based upon alkoxyaluminates, [Al­(ORF)4]− (e.g., ORF = OC­(CF3)3, OCH­(CF3)2), or arylborates, e.g., [BArF 4]− (ArF = 3,5-(CF3)2C6H3), Chart .

1. Common WCAs and the [FPB]− Anion.

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We have used the [BArF 4]− anion extensively in single-crystal to single-crystal (SC-SC) solid-state molecular organometallic chemistry (SMOM), − where partnering with a reactive transition metal cation allows for the isolation of solution-unstable complexes by solid/gas reactivity. For example, the stable σ-alkane complex [Rh­(Cy2PCH2CH2PCy2)­(endo-η2η2-NBA)]­[BArF 4] [1-NBA]­[BAr F 4 ] (NBA = norbornane) results from reaction of a norbornadiene precursor with H2. The framework of [BArF 4]− anions supports metal-centered reactivity, − has CF3 groups that promote substrate diffusion , and provides stability from noncovalent interactions. , In this system, different anions, such as [Al­(ORF)4]−  or [B­(3,5-Cl2–C6H3)4]−  either do not support SC-SC reactivity or result in NBA displacement to ultimately form an arene-coordinated zwitterion.

The multistep synthesis of solvent-free precursor M­[BArF 4] (M = Li+, Na+, K+) creates a motivation to identify alternative anions that can facilitate SC-SC transformations. These anions should be cost-competitive and easily prepared on the gram scale as solvate-free salts of group 1 cations. The perfluoropinacol borate-based anion [B­(O2C2(CF3)4)2]−, [FPB] – (Chart ), offers these advantages. While its potential for use in organometallic chemistry has been suggested, this has not been reported outside of a patent disclosing its use in olefin polymerization. The stability of its group 1 salts, however, is demonstrated in its use as battery electrolytes. − We show here that [FPB] – can be used as a WCA for SMOM, by its use in the synthesis of a σ-alkane complex using SC-SC methods.

Solvate-free Na­[FPB] is synthesized from commercial perfluoropinacol and Na­[BH4] in THF solvent, using the reported method for group 1 cation salts, which have been structurally characterized as etherate solvates. − Heating the resulting solid under dynamic vacuum (10–2 mbar) at 80 °C (18 h) forms free-flowing powdered Na­[FPB]. A single-crystal X-ray diffraction (SCXRD) study of Na­[FPB] (crystals isolated from hot 1,2-dichloroethane) revealed a 1D coordination polymer with B–O···Na linkages, Scheme A. Solution NMR data (THF-d 8) showed no significant resonances associated with THF-h 8 in the 1H NMR spectrum. The cost of preparing Na­[FPB] is competitive with Na­[BArF 4], ∼£10 versus £7/mmol (January 2025 online prices); while the Process Mass Intensity metric (PMI) favors Na­[FPB], 22.6 versus 52.6. [Caution!] These advantages, however, should be balanced with the toxicity associated with the starting perfluoropinacol reagent; see the Supporting Information.

1. Synthesis of Na­[FPB] and [H­(OEt2)2]­[FPB].

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The synthetically useful oxonium acid , [H­(OEt 2 ) 2 ]­[FPB] can be prepared as a free-flowing white microcrystalline powder by addition of HCl to Na­[FPB] in Et2O solvent, following by filtration and removal of solvent [δ­(1H) = 16.6 H(OEt2)2, CD2Cl2], Scheme B. Addition of an excess of Proton Sponge (PS, 1,8-bis­(dimethylamino)­naphthalene) to [H­(OEt 2 ) 2 ]­[FPB] resulted in a mixture of unchanged PS and [PS-H]­[FPB]/Et2O, from which reliable integrals could be obtained in the 1H NMR spectrum. This allows for the determination of Et2O solvation of the oxonium acid in the bulk powder, i.e., [H­(OEt2)2]+. Analysis by SCXRD (crystals grown from CH2Cl2/Et2O) revealed two polymorphs, one of which gave a solvable structure (Supporting Information). This reveals this polymorph to have only one Et2O solvent molecule, which sandwiches the proton with the [FPB] – anion (see Figure S56).

The utility of Na­[FPB] as a supporting anion for SC-SC solid–gas reactivity is demonstrated by the synthesis of [Rh­(Cy2PCH2CH2PCy2)­(NBD)]­[FPB], [1-NBD]­[FPB] (NBD = norbornadiene), and its onward solid/gas reactivity with H2 to form the indefinitely stable σ-alkane complex [Rh­(Cy2PCH2CH2PCy2)­(exo-η2η2-NBA)]­[FPB], [1-NBA]-[FPB], Figure A. Complex [1-NBD]­[FPB] is isolated as block-like red crystals by a straightforward route using Na­[FPB], [Rh­(NBD)­Cl2]2 and Cy2PCH2CH2PCy2. The SCXRD structure shows an orthobifastigium arrangement of [FPB]− anions surrounding two crystallographically equivalent [Rh­(Cy2PCH2CH2PCy2)­(NBD)]+ cations, in which the NBD ligands are directed toward each other (Figure S52). Solution and solid-state NMR (SSNMR) data support this formulation. Addition of H2 (1 bar) to crystals of [1-NBD]­[FPB] (50 mg scale) over 80 min results in the quantitative formation of [1-NBA]­[FPB] in a SC-SC transformation. The resulting 31P­{1H} SSNMR spectrum shows a characteristic downfield shift and increase in J(RhP) on formation of the σ-alkane complex (155 and 195 Hz), Figure B. In the 13C­{1H} SSNMR spectrum of [1-NBA]­[FPB] signals due to NBD are absent, with broad signals due to the anion observed at δ 121.8 (vbr), 86.4 (br).

1.

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A) Synthesis of [1-NBA]­[BAr F 4 ]. B) 31P­{1H} SSNMR of [1-NBD]­[BAr F 4 ] and [1-NBA]­[BAr F 4 ]. C) Molecular structure of the asymmetric unit in [1-NBA]­[BAr F 4 ]. Displacement ellipsoids at the 40% level. D) Packing arrangement of anions, van der Waals surface. E) Reaction of [1-NBA]­[BAr F 4 ] with 1,2-F2C6H4.

The SCXRD structure of [1-NBA]­[FPB] (Figure C) shows a NBA-alkane ligand binding through two 3c-2e Rh···H–C interactions [Rh···C 2.385(3) and 2.363(2) Å] to give a formally d 8 , 16-electron Rh­(I) center, similar to [1-NBA]­[BAr F 4 ]. The H atoms associated with this interaction were located. However, in contrast with [1-NBA]­[BAr F 4 ], the chelating NBA ligand binds through two exo-C–H groups, the same as observed for the metastable [1-NBA]­[B­(3,5-Cl 2 -C 6 H 3 ) 4 ]. As for [1-NBD]­[FPB] the anions adopt an orthobifastigium arrangement (space group P-1, Figure D), and there is no significant change in the unit cell volume (3% difference). Uniquely for σ-alkane complexes synthesized using SMOM methods, − this packing directs the alkane ligands in the crystallographically equivalent cations to face one another, and the metal centers to be relatively close (∼10 Å). As for [1-NBA]­[BAr F 4 ], complex [1-NBA]­[FPB] is stable at 298 K under an Ar atmosphere (1 month by 31P­{1H} SSNMR). It reacts rapidly with 1,2-F2C6H4 solvent to displace the alkane to form the arene-adduct, [1-F 2 C 6 H 4 ]­[FPB], as characterized by SCXRD (Figure E, Figure S54).

[1-NBA]­[FPB] was characterized computationally using periodic-DFT calculations. Geometry optimization relaxing the H atom positions shows elongation of the C1–H1A and C2–H2B bonds to 1.17 Å, and NBO calculations indicate dominant C–H → Rh σ-donation (Figure A). These, and other computed metrics (Figures S33–S36), all signal a σ-alkane complex. The η2 C–H binding mode is also reflected in the computed Rh···H–C angles (102°) and the Independent Gradient Model plot based on Hirshfeld partitioning (IGMH, Figure B).

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A) Key donor–acceptor interactions for the Rh-NBA interaction in the cation of [1-NBA]­[FPB] as quantified via second-order perturbation NBO analyses. B) IGMH plot for the cation highlighting the Rh-NBA interaction (sign­(λ2)­ρ-colored isosurfaces with δGinter = 0.003 au) C) IGMH plot of the cation–anion pair and D) IGMH plot of cation–cation pair (atoms colored by %δGatom). See Supporting Information for IGMH plots of other nearest-neighbor ion pairs.

The NBA solid-state binding energy in [1-NBA]­[FPB] (i.e., the energy required to remove one NBA ligand from the unit cell) is computed to be 49.5 kcal/mol and compares with a molecular binding energy (for the isolated cation) of 36.5 kcal/mol. This gives a solid-state stabilization energy (SSSE) of 12.5 kcal/mol, similar to that computed for the endo-bound NBA in [1-NBA]­[BAr F 4 ] (14.0 kcal/mol). For [1-NBA]­[BAr Cl 4 ] (which also exhibits exo-NBA binding), the NBA solid-state binding energy is 50.7 kcal/mol, and the SSSE is 14.9 kcal/mol. Thus, the [BArF 4]− and [FPB]− lattices in [1-NBA]­[BAr F 4 ] and [1-NBA]­[FPB] have similar influence on alkane binding across different topologies and NBA binding modes. In [1-NBA]­[BAr Cl 4 ] the computed exo to endo rearrangement of one NBA ligand within the unit cell resulted in a destabilization of +2.3 kcal/mol. In [1-NBA]­[FPB] this is computed to be +8.4 kcal/mol, while in the absence of any solid-state environment this difference collapses to 0.3 kcal/mol. This emphasizes the role of the solid-state 2° microenvironment in controlling structure and stability. ,

The role of the 2° microenvironment was further probed via IGMH plots of nearest neighbor ion pairs, in particular those highlighting interactions between the NBA ligand and the adjacent [FPB] – anion (Figure C) and between NBA ligands arising from the unusual back-to-back cation packing motif (Figure D). In both cases, green isosurfaces indicate regions of stabilizing dispersion interactions, while the atom color coding highlights the largest % atomic contributions in red. H6B···F1/F2 contacts contribute most to NBA···[FPB] – dispersion while the H3···H6A contacts are most prominent in the cation–cation pair. These interactions reflect correspondingly short computed nonbonded distances (H6A···F1 = 2.39 Å; H6A···F2 = 2.61 Å; H3···H7B = 2.49 Å). In total, forty-two short H···F contacts (i.e., <ΣvdW radii + 10%) are present around the cation in [1-NBA]­[FPB]. A further estimate of interion dispersion comes from the computed cation–anion interaction energies: with PBE+D3 these range from 32 to 68 kcal/mol and these drop by 5–15% when recomputed without the D3 correction (Figures S38–S45).

In conclusion, we show that the [FPB] – anion supports SC-SC solid/gas reactivity at reactive cationic metal centers. The ease of synthesis of a variety of synthetically useful salts, robustness, and low cost of [FPB] – mean that it perhaps should be considered more widely in the toolbox of organometallic and main-group chemistry more generally, when suitable precautions are taken for the safe handling of perfluoropinacol and its derivatives.

Supplementary Material

om5c00112_si_001.pdf (4MB, pdf)
om5c00112_si_002.xyz (158.3KB, xyz)

Acknowledgments

EPSRC (EP/W015552/1 and EP/W015498/1). This work used the ARCHER2 UK National Supercomputing Service (https://www.archer2.ac.uk).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.organomet.5c00112.

  • Full experimental details, characterization, NMR spectra, cost analysis, computational details, electronic structure analyses, and X-ray crystallographic figures (PDF)

  • Computed geometries (XYZ)

The authors declare no competing financial interest.

References

  1. Riddlestone I. M., Kraft A., Schaefer J., Krossing I.. Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions. Angew. Chem., Int. Ed. 2018;57:13982–14024. doi: 10.1002/anie.201710782. [DOI] [PubMed] [Google Scholar]
  2. Bochmann M.. The Chemistry of Catalyst Activation: The Case of Group 4 Polymerization Catalysts. Organometallics. 2010;29:4711–4740. doi: 10.1021/om1004447. [DOI] [Google Scholar]
  3. Ittel S. D., Johnson L. K., Brookhart M.. Late-Metal Catalysts for Ethylene Homo- and Copolymerization. Chem. Rev. 2000;100:1169–1204. doi: 10.1021/cr9804644. [DOI] [PubMed] [Google Scholar]
  4. Engesser T. A., Lichtenthaler M. R., Schleep M., Krossing I.. Reactive p-block cations stabilized by weakly coordinating anions. Chem. Soc. Rev. 2016;45:789–899. doi: 10.1039/C5CS00672D. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Krossing I.. The Facile Preparation of Weakly Coordinating Anions: Structure and Characterisation of Silverpolyfluoroalkoxyaluminates AgAl­(ORF)4, Calculation of the Alkoxide Ion Affinity. Chem.Eur. J. 2001;7:490–502. doi: 10.1002/1521-3765(20010119)7:2&#x0003c;490::AID-CHEM490&#x0003e;3.0.CO;2-I. [DOI] [PubMed] [Google Scholar]
  6. Brookhart M., Grant B., Volpe A. F. Jr.. [(3,5-(CF3)2C6H3)4B]−[H­(OEt2)2]+: a convenient reagent for generation and stabilization of cationic, highly electrophilic organometallic complexes. Organometallics. 1992;11:3920–3922. doi: 10.1021/om00059a071. [DOI] [Google Scholar]
  7. Pike S. D., Chadwick F. M., Rees N. H., Scott M. P., Weller A. S., Krämer T., Macgregor S. A.. Solid-State Synthesis and Characterization of σ-Alkane Complexes, [Rh­(L2)­(η2,η2-C7H12)]­[BArF 4] (L2 = Bidentate Chelating Phosphine) J. Am. Chem. Soc. 2015;137:820–833. doi: 10.1021/ja510437p. [DOI] [PubMed] [Google Scholar]
  8. Chadwick F. M., McKay A. I., Martinez-Martinez A. J., Rees N. H., Krämer T., Macgregor S. A., Weller A. S.. Solid-state molecular organometallic chemistry. Single-crystal to single-crystal reactivity and catalysis with light hydrocarbon substrates. Chem. Sci. 2017;8:6014–6029. doi: 10.1039/C7SC01491K. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Bukvic A. J., Burnage A. L., Tizzard G. J., Martínez-Martínez A. J., McKay A. I., Rees N. H., Tegner B. E., Krämer T., Fish H.. et al. A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane. J. Am. Chem. Soc. 2021;143:5106–5120. doi: 10.1021/jacs.1c00738. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Sajjad M. A., Macgregor S. A., Weller A. S.. A comparison of non-covalent interactions in the crystal structures of two σ-alkane complexes of Rh exhibiting contrasting stabilities in the solid state. Faraday Discuss. 2023;244:222–240. doi: 10.1039/D3FD00009E. [DOI] [PubMed] [Google Scholar]
  11. Chadwick F. M., Rees N. H., Weller A. S., Krämer T., Iannuzzi M., Macgregor S. A.. A Rhodium–Pentane Sigma-Alkane Complex: Characterization in the Solid State by Experimental and Computational Techniques. Angew. Chem., Int. Ed. 2016;55:3677–3681. doi: 10.1002/anie.201511269. [DOI] [PMC free article] [PubMed] [Google Scholar]
  12. Gyton M. R., Sajjad M. A., Storm D. J., Altus K. M., Goodall J. C., Johnson C. L., Page S. J., Edwards A. J., Piltz R. O.. et al. An Operationally Unsaturated Iridium-Pincer Complex That C–H Activates Methane and Ethane in the Crystalline Solid-State. J. Am. Chem. Soc. 2025;147:8706–8719. doi: 10.1021/jacs.4c18122. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Martínez-Martínez A. J., Rees N. H., Weller A. S.. Reversible Encapsulation of Xenon and CH2Cl2 in a Solid-State Molecular Organometallic Framework (Guest@SMOM) Angew. Chem., Int. Ed. 2019;58:16873–16877. doi: 10.1002/anie.201910539. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Vitórica-Yrezábal I. J., Libri S., Loader J. R., Mínguez Espallargas G., Hippler M., Fletcher A. J., Thompson S. P., Warren J. E., Musumeci D.. et al. Coordination Polymer Flexibility Leads to Polymorphism and Enables a Crystalline Solid–Vapour Reaction: A Multi-technique Mechanistic Study. Chem.Eur. J. 2015;21:8799–8811. doi: 10.1002/chem.201500514. [DOI] [PMC free article] [PubMed] [Google Scholar]
  15. McKay A. I., Martínez-Martínez A. J., Griffiths H. J., Rees N. H., Waters J. B., Weller A. S., Krämer T., Macgregor S. A.. Controlling Structure and Reactivity in Cationic Solid-State Molecular Organometallic Systems Using Anion Templating. Organometallics. 2018;37:3524–3532. doi: 10.1021/acs.organomet.8b00215. [DOI] [Google Scholar]
  16. Martínez-Martínez A. J., Weller A. S.. Solvent-free anhydrous Li+, Na+ and K+ salts of [B­(3,5-(CF3)2C6H3)4]−, [BArF 4]−. Improved synthesis and solid-state structures. Dalton Trans. 2019;48:3551–3554. doi: 10.1039/C9DT00235A. [DOI] [PubMed] [Google Scholar]
  17. Krossing I., Raabe I.. Noncoordinating AnionsFact or Fiction? A Survey of Likely Candidates. Angew. Chem., Int. Ed. 2004;43:2066–2090. doi: 10.1002/anie.200300620. [DOI] [PubMed] [Google Scholar]
  18. Gao, X. ; Jaber, I. . Novel Borate Activator. US Patent US20060009596A1, 2006.
  19. Ni L., Zhang S., Li C., Lu J., Li J., Wang J., Zhang S., Chen G., Zhang Z.. et al. Tailored Cation–Anion Coordination in Carbonate Electrolyte Enabling a Rigid-Flexible Compact Solid-Electrolyte Interphase for Potassium Batteries. Adv. Funct. Mater. 2024;34:2400570. doi: 10.1002/adfm.202400570. [DOI] [Google Scholar]
  20. Chen G., Qiao L., Xu G., Li L., Li J., Li L., Liu X., Cui Z., Zhang S.. et al. A Highly-Fluorinated Lithium Borate Main Salt Empowering Stable Lithium Metal Batteries. Angew. Chem., Int. Ed. 2024;63:e202400797. doi: 10.1002/anie.202400797. [DOI] [PubMed] [Google Scholar]
  21. Murugan S., Klostermann S. V., Frey W., Kästner J., Buchmeiser M. R.. A sodium bis­(perfluoropinacol) borate-based electrolyte for stable, high-performance room temperature sodium-sulfur batteries based on sulfurized poly­(acrylonitrile) Electrochem. Commun. 2021;132:107137. doi: 10.1016/j.elecom.2021.107137. [DOI] [Google Scholar]
  22. Ould D. M. C., Menkin S., Smith H. E., Riesgo-Gonzalez V., Jónsson E., O’Keefe C. A., Coowar F., Barker J., Bond A. D.. et al. Sodium Borates: Expanding the Electrolyte Selection for Sodium-Ion Batteries. Angew. Chem., Int. Ed. 2022;61:e202202133. doi: 10.1002/anie.202202133. [DOI] [PMC free article] [PubMed] [Google Scholar]
  23. Jimenez-Gonzalez C., Ponder C. S., Broxterman Q. B., Manley J. B.. Using the Right Green Yardstick: Why Process Mass Intensity Is Used in the Pharmaceutical Industry To Drive More Sustainable Processes. Org. Process Res. Dev. 2011;15:912–917. doi: 10.1021/op200097d. [DOI] [Google Scholar]
  24. Laird T.. The Dangers of the Unknown. Org. Process Dev. 2003;7:225. doi: 10.1021/op034047r. Perfluoropinacol should be treated as acutely toxic and is fatal if contacted with the skin. We can find no available data for the corresponding [FPB]− anion, despite its widespread adoption in battery research. We thank Professor Ingo Krossing (University of Freiburg) in alerting us to this paper: [DOI] [Google Scholar]
  25. Krossing I., Reisinger A.. Perfluorinated Alkoxyaluminate Salts of Cationic Brønsted Acids: Synthesis, Structure, and Characterization of [H­(OEt2)2]­[Al­{OC­(CF3)3}4] and­[H­(THF)2]­[Al­{OC­(CF3)3}4] Eur. J. Inorg. Chem. 2005;2005:1979–1989. doi: 10.1002/ejic.200400436. [DOI] [Google Scholar]
  26. Furfari S. K., Tegner B. E., Burnage A. L., Doyle L. R., Bukvic A. J., Macgregor S. A., Weller A. S.. Selectivity of Rh···H–C Binding in a σ-Alkane Complex Controlled by the Secondary Microenvironment in the Solid State. Chem.Eur. J. 2021;27:3177–3183. doi: 10.1002/chem.202004585. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Krämer T., Chadwick F. M., Macgregor S. A., Weller A. S.. Solid-State Confinement Effects in Selective exo-H/D Exchange in the Rhodium σ-Norbornane Complex [(Cy2PCH2CH2PCy2)­Rh­(η2:η2-C7H12)]­[BArF 4] Helv. Chim. Acta. 2023;106:e202200154. doi: 10.1002/hlca.202200154. [DOI] [Google Scholar]
  28. Alvarez S.. A cartography of the van der Waals territories. Dalton Trans. 2013;42:8617–8636. doi: 10.1039/c3dt50599e. [DOI] [PubMed] [Google Scholar]
  29. Beckett, G. ; Beech-Brandt, J. ; Leach, K. ; Payne, Z. ; Simpson, A. ; Smith, L. ; Turner, A. ; Whiting, A. . ARCHER2 Service Description; Zenodo, 2024, 10.5281/zenodo.14507040. [DOI] [Google Scholar]

Associated Data

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Supplementary Materials

om5c00112_si_001.pdf (4MB, pdf)
om5c00112_si_002.xyz (158.3KB, xyz)

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