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

TL;DR

This paper explores a new use for a specific anion in solid-state chemistry to create and study a unique organometallic complex.

Contribution

The paper introduces a new application of the [FPB]− anion in solid-state molecular organometallic chemistry.

Findings

A σ-alkane complex was isolated and characterized using [FPB]−.

Na+[FPB]− and [H(OEt2)2][FPB] were synthesized and reported.

Periodic DFT and IGMH analysis supported the structural characterization.

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.

Figures

Full text

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(OR^F^)4]^−^ (e.g., OR^F^ = OC­(CF_3_)3, OCH­(CF_3_)2),? or arylborates, e.g., [BAr^F^ 4]^−^ (Ar^F^ = 3,5-(CF_3_)2_C_6_H_3),? Chart.

We have used the [BAr^F^ 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(Cy_2_PCH_2_CH_2_PCy_2_)­(endo-η^2^η^2^-NBA)]­[BAr^F^ 4] [1-NBA]­[BAr ^ F ^ _ 4 _ ] (NBA = norbornane) results from reaction of a norbornadiene precursor with H_2_.? The framework of [BAr^F^ 4]^−^ anions supports metal-centered reactivity, ?−? ? has CF_3_ groups that promote substrate diffusion ?,? and provides stability from noncovalent interactions. ?,? In this system, different anions, such as [Al­(OR^F^)4]^− ^ ? or [B­(3,5-Cl_2_–C_6_H_3_)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[BAr^F^ 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(O_2_C_2_(CF_3_)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[BH_4_] 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, SchemeA. Solution NMR data? (THF-d 8) showed no significant resonances associated with THF-h 8 in the ^1^H NMR spectrum. The cost of preparing Na­[FPB] is competitive with Na­[BAr^F^ 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.

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 Et_2_O solvent, following by filtration and removal of solvent [δ­(^1^H) = 16.6 H(OEt_2_)2, CD_2_Cl_2_], SchemeB. 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]/Et_2_O, from which reliable integrals could be obtained in the ^1^H NMR spectrum. This allows for the determination of Et_2_O solvation of the oxonium acid in the bulk powder, i.e., [H­(OEt_2_)2]^+^. Analysis by SCXRD (crystals grown from CH_2_Cl_2_/Et_2_O) revealed two polymorphs, one of which gave a solvable structure (Supporting Information). This reveals this polymorph to have only one Et_2_O 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(Cy_2_PCH_2_CH_2_PCy_2_)­(NBD)]­[FPB], [1-NBD]­[FPB] (NBD = norbornadiene), and its onward solid/gas reactivity with H_2_ to form the indefinitely stable σ-alkane complex [Rh­(Cy_2_PCH_2_CH_2_PCy_2_)­(exo-η^2^η^2^-NBA)]­[FPB], [1-NBA]-[FPB], FigureA. Complex [1-NBD]­[FPB] is isolated as block-like red crystals by a straightforward route using Na­[FPB], [Rh­(NBD)­Cl_2_]2 and Cy_2_PCH_2_CH_2_PCy_2_. The SCXRD structure shows an orthobifastigium arrangement of [FPB]^−^ anions surrounding two crystallographically equivalent [Rh­(Cy_2_PCH_2_CH_2_PCy_2_)­(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 H_2_ (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 ^31^P­{^1^H} SSNMR spectrum shows a characteristic? downfield shift and increase in J(RhP) on formation of the σ-alkane complex (155 and 195 Hz), FigureB. In the ^13^C­{^1^H} 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).

The SCXRD structure of [1-NBA]­[FPB] (FigureC) 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, FigureD), 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 ^31^P­{^1^H} SSNMR). It reacts rapidly with 1,2-F_2_C_6_H_4 solvent to displace the alkane to form the arene-adduct, [1-F _ 2 _ C _ 6 _ H _ 4 _ ]­[FPB],? as characterized by SCXRD (FigureE, 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 (FigureA). These, and other computed metrics (Figures S33–S36), all signal a σ-alkane complex. The η^2^ 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, FigureB).

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 [BAr^F^ 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 (FigureC) and between NBA ligands arising from the unusual back-to-back cation packing motif (FigureD). 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., <Σ_vdW 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