cis

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Letter 12

122-

The Interaction of B F with All-cis 1,2,3,4,5,6 Hexafluorocyclohexane in the Gas Phase Michael J Lecours, Rick A Marta, Vincent Steinmetz, Neil S. Keddie, Eric Fillion, David O'Hagan, Terrance B. McMahon, and William Scott Hopkins J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02629 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 12, 2016

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The Journal of Physical Chemistry Letters

The Interaction of B12F122− with All-cis 1,2,3,4,5,6 Hexafluorocyclohexane in the Gas Phase Michael J. Lecours,[a] Rick A. Marta,[a] Vincent Steinmetz,[b] Neil Keddie,[c] Eric Fillion,[a], David O’Hagan,[c]* Terrance B. McMahon,[a]* W. Scott Hopkins[a]* [a] Department of Chemistry, University of Waterloo, 200 University Ave West, Waterloo, ON, N2L 3G1 Canada [b] Laboratoire Chimie Phyique, CLIO/LCP, Bâtiment 201, Campus Universitaire d’Orsay, 91405 France [c] EastCHEM School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Ky16 9ST, UK AUTHOR INFORMATION Corresponding Authors *Scott Hopkins: [email protected]; Terry McMahon: [email protected]; David O’Hagan: [email protected]

ACS Paragon Plus Environment

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ABSTRACT:

Clusters

of

all-cis

1,2,3,4,5,6-hexafluorocyclohexane

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and

the

dodecafluorododecaboron dianion, [C6F6H6]n[B12F12]2− (n = 0–4), are investigated in a combined experimental and computational study. DFT calculations and IRMPD spectra in the region of 800–2000 cm−1 indicate that C6H6F6 binds to open trigonal faces of B12F122− via a three-point interlocking binding motif. Calculated binding interactions reveal substantial contributions from C-H•••F hydrogen-bonding and binding energies that are amongst the strongest observed for a neutral-anion system.

TOC GRAPHICS

Janus-like complexation ability: Facially polarized all-cis C6H6F6 binds via strong electrostatic interactions and, in the case of B12F122−, C–H•••F hydrogen bonding. Binding strengths in excess of 160 kJ mol−1 are amongst the largest observed for neutral-anion systems.


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The physicochemical properties of, e.g., organic polymers, pharmaceuticals, and agrochemicals are often tuned via fluorination.1-3 Consequently, a great deal of effort has gone into developing fluorination reaction methodologies which exert a high degree of control and selectivity.4-8 A high-water mark in this regard was recently achieved by Keddie et al., who, in a synthetic tour-de-force, were able to prepare all-cis 1,2,3,4,5,6-hexafluorocyclohexane (hereafter 1).9 This facially polarized ring is highly unusual in organic chemistry, and it is thought to exhibit the highest dipole of any non-ionic, or aliphatic compound when in its ground state chair conformation.9 Recently, we reported on the gas phase interactions within clusters containing 1 and monoatomic ions.10 Owing to its very large dipole moment (ca. 7 D), 1 forms strongly (largely electrostatically) bound complexes with both anions and cations; in fact, only polyethers such as 18-crown-6 and diglyme bind Na+ more strongly than 1.10-12 Here, we continue our investigation of the properties of 1 by detailing the first study of its interactions with a polyatomic ion, B12F122−. As a result of the strong B–F covalent bonds and the centralized gu symmetry HOMO of the boron cage, B12F122− is expected to be very weakly interacting. For this reason, B12F122− is often referred to as a “superweak” or “weakly coordinating” anion.13-17 Consequently, it has been suggested that sodium salts of B12F122− might exhibit superionicity and find utility as solid-state Na superionic conductors for use in sodium-based batteries, as has been explored for the analogous Na2B12H12 salt.18 Thus, investigations of clusters which contain B12F122− anions are interesting in their own right, since the interactions of B12F122− with other chemical species are largely unexplored. Recent work from this lab has shown that the charge-transfer properties of transition metal B12F122− complexes can be tailored by varying the transition metal cation, which essentially tunes the overlap of the cation accepting orbital and the B12F122− HOMO.15 In the case


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of [1n•B12F12]2− clusters, one would expect that bonding should be strong and predominantly associated with the charge-dipole interaction. However, owing to the high degree of polarization of the C–H bonds in 1, it is possible that C–H•••F hydrogen bonding might also play a role in cluster binding. To explore the structures and bonding of uncomplexed B12F122− and the [1n•B12F12]2− (n = 1–4) clusters, experiments were conducted at the Centre Laser Infrarouge d’Orsay (CLIO) free electron laser (FEL) facility. The experimental apparatus has been previously described in detail.19-20 [1n•B12F12]2− (n = 1–3) clusters were generated by injecting a solution of 100 µmol L−1 Ag2B12F12 and 1 in 50:50 CH3OH:H2O into vacuum via an ESI source operating in negative ion mode. The resulting gas phase clusters were transferred to a Bruker Esquire 3000+ ion trap mass spectrometer where they were characterized and subsequently irradiated with the tunable output of the infrared FEL. Figure 1 shows a typical mass spectrum of the [1n•B12F12]2− clusters. We see no evidence for clusters that are larger than [13•B12F12]2− (m/z = 467.11 amu), thus suggesting that three molecules of 1 fill the first solvation shell of B12F122−.


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Figure 1. A mass spectrum of uncomplexed B12F122− and the [1n•B12F12]2− (n = 1–3) clusters obtained from a Bruker 3000+ quadrupole ion trap. [14•B12F12]2− is not observed. Each [1n•B12F12]2− (n = 1–3) cluster size was isolated and probed using infrared multiple photon dissociation (IRMPD) spectroscopy. The only observed product channel corresponded to loss of 1 from the parent cluster. The results of IRMPD experiments are shown in the black traces in Figure 2. The spectrum of uncomplexed B12F122− (Figure 2A) exhibits a single, broad feature centered at ca. 1250 cm−1, which arises due to excitation of the T1u normal mode of the icosahedral boron cage.15, 21 The breadth of this feature stems predominantly from the vibrational isotope effect associated with the various

10

B and

11

B isotopologues and isotopomers (i.e.,

isotopic isomers) present in the probed sample; harmonic frequency calculations for the all-10B and all-11B isotopologues indicate a shift of ca. 50 cm−1, which accords well with experimental observations .15, 21 Figures 2B–D show the spectra of the [1n•B12F12]2− (n = 1–3) clusters. Upon addition of the first molecule of 1, two new peaks appear in the vibrational spectrum at 1065 cm−1 and 1160 cm−1. As additional molecules of 1 are added, these peaks grow in intensity relative to the B12F122− T1u peak (owing to the additional C–F bonds and enhanced absorption cross section), but their wavenumber remains unchanged. This suggests that the second and third molecules of 1 bind to B12F122− in a similar fashion to the first. Also shown in Figure 2 (red traces) are vibrational spectra for the [1n•B12F12]2− (n = 0–3) clusters, which have been calculated at the B3LYP/6-311++G(d,p) level of theory. Owing to the fact that the isotopically pure 11

B1219F122− species was employed in the calculations, the predicted T1u vibrational peak (1216

cm−1) is much narrower than the experimentally observed transition. The intense peaks associated with 1 (calculated to lie at 1046 cm−1 and 1143 cm−1) arise from C–F stretching


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motions, which are analogous to the C–F modes of E and A1 symmetry in the free 1 molecule, respectively. The geometries of the [1n•B12F12]2− (n = 1–3) clusters are shown in Figures 3 and 4. As expected based on charge-dipole interaction arguments, it is the partially positive hydrogen face of the 1 molecules which is oriented towards the B12F122− dianion. This accords well with the observed invariance of the C–F stretching motions with increasing complexation, since the F atoms of 1 are oriented away from the B12F122− moiety and out into vacuum for [1n•B12F12]2− (n = 1–3). Interestingly, the size of B12F122− and 1 are such that the three axial hydrogen atoms of 1 can interlock with three fluorine atoms on a B12F122− trigonal face in a three-point binding orientation (see Figure 3D; XYZ coordinates are provided in supporting information). It should be noted that our calculations only identified clusters in which 1 was in a chair conformation. The twist-boat conformation of the free 1 molecule is ca. 10 kJ mol−1 higher in energy than the chair structure and,10 due to the C–H•••F hydrogen bonding interactions (vida infra), [1n•B12F12]2− (n = 1–3) clusters containing a single twist-boat conformer are expected to lie at least 20 kJ mol−1 above the global minima. In comparing the adjacent F–F distances in the free B12F122− species to those in the interacting trigonal B12F122− face of [1•B12F12]2−, we find contraction from 3.26 Å to 3.17 Å, suggestive of a significant interaction between the fluorine atoms of B12F122− and the axial hydrogen atoms of the bound 1. Indeed, a concomitant elongation of the axial H–H distance from 2.51 Å to 2.57 Å is observed in 1 upon complexation. We also find an average C–H•••F interaction distance of 2.41 Å, which is considerably shorter than the sum of the van der Waals radii of H (1.20 Å) and F (1.47 Å), 2.67 Å. This interaction distance is consistent with the formation of relatively strong C–H•••F hydrogen bonds,22 and is comparable to some of the shorter C–H•••F–B distances observed.23 The interaction between the B12F122− and


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1 in [1•B12F12]2− was investigated computationally by conducting a Natural Bonding Orbital (NBO) analysis.24 NBO interaction energies are calculated via second-order perturbation as described previous works.15,25 A total electron donation interaction energy of Edonation = 30.9 kJ mol−1 is calculated, of which C–H•••F hydrogen bonding accounts for 27.8 kJ mol−1. This represents the energy of interaction arising from electron density donation between the two moieties. This magnitude of C–H•••F hydrogen bonding is observed for each molecule in the larger clusters (see Table S2 in supporting information), which suggests that, once bound, molecules of 1 do not freely move over to adjacent sites on the surface of the B12F122− moiety. Given the energy of electron density donation, one can estimate the strength of the charge-dipole interaction (EC-D) by subtracting Edonation from the calculated ground state dissociation energy, D0. Calculations at the B3LYP/6-311++G(d,p) level of theory yield a value of D0 = 132.9 kJ mol−1 for [1•B12F12]2− (D0 = 163.5 kJ mol−1 upon inclusion of the GD3 empirical dispersion correction; see table S3 in the Supporting Information).25 This results in a value of EC-D = 102.0 kJ mol−1 (EC-D = 132.6 kJ mol−1 upon inclusion of the GD3 empirical dispersion correction), which accords well with the classical charge-dipole interaction energy of 134 kJ mol−1 that is calculated using the B3LYP optimized geometry and the CCSD(T)/6-311++G(d,p) dipole moment of 1 (µ = 7.30 D). Table 1 provides the calculated binding energies for each of the [1n•B12F12]2− (n = 1–4) clusters. Note that basis set superposition error (BSSE) should reduce these binding energies. However, BSSE was not accounted for in these calculations since, owing to the long range of interaction and small number of atomic orbitals involved in the C–H•••F interaction, BSSE is expected to be relatively small.


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Figure 2. Experimental IRMPD spectra (black) for (A) B12F122−, (B) [1•B12F12]2−, (C) [12•B12F12]2−, and (D) [13•B12F12]2−. Calculated vibrational spectra are displayed beneath experimental traces in color and have been convoluted with a Gaussian function with FWHM = 4 cm−1. Vertical dashed lines indicate the calculated wavenumbers of the C–F stretches in free C6H6F6. Calculations were conducted at the B3LYP/6-311++G(d,p) level of theory. Calculations identified two low-lying isomers of [12•B12F12]2− (see Figure 4); a bent structure was found to lie ∆G° = 3.7 kJ mol−1 above the global minimum linear geometry. Assuming a Boltzmann thermal distribution at 298 K, we expect that the two isomers exist in a ca. 4:1 relative population within the probed ensemble. This is consistent with observed breadth and structure of the B12F122− feature shown in Figure 2C. The IMRPD spectrum for the heaviest


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isotopologue, [12•11B12F12]2−, is provided in the supporting information; the B12F122− feature in this spectrum is not broadened by vibrational isotope effects arising from isotopic isomers, and provides a clearer example of the structural features arising from the bent and linear isomeric species. In comparing the structure of [12•B12F12]2− with that of [1•B12F12]2−, we see that the second molecule of 1 also adopts the interlocking H•••F•••H binding scheme and it prefers to bind at least three trigonal facial sites away from the first 1 – the minimum binding distance at which no B12F122− fluorine is shared between the two molecules of 1.

Figure 3. (A) Optimized geometry of B12F122−. (B) front and (C), side view of the [1•B12F12]2− cluster. (D) A skeletal drawing showing the interlocking three-point bonding interaction. Calculations were conducted at the B3LYP/6-311++G(d,p) level of theory.


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Table 1. Calculated binding energies for loss of a single molecule of 1 from [1n•B12F12]2− (n = 1–4). Thermodynamic corrections were calculated at a temperature of T = 298 K. Calculations were conducted at the B3LYP/6-311++G(d,p) level of theory. D0 / kJ mol−1

∆H° / kJ mol−1

∆G° / kJ mol−1

S° / J mol−1 K−1

132.9

129.5

84.6

855.6

117.8

114.1

72.7

1134.9

Bent [12•B12F12]2−

115.3

111.7

69.1

1132.2

Planar [13•B12F12]2−

99.9

96.0

56.5

1422.6

Pyramidal [13•B12F12]2−

98.6

94.8

55.1

1422.1

Tetrahedral[14•B12F12]2−

90.9

83.9

38.6

1689.5

Species [1•B12F12]2− 2−

Linear [12•B12F12]

Two structures of [13•B12F12]2− were also identified and are shown in Figure 4. The isomer with three 1 molecules oriented in a trigonal planar fashion was calculated to lie ∆G° = 1.4 kJ mol−1 below the trigonal pyramidal geometry. As was the case with the addition of the second 1 molecule, the third 1 molecule prefers to bind at least three trigonal facial sites away from the first two (i.e., prefers not bind to F atoms that are already interacting with another 1 molecule). Consequently, a third molecule of 1 can be added only to the bent isomer of [12•B12F12]2−; the linear isomer does not have an open B12F122− trigonal face containing three unbound fluorine atoms (see Figure 4). This propensity for 1 to bind only to the open, uncomplexed trigonal faces of B12F122− also explains the non-observation of [1n•B12F12]2− (n ≥ 4) clusters, since (given the observed binding motif) it is not possible to add a fourth 1 molecule to the global minimum structure of [13•B12F12]2−. Assuming stepwise addition of 1, [14•B12F12]2− can only be produced from complexation of 1 with the energetically disfavored pyramidal structure of [13•B12F12]2−, which can only be generated from the energetically disfavored bent structure of [12•B12F12]2−.


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Moreover, the binding energy of [14•B12F12]2− is considerably less than the smaller clusters, and [14•B12F12]2− clusters that do form are much less likely to survive the ESI process.26

Figure 4. Optimized geometries for the (top left) linear isomer of [12•B12F12]2−, (top right) bent isomer of [12•B12F12]2−, (bottom left) planar isomer of [13•B12F12]2−, and (bottom right) pyramidal isomer of [13•B12F12]2−. Calculations were conducted at the B3LYP/6-311++G(d,p) level of theory. In summary, by examining the structures of the [1n•B12F12]2− (n = 1–4) clusters, we have garnered a microscopic view of the evolution of the first solvation shell for B12F122− in all-cis 1,2,3,4,5,6-hexafluorocyclohexane. While ion-solvent binding is largely electrostatic in nature in these clusters, there is also a substantial contribution from C–H•••F hydrogen bonding. This further underscores the unusual properties of 1 and highlights its potential as a complexing agent – for both cations and anions. It is noteworthy that 1 binds B12F122− (∆H° = 129.5 kJ mol−1; ∆HGD3° = 160.1 kJ mol−1) as strongly as it does Cl− (∆HGD3° = 156.5 kJ mol−1),10 despite the fact that B12F122− is considered a “weakly-coordinating” anion. For comparison, the largest Cl−


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molecular binding enthalpy in the NIST database (to p-cyanophenol) is 148 kJ mol−1.11, 27 The calculated gas phase binding energies for the [1n•B12F12]2− (n = 1–3) clusters are also substantially larger than those calculated for B12F122− complexes with transition metal cations, where fragmentation proceeds via a low-lying charge-transfer threshold.15 Again, this demonstrates the remarkable complexation ability of 1. The structures and physicochemical properties of complexes of 1 with common polyatomic anions and cations are currently unknown, and several of these are currently under investigation. ASSOCIATED CONTENT Supporting Information. Cluster XYZ atomic coordinates, calculated spectra, NBO calculation details, and thermochemical data are provided free of charge. ACKNOWLEDGMENT The authors gratefully acknowledge the Centre Laser Infrarouge d’Orsay (CLIO) team and technical support staff, for their valuable assistance and kind hospitality. Furthermore, we acknowledge high performance computing support from the SHARCNET consortium of Compute Canada. The generous financial support of this work from the Natural Sciences and Engineering Research Council of Canada is also acknowledged. REFERENCES (1) Begue, J.-P.; Bonnet-Deplon, D. Bioorganic and medicinal chemistry of fluorine. John Wiley & Sons: Hoboken, N.J., 2008. (2) Fier, P. S.; Hartwig, J. F. Selective C-H Fluorination of Pyridines and Diazines Inspired by a Classic Amination Reaction. Science 2013, 342, 956-960. (3) Muller, K.; Faeh, C.; Diederich, F. Fluorine in pharmaceuticals: Looking beyond intuition. Science 2007, 317, 1881-1886.


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1 2

Page 15 ofThe 19 Journal of Physical Chemistry Letters

1

2

3

4

6

n = 0

Io n C o u n t / 1 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

8 4 (x 1 0 0 )

0 ACS Paragon Plus Environment

2 0 0

4 0 0

m /z (a m u )

6 0 0

The Journal of Physical Chemistry Page Letters 16 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23


Page 17 ofThe 19 Journal of Physical A CChemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

B

F

D

F F

F F

F H H

F ACS Paragon Plus Environment F B B

H

F B

The Journal of Physical Chemistry Letters Page 18 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

2– [12•B12F12] linear isomer

[12•B12F12]2– bent isomer

ACS Paragon Plus Environment [13•B12F12]2– planar isomer

[13•B12F12]2– pyramidal isomer

Journal Page 19 of of Physical 19 Chemistry Le

1 2 3 4 5 6 Paragon Plus Environment ACS 7 8 9

cis

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