Pentaborate ( 1-) salts templated by substituted pyrrolidinium cations : synthesis , structural characterization , and modelling of solid-state H-bond interactions by DFT calculations

Dyfyniad o'r fersiwn a gyhoeddwyd / Citation for published version (APA): Beckett, M. A., Coles, S., Davies, R. A., Horton, P., & Jones, C. L. (2015). Pentaborate(1-) salts templated by substituted pyrrolidinium cations: synthesis, structural characterization, and modelling of solid-state H-bond interactions by DFT calculations. Dalton Transactions, 44, 70327040. https://doi.org/10.1039/C5DT00248F


Introduction
Many organic bases react with B(OH) 3 in aqueous solution to yield pentaborate(1−) salts, [NMC][B 5 O 6 (OH) 4 ], in which the protonated organic base is a non-metal cation (NMC). 1 On rare occasions salts containing three, 2 four, 3 seven, 4 eight, 5 nine, 6 fourteen 7 and fifteen 8 B atoms have been obtained.Variations arise since B(OH) 3 in basic aqueous solution forms a dynamic combinatorial library 9 (DCL) of polyborate anions whose concentrations are pH and boron concentration dependent. 10owever, in mildly basic solutions it is estimated that <5% of the total boron is in the form of the [B 5 O 6 (OH) 4 ] − anion, with [B 3 O 3 (OH) 4 ] − and [B 4 O 5 (OH) 4 ] 2− the dominant species. 11We have recently performed DFT calculations (gas-phase) 12 on the relative stabilities of the polyborate anions and concluded that in isolation the order of stability follows monoborate(1−) > triborate(1−) > pentaborate(1−) > triborate(2−) > tetraborate(2−).However, contrary to this order of stability, pentaborate(1−) salts are readily crystallized from aqueous solutions.The cations in these polyborate salts are structure directing and actively template the architecture of the NMC polyborate salts.The cations can influence the structures by their size, charge, and in some cases by their ability to form strong H-bond interactions.H-bonds are ranked high for intermolecular interaction energies in crystal engineering. 13H-bond interactions between hydrated polyborate anions are ubiquitous 14 in polyborate salts, although cation-anion interactions will also play a significant role in the solid-state energetics.In this manuscript we prepare four (substituted) pyrrolidinium cation pentaborate salts, and confirm the structures by X-ray crystallography.We also examine their solid-state H-bond interactions and calculate (DFT) energies of the anion-anion interactions found in these structures.For completeness, we also calculate H-bond energies for anion-anion H-bonding motifs found in other pentaborate structures, and propose an explanation as to why pentaborate salts are so readily formed.
Salts 1-4 were characterized by elemental composition, spectroscopy (NMR and IR), and thermal analysis.These data indicated that they were pentaborates and their structures were confirmed by single-crystal XRD studies.5 These species arise due to the complex borate equilibria present in aqueous solution. 10,16 11B NMR spectra obtained under very dilute conditions can give some diagnostic information.Under these conditions, the formation of polyborate species is suppressed, and a single peak is observed due to equilibrium monoborate (B(OH) 3 /[B(OH) 4 ] − ) species, and the observed chemical shift is dependent upon the relative proportions of B trig and B tet in solution. 17Thus, the pentaborate(1−) anion should show, at 'infinite' dilution, one peak at 16.1 ppm, and the pentaborate salts 1-4 all give a signal at this chemical shift when in dilute solution.The total B/charge ratio can be calculated from an observed chemical shift for dilute solutions (see experimental).
This chemical shift value is not often noted but may have utility in helping to formulate products of unknown composition. 1 H and 13 C spectra (in D 2 O) were fully consistent with those expected for pyrrolidinium cations, with the NH (1, 3, 4), OH (4) and BOH protons overlapping as represented by a broad signal at ∼4.7 ppm due to rapid exchange.IR spectra of 1-4 clearly all show the diagnostic band of pentaborate salts at ∼925 cm −1 . 18The recrystallized sample of 3 from acetone-H 2 O afforded a solvated species 3•1/2CH

Thermal properties
The thermal properties of the non-metal polyborate salts 1-4 were examined by TGA (in air) and DSC analysis.Previous studies on non-metal pentaborate salts has shown that they usually thermally dehydrate at temperatures up to 250 °C (via an endothermic process) to afford anhydrous non-metal cation pentaborate salts. 1,19At higher temperatures (up to 800 °C) in air exothermic processes occur (consistent with oxidation of the cation) and leaving B 2 O 3 as a glassy residual solid, via an expanded intumesced material. 5,20B 2 O 3 is also observed as the final product if the DSC thermolysis is recorded in an inert (Ar/N 2 ) atmosphere. 1,21Compounds 1-4 all followed this expected path of decomposition, with observed weight losses for the dehydration, and residual masses of B 2 O 3 after oxidation being consistent with calculated values (see experimental section).This is illustrated for 1 in eqn ( 5) and (6).
Samples of 1 and 2 were each separately calcined in air at 250 °C, 500 °C and 750 °C for 24 h in order to obtained significant quantities of the 'anhydrous', 'intumesced', and 'residual' materials.BET analysis of the 6 calcined materials showed that they were all essentially non-porous with porosities of <1.0 m 2 g −1 .These data are in agreement with BET analysis of thermal materials derived from other NMC pentaborates.0][21][22] The bond lengths and internuclear angles are also within ranges found in related boroxole (B 3 O 3 ) structures which also contain both 4-coordinate and 3-coordinate B centres bound to O. 23 Structures 1-4 all possess giant H-bond anionic lattices, with cations (and co-crystallized species) situated within 'cavities' of the lattice.It is informative to compare the structures of 1 and 2. The unsubstituted cation (in 1) is smaller and able to partake in H-bonding interactions whereas the dimethylated cation (in 2) is larger and is unable to partake in H-bond interactions.Despite these differences, 1 and 2 both crystallize in the same space group with triclinic unit cells, and have very similar supramolecular giant structures.Appropriately, the unit cell of 2 is expanded by 13.3% to accommodate the larger dimethylated cation.The anion-anion H-bond interactions in both of these structures may be described 5,20,24  ], which has significantly non-planar boroxole rings. 25The rotamer which is most commonly observed has one H-atom pointing away from the 4-coordinate B centre towards the γ-O atoms (coplanar with the boroxole rings and no bond critical point) and 3 H atoms pointing inwards.This rotamer is 4 kJ mol −1 higher energy 12 and is found as a basis for interanionic interactions in 1-4 and in most other reported pentaborate structures.The anionanion H-bond interactions found in 1-4 are illustrated in Fig. 6.Each pentaborate is involved with three R 2 2 (8) interactions involving reciprocal-α sites and one C(8) interaction to a β site. 24The 'outward' pointing H-atom is involved in this chain interaction.The gas-phase 3 'inward'/1 'outward' rotamer (iiio) was used as a starting geometry for DFT calculations involving pairing of anions in the geometries appropriate for the R 2 2 (8) and C (8)   interactions.Initially, attempts to pair the anions resulted in endothermic rather than exothermic interactions, presumably a result of unfavorable coulombic forces.We attempted to solve this issue by protonating the pentaborate anions on γ-O atoms on the boroxole rings not involved in H-bonding.The interactions now became exothermic but minimised structures were considered unrepresentative since they contained borox- ole rings which were distorted away from their idealised planar conformations.An alternative procedure, which we believe was successful, involved using 'solvated' rather than 'gas-phase' DFT energies in the calculations.Fang and co-workers 26 have recently calculated solvated energies of the pentaborate anion (but did not specify the rotamer) at a lower computational level.Our data for the solvated iiio isomer was similar to their calculated value but not directly comparable since different basis sets were used.The solvated rotamers were dimerized and exothermic energies were computed, without boroxole distortions.The data for these two interactions are given in Table 3. which were computed at a lower level.It is difficult to compare the calculated values with those observed by X-ray crystallogra-phically since the O-H distance in structures 1-4 was crystallographically fixed at 0.84 Å.Despite this, the calculated data does agree (with the exception for the OHO angle which is 1.6°l arger than the range) within the observed ranges for the structural data available for 1-4 (Table 2).However, this OHO angle is within the range of structures published elsewhere. 20This leads us to conclude that our approach is valid and that the reciprocal-α H-bonds in these systems are relatively strong, and strongly influence the structure.
For completeness these were calculated by the same methods and their data are included in Table 3.The H-bond strengths for the R 2 2 (8) reciprocal-γ interaction is comparable to that of a C(8) β-chain, and is favoured over that of the R 2 2 (12) reciprocal-β interaction.QTAIM calculations (Fig. 8 25 and QTAIM analysis indicates that in addition to the two H-bonds, there exists a further bond critical point between these two O atoms.ρ b for these H-bonds are the lowest of the 4 calculated H-bond interactions and this is in agreement with less favourable H-bond energies.

Conclusion
As noted in sections 1 and 2.3 the majority of polyborate salts are pentaborates and the majority of pentaborate salts crystallize with either a 'brickwall' or a 'herringbone' giant H-bonded   Cation-anion interactions (as observed in 1, 3 and 4) can further stabilize the structure but do not necessarily outweigh the anion-anion contributions, which primarily arise through the reciprocal-α H-bonds.We surmise that given the energetically favoured pentaborate lattice, polyborates other than pentaborates would only be formed when the lattice cannot be stretched to accommodate the cations, and/or when there is sufficient cation-anion interactions to dominate the energetics.

General
All chemicals were obtained commercially from Sigma Aldrich (UK) or Lancaster Synthesis (UK) and were used as supplied.N,N-Dimethyl pyrrolidinium iodide was prepared from N-methylpyrrolidine by use of MeI following standard procedures.NMR spectra were recorded at room temperature (298 K) on a Bruker Ultrashield™ Plus 400, using TopSpin™ 3.2 software package; spectra were further analysed using MestReNova v6.0.2-5475. 11B, 13 C and 1 H NMR spectra were obtained at 400 MHz  mounted at the window of an FR-E-Superbright molybdenum anode generator with VHF Varimax optics (70 mm focus).Cell determination and data collection, data reduction, cell refinement and absorption correction were carried out using Crystal-Clear, 33 structure solution and refinement using SHELX programs. 34 B NMR spectra of moderately concentrated aqueous solutions (D 2 O) of these salts displayed the 3 characteristic signals at ∼18, 13 and 1 ppm which are assigned to B(OH) 3 /[OH], [B 3 O 3 (OH) 4 ] − and the 4-coordinate centre of [B 5 O 6 (OH) 4 ] − , respectively.
The R 2 2 (8) α-reciprocal dimer is considerably more favoured per H-bond (−21 kJ mol −1 ) than the β-chain (−16 kJ mol −1 ).Durka et al. have calculated H-bond energies for boronic acid dimers, which also contains a R 2 2 (8) ring, and have reported an energy of −23.7 kJ mol −1 . 27Our calculated structural data for the R 2 2 (8) system for D⋯A, angle OHO, H⋯O and H-O are 2.78 Å, 178.1°, 1.77 Å and 0.98 Å and these agree well with Durka's values (2.73 Å, 176.8°, 1.73 Å, 0.99 Å) and ESI †) on all H-bonded systems show that the H-bonds have bond critical points, with the energies of the H-bonds mirroring the electron density (ρ b ) at their bond critical points.There is also a red-shift in calculated O-H (donor) stretching vibrational frequencies of up to 450 cm −1 , which correlates with the relative energies of the H-bonds.The calculated R 2 2 (12) reciprocal-β interaction has a close O⋯O contact (3.04 Å) which is similar to that observed in [2-i PrN 2 C 3 H 4 ][B 5 O 6 (OH) 4 ] (2.98 Å)

Fig. 8
Fig.8QTAIM analysis of the H-bond interactions between pairs of pentaborate anions.Bond critical points (small red spheres) and ring critical points (small yellow spheres) are shown.
128)interactions (involving α acceptor sites).The unsubstituted pyrrolidinium cation in 1 is involved in H-bonding to both an α (O1) and a β (O8) pentaborate acceptor site.Details of these H-bond interactions are given in Table2.The inferences from this are that whilst additional H-bond interactions in 1 may further stabilize its solid-state structure, the brickwall structure is sufficiently flexible to accommodate larger cations, and that the pentaboratepentaborate H-bond interactions dominate the energetics.These anion-anion H-bond interactions (and others commonly encountered in pentaborate structures) are discussed in section 2.4 in a computational study.2.4.DFT calculations on solid-state H-bonding motifs observed in pentaborate saltsGiven that pentaborate(1−) salts are most commonly crystallized from the DCL of polyborate anions available in aqueous solution, and that anion-anion H-bond interactions are found in all pentaborate structures, the energetics of these interactions have been examined computationally.Our QTAIM studies on gas-phase polyborate anions12noted that H atoms are at a minimum energy when in the plane of a boroxole ring and that the pentaborate(1−) anion has 4 low energy rotamers which vary in energy by 22 kJ mol −1 ; the lowest energy rotamer having all four H atoms directed inwards towards α-O atoms (no bond critical points) and coplanar with the boroxole rings.