Ionic Liquids in Wonderland: From Electrostatics to Coordination

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C: Physical Processes in Nanomaterials and Nanostructures

ILs in Wonderland: From Electrostatics to Coordination Chemistry Tiago F. C. Cruz, Karina Shimizu, José M. S. S. Esperança, Vânia André, M. Teresa Duarte, Luis Paulo N. Rebelo, Pedro T. Gomes, and Jose Nuno Canongia Lopes J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00987 • Publication Date (Web): 08 Feb 2019 Downloaded from http://pubs.acs.org on February 9, 2019

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The Journal of Physical Chemistry

ILs in Wonderland: From Electrostatics to Coordination Chemistry Tiago F. C. Cruz,a Karina Shimizu,a José M. S. S. Esperança,b,c Vânia André,a M. Teresa Duarte,a Luís P. N. Rebelo,*b,c Pedro T. Gomes*a and José N. Canongia Lopes*a a

Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal. b LAQV, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829516 Caparica, Portugal. c Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, 2780 – 157 Oeiras, Portugal. KEYWORDS: Inverted Ionic Liquids, Nano-segregated systems ABSTRACT: This work represents the first systematic study comparing a homologous series of alkali-based ionic liquids — potassium 1-alkyl-3-methylcyclopentadienyl, K[CnC1Cp]— with their charge-inverted counterparts of the 1-alkyl-3methylimidazolium chloride series, [CnC1Im]Cl. Three new compounds of the K[CnC1Cp] (n= 2, 8 or 10) were synthesised, purified and analysed by NMR, completing the series of previously reported derivatives (n= 4, 6). Further characterisation involved differential scanning calorimetry and powder X-ray diffraction determinations. The results show that most of the alkali-based salts exhibit melting temperature values in the range commonly observed for halide-based ionic liquids. However, striking structural differences were revealed by both X-ray diffraction and molecular dynamics results. These findings are consistent with the fact that alkali metal cations are efficient electron acceptors that can participate in interactions with a significant covalent character, namely with aromatic moieties present in the cyclopentadienyl anions. This new concept extends the boundaries of ionic liquids from the realm of non-covalent electrostatic systems to that of coordination chemistry.

INTRODUCTION

In a recent work, presented at the 2017 Faraday Discussions Meeting on Ionic Liquids,1 we have introduced the concept of charge-inverted ionic pairs in order to explore the relations between electrostatic interactions, structure and the occurrence of ionic liquids (ILs). Those very preliminary results suggest marked structural differences that bridge the gap between coordination compounds and ionic systems. The present work represents the first study comprising two families of chargeinverted ionic pairs — potassium 1-alkyl-3methylcyclopentadienyl versus 1-alkyl-3-methylimidazolium chloride systems — in order to address issues of coordination and structure. The cyclopentadienyl anion ([Cp]–=[C5H5]–) has been a historic and important ligand in the fields of coordination and organometallic chemistry, being thoroughly employed and studied with most d-block, f-block and main group elements. In particular, cyclopentadienyl alkali metal compounds (M[Cp]) are the most important synthetic precursors to other organometallic Cp complexes, namely the metallocenes.2 M[Cp] salts are very frequently obtained as adducts of these moieties with N- or O-containing ligands (including solvent molecules) coordinated to the metal atoms.3-5 A notable finding was made when the solid-state structures of solvatefree M[Cp] (M=Li, Na, K, Rb, Cs) compounds were reported by X-ray powder diffraction,6-8 revealing remarkable 1-D

coordination polymeric ‘multidecker’ structures (‘string of pearls’, for Li and Na, and zigzag chains, for K, Rb and Cs), which result from the self-assembling of multiple M[Cp] units via dual η5 coordination between the metal ions and the Cp rings. M[Cp] salts with substituted Cp rings are scarcely reported in the literature, especially in the case of K derivatives.9-13 In particular, the potassium 1-alkyl-3methylcyclopentadienyl derivatives, K[CnC1Cp] (Scheme 1, A), were unknown until very recently, when we reported the synthesis and characterisation of the derivatives with n = 4 and 6.1 Potassium 1-alkyl-3-methylcyclopentadienyl salts, K[CnC1Cp], are isoelectronic and isostructural to their 1-alkyl3-methylimidazolium chloride equivalents, [CnC1Im]Cl, (Scheme 1, B), which are well-known and seminal ionic liquids.14 K[CnC1Cp]) and [CnC1Im]Cl are charge-inverted ionic pairs: the potassium cation is the isoelectronic counterpart of the chloride anion; [CnC1Cp] anions are the isoelectronic and isostructural counterparts of [CnC1Im] cations (Scheme 1). One can go conceptually from A to B in Scheme 1, simply by “removing” two protons and two neutrons from K+ (which becomes Cl–) and “adding” them to the two alkyl-substituted carbon atoms of the Cp ring (which becomes the imidazolium ring with two alkyl-substituted nitrogen atoms). If one discounts the isotopic abundance of each element, the two salts should also have the same overall mass.

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Scheme 1 Isoelectronic and isostructural equivalence between potassium 1-alkyl-3-methylcyclopentadienyl (K[CnC1Cp], A) and 1-alkyl-3-methylimidazolium chloride ([CnC1Im]Cl, B) The concept of charge-inverted ionic pairs is more or less straightforward in the case of inorganic salts composed of atomic ions (e.g. NaCl/KF or MgS/CaO are two such pairs). Things get more complicated in the case of ionic liquids that contain at least one molecular ion combined with an atomic counter-ion. A simple search at the ILThermo database15-17 reveals that, whereas there are more than 400 entries concerning ionic liquids based on the chloride anion, there is only one entry concerning a potassium-based ionic liquid (potassium bis(fluorosulfonyl)amide). In fact, alkali metals combined with bis(fluorosulfonyl)amide or bis(trifluoromethylsulfonyl)amide yield salts that bridge the melting-point temperature gap that exists between lowtemperature ionic liquids and traditional inorganic salts.18 However, the corresponding charge-inverted ionic pairs do not exist for those cases. So far, the K[CnC1Cp]/[CnC1Im]Cl pair is truly unique as far as ionic liquids are concerned. Our first results on K[C4C1Cp] and K[C6C1Cp] have shown that these two salts are indeed ionic liquids with melting point temperatures below 373 K.1 Molecular dynamics (MD) simulations have also shown that, due to the diverse nature of the interactions of imidazolium and cyclopentadienyl aromatic rings with their respective counterions, the predicted structures of K[C4C1Cp] and [C4C1Im]Cl should be very different. In the present article a set of three additional K[CnC1Cp] salts (n = 2, 8, 10) was synthesised and characterised by solution 1H and 13C{1H} Nuclear Magnetic Resonance (NMR) spectroscopy, completing now, along with the salts reported before,1 a family of five salts in which n = 2, 4, 6, 8, 10. The structure of K[CnC1Cp] (n = 2, 4, 6) samples were studied by powder X-Ray Diffraction and the obtained structure factor functions compared with data obtained from MD simulations. Thermal analyses of all systems were carried out by Differential Scanning Calorimetry (DSC) up to 450 K. Finally, the K[CnC1Cp] and [CnC1Im]Cl families are compared in terms of their unique coordination patterns that lead to the emergence of 1-D and 3-D ionic networks, respectively.

RESULTS AND DISCUSSION Synthesis and NMR spectroscopy characterisation The 1-R-3-methylcyclopentadienyl potassium derivatives K[CnC1Cp] (2a, 2d and 2e), in which the alkyl group R is ethyl (K[C2C1Cp], 2a), n-octyl (K[C8C1Cp], 2d), and n-decyl

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(K[C10C1Cp], 2e) were prepared by a deprotonation reaction of a 1-R-3-methylcyclopentadiene mixture of isomers (1a, 1d or 1e, respectively) with KH in THF, at 50 °C (Scheme 2), in a similar procedure used for the previously reported n-butyl (K[C4C1Cp], 2b) and n-hexyl (K[C6C1Cp], 2c) derivatives.1 After workup, the potassium salts 2a, 2d and 2e were obtained as off-white (2a) and pale orange (2d and 2e) deliquescent pyrophoric powders. Similarly to 2b and 2c, compounds 2a, 2d and 2e are insoluble in hydrocarbons, but are extremely soluble in coordinating solvents, such as THF or pyridine. Compounds 2a, 2d and 2e were characterised by 1H and 13 C{1H} NMR spectroscopy in pyridine-d5 (Figs. S1-S6 in SI), displaying the expected resonances and corresponding chemical shifts of cyclopentadienyl anions coordinated to potassium atoms in a η5-fashion, and free of coordinated solvent molecules or co-solvates. A marked difference for the n-alkyl moieties is observed between 2a and the remaining compounds, whereby the methyl group of the C1 substituent is upfield shifted, from 1.40 ppm in 2a to 0.96-0.89 ppm in 2b-e (see also ref. 1). Increasing the number of carbons in the nalkyl chain leads to a broad superimposition of alkyl 1H resonances.

Powder X-ray diffraction To obtain some insights into the structure of the potassium 1-alkyl-3-methylcyclopentadienyl derivatives, powder X-ray diffraction (PXRD) data on selected samples was collected under high-resolution synchrotron radiation at the European Synchrotron Radiation Facility (ESRF) in Grenoble.

Scheme 2 Synthesis of the family of salts K[CnC1Cp] (2a-e). Thus, compounds 2a-c were characterised by PXRD at lowq values, revealing that the powder samples have limited longrange crystallinity. Also, by increasing the number of carbon atoms of the n-alkyl chain, the degree of crystallinity decreases dramatically, in the order R = Et (2a) > R = n-Bu (2b) > R = n-Hex (2c). Figure 1 shows the low-q powder Xray diffractograms of complexes 2a-c along with structure factor functions in the q-value range, calculated from MD simulations. The correspondence between the positions of the different diffractogram/structure factor peaks and the structural characteristics of the compounds at a nanoscopic level are analysed in the MD discussion section.


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The Journal of Physical Chemistry with lower Tm values for ionic liquids with intermediate alkyl side chains in the molecular ion (C6 or C8); iii) one member of each series, K[C2C1Cp] and [C4C1Im]Cl, exhibit two solidliquid phase transitions that are reversible and suggest the presence of enantiotropic solid-solid transformations47; iv) the [CnC1Im]Cl (4≤even n≤10) salts can exist as metastable liquids and their glass transition temperatures have been measured.

Figure 1 Three bottom lines: powder X-ray diffraction data for K[C2C1Cp] (2a, blue), K[C4C1Cp] (2b, orange), K[C6C1Cp] (2c, grey). Three top lines: corresponding structure factor data from MD simulations obtained for the same series of compounds using the same colour coding. The MD data correspond to samples at 400 K (liquid or amorphous states).

Differential Scanning Calorimetry Data The DSC results depicted in Figure 2 confirm the liquid nature of the K[C4C1Cp] (2b) and K[C6C1Cp] (2c) salts around 100 ºC, with melting point temperatures of 95 and 89 ºC, respectively. On the other hand, the K[C8C1Cp] (2d) and K[C10C1Cp] (2e) salts remain solid in the entire temperature range covered by the corresponding DSC experiments (from room temperature to around 230 ºC). Finally, the K[C2C1Cp] (2a) salt exhibits two phase transitions (both during the heating and cooling cycles) at 127 and 142 ºC. The processes are reversible as can be attested by the superimposition of the different heating/cooling cycles after the first heating ramp. Such fact also confirms the stability of the samples under inert atmosphere within the studied temperature range. Enthalpies of fusion were also calculated from the DSC plots. Estimated values of 3.0±0.6, 3.4±0.4 and 3.7±0.3 kJ/mol were found for K[C2C1Cp] (2a), K[C4C1Cp] (2b) and K[C6C1Cp] (2c), respectively. The enthalpy value obtained for the K[C2C1Cp] corresponds to the heat exchanged during the two phase transitions that occurred during each heating cycle.

Phase-transition temperature analysis Figure 3 compares the results obtained for the K[CnC1Cp] series with selected phase-change data19‒47 for the corresponding charge-inverted ionic series, [CnC1Im]Cl. A few analogies and differences can be inferred from the data: i) the melting point temperatures, Tm, of the alkali-based series are higher than the corresponding values for the halide-based family (ca. 55 K and 30 K differences in the case of the ethyland butyl-substituted analogues, respectively); ii) the Tm trends along both series (dotted lines in Fig. 3) are not monotonic,

Figure 2 Three cycles of DSC experiments with five K[CnC1Cp] samples (C2, C4, C6, C8 and C10, 2a-e). All cycles are superimposed but the first is coloured orange (heating) and cyan (cooling), whereas the 2nd and 3rd cycles are coloured red (heating) and blue (cooling). The heat flows were normalised to take into account the different cooling/heating rates. Melting point temperatures were estimated from the onset of the corresponding exothermal/endothermal processes and are presented in the relevant DSC plots.

The same is not true for members of the K[CnC1Cp] series that easily undergo first order liquid-solid phase transitions. Conversely, [C6C1Im]Cl does not crystalize and only its glass transition temperature is known. The traits shared by both series (lower melting point temperatures for ILs with intermediate alkyl side chain lengths; existence of solid polymorphs that lead to secondary solid-liquid transitions) are also common in other ionic liquid families with varying sizes of alkyl side chains in one of the ions48-54. Such behaviour represents the hindrance of ionic packing caused by non-polar moieties (alkyl side chains in the present case) that can occupy non-negligible volumes and can exhibit different conformational orientations.



Figure 3 Phase transition temperatures, t/ºC, of the K[CnC1Cp] (circles) and [CnC1Im]Cl (squares) charge-inverted IL series as a function of the alkyl side chain length of the molecular ion, Cn. The empty symbols represent melting point temperatures (crystalliquid phase transitions); the black symbols represent melting point temperatures between a second (less-stable) crystalline form and the liquid; the grey squares represent glass-transition temperatures. The K[CnC1Cp] data (circles) were measured in this work. The [CnC1Im]Cl data are averages from multiple sources,1947 given as supporting information in SI.

For large alkyl side chains such packing problems are partially overcome by a better segregation between ionic and non-polar domains, with the emergence of layering patterns that can even lead to mesophases such as smectic liquidcrystalline phases.55-57 The non-monotonic behaviour found for the melting point temperatures is thus a consequence of the two trends: increasing crystal destabilisation due to the presence of longer alkyl side chains until a point when those chains are so long that they can start to interact preferentially with each other in very effective ways, segregating from the polar parts and stabilising the resulting crystal/liquid crystal. The two main differences between the K[CnC1Cp] and [CnC1Im]Cl series —the inexistence of melting for the C8 and C10 members of the former series in the studied temperature range and the existence of glass transition temperatures for most members of the latter— can be interpreted as a sign of structural dissimilarities in interionic packing in the two series. It is also interesting to note that the enthalpy of fusion values for K[C2C1Cp], K[C4C1Cp] and K[C6C1Cp] (in the 3 to 4 kJ/mol range) are much lower than the corresponding values for [C2C1Im]Cl and [C4C1Im]Cl (around 15 and 23 kJ/mol, respectively26,29,34,41,45,47,58,59). Such disparity can also be attributed to structural differences between the two salts.

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dynamics (MD) simulations. Details concerning the modelling of the salts under discussion and the simulation of the MD trajectories are given in SI. To compare the results obtained from both the PXRD and MD sources, we have converted the original PXRD data from 2θ units (in degrees) to q-values (in nm-1) and used Fourier transforms on the appropriate pair correlation functions obtained by MD to yield Structure Factor functions, S(q), in the same units. The results are compared in Fig. 1. The first point to be made is that the comparisons are being done at low q-values in the 2 < q/nm-1 < 16 range (corresponding to interionic features ranging from 3 to 0.4 nm) between crystallites in the XRD experiments and amorphous (liquid) systems in the MD simulations. The small but still sharp XRD peaks in the low-q region reflect the existence of micro crystallites; the broad peaks of the MD S(q) functions correspond to a liquid phase. Nevertheless, the main features of the XRD diffractograms and MD S(q) functions have a good degree of correspondence: i) the two peaks at 12.6 and 13.8 nm-1 in the XRD plots for K[C2C1Cp] and K[C4C1Cp] show up as a broad S(q) peak centred at 13.0 nm-1 (red shaded area in Fig. 1); ii) the XRD peaks at 7.7, 6.2 and 3.0 nm-1 of K[C2C1Cp], K[C4C1Cp] and K[C6C1Cp], respectively, can be associated with the broad S(q) peaks in the corresponding S(q) plots of the same salts (circle-capped lines in Fig. 1). Feature i) is independent of the alkyl side chain length in the IL anion; feature ii) shifts to lower q-values as the alkyl chain length increases. Unfortunately, the relatively low crystallinity of the powder samples used in the XRD experiments does not allow a full analysis to resolve the crystalline structures along the K[CnC1Cp] series. However, approximate structures of those salts can be inferred taking into account the present XRD results, the MD-calculated S(q) functions and the available structural information for the K[Cp] salt. As mentioned in the introduction, the solid state structure of K[Cp] and other alkali metal cyclopentadienyl salts can be described as a multidecker structure with two Cp rings η5 coordinated to each alkali metal atom (Fig. 4). The structures of Li[Cp] and Na[Cp] are linear (sometimes dubbed as a “string of pearls”6), whereas that of K[Cp] and other heavier alkali metals (Rb, Cs) form zigzag chains. In the case of K[Cp] the angle between three consecutive potassium atoms is 138°.8 In all cases crystals are formed by the compact and ordered bundling of these chains. The XRD spectra for the K[Cp]8 shows peaks at 8.6, 12.5 and 14.0 nm-1, corresponding to inter-plane distances of 0.73, 0.50 and 0.45 nm and Miller indices of (1 0 1), (2 0 0) and (2 1 0), respectively. The zig-zag chains are oriented along the c unit-cell vector, which means that the second and third peaks reflect distances between different chains (inter bundle distances), and only the first peak contains information concerning distances along the chain. A wavelength of around 1.05 nm along the c unit-cell vector (cf. Fig. 4) can be calculated from the 8.6 nm-1 peak with a Miller index of (1 0 1) if one deducts the contribution from the a unit-cell vector.

Structural Analysis of the K[CnC1Cp] family In order to probe from a molecular point of view the structure of the K[CnC1Cp] series, we have decided to complement the experimental evidence obtained from powder X-ray diffraction experiments (PXRD) with molecular


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Structural differences between the K[CnC1Cp] and [CnC1Im]Cl families

Figure 4 The multidecker structures of Li[Cp] and K[Cp] as strings of intercalated alkali metal cations and cyclopentadienyl anions. Note the linear and zig-zag nature of the Li[Cp] and K[Cp] strings, respectively.6,8

Such distance is related to the broad peak centred at 13.0 nm-1 that shows up in the S(q) function of the MD simulated K[C2C1Cp] liquid. It is also reflected by the peaks at 12.6 and 13.8 nm-1 in the XRD plots for solid K[C2C1Cp] and K[C4C1Cp]. In ionic liquids such peak is commonly dubbed as chargeordering peak (COP) and mirrors the characteristic distances found between two ions of equal charge that are intercalated by an ion of opposite charge within the polar network. In the present case the polar network are the zig-zag chains of intercalated potassium and cyclopentadienyl rings, with a COP that corresponds to half the wavelength of the K[Cp] zig-zag chain (1.05/2 = 0.50 nm ≈ 2π/13 nm-1). The position of the COP does not change along the K[CnC1Cp] series since the increase in the alkyl side chains in the cyclopentadienyl rings does not change the K-Cp spacing along the chains but only the distance between different chains. However, the existence of alkyl substituents in the 1 and 3 positions of the cyclopentadienyl ring (and the loss of symmetry of the ring) causes dramatic changes in the packing/bundling of different zig-zag chains, and these are reflected in the shifts of the other peak present in the S(q) functions at lower q values as one of the alkyl side chains in the K[CnC1Cp] series is increased. Obviously, longer alkyl side chains will cause different chains to be bundled further apart. It is also important to stress that the asymmetry of the substituted rings can also cause loss of symmetry along each multidecker chain: i) the alkyl substituents that emerge from the chains may be oriented in different directions and ii) the regular zig-zag pattern of the chain may be deeply perturbed. This situation is analogous to problems of tacticity in polymer chains. Such analogy can be further explored if one recalls that atactic polymers tend to bundle in amorphous structures, a situation that may explain the limited crystallinity of the samples used in the XRD experiments. Such loss of crystallinity should be more obvious when the alkyl side chains are longer, a fact also verified experimentally along the sequence K[C2C1Cp] to K[C6C1Cp].

The structures of the K[CnC1Cp] and [CnC1Im]Cl chargeinverted series are dramatically different: the polar network of the former is constituted by the above-mentioned bundling of multidecker chains; the polar network of the latter is threedimensional in nature, with each imidazolium cation interacting directly with more than four chloride anions.60 Moreover, the potassium cations in the K[CnC1Cp] series interact above and below the aromatic plane of the cyclopentadienyl anion, whereas the chloride anions in the [CnC1Im]Cl series interact mostly around the aromatic plane of the imidazolium ring. Such state of affairs is confirmed, for instance, by the available structures of the charge-inverted crystals [Im]Br and Rb[Cp]61,62. Also, the distances between the centroid of the cyclopentadienyl ring and the rubidium cations (0.295 nm) correspond to η5 bonds between the alkali metal and the cyclopentadienyl ring, whereas the distances between the bromide anions and the hydrogen atoms attached to the imidazolium ring (in the 0.245 to 0.283 nm range) correspond to weak hydrogen bonds. The MD simulations on disordered K[CnC1Cp] and [CnC1Im]Cl systems also confirm that the number of contact neighbours between the charged moieties of the ions is always 2.0 in the case of all K[CnC1Cp] salts and decreases from 5.7 to 4.5 as the alkyl side chains grow larger in the [CnC1Im]Cl series (Fig. 5).

Figure 5 Aggregate analyses on the structure of K[CnC1Cp] and [CnC1Im]Cl systems obtained by MD simulation. The empty squares in the two graphics depict the average number, Ni, of atomic ions Cl and K that are direct contact neighbours of the Im and Cp rings, respectively. The filled circles represent the average number of contact neighbours of a given alkyl side chain that are also alkyl side chains. The two figure insets illustrate the very different interactions between ions that prevail in each type of system.

Conversely, the average number of contacts between alkyl side chains in both series rises from 2 to around 7 as the length of the alkyl side chains increases. Such increase leads to larger average distances between mutidecker chains in the K[CnC1Cp] systems (and shifts to lower q-values of the corresponding S(q) peaks) and to larger non-polar domains in the [CnC1Im]Cl systems that eventually percolate and form a second continuous non-polar sub-phase (with the concomitant



appearance of a low-q peak in the corresponding S(q) functions, as observed in other IL systems based on imidazolium cations63,64). The small figure insets in Fig. 5 illustrating the strikingly different inter-ion spatial arrangements in the K[CnC1Cp] and [CnC1Im]Cl series were taken directly from known structures deposited at the CCDC.8,60 Interestingly, the rendering algorithms (based on the van der Waals radii of each atomic species) show the K-Cp interactions as covalent in character (η5 coordination) and the Cl-Im interactions just as intermolecular contacts. This means that in spite of being the charge-inverted analogues of common ionic liquids, the members of the K[CnC1Cp] series should also be reckoned as systems where coordination takes a central role. This overlap between covalently bound species and electrostatic interactions may be of great significance in the context of pushing the boundaries of ionic liquids as a novel class of compounds. The difference between K[CnC1Cp] and [CnC1Im]Cl is further illustrated in the two simulation snapshots of K[C10C1Cp] and [C10C1Im]Cl presented in Fig. 6, where only the direct contacts between the atomic ions and the centroids of the aromatic rings are rendered as wireframe connections. The omission of the alkyl chains that form the non-polar domains allows the visualisation of the two very different types of connectivity in the two polar networks. The MD results are similar in terms of morphology of the polar networks for all the other members of the two series. Fig. 6 also shows that the zig-zag patterns present in the chains of K[Cp] crystals no longer exist in the amorphous structures of the simulated K[CnC1Cp] series: the chains are flexible enough to assume different conformations and adapt themselves to the presence of the non-polar domains formed by the alkyl side chains emerging from the cyclopentadienyl rings.

The polymeric nature of the K-Cp multidecker chains As stated before, the concept of covalently bound chains with repeating units resonates with many of the ideas commonly discussed in polymer science. In this context aggregate analyses performed on the MD trajectories were used to obtain chain size distributions. These are depicted in Fig. 7 in the form of number-weighted and mass-weighted probability distribution functions, pnf and pmf. The figure shows the distributions obtained for the K[C4C1Cp] system but similar results were found for all other systems within the statistical uncertainty inherent to the simulations.

Figure 7 Discrete probability distribution functions of the size of the potassium-cyclopentadienyl chains in the K[C4C1Cp] system, obtained from aggregate analyses on MD trajectories at 400 K. The blue and green interconnected dots represent number- and mass-weighted probability distribution functions, pnf and pmf, respectively. The red curve represents a mass-weighted probability function calculated using Schulz-Flory distribution with a fraction of uncoordinated ion pairs of 0.64%.

Figure 6 Two MD simulation snapshots of the K[C10C1Cp] (left panel) and [C10C1Im]Cl (right panel) systems. Each simulation box (shaded areas) has a side of around 7 nm. Only the atomic ions (potassium in pink; chloride in green) and the centroids of the aromatic rings (in grey) have been rendered as a wireframe mesh. The linear strands present in K[C10C1Cp] contrast vividly with the branched network present in [C10C1Im]Cl . It must be stressed that boundary conditions are present in all directions, which means that the interruptions of the mesh at the edges of the box (including the front and back) are artefacts of the rendering process.

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From the pnf and pmf distributions it was possible to calculate the number-average and mass-average molar mass of the chains in the systems (for the K[C4C1Cp] system Mn = ∑i NiMi / ∑i Ni = 13490 a. m. u. ; Mw = ∑i NiMi2 / ∑i NiMi = 25200 a. m. u.) and the corresponding dispersity index, Ð = Mw / Mn = 1.87. The pwf results can also be fitted to a Schulz-Flory distribution if one assumes that the formation of the chains occurs as in an ideal step-growth aggregation process and that in equilibrium the fraction of uncoordinated ion pairs is only 0.64%. These are the most speculative results of the present work since they are obviously dependent on the models used to describe the interactions between ions in the MD-simulated systems. In fact, the covalent character associated to the establishment of a η5 coordination between a given potassium ion and a cyclopentadienyl ring is not considered explicitly by the model, which in terms of intermolecular interactions only assumes repulsive, dispersive and electrostatic interactions between multiple isotropic interaction centres. The parameterisation of those interactions takes into account information obtained from quantum mechanical calculations on selected molecular species present in the systems but the


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fitting process is empirical and averages-out much of the intraatomic information necessary to describe correctly the emergence of a covalent bond. Nevertheless, the model is able to capture the extensive formation of K-Cp chains, in agreement with the structure previously reported for the analogous K[Cp] crystals and inferred for the K[CnC1Cp] systems by the PXRD diffractograms obtained in this work. Future modelling work where the covalent character of the K-Cp interactions is explicitly taken into account using ab initio molecular dynamics techniques (AIMD) is currently being developed.

Conclusions The present work is the first systematic study covering an entire homologous series of alkali-based ionic liquids, potassium 1-alkyl-3-methylcyclopentadienyl (K[CnC1Cp]), that are the charge-inverted analogues of the well-known 1alkyl-3-methylimidazolium chloride ([CnC1Im]Cl) ionic liquid series. Due to their lack of intrinsic stability to air or moisture, this novel ionic liquid family will never reach the same level of popularity or lead to numerous potential applications as its halide-based counterpart. Nevertheless its synthesis and characterisation proved three important facts: i) There is no fundamental reason that prevents the existence of alkali-based ionic liquids. If alkali-metal cations are combined with suitable (and stable) molecular anions, the resulting salts will exhibit melting temperatures in the ranges commonly observed for halide-based ionic liquids; ii) Alkali metal cations are efficient electron acceptors that can participate in interactions with a significant covalent character (the η5 coordination observed in the present case between the K cations and Cp anions). This extends the boundaries of ionic liquids from the realm of non-covalent electrostatic systems to that of coordination chemistry; and iii) Aromatic systems interact very differently with atomic anions (halides) and cations (alkali metals): K[CnC1Cp] and [CnC1Im]Cl are composed of isoelectronic and isostructural ions that nevertheless yield salts with striking structural differences. Future work on charge-inverted ionic liquid pairs should address this last issue via the synthesis of analogous molecular ions with no aromatic moieties.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedures, 1H and 13C{1H} NMR spectra of K[CnC1Cp], Powder X-ray diffraction and differential scanning calorimetry (DSC) methodologies, molecular dynamics (MD) simulation details, Phase-transition temperature and standard molar enthalpy of fusion data along the [CnC1Im]Cl series (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS We thank Fundação para a Ciência e a Tecnologia, Portugal, for financial support (Projects UID/QUI/00100/2013, Lisboa/01/0145/FEDER/028367, PTDC/QUI-QFI/29527/2017, UID/QUI/50006/2013, and UID/Multi/04551/2013), for fellowship to T.F.C.C. (PD/BD/52372/2013, CATSUS PhD program), for a CEEC contract to K.S. (IST-ID/100/2018), and for a FCT Investigator contract to J.M.S.S.E (IF/00355/2012). We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and we would like to thank Dr. Ana Gutiérrez and Dr. Germán R. Castro for assistance in using beamline BM2.

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