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Article

Crystal Engineering of Cation-Radical Salts with Weakly Coordinating Carbadodecaborate Anions

by
Emmanuel Adeniyi
1,
Matthias Zeller
2 and
Sergiy V. Rosokha
1,*
1
Department of Chemistry, Ball State University, Muncie, IN 47306, USA
2
Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA
*
Author to whom correspondence should be addressed.
Crystals 2023, 13(1), 99; https://doi.org/10.3390/cryst13010099
Submission received: 17 December 2022 / Revised: 26 December 2022 / Accepted: 1 January 2023 / Published: 5 January 2023
(This article belongs to the Special Issue Feature Papers in Crystal Engineering in 2022)
Browse Figures
Versions Notes

Abstract

:
An X-ray structural analysis revealed that the salts of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), N-methylphenothiazine (MPTZ), and octamethylbiphenylene (OMB), with dodecamethyl- or hexabromo-substituted carbadodecaborate anions, comprise layers of the (partially) oxidized organic donors separated by sheets of the bulky counter-ions. The cationic layers comprise either well-separated TMPD+ or MPTZ+ cation radicals or π-stacks of partially oxidized OMB moieties consisting of more or less distinct (OMB)2+ units. Quantum mechanical calculations revealed that the formation of essentially isolated cation-radical or π-bonded associations in the salts with these weakly coordinating anions is correlated with the strength of the multicenter π-bonding between cation radicals. This pancake bonding is determined by the balance of the electrostatic repulsion between cationic counterparts and attractive dispersion and weakly covalent interactions.

1. Introduction

Following the discovery of the (super-)conductive and magnetic properties of the cation-radical salts of some planar organic donors, the preparation and characterization of such compounds became one of the central themes of structural and material chemistry [1,2,3,4,5]. The variation of counter-ions and stoichiometry led to wide-range modifications of the crystal architecture of the salts with a certain cation radical. It allowed modulating the 3D arrangement of the networks formed by the ion-radical species and the strength of their interactions, which is critical for the material-science properties [6,7,8]. Indeed, the substantial electronic coupling between the π-stacked moieties led to electron delocalization, and the charge carrier’s mobility within the array was related to the conduction bandwidth. However, strong π-bonding between a pair of cation radicals can also lead to diamagnetic dimers, and thus, it resulted in non-conducting (insulator) states [7].
Previous studies have mostly focused on the cation radicals of tetrathiafulvalene (TTF) analogs (such as bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) or tetramethyltetraselenafulvalene (TMTSF)) [9]. While these systems are considered the “brick and mortar of organic electronics”, [10] further advances in this area require the development of methods for the preparation of salts with diverse types of species, e.g., planar hydrocarbon- or nitrogen-containing cation radicals [11,12]. Since the latter are frequently more reactive than the tetrathiafulvalene cation radicals, crystal engineering of their salts requires a judicious choice of counter-ions [13]. Weakly coordinating anions (WCA), such as the carbadodecaborate derivatives dodecamethyl-closo-1-carbododecaborate (Me12CAR−) or 7,8,9,10,11,12-hexabromo-closo-1-carbododecaborate (Br6CAR−) illustrated in Figure 1, have appeared as very promising building blocks for such materials [14,15,16,17,18].
The non-nucleophilic nature of these anions facilitates the stability of the reactive cationic species in the solid state and in solution. Previous works demonstrated that the utilization of these anions allows the preparation of crystalline salts of benzenium, vinyl, or tert-butyl cations, which are persistent at room temperature [19,20,21]. Carbododecaborate anions were also fruitfully utilized for the isolation of unstable organometallic intermediates [22,23]. Aside from this, the use of bulky anions with delocalized charge allows the electrostatic interactions between counter-ions to be minimized. Therefore, the formation of dimeric (or higher order) associations of cation radicals is predominantly determined by the intrinsic properties of the latter for π-bonding.
We have previously shown that the use of carbadodecaborate anions (Figure 1) allows the preparation of persistent cation-radical salts with targeted stoichiometry and various architectures via chemical oxidation of the neutral tetrathiafulvalene derivatives [24,25,26]. In particular, the crystalline cation-radical salts with Br6CAR− counter-ions were prepared through the interaction of neutral organic donors, D, with tris-(4-bromophenyl)aminium (TBPA+•) salts of this anion, (TPBA)Br6CAR, in dichloromethane (Equation (1)):
D + (TPBA)Br6CAR → D+• + Br6CAR− + TPBA
Alternatively, such salts were prepared by the oxidation of donors with the Me12CAR• radical (Equation (2))
D + Me12CAR• → D+• + Me12CAR− + TPBA
Depending on the donor and stoichiometric ratios of reactants, the structures of these salts comprised isolated cation radicals, dicationic dimers or trimers, or π-stacks of partially oxidized moieties. The variations of the crystal architecture were accompanied by modulation of the spin–spin interactions, and therefore, changes in the magnetic properties of these substances. [25,26] To clarify the applicability and limitations of these synthetic methods, as well as to evaluate the factors determining the crystal architectures of the resulting salts, in the current work, we explored if similar reactions will allow the preparation of cation-radical salts of hydrocarbon or nitrogen-containing donors, such as N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), 10-methylphenothiazine (MPTZ), and octamethylbiphenylene (OMB), as shown in Chart 1. Earlier studies indicated the possibility of the formation of π-bonded dimers of these cation radicals [27,28,29]. As such, we also explored in this work the factors determining the formation of π-bonded associates (which are critical for the properties of the substances) in the salts with minimized interaction with the counter-ions.

2. Materials and Methods

2.1. Materials

N,N,N′,N′-Tetramethyl-p-phenylenediamine (from Sigma Aldrich, Burlington, MA, USA) was purified by sublimation. 10-Methylphenothiazine, tris(4-bromophenyl)amine (both from Sigma Aldrich, Burlington, MA, USA), cesium dodecamethyl-closo-1-carbododecaborate, CsMe12CAR, (from Katchem, Praha, Czech Republic), and silver 7,8,9,10,11,12-hexabromo-closo-1-carbododecaborate, AgBr6CAR (from Strem, Newburyport, MA, USA) were used without additional purification. Octamethylbiphenylene was prepared by the reaction of 1,2-dibromotetramethylbenzene with n-butyl lithium (both from Sigma Aldrich, Burlington, MA, USA) [30]. Dodecamethylcarboranyl radicals, Me12CAR•, were prepared by the reaction of CsMe12CAR with PbO2 in acetonitrile [31]. (TBPA)Br6CAR, was synthesized by the addition of Br2 to a solution of AgBr6CAR and TBPA in anhydrous dichloromethane under an N2 atmosphere at −78 °C, as described earlier [32]. A similar reaction using half of the stoichiometric ratio of Br2 resulted in the formation of (TBPA)Ag(Br6CAR)2. The purity of these salts (>98%) was confirmed via UV–vis measurements (using the characteristic absorption band of the TBPA+• cation radical at λmax = 704 nm with ε = 3.3 × 104 M −1cm−1 [32]).

2.2. Crystallization and X-ray Structural Analysis

The cation-radical salts with Me12CAR−, Br6CAR− or Ag(Br6CAR)2− counter-ions were prepared by the addition of a solution of either AgBr6CAR, (TBPA)Br6CAR or CAR• to the dichloromethane solutions of the organic donors under an argon atmosphere. The addition of hexane and partial evaporation of dichloromethane led to the formation of dark-colored crystalline salts of the cation radicals. These salts were recrystallized by slow evaporation of their solutions in dichloromethane:hexane (5:1) mixtures or from acetonitrile. Such recrystallizations produced crystals suitable for X-ray crystallography analysis. The purity of the salts (>97%) was established using UV–vis spectral measurements by comparison of the intensities of the absorption maxima of the solutions prepared from the bulk samples with those predicted based on the characteristic absorption bands of the corresponding cation radicals in the visible range [27,28,29,32]. The FT-IR spectra of the cation-radical salts and parent neutral molecules are shown in Figures S1–S3 in the Supporting Information.
Specifically, to prepare (TMPD)Br6CAR, 66 mg (0.060 mmol) of (TBPA)Br6CAR in 5 mL CH2Cl2 was added to a solution of 17.0 mg (0.10 mmol) of TMPD in 5 mL of CH2Cl2 in a Schlenk tube under argon atmosphere. After the mixture was stirred for 10 min, 10 mL of hexane was added and about 60% of the solution was evaporated under reduced pressure. The dark precipitate was filtered off, washed with pentane, and dried under vacuum (45 mg, yield 82%). This material was dissolved in 10 mL of a 2:1 dichloromethane/hexane mixture. Slow evaporation of this solution resulted in the formation of dark plates of (TMPD)Br6CAR suitable for a single crystal X-ray analysis (vide infra). (MPTZ)Br6CAR and (OMB)2Br6CAR salts were prepared in a similar way through the interaction of 20 mg (0.090 mmol) of MPTZ or 24 mg (0.091 mmol) of OMB with 66 mg (0.060 mmol) of (TBPA)Br6CAR. (OMB)2 [Ag(Br6CAR)2] was prepared by mixing the solutions of 14.5 mg (0.055 mmol) of OMB with 44.3 mg (0.024 mmol) of TBPA[Ag(Br6CAR)2]. To prepare (TMPD)Me12CAR, a solution of 13 mg (0.042 mmol) of CAR• in pentane was added to the solution of 7.1 mg (0.043 mmol) of TMPD in 5 mL of CH2Cl2 in a Schlenk tube, and the solution was stirred for 10 min. Hexane (20 mL) was added to this solution, and about 60% of the solution was evaporated under reduced pressure. The dark (TMPD)Me12CAR precipitate was filtered off, washed with pentane, and dried. (MPTZ)Me12CAR was similarly prepared by the interaction of 13 mg (0.042 mmol) of CAR• with 10.0 mg (0.045 mmol) of MPTZ.
The single crystal structures of all salts except (TMPD)Me12CAR were examined on a Bruker Quest diffractometer (Bruker AXS, LLC, Madison, WI, USA)with a fixed chi angle, a sealed tube fine-focus X-ray tube, a single crystal curved graphite incident-beam monochromator (Bruker AXS, LLC, Madison, WI, USA), and a Photon100 or PhotonII area detector (Bruker AXS, LLC, Madison, WI, USA). Examination and data collection were performed with Mo Kα radiation (λ = 0.71073 Å). A single crystal of (TMPD)Me12CAR was analyzed with a Bruker Quest diffractometer (Bruker AXS, LLC, Madison, WI, USA) with a PhotonII area detector with microsource Cu Kα radiation (λ = 1.54178 Å). Both instruments were equipped with an Oxford Cryosystems low-temperature device, and all data were collected at 150 K. Reflections were indexed and processed, and the files were scaled and corrected for absorption using APEX3 or APEX4 [33]. The space groups were assigned using XPREP within the SHELXTL suite of programs [34], and the structures were solved by direct or dual space methods and refined by full-matrix least-squares against F2 with all reflections using Shelxl2018 [35,36], using the graphical interface Shelxle [37].
(TMPD)Br6CAR (1). Chemical formula C11H22B11Br6N2, M = 780.67 g/mol. Orthorhombic, space group Pnma, a = 12.2897 (17) Å, b 19.147 (3) Å, c = 10.683 (2) Å, V = 2513.7 (7) Å3, Z = 4, T = 150 K, μ(Mo Kα) = 9.59 mm−1, 25,820 reflections measured, 4301 unique (Rint = 0.084). The final R1 was 0.033 (I > 2σ(I)) and wR2 was 0.080 (all data).
(TMPD)Me12CAR (2). Chemical formula C23H52B11N2, M = 475.57 g/mol. Monoclinic, space group P21/c, a = 9.4382 (5) Å, b = 16.2939 (8) Å, c = 10.0320 (5) Å, β = 99.988 (2)°, V = 1519.39 (13) Å3, Z = 2, T = 150 K, μ(CuKα) = 0.38 mm−1, 23,308 reflections measured, 5240 unique (Rint = 0.078). The final R1 was 0.069 (I > 2σ(I)) and wR2 was 0.206 (all data).
(MPTZ)Br6CAR·CH2Cl2 (3). Chemical formula C15H29B11Br6 Cl2NS, M = 914.64 g/mol. Monoclinic, space group P21/c, a = 14.5114 (6) Å, b = 11.9307 (5) Å, c = 17.2932 (7) Å, β = 92.143 (3)°, V = 2991.9 (2) Å3, Z = 4, T = 150 K, μ(Mo Kα) = 8.31 mm−1, 67,062 reflections measured, 8757 unique (Rint = 0.117). The final R1 was 0.045 (I > 2σ(I)) and wR2 was 0.116 (all data).
(MPTZ)Me12CAR (4). Chemical formula C13H47B11NS, M = 524.61 g/mol. Triclinic, space group P 1 ¯ , a = 9.1947 (7) Å, b 9.6197 (7) Å, c = 9.8672 (6) Å, α = 92.220 (4) °, β = 108.427 (4)°, γ = 107.029 (4)°, V = 783.37 (10) Å3, Z = 1, T = 150 K, μ(Mo Kα) = 0.102 mm−1, 31,192 reflections measured, 5974 unique (Rint = 0.046). The final R1 was 0.058 (I > 2σ(I)) and wR2 was 0.188 (all data).
(OMB)2Br6CAR·CH3CN (5). Chemical formula C43H57B11Br6N, M = 1186.26 g/mol. Monoclinic, space group P21/m, a = 8.5334 (10) Å, b = 28.567 (2) Å, c = 10.1051 (9) Å, β = 102.614 (6)°, V = 2403.9 (4) Å3, Z = 2, T = 150 K, μ(Mo Kα) 5.04 mm−1, 30,815 reflections measured, 5940 unique (Rint = 0.070). The final R1 was 0.060 (I > 2σ(I)) and wR2 was 0.134 (all data).
(OMB)2·[Ag(Br6CAR)2 ·CH2Cl2 (6). Chemical formula C44H64AgB22 Br12Cl4, M = 2039.24 g/mol. Triclinic, space group P 1 ¯ , a = 7.4994 (11) Å, b = 14.880 (2) Å, c = 16.050 (2) Å, α = 79.284 (6)°, β = 84.942 (6)°, γ = 81.080 (6)°, V = 1735.2 (4) Å3, Z = 1, T = 150 K, μ(Mo Kα) 7.39 mm−1, 58,189 reflections measured, 13,229 unique (Rint = 0.054). The final R1 was 0.040 (I > 2σ(I)) and wR2 was 0.105 (all data).
Details of the refinement of disorders or twinning in structures 2, 4 and 5 are presented in the Supporting Information. Complete crystallographic data, in CIF format, have been deposited with the Cambridge Crystallographic Data Centre. CCDC 2224556–2224561 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 15 December 2022).

2.3. Computations

Geometries of the monomers and dimers were optimized without constraints via DFT calculations with the UM06L functional and a def2tzvpp basis set using the Gaussian 09 suite of programs [38,39]. Previous work showed that these methods provide a good model for π-bonded associations of ion radicals at a reasonable computational cost [40]. Moreover, the use of UM06L/def2tzvpp calculations in acetonitrile and/or dichloromethane allows comparing the stabilities of the π-bonded dimers of the cation radicals of OMB, TMPD, and MPTZ with those formed by the TTF derivatives which we reported earlier [25,26]. Calculations with acetonitrile or dichloromethane as a medium were carried out using a polarizable continuum model [41]. The absence of imaginary frequencies confirmed that the optimized structures represent true minima. Values of the formation energies, ΔE, of the dicationic dimers were determined as: ΔE = Edim − 2ECR, where Edim and ECR are the energies (including ZPE) of the optimized dimers and monomer cation radicals, respectively. The ΔE values for the monocationic dimers were determined as: ΔE = Edim − ECR − EN, where EN is the energy of the optimized neutral organic donor. The energies of dimeric and monomeric species are listed in Table S1 in the Supporting Information. ΔE values in dichloromethane and vacuum were estimated in the same way using energies obtained via single-point calculations, using coordinates of the species optimized in acetonitrile. The singlet–triplet energy gap, ΔETS, was determined as the difference in the energies of the optimized dimers in their (ground-state) singlet states and the energies of the triplet states obtained via single-point calculations using the same geometry [42]. Following Winter et al. [43], the dispersion components, EDIS, were estimated as the difference between the energies of the calculated dimers (with the def2tzvpp basis set and the geometries obtained from the UM06L optimizations), using the B97D functional (which includes a dispersion correction) and the B98 functional (without dispersion correction), EDIS = ΔE(B98) − ΔE(B97D), (Table S2 in the Supporting Information). The electrostatic interaction energies between monomers in the dimers were estimated by summarizing the electrostatic interaction (in vacuum) between the charges located at the positions of each atom of one of the monomers and the charges located at the positions of each atom of their counterparts (i.e., summation is limited to atomic pairs that belong to different monomers): EES = 1 2 ∑ i ∑ j k qiqj/rij, where k is the Coulomb constant, qi and qj are the values of charges (calculated as the ESP charges) of the atoms in two monomers, and rij are distances between these atoms calculated from their coordinates in the dimers (½ coefficient is included to avoid double-counting of interactions between each pair of atoms (Table S3 in the Supporting Information)). The quantum theory of atoms in molecules (QTAIM) and non-covalent indices (NCI) analyses were performed with the Multiwfn program [44]. They were visualized using the VMD program [45].

3. Results and Discussion

3.1. X-ray Structural Characterization of the Cation-Radical Salts

The addition of a colorless solution of an organic donor D from Chart 1 in dichloromethane to the solutions of (TBPA)Br6CAR or CAR• resulted in the rapid change of color of the mixture. UV-vis spectral measurements indicated that these changes are related to the disappearance of the strong absorption band of the TBPA+• cation radical at 704 nm, or the weak band of CAR• at 900 nm, and the formation of the bands of the corresponding cation radicals D+•. The addition of hexane and partial evaporations of the solutions resulted in the precipitation of the salts of D+• with Br6CAR− or CAR− in an essentially quantitative yield. Recrystallization of the salts from dichloromethane/hexane mixtures produced crystalline materials suitable for a single-crystal X-ray structural analysis.
In particular, the X-ray analysis revealed that the oxidation of TMPD with CAR• radicals produced monoclinic crystals (P21/c space group) of (TMPD)Me12CAR. This salt comprised layers of Me12CAR− anions and TMPD+ cation radicals parallel to the bc plane (Figure 2A).
The center-to-center distance between TMPD moieties in these layers is large (9.43 Å), and they are surrounded by the Me6CAR− counter ions. As such, there are no short contacts between essentially isolated TMPD+• moieties. Average bond lengths within practically planar (centrosymmetric) TMPD+• in this salt are listed in Table 1. The comparison of their values with the reported earlier characteristics of the corresponding neutral molecule [46] showed that the bonds adjacent to the amino-substituents (designated as b) became longer, while bonds designated a became shorter than that in the neutral molecule (which shows all bonds within the aromatic ring in the 1.39 ± 0.01 Å range). The C-N bond lengths were also about 0.06 Å shorter than that in the neutral molecules. All these differences are consistent with the structural features of earlier reported TMPD+•, and they indicate that oxidation led to a substantial quinonoidal distortion of the aromatic TMPD moieties.
Oxidation of the same TMPD molecules with TBPA+•, taken as a salt with Br6CAR− anions (Equation (1)), produced orthorhombic crystals of (TMPD)Br6CAR. They contain layers of TMPD+• and Br6CAR− moieties parallel to an ac plane (Figure S4 in the Supporting Information). Similar to (TMPD)Me12CAR crystals, TMPD+• cation radicals are substantially separated from each other (with a center-to-center distance of 8.14 Å) and show short contacts only with Br6CAR− anions. The geometric characteristics of the essentially planar TMPD+• were also quite close to those in the (TMPD)Me12CAR salt (Table 1).
The crystallization of N-methylphenothiazine cation radicals with Br6CAR− anions produced monoclinic crystals in the P21/c space group. The X-ray structural analysis showed that these crystals comprised double chains of MPTZ+• moieties surrounded by Br6CAR− anions (Figure 3A). These double chains consist of parallel rows of MPTZ+• cations arranged at a distance of about 3.2 Å (Figure 3B).
The MPTZ+• cations in one of the rows were turned by 180° and shifted by 4.7 Å with respect to the moieties in the other row, so a central N-S axis of a molecule in one row was located above the gap between two molecules in the other row. As a result, the aromatic rings of the MPTZ+• moieties in one row were stacked over the aromatic rings of two of their counterparts in the other row. However, no contacts were shorter than the van der Waals separations between MPTZ+• in the same or different rows. A comparison of the geometry of the cationic and neutral MPTZ showed that the angle between rings of 14.5° in the cation radical is much smaller than that in the neutral molecule (36.3°) [47]. The geometric characteristics listed in Table 1 also indicate that besides substantial planarization of the MPTZ moiety, its oxidation led to the shortening of S-C and N-C bonds in the central ring. Moreover, the side (aromatic) rings became more distorted, with bonds designated b being noticeably shorter than the other bonds.
The interaction of MPTZ with Me12CAR• radicals produced triclinic crystals in the space group P 1 ¯ . Similar to the salt with Br6CAR−, these crystals comprised layers of MPTZ+• separated by sheets of Me12CAR− (Figure S5A in the Supporting Information). In this case, the MPTZ+• were disordered. Most notably, they were also arranged at significant separations from each other (with the center-to-center distance between neighboring moieties of more than 9 Å). As such, no π-stacking was observed, and there were short contacts only between the side carbon atoms in the MPTZ aromatic rings (Figure S5B in the Supporting Information).
In contrast to the reactions with TMPD or MPTZ, the interaction of (TBPA)Br6CAR with OMB resulted in the crystallization of a salt with 2:1 OMB to Br6CAR stoichiometry. The X-ray analysis showed that the monoclinic (OMB)2Br6CAR crystals (obtained by slow cooling of their solution in acetonitrile from room temperature to −30 °C) comprised layers of cationic OMB moieties separated by layers of anions and molecules of acetonitrile (Figure 4A).
The cationic layers showed stacks of co-planar OMB moieties consisting of distinct centrosymmetric pairs of partially oxidized electron donors. The interplanar distance between OMB moieties in these dyads of 3.38 Å was noticeably smaller than the distance of 3.58 Å between molecules from the neighboring pairs. Moreover, the OMB molecules slightly deviated from planarity, and their central parts were shifted towards each other. Accordingly, the pairs showed multiple intermolecular C-C distances (shown as blue lines in Figure 4B), which were shorter than the sum of the van der Waals radii of carbons of 3.40 Å (with the shortest C-C separation being 3.292 Å). In comparison, there were no such short separations between OMB molecules from the neighboring dyads. All these data indicate that multicenter π-bonding (referred to as pancake bonding [48,49,50]) between neighboring species resulted in the formation of distinct monocationic dimers. Furthermore, the stoichiometry of the salt and symmetry of the paramagnetic (OMB2)+• pairs (which are frequently called “pimers” [51]) indicate that positive charge was delocalized over both OMB moieties. In accordance with these 0.5+ charges, the average bond lengths of the OMB cores in Table 1 are intermediate between those in the reported neutral molecule and its cation radical.
The crystals resulting from the oxidation of OMB with Me12CAR• radicals were not suitable for the X-ray structural analysis. Thus, to verify the generality of the formation of π-stacks of OMB moieties showing pancake bonding in the salts with bulky weakly coordinating anions, the salt of TPBA+• with Ag(Br6CAR)2− anions was prepared (see Materials and Methods for details). Interaction of this oxidant with octamethylbiphenylene produced a salt comprising OMB and Ag(Br6CAR)2− in a 2:1 stoichiometry. The X-ray structural analysis revealed that the triclinic crystals of this salt contain layers of OMB molecules together with CH2Cl2 solvate (Figure 5A). They were separated by layers of anionic [Ag(Br6CAR)2] complexes (consisting of two Br6CAR anions coordinated via three of their bromine substituents to silver cations, similar to the earlier reported complex [52].
Similar to the (OMB)2Br6CAR salt, the OMB layers comprised stacks of co-planar OMB moieties, in which the neighbors were shifted relative to each other by about 1.5 Å. Their identical geometries indicate an equal +0.5 charges on each OMB moiety. The average bond lengths within the OMB core were also intermediate between those in the neutral and monocationic species, and they were close to those in the salt with Br6CAR− anions. The interplanar separations between the OMB molecule and its two neighbors of about 3.47 Å and 3.60 Å were also somewhat different. However, since the smaller of these separations was more than 3.40 Å, no intermolecular C-C contacts were shorter than the sum of the van der Waals radii.

3.2. Quantum-Mechanical Computations of the Dimers of Cation Radicals

To evaluate reasons for the crystallization of the π-bonded associations or essentially isolated cation radicals, we employed quantum-mechanical calculations of the TMPD22+, MPTZ22+, and OMB22+ dimers. The UM06L/def2tzvpp calculations (with acetonitrile as medium) produced stable π-bonded dimers. The structures of these dicationic associations are illustrated in Figure 6.
The TMPD22+ dyad showed two co-planar TMPD moieties, shifted relative to each other perpendicular to their main axes (the dimer in which counterparts were shifted parallel to the main axes showed somewhat higher energy). The interplanar separation was about 3.15 Å, and there were a number of interatomic C-C distances of about 3.20 Å (which are shorter than the sum of the van der Waals separations of 3.40 Å for carbon atoms). The MPTZ2+ dimer comprised two molecules turned about 90° relative to each other (a similar crossed decker arrangement was observed earlier in the dicationic trimers of thianthrene [53]). These dimers also showed several S-C contacts in the 3.15–3.30 Å range (which are shorter than the corresponding sum of the van der Waals radii of 3.50 Å [54]). The OMB22+ dimer consisted of two moieties shifted relative to each other along their main axes. This dimer also showed multiple C-C contacts in the 3.0–3.2 Å range. Such arrangements of monomers in all three dimers suggests strong pancake bonding between the π-stacked cationic moieties. The QTAIM analysis revealed multiple (3,−1) bond critical points (BCPs) and the corresponding bond paths between the counterparts corroborating such a multicenter π-bonding (electron densities at the BCPs along bond paths connecting two fragments are listed in Table S5 in the Supporting Information). The interaction energy values resulting from the M062X/def2tzvpp calculations of these dimers are listed in Table 2.
The ΔE values in Table 2 indicate that the OMB22+ dimers are substantially more stable than their TMPD22+ or MPTZ22+ analogs. Comparison with the values (calculated in the same way) reported earlier for π-dimers of the tetrathiafulvalene derivatives and the crystal structures of their cation radicals [25,26] demonstrated that the formation of the π-bonded associations in salts with the weakly coordinating anions is highly correlated with the strength of pancake bonding. Indeed, the bonding involving TMPD+• or MPTZ+• was weaker than that reported earlier between TTF+• cation radicals (which show ΔE of −38.9 kJ/mol). Accordingly, similar to the TTF, salts of TMPD and MPTZ with weakly-coordinating Me12CAR− and Br6CAR− anions comprised isolated cation radicals. The ΔE values for the OMB22+ dimers were more negative than those determined for analogous BEDT-TTF22+ or TMTSF22+ dyads (−60.2 and −67.7 kJ/mol, respectively [25]). In agreement with the strong pancake bonding, the salts of all these derivatives with Br6CAR− or Me12CAR− (or other weakly-coordinating anions) comprised π-bonded associations.
To establish the reasons for the differences in strength of the pancake bonding of cation radicals of TMPD, MPTZ, and OMB (and therefore the crystal architectures of their salts), we estimated electrostatic, dispersion and weakly covalent components in their interactions. The data in Table 2 show that while electrostatic repulsion between cationic counterparts in these associations is comparable, the dispersion interaction in OMB22+ dimers was higher than in its TMPD and MPTZ analogs. Moreover, the difference in energy between triplet and singlet states in the OMB22+ associations (which reflects the strength of molecular–orbital interaction [42]) was also much larger than that in the TMPD22+ or MPTZ22+ analogs. Thus, more favorable dispersion and molecular–orbital interactions result in the substantially higher strength of the pancake bonding involving the OMB cation radicals.
Finally, to determine the reason for the formation of the (OMB)2+• associations (all salts were prepared under the condition of an excess of organic donors), we compared the stability of the monocationic and dicationic dimers of OMB. The ΔE values for these associations resulting from calculations with polar acetonitrile as a medium were similar. However, earlier studies demonstrated that the magnitudes of the (negative) ΔE values for the dicationic dimers in moderately polar dichloromethane are substantially smaller than those in acetonitrile, and they become positive if calculations are carried out in vacuum [26]. The dicationic dimers under study in the current work showed similar tendencies (Table S4 in the Supporting Information). In comparison, the changes in the ΔE values for monocationic dimers with polarity of the medium were much smaller, and these associations remained stable even in vacuum. Apparently, the environment in the salts with the weakly coordinating anions is best modeled by the relatively low-polar solvents; therefore, it is best suited for the formation of monocationic (OMB)2+• associations.

4. Conclusions

The electrostatic interaction between counter-ions is a dominant factor in the formation of the ionic compounds, and it plays an essential role in their crystal architectures. Therefore, the variation of counter-ions frequently (and unpredictably) led to a switch from the crystallization of isolated ion radicals to the formation of the π-dimers. The use of weakly coordinated anions allowed minimizing such electrostatic forces. As a result, the appearance of the π-bonded associations in the WCA salts with the variety of cation radicals was determined by the strength of the pancake bonding of the latter. Indeed, the six salts prepared in the current work showed similar layers of oxidized donors separated by layers of bulky anions. However, while the layers of TMPD and MPTZ salts comprised practically isolated cation radicals, the OMB salts contained π-stacked associations. A similar switch from the crystallization of the isolated cation radical to the formation of the solid-state dimer with the increase in the strength of the (calculated) pancake bonding was observed earlier in the salts of WCA with the oxidized TTF derivatives. The identification of such trends allows targeted preparation of the salts with the distinct arrangements of ion radicals, thus, enabling exploration of the effects of the wide variations of interaction between them on the spectral, conducting, and magnetic properties of these solid-state materials.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/cryst13010099/s1: Details of X-ray structures’ analysis; Figures S1–S3: FT-IR spectra; Figure S4: X-ray structure of (TMPD)Br6CAR; Figure S5: X-ray structure of (MPTZ)Me12CAR; Table S1: Calculated energies of the monomers and dimers; Table S2: Details of calculations of dispersion; Table S3: Coordinates and atomic charges; Table S4: Energies of the π-bonded dimers calculated in vacuum and in dichloromethane; Table S5: Electron density at the BCPs in the π-bonded dimers.

Author Contributions

Conceptualization and methodology, S.V.R.; crystal preparation, E.A.; X-ray crystallographic analysis, M.Z.; quantum mechanical computations and data analysis, visualization, S.V.R.; writing—original draft preparation, S.V.R.; writing—review and editing, E.A., M.Z. and S.V.R.; supervision, S.V.R.; funding acquisition, S.V.R. and M.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Petroleum Research Fund of the American Chemical Society (grant 56516-UR). X-ray structural measurements were supported by the National Science Foundation through the Major Research Instrumentation Program under Grant No. CHE 1625543 (funding for the single crystal X-ray diffractometer). Calculations were performed on Ball State University’s Beowulf cluster, which is supported by the National Science Foundation (MRI-1726017) and Ball State University.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre and can be obtained free of charge (see above). Atomic coordinates and energies of the calculated complexes are available in the Supplementary Information.

Acknowledgments

We thank S. Dunlap and C. Kunk for the synthesis of OMB.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 13 00099 g001 550
Figure 1. Structures and abbreviations of carbadodecaborate anions.
Figure 1. Structures and abbreviations of carbadodecaborate anions.
Crystals 13 00099 g001
Crystals 13 00099 ch001 550
Chart 1. Structures and abbreviations of the organic donors.
Chart 1. Structures and abbreviations of the organic donors.
Crystals 13 00099 ch001
Crystals 13 00099 g002 550
Figure 2. The crystal lattice of (TMPD)Me12CAR (A) comprising essentially isolated TMPD+• cation radicals (B). Light blue lines indicate contacts shorter than the van der Waals separations.
Figure 2. The crystal lattice of (TMPD)Me12CAR (A) comprising essentially isolated TMPD+• cation radicals (B). Light blue lines indicate contacts shorter than the van der Waals separations.
Crystals 13 00099 g002
Crystals 13 00099 g003 550
Figure 3. The crystal lattice of (MPTZ)Br6CAR (A) comprising essentially isolated MPTZ+• cation radicals (B). Light blue lines indicate contacts shorter than the van der Waals separations.
Figure 3. The crystal lattice of (MPTZ)Br6CAR (A) comprising essentially isolated MPTZ+• cation radicals (B). Light blue lines indicate contacts shorter than the van der Waals separations.
Crystals 13 00099 g003
Crystals 13 00099 g004 550
Figure 4. Crystal lattice of (OMB)2Br6CAR (A) comprising stacks of partially oxidized OMB moieties (B). Light blue lines indicate contacts shorter than the van der Waals separations.
Figure 4. Crystal lattice of (OMB)2Br6CAR (A) comprising stacks of partially oxidized OMB moieties (B). Light blue lines indicate contacts shorter than the van der Waals separations.
Crystals 13 00099 g004
Crystals 13 00099 g005 550
Figure 5. The crystal lattice of (OMB)2 [Ag(Br6CAR)2] (A) comprising stacks of partially oxidized OMB moieties (B). Light blue lines indicate contacts shorter than the van der Waals separations.
Figure 5. The crystal lattice of (OMB)2 [Ag(Br6CAR)2] (A) comprising stacks of partially oxidized OMB moieties (B). Light blue lines indicate contacts shorter than the van der Waals separations.
Crystals 13 00099 g005
Crystals 13 00099 g006 550
Figure 6. Optimized structures of TMPD22+ (A), MPTZ22+ (B) and OMB22+ (C) dimers showing bond critical points (small orange spheres) and bond paths (orange lines) between cationic fragments (from QTAIM analysis).
Figure 6. Optimized structures of TMPD22+ (A), MPTZ22+ (B) and OMB22+ (C) dimers showing bond critical points (small orange spheres) and bond paths (orange lines) between cationic fragments (from QTAIM analysis).
Crystals 13 00099 g006
Table
Table 1. Average bond length (in Å) in the D cores in the salts with WCA counter-ions.
Table 1. Average bond length (in Å) in the D cores in the salts with WCA counter-ions.
Crystals 13 00099 i001
Salt ia iib iic iid iie iif ii
(TMPD)Me12CAR1.3571.4211.3421.453
(TMPD)Br6CAR1.3541.4241.3501.460
(MPTZ)Br6CAR1.4031.3591.4081.3931.7231.391
(MPTZ)Me12CAR iii1.3991.3661.4081.4111.7151.389
(OMB)2Br6CAR1.4211.4221.3761.4341.501
(OMB)2[Ag(Br6CAR)2]1.4181.4221.3781.4261.4915
i Solvate molecules are not listed. ii See the structures above. iii Disordered MPTZ moiety.
Table
Table 2. Formation energies and its components (in kJ/mol) of the calculated π-dimers.
Table 2. Formation energies and its components (in kJ/mol) of the calculated π-dimers.
π-DimerΔE aEST bEdisp cEES d
TMPD22+−14.5−34.0−101.9245.8
MPTZ22+−30.7−37.2−92.8239.0
OMB22+−74.0−71.5−177.0216.7
a From M062X/def2tzvpp calculations, with CH3CN as the medium. b Singlet–triplet energy gap. c Dispersion interaction. d Electrostatic interaction between monomers in the π-dimers (see Materials and Methods and Supplementary Information for details).
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Adeniyi, E.; Zeller, M.; Rosokha, S.V. Crystal Engineering of Cation-Radical Salts with Weakly Coordinating Carbadodecaborate Anions. Crystals 2023, 13, 99. https://doi.org/10.3390/cryst13010099

AMA Style

Adeniyi E, Zeller M, Rosokha SV. Crystal Engineering of Cation-Radical Salts with Weakly Coordinating Carbadodecaborate Anions. Crystals. 2023; 13(1):99. https://doi.org/10.3390/cryst13010099

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Adeniyi, Emmanuel, Matthias Zeller, and Sergiy V. Rosokha. 2023. "Crystal Engineering of Cation-Radical Salts with Weakly Coordinating Carbadodecaborate Anions" Crystals 13, no. 1: 99. https://doi.org/10.3390/cryst13010099

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