Halocyclopropenium-Halide Halogen-Bonded Ion Pairs and Their Hydrogen-Bonded Halide Solvates Mohammed S. Abdelbassit,a Owen J. Curnow,*a and Mark R. Waterland*b a School of Physical and Chemical Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand, e-mail: owen.curnow@canterbury.ac.nz b School of Natural Sciences, Massey University, Private Bag 11–222, Palmerston North 4442 (New Zealand), e-mail: M.Waterland@massey.ac.nz © 2022 The Authors. Helvetica Chimica Acta published by Wiley-VHCA AG. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. A series of salts with a diaminohalocyclopropenium cation and halide anion [C3(N iPr2)2X]X (X=Cl ([1]Cl) or Br ([2]Br) were isolated with a variety of solvates and, in one case, as a co-crystal with hydronium chloride. In particular, the initial synthesis of [1]Cl formed a co-crystal with hydronium and with CH2Cl2 solvate ([1]2[OH3Cl3] · CH2Cl2) upon isolation from acetone/CH2Cl2. Recrystallization of this from chloroform gave a dichloroform adduct [1]Cl · 2CHCl3, whereas treatment with ICl formed an octahalide cluster [1]2I4Cl4. The bromine salt [2]Br ·C2H4Br2 was prepared by treatment of [1]Cl with dibromoethane and was isolated as a solvate. The hydronium cation was found as part of a hydronium trichloride cluster [OH3Cl3] 2� and this, along with a partially-deuterated analogue of [OHD2Cl3] 2� and [OD3Cl3] 2� , was studied computationally and by mid- and far-infrared spectroscopy. Significant halogen bonds were found between 1+ or 2+ and chloride or bromide, respectively. On the other hand, the distance to the octahalide [I4Cl4] 2� is too long for a halogen bond. Hydrogen bonding from the halides to the halomethane solvates is also significantly stronger than to the cation isopropyl groups. The geometries formed at the halide ions with respect to the halogen bond and strong hydrogen bonds are pyramidal with approximately orthogonal angles. Keywords: halides, halogens, halogen bonding, hydrogen bonds, X-ray diffraction. Introduction Halogen bonding can result when a halogen atom forms a σ bond, R� X; a region of positive charge (a σ hole) will then form on the halogen opposite the σ bond and this halogen bond donor atom can then form an electrostatic interaction with a lone pair, or region of negative charge, from some other atom, the halogen bond acceptor.[1–4] HOMO–LUMO interac- tions are also important, in addition to some contribu- tion from dispersive forces.[3,5,6] The magnitude of this σ hole increases down the halogen group and is thus most significant for bromine and iodine. It also increases with increasing electronegativity of the group attached to the halogen, for example, N� Cl versus C� Cl.[7–11] It should also be noted that there is a region of negative charge in a ring around the halogen and orthogonal to the σ hole, thus the halogen atom can potentially act as both a halogen- bond donor and halogen-bond acceptor. In the case of the polyhalides, this frequently results in combina- tions of linear and orthogonal bond angles, such as is found in the Z-shaped octahalides X8 2� (X=Br, I).[12–14] Halogen bond donor strengths can be increased by use of halogenated cations, particularly of brominated and iodinated imidazolium compounds.[15,16] Notably, Holthoff et al. have also reported ‘anti-electrostatic’ halogen bonding, in which the halogen bond donor and acceptor have the same non-zero charge, be- tween the iodinated bis(dicyanometh- ylene)cyclopropanid derivative [C3(C(CN)2)2I] � and I� .[17] The halogen bond donor ability of organo- Supporting information for this article is available on the WWW under https://doi.org/10.1002/hlca.202200163 doi.org/10.1002/hlca.202200163 RESEARCH ARTICLE Helv. Chim. Acta 2023, 106, e202200163 © 2022 The Authors. Helvetica Chimica Acta published by Wiley-VHCA AG Wiley VCH Freitag, 13.01.2023 2301 / 279119 [S. 93/104] 1 http://orcid.org/0000-0002-2812-2400 http://orcid.org/0000-0001-7479-856X http://orcid.org/0000-0002-8493-9407 https://doi.org/10.1002/hlca.202200163 http://crossmark.crossref.org/dialog/?doi=10.1002%2Fhlca.202200163&domain=pdf&date_stamp=2022-12-12 chlorides, however, is significantly less than that of analogous bromides and iodides. Kukushkin and co- workers found that the maximum electrostatic poten- tial on Cl in CH2Cl2 is about one tenth that of I in CH2I2, Cl in CHCl3 is about one third that of I in CHI3 and Cl in CCl4 is about half that of I in CI4. [18] Consequently, despite CH2Cl2 and CHCl3 ranking as second- and fifth- most common solvents, respectively, in crystal struc- tures, the number of observed halogen bonds with these molecules is relatively small.[18,19] The chloride anion on the other hand is potentially a good halogen-bond acceptor; however, its interactions tend to be dominated by its preference for hydrogen bonding, so the number of halogen bonds with chloride is limited. The chloride–water hydrogen bond, for example, is significantly stronger than the water–water hydrogen bond.[20] Similarly, chloroal- kanes such as CH2Cl2 and CHCl3 much prefer to form hydrogen bonds to chloride than halogen bonds.[21] Indeed, hydrogen bonding to chloride is of particular interest for its role in biological systems, where it is prolific, as well as its applications in asymmetric catalysis and in anion receptors.[22–28] A search of the Cambridge Structural Database (CSD) by Awwadi et al. in 2007 found just 13 C� Cl···Cl� halogen bonding examples.[29] In the work reported here, we will describe the halogen bond donor ability of halogen-substituted (both Cl and Br) diaminocyclopropenium cations to halide ions (Cl� and Br� ) along with hydrogen bonds to these halide ions, including a rare example of a hydronium trichloride cluster. Results and Discussion Synthesis [C3(N iPr2)2Cl]Cl ([1]Cl) has been known since work on triaminocyclopropenium (TAC) salts began in the 1970s.[19] It was found then that bulky secondary amines such as HNiPr2 would not form a triaminocyclo- propenium cation (although we later showed that it can form) but would instead lead to a diaminochlor- ocyclopropenium cation (1+). The cation was initially isolated as the perchlorate salt [1]ClO4 by Yoshida and Tawara.[30,31] Weiss and coworkers reported some early studies on its use in a variety of transformations, including the preparation of a cyclopropenylidene complex.[32–35] Amongst its more recent uses, Bertrand has used it to isolate cyclopropenylidenes[36] and Lambert as a ‘ClickabIL’ reagent to prepare poly(ionic liquids).[37] Although it has been used frequently, reports on its isolation or properties are scarce and references to its synthesis frustratingly lead back to Yoshida and Tawara’s initial report of the perchlorate salt, rather than the chloride salt. Taylor reported NMR and crystallographic data for the perchlorate salt.[38,39] We initially attempted to isolate [1]Cl by addition of acetone to remove [iPr2NH2]Cl by precipitation fol- lowed by crystallization from CH2Cl2 (Scheme 1). How- ever, this was found to co-crystallize with hydronium chloride (generated from water in the undried acetone and H+ from the ammonium salt side product) and CH2Cl2 solvate as [1]2[OH3Cl3] · CH2Cl2. Crystallization of this from a chloroform solution was found to give the dichloroform solvate [1]Cl · 2CHCl3. The solid-state structures of both [1]2[OH3Cl3] · CH2Cl2 and [1]Cl · 2CHCl3, as discussed in detail below, were found to have a halogen bond between the chloro substitu- ent on the cation and a chloride anion. Previously, we have converted TAC chloride salts to bromide or iodide salts by heating them to reflux in a solution of an alkyl halide, such as dibromoethane or iodoethane, respectively.[40] When we similarly treated [1]Cl with dibromoethane, we found that not only was the chloride replaced by bromide, but the chloro substituent was also replaced by bromine to give [C3(N iPr2)2Br]Br ([2]Br; Scheme 2). This salt was then isolated as the dibromoethane solvate [2]Br ·C2H4Br2 and found to have bromo-bromide halogen bonding. As part of our studies on polyhalides,[41–44] we treated [1]2[OH3Cl3] with two equivalents of ICl under reflux in CH2Cl2. We were hoping that the C2v- symmetric cation would favor the crystallization of a Scheme 1. Syntheses of [C3(N iPr2)2Cl]Cl co-crystallized with hydronium chloride and CH2Cl2 solvate or crystallized with CHCl3 solvate. Scheme 2. Proposed route for the synthesis of the bromo- bromide salt [C3(N iPr2)2Br]Br ([2]Br). Helv. Chim. Acta 2023, 106, e202200163 www.helv.wiley.com (2 of 12) e202200163 © 2022 The Authors. Helvetica Chimica Acta published by Wiley-VHCA AG Wiley VCH Freitag, 13.01.2023 2301 / 279119 [S. 94/104] 1 15222675, 2023, 1, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/hlca.202200163 by M assey U niversity L ibrary, W iley O nline L ibrary on [10/01/2024]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense www.helv.wiley.com C2v-symmetric pentahalide I2Cl3 � . Instead, it was found to give the octahalide-containing salt [C3(N iPr2)2Cl][I4Cl4] ([1]2I4Cl4) which possibly exhibits very weak halogen bonding between the cation and the anion (Scheme 3). We recently reported a series of iodine-chlorine octahalides, including [C3(NEt2)3]2[I4Cl4] which was prepared by treatment of [C3(NEt2)3]ICl2 with I2. [41] The formation of the central I2 in [1]2I4Cl4 is due to halogen-redistribution reactions. Solid State Structures [C3(N iPr2)2Cl]2[OH3Cl3] ·CH2Cl2 crystallizes in the mono- clinic space group P21/c. The asymmetric unit contains two cations of 1+, a hydronium cation, three chlorides and one CH2Cl2 solvate molecule. Figure 1 illustrates the atomic labeling scheme and asymmetric unit. The presence of an electronegative and weak π donor Cl ring substituent in the cation leads to a change in the C3 ring bond distances in which C1� C3 and C2� C3 are slightly shortened (1.360(3)–1.366(3) Å) compared to symmetrical TAC cations at ca. 1.38 Å,[45] but similar to those in [C3Cl3][AlCl4] which average 1.356 Å.[46] C1� C2, however, is elongated compared to symmetric TAC cations (1.432(3) Å and 1.429(3) Å). This variation in the bond distances can be attributed to increased π donation into the C3 ring from the planar diisopropy- lamino groups (the sum of the angles around N1 and N2 are >359°). The C� Cl distances for the two independent cations are essentially the same (1.690(2) and 1.688(2) Å for C3� Cl5 and C3 A� Cl6, resp.) and are shorter than in the CH2Cl2 solvate (1.762(3) and 1.785(2) Å), but longer than in [C3Cl3][AlCl4] (mean= 1.631 Å) which implies reduced π donation from the chloro atom in 1+.[46] Each cation of 1+ uses a σ hole on the chloro substituent to form a weak halogen bond to a chloride. The two halogen bonding Cl� � Cl distances are very different, 3.2718(6) Å for Cl1� Cl5 and 3.7359(7) Å for Cl2� Cl6, but are both close to having linear C� Cl� Cl angles (177.59(8)° and 171.40(7)°, resp.). Halogen-bonding parameters for the compounds described in this article are collected in Table 1. A ratio (RD) of the interatomic distance and the sum of the atomic and ionic radii is a useful measure of the strength of the halogen bond, with an RD of greater than one implying that there is no halogen bond. Cl has an atomic radius of 1.75 Å whereas the chloride ion has an ionic radius of 1.81 Å,[47,48] and on this basis the long Cl2� Cl6 distance would not be considered a halogen bond whereas the shorter Cl1� Cl5 distance would be. These distances can be compared to the longer anti-electrostatic Cl� ClScheme 3. Synthesis of the octahalide-containing salt [C3(N iPr2)2Cl]2I4Cl4 ([1]2I4Cl4). Figure 1. Thermal ellipsoid plot of [1]2[OH3Cl3] ·CH2Cl2 illustrating the asymmetric unit and the atomic-labelling scheme. Helv. Chim. Acta 2023, 106, e202200163 www.helv.wiley.com (3 of 12) e202200163 © 2022 The Authors. Helvetica Chimica Acta published by Wiley-VHCA AG Wiley VCH Freitag, 13.01.2023 2301 / 279119 [S. 95/104] 1 15222675, 2023, 1, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/hlca.202200163 by M assey U niversity L ibrary, W iley O nline L ibrary on [10/01/2024]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense www.helv.wiley.com halogen bond distances formed between ICl2 � and I2Cl3 � of 3.497 Å (RD=0.982) and BrCl2 � and Br2Cl3 � of 3.359 Å (RD=0.944).[41,42] In those cases, the σ hole on the chlorine atom is formed by the attached electro- negative halogen, whereas, in the case of 1+, it is formed by the electron-withdrawing effects of the attached cyclopropenium cation. Also of relevance are the organochloro chloride salts 2-chloropyridinium chloride (Cl···Cl� =3.507, RD=0.985),[49] 3-chloropyridi- nium chloride (Cl···Cl� =3.479, RD=0.977),[49] 4-chlor- opyridinium chloride (Cl···Cl� =3.3352, RD=0.937),[50] and 1,3-dimethyl-4,5-dichloroimidazolium chloride in which the chloride has two halogen bonds, at 3.2688(8) and 3.2232(8) Å, RD=0.918 and 0.905, respectively.[51] The RD values for the latter imidazo- lium salt are most similar to that of the chloride salts described here. Presumably, the longer of the two Cl� � Cl distances in [1]2[OH3Cl3] is caused by crystal- packing effects (a weak halogen bond would be expected to have a relatively wide and shallow potential well and thus be easily compressed or elongated/broken), and the shorter distance is prob- ably a more accurate reflection of the potential halogen-bonding strength between a chloro-TAC cati- on and a chloride. Intriguingly, Kukushkin and co- workers reported a series of CH2Cl2 solvates of chloride salts in which most formed CH� Cl� hydrogen bonds, however, two formed a CCl� Cl� halogen bond with Cl� Cl distances of 3.276(3) Å (RD=0.920) and 3.236(2) Å (RD=0.909).[18] The authors suggested that the halogen-bond formation, rather than hydrogen-bond formation, was due to crystal-packing constraints. They also identified nine similar structures in the CCDC database with Cl� Cl distances in the range 3.26– 3.50 Å. For Cl� Cl halogen bonds between neutral molecules, Allen found 63 structures with CH2Cl2� Cl� C halogen bonds in the Cambridge Structural Database (CSD).[19] The Cl� Cl distances range 3.03–3.50 Å with a mean of 3.39 Å (RD=0.969). Similarly, for HCCl3� Cl� C halogen bonds, they found 134 examples with a range of 2.62–3.50 Å and a mean of 3.38 Å (RD=0.966). On that basis, the CCl+� Cl� halogen bonds with 1+ appear to be only slighter shorter than typical CCl� Cl� halogen bonds for neutral molecules. As well as the halogen bonds, these chlorides are also bridged through hydrogen bonding to the hydro- nium cation and the CH2Cl2 solvate. As would be expected, the hydrogen bonds to the hydronium (Cl� � H=2.0760(5) and 2.0746(5) Å) are significantly stronger/shorter than the hydrogen bonds to CH2Cl2 (Cl� � H=2.7264(5) and 2.5584(5) Å). The hydrogen- bonding parameters are given in Table 2. Allen found Table 1. Halogen-bonding parameters. C� Cl Distance [Å] Cl� Cl Distance [Å] RD [a] C� Cl� Cl Angle [°] [1]2[OH3Cl3] ·CH2Cl2 C3� Cl5 1.690(2) Cl1� Cl5 3.2718(6) 0.919 C3� Cl5� Cl1 177.59(8) C3 A� Cl6 1.688(2) Cl2� Cl6 3.7359(7)[b] 1.049 C3 A� Cl6� Cl2 171.40(7) [1]Cl · 2CHCl3 C1� Cl0 1.673(4) Cl1� Cl0 3.1290(13) 0.879 C1� Cl0� Cl1 168.39(13) C1 A� Cl0 A 1.675(4) Cl2� Cl0 A 3.2514(12) 0.913 C1 A� Cl0 A� Cl2 170.40(12) C1B� Cl0B 1.677(4) Cl3� Cl0B 3.2602(11) 0.916 C1B� Cl0B� Cl3 171.09913) [2]Br.C2H4Br2 C3� Br2 1.852(4) Br1� Br2 3.2242(6) 0.846 C3� Br2� Br3 173.93(12) [1]2I4Cl4 C3� Cl3 1.693(6) Cl2� Cl3 3.952(3)[b] 1.110 C� Cl3� Cl2 171.4(2) [a] Ratio of X� X distance and sum of the van der Waals and ionic radii. [b] Distance is too long to normally be considered a halogen bond. Table 2. Hydrogen-bonding parameters for [1]2[OH3Cl3] ·CH2Cl2. Hydrogen bond (D� H� A) D� H [Å] H� A [Å] D� A [Å] D� H� A [Å] O1-H1a� Cl1 0.8500(17) 2.0760(5) 2.9215(17) 173.01(11) O1-H1b� Cl2 0.8498(17) 2.0746(5) 2.9232(18) 176.46(12) O1-H1c� Cl3 0.99(4) 1.87(4) 2.8619(18) 176(3) C4-H4b� Cl2 0.970(2) 2.5584(5) 3.523(2) 172.80(14) C4-H4a� Cl1 0.970(2) 2.7264(5) 3.623(3) 153.94(14) Helv. Chim. Acta 2023, 106, e202200163 www.helv.wiley.com (4 of 12) e202200163 © 2022 The Authors. Helvetica Chimica Acta published by Wiley-VHCA AG Wiley VCH Freitag, 13.01.2023 2301 / 279119 [S. 96/104] 1 15222675, 2023, 1, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/hlca.202200163 by M assey U niversity L ibrary, W iley O nline L ibrary on [10/01/2024]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense www.helv.wiley.com 53 structures with CCl2H2� Cl � hydrogen bonds in the CSD.[19] The H� Cl distances ranged from 2.33–2.95 Å with a mean of 2.57 Å. Another notable feature of the halogen-bonded chloride ions is their pyramidal geometries; the Cl� Cl� H angles being either very acute (77.222(16)° and 76.652(15)°) or nearer 100° (99.598(17)° and 107.91(2)°) while the H� Cl� H angles are both acute (81.488(16)° and 81.444(18)°). This is in fact a common feature of chloride geometries involving weak bonds,[52] as we will also see in the other salts described here. The third chloride ion in the asymmetric unit, Cl3, has only one strong hydrogen bond, which is to the hydronium cation. The Cl� O distance is consequently shorter than to the other two chlorides (2.8619(18) Å versus 2.9215(17) Å and 2.9232(18) Å) and so the Cl� � H distance is shorter (1.87(4) Å) and the O� H distance longer (0.99(4) Å versus 0.850(2) Å). In terms of the weaker CH� Cl� hydrogen bonds to the cations, Cl3 has four Cl� � H distances in the range 2.70–2.90 Å whereas Cl1 and Cl2 have only one each. The hydronium cation has the expected trigonal pyramidal geometry and, with the three hydrogen- bound chloride ions, forms an [OH3Cl3] 2� cluster. The Cl� O� Cl angles are near tetrahedral (Cl1� O1� Cl2= 114.27(5)°; Cl1� O1� Cl3=108.31(6)°; Cl2� O1� Cl3= 114.38(6)°; sum=337.0°). Whereas there are many reports of crystallographically-characterized hydro- nium salts (see references [37–40] for some examples), we know of only two other discrete structures of hydronium trichloride [OH3Cl3] 2� . Willey and co-work- ers synthesized [GeCl3(L 1)]2[H3O]Cl3 ·MeCN, where L 1 is 1,4,7-trimethyl-1,4,7-triazacyclononane.[53] The Cl� O distances are 2.852(2), 2.860(2) and 2.888(2) Å and the Cl� O� Cl angles are 112.53(8)°, 111.93(8)° and 107.12(7)° (sum=331.6°). In 2011, Denton and co- workers reported an alkoxyphosphonium chloride in which a discrete hydronium trichloride cluster was co- crystallized.[54] The Cl� O distances are 2.855(2), 2.871(2) and 2.892(2) Å and the Cl� O� Cl angles are 108.73(7), 106.69(6) and 126.73(7)° (sum=342.2°). Thus, it can be seen that our structure has two Cl� O distances that are longer than all of the others, and this can probably be attributed to the additional halogen bonding to those chlorides. The salt [C3(N iPr2)2Cl]Cl · 2CHCl3 ([1]Cl · 2CHCl3) crys- tallizes in the orthorhombic space group P212121, and the asymmetric unit shows three independent clusters of [1]Cl · 2CHCl3. The most notable feature of these clusters is the strong halogen bonding between the chloride and the chloro substituent of the cations (Figure 2); the Cl� � Cl distances are 3.1290(13), 3.2514(12) and 3.2602(11) Å (Table 1). Again, the C� Cl� Cl angles are approximately linear (168–171°). These Cl� Cl distances are all shorter, and have smaller RD values, than the halogen bonds in [1]2[OH3Cl3] ·CH2Cl2 and this may be attributed to the much weaker hydrogen bonds of the chloride to the chloroform solvate, compared to the hydronium. There is essentially no effect on the structural features of the cation compared to the cations in [1]2[OH3Cl3] ·CH2Cl2, with the C� Cl distances ranging from 1.673(4) to 1.677(4) Å. The chloride-chloroform Cl� H hydrogen bond distances (Table 3) lie in the range of 2.33– 2.47 Å (mean=2.41 Å) which is shorter than in Figure 2. (a) Thermal ellipsoid plot of [1]Cl · 2CHCl3 illustrating one of three similar clusters in the asymmetric unit (see Supporting Information); (b) Illustration of the chloroform–chloroform interaction between Cl6 and Cl13. Helv. Chim. Acta 2023, 106, e202200163 www.helv.wiley.com (5 of 12) e202200163 © 2022 The Authors. Helvetica Chimica Acta published by Wiley-VHCA AG Wiley VCH Freitag, 13.01.2023 2301 / 279119 [S. 97/104] 1 15222675, 2023, 1, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/hlca.202200163 by M assey U niversity L ibrary, W iley O nline L ibrary on [10/01/2024]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense www.helv.wiley.com [1]2[OH3Cl3] ·CH2Cl2, probably due to a combination of chloroform being a better hydrogen-bond donor and the lack of a competing chloride-hydronium hydrogen bond making the chloride a better hydrogen-bond acceptor. Allen found 38 examples of chloroform- chloride hydrogen bonding, with H� Cl=2.26–2.80 Å and also with a mean of 2.41 Å.[19] The chlorides also have some weaker hydrogen bonds to isopropyl groups (Cl� H distances of 2.80–2.85 Å). Additionally, as seen in Figure 2,b, there is a halogen bond between two chloroform molecules (Cl6� Cl13); this has a Cl� Cl distance of 3.3443(19) Å (RD=0.956).[55] As noted above, Allen found a mean distance of 3.38 Å for related halogen bonds with chloroform molecules.[19] Kukushkin similarly reported Cl� Cl halogen bonds between chloride-chloroform clusters, with Cl� Cl distances of 3.35–3.54 Å.[21] Nota- bly, the chloroform-chloroform halogen bonds in [1]Cl · 2CHCl3 are not as linear (C5� Cl6� Cl13= 154.46(14)° and C6� Cl13� Cl6=135.93(15)°) as those to chloride ions. Two linear C� Cl� Cl angles would involve an unfavorable electrostatic interaction between two positive σ holes. An electrostatically optimum arrange- ment would be one linear angle and one nearly orthogonal. Our example appears to be in between, which possibly reflects the influence of crystal-packing effects. Indeed, Desiraju and coworkers classified these as Type I halogen bonds and found that they are largely due to dispersive interactions.[56,57] Similar to the chloride ions in [1]2[OH3Cl3] ·CH2Cl2, all three chloride ions in the chloroform solvate have a trigonal pyramidal coordination geometry with acute angles between the halogen bond and hydrogen bonds, varying between 66° and 89° (Table 8S). The salt [C3(N iPr2)2Br]Br ·C2H4Br2 crystallizes in the monoclinic space group P21/n. The asymmetric unit contains one cation, one bromide and one 1,2- dibromoethane solvate (Figure 3). The cation 2+ has a very similar structure to that of 1+ in the cations described above. The ring C� C distances are essen- tially the same: C1� C2=1.418(5) Å versus 1.429(3)– 1.434(5) Å; and C2� C3 and C1� C3=1.368(5) Å versus 1.360(3)–1.379(5) Å. The C� Br distance of 1.852(4) Å is slightly shorter than in the dibromoethane solvate (1.885(7) and 1.946(6) Å). The bromide anion has a halogen bond with 2+ (Br1� Br2=3.2242(6) Å; Br1� Br2� C3 173.93(12)°) along with weak hydrogen bonds to dibromoethane (Br1� H5 A=2.7524(4) Å) and an isopropyl CH group (Br1� H11=2.8695(4) Å) (Fig- ure 4, Table 11S). Other hydrogen bonds to the bromide are much weaker (there are five Br� H distances to methyl protons in the range of 3.0–3.3 Å). The σ hole on a bromo substituent is significantly greater than for a chloro substituent, and, conse- quently, the halogen bonds are stronger and usually shorter, despite the larger size of Br versus Cl. The Br� � Br halogen bond here is similar to what we have observed in a variety of octahalides (2.914– 3.314 Å)[42–44] while Kukushkin reported a halogen bond distance of 3.3137(8) Å between Br� and CH2Br2. [18] Beer and co-workers reported similar Br� Br halogen-bond distances in bromo-imidazolium bromide salts of 3.2968(4), 3.2652(13) and 3.1888(14) Å (RD=0.865, 0.857 and 0.837, resp.).[16] With an atomic Table 3. Hydrogen-bonding parameters for [1]Cl · 2CHCl3. Hydrogen bond (D� H� A) D� H [Å] H� A [Å] D� A [Å] D� H� A [°] C4� H4� Cl1 0.980(4) 2.3784(9) 3.347(4) 169.6(2) C5� H5� Cl1 0.980(4) 2.4671(9) 3.401(4) 159.3(3) C6� H6� Cl2 0.981(4) 2.4399(9) 3.417(4) 174.5(2) C7� H7� Cl2 0.980(4) 2.3824(9) 3.336(4) 164.3(3) C8� H8� Cl3 0.980(4) 2.3379(7) 3.306(4) 169.5(3) C9� H9� Cl3 0.981(4) 2.4676(7) 3.430(4) 166.9(3) Figure 3. Thermal ellipsoid plot of the asymmetric unit of [2]Br ·C2H4Br2 with the atomic-labelling scheme. Helv. Chim. Acta 2023, 106, e202200163 www.helv.wiley.com (6 of 12) e202200163 © 2022 The Authors. Helvetica Chimica Acta published by Wiley-VHCA AG Wiley VCH Freitag, 13.01.2023 2301 / 279119 [S. 98/104] 1 15222675, 2023, 1, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/hlca.202200163 by M assey U niversity L ibrary, W iley O nline L ibrary on [10/01/2024]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense www.helv.wiley.com radius of 1.85 Å for Br and an effective ionic radius of 1.96 Å for Br� ,[47,48] RD (0.846) in [2]Br is indeed significantly smaller than for the chloro-chloride halo- gen bonds (Table 1). The bromide coordination geometry is again trigonal pyramidal for the three strongest halogen- and hydrogen-bonding interactions: Br2� Br1� H5 A= 80.062(11)°; Br2� Br1� H11=96.636(14)°; H5 A� Br1� H11=59.466(8)° (sum=236.2°). The octahalide salt [1]2I4Cl4 was found to crystallize in the monoclinic space group I2/a with one cation (Figure 5) and half of an octahalide in the asymmetric unit, with the other half generated by a C2 axis (Figure 6,a). The structural parameters of the 1+ cation are essentially identical to the other examples of 1+ described here. The anion is an [I4Cl4] 2� octahalide in which the central I2 is disordered over two positions so as to effectively give a hybrid structure between an idealized symmetric octahalide with ordered and equivalent central I atoms, such as that which we reported earlier,[41] and an idealized pentahalide- trihalide structure, which would have symmetric trihalide and symmetric pentahalide fragments. The central I2 distance of 2.719(2) Å is the same as in the symmetric anion 2.7245(10) Å, but the Cl� I halogen- bond distances of 3.430(3) Å and 2.910(3) Å are longer and shorter, respectively, than in the symmetric octahalide (3.125(2) Å). The terminal trihalide units have similar I� Cl distances (2.583(2) Å for I1� Cl1 and 2.512(2) Å for I1� Cl2) to the symmetric octahalide (2.592(2) Å and 2.504(2) Å). Another difference is that Figure 4. Coordination environment of the bromide ion in [2]Br.C2H4Br2. Figure 5. Atomic-labelling scheme for the cation in [1]2I4Cl4. Figure 6. Thermal ellipsoid plots of [1]2I4Cl4: (a) the halogen- bonding and halogen-halogen distances of I4Cl4 2� . (b) View along the I2Cl2 axis (I1� Cl1� Cl1’� I1’=120.55(10)°). Helv. Chim. Acta 2023, 106, e202200163 www.helv.wiley.com (7 of 12) e202200163 © 2022 The Authors. Helvetica Chimica Acta published by Wiley-VHCA AG Wiley VCH Freitag, 13.01.2023 2301 / 279119 [S. 99/104] 1 15222675, 2023, 1, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/hlca.202200163 by M assey U niversity L ibrary, W iley O nline L ibrary on [10/01/2024]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense www.helv.wiley.com whereas the symmetric octahalide is planar (this is the lowest energy conformation),[58] this example is twisted about the I2Cl2 axis (Figure 6,b). The barrier to this twist is typically very low.[58] The distance between the chloro substituent of 1+ and the terminal chloride of the octahalide is very long at 3.952(3) Å (Cl2� Cl3) and would not be considered as a halogen bond (RD=1.110). However, the σ hole on the chloro group points at the terminal chloride (C3� Cl3� Cl2=171.4(2)°) and the Cl3� Cl2� I1 angle of 74.90(6)° is also consistent with a σ hole interaction, just as the I1� Cl1� I2/I2 A angles are similarly perpendicular at 91.65(9)° and 89.32(8)°, respectively. Indeed, the terminal chloride Cl2 can be considered to have a very strong halogen bond with ICl and, consequently, a very weak halogen bond to 1+. Nonetheless, further evidence against this being a halogen bond is that there is also a chloro-iodine interaction (Cl3� I2 A) with a similar distance of 3.987(3) Å (the sum of the Cl and I van der Waals radii is 3.73 Å). Although this is perpendicular to both the I� I axis (Cl3� I2 A� I2=85.80(7)°) and the C� Cl axis (C3� Cl3� I2 A=88.6(2)°) which would not normally be favorable for a halogen bond. Infrared Spectroscopy of the Hydronium Trichloride The hydronium trichloride cluster was also studied by mid- and far-infrared spectroscopy (Figure 7, Table 4). To assist in the band assignments, a partially deuter- ated sample containing a mixture of D3O + and HD2O + cations was also prepared. An idealized [OH3Cl3] 2� cluster would have C3v symmetry and would display two bands in the ν(OH) region: a symmetric A1 band ν(OHsym) and a degenerate E band ν(OHasym). These are ν10 and ν9, respectively, in Figure 8. The cluster in [1]2[H3O]Cl3 ·CH2Cl2 has crystallographic C1 symmetry, but is close to Cs symmetry with one short hydrogen bond and two long hydrogen bonds. Therefore, we might expect to see splitting of the E band. The vibrational bands were also calculated for a gas phase [OH3Cl3] 2� cluster, as well as all the H/D isotopomers, at the MP2/6-311+ +G(3d,2p) level (Table 4). Full details are given in the Supporting Information. The ν(OHsym) band is calculated to be at 2690 cm � 1. A weak band at 2876 cm� 1 does not change in intensity upon deuteration, so we instead assign the band at 2811 cm� 1, which is lost upon deuteration, to this transition. The most intense band occurs for the degenerate ν(OHasym) E stretching mode ν9 and is calculated to be at 2481 cm� 1, which we observe experimentally at 2681 cm� 1 with a significant side band at 2656 cm� 1 that may be due to splitting because of the lower symmetry in the solid state. A strong sharp band at 2482 cm� 1 is assigned to a combination band (see below). Note that a significant band appears for a vibration of the cyclopropenium C3 ring at 1911 cm � 1. Stoyanov et al. reported quite different and very broad and overlapping IR bands for the chlorinated carborane salt [H3O][CHB11Cl11] at 3224 and 2911 cm � 1 as well as a weak bending mode at 1602 cm� 1.[59] They also reported an inverse relationship between the stretching frequency and the bending frequency for a Figure 7. Mid (top) and far (bottom) infrared spectra of [1]2[H3O]Cl3 ·CH2Cl2 and [1]2[HD2O]Cl3 ·CH2Cl2. Helv. Chim. Acta 2023, 106, e202200163 www.helv.wiley.com (8 of 12) e202200163 © 2022 The Authors. Helvetica Chimica Acta published by Wiley-VHCA AG Wiley VCH Freitag, 13.01.2023 2301 / 279119 [S. 100/104] 1 15222675, 2023, 1, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/hlca.202200163 by M assey U niversity L ibrary, W iley O nline L ibrary on [10/01/2024]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense www.helv.wiley.com series of hydronium salts. On the other hand, Desbat and Huong reported Raman bands at 2895, 2630 and 2525 cm� 1 for [H3O]Cl, which are at similar energies to the bands we observe due to also having strong hydrogen bonding to chloride ions.[60] Additionally, they report bending vibrational modes at 1650 and 1615 cm� 1. We do observe a weak band at 1640 cm� 1, however, this does not disappear upon deuteration and is also present in the infrared spectrum of [1]Cl · 2CHCl3 (Supporting Information), so it is unlikely to be this band. The calculated intensity for the degenerate H3O + bending mode ν8 at 1598 cm � 1 is also extremely low and is probably why we do not see it. A significant band is calculated to occur at 1178 cm� 1 due to the A1-symmetric H3O + bending mode ν7. It is possible that the band at 2482 cm � 1 is an overtone of ν7 ([ClHOH] � shows a strong overtone of the H2O bending mode).[61] However, we favor a combination band from the ν8 H3O + asym bending mode at 1598 cm� 1 and the ν6 mode calculated to be at 753 cm� 1 (sum=2351 cm� 1). These bands are very similar in nature with both being a mix of degenerate bending and stretching modes, but with different relative amounts. The partially-deuterated [OHD2Cl3] 2� cluster would be expected to have a ν(OH) band intermediate between ν(OHsym) and ν(OHasym) of [OH3Cl3] 2� , and this appears in the expected place at 2725 cm� 1. Statistically, we would also expect to see some of either [OH2DCl3] 2� or [OD3Cl3] 2� , however, there are no other bands in the ν(OH) region, thus ruling out the presence of [OH2DCl3] 2� . In the ν(OD) region, there are three bands, at 2180, 2118 and 2083 cm� 1. We assign the 2180 and 2118 cm� 1 bands to ν(OD)sym and ν(OD)asym, respectively, of [OHD2Cl3] 2� and the 2083 cm� 1 band to the E band, ν(OD)asym, of [OD3Cl3] 2� . The symmetric stretching band for this Table 4. Experimental and calculated vibrational frequencies for [OH3Cl3] 2� and [OHD2Cl3] 2� /[OD3Cl3] 2� . [OH3Cl3] 2� [OHD2Cl3] 2� /[OD3Cl3] 2� Experimental Calc.[a] Assignment Experimental Calc.[a] Assignment 2811 2690 ν10 2725 2589 ν(O� H) 2681, 2656 2481 ν9 2180 1890 ν(ODsym) 2118 1848 ν(ODasym) 2482 2351 ν6 + ν8 2083 1842 ν9 [OD3Cl3] 2� 514 2ν4 486 2ν4 [OHD2Cl3] 2� 465 2ν4 [OD3Cl3] 2� 393 ν3 + ν4 375 ν3 + ν4 [OHD2Cl3] 2� 365 ν3 + ν4 [OD3Cl3] 2� 283 269 ν4 282 259 ν4 252 269 ν4 249 259 ν4 200 Cation 200 Cation 180 230 ν3 176 226 ν3 [a] Calculated (and scaled) at the MP2/6-311+ +G(3d,2p) level. Figure 8. Calculated fundamental vibrational modes of [OH3Cl3] 2� . Helv. Chim. Acta 2023, 106, e202200163 www.helv.wiley.com (9 of 12) e202200163 © 2022 The Authors. Helvetica Chimica Acta published by Wiley-VHCA AG Wiley VCH Freitag, 13.01.2023 2301 / 279119 [S. 101/104] 1 15222675, 2023, 1, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/hlca.202200163 by M assey U niversity L ibrary, W iley O nline L ibrary on [10/01/2024]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense www.helv.wiley.com cluster would be weaker and probably overlapping with the [OHD2Cl3] 2� bands. In the far-infrared, fundamental bands below 500 cm� 1 are calculated to be very similar in energy for all isotopomers. Asymmetric and symmetric H3O + � Cl3 liberational stretching modes ν4 and ν3, respec- tively, are calculated to occur at 269 and 230 cm� 1, respectively, compared to ca. 200 cm� 1 for [ClHOH]� .[45] We possibly see a splitting of the degenerate ν4 into bands at 283 and 252 cm� 1 for [OH3Cl3] 2� and, similarly, 282 and 249 cm� 1 for [OHD2Cl3] 2� . We assign v3 to the band at 180 cm� 1 due to its change of intensity (possibly due to the mixture of OHD2 + and OD3 +). Weaker bands at 514 and 393 cm� 1 in the spectrum of [OH3Cl3] 2� (which disappear upon deuteration) may be overtone and combination bands 2ν4 and ν3 + ν4, respectively. These bands appear to split in the spectrum of [OHD2Cl3] 2� /[OD3Cl3] 2� to bands at 486/465 and 375/365 cm� 1, respectively, due to the presence of the two isotopomers [OH2DCl3] 2� and [OD3Cl3] 2� . Conclusion We have presented four structures containing halo- cyclopropenium cations, three of which have signifi- cant chloro-chloride or bromo-bromide CX+� X� halo- gen bonds and one which probably does not have a halogen bond to an [I4Cl4] 2� octahalide. The degree to which the co-crystallites of [1]Cl impact on the Cl···Cl� halogen bond tentatively appears to depend on two factors: whether crystal-packing constraints allow the formation of a halogen bond (in the hydronium case one of the two independent cations does not form a halogen bond, despite the near linear C� Cl···Cl� angle), and some dependence on the strength of other intermolecular interactions, such as hydrogen bonds, to the acceptor Cl� ion. Note that the terminal chloride in the [I4Cl4] 2� cluster can be considered to have a strong halogen bond to an ICl fragment which weakens the halogen bond to the cation. The halide ions have significant hydrogen bonds with alkylhalides and, in one case, a hydronium cation. In the absence of significant geometrical constraints, the coordination geometries of the halides involve approximately orthogonal angles, frequently less than 90°. The vibrational modes of a hydronium trichloride cluster cation were studied by both computational studies and infrared spectroscopy, aided by the spectra of a partially-deuterated sample, which gave very good agreement with the calculated spectra. Supporting Information Synthesis and characterization details; crystallographic data, bond lengths and angles; Computational details (geometry and calculated frequencies); IR spectra. Crystallographic data (CDCC 2201393–2201396) is also available free of charge from the Cambridge Crystallo- graphic Data Centre, 12, Union Road, Cambridge, CB2 1EZ, UK; E-mail: deposit@ccdc.cam.ac.uk. Acknowledgements Open Access publishing facilitated by University of Canterbury, as part of the Wiley - University of Canterbury agreement via the Council of Australian University Librarians. 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Helvetica Chimica Acta published by Wiley-VHCA AG Wiley VCH Freitag, 13.01.2023 2301 / 279119 [S. 104/104] 1 15222675, 2023, 1, D ow nloaded from https://onlinelibrary.w iley.com /doi/10.1002/hlca.202200163 by M assey U niversity L ibrary, W iley O nline L ibrary on [10/01/2024]. See the T erm s and C onditions (https://onlinelibrary.w iley.com /term s-and-conditions) on W iley O nline L ibrary for rules of use; O A articles are governed by the applicable C reative C om m ons L icense www.helv.wiley.com