From Missing Links to New Records: A Series of Novel Polychlorine Anions

Herein we report the synthesis and structural characterization of four novel polychloride compounds. The compounds [CCl(NMe 2 ) 2 ][Cl(Cl 2 ) 3 ] and [NPr 4 ][Cl(Cl 2 ) 4 ] have been obtained from the reaction of the corresponding chloride salts with elemental chlorine at low temperature. They are the missing links in the series of polychloride monoanions [Cl(Cl) n ] (cid:0) ( n = 1– 6). Additionally, the reaction of decamethylferrocene with elemental chlorine was studied yielding [Cp* 2 Fe] 2 [Cl 20 ], which contains the largest known polychloride [Cl 20 ] 2 (cid:0) to date, and [Cp* 2 Fe][Cl(Cl 2 ) 4 (HF)], which is the first example of a polychlor-ide-HF network stabilized by strong hydrogen and halogen bonding. All compounds have been characterized by single-crystal X-ray diffraction, Raman spectroscopy and quantum-chemical calculations.


Introduction
In recent years, polyhalides have received much attention due to various possible applications. [1]They exist as either solids or low viscous ionic liquids with significantly lower vapor pressure compared to the neat halogens. [2]Therefore, trihalides as well as higher polyhalides have been applied as easy-to-handle halogenation reagents which react with ketones, [3] alkenes [4] and alkynes. [3]Additionally, polychlorides have shown to be efficient and safe materials for the storage of elemental chlorine. [5]Recently it was shown that ionic liquids based on polyhalogen-and polyinterhalogen anions can be used for the oxidative dissolution of metals and alloys.Since even noble elements can be dissolved, an application in metal recycling and urban mining might be possible. [6]Besides their potential as oxidation reagents, ionic liquids based on polyhalogen anions possess a surprisingly high conductivity [2] and it was shown that polybromides are present in zinc-bromine (redox flow) batteries. [7]olybromides and polyiodides display a structurally rich and diverse chemistry and large anions up to [Br 24 ] 2À , [I 26 ] 4À and [I 29 ] 3À can be isolated (Figure 1). [8]In contrast, the more challenging handling of gaseous chlorine as well as the lower stability of polychlorides have so far hampered the exploration of the structural chemistry of polychlorides. [9]Polyfluorides have only been detected under matrix conditions. [10]So far, the only structurally characterized polychlorine monoanions were [Cl 3 ] À , [Cl(Cl 2 ) 2 ] À [11] and only very recently crystal structures of [Cl(Cl 2 ) 5 ] À and [Cl(Cl 2 ) 6 ] À were reported. [12]11b] [Cl(Cl  Overview of experimentally known homoatomic polyhalogen mono-and dianions.These anions have been crystallographically characterized except for the shaded ones, which have only been observed spectroscopically.Reproduced from ref. [1] the highest chlorine content, [Cl(Cl 2 ) 6 ] À , has an octahedral geometry in the solid state. [12]So far, only two polychlorine dianions are known in the literature.The [Cl 8 ] 2À ([CCl (NMe) 2 ] 2 [Cl 8 ] [13] , [CCl(NMe) 2 ] + = tetramethylchloroamidinium) which shows a discrete, Z-shaped structure and the [Cl 12 ] 2À ([NMe 3 Ph] 2 [Cl 12 ] [12] , [NMe 3 Ph] + = trimethylphenylammonium) which possesses inversion symmetry and can be described as two [Cl(Cl 2 ) 2 ] À units which are connected by a Cl 2 molecule.The [Cl 12 ] 2À units are further connected to form a three-dimensional network resembling a honeycomb.In 2015 the molecular structure of a 2D layered infinite polychlorine structure consisting of [Cl 3 ] À and Cl 2 building blocks ([NEt 4 ][Cl 3 ) 2 • Cl 2 ], [NEt 4 ] + = tetraethylammonium) was described. [14]he bonding situation of such higher polyhalogen anions can be described as a Lewis acid-base interaction between the halide (Lewis base) and the dihalogen (Lewis acid).Two major interactions contribute to the bonding: First there is charge transfer from the HOMO of the halide ion (lone pair) into the LUMO of the dihalogen (σ*(XÀ X)) which leads to an elongation of the XÀ X bond.Additionally, the electrostatic potential of the dihalogen is anisotropic.This results in a belt of higher electron density perpendicular to the molecule's bonding axis and an area of lower electron density along the extension of the covalent bond, the so called σ-hole.Between the negatively charged halide ion and the positive σ-hole there is an attractive electrostatic interaction. [15]Very recently the bonding situation of the [Cl 3 ] À has been studied by experimental and computed electron densities determination.It was shown that a genuine ppσ bond is formed if a chlorine molecule interacts with a "naked" chloride ion.Furthermore, it appears to be a smooth transition from the asymmetric to the symmetric compound which is a crucial property for its chemical reactivity. [16]erein, we report the synthesis and characterization of four hitherto unknown polychloride compounds.It was possible to obtain the first molecular structures of a heptachloride ([CCl (NMe 2 ) 2 ][Cl(Cl 2 ) 3 ]) and a nonachloride ([NPr 4 ][Cl(Cl 2 ) 4 ]).In addition, the first polychloride-HF network was synthesized ([Cp* 2 Fe] 2 [Cl(Cl 2 ) 4 (HF)]) and the largest known polychloride, the [Cl 20 ] 2À could be obtained by oxidation of Cp* 2 Fe with a large excess of chlorine.

Results and Discussion
The Heptachloride [Cl(Cl 2 ) 3 ] À An anion, which can formally be described as a [Cl(Cl 2 ) 3 ] À , was obtained by the reaction of the Vilsmeier salt [CCl(NMe 2 ) 2 ]Cl with a large excess of chlorine without additional solvent (Equation 1).
The anion within [CCl(NMe 2 ) 2 ][Cl(Cl 2 ) 3 ] is best described as a polychloride network in which the central chloride is coordinated by five chlorine molecules, four of them are bridging to the next chloride ion while one is terminal.Four chloride ions connected by four chlorine molecules form a bent rectangle, with the fifth chlorine alternatingly pointing up and downwards.The bent rectangles are connected by all four edges to form an extended network (Figure 3).
The distance between the central halide and the terminal Cl 2 unit (275.1(1)pm) as well as the Cl 2 bond length (204.9(1)pm) are in good agreement to the bond length calculated on SCS-MP2/def2-TZVPP for a square pyramidal [Cl(Cl) 5 ] À anion (277.8-282.2pm, 203.9-204.8pm).In comparison to crystalline Cl 2 (R(ClÀ Cl) = 198.4(1)pm) the Cl 2 bond length is elongated by 6.5(2) pm which can be explained by the aforementioned donation of electron density into the σ*-orbital of the ClÀ Cl bond which leads to a bond weakening effect.The distance between the central halide and the bridging Cl 2 units is significantly longer (287.8(1)-302.5(1)pm).Nevertheless, the  weakening of the ClÀ Cl bond is comparable for the bridging Cl 2 units (R(ClÀ Cl) = 204.0(1)-204.5pm) which is due to a charge transfer from two chloride ions.The shortest distances between cation and anion are a ClÀ H distance of 274.0(3) pm and a ClÀ Cl distance of 339.1(1) pm which indicate moderate hydrogen and halogen bonding interactions.
The Raman spectrum of the single crystal of [CCl(NMe 2 ) 2 ][Cl-(Cl 2 ) 3 ] shows three bands at 471, 451 and 430 cm À 1 (see Figure S8).Those bands correspond to the stretching modes of the three crystallographic independent Cl 2 units in the crystal.The experimentally obtained spectra is rather similar to the computed spectra at SCS-MP2/def2-TZVPP level for a C 4v symmetric [Cl(Cl 2 ) 5 ] À anion in the gas phase while no exact match can be expected due to the network structure in the solid state (see Figure S9).2).

Reactions of [Cp* 2 Fe] with Cl 2
The reactions of ferrocene with dihalogens are known for a long time. [21]Ferrocenium is well known to form a series of polyiodide complexes, from the triiodide [Cp 2 Fe][I 3 ] [21] , [Cp 2 Fe][I 7 ] [22] , [Cp 2 Fe] 2 [I 16 ] [23] to [Cp 2 Fe] 3 [I 29 ] [8c] (Cp 2 Fe = ferrocene, Cp* 2 Fe = decamethylferrocene) .However, elemental bromine or chlorine are so reactive, that they partially or completely destroy the ferrocene moiety, leading to either [Cp 2 Fe][FeX 4 ] or pentahalocyclopentane C 5 H 5 X 5 (X=Cl, Br). [24]Only a small number of ferrocenium derivatives with tribromide counter anions have been isolated so far. [25]ecently it was shown that ferrocene derivatives can be oxidized twice to form stable compounds containing tetravalent iron ([Cp* 2 Fe] 2 + , [Cp* 2 FeCO] 2 + and [Cp 2 FeH + ]). [26]Therefore it was attempted to oxidize [Cp* 2 Fe] using an excess of the strong oxidizer Cl 2 to form either [Cp* 2 Fe] 2 + or the unknown Co-crystalized HF is most likely a result of adventitious traces of HF which were present since the reaction was performed in a PFA reactor on a steel line which was previously used for reactions involving HF.To rule out a contamination with water (differentiation between HF and H 2 O is not always possible by XRD) the reaction was repeated, and stoichiometric amounts of HF and H 2 O were added, respectively.The addition of milligram amounts of HF led to the formation of the same crystals, while addition of water was inconclusive.
The structure of the anion can be described as an intermediate between a square pyramidal structure with HF on the top of the pyramid (C 4v ) and a trigonal bipyramidal (C 2v ) structure with HF in an equatorial position.This interpretation is further supported by the structural parameter τ which was calculated to be 0.56 (τ = (β-α/60°, α and β are the largest angle in the coordination sphere; τ = 0 for perfect square-pyramidal structure and 1 for perfect bipyramidal symmetric structure) [27] .Quantum-chemical calculations in the gas-phase predict both structures to be very close in energy.On B3LYP(D3BJ)/def2-TZVPP level of theory the C 2v structure is favored by 0.5 kJ mol À 1 with the C 4v being a transition state while on SCS-MP2 level of theory the C 4v symmetric structure is equal in energy both being minima, indicating a very shallow potential energy surface.The bond length of the coordinated Cl 2 is only slightly elongated by 2.7(2) to 4.3(2) pm when compared to crystalline Cl 2 (R(ClÀ Cl) = 198.4(1)pm) [17] while the short F1À Cl1 distance of 289.1(2) pm indicate strong hydrogen bonding interaction.A comparison with the molecular structure of [PPN][Cl(Cl 2 ) 5 ] • Cl 2 , in which the central chloride also has a coordination number of five, shows a comparable weakening of the ClÀ Cl bond with bond length of 201.5(1) to 202.6(1) pm. [12]This indicates that the Lewis acidity of the HF molecule is at least comparable to the Lewis acidity of Cl 2 .Quantum-chemical calculations support this observation since the addition of a HF molecule to [Cl(Cl 2 ) 4 ] À was calculated (B3LYP(D3BJ)/def2-TZVPP and SCS-MP2/def2-TZVPP (value in parentheses) to be 8.6 (13.3) kJ mol À 1 exergonic while the exchange of one Cl 2 unit from [Cl(Cl 2 ) 5 ] À against one HF molecule is 17.2 (32.0) kJ mol À 1 exergonic (Scheme 1).
This also explains why a mixed anion containing HF and Cl 2 is obtained even when an excess of chlorine was used.Further quantum-chemical calculations show that in principle all exchange reactions of Cl 2 against HF are exergonic for the system [Cl(Cl 2 ) 5-n (HF) n ] + HF![Cl(Cl 2 ) 5-n-1 (HF) n + 1 ] À + Cl 2 (n = 0-4, see Table S6) while an exchange of Cl 2 in [Cl(Cl 2 ) 5 ] À against HCl is also exergonic (Table S7).
The Raman spectrum of the single crystal of [Cp* 2 Fe][Cl(Cl 2 ) 4 (HF)] shows three bands at 507, 488 and 477 cm À 1 .Those bands agree well to the bands calculated on SCS-MP2/def2-TZVPP level of theory for a C 2v symmetric [Cl(Cl 2 ) 4 (HF)] À (499, 480, 477 cm À 1 ) while again no exact match can be expected due to the reduced symmetry in the crystal and the formation of the polyhalide network (see Figure S9).
When an even larger excess of Cl 2 (21 equiv.) is used in absence of HF slow cooling of the reaction mixture to À 75 ] crystallizes in the monoclinic space group C2/ c (Figure 7).
The structure of the anion is interpreted as two [Cl(Cl 2 ) 4 ] À units which are connected by a single bridging Cl 2 molecule.Overall, the structure of the anion is rather asymmetric with large differences in the distance between the central halide and the coordinated Cl 2 unit (R(ClÀ Cl) = 271.4(1) to 299.2(1) pm).This might be due to further inter-molecular interactions to other [Cl 20 ] 2À units.The Cl 2 bond length of the coordinated Cl 2 units correlates well to the distance to the central halide.It increases when the distance is shorter which can be explained by a stronger interaction and therefore more donation of electron density into the σ* orbital of the ClÀ Cl bond.The only exception is the bridging Cl 2 which has a significantly elongated bond (203.6(1)pm) even though the distance to the chloride ion is rather large (299.2(1)pm).This is due to the interaction with two Cl À ions.The Raman spectrum shows five bands between 490 and 419 cm À 1 (see Figure S8).This is in agreement with the symmetry of the anion as the [Cl 20 ] 2À exhibits inversion , five bands with A g symmetry are computed on B3LYP(D3BJ)-COSMO/def2-TZVPP and SCS-MP2-COSMO/def2-TZVPP (ɛ rel = 100) level of theory (see Figure S9).The broad area of wavenumbers over which the bands are spread can be explained by the rather large differences in the ClÀ Cl bond length (R(ClÀ Cl) = 202.0(1)-205.7(1)pm).
Both the [Cl(Cl 2 ) 4 (HF)] À and the [Cl 20 ] 2À form three-dimensional networks in the solid state.The [Cl(Cl 2 ) 4 (HF)] À forms hexagonal prisms via short inter-molecular FÀ Cl distances of F1À Cl3 273.9(2) pm, F1À Cl5 287.8(2) pm and F1À Cl7 283.2(2) pm (Σ vdW radii = 322 pm [20] , see Figure 8).This gives a coordination number of five for the central chloride as well as for the fluorine of the HF molecule.In this case the fluorine acts as a halogen bond acceptor (Lewis base) to connect the [Cl(Cl 2 ) 4 (HF)] À units.The center of the hexagonal prism is occupied by a [Cp* 2 Fe] + cation with the prisms being connected via all eight phases to translate in all three dimensions (see Figure S5.)For the [Cl 20 ] 2À the same hexagonal prismatic arrangement is observed in the solid state, even though the prism is heavily distorted (See Figure 8).The Hirshfeld surface of the [Cl 20 ] 2À unit shows significant interactions between the anions while only weak interactions with the cations are observed (See Figure S7).Nevertheless, a description as discrete [Cl 20 ] 2À units is reasonable since the inter-molecular distances of R(Cl6À Cl3) = 326.9(1)pm, R-(Cl6À Cl5) = 336.9(1)pm, R(Cl6À Cl9) = 324.8(1)pm and R-(Cl7À Cl7) = 321.8(2)pm (Σ vdW radii = 350 pm) [20] are significantly longer than the longest intramolecular distance (at least 22.6 pm).Additionally, the [Cl(Cl 2 ) 4 (HF)] À anion is the first example for a polychloride-HF network which is stabilized by strong hydrogen and halogen bonding interaction.Overall, it could be shown that polychloride chemistry shows a surprising structural diversity, which is comparable to that of the heavier polyhalogen anions.

Apparatus and Materials
All substances sensitive to water and oxygen were handled under an argon atmosphere using standard Schlenk techniques and oil pump vacuum up to 10 À 3 mbar.Some reactions were performed on a stainless steel vacuum line in self-built reactors consisting of 8 mm o. d.PFA (perfluoroalkoxy alkanes) tubing which where heat sealed on one end and connected to a steel valve on the other end.Dry DCM was obtained by storage over activated 3 Å molecular sieves.Commercially available [NPr 4 ]Cl and Cp* 2 Fe were used without further purification.Chlorine (Linde, purity 2.8) was passed through calcium chloride to remove traces of water.
[CCl(NMe 2 ) 2 ]Cl [28] was prepared according to literature procedures.All salts were dried in vacuo at 100 °C for 10 min prior to use.Raman spectra were recorded on a Bruker (Karlsruhe, Germany) MultiRAM II equipped with a low-temperature Ge detector (1064 nm, 100-180 mW, resolution of 4 cm À 1 ).Spectra of single crystals were recorded at À 196 °C using the Bruker RamanScope III (see supporting information part g for description of the used method).X-ray diffraction data were collected on a Bruker D8 Venture CMOS area detector (Photon 100) diffractometer with Mo Kα radiation.Single crystals were coated with perfluoroether oil at low temperature (À 40/À 80 °C) and mounted on a 0.1-0.2mm Micromount.The structures were solved with the ShelXT [29] structure solution program using intrinsic phasing and refined with the ShelXL [30] refinement package using least squares on weighted F2  values for all reflections using OLEX2 [31] .For quantum chemical calculations (structure optimization (with and without solvent model COSMO [32] ) and frequency calculations (including Raman intensities)) the program package TURBOMOLE 7.3 [33] was used.Functionals (B3LYP(D3BJ) [34] and SCS-MP2 [35] ) and the basis set (def2-TZVPP) [36] were used as implemented in TURBOMOLE.Minima on the potential energy surface were characterized by harmonic vibrational frequency analysis.Thermochemistry was provided with zero-point vibration correction, ΔG values were calculated at 298.15 K and 1.0 bar.Representations of Hirshfeld surfaces were generated using CrystalExplorer17 [37] [CCl(NMe 2 ) 2 ][Cl(Cl 2 ) 3 ]  Inside a 8 mm PFA tube connected to a stainless steel valve, 32 mg (0.1 mmol) Cp* 2 Fe were dissolved in 1.5 ml dichloromethane.90 mg Cl 2 (1.25 mmol) were condensed in at À 196 °C.The mixture was warmed to À 70 °C and mechanically agitated resulting in a dark green suspension.After some minutes 3 mg anhydrous hydrogen fluoride were condensed in at À 196 °C.The mixture was warmed to approximately À 35 °C, resulting in a clear dark-green solution.The mixture was frozen at À 196 °C, and the PFA tube was flame-sealed in vacuum.The sealed-off tube was placed inside a Dewar filled with 1-2 liters of À 35 °C cold ethanol and placed in a À 75 °C freezer.After one day, dark-green crystals had formed.Inside a 8 mm PFA tube connected to a stainless steel valve, 32 mg (0.1 mmol) Cp* 2 Fe were dissolved in 1.5 ml dichloromethane.180 mg Cl 2 (2.5 mmol) were condensed in at À 196 °C.The mixture was warmed to À 70 °C and mechanically agitated resulting in a dark green suspension.After some minutes, the mixture was warmed to À 50 °C, resulting in a clear dark-green solution.The mixture was frozen at À 196 °C, and the PFA tube was flame-sealed in vacuum.The sealed-off tube was placed inside a Dewar filled with 1-2 liters of À 50 °C cold ethanol and placed in a À 75 °C freezer.After one day, dark-green crystals had formed.

Figure 1 .
Figure1.Overview of experimentally known homoatomic polyhalogen mono-and dianions.These anions have been crystallographically characterized except for the shaded ones, which have only been observed spectroscopically.Reproduced from ref.[1]

Figure 7 .
Figure 7. Molecular structure of [Cp* 2 Fe] 2 [Cl 20 ] in the solid state with thermal ellipsoids shown at 50 % probability.Only one cation and no hydrogen atoms are shown for clarity.