Cs[Cl3F10]: A Propeller‐Shaped [Cl3F10]− Anion in a Peculiar A[5]B[5] Structure Type

Abstract Reaction of CsF with ClF3 leads to Cs[Cl3F10]. It contains a molecular, propeller‐shaped [Cl3F10]− anion with a central μ3‐F atom and three T‐shaped ClF3 molecules coordinated to it. This anion represents the first example of a heteropolyhalide anion of higher ClF3 content than [ClF4]− and is the first Cl‐containing interhalogen species with a μ‐bridging F atom. The chemical bonds to the central μ3‐F atom are highly ionic and quite weak as the bond lengths within the coordinating XF3 units (X = Cl, and also calculated for Br, I) are almost unchanged in comparison to free XF3 molecules. Cs[Cl3F10] crystallizes in a very rarely observed A[5]B[5] structure type, where cations and anions are each pseudohexagonally close packed, and reside, each with coordination number five, in the trigonal bipyramidal voids of the other.

Single crystals of the compound were obtained by reaction of CsF with excess liquid chlorine trifluoride at temperatures at or below 0 8C, see Equation 1.
These crystals were directly isolated from cold ClF 3 suspensions and handled at low temperatures under a nitrogen atmosphere. The crystals rapidly deteriorate upon warming to room temperature and/or moisture from air contact, releasing ClF 3 and/or hydrolysis products. Further details on the synthesis are given in the Supporting Information. Singlecrystal X-ray diffraction on Cs[Cl 3 F 10 ] shows it to crystallize in the monoclinic space group P12 1 /n1 (mP56, 14, e 14 ). The lattice parameters are a = 9.0949(7), b = 15.3652 (12), c = 6.8841(5) , b = 91.314(2)8, V = 961.77(13) 3 , and Z = 4 at T = 100 K. Further crystallographic details are given in the Supporting Information, Tables S1-S5.
The compound contains the propeller-shaped [Cl 3 F 10 ] À anion in C 1 symmetry ( Figure 1). However, the deviation of the molecule from C 3 symmetry is small. The anion can be thought of as a central fluoride ion which is coordinated by the Cl atoms of three surrounding ClF 3 molecules. The central m 3 -bridging F atom (F(1) in Figure 1) lies 0.522(2) above a virtual plane formed by the Cl atoms with Cl-m 3 -F-Cl angles from 113.53(9) to 116.12(8)8. Its structure is correctly predicted by VSEPR theory and there seems to be an additional interaction to the Cs + counterion residing above the apex of the trigonal-pyramidal anion ( Figure 1).
In contrast, the quantum-chemically calculated gas-phase minimum structure is D 3 -symmetric (TURBOMOLE, DFT-PBE0/TZVP, see Supporting Information), that is the m 3 -F atom and the Cl atoms reside within a plane and the Cl-m 3 -F-Cl angles are 1208. [19][20][21][22] Even when the geometry optimization is started from C 3 symmetry, where the m 3 -F atom is displaced from the virtual plane formed by the Cl atoms, the D 3symmetric structure is reobtained. Figure 1. The Lewis structure of the [Cl 3 F 10 ] À anion and its structure in the salt Cs[Cl 3 F 10 ]. The displacement ellipsoids are shown at the 70 % probability level at 100 K. Fluorine atoms drawn in yellow, chlorine atoms in green, and caesium atoms in sky blue. [41] In the crystal structure, the ClF 3 units are essentially planar and are rotated around the m 3 -F À Cl bond in the same direction, which leads to the propeller shape of the anion. That is, the ClF 3 units are tilted in comparison to the virtual plane formed by the Cl atoms by 33.34(7), 36.41 (6), and 44.77(6)8. The anion is thus similarly shaped as the [Br 3 F 10 ] À anion, see below. [10] The Cl atoms are surrounded by four F atoms each in a quadrilateral, almost planar manner. The m 3 -F À Cl distances are in the range of 2.243(2) to 2.265 (2) . The F atoms cis-bound relative to m 3 -F show F cis À Cl distances from 1.730(2) to 1.747(2) , while the trans-bound F atoms show F trans ÀCl distances of 1.600(2) to 1.615 (2) . These are comparable with the FÀCl distances reported for the orthorhombic modification of ClF 3 (1.716 and 1.621 ) and thus support the description of the anion having a central fluoride anion coordinated by three ClF 3 molecules. [23] We will see below that the chemical bond between the m 3 -F atom and the three chlorine atoms is almost ionic. Also, a comparison with FÀCl distances in [ClF 4 ] À salts (1.794(4) to 1.8034(9) ) shows significant differences which we attribute to the same reason. [17,18] In the structurally closely related [Br 3 F 10 ] À anions, the m 3 -F À Br distances range from 2.238(10) to 2.248(2) , the F cis À Br distances from 1.824(10) to 1.878 (7) , and the F trans À Br distances from 1.745(2) to 1.767 (7) . [10] So, unexpectedly, the observed m 3 -F-X distances are essentially similar. This is likely to be due to the direct interaction of the Cs + cation with the m 3 -F atom in Cs[Cl 3 F 10 ] leading to coordination number 3 + 1 for it, while such an interaction is absent in the crystal structure of Cs[Br 3 F 10 ] and the coordination number of that m 3 -F atom is only three. The F cis -X and F trans -X atomic distances are circa 0.1 longer for X = Br compared to the Cl-case, which agrees with the expectation.
As mentioned above, quantum-chemical gas-phase calculations for the homologous anions [X 3 F 10 ] À (X = Cl, Br, I) show them to be D 3 -symmetric even when the geometry optimization is started from the pyramidal C 3 symmetry around the m 3 -F atom as in the crystal structure. Except for the pyramidalization around the m 3 -F atom, the calculated structure of the D 3 -symmetric [Cl 3 F 10 ] À anion nicely agrees with the atomic distances and angles reported here. The same is true for X = Br with the previously reported [Br 3 F 10 ] À anion in its Rb and Cs salts. [10] To the best of our knowledge, a compound containing [I 3 F 10 ] À anions is not known yet. The calculated X-F distances within the XF 3 subunits are very similar to those in the solid-state structures of the halogen trifluorides. [23][24][25] This indicates a rather weak interaction of the XF 3 units with the central m 3 -F atom. The calculated X-m 3 -F distances of 2.22, 2.30 and 2.44 for X = Cl, Br, I, respectively, also support this.
To obtain a qualitative picture of the bonding situation in these anions we carried out an intrinsic atomic orbital (IAO) analysis to obtain partial atomic charges (Table S6) and generated the intrinsic bonding orbitals (IBOs), see Figure 2 and Figures S2-S4, which give information about the polarization and contribution of the atoms to the respective bond. [26,27] Further details are given in the Supporting Information. The obtained partial atomic charges from the IAO analysis show that the m 3 -F atom has the most negative partial charge in all three anions, followed by the F cis atom of the XF 3 units. The F trans atoms are the least negatively partially charged F atoms. The partial charges of the atoms follow the expected trend in line with decreasing electronegativities c AR (Cl) > c AR (Br) > c AR (I), that is, the iodine atoms are more positively partially charged than the Cl atoms and the F atoms are most negatively partially charged in [I 3 F 10 ] À . [28] The two IBOs representing the Cl-m 3 -F-Cl bond in the [Cl 3 F 10 ] À anion are shown in Figure 2. Further IBOs showing the other F À Cl bonds, as well as the respective IBOs for [Br 3 F 10 ] À and [I 3 F 10 ] À are reported in the Supporting Information. The percentages next to the atoms show the contribution of each atom to the IBO. The higher the contribution of an atom to the IBO in %, the more ionic is the bond. The contribution of the m 3 -F atom to the respective IBO is above 90 % in all three anions, showing a highly polarized covalent, that is essentially ionic, bond ( Figure 2 and Figures S2-S4). The heavier halogen atoms only have minor contributions to the IBO. Within the series, the contributions of the atoms follow the expected trend, which is due to the decrease of the electronegativities from Cl to I.
After the description of the molecular structure of the anion we come to the description of the crystal structure of Cs[Cl 3 F 10 ].
Surprisingly, Cs[Cl 3 F 10 ] can be regarded as a simple AB structure type. The Cs + cations ("A") are coordinated by five [Cl 3 F 10 ] À anions ("B"), that is C. N. (Cs + ) = 5, and that exclusively via the F cis and m 3 -F atoms. The arrangement of the Cs cations corresponds to the hexagonal-close packing with the layers arranged perpendicular to the c axis ( Figure 3). Into the empty channels (parallel to the c axis), the terminally bound F ter atoms protrude (see center of Figure 3). The m 3 -F atoms, which may be seen as the centers of gravity of the [Cl 3 F 10 ] À anions ("B"), are also arranged in a hexagonal-close packed manner with the layers also perpendicular to the c axis. Their coordination number is also five. Thus, the two pseudohexagonal-close packed lattices of A and B interpenetrate in such a way that each ion is surrounded by five of its counterions in a trigonal bipyramidal manner. That is, the Figure 2. The two intrinsic bond orbitals (IBOs) showing the Cl-m 3 -F-Cl bonds in the [Cl 3 F 10 ] À anion. [27] The percentages indicate the contribution of the Cl atoms and of the m 3 -F atom to the IBO. The larger the percentage, the more polarized the bond. For comparison: For a gasphase NaF molecule, in which the bond should be highly ionic, the contribution of the F atom is 96 %. In the H 2 molecule, which has a purely covalent bond, the contribution of each atom is 50 %. If the summation of the percentages does not add up to 100 %, then other atoms contribute-less than 1 %-to the IBO. For further details, see the Supporting Information. F atoms: yellow, Cl atoms: green.
packing of Cs[Cl 3 F 10 ] can be described as a simple A [5] B [5] structure type. Of course, the situation is somewhat more complicated because the [Cl 3 F 10 ] À ion is not spherical. But if we take idealized planar [Cl 3 F 10 ] À ions, the resulting space group is that of the hexagonal-close packing, P6 3 /mmc (aristotype). Distortion of the planar [Cl 3 F 10 ] À ions to their real structure entails a symmetry reduction to the actual space group P12 1 /n1; that can be represented by the Bärnighausen tree [29][30][31][32][33] shown in Figure S1. The crystal structure of the compound and its relation to the hexagonal aristotype is shown in Figure 3 and explained in detail in the Supporting Information.
A related structural motif of an A [5] B [5] structure type is known to occur as a substructure in the Ni 2 In structure type, where one kind of Ni atoms reside in the octahedral voids and the other kind of Ni atoms in the trigonal bipyramidal voids of hexagonal-close packed In atoms. [34] The reported orthoselenate Li 4 [5] Se [5] O 5 [5] is an ordered variant of the A [5] B [5] aristotype. [35,36] Also, such a 5-5 coordination has been quantum-chemically predicted for a low temperature modification of sodium chloride Na [5] Cl [5] , and later also for magnesium oxide Mg [5] O [5] . [37,38] To the best of our knowledge, no binary compound with such a A [5] B [5] crystal structure exists yet, and Cs[Cl 3 F 10 ] appears to be one of the very rare examples of this type.
The recorded Raman spectrum of Cs[Cl 3 F 10 ] along with a calculated one obtained from quantum-chemical solid-state calculations (CRYSTAL17, DFT-PBE0/TZVP) is shown in Figure 4. [21,22,39] A detailed comparison of the Raman spectra with the ones of Cs[ClF 4 ] and liquid ClF 3 as well as further details are given in the Supporting Information, Table S8 and Figure S5. The calculated Raman spectrum nicely agrees with the measured one, indicating that the main product obtained under the experimental conditions is Cs[Cl 3 F 10 ]. The two main features of the Raman spectrum are the bands above 690 cm À1 and the band at approximately 500 cm À1 . The first can be attributed to the Cl-F trans symmetric stretching mode and the latter to the Cl-F cis symmetric stretching mode.
In summary, we obtained the compound Cs[Cl 3 F 10 ] which crystallizes in the very rarely observed A [5] B [5] structure type. The cations and anions of Cs[Cl 3 F 10 ] form interpenetrated hexagonal-close packed layers. The coordination number of each cation and anion is five. The [Cl 3 F 10 ] À anion is propellershaped with a symmetry close to C 3 as the central m 3 -F atom is surrounded in trigonal-pyramidal manner by the three Cl atoms, while quantum-chemical gas-phase calculations predict the anion to be D 3 -symmetric with the m 3 -F atom in plane with the Cl atoms. The recorded Raman spectrum of Cs-[Cl 3 F 10 ] is in excellent agreement with the quantum-chemically calculated one for the solid-state. So far there is no experimental evidence for the existence of a [Cl 2 F 7 ] À anion Figure 3. The crystal structure of Cs[Cl 3 F 10 ] viewed along the c axis and its relation to the hexagonal aristotype. Cs atoms are shown in skyblue, Cl atoms in green, and F atoms in yellow. All atoms are drawn as spheres with arbitrary radii. Note the pseudohexagonal arrangement of the Cs + cations and the [Cl 3 F 10 ] À anions. If the atoms in the pseudohexagonal cell would fulfil trigonal symmetry, the space group would be P3. [41]  that would be the analogue to the known [Br 2 F 7 ] À anion, or for anions with an even higher ClF 3 content than [Cl 3 F 10 ] À , such as a [Cl 4 F 13 ] À anion. Investigations to obtain compounds containing such anions are ongoing, as well as analogous ones that derive from ClF 5 .