Stabilizing the Homopolycation (I4)2+ with a Hexasulfate in (I4)[S6O19] and a Borosulfate in (I4)[B(S2O7)2]2

Abstract The reaction of elemental iodine and SO3 in a sealed glass ampoule yielded a turquoise‐colored solution. At temperatures below 7 °C, deep red crystals of (I4)[S6O19] grow. With the addition of B2O3 and pyridine‐SO3 complex red crystals of (I4)[B(S2O7)2]2 can be obtained after heating the mixture to 120 °C. The combination of an (I4)2+ cation with oxoanions has previously not been observed. Both anions have a significant but different influence on the structural properties of the (I4)2+ cation.


Introduction
The formation of red and green coloured solutions when concentrated sulfuric acid reacts with tellurium and selenium, respectively, is well known and a typical show experiment in freshman courses. In fact, these reactions can be traced back to the early days of chemistry and Klaproth was the first to report the tellurium reaction. [1,2] Today we know that the observed colour is caused by the formation of polycations, in this case [Te 4 ] 2 + and [Se 8 ] 2 + , respectively. [3,4] Moreover, quite a number of further chalcogen polycations have been reported up to date, significantly inspired by the work of Gillespie et al. in the early 1970s. [5,6] Interestingly, most of these cations are not stabilized as oxoanionic salts, although their discovery originates from reactions in sulfuric acid. Usually the polycations are crystallized as fluoro-antimonates or -arsenates, and (Se 4 )[HS 2 O 7 ], obtained from Se and oleum, is one of the few exceptions. [7] The early discovery of chalcogen cations has certainly triggered investigations aiming at the preparation of similar cations, including the heavier halogens. Thus, the observation of iodine cations was also reported more than 80 years ago. [8] Meanwhile, several polyiodine cations such as, I 3 + and I 5 + have been identified, however, again, not as sulfates or polysulfates, but as [AsF 6 ] À and [SbF 6 ] À salts, respectively. [9][10][11][12][13] The reason for the preference of the anions is, on the one hand that they are weak ligands and on the other, the respective pentafluorides AsF 5 and SbF 5 are suitable oxidizers which allow oxidation of the heavier chalcogens and halogens in a defined way. Moreover, the pentafluorides can be used in liquid SO 2 as a solvent, so that crystallization is facilitated. Another oxidant that has gained popular interest for the preparation of halogen polycations is peroxydisulfuryl difluoride, S 2 O 6 F 2 . After oxidation it provides the [SO 3 F] À anion for the stabilization of the generated polycations. [14] In the case of iodine the reaction leads to the (I 2 ) + cation, however, some disproportionation to (I 3 ) + and I(SO 3 F) 3 is observed. [15] Furthermore, there was clear evidence that the I 2 + cation undergoes dimerization under formation of a (I 4 ) 2 + cation at lower temperature in this medium. [16] Optically, the dimerization is accompanied by a colour change of the solution from dark blue to red upon cooling.  6 ]. [17] It is noteworthy to mention, that this cation also has not been stabilized in an oxoanionic environment. In general, it can be summarized that the preparation and stabilization of chalcogen and halogen polycations is best achieved using a combination of a sufficiently powerful oxidizer and a weak coordinating anion of a superacid. In retrospect, the initial observations that resulted from experiments in H 2 SO 4 and H 2 SO 4 /SO 3 mixtures (oleum) are consistent, as these media meet the requirements perfectly. Furthermore, the combination of iodine and oleum (65 % SO 3 ) has been investigated as a potential medium. [18,19] The formation of a blue solution has been reported but crystallization attempts were strongly hampered by the viscosity of the solution.
In course of our investigations of the H 2 SO 4 /SO 3 system we reported several important findings. Firstly, the extraordinary oxidation power could be shown by the oxidation of platinum metals leading to several unique compounds. [20][21][22][23][24][25][26][27][28][29][30] Secondly, we could demonstrate that the assumed infinite chain growth of polysulfuric acids H 2 S n O 3n + 1 , that is with increasing SO 3 content, is doubtful. [31,32] Nevertheless, the assumption that the Brønsted acidity of H 2 SO 4 /SO 3 mixtures increases with increasing SO 3 content seems reasonable. [33] Thus, we once more investigated the oxidation of iodine in this medium. In particular, we choose the reaction of I 2 with SO 3 as the initial experiment. It is known that liquid γ-SO 3 (mp. 16.8°C) is extremely prone to polymerization in the presence of water, even at very low H 2 O concentrations (> 10 À 6 mol %). [34] Therefore, the reaction should provide a long chained polysulfate anion, as SO 3 is most certainly a sufficiently strong oxidizer.

Results and Discussion
The reaction was carried out in such a fashion that elemental iodine and SO 3 were heated to 80°C in a sealed glass ampoule. Upon cooling to room temperature, a homogeneous turquoise coloured solution was obtained (see Figure S1 in the Supporting Information). After storing the ampoule in a refrigerator at a temperature below 7°C, a large number of red crystals grew. The agglomerates of these crystals were located next to needle shaped asbestos type SO 3 crystals, which grew simultaneously. Prosaically speaking, the red crystals appear as blossoms at the branches formed by asbestos type SO 3 crystals ( Figure 1). The red crystals disappeared if the ampoule is warmed above 7°C. Moreover, they decompose rapidly after opening of the ampoule, even at low temperature. Thus, the manipulation of the single crystals and their preparation for the measurement has to occur rapidly and under keeping under constant cooling (see Supporting Information). Finally, a single crystal could be measured at the P24 beamline at the DESY facility in Hamburg. The structure analysis revealed that (I 4 )[S 6 O 19 ] has formed.
Neither reducing the amount of SO 3 nor increasing the temperature of the afore introduced mixture led to another product. Thus, with the aim of studying the cation with another anion that still features a polysulfate fragment of the chain, we managed to stabilize the (I 4 ) 2 + cation with a borosulfate anion, namely the bis-disulfatoborate anion [B(S 2 O 7 ) 2 ] À . As for (I 4 )[S 6 O 19 ], elemental iodine was the starting material and was heated with a mixture of SO 3 , B 2 O 3 and the pyridine-SO 3 complex (py-SO 3 ) to 120°C. (I 4 )[B(S 2 O 7 ) 2 ] 2 does not crystallize directly from the homogeneous turquoise coloured solution while cooling. However, several days later after having inverted the ampoule, red crystals could be observed on the inner wall ( Figure 2).
It is the first time that the (I 4 ) 2 + cation could be isolated in form of oxoanionic salts. Interestingly, the stability of the structure results not only from the monovalent anion [B-(S 2 O 7 ) 2 ] 2 À but also thanks to a divalent hexasulfate anion. To date, especially weakly coordinating monovalent anions of superacids were considered as ideal candidates. However, our findings corroborate the above-mentioned theoretical predictions, that the acidity of polysulfuric acids H 2 S n O 3n + 1 increases with growing chain lengths. In fact, (I 4 )[S 6 [32] Not unknown but also rare is the [B(S 2 O 7 ) 2 ] 2 À anion. Examples are known, for example, for the alkaline metals and silver in M[B(S 2 O 7 ) 2 ] (M = Li, Na, K, Ag). [35,36,37] and even more recently we reported on two highly untypical heteropoly AuÀ Cl cations stabilized by [B-(S 2 O 7 ) 2 ] 2 À anions. [38] As described for the fluorides (  iodine atoms that are coordinated by the μ 2 -oxygen atom have two and three additional IÀ O connections with only little more than 300 pm distance, respectively. In principle, the formation and the structure of the (I 4 ) 2 + cation is theoretically well understood: In a first step the oxidation of the I 2 molecule leads to shortening of the IÀ I bond under formation of an (I 2 ) + cation. The latter shows an unpaired electron in a π* orbital and pairing of two of these radicals leads to the (I 4 ) 2 + ion (see also Supporting material). [39][40][41] (  [32] The situation for the hexasulfate ion in (I 4 )[S 6 O 19 ] is significantly different. It is still observed that the terminal sulfur oxygen bond (S3À O231) is the longest within the anion, however, with a value of 177.6(4) pm this bond is dramatically shorter than that found for the rubidium and ammonium compounds, respectively (Figure 4). The difference of more than 40 pm is remarkable and the observed distance is also significantly shorter than the value predicted by theory (195.3). [32,42] Even if the data known so far is scarce, there seems to be clear evidence that the nature of the respective counter cation has a great influence on the bond lengths observed in the hexasulfate anion. The cation in (I 4 )[S 6 O 19 ] is highly unusual compared to the spherical ions used so far. The (I 4 ) 2 + cation is connected by only two quite short bonds involving the atoms O32 and O33 of the terminal [SO 4 ] groups of the [S 6 O 19 ] 2À ion (Figure 4 and 5). We assume that these moieties donate electron density to the π*-orbital of the respective (I 2 ) + units, causing the significantly different IÀ I distances of the (I 4 ) 2 + cation. These strong interactions are also reflected by the bond lengths S3À O32 and S3À O33 (144.6(4) and 144.1(4) pm) which are significantly longer than those observed in the hexasulfate anions known so far ( � 140 pm).
In contrast to (I 4 )[S 6 O 19 ], which exhibits a comparatively symmetrical surrounding of the (I 4 ) 2 + cation, the coordination sphere of the (I 4 ) 2 + cation in (I 4 )[B(SO 4 ) 2 ] 2 is highly unsym-  Table 1 and the Supporting Information.