Isolation and Properties of the Long Elusive Deep Blue Soluble [K3{(N t Bu)3S}2]. Cage Radical

Abstract The reaction of potassium metal with sulfurtriimide S(N t Bu)3 (II) gives the long elusive deep blue cage radical [K 3 {(N t Bu) 3 S} 2 ]. (1_K) that crystallizes at −35 °C from toluene solution. The subsequent physical characterization via X‐ray structure analysis, UV/Vis‐, and EPR spectroscopy from solution reveals the existence of one unpaired electron delocalized within the whole cage, i.e. coupling with the six nitrogen atoms, as well as the three potassium atoms caped by the two SN3 ligands. The present X‐ray structure analysis further supports previous assumptions made on the parent compound 1_Li obtained from [Li4{(N t Bu)3S}2] (I) and finally elucidates the structural arrangement of the SN3 caps and alkali metals in such radical cage species.

S1. Synthesis and characterization S1.1 Generalities All reactions were carried out with strict exclusion of air and moisture under inert atmosphere, either with Schlenk techniques or in an Argon glovebox. Highly pure potassium metal was purchased from Sigma-Aldrich and used as received. I, II and 1_Li were synthesized as previously reported .
[S1] All solvents were dried and distilled from Na, K or Na/K prior to use. Elemental analyses were performed on a Vario EL3 at the Mikroanalytische Labor, Institut für Anorganische Chemie, University of Göttingen.

Synthesis of 1_K:
In an argon glovebox, II (2eq, 300mg, 1.2mmol) and potassium metal (3eq, 72mg, 1.8 mmol) were reacted in 2-3 mL toluene. Upon heavy stirring for one hour while the mixture turned troubled and light blue, the potassium was triturated in order to dissolve the blue product forming on its surface. The solution then turned deep blue, was filtered and the solvent removed in vacuo. Crystals were grown from a highly concentrated toluene solution at -35°C. Crystalline yield: 35%. Attempts to improve the yield by stirring longer after trituration resulted in solution color loss after a few minutes. Anal. Calc. for C48H108K3N12S4 (the best fit was found for one radical molecule and two II molecules): M = 1097.66 g/mol. C,52.46;H,9.91;N,15.29;S,11.67. Found: C,53.05;H,9.65;N,14.98;S,11.97.vis (toluene): λmax = 635 nm (broad).

S1.2 Vis spectroscopy
Vis spectra were recorded on an Agilent Cary 60 or an Agilent Cary 50 spectrometer using quartz cuvettes fitted with Young-type Teflon-valves. IR data were measured on neat samples with a Bruker ALPHA FT-IR spectrometer with Platinum ATR module and visualized with the Opus/Origin programs. 10 -4 M toluene solutions of 1_K (blue) and I (colorless) were freshly prepared inside of an argon glove box and measured ASAP on the instrument. For I, a first spectrum was recorded prior to O2 exposure. Upon removal of the sealing cap, the absorption spectra were recorded in 0.1 min steps, while stirring to ensure homogeneity and O2 diffusion into the solution, until the transient product 1_Li entirely decomposed.

S2. X-ray measurements
The datasets were collected on an Incoatec Mo Microsource [S2] with mirror optics and an APEX II detector with a D8 goniometer. The data were integrated with SAINT. [S3] A multi-scan absorption correction was applied using SADABS [S4] or TWINABS [S5] . The structures were solved by SHELXT [S6] and refined on F 2 using SHELXL [S7] in the graphical user interface ShelXle. [S8] For the refinement of disordered moieties, distance restraints and restraints for the anisotropic displacement parameters were used. [S9] The crystals of 1_K are extremely sensitive and difficult to handle. Data sets were collected on two crystals. The first dataset was measured at 100 K. The structure crystallizes in space group is P21/c with two molecules in the asymmetric unit. A small amount of non-merohedral twinning could be detected, but the twin integration does not improve the model. In both molecule three of the six t-butyl groups are disordered. Both molecules have very similar geometry and both show the described difference in the two caps: The S-N bond lengths are identical within 3  for cap B, while in cap A, two S-N bond lengths are shorter while the third one is similar to the S-N lengths of cap B.
In the endeavour to improve the structure quality data of a second crystalline sample were collected at 120 K. Here the cell volume is halved and, in the space group P21/n, only one molecule is present in the asymmetric unit. However, the two cells are related. The smaller cell corresponds to the primitive cell which can be derived from the larger one, as a B centred cell. No intermediate reflections violating the B centring could be found. Additionally, also in this data set a small amount of twinning could be detected, but the twin integration does not lead to an improvement of the model. Now all t-butyl groups are disordered. The ADPs of the nitrogen atoms are elongated but the atoms are not included in the disorder, which could have a small influence on the S-N bond lengths. Now the differences between cap A and B are even more pronounced. In cap A the two smaller bond lengths are even smaller and the longer one is significantly longer than the S-N bond lengths of cap B.
Afterwards we performed refinements against the 120K data in the larger cell and against the 100 K data in the smaller cell. For both data sets the new refinements showed the same behaviour of the S-N bond lengths as before but the quality of the models decreases (see Table S2.1).
The difference between the two data sets could be either explained by the different temperature or a different polymorph. In both crystals the t-butyl groups are strongly vibrating and are also statistically disordered, with a lower temperature two positions for some of them could be frozen, which could result in the intermediate reflections.

S4. Theoretical calculations
TD-DFT and spin density calculations were performed in this order: -1_K_UNSYMM: coordinates from the X-ray structure, without symmetry restrictions, at the theory level def2-svp in the orca suite [S10]. -1_K_UNSYMM coordinates from the previous calculation, at the theory level def2-TZVP in the orca suite. -1_K_SYMM: modified coordinates to match a Cs symmetry (based on the EPR results), using the orca software suites, at the theory level def2-TZVP.
The results were visualized with the Chemcraft software [S11] and are reported in the following sections.  Figure S4.1.2: TD-DFT difference density plots for the states responsible for the absorption bands at 578, 572 and 388 nm for 1_K. Top and side views are shown for better clarity, while carbon and hydrogen atoms are omitted for clarity. Positive and negative densities are depicted in orange and blue, respectively (isosurfaces at 0.005 level (a.u.)).

S4.3 Spin density calculations
The EPR simulation was performed with the EPR simulator developed by Viktor Chechik at the University of York (victor.chechik@york.ac.uk) based on EasySpin . [S12] and as well as with SPINFIT (Bruker software).
The spin density was furthermore calculated on the level of theory PBE0 D3BJ def2tzvp def2/J RIJCOSX in Orca. [S10] The same coordinates reported above were used (UNSYMM) Spin density plots were generated with the Chemcraft software. [S11] The following spin density plot was obtained : Figure S4.3.1: Calculated spin density plots for 1_K_UNSYMM. Positive and negative densities are depicted in orange and blue, respectively (isosurfaces at 0.005 level). Both top-down (left) and side views (right) are depicted for better clarity. Hydrogens are omitted for clarity.
The results depicted in Figure S4.3.1 suggest that the spin density is located only on three nitrogen atoms of the same cap. The experimental EPR data, however, clearly suggest hyperfine coupling to six equivalent nitrogen atoms and thus two equivalent sulfurtriimide units. When the structure of the radical was computationally freely optimized (gas phase conditions) the 'asymmetric' (cap A/cap B) type of structure observed in the crystal structure was retained. Spin density in these structures was almost entirely located on one sulfurtriimide fragment. In order to computationally probe a species that reflects the EPR-spectroscopically observed features, a symmetry restricted optimization enforcing (C3h, or Cs) symmetry was performed. Spin density of these Cs symmetric molecules is then equally distributed between both sulfurtriimide fragments.  Let us assume the start is the localisation of the electron in red at the upper sulfur atom (a). This results in four electrons at that sulfur, one each from the S-N single bonds plus the unpaired electron, causing a double positive formal charge. Now the electron moves to the first nitrogen atom at the left (b), formally by one nitrogen lone pair to form a S=N double bond first and further giving the lone pair at sulfur. That reduced the sulfur formal charge to one and the negative charge at the nitrogen atom, hosting the unpaired electron, vanishes. Next the electron moves on to a potassium cation (c), formally generating an uncharged alkali metal atom while the nitrogen atom only holds two covalent bonds plus a single lone pair. Hence that nitrogen is now formally positive charged. The unpaired electron can move on to the lower SN3-cap in the same fashion (d) and reaches eventually the sulfur atom in the lower cap (e). Obviously all three nitrogen atoms in the upper cap can be engaged in the same way (h, g) and the electron can couple to any of the potassium cations (f). Of course, the same is valid for the lower cap, resulting of a delocalised unpaired electron in the whole S2N6K3 uncharged cage.