s-Block metal scorpionates – A new sodium hydrido-tris(3,5-dimethyl-1-pyrazolyl)borate salt showing an unusual core stabilized by bridging and terminal O-bonded DMSO ligands

Dissolution of [(μ-Me2CO)3(NaTp)2] (1) (Tp*  = hydrido-tris(3,5-dimethyl-1-pyrazolyl)borate) in DMSO at room temperature leads to the growth of colourless crystals characterized as the new salt [Na2Tp(μ-Me2SO)3(Me2SO)3] [NaTp2] (2). 2 crystallized in the trigonal space group R3 with Z = 3, a = 14.1227(2) Å, b = 14.1227(10) Å, c = 33.9685(2) Å, and V  =  5867.35(17) Å3. Interestingly, anion and cation of 2 both contain the Tp* ligand. Moreover, the cationic moiety highlights an unusual sodium atom hexacoordinated by six DMSO molecules acting as O-bonded ligands. Three of which exhibit a bridging coordination mode and three are in terminal position. To the best of our knowledge, the framework of [Na2Tp(μ-Me2SO)3(Me2SO)3] is unprecedented.


Results and discussion
The framework of 1 was described as two hydrido-tris(3,5dimethyl-1-pyrazolyl)borate sodium moieties bridged by three acetone molecules acting as oxygen-based donors (Plasseraud and Cattey, 2014). When crystals of 1 are stored at 4°C in the crystallization solution, they remain stable several weeks. However at room temperature they gradually degrade. The thermogravimetric analysis performed on fresh crystalline sample of 1 demonstrated this relative fragility by showing a significant weight loss starting at relatively low temperature, from 70-75°C ( Figure 1). This alteration can be explained by the facile breaking of the three bridges connecting the two sodium atoms and resulting from the release of the coordinated acetone molecules. The infrared spectrum recorded from crystals of 1 stored in an oven at 100°C during 1 h, corroborates this observation by clearly demonstrating the complete disappearance of the carbonyl band (acetone ligands) located at 1698 cm −1 (Figures 2a,b). However as shown in the spectrum 1b, the other significant absorption bands are relatively unaffected, the C-H (pyrazole ring) and B-H elongations are still present around 3110 and 2450 cm −1 respectively. Moreover, the fingerprint in the range of 1600 to 500 cm −1 undergoes few changes, especially as regards the characteristic bands of the pyrazole ring and the B-N bond (Dehnicke and Fernández, 1976). Thus, this would imply the preservation of the Tp* ligand.
Based on these observations and considering 1 as a promising reaction synthon, we investigated its reactivity toward molecules able to act as O-donor ligands with the aim to access to original frameworks of sodium scorpionates. Compound 1 is in particular well soluble in DMSO, tetrahydrofuran (THF), dimethyl ether (DME) and diethyl ether (Et 2 O). Herein, we describe the result of the recombination of 1 in the presence of DMSO. The reaction was firstly monitored by 1 H NMR spectroscopy in DMSOd 6 . After several days at room temperature we observed the clear decrease in the intensity of the signal corresponding to the acetone ligands (Me 2 CO) of 1 while Tp* signals are still present (Figure 3). The same observation was made by 13 C{ 1 H} NMR while the 11 B spectrum did not show any differences (broad signal centred at −9 ppm).
Concomitantly colourless prismatic crystals grown at the bottom of the NMR tube. They were then characterized as being the new salt [Na 2 Tp*(μ-Me 2 SO) 3 (Me 2 SO) 3 ][NaTp * 2 ] (2). On the infrared spectrum of 2 (ATR analysis, Figure 2c), two absorption bands (weak) attributed to ν B−H are observed at 2513 and 2252 cm −1 , which effectively support the presence of two distinct types of coordination for Tp* ligands. However, no differentiation was possible in solution. Only one set of signals was observed for Tp* by 1 H NMR spectroscopy. Compare to 1, the IR spectrum is more abundant in the range of 1100−950 cm −1 , highlighting the presence of additional absorption bands (at 1051 and 1024 cm −1 ) attributable to ν S−O elongations and which supports the coordination of DMSO molecules (Calligaris, 2004). Furthermore, the CHN elemental analysis performed on crystals also supports  the formula of 2. Thus, the displacement of the acetone coordinating ligands of 1 by an excess of DMSO molecules and leading to 2, can be summarized according to Equation 1. DMSO exhibits a higher Gutmann's donor number (DN) than acetone, 29.8 kcal·mol -1 against 17 kcal·mol -1 (Gutmann, 1976), which can be viewed as the driving force of the exchange, also supported by the reaction conditions (using a large excess of DMSO). DMSO molecule is known to show a high coordination ability. Thus, in recent decades and on several occasions, metal complexes with DMSO ligands were the subject of review articles focusing on structural aspects and coordination modes but also on their possible use in homogeneous catalysis (Alessio, 2004;Calligaris, 2004;Sipos et al., 2015).
An X-ray crystallographic analysis on suitable crystals of 2 confirmed the ionic structure of this new compound. Interestingly, the anionic and cationic components both incorporate Tp*Na moieties. One free molecule of DMSO complete the unit cell of 2. Crystallographic data and refinement details are summarized in Table 1. An Ortep view, together with selected bonds lengths and angles, is shown in Figure 4. The [NaTp * 2 ] − anion results on the hexacoordination of a sodium atom by two Tp* ligands according to a sandwich-type structure. The sodium atom is in a distorted octahedral environment while Tp* ligands adopt a propeller-like structure. The Na−N distances are in the range of 2.566(3)−2.586(3) Å. The N-Na-N intra ligand angles measure less than 90° [N3−Na3−N3 = 74.89(11)°] while N-Na-N inter ligand angles are larger than 90° [N3−Na3−N5 = 103.67(10)°]. To our knowledge, this is only the second time that such a structure of a discrete anionic [NaTp 2 ] − complex ion is described in the literature. The first example was reported by Reglinski et al. (1999) for the salt [Tm 2 Bi][Tp 2 Na] (Tm = hydrotris(methimazolyl)borate), isolated during the treatment of BiCl 3 by successive additions of NaTm, and then NaTp. More recently, a comparable arrangement for the anion was revealed by Bailey et al. (2001) in the hydridro-tris(thioxotriazolyl) borate (Tt) sodium salt [Tt 2 Na]Na(H 2 O) 6 .
From a supramolecular interest, a view of the crystalline packing of 2 is depicted in Figure 5. The organization can be considered as a columnar-like arrangement oriented along the c-axis. It is based on the alignment and the repetition of sequences involving a free molecule of DMSO, the cation [Na 2 Tp * (μ-Me 2 SO) 3 (Me 2 SO) 3 ] + and the anion [NaTp 2 ] − . In conclusion, a new sodium hydrido-tris(3,5-dimethyl-1pyrazolyl)borate salt was elucidated describing, for both the anion and the cation, unusual structures. Other studies are in progress to extend the reactivity of compound 1 to other O-donor molecules.

] (2)
Initially, the salt 2 was isolated by preparing an NMR tube based on a sample of 1 dissolved in DMSO-d 6 (Sigma-Aldrich). The storage of the sample in air and at room temperature led after several days to the growth of prismatic colourless single crystals which were characterized as 2. The isolation of 2 as crystalline material was also obtained

X-ray crystallography
A suitable single crystal of 2 was mounted on a mylar loop, and the diffraction data were collected at 100 K on a Bruker D8 Venture triumph Mo equipped with a nitrogen jet stream low-temperature system (Oxford Cryosystems). The X-ray source was graphite-monochromated Mo-K α radiation (λ = 0.71073 Å) from a sealed tube. The total number of runs and images was based on the strategy calculation from the program Apex3 (Bruker, 2015) and the unit cell was refined using Saint (Bruker, V8.34A, after 2013). Data reduction, scaling and absorption corrections were performed using Saint (Bruker, V8.34A, after 2013). A multi-scan absorption correction was performed using SADABS-2012/1 (Bruker, 2012) was used for absorption correction. Using Olex2 (Dolomanov et al., 2009), the structure was solved by Intrinsic Phasing using the Shelxt program (Sheldrick, 2015) and refined with full-matrix least-squares on F 2 using the version 2018/3 of Shelxl program (Sheldrick, 2008(Sheldrick, , 2015. Programs used for the representation of the molecular and crystal structures: Olex2 (Dolomanov et al., 2009), Mercury (Macrae et al., 2008). All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were included in their calculated positions and refined as riding atoms.
CCDC 1985584 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.