Uranium(iv) terminal hydrosulfido and sulfido complexes: insights into the nature of the uranium–sulfur bond

We report the synthesis and characterization of terminal uranium(iv) hydrosulfido and sulfido complexes, supported by the hexadentate, tacn-based ligand (Ad,MeArO)3tacn3–.


Synthesis of [(( Ad,Me ArO) 3 tacn)U≡S···K(db-18-c-6)] (3).
A vial was charged with [(( Ad,Me ArO) 3 tacn)U-SH] (50 mg, n = 0.04 mmol) and 2 mL of pyridine, resulting a cloudy cyan solution. First, dibenzo-18crown-6 (14 mg, n = 0.04 mmol) was dissolved in 1 mL pyridine and added to the reaction solution, followed by the dropwise addition of K(N(SiMe 3 ) 2 ) (8 mg, n = 0.04 mmol) in 1 mL pyridine. The solution turns immediately yellow-orange and gets clear. The solution was allowed to stir for 15 min to assure complete reaction, with subsequent removal of the solvent under reduced pressure. The residue was washed three times with cold diethylether to obtain [(( Ad,Me ArO) 3 tacn)U≡S···K(db-18c6)] as orange solid. Crystals suitable for X-ray diffraction were obtained by diffusion of diethylether into a concentrated solution of [(( Ad,Me ArO) 3

Synthesis of [K(2.2.2-crypt)][(( Ad,Me ArO) 3 tacn)U≡S] (4).
A vial was charged with [(( Ad,Me ArO) 3 tacn)U-SH] (98 mg, n = 0.08 mmol) and 4 mL of THF, resulting a cloudy cyan solution. First, 2.2.2-cryptand (30 mg, n = 0.08 mmol) was dissolved in 1 mL THF and added to the reaction solution, followed by the dropwise addition of K(N(SiMe 3 ) 2 ) (16 mg, n = 0.08 mmol) in 1 mL THF. The solution turns bright orange and after 15 min an orange solid precipitates out of solution. The reaction mixture was allowed to stir for another 15  Suitable single crystals of the investigated compounds were embedded in protective perfluoropolyalkyether oil on a microscope slide and a single specimen was selected and subsequently transferred to the cold nitrogen gas stream of the diffractometer. Intensity data were collected using MoK α radiation on either a Bruker Smart APEX 2 diffractometer (λ = 0.71073 Å, graphite monochromator) for 2 or a Bruker Kappa APEX 2 IS Duo diffractometer (λ = 0.71073 Å, QUAZAR focussing Montel optics) for 3 and 4. Data were corrected for Lorentz and polarization effects, semiempirical absorption corrections were performed on the basis of multiple scans using SADABS. 4 The structures were solved by direct methods (SHELXTL NT 6.12) 5 and refined by full-matrix least-squares procedures on F 2 using SHELXL 2014/6. 6 All non-hydrogen atoms were refined with anisotropic displacement parameters. Tentative positions for the sulfur bound hydrogen atoms in the two independent complex molecules in the crystal structure of 2 were derived from a difference Fourier synthesis and were subsequently treated using a riding model. All other hydrogen atoms were placed in positions of optimized geometry. The isotropic displacement parameters of all H atoms were tied to those of the corresponding carrier atoms by a factor of either 1.2 or 1.5. Olex2 was used to prepare material for publication. 7 Crystallographic data, data collection, and structure refinement details are given in Table S1.
Compound 2 crystallized with two independent molecules of the U complex, each being situated on a crystallographic threefold axis. The asymmetric unit contained a number of disordered CH 2 Cl 2 solvent molecules. Two alternative orientations were refined in the case of two of these solvent molecules resulting in site occupancies of 65(2) and 35(2) % for the atoms C100, Cl11, Cl12 and C110, Cl13, Cl14 and of 57(2) and 43(2) % for the atoms C200, Cl21, Cl22 and C210, Cl23, Cl24, respectively. A third CH 2 Cl 2 solvent molecule was situated on a crystallographic threefold axis and was accordingly disordered. Here, no hydrogen atoms were included. DFIX, SAME, SIMU, and ISOR restraints were applied in the refinement of the disorder. Compound 3 crystallized with a total of 0.62 molecules of benzene and 0.38 molecules of Et 2 O per formula unit. The two solvent molecules shared a common crystallographic site. Similarity restraints were applied to the anisotropic displacement ellipsoids of the disordered atoms (i.e. the two solvent molecules). In compound 4 both the uranium complex and the [K(2,2,2-crypt)] moiety were situated on threefold crystallographic axes.
A significant difference in the U-S bond distance of the two molecules of 2 (0.06 Å) was observed. Additionally, the anisotropic displacement parameters (a.d.p.'s) for the two independent S atoms look very different. We made a number of attempts to explain this different behavior by a slight deviation of the sulfur atoms from the crystallographic threefold rotation axis. Allowing the sulfur atoms to deviate from this special position and free refinement of all three coordinates does not change the overall picture of the a.d.p.'s, nor does it significantly change the U-S bond distances. In order to get a more isotropic behavior of sulfur atom S2, SIMU and ISOR restraints with very small e.s.d's proved to be necessary. A refinement of the site occupancy factors of the two sulfur atoms (as free variables) resulted in values of 0.324(5) for S1 and 0.338(5) for S2, which is very close to the expected value of ⅓ for the situation on a threefold axis. To conclude, the difference in the U-S bond distances is caused by packing forces. In the crystalline packing, one can observe distinct differences in the packing of the two independent molecules. The packing of the U1 molecules is characterized by an offset of two consecutive molecules with additional solvent molecules in between (see Figure  S1), thus leaving the S atom as far as possible unaffected from neighbored complex or solvent molecules.
The shortest S···C distance here is observed between S1 and an adjacent dichloromethane solvent (C100) at a distance of 3.933 Å. In contrast to this, consecutive U2 molecules are arranged in a head-to-tail fashion with no offset and without having additional solvent molecules in between. The S2 atoms are pointing to the tacn moiety of the next U2 molecule with short distances of 2.75 Å between sulfur and the tacn hydrogen atoms at C21A, C21B and C21C (distance S2···C21A is 3.716 Å). The S-H···H21X distance amounts to 1.89 Å (see Fig S2). There is obviously some steric pressure exerted on the U2-S2 bond by this short intermolecular distances.  A striking difference in U-S bond length in 3 and 4 was observed. There is without any doubt a S···K bonding interaction in 3 (the observed S···K distance of 3.136(2) Å is significantly shorter than the sum of the corresponding van der Waals radii of 4.60 Å, the sum of covalent radii is 3.07 Å). This S···K bond is on the short side of bonding distances for bonding S···K(db18-c-6) interactions (a CSD search revealed a range from 3.051 -3.700 Å for this type of bond/interaction), which indicates a fairly strong bond in 3. This, in turn, brings the whole potassium diphenyl-18-crown-6 moiety close to the uranium complex. Its steric influence is visible by the significant differences between the U-N tacn bonds. One short U-N bond of 2.704(3) Å (U1-N1) is observed at the side of the ( Ad,Me ArO) 3 tacn ligand, where the potassium diphenyl-18-crown-6 moiety enters the uranium complex' periphery. The other two U-N tacn bonds on the opposite side amount to 2.883(3) Å for U1-N2 and 2.870(3) Å for U1-N3. In a similar way, the ArO ring on the cation's side (O1, C8 -C13) is pushed away from the potassium diphenyl-18-crown-6 moiety, as can be seen from the torsion angle U1-O1-C9-C8 (-17.3(6)°), while the corresponding torsion angles on the other side of the uranium complex are significantly larger by more than 5° (U1-O2-C27-C26: 25.8(7)° and U1-O3-C45-C44: 22.2(8)°). Last but not least, the U center in 3 lies to a minor amount below the plane formed by the three oxygen donor atoms of the ( Ad,Me ArO) 3 tacn ligand than in 4 (out-of-plane shifts are -0.055 Å in 3 and -0.086 Å in 4). Therefore, the striking differences observed for the U-S bond distances in 3 and 4 might again be explained by the steric situation in the two complexes. In compound 4, the sulfur atom is only bonded to the uranium center without further steric constraints. In 4, however, the sulfur atom is bonded both to the uranium center of the complex and to the potassium of the complex cation. In this situation one would, at first sight, expect an elongation of the U-S bond but a shortening (compared to the observed U-S bond in 4) is observed. The dibenzo-18-crown-6 moiety exerts a considerable steric strain that might push the sulfur atom slightly deeper into the cavity of the [(( Ad,Me ArO) 3 tacn)U] moiety, while at the same time the uranium reduces its negative out-of-plane shift and moves closer to the sulfur atom, in order to accommodate the sterically demanding potassium dibenzo-18-crown-6 moiety in the complex periphery. As we could already see from the differences in the U-S bond distances in the two independent molecules in 2, it can be attributed to the packing forces and, therefore, the steric situation that causes significant changes in the length of the observed U-S bond. Figure S3: Ball-and-stick representation of the molecular structure of 3 highlighting the steric pressure exerted by the potassium diphenyl-18-crown-6 moiety on the left side of the complex molecule (hydrogen atoms and solvent molecules omitted for clarity).  Figure S4: Molecular structures of the two crystallographically independent molecules per asymmetric unit in crystals of 2 · 3.25 CH 2 Cl 2 . Non-sulfur bound hydrogen atoms and co-crystallized solvent molecules are omitted for clarity. Thermal ellipsoids are drawn at 50% probability.

IR spectroscopy
In order to get insight into the strength of the uranium-sulfido bond, infrared vibrational spectroscopy was performed on complexes 2-4. Additionally, IR spectroscopy was expected to be a useful tool to prove the presence of the SH species, because the weak (SH) band should be Î _ observable between 2600-2300 cm -1 , a region were typically no other absorptions are observed. However, the (SH) band is not detectable for hydrosulfido complex 2, an observation that was

Computational Study
The Gaussian09 program suite was used for performing all the quantum-chemical calculations. 17 As functional we have used the Becke's 3-parameter hybrid one, 18 combined with the non-local correlation functional provided by Perdew/Wang , 19 denoted as B3PW91. The relativistic energy-consistent small-core pseudopotential of the Stuttgart-Köln ECP library was used in combination with its adapted segmented basis set to represent uranium atom, going from +III to +IV to +V complexes. [20][21][22] For the potassium and sulphur, the quasi-relativistic energy-adjusted ab-initio pseudopotentials wasused, along with its corresponding energy-optimized valence basis sets, 23,24 augmented by a d polarization function, for the case of silicon atoms. 25 For all the atoms, the 6-31G(d) basis set was used, 26,27 In all computations no constrains were imposed on the geometry. Full geometry optimization was performed for each structure using Schlegel's analytical gradient method 28