Reactivity of uranium(iii) with H2E (E = S, Se, Te): synthesis of a series of mononuclear and dinuclear uranium(iv) hydrochalcogenido complexes

Reaction of [((AdArO)3N)UIII(DME)] with EH2 (E = S, Se, Te) yields a complete series of mono- and dinuclear uranium(iv) hydrochalcogenide complexes.


Results and discussion
Recently, we reported the synthesis of the uranium(III) complex [(( Ad ArO) 3 N)U(DME)] supported by the exible, N-anchored chelator ( Ad ArO) 3 N 3À (with ( Ad ArO) 3 N 3À ¼ tris(2-hydroxy-3-adamantyl-5-methylbenzyl)amine). 54 This reactive, low-valent complex can undergo reactions such as the activation of CO 2 and its heterocumulene analogs COS and CS 2 , 55-57 as well as the activation of elemental chalcogens. 58 The starting material [(( Ad ArO) 3 N)U(DME)] exhibits a half-step potential of À1.879 V for the uranium(III/IV) redox couple (see ESI †), supporting the fact that this complex is a potent reductant, whichin partexplains its observed reactivity with the H 2 E substrates. Furthermore, even uranium(IV) complexes supported by the ( Ad ArO) 3 N 3À ligand can show remarkable reactivity under the right conditions, such as the formation of polychalcogenido complexes through stepwise addition of stoichiometrically precise amounts of elemental chalcogen. 59 H 2 S and its heavier congeners H 2 Se and H 2 Te are known and well-documented precursors for the synthesis of hydrochalcogenides, however, the resultant hydrochalcogenides are rarely stable compounds and subsequent reactions oen lead to the formation of metal polychalcogenide clusters or binary metal salts instead. 1,9 Furthermore, synthetic work is greatly complicated by the high toxicity and malodor of these gases. With the commercial availability of H 2 S as a solution in THF, reactions with this gas can be carried out under much simpler conditions. That said, no such reagents exist for its selenium and tellurium analogs. To solve these synthetic challenges, we created concentrated solutions of H 2 Se and H 2 Te by condensing the respective gas formed from Al 2 E 3 (E ¼ Se, Te) and sulfuric acid into a THF solution at À70 C. 60 These solutions can be stored under an inert atmosphere in a freezer at À35 C for several weeks. This procedure greatly simplies handling H 2 Se and H 2 Te.
Synthesis and molecular structures of mononuclear uranium(IV) hydrochalcogenido complexes 1-EH (E ¼ S, Se, Te) The mononuclear thiolato complex [(( Ad ArO) 3 N)U(DME)(SH)] (1-SH) has been synthesized by reacting the uranium(III) starting material [(( Ad ArO) 3 N)U(DME)] in THF with an excess amount of H 2 S via dropwise addition of a solution of H 2 S in THF until the reaction mixture turned green. Within ve minutes, a light green precipitate formed and 1-SH was obtained in 70% yield aer ltration and drying of the solids in vacuo (Scheme 1). Recrystallization of the solid via diffusion of n-hexane into a concentrated DME solution yielded crystals suitable for X-ray diffraction analysis. The solid state structure of 1-SH reveals a mononuclear complex [(( Ad ArO) 3 N)U(DME)(SH)] with a sevencoordinate uranium center in a distorted, monocapped trigonal prismatic coordination environment, in which one molecule of DME is coordinated to the uranium center in a bidentate fashion (Fig. 1, le). The U-S bond distance of 2.797(1)Å is clearly longer than reported U]S double bonds (2.382(11)-2.481(1)Å) and is in good agreement with a U-S single bond (2.588(1)-2.907(3)Å, Table 1). 58,59,[61][62][63][64][65] Furthermore, the chalcogen-bound hydrogen could be located in the difference Fourier map and was subsequently treated using a riding model. The U-N distance of 2.616(2)Å and the U-O avg. bonds of 2.171Å are comparable to other complexes supported by the N-anchored ligand ( Ad ArO) 3 N 3À . 54,58,59 Additionally, the SH À ligand is not coordinated in the axial position directly trans to the nitrogen anchor, which is clearly shown in the N-U-S bond angle of 136.24 (4) .
Applying a similar synthetic procedure, the mononuclear selenolato complex [(( Ad ArO) 3 N)U(DME)(SeH)] (1-SeH) can be synthesized in 72% yield (Scheme 1). The molecular structure of 1-SeH, obtained from crystals grown from a concentrated DME solution, compares well to that of 1-SH ( Fig. 1, center). Noteworthy, the asymmetric unit of the cell in 1-SeH contains two independent but structurally very similar molecules of the complex, and hence, only the values of 1-SeH A are given in Table 1 (for more details see ESI †). According to the 3s criterion, the U-N (2.605(3)Å) and U-O avg. bond lengths (2.170Å) are the same as those observed in 1-SH. The U-Se bond of 2.936(1)Å is signicantly longer than a distinctive U]Se double bond (2.533(1)-2.646(1)Å) and can be compared to other complexes with U-Se single bond distances varying from 2.719(1) to 3.125(1)Å. [57][58][59][61][62][63]66,67 As in complex 1-SH, the N-U-Se angle is strongly bent with 133.44 (7) . In contrast to 1-SH, the seleniumbound hydrogen could not be located in the difference Fourier map, but was conrmed in 1 H and 2 H{ 1 H} NMR experiments. The 1 H NMR spectrum of 1-SeH recorded in pyridine-d 5 shows a total of ten signals, ranging from À63 to 11 ppm, however, the unambiguous assignment of the hydrogen resonance of the SeH À ligand is severely hampered due to the complicated nature of the supporting ligand system. As a result, the deuterated analog of 1-SeH, namely [(( Ad ArO) 3 N)U(DME)(SeD)] (1-SeD), was synthesized in a reaction of the uranium(III) starting material [(( Ad ArO) 3 N)U(DME)] with D 2 Se. As expected, the 1 H NMR spectrum of 1-SeD shows only nine signals, and thus, unequivocally designates the signal at À62.86 ppm to the SeH À hydrogen. Additional 2 H{ 1 H} NMR experiments in pyridine revealed a single deuterium signal at À62.69 ppm that originates from the SeD À moiety, further supporting the previous assignment in the 1 H NMR spectrum and conrming the presence of the chalcogen-bound hydrogen.
In contrast to the closely related syntheses of 1-SH and 1-SeH, the preparation of the tellurium analog complex [(( Ad ArO) 3 N)U(DME)(TeH)] (1-TeH), requires special precautions, since H 2 Te is very susceptible to light and quickly decomposes at temperatures above 0 C. As a consequence, the reaction needs to be carried out in cooled solvents under the rigorous exclusion of light to obtain 1-TeH in 78% yield (Scheme 1). Remarkably, the resulting complex 1-TeH is both stable at elevated temperatures up to 80 C and in the presence of light, which contrasts with the very few reported, usually rather unstable, transition metal complexes featuring a TeH À ligand. 1,9 Single crystals suitable for X-ray diffraction were obtained via diffusion of n-hexane into a mixture of benzene and DME (7 : 3). The molecular structure of 1-TeH is distinctly different from those of 1-SH and 1-SeH, as it shows a six-coordinate uranium center, located in a distorted octahedral coordination environment (Fig. 1, right). Furthermore, and consequently, the DME solvent molecule is now coordinated in a monodentate fashion. The U-N (2.575(1)Å) and U-O avg. bond lengths (2.130Å) are slightly shorter than those observed in 1-SH and 1-SeH. The U-Te bond distance of 3.122(1)Å is in accordance with the formulation of a U-Te single bond (Table 1)  The heavier chalcogenido complexes 2-SeH and 2-TeH exhibit the same molecular structures as 2-SH and feature comparable U-O avg. and U-N bond lengths (see Table 1). The U-Se and U-Te bonds range from 2.989(1) to 3.094(1)Å and 3.119(2) to 3.296(2)Å, respectively, which is slightly longer compared to the bis-m-chalcogenido complexes [Na(DME) 3 TeH)), which is in accordance with the larger atomic radii of selenium and tellurium that lead to slightly higher steric repulsion within the complexes. Noteworthy, the mononuclear complexes 1-EH and their dinuclear counterparts 2-EH showed identical 1 H NMR spectra in deuterated benzene. Complexes supported by the highly exible N-anchored ligand ( Ad ArO) 3 N 3À as well as the oen highly nucleophilic EH À functional groups show a strong tendency to form dinuclear species, hence, it is within expectation that 1-EH can dimerize to 2-EH in noncoordinating solvents, such as benzene, with the loss of coordinating solvent. 1,[54][55][56][57][58][59] This conclusion is further supported by the presence of broadened signals at approximately 3 ppm in all spectra, which are attributed to uncoordinated DME. Likewise, the dinuclear species 2-EH dissociates in DME to form the mononuclear complexes 1-EH.

UV/vis/NIR spectroscopy of complexes 1-EH and 2-EH
The vis/NIR spectra of complexes 1-EH, recorded in DME, are all very similar and show a number of low-intensity f-f transitions with small molar extinction coefficients of 10-35 M À1 cm À1 between 500 and 2200 nm (see Fig. 3, le), characteristic for tetravalent complexes of uranium. 91 While the spectra of 1-SH and 1-SeH are almost identical, the spectrum of 1-TeH clearly shows the hypsochromic shi of two bands at 1011 nm (3 ¼ 24 M À1 cm À1 ) and 1815 nm (3 ¼ 20 M À1 cm À1 ). This observation can be rationalized by the discrepancy in the ligand-eld splitting that should be expected due to the structural difference of 1-TeH compared to 1-SH and 1-SeH. The UV/vis region shows one intense charge-transfer band centered at 287 nm for each compound with molar extinction coefficients ranging from 14-  (2-TeH) at 2 K (Fig. 4, bottom). These observations are in agreement with tetravalent uranium centers with a nonmagnetic 3 H 4 ground state. 58,98,99 An interesting feature to note, is the difference in the plot of c M vs. T of 1-EH and 2-EH. While the mononuclear complexes 1-EH show temperature-independent paramagnetism (TIP) over a wide temperature range from 10 to 62 K (for 1-SH and 1-SeH) and 101 K (1-TeH), followed by a steady decrease of the molar susceptibility, this feature is slightly less pronounced in the dinuclear complexes 2-SH and 2-SeH (TIP below 50 K, see ESI †) and shows a signicant difference in 2-TeH (TIP below 55 K).

Conclusion
The reaction of H 2 E (E ¼ S, Se, Te) with a reducing metal center is a viable synthetic route to obtain metal hydrochalcogenido complexes, however, handling of these gases is greatly complicated due to their high toxicity. With the use of concentrated and cooled solutions of these gases in THF, we were able to synthesize the new uranium(IV) hydrochalcogenido complexes [(( Ad ArO) 3 N)U(DME)(EH)] (1-EH, E ¼ S, Se, Te), using simple glovebox techniques. These compounds yield the dinuclear complexes [{(( Ad ArO) 3 N)U} 2 (m-EH) 2 ] (2-EH, E ¼ S, Se, Te) in noncoordinating solvents, such as benzene, with the loss of coordinating DME. UV/vis/NIR spectroscopy and SQUID magnetization measurements further conrm that complexes 1-EH and 2-EH can be clearly distinguished both in solution and in the solid state. We are currently trying to access terminal monochalcogenido species from the herein presented hydrochalcogenido complexes, in order to establish a series of chalcogenido complexes with U-E single and U]E double bonds with similar coordination environments. This series should provide an excellent opportunity to gain more insight into the covalency and f-orbital participation of the U-E bond in uranium chalcogenido complexes. Fig. 3 Vis/NIR spectra of hydrochalcogenido complexes 1-SH (9.2 mM, black), 1-SeH (9.7 mM, red), and 1-TeH (4.1 mM, blue) recorded in DME at 25 C (left) and vis/NIR spectra of hydrochalcogenido complexes 2-SH (2.0 mM, black), 2-SeH (6.6 mM, red), and 2-TeH (6.5 mM, blue) recorded in benzene at 25 C (right). The molar extinction coefficients for complexes 2-EH were calculated per uranium center for a better comparison.