Reactivity of uranium ( IV ) bridged chalcogenido complexes UIV – E – UIV ( E 1⁄4 S , Se ) with elemental sulfur and selenium : synthesis of polychalcogenido-bridged uranium complexes †

We report the syntheses, electronic properties, and molecular structures of a series of polychalcogenidobridged dinuclear uranium species. These complexes are supported by the sterically encumbering but highly flexible, single N-anchored tris(aryloxide) chelator (ArO)3N 3 . Reaction of an appropriate uranium precursor, either the U(III) starting material, [((ArO)3N)U(DME)], or the dinuclear monochalcogenido-bridged uranium(IV/IV) compounds [{((ArO)3N)U(DME)}2(m-E)] (E 1⁄4 S, Se), with elemental sulfur or selenium, yields new complexes with a variety of bridging chalcogenide entities m-Em n (E 1⁄4 S, m 1⁄4 2, n 1⁄4 1 or 2 and E 1⁄4 Se, m 1⁄4 2, 4; n 1⁄4 2). Activation of the heavy chalcogens typically requires either a coordinatively unsaturated, strongly-reducing metal complex or a compound with a metal– metal bond. Since uranium complexes in the +IV oxidation state, are generally considered rather unreactive, the observed reaction of the here employed uranium(IV)/(IV) species with elemental chalcogens is fairly remarkable.


Introduction
5][16][17][18][19][20][21][22] This renewed interest in actinide reactivity with the chalcogens and chalcogenides canto some extentbe attributed to the importance of solid-state materials that are obtained from controlled pyrolysis of inorganic or organometallic chalcogenolates. 234,19,37 However, by utilizing an ylide-masked U(III) starting material that slows the rate of comproportionation, and thus prevents the formation of bridging chalcogenide complexes, the monomeric, terminal chalcogenide species [H 3 CPPh 3 ][(R 2 N) 3 U(E)] (E ¼ S, Se, Te; R ¼ SiMe 3 ) were synthesized recently by Hayton et al. 38 Surprisingly, despite the propensity of the heavier chalcogenides to catenate to rings and chains of various sizes, 32,33 there are only a few actinide complexes featuring polychalcogenide ligands.
The few existing complexes include the thorium penta-suldo complex [Cp* 2 ThS 5 ], that was obtained via salt metathesis reaction of [Cp* 2 ThCl 2 ] with Na 2 S 5 by Sattelberger et al., 2 and the homoleptic diselenido complex K 4 [U(Se 2 ) 4 ], synthesized by Kanatzidis et al. via molten salt synthetic techniques. 39dditionally, Boncella et al. were able to synthesize the two tetrachalcogenido complexes [{( t Bu 2 bpy)U(N t Bu) 2 (I)} 2 (m-h 2 :h 2 -E 4 )] (E ¼ S, Se) by activating elemental sulfur and selenium, respectively, with a dinuclear uranium(V/V) halide complex that serves as a two-electron reductant. 37erein, we report the syntheses, electronic properties, and molecular structures of a series of dinuclear uranium polychalcogenido complexes of the type [{(( Ad ArO) These new compounds can be synthesized in a remarkably controlled manner just by the appropriate choice of uranium precursor and the stoichiometric addition of the elemental chalcogen.

2À
-U(IV) entity adopts a M 2 L 2 -buttery structural motif, with a torsion angle of 125.02 and is thus strongly bent compared to the structurally similar bis-m-selenido complex [Na(DME) 3 ] 2 [{(( Ad ArO) 3 N)U IV } 2 (m-Se) 2 ] that has a torsion angle of 160.8 between the two uranium centers and the two Se 2À ligands. 14The average U-O Ar bond distances of 2.149 Å are slightly shorter than those observed in 2 (2.178, 2.179 Å) but are still within the range of U-O Ar bond distances in other complexes supported by the ( Ad ArO) 3 N 3À ligand. 14,20,21n attempts to further investigate the synthesis of complex 3, we added increasingly more than 2 equiv. of Se, ultimately leading to the formation of yet another, olive-green product.An X-ray diffraction analysis of single crystals obtained by slow diffusion of hexane into a saturated toluene solution revealed a dinuclear tetravalent uranium complex, namely [{(( Ad ArO) 3 N) U IV } 2 (m-h 3 :h 3 -Se 4 )] (4); now bridged by four selenium atoms with a formal oxidation state of Se 4 2À (Fig. 1, bottom).Hence, we adjusted the reaction conditions appropriately and added exactly 4 equiv. of selenium powder to [(( Ad ArO) 3 N)U III (DME)] and stirred the reaction solution for 3 days aer which olivegreen 4 forms (Scheme 1, bottom).Complex 4 was obtained aer ltration and drying of the solids in vacuo in 90% yield.1).2][53] Similar bond properties are known for polysulde ligands and can be attributed to charge localization at the rst and last selenium atom of the chain. 33,54,55Only three selenium atoms of the Se 4 2À ligand are coordinated to each uranium center in a m-h 3 :h 3 fashion.The U-Se bonds are all slightly asymmetric and range from 3.052(2) to 3.125(1) Å, which is marginally longer than the bond distances in 3 and other reported U-Se distances.and is structurally very similar to the bridging tetraselenido complex [( t Bu 2 bpy)U(N t Bu) 2 (I)] 2 (m-h 2 :h 2 -Se 4 ) synthesized by Boncella et al. 37 The dinuclear complex is situated on a crystallographic inversion center and exhibits C i molecular symmetry.Each uranium center in 5 adopts a distorted mono-capped trigonal prismatic coordination geometry.The U-Se bond lengths of 2.913(1) Å and 3.178(1) Å are indicative of a strong anionic U-Se1 bond and a weaker dative interaction U-Se2, respectively, an observation that is identical to that reported by Boncella and coworkers. 37Furthermore, similar to 4, compound 5 exhibits two shorter Se-Se bonds at 2.288(1) Å and one longer bond at 2.420(0) Å between Se2-Se3; the average U-O Ar bond distances of 2.135 Å are comparable with those observed for compound 4 (Table 1).Complexes 4 and 5 are unambiguously distinguishable by 1 H NMR spectroscopy.The signals of 4 are very sharp compared to the broadened signals of 5, which is likely due to the uxional nature of the coordinated THF solvent molecules.It should be noted that a second minor species (5.5%) in the investigated crystal of 5 was identied as [{(( Ad ArO) 3 N) U IV (THF)} 2 (m-h 2 :h 2 -Se 3 )] with a bridging Se 3 2À ligand (see ESI †). 56Unfortunately, this compound could not be synthesized reproducibly nor isolated in pure form.Attempts to isolate different products by adding even more equivalents of elemental selenium to compounds 4 and 5 have not been successful.This observation might be attributed to the decreasing solubility of complexes 4 and 5.

Synthesis and molecular structure of polysuldo-bridged uranium complexes
The m-suldo complex [{(( Ad ArO) 3 N)U IV (DME)} 2 (m-S)] ( 6) exhibits similar reactivity to the m-selenido bridged complex [{(( Ad ArO) 3 N)U IV (DME)} 2 (m-Se)] (2) if treated with 0.375 equiv. of elemental sulfur in benzene (Scheme 3).Immediate colour change of the reaction mixture to dark brown was observed and, within 5 minutes of further stirring, a dark brown solid precipitated.Aer 2 hours of stirring, the solution was ltered and the precipitate was washed with benzene and dried in vacuo to yield complex 7 analytically pure in 88% yield.An X-ray diffraction analysis of crystals grown by diffusing DME into a saturated THF solution revealed two crystallographically independent but chemically equivalent molecules in the asymmetric unit (Z ¼ 2), namely two dinuclear uranium complexes [{(( Ad ArO) 3 N)U} 2 (m-h 2 :h 2 -S 2 ) 2 ] (7A) (Fig. 3, top) and [{(( Ad ArO) 3 N)-U} 2 (m-h 2 :h 2 -S 2 )(m-h 1 :h 2 -S 2 )] (7B) (Fig. 3, bottom) that contain four bridging sulfur atoms.However, in contrast to 4, the bridging unit in both complexes 7A and 7B is not comprised of a S 4 2À chain-type ligand but of two independent S 2 moieties.In  2).These bond distances are at the shorter end of known S-S bond lengths observed in other uranium 1,19,50,[57][58][59][60][61] and transition metal complexes, [62][63][64][65][66] featuring the persuldo ligand S 2 2À .However, the very rare supersuldo complexes of the transition metals have shown a wider range for the S-S bond length, ranging from 1.944-2.0238][69][70] Accordingly, based on the bond metric of the S 2 moiety, an exact assignment (persulde vs. supersulde) remains impossible.Both ligands  6) and other bridging suldo complexes reported in the literature. 6,19,37he average U-O Ar bond distances of 2.104 Å and 2.106 Å, respectively, are signicantly shorter than those observed in 6, which is an observation that was also made for complexes 4 and 5. Complex 7B is slightly different from 7A as only one S 2 unit features the h 2 :h 2 -binding mode that was already observed in 7A.The second S 2 moiety exhibits a m-h 1 :h 2 binding mode, which results in both uranium centers being situated in a different coordination environment.The U1 ion has a coordination number of eight and features a distorted squareantiprismatic coordination geometry, while U2 is only sevencoordinate and adopts a distorted mono-capped octahedral coordination environment.The U-S bond lengths (2.684(2)-2.860(2)Å) and S-S bond distances (2.051(2), 2.044(2) Å) are all in the same range as observed in 7A (Table 2).Finally, the average U-O Ar bond distances of 2.110 Å and 2.087 Å can be compared to the observations made in complexes 4, 5, and 7A.We investigated if the persuldo complex [{(( Ad ArO) 3 N)U IV } 2 -(m-S 2 )], which might be a possible intermediate in the formation of 7, could be synthesized similarly to 3.However, all attempts to form a persuldo complex lead to the precipitation of 7.This selective formation of 7 likely is due to the S 8 ring structure of elemental sulfur as well as the low solubility of complex 7, which leads to an excess of sulfur atoms in the coordination vicinity of the uranium center and subsequent precipitation of 7. Likewise, trying to form the persuldo complex by reacting 7 with stoichiometric amounts of 1 in a comproportionation reaction only lead to intractable mixtures of different products, from which no pure compound could be isolated.

Electronic structure of polychalcogenido bridged U(IV/IV) complexes
In order to probe the formal oxidation state of the uranium ions and their electronic structures, we performed variable temperature (2-300 K) dc magnetization measurements on compounds 3, 4, and 5. SQUID measurements reveal magnetic moments of m eff ¼ 3.81 (3), 3.25 (4) and 3.99 ( 5) m B at 300 K, which decrease with decreasing temperature to m eff ¼ 0.43 (3), 0.55 (4), and 0.46 (5) m B at 2 K (Fig. 4).Accordingly, the observed magnetic moments and their temperature-dependency is characteristic of uranium(IV) complexes with an f 2 electron conguration and a 3 H 4 ground state. 14,21,71 investigation of the temperature-dependent behaviour of disuldo bridged complex 7 revealed a magnetic moment of m eff ¼ 2.33 m B at 300 K (per formula unit) that decreases to 0.37 m B at 2 K upon decreasing temperature (Fig. 5, top).While a lowtemperature magnetic moment of $0.4-0.5 m B is typically observed in uranium(IV) compounds, the unusually low magnetic moment of 2.33 m B at room temperature is more indicative of a uranium(V) species. 72Antiferromagnetic exchange coupling between two uranium centers can signicantly decrease the overall magnetic moment and is typically observed as a maximum in the plot of c M vs. T. 14,73 The magnetic susceptibility of 7, however, does not show a maximum (Fig. 5, bottom); and hence, antiferromagnetic exchange coupling is not evident for this compound but cannot be ruled out entirely.Furthermore, increased covalency of the U-Se bonds in 3-5 and the U-S bonds in 7, respectively, can also reduce the overall magnetic moment. 74The observed non-magnetic ground state of 7 at low temperature (m eff ¼ 0.37 B.M. at 2 K) allows for different interpretations, arising from several possible electronic congurations and magnetic exchange interactions within the [U(S 2 ) 2 U] unit: the ground state could result from non-interacting U(IV) f 2 ions (singlet G 1 ground state within the 3 H 4 ground manifold) with through-space, antiferromagnetically coupled supersulde (S 2 c À ) ligands or an entirely antiferromagnetically coupled system of U(IV) and bridging radical anions.Alternatively, and assuming that the bridging S 2 ligands are further reduced to diamagnetic persuldes, S 2 2À , the U(V) f 1 ions could be strongly antiferromagnetically coupled to yield the observed non-magnetic ground state at 2 K. Closer inspection of the c M vs. T plot 5, bottom) indeed reveals a plateau at approx.6][77] However, the current magnetization study does not allow for an unambiguous assignment of the U ions' and bridging S 2 ligands' oxidation states.Accordingly, CW X-band EPR spectroscopic studies were performed on samples of 7 in liquid and frozen toluene solution at room temperature and 7 K, respectively.However, samples of 7 are EPR silent under these conditions.This is not unusual and in principlenot in contrast with the formulation of 7 as U(IV/IV) with two bridging, radical anionic, supersuldo ligands, nor as paramagnetic U(V/V) with bridging, diamagnetic persul-do ligands.Previous studies have shown that both U(V) species (e.g., U(V) imidos) 78 and complexes of U(IV) with coordinated radical anions, such as carbon dioxide, 79 diphenyldiazomethane, 80 or benzophenone 81 are EPR inactive.These observations render attempts to assign the exact oxidation state of the uranium ions as well as the bridging ligands in complexes 7A and 7B difficult.Full understanding of the magnetic behavior of this compound and comparison to 1-6 requires collaborative XAS spectroscopic and DFT theoretical studies that are currently being planned and will be reported in due time.

Conclusion
Polychalcogenido complexes of actinidesespecially those featuring the heavier group 16 elements S, Se, and Teare an exceedingly rare class of compounds.Generally, low-valent, strongly-reducing metals are essential for the activation of elemental chalcogens.Herein we demonstrated, however, that by carefully choosing the reaction conditions, even relatively unreactive uranium(IV) complexes can be employed for controlled and stoichiometric elemental chalcogen activation; given the chelating ligand at the U(IV) ion provides sufficient stability and exibility.Thus, employing the sterically highly accommodating ( Ad ArO) 3 N 3À chelate, polyselenido complexes 3, 4, and 5 were synthesized via different pathways, starting from either the U(IV) complex (2 and 6) or the U(III) complex (1), respectively.These complexes feature m-Se  themselves and is independent from the metal centers that are basically serving as a stage for chalcogenide transformation.The reactivity of elemental sulfur with the respective U(III) and U(IV) complexes is distinctly different from that of selenium.The resulting complex [{(( Ad ArO) 3 N)U} 2 (m-S 2 ) 2 ] (7) features two bridging S 2 units instead of a single S 4 2À ligand as observed with selenium.These novel uranium polychalcogenido complexes are currently being investigated with respect to chalcogen atom transfer chemistry.
Alternatively, [{(( Ad ArO) 3 N)U IV (DME)} 2 (m-Se)] (2) can be treated with just 3 equiv.of selenium powder to obtain 4 with a similar yield.Evidently, 3 is still reactive towards elemental selenium powder and is able to insert two more selenium atoms into the bridging Se 2 2À unit of 3 to form the Se 4 2À ligand found in 4.This reactivity clearly shows the proclivity of selenium to catenate to chains as there is no redox chemistry involving the uranium centers and their oxidation state of +IV is retained (vide infra).The molecular structure of 4 shows two tetravalent uranium centers in a distorted mono-capped trigonal prismatic geometry.Two Se atoms of the Se 4 2À moiety are located trans and one Se atom is situated cis to the nitrogen anchor.In contrast to 3, the uranium centers in 4 are not coordinated by additional solvent molecules.The bridging Se 4 2À moiety features alternating Se-Se bond distances with two very similar bond lengths with Se1-Se2 at 2.316(2) and Se3-Se4 at 2.310(2) Å as well as a slightly larger bond distance of 2.438(1) Å for Se2-Se3 (Table

contrast to complexes 3 - 5 , 2 À
and within the scope of the present study, the oxidation state of the uranium centers (and chalcogenide ligands) in 7A and 7B cannot be unambiguously iden-tied (vide infra).Complexes 7A and 7B could be formulated either as two U(IV) centers with two bridging supersuldo S units or as two U(V) centers bridged by two persuldo S 2 2À ligands.Each of the eight-coordinate uranium centers in 7A adopts a distorted square-antiprismatic coordination geometry.The bridging S 2 fragments in 7A both feature an h 2 :h 2 -binding mode and show two almost identical S1-S2 and S3-S4 bond distances of 2.050(2) Å and 2.053(2) Å, respectively (Table

Fig. 3
Fig. 3 Molecular structures of uranium disulfide complex 7A (top) and uranium disulfide complex 7B (bottom) in crystals of 7$5DME.Adamantyl groups, H-atoms and co-crystallized solvent molecules are omitted for clarity.Thermal ellipsoids are at 50% probability.

Fig. 5
Fig. 5 Temperature-dependent SQUID magnetization data of polysulfido-bridged compound 7 as a plot of m eff vs. T (top) and c M vs. T (bottom).Data were corrected for underlying diamagnetism.