The utility of the sterically bulky terphenylthiolate ligand, (SAr iPr6 ) 1À in expanding uranium reductive chemistry has been explored.

Reduction of U(SAr iPr6 ) 2 I forms the U(II) complex, U(SAr iPr6 ) 2 , in which the metal is protected by the flanking arene rings of the ligand, but they move out of the way to accommodate the four electron reduction of PhNQ Q QNPh to form the U(VI) bis(imido) product U(SAr iPr6 ) 2 (Q Q QNPh) 2 (THF) 2 . The KC 8 reduction of U(SAr iPr6 ) 2 generates a more reduced complex, KU(l-SAr iPr6 ) 2 , initially identified by a À2.55 V vs. Fc + /Fc electrochemical reduction event in THF.

The reductive redox chemistry of uranium is receiving increasing amounts of attention following the isolation of numerous examples of U(II) complexes [1][2][3][4][5][6][7][8] and their reduction products identified as U(I) synthons. [8][9][10] In addition, a U(I) metallocene anion, [(C 5 i Pr 5 ) 2 U] 1À , has been reported. 11 As shown in Fig.

1, all of these ''U(I)'' examples have involved ligands with first row main group donor atoms, C, N, or O. In efforts to extend the coordination chemistry of low oxidation state actinides to second row donor atoms, we have examined the chemistry of uranium with the sterically bulky terphenylthiolate ligand, [SC 6 H 3 -2,6-(C 6 H 2 -2,4,6-i Pr 3 ) 2 ] 1À , (SAr iPr6 ) 1À .

The (SAr iPr6 ) 1À ligand and related (OAr iPr6 ) 1À and (NHAr iPr6 ) 1À ligands were extensively studied since the 1990s by Power, et al. [12][13][14] with transition metals [15][16][17][18][19][20][21] and main group elements 22,23 to stabilize low oxidation state, low coordinate complexes. Niemeyer, et al. demonstrated in the early 2000s that the (SAr iPr6 ) 1À ligand stabilizes unsolvated f-block complexes Ln(SAr iPr6 ) 2 for Sm(II), Eu(II), and Yb(II) 24,25 that have Z 6 -interactions with the flanking aryl rings. The rare-earth complexes Sm(OAr iPr6 ) 2 26 and Ln[N(H)Ar iPr6 ] 2 (Ln = Sc, Y La, Sm, Eu, Tm, and Yb) 27,28 have similar structures. We recently employed the (SAr iPr6 ) 1À ligand to isolate the first examples of Ln(II) complexes of La, Nd, and Tm 29 with second row main group donor atoms and now extend this chemistry to uranium since the only U(II) complex with this type of ligand was U[N(H)Ar iPr6 ] 2 . 4 While this work was in progress, U(IV) and U(III) complexes of (SAr iPr6 ) 1À were published. 30 The reaction of [K(m-SAr iPr6 )] 2 12 with UI 3 31 in Et 2 O affords the purple uranium(III) complex U(SAr iPr6 ) 2 I, 1, in 74% yield, eqn (1). Analogous reactions by Goodwin, et al. with chloride and borohydride starting materials were shown to be more complicated although U(SAr iPr6 ) 2 (BH 4 ) was readily prepared. 30 The X-ray crystal structure of 1, Fig. S1 (ESI †), shows the uranium atom sandwiched by two Z 6 -arene rings with the iodide and two sulfur donor atoms in between in a trigonal arrangement. The complexes Ln(EAr iPr6 ) 2 X (E = N(H), S, Se; X = Cl, I), 24,[27][28][29]32 U[N(H)Ar iPr6 ] 2 I, 4 and U(SAr iPr6 ) 2 (BH 4 ) 30 share this structural motif. Metrical parameters are discussed in the ESI. †

Stoichiometric reduction of purple 1 with KC 8 in Et 2 O, eq. 2, generated the dark green neutral uranium(II) thiolate, U(SAr iPr6 ) 2 , 2, in 84% yield. Slow evaporation of an n-hexane solution of the highly soluble 2 provided green-brown needles suitable for X-ray crystallography.

(2)

The structure of 2, Fig. 2, contains 2.763(4) and 2.772(4) Å U-S bond lengths similar to the 2.7590 (5) 4 The closer ring has a boat-like distortion from planarity with a fold angle of 191. The C-C bond distances in the two Z 6 -arene rings range between 1.39(2)-1.46(2) Å, but the large error limits preclude further analysis.

The electronic spectrum of 1 features an absorbance maximum at 475 nm while that of 2 shows only a broad absorbance across the visible spectrum that sharply increases starting at ca. 500 nm (see ESI †). These are similar to the amide analogs. 4 The Evans method 298 K solution magnetic moment of 2 was found to be 2.7-2.8 m B , which can be compared with the 2.3-2.4 m B moments reported for 5f 4 U(II) compounds 2,4,8 and the 2.26 m B moment reported for 5f 3 6d 1 {[(C 5 H 3 (SiMe 3 ) 2 )] 3 U II } 1À . 3 The ability of 2 to participate in multielectron reduction chemistry was demonstrated by its reaction with azobenzene, eq 3. This reaction yielded brown crystals of the U(VI) transbis(imido) product U(SAr iPr6 ) 2 (=NPh) 2 (THF) 2 , 3, in a reaction which is a formal U(II) to U(VI) four-electron transformation. The four-electron reduction of PhNQNPh at a single metal was first observed for a 5d 4 W(II) Z 6 -arene complex 33 and was recently extended to f-element chemistry by the reaction of [{(Me 3 Si) 2 N} 3 U II ] 1À and ''masked U(II)'' species with azobenzene. [34][35][36] (3)

The structure of 3, Fig. 3, is inversion symmetric with an approximately octahedral geometry around the U atom. The short 1.928(3) Å UQN bonds are typical of U(VI) bis(imido) compounds [37][38][39][40][41] and the 2.7081(6) Å U-S bonds are close to the 2.7297(7) Å U-S distances in U(SPh) 2 (QN t Bu) 2 (py) 2 . 39 Notably, the flanking Z 6 -arene ligands have completely disassociated from the U atom, demonstrating that the steric bulk protecting the U(II) center in 2 can readily move out of the way to accommodate reactivity.

The structure of 4, Fig. 4, is strikingly similar to the group 1 thiolate dimers {M(m-SAr iPr6 )} 2 (M = Li-Cs) 12 in which the metal atoms of the M 2 (m-S) 2 core are complexed by flanking aryl rings from each terphenyl ligand. The S-U-S angle has narrowed substantially to 62.78(6)1 from 132.51 in 2. The 2.887(2) and 2.902(2) Å U-S distances in 4 are ca. 0.1-0.2 Å longer than those in 1, 2, 3, and the related U(IV) and U(III) (SAr iPr6 ) 1À compounds 30 as expected for bridging ligands.

The K and U in 4 are structurally distinguished by the nature of their coordinated terphenyl flanking arenes. The rings near K in 4 retain their planarity with long 3.13(1) and 3.15(1) Å K-Cnt distances, while those oriented toward U have shorter U-Cnt distances of 2.32(1) and 2.42(1) Å and have undergone boat-like distortions with fold angles of 22.7(7)1 and 7.4(7)1, respectively. This distortion is consistent with d-back-bonding from the U orbitals to the arene p* orbitals and partial reduction of the arene ring. 9,28,29,[44][45][46][47] As in 2, the error limits on the 1.39(1) to 1.45(1) C-C bond distances in the rings are too large to provide statistically meaningful comparisons.

The stepwise reduction of 1 to 2 and then 4 with KC 8 is in marked contrast to the synthesis of U[N(H)Ar iPr6 ] 2 which was prepared in high yield from U[(N(H)Ar iPr6 ) 2 I] using an excess of KC 8 . 4 This difference could be due to increased flexibility of the C-S-U linkages compared to C-N(H)-U which allows for each metal to be complexed by two arene groups and bridging thiolate groups. Differences in the structures of the potassium thiolate and amide salts are in support of this. While [K(m-SAr iPr6 )] 2 has a dimeric structure, 12 the potassium amide crystallizes as the monomer [K(Et 2 O) 2 (NHAr iPr6 )]. 48 The UV-visible spectrum of 4 showed only an absorbance at 280 that tails into the visible with a shoulder feature at ca. 400 nm. No resonances attributable to 4 were observed in the 1 H NMR spectrum in benzene and the 298 K Evans method magnetic moment was found to be 2.3-2.4 m B . This is similar to the 2.20-2.46 m B moments reported for the formally ''U(I)'' species with reduced arene ligands, 8-10 whereas [(C 5 i Pr 5 ) 2 U I ] À has a 5.35 m B moment at 300 K and was assigned a 5f 3 6d 1 (7s/6d) 1 ground state. 11 Electronic structure calculations using density functional theory (DFT) provided optimized structures that matched the X-ray data for 2 and 4 (see ESI † for details). Calculations on 2 indicated that the disparate U-Cnt distances and UV-visible spectrum were consistent with a quintet ground state. Natural population analysis (NPA) of the spin density indicated unpaired electron populations of 2.83 in U f orbitals, 0.39 in U d orbitals, and 0.35 in the proximal arene ring. Quartet and sextet ground states for 4 were found to be within 5 kcal mol À1 with the quartet being lowest and providing the best match to the X-ray and UV-visible data. The quartet ground state features three electrons in U 5f orbitals that interact with the arene rings, and a doubly-occupied d-bonding combination, Fig. 5, derived from two 6d orbitals with strong overlap with arene p* orbitals in the proximal ring. As recently pointed out in a review of uranium arene interactions, 45 more extensive physical measurements will be needed in the future to establish the detailed nature of the uranium arene interactions in 4.

In conclusion, we have shown that the hexa-isopropylterphenylthiolate ligand allows the isolation of a neutral uranium(II) thiolate complex which can effect four electron reduction chemistry. Complex 2 is readily reduced further by one electron to the bimetallic thiolate complex KU(m-SAr iPr6 ) 2 , 4. Complex 4 has significant interactions between U and the p* orbitals of the coordinated arene rings which are distorted from planarity that complicate the definitive identification of formal oxidation state. Further investigations into the electronic description of this complex and its utility in multi-electron reactivity are underway.