[UIII{N(SiMe2tBu)2}3]: A Structurally Authenticated Trigonal Planar Actinide Complex

We report the synthesis and characterization of the uranium(III) triamide complex [UIII(N**)3] [1, N**=N(SiMe2tBu)2−]. Surprisingly, complex 1 exhibits a trigonal planar geometry in the solid state, which is unprecedented for three-coordinate actinide complexes that have exclusively adopted trigonal pyramidal geometries to date. The characterization data for [UIII(N**)3] were compared with the prototypical trigonal pyramidal uranium(III) triamide complex [UIII(N“)3] (N”=N(SiMe3)2−), and taken together with theoretical calculations it was concluded that pyramidalization results in net stabilization for [UIII(N“)3], but this can be overcome with very sterically demanding ligands, such as N**. The planarity of 1 leads to favorable magnetic dynamics, which may be considered in the future design of UIII single-molecule magnets.

Ab initio calculations on [An III (CH 3 ) 3 ] (An = U, Np, Pu) [33] and [An III (NH 2 ) 3 ] (An= U, Np) [34] have shown that the involvement of An 6d orbitals in the UÀX (X= C, N) s components may be associated with pyramidalization in the absence of steric contributions. Thus, given the similar bonding within 1 and [U III (N") 3 ] together with the small U 6d/5f contributions to the UÀN s and p components, we suggest that the experimentally determined trigonal planar geometry of 1 results from steric interactions involving the large N** ligands. These interactions could predominate over crystal packing forces, which are often only approximately 10 kJ mol À1 . [35] We conclude that there are minor differences in bonding between 1 and [U III (N") 3 ], therefore, the planar geometry of 1 derives principally from steric effects involving the ligands.
The solution magnetic moment of 1 was calculated to be 2.59 m B in [D 6 ]benzene at 298 K by using the Evans method. [36] Magnetometry measurements on a powdered sample of 1 suspended in eicosane gave a magnetic susceptibility temperature product, cT, of 1.07 cm 3 Kmol À1 (2.92 m B ) at 298 K, [21] which corresponds well with the solution measurement considering weighing errors and the difference in phase. These values are lower than for a free-ion 5f 3 4 I 9/2 ground state (3.69 m B ), because not all crystal field levels are thermally occupied, [37] but are typical for U III complexes described in the literature (range 2.13-4.63 m B ). [8,15,22,25,26,30,38] The cT value of 1 decreases to 0.41 cm 3 Kmol À1 at 2 K; ac measurements give a low-temperature plateau in the in-phase c'T at 0.48 cm 3 Kmol À1 [21] consistent with thermal depopulation into a Kramers doublet ground state. [3,13] Low-temperature EPR spectra of 1 are consistent with U III , [27] and simulation gives g eff = 3.55, 2.97, and 0.553 for the ground Kramers doublet (the latter is observed at high field at X-band, but is beyond the magnetic field range at Q band; Figure 2 a).
Compound [U III (N") 3 ] is an SMM, [15] hence, we have performed low-temperature ac measurements on 1 to probe differences in the dynamic magnetic behavior as a result of the higher symmetry. Compound 1 is also an SMM, with clear frequency-dependent behavior (Figure 2 c and d). [21] Under the optimal dc field of 600 G, the magnetization relaxes much slower than in [U III (N") 3 ], and maxima in the out-of-phase susceptibility c''(T) are seen to significantly higher temperatures for 1 than for [U III (N") 3 ] at equivalent frequencies (e.g., 3.5 vs. 2.1 K, respectively, for 1.4 kHz). An Arrhenius treatment [21] of the higher-temperature ac data gives an energy barrier of U eff = 21.4 AE 0.2 K for 1. Although this is lower than that reported for [U III (N") 3 ] (31 K), the latter value was derived from an extremely limited temperature range [15] and should be treated with some caution. The relaxation time (t) at 2 K is 2.6 ms for 1; from the previously reported data [15] we find 0.3 ms for [U III (N") 3 ] at 2 K, an order of magnitude quicker. The pre-factor t 0 for 1 is greater by four orders of magnitude (3.1 10 À7 cf. 10 À11 s for [U III (N") 3 ]). [15] Moreover, the frequency dependence of c' and c" at 1.8 K for 1 [21] reveal a single relaxation process with a narrow distribution in relaxation times (a = 0.001-0.03 from Cole-Cole analysis), an order of magnitude lower than in [U III (N") 3 ] (a = 0.09-0.34). [15] In fact, the difference in dynamics is sufficient that magnetization hysteresis is observed for 1 at 1.8 K on a conventional superconducting quantum interference device (SQUID) magnetometer (Figure 2 In the trigonal planar geometry of 1, with no axial ligands, we expect a low J z state of U III to be stabilized by the crystal field. This is supported by the EPR analysis: if we assume a 4 I 9/2 ground term, [39] with g J = 8/11, the J z = AE 1/2 doublet is calculated to have g x,y = 3.65, g z = 0.73 (all other doublets have g x,y = 0), in good agreement with experiment. j J z j = 1/2 is also the ground doublet of the (pyramidal) 4f 3 complex [Nd III (N") 3 ] from optical studies. [40] Hence, 1 and [U III (N") 3 ] are SMMs despite their easy-plane anisotropy: this highlights the complexity of interpreting f-block relaxation data, [41] particularly when relatively low (tens of K) energy barriers are involved. At this stage, we can speculate that the "cleaner" and slower relaxation of 1 compared with [U III (N") 3 ] on flattening the geometry is because of quenched mixing. In D 3h j J z j = 1/2 cannot mix with any other doublet within the 4 I 9/2 term, whereas in C 3v , it can mix with both j J z j = 5/2 and 7/2.
To conclude, we have prepared and fully characterized an unprecedented trigonal planar actinide triamide complex. Differences in the spectroscopic and magnetic data between 1 and [U III (N") 3 ] can be attributed to differences in symmetry that may be useful to consider in the future design of U III SMMs with greater relaxation times. Computational analyses of 1 and [U III (N") 3 ] have shown only minor differences in their calculated bonding schemes, therefore, the energy gained by pyramidalization, which leads to favorable agostic M···SiÀC g interactions in [U III (N") 3 ], [8d, 32,33] can be overcome by sterically demanding ligands, such as N**.

Experimental Section
Synthesis of 1: THF (20 mL) was added to a precooled (À78 8C) mixture of [K{N(SiMe 2 tBu) 2 }] 2 (1.007 g, 1.5 mmol) and [U(I) 3 (THF) 4 ] (0.907 g, 1 mmol). The reaction mixture was allowed to warm to RT slowly with stirring over 48 h, with precipitation of a pale solid. Volatiles were removed in vacuo, and the dark purple solid was extracted with hexanes (3 10 mL). Recrystallization from hexanes (5 mL) at À30 8C gave 1 as dark purple needles (0.605 g, 62 %). 1  Low carbon values were obtained upon repeating the analysis multiple times on different batches and is ascribed to 1 being a silicon-rich molecule, as was observed previously. [42]