Characterizing Pressure-Induced Uranium C=H Agostic Bonds

The diuranium(III) compound [UN′′2]2(μ-η6:η6-C6H6) (N′′=N(SiMe3)2) has been studied using variable, high-pressure single-crystal X-ray crystallography, and density functional theory. In this compound, the low-coordinate metal cations are coupled through π- and δ-symmetric arene overlap and show close metal=CH contacts with the flexible methyl CH groups of the sterically encumbered amido ligands. The metal–metal separation decreases with increasing pressure, but the most significant structural changes are to the close contacts between ligand CH bonds and the U centers. Although the interatomic distances are suggestive of agostic-type interactions between the U and ligand peripheral CH groups, QTAIM (quantum theory of atoms-in-molecules) computational analysis suggests that there is no such interaction at ambient pressure. However, QTAIM and NBO analyses indicate that the interaction becomes agostic at 3.2 GPa.


Synthetic details
[N''2U]2(μ-C6H6) 1 was synthesized as previously described (N'' = N(SiMe3)2. [1] All crystal manipulations were carried out under a dry, oxygen-free dinitrogen atmosphere using standard Schlenk techniques and coated in Fluorinert F70 oil before being transferred rapidly to the pressure cell.

Crystallographic details
CIF data are deposited with the CCDC, codes 1032195-9.
High-pressure single crystal experiments were carried out using a Merrill-Bassett diamond anvil cell (half-opening angle 40°), [2] equipped with Boehler-Almax diamonds with 600 µm culets and a tungsten gasket. [3] Fluorinert F70 was used as hydrostatic medium and a small ruby chip was loaded into the cell as the pressure calibrant with the ruby fluorescence used to measure the pressure. [4] Data collections were taken at ambient pressure, 0.24, 0.45, 1.15, 2.20, 3.35, 4.8 and 6.0 GPa for 1. Starting models for refinement of the high-pressure structures were taken from the ambient-pressure results. The program CRYSTALS [9] was used to refine the structures of 1 against F using the reflections with I > 2σ(I). Due to the low completeness of the data sets (a result of shading of reciprocal space by the pressure cell), thermal and vibrational similarity restraints were used for all the non-H and non-U atoms. All the H atoms were placed geometrically, and all the aromatic rings in the aryloxide and hydrazonido ligands for compound 1 refined as rigid groups. Compound 1 becomes twinned after the phase transition and the consequent symmetry loss meant all the C atoms in the new phase had to be refined isotropically. Additional details are reported in Table SI1.
The peripheral X ligands in 1 adopt an eclipsed geometry. This is in contrast to the bis aryloxide complexes, [U(ODtbp)2]2(C6H5R) (R = H, Me) for which a variety of structures that differ in the identity of the central arene and the presence and identity of lattice solvent have been collected and show a range in the angle between the two UX2 planes from 53.5° to 89.49°. This suggests that relatively subtle changes in crystal packing can influence the geometry at the metal.

Details for the animation of the compression of [UN" 2 ] 2 (μ-C 6 H 6 ) 1
The movie associated with the SI shows the compression series for 1. C4 and C10 are highlighted in orange. The first effect of the phase change is to collapse interstitial voids. C4 and 10 face each other across the void, and the N" ligands, which were quite distant, are forced to 'butt' into each other. In particular, the methyl groups C5 and C12, which are positioned on the upper surface of the plots, are pushed right into each other. C4 and C10, which are part of the same SiMe3 groups are pushed closer to the U centres in order to accommodate this strain. The data are insufficiently precise to identify whether the C-C bond lengths in the arene in 1 are lengthened by any degree.

Quantum chemical calculations
Spin-unrestricted density functional theory calculations were performed on 1 with the Gaussian 09 code, Revision C.01. [10] A (14s 13p 10d 8f 6g)/[10s 9p 5d 4f 3g] segmented valence basis set with Stuttgart-Bonn variety relativistic pseudopotential was used for uranium, [11] and the cc-pVTZ basis sets of Dunning for the other elements. The PBE functional was employed, [12] in conjunction with the ultrafine integration grid and the standard SCF convergence criterion (10 -8 ).
For each of the structures at the six different pressures (ambient, 0.8, 1.3, 1.8, 2.3 and 3.2 GPa), the positions of the heavy atoms were fixed to those obtained experimentally, and those of the H atoms were optimized using the standard geometry convergence criteria. QTAIM analyses were performed using the AIMALL program package, [13] with .wfx files generated in Gaussian used as input. Natural bond orbital analyses were performed using the GenNBO6 code, [14] using .47 files from G09 as input.

Computational coordinates
Converged