Contrasting behaviour under pressure reveals the reasons for pyramidalization in tris(amido)uranium(III) and tris(arylthiolate) uranium(III) molecules

Abstract

A range of reasons has been suggested for why many low-coordinate complexes across the periodic table exhibit a geometry that is bent, rather a higher symmetry that would best separate the ligands. The dominating reason or reasons are still debated. Here we show that two pyramidal UX3 molecules, in which X is a bulky anionic ligand, show opposite behaviour upon pressurisation in the solid state. UN″3 (UN3, N″ = N(SiMe3)2) increases in pyramidalization between ambient pressure and 4.08 GPa, while U(SAr)3 (US3, SAr = S-C6H2-tBu3−2,4,6) undergoes pressure-induced planarization. This capacity for planarization enables the use of X-ray structural and computational analyses to explore the four hypotheses normally put forward for this pyramidalization. The pyramidality of UN3, which increases with pressure, is favoured by increased dipole and reduction in molecular volume, the two factors outweighing the slight increase in metal-ligand agostic interactions that would be formed if it was planar. The ambient pressure pyramidal geometry of US3 is favoured by the induced dipole moment and agostic bond formation but these are weaker drivers than in UN3; the pressure-induced planarization of US3 is promoted by the lower molecular volume of US3 when it is planar compared to when it is pyramidal.

Synthesis and Characterization of Uranium Complexes Supported by Substituted Aryldimethylsilylanilide Ligands

“…The U–N distance must elongate for the arene ring to bind to U due to the aforementioned strain. The U–N distance of the arene-free ligand most likely shortens due to electrostatic compensation; however, both of the U–N distances are very similar to other U­(III)–N distances: U­(HMDS) 3 , 2.320(4) Å; , I , 2.295(10) to 2.361(9) Å; J , 2.340(19) Å. The structure from the purple plates, 1 · ( n -hexane ) , contains 1 equiv n -hexane disordered across an inversion center of the P -1 space group in the asymmetric unit.…”

An Iron–Sulfur Cluster with a Highly Pyramidalized Three-Coordinate Iron Center and a Negligible Affinity for Dinitrogen

“…Although somewhat planarized compared with a typical Fe in an Fe–S cluster, the pyramidalization of the three-coordinate Fe site is still notable from a broader coordination chemistry perspective. Pyramidalized geometries are common for d 0 and d 10 transition-metal, lanthanide, and actinide complexes, and have been attributed to several factors including imbuing an increased dipole moment and stabilizing dispersion interactions. − In constrast, pyramidalized geometries for three-coordinate Fe centers are rare, and the trigonal pyramidal site in 2 is the most pyramidalized three-coordinate Fe site reported in the Cambridge Structural Database . For the unique site in 2 , full planarization is likely disfavored by the constraints imposed by the remainder of the cluster; a planar Fe site would be forced to be unreasonably close to the other Fe centers in the cluster and would have unreasonably acute Fe–S–Fe angles at the S atoms bound to the unique site.…”

Pressure-Induced Coordination Number Transition in Lanthanide Mellitate Coordination Polymers: Structure and Spectroscopy

“…Although high-pressure structural studies of coordination complexes are scant, especially for f-elements, due to the difficulties associated with the sample being enclosed in a diamond anvil cell (DAC), there has been recent interest and advances in using high-pressure crystallographic techniques to characterize pressure-induced structural changes. − In some cases, computational structural optimizations within theoretical or experimental unit cell parameters have been used to predict changes in bond distances; , however, with specially designed DACs having a compact form and large opening angles, it is possible to obtain crystal structures at elevated pressures. , Due to the nature of this process, high-pressure SCXRD data typically suffer from low completeness and redundancy; however, with careful set-up and processing of data, it is possible to acquire meaningful results. − In this paper, we present the high-pressure SCXRD characterization of a cerium mellitate coordination polymer and measurement of the metal–ligand bond length changes as well as an unexpected coordination number transition.…”