Characterizing Pressure‐Induced Uranium CH Agostic Bonds

Abstract

The diuranium(III) compound [UN'' 2 ] 2 (m-h 6 :h 6 -C 6 H 6 )( N '' = N(SiMe 3 ) 2 )h as been studied using variable, high-pressure single-crystal X-ray crystallography,and density functional theory .Inthis compound, the low-coordinate metal cations are coupled through p-and d-symmetric arene overlap and show close metalÀCH contacts with the flexible methyl CH groups of the sterically encumbered amido ligands.The metalmetal separation decreases with increasing pressure,b ut the most significant structural changes are to the close contacts between ligand CH bonds and the Ucenters.A lthough the interatomic distances are suggestive of agostic-type interactions between the Ua nd ligand peripheral CH groups,Q TAIM (quantum theory of atoms-in-molecules) computational analysis suggests that there is no such interaction at ambient pressure.H owever,Q TAIM and NBO analyses indicate that the interaction becomes agostic at 3.2 GPa.Simple,low-coordinate U III complexes have been recently reported to show arich reactivity with inert small molecules. [1] This reactivity is despite the fact that in the solid state, structural studies show close contacts between the metal and peripheral ligand Cand Hatoms that sometimes protect ap otential coordination site and sometimes simply block further reactions.F or example,h ydrocarbon solutions of [U(N'') 3 ](N'' = N(SiMe 3 ) 2 ) [2] (containing three UÀC SiMe3 close contacts,a verage distance = 3.047 ) reductively couple CO, [3] whereas those of [U{N(SiMe 2 Ph) 2 } 3 ]( containing three U À C ipsoPh close contacts,average distance = 3.093 ) [4] do not.Arguably the most useful homogeneous CÀHb ond functionalization reactions currently being developed for dblock metal catalysts rely on CH metallation of ab ound substrate. [5] Similarly,U complexes that show CH metallation [6] have been developed into catalysts for N-heterocycle coupling,f or example. [7] TheU III aryloxide [{(ArO) 3 tacn}U-(cC 6 H 12 )] (tacn = 1,4,7-triazacyclonane) shows an intermolecular CÀHc ontact to am olecule of cyclohexane solvent with aU À Cd istance of 3.864 (7) a nd an h 2 -CH interaction suggested by calculations. [8] More generally,a gostic interactions between uranium centers and ligand CH groups are often invoked from inspection of close metal-ligand contacts in X-ray structures. [9] No routine method exists for assessing the strength and influence of weak interactions to confirm ag enuine agostic interaction in paramagnets,w here traditional NMR spectral methods do not work [10] and neutron diffraction studies are not readily available. [11] Computationally,t he electron-density-basedq uantum theory of atoms-inmolecules (QTAIM), with its simple definition of achemical bond, has been successfully employed to identify agostic interactions. [12] Systems in which energetically competitive structures are related by subtle changes,such as agostic interactions,may be sensitive to external conditions such as pressure.H igh pressure has previously shown unconventional behavior in coordination compounds...

Instantaneous and Phosphine-Catalyzed Arene Binding and Reduction by U(III) Complexes

“…16 We then used variable pressure crystallography to study the compressibility of the structure, recognizing that the covalent bonds in diamond are extremely incompressible whilst ionic sodium chloride is highly compressible; even at gigapascals pressure the solid state structure showed only a minimal shortening of the metal-arene distance, indicative of a less compressible covalent interaction. 42 At the other extreme, arguments based on a combination of principally ionic bonding and simple Hückel aromaticity theory would suggest complexes containing a 4 -(10 π-electron) charge on the bridging arene and higher metal oxidation state (U V ), i.e. X3U V (η 6 -arene)U V X3 and a tetraanionic bridging C6 ring.…”

Expanding the Library of Uranyl Amide Derivatives: New Complexes Featuring the tert-Butyldimethylsilylamide Ligand

“…The strong Si−H coupling constant indicates that despite the proximity of the Si−H bonds to the uranium center, no secondary Si−H interactions are noted in this case. This is surprising, given the observation of Si−H secondary interactions on lanthanide derivatives of this ligand, 19,20 as well as of C−H agostic bonds of −N(SiMe 3 ) 2 on low-valent uranium, 27 both of which give insight into unique f-orbital participation in bonding. 28 To remove the lithium countercation from the coordination sphere, 1 was treated with 12-crown-4, which produced [Li(12crown-4) 2 ][UO 2 (N(SiHMe 2 ) t Bu) 3 ] (1-crown) as an orange solid in quantitative yield.…”

Tuning the Kinetic Inertness of Bi3+ Complexes: The Impact of Donor Atoms on Diaza-18-Crown-6 Ligands as Chelators for 213Bi Targeted Alpha Therapy

“…88,89 To avoid this issue, the energy density parameters V(r), G(r), and H(r), the ratio of potential and kinetic energies |V|/G, and the normalized total energy density H/ρ are commonly used to interpret the nature of bonding interactions in heavy metal complexes. [90][91][92][93][94][95] Complete QTAIM results are compiled in Tables 5 and S3-S8. In agreement with our X-ray crystallographic and NBO analysis, the interactions of the Bi 3+ center with the crown ether oxygen (O1-O4) and amine nitrogen (N1 and N3) atoms of the diaza-18-crown-6 core are almost purely ionic nature, as evidenced by the values of H(r) ≥ 0 and |V|/G < 1 (Table S3- S8).…”