Most homogeneous catalysis relies on the design of metal complexes to trap and convert substrates or small molecules to value-added products. Organometallic lanthanide compounds first gave a tantalizing glimpse of their potential for catalytic CH bond transformations with the selective cleavage of one CH bond in methane by bis(permethylcyclopentadienyl)lanthanide methyl [(η⁵-C₅Me₅)₂Ln(CH₃)] complexes some 25 years ago. Since then, numerous metal complexes from across the periodic table have been shown to selectively activate hydrocarbon CH bonds, but the challenges of closing catalytic cycles still remain; many f-block complexes show great potential in this important area of chemistry.

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 metalCH 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.

Homogene Katalyse hängt zu erheblichen Teilen von der Synthese von Metallkomplexen ab, die in der Lage sind, (niedermolekulare) Substrate zu binden und zu wertvolleren Produkten umzusetzen. Einen ersten Vorgeschmack der katalytischen C-H-Aktivierung mit metallorganischen Lanthanoidkomplexen gab es vor etwa 25 Jahren, als es gelang, mit Bis(pentamethylcyclopentadenyl)lanthanoid-Methylkomplexen [(η⁵-(C₅Me₅)₂Ln(CH₃)] eine C-H-Bindung von Methan zu spalten. In der Zwischenzeit wurden zahlreiche Metallkomplexe aus dem ganzen Periodensystem gefunden, die selektiv C-H-Bindungen aktivieren können, aber die Herausforderung, einen geschlossenen Katalysezyklus zu etablieren, bleibt bestehen. Viele f-Block-Komplexe scheinen aber über großes Potenzial auf diesem wichtigen Gebiet zu verfügen.

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 metalCH 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.

Simple and versatile routes to the functionalization of uranyl-derived U^V–oxo groups are presented. The oxo-lithiated, binuclear uranium(V)–oxo complexes [{(py)₃LiOUO}₂(L)] and [{(py)₃LiOUO}(OUOSiMe₃)(L)] were prepared by the direct combination of the uranyl(VI) silylamide “ate” complex [Li(py)₂][(OUO)(N”)₃] (N”=N(SiMe₃)₂) with the polypyrrolic macrocycle H₄L or the mononuclear uranyl (VI) Pacman complex [UO₂(py)(H₂L)], respectively. These oxo-metalated complexes display distinct UO single and multiple bonding patterns and an axial/equatorial arrangement of oxo ligands. Their ready availability allows the direct functionalization of the uranyl oxo group leading to the binuclear uranium(V) oxo–stannylated complexes [{(R₃Sn)OUO}₂(L)] (R=nBu, Ph), which represent rare examples of mixed uranium/tin complexes. Also, uranium–oxo-group exchange occurred in reactions with [TiCl(OiPr)₃] to form U-OC bonds [{(py)₃LiOUO}(OUOiPr)(L)] and [(iPrOUO)₂(L)]. Overall, these represent the first family of uranium(V) complexes that are oxo-functionalised by Group 14 elements.

Oxidative halogenation with trityl chloride provides convenient access to Ce^IV and U^IV chloroamides [M(N{SiMe₃}₂)₃Cl] and their N-heterocyclic carbene derivatives, [M(L)(N{SiMe₃}₂)₂Cl] (L=OCMe₂CH₂(CNCH₂CH₂NDipp) Dipp=2,6-iPr₂C₆H₃). Computational analysis of the bonding in these and a fluoro analogue, [U(L)(N{SiMe₃}₂)₂F], provides new information on the covalency in this relative rare oxidation state for molecular cerium complexes. Computational studies reveal increased Mayer bond orders in the actinide carbene bond compared with the lanthanide carbene bond, and natural and atoms-in-molecules analyses suggest greater overall ionicity in the cerium complexes than in the uranium analogues.

The reaction of a chiral racemic bidentate ligand HL¹ (tBu₂P(O)CH₂CH(tBu)OH) with mid to late trivalent lanthanide cations affords predominantly homochiral lanthanide complexes (RRR)-[Ln(L¹)₃] and (SSS)-[Ln(L¹)₃]. A series of reactions are reported that demonstrate that the syntheses are under thermodynamic control, and driven by a ligand ‘self-recognition’ process, in which the large asymmetric bidentate L¹ ligands pack most favourably in a C₃ geometry around the lanthanide cation. The synthesis of bis(L¹) adducts [Ln(L¹)₂X] (X=N(SiMe₃)₂, OC₆H₃tBu-2,6) is also reported. Analysis of the diastereomer mixtures shows that homochiral (L¹)₂ complexes are favoured but to a lesser extent. The complexes Ln(L¹)₃ and [Ln(L¹)₂(OC₆H₃tBu-2,6)] have been studied as initiators for the polymerization of ε-caprolactone and its copolymer with lactide, glycolide and its copolymer with lactide, and ε-caprolactam.

A new and modular route to bidentate ligands that combines an alkoxide with a saturated backbone N-heterocyclic carbene (NHC) is presented. The bi(heterocyclic) compounds are formally the addition product of a saturated NHC and the alcohol group of the N-functionalised arm. Using these compounds, the synthesis and structural characterisation of the first electropositive metal complexes of saturated N-heterocyclic carbenes has been achieved, and examples structurally characterised for the yttrium(III) and the uranyl [UO₂]²⁺ cations.