Our knowledge of actinide chemical bonds lags far behind our understanding of the bonding regimes of any other series of elements. This is a major issue given the technological as well as fundamental importance of f-block elements. Some key chemical differences between actinides and lanthanides-and between different actinides-can be ascribed to minor differences in covalency, that is, the degree to which electrons are shared between the f-block element and coordinated ligands. Yet there are almost no direct measures of such covalency for actinides. Here we report the first pulsed electron paramagnetic resonance spectra of actinide compounds. We apply the hyperfine sublevel correlation technique to quantify the electron-spin density at ligand nuclei (via the weak hyperfine interactions) in molecular thorium(III) and uranium(III) species and therefore the extent of covalency. Such information will be important in developing our understanding of the chemical bonding, and therefore the reactivity, of actinides.

The lithium silylamides [Li(μ 3 -NHSiMe 2 Bu t )] 6 (1) and [Li(μ-NHSiPr i 3 )(THF)] 2 (2) were reacted with ClSiMe 3 , ClSiMe 2 Bu t , or ClSiPr i 3 to prepare a series of secondary silylamines by salt metathesis reactions. These were deprotonated with KH to afford the group 1 transfer agents [K{μ-N(SiMe 2 Bu t )(SiMe 3 )}(C 7 H 8 )] 2 (3), [{K[μ-N(SiPr i 3 )(SiMe 3 )]} 2 ] ∞ (4), [{K[μ-N(SiMe 2 Bu t ) 2 ]} 2 (C 7 H 8 )] ∞ (5), [K{N(SiPr i 3 )(SiMe 2 Bu t )}] ∞ (6), [K{N(SiPr i 3 ) 2 }] ∞ (7), and [K{N(SiPr i 3 ) 2 }(THF) 3 ] (8). The synthetic utility of these group 1 transfer agents has been demonstrated by their reactions with [Ln(I) 3 (THF) 4 ] (Ln = La, Ce) in various stoichiometries to yield heteroleptic [La{N(SiMe 2 Bu t )(SiMe 3 )} 2 (μ-I)] 2 (9) and homoleptic [Ln{N(SiMe 2 Bu t )(SiMe 3 )} 3 ] (Ln = La 10, Ce 11) and [La{N(SiMe 2 Bu t ) 2 } 3 ] (12). The very bulky silylamide ligands described herein can impart unusual geometries to their lanthanide complexes. Complexes 10−12 remarkably exhibit approximate planarity in the solid state rather than the more common trigonal pyramidal shapes observed in previously reported neutral homoleptic lanthanide silylamide complexes. Complexes 1−12 have been variously characterized by X-ray crystallography, NMR spectroscopy, FTIR spectroscopy, and CHN microanalysis. ■ INTRODUCTIONBulky monodentate alkali metal secondary amides have been employed ubiquitously in diverse research fields. 1 This can in part be attributed to their utility as strong bases, with favorable properties including relatively low nucleophilicity and ease of handling, commercial availability of precursors, and high solubility in hydrocarbon solvents. 2 Of these reagents, the silylamide {N(SiMe 3 ) 2 } − (N″) has received considerable attention as both a base and a ligand since the disclosure of synthetic routes to HN″, 3 LiN″, 4 NaN″, 5 and KN″. 5 The group 1 transfer agents have been widely used to prepare homoleptic three-coordinate p-, d-, and f-block complexes of the general formula [M III (N″) 3 ], as the bulky silyl groups engender low coordination numbers, even for relatively large M III cations. These coordinatively unsaturated complexes can exhibit interesting reactivity profiles. 6 In the solid state, these complexes are trigonal planar D 3h for group 13 (M = Al, Ga, In, Tl) 7 and the first-row transition metals (Ti−Co) 8 and trigonal pyramidal C 3v for group 15 (M = P, As, Sb, Bi), 9 lanthanides (M = Sc, Y, La, Ce−Lu; the group 3 metals are included as lanthanide mimics), 10 and actinides (M = U, Pu). 11 [Ln(N″) 3 ] (Ln = lanthanide) complexes exhibit a zero dipole moment in solution, indicating that they are trigonal planar in this phase, 10g and the scandium homologue, [Sc(N″) 3 ], is trigonal planar in the gas phase and pyramidal in the solid state, which is attributed to crystal packing effects. 12 Germane to this discussion, the related Ln(II) "ate" complexes [MLn(μ-N″) 2 (N″)] (M = Li, Na, K; Ln = Sm, Eu, Yb) 13 exhibit approximately trigonal planar geometries about the lanthani...

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Although the molecular chemistry of thorium is dominated by the +4 oxidation state accounts of Th(iii) complexes have continued to increase in frequency since the first structurally characterised example was reported thirty years ago. The isolation of the first Th(ii) complexes in 2015 and exciting recent Th(iii) and Th(ii) reactivity studies both indicate that this long-neglected area is set to undergo a rapid expansion in research activity over the next decade, as previously seen since the turn of the millennium for analogous U(iii) small molecule activation chemistry. In this perspective article, we review synthetic routes to Th(iii) and Th(ii) complexes and summarise their distinctive physical properties. We provide a near-chronological discussion of these systems, focusing on structurally characterised examples, and cover complementary theoretical studies that rationalise electronic structures. All reactivity studies of Th(iii) and Th(ii) complexes that have been reported to date are described in detail.

The redox chemistry of uranium is burgeoning and uranium(III) complexes have been shown to promote many interesting synthetic transformations. However, their utility is limited by their reduction potentials, which are smaller than many non-traditional lanthanide(II) complexes. Thorium(III) has a greater redox potential so it should present unprecedented opportunities for actinide reactivity but as with uranium(II) and thorium(II) chemistry, these have not yet been fully realized. Herein we present reactivity studies of two equivalents of [Th(Cp'') ] (1, Cp''={C H (SiMe ) -1,3}) with 4,4'-bipyridine or two equivalents of pyridine to give [{Th(Cp'') } {μ-(NC H ) }] (2) and [{Th(Cp'') } {μ-(NC H ) }] (3), respectively. As relatively large reduction potentials are required to effect these transformations we have shown that thorium(III) can promote reactions that uranium(III) cannot, opening up promising new reductive chemistry for the actinides.