In recent work we have reported the synthesis and physical properties of near-linear Ln(II) (Ln = lanthanide) complexes utilizing the bulky bis(silylamide) {N(SiiPr3)2}. Herein, we synthesize trigonal-planar Ln(II) complexes by employing a smaller bis(silylamide), {N(SitBuMe2)2} (N**), to study the effects of this relatively rare Ln geometry/oxidation state combination on the magnetic and optical properties of complexes. We show that the charge-separated trigonal-planar Ln(II) complexes [K(2.2.2-cryptand)][Ln(N**)3] (Ln = Sm (1), Eu (2), Tm (3), Yb (4)) can be prepared by the reaction of 1.5 equiv of [{K(N**)}2] with LnI2THF2 (Ln = Sm, Yb) or LnI2 (Ln = Eu, Tm) and 1 equiv of 2.2.2-cryptand in Et2O. Complex 3 is the first structurally characterized trigonal-planar Tm(II) complex. In the absence of 2.2.2-cryptand, [K(DME)3][Sm(N**)3] (5) and [Ln(N**)2(μ-N**)K(toluene)2] (Ln = Sm (6), Eu (7)) were isolated in the presence of DME (dimethoxyethane) or toluene, respectively. The 1:1 reaction of [{K(N**)}2] with LnI2THF2 (Ln = Sm, Yb) in THF gave the four-coordinate pseudo-tetrahedral Lewis base adducts [Ln(N**)2(THF)2] (Ln = Sm (8), Yb (9)) and the cyclometalated complex [Yb(N**){N(SitBuMe2)(SitBuMeCH2)}(THF)] (10). Complexes 1–10 have been characterized as appropriate by single-crystal XRD, magnetic measurements, multinuclear NMR, EPR, and electronic spectroscopy, together with CASSCF-SO and DFT calculations. The physical properties of 1–4 are compared and contrasted with those of closely related near-linear Ln(II) bis(silylamide) complexes.

A series of lanthanide complexes bearing organic radical ligands, [Ln(CpR)2(bipy·–)] [Ln = La, CpR = Cptt (1); Ln = Ce, CpR = Cptt (2); Ln = Ce, CpR = Cp″ (3); Ln = Ce, CpR = Cp‴ (4)] [Cptt = {C5H3tBu2-1,3}−; Cp″ = {C5H3(SiMe3)2-1,3}−; Cp‴ = {C5H2(SiMe3)3-1,2,4}−; bipy = 2,2′-bipyridyl], were prepared by reduction of [Ln(CpR)2(μ-I)]2 or [Ce(Cp‴)2(I) (THF)] with KC8 in the presence of bipy (THF = tetrahydrofuran). Complexes 1–4 were thoroughly characterized by structural, spectroscopic, and computational methods, together with magnetism and cyclic voltammetry, to define an unambiguous Ln(III)/bipy·– radical formulation. These complexes can act as selective reducing agents; for example, the reaction of 3 with benzophenone gives [{Ce(Cp”)2(bipy)}2{κ2-O,O′-OPhC(C6H5)CPh2O}] (7), a rare example of a “head-to-tail” coupling product. We estimate the intramolecular exchange coupling for 2–4 using multiconfigurational and spin Hamiltonian methods and find that the commonly used Lines-type isotropic exchange is not appropriate, even for single 4f e–/organic radical pairs.

Following our report of the first near-linear lanthanide (Ln) complex, [Sm(N††)2] (1), herein we present the synthesis of [Ln(N††)2] [N†† = {N(SiiPr3)2}; Ln = Eu (2), Tm (3), Yb (4)], thus achieving approximate uniaxial geometries for a series of “traditional” LnII ions. Experimental evidence, together with calculations performed on a model of 4, indicates that dispersion forces are important for stabilization of the near-linear geometries of 1–4. The isolation of 3 under a dinitrogen atmosphere is noteworthy, given that “[Tm(N″)(μ-N″)]2” (N″ = {N(SiMe3)2}) has not previously been structurally authenticated and reacts rapidly with N2(g) to give [{Tm(N″)2}2(μ-η2:η2-N2)]. Complexes 1–4 have been characterized as appropriate by single-crystal X-ray diffraction, magnetic measurements, electrochemistry, multinuclear NMR, electron paramagnetic resonance (EPR), and electronic spectroscopy, along with computational methods for 3 and 4. The remarkable geometries of monomeric 1–4 lead to interesting physical properties, which complement and contrast with comparatively well understood dimeric [Ln(N″)(μ-N″)]2 complexes. EPR spectroscopy of 3 shows that the near-linear geometry stabilizes mJ states with oblate spheroid electron density distributions, validating our previous suggestions. Cyclic voltammetry experiments carried out on 1–4 did not yield LnII reduction potentials, so a reactivity study of 1 was performed with selected substrates in order to benchmark the SmIII → SmII couple. The separate reactions of 1 with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), azobenzene, and benzophenone gave crystals of [Sm(N††)2(TEMPO)] (5), [Sm(N††)2(N2Ph2)] (6), and [Sm(N††){μ-OPhC(C6H5)CPh2O-κO,O′}]2 (7), respectively. The isolation of 5–7 shows that the SmII center in 1 is still accessible despite having two bulky N†† moieties and that the N-donor atoms are able to deviate further from linearity or ligand scrambling occurs in order to accommodate another ligand in the SmIII coordination spheres of the products.

The preparation of silylamide Hauser base (R2NMgX; X = halide) and amido-Grignard (R2NMgR) complexes from simple Grignard reagents using [K{N(SiMe2tBu)2}]n, [K{N(SiMe2tBu)(SiiPr3)}]n and [K{N(SiiPr3)2}]n, and their parent silylamines, was explored. Both salt metathesis and protonolysis routes proved ineffective with allylmagnesium chloride as a starting material due to complex Schlenk equilibria, with [Mg(NRR′)(μ-Cl)(THF)]2 (NRR′ = {N(SitBuMe2)2}−, 1; {N(SitBuMe2)(SiiPr3)}−, 2; {N(SiiPr3)2}−, 3) and [Mg{N(SiiPr3)2}(μ-C3H5)]∞ (4) identified as minor products. In contrast, salt metathesis protocols using potassium silylamides and methylmagnesium iodide gave [Mg(NRR′)(μ-CH3)]2 (NRR′ = {N(SitBuMe2)2}−, 7a; {N(SitBuMe2)(SiiPr3)}−, 8; {N(SiiPr3)2}−, 9) and [Mg{N(SitBuMe2)2}(CH3)(DME)] (7b), with [Mg{N(SitBuMe2)2}(μ-I)(THF)]2 (10) isolated as a side-product during the preparation of 7a. Unusually, methylmagnesium iodide, di-n-butylmagnesium and 7–9 did not react with HNRR′ under the conditions we employed. The synthesis of [Na{N(SitBuMe2)2}(THF)]2 (5a) and [Na{N(SitBuMe2)2}(DME)2] (5b) from benzyl sodium and HN(SitBuMe2)2, and a solvent-free structure of [K{N(SitBuMe2)2}] (6), are also reported. Complexes 1, 5b, 7a, 7b, 8, 9 and 10 are fully characterised by single crystal XRD, multinuclear NMR and IR spectroscopy and elemental analysis, whereas complexes 2–4, 5a and 6 were identified by XRD only.

[Th(Cp′′)3] (Cp′′ = {C5H3(SiMe3)2-1,3}) activates P4 to give [{Th(Cp′′)3}2(μ–η1:η1-P4)] (1), which has an unprecedented cyclo-P4 binding mode. DFT studies were performed on a model of 1 to probe the bonding in this system.

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.

We report the first near-linear bis(amide) 4f-block compound and show that this novel structure, if implemented with dysprosium(III), would have unprecedented single molecule magnet (SMM) properties with an energy barrier, Ueff, for reorientation of magnetization of 1800 cm−1.

The substituted cyclopentadienyl group 1 transfer agents KCp′′, KCp′′′ and KCptt (Cp′′ = {C5H3(SiMe3)2-1,3}−; Cp′′′ = {C5H2(SiMe3)3-1,2,4}−; Cptt = {C5H3(tBu)2-1,3}−) were prepared by modification of established procedures and the structure of [K(Cp′′)(THF)]∞·THF (1) was obtained. KCp′′ and KCptt were reacted variously with [Ln(I)3(THF)4] (Ln = La, Ce) in 2:1 stoichiometries to afford monomeric [La(Cp′′)2(I)(THF)] (2a·THF) and the dimeric complexes [La(Cp′′)2(μ-I)]2 (2a), [Ce(Cp′′)2(μ-I)]2 (2b) and [Ce(Cptt)2(μ-I)]2 (3). KCp′′′ was reacted with [Ce(I)3(THF)4] to afford the mono-ring complex [Ce(Cp′′′)(I)2(THF)2] (4), regardless of the stoichiometric ratio of the reagents. Complex 4 was reacted with [KN(SiMe3)2] to yield [Ce(Cp′′′)2(I)(THF)] (5), [Ce(Cp′′′){N(SiMe3)2}2] (6) and [Ce{N(SiMe3)2}3] by ligand scrambling. Complexes 1–6 have all been structurally authenticated and are variously characterised by other physical methods.

The lithium silylamides [Li(μ3-NHSiMe2But)]6 (1) and [Li(μ-NHSiPri3)(THF)]2 (2) were reacted with ClSiMe3, ClSiMe2But, or ClSiPri3 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(SiMe2But)(SiMe3)}(C7H8)]2 (3), [{K[μ-N(SiPri3)(SiMe3)]}2]∞ (4), [{K[μ-N(SiMe2But)2]}2(C7H8)]∞ (5), [K{N(SiPri3)(SiMe2But)}]∞ (6), [K{N(SiPri3)2}]∞ (7), and [K{N(SiPri3)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(SiMe2But)(SiMe3)}2(μ-I)]2 (9) and homoleptic [Ln{N(SiMe2But)(SiMe3)}3] (Ln = La 10, Ce 11) and [La{N(SiMe2But)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.

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.

[Th(Cp′′)3] (Cp′′ = {C5H3(SiMe3)2-1,3}) activates P4 to give [{Th(Cp′′)3}2(μ–η1:η1-P4)] (1), which has an unprecedented cyclo-P4 binding mode. DFT studies were performed on a model of 1 to probe the bonding in this system.

The preparation of silylamide Hauser base (R2NMgX; X = halide) and amido-Grignard (R2NMgR) complexes from simple Grignard reagents using [K{N(SiMe2tBu)2}]n, [K{N(SiMe2tBu)(SiiPr3)}]n and [K{N(SiiPr3)2}]n, and their parent silylamines, was explored. Both salt metathesis and protonolysis routes proved ineffective with allylmagnesium chloride as a starting material due to complex Schlenk equilibria, with [Mg(NRR′)(μ-Cl)(THF)]2 (NRR′ = {N(SitBuMe2)2}−, 1; {N(SitBuMe2)(SiiPr3)}−, 2; {N(SiiPr3)2}−, 3) and [Mg{N(SiiPr3)2}(μ-C3H5)]∞ (4) identified as minor products. In contrast, salt metathesis protocols using potassium silylamides and methylmagnesium iodide gave [Mg(NRR′)(μ-CH3)]2 (NRR′ = {N(SitBuMe2)2}−, 7a; {N(SitBuMe2)(SiiPr3)}−, 8; {N(SiiPr3)2}−, 9) and [Mg{N(SitBuMe2)2}(CH3)(DME)] (7b), with [Mg{N(SitBuMe2)2}(μ-I)(THF)]2 (10) isolated as a side-product during the preparation of 7a. Unusually, methylmagnesium iodide, di-n-butylmagnesium and 7–9 did not react with HNRR′ under the conditions we employed. The synthesis of [Na{N(SitBuMe2)2}(THF)]2 (5a) and [Na{N(SitBuMe2)2}(DME)2] (5b) from benzyl sodium and HN(SitBuMe2)2, and a solvent-free structure of [K{N(SitBuMe2)2}] (6), are also reported. Complexes 1, 5b, 7a, 7b, 8, 9 and 10 are fully characterised by single crystal XRD, multinuclear NMR and IR spectroscopy and elemental analysis, whereas complexes 2–4, 5a and 6 were identified by XRD only.

We report the first near-linear bis(amide) 4f-block compound and show that this novel structure, if implemented with dysprosium(III), would have unprecedented single molecule magnet (SMM) properties with an energy barrier, Ueff, for reorientation of magnetization of 1800 cm−1.