Uranium(III) and Uranium(IV) meta-Terphenylthiolate Complexes

We report the synthesis and characterization of crystalline m-terphenylthiolate uranium complexes supported by the bulky ligand system, SAriPr6 (SAriPr6 = {SC6H3-2,6-(Tripp)2}; Tripp = 2,4,6-iPr-C6H2). Treatment of UIVCl4 with 2 equiv of KSAriPr6 in Et2O afforded both [UIV(SAriPr6)2(Cl)2] (1) and the Et2O adduct, [UIV(SAriPr6)2(Cl)2(Et2O)2] (1·Et2O) in poor yield. The reaction between [UIV(BH4)4] and 1 equiv of KSAriPr6 in toluene gave several crystals of the double salt, [UIV(μ-SAriPr6)(BH4)2(μ-BH4)(μ3-BH4)K]2 (2), and exposing the crude reaction mixture to Et2O gave the disulfide dimer, (SAriPr6)2. The reaction between [UIV(BH4)4] and 1 equiv of HSAriPr6 in hot toluene gave [UIII(H3B·SAriPr6 κS,H,H)(BH4)2] (3) which proved resistant to further substitution using either HSAriPr6 or KSAriPr6. Two U(III) mono-terphenylthiolates, [UIII(SAriPr6)(BH4)2] (4a) and [{UIII(SAriPr6)(BH4)}2{μ-B2H6}] (4b), were isolated as a mixture from the reaction between [UIII(BH3)3(toluene)] and 1 equiv of KSAriPr6, while using 2 equiv of KSAriPr6 gave the bis-terphenylthiolate complex [UIII(SAriPr6)2(BH4)] (5). Complex 4b is a rare example of a nido-metalloborane. Complexes 1-5 have been characterized variously by single-crystal and powder X-ray diffraction, multinuclear NMR spectroscopy, infrared spectroscopy, UV-Vis-NIR spectroscopy, SQUID magnetometry, and elemental analyses as appropriate. Quantum chemical calculations have been employed to interpret the nature of the U-S bonding interactions across these complexes.

Proton-Coupled Electron Transfer at the Pu5+/4+ Couple

The synthesis and solution and solid-state characterization of [Pu4+(NPC)4], 1-Pu, (NPC = [NPtBu(pyrr)2]-; tBu = C(CH3)3; pyrr = pyrrolidinyl) and [Pu3+(NPC)4][K(2.2.2.-cryptand)], 2-Pu, is described. Cyclic voltammetry studies of 1-Pu reveal a quasi-reversible Pu4+/3+ couple, an irreversible Pu5+/4+ couple, and a third couple evincing a rapid proton-coupled electron transfer (PCET) reaction occurring after the electrochemical formation of Pu5+. The chemical identity of the product of the PCET reaction was confirmed by independent chemical synthesis to be [Pu4+(NPC)3(HNPC)][B(ArF5)4], 3-Pu, (B(ArF5)4 = tetrakis(2,3,4,5,6-pentafluourophenyl)borate) via two mechanistically distinct transformations of 1-Pu: protonation and oxidation. The kinetics and thermodynamics of this PCET reaction are determined via electrochemical analysis, simulation, and density functional theory. The computational studies demonstrate a direct correlation between the changing nature of 5f and 6d orbital participation in metal-ligand bonding and the electron density on the Nim atom with the thermodynamics of the PCET reaction from Np to Pu, and an indirect correlation with the roughly 5-orders of magnitude faster Pu PCET compared to Np for the An5+ species.

Tris-Silanide f-Block Complexes: Insights into Paramagnetic Influence on NMR Chemical Shifts

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on 1H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29Si NMR study of an isostructural series of locally D 3h -symmetric early f-block metal(III) tris-hypersilanide complexes, [M{Si(SiMe3)3}3(THF)2] (1-M; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR, and UV-vis-NIR spectroscopies, SQUID magnetometry, and elemental analysis. Only one SiMe3 signal was observed in the 29Si ssNMR spectra of 1-M, while two SiMe3 signals were seen in solution 29Si NMR spectra of 1-La and 1-Ce. This is attributed to dynamic averaging of the SiMe3 groups in 1-M in the solid state due to free rotation of the M-Si bonds and dissociation of THF from 1-M in solution to give the locally C 3v -symmetric complexes [M{Si(SiMe3)3}3(THF) n ] (n = 0 or 1), which show restricted rotation of M-Si bonds on the NMR time scale. Density functional theory and complete active space self-consistent field spin-orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental 29Si NMR data for diamagnetic 1-La, suggesting n = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29Si nuclei could only be reproduced for diamagnetic 1-La, showing the current limitations of pNMR calculations for larger nuclei.

Solution and solid-state characterization of rare silyluranium(III) complexes

A uranium(III) silylate complex [K(DME)4][UI2{(Si(SiMe3)2SiMe2)2O}] (1) was stabilized by the addition of 18-crown-6, forming [K(18-crown-6)][UI2{(Si(SiMe3)2SiMe2)2O}] (1-crown). Crystallization under multiple conditions resulted in three distinct molecular structures. Compound 1-crown was further characterized in the solution state via1H, 13C, and 29Si NMR spectroscopy, and electronic absorption spectroscopy.

Electronic structure comparisons of isostructural early d- and f-block metal(iii) bis(cyclopentadienyl) silanide complexes

We report the synthesis of the U(iii) bis(cyclopentadienyl) hypersilanide complex [U(Cp'')2{Si(SiMe3)3}] (Cp'' = {C5H3(SiMe3)2-1,3}), together with isostructural lanthanide and group 4 M(iii) homologues, in order to meaningfully compare metal-silicon bonding between early d- and f-block metals. All complexes were characterised by a combination of NMR, EPR, UV-vis-NIR and ATR-IR spectroscopies, single crystal X-ray diffraction, SQUID magnetometry, elemental analysis and ab initio calculations. We find that for the [M(Cp'')2{Si(SiMe3)3}] (M = Ti, Zr, La, Ce, Nd, U) series the unique anisotropy axis is conserved tangential to ; this is governed by the hypersilanide ligand for the d-block complexes to give easy plane anisotropy, whereas the easy axis is fixed by the two Cp'' ligands in f-block congeners. This divergence is attributed to hypersilanide acting as a strong σ-donor and weak π-acceptor with the d-block metals, whilst f-block metals show predominantly electrostatic bonding with weaker π-components. We make qualitative comparisons on the strength of covalency to derive the ordering Zr > Ti ≫ U > Nd ≈ Ce ≈ La in these complexes, using a combination of analytical techniques. The greater covalency of 5f3 U(iii) vs. 4f3 Nd(iii) is found by comparison of their EPR and electronic absorption spectra and magnetic measurements, with calculations indicating that uranium 5f orbitals have weak π-bonding interactions with both the silanide and Cp'' ligands, in addition to weak δ-antibonding with Cp''.

Synthesis and Characterization of Yttrium Methanediide Silanide Complexes

The salt metathesis reactions of the yttrium methanediide iodide complex [Y(BIPM)(I)(THF)₂] (BIPM = {C(PPh₂NSiMe₃)₂}) with the group 1 silanide ligand-transfer reagents MSiR₃ (M = Na, R₃ = ^tBu₂Me or ^tBu₃; M = K, R₃ = (SiMe₃)₃) gave the yttrium methanediide silanide complexes [Y(BIPM)(Si^tBu₂Me)(THF)] (1), [Y(BIPM)(Si^tBu₃)(THF)] (2), and [Y(BIPM){Si(SiMe₃)₃}(THF)] (3). Complexes 1-3 provide rare examples of structurally authenticated rare earth metal-silicon bonds and were characterized by single-crystal X-ray diffraction, multinuclear NMR and ATR-IR spectroscopies, and elemental analysis. Density functional theory calculations were performed on 1-3 to probe their electronic structures further, revealing predominantly ionic Y-Si bonding. The computed Y-Si bonds show lower covalency than Y═C bonds, which are in turn best represented by Y⁺-C^- dipolar forms due to the strong σ-donor properties of the silanide ligands investigated; these observations are in accord with experimentally obtained ¹³C{¹H} and ²⁹Si{¹H} NMR data for 1-3 and related Y(III) BIPM alkyl complexes in the literature. Preliminary reactivity studies were performed, with complex 1 treated separately with benzophenone, azobenzene, and N,N'-dicyclohexyl-carbodiimide. ²⁹Si{¹H} and ³¹P{¹H} NMR spectra of these reaction mixtures indicated that 1,2-migratory insertion of the unsaturated substrate into the Y-Si bond is favored, while for the latter substrate, a [2 + 2]-cycloaddition reaction also occurs at the Y═C bond to afford [Y{C(PPh₂NSiMe₃)₂[C(NCy)₂]-κ⁴C,N,N',N'}{C(NCy)₂(Si^tBu₂Me)-κ²N,N'}] (4); these reactivity profiles complement and contrast with those of Y(III) BIPM alkyl complexes.

Crystal structures of metallocene complexes with uranium-germanium bonds

The first structural examples of complexes with uranium-germanium bonds are presented, namely, bis-[3,5-bis-(tri-fluoro-meth-yl)phenyl-2κC 1](hydrido-2κH)(iodido-1κI)bis-[1,1(η5)-penta-methyl-cyclo-penta-dien-yl]germaniumuranium(Ge-U), [GeU(C10H15)2(C8H3F6)2HI], and bis-[3,5-bis-(tri-fluoro-meth-yl)phenyl-2κC 1](fluorido-1κI)(hydrido-2κH)bis-[1,1(η5)-penta-methyl-cyclo-penta-dien-yl]germaniumuranium(Ge-U), [GeU(C10H15)2(C8H3F6)2FH]. The two complexes both have a long U-Ge bond [distances of 3.0428 (7) and 3.0524 (7) Å].

29Si NMR Spectroscopy as a Probe of s- and f-Block Metal(II)-Silanide Bond Covalency

We report the use of ²⁹Si NMR spectroscopy and DFT calculations combined to benchmark the covalency in the chemical bonding of s- and f-block metal-silicon bonds. The complexes [M(Si^tBu₃)₂(THF)₂(THF)_x] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(Si^tBu₂Me)₂(THF)₂(THF)_x] (2-M: M = Mg, x = 0; M = Ca, Sm, Eu, Yb, x = 1) have been synthesized and characterized. DFT calculations and ²⁹Si NMR spectroscopic analyses of 1-M and 2-M (M = Mg, Ca, Yb, No, the last in silico due to experimental unavailability) together with known {Si(SiMe₃)₃}^--, {Si(SiMe₂H)₃}^--, and {SiPh₃}^--substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound ²⁹Si NMR isotropic chemical shifts, δ_Si, span a wide (∼225 ppm) range when the metal is kept constant, and direct, linear correlations are found between δ_Si and computed delocalization indices and quantum chemical topology interatomic exchange-correlation energies that are measures of bond covalency. The calculations reveal dominant s- and d-orbital character in the bonding of these silanide complexes, with no significant f-orbital contributions. The δ_Si is determined, relatively, by paramagnetic shielding for a given metal when the silanide is varied but by the spin-orbit shielding term when the metal is varied for a given ligand. The calculations suggest a covalency ordering of No(II) > Yb(II) > Ca(II) ≈ Mg(II), challenging the traditional view of late actinide chemical bonding being equivalent to that of the late lanthanides.

Synthesis and Characterisation of Molecular Polarised-Covalent Thorium-Rhenium and -Ruthenium Bonds

Separate reactions of [Th{N(CH2CH2NSiMe2But)2(CH2CH2NSi(Me)(But)(μ-CH2)]2 (1) with [Re(η5-C5H5)2(H)] (2) or [Ru(η5-C5H5)(H)(CO)2] (3) produced, by alkane elimination, [Th(TrenDMBS)Re(η5-C5H5)2] (ThRe, TrenDMBS = {N(CH2CH2NSiMe2But)3}3-), and [Th(TrenDMBS)Ru(η5-C5H5)(CO)2] (ThRu), which were isolated in crystalline yields of 71% and 62%, respectively. Complex ThRe is the first example of a molecular Th-Re bond to be structurally characterised, and ThRu is only the second example of a structurally authenticated Th-Ru bond. By comparison to isostructural U-analogues, quantum chemical calculations, which are validated by IR and Raman spectroscopic data, suggest that the Th-Re and Th-Ru bonds reported here are more ionic than the corresponding U-Re and U-Ru bonds.

Crystallographic characterization of (C5H4SiMe3)3U(BH4)

New syntheses have been developed for the synthesis of (borohydrido-κ3 H)tris-[η5-(tri-methyl-sil-yl)cyclo-penta-dien-yl]uranium(IV), [U(BH4)(C8H13Si)3] or Cp'3U(BH4) (Cp' = C5H4SiMe3) and its structure has been determined by single-crystal X-ray crystallography. This compound crystallized in the space group P and the structure features three η 5-coordinated Cp' rings and a κ 3-coordinated (BH4)- ligand.

f-Element silicon and heavy tetrel chemistry

The last three decades have seen a significant increase in the number of reports of f-element carbon chemistry, whilst the f-element chemistry of silicon, germanium, tin, and lead remain underdeveloped in comparison. Here, in this perspective we review complexes that contain chemical bonds between f-elements and silicon or the heavier tetrels since the birth of this field in 1985 to present day, with the intention of inspiring researchers to contribute to its development and explore the opportunities that it presents. For the purposes of this perspective, f-elements include lanthanides, actinides and group 3 metals. We focus on complexes that have been structurally authenticated by single-crystal X-ray diffraction, and horizon-scan for future opportunities and targets in the area.