Rare Earth and Main Group Metal Poly(hydrosilyl) Compounds

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.

Metallacyclosilanes of Calcium, Yttrium, and Iron

Utilizing a choice of α,ω-oligosilanylene diides, it is possible to synthesize a number of heterocyclosilanes with heteroelements of calcium, yttrium, and iron by metathesis reactions with respective metal halides CaI₂, YCl₃, and FeBr₂. ²⁹Si NMR spectroscopic analysis of the calcacyclosilanes suggests that these compounds retain a strong oligosilanylene dianion character, which is more pronounced than in the analogous magnesacyclosilanes. As the electronegativity of calcium lies between potassium and magnesium, silyl calcium reagents should be considered as building blocks with an attractive reactivity profile. Reaction of a 1,4-oligosilanylene diide with YCl₃ gave the five-membered yttracyclosilane as an ate-complex with two chlorides still attached to the yttrium atom. Reaction of the obtained compound with two equivalents of NaCp led to another five-membered yttracyclosilane ate-complex with an yttracene fragment. When using a dianionic oligosilanylene ligand containing a siloxane unit, the siloxane oxygen acted as an additional coordination site for Ca and Y. When the same ligand was used to prepare a cyclic 1-ferra-4-oxatetrasilacyclohexane, an analogous transannular interaction between the iron and oxygen atoms is missing.

Reactions of some α,ω-oligosilanylene diides with CaI₂, YCl₃, and FeBr₂ allow convenient access to metallocyclosilanes with Ca, Y, and Fe as heteroatoms. The calcium and yttrium compounds resemble previously prepared magnesacyclosilanes, retaining a strong silanide character. The only related compounds to the described ferracyclosilanes are acyclic examples.

Early Lanthanide(III) Ate Complexes Featuring Ln-Si Bonds (Ln = La, Ce): Synthesis, Structural Characterization, and Bonding Analysis

While research on lanthanide (Ln) complexes with silyl ligands is receiving growing attention, significantly unbalanced efforts have been devoted to different Ln elements. In comparison with the intense investigations on Ln elements such as Sm and Yb, the chemistry of silyl lanthanum and cerium complexes is much slower to develop, and no solid-state structure of a silyl lanthanum complex has been reported so far. In this research, four types of ate complexes, including [(DME)₃Li][Cp₃LnSi(H)Mes₂], [(18-crown-6)K][Cp₃LnSi(CH₃)Ph₂], [(DME)₃Li][Cp₃LnSiPh₃], and [(12-crown-4)₂Na] [Cp₃LnSi(Ph)₂Si(H)Ph₂] (Ln = La, Ce), were synthesized by reacting [(DME)₃Na][Cp₃La(μ-Cl)LaCp₃] or Cp₃Ce(THF) with alkali metal silanides. All of the synthesized silyl Ln ate complexes were structurally characterized. La-Si bond lengths are in a range of 3.1733(4)-3.1897(10) Å, and the calculated formal shortness ratios of the La-Si bonds (1.071.08) are comparable to those in the reported silyl complexes having other Ln metal centers. The Ce-Si bond lengths (3.1415(6)-3.1705(9) Å) are within the typical range of reported silyl cerium ate complexes. ²⁹Si solid-state NMR measurements on the diamagnetic silyl lanthanum complexes were conducted, and large one-bond hyperfine splitting constants arising from = 7/2) were resolved. Computational studies on these silyl lanthanum and cerium complexes suggested the polarized covalent feature of the Ln-Si bonds, which is in line with the measured large ¹J_¹³⁹La-Si splitting constants.

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.

Rare-Earth-Silyl ate-Complexes Opening a Door to Selective Manipulations

The reactions of a number of rare-earth (RE) trichlorides and an oligosilanylene diide containing a siloxane unit in the backbone in DME are described. The formed products of the type [(DME)4·K][(DME)·RE(Cl)2{Si(SiMe3)2SiMe2}2O] (RE = Y, La, Ce, Pr, Sm, Tb, Dy, and Er) are disilylated dichloro metalate complexes and include the first examples of Si-La and Si-Pr compounds as well as the first structurally characterized example of a Si-Dy complex. A most intriguing aspect of the synthesis of these complexes is that they offer entry into a systematic study of the still largely unexplored field of silyl RE complexes by the possibility of ligand exchange reactions under preservation of the Si-RE interaction. This was demonstrated by the conversion of [(DME)4·K][(DME)·RE(Cl)2{Si(SiMe3)2SiMe2}2O] to [(DME)4·K][Cp2Y{Si(SiMe3)2SiMe2}2O].

Metal-metal bonded alkaline-earth distannyls

The first families of alkaline-earth stannylides [Ae(SnPh3)2·(thf) x ] (Ae = Ca, x = 3, 1; Sr, x = 3, 2; Ba, x = 4, 3) and [Ae{Sn(SiMe3)3}2·(thf) x ] (Ae = Ca, x = 4, 4; Sr, x = 4, 5; Ba, x = 4, 6), where Ae is a large alkaline earth with direct Ae-Sn bonds, are presented. All complexes have been characterised by high-resolution solution NMR spectroscopy, including 119Sn NMR, and by X-ray diffraction crystallography. The molecular structures of [Ca(SnPh3)2·(thf)4] (1'), [Sr(SnPh3)2·(thf)4] (2'), [Ba(SnPh3)2·(thf)5] (3'), 4, 5 and [Ba{Sn(SiMe3)3}2·(thf)5] (6'), most of which crystallised as higher thf solvates than their parents 1-6, were established by XRD analysis; the experimentally determined Sn-Ae-Sn' angles lie in the range 158.10(3)-179.33(4)°. In a given series, the 119Sn NMR chemical shifts are slightly deshielded upon descending group 2 from Ca to Ba, while the silyl-substituted stannyls are much more shielded than the phenyl ones (δ 119Sn/ppm: 1', -133.4; 2', -123.6; 3', -95.5; 4, -856.8; 5, -848.2; 6', -792.7). The bonding and electronic properties of these complexes were also analysed by DFT calculations. The combined spectroscopic, crystallographic and computational analysis of these complexes provide some insight into the main features of these unique families of homoleptic complexes. A comprehensive DFT study (Wiberg bond index, QTAIM and energy decomposition analysis) points at a primarily ionic Ae-Sn bonding, with a small covalent contribution, in these series of complexes; the Sn-Ae-Sn' angle is associated with a flat energy potential surface around its minimum, consistent with the broad range of values determined by experimental and computational methods.

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.

β-Amino- and Alkoxy-Substituted Disilanides

Our recent study on formal halide adducts of disilenes led to the investigation of the synthesis and properties of β-fluoro- and chlorodisilanides. The reaction of the functionalized neopentasilanes (Me3Si)3SiSiPh2NEt2 and (Me3Si)3SiSiMe2OMe with KOtBu in the presence of 18-crown-6 provided access to structurally related β-alkoxy- and amino-substituted disilanides. The obtained Et2NPh2Si(Me3Si)2SiK·18-crown-6 was converted to a magnesium silanide and further on to Et2NPh2Si(Me3Si)2Si-substituted ziroconocene and hafnocene chlorides. In addition, an example of a silanide containing both Et2NPh2Si and FPh2Si groups was prepared with moderate selectivity. Also, the analogous germanide Et2NPh2Si(Me3Si)2GeK·18-crown-6 could be obtained.

Chemistry of a 1,5-Oligosilanylene Dianion Containing a Disiloxane Unit

Synthesis of a number of disiloxane containing cyclo- and bicyclooligosilanes is described starting from the dipotassium 1,5-oligosiloxanylene diide derived from 1,3-bis[tris(trimethylsilyl)silyl]tetramethyldisiloxane. In addition, the use of this particular fragment as ligand for zinc and group 4 metallocene complexes was studied. Both types of compounds exhibit marked structural differences compared to related compounds containing Si-Si-Si units instead of the Si-O-Si fragment.

Calcium stannyl formation by organostannane dehydrogenation

Reaction of the dimeric calcium hydride, [(BDI)CaH]₂ (1), with Ph₃SnH ensues with elimination of H₂ to provide [(BDI)Ca-μ₂-H-(SnPh₃)Ca(BDI)] (3) and [(BDI)Ca(SnPh₃)]₂ (4) alongside dismutation to Ph₄Sn, H₂ and Sn(0).