Synthesis, characterization and reactivity of yttrium and gadolinium silyl complexes

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.

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.

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].

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.

Group 4 Metal and Lanthanide Complexes in the Oxidation State +3 with Tris(trimethylsilyl)silyl Ligands

A number of paramagnetic silylated d¹ group 4 metallates were prepared by reaction of potassium tris(trimethylsilyl)silanide with group 4 metallates of the type K[Cp₂MCl₂] (M = Ti, Zr, Hf). The outcomes of the reactions differ for all three metals. While for the hafnium case the expected complex [Cp₂Hf{Si(SiMe₃)₃}₂]⁻ was obtained, the analogous titanium reaction led to a product with two Si(H)(SiMe₃)₂ ligands. The reaction with zirconium caused the formation of a dinuclear fulvalene bridged complex. The desired [Cp₂Zr{Si(SiMe₃)₃}₂]⁻ could be obtained by reduction of Cp₂Zr{Si(SiMe₃)₃}₂ with potassium. In related reactions of potassium tris(trimethylsilyl)silanide with some lanthanidocenes Cp₃Ln (Ln = Ce, Sm, Gd, Ho, Tm) complexes of the type [Cp₃Ln Si(SiMe₃)₃]⁻ with either [18-crown-6·K]⁺ or the complex ion [18-crown-6·K·Cp·K·18-crown-6] as counterions were obtained. Due to d¹ or fⁿ electron configuration, unambiguous characterization of all obtained complexes could only be achieved by single crystal XRD diffraction analysis.

Reactions of (Me₃Si)₃SiK with K[Cp₂MCl₂] (M = Ti, Zr, Hf) gave different paramagnetic silylated d¹ Group 4 metal ate complexes. For Hf the complex [Cp₂Hf{Si(SiMe₃)₃}₂]⁻ was formed, but for Ti a product with two Si(H)(SiMe₃)₂ ligands was isolated. With Zr a dinuclear fulvalene bridged complex formed but [Cp₂Zr{Si(SiMe₃)₃}₂]⁻ could be obtained by reduction of the neutral complex. In reactions of (Me₃Si)₃SiK with Cp₃Ln (Ln = Ce, Sm, Gd, Ho, Tm) complexes of the type [Cp₃LnSi(SiMe₃)₃]⁻ were observed.

Neutral "Cp-Free" Silyl-Lanthanide(II) Complexes: Synthesis, Structure, and Bonding Analysis

Complexes featuring lanthanide silicon bonds represent a research area still in its infancy. Herein, we report a series of Cp-free lanthanide (+II) complexes bearing σ-bonded silyl ligands. By reactions of LnI₂ (Ln = Yb, Eu, Sm) either with a 1,4-oligosilanyl dianion [K-Si(SiMe₃)₂SiMe₂SiMe₂Si(SiMe₃)₂-K)] (1) or with 2 (Me₃Si)₃SiK (3) the corresponding neutral metallacyclopentasilanes ({Me₂Si(Me₃Si)₂Si}₂)Ln·(THF)₄ (Ln = Yb (2a), Eu (2b), Sm (2c)), or the disilylated complexes ({Me₃Si}₃Si)₂Ln·(THF)₃ (Ln = Yb (4a), Eu (4b), Sm (4c)), were selectively obtained. Complexes 2b, 2c, 4b, and 4c represent the first examples of structurally characterized Cp-free Eu and Sm complexes with silyl ligands. In both series, a linear correlation was observed between the Ln–Si bond lengths and the covalent radii of the corresponding lanthanide metals. Density functional theory calculations were also carried out for complexes 2a–c and 4a–c to elucidate the bonding situation between the Ln(+II) centers and Si.

By reactions of LnI₂ (Ln = Yb, Eu, Sm) with oligosilanyl anions or dianions, either neutral disilylated complexes or metallacyclopentasilanes were obtained.

Open-shell lanthanide(II+) or -(III+) complexes bearing σ-silyl and silylene ligands: synthesis, structure, and bonding analysis

Complexes featuring lanthanide (Ln)–Si bonds represent a highly neglected research area. Herein, we report a series of open-shell Ln^II+ and Ln^III+ complexes bearing σ-bonded silyl and base-stabilized N-heterocyclic silylene (NHSi) ligands. The reactions of the Ln^III+ complexes Cp₃Ln (Ln = Tm, Ho, Tb, Gd; Cp = cyclopentadienide) with the 18-crown-6 (18-cr-6)-stabilized 1,4-oligosilanyl dianion [(18-cr-6)KSi(SiMe₃)₂SiMe₂SiMe₂Si(SiMe₃)₂K(18-cr-6)] (1) selectively afford the corresponding metallacyclopentasilane salts [Cp₂Ln({Si(SiMe₃)₂SiMe₂}₂)]⁻[K₂(18-cr-6)₂Cp]⁺ [Ln = Tm (2a), Ho (2b), Tb (2c), Gd (2d)]. Complexes 2a–2d represent the first examples of structurally characterized Tm, Ho, Tb, and Gd complexes featuring Ln–Si bonds. Strikingly, the analogous reaction of 1 with the lighter element analogue Cp₃Ce affords the acyclic product [Cp₃CeSi(SiMe₃)₂SiMe₂SiMe₂Si(SiMe₃)₂-Cp₃Ce]^2–2[K(18-cr-6)]⁺ (3) as the first example of a complex featuring a Ce–Si bond. In an alternative synthetic approach, the aryloxy-functionalized benzamidinato NHSi ligand Si(OC₆H₄-2-tBu){(NtBu)₂CPh} (4a) and the alkoxy analogue Si(OtBu){(NtBu)₂CPh} (4b) were reacted with Cp*₂Sm(OEt₂), affording, by OEt₂ elimination, the corresponding silylene complexes, both featuring Sm^II+ centers: Cp*₂Sm ← :Si(O–C₆H₄-2-tBu){(NtBu)₂CPh} (6) and Cp*₂Sm ← :Si(OtBu){(NtBu)₂CPh} (5). Complexes 5 and 6 are the first four-coordinate silylene complexes of any f-block element to date. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analysis. In the series 2a–2d, a linear correlation was observed between the Ln–Si bond lengths and the covalent radii of the corresponding Ln metals. Moreover, in complexes 5 and 6, notably long Sm–Si bonds are observed, in accordance with a donor–acceptor interaction between Si and Sm [5, 3.4396(15) Å; 6, 3.3142(18) Å]. Density functional theory calculations were carried out for complexes 2a–2d, 5, and 6 to elucidate the bonding situation between the Ln^II+ or Ln^III+ centers and Si. In particular, a decrease in the Mayer bond order (MBO) of the Ln–Si bond is observed in the series 2a–2d in moving from the lighter to the heavier lanthanides (Tm = 0.53, Ho = 0.62, Tb = 0.65, and Gd = 0.75), which might indicate decreasing covalency in the Ln–Si bond. In accordance with the long bond lengths observed experimentally in complexes 5 and 6, comparatively low MBOs were determined for both silylene complexes (5, 0.24; 6, 0.25) .

A series of open-shell Ln^II+ and Ln^III+ complexes bearing σ-bonded silyl and base-stabilized N-heterocyclic silylene (NHSi) ligands are reported. By reactions of Cp₃Ln (Ln = Tm, Ho, Tb, Gd) with a 1,4-oligosilanyl dianion, the metallacyclopentasilane salts [Cp₂Ln({Si(SiMe₃)₂SiMe₂}₂)]⁻[K₂(18-cr-6)₂Cp]⁺ were obtained. Alternatively, two NHSi ligands were reacted with Cp*₂Sm(OEt₂), affording the corresponding silylene complexes, both featuring Sm^II+ centers. Density functional theory calculations were conducted for all complexes to study the bonding situation.

Synthesis, structural characterization and thermal properties of copper and silver silyl complexes

A series of copper and silver-silyl complexes containing N-heterocyclic carbene or N-donor ligands were synthesized and characterized in the solid state. A number of different structural forms were observed and many compounds were shown to be volatile.