Open-Shell Lanthanide(II+) or -(III+) Complexes Bearing σ-Silyl and Silylene Ligands: Synthesis, Structure, and Bonding Analysis

[Image: see text] 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 Cp(3)Ln (Ln = Tm, Ho, Tb, Gd; Cp = cyclopentadienide) with the 18-crown-6 (18-cr-6)-stabilized 1,4-oligosilanyl dianion [(18-cr-6)KSi(SiMe(3))(2)SiMe(2)SiMe(2)Si(SiMe(3))(2)K(18-cr-6)] (1) selectively afford the corresponding metallacyclopentasilane salts [Cp(2)Ln({Si(SiMe(3))(2)SiMe(2)}(2))](−)[K(2)(18-cr-6)(2)Cp](+) [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 Cp(3)Ce affords the acyclic product [Cp(3)CeSi(SiMe(3))(2)SiMe(2)SiMe(2)Si(SiMe(3))(2)-Cp(3)Ce](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(OC(6)H(4)-2-tBu){(NtBu)(2)CPh} (4a) and the alkoxy analogue Si(OtBu){(NtBu)(2)CPh} (4b) were reacted with Cp*(2)Sm(OEt(2)), affording, by OEt(2) elimination, the corresponding silylene complexes, both featuring Sm(II+) centers: Cp*(2)Sm ← :Si(O–C(6)H(4)-2-tBu){(NtBu)(2)CPh} (6) and Cp*(2)Sm ← :Si(OtBu){(NtBu)(2)CPh} (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) .