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

Rainer Zitz,Burgert Blom,Henning Arp,Judith Baumgartner,Małgorzata Walewska,Christoph Marschner,Johann Hlina,Tibor Szilvási

doi:10.1021/ic502991p

Abstract

Complexes featuring lanthanide (Ln)–Si bonds represent a highly neglected research area. Herein, we report a series of open-shell LnII+ and LnIII+ complexes bearing σ-bonded silyl and base-stabilized N-heterocyclic silylene (NHSi) ligands. The reactions of the LnIII+ 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 SmII+ 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 LnII+ or LnIII+ 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) .

Highlights

Compounds featuring lanthanide (Ln)−Si bonds are somewhat rare in contemporary literature,[1−3] further highlighted by the fact that, for the elements Ce, Tb, and Pm, no examples exist
We report a series of open-shell LnII+ and LnIII+ complexes bearing σbonded silyl and base-stabilized N-heterocyclic silylene (NHSi) ligands
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

Summary

■ INTRODUCTION

Compounds featuring lanthanide (Ln)−Si bonds are somewhat rare in contemporary literature,[1−3] further highlighted by the fact that, for the elements Ce, Tb, and Pm, no examples exist. All complexes 2a−2d and 3 are thermally stable but extremely sensitive to moisture and air, as expected for organometallic Ln complexes In addition to these lanthanide silyl complexes, we are interested in the synthesis of base-stabilized silylene lanthanide metal complexes because instead of featuring a σ-bonded silyl group they would rather exhibit coordinative (donor−acceptor) bonds between Si and the Ln element, which would be of interest as a comparison. The 13C NMR spectra for both complexes reveal most of the signals in the expected diamagnetic spectral range, some again somewhat line-broaded, the resonance signals corresponding to the ring atoms of the coordinated Cp* ligand could not be observed in both complexes, akin to the Cp complexes described above. MBOs indicate very low bond order of 0.24 and 0.25 for 5 and 6, respectively

■ CONCLUSION AND SUMMARY

■ ACKNOWLEDGMENTS

■ REFERENCES