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

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) .

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

“… 14 , 27 , 29 − 31 The bond distances of the Eu–Si bonds in 2b (3.2052, 3.2162 Å) and 4b (3.1497 Å) are comparable to that found in Cp* 2 Eu II SiH 3 K(THF) 2 (3.239 Å). 13 For 2c (3.2132 and 3.2444 Å) and 4c (3.1716 Å) the Si–Sm distances are in good agreement with the few published Sm(III)–Si bond lengths (3.133, 3.106, and 3.174 Å) 9 and the known Sm(II)-silylene complexes (3.189 10 and 3.299 11 Å). In general compounds 4a – c with the two bulky tris(trimethylsilyl)silyl groups exhibit the shorter bond lengths compared to the metallacyclopentasilanes 2a – c .…”

“…The few reported samarium silyl compounds, which have also been characterized by X-ray diffraction analysis, feature Sm with Cp* ligands almost exclusively in oxidation state 3+ 7 − 9 with the exception of some divalent silylene complexes reported by Evans and co-workers 10 and our group. 11 The reaction of Cp* 2 Yb·(Et 2 O) with (Me 3 Si) 3 SiLi was shown to result in the formation of Cp*YbSi(SiMe 3 ) 3 ·(THF) 2 accompanied by elimination of Cp*Li. 12 Reacting Cp*YbSi(SiMe 3 ) 3 ·(THF) 2 with excess (Me 3 Si) 3 SiLi led to the formation of some 30% of [(Me 3 Si) 3 Si] 2 Yb·(THF) x , which could not be isolated.…”

Synthesis and Structure of Uranium‐Silylene Complexes

“…An X‐ray diffraction study on single crystals of 1 revealed a uranium–silicon bond length of 3.1637(1) Å (Figure ). While this distance is longer than the sum of the covalent radii of uranium and silicon as tabulated by Pyykkö (2.86 Å) or Alvarez (3.07 Å), bond lengths between f ‐elements and heavy main‐group atoms have often exceeded the sums of the relevant covalent radii, suggesting that these radii may not be the best predictor of main‐group–f element bond length . Compared to its ‘base‐free’ solid‐state structure, the (CpSiMe 3 ) 3 U fragment has reorganized to minimize unfavorable steric interactions: the trimethylsilyl (TMS) groups on the cyclopentadienyl rings of (CpSiMe 3 ) 3 U are oriented away from the fourth ligand, an arrangement that has only been observed previously in some dimeric structures containing bridging ligands .…”

“…The 2010 discovery by Roesky and co-workers that amidinate-supported silylenes could be prepared in high yield throughd ehydrochlorinationp athways hasc aused a surge of growth in their research and application; [18,19] these silylenesh ave been bound to metals from nearly every group in the transition-metal series [20] as well as to lanthanides. [21] The growinga pplication of these silylenes is largely due to the ease with which the parent chlorosilylene Si[PhC(NtBu) 2 ]Cl can be derivatizedt of eature substituents such as alkoxy, amino, phosphino, and even alkyl groups, thus accessing an array of silylenes with varying steric and electronic properties. [20,22,23] In 2009, we showed that the uranium(III) complex (CpSi-Me 3 ) 3 Uc ould be employed to generate speciescontaining actinide-group 13 bonds with aluminum(I) and gallium(I).…”

“…Analogous reactivity toward Si[PhC(NtBu) 2 ]Cl has been observed for as amarium(II)metallocene. [21] The reduction from silicon(II)t os ilicon(I)h ighlights the well-known reducing powero fu ranium(III) organometallic complexes.…”