²⁹Si NMR Spectroscopy as a Probe of s- and f-Block Metal(II)–Silanide Bond Covalency

Abstract

We report the use of 29 Si 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 t Bu 3 ) 2 (THF) 2 (THF) x ] (1-M: M = Mg, Ca, Yb, x = 0; M = Sm, Eu, x = 1) and [M(Si t Bu 2 Me) 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 29 Si 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(SiMe 3 ) 3 } − -, {Si(SiMe 2 H) 3 } − -, and {SiPh 3 } − -substituted analogues provide 20 representative examples spanning five silanide ligands and four divalent metals, revealing that the metal-bound 29 Si 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.

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

“…Clear resonances in the octet pattern arising from the splitting of 139 La ( I = 7/2) were observed in the 29 Si solid-state NMR spectra of 2-La ∼ 4-La . The measured large one-bond coupling constants ( 1 J 139 La–Si = 335 Hz for 2-La , 1 J 139 La–Si = 337 Hz for 3-La , and 1 J 139 La–Si = 318 Hz for 4-La ) indicated significant covalency of the La–Si interactions in these complexes . It is worth noting that the hyperfine splitting of 139 La was not resolved in the 29 Si solid-state NMR spectrum of 5-La , probably due to the relatively significant quadrupolar effect in this complex .…”

“…The measured large onebond coupling constants ( 1 J 139 La−Si = 335 Hz for 2-La, 1 J 139 La−Si = 337 Hz for 3-La, and 1 J 139 La−Si = 318 Hz for 4-La) indicated significant covalency of the La−Si interactions in these complexes. 39 It is worth noting that the hyperfine splitting of 139 La was not resolved in the 29 Si solid-state NMR spectrum of 5-La, probably due to the relatively significant quadrupolar effect in this complex. 40 The unequal integrations of the two 29 Si resonances of 5-La were likely a consequence of the crosspolarization of Si atoms and 1 H decoupling.…”

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

“…A multitude of other ligands have also been employed in salt elimination reactions with LnI 2 salts, which include aromatic ligands (Figure , A , B ), , mono- and multidentate dentate alkyls ( C – H ), ,,− multidentate N- donors ( I – M ), ,− phosphides ( N ), silanides ( O ), , and gallyls. , A major challenge in Ln­(II) chemistry is the stabilization of heteroleptic complexes of the type “Ln­(L)­(I)” directly from salt elimination reactions, owing to the large steric demands of divalent Lns and the tendency in some cases to rearrange to homoleptic Ln­(L) 2 and LnI 2 . This interest originates from the possibility of further functionalizing the complexes by substituting the iodide ligand via metathetical reactivity.…”

Metallacyclosilanes of Calcium, Yttrium, and Iron

“…22 More recently, Okuda and co-workers prepared (Ph 3 Si) 2 Ca (V) from Ph 3 SiK, 23 and Mills and Liddle reported the synthesis of THF adducts of ( t Bu 3 Si) 2 Ca and (tBu 2 MeSi) 2 Ca (VI) from the respective sodium silanides (Chart 1). 24 Silyl Yttrium Compounds. The origin of the organic chemistry of rare-earth metals dates back to the seminal work of Wilkinson and co-workers on metallocenes in the 1950s.…”