Structural Distortions in M[E(SiMe₃)₂]₃ Complexes (M = Group 15, f-Element; E = N, CH): Is Three a Crowd?

Abstract

The tris(bistrimethylsilylamido) species P[N(SiMe3)2]3 (1) and As[N(SiMe3)2]3 (2) have been prepared through halide metathesis in high yield. Their single crystal X-ray structures, along with that of Sb[N(SiMe3)2]3 (3), complete the series of structurally authenticated group 15 M[N(SiMe3)2]3 complexes (the bismuth analogue (4) has been previously reported). All four complexes possess the expected pyramidal geometries, with progressively longer M-N bond distances from P to Bi but closely similar N-M-N angles (107-104°). The structures of 1-4 also display distortions that are similar to those in f-element M[N(SiMe3)2]3 and M[CH(SiMe3)2]3 complexes, in which M···(β-Si-C) interactions have been identified. Such structural features include distorted M-(N,CH)-Si and (N,CH)-Si-C angles and close M···C and M···Si contacts. DFT calculations confirm that there are no M···(β-Si-C) interactions in 1-4; the bond distortions appear to result from the particular steric crowding that arises in pyramidal M[(N,CH)(SiMe3)2]3 complexes. This is likely the source of the most of the distortions in the structures of the f-element analogues as well, even though the latter possess attractive M···Si-C interactions.

The role of the bis-trimethylsilylamido ligand, [N{SiMe3}2]−, in main group chemistry. Part 2: Structural chemistry of the metallic p-block elements

“…Despite the history associated with this molecular fragment, it was not until 2014 that the homoleptic arsenic compound As(N{SiMe 3 } 2 ) 3 (167) was reported, from the reaction of AsI 3 with KN{SiMe 3 } 2 [135]. The molecular structure showed a monomeric species with a pyramidal As centre that was disordered over two positions.…”

“…It was structurally characterized in 2014 as part of a study of the series of homoleptic E(N{SiMe 3 } 2 ) 3 compounds (vide supra) [135]. Few other compounds containing the Sb-N{SiMe 3 } 2 unit have been reported.…”

Reversible temperature-induced polymorphic phase transitions of [Y(OAr)3] and [Ce(OAr)3] (Ar = 2,6-tBu2-4-MeC6H2): interconversions between pyramidal and planar geometries

“…1). [18][19][20][21][22][23][24] These explanations are also likely to underpin the non-linearity of several two-coordinate group 2 compounds as well. 19,[25][26][27][28][29] Despite many years of disagreement, it has more recently been proposed that the polarisable-ion model (Fig.…”

“…31,32 Dispersion forces have been highlighted as important contributors to the bent geometry in SmCp 2   * 27 and CaCp 2   * , 33 but were found to be less important when using a different set of techniques in the study of YbCp 2   contribution. 18 The shallow nature of the potential energy surface and the small amount of energy required for these deformations/interconversions has been alluded to. 20 Metal C-H agostic interactions (5) are another potential complication to the system, 33 but for [Ln{CHIJSiMe 3 ) 2 } 3 ], the influence of these interactions has been suggested by Boyde et al to have been overstated, 18 with more evidence for β-agostic Si-C bonds than γ-agostic C-H bonds.…”

“…18 The shallow nature of the potential energy surface and the small amount of energy required for these deformations/interconversions has been alluded to. 20 Metal C-H agostic interactions (5) are another potential complication to the system, 33 but for [Ln{CHIJSiMe 3 ) 2 } 3 ], the influence of these interactions has been suggested by Boyde et al to have been overstated, 18 with more evidence for β-agostic Si-C bonds than γ-agostic C-H bonds. 23,34 Crystal-packing effects must also be taken into account (6).…”