Thomas Müller

The synthesis and isolation of 12 α-aryl, β, β‘-disilyl-substituted vinyl cations 1b−l, 7, and 8 with the tetrakis(pentafluorophenyl)borate counteranion is reported. The vinyl cations are characterized by NMR spectroscopy and are identified by their specific NMR chemical shifts (δ ¹³C(C⁺) = 178.1−194.5; δ ¹³C(C^β) = 83.3−89.9; δ ¹³C(C^ipso) = 113.6−115.2; δ ²⁹Si = 25.0−12.0), supported by density functional calculations at the B3LYP/6-311G(2d,p)//B3LYP/6-31G(d) level. All cations are found to be stable at room temperature in solution and in the solid state. The NMR chemical shifts as well as J-coupling data indicate for vinyl cations, 1b−l, 7, and 8, the occurrence of substantial stabilization through π-resonance via the aryl substituents and through σ-delocalization via the β-silyl groups. For vinyl cation 8, the free enthalpy of stabilization via π-resonance by the α-ferrocenyl substituent is determined by temperature-dependent ²⁹Si NMR spectroscopy to be ΔG^⧧ = (48.9 ± 4.2) kJ mol^-1. A Hammett-type analysis, which relates the ¹J(SiC^β) coupling constant and the low-field shift of the ²⁹Si NMR signal upon ionization, Δδ ²⁹Si, with the electron-donating ability of the aryl group, indicates an inverse relation between the extent of Si−C hyperconjugation and π-donation. The computed structures (at B3LYP/6-31G(d)) of the vinyl cations 1a−l, 7, and 8 reveal the consequences of Si−C hyperconjugation and of π-resonance interactions with the aryl groups. The structures, however, fail to express the interplay between σ-delocalization and π-conjugation in that the calculated Si−C bond lengths and the C⁺-C^ipso bond lengths do not vary as a function of the substituent.

Norbornyl cations of the group 14 elements Si → Pb have been synthesized from substituted 3-cyclopentenemethyl precursors by intramolecular addition of transient cations to the CC double bond of the 3-cyclopentenemethyl substituent (π-route to norbornyl cations). The norbornyl cations 4a (E = Si, R = Me), 4e (E = Si, R = Et), 4f (E = Si, R = Bu), 4g (E = Ge, R = Bu), 4h (E = Sn, R = Bu), and 4i (E = Pb, R = Et) have been identified by their characteristic NMR chemical shifts (4a,e,f, δ(²⁹Si) = 80−87, δ(¹³C)(CH) = 149.6−150.6; 4g, δ(¹³C)(CH) = 144.8; 4h, δ(¹¹⁹Sn) = 334, δ(¹³C)(CH) = 141.5; 4i, δ(²⁰⁷Pb) = 1049, δ(¹³C)(CH) = 138). The significant deshielding of the vinylic carbon atoms (Δδ(¹³C)) relative to those of the precursor (Δδ(¹³C) = 19.3−20.3 (4a,e,f), Δδ(¹³C) = 14.6 (4g), Δδ(¹³C) = 11.1 (4h), Δδ(¹³C) ≈ 8 (4i)) and the small J coupling constants between the element and the remote vinyl carbons in the case of 4h and 4i (J(CSn) = 26 Hz, J(CPb) = 16 Hz) give experimental evidence for the intramolecular interaction and the charge transfer between the positively charged element and the remote CC double bond. The experimental results are supported by quantum mechanical calculations of structures, energies, and magnetic properties for the norbornyl cations 4a,b (E = Ge, R = Me), 4c (E = Sn, R = Me), 4d (E = Pb, R = Me), and 4e,f at the GIAO/B3LYP/6-311G(3d,p)//MP2/6-311G(d,p) (Si, Ge, C, H), SDD (Sn, Pb) level of theory. The calculated ²⁹Si NMR chemical shifts for the silanorbornyl cations 4a,e,f (δ(²⁹Si) = 77−93) agree well with experiment, and the calculated structures of the cations 4a−f reveal their bridged norbornyl cation nature and suggest also for the experimentally observed species 4a,e−i a formally 3 + 1 coordination for the element atom with the extra coordination provided by the CC double bond. This places five carbon atoms in the close vicinity of the positively charged element atom. The group 14 element norbornyl cations 4a,e−i exhibit only negligible interactions with the aromatic solvent, and they are, depending on the nature of the element group, stable at room temperature in aromatic solvents for periods ranging from a few hours to days. In acetonitrile solution, the intramolecular interaction in the norbornyl cations 4a,e−h breaks down and nitrilium ions with the element in a tetrahedral environment are formed. In contrast, reaction of acetonitrile with the plumbyl cation 4i forms an acetonitrile complex, 10i, in which the norbornyl cation structure is preserved. The X-ray structure of 10i reveals a trigonal bipyramidal environment for the lead atom with the CC double bond of the cyclopentenemethyl ligand and the nitrogen atom of the acetonitrile molecule in apical positions. Density functional calculations at the B3LYP/6-311G(2d,p)//(B3LYP/6-31G(d) (C, H), SDD (Si, Ge, Sn, Pb)) + ΔZPVE level indicate that the thermodynamic stability of the group 14 norbornyl cations increases from Si to Pb. This results in a relative stabilization for the plumbanorbornyl cation 4d compared to tert-butyl cation of 52.7 kcal mol^-1. In contrast, the intramolecular stabilization energy E_A of the norbornyl cations 4a−d decreases, suggesting reduced interaction between the CC double bond and the electron-deficient element center in the plumbacation compared to the silacations. This points to a reduced electrophilicity of the plumbacation compared to its predecessors.

In this paper we predict, using quantum mechanical calculations, which diaminosilylenes would dimerize to produce strongly bound tetraaminodisilenes, which so far have proven to be elusive. The central idea is that diaminosilylenes with a small singlet−triplet energy difference would dimerize to strongly bonded disilenes. Calculations at the B3LYP/6-311++G(3df,2p)//MP2/6-31G(d) level of theory showed that the energy difference between the singlet and the triplet states (ΔE_ST) of diaminosilylenes (R₂N)₂Si: (1) strongly depends on (i) the twist angle φ between the SiN₂ and the R₂N planes and (ii) the NSiN bond angle α at the divalent silicon. ΔE_ST decreases with increased twisting (larger φ) and with widening of α. ΔE_ST is reduced from 70.7 kcal mol^-1 for planar (H₂N)₂Si: (1a) to ΔE_ST = 21.7 kcal mol^-1 when φ is held at 90°. Likewise, the bicyclic diaminosilylenes 1,4-diaza-7-silabicyclo[2.2.1]hepta-7-ylidene and 1,5-diaza-9-silabicyclo[3.3.1]nona-9-ylidene (4a,b), with the nitrogens in the bridgehead positions (φ = 90°), have ΔE_ST values of 45.1 and 38.3 kcal mol^-1, respectively. When dimerized, these silylenes form strongly bonded disilenes 5 (E_dim = −32.2 kcal mol^-1 (4a) and −41.3 kcal mol^-1 (4b)) with SiSi bond lengths of 2.239 Å (4a) and 2.278 Å (4b) (MP2/6-31G(d)//MP2/6-31G(d)). These theoretical predictions pave the way for the synthesis of the first strongly bonded tetraaminodisilene. Due to the steric requirements, also silyl substitution at nitrogen has a significant effect on ΔE_ST and [(H₃Si)₂N]₂Si: (1d) is predicted to form a stable SiSi bonded dimer (E_dim= −24.1 kcal mol^-1). However, the larger size of the Me₃Si substituent prevents the formation of a SiSi bonded dimer of [(Me₃Si)₂N]₂Si: (1e).