Norbornyl Cations of Group 14 Elements

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 CC 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 CC 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 CC 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 CC 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 EA of the norbornyl cations 4a−d decreases, suggesting reduced interaction between
the CC 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.