Synthesis and Characterization of
Bissilylated Onium Ions of Group 15 Elements
Robin Panisch and Thomas Müller*
Institut für Anorganische Chemie der Goethe Universität Frankfurt, Marie Curie-Str. 11, D-60439 Frankfurt, Germany
Bisilylated onium ions of the elements N-Sb are formed by
Synthesis
intramolecular addition of transient silylium ions to EPh2
[Ph3C]+
H3C
Si
H3C
H
groups (E = N-Sb). Solutions of the onium ions in aromatic
[B(C6F5)4]-
CH
Si CH3
3
EPh2
CH
Si CH3
3
EPh2
H3C
Si
H3C +
Benzene, 25ºC
Me2Si
hydrocarbons are stable at r.t. for days, with the execption
+ SiMe2
E
Ph2
of the stibonium ion, which decomposes slowly. The
-
cations are isolated as their [B(C6F5)4] salts and are
E : N, P, As, Sb
characterized by NMR spectroscopy supported by
quantum mechanical calculations.
An Example: NMR Characterization of the Phosphonium Ion
A symmetric cation is formed upon
Calculated structure and NMR parameters
ionization of the phosphino-substituted
31
1
P{ H}
29
Me2Si
21.7 Hz
1
Si { H} INEPT
disilane as it is shown by its NMR spectra.
H
29
The down-field shift of the single
SiMe2
PPh2
3.5
3.0
2.5
Si
resonance in the cation indicates some
1.5
2.0
uptake of positive charge and the
d29Sicalc = 10.9
1
decrease of the JSiP coupling constant is
Ph3C
+
in qualitative agreement with a weaker
106.2°
SiP bond in the cation. Quarternization of
11.3 Hz
the phosphor in the cation, interestingly,
232.1 pm
31
has only small influence on the P NMR
Me2Si
+
P
Ph
d31Pcalc = -55.2
chemical shift.
SiMe2
Ph
9.5
9.0
8.5
8.0
Cs
Quantum mechanical calculations of
7.5
B3LYP/6-31G(d), NMR chemical shift at GIAO/B3LYP/6-311G(3d,p)
structure and NMR chemical shifts for a
50
0
-50
-100
80
60
40
20
0
-20
-40
close model are in good agreement with
-80
-60
the experimental data. This confirms the
validility of the calculated structure.
Bonding Situation in Bissilylated Onium Ions
Synthesis and Characterization of Group 15 Onium Ions
Experiment
intra
Intramolecular Stabilization Energy E
Theory
29
d Si = 44
29
E
Si{1H} Inept
112.1°
E=N
Me2Si
+
N
Ph
SiMe2
Eintra, the energy difference between the
(kcal mol-1)
open silylium ion and the cyclic onium
(H3C)2Si
+
d29Sicalc = 51
ion, is maximal for E = P and it markedly
Si(CH3)2
E(CH3)2
decreases from P to Bi.
193.9 pm
Ph
E
intra
+
E
(H3C)2Si
H3C
The calculated atomic charge at silicon
Si(CH3)2
is largest for the amino substituent and it
CH3
d29Sicalc = 11
d Si = 9
29
E=P
decreases by ca. 0.5 a.u. for the phos-
106.2°
Me2Si
+
P
Ph
SiMe2
N
P
As
Sb
Bi
Eintra
49.6
55.2
48.4
42.6
31.6
232.1 pm
Ph
phonium ion, while it remains nearly
E
constant for the arsonium and stibonium
B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d)
ion.
Me2Si
H
SiMe2
d Si = 18
29
d29Sicalc = 14
EPh2
Me2Si
E = As
+
As
Ph
Calculated atomic and group charges (in a.u., NBO)
106.7°
SiMe2
239.0 pm
Ph
1.83
(H3C)2Si
1.39
+
Si(CH3)2
1.39
(H3C)2Si
N
Me2Si
E = Sb
Ph
+
Sb
1.31
+ Si(CH3)2
As
(H3C)2Si
Si(CH3)2
H3C
H3C
CH3
-0.40
d29Sicalc = 21
d29Si = 18
+
P
H3C
CH3
0.32
0.32
0
-40
CH3
0.52
101.3°
1.0
Charge transfer from the EMe2-
0.9
-80
B3LYP/6-31G(d) (C, H, Si, N, P, As) SDD (Sb)
NMR at GIAO/B3LYP(6-311G(3d,p) (C, H, Si, N, P, As) SDD (Sb)
The tetravalent onium ions of the elements N->Sb could be synthesized in clean reactions
between the precursor silanes and trityl cation. The cations are identified by multinuclear NMR
spectroscopy and were structurally characterized by quantum mechanical calculations for close
models of the cations in the gas phase.
Electron Transfer [a.u.]
40
+ Si(CH3)2
Sb
H3C
SiMe2
Ph
262.4 pm
80
CH3
(H3C)2Si
Me2Si
+
Me
0.2
0.0
Me
very small for the amino group
Me2Si
+
Me2Si
+
SiMe2
N
Me
Me
0.07
Me
but it is considerably larger for
SiMe2
As
0.33
predicted by an NBO analysis is
Me
Me
0.5
0.4
SiMe2
Sb
SiMe2
P
0.6
0.3
+
Me2Si
0.7
0.1
* Financial Support from the German-Israeli Foundation (GIF)
unit to the Me2Si+-R group as
0.8
0.38
Me
0.25
the phosphino group. It remains
approximatively constant for the
arsonium and stibonium cations.