A general synthetic route to a novel type of triamino-substituted planar carbenium ions (5) is reported. The synthetic method is based on a facile and selective nucleophilic aromatic substitution on the tris(2,4,6-trimethoxyphenyl)carbenium ion (1) with amines and gives access to a wide variety of more complex aminosubstituted carbenium ions. X-ray crystallography shows that the 2,6,10-tris(N-pyrrolidinyl)-4,8,12-trioxatriangulenium ion (5b) is planar and forms segregated stacks of cations and PF 6 anions in the solid phase. The stability of the 2,6,10-tris(diethylamino)-4,8,12-trioxatriangulenium ion 5a is expressed as the pK R+ value, which is determined in strongly basic nonaqueous solution on the basis of a new acidity function C_. The pK R+ value of 5a is measured to be 19.7, which is 10 orders of magnitude higher than the values found for the most stable carbenium ions previously reported. Electrochemical reduction of compound 5a leads to rapid dimerization. Two consecutive one-electron oxidations are identified by cyclic voltammetry.
Synthetic procedures and characterization data S2-S9 1 H-NMR and 13 C-NMR spectra S10-S20 General methods. Molecular weights were determined using size exclusion chromatography in HPLC-grade o-dichlorobenzene (ODCB) at 80 °C against polystyrene standards on a Polymer Laboratories-GPC 120 high temperature chromatograph, a PD 2040 high-temperature light scattering detector, and a Midas autosampler. A mixed-C 300 × 7.5 mm column was used, together with a precolumn. The flow rate was 1 mL/min and the injection volume 100 μL. UV-vis absorption spectra were measured with a Perkin-Elmer Lambda 900 spectrometer. TGA experiments were performed with a dynamic heat rate (10 °C/min) under an argon atmosphere (50 ml/min) in the temperature range 50-500 °C. Unless stated otherwise all reagents and solvents were obtained from Aldrich and used without further purification. Dichloromethane, DMF and toluene were dried with molecular sieves (3 Å) and used directly without filtration or distillation. NBS was recrystallised from water and dried at 70 °C in vacuum. Evaporation was performed on a rotary evaporator at 40 °C. NMR spectra were obtained on Bruker 500 MHz or 250 MHz spectrometers. High resolution S1mass spectra were recorded on a tandem mass spectrometer. Melting points were determined on an electrothermal instrument and are uncorrected. The samples were dried at 50 °C for 24 hours in a vacuum oven prior to analysis. 5-bromothiophene-3-carboxylic acid 1 was prepared according to literature procedures.1,2-bis(tetradecyloxy)benzene 2 (1). To a solution of catechol (10 g, 0.091 mol) in dry DMF (50 ml) was added 1-bromotetradecane (0.209 mol, 58 g, 62 mL) and K 2 CO 3 (38 g, 0.27 mol). The mixture was stirred at 100 °C under a nitrogen atmosphere for 40 hours. After cooling the mixture to room temperature (RT), 100 ml of water were added. The organic layer was separated and the aqueous layer was extracted with DCM. The combined organic phase was dried over MgSO 4 . After filtration, the mixture was concentrated under vacuum. The product was recrystallized twice from acetone. Yield: 41 g (90%), white needlelike crystals.
The title compound (4) was synthesized, and its
crystalline structure was determined. The molecule
has
C
3
v
point symmetry and
crystallizes in the trigonal space group R3m.
Crystal data for 4: a = 16.6710(13)
Å, b =
16.6710(13) Å, c = 4.2590(3) Å, α = β
= 90°, γ = 120°, Z = 3,
R(F) = 0.0234. The material has a
permanent
polarization and consequent pyroelectric properties. The room
temperature pyroelectric coefficient was found to be
−3 ± 2 μC m-2
K-1, which is in accordance with a calculated
value of −3.2 μC m-2
K-1. The molecular dipole
moment was determined to be 3.3 ± 0.2 D, the direction of which was
unambiguously assigned with respect to the
molecular coordinates. The thermal expansivity was determined at
temperatures in the range −93 to 200 °C. The
relative dielectric permittivity tensor was obtained at optical
frequencies (ε11 and ε22 = 3.16 and
ε33 = 2.48) and in
the microwave region at 35 GHz (ε11 and ε22
= 5.2 ± 0.6 and ε33 = 2.9 ± 0.2), and at low
frequencies (120 Hz and
1 kHz), the isotropic permittivity was determined
(ε120
Hz = 4.7 ± 0.8 and
ε1
kHz = 4.7 ± 1.1). Finally, an
estimate
of the molecular heat capacity was calculated
(C
p
= 900 J
Kg-1 K-1) and the
material was considered for potential
use in infrared detection as its detectivity merit factor,
M
r, was determined (M
r
= 8.8 × 10-2 m2
C-1).
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