Preparation and X-ray structure of 4-N,N′-bis(trimethylsilyl)-amino-3,5-diisopropylphenylselenium trichloride

Article abstract of DOI:10.1515/znb-1999-0913

The reaction of SeCl₄ or SeCl₂ with N,N′-bis(trimethylsilyl)-2,6-diisopropylaniline occurs not at the nitrogen atom but by electrophilic aromatic substitution at C-4 of the phenyl ring to give [(CH₃)₃Si]₂

Full text of DOI:10.1515/znb-1999-0913

Preparation and X-Ray Structure of 4-N,N'-Bis(trimethylsilyl)-
amino-3,5-diisopropylphenylselenium Trichloride
Arto Maaninena, René T. Boeré*,b, Tristram Chivers0, and Masood Parvezc
d Department of Chemistry, University of Oulu, Linnanmaa, FIN-90570, Finland
b Department of Chemistry and Biochemistry, University of Lethbridge,
Lethbridge, Alberta, Canada TI K 3M4
LDepartment of Chemistry, University of Calgary, Calgary, Alberta, Canada T2N 1N4
* Reprint requests to Prof. Dr. R. T. Boere. Fax: 001-403-329-2057. E-mail: boere@uleth.ca
Z. Naturforsch. 54 b, 1170-1174 (1999); received April 22, 1999
Crystal Structure, 2,6-Diisopropylaniline, Aryl Selenium Trichloride, Bridging Chloride,
NMR Data
The reaction of SeCU or SeCF with /V,/V’-bis(trimethylsilyl)-2,6-diisopropylaniline occurs
not at the nitrogen atom but by electrophilic aromatic substitution at C-4 of the phenyl ring
to give [(CH
triclinic system, space group Pi, a - 10.2598(17), b = 13.665(3), c = 9.7838(10) A, a =
90.056(13), ß = 102.439(11), = 70.922(14)°, V = 1262.3(4) A \Z = 1. The dimer contains
an essentially planar CUSe^-Cl^SeCU unit, with trans apical (Me SihNC H2('Pr)2 groups,
3
)
3
Si]
2
NC
6
H2
('Pr:)SeCl
3
, which crystallizes as the chloro-bridged dimer in the
7
3
6
resulting in approximately square pyramidal geometry at Se. The bridging Se-Cl distances are
unequal at 2.587(2) and 2.749(2) A.
Introduction
only the third reported for RSeCU compounds, each
of which has a different type of dimeric structure.
The reactions of selenium halides, in particular
SeCl2, with primary amines are being investigated
in our laboratories [1]. The preparation of cyclic
or linear selenium imides is one goal of this re-
search. We have examined the reactions of a num-
ber of amines and their trimethylsilylated deriva-
tives, and find that the silylated analogues give
cleaner reactions, presumably because of the ab-
sence of side reactions due to generated HC1. 2,6-
Diisopropylaniline (DipNFb) is a sterically con-
gested primary amine, which may be expected to
produce unusual selenium imide structures com-
pared to the less bulky amines investigated so far.
Although the monosilylated N-trimethylsilyl-2,6-
diisopropylaniline has been reported [2], and its
lithium derivative has been prepared and struc-
turally characterized [3], the fully silylated analogue
has not been reported to our knowledge. In this work
we report on the preparation of N,/V-bis(trimethyl-
silyl)-2,6-diisopropylaniline (1) and studies of its
reactions with SeCU and SeCU. The initial product
of the reaction with SeCU is a yellow crystalline
solid [(CH3)3Si]2NC6H2(,'Pr2)SeCl3 (2), shown by
crystallography to have an SeCU group at the C-4
position of the aromatic ring. The structure of 2 is
Results and Discussion
The silylated aniline was prepared in a straight-
forward manner from the corresponding aniline us-
ing two equivalents of ;?BuLi, followed by refluxing
the anilide with excess ClSi(CH3)3 in diethyl ether,
according to ( 1) and (2).
DipNH2 + 2 »BuLi -> [Li+]2DipN2~
[Li+]2DipN2- + 2 ClSi(CH3)3
(1)
-> DipN[Si(CH3)3]2 + 2 LiCl
(2)
DipN[Si(CH3)3]2 (1) is easily purified by vacuum
sublimation to give colourless crystals that have
been fully characterized by elemental analysis, mass
spectrometry and solution NMR. The structure of 1
has been calculated using AM 1 semi-empirical MO
techniques, and is presented with relevant M O ’s in
Fig. 1. Of particular note is the perpendicular ori-
entation of the aromatic ring to the Si-N-Si plane.
This structural feature is also found in the crystal
structure of 2. A co-planar structure, which is pre-
dicted by AMI calculations for the parent DipNFU,
is not stable to structure minimization for 1, but a
K
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A. Maaninen et al. • 4-N,N'-Bis(trimethylsilyl)amino-3,5-diisopropylphenylselenium Trichloride
1171
(b)
In the solid state at 25 °C and in CH2CI2 so-
lution, even at -2 0 °C, the trichloride 2 decom-
poses to give the corresponding monochloride,
[(CH3)3Si]2NC6H2 ('Pr2)SeCl (3):
[(CH3)3Si]2NC6H2 ('Pr2)SeCl3
-> [(CH3)3Si]2NC6H2 ('Pr2)SeCl + Cl2.
(4)
Although both TeCl4 and SeCl4 undergo para-
directed electrophilic aromatic substitution in reac-
tions with CH3OC6H5 to give 4-CH3OC6H5TeCl3
[5] and (4-CH3OC6H5)2SeCl2 [6], respectively, the
preference for this reaction pathway over Si-N bond
cleavage with 1 was unexpected. A reasonable ex-
planation for this preference is the fact that the N
lone-pair orbital of 1 (Fig. lc) is buried below the
two higher lying tt orbitals of the aromatic ring, one
of which is the HOMO (Fig. lb). The AMI cal-
culations show that the highest charge density on
the aromatic ring is at C-4. Normally, amino groups
are 0 ,/7-directing (leading to substitution at C-4, as
observed), while alkyl groups are also o,p directors,
but of weaker activating character (consistent with
non-substitution at C-3,5). However, most of the
activating character of the amino group is usually
attributed to 7r-delocalization into the N lone pair or-
bital. This mechanism cannot operate in 1 because
of its twisted structure. It is known, however, that
large alkyl substituents can exert considerable steric
influence over the regioselectivity of electrophilic
aromatic substitution. Certainly in the crystal struc-
ture of 2 (see below) the isopropyl groups overhang
the two meta sites of the substituted benzene ring.
Fig. 1. (a) Line drawing of AMI calculated structure of
1 and topological sketch of (b) the HOMO and (c) the
HOMO-3 orbitals, the latter corresponding to the “nitro-
gen lone pair”.
static calculation places it several hundred kJ mol-1
higher in energy than the perpendicular form. The
origin of this conformational preference seems to be
steric crowding by the 2,6-diisopropyl groups. As
a consequence the HOMO of 1 is one of the occu-
pied 7r-orbitals of the aromatic ring, rather than the
nitrogen atom lone pair. The planar N p; orbital is
fully delocalized into the Si-C anti-bonding orbitals
in an MO that is highly reminiscent of the HOMO
of N[Si(CH3)3]3, and is the third-highest orbital in
the stack, located 0.46 eV below the HOMO.
The SeCl3 derivative 2 was first obtained from a
reaction of 1 with SeCb, which implies that SeCL
disproportionates into SeCl4 and Se2Cl2 under the
reaction conditions. Indeed, the concomitant for-
mation of Se2Cl2 is evidenced by a peak at 6 1279
in the 77Se NMR spectrum in CH2CL solution (cf.
b 1279.5 in the same solvent relative to external
(CH3)2Se) [4], The 77Se NMR spectrum of 2 dis-
plays a peak at 6 940. The same product was ob-
tained more cleanly when SeCl4 was used instead
of SeCl2 . The 1H NMR spectrum of 2 revealed that
the Si(CH3 )2 groups have been retained, and the sin-
gle aromatic resonance integrating to 2H suggested
that para substitution of the aromatic ring has taken
place. Slow crystallization of 2 from pentane pro-
vided X-ray quality crystals, and the crystal struc-
ture confirmed that electrophilic aromatic substitu-
tion has occurred to give 4-N,N'-bis(trimethylsilyl)-
amino-3,5-diisopropylphenylselenium trichloride:
In the solid state, 2 exists as a chloro-bridged
dimer (Fig. 2 and Table I). In a tetra-substituted
RSe(IV)X3 derivative the geometry at Se is expected
to be trigonal bipyramidal with the aryl ring in one
of the equatorial sites. However, in the dimer there
is an essentially planar Cl2Se(/./-Cl)2 SeCl2 unit with
trans apical (Me3Si)2NC6H2 ('Pr)2 groups, resulting
in an approximately square pyramidal geometry at
Se. The central (p-C 1)2 bridge is unsymmetrical,
with short and long Se-Cl distances of 2.587(2) and
2.749(2) A. Although ArSeCl3 compounds are well-
known [7 and references therein], there are only
two for which structures are listed in the Cambridge
Crystallographic Database (Fig. 3) [8]. Both 4 and 5
have folded Se(/7-Cl)2Se units. In 4 the Se-C bonds
are arranged cis and inward-pointing, as required by
the orr/20-disubstituted benzene ring [9], whereas in
DipN[Si(CH3)3]2 + SeCl4
-»• [(CH3)3Si]2NC6H2 ('Pr2)SeCl3 + HC1.
(3)
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1172
A. Maaninen et al. • 4-N,N'-Bis(trimethylsilyl)amino-3,5-diisopropylphenylselenium Trichloride
Table I. Selected bond lengths (A) and angles (deg) for 2.
of the benzene backbone in 4. The bridge in 5 is
symmetrical on one side (Se-Cl distances 2.65(2) A)
and asymmetrical on the other (2.51(2), 2.75(2) A),
but on average they are only marginally shorter than
in 2. The iminophosphine derivative Ph^P^NSeCl^
(6 ) exhibits a dimeric structure similar to that of 2 in
the solid state [11]. The Ph^P^N groups are in trans
positions and the bridging Se-Cl distances in the
almost planar C LSe^-C l^SeC b unit are 2.634(2)
and 2.983(2) A.
There are numerous ArTeCl^ structures in the lit-
erature. The structural motifs observed are indicated
by representative examples in Fig. 3. A trans-pla-
nar structure, very similar to that of 2 , is observed in
7 [12]. Like 2, this compound crystallizes about an
inversion centre, and it is likely that the planar struc-
tures of both compounds are determined by crystal
symmetry. However, in 7 there are additional long
contacts between the Te and Cl atoms of neighbour-
ing dimers, thus completing a roughly octahedral
coordination at Te. In 2 such tertiary interactions are
prevented by the bulk of the aryl substituent. The
other tellurium structural types include polymeric
CöHsTeCli (8 ), which shares the square pyramidal
local geometry of 7, but with approximately lin-
ear bridging Cl atoms in an infinite array [13], In
addition there are many examples of compounds
internally chelated by aryl groups bearing N, O, S
and even Se donor groups, of which 9 is a typical
example [14], Such compounds always crystallize
as discrete molecules.
Bond lengths:
2.257(2)
2.749(2)
1.764(5)
1.868(7)
1.834(8)
1.855(8)
1.442(7)
2.206(2)
2.587(2)
1.955(6)
1.736(5)
1.861(8)
1.858(8)
1.851(8)
Se( 1)-Cl(2)
Se(l)-Cl(3*)
Si( 1)-N( 1)
Si( 1)-C( 13)
Si( 1)-C( 15)
Si(2)-C( 17)
N(l)-C(l)
Se(l)-Cl(l)
Se(l)-Cl(3)
Se(l)-C(4)
Si(2)-N(l)
Si( 1)-C( 14)
Si(2)-C( 16)
Si(2)-C( 18)
Bond angles:
89.67(9)
96.3(2)
90.73(8)
85.42(7)
91.7(2)
124.5(3)
118.5(4)
118.8(5)
Cl(l)-Se(l)-Cl(2)
Cl(l)-Se(l)-Cl(3*) 170.61(8)
Cl(l)-Se(l)-Cl(3)
Cl(l)-Se(l)-C(4)
Cl(2)-Se(l)-Cl(3*)
Cl(3)-Se(l)-Cl(3*)
Cl(3*)-Se(l)-C(4)
93.79(9)
Cl(2)-Se(l)-Cl(3)
Cl(2)-Se(l)-C(4)
Cl(3)-Se(l)-C(4)
Se( 1)-Cl(3)-Se( 1)
Si(l)-N(l)-C(l)
Se(l)-C(4)-C(3)
175.39(9)
92.7(2)
90.0(2)
94.58(7) Si( 1)-N( 1)-Si(2)
117.0(4)
119.0(5)
Si(2)-N(l)-C(l)
Se( 1)-C(4)-C(5)
Experimental Section
General
Solvents were dried and distilled twice under an ar-
gon atmosphere from P O10 (dichloromethane and pen-
4
tane) and sodium/benzophenone (ether). All reactions and
the manipulations of air- and moisture-sensitive reagents
were carried out under an argon atmosphere passed
through P O10. DipNH:, CISiMe? (Aldrich) and selenium
4
tetrachloride (Alfa) were used without further purifica-
tion. 'H, 1?C and 77Se NMR spectra were recorded on
a Bruker AM 400 spectrometer operating at 400.135,
100.614 and 76.312 MHz, respectively. The spectral
widths were 4.50, 23.81 and 100.00 kHz, yielding the re-
spective resolutions of 0.55, 1.45 and 6.10 Hz/data point.
The 'H pulse width was 3.0 ^/s, for l3C 3.5 /us and for
77Se 9.0 /zs. The 1H accumulations contained 16, l?C 400
- 1000 and77Se 5000 - 30000 transients. Relaxation times
used were: 'HOs, 13C 5.0 s and77Se 2.0 s. The 'Hand l3C
chemical shifts b [ppm] are referenced to TMS. The 77Se
Fig. 2. ORTEP drawing of the dimeric structure of 2.
5 the cis CF3 groups point outward [10]. In 4, the dis-
tances to the bridging Cl atoms are also unequal at
2.497(3) and 2.697(4) Ä, on average 0.07 A shorter
than in 2 . which may be attributed to the constraint
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A. Maaninen et al. •4-N,N'-Bis(trimethylsilyl)amino-3,5-diisopropylphenylselenium Trichloride
1173
Fig. 3. Structural motifs of RSeCl and
3
RTeCb as determined by X-ray crystal-
lography.
9
Table II. Crystal data for 2.
lock solvents. Mass spectra and elemental analyses were
undertaken in the Department of Chemistry, University
of Calgary and in the Fachbereich Chemie, Universität
Kaiserslautern (Germany).
Formula
Fw.
Crystal system
C
36
H
68
N
4
Cl
6
Si4
Se
2
1039.94
triclinic
Space group
« [A]
MA]
Pi (No. 2)
10.2598(10)
13.665(3)
Preparation of N,N'-bis(trimethylsilyl)-2,6-diisopropyl-
aniline (\)
A solution of 5.0 g (28 mmol) diisopropylaniline
in 1 0 0 ml of anhydrous diethyl ether was treated with
59.2 mmol of 2.5 M nBuLi in hexanes at 0 °C, and stirred
for an additional 20 min at the same temperature. 7.66 g
9.7838(10)
90.056(13)
102.439(11)
70.922(14)
c [A]
a [deg]
ß [deg]
7 [deg]
V [Ä3]
Z
dcalc [g cm "’]
Crystal dim. [mm]
Cell detn, refl
Cell detn, 29 range,Jdeg]
Radiation, MoKQ [A]
Temperature [K]
p [cm-1]
Absorption correction
Absorptions, range
Total reflections
1262.3(4)
1
(70.5 mmol) ClSiMe was added dropwise with stirring.
3
After completion of the addition, the mixture was warmed
to room temperature and then heated to reflux for 4 h,
whereupon it was hot-filtered, the solvent removed un-
der vacuum and the crude product sublimed (80 °C, 4
1.368
0.40x0.35x0.30
24
18.5-24.5
0.71069
170
x
1 0 - 2 mbar) to give the product as colourless crystals,
m. p. 141 - 143 °C. Yield: 7.2 g, 22 mmol, 80%.
Analysis for Ci (321.66)
18.91
8
H
35NSi
2
empirical
0.9385- 1.0000
4735
4461
2256
226
0.045; 0.036
1.91
0 . 0 1
Calcd C 67.2 H 11.0 N4.4%,
Found C 67.2 H 10.9 N 4.2%.
'H NMR (CDCI ): 6 0.11 (s, 18H), 1.21 (d, 7(H-H)
3
Unique reflections
Data with I > 3cr(I)
Parameters refined
6 . 8 Hz, 12H), 3.49 (sept, 7(H-H) 6 . 8 Hz, 2H), 8 7.07
(m, 3H). I3C NMR: <52.79, 25.28, 27.71, 123.76, 124.43,
143.47, 147.24. MS(70eV): m/z 306 (M-CH3+, 100%);
264 (M-'Pr+, 20%); 232 (M-Si[CH3]4+, 53%); 218 (M-
NSi[CH3]4+, 75%); 73 (Si[CH3]3+, 33%).
R\ Rn
GOF
Largest Ala
Final diff. map [e Ä- ’]
F(000)
0.53, -0.47
538
Preparation of 4-N,N'-bis(trimethylsily1)amino-3,5-di-
isopropylphenyl selenium trichloride (2) and 4-N,N'-
bis(trimethylsilyl)amino-3,5-diisopropylphenyl selenium
chloride (3)
chemical shifts were measured relative to external SeOi
(saturated aqueous solutions), but are reported relative to
neat Me2Se at room temperature {
6
(Me
2
Se) = <
5
(Se
0
2
)
A solution of
1
(0.643 g, 2.0 mmol) in CH
2
CI2 was
+ 1302.6}. CDCI and CD CI were used as internal 2H
3
2
2
added to a slurry of selenium tetrachloride (0.442 g,
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1174
A. Maaninen et al. ■4-N,N'-Bis(trimethylsilyl)amino-3,5-diisopropylphenylselenium Trichloride
CI (10 ml) at room temperature. The re- The non-hydrogen atoms were refined anisotropically,
2.0 mmol) in CH
2
2
action mixture was stirred for 2 h, after which the reddish-
and the H atoms were constrained to idealized posi-
brown solution was evaporated to dryness. The solid was tions (C-H = 0.95 A). Conventional atomic scattering
extracted with pentane, filtered, and the extract cooled
to -20 °C to give yellow crystals of 2 suitable for X-
ray crystallography. Yield: 0.52 g, 1.0 mmol, 51%. 'H
NMR (CH2C12/CD2C12): <50.13 (s, 18H), 1.24 (d, /(H-H)
6.6 Hz, 12H), 3.55 (sept, 7(H-H) 6.6 Hz, 2H), 7.98 (s,
factors, corrected for anomalous dispersion, were used
[16], Crystallographic data (excluding structure factors)
for the structures in this paper have been deposited with
the Cambridge Crystallographic Data Centre as supple-
mentary publication no. CCDC 127456. Copies of the
2H). n C NMR (CH2Cl2/CD2Cl2): b 2.45, 24.78. 28.65. data can be obtained, free of charge, on application to
124.00, 145.03, 149.60.77Se NMR (CH
940 ppm.
2
CI
2
/CD
2
CI
2
): b CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax:
+44 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk).
Satisfactory CHN analyses for 2 could not be ob-
tained owing to facile decomposition to the corresponding
monochloride 3.
Electronic structure calculations
All calculations were performed using the AMI
method as implemented in HyperChem 5.1 running on
a Pentium II computer under Windows95 [17],
Analysis for CigH3
4
ClNSeSi (435.06)
2
Calcd C 49.69 H 7.87 N 3.22%,
Found C 48.60 H 7.68 N3.12%.
'H NMR (CD:C12): 6 0.08 (s, 18H), 1.17 (d, 12H),
3.45 (sept, 2H), 7.03 (s, 2H). I3C NMR (CD2C12): 6
2.79, 25.37, 28.00, 124.07, 131.96, 149.73. 77Se NMR
Acknowledgments
We thank the Natural Sciences and Engineering Re-
search Council of Canada for financial support (RTB and
TC), the Academy of Finland for an exchange scholar-
ship (AM), K. A. Coles for preparation of 1, and Dr. G.
Schatte for some of the NMR spectra. RTB acknowledges
support from the Alexander von Humboldt Foundation
during a sabbatical leave at Universität Kaiserslautern,
Kaiserslautern, Germany.
(CD
2
CI2 ): b 1060 ppm.
X-ray analysis
The crystal structure of 2 was determined using a
Rigaku AFC6S diffractometer. Experimental details are
summarized in Table II. The structure was solved by di-
rect methods and expanded using Fourier techniques [15],
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