Solid state, structural and solution studies on bis(2-methylbenzyl)-selenide, methyl(2,4,6-tri-t-butylphenyl)-selenide, bis(2,4,6-tri-methylphenyl)-diselenide, and bis(2,4,6-tri-t-butylphenyl)-diselenide

Article abstract of DOI:10.1016/S0022-328X(99)00343-5

Two selenides, (2-MeBz)₂Se (1) and MeMes*Se (2) and two diselenides, Mes₂Se₂ (3) and Mes*₂Se₂ (4) have been prepared and characterized by single-crystal X-ray diffraction and by NMR spectroscopy. The

Full text of DOI:10.1016/S0022-328X(99)00343-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 588 (1999) 42–50
Solid state, structural and solution studies on
bis(2-methylbenzyl)-selenide,
methyl(2,4,6-tri-t-butylphenyl)-selenide,
bis(2,4,6-tri-methylphenyl)-diselenide, and
bis(2,4,6-tri-t-butylphenyl)-diselenide
Paul M. Dickson, Margaret A.D. McGowan, Burl Yearwood, Mary Jane Heeg,
John P. Oliver *
Department of Chemistry, Wayne State Uni6ersity, Detroit, MI 48202, USA
Received 11 February 1999
Abstract
Two selenides, (2-MeBz)2Se (1) and MeMes*Se (2) and two diselenides, Mes2Se2 (3) and Mes*2 Se2 (4) have been prepared and
characterized by single-crystal X-ray diffraction and by NMR spectroscopy. The structure of 1 was solved in the orthorhombic
,
,
space group Pbca (no. 61) with cell constants a=b=13.944(10) A, c=14.30(2) A. The structure of 2 was determined in space
(
,
,
,
group P1 (no. 2) with cell constants a=5.9443(5) A, b=18.665(2) A, c=26.372(3) A, h=99.393(2)°, i=90.890(2)° and
k=92.703(2)°. The structures of 3 and 4 were determined in the monoclinic space group P21/c (no. 14) with unit cell parameters
,
,
,
,
,
for 3 of a=6.333(1) A, b=18.370(4) A, c=5.122(3) A, i=97.42(3)° and for 4, a=11.359(5) A, b=10.520(8) A, c=30.22(2)
,
A and i=98.94(4)°. A rotational process has been studied in 4 and the kinetics and thermodynamic parameters for this process
are determined from DNMR measurements using three different methods, measurement of coalescence temperatures, calculation
of kinetic parameters using the line half-width approximation, and by full line shape analysis. The results from these three
methods were compared. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Selenium; Structure; Kinetics; Mechanism; Rotational barrier; NMR
substituents have been reported [1–11], and from these
studies, it has been suggested that diselenides relieve
1. Introduction
steric strain by increasing the dihedral angle about the
SeꢀSe bond rather than by increasing the SeꢀSe and
SeꢀC bond lengths [3]. The effect of changing the
dihedral angle on UV spectra also has been examined
[3]. The substituent bound to the Se or Se2 moiety alters
the 77Se chemical shift [12], and with very bulky sub-
stituents present on the diselenides, gives rise to a slow
rotational process [13]. In this paper, we offer further
proof that this is the case by examining the crystal
structures of Mes2Se2, 3, and Mes2*Se2, 4 (Mes=2,4,6-
trimethylphenyl; Mes*=2,4,6-tri-t-butylphenyl). It also
will be seen that, in the case of 4, not all of the strain
is relieved by this increase in dihedral angle but is
manifested in other structural distortions. The dise-
Our interest in the reactions of the higher chalcogens,
S, Se, and Te, and their organic derivatives with Group
13 organometallic species necessitated the preparation
and characterization of a number of organoselenides
and -diselenides. In this paper we report the syntheses
and structures of two selenides and two aryl diselenides,
their characterization by NMR spectroscopy and by
X-ray diffraction.
A large number of organoselenides and diselenides
have been prepared, but only a few have been well
characterized by modern spectroscopic and structural
methods. The structures of diselenides with different
* Corresponding author.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00343-5

P.M. Dickson et al. / Journal of Organometallic Chemistry 588 (1999) 42–50
43
lenides were characterized by 1H-, 13C-, 77Se-, and
variable-temperature 1H-NMR in order to probe the
structure of the compounds in solution and to reexam-
ine the barrier to rotation about the SeꢀSe bond in 4
first reported by Kessler and Rundel [13]. The structure
of the first symmetric alkyl selenide, bis(2-methylben-
zyl)selenide, 1, is described.
in the freezer (−30°C) to crystallize. Colorless crystals
of 1 were obtained in quantitative yield. m.p. 74–76°C;
1H-NMR (C6D6) l 2.072 (s, 3H, CH3), 3.466 (s, 2H,
CH2), 7.05–6.90 (m, 4H, C6H4); 13C {1H}(C6D6) 26.2
(s, CH3), l 66.4 (CH2), l 122–141 (C6H4).
2.3. Preparation of MeMes*Se (2)
MeMes*Se, 2, was obtained as a product in the
2. Experimental
preparation
of
Me2InSeMes*.
Bis(2,4,6-tri-t-
butylphenyl) diselenide (8.26 g, 0.0127 mol) was added
dropwise from a syringe to a solution of Me3In·OEt2
(40 ml, 0.212 mol) in 60 ml of hexane. The orange color
of the diselenide disappeared on addition to the
trimethylindium. The mixture was stirred for 2 days at
room temperature. A yellow precipitate was formed.
The solvent was removed by vacuum to yield both
colorless and yellow crystals. NMR analysis shows the
yellow crystals are the same as the colorless crystals
except coated with a yellow diselenide layer. The
Me2InSeMes* product was removed with the solvent
and yielded no crystals. The colorless crystals of 2 were
used for diffraction studies. Yield 60% (yellow/colorless
2.1. General experimental procedures
All manipulations were performed in the absence of
water and air using standard Schlenk line and dry box
techniques [14], but the final products are air-stable.
They are, however, toxic and have a very strong stench,
so they should be kept in sealed containers at all times.
Argon was purified by passing it through a series of
columns containing Deox catalyst (Alfa), calcium sul-
fate, and phosphorus pentoxide. All solvents used were
dried using standard techniques, and all glassware was
oven-dried. 2-Bromomesitylene (Aldrich) and 2-methyl-
benzyl chloride (Lancaster) were used as received. 1-
1
crystals). m.p. 190–192°C; H-NMR (C6D6; l, ppm):
Bromo-2,4,6-tri-t-butylbenzene
was
synthesized
1.24 (s, 9H, p-C(CH3)3 of Mes*), 1.31 (s, 18H, o-
C(CH3)3 of Mes*), 1.37 (s, 3H, CH3ꢀSe), 7.38 (s, 2H,
aryl of Mes*). 13C {1H} (C6D6; l, ppm): 31.44 (p-
C(CH3)3), 31.78 (CH3ꢀSe), 32.86 (o-C(CH3)3), 33.11
(p-t-Bu), 38.97 (o-t-Bu), 122.71 (CH), 150.51 (p-C-t-
Bu).
according to published procedures [15]. H- and 13C-
NMR spectra were recorded on either a Gemini-300 or
a Varian-U500 NMR spectrometer. The chemical shifts
were referenced to the residual proton line from ben-
1
1
zene-d6 (l=7.15 ppm for H; l=128.0 for 13C). 77Se-
NMR were obtained at 20°C in C6D6 on
a
Varian-U500 spectrometer at 95.349 MHz and were
referenced to an external standard of Me2Se2 (2.48 M
solution in C6D6 270 ppm). Variable-temperature NMR
spectra were obtained on GN-300 and U-500 spectrom-
eters on a 0.077 M solution of 4 in CDCl3 over a 120°C
range (−60 to +60°C). The syntheses of 3 and 4 were
modified from the literature procedures for the synthe-
sis of the disulfides [16,17] and diselenides [18,19]. Other
procedures can be found in the paper by du Mont et al.
[20] and references therein.
2.4. Synthesis of Mes2Se2 (3)
A total of 5 g (25.1 mmol) of 2-bromomesitylene was
added to 0.74 g (30 mmol) of Mg in 200 ml of ethyl
ether and maintained at reflux until all of the bromide
had reacted (about 8 h). The solution was decanted into
a three-necked flask equipped with an argon inlet,
a condenser, and a bent side arm containing 1.98 g
(25 mmol) of gray selenium. The selenium was added
over a period of 6 h in small portions by slowly tipping
up the side arm. This yellowish solution was decanted
from any unreacted selenium and cooled to 0°C. To
this solution was added 10 ml concentrated HCl and
then oxygen was bubbled through the solution to afford
the bright orange, foul-smelling diselenide which was
extracted with ether. X-ray-quality crystals were ob-
tained by recrystallization from toluene at ambient
temperature. Yield 90%. 1H-NMR in C6D6 1.990 (s,
3H, p-CH3), 2.281 (s, 6H, 2,6-CH3), 6.636 (s, 2H,
C6H2); 13C{1H} 20.62 (p-CH3), 24.06 (2,6-CH3), 128.38
(CH), 128.98 (CꢀSe), 138.83 (p-CꢀMe), 143.53 (2,6-
CꢀMe); 77Se{1H} 0.16 M solution in C6D6 at 21°C 368
ppm.
2.2. Synthesis of (2-MeBz)2Se (1)
A solution of 2-methylbenzyl magnesium chloride
(0.056 mol in 200 ml of ether) was added dropwise to a
suspension of 4.45 g (0.056 mol) of selenium in 150 ml
of ether. The mixture was maintained at reflux for 12 h,
yielding the 2-methylbenzylseleno Grignard reagent. At
this point 5.9 ml (0.045 mol) of 2-methylbenzyl chloride
was added dropwise to the resulting colorless solution
which was stirred for about an hour during which time
a precipitate of magnesium chloride formed. The result-
ing colorless solution was decanted from the remaining
selenium; the volume of solvent was reduced and placed

44
P.M. Dickson et al. / Journal of Organometallic Chemistry 588 (1999) 42–50
2.5. Synthesis of Mes*Se2 (4)
[21]. Data for 2 were collected at room temperature on
a Siemens P4/CCD SMART system [22]. A total of 1390
frames was collected at a distance of 5 cm at 10 s
frame−1 yielding 14 558 integrated reflections. Data
were processed and unit cell constants were refined with
the SAINT program utilizing 2482 reflections [22]. Ab-
sorption corrections were rejected because they did not
improve the quality of data. The structure of 1 was
determined successfully in the orthorhombic space
group Pbca (no. 61). Compound 2 was found to be in
the triclinic cell system and was solved in space group
2
A total of 3 g (9.2 mmol) of 1-bromo-2,4,6-tri-t-
butylbenzene was added to 0.22 g (9.2 mmol) of Mg in
200 ml of THF and maintained at reflux until all of the
Mg had reacted (about 20 h). The solution was de-
canted to a three-necked flask equipped with an argon
inlet, a condenser, and a side arm containing 0.72 g (9.1
mmol) of gray selenium which was added over a period
of 6 h in small portions. This yellowish solution was
decanted from any unreacted selenium and cooled to
0°C. To this solution was added 10 ml concentrated
HCl and then oxygen was bubbled through the solution
to afford the bright orange, foul-smelling diselenide
which was extracted with THF. X-ray-quality crystals
were obtained by recrystallization from toluene at am-
(
P1 (no. 2). There are three independent molecules in
the asymmetric unit. Compounds 3 and 4 were assigned
to the space group P21/c (no. 14) based on systematic
absences. Relevant crystallographic parameters for 1–4
are given in Table 1. Structure solution and refinement
was carried out using SHELXS-86 [23] and SHELXL-93
[24]. No corrections were made for secondary extinc-
tion. Absorption corrections made on 1, 3, and 4, using
semiempirical Psi scans, resulted in little or no improve-
ment of the structures. No absorption correction was
made on 2. The solutions for 1–4 were obtained using
direct methods which allowed for assignment of the
selenium atoms. The carbon atoms were located in
subsequent refinement cycles. The hydrogen atom posi-
tions were calculated and riding on the carbon atoms to
bient temperature. Yield 50%. H- and 13C-NMR are
similar to those reported by du Mont [20]. H-NMR in
1
1
C6D6 1.248 (s, 9H, p-C(CH3)3), 1.428 (broad s, 18H,
2,6-C(CH3)3), 7.381 (broad s, 2H, C6H2); 13C{1H} 31.43
(p-C(CH3)3), 33.50 (2,6-C(CH3)3), 35.05 (p-C(CH3)3),
39.30 (2,6-C(CH3)3), 122.64 (CH), 150.45 (p-C-t-Bu),
156.48 (2,6-C-t-Bu); 77Se{H} 0.077 M solution in C6D6
at 21°C 521 ppm.
,
which they were bound (dCH=0.96 A).
3. Crystallographic experimental
Crystals of 1–4 were grown as described in Section 2.
In each case, a suitable crystal was glued to the end of
a glass fiber or sealed in a thin-walled capillary tube,
and mounted on a goniometer head. 1, 3 and 4 were
placed on a Nicolet P21 diffractometer for data collec-
tion. Data reduction was carried out using SHELXTL-PC
4. Results and discussion
The two diorganoselenides, (2-MeBz)2Se (1) and
MeMes*Se (2) and the diorganodiselinides Mes2Se2 (3)
and Mes2*Se2 (4) were prepared as described. Com-
Table 1
Selected experimental parameters for the X-ray diffraction studies of (2-MeBz)2Se (1), MeMes*Se (2), Mes2Se2 (3) and Mes2*Se2 (4)
Compound
1
2
3
4
Formula
C16H18Se
289.26
Orthorhombic
Pbca (no. 61)
13.944(10)
13.944(10)
14.30(2)
90
90
90
2779(4)
8
1.383
C19H32Se
339.41
Triclinic
C
18H22Se2
C36H58Se2
648.74
Monoclinic
P2(1)/c (no. 14)
11.359(5)
10.520(8)
30.22(2)
90
98.94(4)
90
3567(4)
4
Formula weight
Crystal structure
Space group
396.28
Monoclinic
P2(1)/c (no. 14)
6.3330(10)
18.370(4)
15.122(3)
90
97.42(3)
90
1744.5(6)
4
(
P1 (no. 2)
,
a (A)
5.9443(5)
18.665(2)
26.372(3)
99.393(2)
90.890(2)
92.703
2882.7(5)
6
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
3
,
V (A )
Z
Dcalc. (g cm−3
F(000)
)
1.173
1080
1.509
792
1.208
1368
1184
R1 (I\2|I)
wR2 (I\2|I)
R1 (all data)
wR2 (all data)
0.0652
0.1068
0.182
0.141
0.898
0.0778
0.1500
0.230
0.218
0.922
0.0437
0.0964
0.0638
0.1046
1.071
0.0608
0.0886
0.1425
0.1077
0.888
Goodness-of-fit

P.M. Dickson et al. / Journal of Organometallic Chemistry 588 (1999) 42–50
45
mined and are shown in Figs. 3 and 4 with selected
bond distances and bond angles given in Table 3. The
SeꢀSe and SeꢀC bond distances fall within the expected
range.
It has been reported that diselenides relieve steric
strain by increasing the CꢀSeꢀSeꢀC dihedral angle
rather than by increasing the SeꢀSe and SeꢀC bond
lengths [3]. The current studies support this with the
CꢀSeꢀSeꢀC dihedral angle in 3 as being 83.5°, which is
well within the range of 73–87° reported for many of
the less bulky diselenides, while that of 4, 93.3°, lies
between these and the more sterically hindered dise-
lenides such as [(Me3Si)3C]2Se2, (2,6-Mes2C6H3)2Se2,
and [(t-Bu)2CH]2Se2 which have dihedral angles ranging
from 102.4 to 180° (see Table 4).
Fig. 1. Thermal ellipsoid diagram (50% thermal ellipsoids) of (2-
MeBz)2Se (1). Hydrogen atoms have been omitted for clarity.
Further support for this hypothesis comes from a
comparison of 3 with bis-(2,4,6-tris-trifluoromethyl-
phenyl)-diselenide [4], which shows the effects of substi-
tuting F atoms for H atoms on the methyl groups in the
2, 4 and 6 positions. This leads to an increase in the
dihedral angle from 83.5° for 3 to 104° for the fluori-
nated derivative. This increase in dihedral angle ap-
pears to be due to the increase in steric interactions
rather than to a change in the electronic proper-
Fig. 2. Thermal ellipsoid diagram (50% thermal ellipsoids) of
MeMes*Se (2). Hydrogen atoms have been omitted for clarity.
Table 2
,
Selected bond distances (A) and angles (°) for (2-MeBz)2Se (1), and
MeMes*Se (2)
(2-MeBz)2Se (1)
MeMes*Se (2)
Bond length
Se(1)ꢀC(9)
Se(1)ꢀC(1)
1.949(10)
1.983(11)
Se(1)ꢀC(1)
Se(1)ꢀC(19)
1.949(7)
1.955(8)
Fig. 3. Thermal ellipsoid diagram (50% thermal ellipsoids) of Mes2Se2
(3). Hydrogen atoms have been omitted for clarity.
Bond angle
C(9)ꢀSe(1)ꢀC(1)
98.6(4)
C(1)ꢀSeꢀC(19)
96.3(3)
pound 1 is the first dialkyl selenide without a ketone or
other functional group attached at the C(2) position, 2
is a simple selenide with one substituent, the very bulky
super mesityl group. Both were characterized by X-ray
diffraction. Diagrams of 1 and 2 with atom labeling
schemes are shown in Figs. 1 and 2. Selected bond
distances and angles are given in Table 2. The observed
bond distances and angles are similar to those reported
for other aryl and alkyl selenides [25,26].
Compounds 3 and 4 have similar substituents bound
to the SeꢀSe backbone with 4 again containing the very
bulky Mes* group. Their structures have been deter-
Fig. 4. Thermal ellipsoid diagram (50% thermal ellipsoids) of
Mes2*Se2 (4). Hydrogen atoms have been omitted for clarity.

46
P.M. Dickson et al. / Journal of Organometallic Chemistry 588 (1999) 42–50
Table 3
,
Selected bond distances (A) and angles (°) for Mes2Se2 (3) and
Mes*Se2 (4)
2
Mes2Se2 (3)
Mes*Se2 (4)
2
Bond length
Se(1)ꢀC(1)
Se(1)ꢀSe(2)
Se(2)ꢀC(10)
Scheme 1. View down the SeꢀSe bond of a typical diselenide.
1.925(6) Se(1)ꢀC(1)
2.328(1) Se(1)ꢀSe(2)
1.929(6) Se(2)ꢀC(7)
1.932(7)
2.345(2)
1.954(7)
Bond angle
C(1)ꢀSe(1)ꢀSe(2)
C(10)ꢀSe(2)ꢀSe(1)
C(2)ꢀC(1)ꢀSe(1)
101.1(2) C(1)ꢀSe(1)ꢀSe(2)
100.5(2) C(7)ꢀSe(2)ꢀSe(1)
119.6(5) C(6)ꢀC(1)ꢀSe(1)
119.1(5) C(2)ꢀC(1)ꢀSe(1)
119.9(5) C(12)ꢀC(7)ꢀSe(2)
118.9(5) C(8)ꢀC(7)ꢀSe(2)
83.4(3) C(1)ꢀSe(1)ꢀSe(2)ꢀC(7)
103.1(2)
102.8(2)
120.3(6)
120.7(6)
119.6(6)
118.8(6)
93.3(3)
C(6)ꢀC(1)ꢀSe(1)
C(15)ꢀC(10)ꢀSe(2)
C(11)ꢀC(10)ꢀSe(2)
C(1)ꢀSe(1)ꢀSe(2)ꢀC(10)
Se(1)ꢀC(1)ꢀC(2)ꢀC(7)
Se(1)ꢀC(1)ꢀC(6)ꢀC(9)
Scheme 2. SeꢀCꢀCꢀC torsion angles Seꢀ1ꢀ2ꢀ7 and Seꢀ1ꢀ6ꢀ6.
2.2(8) Se(1)ꢀC(1)ꢀC(2)ꢀC(13) 23.0(10)
1.4(8) Se(1)ꢀC(1)ꢀC(6)ꢀC(21) 22.2(10)
Se(2)ꢀC(10)ꢀC(11)ꢀC(16) 3.8(8) Se(2)ꢀC(7)ꢀC(8)ꢀC(25) 25.2(10)
Se(2)ꢀC(10)ꢀC(15)ꢀC(18) 2.9(8) Se(2)ꢀC(7)ꢀC(12)ꢀC(33) 21.3(10)
When the bulky substituent Mes* is present, the
structures are distorted. In 2, the SeꢀC(1)ꢀC(6)ꢀC(15)
and the SeꢀC(1)ꢀC(2)ꢀC(7) torsion angles are 21.9 and
21.0°, and the corresponding torsion angles in 4 are
18.8 and 19° (see Scheme 2). This indicates that there
is substantial steric interaction resulting in the Se atom
and ortho tertiary butyl groups being bent out of the
plane of the phenyl ring in opposite directions.
Another way to describe this is to note that the
t-butyl groups in the ortho positions are bent away
from the ring plane by nearly 10° while the para
t-butyl group is a less severe 4° out of the plane.
These distortions are seen graphically in Scheme 3. In
all of the other structurally characterized aromatic dis-
elenides with ortho groups, this angle ranges from 0 to
7°. Even greater distortions are found in the structure
of Mes2*Te2 with the Te atoms and the t-butyl groups
bent out of the plane of the phenyl ring, giving
TeꢀCꢀCꢀC torsion angles of 29.0 and 26.2°.
ties of the compound. The electronic effects appear to
have the opposite effect as shown in the comparison
of diphenyl diselenide and bis(perfluorophenyl) dise-
lenide, which shows a decrease in the dihedral angle
from 82 to 75.3° [9,27]. This may be the result of the
fluorinated aromatic ring removing electron density
from the lone pairs on the selenium atoms, allowing
the system to adopt a more eclipsed configuration
(Scheme 1).
The structure of 3 can also be compared to the
tellurium analog [28]. The ditelluride has a dihedral
angle of 93.9° compared to 83.5° for 3. In general, the
dihedral angle in a given species increases as the
chalcogen is changed from S to Se to Te.
Table 4
CꢀSeꢀSeꢀC torsion angle and SeꢀSe bond distance in selected diselenides
,
Compound
Torsion angle (°)
SeꢀSe bond distance (A)
Reference
[(Me3Si)3C]2Se2
(2,6-Mes2Ph)2Se2
[(t-Bu)2CH]2Se2
[2,4,6-(CF3)3Ph]2Se2
(2-NO2Ph)2Se2
(p-CH3Ph)Se2
Mes2*Se2
(p-NO2Ph)Se2
Mes2Se2
Ph2Se2
(Ph2CH)2Se2
{2,6-[1-(EtO)Et]2Ph}2Se2
(p-ClPh)2Se2
(F5C6)2Se2
(2,6-i-Pr2Ph)2Se2.I2
180.0
128.2
112.0
104.0
102.4
99.7
93.3
87.8
83.5
82.5
82.0
77.2
74.2
73.6
73.0
2.387
2.339
2.315
2.310
2.330
2.328
2.347
2.302
2.330
2.287
2.285
2.328
2.332
2.319
2.354
[1]
[2]
[3]
[4]
[5]
[6]
This work
[7]
This work
[9]
[10]
[34]
[8]
[27]
[11]

P.M. Dickson et al. / Journal of Organometallic Chemistry 588 (1999) 42–50
47
are relatively low and that the mesityl group is not
sufficiently bulky to hinder rotation in this molecule.
In 4, the 13C- and 77Se-NMR spectra show the ex-
pected patterns and are reported in Section 2. Fig. 5
1
shows a stacked plot of the 500 MHz H-NMR spec-
Scheme 3. Chem 3D drawing of MeMes*Se (2), viewed along phenyl
ring edge.
2
trum of the t-butyl region as a function of temperature.
The spectrum at 55°C represents the fast exchange limit
and shows the pattern expected for an Mes* group. The
slow exchange limit is best seen in the −40°C spectrum
where the ortho t-butyl groups become non-equivalent;
similarly, the meta ring protons are non-equivalent at
this temperature. In the intermediate region, the spec-
trum exhibits the normal behavior for an exchanging
system.
The mechanism proposed for this process has been
described by Kessler and Rundel [13] and is shown in
Scheme 4. It consists of two rotational processes, one
about the SeꢀSe bond and the second about the SeꢀC
bond. The barrier measured is for the rate-determining
process. Thermodynamic and activation energy
parameters were calculated from variable-temperature
NMR data using three different methods: (1) from the
coalescence temperature [13]; (2) calculation of the rate
parameters from the half width [30]; and (3) by a
complete line shape analysis using the WinDNMR
program [31].
4.1. NMR studies
It has been shown that some hindered disulfides,
diselenides [13], and ditellurides [29] exhibit slow rota-
tion about the EꢀE and/or the EꢀC bonds which can be
seen as the alkyl groups in the 2 and 6 positions are
locked in a conformation where they are non-equiva-
lent. Kessler and Rundel [13] have discussed this in
some detail and have reported thermodynamic and
kinetic parameters for several systems, including 4, and
du Mont et al., have discussed this for the analogous
ditelluride [29].
With the availability of a high-field NMR spectrome-
ter, we decided to reexamine this process and studied
1
both 3 and 4 by H-, 13C-, 77Se-, and variable-tempera-
ture NMR spectroscopy. Compound 3 shows the pre-
1
dicted NMR patterns in the H and 13C spectra and a
77Se signal at 368 ppm, consistent with other dise-
lenides, but does not show any evidence for slowing the
rotational processes at −40°C, indicating that the bar-
riers to rotation about the SeꢀSe and SeꢀC bonds in 3
The simplest approach is to use the coalescence tem-
perature. By performing the experiment on both 300
and 500 MHz spectrometers, using the coalescence of
Fig. 5. A stacked plot showing the change in line shape of the 500 MHz 1H-NMR spectrum as a function of temperature (left) and the simulated
spectrum using a two-site model (right).

48
P.M. Dickson et al. / Journal of Organometallic Chemistry 588 (1999) 42–50
The second method used to estimate the rate con-
stant is from the half width (w1/2) of the lines associated
with the exchange process. To improve the results from
this method, line widths were measured using a decon-
volution program which provided a best fit simulation
of the desired peak. The average of the two line widths
were used in the calculation of the rate constant. This
approach is limited to the slow exchange region where
the two exchanging sites give rise to separate reso-
nances that lie between 10 and −30°C for this system.
The third method used was to simulate the variable-
temperature NMR spectra using the program WinD-
NMR [31]. The advantage of this technique is that rate
constants can be obtained over a wider temperature
range, including both the slow and fast regions. Simu-
lated spectra were obtained using a two-site exchange
model (A and B) representing the two observed ortho
signals. The rate constant kab+kba was varied in order
to match the experimental spectrum. Halving this value
provided the rate constant kab for the process under
investigation. Non-exchange line widths of 0.5 Hz were
used at temperatures above −30°C, while for tempera-
tures of −30°C and below it was necessary to increase
Scheme 4.
the meta proton and the t-butyl groups, we were able to
obtain rate constants at four different temperatures.
Fig. 6. Arrhenius and Eyring plots for the rotational process observed in Mes*Se2 (4). The lifetimes were obtained from the complete line shape
2
analysis.

P.M. Dickson et al. / Journal of Organometallic Chemistry 588 (1999) 42–50
49
Fig. 7. Arrhenius plots for the rotational process illustrating the differences between the coalescence temperature measurements, the w1/2
approximation, and the complete line shape analysis.
the non-exchange line width to account for line broad-
ening due to sample viscosity.
phenyl ring. The distance separating the t-butyl group
from the ring can be estimated roughly from the chem-
ical shift difference between the t-butyl group opposite
the benzene ring and the unaffected t-butyl group [33]
and compared to that in the solid state. The difference
in chemical shift for the ortho t-butyl groups at −60°C
is 0.60 ppm while for the meta protons, which are out
of the shielding region, the difference is 0.20 ppm.
Based on the change in chemical shift, it is estimated
that the distance between the aromatic group and the
methyl protons of the t-butyl group is greater than 4.4
All three methods give information about the kinetics
for the processes involved. With the data available,
both Arrhenius and Eyring plots can be generated.
These are both shown in Fig. 6, using the rate constants
obtained from the full line shape analysis.
Comparison of the Arrhenius plots obtained from
the three methods of analysis is shown in Fig. 7. The
results from the half-width approximation and the com-
plete line shape analysis are in relatively good agree-
ment, while the results for these parameters from the
coalescence approximation deviate sharply. It can be
seen from Table 5 that the most uncertainty is found in
the entropy parameter where there is little agreement
between the three methods. The activation parameters
obtained from this experimental study are comparable
to the energy barriers calculated for the rotation about
the SeꢀSe bond in Me2Se2 [32], but the hindered rota-
tion can only be observed when both SeꢀSe and CꢀSe
bond rotations are slow. From the data available, we
cannot determine which process is rate determining, but
it is clear that the best method is to use the complete
line shape analysis where possible for both the kinetic
and thermodynamic parameters.
,
A in solution at −60°C. In the solid state this distance
,
is on the order of 3.5 A. It should be noted that the
shift in the NMR signal will be the average of the three
methyl groups of the t-butyl moiety due to the free
rotation about the CꢀC bond in solution. In the solid
,
state structure this average distance is 4.7 A. Therefore
,
an average distance of 4.4 A above the phenyl ring is a
reasonable result and shows that the structure in solu-
tion at low temperature is quite similar to that in the
solid state.
5. Supplementary material
The chemical shift differences observed for the t-
butyl groups at low temperature arise from their prox-
imity to the shielding or deshielding regions of the
Complete crystal data, bond distances and angles,
anisotropic displacement parameters for all non-hydro-
gen atoms, atomic coordinates and isotropic thermal

50
P.M. Dickson et al. / Journal of Organometallic Chemistry 588 (1999) 42–50
Table 5
[6] G. Van den Bossche, M.R. Spirlet, O. Dideberg, L. Dupont,
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[7] G.D. Morris, F.W.B. Einstein, Acta Crystallogr. C (Cryst. Struc.
Commun.) 42 (1986) 1433.
Kinetic and thermodynamic parameters for the three methods of
analysis of Mes*Se2 (4)
2
Analysis method
Coalescence
4
Half-width
7
WinDNMR
13
[8] F.H. Kruse, R.E. Marsh, J.D. McCullough, Acta Crystallogr. 10
(1957) 201.
[9] R. Marsh, Acta Crystallogr. 5 (1952) 458.
[10] H.T. Palmer, R.A. Palmer, Acta Crystallogr. Sect. B 25 (1969)
1090.
[11] W.-W. du Mont, A. Martens, S. Pohl, W. Saak, Inorg. Chem. 29
(1990) 4847.
[12] K.-S. Tan, A.P. Arnold, D.L. Rabenstein, Can. J. Chem. 66
(1988) 54.
[13] H. Kessler, W. Rundel, Chem. Ber. 101 (1968) 3350.
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thesis (1976) 621.
Number data
points
DH‡ (kJ mol−1
)
73
65
48.8
20.4
52.9
6.92
DS‡
−1
(J mol−1
°
)
DG‡ at +50°C
52
55
58
76
42.2
43.2
44.2
51.0
50.7
51.0
51.3
55.2
(kJ mol−1
DG‡ at 0°C
(kJ mol−1
)
)
DG‡ at −50°C
(kJ mol−1
Ea (kJ mol−1
)
)
[16] W. Rundel, Chem. Ber. 101 (1968) 2956.
[17] M. Bochmann, K.J. Webb, M.B. Hursthouse, M. Mazid, J.
Chem. Soc. Dalton Trans. (1991) 2317.
[18] D.G. Foster, in: E.C. Horning (Ed.), Selenophenol (Benzenese-
lenol), vol. 3, 1955, pp. 771–773.
[19] H.J. Reich, M.L. Cohen, P.S. Clark, in: W.E Noland (Ed.),
Reagents for the Synthesis of Organoselenium Compounds:
Diphenyl Diselenide and Benzene-selenyl Chloride, vol. 6, Wiley,
New York, 1988, pp. 533.
parameters for all non-hydrogen atoms, and hydrogen
atom coordinates and isotropic displacement para-
meters have been deposited with the Cambridge Crys-
tallographic Data Center (CCDC). These data can be
obtained free of charge from the Director, Cambridge
Crystallographic Data Center, 12 Union Road, Cam-
bridge, CB2 1EZ, UK, citing the deposition numbers
124682, 124683, 124684 and 124685 for compounds 1,
2, 3 and 4, respectively, the authors and the reference
(e-mail: deposit@ccdc.cam.ac.uk).
[20] W.-W. du Mont, S. Kubiniok, L. Lange, S. Pohl, W. Saak, I.
Wagner, Chem. Ber. 124 (1991) 1315.
[21] SHELXTL PC
Madison, WI, 1990.
[22] SMART Siemens Analytical X-Ray Instruments, Inc.
, Siemens Analytical X-Ray Instruments, Inc.
/
SAINT,
Madison, WI, 1996.
[23] G.M. Sheldrick, SHELX-86, University of Go¨ttingen, Go¨ttingen,
FRG, 1986.
[24] G.M. Sheldrick, SHELXL-93, University of Go¨ttingen, Go¨ttingen,
FRG, 1993.
Acknowledgements
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(1992) 305.
Acknowledgment is made to the donors of the
Petroleum Research Fund administered by the Ameri-
can Chemical Society for the support of this research.
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