Reactions of organolithium complexes with elemental selenium: Insertion producing [PhCCSeLi · TMEDA · THF] 1 and cyclisation producing [{PhC}4Se] 2 (TMEDA=[Me2NCH2]2)1

Article abstract of DOI:10.1016/S0022-328X(98)00830-4

The reaction of [PhCCLi]_n with Se metal (1:1 equivalents) in THF/TMEDA gives the insertion product [PhCCSeLi · TMEDA · THF] 1, which is monomeric in the solid state. The reaction of PhCCPh with Li metal, generating dilithium tetraphenylbutadien

Full text of DOI:10.1016/S0022-328X(98)00830-4

Journal of Organometallic Chemistry 573 (1999) 267–271
Reactions of organolithium complexes with elemental selenium:
insertion producing [PhCꢀCSeLi · TMEDA · THF] 1 and cyclisation
1
producing [{PhC} Se] 2 (TMEDA=[Me NCH ] )
4
2
2 2
Michael A. Beswick, Christopher N. Harmer, Paul R. Raithby, Alexander Steiner,
Mustafa Tombul, Dominic S. Wright *
Department of Chemistry, Uni6ersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK
Received 16 April 1998
Abstract
The reaction of [PhCꢀCLi]n with Se metal (1:1 equivalents) in THF/TMEDA gives the insertion product
PhCꢀCSeLi · TMEDA · THF] 1, which is monomeric in the solid state. The reaction of PhCꢀCPh with Li metal, generating
[
dilithium tetraphenylbutadiene in situ, followed by the addition of elemental Se gives the selenophene [{PhC} Se] 2, as a result
4
of insertion of Se followed by the elimination of Li Se. The latter provides a clean and direct route to compounds of this type.
2
©
1999 Elsevier Science S.A. All rights reserved.
Keywords: Organolithium; Organoselenides; Selenophene
1
. Introduction
undemanding organic groups (R). Prompted by the
continued scarcity of structurally characterised alkali
metal organo-selenides (containing C–Se–M grouping)
[4–7] and particularly those containing less sterically
demanding organic substituents, we report here the
investigation of the insertion reactions of Se in two
related organolithium systems, the lithium pheny-
lacetylide, [PhCꢀCLi], and dilithium tetraphenylbutadi-
Despite the fact that oligomeric alkali metal alkox-
ides and sulphides ([REM] ; M=Li–Cs, E=O, S)
n
have been the focus for many synthetic and structural
studies in the past twenty years [1,2], it is only recently
that attention has turned to the related Se [3–8] and Te
[9–13] complexes. Studies to date have concentrated on
…
the structures of complexes produced by metallation of
sterically encumbered organic acids REH (E=Se, Te)
with organolithium reagents and the insertion reactions
of elemental Se and Te into C–M or Si–M bonds. The
latter route has significant advantages over metallation
in that it avoids the synthesis of the intermediate
organoselenides and tellurides and, in so doing, gives
potentially easy access to species containing sterically
ene, [PhC—C(Ph)] Li [15–18].
2
2
The reaction of [PhCꢀCLi] with Se metal (1:1 equiva-
lents) in THF followed by removal of the solvent and
replacement with Et O and TMEDA [=(Me NCH ) ]
2
2
2 2
(
1 equivalent) gives [PhCꢀCSeLi · TMEDA · THF] (1),
as a result of insertion of Se into the C–Li bond
Scheme 1). In contrast to the simple insertion reaction
(
observed in the formation of 1, the reaction of the
…
dilithium complex [PhC—C(Ph)] Li in Et O with
2
2
2
Semetal (1:2 equivalents) gives tetraphenyl selenophene,
*
Corresponding author. Fax: +44 022 3336362.
Dedicated to Professor Brian Johnson on the occasion of his 60th
1
[{PhC} Se] (2), as a result of insertion followed by the
4
birthday.
elimination of [Li Se] (Scheme 2). Although the synthe-
2
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII S0022-328X(98)00830-4

268
M.A. Beswick et al. / Journal of Organometallic Chemistry 573 (1999) 267–271
Table 1
Crystal data for 1 and 2
Empirical formula
C H14.5Li0.5N0.5O0.5Se0.5 C24H28OSe
9
Scheme 1.
1
2
Formula weight
Crystal system
Space group
187.67
Triclinic
P1
411.36
Monoclinic
Cc
sis of the same selenophene has been achieved using
nucleophilic substitution of with the
dianion [17,18] direct reaction with
Se provides a cleaner and more convenient pathway.
SeCl2
…
2−
3
[
PhC—C(Ph)]
Crystal size (mm )
0.50×0.50×0.40
0.20×0.20×0.30
2
Unit cell dimensions
˚
a (A)
8.320(2)
10.400(2)
12.180(2)
109.70(3)
98.57(3)
93.31(3)
974.5(3)
4
13.349(3)
14.966(3)
12.807(3)
—
96.81(3)
—
2540(1)
4
1.055
However, as with the reaction involving SeCl the yield
2
˚
b (A)
of 2 is highly variable (ca. 20–40%) as a result of the
˚
c (A)
…
2−
inherent difficulty in generating the [PhC—C(Ph)]2
h (°)
i (°)
k (°)
dianion from PhCꢀCPh and Li without significant for-
mation of the naphthylene byproduct [16–18]. In this
regard, the reaction time, the nature of the Li metal
used (powdered or chunks) and the nature and volume
of solvent used in the reaction are critical. We have
found that for a 15 mmol scale reaction the use of
˚
3
U (A )
Z
Dcalc. (mg m−3
)
1.279
4.00–22.50
2767
q range (°)
Refections collected
Independent reflections 2518
4.11–22.50
2663
1669
lithium powder in 50 ml of Et O with a reaction time of
2
(
Rint
R indices [F\
|(F)](reflections)
R indices (all data)
)
(0.120)
0.040; 0.097
2.5 h gives the most consistent yields of 2.
0.069; 0.174
Prior to their structural characterisation, 1 and 2
4
were fully characterised using analytical and spectro-
scopic means. These studies reveal that the THF ligand
of 1 is highly labile and isolation of the crystalline
0.049; 0.103
0.092; 0.195
0.812; −0.634
Largest difference peak 0.493; −0.655
˚
−3
and hole (e A
)
−
1
complex under vacuum (10 min, 10
atm) leads to
˚
Details in common: u=0.71073 A, [R =SꢀF −F ꢀ/SF and wR =
1
o
c
o
2
2
amorphous powder of formula
[PhCꢀCSe-
2
2 2
c
2 2 0.5
2
2
2
(
Sw(F −F ) /Sw(F ) )
c
,
w=1/[| (F )+(yP) +xP], P=F +
o o
o
o
2
Li · TMEDA]. Although 2 also contains a molecule of
THF per molecular unit in the lattice, here too isolation
under vacuum gives solvent-free material. Details of the
subsequent structural refinements of 1 and 2 are given
in Table 1 and Table 2 lists key bond lengths and
angles in both complexes.
(2F /3)].
and related monomeric tellurides, such as [(2-
Me N)C H TeLi · DME] [DME=(MeOCH ) ] [12], in
2
6
4
2 2
which the alkali metal centres are also pseudo-tetrahe-
dral. However, 1 is the first example of an organo–se-
lenide or telluride containing a non-aromatic organic
substituent.
The Se–Li bond length in 1 [2.595(7) A] is similar to
those present in the other structurally characterised
The low-temperature (153 K) X-ray structural deter-
mination of 1 shows it to exist as monomers in the solid
+
state in which the Li cation is bonded to the anionic
˚
Se centre and coordinated by a TMEDA and THF
ligand (Fig. 1). To our knowledge the only examples of
alkali metal organoselenides to have been structurally
t
monomeric selenides [2,4,6- Bu C H SeLi · 3THF]
˚
[2.57(2) A] [5] and [2,4,6-Me C H SeLi · 3THF] (2.56(1)
3 6 2
3
6
2
t
characterised are the monomers [2,4,6- Bu C H SeLi ·
3
6
2
˚
3
THF] [5] and [2,4,6-Me C H SeLi · 3THF] [6], the
and 2.58(1) A in the two independent molecules)
[6]. The more acute C–Se–Li angle in 1 [98.2(2)°]
compared to those observed in the latter (range
101.4(3)–103.8(4)°) [5,6] is probably a consequence of
the lesser steric demands of the essentially linear
PhCꢀC– group as compared to sterically encum-
3
6
2
dimers [PhSeLi · 2,2%-bipy] and [2-PyrSeLi · 2,2%-bipy]2
2
(
2,2%-bipy=2,2%-bipyridine) [7], and the trimer [2,4,6-
t
Bu C H SeLi · THF] · toluene [4]. The overall struc-
3
6
2
3
ture of 1 is similar to that adopted in the previously
characterised monomeric arylselenide complexes [5,6]
Scheme 2.

M.A. Beswick et al. / Journal of Organometallic Chemistry 573 (1999) 267–271
269
Table 2
compound is of lesser importance apart from confirm-
ing the outcome of the reaction between the
˚
Selected bond lengths (A) and angles (°) for 1 and 2
…
2−
[
PhC—C(Ph)]2
dianion and elemental Se unequivo-
Compound 1
Bond lengths (A)
Se(1)–Li(1)
Se(1)–C(11)
C(11)–C(12)
C(12)–C(13)
˚
cally. The bond lengths and angles within 2 are as
expected on the basis of the structures of other se-
lenophenes, [20] with the presence of relatively long
2.595(7) N(1)–Li(1)
1.813(4) N(2)–Li(2)
1.219(6) Li(1)–O(1)
1.433(6)
2.106(7)
2.125(7)
1.937(7)
˚
˚
Se–C bonds (av. 1.87 A; cf ca. 1.94 A expected for a
single C–Se bond) indicating that the extent of delocal-
isation in this system is low.
Bond angles (°)
O(1)–Li(1)–N(1) 118.1(3)
O(1)–Li(1)–N(2) 110.0(3)
O(1)–Li(1)–Se(1) 108.2(3)
Se(1)–Li(1)–N(1) 117.9(3)
Se(1)–Li(1)–N(2)
Li(1)–Se(1)–C(11)
C(12)–C(11)–Se(1)
113.0(3)
98.2(2)
179.2(3)
The work presented in this paper provides easy ac-
cess to two useful ligand groups. Selenophenes have
been employed as 6e ligands to low oxidation state
Compound 2
Bond lengths (A)
Se(1)–C(4)
Se(1)–C(1)
C(1)–C(2)
transition metal centres (Scheme 3a) [21] and the
˚
−
[
PhCꢀCSe] anion can potentially function as a |/y
1.82(3) C(2)–C(3)
1.92(3) C(3)–C(4)
1.35(4) Range C–Ph
1.38(2)
1.46(5)
1.31(4)–1.59(3)
ligand to transition metals (Scheme 3b).
Bond angles (°)
C(1)–Se(1)–C(4)
C(2)–C(1)–Se(1)
C(3)–C(2)–C(1)
88.7(7)
109(2)
119(4)
C(4)–C(3)–C(2)
C(3)–C(4)–Se(1)
114(4)
110(2)
2. Experimental section
2
.1. General preparati6e techniques
All the reactions were undertaken under dry, O -free
2
argon using a vacuum line and standard inert-atmo-
sphere techniques. The THF and Et O were dried by
2
distillation over Na/benzophenone. TMEDA was dried
using molecular sieve (13X). Complexes 1 and 2 were
isolated and characterised with the aid of an argon-
filled glove box (Miller–Howe, fitted with a Belle inter-
nal circulation system). Melting points were determined
using a conventional apparatus and sealing samples in
capillaries under argon. Elemental analyses (C, H and
N) were performed by firstly sealing samples in air-tight
aluminium boats (1–2 mg) prior to analysis using a
Perkin–Elmer 240 elemental analyser. H- and C-
NMR spectra were recorded on a Bruker AP/DPX 250
MHz spectrometer, using the NMR solvent of DMSO
as internal reference standards.
bered aryl substituents. Comparison of the structural
and IR spectroscopic data for 1 with that for the
loosely-linked dimer [TMEDA · Cd(SeCꢀCPh) ] [19]
2
2
−
containing the same [PhCꢀCSe] ligand, provides some
measure of the degree of charge dispersion from the
anionic Se centre to the PhCꢀC– substituent in 1.
Although there is no significant difference in the CꢀC
bond lengths in 1 and [TMEDA · Cd(SeCꢀCPh) ] , the
2
2
1
13
˚
C–Se bond [1.813(4) A] in 1 is shorter than the termi-
nal or v-bridged PhCꢀCSe groups (1.833(7) and
˚
1
[
.851(7) A, respectively) in [TMEDA Cd(SeCꢀCPh) ]
2
2
19]. In addition, there is a small decrease in the
CꢀC stretching frequency in the IR spectrum of 1
2
.2. Synthesis of 1
−
1
(
2122 cm
)
compared to that in [TMEDA ·
−
1
Cd(SeCꢀCPh) ] (2130 cm ) [19] and PhCꢀCH (2144
2
2
To a solution of PhCꢀCH (1.02 g, 10 mmol) in THF
−
1
cm ). These structural and spectroscopic trends are
consistent with the slightly greater charge separation
for the Se–Li bond in 1, anticipated on the grounds of
the greater electronegativity of Li compared to Cd. The
greater concentration of negative charge on the Se
centre leads to a slightly greater dispersion of electron
density towards the PhCꢀC– group in 1 than in
n
−3
(
20 ml) was added BuLi (6.7 ml, 1.5 mol dm
in
hexanes, 10 mmol) at 0°C. The tawny-brown solution
produced after stirring briefly at 25°C was reacted with
elemental Se (0.780 g, 10 mmol). The solution turned
an orange-red colour and stirring was continued at
2
5°C (ca. 1 h) at which time no Se remained. The
solution was filtered (celite, porosity 3) and the THF
solvent removed and replaced with Et O (15 ml).
TMEDA was added (1.51 ml, 10 mmol) and the yellow
precipitate was dissolved by the addition of a minimum
volume of THF (ca. 2 ml). Storage at −35°C gave
large orange crystals of 1. Isolation under vacuum prior
[TMEDA · Cd(SeCꢀCPh)2]2 (although the extent of
such delocalisation is clearly small in both complexes).
The low-temperature X-ray structure determination
of 2 shows it to have the expected heterocyclic ring
2
structure [{PhC} Se] (Fig. 2). In addition, there is one
4
disordered THF molecule per formula unit in the lat-
tice. In itself the structural characterisation of this
−
1
to analysis (15 min, 10
atm) removes all the Li-

270
M.A. Beswick et al. / Journal of Organometallic Chemistry 573 (1999) 267–271
Fig. 1. Molecular structure of 1. H atoms have been omitted for clarity.
bonded THF and gives an amorphous material of
formula [PhCꢀCSeLi · TMEDA]. The following data
refers to this material; yield (0.69 g, 31%); m.p. 138°C;
30 mmol) in Et O (30 ml). The reaction changes to a
2
deep red colour within ca. 5 min. Stirring at r.t. was
continued for 2.5 h. Some orange solid is normally
present at this stage and the Li metal has not reacted
completely. The reaction was filtered and Se metal (2.34
g, 30 mmol) was added. The reaction was stirred for 12
h at r.t. Filtration gave an orange/yellow solution
which was reduced to ca. 15 ml to give a yellow
precipitate of 2. Crystals are readily obtained by heat-
ing the precipitate into solution followed by storage of
the resulting solution at 5°C; yields over several reac-
tions 1.30–2.34 g (20–36%); m.p. 130–135°C; IR (Nu-
IR (Nujol), 3072(w) (C–H aryl str.), 2122(s) (CꢀC str.),
…
1
1
1
2
596(m) (aryl C—C str.), other bands at 1290(m),
180(w), 1160(w), 1128(w), 1098(w), 1066(m), 1034(s),
1
019(s), 952(s), 792(s), 752(s), 691(s); H-NMR (+
5°C, 250 MHz, d -benzene), 7.63 (d.d., o-C–H), 7.15
6
13
(
t.d., p-C–H), 7.09 (mult., m-C–H); C-NMR (62.895
MHz, +25°C, D -DMSO), 131.7 [Ph, p-], 128.8 [Ph,
6
C(h)], 128.5 [Ph, m-], 125.4 [Ph, o-], 85.2 [Ph–CꢀC],
8
4.7 [Ph–CꢀC], 56.8 [–CH , TMEDA], 46.4 [Me N,
2
2
TMEDA]; Elemental Analysis; found, C 54.3, H 6.9, N
.4; calc. for [PhCꢀCSeLi · TMEDA] , C 55.4, H 6.9,
jol), major bands at 1148(m), 1070(m), 1029(m),
9
1
n
9
2
7
7
13(m), 860(m), 791(m), 743(s), 694(s); H-NMR (+
N 9.2.
5°C, 250 MHz, D -DMSO), 7.55 (mult.), 7.44 (mult.),
.20 (mult.), 7.0 (mult.); Elemental Analysis; found, C
7.8, H 4.8; calc. for [{PhC} Se], C 77.4, H 4.6.
6
2.3. Synthesis of 2
5
To a suspension of Li powder (0.22 g, 31 mmol) in
Et O (20 ml) was added a solution of PhCꢀCPh (3.56 g,
2
2
.4. X-ray structure determinations
Crystals were mounted directly from solution under
argon using a perfluorocarbon oil which protects them
from atmospheric O and moisture. The oil freezes at
2
reduced temperatures and holds the crystal static in the
X-ray beam [22]. Data for 1 and 2 were collected on a
Stoe–Siemens AED four circle diffractometer. The
structures of both were solved by direct methods and
Fig. 2. Molecular structure of 2. H atoms have been omitted for
clarity.
Scheme 3.

M.A. Beswick et al. / Journal of Organometallic Chemistry 573 (1999) 267–271
271
2
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[
[
5
0:50 sites. Details of the structure refinements for 1
and 2 are shown in Table 1. Atom coordinates, bond
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[
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(
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(
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