In further pursuit of the carbene analogy: Preparation and crystal structure of (N,N,N′,N′-tetramethylethylenediamine)potassium [cis-ethene-1,2-di(tert-butylamido)]gallate(I)

Article abstract of DOI:10.1039/b101016f

The preparation and crystal structures of (N,N,N′,N′-tetramethylenediamine)potassium[cis-ethene-1,2-di (tert-butylamido)]gallate(I) were discussed. The structure of the compound consisted of dimeric structure with the potassium cations η⁵-coord

Full text of DOI:10.1039/b101016f

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In further pursuit of the carbene analogy: preparation and crystal
structure of (N,N,NЈ,NЈ-tetramethylethylenediamine)potassium
cis-ethene-1,2-di(tert-butylamido)]gallate(I)
[
Eva Susanne Schmidt, Annette Schier and Hubert Schmidbaur*
Anorganisch-chemisches Institut, Technische Universität München, Lichtenbergstrasse 4,
D-85747 Garching, Germany
Received 27th November 2000, Accepted 30th January 2001
First published as an Advance Article on the web 9th February 2001
Reduction
of
cis-ethene-1,2-di(tert-butylamido)-
mutual approach of two negatively charged gallium atoms (E)
as observed previously with anionic compounds of low-
coordinate gallium, the Ga–Ga bond order of which is highly
gallium(III) chloride with metallic potassium in the
presence of N,N,NЈ,NЈ-tetramethylethylenediamine gives
the title compound which has a novel dimeric structure
with the potassium cations ꢀ -coordinated to the pseudo-
carbene ring system of one monomer and ꢀ -bonded to the
8
controversial.
5
The reaction of 1,4-dilithio-1,4-di-tert-butyl-1,4-diazabutene
1
t
(1, R = Bu), obtained from the reaction of diazabutene and
gallium atom of the other.
lithium metal in tetrahydrofuran, with equimolar quantities of
anhydrous gallium trichloride in hexane at 0 ЊC is known to
7
t
Diazabutadiene derivatives of low-valent carbon (“Arduengo
afford the unsymmetrical dinuclear complex 2, [( BuNCH) ] -
2 2
9
1
carbenes”, A, E = C) are a well-established class of hetero-
Ga Cl , the structure of which has been determined. Reduction
2 2
of this compound at a potassium mirror on the walls of a glass
cycles which show great potential as synthons and as ligands in
2
synthetic and coordination chemistry. The corresponding
reaction vessel at 20 ЊC in two steps, after 10 d leads first to
3
4
10
silylene and germylene heterocycles (A, E = Si, Ge) have also
been described and were shown to have similar characteristics
of structure and bonding. Arduengo carbenes are the mono-
meric units of Wanzlick’s fulvalene-type compounds (B,
the Ga–Ga bonded gallium() compound 3, which after
addition of N,N,NЈ,NЈ-tetramethylethylenediamine (TMEDA,
instead of the crown ether used previously), and another 4 d
reaction time, is finally converted into a new gallium()
compound 4 (lemon-yellow crystals from THF–TMEDA,
18% yield, Scheme 1). The product is soluble in tetrahydro-
5
E = C), and in fact these dimers and monomers may be
interconverted. Monomer–dimer equilibria are also known for
certain silylenes and germylenes (B, E = Si, Ge) and in particu-
lar for the related stannylenes and plumbylenes (A/B, E = Sn,
Pb), but the nature of the E᎐E double bond is a subject of
᎐
controversy owing to significant deviation of the ligand atoms
6
from the molecular plane.
In recent work we were able to show that analogous anionic
systems with low-valent gallium (C, E = Ga) can also be
7
obtained, which are isoelectronic to compounds of type A
t
Scheme 1
(
E = Ge, R = Bu). None of the corresponding discrete boron or
ϩ
aluminium heterocycles are known. With the counter ion K
completely protected by an 18-crown-6 ligand and two tetra-
hydrofuran solvent molecules, the anion is an independent
monomeric unit in the crystal lattice with a completely planar
skeleton including the central carbon atoms of the tert-butyl
1
13
1
furan, and the H and C-{ H} NMR spectra of the solutions
show the resonances of symmetrically bound TMEDA and
t
( BuNCH)2 ligands in a 1:1 ratio. The elemental analysis
is also in agreement with the composition KGa(TMEDA)-
( Bu N C H ).†
Crystals of compound 4 are triclinic, space group P1, with
Z = 2 monomeric units in the unit cell.‡ These monomers have
no crystallographically imposed symmetry but obey quite
closely the requirements of a mirror plane passing through the
7
t
groups.
2
2
2
2
¯
We now describe the result of an attempt to induce dimeriz-
ation of the anions (to give D) by partially stripping the potas-
sium cations of their ligand coating. The exposure of cationic
charge to the anions may give “Coulomb assistance” to the
DOI: 10.1039/b101016f
J. Chem. Soc., Dalton Trans., 2001, 505–507
This journal is © The Royal Society of Chemistry 2001
505

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the potassium atoms (with their TMEDA ligands) to move
from the positions above and below the middle of the Ga–Ga
5
bond to their new η positions above and below the rings retain-
ing the inversion symmetry. Quantum-chemical calculations
with simplified ligands have shown that models E and 4 are
both stable structures with a rather small difference in the total
energy favouring model 4, mainly owing to the negative charge
11
of the heterocyclopentadienyl ring.
The nucleophilic properties of anions of type C with their
lone pair of electrons at Ga() are readily demonstrated in the
7
reactions of the (18-crown-6)(THF) analogue with methyl
2
triflate, which give good yields of the corresponding dimeric
t
gallium() compound [( BuNCH) GaMe] and potassium
2
2
triflate.§
Acknowledgements
This work was supported by Deutsche Forschungsgemein-
schaft, Fonds der Chemischen Industrie and Deutscher
Akademischer Austauschdienst.
Notes and references
Fig. 1 Monomeric unit of compound 4 with atomic numbering.
14
(
ORTEP, 50% probability ellipsoids, hydrogen atoms omitted for
† All experiments were carried out in a dry-box filled with ultrapure
t
clarity). Selected bond lengths (Å) and angles (Њ) Ga–N(1) 1.9834(16),
Ga–N(2) 1.9844(17), Ga–K 3.4681(5), K–C(1) 3.002(2), K–C(2)
argon. A solution of (Li BuNCH) in tetrahydrofuran (15 mL) was
2
prepared from diazabutadiene (1.68 g, 10 mmol) and lithium metal
(0.139 g, 20 mmol) and reacted with a solution of anhydrous GaCl3
(1.760 g, 10 mmol) in hexane (15 mL) at 0 ЊC. On warming the
reaction mixture to 20 ЊC the colour of the solution turned from red
via brown to green and a yellow precipitate was formed. The mixture
was filtered and the filtrate transferred to a reaction vessel in which
metallic potassium (0.39 g, 10 mmol) was deposited as a mirror at
the inner walls through distillation in a vacuum at 500 ЊC. After 10 d
of efficient stirring the potassium was consumed. The mixture was
again filtered and transferred into another flask covered with the
same amount (0.39 g) of fresh potassium mirror at Ϫ78 ЊC, loaded
with TMEDA (1.162 g, 10 mmol) and stirred for another 4 d at
2
.996(2), K–N(3) 2.899(2), K–N(4) 2.8788(19), C(1)–C(2) 1.349(3),
N(1)–C(1) 1.389(3), N(2)–C(2) 1.388(3); N(1)–Ga–N(2) 82.05(7).
2
0 ЊC. A yellow precipitate was formed which was separated by filtra-
tion and recrystallized from TMEDA–THF [v/v 1:1] (0.7 g, 18%
yield). NMR (THF-d , 20 ЊC), H (399.8 MHz): δ 1.40 (s, 18H, Me C),
1
8
3
1
1
3
2
.15 (s, 12H, Me N); 2.29 (s, 4H, CH ), 6.31 (s, 2H, CH); C-{ H}
2
2
(
(
(
100.5 MHz): δ 33.84 (s, CMe ), 45.13 (s, Me N), 53.34 (s, CH ), 57.81
3
2
2
s, CMe ), 115.5 (s, CH). Anal. calc. (found) for C H GaN : C, 48.80
3
16 36
4
49.92); H, 9.22 (9.79); N, 14.20 (13.99%).
The crystalline sample of 4 was placed in inert oil, mounted on a glass
‡
pin, and transferred to the cold gas stream of the diffractometer.
Crystal data were collected and integrated using a Nonius DIP2020
system with monochromated Mo-Kα (λ = 0.71073 Å) radiation at
Ϫ130 ЊC. The structure was solved by direct methods using SHELXS-
1
2
2
9
7
and refined by full matrix least-squares calculations on F using
13
SHELXL-97. Non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were placed in idealized positions
and refined using a riding model with fixed isotropic contributions
Fig. 2 The dimer 4 with Ga ؒ ؒ ؒ K contacts between the monomeric
units. (TMEDA methyl groups and tert-butyl groups are only indicated
by thin lines; ORTEP for all other atoms.)
[
Uiso(fix) = 1.5 × Ueq of the attached C]. The disorder of the two central
carbon atoms C5 and C6 of the TMEDA could be resolved using split
positions with equal populations.
two metal atoms and through the middle of the C1–C2 and C5–
Crystal data for C H Ga K N . M = 786.62, triclinic, a =
3
2
72
2
2
8
C6 bonds (Fig. 1). The pseudo-carbene heterocycle is planar
10.3925(2), b = 10.7648(2), c = 10.9534(2) Å, α = 101.728(1), β =
t
3
92.051(1), γ = 114.450(1)Њ, space group P1, Z = 1, U = 1082.28(4) Å ,
¯
Ϫ1
including the central carbon atoms of the Bu groups. The
µ(Mo-Kα) = 14.66 cm
, 4132 measured and unique reflections,
dimensions of the five-membered ring are identical to those
wR2 = 0.0740, R = 0.0309 for 3906 reflections [I ≥ 2σ(I)] and 217
parameters. CCDC reference number 154913. See http://www.rsc.org/
suppdata/dt/b1/b101016f/ for crystallographic data in CIF or other
electronic format.
found for the anions in the [(18-crown-6)(THF) ]potassium
2
7
salt. However, in contrast to the isolated nature of the anions
in the reference compound, the potassium atoms in 4 are
5
η -bonded to the heterocycle and chelated only by a TMEDA
§
A solution of (18-crown-6)(THF) potassium [cis-ethene-1,2-di-
2
7
molecule. The K–Ga distance is 3.468(1) Å and the K–C and
K–N distances are in the range 2.879(2)–3.113(2) Å.
(
tert-butylamido)]gallate() (0.2 g, 0.3 mmol) in THF was cooled to
Ϫ78 ЊC and treated with methyl triflate (0,049 g, 0.3 mmol). The mix-
ture was warmed to 20 ЊC and stirred for another 20 h. After the solvent
was removed in a vacuum the residue was extracted with hexane. A
The resulting monomeric molecular units are further aggre-
gated into centrosymmetrical dimers not through Ga–Ga*
bonding as proposed in D or E, but through Ga ؒ ؒ ؒ K* contacts
with a distance [3.438(1) Å], even shorter than the intra-
colourless oily liquid remained after evaporation of the solvent. NMR
1
(
C D , 20 ЊC), H: δ Ϫ0.58 (br s, 6H, GaMe), 1.11 (s, 18H, Me C), 1.22
6
6
3
5
(s, 18H, Me
3
C), 4.93 (m, 2H, CH), 6.45 (m, 2H, CH). MS (CI): m/z 504
molecular η contact Ga–K (Fig. 2). The line connecting the Ga
t
ϩ
t
ϩ
t
{
[( BuNCH) ] Ga Me } , 252 {( BuNCH) GaMe} , 237 {( BuNCH) -
2 2 2 2 2 2
ϩ
and K* atoms forms an angle of only 20.8Њ with the C N Ga
2
2
Ga} .
ring plane, indicating that the lone pair of electrons at the
gallium atom is oriented towards the potassium cation.
1
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It should be noted that the structure of 4 can be constructed
by shifting the two C N Ga rings which are coplanar in model
1
2
2
2 W. A. Herrmann and C. Köcher, Angew. Chem., Int. Ed. Engl., 1997,
36, 2163.
D to two parallel planes with a distance of 4.0 Å and allowing
5
06 J. Chem. Soc., Dalton Trans., 2001, 505–507

View Article Online
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