Synthesis, structure and reactions of an amidinate stabilised trigallane

Article abstract of DOI:10.1039/c1dt10209e

The neutral trigallane R₃Ga₃I₂ was obtained in high yields by reaction of "GaI" with the moderately sterically demanding lithium N,N′-dicyclohexylneopentylamidinate. Its chemical reactions with N-lithio-2-propanimine, sodium superhydride and elemental iodine were investigated. RI-DFT calculations have been performed to confirm the bonding situation. All compounds are characterised by crystal structure analysis and NMR-spectroscopy. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt10209e

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PAPER
Synthesis, structure and reactions of an amidinate stabilised trigallane†‡
Gerald Linti* and Thomas Zessin
Received 8th February 2011, Accepted 3rd March 2011
DOI: 10.1039/c1dt10209e
The neutral trigallane R3Ga3I2 was obtained in high yields by reaction of “GaI” with the moderately
sterically demanding lithium N,N¢-dicyclohexylneopentylamidinate. Its chemical reactions with
N-lithio-2-propanimine, sodium superhydride and elemental iodine were investigated. RI-DFT
calculations have been performed to confirm the bonding situation. All compounds are characterised
by crystal structure analysis and NMR-spectroscopy.
metal chemistry, as well.22 Using an amidinate substituent a ger-
1
Introduction
manium(I) compound with a Ge–Ge-bond (GeAmCl)2 has been
synthesized.23 The bis(alkylamidinate) Group 4 metal complexes
of the general formula [RC(NCy)2]2MCl2 (M = Ti, Zr, Hf; R = Me,
tBu) are active ethylene polymerization catalysts in the presence of
methylaluminoxane (MAO) as a cocatalyst.24
Here we examine the reactions of chelating amidinates with gal-
liumsubiodide resulting in corresponding functional digallane and
trigallane compounds. Using a moderately sterically demanding
lithium amidinate provides a chain, composed of three gallium
atoms – stabilised by three amidinate residues, shielding the
central gallium atom – and two terminal iodine atoms, in excellent
yields (>80%). These iodine atoms are perfect groups for further
metathesis reactions. These have been performed by adding lithium
propan-2-ylideneamide or sodium superhydride, respectively.
Low-valent gallium compounds have been of wide interest in
the last few years. Mostly, these compounds form clusters, cages
or rings.1–4 There are only a few examples of open gallium
chains. A large variety of digallanes(4) of type Ga2R4 have
been prepared with various substituents ranging from alkyl-5
and aryl groups6 to silyl-,7 amino-8 and alkoxy groups.9 With
Lewis-basic substituents oligomerisation is possible. For example,
[(Me3Si)3SiGaCl]4 and (tBuO)8Ga4 with digallane units in cage
molecules have been synthesized. By reaction of “GaI” and
triethylphosphane [PEt3GaI2-GaIPEt3-GaI2PEt3]10 was obtained.
We isolated the linear, anionic [(Ph3Ge)3Ga–Ga–Ga(GePh3)3]- as
a product from the reaction of Li(thf)3GePh3 with “GaI”.11 Less
highly substituted trigallanes form rings like [Aryl3Ga3]2- 12 (Aryl =
2,6-dimesitylphenyl) and (tBu3Si)4Ga3.13 Curing the electron de-
ficiency by incorporating donors into the substituent leads to
alkane-like chains with gallium–gallium-bonds: From the reac-
7
9
2
Experimental
tion of the anionic heterocyclic compound [{CHNdipp}2Ga]-
14
with GaH3·quinuclidine the trigallanate [{CHNdipp}2Ga–GaH2–
Ga{CHNdipp}2]-15 could be isolated, which is bent, due to
tetrahedral coordination at the central gallium atom (dipp = 2,6-
diisopropylphenyl). [(nacnac)ClGa–GaCl–GaCl(nacnac)] and
[(nacnac)MeGa–GaMe–GaMe(nacnac)]16 were obtained by ox-
idative addition reactions of GaCl3 and GaMe3 to Ga(nacnac)17
(nacnac = HC{C(Me)N(dipp)}2). Using the structurally related
guanidinate instead of amidinate it has been possible to isolate a
gallium(I) species, the [Ga({dippN}2CNCy2)]18 (Cy = cyclohexyl).
This has been shown to react with complex fragments of plat-
inum, iron, cobalt, nickel and molybdenum as a CO-analog.19–21
Amidinates are valuable substituents in main group and transition
All procedures were performed under purified argon or in vacuum
using Schlenk techniques. Starting materials were prepared ac-
cording to literature or used as purchased. Elemental analyses
were done in the microanalytical laboratory of the Institute
of Inorganic Chemistry, University of Heidelberg, Heidelberg,
Germany. NMR spectra were recorded on a BRUKER DRX200,
a Bruker Advance II 400 or a BRUKER AVANCE III 600
instrument. X-ray crystallography: suitable crystals were mounted
with perfluorinated polyether oil on the tip of a glass fibre and
cooled down to 200 K immediately on the goniometer head. Data
collection was performed on a STOE IPDS I diffractometer with
˚
Mo-Ka radiation (l = 0.71073 A). Structures were solved and
refined with the Bruker AXS SHELXTL 5.1 program package.25
Refinement was in full matrix against F2. All carbon-bonded
hydrogen atoms were included as riding models with fixed isotropic
U values in the final refinement. Due to the quality of the crystals
of 4 the NCMe2-group had to be treated with restraints (DELU,
DFIX C52 C54). In all structures the largest remaining peaks are
located near a heavy atom (I or Ga).
Anorganisch-Chemisches Institut, Im Neuenheimer Feld 270, 69120, Hei-
delberg, Germany. E-mail: gerald.linti@aci.uni-heidelberg.de; Fax: +49-
(0)6221-546617; Tel: +49-(0)6221-548468
† Dedicated to Professor Hansgeorg Schno¨ckel on the occasion of his 70th
birthday.
‡ Electronic supplementary information (ESI) available. CCDC reference
numbers 811974–811982 for 2a, 3a, 3b and 3c, respectively. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10209e
Weighting scheme:
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(600 MHz, C6D6): d = 14.8 (N–C(Me) N), 22.7 (CHMe), 24.9
(CHMe), 29.1 (CHMe2), 124.6 (m-Ar–C), 129.9 (p-Ar–C), 146.0
(i-Ar–C), (NCN and o-Ar–C n.o).
1
w =
s 2(F2 ) +(w1P)2
o
where
Synthesis of 3. In a typical experiment 2.06 g (10 mmol) of
dicyclohexylcarbodiimide were dissolved in 20 mL diethyl ether
and cooled to 0 ◦C. 6.8 mL tert-butyl lithium solution (15% in
n-hexane) were added. Almost immediately the liquid turned into
a gel. After overnight stirring a white suspension was obtained. All
volatiles were removed in vacuum. The resulting 1b was suspended
1
P = (F2 + 2F2 )
o
c
3
For further data, refer to Table 1.
Synthesis of 2. 3.63
g
(10 mmol) of bis-2,6-
◦
diisopropylphenylcarbodiimide were dissolved in 20 mL
diethyl ether and cooled to 0 ◦C. 6.5 mL methyl lithium solution
(5% in diethyl ether) were added. After overnight stirring, all
volatiles were removed in vacuum. The residue 1a was suspended
in toluene and added to 20 mmol “GaI” in toluene at -78 ◦C.
After overnight stirring, the solution had darkened and elemental
gallium had precipitated. All volatiles were removed in vacuum.
The residue was extracted with thf. Upon cooling to -20 ◦C this
solution yielded colourless crystals of 2. Yield: 2.00 g (35%); m.p.
>240 ◦C (dec.).
in toluene and added to 16.7 mmol “GaI” in toluene at -78 C.
After overnight stirring, the solution darkened and elemental
gallium precipitated. All volatiles were removed in vacuum. The
residue was first extracted with n-hexane, then with toluene and
finally with thf. The n-hexane-fraction yielded crystals of 3a (3·n-
hexane), the toluene-fraction yielded crystals of 3b (3·toluene)
and the thf-fraction yielded crystals of 3c (3·thf). Depending on
temperature and concentration solvent free 3 could be obtained
from the toluene-fraction. The total yield of trigallane 3 was 3.43 g
(82%), m.p. > 330 ◦C (dec).
3: C51H93Ga3I2N6 (1253.3 g mol-1) calcd. C 48.87, H 7.48, N
6.71, Ga 16.69, I 20.25, found C 49.35, H 7.37, N 6.10. 1H-NMR
(200 MHz, C6D6): d [ppm] = 1.08–1.86 (m, 60 H, chex), 1.20 (s, 9 H,
N–C(–CMe3) N), 1.21 (s, 9 H, N–C(–CMe3) N), 1.27 (s, 9 H,
N–C(–CMe3) N), 3.75 (m, 6 H, i-H chex). 13C-NMR (400 MHz,
C6D6): d = 25.3–25.9 (chex, m-C), 26.7–26.8 (chex, p-C), 29.4–30.0
2: C52H74Ga2I2N4 (1148.4 g mol-1) calcd. C 54.38, H 6.49, Ga
1
12.14, I 22.10, N 4.88, found C 54.64, H 6.58, N 4.99. H-NMR
(400 MHz, C6D6): d [ppm] = 1.07 (d, 24 H, 3JH–H = 6.9 Hz, CHMe),
1.21 (d, 24 H, 3JH–H = 6.9 Hz, CHMe), 1.91 (s, 6 H, N–C(Me) N),
3.23 (sept, 3JH–H = 6.9 Hz, 8 H, CHMe2), 6.95 (d, 3JH–H = 7.7 Hz,
8 H, m-Ar–H), 7.09 (t, JH–H = 7.7 Hz, 4 H, p-Ar–H). 13C-NMR
3
Table 1 Crystallographic data of 2, 3, 4, 5 and 6
Compound
2
3
4
5
6
Empirical formula
Formula weight/g mol-1
Crystal size/mm
Crystal system
C26H37GaIN2
574.20
0.33 ¥ 0.22 ¥ 0.19
Monoclinic
P 21/c
10.364(2)
17.629(4)
15.399(3)
90.00
107.90(3)
90.00
C51H93Ga3I2N6
1253.27
C66H123Ga3IN7O3
1398.77
0.26 ¥ 0.20 ¥ 0.09
Monoclinic
Cc
13.184(3)
30.314(6)
17.950(4)
90.00
95.17(3)
90.00
7145(2)
4
1.300
1.601
Numerical
0.5012/0.8727
2952
1.76 . . . 24.26
-15 £ h £ 15
-34 £ k £ 33
-20 £ l £ 20
23360
C53H99Ga3N6
1029.54
0.27 ¥ 0.26 ¥ 0.16
Monoclinic
P 21/c
12.634(3)
20.821(4)
21.296(4)
90.00
90.57(3)
90.00
C17H31GaI2N2
586.96
0.45 ¥ 0.18 ¥ 0.16
Monoclinic
C 2/c
27.952(6)
10.316(2)
17.640(4)
90.00
123.05(3)
90.00
0.44 ¥ 0.35 ¥ 0.08
Triclinic
¯
Space group
P1
˚
a/A
12.293(3)
13.507(3)
18.197(4)
81.06(3)
77.42(3)
69.29(3)
2748.2(10)
2
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
2677.3(9)
4
1.425
5601.7(19)
4
1.221
4263.4(15)
8
1.829
Z
D
calc/g cm-3
1.515
2.622
m (Mo-Ka)/mm-1
2.195
1.470
4.187
Absorption correction
Numerical
0.5928/0.6829
1164
2.37 . . . 26.07
-12 £ h £ 12
-21 £ k £ 21
-19 £ l £ 18
21208
Numerical
0.3697/0.7698
1280
2.30 . . . 28.16
-16 £ h £ 16
-17 £ k £ 17
-23 £ l £ 23
26791
Numerical
0.7026/0.8234
2208
2.49 . . . 28.17
-16 £ h £ 16
-27 £ k £ 27
-27 £ l £ 27
54075
Numerical
0.2492/0.5350
2272
2.29 . . . 28.00
-36 £ h £ 36
-13 £ k £ 13
-23 £ l £ 22
20006
T
min/Tmax
F(000)/e
q/◦
Index ranges
Reflections collected
Independent reflections
Observed refl. [I > 2s(I)]
Completeness of data
Data/restraints/parameters
Goodness of fit (S) on F2
Final R indices [I > 2s(I)]
5269
3806
12307
5380
10854
4701
13382
4061
5052
3457
q = 26.07◦; 99.4%
5269/0/271
0.880
q = 28.16◦; 91.2%
12307/0/559
0.656
q = 24.26◦; 98.6%
10854/40/721
0.769
q = 28.17◦; 97.3%
13382/0/563
0.780
q = 28.00◦; 98.1%
5052/0/199
0.937
0.0295
0.0621
0.0362
0.0672
0.0676
0.1358
0.0611
0.1179
0.0432
0.1044
R indices (all data)
0.0466
0.0650
0.531/- 0.392
0.0355
0.1079
0.0765
0.946/- 0.878
0.0232
0.1440
0.1640
1.612/- 0.845
0.0324
0.1995
0.1617
0.688/- 0.555
0.0491
0.0668
0.1122
1.159/- 1.101
0.0572
-3
˚
Max diff. peak/hole [e A ]
w1
CCDC reference number
811974
811976
811980
811981
811982
5592 | Dalton Trans., 2011, 40, 5591–5598
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(CMe3), 35.1–36.2 (chex, o-C), 38.6–39.9 (CMe3), 53.7–55.8 (chex,
i-C), 175.1 (N–C(-CMe3) N).
(1)
Synthesis of 4. 2 mL (3 mmol) of a methyl lithium solution
(5 w% in diethyl ether) were added to 10 mL of acetonitrile. A
white precipitate formed. All volatiles were removed in vacuum
◦
and the precipitate was dissolved in thf. At -78 C this solution
was added to 3.80 g (3 mmol) of 3 in toluene. After overnight
stirring all volatiles were removed in vacuum. The residue was
extracted with thf. This solution yielded some colourless crystals
of 4 which were used for X-ray diffraction and NMR, m.p. >
260 ◦C (dec.).
1
4: H-NMR (600 MHz, C6D6): d [ppm] = 1.00–1.97 (m, 87 H,
(2)
chex, tBu), 2.00–2.22 (m, N CMe2), 3.77 (m, 6 H, i-Hchex). 13C-
NMR (600 MHz, C6D6): d [ppm] = 25.5–26.4 (chex, m-C, p-C), 28.7
(N CMe2), 29.1 (N CMe2), 29.2–29.6 (CMe3), 34.8–35.9 (chex,
o-C), 38.6–39.5 (CMe3), 53.4–55.8 (chex, i-C), 159.5 (N CMe2).
Reducing the steric demand of the amidinato group by using
dicyclohexylamidinates the trigallane 3 is obtained (eqn (3)) in
good yield.
Synthesis of 5. 4.5 mL (4.5 mmol) of a solution of Na[BEt3H]
(c = 1 mol l-1) in toluene were added at -78 ◦C into a solution
of 2.82 g (2.25 mmol) of 3 in 20 mL toluene. After stirring for an
hour the solution turned yellowish. After further overnight stirring
all volatiles were removed in vacuum. The residue was extracted
with n-hexane. This solution yielded colourless crystals of 5 upon
cooling to -20 ◦C.
1
5: H-NMR (400 MHz, CDCl3): d [ppm] = 0.90–1.75 (m, 92
(3)
c
t
3
c
H, hex, Bu, Et), 5.28 (m, JH–H = 1.01 Hz, 6 H, i-H hex), 5.4
(s, 1 H, GaH, broad peak).- 13C-NMR (400 MHz, CDCl3): d =
7.7 (CH2CH3), 17.4 (CMe3), 21.4 (CH2CH3), 25.6–25.7 (m-C, p-
c
c
C hex), 29.2–29.9 (CMe3), 36.6–36.9 (o-C hex), 54.1–54.7 (i-C
chex), 175.1 (N–C(tBu) N). IR (liquid cell, in n-hexane): n =
˜
1755 cm-1 (terminal Ga–H).
It has been demonstrated that in the functional digallane
7
[(Me3Si)3SiGaCl]4 attempted substitution reactions resulted in
Synthesis of 6. A solution of 1.25 g (1 mmol) of 3 in
20 mL toluene was titrated with 20 mL of a 0.1 M iodine
standard solution in toluene. During the titration the solution
discolours immediately. An additional drop of iodine solution
is not discoloured. The concentrated solution yielded colourless
crystals of 6 upon cooling to -20 ◦C.
cleavage of the gallium–gallium bond. In order, to examine the
stability of the shielded trigallium unit of 3 substitution reactions
of the terminal iodo atoms were performed. By metathesis reaction
of 3 with N-lithio-2-propanimine the mono substituted 4 is
obtained, by preserving the trigallane unit (eqn (4)). The second
iodine atom could not be exchanged, probably due to steric
inhibition.
1
6: H-NMR (200 MHz, C6D6): d [ppm] = 1.15–1.78 (m, 20 H,
c
chex), 1.07 (s, 9 H, N–C(-CMe3) N), 3.66 (m, 2 H, i-H hex).
3
Results and discussion
3.1 Synthesis
(4)
Lithium amidinates are obtained by the reaction of suitable
carbodiimides with methyl lithium or tert-butyl lithium
(eqn (1)). Reaction of “GaI”26 with 1a (eqn (2)) yields
the functional diiodobis(amidinato)digallane 2. In 2006
Jones et al.27 reported the structures of di-iodo-bis(N,N¢-
bis(2,6-diisopropylphenyl)imidoformamido)-di-gallium(II) and
diiodo-bis(N,N¢-bis(2,6-diisopropylphenyl)neopentylamidinato)-
di-gallium(II) as the products of the reaction of “GaI” and
lithium-N , N¢-bis(2, 6-diisopropylphenyl)neopentylamidinate
When reacting 3 with sodium triethylhydridoboranate, the first
gallium(II)hydride 5 is obtained. One of the iodine atoms in 3
was substituted by a hydride, the other one by an ethyl group
(eqn (5)). Obviously, the hydridoboranate is an alkylating reagent,
too, in this case. 11B-NMR of the reaction solution indicates the
formation of Et2BI among other products.28
and
lithium-N,N¢-bis(2,6-diisopropylphenyl)formamidinate,
respectively. This shows that the alkyl group linking the nitrogen
atoms has little effect on the structure.
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far this fits the 1H-NMR-spectrum of 3. In addition there are the
signals of the methyl groups of the N CMe2-group at d = 2.00
and d = 2.22 which could be observed.
The 13C-NMR-spectrum of 4 complies with 3, though the
quaternary carbons in the N2C-groups could not be observed.
In addition there are clear signals for the N CMe2-substituent
in terms of the methyl groups at d = 28.7 and d = 29.1 and the
quaternary carbon at d = 159.5.
(5)
The 1H-NMR-spectrum of 5 reveals a broad signal in the typical
range29 for gallium-bound hydrides at d = 5.4 (1 H). The broad
multiplet at d = 0.90–1.75 is caused by the overlapping signals
of the tert-butyl-, the ethyl- and the methylene groups in the
cyclohexyl groups. The tertiary hydrogen atoms in the cyclohexyl
groups are to be found at d = 5.28.
The reaction of 3 with elemental iodine in toluene at room
temperature occurs immediately. It yields the gallium(III) com-
pound 6 (eqn (6)). That allows a redox titration with an iodine
standard solution in toluene. The consumption of iodine solution
corresponds with a four electron reaction. This matches with the
presence of two gallium(II) and one gallium(I) in 3.
The 13C-NMR-spectrum of 5 shows the usual signals of 3.
Additionally signals for the gallium-bound ethyl group are found
at d = 7.7 (CH2CH3) and d = 21.4 (CH2CH3).
The IR-spectrum (liquid cell in n-hexane) shows a terminal
-1 29
˜
(6)
Ga–H vibration at n = 1755 cm .
In summary, 1H-NMR- and 13C-NMR-spectra of 3, 4 and
5 reveal three sets of signals, one for each amidinate and the
respective substituents. This indicates that the Ga3-unit is not only
stable in the solid state but also in solution.
3.2 Spectroscopic characterization
1
The H-NMR-spectrum of 6 shows a broad multiplet at d =
The 1H-NMR-spectrum of 2 shows two doublets (d = 1.07 (24 H)
and d = 1.21 (24 H)) and one septet (d = 3.23 (8 H)) for the iso-
propyl groups which is the usual pattern. The methyl group inside
the amidinate creates a singlet at d = 1.91 (6 H). The aromatic
hydrogen atoms are found at d = 6.95 (d, 8 H, meta) and d = 7.09
(t, 4 H, para).
1.15–1.78 (20 H, CH2 chex). The methyl groups of the tert-butyl
group are found at d = 1.07 (s, 9 H). The methine hydrogen atoms
in the cyclohexyl groups correspond to the quintet at d = 3.66.
The spectrum matches, in general, the one from gallium-N,N¢-
dicyclohexylneopentylamidinate dichloride.30
The 13C-NMR-spectrum of 2 also shows for the iso-propyl
groups two signals for the methyl groups at d = 22.7 and d = 24.9
and one signal for the methine group d = 29.1. The methyl carbon
atoms inside the amidinate show up at d = 14.8. The aromatic
meta-, para- and ipso-carbon atoms are found at d = 124.6, d =
129.9 and d = 146.0. The quaternary carbon atoms NCN and
ortho-C could not be observed.
3.3 Crystal structure determination
2 is obtained as colourless monoclinic crystals, space group
P 21/c (Fig. 1). In the centrosymmetric molecule the amidinate
substituents bind to each of the gallium atoms in a chelating mode.
◦
˚
The bite angle is 66.5(1) and the Ga–N distances are 1.978(2) A
˚
(Ga1–N1) and 1.986(2) A (Ga1–N2), respectively. Each gallium
1H-NMR- and 13C-NMR-spectra of 2 are consistent with a
structure with an asymmetrical substituted gallium and 2/m
symmetry.
atom is bonded to an iodine atom with a distance of 2.533(1) A
(Ga1–I1). The gallium–gallium-bond measures 2.430(1)
˚
˚
A
(Ga1–Ga1¢). This is similar to the bond length in the bis(N,N¢-
bis(2,6-diisopropylphenyl)neopentylamidinate stabilised di-
gallane and in diiodo-bis(N,N¢-bis(2,6-diisopropylphenyl)-
formamidinato)-di-gallium(II).27 The methyl group attached to
the NCN fragment has a typical C–C single bond distance of
1
The H-NMR-spectrum of 3 shows a broad multiplet at d =
1.08–1.86 that corresponds with the 60 hydrogen atoms of the
secondary carbon atoms in the cyclohexyl groups. The tert-butyl
groups are found as three singlets at d = 1.20, 1.21 and 1.27 (9 H
each). The six hydrogen atoms in ipso-position in the cyclohexyl
groups are found at d = 3.75 as a quintet.
˚
1.483(4) A. As expected the C–N bonds in the N2CGa-ring are of
˚
the same length within standard deviation: 1.319(3) A (N1–C1),
The 13C-NMR-spectrum of 3 shows three signals for the carbon
atoms in meta-position at d = 25.3–25.9, the carbon atoms in
para-position at d = 26.7–26.8, the carbon atoms in ortho-position
d = 35.1–36.2 and for the carbon atoms in ipso-position at d =
53.7–55.8. The methyl groups of the tert-butyl groups show up as
a group of three signals at d = 29.4–30.0. The three quaternary
carbon atoms in the tert-butyl groups are found at d = 38.6–39.9.
The quaternary carbon atoms in the N2C-groups are at d = 175.1.
Here only one signal could be observed, the other two were lost in
noise.
1.315(3) A (N2–C1). This indicates delocalization over the NCN
˚
fragment, which has been confirmed by quantum chemical
calculations (see below). The phenyl rings are rotated 75◦–78◦
to the NCN-plane. The isopropyl groups point away from the
amidinate group.
If 2 is crystallized from thf, colourless, triclinic crystals of 2a,
¯
space group P1, are obtained, which contain two molecules thf
together with 2 in the asymmetric unit. Due to these thf molecules
the digallane gets slightly twisted (torsion angle I1–Ga1–Ga2–I2
-169.5(1)◦; see the Quantum chemical calculations section, too).
The remaining structure is not affected by the solvent molecules.
Solvent free 3 was obtained by crystallization from toluene
1
The H-NMR-spectrum of 4 shows a broad multiplet at d =
1.00–1.97 (87 H, tBu, CH2 chex). The hydrogen atoms in the ipso-
position in the cyclohexyl groups are found at d = 3.77 (q, 6 H). So
¯
as colourless triclinic plates, space group P1 (Fig. 2). The core
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Fig. 1 Crystal structure of 2. The thermal ellipsoids are given at
Fig. 2 Crystal structure of 3. The thermal ellipsoids are given at the
the 50% probability level. Hydrogens are omitted for clarity. Se-
◦
50% probability level. H◦ydrogens are omitted for clarity. Selected bond
˚
lected bond lengths [A] and angles [ ]: Ga1–N1 1.978(2), Ga1–N2
˚
lengths [A] and angles [ ]: I1–Ga1 2.594(2), I2–Ga3 2.565(2), Ga1–N1
1.986(2), Ga1–Ga1¢ 2.430(1), Ga1–I1 2.533(1), N1–C1 1.319(3), N1–C3
1.420(3), N2–C1 1.315(3), N2–C15 1.414(3), C1–C2 1.483(4); N1–Ga1–N2
66.5(1), N1–Ga1–Ga1¢ 120.9(1), N2–Ga1–Ga1¢ 119.4(1), N1–Ga1–I1
107.7(1), N2–Ga1–I1 109.5(1), Ga1¢–Ga1–I1 120.7(1), C1–N1–C3
123.9(2), C3–N1–Ga1 144.8(2), C1–N2–C15 126.9(2), C1–N2–Ga1
91.1(1), C15–N2–Ga1 141.6(2), N2–C1–N1 111.2(2).
1.953(4), Ga1–N2 1.963(4), Ga1–Ga2 2.420(1), Ga2–N3 1.975(4),
Ga2–N4 1.978(4), Ga2–Ga3 2.415(1), Ga3–N5 1.950(4), Ga3–N6
1.976(4); N1–Ga1–N2 66.0(2), N1–Ga1–Ga2 127.9(2), N2–Ga1–Ga2
127.9(2), N1–Ga1–I1 105.5(2), N2–Ga1–I1 107.1(2), Ga2–Ga1–I1
113.32(3), N3–Ga2–N4 65.4(2), N3–Ga2–Ga3 114.0(2), N4–Ga2–Ga3
112.7(1), N3–Ga2–Ga1 114.5(2), N4–Ga2–Ga1 114.9(2), Ga3–Ga2–Ga1
122.2(1), N5–Ga3–N6 66.5(2), N5–Ga3–Ga2 125.7(2), N6–Ga3–Ga2
126.7(2), N5–Ga3–I2 109.1(1), N6–Ga3–I2 109.1(1), Ga2–Ga3–I2
112.1(1), C1–N1–Ga1 93.7(3), C1–N2–Ga1 93.0(3), C18–N3–Ga2 94.8(3),
C18–N4–Ga2 94.1(3), C35–N5–Ga3 93.7(3), C35–N6–Ga3 92.0(3),
N1–C1–N2 107.2(4), N3–C18–N4 105.7(4).
of a molecule 3 consists of a chain of three gallium atoms
˚
˚
with distances of 2.420(1) A and 2.415(1) A. The Ga–Ga–Ga
angle at the central gallium atom is 122.2(1)◦. Each gallium
is bonded to a dicyclohexylneopentylamidinate by its nitrogen
˚
˚
atoms (1.950(4) A–1.978(4) A). The Ga–N bonds of Ga(1) and
˚
Ga(3) are shorter by 0.02 A compared to the central gallium
distorted tetrahedral coordination and typical distance (Ga(1)–
atom’s bond lengths. This is conforming to Bent’s rule.31,32 The
terminal gallium atoms in oxidation state +II complete their
distorted tetrahedral coordination with one iodine, each. The Ga–
I(1) 2.566(2) A). The other Ga(II) in the molecule is bonded to
˚
˚
an NCMe2 group with a Ga(3)–N(7) distance of 2.02(1) A and
angles of 113.7(3)◦ (Ga(2)–Ga(3)–N(7)) and 115.0(4)◦ (C(35)–
Ga(3)–N(7)). The Ga(3)–N(7)–C(52) unit is not linear (Ga(3)–
N(7)–C(52) 133.9(10)◦). It points away from the centre of the
structure, in virtue of C(35)–Ga(3)–N(7)–C(52) 4(1)◦ and Ga(3)–
N(7)–C(52)–C(53) 15(2)◦.
˚
˚
I-distances (2.594(1) A–2.565(1) A) are in the typical range. The
amidinate groups binding to them have the same bite angle as in 2
within the standard deviation (66.3(2)◦). The NCN-angles deviate
significantly from the ideal 120◦ for a triple coordinated carbon
(NCN 107.5(4)◦ (avg)). This is in line with the acute NGaN angles
in the planar four membered GaN2C rings. The amidinate bonded
to the central Ga(2) (oxidation state +I) differs slightly, since the
bite angle with 65.3(2)◦ and the NCN-angle with 105.7(4)◦ are a
bit sharper, even.
We were able to obtain 3 cocrystallized with n-hexane (3a,
colourless, triclinic crystals, space group P1), toluene (3b, colour-
less, triclinic crystals, space group P1) and thf (3c, colourless,
monoclinic crystals, space group P21/n). The structures of 3a and
3b do not differ very much from 3, but 3c does. While the torsion
angle I1–Ga1–Ga3–I2 in 3, 3a and 3b is approximately 125–126◦,
in 3c it is -88◦. There is no close contact between the solvent
molecules and the compound. A possible answer for this twist will
be given in the Quantum chemical calculations section.
5 was crystallized from n-hexane as colourless, monoclinic rods
in space group P 21/c (Fig. 4). Molecules of 5 contain a chain of
three gallium atoms which form almost the same angle like in 3
(121.3(1)◦). The Ga–Ga-distances are slightly elongated compared
˚
˚
to 3 (Ga(1)–Ga(2) 2.457(2) A and Ga(2)–Ga(3) 2.463(2) A), which
is in the line with Bent’s rule.31,32 Each of the gallium atoms
is chelated by one dicyclohexylneopentylamidinate, with Ga–N-
¯
¯
˚
˚
distances of 2.006(7) A to 2.029(6) A, which is slightly longer
than in 3. Due to this elongation the bite angles have to decrease
to 64.9(2)◦ to 65.1(2)◦. The distorted tetrahedral coordination
sphere at Ga(1) at the one side of the chain is completed by a
˚
hydride (H(1)) (Ga(1)–H(1) 1.54(6) A). Ga(3) at the other side of
the chain is surrounded in a distorted tetrahedral manner by an
˚
amidinate, Ga(2) and an ethyl group (Ga(3)–C(52) 1.988(9) A).
4 was obtained by crystallization from thf as colourless,
monoclinic rods, space group Cc (Fig. 3). The unit cell contains
three molecules of thf per molecule 4. The Ga3-unit forms an
This ethyl group points away from the molecule in a way, that the
ethyl carbons, Ga(3) and the linking carbon of the amidinate rest
are coplanar (torsion angle C(53)–C(52)–Ga(3)–C(35) -0.5(7)◦).
6 was crystallized from toluene as colourless, monoclinic plates
in space group C 2/c (Fig. 5). It is composed by a gallium atom
in oxidation state +III distorted tetrahedral coordinated by two
iodine atoms and chelated by one dicyclohexylneopentylamid-
◦
˚
angle of 124.5(1) , with distances of Ga(1)–Ga(2) 2.417(2) A
˚
and Ga(2)–Ga(3) 2.436(2) A. All gallium atoms are chelated
by dicyclohexylneopentylamidinates, with Ga–N distances o◦f
◦
˚
˚
1.939(8) A–1.984(7) A and bite angles of 66.6(3) (Ga(1)), 65.9(3)
(Ga(2)) and 68.2(3)◦ (Ga(3)). Ga(1) carries an iodine atom in
˚
˚
inate (Ga(1)–I(1) 2.508(1) A, Ga(1)–I(2) 2.491(1) A, Ga(1)–N
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Fig. 4 Crystal structure of 5. The thermal ellipsoids are given at
Fig. 3 Crystal structure of 4. The thermal ellipsoids are given at
the 25% probability level. Carbon bound hydrogens are omitted for
the 50% probability level. Hydrogens and thf-molecules are omit-
◦
◦
˚
clarity. Selected bond lengths [A] and angles [ ]: Ga1–H1 1.54(6),
Ga1–N1 2.009(7), Ga1–N2 2.021(6), Ga1–Ga2 2.457(2), Ga2–N4
2.013(6), Ga2–N3 2.029(6), Ga2–Ga3 2.463(2), Ga3–C52 1.988(9),
Ga3–N6 2.006(7), Ga3–N5 2.026(6), N1–C1 1.346(9), N2–C1 1.339(9),
N3–C18 1.34(1), N4–C18 1.346(9), N5–C35 1.337(9), N6–C35 1.344(8),
C52–C53 1.53(2), Ga1–Ga3 4.288(2); H1–Ga1–N1 107(2), H1–Ga1–N2
112(2), N1–Ga1–N2 65.1(2), H1–Ga1–Ga2 121(2), N4–Ga2–N3
64.9(2), Ga1–Ga2–Ga3 121.3(1), C52–Ga3–N6 108.8(3), C52–Ga3–N5
107.4(3), N6–Ga3–N5 65.0(2), C1–N1–C6 134.7(7), C1–N1–Ga1 93.7(5),
C1–N2–Ga1 93.4(4), C18–N3–Ga2 93.5(5), C18–N4–Ga2 94.1(5),
C35–N5–Ga3 93.2(4), C35–N6–Ga3 93.9(5), N3–C18–N4 107.5(5),
N5–C35–N6 107.8(6), C53–C52–Ga3 115.3(7).
˚
ted for clarity. Selected bond lengths [A] and angles [ ]: Ga1–N1
1.939(8), Ga1–N2 1.981(9), Ga1–Ga2 2.417(2), Ga1–I1 2.567(2), Ga2–N3
1.958(8), Ga2–N4 1.984(7), Ga2–Ga3 2.436(2), Ga3–N5 1.947(8),
Ga3–N6 1.974(8), Ga3–N7 2.02(1), C52–N7 1.14(2); N1–Ga1–N2
66.6(3), N1–Ga1–Ga2 125.0(2), N2–Ga1–Ga2 129.3(2), N1–Ga1–I1
107.1(2), N2–Ga1–I1 109.0(2), Ga2–Ga1–I1 111.7(1), N3–Ga2–N4
65.9(3), N3–Ga2–Ga1 112.7(3), N4–Ga2–Ga1 112.4(2), N3–Ga2–Ga3
112.5(3), N4–Ga2–Ga3 114.5(2), Ga1–Ga2–Ga3 124.5(1), N5–Ga3–N6
68.2(3), N5–Ga3–N7 109.1(4), N6–Ga3–N7 109.7(4), N5–Ga3–Ga2
125.2(2), N6–Ga3–Ga2 122.9(2), N7–Ga3–Ga2 113.7(3), C1–N1–Ga1
94.4(6), C1–N2–Ga1 93.2(6), C35–N5–Ga3 93.0(6), C35–N6–Ga3 92.9(7),
N4–C18–N3 105.0(7), C52–N7–Ga3 133.9(10).
◦
˚
1.943(4) A). The bite angle accounts 68.1(2) . The N2CC-group
is planar with an angle sum of 359.9◦. The gallium atom is
almost coplanar with the N2CC-group (Ga(1)–N(1)C(1)N(2)C(2)
5.4(1) pm). The angle I(1)–Ga(1)–I(2) differs with 111.8(1)◦
slightly from the ideal 109.5◦ which is in the line with the Bent’s
rule.31,32
3.4 Quantum chemical calculations
All DFT calculations have been performed with the TURBO-
MOLE (VERSION 6-1) package using an RI approximation with
a BP86 functional and def2-TZVP basis set.33–41 The coordinates
from the crystal structure analysis of 3, 4 and 5 were used as
starting points in geometry optimization and following Ahlrich-
Heinzmann population analysis based on occupation numbers
(SEN).42 The coordinates from the crystal structure analysis of 2
and 2a were used for single point calculations in order to determine
the dipole momentum.
The SEN for 3 (Table 2) show that there is a covalent bond
with bond order one between the gallium atoms. The 2c-SEN for
the N–C N group lie in the typical range of delocalized double
bonds, which agrees with the bond lengths being equal.
Fig. 5 Crystal structure of 6. The thermal ellipsoids are given at the 50%
probability level. Hydrogens are omitted for clarity. Selected bond lengths
◦
˚
[A] and angles [ ]: Ga1–I1 2.508(1), Ga1–I2 2.491(1), Ga1–N1 1.939(4),
Ga1–N2 1.946(4), N1–C1 1.339(6), N1–C6 1.456(5), N2–C1 1.359(6),
N2–C12 1.471(5), C1–C2 1.521(7); N1–Ga1–N2 68.1(2), I2–Ga1–I1
111.8(1), N1–C1–N2 107.4(4), N1–C1–C2 124.2(4), N2–C1–C2 128.3(4),
N1–Ga1–I2 115.7(2), N2–Ga1–I2 117.4(2), N1–Ga1–I1 119.3(2),
N2–Ga1–I1 118.4(2),C1–N1–Ga1 92.7(3), C1–N2–Ga1 91.8(3).
When looking at the charge distribution (Table 3) we see a
positive cloud around the carbon atoms (except the methyl carbon
atoms) and a negative cloud around the gallium atoms and iodine
atoms. This results in a calculated dipole momentum of 5.2 Debye.
For comparison only, the dipole momentum of thf (the most polar
solvent we use) is about 1.8 Debye, according to calculations on
the same level.
The dipole momentum of 3c adds up to 8.3 Debye. So this could
explain the uncommon dihedral angle in 3c: the thf molecules
create an electrostatic field which makes the trigallane to twist a
little to become more polar.
Table 2 2-centre Shared Electron Numbers in 3
Bond
2c-SEN
Ga–Ga
Ga–I
1.32 . . . 1.33
0.91
Ga–N (terminal)
Ga–N (centre)
0.82 . . . 0.84
0.82 . . . 0.83
1.62 . . . 1.63
N–C
N
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Table 3 Atomic charges with multicentre corrections in 3
Table 6 2-centre Shared Electron Numbers in 5
Atom
Charge
Bond
2c-SEN
Ga (centre)
Ga (terminal)
I
N (centre)
N (terminal)
-0.06
HGa–Ga
EtGa–Ga
EtGa–N
Ga–N (remaining)
N–C N
Ga–H
Ga–C
1.35
1.37
0.77 . . . 0.79
0.79 . . . 0.85
1.62 . . . 1.63
1.28
+0.04 . . . +0.05
-0.25
-0.08
-0.09 . . . -0.08
+0.13 . . . +0.15
+0.03
N–C
N
N–C(–C)
N
1.27
N2CC(CH3)3
C6H11 (ipso-C)
C6H11 (other-C)
H
-0.04 . . . -0.03
+0.10 . . . +0.12
+0.01 . . . +0.03
-0.02 . . . +0.02
Table 7 Atomic charges with multicentre corrections in 5
Atom
Charge
Table 4 2-centre Shared Electron Numbers in 4
Ga (centre)
Ga (HGa)
Ga (EtGa)
N
-0.10
-0.06
Bond
2c-SEN
+0.05
-0.09 . . . -0.06
C (GaEt)
-0.13
Ga–GaI
Ga–GaNCMe2
Ga–I
Ga–N (centre)
Ga–N (Ga–I)
Ga–N (Ga-(N2C)-NCMe2)
Ga–N (Ga-NCMe2)
1.32
1.33
0.89
0.80 . . . 0.84
0.81 . . . 0.83
0.73 . . . 0.82
1.16
N–C
N
+0.12 . . . +0.13
+0.04
N–C(–C)
N
H (gallium bound)
H (carbon bound)
-0.10
-0.02 . . . +0.03
a disproportionation of Ga(I) to Ga(II) and Ga(0) is stable
in metathesis reactions. This is the first example of amidinate
stabilised gallium(I) compounds. Using sodium superhydride it
has been possible to obtain the first example of a gallium(II)
hydride. The oxidation state of the trigallane has been confirmed
by redox-titration. These functional di- and trigallanes offer a wide
perspective for preparing rings and cages with oligogallium cores.
N–C
N
1.62 . . . 1.64
Table 5 Atomic charges with multicentre corrections in 4
Atom
Charge
Ga (centre)
Ga (Ga–I)
-0.07
+0.03
Ga (Ga-NCMe2)
I
N (centre)
N (Ga–I)
N (Ga-(N2C)-NCMe)
N (Ga-NCMe2)
+0.04
-0.26
-0.08
-0.09 . . . -0.08
-0.11 . . . -0.07
-0.14
+0.13 . . . +0.15
+0.03 . . . +0.04
-0.02 . . . +0.03
Acknowledgements
We would like to thank the DFG for funding and Philipp Butzug
for the collection of the crystallographic data sets.
N–C
N
N–C(–C)
N
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