Insertion of benzonitrile into Al-N and Ga-N Bonds: Formation of fused carbatriaza-gallanes/alanes and their subsequent synthesis from amidines and trimethyl-gallium/aluminum

Article abstract of DOI:10.1021/ic5029096

Insertion of aromatic nitriles into Al-N and Ga-N bonds are reported. Sterically less hindered aluminum amide [PhNHAlMe₂]₂ (1) undergoes C≡N insertion with benzonitrile to give an isomeric mixture of tetracyclic triazaalanes {[PhNC(P

Full text of DOI:10.1021/ic5029096

Article
pubs.acs.org/IC
Insertion of Benzonitrile into Al−N and Ga−N Bonds: Formation of
Fused Carbatriaza-Gallanes/Alanes and Their Subsequent Synthesis
from Amidines and Trimethyl-Gallium/Aluminum
K. Maheswari, A. Ramakrishna Rao, and N. Dastagiri Reddy*
Department of Chemistry, Pondicherry University, Pondicherry 605014, India
S
* Supporting Information
ABSTRACT: Insertion of aromatic nitriles into Al−N and Ga−N bonds
are reported. Sterically less hindered aluminum amide [PhNHAlMe2]2
(1) undergoes CN insertion with benzonitrile to give an isomeric
mixture of tetracyclic triazaalanes {[PhNC(Ph)N]3[PhNC(Ph)NH]Al-
[AlMe][AlMe2]2} (2 and 3). A similar reaction with analogous gallium
amide affords a tetracyclic triazagallane {[PhNC(Ph)N]3[PhNC(Ph)-
NH]Ga[GaMe][GaMe2]2} (6) along with a novel bowl shaped carbon
containing Ga−N cluster {[PhNC(Ph)N][PhN][GaMe]2}3 (5). On the
other hand, when sterically bulky gallium amide (Dipp on N, Dipp = 2,6-
diisopropylphenyl) is employed, a tetrameric gallium amidinate
{[(Dipp)NC(Ph)N]GaMe}4 (8) is obtained. Tetracyclic triazagallazane
6 is also synthesized from the condensation reaction of N-phenyl-
benzamidine with GaMe3·OEt2. Unlike AlMe3, this reaction produces
only one isomer. In case of amidines with bulkier substituents on N such
as Dipp, formation of a bicyclic triazagallane {[(Dipp)NC(Ph)NH]2[(Dipp)NC(Ph)N][GaMe]2} (14) is also observed along
with tetrameric gallium amidinate 8, whereas N-tert-butylbenzamidine affords exclusively a tetrameric gallium amidinate {[(tert-
Bu)NC(Ph)N]GaMe}4 (15) similar to its Al analogue. However, treating N-(Dipp)acetamidine with GaMe3·OEt2 gives only a
bicyclic triazagallane {[(Dipp)NC(Me)NH]2[(Dipp)NC(Me)N][GaMe]2} (16). An intermediate [(tert-Bu)N(H) C(Ph)-
NGaMe2]2 (17), which is involved in the formation of tetrameric gallium amidinate 15, is also characterized. A comparison of the
structural parameters of Ga−N−C and Al−N−C frameworks synthesized in this study is reported.
gallium reagents and noticed that there had been no reports in
the literature on the insertion of nitriles into the Ga−N bond. A
thorough literature search on group 13 amidinates revealed
reports only on N,N′-disubstituted amidinates but not on N-
monosubstituted amidinates.5 Therefore, we also explored the
reactions of nitriles with a gallium amide and the condensation
reactions of amidines with GaMe3·OEt2. The results are reported
herein.
INTRODUCTION
■
A diverse family of rings and clusters of group 13 elements has
been extensively investigated due to their interesting structure
and bonding aspects.1 A great deal of interest in the synthesis of
M−N frameworks (M = group 13 element) is not only because of
the novel structural diversity but also because of their
applications in advanced materials.2 Though Al and Ga belong
to the same group in the periodic table, the variation in their sizes
and Lewis acidity make them distinct and quite often their
compounds are structurally different. There are a few articles
reported in the literature, which emphasize the dissimilarities in
the formation of Al−N and Ga−N cages/heterocycles.3
Recently, we reported the synthesis of several fused polycyclic
Al−N−C heterocycles with diverse structural facets via insertion
of nitriles into Al−N bonds and the condensation of amidines
with AlMe3.4 The structure of the Al−N frameworks formed in
these studies varied depending upon the steric bulkiness of the
nitrile and the amidine. Though the nitriles with varying degree
of steric bulk were employed, in insertion reactions, the Al−N
precursor was limited to only one aluminum amide, which
contained a sterically bulky 2,6-diisopropylphenyl (Dipp) group.
In this report, we explored the reactions of a sterically
nondemanding aluminum amide with nitriles. Further, we were
also curious to compare these reactions with those of analogous
RESULTS AND DISCUSSION
■
Insertion of Nitriles into Al−N Bonds. Heating an
equimolar mixture of N-phenylaluminum amide (1) and
benzonitrile at 170 °C for 2 h resulted in the formation of a
solid residue, which upon recrystallization from the toluene/
hexane mixture yielded a mixture of isomers of a tetracylic
triazaalane, 2 and 3 (Scheme 1) in 4:1 ratio in good yield.
Interestingly, compounds 2 and 3 were also formed when N-
phenylbenzamidine was treated with AlMe3.4b It is noteworthy
that both methods produced the isomers in a 4:1 ratio.
Received: December 9, 2014
© XXXX American Chemical Society
A
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Scheme 1. Reaction of N-Phenylaluminum Amide with Benzonitrile
Scheme 2. Reaction of N-Phenylgallium Amide with Benzonitrile
Insertion of Nitriles into Ga−N Bonds. Similarly, heating
an equimolar mixture of analogous gallium amide 4 with
benzonitrile at 170 °C for 2 h resulted in a glassy residue, which
upon recrystallization from hexane afforded a few crystals of a
bowl shaped Ga−N−C cluster 5 as the first crop. Concentrating
the mother liquor yielded colorless crystals of a tetracyclic
triazagallane 6 in 51% yield (Scheme 2).
angles at the C and N atoms present in the periphery of the bowl
are of sp2, those at peripheral Ga vary significantly.
Unlike aluminum amide, gallium amide gives only one isomer
6, in its reaction with benzonitrile. The 1H NMR spectrum of 6
shows only one set of resonances. Five singlets for Ga−Me
ranging from 0.03 to −2.02 ppm, one singlet for NH at 5.21 ppm,
and a few multiplets for aromatic protons are observed. The
formation of another isomer of tetracyclic compound similar to 3
was not observed in this reaction. The ORTEP diagram of
compound 6 and its selected bond parameters are given in Figure
2. The molecular structure of 6 shows that it is isostructural with
its Al analogue 2. All the Ga−N bonds follow similar trends in
variation of bond parameters as with its Al analogue. The shortest
Ga−N bond is observed for Ga4−N3 (1.8735(17) Å), and the
longest bond is seen for Ga2−N2 (2.0441(18) Å). All the Ga−N
bond distances are similar to those reported for Ga−N clusters.6,7
Similarly, sterically demanding gallium amide (2,6-diisopro-
pylphenyl group on N) 7 reacted with benzonitrile in the same
fashion as analogous aluminum amide4a and afforded a tetrameric
gallium amidinate 8 in 38% yield (Scheme 3).
Because of the formation of only a few crystals, 5 was
characterized only by single crystal X-ray technique. Several
attempts to obtain pure 5 by repeating the reaction were
unsuccessful. The molecular structure of 5 along with selected
bond parameters is given in Figure 1. The C3 symmetric molecule
has a unique structure among the Ga−N frameworks so far
reported. The crystal structure shows a bowl shaped Ga−N−C
framework comprising three Ga2CN3 six-membered and three
Ga2N2 four-membered rings alternatively fused to a six-
membered Ga3N3 ring. The gallazane ring (Ga3N3), which
forms the bottom of the bowl, is almost planar and the Ga and N
atoms are placed 0.11 Å above and below the mean plane.
However, Ga2CN3 six-membered and Ga2N2 four-membered
rings are highly puckered. Ga−N bond lengths in the central
Ga3N3 ring are not equal. While a bond length of 1.953(2) Å is
observed for those shared by Ga2CN3 rings, the Ga−N that are
shared by Ga2N2 rings are placed at a distance of 2.003(2) Å. All
the Ga−N bonds involving tricoordinated N are shorter (Ga2−
N3 1.936(2) Å). The longest is the one which is transverse to the
gallazane ring (Ga2−N1 2.038(2) Å). These bond lengths are
within the range of those reported for Ga−N clusters.6,7 Ga−N−
Ga and N−Ga−N bond angles in the Ga3N3 ring are close to
120° (118.56(12)° and 119.15(12)°, respectively). While bond
1H NMR spectrum of 8 in CDCl3 is similar to that of its Al
analogue except for a slight downfield shift of Ga−Me resonances
(by 0.46 ppm). Formation of 8 was further confirmed by single
crystal X-ray studies. The crystal structure of 8 and its selected
bond lengths and bond angles are given in Figure 3. The central
Ga−N eight membered ring adopts a highly puckered boat
conformation. The degree of puckering is found to be greater for
the Ga compound compared to its Al analogue (see Supporting
Information Table S1). The extent of distortion of the eight
membered ring from an ideal boat can be assessed from their
torsion angles, M····M distances (1,5 positions), angles of
B
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Figure 2. Molecular structure of 6. All hydrogen atoms are omitted for
clarity except on N8. Thermal ellipsoids are drawn at 50% probability
level. Selected bond lengths [Å]: Ga1−N8 1.977(2), Ga4−N1 1.890(2),
Ga1−N1 1.907(2), Ga4−N7 1.965(2), Ga1−C5 1.953(2), N7−C4
1.332(3), Ga3−C9 2.054(3), Ga2−C7 1.968(3), Ga2−N3 1.973(2),
Ga2−N2 2.044(2), N2−C1 1.329(3), N1−C1 1.324(3), Ga4−N3
1.874(2), N4−C2 1.336(3), Ga4−N5 1.897(2), N3−C2 1.322(3),
Ga3−N5 1.982(2), and Ga3−N4 2.042(2). Selected bond angles [o]:
N1−Ga1−N8 98.53(8), N3−Ga2−N2 97.29(7), N5−Ga3−N4
100.15(2), C2−N3−Ga4 122.93(2), N3−C2−N4 123.60(2), C1−
N1−Ga4 123.88(1), N2−C1−N1 122.44(2), N1−Ga1−C5 124.92(9),
N3−Ga4−N1 110.53(8), Ga1−N8−C4 130.71(2), Ga2−N2−
C1123.99(1), Ga4−N3−Ga2 108.09(8), Ga4−N5−Ga3 109.45(8),
N7−C4−N8 121.80(2), N5−Ga3−C9 105.94(9), N3−Ga2−C7
115.75(1), Ga4−N1−Ga1 103.33(8), and Ga4−N7−C4 125.72(2).
Scheme 3. Reaction of N-(Dipp)gallium Amide with
Benzonitrile
Figure 1. Molecular structure of 5. (a) Top view. (b) Side view. All
hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at
50% probability level. Selected bond lengths [Å]: Ga1−N1 2.003(2),
Ga1−N3 1.942(2), Ga2−N3 1.936(2), N1−Ga1′ 1.953(2), Ga1−C1
1.966(3), N2−C3 1.365(4), Ga2−C2 1.956 (3), and Ga2−N1 2.038(2).
Selected bond angles [o]: N1−Ga2−N3 83.60(10), N1−Ga1−N1′
119.15(12), N3−Ga1−N1′ 108.73(10), N3−Ga1−C1 112.15(12),
Ga1−N1−Ga1′ 118.56(12), C2−Ga2−N1 114.78(12), Ga2−N1−
Ga1 90.39(9), N3−Ga1−N1 84.39 (10), N2−Ga2−N3 109.80(10),
and N2−Ga2−C2 112.48(13).
(6 and 11, Scheme 4) in 70−80% yield, whereas AlMe3 gave two
isomers in each case.4b Compounds 6 and 11 are isostructural
with the respective aluminum analogues.
amidinate-C····M−N atoms, and dihedral angles between the
planes. Similar to its Al analogue, 8 also shows longer Ga−N
distances (av. 2.03 Å) between Ga and the Dipp substituted N
and shorter Ga−N distances for those which are exocyclic to the
amidinate ring (av. 1.88 Å). All the bond lengths are in agreement
with those reported in the literature.6−8
Reactions of N-Monosubstituted Amidines with
GaMe3·OEt2. In our earlier studies, reactions of AlMe3 with
several N-monosubstituted amidines resulted in formation of
structurally diverse Al−N−C frameworks.4b Enthused by these
results, we conducted similar reactions with GaMe3·OEt2. When
an equimolar mixture of GaMe3·OEt2 and the corresponding N-
phenylamidine was subjected to heating at 170 °C for 2 h,
tetracyclic Ga−N−C heterocycles 6 and 11 were formed in good
yields. Though GaMe3·OEt2 reacted with N-phenylamidines in a
similar fashion to AlMe3, it gave only a single isomer in both cases
The formation of 11 was further confirmed by single-crystal X-
ray studies. The molecular structure of 11 along with its bond
parameters is shown in Figure 4. Structural aspects of 11 are
closely related to its Al analogue. The six membered M2CN3
rings, in all the polycyclic compounds made in our study, are
puckered. In these rings, five atoms are almost in one plane and
one atom (X) is away from the plane. Figure 5 illustrates the
extent of deviation of such atom from the mean plane of the rest
of the atoms. In the case of benzamidinate heterocycles, 6 and its
Al analogue 2, the extent of deviation of the atom X in all the
rings is almost similar (1.057−0.956 Å in 6 and 1.007−0.908 Å in
2), whereas in the case of acetamidinate heterocycles, 11 and its
Al analogue,4b ring B is almost planar with X deviating only by
0.270 Å (in 11) and 0.236 Å (in Al analogue) away from the
C
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Figure 3. Molecular structure of 8. All hydrogen atoms are omitted for
clarity. Thermal ellipsoids are drawn at 50% probability level. Selected
bond lengths [Å]: Ga1−N2 2.027(2), Ga1−N1 1.970(2), Ga2−N3
1.985(2), Ga2−N4 2.033(2), Ga2−N1 1.880(2), Ga1′−N3 1.985(2),
Ga2−C3 1.951(3), Ga1−C1 1.958(3), N1−C2 1.329(3), and N2−C2
1.349(3). Selected bond angles [o]: N1−Ga1−N2 67.46(9), N1−Ga1−
N3′ 103.53(1), N3′−Ga1−N2 110.23(9), N3−Ga1−C1 122.38(1),
Ga2−N1−Ga1 129.75(1), C1−Ga1−N1 122.05(1), C1−Ga1−N2
118.31(1), N1−Ga2−N4 110.25(9), N1−Ga2−N3 103.75(9), N1−
Ga2−C3 126.55(1), N3−Ga2−C3 118.77(1), Ga2−N1−C2
137.90(2), and N2−C2−N1 112.01(2).
Figure 4. Molecular structure of 11. All hydrogen atoms, except those
on N8, are omitted for clarity. Thermal ellipsoids are drawn at 50%
probability level. Selected bond lengths [Å]: Ga1−N8 1.957(4), Ga4−
N1 1.896(4), Ga1−N1 1.913(4), Ga4−N7 1.977(4), Ga1−C5
1.952(5), N7−C4 1.339(6), Ga3−C9 1.987(5), Ga2−C7 1.978(5),
Ga2−N3 1.961(4), Ga2−N2 2.027(4), N2−C1 1.332(6), N1−C1
1.320(6), Ga4−N3 1.880(4), N4−C2 1.336(6), Ga4−N5 1.899(4),
N3−C2 1.333(6), Ga3−N5 1.973(4), and Ga3−N4 2.040(4). Selected
bond angles [o]: N1−Ga1−N8 98.29(2), N3−Ga2−N2 99.76(2), N5−
Ga3−N4 102.85(2), C2−N3−Ga4 123.31(3), N3−C2−N4 123.80(4),
C1−N1−Ga4 127.90(3), N2−C1−N1 122.51(4), N1−Ga1−C5
128.28(2), N3−Ga4−N1 111.21(2), Ga1−N8−C4 132.00(3), Ga2−
N2−C1 125.31(3), Ga4−N3−Ga2 107.28(2), Ga4−N5−Ga3
112.49(2), N7−C4−N8 120.11(4), N5−Ga3−C9 109.71(2), N3−
Ga2−C7 118.41(2), Ga4−N1−Ga1 101.80(2), and Ga4−N7−C4
125.90(3).
Scheme 4. Reactions of GaMe3·OEt2 with N-Phenylamidines
Interestingly, under similar conditions, N-(Dipp)acetamidine
produced only a bicyclic triazagallane 16 similar to 14 in 67%
yield (Scheme 6). The molecular structure of 16 along with the
bond parameters is given in Figure 7. Compound 16, which
crystallizes in the space group P212121, is a bicyclic
carbagallazane. Two different coordination modes (μ−
1κN,2κN′ and μ−1:2κ2N) for the acetamidinate ligand are
observed. In coordination mode μ−1:2κ2N, the amidinate-C3−
N6 (1.291(3) Å) bond distance is significantly lower than the
other C−N bond distances (C2−N4 1.335(3), C3−N5
1.355(3), and N2−C1 1.324(3) Å) indicating a double bond
character. There is also a variation in Ga−N bond lengths. The
longest is shown in μ−1κN,2κN′ coordination (Ga1−N1
2.019(2) Å), and the shortest is found for μ−1:2κ2N
coordination (Ga2−N5 1.875(2) Å). All the bond lengths are
within the range of those of gallium heterocycles reported in this
study. The bridging-N atom (N5) deviates from the mean planes
of Ga1,N3,C2,N4,Ga2 and Ga1,N1,C1,N2,Ga2 by 0.91 Å. The
mean plane of the rest of the ring atoms. When a comparison is
made between respective Al and Ga heterocycles, the extent of
deviation is found to be slightly larger for Ga heterocycles.
Unlike AlMe3, GaMe3·OEt2 gave a mixture of products 8 and
14 when treated with N-(Dipp)benzamidine (Scheme 5). When
the reaction mixture was recrystallized from the toluene/hexane
mixture at −10 °C, the tetrameric gallium amidinate 8 was
isolated as the first fraction (23%), and further concentrating the
mother liquor yielded a good amount of colorless crystals, which
were later identified as bicycic Ga-amidinate (49%), 14. In
contrast, N-tert-butylbenzamidine afforded a quantitative yield of
15 exclusively. The crystals of 14 did not diffract well, and hence,
the structural parameters are not discussed here, though the data
was good enough for determining the structure (see Supporting
Information Figure S3). The molecular structure of 15 and its
selected bond parameters are shown in Figure 6. The structural
parameters of 15, which are given in the Supporting Information,
Table S2, indicate that the Ga−N eight membered ring is more
symmetrical and less puckered than the related Ga-amidinate 8.
The 1H NMR spectrum of 15 is in agreement with its solid state
structure.
1
dihedral angle between the aforesaid planes is 107.7°. The H
and 13C NMR spectra in CDCl3 support that the solid state
structure is retained in solution.
In order to identify the intermediates in these thermolysis
reactions, we carried out a few experiments under mild reaction
conditions such as low temperature and reflux in solvents. When
an equimolar mixture of N-tert-butylbenzamidine (13) and
GaMe3·OEt2 was stirred in hexane at 0 °C for 30 min, a four-
membered gallium amide 17 was formed in 56% yield. 17 was
further subjected to thermolysis at 170 °C for about 2 h to obtain
the tetrameric gallium amidinate 15 in quantitative yield. This
indicates that the formation of 15 from N-tert-butylbenzamidine
and GaMe3·OEt2 involves a stepwise elimination of methane and
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Figure 5. (a) Representation of deviated atoms (in red) in tetracyclic Al/Ga heterocycles. (b) Representation of deviated atom from the mean plane.
Scheme 5. Reaction of GaMe3·OEt2 with N-Dipp and N-tert-Butylbenzamidines
Scheme 6. Reaction of GaMe3·OEt2 with N-
(Dipp)acetamidine
intermediate products from the other amidines were unsuccess-
ful.
Figure 6. Molecular structure of 15. All hydrogen atoms are omitted for
clarity. Thermal ellipsoids are drawn at 50% probability level. Selected
bond lengths [Å]: Ga1−N2 2.008(2), Ga1−N1 1.979(2), Ga2−N3
1.980(2), Ga2−N4 2.014(2), Ga2−N1 1.872(2), Ga1′−N3 1.874(2),
Ga2−C3 1.957(2), Ga1−C1 1.955(2), N1−C2 1.330(3), and N2−C2
1.330(3). Selected bond angles [o]: N1−Ga1−N2 67.50(7), N1−Ga1−
N3′ 105.24(7), N3′−Ga1−N2 107.09(8), N3−Ga1−C1 123.58(9),
Ga2−N1−Ga1 132.95(9), C1−Ga1−N1 122.24(9), C1−Ga1−N2
117.55(9), N1−Ga2−N4 106.85(8), N1−Ga2−N3 103.65(7), N1−
Ga2−C3 124.33(1), N3−Ga2−C3 122.33(1), Ga2−N1−C2
136.32(2), and N2−C2−N1 112.68(2).
An ORTEP diagram of 17 along with selected bond
parameters is shown in Figure 8. The compound 17 crystallizes
in P21/n space group as a centrosymmetric molecule. It contains
a planar four membered gallazane ring. There are no noticeable
deviations in the bond parameters from similar four membered
Ga−N rings reported in the literature.9
On the other hand, refluxing an equimolar mixture of N-
(Dipp)benzamidine and GaMe3·OEt2 in toluene for 2 h afforded
a mixture of products. Attempts to recrystallize the crude from
hexane resulted in formation of a few crystals of a tricyclic
compound 18 (Figure 9), which is presumably formed due to the
presence of traces of water in the solvent used for crystallization.
Crystal data and the molecular structure of 18 along with the
dimerization of the resultant unstable species as one of the
plausible pathways as shown in Scheme 7. Attempts to isolate any
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Figure 8. Molecular structure of 17. All hydrogen atoms, except those
on N2 and N2′, are omitted for clarity. Thermal ellipsoids are drawn at
50% probability level. Selected bond lengths [Å]: Ga1−N1 1.974(3),
Ga1−C1 1.977(4), C3−N2 1.390(5), Ga1−C2 1.969(5), Ga1−N1′
2.005(3), C3−N1 1.273(5), and Ga1−Ga1′ 3.005(8). Selected bond
angles [o]: N1−Ga1−N1′ 81.91(14), C2−Ga1−N1 110.59(18), C2−
Ga1−C1 122.00(19), Ga1−N1−Ga1′ 98.09(14), N1−C3−N2
121.5(4), Ga1−N1−C3 131.9(3), and C3−N1−Ga1′ 128.9(3).
Figure 7. Molecular structure of 16. All hydrogen atoms, except those
on N2 and N3, are omitted for clarity. Thermal ellipsoids are drawn at
50% probability level. Selected bond lengths [Å]: Ga1−N1 2.019(2),
Ga1−N3 1.976(2), Ga1−N5 1.890(2), Ga2−C5 1.955(3), C2−N4
1.335(3), C3−N6 1.291(3), C3−N5 1.355(3), Ga2−N5 1.875(2), N2−
C1 1.324(3), and Ga2−N4 2.001(19). Selected bond angles [o]: N3−
Ga1−N1 104.37(9), C4−Ga1−N3 108.06(11), N5−Ga1−N3
99.81(9), N5−Ga2−N2 101.85(9), Ga2−N5−Ga1 107.26(10), C2−
N3−Ga1 134.00(16), N5−Ga1−C4 132.71(10), Ga1−N5−C3
117.15(16), N5−C3−N6 119.8(2), and C5−Ga2−N4 110.52(10).
selected bond parameters are given in the Supporting
Information.
Treating N-phenyl amidines such as N-phenylbenzamidine
and N-phenylacetamidine with GaMe3·OEt2 under reflux in
toluene for 4 h afforded tetracyclic triazagallanes, 6 and 11, in
good yield. No intermediates were isolated.
Figure 9. Oxo bridged tricyclic Ga−N−C heterocycle.
A comparison between N-monosubstituted Al and Ga
amidinates revealed that sterically less demanding N-phenyl-
amidinates, which are having wider bite angles, favored bridging
coordination for both the metals and produced structurally
analogous heterocycles. However, when the amidinates having
sterically demanding groups were employed, certain structural
dissimilarities between Al and Ga compounds were found. In
case of Al, N-(Dipp) and N-tert-butyl amidinates preferred four-
membered chelate complexes, which are tetrameric. With the
heavier analogue gallium, such preference for chelation was
found only for N-tert-butylbenzamidinate, while the other two
Scheme 7. Isolation of Intermediate (17) from a Reaction between GaMe3·OEt2 and N-tert-Butylbenzamidine at 0 °C and Its
Further Thermolysis
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Table 1. Crystal Data for Compounds 5, 6, 8, 11, and 15−18
5
6·C7H8
C64H64Ga4N8
8·2C7H8
C94H116Ga4N8
11·C7H8
C81H104Ga8N16
empirical formula
formula wt
temp (K)
cryst syst
space group
a (Å)
C63H63Ga6N9
1364.54
1224.11
149 (2)
triclinic
1636.83
1859.56
140 (2)
triclinic
150.00 (2)
trigonal
120.1 (2)
monoclinic
C2/c
R3
̅
P1
̅
P1
̅
19.2352(4)
19.2352(4)
33.3633(9)
90.00
14.1348(5)
14.1580(4)
16.4427(5)
93.330(2)
113.944(3)
101.566(3)
2910.33(17)
2
29.0426(12)
10.3599(4)
31.2885(17)
90.00
13.0751(6)
13.6722(6)
14.7834(6)
71.830(4)
65.638(4)
62.669(5)
2114.07(16)
1
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
90.00
98.561(4)
90.00
120.00
V (Å3)
10690.4(4)
6
9309.1(7)
4
Z
ρcalcd (Mg m−3
)
1.272
1.397
1.168
1.461
μ (mm−1
)
2.275
1.878
1.191
2.560
F(000)
4140.0
1256.0
3440.0
950.0
cryst size (mm)
0.33 × 0.26 × 0.18
3.67−29.10
0.36 × 0.24 × 0.21
3.71−29.24
0.40 × 0.34 × 0.23
2.63−25.0
0.38 × 0.25 × 0.21
3.58−29.18
θ range (deg)
no. of collected/unique rflns 33439/5892 (R(int) = 0.0645) 31236/13415 (R(int) = 0.0287) 26977/8203 (R(int) = 0.0560) 19783/9648 (R(int) = 0.0492)
no. of data/restraints/params 5892/0/237
13415/0/695
0.0314, 0.0716
0.0470, 0.0788
1.047
8203/0/489
0.0405, 0.0917
0.0614, 0.0990
1.031
9648/0/476
0.0514, 0.1092
0.0924, 0.1292
1.016
a
R1, wR2 (I > 2σ(I))
0.0390, 0.0893
0.0717, 0.0963
0.992
a
R1, wR2 (all data)
GOF
Δρmax/Δρmin (e Å−3
)
0.54/−0.63
0.99/−0.59
0.53/−0.41
16·C6H14
1.30/−0.81
17
15
empirical formula
formula wt
temp (K)
cryst syst
space group
a (Å)
C48H68Ga4N8
C50H83Ga2N6
C26H42Ga2N4
550.08
1035.98
149.8(2)
monoclinic
I2/a
907.66
151.0
150.0(2)
orthorhombic
P212121
monoclinic
P21/n
23.0044(6)
11.3834(2)
23.2147(8)
90.00
13.58923(2)
18.9059(2)
20.0649(2)
90.00
10.6745(7)
9.7298(5)
13.8119(7)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
117.671(4)
90.00
90.00
92.884(6)
90.00
90.00
V (Å3)
5383.9(2)
4
5155.05(10)
4
1432.69(13)
2
Z
ρcalcd (Mg m−3
)
1.278
1.170
1.275
μ (mm−1
)
2.017
1.082
1.899
F(000)
2144.0
1948.0
576
cryst size (mm)
0.42 × 0.35 × 0.28
3.73−29.21
33931/6636 (R(int) = 0.0354)
0.35 × 0.30 × 0.27
2.74−29.37
36228/12217 (R(int) = 0.0418)
12217/1/546
0.0339, 0.0772
0.0438, 0.0832
1.046
0.42 × 0.35 × 0.22
3.82−28.71
9475/3288 (R(int) = 0.0305)
3288/0/150
0.0519, 0.1345
0.0620, 0.1385
1.337
θ range (deg)
no. of collected/unique rflns
no. of data/restraints/params
6636/0/281
a
R1, wR2 (I > 2σ(I))
0.0313, 0.0680
0.0461, 0.0745
1.086
a
R1, wR2 (all data)
GOF
Δρmax/Δρmin (e Å−3
R1 = Σ∥F0| − |Fc∥/Σ|F0|; wR2 = [Σw(F0 − Fc2)2/Σw(F0 )2]0.5.
)
0.94/−0.37
2
0.64/−0.61
1.28/−0.59
a
2
amidinates (N-(Dipp)benzamidinate and N-(Dipp)-
acetamidinate) preferred bridging coordination mode.
condensation reactions explored in this study, Al and Ga reagents
produced analogous compounds except in a few cases (5, 14, and
16). Sterically less hindered aluminum amide (PhNHAlMe2)2 in
its insertion reaction with PhCN produced a mixture of
tetracyclic aluminum amidinate isomers 2 and 3. A similar
reaction employing analogous gallium amide (PhNHGaMe2)2
afforded a single isomer of tetracyclic gallium amidinate 6 along
with a bowl shaped carbon containing Ga−N cluster 5. On the
other hand, sterically bulky gallium amide (Dipp on N) reacted
CONCLUSION
■
In summary, insertion of aromatic nitriles into Al−N and Ga−N
bonds led to the formation of Al−N−C and Ga−N−C
heterocycles, respectively. These heterocycles could also be
obtained by the condensation reactions of N-monosubstituted
amidines with AlMe3 or GaMe3·OEt2. In these insertion and
G
DOI: 10.1021/ic5029096
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
With N-Phenylgallium Amide. N-Phenylgallium amide (0.23 g, 0.59
mmol), benzonitrile (0.06 g, 0.58 mmol), and hexane (20 mL). The
filtrate afforded a few crystals of 5 as the first fraction. Concentrating the
mother liquor further yielded colorless crystals of 6. Yield: 0.17 g (51%).
with PhCN in a similar fashion as its Al analogue did and
produced a tetrameric gallium amidinate 8.
Condensation reactions of GaMe3·OEt2 with sterically less
bulky amidines occurred similar to corresponding reactions of
AlMe3 and yielded tetracyclic gallium amidinates 6 and 9. In case
of amidines with bulkier substituents on N such as Dipp,
formation of a simple bicyclic gallane 14 was also observed along
with a tetrameric gallium amidinate 8, whereas N-tert-
butylbenzamidine afforded exclusively a tetrameric gallium
amidinate 15 similar to its reaction with AlMe3. On the other
hand, treating N-(Dipp)acetamidine with GaMe3·OEt2 resulted
in a bicyclic gallane 16. Comparison of the structural parameters
of Ga−N−C heterocycles with those of similar Al−N−C
frameworks revealed that the Ga analogues are more puckered
than their Al counterparts. Isolation of 17 proves that the
tetrameric amidinates are formed via stepwise elimination of
methane followed by the aggregation of subsequent intermedi-
ates.
1
Mp: 193−194 °C. H NMR (400 MHz, CDCl3): δ −2.02 (s, 3H,
GaMe), −1.65 (s, 3H, GaMe), −1.56 (s, 3H, GaMe), −0.07 (s, 3H,
GaMe), 0.03 (s, 3H, GaMe), 5.21 (s, 1H, NH), 6.08 (m, 2H, ArH),
6.64−7.31 (set of multiplets, 37H, ArH), 7.51 (m, 1H, ArH). 13C NMR
(100 MHz, CDCl3): δ −12.47, −8.79, −6.55, −5.85, −3.34, 122.65,
123.57, 124.26, 125.58, 126.96, 127.29, 127.38, 127.43, 127.67, 127.71,
127.78, 128.10, 128.16, 128.22, 128.25, 128.35, 128.60, 128.88, 129.09,
137.88, 139.62, 140.99, 141.04, 145.94, 147.90, 148.69, 149.14, 173.49,
173.96, 174.38, 177.09 Anal. Calcd for C57H56Ga4N8: C, 60.48; H, 4.99;
N, 9.90. Found: C, 60.69; H, 5.18; N, 9.73.
With N-(Dipp)gallium Amide. N-(Dipp)gallium amide (0.48 g, 0.87
mmol), benzonitrile (0.09 g, 0.87 mmol), and toluene (15 mL). The
filtrate yielded colorless crystals of compound 8 at 0 °C overnight. Yield:
0.24 g (38%). Mp: 236 °C. 1H NMR (400 MHz, CDCl3): −0.24 (s, 12H,
GaMe), 0.72 (d, 12H, CH(CH3)2), 0.99 (m, 24H, CH(CH3)2), 1.20 (d,
12H, CH(CH3)2), 3.27 (m, 4H, CH(CH3)2), 3.62 (m, 4H, CH(CH3)2),
6.94 (m, 4H, ArH), 6.99 (m, 16H,ArH), 7.02 (m, 8H, ArH), 7.08 (m,
4H, ArH). 13C NMR (100 MHz, CDCl3): δ −6.87, 23.11, 23.36, 25.93,
26.19, 27.74, 28.15, 123.21, 123.42, 124.90, 127.92, 128.63, 129.56,
135.31, 140.12, 144.11, 144.67, 178.25. Anal. Calcd for C80H100Ga4N8:
C, 66.15; H, 6.94; N, 7.71. Found: C, 66.29; H, 7.13; N, 7.59.
General Procedure for the Reactions of GaMe3·OEt2 with N-
Monosubstituted Amidines. GaMe3·OEt2 (1 equiv) was added to
the preheated amidine (1 equiv) at 120 °C, and the temperature was
raised and maintained at 170 °C for 2 h. After the reaction mixture was
cooled to room temperature, the resultant residue was recrystallized
from hexanes or toluene/hexane mixtures.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out under
N2 atmosphere using a Schlenk line and a glovebox. Trimethylgallium
ether adduct,10 N-phenylgallium amide,3a N-(Dipp)gallium amide,3a N-
phenylbenzamidine,11 N-(Dipp)-benzamidine,4a N-phenylacetami-
dine,12 and N-tert-butylbenzamidine12 were prepared by following
literature procedures. Hexane and toluene (from Na/benzophenone
ketyl) were distilled fresh as and when required. 1H and 13C spectra were
recorded on a Bruker 400 MHz instrument. Elemental analyses were
performed using a Flash 2000 organic elemental analyzer.
With N-Phenylacetamidine. N-Phenylacetamidine (0.25 g, 1.86
mmol), GaMe3·OEt2 (0.35 g, 1.85 mmol), and hexane (30 mL).
Colorless crystals of 11 were obtained from the filtrate at room
Structural Determination for 5, 6, 8, 11, and 14−18. Single
crystals of 5, 6, 8, 11, and 14−18 were mounted on a glass fiber in
paraffin oil and then brought into the cold nitrogen stream of a low-
temperature device so that the oil solidified. Data collection was
performed on an OXFORD XCALIBUR diffractometer, equipped with
CCD area detector, using graphite-monochromated Mo Kα (λ =
0.71073 Å) radiation. All calculations were performed using SHELXS-
97 and SHELXL-97.13 The structures were solved by direct methods
and successive interpretation of the difference Fourier maps, followed by
full matrix least-squares refinement (against F2). All non-hydrogen
atoms and solvent molecules were refined anisotropically, except for the
solvent molecules in 11. In 5, solvent molecules were severely
disordered and the SQUEEZE program was used to eliminate the
residues. The contribution of the hydrogen atoms, in their calculated
positions, was included in the refinement using a riding model. Upon
convergence, the final Fourier difference map of the X-ray structures
showed no significant peaks. All the data sets were collected to 2Θ values
>50°. Relevant data concerning crystallographic data, data collection,
and refinement details for compounds 5, 6, 8, 11, and 15−17 are
summarized in Table 1, and for compounds 14 and 18, they are given in
Table S3 (see the Supporting Information). Crystallographic data
(excluding structure factors) for the structures reported in this paper
have also been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC 1003875−1003878,
1003881−1003883, 1003885, and 1004150. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K. (Fax: (+44) 1223-336-033; E-mail:
deposit@ccdc.cam.ac.uk).
1
temperature after 1 day. Yield: 0.31 g (76%). Mp: 172−173 °C. H
NMR (400 MHz, CDCl3): δ −0.93 (s, 3H, GaMe), −0.87 (s, 3H,
GaMe), −0.65 (s, 3H, GaMe), −0.36 (s, 3H, GaMe), −0.31 (s, 3H,
GaMe), 1.36 (s, 3H, CH3), 1.83 (s, 3H, CH3), 1.84 (s, 3H, CH3), 1.91 (s,
3H, CH3), 4.87 (s, 1H, NH), 6.34 (m, 2H, ArH), 6.97−7.31 (set of
multiplets, 18H, ArH). 13C NMR (100 MHz, CDCl3): δ −7.61, −5.50,
−5.43, −5.07, 24.43, 24.61, 25.05, 25.43, 124.21, 124.73, 125.17, 125.46,
125.97, 127.57, 127.93, 128.10, 128.70, 129.13, 129.19, 146.10, 148.96,
149.17, 149.46, 170.61, 171.20, 171.67, 175.36. Anal. Calcd for
C37H48Ga4N8: C, 50.29; H, 5.47; N, 12.68. Found: C, 50.13; H, 5.63;
N, 12.81.
With N-Phenylbenzamidine. N-Phenylbenzamidine (0.26 g, 1.32
mmol) and GaMe3·OEt2 (0.31 g, 1.37 mmol). After completion of the
reaction, 30 mL of hexane was added to the residue. Only half of it was
dissolved in hexane. The mixture was filtered, and the filtrate was kept at
room temperature. Colorless crystals of 6 were obtained in 1 day. The
1H NMR spectrum of the precipitate was found to be the same as that of
6. Yield: 0.29 g (72%).
With N-(Dipp)benzamidine. N-(Dipp)benzamidine (0.51 g, 1.82
mmol), GaMe3·OEt2 (0.35 g, 1.85 mmol), toluene (20 mL), and hexane
(5 mL). Keeping the filtrate at −10 °C for 2 days afforded colorless
crystals of 8. Yield: 0.15 g (23%). Further concentrating the mother
liquor to 15 mL and storing it at −10 °C for 1 day afforded colorless
crystals of 14. Yield: 0.30 g (49%). Mp: > 280 °C dec. 1H NMR (400
MHz, CDCl3): δ −0.90 (s, 6H, GaMe), 0.35 (m, 3H, CH(CH3)2), 0.81
(m, 6H, CH(CH3)2), 0.86 (m, 3H, CH(CH3)2), 1.06 (m, 9H,
CH(CH3)2), 1.25 (m, 3H, CH(CH3)2), 1.30 (m, 9H, CH(CH3)2),
1.44 (m, 3H, CH(CH3)2), 2.95 (m, 1H, CH(CH3)2), 3.36 (m, 3H,
CH(CH3)2), 3.55 (m, 2H, CH(CH3)2), 5.18 (s, 2H, NH), 6.78 (m, 2H,
ArH), 6.88 (m, 2H, ArH), 7.10 (m, 8H, ArH), 7.23(m, 12H, ArH). 13C
NMR (100 MHz, CDCl3): δ 21.61, 22.45, 22.71, 23.63, 23.84, 25.16,
25.59, 27.01, 27.71, 28.79, 29.51, 120.03, 121.15, 123.67, 123.72, 125.45,
126.27, 127.10, 127.17, 127.62, 128.04, 128.38, 129.15, 129.25, 129.65,
132.31, 137.68, 138.03, 138.94, 139.65, 140.22, 143.26, 147.75, 164.90.
Anal. Calcd for C59H74Ga2N6: C, 70.39; H, 7.41; N, 8.35. Found: C,
70.52; H, 7.23; N, 8.17.
General Procedure for the Reactions of Benzonitrile with N-
Arylmetal Amides. In a typical reaction, a mixture of N-arylmetal
amide (1 equiv) and benzonitrile (1 equiv) was taken in a Schlenk flask
and heated to 170 °C for 2 h. After the reaction mixture was cooled to
room temperature, the residue was dissolved in hexane or toluene and
filtered using Celite.
With N-Phenylaluminum Amide. N-Phenylaluminum amide (0.53
g, 1.77 mmol), benzonitrile (0.18 g, 1.78 mmol), toluene (10 mL), and
hexane (5 mL). The filtrate afforded a mixture of crystals of 2 and 3 in
4:1 ratio (0.59 g, 69%). Characterization details of these compounds are
reported in our earlier publication.3b
H
DOI: 10.1021/ic5029096
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
With N-tert-Butylbenzamidine. N-tert-Butylbenzamidine (0.47 g,
2.70 mmol), GaMe3·OEt2 (0.51 g, 2.69 mmol), and hexane (25 mL).
Storing the filtrate afforded X-ray quality crystals of 15 in quantitative
yield. Mp: >300 °C dec. 1H NMR (400 MHz, CDCl3): δ −0.89 (s, 12H,
GaMe), 0.92 (s, 36H, C (CH3)3), 7.21 (m, 8H, ArH), 7.36 (m, 12H,
ArH). 13C NMR (100 MHz, CDCl3): δ −9.35, 32.38, 51.20, 127.64,
127.83, 140.51, 177.87. Anal. Calcd for C48H68Ga4N8: C, 55.65; H, 6.62;
N, 10.82. Found: C, 55.49; H, 6.81; N, 10.94.
With N-(Dipp)acetamidine. N-(Dipp)acetamidine (0.58 g, 2.66
mmol), GaMe3·OEt2 (0.50 g, 2.64 mmol), and hexane (25 mL), storing
the filtrate at 0 °C gave colorless crystals of compound 16. Yield: 0.49 g
(67%, based on the amidine). Mp: 233−235 °C. 1H NMR (400 MHz,
CDCl3): δ −0.62 (s, 6H, GaMe), 1.08−1.25 (set of multiplets, 36H,
CH(CH3)2), 1.69 (s, 3H, CH3), 1.81 (s, 6H, CH3), 3.04 (m, 1H,
CH(CH3)2), 3.19 (m, 3H, CH(CH3)2), 3.36 (m, 2H, CH(CH3)2), 4.78
(s, 2H, NH), 6.88 (m, 1H, ArH), 7.04 (m, 2H, ArH), 7.16 (m, 4H, ArH),
7.21(m, 2H, ArH). 13C NMR (100 MHz, CDCl3): δ −7.86, 21.07, 22.85,
23.08, 23.59, 23.83, 23.89, 23.94, 24.03, 24.08, 24.27, 24.91, 25.01, 27.65,
27.68, 28.42, 28.66, 120.11, 122.05, 122.71, 123.43, 123.88, 124.14,
126.42, 139.24, 139.67, 142.41, 143.94, 144.36, 149.54, 163.53, 170.31.
Anal. Calcd for C44H69Ga2N6: C, 64.33; H, 8.47; N, 10.23. Found: C,
64.47; H, 8.29; N, 10.09.
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Low Temperature Reaction between N-tert-Butylbenzamidine
and GaMe3·OEt2. To a solution of N-tert-butylbenzamidine (0.35 g,
2.01 mmol) in hexane at 0 °C was added GaMe3·OEt2 (0.38 g, 2.01
mmol). The reaction mixture was slowly brought to room temperature
and stirred for 30 min. Colorless, X-ray quality crystals of 17 were
obtained by storing the filtrate at −10 °C. Yield: 0.33 g (56%). Mp:
1
164−166 °C. H NMR (400 MHz, CDCl3): δ −0.80 (s, 6H, GaMe),
1.49 (s, 9H, C(CH3)3), 7.42 (m, 3H, ArH), 7.51 (m, 2H, ArH). 13C
NMR (100 MHz, CDCl3): δ −4.93, 29.29, 31.64, 127.93, 128.65,
131.33. Anal. Calcd for C26H42Ga2N4: C, 56.77; H, 7.70; N, 10.19.
Found: C, 56.85; H, 7.57; N, 10.31.
ASSOCIATED CONTENT
* Supporting Information
■
S
Packing diagrams of 5 showing C−H···π interactions, crystal data
and molecular structures of 14 and 18 and their selected bond
parameters, table containing the structural parameters of 8 and
15, and the crystallographic information files (CIF) for 5, 6, 8,
11, and 14−18. This material is available free of charge via the
Internet at http://pubs.acs.org.
(13) (a) SHELXTL v6.12; BrukerAXS Instruments Inc.: Madison,WI,
2000. (b) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ndreddy.che@pondiuni.edu.in.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the Department of Science and
Technology, New Delhi, for financial support and Alexander von
Humboldt Stiftung, Germany, for donation of a glovebox. We
thank the Central Instrumentation Facility, Pondicherry
University, for NMR spectra and DST-FIST for the single
crystal X-ray diffraction facility. K.M. thanks CSIR, India, for a
senior research fellowship.
REFERENCES
■
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I
DOI: 10.1021/ic5029096
Inorg. Chem. XXXX, XXX, XXX−XXX