Reactions of N-monosubstituted amidines with AlMe3 and AlMeCl2: Formation of fused triazaalane heterocycles

Article abstract of DOI:10.1021/om2007375

Reactions of N-monosubstituted amidines of the formula HN=C(R)-NH(R′) (R = Ph, 4-tert-butylphenyl, Me; R′ = 2,6-diisopropylphenyl, Ph) with AlMe₃ and AlMeCl₂ are reported. All the N-(Dipp)amidines (Dipp = 2,6-diisopropylphenyl) react with AlMe3 in a 1:1 ratio to yield tetrameric aluminum amidinates (1, 2, and 3) in good yields. In these compounds, the amidinate ligand chelates to Al while bridging to another Al. However, when N-phenylamidines are employed, tetracyclic (4-9) and pentacyclic (10) Al-N-C heterocycles are formed. In the case of N-phenylbenzamidine, formation of a hexacyclic Al-N-C heterocycle (11) is observed when a slight excess of AlMe ₃ (1:1.2 equiv) is used. In these compounds, the amidinate ligand coordinates to Al atoms in a bridging fashion. Further, N-(Dipp)acetamidine and N-(Dipp)benzamidine are also treated with AlMeCl₂. Whereas N-(Dipp)acetamidine affords an ionic 15-membered aluminum amidinate chain (12), N-(Dipp)benzamidine gives a bicyclic heterocycle (13) and the AlCl₃ adduct of N-(Dipp)benzamidine (14). However, from a reaction between N-phenylbenzamidine and AlMeCl₂, only the AlCl₃ adduct, 15, was isolated. Compounds 3, 4, 6, 8, and 10-15 have been structurally characterized.

Full text of DOI:10.1021/om2007375

Article
pubs.acs.org/Organometallics
Reactions of N-Monosubstituted Amidines with AlMe3 and AlMeCl2:
Formation of Fused Triazaalane Heterocycles
K. Maheswari and N. Dastagiri Reddy*
Department of Chemistry, Pondicherry University, Pondicherry 605014, India
S
* Supporting Information
ABSTRACT: Reactions of N-monosubstituted amidines of
the formula HNC(R)−NH(R′) (R = Ph, 4-tert-butylphenyl,
Me; R′ = 2,6-diisopropylphenyl, Ph) with AlMe3 and AlMeCl2
are reported. All the N-(Dipp)amidines (Dipp = 2,6-
diisopropylphenyl) react with AlMe3 in a 1:1 ratio to yield
tetrameric aluminum amidinates (1, 2, and 3) in good yields.
In these compounds, the amidinate ligand chelates to Al while
bridging to another Al. However, when N-phenylamidines are
employed, tetracyclic (4−9) and pentacyclic (10) Al−N−C
heterocycles are formed. In the case of N-phenylbenzamidine,
formation of a hexacyclic Al−N−C heterocycle (11) is
observed when a slight excess of AlMe3 (1:1.2 equiv) is
used. In these compounds, the amidinate ligand coordinates to Al atoms in a bridging fashion. Further, N-(Dipp)acetamidine and
N-(Dipp)benzamidine are also treated with AlMeCl2. Whereas N-(Dipp)acetamidine affords an ionic 15-membered aluminum
amidinate chain (12), N-(Dipp)benzamidine gives a bicyclic heterocycle (13) and the AlCl3 adduct of N-(Dipp)benzamidine
(14). However, from a reaction between N-phenylbenzamidine and AlMeCl2, only the AlCl3 adduct, 15, was isolated.
Compounds 3, 4, 6, 8, and 10−15 have been structurally characterized.
characterization of tetrameric four-membered aluminum
amidinates and fused six-membered carbaalumazanes (carbon-
containing triazaalanes).
INTRODUCTION
■
Aluminum amidinates have been the theme of interest since the
discovery of their use as catalysts for polymerization of olefins.1
It has also been reported that these compounds are good
precursors for atomic layer deposition (ALD) of aluminum
nitride.2 There has been a plethora of N,N-disubstituted
amidinates of aluminum reported in the literature.3 Two major
synthetic routes for N,N-disubstituted aluminum amidinates are
(i) insertion of carbodiimides into Al−C bonds4 and (ii)
condensation reactions of amidines with aluminum reagents.3
Mononuclear Al amidinates, in which the amidinate ring
chelates to Al, are most commonly found as mono(amidinate)
complexes and rarely as bis- and tris(amidinate) complexes.3a,5
Amidinates can also adopt a bridged coordination with Al,
leading to multinuclear complexes.4,5g,6 Chelate versus bridged
coordination of amidinates to Al depends on the steric bulk on
N atoms. Bulkier substituents favor chelation, and sterically
smaller substituents favor bridging. In contrast, N-monosub-
stituted amidines have not been thoroughly explored for their
reactivity toward Al reagents. Having two acidic protons and
comparatively less steric bulk, N-monosubstituted amidines are
promising motifs for the synthesis of diverse and complex Al−
N frameworks. Recently, we reported the synthesis of a
tetrameric aluminum amidinate from N-(Dipp)benzamidine
(Dipp = 2,6-diisopropylphenyl) and AlMe3.7 In continuation,
we explored the reactions of a few N-(Dipp)- and N-
phenylamidines with trimethylaluminum and methylaluminum
dichloride. Herein, we report the synthesis and structural
RESULTS AND DISCUSSION
■
Recently, we have found that heating an equimolar mixture of
N-(Dipp)benzamidine and AlMe3 gives a tetrameric aluminum
amidinate, 1, in good yield (Scheme 1). Because this reaction is
Scheme 1. Reaction of AlMe3 with N-(Dipp)amidines
the only example of N-monosubstituted amidines with AlMe3,
it is interesting to examine whether analogous amidines with
Received: August 11, 2011
Published: December 21, 2011
© 2011 American Chemical Society
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similar steric bulk react with AlMe3 in a similar way. N-(Dipp)-
4-tert-butylbenzamidine and N-(Dipp)acetamidine, which differ
from N-(Dipp)benzamidine only at the central carbon atom,
have been employed and treated with AlMe3 at 170 °C for 2 h.
In both cases, facile elimination of methane was observed and
tetrameric aluminum amidinates (2 and 3 in Scheme 1) were
collected in quantitative yields.
The aforesaid amidines have large Dipp groups on one of the
N atoms, and hence, the chelation has been observed. Reducing
the steric bulk on the N atom can increase the bite angle and
might lead to bridged coordination. Therefore, the Dipp group
has been replaced by a phenyl group, and N-phenyl-
benzamidine, N-phenyl-4-tert-butylbenzamidine, and N-phenyl-
acetamidine have been explored for their reactivity with AlMe3.
Heating an equimolar mixture of AlMe3 and N-phenyl-
benzamidine at 170 °C for 2 h resulted in a solid residue,
which, upon recrystallization from a toluene/hexane mixture,
yielded a mixture of isomers of a tetracylic triazaalane, 4 and 5
(Scheme 2), in a 4:1 ratio in good yield. Further
recrystallizations did not result in separation of these isomers.
The 1H NMR spectrum clearly shows the presence of a
mixture. There are two sets of Al−Me peaks (five singlets for 4
and five singlets for 5) and two N−H peaks present in the
spectrum, and the intensity of two sets is in a 4:1 ratio.
Only the crystals of 4 were suitable for single-crystal X-ray
diffraction studies, and it has been structurally characterized. An
ORTEP diagram with selected bond parameters is provided in
Figure 2. The molecular structure of 4 shows a tetracylic
Al4C4N8 framework. Three six-membered Al2CN3 rings, which
are fused together like C6 rings in phenylene, form a highly
puckered tricyclic structure with Al in the center of the three
rings. An amidinate bridge between the central Al atom and
one of the Al atoms in the periphery provides the fourth
Al2CN3 ring. While Al atoms are in a distorted tetrahedral
geometry, all skeletal C and N atoms are three-coordinated in a
planar arrangement. Al−N distances vary from 1.8236(18) Å
(Al4−N3) to 1.9611(19) Å (Al2−N2) and are as with the Al−
N compounds reported in the literature.3−8 One of the Al
atoms (Al4) is connected to four N atoms, and the Al4−N8
bond (1.9097(18) Å), which is perpendicular to the mean plane
of the three Al2CN3 rings, is slightly longer than the rest of the
Al4−N bonds (Al4−N1 1.8383(18), Al4−N3 1.8236(18),
Al4−N5 1.8564(18) Å).
The 1H NMR spectra of 2 and 3, which show two multiplets
and four doublets for isopropyl groups, and a singlet for Al−Me
group, are quite similar to that of 1. In addition to these
resonances, singlets for the 4-tert-butyl group (2) and methyl
group on the amidinate carbon (3) are also present in the
aliphatic region. Single crystals of 3 were grown from hexane
solutions and subjected to X-ray analysis. An ORTEP diagram
of 3 along with selected bond parameters is given in Figure 1.
Figure 1. Molecular structure of 3. All hydrogen atoms are omitted for
clarity. Thermal ellipsoids are drawn at the 30% probability level.
Selected bond lengths [Å]: Al1−N1 1.912(3), Al1−N2 1.917(3),
Al1−N7 1.838(3), Al2−N1 1.828(3), Al2−N3 1.911(3), Al2−N4
1.939(3), Al3−N3 1.825(3), Al3−N5 1.901(3), Al3−N6 1.920(3),
Al4−N5 1.827(3), Al4−N7 1.904(3), Al4−N8 1.918(3), Al1−C5
1.950(4), Al2−C6 1.947(4), Al3−C7 1.950(4), Al4−C8 1.942(4),
C1−N1 1.333(4), C1−N2 1.349(5), C2−N3 1.324(5), C2−N4
1.341(5), C3−N5 1.327(4), C3−N6 1.339(5), C4−N7 1.330(5),
C4−N8 1.344(5). Selected bond angles [deg]: N2−C1−N1 112.1(3),
N2−Al1−N1 71.05(13), Al1−N2−C1 88.1(2), Al1−N1−C1 88.7(2),
N7−Al1−N1 108.62(14), N7−Al1−N2 116.03(14), N7−Al1−C5
117.46(15), N2−Al1−C5 114.01(17), N1−Al1−C5 121.45(19), N4−
C2−N3 113.7(3), N3−Al2−N4 70.86(13), Al2−N3−C2 88.5(2),
Al2−N4−C2 86.8(2), N3−Al2−N1 108.03(15), N4−Al2−N1
116.56(14), C6−Al2−N1 117.84(16), C6−Al2−N3 119.02(16),
C6−Al2−N4 115.73(16).
Under similar reaction conditions, N-phenyl-4-tert-butylben-
zamidine afforded a residue, which, upon recrystallization from
hexane, yielded an isomeric mixture of a tetracyclic amidinate, 6
1
and 7, in a 3:1 ratio as a first crop. The H NMR spectrum is
similar to that of the isomeric mixture of 4 and 5. Two sets of
singlets (four in each set) for tert-butyl groups are also
observed. Formation of 6 has been further confirmed by single-
crystal X-ray studies. The molecular structure of 6 with selected
bond parameters is given in the Supporting Information
(Figure S1). The mother liquor afforded a further crop of
crystals, whose 1H NMR spectrum showed an additional set of
six resonances with low intensity in the Al−Me region (see the
Supporting Information, Figure S4). A thorough single-crystal
X-ray analysis of these crystals revealed the presence of a
pentacyclic triazaalane 10. The crystal structure of 10 along
with selected bond parameters is given in Figure 3. 10
The molecular structure of 3 is essentially the same as that of
1.7 It is a cyclic tetramer of four-membered aluminum
amidinate, and the bond parameters are as with 1. The central
eight-membered Al−N ring adopts a puckered boat con-
formation, and the adjacent Al−Me groups are trans to each
other. The N atoms (N2, N4, N6, and N8), which are not part
of the eight-membered ring, lie in the mean plane of the eight-
membered ring. The average distance of Al atoms from the
mean plane is 0.906 Å, and that of N atoms (N1, N3, N4, and
N7) of the eight-membered ring is 0.279 Å. Because the Al
atoms are in a tetrahedral geometry, the average dihedral angle
between the adjacent four-membered rings is as high as 57.8°.
crystallizes in the P1 space group, and there are four dianionic
̅
amidinate groups and one monoanionic amidinate group
involved in the formation of the molecule. The structure is
made of five Al2CN3 rings fused together and looks like an
extended structure of 6. There are, in total, five Al atoms in the
molecule. Two (Al4 and Al2) are connected to four N atoms,
and three (Al1, Al3, and Al5) are connected to two C and two
N atoms. The Al−N bonds arising from Al4 and Al2 (av. 1.858
Å) are shorter than the rest of the Al−N bonds (av. 1.940 Å) in
the molecule. The sum of the bond angles around each skeletal
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Scheme 2. Reaction of AlMe3 with N-Phenylamidines
4:1 ratio. 8 has been structurally characterized, and the
molecular structure along with selected bond parameters is
given in the Supporting Information (Figure S2). Formation of
a pentacyclic triazaalane, similar to 10, has not been observed.
In these reactions, among all the tetracyclic compounds, only
one type of isomer (4, 6, and 8) has been characterized
structurally. Several attempts have been made to isolate the
crystals of the other isomer (5, 7, and 9), but they were
unsuccessful. However, in the case of N-phenyl-4-tert-
butylbenzamidine, we were able to isolate 6 in pure form by
fractional crystallization of a mixture of 6 and 7 from hexane.
1
The H NMR spectrum of this isomer (6) shows only one set
of peaks (see the Supporting Information, Figure S5).
Interestingly, when a mixture of N-phenylbenzamidine and a
slight excess of AlMe3 (1:1.2) was heated at 170 °C, a
hexacyclic Al−N−C heterocycle, 11, was formed in moderate
yield (Scheme 3). Large blocks of crystals, which were
subjected to X-ray analysis, were grown from toluene. The
molecular structure of 11 along with selected bond parameters
is provided in Figure 4.
The basic structure of 11 contains a highly puckered
benzo[cd]pyrene-type framework made up of five Al2CN3
rings, and a C−N bridge connecting one of the peripheral Al
atoms with the central N atom, which makes the structure
hexacyclic. C−N bond distances throughout the molecule,
except between C12 and N9, are in between single and double
bond distances. It is noteworthy that N9, which is connected to
three Al atoms, forms the longest C−N bond (1.408(2) Å) of
all the amidine/-ate C−N bonds reported in this study. The 1H
NMR spectrum of 11 shows eight singlets of equal intensity
ranging from δ −3.02 to 0.73 ppm, which can be assigned to
eight Al−Me protons.
Figure 2. Molecular structure of 4. All hydrogen atoms, except the one
on N7, are omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level. Selected bond lengths [Å]: Al4−N1 1.8383(18),
Al4−N3 1.8236(18), Al4−N5 1.8564(18), Al4−N8 1.9097(18), Al1−
N1 1.8515(17), Al1−N6 1.9153(18), Al1−N7 1.9029(19), Al1−C5
1.941(2), N7−C4 1.326(3), N8−C4 1.330(3), Al3−C9 1.975(2),
Al2−C7 1.957(2), Al2−N3 1.9143(17), Al2−N2 1.9611(19), N2−C1
1.328(3), N1−C1 1.340(2), N4−C2 1.339(3), N3−C2 1.326(3),
Al3−N5 1.9120(19), Al3−N4 1.972(2). Selected bond angles [deg]:
N1−Al1−N7 100.15(8), N3−Al2−N2 98.93(8), N5−Al3−N4
101.59(8), C2−N3−Al4 123.84(14), N3−C2−N4 122.5(2), C1−
N1−Al4 124.02(15), N2−C1−N1 121.58(19), N1−Al1−C5
122.81(9), N3−Al4−N1 109.97(8), Al1−N7−C4 129.00(16), Al2−
N2−C1 123.53(14), Al4−N3−Al2 108.43(10), Al4−N5−Al3
110.84(9), N7−C4−N8 120.4(2), N5−Al3−C9 106.05(10), N3−
Al2−C7 116.83(10), Al4−N1−Al1 103.81(8), Al4−N8−C4
127.03(15), N1−Al4−N8 106.95(8), N3−Al4−N5 113.45(8).
Further, we investigated the reactions of N-monosubstituted
amidines with AlMeCl2. An equimolar homogeneous mixture of
AlMeCl2 and N-(Dipp)acetamidine was heated to 170 °C, and
the residue was crystallized from toluene at rt to obtain X-ray
quality crystals of an acyclic ionic amidinate, 12 (Scheme 4).
Formation of ionic species, 12, is quite surprising, but it has
been reproduced consistently. An ionic species, which was
formed in low yield when an N,N-diisopropylamidine was
C and N atom, which is equal to ∼360°, suggests that they are
in a trigonal-planar geometry.
Similar to N-phenylbenzamidine, N-phenylacetamidine also
reacts with AlMe3 and gives an isomeric mixture of 8 and 9 in a
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Figure 3. Molecular structure of 10. All hydrogen atoms, except the one on N10, are omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level. Selected bond lengths [Å]: Al1−N1 1.924(3), Al1−N2 1.955(3), Al2−N1 1.863(3), Al2−N3 1.829(3), Al2−N5 1.858(3), Al2−N7
1.885(3), Al3−N3 1.914(3), Al3−N4 1.972(3), Al4−N5 1.833(3), Al4−N6 1.887(3), Al4−N8 1.854(3), Al4−N10 1.859(3), Al5A−N8 1.889(3),
Al5A−N9 1.992(3), Al5B−N8 2.013(3), Al5B−N9 2.030(3), N2−C1 1.354(4), N3−C1 1.328(4), N4−C2 1.335(4), N5−C2 1.325(4), C5−N10
1.332(4), N9−C5 1.314(4), N6−C3 1.381(4), N1−C3 1.303(4), N8−C4 1.317(4), N7−C4 1.369(4), Al1−C6 1.964(4), Al1−C7 1.974(4), Al3−
C8 1.974(4), Al3−C9 1.960(4), Al5A−C10A 1.9448, Al5A−C11A 1.9660, Al5B−C10B 1.9162, Al5B−C11B 1.9593. Selected bond angles [deg]:
N1−Al1−N2 100.64(12), N3−Al3−N4 100.58(12), N8−Al5A−N9 102.82(13), N8−Al5B−N9 97.25(13), N3−Al2−N1 112.00(13), N3−Al2−N5
111.44(13), N1−Al2−N7 110.73(12), N5−Al2−N7 103.46(13), N3−Al2−N7 114.32(12), N5−Al4−N10 120.58(13), N10−Al4−N6 109.66(13),
N10−Al4−N8 107.20(13), N3−C1−N2 121.7(3), C1−N2−Al1 122.6(2), Al2−N3−Al3 111.68(15), C2−N4−Al3 126.5(2), N5−C2−N4
123.7(3), N9−C5−N10 120.6(3), C5−N10−Al4 130.0(2), C5−N9−Al5A 129.7(2), C5−N9−Al5B 114.5(2), N8−C4−N7 123.2(3), Al4−N5−Al2
100.39(13), Al2−N1−Al1 110.44(14), Al4−N8−Al5A 118.20(15), Al4−N8−Al5B 107.34(14).
Scheme 3. Reaction of AlMe3 with N-Phenylbenzamidine in
a 1.2:1 Ratio
presumably due to the binding of chloride ion to only one side
of the chain (asymmetric binding). Addition of 1 equiv of
AgBF4 to the NMR tube makes the spectrum simpler. Although
the peaks related to the C2 symmetric structure are intact, the
other set of peaks reduces to a great extent.
Under similar conditions, N-(Dipp)benzamidine reacted
with AlMeCl2 to give a mixture of a bicyclic triazaalane, 13,
and an N-(Dipp)benzamidine adduct of AlCl3, 14 (Scheme 5).
One of the Al atoms in 13 is bonded to only one Cl, even
though there are two Cl atoms on Al in the starting AlMeCl2.
Formation of 13 requires AlMe2Cl, which is presumably formed
due to the exchange of Me and Cl during the course of the
reaction. To confirm this, a reaction between the N-
(Dipp)benzamidine adduct of AlCl3, 14, and AlMe3 was carried
out. When a mixture of 14 and AlMe3 was heated to 170 °C for
2 h, 13 was formed in moderate yield (Scheme 6). Formation
of 13 was also observed in a reaction between N-(Dipp)-
benzamidine and AlMe2Cl under similar conditions. However,
only the AlCl3 adduct, 15, was isolated from a reaction between
N-phenylbenzamidine and AlMeCl2 (Scheme 7). The com-
pounds 13−15 have been structurally characterized, and
ORTEP diagrams with selected bond parameters are given in
Figures 6, 7, and S3 (Supporting Information), respectively.
treated with EtAlCl2, has been reported in the literature.3c The
single-crystal X-ray structure and selected bond parameters of
12 are given in Figure 5. The solid-state structure shows a 15-
membered Al3C4N8 amidinate chain, consisting of two
amidinate, two amidine, and three AlCl2 moieties. There is
also a chloride ion, which brings the two ends of the chain
closer through hydrogen bonding to amidine hydrogens,
present in the molecule. Because of the presence of six NH
hydrogen bond donors, 12 has the potential to act as an anion
receptor. Several compounds having NH groups have been
reported to show an anion binding property toward various
9
1
Compound 13, which crystallizes in the P1 space group, is a
anions. The H NMR spectrum of 12 in CDCl3 shows two
sets of resonances. One set of resonances is in accordance with
the solid-state structure (C2 symmetric). Another set of peaks
suggests an asymmetric structure, which is formed in solution
̅
bicyclic triazaalane. The Al2N3C rings are highly puckered and
1
together form a boatlike structure. The H NMR spectrum of
13 in CDCl3 is in good agreement with the solid-state structure.
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Figure 5. Molecular structure of 12. All hydrogen atoms, except those
on N atoms, are omitted for clarity. Thermal ellipsoids are drawn at
the 50% probability level. Selected bond lengths [Å]: Al1−N1
1.8603(15), Al1−Cl1 2.1243(8), N1−C1 1.323(2), N2−C1 1.333(2),
Al2−N2 1.8867(16), Al2−Cl3 2.1083(7), Al2−N3 1.8509(17), C2−
N3 1.318(2), C2−N4 1.321(2). Selected bond angles [deg]: N1−
Al1−N1′ 113.20(10), Cl1−Al1−Cl1′ 115.10(6), Al−N1−C1
132.27(13), N1−C1−N2 123.58(16), Al2−N2−C1 127.18(12),
N3−Al2−N2 102.09(7), Al2−N3−C2 134.27(13), N3−C2−N4
119.96(17), Cl3−Al2−Cl2 109.67(3).
Figure 4. Molecular structure of 11. All hydrogen atoms are omitted
for clarity. Thermal ellipsoids are drawn at the 50% probability level.
Selected bond lengths [Å]: Al4−C9 1.966(3), Al4−N6 1.9193(19),
Al6−N6 1.8656(19), Al6−N8 1.8794(18), N8−C4 1.326(3), N7−C4
1.356(3), Al4−N7 1.9706(19), N6−C3 1.318(3), N5−C3 1.362(3),
Al3−N5 1.903(2), Al3−N9 1.9456(18), Al6−N9 1.9962(18), Al3−N4
1.9205(18), N4−C2 1.352(3), N3−C2 1.318(3), Al5−N3 1.8499(18),
Al5−N9 1.9641(18), Al3−C8 1.951(2), Al2A−N3 1.939(2), Al2B−
N3 1.894(9), Al2A−N2 1.987(2), Al2B−N2 1.974(9), N2−C1
1.345(3), N1−C1 1.324(3), Al5−N1 1.8750(18), Al1−N1
1.8689(18), Al1−N8 1.8728(18), N9−C12 1.408(2), N10−C12
1.298(3), Al1−C5 1.949(2), Al1−N10 1.9829(18). Selected bond
angles [deg]: N6−Al6−N8 111.10(8), N8−Al6−N9 102.70(8), N1−
Al1−N8 106.89(8), C9−Al4−C10 119.79(12), Al1−N10−C12
124.81(14), N10−C12−N9 119.70(18), Al4−N7−C4 124.99(14),
N8−C4−N7 121.26(19), N6−Al4−C9 112.08(10), Al4−N6−C3
125.77(16), Al6−N6−Al4 108.28(9), Al6−N8−C4 114.45(14),
Al5−N1−C1 112.45(14), N1−C1−N2 120.60(19), C6A−Al2A−
C7A 115.9(2), C6B−Al2B−C7B 119.4(12), N3−Al2A−N2
100.22(10), N3−Al2B−N2 102.3(4), N3−C2−N4 124.23(19), Al6−
N6−C3 125.17(15), Al5−N9−Al6 108.95(8), N1−Al5−N9
103.02(8), Al1−N1−Al5 115.40(9), Al1−N8−Al6 114.67(9), N1−
Al1−N10 97.22(8), Al5−N3−C2 127.04(15), Al3−N4−C2
131.60(15), Al5−N3−Al2 108.73(10), N6−Al6−N9 102.27(8),
Al3−N9−Al6 102.13(8), N5−Al3−N9 104.11(8), Al3−N5−C3
129.56(15), N6−C3−N5 124.7(2), N4−Al3−N9 101.77(8), Al3−
N9−Al5 106.96(8).
Scheme 5. Reaction of AlMeCl2 with N-(Dipp)benzamidine
Scheme 6. Reaction of AlMe3 with N-
(Dipp)benzamidine·AlCl3 Adduct
Scheme 4. Reaction of AlMeCl2 with N-(Dipp)acetamidine
resonance at 0.20 ppm, which is quite high field for methyl
protons of the isopropyl group.
In compounds 1−15, the amidinate or amidine ligands show
several coordination modes (Figure 8). Mono- and dianionic
N-monosubstituted amidinates show two and three types of
coordination modes, respectively, whereas the neutral amidine
It can be seen in the crystal structure that one of the methyl
groups is oriented toward the center of a phenyl ring and
presumably responsible for the appearance of a doublet
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Scheme 7. Reaction of AlMeCl2 with N-Phenylbenzamidine
Figure 7. Molecular structure of 14. All hydrogens except on N atoms
are omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level. Selected bond lengths [Å]: Al1−N1 1.8735(15),
N1−C1 1.316(2), N2−C1 1.327(2), Al1−Cl1 2.1263(7). Selected
bond angles [deg]: N1−C1−N2 121.57(15), Al1−N1−C1
136.35(12), N1−Al1−Cl1 108.40(5).
Figure 6. Molecular structure of 13. All hydrogens except on N atoms
are omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level. Selected bond lengths [Å]: N2−C1 1.342(3), N3−
C1 1.329(2), Al2−N3 1.8580(17), Al2−N1 1.8668(18), Al1−N1
1.8872(18), Al1−N2 1.8881(18), Al1−Cl1 2.1304(9), Al3−Cl5
2.0993(9), N1−C3 1.304(3), Al3−N3 1.8728(18), Al3−N4
1.9253(18), N4−C2 1.328(3), N5−C2 1.332(3), Al2−N5
1.8642(18), Al2−Cl3 2.1234(8). Selected bond angles [deg]: Al1−
N1−C3 122.70(14), Al1−N2−C1 122.06(14), Al2−N1−Al1
110.50(9), N3−Al2−N1 106.12(8), N3−Al3−N4 103.81(8), N1−
Al1−N2 110.28(8), N3−C1−N2 121.74(18), Al2−N1−C3
125.74(14), Al2−N3−C1 123.95(14), N2−Al1−Cl1 110.03(6),
N3−Al3−Cl5 116.27(6), N3−Al2−N5 105.24(8), Al2−N5−C2
132.83(14), N4−C2−N5 119.13(18), Al3−N4−C2 119.37(14),
Al2−N3−Al3 109.34(8), N1−C3−N6 123.02(19).
Figure 8. Coordination modes of N-monosubstituted amidinates/
amidines with Al observed in this study.
works. In the case of the amidines having a Dipp group on the
N atom, formation of tetrameric four-membered AlN2C rings
has been observed exclusively. When a less-bulky group, phenyl,
is present, fused six-membered Al2N3C rings have been formed.
These six-membered heterocycles, fused into bi-, tetra-, penta-,
and hexacyclic frameworks, are the first examples for polycyclic
carbatriazaalanes. Coordination modes of N-monosubstituted
amidinates, observed in this study, can provide an insight into
the coordination behavior of these amidinates.
shows only one type of coordination mode. Compounds 1−3
contain the amidinate in a μ−η1,η2 bridge mode, and in 11, one
of the benzamidinate units bridges four Al atoms in a μ4−η1,η3
fashion. Three different coordination modes (μ3−η1,η2;
μ−η0,η2; and μ−η1,η1) have been observed in 13, which
contains three amidinate ligands and three Al atoms. As
expected, the amidinate N−C−N bond angle in the μ−η1,η2
bridge mode is lower (e.g., av. N−C−N bond angle in 1 is
112.79°) than that found in other coordination modes, which is
around 120°. In the μ4−η1,η3 bonding mode, which is found
only in 11, the amidinate C−N and C−N′ distances vary to a
greater extent (C12−N9 1.408(2) and C12−N10 1.298(3) Å)
and almost represent single and double bonds, as depicted in
Figure 8. In the rest of the coordination modes, these bonds fall
in between C−N single and double distances.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out
under a N2 atmosphere using a Schlenk line and a glovebox.
Trimethylaluminum, methylaluminum dichloride, dimethylaluminum
chloride, 4-tert-butylbenzonitrile, acetonitrile, and 2,6-diisopropylani-
line were procured from Aldrich and used as received. N-(Dipp)-
benzamidine7 and N-phenylbenzamidine10 were prepared by following
literature procedures. Hexane and toluene (from Na/benzophenone
ketyl) were distilled fresh as and when required. H and 13C spectra
1
were recorded on a Bruker 400 MHz instrument. Elemental analyses
were performed using an Elementar Vario EL III analyzer.
Structural Determination for 3, 4, 6, 8, and 10−15. The single
crystals of 3, 4, 6, 8, and 10−15 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
CONCLUSION
■
Unlike N,N-disubstituted amidines, when treated with AlMe3,
N-monosubstituted amidines give complex Al−N−C frame-
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Table 1. Crystal Data for Compounds 3, 4, and 10−14
1
3·C7H8
4·C7H8
C64H64Al4N8
10·1/2C6H14 & /2C5H10
11·2C7H8
C87H90Al6N10
empirical formula
formula wt
temp (K)
cryst syst
space group
a (Å)
C127H192Al8N16
2158.81
C96.50H122Al5N10
1556.94
1053.15
150(2)
triclinic
1437.57
150(2)
293(2)
293(2)
triclinic
triclinic
monoclinic
C2/c
P1
̅
P1
P1
̅
̅
18.5489(3)
18.6181(3)
21.6897(4)
90.704(2)
105.772(2)
103.897(2)
6973.3(2)
2
14.1228(6)
14.1451(7)
16.4352(8)
94.097(4)
114.481(4)
101.097(4)
2889.9(2)
2
15.0411(4)
15.6755(5)
21.7746(6)
89.763(2)
77.295(2)
87.019(2)
5001.3(2)
2
42.1990(12)
14.0190(3)
27.7014(8)
90
b (Å)
c (Å)
α (deg)
β (deg)
101.977 (3)
90
γ (deg)
V (Å3)
16031.1(7)
8
Z
ρcalcd (mg m−3)
μ (mm−1)
F(000)
1.028
1.210
1.034
1.191
0.107
0.128
0.101
0.131
2340
1112
1672
6080
cryst size (mm)
θ range (deg)
0.55 × 0.45 × 0.25
2.75−25.00
0.40 × 0.26 × 0.12
2.62−29.33
0.14 × 0.12 × 0.12
2.51−25.00
0.18 × 0.09 × 0.06
2.64−29.32
no. of collected/unique
reflns
64251/24508 (R(int) =
0.0531)
24141/13281 (R(int) =
0.0533)
41481/17578 (R(int) =
0.0576)
38473/18601 (R(int) =
0.0317)
no. of data/restraints/params 24508/6/1354
13281/0/691
0.0512, 0.0921
0.1278, 0.1043
0.837
17578/30/957
0.0755, 0.2118
0.1221, 0.2393
1.037
18601/104/907
0.0536, 0.1420
0.0924, 0.1546
1.030
a
R1, wR2 (I > 2σ(I))
0.0743, 0.2259
0.1110, 0.2540
1.042
a
R1, wR2 (all data)
GOF
−3
Δρmax/Δρmin (e Å )
1.725/−0.534
0.305/−0.323
0.843/−0.483
13·2C7H8
0.699/−0.737
14
12·C7H8
empirical formula
formula wt
temp (K)
cryst syst
space group
a (Å)
C63H94Al3Cl7N8
C71H84Al3Cl5N6
C19H24AlCl3N2
413.73
1292.55
1279.63
150(2)
triclinic
150(2)
150(2)
monoclinic
C2/c
triclinic
P1
̅
P1
̅
8.2030(4)
26.8732(7)
15.3876(3)
19.2603(9)
90
14.8034(5)
15.4522(6)
17.9574(6)
105.641(3)
108.308(3)
104.256(3)
3500.6(2)
b (Å)
9.3135(4)
c (Å)
13.9156(6)
85.993(4)
α (deg)
β (deg)
112.737(4)
90
88.458(4)
γ (deg)
V (Å3)
89.708(4)
7345.5(4)
4
1060.14(8)
2
Z
2
ρcalcd (mg m−3)
1.169
1.214
1.296
−1
μ (mm )
0.347
0.289
0.478
F(000)
2744
1352
432
cryst size (mm)
0.40 × 0.32 × 0.24
2.64−28.76
28443/8552 (R(int) = 0.0313)
8552/68/394
0.0461, 0.1164
0.0646, 0.1283
1.042
0.28 × 0.28 × 0.20
2.73−29.35
29327/16036 (R(int) = 0.0371)
16036/0/780
0.0467, 0.1221
0.0818, 0.1312
0.969
0.22 × 0.11 × 0.03
2.72−29.31
8527/4843 (R(int) = 0.0289)
4843/0/230
0.0355, 0.0789
0.0619, 0.0852
0.984
θ range (deg)
no. of collected/unique reflns
no. of data/restraints/params
a
R1, wR2 (I > 2σ(I))
a
R1, wR2 (all data)
GOF
Δρmax/Δρmin (e Å−3)
0.599/−0.417
0.566/−0.813
0.326/−0.245
a
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]0.5.
2
2
with a CCD area detector, using graphite-monochromated Mo Kα (λ
= 0.71073 Å) radiation and a low-temperature device. All calculations
were performed using SHELXS-97 and SHELXL-97.11 The structures
were solved by direct methods and successive interpretation of the
difference Fourier maps, followed by full-matrix least-squares refine-
ment (against F2). All non-hydrogen atoms were refined anisotropi-
cally (except one of the tert-Bu groups in 6 and most of the solvent
molecules). One of the tert-Bu groups in 6, one AlMe2 group and one
tert-Bu group in 10, and four carbon atoms of a phenyl group and one
AlMe2 group in 11 were disordered. These groups were modeled by
using occupancy factors, SADI and EADP instructions. In 6 and 10,
solvent molecules were severely disordered and the SQUEEZE
program was used to eliminate the residues. Details of refinement of
the disordered groups and solvent molecules are given in the
Supporting Information. 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.
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All the data sets were collected to 2Θ values > 50°, but they were
cut off to 2Θ = 50° during the refinement. Relevant data concerning
crystallographic data, data collection, and refinement details for 3, 4,
and 10−14 are summarized in Table 1, and those for 6, 8, and 15 are
given in Table S1 (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 821219−
821225, 831925, 831926, and 836071. 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).
General Procedure for the Reaction of Amidines with AlMe3.
A mixture of amidine (1 equiv) and trimethylaluminum (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 either in hexane or in a toluene−hexane mixture and filtered
using Celite.
N-(Dipp)-4-tert-butylbenzamidine with AlMe3. N-(Dipp)-4-
tert-butylbenzamidine (0.49 g, 1.48 mmol), AlMe3 (0.74 mL, 2 M in
toluene, 1.48 mmol), hexane (20 mL). The filtrate afforded colorless
1
crystals of 2 in quantitative yield. mp: 242−245 °C. H NMR (400
MHz, CDCl3): δ −0.69 (s, 12H, AlMe), 0.66 (d, 12H, CH(CH3)2),
0.93 (d, 12H, CH(CH3)2), 1.01 (d, 12H, CH(CH3)2), 1.16 (s, 36H,
C(CH3)3), 1.19 (d, 12H, CH(CH3)2), 3.21 (m, 4H, CH(CH3)2), 3.56
(m, 4H, CH(CH3)2), 6.95 (m, 8H, ArH), 6.99 (m, 4H, ArH), 7.05 (m,
12H, ArH), 7.12 (m, 4H, ArH). 13C NMR (100 MHz, CDCl3): δ
−10.73, 23.13, 23.58, 25.76, 25.84, 27.86, 28.23, 31.16, 34.77, 123.50,
123.66, 124.87, 125.18, 128.30, 132.60, 139.36, 144.08, 144.61, 153.31,
180.14. Anal. Calcd for C102H132Al4N8: C, 77.63; H, 8.43; N, 7.10.
Found: C, 77.50; H, 8.64; N, 7.17.
Synthesis of N-Phenyl-4-tert-butylbenzamidine. To a mixture
of aniline (2.99 g, 32.21 mmol) and 4-tert-butylbenzonitrile (5.12 g,
32.20 mmol) was added AlCl3 (4.28 g, 32.23 mmol) at 0 °C. The
mixture was stirred at this temperature for 10 min and heated at 180
°C for 30 min. The molten mixture was poured into a thoroughly
mixed solution of 1 mL of HCl and 80 mL of water. To this mixture
were added 60 mL of CH2Cl2 and 20 mL of saturated NaHCO3, and
the organic layer was separated. The aqueous layer was washed three
times with CH2Cl2. All the organic layers were combined and dried
over Na2SO4. After removal of volatiles using a rotary evaporator, a
viscous liquid was obtained. This was dissolved in 60 mL of hexane
and kept in the refrigerator to obtain a colorless crystalline material
(4.93 g, 61%). The product was further purified by vacuum
N-(Dipp)acetamidine with AlMe3. N-(Dipp)acetamidine (0.83 g,
3.81 mmol), AlMe3 (1.90 mL, 2 M in toluene, 3.80 mmol), hexane (30
mL). The filtrate afforded a crystalline material of compound 3 in
quantitative yield. mp: 240 °C (dec). 1H NMR (400 MHz, CDCl3): δ
−0.45 (s, 12H, AlMe), 1.11 (d, 12H, CH(CH3)2), 1.16 (d, 12H,
CH(CH3)2), 1.22 (m, 24H, CH(CH3)2), 1.76 (s, 12H, CH3), 3.19 (m,
4H, CH(CH3)2), 3.31 (m, 4H, CH(CH3)2), 7.13 (m, 2H, ArH), 7.15
(m, 4H, ArH), 7.18 (m, 6H, ArH). 13C NMR (100 MHz, CDCl3): δ
−11.65, 20.29, 23.75, 23.87, 25.07, 25.24, 27.79, 28.23, 123.43, 123.58,
125.74, 139.10, 144.63, 145.20, 182.00. Anal. Calcd for C60H92Al4N8:
C, 69.74; H, 8.97; N, 10.84. Found: C, 69.62; H, 8.93; N, 10.86.
N-Phenylbenzamidine with AlMe3 (1:1 Ratio). N-Phenyl-
benzamidine (0.25 g, 1.27 mmol), AlMe3 (0.64 mL, 2 M in toluene,
1.28 mmol), toluene (10 mL), hexane (4 mL). The filtrate afforded a
mixture of crystals of 4·C7H8 and 5 in a 4:1 ratio (0.24 g, 78%).
Removing the solvents from the mother liquor gave a crystalline
residue (0.06 g), which was found to be a mixture of 4 and 5 with a
slight amount of impurities. 1H NMR (400 MHz, CDCl3): the
intensity of peaks for 4 and 5 are in a 4:1 ratio. δ −2.45 (s, 3H, AlMe
(4)), −2.29 (s, 3H, AlMe (5)), −2.14 (s, 3H, AlMe (4) and 3H, AlMe
(5)), −2.10 (s, 3H, AlMe (4)), −1.96 (s, 3H, AlMe (5)), −0.45 (s,
3H, AlMe (5)), −0.44 (s, 3H, AlMe (4) and 3H, AlMe (5)), −0.31 (s,
3H, AlMe (4)), 2.36 (s, 3H, CH3(tol.)), 5.36 (s, 1H, NH (4)), 5.68 (s,
1H, NH (5)), 6.09 (d, 2H, ArH (4)), 6.67−7.40 (m, 37H, ArH (4),
5H, ArH (tol) and 40H (5)), 7.63 (d, 1H, ArH (4)). 13C NMR (100
MHz, CDCl3): δ −10.74, −9.16, −8.17, −7.80, −6.65, 21.62, 123.57,
124.25, 125.05, 125.45, 126.40, 127.04, 127.15, 127.19, 127.50, 127.56,
127.67, 127.68, 127.83, 127.91, 128.00, 128.19, 128.26, 128.30, 128.38,
128.43, 128.46, 128.68, 128.97, 128.19, 129.35, 129.51, 137.30, 137.75,
138.03, 139.64, 139.81, 140.56, 140.83, 140.90, 141.40, 144.46, 145.18,
146.08, 146.13, 147.25, 147.48, 147.78, 147.92, 175.86, 176.62, 176.74,
177.11, 178.53, 178.74, 179.67, 180.98. A sample for elemental analysis
was prepared by heating a mixture of 4 and 5 at 80 °C for 2 h under
high vacuum. Anal. Calcd for C57H56Al4N8: C, 71.24; H, 5.87; N,
11.66. Found: C, 71.02; H, 5.93; N, 11.70.
N-Phenyl-4-tert-butylbenzamidine with AlMe3. N-Phenyl-4-
tert-butylbenzamidine (0.26 g, 1.03 mmol), AlMe3 (0.50 mL, 2 M in
toluene, 1.00 mmol), hexane (15 mL). The filtrate was allowed to
evaporate slowly under nitrogen to obtain colorless crystals of 6 and 7
(0.25 g, 83%). 1H NMR (400 MHz, CDCl3): the intensity of peaks for
6 and 7 are in a 3:1 ratio. δ −2.49 (s, 3H, AlMe (6)), −2.32 (s, 3H,
AlMe (7)), −2.22 (s, 3H, AlMe (7)), −2.17 (s, 3H, AlMe (6)), −2.16
(s, 3H, AlMe (6)), −2.02 (s, 3H, AlMe (7)), −0.44 (s, 6H, AlMe (6
and 7)), −0.40 (s, 3H, AlMe (7)), −0.33 (s, 3H, AlMe (6)), 1.09 (s,
9H, C(CH3)3 (6)), 1.13 (s, 9H, C(CH3)3 (7)), 1.17 (s, 9H, C(CH3)3
(7)), 1.18 (s, 9H, C(CH3)3 (7)), 1.19 (s, 9H, C(CH3)3 (6)), 1.21 (s,
9H, C(CH3)3 (6)), 1.27 (s, 9H, C(CH3)3 (6)), 1.30 (s, 9H, C(CH3)3
(7)), 5.33 (s, 1H, NH (6)), 5.65 (s, 1H, NH (7)), 6.12 (d, 2H, ArH
(6)), 6.65−7.60 (set of multiplets, 32H, ArH (6) and 36H, ArH (7)).
13C NMR (100 MHz, CDCl3): δ −10.86, −9.24, −8.13, −7.77, −6.50,
11.59, 18.93, 20.60, 20.87, 22.81, 25.44, 27.08, 27.83, 29.22, 30.92,
1
sublimation (0.05 mbar, 120 °C). mp: 150−151 °C. H NMR (400
MHz, CDCl3): δ 1.35 (s, 9H, C(CH3)3), 4.81 (s, 2H, NH), 6.99 (m,
2H, ArH), 7.06 (m, 1H, ArH), 7.35 (m, 2H, ArH), 7.46 (m, 2H, ArH),
7.79 (m, 2H, ArH). 13C NMR (100 MHz, CDCl3): δ 31.35, 34.95,
121.83, 123.05, 125.58, 126.66, 129.61, 133.00, 150.12, 154.09, 181.57.
Synthesis of N-(Dipp)-4-tert-butylbenzamidine. The same
procedure as mentioned for the synthesis of N-phenyl-4-tert-
butylbenzamidine, except for the reaction time of 2 h in this case,
was followed, using 2,6-diisopropylaniline (5.12 g, 28.92 mmol), 4-tert-
butylbenzonitrile (4.60 g, 28.89 mmol), and AlCl3 (3.84 g, 28.87
mmol). The product was further purified by vacuum sublimation (0.05
mbar, 160 °C). Yield: 57% (5.49 g). mp: 193−194 °C. 1H NMR (400
MHz, CDCl3): δ 1.19 (d, 12H, CH(CH3)2), 1.38 (s, 9H, C(CH3)3),
3.06 (m, 2H, CH(CH3)2), 4.60 (s, 2H, NH), 7.09 (m, 1H, ArH), 7.17
(m, 2H, ArH), 7.52 (m, 2H, ArH), 7.89 (m, 2H, ArH). 13C NMR (100
MHz, CDCl3): δ 23.72, 23.85, 28.31, 31.39, 34.98, 123.47, 125.67,
126.63, 132.94, 139.48, 144.36, 153.24, 153.93.
Synthesis of N-Phenylacetamidine. This procedure has been
adopted from the literature.12 To a mixture of aniline (15.36 g, 165.16
mmol) and acetonitrile (6.77 g, 165.16 mmol) was added AlCl3 (21.96
g, 165.16 mmol) at 0 °C over a period of 30 min. The mixture was
stirred at this temperature for 10 min and heated at 100 °C for 30 min.
After the dropwise addition of water to the cooled crude product, the
amidine base was liberated by the addition of NaOH solution (24 g in
160 mL of water) and the mixture was extracted three times with
CH2Cl2. All the organic layers were combined and dried over Na2SO4.
After removal of the volatiles using a rotary evaporator, a dark brown
oil was obtained. Vacuum distillation of this crude product gave a
yellow oil, which was solidified after a day. Yield: 76% (16.72 g). mp:
61−63 °C. 1H NMR (400 MHz, CDCl3): δ 2.05 (s, 3H, CH3), 4.56 (s,
2H, NH), 6.85 (m, 2H, ArH), 6.99 (m, 1H, ArH), 7.28 (m, 2H, ArH).
13C NMR (100 MHz, CDCl3): δ 22.99, 115.15, 121.92, 122.79,
129.35, 149.75.
Synthesis of N-(Dipp)acetamidine. The same procedure as
mentioned for the synthesis of N-phenylacetamidine was followed,
using 2,6-diisopropylaniline (9.31 g, 52.54 mmol), acetonitrile (2.15 g,
52.54 mmol), and AlCl3 (6.98 g, 52.54 mmol). The crude product was
recrystallized from hexane, and the crystalline material was further
purified by vacuum sublimation (0.05 mbar, 100 °C). Yield: 73% (8.35
g). mp: 120−121 °C (reported 118−120 °C).13 1H NMR (400 MHz,
CDCl3): δ 1.16 (m, 12H, CH(CH3)2), 2.12 (s, 3H, CH3), 2.99 (m,
2H, CH(CH3)2), 4.28 (s, 2H, NH), 7.03 (m, 1H, ArH), 7.10 (m, 2H,
ArH). 13C NMR (100 MHz, CDCl3): δ 22.11, 23.86, 28.08, 122.99,
123.39, 139.66, 143.89, 153.46.
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31.09, 31.21, 31.25, 31.31, 34.46, 34.54, 34.64, 34.68, 34.83, 34.85,
34.97, 36.24, 41.51, 123.30, 123.81, 124.01, 124.20, 124.24, 124.72,
125.20, 126.18, 127.08, 127.42, 127.52, 127.60, 127.70, 128.12, 128.38,
128.51, 128.93, 129.15, 129.27, 134.60, 137.01, 138.75, 144.66, 146.43,
147.55, 147.80, 148.09, 150.45, 151.10, 151.13, 152.73, 175.88, 176.36,
176.99, 178.71, 178.92, 179.72, 180.84. A sample for elemental analysis
was prepared by heating a mixture of 6 and 7 at 80 °C for 2 h under
high vacuum. Anal. Calcd for C73H88Al4N8: C, 73.96; H, 7.48; N, 9.45.
Found: C, 72.90; H, 7.51; N, 9.49. The mother liquor gave another
crop of crystals, which were found to be a mixture of 6, 7, and 10. 1H
NMR (400 MHz, CDCl3) data for 10: only Al−Me resonances are
clear. The other ones are largely masked by the resonances of 6 and 7:
−2.48, −2.28, −2.27, −0.70, −0.37, and −0.17 (six singlets of equal
intensity, AlMe).
CH(CH3)2), 5.32 (s, 2H, NH), 7.12−7.40 (set of multiplets, 17H,
ArH), 8.84 (s, 2H, NH), 10.73 (s, 2H, NH). 13C NMR (100 MHz,
CDCl3): 17.86, 21.60, 22.34, 23.51, 23.73, 23.84, 24.45, 25.09, 28.95,
28.98, 124.25, 124.30, 125.44, 128.37, 129.18, 129.54, 130.01, 130.68,
133.47, 138.01, 145.88, 173.48, 174.62. Anal. Calcd for
C63H86Al3Cl7N8: C, 58.91; H, 6.75; N, 8.72. Found: C, 58.78; H,
6.83; N, 8.65.
N-(Dipp)benzamidine with AlMeCl2. N-(Dipp)benzamidine
(0.29 g, 1.04 mmol), AlMeCl2 (1.00 mL, 1 M in hexanes, 1.00
mmol), toluene (15 mL), hexane (2 mL). The filtrate afforded X-ray
quality crystals of 13·2C7H8 (0.12 g, 32%). Characterization data for
1
13·2C7H8: mp: 237 °C (dec). H NMR (400 MHz, CDCl3): δ 0.20
(d, 3H, CH(CH3)2), 0.91 (d, 3H, CH(CH3)2), 1.00 (m, 9H,
CH(CH3)2), 1.20 (d, 3H, CH(CH3)2), 1.25 (d, 3H, CH(CH3)2),
1.31 (d, 3H, CH(CH3)2), 1.46 (d, 3H, CH(CH3)2), 1.50 (d, 9H,
CH(CH3)2), 2.36 (s, 6H, CH3 (tol.)), 2.78 (m, 1H, CH(CH3)2), 3.05
(m, 1H, CH(CH3)2), 3.39 (m, 2H, CH(CH3)2), 3.50 (m, 1H,
CH(CH3)2), 3.63 (s, 1H, NH), 3.80 (m, 1H, CH(CH3)2), 6.77 (m,
1H, ArH), 6.84 (m, 2H, ArH), 6.95 (m, 5H, ArH), 7.11−7.30 (set of
multiplets, 26H, ArH). 13C NMR (100 MHz, CDCl3): δ 21.53, 21.61,
22.53, 22.89, 24.19, 24.45, 24.96, 25.24, 25.62, 26.09, 26.19, 27.27,
27.87, 28.44, 28.80, 29.16, 123.44, 123.86, 123.91, 124.32, 124.36,
124.47, 125.46, 126.62, 127.80, 127.85, 128.11, 128.39, 129.20, 129.71,
130.13, 131.00, 131.14, 131.30, 133.95, 136.21, 136.87, 137.44, 138.03,
138.53, 144.25, 144.61, 145.33, 145.57, 145.92, 146.05, 173.47, 175.14,
185.95. Anal. Calcd for C71H84Al3Cl5N6: C, 66.64; H, 6.62; N, 6.57.
Found: C, 66.54; H, 6.59; N, 6.50. The mother liquor gave a further
crop of colorless crystals, which were found to be of 14 (0.18 g, 43%).
N-Phenylacetamidine with AlMe3. N-Phenylacetamidine (0.50
g, 3.73 mmol), AlMe3 (1.86 mL, 2 M in toluene, 3.72 mmol), and
hexane (15 mL). The filtrate afforded a mixture of 8 and 9 in 4:1 ratio
1
(0.48 g, 73%) as colorless crystals after 5 days. H NMR (400 MHz,
CDCl3): the intensity of peaks for 8 and 9 are in 4:1 ratio. δ −1.42 (s,
3H, AlMe (9)), −1.35 (s, 3H, AlMe (8)), −1.30 (s, 3H, AlMe (8)),
−1.26 (s, 3H, AlMe (9)), −1.22 (s, 3H, AlMe (9)), −1.07 (s, 3H,
AlMe (8)), −0.80 (s, 3H, AlMe (9)), −0.78 (s, 3H, AlMe (8)), −0.68
(s, 3H, AlMe (9)), −0.66 (s, 3H, AlMe (8)), 1.39 (s, 3H, CH3 (8)),
1.83 (s, 3H, CH3 (9)), 1.88 (s, 3H, CH3 (8)), 1.88 (s, 3H, CH3 (9)),
1.89 (s, 3H, CH3 (9)), 1.90 (s, 3H, CH3 (8)), 1.91 (s, 3H, CH3 (8)),
1.96 (s, 3H, CH3 (9)), 5.05 (s, 1H, NH (8)), 5.21 (s, 1H, NH (9)),
6.42 (m, br, 2H, ArH (8)), 7.00 − 7.35 (set of multiplets, 18H, ArH
(8) and 20H, ArH (9)). 13C NMR (100 MHz, CDCl3): δ −9.05,
−8.75, −7.68, −6.95, 21.61, 24.34, 24.55, 25.28, 24.41, 25.44, 25.46,
25.83, 25.94, 26.25, 125.09, 125.40, 125.46, 125.67, 125.77, 126.51,
126.64, 127.71, 127.81, 128.04, 128.33, 128.38, 128.49, 128.66, 128.74,
128.94, 129.19, 129.28, 129.36, 129.46, 138.03, 144.99, 145.73, 147.07,
147.30, 147.35, 147.89, 174.15, 174.75, 175.19, 175.69, 176.31, 179.62,
180.00. Sample for elemental analysis was prepared by heating a
mixture of 8 and 9 at 80 °C for 2 h under high vacuum. Anal. Calcd for
C37H48Al4N8: C, 62.35; H, 6.79; N, 15.72. Found: C, 62.26; H, 6.81;
N, 15.78.
N-Phenylbenzamidine with AlMe3 (1:1.2 Ratio). N-Phenyl-
benzamidine (0.26 g, 1.33 mmol), AlMe3 (0.80 mL, 2 M in toluene,
1.60 mmol), toluene (15 mL). Large blocks of colorless crystals of
11·2C7H8 (0.19 g, 57%) were obtained after 1 day at rt. After drying
under vacuum, 11·2C7H8 lost a molecule of toluene. mp: 306−308 °C.
1H NMR (400 MHz, CDCl3): δ −3.02 (s, 3H, AlMe), −2.07 (s, 3H,
AlMe), −1.77 (s, 3H, AlMe), −1.18 (s, 3H, AlMe), −0.62 (s, 3H,
AlMe), −0.37 (s, 3H, AlMe), 0.11 (s, 3H, AlMe), 0.73 (s, 3H, AlMe),
2.36 (s, 3H, CH3 (tol.)), 4.66 (m, 1H, ArH), 5.24 (m, 1H, ArH), 6.19
(m, 1H, ArH), 6.43 (m, 2H, ArH), 6.53 (m, 1H, ArH), 6.78−7.44 (set
of multiplets, 46H, ArH), 7.53 (m, 1H, ArH), 7.65 (m, 1H, ArH), 7.98
(m, 1H, ArH). 13C NMR (100 MHz, CDCl3): δ −12.91, −9.21, −8.82,
−8.17, −6.51, −5.57, −3.76, 21.61, 124.15, 124.23, 124.45, 124.66,
125.46, 125.79, 125.91, 126.27, 126.57, 127.02, 127.11, 127.19, 127.25,
127.44, 127.52, 127.59, 127.87, 127.92, 128.15, 128.19, 128.32, 128.38,
128.64, 128.66, 128.78, 128.79, 128.85, 128.94, 129.02, 129.19, 129.82,
129.96, 130.87, 131.76, 135.28, 138.03, 138.98, 139.19, 139.70, 141.07,
144.08, 144.94, 145.84, 147.06, 147.92, 176.97, 177.05, 179.77, 180.86,
185.25. Anal. Calcd for C80H82Al6N10: C, 71.41; H, 6.14; N, 10.41.
Found: C, 71.48; H, 6.18; N, 10.35.
1
Characterization data for 14. mp: 178−179 °C. H NMR (400 MHz,
CDCl3): two sets of peaks in a 1:3 ratio were observed. δ 1.01 (d, 6H,
CH(CH3)2 minor), 1.22 (d, 6H, CH(CH3)2 minor), 1.26 (d, 6H,
CH(CH3)2 major), 1.35 (d, 6H, CH(CH3)2 major), 3.02 (m, 2H,
CH(CH3)2 major), 3.10 (m, 2H, CH(CH3)2 minor), 5.80 (s, 1H, NH,
major), 6.05 (s, 1H, NH minor), 7.12 (m, 2H, ArH minor), 7.35 (set of
multiplets, 3H, ArH, NH major and 5H, ArH minor), 7.50 (m, 1H,
ArH minor), 7.56 (m, 1H, ArH major), 7.65 (m, 2H, ArH major), 7.74
(m, 1H, ArH major), 7.91 (m, 2H, ArH major), 8.80 (s, 1H, NH
minor). 13C NMR (100 MHz, CDCl3): δ 21.91, 23.49, 24.41, 25.36,
28.96, 29.20, 124.40, 125.61, 126.15, 128.07, 128.32, 129.19, 129.83,
130.12, 130.66, 131.86, 133.97, 145.32, 146.82, 170.90, 171.08. Anal.
Calcd for C19H24AlCl3N2: C, 55.16; H, 5.85; N, 6.77. Found: C, 55.10;
H, 5.90; N, 6.70.
N-Phenylbenzamidine with AlMeCl2. N-Phenylbenzamidine
(0.32 g, 1.63 mmol), AlMeCl2 (1.60 mL, 1 M in hexanes, 1.60
mmol), toluene (15 mL). The filtrate afforded colorless, X-ray quality
1
crystals of 15 (0.29 g, 55%). mp: 199−200 °C. H NMR (400 MHz,
CDCl3): δ 6.05 (s, 1H, NH), 6.46 (s, 0.7H, NH), 6.91 (m, 2H, ArH),
7.24 (m, 2H, ArH), 7.40 (m, 6H, ArH), 7.54 (m, 2H, ArH), 7.62 (m,
4H, ArH), 7.73 (m, 1H, ArH), 7.91 (m, 2H, ArH), 9.32 (s, 1H, NH).
13C NMR (100 MHz, CDCl3): δ 124.96, 125.54, 127.88, 128.45,
128.63, 129.61, 129.79, 130.24, 131.39, 133.35, 134.19. Anal. Calcd for
C13H12AlCl3N2: C, 47.37; H, 3.67; N, 8.50. Found: C, 47.17; H, 3.77;
N, 8.45.
N-(Dipp)benzamidine·AlCl3 Adduct with AlMe3. AlMe3 (0.24
mL, 2 M in toluene, 0.48 mmol) was added dropwise to a solution of
N-(Dipp)benzamidine·AlCl3 adduct (0.20 g, 0.48 mmol) in toluene (2
mL) at room temperature and stirred for 15 min. All the volatiles were
removed under vacuum, and the residue was heated to 170 °C for 2 h.
The reaction mixture was cooled to room temperature, dissolved in a
mixture of 15 mL of toluene and 3 mL of hexane, and filtered. The
filtrate afforded colorless crystals of 13 (0.08 g, 44%) overnight.
General Procedure for the Reaction of Amidines with
AlMeCl2. To a solution of amidine (1 equiv) in a minimum amount
of toluene, AlMeCl2 (1 equiv) was added dropwise at room
temperature and stirred for 15 min. All the volatiles were removed
under vacuum, and the residue was heated to 170 °C for 2 h. The
reaction mixture was cooled to room temperature, dissolved in toluene
or a toluene−hexane mixture, and filtered using Celite.
N-(Dipp)acetamidine with AlMeCl2. N-(Dipp)acetamidine (0.48
g, 2.20 mmol), AlMeCl2 (2.20 mL, 1 M in hexanes, 2.20 mmol),
toluene (15 mL). The filtrate afforded X-ray quality crystals of
12·C7H8 (0.29 g, 41%). mp: 168−170 °C. 1H NMR (400 MHz,
CDCl3): δ 1.21 (m, 48H, CH(CH3)2), 1.98 (s, 6H, CH3), 2.35 (s, 3H,
toluene CH3), 2.86 (m, 10H, CH3 and CH(CH3)2), 3.01 (m, 4H,
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic information files (CIF) for 3, 4, 6, 8, and 10−
1
15; ORTEP diagrams and crystal data of 6, 8, and 15; and H
NMR spectra of 6, 7, and 10 (Al−Me region). This material is
available free of charge via the Internet at http://pubs.acs.org.
205
dx.doi.org/10.1021/om2007375 | Organometallics 2012, 31, 197−206

Organometallics
Article
Commun. 2008, 6321−6323. (c) Yoon, D.-W.; Gross, D. E.; Lynch, V.
M.; Lee, C.-H.; Bennett, P. C.; Sessler, J. L. Chem. Commun. 2009,
1109−1111. (d) Gale, P. A.; Tong, C. C.; Haynes, C. J. E.; Adeosun,
O.; Gross, D. E.; Karnas, E.; Sedenberg, E. M.; Quesada, R.; Sessler, J.
L. J. Am. Chem. Soc. 2010, 132, 3240−3241. (e) Shionoya, M.; Furuta,
H.; Lynch, V.; Harriman, A.; Sessler, J. L. J. Am. Chem. Soc. 1992, 114,
5714−5722.
(10) Cooper, F. C.; Partridge, M. W. Org. Synth. 1956, 36, 64−65.
(11) (a) SHELXTL v6.12; BrukerAXS Instruments Inc.: Madison,WI,
2000. (b) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
(12) Oxley, P.; Partridge, M. W.; Short, W. F. J. Chem. Soc. 1947,
1110−1115.
(13) Barrett, A. G. M.; Gray, A. A.; Hill, M. S.; Hitchcock, P. B.;
Procopiou, P. A.; White, A. J. P. Inorg. Chem. 2006, 45, 3352−3358.
AUTHOR INFORMATION
Corresponding Author
*E-mail: ndreddy.che@pondiuni.edu.in.
■
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.
DEDICATION
■
Dedicated to Prof. Anil K. Bhatnagar on the occasion of his
70th birthday.
REFERENCES
■
(1) Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1997, 119, 8125−
8126.
(2) Brazeau, A. L.; Barry, S. T. Chem. Mater. 2008, 20, 7287−7291.
(3) (a) Coles, M. P.; Swenson, D. C.; Jordan, R. F.; Young, V. G. Jr.
Organometallics 1997, 16, 5183−5194. (b) Clare, B.; Sarker, N.;
Shoemaker, R.; Hagadorn, J. R. Inorg. Chem. 2004, 43, 1159−1166.
(c) Schmidt, J. A. R.; Arnold, J. Organometallics 2002, 21, 2306−2313.
(d) Jones, C.; Aldridge, S.; Gans-Eichler, T.; Stasch, A. Dalton Trans.
2006, 5357−5361. (e) Mansfield, N. E.; Coles, M. P.; Hitchcock, P. B.
Dalton Trans. 2005, 2833−2841. (f) Aeilts, S. L.; Coles, M. P.;
Swenson, D. C.; Jordan, R. F. Organometallics 1998, 17, 3265−3270.
(g) Brazeau, A. L.; DiLabio, G. A.; Kreisel, K. A.; Monillas, W.; Yap, G.
́
P. A.; Barry, S. T. Dalton Trans. 2007, 3297−3304. (h) Boere, R. T.;
Cole, M. L.; Junk, P. C. New J. Chem. 2005, 29, 128−134. (i) Grundy,
J.; Mansfield, N. E.; Coles, M. P.; Hitchcock, P. B. Inorg. Chem. 2008,
47, 2258−2260. (j) Abeysekera, D.; Robertson, K. N.; Cameron, T. S.;
Clyburne, J. A. C. Organometallics 2001, 20, 5532−5536.
(4) Chang, C.-C.; Hsiung, C.-S.; Su, H.-L.; Srinivas, B.; Chiang, M.
Y.; Lee, G.-H.; Wang, Y. Organometallics 1998, 17, 1595−1601.
(5) (a) Hitchcock, P. B.; Lappert, M. F.; Merle, P. G. Dalton Trans.
2007, 585−594. (b) Brazeau, A. L.; Wang, Z.; Rowley, C. N.; Barry, S.
T. Inorg. Chem. 2006, 45, 2276−2281. (c) Ritter, C.; Chizmeshya, A.
V. G.; Gray, T. L.; Kouvetakis, J. Appl. Organomet. Chem. 2007, 21,
595−600. (d) Brazeau, A. L.; DiLabio, G. A.; Kreisel, K. A.; Monillas,
W.; Yap, G. P. A.; Barry, S. T. Dalton Trans. 2007, 3297−3304.
(e) Davies, R. P.; Linton, D. J.; Schooler, P.; Snaith, R.; Wheatley, A. E.
H. Eur. J. Inorg. Chem. 2001, 619−622. (f) Rowley, C. N.; DiLabio, G.
A.; Barry, S. T. Inorg. Chem. 2005, 44, 1983−1991. (g) Cole, M. L.;
Jones, C.; Junk, P. C.; Kloth, M.; Stasch, A. Chem.Eur. J. 2005, 11,
4482−4491. (h) Duchateau, R.; Meetsma, A.; Teuben, J. H. Chem.
Commun. 1996, 223−224.
(6) (a) Hausen, H. D.; Gerstner, F.; Schwarz, W. J. Organomet. Chem.
1978, 145, 277−284. (b) Barker, J.; Aris, D. R.; Blacker, N. C.;
Errington, W.; Phillips, P. R.; Wallbridge, M. G. H. J. Organomet.
Chem. 1999, 586, 138−144. (c) Dagorne, S.; Guzei, I. A.; Coles, M. P.;
Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 274−289. (d) Wasslen, Y.
A.; Kurek, A.; Johnson, P. A.; Pigeon, T. C.; Monillas, W. H.; Yap, G.
P. A.; Barry, S. T. Dalton Trans. 2010, 39, 9046−9054.
(7) Maheswari, K.; Rajendran, N. M.; Meyer, J.; Reddy, N. D.
Organometallics 2010, 29, 3799−3807.
(8) (a) Reddy, N. D.; Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-
G. Inorg. Chem. 2002, 41, 2374−2378. (b) Reddy, N. D.; Kumar, S. S.;
Roesky, H. W.; Vidovic, D.; Magull, J.; Noltemeyer, M.; Schmidt, H.-
G. Eur. J. Inorg. Chem. 2003, 442−448. (c) Kumar, S. S.; Reddy, N. D.;
Roesky, H. W.; Vidovic, D.; Magull, J.; Winter, R. F. Organometallics
2003, 22, 3348−3350. (d) Uhl, W.; Er, E.; Hepp, A.; Kosters, J.;
̈
Grunenberg, J. Organometallics 2008, 27, 3346−3351.
(9) (a) Li, Z.; Zhang, C.; Ren, Y.; Yin, J.; Liu, S. H. Org. Lett. 2011,
13, 6022. (b) Tong, C. C.; Quesada, R.; Sessler, J. L.; Gale, P. A. Chem.
206
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