Reactions of an aluminum amide with aromatic nitriles: Formation of Al-N-C frameworks

Article abstract of DOI:10.1021/om100406y

Reactions of a four-membered Al-N ring, [(Dipp)HNAlMe₂] ₂ (2; Dipp =2,6-diisopropylphenyl), with the aromatic nitriles PhCN, TbpCN (Tbp =4-tert-butylphenyl), DmpCN (Dmp = 2,6-dimethylphenyl), DepCN (DepCN = 2,6-diethylphenyl), and DippCN are reported. 2 reacts with PhCN to give a 1:2 insertion product, the tetrameric aluminum amidinate 3, whereas with TbpCN it gives the 1:1 insertion products 4-6. Under similar conditions, DmpCN, DepCN, and DippCN do not undergo insertion but form simple adducts of alumazene (1), 7a-c and 8a-c (2:1 and 1:1 nitrile/alumazene), in which the nitriles coordinate to Al. Compounds 3-6 and 7a-c have been structurally characterized. 3 is comprised of four four-membered aluminum amidinate rings. 5 features a core made of three Al₂N₂ and two AlCN₂ four-membered rings fused together. The structures of 4 and 6 consist of a dimer of a six-membered Al₂CN₃ framework. A high-yield synthesis of 3 from N-(Dipp)benzamidine and AlMe₃ has also been reported.

Full text of DOI:10.1021/om100406y

Organometallics 2010, 29, 3799–3807 3799
DOI: 10.1021/om100406y
Reactions of an Aluminum Amide with Aromatic Nitriles: Formation of
Al-N-C Frameworks†
K. Maheswari,‡ N. M. Rajendran,‡ Jens Meyer,§ and N. Dastagiri Reddy*,‡
‡Department of Chemistry, Pondicherry University, Pondicherry 605014, India, and
§Institut fu€r Anorganische Chemie, Karlsruhe Institute of Technology (KIT), 76313 Karlsruhe, Germany
Received May 2, 2010
Reactions of a four-membered Al-N ring, [(Dipp)HNAlMe2]2 (2; Dipp =2,6-diisopropylphenyl),
with the aromatic nitriles PhCN, TbpCN (Tbp =4-tert-butylphenyl), DmpCN (Dmp = 2,6-
dimethylphenyl), DepCN (DepCN = 2,6-diethylphenyl), and DippCN are reported. 2 reacts with
PhCN to give a 1:2 insertion product, the tetrameric aluminum amidinate 3, whereas with TbpCN it
gives the 1:1 insertion products 4-6. Under similar conditions, DmpCN, DepCN, and DippCN do not
undergo insertion but form simple adducts of alumazene (1), 7a-c and 8a-c (2:1 and 1:1 nitrile/
alumazene), in which the nitriles coordinate to Al. Compounds 3-6 and 7a-c have been structurally
characterized. 3 is comprised of four four-membered aluminum amidinate rings. 5 features a core made
of three Al2N2 and two AlCN2 four-membered rings fused together. The structures of 4and 6consist of a
dimer of a six-membered Al2CN3 framework. A high-yield synthesis of 3 from N-(Dipp)benzamidine
and AlMe3 has also been reported.
Introduction
The functional group CtN (as in nitriles and the dicya-
namide anion) has been extensively employed in the synth-
esis of carbon-containing inorganic heterocycles.1 Reactions
of alanes with nitriles and isonitriles have been well
documented,2 and these hydroalumination reactions lead
to the formation of simple Al-N heterocycles, in the case
Figure 1
of bulky alanes,3 or three-dimensional Al-N networks, in
the case of simple alanes.4 Recently, there has been increased
† Dedicated to Prof. Dr. h.c. mult. Herbert W. Roesky on the occasion of his
75th birthday.
interest in the synthesis of Al-N and Al-C heterocycles/
clusters from hydrazines and alkynes.5 The strategy of
*To whom correspondence should be addressed. E-mail: ndreddy.che@
pondiuni.edu.in.
(1) For a review on carbaphosphazenes, see: (a) Elias, A. J.; Jain, M.;
Reddy, N. D. Phosphorus, Sulfur, Silicon Relat. Elem. 1998, 140, 203–
inserting C-N multiple bonds into M-N or M-C (M =
Li, Al)6 bonds has been utilized in the synthesis of the
226. For synthesis of carbathiazenes, see: (b) Voss, G.; Fischer, E.; Rembarz,
G.; Schramm, W. Z. Chem. 1976, 16, 358–359. (c) Geevers, J.; Hackmann, J.
Th.; Trompen, W. P.; Young, V. G. J. Chem. Soc. C 1970, 875–878. For
corresponding metallacycles. While the insertion of nitriles
into Al-N bonds was reported as early as 1969,7 and the
method was used for the synthesis of amidines in 1990,8 so far
there have been no reports on the structural characterization
of aluminum amidinates involved in those reactions.
€
carbaborazines, see: (d) Klofkorn, C.; Schmidt, M.; Spaniol, T.; Wagner, T.;
Costisor, O.; Paetzold, P. Chem. Ber. 1995, 128, 1037–1043.
(2) For an overview, see: Kumar, S. S.; Roesky, H. W. Dalton Trans.
2004, 3927–3937.
Although borazine has been known since 1926, its heavier
congener, alumazene, was not known until 1988.9 The
(3) (a) Uhl, W.; Matar, M. Z. Naturforsch. 2004, 59b, 1214–1222. (b)
Wehmschulte, R. J.; Power, P. P. Inorg. Chem. 1998, 37, 6906–6911.
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Roesky, H. W.; Vidovic, D.; Magull, J.; Noltemeyer, M.; Schmidt, H.-G. Eur.
J. Inorg. Chem. 2003, 442–448. (c) Peng, Y.; Rong, J.; Vidovic, D.; Roesky,
H. W.; Labahn, T.; Magull, J.; Noltemeyer, M.; Schmidt, H.-G. J. Fluorine
Chem. 2004, 125, 951–957. (d) Kumar, S. S.; Reddy, N. D.; Roesky, H. W.;
Vidovic, D.; Magull, J.; Winter, R. F. Organometallics 2003, 22, 3348–3350.
(5) (a) Uhl, W.; Heller, D. Z. Anorg. Allg. Chem. 2010, 636, 581–588.
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2002, 41, 2014–2025. (b) Clare, B.; Sarker, N.; Shoemaker, R.; Hagadorn,
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I.; W€urthwein, E.-U. Chem. Eur. J. 2008, 14, 11557–11564. (f) Uhl, W.; Er,
(8) Garigipati, R. S. Tetrahedron Lett. 1990, 31, 1969–1972.
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r
2010 American Chemical Society
Published on Web 08/12/2010
pubs.acs.org/Organometallics

3800 Organometallics, Vol. 29, No. 17, 2010
Maheswari et al.
difficulty in synthesizing Al-N heterocycles, containing
three-coordinated Al and N atoms, arises from the tendency
of both Al and N to form four-coordinated species. The only
known example of alumazene, (MeAlNDipp)3 (1; Dipp =
2,6-diisopropylphenyl), was prepared by a two-stage
methane elimination reaction between AlMe3 and a bulky
amine, DippNH2. Replacing one or two Al atoms from the
skeletal framework of 1 by C leads to a new class of
heterocycles, carbaalumazenes (Figure 1), structurally si-
milar to carbaphosphazenes10 and carbathiazenes,11 which
have attracted much attention because of their applica-
tions. The boron analogue of such heterocycles, a six-
membered carbaborazine (triazadiboracyclohexane), has
been synthesized from a reaction between 2 equiv of an
Scheme 1. Reaction of Aluminum Amide with Benzonitrile
i
iminoborane and 1 equiv of PrCN.1d In this context, we
explored the reactions of the sterically encumbered alumi-
num amide 2 with some aromatic nitriles and the results are
reported herein.
Scheme 4. Synthesis of 3 from N-(Dipp)benzamidine
Scheme 2. Reactions of Aluminum Amide with 4-tert-Butylbenzonitrile
Scheme 3. Plausible Transformations Involved in the Formation of 3, 5, and 6

Article
Organometallics, Vol. 29, No. 17, 2010 3801
Scheme 5. Reactions of Aluminum Amide with 2,6-Dialkylbenzonitriles
Scheme 6. Reactions of Alumazene(1) with 2,6-Dialkylbenzonitriles
Results and Discussion
Reactions of the aluminum amide 2 with aromatic nitriles
were carried out at 170 °C in 1:1 and 1:2 molar ratios. In all
these reactions, evolution of methane was observed around
100 °C and the resultant residue was recrystallized from
hexanes. A 1:1 or 1:2 molar mixture of 2 and PhCN afforded
the tetrameric aluminum amidinate 3 in low yields (Scheme 1).
3, which is an insertion product of two molecules of PhCN
per molecule of 2, is the only isolated product from both 1:1
and 1:2 reactions.
Figure 2. Single-crystal X-ray structure of 3. Pentane and hydrogen
atoms are not shown for clarity. Thermal ellipsoids are drawn at the
50% probability level. Selected bond lengths (A):Al1-N1=1.914(7),
Al1-N10 = 1.937(7), Al1-C5 = 1.938(9), N1-C1 = 1.333(10),
N10-C1 = 1.330(10), Al1-N4 = 1.827(7), Al4-N4 = 1.916(7),
Al4-N3 = 1.821(7), Al3-N3 = 1.899(7), Al3-N2 = 1.823(7),
Al2-N2=1.897(7),Al2-N1 = 1.826(7). Selected bond angles (deg):
N4-Al1-N1 = 109.1(3), Al1-N1-Al2 = 140.9(4), N4-Al1-
C5 = 122.5(4), N1-C1-N10 = 113.3(7), N1-Al1-N10 =
70.6(3), Al1-N1-C1 = 88.4(5).
A thorough 1H NMR analysis of the mother liquor
showed a mixture of Al compounds (based on Al-Me and
iPr resonances), including 3, and our attempts to isolate any
further pure compounds from this mixture were unsuccess-
ful. Pentane, presumably from hexanes, was present in the
crystal lattice and could not be removed even after drying the
crystals under high vacuum for several hours.
Interestingly, in the case of TbpCN, the three products 4
and 5 (from a 1:1 molar ratio) and 6 (from a 1:2 molar ratio)
were obtained, depending on the stoichiometry of 2 and the
nitrile (Scheme 2). All the compounds are formed due to
insertion of one CN (CN = nitrile functional group) per
molecule of 2. The second molecule of TbpCN, in the case of
a 1:2 (2/nitrile) mixture, does not undergo insertion but
simply coordinates to one of the Al atoms, giving 6. Slow
evaporation of a hexane solution of the residue obtained
from the 1:1 reaction initially afforded colorless crystals of 4
in very low yield. Further evaporation of the mother liquor
yielded a good amount of yellow crystals, which were later
identified as 5. Sublimation of the final residue gave some
amount of 1. Compound 5 is the major product among the
isolated species. In the case of a 1:2 (2/nitrile) reaction,
˚
colorless, rhombic-prismatic crystals of 6 were obtained by
storing a solution of the reaction mixture either in hexanes or
in toluene overnight at room temperature. No further com-
pounds were isolated from the mother liquor, either by
crystallization or by sublimation. Compound 6 is highly
insoluble in common hydrocarbon solvents, and therefore
no NMR could be obtained. 6 can readily be prepared by the
addition of excess of TbpCN to 5 in quantitative yield. When
a solution of 5 in hexanes was treated with TbpCN, colorless
crystals of 6 were formed overnight. The formation of 6 in
this reaction was confirmed by comparing single-crystal
1
X-ray structural data. H NMR spectra of 4 and 5 agree
well with their solid-state structures, but resonances related
to some of the iPr groups in 5 are broad.
As illustrated in Scheme 3, formation of compounds 3, 5,
and 6 can be explained in terms of (1) elimination of methane
and (2) insertion of CN into Al-N bonds, followed by (3)
oligomerization of intermediates and further coordination.
Either simultaneous or stepwise occurrence of the first two
(10) Allcock, H. R.; Manners, I.; Renner, G.; Nuyuken, O. U.S.
Patent US 5093438, 1992.
ꢀ
(11) (a) Oakley, R. T. Prog. Inorg. Chem. 1988, 36, 299–391. (b) Boere,
R. T.; Oakley, R. T.; Reed, R. W.; Westwood, N. P. C. J. Am. Chem. Soc.
1989, 111, 1180–1185. (c) Haynes, P. J.; Oakley, R. T.; Cordes, A. W.;
Pennington, W. T. J. Am. Chem. Soc. 1985, 107, 1346–1351.

3802 Organometallics, Vol. 29, No. 17, 2010
Maheswari et al.
Figure 3. (A) Single-crystal X-ray structure of 5. Hexane and hydrogen atoms are not shown for clarity. Thermal ellipsoids are drawn
˚
at the 50% probability level. Selected bond lengths (A): Al1-N1 = 2.0113(19), Al3-N2 = 2.0133(19), Al1-N2 = 1.918(2), Al3-
N1 = 1.910(2), Al1-N3 = 1.831(2), Al3-N4 = 1.833(2), Al2-N3 = 1.809(2), Al4-N4 = 1.804(2), Al2-N5 = 1.940(2), Al4-N6 =
1.941(2), Al2-N1 = 2.023(2), Al4-N2 = 2.038(2), Al1-C1 = 1.953(3), C3-N5 = 1.323(3), C3-N1 = 1.367(3). Selected bond angles
(deg): Al1-N1-Al3 = 94.43(8), N1-Al1-N2 = 83.61(9), Al1-N1-Al2 = 83.12(8), Al1-N3-Al2 = 94.66(10), N1-C3-N5 =
112.9(2), N1-Al2-N5 = 68.89(8), Al2-N1-Al3 = 125.33(10), N3-Al1-N2 = 113.08(10), N5-Al2-C2 = 112.86(10),
N1-Al2-C2 = 116.76(11), N3-Al2-C2 = 129.83(10). (B) Three fused four-membered Al2N2 rings found (i) in 5 and (ii) in
compounds reported in refs 15a-15d. Angles between the mean planes of the rings are shown.
processes presumably results in intermediates containing
two coordinated N and three coordinated Al atoms; hence,
they have enough room to associate themselves into inter-
esting structures. In the case of TbpCN, three-coordinated
Al atoms, present in the dimerized product, are further
coordinated either by internal N atoms to form 5 or by
additional TbpCN molecules to form 6. The presence of only
one Me group on each Al atom and the absence of N-H
units in these compounds suggest the elimination of
methane, which was confirmed by trapping it from a reaction
between 2 and PhCN.
After 3 was fully characterized, it was felt that it could be
synthesized from an appropriate benzamidine. N-(Dipp)ben-
zamidine was prepared from PhCN and DippNH2 and subse-
quently treated with AlMe3 at 150 °C for 3 h. The resultant
crystalline substance was recrystallized from toluene to give
colorless crystals of 3 in 88% yield (Scheme 4). There are several
examples of a reaction between AlMe3 and N,N0-disubstituted
amidines known in the literature.6b,12 However, to the best of
our knowledge, there have been no reactions of AlMe3 with
N-monosubstituted amidines reported so far.
positions of the benzonitrile in addition to having large Dipp
substitutions on the amido N of 2. Under the reaction
conditions, 2 underwent thermolysis to give 1 and the nitriles
coordinated to Al atoms, forming 1:2 and 1:1 adducts (7 and
8, Scheme 5).
Surprisingly, 1H NMR spectra of the isolated 1:2 adducts
7a-c at room temperature were simple and do not correlate
with the solid-state structures. For example, 7c is expected to
show two resonances for Al-Me protons and two for
methine protons of 1, but the spectrum shows only one
resonance for each type, typical of 1 (D3h). The whole
spectrum looks like that of a simple 1:2 mixture of 1 and
DippCN with a slight upfield shift of iPr CH protons (about
0.3 ppm) of DippCN and a downfield shift of iPr CH protons
(about 0.8 ppm) and Al-Me protons (about 0.2 ppm) of 1.
However, upon cooling, all the peaks related to 1 became
broad and at -30 °C the Al-Me peak splits into two (1:2
ratio) while the iPr CH peak divides into three broad peaks.
Similar behavior was also observed in the cases of 7a,b. From
these observations we infer that these adducts exhibit a
dynamic interaction between 1 and the nitriles in solution
at room temperature. However, other evidence for this
dynamic behavior came from NMR tube experiments car-
ried out between 1 (independently prepared) and DippCN in
1:1, 1:2, and 1:3 molar ratios. All of them showed the same
number of resonances with the same chemical shifts as
observed for 7c, but with appropriate integral ratios.
In all the cases, the mother liquor shows a 1H NMR
spectrum similar to that of 7 but with different integral ratios
Reactions of 2 with the sterically demanding 2,6-dialkyl-
benzonitriles DmpCN, DepCN, and DippCN are straight-
forward and simple. No CN insertion products were isolated
in any of these reactions. This can be attributed to the steric
encumbrance caused by the alkyl groups present in ortho
ꢀ
(12) (a) Boere, R. T.; Cole, M. L.; Junk, P. C. New J. Chem. 2005, 29,
128–134. (b) Schmidt, J. A. R.; Arnold, J. Organometallics 2002, 21, 2306–
2313.

Article
Organometallics, Vol. 29, No. 17, 2010 3803
Figure 5. Single-crystal X-ray structure of 6. Dipp groups are
represented by ipso carbon atoms, and toluene molecules and
hydrogen atoms are not shown for clarity. Thermal ellipsoids are
Figure 4. Single-crystal X-ray structure of 4. Dipp groups on
skeletal N are represented by ipso carbon atoms, and hydrogen
atoms (except on N) are not shown for clarity. Thermal ellipsoids
are drawn at the probability level. Al1-N1 = 1.881(2), Al1-N10 =
1.894(2), N1-C1 = 1.338(3), N2-C1 = 1.348(3), N2-Al2 =
1.957(2), Al2-N3 = 1.998(2), Al1-N3 = 1.954(2), Al2-N4 =
1.814(2), Al2-C3 =1.947(3), Al1-C2 = 1.944(3). Selected bond
angles (deg): N1-Al1-N10 =86.32(9), Al1-N1-Al10 =93.68(9),
N1-Al1-N3 = 94.63(10), N1-C1-N2 = 122.9(2), C1-N2-
Al2 = 125.38(18), N2-Al2-N3 = 95.85(9), Al1-N3-Al2 =
106.24(11), N2-Al2-N4 = 117.06(11), N3-Al2-N4 = 104.42(11),
N4-Al2-C3 = 116.66(13), N2-Al2-C3 = 106.48(12), N3-Al2-
C3=114.72(12).
˚
drawn at the 50% probability level. Selected bond lengths (A): Al1-
0
N1=1.918(3), Al1-N1 =1.895(2), N1-C1=1.306(4), N2-C1=
1.373(3), N2-Al2=1.892(2),Al2-N3=1.808(3),N3-Al1=1.825(2),
Al2-N4=2.009(3), Al2-C3=1.944(3), Al1-C2=1.971(3). Selected
0
bond angles (deg): N1-Al1-N1 = 86.67(11), Al1-N1-Al10 =
93.33(11), N1-Al1-N3 = 105.23(11), N1-C1-N2 = 118.7(3),
C1-N2-Al2 = 118.6(2), N2-Al2-N3 = 114.36(11), N4-Al2-
N2 = 97.53(11), N4-Al2-C3 = 104.73(13), N4-Al2-N3 =
107.77(12), C3-Al2-N3 = 117.79(13), C3-Al2-N2 = 111.99(13),
Al1-N3-Al2=109.44(13).
of 1 to nitrile resonances, which can be interpreted for a
mixture of 7 and 8. Alternatively, 7a-c were synthesized as
crystalline solids in good yields from the reaction between 1
and the appropriate 2,6-dialkylbenzonitriles in toluene
(Scheme 6). In a typical reaction, the mixture was stirred in
toluene for 1 h at room temperature, the solvent was
evaporated, and the residue was dissolved in the minimum
amount of hexanes. Colorless crystals of 7a-c were obtained
in a few minutes. Alumazene (1) is highly reactive compared
to its boron analogue, and quite a few reactions of 1 have
been reported in the literature.13
Compounds 3-6 and 7a-c have been structurally charac-
terized, and their ORTEP diagrams are given in Figures 2-8,
respectively. Bond lengths and bond angles for compounds
3-6 are provided in the captions to Figures 2-5, and those for
compounds 7a-c are given in Table 1. The basic structure of 3
can be constructed by connecting four four-membered alumi-
num amidinate rings through imido N (N1, N2, N3, and N4)
€
(13) (a) Lobl, J.; Timoshkin, A. Y.; Cong, T.; Necas, M.; Roesky,
€
H. W.; Pinkas, J. Inorg. Chem. 2007, 46, 5678–5685. (b) Lobl, J.; Pinkas, J.;
€
Roesky, H. W.; Plass, W.; Gorls, H. Inorg. Chem. 2006, 45, 6571–6573. (c)
€
Lobl, J.; Necas, M.; Pinkas, J. Main Group Chem. 2006, 5, 79–88. (d)
Figure 6. Single-crystal X-ray structure of 7a. Another inde-
pendent molecule from the asymmetric unit and hydrogen
atoms are eliminated for clarity. Dipp groups of alumazene
are represented by ipso carbons. Thermal ellipsoids are drawn at
the 50% probability level.
Wessel, H.; Rennekamp, C.; Waezsada, S.-D.; Roesky, H. W.; Montero,
M. L.; Uson, I. Organometallics 1997, 16, 3243–3245. (e) Hohmeister, H.;
Wessel, H.; Lobinger, P.; Roesky, H. W.; M€uller, P.; Uson, I.; Schmidt, H.-G.;
Noltemeyer, M.; Magull, J. J. Fluorine Chem. 2003, 120, 59–64. (f) Pinkas,
J.; Lobl, J.; Dastych, D.; Necas, M.; Roesky, H. W. Inorg. Chem. 2002, 41,
6914–6918. (g) Pinkas, J.; Wessel, H.; Yang, Y.; Montero, M. L.; Noltemeyer,
M.; Froba, M.; Roesky, H. W. Inorg. Chem. 1998, 37, 2450–2457. (h)
Wessel, H.; Rennekamp, C.; Roesky, H. W.; Montero, M. L.; M€uller, P.; Uson,
ꢀ
ꢀ
€
€
and Al on each ring. The eight-membered ring, comprised of
alternating Al and N atoms, is in a boat conformation with
ꢀ
I. Organometallics 1998, 17, 1919–1921.

3804 Organometallics, Vol. 29, No. 17, 2010
Maheswari et al.
Figure 8. Single-crystal X-ray structure of 7c. Another inde-
pendent molecule from the asymmetric unit and hydrogen
atoms are eliminated for clarity. Dipp groups of alumazene
are represented by ipso carbon atoms. Thermal ellipsoids are
drawn at the 50% probability level.
Figure 7. Single-crystal X-ray structure of 7b. Only one of fthe
our independent molecules from the asymmetric unit is shown.
Dipp groups are represented by ipso carbons. Hydrogen atoms
are eliminated for clarity. Thermal ellipsoids are drawn at the
50% probability level.
˚
The longest one (Al3-N2 = 2.0133(19) A) is between the
four-coordinated atoms.
The basic structures of centrosymmetric molecules 4 and 6
(Figures 4 and 5) are closely similar, and both contain a
dimer of the Al2CN3 ring coordinated externally by two
DippNH units (in 4) or two TbpCN molecules (in 6). Six-
membered rings in 4 are highly puckered compared to those
in 5. While DippNH-coordinated Al atoms are in a distorted-
tetrahedral geometry, TbpCN-coordinated Al atoms are in a
trigonal-pyramidal geometry. Variation in Al-N bond
lengths is found in both molecules. The externally coordi-
nated amido nitrogen forms the shortest Al-N bond
methyl groups on the Al atoms projecting out on either side of
the ring. While all the N atoms are three-coordinated in a
planar geometry, all the Al atoms are four-coordinated in a
distorted-tetrahedral geometry. Despite the fact that the co-
ordination numbers for Al and N are constant throughout the
molecule, there is a variation in Al-N bond length. The
shortest is that which is exocylic to the amidinate ring
˚
(average 1.824 A). The longest is between Al and the Dipp-
˚
substituted N (average 1.931 A). Bond angles and bond lengths
within the amidinate ring are in agreement with those reported
in the literature.6b,c,12c,14 An interesting feature of this molecule
is the fact that the bridging amidinate N atoms are three-
coordinate and no such compounds have been reported so far.
The 1H and 13C NMR spectra of 3 (C6D6 or CDCl3) support
the position that the solid-state structure is retained in solution.
5 crystallizes in the space group P21/n with a molecule of
hexane. It consists of three fused four-membered Al2N2
rings. There is a four-membered amidinate ring fused to
each terminal Al2N2 ring sideways (Figure 3). There are five
structures with three fused Al2N2 rings reported in the
literature.15 All of them contain the rings fused in a chair
conformation (“Z” shaped), but in 5 they adopt a boat
conformation (“U” shaped). The angles between the mean
planes of the rings have been compared with those reported,
and they are found to be in a similar range except for one
structure15e (143°). The central Al2N2 ring is slightly puck-
ered, while the terminal rings are planar but rhomboid. As
with the previously reported structures, different Al-N
˚
(Al2-N4 = 1.814(2) A) in 4, whereas the bond between
nitrilic nitrogen and Al is the longest in 5 (Al2-N4 =
˚
2.009(3) A). There is also a variation in ring C-N bonds.
Though the variation is small compared to that of the Al-N
bond, it is more pronounced in 6 than in 4. In 6, the bond
˚
between C and imido N (C1-N1 1.306(4) A) is shorter than
that between C and the Dipp-substituted N (C1-N2 =
1.373(3) A). The four-membered Al2N2 rings in both struc-
˚
tures are planar and slightly rhomboid. All the ring N and C
atoms in 6 are in a plane with their neighboring atoms. The
sum of bond angles around these atoms is equal to 360°.
The molecular structures of 7a-c feature a puckered
Al3N3 ring in which two of the Al atoms are coordinated
by nitriles. In 7a, both DmpCN molecules occupy axial
positions in a cis fashion. However, in 7b,c, the nitriles are
arranged in a trans geometry with one nitrile in an axial
position and the other in an equatorial position. Similar
adducts of 1 with pyridine have been reported in the
literature.13a In the same article, formation of weak cis and
trans adducts of nitriles with 1 was cited as unpublished
results. In all the cases (7a-c), one of the two CtN-Al bond
angles (equatorial in the case of trans adducts) deviates from
linearity by 25-30°. All bond angles and bond lengths in the
six-membered Al3N3 ring are within the range of those
reported in ref 13a.
˚
bonds were found. The shortest bond (Al4-N4 = 1.804(2) A)
is between three-coordinated N and four-coordinated Al.
(14) (a) Abeysekera, D.; Robertson, K. N.; Cameron, T. S.;
Clyburne, J. A. C. Organometallics 2001, 20, 5532–5536. (b) Dagorne,
S.; Guzei, I. A.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122,
274–289.
(15) (a) Uhl, W.; Molter, J.; Koch, R. Eur. J. Inorg. Chem. 1999,
2021–2027. (b) Robinson, G. H.; Moise, F.; Pennington, W. T.; Sangokoya,
S. A. Polyhedron 1989, 8, 1279–1283. (c) Horchler, S.; Parisini, E.; Roesky,
H. W.; Schmidt, H.-G.; Noltemeyer, M. J. Chem. Soc., Dalton Trans. 1997,
2761–2763. (d) von Hanisch, C.; Stahl, S. Z. Anorg. Allg. Chem. 2008, 634,
701–703. (e) Silverman, J. S.; Abernethy, C. D.; Jones, R. A.; Cowley, A. H.
Chem. Commun. 1999, 1645–1646.
Conclusion
The present study was aimed at synthesizing six-membered
carbaalumazenes by inserting CN into Al-N bonds. When
sterically nondemanding nitriles such as PhCN and TbpCN
€

Article
Organometallics, Vol. 29, No. 17, 2010 3805
Table 1. Selected Bond Lengths and Bond Angles for Compounds 7a-c
7a
7b
7c
7a
7b
7c
˚
Bond Lengths (A)
Al1-N1
Al1-N3
Al1-N4
Al3-N3
Al3-N2
Al2-N2
1.822(5)
1.820(6)
2.058(6)
1.806(6)
1.777(6)
1.820(6)
1.812(5)
1.841(5)
2.032(6)
1.787(5)
1.788(6)
1.832(6)
1.810(4)
Al2-N1
Al2-N5
Al1-C1
Al2-C2
Al3-C3
1.791(6)
2.063(7)
1.967(7)
1.973(6)
1.959(7)
1.808(5)
2.012(6)
1.958(6)
1.972(6)
1.948(6)
1.811(4)
2.003(5)
1.945(5)
1.973(5)
1.949(5)
1.830(4)
2.030(4)
1.771(4)
1.765(4)
1.838(4)
Bond Angles (deg)
N1-Al1-N3
C1-Al1-N4
N1-Al1-N4
N3-Al1-N4
Al3-N3-Al1
N3-Al3-N2
Al3-N2-Al2
113.6(3)
99.8(3)
112.2(2)
99.0(3)
111.33(18)
97.80(19)
101.85(17)
103.28(17)
123.4(2)
Al1-N1-Al2
N2-Al2-N1
N1-Al2-N5
C2-Al2-N5
N2-Al2-N5
Al1-N4-C4
Al2-N5-C5
126.0(3)
109.5(3)
110.1(2)
91.5(3)
101.9(3)
177.0(6)
154.6(6)
119.7(2)
111.4(3)
106.7(2)
94.1(3)
106.4(3)
175.1(6)
155.8(6)
119.9(2)
111.56(18)
106.95(19)
94.0(2)
105.71(18)
169.7(4)
150.8(4)
103.0(3)
101.0(2)
120.6(3)
116.9(3)
126.6(3)
103.3(3)
100.4(2)
122.9(3)
117.0(3)
121.9(3)
118.58(19)
120.4(2)
were employed, CN insertion took place but dimers and
tetramers were formed. These associations were anticipated
on the basis of the fact that simple CN insertions would lead
to the formation of fugitive species with two-coordinated N
and three-coordinated Al. Nonetheless, compounds 3-6
offer unique and interesting structural motifs. On the other
hand, no CN insertion was observed in the case of 2,6-
dialkylbenzonitriles, where the CN group is sterically hin-
dered by ortho substitutions. Formation of 3 in high yield in
a reaction between N-(Dipp)benzamidine and AlMe3 gives
an insight into how an N-monosubstituted amidine reacts
with AlMe3.
temperature. Data collection for 3 and 7c was performed on a
STOE STADI 4 diffractometer and for 4-6 and 7a,b on an
OXFORD XCALIBUR diffractometer, both equipped with a
CCD area detector, using graphite-monochromated Mo KR
˚
(λ = 0.710 73 A) radiation and a low-temperature device. All
calculations were performed using SHELXTL (version 6.12) or
SHELXS-97 and SHELXL-97.18 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 in 3-6 and 7c were refined
anisotropically (except the pentane molecule in 3 and one
toluene molecule in 6). In 7a only Al atoms were given aniso-
tropic treatment. One DepCN moiety of 7b was refined iso-
tropically. 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.
There are four molecules in the asymmetric unit of 7b. One
DepCN unit in one of these molecules is severely disordered and
could not be modeled properly. All the other atoms in the cell
were refined anisotropically. Three sets of data were collected
using crystals from three different batches. All the data sets gave
similar results. The asymmetric unit of structure 7c contains two
molecules, each showing somewhat larger displacement ellip-
soids of some substituents on the aryl rings. Several attempts to
solve the structure of 7c in a centrosymmetric space group failed.
All the data sets were collected to 2θ values >50°. Relevant data
concerning crystallographic data, data collection, and refine-
ment details are summarized in Table 2. Crystallographic data
(excluding structure factors) for the structures reported in this
paper have also been deposited with the Cambridge Crystal-
lographic Data Centre as Supplementary Publication Nos.
CCDC 781145-781151. Copies of the data can be obtained
free of charge on application to the CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K. (fax, (þ44) 1223-336-033; e-mail,
deposit@ccdc.cam.ac.uk).
Experimental Section
General Considerations. All manipulations were carried out
under an N2 atmosphere using a Schlenk line and a glovebox.
Trimethylaluminum was procured from Aldrich Chemical Co.
and used as received. 2,6-Diisopropylaniline (CaH2), PhCN
(from P2O5), and hexanes and toluene (from Na/benzophenone
ketyl) were distilled fresh when required. Benzene-d6 and
toluene-d8 were condensed from Na/benzophenone ketyl and
stored in a glovebox. [Me2AlNH(Dipp)]2,9 [MeAlN(Dipp)]3,9
2,6-dialkylbenzonitriles,16 and TbpCN17 were prepared using
literature procedures. 1H and 13C spectra were recorded on
Bruker 500 or 400 MHz instruments. Elemental analyses
€
were performed by the Analytisches Labor des Institut fur
€
€
Anorganische Chemie der Universitat Gottingen.
Structural Determination for 3-6 and 7a-c. Crystals of 3 and
7a-c, suitable for X-ray diffraction, were obtained by storing
solutions of reaction mixtures in hexane at room temperature
for 2 days, and those of 4 and 5 were grown by slow evaporation
of a hexane solution of the reaction mixture. Rhombic-prismatic
single crystals of 6 were obtained from toluene solution at room
temperature. In order to avoid quality degradation, the single
crystals (except 4) were mounted on a glass fiber in perfluoro-
polyalkyl ether oil/paraffin oil and then brought into the cold
nitrogen stream of a low-temperature device so that the oil
solidified. A paraffin oil coated crystal of 4 was mounted on a
glass fiber using epoxy glue, and the data were collected at room
Reaction of 2 with Nitriles. In a typical reaction, a mixture of 2
(1 equiv) and nitrile (1 or 2 equiv) was taken in a Schlenk flask
and heated to 170 °C for 3 h. After the reaction mixture was
cooled to room temperature, the residue was dissolved in
hexanes and filtered using Celite.
With PhCN. 2 (3.00 g, 6.4 mmol), PhCN (0.66 g, 6.4 mmol),
hexane (20 mL). The filtrate afforded X-ray-quality colorless
(16) 2,6-Dialkylbromobenzene from 2,6-dialkylaniline: (a) Schrock,
R. R.; Wesolek, M.; Liu, A. H.; Wallace, K. C.; Dewan, J. C. Inorg.
Chem. 1988, 27, 2050–2054. 2,6-Dialkylbenzonitrile from 2,6-dialkylbro-
1
crystals of 3 1/2C5H12 (0.50 g, 12%). Mp: >300 °C dec. H
3
NMR (400 MHz, C6D6): δ -0.32 (s, 12H, AlMe), 1.74 (d, 12H,
CH(CH3)2), 0.86 (m, 3H, pentane CH3), 1.13 (d, 12H, CH-
(CH3)2), 1.23 (m, 15H, CH(CH3)2 and pentane CH2), 1.40 (d,
€
€
mobenzene: (b) Knorr, R.; Ruhdorfor, J.; Bohrer, P.; Bronberger, H.; Rapple,
E. Liebigs Ann. Chem. 1994, 433–438.
(17) TbpBr from tert-butylbenzene: (a) Furniss, B. S; Hannaford,
A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook of Practical
Organic Chemistry, 5th ed.; Pearson Education: Singapore, 1989; p 861.
TbpCN from TbpBr: (b) Reference 16b.
(18) (a) SHELXTL v6.12; Bruker AXS Instruments Inc., Madison, WI,,
2000. (b) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–122.

3806 Organometallics, Vol. 29, No. 17, 2010
Maheswari et al.

Article
Organometallics, Vol. 29, No. 17, 2010 3807
12H, CH(CH3)2), 3.38 (m, 4H, CH(CH3)2), 3.99 (m, 4H, CH-
(CH3)2), 6.72 (m, 8H, Ar H), 7.00 (m, 4H, Ar H), 7.12 (m, 12H,
Ar H), 7.42 (m, 8H, Ar H). Anal. Calcd for C82.5H106Al4N8: C,
75.20; H, 8.11; N, 8.50. Found: C, 75.01; H, 7.09; N, 8.17.
With TbpCN (1:1 Ratio). 2 (2.00 g, 4.3 mmol), TbpCN (0.68 g,
4.3 mmol), hexane (20 mL). The filtrate was allowed to evapo-
rate slowly under nitrogen to obtain an initial crop of colorless
crystals of 4 (0.10 g), and further evaporation of the mother
liquor gave yellow crystals of 5 (0.64 g, 25%). Characterization
data for 4 are as follows. Mp: 190 °C dec. 1H NMR (400 MHz,
C6D6): δ -1.11 (s, 6H, AlMe), 0.20 (s, 6H, AlMe), 0.86 (d, 6H,
CH(CH3)2), 0.95 (d, 6H, CH(CH3)2), 1.00 (s, 18H, C(CH3)3),
1.13 (d, 6H, CH(CH3)2), 1.15 (d, 12H, CH(CH3)2), 1.21 (m, 18H,
CH(CH3)2), 1.30 (m, 12H, CH(CH3)2), 1.36 (d, 6H, CH(CH3)2),
1.87 (d, 6H, CH(CH3)2), 2.65 (m, 2H, CH(CH3)2), 2.99 (m, 2H,
CH(CH3)2), 3.26 (m, 2H, CH(CH3)2), 3.43 (m, 2H, CH(CH3)2),
3.53 (m, 2H, CH(CH3)2), 3.86 (m, 4H, NH and CH(CH3)2), 4.35
(m, 2H, NH), 6.62 (m, 4H, Ar H), 6.85 (m, 8H, Ar H), 7.08 (m,
8H, Ar H), 7.26 (m, 4H, Ar H), 7.80 (m, 2H, Ar H). Character-
ization data for 5 are as follows. Mp: 270 °C dec. 1H NMR (400
MHz, C6D6): δ -0.08 (s, 6H, AlMe), 0.39 (m, 12H, AlMe and
CH(CH3)2), 0.80 (br, 6H, CH(CH3)2), 0.89 (m, 12H, CH-
(CH3)2), 0.99 (s, 18H, C(CH3)3), 1.31 (br, 6H, CH(CH3)2),
1.46 (m, 12H, CH(CH3)2), 1.54 (br, 6H, CH(CH3)2), 2.72 (br,
2H, CH(CH3)2), 2.92 (m, 2H, CH(CH3)2), 3.16 (br, 2H, CH-
(CH3)2), 3.96 (m, 2H, CH(CH3)2), 6.80 (m, 4H, Ar H), 6.98 (m,
4H, Ar H), 7.10 (m, 4H, Ar H), 7.26 (d, 4H, Ar H), 7.95 (d, 4H,
Ar H). Anal. Calcd for C74H106Al4N6: C, 74.84; H, 9.00; N, 7.08.
Found: C, 74.28; H, 9.08; N, 7.00.
Synthesis of N-(Dipp)benzamidine. Recently, a synthetic pro-
cedure for this compound was reported.19 However, we adopted
a literature procedure published for the preparation of N-
phenylbenzamidine.20 To a mixture of 2,6-diisopropylaniline
(10.0 g, 56.4 mmol) and benzonitrile (5.8 g, 56.4 mmol) was
added AlCl3 (7.5 g, 56.4 mmol) at 0 °C. The mixture was stirred
at this temperature for 10 min and heated at 200 °C for 4 h. The
molten mixture was poured into a thoroughly mixed solution of
3.2 mL of HCl and 250 mL of H2O. After the addition of 3.2 g of
activated carbon the suspension was stirred for 15 min and
filtered. The filtrate was treated with NaOH solution (35 g in
200 mL of water) to precipitate the crude product, which was
filtered and dried in air for several hours. The compound was
recrystallizedfroma hexane/toluene mixture. Yield: 10.5g (66%).
Mp: 159-160 °C. 1H NMR (400 MHz, CDCl3): δ 1.28 (dd, 12H,
CH(CH3)2), 3.07 (m, 2H, CH(CH3)2), 4.63 (s, 2H, NH), 7.11 (m,
1H, Ar H), 7.19 (m, 2H, Ar H), 7.47 (m, 3H, Ar H), 7.94 (m, 2H,
Ar H).
Synthesis of 3 from N-(Dipp)benzamidine. To N-(Dipp)benzamidine
(0.46 g, 1.6 mmol) placed in a Schlenk flask was added a solution of
AlMe3 (0.82 mL, 1.6 mmol, 2 M in toluene); the mixture was
stirred well for 10 min and the flask placed in an oil bath heated at
150 °C for 3 h. The crystalline material was dissolved in toluene
and the solution filtered. When it was cooled to 0 °C, the filtrate
gave 3 (0.44 g, 88%) as colorless crystals overnight.
Synthesis of 7a-c from Alumazene. A mixture of 1 (1 equiv)
and 2,6-dialkylbenzonitrile (2 equiv) was stirred in toluene at
room temperatuer for 1 h and filtered, vthe olatiles were
removed under vacuum, and the residue was dissolved in the
minimum amount of hexane. The crystalline material, which
crashed out of solution instantaneously, was separated out and
washed further with cold hexane.
With TbpCN (1:2 Ratio). 2 (0.50 g, 1.1 mmol), TbpCN (0.34 g,
2.1 mmol), hexane (15 mL). The filtrate afforded colorless
crystals of 6 (0.15 g, 19%) in a few hours at room temperature.
Mp: >300 °C dec. Anal. Calcd for C96H132Al4N8: C, 76.56; H,
8.83; N, 7.44. Found: C, 76.14; H, 8.85; N, 7.22.
7a: 1 (0.46 g, 0.71 mmol), DmpCN (0.27 g, 0.21 mmol). Yield:
0.39 g (60%).
With DmpCN. 2 (0.51 g, 1.1 mmol), DmpCN (0.14 g, 1.1
mmol), hexane (15 mL). Colorless crystals of 7a (0.29 g, 43%)
were obtained by storing the filtrate for 2 days at room tem-
perature. Mp: >300 °C dec. 1H NMR (400 MHz, C6D6):
δ -1.14 (s, 9H, AlMe), 1.18 (d, 36H, CH(CH3)2), 2.46 (s, 12H,
CH3), 3.99 (m, 6H, CH(CH3)2), 6.87 (t, 3H, Ar H), 7.00 (d, 6H,
Ar H), 7.18 (d, 4H, Ar H), 7.47 (t, 2H, Ar H). Anal. Calcd for
C57H78Al3N5: C, 74.89; H, 8.60; N, 7.66. Found: C, 74.18; H,
8.60; N, 7.37.
With DepCN. 2 (0.30 g, 0.6 mmol), DepCN (0.10 g, 0.6 mmol),
hexane (10 mL). Keeping the filtrate at room temperature for 2
days afforded colorless crystals of 7b (0.15 g, 38%). Mp: >300 °C
dec. 1H NMR (400 MHz, C6D6): δ -0.55 (s, 9H, AlMe), 0.97 (t,
12H, CH2CH3), 1.41 (d, 36H, CH(CH3)2), 2.44 (q, 8H, CH2CH3),
4.41 (m, 6H, CH(CH3)2), 6.54 (d, 4H, Ar H), 6.85 (t, 2H, Ar H),
7.07 (t, 3H, Ar H), 7.21 (d, 6H, Ar H). Anal. Calcd for
C61H86Al3N5: C, 75.51; H, 8.93; N, 7.22. Found: C, 75.11; H,
8.95; N, 7.01.
7b: 1 (0.30 g, 0.46 mmol), DepCN (0.15 g, 0.92 mmol). Yield:
0.32 g (77%).
7c: 1 (0.51 g, 0.8 mmol), DippCN (0.29 g, 1.6 mmol). Yield:
0.42 g (52%).
Acknowledgment. We gratefully acknowledge the
Department of Science and Technology, New Delhi,
India, for financial support and the Alexander von Hum-
boldt Stiftung of Germany for donation of a glovebox.
We thank Prof. Dr. H. W. Roesky for providing elemen-
tal analysis, Profs. Dr. F. Breher and Dr. B. K. Saha for
their help concerning the X-ray structures, the Central
Instrumentation Facility, Pondicherry University for 1H
and 13C NMR spectra, DST-FIST for single crystal X-ray
diffraction facility, and UGCSAP for providing infra-
structure.
With DippCN. 2 (0.51 g, 1.1 mmol), DippCN (0.20 g, 1.1
mmol), hexane (10 mL). Large blocks of colorless crystals of 7c
(0.09 g, 12%) were obtained from the filtrate at room temper-
atue after 1 day. Mp: >300 °C dec. H NMR (400.13 MHz,
C6D6): δ -0.63 (s, 9H, AlMe), 1.05 (d, 24H, CH(CH3)2), 1.37 (d,
36H, CH(CH3)2), 3.16 (m, 4H, CH(CH3)2), 4.40 (m, 6H, CH-
(CH3)2), 6.77 (d, 4H, Ar H), 7.02 (t, 2H, Ar H), 7.07 (t, 3H, Ar
H), 7.22 (d, 6H, Ar H). Anal. Calcd for C65H94Al3N5: C, 76.06;
H, 9.23; N, 6.82. Found: C, 75.12; H, 9.44; N, 6.77.
Supporting Information Available: CIF files giving crystallo-
graphic data for 3-6 and 7a-c and a listing giving 13C NMR
data for all the compounds (except for 6) reported in this work.
This material is available free of charge via the Internet at http://
pubs.acs.org.
1
(19) Wang, J.; Xu, F.; Cai, T.; Shen, Q. Org. Lett. 2008, 10, 445–448.
(20) Cooper, F. C.; Partridge, M. W. Org. Synth. 1956, 36, 64–65.