m-Terphenyl-substituted amidinates: Useful ligands in the preparation of robust aluminum alkyls

Article abstract of DOI:10.1021/om010752h

The syntheses of m-terphenyl-substituted amidines and their corresponding dialkylaluminum amidinate complexes are reported. The amidines are prepared in a one-pot reaction using 2,4,6-triphenylphenyllithium (formed in situ by the reaction of 2,4,6-triphenylphenylbromide with n-butyllithium) with dialkylcarbodiimides (RN=C=NR, R = cyclohexyl or isopropyl). An aqueous workup of the reaction mixtures results in the formation of sterically demanding amidines 2H and 3H. The amidines react readily with trimethylaluminum to form dialkylaluminum amidinate complexes. Two complexes have been prepared and comprehensively characterized in the solid state using single-crystal X-ray crystallography. In the crystalline state, dialkylaluminum amidinates are robust complexes that can be easily handled in air for short periods of time without noticeable decomposition.

Full text of DOI:10.1021/om010752h

5532
Organometallics 2001, 20, 5532-5536
m -Ter p h en yl-Su bstitu ted Am id in a tes: Usefu l Liga n d s in
th e P r ep a r a tion of Robu st Alu m in u m Alk yls
Deepa Abeysekera,† Katherine N. Robertson,§ T. Stanley Cameron,*,§ and
J ason A. C. Clyburne*,†,‡,
Department of Chemistry, Acadia University, Wolfville, Nova Scotia, B0P 1X0, Canada,
Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4J 3, Canada, and
Department of Chemistry, Simon Fraser University, Burnaby,
British Columbia, V5A 1S6, Canada
Received August 15, 2001
The syntheses of m-terphenyl-substituted amidines and their corresponding dialkylalu-
minum amidinate complexes are reported. The amidines are prepared in a one-pot reaction
using 2,4,6-triphenylphenyllithium (formed in situ by the reaction of 2,4,6-triphenylphen-
ylbromide with n-butyllithium) with dialkylcarbodiimides (RNdCdNR, R ) cyclohexyl or
isopropyl). An aqueous workup of the reaction mixtures results in the formation of sterically
demanding amidines 2H and 3H. The amidines react readily with trimethylaluminum to
form dialkylaluminum amidinate complexes. Two complexes have been prepared and
comprehensively characterized in the solid state using single-crystal X-ray crystallography.
In the crystalline state, dialkylaluminum amidinates are robust complexes that can be easily
handled in air for short periods of time without noticeable decomposition.
In tr od u ction
bonding mode over the bridging bonding mode. On the
other hand, nonbulky hydrogen and methyl groups on
the carbon center favor the bridging or monodentate
bonding modes (Figure 2).
The four-electron donor, monoanionic amidinate ligand
1 (Figure 1) is an important substituent in organome-
tallic chemistry. It binds readily to numerous main
group elements, transition metals, lanthanide, and
actinide complexes.1 Amidine metal complexes have
been shown to catalyze a wide variety of reactions
including olefin. For example, aluminum-based amidi-
nate complexes, which are of particular relevance to this
paper, have been used as efficient homogeneous cata-
lysts in olefin polymerization reactions.2-4 In addition,
some interest has been focused on the applicability of
aluminum amidinate complexes to serve as single-
source precursors to materials containing the nitride
ion.5 Amidinate complexes are easily prepared from
their corresponding amidines 1H.6
Bulky substituents, such as adamantyl or 2,6-diiso-
propylphenyl, on the nitrogen center confer steric
hindrance primarily in the amidine plane.10,11 A steri-
cally demanding m-terphenyl group on the central
carbon atom provides steric protection in the plane of
the ligand as well as above and below that plane. This
creates a bowl-shaped environment for the amidinate
ligand (Figure 3) that can stabilize unusual coordination
patterns.10 An example is the yttrium mono-amidinate
complex isolated with the incorporation of an m-ter-
phenyl on the central carbon of N,N′-diisopropylami-
dine. Yttrium bis-amidinate complexes are isolated
when smaller substituents are used.10
In amidinate complexes, the coordination environ-
ment at the metal center can be modified by attaching
different substituents to the ligand.7-9 Sterically de-
manding substituents on the carbon and nitrogen atoms
tend to push the lone pairs of electrons on the nitrogen
atoms toward the metal center to favor the chelating
Here we report a simple preparation of the bulky
amidine ligands, N,N′-diisopropyl-2,4,6-triphenylbenz-
amidine 2H and N,N′-dicyclohexyl-2,4,6-triphenylbenz-
amidine 3H, and the synthesis of their corresponding
dimethylaluminum amidinate complexes 2AlMe2 and
3AlMe2.12
* Corresponding authors.
† Acadia University.
§ Dalhousie University.
(7) Foley, S. R.; Bensimon, C.; Richeson, D. S. J . Am. Chem. Soc.
1997, 119, 10359.
‡ Simon Fraser University.
Current address Simon Fraser University. E-mail: clyburne@sfu.ca.
(1) Barker, J .; Kilner, M. Coord. Chem. Rev. 1994, 133, 219.
(2) Coles, M. P.; J ordan, R. F. J . Am. Chem. Soc. 1997, 119, 8125.
(3) Coles, M. P.; Swenson, D. C.; J ordan, R. F. Organometallics 1997,
16, 5183.
(4) Dagorne, S.; Guzei, I. A.; Coles, M. P.; J ordan, R. F. J . Am. Chem.
Soc. 2000, 122, 274.
(8) Dagorne, S.; J ordan, R. F.; Young, V. G., J r. Organometallics
1999, 18, 4619.
(9) Kincaid, K.; Gerlach, C. P.; Giesbrecht, G. R.; Hagadorn, J . R.;
Whitener, G. D.; Shafir, A.; Arnold, J . Organometallics 1999, 18, 5360.
(10) Schmidt, J . A. R.; Arnold, J . Chem. Commun. 1999, 2149.
(11) Boere´, R. T.; Klassen, V.; Wolmersha¨user, G. J . Chem. Soc.,
Dalton Trans. 1998, 4147.
(5) Barker, J .; Blacker, N. C.; Phillips, P. R.; Alcock, N. W.;
Errington, W.; Wallbridge, M. G. H. J . Chem. Soc., Dalton Trans. 1996,
431.
(6) Dagorne, S.; J ordan, R. F.; Young, V. G., J r. Organometallics
1999, 18, 4619.
(12) For the purposes of this paper, the 2,4,6-triphenylpheny ligand
may be considered to be a substituted m-terphenyl ligand For reviews,
see: Twamley, B.; Haubrich, S. T.; Power, P. P. Adv. Organomet. Chem.
1999, 44, 1. Clyburne, J . A. C.; McMullen, N. Coord. Chem. Rev. 2000,
210, 73.
10.1021/om010752h CCC: $20.00 © 2001 American Chemical Society
Publication on Web 11/29/2001

m-Terphenyl-Substituted Amidinates
Organometallics, Vol. 20, No. 26, 2001 5533
F igu r e 1. Amidinate and amidine ligands.
7.33 (m, 17H), 3.00 (s, 2H), 0.48 (s, 12H), -0.82 (s, 6H). 13C-
{1H}NMR (CD2Cl2): δ 142.0, 140.6, 129.7, 129.4, 128.8, 128.2,
127.6, 25.0, -10.2 (v br). 27Al NMR (C6D6): δ 170 (v br). IR
(Nujol, cm-1): 3080w, 3040w, 3020w, 1490m, 1370m, 1340m,
1270w, 1180w, 1120w, 890w, 770m, 760m, 725m, 700s, 680m,
665m.
[(C6H2P h 3)C(NCy)2Al(CH3)2] (3AlMe2). The synthesis of
this compound is similar to that of 2AlMe2; however, the solid
was recrystallized from toluene. A 1.00 g sample of 3H yielded
0.70 g (1.23 mmol, 64%) of 3AlMe2. Mp: 222-225 °C. Anal.
Calcd for C39H45AlN2: C, 82.36; H, 7.97; N, 4.93; Al, 4.74.
Found: C, 81.89; H, 7.84; N, 4.50. 1H NMR (CD2Cl2): δ 7.76-
7.33 (m, 17H), 1.38 (s, 2H), 0.72 (s, 20H), -0.77 (s, 6H). 13C-
{1H}NMR (CD2Cl2): δ 170.3, 142.7, 142.0, 140.9, 129.6, 129.4,
128.8, 128.8, 128.4, 128.2, 127.5, 125.6, 35.7, 25.9, 25.8, -8.0
(very broad). 27Al NMR (C6D6): δ 184 (v br). IR (Nujol, cm-1):
1630m, 1590w, 1255m, 1075m, 1030m, 890m, 800m, 760m,
750m, 700s, 680m.
F igu r e 2. Steric effects of ligand substituents.
X-r a y Cr ysta llogr a p h ic Str u ctu r a l Deter m in a tion . A
single crystal of X (X ) 2H, 3H, 2AlMe2, or 3AlMe2) was
mounted on a Rigaku AFC5R diffractometer equipped with a
rotating anode generator and utilizing graphite-monochro-
mated Cu KR radiation (2H, 2AlMe2, 3AlMe2) or Mo KR
radiation (3H). Cell constants and an orientation matrix for
data collection were obtained from a least squares refinement
using the setting angles of 25 carefully centered reflections.
Data were collected at room temperature (23 °C) and were
corrected for Lorentz and polarization effects. The structure
was solved by direct methods13 and expanded using Fourier
techniques.14 Full matrix least squares refinement was carried
out using SHELXL97.15 The non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were placed in their geo-
metrically calculated positions and allowed to ride on the heavy
atoms to which they were bonded, with Uiso equal to 1.2Ueq of
the heavy atom (1.5Ueq for methyl hydrogens). Full details of
the structures and the refinements are given in the Supporting
Information.
F igu r e 3. Bowl-shaped environment of the m-terphenyl-
substituted amidinate ligand.
Exp er im en ta l Section
Gen er a l P r oced u r es. An MBraun UL-99-245 drybox and
standard Schlenk techniques on a double manifold vacuum
line were used in the manipulation of air and moisture
sensitive compounds. Anhydrous solvents were used as re-
ceived from Aldrich Chemical Co. NMR spectra were recorded
on a Bruker AC 250 spectrometer in five millimeter quartz
1
tubes at the Atlantic Region Magnetic Resonance Center. H
and 13C{1H} chemical shifts are reported in parts per million
(ppm) downfield from tetramethylsilane (TMS) and are cali-
brated to the residual signal of the solvent. The 27Al NMR were
obtained on a Bruker AMX 400 spectrometer in a 10 mm tube
with [Al(H2O)6]3+ as external reference. Infrared spectra were
obtained using a Perkin-Elmer Model 683 spectrometer with
the % transmittance values reported in cm-1. Melting points
were measured using a Mel-Temp apparatus and are uncor-
rected. Electron impact mass spectral data were obtained at
Simon Fraser University (Burnaby, B.C.) using an HP 5985
GC mass spectrometer. Full experimental details for the
preparation of 2H and 3H can be found in the Supporting
Information.
[(C6H2P h 3)C(Ni-P r )2Al(CH3)2] (2AlMe2). N,N′-Diisopro-
pyltriphenylbenzamidine (1.00 g, 2.31 mmol) was dissolved in
ca. 20 mL of toluene. A 2 mL aliquot (4.0 mmol) of 2 M AlMe3
in toluene was added dropwise and stirred for 12 h. The
solvents were removed in vacuo, and the resulting solid was
dissolved in diethyl ether and filtered to remove a small
amount of insoluble material. Upon standing for 1 day, large
cube-shaped crystals were isolated and characterized as
2AlMe2. Yield: 0.55 g, 1.12 mmol, 50%, mp T >250 °C (dec).
Anal. Calcd for C33H37AlN2: C, 81.11; H, 7.63; N, 5.73; Al, 5.52.
Found: C, 80.32; H, 7.55; N, 5.52. 1H NMR (CD2Cl2): δ 7.81-
Resu lts a n d Discu ssion
Syn th esis of Am id in es. Amidines with bulky sub-
stituents on the central carbon atom are sterically
protected in the amidine plane. The use of the m-
terphenyl group extends this steric protection both
above and below the amidine plane. Most amidine
preparations previously reported produce amidines with
substituents such as H, CH3, or C6H5 on the central
carbon. Recently, a report has described an elegant
synthetic route to the preparation of amidinate com-
plexes with bulky m-terphenyl substituents on the
central carbon of the amidinate ligand.10 This prepara-
tion involves the generation of an m-terphenyl-substi-
tuted amidine, which is lithiated and reacted with metal
halides to obtain the amidinate complexes.10 In the
(13) SIR-92: Altomare, A.; Cascarano, M.; Giacovazzo, C.; Gua-
gliardi, A. J . Appl. Crystallogr. 1994, 26, 343.
(14) DIRDIF-94: Beurskens, P. T.; Admiraal, G.; Beurskens, G.;
Bosman, W. P.; de Gelder, R.; Israel, I.; Smits, J . M. M. The DIRDIF-
94 program system; Technical Report of the Crystallography Labora-
tory, University of Nijmegen: The Netherlands, 1994.
(15) Sheldrick, G. M. SHELXL-97; 1997.

5534 Organometallics, Vol. 20, No. 26, 2001
Sch em e 1. P r ep a r a tion of Bu lk y Am id in es via Lith ia ted Ter p h en yls
Abeysekera et al.
Sch em e 2. P r ep a r a tion of Alu m in u m Am id in a te Com p lexes
current research project, amidines with the triph-
enylphenyl substituent were prepared according to
Scheme 1.
The substituted m-terphenyl, 2,4,6-triphenylbromoben-
zene, is easily metalated by combining it with 1.6 M
n-BuLi in hexane and stirring for 4 h.16 Triphenylphen-
yllithium reacts readily with a carbodiimide to produce
a lithiated amidinate. The amidinate is quenched with
water followed by extraction with CH2Cl2 and recrys-
tallization from toluene to produce the neutral amidine.
The identities of N,N′-diisopropyl-2,4,6-triphenylbenz-
amidine (2H) and N,N′-dicyclohexyl-2,4,6-triphenyl-
benzamidine (3H) have been confirmed by spectroscopic
and X-ray crystallographic analysis.
The N-H, CdN, and C-N stretching frequencies are
in agreement with IR spectral frequencies reported for
other amidines.17 NMR solution studies (1H and 13C) in
CDCl3 confirm the presence of isopropyl and cyclohexyl
groups in two different chemical environments, namely,
as an imine nitrogen substituent and as an amine
nitrogen substituent. However, the signals for these
resonances are broadened due to interconversion be-
tween Z-syn and E-syn isomers that occur on an NMR
time scale. This phenomenon has been observed in
amidines with m-terphenyl substituents on the central
carbon atom,10 as well as in amidines with bulky
substituents such as 2,6-diisopropylphenyl on the ni-
trogen atom.11
X-ray crystallographic studies were performed on the
compounds 2H and 3H to evaluate the steric demands
of the m-terphenyl group with respect to the amidine
fragment. An ORTEP view of 2H is shown in Figure 4
along with selected bond lengths and bond angles. The
molecular structures of the amidine compounds show
that they exist in the E-syn configuration. In general,
F igu r e 4. ORTEP view of the molecular structure of 2H
showing the labeling scheme. Thermal ellipsoids are shown
at 50% probability level; hydrogen atoms, except H(1), have
been removed for clarity. Selected bond lengths [Å]: N(1)-
C(1) 1.368(4), N(1)-C(2) 1.462(4), N(1)-H(1) 0.8900, N(2)-
C(1) 1.282(4), N(2)-C(5) 1.460(4), C(1)-C(8) 1.513(4).
Selected bond angles [deg]: C(1)-N(1)-C(2) 122.5(3), C(1)-
N(1)-H(1)121.8, C(2)-N(1)-H(1) 112.2, C(1)-N(2)-C(5)
120.7(3), N(2)-C(1)-N(1) 120.2(3), N(2)-C(1)-C(8) 126.4-
(3), N(1)-C(1)-C(8) 113.4(3); selected torsion angle [deg]
N(1)-C(1)-C(8)-C(9) 70.75(0.39).
the E conformation is energetically more favored than
its Z counterpart.17
Syn th eses of Dia lk yla lu m in u m Am id in a te Com -
p lexes. To prepare robust aluminum amidinate com-
plexes, the amidines 2H and 3H were dissolved in
toluene and reacted with an excess of Al(CH3)3 in
hexane and stirred overnight (Scheme 2). After solvent
removal in vacuo the solid materials were recrystallized
from Et2O and toluene, respectively. The resulting well-
defined block crystals were identified as 2AlMe2 and
3AlMe2 by spectroscopic and X-ray crystallographic
analyses.
(16) Olmstead, M. M.; Power, P. P. J . Organomet. Chem. 1991, 119,
1460.
(17) Patai, S.; Rappoport, Z. The Chemistry of Amidines and
Imidates; J ohn Wiley & Sons: New York, 1991; Vol. 2, pp 1-100.

m-Terphenyl-Substituted Amidinates
Organometallics, Vol. 20, No. 26, 2001 5535
Ta ble 1. Selected Bon d Len gth s a n d Bon d An gles of Am id in es a n d Cor r esp on d in g Am id in a tes
2H
3H
2AlMe2
3AlMe2
t-Bu-C{(N-Cy)2AlMe2
Selected Bond Lengths (Å)
N(1)-C(1)
N(2)-C(1)
C(1)-R1
N(1)-R
1.368(4)
1.282(4)
1.513(4)
1.462(4)
1.460(4)
1.379(4)
1.325(3)
1.325(3)
1.506(5)
1.465(4)
1.465(4)
1.947(3)
1.917(3)
1.947(3)
1.917(3)
1.325(7)
1.328(7)
1.508(7)
1.447(7)
1.458(7)
1.949(7)
1.936(5)
1.944(7)
1.942(5)
1.343(2)
1.339(2)
1.540(2)
1.456(2)
1.454(2)
-
1.283(3)
1.498(4)
1.450(4)
1.462(4)
N(2)-R
Al-C(3)
Al-N(1)
Al-C(2)
Al-N(2)
-
1.958(2)
1.9124(14)
Selected Bond Angles (deg)
N(1)-C(1)-N(2)
R1-C(1)-N(1)
R1-C(1)-N(2)
C(1)-N(1)-R
C(1)-N(1)-H or Al
R-N(1)-H or Al
C(1)-N(2)-R
C(1)-N(2)-Al
R-N(2)-Al
N(1)-Al-N(2)
N(1)-Al-C(3)
N(2)-Al-C(2)
C(3)-Al-C(2)
120.2(3)
113.4(3)
126.4(3)
122.5(3)
121.8
119.8(3)
114.0(2)
126.2(3)
122.9(2)
118.6
109.3(4)
125.4(2)
125.4(2)
127.8(3)
91.1(2)
141.2(2)
127.8(3)
91.1(2)
141.2(2)
68.6(2)
115.32(13)
115.32(13)
117.4(2)
110.8(6)
124.2(6)
125.0(6)
126.6(6)
90.5(4)
142.8(4)
126.0(5)
90.1(4)
143.8(4)
68.5(2)
114.8(3)
115.6(3)
119.2(3)
107.8
131.4
91.3
136.5
132.5
92.1
112.2
120.7(3)
118.6
120.5(2)
133.4
68.7
116.2
Ch a r t 1
groups become equivalent, and the Al-CH3 resonance
occurs at ca. 0.1 ppm. These observations are consistent
1
with H NMR studies of similar compounds with sym-
metric dialkylaluminum fragments.3 The 13C NMR
signal for Al-C is broad due to the 27Al quadrupole.
Similar to previously characterized aluminum amidi-
nate derivatives, the 27Al NMR signal that occurs
between 184 and 170 ppm is also broad.3
Solid -Sta te Str u ctu r es of 2AlMe2 a n d 3AlMe2.
The molecular structure and atom-labeling scheme of
the aluminum amidinate complexe 2AlMe2 is presented
in Figure 5. The structures of the complexes consist of
discrete molecules that do not display unusual inter-
molecular interactions. Table 1 summarizes some se-
lected bond lengths and bond angles of the amidines 2H
and 3H and their corresponding aluminum amidinate
complexes, 2AlMe2 and 3AlMe2. Also included are the
data for an aluminum amidinate complex, t-Bu-C{(N-
Cy)2AlMe2.3 For clarity, the numbering of the data in
the table refers to the amidine and amidinate structures
given in Chart 1.
F igu r e 5. ORTEP view of the dialkylaluminum amidinate
complex 2AlMe2 showing the labeling scheme. Thermal
ellipsoids are shown at 50% probability level; hydrogen
atoms have been removed for clarity; the symmetry (1-x,
y, 0.5-z) is required to generate the second half of the
molecule. Selected bond lengths [Å]: Al(1)-N(1) 1.917(3),
Al(1)-C(19) 1.947(3), Al(1)-C(1) 2.351(4), N(1)-C(1) 1.325-
(3), N(1)-C(2) 1.465(4), C(1)-C(5) 1.506(5). Selected bond
angles [deg]: N(1)-Al(1)-N(1A) 68.6(2), C(19)-Al(1)-
C(19A) 117.4(2), C(1)-N(1)-C(2) 127.8(3), C(1)-N(1)-Al-
(1) 91.1(2), C(2)-N(1)-Al(1) 141.2(2) N(1)-C(1)-C(5) 125.4-
(2). Selected torsion angle [deg]: N(1)-C(1)-C(5)-C(6)
-64.24(0.20).
The formation of a four-membered Al-N-C-N ring
system delocalizes the N-CdN bonding in the amidi-
nates. In the amidines the bonds from the nitrogen
atoms to the central carbon, N(1)-C(1) [C-N] and
N(2)-C(1) [CdN], are localized. As revealed both by
In the IR spectra of the reaction products, 2AlMe2
and 3AlMe2, the N-H stretching frequency of the
amidine (3420-3400 cm-1) is absent. This is consistent
with reaction Scheme 2. Likewise, NMR studies show
the disappearance of the signal attributed to the N-H
resonance. In addition, the isopropyl or cyclohexyl
1
crystallographic studies and H NMR studies in solu-
tion, these bonds are equivalent in the amidinate
complexes. For example, in 2AlMe2, which is cen-
trosymmetric, the N(1)-C(1) distance of 1.325(3) Å is

5536 Organometallics, Vol. 20, No. 26, 2001
Abeysekera et al.
between the single and double bond lengths, 1.368(4)
and 1.282(4) Å, respectively, of the precursor amidine
2H. This is consistent with a bond order of 1.5 in the
delocalized N-C-N bonding systems. Furthermore, the
bond lengths from the aluminum center to the two
nitrogen atoms are crystallographically equal in both
complexes. This confirms the symmetrical bonding
pattern of the amidinate complexes 2AlMe2 an d 3AlMe2.
There is very little change in the bond distances, C-R1,
N(1)-R, and N(2)-R, of the amidinates compared to
those of the corresponding amidines.
at the Al center has been affected by the steric interac-
tions of the substituents on the amidinate ligand.
Hydrolysis of the aluminum amidinate complexes,
after exposure to air, results in the regeneration of the
amidine compounds. Nevertheless, a timed assay of the
IR spectra shows that the N-H stretch at ca. 3420 cm-1
reappears only after the amidinate complexes have been
exposed to air for at least 15 min. This is in sharp
contrast to other aluminum alkyl compounds, where the
high sensitivity of Al-C bonds to air and moisture often
results in their being spontaneously flammable in
air.19,20 In fact, a strong OH band at 3500 cm-1 is
observed in the IR spectrum of Al(CH)3 in hexane after
it has been exposed to air for only 1 min. The stability
of the aluminum amidinate complexes studied here can
be attributed to the steric protection imparted by the
m-terphenyl group attached to the central carbon atom
of the amidinate ligand.
In the amidine compounds, the sum of the three bond
angles around C(1) is 360°. This confirms a trigonal
planar geometry, consistent with the anticipated sp2
geometry. However, the ∼120° N(1)-C(1)-N(2) angle
in the amidines is reduced to a tight ∼109° in the
amidinates due to the presence of the four-membered
ring. This is compensated for by an increase in the
R1-C(1)-N(1) angle. As expected, the C(1)-N(1)-R,
R-N(1)-Al, C(1)-N(2)-R, and R-N(2)-Al angles are
increased in comparison to the C(1)-N(1)-R and
R-N(1)-H angles of the amidines. Compared with the
C(1)-N(1)-H angle of ca. 120° in the amidines, the
C(1)-N(1)-Al and C(1)-N(2)-Al angles are ∼90°.
These changes can be explained by the steric interac-
tions between the m-terphenyl groups on C(1) and the
R groups on the nitrogen atoms, which force the lone
pair on the nitrogen atoms toward the center of the
amidinate ligand. In addition, the m-terphenyl group
on the central carbon atom provides steric protection
above and below the plane of the amidinate ligand.
The Al-C(methyl) bond lengths observed in 2AlMe2
and 3AlMe2 closely resemble the terminal Al-C(meth-
yl) bond distance observed in (Me3Al)2.18 However, the
bond angles around the Al centers are not characteristic
of a regular tetrahedron. The acute N(1)-Al-N(2) angle
(∼69°) indicates considerable strain around the Al
center in both 2AlMe2 and 3AlMe2. The changes in the
bond angles confirm that the coordination environment
Ack n ow led gm en t. The authors gratefully acknowl-
edge the financial support of Dalhousie University,
Acadia University, Simon Fraser University, and the
Natural Sciences and Engineering Research Council of
Canada. The authors also acknowledge the Atlantic
Region Magnetic Resonance Centre at Dalhousie Uni-
versity for NMR data, and especially Dr. Don Hooper
1
and Dr. Mike Lumsden for their help in obtaining H
and13C NMR spectra. Thanks are also expressed to Dr.
Mel Schriver (Atlantic Baptist University) for the
generous loan of glassware.
Su p p or tin g In for m a tion Ava ila ble: Tables of X-ray
structural data for compounds 2H, 3H, 2AlMe2, a n d 3AlMe2
are available free of charge via the Internet at http://pubs.
acs.org.
OM010752H
(19) Omae, I. Applications of Organometallic Compounds; J ohn
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(20) Air stable dialkylaluminum species with diketimate ligands
have been reported: Qian, B: Ward, D. L.; Smith, M. R., III.
Organometallics 1998, 17, 3070.
(18) Vranka, R. G.; Amma, E. L J . Am. Chem. Soc. 1967, 89, 3121.