Molecularly simple dimethylaminomethyl compounds of aluminum, gallium, and indium

Article abstract of DOI:10.1021/om020687r

The dimethylaminomethyl compounds (Me₂AlCH₂NMe₂)₂ (1), (Me₂GaCH₂NMe₂)₂ (2), (Me₂InCH₂NMe₂)₂ (3), (Me₂AlCH₂NMe₂)·(MeClAlCH₂NMe ₂) (4), [ClAl(CH₂NMe₂)₂]₂ (5), and [Al(CH₂NMe₂)₃]₂ (6) have been prepared by reacting LiCH₂NMe₂ with the corresponding element or organoelement chlorides. Characterization was undertaken by means of elemental analyses, NMR spectroscopy (¹H, ¹³C, ²⁷Al), and mass spectrometry. An unusually low ²⁷Al NMR chemical shift of 6 (62 ppm) was observed indicating a possible coordination number higher than four at aluminum in solution. The crystal structures of all compounds but 3 have been determined and confirm that all form dimers with six-membered M₂C₂N₂ rings in the chair conformation. The mass spectra indicate that the dimers are retained in the gas phase, and for 1 the molecular dynamics of ring inversion was monitored in solution by variable-temperature NMR spectroscopy, showing that the barrier for the inversion process (9 kcal mol^-1) is comparable to that of cyclohexane. Calculations at the B3LYP/6-311G(d) level of theory confirm that the molecular structures of 1 and 2 are almost undistorted in the solid state. DFT calculations (B3LYP/6-311G(d)) for the monomeric three-membered ring isomers of 1 and 2 show these to be much higher in energy (81 and 77 kJ mol^-1 per monomer) than the corresponding six-membered ring dimers.

Full text of DOI:10.1021/om020687r

242
Organometallics 2003, 22, 242-249
Molecu la r ly Sim p le Dim eth yla m in om eth yl Com p ou n d s of
Alu m in u m , Ga lliu m , a n d In d iu m
Christian Lustig and Norbert W. Mitzel*
Institut fu¨r Anorganische und Analytische Chemie, Westfa¨lische Wilhelms-Universita¨t
Mu¨nster, Wilhelm-Klemm-Strasse 8, 48149 Mu¨nster, Germany
Received August 20, 2002
The dimethylaminomethyl compounds (Me2AlCH2NMe2)2 (1), (Me2GaCH2NMe2)2 (2), (Me2-
InCH2NMe2)2 (3), (Me2AlCH2NMe2)‚(MeClAlCH2NMe2) (4), [ClAl(CH2NMe2)2]2 (5), and [Al-
(CH2NMe2)3]2 (6) have been prepared by reacting LiCH2NMe2 with the corresponding element
or organoelement chlorides. Characterization was undertaken by means of elemental
analyses, NMR spectroscopy (1H, 13C, 27Al), and mass spectrometry. An unusually low 27Al
NMR chemical shift of 6 (62 ppm) was observed indicating a possible coordination number
higher than four at aluminum in solution. The crystal structures of all compounds but 3
have been determined and confirm that all form dimers with six-membered M2C2N2 rings
in the chair conformation. The mass spectra indicate that the dimers are retained in the
gas phase, and for 1 the molecular dynamics of ring inversion was monitored in solution by
variable-temperature NMR spectroscopy, showing that the barrier for the inversion process
(9 kcal mol-1) is comparable to that of cyclohexane. Calculations at the B3LYP/6-311G(d)
level of theory confirm that the molecular structures of 1 and 2 are almost undistorted in
the solid state. DFT calculations (B3LYP/6-311G(d)) for the monomeric three-membered ring
isomers of 1 and 2 show these to be much higher in energy (81 and 77 kJ mol-1 per monomer)
than the corresponding six-membered ring dimers.
The presence of geminal donor and acceptor sites in
ferth reagent Ph-Hg-CCl2Br.9 Other compounds with
the geminal donor acceptor motif do not extrude the
linking unit and behave in a more classical sense, for
example the reagents for nucleophilic aminomethyla-
tion10 and phosphinomethylation.11
In recent years investigations with focus on the
structural chemistry of such compounds revealed that
there are a number of ways for the acceptor atoms in
such compounds to saturate their demand for electron
density by either intramolecular coordination leading
to three-membered rings or intermolecularly by forma-
tion of higher aggregates, the simplest being six-
membered ring dimers.
Many but not all of the three-membered ring systems
undergo such extrusion reactions, but so far the body
of structural information is still not large enough for a
sound structure-based reactivity prediction, in particu-
lar in the light of so many possible different combina-
tions of donors, acceptors, and linking units. Neverthe-
less, it would have far-reaching consequences if one
could find a unified plan for numerous reactions of
geminal systems based on their molecular structures
and dynamics.12
the same molecule is often the source of characteristic
types of reactivity. Important examples based on this
principle include the carbenoid reactivity of Ko¨brich’s
R-halogen-methyllithium reagents,1 the carbenoids of
the type Li-C-O,2 and corresponding nitrenoids3 and
oxenoids4 containing Li-N-O and Li-O-O units.
Extrusion of the bridging unit under final formation of
a chemical bond between the geminal donor and accep-
tor centers is also reported for silicon-containing species
such as the nitrene generators R3Si-N(R)-OSiMe35 and
the carbene-generating R-fluoromethylsilanes R3Si-
CH2-F.6 Recently Steinborn and co-workers found the
compound LiCH2SPh to show carbene transfer capabil-
ity.7 Examples involving transition metal acceptor
atoms also include the well-known Simmon-Smith
reagent8 I-CH2-Zn-I or the carbene-generating Sey-
* Corresponding author. E-mail: Mitzel@uni-muenster.de.
(1) (a) Kirmse, W. Angew. Chem. 1965, 77, 1; Angew. Chem., Int.
Ed. Engl. 1965, 4, 1. (b) Ko¨brich, G. Angew. Chem. 1972, 84, 557;
Angew. Chem., Int. Ed. Engl. 1972, 11, 473. (c) Siegel, H. Top. Curr.
Chem. 1982, 106, 55.
(2) Boche, G.; Bosold, F.; Lohrenz, J . C. W.; Opel, A.; Zulauf, P.
Chem. Ber. 1993, 126, 1873.
(3) (a) Boche, G.; Boie, C.; Bosold, F.; Harms, K.; Marsch, M. Angew.
Chem. 1994, 106, 90; Angew. Chem., Int. Ed. Engl. 1994, 33, 115. (b)
Schverdina, N. I.; Kotscheschkov, Z. J . Gen. Chem. USSR 1938, 8,
1835. (c) Beak, P.; Kokko, B. J . J . Org. Chem. 1982, 47, 2822. (d) Erdik,
E.; Ay, M. Chem. Rev. 1989, 89, 1947.
(4) (a) Boche, G.; Bosold, F.; Lohrenz, J . C. W. Angew. Chem. 1994,
106, 1228; Angew. Chem., Int. Ed. Engl. 1994, 33, 1161. (b) Boche, G.;
Mo¨bus, K.; Harms, K.; Lohrenz, J . C. W.; Marsch, M. Chem. Eur. J .
1996, 2, 604.
Only a few representatives of organometallic com-
pounds with group 13 acceptors and geminal donor
atoms occurring as three-membered rings are known.
Among these are boron compounds of the formula R2B-
(8) Simmons, H. E.; Smith, R. D. J . Am. Chem. Soc. 1959, 81, 4256.
(9) Seyferth, D. Acc. Chem. Res. 1972, 5, 65.
(5) Chang, Y. H.; Chiu, F.-T.; Zon G. J . Org. Chem. 1981, 46, 352.
(6) Beckers, H.; Bu¨rger, H. J . Organomet. Chem. 1990, 385, 207.
(7) Ru¨ffer, T.; Bruhn, C.; Maulitz, A. H.; Stro¨hl, D.; Steinborn, D.
Organometallics 2000, 19, 2829.
(10) Peterson, D. J . J . Org. Chem. 1967, 32, 1717.
(11) Karsch, H. H.; Schmidbaur, H. Z. Naturforsch. 1977, 32b, 762.
(12) Kostyanovskii, R. G.; Prokov’ev, A. K. Dokl. Akad. Nauk SSSR
1965, 164, 1054.
10.1021/om020687r CCC: $25.00 © 2003 American Chemical Society
Publication on Web 12/20/2002

Dimethylaminomethyl Compounds of Al, Ga, and In
Organometallics, Vol. 22, No. 2, 2003 243
AlCH2N). 27Al NMR (C6D6): δ 166 (ν1/2 ) 950 Hz). CI-MS [m/z
(%)]: 215 (100), [M+ - Me]; 172 (39) [M+ - CH2NMe2], 115
CR′2-NR′′2. Two of them have been structurally eluci-
dated in the solid state and represent three-membered
ring compounds with BCN rings.13,14 Six-membered ring
dimers of systems containing geminal acceptors and
donors are found in a few representatives including the
compounds with B-C-S,15 Al-C-S,16,17 Ga-C-S,17
Al-C-P,18 and In-C-P19 linkages.
In this contribution we present investigations on
simple systems containing the saturated linkage M-C-N
for M being an earth metal atom. Apart from the
heteronorbornane type compounds [Me2MCH2N(Me)]2-
CH2 (M ) Al, Ga) we reported recently20 such com-
pounds are so far unknown. In contrast to the saturated
systems, compounds of the general formula R2Al-CHd
NR′ are known and said to be six-membered ring
dimers.21 There also exists a compound with Al-CdN
linkages and an Al-C-N three-membered ring result-
ing from insertion of an isonitrile into an Al-Al bond,
which however should be viewed as nonclassically
bonded. 22
+
(14) [Me2AlCH2NMe2+], 100 (20) [Me2AlCH2NMe2 - Me].
Anal. Calcd for C10H28Al2N2: C 52.15, H 12.25, N 12.16.
Found: C 52.08, H 11.98, N 11.87.
(Dim eth yla m in om eth yl)d im eth ylga lliu m (Me2Ga CH2-
NMe2)2 (2). The synthesis of (Me2GaCH2NMe2)2 is analogous
to the one for (Me2AlCH2NMe)2, but employing dimethylami-
nomethyllithium (6.15 mmol, 0.40 g) and dimethylgallium
chloride (6.14 mmol, 0.83 g). Yield: 0.67 g (69%), mp 118 °C;
sublimation point 70 °C (0.01mbar). 1H NMR (C6D6): δ -0.36
(s, GaCH3), 1.78 (s, GaCH2N), 1.99 (s, NCH3). 13C{1H} NMR
(C6D6): δ -7.57 (s, GaCH3), 50.72 (s, NCH3), 54.12 (s,
GaCH2N). CI-MS [m/z (%)]: 301 (100), [M+ - Me]; 258 (63)
[M+ - CH2NMe2], 156 (28) [Me2GaCH2NMe2+]. Anal. Calcd
for C10H28Ga2N2: C 38.03, H 8.94, N 8.87. Found: C 38.01, H
8.93, N 8.86.
(Dim eth yla m in om eth yl)d im eth ylin d iu m (Me2In CH2N-
Me2)2 (3). At -78 °C 45 mL of diethyl ether was condensed
onto a mixture of dimethylaminomethyllithium (6.31 mmol,
0.41 g) and dimethylindium chloride (6.31 mmol, 1.13 g). The
mixture was allowed to warm slowly to room temperature. The
solid residue was separated by centrifugation. Filtration,
concentration, and storage at -78 °C yielded a colorless
powder. Yield: 0.61 g (48%), mp 187 °C. 1H NMR (C6D6): δ
-0.21 (s, InCH3), 1.93 (s, InCH2N), 2.02 (s, NCH3). 13C{1H}
NMR (C6D6): δ -8.66 (s, InCH3), 51.94 (s, NCH3), 56.70 (s,
InCH2N). CI-MS [m/z (%)]: 391 (100) [M+ - Me]; 348 (95)
[M+ - CH2NMe2], 318 (32) [M+ - 2Me, CH2NMe2], 202 (10)
[Me2InCH2NMe2+], 145 (21) [InMe2+], 115 (31) [In+]. Anal.
Calcd for C10H28In2N2: C 29.59, H 6.95, N 6.90. Found: C
29.34, H 6.98, N 6.88.
Exp er im en ta l Section
Gen er a l Meth od s. All experiments were carried out under
a dry nitrogen atmosphere with standard Schlenk and high-
vacuum techniques or in a glovebox operated under argon.
Solvents were purified and dried by standard techniques.
Dimethylaluminum chloride was prepared from trimethyl-
aluminum and aluminum trichloride. Dimethylgallium chlo-
ride23 and dimethylindium chloride24 were obtained according
to literature procedures. Pure dimethylaminomethyllithium
was obtained via transmetalation of dimethylaminomethyl-
tri-n-butyltin25 with n-butyllithium. CAUTION: LiCH2NMe2
spontaneously ignites and burns vigorously on contact with
air. Chemical ionization (CI) mass spectra were obtained with
a Varian MAT 311A spectrometer. All NMR spectra were
recorded on a J EOL J NM-LA400 spectrometer (400.05 MHz
1H, 100.50 MHz 13C, 104.05 MHz 27Al) in C6D6 or toluene-d8
as solvent dried over K/Na alloy.
(Dim et h yla m in om et h yl)d im et h yla lu m in u m (Me2Al-
CH2NMe2)2 (1). To a stirred suspension of dimethylamino-
methyllithium (6.92 mmol, 0.45 g) in 45 mL of n-pentane was
slowly added a solution of dimethylaluminum chloride (6.88
mmol, 6.88 mL of an 1 M solution in n-hexane) at -78 °C.
The reaction mixture was slowly warmed to room temperature.
Colorless crystals were obtained after filtration, concentration
to 10 mL, and storage at -78 °C. Yield: 0.49 g (62%), mp 121
°C; sublimation point 65 °C (0.01 mbar). 1H NMR (C6D6): δ
-0.65 (s, AlCH3), 1.56 (s, AlCH2N), 1.97 (s, NCH3). 13C{1H}
NMR: δ -9.92 (br s, AlCH3), 50.45 (s, NCH3), 51.78 (br s,
(Dim eth yla m in om eth yl)d im eth yla lu m in u m (d im eth yl-
a m in om eth yl)(ch lor o)m eth yla lu m in u m [Cl(Me)AlCH 2N-
Me2‚Me2AlCH2NMe2] (4). The synthesis of [Cl(Me)AlCH2-
NMe2‚Me2AlCH2NMe2] is analogous to the one for (Me2Al-
CH2NMe2)2, but employing dimethylaminomethyllithium (5.23
mmol, 0.34 g) and trichlorotrimethyldialuminum (5.23 mmol,
1.07 g). Yield: 0.31 g (48%), mp 122 °C; sublimation point 65
°C (0.01 mbar). The following description of NMR data
represents the spectroscopic data obtained from a solution of
single crystalline material, and obviously there are complex
mixtures in solution. 1H NMR (C6D6): δ -0.74 (s, AlCH3), -0.
67 (s, AlCH3), -0.63 (s, AlCH3), -0.61 (s, AlCH3), -0.57 (s,
AlCH3), -0.50 (s, AlCH3), 1.32 (s, AlCH2N), 1.37 (s, AlCH2N),
1.44 (s, AlCH2N), 1.47 (s, AlCH2N), 1.48 (s, AlCH2N), 1.51 (s,
AlCH2N), 1.56 (s, AlCH2N), 1.57 (s, AlCH2N), 1.59 (s, AlCH2N),
1.60 (s, AlCH2N), 1.62 (s, AlCH2N), 1.66 (s, AlCH2N), 1.71 (s,
AlCH2N), 1.72 (s, AlCH2N), 1.76 (s, NCH3), 1.82 (s, NCH3), 1.9
(s, NCH3), 1.95 (s, NCH3) 1.96 (s, NCH3), 1.99 (s, NCH3), 2.14
(s, NCH3), 2.19 (s, NCH3). 13C{1H} NMR: δ -9.7 (vbr s, AlCH3),
49.32 (br s, AlCH2N), 49.81 (s, NCH3), 49.93 (s, NCH3), 49.97
(s, NCH3), 50.11 (s, NCH3), 50.36 (s, NCH3), 50.49 (s, NCH3)
50.74 (s, NCH3). 27Al NMR: δ 148 (ν1/2 ) 2400 Hz), 166 (ν1/2
950 Hz).
)
(13) Ansorge, A.; Brauer, D. J .; Bu¨rger, H.; Hagen, T.; Pawelke, G.
Angew. Chem., Int. Ed. Engl. 1993, 32, 384
(14) Brauer, D. J .; Bu¨rger, Buchheim-Spiegel, S., T.; Pawelke, G.
Eur. J . Inorg. Chem. 1999, 255.
Bis(d im eth yla m in om eth yl)a lu m in u m Ch lor id e [ClAl-
(CH2NMe2)2]2 (5) a n d Tr is(d im eth yla m in om eth yl)a lu m i-
n u m [Al(CH2NMe2)3]2 (6). The various experiments can
generally be described as follows. To stirred suspensions of
dimethylaminomethyllithium in an inert solvent (pentane,
hexane, toluene) was slowly added an understoichiometric
amount of aluminum chloride at temperatures between -78
°C and ambient temperature. After different conditions for
reaction time (12 h to 1 week) and temperature (RT to reflux)
the resulting suspensions were filtered and the solvent was
removed from the filtrate to yield a solid residue, which was
(15) No¨th, H.; Sedlak, D. Chem. Ber. 1983, 116, 1479.
(16) (a) Dickson, P. M.; Oliver, J . P. Organometallics 1994, 13, 3300.
(b) Ru¨ffer, T. Doctoral Thesis, Martin-Luther-Universita¨t Halle-
Wittenberg 1999. (c) Ru¨ffer, T.; Bruhn, C.; Brusanov, E.; Nordhoff, K.;
Steinborn, D. Z. Anorg. Allg. Chem. 2002, 628, 421.
(17) Lustig, C.; Mitzel, N. W. Organometallics 2002, 21, 3471.
(18) (a) Karsch, H. H.; Appelt A. Phosphorus Sulfur Related Ele-
ments 1983, 18, 287. (b) Karsch, H. H.; Appelt, A.; Ko¨hler, F. H.; Mu¨ller,
G. Organometallics 1985, 4, 231.
(19) Beachley, O. T.; Banks, M. A.; Churchill, M. R.; Feighery, W.
G.; Fettinger, J . C. Organometallics 1991, 10, 3036.
(20) Lustig, C.; Mitzel, N. W. Chem. Commun. 2000, 1393.
(21) Hoberg, H.; Bukowski, P. Liebigs. Ann. Chem. 1975, 1124.
(22) Uhl, W.; Schu¨tz, U.; Hiller W.; Heckel, M. Chem. Ber. 1994,
127, 1587.
1
always a mixture of different compounds, giving complex H
and 13C NMR spectra. From hexane solutions of these mixtures
a
few crystals of 5 and 6 besides precipitates of other
(23) Armer, B.; Schmidbaur, H. Chem. Ber., 1967, 100, 1521.
(24) Clark, H. C.; Pickard, A. L. J . Organomet. Chem., 1967, 9, 427.
(25) Peterson, D. J . J . Am. Chem. Soc. 1971, 93, 4027.
compounds were isolated and identified by crystal structure
analyses (see below). This material did not contain significant

244 Organometallics, Vol. 22, No. 2, 2003
Ta ble 1. Da ta for th e Sin gle Cr ysta l X-r a y Diffr a ction Exp er im en ts
Lustig and Mitzel
1
2
4
5
6
formula
Mr
C
10H28N2Al2
C10H28N2Ga2
315.78
C9H25N2Al2Cl
125.36
C12H32N4Al2Cl2
178.64
C18H48N6Al2
201.29
115.15
T [K]
143(2)
133(2)
123(2)
148(2)
143(2)
cryst syst
space group
a [Å]
b [Å]
c [Å]
triclinic
P1h
triclinic
P1h
triclinic
P1h
monoclinic
P21/c
21.910(4)
7.652(1)
12.041(2)
90
monoclinic
P21/n
10.053(2)
12.629(1)
10.718(2)
90
109.31(1)
90
1284.2(4)
2, 1.041
6.8100(4)
7.664(2)
9.0129(11)
94.21(2)
110.81(1)
116.20(2)
379.37(12)
1, 1.008
0.3 × 0.2 × 0.2
1856/1654
0.0195
6.7886(4)
7.6420(5)
9.0194(7)
94.553(4)
110.708(4)
116.117(4)
377.34(4)
1, 1.390
6.749(2)
7.229(2)
8.883(3)
75.89(2)
68.94(3)
64.81(2)
363.85(19)
1, 1.144
0.3 × 0.2 × 0.1
3192/1596
0.0174
R [deg]
â [deg]
γ [deg]
V [Å3]
99.470(10)
90
1991.2(6)
4, 1.192
0.4 × 0.3 × 0.3
6038/4333
0.0368
Z, Dcalc [g cm-3
]
cryst size [mm]
reflnscollected/unique
Rint
0.4 × 0.2 × 0.2
11608/1450
0.045
0.7 × 0.5 × 0.4
2936/2792
0.0449
2θmax
27.0
99.7
1654/0/120
26.6
91.6
1450/ 0/120
27.0
99.5
1596/1/97
27.0
99.4
4333/0/309
27.0
99.4
completeness [%]
no. of data/restraints/params
extinction coeff
R1/wR2
2792/0/215
0.011(6)
0.0526/0.1539
0.0623/0.1637
0.441/-0.333
0.0257/0.0665
0.0340/0.0697
0.290/-0.125
0.0309/0.0779
0.0321/0.0788
0.521/-0.641
0.0411/0.1115
0.0435/0.1131
0.529/-0.241
0.0329/0.0735
0.0705/0.0853
0.363/-0.231
R1/wR2 (all data)
∆Ffin [e Å-3
]
190388 (Me2GaCH2NMe2)2 (2), CCDC-190389 (Me2AlCH2-
NMe2)‚(MeClAlCH2NMe2) (4), CCDC-190390 [ClAl(CH2NMe2)2]2
(5), and CCDC-190391 [Al(CH2NMe2)3]2 (6). Copies of the data
can be obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: (+44)1223-336-
033; e-mail: deposit@ccdc.cam.ac.uk).
Resu lts a n d Discu ssion
P r ep a r a tion a n d Ch a r a cter iza tion of th e Com -
p ou n d s. The dimethylaminomethyl compounds (Me2-
AlCH2NMe2)2, (Me2GaCH2NMe2)2, and (Me2InCH2-
NMe2)2 were prepared by reacting dimethylaminometh-
yllithium LiCH2NMe2 with the corresponding dimeth-
ylmetal chlorides in hexane (eq 1).
F igu r e 1. 27Al NMR spectrum of a sample (dissolved in
C6D6) resulting from an attempt to prepare Al(CH2NMe2)3
from LiCH2NMe2 (excess) and AlCl3.
amounts of lithium and also gave complex NMR spectra with
too many lines to be interpretable. The 27Al NMR of one of
such samples is shown in Figure 1. Dissolution of the crys-
2 Me2NCH2Li + 2 Me2MCl f
(Me2MCH2NMe2)2 + 2 LiCl (1)
1
tallographically examined crystals also gave complex H and
13C spectra, but 27Al NMR spectra with clearly enhanced
signals at 125 ppm for 5 and 62 ppm for 6, while the other
signals shown in Figure 1 were always present, but less strong.
Cr ysta l Str u ctu r e Deter m in a tion s. Single crystals of
compounds 1, 2, 4, 5, and 6 were mounted under inert
perfluoropolyether at the tip of a glass fiber and cooled in the
cryostream of the diffractometer. Data were collected on a
Nonius Turbo-CAD4 diffractometer for all structures but (Me2-
GaCH2NMe2)2 (2), for which a Nonius DIP 2020 was used. Both
diffractometers were operated with monochromatic Mo KR
radiation (λ ) 0.71073 Å). No absorption corrections were
applied for 1, 4, 5, and 6, and the scattering intensities of 2
were corrected by the program SCALEPACK.26 The structure
solutions were carried out using direct methods, and the
refinements of the structure were undertaken with the pro-
gram SHELXTL 5.01.27 Non-hydrogen atoms were refined with
anisotropic thermal displacement parameters, hydrogen atoms
isotropically. Further details of data collection and refinement
are listed in Table 1.
M ) Al (1), Ga (2), In (3)
The Al, Ga, and In compounds 1, 2, and 3 were
identified by elemental analyses and NMR spectroscopy
of the nuclei 1H, 13C (and 27Al for 1). Mass spectra
proved the presence of dimers in the gas phase. The
occurrence of mass peaks corresponding to the monomer
cations is also observed. These can be formed either by
ionization of monomers present in the gas phase and/
or by decay upon ionization of the dimers.
The Al and Ga compounds 1 and 2 were obtained as
crystalline materials upon cooling solutions in hydro-
carbon solvents. The new compounds are all well soluble
in nonpolar solvents such as pentane, hexane, and
toluene and even better in ethers. The indium compound
(Me2InCH2NMe2)2 (3) is much less soluble in nonpolar
solvents, but dissolves to some extent in diethyl ether
and easily in THF. The compounds are thermally stable
up to about 100 °C in the solid state and in solution.
The compounds 1 and 2 can be sublimed at 65 °C/70 °C
in a vacuum (10-2 mbar).
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publications no. CCDC-190387 (Me2AlCH2NMe2)2 (1), CCDC-
In one experiment according to eq 1, a mixture of Me2-
AlCl and MeAlCl2 was employed close to a stoichiometry
of Me2AlCl‚MeAlCl2. This reaction yielded among other
compounds an aggregate of the formula [(Me2AlCH2-
(26) SCALEPACK: Otwinowski, Z.; Minor, W. Methods Enzymol.
1997, 276, 307.
(27) SHELXTL; Siemens Analytical X-Ray Instrumentation Inc.:
Madison, WI, 1995. Refinement: Sheldrick, G. M. SHELXL93; Uni-
versita¨t Go¨ttingen: Germany, 1993.

Dimethylaminomethyl Compounds of Al, Ga, and In
Organometallics, Vol. 22, No. 2, 2003 245
Ta ble 2. 27Al NMR Sp ectr oscop y Da ta for th e F ive
New Com p ou n d s a n d Tw o Refer en ce Com p ou n d s
in Solu tion (C6D6) a t 21 °C
NMe2)‚(MeClAlCH2NMe2)] (4, eq 2), which was obtained
as a crystalline product described by its crystal structure
1
below. The H and 13C NMR spectra are not as simple
chemical shift [ppm]
as the formula [(Me2AlCH2NMe2)‚(MeClAlCH2NMe2)]
suggests and consist of a number of lines, which indicate
the presence of at least three different species, namely,
(Me2AlCH2NMe2)2, [(Me2AlCH2NMe2)‚(MeClAlCH2N-
Me2)], and (MeClAlCH2NMe2)2. However, as there are
more lines than would be possible for the three species,
we have to assume the presence of further compounds
in this mixture, which could not be separated, as the
only compound we could crystallize from such solutions
is 4. Dissolution of crystals of 4 leads to the same
distribution of products, which obviously result from
redistribution reactions of the monomers with one
another. The simple appearance of the 27Al NMR
spectrum does not reflect this complexity in solution,
compound
(half width)
(Me2AlCH2NMe2)2 (1)
(Cl(Me)AlCH2NMe2)‚
(Me2AlCH2NMe2) (2)
[ClAl(CH2NMe2)2]2 (5)
[Al(CH2NMe2)3]2 (6)
[Cl2AlCH2NMe2]2 (7)
Me3Al‚NMe3
166 (ν1/2 ) 950 Hz)
148 (ν1/2 ) 2400 Hz),
166 (ν1/2 ) 950 Hz)
141 (ν1/2 ) 2500 Hz)
62 (ν1/2 ) 250 Hz)
125 (ν1/2 ) 1200 Hz)
179 (ν1/2 ) 1000 Hz)
109 (ν1/2 ) 100 Hz)
Cl3Al‚NMe3
All compounds reported in this section are sensitive
to oxygen and water, with the sensitivity decreasing in
the series aluminum, gallium, and indium for the
compounds (Me2MCH2NMe2)2. Solutions in hydrocar-
bons contain the dimers as described in more detail
below in a section dealing with the molecular dynamics
in solution. The dimeric structure of the compounds
seems also to be retained in ethers or upon adding
triethylamine to the solutions, as the NMR chemical
shifts do not change significantly, with respect to C6D6
solutions without base addition. This is in contrast to
the corresponding sulfur compounds (Me2MCH2SMe)2,17
which undergo ring opening and coordinate the stronger
bases ethers and trialkylamines to their metal atoms
upon addition of those. Hence we can conclude that
either the geminal nitrogen atoms are stronger donors
or the formation of six-membered ring dimers is other-
wise thermodynamically favored.
as it comprises only two broad lines at δ ) 148 (ν1/2
)
2400 Hz) and 166 ppm (ν1/2 ) 950 Hz), which stem from
Me2AlCH2 and MeClAlCH2 units.
2 Me2NCH2Li + Me2AlCl‚MeAlCl2 f
[(Me2AlCH2NMe2)‚(MeClAlCH2NMe2)] + 2 LiCl (2)
4
To prepare the homoleptic compound [Al(CH2NMe2)3]2,
we reacted aluminum trichloride and Me2NCH2Li in the
stoichiometric ratio 1:3. However, a complex mixture of
compounds resulted from these synthetic attempts. At
least two of the compounds could be obtained in crystal-
line form and are (MeClAlCH2NMe2)2 (5) and [Al(CH2-
NMe2)3]2 (6), which were identified by X-ray crystallog-
raphy (vide infra). Variation of the reaction conditions
resulted in a variation of the ratio of the products
formed. The major problem with these reactions is the
limited solubility of Me2NCH2Li in hydrocarbon solvents
(pentane, hexane, toluene) and diethyl ether and the
slow decomposition in THF. At temperatures of -78 °C
or lower both Me2NCH2Li and AlCl3 are not well soluble
even in ethers, and despite applying stoichiometric
amounts of the reactants, the reactions remained in-
complete with respect to the desired homoleptic com-
pound. The search for optimum conditions (solvents,
temperatures, reaction times (varied from 12 h to 1
week) proved to be rather difficult, and finally we cannot
provide a simple recipe to prepare large amounts of
(MeClAlCH2NMe2)2 (5) or [Al(CH2NMe2)3]2 (6) in a
selective way and high yields. The use of aluminum
bromide and iodide also did not lead to an improvement
of the preparation procedures. Furthermore are there
no suitable separation procedures for the products like
distillation or sublimation, and the rather incomplete
fractional crystallization is the only way we found so
far to solve this problem in part.
27Al NMR Sp ectr oscop y. For those compounds
containing aluminum 27Al NMR spectroscopy has proven
to be a valuable tool in identification of products and in
obtaining information about the coordination of the
metal atoms in solution. Table 2 shows a list of 27Al
NMR data for the compounds described above and some
relevant reference compounds.
The more alkyl groups bound to the Al atoms, the
higher the frequency of the NMR signal. This trend is
manifest from the data for Me3Al‚NMe3 (179 ppm) and
(Me2AlCH2NMe2)2 (166 ppm). More chlorine substitu-
ents shift the signals to lower frequencies, as shown for
[ClAl(CH2NMe2)2]2 (141 ppm) and Cl3Al‚NMe3 (109
ppm). The resonance at 125 ppm in mixtures resulting
from reactions of Me2NCH2Li with AlCl3 (one example
displayed in Figure 1) could be used to tentatively assign
it to [Cl2Al(CH2NMe2)]2 (7). The only exception in the
series of chemical shifts is the value of 62 ppm, which
was assigned [Al(CH2NMe2)3]2 (6), as samples contain-
ing crystallographically identified [Al(CH2NMe2)3]2 dis-
played signals with this chemical shift beside others
when dissolved in C6D6. Resonances at 62 ppm are also
always present in various contributions in the spectra
of mixtures resulting from preparation attempts of 6,
as described above and shown in Figure 1. A value of
62 ppm indicates that the aluminum atoms should
adopt coordination numbers higher than four in solu-
tion, which is not too difficult to imagine in the presence
of three dimethylaminomethyl groups per aluminum
atom. For the homoleptic Al(CH2SMe)3 five-coordinate
Al atoms were also found in the solid state structure.16
Crystallographic investigation of the compound [Al(CH2-
Me2NCH2Li + AlCl3 f
[ClAl(CH2NMe2)2]2, [Al(CH2NMe2)3]2 (3)
5
6
It should also be noted that other attempts to prepare
the Al-chlorinated compounds by chlorination of the
compounds Me2MCH2NMe2 [M ) Al (1), Ga (2)] with
trimethyltin chloride according to a recently reported
procedure28 failed, and only a number of products were
produced, which could neither be separated nor identi-
fied.
(28) Schnitter, C.; Klimek, K.; Roesky, H. W.; Albers, T.; Schmidt,
H.-G.; Ro¨pken, C.; Parisini, E. Organometallics 1998, 17, 2249.

246 Organometallics, Vol. 22, No. 2, 2003
Lustig and Mitzel
F igu r e 2. 1H NMR spectra (resolution 0.4 Hz) at various
temperatures of (Me2AlCH2NMe2)2 dissolved in [D8]toluene.
The signals marked with an asterisk are due to toluene
(2.2 ppm) and an impurity (0.8 ppm).
F igu r e 3. Molecular structure of (Me2AlCH2NMe2)2 (1) as
obtained by low-temperature X-ray crystallography. (Me2-
GaCH2NMe2)2 (2) is isostructural, and a plot is given in
the Supporting Information.
Cr yst a l St r u ct u r es of (Me2AlCH 2NMe2)2 a n d
(Me2Ga CH2NMe2)2. The compounds (Me2AlCH2NMe2)2
(1) and (Me2GaCH2NMe2)2 (2) are isomorphous and both
crystallize in the triclinic space group P1h with Z ) 1
formula unit of the dimer in the unit cell. The structures
of both compounds are illustrated in Figure 3. The
dimers have a crystallographic inversion center at the
middle of the six-membered M2C2N2 rings formed by
Al-N and Ga-N donor-acceptor bonds. The rings
adopt chair conformations. This formation of six-mem-
bered rings for saturated M-C-N units was already
described for the heteronorbornane systems [(Me2MC-
H2NMe)2CH2] (M ) Al, Ga), but in these cases the
M2C2N2 rings have boat conformations.20 Note that
compounds containing the lighter homologous element
boron [(F3C)2B-CR2-NR′2] have been found to crystal-
lize as monomers with an intramolecular B‚‚‚N donor-
acceptor bond leading to a three-membered ring.13,14
More similar to the structures of (Me2AlCH2NMe2)2 and
(Me2GaCH2NMe2)2 are the sulfur analogous compounds
(Me2AlCH2SMe)2 and (Me2GaCH2SMe)2, which have the
same chairlike six-membered ring motif and were
described by us earlier.17
NMe2)3]2 (6) in the solid state reveals the aluminum
centers to adopt the coordination number four (vide
infra), and we have to assume this to be the primary
coordination type also in solution. There is, however,
no basis for a decision whether the remaining dimeth-
ylaminomethyl groups coordinate intra- or intermolecu-
larly.
For the homoleptic sulfur compound Al(CH2SMe)3 a
27Al NMR chemical shift of 160 ppm was reported in
[D8]toluene,16c but in the thesis of Ru¨ffer16b this value
is discussed more controversially, as the spectrum
illustrated in this work shows only a very weak peak,
while for Al(CH2SPh)3 no 27Al NMR spectrum could be
recorded at all.16b The proton NMR data of 6 and Al-
(CH2SMe)3 cannot be compared, as samples of 6 were
not pure and thus gave complex spectra, while the
proton NMR spectrum of Al(CH2SMe)3 was recorded in
[D8]THF, in which Al-S donor bonds are broken and
Al(CH2SMe)3 is monomeric and coordinated to THF. In
this light the coordination of both the homoleptic sulfur
and nitrogen compound remains unclear in solution.
Contamination of the samples of [Al(CH2NMe2)3]2 (6)
by ate-complexes resulting from the reaction of 6 with
an excess of Me2NCH2Li could be excluded on the basis
That the M-C-N angles in both compounds are
distinctly larger [121.0(1)° and 119.7(2)°] than the
tetrahedral angle is consistent with the absence of any
significant attractive interaction between the acceptor
centers (Al, Ga) and the donor atoms (N) in the same
Me2MCH2NMe2 monomer unit. The sums of angles
C-M-C about the metal atoms are larger than expected
for tetrahedral geometries and are 339.7° for (Me2-
AlCH2NMe2)2 and 343.9° for (Me2GaCH2NMe2)2. Re-
markable are the very different C-M-C angles, with
the C1-M-C3 angles (ring to the exocyclic axial methyl
group) being the largest [117.0(1)° and 118.8(2)°], the
angles C2-M-C3 enclosed by the metal atoms and the
two exocyclic carbon atoms being intermediate [113.9-
(1)° and 114.7(2)°], and the C1-M-C2 angles involving
the equatorial methyl groups being the smallest [108.8-
(1)° and 110.4(1)°].
7
of chemical analyses and Li NMR spectroscopy, which
both showed the absence of lithium in the samples
under investigation.
Molecu la r Dyn a m ics of (Me2AlCH2NMe2)2 in So-
lu tion . The dynamic behavior of the heterocyclohexane
ring systems was examined with a sample of (Me2Al-
CH2NMe2)2 dissolved in [D8]toluene by recording proton
NMR spectra at various temperatures. A graphical
representation of these results is depicted in Figure 2.
At low temperatures (178 K) signals are apparent,
which can clearly be assigned to the chemically distin-
guishable axial and equatorial methyl groups at both
nitrogen and aluminum atoms. The signal for the
methylene group is also split at low temperature, but
not resolved into the expected two doublets at the lowest
temperature of our experiments, which was 178 K. At
higher temperatures these signals tend to merge and
finally are mean signals representing the rapid ex-
change of equatorial and axial sites. The coalescence
temperature is found at 187 K, a value that can be
converted by the Eyring equation into a barrier of
inversion of 9.0 ( 0.3 kcal mol-1 for the ring inversion
process. This value is remarkably close to that of
cyclohexane (10.2 ( 0.2 kcal mol-1).29
The M-N distances in the rings of crystalline (Me2-
AlCH2NMe2)2 and (Me2GaCH2NMe2)2 are 2.010(1) and
2.093(2) Å. For the aluminum compound this may be
compared with solid Me3N‚AlH3 [2.063(7) Å]30 or solid
Me3N‚AlMe3 [2.045(1) Å].31 The Ga-N distance is
slightly longer than in solid Me3N‚GaH3 [2.081 Å]32 and
equal to that of gaseous Me3N‚GaMe3 [2.09(5) Å].33
(30) (a) Warner, H. E.; Wang, Y.; Ward, C.; Gillies, C. W.; Interrante,
L. J . Phys. Chem. 1994, 98, 12215. (b) Almenningen, A.; Gundersen,
G.; Haugen, T.; Haaland, A. Acta Chem. Scand. 1972, 26, 3928.
(29) Friebolin, H.; Ein- und zweidimensionale NMR-Spektrosko-
pie: e. Einf.; VCH: Weinheim, 1988; p 252.

Dimethylaminomethyl Compounds of Al, Ga, and In
Organometallics, Vol. 22, No. 2, 2003 247
F igu r e 4. Molecular structure of [ClAl(CH2NMe2)2]2 (5)
as obtained by low-temperature X-ray crystallography.
Cr ysta l Str u ctu r e of (Me2AlCH2NMe2)‚(MeClAl-
CH2NMe2). The unsymmetrical heterocyclohexane (Me2-
AlCH2NMe2)‚(MeClAlCH2NMe2) crystallizes in the cen-
trosymmetric space group P1h, which requires the
molecule to be disordered about the center of inversion.
It is only the axial sites of the cyclohexane skeleton that
are adopted by the chlorine atoms. An alternative
refinement in the space group P1 with the assumption
of a racemic twin turned out to be a less suitable
description of the situation, resulting in a slightly worse
fit to the experimental scattering intensities. The dif-
fraction experiment was carried out two times with
different crystals and led to the following values for the
occupation factors for Cl and CH3 groups at the sites of
disorder: first experiment 0.46/0.54, second experiment
0.58/0.42, which finally led us to fix the occupation site
factors to 0.5. The desired separation of the electron
density maxima between carbon and chlorine positions
was achieved by tying the Al-C distance of the disor-
dered methyl group to the Al-C distance of the adjacent
methyl group.
Although the total fit of the structure model to the
experimental data and the estimated standard devia-
tions are good, the disorder does not allow deriving
accurate structural parameters for the molecule, as the
structural parameters of the corresponding units in the
(Me2AlCH2NMe2) and (MeClAlCH2NMe2) parts of the
molecule are necessarily averaged. A figure of the
structure and these mean values are given in the
Supporting Information. Nevertheless, the structure is
rather similar to that of (Me2AlCH2NMe2)2, vide supra.
In particular the Al-C-N angles are very similar, at
121.0(1)° in (Me2AlCH2NMe2)2 and 122.2(1)° in the
monochlorine derivative. Expectedly, the Al-N bonds
are slightly shorter in (Me2AlCH2NMe2)‚(MeClAlCH2-
NMe2) due to the electronegative chlorine atom attached
to the Lewis acidic Al center.
F igu r e 5. Molecular structure of [Al(CH2NMe2)3]2 (6) as
obtained by low-temperature X-ray crystallography.
the heterocyclohexane ring, which again adopts a chair
conformation. Consistent with the solution 27Al NMR
spectra is that the aluminum atoms are only four- and
not five-coordinate, despite the presence of two nitrogen
donor sites per aluminum center. Therefore, two types
of Me2NCH2 groups are present: one involved in ring
formation with the nitrogen atom being tetracoordinate
and bound to the aluminum atom of the second mono-
mer and one “free” dimethylaminomethyl group, which
has a free donor site at its nitrogen center. The endocy-
clic methylene group has a wider Al-C-N valence angle
of 122.8(1)° (similar to the three compounds described
above) than the exocyclic one, which is 116.6(1)°.
However, this is still larger than an ideal tetrahedral
angle, but cannot be attributed to a deformation occur-
ring upon ring formation. It is best compared to the also
widened B-C-C angle in triethylborane, which was
rationalized by hyperconjugation of the p-orbital at
boron into C-H orbitals.34
Due to the electronegative chlorine substituent, the
lengths of other bonds to aluminum are shorter than
in (Me2AlCH2NMe2)2, i.e., the Al-C bonds to the en-
docyclic methylene groups [1.989(2) vs 2.016(1) Å] and
the Al-N bonds [1.974(2) vs 2.010(1) Å].
Cr ysta l Str u ctu r e of [Al(CH2NMe2)3]2. The homo-
leptic compound [Al(CH2NMe2)3]2 (6) crystallizes in the
monoclinic space group P21/n, with the molecules sur-
rounding the crystallographic center of inversion (Figure
5). The salient point in this structure is the coexistence
of coordinate and noncoordinate nitrogen atoms, and the
formation of the central six-membered ring with four
coordinate Al atoms despite the NMR experiments in
C6D6 solution seems to indicate higher coordination
numbers at Al and therefore involvement of more than
one of the three CH2NMe2 groups coordinating the Al
centers via their nitrogen functions. In common with
the crystal structures described above, the six-mem-
bered ring adopts a chair conformation and has wide
Al-C-N angles of 121.8(1)°, whereas the exocyclic Al-
C-N angles are much closer to the tetrahedral angle
at 114.9(1)°. Accordingly the endocyclic (H2)C-N bond
[1.514(2) Å] is longer, while the exocyclic ones [1.466-
(2), 1.481(2) Å] resemble more typical values found for
instance in the N-C bond lengths to the methyl groups
[1.448(3)-1.455(3) Å] at the exocyclic N atoms.
Cr ysta l Str u ctu r e of [ClAl(CH2NMe2)2]2. The com-
pound [ClAl(CH2NMe2)2]2 (5) crystallizes in the mono-
clinic space group P21/c, whereby the molecules are
arranged about the center of inversion (Figure 4). As
in (Me2AlCH2NMe2)‚(MeClAlCH2NMe2) (4) the chlorine
substituents are only found to occupy the axial sites of
(31) (a) Crystal structure: Gelbrich, T.; Sieler, J .; Du¨michen, U. Z.
Kristallogr. 2000, 215, 127. Gas-phase structures: (b) Anderson, G.
A.; Forgaard, F. R.; Haaland, A. Acta Chem. Scand. 1972, 26, 1947.
(c) Anderson, G. A.; Forgaard, F. R.; Haaland, A. J . Chem. Soc. D 1971,
480.
(32) (a) Shriver, D. F.; Nordman, C. E. Inorg. Chem. 1963, 2, 1298.
(b) Andrews, P. C.; Gardiner, M. G.; Raston, C. L.; Tolhurst, V. A. Inorg.
Chim. Acta 1997, 259, 249. (c) Brain, P. T.; Brown, H. E.; Downs, A.
J .; Greene, T. M.; J ohnsen, E.; Parsons, S.; Rankin, D. W. H.; Smart,
B. A.; Tang, C. Y. J . Chem. Soc., Dalton Trans. 1998, 3685.
(33) Mastryukov, V. S.; Novikov, V. P.; Vilkov, L. V.; Golubinskii,
A. V.; Golubinskaya, L. M.; Bregadze, V. I. Zh. Strukt. Khim./ J . Struct.
Chem. (Engl. Transl.) 1987, 28, 143/122.
(34) Boese, R.; Bla¨ser, D.; Niederpru¨m, N.; Nu¨sse, M.; Brett, W. A.;
Schleyer, P. v. R.; Bu¨hl, M.; van Eikema Hommes, N. J . R. Angew.
Chem., Int. Ed. Engl. 1992, 31, 314.

248 Organometallics, Vol. 22, No. 2, 2003
Lustig and Mitzel
Sch em e 1
Ta ble 3. Bon d Len gth s [Å] a n d An gles [d eg] for
[Me2AlCH2NMe2]2 (1) a n d [Me2Ga CH2NMe2]2 (2) a s
Obta in ed fr om Low -Tem p er a tu r e X-r a y
Cr ysta llogr a p h y a n d Qu a n tu m Ch em ica l
Ca lcu la tion s
[Me2AlCH2NMe2]2
[Me2GaCH2NMe2]2
B3LYP/
B3LYP/
XRD
6-311G(d)
XRD
6-311G(d)
M(1)-C(1)
M(1)-C(2)
M(1)-C(3)
M(1)-N(1a)
N(1)-C(5)
2.016(1)
1.979(1)
1.980(1)
2.010(1)
1.482(2)
1.486(2)
1.509(1)
113.9(1)
2.042
1.994
1.996
2.071
1.483
1.485
1.509
115.1
106.3
105.1
108.5
117.1
103.5
108.2
110.0
110.4
110.0
110.4
107.9
112.0
2.019(3)
1.994(3)
1.998(4)
2.093(2)
1.475(4)
1.487(4)
1.459(4)
114.7(2)
104.6(1)
103.5(1)
110.4(1)
118.8(2)
102.8(1)
108.5(4)
110.2(2)
109.7(2)
110.6(2)
109.2(2)
108.6(2)
119.7(2)
2.051
2.010
2.012
2.174
1.477
1.481
1.499
116.4
104.3
103.0
109.5
119.0
102.2
108.5
110.6
110.2
110.3
108.6
108.7
121.2
F igu r e 6. Molecular structures of the three-membered
ring isomer Me2AlCH2NMe2 (1a ) as calculated at the
B3LYP/6-311G(d) level of theory.
N(1)-C(4)
Ta ble 4. Bon d Len gth s a n d An gles for
[Cl(Me2NCH2)2Al]2 (5)
N(1)-C(1)
C(2)-M(1)-C(3)
distance [Å]
value
angle [deg]
value
C(2)-M(1)-N(1a) 106.8(1)
C(3)-M(1)-N(1a) 105.2(1)
Al(1)-C(2)
Al(1)-N(1a)
Al(1)-C(1)
Al(1)-Cl(1)
N(1)-C(12)
N(1)-C(11)
N(1)-C(1)
N(1)-Al(1a)
N(2)-C(22)
N(2)-C(21)
N(2)-C(2)
1.973(2)
1.974(2)
1.989(2)
2.171(1)
1.486(3)
1.489(3)
1.510(2)
1.974(2)
1.449(3)
1.456(3)
1.482(2)
C(2)-Al(1)-N(1a)
C(2)-Al(1)-C(1)
N(1a)1-Al(1)-C(1)
C(2)-Al(1)-Cl(1)
N(1a)-Al(1)-Cl(1)
C(1)-Al(1)-Cl(1)
C(12)-N(1)-C(11)
C(12)-N(1)-C(1)
C(11)-N(1)-C(1)
C(12)-N(1)-Al(1a)
C(11)-N(1)-Al(1a)
C(1)-N(1)-Al(1a)
N(1)-C(1)-Al(1)
C(22)-N(2)-C(21)
C(22)-N(2)-C(2)
C(21)-N(2)-C(2)
N(2)-C(2)-Al(1)
108.5(1)
109.8(1)
106.2(1)
114.7(1)
104.0(1)
113.1(1)
107.5(2)
110.0(2)
110.2(2)
112.7(1)
107.8(1)
108.6(1)
122.8(1)
109.2(2)
110.7(2)
111.2(2)
116.6(1)
C(2)-M(1)-C(1)
C(3)-M(1)-C(1)
108.8(1)
117.0(1)
N(1a)-M(1)-C(1) 104.0(1)
C(5)-N(1)-C(4)
C(5)-N(1)-C(1)
C(4)-N(1)-C(1)
108.1(1)
109.5(1)
110.0(1)
C(5)-N(1)-M(1a) 110.9(1)
C(4)-N(1)-M(1a) 110.7(1)
C(1)-N(1)-M(1a) 107.6(1)
N(1)-C(1)-M(1)
121.0(1)
Th r ee-Mem b er ed Mon om er s vs Six-Mem ber ed
Rin g Dim er s: Ca lcu la tion s. The formation of six-
membered ring dimers as described in the preceding
section is only one of a number of alternatives one can
imagine to saturate the coordinative demand of the
Lewis acidic earth metal centers (Scheme 1). In par-
ticular the highly strained three-membered rings with
an intra-monomer donor-acceptor bond have been
found to exist in compounds with B-C-N13,14 and Al-
N-N35 units (and also in Si-O-N compounds).36 The
differences in energies between the six-membered ring
dimers and the three-membered ring monomers should
be dominated by the ring strain. We optimized both
types of geometries for the Al and Ga compounds 1 and
2 and their three-membered ring monomers 1a and 2a
with HF and DFT methods up to the B3LYP/6-311G(d)
level of theory. The geometries of the compounds 1 and
2 are so close to their crystal structures that they do
not need to be displayed separately. Their geometry
parameters may be compared with the experimental
results in Table 3. The small ring monomer geometry
of 1a is shown in Figure 6, and the bond lengths and
angles of the Me2AlCH2NMe2 and Me2GaCH2NMe2
monomers (1a and 2a ) are listed in Table 6. The
structure of 1a was also calculated at the higher MP2/
6-311G(d,p) level, but the geometry is quite close to that
optimized at the B3LYP/6-311G(d) level of theory,
making us confident that the DFT method is suitable
to predict the structures with suitable accuracy.
Concerning the bonding situation in 1a and 2a , a
closer look at the geometries reveals that the C2M-CH2
units in 1a and 2a are almost planar, which seems
surprising at first glance. However, this geometry
compares nicely to the crystal structure data of the
related compounds (F3C)2BC(SiMe3)(CH2Ph)NMe213 and
(F3C)2BC(Ph)2NMe2,14 where it was also noted that the
substituent planes BCC and CCSi do not bisect the
corresponding angles of the three-membered rings. In
this case it was attributed to steric interactions between
the substituents at the ring atoms B and C, but in light
of our quantum chemical results for the closely related
1a we have to favor electronic interactions as the
superior way to rationalize this behavior.
(35) (a) Uhl, W.; Molter, J .; Neumu¨ller, B. Eur. J . Inorg. Chem. 2001,
1510. Related compounds can be found in: (b) Uhl, W.; Molter, J .;
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Uhl, W.; Molter, J .; Koch, R. Eur. J . Inorg. Chem. 1999, 2021. (d) Uhl,
W.; Molter, J .; Koch, R. Eur. J . Inorg. Chem. 2000, 2255. (e) Uhl, W.;
Molter, J .; Neumu¨ller, B.; Saak, W. Z. Anorg. Allg. Chem. 2000, 626,
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19, 4422. (g) Uhl, W.; Molter, J .; Neumu¨ller, B. Inorg. Chem. 2001,
20, 2011. (h) Uhl, W.; Molter, W.; Neumu¨ller, B. Chem. Eur. J . 2001,
7, 1510.
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Angew. Chem., Int. Ed. Engl. 1997, 36, 2807. (b) Losehand, U.; Mitzel,
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Dimethylaminomethyl Compounds of Al, Ga, and In
Organometallics, Vol. 22, No. 2, 2003 249
Ta ble 5. Bon d Len gth s a n d An gles for
[(Me2NCH2)3Al]2 (6)
bonds in the three-membered ring SiON compounds
with Si‚‚‚N donor-acceptor interactions recently inves-
tigated by our group.36
distance [Å]
value
angle [deg]
value
The coordination sphere MC3 about the Al and Ga
atoms in 1a and 2a are flat, as indicated by the sum of
angles CMC. These are 358.7° and 358.8° for 1a and
2a . Together with the equally long M-C distances this
could be interpreted as a three-coordinate metal atom,
with the geminal nitrogen center being attracted by
purely electrostatic forces, i.e., not leading to a pyra-
midalization under formation of an at least partially
covalently bonded adduct.
The energy differences 1/1a and 2/2a (81 and 77 kJ
mol-1 per monomer) give a rough estimate for the
driving force of preference for the dimeric six-membered
ring structures.
We have prepared simple organometallic compounds
containing saturated M-C-N linkages for the metals
aluminum, gallium, and indium. Like the simple boron
compound H2BCH2NMe2,38 our new compounds are
dimeric with M2C2N2 rings. Electronegative or large
substituents at boron have allowed the isolation of
compounds with BCN three-membered rings. Calcula-
tions on the alternative aggregation motifs for Al-C-N
and Ga-C-N compounds favor clearly the dimerization
into six-membered rings over three-membered mono-
mers. At present we are going to explore the effects of
substituents at the metal and the nitrogen atom for the
aluminum, gallium, and indium compounds. The pos-
sibility for higher aggregation than into dimers or
intramolecular coordination of aminomethyl groups to
the metal atoms as is evident for Al(CH2NMe2)3 in
solution still remains incompletely understood and
deserves further investigations.
Dimerization rather than three-membered ring for-
mation is found to be the preferred way of aggregation
in a variety of dimethylaminomethyl compounds of Al,
Ga, and In. These are compounds with geminal nitrogen
donor and metal acceptor centers, which were prepared
and their coordination investigated in different phases,
accompanied by studies of the dynamic behavior by
NMR and theoretical studies.
Al(1)-N(1)
Al(1)-C(2)
Al(1)-C(1)
Al(1)-C(3)
N(1)-C(11)
N(1)-C(12)
N(1)-C(1a)
C(1)-N(1a)
C(2)-N(2)
N(2)-C(21)
N(2)-C(22)
N(3)-C(32)
N(3)-C(31)
N(3)-C(3)
1.999(2)
2.009(2)
2.009(2)
2.013(2)
1.484(2)
1.484(2)
1.514(2)
1.514(2)
1.466(2)
1.448(3)
1.455(3)
1.450(3)
1.455(3)
1.481(2)
N(1)-Al(1)-C(2)
N(1)-Al(1)-C(1)
C(2)-Al(1)-C(1)
N(1)-Al(1)-C(3)
C(2)-Al(1)-C(3)
C(1)-Al(1)-C(3)
C(11)-N(1)-C(12)
C(11)-N(1)-C(1a)
C(12)-N(1)-C(1a)
C(11)-N(1)-Al(1)
C(12)-N(1)-Al(1)
C(1a)-N(1)-Al(1)
N(1a)-C(1)-Al(1)
N(2)-C(2)-Al(1)
C(21)-N(2)-C(22)
C(21)-N(2)-C(2)
C(22)-N(2)-C(2)
C(32)-N(3)-C(31)
C(32)-N(3)-C(3)
C(31)-N(3)-C(3)
N(3)-C(3)-Al(1)
104.5(1)
104.8(1)
116.0(1)
104.3(1)
120.2(1)
105.5(1)
108.0(1)
110.1(1)
109.1(1)
110.5(1)
110.7(1)
108.5(1)
121.8(1)
114.9(1)
110.0(2)
110.5(2)
111.2(2)
109.1(2)
109.6(2)
112.0(2)
114.9(1)
Ta ble 6. Geom etr y P a r a m eter s for th e
Hyp oth etica l Th r ee-Mem ber ed Rin g Com p ou n d s
Me2AlCH2NMe2 (1a ) a n d Me2Ga CH2NMe2 (1b) a s
Ca lcu la ted Qu a n tu m Ch em ica llya
Me2AlCH2NMe2
Me2GaCH2NMe2
MP2/
6-311G(d,p)
B3LYP/
6-311G(d)
B3LYP/
6-311G(d)
compound
M-CH3
1.973
1.972
1.533
1.465
2.035
69.7
44.9
65.4
119.6
120.5
112.7
110.8
1.972
1.979
1.538
1.466
2.068
71.0
45.9
64.4
119.9
119.9
113.5
111.4
1.992
1.993
1.513
1.459
2.282
80.0
40.7
59.3
118.9
122.0
113.6
112.0
M-CH2
N-CH2
N-CH3
M-N
M-C-N
H2C-M-N
H2C-N-M
H2C-M-CH3
H3C-M-CH3
H2C-N-CH3
H2C-N-CH3
a
Bond lengths are given in Å and angles in deg.
A further compound that may be compared with 1a
is the three-membered BNN ring in (Mes)ClBN[BCl-
(Mes)]NMe2,37 which also has the substituent plane
BClC at the ring boron atom deviating substantially
from the bisector of the ring valence angle at this boron
atom.
The metal-carbon distances for the bonds to the
methyl groups and the methylene units are very similar,
unlike in the structures of the dimers, where the
M-CH2 distances are slightly longer. However, the
endocyclic N-C distances are predicted to be 1.538 and
1.513 Å for 1a and 2a and are thus substantially
widened with respect to the exocyclic N-C distances of
1.466 and 1.459 Å, which are in the normal range for
such bonds. This parallels the widening of the N-O
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft, Fonds der Chemis-
chen Industrie, and the Leonhard-Lorenz-Stiftung. We
are grateful to Professor H. Schmidbaur for generous
support.
Su p p or tin g In for m a tion Ava ila ble: Crystal structure
plots of compounds 2 and 4, a table of structural parameters
for 4, tables of crystal data, atomic coordinates, and equivalent
isotropic thermal parameters for all crystal structures. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM020687R
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