Synthesis and Structural Characterization of an Exclusively N-Based Tetrameric Aluminum(I) Compound

Article abstract of DOI:10.1021/om0302793

The synthesis and X-ray structure of the first N-based tetrameric complex were reported. Single-crystal X-ray structural analysis showed a nearly perfect tetrahedral Al₄ framework. The ¹H NMR spectrum recorded at room temperature showed broad signals, which can be attributed to the hindered rotation of the sterically rotation of the sterically bulky groups.

Full text of DOI:10.1021/om0302793

Organometallics 2003, 22, 3637-3638
3637
Syn th esis a n d Str u ctu r a l Ch a r a cter iza tion of a n
Exclu sively N-Ba sed Tetr a m er ic Alu m in u m (I) Com p ou n d
Marcus Schiefer, N. Dastagiri Reddy, Herbert W. Roesky,* and Denis Vidovic
Institut fu¨r Anorganische Chemie, Universita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany
Received April 15, 2003
Sch em e 1
Summary: The synthesis and X-ray structure of the first
exclusively N-based tetrameric complex [RAl]4 (1; R )
2,6-iPr2C6H3N(SiMe3)) are reported. 1 was prepared by
the reduction of [RAlI2]2 (2) with Na/ K alloy.
Even though the synthesis of the first Al(I) tetramer,
(RAl)4 (R ) Cp*), was reported in 1991, only a few
compounds of this type have been synthesized to date,1
despite their ability to form novel coordination com-
plexes with d-block metals which involve σ(Al-M) donor
bonds.2 One of the reasons may be the difficulty involved
in their syntheses, due to the tendency of these com-
pounds to disproportionate to elemental aluminum and
AlR3 in the absence of a suitable stabilizing R group.
Therefore, choosing an appropriate R group has always
been a key factor in the synthesis of aluminum(I)
compounds of the general type (RAl)4. In the structur-
ally characterized compounds, the ligands Cp*,1a,c,f,g tris-
(trimethylsilyl)methyl,1b tris(trimethylsilyl)silyl,1e and
tri-tert-butylsilyl1d have been employed to stabilize the
aluminum tetrahedron. It has also been reported that
one of the Cp* groups in (Cp*Al)4 can be substituted by
a N-based ligand to obtain the unsymmetrical compound
[(Cp*Al)3AlN(SiMe3)2].1c In our continuing efforts to
explore this class of compounds, we have chosen a ligand
to obtain an exclusively N-based aluminum tetrahedron,
and herein we report the synthesis and crystal structure
of [RAl]4 (1) and its precursor [RAlI2]2 (2) (R ) 2,6-
iPr2C6H3N(SiMe3)).
num(III) diiodide (2). Addition of iodine to a solution of
[RAlMe2]2 in toluene afforded 2 in good yield (Scheme
3
1).4 The X-ray structural analysis of 25 revealed that
the molecule exists as a dimer in the solid state, forming
an Al2I2 four-membered ring and thereby distorting the
tetrahedral geometry around the Al atoms (Figure 1).
The I(1A)-Al(1)-I(1) angle is only 93.83(7)°, while the
N(1)-Al(1)-I(2) angle is 120.59(19)°. As with the previ-
ously studied structures,6 the bond distances between
the Al atoms and the bridging I atoms are longer (2.645-
(2) and 2.687(2) Å) than those of known RAlI2 com-
pounds,7 while the bond distances between Al and the
terminal I atoms (2.464(2) Å) are shorter.
Reduction of 2 with Na/K alloy in hexane gave the
pale yellow product 1 (Scheme 2).8 The compound is
stable at room temperature and decomposes at 270 °C.
Al(I) compounds of the composition (RAl)4 may be
obtained by the reductive dehalogenation of aluminum-
(III) diiodides. Previously, we have reported an alumi-
num(III) diiodide with coordinated THF as starting
material that lowers the reactivity and decreases the
yield, due to the formation of THF ring-opening products
during the reduction process.1b Therefore, in the present
work, we have employed a donor-free dimeric alumi-
(3) Waezsada, S. D.; Liu, F.-Q.; Murphy, E. F.; Roesky, H. W.;
Teichert, M.; Uso´n, I.; Schmidt, H.-G.; Albers, T.; Parisini, E.; Nolte-
meyer, M. Organometallics 1997, 16, 1260-1264.
(4) Synthesis of (RAlI2)2 (2; R ) 2,6-iPr2C6H3N(SiMe3)): to a solution
of (2,6-iPr2C6H3N(SiMe3)AlMe2)23 (12.22 g, 20 mmol) in toluene (30 mL)
was added slowly a solution of iodine (20.30 g, 80 mmol) in toluene
(200 mL) at -78 °C. The reaction mixture was stirred at this
temperature for 2 h and then slowly warmed to room temperature with
continued stirring overnight while it was protected from light. The
volatiles were removed under vacuum, and to the residue was added
cold pentane (75 mL). After filtration, removal of pentane from the
filtrate afforded compound 2 as a colorless solid in good yield (16.5 g,
78%). Suitable crystals for X-ray analysis were grown by slow evapora-
tion of a toluene solution. Mp: 167 °C dec. 1H NMR (200 MHz, C6D6):
δ 7.00-7.10 (m, 3 H, Ar H), 3.55 (sept, J ) 6.9 Hz, 2 H, CH(CH3)2),
1.35 (d, J ) 6.9 Hz, 6 H, CH(CH3)2), 1.22 (d, J ) 6.9 Hz, 6 H, CH-
(CH3)2), 0.34 (s, 9 H, SiMe3). MS (EI) corresponds to the monomer:
m/z (%) 529 (M+, 70), 402 ([M+ - I - H], 100). IR (Nujol): ν˜ 1614,
* To whom correspondence should be addressed. Fax: (+49)551-
393373. E-mail: hroesky@gwdg.de.
(1) (a) Dohmeier, C.; Robl, C.; Tacke, M.; Schno¨ckel, H. Angew.
Chem. 1991, 103, 594-595; Angew. Chem., Int. Ed. Engl. 1991, 30,
564-565. (b) Schnitter, C.; Roesky, H. W.; Ro¨pken, C.; Herbst-Irmer,
R.; Schmidt, H.-G.; Noltemeyer, M. Angew. Chem. 1998, 110, 2059-
2062; Angew. Chem., Int. Ed. 1998, 37, 1952-1955. (c) Sitzmann, H.;
Lappert, M. F.; Dohmeier, C.; U¨ ffing, C.; Schno¨ckel, H. J . Organomet.
Chem. 1998, 561, 203-208. (d) Purath, A.; Dohmeier, C.; Ecker, A.;
Schno¨ckel, H.; Amelunxen, K.; Passler, T.; Wiberg, N. Organometallics
1998, 17, 1894-1896. (e) Purath, A.; Schno¨ckel, H. J . Organomet.
Chem. 1999, 579, 373-375. (f) Schormann, M.; Klimek, K. S.; Hatop,
H.; Varkey, S. P.; Roesky, H. W.; Lehmann, C.; Ro¨pken, C.; Herbst-
Irmer, R.; Noltemeyer, M. J . Solid State Chem. 2001, 162, 225-236.
(g) Schulz, S.; Roesky, H. W.; Koch, H. J .; Sheldrick, G. M.; Stalke, D.;
Kuhn, A. Angew. Chem. 1993, 105, 1828-1830; Angew. Chem., Int.
Ed. Engl. 1993, 32, 1729-1731.
1554, 1313, 1250, 1164, 1101, 1041, 925, 888, 869, 801, 746, 540 cm-1
.
Anal. Calcd for C30H52Al2I4N2Si2 (1058.51): C, 34.04; H, 4.95; N, 2.65.
Found: C, 33.76; H, 4.81; N, 2.83.
(5) Crystallographic data for compound 2:
C15H26AlI2NSi (fw )
529.24), monoclinic, space group P21/c, a ) 12.957(5) Å, b ) 9.282(2)
Å, c ) 17.541(14) Å, â ) 96.21(5)°, V ) 2.110(10) Å3, Z ) 4, Fcalcd
)
1.676 g cm-3, µ(Mo KR) ) 3.091 mm-1, T ) 200(2) K, R1 (wR2) (I >
2σ(I)) ) 0.0539 (0.1385), R1 (wR2) (all data) ) 0.0602 (0.1463).
(6) Allegra, G.; Perego, G.; Immirzi, A. Makromol. Chem. 1963, 61,
69-79.
(2) Fischer, R. A.; Weiss, J . Angew. Chem. 1999, 111, 3002-3022;
Angew. Chem., Int. Ed. 1999, 38, 2831-2850.
10.1021/om0302793 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 08/06/2003

3638 Organometallics, Vol. 22, No. 18, 2003
Communications
F igu r e 1. Molecular structure of 2. For clarity, all the
hydrogen atoms are omitted. Non-hydrogen atoms are
represented by thermal ellipsoids drawn at 50% probability
level. Selected bond lengths (Å) and angles (deg): Al(1)-
N(1) ) 1.784(6), Al(1)-I(2) ) 2.464(2), Al(1)-I(1A) ) 2.645-
(2), Al(1)-I(1) ) 2.687(2), I(1)-Al(1A) ) 2.645(2); N(1)-
Al(1)-I(2) ) 120.59(19), N(1)-Al(1)-I(1A) ) 113.34(19),
I(2)-Al(1)-I(1A) ) 106.81(8), N(1)-Al(1)-I(1) ) 114.7(2),
I(2)-Al(1)-I(1) ) 103.92(8), I(1A)-Al(1)-I(1) ) 93.83(7).
F igu r e 2. Molecular structure of 1. For clarity, all the
hydrogen atoms are omitted. Non-hydrogen atoms are
represented by thermal ellipsoids drawn at 50% probability
level. Selected bond lengths (Å) and angles (deg): Al(2)-
N(2) ) 1.808(2), Al(1)-N(1) ) 1.821(2), Al(2)-Al(3) )
2.601(1), Al(1)-Al(3) ) 2.635(1); N(3)-Al(3)-Al(2) ) 141.79-
(7), N(1)-Al(1)-Al(3) ) 149.54(8), Al(2)-Al(4)-Al(3) )
59.37(3), Al(3)-Al(2)-Al(1) ) 60.79(3).
Sch em e 2
protons could be determined. A partial dissociation of
1 to the monomeric species in solution at 70 °C cannot
be ruled out.
Ack n ow led gm en t. This work was supported by the
Deutsche Forschungsgemeinschaft and the Go¨ttinger
Akademie der Wissenschaften. N.D.R. thanks the Al-
exander von Humboldt-Stiftung for a research fellow-
ship.
Su p p or tin g In for m a tion Ava ila ble: Tables giving X-ray
crystallographic data for the structures of 1 and 2. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Single-crystal X-ray structural analysis9 showed a
nearly perfect tetrahedral Al4 framework (Figure 2). No
Al-Al-Al angle differs more than 1° from the ideal
angle of 60°. The average Al-Al bond length (2.619 Å)
is relatively short compared to (Cp*Al)4 (2.769 Å) but
is in good agreement with the calculated length (SCF)
for the model compound (AlNH2)4 (2.646 Å).1c The EI
mass spectrum of 1 shows the molecular ion peak with
a relative intensity of 16%. This result is quite surpris-
ing, since the mass spectrum of (Cp*Al)4 exhibits only
the monomeric species Cp*Al under similar conditions.1g
OM0302793
(8) Synthesis of [RAl]4 (1): compound 2 (0.53 g, 0.50 mmol) in hexane
(60 mL) was treated with Na/K alloy, made from sodium (0.024 g, 1
mmol) and potassium (0.041 g, 1 mmol). The mixture was stirred for
12 days at room temperature and filtered through Celite. The yellow
residue, which was obtained after the removal of hexane from the
filtrate, was treated with dimethoxyethane (4 mL), and the white solid
was filtered through Celite. The solvent was removed under vacuum,
diisoproyl ether (0.7 mL) was added, and the mixture was stored for 3
days at 12 °C to obtain yellow crystals of 1‚(diisopropyl ether) (0.096
g, 32%). Mp: 270 °C dec. 1H NMR (200 MHz, C6D6): δ 7.05 (AB2, 12
H, Ar H), 3.68 (br, 8 H, CH(CH3)2), 3.44 (sept, J ) 6.1 Hz, 2 H,
OCH(CH3)2), 1.27 (d, br, 24 H, CH(CH3)2), 1.20 (d, J ) 6.1 Hz, 12 H,
CH(CH3)2), 1.14 (d, br, 12 H, CH(CH3)2), 1.06 (d, br, 12 H, CH(CH3)2),
0.20 (s, br, 36 H, Si(CH3)3). 1H NMR (200 MHz, C6D6, 70 °C): δ 7.15
(AB2, 12 H, Ar H), 3.67 (sept, J ) 6.9 Hz, 8 H, CH(CH3)2), 3.45 (sept,
J ) 6.1 Hz, 2 H, OCH(CH3)2), 1.24 (d, J ) 6.9 Hz, 24 H, CH(CH3)2),
1.10 (d, J ) 6.9 Hz, 24 H, CH(CH3)2), 1.06 (d, J ) 6.1 Hz, 12 H, CH-
(CH3)2), 0.21 (s, 27 H, Si(CH3)3). 13C NMR (125 MHz, C6D6): δ 146.2,
138.8, 124.7, 124.1 (arom C), 28.1, 27.5 (CH), 23.8, 23.0 (CH3), 3.9 (Si-
(CH3)3). MS (EI): m/z (%) 1100 (M+, 16), 550 (1/2 M+, 100). IR (Nujol):
ν˜ 3176, 1587, 1433, 1363, 1314, 1249, 1237, 1176, 1103, 1052, 1041,
884, 836, 796, 749, 680, 539, 435 cm-1. Anal. Calcd for C66H118Al4N4-
OSi4 (1203.96): C, 65.84; H, 9.88; N, 4.65. Found: C, 64.48; H, 9.09;
N, 4.19.
1
The H NMR spectrum recorded at room temperature
shows broad signals, which can be attributed to the
hindered rotation of the sterically bulky 2,6-iPr2C6H3N-
(SiMe3) groups. However, at 70 °C the signals are so
resolved that the coupling constants of the isopropyl
(7) (a) Schnitter, C.; Klimek, K.; Roesky, H. W.; Albers, T.; Schmidt,
H.-G.; Ro¨pken, C.; Parisini, E. Organometallics 1998, 17, 2249-2257.
(b) Ecker, A.; Ko¨ppe, R.; U¨ ffing, C.; Schno¨ckel, H. Z. Anorg. Allg. Chem.
1998, 624, 817-822. (c) U¨ ffing, C.; Baum, E.; Ko¨ppe, R.; Schno¨ckel,
H. Angew. Chem. 1998, 110, 2488-2491; Angew. Chem., Int. Ed. 1998,
37, 2397-2400. (d) Krossing, I.; No¨th, H.; Schwenk-Kircher, H. Eur.
J . Inorg. Chem. 1998, 927-939. (e) Bo¨ker, C.; Noltemeyer, M.;
Gornitzka, H.; Kneisel, B. O.; Teichert, M.; Herbst-Irmer, R.; Meller,
A. Main Group Met. Chem. 1998, 21, 565-579. (f) Ecker, A.; Baum,
E.; Friesen, M. A.; J unker, M. A.; U¨ ffing, C.; Ko¨ppe, R.; Schno¨ckel, H.
Z. Anorg. Allg. Chem. 1998, 624, 513-516.
(9) Crystallographic data for compound 1‚(diisopropyl ether): C66H118
-
Al4N4Si4 (fw ) 1187.92), triclinic, space group P1h, a ) 13.0801(9) Å, b
) 13.7352(10) Å, c ) 20.8205(15) Å, R ) 83.173(6)°, â ) 96.21(5)°, γ )
81.883(6)°, V ) 3669.6(5) Å3, Z ) 2, Fcalcd ) 1.075 g cm-3, µ(Mo KR) )
0.167 mm-1, T ) 133(2) K, R1 (wR2) (I > 2σ(I)) ) 0.0492 (0.1153), R1
(wR2) (all data) ) 0.0714 (0.1209).