Stepwise hydrolysis of aluminum chloride iodide LAlClI (L = HC[(CMe)(NAr)]2, Ar = 2,6-iPr2C6H3) in the presence of N-heterocyclic carbene as hydrogen halide acceptor

Article abstract of DOI:10.1021/om049339a

The aluminum chloride iodide LAlClI (2, L = HC[(CMe)(NAr)]₂, Ar = 2,6-iPr₂C₆H₃) has been synthesized to study the stepwise hydrolysis of this compound in the presence of 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene as hydrogen halide acceptor. The isolation and characterization of the aluminum chloride hydroxide [LAlCl(μ-OH)]₂ (3) and the dihydroxide LAl(OH)₂ (4) demonstrate the realization of a controlled hydrolysis.

Full text of DOI:10.1021/om049339a

380
Organometallics 2005, 24, 380-384
Stepwise Hydrolysis of Aluminum Chloride Iodide
LAlClI (L ) HC[(CMe)(NAr)]2, Ar ) 2,6-iPr2C6H3) in the
Presence of N-Heterocyclic Carbene as Hydrogen Halide
Acceptor†
Hongping Zhu, Jianfang Chai, Cheng He, Guangcai Bai, Herbert W. Roesky,*
Vojtech Jancik, Hans-Georg Schmidt, and Mathias Noltemeyer
Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen, Tammannstrasse 4,
D-37077 Go¨ttingen, Germany
Received August 24, 2004
The aluminum chloride iodide LAlClI (2, L ) HC[(CMe)(NAr)]2, Ar ) 2,6-iPr2C6H3) has
been synthesized to study the stepwise hydrolysis of this compound in the presence of 1,3-
diisopropyl-4,5-dimethylimidazol-2-ylidene as hydrogen halide acceptor. The isolation and
characterization of the aluminum chloride hydroxide [LAlCl(µ-OH)]2 (3) and the dihydroxide
LAl(OH)2 (4) demonstrate the realization of a controlled hydrolysis.
Introduction
water or hydrated inert salts afforded some aggregated
alumoxanes [RAlO]n, aluminum hydroxides, and oxide
hydroxides. On the other hand, it has been shown that
aluminum hydrides,8 alkyls,9 or aluminum amino-
amides10 stabilized by bulky organic ligands can be used
as precursor for the synthesis of organoalumoxanes,
organoalumoxane hydrides, and hydroxides by employ-
ing H2O or water-containing compounds (for example,
H2O‚B(C6F5)3). In most cases the bulky ligand at the
aluminum center hinders the aggregation of the hy-
drolysis products. However, it has an effect on the
stabilization of the metal, whether electronic or steric,
and can result in the formation of unusual complexes,
such as L′AldO‚B(C6F5)3 (L′ ) Et2NCH2CH2NC(Me)-
CHC(Me)NCH2CH2NEt2).9 More recently, we have re-
ported the reaction of LAlI2 (L ) HC[(CMe)(NAr)]2, Ar
) 2,6-iPr2C6H3) with KOH containing a small amount
of H2O and KH in a liquid ammonia/toluene two-phase
system and successfully isolated the first terminal
aluminum dihydroxide LAl(OH)2 supported by the bulky
L ligand.11 An improved route to LAl(OH)2 was realized
by using a strong nucleophilic reagent, N-heterocyclic
carbene, as a HCl acceptor for the reaction of LAlCl2
and stoichiometric amounts of water.12 In the course of
the synthesis of LAl(OH)2 from LAl(Hal)2 (Hal ) I, Cl),
a stepwise process was proposed.11 However, there was
no experimental evidence for the formation of LAl(OH)-
Cl or LAl(OH)I as an intermediate. In this paper we
report the preparation of aluminum chloride iodide
The controlled hydrolysis of organoaluminum com-
pounds is of great interest since it can lead to the
formation of alumoxanes, which are used as active
catalysts and cocatalysts for the polymerization of a
wide range of organic monomers.1-4 Furthermore, it can
be useful to generate organoaluminum hydroxides,
oxides, or oxide hydroxides.5 On one hand, the reaction
of AlR3 compounds (R ) Me, tBu, Mes, or Ph)6,7 with
† Dedicated to Professor Alfred Schmidpeter on the occasion of his
75th birthday.
* To whom correspondence should be addressed. E-mail: hroesky@
gwdg.de.
(1) (a) Colclough, R. O. J. Polym. Sci. 1959, 34, 171-178. (b)
Colclough, R. O.; Gee, G.; Jagger, A. H. J. Polym. Sci. 1960, 48, 270-
275. (c) Vandenberg, E. J. J. Polym. Sci. 1960, 47, 486-489.
(2) (a) Ishida, S. I. J. Polym. Sci. 1962, 62, 1-14. (b) Longiave, C.;
Castelli, R. J. Polym. Sci. 1963, 4C, 387-398.
(3) (a) Sishta, C.; Hathorn, R. M.; Marks, T. J. J. Am. Chem. Soc.
1992, 114, 1112-1114. (b) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt,
R.; Rieger, B.; Waymouth, R. Angew. Chem. 1995, 107, 1255-1283;
Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170. (c) Koide, Y.; Bott,
S. G.; Barron, A. R. Organometallics 1996, 15, 2213-2226.
(4) (a) Sinn, H.; Kaminsky, W.; Vollmer, H. J.; Woldt, R. Angew.
Chem. 1980, 92, 396-402; Angew. Chem., Int. Ed. Engl. 1980, 19, 390-
392. (b) Sinn, H.; Kaminsky, W. Adv. Organomet. Chem. 1980, 18, 99-
149.
(5) Besides the hydrolytic synthesis,2b,5a,b aluminum hydroxides and
oxides are usually prepared by the reaction of species containing
reactive oxygen atoms such as CO2, (Me2SiO)3, dioxane, Ag2O, and
LiOH:5c-f,7b (a) Storr, A.; Jones, K.; Laubengayer, A. W. J. Am. Chem.
Soc. 1968, 90, 3173-3177. (b) Bolesławski, M.; Pasynkiewicz, S.;
Minorska, A.; Hrynio´w, W. J. Organomet. Chem. 1974, 65, 165-167.
(c) Zieglar, K.; Krupp, F.; Weyer, K.; Larbig, W. Liebigs Ann. Chem.
1960, 629, 251-256. (d) Wehmschulte, R. J.; Power, P. P. J. Am. Chem.
Soc. 1997, 119, 8387-8388. (e) Zheng, W.; Mo¨sch-Zanetti, N. C.;
Roesky, H. W.; Noltemeyer, M.; Schmidt, H.-G. Angew. Chem. 2000,
112, 4446-4449; Angew. Chem., Int. Ed. 2000, 39, 4276-4279. (f)
Kuhn, N.; Fuchs, S.; Niquet, E.; Richter, M.; Steimann, M. Z. Anorg.
Allg. Chem. 2002, 628, 717-718.
(6) (a) Mason, M. R.; Smith, J. M.; Bott, S. G.; Barron, A. R. J. Am.
Chem. Soc. 1993, 115, 4971-4984. (b) Harlan, C. J.; Mason, M. R.;
Barron, A. R. Organometallics 1994, 13, 2957-2969. (c) Landry, C.
C.; Harlan, C. J.; Bott, S. G.; Barron, A. R. Angew. Chem. 1995, 107,
1315-1317; Angew. Chem., Int. Ed. Engl. 1995, 107, 1201-1203.
(7) (a) Storre, J.; Schnitter, C.; Roesky, H. W.; Schmidt, H.-G.;
Noltemeyer, M.; Fleischer, R.; Stalke, D. J. Am. Chem. Soc. 1996, 118,
1380-1386. (b) Storre, J.; Schnitter, C.; Roesky, H. W.; Schmidt, H.-
G.; Noltemeyer, M.; Fleischer, R.; Stalke, D. J. Am. Chem. Soc. 1997,
119, 7505-7513.
(8) (a) Zheng, W.; Roesky, H. W.; Noltemeyer, M. Organometallics
2001, 20, 1033-1035. (b) Peng, Y.; Bai, G.; Fan, H.; Vidovic, D.; Roesky,
H. W.; Magull, J. Inorg. Chem. 2004, 43, 1217-1219.
(9) Neculai, D.; Roesky, H. W.; Neculai, A. M.; Magull, J.; Walfort,
B.; Stalke, D. Angew. Chem. 2002, 114, 4470-4472; Angew. Chem.,
Int. Ed. 2002, 41, 4294-4296.
(10) McMahon, C. N.; Barron, A. R. J. Chem. Soc., Dalton Trans.
1998, 3703-3704.
(11) Bai, G.; Peng, Y.; Roesky, H. W.; Li, J.; Schmidt, H.-G.;
Noltemeyer, M. Angew. Chem. 2003, 115, 1164-1167; Angew. Chem.,
Int. Ed. 2003, 42, 1132-1135.
(12) Jancik, V.; Pineda, L. W.; Pinkas, J.; Roesky, H. W.; Neculai,
D.; Neculai, A. M.; Herbst-Irmer, R. Angew. Chem. 2004, 116, 2194-
2197; Angew. Chem., Int. Ed. 2004, 43, 2142-2145.
10.1021/om049339a CCC: $30.25 © 2005 American Chemical Society
Publication on Web 12/30/2004

Hydrolysis of an Organoaluminum Compound
Organometallics, Vol. 24, No. 3, 2005 381
with n-hexane (10 mL). The extract was kept at 4 °C to afford
colorless crystals of 3 (0.35 g, 70%). Mp > 177 °C (dec). 1H
NMR (500.13 MHz, C6D6, 298 K, ppm): δ 0.72 (s, 2 H, OH),
LAlClI (2) and its stepwise reaction with water by using
a N-heterocyclic carbene as hydrogen halide acceptor,
as well as isolation of the intermediate [LAlCl(µ-OH)]2.
3
1.12 (d, 2 × 3 H, JHH ) 6.8 Hz, CH(CH3)2), 1.13 (d, 2 × 3 H,
3
3JHH ) 6.8 Hz, CH(CH3)2), 1.33 (d, 2 × 3 H, JHH ) 6.8 Hz,
3
Experimental Section
CH(CH3)2), 1.48 (d, 2 × 3 H, JHH ) 6.8 Hz, CH(CH3)2), 1.66
3
(s, 2 × 3 H, â-Me), 3.42 (sept, 2 × 1 H, JHH ) 6.8 Hz,
General Procedures. All manipulations were carried out
under a purified nitrogen atmosphere using Schlenk tech-
niques or inside a Mbraun MB 150-GI glovebox. All solvents
were distilled from Na/benzophenone ketyl prior to use.
Commercially available chemicals were purchased from Ald-
rich or Fluka and used as received. LH (L ) HC[(CMe)(NAr)]2,
Ar ) 2,6-iPr2C6H3)13,14 and [CN(iPr)C2Me2N(iPr)] (:C)15 were
prepared as described in the literature. Elemental analyses
were performed by the Analytisches Labor des Instituts fu¨r
Anorganische Chemie der Universita¨t Go¨ttingen. 1H (200.13,
300.13, and 500.13 MHz) and 27Al (78.20 MHz) NMR spectra
were recorded on Bruker AM 200, 300, and 500 spectrometers
and IR spectra on a Bio-Rad Digilab FTS-7 spectrometer. EI
mass spectra were measured on a Finnigan MAT 8230 or a
Varian MAT CH5 instrument. Melting points were measured
in sealed glass tubes and were not corrected.
Synthesis of LAlClMe (1). To a toluene solution (80 mL)
of LH (8.36 g, 20 mmol) at 0 °C was added dropwise n-BuLi (2
M, 10 mL, 20 mmol). The mixture was stirred and allowed to
warm to room temperature. After additional stirring for 12 h,
the solution was cooled to -78 °C and AlCl2Me (1 M, 20 mL,
20 mmol) was added. The resulting suspension was allowed
to warm to room temperature and stirred for 12 h. After
workup, the insoluble LiCl was removed by filtration and the
filtrate was dried in vacuo and washed with n-hexane to yield
crystalline solid 1 (9.1 g, 92%). Mp: 191-192 °C. 1H NMR
(300.13 MHz, C6D6, 298 K, ppm): δ -0.64 (s, 3 H, Al-Me), 1.00
CH(CH3)2), 3.47 (sept, 2 × 1 H, 3JHH ) 6.8 Hz, CH(CH3)2), 4.90
(s, 1 H, γ-CH), 7.04-7.12 (m, 6 H, Ar-H). 1H NMR (200.13
MHz, D8-toluene, 298 K, ppm): δ 0.69 (s, 2 H, OH), 1.12 (d, 4
× 3 H, 3JHH ) 6.8 Hz, CH(CH3)2), 1.33 (d, 2 × 3 H, 3JHH ) 6.8
3
Hz, CH(CH3)2), 1.46 (d, 2 × 3 H, JHH ) 6.8 Hz, CH(CH3)2),
3
1.58 (s, 2 × 3 H, â-Me), 3.40 (sept, 4 × 1 H, JHH ) 6.8 Hz,
CH(CH3)2), 4.94 (s, 1 H, γ-CH), 6.96-7.20 (m, 6 H, Ar-H). 27Al
NMR (78.20 MHz, C6D6, 298 K, ppm): no resonances were
observed. EI-MS: m/z (%) 496.3 (50, [M+/2]), 478.3 (16, [M+/2
- OH - 1], 461.3 (100, [M+/2 - Cl]). IR (Nujol mull, cm-1): ν
3459 (broad, OH). Anal. Calcd for C58H84Al2Cl2N4O2 (Mr
)
994.21): C, 70.07; H, 8.52; N, 5.64. Found: C, 69.34; H, 8.69;
N, 5.34. X-ray quality crystals of 3 were grown from n-hexane
and contained one molecule of n-hexane.
Synthesis of LAl(OH)2 (4). The synthetic procedure for 4
resembled that of 3 with the starting materials 3, [CN(iPr)-
C2Me2N(iPr)], and H2O in molar ratio of 1:2:2. After workup
by filtration to remove insoluble solids and drying in vacuo
the n-hexane extract was kept at 4 °C for 3 days to afford
colorless crystals of 4 (25%). The spectrometric and spectro-
scopic data (EI-MS, IR, 1H NMR) are almost the same as those
described in the literature.11,12 The characteristic data for the
Al-OH functionality are listed here: 1H NMR (300.13 MHz,
D8-toluene, 298 K, ppm): δ 0.27 (s, 2 × 1 H, OH). IR (Nujol
mull, cm-1): ν˜ 3448 (broad, OH). The X-ray single-crystal
diffraction analysis confirms the same set of data reported for
complex 4.11
X-ray Structure Determination and Refinement. The
crystallographic data for compounds 2 and 3‚n-hexane were
collected on a Stoe IPDS II-array detector system with
graphite-monochromated Mo KR radiation (λ ) 0.71073 Å).
These two structures were solved by direct methods (SHELXS-
96)16 and refined against F2 using SHELXL-97.17 All non-
hydrogen atoms were located by difference Fourier synthesis
and refined anisotropically, and hydrogen atoms were included
using the riding model with Uiso related to the Uiso of the parent
atoms except for the Al-µ-OH hydrogen atom, which was
located by difference Fourier synthesis and refined isotropi-
cally. A summary of cell parameters, data collection, and
structure solution and refinement is given in Table 1.
3
3
(d, 2 × 3 H, JHH ) 6.8 Hz, CH(CH3)2), 1.19 (d, 2 × 3 H, JHH
3
) 6.8 Hz, CH(CH3)2), 1.28 (d, 2 × 3 H, JHH ) 6.8 Hz, CH-
(CH3)2), 1.46 (d, 2 × 3 H, 3JHH ) 6.8 Hz, CH(CH3)2), 1.54 (s, 2
3
× 3 H, â-Me), 3.21 (sept, 2 × 1 H, JHH ) 6.8 Hz, CH(CH3)2),
3
3.77 (sept, 2 × 1 H, JHH ) 6.8 Hz, CH(CH3)2), 4.98 (s, 1 H,
γ-CH), 7.00-7.10 (m, 6 H, Ar-H). EI-MS: m/z (%) 494.3 (3,
[M+]), 479.3 (100, [M+ - Me]). Anal. Calcd for C30H44AlClN2
(Mr ) 495.13): C, 72.77; H, 8.96; N, 5.66. Found: C, 72.32; H,
8.74; N, 5.71.
Synthesis of LAlClI (2). A toluene solution (80 mL) of 1
(7.43 g, 15 mmol) and I2 (3.81 g, 15 mmol) was stirred for 5
days. The solution was concentrated (ca. 20 mL) and kept at
4 °C for 24 h. Very light yellow crystals of 2 were formed and
collected (5.5 g). The mother liquor was concentrated (ca 5 mL)
to afford a second crop of 2 (1.7 g). Total yield: 7.2 g (79%).
1
Results and Discussion
Mp: 200-203 °C. H NMR (300.13 MHz, C6D6, 298 K, ppm):
δ 1.02 (d, 2 × 3 H, 3JHH ) 6.8 Hz, CH(CH3)2), 1.12 (d, 2 × 3 H,
Compound LAlClI (2) was prepared according to the
procedures given in eqs 1 and 2. The toluene/n-hexane
solution of LLi prepared from LH and n-BuLi was
directly used for the reaction with AlCl2Me. Compound
LAlClMe (1) was obtained as a crystalline solid, and its
composition was confirmed by EI-MS and 1H NMR
measurements as well as by elemental analysis. The
reaction of 1 with 1 equiv of I2 within 5 days at room
temperature resulted in the formation of compound 2
as very light yellow crystals. Compound 2 melts at 200-
203 °C, and the EI-MS indicates its monomeric nature
3
3JHH ) 6.8 Hz, CH(CH3)2), 1.41 (d, 2 × 3 H, JHH ) 6.8 Hz,
3
CH(CH3)2), 1.42 (d, 2 × 3 H, JHH ) 6.8 Hz, CH(CH3)2), 1.52
3
(s, 2 × 3 H, â-Me), 3.28 (sept, 2 × 1 H, JHH ) 6.8 Hz,
CH(CH3)2), 3.64 (sept, 2 × 1 H, 3JHH ) 6.8 Hz, CH(CH3)2), 4.96
(s, 1 H, γ-CH), 7.04-7.14 (m, 6 H, Ar-H). EI-MS: m/z (%) 606
(1, [M+]), 571 (4, [M+ - Cl], 479 (100, [M+ - I]). Anal. Calcd
for C29H41AlClIN2 (Mr ) 606.97): C, 57.34; H, 6.81; Cl, 5.32;
I, 20.91; N, 4.62. Found: C, 58.00; H, 6.86; Cl, 5.14; I, 19.23;
N, 4.70.
Synthesis of [LAlCl(µ-OH)]2 (3). To a mixture of 2 (0.61
g, 1 mmol) and [CN(iPr)C2Me2N(iPr)] (:C, 0.18 g, 1 mmol) in
toluene (40 mL) at 0 °C was added distilled H2O (18 µL, 1
mmol). The suspension was allowed to warm to room temper-
ature and stirred for 12 h. The insoluble solid was removed
by filtration, and the filtrate was dried in vacuo and extracted
in the gas phase (m/z (%): 606 (1, [M+]), 571 (4, [M+
-
1
Cl], 479 (100, [M+ - I]). The H NMR spectrum of 2
shows two separated groups of septets (3.28, 3.64 ppm)
and four groups of doublets (1.02, 1.12, 1.41, 1.42 ppm)
(13) Qian, B.; Ward, D. L.; Smith, M. R.; III. Organometallics 1998,
17, 3070-3076.
(16) Sheldrick, G. M. SHELXS-90, Program for Structure Solution.
Acta Crystallogr., Sect. A 1990, 46, 467.
(17) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure
Refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(14) Budzelaar, D. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J. Inorg.
Chem. 1998, 1485-1494.
(15) Kuhn, N.; Kratz, T. Synthesis 1993, 561-562.

382 Organometallics, Vol. 24, No. 3, 2005
Zhu et al.
Table 1. Crystallographic Data for Compounds 2
and 3‚n-hexane
2
3‚n-hexane
formula
fw
C
29H41AlClIN2
C64H98Al2Cl2N2O2
1080.32
606.97
temp (K)
cryst syst
space group
a (Å)
133(2)
monoclinic
P2(1)/n
133(2)
monoclinic
P2(1)/c
12.594(1)
19.493(1)
13.502(1)
116.47(1)
2967(1)
13.567(2)
13.263(2)
17.359(5)
103.76(2)
3034(1)
b (Å)
c (Å)
â (deg)
V (Å3)
Z
4
2
Fc (Mg/m3)
1.359
1.183
µ (mm-1
F(000)
)
1.218
0.182
1248
1172
θ range (deg)
index ranges
1.84-24.86
-14 e h e 14
-22 e k e 22
-15 e l e 15
28 829
1.55-25.40
-16 e h e 16
-15 e k e 15
-20 e l e 20
46 371
Figure 1. Molecular structure of 2. Thermal ellipsoids are
drawn at the 50% level, and the hydrogen atoms of the L
ligand are omitted for clarity.
no. of reflns collected
no. of indep reflns (Rint
no. of data/restraints/
params
)
5005 (0.0343)
5005/0/317
5559 (0.0802)
5559/0/376
Table 2. Selected Bond Distances (Å) and Angles
(deg) for Compound 2
GoF/F2
1.092
0.923
R1a, wR2b(I > 2σ(I))
R1a, wR2b(all data)
largest diff peak/hole
0.0322, 0.0939
0.0368, 0.0959
0.853/-0.935
0.0338, 0.0729
0.0556, 0.0782
0.229/-0.190
Al(1)-N(1)
Al(1)-Cl(1)
1.879(3)
2.151(1)
Al(1)-N(2)
Al(1)-I(1)
1.863(3)
2.473(1)
(e‚Å-3
)
N(1)-Al(1)-N(2) 100.04(12) N(1)-Al(1)-Cl(1) 108.18(9)
N(1)-Al(1)-I(1)
N(2)-Al(1)-I(1)
115.35(9)
111.57(9)
N(2)-Al(1)-Cl(1) 109.17(9)
Cl(1)-Al(1)-I(1) 111.82(5)
2
2
a R ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo - Fc )2/∑w(Fo2)]1/2
.
for the respective isopropyl methine and methyl proton
resonances of the Ar substituents, evidencing the asym-
metric steric environment of the Ar isopropyl groups
induced by Al-Cl and Al-I.
equiv of 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene
(abbreviated as :C) in toluene solution from 0 °C to room
temperature (eq 3). The removal of solvent in vacuo
followed by extraction with n-hexane gave partial hy-
drolysis product [LAlCl(µ-OH)]2 (3) in 70% yield. The
mass spectrum of 3 indicates the monomeric composi-
tion (m/z (%): 496.3 (50, [M+/2]), 478.3 (16, [M+/2 - OH
1
- 1], 461.3 (100, [M+/2 - Cl]). However the H NMR
spectrum of 3 recorded at 200.13 MHz shows one septet
(3.40 ppm) for the isopropyl methine resonances of the
Ar substituents, while two groups of incompletely
separated septets (3.42, 3.47 ppm) were observed at
500.13 MHz. These are markedly different from those
observed in the NMR spectrum of compound 2 (3.28,
3.64 ppm) and imply a possible oligomerization in
solution. One singlet at 0.72 (in C6D6) and alternatively
at 0.69 (in D8-toluene) ppm could be assigned to the Al-
µ-OH proton resonance, comparable to the â-diketimi-
nato aluminum dihydroxide LAl(OH)2 (0.22 ppm).12 One
broad absorption centered at 3459 cm-1 corresponds to
νOH in the IR spectrum of 3.
The X-ray crystal structural analysis of 2 showed a
mononuclear compound with an aluminum center co-
ordinated to the chelating â-diketiminato ligand and one
Cl and one I atom in a distorted tetrahedral geometry.
The molecular structure of 2 is shown in Figure 1.
Selected bond lengths and angles are listed in Table 2.
The terminal Al-I bond length (2.473(1) Å) is a little
shorter than those in LAlI2 (2.501(3), 2.543(3) Å)18 and
close to those in [RAlI(µ-I)]2 (R ) 2,6-iPr2C6H3N(SiMe3))
(2.464(2) Å).19 The Al-Cl distance (2.151(1) Å) is in good
agreement with those in the terminal aluminum chlo-
rides [3,5-tBu2NCHdC(SiMe3)‚pz]AlCl2 (2.135(1) Å),
[3,5-tBu2NCHdC(SiMe3)‚pz]AlCl(3,5-tBu2pz) (2.153(1)
Å),20 and [(C7H13N2)AlCl]2(µ-O) (2.164(1) Å).5f
The stepwise hydrolysis of compound 2 was first
carried out with 1 equiv of H2O in the presence of 1
(18) The structural data for LAlI2 come from: Cui, C. Ph.D. Thesis,
Go¨ttingen, 2001.
(19) Schiefer, M.; Reddy, N. D.; Roesky, H. W.; Vidovic, D. Orga-
nometallics 2003, 22, 3637-3638.
(20) Zheng, W.; Mo¨sch-Zanetti, N. C.; Blunck, T.; Roesky, H. W.;
Noltemeyer, M.; Schmidt, H.-G. Organometallics 2001, 20, 3299-3303.
The X-ray structural analysis unambiguously con-
firms compound 3 as a dimer in the solid state, which

Hydrolysis of an Organoaluminum Compound
Organometallics, Vol. 24, No. 3, 2005 383
Scheme 1. Possible Formation of 3
results in the generation of [H:C]+I- as insoluble mi-
crocrystals,22 indicating the capability of proton abstrac-
tion of :C from the â-diketiminato ligand. A reference
experiment of :C with H2O in THF monitored by the
1H NMR spectrum shows no characteristic resonance
for [H:C]+ and, therefore, indicates the incapability of
:C to abstract a proton from uncoordinated H2O. Ac-
cordingly, we assume that the formation of 3 from the
reaction of 2, :C, and H2O may proceed through the
initial coordination of H2O to aluminum (Scheme 1).
This assumption is in accordance with the formation of
Al(OH2)6Cl3 from AlCl3 by adding controlled amounts
of water.23 Comparable examples are the triarylalumi-
num water adduct Mes3Al‚OH2‚nTHF7a and the struc-
turally characterized (C6F5)3Al‚OH2.24 The coordinated
H2O in A is deprotonated by :C to give the cation [H:C]+,
which subsequently combines with I- to form the
insoluble [H:C]+I-. The generation of [H:C]+I- rather
than [H:C]+Cl- apparently indicates the easier removal
of I- compared to that of Cl-.
Further hydrolysis of 3 was accomplished with 2 equiv
of H2O and :C (eq 4). As expected the aluminum
dihydroxide LAl(OH)2 (4) was formed, however in a
relatively low yield (25%). Such low yield might be due
to the complete hydrolysis of partial 3, which resulted
in an insoluble aluminum oxide or hydroxide, and HL,11
although a high yield was achieved by the direct
hydrolysis of LAlCl2 in the presence of :C.12 The
hydrolytic procedure to 4 is similar to that for the
conversion of 2 to 3, but during the reaction undergoes
a dissociation from dimeric 3 to monomeric 4 compared
to an association from monomeric 2 to dimeric 3 in the
first step of the hydrolysis.
Figure 2. Molecular structure of 3‚n-hexane. Thermal
ellipsoids are drawn at the 50% level, and the hydrogen
atoms of the L ligand and one molecular n-hexane are
omitted for clarity.
Table 3. Selected Bond Distances (Å) and Angles
(deg) for Compound 3‚n-hexane
Al(1)-N(1)
1.977(1)
2.189(1)
1.886(1)
90.05(6)
Al(1)-N(2)
Al(1)-O(1)
1.984(1)
1.875(1)
Al(1)-Cl(1)
Al(1)-O(1A)
N(1)-Al(1)-N(2)
N(1)-Al(1)-O(1)
N(2)-Al(1)-Cl(1)
N(1)-Al(1)-Cl(1)
151.96(6) N(1)-Al(1)-O(1A) 92.91(6)
101.75(5)
100.90(5) N(2)-Al(1)-O(1)
93.17(6)
N(2)-Al(1)-O(1A) 153.56(6) O(1)-Al(1)-O(1A) 72.46(7)
O(1)-Al(1)-Cl(1) 104.94(5) O(1A)-Al(1)-Cl(1) 104.18(5)
is consistent with its solution behavior; however, this
is in contrast to the EI-MS results. The molecular
structure of 3 is depicted in Figure 2. Selected bond
lengths and angles are listed in Table 3. Each Al center
is coordinated to one â-diketiminato ligand, one Cl, and
two OH groups and adopts a distorted tetragonal-
pyramidal geometry with the Cl atom located at the
apical position and 2 N and 2 O atoms forming the basal
plane (the least-squares plane ∆ ) 0.012 Å). The Al2O2
core is a perfectly planar four-membered ring due to the
symmetry. The two H atoms of the OH groups are
located within this plane (∆ ) 0.009 Å). The Al-OH
bond lengths (1.875(1), 1.886(1) Å) fall within the range
1.787(3)-2.086(4) Å observed for bridging aluminum
hydroxides6,7 and are longer than the terminal ones in
LAl(OH)2 (1.697(2) and 1.711(2) Å)11 and [LAl(OH)]2(µ-
O) (1.738(3) and 1.741(3) Å).21 The two Al-Cl bonds are
in trans position toward the Al2O2H2 plane with bond
distances (2.190(1) Å) slightly longer than that in 2.
Conclusion and Remarks
In summary we have reported on the synthesis of
LAlClI (2) and its stepwise hydrolysis in the presence
of 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (:C).
The difference in the bond strength of Al-I and Al-
Cl25 allows the controlled stepwise hydrolysis. The
Attempts to obtain monomeric 3 by recrystallization
from THF or by the use of THF as a solvent during the
preparation were not successful, only the dimer 2 being
formed. This behavior demonstrates the strong Lewis
acidic Al center in the monomeric LAlCl(OH), which
leads to an association despite the bulky ligand. This is
in contrast to the monomeric species LAlMe2,13 LAlI2,18
LAl(NH2)2,12 and LAlClI.
(22) Analytic data for compound [H:C]+I-: Mp 173-174 °C. 1H NMR
(500.13 MHz, CD3CN, 298 K, ppm): δ 1.51 (d, 4 × 3 H, 3JHH ) 6.7 Hz,
3
CH(CH3)2), 2.23 (s, 2 × 3 H, C(CH3)), 4.49 (sept, 2 × 1 H, JHH ) 6.7
Hz, CH(CH3)2), 8.63 (s, 1 H, H:C). EI-MS m/z (%): 181 (100, [H:C]+),
128 (20, [I-]). Anal. Calcd for C11H21IN2 (Mr ) 308.1): C, 42.87; H,
6.87; N, 9.09; I, 41.17. Found: C, 43.19; H, 6.83; N, 8.95; I, 40.45. The
proton source of [H:C]+I- might come from the â-methyl groups of the
L ligand backbone. Related studies of deprotonation of such methyl
group are shown in: (a) Ding, Y.; Hao, H.; Roesky, H. W.; Noltemeyer,
M.; Schmidt, H.-G. Organometallics 2001, 20, 4806-4811. (b) Ding,
Y.; Ma, Q.; Roesky, H. W.; Herbst-Irmer, R.; Uso´n, I.; Noltemeyer, M.;
Schmidt, H.-G. Organometallics 2002, 21, 5216-5220. (c) Harder, S.
Angew. Chem. 2003, 115, 3553-3556; Angew. Chem., Int. Ed. 2003,
42, 3430-3434.
It is interesting to mention that a direct interaction
between 2 and N-heterocyclic carbene :C in THF solvent
(23) Holleman A. F.; Wiberg, E.; Wiberg, N. Lehrbuch der Anorga-
nischen Chemie Walter de Gruyter; Berlin, 1995; p 1075.
(24) Chakraborty, D.; Chen, E. Y.-X. Organometallics 2003, 22, 207-
210.
(21) Bai, G.; Roesky, H. W.; Li, J.; Noltemeyer, M.; Schmidt, H.-G.
Angew. Chem. 2003, 115, 5660-5664; Angew. Chem., Int. Ed. 2003,
42, 5502-5506.

384 Organometallics, Vol. 24, No. 3, 2005
Zhu et al.
deprotonation of the coordinated H2O by :C with the
formation of the insoluble [H:C]+I- or [H:C]+Cl- salts
leads to [LAlCl(µ-OH)]2 (3) and LAl(OH)2 (4), respec-
tively, and demonstrates the powerful acceptor proper-
ties of the N-heterocyclic carbene. Accordingly, we were
able to confirm this stepwise process, since there are
no structurally characterized precedents known con-
taining the Al(OH)Cl moiety, although this type of
intermediate is assumed in any hydrolysis product of
AlCl3. However, the stepwise hydrolysis from 2 to 3 and
finally to 4 is accompanied by an association followed
by a dissociation process of LAlCl(OH) species. This
observation is beyond our scope of the direct hydrolysis
of LAl(Hal)2 (Hal ) I, Cl) to 4,11 although it is not clear
whether this pathway is involved in the latter forma-
tion. This may reflect the complexity of the stepwise
hydrolysis process. Moreover the advantage of the
[H:C]+X- (X ) Cl, I) is that it can be completely
recovered by filtration and in the presence of a strong
base such as tBuOK quantitatively recycled to the
N-heterocyclic carbene. This method of hydrolysis in the
presence of N-heterocyclic carbene will be further ap-
plied to the preparation of Bro¨nsted acidic sensitive
systems.23 Compound 3 contains Al-Cl and Al-OH
functionalities, of which the former can react by me-
tathesis, and the OH acting as a Bro¨nsted acidic proton
has been documented.21 Further investigations on the
functional reactions of 3 are in progress.
Acknowledgment. We are grateful for the financial
support by the Deutsche Forschungsgemeinschaft and
the Go¨ttinger Akademie der Wissenschaften.
Supporting Information Available: CIF files for com-
pounds 2 and 3‚n-hexane are available free of charge via the
Internet at http://pubs.acs.org.
(25) D°298(Al-I) ) 369.9 ( 2.1 kJ/mol; D°298(Al-Cl) ) 511.3 ( 0.9
kJ/mol. Handbook of Chemistry and Physics; Lide, D. R.. Ed.; 2003-
2004, Vol. 84, pp 9-19.
OM049339A