Synthesis and properties of calcium tetraorganylalanates with [Me 4-nAlPhn]- anions

Article abstract of DOI:10.1021/om800509p

Triphenylalane yields in THF or Et₂O the corresponding ether complexes [(thf)AlPh₃] (1a) and [(Et₂O)-AlPh₃] (1b). The reaction of these triphenylalanes with phenylcalcium iodide in THF yielded quantitatively [(thf)₅CaI][AlPh₄] (2), which can be recrystallized from diethyl ether/THF mixtures without ether exchange reactions. The reaction of PhCa(thf)₄I with trimethylalane in THF in an equimolar ratio leads to the formation of solvent-separated [(thf) ₆Ca][AlMe₃Ph]₂ (5), which immediately shows ligand redistribution. Therefore, a fractionated crystallization gives [(thf)₆Ca][AlMe₂Ph₂]₂ (4) at 4°C, [(thf)₄CaI₂] at -20°C, and after reduction of the volume of the mother liquor [(thf)₆Ca][AlMe₃Ph] ₂ (5) at -40°C and [(thf)₆Ca][AlMe₄] ₂ (6) at -78°C. The formation of (thf)₄Cal₂ confirms that a Schlenk equilibrium is operative besides the ligand redistribution reactions. A solution of crystalline [(thf)₆Ca] [AlMe₂Ph₂]₂ (4) in THF shows 4 as the major component besides [(thf)₆Ca][AlMe₃Ph]₂ (5) and [(thf)₆Ca][AlMePh₃]₂ (3). With an increasing number of methyl groups the melting points decrease from 210°C for the tetraphenylalanate 2 to 20°C for the tetramethylalanate 6.

Full text of DOI:10.1021/om800509p

5052
Organometallics 2008, 27, 5052–5057
Synthesis and Properties of Calcium Tetraorganylalanates with
[Me4-nAlPhn]- Anions
Sven Krieck, Helmar Go¨rls, and Matthias Westerhausen*
Institute of Inorganic and Analytical Chemistry, Friedrich-Schiller-UniVersita¨t Jena,
August-Bebel-Strasse 2, D-07743 Jena, Germany
ReceiVed June 3, 2008
Triphenylalane yields in THF or Et2O the corresponding ether complexes [(thf)AlPh3] (1a) and [(Et2O)-
AlPh3] (1b). The reaction of these triphenylalanes with phenylcalcium iodide in THF yielded quantitatively
[(thf)5CaI][AlPh4] (2), which can be recrystallized from diethyl ether/THF mixtures without ether exchange
reactions. The reaction of PhCa(thf)4I with trimethylalane in THF in an equimolar ratio leads to the
formation of solvent-separated [(thf)6Ca][AlMe3Ph]2 (5), which immediately shows ligand redistribution.
Therefore, a fractionated crystallization gives [(thf)6Ca][AlMe2Ph2]2 (4) at 4 °C, [(thf)4CaI2] at -20 °C,
and after reduction of the volume of the mother liquor [(thf)6Ca][AlMe3Ph]2 (5) at -40 °C and
[(thf)6Ca][AlMe4]2 (6) at -78 °C. The formation of (thf)4CaI2 confirms that a Schlenk equilibrium is
operative besides the ligand redistribution reactions. A solution of crystalline [(thf)6Ca][AlMe2Ph2]2 (4)
in THF shows 4 as the major component besides [(thf)6Ca][AlMe3Ph]2 (5) and [(thf)6Ca][AlMePh3]2 (3).
With an increasing number of methyl groups the melting points decrease from 210 °C for the
tetraphenylalanate 2 to 20 °C for the tetramethylalanate 6.
reaction of Ca, HgR′2, and AlR3 (R ) Et, Pr, Ph; R′ ) Et, Pr,
Ph, Tol)9,10 as well as the metathesis reaction of CaCl2 with
Na(AlEt4).9 Transmetalation reactions of freshly prepared Ca
turningswithAlEt3 alsoofferedasuitableaccesstoCa(AlEt4)2.11,12
The reaction of Ca(AlEt4)2 (prepared from Ca, HgPh2, and
AlEt3)10 with benzophenone yielded at room temperature the
Ph2CO adduct, whereas at 90 °C in dodecane triphenylmethanol
was formed.13 From NMR experiments it was deduced that
Ca[AlEt4]2 exists as an equilibrium between the ions Ca2+/2
Introduction
Alanates have attracted the interest of many research groups
for many decades, and the first review articles appeared already
more than 40 years ago.1,2 Recent review articles summarize
the latest developments in this field.3,4 On one hand, the
formation of “ate” complexes of the type M(M′Rn) with M being
a more electropositive metal than M′ can lead to an increased
reactivity compared to the starting homonuclear organometallics
MR and M′Rn-1. Already in 1974, Chastrette and Gauthier
reported that the “ate” complex Ca(ZnR4) is much more reactive
toward ketones than either CaR2 or ZnR2.5 On the other hand,
lower reactivity was observed for Li(AlPh4) compared with
phenyllithium and triphenylalane.2 Wheras LiPh and AlPh3 form
triphenylmethanol with benzophenone, no reaction is observed
between Li(AlPh4) and benzophenone. Already in 1913 triph-
enylalane, with a melting point of 196-200 °C, was prepared
from HgPh2 and Al chips in the absence of air and moisture.6
A tetraphenylalanate was synthesized via the addition of
phenyllithium to AlPh3, yielding [(Et2O)2Li][AlPh4] or [(thf)Li]-
[AlPh4] depending on the solvent.7
-
AlEt4 and the trinuclear molecule [Ca{(µ-Et)2AlEt2}2].14
Molecular [R2NCa(µ-CH2R)2Al(CH2R)2]2 (R ) SiMe3) formed
from the reaction of [Ca(NR2)2]2 with Al(CH2R)3.15 Incorpora-
tion of additional donor atoms supports the formation of
molecular calcium bisalanates as shown for [Ca{(µ-2-Py)3-
AlMe}2] with a hexacoordinate calcium center.16 Solvent-
separatedionswereobtainedfromthereactionof(dme)2Ca(NPh2)2
and AlMe3 in tetrahydrofuran, yielding [(thf)6Ca][Me3Al-
NPh2]2.17
The availability of phenylcalcium iodide,18-22 our interest
in heterobimetallic compounds,4 and the interesting and
unexpected properties of reported calcium bisalanates1,2
Calcium bis(tetraorganylalanates) have also been known for
many decades. Lehmkuhl and Eisenbach prepared Ca(AlEt4)2
from AlEt3 and calcium bis(alkoxides);8 this pyrophoric calcium
bisalanate showed nearly no conductivity and a surprising low
melting point of 40 °C. Other synthetic procedures include the
(9) Lehmkuhl, H.; Eisenbach, W. Liebigs Ann. Chem. 1967, 705, 42–
53.
(10) Ivanov, L. L.; Zavizion, S. Y.; Zakharkin, L. I. Zh. Obshchei Khim.
1975, 45, 1060–1065.
(11) Ivanov, L. L.; Zavizion, S. Y.; Zakharkin, L. I. Synth. Inorg. Met.
Org. Chem. 1973, 3, 323–327.
(12) Ivanov, L. L.; Zavizion, S. Y.; Zakharkin, L. I. Zh. Obshchei Khim.
1973, 43, 2254–2258.
(13) Zakharkin, L. I.; Ivanov, L. L.; Zavizion, S. Y.; Sorokina, T. G.
IzV. Akad. Nauk SSSR 1976, 2557–2560.
(14) Bresler, L. S.; Kulvelis, J.; Lubnin, A. V. Zh. Obshchei Khim. 1984,
54, 1306–1312.
(15) Westerhausen, M.; Birg, C.; No¨th, H.; Knizek, J.; Seifert, T. Eur.
J. Inorg. Chem. 1999, 220, 9–2214.
(16) Garcia, F.; Hopkins, A. D.; Kowenicki, R. A.; McPartlin, M.;
Rogers, M. C.; Silvia, J. S.; Wright, D. S. Organometallics 2006, 25, 2561–
2568.
* Corresponding author. Fax: +49 (0) 3641 9-48-102. E-mail: m.we@
uni-jena.de.
(1) Lehmkuhl, H. Angew. Chem. 1963, 75, 1090–1097.
(2) Tochtermann, W. Angew. Chem. 1966, 78, 355–375.
(3) Linton, D. J.; Schooler, P.; Wheatley, A. E. H. Coord. Chem. ReV.
2001, 223, 53–115.
(4) Westerhausen, M. Dalton Trans. 2006, 4755–4768.
(5) Chastrette, M.; Gauthier, R. J. Organomet. Chem. 1974, 71, 11–15.
(6) Hilpert, S.; Gruttner, G. Ber. Dtsch. Chem. Ges. 1913, 45, 2828–
2832.
(7) Wittig, G.; Bub, O. Liebigs Ann. Chem. 1950, 566, 113–129.
(8) Lehmkuhl, H.; Eisenbach, W. Angew. Chem. 1962, 74, 779–780.
(17) Krieck, S.; Go¨rls, H.; Westerhausen, M. Inorg. Chem. Commun
2008, 11, 911–913.
10.1021/om800509p CCC: $40.75
2008 American Chemical Society
Publication on Web 09/06/2008

Synthesis of Calcium Tetraorganylalanates
Organometallics, Vol. 27, No. 19, 2008 5053
Table 1. 27Al{1H} NMR Data (chemical shift δ(27Al) and half-height
width ω1/2) of the Alanate Anions of the Type [Me4-nAlPhn]- [n ) 4
(2), 3 (3), 2 (4), 1 (5), and 0 (6)] ([D8]THF solutions, 25°C)a
initiated our reinvestigation of calcium bisalanates of the type
[Ca(Me4-nAlPhn)2][n ) 4 (2), 3 (3), 2 (4), 1 (5), and 0 (6)].
For comparison reasons the molecular structures of [(L)-
AlPh3] (1) were determined. In the past only homoleptic
alanate anions were prepared with one exception employing
the reaction of Ca, HgR′2, and AlR3.10
alanate
(AlPh3)2
compd
cation
δ(27Al) (ppm)
ω1/2 (Hz)
140
148
147
132.9
138.8
144.6
149.8
154.4
3700
3300
3300
31
91
91
[(thf)AlPh3]
[(Et2O)AlPh3]
[AlPh4]-
1a
1b
2
3
4
5
6
[(thf)5CaI]+
[(thf)6Ca]2+
[(thf)6Ca]2+
[(thf)6Ca]2+
[(thf)6Ca]2+
[AlMePh3]-
[AlMe2Ph2]-
[AlMe3Ph]-
[AlMe4]-
Results and Discussion
73
36
Synthesis. Triphenylalane was dissolved in THF or Et2O,
yielding the corresponding ether complexes [(thf)AlPh3] (1a)
and [(Et2O)AlPh3] (1b). The reaction of these organoalu-
minium compounds with phenylcalcium iodide in THF
yielded quantitatively the tetraphenylalanates (eq 1). The
solvent-separated ion pair [(thf)5CaI][AlPh4] (2) can be
recrystallized from diethyl ether/THF mixtures without ether
exchange reactions. In order to investigate equilibriums in
solution, heteroleptic alanates containing methyl and phenyl
groups were synthesized.
a For comparison reasons the parameters of AlPh3 and its ether ad-
ducts are included as well.
lanate 6. This low melting point for a calcium bis(tetralkyla-
lanate) is in agreement with the findings of Lehmkuhl and
Eisenbach.8
27Al{1H} NMR Spectroscopy. The 27Al{1H} NMR param-
eters of the compounds described above are summarized in
Table 1. The chemical shift correlates with the coordination
geometry of the complex and therefore indicates the coordination
number of the aluminum centers in solution. Due to this fact,
the chemical shifts of the 27Al{1H} resonances clearly lie in
the characteristic range of 125-180 ppm for compounds with
tetracoordinate aluminum atoms.23-25 Due to the rather high
symmetry, the half-height widths ω1/2 of the alanate anions are
rather small, whereas the dimer (AlPh3)2 or the ether adducts
of AlPh3 show ω1/2 values of more than 3000 Hz. Furthermore,
the half-height widths are dependent on the bulkiness of the
substituents at aluminum.24,25 The 27Al{1H} NMR experiments
of [(thf)5CaI][AlPh4] (2) showed a chemical shift of δ(27Al) )
132.9 with a half-height width of only 31.3 Hz. In order to
confirm that this compound consists of solvent-separated ion
pairs in solution and that therefore the nature of the cation is
irrelevant, [Li(AlPh4)] was prepared for comparison reasons and
dissolved in DME and THF; this solution also showed a value
of δ(27Al) ) 132.9 with a half-height width of 20 Hz and a
The reaction of PhCa(thf)4I with trimethylalane in THF in a
stoichiometric ratio of 1:1 led to the formation of the solvent-
separated addition product [(thf)6Ca][AlMe3Ph]2 (5) (eq 2),
which immediately shows a ligand redistribution. Therefore,
other alanate anions were also present. A fractionated crystal-
lization gave [(thf)6Ca][AlMe2Ph2]2 (4) at 4 °C. Further cooling
of the mother liquor to -20 °C after removal of 4 led to
precipitation of [(thf)4CaI2]. After reduction of the solvent and
cooling to -40 °C [(thf)6Ca][AlMe3Ph]2 (5) crystallized. Finally,
cooling of the mother liquor to -78 °C afforded the precipitation
of [(thf)6Ca][AlMe4]2 (6). The formation and isolation of (thf)4-
CaI2 confirmed that a Schlenk equilibrium was operative in these
solutions besides the ligand redistribution reactions.
1
coupling constant of J(27Al-13C) ) 94 Hz. The exchange of
phenyl groups by methyl substituents leads to a gradual low-
field shift of the 27Al{1H} NMR resonances of approximately
5 ppm. In Figure 1 the 27Al{1H} NMR spectra of [D8]THF
solutions of crystalline [(thf)6Ca][AlMe2Ph2]2 (4) and [(thf)6Ca]-
[AlMe3Ph]2 (5) are displayed. The relative intensities as shown
in these spectra remain unchanged and indicate that an equi-
librium between these species is operative.
Molecular Structures. For comparison reasons and in order
to evaluate the influence of negative charge on the structural
parameters of the alanates, the crystal structures of [(thf)AlPh3]
(1a) (Figure 2) and [(Et2O)AlPh3] (1b) were determined. The
centrosymmetric dimer Ph2Al(µ-Ph)2AlPh2 contains three-center
two-electron Al-C-Al bonds with a rather large average Al-C
bond length of 2.182 Å, whereas the average Al-C distance of
A solution of crystalline [(thf)6Ca][AlMe2Ph2]2 (4) in THF
showed 4 as the major component; however, also minor amounts
of [(thf)6Ca][AlMe3Ph]2 (5) and [(thf)6Ca][AlMePh3]2 (3) were
present (eq 3). Thus, the whole series of calcium bisalanates of
the type [Ca(Me4-nAlPhn)2] with n ) 4 (2), 3 (3), 2 (4), 1 (5),
and 0 (6) were obtained and investigated. With an increasing
number of methyl groups the melting points decrease from 210
°C for the tetraphenylalanate 2 to 20 °C for the tetramethyla-
(18) (a) Westerhausen, M.; Ga¨rtner, M.; Fischer, R.; Langer, J. Angew.
Chem. 2007, 119, 1994–2001; Angew. Chem. Int. Ed. 2007, 46, 1950-
1956.
(19) Westerhausen, M. Chem.-Eur. J. 2007, 13, 6292–6306.
(20) Westerhausen, M. Coord. Chem. ReV 2008, 252, 1516–1531.
(21) Fischer, R.; Ga¨rtner, M.; Go¨rls, H.; Westerhausen, M. Organome-
tallics 2006, 25, 3496–3500.
(23) Benn, R.; Rufinska, A.; Lehmkuhl, H.; Janssen, E.; Kru¨ger, C.
Angew. Chem. 1983, 95, 808–809; Angew. Chem., Int. Ed. Engl. 1983, 22,
774-775.
(24) Benn, R.; Rufinska, A. Angew. Chem. 1986, 98, 851–971; Angew.
Chem., Int. Ed. Engl. 1986, 25, 861-881.
(22) Ga¨rtner, M.; Go¨rls, H.; Westerhausen, M. Synthesis 2007, 725–
730.
(25) Benn, R.; Janssen, E.; Lehmkuhl, H.; Rufinska, A. J. Organomet.
Chem. 1987, 333, 155–168.

5054 Organometallics, Vol. 27, No. 19, 2008
Krieck et al.
Figure 3. Molecular structure of [(thf)5CaI][AlPh4] (2). The ell-
ipsoids represent a probability of 40%. The H atoms are omitted
for clarity reasons.
Figure 1. 27Al{1H} NMR spectra of [D8]THF solutions of [(thf)6Ca]-
[Me3AlPh]2 (5) at the top and of [(thf)6Ca][Me2AlPh2]2 (4) at the
bottom (25 °C, 104.28 MHz).
Table 2. Selected Structural Parameters (bond lengths [Å], angles
[deg]) of the THF and Et2O adducts of AlPh3 and of the
Tetraorganylalanates [Me4-nAlPhn]- [n ) 4 (2), 2 (4), and 1 (5)] of
Calcium
1a
1b
2
4
5
Al-CPh
Al-CMe
1.976(2)
1.981(2)
1.982(2)
1.978(2)
1.984(2)
1.985(2)
2.006(4)
2.011(4)
2.017(4)
2.020(3)
2.029(3)
2.031(2)
2.021(2)
1.998(2)
2.003(3)
1.997(2)
1.999(2)
2.006(2)
Al-O
1.893(1)
104.0
1.920(1)
103.6
O-Al-
a
CPh
CPh-Al- 114.4
114.6
109.5
111.8(1)
a
CPh
CMe-Al-
114.9(1)
107.6
111.0
a
CMe
CPh-Al-
107.9
233.9
a
CMe
Figure 2. Molecular structure [(thf)AlPh3] (1a). The ellipsoids
represent a probability of 40%; H atoms are omitted for clarity
reasons.
Ca-Oa
Ca-I
236.4
305.81(8)
234.1
a Average value if no esd is given.
the terminally bound phenyl groups shows a value of 1.958 Å.26
The tetrahedral environment of the tetracoordinate aluminum
atoms is distorted due to the small endocyclic C-Al-C bond
angle of 103.5°. In the ether adducts 1a and 1b average Al-C
bond lengths of 1.979 and 1.982 Å, respectively, are found.
Comparable values were also published for the Et2O adduct of
tri(o-tolyl)alane (Al-C 1.998(3) Å, Al-O 1.928(3) Å)27 and
for the complex [Ph3Al · P(SiMe3)3] (Al-C 1.990(5) Å, Al-P
2.514(2) Å).28 The Al-O distances of the THF and diethyl ether
adducts 1a (Al-O1 1.893(1) Å) and 1b (Al-O1 1.920(1) Å)
differ slightly, in agreement with the larger basicity (e.g.,
Gutmann donor number)29 of THF and the larger steric demand
of Et2O. In both complexes the C-Al-C bond angles are more
than 10° larger than the O-Al-C values, in agreement with
the VSEPR model.30 Enhanced steric strain in [(thf)AlMes3]
(Mes ) 2,4,6-trimethylphenyl, mesityl) leads to larger Al-O
and Al-C distances of 1.969(5) and 2.017(7) Å, respectively.31
These triaryl(tetrahydrofuran)alanes were successfully employed
in catalytic arylation of aldehydes32 and palladium-mediated
cross-coupling reactions.33
Molecular structure and numbering scheme of [(thf)5CaI]-
[AlPh4] (2) are represented in Figure 3. The [(thf)5CaI] cation
contains a hexacoordinate calcium atom in an octahedral
environment. The Ca-I bond length of 3.0581(8) Å is smaller
than that observed for (thf)4CaI234,35 and for (thf)4Ca(Aryl)I21,36
with hexacoordinate calcium atoms. Larger coordination num-
bers lead to larger Ca-I distances, as observed, for example,
for (dme)2(thf)CaI2.37 The average Ca-O distance of 2.364 Å
in 2 lies in the characteristic range.
The tetraphenylalanate anion contains a tetrahedrally coor-
dinated aluminum atom with an average Al-C bond length of
2.014 Å (Table 2). The elongation results from steric and
electrostatic repulsion between the four phenyl anions. In the
anion of [Cp2Ta(CH2)(Me)Al(C6F5)2][Al(C6F5)4] similar Al-C
bond lengths of 2.016(2) Å were observed.38 Despite different
acceptor abilities of AlPh3 and Al(C6F5)3 (complexation energies
(32) Wu, K.-H.; Gau, H.-M. J. Am. Chem. Soc. 2006, 128, 14808–14809.
(33) Ku, S.-L.; Hui, X.-P.; Chen, C.-A.; Kuo, Y.-Y.; Gau, H.-M. Chem.
Commun. 2007, 384, 7–3849.
(34) Tesh, K. F.; Burkey, D. J.; Hanusa, T. P. J. Am. Chem. Soc. 1994,
116, 2409–2417.
(26) Malone, J. F.; McDonald, W. S. J. Chem. Soc., Dalton Trans. 1972,
2646–2648.
(27) Barber, M.; Liptak, D.; Oliver, J. P. Organometallics 1982, 1, 1307–
1311.
(28) Laske Cooke, J. A.; Rahbarnooki, H.; McPhail, A. T.; Wells, R. L.
Polyhedron 1996, 15, 3033–3044.
(35) Henderson, K. W.; Rood, J. A.; Noll, B. C. Acta Crystallogr. 2005,
E61, m2006–m2007.
(36) Fischer, R.; Ga¨rtner, M.; Go¨rls, H.; Westerhausen, M. Angew. Chem.
2006, 118, 624–627; Angew. Chem. Int. Ed. 2006, 45, 609612.
(37) Ga¨rtner, M.; Go¨rls, H.; Westerhausen, M. Acta Crystallogr. 2007,
E63, m3169.
(29) Gutmann, V.; Resch, G.; Linert, W. Coord. Chem. ReV. 1982, 43,
133–164.
(30) Gillespie, R. J. Chem. Soc. ReV. 1992, 21, 59–69.
(31) De Mel, V. S. J.; Oliver, J. P. Organometallics 1989, 8, 827–830.

Synthesis of Calcium Tetraorganylalanates
Organometallics, Vol. 27, No. 19, 2008 5055
alanate anions (Me4-nAlPhn)- are observed. The ratio of phenyl
to methyl groups strongly influences the melting point, leading
to larger values for phenyl-rich alanates. All these calcium
bis(alanates) form yellow solutions with THF, and even at
elevated temperatures, no ether cleavage reactions are observed.
This observation is in contrast to the “simple” phenylcalcium
iodides, which cleave THF at rather low temperatures.18-22,41
In the solid state and in solution, solvent-separated ions with
tetrahedrally coordinated aluminum atoms are observed. In the
27Al{1H} NMR spectra a decreasing number of phenyl groups
leads to a low-field shift of the resonances. In addition, the
signals become narrower with a decreasing number of phenyl
groups. However, the smallest half-height widths were observed
for the homoleptic alanates [AlPh4]- of 2 and [AlMe4]- of 6.
The tetraorganylalanate anions show only slightly distorted
tetrahedrally coordinated aluminum atoms. The Al-C bond
lengths are elongated compared to the neutral ether adducts of
AlPh3. The cations consist of calcium cations in octahedral
environments. The Ca-O distances of [(thf)6Ca]2+ exhibit
values around 2.34 Å, whereas the substitution of one THF
ligand by an iodide anion leads to an elongation of the remaining
Ca-O bonds of [(thf)5CaI]+ due to the reduced overall charge
and, hence, reduced electrostatic attraction between Ca and O.
Figure 4. Molecular structure and numbering scheme of [(thf)6Ca]-
[Me2AlPh2]2 (4). Symmetry-related atoms (-x, -y, -z+1) are
marked with “A”. The ellipsoids represent a probability of 40%.
Hydrogen atoms are not shown for clarity reasons.
Figure 5. Molecular structure and numbering scheme of [(thf)6Ca]-
[Me3AlPh]2 (4). Symmetry-related atoms (-x+1, -y, -z) are
marked with “A”. The ellipsoids represent a probability of 40%.
Hydrogen atoms are omitted for clarity reasons.
Experimental Section
General Remarks. All manipulations were carried out under
an argon atmosphere using standard Schlenk techniques. The
solvents were dried according to common procedures and distilled
under argon; deuterated solvents were dried over sodium, degassed,
and saturated with argon. The 1H and 13C{1H} NMR spectra were
obtained on a Bruker AC 400 MHz spectrometer. 27Al{1H} NMR
spectra were recorded on a Bruker Avance 400 at 104.27 MHz,
and the spectra were externally referenced to a saturated [D8]toluene
solution of AlCl3, at δ ) 100.0. Mass spectra were obtained on a
Finnigan MAT SSQ 710 system, and IR measurements were carried
out using a Perkin-Elmer System 2000 FTIR. The IR spectra were
taken as Nujol mulls between KBr windows. Melting and decom-
position points were measured with a Reichert-Jung apparatus type
302102 and are uncorrected. For the elemental analysis V2O5 was
added to the samples in order to enhance combustion; nevertheless,
carbon values are low most probably due to carbonate formation
during combustion. AlMe3 (2 M toluene solution) was purchased
from Alfa Aesar. PhCaI(thf)4,21,22 Li(AlPh)4,7 [(thf)AlPh3] (1a),42
and [(Et2O)AlPh3] (1b)43 were prepared according to literature
procedures.
Synthesis of (Et2O)AlPh3 (1b) and AlPh3. To a stirred sus-
pension of AlCl3 (74.98 mmol, 10.0 g; freshly sublimate) in Et2O
(50 mL) at -78 °C was slowly added a solution of PhLi (249.92
mmol, 142.0 mL, 1.76 M) in Et2O. The mixture was stirred at -78
°C for 30 min and then allowed to warm to room temperature. The
mixture was stirred for an additional 4 h and filtered. The volume
of the filtrate was reduced to one-quarter and cooled to -40 °C.
All solid materials were collected on a frit and extracted with
toluene. Storage of the mother liquor at -40 °C led to precipitation
of large colorless needles. Filtration and drying under vacuum at
50 °C yielded 11.23 g (33.78 mmol, 44%) of (Et2O)AlPh3 (1b).
-
for AlPh3 with H2O and CH3 63.8 and 401.1 kJ mol-1, for
-
Al(C6F5)3 with H2O and CH3 112.0 and 521.5 kJ mol-1 39
)
very similar bond lengths of the Al-C bonds in these alanate
anions were observed and can be explained by steric repulsion.
In the contact ion pair [Ca{(µ-2-Py)3AlMe}2] comparable Al-C
distances (av value 2.021 Å) to the pyridyl moieties were found,
whereas the Al-CMe distance showed a smaller value of
1.978(2) Å.16
Molecular structure and numbering scheme of [(thf)6Ca]-
[AlMe2Ph2]2 (4) are displayed in Figure 4. The hexacoordinate
calcium atom shows an average Ca-O distance of 2.341 Å.
The Al-C bond lengths of the methyl groups (av Al-CMe 2.001
Å) of the dimethyldiphenylalanate anion are smaller than those
of the phenyl substituents (av Al-CPh 2.030 Å), which is in
agreement of the observation at [Ca{(µ-2-Py)3AlMe}2].16 In the
contact ion pair [R2NCa(µ-CH2R)2Al(CH2R)2]2 (R ) SiMe3),
average Al-C distances of 1.993 and 2.075 Å were found for
the terminal and bridging trimethylsilylmethyl groups, respec-
tively.15
Molecular structure and numbering scheme of [(thf)6Ca]-
[AlMe3Ph]2 (5) are shown in Figure 5. The distortions of the
anion are as expected. The methyl groups cause slightly larger
steric strain than phenyl groups, and therefore, the CMe-Al-CMe
bond angles are larger (av value 111.0°) than the average
CMe-Al-CPh angle of 107.9°. The Al-C4 bond length of
2.021(2) Å to the phenyl substituent is also larger than the
average Al-CMe value of 2.001 Å.
Summary
(38) Mariott, W. R.; Gustafson, L. O.; Chen, E. Y.-X. Organometallics
2006, 25, 3721–3729.
(39) Timoshkin, A. Y.; Frenking, G. Organometallics 2008, 27, 371–
380.
(40) Naka, H.; Uchiyama, M.; Matsumoto, Y.; Wheatley, A. E. H.;
McPartlin, M.; Morey, J. V.; Kondo, Y. J. Am. Chem. Soc. 2007, 129, 1921–
1930.
(41) Ga¨rtner, M.; Go¨rls, H.; Westerhausen, M. J. Organomet. Chem.
2008, 693, 221–227.
Alanates represent a compound class with enormous potential
for applications in organic synthesis.40 The calcium bis(tetraor-
ganylalanates) of the type [Ca(Me4-nAlPhn)2][n ) 4 (2), 3 (3),
2 (4), 1 (5), and 0 (6)] are accessible via the addition of
phenylcalcium iodide in THF to AlPh3 (yielding 2) or AlMe3
(yielding 5). The Schlenk equilibrium leads to the formation of
(thf)4CaI2 and the calcium bisalanates, whereas due to the
redistribution of the phenyl and methyl substituents all possible
(42) Mole, T. Aust. J. Chem. 1963, 16, 794–800.
(43) Mole, T. Aust. J. Chem. 1963, 16, 801–806.

5056 Organometallics, Vol. 27, No. 19, 2008
Krieck et al.
Table 3. Crystal Data and Refinement Details for the X-ray Structure Determinations of 1, 2, and 4
1a
1b
2
4
5
formula
fw (g · mol-1
T/°C
cryst syst
space group
a/Å
C22H23AlO
330.38
-90(2)
monoclinic
P21/c
9.5777(3)
12.7810(6)
16.0315(8)
90.00
105.089(3)
90.00
1894.80(14)
4
1.158
1.12
12 581
2782
4312/0.0571
0.1309
0.0501
1.020
0.245/-0.233
none
690007
C22H25Al O
332.40
-90(2)
monoclinic
P21/c
12.1260(4)
7.5520(3)
21.1946(9)
90.00
100.231(3)
90.00
1910.05(13)
4
1.156
1.11
13 073
3125
4352/0.0387
0.1238
0.0470
1.015
0.249/-0.209
none
690008
C48H68Al CaIO6
934.98
C52H80Al2 CaO6
895.20
-90(2)
monoclinic
P21/n
12.8483(4)
15.8241(5)
12.9309(4)
90.00
90.000(3)
90.00
2629.02(14)
2
1.131
1.97
18 456
4009
6025/0.0535
0.1461
0.0550
1.031
0.528/-0.510
none
C56H92Al2CaO6
955.34
)
-90(2)
-90(2)
monoclinic
P21/n
12.5891(3)
16.7864(3)
13.8631(3)
90.00
93.245(1)
90.00
2924.93(11)
2
1.085
1.81
20 524
5121
6676/0.0373
0.1254
0.0455
triclinic
j
P1
9.5544(3)
14.2629(5)
19.1425(5)
108.701(2)
102.449(2)
92.823(2)
2392.96(13)
2
1.298
8.39
16 920
7710
10 863/0.0331
0.1356
0.0535
0.961
1.172/-0.660
none
b/Å
c/Å
R/deg
ꢀ/deg
γ/deg
V/Å3
Z
F (g · cm-3
)
µ (cm-1
)
no. measd data
no. data with I > 2σ(I)
no. unique data/Rint
wR2 (all data, on F2)a
R1 (I > 2σ(I))a
sb
0.992
0.490/-0.312
none
res dens/e · Å-3
absorpt method
CCDC No.
690009
690010
690011
a Definition of the R indices: R1 ) (∑||Fo| - |Fc||)/∑|Fo|. wR2 ) {∑[w(Fo - Fc ) ]/∑[w(Fo2)2]}1/2 with w-1 ) σ2(Fo2) + (aP)2. b s ) {∑[w(Fo
-
2
2 2
2
Fc ) ]/(No - Np)}1/2
2 2
.
The solid was then dried under vacuum about 150 °C, extracted
with boiling toluene, and filtered off. The extract was stored at -40
°C, and colorless needles were grown, which were collected and
dried at 160 °C under vacuum for 2 h to yield 5.85 g of AlPh3
(22.65 mmol, 30%).
128.9 (2C, m-C), 138.8 (2C, o-C), 148.6 (1C, i-C). 27Al NMR
(104.28 MHz, 25 °C, [D6]benzene): δ 147.6 (w1/2 ) 3300 Hz). MS
(EI, m/z, [%]): 26 (C2H2) [80], 39 (C3H3) [70], 78 (PhH) [100],
176 (M - 2Ph) [10], 181 (M - Ph - thf) [8], 253 (M - Ph) [10].
IR (Nujol, KBr, cm-1): 2924 vs (br), 1954 w, 1880 m, 1577 m,
1457 s, 1419 s, 1377 m, 1299 w, 1248 m, 1189 w, 1153 w; 1084 s,
1042 m, 1001 m, 994 m, 955 w, 919 m, 851 vs, 732 s, 706 vs, 680
vs, 473 vs, 453 m.
Physical Data for 1b. Decomposition above 124 °C. Anal. Calcd
for C22H25AlO (332.42 g mol-1): Al 8.11. Found: Al 7.92. 1H NMR
(200.13 MHz, 25 °C, [D6]benzene): δ 1.13 (CH3, Et2O), 3.40 (CH2,
Et2O), 7.20-7.28 (9H, m, m-H + p-H), 7.77 (6H, dd, o-H). 13C{1H}
NMR (50.33 MHz, 25 °C, [D6]benzene): δ 15.6 (CH3, Et2O), 66.2
(CH2, Et2O), 127.6 (6C, m-C), 127.9 (3C, p-C), 138.8 (6C, o-C),
148.4 (3C, i-C). 27Al NMR (104.28 MHz, 25 °C, [D6]benzene): δ
147.6 (w1/2 ) 3300 Hz). MS (EI, m/z, [%]): 39 (C3H3) [15], 78
(PhH) [100], 181 (M - Ph - Et2O) [60], 255 (M - Ph) [30], 332
(M) [8]. IR (Nujol, KBr, cm-1): 2924 vs (br), 1954 w, 1880 m,
1577 m, 1457 s, 1419 s, 1377 m, 1299 w, 1248 m, 1189 w, 1153 w,
1084 s, 1042 m, 1001 m, 994 m, 955 w, 919 m, 851 vs, 732 s, 706
vs, 680 vs, 473 vs, 453 m.
Physical Data for AlPh3. Melting point 223 °C. Anal. Calcd
for C18H15Al (258.29 g mol-1): Al 10.45. Found: Al 10.13. 1H
NMR (400.25 MHz, 25 °C, [D6]benzene): δ 7.23 (3H, m, m-H,
p-H), 7.76 (2H, dd, 3JH-H ) 7.6 Hz, o-H). 13C{1H} NMR (100.65
MHz, 25 °C, [D6]benzene): δ 127.6 (1C, p-C), 127.9 (2C, m-C),
138.8 (2C, o-C), 148.3 (1C, i-C). 27Al NMR (104.28 MHz, 25 °C,
[D6]benzene): δ 140.3 (w1/2 ) 3700 Hz). MS (EI, m/z, [%]): 78
(PhH) [100], 104 (M - 2Ph) [24], 181 (M - Ph) [60], 258 (M)
[40]. IR (Nujol, KBr, cm-1): 2925 vs (br), 1958 w, 1889 w, 1836 w,
1774 w, 1595 w, 1578 m, 1465 vs, 1419 vs, 1388 s, 1377 s, 1323 m,
1283 w, 1246 m, 1192 m, 1148 m, 1086 vs, 1018 vs, 993 s, 888 s,
833 m, 722 vs, 728 vs, 704 vs, 680 vs.
Synthesis of [(thf)5CaI][AlPh4] (2). Solid AlPh3 (0.74 g, 2.86
mmol) was added in small portions within 30 min to a stirred
solution of PhCaI(thf)4 (33.0 mL, 2.87 mmol, 0.087 M) in THF at
-40 °C. Then the reaction mixture was warmed to room temper-
ature, stirred for additional 4 h, and filtered, and all volatiles were
removed under vacuum. The solid residue was washed with 30 mL
of toluene, dried under vacuum, and suspended in a mixture of 15
mL of Et2O and 20 mL of THF. The solvent was reduced until the
cloudy solution became clear. Storage at -40 °C led to the
precipitation of colorless prisms, which were collected on a cooled
frit and dried gently under vacuum, yielding 1.23 g of 2 (1.68 mmol,
59%). Decomposition above 210 °C. Anal. Calcd for
C44H60AlCaIO5 (862.91): C 61.24, H 7.01, Al 3.13, Ca 4,64. Found:
C 59.08, H 7.18, Al 2.92, Ca 4.25. 1H NMR (200.13 MHz, 25 °C,
3
[D8]THF): δ 1.74 (CH2, thf), 3.56 (CH2O, thf), 6.89 (1H, t, JH-H
) 7.4 Hz, p-H), 7.05 (2H, t, 3JH-H ) 7.3 Hz, m-H), 7.62 (2H, dd,
3JH-H ) 7.4 Hz, o-H). 13C{1H} NMR (50.33 MHz, 25 °C,
[D8]THF): δ 26.3 (CH2, thf), 68.2 (CH2O, thf), 126.8 (1C, p-C),
128.9 (2C, m-C), 138.9 (2C, o-C), 139.8 (1C, i-C). 27Al NMR
(104.28 MHz, 25 °C, [D6]benzene/[D8]THF ) 2:1): δ 132.9 (w1/2
) 31 Hz). MS (FAB-, m/z, [%]): 335 (AlPh4) [100]. MS (FAB+,
m/z, [%]): 210 ((thf)CaI-CO) [100], 238 ((thf)CaI) [20], 257
((thf)2Ca) [23], 283 ((thf)2CaI-CO) [50], 310 ((thf)2CaI) [25], 527
((thf)5CaI) [3]. IR (Nujol, KBr, cm-1): ν 3045 m, 2925 vs (br),
1459 s, 1417 m, 1377 m, 1244 w, 1180 w, 1075 m, 1023 s, 916 m,
870 m, 730 m, 706 s, 665 m, 475 m.
Synthesis of (thf)AlPh3 (1a). Solid AlPh3 (0.18 g, 0.70 mmol)
was dissolved in THF (5 mL) and stirred at room temperature for
1 h. Addition of Et2O (5 mL) and cooling at -40 °C yielded large
colorless needles, which were collected on a cooled frit and dried
gently under vacuum to obtain 0.21 g of 1a (0.64 mmol, 91%).
Physical Data for 1a. Mp: decomposition above 120 °C. Anal.
Calcd for C22H23AlO (330.41 g mol-1): C 79.97, H 7.02, Al 8.17.
Synthesis of [(thf)6Ca][AlMe2Ph2]2 (4), [(thf)6Ca][AlMe3Ph]2-
(PhMe)2 (5), and [(thf)6Ca][AlMe4]2 (6). A 2 M solution of AlMe3
in toluene (4.95 mL, 9.90 mmol, 2 M) was added dropwise to a
solution of PhCaI(thf)4 (30.0 mL, 9.90 mmol, 0.33 M) in THF at
-78 °C and stirred for 30 min. Then the mixture was warmed to
room temperature (at about -10 °C a clear solution formed) and
stirred for an additional 12 h. Reduction of the volume of the
reaction mixture to 20 mL, filtration, and storage of the yellowish
1
Found: C 77.86, H 6.89, Al 7.58. H NMR (200.13 MHz, 25 °C,
3
[D8]THF): δ 1.75 (CH2, thf), 3.61 (CH2O, thf), 7.23 (2H, t, JH-H
) 6.6 Hz, m-H), 7.26 (1H, td, 3JH-H ) 5.7 Hz, p-H), 7.76 (2H, dd,
3JH-H ) 7.6 Hz, o-H). 13C{1H} NMR (50.33 MHz, 25 °C,
[D8]THF): δ 25.6 (CH2, thf), 67.3 (CH2O, thf), 127.7 (1C, p-C),

Synthesis of Calcium Tetraorganylalanates
Organometallics, Vol. 27, No. 19, 2008 5057
solution at +4 °C led to precipitation of large colorless needles.
Collection on a cooled frit and drying gently under vacuum gave
1.07 g of 4 (1.20 mmol, 24%). Storage of the mother liquor at
-20 °C led to crystallization of 1.29 g of (thf)4CaI2 (2.22 mmol,
22%). After removal of this compound, the volume of the solution
was reduced to 4.5 mL. Cooling of this mother liquor to -40 °C
afforded the precipitation of 0.81 g (0.84 mmol, 19%) of 5. Further
cooling of the mother liquor at -78 °C led to formation of an
amorphous white powder of 6 (0.26 g, 0.04 mmol, 1%). The yields
refer to the amount of employed calcium.
Physical Data for 6. Mp: 20 °C (dec). Anal. Calcd for
C40H96Al2CaO6 (646.95 g mol-1): This compound is extremely air
and moisture sensitive and loses coordinated THF once isolated.
For this reason it was impossible to weigh out a definite amount
because the weight of the substance changed permanently during
1
handling and weighing in the glovebox. H NMR (200.13 MHz,
25 °C, [D6]benzene): δ -0.44 (24H, s, Al-CH3), 1.32 (CH2, thf),
3.53 (CH2O, thf). 13C{1H} NMR (50.33 MHz, 25 °C, [D6]benzene):
δ -8.2 (8C, broad, Al-CH3), 25.6 (CH2, thf), 68.4 (CH2O, thf).
27Al NMR (104.28 MHz, 25 °C, [D6]benzene): δ 155.0 ([AlMe4]-,
w1/2 ) 125 Hz). IR (Nujol, KBr, cm-1): 3040 w, 2923 vs, 2854
vs, 1460 s, 1417 w, 1377 m, 1296 w, 1243 w, 1165 w, 1071 m,
1028 s, 918 m, 874 s, 711 m, 674 m, 628 w, 612 w.
Physical Data for 4. Mp: 53 °C (dec). Anal. Calcd for
C52H80Al2CaO6 (895.23 g mol-1): This compound is extremely air
and moisture sensitive and loses coordinated THF once isolated.
For this reason it was impossible to weigh out a definite amount
because the weight of the substance changed permanently during
X-ray Structure Determinations. The intensity data for the
compounds 1a, 1b, 2, 4, and 5 were collected on a Nonius
KappaCCD diffractometer using graphite-monochromated Mo KR
radiation. Data were corrected for Lorentz and polarization effects
but not for absorption effects.44,45 The structures were solved by
direct methods (SHELXS)46 and refined by full-matrix least-squares
1
handling and weighing in the glovebox. H NMR (200.13 MHz,
25 °C, [D8]THF): δ -0.94 (6H, s, Al-CH3), 1.74 (CH2, thf), 3.61
(CH2O, thf), 6.80 (2H, t,3JH-H ) 6.8 Hz, p-H), 6.86 (4H, tt, 3JH-H
) 7.6 Hz, m-H), 7.53 (2H, d, 3JH-H ) 6.2 Hz, o-H). 13C{1H} NMR
(50.33 MHz, 25 °C, [D8]THF): δ -8.7 (2C, broad, Al-CH3), 25.2
(CH2, thf), 68.2 (CH2O, thf), 123.9 (2C, p-C), 125.7 (4C, m-C),
138.7 (4C, o-C), 151.1 (2C, i-C). 27Al NMR (104.28 MHz, 25 °C,
2
techniques against Fo (SHELXL-97)47 (Table 3). The hydrogen
atoms were included at calculated positions with fixed thermal
parameters. All non-hydrogen atoms were refined anisotropically.47
XP (SIEMENS Analytical X-ray Instruments, Inc.) was used for
structure representations.
[D8]THF):
δ ) 91 Hz), 144.6
138.8 ([AlMePh3]-, w1/2
([AlMe2Ph2]-, w1/2 ) 91 Hz), 149.8 ([AlMe3Ph]-, w1/2 ) 73 Hz),
154.4 ([AlMe4]-, w1/2 ) 36 Hz). MS (FAB-, m/z, [%]): 211
(AlMe2Ph2) [10], 335 [100]. MS (FAB+, m/z, [%]): 210 [100],
257 ((thf)2Ca) [22], 328 ((thf)4Ca) [18], 372 ((thf)5Ca-C2H6) [15].
IR (Nujol, KBr, cm-1): 3040 m, 2923 vs (br), 1460 s, 1417 m,
1377 m, 1344 w, 1296 w, 1243 m, 1165 m, 1071 m, 1019 s, 918 m,
868 s, 733 m, 711 s, 674 s, 628 m, 612 m, 528 m, 464 w.
Physical Data for 5. Mp: 34 °C (dec). Anal. Calcd for
C56H92Al2CaO6 (955.37 g mol-1): This compound is extremely air
and moisture sensitive and loses coordinated THF once isolated.
For this reason it was impossible to weigh out a definite amount
because the weight of the substance changed permanently during
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft (DFG, Bonn-Bad Godes-
berg, Germany) and the Fonds der Chemischen Industrie
(Frankfurt/Main, Germany). S.K. is grateful to the Verband
der Chemischen Industrie (VCI/FCI) for a Ph.D. grant.
Supporting Information Available: CIF files giving data
collection and refinement details as well as positional coordinates
of all atoms. This material is available free of charge via the Internet
at http://pubs.acs.org. In addition, crystallographic data (excluding
structure factors) has also been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication CCDC-
690007 for 1a, CCDC-690008 for 1b, CCDC-690009 for 2, CCDC-
690010 for 4, and CCDC-690011 for 5. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK [e-mail: deposit@ccdc.cam.ac.uk].
1
handling and weighing in the glovebox. H NMR (200.13 MHz,
25 °C, [D6]benzene/[D8]THF (2:1)): δ -0.574 (18H, s, Al-CH3),
1.72 (CH2, thf), 2.29 (6H, s, CH3 (toluene)), 3.69 (CH2O, thf),
7.08-7.32 (14H, m-H + p-H + toluene), 7.98 (2H, d, o-H).
13C{1H} NMR (50.33 MHz, 25 °C, [D6]benzene/[D8]THF (2:1)):
δ -8.8 (6C, broad, Al-CH3), 21.5 (2C, CH3 (toluene)), 25.6 (CH2,
thf), 68.7 (CH2O, thf), 124.4 (2C, p-C), 126.1 (2C, C4 (toluene)),
126.3 (4C, m-C) 128.8 (4C, C3/5 (toluene)), 129,8 (4C, C2/6
(toluene)), 138.4 (2C, C1 (toluene)) 138.6 (2C, o-C), 151.2 (2C,
i-C). 27Al NMR (104.28 MHz, 25 °C, [D6]benzene/[D8]THF (2:
1)): δ 138.8 ([AlMePh3]-, w1/2 ) 122 Hz), 144.7 ([AlMe2Ph2]-,
w1/2 ) 192 Hz), 150.1 ([AlMe3Ph]-, w1/2 ) 130 Hz), 154.7
([AlMe4]-, w1/2 ) 104 Hz). MS (FAB-, m/z, [%]): 149 (AlMe3Ph)
[13]. IR (Nujol, KBr, cm-1): 3039 m, 2923 vs (br), 2243 w, 2126 w,
1460 vs, 1377 s, 1296 w, 1243 w, 1166 m, 1102 m, 1071 m, 1018 s,
918 m, 870 m, 824 m, 710 s, 677 s, 628 m, 612 m.
OM800509P
(44) COLLECT, Data Collection Software; Nonius B.V.: Netherlands,
1998.
(45) Otwinowski, Z.; Minor, W. Processing of X-Ray Diffraction Data
Collected in Oscillation Mode. In Methods in EnzymologyVol. 276,
Macromolecular Crystallography, Part A, Carter, C. W.; Sweet, R. M., Eds.;
Academic Press: San Diego, 1997; pp 307-326.
(46) Sheldrick, G. M. Acta Crystallogr. 1990, A 46, 467–473.
(47) Sheldrick, G. M. SHELXL-97 (Release 97-2); University of
Go¨ttingen: Germany, 1997.