The anion [Mg6Cl8Cp*5]-: A final intermediate on the way to the molecular donor-free grignard compound MgClCp*?

Article abstract of DOI:10.1021/om051074s

The reaction at - 70 °C of decamethylmagnesocene MgCp* ₂ (Cp* = pentamethylcyclopentadienyl) with monovalent aluminum chloride, AlCl, stabilized with diethyl ether and dissolved in toluene, results in tetrameric tetrahedrotetrakis- (pentamethylcyclopentadienylaluminum), [AlCp*]₄, and the dimeric bis(pentamethylcyclopentadienylmagnesium chloride diethyl etherate), [MgCp*Cl·Et₂O]₂. At a slightly higher temperature and by stepwise decrease of the diethyl ether concentration before reaction, a preferred formation of metal clusters such as dodekakispentamethylcyclopentadienylpentacontaaluminum, Al ₅₀Cp*₁₂, was frequently observed. Interestingly, after working up the residue of this reaction, a new kind of Grignard compound could be isolated. We report here on the synthetic details and X-ray structure of a donor-free Grignard compound with an inverse magnesocene motif, μ-Cp*Mg₂, in the anionic part of the compound, [Mg ₆Cl₈Cp*₅][AlCp*2]·C ₆H₆(1).

Full text of DOI:10.1021/om051074s

Organometallics 2006, 25, 2101-2103
2101
The Anion [Mg6Cl8Cp*5]-: A Final Intermediate on the Way to the
Molecular Donor-Free Grignard Compound MgClCp*?
Jean Vollet,† Jens R. Hartig,† Katarzyna Baranowska,‡ and Hansgeorg Schno¨ckel*,†
Institute for Inorganic Chemistry, UniVersity of Karlsruhe, Engesserstrasse 15,
76131 Karlsruhe, Germany, and Faculty of Chemistry, Gdansk UniVersity of Technology,
G. Narutowicza Street 11/12, 80-952 Gdansk, Poland
ReceiVed December 16, 2005
Summary: The reaction at -70 °C of decamethylmagnesocene
MgCp*2 (Cp* ) pentamethylcyclopentadienyl) with monoValent
aluminum chloride, AlCl, stabilized with diethyl ether and
dissolVed in toluene, results in tetrameric tetrahedrotetrakis-
(pentamethylcyclopentadienylaluminum), [AlCp*]4, and the
dimeric bis(pentamethylcyclopentadienylmagnesium chloride
diethyl etherate), [MgCp*Cl‚Et2O]2. At a slightly higher tem-
perature and by stepwise decrease of the diethyl ether concen-
tration before reaction, a preferred formation of metal clusters
such as dodekakispentamethylcyclopentadienylpentaconta-
aluminum, Al50Cp*12, was frequently obserVed. Interestingly,
after working up the residue of this reaction, a new kind of
Grignard compound could be isolated. We report here on the
synthetic details and X-ray structure of a donor-free Grignard
compound with an inVerse magnesocene motif, µ-Cp*Mg2, in
the anionic part of the compound, [Mg6Cl8Cp*5][AlCp*2]‚C6H6
(1).
reduced stepwise via evacuation. This low concentration of, for
example, diethyl ether may be the reason for the formation of
donor-poor Grignard compounds, i.e., of oligomers in which
the low coordination number of magnesium is compensated by
self-aggregation and not via classical donor stabilization. Herein
we report an example of this kind, a compound containing the
donor-free anion [Mg6Cl8Cp*5]-.
Results and Discussion
A metastable solution of AlCl in an ether-toluene mixture
(1:3 v/v)7 was concentrated in a vacuum, to reduce the amount
of ether, at -30 °C. The remaining solution was reacted with
MgCp*2. Measuring a NMR sample of the mixture, a high-
shifted NMR signal at - 272 ppm in the 27Al NMR spectra
indicates the beginning of cluster formation assigned to the inner
shell of Al50Cp*12,8 but this time only the solid compounds
5
6
(AlCp*)4 and (MgCp*Cl‚Et2O)2 were obtained from the
reaction mixture by fractional crystallization. After removal of
the remaining solvent in vacuo and extraction of the solid residue
with benzene, colorless crystals of [Mg6Cl8Cp*5][AlCp*2]‚C6H6
(1) were obtained. The crystals of 1 were suitable for X-ray
analysis. Compound 1 crystallizes in the orthorhombic space
group P212121 with two molecules per unit cell. The X-ray
structure analysis of 1 is presented in Figure 1. Relevant
crystallographic data are given in Table 1, with selected bond
lengths and angles listed in Table 2.
The anion 1a contains the heterocubane-like fragment of a
donor-free Grignard compound, and the 1b ion is the sandwich-
like aluminocenium cation [AlCp*2]+, which we synthesized
some years ago.9 The aluminocenium cation 1b exhibits
structural and NMR parameters very similar to those described
earlier.9 The formation of 1b is evidence for the internal
disproportion process of the metastable AlCl species or AlCp*
oligomers as intermediates.
The most remarkable feature of the structure of 1a is its
inverse sandwich-like MgCp*Mg moiety, which, as far as we
know, has not been observed for magnesium before, although
it is known for other main group elements.10 This dimerization
via a bridging Cp* ligand seems to be favored because of a
deficiency of Cp* ligands and magnesium ions in this solution.
The same situation is observed for MgX2 dihalides, when
monomeric species with 4-fold-coordinated Mg atoms are
present due to the presence of an excess of ether [MgX2‚2D]
and when solid MgX2 as an ionic lattice (CdCl2 structure) with
Introduction
1,2
Magnesium organyl compounds such as MgCp2
and
MgCp*2 have proved to be powerful starting materials in
synthetic chemistry, although the X-ray structure of MgCp*2
was determined only recently.3 For instance, MgCp*2 was an
essential precursor for the preparation of the first Al(I) organic
compound (AlCp*)4 via a substitution reaction of metastable
AlCl.4,5 Since only one Cp* ligand of MgCp*2 could be
transferred to the Al(I) species, in every case the Grignard
compound [MgCp*X‚D]2 (D ) donor solvent)6 as the donor-
stabilized dimer was obtained. To vary the synthesis of AlCp*
in favor of the formation of metal clusters,7 e.g., the Al50Cp*12
cluster,8 the donor concentration during the reaction must be
* To whom correspondence should be addressed. Tel: +49-721-6082981.
Fax:
+49-721-6084854.
E-mail:
hansgeorg.schnoeckel@
chemie.uni-karlsruhe.de.
† University of Karlsruhe.
‡ Gdansk University of Technology.
(1) Duff, W. A.; Hitchcock, P. B.; Lappert, M. F.; Taylor, R. G.; Segal,
J. A. J. Organomet. Chem. 1985, 293, 271.
(2) Lindsell, W. E. In ComprehensiVe Organometallic Chemistry;
Wilkinson, G., Ed.; Pergamon Press: Oxford, 1982; Vol. 1, Chapter 4.
(3) Vollet, J.; Baum, E.; Schno¨ckel, H. Organometallics 2003, 22, 2525.
(4) Dohmeier; C., Robl, C.; Tacke, M.; Schno¨ckel, H. Angew. Chem.
1991, 103, 594; Angew. Chem., Int. Ed. Engl. 1991, 30, 564.
(5) Dohmeier, C.; Loos, D.; Schno¨ckel, H. Angew. Chem. 1996, 108,
141; Angew. Chem., Int. Ed. Engl. 1996, 35, 129.
(6) Dohmeier, C.; Loos, D.; Robl, C.; Schno¨ckel, H.: Fenske, D. J.
Organomet. Chem. 1993, 448, 5.
(7) (a) For this special kind of metal clusters (e.g., AlnRm, n > m) we
have introduced the term metalloid clusters: Schnepf, A.; Sto¨sser, G.;
Schno¨ckel, H. J. Am. Chem. Soc. 2000, 122, 9178. (b) Schnepf, A.;
Schno¨ckel, H. Angew. Chem. 2002, 114, 3683; Angew. Chem., Int. Ed. 2002,
41, 3532.
(9) Dohmeier, C.; Schno¨ckel, H.; Robl, C.; Schneider, U.; Ahlrichs, R.
Angew. Chem. 1993, 105, 1714; Angew. Chem., Int. Ed. Engl. 1993, 32,
1655.
(8) Vollet, J.; Hartig, J. R.; Schno¨ckel, H. Angew. Chem. 2004, 116,
3248; Angew. Chem., Int. Ed. 2004, 43, 3186.
(10) Jutzi, P.; Burford, N. In Metallocenes; Togni, A., Haltermann, R.
L., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol. 1, Chapter 1.
10.1021/om051074s CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/15/2006

2102 Organometallics, Vol. 25, No. 8, 2006
Notes
Figure 1. Molecular structure of [Mg6Cl8Cp*5][AlCp*2]‚C6H6 (1)with the anion [Mg6Cl8Cp*5]- (1a) and the aluminocenium cation [AlCp*2]+
(1b) (benzene and H atoms are omitted). Thermal ellipsoids are shown at 50%. Average distances [pm] of 1a: Mg-Cring 237.6, Mg-µ-
Cring 242.2, Mg-Cpringcentroid 205.3, Mg-µ-Cpringcentroid 211.5, Mg-µ3-Cl 253.0, Mg-µ-Cl 241.1, µ-Cring-Cring 139.1, Cring-Cring 140.9,
Cring-Cmethyl 151.2. Average distances [pm] of 1b: Al-Cring 212.4, Al-Cpringcentroid 172.7, Cring-Cring 145.4, Cring-Cmethyl 153.1.
Table 1. Crystal Data for 1
C76H111AlCl8Mg6
Table 2. Selected Bond Lengths [pm] and Angles [deg] for 1
compd‚solvate
fw
Mg(1)-Cl(1)
Mg(1)-Cl(2)
Mg(1)-Cl(3)
Mg(1)-C(11)
Mg(1)-C(12)
Mg(2)-Cl(1)
Mg(2)-Cl(2)
Mg(2)-Cl(4)
Mg(2)-C(21)
Mg(2)-C(22)
Mg(2)-C(23)
Mg(2)-C(22)*
Mg(2)-C(23)*
Mg(3)-Cl(2)
Mg(3)-Cl(3)
251.6(4)
254.6(3)
237.7(4)
239.0(11)
241.7(10)
234.6(3)
251.9(1)
233.0(3)
250.4(5)
236.9(8)
236.8(8)
246.1(8)
240.8(9)
252.3(3)
238.1(4)
Mg(3)-Cl(4)
Mg(3)-C(31)
Mg(3)-C(32)
C(21)-C(22)
C(22)-C(23)
C(23)-C(23)*
C(22)*-C(21)
C(21)-C(26)
C(22)-C(27)
C(23)-C(28)
Al(1)-C(1)
251.3(4)
235.0(9)
235.4(10)
132.2(10)
133.8(11)
163.9(13)
132.2(10)
156.2(11)
162.7(11)
147.2(12)
213.0(9)
216.6(9)
209.9(9)
351.7(4)
361.3(4)
1481.09
temp, K
wavelength, Å
cryst syst
space group
cryst dimens, mm
a, Å
200(2)
0.71073
orthorhombic
P212121
0.50 × 0.30 × 0.15
15.7808(10)
16.3514(13)
16.6649(9)
4300.2(5)
b, Å
c, Å
V, Å3
Z
d
2
Al(1)-C(2)
calcd, g cm-3
1.144
0.353
1576
51.98
Al(1)-C(3)
Mg(1)-Mg(2)
Mg(1)-Mg(3)
abs coeff, mm-1
F(000)
max 2θ, deg
index ranges
Cl(1)-Mg(1)-Cl(2)
Cl(2)-Mg(1)-Cl(3)
Cl(3)-Mg(1)-Cl(1)
Cl(1)-Mg(1)-C(11)
Cl(1)-Mg(1)-C(12)
87.1(1)
Cl(2)-Mg(3)-Cl(3)
84.78(9)
87.1(1)
101.9(1)
114.3(3)
95.0(2)
-20 e h e 20;
-19 e k e 19;
-20 e l e 20
31 907 [0.0546]
8409
84.35(9) Cl(2)-Mg(3)-Cl(4)
101.8(1)
127.9(3)
154.2(3)
Cl(3)-Mg(3)-Cl(4)
Cl(2)-Mg(3)-C(31)
Cl(2)-Mg(3)-C(32)
no. of rflns [Rint
]
no. of indep rflns
Mg(1)-C(11)-C(16) 120.1(7)
Mg(3)-C(31)-C(36) 122.0(6)
no. of data/restraints/params
8409/9/398
0.897
Cl(1)-Mg(2)-Cl(2)
Cl(1)-Mg(2)-Cl(4)
Cl(2)-Mg(2)-Cl(4)
Cl(1)-Mg(2)-C(21)
Cl(1)-Mg(2)-C(22)
91.57(7) Mg(1)-Cl(1)-Mg(2)
104.6(2)
91.23(7) Mg(1)-Cl(2)-Mg(3)
90.09(6) Mg(1)-Cl(3)-Mg(3)
114.0(2)
92.6(1)
goodness of fit on F2
Mg(1)-Cl(2)-Mg(2)
87.93(8)
90.9(1)
98.8(2)
88.16(8)
92.7(1)
final R indices (I > 2σ(I))
largest diff peak and hole, e Å-3
RF ) 0.0457, Rw ) 0.1156
0.823 and -0.299
Mg(2)-Cl(2)-Mg(3)
a Mg coordination number of 6 is preferred in the absence of
diethyl ether. However, if the amount of diethyl ether is
diminished carefully, [MgX2‚Et2O]n is observed in which a
chain-like structure containing five-coordinate magnesium is
present.11 In the system of MgCl2 and diethyl ether we have
recently been successful in obtaining a highly symmetrical
[Mg3Cl5‚6Et2O]+ intermediate12 in which the metal atoms and
the five chlorine and six oxygen atoms of the ligands form a
structural unit, similar but inverse to that of the metal-rich
Cs11O3 suboxide entity.13
In 1a we have a fragment of the thus far unknown structure
of a molecular Grignard compound. However, many years ago
some results in this field were described by Klabunde,14 but
without structural information. Furthermore, a polymeric chain-
like structure containing MgR2 and MgRBr units has been
presented by Bickelhaupt recently.15 The latter compound is,
Mg(2)-C(21)-C(26) 122.72(5) Mg(2)-Cl(4)-Mg(3)
as far as we know, the only structurally characterized donor-
free Grignard-like compound fragment published to date. Further
developments in this field were described quite recently by
Knochel.16
The Grignard-like fragment in 1a resembles two parts of a
heterocubane-like structure. A complete heterocubane structure
of this type is present in the valence-isoelectronic molecule
[AlCp*Se]4.17,18 From this analogy it can be deduced that a
compound, e.g., [MgCp*Cl]4, might exist. Its synthesis may be
possible under conditions such as those used for 1, but with
more MgCp*2 in the solution. A much simpler synthesis of
[MgCp*Cl]4 should be possible starting with the [MgClCp*‚
(15) Markies, P. R.; Schat, G.; Akkerman, O. S.; Bickelhaupt, F.; Smeets,
W. J. J.; Duisenberg, A. J. M.; Spek, A. L. J. Organomet. Chem. 1989,
375, 11.
(16) Krasovskiy, A.; Straub, B. F.; Knochel, P. Angew. Chem. 2006,
118, 165; Angew. Chem., Int. Ed. 2006, 45, 159.
(11) Ecker, A.; U¨ ffing, C.; Schno¨ckel, H. Chem. Eur. J. 1996, 2, 1112.
(12) Loos, D.; Eichkorn, K.; Magull, J.; Ahlrichs, R.; Schno¨ckel, H. Z.
Anorg. Allg. Chem. 1995, 621, 1582.
(13) Simon, A.; Westerbeck, E. Angew. Chem. 1972, 84, 1190; Angew.
Chem., Int. Ed. Engl. 1972, 11, 1105.
(14) Klabunde, K. J.; Efner, H. F.; Satel, L.; Donley, W. J. Organomet.
Chem. 1974, 71, 309.
(17) Schulz, S.; Roesky, H. W.; Koch, H. J.; Sheldrick, G. M.; Stalke,
D.; Kuhn, A. Angew. Chem. 1993, 105, 1828; Angew. Chem., Int. Ed. Engl.
1993, 32, 1729.
(18) Purath, A. Thesis, Universita¨t Karlsruhe (TH), Karlsruhe, 1999.

Notes
Organometallics, Vol. 25, No. 8, 2006 2103
[Mg6Cl8Cp*5][AlCp*2]‚C6H6 (1). The diethyl ether component
of a 43 mL portion of a 0.38 M AlCl‚Et2O toluene solution (25
vol % Et2O, ω(AlCl) ) 92%, 15 mmol) was concentrated to 30
mL in vacuo over a period of 8 h at -50 to -40 °C, and 4.42 g of
MgCp*2 (15 mmol) dissolved in toluene was added to the solution
at -70 °C. The resulting dark red reaction mixture was warmed to
-30 °C and stirred for 3 h. Subsequently, all volatile components
were removed, and the residue was washed with 15 mL of pentane.
The residue was extracted with toluene, and after taking a NMR
sample the extract was stored at -30 °C. An amount of 1.29 g
(1.99 mmol) of yellow crystals of [AlCp*]4 and 0.51 g (0.95 mmol)
of colorless crystals of [MgCp*Cl‚Et2O]2 were isolated from the
toluene extract after several concentrating steps. After removing
the rest of the toluene the residue was washed with benzene. After
2 weeks colorless crystals of 1 were obtained from the benzene
extract stored at ambient temperature. The sample for NMR
spectroscopy was separated from the solution directly after the
reaction had been performed.
Et2O]2 compound mentioned above, performing a dimerization
reaction while the diethyl ether is carefully removed. DFT
calculations for this process (eq 1) show an energy increase of
2 [MgCp*Cl‚Et2O]2 f [MgCp*Cl]4 + 4 Et2O
(1)
only 100 kJ‚mol-1 for the reaction enthalpy ∆RH°. Because of
the increase in ∆RS°, it seems promising to perform these
experiments in order to obtain the first pure molecular donor-
free Grignard compound. These experiments are currently
ongoing.
Summary
We have prepared and structurally characterized a rare
example of a donor-free Grignard compound. The structure of
[Mg6Cl8Cp*5][AlCp*2]‚C6H6 (1) has been confirmed by X-ray
diffraction analysis. Its structure contains the first inverse
magnesocene motif in its anionic part. The compound is of
special interest in terms of its composition: a normal (Cp*AlCp*)
and an inverse (MgCp*Mg) metallocene moiety are both present
in one crystalline compound.
27Al NMR (78 MHz, C6D6, ppm): δ -80 [AlCp*]4, -114
[AlCp*2]+, -272 [Al8Al42Cp*12]. 1H NMR (250 MHz, C6D6,
ppm): δ 1.97 (broad). 13C NMR (63 MHz, C6D6, ppm): δ 10.7
[Me5C5], 111.0 [Me5C5].
Crystallographic Analysis for 1. A crystal of 1 was mounted
on a glass fiber in silicone oil at -73 °C. The data were collected
on a STOE IPDS two-circle diffractometer using graphite-mono-
chromated Mo KR radiation and a Stoe-IPDS area detector. The
data were corrected for absorption by the Stoe IPDS software.
Lorentz polarization and absorption corrections were applied. The
structural solution (calculated by direct methods) and refinement
(on F2, H atoms calculated) were carried out using the Sheldrick
SHELX97 program suite.
CCDC-290653 (1) contains the supplementary crystallographic
data for this note at www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB21EZ, UK; fax: (+44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
Experimental Section
MgCp*2 was synthesized as described in the literature.19 A 0.38
M AlCl‚Et2O solution in toluene was prepared using the co-
condensation technique described earlier.7 Benzene, toluene, diethyl
ether, and n-pentane were dried over sodium, freshly distilled under
argon, and freeze-thaw-degassed prior to use. All manipulations
were carried out under high-purity nitrogen in flame-dried glass-
ware. The quantum-chemical calculations (DFT) were performed
with the TURBOMOLE program package. The BP86 functional
with the SVP basis set was used for the elements.20
(19) (a) Kohl, F. X.; Jutzi, P. Chem. Ber. 1981, 114, 489. (b) Whitesides,
M. G.; Feitler, D. Inorg. Chem. 1976, 15, 466. (c) Duff, A. W.; Hitchcock,
P. B.; Lappert, M. F.; Taylor, R. G.; Segal, J. A. J. Organomet. Chem.
1985, 293, 271.
(20) (a) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346. (b)
Eichkorn, K.; Treutler, O.; O¨ hm, H.; Ha¨ser, M.; Ahlrichs, R. Chem. Phys.
Lett. 1995, 242, 652. (c) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs,
R. Theor. Chim. Acta 1997, 97, 119. (d) Becke, A. D. Phys. ReV. A 1998,
38, 3098. (e) Perdew, J. P. Phys. ReV. B 1996, 33, 8822. (f) Schreckenbach,
G.; T. Ziegler, J. Phys. Chem. 1995, 99, 606. (g) Ha¨ser, M.; Ahlrichs, R.;
Baron, H. P.; Weiss, P.; Horn, H. Theor. Chim. Acta 1992, 83, 455.
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft, the Centre for Functional Nanostructures (CFN),
and the Fonds der Chemischen Industrie for financial support.
Supporting Information Available: Data for 1 can be obtained
free of charge via the Internet at http://pubs.acs.org.
OM051074S