Alkylmagnesium complexes with the rigid dpp-bian ligand {dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene}

Article abstract of DOI:10.1002/ejic.200700816

The reactions between equimolar amounts of (dpp-bian)Na-(Et ₂O)_x (1) {dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino] -acenaphthene} and MeMgBr, EtMgBr and Me₃SiCH₂MgCl in Et₂O afford the correspondi

Full text of DOI:10.1002/ejic.200700816

FULL PAPER
DOI: 10.1002/ejic.200700816
Alkylmagnesium Complexes with the Rigid dpp-bian Ligand {dpp-bian =
1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphthene}
Igor L. Fedushkin,*[a] Alexander G. Morozov,[a] Markus Hummert,[b] and
Herbert Schumann*[b]
Keywords: Magnesium / N ligands / Alkyl complexes / Structure elucidation
The reactions between equimolar amounts of (dpp-bian)Na-
(Et2O)x (1) {dpp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]-
acenaphthene} and MeMgBr, EtMgBr and Me3SiCH2MgCl
in Et2O afford the corresponding alkylmagnesium complexes
[(dpp-bian)MgMe]2 (2), (dpp-bian)MgEt(Et2O) (3) and (dpp-
bian)MgCH2SiMe3(Et2O) (4), respectively. Compound 2 and
tBuOH react with elimination of methane and the formation
of [(dpp-bian)MgOtBu]2 (5). Compounds 2–5, all of which
contain the dpp-bian ligand in its radical monoanionic form,
were characterised by elemental analysis, IR spectroscopy, as
well as by single-crystal X-ray diffraction. Molecular struc-
ture determination revealed that the magnesium atoms in 2–
5 are four-coordinate. The magnesium atoms of complexes 2
and 5 reach the coordination number four by dimerisation
through bridging Me and tBuO groups, respectively, whereas
compounds 3 and 4 reach it by coordination of one molecule
of diethyl ether.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
(Et2O)2 containing radical monoanionic dpp-bian is truly
stable in hexane and Et2O, but it decomposes immediately
when it is dissolved in THF with reductive elimination of
isopropyl radicals (Scheme 2).[3]
Introduction
Recently, we reported on the successful preparation of
monomeric alkylaluminium complexes of the type A, B and
C, all of which contain the dpp-bian ligand in its radical
monoanionic (A)[1] or dianionic state (B and C)
(Scheme 1).[2] Although these complexes are very sensitive
to air and moisture, they are thermally stable and do not
decompose in solution over the course of several weeks.
In contrast, the magnesium complex (dpp-bian)Mg(iPr)-
Scheme 2.
This result demonstrates that reductive elimination pro-
cesses can be realised even with complexes of main group
metals that are not able to change their oxidation state. To
make such processes possible, one has to use ligands that
are able to change their “oxidation state”, as is the case
with the dpp-bian ligand. To get a deeper insight into the
structural peculiarities and the special solution behaviour
of alkylmagnesium complexes coordinated by the dpp-bian
radical anion, we investigated magnesium derivatives of the
type (dpp-bian)MgR (R = Me, Et, tBu, CH2=CMeCH2 and
CH2SiMe3). The present work demonstrates that complexes
with alkyl groups that are less bulky than the isopropyl moi-
ety are stable, whereas tert-butyl- and 2-methylallylmagnes-
ium complexes undergo alkyl radical elimination before
they can be isolated.
Scheme 1.
[a] G. A. Razuvaev Institute of Organometallic Chemistry of Rus-
sian Academy of Sciences,
Tropinina 49, 603950 Nizhny Novgorod, GSP-445, Russia
Fax: +7-831-4627497
E-mail: igorfed@iomc.ras.ru
[b] Institut für Chemie der Technischen Universität Berlin,
Straße des 17. Juni 135, 10623 Berlin, Germany
Fax: +49-30-3142-2168
E-mail: schumann@chem.tu-berlin.de
1584
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 1584–1588

Alkylmagnesium Complexes with the Rigid dpp-bian Ligand
Scheme 3.
Results and Discussion
Synthesis of [(dpp-bian)MgMe]2 (2), (dpp-bian)MgEt(Et2O)
(3), (dpp-bian)MgCH2SiMe3(Et2O) (4) and [(dpp-bian)-
MgOtBu]2 (5)
Scheme 5.
Compounds 2–4 were prepared by salt elimination reac-
tions between the in situ generated sodium complex (dpp-
bian)Na(Et2O)x (1) and the Grignard reagents MeMgBr,
EtMgBr and Me3SiCH2MgCl conducted in diethyl ether
(Schemes 3 and 4). The solution of the starting sodium
complex 1, as well as those of compounds 2–4, are red,
which thus proves the presence of species containing the
dpp-bian radical monoanion.
Compounds 2–5 were characterised by elemental analy-
sis, IR spectroscopy and single-crystal X-ray analysis. The
paramagnetism of all these compounds caused by the pres-
ence of the radical dpp-bian anion prevented NMR spec-
troscopic investigations.
In contrast to 1–4, complex 5 is an effective catalyst for
the ring-opening polymerisation of cyclic esters, especially
of rac- and -lactide. When the polymerisation of rac-lac-
tide with 5 was carried out in a 200:1 molar ratio in THF
at room temperature the conversion of 95% was achieved
within several minutes. The gel-permission chromatography
has proved the formation of a high molecular weight poly-
mer (MW = 7.5ϫ104, PDI 1.36).
Molecular Structures of 2–5
Scheme 4.
The molecular structures of 2, 3, 4 and 5 were deter-
mined by single-crystal X-ray diffraction and are depicted
Whereas the methyl derivative 2 is readily soluble in di- in Figures 1, 2, 3 and 4, respectively. Suitable crystals were
ethyl ether and has to be isolated by crystallisation from obtained from toluene (2, 5) and diethyl ether (3, 4). Se-
toluene, compounds 3 and 4 crystallise after separation of lected bond lengths and bond angles of the four compounds
the precipitated sodium halide from the clear and concen- are shown in Table 1, and the crystal data collections and
trated reaction solutions. Compounds 2–4 are thermally structure refinement data of the respective molecules are
rather stable. They do not decompose, neither in THF at listed in Table 2.
room temperature nor in boiling toluene.
Complexes 2–5 represent four-coordinate magnesium
In contrast to the above reactions, the treatment of 1 species with the dpp-bian radical anion acting as a chelating
with tBuMgCl or CH2=CMeCH2MgBr in diethyl ether ligand. In the case of complexes 2 and 5, the coordination
causes a change in the colour of the solution from red to number 4 of magnesium is reached by formation of dimeric
green, which is thus unambiguously indicative of the forma- molecules through bridging methyl and tert-butoxy groups,
tion of dianionic dpp-bian species. As in the case of the respectively, and in the case of the monomeric complexes 3
reaction of 1 with iPrMgCl,[3] the magnesium complex and 4 by additional coordination of one molecule of diethyl
(dpp-bian)Mg(Et2O)2 containing the dpp-bian dianion pre- ether. The shortening of the Mg–N distances in complex 2
cipitates from the reaction mixture as green crystals.
(av. 2.0584 Å) relative to those in 3 (av. 2.103 Å), 4 (av.
As to be expected, complexes 2–4 are sensitive towards 2.096 Å) and 5 (av. 2.109 Å) reflects a strengthening of the
acidic substances. Thus, treatment of 2 with tert-butyl metal-to-ligand interactions, which thus compensates for
alcohol causes liberation of methane and formation of the the somewhat higher electron deficiency at the Mg atoms
corresponding alkoxide 5 in a yield of 65% (Scheme 5).
in 2 relative to the situation in 3, 4 and 5. In general, all the
Eur. J. Inorg. Chem. 2008, 1584–1588
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1585

I. L. Fedushkin A. G. Morozov, M. Hummert, H. Schumann
FULL PAPER
Figure 1. Molecular structure of 2. The hydrogen atoms are omit-
ted for clarity; thermal ellipsoids are drawn at 30% probability.
Figure 4. Molecular structure of 5. The hydrogen atoms are omit-
ted for clarity; thermal ellipsoids are drawn at 30% probability.
Table 1. Selected bond lengths [Å] and angles [°] for complexes 2–5.
Compound
2
3[a]
4
5
Mg(1)–N(1)
Mg(1)–N(2)
Mg(1)–O(1)
Mg(1)–O(1a)
2.0550(18) 2.100(3)
2.0618(18) 2.107(3)
2.055(3)
2.106(3)
2.085(3)
2.061(3)
2.133(3)
2.084(3)
1.953(2)
1.953(2)
Mg(1)–C(37)
Mg(1)–C(37a)
N(1)–C(1)
N(2)–C(2)
C(1)–C(2)
Mg(1)–N(1)–C(1)
Mg(1)–N(2)–C(2)
O(1)–Mg(1)–N(1)
O(1)–Mg(1)–N(2)
O(1)–Mg(1)–C(1)
N(1)–Mg(1)–N(2)
N(1)–Mg(1)–C(37)
N(2)–Mg(1)-(C37)
2.314(2)
2.254(3)
1.343(3)
1.337(2)
1.431(3)
2.220(3)
2.124(4)
1.338(4)
1.337(5)
1.445(5)
1.333(4)
1.326(5)
1.452(5)
108.6(2)
109.1(2)
1.342(4)
1.339(4)
1.437(5)
104.4(2)
105.6(2)
107.27(13) 106.7(2)
107.06(13) 106.3(2)
114.73(12) 104.77(13) 122.81(12)
109.10(12) 107.47(12) 118.25(11)
128.33(12) 109.58(12) 123.42(10)
82.33(12)
123.44(13) 124.00(14)
119.77(12) 119.79(15)
Figure 2. Molecular structure of 3. The hydrogen atoms are omit-
ted for clarity; thermal ellipsoids are drawn at 30% probability.
84.75(7)
113.11(8)
118.92(8)
82.76(13)
83.19(11)
[a] For one of two crystallographically independent molecules.
all fit well in the series of Mg–C(alkyl) distances of the fol-
lowing N-ligand supported magnesium compounds: [CH(8-
C9H6N)P(iPr2)=N(tBu)]MgEt [2.183(14) Å],[5a] {CH[C-
(Me)N(2,6-iPr2C6H3)]2}MgCH2Ph(THF) [2.1325(18) Å],[5b]
[PhTptBu]MgEt [PhTptBu = phenyltris-(3-tert-butylpyr-
azolyl)borato] [2.163(2) Å][5c] and {2,6-N(R)C5H3[ArN=C-
(Me)]2}MgR (R = Me, Et, iPr) [2.122(2)–2.157(2) Å].[5d]
The Mg–O(tBu) bond lengths in 5 [1.963(3) and 1.953(3) Å]
are fairly similar as the Mg–O distances in dimeric magne-
sium ketiminate [{CH[C(Me)NC6H4-2-OMe]2}MgOtBu]2
[1.944(2) and 1.965(2) Å].[6] As in other metal complexes of
the dpp-bian radical anion,[4] the C(1)–N(1) and C(2)–N(2)
bond lengths in 2–5 [1.324(6)–1.343(3) Å] are longer than
in free dpp-bian [both 1.282(4) Å][7] and in metal complexes
containing neutral dpp-bian ligands, for example, (dpp-
bian)SnCl4 [1.291(8) and 1.286(8) Å][8] and (dpp-bian)SnCl2
[1.283(1) and 1.269(1) Å].[9]
Figure 3. Molecular structure of 4. The hydrogen atoms are omit-
ted for clarity; thermal ellipsoids are drawn at 30% probability.
observed Mg–N bond lengths correspond well with those in
other magnesium complexes containing the dpp-bian radi-
cal anion.[4] The bite angles N(1)–Mg–N(2) of all four
molecules lie in the narrow range from 82.3 to 84.8°. As to
be expected, the Mg–C(alkyl) bond lengths decrease on go-
ing from dimeric 2 (av. 2.284 Å) to the monomeric mole-
Conclusions
In this study we demonstrated that dpp-bian alkylmagne-
cules 3 [2.220(3)/2.1161(4) Å] and 4 [2.124(4) Å], but they sium complexes with less bulky alkyl groups such as Me, Et
1586
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Eur. J. Inorg. Chem. 2008, 1584–1588

Alkylmagnesium Complexes with the Rigid dpp-bian Ligand
[(dpp-bian)MgMe]2 (2): To the ethereal solution of 1, prepared as
described above, was added a solution of MeMgBr (0.5  in diethyl
ether, 4 mL, 2.0 mmol). After stirring the mixture at room tempera-
ture for 1 h, the solvent was removed from the red solution under
reduced pressure. Repeated extraction of the remaining solid with
hot toluene (ca. 500 mL) afforded 2 as deep-red prismatic crystals
and Me3SiCH2 (2–4, respectively), are stable both in solu-
tion and in the solid state. However, dpp-bian-supported
alkylmagnesium complexes with the bulky tert-butyl or 2-
methylallyl group are just as unstable as the recently pub-
lished (dpp-bian)Mg(iPr)(Et2O)2 (1)[3] and are prone to un-
dergo reductive elimination of the respective alkyl radicals.
As we could demonstrate by the reaction of [(dpp-bian)-
MgMe]2 (2) with tert-butyl alcohol yielding [(dpp-bian)-
MgOtBu]2 (5), the reactions of the stable complexes 2–4
(0.79 g, 73%). M.p. Ͼ200 °C. IR (nujol): ν = 1655 (m), 1630 (w),
˜
1588 (m), 1522 (s), 1461 (s), 1380 (s), 1365 (m), 1319 (m), 1253 (m),
1188 (m), 1103 (w), 1080 (w), 1057 (w), 1042 (w), 934 (w), 865 (m),
815 (s), 799 (s), 769 (s), 757 (vs), 730 (s), 699 (s), 669 (m), 607 (m),
with hydrogen acidic substrates may be a suitable method 522 (s), 457 (m) cm–1. C74H86Mg2N4 (1080.08): calcd. C 82.29, H
8.03; found C 81.95, H 8.00.
for the synthesis of novel magnesium complexes supported
by the radical anionic dpp-bian ligand.
(dpp-bian)MgEt(Et2O) (3): To the ethereal solution of 1, prepared
as described above, was added a solution of EtMgBr (1.5  in di-
ethyl ether, 1.4 mL, 2.0 mmol). After stirring the mixture at room
temperature for 1 h, the red solution was filtered off from precipi-
tated NaBr and was then reduced in volume to 15 mL by evapora-
tion of the solvent in vacuo. Crystallisation at room temperature
gave 0.68 g (54%) of 3 as deep-red crystals. M.p. Ͼ200 °C. IR (nu-
jol): ν˜ = 1670 (m), 1627 (w), 1540 (m), 1530 (s), 1458 (s), 1373 (m),
1350 (m), 1310 (m), 1248 (m), 1090 (w), 1076 (w), 920 (w), 853 (w),
807 (s), 764 (s), 750 (vs), 731 (s), 690 (m), 601 (w), 527 (m) cm–1.
C42H55MgN2O (628.19): calcd. C 80.30, H 8.82; found C 80.07, H
8.69.
Experimental Section
General Remarks: All manipulations were carried out in vacuo or
under an atmosphere of nitrogen gas by using Schlenk techniques.
The solvents Et2O and toluene were distilled from sodium/benzo-
phenone prior to use. Melting points were measured in sealed capil-
laries. IR spectra were recorded with an FTIR FSM-1201 spec-
trometer.
(dpp-bian)Na(Et2O)x (1):
A suspension of dpp-bian (1.0 g,
2.0 mmol) in Et2O (40 mL) was added to sodium metal (0.05 g,
2.14 mmol). Vigorous stirring of the mixture at room temperature
for 4 h resulted in the formation of a clear red solution of 1. This
solution was used in situ for the syntheses of 2, 3 and 4. Their
yields were calculated on the basis of the amount of dpp-bian used
in this reaction.
(dpp-bian)MgCH2SiMe3(Et2O) (4): Similar to the preparation of 3,
the ethereal solution of 1, prepared as described above, was treated
with a solution of Me3SiCH2MgCl (2  in diethyl ether, 2 mL,
2.0 mmol). From the red solution, reduced in volume to 5 mL by
evaporation in vacuo, 4 crystallised at room temperature within a
few days as large deep-red prismatic crystals. Yield 1.04 g (76%).
Table 2. Crystal data and structure refinement details for 2–5.
Compound
2
3
4
5
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C74H86Mg2N4
1080.09
monoclinic
P21/n
16.2591(2)
12.4590(1)
16.3895(2)
–
C42H55MgN2O
628.19
monoclinic
P21/c
22.4481(6)
17.1710(4)
21.4873(6)
–
C44H61MgN2OSi
686.35
orthorhombic
P212121
12.3084(3)
15.9415(4)
21.3489(5)
90
C94H114Mg2N4O2
1380.51
triclinic
¯
P1
10.9989(5)
13.6483(7)
14.6897(7)
99.036(2)
α [°]
β [°]
γ [°]
93.018(1)
–
3315.45(6)
4
114.406(1)
–
7542.3(3)
8
1.106
0.080
90
90
105.320(2)
103.652(2)
2009.28(17)
1
1.141
0.081
Volume [Å3]
Z
4188.96(18)
4
1.088
Dcalcd. [gcm–3]
1.082
Absorption coefficient [mm–1] 0.079
0.104
F(000)
Crystal size [mm]
θ range for data collection [°] 1.72 to 25.00
Index ranges
1164
0.78ϫ0.37ϫ0.18
2728
1492
746
0.56ϫ0.52ϫ0.12
1.58 to 25.00
–26ՅhՅ26
–20ՅkՅ17
–23ՅlՅ25
45125
0.58ϫ0.30ϫ0.02
1.59 to 25.00
–14ՅhՅ12
–17ՅkՅ18
–25ՅlՅ25
25822
0.35ϫ0.34ϫ0.15
2.01 to 25.00
–13ՅhՅ13
–16ՅkՅ10
–17ՅlՅ17
12631
–19ՅhՅ19
–14ՅkՅ14
–16ՅlՅ19
19949
Reflections collected
Independent reflections
Max/min transmission
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ(I)]
5835 [Rint. = 0.0657]
0.9859/0.9408
5835/3/378
1.037
R1 = 0.0535
wR2 = 0.1359
R1 = 0.0820
wR2 = 0.1520
–
13253 [Rint. = 0.0968]
0.9905/0.9566
13253/11/873
1.056
R1 = 0.0814
wR2 = 0.2169
R1 = 0.1527
wR2 = 0.2615
–
7354 [Rint. = 0.1326]
0.9938/0.9576
7354/0/456
1.001
R1 = 0.0661
wR2 = 0.1113
R1 = 0.1278
wR2 = 0.1312
0.2(2)
7060 [Rint = 0.0775]
0.9880/0.9722
7060/0/512
0.959
R1 = 0.0673
wR2 = 0.1435
R1 = 0.1606
wR2 = 0.1810
–
R indices (all data)
Absolute structure parameter
Largest diff. peak/hole [eÅ–3] 0.382/–0.424
1.119/–0.410
0.268/–0.337
0.262/–0.328
Eur. J. Inorg. Chem. 2008, 1584–1588
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1587

I. L. Fedushkin A. G. Morozov, M. Hummert, H. Schumann
FULL PAPER
M.p. 151–153 °C. IR (nujol): ν = 1680 (m), 1641 (m), 1595 (s), 1536
˜
(m), 1462 (m), 1368 (m), 1340 (w), 1325 (w), 1270 (s), 1080 (w), [1] a) H. Schumann, M. Hummert, A. N. Lukoyanov, I. L.
Fedushkin, Organometallics 2005, 24, 3891–3896; b) A. N. Lu-
koyanov, I. L. Fedushkin, M. Hummert, H. Schumann, Russ.
Chem. Bull. 2006, 55, 422–428.
910 (m), 860 (s), 839 (m), 802 (vs), 780 (s), 760 (s), 641 (w), 516
(s), 484 (w) cm–1. C44H61MgN2OSi (686.35): calcd. C 77.00, H 8.96;
found C 76.75, H 8.81.
[2] a) A. N. Lukoyanov, I. L. Fedushkin, M. Hummert, H. Schum-
ann, Z. Anorg. Allg. Chem. 2006, 632, 1471–1476; b) H. Schum-
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[3] I. L. Fedushkin, A. A. Skatova, M. Hummert, H. Schumann,
Eur. J. Inorg. Chem. 2005, 1601–1608.
[(dpp-bian)(MgOtBu)]2 (5): To
a suspension of 2 (0.62 g,
0.57 mmol) in toluene (50 mL), cooled to ca. –70 °C, was added
tBuOH (0.093 g, 1.13 mmol) by condensation in vacuo. Slow
warming of the reaction mixture to ambient temperature was ac-
companied with liberation of methane and formation of a clear red
solution, which was reduced in volume to ca. 15 mL by evaporation
in vacuo at 80 °C. Crystallisation at ambient temperature gave
[4] a) I. L. Fedushkin, A. A. Skatova, V. K. Cherkasov, V. A. Chu-
dakova, S. Dechert, M. Hummert, H. Schumann, Chem. Eur.
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0.51 g (65%) of 5 as deep-red crystals. M.p. Ͼ200 °C. IR (nujol): ν
˜
= 1676 (m), 1658 (w), 1606 (s), 1588 (w), 1575 (m), 1384 (s), 1363
(s), 1313 (s), 1293 (m), 1136 (m), 1057 (s), 856 (s), 692 (s), 666 (s),
632 (m) cm–1. C94H114Mg2N4O2 (1380.51): calcd. C 81.78, H 8.32;
found C 81.15, H 8.03.
X-ray Crystallographic Study of 2–5: The crystal data and details
of the data collections are listed in Table 2. The data were collected
by using a SMART CCD diffractometer (graphite-monochromated
Mo-Kα radiation, ω-scan technique, λ = 0.71073 Å) at 173 K. The
structures were solved by direct methods with the use of SHELXS-
97[10] and were refined on F2 by using all reflections with the
SHELXL-97 program.[11] SADABS[12] was used to perform area-
detector scaling and absorption corrections. The absolute structure
of the noncentrosymmetric space groups was determined by using
SHELXL-97 according to Flack.[13] All non-hydrogen atoms were
refined anisotropically. All hydrogen atoms except those of the
CH3Mg groups of compound 2 were placed in calculated positions
by using a riding model. The positions of the hydrogen atoms of
the CH3Mg groups in 2 were deduced from the electron density
map. The coordinated diethyl ether molecule in 3 was refined iso-
tropically because of its high disorder. CCDC-654640 (for 2),
-654641 (for 3), -654642 (4) and -654643 (for 5) contain the supple-
mentary crystallographic data. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[10] G. M. Sheldrick, SHELXS-97: Program for the Solution of
Crystal Structures, University of Göttingen, 1990.
[11] G. M. Sheldrick, SHELXL-97: Program for the Refinement of
Crystal Structures, University of Göttingen, 1997.
[12] G. M. Sheldrick, SADABS: Program for Empirical Absorption
Correction of Area Detector Data, University of Göttingen,
1996.
[13] a) H. D. Flack, Acta Crystallogr., Sect. A 1983, 39, 876; b) G.
Bernardinelli, H. D. Flack, Acta Crystallogr., Sect. A 1985, 41,
500.
Acknowledgments
This work was supported by the Alexander von Humboldt Founda-
tion (Partnership Project between the IOMC RAS and the Institut
für Chemie der Technischen Universität Berlin), the Russian Foun-
dation for Basic Research (Grant No. 07-03-00545), the Fonds der
Chemischen Industrie and the Deutsche Forschungsgemeinschaft
(H. S.).
Received: August 2, 2007
Published Online: February 18, 2008
1588
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Eur. J. Inorg. Chem. 2008, 1584–1588