Reactivity of permethylated magnesium complexes toward β-diimines

Article abstract of DOI:10.1021/om200361m

The peralkylated magnesium complex Mg(AlMe₄)₂ reacts with bis-N,N-diimine PhCH=NCH₂CH₂N=CHPh to yield the compound [PhCH(Me)NCH₂CH₂N=CHPh]Mg(AlMe ₄)(AlMe₃), via an alk

Full text of DOI:10.1021/om200361m

ARTICLE
pubs.acs.org/Organometallics
Reactivity of Permethylated Magnesium Complexes toward
β-Diimines
Olaf Michel,†,‡ Koji Yamamoto,§ Hayato Tsurugi,§ C€acilia Maichle-M€ossmer,† Karl W. T€ornroos,‡
Kazushi Mashima,*,§ and Reiner Anwander*,†,‡
†Institut f€ur Anorganische Chemie, Universit€at T€ubingen, Auf der Morgenstelle 18, D-72076 T€ubingen, Germany
‡Department of Chemistry, University of Bergen, Allꢀegaten 41, N-5007 Bergen, Norway
§Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan
S Supporting Information
b
ABSTRACT: The peralkylated magnesium complex Mg-
(AlMe4)2 reacts with bis-N,N0-diimine PhCHdNCH2CH2Nd
CHPh to yield the compound [PhCH(Me)NCH2CH2NdCHPh]
Mg(AlMe4)(AlMe3), via an alkyl migration. Such a 1,2-addition of
the MgꢀCH3 moiety to the unsaturated NdC imino group is also
observed for [MgMe2], affording dimagnesium complex [{PhCH
(Me)NCH2CH2NdCHPh}Mg(Me)]2. In contrast, treatment of
PhCHdNCH2CH2NdCHPh with AlMe3 gives the donor-adduct
[PhCHdNCH2CH2NdCHPh][(AlMe3)2]. Based on these distinct reactivities a plausible reaction mechanism for the formation of
[PhCH(Me)NCH2CH2NdCHPh]Mg(AlMe4)(AlMe3) is proposed. Donor-adduct formation is also observed when the perethylated
barium aluminate [Ba(AlEt4)2]n is reacted with a β-diimine, affording the complex [Ba(AlEt4)2(PhCdNCH2CH2NdCPh)]n. The scope of
this reaction behavior was further investigated by reacting Mg(AlMe4)2, AlMe3, and [MgMe2] with β-diimines bearing substituted phenyl
rings. Neither the ortho-methyl- nor the meta-tert-butyl-disubstituted proligands show any reaction with [MgMe2], while adducts
[(C6H3Me2-2,6)CHdNCH2CH2NdCH(C6H3Me2-2,6)][(AlMe3)2] and [(C6H3tBu2-3,5)CHdNCH2CH2NdCH(C6H3tBu2-3,5)]-
[(AlMe3)2] are the prevalent reaction products when Mg(AlMe4)2 and AlMe3 are employed.
’ INTRODUCTION
(e.g., AlMe3, ZnMe2) and are currently exploited for the in situ
generation of new monoanionic iminoꢀamido ancillary ligands.16ꢀ19
We are particularly interested in the reactivity of peralkylated
heterobimetallic complexes comprising metal tetraalkylalumi-
nates and tetraalkylgallates.20,21 Although the permethylated
complex Mg(AlMe4)2 was described more than 50 years ago,22,23
reports on its reactivity are scarce.23 We anticipated that Mg-
(AlMe4)2 could be an ideal candidate for evaluating any distinct
alkylating capability of MgꢀCH3 versus AlꢀCH3 moieties.
Moreover, β-diimines, recently shown to be prone to imine
alkylation, were selected as target molecules.19e Herein, we
report on alkyl migrations in the systems Mg(AlMe4)2/
PhCHdNCH2CH2NdCHPh and MgMe2/PhCHdNCH2-
CH2NdCHPh, yielding heteroleptic magnesium methyl complexes
supported by an iminoꢀamido ligand. In contrast simple adduct
formation is found for the reactions of AlMe3/PhCHdNCH2
CH2NdCHPh and [Ba(AlEt4)2]n/PhCHdNCH2CH2NdCHPh.
Ever since their discovery in 1900 by Victor Grignard, organo-
magnesium halide compounds of the type RMgX have emerged
as one of the most eminent metalorganic reagents in organic and
organometallic synthesis.1 Such “Grignard reagents” are still
accessed by the original protocol, reacting organic halides RX
with elemental magnesium in ethereal solvents.2 Elucidation of
the “Schlenk equilibrium” 2 RMgX T R2Mg + MgX23 and the
X-ray structure analyses of solvent- and alkali metal salt-free
homoleptic dialkyl derivatives such as Mg(2,4,6-tBu3C6H2)2,4
Mg[C(SiMe3)3]2,5 Mg[CH(SiMe3)2]2,6 and (MgtBu2)2 mark
7
further milestones in organomagnesium chemistry. Moreover,
heteroleptic Lewis-acidic magnesium alkyl complexes were found
as promising catalysts for several polymerization reactions.8ꢀ10
The reactivity of magnesium dialkyls toward diimines nicely
features distinct reaction pathways dependent on the steric bulk
of the alkyl ligands and the functional groups integrated into the
diimine backbone. In addition to simple chelate coordination of
the diimine (A, Chart 1), single electron transfer (SET, B, Chart 1)
and alkyl transfer products (selective C-alkylation (C, Chart 1) or
N-alkylation (D, Chart 1)) were observed.11ꢀ15 The latter alkyla-
tion capability and concomitant alkyl migration to iminic proligands
(complexes C and D, Chart 1) have also been observed for other
Lewis-acid main group and transition metal alkyl compounds
’ RESULTS AND DISCUSSION
The reaction of Mg(AlMe4)2 with N1,N2-bis(phenylmeth-
ylene)-1,2-diiminoethane 1a in toluene at ambient temperature
gave the bimetallic MgAl2 complex 2, which could be crystallized
Received: April 29, 2011
Published: June 27, 2011
r
2011 American Chemical Society
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Organometallics
ARTICLE
Chart 1. Reactivity of Magnesium Dialkyls toward Diimines
Figure 1. VT 1H NMR spectra (500.13 MHz, d8-toluene) of compound 2,
in the region of metal-bonded CH3 protons.
Scheme 1. Distinct Reaction Pathways between β-Diimine
1a and Group 2 and 13 Metal Alkylsa
Figure 2. Molecular structure of [PhCH(Me)NCH2CH2NdCHPh]
Mg(AlMe4)(AlMe3) (2). Hydrogen atoms are omitted for clarity.
Atoms are represented by atomic displacement ellipsoids at the
50% level.
Table 1. Selected Bond Distances and Angles for [PhCH(Me)
NCH2CH2NdCHPh]Mg(AlMe4)(AlMe3) (2)
bond distances [Å]
angles [deg]
Mg1ꢀN2
2.125(2)
2.122(1)
1.954(1)
2.550(2)
2.265(2)
2.422(2)
1.274(2)
1.469(2)
1.474(2)
1.524(2)
1.494(2)
1.487(2)
1.522(2)
1.519(2)
C11ꢀC1ꢀN2
123.2(2)
118.8(2)
109.9(1)
110.9(1)
114.7(1)
88.01(7)
172.85(7)
91.80(7)
86.63(5)
40.06(7)
131.71(7)
88.73(6)
80.00(6)
Mg1ꢀN5
Al1ꢀN5
Mg1ꢀC01
Mg1ꢀC21
Mg1ꢀC22
C1ꢀN2
C1ꢀN2ꢀC3
C4ꢀN5ꢀC6
N5ꢀC6ꢀC60
N5ꢀC6ꢀC61
C01ꢀMg1ꢀC21
C01ꢀMg1ꢀC22
C21ꢀMg1ꢀC22
N2ꢀMg1ꢀN5
N2ꢀMg1ꢀC21
N5ꢀMg1ꢀC21
N2ꢀMg1ꢀC01
N5ꢀMg1ꢀC01
a (a) Mg(AlMe4)2, toluene, rt, 16 h; (b) MgMe2, toluene, rt, 16 h; (c) 2
equiv of AlMe3, toluene, rt, 16 h; (d) [Ba(AlEt4)2]n, toluene rt, 16 h.
C1ꢀC11
N2ꢀC3
C3ꢀC4
from a saturated toluene solution at ꢀ40 °C in almost 50% yield
1
C4ꢀN5
(Scheme 1, reaction a). The ambient-temperature H NMR
N5ꢀC6
spectrum of complex 2 in C6D6 displayed a resonance at δ 7.21
attributable to an imine group (HCdN). Mutually coupled methine
and methyl proton resonances were observed at δ 4.30 and 1.83
(3JHH = 7.1 Hz), respectively, while one broad resonance at δ ꢀ0.32
could be assigned to aluminum/magnesium-bonded methyl groups.
A variable-temperature (VT) 1H NMR spectroscopic study in
toluene-d8 shed further light on the latter methyl coordination
C6ꢀC60
C6ꢀC61
(Figure 1). At 0 °C the methyl resonance had decoalesced into a
set of two signals in accordance with the presence of Al(CH3)3
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Table 2. Selected Bond Distances and Angles for [{PhCH(Me)
NCH2CH2NdCHPh}Mg(Me)]2 (3)
bond distances [Å]
angles [deg]
Mg1ꢀN1
2.201(1)
2.175(1)
2.084(1)
2.146(2)
2.102(1)
2.221(1)
2.155(1)
2.175(2)
1.481(2)
1.276(2)
1.475(2)
1.471(2)
1.526(2)
1.278(2)
1.479(2)
1.475(2)
1.484(2)
1.524(2)
Mg1ꢀN2ꢀMg2
88.11(5)
89.13(5)
106.45(6)
125.23(6)
115.6(1)
110.8(1)
109.4(1)
112.01(6)
127.61(6)
115.4(1)
110.8(1)
Mg1ꢀN2
Mg1ꢀN4
Mg1ꢀC18
Mg2ꢀN2
Mg2ꢀN3
Mg2ꢀN4
Mg2ꢀC36
N1ꢀC1
Mg1ꢀN4ꢀMg2
C18ꢀMg1ꢀN1
C18ꢀMg1ꢀN2
C1ꢀN1ꢀC11
C2ꢀN2ꢀC3
Figure 3. Molecular structure of [{PhCH(Me)NCH2CH2NdCHPh}
Mg(Me)]2 (3). Hydrogen atoms are omitted for clarity. Atoms are
represented by atomic displacement ellipsoids at the 50% level.
N2ꢀC3ꢀC10
C36ꢀMg2ꢀN3
C36ꢀMg2ꢀN4
C19ꢀN3ꢀC29
C20ꢀN4ꢀC21
N1ꢀC11
N2ꢀC2
N2ꢀC3
C3ꢀC10
N3ꢀC29
N3ꢀC19
N4ꢀC20
N4ꢀC21
C21ꢀC28
“adduct” and Al(CH3)4 aluminate moieties. Upon further cool-
ing to ꢀ70 °C, these signals significantly broadened simulta-
neously, adopting an overlapping four-peak pattern, pointing to a
limited mobility of terminal and bridging AlꢀCH3 methyl
groups. Similar decoalescence phenomena have been observed
previously for alkylated LnAl heterobimetallic complexes.24
The molecular structure of 2 was unequivocally revealed by an
X-ray crystallographic study (Figure 2, Table 1). In the solid state
the coordination geometry around the magnesium atom is best
described as distorted trigonal bipyramidal. The three equatorial
postions are occupied by the N atoms of a chelating ami-
doꢀimino ligand and one methyl group of an η2-coordinated
[AlMe4]ꢀ anion. The second methyl group of this aluminate
moiety and another methyl group of the N(amido)-coordinated
AlMe3 are located in the apical positions. Formation of a four-
membered ring as for the latter Mg(μ-CH3)(μ-NR2)AlMe2 entity
has been identified as a common structural motif in LnAl hetero-
bimetallic amide complexes.25 Apparently, one of the imine moieties
in 1a was selectively methylated to form this monoanionic ami-
doꢀimino ligand.19f The bridging MgꢀC(methyl) bond distances
are in the order aluminate (equatorial, 2.265(2) Å) < aluminate
(apical, 2.422(2) Å) < AlMe3 (apical, 2.550(2) Å) and longer than in
four-coordinate homoleptic Mg(AlMe4)2 (av MgꢀC, 2.208 Å).26
The MgꢀN2(imino) and MgꢀN5(amido) bond lengths of
2.125(2) and 2.122(1) Å, respectively, are almost identical, matching
the average MgꢀN donor bond distance of 2.192 Å in four-
coordinate (Me3Si)2Mg(TMEDA) (TMEDA = tetramethylethyl-
diamine).27 Furthermore, the C1ꢀN2 and C6ꢀN5 bond distances
of 1.274(2) and 1.488(1) Å, respectively, clearly evidence the pre-
sence of imino and amido moieties.
Previously, the occurrence of a donor-induced aluminate
cleavage was reported when Mg(AlEt4)2 and Mg(AlMe4)2 were
treated with 2 equiv of donor (OEt2/THF), forming AlEt3-
(OEt2)/AlMe3(THF) and mixed alkyl-magnesium species
EtMg(AlEt4) and MeMg(AlMe4), respectively.23 It is note-
worthy that exposure of a stirred solution of 2 in toluene to 1
equiv of diethyl ether at ambient temperature for 6 h did not give
the respective AlMe3-cleaved product (as evidenced by the
relative signal intensities in the NMR spectrum). In order to
further investigate the dual reactivity of Mg(AlMe4)2, that is,
imino alkylation and AlMe3ꢀimino adduct formation, we in-
dependently examined the reactions of MgMe2 and AlMe3 with
β-diimine 1a. Treatment of 1a with MgMe2 in toluene afforded a
dark orange solution containing heteroleptic magnesium complex 3,
bearing the same monoanionic amidoꢀimino ligand as complex 2
(Scheme 1, reaction b, upon stirring for 16 h at ambient temperature).
Correspondingly, the 1H NMR spectrum of 3 in C6D6 shows one
Figure 4. Molecular structure of [PhCHdNCH2CH2NdCHPh]
[(AlMe3)2] (4). Hydrogen atoms are omitted for clarity. Atoms are
represented by atomic displacement ellipsoids at the 50% level. Selected
bond lengths (Å) and angles (deg) for 4: Al1ꢀN12 2.062(3), N12ꢀC11
1.475(4), N12ꢀC13 1.283(4), C13ꢀC131 1.458(5); Al1ꢀN12ꢀC11
114.7(2), Al1ꢀN12ꢀC13 129.4(2), C11ꢀN12ꢀC13 115.3(3),
N12ꢀC11ꢀC110 109.1(3), N12ꢀC13ꢀC131 126.7(3).
quartet and one doublet resonance at δ4.30 and 2.25, respectively, for
the NCH(CH3)Ph moiety, giving clear evidence of a methyl group
migration to the unsaturated NdC imino double bond. Also, a
magnesium-bonded methyl group was revealed by singlet signals at
δH ꢀ0.42 and δC ꢀ11.5. Single crystals of 3 suitable for X-ray
diffraction analysis were obtained from a concentrated toluene
solution at ꢀ40 °C. As shown in Figure 3, complex 3 forms a
nitrogen(amido)-bridged dimer in the solid state. The shortest
MgꢀN distances are 2.084(1) and 2.102(1) Å and are observed
for the amido nitrogen atoms, which engage in a chelate bonding with
the other Mg center (Table 2). The terminal Mg1ꢀC18 and
Mg2ꢀC36 bond distances of 2.146(2) and 2.175(2) Å, respectively,
are longer than those in sterically less encumbered [(THF)MgMe
(μ-Me)]2 (MgꢀCterminal = 2.121(2) Å).23
In contrast, treatment of ligand 1a with 2 equiv of AlMe3 in
a stirred toluene solution at ambient temperature for 16 h
yielded after crystallization at ꢀ40 °C the corresponding
1
AlMe3ꢀdiimino adduct 4. The H NMR spectrum of 4 in
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Scheme 2. Proposed Mechanism of Formation of Complex
[PhCH(Me)NCH2CH2NdCHPh]Mg(AlMe4)(AlMe3) (2)
Figure 5. Molecular structure of [(C6H3tBu2-3,5)CHdNCH2CH2Nd
CH(C6H3tBu2-3,5)][(AlMe3)2] (6). Hydrogen atoms are omitted for
clarity. Atoms are represented by atomic displacement ellipsoids at the
50% level. Co-crystallized toluene (one molecule per unit cell) is
not shown.
be noted that the reaction of 3 with 2 equiv of AlMe3 does not
simply form complex 2 as indicated by NMR spectroscopy.
In order to further examine the scope of this reaction behavior,
phenyl-substituted β-diimines 1b and 1c were reacted with
homoleptic Mg(AlMe4)2 (Scheme 3). Interestingly, in toluene
solution only the formation of AlMe3ꢀdiimino complexes 5 and
6 was observed, consistent with the NMR data. Even heating a
mixture of Mg(AlMe4)2 and 1c for a prolonged time in a pressure
tube did not lead to any significant alkylation. The molecular
composition of compound 6 could be confirmed by X-ray
diffraction analysis (Figure 5), although the limited quality of
the crystals precludes a detailed discussion of metrical para-
meters. Not surprisingly, compounds 5 and 6 are also obtained
by adding AlMe3 directly to a toluene solution of the β-diimines
1b and 1c, respectively, at ambient temperature (Scheme 3) or
upon heating in a pressure tube. Moreover, neither the ortho-
methyl-disubstituted proligand 1b nor the meta-tert-butyl-disub-
stituted ligand 1c showed any reaction with [MgMe2]. We
assume that it is mainly the steric hindrance of the proligand29
and/or conformational (im)mobility of the mono-AlMe3-coor-
dinated intermediates that cause the different reaction behavior.
The different conformations of the diimine ligand are nicely
featured by the bis(AlMe3) adducts 4 and 6 (Figures 4 and 5).
The enhanced alkylating power of the MgꢀCH3 moiety was
further corroborated by examining the corresponding reaction of
perethylated barium aluminate, [Ba(AlEt4)2]n, with 1 equiv of
proligand 1a (Scheme 1, reaction d). The absence of any imine
alkylation was suggested by an NMR spectroscopic investigation
of the crystalline product 7 (65%), obtained by cooling the
reaction mixture to ꢀ40 °C, and of the mother liquor. The 1H
NMR spectrum of 7 in C6D6 displays a triplet and a quartet at
1.49 and 0.10 ppm, respectively, with a relative integral ratio of
3:2, assignable to the AlEt4 groups. An additional peak at δ 7.71 is
attributed to the NdCH protons of the donor ligand. The
formation of donor-adduct [Ba(AlEt4)2(PhCdNCH2CH2Nd
CPh)]n (7) was unequivocally proven by an X-ray structure
analysis (Figure 6, Table 3), revealing that the three-dimensional
network structure of [Ba(AlEt4)2]n has been disrupted to an infi-
nite chain structure via chelate formation with the hard β-diimine.
Scheme 3. Reactivity of β-Diimines 1b and 1c toward
Mg(AlMe4)2 and AlMe3
C6D6 showed a single resonance (δ 8.28, relative intensity of 2)
for the imino protons (HCdN) of the ligand backbone, suggest-
ing a highly symmetric and/or fluxional structure. The methylene
and the aluminum-bonded methyl protons display a relative
intensity of 4:18, indicating adduct formation of each imino
nitrogen with one AlMe3 group (Scheme 1, reaction c). The
solid-state structure of 4 was elucidated by single-crystal X-ray
diffraction (Figure 4). The AlꢀN bond distance of 2.062(3) Å is
comparable to those found in similar adduct complexes,25,28
while the NꢀC imino bond distance of 1.283(4) Å is clearly
indicative of a CdN double bond.
Given this distinct reactivity of [MgMe2] and Al2Me6 toward
1a—methylation is observed only for MgMe2—we propose the
following reaction mechanism for the formation of compound 2
(Scheme 2). First, donor-induced cleavage of Mg(AlMe4)2 by
one of the imino donor moieties of β-diimine 1a proceeds to give
heteroleptic MeMg(AlMe4) and (diimine)AlMe3. Subsequent
coordination of the second imino nitrogen atom to the magne-
sium atom and methyl migration onto the CdN bond—1,2-
addition of the MgꢀCH3 moiety to the aluminum-free CdN
group—results in the MgAl2 bimetallic complex 2. It should also
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Organometallics
ARTICLE
Table 4. Crystallographic Data for 2, 3, 4, 6, and 7
2
3
4
chemical formula
Mr
C
24H40Al2MgN2
C36H44Mg2N4
581.37
C22H34Al2N2
380.47
434.85
monoclinic
P21/c
cryst syst
space group
a/Å
triclinic
triclinic
P1
P1
8.8863(8)
18.9418(11)
15.8136(12)
90.00
8.9465(4)
10.0006(4)
19.4059(8)
79.3630(10)
82.6500(10)
73.6380(10)
1631.99(12)
2
6.8629(18)
7.7586(19)
11.250(3)
102.16(2)
91.06(2)
90.75(2)
585.4(3)
1
b/Å
c/Å
R/deg
β/deg
γ/deg
V/Å3
99.651(7)
90.00
2624.1(3)
4
Z
F(000)
T/K
944
624
206
173(2)
1.101
103(2)
173(2)
1.079
F
calcd/g cmꢀ3
1.183
μ/mmꢀ1
R1(obsd)a
wR2(all)b
Sc
0.147
0.104
0.132
Figure 6. Molecular structure of [Ba(AlEt4)2(PhCdNCH2CH2Nd
CPh)]n (7). Hydrogen atoms are omitted for clarity. Atoms are
represented by atomic displacement ellipsoids at the 50% level.
0.0516
0.1046
1.208
0.0368
0.0575
0.0992
0.1382
1.028
1.222
6
7
Table 3. Selected Bond Distances and Angles for [Ba(AlEt4)2
(PhCdNCH2CH2NdCPh)]n (7)
chemical formula
Mr
C
45H76Al2N2
C32H56Al2BaN2
660.09
699.04
monoclinic
C2/c
bond distances [Å]
angles [deg]
cryst syst
space group
a/Å
triclinic
P1
Ba1ꢀN1
2.893(1)
2.848(1)
3.002(1)
3.072(1)
3.140(1)
3.188(1)
3.281(1)
3.6469(4)
3.8016(4)
3.613(1)
3.572(1)
1.464(2)
1.275(2)
1.472(2)
1.276(2)
N1ꢀBa1ꢀN2
59.46(3)
67.99(4)
64.55(3)
149.47(4)
82.21(3)
78.92(4)
143.82(3)
146.64(3)
161.05(4)
126.44(4)
115.72(3)
116.76(8)
123.71(8)
118.4(1)
117.0(1)
Ba1ꢀN2
C17ꢀBa1ꢀC19
C25ꢀBa1ꢀC27
N1ꢀBa1ꢀC19
N2ꢀBa1ꢀC27
C17ꢀBa1ꢀC27
C25ꢀBa1ꢀC310
C27ꢀBa1ꢀC310
N2ꢀBa1ꢀC17
N2ꢀBa1ꢀC19
N1ꢀBa1ꢀC17
Ba1ꢀN1ꢀC3
37.826(4)
11.3329(11)
24.359(2)
90.00
8.5950(2)
11.4426(3)
18.5980(5)
87.5142(2)
89.4389(3)
76.8646(3)
1179.57(8)
2
Ba1ꢀC17
Ba1ꢀC19
Ba1ꢀC25
Ba1ꢀC27
Ba1ꢀC310
b/Å
c/Å
R/deg
β/deg
γ/deg
V/Å3
112.235(9)
90.00
Ba1 Al1
9665.7(16)
8
3 3 3
Ba1
Al2
Z
3 3 3 3
Ba1 C5
F(000)
T/K
3087
688
3 3 3
Ba1 C15
173(2)
0.961
123(2)
3 3 3
F
calcd/g cmꢀ3
1.232
N1ꢀC1
N1ꢀC3
N2ꢀC2
N2ꢀC10
μ/mmꢀ1
R1(obsd)a
wR2(all)b
Sc
0.088
1.186
Ba1ꢀN2ꢀC10
C1ꢀN1ꢀC3
C2ꢀN2ꢀC10
0.1489
0.3116
1.598
0.0204
0.0542
1.099
2
a R1 = ∑(|Fo| ꢀ |Fc|)/∑|Fo|, Fo > 4σ(Fo). b wR2 = {∑[w(Fo ꢀ Fc2)2]/
The seven-coordinate barium center is further surrounded
by two tetraethylaluminate groups, one showing a “terminal”
η2-coordination, while the second bridges two barium centers in
a μ2-η2:η1 mode. The BaꢀC bond distances of the terminal unit
average 3.037 Å and are considerably shorter than those found
for the interconnecting one (3.140(1)ꢀ3.281(1) Å), which
shows the longest distance for the η1-bonded ethyl group. For
comparison, the BaꢀC bond distances of the μ3-η1:η1:η1-
bonded alkylaluminate ligands in homoleptic [Ba(AlEt4)2]n
average 3.125 Å.21h While the μ2-η2:η1 alkylaluminate coordination
mode has been observed for Ln(III)ꢀAlMe4 complexes,30 the
intricate solid-state structure of [M(AlEt4)2]n (M = Sm, Eu, Yb,
Ca) revealed μ2-η3:η1-interconnecting ethylaluminate moieties.23,31
Polymeric lithium tetraethylaluminate shows [Li(AlEt4)]n units,
which are aligned in linear chains via μ2-η2:η2 alkylaluminate
∑[w(Fo ) ]}1/2. c S = [∑w(Fo ꢀ Fc2)2/(no ꢀ np)]1/2
.
2 2
2
coordination.32 Moreover, the β-diimine ligands in 7 twist such that
one of the ortho-carbon ring atoms of each phenyl group seems
to make contact with the barium metal center (Ba C 3.572 and
3.613 Å), formally increasing the coordination number to 9. The
BaꢀN bond distances in 7 average 2.871 Å and are significantly
longer than the BaꢀN(pyrazolyl) bond lengths of 2.808 Å in
eight-coordinate complex Tp0BaI(Hpz) (Tp0 = tris{3-methoxy-1,
1-dimethylpyrazolyl}hydroborate).33
As suggested by elemental analysis and NMR spectros-
copic investigations, similar adduct formation occurs between
[Ba(AlEt4)2]n and tert-butyl-disubstituted proligand 1c; however,
3 3 3
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Organometallics
ARTICLE
donor-adduct Ba(AlEt4)2[(C6H3tBu2-3,5)CHdNCH2CH2NdCH-
(C6H3tBu2-3,5)] (8) did not produce single crystals suitable for an
X-ray diffraction analysis. As proposed in Scheme 2, the key step of
the diimine ligand alkylation for group 2 metal aluminate complexes is
probably the donor-induced tetraalkylaluminate cleavage to generate
a metalꢀalkylꢀaluminate intermediate. Apparently, the large ionic
size and moderate Lewis acidity of the barium(II) center, that is, its
lower hardness compared to the small Mg2+, impede the separation of
triethylaluminum by the diimine ligand, resulting in the formation of
simple donor-adducts 7 and 8. This is in line with the reactivity of
complexes [M(AlEt4)2]n (M = Sm, Eu, Yb, Ca) toward hard Lewis
bases with HSAB interactions and tetraalkylaluminate ion separation
prevailing over donor-induced tetraalkylaluminate cleavage.23,31
1025 m, 1004 m, 964 m, 915 w, 887 w, 857 w, 770 s, 737 w. 1H NMR
(300 MHz, C6D6, 35 °C): δ 8.45 (s, 2H, NdCH), 6.9ꢀ7.0 (m, 2H, p-
Ar), 6.9 (m, 4H, m-Ar), 3.90 (s, 4H, NCH2CH2N), 2.33 (s, 12H,
ArCH3). 13C NMR (75 MHz, C6D6, 35 °C): δ 161.8 (CdN), 137.9
(Ar), 134.2 (Ar), 129.0 (Ar), 128.9 (Ar), 63.6 (NCH2CH2N), 20.9
(ArCH3). HRMS (EI): m/z calcd for C20H24N2 292.1939, found
292.1931.
(C6H3tBu2-3,5)CHdNCH2CH2NdCH(C6H3tBu2-3,5) (1c). To
a solution of 3,5-di-tert-butylbenzaldehyde (1.47 g, 0.67 ꢁ 10ꢀ2 mol) in
ethanol (15 mL) was added ethylenediamine (0.23 mL, 0.33 ꢁ 10ꢀ2
mol) at ambient temperature. The mixture was stirred overnight. After
all volatiles had been removed under reduced pressure, the resulting
residue was washed with water to give 1c (94% yield) as a white powder
upon drying in vacuo. IR (Nujol cmꢀ1): 1641 s, 1592 m, 1288 w, 1269 w,
1246 m, 1210 m, 1168 w, 1018 m, 962 m, 894 w, 875 m 770 w. 1H NMR
(300 MHz, C6D6, 35 °C): δ 8.20 (s, 2H, NdCH), 7.77 (d, 4J = 1.7 Hz,
4H, o-Ar), 7.54 (t, 4J = 1.7 Hz, 2H, p-Ar), 4.03 (s, 4H, NCH2CH2N),
1.25 (s, 36H, ArC(CH3)3). 13C NMR (75 MHz, C6D6, 35 °C): δ 163.0
(CdN), 151.2 (Ar), 136.9 (Ar), 124.7 (Ar), 123.1 (Ar), 62.4
(NCH2CH2N), 31.5 (ArCH3). HRMS (EI): m/z calcd for C32H48N2
460.3817, found 460.3823.
[PhCH(Me)NCH2CH2NdCHPh]Mg(AlMe4)(AlMe3) (2). A so-
lution of 1a (119 mg, 0.50 mmol) in toluene was added to a stirred
solution of Mg(AlMe4)2 (100 mg, 0.50 mmol) in toluene at ambient
temperature. The reaction mixture was allowed to stir for 16 h. The
suspension was centrifuged and filtrated, and the remaining solution
concentrated. Cooling the solution to ꢀ40 °C yielded 2 (49%, 0.25
mmol) as colorless crystals suitable for X-ray diffraction analysis. IR
(Nujol cmꢀ1): 1641 s, 1595 w, 1578 w, 1491 w, 1311 w, 1215 s, 1175 m,
1130 w, 1100 m, 1074 m, 1025 m, 973 w, 919 w, 894 m, 751 m, 644 w,
590 m, 562 w, 505 w, 484 w. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.54
(d, 3J = 7.2 Hz, 2H, o-Ph), 7.33 (m, 2H, m-Ph), 7.26 (m, 2H, o-Ph), 7.24
(s, 2H, m-Ph), 7.21 (s, 1H, NdCHPh), 7.19 (m, 2H, p-Ph), 4.30 (q, 3J =
7.1 Hz, 1H, NCHPhCH3), 3.34 (m, 2H, NCH2CH2N), 2.67 (m, 2H,
NCH2CH2), 1.83 (d, 3J = 7.1 Hz, 3H, NCHPhCH3), ꢀ0.32 (b, 21H,
AlCH3). 13C NMR (100 MHz, C6D6, 25 °C): δ 168.8 (CdN), 144.5
(ipso-Ph), 133.6 (Ph), 129.8 (Ph), 128.7 (Ph), 128.6 (Ph), 61.4 (CH),
59.8 (CH2), 47.3 (CH2), 22.5 (CH3), ꢀ4.6 (Al(CH3)3), ꢀ6.8 (Al(CH3)4).
Anal. Calcd for C24H38Al2MgN2 (432.84): C, 66.60; H, 8.85; N, 6.47.
Found: C, 66.20; H, 9.09; N, 6.37.
’ CONCLUSION
The distinct reactivity of [MgMe2] and Al2Me6 toward PhCd
NCH2CH2NdCPh, affording [{PhCH(Me)NCH2CH2Nd
CHPh}Mg(Me)]2 and [PhCHdNCH2CH2NdCHPh]-
[(AlMe3)2], respectively, clearly documents the enhanced alkylating
power of MgꢀCH3 moieties. The formation of the same iminoꢀ
amido ligand [PhCH(Me)NCH2CH2NdCHPh] from the reaction
of Mg(AlMe4)2 with a β-diimine suggests a donor(imino)-induced
tetramethylaluminate cleavage and formation of [(AlMe4)MgꢀMe]
as the initiating step. Alkyl substitution of the phenyl rings of the
β-diimine proligands crucially affects the alkylating capability of such
Mg(AlMe4)2-derived MgꢀCH3 moieties, resulting predominantly
in donor-adducts of type [β-diimine][(AlMe3)2]. Alkyl migration
onto the imino functionality is also not prominent for the peralkylated
compound [Ba(AlEt4)2]n, featuring the highly electropositive barium
center: the β-diimine partially disrupts the network structure of
[Ba(AlEt4)2]n, forming adducts [(β-diimine)Ba(AlEt4)2]n exclu-
sively. The infinite chain structure of [Ba(AlEt4)2(PhCd
NCH2CH2NdCPh)]n features μ2-η2:η1-bridging alkylaluminate
ligands as the lead structural motif. Ongoing studies in our
laboratories are aimed at a deeper understanding of the reactivity
of metal alkylaluminates toward unsaturated organic molecules.
’ EXPERIMENTAL SECTION
[{PhCH(Me)NCH2CH2NdCHPh}Mg(Me)]2 (3). To a stirred
suspension of MgMe2 (25 mg, 0.46 mmol) in 5 mL of toluene was
added a solution of 1a (109 mg, 46 mmol) in toluene at ambient
temperature. The reaction mixture was allowed to stir for 16 h while its
color turned to orange. The suspension was centrifuged and the
remaining solution filtrated and subsequently concentrated under
reduced pressure. Cooling to ꢀ40 °C yielded single crystals of com-
pound 3 (70%, 0.32 mmol) suitable for X-ray diffraction analysis. IR
(Nujol cmꢀ1): 1634 s, 1578 w, 1337 w, 1309 w, 1292 w, 1274 w, 1217 w,
1105 m, 1079 w, 1042 m, 1025 w, 976 w, 890 m, 833 w, 779 w, 756 s, 728
w, 630 m, 587 m, 557 s, 515 s. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.86
(dd, 3J = 7.7 Hz, 4J = 1.2 Hz, 2H, o-Ph), 7.39 (dd, 3J = 7.4 Hz, 4J = 7.7 Hz,
2H, m-Ph), 7.33 (d, 4J = 1.2 Hz, 1H, NdCHPh), 7.32 (m, 2H, o-Ph),
7.23 (dd, 3J = 7.4 Hz, 4J = 1.2 Hz, 1H, p-Ph), 7.01 (m, 2H, m-Ph), 4.30
General Remarks. Materials and Methods. All reactions
involving air- and moisture-sensitive organometallic compounds were
performed under a dry argon atmosphere using standard Schlenk and
glovebox techniques (MBraun MB250B; < 1 ppm O2, < 1 ppm H2O).
Hexane, thf, and toluene were purified using Grubbs columns (MBraun SPS,
solvent purification system). Deuterated solvents were dried over Na/K. All
solvents were stored inside a glovebox. Complexes [MgMe2],34 Mg-
21h
(AlMe4)2,22 and [Ba(AlEt4)2]n as well as the proligand 1a35 were
synthesized according to the literature. NMR spectra were recorded on a
Varian UNITY INOVA-300, a Bruker-AVII-400, and a Bruker-BIOSPIN-
AV500. 1H and 13C NMR chemical shifts were referenced to internal solvent
resonances reported in parts per million relative to TMS. Infrared spectra
were recorded on a Thermo Scientific Nicolet6700 FTIR spectrometer as
Nujol mulls sandwiched between CsI plates. High-resolution mass spectra
were recorded on a JEOL JMS-700. Elemental analyses were performed on
an Elementar Vario MICRO.
(C6H3Me2-2,6)CHdNCH2CH2NdCH(C6H3Me2-2,6) (1b). To
a mixture of 2,6-dimethylbenzaldehyde (0.75 mL, 2.00 ꢁ 10ꢀ2 mol) in
H2O (20 mL) was added ethylenediamine (0.67 mL, 1.00 ꢁ 10ꢀ2 mol)
at ambient temperature. The mixture was allowed to warm to 50 °C and
stirred overnight. A white solid was isolated by filtration, washed with
water, and dried in vacuo to give 1b (93%). IR (Nujol cmꢀ1): 1695 w,
1641 s, 1590 w, 1278 w, 1210 w, 1189 w, 1166 m, 1093 w, 1046 w,
3
(q, J = 6.7 Hz, 1H, NCHPhCH3), 3.78 (m, 2H, NCH2CH2N), 2.67
(ddd, 2J = 14.0 Hz, 3J = 2.7, 2.9 Hz, 2H, NCH2CH2N), 2.50 (ddd, 2J =
12.9 Hz, 3J = 2.5, 2.7 Hz, 3H, NCHPhCH3), ꢀ0.42 (s, 3H, MgCH3). 13
C
NMR (100 MHz, C6D6, 25 °C): δ 165.5 (CdN), 149.5 (ipso-Ar), 133.2
(ipso-Ar), 132.4 (Ar), 129.2 (Ar), 128.7 (Ar), 128.6 (Ar), 126.6 (Ar),
60.7 (CH), 60.5 (CH2), 50.7 (CH2), 26.1 (CH3), ꢀ11.5 (MgCH3).
Anal. Calcd for C36H44Mg2N4 (581.37): C, 74.37; H, 7.63; N, 9.64.
Found: C, 73.74; H, 7.17; N, 9.61.
General Procedure for the Synthesis of AlMe3ꢀDiimino
Adducts. To a stirred solution of 1 dissolved in toluene were added
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Organometallics
ARTICLE
2 equiv of AlMe3. After being stirred for 16 h at ambient temperature the
mixture was filtrated and the remaining solution concentrated under
reduced pressure. Cooling to ꢀ40 °C afforded single crystals suitable for
X-ray diffraction analysis.
(AlCH2CH3), 7.2 (br, AlCH2CH3). Anal. Calcd for C48H88Al2BaN2
(884.52): C, 65.18; H, 10.03; N, 3.17. Found: C, 65.38; H, 9.338;
N, 3.08.
Crystallographic Data Collection and Refinement. Crystals
of 2, 3, 4, 6, and 7 were grown by standard techniques from saturated
solutions using toluene at ꢀ40 °C. Suitable single crystals of 2, 3, 4, 6,
and 7 were selected in a glovebox and coated with Paratone-N oil
(Hampton Research), fixed in a nylon loop (3, 7) or on a glass fiber
(2, 4, 6). Data collection for 3 and 7 was done on a Bruker SMART 2K
CCD diffractometer using graphite-monochromated Mo KR radiation
(λ = 0.71073 Å) performing 182° ω-scans in four orthogonal ϕ
positions. Raw data were collected using the program SMART36 and
integrated and reduced with the program SAINT.37 Corrections for
absorption effects were applied using SHELXTL and/or SADABS.38
Data collection for 2, 4, and 6 was done on a STOE-IPDS II system. The
structure was solved by direct methods using WinGX suite of programs
including SHELXS and SHELXL for structure solution and refine-
ment.39 Further details of the refinement and crystallographic data are
listed in Table 4 and in CIF files.
[PhCHdNCH2CH2NdCHPh][(AlMe3)2] (4). Following the pro-
cedure described above, 1a (100 mg, 0.42 mmol) and AlMe3 (61 mg,
0.85 mmol) yielded 4 as colorless crystals (35%; 0.15 mmol). IR
(Nujol cmꢀ1): 1616 s, 1234 s, 1016 m, 981 m, 756 m, 627 w, 569 w,
514 w, 484 w, 416 w. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.94 (s, 2H,
NdCH), 7.55 (d, 3J = 7.1 Hz, 4H, Ph), 7.27 (s, 2H, Ph), 6.93 (m, 6H,
Ph), 3.89 (s, 4H, NCH2CH2N), ꢀ0.27 (s, 18H, Al(CH3)3). 13C NMR
(100 MHz, C6D6, 25 °C): δ 176.7 (CdN), 133.8 (Ph), 131.5 (Ph),
131.2 (Ph), 128.5 (Ph), 128.3 (Ph), 127.8 (Ph), 61.7 (CH2), ꢀ5.0
(Al(CH3)3). Anal. Calcd for C22H34Al2N2 (380.48): C, 69.45; H, 9.01;
N, 7.36. Found: C, 69.51; H, 9.04; N, 7.37.
[(C6H3Me2-2,6)CHdNCH2CH2NdCH(C6H3Me2-2,6)][(AlMe3)2]
(5). Following the procedure described above, 1b (100 mg, 0.34 mmol) and
AlMe3 (49 mg, 0.69 mmol) yielded 5 as colorless crystals (15%; 0.02 mmol).
IR (Nujol cmꢀ1): 1637 s, 1595 w, 1297 w, 1253 m, 1173 m, 1030 w, 1011 w,
836 w, 786 m, 630 w. 1H NMR (400 MHz, C6D6, 25 °C): δ 8.28 (s, 2H,
NdCH), 6.95 (t, 3J = 7.7 Hz, 2H, p-ArH), 6.72 (d, 3J = 7.7 Hz, 4H, m-ArH),
4.04 (s, 4H, NCH2CH2N), 1.92 (s, 12H, CH3), ꢀ0.62 (s, 18H, Al(CH3)3).
13C NMR (100 MHz, C6D6, 25 °C): δ 178.9 (CdN), 135.1 (Ar), 132.5
(Ar), 130.6 (Ar), 127.9 (Ar), 59.4 (NCH2CH2N), 19.8 (CH3), ꢀ6.9
(Al(CH3)3). Anal. Calcd for C26H42Al2N2 (436.59): C, 71.53; H, 9.70; N,
6.42. Found: C, 71.15; H, 8.90; N, 6.54.
[(C6H3tBu2-3,5)CHdNCH2CH2NdCH(C6H3tBu2-3,5)][(AlMe3)2]
(6). Following the procedure described above, 1c (50 mg; 0.11 mmol) and
AlMe3 (16 mg, 0.22 mmol) yielded 6 as colorless crystals (55%; 0.06 mmol).
IR (Nujol cmꢀ1): 1655 w, 1618 m, 1595 m, 1374 s, 1360 s, 1243 m, 1196 w,
1185 w, 1053 w, 887 w, 873 w, 630 w. 1H NMR (400 MHz, C6D6, 25 °C):
δ7.95(s, 2H,NdCH),7.47(s, 2H,p-ArH),7.44(s,4H,o-ArH), 3.86 (s, 4H,
NCH2CH2N), 1.11 (s, 36H, C(CH3)3),0.35(s, 18H, Al(CH3)3). 13CNMR
(100 MHz, C6D6, 25 °C): δ 177.9 (br, CdN), 151.4 (Ar), 128.7 (br, Ar),
126.4 (br, Ar), 61.2 (br, NCH2CH2N), 35.0 (C(CH3)3), 31.3 (C(CH3)3),
ꢀ5.7 (br, Al(CH3)3). Anal. Calcd for C38H66Al2N2 (604.91): C, 75.45; H,
11.00; N, 4.63. Found: C, 75.54; H, 11.08; N, 4.65.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files giving full crystallo-
b
graphic data for complexes 2, 3, 4, 6, and 7. This material is
available free of charge via the Internet at http://pubs.acs.org.
Crystallographic data have also been deposited at the Cambridge
Crystallographic Data Centre under CCDC reference numbers
818906ꢀ818909 and 821801 and can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: mashima@chem.es.osaka-u.ac.jp, reiner.anwander@
uni-tuebingen.de.
’ ACKNOWLEDGMENT
[Ba(AlEt4)2(PhCdNCH2CH2NdCPh)]n (7). To a stirred solution
of 1a (60 mg, 0.25 mmol) in 5 mL of toluene was added 1 equiv of
[Ba(AlEt4)2]n (115 mg, 0.25 mmol) at ambient temperature. After being
stirred for 16 h the orange solution was filtrated and its volume reduced
under vacuum. Cooling the solution to ꢀ40 °C yielded 7 as single crystals
suitable for X-ray diffraction analysis (65%, 0.16 mmol). IR (Nujol cmꢀ1):
1639 m, 1576 w, 1336 w, 1306 w, 1224 vw, 1171 m, 1114 vw, 1098 vw, 1072
w, 1037 w, 976 m, 932 m, 845 w, 747 m, 718 m, 702 w, 641 w, 590 m, 534 w,
501 vw. 1H NMR (400 MHz, C6D6, 25 °C): δ 7.71 (br, 2H, CHdN), 7.35
(br, 4H,Ph), 7.27(br, 2H,Ph), 7.22(m, 2H,Ph), 3.19(s, 4H, NCH2CH2N),
1.49 (t, J = 7.7 Hz, 24H, AlCH2CH3), 0.10 (q, J = 7.7 Hz, 16H,
AlCH2CH3). 13C NMR (100 MHz, C6D6, 25 °C): δ 169.5 (CdN),
134.8 (Ph), 133.3 (Ph), 130.4 (Ar), 127.5 (Ar), 61.7 (NCH2CH2N), 11.8
(AlCH2CH3), 7.2 (AlCH2CH3). Anal. Calcd for C32H56Al2BaN2 (660.09):
C, 58.23; H, 8.55; N, 4.24. Found: C, 58.30; H, 8.40; N, 4.07.
Ba(AlEt4)2[(C6H3tBu2-3,5)CHdNCH2CH2NdCH(C6H3tBu2-3,5)]
(8). To a stirred solution of 1c (105 mg, 0.21 mmol) in 5 mL of toluene was
added1equivof[Ba(AlEt4)2]n (100 mg, 0.22 mmol) at ambient temperature.
After being stirred for 16 h the orange solution was filtrated and its volume
reduced under vacuum. Cooling the solution to ꢀ40 °C yielded 8 as a
crystalline precipitate (65%, 0.14 mmol). IR (Nujol cmꢀ1): 1635 m, 1596 m,
1248 w, 1223 w, 1197 w, 1097 w, 985 m, 897 w, 719 m. 1H NMR (400 MHz,
C6D6, 25 °C): δ 7.91 (s, 2H, p-Ph), 7.78 (s, 2H, NdCHPh), 7.48 (s, 4H,
o-Ph), 3.44 (s, 4H, NCH2CH2N), 1.49 (t, 3J = 7.7 Hz, AlCH2CH3), 1.45
(s, 36H, C(CH3)3), 0.06 (q, 3J = 7.7 Hz, 16H, AlCH2CH3). 13C NMR (100
MHz, C6D6, 25 °C): δ 171.8 (CdN), 152.9 (Ar), 134.2 (Ar), 128.2 (Ar),
122.9 (Ar), 62.5 (NCH2CH2N), 35.2 (C(CH3)3), 31.2 (C(CH3)3), 12.01
We thank the Norwegian Research Council (Project No.
182547/I30) for financial support. K.Y. wishes to thank the
Japan Society for the Promotion of Science (Support Program
for Improving Graduate School Education, Integrated Education
System Based on the Promotion of International Student Mobility)
for generously supporting his research stay in Germany.
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