Syntheses and structures of magnesium complexes with reduced α-diimine ligands

Article abstract of DOI:10.1021/om2003199

The reduction of neutral α-diimine ligands bearing different substituents on the N-aryl rings by different amounts of potassium metal and subsequent reaction with anhydrous MgCl₂ in THF afforded a series of magnesium compounds, [(L^iPr

Full text of DOI:10.1021/om2003199

Article
pubs.acs.org/Organometallics
Syntheses and Structures of Magnesium Complexes with Reduced
α-Diimine Ligands
Jing Gao,†,‡ Yanyan Liu,† Yanxia Zhao,†,‡ Xiao-Juan Yang,*,† and Yunxia Sui§
†State Key Laboratory for Oxo Synthesis & Selective Oxidation, Lanzhou Institute of Chemical Physics, CAS, Lanzhou 730000, China
‡Graduate University of Chinese Academy of Sciences, Beijing 100049, China
§Center of Modern Analysis, Nanjing University, Nanjing 210093, China
S
* Supporting Information
ABSTRACT: The reduction of neutral α-diimine ligands bearing different substituents on
the N-aryl rings by different amounts of potassium metal and subsequent reaction with
anhydrous MgCl2 in THF afforded a series of magnesium compounds, [(LiPr)2−Mg-
(THF)2]·THF (2), [(LMes)2−Mg(THF)3] (3), [(LiPr)− Mg]·THF (4), [(LMes)− Mg] (5),
2
2
2−
and [(LMes
)
2Mg(η6:η6-K(THF)2)][K(THF)6]·(THF)2 (6) (LiPr = [(2,6-iPr2C6H3)NC-
(Me)]2), LMes = [(2,4,6-Me3C6H2)NC(Me)]2). Complexes 2−6 have been characterized by
single-crystal X-ray diffraction, elemental analysis, NMR spectroscopy, and EPR studies (for
4 and 5). The noninnocent α-diimine ligands exist as the dianionic form in compounds 2, 3,
and 6 and as a monoanion in 4 and 5. Effects of the ligand substituents and the amount of
the reducing agent on the structure of the product have been discussed.
α-diimine ligands are rather rare considering the wide application
INTRODUCTION
■
of such ligands in the coordination with a variety of metal ions,
from transition11 and rare earth12 to main group metals.13
Moreover, these ligands are redox noninnocent and can accept
one or two electrons to form the monoanion and dianion, which
in turn can serve as reductants in the synthesis of some low-
valent complexes.
Magnesium compounds play indispensable roles in organic and
organometallic chemistry, as they can be employed as con-
venient precursors for the synthesis of other products. The
most important class of organomagnesium compounds is
Grignard reagents, which have been utilized in numerous
synthetic transformations.1,2 In most organomagnesium com-
plexes, the magnesium metal exhibits the formal oxidation state
of +2 because this valence of the alkaline earth metals possesses
the stable noble gas configuration. Recently, alkaline earth
metal complexes in the +1 valence have been developed. The
first stable MgI−MgI-bonded compounds [LMg−MgL] (L =
[(ArN)2CNiPr2], or nacnac, [(ArNCMe)2CH]; Ar =
2,6-iPr2C6H3) were reported in 2007,3a followed by some
nacnac analogues.3b Another example of the MgI−MgI bond,
[(LiPr)2−Mg−Mg(LiPr)2−][K(THF)3]2 (1), which is stabilized
by α-diimine ligands, was reported by our group in 2009.4 The
MgI−MgI bonds in these compounds are kinetically protected
by sterically bulky ligands from disproportionation processes.5
A calcium(I) compound stabilized with an arene system,
[(THF)3Ca{μ-C6H3-1,3,5-Ph3}Ca(THF)3],6 has also been
obtained. These results invoked great interest in the study of
new magnesium organic compounds.
Recently we have used α-diimine ligands to synthesize a series
of transition and main group binuclear (metal−metal-bonded)
and mononuclear metal compounds.4,11,13 The effects of the
ligand substituents and reaction conditions (the reducing agent
and amount, solvent, etc.) on the structure of the metal
complexes have been investigated. It has been found that the
reduced (monoanionic or dianionic) α-diimine ligands can
effectively form complexes with many metal ions, both in normal
and low oxidation states, with variable structures. Very recently,
we reported a series of calcium(II) complexes with reduced α-
diimines.13c To further explore the coordination chemistry of
these noninnocent ligands with group 2 metals, we present
herein the syntheses and structures of five new magnesium
complexes, [(LiPr)2−Mg(THF)2]·THF (2), [(LMes)2−Mg-
(THF)3] (3), [(LiPr)− Mg]·THF (4), [(LMes)− Mg] (5), and
2
2
2−
[(LMes
)
2Mg(η6:η6-K(THF)2)][K(THF)6]·(THF)2 (6). In
The structures and reactivity of MgII complexes with N-donor
ligands, including β-diketiminates (nacnac),7 bis(arylimino)-
acenaphthenes (Ar-BIAN),8 amidinates,9 and diazabutadiene
(DAB),10 have been reported previously, such as the homoleptic
L2Mg and LMg(solvent)n complexes. However, monomeric
magnesium complexes with N-aryl-substituted
contrast to the dimagnesium(I) compound 1, the magnesium
metal shows the oxidation state of +2 in these products. The
mononuclear complexes 2−5 contain dianionic (2 and 3) or
Received: April 14, 2011
Published: October 24, 2011
© 2011 American Chemical Society
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Scheme 1. Synthesis of Compounds 1−6
monoanionic (4 and 5) ligands, respectively, while the bimetallic
compound 6 features dianionic ligands.
orange-red (magnesium compounds), from which the com-
plexes could be isolated as red crystals. As mentioned above,
the α-diimine ligands LiPr and LMes can take one or two
electrons to form the monoanion and dianion, respectively,
which can be reflected by the changes of the C−N and C−C
bond lengths.11,13 On going from the neutral ligand to the
radical monoanion L•− and then to the enediamide dianion L2−,
the two CN double bonds are elongated to single bonds via a
C−N bond of 1.5 bond order, and the central C−C bond is
shortened. As indicated in the crystal structures (see below), in
compounds 1−3 and 6 the ligands appear as dianions, while in
4 and 5 they are the radical-monoanions.
RESULTS AND DISCUSSION
■
Synthesis. The complexes 2−6 were obtained from the
reaction of the reduced ligands and MgCl2 (Scheme 1).
Reduction of the neutral ligands (LiPr or LMes) with one or two
equivalents of potassium metal in THF, followed by addition of
one equivalent of anhydrous MgCl2, yielded the compounds
2−5. Compounds [(LiPr 2−Mg(THF)2]·THF (2) and
)
[(LMes)2−Mg(THF)3] (3) show a metal-to-ligand ratio of 1:1,
in which the Mg2+ ion is further coordinated by two or three
THF molecules. Compound 2 can be converted to 1 by
addition of another equivalent of K, accompanied by the
reduction of MgII to MgI, while 3 has also been isolated by
reduction of LMes with metallic magnesium in THF.13a In the
compounds [(LiPr)− Mg]·THF (4) and [(LMes)− Mg] (5), the
X-ray Crystal Structures. Single crystals of 2−6 suitable
for X-ray diffraction studies were obtained by slow evaporation
of their THF solutions. The molecular structures of 2−6 in the
solid state are depicted in Figures 1−4 and 6. Selected bond
2
2
Mg center is coordinated by two chelating ligands with an M:L
ratio of 1:2. Initially, these two products were obtained from
the reaction of L and MgCl2 in a 1:1 stoichiometry. We thus
used half an equivalent of anhydrous MgCl2 in the reaction and,
as expected, isolated 4 and 5 in a significantly improved yield.
2−
The compound 6, [(LMes
)
2Mg(η 6:η 6-K(THF)2)][K-
(THF)6]·(THF)2, was prepared by a similar process to the
synthesis of 1 (with LiPr) with three equivalents of K metal.4
However, instead of forming a dimagnesium(I) compound like
1, the divalent Mg complex (6) was yielded with two (solvated)
K+ ions incorporated in the structure. This difference may be
caused by the smaller ligand LMes, which cannot provide
sufficient protection for an MgI2 core, as also observed for the
zinc analogues with such ligands.11b
These compounds are highly sensitive to air and moisture,
but are thermally quite stable under argon at room temperature.
They are soluble in THF and diethyl ether and slightly soluble
in toluene and hexane. The formation of 1−6 can be readily
monitored by the color change of the reaction solution from
orange (free ligands) to red (potassium salt complexes), and to
Figure 1. Molecular structure of 2 (thermal ellipsoids at the 20%
probability level; H atoms are omitted for clarity; C atoms of THF are
drawn as smaller spheres).
lengths and angles are listed in Tables 1−3, and the crystal data
collection and structure refinement details are given in Table 4.
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Figure 2. Molecular structure of 3 (thermal ellipsoids at the 20%
probability level; H atoms are omitted for clarity; C atoms of THF are
drawn as smaller spheres).
Figure 4. Molecular structure of 5 (thermal ellipsoids at the 20%
probability level; H atoms are omitted for clarity).
Figure 3. Molecular structure of 4 (thermal ellipsoids at the 20%
probability level; H atoms are omitted for clarity).
[(LiPr)2−Mg(THF)2]·THF (2) and [(LMes)2−Mg(THF)3] (3). In
the monomeric complex 2, the MgII ion sits in a distorted
tetrahedral environment containing one chelating α-diimine
ligand and two THF molecules (Figure 1). The two Mg−N
bonds (1.966(3) and 1.970(3) Å) and the two Mg−O bonds
(2.012(3) and 2.038(2) Å) do not differ markedly in length.
The bond angles around the Mg atom display apparent
distortion from the ideal value of 109.5°, with the N−Mg−N
(88.7°) bite angle and O−Mg−O (93.1°) angle being slightly
acute and others widened (116.3−122.2°). The dihedral angle
of the coordinate MgN2 plane and the C2N2 plane is 13.1°, and
the Mg atom deviates by 0.32 Å from the C2N2 plane.
Figure 5. Room-temperature EPR spectra recorded in THF solution:
(a) simulated; (b) experimental spectrum for 4; and (c) experimental
spectrum for 5.
Switching the ligand substituents from 2,6-diisopropyl to the
less crowded 2,4,6-trimethyl also led to a mononuclear
compound (3). The MgII center is coordinated by one ligand
and three THF molecules in a distorted trigonal-bipyramidal
geometry (Figure 2), which is different from the tetrahedral
coordination of the Mg atom in 2 with one less THF. This
structure was reported previously by us (but it was obtained
from a different synthetic method).13a Compared to 2, the
Mg−N bonds (2.021(2) and 2.051(2) Å) and Mg−O bonds
(2.196(2), 2.080(2), and 2.110(2) Å) in 3 are longer, and the
N−Mg−N bite angle (84.04°) is smaller (88.70° in 2). The Mg
atom is more coplanar (deviating 0.19 Å) with the C2N2 plane.
Some similar magnesium or calcium complexes with related
ligands, such as (dpp-Bian)Mg(THF)3, (dpp-Bian)Ca(THF)4·
(THF)1/2, and (dph-Bian)Ca(THF)3, have been reported in
the literature.8 We have also synthesized the calcium complex
Figure 6. Molecular structure of 6 (thermal ellipsoids at the 20%
probability level; Pr groups and H atoms are omitted for clarity; C
atoms of THF are drawn as smaller spheres).
i
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Moreover, it is known that the reduced α-diimines can
function as reducing agents for the synthesis of other organo-
metallic compounds.4,11,13 We have previously carried out the
reaction of ZnCl2 with the in situ generated [(LiPr)2−Na2] and
obtained the desired compound [(LiPr)−Zn−Zn(LiPr)−]. The
dianion (LiPr)2− acted as an electron donor and reduced the
initial divalent ZnII ion to the ZnI in the product, while the
ligand itself was oxidized to the monoanion (LiPr)−.11c In this
work, the similar reaction of [(LiPr)2−K2] with MgCl2, however,
did not produce the M−M-bonded complex but resulted in the
mononuclear complex 2, and no redox process occurred. In
addition, the reaction of the doubly reduced ligands with CaCl2
also did not cause the reduction of the divalent CaII ion.13c
These results demonstrate the difference of the electropositive
nature and reduction ability of the metal ions.
Table 1. Selected Bond Distances (Å) and Bond Angles (deg)
for 2 and 3
2
3
C(1)−C(2)
C(1)−N(1)
C(2)−N(2)
Mg(1)−N(1)
Mg(1)−N(2)
Mg(1)−O(1)
Mg(1)−O(2)
Mg(1)−O(3)
N(1)−Mg(1)−N(2)
N(1)−Mg(1)−O(1)
N(1)−Mg(1)−O(2)
N(1)−Mg(1)−O(3)
N(2)−Mg(1)−O(1)
N(2)−Mg(1)−O(2)
N(2)−Mg(1)−O(3)
O(1)−Mg(1)−O(2)
O(1)−Mg(1)−O(3)
O(2)−Mg(1)−O(3)
1.366(5)
1.416(4)
1.432(4)
1.966(3)
1.970(3)
2.012(3)
2.038(2)
1.350(3)
1.406(3)
1.426(3)
2.021(2)
2.051(2)
2.196(2)
2.080(2)
2.110(2)
84.04(9)
93.35(9)
143.59(9)
112.49(9)
168.39(8)
95.44(9)
104.50(8)
79.94(8)
86.96(8)
102.92(8)
88.70(12)
116.29(12)
122.21(12)
[(LiPr)− Mg]·THF (4) and [(LMes)− Mg] (5). Compounds 4
2
2
117.87(12)
121.25(12)
and 5 have similar compositions and structures, in which the
Mg center is coordinated by two α-diimine ligands, forming a
distorted tetrahedron. The iPr-substituted compound 4
crystallizes in the monoclinic space group P2/c (Figure 3)
with a lattice THF molecule. The Mg atom deviates by 0.661 Å
from the C2N2 plane, and the average dihedral angle of the
C2N2 plane and the N2Mg plane is 22.1°. The two metallo
C2N2Mg planes are nearly perpendicular to each other (with a
dihedral angle of 79.7°).
93.12(11)
Table 2. Selected Bond Distances (Å) and Bond Angles (deg)
for 4 and 5
The mesityl analogue 5 crystallizes in the orthorhombic
space group Fdd2 with half of the molecule in the asymmetric
unit, and the remaining part can be generated by a C2 axis
(Figure 4). The major difference of the structure of 5 from 4 is
that the five-membered chelate ring C2N2Mg is nearly planar,
with the Mg atom deviating only by 0.038 Å from the C2N2
plane (compared to 0.661 Å in 4). Moreover, the relative
orientation of the two C2N2Mg planes is “flattened” (with a
dihedral angle of 46.9°) in a way that is between the tetrahedral
and square-planar geometries, which is in contrast to the nearly
perpendicular situation of tetrahedral geometry (79.7°) in 4.
These differences can be attributed to the larger steric repulsion
of the LiPr than the LMes ligand. The reason that complex 5
tends to adopt a flattened (instead of tetrahedral) structure may
be that such an arrangement allows for two symmetry-related,
rather strong C−H···π interactions between one of the methyl
groups (C9) and the aryl ring of another ligand at one side of
the molecule (C···centroid distance 3.528 Å, shown in dashed
lines in Figure 4), as well as a π···π stacking interaction between
the two aryl rings at the other side (dihedral angle 2.16°,
centroid···centroid distance 3.953 Å, dotted line in Figure 4).
The 1:2 (metal-to-ligand) complexes 4 and 5 are similar to
the mononuclear alkaline earth metal compounds with related
ligands, such as [(dpp-Bian)2Mg],8g (dipp-nacnac)2Mg, and
(dipp-nacnac)2Ca.7b For both 4 and 5, the bond lengths of
Mg−N (2.078(2)/2.094(3) Å in 4 and 2.080(2)/2.070(2) Å in
5) are comparable to those in (dipp-nacnac)2Mg (2.100(2)/
2.122(2) Å)7b and [(dpp-Bian)2Mg] (2.111(1)/2.102(1) Å).8g
The bond lengths of C−C (1.403(5) Å in 4 and 1.460(3) Å in
5) and C−N (1.358(4)/1.367(4) Å in 4 and 1.344(3)/
1.342(3) Å in 5) of the central C2N2 moieties indicate that the
ligands are the π radical monoanions (L)•− and the Mg is a
divalent cation.
4
5
C(1)−C(2)
C(1)−N(1)
C(2)−N(2)
Mg(1)−N(1)
1.403(5)
1.460(3)
1.344(3)
1.342(3)
2.080(2)
2.070(2)
79.38(8)
108.18(14)
149.78(9)
109.16(14)
1.358(4)
1.367(4)
2.078(2)
Mg(1)−N(2)
2.094(3)
N(1)−Mg(1)−N(2)
N(1)−Mg(1)−N(1A)
N(1)−Mg(1)−N(2A)
N(2)−Mg(1)−N(2A)
81.54(10)
128.50(15)
114.67(10)
143.76(16)
Table 3. Selected Bond Distances (Å) and Bond Angles (deg)
for 6
Mg(1)−N(1)
Mg(1)−N(2)
Mg(1)−N(3)
Mg(1)−N(4)
C(1)−C(2)
C(1)−N(1)
C(2)−N(2)
C(23)−C(24)
C(23)−N(3)
N(1)−Mg(1)−N(2)
N(3)−Mg(1)−N(4)
2.045(4)
2.066(4)
2.035(4)
2.065(4)
1.340(8)
1.430(7)
1.409(7)
1.368(7)
1.411(7)
83.04(17)
83.24(17)
C(24)−N(4)
K(1)−O(1)
K(1)−O(2)
K(2)−O(3)
K(2)−O(4)
K(2)−O(5)
K(2)−O(6)
K(2)−O(7)
K(2)−O(8)
1.417(7)
2.681(5)
2.690(5)
2.660(7)
2.691(5)
2.702(6)
2.744(6)
2.753(8)
2.697(8)
120.67(18)
126.10(16)
N(1)−Mg(1)−N(3)
N(2)−Mg(1)−N(4)
[(LiPr)2−Ca(THF)3] and obtained the preliminary structure
that demonstrated the presence of three THF molecules.
Unfortunately, further refinement of the structure was
unsuccessful due to the poor crystal quality.14 A comparison
of these structures clearly indicates that the ionic radius of the
metal ion (MgII or CaII) and the steric bulk of the ligand are the
key factors that determine the number of THF molecules
coordinated to the metal. The larger calcium atom tends to
accommodate more THF molecules, and the bulkier iPr-
substituted ligand prefers less solvent molecules than the LMes
ligand as in the cases of 2 and 3.
Since complexes 4 and 5 contain the monoanionic ligands,
their EPR spectra have been studied at both room temperature
and low temperature. The room-temperature spectra of 4 and 5
recorded in THF and the simulated spectrum for them are
shown in Figure 5. The simulation gave identical hyperfine
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Table 4. Crystallographic and Structure Refinement Data for 2−6
2
3
4
5
6
empirical formula
C40H64N2O3Mg
645.24
C34H52N2O3Mg
561.09
C60H88N4OMg
905.65
C44H56N4Mg
665.24
C84H136N4O10MgK2
1464.48
fw
cryst syst
space group
a/Å
orthorhombic
Pbca
triclinic
monoclinic
P2/c
orthorhombic
Fdd2
orthorhombic
Pna21
P1
̅
18.5135(16)
20.2977(18)
21.0200(18)
90.00
9.9238(7)
10.9398(7)
16.2205(11)
74.7420(10)
76.3520(10)
82.9480(10)
1647.46(19)
2
13.4746(17)
12.1227(15)
20.8669(19)
90.00
20.497(6)
26.472(7)
14.723(4)
90.00
26.701(3)
12.6843(16)
25.711(3)
90.00
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
Z
90.00
125.462(5)
90.00
90.00
90.00
90.00
90.00
90.00
7898.9(12)
8
2776.3(6)
2
7989(4)
8
8707.9(19)
4
D
calcd/g cm−3
1.085
1.131
1.083
1.106
1.117
cryst size/mm3
0.30 × 0.25 × 0.25
2832
0.25 × 0.20 × 0.20
612
0.25 × 0.25 × 0.20
992
0.30 × 0.25 × 0.20
2880
0.30 × 0.25 × 0.25
3192
F(000)
μ/mm−1
0.081
0.088
0.074
0.079
0.171
θ range
1.78−25.07
38 520
2.06−25.00
11 048
1. 68−24.77
177 76
1.87−25.05
12 999
1.95−24.74
54 019
reflns collected
indep reflns
obsd reflns [I > 2σ(I)]
Rint
6977
5687
4726
3529
14 631
3421
4637
2663
2318
8737
0.0808
0.0188
0.0461
0.0417
0.0686
R1, wR2
[I > 2σ(I)]
0.0653, 0.1765
0.0493, 0.1369
0.0668, 0.1862
0.0429, 0.0971
0.0751, 0.1854
R1, wR2
0.1424, 0.2283
1.004
0.0604, 0.1481
1.029
0.1248, 0.2294
1.034
0.0820, 0.1132
1.021
0.1417, 0.2260
1.053
(all data)
GOF (F2)
coupling constants, aN = aH = 5.58 G, which are very close to
those (5.59 G) of [(LiPr)−Mg(μ-Me)]2 with the same
(LiPr)−Mg fragment.8e The spectrum of 4 is well resolved and
can be explained as originating from the superimposition of
overlapping quintets (due to two equivalent 14N nuclei, I = 1)
imino carbon atoms C(1)−C(1A) and C(2)−C(2A) in 4 and
5, respectively, in the crystal structures and are also comparable
to those in [(dpp-Bian)2Mg] (5.28 and 5.65 Å from EPR8d and
X-ray data,8g respectively). Moreover, evidence for the biradical
nature of 4 and 5 was also obtained from their half-field
(ΔMs = 2) transitions (Figure S1). The extremely weak half-
field transitions (ΔMs = 2) characteristic of the S = 1 triplet
ground state were observed at 1680 and 1685 G (see Figure
S3), respectively, which indicate that the two S = 1/2 spins are
only weakly coupled and the unpaired electrons are primarily
localized on the NCCN backbone.8h
1
and septets (due to six equivalent H nuclei of two methyl
groups, I = 1/2). Since the aN and aH couplings are identical,
the spectrum of 4 appears as an asymmetric 11-lined signal with
ratios 1: 2: 3: 5: 8: 9: 8: 5: 3: 2: 1, which results from the
overlap of the quintets of septets splitting. The experimental
result of 4 is in good agreement with the simulated one and is
also similar to literature reports on related systems.8e In the
case of compound 5, reduced overlap and more splitting was
observed (Figure 5). The differences between the hyperfine
coupling of 4 and 5 may be attributed to the subtle differences
of their structures. Compound 4 (with the larger LiPr ligand)
shows a slightly distorted tetrahedral coordination geometry,
and the two ligand radicals are nearly orthogonal to each other.
In contrast, compound 5 adopts a flattened structure, which
leads to more asymmetric environments for the N atoms and
methyl groups (Figure 4) and results in the reduced overlap of
lines in the EPR spectrum. Nevertheless, the g values (2.0037
for 4 and 5) and coupling patterns suggest that the electrons
are highly delocalized over the ligands, which are consistent
with related compounds containing radical anionic diimine
ligands.8d−f,10,c,c
[(LMes 2−2Mg(η 6:η 6-K(THF)2)][K(THF)6]·(THF)2 (6). In
)
order to gain more insights into the chemistry of the
magnesium compounds with α-diimine ligands, a similar
process employed to synthesize the Mg−Mg-bonded species
1 was adopted in the reaction of the less bulky LMes and
anhydrous MgCl2 with three equivalents of potassium metal.
2−
Interestingly, the compound [(LMes
)
2Mg(η6:η6-K(THF)2)]-
[K(THF)6]·(THF)2 (6) was isolated.
2−
Compound 6 contains an anionic [(LMes
)
2MgK(THF)2]−
unit and a [K(THF)6]+ cation (Figure 6), the former involving
alkali metal cation−π interactions between potassium and
substituted arene groups, which recently emerged as an
important bonding force.15 In the anionic fragment, the Mg2+
center is coordinated by four N atoms of two α-diimine ligands
in a distorted tetrahedral geometry similar to 4 and 5. The Mg
atom deviates by 0.652 and 0.693 Å from the two C2N2 planes.
The average Mg−N bond length (2.040 Å) is shorter than that
in 4 and 5. There is one potassium ion (K1) in the anionic
moiety, which is η6-sandwiched by two flanking N-phenyl rings
from each of the α-diimine ligands and is also coordinated by
two THF molecules, which is different from the situation in
compound 1 (where the K+ ion contacts with the C2N2
backbone of the dianionic ligand).4 The K−C distances vary
The frozen solution (toluene, 150 K) EPR spectra of
compounds 4 and 5 (Figures S1 and S2) further confirmed that
these compounds are biradical species with one unpaired
electron localized on each ligand. From the zero-field splitting
parameters (D, 196 and 185 G), the distances between the
geometric centers of the localization of the unpaired electrons
were calculated to be 5.21 and 5.31 Å. These values are
consistent with the distances of 5.48 and 5.62 Å between the
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CH(CH3)2), 1.29 (THF), 2.03 (s, 6H, CCH3), 3.50 (m, 4H,
CH(CH3)2 and THF), 6.99−7.14 (m, 6H, ArH). Anal. Calcd for
C36H56N2O2Mg·THF (645.24): C, 74.45; H, 9.99; N, 4.34. Found: C,
74.63 ; H, 10.15 ; N, 4.21.
between 3.127 and 3.259 Å, which are comparable with the K−
C bond lengths found in [(η6:η6-K(THF)(η6:η6-K-(THF)2)]-
[Mg{η2-Me2Si(Ndipp)2}2] (Dipp = 2,6-iPr2C6H3) (2.968(3)−
3.298(3) Å) and [{(THF)2K(μ-N(Ph)iPr)2}2Ca] (3.156(2)−
3.259(2) Å) with the slipped η6-coordination.15 The average
distance of K···phenyl ring is about 2.861 Å, while the Mg···K
separation is 4.669 Å. In the countercation fragment, the K(2)+
ion is coordinated by six THF molecules in an octahedral
arrangement with K−O distances ranging from 2.660(7) to
2.753(8) Å. The carbon atoms of the THF molecules are
disordered and have been refined using two split positions and
geometrical constraints. The average bond lengths of C−C
(1.354 Å) and C−N (1.434 Å) in 6 confirm the dianionic
character of the ligand.
Synthesis of [(LMes 2−
) Mg(THF)3] (3). Method A. Complex 3 was
prepared by a similar procedure to that employed for 2, from MgCl2
(0.089 g, 0.936 mmol), LMes (0.300 g, 0.936 mmol), and K (0.073 g,
1
1.87 mmol). Red crystals (0.309 g, 59%) were isolated. H NMR (400
MHz, C6D6, δ/ppm): 1.29 (THF), 2.13 (s, 6H, CCH3), 2.42 (s, 6H, p-
ArCH3), 2.48 (s, 12H, o-ArCH3), 3.50 (THF), 7.13 (s, 4H, m-ArH).
Method B. As reported previously by us.13a
Synthesis of [(LiPr)−2Mg]·THF (4) and [(LMes −
) 2Mg] (5). Com-
plexes 4 and 5 were also prepared by the same method as described
above. For 4: MgCl2 (0.059 g, 0.62 mmol), LiPr (0.500 g, 1.24 mmol),
and K (0.050 g, 1.24 mmol). Red crystals (0.11 g, 22%). Anal. Calcd
for C56H80N4Mg·THF (905.65): C, 79.57; H, 9.79; N, 6.18. Found: C,
79.30; H, 9.71; N, 6.10. For 5: MgCl2 (0.074 g, 0.78 mmol), LMes
(0.500 g, 1.56 mmol), and K (0.061 g, 1.56 mmol). Red crystals (0.11
g, 22%). Anal. Calcd for C44H56N4Mg·THF (741.38): C, 77.76; H,
8.70; N, 7.56. Found: C, 77.22 ; H, 8.22 ; N, 8.06.
A
similar complex of the congener calcium,
[(LiPr)2−2CaK2(THF)]n,13c reported by us also shows a Ca:K:L
ratio of 1:2:2 as in compound 6. However, its structure is
significantly different. Although the main structural motif is the
tetrahedral [L2M] core in both the magnesium compound 6 and
the calcium analogue [(LiPr)2−2CaK2(THF)]n, the location and
coordination mode of the two K+ ions are different. In the
calcium compound, one of the K+ ions is sandwiched by two
phenyl rings in two separated [(LiPr)2−2Ca] units, while the other
K+ cation is η3-coordinated by the phenyl rings from two ligands
within one [(LiPr)2−2Ca] unit. As a result, an infinite left-handed
21 helical chain is formed.13c In the magnesium species 6, one
potassium ion is encapsulated by two phenyl rings within one
[(LiPr)2−2Mg] unit, but the other K+ ion is isolated and does not
form any contacts with the diimine ligand; instead, it is solvated
by six THF molecules. Thus a discrete rather than infinite
structure results.
Synthesis of [(LMes 2−2Mg(η 6:η 6-K(THF)2)][K(THF)6]·(THF)2
)
(6). Three equivalents of potassium (0.180 g, 4.69 mmol) were
added to a THF solution of LMes (0.500 g, 1.56 mmol) at room
temperature. The mixture was stirred for 24 h, and anhydrous MgCl2
(0.150 g, 1.56 mmol) was added. Further reaction for 2 days and
1
subsequent workup yielded red crystals of 6 (0.38 g, 37%). H NMR
(400 MHz, C6D6, δ/ppm): 1.40 (m, 32H, THF), 1.54 (s, 6H, CCH3),
2.01 (broad, 12H, o-ArCH3), 2.33 (s, 6H, p-ArCH3), 3.56 (s, 6H,
THF), 6.70 (m, 4H, ArH). Anal. Calcd for C80H128K2MgN4O8·THF
(1392.40): C, 69.01; H, 9.26; N, 4.02. Found: C, 69.43; H, 9.07; N,
4.32.
X-ray Crystal Structure Determination. Diffraction data for
compounds 2−6 were collected on a Bruker SMART APEX II
diffractometer at 296 K with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å). An empirical absorption correction using
SADABS was applied for all data. The structures were solved by direct
methods using the SHELXS program. All non-hydrogen atoms were
refined anisotropically by full-matrix least-squares on F2 by the use of
the program SHELXL. The hydrogen atoms bonded to carbon were
included in idealized geometric positions with thermal parameters
equivalent to 1.2 times those of the atom to which they were attached.
Crystallographic data and refinement details are listed in Table 4.
CONCLUSION
■
A series of magnesium complexes with reduced α-diimine
ligands have been synthesized by the reaction of the neutral
ligand, MgCl2, and potassium metal. By varying the equivalents
of K (to L), the α-diimine ligands accepted one or two
electrons and formed the dianion (in 2, 3, and 6) or the
monoanion (in 4 and 5) in the resulting complexes. However,
the initial divalent magnesium ion retained its oxidation state of
+2, which is different from the case of the MgI−MgI-bonded
compound. Variable structures were obtained upon changing
the amount of the reducing agent K and ligand substituents,
and the effects of these factors have been discussed.
ASSOCIATED CONTENT
* Supporting Information
■
S
EPR spectra of compounds 4 and 5 and detailed information
on the X-ray crystal structure analysis of compounds 2 and 4−6
(CIF files). This material is available free of charge via the
Internet at http://pubs.acs.org.
EXPERIMENTAL SECTION
AUTHOR INFORMATION
Corresponding Author
*E-mail: yangxj@lzb.ac.cn.
■
■
General Procedures. All manipulations were carried out under an
inert atmosphere of argon using standard Schlenk or drybox
techniques. Tetrahydrofuran was dried by sodium/benzophenone
and distilled under argon prior to use. Anhydrous MgCl2 was
purchased from Alfa Aesar. Benzene-d6 was dried over Na/K alloy.
The reduced ligands, LKn (L = LiPr or LMes; n = 1 or 2), were prepared
as previously described.13a 1H NMR spectra were obtained on a Bruker
DPX-200 NMR spectrometer. Elemental analyses were performed
with an Elementar VarioEL III instrument. EPR spectra were recorded
on a Bruker EMX-10/12 spectrometer.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (20771103 and 20972169).
■
REFERENCES
■
Synthesis of [(LiPr 2−
) Mg(THF)2]·THF (2). Anhydrous MgCl2
(1) (a) Grignard Reagents: New Developments; Ritchey, Jr. H. G., Ed.;
Wiley: Chichester, 2000. (b) The Chemistry of Organomagnesium
Compounds, Part 1; Rappoport, Z., Marek, I., Eds.; Wiley: Chichester,
2008.
(2) (a) Green, S. P.; Jones, C.; Stasch, A. Angew. Chem., Int. Ed. 2008,
47, 9079−9083. (b) Bonyhady, S. J.; Green, S. P.; Jones, C.;
Nembenna, S.; Stasch, A. Angew. Chem., Int. Ed. 2009, 48, 2973−2977.
(c) Sidiropoulos, A.; Jones, C.; Stasch, A.; Klein, S.; Frenking, G.
(0.070 g, 0.741 mmol) was added to a rigorously stirred solution of
[(LiPr)2−K2] in THF [prepared in situ from LiPr (0.300 g, 0.741 mmol)
and two equivalents of potassium (0.058 g, 1.48 mmol)]. The mixture
was stirred for 3 days at room temperature. It was then filtered, and
the filtrate was concentrated to ca. 5 mL. Slow evaporation of the
solution at −20 °C for several days gave the product 2 as red crystals
1
(0.16 g, 38%). H NMR (400 MHz, C6D6, δ/ppm): 1.20 (d, 24H,
6076
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Article
Angew. Chem., Int. Ed. 2009, 48, 9701−9704. (d) Bonyhady, S. J.;
Collis, D.; Frenking, G.; Holzmann, N.; Jones, C.; Stasch, A. Nat.
Chem. 2010, 865−869. (e) Jones, C.; McDyre, L.; Murphy, D. M.;
Stasch, A. Chem. Commun. 2010, 46, 1511−1513.
Maslova, O. V.; Hummert, M.; Schumann, H. Inorg. Chem. 2010, 49,
2901−2910.
(13) (a) Liu, Y.; Yang, P.; Yu, J.; Yang, X.-J.; Zhang, J. D.; Chen, Z.;
Schaefer, H. F.; Wu, B. Organometallics 2008, 27, 5830−5835. (b) Liu,
Y.; Li, S.; Yang, X.-J.; Li, Q.-S.; Xie, Y.; Schaefer, H. F.; Wu, B.
J. Organomet. Chem. 2011, 696, 1450−1455. (c) Liu, Y.; Zhao, Y.;
Yang, X.-J.; Li, S.; Gao, J.; Yang, P.; Xia, Y.; Wu, B. Organometallics
2011, 30, 1599−1606. (d) Fedushkin, I. L.; Skatova, A. A.; Chudakova,
V. A.; Fukin, G. K. Angew. Chem., Int. Ed. 2003, 42, 3294−3298.
(14) Unpublished results. Preliminary crystal data for
[(LiPr)2‑Ca(THF)3]: C80H128N4O6Ca2, fw = 1322.02, monoclinic,
space group P21/n, a = 9.8968(9) Å, b = 38.527(3) Å, c = 22.576(2) Å,
β = 93.157(2)°, V = 8595.1(1) Å3, Z = 4, Dc = 1.022 g cm−3, F(000) =
(3) (a) Green, S. P.; Jones, C.; Stasch, A. Science 2007, 318, 1754−
1757. (b) Bonyhady, S. J.; Jones, C.; Nembenna, S.; Stasch, A.;
Edwards, A. J.; McIntyre, G. J. Chem.Eur. J. 2010, 16, 938−955.
(4) Liu, Y.; Li, S.; Yang, X.-J.; Yang, P.; Wu, B. J. Am. Chem. Soc.
2009, 131, 4210−4211.
(5) (a) Westerhausen, M. Angew. Chem., Int. Ed. 2008, 47, 2185−
2187. (b) Datta, A. J. Phys. Chem. C 2008, 112, 18727−18729.
(c) Koppe, R.; Henke, P.; Schnockel, H. Angew. Chem., Int. Ed. 2008,
̈
̈
47, 8740−8744. (d) Overgaard, J.; Jones, C.; Stasch, A.; Iversen, B. B.
J. Am. Chem. Soc. 2009, 131, 4208−4209. (e) Kan, Y.-H. THEOCHEM
2009, 894, 88−92. (f) Platts, J. A.; Overgaard, J.; Jones, C.; Iversen,
B. B.; Stasch, A. J. Phys. Chem. A 2011, 115, 194−200.
2896, m = 0.179 mm−1, 296(2) K, 6902 unique reflections [Rint
0.0915], R1 = 0.1232, wR2 = 0.3463 [I > 2σ(I)].
=
(15) (a) Twamley, B.; Power, P. P. Angew. Chem., Int. Ed. 2000, 39,
3500−3503. (b) Pu, L.; Phillips, A. D.; Richards, A. F.; Stender, M.;
Simons, R. S.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003,
125, 11626−11636. (c) Richards, A. F.; Brynda, M.; Power, P. P. J. Am.
Chem. Soc. 2004, 126, 10530−10531. (d) Tsai, Y.-C.; Lu, D.-Y.; Lin,
Y.-M.; Hwang, J.-K.; Yu, J.-S. K. Chem. Commun. 2007, 4125−4127.
(e) Zhu, Z.-L.; Wang, X.-P.; Olmstead, M. M.; Power, P. P. Angew.
Chem., Int. Ed. 2009, 48, 2027−2030. (f) Pi, C.; Wan, L.; Gu, Y.; Wu,
H.; Wang, C.; Zheng, W.; Weng, L.; Chen, Z.; Yang, X.; Wu, L.
(6) (a) Krieck, S.; Gorls, H.; Yu, L.; Reiher, M.; Westerhausen, M.
̈
J. Am. Chem. Soc. 2009, 131, 2977−2985. (b) Krieck, S.; Gorls, H.;
̈
Westerhausen, M. J. Am. Chem. Soc. 2010, 132, 12492−12501.
(c) Krieck, S.; Yu, L.; Reiher, M.; Westerhausen, M. Eur. J. Inorg.
Chem. 2010, 197−216. (d) Schulz, S. Chem.Eur. J. 2010, 16, 6416−
6428.
(7) (a) Caro, C. F.; Hitchcock, P. B.; Lappert, M. F. Chem. Commun.
1999, 1433−1434. (b) Harder, S. Organometallics 2002, 21, 3782−
3787. (c) Dove, A. P.; Gibson, V. C.; Hormnirun, P.; Marshall, E. L.;
Segal, J. A.; White, A. J. P.; Williams, D. J. Dalton Trans. 2003, 3088−
3097.
Organometallics 2009, 28, 5281−5284. (g) Glock, C.; Gorls, H.;
̈
Westerhausen, M. Inorg. Chem. 2009, 48, 394−397.
(8) (a) Fedushkin, I. L.; Skatova, A. A.; Chudakova, V. A.; Fukin,
G. K.; Dechert, S.; Schumann, H. Eur. J. Inorg. Chem. 2003, 3336−
3346. (b) Fedushkin, I. L.; Chudakova, V. A.; Skatova, A. A.;
Khvoinova, N. M.; Kurskii, Y. A.; Glukhova, T. A.; Fukin, G. K.;
Dechert, S.; Hummert, M.; Schumann, H. Z. Anorg. Allg. Chem. 2004,
630, 501−507. (c) Fedushkin, I. L.; Skatova, A. A.; Hummert, M.;
Schumann, H. Eur. J. Inorg. Chem. 2005, 1601−1608. (d) Fedushkin, I.
L.; Skatova, A. A.; Chudakova, V. A.; Cherkasov, V. K.; Dechert, S.;
Schumann, H. Russ. Chem. Bull., Int. Ed. 2004, 53, 2142−2147.
(e) Bailey, P. J.; Dick, C. M.; Fabre, S.; Parsons, S.; Yellowlees, L. J.
Dalton Trans. 2006, 1602−1610. (f) Bailey, P. J.; Coxall, R. A.; Dick,
C. M.; Fabre, S.; Parsons, S.; Yellowlees, L. J. Chem. Commun. 2005,
4563−4565. (g) Fedushkin, I. L.; Chudakova, V. A.; Skatova, A. A.;
Fukin, G. K. Heteroat. Chem. 2005, 16, 663−670. (h) Baker, R. J.;
Jones, C.; Mills, D. P.; Murphy, D. M.; Hey-Hawkins, E.; Wolf, R.
Dalton Trans. 2006, 64−72.
(9) (a) Nimitsiriwat, N.; Gibson, V. C.; Marshall, E. L.; Takolpuckdee,
P.; Tomov, A. K.; White, A. J. P.; Williams, D. J.; Elsegood, M. R. J.;
́
Dale, S. H. Inorg. Chem. 2007, 46, 9988−9997. (b) Boere, R. T.; Cole,
M. L.; Junk, P. C. New J. Chem. 2005, 29, 128−134. (c) Xia, A.;
El-Kaderi, H. M.; Heeg, M. J.; Winter, C. H. J. Organomet. Chem. 2003,
682, 224−232.
(10) Gardiner, M. G.; Hanson, G. R.; Henderson, M. J.; Lee, F. C.;
Raston, C. L. Inorg. Chem. 1994, 33, 2456−2461.
(11) (a) Yang, X.-J.; Yu, J.; Liu, Y.; Xie, Y.; Schaefer, H. F.; Liang, Y.;
Wu, B. Chem. Commun. 2007, 2363−2365. (b) Yu, J.; Yang, X.-J.; Liu,
Y.; Pu, Z.; Li, Q.-S.; Xie, Y.; Schaefer, H. F.; Wu, B. Organometallics
2008, 27, 5800−5805. (c) Liu, Y.; Li, S.; Yang, X.-J.; Yang, P.; Gao, J.;
Xia, Y.; Wu, B. Organometallics 2009, 28, 5270−5272. (d) Yang, P.;
Yang, X.-J.; Yu, J.; Liu, Y.; Zhang, C.; Deng, Y.-H.; Wu, B. Dalton
Trans. 2009, 5773−5779.
(12) (a) Makhrova, T. V.; Fukin, G. K.; Cherkasov, A. V.; Trifonov,
A. A. Russ. Chem. Bull., Int. Ed. 2008, 57, 2285−2290. (b) Mahrova, T.
V.; Fukin, G. K.; Cherkasov, A. V.; Trifonov, A. A.; Ajellal, N.;
Carpentier, J.-F. Inorg. Chem. 2009, 48, 4258−4266. (c) Panda, T. K.;
Kaneko, H.; Pal, K.; Tsurugi, H.; Mashima, K. Organometallics 2010,
29, 2610−2615. (d) Trifonov, A. A.; Fedorova, E. A.; Fukin, G. K.;
Baranov, E. V.; Druzhkov, N. O.; Bochkarev, M. N. Chem.Eur. J.
2006, 12, 2752−2757. (e) Trifonov, A. A. Eur. J. Inorg. Chem. 2007,
3151−3167. (f) Fedushkin, I. L.; Maslova, O. V.; Baranov, E. V.;
Shavyrln, A. S. Inorg. Chem. 2009, 48, 2355−2357. (g) Fedushkin, I. L.;
6077
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