A sterically demanding iminopyridine ligand affords redox-active complexes of aluminum(III) and gallium(III)

Article abstract of DOI:10.1021/ic201729b

The combination of an electrophilic metal center with a redox active ligand set has the potential to provide reactivity unique from transition metal redox chemistry. In this report, substituted iminopyridine complexes containing monoanionic and dianionic

Full text of DOI:10.1021/ic201729b

Article
pubs.acs.org/IC
A Sterically Demanding Iminopyridine Ligand Affords Redox-Active
Complexes of Aluminum(III) and Gallium(III)
Thomas W. Myers and Louise A. Berben*
Department of Chemistry, University of California, Davis, California 95616, United States
S
* Supporting Information
ABSTRACT: The combination of an electrophilic metal center with a
redox active ligand set has the potential to provide reactivity unique from
transition metal redox chemistry. In this report, substituted iminopyridine
complexes containing monoanionic and dianionic MeIPMes ligands have
been characterized structurally and electronically. Green (MeIPMes−)AlCl2
(1), (MeIPMes−)AlMe2 (2), and (MeIPMes−)GaCl2 (5) have a doublet
spin state which results from the anion radical form of MeIPMes. Purple
(
(
MeIPMes2−)AlCl(OEt2) (3), (MeIPMes2−)AlMe(OEt2) (4), and
MeIPMes2−)GaCl(OEt2) (6) are each diamagnetic. We have also
investigated the solvent dependence of the decomposition of the MeIPMes
anion radical. Complexes 1 and 2 can be obtained from benzene and hexanes whereas the use of ether solvents results in the
formation of undesirable (CH2IPMes−)AlCl2 (1a) and (CH2IPMes−)AlCl2 (2a) formed by loss of a hydrogen atom from the MeIPMes
−
ligand. Electrochemical measurements indicate that 1, 2, and 5 are redox active.
ligand in the chemistry of the copper(I) complex, (MeIPPh)Cu-
INTRODUCTION
■
(O2CC(O)Ph), the large size of MeIP-Ph provided a platform to
position an aryl ring in close proximity to the site
of putative Cu−O bond formation when (MeIPPh)Cu(O2CC-
(O)Ph) reacts with dioxygen.5 Subsequent to reaction with
dioxygen, hydroxylation of the well-positioned aryl ring in
MeIPPh demonstrated the strong oxidizing power of a proposed
Cu(III)-oxo functional group. In a further example of MeIPPh
chemistry, formation of tetrahedral cobalt complexes was inves-
tigated and yielded (MeIPPh)CoCl2.6 It was shown that this
complex and its derivatives, in conjunction with methylalumi-
numoxide (MAO), are active for the polymerization of ethyl-
ene. The turnover frequencies (TOF) for the polymerization
reactions were shown to be 2 orders of magnitude faster than
had been reported for the square planar complexes of cobalt
formed from the analogous but tridentate 2,6-bis(imino)-
pyridine ligand (MeI2P): (MeI2P)CoCl.7 In all cases that we are
aware of, the bulky MeIPPh ligand participates as a neutral and
redox-innocent ligand.
Electrophilic transition metal complexes of redox-active ligands
have been studied by Heyduk and co-workers. These studies
focused mostly on zirconium and tantalum and, in particular a
zirconium complex which can reductively eliminate biphenyl
was investigated.1 In the present work we make use of a sub-
stituted iminopyridine ligand in conjunction with aluminum-
(III) and gallium(III). Previous work, by both Westerhausen
and co-workers, and by Wieghardt and co-workers, has
demonstrated that the iminopyridine ligand (Chart 1) can
behave as a redox active ligand and can stabilize magnesium
and transition metal complexes, respectively.2 These previous
reports incorporated IP ligands in the neutral, singly reduced,
and doubly reduced oxidation states.2a,3 In general, highly
electrophilic group 13 metal ions are not redox active and
to investigate concurrent electrophilic and redox reactivity we
are investigating aluminum(III) and gallium(III) complexes of
redox active ligands.
We have previously reported the synthesis of aluminum(III)
complexes with the redox-active iminopyridine ligand, 2,6-
bis(isopropyl)-N-(2-pyridinylmethylene)phenylamine (hence-
forth denoted by IP, Chart 1).4 In that work, the redox
chemistry and electronic structure of complexes which have
coordination numbers of four or five were investigated:
(IPn−)2Al or (IPn−)2Al−X (where X = monodentate ligand).
The five-coordinate complexes that we reported were neutral or
anionic, and the four-coordinate complexes were anionic. In the
present work we have lowered the coordination number of the
metal center to increase the electrophilicity of the complexes.
Aryl-substituted iminopyridine ligands (MeIPPh) have pre-
viously been employed as sterically demanding ligands in the
chemistry of transition metal ions. For example, as the ancillary
In the foregoing report, we have shown that MeIPMes can
behave as a redox-active ligand in the same way that we and
others have previously employed the less bulky, unsubstituted
iminopyridine ligand (IP).4 Because of the steric bulk of the
MeIPMes ligand, these complexes contain only one MeIPMes ligand
each, and can be described by the general formulation
(MeIPMesn−)Al-X3−n (X = Cl, Me, n = 1, 2). We have also inves-
tigated the formation of gallium(III) complexes with MeIPMes
.
n−
We demonstrate that tetrahedral complexes of aluminum-
(III) and gallium(III) can be isolated where the (MeIPMes
)
Received: August 8, 2011
Published: January 5, 2012
© 2012 American Chemical Society
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Chart 1. Structures of Ligands Mentioned in the Text with Their Abbreviations
ligand is in the 1- or the 2- oxidation state. We have also found
that reaction conditions must be carefully chosen to prevent
decomposition pathways for the anion radical oxidation state
ligand radical are indeed less stable, under some reaction
conditions, than those we have previously isolated as diradical
complexes. We found that the most successful method for
synthesis of (MeIPMes−)AlCl2 (1), (MeIPMes−)AlMe2 (2), and
(MeIPMes−)GaCl2 (5) employed benzene as reaction solvent and
hexane for workup and crystallization. The ligand MeIPMes was
initially reduced with 1 equiv of sodium metal in benzene after
which time AlCl3, AlMe2Cl, or GaCl3, respectively, were added
as a solid. The resulting dark green products were purified and
crystallized from hexane.
The formation of the aluminum(III) complexes was also
investigated in alternate solvents. When ether is employed as
the reaction solvent, the alkene products, (CH2IPMes−)AlCl2
(1a) and (CH2IPMes−)AlMe2 (2a), respectively, were obtained.
(Chart 1, Scheme 1). The CH2 alkene group in complexes 1a
and 2a was identified by 1H NMR spectroscopy and are
observed as doublets at 4.10 and 4.73 in 1a and at 3.90 and
4.68 in 2a. In addition, the Cim−CMe bond lengths, observed
crystallographically, are distinct for each of the ligand forms
(vide infra). In some experiments, mixtures of 1:1a, or 2:2a
were obtained, and the ratio of the products was depen-
dent on reaction time and the solvents employed for the
purification of the complexes. The same reactivity patterns were
observed when we employed AlI3 as the starting reagent in place
of AlCl3.
of the ligand MeIPMes−, to afford CH2IPMes after loss of a
−
hydrogen atom (Chart 1). The known one-electron reduced
complexes of methyl-substituted iminopyridine (MeIP) and
bis(imino)pyridine (MeI2P) derivatives are also often
reported in the alkene form (CH2IP− and CH2I2P−) and we
show that this decomposition pathway can be circumvented
by judicious choice of solvent. In the absence of decom-
position pathways, aluminum and gallium complexes with
doublet electronic states arise from the singly reduced
ligands, while complexes of the doubly reduced ligands are
diamagnetic.
RESULTS AND DISCUSSION
■
Synthesis of Aluminum and Gallium Complexes 1−6.
Synthesis of aluminum and gallium complexes of the reduced
MeIPMes employed sodium metal as a reductant because we
had found this to be effective in our previous syntheses of
[(IPn−)2Al]m− complexes (Scheme 1).4 In previous work, we
Scheme 1. Synthesis of Aluminum and Gallium Compounds
In the chemistry of methyl-substituted iminopyridine ligand
(
MeIP) and methyl-substituted bis(imino)pyridine ligand
(MeI2P) reported in the literature, both forms of the one
electron reduced ligand that we report here are commonly
observed. Therefore, understanding the conditions and that
lead to formation of each of our ligand forms, MeIPMes−, and
CH2IPMes−, may be of general interest. We have observed that
complexes of the alkene ligand CH2IPMes−, 1a and 2a, are
obtained when an ether is used as the reaction solvent for
the reduction of the ligand with sodium metal. In an
independent reaction of MeIPMes with sodium in ether, single
crystals of Na(CH2IPMes−) (7) were obtained which suggests
that formation of the alkene occurs during ligand reduction
and is independent of the reaction with the aluminum salt
(Figure 1). The alkene nature of 7 was identified crystallo-
graphically, and the bond distances in the compound are
discussed below.
The reduction chemistry of MeIPMes has not been previously
studied. However, we have observed parallels with the corre-
sponding MeI2P chemistry reported in the literature. In addition,
deprotonation of other potentially redox active ligands such as
α-diimines,8 α-iminoketones,9 salens,10 and others11,12 has been
reported by others. On the basis of the reported observations
pertaining to MeI2P and our own observations, we have been
able to identify conditions under which the anion radical form
of MeIPMes (or presumably MeI2P or MeIP) can be obtained
selectively (Chart 1).13−16 A common theme in these reports,
proposed that the stability of the ligand radicals in (IP−)2AlCl
was due at least in part, to exchange coupling through the
aluminum center. In the present work we have found that the
complexes of the form (MeIPMes−)AlX2 which contain only one
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2−
dinitrogen leads to coordination of a bridging N2 ligand and
no deprotonation.14 In a subsequent report, (MeI2P)FeCl2
underwent both reduction and deprotonation in THF under
an N2 atmosphere when exposed to either Na or NaH.15 Given
the precedent for deprotonation of the IP ligand class by strong
bases we speculate that the role of the ether solvent in
deprotonation of MeIPMes is to modify the basicity of the
sodium metal that we employ for ligand reduction. That is, in
noncoordinating benzene or toluene solvents sodium acts as a
reductant only, and in ethereal solvents sodium behaves as both
a reductant and a base.
Syntheses of aluminum complexes in which the MeIPMes li-
gand is reduced by two electrons were attempted by employing
2 equiv of sodium metal in conjunction with MeIPMes and MX3
starting reagents (M = Al, Ga; X = Cl or Me, Scheme 1). Initial
2−
attempts in this endeavor were unsuccessful when (MeIPMes
)
-
AlCl or (MeIPMes2−)AlMe were targeted in nonether solvents.
Preparation of deep purple, two-electron reduced complexes
was successfully achieved when ether was employed to stabi-
lize four coordinate complexes (MeIPMes2−)AlCl(OEt2) (3),
(MeIPMes2−)AlMe(OEt2) (4) and (MeIPMes2−)GaCl(OEt2) (6).
Figure 1. Solid state structure of 7. Yellow, white, and blue ellipsoids
represent Na, C, and N atoms, respectively; ellipsoids are shown at the
50% probability level. H atoms omitted.
To prevent deprotonation of MeIPMes in diethylether, MeIPMes
−
was allowed to complex to the metal salt, AlCl3, AlMe3, or
GaCl3, in ether before reduction by sodium metal. Under these
conditions no alkene resonances were observed in the H NMR
spectrum of both the reaction mixture and the isolated product.
Similar stabilization by coordination of ether has been previously
observed for aluminum complexes of redox active dpp-BIAN
ligands (dpp-BIAN = 1,2-bis[(2,6-diisopropyl phenyl)imino]-
acenaphthene).22
1
and in our own observations, is that an ether solvent (usually
tetrahydrofuran (THF) or diethylether) is used as the reaction
solvent when complexes of CH2IP17 and CH2I2P,18 which are
analogues of CH2IPMes−, are observed. In addition, reduction
of the ligand prior to metal salt addition leads to
decomposition of the anion radical to afford the alkene
ligand form. Synthetic methods in which unreduced ligand,
reductant, and metal salt are mixed concurrently lead more
reliably to formation of he one-electron reduced anion
radical form of the ligand analogous to MeIPMes−. The anion
radical form of MeI2P has also been accessed via trans-
metalation from an iron(II) complex to aluminum(III) in
toluene,19 and gallium complexes of MeI2P− can be accessed
by reduction of the neutral ligand by GaI;20 both of these
situations are variations of a synthetic method in which
concurrent addition of reductant and metal salt to the un-
reduced ligand is employed.
Compounds 1−6 are soluble in alkane, aromatic, and ether
1
solvents. The H NMR spectra for the one-electron reduced
and paramagnetic complexes of the MeIPMes− ligand 1, 2, and 5
display proton resonances that are shifted from the expected
positions and very broad. However, all resonances do occur
between −1 and 11 ppm. For example, proton resonances
corresponding to the pyridine rings fall in the range 7.25−
6.40 ppm. Three methyl resonances are observed; one each for
the methyl group on the imine carbon, the o-mesityl carbon
atoms, and the p-mesityl carbon atom: 2.19, 2.11, and 2.02 ppm,
1
respectively. The H NMR spectra of the diamagnetic com-
Although less common, successful formation of the anion
radical form of the I2P ligand by reduction of the neutral ligand
complex has been demonstrated. Chirik and co-workers
reported that (MeI2P)CoCl2 can be reduced to the radical
anion (MeI2P−)CoCl in toluene by Zn metal, and also by strong
bases such as alkyl lithium reagents.21 In selected instances, the
radical anion can be formed in the presence of ether solvents.
In each of these examples that we found, strong metal−ligand
electronic interactions were invoked.13−15 In a similar vein, we
have previously observed that strong coupling between anion
radical IP ligands through an aluminum center prevents C−C
coupling of IP radicals of adjacent complexes to yield a dimer.4
To our knowledge, deprotonation of MeIP ligands to form
CH2IP complexes in a nonether solvent has not been observed,
and thus ether solvents appear to be a key feature of CH2IP
formation.
plexes, 3 and 4, further support the assignment of the redox
events as ligand-based. The observed chemical shifts corre-
sponding to proton resonances on the pyridine ligands do not
fall within the expected range of aromatic proton resonances
because the pyridine ring is dearomatized upon two-electron
reduction of MeIPMes. For example, in 3 the protons corre-
2−
sponding to the dearomatized pyridine group of MeIPMes are
observed in the range 6.18−4.90 ppm.
Electronic Structure. We have previously reported that
aluminum complexes of the IP− and IP2− are deep green and
deep purple, respectively.1 In the present work we have
observed the same colors for complexes of the equivalent
−
2−
oxidation states of the MeIPMes and MeIPMes ligands. The
intense absorption bands responsible for these colors are
assigned as ligand-based π−π* transitions (Supporting
Information, Figure S1).
When complexes of the intact anion radical ligands are
exposed to a strong base or to excess reducing agent, depro-
tonation to yield the CH2IPMes− form of the ligand can again be
observed. Recently Gambarotta and co-workers have shown
that (MeI2P−)CoCl can be deprotonated by NaH in THF under
argon, although the same reaction run under an atmosphere of
Magnetic susceptibility measurements were performed on
complexes 1−4 (Supporting Information, Figure S2). Data was
collected in an applied field of 1000 Oe between 4 and 300 K.
Measurements were performed on multiple batches of sample
and gave consistent results. Complexes 1, 2, and 5, containing
−
the MeIPMes ligand are paramagnetic with temperature
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Figure 2. Structures of 1, and 1a. Pink, white, green, and blue ellipsoids represent Al, C, Cl, and N atoms ,respectively; ellipsoids are shown at the
50% probability level. H atoms omitted.
independent magnetic moments between 5−300 K consistent
with one unpaired electron per complex; 1.73, 1.66, and
1.68 μB, respectively. As expected, complexes 3, 4, and 6, which
incorporate MeIPMes2−, are diamagnetic.
Solid State Structures of 1−7. Single crystals of com-
pounds containing the one-electron reduced ligand, 1, 2, and 5,
were grown by chilling concentrated solutions of the complexes
in hexanes at −25 °C for between 1 and 5 days each (Figures 2,
3, and Supporting Information, Figure S3). Compounds 3, 4,
Figure 4. Structure of 4. Pink, white, and blue ellipsoids represent Al,
C, and N atoms, respectively; ellipsoids are shown at the 50%
probability level. H atoms omitted.
four-coordinate with a pseudo tetrahedral geometry and
contain one bidentate MeIPMes ligand and two monodentate
ligands (Tables 1−3). In each of the tetrahedral complexes the
bite angle of the bidentate, substituted iminopyridine ligand is
less than the tetrahedral angle, 109.4°; in the one-electron
reduced ligands this angle ranges from 83.92(7)° up to
87.24(6)° and in the two-electron reduced complexes the
bite angle is slightly greater and falls between 89.17(8)° and
90.99(6)°. Similar bite angles were observed in the aluminum
Figure 3. Structure of 5. Light blue, white, green, and blue ellipsoids
complexes of the unsubstituted IP ligands which we have
represent Ga, C, Cl, and N atoms, respectively; ellipsoids are shown at
previously reported.4 In all of the complexes, the largest L−M−
the 50% probability level. H atoms omitted.
L angles are between the IP-based Nim/Npy atoms and the
and 7 which each incorporate two-electron reduced MeIPMes
ligands were crystallized by chilling concentrated ether solu-
tions at −25 °C for approximately 1 day. (Figures 4, 1, and
Supporting Information, Figure S4) Each of the complexes are
monodentate ligands, and these angles range from 113.96(4)°
up to 127.49(5)°. X−M−X angles between monodentate
ligands are in general quite close to tetrahedral and range from
102.86(4)° up to 115.42(9)°.
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a
Table 1. Crystallographic Data for the Complexes 1−2
1
1a
2
2a
formula
C34 H48AlCl2N2
0.34 × 0.27 × 0.20
578.83
C34H46AlCl2N2
0.23 × 0.19 × 0.14
578.97
C33H47AlN2
0.42 × 0.34 × 0.26
498.71
C36H53AlN2
crystal size
formula weight, g mol−1
0.15 × 0.14 × 0.08
540.78
space group
P21/c
P21/c
P21/c
P21/n
a, Å
13.7327(4)
17.9380(5)
17.3440(4)
90
13.6859(11)
17.9072(14)
17.3368(10)
90
11.2859(4)
17.0176(6)
16.6324(6)
90
13.7281(4)
17.9194(5)
13.8370(4)
90
b, Å
c, Å
α, deg
β, deg
128.569(2)
90
128.517(4)
90
105.2380(10)
90
100.6650(10)
90
γ, deg
V, Å3
3340.5(2)
4
3324.4(2)
4
3082.1(2)
4
3345.09(2)
4
Z
T, K
90(2)
90(2)
90(2)
90(2)
ρ, calcd, g cm−3
refl. collected/2θmax
unique refl./ I > 2σ (I)
no. parameters/restraints
λ, Å/μ (Kα), cm−1
1.151
1.157
1.075
1.074
50732/66.52
12242/8607
372
40831/60.26
9533/5097
383
46498/64.70
10916/8185
336
44272/136.86
5890/5598
352
0.71073
0.71073
0.71073
1.54178
0.0630/1.042
0.1817
b
R1/GOF
0.0519/1.042
0.1340
0.0591/1.000
0.1370
0.0542/0.943
0.1385
b
wR2 (I > 2σ(I))
residual density, e Å−3
0.825/−0.786
0.483/−0.499
0.795/−0.563
1.045/−0.397
a
b
Obtained with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = {∑w(Fo − Fc2)2/∑w(Fo )2}1/2
.
2
2
a
Table 2. Crystallographic Data for the Complexes 3, 4, 5, and 7
3
4
5
7
formula
C32H44AlClN2O
0.23 × 0.18 × 0.14
535.12
C33H47AlON2
0.16 × 0.15 × 0.09
511.55
C34H48Cl2 GaN2
0.53 × 0.34 × 0.32
627.35
C36H53N2NaO2
0.23 × 0.19 × 0.15
568.79
crystal size
formula weight, g mol−1
space group
P21/c
P21/n
P21/n
Pbca
a, Å
13.418(3)
15.976(3)
16.537(6)
90
13.680(3)
15.860(3)
14.630(3)
90
13.7380(12)
17.9556(15)
13.8993(12)
90
18.5932(5)
16.5947(4)
22.1689(6)
90
b, Å
c, Å
α, deg
β, deg
121.93(2)
90
108.94(3)
90
102.2780(10)
90
90
γ, deg
V, Å3
90
3008.6(2)
4
3002.3(3)
4
3350.2(2)
4
6840.2(3)
8
Z
T, K
90(2)
90(2)
90(2)
90(2)
ρ, calcd, g cm−3
refl. collected/2θmax
unique refl./ I > 2σ(I)
no. parameters/restraints
λ, Å/μ (Kα), cm−1
1.181
1.132
1.240
1.105
18915/50.74
5437/4966
344
11253/85.00
5347/4819
334
48679/65.20
11482/10125
376
83033/58.30
9216/8057
378
0.71073
0.71073
0.0553/1.040
0.1508
0.71073
0.7107
b
R1/GOF
0.0411/1.040
0.1081
0.0333/0.864
0.0921
0.0402/1.040
0.1117
b
wR2 (I > 2σ(I))
residual density, e Å−3
0.437/−0.293
0.578/−0.355
0.763/−0.781
0.434/−0.351
a
b
Obtained with graphite-monochromated Mo Kα (λ = 0.71073 Å) radiation. R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = {∑w(Fo − Fc2)2/∑w(Fo )2}1/2
.
2
2
The bond lengths which are observed in complexes of
bond lengths are also consistent with the 1− oxidation state
(Chart 1). This is apparent when the bond lengths are compared
with the neutral ligand complex (IP)AlCl3 which we have
previously reported. The Cim−Cpy bond lengths for 1, 2, and 7
shorten a little at 1.435(2), 1.430(2), and 1.436(2) Å, respec-
tively. These metrics are not as short as we have previously
observed in IP− complexes of aluminum (1.405(6) Å), but they
are shorter than others have previously observed in complexes
containing the neutral MeIPMes ligand (1.452(6) Å).5,6 The Cim−
the one-electron reduced MeIPMes ligand 1, 2, and 5 support
assignment of the ligand oxidation state as −1 (Chart 1, Table 3).
The Al−Nim bonds are 1.856(1) Å, 1.906(1) Å, and 1.907(1) Å
respectively, which are even shorter than the Al−Nim bonds
which we observed in one-electron reduced complexes of
aluminum with the unsubsituted IP ligand (Table 4).4 Evidence
for the partial localization of the reduction event on the imine
functionality was observed in the lengthening of the CN
double bonds to 1.370(2) Å, 1.362(2) Å, and 1.364(2) Å, respec-
tively. The bond length trends for the Cim−Cpy and Cim−CMe
C
Me bond lengths remain consistent with single bond character at
1.458(2), 1.459(2), and 1.462(2) Å, respectively. In each of these
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Table 3. Selected Average Interatomic Distances (Å) and Selected Average Angles (deg) in 1−5 and 7
1
(IP)AlCl3
1
1a
2
2a
3
4
5
7
M−Nim
M−Npy
M−X
M−O
Cim−Nim
Cim−Cpy
Cpy−Npy
Cim−CMe
Nim−M−Npy
Npy−M−X
Nim−M−X
X−M−X
Npy−M−O
Nim−M−O
X−M−O
Cpy−Cim−Nim
1.856(1)
1.909(1)
2.1176(6)
n/a
1.856(1)
1.909(1)
2.1176(6)
n/a
1.828(3)
1.923(3)
2.116(1)
n/a
1.906(1)
1.963(1)
1.984(1)
n/a
1.871(2)
1.988(2)
1.969(2)
n/a
1.806(1)
1.824(2)
2.115(1)
1.880(1)
1.425(2)
1.366(2)
1.390(2)
1.494(2)
90.99(6)
127.49(5)
117.78(5)
n/a
1.828(2)
1.847(2)
1.938(3)
1.935(2)
1.422(3)
1.366(3)
1.380(3)
1.494(3)
89.17(8)
126.8(1)
123.0(1)
n/a
1.907(1)
1.954(1)
2.1605(4)
n/a
2.3346(9)
2.4305(9)
n/a
2.4417(9)
1.353(1)
1.501(1)
1.351(1)
1.381(1)
70.18(3)
149.39(3)
126.96(3)
104.68(3)
97.59(3)
110.35(3)
104.68(3)
117.90(8)
1.370(2)
1.435(2)
1.384(2)
1.458(2)
87.24(6)
114.90(5)
113.96(4)
109.73(3)
n/a
1.370(2)
1.435(2)
1.384(2)
1.458(2)
87.24(6)
114.90(5)
113.96(4)
109.73(3)
n/a
1.386(4)
1.474(4)
1.369(4)
1.380(4)
87.1(1)
114.20(9)
115.31(9)
109.42(5)
n/a
1.362(2)
1.430(2)
1.383(2)
1.459(2)
84.31(5)
116.38(5)
114.86(5)
113.44(6)
n/a
1.379(2)
1.492(3)
1.360(3)
1.348(3)
83.92(7)
111.59(8)
114.90(7)
115.42(9)
n/a
1.364(2)
1.436(2)
1.364(2)
1.462(2)
85.87(5)
115.29(4)
114.58(4)
109.68(2)
n/a
106.04(6)
111.05(6)
102.86(4)
119.2(1)
102.75(8)
107.90(8)
104.8(1)
115.1(2)
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
114.7(1)
114.7(1)
113.2(1)
115.16(9)
111.8(2)
115.6(1)
The bond lengths associated with complexes 3 and 4, which
Table 4. Comparison of Ligand Backbone Bond Lengths for
One-Electron and Two-Electron Reduced Ligands from This
Work with Values Reported in the Literature
contain the two-electron reduced MeIPMes ligand, are charac-
2−
teristic of the IP2− oxidation state that we and others have
previously reported.1,23 The foregoing discussion focuses on 3
only, because metrics for 4 are similar (Table 3). In accord with
the higher negative charge on the ligand, the Al−Nim and Al−
Npy bond lengths are both significantly shortened to 1.806(1) Å
and 1.824(2) Å, respectively. The formerly CN double bond
is lengthened even further than observed in complex 1 and is
1.425(2) Å. The Cim−Cpy bond length in this case is shortened
as would be expected, and measures 1.366(2) Å. In further
agreement with the two-electron reduced nature of the ligand,
the pyridine ring is dearomatized and displays alternating bond
lengths that approach the lengths expected in pure single and
double bonds. The Cim−CMe bond has a distance of 1.494(2) Å
consistent with single bond character. Complex 4, which incor-
porates a methyl monodentate ligand instead of the chloro
monodentate ligand in 3, displays the same general trends in bond
literature (Å)1,8,20
current work (Å)
a
CH2IPMes
Cim−Cpy
Cim−NIm
Cim−CCH2
NIm−Al
NPy−Al
Cim−Cpy
Cim−Nim
Cim−CMe
NIm−Al
NPy−Al
Cim−Cpy
Cim−Nim
Cim−CMe
NIm−Al
NPy−Al
1.474(3)
1.370(4)
1.309(3)
1.874(2)
1.953(2)
1.435(3)
1.322(3)
1.492(3)
1.915(1)
2.009(2)
1.356(6)
1.414(6)
n/a
1.483(4)
1.382(4)
1.364(3)
1.850(3)
1.955(2)
1.433(2)
1.366(2)
1.458(2)
1.881(2)
1.936(2)
1.366(3)
1.423(3)
1.494(2)
1.817(2)
−
b
MeIPMes−
c
MeIPMes2−
length changes for the two-electron reduced state of MeIPMes
.
1.844(4)
1.873(4)
1.836(2)
However, these bond length changes are not as pronounced as
we observed in 3. The gallium analogue of these complexes, com-
pound 6, is diamagnetic and was identified by 1H NMR and other
spectroscopic techniques. We have no crystallographic data for this
complex.
a
b
Average bond distances taken from 1a and 2a. Average bond
distances taken from 1 and 2. Literature values for Cim−Cpy, Cim−Nim,
and Cim−Cme taken from MeI2P−, values for Nim−Al and Npy−Al taken
c
from IP−. Average bond distances taken from 3 and 4. Literature
values from IP2−.
−
The sodium salt of the CH2IPMes ligand (7) displays bond
distances and angles very similar to those we report for 1a and
2a. The Cim−Cpy bond has lengthened to full single bond
character at 1.501(1) Å. Additionally, the dihedral angle
between the Cim−Nim and Cpy−Npy bonds is 19.4(1)° indi-
cating that pyridine and imine π systems are no longer in
conjugation; this is also consistent with full Cim−Cpy single
bond character. Both the Cim−Nim and the Cim−CMe bonds
are between single and double bond character at 1.353(1) Å
and 1.381(1) Å, respectively. These distances suggest that the
negative charge is delocalized over Cim, Nim, and CMe but not
over the pyridine ring. The complex also has severe distortions
away from an ideal tetrahedral geometry with Npy−Al−O
angles of 97.59(3)° and 149.39(3)°.
three one-electron reduced complexes the pyridine rings largely
retain their aromatic character.
Complexes 1a and 2a, which were obtained when the reac-
tion solvent is ether, display bond lengths and angles consistent
with the CH2IPMes− formulation for the reduced ligand (Chart 1,
Table 3). Notable differences in the bond lengths described for
the MeIPMes− ligand in 1, 2, and 5 are observed for the Cim−Nim,
Cim−Cpy, and Cim−CMe bonds. The Cim−Cpy bond lengths do
not exhibit structural changes consistent with the shortening
of the C−C single bond. Instead, apparent shortening of the
Cim−CMe bond is observed, and these Cim−CMe bonds are
consistent with double bond character, at 1.380(4) Å and 1.348(3)
Å, respectively. The Cim−Cpy bond lengths are 1.474(4) and
1.492(3) Å. The Cim−Nim bonds lengthen more than was expected
Electrochemical Measurements. Cyclic voltammetry
measurements were performed in 0.3 M Bu4NPF6 THF solu-
tions of complexes 1−6 (Figure 5 and Supporting Information,
Figure S5). For each of the complexes containing the one-
electron reduced ligand, 1, 2, and 5, the cyclic voltammogram
−
for the MeIPMes formulation of the ligand. At 1.386(4) and
1.379(2) Å, respectively, these bond lengths are also consistent
−
with CH2IPMes
.
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Figure 5. CVs for the complexes containing the one-electron reduced ligand, MeIPMes−: 1, 2, and 5. Recorded in 0.3 M Bu4NPF6 THF solution.
(CV) displays one irreversible oxidation wave for the MeIPMes
1−/0
all referenced to the SCE couple, and were determined using decam-
ethylferrocene as an internal standard. The number of electrons passed
couple and one reduction wave for the MeIPMes2−/1− redox couple
2−/1−
(Figure 5). The MeIPMes
redox couple is reversible for com-
in a given redox process was estimated by comparison of the peak
current with the peak current of decamethylferrocene included as an
internal standard. UV−vis spectra were recorded in THF or hexane
solutions using a Varian Cary 1 UV−vis spectrometer. Magnetic mea-
surements were recorded using a Quantum Design MPMS XL magneto-
meter at 0.1 T. The sample was contained under nitrogen in a gelcap
and suspended in the magnetometer in a plastic straw. The magnetic
susceptibility was adjusted for diamagnetic contributions using the
constitutive corrections of Pascal’s constants.
X-ray Structure Determinations. X-ray diffraction studies were
carried out on a Bruker SMART 1000, a Bruker SMART APEXII, or a
Bruker SMART APEX Duo diffractometer equipped with a CCD
detector.24 Measurements were carried out at −175 °C using Mo Kα
(λ = 0.71073 Å) and Cu Kα (1.5418 Å) radiation. Crystals were
mounted on a glass capillary or Kaptan Loop with Paratone-N oil.
Initial lattice parameters were obtained from a least-squares analysis of
more than 100 centered reflections; these parameters were later
refined against all data. Data were integrated and corrected for Lorentz
polarization effects using SAINT and were corrected for absorption
effects using SADABS2.3.
Space group assignments were based upon systematic absences, E
statistics, and successful refinement of the structures. Structures were
solved by direct methods with the aid of successive difference Fourier
maps and were refined against all data using the SHELXTL 5.0
software package. Thermal parameters for all non-hydrogen atoms
were refined anisotropically. Hydrogen atoms, where added, were
assigned to ideal positions and refined using a riding model with an
isotropic thermal parameter 1.2 times that of the attached carbon atom
(1.5 times for methyl hydrogens).
plex 4 and irreversible for complexes 3 and 5. Comparison of the
CVs for 1 and 2 reveals that the redox couples for complex 4 are
in general about 0.5 V more positive than those for complex 3
which suggests that the methyl substituted complex is more easily
reduced. The CVs for the two-electron reduced complexes, 3, 4,
and 6, are significantly less well-defined than those for the one-
electron reduced complexes (Supporting Information, Figure S5).
The aluminum complexes 3 and 4 display no definitive redox
activity, and the gallium complex, 6, reveals two very broad and
irreversible oxidation waves at −1.2 and +0.2 V vs SCE. Both
of the broad oxidation events associated with 6 occur within
0.5 V of the corresponding redox couples observed for the one-
electron reduced complex 5.
Conclusions and Outlook. We have demonstrated that a
bulky mesityl-substituted iminopyridine ligand (MeIPMes) can be
employed as a noninnocent ligand in a −1 or −2 oxidation state
for aluminum(III) and gallium(III). Complexes of the
monoanionic radical and dianionic MeIPMes ligand have been
isolated for aluminum(IIII) and gallium(III), and their solid
state structures demonstrate that the bulky ligand directs the
formation of a complex with a single MeIPMes ligand. This is in
contrast to the [IP2Al] formulation which we have consistently
observed with the smaller IP ligand.4 The solvent dependence
we have observed for formation of complexes containing the
one-electron reduced oxidation state of MeIPMes, demonstrates
that by controlling reactions conditions, especially solvent
choice, the formation of the ligand CH2IPMes which involves loss
of a hydrogen atom, can be avoided. This synthetic control will
facilitate the exploration of the reversible redox processes that
we wish to exploit. Future work will focus on the synthesis of
cationic complexes and three-coordinate complexes of alumi-
num and gallium supported by the MeIPMes ligand. In addition,
complexes analogous to 3 which utilize more weakly
coordinating ligands than ether are being investigated.
Preparation of Compounds. All manipulations were carried out
using standard Schlenk or glovebox techniques under a dinitrogen
atmosphere. Unless otherwise noted, solvents were deoxygenated and
dried by thorough sparging with Ar gas followed by passage through
an activated alumina column. Deuterated solvents were purchased
from Cambridge Isotopes Laboratories, Inc. and were degassed and
stored over activated 3 Å molecular sieves prior to use. The ligand
MeIPMes was prepared according to literature procedures.25 All other
reagents were purchased from commercial vendors and used without
further purification.
(
MeIPMes−)AlCl2 (1). To a solution of MeIPMes (500 mg, 1.26 mmol)
EXPERIMENTAL SECTION
in benzene (15 mL) was added sodium (29.1 mg, 1.26 mmol). The
yellow solution was stirred for 4 h until all the sodium was consumed
and the solution was a dark red color. To this solution aluminum
trichloride (168 mg, 1.26 mmol) was added in portions to yield a dark
green solution and a gray precipitate. The reaction mixture was stirred
overnight. The resulting dark green solution was evaporated to
dryness, extracted into hexane (20 mL), and filtered through Celite to
remove salts. The green filtrate was concentrated to 10 mL and cooled
at −25 °C overnight to yield a dark green powder (343 mg, 70%).
Single crystals suitable for X-ray diffraction were grown by cooling a
■
Physical Measurements. Elemental analyses were performed by
Columbia Analytical. 1H NMR spectra were recorded at ambient
1
temperature using a Varian 400 MHz spectrometer. H NMR spectra
of all paramagnetic compounds were recorded from −80 ppm to
150 ppm. Chemical shifts were referenced to residual solvent. Electro-
chemical measurements were recorded in a glovebox under a dinitro-
gen atmosphere using a CH Instruments Electrochemical Analyzer, a
glassy carbon working electrode, a platinum wire auxiliary-electrode, and
an Ag/AgNO3 nonaqueous reference electrode. Reported potentials are
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1
concentrated hexane solution to −25 °C overnight. H NMR (300
MHz, C6D6) δ 7.44 (br), 5.85 (br), 4.47 (br), 4.27 (br), 3.41 (br), 2.76
(br), 2.63 (br) ppm. IR (KBr): 3054 (m), 2968 (vs), 2930(vs), 2869
(s), 1614 (s) 1573 (s), 1534 (m), 1510 (m), 1465 (vs), 1445 (vs),
1385 (s), 1353 (s), 1324 (s), 1281 (s), 1267 (s), 852 (s), 762 (s), 731
(s), 703 (s), 662 (s), 535 (vs), 495 (vs) cm−1. UV−vis spectrum
(Hexane) λmax (εM): 267 (29 590), 370 (32 180), 402 (16 940), 440
(8760), 738 (1640) nm (L mol−1 cm−1). Anal. Calcd. For
C28H35N2AlCl2: C, 67.74; H, 6.90; N, 5.64. Found C, 67.92; H,
7.01; N, 5.57. μeff = 1.73 μB.
salts. The purple filtrate was concentrated to 10 mL and cooled at −25 °C
overnight to afford a dark purple powder (424 mg, 63%). Single crystals
suitable for X-ray diffraction were grown by cooling a concentrated ether
solution to −25 °C for 3 h. 1H NMR (300 MHz, C6D6) δ 7.20−7.14 (m,
3H, ph), 6.86 (s, 1H, Mes CH), 6.76 (s, 1H, Mes CH), 6.18 (d, J = 9.6,
1H, py), 5.79 (dd, J = 5.7, 9.7, 1H, py), 4.87 (d, J = 5.70, 1H, py), 3.59
(hept, J = 6.84, 1H, CH(CH3) 2), 3.46 (q, J = 7.04, 2H, Ether CH2), 3.41
(q, J = 7.04, 2H, Ether CH2), 3.10 (sept, J = 6.84, 1H, CH(CH3)2), 2.74
(s, 3H, imCH3), 2.35 (s, 3H, Mes p-CH3), 2.17 (s, 3H, Mes o-CH3), 1.71
(s, 3H, Mes o-CH3), 1.44 (d, J = 6.9, 3H, CH(CH3)2), 1.32 (d, J = 6.8,
CH(CH3)2), 1.24 (d, J = 6.9, 3H, CH(CH3)2), 1.04 (d, J = 7.0, 3H,
CH(CH3)2), 0.65 (t, J = 7.04, 6H, Ether CH3) ppm. IR (KBr): 3035 (m),
2961 (vs), 2973 (vs), 2926 (vs), 2867 (s), 1611 (m), 1577 (m), 1540
(m), 1463 (vs), 1441 (vs), 1381 (s), 1342 (s), 1324 (s), 1283 (vs), 1274
(vs), 1201 (s), 1154 (s), 1129 (s), 1101 (s), 1017 (s), 799 (s), 766 (vs),
734 (s), 682 (vs) cm−1. UV−vis spectrum (THF) λmax (εM): 250 (41
900), 322 (23 560), 385 (22 820), 463 (13 210), 744 (2860) nm
(L mol−1 cm−1). Anal. Calcd. For C32H44N2O2AlCl: C, 71.82; H, 8.29; N,
5.23. Found C, 72.03; H, 8.17; N, 5.46.
(
CH2IPMes−)AlCl2 (1a). To a solution of MeIPMes (500 mg, 1.26
mmol) in ether (15 mL) was added sodium (29.1 mg, 1.27 mmol).
The yellow solution was stirred for 4 h until all the sodium was
consumed and the solution was a dark red color. To this solution
aluminum trichloride (169 mg, 1.26 mmol) was added in portions to
afford a dark green solution and a gray precipitate. The reaction
mixture was stirred overnight. The resulting dark green solution was
evaporated to dryness, extracted into hexane (20 mL), and filtered
through Celite to remove salts. The green filtrate was concentrated to
10 mL and cooled at −25 °C overnight to afford a dark green powder
(357 mg, 72%). Single crystals suitable for X-ray diffraction were
grown by cooling a concentrated hexane solution to −25 °C overnight.
1H NMR (300 MHz, C6D6) δ 7.30 (d, J = 7.42, 2H, py), 7.25 (s, 2H,
Mes), 6.90 (m, 1H, ph), 6.83 (d, J = 7.42, 2H, ph), 6.47 (t, J = 8.53, 1H,
py), 6.40 (d, J = 8.53, 1H, py), 4.73 (d, J = 7.66, 1H, im CH2), 4.03 (d, J =
7.66, 1H, im CH2), 3.62 (sept, J = 7.43, 2H, CH(CH3)2), 2.08 (s, 3H,
Mes), 2.02 (s, 6H, Mes), 1.29 (d, J = 7.42, 12H, CH(CH3)2) ppm.
(
MeIPMes2−)AlMe(OEt2) (4). To a solution of MeIPMes (500 mg, 1.26
mmol) in ether (20 mL) methyl aluminum dichloride (1.26 mL, 1 M in
hexane, 1.26 mmol) was added in portions. The yellow solution was
stirred for 30 min. Next sodium (72.7 mg, 3.16 mmol) was added to yield
a dark green solution initially and a gray precipitate. The reaction mixture
was stirred for 24 h until the solution turned purple. The solution
was filtered through Celite to remove salts. The purple filtrate was con-
centrated to 10 mL and cooled at −25 °C overnight to yield a dark purple
powder (380 mg, 59%). Single crystals suitable for X-ray diffraction were
(
MeIPMes−)AlMe2 (2). To a solution of MeIPMes (500 mg, 1.26 mmol)
1
grown by cooling a concentrated ether solution to −25 °C for 3 h. H
in benzene (15 mL) was added sodium (29.1 mg, 1.27 mmol). The
yellow solution was stirred for 4 h until all the sodium was consumed and
the solution was a dark red color. Carefully in portions dimethyl alumi-
num chloride (1.26 mL, 1 M in hexane, 1.26 mmol) was added, and a
dark green solution and a gray precipitate was obtained. The reaction
mixture was stirred overnight. The resulting dark green solution was
evaporated to dryness, extracted into hexane (20 mL) and filtered through
Celite to remove salts. The green filtrate was concentrated to 10 mL and
cooled at −25 °C overnight to afford a dark green powder (451 mg,
78%). Single crystals suitable for X-ray diffraction were grown by cooling a
concentrated hexane solution to −25 °C for 3 days. 1H NMR (300 MHz,
C6D6) δ 7.61 (br), 5.83 (br), 4.41 (br), 4.25 (br), 3.37 (br), 3.07 (br),
2.20 (br), −1.84 (br) ppm. ppm. IR (KBr): 3060 (m), 2967 (vs), 2952
(vs), 2925 (vs), 2870 (s), 1615 (s), 1570 (s), 1531 (s), 1528 (s), 1451
(vs), 1397 (s), 1393 (s), 1377 (s), 1366 (s), 1321 (s), 970 (s), 795 (s),
760 (vs), 726 (s), 701 (s), 688 (s) cm−1. UV−vis spectrum (Hexane) λmax
(εM): 375 (23 950), 427 (8430), 461 (6730), 750 (2310) nm (L mol−1
cm−1). Anal. Calcd. For C30H39N2Al: C, 79.08; H, 8.85; N, 6.15. Found C,
79.45; H, 8.89; N, 6.07. μeff = 1.66 μB.
NMR (300 MHz, C6D6) δ 7.17−7.24 (m, 3H, ph), 6.90 (s, 1H, Mes CH),
6.85 (s, 1H, Mes CH), 6.33 (d, J = 9.6, 1H, py), 5.96 (dd, J = 5.7, 9.7, 1H,
py), 4.91 (d, J = 5.7, 1H, py), 4.36 (hept, J = 6.84, 1H, CH(CH3) 2), 3.92
(sept, J = 6.84, 1H, CH(CH3)2), 3.39−3.49 (m, 4H, Ether CH2), 2.77 (s,
3H, imCH3), 2.40 (s, 3H, Mes p-CH3), 2.23 (s, 3H, Mes o-CH3), 1.85 (s,
3H, Mes o-CH3), 1.42 (d, J = 6.9, 3H, CH(CH3)2), 1.34 (d, J = 6.8,
CH(CH3)2), 1.30 (d, J = 6.9, 3H, CH(CH3)2), 1.12 (t, J = 7.04, 6H, Ether
CH3), 0.95 (d, J = 7.0, 3H, CH(CH3)2), −1.15 (s, 3H, Al−CH3) ppm. IR
(KBr): 3035 (m), 2961 (vs), 2973 (vs), 2926 (vs), 2867 (s), 1615 (m),
1572 (m), 1537 (m), 1463 (vs), 1441 (vs), 1381 (s), 1351 (s), 1323 (s),
1283 (vs), 1274 (vs), 1199 (s), 1154 (s), 1129 (s), 1101 (s), 795 (s), 760
(s), 733 (s), 704 (s), 663 (s), 587 (s) cm−1. UV−vis spectrum (THF) λmax
(εM): 253 (40280), 337 (21900), 381 (24710), 451 (15650), 750 (3290)
nm (L mol−1cm−1). Anal. Calcd. For C33H47N2O2Al: C, 77.00; H, 9.20; N,
5.44. Found C, 77.15; H, 9.24; N, 5.32.
(
MeIPMes−)GaCl2 (5). To a solution of MeIPMes (500 mg, 1.26 mmol)
in benzene (15 mL) was added sodium (29.1 mg, 1.27 mmol). The
yellow solution was stirred for 4 h until all the sodium was consumed and
the solution was a dark red color. To this solution gallium trichloride
(222 mg, 1.26 mmol) was added in portions to yield a dark green solution
and a gray precipitate. The reaction mixture was stirred overnight. The
resulting dark green solution was evaporated to dryness, extracted into
hexane (20 mL) and filtered through Celite to remove salts. The green
filtrate was concentrated to 10 mL and cooled at −25 °C overnight to
afford a dark green powder (365 mg, 68%). Single crystals suitable for
X-ray diffraction were grown by cooling a concentrated hexane solution to
(
CH2IPMes−)AlMe2 (2a). To a solution of MeIPMes (500 mg, 1.26
mmol) in ether (15 mL) was added sodium (29.1 mg, 1.27 mmol). The
yellow solution was stirred for 4 h until all the sodium was consumed and
the solution was a dark red color. Dimethyl aluminum chloride (1.26 mL,
1 M in hexane, 1.26 mmol) was added in portions to yield a dark green
solution and a gray precipitate. The reaction mixture was stirred overnight.
The resulting dark green solution was evaporated to dryness, extracted
into hexane (20 mL), and filtered through Celite to remove salts. The
green filtrate was concentrated to 10 mL and cooled at −25 °C overnight
to yield a dark green powder (421 mg, 74%). Single crystals suitable for
X-ray diffraction were grown by cooling a concentrated hexane solution to
−25 °C for 3 days. 1H NMR (300 MHz, C6D6) δ 7.39 (d, J = 7.67, 1H,
py), 7.29 (s, 2H, Mes), 6.88 (t, J = 7.67, 1H, ph), 6.76 (d, J = 7.44, 2H,
ph), 6.57 (d, J = 8.62, 1H, py), 6.43 (t, J = 8.62, 1H, py), 4.68 (d, J = 7.56,
1H, im CH2), 3.90 (d, J = 7.32, 1H, im CH2), 3.62 (sept, J = 7.67, 2H,
CH(CH3)2), 2.05 (s, 3H, Mes), 1.89 (s, 6H, Mes), 1.38 (d, J = 7.67, 12H,
CH(CH3)2), −0.61 (s, 3H, Al−CH3) ppm.
1
−25 °C overnight. H NMR (300 MHz, C6D6) δ 7.81 (br), 6.17 (br),
5.37 (br), 4.25 (br), 4.03 (br), 3.31 (br), 2.61 (br) ppm. IR (KBr): 3062
(m), 2963 (vs), 2925 (vs), 2870 (s), 2736 (w), 1642 (s), 1614 (s), 1582
(s), 1569 (vs), 1535 (s), 1505 (m), 1464 (vs), 1446 (vs), 1396 (vs), 1388
(vs), 1378 (s), 1363 (s), 1023 (s), 854 (s), 820 (s), 799 (s), 786 (s), 775
(vs) cm−1. UV−vis spectrum (Hexane) λmax (εM): 284 (26400), 370
(22610), 428 (11010), 737 (2930) nm (L mol−1cm−1). Anal. Calcd. C,
62.51; H, 6.36; N, 5.20. Found C, 62.26; H, 6.42; N, 5.07. μeff = 1.68 μB.
(
MeIPMes2−)GaCl(OEt2) (6). To a solution of MeIPMes (500 mg, 1.26
(
MeIPMes2−)AlCl(OEt2) (3). To a solution of MeIPMes (500 mg, 1.26
mmol) in ether (20 mL) was added sodium (72.7 mg, 3.16 mmol).
The yellow solution was stirred for 4 h until all the sodium was
consumed and the solution was a dark brown color. To this solution
gallium trichloride (221 mg, 1.26 mmol) was added in portions
initially to afford a dark green solution and a gray precipitate. The
reaction mixture was stirred for 24 h until the solution turned purple.
mmol) in ether (20 mL) was added aluminum trichloride (169 mg, 1.26
mmol). The yellow solution was stirred for 30 min. Next sodium
(72.7 mg, 3.16 mmol) was added to yield a dark green solution initially
and a gray precipitate. The reaction mixture was stirred for 24 h until the
solution turned purple. The solution was filtered through Celite to remove
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The solution was filtered through Celite to remove salts. The purple
filtrate was concentrated to 10 mL and cooled at −25 °C overnight
and a dark purple powder (407 mg, 61%) was obtained. 1H NMR (300
MHz, C6D6) 7.24−7.31 (m, 2H, ph), 7.19 (s, 2H, Mes), 7.03 (t, J =
7.30, 1H, ph), 6.85 (s, 1H, Mes), 6.36 (t, J = 7.98, 1H, py), 4.74 (q,
J =6.90, 1H, py), 4.39 (t, J =6.60, 1H, py), 3.67 (hept, J = 6.21, 2H,
CH(CH3)2), 3.57 (m, 4H, Ether), 2.79 (hept, J = 6.21, 2H,
CH(CH3)2), 2.31 (s, 3H, im-CH3), 2.11 (s, 3H, Mes), 2.07 (s, 3H,
Mes), 1.97 (s, 3H, Mes), 1.44 (d, J = 7.60, 3H, CH(CH3)2), 1.40 (d,
J = 7.60, 3H, CH(CH3)2), 1.31 (d, J = 7.60, 3H, CH(CH3)2), 1.13 (d,
J = 7.60, 3H, CH(CH3)2) ppm. IR (KBr): 3031 (m), 2968 (vs), 2975
(vs), 2926 (vs), 2865 (s), 2736 (w), 1610 (m), 1573 (m), 15380 (m),
1462 (vs), 1443 (vs), 1380 (s), 1342 (s), 1324 (s), 1281 (vs), 1272
(vs), 1198 (s), 1153 (s), 1132 (s), 1101 (s), 1018 (s), 854 (m), 799
(s), 767 (vs), 734 (s), 682 (vs), 606 (m), 550 (m), 538 (w), 521 (m),
438 (w) cm−1. UV−vis spectrum (THF) λmax (εM): 370 (15 580) nm
(L mol−1cm−1). Anal. Calcd. C, 66.51; H 7.67; N, 4.85. Found C,
66.22; H, 8.02; N, 4.67.
(7) Kleigrewe, N.; Steffen, W.; Blomker, T.; Kehr, G.; Frohlich, R.;
̈ ̈
Wibbeling, B.; Erker, G.; Wasilke, J.; Wu, G.; Bazan, Q. C. J. Am.
Chem. Soc. 2005, 127, 13955.
(8) Kim, Y. H.; Kim, T. H.; Kim, N. Y.; Cho, E. S.; Lee, B. Y.; Shin,
D. M.; Chung, Y. K. Organometallics 2003, 22, 1503.
(9) (a) Chen, Y.; Boardman, B. M.; Wu, G.; Bazan, G. C.
J. Organomet. Chem. 2007, 692, 4745. (b) Kim, H. Y.; Kim, T. H.;
Lee, B. Y. Organometallics 2002, 21, 3082.
(10) Wagler, J.; Bohme, U.; Brendler, E.; Thomas, B.; Goutal, S.;
Mayr, H.; Kempf, B.; Remennikov, G. Y.; Roewer, G. Inorg. Chim. Acta
2005, 358, 4270.
(11) Porter, R. M.; Winston, S.; Danopoulos, A. A.; Hursthouse, M.
B. J. Chem. Soc., Dalton. Trans. 2002, 3290.
(12) Matthias, D.; Yao, S.; Brym, M.; Wullen, C. V. Angew. Chem., Int.
Ed. 2006, 45, 4349.
(13) (a) Vidyaratne, I.; Gambarotta, S.; Korobkov, I.; Budzelaar, P.
H. M. Inorg. Chem. 2005, 44, 1187. (b) deBruin, B.; Bill, E.; Bothe, E.;
Weyhermuller, T.; Wieghardt, K. Inorg. Chem. 2000, 39, 2936.
̈
(
CH2IPMes−)Na(OEt2)2 (7). To a solution of MeIPMes (200 mg, 0.51
(14) Sugirama, H.; Aharonian, G.; Gambarotta, S.; Yap, G. P. A;
mmol) in ether (5 mL) was added sodium (11.6 mg, 0.51 mmol). The
yellow solution was stirred for 4 h until all the sodium was consumed and
the solution was brown. The resulting solution was cooled overnight at
−25 °C, and a brown powder (106 mg, 52%) was collected by filtration,
washed with hexane (5 mL) and dried overnight. Single crystals suitable
for X-ray diffraction were grown by cooling a concentrated ether solution
to −25 °C overnight. 1H NMR (300 MHz, C6D6) 7.41 (m, 1H, py), 7.31
(s, 2H, Mes), 7.03 (m, 1H, ph), 6.84 (m, 2H, pH), 6.63 (d, J = 8.54, 1H,
py), 6.59 (t, J = 8.32, 1H, py), 4.22 (d, J = 7.22, 1H, im-CH2), 3.53 (d, J =
7.13, 1H, im-CH2), 3.38 (m, 2H, CH(CH3)2, 3.04 (m, 8H, ether), 2.10 (s,
3H, Mes), 1.88 (s, 6H, Mes), 1.25 (d, J = 7.45, 12H, CH(CH3)2), 0.89
(m, 12H, ether) ppm. Anal. Calcd. C, 76.02; H 9.39; N, 4.92. Found C,
76.16; H, 9.24; N, 4.77.
Budzelaar. J. Am. Chem. Soc. 2002, 124, 12268.
(15) Vidyaratne, I.; Scott, J.; Gambarotta, S.; Budzelaar, P. H. M.
Inorg. Chem. 2007, 46, 7040.
(16) Scott, J.; Vidyaratne, I.; Korobkov, I.; Gambarotta, S.; Budzelaar,
P. H. M. Inorg. Chem. 2008, 47, 896.
(17) Joung, U. G.; Kim, T. H.; Joe, D. J.; Lee, B. Y.; Shin, D. M.;
Chung, Y. K. Polyhedron 2004, 23, 1587.
(18) (a) Sugiyama, H.; Korobkov, I.; Gambarotta, S. Inorg. Chem.
2004, 43, 5771. (b) Sugiyama, H.; Gambarotta, S.; Yap, G. P. A;
Wilson, D. R.; Thiele, S. K. H. Organometallics 2004, 23, 5054.
(c) Khorobkov, I.; Gambarotta, S.; Yap, G. P. A Organometallics 2002,
21, 3088.
(19) Scott, J.; Gambarotta, S.; Korobokov, I.; Knijnenburg, Q.;
deBruin, B.; Budzelaar., P. H. M. J. Am. Chem. Soc. 2006, 127, 17204.
(20) Jurca, T.; Dawson, K.; Mallow, I.; Burchell, T.; Yap, G. P. A.;
Richeson, D. S. Dalton. Trans. 2010, 39, 1266.
(21) Bowman, A. C.; Milsmann, C.; Atienza, C. C. H.; Lobkovsky, E.;
Weighardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2010, 132, 1676.
(22) Fedushkin, I. L.; Lukoyanov, A. N.; Hummert, M.; Schumann,
H. Z. Anorg. Allg. Chem. 2008, 634, 356.
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files for the structures of 1−7. UV−vis spectra for 1−6.
Depictions of the solid state structures of 2, 2a, 3, and 7. CVs
for 3, 4, and 6. Plots of magnetic susceptibility data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(23) Lu, C. C.; Weyhermuller, T.; Bill, E.; Wieghardt, K. Inog. Chem.
̈
2009, 48, 6055.
(24) (a) SMART Software Users Guide, Version 5.1; Bruker Analytical
X-Ray Systems, Inc.; Madison, WI, 1999. (b) SAINT Software Users
Guide, Version 7.0; Bruker Analytical X-Ray Systems, Inc.: Madison,
WI, 1999. (c) Sheldrick, G. M. SADABS, Version 2.03; Bruker
Analytical X-Ray Systems, Inc.: Madison, WI, 2000. (d) Sheldrick, G.
M. SHELXTL, Version 6.12; Bruker Analytical X-Ray Systems, Inc.:
Madison, WI, 1999. (e) International Tables for X-Ray Crystallography;
Kluwer Academic Publishers: Dordrecht, The Netherlands, 1992;
Vol. C.
AUTHOR INFORMATION
Corresponding Author
*E-mail: laberben@ucdavis.edu.
■
ACKNOWLEDGMENTS
■
We would like to thank the University of California Davis for
funding and Drs. M. Shanmugam and J. C. Fettinger for
experimental assistance.
(25) Laine, T. V.; Klinga, M.; Leskela, M. Eur. J. Inorg. Chem. 1999, 959.
̈
REFERENCES
■
(1) Hameline, M. R.; Heyduk, A. H. J. Am. Chem. Soc. 2006, 128,
8410.
(2) (a) Westerhausen, M.; Bollwein, T.; Makropoulos, N.;
Schneiderbauer, S.; Suter, S.; Noth, H.; Mayer, P.; Piotrowski, H.;
̈
Polborn, K.; Pfitzner, A. Eur. J. Inorg. Chem. 2002, 389. (b) Lu, C. C.;
Bill, E.; Weyhermuller, T.; Bothe, E.; Wieghardt, K. J. Am. Chem. Soc.
̈
2008, 130, 3181.
(3) Lu, C. C.; De Beer George, S.; Weyhermuller, T.; Bill, E.; Bothe,
̈
E.; Wieghardt, K. Angew. Chem., Int. Ed. 2008, 47, 1.
(4) (a) Myers, T. W.; Kazem, N.; Stoll, S.; Britt, R. D.; Shanmugam,
M.; Berben, L. A. J. Am. Chem. Soc. 2011, 133, 8662. (b) Myers, T. W.;
Berben, L. A. J. Am. Chem. Soc. 2011, 133, 11865.
(5) Hong, S.; Huber, S. M.; Gagliardi, L.; Cramer, C. C.; Tolman, W.
B. J. Am. Chem. Soc. 2007, 129, 14190.
(6) Bianchini, C.; Mantovani, G.; Meli, A.; Migliacci, F.
Organometallics 2003, 22, 2545.
1488
dx.doi.org/10.1021/ic201729b | Inorg. Chem. 2012, 51, 1480−1488