Synthesis and characterization of aluminum-α-diimine complexes over multiple redox states

Article abstract of DOI:10.1021/ic5003989

The aluminum complexes (L_Mes^2-)AlCl(THF) (3) and (L_Dipp^-)AlCl₂ (4) (L_Mes = N,N′-bis[2,4,6-trimethylphenyl]-2,3-dimethyl-1,4-diazabutadiene, L _Dipp = N,N′-bis[2,6-diisopropylphenyl]-2,3-dimethyl-1,4- diazabutadiene) were prepared by direct reduction of the ligands with sodium metal followed by salt metathesis with AlCl₃. The (L _Mes^-)AlCl₂ (5) complex was prepared through one-electron oxidative functionalization of 3 with either AgCl or CuCl. Complex 3 was characterized using ¹H and ¹³C NMR spectoscopies. Single-crystal X-ray diffraction analysis of the complexes revealed that 3-5 are all four-coordinate, with 3 exhibiting a trigonal pyramidal geometry, while 4 and 5 exist between trigonal pyramidal and tetrahedral. Notable in the L _Mes complexes 3 and 5 is a systematic lengthening of the C-N _imido bonds and shortening of the C-C bond in the N-C-C-N backbone with increased electron density on the ligand. The geometries of the complexes 3 and 5 were optimized using DFT, which showed primarily ligand-based frontier orbitals, supporting the analysis of the solid-state structural data. The complexes 3-5 were also characterized by electrochemistry. The cyclic voltamogram of complex 3 showed an oxidation processes at -0.94 and -0.03 V versus ferrocene, while complexes 4 and 5 exhibit both reduction (-1.37 and -1.34 V, respectively) and oxidation (-0.62 and -0.73 V, respectively) features.

Full text of DOI:10.1021/ic5003989

Article
pubs.acs.org/IC
Synthesis and Characterization of Aluminum-α-diimine Complexes
over Multiple Redox States
Bren E. Cole,† Jeffrey P. Wolbach,† William G. Dougherty, Jr.,‡ Nicholas A. Piro,‡ W. Scott Kassel,‡
and Christopher R. Graves*,†
†Department of Chemistry & Biochemistry, Albright College, 13th & Bern Street, Reading, Pennsylvania 19612, United States
‡Department of Chemistry, Villanova University, 800 Lancaster Avenue, Villanova, Pennsylvania 19085, United States
S
* Supporting Information
ABSTRACT: The aluminum complexes (LMes2−)AlCl(THF) (3) and (LDipp−)AlCl2
(4) (LMes = N,N′-bis[2,4,6-trimethylphenyl]-2,3-dimethyl-1,4-diazabutadiene, LDipp
=
N,N′-bis[2,6-diisopropylphenyl]-2,3-dimethyl-1,4-diazabutadiene) were prepared by
direct reduction of the ligands with sodium metal followed by salt metathesis with
AlCl3. The (LMes−)AlCl2 (5) complex was prepared through one-electron oxidative
functionalization of 3 with either AgCl or CuCl. Complex 3 was characterized using
1H and 13C NMR spectoscopies. Single-crystal X-ray diffraction analysis of the
complexes revealed that 3−5 are all four-coordinate, with 3 exhibiting a trigonal pyramidal geometry, while 4 and 5 exist between
trigonal pyramidal and tetrahedral. Notable in the LMes complexes 3 and 5 is a systematic lengthening of the C−Nimido bonds and
shortening of the C−C bond in the N−C−C−N backbone with increased electron density on the ligand. The geometries of the
complexes 3 and 5 were optimized using DFT, which showed primarily ligand-based frontier orbitals, supporting the analysis of
the solid-state structural data. The complexes 3−5 were also characterized by electrochemistry. The cyclic voltamogram of
complex 3 showed an oxidation processes at −0.94 and −0.03 V versus ferrocene, while complexes 4 and 5 exhibit both
reduction (−1.37 and −1.34 V, respectively) and oxidation (−0.62 and −0.73 V, respectively) features.
reaction to Al(III), inhibiting the application of the Al(I)/
Al(III) couple in practical catalysis. In order for aluminum
complexes to be applicable for redox transformations,
complexes able to readily undergo reversible oxidation and
reduction chemistries are required.
INTRODUCTION
■
A central challenge in chemistry is the need to develop catalytic
systems for small-molecule transformations that employ
nonprecious metal complexes.1 Aluminum is an attractive
choice for such catalyst development because it is readily
available, inexpensive, and nontoxic.2 Aluminum constitutes
∼8% of the mass of the earth’s crust,3 and at ∼$2.00/kg,4
aluminum is ∼104−105 times less expensive than precious
metals such as Pd, Pt, Rh, and Ir.5 Although aluminum
complexes have a rich history in Lewis-acid catalysis,6
application of aluminum complexes as reagents for the
electrophilic activation and redox transformation of small
molecules is in its infancy.
Aluminum chemistry has historically been defined by the
stability of the tripositive configuration.3 The lack of readily
accessible multielectron redox states has limited the applic-
ability of Al complexes as catalytic species for processes
dependent on oxidative and reductive transformations. Roesky
and co-workers have prepared low-valent Al(I)-carbene
analogues and reported the two-electron redox chemistry of
the complexes.7−9 This work clearly demonstrates that when a
redox process is available, the aluminum center is capable of
facilitating small-molecule activation coupled with redox
transformations. However, molecular complexes supporting
low-valent Al(I) are not common, with only a handful of
known examples.10−15 Additionally, although the Al(I) →
Al(III) oxidation reaction is facile, the corresponding Al(III) →
Al(I) reduction occurs only under highly reducing conditions.
This presents difficulties in the regeneration of Al(I) after
Metal complexes of redox-active ligands are a burgeoning
class of compounds that have been used to great success for
multielectron transformations in recent years.16−18 Recent work
by Berben and co-workers has demonstrated that coordination
of iminopyridine-based ligands (IP) to an Al(III) ion results in
a series of complexes spanning multiple oxidation states. When
utilizing iminopyridine ligands with an unsubstituted pyridine
ring, complexes of the type (IP)AlX3, (IP−)2AlX, and
[M][(IP2−)2Al] (X = monoanionic ligand; M = Na(THF)6
Na(DME)3, NBu4) were prepared.19−21 Increasing the steric
demand of the IP ligand through substitution of the pyridine
ring resulted in a series of complexes with just one IP ligand:
(IP−)AlX2 and (IP2−)AlX(OEt2) (X = Cl, Me).22 The
complexes exhibit multiple redox couples in their cyclic
voltammograms, demonstrating that the redox characteristics
of the ligand were extended to the complexes. Berben also
demonstrated that the redox behavior could be exploited to
facilitate reactivity at the Al(III) ion,19,20 including reduction of
23
2−
CO2 into CO3
.
This work demonstrates that incorporation
of ligands that can access several redox states can result in
aluminum(III) complexes capable of redox properties and that
Received: February 18, 2014
Published: March 24, 2014
© 2014 American Chemical Society
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Figure 1. Redox states of the α-diimine ligand.
the redox activity can be harnessed for novel reaction
chemistry.
redox properties, indicating that the redox properties of the
ligand are also present in the complexes.
We have been investigating the coordination chemistry of the
classical redox-active ligands α-diimines with aluminum(III). α-
Diimines are well known to exhibit rich redox behavior24 and
can be singly or doubly reduced to form the mono- (diimine−)
and dianionic (diimine2−) species, respectively (Figure 1).
Various main group-,25 transition-,26 and rare earth-metal27
complexes incorporating α-diimines across all three ligand
oxidation states have been reported. The α-diimine ligands are
attractive, as both the steric and electronic properties of the
ligand can be readily modified through variation of the carbon
and nitrogen substituents in the N−C−C−N backbone.
Tunability of the ligand through substituent effects provides a
large parameter space for optimization of the aluminum
complexes.
RESULTS AND DISCUSSION
■
Synthesis of Aluminum Complexes 3−5. The aluminum
complex incorporating the doubly reduced α-diimine ligand
LMes (1) was prepared on a 1 g scale. Reduction of 1 with 2
equiv of alkali metal in THF followed by addition of AlCl3 and
stirring at room temperature for 12 h afforded (LMes2−)AlCl-
(THF) (3) as an off-white solid in 81% isolated yield after
Scheme 1. Synthesis of the Al-α-diimine Doubly Reduced
Complex (LMes2−)AlCl(THF) (3)
The coordination chemistry of α-diimine ligands to
aluminum(III) has some precedent. Raston and collaborators
t
have reported the oxidation of aluminum metal with the Bu−
DAB (tBu−DAB = N,N′-di(tert-butyl)-1,4-diazabutadiene)
ligand to give the complex Al(tBu−DAB)2.28 Physical character-
ization of the complex revealed the electronic structure of the
complex to be best described as an aluminum(III) ion
coordinated by a doubly reduced Bu−DAB2− ligand and
t
workup (Scheme 1). Complex 3 was readily characterized by
1H and 13C NMR spectroscopies, with the spectra indicating 3
is a diamagnetic complex. The ortho-methyl groups of complex
3 are symmetry inequivalent and broadened (Δν1/2 = 71 and 86
t
another Bu−DAB− singly reduced radical anion. The Murphy
group has prepared aluminum complexes of the related ligand
Dipp-DAB (Dipp-DAB = N,N′-bis(2,6-diisopropylphenyl)-1,4-
diazabutadiene).29 Reaction of Dipp-DAB with a 1:2 mixture of
AlI3 and Al metal gives (Dipp-DAB−)AlI2, while reaction of the
ligand with just AlI3 gives [(Dipp-DAB)AlI2][I]. Aluminum
complexes with the related Dipp-BIAN ligand (Dipp-BIAN =
1,2-bis(2,6-diisopropylphenylimino)acenaphthylene) have also
been prepared for both the mono- and dianionic forms of the
ligand. Fedushkin and co-workers synthesized compounds with
the singly reduced ligand of the type (Dipp-BIAN−)AlX2, where
1
Hz) at 25 °C in its H NMR spectra, suggesting a hindered
rotation about the N−CAr bond. VT-NMR shows that the two
signals coalesce above 60 °C (see Figure S2). Although 3 can
be stored under nitrogen at −25 °C for several weeks without
noticeable degradation, the compound does slowly decompose
and cannot be indefinitely stored.
Attempts to prepare the corresponding (LDipp2−)AlCl(THF)
complex using a similar pathway to that for 3 implementing the
LDipp (2) ligand proved unsuccessful, resulting in intractable
mixtures of products. Increasing the amount to 4 equiv or
changing the identity (K versus Na) of the alkali-metal reducing
agent did not lead to an improved reaction outcome. However,
a direct reduction route could be employed successfully for the
synthesis of the singly reduced complex: Reduction of 2 with 1
equiv of sodium metal followed by addition of AlCl3 and
stirring at room temperature for 12 h afforded (LDipp−)AlCl2
(4) in 77% yield (Scheme 2).
31
X = Cl30 or an alkyl group (Me, Et, or Bu). The analogous
[Mg2Cl3(THF)6][(Dipp-BIAN2−)AlCl2] compound was also
prepared.30 Recently, Cui and co-workers reported the reaction
of the protonated ene-diamide Ar−NH−C(CH3)C(CH3)−
NH−Ar (Ar = 2,4-iPr2−C6H3, LDippH2) with trialkylaluminum
compounds.32 Reaction of LDippH2 with AlMe3 gave the
(LDippH)AlMe2 compound, while the analogous reaction with
AlEt3 gave (LDipp−)AlEt2.
i
However, a comprehensive structural, electrochemical, and
quantum chemistry study of Al-α-diimine complexes has not
been demonstrated. Herein we report the synthesis and
characterization of aluminum complexes supported by N-aryl-
substituted α-diimine ligands of the type Ar−NC(CH3)-
C(CH3)N−Ar (Ar = 2,4,6-Me3-C6H2, LMes, 1; Ar = 2,6-iPr2−
C6H3, LDipp, 2). We have prepared and characterized Al-α-
diimine complexes with both the singly and doubly reduced
form of the ligands and characterized them, including a
quantum chemical description of the compounds. Importantly,
the cyclic voltammograms of the complexes exhibit reversible
A similar pathway to directly prepare the singly reduced
(LMes−)AlCl2 complex 5 through limiting the amount of alkali
metal employed to 1 equiv yielded multiple products, including
both the doubly and singly reduced complexes and free ligand.
However, as shown in Scheme 3, reaction of 3 with 1 equiv of
AgCl resulted in oxidative functionalization to give (LMes−)-
AlCl2 (5) as an orange solid in 89% yield. Compound 5 could
also be prepared using CuCl as the oxidant; reaction of 3 with 1
equiv of CuCl gave 5 in 65% yield. Although the AgCl reaction
was run for 12 h, the CuCl reaction was stopped after 3 h, as
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Scheme 2. Synthesis of the Al-α-diimine Singly Reduced
Complex (LDipp−)AlCl2 (4)
Scheme 3. Synthesis of Al-α-diimine Singly Reduced
Complex (LMes−)AlCl2 (5) through Oxidative
Functionalization with M−Cl (M = Ag, Cu) Oxidizing
Agents
Figure 2. Solid-state structure of (LMes2−)AlCl(THF) (3) with
ellipsoids at the 30% probability level. H atoms and an interstitial
toluene molecule have been omitted for clarity.
longer reaction times resulted in decreased yields and impure
product. These oxidative functionalization routes demonstrate
that the redox properties of the complex can be coupled to
reaction chemistry at the aluminum ion. Similar reaction
chemistry was observed by the Berben group, who showed that
their reduced Al-iminopyridine complex [Na(THF)6]-
[(IP2−)2Al] could be oxidatively functionalized using ZnX2
compounds to (IP−)2AlX (X = Cl, CCPh, N3, SPh,
NHPh).20 Both complexes 4 and 5 are paramagnetic with a
singly reduced ligand and result in 1H NMR spectra that display
no signals (Figures S4 and S5).
Figure 3. Solid-state structure of (LDipp−)AlCl2 (4). Ellipsoids are
Attempts to prepare aluminum complexes with neutral LMes
and LDipp ligands were unsuccessful. Reaction between 1 or 2
with AlCl3 did not yield the LAlCl3 adducts, and reaction of
complex 4 or 5 with either AgCl or CuCl resulted in isolation
of free neutral diimine ligands. Finally, reaction of 5 with
FcBPh4 in THF produced free LMes ligand, [trans-
(THF)4AlCl2][BPh4],33 and ferrocene. These results suggest
that LMes and LDipp do not effectively bind the AlIII cation in
their neutral forms in the presence of coordinating solvent.34
Solid-State Structures of 3−5. The structures of the
complexes 3−5 were corroborated by X-ray crystallography.
Single crystals of 3 were grown from a toluene solution at −25
°C, which crystallized with a molecule of interstitial toluene
(Figure 2). Single crystals of 4 and 5 were both grown from
hexane solutions at −25 °C (Figures 3 and 4). Crystallographic
data are provided in Table 1, and selected bonding metrics for
the complexes are provided in Table 2.
All three complexes 3−5 are four-coordinate at the
aluminum(III) cation, with a bidentate α-diimine ligand and
two monodentate ligands (either Cl or THF) constituting the
coordination sphere. According to the τ4 parameter introduced
by Houser,35 the geometry of 3 is best described as trigonal
pyramidal with τ4 = 0.82. Conversely, 4 and 5 have τ4 values of
0.92 and 0.90, respectively, and exist between the trigonal
pyramidal and tetrahedral geometries. At 2.1078(6)−2.1278(6)
Å, the Al−Cl bond lengths for the complexes are unremarkable
and are in the range of other terminal Al−Cl bonds reported
projected at 30% probability, and H atoms are omitted for clarity.
Figure 4. Solid-state structure of (LMes−)AlCl2 (5). Ellipsoids are
projected at 30% probability, and H atoms are omitted for clarity.
for four-coordinate aluminum(III) complexes.36 The Al−N
bond lengths are slightly shorter for complex 3 (1.799(3) and
1.803(3) Å), which contains a doubly reduced ligand, versus
complexes 4 (1.8733(9) Å) and 5 (1.8643(14) and 1.8782(15)
Å), which have singly reduced ligands, supporting a relatively
larger electron density on the doubly reduced ligand. A similar
trend was observed for the iminopyridine complexes reported
by Berben, where the Nimido−Al bond distances were in the
range 1.806(1)−1.879(2) Å for the doubly reduced ligands
relative to 1.856(1)−1.989(3) Å for the singly reduced
ligands.19,20,22 The aluminum complexes with the Dipp-BIAN
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workers.25d When L− is LMes−, the imido C−N distances are
1.323(3) and 1.347(3) Å with a C−C distance of 1.432(3) Å,
compared to imido C−N distances of 1.473(4) and 1.379(4) Å
and a C−C distance of 1.354(4) Å when L− is LDipp−. The
binding metrics are also in good agreement with those found
for the (LDipp−)AlEt2 complex, which had imido C−N distances
of 1.375(3) and 1.390(3) Å and a C−C distance of 1.391(3)
Å.32
Table 1. Crystallographic Data for Complexes 3−5
3
4
5
formula
a (Å)
C33H44AlClN2O
23.107(3)
11.7175(14)
23.314(3)
90
C28H40AlCl2N2
12.3683(4)
21.3523(8)
10.4652(4)
90
C22H28AlCl2N2
17.6845(14)
14.1011(10)
18.3747(14)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
100.299(7)
90
90
90
In 3, the α-diimine ligand exists in the enediamide form, with
elongated imido C−N bond lengths of 1.438(4) and 1.440(4)
Å and a shortened C−C distance of 1.341(4) Å. These bonding
parameters are comparable to those reported for other
complexes incorporating LMes in the doubly reduced state.
The (LMes2−)Mg(THF)3 complex reported by Yang, Wu, and
co-workers had C−N distances of 1.426(3) and 1.406(3) Å and
a C−C distance of 1.350(3) Å,25c while their [Na-
(THF)2(LMes2−)Na]2 complex had C−N distances of
1.428(4) and 1.411(4) Å and a C−C distance of 1.356(4) Å.25c
Density Functional Theory (DFT) Studies. The full
geometries of compounds 3 and 5 were optimized using DFT
(Figures S6 and S7 and Tables S1 and S2) with the geometry of
compound 5 constrained to C2v symmetry. The theoretical
bond distances and angles were found to be in good agreement
with those obtained in the X-ray analysis (Table 2). As with the
X-ray data, the theoretical picture shows a shortening of the C−
C bond and a lengthening of the C−N bonds within the
diimine backbone as additional electrons are added to the
ligand system. This assertion is supported by analysis of the
frontier orbitals: The HOMO of 3 and the SOMO of 5 (Figure
5) are primarily ligand based and located on the N−C−C−N
backbone with a bonding interaction between the C−C and an
antibonding interaction between the C−N groups.
90
90
6210.9(12)
8
2763.77(17)
4
4582.1(6)
8
fw (g/mol)
Space group
T (K)
547.13
C2/c
502.50
Pnma
418.34
Pbca
120(2)
0.71073
1.170
100(2)
0.71073
1.208
100(2)
0.71073
1.213
λ (Å)
Dcalc (mg·m−3
)
μ (mm−1
)
0.178
0.285
0.331
R1 (I > 2σ(I))
wR2 (all data)
0.0661
0.1438
1.18
0.0387
0.1108
1.03
0.0430
0.1152
1.13
Tmax/Tmin
GOF (F2)
1.019
1.025
1.044
ligand reported by Fedushkin also show a similar trend in
bonding parameters: The Al−N lengths in the (Dipp-
BIAN−)AlCl2 complex are 1.890(2) and 1.888(2) Å, compared
to 1.844(4) and 1.838(4) Å for the [(Dipp-BIAN2−)AlCl2]
−
anion.30
The oxidation state of the ligand could be further assessed
through the bonding metrics of the N−C−C−N backbone in
the complexes.37 In the free LMes ligand 1, the C−N bond
lengths are 1.278(2) Å, while the C−C bond distance is
1.500(2) Å.30 In comparison, there is a lengthening of the C−N
bonds and shortening of the C−C bonds in the singly reduced
complex 5, supporting addition of electron density into the
LUMO of the ligand.28b The imido C−N bond lengths in 5 are
1.355(2) and 1.348(2) Å, and the C−C bond distance is
1.432(2) Å. Similar bonding metrics were observed for
compound 4, which has an imido C−N distance of
1.3456(13) Å and a C−C distance of 1.466(2) Å. These
bond distances compare well to the metrics obtained for the
[(L−)Ca(μ2-Cl)(THF)2]2 complexes reported by Yang and co-
The charge distribution of complexes 3 and 5 was studied by
the natural bonding orbital (NBO) method, with selected data
given in Table 3. NBO studies show that the α-diimine ligand
on complex 3 is anionic, as the electron acceptors N(1) and
N(2) acquire much of the negative charge, while the
aluminum(III) cation has a larger positive charge. In
comparison, N(1) and N(2) on complex 5 have less negative
charge, as is expected for the singly reduced complex.
Table 2. Selected Calculated and Experimental Bond Distances (Å) and Angles (deg) for 3−5
3exp
3theory
4exp
5exp
5theory
1.896
Al−N
Al−Cl
1.799(3)
1.803(3)
2.1229(14)
1.815
1.817
2.124
1.8733(9)
1.8643(14)
1.8782(15)
2.1215(7)
2.1252(7)
n/a
2.1278(6)
2.1078(6)
n/a
2.132 2.131
Al−O
1.856(2)
1.928
n/a
C−Cbackbone
C−Nimine
1.341(4)
1.363
1.466(2)
1.3456(13)
1.438(2)
1.355(2)
1.348(2)
87.00(6)
114.07(5)
117.38(5)
115.24(5)
114.33(5)
n/a
1.422
1.349
1.438(4)
1.427
1.440(4)
1.429
N−Al−N
N−Al−Cl
93.45(13)
122.38(11)
121.29(10)
93.215
125.045
125.160
86.77(6)
113.44(3)
116.14(3)
86.381
114.599
N−Al−O
108.52(12)
110.50(12)
n/a
106.193
105.892
n/a
n/a
n/a
Cl−Al−Cl
Cl−Al−O
109.50(3)
n/a
107.95(3)
n/a
110.368
n/a
100.51(9)
99.450
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Figure 5. HOMO of (LMes2−)AlCl(THF) (3) and SOMO of
(LMes−)AlCl2 (5). Molecular orbitals are visualized using VMD with
isosurfaces at 0.03 au.
Electrochemistry. Electrochemical characterization of 3 in
THF solution with 0.1 M [n-Pr4N][BArF] (BArF− = B(3,5-
CF3)2-C6H3)4 ) supporting electrolyte showed multiple redox
Figure 6. Cyclic voltammogram of compound 3 recorded in 0.1 M [n-
−
Pr4N][BArF] THF solution.
processes in the cyclic voltammogram (Figure 6 and Table 4).
The rest potential of the complex was −1.25 V, so we assign the
reversible feature centered at E1/2 = −0.94 V versus Fc/Fc+ as
an oxidation process corresponding to the to the L2−/L−
couple. We assign the second, less reversible feature at −0.03
V as the L−/L0 couple. The identity of the feature at ∼−0.64 V
(marked with an *) is as yet unclear, but we speculate that it is
the result of a chemical process or instability of 3 under the
experimental conditions. The feature was persistent across
several measured samples and was observed for analytically
pure material.
Table 4. Electrochemical Potentials (V vs Fc/Fc+) for
Complexes 3−5
diimine2−/diimine−
diimine−/diimine0
ΔE
3
4
5
−0.94
−1.37
−1.34
−0.03
−0.62
−0.73
0.91
0.75
0.61
The cyclic voltammograms of compounds 4 and 5 are shown
in Figure 7 and are cleaner than that for 3, with both
compounds displaying two independent redox features. Both
complexes have a reversible feature that we assign to the L2−/
L− couple at E1/2 = −1.37 V (for 4) and E1/2 = −1.34 V (for 5)
versus Fc/Fc+. Comparison of these features to that in 3 shows
a shift to more reducing potentials by ∼0.40 V. A similar shift
was observed by Berben when comparing the (IP−)2AlCl and
[Na(DME)3][(IP2−)2Al] complexes.21 Complexes 4 and 5 also
have processes that we assign to the L−/L0 couple at E1/2
=
−0.62 V and E1/2 = −0.73 V versus Fc/Fc+, respectively. The
process is quasi-reversible for 4 and reversible for 5 and more
clearly defined than the corresponding feature for complex 3.
Again a shift to more negative potentials was observed.
CONCLUSIONS
■
We have demonstrated that the redox-active α-diimine ligands
can be coordinated to an aluminum(III) ion to afford
complexes in the −1 or −2 oxidation state of the ligand. The
structural and theoretical characterization of the complexes
corroborates ligand oxidation state assignment. The electro-
chemical characterization shows reversible redox properties for
all complexes, suggesting that the aluminum(III) ion can
support all oxidation states of the ligand. We have also shown
that the (LMes2−)AlCl(THF) can be oxidized using either AgCl
or CuCl to give (LMes−)AlCl2, indicating the redox properties of
the complex can be coupled to reactivity at the aluminum(III)
ion. We are currently investigating other reactivity profiles for
the complexes, namely through the development of two-
electron chemistry. We are also exploring the coordination
Figure 7. Cyclic voltammograms of compounds 4 (top) and 5
(bottom) recorded in 0.1 M [n-Pr4N][BArF] THF solution.
Table 3. Charge Distribution of Complexes 3 and 5
Al
Cl(1)
Cl(2)/O(1)
C(1)
C(2)
N(1)
N(2)
3
5
+1.88804
+1.67783
−0.53781
−0.53048
−0.72058
−0.53048
+0.09444
+0.23157
+0.10113
+0.23157
−0.98492
−0.78159
−0.98270
−0.78159
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equipped with a molecular sieves 13X/Q5 Cu-0226S catalyst purifier
system. Glassware was dried overnight at 150 °C before use. C6D6 was
purchased from Sigma Aldrich and was stored over potassium metal
prior to use. Tetrahydrofuran, hexanes, pentane, and toluene were
purchased from Fisher Scientific. These solvents were sparged for 20
min with dry argon and dried using a commercial two-column solvent
purification system comprising columns packed with Q5 reactant and
neutral alumina, respectively (for hexanes, toluene, and pentane), or
two columns of neutral alumina (for THF). Celite was purchased from
Sigma Aldrich and was dried under reduced pressure at 250 °C for 48
h prior to use. The α-diimine ligands LMes (1) and LDipp (2) were
prepared according to literature procedures.47 [n-Pr4N][BArF] was
prepared as reported by Kiplinger et al.48 All other reagents were
purchased from commercial sources and used as received.
chemistry of N-aryl α-diimines that lack substitution in the 2,6
positions of the aryl substituent to aluminum(III) to prepare
stable complexes incorporating the neutral oxidation state of
the ligand.
EXPERIMENTAL SECTION
■
Physical Measurements. 1H and 13C NMR spectra were recorded
at ambient temperature in C6D6 using a Varian 400 MHz spectrometer
1
(399.78 MHz for H, 100.52 MHz for 13C). Chemical shifts were
referenced to residual solvent. Elemental analyses were performed at
the University of California, Berkeley Microanalytical Facility, on a
Perkin-Elmer Series II 2400 CHNS analyzer.
Electrochemical Measurements. CVs were recorded in a
glovebox under a dinitrogen environment using either a BASi
Epsilon-EC potentiostat/galvanostat (for 3) or a CH Instruments
620D electrochemical analyzer/workstation (for 4 and 5). In both
cases, a glassy carbon working electrode, a platinum wire auxiliary
electrode, and a silver wire plated with AgCl as a quasi-reference
electrode were utilized. Potentials were reported versus ferrocene,
which was added as an internal standard for calibration at the end of
each run. Solutions employed during these studies were ∼3 mM in
analyte and 100 mM in [n-Pr4N][BArF] (BArF− = B(3,5-CF3)2-
(LMes2−)AlCl(THF) (3). LMes (1.0 g, 3.12 mmol) in THF (10 mL)
was added to a stirring suspension of sodium metal (0.14 g, 6.24
mmol) in THF (25 mL). After 4 h, AlCl3 (0.42 g, 3.12 mmol) was
added, and the reaction was allowed to stir at room temperature. After
12 h, the reaction was filtered over a Celite-padded frit, and volatiles
were removed from the pale yellow filtrate under vacuum. The crude
reaction product was taken up into toluene (50 mL) and filtered over a
Celite-padded frit, and volatiles were removed from the filtrate. The
resultant oily solid was triterated with pentane (3 × 10 mL) and dried
under reduced pressure to give 3 as an off-white solid. Yield: 1.15 g
(81%). Crystals suitable for X-ray diffraction were obtained from
−
C6H3)4 ) in ∼3 mL of THF. All data were collected in a positive-
feedback IR compensation mode.
1
cooling a saturated toluene solution at −25 °C. H NMR: δ 6.96 (s,
X-ray Structure Determination. X-ray diffraction data were
4H), 3.45 (bm, 4H, THF), 2.69 (bs, 6H, o-CH3), 2.32 (bs, 6H, o-
CH3), 2.26 (s, 6H, p-CH3), 1.78 (s, 6H, CH3-C−N), 0.78 (bs, 4H,
THF). 13C{1H} NMR: δ 166.4, 144.2, 132.7, 128.1, 118.6, 73.2, 25.2,
21.6, 20.4, 14.6. Anal. Calcd for C26H36AlClN2O: C, 68.63; H, 7.98; N,
6.16. Found: C, 68.56; H, 7.82; N, 6.00.
collected on a Bruker-AXS Kappa APEX II CCD diffractometer with
̈
0.71073 Å Mo Kα radiation. Cell parameters were retrieved using
APEX II software38 and further refined on all observed reflections
during integration using SAINT+.39 Each data set was treated with
SADABS40 absorption corrections based on redundant multiscan data.
The structures were solved by direct methods and refined by least-
squares method on F2 using the SHELXTL program package.41 All
non-hydrogen atoms were refined with anisotropic displacement
parameters, and all hydrogen atoms were treated with a riding model.
Details regarding specific solution refinement for each compound are
provided in the following paragraphs.
(LDipp−)AlCl2 (4). LDipp (0.50 g, 1.23 mmol) in THF (10 mL) was
added to a stirring suspension of sodium metal (0.028 g, 1.23 mmol)
in THF (25 mL). After 4 h, AlCl3 (0.16 g, 1.23 mmol) was added, and
the reaction was allowed to stir at room temperature. After 12 h, the
reaction was filtered over a Celite-padded frit, and volatiles were
removed from the pale yellow filtrate under vacuum. The crude
reaction product was taken up into hot toluene (25 mL) and filtered
over a Celite-padded frit, and volatiles were removed from the filtrate.
Yield: 0.48 g (77%). Crystals suitable for X-ray diffraction were
obtained from cooling a saturated hexane solution at −25 °C. Anal.
Calcd for C28H40AlCl2N2: C, 66.92; H, 8.02; N, 5.57. Found: C, 67.45;
H, 8.07; N, 5.25.
X-ray structural analysis for 3: A single colorless plate (0.2 × 0.06 ×
0.02 mm3) was mounted in immersion oil onto a glass fiber, and data
were collected under a nitrogen stream at 120 K. Systematic absences
in the data were consistent with the centrosymmetric, monoclinic
space group C2/c. The asymmetric unit contains one (LMes2−)AlCl-
(THF) molecule and a molecule of toluene solvent. The toluene
solvent was disordered over two positions, which were located from
the difference map and refined using SIMU, DELU, and SAME
commands within SHELXL.
X-ray structural analysis for 4: A single orange block (0.2 × 0.12 ×
0.10 mm3) was mounted in immersion oil onto a glass fiber, and data
were collected under a nitrogen stream at 100 K. Systematic absences
in the data were consistent with the centrosymmetric, orthorhombic
space group Pnma. The asymmetric unit contains one-half molecule of
(LDipp−)AlCl2, as the molecule lies on a crystallographic mirror plane.
X-ray structural analysis for 5: A single orange shard (0.27 × 0.18 ×
0.07 mm3) was mounted in immersion oil onto a glass fiber, and data
were collected under a nitrogen stream at 100 K. Systematic absences
in the data were consistent with the centrosymmetric, orthorhombic
space group Pbca. The asymmetric unit contains one molecule of
(LMes−)AlCl2.
Computational Details. The structure optimization of 3 and 5
was performed with the Gaussian ’09, Revision A.1,42 program using
the M06 density functional43 and the 6-31+G(d,p) basis set.44,45
Geometry operations were carried out in C1 symmetry for compound
3 and C2v symmetry for compound 5. All frequency calculations found
no imaginary frequencies, confirming that the optimized structures
were minima. Binding analysis was performed using NBO 3 as coded
within Gaussian ’09. Molecular orbitals were visualized using Visual
Molecular Dynamics46 with isosurfaces at 0.03 au.
(LMes−)AlCl2 (5). Using AgCl: (LMes2−)AlCl(THF) (3) (0.50 g, 1.10
mmol) was dissolved in toluene (25 mL), and AgCl (0.16 g, 1.10
mmol) was added. The solution was stirred at room temperature for
12 h, after which it was filtered over a Celite-padded frit, and volatiles
were removed from the orange filtrate under vacuum. The crude
reaction product was taken up into boiling hexanes (25 mL) and
filtered over a Celite-padded frit, and volatiles were removed from the
filtrate to give 5 as an orange powder. Yield: 0.41 g (89%).
Using CuCl: (LMes2−)AlCl(THF) (3) (0.25 g, 0.55 mmol) was
dissolved in toluene (15 mL), and CuCl (0.055 g, 0.55 mmol) was
added. The solution was stirred at room temperature for 3 h, after
which it was filtered over a Celite-padded frit, and volatiles were
removed from the orange filtrate under vacuum. The crude reaction
product was taken up into boiling hexanes (25 mL) and filtered over a
Celite-padded frit, and volatiles were removed from the filtrate to give
5 as an orange powder. Yield: 0.15 g (65%).
Crystals suitable for X-ray diffraction were obtained from cooling a
saturated hexane solution at −25 °C. Anal. Calcd for C22H28AlCl2N2:
C, 63.16; H, 6.75; N, 6.70. Found: C, 62.32; H, 6.63; N, 6.32.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR spectra of 3−5. VT-NMR spectra for 3. 13C NMR
spectrum of 3. Tables of coordinates from geometry
optimizations of 3 and 5. Full ref 42. CIF files for complexes
Preparation of Compounds. All reactions and manipulations
were performed under an inert atmosphere (N2) using standard
Schlenk techniques or in a Vacuum Atmospheres, Inc. Nexus II drybox
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Article
Wieghardt, K. Angew. Chem., Int. Ed. 2011, 50, 1652−1655.
(i) Darmon, J. M.; Stieber, S. C. E.; Sylvester, K. T.; Fernandez, I.;
Lobkovsky, E.; Semproni, S. P.; Bill, E.; Wieghardt, K.; DeBeer, S.;
Chirik, P. J. J. Am. Chem. Soc. 2012, 134, 17125−17137.
(17) For recent reivews on the application of redox-active ligand
metal complexes in catalysis, see: (a) Lyaskovskyy, V.; de Bruin, B.
ACS Catal. 2012, 2, 270−279. (b) Luca, O. R.; Crabtree, R. H. Chem.
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3−5. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: cgraves@alb.edu.
■
Notes
The authors declare no competing financial interest.
(18) For recent reviews, see: (a) Kaim, W. Eur. J. Inorg. Chem. 2012,
2012, 343−348. Caulton, K. G. Eur. J. Inorg. Chem. 2012, 2012, 435−
443.
ACKNOWLEDGMENTS
■
(19) Myers, T. W.; Berben, L. A. J. Am. Chem. Soc. 2011, 133,
We thank Albright College and the ACS-Petroleum Research
Fund (PRF 52181-UNI3) for financial support of this work. We
thank the George I. Alden Trust for funding toward the NMR
spectrometer used in this work. B.E.C. thanks Albright College
for a 2012 Summer Research Fellowship and the Goldwater
Foundation. We thank Jerome Robinson and Professor Eric J.
Schelter (both University of Pennsylvania) for helpful
discussions.
11865−11867.
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