Synthesis and reactions of a redox-active α-diimine aluminum complex

Article abstract of DOI:10.1021/om300920y

The reaction of the ene-diamine LH₂ (L = [ArNC(Me)C(Me)NAr] ^2-, Ar = 2,6-iPr₂C₆H₃) with AlMe₃ and AlEt₃ yielded the aluminum dimethyl complex (LH)AlMe₂ (1) and the radical (L^?)AlEt₂ (2; L^? = [ArNC(Me)C(Me)NAr]^?-), respectively. Treatment of 2 with O₂ and TEMPO (2,2,6,6-tetramethylpiperidin-1- oxyl) led to hydrogen abstraction from the methyl group at the ligand backbone to give the diamagnetic species (L*)AlEt₂ (3; L* = [ArN=C(Me)C(CH₂)NAr]^-) and (L*)Al(TEMPO)(Et) (4). In contrast, the reaction of 2 with iodine resulted in the iodination of the Al-C bonds and yielded the radical (L^?)AlI₂ (5). Reaction of 2 with BCl₃ led to electron transfer between the α-diimine radicals with the formation of the aluminum cation [(L⁰)AlCl ₂]⁺[AlCl₄]^- (6; L⁰ = ArN=C(Me)(Me)C=NAr) and the diazaborole LBCl (7). The structures of compounds 2, 3, and 7 have been determined by X-ray single-crystal diffraction, and the paramagnetic species 2 has been characterized by EPR measurements.

Full text of DOI:10.1021/om300920y

Article
pubs.acs.org/Organometallics
Synthesis and Reactions of a Redox-Active α‑Diimine Aluminum
Complex
Jianfeng Li, Kun Zhang, Hanmin Huang, Ao Yu, Hongfan Hu, Haiyan Cui, and Chunming Cui*
State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
S
* Supporting Information
ABSTRACT: The reaction of the ene-diamine LH2 (L =
[ArNC(Me)C(Me)NAr]2−, Ar = 2,6-iPr2C6H3) with AlMe3
and AlEt3 yielded the aluminum dimethyl complex (LH)AlMe2
(1) and the radical (L•)AlEt2 (2; L• = [ArNC(Me)C(Me)-
NAr]•−), respectively. Treatment of 2 with O2 and TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) led to hydrogen abstraction
from the methyl group at the ligand backbone to give the diamagnetic species (L*)AlEt2 (3; L* = [ArN
C(Me)C(CH2)NAr]−) and (L*)Al(TEMPO)(Et) (4). In contrast, the reaction of 2 with iodine resulted in the iodination of
the Al−C bonds and yielded the radical (L•)AlI2 (5). Reaction of 2 with BCl3 led to electron transfer between the α-diimine
radicals with the formation of the aluminum cation [(L0)AlCl2]+[AlCl4]− (6; L0 = ArNC(Me)(Me)CNAr) and the
diazaborole LBCl (7). The structures of compounds 2, 3, and 7 have been determined by X-ray single-crystal diffraction, and the
paramagnetic species 2 has been characterized by EPR measurements.
salts of radical anionic α-diimines with main-group metal
halides.3h,4,5b−d The formation of a paramagnetic aluminum
complex by the transmetalation of a redox-active ligand from a
transition-metal complex has also been reported.5e Herein we
report on the direct generation of the α-diimine diethylalumi-
num radical 2 by the reaction of triethylaluminum with an ene-
diamine (Scheme 1). Moreover, the radical 2 can be involved in
electron-transfer reactions, as demonstrated by its reactions
with dioxygen, TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl),
and BCl3.9
INTRODUCTION
■
The introduction of noninnocent ligands into metal complexes
has attracted intense interest in the past decade, as the
cooperation of a metal ion and the ligand offers a unique
opportunity to modify the reactivity of the corresponding
complexes.1,2 Main-group-element complexes based on ligands
containing α-diimine moieties, such as 1,4-diazabutadiene,3
bis(imino)acenaphthene,4 iminopyridine5 and bipyridine,6 are
typical representatives. The α-diimine backbone can undergo
one- or two-electron reduction to form the corresponding
radical anion and dianionic ene-diamide ligand (Chart 1),5b,7,8
which allows the generally redox-inactive main-group ions to
access multiple redox states.4b,5a
RESULTS AND DISCUSSION
■
The reactions of the ene-diamine LH2 (L = [ArNC(Me)C-
(Me)NAr]2−, Ar = 2,6-iPr2C6H3) with the three different
aluminum alkyls AlMe3, AlEt3, and Al(iBu)3 have been
investigated for the preparation of the desired aluminum
compounds. Reaction of AlMe3 with 1 equiv of LH2 in refluxing
n-hexane yielded the neutral compound (LH)AlMe2 (1) as
orange crystals in good yield (Scheme 1). However, similar
reactions with AlEt3 and Al(iBu)3 resulted in oily mixtures,
from which a product could not be isolated and characterized.
Reaction of LH2 with 2 equiv of AlMe3 also yielded 1 as the
sole isolable product under different conditions. In contrast,
reaction of LH2 with 2 equiv of AlEt3 in refluxing n-hexane
afforded air- and moisture-sensitive red crystals of the radical
(L•)AlEt2 (2; L• = [ArNC(Me)C(Me)NAr]•−) in high yield
(82%) with the formation of aluminum powder.10 A similar
reaction with 2 equiv of Al(iBu)3 led to the formation of a
complicated mixture. Attempts to isolate a single product from
the mixture were unsuccessful to date. These results indicated
Chart 1. Chemical Conversions of Noninnocent α-Diimine
Ligands
Main-group-element complexes supported by a radical
anionic α-diimine ligand, in general, can be prepared by two
different routes: (i) reduction of the adducts formed by neutral
α-diimines with main-group precursors via a single-electron
transfer process3g or reduction with main-group metals and
low-valent main-group halides such as Li, Ba, Al, GaI, InCl, and
Received: September 28, 2012
3c−f,4a
GeCl2
and (ii) salt elimination reactions of alkali-metal
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Complexes 1 and 2
Scheme 2. Proposed Synthetic Pathways of 1 and 2
that the steric factors of aluminum trialkyls have substantial
effects on the reactions.
Since no products can be isolated for the reaction of LH2
with 1 equiv of AlEt3, we proposed that the initial product
(LH)AlEt2 (Scheme 2, A) as formed in the reaction of AlMe3
cannot form the same cyclic structure as 1, due to the steric
effects of ethyl groups in comparison to small methyl groups in
1, and thus another free NH group in (LH)AlEt2 can easily
react with an excess of AlEt3 to give the dimetallic intermediate
B (Scheme 2).11 This dimetallic species subsequently under-
went the homolytic cleavage of one of the Al−N bonds to give
the radical 2.12 The reaction of AlEt3 with the same amount of
isocyanide ligands in Ir2(CNR)4(dmpm) (R = 2,6-Me2C6H3,
dmpm = Me2PCH2PMe2) has been reported to yield the radical
complex Ir2[C2(NR)2AlEt2](CNR)2(dmpm)2.13 The failure to
give the similar radical species in the reaction of AlMe3 may be
attributed to the stable five-membered-ring structure of 1.
Indeed, reaction of 1 with AlMe3 in refluxing n-hexane and even
in hot toluene for several hours only led to the recovery of a
significant amount of 1.
Figure 1. Experimental (in toluene at 298 K) and simulated EPR
spectra of 2.
Table 1. Isotropic g and Hyperfine Coupling Constants
(mT) for 2, (Ar-BIAN•)AlR2 (R = Me, Et, iBu),4c and
[ArNC(H)C(H)NAr•]AlI2,3d Obtained from Computer
a
Simulations of the Corresponding EPR Spectra
hyperfine coupling
b
complex
giso
27Al
14N
1H
127I
c
The structure of 1 was identified by 1H and 13C NMR and IR
spectroscopy and elemental analysis. In the 1H NMR spectrum,
the two methyl groups bonded to the aluminum atom show
two singlets at δ −0.51 and −0.29 ppm and the remaining
proton on one of the nitrogen atoms resonates at δ 6.01 ppm,
which is shifted to low field in comparison to the free amine
due to the nitrogen coordination to the aluminum atom. The
paramagnetic compound 2 has been characterized by EPR
spectroscopy in toluene. Figure 1 shows the EPR spectrum of 2
and the corresponding simulated spectrum. The signals of the
EPR spectrum are well resolved except those at the ends of the
spectrum, which are probably due to the fact that the
resonances at low and high field are beyond the detection
limit of the instrument. The 27Al and 14N nuclei in 2 feature
2
2.0029
2.0031
2.0033
2.0032
2.0038
1.17
0.60
0.62
0.61
0.285
0.94 0.25
d
e
e
e
(Ar-BIAN•)AlMe2
(Ar-BIAN•)AlEt2
(Ar-BIAN•)AliBu2
0.46 0.14 /0.10
d
0.46 0.14 /0.10
d
0.46 0.14 /0.10
f
[ArNC(H)C(H)NAr•]
AlI2
0.67 0.59
0.04
a
b
Ar = 2,6-iPr2C6H3. 14N refers to two equivalent nitrogen nuclei.
c
Refers to six equivalent 1H nuclei in two equivalent methyl groups of
d
ligand L•. Refers to two equivalent 1H nuclei in ortho positions of the
e
naphthalene system (relative to the diimine fragment). Refers to two
equivalent 1H nuclei in para positions of the naphthalene system
f
(relative to the diimine fragment). Refers to two equivalent imine 1H
nuclei.
electron density to the central five-membered ring atoms than
those in (Ar-BIAN•)AlEt2 and [ArNC(H)C(H)NAr•]AlI2,
since the two reported complexes have a naphthalene unit or
two electronegative iodine atoms, which can share electron
density of the unpaired electron via delocalization to the large
aromatic ring and the iodine atoms, respectively.
large hyperfine coupling constants in comparison to those in
4 c
(Ar-BIAN• )AlEt2
(Ar-BIAN = 1, 2-bis[(2, 6-
diisopropylphenyl)imino]acenaphthene) and [ArNC(H)C(H)-
3d
NAr•]AlI2 (Ar = 2,6-iPr2C6H3) (Table 1). These differences
suggested that the unpaired electron in 2 distributes more
B
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1/2 equiv of dioxygen at −78 °C, the solution immediately
turned light yellow. The compound (L*)AEt2 (3; L* = [ArN
C(Me)C(CH2)NAr]−) was isolated in 34% yield after a
standard workup. In addition, small amounts of aluminum
metal and 3,3′,5,5′-tetraisopropyl-1,1′-biphenyl-4,4′-diamine
were isolated. Obviously, 3 was formed by the abstraction of
a hydrogen radical from one of the methyl groups on the ligand
backbone in 2.18 It is quite possible that the radical species 2
initially couples with dioxygen to form a peroxyl radical, which
is highly reactive and rapidly abstracts the hydrogen atom from
the rest of 2 in the reaction system to yield 3 and a
hydroperoxide. The peroxide may degrade to aluminum metal
and some organic fragments, including the isolated compound
3,3′,5,5′-tetraisopropyl-1,1′-biphenyl-4,4′-diamine. The reac-
tions of 2 with 1 equiv or more of dioxygen led to the
formation of an oily mixture containing a number of species
which could not be identified to date. These experiments
support the abstraction of hydrogen by the initially formed
peroxyl radical intermediate rather than by dioxygen itself. The
mechanism is further supported by the reaction of 2 with
TEMPO. The reaction of 2 with TEMPO in toluene from −78
°C to room temperature afforded the compound (L*)Al-
(TEMPO)(Et) (4) (Scheme 3). It appears that the TEMPO
radical acted as a radical trap reagent to abstract hydrogen
radical from one of the methyl groups on the ligand backbone
in 2 to generate TEMPOH and 3 followed by the ethane
elimination reaction of TEMPOH with 3. In order to prove the
last step of the reaction, the reaction of TEMPOH with 3 was
performed. As expected, this reaction indeed yielded 4, as
indicated by NMR analysis.
The structure of 2 in Figure 2 shows that the aluminum atom
is four-coordinate, being bound to the radical anionic ligand in
Figure 2. ORTEP representation of the X-ray structure of 2.
Hydrogen atoms and disordered atom C29′ have been omitted for
clarity. Thermal ellipsoids are drawn at the 30% probability level.
Selected bond lengths (Å) and angles (deg): Al1−N1 = 1.914(2),
Al1−N2 = 1.935(2), Al1−C1 = 1.963(3), Al1−C3 = 1.999(3), N1−
C6 = 1.375(3), N2−C7 = 1.390(3), C5−C6 = 1.503(3), C6−C7 =
1.391(3), C7−C8 = 1.488(3); N1−Al1−N2 = 85.28(8), C1−Al1−C3
= 107.68(12).
an η2 fashion. The aluminum atom in 2 can be best described as
having a distorted-tetrahedral geometry due to the small N1−
Al1−N2 angle (85.28(8)°). This angle is very close to that
observed in (Ar-BIAN•)AlEt2 (85.31(12)°).4c The average Al−
C bond length of 1.981(3) Å in 2 is only slightly longer than
that observed in (Ar-BIAN•)AlEt2 (average 1.962(4) Å); the
average Al−N bond length of 1.925(2) Å in 2 is slightly shorter
than those reported for (Ar-BIAN•)AlEt2 (average 1.957(3) Å).
The bond lengths in the NCCN fragment of the ene-diamine
ligand (C−C = 1.391(3) Å, C−N = 1.383(3) Å (average)) lie
between those of double and single bonds reported for free
ene-diamines (C−C = 1.339(2) Å, C−N = 1.431(2) Å
(average))14 and free α-diimines (C−C = 1.498(3) Å, C−N
= 1.279(3) Å (average)),15 indicating the delocalized five-
membered-ring framework.
The structure of 3 has been confirmed by X-ray single-crystal
diffraction and is shown in Figure 3. The Al1−N1 bond length
of 1.874(2) Å is significantly shorter than Al1−N2 (1.966(2)
Å). In addition, the N2−C15 bond length of 1.295(3) Å and
the C13−C14 bond length of 1.335(4) Å are indicative of N
C and CC double bonds. These parameters suggest a
monoanionic ligand framework featuring an exocyclic CC
1
double bond. The H NMR spectrum of 3 shows two singlets
at δ 4.03 and 4.46 ppm, attributed to exocyclic CH2 resonances.
In 13C NMR spectrum, the two singlets at δ 94.03 and 141.58
ppm can be assigned to the carbon atoms of the exocyclic CH2
and imine carbon in the ring. Compound 4 was characterized
by 1H and 13C NMR, IR, and mass spectroscopy and elemental
analysis. Consistent with the 1H NMR and 13C NMR
spectroscopic data of 3, those of 4 show two singlets (δ 4.14
The reactions of 2 with dioxygen16 and the oxyl radical
TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl)4e,17 were inves-
tigated. When a red solution of 2 in n-hexane was exposed to
Scheme 3. Reactivity of 2
C
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Figure 4. ORTEP representation of the X-ray structure of 7.
Hydrogen atoms have been omitted for clarity. Thermal ellipsoids
are drawn at the 30% probability level. Selected bond lengths (Å) and
angles (deg): B1−N1 = 1.4096(15), B1−N2 = 1.4105(16), B1−Cl1 =
1.7739(14), N1−C1 = 1.4256(14), N2−C2 = 1.4229(14), C1−C2 =
1.3515(16), C1−C15 = 1.4924(16), C2−C16 = 1.4904(16); N1−B1−
N2 = 107.35(10), N1−B1−Cl1 = 126.73(10), N2−B1−Cl1 =
125.93(9).
Figure 3. ORTEP representation of the X-ray structure of 3.
Hydrogen atoms have been omitted for clarity. Thermal ellipsoids
are drawn at the 30% probability level. Selected bond lengths (Å) and
angles (deg): Al1−N1 = 1.874(2), Al1−N2 = 1.966(2), Al1−C29 =
1.965(3), Al1−C31 = 1.956(3), N1−C14 = 1.381(3), N2−C15 =
1.295(3), C13−C14 = 1.335(4), C14−C15 = 1.485(3), C15−C16 =
1.489(4); N1−Al1−N2 = 83.64(9), C29−Al1−C31 = 112.43(12).
CONCLUSION
■
The paramagnetic aluminum diethyl compound 2 has been
prepared in a one-step reaction of the diamine ligand LH2 with
triethylaluminum. The variation of aluminum alkyls did not
yield similar paramagnetic species, indicating that the reaction is
sensitive to the steric factors of aluminum alkyls. The radical 2
could be converted to a diamagnetic species in the presence of
peroxyl and oxyl radicals via the abstraction of a hydrogen atom
from 2. The electron transfer from ligand to ligand in the
different coordination environments may also take place, as
indicated by the reaction of 2 with BCl3. The delocalized radical
ligand in 2 can also act as a spectator ligand in the reaction of 2
with iodine. The results demonstrated that the anionic radical
α-diimine ligand in 2 not only participates in electron transfer
reactions but also acts as a spectator ligand.
(CCHH), 4.49 (CCHH) ppm) in the proton NMR and
two singlets (δ 95.27 (CCH2), 142.95 (CCH2) ppm) in
the 13C NMR spectrum. These data indicated that the ligands
in 3 and 4 have the same coordination mode.
Reaction of 2 with iodine yielded the aluminum diiodide
(L•)AlI2 (5). It has been reported that reactions of aluminum
dimethyl and diethyl complexes with iodine provided an
efficient and facile route to the corresponding aluminum
diiodides.19 The mechanism for this reaction has not been well
established. It is quite possible that the alkyl anion arising from
the highly polarized Al−C bond initially attacks iodine to form
an alkyl iodide followed by the combination of the aluminum
cation with the iodide ion. In the reaction of 2 with iodine, the
delocalized radical is not involved in the reaction and only acts
as a spectator ligand. The paramagnetic species 5 was
characterized by EPR (Figure 1S in the Supporting
Information), IR, and mass spectroscopy and elemental
analysis. The similar paramagnetic diiodide [ArNC(H)C(H)-
NAr•]AlI2 was prepared by the reaction of the corresponding
α-diimine with AlI3 in the presence of aluminum powder in
toluene.3g Reaction of 2 with 1 equiv of BCl3 in n-hexane
yielded the neutral α-diimine-stabilized cationic aluminum
dichloride [(L0)AlCl2]+[AlCl4]− (6; L0 = ArNC(Me)(Me)-
CNAr) and the diazaborole LBCl (7). Compounds 6 and 7
were characterized by 1H and 13C NMR, IR, and mass
spectroscopy and elemental analysis. The structure of 7 was
determined by an X-ray single-crystal analysis (Figure 4).
Compound 7 features the ene-diamide ligand, as indicated by
the planar and delocalized BN2C2 five-membered ring and the
three-coordinate boron atom with a planar geometry. These
structural features are similar to those reported for the
diazaboroles [ArNC(R)C(R)NAr]BCl (R = H, Cl, Ph).3e,20
The formation of 6 and 7 apparently resulted from the electron
transfer between the ligand in the different molecules. The
mechanism for the reaction has yet to be investigated.
However, it is possible that transmetalation of the radical
anionic ligand from aluminum to boron may take place during
the reaction. The driving force for the electron transfer reaction
can be attributed to the high stability of the aromatic
diazaborole ring.
EXPERIMENTAL SECTION
■
All manipulations involving air- and moisture-sensitive compounds
were carried out under an atmosphere of dry argon by using modified
Schlenk-line and glovebox techniques. Elemental analyses were carried
out using an Elementar Vario EL analyzer. The 1H, 11B, and 13C NMR
spectroscopic data were recorded on a Bruker AV400 spectrometer.
Infrared spectra were recorded on a Bio-Rad FTS 6000 spectropho-
tometer. An EPR spectrum was recorded on a Bruker EMX-6/1
spectrometer. Computer simulation was performed using the
WINEPR SimFoniA Bruker software.21 The solvents (THF, toluene,
n-hexane, and diethyl ether) were freshly distilled from sodium and
degassed prior to use. The ene-diamine14 and TEMPOH22 were
synthesized according to the published procedures. Other chemicals
were of analytical grade and were used as received.
Synthesis of (LH)AlMe2 (1). A solution of AlMe3 (5 mL, 1 M in
toluene) in n-hexane (10 mL) was added to a solution of LH2 (2.03 g,
5 mmol) in n-hexane (40 mL). Then the mixture was refluxed for 6 h.
The resulting orange solution was concentrated to 5 mL. Storage at
−35 °C overnight afforded orange crystals of 1 (1.55 g, 67%). Mp:
210−212 °C. Anal. Calcd for C30H47AlN2 (462.69): C, 77.88; H,
1
10.24; N, 6.05. Found: C, 77.35; H, 10.73; N, 5.99. H NMR (C6D6,
400 MHz, 25 °C): δ −0.51 (s, 3H, AlMe), −0.29 (s, 3H, AlMe), 1.02
(d, J = 6.5 Hz, 3H, CHMe2), 1.09 (d, J = 6.7 Hz, 3H, CHMe2), 1.17 (d,
J = 6.5 Hz, 3H, CHMe2), 1.27 (d, J = 6.8 Hz, 3H, CHMe2), 1.31 (d, J =
6.8 Hz, 3H, CHMe2), 1.37 (d, J = 6.7 Hz, 3H, CHMe2), 1.41 (s, 6H,
CMe), 1.42 (d, J = 6.1 Hz, 6H, CHMe2), 2.83 (sept, J = 6.7 Hz, 1H,
CHMe2), 3.61 (sept, J = 6.6 Hz, 1H, CHMe2), 3.69 (sept, J = 6.8 Hz,
1H, CHMe2), 3.93 (sept, J = 6.8 Hz, 1H, CHMe2), 6.01 (s, 1H, NH),
6.91−7.23 (m, 6H, Ar-H). 13C NMR (C6D6, 100 MHz, 25 °C): δ
−9.65 (AlMe), −6.09 (AlMe), 14.56 (CMe), 23.59, 24.95, 25.57, 26.47,
27.73 (CHMe2), 28.37, 28.85 (CHMe2), 101.53 (CN), 124.03, 124.29,
D
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125.87, 126.19, 126.85, 135.40, 140.62, 141.38, 142.87, 147.74, 148.55
(Ar-C). EI-MS: m/z (%) 462.4 (25).
spectroscopy. The corresponding 1H NMR was consistent with that of
4.
Synthesis of (L•)AlI2 (5). A solution of I2 (508 mg, 2 mmol) in
toluene (10 mL) was added to a solution of 2 (490 mg, 1 mmol) in
toluene (20 mL) at 0 °C. The mixture was warmed to room
temperature and stirred continuously for 8 h. The red solution was
concentrated to 3 mL. Storage at −35 °C overnight afforded orange-
red crystals of 5 (0.58 g, 84%). Mp: 265−268 °C. Anal. Calcd for
C28H40AlI2N2 (685.42): C, 48.92; H, 6.16; N, 4.08. Found: C, 48.97;
Synthesis of (L•)AlEt2 (2). A solution of AlEt3 (1.14 g, 10 mmol)
in n-hexane (10 mL) was added to a solution of LH2 (2.03 g, 5 mmol)
in n-hexane (40 mL). Then the mixture was refluxed for 6 h. After
filtration the red solution was concentrated to 5 mL. Storage at −35
°C overnight afforded red crystals of 2 (2.01 g, 82%). Mp: 124−126
°C. Anal. Calcd for C32H50AlN2 (489.72): C, 78.48; H, 10.29; N, 5.72.
Found: C, 77.91; H, 10.30; N, 5.65. EPR (25 °C, toluene): g = 2.0029,
H, 6.57; N, 4.32. IR (cm−1): ν
̃
422 (w), 684 (m), 729 (m), 761 (m),
804 (m), 840 (m), 900 (w), 940 (m), 972 (m), 1101 (m), 1179 (w),
1259 (m), 1302 (w), 1327 (w), 1364 (m), 1487 (m), 1499 (w), 1640
(m), 2869 (s), 2963 (s). EI-MS: m/z (%) 685.3 (27).
AAl = 1.17 mT, AN = 0.94 mT (2N), AH = 0.25 mT (6H). IR (cm−1): ν
̃
685 (m), 762 (m), 766 (m), 935 (m), 1024 (m), 1100 (m), 1117 (m),
1182 (m), 1216 (w), 1259 (m), 1324 (m), 1362 (m), 1382 (m), 1430
(m), 1463 (m), 1588 (m), 1641 (m), 1705 (w), 2722 (w), 2868 (m),
2961 (s), 3062 (m). EI-MS: m/z (%) 489.1 (16).
Synthesis of [(L0)AlCl2]+[AlCl4]− (6) and LBCl (7). BCl3 (1 mL, 1
M in n-hexane) was added to 2 (490 mg, 1 mmol) in ether at −78 °C.
The mixture was warmed to room temperature and stirred
continuously for 8 h. All volatile materials were removed under
vacuum. The yellow residue was extracted with 10 mL of n-hexane to
give a colorless filtrate and a yellow solid. The colorless filtrate was
concentrated and stored at −35 °C overnight to afford colorless
crystals of 7 (108 mg, 24% based on BCl3 consumed). The yellow
solid was extracted with 10 mL of diethyl ether, and the yellow filtrate
was concentrated. Storage at −35 °C overnight afforded orange
crystals of 6 (0.17 g, 47% based on Al). Data for 6 are as follows. Mp:
143−145 °C. Anal. Calcd for C31H48Al2Cl6N2 (715.41): C, 52.04; H,
6.76; N, 3.92. Found: C, 52.36; H, 7.19; N, 4.06. 1H NMR (C6D6, 400
MHz, 25 °C): δ 1.05 (d, J = 6.7 Hz, 12H, CHMe2), 1.20 (d, J = 6.7 Hz,
12H, CHMe2), 2.39 (s, 6H, CMe), 3.25 (sept, J = 7.0 Hz, 4H,
CHMe2), 7.00−7.10 (m, 6H, Ar-H). 13C NMR (C6D6, 100 MHz, 25
°C): δ 15.53 (CMe), 22.88, 23.63 (CHMe2), 29.21 (CHMe2), 124.61,
Synthesis of (L*)AlEt2 (3). Dry O2 gas (22.4 mL, 1 mmol) was
injected by syringe into a solution of 2 (980 mg, 2 mmol) in n-hexane
(20 mL) at −78 °C. The mixture was stirred and warmed to room
temperature. After filtration the yellow solution was concentrated to 3
mL. Storage at −35 °C overnight afforded yellow crystals of 3 (0. 33 g,
34%). Mp: 122−124 °C. Anal. Calcd for C32H49AlN2 (488. 71): C,
78.64; H, 10.11; N, 5.73. Found: C, 78.37; H, 10.19; N, 5.41. 1H NMR
(C6D6, 400 MHz, 25 °C): δ 0.31 (q, J = 9.1 Hz, 4H, AlCH2), 0.87 (d, J
= 6.7 Hz, 6H, CHMe2), 1.16 (t, J = 9.1 Hz, 6H, AlCH2Me), 1.28 (d, J =
6.8 Hz, 6H, CHMe2), 1.32 (d, J = 6.8 Hz, 6H, CHMe2), 1.43 (d, J = 6.8
Hz, 6H, CHMe2), 1.64 (s, 3H, CMe), 2.98 (sept, J = 6.7 Hz, 2H,
CHMe2), 3.60 (sept, J = 6.9 Hz, 2H, CHMe2), 4.03 (s, 1H, CCHH),
4.46 (s, 1H, CCHH), 7.02−7.27 (m, 6H, Ar-H). 13C NMR (C6D6, 100
MHz, 25 °C): δ 0.24 (AlCH2), 9.55 (AlCH2Me), 17.55 (CMe), 23.89,
24.73, 24.89, 26.17 (CHMe2), 28.37, 28.56 (CHMe2), 94.03 (CCH2),
124.43, 124.74, 126.09, 138.71, 141.44, 146.79, 153.12 (Ar-C), 141.58
129.80, 135.77, 139.69 (Ar-C), 171.63 (CN). IR (cm−1): ν
̃
403 (w),
(CCH2), 181.24 (CN). IR (cm−1): ν
̃
442 (m), 561 (m), 566 (m),
528 (w), 683 (m), 763 (m), 802 (m), 936 (w), 1021 (m), 1097 (m),
1181 (w), 1260 (m), 1325 (w), 1363 (m), 1414 (m), 1434 (m), 1464
(m), 1507 (m), 1541 (m), 1590 (m), 1639 (m), 1726 (w), 2871 (m),
2929 (s), 2961 (s). EI-MS: m/z (%) 501.2 (10) [6 − [AlCl4]−]. Data
for 7 are as follows. Mp: 121−123 °C. Anal. Calcd for C28H40BClN2
(450.88): C, 74.58; H, 8.94; N, 6.21. Found: C, 73.97; H, 7.57; N,
623 (m), 642 (s), 744 (m), 795 (s), 978 (m), 1026 (m), 1105 (m),
1183 (m), 1217 (m), 1255 (m), 1318 (m), 1349 (m), 1438 (m), 1464
(m), 1564 (m), 1580 (s), 2864 (s), 2968 (s). EI-MS: m/z (%) 488.2
(31).
Synthesis of (L*)Al(TEMPO)(Et) (4). A solution of TEMPO (156
mg, 1 mmol) in toluene (10 mL) was added to 2 (490 mg, 1 mmol) in
toluene at −78 °C. The mixture was warmed to room temperature and
stirred continuously for 8 h. All volatile materials were removed under
vacuum. The yellow residue was extracted with 10 mL of n-hexane,
and the yellow filtrate was concentrated. Storage at −35 °C overnight
afforded yellow crystals of 4 (0.41 g, 67%). Mp: 198−200 °C. Anal.
Calcd for C39H62AlN3O (615.91): C, 76.05; H, 10.15; N, 6.82. Found:
C, 75.89; H, 10.58; N, 5.98. 1H NMR (C6D6, 400 MHz, 25 °C): δ 0.54
(q, J = 7.9 Hz, 2H, AlCH2), 0.65 (s, 3H, TEMPO-Me), 078 (s, 3H,
TEMPO-Me), 0.94 (d, J = 6.4 Hz, 3H, CHMe2), 0.96 (d, J = 6.4 Hz,
3H, CHMe2), 1.02 (s, 6H, TEMPO-Me), 1.29 (d, J = 7.0 Hz, 3H,
CHMe2), 1.30 (m, 4H, TEMPO-CH2), 1.32 (d, J = 7.0 Hz, 3H,
CHMe2), 1.40 (d, J = 6.8 Hz, 3H, CHMe2), 1.43 (m, 2H, TEMPO-
CH2), 1.45 (d, J = 6.9 Hz, 3H, CHMe2), 1.54 (t, J = 7.9 Hz, 3H,
AlCH2Me), 1.55 (d, J = 6.8 Hz, 6H, CHMe2), 1.62 (d, J = 6.9 Hz, 3H,
CHMe2), 1.66 (s, 3H, CMe), 2.93 (sept, J = 6.7 Hz, 1H, CHMe2), 3.33
(sept, J = 6.7 Hz, 1H, CHMe2), 3.49 (sept, J = 6.9 Hz, 1H, CHMe2),
3.80 (sept, J = 6.9 Hz, 1H, CHMe2), 4.14 (s, 1H, CCHH), 4.49 (s, 1H,
CCHH), 7.01−7.36 (m, 6H, Ar-H). 13C NMR (C6D6, 100 MHz, 25
°C): δ 2.34 (AlCH2), 10.15 (AlCH2Me), 17.76, 17.83 (TEMPO-Me),
19.59 (CMe), 23.89 (TEMPO-CH2), 24.39, 24.40 (TEMPO-Me),
24.53, 24.89, 24.95, 26.23, 26.42, 28.07, 28.15, 28.76 (CHMe2), 29.07,
30.23, 32.62, 32.36 (CHMe2), 40.58, 40.48 (TEMPO-CH2), 58.97,
58.77 (TEMPO-CMe2), 95.27 (CCH2), 123,86, 124.49, 124.69,
124.75, 126.09, 139.93, 141.27, 141.90, 146.55, 147.05, 152.46 (Ar-
1
5.32. H NMR (C6D6, 400 MHz, 25 °C): δ 1.19 (d, J = 6.9 Hz, 12H,
CHMe2), 1.31 (d, J = 6.8 Hz, 12H, CHMe2), 1.65 (s, 6H, CMe), 3.11
(sept, J = 6.8 Hz, 4H, CHMe2), 7.17−7.23 (m, 6H, Ar-H). 13C NMR
(C6D6, 100 MHz, 25 °C): δ 10.76 (CMe), 23.51, 24.78 (CHMe2),
28.84 (CHMe2), 119.89 (CN), 123.80, 135.43, 147.06 (Ar-C). 11B
NMR (C6D6, 128 MHz, 25 °C): δ 21.08. IR (cm−1): ν
̃
444 (m), 517
(w), 531 (w), 588 (w), 620 (w), 646 (w), 679 (w), 741 (m), 757 (w),
786 (w), 817 (w), 938 (m), 1057 (w), 1106 (w), 1178 (w), 1255 (w),
1281 (s), 1316 (s), 1387 (s), 1468 (s), 1587 (w), 1927 (w), 2867 (m),
2960 (s), 3069 (w). EI-MS: m/z (%) 450.1 (56).
X-ray Structural Determination. All intensity data were collected
with a Bruker SMART CCD diffractometer, using graphite-
monochromated Mo Kα radiation (λ = 0.710 73 Å). The structures
were resolved by direct methods and refined by full-matrix least
squares on F2.23 Hydrogen atoms were considered in calculated
positions. All non-hydrogen atoms were refined anisotropically. The
ORTEP-3 program was utilized to draw the molecules.24 Crystal data
and data collection details are collected in Table 1S in the Supporting
Information. Crystals of 2, 3, and 7 suitable for X-ray diffraction were
obtained from n-hexane at −35 °C.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files and a table giving crystal data and data collection
details for compounds 2, 3, and 7 and a figure giving the
experimental EPR spectrum of 5. This material is available free
of charge via the Internet at http://pubs.acs.org.
C), 142.95 (CCH2), 181.19 (CN). IR (cm−1): ν
̃
441 (m), 495 (w),
745 (m), 797 (s), 869 (m), 1027 (m), 1098 (m), 1182 (w), 1214 (m),
1259 (m), 1318 (m), 1352 (m), 1437 (m), 1461 (m), 1563 (m), 1606
(m), 2865 (m), 2964 (s), 3058 (w). EI-MS: m/z (%) 615.4 (29).
Conversion of 3 to 4. In an NMR tube, a mixture of 3 (24 mg,
0.05 mol) and TEMPOH (8 mg, 0.05 mol) was added to C6D6 (0.6
mL), and then the NMR tube was shaken carefully. After the NMR
tube stood at room temperature for 2 h, it was identified by NMR
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: cmcui@nankai.edu.cn.
E
dx.doi.org/10.1021/om300920y | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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̈
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China and 973 program (Grant No.
2012CB821600).
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dx.doi.org/10.1021/om300920y | Organometallics XXXX, XXX, XXX−XXX