Preparation of aluminum(III) (bis(amido)pyridine)(thiolate) complexes: Unexpected transmetalation mediated by LiAlH4

Article abstract of DOI:10.1016/j.ica.2011.09.056

Treatment of an unsymmetrical bis(imino)pyridyl-thiolate zinc(II) complex [Zn^II(LN₃S)(OTf)] (1) with LiAlH₄ results in the double reduction of the two imino groups in the ligand backbone, and at the same time causes a rare

Full text of DOI:10.1016/j.ica.2011.09.056

Inorganica Chimica Acta 382 (2012) 19–26
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Preparation of aluminum(III) (bis(amido)pyridine)(thiolate) complexes: Unexpected
transmetalation mediated by LiAlH4
⇑
Yosra M. Badiei, Yunbo Jiang, Leland R. Widger, Maxime A. Siegler, David P. Goldberg
Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA
a r t i c l e i n f o
a b s t r a c t
Treatment of an unsymmetrical bis(imino)pyridyl-thiolate zinc(II) complex [ZnII(LN3S)(OTf)] (1) with
LiAlH4 results in the double reduction of the two imino groups in the ligand backbone, and at the same
time causes a rare transmetalation reaction to occur. The products formed in this reaction are two novel
aluminium(III) bis(amido)pyridyl-thiolate complexes [(R,S/S,R-[AlIII(LH2N3S)(THF)] (2a) and [(R,R/S,S-
[AlIII(LH2N3S)(THF)] (2b), which are diastereomers of each other. These complexes have been character-
ized by single-crystal X-ray diffraction and 1H NMR spectroscopy. Single crystal X-ray structure analysis
shows that the AlIII ion is bound in an almost idealized square pyramidal geometry in 2a, while being held
in a more distorted square pyramidal geometry in 2b. The major difference between 2a and 2b arises in
the orientation of the terminal methyl groups of the ligand backbone in relation to the AlIIIN3S plane.
These two complexes are crystallized at different temperatures (room temperature versus ꢀ35 °C), allow-
ing for their separate isolation. Structural analysis shows that these complexes are reduced by the formal
addition of one hydride ion to each imino group, resulting in a deprotonated bis(amido)pyridyl-thiolate
ligand. A detailed analysis of metrical parameters rules out the possibility of pure one- or two-electron
Article history:
Received 8 July 2011
Received in revised form 23 September 2011
Accepted 26 September 2011
Available online 6 October 2011
Keywords:
Aluminum
Bis(imino)pyridine
Transmetalation
Zinc
Ligand reduction
Ligand design
reduction of the
p
-conjugated bis(imino)pyridine framework. 1H NMR spectra reveal a rich pattern in
solution indicating that the solution state structures for 2a and 2b match those observed in the solid-
state crystal structures, and reveal that both complexes are severely conformationally restricted. Direct
organic synthetic methods failed to produce the reduced bis(amino)pyridyl-thiol ligand in pure form,
but during the course of these efforts an unusual unsymmetrical aminopyridyl ketone,1-(6-(1-(2,6-diiso-
propylphenylamino)ethyl)pyridin-2-yl)ethanone was synthesized in good yield and can be used as a pos-
sible precursor for further ligand development.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
the latter goal, we are interested in developing mixed N,S donor
ligands for preparing models of non-heme iron enzymes such as
The tridentate 2,6-bis(imino)pyridine (2,6-BIP) platform has
played an important role in a broad range of reactions, including
olefin polymerizations, Fe- and Co-centered dinitrogen reduction,
and alkane oxidation catalysis [1,2]. It also exhibits rich redox
behavior, providing a site for storing reducing equivalents in con-
junction with redox-active metals (e.g., Fe and Ni) [3–10]. Given
the usefulness and versatility of this ligand, significant synthetic
efforts have gone into modifying the steric and electronic proper-
ties of the BIP-type framework, either by substitution of the typical
aryl imino substituents, or other methods [11–17]. A particular
challenge arises when confronted with preparing desymmetrized
analogs of the parent BIP ligand 2,6-(ArN@CMe)2(C5H3N) in which
the Ar substituents are different. These unsymmetrical derivatives
are desirable for applications from catalysis [18–20] to synthetic
bioinorganic analogs of enzyme active sites [21]. With regard to
superoxide reductase and cysteine dioxygenase (CDO) [22–27].
Ligand design to target these centers is non-trivial because of the
difficulties posed in making unsymmetrical N,S-systems. We re-
cently employed the BIP platform to successfully prepare the
unsymmetrical complex [FeII(LN3S)(OTf)] via a metal-assisted tem-
plate reaction, which served as a structural and functional model of
the His3Cys-FeII center found in substrate-bound CDO [26].
To modify, and possibly enhance, the reactivity of our nonheme
iron models with O2, we sought a more electron-rich version of the
LN3S ligand, and therefore targeted the synthesis of the reduced,
bis(amino)pyridine analog. Although bis(amino)pyridine systems
have been sought for their use in catalysis, relatively few such
compounds have been prepared [14,28,29]. For example, the syn-
thesis of 2,6-bis(2,6-diisopropylphenylaminomethyl)pyridine was
accomplished by addition of LiNHAr (Ar @ 2,6-di-isopropylphenyl)
to 2,6-bis(bromomethyl)pyridine [28]. However, this double alkyl-
ation strategy is not practical for the synthesis of unsymmetrical
derivatives.
⇑
Corresponding author.
E-mail address: dpg@jhu.edu (D.P. Goldberg).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.09.056

20
Y.M. Badiei et al. / Inorganica Chimica Acta 382 (2012) 19–26
Herein we report the successful reduction of the bis(imino)
respond to this isomer (2b), indicating it is not present in any sig-
nificant quantity.13C NMR (CD2Cl2) d 163.0, 162.5, 149.5, 148.5,
148.4, 140.4, 130.5, 129.4, 124.4, 123.9, 122.9, 122.8, 119.4,
119.1, 113.4, 107.3, 65.8, 28.1, 27.4, 25.8, 25.5, 25.1, 24.0, 21.1,
20.3, 15.2. LDIMS(+): m/z 458 [2a – THF + H]+. Anal. Calc. for
functionalities in the unsymmetrical LN3S ligand, starting from
the zinc(II) complex [Zn(LN3S)(OTf)] (1). During this work a rare
transmetalation reaction was discovered, leading to the isolation
and structural characterization of the neutral AlIII complexes
[Al(LH2N3S)(THF)] (2a–b). This facile transmetalation process has
potential implications for the role of Al co-catalysts in olefin poly-
merizations with BIP ligands.
C33H45AlN3O1.5S (2a ꢁ 1/2(C4H10O)), C, 69.93; H, 8.00; N, 7.41.
Found C, 69.38; H, 7.45; N, 8.79%.
2.4. Synthesis of R,R/S,S-[AlIII(LH2N3S)(THF)] (2b)
2. Experimental
The mother liquor from crystallizations of 2a was separated and
layered with pentane (Et2O to pentane ratio 8:1), and left to stand
at ꢀ35 °C for 2 d, affording X-ray quality crystals of diastereoiso-
mer 2b. 1H NMR (CD2Cl2) d 7.51 (t, 1H, J = 7.2 Hz, Py-Hp), 7.39 (d,
1H, J = 7.6 Hz, Py-Hm), 7.16 (d, 1H, J = 7.6 Hz, Py-Hm), 7.09 (t, 1H,
J = 6.4 Hz Ar–H), 7.04–6.96 (m, 3H, Ar–H), 6.88 (d, 1H, J = 8.0 Hz,
Ar–H), 6.58 (t, 2H, J = 7.6 Hz, Ar–H), 4.72 (br, 1H, CHPy), 4.20 (br,
1H, CHPy), 3.92 (br, 2H, CH2 (THF), 3.68 (br, 2H, CH2 (THF)), 3.23
(m, 2H, J = 6.8 Hz, CHMe2), 1.82 (br, 4H, CH2(THF)), 1.61 (d, 3H,
J = 6.4 Hz, Me-CHPy), 1.46 (d, 3H, J = 6.4 Hz, Me-CHPy), 1.22 (d,
6H, J = 6.8 Hz, CHMe2), 1.02 (d, 6H, J = 6.8 Hz, CHMe2).
2.1. General procedures
All reactions were carried out under an atmosphere of N2 or Ar
using a drybox or standard Schlenk techniques unless noted other-
wise. All reagents were purchased from commercial vendors and
used without further purification unless noted otherwise. Diethyl
ether and dichloromethane were purified via a Pure-Solv Solvent
Purification System from Innovative Technology, Inc. All solvents
were degassed by repeated cycles of freeze-pump-thaw and then
stored in an N2-filled drybox. Methanol was distilled over CaH2.
The pyridyliminoketone, 2-(O@CMe)-6-(2,6-iPr2-C6H3N@CMe)–
C5H3N was synthesized according to the literature [18]. The zinc
complex ZnII(LN3S)(OTf) (1) was synthesized as described previ-
ously [26].
2.5. Synthesis of 1-(6-(1-(2,6-diisopropylphenylamino)ethyl)pyridin-
2-yl)ethanone (3)
Under ambient conditions, 1-(6-(1-(2,6-diisopropylphenylimi-
no)ethyl)pyridin-2-yl)ethanone (1.0 g, 3.10 mmol) was dissolved
2.2. Physical methods
in
a 1:1 mixture of CHCl3/MeOH before NaBH4 (117 mg,
3.10 mmol) was added in small portions. The reaction was moni-
tored by TLC, and after 2 h the reaction had reached completion.
The reaction mixture was concentrated in vacuo, and the crude
oil was re-dissolved in CHCl3, washed twice with water, dried
and concentrated. Purification by column chromatography on silica
(EtOAc/hexanes) gave 3 as pure yellow solid. Yield 561 mg (56%).
1H NMR (CDCl3) d 8.29 (d, 1H), 7.81 (t, 1H), 7.36 (d, 1H), 7.17 (d,
2H), 7.10 (t, 1H), 4.96 (m, 1H), 4.57 (d, 1H), 2.74 (m, 2H), 2.23 (s,
3H), 1.57 (d, 3H), 1.15 (d, 12H); 13C NMR (CDCl3) d 166.7, 162.1,
154.9, 146.6, 137.7, 136.0, 123.9, 123.3, 121.2, 120.0, 68.8, 28.6,
24.5, 23.5, 23.2, 17.6.
NMR was performed on a BrukerAvance 400 MHz FT-NMR spec-
trometer at 25 °C. Elemental analysis was performed by Atlantic-
Microlab Inc., Norcross, GA. LDI-ToF mass spectra were obtained
using a Bruker Autoflex III Maldi ToF/ToF instrument (Billerica,
MA). Samples were dissolved in Et2O or CH2Cl2 and deposited on
the target plate in the absence of any added matrix. Samples were
irradiated with a 355 nm UV laser and mass-analyzed by ToF mass
spectrometry in the reflectron mode.
2.3. Synthesis of R,S/S,R-[AlIII(LH2N3S)(THF)] ꢁ 1/2Et2O (2a ꢁ 1/2Et2O)
To a chilled slurry of Zn(LN3S)OTf] (1) (430 mg, 0.670 mmol) in
15 mL THF at ꢀ35 °C (the solution was pre-cooled in the drybox
freezer (ꢀ35 °C) for 10 min) was added 0.60 mL of LiAlH4 solution
(1 M) dropwise. Bubbling was observed and the solution turned
from red to a green-yellow color with the formation of a yellow
precipitate as the reaction warmed to room temperature. The solu-
tion was stirred for 2 h at r.t., and then the THF was concentrated in
vacuo to ca. 1 mL. Diethyl ether (10 mL) was added, and the solu-
tion was filtered on a fine frit. The filtrate was concentrated in va-
cuo, and slow evaporation over 1 d yielded 2a as yellow crystals
(30%). 1H NMR (CD2Cl2) d 8.02 (t, 1H, J = 7.7 Hz, Py-Hp), 7.53 (d,
1H, J = 7.7 Hz, Py-Hm), 7.45 (d, 1H, J = 7.7 Hz, Py-Hm), 7.14-7.05
(m, 4H, Ar–H), 6.83 (t, 1H, J = 7.6 Hz, Ar–H), 6.43 (d, 1H,
J = 7.3 Hz, Ar–H), 6.30 (t, 1H, J = 7.3 Hz, Ar–H), 4.82 (q, 1H,
J = 6.4 Hz, CHPy), 4.30 (q, 1H, J = 6.8 Hz, CHPy), 4.00 (br, 2H, CH2
(THF), 3.77 (m, 3H, CH2(THF), one set of CHMe2 resonances is bur-
ied under this THF peak), 2.82 (m, 1H, J = 6.8 Hz, CHMe2), 1.89 (br, 4
H, CH2(THF)), 1.55 (d, 3H, J = 6.4 Hz, Me-CHPy), 1.40 (d, 3H,
J = 6.8 Hz, Me-CHPy), 1.26 (d, 3H, J = 6.8 CHMe2), 1.25 (d, 3H,
J = 6.8 CHMe2), 1.09 (d, 3H, J = 6.8 CHMe2), 1.02 (d, 3H, J = 6.8
CHMe2). Diethylether (0.5 equiv.) is found in the 1H NMR spectrum
(d 3.45 and 1.15) and is in agreement with the Et2O found in the
crystal lattice by X-ray structural analysis. The crystal structure
of 2a also suggests that there should be minor contamination from
the other racemic pair (S,S/R,R) of diastereomers. However, NMR
analysis of bulk crystalline material does not reveal peaks that cor-
2.6. X-ray structure determinations
All reflection intensities were measured at 110(2) K using a
KM4/Xcalibur (detector: Sapphire3) with enhanced graphite-
monochromated Mo K
a radiation (k = 0.71073 Å) under the pro-
gram CrysAlisPro (Version 1.171.33.55, Oxford Diffraction Ltd.,
2010). The program CrysAlisPro (Version 1.171.33.55, Oxford Dif-
fraction Ltd., 2010) was used to refine the cell dimensions. Data
reduction was done using the program CRYSALISPRO (Version
1.171.33.55, Oxford Diffraction Ltd., 2010). The structure was
solved with the program SHELXS-97 (Sheldrick, 2008) and was re-
fined on F2 with SHELXL-97 (Sheldrick, 2008) [30]. Analytical nu-
meric absorption corrections based on a multifaceted crystal
model were applied using CRYSALISPRO (Version 1.171.33.55, Oxford
Diffraction Ltd., 2010). The temperature of the data collection
was controlled using the system Cryojet (manufactured by Oxford
Instruments). The H-atoms were placed at calculated positions (ex-
cept for that located on C14) using the instructions AFIX 13, AFIX
23, AFIX 43 or AFIX 137 with isotropic displacement parameters
having values 1.2 or 1.5 times Ueq of the attached C atoms. The po-
sition and the isotropic temperature factor of the H atom located
on C14 (i.e., H14) were refined freely. The distance C14ꢀH14 was
restrained to be 1.00(1) Å. Relevant crystallographic data and
structure refinement parameters for these complexes are summa-
rized in Table 1.

Y.M. Badiei et al. / Inorganica Chimica Acta 382 (2012) 19–26
21
Table 1
was found in one large void volume located approximately at (½, 0,
1), which includes 65 electrons in a volume of 346 Å3. The void
likely contains unresolved solvent molecules. The contribution of
the unresolved residual electron density was then taken out for
the subsequent stages of the refinement using the program SQUEEZE
[31].
Crystallographic data for complexes 2a–b.
(2a)
(2b)
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
2(C31H40AlN3OS)ꢁC4H10
O
C31H40AlN3OS
529.70
1133.52
triclinic
triclinic
ꢀ
ꢀ
P1 (#2)
P1 (#2)
8.8009 (2)
13.7366 (4)
13.9119 (5)
111.716 (3)
93.621 (2)
91.141 (2)
1557.69 (8)
1
12.54209 (18)
15.8805 (2)
17.5832 (3)
68.5229 (14)
89.6375 (12)
73.0413 (13)
3097.32 (8)
4
b (Å)
c (Å)
3. Results and discussion
a
(°)
b (°)
3.1. Synthesis
c
(°)
V (Å3)
The key desymmetrization for the BIP platform occurs with the
synthesis of the pyridyliminoketone, 2-(O@CMe)-6-(2,6-iPr2-
C6H3N@CMe)-C5H3N (Scheme 1), previously reported by Bianchini
and coworkers [18]. This precursor bears a single 2,6-diisopropyl-
phenyl substituent, which provides steric protection of the subse-
quently derived thiolate-ligated complexes. An obvious strategy to
obtain the bis(amino) derivative involves reductive amination of
the imino-ketone with 2-aminothiophenol, but this reaction failed
to give the desired product under a variety of conditions. Simple
condensation to give the mixed imine was also problematic; the
standard methods for Schiff base formation proved unsuccessful,
giving only mixtures of unidentified products or recovered starting
materials (Scheme 1). A third route involved initial reduction of the
pyridyliminoketone to the pyridylaminoketone 2-(O@CMe)-6-(2,6-
iPr2-C6H3NHCHMe)-C5H3N 3, which proceeded smoothly with
NaBH4 (Scheme 2). However, attempts to form the imine with 2-
aminothiophenol and 3 also either failed to react, or gave only un-
wieldy mixtures of products. Compound 3 also proved to be an
ineffective substrate for the metal-assisted template reaction used
in the synthesis of 1. It is likely that the difficulties encountered in
condensation of 3 with 2-aminothiophenol is a result of the pres-
ence of the nucleophilic thiol group. The 2-aminothiophenol is
known to be problematic when condensing with ketones because
both the N and S groups may add to the carbonyl carbon atom to
give a thiazoline [32]. Thus compound 3 should be useful as a
key precursor to forming other types of unsymmetrical, polyden-
tate pyridine-based ligands.
We next investigated the reduction of the zinc complex (1) as
an alternative strategy for synthesizing a reduced bis(amino)
derivative (Scheme 3). Complex 1 was prepared via a zinc template
reaction, as previously described [26]. Treatment of 1 with LiAlH4
at ꢀ35 °C in THF for 1 h led to a color change from red to yellow-
green with the formation of a yellow precipitate. The solution
was warmed and stirred for an additional 2 h at room temperature.
After work-up and recrystallization from Et2O/THF, yellow crystals
(blocks) suitable for crystal structure determination were obtained.
The crystal structure revealed that a surprising transmetalation
reaction had occurred, and the ZnII ion was replaced by an AlIII
ion bound in the tetradentate N3S(thiolate) site, giving complex
2a. The molecular structure for 2a is shown in Fig. 1. A THF
Z
l
(mmꢀ1
)
0.16
0.16
Crystal size (mm)
Tmin., Tmax.
No. of measured,
independent and
observed [I > 2r(I)]
reflections
0.40 ꢂ 0.25 ꢂ 0.14
0.965, 0.985
15662, 5478, 3770
0.42 ꢂ 0.38 ꢂ 0.21
0.948, 0.974
54514, 12182,
9655
Rint
0.049
0.049, 0.136, 1.02
5478
402
0.056
0.042, 0.124, 1.08
12182
679
0
R[F2 > 2
r
(F2)], wR(F2), S
No. of reflections
No. of parameters
No. of restraints
H-atom treatment
43
H atoms treated by a
mixture of independent and parameters
constrained refinement
0.32, ꢀ0.36
H-atom
constrained
0.41, ꢀ0.47
D
qmax.
,
D
qmin. (eÅꢀ3
)
2.6.1. Complex 2a
The structure of the compound C31H40AlN3OSꢁ1/2(C4H10O) 2a
ꢀ
was solved and refined in the centrosymmetric space group P1with
Z0 = 1. The asymmetric unit contains one molecule of the Al com-
plex and one lattice diethyl ether solvent molecule (with an occu-
pancy factor of 0.5). The lattice solvent molecule is found
disordered over a center of inversion (½, ½, 0). The atom C8/C80
(methyl group located on C7) is disordered over two positions such
that the chirality on C7 is either R or S. The major component of the
disorder (C8) refines to an occupancy of 0.819(6). There is no evi-
dence that suggests disorder for atom C15 (methyl group located
on C14). Disorder on C15 may not occur because of steric effects
from the isopropyl group C22/C23/C24. The C7 and C14 atoms have
opposite chirality (i.e., C7RC14S or C7SC14R) approximately 82% of
the time, and have the same chirality (i.e., C7RC14R or C7SC14S)
approximately the rest of the time.
2.6.2. Complex 2b
The structure of the compound C31H40AlN3OS was solved and
0
ꢀ
refined in the centrosymmetric space group P1 with Z = 2. The
asymmetric unit contains two crystallographically independent
molecules of the Al complex. The diastereoisomers that are found
in the crystal lattice are S,S and R,R. Some residual electron density
Scheme 1. Attempted non-metal-template syntheses.

22
Y.M. Badiei et al. / Inorganica Chimica Acta 382 (2012) 19–26
Scheme 2. Synthesis of 3.
Scheme 3. Synthesis of the AlIII complexes 2a and 2b.
molecule occupies the fifth coordination site at the AlIII center,
while a non-coordinating molecule of Et2O is found in the lattice
in the ratio 2 (2a):1 Et2O. The absence of any counter ions in the
crystal lattice indicates that the ligand must carry a 3ꢀ charge to
balance the trivalent Al cation.
The trianionic charge of the tetradentate ligand may arise from
the reduction of the two imine bonds by addition of two hydride
ions, which would lead to the former imino N donors being con-
verted to two monoanionic amido donors. An alternate configura-
tion would result from the addition of two electrons to the
sp3 hybridization, as opposed to the angles between 113.7° and
115.7° found in the zinc(II) starting material 1, which contains
sp2-hybridized imino carbon atoms. In addition, the methyl groups
in 2a (C(8) and C(15)) do not lie in the same plane as the pyridine
and former Cimine–N groups, as would be expected for mono- or di-
radical ligand structures [8,33,34]. The Cimine–Nimine distances in 1
are 1.278(3) and 1.283(3) Å. These distances are significantly elon-
gated in 2a, C(7)–N(1) = 1.462(3) Å and C(14)–N(3) = 1.465(3) Å,
whereas the C(7/14)–Cipso bonds in 2a are nearly identical to those
in 1 (1.517(4) and 1.520(4) Å for 2a compared to 1.501(4) and
1.497(4) Å in 1). In contrast, pure 1eꢀ- or 2eꢀ-reduction of the
bis(imino)pyridine framework is expected to result in only a mod-
est elongation of the Cimine–Nimine bonds; density functional theory
(DFT) calculations for a hypothetical bis(imino)pyridine zinc(II)
complex reveal 0.03–0.05 Å elongation of the Cimine–Nimine bonds
upon 1eꢀ or 2eꢀ reduction [34]. Similarly, modest contractions of
the Cimine–Cipso single bonds (0.04–0.06 Å) are calculated for the re-
duced ZnII structures. The same trends are observed for the crystal-
lographically characterized 1eꢀ- and 2eꢀ-reduced Fe complexes
[Fe(iPrPDI)(Cl)], [Fe(iPrPDI)(N2)2] and [Fe(iPrPDI)(DMAP)] [34].
Finally, the H atom on C(14) was located in the difference map of
the crystal structure for 2a, supporting the assignment of C(14)
as an sp3-hybridized tertiary carbon center. All of the metrical
parameters for the bis(imino)pyridine backbone in 2a are consis-
tent with hydride addition to the imino bonds in 1, and show no
conjugated
reduction of the conjugated
p
system of the bis(imino)pyridine framework. The
framework in aryl-substituted
p
bis(imino)pyridine metal complexes by one or two electrons to
give monoanionic or dianionic (monoradical or diradical) ligands
has been observed for a number of metal complexes, including AlIII
bis(imino)pyridine complexes [8]. In general this ligand ‘‘non-
innocence’’ is of interest because of fundamental questions related
to electronic structure, as well as for the potential implications
regarding redox-based reactivity of the ligand-metal complex.
Both 1eꢀ- and 2eꢀ-reductions have occurred when [Fe(iPrPDI)(Cl)2]
(iPrPDI @ 2,6-(2,6-iPr2C6H3N@CMe)2C5H3N) was treated with the
hydride donor NaBEt3H, giving mono- or di-radical products
[33,34]. Thus the possibility of electron-transfer to the ligand in
1 following treatment with LiAlH4 was carefully considered (vide
infra).
evidence for 1eꢀ- or 2eꢀ-reduction in the conjugated
p system.
The former analysis strongly indicates that the N3S donor in 2a
is best described as a fully deprotonated, bis(amido)pyridylthiolate
ligand. The short Al–N distances of Al–N(1) = 1.887(2) Å and
Al–N(3) = 1.862(2) Å for 2a are in good agreement with other bis
(amido) donors. For example, a mixed pyridyl(imino)(amido)
3.2. Structural analysis
Relevant bond lengths and angles for 2a are summarized in
Table 2, together with data for related complexes. The angles about
C(7) and C(14) in 2a range between 105.9° and 115.8°, suggesting

Y.M. Badiei et al. / Inorganica Chimica Acta 382 (2012) 19–26
23
donor set toward the axially ligated THF molecule. In comparison,
the ZnII ion for 1 exhibits a more distorted square pyramidal geom-
etry with
plane. The analogous FeII complex [FeII(LN3S)(OTf)] is also distorted
square pyramidal ( = 0.12) with an even larger Fe-to-plane dis-
tance of 0.540(3) Å.
s = 0.09, and sits further away (0.471(4) Å) from the N3S
s
From the former structural data we conclude that complex 1
has been fully reduced by LiAlH4 to give two deprotonated, anionic
amido donors, and the ZnII ion has been replaced by AlIII via a novel
transmetalation reaction. The fate of the ZnII ion in this process is
not known, although we speculate that it is likely released into
solution as zinc triflate/hydride salts that may in part form insolu-
ble byproducts. A white precipitate forms following treatment of 1
with LiAlH4 and warming up of the reaction mixture, and this
material may contain some of the displaced ZnII ion as an insoluble
byproduct.
3.3. Mechanistic considerations
Fig. 1. Displacement ellipsoid plot (50% probability level) of diastereomer 2a. H
atoms and the lattice Et2O solvent molecule are omitted for clarity.
The facile reduction of imine bonds by strong hydride donors
such as LiAlH4 is well known. It is therefore reasonable to conclude
that the reaction of 1 with LiAlH4 occurs through hydride addition
to the imine bonds in 1 in a first step, which is then followed by
transmetalation of ZnII by AlIII. Evidence for this mechanism was
obtained from laser desorption ionization mass spectrometry
(LDI-MS). Analysis of the white solid that is produced during the
reaction was performed by LDI-MS, and revealed a prominent iso-
topic cluster centered at m/z = 496.14 (Fig. S1). The isotope pattern
matches that for [ZnIILH4N3S]+ (theor. m/z = 496.18), and can be as-
signed to the fully reduced ZnII complex, in which both imine
bonds have been reduced and the resulting N donors are proton-
ated (from isolation in air). A small peak is also observed at
m/z = 494.13, which can be assigned to the ZnII product in which
only one of the imine bonds has been reduced ([ZnII(LH2N3S)]+, the-
or. m/z = 494.16). These data indicate that the reduction of the
imine bonds of the zinc(II) starting material most likely occurs
prior to the transmetalation process.
Table 2
Selected bond lengths (Å) and angles (°) for 1, 2a, 2b and [FeII(LN3S)OTf].
2a
2ba
1b
FeII(LN3S)(OTf)
Ref.
this work
1.887(2)
1.967(2)
1.862(2)
2.2798(9)
1.9320(18)
1.462(3)
1.465(3)
79.93(9)
154.19(10)
87.18(7)
97.93(9)
81.33(9)
153.69(8)
100.03(8)
102.34(7)
102.69(8)
104.43(6)
105.9(2)
107.7(2)
this work
1.8763(13)
1.9590(13)
1.8560(13)
2.2796(6)
1.9485(11)
1.4559(19)
1.4714(19)
79.91(5)
146.83(6)
88.84(4)
100.96(5)
80.50(5)
[26]
[26]
Mc–N1
2.202(2)
2.049(2)
2.173(2)
2.2521(8)
2.0940(18)
1.283(3)
1.278(3)
74.54(9)
148.80(8)
85.33(6)
97.56(8)
75.51(9)
143.22(7)
99.75(8)
114.62(6)
95.88(8)
113.38(6)
113.7(2)
115.7(2)
2.188(2)
2.072(2)
2.184(2)
2.2942(8)
2.0870(18)
1.293(3)
1.284(3)
73.89(9)
146.47(8)
82.95(6)
98.91(8)
74.32(8)
139.13(6)
102.77(8)
115.57(6)
98.20(8)
113.92(6)
113.9(2)
115.9(2)
M–N2
M–N3
M–S1
M–O1
N1–C7
N3–C14
N1–M–N2
N1–M–N3
N1–M–S1
N1–M–O1
N2–M–N3
N2–M–S1
N2–M–O1
N3–M–S1
N3–M–O1
S1–M–O1
N1–C7–C9
N3–C14–C13
165.36(4)
92.17(5)
104.84(4)
106.28(6)
99.20(4)
To provide further evidence for the proposed mechanistic se-
quence, we examined the reaction of 1 with a different hydride
source, NaBH4, which does not contain the AlIII ion. Addition of
1.5 equiv. of NaBH4 to 1 in MeOH/EtOH at r.t. led to the formation
of a yellow solid as seen for LiAlH4. This solid was filtered and
isolated by aerobic work-up and analyzed by LDI-MS. A prominent
106.30(12)
106.79(12)
a
Data taken from only one of two distinct, but similar molecules in the asym-
metric unit.
b
Data taken from only one of three distinct, but similar molecules in the
asymmetric unit.
c
M equals Al1 in 2a and 2b, Zn1 in 1, Fe1 in the [FeII(LN3S)(OTf)].
aluminum(III) compound derived from treatment of the parent
2,6-bis(imino)pyridine ligand with AlMe3 reported by Bruce and
coworkers [35] exhibits a short, deprotonated Namido–Al dis-
tance = 1.876(4) Å. These AlIII–N bonds are significantly shorter
than the ZnII–N (2.202(2) and 2.173(2) Å) and FeII–N (2.188(2)
and 2.184(2) Å) bond lengths observed in 1 and [FeII(LN3S)(OTf)],
respectively [27], or the AlIII–N(H)RR0 bond lengths of 2.090(5)
and 2.098(5) Å seen in the complex [((R,R)-R4-salcyan)Al(OH)]
[36], where the amine donor is protonated. Furthermore, the
AlIII–S distance in 2a (2.2798(9) Å) falls in the reported range for
other AlIII–SR bond lengths (2.220(1)–2.5226(6) Å) [37–39].
The geometry of the AlIII ion is nearly an idealized square pyra-
mid (sp) based on its
s value, which is a quantitative angular
parameter that distinguishes between purely square pyramidal
and trigonal bipyramidal (tbp) geometries in 5-coordinate com-
Fig. 2. Displacement ellipsoid plot (50% probability level) of diastereomer 2b. H
atoms and the other crystallographically independent molecule are omitted for
clarity.
plexes. For 2a,
s = 0.01, where ((sp) 0 6 s 6 1 (tbp)) [40]. The Al
atom lies 0.402(3) Å above the basal plane defined by the N3S

24
Y.M. Badiei et al. / Inorganica Chimica Acta 382 (2012) 19–26
Fig. 3. 1H NMR spectrum of 2a.
isotopic cluster at m/z = 494.00 was observed, corresponding to the
singly reduced ZnII complex [ZnII(LH2N3S)]+ (Fig. S2). Although this
procedure is not useful for obtaining the fully reduced product, it
provides clear evidence that imine reduction readily occurs di-
rectly for the ZnII complex, and does not require AlIII. This result
supports our overall proposed mechanism in which reduction of
the imine bonds in 1 is relatively facile, and occurs prior to AlIII
transmetalation.
the non-planarity of C(7a) and C(14a). The Al–N distances of
Al–N(1) = 1.8763(13) and Al–N(3) = 1.8560(13) Å for 2b are in
good agreement with the deprotonated Namido–Al distance. It is
worth noting that the geometry of complex 2b is distorted with
s
values of 0.31 and 0.24 corresponding to the two independent
molecules in the unit cell. In addition, the Al ion is 0.338(2)
and 0.354(2) Å above the basal plane for the two independent
molecules. Thus complex 2b is the second diastereomeric pair
(R,R/S,S) produced in the reduction/transmetalation reaction, and
shows that there is no strong diastereomeric preference for this
reaction. The different orientation of the methyl groups does
not cause any significant changes in bond lengths, but subtle
changes are induced about the AlIII coordination sphere such that
the geometry is now significantly distorted from idealized square
pyramidal, as seen for 2a.
3.4. Stereochemistry
The structural data for the AlIII complex 2a are consistent with
one diastereomeric pair (R,S/S,R) being formed in the reduction
reaction. However, reduction of 1 with LiAlH4 is not expected to
proceed with diastereoselectivity, and we wondered whether
the other R,R/S,S diastereomeric pair was formed in this reaction.
Recrystallization of the mother liquor that was separated from 2a
by layering with pentane at ꢀ35 °C afforded a new batch of yel-
low crystalline blocks (Scheme 2). Structural analysis of these
new crystals revealed that they were not a second crop of 2a,
but a new complex 2b (Fig. 2). Relevant bond lengths and angles
are summarized in Table 2 for one of the two independent mole-
cules in the asymmetric unit (both molecules exhibit similar bond
distances and angles, and therefore only one set of bond distances
and angles are given as representative of the structure). As seen
in 2a, the tetradentate LH2N3S ligand forms the basal plane
around a transmetalated AlIII ion, with a molecule of THF occupy-
ing the axial position. The key difference seen for 2b as compared
to 2a lies in the orientation of the methyl groups attached to the
former imino carbon atoms (C(7a) and C(14a)). In 2b, these
methyl groups can be seen as oriented anti in relation to the
AlN3 plane, whereas for 2a these groups are positioned syn across
this plane. Reduction of both imine bonds of 2b is confirmed by
the former Cimine–Nimine and Cimine–Cipso distances, as well as
3.5. 1H NMR spectra
As seen in the solid-state crystal structure, reduction of the
imine bonds in 1 produces two chiral centers at the ketimine car-
bon atoms, and the structure of 2a consists of the racemate of the
S,R/R,S diastereomer1. The 1H NMR spectrum of 2a in CD2Cl2 is
shown in Fig. 3. The two new C–H resonances resulting from the
reduction of the imino carbon atoms are significantly separated
and can be found as quartets at d 4.82 (J = 6.4 Hz) and 4.30
(J = 6.8 Hz) ppm. The adjacent methyl groups are also distinct, and
shifted upfield as expected from 2.79 and 2.43 ppm in 1 to 1.55
(J = 6.4 Hz) and 1.40 (J = 6.8 Hz) ppm in 2a. The isopropyl methine
1
Some minor positional disorder of C(8) is observed for 2a, and modeling of this
disorder leads to an estimate of the population of the two diastereomers of 82% for
the S,R/R,S pair, and 18% for the S,S/R,R pair. However, 1H NMR data show no evidence
of the S,S/R,R isomer in the bulk solid, suggesting the population of the minor
component is much less than 18%.

Y.M. Badiei et al. / Inorganica Chimica Acta 382 (2012) 19–26
25
Fig. 4. 1H NMR spectrum of 2b.
resonances are even further separated by ꢃ1 ppm, appearing at 3.77
4. Concluding remarks
(overlapped with THF) and 2.82 ppm (septet). This latter ꢃ1 ppm
separation clearly indicates that rotation is highly restricted about
the C(ipso)–N bond of the diisopropylphenyl substituent. Interest-
The synthesis and characterization of two new thiolate-contain-
ing Al(III) bis(amido)pyridylthiolate complexes was accomplished
via reduction of a ZnII bis(imino)pyridylthiolate precursor complex
with LiAlH4. The synthesis of the reduced bis(amino)pyridylthiol li-
gand could not be accomplished by conventional organic synthetic
strategies, and thus the success with the zinc(II) complex 1 as
starting material highlights the usefulness of metal template
chemistry in the preparation of unsymmetrical frameworks such
as 1. Interestingly, during the reduction process a rare transmeta-
lation reaction occurs, producing the AlIII products.
The BIP systems have been extensively studied as supporting
ligands for Fe- and Co-catalyzed olefin oligomerizations and poly-
merizations, and aluminum-containing co-catalysts (methylalumi-
noxane (MAO), AlR3) are essential for these reactions. Significant
efforts have been made to discern the role of the aluminum co-cat-
alysts, but this role remains poorly understood. Evidence has been
presented by other workers to indicate that transmetalation with
Al may be an important factor in the activity of these catalytic sys-
tems [8,35,41]. Of particular relevance here is the work from Gam-
barotta and co-workers, which showed that BIP-FeII complexes
react with AlR3 in an unexpected transmetalation process to give AlIII
products [8]. In this reaction the BIP ligand was reduced by electrons
i
ingly, all four Pr-Me groups also give rise to distinct resonances.
One pair of doublets appears at 1.09 and 1.01 ppm, while the other
pair appears as overlapping doublets at 1.26 ppm. The former data
indicate that the rotational freedom about the iPr groups is hindered,
pointing to the highly crowded nature of the complex. The solution
state structure for 2a matches the solid state, and reveals that the
complex is severely conformationally restricted.
Althoughthe limitedsolubility of 2b in CD2Cl2 leads to a less well-
resolved 1H NMR spectrum (Fig. 4), comparisons can still be made
between the solution state structures of 2a and 2b, revealing signif-
icant differences for the two complexes. The resonances for the new
backbone C–H protons in 2b (d 4.72 and 4.20 ppm) are each shifted
upfield by 0.1 ppm in comparison to 2a, while the adjacent methyl
groups are shifted slightly downfield for 2b to
d 1.61 and
1.46 ppm. Interestingly, large changes in the resonances for the iPr
groups for 2b versus 2a are seen. The iPr-CH methine groups in 2b
are no longer separated as in 2a, but appear as a broad, overlapping
mutliplet at 3.23 ppm. In addition, all four iPr-CH3 doublets are sep-
arated for 2a, but for 2b the iPr-CH3 resonances can be assigned to
two doublets centered at 1.22 and 1.02 ppm. The NMR data for 2b
are consistent with hindered rotation about the diisopropylphenyl
C(ipso)–N bond as seen for 2a, but the fact that the iPr-CH and iPr-
CH3 peaks are not well-separated in 2b are consistent with the dif-
delocalized on to the p-system, resulting in paramagnetic Al com-
plexes. The ability of BIP ligands to exist in such ‘‘non-innocent’’ re-
duced states [5,10] suggests the possibility of a non-innocent
configuration for 2a–b, and thus such a configuration was carefully
considered. However, the structural parameters and diamagnetic 1H
NMR spectra for 2a–b clearly rule out this possibility. The transfor-
mation of 1 to 2a–b is only the second example of an Al-mediated
transmetalation reaction with the BIP system besides that reported
by Gambarotta and co-workers [8], and is the only case in which the
i
ferent solid-state structures of 2b versus 2a. The Pr-C(25) in 2a is
likely more crowded than the corresponding C(25A) in 2b because
of the positioning of the methyl group C(15), which lies on the same
side of the chelate ring as the THF molecule, in proximity to C(25).
From the 1H NMR analysis, it is clear that both 2a and 2b maintain
their solid state structures in solution.

26
Y.M. Badiei et al. / Inorganica Chimica Acta 382 (2012) 19–26
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The financial support from the National Institutes of Health
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