Novel In Situ Manganese-Promoted Double-Aldol Addition

24 Pages·2011·1.38 MB·English

by  Fatemah Habib

Accepted Manuscript
Novel In Situ Manganese-Promoted Double-Aldol Addition
Fatemah Habib, Cyril Cook, Ilia Korobkov, Muralee Murugesu
PII: S0020-1693(11)00947-9
DOI: 10.1016/j.ica.2011.11.023
Reference: ICA 14655
To appear in: Inorganica Chimica Acta
Received Date: 17 September 2011
Revised Date: 8 November 2011
Accepted Date: 9 November 2011
Please cite this article as: F. Habib, C. Cook, I. Korobkov, M. Murugesu, Novel In Situ Manganese-Promoted
Double-Aldol Addition, Inorganica Chimica Acta (2011), doi: 10.1016/j.ica.2011.11.023
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(cid:1)
Novel In Situ Manganese-Promoted Double-Aldol Addition
Fatemah Habib,a Cyril Cook,a Ilia Korobkov,a Muralee Murugesua,b*

a Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, ON, K1N6N5, Canada.
b Centre for Catalysis Research and Innovation, 30 Marie Curie, Ottawa, ON, K1N6N5, Canada.

Abstract

A novel in situ Mnn+-promoted double-aldol reaction is reported. Single crystal X-ray measurements
confirm the addition of acetone to two o-vanillin molecules in an original in situ (cid:1),(cid:1) double aldol
reaction promoted by Mn ions in the presence of base. The newly formed ligands coordinate to four
MnIII ions forming a defect-dicubane core structure (1) bridged exclusively by oxygen-based ligands.
Other 3d metals were employed under the same reaction conditions, however no aldol addition
occurred and tetranuclear cubane-like structures formed using CoII (2) and NiII (3) ions. Magnetic
measurements were carried out on all complexes using SQUID magnetometry. Dominant
ferromagnetic interactions were observed for complexes 1 and 3 with J = 1.8 cm-1, J’ = (cid:1) 2.5 cm-1, g
= 1.95 for 1 and J = 3.1 cm-1, g = 2.17 for 3 while complex 2 exhibited antiferromagnetic exchange
interactions. Notably, complex 1 was shown to exhibit spin frustration rarely seen in {MnIII}
4
systems resulting in an intermediate spin ground state of S = 6.
T

Graphical Abstract

(cid:1)
(cid:1)
Keywords: Manganese-promoted, double aldol addition, transition metal, magnetism, defect-
dicubane, cubane.
Introduction
The aldol reaction has been extensively studied, especially in its asymmetric and catalytic versions,
with the use of asymmetric organo-catalysts or chelating agents in metal complex systems.
Generally, the metals used to promote enolization followed by aldol addition are mainly B, Ti or Sn
ions, due to their specific Lewis acid properties [1-4]. To a lesser extent, other first ro w transition
metals, based on Zr, Co, Ni, Cu or Zn ions, have been used [5-10]. Additionally, we have recently
reported the synthesis of a DyIII compound using an aldol ligand formed in situ by a single aldol
6
addition of acetone to o-vanillin; lanthanide metal ions in this case had promoted the reaction [11].
In our efforts to design new multidentate ligands which have the ability to form novel multinuclear
complexes with intriguing structural features magnetic properties, we are exploring this reaction
using Lewis acid metal assistance and thermodynamic conditions. This advantageous reaction would
allow access to ligands which would otherwise be extremely difficult to isolate via classic anionic
organic chemistry under kinetic conditions. This, in turn, would allow the synthesis of novel
coordination complexes which were previously limited through ligand design. With this in mind, we
have investigated the scope of the in situ aldol reaction between acetone and o-vanillin by employing
first row transition metals as potential promoters, and more specifically, with Mn as it can possess
multiple oxidation states with a wide range of Lewis acidities. To date, the synthesis of aldol
products has been mainly the result of aldol-like type reactions: activation of a C-C triple bond (cid:1) to
the carbonyl of the future enolate partner [12], or activation of an (cid:1)-halocarbonyl to produce the
enolate [13-15]. Alternatively, the use of Mn ions for this reaction has been relatively unexplored;
only once has Mn been used in an aldol reaction where the auto-aldol condensation of benzyl methyl
ketone was induced by MnMe(CO) leading to an (cid:2) -C O mononuclear Mn complex [16]. The
5 5 4
discovery of efficient and novel ways of synthesizing polydentate ligands is highly sought after due
to their high coordination modes resulting in multinuclear clusters [17,18]. Furthermore, clusters
(cid:1)
(cid:1)
with high nuclearities have been the focus of intense research due to their potential applications in
numerous fields such as Single-Molecule Magnets [19-21] and related information storage as well as
fundamental stepping stones for magneto-structural correlations. The desire to understand the
magnetic properties of these complexes has led to the investigation of simpler systems such as
{Mn } which can give important fundamental information regarding the interactions between metal
4
centers as well as the effects of key structural differences on those interactions [22]. Herein we
report a unique one-pot (cid:1),(cid:1) double aldol addition of acetone to two o-vanillin molecules promoted

by Mnn+ ions in situ, leading to a novel multidentate ligand, acetone-divanillin aldol (AcVn )
2
(Scheme 1). Additionally, we report the synthesis and unusual magnetic properties of a rare defect-
dicubane {Mn } complex (1) where all metal centers are in the +3 oxidation state as well as two
4
cubane structures using CoII (2) and NiII (3). To our knowledge, the (cid:1),(cid:1) double aldol addition has
never been reported previously as a one pot reaction nor as a specific Mnn+-promoted reaction.
Scheme 1. Synthesis of the acetone-divanillin aldol ligand (AcVn2).(cid:1)

Experimental Section
Materials and Physical Measurements.
All chemicals and solvents were obtained from commercial sources and were used as received,
without further purification. Infrared analyses were performed on NaCl plates using a Bomem
MB100 IR spectrometer in the 4000-600 cm-1 range.
The variable temperature magnetic susceptibility measurements were obtained using a Quantum
Design SQUID MPMS-XL7 magnetometer which operates between 1.8 and 300 K for Direct
Current (DC) applied fields up to 7 T. Measurements were performed on polycrystalline samples of
16.6 mg for 1, 14.3 mg for 2 and 14.7 mg for 3. Alternating Current (AC) susceptibility
(cid:1)
(cid:1)
measurements were carried out under an oscillating AC field of 3 Oe and AC frequencies ranging
from 1 to 1500 Hz. The magnetic data were corrected for the sample holder and diamagnetic
contributions.

Single Crystal X-Ray Diffraction.
Single dark brown rectangular (1), purple rectangular (2) and green plate-like (3) crystals suitable for
X-ray diffraction measurements were mounted on a glass fibre. Unit cell measurements and intensity
data collections were performed on a Bruker-AXS SMART 1 k CCD diffractometer u s ing graphite
monochromatized Mo K radiation (l = 0.71073 Å). The data reduction included a correction for
a
Lorentz and polarization effects, with an applied multi-scan absorption correction (SADABS). The
crystal data and refinement parameters are listed in table 1. Selected interatomic distances and angles
are listed in tables 2 and 3. The crystal structures were solved and refined using the SHELXTL
program suite [23]. Direct methods yielded all non-hydrogen atoms. All hydrogen atom positions
were calculated geometrically and were riding on their respective atoms.

Synthesis of [MnIII (AcVn ) (µ -OMe) (µ-OMe) (MeOH) ] (1). All three complexes were
4 2 2 3 2 2 2
synthesized using identical reaction conditions, therefore only the synthesis of 1 will be described in
detail. The addition of 0.25 mmol of metal salt (MnCl , 31.5 mg; CoCl (cid:1)6H O, 59.5 mg;
2 2 2
NiCl (cid:1)6H O, 59.4 mg) to a 1:1 MeOH/Acetone (30 mL) solution of o-vanillin (0.50 mmol, 76.1 mg)
2 2
and tetraethylammonium hydroxide (TEAOH, 25% w/w in MeOH, 0.50 mmol, 334 µL) yielded a
light brown solution. The solution was stirred for an hour then filtered. The filtrate was allowed to
stand at room temperature and dark brown rectangular crystals suitable for single crystal X-ray
diffraction analysis were obtained after 3-4 days. Yields = 66% (1), 53% (2, purple crystals), 51%
(3, green crystals).

Infrared Analyses for Complexes 1-3.
(cid:1)
(cid:1)
Selected IR data for 1 (Nujol mull, cm-1): 3386 (br), 1666 (s), 1632 (s), 1568 (m), 1442 (s), 1307
(w), 1245 (s), 1068 (s), 1011 (m), 860 (s), 735 (m), 612 (w). 2 (Nujol mull, cm-1): 3394 (br), 1628
(s), 1541 (m), 1339 (m), 1240 (m), 1208 (s), 1069 (m), 958 (w), 842 (w), 745 (w), 723 (m), 649 (w).
3 (Nujol mull, cm-1): 3389 (br), 1628 (s), 1540 (s), 1342 (m), 1245 (s), 1207 (s), 1068 (m), 961 (s),
861 (m), 750 (m), 723 (m), 651 (w).

Table 1. Crystallographic data for complexes 1-3.
1 2 3
Formula C H Mn O C H CoO C H NiO
44 56 4 20 10 14 5 10 14 5
Crystal color Dark brown Purple Gr e en
FW, g.mol-1 1124.65 273.14 272.92
Crystal system Monoclinic Tetragonal Tetragonal
Space group P2 /c I4 /a I4 /a
1 1 1
T, K 200(2) 296(2) 200(2)
(cid:3) , Å 0.71073 0.71073 0.71073
a , Å 11.939(3) 22.501(10) 22.1262(8)
b , Å 20.901(5) 22.501(10) 22.1262(8)
c , Å 9.318(2) 9.706(5) 9.5352(3)
(cid:1) , ˚ 90 90 90
(cid:4) , ˚ 96.051(14) 90 90
(cid:5) , ˚ 90 90 90
V, Å3 2312.2(10) 4914(4) 4668.1(3)
Z 2 16 16
(cid:6) , g.cm−3 1.615 1.477 1.553
calcd
m (Mo, K(cid:2)), mm-1 1.148 1.399 1.664
F(000) 1160 2256 2272
measd/indep 20095/7190 39799/8079 2865/9371
R1(I > 2 (cid:3) (I)) 0.0496 0.0283 0.0348
wR2 (I > 2 (cid:3) (I)) 0.1267 0.0766 0.0943
GOF on F2 1.027 1.014 1.010
CCDC number 850038 850039 850040
R1 = (cid:4)||Fo| - |Fc||/(cid:4)|Fo|; wR2 = {(cid:4)w[(Fo) - (Fc) ] /(cid:4)w[(Fo) ] }1/2
2 2 2 2 2

(cid:1)
(cid:1)
Results and Discussion
Synthesis.
All three complexes were synthesized using the same procedure consisting of the addition of o-
vanillin to a solution of TEAOH in MeOH/Acetone (15:15 mL) mixture, followed by addition of the
metal salt. TEAOH acts as a base in this reaction forming bridging oxides, methoxides and ligands
after deprotonation. Single crystal X-ray crystallography indicated that the ligand in complex 1,
AcVn , is formed in situ through an (cid:1),(cid:1) double aldol addition of acetone to two molecules of o-
2

vanillin promoted by Mnn+ ions in the presence of base. Our proposed mechanism(cid:1)to rationalize the
formation of AcVn by Mnn+-promoted (cid:1),(cid:1) double aldol additions is described in scheme 2.
2

(cid:1)
(cid:1)
(cid:1)
Scheme 2. Proposed mechanism for the Mnn+-promoted (cid:1),(cid:1) double aldol additions.
Initially, a molecule of acetone is deprotonated by the hydroxide moiety of the base, TEAOH,
yielding enolate i which is stabilized by the keto-enolate mesomeric effect as well as coordination to
Mnn+ (part A). Furthermore, the phenol position of o-vanillin is also deprotonated by the hydroxide
base leading to a Mnn+-phenolate complex ii. The concomitant coordination of the Mnn+ ion to the
neighboring carbonyl oxygen atom of o-vanillin allows the activation of the aldehyde function which
undergoes the first aldol addition of the acetone enolate i yielding the first aldol produc t iii (part B).
The latter can then be further deprotonated either in the (cid:1) or (cid:1)’ position from the carbonyl of the
acetone residue. Deprotonation of iii occurs predominantly in the (cid:1) position due to the
thermodynamic conditions used (medium-strong base at RT) which favor deprotonation on the most
substituted (cid:1) position (part C). The formation of enolate iv, stabilized by the keto-enolate mesomeric
effect, is also promoted by Mnn+ coordination assistance. Enolate iv can undergo another aldol
addition on a second molecule of o-vanillin, activated by its coordination to Mnn+, leading to the
ligand acetone-divanillin aldol v (AcVn ) which is quadruply deprotonated and coordinated to two
2
Mnn+ ions. It is noteworthy that the only product we observe is the (cid:1),(cid:1) double aldol product v. None
of the alternative aldol products that could have resulted from only one aldol addition, (cid:1),(cid:1)’ double
aldol addition or even more than two aldol additions were observed. Moreover, when the same
reaction was carried out with chloride salts of CrIII, FeII, CoII, NiII and ZnII no aldol product was
obtained whether as a crystallized complex or in solution. Crystals were observed for the CoII and
NiII reactions, however only o-vanillin was acting as a ligand in addition to methoxide and methanol
coordinating solvents. The specificity of the reaction towards manganese ions could be explained by
the accessibility of a wide range of oxidation states for the Mnn+ ion (from II to VII) as well as a
strong Lewis acidity. Indeed, Mnn+ ions can be classified as hard Lewis acids while FeII, CoII, NiII
and ZnII ions are considered borderline [24-26]. This hard Lewis acidity allows the Mnn+ ions to
efficiently activate the carbonyl functions of acetone and o-vanillin even when already coordinating
to the phenolate of the o-vanillin leading to this unique (cid:1),(cid:1) double aldol reaction. In addition, this
(cid:1)
(cid:1)
strong Lewis acidity is also in part responsible for the second aldol addition occurring at the (cid:1)
position, as Morris observed [27]. Increasing the Lewis acidity of the metal promoter increases the
(cid:1)/(cid:1)’ ratio of the second aldol addition on a mono-aldol product. The absence of a third (cid:1) or (cid:1)’ aldol
addition could be due to steric hindrance around the acetone residue of v (AcVn ), while the
2
slowness of the reaction may have allowed the {MnIII } complex to form before a third aldol
4
addition process could occur. Moreover, due to their role in the induced aldol additions, the Mnn+
ions are already coordinating while they promote the ligand synthesis. Hence, we can assume that

the complex involving v (AcVn ) is more favorable allowing its crystallization before a possible
2
third aldol addition could take place. It is noteworthy that the reaction to form the {Mn } complex
4
was carried out under air-free conditions to prevent the oxidation of MnII to MnIII in order to probe
the oxidation state of the active species (see Supporting Information). This reaction did not yield any
crystals of the product nor any aldol product in the solution (analyzed by NMR), thereby leading us
to believe that MnIII is more likely promoting the double aldol addition. This is further supported by
an additional experiment carried out using MnF as a direct source of MnIII ions which led to crystals
3
of 1 in the same period of time (see Supporting Information). These observations can be explained
by the flexibility that is inherent in MnIII ions due to Jahn-Teller (JT) elongations which are absent in
MnII ions. Moreover, employing metal ions with similar Lewis acidity but no JT elongations such as
CrIII did not yield any aldol product confirming our belief that flexibility at the metal center is
critical in promoting the double aldol addition. The specificity of this reaction for MnIII ions is
therefore due to both hard Lewis acidity as well as flexible coordination sites along the JT axes.
Based on these experiments, we can assume that MnII species are initially oxidized to MnIII which
can then promote the aldol addition to form AcVn2 and subsequently complex 1. To our knowledge,
this novel reaction is the first of its kind where Mnn+ ions were used to promote an aldol addition
reaction as well as the first reported one-pot (cid:1),(cid:1) double aldol addition.
Structural Analysis.
(cid:1)
(cid:1)
The in situ (cid:1),(cid:1) double aldol addition of acetone to two o-vanillin molecules promoted by Mnn+ ions
leads to the formation of a tetranuclear complex with a defect-dicubane core structure for complex 1
(figure 1). Single crystal X-ray analysis reveals that complex 1 crystallizes in the monoclinic space
group P21/c. The planar core of 1 is composed of four distorted octahedral MnIII ions (confirmed by
bond valence sum calculations, table S1, and charge balance considerations) bridged exclusively by
oxygen atoms originating from two AcVn ligands, two µ -methoxide and two µ-methoxide
2 3
molecules. The remaining axial positions are occupied by two methanol solvent molecules. As

expected, MnIII ions adopt distorted geometries with characteristic elongations along one axis known
as the JT axis. The bridging oxygen atoms form magnetic superexchange pathways between the
metal centers with Mn1-Mn1a, Mn1-Mn2 and Mn2-Mn2a distances of 3.25, 3.12 and 5.34 Å,
respectively. Packing diagrams along the a-, b- and c-axes are shown in figures S1-S3. Along the a-
axis, alternating layers of ABA style can be observed while along the b-axis, molecules are oriented
in the same fashion across all layers. Along the c-axis, packing in ABA style is observed as well
where molecules appear to be in zig-zag formation vertically. The closest intermolecular metal-metal
distance of 6.64 Å is found within the same layer along the b-axis while the closest inter-layer metal-
metal distance was found to be 9.93 Å.
(cid:1)