Bimetallic Bis-anion Cascade Complexes of Magnesium in Nonaqueous Solution

Article abstract of DOI:10.1021/acs.inorgchem.9b03710

Bimetallic magnesium(II) complexes are gaining significant interest in catalysis, yet their fundamental formation and behavior in organic media remain surprisingly unexplored relative to other divalent cations. To understand key principles of their formation, we investigate symmetric ditopic ligands bearing a phenolic backbone and characterize their ability to form dinuclear magnesium(II) cascade complexes with two bridging anions. High-fidelity production of bimetallic magnesium complexes relative to the monometallic complexes is indicative of positive cooperativity. Binding and recognition of analytes or substrates is a key characteristic of metal cascade complexes and relies on anion exchange, but this is also rarely studied with bimetallic magnesium complexes. Investigations with acetate, phosphate, and pyrophosphate reveal exchange of bridging nitrates using the bis-dipicolylamine complex. Rare seven-coordinate magnesium centers are found for the ester complex. The findings in this study provide formative steps to establish design principles for future generations of bimetallic magnesium(II) complexes.

Full text of DOI:10.1021/acs.inorgchem.9b03710

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Bimetallic Bis-anion Cascade Complexes of Magnesium in
Nonaqueous Solution
David Van Craen, Ian G. Flynn, Veronica Carta, and Amar H. Flood*
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ABSTRACT: Bimetallic magnesium(II) complexes are gaining significant interest in
catalysis, yet their fundamental formation and behavior in organic media remain surprisingly
unexplored relative to other divalent cations. To understand key principles of their formation,
we investigate symmetric ditopic ligands bearing a phenolic backbone and characterize their
ability to form dinuclear magnesium(II) cascade complexes with two bridging anions. High-
fidelity production of bimetallic magnesium complexes relative to the monometallic
complexes is indicative of positive cooperativity. Binding and recognition of analytes or
substrates is a key characteristic of metal cascade complexes and relies on anion exchange, but
this is also rarely studied with bimetallic magnesium complexes. Investigations with acetate,
phosphate, and pyrophosphate reveal exchange of bridging nitrates using the bis-
dipicolylamine complex. Rare seven-coordinate magnesium centers are found for the ester
complex. The findings in this study provide formative steps to establish design principles for future generations of bimetallic
magnesium(II) complexes.
ion53,55−57,59,61,62 as well as cooperative binding of two
INTRODUCTION
Magnesium and its complexes are essential for life and are of
■
substrates at the same time.53−56,59,60 Fundamental binding
longstanding importance in chemistry.1,2 Their roles can be
studies describing how magnesium ions bind to ditopic ligands
to produce complexes that are similar to these catalytic systems
is even more rare. As a result, insights into their solution-phase
behavior is severely lacking. Key qualities include their
speciation and the existence of species of intermediate
nuclearity in solution, the degree of cooperative binding of
the two magnesium ions, and the behavior of the bimetallic
magnesium complex upon substrate binding. NMR spectros-
copy and detailed binding studies are often needed to address
these features.
Examples of the coordination chemistry of magnesium are
limited. Highly charged ligands, like ethylene diamine
tetraacetic acid and its analogues,65−68 are frequently used as
magnesium chelators. However, these ligands are almost
exclusively studied in aqueous media on account of their
solubility and, therefore, are not helpful as models for
processes like catalysis that are performed in organic solvents.
Metal-cascade receptors, which are more structurally similar to
bimetallic catalysts and which also serve as receptor-based
sensors for anionic analytes like pyrophosphate or ATP,69−75
are generally used in water and are also not ideal model
seen in photosynthesis, the respiratory chain, and ATP
consuming reactions1−5 as well as the famous Grignard
reaction,6−10 which utilizes the reactivity of organyl-magne-
sium halides. Magnesium is now being considered as a low cost
and low toxicity replacement for transition metals in catalysts
for more environmentally friendly and “green” chemical
processes.11,12 Emerging examples include ring-opening
polymerizations,13−22 cycloadditions,23−27 redox-reactions,28
addition or radical addition reactions,29−33 and hydroboration
reactions34−38 as well as C−H and C−F activation.39,40
Fundamental knowledge of magnesium-ligand coordination
chemistry and the supramolecular chemistry associated with
the binding of substrates and of substrate analogs is a key
aspect for the knowledge-driven design of new ligands. The
catalytic species are generally assumed to be mononuclear
magnesium complexes; however, dimers15,16,18−20,41−44 or
multimers29,45−51 are often suggested. While these applications
would benefit from comprehensive studies of magnesium
complexation, such investigations are rare.52 Here we address
this shortcoming with an investigation into some of the first
cascade complexes of magnesium.
While there are over 150 examples of magnesium-catalyzed
reactions, just 10% of these studies involve bimetallic
magnesium complexes. Of these few, they have shown promise
as catalysts in ring opening53 and addition reactions33,54−56 as
well as in polymerizations.57−64 The close proximity of the
magnesium centers in these dinuclear complexes implies
coordination of the substrate in a bridging fash-
Received: December 21, 2019
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candidates. Cascade complexes consist of metal centers that
define ideal binding sites for counteranions. Negatively charged
analytes can replace these counteranions resulting in spectral
changes which makes them ideal for anion recognition.
Transition metals like zinc(II),69−74,76,77 copper(II),77,78 and
cobalt(III)79 are frequently chosen to combine with bis-
dipicolylamine-based ligands for these sensors instead of
magnesium on account of the poor complex stability of
magnesium complexes in water.76,80
Mononuclear magnesium complexes of dipicolylamine have
been studied previously as fluorescent probes for magnesium
ions in acetonitrile.80 Similar complexes and their dimers have
been applied as catalysts in living lactide polymerizations in
dichloromethane.20 A magnesium complex of a bis-dipicoly-
amine-bearing ligand was briefly mentioned in a patent,81 but
neither NMR nor solid state data was provided.
phosphate, and hydrogen pyrophosphate. Our findings show
that a variety of dinuclear magnesium(II) complexes are
readily accessible and that bridging anions can be readily
exchanged. These bimetallic bis-anion cascade complexes help
take the first steps toward understanding the coordination
chemistry of bimetallic magnesium complexes.
EXPERIMENTAL SECTION
Ligand Preparation. All ligands were prepared using a four-step
pathway (Scheme 1). It is possible to reduce the number of steps
■
Scheme 1. Synthetic Route for the Preparation of
a
Symmetric Ditopic Ligands
For all these reasons, studies of bimetallic magnesium
complexes and their formation in organic media are of
fundamental interest and are needed to help inform the
development and design of next-generation catalysts and
sensors. Investigations of the exchange of coordinating anions
may also serve as surrogates for substrates and analytes to help
close the gap in the fundamental understanding of these
valuable complexes.
Herein we investigate the binding behavior of bimetallic
magnesium complexes composed of ligands (Figure 1) bearing
Figure 1. Binding behavior of the various ligands toward bimetallic
magnesium cascade complexes.
a
DBU is used as base in the final step for ligands substituted with
pyridine, triazole, furan, and thiophene. The IDA (iminodiacetic acid)
ester bearing ligands are obtained using triethylamine as base.
lone-pair donor atoms (N, O, S) and study the influence and
exchange of bridging anions. The binding pockets share a
central phenolic linker to enhance the binding strength of the
magnesium ions. It is not known if coordination of two
magnesium cations would show positive or negative cooper-
ativity. It is also unknown if the metal centers bound by this
ligand scaffold will be bridged by the counteranions of the
magnesium salt used for the complexation or, in the case of
bridging anions, if these anions can be exchanged or if metal
extrusion occurs instead. We find that the binding pockets with
pyridines and the esters enable competent binding with
positive cooperativity favoring formation of the dinuclear
magnesium complexes, while furan and thiophene lack good
binding strengths. The pyridine-bearing complex forms a cleft
between the magnesium centers occupied by two bridging
nitrates. Use of the ester-derived ligands closes the cleft to
generate nonbridging nitrate anions and results in a rare seven-
coordinate pentagonal-bipyramidal coordination around each
magnesium ion. Investigations of the exchange of bridging
nitrates in the dinuclear magnesium complex of the pyridine-
bearing ligand were conducted with acetate, dihydrogen
either by benzylic bromination of 4-bromo-2,6-xylenol bearing a
protected hydroxyl function82,83 or by a Mannich reaction starting
from the commercially available 4-bromophenol 1 and the
corresponding amines.82,84 However, the multistep route82 we
followed involves three well-reported reactions85−87 and offers high
ligand modularity by using simple nucleophilic substitution in the
final step, which can also be easily monitored by 1H NMR
spectroscopy.
Starting with 4-bromophenol 1, a Duff reaction with hexamethy-
lenetetramine in trifluoroacetic acid results in dialdehyde 2.85,86 After
recrystallization, reduction with lithium aluminium hydride affords
trialcohol 3.85 Conversion of the benzylic alcohols into bromide
leaving groups generates ligand precursor 4.87 The three steps leading
to 4 can be performed on multigram scales without resorting to time-
consuming purification methods. The final substitution reaction
affords the target ligands in moderate to good yields. Bis-
dipicolylamine ligand LDPA-H and methyl ester ligand LMe-H are
known compounds.84,88 The triazole-bearing amine is synthesized
using click chemistry with dipropargylamine and phenylazide in good
yields. The ligands bearing the various ester functionalities are
obtained in good to very good yields from iminodiacetic acid (IDA)
after esterification.
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Figure 2. (a) The binding of bis-dipicolylamine ligand LDPA-H upon addition of magnesium salt produces a putative mononuclear complex and the
dinuclear species seen in the solid state. (b) The crystal structure of the bimetallic complex [LDPAMg2(μ2-NO3)2](NO3) is shown with spacefill, (c)
capped sticks, and (d) a side view with capped sticks and omitted nitrate counteranion. A single methanol molecule located in the unit cell is
omitted for clarity in all three cases. (e) Solution binding (1H NMR, 600 MHz) is observed in acetonitrile-d3 upon addition of Mg(NO3)2(H2O)6
to LDPA-H (5 mM, dilution corrected) at room temperature. Plotted curves for the changes are shown in the Supporting Information Figures S2
and S3.
Crystal Structures and Binding Behavior. Single crystals were
grown from a methanol solution (LDPA-H) or an acetonitrile solution
with a 1:2 mixture of bis-dipicolylamine ligand LDPA-H as well as ethyl
iminodiacetate ligand LEt-H and magnesium(II) nitrate hexahydrate
by vapor diffusion of diethyl ether. The binding behavior in solution
was observed using 1H NMR spectroscopy following successive
addition of magnesium(II) nitrate hexahydrate to the ligands. The 1H
NMR studies were carried out using 5 mM solutions of acetonitrile-d3
at room temperature, and each aliquot addition is dilution corrected.
cascade complex in the gas phase using electrospray ionization
mass spectrometry (ESI-MS).
The crystal structure of [LDPAMg2(μ2-NO3)2](NO3) shows
two magnesium cations located in each binding pocket (Figure
2b−d). Each magnesium is coordinated by the three nitrogen
atoms of the dipicolylamine units, and they share the
deprotonated phenolic oxygen in the ligand backbone. The
Mg···N average bond lengths of 2.19 Å are longer than the
Mg···O bonds of 2.05 Å, which is consistent with the different
valence state of the donor atoms. The torsion of the
dipicolylamine units relative to the backbone creates a cleft
between the magnesium centers that enables them to be
bridged by the two nitrate anions. An additional nitrate serves
as an outer-sphere counteranion.
RESULTS AND DISCUSSION
■
Binding Behavior of Ligands Bearing Heterocyclic
Rings. Based on prior studies of either cobalt79 or zinc69 with
similar ligands, it is expected that two magnesium cations will
bind into each of the binding pockets of the bis-dipicolylamine
ligand LDPA-H (Scheme 1). The nitrate counteranions are
expected to bridge the magnesium centers as is seen with zinc
cascade complexes of analytes like pyrophosphate.69 The
complex formed upon addition of magnesium(II) nitrate
hexahydrate to the ligand, [LDPAMg2(μ2-NO3)2]+, fulfilled
these expectations. This complex possesses a single positive
charge (Figure 2a), which is ideal for observing the intact
Magnesium binding in acetonitrile-d3 is consistent with the
solid-state structure (Figure 2e). Two species are observed
during titration with increasing amounts of magnesium nitrate
hexahydrate. The first species generates downfield shifts in the
1H NMR signals (pink, gray, cyan, and red). This shifting set of
peaks is assigned to the formation of an intermediate 1:1
complex [LDPAMg]+ in fast exchange with the free ligand
(Figure S2). The intensity of this signature decreases with
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addition of magnesium and is directly replaced by a slow
exchanging species (green). This signature emerges upon
addition of the initial 0.3 equiv aliquot of magnesium and
grows continuously until saturation at 2 equiv. Splitting of the
pyridyl signals into two sets is consistent with the two different
pyridyl environments around each magnesium center as seen
in the crystal structure. Consistent with lowered symmetry and
slow exchange on the NMR time scale, the aliphatic signals
split into four doublets for proton Hc and two doublets for
proton Hb. Formation of the dinuclear complex is highly
favored relative to the mononuclear complex. The positive
cooperativity is seen in the direct formation of the dinuclear
species from the beginning of the titration and the saturation at
2 equiv (Figures S1 and S3). The noncooperative situation
would have shown formation of the dinuclear species only after
production of the mononuclear, but that behavior was not
observed.
Lthiophene-H, results in nearly no magnesium binding (Figure
S11). This observation is in line with the hard and soft acids
and bases concept. Magnesium, as a hard acid, disfavors sulfur.
Binding Behavior of Ligands Bearing IDA (Iminodi-
acetic Acid) Esters. In general, we observe weaker binding of
magnesium with the IDA ester ligands than with the bis-
dipicolylamine ligand LDPA-H. The methyl ester LMe-H shows
1
only a fast exchange H NMR signature upon magnesium
addition (Figure 3a). This observation leads to the assumption
The ESI-MS verifies the presence of both mononuclear and
dinuclear complexes. Low resolution ESI-MS with substoichio-
metric amounts of magnesium (0.5 and 1 equiv) show signals
of the mononuclear and dinuclear species together with the
unbound ligand as either protonated species or sodium salts.
The intensity of the signal for the dinuclear complex grows
with increasing amounts of the magnesium salt (Figure S35).
High resolution ESI-MS (Figure S38) matches the values and
isotope pattern of the dinuclear magnesium complex
[LDPAMg2(μ2-NO3)2]+ with m/z peaks of 765.1129 Da (79Br
isotope) and 767.1097 Da (81Br isotope). No evidence of a
dimeric species involving two ligands was found by mass
spectrometry. The nearly identical diffusion coefficients of the
free ligand LDPA-H (D = 1.15 × 10−9 m2/s) and the final
species (D = 1.07 × 10−9 m2/s) are consistent with a
nondimeric ligand structure.
The bimetallic complex [LDPAMg2(μ2-NO3)2]+ is stable and
maintained in solution for at least 2 weeks without
decomposition as measured using NMR spectroscopy (Figure
S4). It is also possible to prepare a solid form of the complex
by removal of acetonitrile. The resulting green solid can then
be redissolved in acetonitrile as well as in dichloromethane,
which cannot be used directly to prepare the complex on
account of solubility limitations (Figure S5).
The triazole ligand Ltriazole-H shows a similar albeit weaker
1
magnesium binding behavior (Figure S6). Splitting of the H
NMR signals for the dinuclear complex [LtriazoleMg2(NO3)2]+
is found as soon as 0.3 equiv of magnesium(II) nitrate
hexahydrate is added to the ligand. Those signals display a
slow-exchange signature and grow in intensity during the
titration (Figure S8). The mononuclear complex [LtriazoleMg]+
displays characteristic fast-exchange peaks that shift downfield
until the addition of 1 equiv of the magnesium salt (Figure S7).
A 1:2 ratio between the dinuclear and mononuclear complexes
is present at 5 equiv of added magnesium nitrate indicative of
the lower overall complex stability with triazoles.89−92 Mass
spectrometry signals (Figure S39) are seen for the dinuclear
magnesium complex [LtriazoleMg2(NO3)2]+ at 1029.1990 Da
(79Br isotope) and 1031.1973 Da (81Br isotope).
Figure 3. Magnesium binding of the ligands LMe/Et/iPr-H in
acetonitrile-d3 (5 mM, dilution corrected) at room temperature
observed with 1H NMR (600 MHz) upon addition of Mg-
(NO3)2(H2O)6.
that the major species in solution is the mononuclear
magnesium complex [LMeMg]+ and that the ligand is not
well suited for significant magnesium binding. Crystallization
from either methanol or acetonitrile only results in
magnesium(II) nitrate.
A significant change in the binding behavior is observed
when using the ethyl and isopropyl esters, LEt/iPr-H. This
change in affinity is assumed to be related to the enhanced
inductive effect resulting in a higher electron density at the
carbonyl oxygen of the esters. Upon addition of magnesium, an
extra signal appears in the aromatic region (green box, Figures
3b and 3c). The intensity of this peak increases with the
The furan ligand Lfuran-H shows weak binding of
1
magnesium. The slow-exchanging signature in the H NMR
typical of the dinuclear magnesium complex is not observed
(Figure S9). Consequently, magnesium binding is assumed to
proceed in a 1:1 fashion or to speciate as a mixture. This
assignment is consistent with the small shifts of the observed
NMR signals. Its sulfur analog, the thiophene bearing ligand
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Figure 4. Crystal structure of the dinuclear complex [LEtMg2(NO3)2(H2O)2](NO3) with omitted solvent molecules. (a) Structure of the
crystallized complex. (b) Side view with capped sticks and omitted outer-sphere nitrate counteranion. (c) Top view with spacefill. (d) Seven-
coordinate pentagonal-bipyramidal magnesium center. (e) View along the plane of the phenol backbone showing the difference between the
orientations of the ester chain of [LEtMg2(NO3)2(H2O)2]+ (left) and the pyridine units of [LDPAMg2(μ2-NO3)2]+ (right).
amount of added magnesium(II) salt (Figures S17 and S20)
alongside a fast-exchange process (Figures S16 and S19) that
resembles the signature seen with the methyl ester LMe-H. No
new signals arise in the aliphatic region. Rather they appear to
be too broad to be readily assigned. Aggregation is inconsistent
with the broad aliphatic signals on account of nearly identical
diffusion coefficients for the peaks at 7.32 ppm (green box,
Figure 3c) and 7.38 (pink, Figure 3c). Fortuitously, the broad
aliphatic signals emerge more visibly with the isopropyl ester
complex and sharpen up at lower temperature with consistent
peak intensities (Figure S21). This observation is the first
indication of the dinuclear magnesium complex seen
previously in the slow-exchanging signature with LDPA-H.
Mass spectrometry shows signals of the dinuclear magnesium
complexes in all cases (Figures S40−S42) with m/z values of
689.0271 and 691.0251 Da for [LMeMg2(NO3)2]+, 745.0894
and 747.0877 Da for [LEtMg2(NO3)2]+, and 801.1523 and
803.1506 Da for [LiPrMg2(NO3)2]+. The benzyl ester ligand
LBz-H binds weaker than the ethyl or isopropyl ester LEt/iPr-H
but stronger than the methyl ester LMe-H. The same slow-
exchanging aromatic signal is found for the dinuclear complex
[LBzMg2(NO3)2]+ but with a lower overall intensity (Figures
S22 and S24). Values of 993.1527 and 995.1513 Da are seen in
the ESI-MS (Figure S43) for the dinuclear complex
[LBzMg2(NO3)2]+. A 5-fold excess of magnesium salt was
used for the gas phase experiments with the ester ligands on
account of their weaker binding behavior.
in the inner coordination sphere has been reported93 in the
Cambridge Structure Database, which is composed of an aza-
crown ether complex. Just a dozen similar structures94−102 with
seven-coordinate magnesium centers are known that involve
oxygen and more than one nitrogen atom. The bond lengths
are slightly longer than the ones of the dipicolylamine complex
[LDPAMg2(μ2-NO3)2](NO3) with 2.4 Å (N···Mg) and 2.16 Å
(O···Mg). Torsion of the IDA ester units is observed for the
complex (Figure 4b, c) as was also seen with bis-dipicolyl-
amine [LDPAMg2(μ2-NO3)2](NO3). However, the ester groups
of each IDA ester unit behave very differently in comparison to
the pyridine units of the bis-dipicolylamine structure. An
interplay of electronic and steric effects is assumed to be the
reason for the seven-coordinate magnesium centers. The ester
possesses weaker electron donating properties than pyridine. In
addition, the ester has a strong electron-withdrawing effect,
that is expected to weaken the ligating power of the central
tertiary nitrogen more so than pyridine groups would. The
weaker binding strength of the aliphatic nitrogen atom is
reflected in a 0.2 Å increase of the bond length to magnesium
((CH2)2N···Mg: 2.40 Å in [LEtMg2(NO3)2(H2O)2]+ vs 2.20 Å
in [LDPAMg2(μ2-NO3)2]+). The steric effect reveals itself in the
distorted structure. Within the equatorial plane, the two ester
oxygen atoms that are located trans to each other define a O−
Mg−O angle of 143°, which is much less than the ideal 180°.
Also, the ester chains of both of the IDA ester units are
oriented in a linear fashion at an angle of 171° between the two
planes spanned by the CH2C=O unit of the ester instead of the
orthogonal (84°) arrangement seen with LDPA (Figure 4e).
This span and the 143° O−Mg−O angle in the equatorial
plane provide greater room for the nitrate to coordinate
through two of its oxygen atoms with a tight O−Mg−O bite
angle of 58° instead of just one oxygen atom. Electronic
demands and steric access are thus responsible for expanding
the donor atom set to seven.
We obtained the crystal structure of the ethyl IDA ester
l i g a n d a s
a d i n u c l e a r m a g n e s i u m c o m p l e x
[LEtMg2(NO3)2(H2O)2](NO3). We now see that the coordi-
nating nitrates are no longer bridging the metal centers. The
structure also possesses magnesium centers that are in a rare
seven-coordinate geometry with distorted pentagonal-bipyr-
amidal coordination (Figure 4a, d). In contrast, the structure of
[LDPAMg2(μ2-NO3)2](NO3) displays two six-coordinate octa-
hedral magnesium centers (Figure 2b−d). Only one seven-
coordinate magnesium with six oxygen and one nitrogen atoms
Exchange of the Bridging Nitrate Anions in
[LDPAMg2(μ2-NO3)2]+. Exchange of bridging anions is a key
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Figure 5. (a) Anion exchange reaction of the bridging nitrate groups by acetate added as the tetrabutylammonium salt in acetonitrile-d3. (b) The
crystal structure is shown with omitted solvent in spacefill. The nitrate counteranion is hidden behind the upper pyridine ring. (c) Top view with
capped sticks and (d) side view with omitted nitrate counteranion. (e) 1H NMR solution spectra (600 MHz, 5 mM, dilution corrected) following
the exchange of bridging nitrate for bridging acetate observed in acetonitrile-d3 solution at room temperature.
characteristic of metal-cascade complexes though this behavior
has never been studied previously with bimetallic magnesium
complexes using NMR spectroscopy. The capability of anion
exchange of the bis-dipicolylamine complex [LDPAMg2(μ2-
NO3)2]+ was investigated by evaluating the ability of different
anions to bridge the magnesium cations. Anion exchange
studies were undertaken by starting with the dinuclear nitrate-
bridged magnesium complex [LDPAMg2(μ2-NO3)2]+. Tetrabu-
tylammonium salts of acetate (Figure 5a), dihydrogen
phosphate, and hydrogen pyrophosphate (Figures S25, S30,
S31, and S33) were examined.
Crystals obtained after the addition of 2 equiv of acetate
verify the exchange. Acetate is now occupying the role of
bridging anions in the complex [LDPAMg2(μ2-OAc)2](NO3),
which retains an outer-sphere nitrate as counteranion (Figure
5b−d). The same slightly distorted octahedral geometry
around the magnesium centers is seen with the acetate bridged
structure compared to the nitrate bridged one. Very similar
average N···Mg distances of 2.23 Å and O···Mg distances of
2.02 Å are found. The same cleft is created between the
magnesium centers, which is occupied by the bridging acetate
anions.
of bridging nitrate by acetate. Addition of a large excess of
tetrabutylammonium acetate and nitrate to the acetate-bridged
complex was investigated in order to see different binding
modes or anion exchange back to nitrate. Excess acetate salt
led to the emergence of a new set of signals, which shows that
the binding mode can be changed. However, more than 20
equiv of additional acetate were necessary to observe the new
species, and the original acetate-bridged complex
[LDPAMg2(μ2-OAc)2]+ remains the main species (Figure
S26). Addition of the nitrate salt does not show any change
even after the addition of 50 equiv. As a result, the acetate-
bridged complex is very stable (Figure S27).
Addition of 2 equiv of tetrabutylammonium dihydrogen
phosphate results in a similar trend. Shifted signals show the
exchange of nitrate for phosphate and the formation of the
putative phosphate-bridged species [LDPAMg2(HxPO4)2](−3+2x)
as a major product, but a significant amount of an intermediate
1
species remains visible in the H NMR spectra (Figure S30).
Addition of excess phosphate leads, in contrast to the acetate
salt, to the formation and precipitation of a colorless solid. 31P
NMR matches the findings, and two complexes involving
phosphate are observed (Figure S31). This solid is assumed to
be magnesium phosphate based on the recovery of the signals
for the unbound ligand LDPA-H. Titrations with magnesium
acetate tetrahydrate and magnesium hydrogen phosphate
trihydrate are not possible in the same way as the titrations
conducted with magnesium nitrate hexahydrate on account of
poor solubility. The anion exchange experiments show that the
acetate complex is stable in solution. For this reason it should
be possible to form the acetate complex by direct addition of
The crystal structure is consistent with anion exchange
occurring in solution (Figure 5e). Addition of the acetate salt
leads to the evolution of a new slow-exchanging species
(marked in pink), which is highly favored after addition of the
2 equiv of acetate. With higher equivalents of acetate, none of
the initial nitrate bridged complex [LDPAMg2(μ2-NO3)2](NO3)
(green signals) nor any intermediate species are present
(Figure S25). These observations show the favored exchange
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magnesium acetate. We investigated different methods to
determine if the formation of a magnesium complex occurs.
Magnesium acetate tetrahydrate and magnesium hydrogen
phosphate trihydrate were added into NMR tubes as solids. A
solution of the ligand in deuterated acetonitrile (5 mM) was
added to the salt in different stoichiometries. NMR spectra
show that complex formation with magnesium acetate is very
slow. Sonication and heating for 3 h showed that the same
complex observed by anion exchange can be formed with
magnesium acetate tetrahydrate (Figure S28). A time-depend-
ent study shows that complex formation takes around 3 days if
no external stimulus is applied (Figure S29). The samples with
magnesium hydrogen phosphate trihydrate, on the other hand,
show no complex formation even after applying an external
force (Figure S32). Addition of pyrophosphate (TBA3·HP2O7)
acts in an even more extreme way, which is likely a result of its
higher negative charge. Precipitation of an insoluble salt is
observed with signals for the free ligand LDPA-H dominating
the 1H NMR spectra after addition of 2 equiv of the
pyrophosphate salt (Figure S33).
information for crystal structures (CCDC 1972635−
1972637) (PDF)
Accession Codes
CCDC 1972635−1972637 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Amar H. Flood − Department of Chemistry, Indiana University,
Bloomington, Indiana 47405-7102, United States;
orcid.org/0000-0002-2764-9155; Email: aflood@
indiana.edu
Authors
David Van Craen − Department of Chemistry, Indiana
University, Bloomington, Indiana 47405-7102, United States
Ian G. Flynn − Department of Chemistry, Indiana University,
Bloomington, Indiana 47405-7102, United States
Veronica Carta − Department of Chemistry, Indiana University,
Bloomington, Indiana 47405-7102, United States
CONCLUSION
■
Magnesium binding to a series of ditopic ligands bearing a
shared phenolic oxygen favor dinuclear complexes, and they
can accommodate bridging and exchangeable anions depend-
ing on ligand structure. Putative mononuclear complexes are
less favored and display fast-exchange signatures on the NMR
time scale. Crystal structures reveal two nitrate anions bridging
the two magnesium centers and residing in a central cleft
defined by the dipicolylamine units of ligand LDPA-H. No
bridging occurs in the case of the ethyl ester ligand, which is
attributed to a near linear arrangement of the ester groups on
the periphery of the backbone that closes the cleft between the
metal centers. Anion exchange of the bridging anions in the
dinuclear magnesium complex of dipicolylamine ligand LDPA-H
shows the potential for in situ variation of the coordination
sphere in this magnesium cascade complex. Acetate shows
direct exchange, while phosphate and pyrophosphate lead to
disassembly of the complex upon precipitation of all-inorganic
salts. This fundamental study opens up ways for the in situ
generation of dinuclear magnesium complexes with high
modularity. The difference in the nitrate-bridged
[LD P A Mg2 (μ2 -NO3 )2 ]+ and nonbridged complex
[LEtMg2(NO3)2(H2O)2]+ is of interest for the ligand-
controlled variation in the magnesium coordination sphere as
a result of the closure of the cleft between the metal centers.
The close proximity between the coordinating nitrate anions
and the ester residue in [LEtMg2(NO3)2(H2O)2]+ may serve as
the basis of chiral esters to investigate asymmetric trans-
formations. These types of insights result from the
fundamental investigation of the coordination chemistry and
cascade complexation phenomena of magnesium ions in
nonaqueous solution.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03710
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
D.V.C., I.F., and A.H.F. thank Thermo Fisher Scientific for
financial support of the conducted research. A.H.F. acknowl-
edges support from the National Science Foundation (CHE-
1609672).
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