Synthesis of pyridine-alkoxide ligands for formation of polynuclear complexes

Article abstract of DOI:10.1039/c7nj01845b

We have prepared and characterized a series of novel polydentate N,O-donor ligands derived from our well-studied ligand 2-(2-pyridinyl)-2-propanol (pyalkH), having the general formula Me{C(OH)(2-py)CH₂}_nH, where n = 2 or 3. Like pyalkH, these analogues bind via N and O with deprotonation at the latter, thus extending the strongly donor pyridine-alkoxide chelation power of pyalkH to polydentate forms. The greater denticity allows for more effective binding and polynuclear cluster formation with first-row transition metals. Several stable alkoxo-bridged polynuclear clusters of these ligands with Mn, Cu, Co and Ni have been prepared; all reported ligands and complexes have been characterized, including by X-ray crystallography. We report a one-step synthesis of these ligands, alongside pyalkH, on a multi-gram scale from inexpensive starting materials. We have also developed a new scalable procedure for the isolation of pyalkH that avoids the need for chromatography, making large-scale production of this ligand commercially viable.

Full text of DOI:10.1039/c7nj01845b

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DOI: 10.1039/C7NJ01845B.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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New Journal of Chemistry
Synthesis of Pyridine-Alkoxide Ligands for Formation of
Polynuclear Complexes
Dimitar Y. Shopov,‡ Liam S. Sharninghausen,‡ Shashi Bhushan Sinha,‡ Julia E. Borowski,
Brandon Q. Mercado, Gary W. Brudvig* and Robert H. Crabtree*
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520,
United States.
ABSTRACT:
We have prepared and characterized a series of novel polydentate N,Oꢀdonor ligands derived
from our wellꢀstudied ligand 2ꢀ(2ꢀpyridyl)ꢀisopropanol (pyalkH), having the general formula
Me{C(OH)(2ꢀpy)CH2}nH, where n = 2 or 3. Like pyalkH, these analogues bind via N and O
with deprotonation at the latter, thus extending the strongly donor pyridineꢀalkoxide chelation
power of pyalkH to polydentate forms. The greater denticity allows for more effective binding
and polynuclear cluster formation with firstꢀrow transition metals. Several stable alkoxoꢀbridged
polynuclear clusters of these ligands with Mn, Cu, Co and Ni have been prepared; all reported
ligands and complexes have been characterized, including by Xꢀray crystallography. We report a
oneꢀstep synthesis of these ligands, alongside pyalkH, on a multiꢀgram scale from inexpensive
starting materials. We have also developed a new scalable procedure for the isolation of pyalkH
that avoids the need for chromatography, making largeꢀscale production of this ligand
commercially viable.
TOC TEXT
A series of novel polydentate N,Oꢀdonor ligands strongly favour formation of polynuclear
complexes.
TOC

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INTRODUCTION
Polynuclear complexes of redox active metals are of current interest1 because they can be
effective catalysts for multielectron reactions,2 such as water oxidation. Indeed, the oxygenꢀ
evolving complex in photosynthesis, as well as several synthetic waterꢀoxidation catalysts
(WOCs) contain multiple metal centers.3 Polynuclear complexes of the first row require
appropriate ligand frameworks2b,4 to avoid ligand lability. Multiply chelating polydentate ligands
may help stabilize the cluster and prevent ligand loss on redox cycling.5 At the same time,
however, synthesis of large, intricate ligands can be a very timeꢀconsuming and costly process,
offsetting the benefit of using cheap, firstꢀrow metals. An ideal ligand would bear the desirable
functionality while at the same time be easily synthesized from inexpensive precursors.
The oxidationꢀresistant, bidentate pyalkH ligand, 2ꢀ(2ꢀpyridyl)ꢀisopropanol (Scheme 1), has
proved useful in stabilizing unusually high oxidation states,6 such as Rh(IV),7 in which cases it
deprotonates to give the alkoxide (pyalk) on coordination. Catalysis is also seen in the case of
our iridium pyalk complexes that act as robust homogeneous and heterogenized WOCs.2c,7a This
ligand has several desirable properties,7a including strong donor power, high resistance to
oxidative degradation, amphiphilic solubility, and stable binding.8 However, its bidentate
character limits the ability to promote stable polynuclear clusters.
The previously reported synthetic procedure for pyalkH (Scheme 1) was suboptimal in
needing costly column chromatography, which negated the benefit of employing cheap
precursors.9 We now report an improved procedure for pyalkH that eliminates any
chromatography and allows straightforward moleꢀscale production of the ligand using standard
laboratory equipment. This advancement is particularly significant as pyalkH has found
commercial applications.ref
Scheme 1. Synthesis of pyalkH via Grignard methylation.
In an important advantage of the improved process, we can now isolate a series of new
polydentate ligands related to pyalkH (Scheme 2), which are formed during its synthesis. These
aliphaticꢀbackbone tetradentate dimers and hexadentate trimers have the same functionality as
pyalkH but with higher denticity, allowing for binding of multiple metals. Due to the structural
constraints, it is practically impossible for any of these ligands to bind to a single metal center
with more than 3 out of 4 or 4 out of 6 coordinating groups, which encourages coordination to
additional metal centers. Furthermore, the deprotonated alkoxo groups are very electronꢀrich, and
as a result are commonly observed to coordinate to two metal centers. The combination of these

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two features makes these ligands especially suitable for the preparation of transition metal
clusters.
Several stable oligonuclear complexes of these ligands with firstꢀrow metals (Mn, Co, Ni,
and Cu) have now been characterized, demonstrating robust cluster formation. We report a
procedure for preparing these ligands on a multiꢀgram scale in a single step, along with a
significantly improved isolation procedure for the original pyalkH ligand.
Scheme 2. Isolated oligomeric pyalkH derivatives. Top: Generalized polymer structure
showing the relationship between the ligands: pyalkH (n = 1), mD and rD (n = 2), mmT,
rrT, and mrT (n = 3). Chiral species isolated as racemic mixtures.
RESULTS AND DISCUSSION
Ligand synthesis
We were initially alerted to the existence of the oligomeric pyridinols of Scheme 2 by the
unexpected formation of crystals of 1 (Figure 1) in a reaction between CoCl2 and an excess of
crude pyalkH ligand that had not been purified by column chromatography. Prior reported
ligand separation conditions (ethyl acetate/hexane) result in very broad TLC bands that seem

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consistent with the sole presence of 2ꢀacetylpyridine starting material and pyalkH. Adding 1%
triethylamine to the eluent improved the separation, resulting in the resolution of two new elution
bands following the pyalkH band. A large scale synthesis showed these to be the rac and meso
diastereomers of the methyleneꢀbridged dimer structure (Me{C(OH)pyCH2}nH, n = 2), rD (first
band) and mD (second band) respectively (Scheme 2). At larger scales, three additional lesser
bands are revealed, and proved to be the three trimer diastereomers mmT, rrT, and mrT
(Scheme 2).
N
Co
N
N
Co
N
O
O
O
O
O
O
O
1
Figure 1. Schematic and crystallographic structure of complex 1. Thermal ellipsoids drawn at
50% probability level; hydrogen atoms are omitted for clarity.
Formation of these oligomers can be rationalized in terms of common Grignard side
reactions. Scheme 3 shows how deprotonation of 2ꢀacetylpyridine by the Grignard to form the
enolate anion, followed by an aldol coupling with 2ꢀacetylpyridine could give a
monodeprotonated ketol (aD) that could then be methylated by the Grignard to mD or rD. A
second aldol reaction between aD and another enolate could form a ketoꢀdiol (not isolated)
followed by methylation to give rrT, mrT or mmT (Scheme 4). This aldolꢀchain reaction also
presumably forms heavier oligomers, but these could not be isolated due to diminishing
abundance and increased polarity and isomer multiplicity. In addition to the saturated pyridinols,
product mixtures also contained 2ꢀacetylpyridine and aD, even when a considerable excess of
Grignard reagent was added, implying that their deprotonated states cannot undergo methylation.

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Scheme 3. Proposed reaction mechanism leading to the formation of the dimeric ligands
mD and rD.
Scheme 4. Proposed ligand synthesis reaction roadmap leading to the various observed
products, with ranges of yields based on various reaction conditions attempted.
OH
O
Methylation
MeMgCl
N
N
H2O
Hpyalk,
30-70%
Deprotonation
Dimeric analogues
MeMgCl
OH
OH
O
N
O-
O
O
O
Py
Py
Methylation
MeMgCl
N
N
N
N
mD
, 3-11%
-
H2O
CH2
Py
OH
H2O
Aldol
2-acetylpyridine, 10-
50%
OH
Py
(starting material)
rD
, 5-19%
O
Deprotonation
N
-
Trimeric analogues
MeMgCl
CH2
Aldol
OH
OH
OH
heat
Py
Py
Py
mmT
, <1%
O-
O-
O-
O
-
Py
OH
Py
OH
OH
O
Methylation
N
N
N
N
H2O
MeMgCl
Py
H2O
OH
Py
Py
O
rrT
, <1%
aD,
0-20%
N
OH
Py
OH
Methylation
Aldol
Py
OH
Py
...
Further products not isolated
mrT
, <1%
While we obtain the oligomers from a simple oneꢀstep reaction, their yields are fairly low
and variable. However, despite various attempts to selectively prepare them in greater yields, we
were ultimately unable to improve on the original conditions, yielding 20ꢀ30% combined
oligomers (a more detailed discussion is presented in SI). Given the low cost of the starting
materials, we favored performing the oneꢀstep Grignard reaction on a large scale despite the
limited oligomer yield, since the primary byproduct, pyalkH, is also a useful ligand. However,

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this strategy would not be feasible if the entire product mixture had to be separated by
chromatography, as previously reported.10 We found that largeꢀscale workup is facilitated by first
removing the monomeric species, pyalkH and 2ꢀacetylpyridine (which account for the majority
of the crude material), by distillation under reduced pressure, followed by oligomer separation in
a smallerꢀscale chromatography step. PyalkH and acetylpyridine can then be efficiently
separated on a large scale by exploiting their pKa difference. Titration with acid allows selective
protonation of pyalkH, after which the two can be separated via an aqueous/organic partition
(e.g., H2O/EtOAc). Neutralized pyalkH can be recrystallized (see Experimental), giving highly
pure product, while the 2ꢀacetylpyridine extract can be recycled. This simple and scalable
isolation protocol allows for facile preparation of large quantities of this useful ligand at a multi
mole scale.
The remaining distillation residue could be separated by silica gel chromatography using
an incrementally varied mixture of hexane, ethyl acetate, acetone, and triethylamine, (See
Experimental) giving two dimer and three trimer species, all diastereomers arising from the
chirality of the quaternary carbons. The two dimers are the isotactic meso compound mD and the
syntactic racemo compound rD, while the three trimers are the isotactic meso, meso mmT, the
syntactic racemo, racemo rrT, and the atactic meso, racemo mrT (Scheme 2). Proton and
carbon NMR spectroscopy shows the expected peak patterns, with the methylene protons being
the most diagnostic (see SI). For mD, these give a geminally coupled diastereotopic pattern,
while for rD they appear as a singlet due to the twofold rotational symmetry. Both isomers show
a single pyridine and methyl signature due to their symmetry. Trimer rrT has one set of split
diastereotopic methylene protons, though with smaller splitting than for mmT, while mrT has
one weakly split and one strongly split set. As expected by symmetry, compounds mmT and rrT
show two pyridine and one methyl signals, while mrT gives three pyridyl and two methyl
signals.
Although these compounds are initially isolated as highly viscous yellow or brown oils,
letting the concentrated material stand undisturbed eventually resulted in spontaneous
crystallization, ranging in time from less than one day for mD and mmT to approximately one
year for mrT. Only once this spontaneous crystallization had occurred for a specific compound
could it be readily recrystallized in subsequent batches to give highly pure material (though
uncrystallized material is sufficiently pure for use), no doubt as a result of seeds of the material
being present in the laboratory. Xꢀray crystallography confirmed the expected structures (Figure
2).

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Figure 2. Crystal structures of the oligomeric ligands (50% probability). Hydrogen atoms other
than OH protons have been omitted for clarity.
Synthesis of metal complexes
The ligand series reacted with divalent Cu, Ni, Co and Mn salts to give oligonuclear
complexes of different nuclearities depending on the conditions. As expected10 in all complexes
besides 12 the OH groups are deprotonated and bind as a mixture of terminal and bridging
alkoxides. All compounds are stable to handling at ambient conditions, and the stability of
complexes 1-4, 6, 8, and 12 (shown below) in solution was checked using UVꢀvisible
spectroscopy. Absorption spectra of these complexes remain unchanged even after solutions
have been allowed to stand for one week, highlighting their stability. Another advantage of these
ligands is the favorability of crystallization of their coordination complexes using standard
techniques (See experimental for details), which allows for easy purification.
Copper
Three different Cu(II) complexes (Scheme 5) were accessible by varying the conditions
(see Experimental). For Cu mD complexes, the nuclearity of the product was dependent on the
solvent: acetonitrile gave monomer 2 while methanol gave dinuclear cluster 3, even if the
starting material stoichiometries are swapped. Square planar 2 features an internal OꢀH···O
(H···O; 1.79 Å) hydrogen bond between the pendant mD alcohol groups and the basic
coordinated alkoxide. Dinuclear complex 3 is inversionꢀsymmetric and contains a diamond core
formed by two fiveꢀcoordinate copper centers with bridging alkoxides, bound asymmetrically to
each Cu atom. A strong JahnꢀTeller distortion is present as would be expected, with one Cuꢀ
bridging O bond at 1.925(2) Å and the other at 2.300(2) Å. Coordination with rD instead of mD

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gives 4, in which the two Cu centers are square planar and unbridged rather than fiveꢀcoordinate
(CuꢀCu distance >3 Å). Complexes 2ꢀ4 are pale blue in color, with λmax between 592 and 605 nm
(Table S2). The complexes all display redox features at oxidizing potentials in cyclic
voltammetry experiments (Figure 4, Table S1). While 4, with spatially separated Cu centers,
shows one oxidation wave, 3 shows two separate features, consistent with coupling of the Cu
centers (Figure 4). The monomeric complex 2 also shows only one quasiꢀreversible oxidation
wave (See SI). Based on their potentials, we propose these features are CuII/III couples; however,
work on isolating oxidized species is ongoing,
Scheme 5. Synthesis of copper complexes with mD and rD.
Cu(OAc)2
mD
2 equiv.
acetonitrile
rD
1 equiv.
methanol
1 equiv. mD
methanol
N
O
N
N
O
HO
O
N
Cu
Cu
N
O
O
N
O
Cu
Cu
N
Cu
O
N
O
N
O
O
N
OH
N
N
4
2
3
Figure 3. Crystal structures of the copper complexes 2ꢀ4 (50% probability). Hydrogen atoms
other than OH protons have been omitted for clarity.

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Figure 4. Cyclic voltammograms of complexes 3 and 4. Experiments run in aqueous KNO3
(0.1M) with boron doped diamond working electrode and Ag/AgCl reference electrode.

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Manganese
Manganese complexes of polydentate N,O ligands are effective in catalytic oxygen
evolution,3a,11 therefore we sought to investigate these new ligands. Using mD or mmT and
acetonitrile and methanol as solvents, we prepared five polynuclear Mn complexes from simple
Mn(II) salts (Scheme 6), utilizing H2O2 to afford less labile high oxidation states of Mn. Treating
MnCl2 with mD gives two clusters, 5 and 6, which are separable by extraction and precipitation.
The Mn(III,III,III)ꢀtrimer 5 has a Mn3(mD)3O core reminiscent of the Mn(III)ꢀacetate trimer. It is
a partial cubane and distantly resembles the Mn4O4Ca oxygenꢀevolving complex in
photosystem II. The carbonateꢀbridged dimerꢀofꢀdimers 6 was assigned as a Mn(II, II, III, III)
species, based on charge balance and bond lengths. The Mn(II) and Mn(III) centers are bound to
two and four alkoxides, respectively, and the Mn(III) centers have JahnꢀTeller axes along the
MnꢀN direction. Complex 6 is the thermodynamic product, and can be formed in higher yields
by heating the reaction mixture for several hours. Because we were able to selectively prepare 6,
we explored its redox properties (Figure S1); its cyclic voltammagram shows two quasiꢀ
reversible redox features at 0.714 and 1.270 V vs. NHE, respectively (Table S1). Additionally,
we found that applying a fixed potential of 1 V vs. NHE to 6 in an aqueous KNO3 solution gives
quantitative formation of Mn(II,IV)ꢀdimer 7 (Scheme 6), in which the carbonate bridge has been
hydrolysed. The bond length data and the absence of JahnꢀTeller distortion is consistent with a
Mn(II,IV) rather than a Mn(III,III) assignment. We could also prepare a similar Mn(II,IV)
complex, 8, with bridging acetate groups, by starting with Mn(OAc)2 (see Experimental). Using
the trimeric ligand mmT with MnCl2 gives two products, the linear trinuclear Mn(II,III,III)ꢀ
complex 9 and the related dinuclear Mn(III,III)ꢀcomplex 10. Complexes 6 and 8 were further
characterized by UVꢀvisible spectroscopy, and showed λmax of 411 and and 475 nm, respectively
(Table S2).

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Scheme 6. Formation of manganese complexes with mD and rrT.
2+
N
N
O
IV
Mn
O
II
O
N
N
Mn
O
O
O
O
O
IV
Mn
O
O
II
Mn
N
+
O
N
O
N
N
N
8
N
O
O
N
III
Mn
N
N
III
O
Mn
Cl
Cl
II
Mn
O
O
O
O
III
Mn
O
O
III
mD
1.2 equiv.
X = CH3COO-
O
N
Mn
N
O
III
Mn
O
N
N
N
O
N
N
mT
iv. m
mD
1.2 equiv.
X = Cl
1.2 equ
Cl
X =
9
MnX2
Acetonitrile/Methanol
5
+
+
N
2+
N
N
N
N
N
N
O
III
Mn
N
O
O
N
O
III
O
III
Mn
O
II
Mn
O
Mn
N
O
O
IV
O
O
OH2
OH2
II
Mn
bulk electrolysis
O
O
III
Mn
O
II
Mn
O
O
O
Mn
N
O
O
O
N
O
N
N
N
N
N
N
10
6
7
Figure 5. Crystal structures of the manganese complexes 5ꢀ10 drawn at the 50% probability
level. Hydrogen atoms have been omitted for clarity, except when present on alcohol groups.

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Nickel and Cobalt
We also prepared complexes of mD using Ni and Co. Ni monomer and dimer species
were prepared as for Cu, with the same solvent dependence: acetonitrile gives monomer 11,
while methanol gives dinuclear 12 (Scheme 7). The latter is the only compound in which we find
protonated alcohol ligands. The cyclic voltammogram of dimeric complex 12 contains a
reversible redox feature at 0.870 V vs. NHE and an irreversible feature at 1.164 V vs. NHE
(Table S1).
Treating CoCl2 with mD followed by addition of Na2CO3 and H2O2 gives the Co(III,III)
dimer (1, described above) in good yield. This carbonateꢀcapped complex is remarkably stable,
resisting loss of mD or carbonate in strong acid (pꢀtoluenesulfonic acid) at elevated temperature
(ca. 100oC). Treating Co(OAc)2 with mmT also affords a dinuclear Co(III,III) cluster, 13 (See
SI and Experimental for details). Complexes 1 and 12 were characterized by UVꢀvisible
spectroscopy and give λmax values of 535 and 602 nm, respectively (Table S2).
Scheme 7. Formation of nickel complexes with mD.
+1
N
HO
N
N
Ni
N
O
N
1 equiv.mD
mD
2 equiv.
O
HO
HO
O
O
Ni(OAc)2
Ni
Ni
CH3
O
methanol/
acetonitrile
N
acetonitrile
O
N
OH
N
12
11
Figure 6. Crystal structures of the nickel complexes 11ꢀ12 (50% probability). Hydrogen atoms
other than in OH groups have been omitted for clarity.
Conclusions
We report a family of polynucleating pyridineꢀalkoxide ligands, dimeric and trimeric
relatives of the oxidation resistant pyalkH ligand, which has previously been shown to be very
effective for oxidation catalysis and for stabilization of unusually high oxidation states. We also
report a new purification technique for pyalkH itself, allowing large quantities of pure ligand to
be synthesized from cheap starting materials. Finally, we have successfully synthesized and
characterized several firstꢀrow metal complexes of the new ligands. The ligands are versatile as

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they have been shown to form stable clusters with various metals (Co, Mn, Ni and Cu) under
different conditions.
Our ligand synthesis involves only one step and uses very cheap starting materials (2ꢀ
acetylpyridine and methylmagnesium halide), features noteworthy for the preparation of larger,
polydentate ligands. The primary byproducts of this process are pyalkH, a highly useful ligand
in its own right, and 2ꢀacetylpyridine, recyclable starting material. Thanks to our separation
protocol, these two species can now be isolated on a large scale without chromatography,
simplifying the separation of the oligomeric ligands.
These new ligands have a natural tendency to form polynuclear clusters with firstꢀrow
transition metals. Such polynuclear structures hold promise for the development of firstꢀrow
transition metal catalysts. Redox properties of the clusters presented, higher oxidation states,
catalytic studies will be the subject of future reports.
EXPERIMENTAL METHODS
Physical Methods
NMR Spectroscopy. NMR spectra (1H and 13C {1H}) were recorded on an Agilent DD2 600 MHz
spectrometer equipped with a chilled probe. Chemical shifts are reported after calibration with
solvent residual peaks.
UV−Visible Spectroscopy. Absorption spectra were collected using a Cary 50 spectrophotometer
for 0.1 mM solutions in dichloromethane, water and methanol.
FTIR spectroscopy. All IR spectra were recorded on a Thermo Scientific Nicolet 6700
spectrometer outfitted with a Smart Orbit diamond ATR cell. Powder samples of pure analyte
were pressed onto the cell window.
HighꢀResolution Mass Spectroscopy (HRMS). Mass spectrometry analyses were performed by
the Mass Spectrometry and Proteomics Resource of the W.M. Keck Foundation Biotechnology
Resource Laboratory at Yale University, using a 9.4 T Bruker Qe FTꢀICR MS instrument in
positive ion mode.
Electrochemistry. Cycliv voltammograms were performed using standard threeꢀelectrode
measurements carried out on a Princeton Applied Research VersaSTATꢀ4 or a Pine AFCBP1
bipotentiostat. Aqueous experiments were performed with a boronꢀdoped diamond working
electrode, a platinum wire counter electrode and a saturated aqueous Ag/AgCl reference
electrode (+0.197 V vs. NHE, purchased from Bioanalytical Systems, Inc.). For experiments in
organic solvent, a glassy carbon working electrode and platinum wire counter electrode were
used. A silver wire was used as a pseudoreference electrode and the potential was referenced
using a ferrocene internal standard.

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General
Unless otherwise specified, solvents and reagents were purchased from commercial
sources and used as received. Full crystallographic information for all novel ligands and
complexes is reported in the SI.
Synthesis of pyalkH, aD, mD, rD, mmT, mrT, rrT
To a 3ꢀneck 4ꢀliter round bottom flask were added 112 mL (1 mol) 2ꢀacetylpyridine and
110 mL dry degassed THF. The solution was cooled in an ice bath and 350 mL of a 3 M THF
solution of MeMgCl was added dropwise over 3 hours under a flow of nitrogen. During addition,
the mixture was stirred rapidly (~1000 RPM) with a motor stirrer, while the temperature was
monitored to not exceed 10 ˚C. 200 mL of additional THF was added during the course of
MeMgCl addition to minimize formation of solids and facilitate mixing. Subsequently, solids
were manually scraped off the walls of the flask and readded to the mixture, which was then
stirred for 1 hour at room temperature. Approximately 1 L water was added and the resulting
slurry mixed well, after which the products were extracted with approx. 1 L ethyl acetate. The
organic extract was dried with MgSO4 and the solvent was removed under reduced pressure.
Note: pyalkH is volatile; leaving the mixture under prolonged reduced pressure after the solvent
is removed may result in some product loss.
Isolation of 2-(2-pyridinyl)-2-propanol (pyalkH)
The compounds pyalkH and 2ꢀacetylpyridine were distilled simultaneously from the
o
o
resulting mixture under reduced pressure at ca. 95 C (pressureꢀdependent, ca. 180 C at 150
mbar). The distillate was diluted with ca. 200 mL water and layered with ca. 200 mL ethyl
acetate. The mixture was titrated with H2SO4 until no pyalkH remained in the organic layer; the
aqueous layer was separated and washed twice with ca. 200 mL ethyl acetate. The aqueous layer
was again layered with ethyl acetate and partially neutralized with NaHCO3 until the aqueous
layer was mostly free of 2ꢀacetylpyridine (this step to be performed only if necessary).
Monitoring the composition of the two phases was done by TLC in 30% ethyl acetate and 1%
triethylamine in hexane (2ꢀacetylpyridine elutes first); aqueous aliquots were first neutralized
with excess NaHCO3 and extracted into ethyl acetate.
The aqueous phase was neutralized with excess NaHCO3, extracted into ethyl acetate,
and evaporated under reduced pressure. The viscous oil was diluted with ca. 500 mL of pentane
o
and crystallized by cooling to approx. 10 C and introducing a seed crystal, followed by further
o
cooling to –25 C overnight. Without warming, the supernatant was removed and the crystals
washed three times with cold pentane, warmed to room temperature, and dried under ambient
conditions. Larger crystals of high purity can be obtained by recrystallization in pentane via slow
evaporation at room temperature for several days in the presence of a seed crystal. Note: pyalkH
obtained by distillation may persist in supercooled/supersaturated liquid form in the absence of
seed crystals. Seed crystals can most often be formed in agitated evaporating pentane solutions.
Yield: 43.84 g, 32%.

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Separation of mD, rD, mmT, rrT, mrT
The viscous dark residue remaining after distillation was extracted with several portions
of hexane until only dark insoluble solids remained. This solid was dissolved in minimal DCM,
precipitated with hexane and filtered. This process was repeated 2ꢀ3 times. The solutions were
combined and concentrated in vacuo to give a light brown oil (note: initial reduction of these
colored impurities is nontrivial as they will coꢀelute with the products and interfere with their
final purity and ability to crystallize). The oil (ca. 23 g) was diluted with a minimum of
dichloromethane and loaded into a large (10 x 40 cm) silica gel column presoaked with hexane
and run with a solution of 30% ethyl acetate and 1 % triethylamine in hexane until leftover 2ꢀ
acetylpyridine, followed by pyalkH, eluted. The amount of ethyl acetate in the eluent was
gradually increased to 66% as rD and then mD eluted. For elution of the trimeric species, the
eluent was gradually spiked with acetone up to 50%, while keeping a 2:1 ratio of ethyl
acetate:hexane. Compound rrT elutes first at ca. 10% acetone, followed by mrT and finally
mmT. All of the compound fractions were evaporated under reduced pressure and initially
isolated as yellow or brown oils. The pyalkH was combined with the distilled fraction and
purified by repeated crystallizations from concentrated pentane solution at low temperature. The
compounds were crystallized from a saturated DCM/octane solution. The crystallization
occurred readily only after spontaneous crystallization from the neat oil had occurred. The
timescale of this spontaneous crystallization varied from several hours for mD and mmT to
several weeks or months for the other ligands.
1
mesoꢀ2,4ꢀdi(2ꢀpyridynyl)ꢀ2,4ꢀpentanediol•H2O (mD): Yield 5.03 g, 4.0%. H NMR (600 MHz,
Chloroformꢀd) δ 8.03 (m, 2H, ArꢀH), 7.31 (m, 2H, ArꢀH), 7.17 (m, 2H, ArꢀH), 6.79 (m, 2H, Arꢀ
H), 6.25 (s, 2H, OꢀH), 3.03 (d, J = 14.8 Hz, 1H, HCꢀH), 2.55 (d, J = 14.8 Hz, 1H, HCꢀH), 1.46
(s, 6H, CH3). 13C NMR (151 MHz, Chloroformꢀd) δ 164.43, 146.91, 136.04, 120.89, 120.07,
75.75, 50.55, 32.91. HRMS (FTꢀICR): Calculated [C15H18N2O2]Na+: 281.1266. Found:
281.1270.
1
racemoꢀ2,4ꢀdi(2ꢀpyridynyl)ꢀ2,4ꢀpentanediol•H2O (rD): Yield 8.24 g, 6.5%. H NMR (600 MHz,
Chloroformꢀd) δ 8.50 (m, ArꢀH, 2H), 7.69 (m, ArꢀH, 2H), 7.60 (m, ArꢀH, 2H), 7.16 (m, ArꢀH,
2H), 6.22 (s, OꢀH, 2H), 2.54 (s, CH2, 2H), 1.16 (s, CH3 6H). 13C NMR (151 MHz, Chloroformꢀd)
δ 166.48, 147.56, 137.22, 121.88, 119.61, 75.75, 51.18, 30.74. HRMS (FTꢀICR): Calculated
[C15H18N2O2]H+: 259.1447. Found: 259.1445.
1
meso,mesoꢀ2,4,6ꢀtri(2ꢀpyridynyl)ꢀ2,4,6ꢀheptanetriol (mmT): Yield 440 mg, 0.4%. H NMR (600
MHz, Chloroformꢀd) δ 7.88 – 7.86 (m, ArꢀH, 2H), 7.76 – 7.73 (m, ArꢀH, 1H), 7.25 – 7.22 (m,
ArꢀH, 2H), 7.10 (t, J = 1.0 Hz, ArꢀH, 1H), 7.08 (t, J = 1.1 Hz, ArꢀH, 1H), 6.79 – 6.76 (m, ArꢀH,
1H), 6.69 – 6.66 (m, ArꢀH, 2H), 6.63 – 6.61 (m, ArꢀH, 1H), 6.39 – 6.36 (m, ArꢀH, 1H), 3.11 (d, J
= 14.6 Hz, HCꢀH, 2H), 2.48 (d, J = 14.6 Hz, HCꢀH, 2H), 1.39 (s, CH3, 6H). 13C NMR (151 MHz,
Chloroformꢀd) δ 164.27, 162.33, 146.92, 146.72, 135.53, 134.58, 120.60, 120.37, 119.82,
119.60, 79.82, 75.78, 52.09, 32.90. HRMS (FTꢀICR): Calculated [C22H25N3O3]Na+: 402.1794.
Found: 402.1779.

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1
racemo,racemoꢀ2,4,6ꢀtri(2ꢀpyridynyl)ꢀ2,4,6ꢀheptanetriol (rrT): Yield 730 mg, 0.6%. H NMR
(600 MHz, Chloroformꢀd) δ 8.53 – 8.49 (m, ArꢀH, 1H), 8.43 – 8.38 (m, ArꢀH, 2H), 7.78 – 7.75
(m, ArꢀH,1H), 7.69 – 7.64 (m, ArꢀH,1H), 7.57 (td, J = 7.7, 1.8 Hz, ArꢀH, 2H), 7.39 (dt, J = 7.9,
1.1 Hz, ArꢀH, 2H), 7.15 – 7.11 (m, ArꢀH,1H), 7.07 – 7.02 (m, ArꢀH, 2H), 2.40 (d, J = 14.7 Hz,
HCꢀH, 2H), 2.21 (d, J = 14.8 Hz, HCꢀH, 2H), 1.01 (s, CH3, 6H). 13C NMR (151 MHz,
Chloroformꢀd) δ 166.61, 165.98, 147.67, 147.26, 136.72, 136.66, 121.59, 121.36, 120.40,
119.18, 79.73, 75.44, 51.37, 29.85. HRMS (FTꢀICR): Calculated [C22H25N3O3]Na+: 402.1794.
Found: 402.1783.
1
meso,racemoꢀ2,4,6ꢀtri(2ꢀpyridynyl)ꢀ2,4,6ꢀheptanetriol•H2O (mrT): Yield 890 mg, 0.7% H
NMR (600 MHz, Chloroformꢀd) δ 8.57 – 8.47 (m, ArꢀH, 1H), 8.20 – 8.11 (m, ArꢀH, 1H), 8.01 –
7.91 (m, ArꢀH, 1H), 7.69 (m, 1H), 7.55 (m, ArꢀH, 1H), 7.29 (m, ArꢀH, 1H), 7.23 – 7.13 (m, Arꢀ
H, 4H), 6.79 – 6.71 (m, ArꢀH, 2H), 2.89 (d, J = 14.6 Hz, CH2, 1H), 2.59 (d, J = 14.7 Hz, CH2,
1H), 2.48 (d, J = 14.7 Hz, CH2, 1H), 2.13 (d, J = 14.6 Hz, CH2, 1H), 1.25 (s, CH3, 3H), 1.01 (s,
CH3, 3H). 13C NMR (151 MHz, Chloroformꢀd) δ 166.60, 164.84, 164.22, 147.61, 147.51,
147.12, 137.14, 135.79 (d, J = 4.9 Hz), 121.81, 120.87, 120.67, 120.60, 119.80, 119.46, 80.10,
75.90, 75.83, 52.79, 50.61, 33.09, 29.93, 29.85.
Attempted isolation of aD
An aliquot (ca. 50 mg) of crude reaction product (prior to distillation) was loaded on a 1
mm thick silica gel TLC plate and eluted with 1:1 ethyl acetate/hexane with 1% triethylamine.
The third major band (poorly separated aD and rD) was scraped and extracted with acetone, and
the resulting residue eluted a second time under the same conditions. The leading edge of the
major band pair (rD trailing) was collected. This yielded aD with ca. 20% 2ꢀacetylpyridine.
While the two constituents separate very distinctly, obtaining a pure sample of aD was not
possible as it partially degrades to 2ꢀacetylpyridine under ambient conditions. NMR spectra of
the mixture were compared to those of 2ꢀacetylpyridine to identify the peaks of aD, listed below.
3ꢀhydroxyꢀ1,3ꢀdi(pyridinꢀ2ꢀyl)butanꢀ1ꢀone (aD): 1H NMR (400 MHz, CDCl3) δ = 8.68 (ddd, J =
4.8, 1.7, 0.9 Hz, 1H, ArꢀH), 8.40 (ddd, J = 4.9, 1.7, 1.0 Hz, 1H, ArꢀH), 7.92 (dt, J = 7.9, 1.1 Hz,
1H, ArꢀH), 7.81 (td, J = 7.7, 1.8 Hz, 1H, ArꢀH), 7.71ꢀ7.63 (m, 2H, ArꢀH), 7.45 (ddd, J = 7.5, 4.8,
1.2 Hz, 1H, ArꢀH), 7.08 (ddd, J = 7.1, 4.9, 1.5 Hz, 1H, ArꢀH), 5.47 (s, 1H, OH), 4.29 (d, J = 16.6
Hz, 1H, CH2), 3.56 (d, J = 16.7 Hz, 1H, CH2), 1.61 (s, 3H, CH3).13C{1H} NMR (101 MHz,
CDCl3) δ = 201.87 (C=O), 166.02 (Ar), 153.54 (Ar), 148.90 (Ar), 148.05 (Ar), 137.18 (Ar),
136.75 (Ar), 127.35 (Ar), 122.07 (Ar), 121.75 (Ar), 119.45 (Ar), 74.91 , 48.74 , 29.97. HRMS
(FTꢀICR) calc. [C14H14N2O2]H+: 243.1134 Found: 243.1129.
Preparation of complexes
Preparation of Co2(mD)2CO3•2H2O (1). To a stirred solution of mD (62 mg, 0.24 mmol) in
acetonitrile (2 mL), a solution of CoCl2.2H2O (33 mg, 0.2 mmol) in acetonitrile was added
dropwise. 2ꢀ4 drops of methanol were added to completely dissolve the reaction mixture. The

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reaction mixture was stirred for 15 min and afterwards a 30% H2O2 solution (0.03 mL) was
added. The color of the solution changed to blue. After stirring for 20 min, K2CO3 (83 mg, 0.6
mmol) was added to the reaction mixture. The solution was stirred overnight and the color
changed to yellow brown. The solvent was removed and the crude product was dissolved in
dichloromethane and filtered. The volume of the filtrate was reduced and brown crystals of 1
1
were grown by layering the filtrate with pentane at room temperature. Yield: 52 mg, 70%. H
NMR (600 MHz, Methylene Chlorideꢀd2) δ 8.96 (d, J = 5.8, ArꢀH, 2H), 8.71 (d, J = 5.5 Hz, Arꢀ
H, 2H), 7.59 (m, ArꢀH, 2H), 7.53 (m, ArꢀH, 2H), 7.25 (m, ArꢀH, 2H), 7.04 – 6.99 (m, ArꢀH, 4H),
6.60 (d, J = 7.9 Hz, ArꢀH, 2H), 2.32 (d, J = 15.0 Hz, HCꢀH, 2H), 2.10 (d, J = 15.0 Hz, HCꢀH,
2H), 1.48 (s, CH3, 7H), 1.36 (s, CH3, 6H).
Preparation of Cu(mD)2 (2). To a stirred solution of mD (51 mg, 0.2 mmol) in acetonitrile (2
mL), a solution of Cu(OAc)2•H2O (20 mg, 0.1 mmol) in acetonitrile was added dropwise. The
reaction mixture was stirred for 20 min and K2CO3 (41 mg, 0.3 mmol) was added to the reaction
mixture. The solution was stirred overnight. The final solution was light blue in color. The
solvent was removed and the crude product dissolved in acetonitrile and filtered. The volume of
the filtrate was reduced and blue crystals of 2 were grown by slow evaporation at room
temperature. Yield: 29 mg, 50%.
Preparation of Cu2(mD)2•2H2O (3). To a stirred solution of mD (62 mg, 0.24 mmol) in methanol
(2 mL), a solution of Cu(OAc)2•H2O (40 mg, 0.2 mmol) in methanol was added dropwise. The
reaction mixture was stirred for 20 min resulting in a blue solution. K2CO3 (83 mg, 0.6 mmol)
was added to the reaction mixture and left to stir overnight. The final solution was dark blue in
color. The solvent was removed and the crude product dissolved in dichloromethane and filtered.
The volume of the filtrate was reduced and purple crystals of 3 were grown by layering the
filtrate with pentane at room temperature. Yield: 30 mg, 45%. HRMS (FTꢀICR): Calculated
[C30H34CuN4O4]Na+: 600.1774. Found: 600.1156.
Preparation of Cu2(rD)2•2H2O (4). To a stirred solution of rD (62 mg, 0.24 mmol) in methanol
(2 mL), a solution of Cu(OAc)2•H2O (40 mg, 0.2 mmol) in methanol was added dropwise. The
reaction mixture was stirred for 20 min resulting in a blue solution. K2CO3 (83 mg, 0.6 mmol)
was added to the reaction mixture and left to stir overnight. The final solution was dark blue in
color. The solvent was removed and the crude product dissolved in dichloromethane and filtered.
The volume of the filtrate was reduced and light purple crystals of 3 were grown by layering the
filtrate with pentane at room temperature. Yield: 25 mg, 40%. HRMS (FTꢀICR): Calculated
[C30H32Cu2N4O4]H+: 639.1094. Found: 639.1085.
Preparation of [Mn3(mD)3µ3ꢀO][BArF]•H2O (5). To a stirred solution of mD (62 mg, 0.24
mmol) in acetonitrile/methanol (6:1; 2 mL), a solution of MnCl2ꢀ2 H2O (32 mg, 0.2 mmol) in
methanol was added dropwise. The reaction mixture was stirred for 20 min resulting in a
red/brown solution. H2O2 (6 eq.) and K2CO3 (83 mg, 0.6 mmol) were added and the reaction
mixture was stirred at room temperature for 2 hours. The solvent was removed and the residue
was dissolved in dichloromethane, filtered, and stirred with excess NaBArF4 in dichloromethane
for 1 h. The mixture was filtered, reduced in volume and layered with pentane. Crystals of 5
formed as red plates after slow diffusion over several days at room temperature. Yield: <5%.

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Preparation of Mn4(mD)4CO3•10H2O (6). To a stirred solution of mD (62 mg, 0.24 mmol) in
Acetonitrile/methanol (6:1; 2 mL), a solution of MnCl2•2H2O (32 mg, 0.2 mmol) in methanol
was added dropwise. The reaction mixture was stirred for 20 min resulting in a red/brown
solution. H2O2 (6 eq.) and K2CO3 (83 mg, 0.6 mmol) were added and the reaction mixture was
heated to 45 oC and left to stir overnight. The solvent was removed and the residue was dissolved
in dichloromethane and filtered. The volume of the filtrate was reduced and brown crystals of 6
were grown by layering the filtrate with pentane at room temperature. Yield: 30 mg, 40%.
Preparation of [Mn2(mD)2(H2O)2][NO3]2•6H2O (7). A solution of 6 in 0.1 M KNO3 buffer was
electrolyzed at 1 V vs. NHE using a platinum mesh working electrode for ~1 hour. Red crystals
were grown by slow evaporation of the solution over several days.
Preparation of [Mn4(mD)4(CH3COO)2][PF6]2 (8). To a stirred solution of mD (62 mg, 0.24
mmol) in Acetonitrile/methanol (6:1; 2 mL), a solution of Mn(OAc)2•4H2O (49 mg, 0.2 mmol) in
methanol was added dropwise. The reaction mixture was stirred for 20 min resulting in a
red/brown solution. H2O2 (6 eq.) and K2CO3 (83 mg, 0.6 mmol) were added and the reaction
mixture was stirred overnight at room temperature. The solvent was removed and the residue
was dissolved in dichloromethane and filtered. Excess KPF6 was added and the mixture was
stirred for 1 hour and filtered. The volume of the filtrate was reduced and brown crystals of 8
were grown by layering the filtrate with pentane at room temperature. Yield: 20 mg, 22%.
Preparation of Mn3(mmT)2Cl2 (9) and Mn2(mmT)2 (10). To a stirred solution of mmT (91 mg,
0.24 mmol) in Acetonitrile/methanol (6:1; 2 mL), a solution of MnCl2•2H2O (32 mg, 0.2 mmol)
in methanol was added dropwise. The reaction mixture was stirred for 20 min resulting in a
red/brown solution. H2O2 (6 eq.) and K2CO3 (83 mg, 0.6 mmol) were added and the reaction
mixture was stirred overnight at room temperature. The solvent was removed and the residue
was dissolved in dichloromethane and filtered. The volume of the filtrate was reduced and
crystals of 9 and 10 were grown by layering the filtrate with pentane at room temperature. No
attempt was made to separate 9 and 10 in bulk from the crude mixture (formed in moderate
yield), thus no yields were recorded.
Preparation of Ni(mD)2 (11). To a stirred solution of mD (62 mg, 0.24 mmol) in acetonitrile (2
mL), a solution of NiBF4•6H2O (68 mg, 0.2 mmol) in acetonitrile was added dropwise. The
reaction mixture was stirred for 20 min and K2CO3 (83 mg, 0.6 mmol) was added to the reaction
mixture and left to stir overnight. The final solution was tan in color. The solvent was removed
and the crude product dissolved in dichloromethane and filtered. The volume of the filtrate was
reduced and tan colored crystals of 11 were grown by layering the filtrate with pentane at low
temperature. Yield: ~30 mg, 30%.
Preparation of [Ni2(mD)2(CH3COO)][CH3COO] (12). To a stirred solution of mD (62 mg, 0.24
mmol) in acetonitrile/methanol (3:1) (2 mL), a solution of Ni(OAc)2•4H2O (50 mg, 0.2 mmol) in
acetonitrile/methanol (3:1) was added dropwise. The reaction mixture was stirred for 15 min and
afterwards a 30% H2O2 solution (0.03 mL) was added. After stirring for another 20 min,
NaHCO3 (59 mg, 0.7 mmol) was added to the reaction mixture. The solution was stirred
overnight at 65 °C. The solvent was removed and the crude product was dissolved in
dichloromethane and filtered. The volume of the filtrate was reduced and blue crystals of 12

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were grown by slow evaporation of this solution layered with octane at room temperature. Yield:
38 mg, 50%.
Preparation of Co2(mmT)2•2H2O (13). To a stirred solution of mmT (76 mg, 0.2 mmol) in
acetonitrile (2 mL), a solution of Co(OAc)2.2H2O (49 mg, 0.2 mmol) in acetonitrile was added
dropwise. 2ꢀ4 drops of methanol were added to completely dissolve the reaction mixture. The
reaction mixture was stirred for 15 min and afterwards a 30% H2O2 solution (0.03 mL) was
added. The color of the solution changed to brownish red. After stirring for 20 min, K2CO3 (83
mg, 0.6 mmol) was added to the reaction mixture, and the solution was stirred overnight. The
solvent was removed and the crude product was dissolved in dichloromethane and filtered. The
volume of the filtrate was reduced and brown crystals of 1 were grown were grown by slow
evaporation of this solution layered with octane at room temperature. Yield: 15ꢀ20%.
Supporting Information
NMR and HRMS spectra, cyclic voltammograms, and crystallographic details are
available free of charge via the internet at http://pubs.acs.org.
Acknowledgments
This work (L.S.S, S.B.S, and D.Y.S) was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences under Award Number DEꢀSC0001059 as part of the Argonneꢀ
Northwestern Solar Energy Research (ANSER) Energy Frontier Research Center (spectroscopy
and characterization) and under Award Number DEFG02ꢀ07ER15909 (synthesis).
‡ These authors contributed equally
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