Recyclable chemosensor for oxalate based on bimetallic complexes of a dinucleating bis(iminopyridine) ligand

Article abstract of DOI:10.1039/c4dt00577e

Herein we describe bimetallic di-nickel and di-copper complexes [Ni ₂(L)Br₄] (1) and [Cu₂(L)Br₄(NCMe) ₂] (2) (L = (1E,1′E)-N,N′-(1,4-phenylenebis(methylene)) bis(1-(6-(2,4,6-triisopropylphenyl)pyridin-2-yl)methanimine)) that bind oxalate intramolecularly to form [Ni₂(L)Br₂(C₂O ₄)(NCMe)] (3) and [Cu₂(L)Br₂(C ₂O₄)] (4). For the di-nickel complex 1, oxalate incorporation is accompanied by a significant colour change, from red-pink (1) to deep green (3). Mass spectrometric experiments demonstrate that the compound 1 is selective for oxalate versus related mono- and di-carboxylates tested. Oxalate can be released by the addition of slight excess of calcium bromide that forms insoluble calcium oxalate and restores the original Ni ₂(L)Br₄ species. The product of the oxalate release was crystallized as [Ni₂(L)Br₄]CaBr₂(THF) ₄ species. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00577e

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Recyclable chemosensor for oxalate based
on bimetallic complexes of a dinucleating
bis(iminopyridine) ligand†
Cite this: Dalton Trans., 2014, 43,
7979
J. W. Beattie, D. S. White, A. Bheemaraju, P. D. Martin and S. Groysman*
Herein we describe bimetallic di-nickel and di-copper complexes [Ni2(L)Br4] (1) and [Cu2(L)Br4(NCMe)2]
(2) (L = (1E,1’E)-N,N’-(1,4-phenylenebis(methylene))bis(1-(6-(2,4,6-triisopropylphenyl)pyridin-2-yl)metha-
nimine)) that bind oxalate intramolecularly to form [Ni2(L)Br2(C2O4)(NCMe)] (3) and [Cu2(L)Br2(C2O4)] (4).
For the di-nickel complex 1, oxalate incorporation is accompanied by a significant colour change, from
red-pink (1) to deep green (3). Mass spectrometric experiments demonstrate that the compound 1 is
selective for oxalate versus related mono- and di-carboxylates tested. Oxalate can be released by the
addition of slight excess of calcium bromide that forms insoluble calcium oxalate and restores the original
Ni2(L)Br4 species. The product of the oxalate release was crystallized as [Ni2(L)Br4]·CaBr2(THF)4 species.
Received 24th February 2014,
Accepted 1st April 2014
DOI: 10.1039/c4dt00577e
www.rsc.org/dalton
flexible nature of the ligands will enable reversible binding of
oxalate. For the current study, we chose ligand L featuring
Introduction
Chemical detection of oxalate is of considerable interest due 2,4,6-triisopropylphenyl substituents in the ortho positions of
to the important roles that oxalate plays in food chemistry and the pyridine as its complexes demonstrated significantly
in biological processes.1 Oxalate is frequently encountered as a better solubility than complexes of other bis(iminopyridine)
bridging ligand in transition metal chemistry.2 Therefore, ligands.12c In addition, we anticipated that the bulky groups
bimetallic complexes that are designed to bind oxalate intra- in the 2′ position will lead to the well-defined bimetallic
molecularly can be envisioned as efficient oxalate sensors. The species.13 We focused on the nickel(II) and copper(II) metal
vast majority of bimetallic oxalate complexes feature inter- centres since they are capable of binding oxalate, yet not oxo-
molecular coordination of oxalate.3 Several systems have been philic enough to prevent its removal.
recently reported that bind oxalate intramolecularly.4–8 The
majority of these systems rely on fluorescence indicators. The
design of a system that enables detection of oxalate without
the use of fluorescence indicators9,10 and is also capable of
regeneration is of considerable interest. Toward this end, we
report bimetallic complexes that enable simple colourimetric
detection of oxalate and can be easily recycled.
We and others are investigating bimetallic complexes for
the cooperative binding and activation of small molecules.11
We have recently reported a new family of dinucleating bis-
(iminopyridine) ligands linked by a para-xylylene bridge.12 We
hypothesized that the resulting bimetallic complexes possess
an appropriate cavity size to bind oxalate between the two tran-
sition metals. Furthermore, we postulated that the highly
Results and discussion
Synthetic transformations described in this paper are summar-
ized in Fig. 1. To access di-nickel(II) and di-copper(II) com-
plexes anticipated to serve as oxalate sensors, we treated one
equivalent of the ligand L in tetrahydrofuran (THF) with two
equivalents of the NiBr2(dme) (dme = dimethoxyethane) or
CuBr2 in THF or THF–CH3CN (CH3CN = acetonitrile). The
compounds Ni2(L)Br4 (1) and Cu2(L)Br4 (2) were obtained as
pink (1) and green (2) crystals by vapour diffusion of ether into
the saturated THF and CH3CN solutions in 82% and 75%
yields, respectively. Compounds 1 and 2 were characterized by
1H NMR, solution magnetic measurements (using Evans
method), elemental analysis, X-ray crystallography, IR spectro-
scopy, and mass spectrometry (see below). Well-resolved 1H
NMR spectrum of 1 (Fig. S3†) demonstrates five paramagneti-
cally shifted signals (in the range of 10–77 ppm) that can be
attributed to the five different types of aromatic protons of
Department of Chemistry, Wayne State University, Detroit, Michigan 48202, USA.
E-mail: groysman@chem.wayne.edu
†Electronic supplementary information (ESI) available: Crystallographic data in
CIF format, mass spectra, NMR spectra, UV-vis spectra, and details on the deter-
mination of oxalate binding constant for 3. CCDC 987904–987906,
987908–987909. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt00577e
1
L. In contrast, the H NMR spectrum of 2 is significantly less
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Fig. 2 Structures of complexes 1 and 2. Selected distances and angles
for 1: Ni–Ni 11.309(1) Å, Ni–Br1 2.312(1) Å, Ni–Br2 2.318(1) Å, Ni–N1
2.006(3) Å, Ni–N2 1.980(3) Å, Br1–Ni–Br2 135.0(1)°, N1–Ni–N2 82.9(1)°.
Selected distances and angles for 2: Cu–Cu 11.38(1) Å, Cu–Br1 2.474(1)
Å, Cu–Br2 2.425(1) Å, Cu–N1 1.970(3) Å, Cu–N2 2.227(3) Å, Cu–N3 1.952(3)
Å, Br1–Cu–Br2 151.1(1)°, N1–Cu–N2 79.2(1)°, N1–Cu–N3 174.6°.
Next, we investigated the reactivity of complexes 1 and 2
with oxalate. For the preliminary investigation, we synthesized
(NBu4)2[C2O4],14 that is soluble in polar organic solvents
(e.g. CH3CN). The addition of one equivalent of (NBu4)2[C2O4]
in CH3CN to the red-pink solution of 1 in THF immediately
results in a colour change to green. Subsequent work-up and
crystallization from CH3CN–ether yielded green crystals of the
oxalate-bridged complex [Ni2(L)Br2(μ2-C2O4)(NCMe)] (3) in 78%
yield. Similarly, the reaction of (NBu4)2[C2O4] with bright
yellow-green THF–CH3CN solution of 2 yields dark green com-
pound 4 (83% yield). Compounds 3 and 4 were also character-
ized by elemental analysis, X-ray crystallography, IR
spectroscopy and mass spectrometry. As both 3 and 4 are only
sparingly soluble in common deuterated solvents, we were not
able to determine their solution magnetic moments reliably.
The X-ray structures of 3 and 4 are displayed in Fig. 3. Both
compounds demonstrate intramolecular binding of the
oxalate, with the metal–metal distances being 5.36 Å for 3 and
5.20 Å for 4, that are typical metal–metal separations in the
intermolecular complexes of the corresponding metal–oxalate
complexes.5 Interestingly, the di-nickel complex 3 demon-
strates two different coordination environments at the nickel
centres. Ni1 is hexa-coordinate, due to the coordinated CH3CN
ligand. Ni2 is penta-coordinate, featuring distorted square-
pyramidal geometry. Both copper centers in 4 are penta-
coordinate. However, they demonstrate two different
coordination geometries, with Cu1 being approximately square
Fig. 1 Synthesis of compounds 1 and 2 and their reactivity with oxalate
salts to afford compounds 3 and 4.
informative (Fig. S4†). Solution magnetic moment of 1 (deter-
mined by Evans method) is consistent with four unpaired elec-
trons per molecule of 1 (µobs = 4.83 versus µcalc = 4.90). For the
di-copper complex 2, the solution magnetic moment was deter-
mined to be 2.16 (µcalc = 2.83).
X-ray structures of compounds 1 and 2 are presented in
Fig. 2. X-ray structures of both complexes (see Fig. 2 for the
structures of 1 and 2) notably demonstrate anti geometry of
the two sides of a complex. Accordingly, the compounds
exhibit large metal–metal separations (Ni–Ni is 11.309 Å,
Cu–Cu is 11.380 Å). Both compounds are centrosymmetric
ˉ
(space groups are P21/c and P1), so only half of the molecule
constitutes an asymmetric unit. Whereas both compounds can
be synthesized in the presence of CH3CN, only the di-copper
complex features CH3CN binding to the metal centres. Accord-
ingly, the di-copper complex 2 features distorted square-
pyramidal geometry at the copper(II) centres, whereas the
di-nickel complex 1 has four-coordinate metal centres. CH3CN
ligands bound to the copper centres are found in two alternate
conformations; only one is shown for clarity.
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Fig. 5 Experimental and calculated ESI-MS spectrum of complex 3
in the 1000–1015 region, demonstrating the peak attributed to
[3-Br-CH3CN]+.
notable exception of the strong peak around 1635 cm−1 associ-
ated with the oxalate stretch (1645 cm−1 in sodium oxalate).15
To gain additional information on the nature of com-
pounds 1–4 in solution, we have characterized them by mass
spectrometry (ESI-MS). Di-nickel complexes appear to be sig-
nificantly more stable under MS conditions, allowing the
detection of molecular ions. Thus, compound 1 gives rise to
the molecular ion peak at m/z 1071.5 that is attributed to the
[Ni2(L)Br3]+ (1-Br) (Fig. S5†). In contrast, no corresponding
[Cu2(L)Br3]+ (1-Br) is observed. Instead, only the monometallic
species [Cu(L)Br]+ is detected at m/z 860.3. Compound 3
appears to be particularly stable under MS conditions, giving
rise to two prominent peaks consistent with the mono-cation
Fig. 3 Structures of complexes 3 and 4. Selected distances and angles
for 3: Ni1–Ni2 5.36 Å, Ni1–Br1 2.516(1) Å, Ni2–Br2 2.425(1) Å, Ni1–N1 [Ni2(L)(C2O4)Br]+ ([3-Br-CH3CN]) and the di-cation [Ni2(L)-
(C2O4)]2+ ([3-2Br-CH3CN]). To confirm the nature of these ions,
we further characterized this compound by high-resolution
2.062(2) Å, Ni1–N2 2.170(2) Å, Ni1–N3 2.096(2) Å, Ni1–O1 2.064(3) Å,
Ni1–O2 2.081(3) Å, Ni2–N3 2.033(2) Å, Ni2–N4 2.095(2) Å, Ni2–O3
2.029(3) Å, Ni1–O4 2.033(3) Å, O3–Ni2–Br2 167.6(1)°, O4–Ni2–N2 163.6(1)°.
Selected distances and angles for 4: Cu1–Cu2 5.20 Å, Cu1–Br1 2.396(1)
mass spectrometry, and correlated the obtained spectra with
the expected isotopic distribution. The peak at m/z 1001.2656
Å, Cu2–Br2 2.362(1) Å, Cu1–N1 2.240(2) Å, Cu1–N2 2.027(2) Å, Cu2–N3
2.049(2) Å, Cu2–N4 2.223(2) Å, Cu2–O3 2.019(2) Å, Cu2–O4 1.991(2) Å, (Fig. 5) is consistent with the mono-cation [Ni2(L)(C2O4)Br]+.
O2–Cu2–N2 171.6(1)°, O4–Cu2–N4 132.1(1)°.
The peak at m/z 461.1706 is consistent with the di-cation
[Ni2(L)(C2O4)]2+ (see Fig. S6†).
As compound 1 demonstrates more significant colour
change than compound 2, and can be easily detected in situ by
mass spectrometry, we decided to focus on further exploration
of its reactivity. The reactivity of
1 is not limited to
(NBu4)2[C2O4] in organic solvents. Addition of MeOH, or
MeOH–H2O (4 : 1) solutions of either K2C2O4 or Na2C2O4 to a
1 : 1 solution of 1 in THF–CH3CN leads to the formation of
green solutions containing 3 (based on mass spectrometry),
from which pure 3 can be isolated upon recrystallization.
We followed oxalate incorporation using UV-vis spectroscopy.
UV-vis spectra of compounds 1 and 3 are presented in Fig. 6.
The spectra (inset) clearly demonstrate significant differences
between 1 and 3. Upon titration of 1 with (NBu4)2[C2O4] a
clean disappearance of 1 and emergence of 3 are observed
(Fig. 6).
Fig. 4 IR spectra of complexes 1–4.
pyramidal, and Cu2 being trigonal bipyramidal. For the
Following our studies on the oxalate binding by complex 1,
selected bonds and angles, see Fig. 3. We have also we have interrogated the selectivity of 1 for oxalate, versus
characterized complexes 1–4 by IR spectroscopy (Fig. 4). The other simple mono- and di-carboxylates: formate, acetate, suc-
complexes demonstrate very similar IR spectra with the cinate, malonate, and glutarate. We decided to use mass
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Fig. 6 Spectrophotometric titration of 1 with (NBu4)2[C2O4] in CH3CN–
THF. Inset: spectrum of the starting material (1, red line); spectrum of
the product (3, green line).
spectrometry as a tool to determine selectivity of the di-nickel
system 1 toward oxalate, versus other carboxylates, due to
several reasons. Most importantly, compound 3 gives rise to an
easily recognizable ESI-MS fingerprint at 1001 (highest peak at
1004 in the low-resolution mass spectrum) corresponding to
the mono-cation [Ni2(L)(C2O4)Br]1+ (Fig. 5), which allows its
easy and reliable detection. In addition, the UV-vis spectra
of the products of the reaction of compound 1 with other
carboxylates are very similar to that of compound 3 (see
Fig. S22–S26† for details). Whereas mass spectrometry does
not provide a quantitative tool, it nevertheless allows direct
observation of the products of competition experiments given
their stability under the analytic conditions.16 Two series of
experiments were conducted. In the first series of experiments,
compound 1 was treated with the equimolar amounts of the
respective mono- or dicarboxylate (formate, acetate, malonate,
succinate, and glutarate). Next, the products of these reactions
were analysed by electrospray (ESI) mass spectrometry. The
results of these experiments are displayed in Fig. 7. Only the
most significant region (950–1100 m/z) is shown in Fig. 7; full
mass spectra are given in ESI.† Mass spectra clearly demon-
strate the expected products of the reaction of 1 with the
respective carboxylate. The addition of formate gives rise to
the two peaks at m/z = 1040 and 1076 ([Ni2(L)(O2CH)Br2]1+ and
[Ni2(L)Br3]1+, respectively). Similarly, the reaction of compound
1 with acetate gives rise to the peaks at m/z = 1034, 1056, and
1076, corresponding to [Ni2(L)(O2CCH3)2Br]1+, [Ni2(L)(O2CCH3)-
Br2]1+ and [Ni2(L)Br3]1+, respectively. The addition of malonate
to the solutions of 1 leads to the signal at m/z = 1017, consist-
ent with [Ni2(L)(O2CCH2CO2)Br]1+. Treatment of 1 with succi-
nate gives rise to the signal at m/z = 1032 [Ni2(L)(O2C-
(CH2)2CO2)Br]1+. The reaction of 1 with glutarate leads to the
signal at m/z of 1047, indicating [Ni2(L)(O2C(CH2)3CO2)Br]1+.
In the second series of experiments, the mixtures of 1 with
the respective mono- or dicarboxylates (designated as [1 + car-
boxylate] in Fig. 7) were treated with one equivalent of oxalate;
the resulting products were also analysed by ESI-MS (Fig. 8).
Treatment of the solution of [1 + formate] with oxalate leads to
a single signal around m/z = 1004, indicative of the formation
of the oxalate complex [Ni2(L)(O2CCO2)Br]1+ only. Treatment of
the solution of [1 + acetate] with oxalate leads to the detection
Fig. 7 ESI mass spectra (in the range of m/z 950–1100) of the reaction
mixtures of complex 1 with formate, acetate, malonate, succinate, and
glutarate.
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of two products in 950–1100 region: [Ni2(L)(O2CCO2)Br]1+
(1004) and [Ni2(L)(O2CCO2)(O2CCH3)]1+ (982). This result is
consistent with the previous observation of [Ni2(L)Br3]1+,
[Ni2(L)(O2CCH3)Br2]1+, [Ni2(L)(O2CCH3)2Br]1+ species in the
mixture of [1 + acetate] (Fig. 7). The reaction of [1 + malonate]
with oxalate forms [Ni2(L)(O2CCO2)Br]1+ as the only product
detected by mass spectrometry. Treatment of [1 + succinate]
with oxalate leads in the appearance of the prominent oxalate
signal at m/z = 1004, although a minor signal attributable to
[Ni2(L)(O2C(CH2)2CO2)Br]1+ is also observed at m/z = 1032.
Finally, treatment of [1
+ glutarate] with stoichiometric
amounts of oxalate gives rise to the signal at m/z = 1004,
although other signals are also detected around m/z = 989 and
m/z = 1041. We were not able to assign these signals.
We have also determined binding affinity of the di-nickel
system for oxalate. The binding constant was determined by
the spectrophotometric titration of the metal complex (1) with
oxalate, followed by fitting the data to the non-linear curve as
previously described.17 The obtained binding constant of 5.2 ×
102 M−1 appears to be two to three orders of magnitude lower
than those previously reported for the dinuclear complexes.5–10
It is possible that the significant flexibility of our system is
responsible for the decreased value of the binding constant,
versus previously reported systems. We surmised that such
flexibility may also lead to the reversibility in oxalate coordi-
nation within the intra-complex cavity. Thus, we investigated
next whether the oxalate can be released from 3. Toward this
goal, compound 3 was treated with slight excess of calcium
bromide, as calcium salts are known to form insoluble
calcium oxalate.18 Addition of the colourless acetonitrile solu-
tion of CaBr2 (2 equiv.) to the stirred green solution of 3
immediately results in the colour change to red and the for-
mation of white precipitate. Recrystallization of the red residue
from THF demonstrates that the starting material Ni2(L)Br4 (1)
has been restored. Interestingly, 1 co-crystallizes with one
equivalent of CaBr2(THF)4 (Fig. 9).
Fig. 8 ESI mass spectra (in the range of m/z 950–1100) of the compe-
tition experiments involving addition of oxalate to the reaction mixtures
of complex 1 with formate, acetate, malonate, succinate, and glutarate.
Fig. 9 X-ray structure of 1·CaBr2(THF)4 (50% probability ellipsoids).
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The structure of 1·CaBr2(THF)4 demonstrates slightly Elemental analyses were performed under ambient
different relative conformation of metal centres in Ni2(L)Br4 conditions by Midwest Microlab LLC or Columbia Analytical
(Ni–Ni separation of 10.22 Å). However, bond metrics around Services, Inc.
Ni centers are overall similar to those in compound 1. We have
also attempted to extract oxalate from compound 4. The
Synthesis and characterization of compounds
addition of the acetonitrile solution of CaBr2 to compound 4
Synthesis of Ni2(L)Br4 (1). To a 3 mL solution of NiBr2DME
has also led to the formation of calcium oxalate. However, we (17.2 mg, 0.056 mmol) in THF a slurry of L (20.0 mg,
were not able to isolate the copper products of this reaction.
0.028 mmol) in 3 mL of THF was added. The mixture was
allowed to stir for one hour. The resulting pink solution was
filtered. The solvent of the filtrate was removed in vacuo. X-ray
quality pink crystals were obtained by vapor diffusion of
diethyl ether into a concentrated THF–CH3CN solution yield-
Conclusions
We demonstrated that the highly flexible di-nickel and di- ing 26.0 mg of Ni2(L)Br4 (1, 82%). M.p. 215 °C (decompo-
copper complexes tethered by an open-chain bis(iminopyri- sition). ESI-MS Calcd for [C50H62N4Ni2Br3]+ (1-Br) 1071.1,
dine) ligand bind oxalate in the intramolecular cavity. The di- found 1071.5. Anal. Calcd for C50H62N4Ni2Br4: C, 52.0; H,
nickel complex allows colourimetric detection of oxalate as 5.4; N, 4.9. Found: C, 52.3; H, 5.3; N, 5.0. The structure was
oxalate incorporation is accompanied by a substantial colour also confirmed by X-ray structure determination (Fig. 2,
change. Oxalate recognition by the di-nickel complex is selec- Table S1†). 1H NMR (CD2Cl2, 400 MHz) δ 1.3, 1.6, 2.9, 10.7,
tive as oxalate replaces other simple mono- and di-carboxylate 16.0, 18.9, 45.5, 77.4 ppm. μeff (Evans, CD2Cl2, 298 K): 4.83μB.
tested. The highly flexible nature of the system leads to the IR (cm−1): 2956 (s), 1458 (s), 1369 (m), 1170 (m), 1050 (m),
relatively low oxalate-binding constant compared with other 880 (m).
(mostly pre-organized macrocyclic) systems previously
Synthesis of Cu2(L)Br4(CH3CN)2 (2). To a 3 mL solution of
reported. However, this flexibility enables reversible incorpor- CuBr2 (8.0 mg, 0.056 mmol) in THF a slurry of L (20.0 mg,
ation of oxalate, making our sensor recyclable.
0.278 mmol) in 3 mL of THF was added. Upon the addition,
the color changed to green-yellow. The mixture was allowed to
stir for one hour. The solvent was removed under vacuum.
X-ray quality green crystals were obtained by vapour diffusion
of Et2O into a concentrated CH3CN solution yielding Cu2(L)-
Br4(CH3CN)2 (2, 24.0 mg, 75%). M.p. = 90 °C (decomposition).
Experimental
General methods and procedures
Nickel bromide dimethoxyethane complex (NiBr2(DME)), Anal. Calcd for C54H68Br4N6Cu2: C, 51.9; H, 5.5; N, 6.7 Found:
copper(II) bromide, oxalic acid dihydrate, tetrabutylammonium C, 51.3; H, 5.4; N, 6.4. ESI-MS Calcd for [C50H62N4Cu2Br2]+
methoxide solution, potassium oxalate dihydrate, sodium (1-2Br) 1002.2, found. 1002.6. The structure was confirmed by
oxalate, ammonium formate, ammonium acetate, succinic X-ray structure determination (Fig. 2, Table S1†). μeff (Evans,
acid disodium salt, malonic acid and glutaric acid were pur- CD2Cl2, 298 K): 2.16μB. IR (cm−1): 2956 (s), 2360 (w), 1458 (s),
chased from Aldrich, Strem or TCI America and used as 1369 (m), 1170 (m), 1050 (m), 880 (m).
received. L has been prepared as previously described.12c All
Synthesis of (NBu4)2[C2O4]. This procedure is a modifi-
solvents were purchased from Fisher scientific and were of cation of a previously reported method.14 To a 20 mL solution
HPLC grade. The solvents were purified using an MBraun of oxalic acid dihydrate (0.500 g, 3.976 mmol) 13.3 mL of
solvent purification system and stored over 3 Å molecular (NBu4)(OMe) (20% in MeOH) solution was added. The reaction
sieves. Compounds were routinely characterized by 1H NMR was allowed to stir for 4 hours. The solvent was removed under
spectroscopy, Evans method, X-ray crystallography, IR spec- vacuum. The resulting solid was dried under high vacuum for
1
troscopy, elemental analyses, and mass spectrometry (ESI). 24 h yielding 2.22 g (98%) of (NBu4)2[C2O4]. H NMR (CD3CN,
NMR spectra were recorded at the Lumigen Instrument Center 400 MHz) δ 3.15 (t, J = 8.0, 16H), 1.59 (q, J = 6.4, 16H), 1.33
(Wayne State University) on a Varian Mercury 400 NMR (q, J = 7.2, 16H), 0.93 (t, J = 6.8, 24H); 13C NMR (CD3CN,
Spectrometer in CD3CN at room temperature. Chemical shifts 75 MHz) δ 176.72, 59.20, 24.40, 20.34, 13.87.
and coupling constants (J) were reported in parts per million
Synthesis of Ni2(L)(μ-C2O4)Br2(NCMe) (3). To a 3 mL of
(δ) and Hertz respectively. IR spectra of powdered samples solution of 1 (26.3 mg, 0.023 mmol) in THF a 2 mL solution of
were recorded on a Shimadzu IR Affinity-1 FT-IR Spectrometer (NBu4)2[C2O4] (13.0 mg. 0.023 mmol) in CH3CN was added,
outfitted with a MIRacle10 attenuated total reflectance acces- resulting in green solution. The mixture was allowed to stir for
sory with a monolithic diamond crystal stage and pressure one hour, filtered, and the volatiles were removed in vacuo.
clamp. Low resolution mass spectra were obtained at the Green X-ray quality crystals were obtained by slow diffusion of
Lumigen Instrument Center utilizing a Waters Micromass ZQ ether into CH3CN yielding 20 mg of Ni2(L)(μ-C2O4)Br2(NCMe)
mass spectrometer (direct injection, with capillary at 3.573 (3, 78%). M.p. = 207 °C (decomposition). ESI-MS Calcd for
(kV) and cone voltage of 20.000 (V)). Only selected peaks in the [C50H62N4Ni2C2O4Br]+ (3-Br-CH3CN) 1001.3, found 1001.3.
mass spectra are reported below. UV-visible spectra Anal. Calcd for C54H65N5Ni2Br2O4: C, 57.6; H, 5.8; N, 6.2.
were obtained on
a
Shimadzu UV-1800 spectrometer. Found: C, 56.6; H, 5.8; N, 6.0. The structure was also con-
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firmed by X-ray structure determination (Fig. 3, Table S1†). shaken for 2 min. An aliquot of the resulting solution was
IR (cm−1): 2956 (s), 1647 (s), 1607 (s), 1458 (s), 1369 (m), 1310 then injected into the ESI-MS for analysis.
(m), 1170 (m), 1050 (m), 880 (m).
Synthesis of Ni2(L)(μ-C2O4)Br2(CH3CN) (3) using NaC2O4 or
KC2O4. To a 3 mL of solution of 1 (26.0 mg, 0.028 mmol) in X-ray crystallographic details
THF K2C2O4·2H2O (5.0 mg, 0.028 mmol) in 1 mL of MeOH was
Structures of compounds 1, 2, 3, 4, and 1·CaBr2(THF)4 were
added. Upon the addition, the color changed to dark green.
confirmed by X-ray structure determination. The crystals were
The mixture was allowed to stir for 1 h then filtered. The
mounted on a Bruker APEXII/Kappa three circle goniometer
resulting green solution was concentrated under vacuum.
platform diffractometer equipped with an APEX-2 detector.
A graphic monochromator was employed for wavelength selec-
tion of the Mo Kα radiation (λ = 0.71073 Å). The data were pro-
cessed and refined using the APEX2 software. Structures were
solved by direct methods in SHELXS and refined by standard
difference Fourier techniques in the SHELXTL program suite
(6.10 v., Sheldrick G. M., and Siemens Industrial Automation,
2000). Hydrogen atoms were placed in calculated positions
Green X-ray quality crystals were obtained by slow diffusion of
ether into CH3CN yielding Ni2(L)(μ-C2O4)Br2(CH3CN) (18 mg,
72%) as green crystals. The nature of the product was con-
firmed by the unit cell determination (identical to the unit cell
of 3). The compound 3 can be obtained in a similar fashion
using the solution of NaC2O4 (3.7 mg. 0.0278 mmol) in a
1 : 1 mixture of MeOH–H2O.
Synthesis of Cu2(L)(C2O4)Br2 (4). To a 3 mL of solution of 2
using the standard riding model and refined isotropically; all
other atoms were refined anisotropically. Some of the para-iPr
(66 mg, 0.0556 mmol) in CH3CN a 2 mL solution of
(NBu4)2[C2O4] (32 mg, 0.056 mmol) in CH3CN was added.
groups displayed large wagging motion which in selected cases
Upon the addition, the colour changed to dark green. The
(1) was successfully modelled as two different conformations.
mixture was allowed to stir for 1 h then filtered. The resulting
In contrast, we had only limited success in modelling the dis-
green solution was concentrated under vacuum. Green X-ray
order of the para-iPr groups in the structures of 2 and 3. Even
quality crystals were obtained by slow diffusion of ether into
though these structures were collected at 100 K the thermal
CH3CN yielding Cu2(L)(C2O4)Br2 (4, 50 mg, 83%). M.p. =
parameters for some of these groups were very high. The con-
152 °C (decomposition). Anal. Calcd for C52H64Br2N4O4Cu2: C,
clusion is that these groups are not well defined and thus
57.1 H, 5.7 N, 5.1 Found: C, 57.1 H, 5.7 N, 5.2. The structure
some were refined isotropically. The isotropic refinement of
was also confirmed by X-ray structure determination (Fig. 3,
these atoms does not significantly alter the R-factor, and does
Table S1†). IR (cm−1): 2956 (s), 1647 (s), 1607 (s), 1458 (s),
not alter the conclusions of this paper in any way. Structures
1369 (m), 1310 (m), 1170 (m), 1050 (m), 880 (m).
of 3 and 4 contained one molecule of ether solvent, and one
molecule of acetonitrile in the asymmetric unit. Structure of
Oxalate extraction from Ni2(L)(μ-C2O4)Br2(CH3CN) (3) to
form Ni2(L)Br4·CaBr2THF4 (1·CaBr2THF4). To a green 3 mL of
1·CaBr2(THF)4 contained one molecule of ether solvent in the
solution of 3 (12 mg, 0.011 mmol) in CH3CN a colourless 2 mL
asymmetric unit. The acetonitrile ligands in the structures of
solution of CaBr2 in CH3CN (4 mg. 0.02 mmol) was added.
2 and 3 were disordered over two positions. In addition, the
structure of 2 contained acetonitrile solvent disordered over
two positions in the asymmetric unit. Detailed crystal and
structure refinement data are given in Table S1.†
The reaction color immediately turned red-pink, and white
precipitate formed. The mixture was allowed to stir for 1 h
then filtered. The resulting red-pink solution was concentrated
under vacuum. Pink X-ray quality crystals were obtained by
slow diffusion of ether into THF solution yielding Ni2(L)-
Br4·CaBr2THF4 (1·CaBr2THF4, 14 mg, ca. 90%). The nature of
the product was established by the X-ray structure determi-
Acknowledgements
nation (Fig. 9).
We thank Wayne State University for startup funding, Dr Lew
Hryhorczuk and Dr Yuriy Danylyuk for the experimental assist-
ance. JWB thanks the Department of Chemistry for the
Continuing Graduate Rumble Fellowship. DSW thanks Under-
General procedure for obtaining the ESI-MS of the products
of the reaction of 1 with mono- and di-carboxylates. To a solu-
tion of L (20 mg, 0.278 mmol) and 2 equiv. NiBr2DME (17 mg,
0.556 mmol) in THF a mixture of 1 equiv. of the corresponding
graduate Research Opportunities Program of Wayne State
mono- or dicarboxylate ion (formate, acetate, succinate, malo-
University for the Undergraduate Research and Creative Pro-
nate, and glutarate) in MeOH was added and shaken for
jects Award.
2 min. An aliquot of the resulting solution was then injected
into the ESI-MS for analysis.
General procedure for obtaining the electrospray mass
spectra of the products of the reaction of 1 with the mixtures
Notes and references
of mono- and dicarboxylates with oxalate. To a solution of L
(20 mg, 0.278 mmol), 2 equiv. NiBr2DME (17 mg, 0.556 mmol)
in THF and 1 equiv. of the competitor ion (formate, acetate,
succinate, malonate, and glutarate) in MeOH a mixture of
1 equiv. of (NBu4)2[C2O4] (16 mg, 0.278 mmol) was added and
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7986 | Dalton Trans., 2014, 43, 7979–7986
This journal is © The Royal Society of Chemistry 2014