A versatile dinucleating ligand containing sulfonamide groups

Article abstract of DOI:10.1021/ic402599e

Copper, iron, and gallium coordination chemistries of the new pentadentate bis-sulfonamide ligand 2,6-bis(N-2-pyridylmethylsulfonamido)-4-methylphenol (psmpH₃) were investigated. PsmpH₃ is capable of varying degrees of deprotonation, and notably, complexes containing the fully trideprotonated ligand can be prepared in aqueous solutions using only divalent metal ions. Two of the copper(II) complexes, [Cu₂(psmp)(OH)] and [Cu₂(psmp)(OAc)₂]^-, demonstrate the anticipated 1:2 ligand/metal stoichiometry and show that the dimetallic binding site created for exogenous ligands possesses high inherent flexibility since additional one- and three-atom bridging ligands bridge the two copper(II) ions in each complex, respectively. This gives rise to a difference of 0.4 A in the Cu···Cu distances. Complexes with 2:3 and 2:1 ligand/metal stoichiometries for the divalent and trivalent metal ions, respectively, were observed in [Cu₃(psmp)₂(H ₂O)] and [M(psmpH)(psmpH₂)], where M = Ga^III, Fe^III. The deprotonated tridentate N-2-pyridylsulfonylmethylphenolato moieties chelate the metal ions in a meridional fashion, whereas in [Cu ₃(psmp)₂(H₂O)] the rare μ₂-N- sulfonamido bridging coordination mode is observed. In the bis-ligand mononuclear complexes, one picolyl arm of each ligand is protonated and uncoordinated. Magnetic susceptibility measurements on the doubly and triply bridged dicopper(II) complexes indicate strong and medium strength antiferromagnetic coupling interactions, with J = 234 cm^-1 and 115 cm^-1 for [Cu₂(psmp)(OH)] and [Cu₂(psmp)(OAc) ₂]^-, respectively (in H_HDvV =...+JS ₁S₂ convention). The trinuclear [Cu₃(psmp) ₂(H₂O)], in which the central copper ion is linked to two flanking copper atoms by two μ₂-N-sulfonamido bridges and two phenoxide bridges shows an overall magnetic behavior of antiferromagnetic coupling. This is corroborated computationally by broken-symmetry density functional theory, which for isotropic modeling of the coupling predicts an antiferromagnetic coupling strength of J = 70.5 cm^-1.

Full text of DOI:10.1021/ic402599e

Article
pubs.acs.org/IC
A Versatile Dinucleating Ligand Containing Sulfonamide Groups
Jonas Sundberg,† Hannes Witt,† Lisa Cameron,†,‡ Mikael Hakansson,§ Jesper Bendix,⊥
̊
and Christine J. McKenzie*,†
†Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark
‡School of Chemistry, The University of Sydney, NSW 2006, Australia
§Department of Chemistry and Molecular Biology, University of Gothenburg, SE-412 96 Gothenburg, Sweden
⊥Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark
S
* Supporting Information
ABSTRACT: Copper, iron, and gallium coordination chemistries
of the new pentadentate bis-sulfonamide ligand 2,6-bis(N-2-
pyridylmethylsulfonamido)-4-methylphenol (psmpH3) were inves-
tigated. PsmpH3 is capable of varying degrees of deprotonation,
and notably, complexes containing the fully trideprotonated ligand
can be prepared in aqueous solutions using only divalent metal
ions. Two of the copper(II) complexes, [Cu2(psmp)(OH)] and
[Cu2(psmp)(OAc)2]−, demonstrate the anticipated 1:2 ligand/
metal stoichiometry and show that the dimetallic binding site
created for exogenous ligands possesses high inherent flexibility
since additional one- and three-atom bridging ligands bridge the
two copper(II) ions in each complex, respectively. This gives rise to a difference of 0.4 Å in the Cu···Cu distances. Complexes with 2:3
and 2:1 ligand/metal stoichiometries for the divalent and trivalent metal ions, respectively, were observed in [Cu3(psmp)2(H2O)] and
[M(psmpH)(psmpH2)], where M = GaIII, FeIII. The deprotonated tridentate N-2-pyridylsulfonylmethylphenolato moieties chelate the
metal ions in a meridional fashion, whereas in [Cu3(psmp)2(H2O)] the rare μ2-N-sulfonamido bridging coordination mode is observed.
In the bis-ligand mononuclear complexes, one picolyl arm of each ligand is protonated and uncoordinated. Magnetic susceptibility
measurements on the doubly and triply bridged dicopper(II) complexes indicate strong and medium strength antiferromagnetic coupling
interactions, with J = 234 cm−1 and 115 cm−1 for [Cu2(psmp)(OH)] and [Cu2(psmp)(OAc)2]−, respectively (in HHDvV =...+JS1S2
convention). The trinuclear [Cu3(psmp)2(H2O)], in which the central copper ion is linked to two flanking copper atoms by two
μ2-N-sulfonamido bridges and two phenoxide bridges shows an overall magnetic behavior of antiferromagnetic coupling. This is
corroborated computationally by broken-symmetry density functional theory, which for isotropic modeling of the coupling predicts an
antiferromagnetic coupling strength of J = 70.5 cm−1.
Scheme 1
INTRODUCTION
■
Acyclic dinucleating ligands are of continued interest for the
preparation of complexes that bind exogenous molecules at
preorganized “dimetallic sites.” Coordination at these sites has,
in some cases, resulted in the labile exogenous guests showing
enhanced reactivity, and some proposed mechanisms of
reactions are reminiscent of those for the dinuclear metallo-
enzymes.1 Clearly features of the supporting ligand, like donor
atoms and charge, denticity, chelate ring size, bite angles, and
other steric factors will influence the metal ion geometries and
M···M distances and consequently tune the coordination
chemistry of a preorganized dimetallic active site. One of the
largest classes of acyclic and macrocyclic dinucleating ligands
are constructed around bis-ortho functionalized, potentially
bridging phenolato groups. Readily accessible frameworks for
acyclic systems of this type of ligand utilize a Schiff base
condensation of 2,6-diformyl-4-methylphenol with amines con-
taining other donor groups, for example 2-(aminomethyl)-pyridine
(Scheme 1a).2 We were interested in conserving this particular
framework, but to modify it by including anionic donors trans to
the sites in which the exogenous ligands will bind in derived
dimetallic complexes. The intention was to subsequently exploit
the resultant lower overall positive charge of the complex and trans
effect in influencing the reactivity of coordinated guests. This
construction might be expected to give scope for accessing
dinuclear complexes with high-valent metal ions. A desirable
outcome is opening up new metal cooperativity vistas in the field
of the catalysis of oxidation reactions by first-row transition metals.
Received: October 15, 2013
Published: February 28, 2014
© 2014 American Chemical Society
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By analogy dianionic macrocycles and chelates like porphyrins and
salens have been benchmarks in the history of the development of
monometallic catalysts for oxidation reactions. At the other end of
the spectrum, the coordination of low-valent metal ions by such
ligands might have potential for the activation of electrophilic
guests bound at the dimetallic site. To this end we speculated that
the fully deprotonated forms of the ligands in Scheme 1b−d were
candidates for the construction of dinuclear complexes containing
the same donor set, but with 3 times the negative charge of the
deprotonated ligand in Scheme 1a.
Scheme 2
Reactions of psmpH3 with 2 equiv of copper(II) acetate in
CH2Cl2, CH3CN, MeOH, EtOH, or water all resulted in the
immediate formation of relatively insoluble solids with various
greenish-blue hues. Bands between 1000 and 1400 cm−1 in
their IR spectra can be associated with the sulfonamide groups.
Small variations could be correlated to differences in color.
Inspection of the crystalline solids using a light microscope
showed that solids were in general mixtures containing two or
three different crystal types. The least soluble of these solids,
oblong dark blue crystals, were the most thermodynamically
stable crystalline product. Mixtures of crystals that redissolved
in mother liquor or were dissolved in fresh solvent were prone
to convert to this form if allowed to stand for a prolonged
period of time under ambient conditions. This process is
accelerated by addition of small amounts of NaOH (1 equiv per
ligand) or by diffusion of ammonia into the solutions. An olive-
green solid obtained from the reactions performed in
dichloromethane was the only material that showed vibrations
that could be attributed to the incorporation of an acetato
ligand, despite the presence of acetate in all reactions. At 1601
and 1410 cm−1, for asymmetric and symmetric carboxylato
stretching, respectively, the difference of 191 cm−1 was consistent
with a μ2-acetato group.12 Corroborating this, the negative-ion
electrospray ionization mass spectrometry (ESI-MS) shows a
base peak at m/z 690.96, which can be assigned to [Cu2(psmp)-
(OAc)2]−, suggesting the presence of this ion in the material.
The issue of a charge-balanced formulation was first resolved by
single-crystal X-ray diffraction, vide infra, showing that the
unexpected countercation, [N(Et)3CH2Cl]+, had been formed
from the reaction between triethylamine and CH2Cl2.13 Despite
this, the preparation of [N(Et)3CH2Cl][Cu2(psmp)(OAc)2] is
entirely repeatable. Attempts to prepare [Cu2(psmp)(OAc)2]− in
salts with alternative, deliberately added, and similarly sized
counterions, for example, Et4N+, by reactions where triethyl-
amine and/or CH2Cl2 were omitted, produce powders that,
according to spectroscopy, contained the same complex anion.
As mentioned above acetato bands were absent in the IR
spectra of dark-blue and grass-green crystals obtained from the
aforementioned reactions. What appeared to be very similar
materials by comparisons of their IR spectra could also be
obtained using copper(II) nitrate and copper(II) sulfate as the
starting materials. Thus it could be concluded that none of the
acetate, nitrate, or sulfate were components of these two
different products, and that they were most likely neutral
compounds. Positive-ion ESI mass spectra were obtained only
for the slightly more soluble grass-green materials; however, the
spectra were not particularly informative inasmuch as they
contained several ions assignable to species containing 1:1, 1:2,
and 3:2 ligand/copper stoichiometries. Optimization of
reaction conditions finally produced pure batches of well-
defined crystalline materials of the dark-blue and grass-green
copper complexes.
It is reasonable to assume that the deprotonation of the
known bis-secondary amine ligand3 (Scheme 1b) is unlikely to
occur in the presence of water, and the use of nonaqueous
solvents, anhydrous metal sources, and prior deprotonation by
very strong bases (e.g., alkyllithium reagents) are required for
the preparation of metal complexes. Even if complexes were
isolated these are likely to be easily hydrolyzed, and
protonation of the amido groups would occur. Thus we
considered that the approach of introducing secondary amide
groups is more practical. It is well-established that deprotona-
tion and N coordination of secondary carboxamido groups in
chelating and macrocyclic ligands to metal ions is facile even in
water, despite unfavorable pKa values. The strong amido−N−σ
donor capability is important for the tetraamido macrocycles,
developed by Collins and co-workers, which have proved to be
important ligands for the purpose of stabilizing mononuclear
high-valent iron complexes.4,5 Using this class of ligand,
Fe(IV)oxo4 and the first Fe(V)oxo5 species have been identified,
and these complexes have been exploited for the catalysis of the
oxidation of organic substrates. The early studies by Stephens and
Vagg and co-workers demonstrated that tetradentate diamide
ligands could be deprotonated and employed successfully under
aqueous conditions for the formation of mononuclear inter-
mediate valent complexes.6 The dicarboxamide pro-ligand in
Scheme 1c was, however, not deprotonated in a zinc complex.7
Given this outcome, and the fact that the pKa values of neutral
primary and secondary sulfonamides (pKa approximately 7−16)
can be several orders of magnitude lower than for the analogous
carboxamides (pKa approximately 12−27), we turned our sights to
the synthesis of the previously unknown bis-sulfonamide pro-
ligand, Scheme 1d. Apart from the more favorable acid−base
chemistry, sulfonamide groups have not been extensively
employed as central −R−SO2−N− components in chelating
ligands. They must, however, be attractive in terms of their
charge versatility; sulfonamide donors can be dianionic,8,9
monoanionic,10 or neutral.11 The majority of the known
sulfonamide-containing ligands are derived from tosylation of
the parent amine-functionalized ligands; consequently, the −R−
SO2−N− moiety terminates and does not form part of a chelate.
Therefore, known complexes do not provide predictive models
for the way in which the potentially dinucleating bis-sulfonamide
pro-ligand, 2,6-bis(N-2-pyridylmethylsulfonamido)-4-methylphenol
(psmpH3), in Scheme 1d might be expected to coordinate in
metal complexes. We report here the synthesis of psmpH3 and
the characterization of a remarkable range of topologies in its
coordination complexes.
RESULTS AND DISCUSSION
■
The pro-ligand 2,6-bis(N-2-pyridylmethylsulfonamide)-4-methyl-
phenol (psmpH3) is prepared by a 2:1 condensation of
2-aminopyridine and 2,6-dichlorosulfone-4-methylphenol
(Scheme 2) and is isolated in good yields as an air- and
moisture-stable off-white powder.
The structures of the three different copper(II) compounds,
which in practice can cocrystallize from the 2:1 reaction of
copper acetate and psmpH3, were determined by single-crystal
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X-ray diffraction to be [Cu2(psmp)(OH)] (blue oblong
crystals), [N(Et)3CH2Cl][Cu2(psmp)(OAc)2]·2CH2Cl2 (olive-
green crystals), and [Cu3(psmp)2(H2O)]·H2O (grass-green
crystals). Their molecular structures are shown in Figure 1a−c.
that the S−O distances are essentially unaffected by
coordination of sulfonamide. Accordingly many of the bands
in the relevant region of the IR spectra are similar throughout
the series, and this technique was not particularly diagnostic of
the structure of these relatively insoluble compounds.
X-RAY CRYSTAL STRUCTURES
■
The molecular structures of psmpH3, [Cu2(psmp)(OH)],
[Cu2(psmp)(OAc)2]−, [Cu3(psmp)2(H2O)], [Ga(psmpH)-
(psmpH2)], and [Fe(psmpH)(psmpH2)] were determined by
single-crystal X-ray diffraction and are shown in Figures 2−6. A
Figure 1. Chemical diagrams of the complexes of fully or partially
deprotonated 2,6-bis(N-2-pyridylmethyl-sulfonamide)-4-methylphenol
(psmpH3) found in (a) [Cu2(psmp)(OH)], (b) [N(Et)3CH2Cl]-
[Cu2(psmp)(OAc)2]·2CH2Cl2, (c) [Cu3(psmp)2(H2O)]·H2O,
and (d) [M(psmpH)(psmpH2)]. R = −SO2NCH2py, M = FeIII
or GaIII.
Figure 2. Molecular structure of psmpH3. Anisotropic displacement
parameters are drawn at the 50% probability level.
These contrasting structures conclusively demonstrate that all of
the potential sulfonamide donors of psmpH3 are deprotonated
and that the N atom is coordinated to the copper(II) ions. As
mentioned above the addition of hydroxide or ammonia to any of
the reaction mixtures leads ultimately to the formation of the blue
compound, which we could now assign to the very insoluble
[Cu2(psmp)(OH)]. Thus it can be surmised that deprotonation
of the water ligand of the neutral 2:3 ligand/metal complex
[Cu3(psmp)2(H2O)] does not produce its deprotonated congener
[Cu3(psmp)2(OH)]−; rather, a rearrangement occurs, and the
neutral 1:2 ligand/metal species [Cu2(psmp)(OH)] is formed,
consistent with the fact that hydroxide ligands prefer bridging over
terminal coordination.
The outcome of the reactions of gallium(III) and iron(III)
salts with psmpH3 contrasts to the reactions with the divalent
metal ions. Even in the presence of more than two metal ions
per psmpH3 the neutral bis-ligand mononuclear complexes
[M(psmpH)(psmpH2)] are isolated (Figure 1d). In these, one
tridentate NpyNsulfonamideOphenol half of each ligand binds the
octahedral ion meridionally, while the other half of each ligand
is uncoordinated and protonated. This result was unanticipated
given that the more highly electropositive metals ions,
compared to Cu2+, might be expected to outcompete protons.
The crystal structure of [Cu3(psmp)2(H2O)] shows an
elongation of the S−N bond of the bridging sulfonamides
compared to the sulfonamide involved in meridional chelation
of one metal ion. The former bond distance is comparable to
the one found in the structure of the free ligand, as well as the
uncoordinated sulfonamides in [M(psmpH)(psmpH2)]
(M = GaIII or FeIII). Otherwise the crystal structures show
Figure 3. Molecular structure of [Cu2(psmp)(OH)]. Anisotropic
displacement parameters are drawn at the 50% probability level. Apart
from that bound to O6, the hydrogen atoms are omitted for clarity.
summary of bond distances and angles for the complexes can
be found in Tables 1−3. The structure of the free ligand,
psmpH3, shows a conformation in which the pyridyl arms are
twisted approximately 180° relative to each other above and
below the plane of the phenol ring to give a slightly offset
intramolecular π-stacking (Figure 2). The phenol hydroxyl
H atom H-bonds to a sulfonyl O (2.770 Å). The structures
of neutral [Cu2(psmp)(OH)] (Figure 3) and anionic
[Cu2(psmp)(OAc)2]− (Figure 4) demonstrate conclusively
that this new bis-sulfonamide ligand forms the intended
dinuclear complexes. The conformation of the ligand has
changed dramatically compared to its protonated precursor.
Furthermore the [M2(psmp)]+ scaffold is flexible enough to
accommodate the extremes of auxiliary one-atom and three-atom
bridging groups. The incorporation of a hydroxido or two acetato
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Figure 4. Molecular structure of the of the anion in [N(Et)3CH2Cl]-
[Cu2(psmp)(OAc)2]. Anisotropic displacement parameters are drawn
at the 50% probability level. Hydrogen atoms omitted for clarity.
Figure 5. Molecular structure of [Cu3(psmp)2(H2O)]. Anisotropic
displacement parameters are drawn at the 50% probability level. The
noncoordinated water molecule site occupation is 50%. Hydrogen
atoms omitted for clarity.
bridges is associated with square planar and distorted square
pyramidal geometries and intermetallic distances of 2.9452(3)
and 3.350(4) Å, respectively. The former distance is similar to
that found in dicopper(II) complexes of the Schiff base ligand in
Scheme 1a, while the latter is significantly larger and consistent
with the presence of two three-atom bridgesa construction
that in fact has not yet been identified for the Schiff base ligand in
Scheme 1a. The deprotonated sulfonamide groups central in the
tridentate meridional arrangement of each arm coordinate via the
N atoms. The dihedral angle between the basal CuN2O2 planes
of the distorted square pyramidal copper ions in [Cu2(psmp)-
(OAc)2] is approximately 95°.
Figure 6. (a) Molecular structure of [Ga(psmpH)(psmpH2)].
Anisotropic displacement parameters are drawn at the 50% probability
level. Hydrogen atoms omitted for clarity. (b) Packing diagram of
[Ga(psmpH)(psmpH2)] showing H-bonded chains along the a axis.
(c) Molecular overlay of (red) [Fe(psmpH)(psmpH2)] and (blue)
[Ga(psmpH)(psmpH2)].
The structure of [Ga(psmpH)(psmpH2)] (Figure 6a) shows
the Ga(III) ion is octahedrally coordinated by two tridentate
meridional pyCH2NS(O)2phenolato groups of the two ligands.
The deprotonated sulfonamide N atoms of each ligand are
orientated trans to each other. The phenolato O of one ligand
and pyridine N donors of the second are trans to each other.
The uncoordinated sulfonamide and pyridine N donors are
protonated and are involved in a H-bonded chain of molecules
along the a axis (Figure 6b). The lattice water is encased in a
cavity surrounded by a sulfonamide O atom (H2O···O4 =
2.956(7)Å), the phenolato O atom (H2O···O1 = 3.055(7)Å),
and a pyridine CH group (H2O···C = 3.270(1)Å), all belonging
to a single psmp unit. The iron complex is structurally identical,
with an analogous hydrogen-bonding pattern. An overlay of the
crystal structures of the Ga(III) and Fe(III) complexes is shown
in Figure 6c.
In the structure of [Cu3(psmp)2(H2O)], the two psmp3−
ligands bind to all three linearly oriented copper ions by forming a
double helix around them (Figure 5). A deprotonated sulfonamide
N atom from each half of each ligand forms μ2 bridges between
the central copper and one flanking copper ion. The other half of
each ligand meridionally chelate each flanking copper(II) ion. The
basal planes of the three copper ions are coplanar and linearly
orientated, with the apical positions filled by the pyridine N for the
two terminal copper ions and an exogenous water ligand for the
central copper ion. The Cu···Cu distances are 3.0586(6)Å. The
coordinated water group is contained within a well-defined cleft
surrounded by two pyridyl rings and two sulfonamide groups, a
second water molecule situated between the coordinated H2O and
surrounding sulfonamide oxygen atom stabilizes the conformation
via hydrogen bonding.
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Table 1. Selected Bond Distances (Å) and Angles (deg) for
[Cu2(psmp)(OH)] and [Cu2(psmp)(OAc)2]−
̂
̂
̂
̂
̂
H = μBB·[gx(S1,x + S2,x) + gy(S1,y + S2,y)
̂
̂
̂
̂
+ gz(S1,z + S2,z)] + JS1·S2
(1)
[Cu2(psmp)(OH)]
[Cu2(psmp)(OAc)2]−
Cu1−O1
Cu2−O1
Cu1···Cu2
Cu1−N1
Cu1−N2
Cu2−N3
Cu2−N4
S1−O2
S1−O3
S2−O4
S2−O5
S1−N1
2.0102(10)
1.9499(11)
2.9452(3)
1.8989(13)
1.9709(13)
1.9183(12)
1.9632(12)
1.4459(11)
1.4523(12)
1.4588(11)
1.4498(12)
1.585(4)
2.003(3)
2.018(3)
3.350(4)
1.944(3)
2.034(3)
1.943(4)
2.037(3)
1.443(4)
1.451(3)
1.455(3)
1.451(4)
1.570(4)
1.576(4)
97.48(12)
93.59(13)
170.81(13)
80.92(14)
97.48(12)
93.05(13)
169.30(13)
80.94(14)
112.86(13)
gives the relatively strong antiferromagnetic coupling of J =
234 cm−1. Not unexpectedly, because of the square pyramidal
geometries, acetate bridges, and larger Cu−Cu separation, the
[N(Et)3CH2Cl][Cu2(psmp)(OAc)2]·2CH2Cl2 compound
shows weaker antiferromagnetic coupling, with J = 115 cm−1
and a χmT value approaching at RT that for two uncoupled
copper(II) ions.
Only a handful of complexes containing N-sulfonamide-
bridged metal ions are known, and most are diamagnetic.14 For
this reason the extent to which this unusual bridging group can
mediate superexchange pathways for magnetic interactions has
not been explored. We found only one system that is
comparable to ours, namely, a doubly μ2-Nsulfonamide bridged
dicopper(II) complex.15 This complex displayed a ferromag-
netic interaction (J = −54 cm−1) between the square planar d9
centers and weak intermolecular antiferromagnetic interactions
(J = 0.66 cm−1). It was thus of some interest to investigate the
magnetic properties of [Cu3(psmp)2(H2O)]·H2O. Overall
antiferromagnetism is evident in the χmT product of this
complex, and the temperature dependence of the susceptibility
varies between 0.47 and 1.15 cm3 mol−1 K as expected for one
and just below three unpaired electrons (with giso ≈ 2.2, cf. also
Supporting Information) at low and high temperature,
respectively. Dominant antiferromagnetic coupling is in agree-
ment with density functional theory (DFT) calculations,
vide infra.
S2−N3
1.596(4)
O1−Cu1−O6
O1−Cu1−N1
O1−Cu1−N2
N1−Cu1−N2
O1−Cu2−O6
O1−Cu2−N3
O1−Cu2−N4
N3−Cu2−N4
Cu1−O1− Cu2
79.87(4)
94.06(5)
172.46(5)
84.21(5)
80.89(4)
97.62(5)
171.07(5)
84.23(5)
96.08(4)
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[Cu3(psmp)2(H2O)]
[Cu3(psmp)2(H2O)]
Cu1−N1
Cu1−N2
Cu1−N3
Cu1−N4
Cu1−O3
Cu1···Cu2
Cu2−O3
Cu2−N4
Cu2−O6
Cu1···Cu2
S1···N2
S2···N4
Cu1−N4−Cu2
Cu1−O3−Cu2
N4−Cu2−O3
N1−Cu1−N4
N1−Cu1−N2
N2−Cu1−O3
N4−Cu1−03
N4−Cu2−O4
2.085(3)
1.937(3)
2.122(3)
2.066(3)
2.091(3)
3.0586(6)
1.953(3)
2.059(3)
2.239(4)
3.0586(6)
1.572(3)
1.630(3)
95.73(9)
98.24(9)
94.57(9)
104.03(9)
79.10(9)
92.98(9)
80.98(9)
84.54(9)
THEORETICAL MODELING
■
The neutral compound [Cu3(psmp)2(H2O)] was modeled
computationally with a Zn substituted for one of the terminal
copper ions. The hydrogen atoms on the central water ligand
were placed in calculated positions. A broken-symmetry (BS)
DFT (S = 0,1) calculation was performed to determine the
magnetic coupling. The computed value for the isotropic exchange
coupling parameter was J = 70.48 cm−1 (+JS1S2 convention).
Interestingly, the shapes and magnitudes of the Mulliken spin
densities were determined to be quite similar for the bridging
oxygen from the phenolate (+0.026) and the nitrogen from the
sulfonamide (+0.023) (Figure 9).
ELECTROCHEMISTRY
■
A cyclic voltammogram of [Cu3(psmp)2(H2O)], Figure 10,
suggests irreversible redox chemistry. It is likely also complicated
by the rearranged species that can exist, as shown through ESI-MS
and isolation (e.g., “recrystallization” of [Cu3(psmp)2(H2O)] gives
[CuII (psmp)(OH)]). However, a dominating process is assigned
2
to an irreversible Cu(II) to Cu(III) oxidation at 1.276 V. We
assume this to be associated with the central copper ion and that
this is broadened because the auxiliary water ligand found in the
solid state is labile, and the closely related species [Cu3(psmp)2-
(H2O)], [Cu3(psmp)2(CH3CN)], and [Cu3(psmp)2] may well be
present and all oxidized to a Cu(II)Cu(III)Cu(II) complex at
about the same potential. Lability and Cu(III)−OH2 acidity would
be consistent also with the presence of irreversible reductions at
−0.717, −0.368, −0.089 V versus Fc+/Fc. When a reductive scan
starting at 0 V was run, the irreversible reductions were not
observed (see Supporting Information, Figure S22). In the case of
processes involving the crystallographically characterized ion
[Cu3(psmp)2(H2O)] a spontaneous deprotonatation of the
MAGNETIC SUSCEPTIBILITY
■
Variable-temperature magnetic susceptibility measurements for
the three copper compounds were measured over the range of
2.0−300 K. The plots of χmT versus T show that the systems
display medium to strong antiferromagnetic interactions
(Figures 7 and 8). The plot for [Cu2(psmp)(OH)] shows
χmT ≈ 0.52 cm3 mol−1 K at room temperature (RT), which is
significantly lower than 0.75 cm3 mol−1 K, the high-temperature
value expected for two uncoupled copper(II) ions. The fit
employing the spin-Hamiltonian of eq 1
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Table 3. Selected Bond Distances (Å) and Angles (deg) for [Ga(psmpH)(psmpH2)] and [Fe(psmpH)(psmpH2)]
[Ga(psmpH)(psmpH2)]·H2O
[Fe(psmpH)(psmpH2)]·3H2O
M−O1
M−O6
M−N3
M−N4
M−N5
M−N6
S1−O2
S1−O3
S2−O4
S2−O5
S1−N1
S2−N3
O1−M−N3
O1−M−N4
O1−M−N5
O1−M−N6
O1−M−O6
O6−M−N3
O6−M−N4
O6−M−N5
O6−M−N6
1.964(4)
1.951(4)
2.020(5)
2.123(5)
2.002(5)
2.105(5)
1.433(4)
1.441(5)
1.458(4)
1.454(4)
1.618(5)
1.582(5)
92.07(19)
169.97(18)
92.77(19)
90.84(17)
93.90(16)
94.34(18)
90.46(17)
91.76(17)
170.25(17)
1.9559(19)
1.9419(19)
2.059(2)
2.166(2)
2.064(2)
2.157(3)
1.435(2)
1.436(2)
1.450(2)
1.449(2)
1.617(3)
1.589(2)
89.34(8)
164.25(9)
95.17(8)
91.89(9)
96.78(8)
97.89(9)
91.16(9)
88.49(9)
163.57(9)
Figure 7. Variable-temperature magnetic susceptibility for (a)
[Cu2(psmp)(OH)] and (b) [N(Et)3CH2Cl][Cu2(psmp)(OAc)2].
◇
The diamonds ( ) represent the experimental results, and the solid
Figure 9. Computed spin densities for the BS state of the dinuclear
model [Cu2Zn(psmp)2(H2O)] with modeled water ligand. Plot is
shown with isosurface values of 1.3 × 10−3 Å−3.
lines represent the best fits. Parameter values: (a) gz = 2.14, gx =
gy = 2.00 (fixed), J = 234 cm−1; (b) gz = 2.19, gx = gy = 2.00 (fixed), J =
115 cm−1.
Figure 8. Variable-temperature magnetic susceptibility for
[Cu3(psmp)2(H2O)]·H2O.
Figure 10. Cyclic voltammogram of [Cu3(psmp)2(H2O)] in
acetonitrile. Scan rate 100 mV s−1, reference Fc+/Fc.
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water ligand after oxidation would give rise to irreversibility. The
relevant reactions would be:
constructed with two pyridine groups and two sulfonyl O
atoms. Substitution of the water ligand with a sterically
appropriate H-donor guest might lead to supramolecular
activation chemistry since the guest can interact with the
metal complex host via five contacts; one through copper ion
coordination, two through π interactions with pyridyl rings, and
two through H bonding or electrophilic interactions
with sulfonyl O atoms. We will search for an appropriate
guest. The structures of the 2:3 and 2:1 ligand/metal complexes
[Cu3(psmp)2(H2O)] and [M(psmpH)(psmpH2)], where M =
Ga(III), Fe(III), suggest the accessibility of bis-ligand mixed
metal complexes using self-assembly and stepwise methods.
The trinuclear complex [Cu3(psmp)2(H2O)] contains two
chemically different metal coordination environments. This
arrangement hints at the possible accessibility of structurally
related heterotrimetallic complexes with the [Cu2M-
(psmp)2(X)] formulation. For example, replacing the central
Cu(II)−OH2 (M−X) unit with a topologically similar VO
can be envisaged to give the analogously neutral and
topologically similar [Cu2VO(psmp)2]. The preparation of
this complex and other heterotrinuclear Cu2/M and M′2/M
combinations may be of interest in the future development of
the chemistry of this new class of ligand.
[CuII (psmp)2(H2O)]
→3[CuII CuIII(psmp)2(H2O)]+ + e−
2
[CuII CuIII(psmp)2(H2O)]+
→2[CuII CuIII(psmp)2(OH)] + H+
2
Thus on re-reduction protonation of hydroxide ligand would
subsequently occur:
[CuII CuIII(psmp)2(OH)] + e− → [CuII (psmp)2(OH)]−
2
3
[CuII (psmp)2(OH)]− + H+ → [CuII (psmp)2(OH2)]
3
3
Several redox processes are observed in acetonitrile solutions
prepared from [N(Et)3CH2Cl][Cu2(psmp)(OAc)2] (see Sup-
porting Information, Figure S23). Acetate lability, and
rearrangements to [Cu3(psmp)2(H2O)] and [Cu2(psmp)-
(OH)], as evident in the ESI-MS are feasible, and assignment
was not attempted. [Cu2(psmp)(OH)] was too insoluble, and
no signals could be observed. [Fe(III)(psmpH)(psmpH2)]
shows an irreversible oxidation at 1.790 V and a quasi-reversible
reduction centered at −1.15 V (see Supporting Information,
Figure S24). The first process is not present in the cyclic
voltammogram (CV) of the isovalent Ga complex and is
therefore assigned to a Fe(III)/Fe(IV) process with irrevers-
ibility due to second coordination sphere protonation change.
The latter process is present in the Ga(III) complex and is
assumed to be sulfonamide-based.
EXPERIMENTAL SECTION
■
Physical Measurements. ESI-MS spectra were recorded on a
Bruker microTOF-QII. IR spectra were measured on a PerkinElmer
Spectrum 65 equipped with a universal attenuated total reflection
(ATR) sampling accessory. Solution 1H and 13C spectra were recorded
on a Bruker 400 MHz using solvent residual peak as internal standard.
Elemental analyses were performed at the Department of Chemistry,
University of Copenhagen, Denmark. Melting point (m.p.) was
CONCLUSIONS
■
determined on a Buchi 535. CVs were recorded in acetonitrile solution
̈
Our initial survey of the coordination chemistry of the new bis-
sulfonamide ligand reveals that a wide variety of nuclearities,
geometries, and protonation states are represented by the
complexes reported here. Some of their features are worth
emphasizing: All three deprotonated forms (psmp3−, psmpH2−,
using an Autolab system (Eco Chemie, The Netherlands), controlled
by GPES software. The working electrode was a Pt disk, the auxiliary
electrode was a platinum wire, and the reference electrode was
Ag/AgNO3. As electrolyte, 0.1 M TBAPF6 (TBA = tetrabutylammonium)
was used. All measurements were calibrated versus the ferrocene/
ferrocenium (Fc0/+) redox couple E1/2 = 0.44 V. CV spectra were
recorded at a scan rate of 100 mV/s. The magnetic characterization was
conducted on a Quantum Design MPMS-XL SQUID magnetometer
equipped with a 5 T dc magnet. Dc susceptibility measurements were
conducted on polycrystalline samples in polycarbonate capsules with a dc
static field of 1000 Oe. The susceptibility was corrected for diamagnetic
contributions from the compound by means of Pascal constants16 and
from the capsule (−5 × 10−7 cm3 g−1). The electron paramagnetic
resonance (EPR) spectra were recorded on a Bruker Elexsys E500
equipped with a Bruker ER 4116 DM dual mode cavity, an EIP 538B
frequency counter, an ESR9 cryostat, and a ER035 M NMR Gauss-meter.
General Information. Reagents and solvents were obtained
commercially from Sigma Aldrich and used as received. The
intermediate 2,6-dichlorosulfone-4-methylphenol was prepared using
literature methods.17
−
and psmpH2 ) of the ligand have been observed, and notably,
these can be prepared in the presence of water, even for the
fully trideprotonated ligand when a divalent metal ion is
employed. Apart from its anticipated ability to be involved in
chelation, the deprotonated sulfonamide nitrogen atom can act
as a bridging donor atom. Magnetic susceptibility measure-
ments on [Cu3(psmp)2(H2O)]·H2O suggest that this rare
bridging group is similar to a phenolato group in its ability to
commute magnetic exchange coupling. Cyclic voltammetry
suggests the +3 and +4 oxidation states may be accessible in the
copper and the iron complexes, respectively.
The relatively low solubility of complexes and a propensity
toward metal/ligand stoichiometry rearrangements has so far
limited an exploration for the potential of these complexes for
small molecule activation at their dimetallic sites. However, the
series shows topological and structural properties relevant to
these aims. The two dimetallic complexes, [Cu2(psmp)(OH)]
and [Cu2(psmp)(OAc)2]− accommodate the two extremes in
co-bridging ligands so far observed for a single μ2-phenolato
dimetal system: A single one-atom co-bridge (μ2−OH) and
two three-atom co-bridges (μ2-OAc). Such a structural
flexibility is interesting with respect to the potential of guest
activation by dimetallic systems.1 Furthermore the tricopper
complex [Cu3(psmp)2(H2O)] shows an interesting topology.
The auxiliary water ligand bound to the central copper ion is
contained in a well-defined cleft whose four walls are
Syntheses. 2,6-Bis(N-2-pyridylmethylsulfonamido)-4-methyl-
phenol (psmpH3). A solution of 2-aminopyridine (1.4 mL, 1.5 g,
13 mmol) and triethylamine (1.8 mL, 1.3 g, 13 mmol) in
dichloromethane (50 mL) was added slowly to a suspension of 2,6-
dichlorosulfone-4-methylphenol (2.0 g, 6.5 mmol) in dichloromethane
(50 mL). The resulting brown solution was stirred for approximately
16 h at RT, and the solvent was removed in vacuo. The residue was
dissolved in a mixture of water (50 mL) and ethyl acetate (100 mL).
The phases were separated, and the water phase was extracted with
ethyl acetate (3 × 100 mL). The combined organic phases were dried
over MgSO4, and the solvent was removed in vacuo to yield 2,6-bis(N-
2-pyridylmethylsulfonamido)-4-methylphenol (2.3 g, 5.2 mmol, 80%)
as an off-white powder. m.p. 118.7−122.6°. Recrystallization from a
mixture of EtOH and H2O gave platelike X-ray quality crystals. 1H NMR
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(400 MHz, CDCl3): δ (ppm) = 2.15 (s, 3H, CH3), 4.07 (s, 4 H, CH2),
7.02−7.04 (m, 2H, arom. H), 7.25 (d, 2H, J = 7.8 Hz, arom. H), 7.52−
7.57 (m, 4H, arom. H), 8.31−8.33 (m, 2H, arom. H). 13C NMR (100
MHz, CDCl3): δ (ppm) = 19.9 (CH3), 48.5 (CH2), 120.5 (arom. C),
122.4 (arom. C), 122.6 (arom. C), 123.0 (arom. C), 127.5 (arom. C),
134.4 (arom. C), 136.7 (arom. C), 149.0 (arom. C), 156.1 (arom. C).
ESI-MS pos. calcd (found) (m/z) = 449.10 (449.09, [psmpH3 + H]+,
C19H21N4O5S2, 100%). IR (Fourier transform (FT)-ATR diamond anvil)
ν (cm−1) = 1594 m, 1573 w, 1474 m, 1437 m, 1397 w, 1308 s, 1265 w,
1195 m, 1134 s, 1077 w, 1047 w, 997 w, 922 w, 887 w, 800 m, 753 s, 624
m, 565 s, 509 m, 478 w.
[Cu2(psmp)(OH)]. Triethylamine (1 mL) was added under stirring
to a suspension of psmpH3 (100 mg, 0.22 mmol) in water (10 mL).
The resulting solution was mixed with a solution of Cu(NO3)2·3H2O
(106 mg, 0.44 mmol) in water (10 mL) and allowed to stand
overnight to form [Cu2(psmp)(OH)] (58 mg, 0.1 mmol, 45%) as blue
needles, which were washed with acetone and air-dried. Anal. Calcd for
C19H18Cu2N4O6S2: C, 38.71; H, 3.08; N, 9.50. Found: C, 38.54; H,
3.04; N, 9.65%. IR (FT-ATR diamond anvil) ν (cm−1) = 1608 m, 1485
w, 1433 s, 1354 w, 1283 w, 1264 m, 1252 m, 1234 m, 1220 m, 1140 m,
1100 s, 1081 s, 1050 m, 949 w, 876 w, 817 m, 777 m, 763 m, 743 m,
719 w, 683 m, 674 w, 660 m, 642 m, 603 w, 584 m, 559 w, 544 m,
523 m, 490 w, 465 m, 444 w, 418 m.
[N(Et)3CH2Cl][Cu2(psmp)(OAc)2]. Cu(OAc)2 (175.6 mg, 0.88 mmol)
was added to a solution of psmpH3 (200 mg, 0.44 mmol) and
triethylamine (3 mL) in dichloromethane (20 mL). The resulting
suspension was refluxed overnight to form a dark greenish-brown
solution. After cooling to RT the precipitate was collected by filtration,
washed with ethanol, and air-dried to yield [N(Et)3CH2Cl]-
[Cu2(psmp)(OAc)2] (177 mg, 0.21 mmol, 95%) as a bright-green
powder. X-ray quality crystals of the dichloromethane solvate were
formed by slow evaporation of the mother liquor. Anal. Calcd for
C30H40ClCu2N5O9S2: C, 42.83; H, 4.79; N, 8.32. Found: C, 42.33; H,
4.61; N, 8.60%. ESI-MS neg. calcd (found) (m/z) = 690.95 (690.96,
[Cu2(psmp)(OAc)2], Cu2C23H23N4O9S2, 100%), 630.93 (630.93,
[Cu2(psmp)(OAc)H], Cu2C21H19N4O7S2, 41%), 606.88 (606.89,
[Cu2(psmp) + HCl], Cu2C19H16N4O5S2Cl, 23%). IR (FT-ATR
diamond anvil) ν (cm−1) = 1614 m, 1601 m, 1479 w, 1439 m, 1410 s,
1336 w, 1279 w, 1244 m, 1148 m, 1115 w, 1096 s, 1049 w, 1049 w,
1028 w, 951 w, 880 w, 838 w, 814 w, 777 m, 765 m, 720 w, 678 m,
657 s, 596 m, 556 s, 537 w, 500 w, 453 w, 420 w.
[Cu3(psmp)2(H2O)]·H2O. A solution of NaOH (10%) in water
(5 mL) was added to a solution of CuSO4·5H2O (83 mg, 0.33 mmol)
in water (4 mL). The bright blue precipitate was collected by
centrifugation, washed with cold water (5 × 6 mL), and suspended in
MeOH (5 mL). This suspension was added to a solution of psmpH3
(100 mg, 0.22 mmol) in MeOH (5 mL) and was allowed to stand
without stirring for 3 d to form [Cu3(psmp)2(H2O)] (65 mg, 0.06
mmol, 55%) as green crystals, which were separated from byproducts
and remaining copper hydroxide by hand, washed with ethanol, and
air-dried. Anal. Calcd for C38H38Cu3N8O12S4: C, 40.84; H, 3.43; N,
10.03. Found: C, 41.33; H, 3.69; N, 9.34%. ESI-MS pos. calcd (found)
(m/z) = 1103.91 (1103.91, [Cu3(psmp)2 + Na]+, Cu3C38H34N8O10S4Na,
36%), 1081.92 (1081.92, [Cu3(psmp)2 + H]+, Cu3C38H35N8O10S4,
100%), 1042.99 (1042.99, [Cu2(psmpH)2 + Na]+, Cu2C38H36N8O10S4Na,
88%), 1021.01 (1021.00, [Cu2(psmpH)2 + H]+, Cu2C38H37N8O10S4,
85%), 610.92 (610.91, [Cu2(psmp)(OH) + Na]+, Cu2C19H18N4O6S2Na,
20%), 588.93 (588.93 [Cu2(psmp)(H2O)]+, Cu2C19H19N4O6S2, 17%),
531.99 (531.99, [Cu(psmpH) + Na]+, CuC19H18N4O5S2Na, 54%),
510.01 (510.00, [Cu(psmpH) + H]+, CuC19H19N4O5S2, 38%). IR (FT-
ATR diamond anvil) ν (cm−1) = 1608 w, 1439 s, 1284 m, 1265 m, 1206
w, 1131 s, 1078 m, 1029 w, 939 w, 884 w, 811 m, 782 m, 757 s, 651 w,
631 w, 583 s, 529 m, 457 m.
formed. The mixture was allowed to slowly cool to −40 °C, the
mother liquor was decanted off, and the product was washed with ice-
cold MeOH (3 × 5 mL) and air-dried (163.2 mg, 0.17 mmol, 42.7%).
Anal. Calcd for C38H39GaN8O11S4: C, 46.49; H, 4.00; N, 11.41.
Found: C, 46.06; H, 3.42; N, 11.15%. ESI-MS pos. calcd (found)
(m/z) = 963.08 (963.08, [Ga(psmpH2)2]+, C38H38GaN8O10S4, 100%),
985.07 (985.06, [Ga(psmpH)(psmpH2) + Na]+, C38H37Ga-
N8NaO10S4, 21%). IR (FT-ATR diamond anvil) ν (cm−1) = 1602 w,
1573 w, 1442 m, 1322 m, 1275 m, 1205 w, 1125 s, 1052 m, 1025 w,
1008 w, 925 w, 860 m, 835 w, 805 m, 766 m, 652 m, 569 s, 533 m,
510 m.
[Fe(psmpH)(psmpH2)]·2H2O. NH4Fe(SO4)2·12H20 (200.0 mg,
0.40 mmol) was dissolved in a mixture of MeOH (20 mL) and
H2O (10 mL). The bright yellow solution was combined with psmpH3
(100.0 mg, 0.21 mmol) and triethylamine (0.1 mL) in MeOH (10 mL),
whereupon it immediately turned dark red. Slow evaporation yielded
needle-like red crystals, which were collected by filtration, washed with
ice-cold MeOH (2 × 5 mL), and air-dried (62.7 mg, 0.06 mmol,
30.8%). Anal. Calcd for C38H41FeN8O12S4: C, 46.30; H, 4.19; N,
11.37. Found: C, 46.86; H, 4.38; N, 11.20%. ESI-MS pos. calcd
(found) (m/z) = 950.09 (950.09, [Fe(psmpH2)2]+, C38H38FeN8O10S4,
100%), 972.08 (972.08, [Fe(psmpH)(psmpH2) + Na]+, 48%),
1004.01 (1004.01 [Fe2(psmpH)2]+, 76%). IR (FT-ATR diamond
anvil) ν (cm−1) = 1645 w, 1595 m, 1439 s, 130 m, 1210 m, 1150 m,
1105 s, 925 m, 815 m, 758 m, 665 w, 571 m.
Computational Modeling. For evaluation of the exchange
couplings, the BS approach of Noodleman18 as implemented in the
ORCA version 2.8 suite of programs19,20 was employed. The
formalism of Yamaguchi,21 which employs calculated expectation
values ⟨S2⟩ for both high-spin and BS states, was used. Calculations
related to magnetic interactions have been performed using the PBE0
functional. The def2-TZVP basis function set from Alrichs was used.22
Spin densities were visualized using the UCSF Chimera program
version 1.5.3. The modeling of the magnetic data was performed with
the MagProp program as included in the DAVE suite.23
Single-Crystal X-ray Diffraction. Selected crystallographic data
are presented in Table 4. Diffraction data were collected using a
Bruker-Nonius X8 APEX-II instrument (Mo Kα radiation, graphite
monochromated fine-focused sealed tube), Oxford Agilent Supernova
(Mo Kα radiation), or a Rigaku R-Axis IIC image-plate system
(graphite monochromated Mo Kα radiation from a Rigaku RU-H3R
rotating anode). Structure solutions were carried out with either
SHELXS-97/201324 or SIR-9225 and were refined against F2 by full
matrix least-squares using SHELXL-97/2013.24 For psmpH3, all
hydrogen atoms were located in a difference Fourier map and refined
with isotropic displacement parameters Uiso(H) = 1.5Ueq when
connected to hydroxyl or methyl groups and Uiso(H) = 1.2Ueq for
others. Sulfonamide H-atoms were constrained to a distance N···H of
0.91 0.01 Å, and hydroxyl H atom was constrained to 0.84 0.01 Å.
For all other structures H atoms on C atoms were placed at calculated
positions and allowed to ride during refinement. When possible, H
atoms on sulfonamide/amine, coordinated hydroxo and/or water
molecules were located in difference Fourier maps, and their positions
were refined with restrained N/O−H distances or placed at calculated
positions to form reasonable H bonds; subsequently, they were
allowed to ride on the parent atom. Residual electron density,
attributed to cocrystallized solvent molecules that was not possible to
model satisfactorily in the structure of [Ga(psmpH)(psmpH2)], was
removed using the SQUEEZE-routine in PLATON.26 Crystallographic
data (excluding structure factors) for structures within this Paper have
been deposited to the Cambridge Crystallographic Data Centre
(CCDC 964331−964336).
[Ga(psmpH)(psmpH2)]·H2O. A solution of Ga(NO3)3·H2O (100.0 mg,
0.39 mmol) in MeOH (5 mL) was added to a mixture of pmspH3
(350.8 mg, 0.78 mmol, 2 equiv) and NaOAc·3H2O (319.3 mg,
2.35 mmol) in MeOH/H2O (20:5 v/v). The resulting clear yellow
solution was allowed to stand in an open beaker at RT for 2 h,
whereupon it started becoming cloudy. The beaker was sealed and left
undisturbed overnight, after which an off-white crystalline mass had
ASSOCIATED CONTENT
■
S
* Supporting Information
Complete crystallographic data (as CIF-files) for structures 1−
6; NMR, FT-IR, and ESI-MS spectra; discussion of magnetic
properties of [Cu3(psmp)2(H2O)]·H2O. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Johansson, F. B.; Veltze,
Dalton Trans. 2011, 40, 3336−3345.
(12) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Part B; Wiley Interscience: Hoboken, NJ,
1997; p 60.
́
S.; Skou, E.; Bond, A. D.; McKenzie, C. J.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mckenzie@sdu.dk. Phone: 45 6550 2518. Fax: (+45)
6615 8760.
(13) (a) Wrigth, D.; Wulff, C. J. Org. Chem. 1970, 35, 4252−4252.
(b) Almarzoqi, B.; George, A. V.; Isaacs, N. S. Tetrahedron 1986, 42,
601−601. (c) Mas, M.; Sola, J.; Solans, X.; Aguilo, M. Inorg. Chim.
Acta. 1987, 133, 217−219.
Funding
The authors declare no competing financial interest in funding
sources.
(14) (a) Menabue, L.; Saladini, M. Inorg. Chem. 1991, 30, 1651−
1655. (b) Goodwin, J. M.; Chiang, P.-C.; Brynda, M.; Penkina, K.;
Olmstead, M. M.; Patten, T. E. Dalton Trans. 2007, 3086−3092.
(c) Ishiwata, K.; Kuwata, S.; Ikiraya, T. J. Am. Chem. Soc. 2009, 131,
5001−5009.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(15) Sanmartín, J.; Novio, F.; García-Deibe, A. M.; Fondo, M.;
Ocampo, N.; Bermejo, M. R. Eur. J. Inorg. Chem. 2008, 1719−1726.
(16) (a) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532−536.
(b) Gutierrez, L.; Alzuet, G.; Real, J. A.; Cano, J.; Borras, J.;
Castineiras, A. Inorg. Chem. 2000, 39, 3608−3614. (c) Gutierrez, L.;
Alzuet, G.; Borras, J.; Liu-Gonzalez, M.; Sanz, F.; Castineiras, A.
Polyhedron 2001, 20, 703−709. (d) Congreve, A.; Kataky, R.; Knell,
M.; Parker, D.; Puschmann, H.; Senanayake, K.; Wylie, L. New J.
Chem. 2003, 27, 98−106. (e) Duran, M. L.; Garcia-Vazquez, J. A.;
Gomez, C.; Sousa-Pedrares, A.; Romero, J.; Sousa, A. Eur. J. Inorg.
Chem. 2002, 2348−2354.
This work was supported by the Danish Council for
Independent Research | Natural Sciences. We thank Dr. Jason
Price for assistance with part of the crystallographic studies.
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