"Scorpionate-like" complexes that are held together by hydrogen bonds: Crystallographic and spectroscopic studies of (3-NH(t-butyl)-5-methyl-pyrazole)nMX2 (M = Zn, Ni, Co, Mn; N = 3, 4; X = Cl, Br)

Article abstract of DOI:10.1016/j.poly.2015.10.003

The hydrogen bonding ligand, 3-NH(t-butyl)-5-methyl-pyrazole, forms "scorpionate-like" first row transition metal complexes that are held together by hydrogen bonds rather than covalent bonds. The formulae of these complexes are (LH)_nMX₂, where n = 3, 4; X = Cl, Br; and LH = 3-NH(t-butyl)-5-methyl-pyrazole. The amino-substituted pyrazole can hydrogen bond via both the amino group and the pyrazole NH to form intramolecular NH to halide hydrogen bonds. These complexes have been well characterized and show a 3:1 ratio of ligand to metal for zinc and cobalt (1 and 2), and a 4:1 ratio of ligand to metal for manganese and nickel (3 and 4). The hydrogen bonding interactions appear to be stronger for the 3:1 complexes. The crystallographic and spectroscopic studies (EPR and NMR) have shown that these hydrogen-bonding interactions are strong enough to perturb metal halogen bond distances and, with non-hydrogen bonding solvents, the hydrogen bonds appear to hold these complexes together in solution.

Full text of DOI:10.1016/j.poly.2015.10.003

Polyhedron xxx (2015) xxx–xxx
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Polyhedron
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‘‘Scorpionate-like” complexes that are held together by hydrogen bonds:
Crystallographic and spectroscopic studies of (3-NH(t-butyl)-5-methyl-
pyrazole)nMX2 (M = Zn, Ni, Co, Mn; n = 3, 4; X = Cl, Br)
Lee Serpas a, Robert R. Baum c, Alyssa McGhee c, Ismael Nieto a, Katherine L. Jernigan e, Matthias Zeller b,
Gregory M. Ferrence d, David L. Tierney c, Elizabeth T. Papish a,e,
⇑
a Drexel University, Dept. of Chemistry, 3141 Chestnut St., Philadelphia, PA 19104, United States
b Youngstown State University, Dept. of Chemistry, One University Plaza, Youngstown, OH 44555, United States
c Miami University, Dept. of Chemistry and Biochemistry, 701 East High Street, Oxford, OH 45056, United States
d Illinois State University, Dept. of Chemistry, CB 4160, Normal, IL 61790, United States
e The University of Alabama, Dept. of Chemistry, 250 Hackberry Lane, Tuscaloosa, AL 35401, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The hydrogen bonding ligand, 3-NH(t-butyl)-5-methyl-pyrazole, forms ‘‘scorpionate-like” first row
transition metal complexes that are held together by hydrogen bonds rather than covalent bonds. The
formulae of these complexes are (LH)nMX2, where n = 3, 4; X = Cl, Br; and LH = 3-NH(t-butyl)-5-
methyl-pyrazole. The amino-substituted pyrazole can hydrogen bond via both the amino group and
the pyrazole NH to form intramolecular NH to halide hydrogen bonds. These complexes have been well
characterized and show a 3:1 ratio of ligand to metal for zinc and cobalt (1 and 2), and a 4:1 ratio of ligand
to metal for manganese and nickel (3 and 4). The hydrogen bonding interactions appear to be stronger for
the 3:1 complexes. The crystallographic and spectroscopic studies (EPR and NMR) have shown that these
hydrogen-bonding interactions are strong enough to perturb metal halogen bond distances and, with
non-hydrogen bonding solvents, the hydrogen bonds appear to hold these complexes together in
solution.
Received 4 July 2015
Accepted 5 October 2015
Available online xxxx
Keywords:
Hydrogen-bonds
Scorpionates
Transition metal complexes
Pyrazoles
Anion binding
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
scaffold. These studies have shown that hydrogen bonds can tune
the redox potentials of the bound metal [10,11], allow for the iso-
Hydrogen bonding motifs are found in many enzymes and are
often responsible for determining biological function [1]. They also
provide an important design principle for chemists wishing to
mimic Nature’s selectivity and efficiency. For example, key differ-
ences in the presence or absence of hydrogen bond donors and
acceptors determine why oxygen-bound peroxidase enzymes
cleave the O–O bond but the globins bind and release oxygen, leav-
ing the O–O bond intact [2,3]. Several different types of ligands
have been designed to provide hydrogen bonds near the metal cen-
ter [4,5]. Amide and urea groups have been used by Masuda [6] and
Borovik [7], respectively, and these groups offer strong hydrogen
bonds near the metal center on a tripodal scaffold (Scheme 1a
and b). Berreau [8] and Mareque-Rivas [9] (Scheme 1c and d) have
used amine hydrogen bonding groups to construct a similar
lation of rare metal oxo complexes [12], and aid in the activation of
water and CO2 [13–15]. Recently, during the later stages of our
study, a new ligand with a tripodal network of hydrogen bonds
near the metal has been reported by Szymczak (Scheme 1e) [16].
However, all of the above species have supported trigonal–
bipyramidal geometries at the metal center. We were intrigued
with the possibility of a non-coordinating atom anchoring the
hydrogen-bonding scaffold.
We have had a long-standing interest in creating scaffolds that
place hydrogen bonds near and far from the metal center [17–24].
The work herein describes the synthesis of a novel tripodal,
monoanionic ligand based upon amino substituted pyrazole rings
(LH) that offers hydrogen bonds near the metal center. In the
course of our synthetic efforts towards novel tripodal ligands, we
discovered that 3-(NH-t-Butyl)-5-methyl-pyrazole (LH) can sup-
port metal complexes ((LH)nMX2) which contain a network of six
to eight intramolecular hydrogen bonds. In particular, this study
shows that hydrogen bonds alone can be used to bind anions,
⇑
Corresponding author at: The University of Alabama, Dept. of Chemistry, 250
Hackberry Lane, Tuscaloosa, AL 35401, United States.
E-mail address: etpapish@ua.edu (E.T. Papish).
http://dx.doi.org/10.1016/j.poly.2015.10.003
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
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2
L. Serpas et al. / Polyhedron xxx (2015) xxx–xxx
Scheme 1. Hydrogen bonding ligand scaffolds designed by Masuda (a), Borovik (b), Berreau (c), Mareque-Rivas (d), and Szymczak (e).
and metal to halide bonds are lengthened by hydrogen bonding
interactions.
encouraged by a report by Aullón et al. that demonstrated compu-
tationally that three-position amine groups (NH2 in their case)
were helpful for oxygen activation [25]. We chose instead to use
NH(t-butyl) groups for steric protection of the metal center. We
initially aimed to synthesize chelates, as described in the support-
ing information, but these initial efforts were unsuccessful. In the
course of these studies we discovered that 3-NH(t-butyl)-5-
methyl-pyrazole (LH) supports structures with intramolecular
hydrogen bonds that are interesting in their own right.
2. Results and discussion
2.1. Synthesis and ligand design
We used pyrazole-based ligands with amine groups at the three
position for formation of a hydrogen bonding network. We were
Scheme 2. The synthesis of 3-NH(t-butyl)-5-methyl-pyrazole.
Fig. 1. Structural diagram for the tetramer of 3-(NH-t-Butyl)-5-methyl-pyrazole (LH). Ellipsoids are shown at 30% probability. Hydrogen bonds (red dashed lines) connect
four pyrazole molecules (two asymmetric units). Non-hydrogen bonding hydrogen atoms are omitted for clarity. (Colour online.)
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L. Serpas et al. / Polyhedron xxx (2015) xxx–xxx
3
Scheme 3. The products formed from attempts to coordinate Tl(L3BH) to a transition metal. Combining excess LH and MX2 in solution also forms these products. Hydrogen
bonds are shown in red. Note that for complex 4, short contacts between NH and Br are present, but these are too long to be hydrogen bonds. (Colour online.)
We began by synthesizing the pyrazole ring (LH) as described in
the literature (Scheme 2) [26]. Steps 1 and 2 were achieved in 95%
and 96% yields, respectively. The product pyrazole was character-
ized by 1H and 13C-NMR, IR and MS, and all spectra matched the
literature report. This pyrazole was crystallized from toluene, and
the X-ray crystal structure is described below (see Section 2.2,
Fig. 1).
The (LH)nMX2 products (1–4) can be formed in high yields and
remarkably, they resemble scorpionate complexes but they are
held together only by hydrogen bonds and coordinate-covalent
ligand to metal interactions (Scheme 3). These complexes can also
be regarded as self-assembled molecular structures. The M = ZnII,
CoII complexes (1, 2, respectively) show a three to one ratio of
ligand to metal and are characterized by spectroscopic and analyt-
ical methods (with crystal structures obtained). The M = MnII, NiII
complexes (3, 4, respectively) show a four to one ratio of ligand
to metal and all are characterized by spectroscopic, analytical,
and crystallographic methods. The reasons for these coordination
preferences appear to be related to the size of the ionic radii (ZnII
is the smallest ion and prefers a tetrahedral structure; conversely
MnII is largest). Crystal field stabilization also reaches a maximum
for NiII among first row divalent ions [27–31] and would favour a
Table 1
Crystallographic experimental details. Experiments were carried out at 100 K with graphite monochromated Mo Ka radiation on Bruker AXS SMART APEX CCD or APEX-II CCD
diffractometers.
LH
1 = (LH)3ZnBr2
2 = (LH)3CoCl2
3 = (LH)4MnCl2
4EtOH = (LH)4NiBr2ÁEtOH
Chemical formula
Mr
Crystal system, space
group
C8H15N3
153.23
monoclinic, P21/c
C24H45Br2N9Zn
684.88
trigonal, R3c:H
C24H45ClCoN9ÁCl
589.52
monoclinic, P21/c
C32H60ClMnN12ÁCl
C32H60Br2N12NiÁC2H6O
738.76
877.51
ꢀ
monoclinic, P21/n
triclinic, P1
a, b, c (Å)
9.030 (5), 10.662 (7),
18.516 (11)
90, 91.78 (2), 90
13.850 (3), 13.850,
27.831 (7)
90, 90, 120
9.1109 (12), 28.167 (4),
12.1977 (16)
90, 101.607 (2), 90
12.4871 (6), 12.4903
(6), 14.1462 (7)
72.856 (3), 78.438 (3),
89.997 (3)
2061.42 (18)
2
11.9812 (9), 29.808 (2),
12.0344 (10)
90, 90.316 (2), 90
a
, b,
c
(°)
V (Å3)
1781.7 (19)
8
0.07
4623 (3)
6
3.42
3066.2 (7)
4
0.76
4297.8 (6)
4
2.35
Z
l
(mmÀ1
)
0.49
Crystal shape
rod
block
plate
plate
block
Colour
colourless
0.44 Â 0.20 Â 0.10
Multi-scan, Apex2
0.648, 0.746
colourless
0.41 Â 0.21 Â 0.18
Multi-scan, Apex2
0.581, 0.746
7226, 2636, 2093
green
colourless
green
Crystal size (mm)
Absorption correction
Tminimum, Tmaximum
0.35 Â 0.16 Â 0.08
Multi-scan, Apex2
0.616, 0.746
25275, 9132, 6893
0.27 Â 0.26 Â 0.04
Multi-scan, TWINABS
0.565, 0.746
12873, 12873, 8614
0.55 Â 0.55 Â 0.38
Multi-scan, Apex2
0.523, 0.746
100363, 13513, 10798
No. of meas., indep., obs. 8728, 4352, 2771
[I > 2 (I)] refl.
Rint
r
0.046
0.667
0.031
0.719
0.037
0.730
0.0859
0.741
h = À17 ? 18, k = -
17 ? 18, l = 0 ? 20
0.056, 0.132, 1.02
12,873, 454, 6
0.042
0.739
(sin h/k)max (ÅÀ1
Range of h, k, l
)
h = À11 ? 12, k = À14 ?
13, l = À24 ? 20
0.059, 0.148, 1.01
4352, 219, 4
h = À19 ? 14, k = À5 ?
19, l = À25 ? 39
0.025, 0.065, 1.05
2636, 114, 1
h = À13 ? 13, k = À40 ?
39, l = À17 ? 14
0.042, 0.106, 1.03
9132, 365, 14
h = À17 ? 17, k = À43 ?
43, l = À17 ? 17
0.042, 0.103, 1.08
13,513, 773, 277
R[F2 > 2 (F2)], wR(F2), S
r
No. reflections,
parameters.,
restraints
D
qmaximum
,
D
qminimum
0.28, À0.26
0.44, À0.32
0.76, À0.46
0.57, À0.55
1.58, À0.76
(e ÅÀ3
)
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4
L. Serpas et al. / Polyhedron xxx (2015) xxx–xxx
Fig. 2. Structural diagram for (LH)3ZnBr2 (1). Ellipsoids are shown at 30%
probability. Hydrogen bonds are shown with red dashed lines. Non-hydrogen
bonding hydrogens are omitted for clarity. (Colour online.)
Fig. 3. Structural diagram for (LH)3CoCl2 (2). Ellipsoids are shown at 30% proba-
bility. Hydrogen bonds are shown with red dashed lines. Non-hydrogen bonding
hydrogens are omitted for clarity. The pyrazole ring on the left shows disorder in
the NH-t-Bu unit. (Colour online.)
(nearly) octahedral geometry. Regardless of the metal to ligand
ratio used in the synthesis the same complex is formed. For exam-
ple, the (LH)4NiBr2 product is formed from a three to one ratio of
ligand to metal with some unreacted metal salt leftover in this
case. These crystal structures are described below. The metal com-
plexes (1–3) made from LH show hydrogen bonding interactions
that are summarized and compared in Table 3; the influence of
the geometry on the strength of the hydrogen bonds and the
M–X distances is readily apparent and is discussed below.
and eight LH molecules are in the unit cell. Each pyrazole moiety
shows three hydrogen bonds to neighboring pyrazole molecules
in the crystal phase. There are six weak[32] N–HÁ Á ÁN hydrogen
bonds in this structure (all NÁ Á ÁN distances are above 2.914(3) Å);
N-H1 and N-H3 are hydrogen bond donors and N2 is a hydrogen
bond acceptor. However, as shown below, N2 can also bind to a
metal.
The zinc complex with 3-(NH-t-butyl)-5-methyl-pyrazole (LH),
Fig. 2, binds two bromide ions forming a ditopic salt with the for-
mula (LH)3ZnBr2 (1). One bromide (Br1) is bound by Zn and three
weak hydrogen bonds (NÁ Á ÁBr = 3.441(3) Å, N–HÁ Á ÁBr = 161.9(2)°)
[33]. The other bromide (Br2) is bound by three moderate strength
hydrogen bonds [33] (NÁ Á ÁBr = 3.281(3) Å, N–HÁ Á ÁBr = 173.6(2)°);
Table 3 shows that the moderate strength hydrogen bonds exhibit
NÁ Á ÁBr distances that are less than the distance predicted from
the ionic radius of bromide and the covalent radii of N and H
2.2. Crystal Structures
The pyrazole ligand is a known compound but the X-ray crystal
structure shown in Fig. 1 is new. The crystallographic experimental
data for this compound and all other compounds is given in Table 1.
3-NH-t-Butyl-5-methyl-pyrazole (LH) shows
hydrogen bonds that connect four pyrazole molecules in the crys-
tal phase (Fig. 1). Two LH molecules are in the asymmetric unit,
a
network of
Scheme 4. The closest literature analogs to our complexes 1–4 are (a) (3-t-butyl-pyrazole)3ZnBr2 [34] and (b) (3-amino-5-phenyl-pyrazole)3ZnCl2 [35]. These structures
show intramolecular hydrogen bonds.
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L. Serpas et al. / Polyhedron xxx (2015) xxx–xxx
5
atoms. Notably, this structure is dominated by intramolecular
hydrogen bonds, and intermolecular hydrogen bonds are not
present in the crystal phase. A close analog to this structure in
the literature is (3-t-butyl-pyrazole)3ZnBr2 (Scheme 4a) in which
three hydrogen bonds encapsulate a halide ion [34]; interestingly
this complex has the t-butyl groups turned away from the metal
center unlike the arrangement seen for 1. Also similar to 1 is the
(2)° in the empty cavity above the metal. Given the structure type
(which constrains N to Cl distances) the angles are a better
measure of hydrogen bond strength than the distances [32].
The Mn–Cl1 bond and the Mn to Cl2 ionic interaction forces the
chlorides to be close to the NH of the pyrazole co-ligands regard-
less of whether (or not) hydrogen bonds are present. Thus, these
non-linear angles (Table 3) indicate very weak hydrogen bonds
are present. Around the Mn–Cl1 bond, the hydrogen bonds appear
more linear with N–HÁ Á ÁCl1 angles of 167(3)–171(3)° but N to Cl1
distances are longer and range from 3.306(3) Å to 3.320(3) Å.
Through comparison with the cobalt structure (Fig. 3), it appears
that a 4:1 ratio of ligand to metal leaves the hydrogen bonds
around the M–Cl bond relatively unperturbed (which use NHt-
butyl), but the hydrogen bonds from the NH of the pyrazole ring
are much weaker due to angle deviations. The Mn–Cl1 distance
is 2.470(1) Å, a value that is much longer than typical Mn–Cl dis-
tances for five coordinate complexes (2.358 Å marks the 75th per-
centile for Mn–Cl distances [38] and 2.47 Å is the sum of the ionic
radii for MnII and chloride). This Mn–Cl1 bond lengthening could
be due to the NH hydrogen bonds or due to a weak Mn–Cl2 ionic
interaction (2.954(1) Å shows a short contact but not a bond).
Unlike complexes 1–3, the nickel complex, (LH)4NiBr2 (4)
(Fig. 5), does not show any NH to Br hydrogen bonds. Complexes
1–3 lack solvent molecules in the crystal phase, but complex 4 con-
tains ethanol solvent in the structure. In 4, the NÁ Á ÁBr distances are
too long (3.336(4)-3.454(2) Å) for hydrogen bonding interactions,
and the N–HÁ Á ÁBr angles are non-linear (Table 3). Furthermore,
the pyrazole rings are tilted at an angle (relative to the Ni–Br axis
with Br–Ni–N–(C/N) torsion angles of 36–46°) forcing the NH
groups to tilt away from the bromides. In contrast, analogous
halide–M–N–(C/N) torsion angles are ꢀ11–16° for the Mn complex
3, ꢀ0–7° for the Co complex 2, and ꢀ1° for the Zn complex 1. Thus,
(LH)4NiBr2 is the first complex in this series to show the pyrazole
rings canted (relative to the Br–Ni–Br axis) since the NH to halide
hydrogen bonding interaction is not present. The presence of disor-
dered ethanol solvent (not shown in Fig. 5) which hydrogen bonds
to bromide (Br2) in the crystal phase likely explains why this struc-
ture lacks NH to bromide hydrogen bonds. This interaction likely
explains why the Ni1 to Br2 distance is so long; at 2.809(1) Å it
is well above the 75th percentile value of 2.572 Å [38]. The Ni1
to Br1 distance is more typical at 2.568(1) Å, which is less than
the sum of the ionic radii (2.65 Å). This is the only structure that
shows two M–X bonds, although one is weak.
reported
structure
of
(3-amino-5-phenyl-pyrazole)3ZnCl2
(Scheme 4b) which also features N–H to Cl hydrogen bonding
interactions [35]. Halides seem especially well suited for hydrogen
bonding in this geometry as, notably, some Zn(II) complexes of
3-amino substituted pyrazole ligands that lack halides do not exhi-
bit such hydrogen bonding arrangements [36,37]. The Zn–Br dis-
tance in 1 is very long, at 2.441(1) Å (cf. Zn–Br = 2.3557(3) Å in
(3-t-butyl-pyrazole)3ZnBr2 [34] and 2.399 Å defines the start of
the upper quartile for four coordinate Zn–Br bond distances in
Orpen’s review [38]). However, a Zn–Br bonding interaction is still
present in 1 as shown by a distance that is less than 2.56 Å,
which is the sum of the ionic radii for ZnII and bromide (Table 3).
The Zn–Br bond lengthening (relative to average Zn–Br distances)
is a result of three weak hydrogen bonds. Interestingly, the struc-
ture in Fig. 2 resembles a scorpionate complex, but the ligands
are tethered by three hydrogen bonds to a halide rather than three
covalent bonds.
Similarly, the cobalt complex, (LH)3CoCl2 (2) (Fig. 3), shows six
hydrogen bonds that bind two chloride anions. Again, there are
strong hydrogen bonds (ranging from 3.122(2) to 3.132(2) Å for
N to Cl2 distances with nearly linear angles ranging from 175.5
(1) to 178.6(1)°) away from the metal and weak hydrogen bonds
near the metal (3.318(2) to 3.338(3) Å for N to Cl1 distances, and
164(2) to 170(2)° for N–HÁ Á ÁCl angles). The Co1–Cl1 distance is rel-
atively long (at 2.302(1) Å), indicating that the three hydrogen
bonds have weakened the chloride to metal interaction [38]. Like
the zinc complex (Fig. 2), the cobalt complex in Fig. 3 resembles
a scorpionate complex but is held together by the ligand to metal
interactions and hydrogen bonds. As shown in Tables 2 and 3,
complexes 1 and 2 have very similar metrical parameters (espe-
cially when compared to the calculated values) and both indicate
hydrogen bonds that weaken somewhat the M–X bond.
In contrast, the manganese complex, (LH)4MnCl2 (3) (Fig. 4), has
a ligand to metal ratio of 4:1. This arrangement appears to make
the hydrogen bonds weaker with N to Cl2 distances ranging from
3.114(3) Å to 3.128(3) Å with N–HÁ Á ÁCl2 angles 137.0(2)–138.2
Table 2
Bond lengths (Å) and angles (°) for complexes 1–4. Complex 1 displays C3 symmetry and thus there are only 2 unique bond lengths and angles around the metal center.
Comparable distances are shown in grey.
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L. Serpas et al. / Polyhedron xxx (2015) xxx–xxx
Table 3
A comparison of bond lengths and angles relevant to hydrogen bonding in complexes 1–4. Nearly linear N–H–X angles are an indication of strong hydrogen bonding interactions
[32]. The directly comparable data is shaded in the same colour for ease of comparison of experimental and calculated data.
aThe calculated values of M–X come from the ionic radii of the bound ions to estimate M–X distances as the sum of the ionic radii. A significant bonding interaction should
decrease this distance. Conversely, hydrogen bonding to the halide can lengthen this distance.
bThe minimal N–X distance involving a hydrogen bonded (NHÁ Á ÁX) interaction, as calculated from the covalent and ionic radii of the atoms (N, H) and ions (X = ClÀ or BrÀ)
involved in hydrogen bonding.
2.3. Mass spectrometry (MS)
one pyrazole ring (LH) when using the softer LIFDI method. Similar
to the manganese complex 3, the relatively weak interactions in 4
are shown by loss of LH and bromide by the FAB MS technique, and
the loss of LH in the highest molecular weight peak when using the
softer LIFDI technique.
The strength of the hydrogen bonds for 1 and 2 is reflected by
the MS results. Complex 1 is stable enough to be detected in the
mass spectrometer, reflecting the strength and persistence of the
bonds in the complex. Therefore, the complex is effective at anion
binding, as shown here with bromide. Other examples of anion
binding through hydrogen bonds have been reported in the litera-
ture [39–43]. Complex 2 is also relatively stable in the mass spec-
trometer, with the loss of an HCl unit occurring before it reaches
the detector and the observed ion corresponds to [(LH)2(L)CoCl]+.
For complex 3, the relative weakness of hydrogen bonds and metal
to pyrazole interactions is shown by the loss of L and Cl prior to
reaching the detector with the FAB MS method, and the loss of
2.4. Spectroscopy
Complexes 1–4 present two distinct structure types – (LH)4MX2
(Ni and Mn) and (LH)3MX2 (Co and Zn). We will discuss the struc-
ture types independently, focusing on the more spectroscopically
accessible Ni and Co complexes. The d5 Mn(II) ion gives very weak
optical spectra, and its electron relaxation properties generally
make 1H NMR unobservable. Consequently, its complexes can only
Fig. 5. Structural diagram for (LH)4NiBr2 (4). Ellipsoids are shown at 30% proba-
bility. Most hydrogens and ethanol solvent molecule are omitted for clarity.
Disordered atoms have been hidden for clarity, see the Supporting Information for a
view that includes disordered ethanol and pyrazole rings.
Fig. 4. Structural diagram for (LH)4MnCl2 (3). Ellipsoids are shown at 30%
probability. Hydrogen bonds are shown with red dashed lines. Non-hydrogen
bonding hydrogens are omitted for clarity. (Colour online.)
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L. Serpas et al. / Polyhedron xxx (2015) xxx–xxx
7
Fig. 6. (left) UV–vis-NIR spectra in CHCl3 (solid line) and CH3OH (dashed line) and (right) 200 MHz 1H NMR of (LH)4NiBr2 (4) in different solvents.
Fig. 7. UV–vis-NIR (left) and 200 MHz 1H NMR in CDCl3 (right) of (LH)3CoCl2 (2).
be probed using EPR. Similarly, the d10 Zn(II) ion is neither optically
nor EPR active, and its complexes give NMR spectra that are often
difficult to distinguish from those of the uncomplexed ligand. In
contrast, the d8 Ni(II) ion gives meaningful optical spectra and its
complexes are amenable to NMR, while complexes of the d7 Co
(II) ion give readily observable signals in optical, NMR and EPR
spectroscopies.
As discussed above, the two (LH)4MX2 complexes yielded differ-
ent crystal structures, dependent on the presence, or absence, of a
hydrogen-bonding solvent in the crystal lattice. To determine if
these two substructures could interconvert, we examined the sol-
vent-dependence of their spectroscopy. The optical spectrum of
(LH)4NiBr2 (4) in chloroform shows a strong Br ? Ni CT band and
weak, but measurable ligand-field intensity (Fig. 6, left). Although
not conclusive, the ligand-field extinction coefficients are consis-
tent with previously reported five-coordinate Ni(II) Tp complexes
[44,45] and larger than expected for a six-coordinate Ni(II) [46].
The CT band shifts to much higher energy in methanol, and the
ligand-field bands are almost entirely attenuated, suggesting the
H-bonding solvent alters, or possibly disrupts the structure of the
complex. 1H NMR of the NiII complex is also highly solvent-depen-
dent (Fig. 6, right). In CDCl3, the NiII complex gives a very broad
envelope of resonances from 15 to 65 ppm, all of which virtually
pattern at g = 2.0 (A(55Mn) = 82 G), and lower-field transitions,
consistent with the outer doublets of an S = 5/2 MnII ion under
the influence of a small zero-field splitting (D ꢀ 0.1 cmÀ1) [47].
These values of g, A, and D are similar to well-characterized octahe-
dral complexes of Mn(II), including MnN4Cl2 complexes [48], sug-
gesting the Mn(II) complex may be six-coordinate in frozen
solution, with the possible coordination of chloride in the sixth
site. Although poorly resolved, 14N hyperfine couplings clearly
indicate that the pyrazole ligands remain coordinated. However,
at present, it is difficult to determine if the Mn(II) complex retains
the five-coordinate structure seen in the crystal structure in solu-
tion, or collapses to the canted structure shown by the Ni(II) com-
plex. Overall, the spectroscopy is consistent with intact Ni(II) and
Mn(II) complexes in both aprotic and protic solvents, and leaves
open the possibility that these structures will interconvert, depen-
dent on the properties of the solvent.
The Co(II) complex, (LH)3CoCl2, representing the other structure
type, shows similar solvent dependence, with protic solvents
appearing to disrupt the structure. In chloroform, the CoII complex
(2) shows a strong Cl ? Co CT band at k ꢀ 390 nm and ligand-field
transitions that extend from k ꢀ 475 to 700 nm, along with a
weaker ligand-field transition near 1100 nm (Fig. 7, left), and
extinction coefficients consistent with four coordination. The CT
band shifts to higher energy in methanol, and the intensity in the
ligand-field region is dramatically attenuated, suggesting
the hydrogen-bonding solvent may either alter the structure of
the complex itself, or alter the hydrogen-bonding interaction
disappear on introduction of 20 ll of D2O (and are nearly absent
in CD3OD). In CD3CN, the 1H NMR spectrum sharpens significantly,
accompanied by small changes in chemical shift. This may reflect
replacement of one or both bromide ions with coordinated acetoni-
trile, or it may reflect the conversion of (4) into the H-bonded
structure demonstrated by the Mn(II) complex. We were unable
to examine the temperature-dependence of the Ni(II) complex’s
NMR, due to significant precipitation at lower temperatures. EPR
of the Mn complex (3) (Fig. S10) in 70/30 toluene/chloroform
with the halide ion. Conversion to
a higher-coordination
CoClx(LH)y(MeOH)z complex would also allow for retention of the
Cl ? Co CT band and account for attenuated ligand-field transi-
tions. Proton NMR in CDCl3 shows sharp features that are consis-
tent with retention of the H-bonded structure (Fig. 7, right), and
temperature-dependent studies (Fig. S7) show evidence of
(aprotic, non-coordinating solvents) shows
a common 6-line
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8
L. Serpas et al. / Polyhedron xxx (2015) xxx–xxx
dynamic behavior, including coalescence of a the pair of peaks near
20 ppm (most likely from the t-butyl groups) at low temperature,
and a sharp-to-broad-to-sharp pattern displayed by the most
downfield shifted resonance. While the nature of these dynamics
is at present unclear, and is the subject of further study, the tem-
perature-dependent 1H NMR of the CoII complex clearly indicates
that the complex remains intact in CDCl3, and that it is in motion.
EPR of the CoII complex further supports its description as an intact
tetrahedral complex based on weak intensity and its line shape
(Fig. S8) [49]. The 1H NMR spectrum of the ZnII complex (1) in
CDCl3 (Fig. S1) shows only resonances attributable to an intact
cluster, including a very broad solvent exchangeable resonance.
Overall, spectroscopy of the (LH)3MX2 complexes suggests that
indeed the two structure types may interconvert.
4.2. Synthesis of 3-NH(t-butyl)-5-methyl-pyrazole (LH)
This compound was synthesized according to a literature
procedure [21].
4.3. Synthesis of (LH)3ZnBr2 (1)
The pyrazole LH (1.001 g, 6.533 mmol) was treated with ZnBr2
(0.491 g, 2.18 mmol) in 50 mL of ethanol and allowed to stir for
20–24 h. Then, the solvent was removed under vacuum to produce
a yellow oil. The product was recrystallized from CH2Cl2 and pen-
tane (1:2) which allowed for the isolation of a crystalline precipi-
tate (95% yield, 1.418 g, 2.071 mmol) that was dried under
vacuum. 1H NMR in CDCl3 (d, ppm): 1.293 (tBu, s, 27H), 2.266
(Me, s, 9H), 5.569 (4-pz, s, 3H). 13C NMR in CDCl3 (d, ppm): 11.74
(Me), 29.71 (C(CH3)3), 51.59 (C(CH3)3), 93.39 (4-pz), 144.22 and
3. Conclusions
155.63 (3-pz and 5-pz). IR (m
, cmÀ1): 3370, 3151, 3082, 2978,
2877, 1585, 1514, 1397, 1367, 1230, 1212, 1040, 957, 774. FAB
MS (m/z): 684.1 [M]+, 604.2 [MÀHBr]+, 451.1 [MÀHBrÀLH]+, 298.0
[MÀHBrÀ2LH]+. High res. MS: (m/z) = 602.2297 expt. (602.2273
calcd.), 604.2234 expt. (604.2252 calcd.). In both MS experiments,
all peaks showed the expected isotopic pattern. Elemental Anal. expt.
Calcd.: C, 43.04 (42.09); H, 6.74 (6.62); N, 18.57 (18.41).
The complexes described herein are composed of ‘‘scorpionate
like” chelates in which the chelating ligand (which is composed
of three or four 3-NH(t-butyl)-5-methyl-pyrazole molecules) is
held together by hydrogen bonds. Thus, complexes 1, 2, and 3 have
NH to halide hydrogen bonds that support a structure with pyra-
zole to metal ratios of 3:1 or 4:1. In contrast, intramolecular hydro-
gen bonds are absent from 4, the 4:1 nickel complex with the same
pyrazole ligand. However, this may be due to ethanol solvent being
present for 4. These hydrogen-bonded self-assembled structures
hold together in the solid state and in relatively non-polar
solutions (e.g. chloroform). However, spectroscopic studies in
methanol suggest that these scorpionate-like hydrogen bonded
complexes are disrupted in polar solution, presumably because
hydrogen bonds to solvent can replace the intramolecular
hydrogen bonds. These complexes could be applied towards the
development of sensors for halide ions in organic solutions,
whereby the binding of halide ions could be detected by changes
in the UV–Visible spectral features [50]. Furthermore, this pyrazole
ligand shows an ability to form hydrogen bonds to anions that in
some ways resemble anion-binding catalysis in synthetic systems
[51,52] and in natural enzymes [53].
4.4. Synthesis of (LH)3CoCl2 (2)
The pyrazole LH (0.100 g, 0.653 mmol) was treated with CoCl2
(0.028 g, 0.22 mmol) in 5 mL of ethanol and allowed to stir for
3 days. Then, the solvent was removed under vacuum to produce
a solid. The product was recrystallized from the diffusion of hex-
anes into a methanol solution using the vapor diffusion recrystal-
lization technique. The crystalline product was formed in 96%
yield (0.122 g, 0.207 mmol) and was dried under vacuum. IR (m,
cmÀ1): 3362, 3350, 3326, 3153, 3045, 2968, 2923, 2873, 1652,
1589, 1549, 1515, 1400, 1366, 1298, 1228, 1211, 1141, 1040,
956, 776, 749. FAB MS (m/z): 553.3 [MÀHCl]+, 400.2 [MÀHClÀLH]+,
247.0 [MÀHClÀ2LH]+; all peaks showed the expected isotopic pat-
tern. High res. MS: (m/z) = 553.2816 expt. (553.2818 calcd.); the
expected isotopic pattern was observed. Elemental Anal. expt. Calcd.:
C, 49.03 (48.90); H, 7.80 (7.69); N, 21.34 (21.38).
4. Experimental
4.5. Synthesis of (LH)4MnCl2 (3)
4.1. General
The pyrazole LH (0.100 g, 0.653 mmol) was treated with MnCl2
(0.020 g, 0.16 mmol) in 5 mL of ethanol and allowed to stir for
3 days. Then, the solvent was removed under vacuum to produce
a solid. The product was recrystallized from CH2Cl2 and hexanes
(3:1) which allowed for the isolation of a crystalline precipitate
(96% yield, 0.114 g, 0.154 mmol) that was dried under vacuum.
All commercially available reagents were used as received. The
reactions to form 1–4 were generally not air sensitive and reac-
tions were run both in air and under nitrogen giving identical
results, and most purification and isolation procedures were done
open to air. Anhydrous MX2 salts were weighed out in a glovebox
to prevent moisture absorption so an accurate mass could be
obtained. Organic solvents were used as received. Proton and car-
bon NMR spectra were recorded using either a 300 MHz (300 MHz
is the frequency for 1H NMR spectra, 75 MHz for 13C NMR spectra
on this instrument) or a 500 MHz (used primarily for 1H NMR) Var-
ian Unity Inova NMR spectrophotometer at Drexel University. 1H
NMR spectroscopy experiments were also done at Miami Univer-
sity on a 200 MHz NMR instrument. Infrared spectra were col-
lected on a Perkin Elmer Spectrum One FT-IR spectrometer using
a universal ATR sampling accessory. High resolution (HR) mass
spectrometry was performed on a VG70SE double focusing, triple
quadrupole mass spectrometer equipped with FAB or CI ionization
capability. Electrospray ionization (ESI) mass spectrometry was
IR (m
, cmÀ1): 3320, 2971, 1576, 1539, 1512, 1393, 1362, 1289,
1233, 1150, 1017, 956, 809, 747, 662. FAB MS (m/z): 550.1 (weak)
[MÀLÀCl]+, this complex fragments quickly in the MS and all other
peaks are LH or a dimer or trimer of LH; all peaks showed the
expected isotopic pattern. High res. MS: (m/z) = 551.3024 expt.
(551.3023 calcd.) [M+HÀLÀCl]+; these and other peaks showed
the expected isotopic pattern. Due to fast fragmentation and loss
of one L prior to reaching the detector, we also studied this com-
pound by LIFDI MS, which is a softer technique but we saw similar
results: (m/z) = 584 [MÀLH]+, 549 [MÀLHÀCl]+, 431 [MÀ2LH]+.
4.6. Synthesis of (LH)4NiBr2 (4)
The pyrazole LH (0.200 g, 1.31 mmol) was treated with NiBr2
(0.071 g, 0.33 mmol) in 10 mL of ethanol and allowed to stir for
2 days. Then, the solvent was removed under vacuum to produce
a solid. The product was recrystallized from ethanol which allowed
performed on
a SCIEX API3000 mass spectrometer for fast
atom bombardment (FAB, cesium ion). Elemental analysis was
performed by Robertson Microlit, Ledgewood, NJ.
Please cite this article in press as: L. Serpas et al., Polyhedron (2015), http://dx.doi.org/10.1016/j.poly.2015.10.003

L. Serpas et al. / Polyhedron xxx (2015) xxx–xxx
9
for the isolation of a crystalline precipitate (67% yield, 0.181 g,
Appendix A. Supplementary data
0.218 mmol) that was dried under vacuum. The structure is
(LH)4NiBr2 when rigorously dried under vacuum, although
CCDC 1048804–1048808 contain the supplementary crystallo-
graphic data for compounds LH and 1–4. These data can be
obtained free of charge from the Cambridge Crystallographic Data
[(LH)4NiBr2]EtOH is observed crystallographically. IR (m
, cmÀ1):
3315, 2972, 2930, 1579, 1508, 1393, 1361, 1238, 1025, 739. FAB
MS (m/z): 598.4 [MÀLHÀHBr]+, 445.2 [MÀ2LHÀHBr]+; all peaks
showed the expected isotopic pattern. Due to fast fragmentation
and loss of one L prior to reaching the detector, we also studied this
compound by LIFDI MS, which is a softer technique but we saw
similar results: (m/z) = 678 [MÀLH]+, 598 [MÀLHÀHBr]+, 524
[MÀ2LHÀH]+, 445 [MÀ2LHÀHBr]+.
Centre via https://summary.ccdc.cam.ac.uk/structure-summary?
ccdc=1048804-1048808. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.poly.2015.10.003.
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Acknowledgements
We thank Eric J. Schelter, Robert C. Scarrow, and Deidra
L. Gerlach for helpful suggestions. We thank NSF CAREER (Grant
CHE-0846383 and CHE-1360802 to E.T.P. and her group); NSF
(CHE-1152755 to DLT); and NSF REU supplement (to L.S. via E.T.
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