Synthesis and characterization of a series of diphenyldipyrazolylmethane complexes with zinc(II)

Article abstract of DOI:10.1016/j.ica.2008.10.024

The zinc(II) coordination chemistry of a series of diphenyldipyrazolylmethane ligands was explored using ¹H NMR and single crystal X-ray diffraction. Unsubstituted diphenyldipyrazolylmethane (dpdpm), diphenylbis(3-methylpyrazolyl)methane (dpdp′

Full text of DOI:10.1016/j.ica.2008.10.024

Inorganica Chimica Acta 362 (2009) 2396–2401
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Inorganica Chimica Acta
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Synthesis and characterization of a series of diphenyldipyrazolylmethane
complexes with zinc(II)
*
Janet L. Shaw , Kevin P. Gwaltney, Nikky Keer
Kennesaw State University, Department of Chemistry and Biochemistry, Kennesaw, GA 30144-5591, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 July 2008
Received in revised form 17 October 2008
Accepted 23 October 2008
Available online 30 October 2008
The zinc(II) coordination chemistry of a series of diphenyldipyrazolylmethane ligands was explored using
1H NMR and single crystal X-ray diffraction. Unsubstituted diphenyldipyrazolylmethane (dpdpm),
diphenylbis(3-methylpyrazolyl)methane (dpdp0m), and diphenylbis(3,5-dimethylpyrazolyl)methane
(dpdp00m) were reacted with Zn(NO3)2 to afford Zn(dpdpm)(NO3)2, Zn(dpdp0m)(NO3)2 and Zn(Pz00)2(NO3)2
where Pz00 = 3,5-dimethylpyrazole, respectively. All attempts to isolate Zn(dpdp00m)(NO3)2 with the intact
dpdp00m ligand were unsuccessful due to decomposition of the ligand. These bidentate ligands support
the formation of 1:1 ligand to metal complexes and structurally model the two histidine coordination
mode common in zinc proteins.
Keywords:
X-ray crystal structures
1H NMR
Published by Elsevier B.V.
Diphenyldipyrazolylmethane
Zinc(II) complexes
Scorpionates
Chelating ligands
1. Introduction
diphenyldipyrazolylmethane complexes have been reported by
Baho and Zargarian [5].
Diphenyldipyrazolylmethane is a bidentate, neutral ligand con-
taining two pyrazole moieties available for coordination to metal
centers. The sterics and electronics of diphenyldipyrazolymethane
can be adjusted by employing substituted pyrazole moieties during
ligand synthesis [1]. This control over ligand design can be used to
enforce a desired geometry at a metal center and to tune the reac-
tivity of the metal ligand complex.
In the literature, diphenyldipyrazolylmethane ligands have re-
ceived relatively little attention compared to their polypyrazolylb-
orate counterparts. In 1993, Shiu et al. isolated three molybdenum
complexes with diphenyldipyrazolylmethanes [2], and in 1999 Jor-
dan et al. synthesized cationic palladium(II) alkyl organometallic
complexes for use in polymerization catalysis [3]. In addition, Re-
ger et al. used these ligands in 2004 to gain a fundamental under-
An investigation of the zinc(II) coordination chemistry of
unsubstituted diphenyldipyrazolylmethane (dpdpm), diphenyl-
bis(3-methylpyrazolyl)methane (dpdp0m), and diphenylbis(3,5-
dimethylpyrazolyl)methane (dpdp00m) is presented here. Zinc is a
biologically relevant metal that serves catalytic and structural roles
in more than 300 enzymes [6]. A common amino acid for zinc liga-
tion in these enzymes is histidine, and diphenyldipyrazolylme-
thane ligands may serve as accurate structural models for the 2
His coordination environment found in carboxypeptidase, therm-
olysin, and neutral protease [6c]. Here, the reactivity of these li-
gands and their coordination chemistry with zinc is explored
using 1H NMR and single crystal X-ray diffraction in order to deter-
mine if diphenyldipyrazolylmethanes can be used to model the ac-
tive site structures of zinc containing enzymes.
standing of metal cation-p phenyl interactions in complexes with
Ag(I) [4]. More recently, attention has shifted to generating diph-
enyldipyrazolylmethane complexes with the first row transition
metals copper(II) and nickel(II). With regards to the copper(II)
chemistry, these ligands have been used to mimic histidine coordi-
nation which is common in metalloproteins [1a], and have been
used in oxalate bridged complexes to study electronic communica-
tion between d9 metal centers [1b]. Most recently, interesting sol-
vato-, vapo-, and thermochromic properties of a series of nickel(II)
2. Experimental
All syntheses were carried out in air and the reagents and sol-
vents were obtained commercially and used as received. Elemental
analysis was performed by Atlantic Microlabs Inc. Fourier trans-
form infrared spectroscopy (FTIR) was performed on powdered sol-
ids using a Perkin–Elmer SpectraOne spectrophotometer fitted
with a diamond attenuated total reflectance stage. NMR spectra
were recorded using a Bruker AVANCE 300 MHz instrument. Melt-
ing points (m.p.) were measured using a Mel-TempÒ instrument.
Syntheses of these ligands were reported previously [1a].
* Corresponding author. Tel.: +1 770 499 3428; fax: +1 770 423 6744.
E-mail address: jshaw22@kennesaw.edu (J.L. Shaw).
0020-1693/$ - see front matter Published by Elsevier B.V.
doi:10.1016/j.ica.2008.10.024

J.L. Shaw et al. / Inorganica Chimica Acta 362 (2009) 2396–2401
2397
Single crystal X-ray diffraction data was collected at the Uni-
versity of Akron. The X-ray intensity data for all compounds
were measured at 100 K (Bruker KRYO-FLEX) on a Bruker SMART
APEX CCD-based X-ray diffractometer system equipped with a
Mo-target X-ray tube (k = 0.71073 Å) operated at 2000 W power.
Crystals were mounted on a cryoloop using Paratone N-Exxon oil
and placed under a stream of nitrogen. The detector was placed
at a distance of 5.009 cm from the crystals. Frames were col-
1383 (m), 1271 (vs), 1253 (m), 1226 (w), 1209 (m), 1198 (m),
1187 (m), 1174 (w), 1163 (w), 1103 (m), 1086 (m), 1066 (s),
1016 (s), 1003 (m), 991 (m), 941 (w), 923 (w), 891 (w), 860
(w), 839 (w), 814 (m), 777 (s), 760 (s), 746 (vs), 699 (s), 657
(m). M.p. 187–195 °C. Anal. Calc. for ZnC19H16N6O6: C, 46.58;
H, 3.30; N, 17.16. Found: C, 46.52; H, 3.29; N, 17.08%.
2.2. Zn(dpdp0m)(NO3)2
lected with a scan width of 0.3° in
x. Analyses of the data sets
showed negligible decay during data collection. The data were
corrected for absorption with the SADABS program. The structures
were refined using the Bruker SHELXTL Software Package (Version
6.1), and were solved using direct methods until the final aniso-
tropic full-matrix, least squares refinement of F2 converged [7].
The PATTERSON program was used to solve the structure of
Zn(dpdpm)(NO3)Cl.
Dpdp0m (0.501 g, 1.53 mmol) was dissolved in 200 mL of warm
ethanol in a round bottom flask with stir bar. Solid zinc nitrate
hexahydrate (0.599 g, 2.01 mmol) was then added to clear the
solution. After 2 h of stirring at room temperature a white precip-
itate began to form. After six hours of stirring the reaction mix-
ture was filtered to isolate the white solid product (0.156 g,
20%). The solid was re-dissolved in dichloromethane and slow
evaporation of the solvent produced clear, colorless crystals of
X-ray quality. The crystal data and structure refinement parame-
ters are summarized in Table 1. 1H NMR (300 MHz, CDCl3, 22 °C)
7.61 (t, J = 7.4 Hz, 2H, Ph), 7.49 (t, J = 7.7 Hz, 4H, Ph), 7.35 (d,
J = 2.7 Hz, 2H, pyrazole), 6.65 (d, J = 5.9 Hz, 4H, Ph), 6.32 (d,
J = 2.7 Hz, 2H, pyrazole), 2.50 (s, 6H, methyl). IR (cmꢀ1): 1527
(w), 1483 (s), 1449 (m), 1382 (m), 1351 (w), 1301 (s), 1280
(vs), 1186 (s), 1076 (s), 1046 (w), 1035 (w), 1009 (s), 937 (w),
914 (w), 886 (m), 862 (m), 835 (w), 809 (m), 793 (m), 779 (m),
765 (s), 749 (s), 713 (s), 693 (s), 656 (m). M.p. 188–195 °C. Anal.
Calc. for ZnC21H20N6O6: C, 48.70; H, 3.90; N, 16.23. Found: C, 48.30;
H, 3.88; N, 16.00%.
2.1. Zn(dpdpm)(NO3)2
Dpdpm (1.26 g, 4.20 mmol) was dissolved in 300 mL of etha-
nol in a round bottom flask with stir bar. Solid zinc nitrate hexa-
hydrate (1.50 g, 5.04 mmol) was then added to the clear solution.
The reaction was allowed to stir overnight at room temperature.
Then, the excess solvent was removed via rotary evaporator
yielding a white solid. The solid was re-dissolved in dichloro-
methane and filtered to remove insoluble materials. Slow evapo-
ration of the solvent produced clear, colorless crystals (1.55 g,
76%). X-ray quality crystals were grown from dichloromethane,
and crystal data and structure refinement parameters are sum-
marized in Table 1. One of the nitrates is replaced during recrys-
tallization to yield Zn(dpdpm)(NO3)Cl in the solid state. 1H NMR
(300 MHz, CDCl3, 22 °C) 8.17 (d, J = 2.1 Hz, 2H, pyrazole), 7.61 (t,
J = 7.4 Hz, 2H, Ph), 7.49 (m, 6H, 4 Ph and 2 pyrazole), 6.62 (d,
J = 7.2 Hz, 4H, Ph), 6.57 (t, J = 2.5 Hz, 2H, pyrazole). IR (cmꢀ1):
3394 (w), 3105 (w), 1477 (s), 1450 (m), 1436 (m), 1405 (m),
2.3. Zn(Pz00)2(NO3)2
This compound was isolated as a decomposition product in the
attempt to synthesize Zn(dpdp00m)(NO3)2. Dpdp00m (0.500 g,
1.40 mmol) was dissolved in 200 mL of warm ethanol and solid
zinc nitrate hexahydrate (0.509 g, 1.71 mmol) was added. The
reaction was allowed to stir at reflux overnight. In the morning,
the clear, colorless solution was evaporated to dryness using a ro-
tary evaporator. The resulting oil was stored in a vacuum desicca-
tor under reduced pressure for 1 week. NMR analysis of the oil in
deuterated chloroform indicated that the dpdp00m ligand had bro-
ken apart as evidenced by the single peak at 2.25 ppm indicating
equivalent methyl groups on the pyrazole rings. Attempts to sepa-
rate the mixture of products from this oil for complete analysis
were unsuccessful. However, crystals eventually formed in the oil
and were analyzed using single crystal X-ray diffraction. The crys-
tal data and structure refinement parameters are summarized in
Table 1.
Table 1
Selected crystallographic data for all three zinc complexes.
Molecular formula
ZnC19H16N5O3Cl ZnC21H20N6O6 ZnC10H16N6O6 ꢁ CHCl3
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
463.19
517.82
501.03
triclinic
orthorhombic monoclinic
P2(1)2(1)2(1)
ꢀ
P1
P2(1)/n
10.2143(13)
10.2746(13)
10.4222(13)
74.878(2)
73.513(2)
68.444(2)
2
12.6101(13)
12.8930(13)
13.6505(14)
90
90
90
4
18.043(8)
12.277(6)
18.599(8)
90
101.047(7)
90
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
To isolate pure Zn(Pz00)2(NO3)2 for complete analysis another
synthetic protocol was necessary. The free 3,5-dimethylpyrazole
(Pz00, 0.505 g, 5.25 mmol) was dissolved in 150 mL of ethanol at
room temperature. To this clear solution was added solid zinc
nitrate hexahydrate (0.892 g, 3.00 mmol). The reaction was al-
lowed to stir overnight. Then, the excess solvent was removed
via rotary evaporation to yield a colorless oil. The oil was placed
in a vacuum desiccator for 1 week to dry under reduced pres-
sure. The resulting solid was re-dissolved in dichloromethane
and filtered to remove insoluble impurities. Upon slow evapora-
tion of the solvent a light yellow solid was obtained (0.719 g,
72%). 1H NMR (300 MHz, CD3OD, 22 °C) 6.14 (s, 2H, pyrazole),
2.31 (s, 12H, methyl). IR (cmꢀ1): 3356 (m), 3252 (m), 3151
(w), 1606 (w), 1576 (m), 1505 (m), 1460 (s), 1414 (m), 1279
(vs), 1176 (m), 1147 (m), 1046 (s), 1004 (s), 812 (m), 806 (m),
756 (w), 744 (w), 656 (m). M.p. 37–42 °C. Anal. Calc. for
ZnC10H16N6O6: C, 31.47; H, 4.23; N, 22.01. Found: C, 31.60; H,
4.19; N, 21.78%.
Z
8
Volume (Å3)
960.3(2)
1.450
2219.3(4)
1.158
4044(3)
1.651
Absorbtion coefficient
lcalc (mmꢀ1
)
F(000)
472
1.602
1064
1.550
2032
1.646
dcalc (Mg/m3)
h Range for data
collection (°)
2.07–27.00
2.17–28.28
1.44–26.75
Reflections collected/
8044/4109
19038/5281
31927/8615
unique
[Rint
]
0.0266
0.0597
0.1148
Data/restraints/
parameters
4109/0/262
5281/0/309
8615/0/495
Goodness-of-fit (GOF)
Final R indices
[I > 2r(I)]
1.057
1.001
0.907
R1 = 0.0330
wR2 = 0.0753
R1 = 0.0391
wR2 = 0.0774
0.479 and
ꢀ0.314
R1 = 0.0335
wR2 = 0.0755
R1 = 0.0373
wR2 = 0.0771
1.056 and
ꢀ0.290
R1 = 0.0524
wR2 = 0.1246
R1 = 0.1304
wR2 = 0.1346
0.539 and ꢀ0.924
R indices (all data)
Largest differences in
peak and hole
0
(e ÅAꢀ3
)

2398
J.L. Shaw et al. / Inorganica Chimica Acta 362 (2009) 2396–2401
47 °C temperature range. At 47 °C, the doublet near 6.6 ppm, as-
3. Results and discussion
3.1. Synthesis and spectroscopy
signed to the four phenyl protons labeled Ha, sharpens and is much
more resolved from the H4 protons of the pyrazole rings, as ex-
pected, due to rapid interconversion. As the sample is cooled, this
doublet broadens and shifts upfield. At ꢀ13 °C, this peak becomes a
featureless broad resonance spanning one ppm, superimposed
with the peak from the H4 protons of the pyrazole rings. By
ꢀ53 °C, this peak has narrowed significantly and shifted to
6.0 ppm. The depleted integration (2H), as well as significant
changes in the resonances spanning 7.3–8.0 ppm, confirms that
the four protons labeled Ha are not equivalent at ꢀ53 °C. These
variable temperature 1H NMR results follow trends observed in the
The room temperature 1H NMR spectra of Zn(dpdpm)(NO3)2
and Zn(dpdp0m)(NO3)2 are very similar so only Zn(dpdpm)(NO3)2
will be discussed here to avoid redundancy. In the spectrum of
the free dpdpm ligand the phenyl protons appear as two sets of
multiplets. One multiplet is centered at 7.1 ppm (4H) and the other
is centered at 7.4 ppm (6H) [1]. Upon metallation with zinc(II), the
phenyl protons of dpdpm separate into three distinct groups. The
1H NMR spectrum for Zn(dpdpm)(NO3)2 and the labeling scheme
used for the protons of the ligand are shown in Figs. 1 and 2. Trip-
lets located at 7.6 ppm (Hc protons) and 7.5 ppm (Hb protons) are
shifted slightly downfield relative to the free ligand. The most dra-
matically shifted phenyl resonance upon metallation is the doublet
at 6.6 ppm (Ha protons). The significant upfield shift of this reso-
nance results from anisotropic shielding of these protons due to
their proximity to the face of the neighboring phenyl ring. This res-
onance is also broadened relative to the other peaks in the spec-
trum, and has been shown in the literature to separate into
multiple peaks at low temperature due to the constrained orienta-
tion of the phenyl rings with respect to each other [3].
literature
for
similar
metal
complexes
with
diph-
enyldipyrazolylmethane ligands. Both the room temperature and
variable temperature 1H NMR spectra support the solution state
structure of Zn(dpdpm)(NO3)2.
A variety of reaction conditions were employed in attempts to
isolate Zn(dpdp00m)(NO3)2. Several dry solvents were tested under
anaerobic conditions, and different metal starting salts such as
anhydrous zinc chloride and zinc acetate were also employed.
However, all attempts to isolate Zn(dpdp00m)(NO3)2 were unsuc-
cessful due to decomposition of the ligand.
Room temperature 1H NMR was used to study the decomposi-
tion of dpdp00m and the subsequent formation of Zn(Pz00)2(NO3)2.
To begin, dpdp00m was dissolved in d-MeOH and a baseline spec-
trum was recorded. Some of the ligand remained undissolved in
the NMR tube, but heat was not applied at this point in the exper-
iment. Then, solid Zn(NO3)2 ꢂ 6H2O was added and spectra were
recorded periodically over the next 24 h. To observe the decompo-
sition of dpdp00m it is easiest to follow the methyl resonances of the
intact ligand at 2.08 and 1.45 ppm because, upon ligand decompo-
sition, these two peaks are replaced by a single resonance which
appears slightly downfield. This new singlet results from the six
hydrogen atoms of the magnetically equivalent methyl groups of
3,5-dimethylpyrazole (Pz00).
Variable temperature 1H NMR experiments were also per-
formed on Zn(dpdpm)(NO3)2 (see Supplementary Materials). Spec-
tra were recorded in deuterated chloroform spanning the ꢀ53 °C to
Hb
Hb
Hc
Hb
Ha
Ha
Hc
Hb
Ha
H5
Fig. 3 shows representative 1H NMR spectra from this experi-
ment. The first indication of ligand decomposition occurs after
2.5 h with a tiny peak growing in at 2.1 ppm. After about 24 hours
at room temperature the ratio of the methyl peaks for Pz00 and in-
tact dpdp00m is 1:2:2, respectively. At this point the contents of the
NMR tube were warmed on a hotplate for 15 min. Heating in-
creased the solubility of dpdp00m, and a spectrum was recorded
immediately following which showed that the decomposition
Ha
H5
N
N
N
N
H4
H4
H3
H3
Fig. 1. Labeling scheme of diphenyldipyrazolylmethane used for 1H NMR.
Fig. 2. Room temperature 1H NMR spectrum for Zn(dpdpm)(NO3)2 run in CDCl3.

J.L. Shaw et al. / Inorganica Chimica Acta 362 (2009) 2396–2401
2399
1
Fig. 3. Selected room temperature H NMR spectra from the dpdp00m decomposition study (performed in d-MeOH).
product had dramatically increased in concentration giving a ratio
Table 2
of 19:1:1 for the methyl peaks of Pz00 and dpdp00m. An upfield shift
of all three peaks is also observed in the spectrum following
heating.
Selected bond angles (°) and lengths (Å) for all three zinc complexes.
ZnC19H16N5O3Cl
Zn–O1
Zn–O2
Zn–N2
Zn–N4
Zn –Cl
2.0016(16)
2.4768(17)
2.0417(18)
2.0179(18)
2.2070(6)
O1–Zn–N2
O2–Zn–N2
O1–Zn–N4
O2–Zn–N4
O1–Zn–O2
N2–Zn–N4
O1–Zn–Cl
O2–Zn–Cl
N2–Zn–Cl
N4–Zn–Cl
103.07(7)
Decomposition of similar pyrazole based ligands upon reaction
with metal salts has been reported in the literature [8]. The pro-
posed mechanism involves metal-mediated hydrolysis of the pyra-
zole nitrogen-X bond where X may be boron, as is the case with
pyrazolylborates, or carbon, as is the case in this report [1a]. For
the pyrazolylborates substitution at the 5-position of the pyrazole
ring often helps protect B–N bonds from hydrolysis [9]. However,
here the addition of a methyl substituent at the 5-position of the
pyrazole ring decreases the stability of dpdp00m in the presence
of Zn(NO3)2 relative to the 3-methylated dpdp’m ligand for which
a zinc complex was obtained. This trend in reactivity was also re-
ported previously in a paper employing Cu(NO3)2 [1a]. Thus far,
only molybdenum carbonyl has been successfully incorporated
into a complex with the intact dpdp00m ligand according to the lit-
erature [2].
154.36(6)
129.13(7)
92.63(6)
56.57(6)
89.85(7)
108.68(5)
94.89(4)
107.40(5)
113.83(5)
ZnC21H20N6O6
Zn–O1
Zn–O2
Zn–O4
Zn–N2
2.4730(18)
2.005(16)
1.9605(17)
2.0189(18)
1.9959(18)
O2–Zn–N2
O4–Zn–N2
O2–Zn–N4
O4–Zn–N4
O1–Zn–O4
O2–Zn–O4
O1–Zn–N2
O1–Zn–N4
O1–Zn–O2
N2–Zn–N4
104.91(7)
188.04(7)
125.56(7)
122.40(7)
88.18(7)
96.76(7)
150.99(7)
86.01(7)
56.67(6)
89.37(7)
Zn–N4
3.2. X-ray crystallography
ZnC10H16N6O6 ꢂ CHCl3
Zn1–O1
Zn1–O3
Zn1–O4
Zn1–O5
Suitable crystals for single crystal X-ray diffraction studies were
obtained for each of the zinc complexes. Crystallographic data are
located in Table 1, and selected bond distances and angles are pro-
vided in Table 2. Figs. 4–6 contain perspective views of the three
zinc complexes.
Dpdpm forms a 1:1 metal-to-ligand complex with zinc(II) in the
solid state. One of the two ligated nitrates in the Zn(dpdpm)(NO3)2
complex (supported by elemental analysis) is replaced by a chlo-
ride during recrystallization in methylene chloride to yield
Zn(dpdpm)(NO3)Cl (Fig. 4). Because of the flexibility in nitrate
coordination, and the fact that nitrate is isoelectronic with bicar-
bonate which is important in the catalytic cycle of carbonic anhy-
drase, a way to quantitate the denticity of metal bound nitrates is
commonly used in the literature. The difference between the two
2.064(4)
2.553(4)
2.427(4)
2.055(4)
2.009(4)
1.995(4)
O1–Zn1–N1
O4–Zn1–N1
O5–Zn1–N1
O1–Zn1–N3
O4–Zn1–N3
O5–Zn1–N3
O1–Zn1–O4
O1–Zn1–O5
O4–Zn1–O5
N1–Zn1–N3
O7–Zn2–N5
O7–Zn2–N7
O10–Zn2–N5
O10–Zn2–N7
O10–Zn2–O7
N5–Zn2–N7
104.72(17)
154.49(15)
98.77(16)
98.83(16)
93.29(15)
116.31(16)
88.06(14)
129.83(15)
57.05(13)
106.11(18)
108.49(17)
104.53(16)
136.01(16)
97.38(17)
101.58(15)
105.051(19)
Zn1–N1
Zn1–N3
Zn2–O7
Zn2–O8
Zn2–O10
Zn2–N5
Zn2–N7
2.031(4)
2.582(4)
1.993(4)
1.961(4)
2.009(4)

2400
J.L. Shaw et al. / Inorganica Chimica Acta 362 (2009) 2396–2401
Fig. 4. Structure of Zn(dpdpm)(NO3)Cl with 50% thermal ellipsoids and hydrogen atoms omitted for clarity.
Fig. 6. Structure of Zn(Pz00)2(NO3)2 with 50% thermal ellipsoids. Chloroform
molecules and hydrogen atoms omitted for clarity.
The six-membered ring formed by the chelating dpdpm ligand
is in an approximate boat conformation as indicated by the
169.03° angle between the N–Zn–N and the N–N–N–N planes
(Table 3) [3]. One phenyl ring of dpdpm points towards the zinc,
and the closest Zn–Cph interactions are at 3.37 and 3.89 Å. As stated
in the literature, these M–Cph interactions are most likely due to
steric congestion around the quarternary carbon of the ligand
which results in constrained ligand geometry; rather than metal
Fig. 5. Structure of Zn(dpdp0m)(NO3)2 with 50% thermal ellipsoids and hydrogen
atoms omitted for clarity.
M—ONO3 distances (
N angles ( h) are used to classify nitrate denticity (
h < 14° for bidentate; 0.3 < d < 0.6 Å and 14 < h < 28° for ani-
sodentate; d > 0.6 Å and h > 28° for monodentate) [6c]. Using
this method, the binding mode of the nitrate in Zn(dpdpm)(NO3)Cl
is best described as anisobidentate ( d = 0.48 Å and h = 21.37°).
Therefore, the zinc center in Zn(dpdpm)(NO3)Cl is five coordi-
nate, and with = 0.42 the geometry at the metal is best described
D
d) and the difference between the two M–O–
D
D
d < 0.3 Å and
cation-p interactions [4]. The phenyl rings of dpdpm may help
D
D
D
the isolation of 1:1 metal-to-ligand complexes with zinc(II) in
the solid state; whereas bis-(pyrazolyl)alkane derivatives and
tris-(pyrazolyl)methane ligands have been shown to produce 1:2
complexes upon reaction with Zn(NO3)2 in the literature [12].
The X-ray crystal structure of Zn(dpdp0m)(NO3)2 (Fig. 5) com-
pares nicely to that of Zn(dpdpm)(NO3)Cl with only subtle differ-
D
D
D
D
s
as distorted square pyramidal [10]. The chloride is best treated as
the axial ligand in this geometry, leaving the anisobidentate nitrate
and the chelating dpdpm to complete the basal plane (Fig. 4). The
small bite angle of the nitrate (ꢃ56°) leads to significant distortion
in the basal plane, and the zinc ion is displaced 0.60 Å from the
mean plane generated by N2–N4–O2–O1 towards the axial chlo-
ride. The chloride leans slightly off-axis away from the dpdpm li-
gand with Cl–Zn–N2 and Cl–Zn–N4 angles of around 107° and
114°, respectively. The Zn–Npz and Zn–Cl bond lengths are close
to those found in similar complexes [11].
Table 3
Angles generated by selected planes.
Zn(dpdpm)(NO3)Cl
Zn(dpdp0m)(NO3)2
N–Zn–N/N–N–N–N
N–C–N/N–N–N–N
Pz/pz
N–Zn–N/pz
N–N–N–N/pz
169.03
127.63
131.02
27.57/23.63
27.37/22.02
155.62
130.58
135.82
32.36/30.06
27.06/17.23

J.L. Shaw et al. / Inorganica Chimica Acta 362 (2009) 2396–2401
2401
ences owing to the 3-methyl substituents of the pyrazole moieties.
4. Conclusions
One nitrate in Zn(dpdp0m)(NO3)2 is bound to zinc in an anisobiden-
tate fashion (0.47 Å and 20.42°) while the other is clearly monoden-
Three new zinc(II) complexes were synthesized using diph-
enyldipyrazolylmethane ligands. Zn(dpdpm)(NO3)Cl and
tate(0.79 Åand36.49°)basedonthecalculatedvaluesfor
D
dand
Dh.
Zn(dpdp0m)(NO3)2 were isolated as 1:1 metal to ligand complexes
in the solid state. Attempts to isolate Zn(dpdp00m)(NO3)2 were
unsuccessful due to decomposition of the dpdp00m ligand in the
presence of metal. Instead, Zn(Pz00)2(NO3)2 was obtained and
characterized. Future work will include a detailed NMR study of
the conditions leading to dpdp00m ligand decomposition. We
also plan to continue exploring the metal chemistry of diph-
enyldipyrazolylmethane ligands in hopes of synthesizing models
of protein active sites.
Therefore, the zinc center is five coordinate, and with
s
= 0.42
the geometry of Zn(dpdp0m)(NO3)2 is also distorted square pyrami-
dal. The monodentate nitrate is best treated as the axial ligand in
this complex, leaving the anisobidentate nitrate and dpdp0m to
complete the basal plane. The small bite angle of the anisobiden-
tate nitrate (ꢃ57°) leads to significant distortion in the basal plane
with angles deviating significantly from the ideal 90°. The zinc ion
is displaced 0.66 Å from the mean plane generated by N2–N4–O2–
O1 towards the axial nitrate. The axial nitrate is leaning off-axis
away from the dpdp’m ligand with O4-Zn-N2 and O4–Zn–N4 an-
gles of around 118° and 122°, respectively. This deviation is more
pronounced than in Zn(dpdpm)(NO3)Cl due to steric interactions
with the 3-methyl substituents of the pyrazole rings.
Acknowledgements
J.L.S. would like to thank Dr. Chris Ziegler at the University of
Akron for graciously collecting the single crystal X-ray diffraction
data and the Center for Excellence in Teaching and Learning Incen-
tive Grant at Kennesaw State University for financial support. We
wish to also acknowledge NSF Grant CHE-0116041 for funds used
to purchase the Bruker-Nonius diffractometer.
One phenyl ring of dpdp0m points towards the zinc center, and
the closest metal–Cph interactions are at 3.25 and 3.61 Å. The boat
conformation formed by the chelating dpdp0m ligand is more pro-
nounced in Zn(dpdp0m)(NO3)2 relative to Zn(dpdpm)(NO3)Cl with
a smaller angle between the N–Zn–N and N–N–N–N planes
(155.62° versus 169.03°). This is due to steric crowding associated
with the methyl substituents on the pyrazole rings.
Appendix A. Supplementary material
The average Zn–Npz bond length of 2.02 Å in Zn(dpdp0m)(NO3)2
and Zn(dpdpm)(NO3)Cl compares nicely with the Zn–NHis bond
length of around 2.1 Å found in many zinc metalloproteins [13].
For example, in carboxypeptidase A the Zn–NHis bond lengths are
2.00 and 2.08 Å [14]. In addition, the average bond lengths of the
anisobidentate nitrates of 2.00 and 2.48 Å from these two com-
plexes are close to the observed Zn–OGlu bond distances of around
2.1 and 2.5 Å in neutral protease from Bacillus cereus [15]. The Zn–
Npz and Zn–Omono/aniso bond lengths are also close to those seen in
similar coordination complexes such as Zn(acetate)2(2,9-dimethyl-
o-phenanthroline)ꢁ3H2O (Zn–N = 2.06 and 2.09 Å; Zn–Omono = 1.91,
Zn–Oaniso = 2.08 and 2.36 Å) [14].
CCDC 690653, 690654 and 690655 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.ica.2008.10.024.
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