New zinc complexes bearing κ2-heteroscorpionate ligands: Influence of second-sphere bonding interactions on reactivity and properties

Article abstract of DOI:10.1021/ic901091e

New zinc complexes (LOMe)ZnCl₂ (1) and (LOH)ZnCl₂(2) of the heteroscorplonate ligands 1-[(3,5-di-/erf-butyl-2methoxyphenyl)(3,5- dlmethyl-pyrazol-1 -yl)methyl)]-3,5-dlmethyl-pyrazole (LOMe) and 2,4-di-/ert-butyl-6-[bis(3,5dimethyl-py

Full text of DOI:10.1021/ic901091e

9510 Inorg. Chem. 2009, 48, 9510–9518
DOI: 10.1021/ic901091e
New Zinc Complexes bearing K2-Heteroscorpionate Ligands: Influence of
Second-Sphere Bonding Interactions on Reactivity and Properties
Stefano Milione,* Carmine Capacchione, Cinzia Cuomo, Maria Strianese, Valerio Bertolasi,† and Alfonso Grassi
ꢀ
Dipartimento di Chimica, Universita di Salerno, via Ponte don Melillo, I-84084 Fisciano (SA), Italy, and
†
ꢀ
Universita di Ferrara, Centro di Strutturistica Diffrattometrica and Dipartimento di Chimica, Via L. Borsari,
46, I-44100 Ferrara, Italy
Received June 5, 2009
New zinc complexes (LOMe)ZnCl2 (1) and (LOH)ZnCl2(2) of the heteroscorpionate ligands 1-[(3,5-di-tert-butyl-2-
methoxyphenyl)(3,5-dimethyl-pyrazol-1-yl)methyl)]-3,5-dimethyl-pyrazole (LOMe) and 2,4-di-tert-butyl-6-[bis(3,5-
dimethyl-pyrazol-1-yl)methyl]phenol (LOH) have been synthesized. The X-ray molecular structure of 2 was reported
and compared with the one of the iron(II) complex (LOH)FeI2 (3). The complexes 2-3 adopt a tetrahedral structure in
the solid state in which the LOH ligand is κ2-coordinated to the metal via the imino nitrogens of the two pyrazolyl rings.
The hydroxyl phenyl group is not coordinated to the metal but found to be involved in an intermolecular hydrogen bond.
The solution structures of 1 and 2 are consistent with this tetrahedral CS symmetric geometry. Dilution and 1H-1H
Nuclear Overhauser Effect Spectroscopy (NOESY) experiments revealed that the free ligands LOMe and LOH are
involved in intra- and intermolecular hydrogen bonding interactions. Coordination of LOMe and LOH to ZnCl2 was
investigated by NMR titration methods. Association constants(Ka) of (8.6 ( 0.4) ꢀ 102 M-1 and (7.8 ( 0.3) ꢀ 102 M-1
were obtained in methanol/water solutions (95:5) for LOMe and LOH, respectively. Coordination of bis(3,5-dimethyl-
pyrazol-1-yl)methane (bpm) ligand to ZnCl2 is weaker, as evidenced by the lower value of the association constant
(5.3 ( 0.3) ꢀ 102 M-1. When bpm was added to solutions of 1 or 2, an equilibrium shifted toward the (bmp)ZnCl2
species was observed. The thermodynamic parameters for this reaction were determined by VT NMR analysis. The
optical properties ofthe ligands(LOMe, LOH) and of the corresponding zinc complexes 1 and 2 were also investigated by
means of UV-vis and fluorescence spectroscopy to assess the potential use of these ligands as fluorescent sensors for
Zn2þ detection.
Introduction
substrate-metal interactions in analogy with enzymatic and
organo catalysis.3 The presence of functionalities suitable for
intermolecular noncovalent interactions in the second coor-
dination sphere may also lead to self-aggregation of transi-
tion-metal complexes.4 Among these weak noncovalent
interactions, hydrogen bonding (HB) is probably the most
important.5 Indeed it strongly contributes to determine
structure and reactivity of coordination compounds.6
We were interested in exploring the coordination behavior
of neutral bispyrazolylmethane ligands containing HB donor
or acceptor sites toward Zinc ion. Zinc is the second most
abundant transition-metal ion in the human body and serves
as active site of several metalloproteins found in nature such
as carboxypeptidase A, thermolysin, and leucine aminopep-
tidase.7 Their active sites exhibit a common structural motif
The investigation of the second-sphere bonding interac-
tions of coordination compounds is an emerging area of
research in the scientific literature: non-coordinating active-
sites can have important role in the molecular recognition
and activation processes involving catalysts. These interac-
tions have been exploited to optimize the recognition process
between the substrate and the catalyst1 and to impart or to
improve enantioselectivity to the catalyst itself.2 It has also
been proposed that these interactions may be solely respon-
sible for the activation process without the necessity of
*To whom correspondence should be addressed. E-mail: smilione@
unisa.it.
(1) Das, S.; Incarvito, C. D.; Crabtree, R. H.; Brudvig, G. W. Science
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(b) Kramer, R. Angew. Chem., Int. Ed. 2006, 45, 858–860. (c) Roelfes, G.;
Feringa, B. L. Angew. Chem., Int. Ed. 2005, 44, 3230–3232.
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7931–7944. (b) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122,
1466–1478. (c) Menashe, N.; Shvo, Y. Organometallics 1991, 10, 3885–3891.
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r
pubs.acs.org/IC
Published on Web 08/31/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9511
composed of a pseudo-tetrahedral Zinc center in combina-
tion with nitrogen, oxygen, and/or sulfur donors.8 To under-
stand how the [NxOySz] donor array modulates the chemistry
around the Zinc center, a large number of small synthetic
analogues to these Zinc metalloproteins have been synthe-
sized and characterized in the past years.9
In particular we considered the 1-[(3,5-di-tert-butyl-2-
methoxyphenyl)(3,5-dimethyl-pyrazol-1-yl)methyl]-3,5-di-
methyl-pyrazole (LOMe) and the corresponding hydroxyl
derivative 2,4-di-tert-butyl-6-[bis(3,5-dimethyl-pyrazol-1-yl)-
methyl]phenol (LOH) in which the methoxy-phenyl and the
hydroxy-phenyl group can act as HB acceptor and donor
site, respectively. Because of the spatial arrangement of the
donor atoms, these ligands can be considered heteroscorpio-
nate ligands10 in which the bis(pyrazolyl)methane skele-
ton exhibits R0 substituent, HC(pz)2R0, such as , for example,
phenol,11 thiophenol,12 carboxyl,13 or dithiocarboxyl group.14
These ligands have been widely utilized for developing
metalloprotein models; in fact, they can be considered as
synthetic models of the 2-His-1-carboxylate triad.
Herein we describe the synthesis, the solution properties,
and the reactivity of new Zinc complexes bearing the LOMe
and LOH ligands. The supramolecular structures of both
ligands and complexes were investigated in solution by means
of NMR experiments focusing the attention on the role of the
pendant methoxy/hydroxy-phenyl group. The peculiar be-
havior of these complexes was compared with that of an
analogous Zinc complex bearing a bis-pyrazolyl ligand lack-
ing the pendant methoxy/hydroxy-phenyl group.
Moreover we describe the solution and the optical proper-
ties of these two heteroscorpionate ligands (LOH, LOMe)
whose overall characteristics (e.g., chemical stability and easy
of synthesis) make them potential candidates for the devel-
opment of fluorescent-based Zn2þ sensors.
previously reported.15 C6D6 and CD2Cl2 were degassed by three
successive “freeze-pump-thaw” cycles, dried over molecular
sieves (4 A) and stored in a glovebox prior to use. NMR
spectra were recorded on a Bruker AM300 and Bruker
1
AVANCE 400 operating at 300 and 400 MHz for H, respec-
1
tively. The H and 13C chemical shifts are referred to SiMe4
using the residual protio impurities of the deuterated solvents.
Mass spectrometry analyses were carried out using a Micro-
mass Quattro micro API triple quadrupole mass spectro-
meter equipped with an electrospray ion source (Waters,
Milford, MA).
Synthesis of 1-((3,5-Di-tert-butyl-2-methoxyphenyl)(3,5-dimeth-
yl-pyrazol-1-yl)methyl)-3,5-dimethyl-pyrazole (LOMe). A solu-
tion of LOH (900 mg, 2.2 mmol) in acetone (80 mL) was added
to a suspension of K2CO3 (910 mg, 6.6 mmol) in acetone (80 mL).
The resulting mixture was refluxed under stirring. Iodomethane
(2.1 g, 11 mmol) was added dropwise by a siringe, and the
resulting mixture refluxed for 3 h. All volatiles were then
removed in vacuo, and the product was extracted with water
and diethylether. The organic layer was dried over Na2SO4, and
the solvent removed by distilling in vacuo. Recrystallization of
the crude product from acetone afforded LOMe as a colorless
bright microcrystalline solid. Yield: 620 mg (67%). Anal. Calcd
for C26H38N4O: C, 73.89; H, 9.06; N, 13.26. Found: C, 73.94; H,
9.17; N, 13.19. 1H NMR (400 MHz, CD2Cl2, 25 °C): δ 1.19 (s,
9H, 5-tBu-Ph), 1.38 (s, 9H, 3-tBu-Ph), 2.05 (s, 6H, 5-CH3-Pz),
2.17 (s, 6H, 3-CH3-Pz), 3.44 (s, 3H, OCH3), 5.86 (s, 2H, Pz-H),
6.94 (d, 1H, J = 2.4 Hz, 6-Ph), 7.36 (d, 1H, J = 2.4 Hz, 4-Ph),
7.61 (s, 1H, -CH-). 13C{1H} NMR data (100 MHz, CD2Cl2,
25 °C): 11.5, 14.1, 31.4, 31.5, 34.7, 35.5 61.9, 70.6, 106.9, 125.1,
125.5, 129.7, 140.5, 141.8, 145.7, 148.1, 155.4. MS (ESI acet-
onitrile): m/z (%) 445.8 (100) [Mþ þ Na], 327.7 (18) [Mþ
-
ZnCl2 - 3,5-Me2-pz].
Synthesis of (LOMe)ZnCl2 [LOMe = 1-((3,5-Di-tert-butyl-2-
methoxyphenyl)(3,5-dimethyl-pyrazol-1-yl)methyl)-3,5-dimethyl-
pyrazole] (1). A solution of LOMe (63 mg, 0.150 mmol) in
dichloromethane (1.0 mL) was added to a suspension of ZnCl2
(20 mg, 0.150 mmol) in dichloromethane (1.0 mL). The mixture
was stirred overnight. All volatiles were then removed in vacuo.
Recrystallization of the crude product from dichloromethane-
pentane 1:1 (v/v) afforded 1 as a bright microcrystalline solid.
Yield: 60 mg (72%). Anal. Calcd for C26H38N4OZnCl2: C,
56.07; H, 6.88; N, 10.06. Found: C, 56.23; H, 7.01; N, 9.98. 1H
NMR (400 MHz, CD2Cl2, 25 °C): δ 1.22 (s, 9H, 5-tBu-Ph), 1.38
(s, 9H, 3-tBu-Ph), 2.48 (s, 6H, 5-CH3-Pz), 2.50 (s, 6H, 3-CH3-
Pz), 3.33 (s, 3H, OCH3), 6.18 (s, 2H, Pz-H), 6.42 (d, 1H, J =
2.4 Hz, 6-Ph), 7.49 (d, 1H, J = 2.4 Hz, 4-Ph) 7.50 (s, 1H, -CH-).
13C{1H} NMR data (100 MHz, CD2Cl2, 25 °C): δ 11.9, 14.3,
31.3, 32.2, 35.1, 36.1 63.5, 66.4, 108.5, 123.5, 128.1, 129.6, 142.8,
144.8, 147.7, 154.9, 155.1. MS (ESI acetonitrile): m/z (%)
521.6 (50) [Mþ - Cl], 445.7 (50) [Mþ - ZnCl2 þ Na], 327.7 (100)
[Mþ - ZnCl2 - 3,5-Me2-pz].
Synthesis of (LOH)ZnCl2 [LOH = 2,4-di-tert-butyl-6-(bis-
(3,5-dimethyl-pyrazol-1-yl)methyl)phenol] (2). A solution of
LOH (61 mg, 0.150 mmol) in dichloromethane (1.0 mL) was
added to a suspension of ZnCl2 (20 mg, 0.150 mmol) in
dichloromethane (1.0 mL). The mixture was stirred overnight.
A bright crystalline solid was isolated after cooling of the filtrate
solution at -20 °C. Yield: 55 mg (68%). The crystalline sample
spontaneously and slowly releases dichloromethane molecules
clathrated in the lattice. Anal. Calcd for C25H36N4OZnCl2: C,
55.31; H, 6.68; N, 10.32. Found: C, 55.65; H, 6.89; N, 10.15. 1H
NMR (400 MHz, CD2Cl2, 25 °C): δ 1.22 (s, 9H, 5-tBu-Ph), 1.40
(s, 9H, 3-tBu-Ph), 2.48 (s, 6H, 5-CH3-Pz), 2.51 (s, 6H, 3-CH3-
Pz), 5.15 (s, 2H, Pz-H), 6.12 (s, 1H, -OH), 6.54 (d, 1H, J =
2.4 Hz, 6-Ph), 7.34 (d, 1H, J = 2.4 Hz, 4-Ph), 7.59 (s, 1H, -CH-).
13C{1H} NMR data (100 MHz, CD2Cl2, 25 °C): δ 11.9,
14.3, 30.8, 31.4, 34.3, 34.9, 66.0, 107.9, 122.5, 123.3, 126.8,
134.5, 144.2, 145.1, 149.6, 154.8. MS (ESI acetonitrile): m/z
Experimental Section
General Procedures. All experiments were performed
under nitrogen atmosphere using standard Schlenk-type
techniques or MBraun glovebox. Toluene, hexane, and tetra-
hydrofuran (THF) were distilled over sodium/benzophenone;
dichloromethane was distilled over CaH2. 2,4-Di-tert-butyl-
6-[bis(3,5-dimethyl-pyrazol-1-yl)methyl]phenol (LOH) and bis-
(3,5-dimethyl-pyrazol-1-yl)methane (bpm) were prepared as
(8) Lipscomb, W. N.; Strater, N. Chem. Rev. 1996, 96, 2375–2434.
(9) (a) Parkin, G. Chem. Rev. 2004, 104, 699–768. (b) Vahrenkamp, H.
Dalton Trans. 2007, 4751–4759. (c) Penkova, L. V.; Maciag, A.; Rybak-
Akimova, E. V.; Haukka, M.; Pavlenko, V. A.; Iskenderov, T. S.; Kozlowski,
H.; Meyer, F.; Fritsky, I. O. Inorg. Chem. 2009, 48, 6960–6971. (d) Smith, J. N.;
Hoffman, J. T.; Shirin, Z.; Carrano, C. J. Inorg. Chem. 2005, 44, 2012–2017.
(e) Chiou, S.-J.; Riordan, C. G.; Rheingold, A. L. Proc. Natl. Acad. Sci. U.S.A.
2003, 100, 3695–3700. (f) Chiou, S.-J.; Innocent, J.; Riordan, C. G.; Lam, K.-C.;
Liable-Sands, L.; Rheingold, A. L. Inorg. Chem. 2000, 39, 4347–4353.
(10) Trofimenko, S. Scorpionates: The coordination Chemistry of Poly-
pyrazolylborate Ligands; Imperial College Press: London, 1999.
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(12) Higgs, T. C.; Ji, D.; Czernuszewicz, R. S.; Matzanke, B. F.;
Schunemann, V.; Trautwein, A. X.; Helliwell, M.; Ramirez, W.; Carrano,
C. J. Inorg. Chem. 1998, 37, 2383–2392.
ꢁ
~
(13) Otero, A.; Fernandez-Baeza, J.; Tejeda, J.; Antinolo, A.; Carrillo-
ꢁ ꢁ ꢁ
Hermosilla, F.; Dıez-Barra, E.; Lara-Sanchez, A.; Fernandez-Lopez, M.;
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Lanfranchi, M.; Pellinghelli, M. A. J. Chem. Soc., Dalton Trans. 1999, 3537–
3539.
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ꢁ
ꢁ
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F.; Tejeda, J.; Lara-Sanchez, A.; Sanchez-Barba, L.; Fernandez-Lopez, M.;
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9512 Inorganic Chemistry, Vol. 48, No. 19, 2009
Milione et al.
Table 1. Crystallographic Data
(LOH)ZnCl2 (2)
(LOH)FeI2 (3)
formula
M
space group
crystal system
a/A
C25H36Cl2N4OZn 2(CH2Cl2)
714.70
P21/c
monoclinic
8.9520(2)
19.7884(5)
19.6539(5)
90.510(1)
3481.5(2)
4
295
1.364
1480
11.92
23888
6737
0.0663
4865
C25H36FeI2N4O
718.23
P21/c
monoclinic
12.0259(2)
19.3743(4)
14.5478(3)
112.254(1)
3137.1(1)
4
295
1.521
1416
24.70
26090
6780
0.0362
4386
3
b/A
c/A
β/deg
U/A3
Z
T/K
Dc/g cm-3
F(000)
μ(Mo KR)/cm-1
measured reflections
unique reflections
Rint
obs. refl.ns [I g 2σ(I)]
θmin - θmax/deg
hkl ranges
R(F2) (obs.refl.ns)
wR(F2) (All Refl.ns)
no. variables
goodness of fit
ΔFmax; ΔFmin /e A-3
3.26-26.00
-11,11;-24,24;-24,24
0.0754
0.2248
366
1.042
0.97; -0.90
2.82-27.00
-14,15;-22,24;-17,18
0.0456
0.1126
312
1.046
1.07; -1.25
(%) 529.6 (38) [Mþ - HCl þ Na], 507.6 (8) [Mþ - Cl], 431.7 (84)
[Mþ - ZnCl2 þ Na], 335.7 (100) [Mþ- HCl -ZnCl - 3,5-Me2-
pz þ Na].
with graphite monochromated Mo KR radiation. The data sets
were integrated with the Denzo-SMN package17 and corrected
for Lorentz, polarization and absorption effects (SORTAV).18
The structure was solved by direct methods (SIR97)19 and
refined using full-matrix least-squares with all non-hydrogen
atoms anisotropically and hydrogens included on calculated
positions, riding on their carrier atoms. The asymmetric unit of
complex (LOH)ZnCl2 (2) contains two solvent molecules of
CH2Cl2 which display some disorder. Furthermore, the 2-tBu
group in complex (2) was found disordered and the three methyl
groups were refined isotropically over two positions with occu-
pation factors of 0.60 and 0.40, respectively.
Synthesis of (LOH)FeI2 [LOH = 2,4-Di-tert-butyl-6-(bis(3,5-
dimethyl-pyrazol-1-yl)methyl)phenol] (3). A solution of LOH
(500 mg, 1.2 mmol) in dichloromethane (25 mL) was added to
a suspension of FeI2 (380 mg, 1.2 mmol) in dichloromethane
(25 mL). The mixture was stirred overnight. Recrystallization of
the crude product from diethylether/pentane afforded 3 as a pale
brown microcrystalline solid. Yield: 390 mg (38%). Anal. Calcd
for C25H36N4OFeI2: C, 41.81; H, 5.05; N, 7.80. Found: C, 41.99;
H, 5.21; N, 7.67. μeff = 3.7 μB (CD3CN, room temperature (rt)).
NMR Titrations. Dilution 1H NMR titrations for LOH were
performed at 25 °C in C6D6. A total of 16 spectra were registered
using a ligand concentration in the range of 0.200-0.005 M.
Speciation of ZnCl2/LOR (R = H; Me) system was investi-
gated in CD3OD-D2O 95:5 (v/v) mixture at 25 °C by collecting
1H NMR spectra of a 0.0050 M ligand solution titrated with
aliquots of a 0.10 M ZnCl2 solution. The equilibrium between
LOR and (LOR)ZnCl2 occurs in rapid exchange on the NMR
time scale. The variation of the chemical shift (Δδ) for the
resonance of 2-tBu group of the ligand (Scheme 2) was mon-
itored during the titration. The association constant (Ka) was
determined by fitting the plot of Δδ versus [ZnCl2]/[LOR].16
Complexation of ZnCl2 with bpm ligand was studied in the same
experimental conditions and with the same procedure.
Absorbance and Fluorescence Measurements. Absorption
spectra were recorded on a Perkin-Elmer Lambda EZ201
instrument, using 1 cm quartz cell and PESSW 1.2 Revision E
software. Fluorescence spectra were recorded on a Cary Eclipse
Spectrophotometer in a 10 ꢀ 10 mm airtight quartz fluorescence
cuvette (Hellma Benelux bv, Rijswijk, Netherlands). Both ab-
sorption and fluorescence measurements were performed at
room temperature.
All calculations were performed using SHELXL-9720 and
PARST21 implemented in the WINGX22 system of programs.
The crystal data are given in Table 1. Selected bond distances
and angles are given in Table 2.
Crystallographic data (excluding structure factors) have been
deposited at the Cambridge Crystallographic Data Centre and
allocated the deposition number CCDC 735023-735024. These
data can be obtained free of charge via www.ccdc.cam.ac.
uk/conts/retrieving.html or on application to CCDC, Union
Road, Cambridge, CB2 1EZ, U.K. [fax: (þ44)1223-336033,
e-mail: deposit@ccdc.cam.ac.uk].
Results and Discussion
The heteroscorpionate ligand [2,4-di-tert-butyl-6-(bis(3,5-
dimethyl-pyrazol-1-yl)methyl]phenol (LOH) was prepared
according to literature procedures based on the reaction of
1,10-carbonyldipyrazoles with carbonyl compounds cata-
lyzed by CoCl2.15 The corresponding methylether derivative
1-[(3,5-di-tert-butyl-2-methoxyphenyl)(3,5-dimethyl-pyrazol-
1-yl)methyl]-3,5-dimethyl-pyrazole (LOMe) was obtained by
Crystal Structure Determinations. The crystal data of com-
pounds (LOH)ZnCl2 (2) and (LOH)FeI2 (3) were collected at
room temperature using a Nonius Kappa CCD diffractometer
(18) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51(Pt 1), 33–38.
(19) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Gia-
covazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J.
Appl. Crystallogr. 1999, 32, 115–119.
(16) Fielding, L. Tetrahedron 2000, 56, 6151–6170.
(20) Sheldrich, G. M. SHELXTL-97, Program for Crystal Structure
::
::
(17) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data
Collected in Oscillation Mode. In Methods in Enzymology: Macromolecular
Crystallography, part A; Academic Press: San Diego, 1997; pp 307-326.
Refinement; University of Gottingen: Gottingen, Germany, 1997.
(21) Nardelli, M. J. Appl. Crystallogr. 1995, 28, 659.
(22) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9513
(LOH)FeI2 (3)
Table 2. Selected Geometrical Parameters (A and deg)
(LOH)ZnCl2 (2)
Distances
Angles
Zn1-Cl1
Zn1-Cl2
Zn1-N1
Zn1-N3
2.230(2)
2.219(2)
2.038(4)
2.040(4)
Fe1-I1
Fe1-I2
Fe1-N1
Fe1-N3
2.5843(8)
2.5763(7)
2.063(3)
2.087(3)
Cl1-Zn1-Cl2
Cl1-Zn1-N1
Cl1-Zn1-N3
Cl2-Zn1-N1
Cl2-Zn1-N3
N1-Zn1-N3
112.74(6)
114.6(1)
114.6(1)
110.7(1)
109.7(1)
92.9(2)
I1-Fe1-I2
117.71(3)
I1-Fe1-N1
I1-Fe1-N3
I2-Fe1-N1
I2-Fe1-N3
N1-Fe1-N3
114.6(1)
114.4(1)
107.6(1)
108.4(1)
90.8(1)
Torsion Angles
Dihedral Angle
C1-C6-C7-N2
C1-C6-C7-N4
76.6(5)
-157.0(4)
C1-C6-C7-N2
C1-C6-C7-N4
158.1(3)
-74.8(4)
N1-N2-C8-C9-C10
N3-N4-C13-C14-C15
N3-N4-C13-C14-C15
138.0(2)
134.3(2)
N1-N2-C8-C9-C10
O-H X-M H-bond*
3 3 3
O1 Cl1(x-1, y, z)
3.246(4)
2.61
125
O1 I1(1-x, y-1/2, 3/2-z)
3 3 3
3.528(3)
3.01
118
3 3 3
H1 Cl1(x-1, y, z)
H1 I1(1-x, y-1/2, 3/2-z)
3 3 3
3 3 3
O1-H1 Cl1
O1-H1 I1
3 3 3
3 3 3
H1 Cl1-Zn1
114
H1 I1-Fe1
3 3 3
122
3 3 3
* The O-H distances were normalized.
Scheme 1
result in the same structural motif. For comparative purpose
we investigated the reaction of LOH with FeI2: the same
coordination mode was observed for the corresponding
(LOH)FeI2 (3) complex.
Solid-State Structures. The Oak Ridge Thermal Ellip-
soid Plot (ORTEP)23 views of the complexes 2 and 3 are
shown in Figures 1 and 2. The Zn(II) in 2 is tetrahedrally
κ2-N,N coordinated to two pyrazolyl rings of LOH and to
two chloride ligands. The distortion of the Zn(II) from the
ideal tetrahedral coordination is mainly determined by
the N-Zn-N angle of 92.9(2)° constrained by the bite of
the bis-chelating LOH ligand. The Zn-Cl distances of
2.230(2) and 2.219(2) A and Zn-N (pyrazole) ones of
2.038(4) and 2.040(4) A are in agreement with the values
found in similar complexes.24
The Fe(II) tetrahedral coordination in 3 is similar to
that exhibited by Zn(II) of the previous complex, with
Fe(II) κ2-N,N bound to two pyrazolyl rings of LOH
ligand and to two iodide ligands. The distortion of the
tetrahedral Fe(II) is mainly determined by the N-Fe-N
angle of 90.8(1)° constrained by the bite of the bis-
chelating LOH ligand.
reaction of LOH with iodomethane in the presence of
potassium carbonate.
The complexes (LOR)ZnCl2 [R = Me (1), H (2)] were
prepared in high yield by reacting ZnCl2 with the neutral
ligand in 1:1 molar ratio in dichloromethane (Scheme 1). The
two complexes are soluble in halogenated or oxygenated
solvent and slightly soluble in aromatic solvents.
In both complexes (2) and (3) the metal bonded halogen
atoms Cl and I are strongly polar and good acceptors of
HB.25,6 Accordingly, the uncoordinated hydroxy phenyl
As far as the coordination capabilities of the ligands are
concerned, the two pyrazolyl ring and the phenol group could
provide the N2O donor set required for a hypothetical
pentacoordinate complex. Nevertheless, the reaction of these
heteroscorpionate ligands with ZnCl2 led to mononuclear
tetrahedral complexes in which the heteroscorpionate ligand
resulted κ2-coordinated to the metal center through the imino
nitrogen atoms of the two pyrazolyl rings. Iron(II) and
Zinc(II) have similar atomic radii; therefore, one can expect
that the reaction of Zinc and Iron salts with a givenligand can
(23) Burnett, M. N.; Johnson, C. K. ORTEP III: Oak Ridge Thermal
Ellipsoids Plot Program for Crystal Structure Illustration, ORNL-6895; Oak
Ridge National Laboratory: Oak Ridge, TN, 1996.
(24) Schofield, A. D.; Barros, M. L.; Cushion, M. G.; Schwarz, A. D.;
Mountford, P. Dalton Trans. 2009, 1, 85–96.
(25) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural
Chemistry and Biology; Oxford University Press: Oxford, 1999.

9514 Inorganic Chemistry, Vol. 48, No. 19, 2009
Milione et al.
Figure 1. ORTEP view of the complex (LOH)ZnCl2 (2) showing the
thermal ellipsoids at 30% probability level.
Figure 3. Diagram of molecules of complexes (LOH)ZnCl2 (2) and
(LOH)FeI2 (3) to illustrate the O-H Cl (a) and O-H I (b) H-bond
interactions.
3 3 3 3 3 3
oxygen of the phenolate group.24,28 Conversely, the com-
plexes reported in the present study represent a class of
compounds where the phenolic group of the heteroscor-
pionate ligand is not involved in the coordination mode
with the central metallic core.11,29
Solution Properties. The solution structures of LOR
(R = Me, H) ligands and complexes 1-2 were investi-
gated by means of NMR spectroscopy. The assignments
of the proton resonances were based on previously re-
ported data28 and corroborated by means of mono- and
bidimensional NMR experiments.
Dilution and 1H-1H Nuclear Overhauser Effect Spec-
troscopy (NOESY) experiments were carried out to dis-
criminate the presence of intra- or intermolecular HB
interactions. The chemical shift of the proton resonan-
ces did not change in dilution experiments of LOMe in
C6D6 from 200 to 5 mM (ΔδCH<0.01) indicating that no
intermolecular interaction occurs. NOESY of LOMe (in
C6D6 solution) showed, in addition to the interactions of
the neighboring protons, a strong contact between the
aliphatic methine proton (CH, Scheme 2) and the meth-
oxy group (Figure 4). This suggested that the phenyl ring
has a preferred orientation in which the MeO group is
roughly opposite to the pyrazol rings. This conformation
is probably stabilized by a labile intramolecular HB
between the acidic CH proton and the Lewis basic
methoxy group. However, the reorientation of the phenyl
ring is not hampered, and other conformations for the
molecule are possible as indicated by the presence of a
Figure 2. ORTEP view of the complex (LOH)FeI2 (3) showing the
thermal ellipsoids at 30% probability level.
groups are weakly hydrogen bonded to adjacent mole-
cules with Cl H and I H distances of 2.61 and 3.01 A
shorter than the sums of van der Waals radii of 2.95
3 3 3
3 3 3
and 3.18 A26 (Table 2, Figures 3). Furthermore, the
H
X-M angles of 114 and 122° are in agreement with
3 3 3
the hypothesis that the O-H X-M hydrogen bonds in
crystals are almost exclusively donated roughly perpen-
3 3 3
dicular to the M-X bond.27
These heteroscorpionate ligands were mainly found as
tri- or bidentate anionic ligands which coordinate metal
centers through one or two pyrazolyl nitrogens and the
(28) (a) Milione, S.; Bertolasi, V.; Cuenca, T.; Grassi, A. Organometallics
2005, 24, 4915–4925. (b) Milione, S.; Grisi, F.; Centore, R.; Tuzi, A. Organo-
metallics 2006, 25, 266–274.
(29) (a) Shirin, Z.; Hammes, B. S.; Warthen, C. R.; Carrano, C. J. J.
Chem. Crystallogr. 2003, 33, 431–436. (b) Shirin, Z.; Carrano, C. J. Polyhedron
2004, 23, 239–244. (c) Elflein, J.; Platzmann, F.; Burzlaff, N. Eur. J. Inorg.
Chem. 2007, 5173–5176.
(26) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.
(27) (a) Yap, G. P. A.; Rheingold, A. L.; Das, P.; Crabtree, R. H. Inorg.
ꢁ
Chem. 1995, 34, 3474–3476. (b) Aullon, G.; Bellamy, D.; Brammer, L.; Bruton,
E.; Orpen, A. G. Chem. Commun. 1998, 653–654.

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9515
Figure 5. Stacking plot of partial 1H NMR spectra of LOH in C6D6 at
different concentrations. The drift of the aliphatic methine proton (CH)
resonance is shown.
Figure 4. Section of 1H-1H NOESY spectrum of LOMe (C6D6, rt,
400 MHz). The proton resonances are labeled according to the Scheme 2.
Scheme 2
Scheme 3
weaker contact between the 6-Ph and the 5-Me-Pz pro-
tons (see Scheme 2).
assembled via HB, are formed in concentrated C6D6
solutions (Scheme 3). In polar solvents bearing HB
acceptor sites these intermolecular interactions are miss-
ing, and no chemical shift difference was actually ob-
served in dilution experiments carried out for LOH in
CD2Cl2 solution.
In the X-ray structures of neutral heteroscorpionate
ligands featuring a hydroxyl phenyl group an intramole-
cular HB between the hydroxyl and pyrazolyl groups can
be observed.30 In dilution experiments carried out in
1
The 1H and 13C NMR spectra of complexes 1 and 2 are
consistent with a tetrahedral CS symmetric structure
resulting from the κ2-N,N coordination of the ligands to
the metal center. The resonances are slightly shifted when
compared to the corresponding free ligand. The only
exception is the resonance of the hydroxyl group that
moved from 10.15 ppm in LOH to 5.15 ppm in 2 (CD2Cl2,
rt). In the complexes 1-2 the nitrogen HB acceptor sites
are replaced by chlorine atoms. In this way the behavior
expected for the complexes is the same observed for the
heteroscorpionate ligands. Unfortunately the search
for the intermolecular bond in solution was hampered
by the low solubility of these complexes in C6D6. Dilu-
tion experiments in the narrow concentration range
0.001-0.005 M showed no significant changes in the
chemical shifts.
C6D6 the invariance of the H NMR chemical shift for
the proton of the hydroxyl phenyl (OH) group suggested
that the same intramolecular interaction occurs in solu-
tion. Interestingly, an upfield drift in the chemical shift of
the aliphatic methine proton (CH) was observed when the
analytical concentration of LOH was decreased from
200 mM to 5 mM (Figure 5), revealing the presence of
an intermolecular HB between the CH and the imine
nitrogen of one pyrazolyl ring (the only HB acceptor site
of LOH) of another LOH molecule. Over the course of
dilution, the concentration of the hydrogen bonded spe-
cies decreases and the population of the shielded proton
of the aliphatic methine (CH) increases determining an
1
upfield shift of this H signal. The NOESY experiment
showed a strong contact between the latter proton and the
6-Ph proton confirming that the orientation of the phenyl
ring in LOH is so that the hydroxyl group and the pyrazol
rings are on the same side. As each molecule can be bound
to two adjacent molecules, long chains of molecules,
Owing to the paramagnetism of the metal center (μeff
=
3.7 μB in CD3CN at room temperature), the 1H NMR of 3
displayed broad and featureless resonances.
Coordination Studies. Speciation of LOR/ZnCl2 system
was evaluated in anhydrous acetonitrile by adding ali-
quots of a 0.10 M ZnCl2 solution to a 0.0050 M LOR
solution. The equilibrium between the free ligand and
(LOR)ZnCl2 occurs in the slow exchange regime on the
(30) (a) Hubner, E.; Hass, T.; Burzlaff, N. Eur. J. Inorg. Chem. 2006,
4989–4997. (b) Otero, A.; Fernandez-Baeza, J.; Antinolo, A.; Tejeda, J.; Lara-
Sanchez, A.; Sanchez-Barba, L. F.; Sanchez-Molina, M.; Franco, S.; Lopez-
Solera, M. I.; Rodrguez, A. M. Inorg. Chem. 2007, 46, 8475–8477.

9516 Inorganic Chemistry, Vol. 48, No. 19, 2009
Milione et al.
NMR time scale. The integration of diagnostic reso-
nances revealed that the amount of the formed complex
is equivalent to the amount of added ZnCl2 suggesting
that the association constant (Ka) in anhydrous acetoni-
trile is over the detectable limit by NMR titration meth-
ods (>105 M-1).16 In a methanol/water (95:5) solution
the equilibrium between LOR and (LOR)ZnCl2 occurs in
rapid exchange. The association constant (Ka) was deter-
mined by fitting the plot of Δδ of the 2-tBu 1H-resonance
versus [ZnCl2]/[LOR].16 Ka values of (8.6 ( 0.4) ꢀ 102 M-1
for the association of LOMe to ZnCl2 and of (7.8 ( 0.3) ꢀ
102 M-1 in the case of LOH were determined. When these
values were compared to data reported in the literature
for similar ligands,31 they appeared to be lower. This
finding can be traced back to the presence of water in the
sample solution that probably competes with the ligand in
coordinating the acidic Zn center.
As the coordination mode of the heteroscorpionate
ligands LOR to acidic Zn2þ center is very similar to that
found for the bis(3,5-dimethyl-pyrazol-1-yl)methane
(bpm) ligand,32 we were interested in comparing their
coordination capabilities. The average Zn-N bond
length found in the X-ray structure for 2 (2.039 A, Zn1-
N1 = 2.038(4) A, Zn1-N3 = 2.040(4) A, Table 2) is signi-
ficantly shorter than that for (bpm)ZnCl2 (2.060 A,
Zn1-N1 = 2.057(1) A, Zn1-N3 = 2.062(1) A).32 More-
over, the association constant for the bpm/ZnCl2 system,
determined in the same experimental conditions, is lower,
Figure 6. Section of 1H NMR spectra (CD2Cl2, rt, 400 MHz) of 1/bpm
(top) and 2/bpm (bottom). The resonances are labeled according to the
Scheme 2; the signals marked with an asterisk are due to the zinc
complexes 1 (top) and 2 (bottom). The broad resonances between 5.9
and 6.3 ppm are due to (bpm)ZnCl2.
is equal, the equilibrium constants Keq were determined at
different temperatures according to the following eq 2:
(5.3 ( 0.3) ꢀ 102 M-1
.
Keq ¼ ½LORꢁ½ðbpmÞZnCl2ꢁ=½ðLORÞZnCl2ꢁ½bpmꢁ
The stronger coordination to ZnCl2 of the LOR ligands
compared to the bpm ligand can be ascribed to the
presence of the methoxy/hydroxy-phenyl group on the
methylene bridge that produces an increase in the basicity
of the pyrazolyl nitrogen as a result of the electron-
donating properties of these groups.
When a CD2Cl2 solution of 1 or 2 reacted with equi-
molar amount of bpm ligand, an exchange process oc-
curred according to the equilibrium (1):
2
¼ R2=ð1 - RÞ
ð2Þ
The thermodynamic parameters were thus extrapo-
lated from the inverse temperature plot of ln(Keq). The
standard enthalpy (ΔH°) and entropy (ΔS°) values of
-2.7 ( 0.2 kcal mol-1 and -3.0 ( 0.7 cal mol-1 K-1 were
determined for the substitution of LOMe in 1 by bpm;
analogously the ΔH° and ΔS° values of -3.2 ( 0.3 kcal.
mol-1 and -3.0 ( 0.9 cal mol-1 K-1 were determined for
the substitution of LOH.
The negative values of ΔH° indicate that the substitu-
tion of the heteroscorpionate ligands for bpm is thermo-
dynamically favored. As (bpm)ZnCl2 is less stable than 1
and 2, this enthalpic gain originates from the intra- and
intermolecular HB interactions previously described for
the heteroscorpionate ligands LOR which stabilize the
free ligand and promote its substitution. This is con-
firmed by the negative values found for ΔS° in the same
reaction.
The higher value of R obtained for the substitution of
LOH compared to that of LOMe is probably due to the
higher capability of the hydroxy-phenyl group to estab-
lish HB interactions. To verify this hypothesis we inves-
tigated the exchange reactions of Zinc complexes 1-2
promoted by the heteroscorpionate ligands. When 1 was
treated with 1 equiv of LOH (eq 3) in CD2Cl2 solution, a
mixture of 1, 2, LOH, and LOMe was obtained where 1
and LOH were in slight excess.
ðLORÞZnCl2 þ bpm / LOR þ ðbpmÞZnCl2
ð1Þ
The area ratio of the 1H resonances diagnostic for the
free and coordinated LOR ligand permitted the conver-
sion grade (R) to be determined as follows:
R ¼ ALOR=ðALOR þ ALORZnCl2
Þ
where ALOR and ALORZnCl2 are the areas of the aliphatic
methine proton (CH) signals in the free and coordinated
ligand, respectively. Surprisingly the conversion grades
were 0.83 and 0.88 at 20 °C for 1 and 2, respectively,
suggesting that the equilibrium is strongly shifted toward
the (bpm)ZnCl2 species in both cases. It is worth noting
that the proton resonances of (bpm)ZnCl2 are broad,
indicating a rapid exchange equilibrium between the free
and the coordinated bpm ligand as shown in the NMR
spectra of Figure 6. VT NMR analysis was carried out to
evaluate the thermodynamic parameters for these equili-
bria. When the starting concentration of the two reagents
ðLOMeÞZnCl2 þ LOH / LOMe þ ðLOHÞZnCl2 ð3Þ
The conversion grade lower than 0.50, namely, 0.46,
(31) Gennari, M.; Tegoni, M.; Lanfranchi, M.; Pellinghelli, M. A.;
Marchi, L. Inorg. Chem. 2007, 46, 3367–3377.
(32) Cheng, M.-L.; Li, H.-X.; Zang, Y.; Lang, J. P. Acta Crystallogr.,
Sect. C 2006, C62, m74–m77.
1
confirmed the previous hypothesis. The H-1H EXSY

Article
Inorganic Chemistry, Vol. 48, No. 19, 2009 9517
Figure 8. Emission spectrum of LOH (black trace) and LOHZnCl2
(gray trace). (rt, 70 μM in CH2Cl2).
two bands centered at 241 and 280 nm. The first band was
ascribed to a π-π* transition of the pyrazolyl rings, the
same observed in the absorption spectrum of the bpm
ligand at 223 nm;36 the second one was attributed to a
π-π* transition of the phenyl rings. The coordination of
the ligands to the metal ion induces no significant varia-
tions in the absorption spectra.
Both LOMe and LOH ligands are fluorescent and
exhibit an emission band in the near UV region, around
310 nm. The fluorescence intensity is affected by the
solvent: in methanol solution the fluorescence intensity
displayed by LOH resulted 95% higher than that found in
dichloromethane. In the same experimental conditions,
the fluorescence intensity of LOMe did not show any
solvent dependent effect. In THF solutions the fluores-
cence intensity displayed by LOH resulted 30% higher
than that registered in dichloromethane, whereas that one
displayed by LOMe resulted unchanged under the same
conditions.
Figure 7. 1H-1H EXSY spectrum of 1, 2, LOH, and LOMe mixture (rt,
mixing time = 0.8 s). The exchange of the Pz protons for 1/LOMe and 2/
LOH is evidenced.
spectrum of this mixture (CD2Cl2, τm = 0.8 s), along with
the negative cross-peaks deriving from cross-relaxation
(NOE), showed a set of positive cross-peaks (same diag-
onal sign) revealing a chemical exchange between 1 and
LOMe and between 2 and LOH (Figure 7). The inten-
sity of the correlation peaks found for the two couples
1/LOMe and 2/LOH permitted the rate constant (k) to be
determined. The values of 0.12 and 0.07 s-1 were obtained
for the exchange of 1-LOMe and 2-LOH respectively.
These values suggest a slow exchange of the ligands at
room temperature in halogenated solvents.
Optical Properties. Zinc is active in a variety of biolo-
gical processes such as gene transcription, regulation of
metalloenzymes, neural signal transmission.33 Determi-
nation of Zn2þ in biological samples, within the range of
1 fM in bacterial cells to 0.1 mM in some vesicles,34 is of
great significance. Currently, direct detection of Zn2þ is
particularly challenging because of the spectroscopically
silent d10 electronic configuration. The first used techno-
logies to monitor Zn2þ levels were mainly based on
histochemical procedures (e.g., use of the colorimetric
indicator dithizone and autometallography). Because
of their invasiveness, these systems are not ideally suited
for studying Zn2þ-related biochemistry. To avoid some
of the limitations of the existing methodologies35 recently
fluorescence-based methods have been proposed as
one of the most effective way for Zn2þ detection. To
assess the potential use of ligands LOR as fluorescent
sensors for Zn2þ detection, we investigated their optical
properties and the ones of the corresponding Zn com-
plexes by means of UV-vis and fluorescence spectro-
scopies.
Interestingly, coordination of Zn2þ induces a fast en-
hancement of LOH fluorescence while it quenches that
one of LOMe. In other words, Zn2þ “turns-on” the
fluorescence of LOH and “turns-off” the one of LOMe.
In dichloromethane solutions a fluorescence switching of
90% was found in the case of LOH (Figure 8), and of 65%
for LOMe (Figure 9). The observed differences in the
fluorescent behavior of LOMe and LOH are most likely
due to the different tendency to establish intra- or inter-
molecular HB interactions. In the case of LOH, the
hydroxyl phenyl group is HB bound to the imine
nitrogen of one pyrazolyl group. When this interaction
is absent, such as in MeOH or THF solution or by coordi-
nation of Zn2þ, an enhancement of LOH fluorescence
is observed. A possible explanation for this finding is the
interruption of photoinduced electron transfer (PET) of
electronic density from the lone pairs on the oxygen atom
to the π-system of the fluorophore.37
The fluorescence intensities of LOMe and LOH were
measured in the presence of a large excess of the most
biologically relevant, potentially competing, mobile me-
tal ions. Different results were found for the two hetero-
scorpionate ligands investigated. Ca2þ, Naþ, and Ni2þ
did not perturb LOH fluorescence while they quenched
The absorption spectra of the heteroscorpionate li-
gands, LOMe and LOH, are very similar and features
(33) Liu, Y.; Zhang, N.; Chen, Y.; Wang, L. H. Org. Lett. 2007, 9, 315–
318.
(34) Wang, H. H.; Gan, Q.; Wang, X. J.; Xue, L.; Liu, S. H.; Jiang, H.
Org. Lett. 2007, 9, 4995–4998.
(35) (a) Zhang, X. A.; Lovejoy, K. S.; Jasanoff, A.; Lippard, S. J. Proc.
Natl. Acad. Sci. U.S.A 2007, 104, 10780–10785. (b) Burdette, S. C.; Walkup, G.
K.; Spingler, B.; Tsien, R. Y.; Lippard, S. J. J. Am. Chem. Soc. 2001, 123, 7831–
7841. (c) Hirano, T.; Kikuchi, K.; Urano, Y.; Higuchi, T.; Nagano, T. J. Am.
Chem. Soc. 2000, 122, 12399–12400.
(36) Ferro, R.; Milione, S.; Caruso, T.; Grassi, A. J. Mol. Catal. A: Chem.
2009, 307, 128–133.
(37) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J.
M.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 1515–
1566.

9518 Inorganic Chemistry, Vol. 48, No. 19, 2009
Milione et al.
methylene bridge of the heteroscorpionate ligands strength-
ens the coordination of the pyrazolyl rings. Notwithstanding
this, the bis-pyrazolyl-methane replaces the heteroscorpio-
nate ligands from the coordination sphere of Zinc. As matter
of fact an exchange process strongly shifted toward
(bmp)ZnCl2 species was observed when bpm was added to
a solution of 1 or 2. The analysis of the thermodynamic
parameters suggests that the driving force of the reaction is
the possibility for the free heteroscorpionate ligands to
establish noncovalent interactions in solution.
The higher value of ΔH° observed for the substitution of
LOH compared to LOMe is probably due to the higher
capability of the hydroxy-phenyl group (hydrogen-bond
donor) to establish HB interactions compared with that of
the methoxy-phenyl group (hydrogen-bond acceptor). This
feature was also evident in the substitution reactions of Zinc
complexes 1-2 promoted by the heteroscorpionate ligands.
Finally, the present data provide preliminary results on a
proof-of-principle method for monitoring Zn2þ by using
easily synthesized and robust organic ligands. Within this
contribution we are providing guiding principles for design-
ing fluorescence-based zinc sensors. A theoretical under-
standing of their mechanisms will be undertaken at a later
stage in our investigations. Such an achievement, in fact,
would be helpful for further rational design of zinc sensors.
A fine tailoring of the ligand skeleton aiming at enhancing
the scarce solubility in aqueous solution at neutral pH
of both LOH and LOMe is currently under way in our
laboratory.
Figure 9. Emission spectrum of LOMe (gray trace) of LOMeZnCl2
(black trace). (rt, 70 μM in CH2Cl2).
the one displayed by LOMe. Co2þ, as commonly reported
in the case of divalent first-row transition metal ions,32
turned off the fluorescence emission of both LOH and
LOMe. Unfortunately, measurements under simulated
physiological conditions could not be performed because
of the scarce solubility of both LOH and LOMe in
aqueous solutions.
Conclusions
New Zinc and Iron complexes 1-3 of two heteroscorpio-
nate ligands have been synthesized and characterized by
X-ray diffraction analysis and NMR spectroscopy. The
complexes exhibit a tetrahedral geometry in which the ligand
is κ2-coordinated to the metal. The hydroxyl phenyl group of
LOH is not coordinated but bound to an adjacent molecule
giving long hydrogen bound chains. The solution structures
of both ligands and complexes were also investigated by
means of NMR spectroscopy to assess the presence of
noncovalent interactions in solution.
Acknowledgment. The authors thank Patrizia Oliva and
Dr. Patrizia Iannece of the Department of Chemistry of
the University of Salerno for their technical assistance.
Supporting Information Available: Crystallographic data in
CIF file format. This material is available free of charge via the
Internet at http://pubs.acs.org.
In comparison with the bis-pyrazolyl-methane ligand,
the electron-donor methoxy/hydroxy-phenyl group on the