Heteroscorpionate-based Co2+,Zn2+, and Cu 2+ complexes: Coordination behavior, aerobic oxidation, and hydrogen sulfide detection

Article abstract of DOI:10.1021/ic101467s

The coordination behavior and reactivity of the phenol-substituted bis(pyrazolyl)methane ligands, (3,5-tBu₂-2-phenol)-bis(3,5-Me ₂-pyrazol-1-yl)methane (L1-H) and 2-phenol-bis(3,5-Me ₂-pyrazol-1-yl)methane (L2-H) have been investigated in the metal complexes (L1-H)CoCl₂ (l), (L1-H)ZnCl₂ (2), (L3)CuCl ₂ (3), (L2)₂Co₂Cl₂ (4)(L2-H)ZnCl₂ (5), and (L2-H)CuCl₂ · H ₂O (6). The mononuclear tetrahedral cobalt complex 1 was isolated and fully characterized by X-ray single crystal diffraction and 1H NMR spectroscopy and relaxometry. The neutral L1-H is κ2;-coordinated to the metal center whereas the not coordinated hydroxy-phenyl group is involved in extended intermolecular hydrogen bonds. Aerobic oxidation of L1-H was observed in the reaction of this ligand with CuCl₂ to yield the para-quinone derivative L3 (L3 = 2-tBu-6-(bis(3,5-Me₂-pyrazol-1-yl)methyl) cyclohexa-2,5-diene-1,4-dione). Upon oxidation L3 resulted κ2-coordinated to the tetrahedral Cu(II) metal center, affording 3. The reaction of L2-H with CoCl₂ 6H₂O produced the elimination of 1 equiv of hydrochloric acid and the formation of the binuclear complex 4 in which one cobalt is in an octahedral environment featuring two κ3-coordinated deprotonated ligands whereas the second cobalt center is detected in tetrahedral coordination geometry, bound to the octahedral cobalt via two phenoxo bridging moieties. Interestingly L2-H, (3-tBu-2-phenol)bis(3,5-Me₂-pyrazol-1- yl)methane (L4-H),or (5-tBu-2-phenol)b/s(3,5-Me₂-pyrazol-1-yl)methane (L5-H) were not oxidized in the reaction with CuCl₂. The reaction of the ligand L2-H with ZnCl₂ and CuCl₂ 2H₂O yielded the κ2-coordinated tetrahedral complex 5 and the square planar complex 6, respectively. The application of the cobalt complex 1 as molecular dosimeter for H₂S was explored and compared to that of the zinc analogue 2. Density functional theory (DFT) calculations and NMR experiments to assess the possible mechanisms of H₂S detection by both 1 and 2 are also described.

Full text of DOI:10.1021/ic101467s

900 Inorg. Chem. 2011, 50, 900–910
DOI: 10.1021/ic101467s
Heteroscorpionate-Based Co2þ, Zn2þ, and Cu2þ Complexes: Coordination
Behavior, Aerobic Oxidation, and Hydrogen Sulfide Detection
Maria Strianese, Stefano Milione,* Valerio Bertolasi,† Claudio Pellecchia, and Alfonso Grassi
ꢀ
Dipartimento di Chimica, Universita di Salerno, via Ponte don Melillo, I-84084 Fisciano (SA), Italy.
†
ꢀ
Universita di Ferrara, Centro di Strutturistica Diffrattometrica e Dipartimento di Chimica,
Via L. Borsari, 46, I-44100 Ferrara, Italy
Received July 22, 2010
The coordination behavior and reactivity of the phenol-substituted bis(pyrazolyl)methane ligands, (3,5-tBu2-2-phenol)-
bis(3,5-Me2-pyrazol-1-yl)methane (L1-H) and 2-phenol-bis(3,5-Me2-pyrazol-1-yl)methane (L2-H) have been
investigated in the metal complexes (L1-H)CoCl2 (1), (L1-H)ZnCl2 (2), (L3)CuCl2 (3), (L2)2Co2Cl2 (4) (L2-H)ZnCl2
(5), and (L2-H)CuCl2 H2O (6). The mononuclear tetrahedral cobalt complex 1 was isolated and fully characterized by
3
X-ray single crystal diffraction and 1H NMR spectroscopy and relaxometry. The neutral L1-H is κ2-coordinated to the
metal center whereas the not coordinated hydroxy-phenyl group is involved in extended intermolecular hydrogen
bonds. Aerobic oxidation of L1-H was observed in the reaction of this ligand with CuCl2 to yield the para-quinone
derivative L3 (L3 = 2-tBu-6-(bis(3,5-Me2-pyrazol-1-yl)methyl)cyclohexa-2,5-diene-1,4-dione). Upon oxidation L3
resulted κ2-coordinated to the tetrahedral Cu(II) metal center, affording 3. The reaction of L2-H with CoCl2 6H2O
3
produced the elimination of 1 equiv of hydrochloric acid and the formation of the binuclear complex 4 in which one
cobalt is in an octahedral environment featuring two κ3-coordinated deprotonated ligands whereas the second cobalt
center is detected in tetrahedral coordination geometry, bound to the octahedral cobalt via two phenoxo bridging
moieties. Interestingly L2-H, (3-tBu-2-phenol)bis(3,5-Me2-pyrazol-1-yl)methane (L4-H), or (5-tBu-2-phenol)bis(3,5-
Me2-pyrazol-1-yl)methane (L5-H) were not oxidized in the reaction with CuCl2. The reaction of the ligand L2-H with
ZnCl2 and CuCl2 2H2O yielded the κ2-coordinated tetrahedral complex 5 and the square planar complex 6,
3
respectively. The application of the cobalt complex 1 as molecular dosimeter for H2S was explored and compared to
that of the zinc analogue 2. Density functional theory (DFT) calculations and NMR experiments to assess the possible
mechanisms of H2S detection by both 1 and 2 are also described.
Introduction
heteroscopionates are based on substituted bis(pyrazolyl)-
methane, HC(pz)2R0, where the R0 group is, for example,
phenol,3 thiophenol,4 carboxyl,5or dithiocarboxyl group.6
In our research group we devoted particular attention to
the phenol-substituted bis(pyrazolyl)methane derivatives.7-14
Recently the coordination of (3,5-tBu2-2-phenol)bis(3,5-
Me2-pyrazol-1-yl)methane ligand (L1-H) to the acidic Zn2þ
center has been described.7 The complexes exhibit a tetrahedral
Heteroscorpionate ligands have been widely investigated
for their application in coordination, organometallic, and
bioinorganic chemistry. These ligands are derivatives of
poly(pyrazolyl)borates, or scorpionates, ligands in which
one of the three pyrazole groups is replaced with a different
substituted pyrazolyl group or with a coordinating tethered
group, containing a coordinating donor, for example, -SR,
-NR2, -Ar. The term heteroscorpionate also applies to trident-
ate ligands in which a main group element, for example, Ga,
In, Al, C, Si, replaces B. A large variety of tridentate, neutral,
or anionic ligands, where the tethered coordinating site or
the atom bridging the two pyrazolyl moieties were changed,
have been synthesized.1,2 Typical examples of tridentate
(3) Higgs, T. C.; Carrano, C. J. Inorg. Chem. 1997, 36, 298–306.
(4) 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.
(5) Otero, A.; Fernandez-Baeza, J.; Tejeda, J.; Antinolo, A.; Carrillo-
Hermosilla, F.; ez-Barra, E.; Lara-Sanchez, A.; Fernandez-Lopez, M.;
Lanfranchi, M.; Pellinghelli, M. A. J. Chem. Soc.,-Dalton Trans. 1999,
3537–39.
(6) Otero, A.; Fernandez-Baeza, J.; Antinolo, A.; Carrillo-Hermosilla, F.;
Tejeda, J.; Lara-Sanchez, A.; Sanchez-Barba, L.; Fernandez-Lopez, M.;
Rodriguez, A. M.; Lopez-Solera, I. Inorg. Chem. 2002, 41, 5193–5202.
(7) Milione, S.; Capacchione, C.; Cuomo, C.; Strianese, M.; Bertolasi, V.;
Grassi, A. Inorg. Chem. 2009, 48, 9510–9518.
(8) Cuomo, C.; Milione, S.; Grassi, A. Macromol. Rapid Commun. 2006,
27, 611–618.
*To whom correspondence should be addressed. E-mail: smilione@unisa.
it.
(1) Trofimenko, S. Scorpionates: The coordination Chemistry of Polypyr-
azolylborate Ligands; Imperial College Press: London, 1999.
(2) Pettinari, C. Scorpionates: II chelating borate ligands; Imperial College
Press: London, 2008.
r
pubs.acs.org/IC
Published on Web 01/07/2011
2011 American Chemical Society

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 901
geometry in which the ligand is N,N-κ2-coordinated to the
metal and the hydroxyl phenyl group is not coordinated but
is involved in extended intermolecular hydrogen bonding.
An unexpected exchange reaction promoted by bis-pyrazolyl-
methane was observed revealing the influence of the non-
coordinating active sites on the stability/reactivity of these
complexes.7
In this paper we also explored, as test case, the potential of
the cobalt (L1-H)CoCl2 (1) and zinc (L1-H)ZnCl2 (2) com-
plexes as H2S molecular recognition elements. According to
the HSAB (Hard and Soft Acids and Bases) theory, both
cobalt and zinc ions are “borderline” soft Lewis acids20 and
therefore good candidates for binding soft Lewis bases like
H2S.
In this study we extended our investigation to the middle
and late transition metals, cobalt and copper. These metals
have been widely used to obtain metal-substituted proteins as
spectroscopic surrogates of Zn(II) analogues.15 Comparisons
of Zn, Co, and Cu complexes featuring biologically relevant
“N2O” coordination sphere are of great interest. The relative
ability of surrogate metal ions to act as functional replace-
ments is still obscure as in the case of metal-substituted
enzymes in which the activity can be retained or lost at
variance with the metal center. As explanatory example the
substitution of Cu(II) or Co(II) for Zn in the metallopro-
teases astacin and serralysin gives active enzymes whereas
that with Ni(II) or Hg(II) does not.16
The coordination ability of bis(pyrazol-1-yl)-2-phenyl-
methane ligands to Co(II), Cu(II), and Zn(II) ions has
already been explored to some extent.17-19 In these studies
the heteroscorpionate ligands were mainly employed as anionic
ligands to coordinate the metal center through two pyrazole
nitrogens and the oxygen of the phenolate group, resulting in
a facial coordination of the mixed-donor set. In most of these
complexes, Zn(II) is reliably tetrahedral while Co(II) and
Cu(II) show a much stronger tendency to expand their co-
ordination sphere and to adopt an octahedral geometry. Mono-
nuclear “sandwich” complexes or homometallic di- and
trinuclear species were obtained depending on the degree of
substitution on the pyrazole arms and on the experimental
conditions.19 The coordination behavior and the reactivity of
the corresponding neutral ligands is not common and easily
predictable: the OH phenyl group could be either involved or
not in the coordination to the metal center. Furthermore, it
could undergo chemical modifications (e.g., deprotonation
or redox reactions). We also aimed at shedding light on the pos-
sible role of the substituents on the reactivity of the pendant
phenol group. For these reasons three differently phenol sub-
stituted bis-pyrazolyl-methane ligands (2-phenol-bis(3,5-Me2-
pyrazol-1-yl)methane (L2-H), (3-tBu-2-phenol)bis(3,5-Me2-
pyrazol-1-yl)methane (L4-H), and (5-tBu-2-phenol)bis(3,
5-Me2-pyrazol-1-yl)methane (L5-H)) were included in this work.
Experimental Section
Reagents and Materials. All syntheses were carried out using
reaction grade chemicals as received, unless otherwise noted.
(3,5-tBu2-2-phenol)bis(3,5-Me2-pyrazol-1-yl)methane (L1-H),21
2-phenol-bis(3,5-Me2-pyrazol-1-yl)methane17 (L2-H), and (L1-H)-
7
ZnCl2 (2) were prepared as previously reported. Elemental
analyses were performed with a PERKIN-Elmer 240-C. Mass
spectrometry analyses were carried out using a Micromass
Quattro micro API triple quadrupole mass spectrometer equip-
ped with an electrospray ion source (Waters, Milford, MA).
Room temperature 1H NMR spectra were recorded on a Bruker
AVANCE 400 NMR instrument. All spectra were recorded in
5 mm o.d. NMR tubes. The chemical shifts were reported in
δ (ppm) referenced to SiMe4 using the residual proton impurities
of the deuterated solvent. Typically, 6 mg of the complexes in
0.6 mL of CD2Cl2 were used for each experiment. An inver-
sion-recovery pulse sequence was used to measure longitudinal
relaxation times. The T1 values were obtained from a two-
parameter fit of the data to an exponential recovery function.
Synthesis of (3-tBu-2-phenol)bis(3,5-Me2-pyrazol-1-yl)methane
(L4-H). Bis(3,5-Me2-pyrazol-1-yl)ketone17 (1.00 g, 4.58 mmol),
3-tBu-salicylaldehyde (0.82 g, 4.58 mmol), and CoCl2 6H20
3
(0.02 g, 0.08 mmol) were placed in a 100 cm3 round-bottomed
flask. The mixture was then heated to 120 °C. During this time
the mixture turned to dark blue and evolution of CO2 was
observed. The mixture was cooled to RT, CH2Cl2 (40 cm3)
was added, and the flask shaken until a pink solution formed.
This solution was extracted two times with H2O (2 ꢀ 40 cm3),
separated and dried over MgSO4 for 16 h. Hexane (60 cm3) was
then added and CH2Cl2 was removed by rotary evaporation,
causing the precipitation of a pale beige solid which was
collected by filtration, washed with hexane (20 cm3), and dried
in vacuo. Yield: 0.79 g, 49%. Anal. Calcd for L4-H C21H28N4O:
C, 71.56; H, 8.01; N, 15.90. Found: C, 72.03; H, 8.56; N, 15.84.
1H NMR (400 MHz, CDCl3, 20 °C): δ 1.41 (s, 9H, 3-tBu-Ph),
2.09 (s, 6H, 5-CH3-Pz), 2.19 (s, 6H, 3-CH3-Pz), 5.85 (s, 2H, Pz-
H), 6.79 (m, 2H, 5-Ph þ 6-Ph), 7.30 (s, 1H, -CH-), 7.32 (dd,
1H, J = 7.1 Hz, J = 2.4 Hz 4-Ph). MS (ESI acetonitrile): m/z
(%) 728.02 (70) [(L4-H)2]Na þ, 375.67 (35) [(L4-H)]Naþ.
Synthesis of (5-tBu-2-phenol)bis(3,5-Me2-pyrazol-1-yl)methane
(L5-H). The procedure was the same used for L4-H. The reagents
and reactant ratios were bis(3,5-Me2-pyrazol-1-yl)ketone (1.00 g,
4.58 mmol), 5-tBu-salicylaldehyde (0.82 g, 4.58 mmol), and CoCl2
(9) Milione, S.; Montefusco, C.; Cuenca, T.; Grassi, A. Chem. Commun.
2003, 1176–1177.
(10) Milione, S.; Bertolasi, V.; Cuenca, T.; Grassi, A. Organometallics
2005, 24, 4915–4925.
(11) Milione, S.; Grisi, F.; Centore, R.; Tuzi, A. Organometallics 2006, 25,
266–274.
(12) Milione, S.; Cuomo, C.; Grassi, A. Top. Catal. 2006, 40, 163–172.
(13) Paolucci, G.; Bortoluzzi, M.; Milione, S.; Grassi, A. Inorg. Chim.
Acta 2009, 362, 4353–4357.
3
6H20 (0.02 g, 0.08 g mmol). Yield: 0.87 g, 53%. Anal. Calcd for L5-
H C21H28N4O: C, 71.56 H, 8.01; N, 15.90. Found: C, 72.06; H,
8.39; N, 15.79. 1H NMR (400 MHz, CDCl3, 20 °C): δ 1.24 (s, 9H,
5-tBu-Ph), 2.14 (s, 6H, 5-CH3-Pz), 2.18 (s, 6H, 3-CH3-Pz), 5.83 (s,
2H, Pz-H), 6.91 (d, 1H, J = 8.6 Hz, 3-Ph), 6.98 (d, 1H, J = 2.2 Hz,
6-Ph), 7.20 (s, 1H, -CH-), 7.27 (dd, 1H, J = 8.8 Hz, J = 2.2 Hz,
4-Ph). MS (ESI acetonitrile): m/z (%) 762.96 (100) [(L5-H)2-
(H2O)]Kþ, 375.74 (10) [(L5-H)]Naþ.
(14) Silvestri, A.; Grisi, F.; Milione, S. J. Polym. Sci., Polym. Chem. 2010,
48, 3632–3639.
(15) Salgado, J.; Kalverda, A. P.; Diederix, R. E. M.; Canters, G. W.;
Moratal, J. M.; Lawler, A. T.; Dennison, C. J. Biol. Inorg. Chem. 1999, 4,
457–467.
(16) Auld, D. S. Proteolytic Enzymes: Aspartic and Metallo Peptidases;
Barrett, A. J., Ed.; Academic Press: San Diego, CA, 1995; Vol. 248, pp 228-242.
(17) Higgs, T. C.; Dean, N. S.; Carrano, C. J. Inorg. Chem. 1998, 37, 1473–
1482.
Synthesis of (L1-H)CoCl2 (1). A solution of L1-H (200 mg,
0.489 mmol) in tetrahydrofuran (THF, 1.5 mL) was added to
a solution of CoCl2 6H2O (116 mg, 0.489 mmol) in THF
(1.5 mL). The mixture was stirred for 1 h at room temperature
(RT). A bright blue crystalline solid was isolated after cooling
3
(18) Higgs, T. C.; Spartalian, K.; O’Connor, C. J.; Matzanke, B. F.;
Carrano, C. J. Inorg. Chem. 1998, 37, 2263–2272.
(19) Higgs, T. C.; Carrano, C. J. Inorg. Chem. 1997, 36, 291–297.
(20) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533–3539.
(21) Hammes, B. S.; Carrano, C. J. Inorg. Chem. 1999, 38, 3562–3568.

902 Inorganic Chemistry, Vol. 50, No. 3, 2011
Strianese et al.
the filtrate solution down to -20 °C. Yield: 145 mg (55%). X-ray
crystals were grown from a THF/CH2Cl2 solution at -20 °C.
Anal. Calcd for 1 THF CH2Cl2 C30H46N4O2CoCl4: C, 51.81;
(ESI acetonitrile): m/z (%) 449.49 (40) [(L4-H)CuCl]þ, 490.61
(75) [(L4)CuCl(H2O)]Naþ.
Synthesis of (L5-H)CuCl2 (9). The same procedure as for (L4-
H)CuCl2 (8) (vide supra) was used with the following amounts
of reagents: L5-H (100 mg, 0.283 mmol), CuCl2 2H2O (48 mg,
3
3
1
H, 6.68; N, 8.06. Found: C, 51.20; H, 6.48; N, 8.54. H NMR
(400 MHz, CD2Cl2, 20 °C): δ -18.97 (s, 6H, 3-CH3-Pz), 0.12 (s,
9H, 5-tBu-Ph), 2.64 (s, 9H, 3-tBu-Ph), 9.57 (s, 1H, 4-Ph), 9.88 (s,
1H, -OH), 21.09 (s, 1H, -CH-), 38.85 (s, 2H, Pz-H), 42.93 (s,
6H, 5-CH3-Pz). MS (ESI acetonitrile): m/z (%) 507.49 (100)
[(L1-H)Co(H2O)]Naþ, 562.52 (15) [(L1-H)Co(H2O)4]Naþ.
Synthesis of (L3)CuCl2 (3) (L3 = 2-tBu-6-(bis(3,5-Me2-pyrazol-
1-yl)methyl)cyclohexa-2,5-diene-1,4-dione). A solution of L1-H
(200 mg, 0.489 mmol) in THF (1.0 mL) was slowly added to a
3
0.283 mmol), THF (0.5 mL). Yield: 90 mg (66%). Anal. Calcd
for 9 C21H28N4OCuCl2: C, 51.80; H, 5.80; N, 11.51. Found: C,
53.06; H, 6.14; N, 10.83. MS (ESI acetonitrile): m/z (%) 449.55
(30) [(L5-H)CuCl]þ, 490.57 (70) [(L5)CuCl(H2O)]Naþ.
H2S-Saturated Solutions. Saturated dichloromethane solu-
tions of H2S were prepared by bubbling H2S gas through 5 mL
of CH2Cl2 for 1 h resulting in an approximate concentration of
0.53 M at 20 °C.22 H2S gas was produced in situ according to
literature procedures.23
solution of CuCl2 6H2O (83 mg, 0.489 mmol) in THF (1.0 mL).
3
The mixture was stirred for 5 h at RT. Pale yellow crystals were
isolated after cooling the filtrate solution down to -20 °C.
Yield: 150 mg (61%). X-ray crystals were grown by slow dif-
fusion of Et2O in a CH3CN solution of 3. Anal. Calcd for 3
C21H26N4O2CuCl2: C, 50.35; H, 5.24; N, 11.19. Found: C,
50.68; H, 5.12; N, 10.98. MS (ESI acetonitrile): m/z (%)
469.86 (100) [(L3)Cu(H2O)]Naþ, 388.91 (30) [(L3)]Naþ.
Absorbance and Fluorescence Measurements. Absorption
spectra were recorded on a Cary-50 Spectrophotometer, using
a 1 cm quartz cuvette (Hellma Benelux bv, Rijswijk, Netherlands)
and a slit-width equivalent to a bandwidth of 5 nm. Binding
constants for 1:1 complexation were obtained by a nonlinear
least-squares fit24 of the absorbance (A) versus the concentra-
tion of the ligand L1-H added, according to the following
equation:
Synthesis of (L2)2Co2Cl2 (4). A suspension of L2-H (200 mg,
0.675 mmol) in THF (5.5 mL) was added to a solution of
CoCl2 6H2O (160 mg, 0.675 mmol) in THF (1.5 mL). The
3
A ¼ A0 þ ½ðAlim - A0Þ=2c0ꢁ½c0 þ cL þ 1=Ka
mixture was stirred overnight at RT. A bright blue crystalline
solid was isolated. Yield: 200 mg (40%). X-ray crystals were grown
2
1=2
- ½ðc0 þ cL þ 1=KaÞ - 4c0cLꢁ
ꢁ
from CH2Cl2 solution at -20 °C. Anal. Calcd for 4 CH2Cl2
3
C35H40Cl4Co2N8O2: C, 48.63; H, 4.67; N, 12.97. Found: C,
48.52; H, 4.73; N, 12.85. MS (ESI acetonitrile): m/z (%) 650.74
(100) [(L2-H)(L2)Co]þ.
where A0 and A are the absorbances of the complex at a selected
wavelength in the absence and presence of the ligand L1-H,
respectively; c0 is the total concentration of the Co2þ ion;
cL is the concentration of L1-H; Alim is the limiting value of
the absorbance in the presence of a large excess of L1-H, and Ka
is the stability constant.
Synthesis of (L2-H)ZnCl2 (5). A solution of L2-H (100 mg,
0.340 mmol) in CH2Cl2 (5 mL) was added to a solution of ZnCl2
(45 mg, 0.340 mmol) in THF (0.5 mL). The mixture was stirred
for 1 h at RT. A colorless crystalline solid was isolated after
cooling the filtrate solution down to -20 °C. Yield: 90 mg
(60%). X-ray crystals were grown from a THF/CH2Cl2 solution
at -20 °C. Anal. Calcd for 5 C17H20Cl2N4OZn: C, 47.19; H,
4.67; N, 12.95. Found: C, 47.11; H, 4.61; N, 12.80. 1H NMR (400
MHz, CD2Cl2, 20 °C): 1H NMR (400 MHz, CD2Cl2, 20 °C): δ
2.48 (s, 6H, 5-CH3-Pz), 2.50 (s, 6H, 3-CH3-Pz), 6.11 (s, 2H, Pz-
H), 6.73 (d, 1H, J = 8.2 Hz, 3-Ph), 6.92 (t, 1H, J = 8.4 Hz, 4-Ph)
7.13 (d, 1H, J = 7.5 Hz, 6-Ph), 7.22 (t, 1H, J = 7.3 Hz, 5-Ph),
7.62 (s, 1H, -CH-). MS (ESI acetonitrile): m/z (%) 395.64
(100) [(L2-H)ZnCl]þ.
Fluorescence spectra were measured on a Cary Eclipse Spec-
trophotometer in a 10 ꢀ 10 mm2 airtight quartz fluorescence
cuvette (Hellma Benelux bv, Rijswijk, Netherlands) with an
emission band-pass of 10 nm and an excitation band-pass of
5 nm. Both absorption and fluorescence measurements were
performed in dichloromethane solutions at RT. The H2S titra-
tion experiments were performed as follows: the cuvette was
filled with sample solutions in dichloromethane. Then microliter
amounts of H2S-saturated dichloromethane solutions (to the
end concentrations specified in the figure captions) were injected
via a gastight syringe at intervals of 1 min between subsequent
additions. The experiment ended when no changes in the fluore-
scence intensities could be detected upon H2S addition.
1H NMR H2S titrations for 1 and 2 were performed at 25 °C in
CD2Cl2. A total of 6 spectra were registered varying the H2S
concentration in the range of 0.150-0.350 M. The complex
concentration was 0.010 M.
Synthesis of (L2-H)CuCl2 H2O (6). A solution of L2-H (50
3
mg, 0.177 mmol) in CH2Cl2 (2.5 mL) was added to a solution of
CuCl2 2H2O (30 mg, 0.177 mmol) in THF (0.5 mL). The mix-
3
ture was stirred for 1 h at RT. A bright green crystalline solid was
isolated after cooling the filtrate solution down to -20 °C.
Yield: 30 mg (40%). X-ray crystals were grown from a THF/
CH2Cl2 solution at -20 °C. Anal. Calcd for. 6 2(H2O) C17H26-
Crystal Structure Determinations. The crystal data of com-
pounds (L1-H)CoCl2 (1), (L3)CuCl2 (3), (L2)2Co2Cl2 (4), (L2-
H)ZnCl2 (5), and (L2-H)CuCl2 (6) were collected at RT using a
Nonius Kappa CCD diffractometer with graphite monochro-
mated Mo-KR radiation. The data sets were integrated with the
Denzo-SMN package25 and corrected for Lorentz, polarization
and absorption effects.26 The structures were solved by direct
methods27 and refined using full-matrix least-squares with all
3
Cl2CuN4O4: C, 42.11; H, 5.41; N, 11.56. Found: C, 42.03; H,
5.48; N, 12.04. MS (ESI acetonitrile): m/z (%) 654.65 (100) [(L2-
H)(L2)Cu], 400.46 (35) [(L2-H)Cu(H2O)]Naþ.
Synthesis of (L1-H)CuCl2 (7). In drybox, a solution of L1-H
(300 mg, 0.734 mmol) in anhydrous and deareated THF (2.0 mL)
was slowly added to a solution of CuCl2 2H2O (125 mg,
3
0.734 mmol) in degassed THF (1.0 mL). The mixture was stirred
for 5 h at RT. A pale green powder was isolated after cooling the
filtrate solution down to -20 °C. Yield: 225 mg (56%). Anal.
Calcd for 7 C25H36N4OCuCl2: C, 55.29; H, 6.68; N, 10.32.
Found: C, 56.16; H, 7.01; N, 9.87. MS (ESI acetonitrile): m/z
(%) 512.70 (20) [(L1)Cu(H2O)]Naþ.
(22) Barnabas, F. A.; Sallin, D.; James, B. R. Can. J. Chem. 1989, 67,
2009–2015.
(23) Mattson B.; Anderson M.; Mattson S. Microscale Gas Chemistry,
(book) 4th ed.; Educational Innovations: Norwalk, CT, 2003.
(24) Bourson, J.; Pouget, J.; Valeur, B. J. Phys. Chem. 1993, 97, 4552–57.
(25) 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, CA, 1997; pp 307-326.
(26) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51(Pt 1), 33–38.
(27) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo,
C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl.
Crystallogr. 1999, 32, 115–19.
Synthesis of (L4-H)CuCl2 (8). A solution of L4-H (100 mg,
0.283 mmol) in CH2Cl2 (0.8 mL) was slowly added to a solution
of CuCl2 2H2O (48 mg, 0.283 mmol) in THF (0.5 mL). The
3
mixture was stirred for 5 h at RT. A pale green powder was
isolated after cooling the filtrate solution down to -20 °C.
Yield: 75 mg (55%). Anal. Calcd for 8 C21H28N4OCuCl2: C,
51.80; H, 5.80; N, 11.51. Found: C, 53.01; H, 6.20; N, 10.97. MS

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 903
Table 1. Crystallographic Data
compound
formula
(L1-H)CoCl2 (1)
C25H36Cl2CoN4O
CH2Cl2 C4H8O
695.44
P21/c
monoclinic
9.0399(4)
20.0369(8)
19.6152(9)
90
92.463(2)
90
3549.6(3)
4
295
1.301
1460
8.16
(L3)CuCl2 (3)
(L2)2Co2Cl2 (4)
(L2-H)ZnCl2 (5)
(L2-H)CuCl2 H2O (6)
3
C21H26Cl2CuN4O2
C34H38Cl2Co2N8O2
CH2Cl2
864.41
R3c
trigonal
19.6685(3)
19.6685(3)
53.1546(6)
90
C17H20Cl2N4OZn
C17H22Cl2CuN4O2
3
3
3
2(H2O)
484.86
P21/n
3
M
space group
crystal system
500.90
Cc
432.64
Cc
monoclinic
9.4281(3)
18.9355(7)
13.3689(5)
90
91.056(2)
90
2386.3(2)
4
295
1.394
1036
11.63
11593
2821
monoclinic
9.1544(8)
14.4953(13)
15.0072(14)
90
103.606(4)
90
1935.5(3)
4
295
1.485
888
15.57
5614
2988
0.0870
2401
monoclinic
8.6189(2)
14.6164(5)
16.9787(6)
90
101.775(1)
90
2093.9(1)
4
295
1.538
1004
15.38
13583
4813
0.0320
3668
˚
a/A
˚
b/A
˚
c/A
R/deg
β/deg
γ/deg
90
120
3
˚
U/A
17807.9(4)
18
295
1.451
7992
11.50
21049
4317
0.0357
3291
Z
T/K
Dc/g cm-3
F(000)
μ(Mo-KR)/cm-1
measured reflections 10325
unique reflections
Rint
obs. reflns [I g 2σ(I)] 3528
6050
0.0463
0.0318
2433
θmin-θmax/deg
hkl ranges
2.72-25.00
5.21-28.00
2.37-27.00
5.59-25.32
3.37-27.55
-10,10;-23,21;-23,23 -12,12;-21,24;-17,17 -25,19;-21,21;-67,67 -10,10;-17,17;-17,17 -11,11;-16,19;-22,22
R(F2) (obs.reflns)
wR(F2) (all reflns)
no. variables
0.0734
0.2279
364
0.0365
0.0948
278
0.0530
0.1637
236
0.0636
0.1717
230
0.0385
0.1018
269
goodness of fit
ΔFmax; ΔFmin /e A
1.024
0.58; -0.45
1.053
0.41;-0.35
1.057
1.18;-0.92
1.024
0.66;-0.43
1.035
0.33;-0.50
-3
˚
˚
Table 2. Selected Geometrical Parameters (A and degrees) for Compounds (1)
and (3)a
non-hydrogen atoms anisotropically and hydrogens included on
calculated positions, riding on their carrier atoms. (L1-H)CoCl2
(1) contains two solvent molecules, CH2Cl2 and THF, which
display some disorder. Furthermore, the methyl groups in a
tButyl group were found disordered and refined isotropically
over two positions with occupation factors of 0.50 each. The
asymmetric unit of (L2)2Co2Cl2 (4) contains one-half of the
dimetallic molecule, both Co1 and Co2 atoms being located on a
2-fold C2 rotation axis, and one-half of the solvent molecule
CH2Cl2 located on a 2-fold axis passing through the Carbon
atom. The asymmetric unit of (L2-H)CuCl2 (6) contains the
square pyramidal complex of Cu(II) with a water molecule
coordinated in apical position and two other non-coordinated
water molecules.
(L1-H)CoCl2 (1)
(L3)CuCl2 (3)
Distances
Co1-Cl1
Co1-Cl2
Co1-N1
Co1-N3
2.227(2) Cu1-Cl1
2.231(2) Cu1-Cl2
2.019(5) Cu1-N2
2.028(4) Co1-N4
2.217(1)
2.188(2)
2.091(4)
1.975(4)
Angles
Cl1-Co1-Cl2
Cl1-Co1-N1
Cl1-Co1-N3
Cl2-Co1-N1
Cl2-Co1-N3
N1-Co1-N3
111.2(1) Cl1-Cu1-Cl2
116.0(1) Cl1-Cu1-N2
116.2(1) Cl1-Cu1-N4
109.9(1) Cl2-Cu1-N2
108.9(1) Cl2-Cu1-N4
93.4(2) N2-Cu1-N4
101.2(1)
136.2(1)
96.6(1)
104.8(1)
137.5(1)
87.4(2)
All calculations were performed using SHELXL-9728 and
PARST29 implemented in the WINGX30 system of programs.
The crystal data are given in Table 1. Selected bond distances
and angles are given in Tables 2, 3, and 4.
Torsion Angles
Crystallographic data (excluding structure factors) have been
deposited at the Cambridge Crystallographic Data Centre and
allocated the deposition numbers CCDC 773833: (L1-H)CoCl2
(1); 777333: (L3)CuCl2 (3); 784827: (L2)2Co2Cl2 (4); 784828:
(L2-H)ZnCl2 (5); 784829: (L2-H)CuCl2(6). These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retriev-
ing.html or on application to CCDC, Union Road, Cambridge,
CB2 1EZ, U.K. [fax: (þ44)1223-336033, e-mail: deposit@ccdc.
cam.ac.uk]
C1-C6-C7-N2
C1-C6-C7-N4
75.3(6) C1-C6-C7-N1
-157.9(5) C1-C6-C7-N3
170.7(4)
47.9(5)
Dihedral Angles
Cl1-Co1-Cl2 ∧
N1-Co1-N3
89.5(1) Cl1-Cu1-Cl2 ∧
N2-Cu1-N4
59.1(1)
N1-N2-C8-C9-C10 ∧ 138.6(2) N1-N2-C8-C9-C10 ∧ 130.0(2)
N3-N4-C13-C14-C15
N3-N4-C13-C14-C15
a (∧) stands for dihedral angle.
Results and Discussion
Direct treatment of (3,5-tBu2-2-phenol)bis(3,5-Me2-pyra-
at -20 °C (Table 1); the ORTEP31 view of complex 1 is shown
in Figure 1. The cobalt(II) metal center is tetrahedrally κ2-N,
N coordinated by two pyrazole rings of L1-H and two
chloride ligands. The dihedral angle between the Cl1-Co-
Cl2 and N1-Co-N2 planes is of 89.5(1)°, a value very close
to the ideal one of 90°. The slight distortion from the ideal
tetrahedral coordination is mainly determined by the
N-Co-N angle of 93.4(2)° constrained by the bite of bis-
chelating ligand. The Co-Cl distances of 2.227(2) and
zol-1-yl)methane (L1-H) with 1 equiv of CoCl2 6H2O at RT
3
afforded the mononuclear tetrahedral complex (L1-H)CoCl2
(1) in good yield (Scheme 1). Single crystal of 1 suitable for
X-ray diffraction were grown from a THF/CH2Cl2 solution
(28) Sheldrich, G. M. SHELXTL-97, Program for the Solution of Crystal
€
€
Structure Refinement; University of Gottingen: Gottingen, Germany, 1997.
(29) Nardelli, M. J. Appl. Crystallogr. 1995, 28, 659.
(30) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–838.

904 Inorganic Chemistry, Vol. 50, No. 3, 2011
Strianese et al.
a
˚
Table 3. Selected Geometrical Parameters (A and degrees) for Compound (4)
Scheme 1
(L2)2Co2Cl2 (4)
Distances
Co1-O1
Co1-N2
Co1-N4
2.094(3)
2.157(3)
2.127(3)
Co2-Cl1
Co2-O1
2.224(1)
1.981(2)
Angles
O1-Co1-N2
O1-Co1-N4
O1-Co1-N40
O1-Co1-O10
N2-Co1-N4
N2-Co1-N20
N4-Co1-N40
85.7(1)
92.1(1)
168.5(1)
76.9(1)
83.7(1)
176.8(1)
99.0(1)
O1-Co2-Cl1
O1-Co2-Cl10
O1-Co2-O10
Cl1-Co2-Cl10
Co1-O1-Co2
111.2(1)
116.6(1)
82.2(1)
115.0(1)
100.4(1)
Torsion Angles
C1-C6-C7-N1
47.5(5)
C1-C6-C7-N3
-80.9(4)
Dihedral Angles
∧
Cl1-Co2-Cl10
90
N1-N2-C8-C9-C10∧
N3-N4-C13-C14-C15
121.9(2)
O1-Co2-O10
a (∧) stands for dihedral angle.
lar to those observed in the zinc complex of the same
heteroscorpionate ligand (L1-H)ZnCl2 (2).7
˚
Table 4. Selected Geometrical Parameters (A and degrees) for Compounds (5)
and (6)a
To our surprise, the treatment of L1-H with 1 equiv of
(L2-H)ZnCl2 (5)
(L2-H)CuCl2 H2O (6)
3
CuCl2 2H2O afforded the complex (L3)CuCl2 (3) (L3 =
3
2-tBu-6-(bis(3,5-Me2-pyrazol-1-yl)methyl)cyclohexa-2,5-diene-
1,4-dione). In this reaction the phenol moiety was trans-
formed into the corresponding para-benzoquinone residue
(Scheme 1, results of an investigation into this reaction are
presented in a subsequent paragraph). The heteroscorpionate
ligand L3 is κ2-coordinated to the tetrahedral metal center
through the imino nitrogen atoms of the two pyrazolyl rings.
The ORTEP view of the complex 3 is shown in Figure 2. The
geometry of the copper atom resembles a tetrahedral ar-
rangement; however, the plane defined by Cu1, Cl1, and Cl2
atoms makes an angle of 121.9(2)° with the plane defined by
Cu1, N2, and N4 atoms. Hence, the geometry at the copper is
intermediate between square planar and tetrahedral and is
better described as distorted tetrahedral because the twist
angle is much closer to the 90° angle expected for a tetra-
hedron as compared to the 180° angle for the square planar
description. Distortion from planarity is unusual for Cu(II)
complexes, but more common for complexes with large steric
hindrance.34-43 Here, the distorted coordination geometry
observed in 3 could be attributed to the steric repulsion
between the coordinated Cl and the methyl groups of the pyra-
zoles. The other structural parameters are in good agreement
with those observed in CuCl2 complexes17 exhibiting dis-
torted pseudotetrahedral geometry.34-43
Distances
Zn1-Cl1
Zn1-Cl2
Zn1-N2
Zn1-N4
2.194(3) Cu1-Cl1
2.269(3) Cu1-Cl2
2.028(8) Cu1-N2
2.026(9) Cu1-N4
Cu1-O1w
2.291(1)
2.316(1)
2.037(2)
2.039(2)
2.244(2)
Angles
Cl1-Zn1-Cl2
Cl1-Zn1-N2
Cl1-Zn1-N4
Cl2-Zn1-N2
Cl2-Zn1-N4
N2-Zn1-N4
113.9(1) Cl1-Cu1-Cl2
119.8(2) Cl1-Cu1-N2
115.6(2) Cl1-Cu1-N4
106.0(2) Cl2-Cu1-N2
105.5(2) Cl2-Cu1-N4
93.4(3) N2-Cu1-N4
Cl1-Cu1-O1w
89.82(3)
91.55(6)
170.88(6)
164.96(7)
90.65(5)
85.67(8)
94.03(7)
95.45(7)
99.39(9)
94.99(9)
Cl2-Cu1-O1w
N2-Cu1-O1w
N4-Cu1-O1w
Torsion Angles
C1-C6-C7-N1
C1-C6-C7-N3
-59.1(11)C1-C6-C7-N1
174.2(8) C1-C6-C7-N3
-57.2(3)
179.6(2)
Dihedral Angles
Cl1-Zn1-Cl2 ∧
N1-Zn1-N3
90.9(2) Cl1-Cu1-Cl2 ∧
N2-Cu1-N4
162.93(5)
N1-N2-C8-C9-C10∧ 136.4(4) N1-N2-C8-C9-C10∧ 114.20(8)
N3-N4-C13-C14-C15
N3-N4-C13-C14-C15
a (∧) stands for dihedral angle.
(34) Riggio, I.; van Albada, G. A.; Ellis, D. D.; Mutikainen, I.; Spek,
A. L.; Turpeinen, U.; Reedijk, J. Polyhedron 2001, 20, 2659–2666.
(35) Schuitema, A. M.; Engelen, M.; Koval, I. A.; Gorter, S.; Driessen,
W. L.; Reedijk, J. Inorg. Chim. Acta 2001, 324, 57–64.
(36) Stibrany, R. T.; Schulz, D. N.; Kacker, S.; Patil, A. O.; Baugh, L. S.;
Rucker, S. P.; Zushma, S.; Berluche, E.; Sissano, J. A. Macromolecules 2003,
36, 8584–8586.
˚
2.231(2) A and Co-N (pyrazole) ones of 2.019(5) and
˚
2.028(4) A are in agreement with the values found in similar
complexes.19,32,33 The geometrical parameters are very simi-
(31) 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.
(32) Santillan, G. A.; Carrano, C. J. Dalton Trans. 2008, 3995–4005.
(33) Li, Q.-Y.; Zhang, W.-H.; Li, H.-X.; Tang, X.-Y.; Lang, J.-P.; Zhang,
Y.; Wang, X.-Y.; Gao, S. Chin. J. Chem. 2006, 24, 1716–1720.
(37) Sbai, F.; Regragui, R.; Essassi, E.; Kenz, A.; Pierrot, M. Acta
Crystallogr.; Sect. C 2003, 59, m334–m336.
(38) Shaw, J. L.; Cardon, T. B.; Lorigan, G. A.; Ziegler, C. J. Eur. J. Inorg.
Chem. 2004, 1073–1080.
(39) van Albada, G. A.; Mutikainen, I.; Turpeinen, U.; Reedijk, J. J. Chem.
Crystallogr. 2007, 37, 489–496.

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 905
Scheme 2
Figure 1. ORTEP view of complex 1 showing the thermal ellipsoids at
30% probability level.
two phenolate oxygens. The ORTEP view of complex 4 is
shown in Figure 3. This species had been already obtained by
a different synthetic route, that is, by treating the phenolate-
L2 derivative with 1 equiv of CoCl2.17 Our derivative belongs
to a different space group (R3c instead of P21/c) and contains
a different solvent molecule (CH2Cl2 instead of CH3CN) per
asymmetric unit.
The nuclearity of the cobalt complexes seems to depend on
the steric hindrance of the ligand in close proximity to the
metal center. Actually, when the methyl groups are replaced
by isopropyl groups, as in the case of 2-phenol-bis(3,5-iPr2-
pyrazol-1-yl)methane reported by Higgs and Carrano,19 the
mononuclear tetrahedral complex is obtained.
Interestingly deprotonation of the ligand was not observed
in the reaction of L2-H with ZnCl2 or CuCl2 2H2O. The mono-
3
nuclear complexes (L2-H)ZnCl2 (5) and (L2-H)CuCl2 H2O
3
(6) bearing the κ2-coordinated L2-H were indeed iso-
lated from the reaction mixture (Scheme 2). Most likely,
the less acidic character of the zinc(II) and copper(II) cations
hampers the elimination of hydrochloric acid during the reac-
tion under the conditions employed. Interestingly, the oxida-
tion of the phenol group to yield the corresponding para-
Figure 2. ORTEP view of complex 3 showing the thermal ellipsoids at
30% probability level.
The reaction of CoCl2 6H2O, ZnCl2, and CuCl2 2H2O
3
3
quinone did not occur in the reaction of L2-H with CuCl2
2H2O.
with the less sterically hindered L2-H ligand led to different
products (Scheme 2). Indeed the treatment of L2-H with 1
3
The ORTEP view of5 is shown inFigure 4. The zinc(II) ion
displays an almost perfect tetrahedral geometry. The struc-
tural parameters are in perfect agreement with those of the
analogous complexes. The molecules form, in the crystal,
equiv of CoCl2 6H2O at RT afforded the homometallic
3
dimer (L2)2Co2Cl2 (4) in which two cobalt atoms exhibit
octahedral and tetrahedral coordination environments, re-
spectively. One cobalt atom is coordinated by two tridentate
heteroscorpionate ligands through four pyrazole nitrogen
atoms and by two phenolate oxygens while the second cobalt
atom is coordinated by two chlorine atoms and by the same
extended chains bound by means of OH(phenolic) Cl
3 3 3
˚
hydrogen bonds [O1 Cl2 = 3.115 A].
3 3 3
The ORTEP view of 6 is shown in Figure 5. The copper ion
is in a square pyramidal environment with two pyrazole
nitrogen atoms and two chlorine atoms on the basal plane
and a water molecule in the apical position. This molecule
together with other two non-coordinated water molecules
and the phenol group form an extended network of
hydrogen bonds in the crystals (Supporting Information,
Figure S1).
(40) Santillan, G. A.; Carrano, C. J. Inorg. Chem. 2007, 46, 1751–1759.
(41) Kozlevcar, B.; Pregelj, T.; Pevec, A.; Kitanovski, N.; Costa, J. S.; van
Albada, G.; Gamez, P.; Reedijk, J. Eur. J. Inorg. Chem. 2008, 4977–4982.
(42) Drabina, P.; Valenta, P.; Jansa, P.; Ruzicka, A.; Hanusek, J.; Sedlak,
M. Polyhedron 2008, 27, 268–274.
(43) Fujisawa, K.; Kanda, R.; Miyashita, Y.; Okamoto, K. Polyhedron
2008, 27, 1432–1446.

906 Inorganic Chemistry, Vol. 50, No. 3, 2011
Strianese et al.
Figure 5. ORTEP view of complex 6 showing the thermal ellipsoids at
30% probability level.
Figure 3. ORTEP view of complex 4 showing the thermal ellipsoids at
30% probability level.
Figure 4. ORTEP view of complex 5 showing the thermal ellipsoids at
30% probability level.
Figure 6. 1H NMR spectrum (2 mM, CD2Cl2, 293 K) of 1. See text for
further discussion and assignments. The signal labeled with one star is due
to the protio impurities of the deuterated solvent; the signals labeled with
two stars are due to THF molecules clathrated in the crystalline com-
pound.
NMR Spectroscopy of 1-6. Despite the paramagnet-
ism of the metal center, the 1H NMR spectra of cobalt(II)
complexes exhibit relatively sharp but paramagnetically
shifted resonances because of the favorable electronic
1
spin relaxation times (τs ≈ 10-11 s).44 The H NMR of
probably as a result of different spin polarization effects
in the pyrazolyl rings.45 The methyl resonances of pyr-
azolyl rings have been distinguished by their different
longitudinal relaxation times, as the 3-methyl groups are
closer to the cobalt(II) ion and, hence, have T1 values
shorter than those of the 5-methyl protons. The signal of
the phenol proton has been unambiguously identified at
9.88 ppm upon addition of D2O which yields to the dis-
appearance of this signal in the spectrum owing to a
proton-deuterium exchange. The resonance of the H9
of the phenol group was not observed in the NMR
spectrum, probably because of the extreme broadening
of the signal due to the small distance between this
(L1-H)CoCl2 (1) exhibits well resolved signals in the
1
range -20 to 45 ppm. The assignments of the H NMR
signals in Figure 6 have been made on the basis of the
paramagnetic shift resulting from the proximity of the
proton nuclei to the metal center, the integral ratios and
T1 values. The pattern of resonances is consistent with the
presence of a plane of symmetry rendering equivalent the
two pyrazole rings of the ligand. The outermost shifted
signal at 42.53 ppm was assigned to the 5-methyl groups
of the pyrazolyl rings whereas the 3-methyl groups of the
same rings were found downfield shifted at -18.80 ppm,
(44) La Mar, G. N.; Horrocks, W.DeW.; Holm, R. H. NMR of Para-
magnetic Molecules; Principles and Applications; Academic Press: New York,
1973.
(45) Bertini, I.; Luchinat, C. Coord. Chem. Rev. 1996, 150, 29–75.

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 907
atom and the paramagnetic cobalt center (rH-Co = 3.20 A,
based on the X-ray structure).
Assuming a predominant dipolar relaxation mecha-
investigation by Wagner and co-workers.50 These authors
reported the efficient oxidative demethylation of para-
dimethoxyphenyl-substituted bis(pyrazol-1-yl)methane
ligands catalyzed by [Ce(NH4)2(NO3)6].
˚
nism, the Co H contact distance r should be propor-
3 3 3
tional to T11/6. Dipolar relaxation is defined by
To shed light on the factors governing the oxidation of
L1-H, investigations into the formation of the para-
quinone ligand to L3 were carried out. At first, we devel-
oped a procedure that let us to easily identify the product
outcome. The copper(II) complexes are paramagnetic
and no resonance can be observed in the corresponding
1H NMR spectrum. We thought to add an equivalent of
N,N,N0,N0,N00-pentamethyl-diethylenetriamine (PMDETA)
to an appropriate deuterated solution of the heterosco-
pionate copper complex for allowing the following sub-
stitution reaction:
T1 ¼ a r6
ð1Þ
3
where r is the distance to the paramagnetic center, and a is
a constant, which is equal for all the protons in the mole-
cule. Equation 1 leads to a linear dependence between T1
and r in a bilog plot, with the slope equal to 6. The T1
dependence on the crystallographic distances for 1 is
fairly accurate (see Supporting Information, Figure S2)
and suggests that the solid state structure is retained in
solution.
1
The H NMR of (L2)2Co2Cl2 (4) was previously re-
ðLigÞCuCl2 þ PMDETA f ðPMDETAÞCuCl2 þ Lig
ported by Higgs and Carrano.17 They evidenced a partial
dissociation of [(L2)2Co2Cl2] MeCN in dichloromethane
In the NMR spectrum of this mixture, since (PMDETA)
CuCl2 is a paramagnetic species, only the free ligand is
detectable. To test this procedure, we prepared a THF-d8
solution of L3CuCl2 and we added 1 equiv of PMDETA
3
solution at 25 °C producing the monomeric sandwich
species (L2)2Co and solvated CoCl2. While the 1H NMR
spectrum of the monomeric compound is completely
within the -60 to þ60 ppm range, the 1H NMR spectrum
of the dimer is spread out over the range -120 to þ120 ppm.
1
into the NMR tube. In the corresponding H NMR
spectrum the pattern of resonances of L3 was clearly
observed. The aromatic protons on the para-quinone
fragment give rise to a multiplet at 5.75 ppm and
a doublet at 6.56 ppm. The resonance of the methine
proton appears at 7.48 ppm. In the 13C NMR spec-
trum the presence of the benzoquinone moiety is
accounted by two low-field signals at 187.2 ppm and
185.6 ppm.
1
The H NMR spectra of the diamagnetic zinc com-
plexes 2 and 5 are consistent with a high symmetric struc-
ture resulting from the κ2-N,N coordination of the ligands
to the metal center. The two pyrazolyl rings appeared
equivalent showing that the structure is not rigid and the
reorientation of the phenol ring is rapid on NMR time
scale. The resonances of the pyrazol rings are generally
downshifted when compared to those of the correspond-
ing free ligands; for complex 2 the variation of chemical
shifts (Δδ) is in the range 0.25-0.41 ppm. This is con-
sistent with a decrease in electron density due to coordi-
nation at the metal center. Also in these cases the solid
state structures are retained in solution.
To determine whether the source of the oxygen atom in
our oxidation reaction is dioxygen or water, we carried
out the reaction between L1-H and CuCl2 2H2O under
3
nitrogen atmosphere in deaerated solvent. The product
reaction was isolated and identified as (L1-H)CuCl2 (7)
1
by means of elemental analysis, ESI-MS and H NMR
For mononuclear copper(II) complexes, 1H NMR
signals are generally not observed owing to the relatively
long electronic relaxation time (τs ≈ 10-9 s) for Cu2þ ion44
which leads to line broadening. As matter of fact the 1H
NMR spectra of (L3)CuCl2 (3) and of (L2-H)CuCl2 H2O
(6) show featureless broad resonances.
Investigation into the Formation of the para-Quinone
Ligand L3. The aerobic oxidation of alkyl-phenols cata-
lyzed by copper(II) salts, in the presence of nitrogen based
compounds, is well established in the scientific litera-
ture.46,47 Sun and co-workers reported the oxidation of
2,3,6-trimethylphenol to 2,3,6-trimethyl-1,4-benzoquin-
one48 and proposed that this reaction is catalyzed by an
oxotetracuprate cluster isolated from the reaction solu-
tion. Differently, Karlin and co-workers characterized an
end-on bound superoxo copper(II) complex and demon-
strated the catalytic properties of this species in the
oxidation of 2,4,6-tri-tert-butylphenol to 2,6-di-tert-butyl-
1,4-benzoquinone.49 Very recently para-quinone-containing
bis(pyrazol-1-yl)methane ligands were the subject of
(Supporting Information, Figure S3) spectroscopy. Sub-
sequently a known amount of 7 was dissolved in THF-d8
([10 mM]) and the corresponding solution was bubbled
with dry oxygen for 2 h ([1 mM]51). The corresponding 1H
NMR spectrum of this solution treated with 1 equiv of
PMDETA clearly revealed the presence of L3. However,
in solid state, 7 is quite stable: the exposure of the solid to
air (two days) does not cause any morphologic change,
the subsequent treatment of a solution of this sample with
1 equiv of PMDETA in CDCl3 showed no evidence for
the oxidation of L1-H to L3.
3
The kinetics of the oxidation reaction leading to 3 was
investigated. To this end air was admitted to a mixture of
L1-H and CuCl2 2H2O in THF-d8 (30 mM). The reaction
3
was tracked by analyzing the product mixtures sampled
from the reactor at certain reaction times. Each aliquot of
this solution was treated with PMDETA and analyzed by
1H NMR spectroscopy (Supporting Information, Figure
S4). After an induction time of 30 min, in which hydro-
lysis of about 15 mol % of L1-H to 3,5-tBu2-salicylalde-
hyde was observed, the oxidation of L1-H to L3 started.
After 2 h, L1-H was no longer present, and L3 was the
(46) Punniyamurthy, T.; Rout, L. Coord. Chem. Rev. 2008, 252, 134–154.
(47) Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047–1076.
(48) Sun, H. J.; Harms, K.; Sundermeyer, J. J. Am. Chem. Soc. 2004, 126,
9550–9551.
(49) Maiti, D.; Fry, H. C.; Woertink, J. S.; Vance, M. A.; Solomon, E. I.;
Karlin, K. D. J. Am. Chem. Soc. 2007, 129, 264–265.
(50) Blasberg, F.; Bats, J. W.; Bolta, M.; Lerner, H. W.; Wagner, M.
Inorg. Chem. 2010, 49, 7435–7445.
(51) Battino, R.; Rettich, T. R.; Tominaga, T. J. Phys. Chem. Ref. Data
1983, 12, 163–178.

908 Inorganic Chemistry, Vol. 50, No. 3, 2011
Strianese et al.
principal product (70%). The composition of the mixture
do not change further with time.52
Finally we explored the role of the tert-butyl substitu-
ents of the phenol ring in the ligand oxidation. Two new
ligands featuring only one tert-butyl group in ortho or
para position, that is, (3-tBu-2-phenol)bis(3,5-Me2-pyrazol-
1-yl)methane (L4-H) and (5-tBu-2-phenol)bis(3,5-Me2-
pyrazol-1-yl)methane (L5-H) were properly synthesized
and allowed to react with CuCl2 2H2O under the same
3
experimental conditions employed for 3. The reaction pro-
ducts (L4-H)CuCl2 (8) and (L5-H)CuCl2 (9) were isolated
and characterized by elemental analysis and electrospray
ionization mass spectroscopy (ESI-MS). In both cases no
pieces of evidence of ligand oxidation were found. To
corroborate these results, a known amount of both com-
plexes was dissolved in CDCl3 and treated with 1 equiv of
PMDETA. In both cases the starting ligands L4-H and
L5-H were observed in the 1H NMR spectrum. We thus
concluded that both tert-butyl groups in ortho and para
positions of phenol group are required to ensure the
oxidation of L1-H to L3.
Optical Properties and H2S Detection by 1 and 2. H2S is
a colorless, flammable gas with a characteristic odor of
rotten eggs and is considered one of the most dangerous
environmental toxic and crude metabolic poisons.53,54 It
is more toxic than hydrogen cyanide and exposure to as
little as 300 ppm in air for just 30 min can be fatal in
humans.55 In 2008 experimental evidence indicated that
like carbon monoxide and nitric oxide, H2S is an impor-
tant signaling molecule in biology, and it may find a role
in medicine.56 Many natural sources produce H2S, in-
cluding volcanic gases, natural gas and coal reserves,
sulfur springs, putrefying vegetable and animal matter.
Several industrial processes also generate H2S as a by-
product. Easy and cheap monitoring of hydrogen sulfide
is especially challenging. Cobalt and zinc ions are “bor-
derline” soft Lewis acids20 and therefore they should
display a good affinity for H2S. This prompted us to
investigate the use of 1 and of the zinc analogue 2 in H2S
monitoring.
Figure 7. Absorption titration of CoCl2 6H2O with L1-H (RT, THF).
3
[CoCl2 6H2O] = 0.6 mM.
3
plot exhibited the inflection point at 0.5, which clearly
indicates the formation of 1 in solution (see Supporting
Information, Figure S5). An association constant Ka of
125 ( 36 ꢀ 103 M-1 was obtained by a nonlinear least-
squares fit24 (see Supporting Information, Figure S6).
Speciation of L1-H/Zn2þ system had been previously
evaluated by means of NMR spectroscopy.7 The associa-
tion constant was determined by fitting the plot of the
variation of the chemical shift (Δδ) observed for the 2-tBu
resonance versus [Zn2þ]/[L1-H]. The Ka value was 7.8 (
0.3 ꢀ 102 M-1.7 The lower Ka value found for Zn(II) can
be ascribed to the lower acidic character of this cation
compared to Co(II).
Both the complexes 1 and 2 are fluorescent and exhibit
an emission band in the near UV region (around 320 nm).
Interestingly the coordination to Co2þ and Zn2þ induces
a fast enhancement of L1-H fluorescence in dichloro-
methane solution (switching ratio SR = 85% (1), 90%
(2); Supporting Information, Figures S7 and S8), while a
fast quenching was observed in THF solution (SR = 56%
(1), 66% (2); Supporting Information, Figures S9 and
S10). Thus the fluorescence of the L1-H ligand is turned-
on by coordination to Zn2þ or Co2þ in dichloromethane,
and turned-off in THF. This solvent dependent fluores-
cence intensity of L1-H was previously interpreted in the
light of the different tendency to establish intra- or
intermolecular hydrogen bonding (HB) interactions. In-
deed, in THF solution the fluorescence intensity of L1-H
resulted 30% higher than that registered in dichloro-
methane (Supporting Information, Figure S11).7 Most
likely, this finding could be the possible explanation for
the different fluorescence trend observed for 1 and 2 in
dichloromethane solutions and in THF solutions.
Then H2S detection by 1 and 2 was investigated by
means of fluorescence spectroscopy. Intriguingly, the
bubbling of H2S gas through a dichloromethane solution
of 1 induces a fast and sizable enhancement of the fluo-
rescence intensity, while it quenches the one of 2, under
the same experimental conditions (Figures 8 and 9). In
other words, when H2S is added to the solution the fluo-
rescence of 1 “turns on” and that one of 2 “turns off”. In
dichloromethane (λexc = 290 nm) a fluorescence switch-
ing of 70% and 95% was found for 1 and 2, respectively.
This finding was corroborated adding increasing amounts
of H2S. In the case of 1 monitoring the fluorescence inten-
sities a progressive enhancement was observed (Figure 8).
Differently a progressive quenching was registered in the
case of 2 (Figure 9). In both cases when plotting the fluo-
rescence intensity values against the H2S concentrations,
We first quoted the binding affinities of L1-H for Co(II)
and Zn(II). Speciation of L1-H to cobalt(II) was deter-
mined by means of UV-vis spectroscopy. The absorption
spectrum of a colorless THF solution of CoCl2 6 H2O
3
was monitored while adding increasing amounts of L1-H
dissolved in THF (Figure 7). During the addition, the
absorption peaks at 670 and 615 nm gradually increased
together with a new band at 550 nm. The intensity of these
peaks increased almost linearly until the [L1-H]/[Co2þ
]
molar ratio reached the value of one, corresponding to the
formation of 1. Meanwhile, additional absorption bands
at 635 and 570 nm decreased to a limit value and were
found slightly red-shifted. The presence of well-defined
isosbestic points at 568, 591, 626, 643 nm indicated that
only two species coexisted in the equilibrium. The Job’s
(52) Attemps to identify the other products of the reaction met with
failure.
(53) Smith, R. P. Am. Sci. 2010, 98, 6–9.
(54) Smith, R. P. Can. Med. Assoc. J. 1978, 118, 775–776.
(55) Li, L.; Moore, P. K. Biochem. Soc. Trans. 2007, 35, 1138–1141.
(56) Yang, G.; Wu, L.; Jiang, B.; Yang, W.; Qi, J.; Cao, K.; Meng, Q.;
Mustafa, A. K.; Mu, W.; Zhang, S.; Snyder, S. H.; Wang, R. Science 2008,
322, 587–590.

Article
Inorganic Chemistry, Vol. 50, No. 3, 2011 909
tool of recognition in host-guest and solvent-solute
systems.57,58
When H2S was added to a CD2Cl2 solution of 2 the
progressive hydrolysis of the coordinated ligand was
observed. This was evidenced by the progressive increase
of the signal at 11.37 ppm due to the aldehydic proton of
3,5-di-tert-butyl-salicylaldehyde (Supporting Informa-
tion, Figure S15). Most likely the progressive quenching
of the fluorescence observed for 2 upon addition of H2S is
due to the fact that when the starting ligand hydrolyzes it
turns into species with lower fluorescence intensity. In
view of these results, both 1 and 2 act as H2S dosimeters,
that is, irreversible devices, which progressively accumu-
late the dose, each time adding up the signal.59,60
Figure 8. Emission spectra of 1 after addition of H2S in the range of
0-151 mM (RT, CH2Cl2). [1] = 0.3 mM.
All the H2S dosimeters reported in the literature are
substantially different than ours since they do not rely on
coordination complexes. One of the first most efficient
examples of H2S dosimeters was reported at the end of the
1990s.61 The method is based on trapping the H2S in
a solution with a chromophore. H2S quantification is
then made by monitoring the product.61 A more sensitive
variant of this type of methodology is the colori-
metric tube.62 Air is sucked through tubes which have been
packed with silica gel impregnated with a silver-gelatin
complex. In the present case the H2S is detected by
monitoring the formation of Ag2S.62 Chemically treated
papers have been also widely used as H2S dosimeters.63
When these papers are exposed to hydrogen sulfide gas, a
surface reaction on paper causes accumulation of the
analyte, depending on the time of exposure and the con-
centration of hydrogen sulfide.63
Figure 9. Emission spectra of 2 after addition of H2S in the range of
3.5-88 mM (RT, CH2Cl2). [2] = 0.3 mM.
a trend could be found (Supporting Information, Figures
S12 and S13).
When bubbling Ar through the solutions of 1 and 2 the
initial fluorescence intensity values could not be restored.
We tried to assess the reasons why the fluorescence
intensity of 1 and 2 changes upon H2S addition. First we
checked if 1 or 2 can coordinate H2S, that is, if there is a
minimum in the energy surface corresponding to the
approach of the H2S to the metal center. We performed
a scan of the potential energy surface of the system
composed of (L1-H)MCl2 þ H2S for both cobalt and
zinc. Constrained geometry optimizations at the BP86
level were performed by keeping fixed the M-H2S dis-
Conclusion
In the present paper we reported on the synthesis and
characterization of middle (CoII) as well as late transition
metal (ZnII, CuII) complexes bearing phenol-substituted bis-
(pyrazolyl)methane ligands. At variance of the type of sub-
stituents in ortho and para position of the phenol moiety
different coordination modes and reactivity were found. For
the cobalt complexes, a mononuclear tetrahedral complex 1
or a dinuclear octahedral/tetrahedral complex 4 was obtained.
In these complexes the heteroscorpionate ligand resulted κ2-
or κ3-coordinated to the metal center, respectively. More
interestingly, when reacting L1-H with copper, strong evi-
dence indicated the aerobic oxidation of L1-H to the para-
quinone derivative L3 in which one tert-butyl group was
replaced by an oxygen atom. In an effort to gain a better
picture on the mechanism of the above oxidation reaction we
repeated the reaction under an inert gas atmosphere in deaerated
solvents, and we investigated ligands with different substitu-
tion patterns (L1-H, L4-H, L5-H). These experiments led to
˚
tance at selected values in the range of 2.0-4.0 A and
relaxing all the other geometrical parameters. No mini-
mum was observed in the corresponding curves (Support-
ing Information, Figure S14). This indicates that hetero-
scorpionate H2S-complexes of both cobalt and zinc are
not prone to form. The same conclusion was obtained by
1H NMR spectroscopy when adding increasing amounts
of H2S to the solutions of 1 and 2. In the case of 1, the
addition of increasing amounts of H2S caused a slight
shift of the proton NMR pattern. No resonances attri-
butable to H2S coordination were observed. When a large
amount of H2S was added to the NMR tube, a dark
product precipitated and the pattern of the initial ligand
begun to appear.
(57) Kamlet, M. J.; Abboud, J. L.; Abraham, M. H.; Taft, R. W. J. Org.
Chem. 1983, 48, 2877–2887.
(58) de Silva, A. P.; Gunaratne, H. Q.; Gunnlaugsson, T.; Huxley, A. J.;
McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515–
1566.
(59) Boiocchi, M.; Fabbrizzi, L.; Licchelli, M.; Sacchi, D.; Vazquez, M.;
Zampa, C. Chem. Commun. 2003, 1812–1813.
(60) Duong, T. Q.; Kim, J. S. Chem. Rev. 2010, 110, 6280–6301.
(61) Balasubramanian, N.; Kumar, B. S. M. Analyst 1990, 115, 859–863.
(62) Pal, T.; Ganguly, A.; Malty, D, S. Analyst 1986, 111, 691–693.
(63) Hayes, E. T.; Ataman, O. Y.; Karagozler, A. E.; Zhang, Y. L.;
Hautman, D. P.; Emerich, R. T.; Ataman, A. G.; Zimmer, H.; Mark, H. B.,
Jr. Microchem. J. 1990, 41, 98–105.
As observed above the fluorescence intensity of 1 is
affected by the solvent, so the variation of fluorescence
observed for 1 upon addition of H2S could be ascribed to
the change of the environment, that is, the non-covalent
interactions that are necessarily established between the
solute and the solvent. Indeed occurrence of non-covalent
interactions as hydrogen-bonding is a well established

910 Inorganic Chemistry, Vol. 50, No. 3, 2011
Strianese et al.
the conclusion that the L3 formation requires O2 as oxidant
and the presence of both tert-butyl substituents of L1-H.
The potential application of cobalt 1 and zinc 2 complexes
as molecular recognition elements for H2S was also explored.
Addition of H2S induced a modification of the fluorescence
intensity in 1 or 2. Interestingly, the presence of H2S en-
hanced the fluorescence of 1 whereas it quenched that of 2. In
case of 1, the variation of fluorescence intensity could be
ascribed to the change of the environment, whereas, in case of
2, it was originated from the hydrolysis reaction occurring
upon addition of H2S. The irreversibility of the process led us
to the definition of 1 and 2 as dosimeters for H2S, by com-
parison with literature nomenclature.59,60 Design of dosi-
meters has recently emerged as an active research area of
significant importance. Although the systems presented here
have limited applications, our findings might be significant as
a starting point for possible developments in the fast growing
field of chemo-sensing and more specifically of the H2Sdetection.
Acknowledgment. The authors thank Prof. Maurizio
Licchelli of the Department of Chemistry (University of
Pavia) for critically discussing the manuscript and Dr.
Patrizia Iannece and Dr. Patrizia Oliva of the Department
of Chemistry (University of Salerno) for their technical
assistance.
Supporting Information Available: Figures S1-S15 (1H NMR,
absorption, fluorescence spectra and computational details) and
X-ray crystallographic files (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.