Structure and physical properties of several pseudotetrahedral zinc complexes containing a new alkyl thiolate heteroscorpionate ligand

Article abstract of DOI:10.1039/b004037l

A series of pseudotetrahedral and octahedral zinc(n) complexes of the heteroscorpionate ligand bis(3, 5-dimethylpyrazolyl)(l-methyl-l-sulfanylethyl)methane (L3SH) have been synthesized and most characterized by X-ray crystallography. Pseudotetrahedral complexes isolated include [Zn(L3S)(CH3)]( [Zn(L3S)(Cl)], [Zn(L3S)(OAc)], [Zn(L3S)(SPhF5)], [Zn(L3S)(SBz>] (Bz = CH2Ph), [Zn(L3S)(OPh''-N00]) and [Zn(L3S)2]. In addition, the octahedral complex [Zn2{(L3S)2Zn} {O2P(OPh)2}, J was also synthesized. Comparisons between these complexes and those of the corresponding tris(pyrazolyl)borate, Tp~, or aromatic phenol or benzenethiolate ligands (L1O)~ and (L2S)~ reveal significant differences. Among these is a change in binding of acetate from pure unidentate to anisobidentate as the ligand donor is changed from S to N to 0. Trends such as these may aid in understanding how the donor set affects the structure and reactivity at pseudotetrahedral zinc centers in metalloproteins. The .Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b004037l

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Structure and physical properties of several pseudotetrahedral zinc
complexes containing a new alkyl thiolate heteroscorpionate ligand
Brian S. Hammes and Carl J. Carrano*
The Department of Chemistry, Southwest Texas State University, San Marcos, TX 78666,
USA
Received 19th May 2000, Accepted 2nd August 2000
First published as an Advance Article on the web 13th September 2000
A series of pseudotetrahedral and octahedral zinc() complexes of the heteroscorpionate ligand bis(3,5-dimethyl-
pyrazolyl)(1-methyl-1-sulfanylethyl)methane (L3SH) have been synthesized and most characterized by X-ray
crystallography. Pseudotetrahedral complexes isolated include [Zn(L3S)(CH3)], [Zn(L3S)(Cl)], [Zn(L3S)(OAc)],
[Zn(L3S)(SPhF5)], [Zn(L3S)(SBz)] (Bz = CH2Ph), [Zn(L3S)(OPhp-NO )], and [Zn(L3S)2]. In addition, the octahedral
2
complex [Zn2{(L3S)2Zn}{O2P(OPh)2}4] was also synthesized. Comparisons between these complexes and those
of the corresponding tris(pyrazolyl)borate, TpϪ, or aromatic phenol or benzenethiolate ligands (L1O)Ϫ and (L2S)Ϫ
reveal significant differences. Among these is a change in binding of acetate from pure unidentate to anisobidentate
as the ligand donor is changed from S to N to O. Trends such as these may aid in understanding how the donor set
affects the structure and reactivity at pseudotetrahedral zinc centers in metalloproteins.
varying only in the donor set, i.e. N3, N2O, N2S.28–30 Now we
Introduction
continue our investigation of novel heteroscorpionate ligands
by introducing a new alkyl thiol heteroscorpionate ligand
bis(3,5-dimethylpyrazolyl)(1-methyl-1-sulfanylethyl)methane
(L3SH). Reported herein are the synthesis and structural and
physical properties for several pseudotetrahedral zinc() com-
plexes containing the alkyl thiolate heteroscorpionate ligand
(L3S)Ϫ along with comparisons to previous members of this
family.
Zinc is the active metal in the largest group of metalloproteins
found in nature. While the biological functions of zinc in these
proteins varies considerably, their active sites exhibit a common
structural motif.1 Thus, most active sites are comprised of
a pseudotetrahedral zinc center containing a combination
of nitrogen, oxygen, and/or sulfur donors, originating from
solvent water, and/or histidine, tyrosine, aspartic or glutamic
acids, and cysteine residues of the protein itself. It is therefore
particularly important to understand how the [NxOySz] donor
array modulates the chemistry around a pseudotetrahedral zinc
center.
One way to approach this question is to synthesize, struc-
turally characterize, and perform reactivity studies on small
synthetic analogs to these zinc metalloproteins. Thus far, the
most successful metalloprotein models have utilized tris(pyr-
azolyl)borate ligands, TpR, as a template, which provides a
facial array of three nitrogen donors from which a number of
pseudotetrahedral zinc complexes of the form LZnX have
been prepared.2–16 However, the tris(pyrazolyl)borate ligand is
restricted to providing an all nitrogen (N3) donor set around the
zinc ion, inappropriate for many zinc-containing biomolecules.
Recently efforts to synthesize unsymmetrical ligands with
mixed N, O, S donor atoms has been undertaken with some
success. Vahrenkamp and co-workers have reported a number
of tripodal N3O or N3S ligands that incorporate carboxylate,
phenolate, or thiolate moieties,17–20 while Parkin and co-workers
have synthesized several facially coordinating N2O or N2S
heteroscorpionate ligands containing thioethers, thioimid-
azolyl, or carboxylate moieties attached to the B–H group.21–23
In addition, Riordan and Fenton and co-workers have also
explored other tripodal ligands in an effort to achieve the same
goal.24,25 Nevertheless, so far, no single family of easily syn-
thesized complexes containing all the biologically relevant
donor atoms have been reported. The synthesis of zinc()
complexes with thiolate ligands has been especially difficult due
to the propensity of the sulfur atoms to bridge and produce
oligomeric complexes.26,27
Experimental
All syntheses were carried out in air and the reagents and
solvents purchased from commercial sources and used as
received unless otherwise noted. Toluene was distilled under
argon over Na–benzophenone. The ligand precursor, bis(3,5-
dimethylpyrazolyl) ketone, was prepared using previously
reported procedures.31
Syntheses
L3SH 1. 2-(Methyldisulfanyl)isobutyraldehyde (5.8 g, 39
mmol) was combined with bis(3,5-dimethylpyrazolyl) ketone
(8.43 g, 39 mmol) and CoCl2ؒ6H2O (0.13 g, 0.46 mmol) in a 100
cm3 round-bottom flask. The mixture was warmed to 80 ЊC and
stirred for 3 h or until CO2 evolution had ceased. It was then
cooled to room temperature, dissolved in dichloromethane and
washed two times with water and once with a saturated solution
of NaCl. The organic layer was collected and dried over
anhydrous MgSO4. Volatiles were removed under vacuum and
the residue was taken up in pentane. Cooling to Ϫ20 ЊC over-
night gave 7.1 g (56%) of pure disulfide (L3SSMe). The
disulfide intermediate was dissolved in THF and added drop-
We have recently described the synthesis and characterization
of two new heteroscorpionate ligands (L1OH and L2SH)
which, along with the tris(pyrazolyl)borates, form a family of
ligands with the same coordination geometry, charge, etc.
3304
J. Chem. Soc., Dalton Trans., 2000, 3304–3309
This journal is © The Royal Society of Chemistry 2000
DOI: 10.1039/b004037l

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wise to a solution of LiAlH4 (23 ml, 23 mmol) in THF. After
addition was complete the mixture was refluxed for 1 h, after
which the excess of LiAlH4 was destroyed by careful dropwise
addition of water (CAUTION: foaming due to H2 evolution!!).
The reaction mixture was then filtered through Celite, and
washed with THF and water. Most of the THF was removed
under reduced pressure and the resulting oil acidified, extracted
with dichloromethane and dried over anhydrous MgSO4. The
volatiles were removed under reduced pressure to yield 4.6 g
(41% based on 2-(methyldisulfanyl)isobutyraldehyde) of pure
L3SH as a colorless oil. Calc. (found) for C14H22N4Sؒ0.25 H2O:
(s, 6 H, Pz CH3), 2.39 (s, 6 H, Pz CH3) and 1.34 (s, 6 H,
C(CH3)2). 13C NMR (CDCl3): δ 149.96, 140.93, 106.48, 72.90,
47.98, 31.55, 22.62, 14.08, 13.16 and 11.35.
[Zn(L3S)(SBz)] 6. A solution of complex 2 (0.22 g, 0.60
mmol) in 20 mL of CH2Cl2 was treated with a CH2Cl2 solution
of PhCH2SH (0.075 g, 0.60 mmol). The resulting solution was
stirred for 6 h, dried under reduced pressure and crystallized by
layering a CH2Cl2 solution of the [Zn(L3S)(SBz)] with hexane
to yield 0.19 g (67%). Calc. (found) for [Zn(L3S)(SBz)]ؒ
0.25H2O, C21H28.5N4O0.25S2Zn: C, 53.61 (53.38); H, 6.12 (5.85);
1
1
N, 11.91 (11.92)%. H NMR (CDCl3): δ 7.42 (d, 2 H, J 7,
C, 59.80 (59.43); H, 7.72 (8.03); N, 19.51 (19.80)%. H NMR
SCH2C6H5), 7.24 (t, 2 H, J 7, SCH2C6H5), 7.12 (t, 1 H, J 7 Hz,
SCH2C6H5), 5.95 (s, 1 H, CH), 5.93 (s, 2 H, Pz H), 3.88 (s, 2 H,
SCH2Ph), 2.38 (s, 6 H, Pz CH3), 2.36 (s, 6 H, Pz CH3) and 1.38
(s, 6 H, C(CH3)2). 13C NMR (CDCl3): δ 149.92, 145.50, 140.52,
128.35, 128.17, 125.60, 106.21, 72.84, 47.96, 33.84, 30.57, 13.11
and 11.32.
(CDCl3): δ 6.15 (s, 1 H, CH), 5.79 (s, 2 H, Pz H), 2.85 (s, 1 H,
SH), 2.23 (s, 6 H, Pz CH3), 2.12 (s, 6 H, Pz CH3) and 1.69 (s,
6 H, C(CH3)2). 13C NMR (CDCl3): δ 147.71, 140.51, 106.22,
77.92, 49.14, 29.84, 13.77 and 11.53. FTIR (NaCl, cmϪ1): νSH
2549.
[Zn(L3S)(CH3)] 2. A solution of L3SH (0.60 g, 2.2 mmol) in
25 mL of toluene was treated with 1.3 mL of a toluene solution
of Zn(CH3)2 (0.24 g, 2.6 mmol) under argon. The reaction mix-
ture containing the precipitated product was stirred for 1 h and
subsequent work-up was performed in air. The white product
was filtered off and washed with ether to yield 0.64 g (83%)
of [Zn(L3S)(CH3)]. Calc. (found) for [Zn(L3S)(CH3)]ؒ0.28C6H5-
CH3ؒ0.25H2O, C17H26.7N4O0.25SZn: C, 52.54 (52.16); H, 6.94
[Zn(L3S)(OPhp-NO )] 7. A solution of complex 2 (0.18 g, 0.46
2
mmol) in 20 mL of CH2Cl2 was treated with a CH2Cl2 solution
of p-nitrophenol (0.069 g, 0.46 mmol). The resulting solution
was stirred for 24 h and dried under reduced pressure to give
[Zn(L3S)(OPhp-NO )] as a light yellow solid. The product was
2
crystallized by layering a concentrated CH2Cl2 solution of the
complex with hexane to yield 0.19 g (87%). Calc. (found) for
[Zn(L3S)(OPhp-NO )], C20H25N5O3SZn: C, 49.95 (49.73); H, 5.25
2
1
(6.55); N, 14.42 (14.12)%. H NMR (CDCl3): δ 5.95 (s, 1 H,
(5.12); N, 14.56 (14.40)%. 1H NMR (CDCl3): δ 8.03 (d, 2 H, J 7,
OC6H4NO2), 6.75 (d, 2 H, J 7 Hz, OC6H4NO2), 6.03 (s, 1 H,
CH), 6.01 (s, 2 H, Pz H), 2.42 (s, 6 H, Pz CH3), 2.29 (s, 6 H, Pz
CH3) and 1.40 (s, 6 H, C(CH3)2). 13C NMR (CDCl3): δ 149.64,
140.66, 126.54, 118.15, 106.35, 72.62, 48.77, 33.57, 12.98 and
11.31.
CH), 5.90 (s, 2 H, Pz H), 2.37 (s, 6 H, Pz CH3), 2.30 (s, 6 H, Pz
CH3), 1.35 (s, 6 H, C(CH3)2) and Ϫ0.55 (s, 3 H, CH3). 13C NMR
(CDCl3): δ 148.85, 140.00, 105.84, 72.81, 47.05, 33.88, 13.06,
11.22 and Ϫ15.69.
[Zn(L3S)Cl] 3. A solution of complex 2 (0.31 g, 0.86 mmol)
in 30 mL of CH2Cl2 was treated dropwise with 1 equivalent of
conc. HCl (0.031 g, 0.86 mmol as a solution in CH2Cl2). The
resulting reaction mixture was stirred for 1 h and concentrated
under reduced pressure to yield 0.264 g (81%) of the zinc
complex [Zn(L3S)Cl]. Recrystallization was accomplished by
slow evaporation of a 1.5:1 CH2Cl2–hexane solution. Calc.
(found) for [Zn(L3S)Cl]ؒ0.75H2O, C14H22.5ClN4O0.75SZn: C,
42.92 (43.08); H, 5.80 (5.48); N, 14.30 (14.08)%. 1H NMR
(CDCl3): δ 5.99 (s, 1 H, CH), 5.98 (s, 2 H, Pz H), 2.44 (s, 6 H, Pz
CH3), 2.40 (s, 6 H, Pz CH3) and 1.37 (s, 6 H, C(CH3)2). 13C
NMR (CDCl3): δ 149.95, 141.05, 106.31, 72.65, 48.21, 33.66,
12.95 and 11.35.
[Zn(L3S)2] 8. A solution of complex 4 (0.22 g, 0.54 mmol) in
20 mL of CH2Cl2 was treated with a 0.1 M aqueous NaOH
solution (4 mL). The resulting solution was stirred for 20 min
before separation. The organic layer was dried over anhydrous
MgSO4, filtered, dried under reduced pressure and recrystal-
lized from a hexane–CH2Cl2 solution to give 0.14 g, 82% of
[Zn(L3S)2]. (Calc. (found) for [Zn(L3S)2]ؒ0.25 H2O, C28H42.5
-
N8O0.25S2Zn: C, 53.44 (53.69); H, 6.82 (6.75); N, 17.81 (17.64)%.
1H NMR (CDCl3): δ 6.05 (s, 1 H, CH), 5.82 (s, 2 H, Pz H), 2.40
(s, 6 H, Pz CH3), 2.31 (s, 6 H, Pz CH3) and 1.59 (s, 6 H,
C(CH3)2). 13C NMR (CDCl3): δ 148.75, 140.17, 105.58, 74.06,
49.29, 33.51, 13.73 and 11.87.
[Zn(L3S)(OAc)] 4. A solution of complex 2 (0.41 g, 1.1
mmol) in 30 mL of CH2Cl2 was treated with HOAc (0.068 g, 1.1
mmol) as a solution in CH2Cl2. The resulting solution was
stirred for 1 h, concentrated under reduced pressure, and
layered with hexane. Over a 2 day period [Zn(L3S)(OAc)]
crystallized as colorless blocks, which were filtered off and
dried under reduced pressure to yield 0.31 g, 69%. Calc. (found)
for [Zn(L3S)(OAc)], C16H24N4O2SZn: C, 47.82 (47.75); H, 6.03
(6.03); N, 13.94 (13.78)%. FTIR (KBr, cmϪ1): νCO(OAcϪ) 1623,
1375. 1H NMR (CDCl3): δ 5.95 (s, 1 H, CH), 5.94 (s, 2 H, Pz H),
2.38 (s, 6 H, Pz CH3), 2.36 (s, 6 H, Pz CH3), 2.14 (s, 3 H,
OC(O)CH3) and 1.36 (s, 6 H, C(CH3)2). 13C NMR (CDCl3):
δ 179.08, 149.71, 140.70, 106.27, 72.68, 47.90, 33.82, 12.87 and
11.39.
[Zn2{(L3S)2Zn}{O2P(OPh)2}4] 9. A solution of complex 2
(0.20 g, 0.55 mmol) in 20 mL of CH2Cl2 was treated with a
CH2Cl2 solution of diphenyl phosphate (0.14 g, 0.55 mmol).
The resulting solution was stirred for 1 h and dried under
reduced pressure to give a sticky white solid. Crystals suitable
for X-ray diffraction were obtained by slow evaporation of a
1.3:1 CH2Cl2–hexane solution to yield 0.13 g (27%) of the
zinc() complex. Calc. (found) for [Zn2{(L3S)2Zn}{O2P-
(OPh)}4], C76H82N8O16P4S2Zn3: C, 52.22 (52.19); H, 4.74 (4.86);
N, 6.41 (6.48)%. 1H NMR (CDCl3): δ 7.21 (br, 32 H, Ar H), 7.03
(br, 8 H, Ar H), 5.94 (s, 2 H, CH), 5.91 (s, 4 H, Pz H), 2.36 (s,
12 H, Pz CH3), 2.33 (s, 12 H, Pz CH3) and 1.59 (s, 12 H, C(CH3)2).
13C NMR (CDCl3): δ 152.20, 150.29, 141.34, 129.18, 123.55,
120.29, 120.23, 106.42, 72.62, 48.49, 33.56, 12.90 and 11.37.
[Zn(L3S)(SPhF5)] 5. A solution of complex 2 (0.17 g, 0.48
mmol) in 30 mL of CH2Cl2 was treated 1 equivalent of HSPhF5
(0.095 g, 0.48 mmol). The resulting solution was stirred for 1 h,
concentrated under reduced pressure and crystallized by layer-
ing the concentrated CH2Cl2 solution with hexane to give
[Zn(L3S)(SPhF5)] as colorless crystals (0.20 g, 79%). Calc.
(found) for [Zn(L3S)(SPhF5)]ؒ0.33C6H12ؒH2O, C22H27F5N4-
Physical methods
Elemental analyses were obtained from Quantitative Technolo-
gies, Inc., Whitehouse, NJ. All samples were dried in vacuo prior
to analysis. The presence of solvates was corroborated by
1
1
FTIR, H NMR, or X-ray crystallography. H and 13C NMR
spectra were collected on a Varian UNITY INOVA 400 MHz
NMR spectrometer. Chemical shifts are reported in ppm rela-
tive to an internal standard of TMS. The 13C quaternary carbon
1
OS2Zn: C, 44.94 (45.09); H, 4.64 (4.35); N, 9.52 (9.37)%. H
NMR (CD2Cl2): δ 5.98 (s, 2 H, Pz H), 5.97 (s, 1 H, CH), 2.42
J. Chem. Soc., Dalton Trans., 2000, 3304–3309
3305

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peaks that are not observed are a result of either poor solubility
and/or overlapping signals. IR spectra were recorded as KBr
disks on a Perkin-Elmer 1600 Series FTIR spectrometer and are
reported in wavenumbers.
Crystallographic structure determination
Crystal, data collection, and refinement parameters for
complexes 3, 4, 5, 8 and 9 are given in Table 1. Crystals of
all complexes, except 8, were sealed in thin-walled quartz
capillaries, mounted on a Siemens P4 diffractometer with a
sealed-tube molybdenum X-ray source (λ 0.71073 Å) controlled
via PC computer running Siemens XSCANS 2.1. Crystals of 8
were mounted on a Nonius Kappa CCD diffractometer. The
structures were solved using direct methods or via the Patterson
function, completed by subsequent Fourier difference
syntheses, and refined by full-matrix least-squares procedures
on F2. Complex 3 crystallized with two crystallographically
independent, but virtually identical, molecules per unit cell.
The metrical parameters quoted in the text refer to only one
of the molecules but structural parameters of both crystal-
lographically independent molecules are available in the
Supporting Information. The asymmetric unit of 9 contains 2
half molecules of CH2Cl2, while 5 contains a full molecule of
H2O. All non-hydrogen atoms were refined with anisotropic
displacement coefficients, and treated as idealized contributions
using a riding model except where noted. All software
and sources of the scattering factors are contained in the
SHELXTL 5.0 program library.32
Fig.
1
An ORTEP33 diagram with 30% thermal ellipsoids for
[Zn(L3S)Cl] showing atomic labeling for the coordination sphere only.
CCDC reference number 186/2130.
See http://www.rsc.org/suppdata/dt/b0/b004037l/ for crystal-
lographic files in .cif format.
Results
Synthesis and reactivity
Fig. 2 An ORTEP diagram for [Zn(L3S)(OAc)]. Details as in Fig. 1.
The synthesis of L3SH begins with the corresponding hetero-
disulfide, obtained in reasonable yield via the route previously
used to prepare other members of this series.28 Thus bis(3,5-
dimethylpyrazolyl) ketone reacted readily with 2-methyl-
disulfanylisobutyraldehyde as a melt at 80 ЊC in the presence of
CoCl2 as a catalyst. The thio-protected intermediate, L3SSMe,
was cleanly reduced with LiAlH4 to yield the desired product.
Although the Liϩ salt precipitated from the THF reaction
mixture following the reduction, it was contaminated with Al
containing solids and difficult to purify. The protonated ligand,
L3SH, was therefore isolated by removal of the THF, acidifi-
cation, and extraction with dichloromethane. Both the lithium
salt and the free thiol ligand are sensitive to oxygen and were
stored in a dry box.
The complex [Zn(L3S)(CH3)] proved to be easily prepared
and a versatile synthon for the formation of a variety of other
pseudotetrahedral [Zn(L3S)X] complexes since it is precipitated
in nearly pure form and in >80% yield when dimethylzinc (ca.
2 M, in toluene, 10% excess) is added dropwise to a solution of
L3SH in toluene. The methyl derivative reacts cleanly with
exogenous protic reagents, HX, to liberate methane and form
the corresponding [Zn(L3S)X] complex. However, success of
the reaction depends on the pKa of the protic ligand used.
Fig. 3 An ORTEP diagram for [Zn(L3S)(SPhF5)]. Details as in Fig. 1.
Molecular structures
Hence, while reactions with HCl, HOAc, p-nitrophenol
F5
(HOPhNO ) or pentafluorobenzenethiol (HSPh ) proceeded
Single-crystal X-ray diffraction studies on [Zn(L3S)Cl] 3,
[Zn(L3S)(OAc)] 4, [Zn(L3S)(SPhF5)] 5, [Zn(L3S)2] 8 and
[Zn2{(L3S)2Zn}{O2P(OPh)2}4] 9 confirm that (L3S)Ϫ, formed
by deprotonation of the thiolate sulfur in L3SH, binds to
the zinc ion in a tridentate fashion. Selected bond distances and
angles for the zinc complexes are shown in Table 2 and Figs.
1–5 show the thermal ellipsoid plots. Additional structural
parameters are available in the Supporting Information.
2
smoothly to produce the desired [Zn(L3S)X] product, water or
phenol itself did not. Previously, work done in our laboratory
has determined that protic reagents with pKa <9 (as deter-
mined in water) proceeded smoothly to product regardless of
donor atom.28 However for ligands with pKa at or above 9 the
success of the reaction depends upon the thiophilicity of the
zinc ion. Thus while PhCH2CSH, pKa ca. 10–11, reacts cleanly
to produce the desired products within hours, reactions with
phenol or water produced no reaction over 3 d.
The molecular structures of complexes 3–5 confirm the
pseudo-tetrahedral coordination geometry. The two pyrazolyl
3306
J. Chem. Soc., Dalton Trans., 2000, 3304–3309

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Table 1 Summary of crystallographic data and parameters for [Zn(L3S)Cl] 3, [Zn(L3S)(OAc)] 4, [Zn(L3S)(SPhF5)]ؒH2O 5ؒH2O, [Zn(L3S)2] 8, and
[Zn{(L3S)2Zn2}{O2P(OPh)2}4]ؒCH2Cl2 9ؒCH2Cl2
3
4
5ؒH2O
8
9ؒCH2Cl2
Molecular formula
M
C14H21ClN4SZn
378.23
C16H24N4O2SZn
401.82
C20H23F5N4OS2Zn
559.98
C28H42N8S2Zn
620.19
C82H76N8O16S2P4Zn3
1838.9
T/K
Crystal system
Space group
293(2)
293(2)
Orthorhombic
Cmc21
293(2)
Monoclinic
P21/n
153(2)
Monoclinic
P21/c
293(2)
Triclinic
Triclinic
¯
¯
P1
P1
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
9.905(2)
13.758(2)
14.787(3)
67.815(12)
87.213(14)
71.473(14)
4
1763.4(5)
1.661
4499
11.889(2)
11.694(2)
13.953(3)
8.563(2)
33.701(5)
9.454(3)
8.7338(2)
14.6710(5)
24.6109(6)
10.996(2)
14.959(2)
26.801(3)
83.64(4)
84.41(4)
71.78(2)
4
4476(3)
1.039
11259
93.58(2)
96.225(2)
Z
4
4
4
V/Å3
1939.9(6)
1.389
832
828
0.0303
0.0811
2722.8(11)
1.127
3561
3297
0.0717
0.2177
3134.9(2)
0.948
17232
4884
0.0418
0.1077
µcalc/mmϪ1
No. reflections collected
No. unique reflections
R( F )
4198
0.0371
0.0940
10983
0.1215
0.3089
Rw(F2)
Table 2 Selected bond distances (Å) and angles (Њ) for complexes 3–5
and 8a
3
4
5
8
Zn(1)–N(1)
Zn(1)–N(1)#1
Zn(1)–N(3)
Zn(1)–S(1)
Zn(1)–O(1)
Zn(1)–S(2)
Zn(1)–Cl(1)
2.079(3)
2.078(3)
2.054(3) 2.080(8)
2.054(2)
2.069(2)
2.034(8)
2.087(3)
2.2744(11)
2.2488(13) 2.242(2) 2.252(3)
1.928(5)
2.276(3)
2.2306(8)
2.1907(14)
N(1)–Zn(1)–N(3)
N(1)–Zn(1)–N(1)#2
N(1)–Zn(1)–S(1)
N(1)#2–Zn(1)–S(1)
N(3)–Zn(1)–S(1)
N(1)–Zn(1)–S(2)
N(3)–Zn(1)–S(2)
N(1)–Zn(1)–O(1)
N(1)#2–Zn(1)–O(1)
N(1)–Zn(1)–Cl(1)
N(3)–Zn(1)–Cl(1)
S(1)–Zn(1)–Cl(1)
S(1)–Zn(1)–O(1)
S(1)–Zn(1)–S(2)
Zn(1)–O(1)–C(9)
86.65(13)
98.13(10)
97.41(10)
88.4(3)
95.4(2)
86.31(10)
95.42(9)
90.8(2)
97.4(2)
97.4(2)
98.2(2)
127.8(2)
125.6(2)
97.48(9)
125.11(8)
114.44(8)
121.2(2)
121.2(2)
117.16(11)
118.11(11)
129.67(6)
Fig. 4 An ORTEP diagram for [Zn(L3S)2]. Details as in Fig. 1.
121.8(2)
115.3(5)
114.04(11) 127.99(3)
a Numbers in parentheses are estimated standard deviations.
largest internal angle. The Zn(1)–S(1) bond distances for 3 and
4 are statistically indistinguishable, 2.249(1) and 2.242(2) Å,
respectively; that for 5 was slightly longer at 2.252(3) Å. The
Zn(1)–Npz and Zn(1)–S(1) bond distances in 3–5 are not
unusual and are similar to those reported for other ZnII–Npz
and ZnII–Sthiolate complexes.28 The “axial” ligands are positioned
perpendicular to the trigonal plane formed by the two pyrazolyl
nitrogens and the thiolate sulfur of (L3S)Ϫ, resulting in Zn(1)–
Cl(1) for 3 and Zn(1)–S(2) for 5 of 2.1907(14) and 2.276(3) Å,
respectively. In 5 the S(1)–Zn(1)–S(2) and average Npz–Zn(1)–
S(2) bond angles are 114.04(11) and 126.7(1)Њ, respectively. In
complex 4 the acetate is bound in a nearly complete uniden-
tate mode to the zinc ion with a Zn(1)–O(1) bond distance of
1.928(5) Å and a Zn(1)–O(1)–C(9) bond angle of 115.3(5)Њ. The
second acetate oxygen, O(2), is essentially uncoordinated at
2.875 Å away from the zinc.
Fig. 5 Structural diagram for [Zn2{(L3S)2Zn}{O2P(OPh)2}4] at its
present state of refinement. Phenyl rings from the diphenyl phosphates
have been removed for clarity.
nitrogens and one thiolate sulfur donor of (L3S)Ϫ constitute the
trigonal face of the tetrahedron. The Npz–Zn(1)–Npz bond
angles range from 86.65(13)Њ for [Zn(L3S)Cl] to 90.8(2)Њ for
[Zn(L3S)(OAc)], while the average Npz–Zn(1)–S(1) bond angles
range from 96.8(1)Њ for [Zn(L3S)(SPhF5)] to 97.77(7)Њ for
[Zn(L3S)Cl]. All angles are less than the 109.5Њ expected for
idealized tetrahedral geometry due to the “bite” of the tri-
dentate ligand and in all cases the thiolate sulfur supports the
The X-ray structural analyses of [Zn(L3S)2] 8 and [Zn2{(L-
3S)2Zn}{O2P(OPh)}4] 9 show two different coordination modes
for complexes of basic chemical formula [Zn(L)2]. For [Zn-
(L3S)2] the molecular structure shows an unexpected pseudo-
tetrahedral coordination around the zinc() ion rather than the
J. Chem. Soc., Dalton Trans., 2000, 3304–3309
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can be described as being anisobidentate while that of the
copper and nickel analogs as symmetrical bidentate.34,35 With
the recent discovery of CAs with donor spheres around the
zinc that are not N3O,37 it becomes of interest to determine if
the donor set could also affect the reactivity of the enzyme via
a changing coordination mode of the bicarbonate ion. Our
results with a series of LZn(OAc) complexes show a clear trend.
As the donor sphere is changed from N2SO to N3O to N2O2
(L = L3S, Tpt-butyl,H and L1O) ∆d/Å varies from 0.95 to 0.53 and
∆θ/Њ from 44.2 to 25.9, that is from pure unidentate to aniso-
bidentate.36 This would suggest that the addition of sulfur to
the donor sphere should lead to an inherently more reactive
CA. Interestingly the recently structurally characterized CA
from Porphyridium purpureum has been found to have an NS2O
environment around the zinc.37
Fig. 6 Parameters used in assigning the coordination mode in acetate
complexes.
octahedral geometry implied by the formula. Two pyrazolyl
nitrogens and the thiolate sulfur donor from one molecule of
(L3S)Ϫ constitute the trigonal face of the tetrahedron. The
N(1)–Zn(1)–N(3) bond angle is 86.31(10)Њ while the average
Npz–Zn(1)–S(1) bond angle is 96.45(6)Њ. The Zn(1)–S(1) and
average Zn(1)–Npz bond distances for 8 are 2.2744(11) and
2.078(2) Å, respectively. The second (L3S)Ϫ ligand, which
occupies the fourth site in the tetrahedral geometry, is coordin-
ated to the zinc ion only through the thiolate sulfur with a
Zn(1)–S(2) bond distance of 2.2306(8) Å and S(1)–Zn(1)–S(2)
and average Npz–Zn(1)–S(2) bond angles of 127.99(3) and
119.78(6)Њ, respectively. Surprisingly the two pyrazole rings
from the second (L3S)Ϫ molecule remain uncoordinated and
are rotated away from the metal.
For [Zn2{(L3S)2Zn}{O2P(OPh)2}4] 9 the asymmetric unit
of the crystal structure contains one zinc ion octahedrally
encapsulated by two tridentate L3S moieties with the sulfur
donors in a cis arrangement. This cis arrangement of sulfur
donors allows them to bridge to a (PhO)2PO2Zn(µ-(PhO)2PO2)2-
Zn(PhO)2PO2 unit to form a trinuclear complex. Unfortunately
the only available crystals of 9 diffracted extremely weakly
giving a final structure with weighted R values well outside the
acceptable limits. Thus we choose not to report the structure in
detail with full bond lengths and angles, which are in any event
unexceptional. Nevertheless the core structure refined nicely
and the atom connectivity is unambiguous.
In addition to the structural trends noted above, differences in
reactivity at N2X coordinated zinc centers as the donor atom is
changed from N to O to S were also evident. Some of these will
be reported in a separate publication30 but noteworthy in this
context is our complete inability to isolate stable zinc hydroxo
complexes of the (L1O)Ϫ, (L2S)Ϫ or (L3S)Ϫ ligands using any of
the methods that work for the analogous (Tp)Ϫ complexes.2,3,11
This despite the fact that such species appear to be the thermo-
dynamic “sink” for the zinc-tris(pyrazolyl)borates in the
presence of water or alkali. Thus upon treatment with NaOH,
[Zn(L)X], where L = (L1O)Ϫ, yields only Zn(OH)2 and “free”
ligand, while with (L2S)Ϫ and (L3S)Ϫ rearrangement products
of the form [Zn(L)2] were isolated. It seems likely that hydroxo
complexes are in fact initially formed in all cases but that they
are much more reactive than those of the corresponding (Tp)Ϫ
complex. For (L1O)Ϫ it appears that the weakly bound phen-
oxide simply cannot compete with OHϪ as a ligand for the
metal while with (L2S)Ϫ and (L3S)Ϫ the rearrangement is driven
by the enormous preference for zinc to adopt an N2S2 donor
sphere,38 i.e. 2(LS)ZnX ϩ 2OHϪ → 2(LS)Zn(OH) →
(LS)2Zn ϩ Zn(OH)2. Interestingly the exact nature of the [Zn-
(LS)2] complex formed was different for each of the two thio
ligands. In the case of (L2S)Ϫ the final product was one where
the zinc was tetrahedrally encapsulated by two ligands where
each one coordinated in an unexpected NS bidentate mode
leaving one pyrazole arm free. For (L3S)Ϫ the tetrahedrally
coordinated N2S2Zn complex in the solid state is composed of
one tripodally coordinated ligand with the other monodentate
through the sulfur. In solution, however, NMR shows that all
the pyrazoles are equivalent, a situation incompatible with the
solid state structure. At least two possibilities to account for this
discrepancy exist: (1) the structure in solution is the octa-
hedrally coordinated L2Zn species where each ligand is tri-
podally coordinated and the two sulfur donors are trans to one
another giving a plane of symmetry or (2) The tridentate and
monodentate ligands in the tetrahedral complex seen in the
solid state exist in solution but are in rapid equilibrium. The
latter explanation is supported by the somewhat broad lines
seen in the NMR spectra of 8.
Discussion
With the introduction of (L3S)Ϫ we now a have family of iso-
structural, isosteric and isoelectronic tripodal ligands providing
N3 (Tp)Ϫ, N2Ophenol(L1O)Ϫ and N2Salkyl/aryl(L3S)Ϫ or (L2S)Ϫ
donor spheres to zinc. One of the goals of our research is to see
how changing the donor atom, keeping all other variables con-
stant, affects the structure/reactivity at a pseudotetrahedral zinc
center. We have now accumulated enough data to begin to
describe some potential trends.
In carbonic anhydrase (CA), an enzyme involved in the
reversible hydration of CO2, substitution of the native ZnII by
other divalent metals leads to enzymes with decreasing or no
activity following the approximate order Zn > Co ӷ Cu,Ni,Cd
or Hg.34,35 The mode of binding (monodentate vs. bidentate) of
the product, bicarbonate ion, to the metal center has been pro-
posed to be important to this changing reactivity. The structure
of human CA I with bicarbonate has shown that the anion
binds in a pure unidentate mode while the binding in the
cobalt() analog is better described as anisobidentate.34,35 Thus
Parkin and co-workers suggested that the varying activity of
metal substituted CA can be attributed to a change in bonding
of bicarbonate from anisobidentate in the active zinc() and
cobalt() substituted enzymes to a symmetric bidentate mode
in the inactive analogs of CuII, NiII, CdII and HgII. This pro-
posal is based on the structures found for a series of TpM
complexes where M = Zn, Co, Ni or Cu, with the nitrate ion
(which is isoelectronic with bicarbonate).34,35 A scheme to
describe the denticity of potentially mono- or bi-dentate lig-
ands such as nitrate, bicarbonate or by analogy acetate has been
devised based on a combination of M–O distances d2 Ϫ d1/Å
and the M–O–X angles θ1 Ϫ θ2/Њ (Fig. 6).36 Using these criteria,
the binding of nitrate to complexes of Zn and Co of sterically
bulky tris(pyrazolyl)borate or tris(imidazolyl)phosphine ligands
Further differences in reactivity between the two thio ligands
were also noted. For example the lack of reactivity of complex
2 with water stands in stark contrast to what was seen for the
aromatic analog, (L2S)Ϫ, which reacted rapidly with even traces
of water in solvents.28 It is somewhat difficult to attribute this
vast difference in reactivity of the zinc methyl derivative of
(L3S)Ϫ and (L2S)Ϫ to the nature of the donor (aliphatic vs.
aromatic thiol). Rather it is more likely that it is the difference in
the S-donor chelate ring size, i.e. 6-membered for (L3S)Ϫ and 7
for (L2S)Ϫ, that destabilizes the latter.
Acknowledgements
This work was supported by Grants AI-1157 from the Robert
A. Welch Foundation and CHE-9726488 from the NSF. The
NSF-ILI program grant USE-9151286 is acknowledged for
3308
J. Chem. Soc., Dalton Trans., 2000, 3304–3309

View Article Online
17 R. Burth, A. Stange, M. Schäfer and H. Vahrenkamp, Eur. J. Inorg.
partial support of the X-ray diffraction facilities at Southwest
Texas State University. We thank Dr. Vincent Lynch, Depart-
ment of Chemistry and Biochemistry, University of Texas at
Austin for data collection on compound 8.
Chem., 1998, 1759.
18 A. Trösch and H. Vahrenkamp, Eur. J. Inorg. Chem., 1998, 827.
19 A. Abufarag and H. Vahrenkamp, Inorg. Chem., 1995, 34, 2207.
20 A. Abufarag and H. Vahrenkamp, Inorg. Chem., 1995, 34, 3279.
21 P. Ghosh and G. Parkin, J. Chem. Soc., Dalton Trans., 1998, 2281.
22 C. Kimblin, T. Hascall and G. Parkin, Inorg. Chem., 1997, 36, 5680.
23 P. Ghosh and G. Parkin, Chem. Commun., 1998, 413.
24 C. O. Rodriguez de Bardarin, N. A. Bailey, D. E. Fenton and Q. He,
J. Chem. Soc., Dalton Trans., 1997, 161.
References
1 W. N. Lipscomb and N. Straeter, Chem. Rev., 1996, 96, 2375.
2 K. Kitajima, S. Hikichi, M. Tanaka and Y. Moro-oka, J. Am. Chem.
Soc., 1993, 115, 5496.
3 A. Looney, R. Han, K. McNeill and G. Parkin, J. Am. Chem. Soc.,
1993, 115, 4690.
25 S. J. Chiou, P. Ge, C. G. Riordan, L. Liable-Sands and A. L.
Rheingold, Chem. Commun., 1999, 159.
26 I. G. Dance, Polyhedron, 1996, 5, 1037.
4 C. Bergquist and G. Parkin, J. Am. Chem. Soc., 1999, 121, 6322.
5 R. Walz, K. Weis, M. Ruf and H. Vahrenkamp, Chem. Ber., 1997,
130, 975.
6 M. Ruf, K. Weis and H. Vahrenkamp, J. Am. Chem. Soc., 1996, 118,
9288.
7 R. Alsfasser, A. K. Powell and H. Vahrenkamp, Angew. Chem., Int.
Ed. Engl., 1990, 29, 898.
27 R. H. Prince, in Comprehensive Coordination Chemistry, eds.
G. Wilkinson, R. D. Gillard and J. A. McCleverty, 1988, vol. 5,
Pergamon, London, p. 925.
28 B. S. Hammes and C. J. Carrano, Inorg. Chem., 1999, 38, 3562.
29 B. S. Hammes and C. J. Carrano, Inorg. Chim. Acta, 2000, 300, 427.
30 B. S. Hammes, C. R. Warthen, D. C. Crans and C. J. Carrano,
J. Inorg. Biochem., 2000, in press.
8 R. Alsfasser, S. Trofimenko, A. Looney, G. Parkin and H.
Vahrenkamp, Inorg. Chem., 1991, 30, 4098.
9 M. Ruf, F. A. Schell, R. Walz and H. Vahrenkamp, Chem Ber., 1997,
130, 101.
10 K. Weiss, M. Rombach, M. Ruf and H. Vahrenkamp, Eur. J. Inorg.
Chem., 1998, 263.
11 M. Ruf and H. Vahrenkamp, Inorg. Chem., 1996, 35, 6571.
12 M. Ruf, K. Weis, I. Brasack and H. Vahrenkamp, Inorg. Chim. Acta,
1996, 250, 271.
13 K. Weis and H. Vahrenkamp, Eur. J. Inorg. Chem., 1998, 271.
14 A. Looney, G. Parkin, R. Alsfasser, M. Ruf and H. Vahrenkamp,
Angew. Chem., Int. Ed. Engl., 1992, 31, 92.
15 S. Hikichi, M. Tanaka, Y. Moro-oka and N. Kitajima, J. Chem. Soc.,
Chem. Commun., 1992, 814.
31 T. C. Higgs and C. J. Carrano, Inorg. Chem., 1997, 36, 291.
32 G. M. Sheldrick, SHELXTL 5.0, Siemens XRD, Madison, WI,
1997.
33 C. K. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge
National Laboratory, Oak Ridge, TN, 1976.
34 R. Han and G. Parkin, J. Am. Chem. Soc., 1991, 113, 9707.
35 C. Kimblin, V. J. Murphy, T. Hascall, B. M. Bridgewater, J. B.
Bonanno and G. Parkin, Inorg. Chem., 2000, 39, 967.
36 G. J. Kleywegt, W. G. R. Weismeijer, G. J. Driel, W. L. Drissen,
J. Reedijk and J. H. Noordik, J. Chem. Soc., Dalton Trans., 1985,
2177.
37 S. Mitsuhashi, T. Mizushima, E. Yamashita, M. Yamamoto,
T. Kumasaka, H. Moriyama, T. Ueki, S. Miyachi and T. Tsukihara
J. Biol. Chem., 2000, 275, 5521.
16 H. Vahrenkamp, Acc. Chem. Res., 1999, 32, 589.
38 R. Burth and H. Vahrenkamp, Inorg. Chim. Acta, 1998, 282, 193.
J. Chem. Soc., Dalton Trans., 2000, 3304–3309
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