Synthesis and characterization of novel mononuclear cadmium thiolate complexes in a sulfur-rich environment

Article abstract of DOI:10.1021/ic0109243

Four different well-defined cadmium monothiolate complexes of general formula (Tm^p-Tol)CdSR (R = Bz, Ph, p-Tol, C₆F₅), where Tm^p-Tol is the bulky tris(2-mercapto-1-p-tolylimidazolyl)borate ligand, have been read

Full text of DOI:10.1021/ic0109243

Inorg. Chem. 2002, 41, 998−1001
Synthesis and Characterization of Novel Mononuclear Cadmium Thiolate
Complexes in a Sulfur-Rich Environment
,‡
,†
Selma Bakbak,† Christopher D. Incarvito,‡ Arnold L. Rheingold,* and Daniel Rabinovich*
Department of Chemistry, The UniVersity of North Carolina at Charlotte,
9201 UniVersity City BouleVard, Charlotte, North Carolina 28223, and
Department of Chemistry and Biochemistry, UniVersity of Delaware,
Newark, Delaware 19716
Received August 27, 2001
Introduction
The tendency to bridge metal centers and form extended
structures is one of the distinctive traits of metal thiolate
coordination chemistry.1 Cadmium thiolate complexes are
no exception, and many intricate cluster and polymeric
species, some of which have interesting optoelectronic
properties, have been prepared in recent years.2 In contrast,
mononuclear cadmium thiolate complexes, structurally au-
thenticated examples of which include trigonal planar
[Cd(SR)3]- and tetrahedral [Cd(SR)4]2- and Cd(SR)2L2
derivatives,3 are less common. Such molecular species have
been synthesized primarily as model compounds in the study
of zinc enzymes4 because of the favorable spectroscopic (e.g.,
NMR) properties of cadmium relative to zinc.5 To prepare
Figure 1. Trofimenko’s tris(pyrazolyl)borate (TpRR′) and Reglinski’s
original tris(2-mercapto-1-methylimidazolyl)borate (TmMe) ligands.
* Author to whom correspondence should be addressed. E-mail:
drabinov@email.uncc.edu.
well-defined monothiolate complexes of cadmium in a sulfur-
rich environment, we decided to use the anionic tris-
(mercaptoimidazolyl)borate (TmR) ligand system, recently
introduced6 as a soft counterpart of the versatile tris-
(pyrazolyl)borate family of ligands (Figure 1).7 Several TmR
ligands have been successfully applied to zinc bioinorganic
chemistry,8 and our own contributions to this area include
the syntheses of the new bulky benzyl- and p-tolyl-substituted
ligands TmBz and Tmp-Tol and their corresponding group 12
metal complexes (TmR)MBr (M ) Zn, Cd).9 In particular,
we considered that the complex (Tmp-Tol)CdBr, because of
† The University of North Carolina at Charlotte.
‡ University of Delaware.
(1) (a) Krebs, B.; Henkel, G. Angew. Chem., Int. Ed. Engl. 1991, 30, 769-
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(5) (a) Summers, M. F. Coord. Chem. ReV. 1988, 86, 43-134. (b)
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Chem. Commun. 1996, 1975-1976.
(7) Trofimenko, S. Scorpionates: The Coordination Chemistry of Poly-
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(8) See ref 6 and: (a) Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.;
Parkin, G. Chem. Commun. 1999, 2301-2302. (b) Bridgewater, B.
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998 Inorganic Chemistry, Vol. 41, No. 4, 2002
10.1021/ic0109243 CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/18/2002

NOTE
its steric bulk, good solubility in common organic solvents,
and ease of preparation in excellent yield, would be an ideal
starting material for the preparation of the desired mono-
nuclear cadmium thiolate compounds. That turned out to be
the case, and we report in this paper the synthesis and
characterization of the first series of neutral monothiolate
complexes of cadmium of general formula (Tmp-Tol)CdSR,
compounds which provide a rare opportunity to compare
directly the simultaneous binding of both thione and thiolate
donor groups at a single metal center.10
Results and Discussion
The cadmium thiolate complexes (Tmp-Tol)CdSR (R ) Bz,
Ph, p-Tol, C6F5) were readily prepared by reacting benzene
or acetonitrile solutions of (Tmp-Tol)CdBr with equimolar
amounts of the corresponding thallium thiolates Tl(SR), as
shown in eq 1. The desired products were isolated in very
Figure 2. Molecular structure of (Tmp-Tol)CdSPh. Selected bond lengths
(Å) and angles (deg): Cd(1)sS(1) 2.529(5), Cd(1)sS(2) 2.477(5), Cd-
(1)sS(3) 2.516(5), Cd(1)sS(4) 2.573(4); S(1)sCd(1)sS(2) 96.01(9), S(1)s
Cd(1)sS(3) 97.91(15), S(1)sCd(1)sS(4) 122.21(11), S(2)sCd(1)sS(3)
99.89(11), S(2)sCd(1)sS(4) 114.04(15), S(3)sCd(1)sS(4) 121.78(6), Cd-
(1)sS(1)sC(3) 101.63(13), Cd(1)sS(2)sC(13) 100.55(18), Cd(1)sS(3)s
C(23) 101.01(16), Cd(1)sS(4)sC(31) 96.62(16).
(Tmp-Tol)CdBr + Tl(SR) C6H6 or MeCN8 (Tmp-Tol)CdSR (1)
-TlBr
good yield (80-90%) after separating by filtration in each
case the insoluble thallous bromide byproduct. While this
protocol seems to work well for primary alkyl and aryl
thiolates, the corresponding reaction with Tl(SBut) failed to
give any isolable products. The new cadmium thiolate
complexes are all fairly air-stable white solids, only mod-
erately soluble in benzene or toluene but very soluble in more
polar solvents such as dichloromethane, chloroform, tetra-
hydrofuran, acetone, acetonitrile, N,N′-dimethylformamide,
and dimethyl sulfoxide. They were characterized by a
combination of analytical and spectroscopic methods, includ-
ing elemental analyses (CHN) and IR and NMR spec-
troscopies. The IR spectra of all three arylthiolate complexes
show single νB-H absorptions of medium intensity close to
2404 cm-1, values which are shifted some 30 cm-1 to lower
frequencies relative to (Tmp-Tol)CdBr and ∼54 cm-1 lower
than the corresponding value seen for the less electron-
Cd)9 and other complexes of general formula (TmR)ZnX,8
with the three mercaptoimidazolyl groups distributed in a
propeller-like arrangement around the pseudo-C3 axis that
contains the boron and cadmium atoms. Even though the
steric bulk provided by the p-tolyl substituents evidently
contributes to the stabilization (i.e., monomeric nature) of
(Tmp-Tol)CdSPh, the phenylthiolate ligand is still considerably
bent at sulfur (CdsSsC ≈ 96.6°).11 In addition, whereas
the average STmsCdsSTm bond angle in (Tmp-Tol)CdSPh
(97.9°) deviates ∼6° further from the ideal tetrahedral angles
than do the corresponding values found in either (TmBz)CdBr
(103.1°) or (TmPh)ZnSPh (105.3°), the average STmsCds
SPh angle in (Tmp-Tol)CdSPh (119.3°) is about 5° larger than
the STmsMsX angles in either (TmBz)CdBr or (TmPh)ZnSPh.
The observation of more distorted structures in the
(TmR)CdX complexes, despite the larger size of cadmium
relative to zinc, can be therefore attributed to the geometric
constraints of the tridentate ligands, a phenomenon that is
also precedented in group 12 metal tris(pyrazolyl)borate
chemistry.12
1
withdrawing benzylthiolate derivative (Tmp-Tol)CdSBz. H
and 13C NMR data for the series (Tmp-Tol)CdSR, excluding
the thiolate signals, are not only virtually identical to those
of (Tmp-Tol)CdBr but also indicative of the magnetic equiva-
lency on the NMR time scale of the three mercaptoimidazolyl
groups in the Tmp-Tol ligands and thus provide evidence for
the existence of mononuclear tetrahedral complexes in
solution.
Single crystals suitable for an X-ray diffraction study of
the phenylthiolate derivative (Tmp-Tol)CdSPh were obtained
by slow evaporation at room temperature of a benzene
solution of the complex. As shown in Figure 2, the four-
coordinate complex exhibits in the solid state a distorted
tetrahedral geometry akin to those of (TmBz)MBr (M ) Zn,
The CdsSTm bond distances, in the range 2.477(5)-2.529-
(5) Å, are slightly shorter than those seen in (TmBz)CdBr
[2.515(1)-2.569(1) Å], the only other structurally character-
ized cadmium tris(mercaptoimidazolyl)borate complex that
we are aware of.13 Furthermore, the CdsSPh bond distance
(11) For comparison, the CdsSsC angles in homoleptic [Cd(SR)4]2-
complexes are in the range 106-110°, and the ZnsSsC angle in
(TmPh)ZnSPh is 102.8°. See refs 3d-g and 8b, respectively.
(12) For example, the average NsMsI angles in the iodo complexes
(TpiPr )MI (M ) Zn,12a Cd12b) are 123.2° and 128.0°, respectively. (a)
2
Han, R.; Looney, A.; McNeill, K.; Parkin, G.; Rheingold, A. L.;
Haggerty, B. S. J. Inorg. Biochem. 1993, 49, 105-121. (b) Looney,
A.; Saleh, A.; Zhang, Y.; Parkin, G. Inorg. Chem. 1994, 33, 1158-
1164.
(9) Bakbak, S.; Bhatia, V. K.; Incarvito, C. D.; Rheingold, A. L.;
Rabinovich, D. Polyhedron 2001, 20, 3343-3348.
(10) The related phenyltris[(tert-butylthio)methyl]borate zinc phenylthiolate
complex (PhTttBu)ZnSPh, which contains both thioether and thiolate
ligands, has recently been prepared as a model compound for the active
center in methionine synthases: Chiou, S.-J.; Innocent, J.; Riordan,
C. G.; Lam, K.-C.; Liable-Sands, L.; Rheingold, A. L. Inorg. Chem.
2000, 39, 4347-4353.
(13) The octahedral bis(mercaptoimidazolyl)(pyrazolyl)borate complex Cd-
(pzBmMe)2 has, in addition to two relatively weak Cd‚‚‚HsB contacts
at ∼2.6 Å, four CdsS bond lengths in the range 2.526(1)-2.572(1)
Å. See: Kimblin, C.; Bridgewater, B. M.; Churchill, D. G.; Hascall,
T.; Parkin. G. Inorg. Chem. 2000, 39, 4240-4243.
Inorganic Chemistry, Vol. 41, No. 4, 2002 999

NOTE
solid, which was washed with pentane (2 × 5 mL) and dried in
vacuo for 1 h (0.20 g, 82%). Mp ) 160 °C (dec). NMR data (in
CD2Cl2): 1H d 2.39 (s, 9 H, NC6H4CH3), 3.59 (br s, 2 H, CH2),
6.80-7.35 (m, 23 H, NC6H4CH3 + imidazole H + SCH2C6H5),
BH not located; 13C d 21.3 (q, 1JC-H ) 127, 3 C, C6H4CH3), 31.7
(t, 1JC-H ) 155, 1 C, CH2), 121.2 (d, 1JC-H ) 199, 3 C, imidazole
[2.573(4) Å] appears to be the longest reported so far for
such interactions in cadmium complexes containing terminal
thiolate ligands, values which are typically between 2.42 and
2.55 Å.2,3 All the remaining interatomic distances (i.e.,
BsN, SsC, CsN, and CsC) are within normal ranges.
1
1
C), 124.6 (d, JC-H ) 201, 3 C, imidazole C), 126.9 (d, JC-H
)
Conclusions
160, 1 C, Cp in SCH2C6H5), 127.1 (d, 1JC-H ) 157, 6 C, Co or Cm
1
In summary, four rare examples of mononuclear cadmium
monothiolate complexes (Tmp-Tol)CdSR (R ) Bz, Ph, p-Tol,
C6F5) have been readily prepared and fully characterized.
The X-ray structure of the phenylthiolate derivative revealed
that all the CdsS bond distances and CdsSsC bond angles
are fairly similar, an observation that lends support to the
notion that the thione groups in TmR ligands can be regarded
as “masked” thiolate groups. We are currently exploring the
synthesis and reactivity (e.g., protonation and alkylation
reactions) of these and additional sulfur-rich complexes
(TmR)MSR′ (M ) Cd, Hg), studies which are primarily
aimed at modeling the biologically relevant scission of
cysteine thiolate residues and concomitant formation of
thioethers or thiols in a variety of zinc metalloenzymes.8b,14
in p-Tol), 128.3 (d, JC-H ) 160, 2 C, Co or Cm in SCH2C6H5),
1
128.7 (d, JC-H ) 160, 2 C, Co or Cm in SCH2C6H5), 129.9 (d,
1JC-H ) 160, 6 C, Co or Cm in p-Tol), 135.5 (s, 3 C, Cp in p-Tol),
138.9 (s, 3 C, Cipso in p-Tol), 158.7 (s, 3 C, CdS). IR data: 2458
(νB-H). Anal. Calcd for C37H35BCdN6S4: C, 54.5; H, 4.3; N, 10.3.
Found: C, 54.4; H, 4.5; N, 10.3%.
Synthesis of (Tmp-Tol)CdSPh.
A yellow suspension of
(Tmp-Tol)CdBr (0.25 g, 0.32 mmol) and Tl(SC6H5) (0.10 g, 0.32
mmol) in benzene (20 mL) was stirred for 2 h, and the resulting
grayish yellow suspension was filtered. The solvent was removed
under reduced pressure from the colorless filtrate to give a white
solid, which was washed with pentane (2 × 8 mL) and dried in
vacuo for 2 h (0.23 g, 90%). Mp ) 232 °C (dec). NMR data (in
CD2Cl2): 1H d 2.40 (s, 9 H, NC6H4CH3), 6.84-7.32 (m, 23 H,
NC6H4CH3 + imidazole H + SC6H5), BH not located; 13C d 21.3
1
1
(q, JC-H ) 127, 3 C, C6H4CH3), 121.4 (d, JC-H ) 198, 3 C,
imidazole C), 122.4 (d, 1JC-H ) 162, 1 C, Cp in SC6H5), 124.8 (d,
1JC-H ) 195, 3 C, imidazole C), 127.1 (d, 1JC-H ) 163, 6 C, Co or
Experimental Section
General Considerations. All reactions were performed under
dry, oxygen-free nitrogen in an Innovative Technology System One-
M-DC glovebox or under argon using a combination of high-
vacuum and Schlenk techniques.15 Solvents were purified and
degassed by standard procedures, and all commercially avail-
able reagents were used as received. Whereas the complex
(Tmp-Tol)CdBr was prepared as published,9 the thallium thiolates
Tl(SR) were obtained using a modification of the procedure reported
for the synthesis of various thallium phenoxides16 by reacting
Tl(OEt) with a slight excess of the corresponding thiols in pentane;
the bright yellow (R ) Bz, Ph, p-Tol) or off-white (R ) C6F5)
solids were isolated in 90-95% yield, dried in vacuo for at least 3
h, and stored in the glovebox. 1H and 13C NMR spectra were
obtained on General Electric QE 300 or Varian Gemini (300 MHz)
FT spectrometers. Chemical shifts are reported in ppm relative to
SiMe4 (δ ) 0 ppm) and were referenced internally with respect to
the solvent resonances (1H, δ 5.32 for CDHCl2; 13C, δ 53.8 for
CD2Cl2); coupling constants are given in hertz. IR spectra were
recorded as KBr pellets on a Bio-Rad 175C FT spectrophotometer
and are reported in reciprocal centimeters. Elemental analyses were
determined by Atlantic Microlab, Inc. (Norcross, GA).
1
Cm in p-Tol), 128.1 (d, JC-H ) 153, 2 C, Co or Cm in SC6H5),
130.0 (d, 1JC-H ) 160, 6 C, Co or Cm in p-Tol), 133.1 (d, 1JC-H
)
157, 2 C, Co or Cm in SC6H5), 135.4 (s, 3 C, Cp in p-Tol), 139.6 (s,
3 C, Cipso in p-Tol), 143.3 (s, 1 C, Cipso in SC6H5), 158.3 (s, 3 C,
CdS). IR data: 2405 (νB-H). Anal. Calcd for C36H33BCdN6S4: C,
54.0; H, 4.2; N, 10.5. Found: C, 54.1; H, 4.1; N, 10.4%.
Synthesis of (Tmp-Tol)CdS-p-C6H4Me. A yellow suspension of
(Tmp-Tol)CdBr (0.28 g, 0.36 mmol) and Tl(S-p-C6H4Me) (0.12 g,
0.36 mmol) in acetonitrile (25 mL) was stirred for 2 h, and the
resulting beige suspension was filtered. Concentration of the
colorless filtrate under reduced pressure to ∼3 mL and addition of
diethyl ether (20 mL) resulted in the separation of a white solid,
which was isolated by decantation, washed with diethyl ether (2 ×
10 mL), and dried in vacuo for 1 h (0.25 g, 85%). Mp ) 242 °C
(dec). NMR data (in CD2Cl2): 1H d 2.21 (s, 3 H, SC6H4CH3), 2.41
(s, 9 H, NC6H4CH3), 6.66-7.30 (m, 22 H, NC6H4CH3 + imidazole
H + SC6H4CH3), BH not located; 13C d 20.9 (q, 1JC-H ) 126, 1 C,
SC6H4CH3), 21.3 (q, 1JC-H ) 127, 3 C, NC6H4CH3), 121.4 (d, 1JC-H
1
) 198, 3 C, imidazole C), 124.8 (d, JC-H ) 199, 3 C, imidazole
C), 127.1 (d, 1JC-H ) 163, 6 C, Co or Cm in NC6H4CH3), 128.9 (d,
1JC-H ) 156, 2 C, Co or Cm in SC6H4CH3), 130.0 (d, 1JC-H ) 160,
Synthesis of (Tmp-Tol)CdSBz.
A yellow suspension of
1
(Tmp-Tol)CdBr (0.23 g, 0.30 mmol) and Tl(SCH2C6H5) (0.10 g, 0.31
mmol) in benzene (25 mL) was stirred for 2 h, and the resulting
brownish yellow suspension was filtered. The solvent was removed
under reduced pressure from the colorless filtrate to give a white
6 C, Co or Cm in NC6H4CH3), 133.1 (d, JC-H ) 161, 2 C, Co or
Cm in SC6H4CH3), 135.5 (s, 3 C, Cp in NC6H4CH3), 139.6 (s, 3 C,
Cipso in NC6H4CH3), 158.4 (s, 3 C, CdS), Cipso and Cp in SC6H4-
CH3 not located. IR data: 2405 (νB-H). Anal. Calcd for C37H35-
BCdN6S4: C, 54.5; H, 4.3; N, 10.3. Found: C, 54.3; H, 4.4; N,
10.1%.
(14) (a) Wilker, J. J.; Lippard, S. J. Inorg. Chem. 1997, 36, 969-978. (b)
Grapperhaus, C. A.; Tuntulani, T.; Reibenspies, J. H.; Darensbourg,
M. Y. Inorg. Chem. 1998, 37, 4052-4058. (c) Brand, U.; Rombach,
M.; Vahrenkamp, H. Chem. Commun. 1998, 2717-2718. (d) Hammes,
B. S.; Carrano, C. J. Inorg. Chem. 2001, 40, 919-927.
(15) (a) Errington, R. J. AdVanced Practical Inorganic and Metalorganic
Chemistry; Blackie Academic & Professional: London, 1997. (b)
Experimental Organometallic Chemistry; Wayda, A. L., Darensbourg,
M. Y., Eds.; American Chemical Society: Washington, DC, 1987.
(c) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-SensitiVe
Compounds, 2nd ed.; Wiley-Interscience: New York, 1986.
(16) Hughes, R. P.; Zheng, X.; Morse, C. A.; Curnow, O. J.; Lomprey, J.
R.; Rheingold, A. L.; Yap, G. P. A. Organometallics 1998, 17, 457-
465.
Synthesis of (Tmp-Tol)CdSC6F5.
A white suspension of
(Tmp-Tol)CdBr (0.20 g, 0.26 mmol) and Tl(SC6F5) (0.11 g, 0.27
mmol) in benzene (10 mL) was stirred for 1.5 h, and the resulting
yellowish suspension was filtered. The solvent was removed from
the colorless filtrate under reduced pressure to give a white solid,
which was washed with pentane (6 × 10 mL) and dried in vacuo
for 4 h (0.20 g, 86%). Mp ) 222 °C (dec). NMR data (in CD2Cl2):
1H d 2.42 (s, 9 H, NC6H4CH3), 7.00-7.33 (m, 18 H, NC6H4CH3
+ imidazole H), BH not located; 13C d 21.3 (q, 1JC-H ) 127, 3 C,
1
C6H4CH3), 121.5 (d, JC-H ) 198, 3 C, imidazole C), 124.9 (d,
1000 Inorganic Chemistry, Vol. 41, No. 4, 2002

NOTE
Table 1. Crystallographic Data for (Tmp-Tol)CdSPh
immediately placed in the cold nitrogen stream. A data set was
collected on a Siemens P4 diffractometer equipped with a SMART/
CCD detector using Mo KR radiation (λ ) 0.710 73 Å). Despite
the appearance of higher symmetry, the monoclinic space group
P21/n was chosen on the basis of photographic and intensity data.
Solution in the reported space group yielded chemically reasonable
and computationally stable results of refinement. The structure was
solved using direct methods, completed by subsequent difference
Fourier syntheses, and refined by full-matrix least-squares proce-
dures. All non-hydrogen atoms were refined with anisotropic
displacement coefficients, and hydrogen atoms were treated as
idealized contributions. All software and sources of the scattering
factors are contained in the SHELXTL (version 5.1) program
library.17
formula
fw
C36H33BCdN6S4
801.13
cryst color, habit
cryst syst
space group
T, K
colorless block
monoclinic
P21/n (no. 14)
173(2)
a, Å
b, Å
c, Å
15.01(4)
15.32(4)
15.18(4)
90.00(5)
3489(14)
4
1.525
9.02
29.90
8055
â, deg
V, Å3
Z
Dc, g cm-3
µ (Mo KR), cm-1
θ
max, deg
no. of data
no. of parameters
R1/wR2 [I > 2σ(I)] a
R1/wR2 (all data) a
441
0.0595/0.1003
0.0719/0.1025
Acknowledgment. We thank the Camille and Henry
Dreyfus Foundation Faculty Start-Up Grant Program for
Undergraduate Institutions, Research Corporation for a
Cottrell College Science Award, the donors of the Petroleum
Research Fund, administered by the American Chemical
Society, and The University of North Carolina at Charlotte
for generous support of this research.
2
2
2
a R1 ) ∑(|Fo| - |Fc|)/∑|Fo|; wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )2}1/2
.
1JC-H ) 190, 3 C, imidazole C), 127.1 (d, 1JC-H ) 163, 6 C, Co or
Cm in p-Tol), 130.0 (d, 1JC-H ) 160, 6 C, Co or Cm in p-Tol), 135.3
(s, 3 C, Cp in p-Tol), 139.8 (s, 3 C, Cipso in p-Tol), 147.5 (d, 1JC-F
) 232, 1 C, Cipso in SC6F5), 157.9 (s, 3 C, CdS); Co, Cm, and Cp
in SC6F5 not located. IR data: 2403 (νB-H). Anal. Calcd for C36H28-
BCdF5N6S4: C, 48.5; H, 3.2; N, 9.4. Found: C, 48.6; H, 3.3; N,
9.3%.
X-ray Structure Determination. A summary of crystal data
collection and refinement parameters for (Tmp-Tol)CdSPh is given
in Table 1. A crystal suitable for data collection was selected and
mounted with epoxy cement on the tip of a fine glass fiber and
Supporting Information Available: X-ray crystallographic file
in CIF format for the structure determination of (Tmp-Tol)CdSPh.
This material is available free of charge via the Internet at
http://pubs.acs.org.
IC0109243
(17) Sheldrick, G. M. Siemens XRD: Madison, WI.
Inorganic Chemistry, Vol. 41, No. 4, 2002 1001