Multiple bonding modes exhibited by heteroscorpionate N2 S(alkylthiolate) ligands with Zn(II) and Fe(II)

Article abstract of DOI:10.1016/j.poly.2003.11.034

Two related heteroscorpionate N₂S(thiolate) ligands L ¹ and L² have been employed in the synthesis of new zinc(II) and iron(II) complexes. These ligands contain two imidazolyl donors and one alkylthiolate donor connected by a central carbon atom. The ligand L¹ was reacted with ZnBr₂ to give the trinuclear zinc complex [(L¹)₃Zn₃Br₂]Br (1). The X-ray structure of 1 reveals one of the zinc ions to be coordinated by two L¹ ligands in a tripodal arrangement resulting in a distorted octahedral geometry, while the other two zinc ions are both held in a distorted tetrahedral geometry and are coordinated to another molecule of L¹ in an unusual tridentate, multibridging coordination mode. Modification of L¹ to include phenyl substituents on the imidazole groups leads to L², and these substituents cause quite a different result to be obtained in the reaction with ZnBr₂. The combination of L ² and ZnBr₂ leads to the dimeric zinc complex (L ²)₂Zn₂Br₂, (2). Characterization of 2 by X-ray diffraction shows this complex to be a dimer with a central Zn ₂(μ-SR)₂ rhomb. Each zinc ion is found in a distorted tetrahedral geometry coordinated by L² in a bidentate, N,S fashion. The ¹H NMR spectrum of 2 provides strong evidence this structure is retained in solution. In comparison, reaction of L² with FeCl ₂ leads to the monomeric complex (L²)₂Fe ^II (3), which has each ligand coordinated in the same bidentate, N,S bonding mode. Interestingly, the methoxy groups of the two ligands in 3 are close enough to the iron center for a possible bonding interaction, suggesting an unexpected N₂O coordination mode.

Full text of DOI:10.1016/j.poly.2003.11.034

Polyhedron 23 (2004) 471–480
www.elsevier.com/locate/poly
Multiple bonding modes exhibited by
heteroscorpionate N2S(alkylthiolate) ligands
with Zn(II) and Fe(II)
a
Vivek V. Karambelkar , Robert C. diTargiani , Christopher D. Incarvito ,
a
b
c
c
d
Lev N. Zakharov , Arnold L. Rheingold , Charlotte L. Stern , David P. Goldberg
a,
*
a
Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA
b
Department of Chemistry, Yale University, New Haven, CT 06520-8107, USA
Department of Chemistry and Biochemistry, University of California, San Diego, CA 92093, USA
c
d
X-ray Crystallography Facility, Northwestern University, Evanston, IL 60208, USA
Received 9 July 2003; accepted 20 August 2003
Abstract
Two related heteroscorpionate N2S(thiolate) ligands L1 and L2 have been employed in the synthesis of new zinc(II) and iron(II)
complexes. These ligands contain two imidazolyl donors and one alkylthiolate donor connected by a central carbon atom. The
ligand L1 was reacted with ZnBr2 to give the trinuclear zinc complex [(L1)3Zn3Br2]Br (1). The X-ray structure of 1 reveals one of the
zinc ions to be coordinated by two L1 ligands in a tripodal arrangement resulting in a distorted octahedral geometry, while the other
two zinc ions are both held in a distorted tetrahedral geometry and are coordinated to another molecule of L1 in an unusual tri-
dentate, multibridging coordination mode. Modification of L1 to include phenyl substituents on the imidazole groups leads to L2,
and these substituents cause quite a different result to be obtained in the reaction with ZnBr2. The combination of L2 and ZnBr2
leads to the dimeric zinc complex (L2)2Zn2Br2, (2). Characterization of 2 by X-ray diffraction shows this complex to be a dimer with
a central Zn2(l-SR)2 rhomb. Each zinc ion is found in a distorted tetrahedral geometry coordinated by L2 in a bidentate, N,S
1
fashion. The H NMR spectrum of 2 provides strong evidence this structure is retained in solution. In comparison, reaction of L2
with FeCl2 leads to the monomeric complex (L2)2FeII (3), which has each ligand coordinated in the same bidentate, N,S bonding
mode. Interestingly, the methoxy groups of the two ligands in 3 are close enough to the iron center for a possible bonding inter-
action, suggesting an unexpected N2O coordination mode.
Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: Heteroscorpionate; Tripodal; Alkylthiolate; Imidazole; Iron; Zinc
1. Introduction
We have specifically targeted the metal active site of the
latter enzyme for synthetic modeling because we find it
especially intriguing from an inorganic point of view; it
utilizes an iron(II) center in vivo yet belongs to the
mononuclear zinc(II) enzyme family in terms of both
structure and function [8b]. In order to mimic the active
site of PDF, which has a His2Cys donor set coordinated
to an M(II) ion in all of its known derivatives
(M(II) ¼ Fe(II), Zn(II), Co(II), Ni(II)), we have made
efforts to synthesize ligands with mixed nitrogen/alkyl-
thiolate donor sets. Accordingly, we recently prepared a
simple linearly disposed N2S(thiolate) ligand called
PATH, which has led to the synthesis of a novel series of
A variety of metalloproteins rely on a single metal ion
cofactor bound to the protein by a combination of
HisxCysy amino acid ligands. Examples include zinc
proteins such as the matrix metalloproteinases [1], zinc
fingers [2], alcohol dehydrogenase [3], farnesyl transfer-
ase [4] and spinach carbonic anhydrase [5], as well as the
blue copper proteins [6] and the non-heme iron enzymes
nitrile hydratase [7] and peptide deformylase (PDF) [8].
* Corresponding author.
E-mail address: dpg@jhu.edu (D.P. Goldberg).
0277-5387/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2003.11.034

472
V.V. Karambelkar et al. / Polyhedron 23 (2004) 471–480
monomeric, pseudotetrahedral N2S(thiolate) complexes
((PATH)MIIL; M ¼ Zn, Co) [9–11]. These complexes
have served as structural models for the generalized
(His)2(Cys)MIIL active site of PDF, and recently we
have shown that a PATH-Zn(II) complex generated in
water acts as a functional model complex by catalyzing
the hydrolysis of an ester substrate [12].
2. Experimental
2.1. Materials and general methods
No special precautions were taken for the synthesis of
1. All synthetic procedures for complexes 2 and 3 were
carried out by using a dry-box or Schlenk techniques.
For the synthesis of complex 3, MeOH and CH2Cl2
were dried by refluxing over CaH2 for 18 h and then
distilled. Traces of oxygen were removed from these
solvents by 3 cycles of freeze–pump–thaw. Ligands L1H
[22] and L2H [23] were prepared as described in the lit-
erature. All other chemicals used in the experiments
were of analytical grade from commercial sources and
used without further purifications.
At the same time, we have also pursued the con-
struction of scorpionate ligands bearing an N2S(thio-
late) donor set. The tris(pyrazolyl)borate (Tp) family of
scorpionate ligands has proven remarkably useful for
the construction of many enzyme model complexes, es-
pecially models of the zinc enzyme family [13–16]. Their
incredible success has spurred the development of many
variations, including systems in which the pyrazolyl N
groups have been replaced by sulfur donors. Such sys-
tems include thioether [17] and methimazolyl-containing
[18–20] ligands. However, to our knowledge there are as
yet no boron-based scorpionate ligands bearing a sulfur
donor derived from an alkylthiolate group, most likely
because of the synthetic difficulties of installing an
alkylthiolate appendage. We have turned to a carbon-
centered framework as a synthetically feasible alterna-
tive to the boron-centered systems. Herein we describe
the coordination behavior of the scorpionate ligands
L1H and L2H (Fig. 1). Both of these ligands rely on a
central carbon atom in place of boron to link the im-
idazolyl and alkylthiolate donors into a tripodal
framework [21]. The ligand L1H was originally synthe-
sized by Bosnich over 20 years ago for use in the prep-
aration of models of blue copper proteins [22]. Still, little
is known about its coordination chemistry, and to our
knowledge no X-ray structures of complexes containing
L1 have been reported to date. The synthesis of the li-
gand L2H was previously communicated by some of us
[23].
2.1.1. Synthesis of [(L1)3Zn3Br2]Br (1)
Methanol (15 mL) was added to L1H (0.050 g, 0.178
mmol), NaOMe (0.011 g, 0.196 mmol) and ZnBr2 (0.044
g, 0.196 mmol) to yield a clear yellow solution. Over-
night a white precipitate formed and the solution was
filtered. This precipitate was dissolved by addition of 2
mL of hot CH3CN. Crystals suitable for X-ray diffrac-
tion were obtained by diffusion of Et2O into a CH3CN
solution of 1. Yield <2%.
2.1.2. Synthesis of [(L2)2Zn2Br2] (2)
To a solution of L2H (0.1 g, 0.17 mmol) in CH2Cl2
(10 mL) was added NaOH (0.008 g, 0.19 mmol) in
MeOH, and then the mixture was bubbled with argon
for 30 minutes to make sure that the solution was oxy-
gen-free. An amount of ZnBr2 (0.039 g, 0.17 mmol) was
dissolved in MeOH (5 mL) and bubbled with argon for
30 minutes. The methanolic solution of ZnBr2 was ad-
ded dropwise to the solution of deprotonated ligand and
the reaction mixture was stirred for 18 h under argon.
The complex 2 precipitated from the reaction mixture as
a white powder, which was filtered, washed with Et2O
and dried under vacuum. Yield: 0.07 g, 56%. Crystals
suitable for X-ray diffraction were obtained by diffusion
of Et2O into a CH2Cl2 solution of 2. Anal. Calc. for
We report here the first X-ray structure of any tran-
sition metal complex of the scorpionate ligand L1. In
addition, the first iron(II) complex of L2 has been syn-
thesized and crystallographically characterized, and a
new dinuclear zinc(II) complex of L2 has been charac-
terized by both 1H NMR spectroscopy and X-ray
crystallography, allowing for a comparison between
solution and solid-state structures. These different
structures reveal that the L1/L2 framework can adopt
three different coordination modes, including the ex-
pected tripodal N2S bonding mode.
C
74:5H71N8O2S2Zn2Br2Cl [2 ꢀ (CH2Cl2)0:5]: C, 59.63; H,
1
4.77; N, 7.47. Found: C, 59.40; H, 4.91; N, 7.29%. H
NMR (CDCl3, ppm): 7.56–7.11 (m, 40H, C6H5); 3.51 (s,
6H, N–CH3); 3.26 (s, 6H, N–CH3); 2.74 (s, 6H, O–CH3);
1.93 (s, 12H, C–CH3).
2.1.3. Synthesis of (L2)2Fe (3)
A 2 mL solution of NaOH (0.009 g, 0.23 mmol) in
MeOH was added to L2H (0.06 g, 0.1 mmol) dissolved
in CH2Cl2 (3 mL) and the mixture was stirred for 30
minutes. The solution of deprotonated ligand was then
added to a solution of FeCl2 (0.014 g, 0.11 mmol) in 3
mL MeOH, which was then stirred for 18 h to get a
green precipitate. This precipitate was dissolved by ad-
dition of 2 mL CH2Cl2 to the reaction mixture. To the
Ph
Ph
N
N
SH
N
N
SH
Ph
Ph
N
N
N
N
OMe
L1H
OMe
L2H
Fig. 1. Schematic presentation of the ligands.

V.V. Karambelkar et al. / Polyhedron 23 (2004) 471–480
473
resulting clear green solution, 3 mL of MeOH was ad-
2.3. X-ray crystallography
ded and colorless crystals were obtained by storing this
solution at )20 °C for four days. Yield 0.03 g, 45%. IR
of 3 ꢀ (CH3OH)2 (KBr, cmꢁ1): 3479, 3051, 2975, 2915,
1603, 1504, 1463, 1443, 1367, 1262, 1229, 1179, 1125,
1075, 1027, 1001, 966, 930, 775, 697, 671, 648, 569, 530.
For comparison the IR of L2H is as follows (KBr,
cmꢁ1): 3062, 2925, 1602, 1504, 1443, 1375, 1303, 1240,
1178, 1128, 1080, 1026, 964, 776, 698, 666, 627, 528.
Anal. Calc. for C76H78N8O4S2Fe [3 ꢀ (CH3OH)2]: C,
70.90; H, 6.11; N, 8.70. Found: C, 71.35; H, 5.79; N,
8.97%.
Diffraction intensity data were collected with a Bru-
ker Smart Apex CCD (1 and 3) and a SMART-1000
CCD (2) diffractometer with graphite monochromated
Mo Ka radiation. Crystal data, data collection, and
refinement parameters for all structures are given in
Table 1. The structures were solved using direct meth-
ods, completed by subsequent difference Fourier syn-
theses, and refined by full matrix least-squares
procedures on F2. SADABS absorption corrections were
applied to 1 and 3 (Tmin, Tmax ¼ 0.390, 0.699 (1) and
0.898, 0.924 (3)). In the case of 2, an analytical ab-
sorption correction was applied. Minimum and maxi-
mum transmission factors were 0.746 and 0.932,
respectively. All non-hydrogen atoms were refined with
anisotropic displacement coefficients, and hydrogen at-
oms were treated as idealized contributions in a riding
group model.
2.2. Physical measurements
Elemental analyses were performed at Atlantic Mi-
crolab, Inc. (Norcross, GA). 1H NMR spectrum in
CDCl3 was recorded on a Varian Unity Plus 400 spec-
trometer (400 MHz) at ambient probe temperature with
tetramethylsilane as the internal reference. IR spectra
were recorded on a Bruker Vector 22 IR spectrometer as
KBr disks.
In the crystal structure of 2 there is a disordered
CH2Cl2 molecule. It was refined isotropically with oc-
cupancy factors of 0.125 for four chlorine atoms, 0.25
Table 1
Crystal data, data collection, and refinement parameters for 1–3
Empirical formula
C41H60Br3N13O3S3Zn3(1 ꢀ CH3CN) C74:5H69Br2
C
80H94FeN8O8S2[3 ꢀ (CH3OH)6]
ClN8O2S2Zn2[2 ꢀ (CH2Cl2)0:5
]
Formula weight
Temperature (K)
ꢀ
Wavelength (A)
1315.4
173(2)
1498.56
153(2)
0.71073
1415.60
150(2)
0.71073
0.71073
monoclinic
P21=c
Crystal system
Space group
Unit cell dimensions
triclinic
ꢁ
triclinic
ꢁ
Pð1Þ
Pð1Þ
ꢀ
a (A)
11.7069(7)
10.784(2)
11.371(2)
29.455(4)
97.326(3)
97.564(2)
96.209(3)
3522.3(8)
2
11.0038(14)
16.164(2)
21.639(3)
78.800(2)
88.549(2)
89.858(2)
3774.3(8)
2
ꢀ
b (A)
22.4018(13)
20.4328(13)
90
ꢀ
c (A)
a (°)
b (°)
c (°)
104.721(1)
90
5182.7(5)
3
ꢀ
Volume (A )
Z
4
1.685
Density (calculated) (g cmꢁ3
Absorption coefficient (mmꢁ1
)
1.413
1.967
1534
1.246
0.315
1504
)
3.862
2656
F (000)
Crystal size (mm)
0.30 ꢂ 0.20 ꢂ 0.10
1.80–27.00
ꢁ14 6 h 6 14
ꢁ28 6 k 6 26
ꢁ15 6 l 6 26
23055
0.17 ꢂ 0.15 ꢂ 0.04
1.41–28.31
ꢁ13 6 h 6 13
ꢁ14 6 k 6 14
ꢁ37 6 l 6 39
32478
0.35 ꢂ 0.30 ꢂ 0.25
1.45–28.30
ꢁ12 6 h 6 14
ꢁ21 6 k 6 19
ꢁ28 6 l 6 26
22764
h Range for data collection (°)
Index ranges
Reflections collected
Independent reflections
Absorption correction
Refinement method
10738 [Rint ¼ 0.0467]
16280 [Rint ¼ 0.044]
16247 [Rint ¼ 0.0390]
SADABS
Analytical
SADABS
full-matrix least-squares on F 2
10738/0/595
1.004
full-matrix least-squares on F 2 full-matrix least-squares on F 2
Data/restraints/parameters
Goodness-of-fit on F 2
8660/0/839
1.60
16247/0/820
1.060
a
Final R indices ½I > 2rðIÞꢃ
R1 ¼ 0.0426 wR2 ¼ 0.1018
R1 ¼ 0.0723, wR2 ¼ 0.1158
0.96 and )0.89
R1 ¼ 0.041 wR2 ¼ 0.088
R1 ¼ 0.066, wR2 ¼ 0.097
0.91 and )0.57
R1 ¼ 0.1071 wR2 ¼ 0.3003
R1 ¼ 0.1428, wR2 ¼ 0.3186
2.58 and )0.76
R indices (all data)a
Largest difference peak and hole (e A
ꢁ3
ꢀ
)
n
.
o
1=2
a
P
P
P
P
2
2
2
R1 ¼ jjFoj ꢁ jFcjj= jFoj; wR2 ¼
½wðFo2 ꢁ Fc2Þ ꢃ
½wðFo2Þ ꢃ
; w ¼ 1=½s2ðFo2Þ þ ðaPÞ þ bPꢃ; P ¼ ½2Fc2 þ maxðFo; 0Þꢃ=3.

474
V.V. Karambelkar et al. / Polyhedron 23 (2004) 471–480
+
for two chlorine atoms and the occupancy factor of 0.5
for the carbon atom to give half a CH2Cl2 molecule per
dimer unit while the remaining non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were in-
cluded in idealized positions but not refined.
-
Br
N
N
N
N
N
MeO
OMe
N
Zn
N
N
SH
N
N
NaOMe
MeOH
ZnBr
+
2
S
S
N
N
OMe
In the crystal structure of 3, there are six MeOH
solvent molecules. Two of those MeOH molecules are
involved in weak hydrogen bonding with the thiolate
sulfur atoms. The other four highly disordered MeOH
molecules were treated as diffuse contributions with the
SQUEEZE program (A. Spek, Platon software package).
Corrections of the X-ray data by SQUEEZE found 159
eꢁ/unit cell which is close to the required value of 144
eꢁ/unit cell for four MeOH molecules. All samples of 3
examined diffracted weakly and diffusely, almost cer-
tainly a result of the loosely fixed solvent molecules,
resulting in a rather high final R factor.
Zn
1
Zn
L H
Br
S
Br
N
N
N
N
OMe
1
Scheme 1.
white precipitate is formed and isolated by filtration.
This solid has limited solubility, but can be redissolved
in hot CH3CN. From Et2O/CH3CN a very small
amount of 1 can be obtained as X-ray quality crystals.
Attempts to scale up the reaction or obtain subsequent
crops of 1 led to the formation of yellow oils or solids
which could not be re-crystallized to give more 1. Thus
the yield of 1 is extremely low, and sometimes no crys-
tals of 1 form, but the few crystals that were obtained by
this method were utilized for X-ray crystallography. The
low yield of 1 precluded us from obtaining other char-
acterization data on 1.
All software and sources of scattering factors for 1
and 3 are contained in the SHELXTL (5.10) program
package (G. Sheldrick, Bruker XRD, Madison, WI).
For 2 all calculations were performed using the TE
crystallographic software package of Molecular Struc-
ture Corporation (TE SAN for Windows version 1.05:
XSAN
X
Crystal Structure Analysis Package, Molecular Struc-
ture Corporation (1997–1998)).
3.2. Crystal structure of [(L1)3Zn3Br2]Br (1)
3. Results and discussion
An ORTEP view of 1 is shown in Fig. 2. Crystallo-
graphic data are summarized in Table 1. Selected bond
distances and angles are given in Table 2. Structural
characterization of 1 has revealed a trinuclear zinc
complex that crystallizes in the monoclinic space group
P21=c. The metal:ligand ratio is 1:1, but the overall
structure is much less symmetric than the simple formula
3.1. Synthesis of [(L1)3Zn3Br2]Br (1)
The heteroscorpionate N2S(thiolate) ligand L1H,
shown in Fig. 1, was originally prepared over 20 years
ago for use in the synthesis of ‘‘blue copper’’ protein
model complexes [22]. We recognized L1 to be an in-
triguing example of a tripodal ligand with an alkylthi-
olate donor, which also had the added advantage of
containing biomimetic imidazole donors. To our
knowledge the blue copper modeling work was the only
published account discussing L1, and there were as yet
no crystallographically characterized compounds con-
taining L1. As pointed out in the Introduction, hetero-
scorpionate ligands bearing an alkylthiolate donor are
extremely rare, and thus we were compelled to see if we
could develop the coordination chemistry of this system.
We reacted L1 with a Zn(II) source to determine its
coordination behavior with this ion, and obtained a
trinuclear zinc complex in which L1 displays two distinct
bonding modes.
For the synthesis of [(L1)3Zn3Br2]Br (1), the three
starting materials L1H, NaOMe, and ZnBr2 are dis-
solved together in MeOH, as shown in Scheme 1. The
ligand L1H is freely soluble in MeOH, unlike L2H, and
therefore the use of CH2Cl2 can be avoided, in contrast
to the synthesis of 2 and 3 (vide infra). Initially a clear
solution is obtained, but after 18 h a small amount of
Fig. 2. Thermal ellipsoid plot of the cation of 1 at 40% probability.
Hydrogen atoms are omitted for clarity.

V.V. Karambelkar et al. / Polyhedron 23 (2004) 471–480
475
Table 2
dral geometry. All of these Zn–N(im) distances are rea-
sonable compared to other octahedral zinc complexes
containing imidazole donors [24,25]. Interestingly, the
ꢀ
Bond lengths (A) and angles (°) for 1
Bond lengths
Zn(1)–S(1)
Zn(1)–S(2)
Zn(1)–N(1)
Zn(1)–N(3)
Zn(1)–N(5)
Zn(1)–N(7)
Zn(2)–Br(1)
Zn(2)–S(1)
Zn(2)–S(3)
Zn(2)–N(9)
Zn(3)–Br(2)
Zn(3)–S(2)
Zn(3)–S(3)
Zn(3)–N(11)
2.6697(11)
2.5110(12)
2.091(3)
ꢀ
Zn(1)–S distances of Zn(1)–S(1) ¼ 2.6697(11) A and
ꢀ
Zn(1)–S(2) ¼ 2.5110(12) A are quite long for a typical
Zn–S bond, and considerably longer than the distances
from the thiolate sulfur atoms to the respective tetrahe-
2.068(4)
2.055(3)
2.141(3)
ꢀ
dral zinc(II) ions: Zn(2)–S(1) 2.3115(12) A and Zn(3)–
ꢀ
S(2) 2.2843(11) A. This observation is counterintuitive to
2.4371(7)
2.3115(12)
2.3575(12)
2.026(3)
the expectation that the Zn–S bonds of the chelate ring
should be shorter than the bridging Zn–S bonds. How-
ever, this same trend is observed for two other trinuclear
zinc complexes, [(MBPA)4Zn3]I2 (MBPAH ¼ N-(2-mer-
captoisobutyl)-(picolyl)amine)[26] and [Zn{Zn(L3)2}2]
(ClO4)2 (L3 ¼ 2-[(2-pyridylmethyl)amino]ethanethiolate)
[27], both of which have 6-coordinate and 4-coordinate
zinc(II) ions bridged by thiolate ligands. In these com-
pounds the sulfur distance to the six-coordinate zinc ion
is significantly longer than the sulfur distance to the four-
2.3931(6)
2.2843(11)
2.3715(12)
2.034(4)
Bond angles
N(5)–Zn(1)–N(3)
N(5)–Zn(1)–N(1)
N(3)–Zn(1)–N(1)
N(5)–Zn(1)–N(7)
N(3)–Zn(1)–N(7)
N(1)–Zn(1)–N(7)
N(5)–Zn(1)–S(2)
N(3)–Zn(1)–S(2)
N(1)–Zn(1)–S(2)
N(7)–Zn(1)–S(2)
N(5)–Zn(1)–S(1)
N(3)–Zn(1)–S(1)
N(1)–Zn(1)–S(1)
N(7)–Zn(1)–S(1)
S(2)–Zn(1)–S(1)
N(9)–Zn(2)–S(1)
N(9)–Zn(2)–S(3)
S(1)–Zn(2)–S(3)
N(9)–Zn(2)–Br(1)
S(1)–Zn(2)–Br(1)
S(3)–Zn(2)–Br(1)
N(11)–Zn(3)–S(2)
N(11)–Zn(3)–S(3)
S(2)–Zn(3)–S(3)
N(11)–Zn(3)–Br(2)
S(2)–Zn(3)–Br(2)
S(3)–Zn(3)–Br(2)
99.28(14)
98.94(14)
84.15(14)
83.42(13)
93.26(14)
176.73(14)
88.92(10)
169.75(10)
100.75(10)
81.50(10)
175.56(11)
84.21(10)
84.11(10)
93.66(9)
coordinate zinc center ([(MBPA)4Zn3]I2: Znoctahedral
ꢀ
–
ꢀ
S ¼ 2.545(3) A, Zntetrahedral–S ¼ 2.365(3) A; [Zn{Zn
(L3)2}2] (ClO4)2: Znoctahedral–S ¼ 2.538(3))2.596(3) A,
ꢀ
ꢀ
Zntetrahedral–S ¼ 2.346(3))2.363(3) A). In both of the
latter complexes the thiolate donor is also part of a
chelate ring that includes the octahedral zinc ion. The
considerably longer Znoctahedral–S distances are likely a
consequence of the increase in coordination number
compared to a tetrahedral zinc site.
87.33(4)
The tetrahedral zinc centers Zn(2) and Zn(3) are not
only bridged by sulfur atoms to the octahedral zinc ion,
but are also connected to each other by the thiolate arm
of a third ligand. Each of these four-coordinate zinc ions
is also bound by a separate imidazole donor from the
same ligand. Thus the imidazolyl groups together with
the bridgehead carbon atom also participate in bridging
the dinuclear zinc fragment. This (im)-C-(im)-type
bridge has been observed previously for the related
N3 scorpionate ligand tris[2-(1-methylimidazolyl)meth-
oxymethane (timm), which has three imidazolyl donors
connected by a central C3 symmetric carbon atom. The
dimeric complexes of formula [Cu(timm)]2[BF4]2 [28]
and [Pd2Cl2(timm)2]Cl2 [29] have structures which show
the ligand bound to one metal ion through two imid-
azole rings while the third imidazole group coordinates
to the other metal ion of the dimer. Interestingly, the
unusual ‘‘1 + 2’’ assembly of an octahedral zinc ion
bridged to a dinuclear fragment of two tetrahedral zinc
ions has been seen before for another N2S(thiolate)
scorpionate ligand, but in this case the tetrahedral zinc
ions are bridged by a (PhO)2POꢁ2 ligand [21b]. The
fourth site of each distorted tetrahedral zinc center in
1 is occupied by a bromide ion. The Zn–N(im) dis-
tances for the four-coordinate zinc sites are 2.026(3)
108.76(10)
90.86(10)
123.06(4)
100.78(9)
108.29(4)
119.96(3)
107.90(10)
99.86(11)
109.06(4)
103.06(10)
121.46(4)
112.93(3)
implies. The ligands exhibit two distinct coordination
modes. One of the zinc centers, Zn(1), is bound by two
ligands each through the expected tripodal N2S coordi-
nation. The geometry at this zinc center is best described
as distorted octahedral with cis angles ranging from 83°
to 101° and trans angles ranging from 170° to 177°. The
two sulfur atoms are in a cis arrangement. The Zn(1)–
ꢀ
N(im) distances range from 2.055(3) to 2.141(3) A. In the
case of the longer Zn(1)–N(im) distances (Zn(1)–
ꢀ
ꢀ
N(7) ¼ 2.141(3) A and Zn(1)–N(1) ¼ 2.091(3) A), the
imidazoles are trans to one another, while the imidazoles
ꢀ
with shorter bond distances (Zn(1)–N(5) ¼ 2.055(3) A
ꢀ
and Zn(1)–N(3) ¼ 2.068(4) A) are trans to the sulfur at-
ꢀ
oms. From these distances it is reasonable to conclude
that N(7) and N(1) lie on axial positions while N(5) and
N(3) lie on equatorial positions in the distorted octahe-
and 2.034(4) A. These Zn–N lengths are slightly shorter
than those found for the octahedral zinc site, as to be
expected from the lower coordination number of the

476
V.V. Karambelkar et al. / Polyhedron 23 (2004) 471–480
tetrahedral centers. The angles around the two four-
coordinate zinc ions are distinct, ranging from 90.9°–
123.1° for Zn(2) to 99.9°–121.5° for Zn(3), making them
both rather distorted from ideal tetrahedral geometry.
CH3CN), the rearranged Zn–N2S2 product was not ob-
tained, but instead the dinuclear complexes [(L2)2Zn2
(CH3CN)2(l-OH)](X)3 (X ¼ BFꢁ4 , CF3SO3ꢁ) were iso-
lated and crystallographically characterized [23]. During
the formation of these dimers an undesirable intramo-
lecular disulfide linkage had formed. These synthetic
observations are summarized in Scheme 2.
3.3. Coordination behavior of L2
Although L1 exhibited the desired tripodal bonding
mode in complex 1, it also demonstrated a propensity to
form both 2:1 octahedral complexes as well as thiolate-
bridged oligomers. Given these results, we sought
methods for synthetically modifying L1 to incorporate
steric bulk near the metal center during tripodal coor-
dination, thereby favoring monomeric (N2S)ZnL com-
plexes. Modification of the imidazole groups was the
most synthetically feasible method of achieving this goal
given the stepwise synthesis of L1H [22], and thus we
targeted the diphenyl-substituted analog L2H, shown in
Fig. 1. The synthesis of L2H has been communicated
previously, and in the same work L2 was shown to react
with Zn(CH3)2 to give the neutral, monomeric zinc alkyl
complex (L2)ZnCH3, as seen in Scheme 2 [23]. The syn-
thesis of this complex demonstrated that L2 coordinates
in the desired tridentate facial mode to give a tetrahedral
(N2S)ZnL complex. This complex was prepared as a
possible starting material for the formation of (L2)ZnOH
by reaction with water. However, upon reaction of
(L2)ZnCH3 with water in toluene, a redistribution of li-
gand:metal stoichiometry took place along with a rear-
rangement of the N2S bonding mode of L2 to a bidentate
N,S mode, resulting in the formation of a four-coordi-
nate N2S2–Zn complex. Alternatively, if (L2)ZnCH3 was
reacted under acidic conditions (HBF4 or CF3SO3H in
In continuing our efforts to determine the coordina-
tion behavior of L2, we have reacted L2 with the metal
salts ZnBr2 and FeCl2, as shown in Scheme 2. The
syntheses of 2 and 3 were carried out under the strict
exclusion of air to prevent the possible oxidation of the
thiolate ligand to a disulfide and in the case of 3, to
prevent oxidation of the Fe(II) product. The ligand L2H
is freely soluble in CH2Cl2 but only sparingly soluble
in MeOH. Thus, L2H was first dissolved in CH2Cl2
and then deprotonated by the addition of NaOH in
MeOH in the synthesis of both 2 and 3. For 2, a solu-
tion of ZnBr2 in MeOH was added dropwise to the
solution of L2 whereas for 3 the addition was reversed
and ligand was added to metal. Complex 2 precipitates
from the reaction mixture as a white powder. After
workup this powder was shown to be pure 2 by its
1H NMR spectrum. Complex 2 is freely soluble in
CH2Cl2, and recrystallization of 2 from vapor diffusion
of Et2O into CH2Cl2 gave the crystals that were used
for the structural determination. In the case of 3, a
precipitate also forms in the reaction mixture, but this
precipitate is not isolated and instead is simply recrys-
tallized directly by the addition of CH2Cl2 to the reac-
tion mixture to redissolve the solid. X-ray quality
crystals of 3 are obtained after storage of this mixture at
)20 °C.
Scheme 2. Conditions: (a) 1.4 Zn(CH3)2, toluene, rt, 3 h. (b) 30 H2O, toluene, rt, 6 d. (c) 2 HBF4 (aq.) or 2 CF3SO3H, CH3CN, rt, 24 h. (d) 1.1
NaOH, MeOH; 1 ZnBr2, MeOH; 18 h. (e) 2.2 NaOH, MeOH; 1.1 FeCl2, MeOH; 18 h.

V.V. Karambelkar et al. / Polyhedron 23 (2004) 471–480
477
3.4. Crystal structure of [(L2)2Zn2Br2] (2)
zinc atom. A bromide ion completes the coordination
sphere of each zinc atom. Each ligand has one imidazole
moiety that is free and not coordinated to a zinc atom,
and thus each ligand binds in a bidentate N,S fashion
through one nitrogen atom and one l-sulfur atom. As
pointed out in Section 3.3, evidence for this type of
bonding mode has been obtained previously for L2. In
comparison, the dimeric complexes of the N3 scorpio-
nate ligand timm, [Cu(timm)]2[BF4]2 [28] and
[Pd2Cl2(timm)2]Cl2 [29], described previously in Section
3.2, have structures which show a bidentate N,N
bonding mode. Thus, the bidentate bonding mode of
these carbon-centered scorpionate ligands is not unique
to mixed N/S systems, and is seen for the homoleptic N3
systems as well.
An ORTEP view of 2 is shown in Fig. 3. Crystallo-
graphic data are summarized in Table 1. Selected bond
distances and angles are given in Table 3. Complex 2
ꢁ
crystallizes in the triclinic Pð1Þ space group. The dimer
does not sit on an inversion center, and therefore all
bonds and angles are crystallographically unique. A
CH2Cl2 molecule of 1/2 occupancy was located per di-
meric unit. Both zinc centers have a distorted tetrahedral
geometry and are bridged by two sulfur atoms from two
ligand molecules to give a Zn2S2 rhomb. In addition to
the bridging thiolates, a nitrogen atom from one of the
N-methyl-4,5-diphenylimidazole rings coordinates each
The Zn–S distances of 2 range between 2.3400(13)
ꢀ
and 2.4034(13) A, which is within the range of other
Zn2(l-SR)2 complexes [30–34], such as the homoleptic
complex [(Et4N)2Zn2(SEt)6] [33] where d(Zn–S) ¼
ꢀ
2.394(1) and 2.456(1) A, or [Zn2(CH3)2({SC6H4(R)-CH
(Me)NMe2)-2})2, where the zinc ions are bound by an
N,S chelate ring and d(Zn–S) ranges between 2.389(1)
ꢀ
and 2.453(2) A [34]. The carbon atoms attached to each
thiolate bridge are on opposite sides of the Zn2S2 plane,
as are the bromine atoms, and thus 2 can be classified in
the anti form [33]. The Zn(1)–N(4) and Zn(2)–N(8)
ꢀ
distances of 2.061(3) and 2.050(4) A, respectively, are
close to other Zntetrahedral–Nimidazole distances [35–37].
ꢀ
The Zn–Br distances of 2.3573(9) and 2.3706(9) A are
also unexceptional [9,35,38]. The angles around the
zinc centers range from 82.59(4)°–129.23(4)°, which
indicates a significant distortion from tetrahedral ge-
ometry. The dihedral angle between the two planes
defined by Zn(1)S2 and Zn(2)S2 is 19.3°, giving a slight
V shape to the Zn2S2 unit. In the literature this dihedral
angle is found to vary from 0° for [(Et4N)2Zn2(SEt)6]
[33] to 29.18° for [(Me)Zn(SBut)(cis-C6H15N3)]2 where
(cis-C6H15N3) ¼ 1,3,5-trimethylhexahydro-1,3,5-triazine
[39]. The reasons for the variance in this fold angle are
not clear. The distance between the two zinc atoms in 2
Fig. 3. Thermal ellipsoid plot of 2 at 40% probability. Hydrogen atoms
are omitted for clarity.
Table 3
ꢀ
Bond lengths (A) and angles (°) for 2
Bond lengths
Br(1)–Zn(1)
Br(2)–Zn(2)
Zn(1)–S(1)
Zn(1)–S(2)
Zn(1)–N(4)
Zn(2)–S(1)
Zn(2)–S(2)
Zn(2)–N(8)
2.3706(9)
2.3573(9)
2.3400(13)
2.3807(13)
2.061(3)
ꢀ
2.4034(13)
2.3787(13)
2.050(4)
is 3.141 A which is also in the normal range for Zn2(l-
SR)2 complexes [33].
1
3.5. H NMR spectrum of [(L2)2Zn2Br2] (2)
Bond angles
Br(1)–Zn(1)–S(1)
Br(1)–Zn(1)–S(2)
Br(1)–Zn(1)–N(4)
S(1)–Zn(1)–S(2)
S(1)–Zn(1)–N(4)
S(2)–Zn(1)–N(4)
Br(2)–Zn(2)–S(1)
Br(2)–Zn(2)–S(2)
Br(2)–Zn(2)–N(8)
S(1)–Zn(2)–S(2)
S(1)–Zn(2)–N(8)
S(2)–Zn(2)–N(8)
Zn(1)–S(1)–Zn2
Zn(1)–S(2)–Zn(2)
114.89(4)
122.77(4)
111.00(9)
94.84(4)
The 1H NMR spectrum for 2 in CDCl3 provides good
evidence that the dinuclear structure obtained from X-
ray crystallography remains intact in solution. The res-
onances for the phenyl protons of 2 appear as a complex
multiplet in the region d 7.11–7.56 ppm, and do not
provide much structural information. However, for the
N–CH3 substituents, two distinct singlets are observed
at 3.51 and 3.26 ppm, consistent with the two different
types of imidazole groups in the dimer. We assign the
upfield peak at 3.26 ppm to the non-coordinated imid-
azole group, whereas the peak at 3.51 ppm is assigned to
99.19(10)
110.77(10)
113.12(4)
129.23(4)
117.78(10)
93.25(4)
104.66(10)
94.06(10)
82.92(4)
82.59(4)

478
V.V. Karambelkar et al. / Polyhedron 23 (2004) 471–480
Table 4
the imidazole group bound to the zinc(II) ion. The latter
assignment is based on the 1H NMR spectra for
(L2)ZnCH3 and [(L2)2Zn2(CH3CN)2(l-OH)](X)3 (X ¼
BFꢁ4 , CF3SOꢁ3 ), both of which have only the coordi-
nated type of imidazolyl group (Scheme 2) and exhibit
downfield-shifted N–CH3 resonances of d 3.87 and 3.95
ppm, respectively [23]. A similar pattern to that of 2 was
seen for (L2)2Zn, which exhibited two peaks at 2.95 and
3.87 ppm, presumed to be the free and bound imidazole
groups, respectively [23]. Curiously, the free ligand itself,
L2H, gives rise to a relatively downfield N–CH3 reso-
nance of 3.53 ppm. However, the precedent for a
coordinated N–CH3 peak being significantly downfield-
shifted is clear from the former data. The singlet at 2.74
ppm in the spectrum of 2 is assigned to the O–CH3
resonance, which is close to the same peak found for
(L2)2Zn (O–CH3: d 2.57 ppm), yet quite distinct from
that of (L2)ZnCH3 (3.57 ppm) or [(L2)Zn–(l-OH)–
Zn(L2)]3þ (3.56 ppm). Finally, the gem-dimethyl pro-
tons for 2 appear as a singlet at d 1.93 ppm, which is
downfield of the same resonance found for the free li-
gand L2H (1.60 ppm) or for (L2)ZnCH3 (1.64 ppm), in
which L2 is bound in a tripodal mode. Thus the 1H
NMR spectrum of 2 appears to be quite characteristic of
the bonding mode of L2 (N,S versus N,N,S for
(L2)ZnCH3 and N,N for [(L2)Zn–(l-OH)–Zn(L2)]3þ).
ꢀ
Bond lengths (A) and angles (°) for 3
Bond lengths
Fe(1)–N(1)
Fe(1)–N(5)
Fe(1)–S(2)
Fe(1)–S(1)
2.157(4)
2.157(4)
2.3363(14)
2.3480(14)
Bond angles
N(1)–Fe(1)–N(5)
N(1)–Fe(1)–S(2)
N(5)–Fe(1)–S(2)
N(1)–Fe(1)–S(1)
N(5)–Fe(1)–S(1)
S(2)–Fe(1)–S(1)
102.77(15)
107.65(11)
99.65(11)
98.68(10)
109.78(11)
134.72(6)
The hydrogen atoms on the MeOH molecules could not
be located by X-ray diffraction, but the distances be-
tween the oxygen and sulfur atoms, O(1S)–S(1) ¼ 3.183
ꢀ
ꢀ
A and O(2S)–S(2) ¼ 3.167 A, indicate an H-bonding
interaction [40]. Despite these putative hydrogen bonds,
ꢀ
the Fe–S distances of Fe–S(1) ¼ 2.3480(14) A and Fe–
ꢀ
S(2) ¼ 2.3363(14) A, are quite normal. Interestingly,
both the methoxy oxygen atoms of the ligands are
oriented towards the iron(II) ion, with distances of
ꢀ
Fe–O(1) ¼ 2.462 and Fe–O(2) ¼ 2.473 A. These dis-
tances are comparable to other Fe–O(ether) distances
for compounds in which an Fe–O(ether) bond has been
assigned. For example, a mean Fe–O distance of
3.6. Crystal structure of (L2)2Fe (3)
2.40 A is found for the crown ether complex {[Fe([12]-
ꢀ
ꢀ
crown-4)2](PF6)2}[41], and (Fe–O)ave ¼ 2.386(3) A for
An ORTEP view of 3 is shown in Fig. 4. Crystallo-
graphic data are summarized in Table 1. Selected bond
distances and angles are given in Table 4. This complex
the calixarene complex [Fe(II–H)](FeCl4)2 [42] [(II–
H) ¼ 5,11,17,23-tetra-tert-butyl-24-hydroxy-26,27,28-tris
(diethyl-carbamoylmethoxy)calix-[4]arene]. These data
suggest that there is a bonding interaction between the
Fe2þ ion and the methoxy oxygen atoms, and the ligand
has adopted the unexpected tripodal N2O bonding
mode. However, the angles do not support an assign-
ment of octahedral geometry in preference to a distorted
tetrahedral geometry, and therefore we conclude that
this O(ether)–FeII interaction is weak at best. The Fe–N
ꢁ
crystallizes in the triclinic space group Pð1Þ. The ferrous
ion adopts a distorted tetrahedral geometry and is co-
ordinated by two ligand molecules through one imid-
azole and one thiolate donor, which makes the complex
neutral in charge. As observed in the case of 2, the other
imidazole donor from each ligand molecule remains
uncoordinated, demonstrating the preference for an N,S
binding mode over an N2S mode for L2. The two MeOH
molecules found in the lattice appear to be involved in
hydrogen bonding with separate thiolate sulfur atoms.
ꢀ
distances of 2.157(4) A are slightly longer than the bond
II
ꢀ
length of 2.095(4) A for the related Fe (N2S2) complex
Fe(S-2,6-i-Pr2C6H3)2(1-MeIm)2 [43]. The X-ray struc-
tures of complexes 2 and 3 together with the synthesis
and spectroscopic characterization of (L2)2Zn (Scheme
2) make it clear that L2 has a particular preference for
coordination via a bidentate N,S bonding mode.
4. Conclusions
We have succeeded in obtaining the first X-ray
structure of a transition metal complex of the N2S(thi-
olate) ligand L1. This trinuclear zinc complex 1 showed
that L1 does bind in the designed tripodal mode, but
also exhibits another multibridging bonding mode in-
volving bridging imidazole and thiolate groups. The
Fig. 4. Thermal ellipsoid plot of 3 at 40% probability. Hydrogen atoms
are omitted for clarity.

V.V. Karambelkar et al. / Polyhedron 23 (2004) 471–480
479
ligand L2 is a modified version of L1 bearing phenyl
substituents, and exhibits a third type of bidentate N,S
bonding mode evidenced by the X-ray structures of the
dinuclear zinc(II) complex 2 and the mononuclear ir-
on(II) complex 3. This bonding mode is likely favored in
the case of L2 because of a combination of strain around
the central carbon atom and the weakened donor power
of the imidazole groups caused by the electron-with-
drawing phenyl substituents. In addition, yet another
type of bonding mode may be operative in 3; there is
some evidence for a weak bond between the methoxy
oxygen atom of L2 and Fe(II), giving rise to tripodal
NSO(ether)-type coordination. Clearly, further modifi-
cations to these heteroscorpionate N2S(thiolate) ligands
are necessary before they can rival the tris(pyraz-
olyl)borates in terms of their ability to enforce a bio-
logically relevant monomeric, tetrahedral geometry at
the metal center. The results obtained here suggest that
replacing the phenyl substituents with electron-donating
groups (e.g. alkyl groups), as well as increasing the steric
encumbrance near the sulfur atom to discourage thiolate
bridges from forming, should favor complexes of the
desired nuclearity and geometry.
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The authors wish to acknowledge the National In-
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