Syntheses, characterization, density functional theory calculations, and activity of tridentate SNS zinc pincer complexes based on bis-imidazole or bis-triazole precursors

Article abstract of DOI:10.1016/j.ica.2011.12.047

A series of tridentate pincer ligands, each possessing two sulfur- and one nitrogen-donor functionalities (SNS), based on bis-imidazole or bis-triazole salts were metallated with ZnCl₂ to give new tridentate SNS pincer zinc(II) complexes [(SNS)ZnCl]⁺. The zinc complexes serve as models for the zinc active site in liver alcohol dehydrogenase (LADH) and were characterized with single crystal X-ray diffraction, ¹H, ¹³C, and HSQC NMR spectroscopies, electrospray mass spectrometry, and elemental analysis. The zinc complexes feature SNS donor atoms and pseudotetrahedral geometry about the zinc center, as is seen for liver alcohol dehydrogenase. The bond lengths and bond angles of the zinc complexes correlate well to those in horse LADH. The SNS ligand precursors were characterized with ¹H, ¹³C, and HSQC NMR spectroscopies, elemental analysis, and cyclic voltammetry, and were found to be redox active. Gaussian calculations were performed and agree with the experimentally observed oxidation potentials for the pincer ligand precursors. The zinc complexes were screened for the reduction of electron-poor aldehydes in the presence of a hydrogen donor, 1-benzyl-1,4-dihydronicotinamide (BNAH), and it was determined that they enhance the reduction of electron-poor aldehydes. The SNS zinc pincer complexes with bis-triazole ligand precursors exhibit higher activity for the reduction of 4-nitrobenzaldehyde than do SNS zinc pincer complexes with bis-imidazole ligand precursors. Quantitative stoichiometric conversion was seen for the reduction of pyridine-2-carboxaldehyde via SNS zinc pincer complexes with either bis-imidazole or bis-triazole ligand precursors.

Full text of DOI:10.1016/j.ica.2011.12.047

Inorganica Chimica Acta 387 (2012) 25–36
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Syntheses, characterization, density functional theory calculations, and activity
of tridentate SNS zinc pincer complexes based on bis-imidazole or
bis-triazole precursors
b
c
d
a
John R. Miecznikowski a, , Wayne Lo , Matthew A. Lynn , Swapan Jain , Lauren C. Keilich ,
Nathan F. Kloczko a, Brianne E. O’Loughlin a, Amanda P. DiMarzio a, Kathleen M. Foley a, George P. Lisi a,
Daniel J. Kwiecien a, Elizabeth E. Butrick a, Erin Powers a, Raed Al-Abbasee d
⇑
a Department of Chemistry and Biochemistry, Fairfield University, 1073 North Benson Road, Fairfield, CT 06824, USA
b Department of Chemistry, Boston College, 140 Commonwealth Avenue, Chestnut Hill, MA 02467, USA
c Department of Science and Mathematics, National Technical Institute for the Deaf, Rochester Institute of Technology, 52 Lomb Memorial Drive, Rochester, NY 14623, USA
d Department of Chemistry, Bard College, P.O. Box 5000, Annandale-on-Hudson, NY 12504-5000, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 August 2011
Received in revised form 22 December 2011
Accepted 23 December 2011
Available online 29 December 2011
A series of tridentate pincer ligands, each possessing two sulfur- and one nitrogen-donor functionalities
(SNS), based on bis-imidazole or bis-triazole salts were metallated with ZnCl2 to give new tridentate SNS
pincer zinc(II) complexes [(SNS)ZnCl]+. The zinc complexes serve as models for the zinc active site in liver
alcohol dehydrogenase (LADH) and were characterized with single crystal X-ray diffraction, 1H, 13C, and
HSQC NMR spectroscopies, electrospray mass spectrometry, and elemental analysis. The zinc complexes
feature SNS donor atoms and pseudotetrahedral geometry about the zinc center, as is seen for liver alco-
hol dehydrogenase. The bond lengths and bond angles of the zinc complexes correlate well to those in
horse LADH. The SNS ligand precursors were characterized with 1H, 13C, and HSQC NMR spectroscopies,
elemental analysis, and cyclic voltammetry, and were found to be redox active. Gaussian calculations
were performed and agree with the experimentally observed oxidation potentials for the pincer ligand
precursors. The zinc complexes were screened for the reduction of electron-poor aldehydes in the pres-
ence of a hydrogen donor, 1-benzyl-1,4-dihydronicotinamide (BNAH), and it was determined that they
enhance the reduction of electron-poor aldehydes. The SNS zinc pincer complexes with bis-triazole
ligand precursors exhibit higher activity for the reduction of 4-nitrobenzaldehyde than do SNS zinc pincer
complexes with bis-imidazole ligand precursors. Quantitative stoichiometric conversion was seen for the
reduction of pyridine-2-carboxaldehyde via SNS zinc pincer complexes with either bis-imidazole or bis-
triazole ligand precursors.
Keywords:
SNS pincer ligand
Mononuclear Zn complexes
X-ray crystallography
Cyclic voltammetry
Density functional theory calculations
Aldehyde reductions
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
and also catalyzes the reverse reaction, which is the reduction of
a ketone or an aldehyde to an alcohol [1,3]. The crystal structure
The synthesis and characterization of complexes that attempt to
mimic natural catalytic behavior have furthered the understanding
of the enzymatic activity of metalloenzyme sites [1]. Model com-
plexes are low molecular mass systems that replicate the metallo-
enzyme in terms of structures, ligand donor atoms, and oxidation
states [2]. Nature is used as a model for the design of highly active
and efficient catalysts, and is also the inspiration behind the syn-
thesis of each model complex in order to investigate structures
and functions of enzymes.
of horse LADH has been solved [4]. The resting enzyme includes
one zinc(II) metal center, which is pseudo-tetrahedrally ligated
with a labile water molecule and so-called ‘‘SNS’’ ligand environ-
ment containing one N-histidine and two S-cysteine side chains.
The nitrogen and sulfur atoms are provided by the histidine and
cysteine residues of a single polypeptide chain [5]. Several LADH
models have been previously reported with the same electron do-
nor atoms as the metalloenzyme [6–13]. However, reactivity data
was either not reported in some cases [14a–e] or it was determined
for zinc LADH model complexes possessing donor atoms that are
different than those within the enzyme’s active site [15].
Liver alcohol dehydrogenase (LADH) is a zinc metalloenzyme
that catalyzes the oxidation of alcohols to aldehydes and ketones,
In a continuous effort to understand the catalytic activity of
metalloenzymes, we have chosen to model the structure and reac-
tivity of the zinc active site in LADH using a new and unique family
⇑
Corresponding author. Tel.: +1 203 254 4000ꢀ2125; fax: +1 203 254 4034.
E-mail address: jmiecznikowski@fairfield.edu (J.R. Miecznikowski).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.12.047

26
J.R. Miecznikowski et al. / Inorganica Chimica Acta 387 (2012) 25–36
of robust SNS pincer ligands. First published in 1976, tridentate
pincer ligands offer several advantages over monodentate ligands
[16]. Primarily, tridentate pincer ligands offer favored metallation
due to a less negative delta entropy of formation in comparison
to monodentate ligands, which makes their metal complexes more
stable [16]. Secondly, tridentate pincer ligands have been shown to
inhibit dimerization of the metal complexes as a whole, a process
that could possibly slow or inhibit catalytic activity [16,17].
Thirdly, the conformational and electronic properties of tridentate
ligands can be tuned by using different starting materials in their
syntheses. Depending largely on the electron count of the metal
center, tridentate pincer ligands can coordinate the metal atom
in either a facial or meridional fashion [16,17].
Pincer ligands have been utilized successfully in organometallic
chemistry to prepare highly catalytically active and robust com-
plexes [17]. The pincer ligand is an excellent system for use in
modeling biological activity since the N-atom of pyridine, is sp2-
hybridized like that of histidine, and the thioimidazolyl S-donors
have been reported by Parkin and Vahrenkamp to model thiol-de-
rived ligands in bio-inspired zinc chemistry [18,15].
To the best of our knowledge, such tridentate pincer ligand sys-
tems have rarely been used in bio-inspired modeling chemistry.
Our group has already had success in preparing tridentate SNS li-
gands that incorporate thione-substituted based imidazole func-
tionalities as well as zinc model complexes that contain these
ligands [19]. The tridentate ligands used in these systems were rel-
atively rigid as the pyridine and the imidazoles were directly
bound to each other. Further, in an effort to understand the pres-
ence and sterics of ancillary alkyl groups on the ability of this mod-
el system to reduce aldehydes, the ligands were prepared with
imidazolyl rings having R groups of various sizes as shown in Fig. 1.
In our current work, we seek to further our understanding of the
catalytic nature of the zinc complexes by tuning the structure of
tridentate SNS-pincer ligands. Previously, we prepared somewhat
rigid ligand systems through the use of 2,6-dibromopyridine as a
ligand precursor. Here, we use the starting material 2,6-(dibro-
momethyl)pyridine to introduce a methylene linker into the pincer
ligand, thereby allowing us to examine the effect of ligand flexibil-
ity on substrate binding toward the goal of better understanding
the catalytic activity of LADH. In a similar vein, Crabtree and co-
workers have shown that such a modification in Pd CNC-pincer
complexes leads to improved catalytic activity in carbon-carbon
bond formation reactions [20]. We expect to fine-tune further
the electronic environment imparted by our ligand set through
the use of imidazole- and triazole-based precursors in the prepara-
tion of the pincer ligand precursor as has been shown previously
by Crabtree and Miecznikowski [21]. Furthermore, sulfur-substi-
tuted triazoline systems(1,2,4-triazolinethiones) have been pre-
pared by others, so we have adapted their synthetic protocol for
use in our current work [22].
in which the ligand set is modified through the placement of a
methylene linker between its pyridinyl and imidazolyl or triazolyl
segments.
2. Experimental
2.1. General procedures
All reagents used are commercially available and were used as
received. Isopropyl imidazole, neopentyl imidazole, 1-isopropyl
triazole, 1-n-butyl triazole, 1-neopentyl triazole, 2,6-bis{[(n-bu-
tyl)-N0-methylene]imidazole}pyridine bromide, were prepared as
reported previously [23–26]. BNA+ and BNAH were prepared as re-
ported previously [27].
NMR spectra were recorded at 25 °C on
a BrukerAvance
300 MHz NMR spectrometer. Spectra were referred to the solvent
residual peak. Electrospray mass spectrometry was performed on
a Micromass ZQ instrument or a Varian LC–MS instrument using
nitrogen as the drying and nebulizing gas. Cyclic voltammetry
experiments were performed using a Cypress Electroanalytical Sys-
tem with a silver wire reference electrode, a glassy carbon working
electrode, and a platinum counter electrode. The supporting elec-
trolyte for the cyclic voltammetry experiments was tetra-N-butyl-
ammonium tetrafluoroborate. The ferrocenium/ferrocene couple
was used as an internal reference; reduction potential values were
corrected by assigning the ferrocenium/ferrocene couple to 0.40 V
versus SCE. When an inert atmosphere was needed, a M-Braun in-
ert atmosphere glove box and standard Schlenk techniques were
used with thoroughly degassed solvents. IR spectra were collected
using a Thermo Nicolet AVATAR 380-FT-IR with a SMART SPECU-
LATR reflectance adaptor. C, H, N elemental analyses were per-
formed by Atlantic Microlab Inc. (Norcross, GA).
2.2. Crystallographic analyses
Crystals of 1, 3, 5b, and 6 were mounted on a glass fiber or loop
and placed in a ꢁ80 °C nitrogen stream on a Bruker diffractometer
equipped with a Smart CCD at Boston College (Chestnut Hill, MA).
Crystallographic data were collected using graphite monochromat-
ed 0.71073 Å Mo K
a radiation and integrated and corrected for
absorption using the Bruker SAINTPLUS software package [28]. The
structures were solved using direct methods and refined using
least-square methods on F-squared [29]. All other pertinent crys-
tallographic details such as h, k, l ranges, 2h ranges, and R-factors
can be found in Table 1.
2.3. Reactivity
In a typical reaction, 0.1 mmol of 4-nitrobenzaldehyde (or pyr-
idine 2-carboxaldehyde), 0.2 mmol of BNAH, and 0.1 mmol of the
zinc complex or 0.2 mmol ZnCl2 were dissolved in 3 mL of CDCl3.
The reaction was heated at reflux. Aliquots of the reaction were ta-
ken at certain times and analyzed using 1H NMR spectroscopy. All
data are averages of at least two runs.
We therefore present here the syntheses, spectroscopic and
electrochemical characterization, computational study, and activ-
ity screening of various tridentate SNS-pincer complexes of zinc
2.4. Gaussian calculations
GAUSSIAN 03 was used to perform single-point calculations and
DFT geometry optimizations using the B3LYP hybrid functional
with 6-31G⁄ basis sets as provided with the software [30]. Calcula-
tions were performed on the ligands alone with R = Me in all cases.
The structures of the ligands were first optimized in the gas phase
as neutral and as cationic species under the Cs and C2 point groups.
Frequency analysis was performed on the optimized structures to
determine whether or not they represented true minima. Small
Fig. 1. Zinc-based SNS model complexes previously prepared by Miecznikowski
et al. [20].

J.R. Miecznikowski et al. / Inorganica Chimica Acta 387 (2012) 25–36
27
Table 1
Crystal and structure refinement data for 1, 3, 5b, and 6.
R = iPr (1)
R = nBu (3)
R = Np (5b)
R = nBu (6)
Formula
C
42H55Cl6N12S4Zn3
C42H56Cl6N10S4Zn3
1238.02
193(2)
0.71073
monoclinic
P2(1)/c
20.2931(8)
21.6238(9)
13.2096(6)
90
107.064(2)
90
5541.4(4)
8
1.484
C21H31N7S2
445.65
193(2)
0.71073
triclinic
C38H54Cl6N14S4Zn4
1380.33
193(2)
0.71073
monoclinic
P2(1)/c
23.7884(13)
11.0040(6)
21.1704(11)
90
99.770(3)
90
5461.4(5)
8
1.679
FW (g/mol)
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
1265.02
193(2)
0.71073
triclinic
ꢀ
P2(1)/c
P1
7.0010(19)
19.310(5)
21.789(6)
70.872(8)
80.971(5)
79.577(5)
2722.0(13)
4
1.543
1.802
1294
0.20 ꢀ 0.10 ꢀ 0.10
0.99–28.46
43409/13420
0.0780
20.655(3)
11.2349(17)
10.6710(16)
90
103.218(3)
90
2410.6(6)
4
1.228
0.243
952
0.20 ꢀ 0.20 ꢀ 0.08
1.01–28.26
35218/5914
0.0659
b (Å)
c (Å)
a
(o)
b (o)
(o)
c
Volume (Å)3
Z
rcalc (gcm3)
Absorbance (mmꢁ1
F(000)
)
1.768
2536
0.10 ꢀ 0.08 ꢀ 0.08
1.87–22.50
31379/7243
0.0713
2.324
2800
0.08 ꢀ 0.06 ꢀ 0.05
1.74–26.31
48222/11044
0.0799
Crystal Size (mm3)
Theta range (°)
Reflections/Unique reflections
Rint
Absorbance correction
Maximum/minimum
Ref. method
none
none
none
none
0.8403/0.7145
full matrix least squares on F2
13420/0/614
0.868
0.8715/0.8430
full matrix least squares on F2
7243/0/562
1.018
0.9809/0.9531
full matrix least squares on F2
5914/0/277
0.627
0.8926/0.8359
full matrix least squares on F2
11044/0/639
0.971
Data/restrictions/parameters
Goodness-of-fit (GOF) on F2
R1 indices (I > 2
r
)
0.0450
0.0913
0.0488
0.0473
wR2
0.0911
0.751 and ꢁ0.765
0.2159
2.055 and ꢁ2.142
0.1357
0.440 and ꢁ0.256
0.0822
0.869 and ꢁ0.673
Peak/hole (eÅꢁ3
)
imaginary frequencies on the order of ꢂꢁ10 cmꢁ1 that are not
indicative of transition-state structures were found only for the
neutral structures of the thiotriazole systems having Cs symmetry.
Single-point SCRF calculations using DMSO via the CPCM solvent
model were then performed using the ‘‘radii = uff’’ and ‘‘nosymm-
cav’’ directives. Oxidation potentials were computed by finding the
difference in the total free energies in solution for the neutral and
13C {1H} NMR (DMSO-d6, 75 MHz), d 153.65 (imidazole NCHN),
138.88 (pyridine CH), 135.50 (pyridine Cipso), 123.30 (imidazole
CH), 122.22 (pyridine CH), 120.45 (imidazole CH), 52.69 (CH2),
52.30 (isopropyl CH), 22.32 (isopropyl CH3).
2.5.1.2. Synthesis of 2,6-bis(N-neopentyl-N0-methyleneimidazole)pyr-
idine bromide (C23H35N5Br2) [2a]. Yield: 3.40 g (quantitative). Anal.
Calc. for C23H35Br2N5ꢃH2O (559.38): C, 49.38; H, 6.67; N, 12.52.
Found: C, 49.07; H, 6.67; N, 12.34%.
cationic species. These
DG values were then referenced to the
absolute SCE potential in DMSO by subtracting 3.83 V (the estab-
lished correction to SHE in DMSO) and 0.241 V (the difference be-
tween SHE and SCE) [31].
1H NMR (DMSO-d6, 300 MHz) d 9.33 (s, 2H, imidazole, CH), 7.99
(m, 1H, pyridine CH), 7.77 (d (3J = 1.5 Hz), 4H, imidazole CH), 7.46
(d (3J = 7.8 Hz), 2H, pyridine CH), 5.59 (s, 4H, CH2), 4.01 (s, 4H, neo-
pentyl CH2), 0.92 (s, 18H, neopentyl CH3).13C {1H} NMR (DMSO-d6,
75 MHz), d 153.71 (pyridine Cipso), 138.90 (pyridine CH), 137.41
(imidazole CH), 123.77 (imidazole CH), 122.66 (imidazole CH),
122.02 (pyridine CH), 59.45 (neopentyl CH2), 52.71 (CH2), 31.93
(neopentylC(CH3)3), 26.63 (neopentyl CH3).
2.5. Syntheses
2.5.1. Syntheses of ligand precursor bromide salts
As an example, a detailed description for the synthesis of 1a is
given. Detailed descriptions for the preparation of all the ligand
precursor bromide salts are given in supporting information.
2.5.1.3. Synthesis of 2,6-bis(N-isopropyl-N0-methylenetriazole)pyri-
2.5.1.1. Synthesis of 2,6-bis(N-isopropyl-N0-methyleneimidazole)pyri-
dine bromide (C19H27N5Br2) [1a]. In a 100 mL round-bottom flask,
1.53 g (5.78 mmol) 2,6-bis(bromomethyl)pyridine was combined
with 1.99 g (18.0 mmol) 1-isopropyl-1,3-imidazole and dissolved
in 10 mL of 1,4-dioxane. The solution was stirred at reflux over-
night during which time, a brown precipitate formed. The solid
was collected with a Buchner funnel and washed with diethyl
ether (3ꢀ, 30 mL each). This solid was then taken up in methanol
(10 mL) and precipitated in diethyl ether. This solution was vac-
uum filtered and the solid product was washed with diethyl ether
(3ꢀ, 30 mL each). Yield: 2.42 g (86.3%). Anal. Calc. for C19H27Br2N5
(485.26): C, 47.03; H, 5.61; N, 14.43. Found: C, 46.95; H, 5.74; N,
14.19%.
dine bromide (C17H25N7Br2) [4a]. Yield: 1.87 g (94%). Anal. Calc. for
C17H25Br2N7 (487.24): C, 41.91; H, 5.17; N, 20.12. Found: C,
42.01; H, 5.19; N, 19.93%.
1H NMR (DMSO-d6, 300 MHz) d 10.37 (s, 2H, triazole CH), 9.24
(s, 2H, triazole CH), 8.01 (m, 1H, pyridine CH), 7.62 (d
(3J = 7.8 Hz), 2H, pyridine CH), 5.69 (s, 4H, CH2), 4.87 (m, 2H, iso-
propyl CH), 1.56 (d (3J = 6.0 Hz), 12H, isopropyl CH3).13C {1H}
NMR (DMSO-d6, 75 MHz), d 152.28 (triazole NCHN), 145.17 (tria-
zole CH), 141.79 (pyridine CH), 138.84 (pyridine Cipso), 122.51 (pyr-
idine CH), 55.16 (CH2), 50.69 (isopropyl CH), 21.15 (isopropyl CH3).
2.5.1.4. Synthesis of 2,6-bis(N-neopentyl-N0-methylenetriazole)pyri-
dine bromide (C21H33N7Br2) [5a]. Yield: 2.05 g (quantitative). Anal.
Calc. for C21H33N7Br2ꢃ0.5 H2O (543.34): C, 45.66; H, 6.20; N,
17.75. Found: C, 45.48; H, 6.18; N, 17.69%.
1H NMR (DMSO-d6, 300 MHz) d 9.47 (s, 2H, imidazole, CH), 7.98
(m, 3H, pyridine CH, and imidazole CH), 7.78 (d (3J = 1.8 Hz), 2H,
imidazole CH), 7.49 (d (3J = 7.5 Hz), 2H, pyridine CH), 5.56 (s, 4H,
CH2), 4.71 (m, 2H, isopropyl CH), 1.49 (d (3J = 6.9 Hz), 12H, CH3).
1H NMR (DMSO-d6, 300 MHz) d 10.43 (s, 2H, triazole, CH), 9.34
(s, 2H, triazole, CH), 8.04 (m, 1H, pyridine CH), 7.63 (d (3J = 7.8 Hz),

28
J.R. Miecznikowski et al. / Inorganica Chimica Acta 387 (2012) 25–36
2H, pyridine CH), 5.76 (s, 4H, CH2), 4.31 (s, 4H, CH2), 0.96 (s, 18H,
CH3).13C{1H} NMR (DMSO-d6, 75 MHz), d 152.37 (pyridine Cipso),
145.01 (triazole CH), 143.61 (triazole CH), 138.94 (pyridine CH),
122.52 (pyridine CH), 61.92 (CH2), 50.89 (CH2), 32.15 (neopen-
tylC(CH3)3), 26.72 (neopentyl C(CH3)3).
1H NMR (DMSO-d6, 300 MHz) d 7.73 (m, 1H, pyridine, CH), 7.21
(d (3J = 2.4 Hz), 2H, imidazole CH), 7.19 (d (3J = 2.4 Hz), 2H, imidaz-
ole CH), 6.91 (d (3J = 7.8 Hz), 2H, pyridine CH), 5.29 (s, 4H, CH2 lin-
ker), 3.98 (t (3J = 7.2 Hz), 4H, butyl CH2), 1.69 (m, 4H, butyl CH2),
1.28 (m, 4H, butyl CH2), 0.94 (t (3J = 7.2 Hz), 6H, butyl CH3), 13C
{1H} NMR (DMSO-d6, 75 MHz), d 161.78 (C@S), 155.83 (pyridine
2.5.1.5. Synthesis of 2,6-bis[N-(n-butyl)-N0-methylenetriazole]pyridine
bromide (C21H31N5Br2) [6a]. Yield: 1.11 g (quantitative). Anal. Calc.
for C19H29Br2N7ꢃ2H2OꢃCH3OH (515.29): C, 42.22, H, 6.58, N, 16.41.
Found: C, 41.82; H, 6.09; N, 15.83%.
Cipso), 137.78 (pyridine CH), 119.97 (pyridine CH), 117.93 (imidaz-
ole CH), 117.64 (imidazole CH), 51.35 (CH2 linker), 46.65 (butyl
CH2), 30.93 (butyl CH2), 22.04 (butyl CH2), 13.54 (butyl CH3). Se-
lected IR bands (reflectance): v
max/cmꢁ1 1129 (C@S).
1H NMR (DMSO-d6, 300 MHz) d 10.30 (s, 2H, triazole, CH), 9.23
(s, 2H, triazole CH), 8.03 (m, 1H, pyridine CH), 7.61 (d (3J = 7.5 Hz),
2H, pyridine CH), 5.70 (s, 4H, CH2), 4.44 (t (3J = 7.2 Hz), 4H, n-butyl
CH2), 1.87 (m, 4H, n-butyl CH2), 1.33 (m, 4H, n-butyl CH2), 0.947
(m, 6H, n-butyl CH3). 13C {1H} NMR (DMSO-d6, 75 MHz), d 152.30
(pyridine CH), 145.31 (pyridine CH), 142.99 (triazole CH), 142.53
(triazole CH), 122.42 (pyridine Cipso), 51.40 (CH2), 48.57 (n-butyl
CH2), 30.01 (n-butyl CH2), 18.75 (n-butyl CH2), 13.28 (n-butyl CH3).
2.5.2.4. Synthesis of 2,6-bis(N-isopropyl-N0-methylenetriazole-2-thi-
one)pyridine (C17H23N7S2) [4b]. Yield: 0.31 g (78.%). Anal. Calc. for
C17H23N7S2ꢃ0.5H2O (389.54): C, 51.23; H, 6.07; N, 24.60. Found:
C, 51.36; H, 5.92; N, 24.43%.
1H NMR (DMSO-d6, 300 MHz) d 8.46 (s, 2H, triazole CH), 7.82 (t
(3J = 7.8 Hz), 1H, pyrdine CH), 7.26 (d (3J = 7.8 Hz), 2H, pyridine
CH), 5.28 (s, 4H, CH2), 4.92 (m, 2H, isopropyl CH), 1.35 (d
(3J = 6.6 Hz), 12H, isopropyl CH3).13C{1H} NMR (DMSO-d6,
75 MHz), d 164.02 (C@S), 154.12 (pyridine Cipso), 141.33 (triazole
CH), 137.95 (pyridine CH), 121.10 (pyridine CH), 49.76 (isopropyl
CH), 48.85 (CH2), 20.50 (isopropyl CH3). Selected IR bands (reflec-
2.5.2. Syntheses of bis-thione ligand precursors
As an example, a detailed description for the synthesis of 1b is
given. Detailed descriptions for the preparation of all the bis-thione
ligand precursors are given in supporting information.
tance): v
max/cmꢁ1 1158 (C@S).
2.5.2.1. Synthesis of 2,6-bis(N-isopropyl-N0-methyleneimidazole-2-
thione)pyridine (C19H25N5S2) [1b]. In a round-bottom flask, 0.24 g
(0.50 mmol) of 1a was added to 0.12 g (1.5 mmol) of sodium ace-
tate. Acetonitrile (20 mL) was added to the solid mixture. The reac-
tion mixture was heated at reflux for a half hour during which, the
solids dissolved in the acetonitrile. To this solution, 0.34 g
(11 mmol) of S8 was added and the mixture was heated at reflux
for seven days. Afterwards, the undissolved solid was filtered out
of the mother liquor. The solvent was evaporated off under re-
duced pressure. The resulting product was oily. The product was
dissolved in dichloromethane and was filtered to remove excess
sodium acetate. The solvent was removed under reduced pressure.
Yield: 0.13 g (70. %). Anal. Calc. C19H25N5S2ꢃ0.5H2O (387.57): C,
57.54; H, 6.61; N, 17.66. Found: C, 57.32; H, 6.37; N, 17.45%.
1H NMR (DMSO-d6, 300 MHz) d 7.73 (t (3J = 7.8 Hz), 1H, pyri-
dine, CH), 7.31 (d (3J = 2.7 Hz), 2H, imidazole CH), 7.22 (d
(3J = 2.4 Hz), 2H, imidazole CH), 6.91 (d (3J = 7.8 Hz), 2H, pyridine
CH), 5.30 (s, 4H, CH2), 4.89 (m, 2H, isopropyl CH), 1.32 (d
2.5.2.5. Synthesis of 2,6-bis(N-neopentyl-N0-methylenetriazole-2-thi-
one)pyridine (C21H31N7S2) [5b]. Yield 0.42 g (82%). Off-white crys-
tals of 5b that were suitable for X-ray diffraction were grown by
allowing diethyl ether to slowly diffuse into a solution of 5b in ace-
tonitrile. Anal. Calc. for C21H31N7S2ꢃ0.5 CH2Cl2: C, 52.90; H, 6.61; N,
20.09. Found: C, 52.54, H, 6.45, N, 20.40%.
1H NMR (DMSO-d6, 300 MHz) d 8.50 (s, 2H, triazole CH), 7.82
(m, 1H, pyridine CH), 7.21 (d (3J = 7.8 Hz), 2H, pyridine CH), 5.29
(s, 4H, CH2), 3.99 (s, 4H, neopentyl CH2), 0.91 (s, 18H, neopentyl
CH3). 13C{1H} NMR (DMSO-d6, 75 MHz), d 167.66 (C@S), 155.08
(pyridine
Cipso), 142.00 (triazole CH), 138.84 (pyridine CH),
121.69 (pyridine CH), 59.23 (neopentyl CH2 linker), 50.21 (CH2 lin-
ker) 34.51 (C(CH3)3), 28.64 (C(CH3)3). Selected IR bands (reflec-
tance): v
max/cmꢁ1 1144 (C@S).
2.5.2.6. Synthesis of 2,6-bis[N-(n-butyl)-N0-methylenetriazole-2-thi-
one]pyridine (C19H29N7S2Br2) [6b]. Yield 0.17 g (46 %). Anal. Calc. for
C
19H27N7S2ꢃ0.5 H2O (417.59): C, 53.49; H, 6.62; N, 22.98. Found: C,
(3J = 6.9 Hz), 12H, CH3). 13C{1H} NMR (DMSO-d6, 75 MHz),
160.92 (C@S), 155.80 (pyridine ipso), 137.80 (pyridine CH),
d
53.45, H, 6.55, N, 22.13%.
C
1H NMR (DMSO-d6, 300 MHz) d 8.47 (s, 2H, triazole CH), 7.80
(m, 1H, pyrdine CH), 7.24 (d (3J = 2.4 Hz), 2H, pyridine CH), 7.23
(d (3J = 2.7 Hz), 2H, pyridine CH), 5.27 (s, 4H, CH2), 4.11 (t,
(3J = 4.8 Hz) 4H, n-butyl CH2), 1.74 (m, 4H, n-butyl CH2), 1.28 (m,
4H, n-butyl CH2), 0.900 (t (3J = 4.8 Hz), 6H, n-butyl CH3). 13C{1H}
NMR (DMSO-d6, 75 MHz), d 165.10 (C@S), 154.09 (pyridine Cipso),
141.35 (triazole CH), 137.95 (pyridine CH), 120.94 (pyridine CH),
49.05 (CH2 linker), 47.99 (n-butyl CH2), 29.56 (n-butyl CH2),
19.04 (n-butyl CH2), 13.44 (n-butyl CH3). Selected IR bands (reflec-
120.08 (pyridine CH), 118.39 (imidazole CH), 114.03 (imidazole
CH), 51.15 (CH2), 48.41 (isopropyl CH), 21.14 (isopropyl CH3). Se-
lected IR bands (reflectance): v
max/cmꢁ1 1129 (C@S).
2.5.2.2. Synthesis of 2,6-bis(N-neopentyl-N0-methyleneimidazole-2-
thione)pyridine (C23H33N5S2) [2b]. Yield: 0.26 g (60%). Anal. Calc.
C
23H33N5S2ꢃ3H2O (443.67): C, 58.69; H, 7.71; N, 14.88. Found: C,
58.86; H, 7.09; N, 14.79%.
1H NMR (DMSO-d6, 300 MHz) d 7.30 (m, 1H, pyridine, CH), 7.22
(d (3J = 2.1 Hz), 2H, imidazole CH), 7.16 (d (3J = 2.1 Hz), 2H, imidaz-
ole CH), 6.86 (d 3J = 7.8 Hz, 2H, pyridine CH), 5.32 (s, 4H, CH2 lin-
ker), 3.92 (s, 4H, neopentyl CH2), 0.96 (s, 18H, neopentyl CH3),
13C{1H} NMR (DMSO-d6, 75 MHz), d 163.54 (C@S), 155.93 (pyridine
tance): v
max/cmꢁ1 1149 (C@S).
2.5.3. Syntheses of zinc complexes
As an example, a detailed description for the synthesis of 1 is gi-
ven. Detailed descriptions for the preparation of all the zinc com-
plexes are given in supporting information.
Cipso), 137.74 (pyridine CH), 119.68 (pyridine CH), 118.81 (imidaz-
ole CH), 117.55 (imidazole CH), 57.03 (neopentyl CH2), 51.63 (CH2
linker), 33.63 (neopentyl C(CCH3)3), 27.82 (neopentyl CH3). Se-
2.5.3.1. Synthesis of chloro-(g3-S,S,N)-[2,6-bis(N-isopropyl-N0-methyl-
eneimidazole-2-thione)pyridine]zinc(II)tetrachlorozincate [C42H55Cl2N12
S4Zn2][ZnCl4] [1]. In a round-bottom flask, 0.13 g (0.34 mmol) of 1b
was combined with 0.098 g (0.72 mmol) of ZnCl2 and dissolved in
10 mL of dichloromethane. The solution mixture was refluxed for
20 h. After this time, the solvent was removed under reduced
pressure. Yield: 0.22 g (quantitative). White single crystals for
lected IR bands (reflectance): v
max/cmꢁ1 1128 (C@S).
2.5.2.3. Synthesis of 2,6-bis[N-(n-butyl)-N0-methyleneimidazole-2-thi-
one]pyridine (C21H29N5S2) [3b]. Yield: 0.10 g (23.%). Anal. Calc. for
C
21H29N5S2ꢃH2O (415.62): C, 58.17; H, 7.21; N, 16.15. Found: C,
57.60; H, 6.66; N, 15.84%.

J.R. Miecznikowski et al. / Inorganica Chimica Acta 387 (2012) 25–36
29
X-ray diffraction were grown by a slow vapor diffusion of diethyl
ether into an acetonitrile solution containing the zinc complex.
Anal. Calc. for C19H27Cl4N5OS2Zn2 (678.15): C, 33.65; H, 4.01; N,
10.33. Found: C, 33.58, H, 3.95, N, 10.54%.
an acetonitrile solution containing the zinc complex. Anal. Calc. for
C
17H25Cl4N7OS2Zn2ꢃ1 H2O (680.13): C, 29.25; H, 3.90; N, 14.04.
Found: C, 29.04 H, 3.64, N, 13.59%.
1H NMR (DMSO-d6, 300 MHz) d 8.44 (s, 2H, triazole CH), 7.81
(m, 1H, pyridine CH), 7.25 (d (3J = 7.8 Hz), 2H, pyridine CH), 5.27
(s, 4H, CH2), 4.91 (m, 2H, isopropyl CH), 1.33 (d (3J = 6.6 Hz), 12H,
isopropyl CH3).13C{1H} NMR (DMSO-d6, 75 MHz), d 164.02 (C@S),
154.11 (pyridine Cipso), 141.34 (triazole CH), 137.96 (pyridine
CH), 121.11 (pyridine CH), 49.77 (isopropyl CH), 48.85 (CH2),
20.50 (isopropyl CH3).
1H NMR (DMSO-d6, 300 MHz) d 7.73 (m, 1H, pyridine CH), 7.31
(d (3J = 2.7 Hz), 2H, imidazole CH), 7.22 (d (3J = 2.4 Hz), 2H, imidaz-
ole CH), 6.90 (d (3J = 7.5 Hz), 2H, pyridine CH), 5.30 (s, 4H, CH2),
4.88 (m, 2H, isopropyl CH) 1.30 (d (3J = 6.6 Hz), 12H, isopropyl
CH3). 13C{1H} NMR (DMSO-d6, 75 MHz), d 160.88 (C@S), 155.83
(pyridine
Cipso), 137.85 (pyridine CH), 120.10 (pyridine CH),
118.45 (imidazole CH), 114.09 (imidazole CH), 51.18 (CH2), 48.45
(isopropyl CH), 21.30 (isopropyl CH3).
1H NMR (MeOH-d4, 300 MHz) d 8.24 (s, 2H, triazole CH), 7.78 (t
(3J = 7.5 Hz), 1H, pyridine CH), 7.34 (d (3J = 7.5 Hz), 2H, pyridine
CH), 5.32 (s, 4H, CH2), 5.05 (m, 2H, isopropyl CH), 1.38 (d
(3J = 4.8 Hz), 12H, isopropyl CH3). 13C{1H} NMR (MeOH-d4,
75 MHz) d 165.55 (C@S), 155.48 (Cipso), 142.29 (CH triazole),
139.15 (CH pyridine), 122.98 (CH pyridine), 51.88 (CH isopropyl),
50.47 (CH2), 20.99 (CH3 isopropyl).
1H NMR (MeOH-d4, 300 MHz) d 7.91 (t (3J = 7.8 Hz), 1H, pyridine
CH), 7.45 (d (3J = 7.8 Hz), 2H, pyridine CH), 7.32 (m, 4H, imidazole
CH), 5.54 (s, 4H, CH2), 4.97 (m, 2H, isopropyl CH) 1.40 (d
(3J = 4.2 Hz), 12H, isopropyl CH3).
2.5.3.2. Synthesis of chloro-(g3-S,S,N)-[2,6-bis(N-neopentyl-N0-meth-
yleneimidazole-2-thione)pyridine]zinc(II)aquatrichlorozincate [C23H33
N5S2ClZn][ZnCl3(OH2)] [2]. Yield: 0.30 g (quantitative). The white
product was purified dissolving the complex into acetonitrile
and allowing diethyl ether vapor to diffuse in to the acetonitrile
solution slowly. Anal. Calc. for C23H35Cl4N5OS2Zn2ꢃ2H2O (734.26):
C, 35.86; H, 5.10; N, 9.09. Found: C, 35.60, H, 4.89, N, 8.99%.
1H NMR (DMSO-d6, 300 MHz) d 7.73 (m, 1H, pyridine CH), 7.21
(d (3J = 2.4 Hz), 2H, imidazole CH), 7.15 (d (3J = 2.4 Hz), 2H, imidaz-
ole CH), 6.84 (d (3J = 7.8 Hz), 2H, pyridine CH), 5.31 (s, 4H, CH2),
3.91 (s, 4H, CH2), 0.95 (s, 18H, CH3).13C{1H} NMR (DMSO-d6,
75 MHz), d 163.70 (C@S), 156.01 (pyridine Cipso), 137.70 (pyridine
CH), 119.85 (pyridine CH), 118.83 (imidazole CH), 117.57 (imidaz-
ole CH), 57.04 (neopentyl CH2), 51.64 (CH2), 33.64 (neopen-
tylC(CH3)3), 27.83 (neopentyl CH3).
2.5.3.5. Synthesis of chloro-(g3-S,S,N)-[2,6-bis(N-neopentyl-N0-meth-
ylenetriazole-2-thione)pyridine]zinc(II)trichlorozincate ([C21H31N7S2ZnCl]
[ZnCl3]) [5]. Yield: 0.25 g (quantitative). The product was purified
by precipitation by dissolving the product in methanol and allow-
ing for a slow vapor diffusion of diethyl ether into the methanol
solution. Anal. Calc. for C21H31Cl4N7OS2Zn2ꢃCH3OH (718.22): C,
35.22; H, 4.70; N, 13.07. Found: C, 35.90 H, 5.20, N, 13.73%.
1H NMR (DMSO-d6, 300 MHz) d 8.49 (s, 1H, triazole CH), 7.81
(m, 1H, pyridine CH), 7.19 (d (3J = 7.8 Hz), 2H, pyridine CH), 5.28
(s, 4H, CH2), 3.98 (s, 4H, neopentyl CH2), 0.97 (s, 18H, neopentyl
CH3).13C{1H} NMR (DMSO-d6, 75 MHz), d 166.75 (C@S), 154.18
(pyridine
Cipso), 141.10 (triazole CH), 137.95 (pyridine CH),
120.80 (pyridine CH), 58.33 (neopentyl CH2), 49.31 (CH2), 33.61
(C(CH3)3), 27.75 (CH3).
1H NMR (MeOH-d4, 300 MHz) d 7.90 (t (3J = 7.8 Hz), 1H, pyridine
CH), 7.42 (d (3J = 7.5 Hz), 2H, pyridine CH), 7.26 (AB doublet
(3J = 1.8 Hz), 2H, imidazole CH), 7.18 (AB doublet (3J = 1.8 Hz), 2H,
imidazole CH), 5.60 (s, 4H, CH2), 4.01 (s, 4H, CH2), 1.01 (s, 18H,
CH3).
1H NMR (MeOH-d4, 300 MHz) d 8.24 (s, 1H, triazole CH), 7.78 (t
(3J = 7.5 Hz), 1H, pyridine CH), 7.32 (d (3J = 7.8 Hz), 2H, pyridine
CH), 5.33 (s, 4H, CH2), 4.05 (s, 4H, neopentyl CH2), 1.02 (s, 18H,
neopentyl CH3).13C{1H} NMR (MeOH-d4, 75 MHz),
d 168.46
(C@S), 155.51 (pyridine Cipso), 141.93 (triazole CH), 139.12 (pyri-
dine CH), 122.77 (pyridine CH), 60.27 (neopentyl CH2), 50.89
(CH2), 34.90 (C(CH3)3), 28.44 (CH3).
2.5.3.3. Synthesis of chloro-(g3-S,S,N)-{2,6-bis[N-(n-butyl)-N0-methyl-
eneimidazole-2-thione]pyridine}zinc(II)]tetrachlorozincate
[C42H56
Cl2N10S4Zn2][ZnCl4] [3]. Yield: 0.11 g (quantitative). Off-white sin-
gle crystals for X-ray diffraction were grown by a slow vapor diffu-
sion of diethyl ether into a methanol solution containing the zinc
complex. Anal. Calc. for C42H58Cl6N10S4Zn3ꢃ2H2O (1240.09): C,
39.53; H, 4.90; N, 10.98. Found: C, 39.60, H, 4.80, N, 10.85%.
1H NMR (DMSO-d6, 300 MHz) d 7.72 (m, 1H, pyridine CH), 7.22
(d (3J = 2.4 Hz), 2H, imidazole CH), 7.19 (d (3J = 2.4 Hz), 2H, imidaz-
ole CH) 6.89 (d (3J = 7.8 Hz), 2H, pyridine CH), 5.28 (s, 4H, CH2),
3.97 (t (3J = 7.5 Hz), 4H, n-butyl CH2), 1.68 (m, 4H, n-butyl CH2),
1.29 (m, 4H, n-butyl CH2), 0.903 (t (3J = 7.2 Hz), 6H, n-butyl CH3).
2.5.3.6. Synthesis of chloro-(g3-S,S,N)-{2,6-bis[(N-(n-butyl)-N0-meth-
ylenetriazole-2-thione]pyridine}zinc(II)bis(l-chlorodichlorozincate)
([C38H54N14S4Zn2Cl2][Zn2Cl6] [6]. Yield: 0.28 g (quantitative). Crys-
tals suitable for X-ray diffraction analysis were grown by allowing
diethyl ether vapor to slowly diffuse into a solution of 6 in metha-
nol. Anal. Calc. for
C
19H29Cl4N7OS2Zn2ꢃH2OꢃCH3OH(708.18): C,
31.68; H, 4.65; N, 12.93. Found: C, 31.51, H, 4.06, N, 12.57%.
1H NMR (DMSO-d6, 300 MHz) d 8.45 (s, 2H, triazole CH), 7.81
(m, 1H, pyridine CH), 7.23 (d (3J = 7.8 Hz), 2H, pyridine CH), 5.26
(s, 4H, CH2), 4.09 (t (3J = 6.9 Hz), 4H, n-butyl CH2), 1.73 (m, 4H,
n-butyl CH2), 1.28 (m, 4H, n-butyl CH2), 0.90 (t (3J = 7.2 Hz),
6H, n-butyl CH3). 13C{1H} NMR (DMSO-d6, 75 MHz), d 165.11
13C{1H} NMR (DMSO-d6, 75 MHz),
(pyridine ipso), 137.77 (pyridine CH), 119.96 (pyridine CH),
d 161.79 (C@S), 155.82
C
117.95 (imidazole CH), 117.65 (imidazole CH), 51.35 (CH2), 46.65
(n-butyl CH2), 30.37 (n-butyl CH2), 19.11 (n-butyl CH2), 13.55
(n-butyl CH3).
(C@S), 154.09 (pyridine
Cipso), 141.36 (pyridine CH), 137.95
1H NMR (MeOH-d4 300 MHz) d 7.90 (m, 1H, pyridine CH), 7.47
(d (3J = 7.8 Hz), 2H, pyridine CH), 7.28 (AB doublet (3J = 2.1 Hz), 2H,
imidazole CH) 7.23 (AB doublet (3J = 2.4 Hz), 2H, imidazole CH),
5.54 (s, 4H, CH2), 4.11 (t (3J = 7.5 Hz), 4H, n-butyl CH2), 1.79 (m,
4H, n-butyl CH2), 1.38 (m, 4H, n-butyl CH2), 0.98 (t (3J = 7.5 Hz),
6H, n-butyl CH3).
(triazole CH), 120.95 (pyridine CH), 49.06 (CH2), 48.00 (n-butyl
CH2), 29.57 (n-butyl CH2), 19.05 (n-butyl CH2), 13.45 (n-butyl
CH3).
1H NMR (MeOH-d4, 300 MHz) d 8.22 (s, 2H, triazole CH), 7.78
(m, 1H, pyridine CH), 7.33 (d (3J = 7.8 Hz), 2H, pyridine CH), 5.31
(s, 4H, CH2), 4.18 (t (3J = 7.2 Hz), 4H, n-butyl CH2), 1.82 (m, 4H, n-
butyl CH2), 1.36 (m, 4H, n-butyl CH2), 0.97 (t (3J = 7.5 Hz), 6H, n-bu-
tyl CH3). 13C{1H} NMR (MeOH-d4 75 MHz), d 166.68 (C@S), 155.40
(pyridine Cipso), 142.29 (pyridine CH), 139.13 (triazole CH), 122.79
(pyridine CH), 50.62 (CH2), 49.96 (n-butyl CH2), 31.16 (n-butyl
CH2), 20.68 (n-butyl CH2), 13.97 (n-butyl CH3).
2.5.3.4. Synthesis of chloro-(g3-S,S,N)-[2,6-bis(N-isopropyl-N0-methyl-
enetriazole-2-thione)pyridine]zinc(II)aquatrichlorozincate [C17H23N7S2
ZnCl][ZnCl3(H2O)] [4]. Yield: 0.24 g (quantitative). The white prod-
uct was precipitated by a slow vapor diffusion of diethyl ether into

30
J.R. Miecznikowski et al. / Inorganica Chimica Acta 387 (2012) 25–36
The 13C NMR of 1b–3b and of 4b–6b, show resonances at d
3. Results and discussion
3.1. Syntheses and spectroscopy
ꢂ162 ppm and d ꢂ166 ppm, respectively, that are consistent with
C@S formation [17]. The ligand precursors 1a–6a contain a reso-
nance at d ꢂ11 ppm in their 1H NMR spectra indicative of an acidic
C–H proton whereas this feature is absent in the spectra of com-
pounds 1b–6b in which the proton has been replaced by a sulfur
atom.
Attenuated total reflectance IR spectra were collected for 1b–
6b. The C@S stretch occurs at 1128–1129 cmꢁ1 for the bis-imidaz-
ole bis-thione precursors (1b–3b) and at 1144–1158 cmꢁ1 for the
bis-triazolebis-thionesystems, 4b–6b. This data is consistent with
the triazole compounds having stronger C@S bonds.
The syntheses of the tridentate SNS ligand precursors and zinc
complexes 1–6 were accomplished following Scheme 1. Different
R groups were employed because Crabtree has reported that mod-
ification of such substituents affects the solubility and catalytic
activity of the metal complexes [24].The alkyl imidazoles or alkyl
triazoles were prepared either by following known routes or were
commercially available [23–26]. These compounds react with 2,6-
bis(bromomethyl)pyridine in 1,4-dioxane to form ligand precursor
salts, 1a–6a, that are soluble in DMSO, methanol, acetonitrile and
water [19,27].
Compounds 1a–6a react with a mild base, sodium acetate, and
elemental sulfur in refluxing acetonitrile to form bis-thione ligand
precursors 1b–6b [23]. As determined by NMR spectroscopy, com-
pounds 1b–6b can be purified by filtering a dichloromethane solu-
tion containing this compound through alumina. The bis-thione
ligand precursors are soluble in DMSO, dichloromethane, chloro-
form, acetone, acetonitrile, and methanol. Off-white crystals of
5b that were suitable for X-ray diffraction were grown by allowing
diethyl ether to slowly diffuse into a solution of 5b in acetonitrile.
The bis-thione ligand precursors subsequently react with ZnCl2
in refluxing CH2Cl2 to afford zinc complexes 1–6. The driving force
for the metallation is the formation of zinc complexes 1–6, which
are sparingly soluble in CH2Cl2. Off-white crystals that were suit-
able for X-ray diffraction were grown by allowing diethyl ether va-
por (1, 3, and 6) to slowly diffuse into an acetonitrile (1) or
methanol (3 or 6) solution containing the zinc complex. All of
the reactions could be carried out in air, and proceeded in yields
at or above 63%. The zinc complexes 1–6 are soluble in DMSO, ace-
tonitrile, methanol, and water and are sparingly soluble in dichlo-
romethane and chloroform. Complexes 4–6 were more soluble in
MeOH-d4 than were 1–3.
Complexes 1–6 were analyzed with 1H, 13C, and HSQC NMR
spectroscopy in DMSO-d6. The 1H and 13C NMR of the zinc com-
plexes are identical to the respective ligand precursors 1b–6b, with
very little shift in the resonances. This was expected since no hydro-
gen or carbon atoms are displaced or added upon metallation with
zinc(II) chloride. In addition, the 1H NMR spectra of complexes 1–6
were acquired in a less polar and weakly coordinating solvent,
MeOH-d4, to verify that the SNS pincer ligands are not displaced
from the coordination sphere of zinc. Electrospray mass spectrom-
etry data, described below, also verifies this result. Zinc complexes
4–6 are more soluble than zinc complexes 1–3 in MeOH-d4. We
were able to prepare NMR samples of complexes 4–6 that were con-
centrated enough in MeOH-d4 so that we could acquire a 13C NMR
spectrum of these complexes. In the 1H NMR spectra that were ac-
quired in MeOH-d4 for complexes 1–3, when compared to the 1H
NMR spectra acquired in DMSO-d6, the resonances generally shifted
at least d 0.10 ppm upfield. For complexes 1–3, the pyridine CH
doublet shifted at least d 0.55 ppm upfield in the proton NMR spec-
tra that were acquired in MeOH-d4, when compared to the 1H NMR
spectra acquired in DMSO-d6.In the 1H NMR spectra that were ac-
quired in MeOH-d4 for complexes 4–6, when compared to the 1H
NMR spectra acquired in DMSO-d6, some of the resonances shifted
upfield and others shifted downfield. The 13C NMR resonances for
complexes 4–6 shifted at least d 0.4 ppm upfield in MeOH-d4 when
compared to the spectra obtained in DMSO-d6.
Ligand precursors 1a–6a and 1b–6b and zinc complexes 1–6
were characterized using 1H, 13C, and HSQC NMR, spectroscopy.
For all compounds, only one set of resonances was detected indi-
cating that the two halves of each molecule are symmetry-related.
ESI-MS spectra for compounds 1–6 were collected with cone
voltages of 0 V and 70 V. The predominant feature in the spectra
Scheme 1. Preparation of 1–6.

J.R. Miecznikowski et al. / Inorganica Chimica Acta 387 (2012) 25–36
31
of these systems at the higher cone voltage is that of the fully li-
gated zinc complex, indicating that the compound is stable and
suggesting that it is unlikely that the ligand is displaced when dis-
solved in a polar solvent. In negative ion mode, the expected m/z
values were seen for [ZnCl3]ꢁ. The isotopic patterns in the mass
spectrometry data fit the assigned structures.
3.3. Cyclic voltammetry of ligand precursors and comparison of cyclic
voltammetry results to Gaussian 03 calculations
Compounds 1b and 5b were studied by cyclic voltammetry in
DMSO as part of their characterizations. The cyclic voltammogram
for 1b (Fig. 6) shows oxidation waves at 976 and 1339 mV (the lat-
ter caused most likely by solvent degradation) while the cyclic vol-
tammogram for 5b (Fig. 7) has a wave at 1178 mV. The oxidation
waves are broad and are located at the same potential across con-
secutive scans, indicating the stability of the ligands with respect
to repeated oxidation and reduction. To understand the nature of
this electrochemical feature, we chose to perform quantum
mechanical calculations using GAUSSIAN 03.
3.2. X-ray crystallography
The solid-state molecular structures of 1, 3, 5b, and 6 are shown
in Figs. 2–5, respectively. Analogous to the LADH structure, com-
plexes 1, 3, and 6 feature a zinc atom that is tetrahedrally coordi-
nated. These complexes also feature SNS donor atoms and
pseudotetrahedral geometry about the zinc center, as is seen for li-
ver alcohol dehydrogenase. The bond lengths and bond angles
compare reasonably well to the active site of horse LADH enzyme
bound to NADH (Table 2) [32].
The Gaussian calculations performed as part of this study agree
quite well with the experimentally observed oxidation potentials
for the pincer ligand and shed light on how the degree of through-
space interaction of the thioimidazole/thiotriazole rings can impact
the observed oxidation potential. The
DG oxidation potentials for
The carbon-sulfur bond lengths for 1, 3, and 6 were between
1.65 and 1.70 Å (Table 2). These bond lengths are between what
is normally associated with a C–S single bond, 1.83 Å, and a C@S
double bond, 1.61 Å [33]. For 1 and 3, the counter-anion is
[ZnCl4]2ꢁ and for 6, the counter ion is [Zn2Cl6]2ꢁ. The counter-ion
is seen even when either one or two molar equivalents of ZnCl2
are used in the preparation of 1–6. In complex 1, two molecules
of acetonitrile co-crystallized within the unit cell. The R-factor
for 3 was 0.09 and was higher than the R-factor reported for 1,
5b, or 6 (Table 1). The higher R-factor for 3 could be attributed
to disorder in the n-butyl groups present in the zinc complex [20].
Various parameters of the GAUSSIAN 03-optimized structures for
the ligands compare favorably with the crystal structure deter-
mined for compound 5b (Fig. 4). In particular, the calculated C@S
bond lengths of 1.675 Å (X = N and R = CH3) and 1.683 Å (X = CH
and R = CH3) are quite close to those found for 5b (1.677 Å and
1.667 Å). Similarly, the N–N bond length calculated for the triazole
ligand (1.375 Å) matches that determined for 5b (1.385 Å) as do
the C–NN (1.362 Å versus 1.353 Å, respectively) and the C–NCH2
(1.390 Å versus 1.373 Å, respectively) bond lengths. The use of
B3LYP/6-31G⁄ method to model the electrochemical behavior of
this ligand as discussed in the following section of this report is
therefore justified.
the solvated bis-thioimidazole system are calculated to be 0.828 V
with the ligand structure constrained to Cs symmetry and 1.234 V
under C2 symmetry. For the bis-thiotriazole compound, the
calculated oxidation potentials are 1.178 V (Cs) and 1.716 V (C2).
For both ligands, the oxidation potential determined under Cs
symmetry correlates well with the experimentally determined
value, with that calculated for the triazole system matching the
experimental value exactly. Also of note are the relative oxidative
potentials, both calculated and experimentally determined,
between the thioimidazole and thiotriazole systems. Replacement
of a C–H unit in the imidazolyl ring with a more electronegative N
atom causes the oxidation to become more difficult by ca. 200 mV.
The imposition of Cs symmetry on the ligand forces the thione-
containing rings to stack while under the C2 point group, the thio-
imidazole/thiotriazole rings are located on opposite sides of the
connecting pyridine ring. For reference, the optimized structures
of the neutral Cs- and C2-symmetry structures are presented in
Fig. 8. Of particular note is the sulfur-sulfur distance between the
two rings, which for the neutral optimized structures is 6.97 Å
for the Cs structure and 10.22 Å for the C2 system. For the cations,
the S–S distance under the Cs point group decreases considerably
to 3.03 Å while it remains largely the same (10.15 Å) for the C2
(1)
Fig. 2. Solid-state structure of complex 1.

32
J.R. Miecznikowski et al. / Inorganica Chimica Acta 387 (2012) 25–36
(3)
Fig. 3. Solid-state structure of complex 3.
(6)
Fig. 5. Solid-state structure of complex 6.
Table 2
(5b)
Selected bond lengths and angles (esd) for 1, 3, and 6 with comparison to LADH–
NADH [33].
Fig. 4. Solid-state structure of complex 5b.
R = iPr (1)
R = nBu (3)
R = nBu (6)
LADH–NADH
2.15
Zn(1)–N(1) (Å)
Zn(1)–Cl(1) (Å)
Zn(1)–S(1) (Å)
Zn(1)–S(2) (Å)
S(1)–C(1) (Å)
N(3)–Zn(1)–Cl(1)
N(3)–Zn(1)–S(1)
Cl(1)–Zn(1)–S(1)
N(3)–Zn(1)–S(2)
Cl(1)–Zn(1)–S(2)
S(1)–Zn(1)–S(2)
2.107(3)
1.972(11)
2.219(5)
2.321(4)
2.321(4)
1.650(12)
99.0(4)
125.0(5)
118.6(2)
100.3(4)
115.25(16)
99.16(15)
2.057(4)
structure. Examination of a contour plot of the HOMO for the bis-
thioimidazole system, shown in Fig. 9, explains this observation.
2.2249(13)
2.3387(14)
2.3510(14)
1.700(4)
107.50(10)
109.76(10)
114.24(5)
114.37(10)
109.55(4)
101.54(5)
2.2500(12)
2.3541(14)
2.3277(14)
1.702(5)
100.39(10)
115.04(11)
111.22(5)
115.71(11)
113.36(5)
101.62(5)
2.32
2.23
This orbital contains S, N, and C
p character and is located entirely
on the thioimidazole rings, with a considerable amount of the orbi-
tal character residing on the S atoms in a r⁄ fashion between the
sulfurs. Removal of an electron from this orbital acts to decrease
the S–S repulsive nature of this orbital, which explains the more
than halving of the S–S distance observed in the geometry optimi-
zations upon oxidation of this system. Similar systems are known
105
114
130

J.R. Miecznikowski et al. / Inorganica Chimica Acta 387 (2012) 25–36
33
25
20
15
10
5
0
-3000 -2500 -2000 -1500 -1000
-500
0
500
1000
1500
2000
-5
-10
-15
-20
Potential (mV)
Fig. 6. Cyclic voltammogram of 1b in DMSO (2 mM) with 0.2 M TBAF. The scan rate was 100 mV/s with ferrocene (E1/2 = 400 mV) used as an internal standard.
25
20
15
10
5
0
-3000 -2500 -2000 -1500 -1000
-500
0
500
1000
1500
2000
-5
-10
-15
-20
Potential (mV)
Fig. 7. Cyclic voltammogram of 5b in DMSO (2 mM) with 0.2 M TBAF. The scan rate was 100 mV/s with ferrocene (E1/2 = 400 mV) used as an internal standard.
Fig. 9. Contour plot of bis-thioimidazole ligand HOMO as viewed from above the
pyridinyl ring. This orbital contains
p-type character on the thioimidazole rings
with a r⁄-like interaction between the p orbitals on the S atoms as seen in the
bottom portion of this contour plot.
to form disulfides upon oxidation [34], which leads us to conclude
that inclusion of a strong S–S interaction through the imposition of
the Cs point group is essential for an acceptable computational
modeling of the oxidation potential. We also note that this interac-
tion, permitted by the flexibility allowed through the introduction
of the methylene linkers, leads to an oxidation that is approxi-
mately 300 mV easier than we observed in our previous report
[19].
3.4. Reactivity
Fig. 8. Optimized structures of thioimidazole systems having Cs (top) and C2
(bottom) symmetry. Hydrogen atoms are white, carbons are grey, nitrogens are
blue, and sulfurs are yellow. (For interpretation of the references in colour in this
figure legend, the reader is referred to the web version of this article.)
With an established synthetic protocol for these metalloenzyme
models and a greater understanding of their structural characteris-

34
J.R. Miecznikowski et al. / Inorganica Chimica Acta 387 (2012) 25–36
tics, our attention was turned to probing their stoichiometric activ-
ity. Zinc complexes 1-6 were screened for activity through the
reduction of 4-nitrobenzaldehyde, an electron-poor aldehyde, in
the presence of a hydrogen donor, 1-benzyl-1,4-dihydronicotina-
mide (BNAH) (Eq. (1)).BNAH was prepared following a known liter-
ature procedure and is the reagent of choice to model NADH [27].
1H NMR was used to follow the disappearance of the aldehyde pro-
ton (d 10.2 ppm) and the shifting of the aromatic C–H protons in
the alcohol product (d 8.24 ppm). The aromatic proton resonances
of the C–H protons in the alcohol product were spectroscopically
distinct from the other product or starting materials resonances
and no overlap of 1H NMR resonances was observed. For all reactiv-
ity experimentation, 0.1 mmol of aldehyde, 0.1 mmol zinc precur-
sor or 0.2 mmol of ZnCl2, and 0.2 mmol BNAH were used. Control
reactions with ZnCl2 were performed using two equivalents of this
salt because complexes 1–6 contain two zinc ions (one in the cat-
ion and one in the anion) per neutral compound. Product formation
was detected by 1H NMR by comparison with authentic material.
In no case was there any indication of reduction of nitro substitu-
ents. Table 3 illustrates the reactivity data for 1–6 as well as for
ZnCl2 and the ligand precursor.
reau and co-workers [35]. Entries 3–5 in Table 3 indicate that ZnCl2
reacts stoichiometrically with electron-poor aldehydes such as 4-
nitrobenzaldhyde to yield alcohol product to a small extent (ca.
18% conversion). We therefore propose that Zn2+ acts as a Lewis
acid catalyst in the reaction where ZnCl2 is utilized. It is plausible
that the Zn2+ in the counteranion contributes to the reactivity that
is shown for 1–6 in Table 3. Experiments are underway to prepare
tridentate zinc SNS pincer complexes with a counter-anion that
does not contain a zinc ion. We also wondered if an excess of ZnCl2
would enhance the rate of conversion of 4-nitrobenzaldehyde. We
saw 42% conversion of 4-nitrobenzaldehyde after 20 h when excess
ZnCl2 (10 eq) was used. Thus, an excess of ZnCl2 and BNAH could
also be used to reduce 4-nitrobenzaldehyde.
Reactivity data for imidazole complexes with the CH2 linker is
consistently lower when compared to previous studies carried
out without the CH2 linker [19] when identical ligand groups were
tested. Imidazole compounds with isopropyl wingtip R groups
without the CH2 linker gave a catalytic activity of 42% compared
to 32% when the CH2 linker compound was tested. Similarly, the
zinc complexes with neopentyl wingtip groups and no CH2 linker
had a greater turnover (48%) than did the analogous compound
ð1Þ
with a methylene linker (23%), and the zinc complexes with n-bu-
tyl wingtip groups and no CH2 linker had a greater turnover (33 %)
and the identical compounds with a CH2 linker (25 %).
As shown in Table 3, zinc complexes 1–6 enhance the rate of the
reaction for the reduction of 4-nitrobenzaldehyde when compared
to that for either ZnCl2 or ligand precursor. Mechanistically, others
have proposed a hydrogen atom transfer between the co-factor
and the substrate upon coordination to the zinc active site based
upon a previously reported mechanism for LADH offered by Ber-
Based on the data presented in Table 3, it appears that the
choice of the alkyl group is important as n-butyl and neopentyl
groups (bis-triazole) gave a higher percent conversion than isopro-
pyl. Previous experimentation without the CH2 linker supported
the claim that isopropyl wingtips yielded the greatest catalytic
activity due to the fact that larger R groups such as n-butyl are
more sterically demanding, when compared to neopentyl or iso-
propyl, and may shield the zinc metal center from interacting with
BNAH and substrate. The opposite trend was observed for both
imidazole and triazole complexes with the CH2 linker. Perhaps
added room for binding provided by the CH2 linker reduces the
wingtip effects previously observed. Furthermore, the larger R
group complexes gave the greatest solubility. The identity of the
wingtip group is therefore a crucial variable to consider when
screening metal complexes for activity [21].
Table 3
Reactivity data for 1–6.
Entry
Zn complex
Time (h)
Conversion (%)
1
2
3
4
5
6
7
8
9
None
20
20
5
<5
<5
15
13
18
32
23
25
37
45
59
42
2b (bisthione ligand precursor)
ZnCl2 (2 eq.)
ZnCl2 (2 eq.)
ZnCl2 (2 eq.)
1
2
3
4
a
20
20
20
20
20
20
20
20
20
The choice of azole rings also is important as CH2 linker com-
plexes with the triazole ring gave consistently greater catalytic
activity for the reduction reaction than identical R group imidazole
complexes. The system with the greatest catalytic efficiency was
complex 6 (59%), which contains triazole rings that possess n-butyl
wingtip groups.
10
11
12
5
6
ZnCl2 (10 eq.)
a
Reaction was run under an N2 environment. The reaction was setup in an inert
atmosphere glove box.

J.R. Miecznikowski et al. / Inorganica Chimica Acta 387 (2012) 25–36
35
Table 4
used in this work have provided new insights in the field of bioin-
organic modeling chemistry. The zinc complexes serve as models
for the zinc active site in liver alcohol dehydrogenase. The SNS zinc
pincer complexes adopt a pseudo-tetrahedral geometry and have a
SNS coordination environment about the zinc center like LADH and
react with BNAH to reduce electron-poor aldehydes. The zinc com-
plexes reported herein react with BNAH to quantitatively reduce
pyridine-2-carboxaldehyde. The zinc complexes reported also re-
duce 4-nitro-benzaldehyde. It remains a challenge to synthesize
a neutral zinc complex with a tridentate ligand with SNS donor
atoms that yield reactivity that is comparable to LADH.
Reactivity data for 1–6 for the reduction ofpyridine-2-carboxaldehyde.
Entry
Zn complex
Time (h)
Conversion (%)
1
2
3
4
5
6
7
8
None
ZnCl2
20
20
20
20
20
20
20
20
<5
19
1
2
3
4
5
6
quantitative
quantitative
quantitative
quantitative
quantitative
quantitative
DFT calculations were performed to examine various structural
and electronic properties of these compounds. The computed oxida-
tion potentials match well with what is observed experimentally
while the calculated reduction potential indicates that the experi-
mentalreductionwavedoesnotcorrespondto asimpleone-electron
reduction of the ligand precursor without other reactivity occurring.
We also tried to reduce another electron-poor aldehyde, pyri-
dine-2-carboxaldehyde, in the presence of a stoichiometric amount
of 1–6 or ZnCl2 (Eq. (2)). 1H NMR was used to follow the disappear-
ance of the aldehyde proton (d 10.2 ppm) and the appearance of
the aromatic C–H protons in the alcohol product (d 8.4 ppm). The
aromatic proton resonance of the alcohol product did not overlap
with the 1H NMR resonances of either the starting materials or
the products or the zinc complex. Table 4 illustrates the reactivity
data for 1–6 as well as for ZnCl2.
Acknowledgements
This work was supported by generous funding from The Fair-
field University Science Institute (J.R.M.), Fairfield University
ð2Þ
As shown in Table 4, zinc complexes 1–6 enhance the rate of the
reaction for the reduction of pyridine-2-carboxaldehyde when com-
pared to ZnCl2. Use of complexes 1–6 results in quantitative conver-
sion of pyridine-2-carboxyaldehyde to pyridin-2-ylmethanol.
As seen in Tables 3 and 4, enhancement for the reduction of 4-
nitrobenzaldehyde or pyridine-2-carboxaldehyde was observed for
complexes 1–6. Stoichiometric conversion to the alcohol product
was observed for pyridine-2-carboxaldehyde. Based on the mech-
anism proposed by Berreau and co-workers [35], the low activity
of complexes 1–6 for the reduction of 4-nitrobenzaldehyde could
be due to the slow hydrogen transfer between the co-factor and
the substrate, which is coordinated to the zinc active site. More
importantly, the alcohol product may inhibit the reaction. As the
alcohol is formed, it may coordinate to the zinc metal center as
the reaction progresses, and thereby hinder the reaction.
Start-up Funding (J.R.M.), Fairfield University Research Grants
(J.R.M.), and an NTID Faculty Evaluation and Development Grant
(M.A.L.). J.R.M. would like to thank Prof. Matthew A. Kubasik, and
Prof. L. Kraig Steffen for helpful suggestions. Thank you also to
the anonymous peer reviewers who helped improve this article.
Appendix A. Supplementary data
The 1H, 13C and HSQC NMR spectra of 1a, 2a, 4a–6a, 1b–6b, and
1–6 and mass spectra of 1–6 are provided. IR spectra for 1b–6b and
crystallographic details of 1, 3, 5b, and 6 are also given. The de-
tailed descriptions for the syntheses of each compound are given
as well. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2011.12.047.
4. Conclusions
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