Examining the impact of steric and electronic variation in N2S scorpionate ligands on the properties of zinc(II) and cadmium(II) complexes

Article abstract of DOI:10.1016/j.ica.2011.07.033

A series of LZn(II)Br (1-4) and LCd(II)Cl complexes (9-11) has been prepared by the reaction of metal halide precursors with the lithium salts of the N₂S^- ligands bis(3,5-diisopropylpyrazol-1-yl) dithioacetate (L¹), bis(3,5-di-tert-butylpyrazol-1-yl)dithioacetate (L²), N-phenyl-2,2-bis(3,5-diisopropylpyrazol-1-yl)thioacetamide (L³) and N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide (L⁴). Characterization by X-ray crystallography and DOSY NMR studies indicate that LZnBr complexes 1-4 are mononuclear both in the solid state and in solution. Steric differences between ligands L¹-L⁴ result in distortion from an ideal tetrahedral geometry for each complex, with the degree of distortion depending on the bulk of the ligand substituents. In contrast, the related complex L³CdCl was shown by X-ray crystallography to dimerize in the solid state to form the chloride-bridged five-coordinate complex [L³CdCl]₂ (10). Despite 10 having a dinuclear structure in the solid state, DOSY NMR studies indicate 9-11 exist as mononuclear LCdCl species in solution. In addition, Zn(II) cyanide complexes of the form LZnCN [L = L¹ (5), L³ (7), L⁴ (8)] have been characterized and the X-ray structure of 8 determined. Moreover, density functional theory calculations have been conducted which yield important insight into the bonding in 1-4 and 5-8 and the electronic impact of ligands L¹-L⁴ on the zinc(II) ion and its ability to function as a Lewis acid catalyst.

Full text of DOI:10.1016/j.ica.2011.07.033

Inorganica Chimica Acta 376 (2011) 562–573
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Examining the impact of steric and electronic variation in N2S scorpionate ligands
on the properties of zinc(II) and cadmium(II) complexes
a,
Nicholas G. Spiropulos a, G.C. Chingas a, Mallory Sullivan a, John T. York b, , Eric C. Brown
⇑
⇑
a Department of Chemistry and Biochemistry, 1910 University Drive, Boise State University, Boise, ID 83725, USA
b Department of Chemistry, 421 N. Woodland Blvd., Stetson University, DeLand, FL 32723, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of LZn(II)Br (1–4) and LCd(II)Cl complexes (9–11) has been prepared by the reaction of metal
halide precursors with the lithium salts of the N2Sꢀ ligands bis(3,5-diisopropylpyrazol-1-yl)dithioacetate
(L1), bis(3,5-di-tert-butylpyrazol-1-yl)dithioacetate (L2), N-phenyl-2,2-bis(3,5-diisopropylpyrazol-1-
yl)thioacetamide (L3) and N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide (L4). Character-
ization by X-ray crystallography and DOSY NMR studies indicate that LZnBr complexes 1–4 are mononu-
clear both in the solid state and in solution. Steric differences between ligands L1–L4 result in distortion
from an ideal tetrahedral geometry for each complex, with the degree of distortion depending on the bulk
of the ligand substituents. In contrast, the related complex L3CdCl was shown by X-ray crystallography to
dimerize in the solid state to form the chloride-bridged five-coordinate complex [L3CdCl]2 (10). Despite
10 having a dinuclear structure in the solid state, DOSY NMR studies indicate 9–11 exist as mononuclear
LCdCl species in solution. In addition, Zn(II) cyanide complexes of the form LZnCN [L = L1 (5), L3 (7), L4 (8)]
have been characterized and the X-ray structure of 8 determined. Moreover, density functional theory
calculations have been conducted which yield important insight into the bonding in 1–4 and 5–8 and
the electronic impact of ligands L1–L4 on the zinc(II) ion and its ability to function as a Lewis acid catalyst.
Ó 2011 Elsevier B.V. All rights reserved.
Received 25 April 2011
Received in revised form 11 July 2011
Accepted 20 July 2011
Available online 28 July 2011
Keywords:
Heteroscorpionate ligands
Zinc
Cadmium
DOSY NMR
X-ray crystal structures
DFT calculations
1. Introduction
zinc(II) and cadmium(II) remain surprisingly unexplored. In partic-
ular, zinc(II)-containing metalloproteins bacteriophage T7 lyso-
The electronic and steric properties of an ancillary ligand can
dramatically influence the reactivity of a metal ion to which it is
bound. As such, extensive efforts have focused on the development
of ligand systems whose steric environment and donor atom types
(e.g. N, O, and S) can be easily modified, with the ultimate goal
being the ability to ‘‘fine-tune’’ a metal complex to produce a de-
sired type of chemical reactivity. One important example of a mod-
ifiable ligand system is the carbon-bridged heteroscorpionate
ligands derived from bis(pyrazol-1-yl)methanes. These tridentate
ligands are similar to tris(pyrazolyl)methanes with one pyrazolyl
group replaced by an anionic coordinating group, such as a carbox-
ylate, dithiocarboxylate, acetamidate or thioacetamidate. Ligands
of this type provide facially coordinating N2Oꢀ and N2Sꢀ donor
environments, which are particularly relevant to the study of
metalloproteins in which this type of coordination environment
is common. As such, they have been used to prepare numerous
transition metal complexes [1,2].
zyme and peptide deformylase (in plants PDF1A) both consist of
a zinc ion bound through a cysteine and two histidine residues
[3,4]. While other examples of N2Sꢀ ligands have been explored
for this type of system, these particular ligands present unique
electronic and steric properties that could potentially affect the
properties and reactivity of the zinc(II) center. Thus, we wanted
to explore potential impacts of variation in these two ligand types
(dithioacetate versus thioacetamide) on the geometry and elec-
tronic structure of zinc(II) complexes. Moreover, because cadmium
substitution of zinc-containing enzymes is often used as a spectro-
scopic probe (via NMR) to study protein active sites, we expanded
the study to also include cadmium [5].
Herein, we report the synthesis and characterization of a series
of sterically bulky bis(pyrazol-1-yl)dithioacetate and bis(pyrazol-
1-yl)thioacetamide ligands (L1–L4) and their lithium salts, which
were subsequently used to prepare LnM-X where n = 1–4, M = Zn,
Cd and X = Br, Cl (Scheme 1). Extensive efforts were used to iden-
tify the structures and nuclearity of these halide complexes both
in the solid state (X-ray crystallography) and in solution (NMR, dif-
fusion ordered spectroscopy (DOSY)). In addition, we describe a
synthetic and computation investigation examining the electronic
properties of ligands L1–L4 and their effect on polarizing cyanide
substrates in LZnCN complexes (where L = L1–L4).
Given the potential usefulness of these ligands in studying bio-
mimetic systems, the coordination chemistry of bis(pyrazol-1-
yl)dithioacetate and bis(pyrazol-1-yl)thioacetamide ligands with
⇑
Corresponding authors. Tel.: +1 208 426 1186; fax: +1 208 426 3027.
E-mail address: ericbrown3@boisestate.edu (E.C. Brown).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.07.033

N.G. Spiropulos et al. / Inorganica Chimica Acta 376 (2011) 562–573
563
Scheme 1. Zinc(II) and cadmium(II) complexes 1–11. Asterisks indicate those complexes characterized by X-ray diffraction.
(DMSO, 150 MHz): d 243.2, 155.4, 151.4, 98.3, 87.5, 27.3, 25.1,
23.5, 23.1, 22.8, 22.8. IR (ATR, cmꢀ1): 2965 (m), 1554 (m), 1457
(m), 1381 (m), 1362 (w), 1299 (s), 1247 (w), 1227 (m), 1178 (m),
1153 (w), 1098 (m), 1076 (m), 1050 (m), 1005 (s), 902 (m), 844
(s), 821 (s), 787 (s), 749 (s), 721 (m), 672 (m). HRMS (ESI, Neg) cal-
culated for [C20H31N4S2ꢀLi]ꢀ: 391.1990, found 391.1996.
2. Experimental
2.1. General
All reactions were performed using standard Schlenk-tube tech-
niques under a nitrogen atmosphere. Solvents and reagents were
obtained from commercial sources and used without further puri-
fication. The solvents tetrahydrofuran (THF), methanol and ethanol
were dried with CaH2 and distilled prior to use. NMR spectra were
recorded on a Bruker AVANCE III 600 NMR. Chemical shifts for 1H
or 13C NMR spectra were referenced to residual solvent peaks.
Infrared spectra were measured using a Perkin–Elmer Spectrum
100 spectrometer. Elemental analyses were performed by Atlantic
Microlab, Inc. of Norcross, GA. High resolution electrospray mass
spectra were recorded on an Applied Biosystems Qstar Elite Quad-
rupole Time of Flight Instrument. 3,5-Diisopropylpyrazole, 3,5-di-
tert-butylpryazole and bis(3,5-di-tert-butylpyrazol-1-yl)methane
were prepared by previously reported procedures [6,7].
2.2.3. Preparation of lithium bis(3,5-di-tert-butylpyrazol-1-yl)dithio-
acetateꢂTHF, Li(L2)ꢂTHF
To
a solution of bis(3,5-di-tert-butylpyrazol-1-yl)methane
(1.0 g, 2.7 mmol) dissolved in dry THF (40 mL) was added nBuLi
(1.6 M, 2.56 mL, 4.1 mmol) in hexane at ꢀ78 °C. After 1 h of stir-
ring, carbon disulfide (0.25 mL, 4.1 mmol) was added to the reac-
tion at ꢀ40 °C, resulting in a red colored solution. The solution
was allowed to reach ambient temperature before the volume
was reduced to 5 mL under reduced pressure. Addition of hexane
(20 mL) resulted in the formation of a orange solid, which was fil-
tered, washed with hexane (2 ꢁ 10 mL) and dried under reduced
pressure (1.15 g, 81%). 1H NMR (CDCl3, 600 MHz): d 7.68 (s, 1H),
5.96 (s, 2H), 3.86 (THF), 1.89 (THF), 1.44 (s, 18H), 1.23 (s, 18H).
13C NMR (CDCl3, 150 MHz): d 248.1, 161.3, 154.1, 101.9, 83.6,
68.7, 32.2, 31.9, 31.1, 30.4, 25.5. IR (ATR, cmꢀ1): 2965 (m), 2871
(w), 1644 (w), 1537 (m), 1460 (m), 1397 (w), 1362 (m), 1318
(m), 1252 (m), 1224 (s), 1126 (w), 1084 (s), 1058 (s), 1035 (m),
1006 (s), 900 (w), 878 (m), 853 (m), 828 (w), 816 (m), 800 (s),
743 (s), 727 (m), 691 (m). HRMS (ESI, Neg) calculated for
[C24H39N4S2ꢀLiꢀTHF]ꢀ: 447.2616, found 447.2634.
2.2. Synthesis and characterization
2.2.1. Preparation of bis(3,5-diisopropylpyrazol-1-yl)methane
Bis(3,5-diisopropylpyrazol-1-yl)methane was prepared using
3,5-diisopropylpyrazole as described for bis(3,5-dimethylpyrazol-
1-yl)methane [8]. Yield: 96%. 1H NMR (CDCl3, 600 MHz): d 6.22
(s, 2H), 5.84 (s, 2H), 3.39 (m, 2H), 2.82 (m, 2H), 1.21 (d, 12H),
1.03 (d, 12H).
2.2.2. Preparation of lithium bis(3,5-diisopropylpyrazol-1-yl)dithio-
acetate, Li(L1)
2.2.4. Preparation of lithium N-phenyl-2,2-bis(3,5-diisopropylpyrazol-
1-yl)thioacetamideꢂTHF, Li(L3)ꢂTHF
To a solution of bis(3,5-diisopropylpyrazol-1-yl)methane (1.0 g,
3.2 mmol) dissolved in dry THF (40 mL) was added nBuLi (1.6 M,
2.94 mL, 4.7 mmol) in hexane at ꢀ78 °C. After 1 h of stirring, car-
bon disulfide (0.28 mL, 4.7 mmol) was added slowly to the reaction
at ꢀ40 °C. The solution was allowed to reach ambient temperature
before the volume was reduced to 5 mL under reduced pressure.
Addition of hexane (20 mL) resulted in the formation of an orange
solid, which was filtered, washed with hexane (2 ꢁ 10 mL) and
dried under reduced pressure (0.98 g, 77%). 1H NMR (DMSO,
600 MHz): d 7.08 (s, 1H), 5.81 (s, 2H), 3.29 (m, 2H), 2.75 (m, 2H),
1.13 (d, 6H), 1.12 (d, 6H), 1.02 (d, 6H), 0.98 (d, 6H). 13C NMR
To a solution of bis(3,5-diisopropylpyrazol-1-yl)methane (1.0 g,
3.2 mmol) dissolved in dry THF (30 mL) was added nBuLi (1.6 M,
2.94 mL, 4.7 mmol) in hexane at ꢀ78 °C. After 1 h of stirring, phe-
nyl isothiocyanate (0.56 mL, 4.7 mmol) was added slowly to the
reaction at ꢀ40 °C. The solution was allowed to reach ambient
temperature before the volume was reduced to 5 mL under re-
duced pressure. Addition of hexane (20 mL) resulted in the forma-
tion of a white solid, which was filtered, washed with hexane
(2 ꢁ 10 mL) and dried under reduced pressure (1.16 mg, 68%). 1H
NMR (CDCl3, 600 MHz): d 7.53 (s, 1H), 7.19 (t, 2H), 6.91 (t, 1H),
6.83 (d, 2H), 5.88 (s, 2H), 3.71 (THF), 3.32 (m, 2H), 2.86 (m, 2H),

564
N.G. Spiropulos et al. / Inorganica Chimica Acta 376 (2011) 562–573
1.78 (THF), 1.51 (d, 6H), 1.28 (d, 6H), 1.21 (d, 6H), 1.19 (d, 6H). 13
C
(m), 971 (w), 934 (w), 894 (w), 878 (m), 827 (s), 814 (m), 799
NMR (CDCl3, 150 MHz): d 181.9, 158.5, 153.0, 151.9, 128.5, 122.6,
122.5, 99.0, 77.0, 68.4, 31.1, 28.0, 26.0, 25.6, 24.2, 23.0, 22.8. IR
(ATR, cmꢀ1): 2964 (m), 2871 (w), 1594 (w), 1543 (s), 1517 (s),
1456 (s), 1384 (m), 1366 (m), 1287 (m), 1222 (m), 1180 (m),
1062 (m), 1046 (s), 1015 (m), 923 (m), 896 (m), 879 (m), 838 (s),
819 (s), 797 (s), 756 (s), 726 (m), 695 (s), 677 (m). HRMS (ESI,
Neg) calculated for [C26H36N5SꢀLiꢀTHF]ꢀ: 450.2691, found
450.2679.
(s), 766 (m), 737 (m), 728 (m), 719 (s), 670 (s). Anal. Calc. for
C24H39BrN4S2Zn: C, 48.61; H, 6.63; N, 9.45. Found: C, 48.51; H,
6.87; N, 9.54%.
2.2.8. Preparation of L3ZnBr (3)
ZnBr2 (0.171 g, 0.76 mmol) was added to Li(L3)ꢂTHF (0.400 g,
0.76 mmol) in dry EtOH (20 mL). The reaction was stirred for
24 h, during which time a white solid formed. The white solid
was collected and dried under reduced pressure (0.390 g, 86%).
Clear crystals suitable for crystallographic characterization were
obtained by Et2O diffusion into THF at room temperature. 1H
NMR (CD3CN, 600 MHz): d 7.32 (t, 2H), 7.09 (t, 1H), 6.77 (d, 2H),
6.37 (s, 2H), 3.45 (m, 2H), 3.29 (m, 2H), 1.42 (d, 6H), 1.39 (d, 6H),
1.30 (d, 6H), 1.26 (d, 6H). 1H NMR (CDCl3, 600 MHz): d 7.30 (t,
2H), 7.26 (s, 1H), 7.06 (t, 1H), 6.85 (d, 2H), 6.10 (s, 2H), 3.52 (m,
2H), 3.18 (m, 2H), 1.45 (d, 6H), 1.39 (d, 6H), 1.31 (d, 6H), 1.25 (d,
6H). 13C NMR (CDCl3, 150 MHz): d 168.9, 161.7, 153.0, 150.4,
128.8, 124.0, 120.3, 100.2, 73.1, 27.1, 26.0, 23.3, 23.3, 22.6, 22.2.
IR (ATR, cmꢀ1): 2970 (m), 2935 (w), 2884 (w), 1550 (s), 1480
(m), 1465 (s), 1450 (m), 1395 (m), 1386 (m), 1372 (w), 1342 (w),
1301 (m), 1289 (m), 1230 (m), 1183 (m), 1096 (m), 1082 (m),
1058 (m), 1026 (m), 973 (w), 918 (w), 878 (m), 824 (s), 814 (s),
801 (s), 776 (s), 733 (s), 722 (m), 694 (s), 669 (m). Anal. Calc. for
2.2.5. Preparation of lithium N-phenyl-2,2-bis(3,5-di-tert-butyl-
pyrazol-1-yl)thioacetamideꢂTHF, Li(L4)ꢂTHF
To
a solution of bis(3,5-di-tert-butylpyrazol-1-yl)methane
(1.0 g, 2.7 mmol) dissolved in dry THF (40 mL) was added nBuLi
(1.6 M, 2.56 mL, 4.1 mmol) in hexane at ꢀ78 °C. After 1 h of stir-
ring, phenyl isothiocyanate (0.49 mL, 4.1 mmol) was added slowly
to the reaction at ꢀ40 °C. The solution was allowed to reach ambi-
ent temperature before the volume was reduced to 5 mL under re-
duced pressure. Addition of hexane (20 mL) resulted in the
formation of a white solid, which was filtered, washed with hexane
(2 ꢁ 10 mL) and dried under reduced pressure (1.15 mg, 73%). 1H
NMR (CDCl3, 600 MHz): d 7.82 (s, 1H), 7.20 (t, 2H), 6.92 (t, 1H),
6.75 (d, 2H), 5.97 (s, 2H), 3.54 (THF), 1.55 (THF), 1.51 (s, 18H),
1.23 (s, 18H). 13C NMR (CDCl3, 150 MHz): d 186.5, 160.5, 154.1,
152.8, 128.7, 122.6, 122.3, 101.6, 80.7, 68.5, 32.2, 32.1, 31.0, 30.5,
25.1. IR (ATR, cmꢀ1): 2958 (m), 1594 (w), 1538 (m), 1513 (s),
1486 (m), 1462 (m), 1363 (m), 1344 (w), 1317 (w), 1255 (m),
1228 (m), 1202 (m), 1071 (m), 1040 (m), 1004 (m), 920 (m), 887
(m), 869 (m), 851 (w), 826 (w), 816 (m), 800 (s), 753 (m), 693
(s). HRMS (ESI, Neg) calculated for [C30H44N5SꢀLiꢀTHF]ꢀ:
506.3318, found 506.3334.
C26H36BrN5SZn: C, 52.40; H, 6.09; N, 11.75. Found: C, 52.27; H,
6.03; N, 11.69%.
2.2.9. Preparation of L4ZnBr (4)
ZnBr2 (0.153 g, 0.68 mmol) was added to Li(L4)ꢂTHF (0.400 g,
0.68 mmol) in dry EtOH (20 mL). The reaction was stirred for
24 h, during which time a white solid formed. The white solid
was collected and dried under reduced pressure (0.376 g, 85%).
Clear crystals suitable for crystallographic characterization were
obtained by hexane diffusion into THF at room temperature. 1H
NMR (CD3CN, 600 MHz): d 7.90 (s, 1H), 7.32 (t, 2H), 7.09 (t, 1H),
6.80 (d, 2H), 6.46 (s, 2H), 1.55 (s, 18H), 1.47 (s, 18H). 1H NMR
(CDCl3, 600 MHz): d 7.85 (s, 1H), 7.29 (t, 2H), 7.04 (t, 1H), 6.89
(d, 2H), 6.19 (s, 2H), 1.56 (s, 18H), 1.49 (s, 18H). 13C NMR (CDCl3,
150 MHz): d 169.7, 164.5, 156.1, 150.4, 128.7, 124.0, 120.7, 104.0,
76.7, 32.8, 32.5, 31.3, 30.6. IR (ATR, cmꢀ1): 2966 (m), 1582 (s),
1541 (m), 1533 (m), 1514 (w), 1463 (m), 1442 (m), 1365 (s),
1352 (m), 1306 (w), 1250 (m), 1218 (s), 1133 (w), 1108 (m),
1063 (m), 1022 (m), 933 (w), 902 (w), 856 (w), 835 (m), 824 (m),
809 (s), 801 (s), 770 (m), 735 (m), 721 (m), 695 (s). Anal. Calc. for
2.2.6. Preparation of L1ZnBr (1)
ZnBr2 (0.225 g, 1.0 mmol) was added to Li(L1) (0.400 g,
1.0 mmol) in dry MeOH (20 mL). The reaction was stirred for
24 h, during which time a pink solid formed. The pink solid was
collected and dried under reduced pressure (0.408 g, 76%). Orange
crystals suitable for crystallographic characterization were ob-
tained by Et2O diffusion into THF at room temperature. 1H NMR
(CD3CN, 600 MHz): d 7.75 (s, 1H), 6.40 (s, 2H), 3.44 (m, 2H), 3.24
(m, 2H), 1.36 (d, 6H), 1.33 (d, 6H), 1.30 (d, 6H), 1.26 (d, 6H). 1H
NMR (CDCl3, 600 MHz): d 7.68 (s, 1H), 6.11 (s, 2H), 3.50 (m, 2H),
3.16 (m, 2H), 1.39 (d, 6H), 1.34 (d, 6H), 1.29 (d, 6H), 1.25 (d, 6H).
13C NMR (CDCl3, 150 MHz): d 235.1, 162.3, 153.8, 100.7, 78.8,
27.2, 26.1, 23.1, 23.0, 22.91, 22.88. IR (ATR, cmꢀ1): 2964 (m),
2930 (w), 1550 (m), 1469 (m), 1395 (w), 1383 (m), 1367 (w),
1297 (m), 1283 (m), 1239 (w), 1222 (m), 1181 (m), 1159 (w),
1089 (m), 1078 (m), 1060 (m), 1040 (s), 1027 (s), 963 (w), 932
(w), 907 (w), 880 (w), 847 (m), 805 (s), 730 (s), 716 (m), 686
(m), 669 (m). Anal. Calc. for C20H31BrN4S2Zn: C, 44.74; H, 5.82; N,
10.43. Found: C, 44.80; H, 5.27; N, 10.01%.
C30H44BrN5SZn: C, 55.26; H, 6.80; N, 10.74. Found: C, 55.19; H,
6.87; N, 10.46%.
2.2.10. Preparation of L1ZnCN (5)
Zn(CN)2 (0.117 g, 1.0 mmol) was added to Li(L1) (0.400 g,
1.0 mmol) in dry MeOH (20 mL). The reaction was stirred for
48 h, during which time a pink solid formed. The pink solid was
collected and dried under reduced pressure (0.180 g, 36%). 1H
NMR (CDCl3, 600 MHz): d 7.64(s, 1H), 6.12 (s, 2H), 3.30 (m, 2H),
3.14 (m, 2H), 1.34 (m, 24H). 13C NMR (CDCl3, 150 MHz): d 234.5,
161.9, 154.1, 134.0 (–CN), 100.8, 78.8, 27.7, 26.1, 23.1, 22.8,
22.74, 22.72. IR (KBr, cmꢀ1): 2969 (s), 2931 (m), 2870 (m), 2079
(w), 1551 (s), 1471 (s), 1384 (m), 1301 (s), 1221 (m), 1182 (m),
1080 (m), 1060 (s), 1044 (s), 1028 (s), 933 (w), 906 (w), 879 (w),
846 (m), 807 (s), 730 (m), 717 (m), 688 (m), 669 (m). Anal. Calc.
for C21H31N5S2ꢂ0.5CH3OH: C, 51.74; H, 6.66; N, 14.03. Found: C,
51.53; H, 6.52; N, 14.11%.
2.2.7. Preparation of L2ZnBr (2)
ZnBr2 (0.171 g, 0.76 mmol) was added to Li(L2)ꢂTHF (0.400 g,
0.76 mmol) in dry MeOH (20 mL). The reaction was stirred for
24 h, during which time a pink solid formed. The pink solid was
collected and dried under reduced pressure (0.425 g, 94%). Orange
crystals suitable for crystallographic characterization were ob-
tained by hexane diffusion into THF at room temperature. 1H
NMR (CD3CN, 600 MHz): d 8.11 (s, 1H), 6.46 (s, 2H), 1.50 (s,
18H), 1.46 (s, 18H). 1H NMR (CDCl3, 600 MHz): d 8.06 (s, 1H),
6.18 (s, 2H), 1.50 (s, 18H), 1.46 (s, 18H). 13C NMR (CDCl3,
150 MHz): d 238.4, 165.2, 156.7, 104.4, 81.1, 32.8, 32.5, 31.1,
30.5. IR (ATR, cmꢀ1): 2963 (m), 1597 (w), 1544 (s), 1459 (s),
1383 (m), 1366 (m), 1338 (w), 1308 (m), 1289 (m), 1252 (s),
1235 (m), 1182 (m), 1154 (w), 1109 (w), 1081 (s), 1058 (s), 1014
2.2.11. Preparation of L3ZnCN (7)
Zn(CN)2 (0.090 g, 0.76 mmol) was added to Li(L3)ꢂTHF (0.400 g,
0.76 mmol) in dry EtOH (20 mL). The reaction was stirred for
48 h, during which time a white solid formed. The white solid

N.G. Spiropulos et al. / Inorganica Chimica Acta 376 (2011) 562–573
565
was collected and dried under reduced pressure (0.236 g, 57%). 1H
NMR (CD3CN, 600 MHz): d 7.30 (t, 3H), 7.06 (t, 1H), 6.73 (m, 2H),
6.34 (s, 2H), 3.25 (m, 4H), 1.39 (d, 6H), 1.35 (d, 6H), 1.31 (d, 6H)
1.27 (d, 6H). 13C NMR (CDCl3, 150 MHz): d 168.4, 161.4, 153.2,
150.3, 134.8 (–CN), 128.9, 124.2, 120.2, 100.3, 73.1, 27.5, 26.0,
23.3, 23.2, 22.7, 22.1. 13C NMR (CD3CN, 150 MHz): d 168.1, 161.2,
154.3, 150.6, 133.7 (–CN), 128.8, 123.8, 119.9, 100.6, 72.8, 27.4,
25.7, 22.5, 22.1, 21.9, 20.9. IR (KBr, cmꢀ1): 2967 (m), 2932 (m),
2871 (m), 1549 (s), 1471 (s), 1368 (m), 1300 (m), 1230 (m), 1184
(m), 1082 (m), 1060 (m), 1044 (m), 1028 (s), 933 (w), 906 (w),
878 (w), 846 (m), 804 (s), 769 (m), 731 (m), 716 (m), 690 (s),
669 (m). Anal. Calc. for C27H36N6S: C, 59.82; H, 6.69; N, 15.5. Found:
C, 59.55; H, 6.73; N, 15.4%.
(w), 1306 (m), 1252 (m), 1235 (m), 1204 (w), 1181 (m), 1154
(w), 1110 (w), 1068 (m), 1057 (m), 1014 (m), 971 (w), 933 (w),
894 (w), 877 (w), 829 (m), 800 (s), 766 (m), 726 (m), 690 (s), 670
(m). Anal. Calc. for C26H36ClN5SCd: C, 52.17; H, 6.06; N, 11.70.
Found: C, 51.93; H, 6.06; N, 11.68%.
2.2.15. Preparation of L4CdCl (11)
CdCl2 (0.124 g, 0.68 mmol) was added to [Li(L4)]ꢂTHF (0.400 g,
0.68 mmol) in dry EtOH (20 mL). The reaction was stirred for
24 h, during which time a white solid formed. The white solid
was collected and dried under reduced pressure (0.307 g, 69%).
1H NMR (CD3CN, 600 MHz): d 7.72 (s, 1H), 7.30 (t, 2H), 7.06 (t,
1H), 6.74 (d, 2H), 6.42 (s, 2H), 1.46 (s, 18H), 1.46 (s, 18H). 13C
NMR (CD3CN, 150 MHz): d 171.1, 165.1, 157.0, 151.0, 128.7,
123.7, 120.1, 104.0, 77.0, 32.3, 32.0, 29.9, 29.6. IR (ATR, cmꢀ1):
2967 (m), 1579 (s), 1542 (m), 1463 (m), 1443 (m), 1366 (s), 1308
(w), 1251 (m), 1225 (s), 1132 (w), 1103 (m), 1063 (m), 1022 (m),
933 (w), 903 (w), 858 (w), 835 (m), 825 (m), 810 (s), 792 (s), 770
(s), 735 (m), 721 (m), 692 (s). Anal. Calc. for C30H44ClN5SCd: C,
55.04; H, 6.77; N, 10.70. Found: C, 55.13; H, 6.85; N, 10.53%.
2.2.12. Preparation of L4ZnCN (8)
Zn(CN)2 (0.080 g, 0.68 mmol) was added to Li(L4)ꢂTHF (0.400 g,
0.68 mmol) in dry EtOH (20 mL). The reaction was stirred for
48 h, during which time a white solid formed. The white solid
was collected and dried under reduced pressure (0.236 g, 54%).
Clear colorless crystals were isolated from slow vapor diffusion
of hexanes into THF at room temperature. 1H NMR (CDCl3,
600 MHz): d 7.85 (s, 1H), 7.30 (t, 3H), 7.05 (t, 1H), 6.84 (d, 2H),
6.19 (s, 2H), 3.72 (EtOH), 1.57 (s, 18H), 1.49 (s, 18H), 1.24 (EtOH).
13C NMR (CDCl3, 150 MHz): d 169.0, 164.1, 156.4, 150.4, 137.8 (–
CN), 128.8, 124.1, 120.4, 104.1, 76.7, 58.6, 32.8, 32.5, 31.4, 30.6,
18.6. IR (KBr, cmꢀ1): 3144 (w), 3056 (w), 2971 (s), 2874 (m),
1581 (s), 1541 (s), 1464 (s), 1442 (s), 1366 (s), 1306 (m), 1250
(s), 1218 (s), 1133 (m), 1108 (m), 1066 (s), 1025 (s), 934 (w), 902
(m), 856 (m), 835 (m), 809 (s), 793 (s), 755 (s), 736 (m), 720 (m),
696 (s), 664 (w), 643 (m), 605 (w), 563 (m), 507 (w). Anal. Calc.
for C31H44N6SꢂC2H5OH: C, 61.52; H, 7.82; N, 13.04. Found: C,
61.23; H, 7.51; N, 13.31%.
2.3. X-ray crystallography
Suitable crystals of L1ZnBr (1), L2ZnBr (2), L3ZnBr (3), L4ZnBr (4),
L4ZnCN (8) and [L3CdCl]2 (10) were mounted on a glass fiber using
hydrocarbon oil and cooled under a nitrogen stream to 150(1) K. A
Nonius Kappa CCD diffractometer (Mo Ka radiation; k = 0.71073 Å)
was used for data collection. Unit cell parameters were determined
from 10 data frames with an oscillation range of 1 deg/frame and
an exposure time of 20 s/frame. Indexing and unit cell refinement
based on the reflections from the initial set of frames were consis-
tent with monoclinic P lattices for 2, 3 and 10, and triclinic P lat-
tices for 1, 4 and 8. The intensity data for each compound was
then collected. These reflections were then indexed, integrated
and corrected for Lorentz, polarization and absorption effects using
DENZO-SMN and SCALEPAC [9]. The space group for each com-
pound was determined from the systematic absences in the dif-
fraction data. The structures were solved by a combination of
direct and heavy atom methods using SIR 97 [10]. Compound 1
crystallized with two crystallographically independent molecules
per unit cell and the carbon atoms of one isopropyl group were
disordered over two positions with a 74:26 occupancy ratio. Com-
pounds 4 and 8 each contain a molecule of disordered cyclohexane
that rests on an inversion center within the asymmetric unit.
Compound 10 contains two molecules of disordered CHCl3. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were placed in ideal positions
and assigned isotropic displacement coefficients U(H) = 1.2U(C)
or 1.5U(Cmethyl) using SHELXL-97 [11]. X-ray crystallographic data
and collection parameters are listed in Table 1 and selected inter-
atomic distances and angles are provided in Tables 2 and 3.
2.2.13. Preparation of L1CdCl (9)
CdCl2 (0.183 g, 1.0 mmol) was added to Li(L1) (0.400 g,
1.0 mmol) in dry MeOH (20 mL). The reaction was stirred for
24 h, during which time an orange solid formed. The orange solid
was collected and dried under reduced pressure (0.380 g, 71%).
1H NMR (CD3CN, 600 MHz): d 7.73 (s, 1H), 6.26 (s, 2H), 3.31 (m,
4H), 1.30 (m, 24H). 1H NMR (CDCl3, 600 MHz): d 7.00 (s, 1H),
6.03 (s, 2H), 3.51 (m, 2H), 3.03 (m, 2H), 1.40 (d, 6H), 1.34 (d, 6H),
1.27 (d, 6H), 1.09 (d, 6H). 13C NMR (solid, 150 MHz): d 244.5,
161.7, 155.7, 98.8, 76.2, 28.6, 26.6, 24.5, 24.5, 24.1, 22.1. IR (ATR,
cmꢀ1): 2967 (m), 2930 (w), 2868 (w), 1552 (m), 1470 (s), 1400
(m), 1382 (m), 1365 (m), 1306 (s), 1293 (s), 1242 (m), 1224 (w),
1182 (m), 1153 (w), 1080 (s), 1055 (s), 1019 (m), 962 (w), 930
(w), 909 (w), 881 (w), 849 (m), 831 (s), 804 (m), 796 (s), 761 (s),
722 (m), 692 (m), 683 (m) 668 (m). Anal. Calc. for C20H31ClN4S2Cd:
C, 44.53; H, 5.79; N, 10.39. Found: C, 44.15; H, 5.91; N, 10.31%.
2.2.14. Preparation of L3CdCl (10)
CdCl2 (0.139 g, 0.76 mmol) was added to [Li(L3)]ꢂTHF (0.400 g,
0.76 mmol) in dry EtOH (20 mL). The reaction was stirred for
24 h, during which time a white solid formed. The white solid
was collected and dried under reduced pressure (0.390 g, 86%).
Clear crystals suitable for crystallographic characterization were
obtained by hexane diffusion into CHCl3 at room temperature. 1H
NMR (CD3CN, 600 MHz): d 7.30 (t, 3H), 7.06 (t, 1H), 6.73 (d, 2H),
6.31 (s, 2H), 3.41 (m, 2H), 3.22 (m, 2H), 1.39 (d, 6H), 1.33 (d, 6H),
1.28 (d, 6H), 1.21 (d, 6H). 1H NMR (CDCl3, 600 MHz): d 7.33 (s,
1H), 7.28 (t, 2H), 7.05 (t, 1H), 6.80 (d, 2H), 6.08 (s, 2H), 3.28 (m,
2H), 3.16 (m, 2H), 1.42 (d, 6H), 1.35 (d, 6H), 1.30 (d, 6H), 1.26 (d,
6H). 13C NMR (CDCl3, 150 MHz): d 169.0, 161.0, 153.4, 150.6,
128.9, 124.0, 120.1, 100.1, 73.5, 27.9, 26.2, 23.51, 23.47, 22.7,
22.4. IR (ATR, cmꢀ1): 2963 (m), 2929 (w), 2870 (w), 1597 (w),
1559 (s), 1548 (s), 1485 (w), 1458 (m), 1383 (m), 1365 (w), 1342
2.4. Diffusion ordered spectroscopy (DOSY)
Spectra were collected on a Bruker Avance III spectrometer
operating at 600 MHz running Topspin 2.1.6. The instrument was
equipped with a 5 mm HCN inverse probe (TXI) having a z-axis gra-
dient coil, having a specified gradient strength of 3.57 G cmꢀ1 Aꢀ1
.
Samples (ꢃ5 mg) were dissolved in deuterated acetonitrile
(ꢃ0.6 mL) and run at 293 K without sample spinning.
The DOSY analysis followed Bruker’s documented procedure
[12]. Diffusion data were obtained using the stebpgp1s Bruker
pulse sequence, a variation of the variable-gradient Stejskal–
Tanner experiment [13]. The reported diffusion coefficients were
calculated from the NMR data using the T1/T2 Relaxation Module
software provided in the Topspin analysis menu. This program

566
N.G. Spiropulos et al. / Inorganica Chimica Acta 376 (2011) 562–573
Table 1
Summary of X-ray crystallographic data and parameters.a
1
2
3
4ꢂ½(C6H12
)
8ꢂ½(C6H12
)
10ꢂ2CHCl3
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C20H31BrN4S2Zn
536.89
C24H39BrN4S2Zn
592.99
monoclinic
P21/n
14.3410(2)
11.9119(2)
16.8844(3)
90
105.3060(11)
90
C26H36BrN5SZn
595.94
monoclinic
P21/m
8.4632(2)
13.8637(2)
11.7280(2)
90
98.1392
90
C33H50BrN5SZn
694.12
C34H50N6SZn
640.23
C54H74Cd2Cl8N10S2
1435.76
monoclinic
P21/n
11.8790(3)
15.0492(2)
18.8828(4)
90
103.8016(7)
90
3278.20(12)
2
triclinic
triclinic
triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
10.02770(10)
13.0702(2)
18.9911(3)
109.5399(6)
93.3334(9)
90.6329(8)
2340.50(6)
4
9.44230(10)
11.26220(10)
17.1048(3)
92.6532(8)
91.3042(7)
108.7771(8)
1718.92(4)
2
9.5212(2)
11.1733(2)
17.0612(3)
91.8074(10)
92.8982(11)
110.1479(9)
1699.46(6)
2
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
2782.03(8)
4
1.416
1362.20(4)
2
1.453
Z
Dcalc (Mg mꢀ3
T (K)
)
1.524
150(1)
1.341
150(1)
1.251
150(1)
1.455
150(1)
150(1)
150(2)
Color
orange
orange
colorless
0.28 ꢁ 0.23 ꢁ 0.08
2.468
2.29–27.48
99.7
colorless
0.30 ꢁ 0.30 ꢁ 0.10
1.966
2.20–27.48
99.7
colorless
0.23 ꢁ 0.18 ꢁ 0.15
0.816
2.39–27.47
99.6
colorless
0.33 ꢁ 0.30 ꢁ 0.10
1.081
2.29–27.53
97.1
Crystal size (mm)
Abs coefficient (mmꢀ1
h Range (°)
0.30 ꢁ 0.23 ꢁ 0.18
0.25 ꢁ 0.20 ꢁ 0.18
)
2.948
2.488
1.14–27.47
99.3
2.12–25.34
99.7
Completeness to h (%)
Reflections collected
19 885
9331
6054
14 948
14 670
13 608
Independent reflections
Parameters
10 630
543
0.0496/0.0914
1.018
0.495/ꢀ0.593
5084
301
0.0754/0.2039
1.158
2.325/ꢀ0.732
3231
250
0.0367/0.0679
1.044
0.378/ꢀ0.463
7854
366
0.0445 /0.0869
1.050
0.639/ꢀ0.618
7753
380
0.0602/0.1058
1.029
0.432/ꢀ0.655
7349
380
0.0501/0.0834
1.057
0.749/ꢀ0.822
R1/wR2 (all data)b
Goodness-of-fit (GOF)
Difference in peak/hole (e Åꢀ3
)
a
Radiation used: Mo K
a
(k = 0.71073 Å).
P
P
P
P
R1
=
||Fo| ꢀ |Fc||/ |Fo|; wR2 = [ [w(F2o ꢀ Fc2)2]/[ (F2o )2]]1/2, where w = 1/[
r
2(F2o ) +(aP)2 + bP].
b
provided the diffusion coefficients by directly fitting the data to a
gaussian profile, in contrast to the alternative linearized Stejskal–
Tanner attenuation plots.
Experiments were performed with identical experimental
parameters: 8 s 1H 90° pulse, 1 ms gradient pulse duration, and
l
96.8 ms diffusion time; pulsed field-gradients were varied from
2% to 95% over 16 steps. The residual solvent signal for CD3CN
was used as the internal reference and its diffusion coefficient
(DCD3CN) was determined to be 4.12 ꢁ 10ꢀ9 m2 sꢀ1 ( 0.19). The
DCD3CN obtained in this study is similar to the value obtained by
Kato et al. (4.3 ꢁ 10ꢀ9 m2 sꢀ1) [14]. The Stokes–Einstein equation
[15] was used to calculate the hydrodynamic radii (r) for the halide
complexes 1–4 and 9–11 from their diffusion coefficients (D), using
a literature value of 0.39 ꢁ 10ꢀ3 kg sꢀ1 mꢀ1 (293 K) for the acetoni-
trile viscosity [16]. The calculated radii (r0) from the solid state
structures were determined by approximating the shape of the
molecules as prolate ellipsoids [17].
Table 2
Selected interatomic distances (Å) and angles (°) for Zn(II) complexes.a
L1ZnBr (1)
Zn(1)–N(2)
Zn(1)–N(3)
Zn(1)–Br(1)
Zn(1)–S(1)
S(1)–C(1)
2.0413(19)
2.0491(19)
2.3031(4)
2.3207(7)
1.717(2)
N(2)–Zn(1)–N(3)
N(2)–Zn(1)–S(1)
N(3)–Zn(1)–S(1)
N(2)–Zn(1)–Br(1)
N(3)–Zn(1)–Br(1)
Br(1)–Zn(1)–S(1)
90.78(7)
95.00(6)
93.92(6)
116.83(5)
118.86(5)
131.95(2)
S(2)–C(1)
1.634(2)
L2ZnBr (2)
Zn(1)–N(1)
Zn(1)–N(3)
Zn(1)–Br(1)
Zn(1)–S(1)
S(1)–C(4)
2.054(7)
2.138(7)
2.3188(12)
2.334(2)
1.714(9)
1.633(8)
N(1)–Zn(1)–N(3)
N(1)–Zn(1)–S(1)
N(3)–Zn(1)–S(1)
N(1)–Zn(1)–Br(1)
N(3)–Zn(1)–Br(1)
Br(1)–Zn(1)–S(1)
86.8(3)
93.1(2)
100.0(2)
128.97(19)
122.16(18)
117.93(7)
2.5. DFT calculations
S(2)–C(4)
All DFT calculations were performed with the GAUSSIAN 03W [18]
suite of programs (Revision E.01) using the restricted B3LYP func-
tional [19]. Geometry optimizations were performed without sym-
metry constraints using atomic coordinates from experimental X-
ray crystal structures as input geometries, when available. For all
optimizations, the Stuttgart/Dresden (SDD) effective core potential
and basis set [20] was used for zinc and the 6-31g(d,p) basis set
L3ZnBr (3)
Zn(1)–N(2)
Zn(1)–N(2A)
Zn(1)–Br(1)
Zn(1)–S(1)
S(1)–C(1)
2.0583(14)
2.0584(14)
2.3093(4)
2.3031(7)
1.751(2)
N(2)–Zn(1)–N(2A)
N(2)–Zn(1)–S(1)
N(2A)–Zn(1)–S(1)
N(2)–Zn(1)–Br(1)
N(2A)–Zn(1)–Br(1)
Br(1)–Zn(1)–S(1)
88.95(8)
97.01(4)
97.01(4)
120.21(4)
120.21(4)
125.31(2)
N(3)–C(1)
1.291(3)
L4ZnBr (4)
Zn(1)–N(4)
Zn(1)–N(1)
Zn(1)–Br(1)
Zn(1)–S(1)
S(1)–C(8)
2.0714(18)
2.1080(18)
2.3172(3)
2.3055(6)
1.741(2)
N(4)–Zn(1)–N(1)
N(4)–Zn(1)–S(1)
N(1)–Zn(1)–S(1)
N(4)–Zn(1)–Br(1)
N(1)–Zn(1)–Br(1)
Br(1)–Zn(1)–S(1)
85.61(7)
99.05(5)
98.26(5)
124.40(5)
125.91(5)
116.470(18)
Table 3
Selected interatomic distances (Å) and angles (°) for [L3CdCl]2 (10).a
[L3CdCl]2 (10)
N(5)–C(8)
1.275(3)
Cd(1)–N(1)
Cd(1)–N(4)
Cd(1)–S(1)
Cd(1)–Cl(1)
Cd(1)–Cl(1A)
N(4)–Cd(1)–N(1)
N(4)–Cd(1)–Cl(1)
N(1)–Cd(1)–Cl(1)
2.3778(19)
2.294(2)
2.4859(6)
2.4541(6)
2.7174(6)
78.50(7)
N(4)–Cd(1)–S(1)
N(1)–Cd(1)–S(1)
Cl(1)–Cd(1)–S(1)
N(4)–Cd(1)–Cl(1A)
N(1)–Cd(1)–Cl(1A)
Cl(1)–Cd(1)–Cl(1A)
S(1)–Cd(1)–Cl(1)
Cd(1)–Cl(1)–Cd(1A)
92.26(5)
89.61(5)
136.89(2)
93.78(5)
172.16(5)
84.30(2)
92.09(2)
95.70(2)
L4ZnCN (8)
Zn(1)–N(4)
Zn(1)–N(1)
Zn(1)–C(8)
Zn(1)–S(1)
S(1)–C(9)
2.0855(17)
2.0527(17)
1.961(2)
2.3056(6)
1.739(2)
N(4)–Zn(1)–N(1)
N(4)–Zn(1)–S(1)
N(1)–Zn(1)–S(1)
N(4)–Zn(1)–C(8)
N(1)–Zn(1)–C(8)
C(8)–Zn(1)–S(1)
85.53(7)
98.59(5)
98.37(5)
125.93(8)
123.98(8)
117.10(7)
130.82(5)
99.65(5)
C(8)–N(5)
1.139(3)
a
a
Estimated standard deviations in parentheses and ‘‘A’’ refers to symmetry-
related atoms.
Estimated standard deviations in parentheses and ‘‘A’’ refers to symmetry-
related atoms.

N.G. Spiropulos et al. / Inorganica Chimica Acta 376 (2011) 562–573
567
[21] was used for all other atoms. All optimized structures were
verified as energetic minima by the absence of imaginary frequen-
cies in subsequent frequency calculations using this same basis set
combination. Natural population analysis using the methods of
Weinhold and co-workers [22] was performed on optimized geom-
etries during single point energy calculations using the SDD effec-
tive core potential and basis set [21] for zinc and the TZVP basis set
of Ahlrichs and co-workers [23] for all other atoms. Compositions
of molecular orbitals and overlap populations between molecular
fragments were calculated using the AOMIX program with this same
basis set combination [24]. Analysis of molecular orbitals using
fragment orbital compositions, charge decomposition analyses
(CDA), and the construction of orbital interaction diagrams were
Zn(II) and Cd(II) halide complexes 1–4 and 9–11 were prepared
in 69–94% yield by treatment of the lithium compounds of L1–L4
with either ZnBr2 or CdCl2 in alcoholic solvent (Scheme 1). Each
of the metal products were precipitated from the reaction mixture
as pure solids, with the choice of alcohol solvent (MeOH or EtOH)
dependent on the insolubility of the specific products. An attempt
to make the cadmium chloride complex using L2 resulted in an
inseparable mixture of products and was not further pursued.
Compounds 1–4 and 9–11 were characterized using 1H and 13C
NMR, IR, elemental analysis (CHN) and, in the case of compounds
1–4 and 10, by single-crystal X-ray crystallography.
Synthetic studies were also undertaken to determine the elec-
tronic properties of ligands L1–L4 by preparing zinc cyanide com-
plexes 5–8 (Scheme 1). The stretching frequencies and 13C NMR
chemical shifts of metal-bound cyanides are known to be sensitive
to the Lewis acidity of a metal center, which is influenced by the
electron-donating properties of the supporting auxillary ligands.
Attempts to prepare complex 6 were unsuccessful and led only
to an inseparable mixture of products. Even though 6 could not
be isolated synthetically, it was studied computationally. Com-
plexes 5, 7 and 8 were characterized by 1H and 13C NMR, IR, ele-
mental analysis and X-ray crystallography. 13C NMR spectroscopy
and X-ray crystallography confirmed that complexes 5, 7 and 8
each contain a coordinated CNꢀ ligand. The 13C NMR spectra of
5–8 show slight differences in chemical shift for the carbon reso-
nance of the bound cyanide anion (d ꢄ 134 ppm for 5 and 7;
d ꢄ 138 ppm for 8), suggesting L1–L4 have similar electron-donat-
ing properties. Surprisingly, the FT-IR spectra of 5–8 display extre-
mely weak mCN stretching vibrations, which prevented their
assignment.
performed using AOMIX-CDA [24].
3. Results and discussion
3.1. Ligand synthesis
The synthetic procedure for the new heteroscorpionate ligands
having dithioacetate or thioacetamide functional groups and bulky
pyrazole substituents is shown in Scheme 1. Deprotonation of the
methylenegroup of eitherbis(3,5-diisopropylpyrazol-1-yl)methane
or bis(3,5-di-tert-butylpyrazol-1-yl)methane with nBuLi in THF at
ꢀ78 °C followed by reaction with carbon disulfide or phenyl isothi-
ocyanate at ꢀ40 °C gave the lithium dithioacetate or thioacetamide
compounds Li(L1) [L1 = bis(3,5-diisopropylpyrazol-1-yl)dithioace-
tate], Li(L2) [L2 = bis(3,5-di-tert-butylpyrazol-1-yl)dithioacetate],
Li(L3) [L3 = N-phenyl-2,2-bis(3,5-diisopropylpyrazol-1-yl)thioace-
tamide], and Li(L4) [L4 = N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-
1-yl)thioacetamide]. The compounds were isolated as air-stable
white or orange colored solids in good yields (ca. 78%). The synthetic
procedure is similar to the strategy reported by Otero et al. [1a,2b].
The lithium salts were characterized by 1H and 13C NMR, IR and
electrospray mass spectrometry (ESI-MS). The 1H and 13C NMR
spectra of each compound show magnetically equivalent pyrazole
rings indicative of complexes having CS-symmetry in solution.
With the exception of Li(L1), each complex contains a THF solvent
molecule, as observed by 1H and 13C NMR. One possible explana-
tion for the absence of a THF molecule in Li(L1) is loss due to drying
under vacuum. For Li(L1) and Li(L2), the 13C NMR signal for the
thiocarbonyl of the dithioacetate group is observed at 243.2 and
248.1 ppm, while, for Li(L3) and Li(L4), the 13C NMR signal for the
imine of the thioacetamide group is observed at 181.9 and
186.5 ppm. The high resolution ESI-MS (negative mode) spectra
of each compound in a chloroform/methanol solution contained
only peaks with m/z values consistent with the molecular ion
[Ln]ꢀ, indicating loss of the lithium ion and THF. Recently, the solid
state structure of Li(L2) was reported by Marchiò et al. to have the
trinuclear composition [Li(L2)]3ꢂ(2.25)THF [1j].
3.3. X-ray structures of zinc bromide complexes 1–4
The solid state structures of L1ZnBr (1) and L2ZnBr (2) with sup-
porting ligands containing the dithioacetate functional group are
shown in Fig. 1. A summary of the X-ray crystallographic data
and selected bond distances and angles are given in Tables 1 and
2. Each compound is monomeric and contains a monoanionic N2S
ligand coordinated to a zinc center through two pyrazolyl nitrogen
atoms and one dithioacetate sulfur atom. In addition, each zinc
atom is coordinated to one bromide ligand in a distorted tetrahe-
dral geometry. Deviation from an idealized tetrahedral geometry
(109.5°) is most noticeable in 2. The extent of the structural devi-
ation in 2 is evident by the differences in the bond angles, with
bond angles varying from 86.8(3)° for the N3–Zn1–N1 angle to
128.97(19)° for the N1–Zn1–Br1 angle. Another measure for deter-
mining the degree of distortion is made by measuring the angle be-
tween the N3–Zn1–N1 and S1–Zn1–Br1 planes. An angle of 90° is
observed in an idealized tetrahedral geometry, while an angle of
0° is observed in a square planar geometry. The angle in 1 and 2
are 89.1° and 84.7°, respectively, illustrating the greater structural
deviation from an idealized tetrahedral geometry in 2 compared to
1. Indeed, the bulky tert-butyl groups on L2 cause the two pyrazole
rings to twist, as evidenced by the angle between the two rings
(41.64°) and the Zn–S–C–C torsion angle of the thioacetate bridge
(24.6(6)°). This twisting also results in two inequivalent Zn–N bond
lengths (2.054(7) and 2.138(7) Å) and a bromide ligand bent to-
wards N3. In contrast the less bulky iPr groups of L1 result in signif-
icantly less structural distortion, with the observed twist angle of
the two pyrazoles at only 3.93° and nearly identical Zn–N bond
lengths (2.0413(19) and 2.0491(19) Å).
3.2. Synthesis of metal complexes 1–11
The N2Sꢀ ligands L1–L4 contain steric and electronic variations
that could potentially have an important impact on the properties
of a bound metal ion. Such variations could be potentially mani-
fested as differences in the electronic properties of the complexes
or in the coordination environment of the metal ions. As noted pre-
viously, this ligand set is particularly relevant to the study of
metalloproteins containing zinc(II) ions. Complexes of Zn(II) were
therefore chosen as ideal candidates for studying the effects of
these ligands. Moreover, because of the larger ionic radius of cad-
mium(II) ions and their ability to substitute for zinc(II) ions in en-
zymes and other coordination complexes, we chose to explore the
analogous chemistry with Cd(II).
The solid state structures of L3ZnBr (3) and L4ZnBr (4) having
the thioacetamide S-donor groups were also determined crystallo-
graphically. Their ORTEP diagrams are shown in Fig. 2 and Table 2
gives selected bond distances and angles. Complexes 3 and 4, also
exhibit facial coordination of their respective ligands and adopt

568
N.G. Spiropulos et al. / Inorganica Chimica Acta 376 (2011) 562–573
Fig. 1. ORTEP diagrams of complexes (a) L1ZnBr (1) and (b) L2ZnBr (2). Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted for clarity.
Fig. 2. ORTEP diagrams of complexes (a) L3ZnBr (3) and (b) L4ZnBr (4). Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted for clarity. ‘‘A’’
refers to symmetry-related atoms.
distorted tetrahedral geometries. The smallest bond angle in both
complexes is the N–Zn–N angle (88.95° in 3, 85.61° in 4), while
the largest angles are the Br–Zn–S angle (125.31°) in 3 and the
N1–Zn1–Br1 angle (125.91°) in 4. The Zn–Br distance
(2.3093(4) Å in 3, 2.3172(3) Å in 4) is similar to the Zn–Br distance
in 1 (2.3031(4) Å) and 2 (2.3188(12) Å) and other tetrahedral Zn(II)
complexes containing a terminal bromide [25]. The average Zn–S
bond length in 3 and 4 (2.3043 Å) is slightly shorter than that in
1 and 2 (2.3274 Å). However these values are all in the range of
analogous four-coordinate Zn(II) complexes (2.25–2.36 Å) with
nonbridging thiolate or thiophenolate ligands [25a,26].
3.4. X-ray structure of zinc cyanide complex 8
The molecular structure of 8 is shown in Fig. 3 and Table 2 gives
selected bond angles and distances. The zinc center adopts a
four-coordinate distorted tetrahedral geometry. The smallest bond
angle is the N4–Zn1–N1 angle (85.53(7)°) and the largest is the
N4–Zn1–C8 angle (125.93(8)°). Compared with 4, which is also
supported by L4, the Zn–N distances in 8 are slightly shorter
(ꢃ0.02 Å) but the Zn–S distances are statistically identical (8,
2.3056(6) Å; 4, 2.3055(6) Å). The C–N bond length of 1.139(3) Å
for the cyanide ligand is slightly longer than that reported for the
related complex {HB(3-Butpz)3}ZnCN (C–N, 1.117(9) Å; Zn–C,
1.962(6) Å), but the Zn–C bond length of 1.961(2) Å in 8 is identical
[25c].
3.5. X-ray structure of [L3Cd(II)Cl]2 (10)
The structure of the representative complex L3Cd(II)Cl was
structurally characterized by X-ray crystallography, with the
structure shown in Fig. 4 and selected crystallographic data listed
Fig. 3. ORTEP diagram of complex L4ZnCN (8). Thermal ellipsoids are drawn at the
50% probability level and hydrogen atoms are omitted for clarity.

N.G. Spiropulos et al. / Inorganica Chimica Acta 376 (2011) 562–573
569
Fig. 4. ORTEP diagram of complex [L3CdCl]2 (10). Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms are omitted for clarity. ‘‘A’’ refers to symmetry-
related atoms.
in Table 3. In contrast to zinc complexes 1–8, the structure of the
cadmium(II) chloride analogue 10 is dimeric. Each cadmium ion
is in a distorted trigonal bipyramidal environment consisting of
two nitrogen atoms and a sulfur atom from the L3 ligand and
two chlorine atoms from the bridging chloride ligands. The equato-
rial plane is defined by the sulfur atom S1, the bridging chlorine
atom Cl1 and the nitrogen atom N4. The equatorial S1–Cd1–N4 an-
gle of 92.26(5)° is smaller than the S1–Cd1–Cl1 and N4–Cd1–Cl1
angles of 136.89(2)° and 130.82(5)°, respectively. The axial posi-
tions consist of the nitrogen atom N1 and the other bridging chlo-
rine atom Cl1A. The N1–Cd1–Cl1A angle is almost linear at
(1.25 ꢁ 10ꢀ9 m2 sꢀ1 for 9, 1.16 ꢁ 10ꢀ9 m2 sꢀ1 for 10 and 1.14 ꢁ
10ꢀ9 m2 sꢀ1 for 11) and hydrodynamic radii (r = 4.40 Å for 9,
4.74 Å for 10, and 4.83 Å for 11) obtained are similar to the values
obtained for complexes 1–4. Indeed, the calculated hydrodynamic
radius of 10 determined from the X-ray crystal structure is 7.7 Å,
ꢃ38% larger than that obtained from the DOSY experiment
(4.74 Å). These data suggest that, like 1–4, the cadmium complexes
9–11 are also monomeric species in solution, despite the dimeric
solid state structure obtained for 10.
3.7. Computational studies
172.16(5)°. The Cd2(l-Cl)2 core is asymmetric, with each cadmium
ion having a long Cd–Cl distance (2.7174(6) Å) and a short Cd–Cl
distance (2.4541(6) Å). The two cadmium centers are separated
by an intramolecular Cd1ꢂꢂꢂCd1A distance of 3.838 Å, which is
similar to values reported for other 5-coordinate, dimeric Cd(II)
complexes with bridging chlorides [27]. The Cd1–S1 distance
(2.4859(6) Å) in 10 is similar to Cd–S distances observed in cad-
mium(II) complexes with aliphatic thiolates (2.47–2.55 Å) [28]
but shorter than cadmium(II) complexes with thioethers (2.58–
2.76 Å) [27b,29].
Zinc(II) complexes, like other Lewis acids, facilitate reactions by
accepting electron density from substrate molecules and by polar-
izing the electronic distribution within these molecules [30–32].
Because the ability of the zinc ion to perform these tasks is directly
related to the charge density of the ion [31], the nature of the sup-
porting auxiliary ligand on the zinc center and its ability to donate
charge to the metal center can have an important impact. To help
elucidate the potential impact of the structural and electronic dif-
ferences in the N2Sꢀ ligands on the electronic nature of the zinc
center, complexes 1–8 were investigated using density functional
theory. DFT-optimized geometries were obtained for all complexes
using the restricted B3LYP functional [19], the SDD basis set and
effective core potential [20] for zinc, and the 6-31g(d,p) basis set
[21] for all other atoms. As shown from the overlay in Fig. 5, the
geometry-optimized structures of the LZnBr complexes 1–4 are
in good agreement with the experimental X-ray crystal structures
(selected geometric parameters are listed in Table S1). Generally,
only minor deviations of ligand alkyl substituents are observed.
One notable exception is the phenyl ring of the thioacetamide li-
gand L3, which is rotated ꢃ56° in the calculated structure of 3 rel-
ative to the crystal structure. The calculated Zn–N bond lengths are
slightly elongated relative to the X-ray structures (elongation of
0.025–0.072 Å; average = 0.059 Å). All calculated Zn–S and Zn–Br
bond lengths are in excellent agreement with the experimental
structures, with all deviations from the experimental structures
of less than 0.02 Å.
3.6. Solution state behavior of zinc and cadmium halide complexes
Diffusion ordered spectroscopy (DOSY) in CD3CN was used to
examine possible differences between the solution and solid state
structures of complexes 1–4 and 9–11. DOSY NMR allows for the
determination of the diffusion coefficient (D) of a molecule in
solution, which is inversely related to the hydrodynamic radius
(r) of the molecule [15]. For 1–4, diffusion coefficients of 1.35 ꢁ
10ꢀ9 m2 sꢀ1 for 1, 1.30 ꢁ 10ꢀ9 m2 sꢀ1 for 2, 1.17 ꢁ 10ꢀ9 m2 sꢀ1 for
3 and 1.28 ꢁ 10ꢀ9 m2 sꢀ1 for 4 were obtained. These diffusion coef-
ficients afforded hydrodynamic radii of r = 4.08 Å for 1, 4.23 Å for 2,
4.70 Å for 3 and 4.30 Å for 4. The radii (r) obtained from the DOSY
experiments do not differ significantly from the calculated radii
(r0), which were determined from the X-ray crystal structures of
1–4 (r0 = 4.3 Å for 1, 4.5 Å for 2, 4.7 Å for 3 and 4.8 Å for 4). The re-
sults of the DOSY and X-ray studies are therefore consistent with
complexes 1–4 having a mononuclear structure both in solution
and in the solid state.
Natural atomic orbital populations and related natural atomic
charges [22] were subsequently computed for the optimized
geometries of 1–4 at the B3LYP level of theory [20] using the
SDD basis set and effective core potential [21] for zinc and the
Analogous DOSY experiments were conducted for cadmium
complexes 9–11, and surprisingly, the diffusion coefficients

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N.G. Spiropulos et al. / Inorganica Chimica Acta 376 (2011) 562–573
Table 4
Calculated natural atomic charges for selected atoms in LZnBr complexes 1–4 and
related ‘‘bare’’ [LZn]+ fragments.a
1
2
3
4
1-barea 2-barea 3-barea 4-barea
Zn
N
0.85
0.88
0.87
0.89
1.12
1.11
ꢀ0.43
ꢀ0.43
ꢀ0.08
N/A
1.16
ꢀ0.41
ꢀ0.41
ꢀ0.24
N/A
1.13
ꢀ0.42
ꢀ0.43
ꢀ0.24
N/A
ꢀ0.35 ꢀ0.37 ꢀ0.34 ꢀ0.36 ꢀ0.42
N0 ꢀ0.35 ꢀ0.37 ꢀ0.34 ꢀ0.36 ꢀ0.42
S
ꢀ0.08 ꢀ0.10 ꢀ0.25 ꢀ0.27 ꢀ0.07
Br ꢀ0.53 ꢀ0.53 ꢀ0.54 ꢀ0.54 N/A
a
Geometries of [LZn]+ fragments used were those obtained from the optimiza-
tion of the full LZnBr complexes.
Fig. 5. Structural overlay of X-ray crystal structures (blue) with geometry
optimized structures (red) for LZnBr complexes 1–4. (For interpretation of the
references in colour in this figure legend, the reader is referred to the web version of
this article.)
Fig. 6. Natural atomic charge distribution for selected atoms within [LZn]+
fragments for dithioacetate and thioacetamide ligands L1 and L3 (hydrogens and
isopropyl groups omitted for clarity).
TZVP basis set [23] for all other atoms. Natural atomic orbitals and
natural atomic charges are useful for quantifying the charge distri-
bution within a metal complex. Thus, comparison of the results
within this series of complexes can provide a means of comparing
the electronic impact of the dithioacetate and thioacetamide li-
gands on the charge on the zinc center. Indeed, the use of natural
population and Mulliken charge analysis on related zinc(II) com-
plexes of N2Sꢀ ligands has been recently shown to yield excellent
insight into the nature of zinc(II)-catalyzed reactions [33]. The cal-
culated natural atomic charges for the zinc ions in 1–4 range from
0.85 to 0.89, consistent with a net delocalization of electron den-
sity (ꢃ1.1 electrons) onto the zinc center from the bromide ion
and the supporting N2Sꢀ ligands. These values are consistent with
the anticipated Lewis acidic role of the zinc(II) ion and the electron
donating natures of the two coordinated anions.
The calculated natural atomic charges for the zinc ions in 1–4
are very similar to one another, suggesting that the variations
within the N2Sꢀ ligands have relatively little impact on the zinc
center. The charge donation to the zinc center in these complexes
is the combined amount from both the Brꢀ and the N2Sꢀ ligands,
however, which complicates the individual quantification of the
N2Sꢀ ligand component. To help quantify the relative electron
donation from ligands L1–L4 without the influence of the bromide
ion, natural atomic charges were similarly computed for the [LZn]+
fragments after removal of the bromide ion from complexes 1–4.1
As shown in Table 4, the natural atomic charges for the ‘‘bare’’ zinc
center in the [LZn]+ fragments are virtually unchanged across the
series (1.11–1.16), again suggesting a similar magnitude of elec-
tron donation from L1–L4 to the zinc ion (0.84–0.89 electrons) de-
spite the changes in both the electronic and structural nature of the
N2Sꢀ ligands. Fig. 6 shows the calculated natural atomic charges for
selected atoms in the [L1Zn]+ and [L3Zn]+ fragments for compari-
son. While slight variations in the zinc natural atomic charges
are observed between the dithioacetate ligands and the thioaceta-
mide ligands, as well as with changes in alkyl group substitution,
the effects are relatively minor.
analysis. Analysis of the natural atomic orbital populations of the
[LZn]+ fragments in 1–4 reveals significant population of the zinc
orbitals resulting from interaction with the N2Sꢀ ligands (Table
S3). The average Zn(4s) orbital population in this series of com-
plexes is approximately 0.58 electrons, and the total Zn(4p) orbital
population is approximately 0.29 electrons. These values are simi-
lar within all four complexes, leading to comparable natural atomic
charges for the zinc centers. These results suggest that, in the bare
[LZn]+ complexes, the Zn(4s) orbital plays the dominant role in
accepting electrons from the N2Sꢀ ligands, while the Zn(4p) orbi-
tals play a lesser role.
Because the strength of a Lewis acid is related not only to the
charge of the metal center, but also to its ability to induce charge
polarization within substrate molecules, the binding of the polariz-
able cyanide (CNꢀ) ligand with the [LZn]+ fragments was also
investigated. Cyanide is a strong
r-donor and a poor p-acceptor,
and comparison of the charge distribution within the CNꢀ ligand
among the series of complexes 5–8 could help quantify the relative
Lewis acidity of the different [LZn]+ fragments. The geometry
optimized structures of the analogous LZnCN complexes were
computed and are shown in Fig. S1, with selected structural
parameters given in Table S2. The DFT calculated geometric param-
eters for complex 8 are in good agreement with the X-ray structure
of this complex, which is the only one for which a crystal structure
was obtained. As in complexes 1–4, the calculated Zn–N bond
lengths in 8 are lengthened by an average of 0.06 Å relative to
the crystal structure, while the calculated Zn–S and Zn–C bond
lengths deviate from the experimental values by less than 0.015 Å.
The calculated mCN values for LZnCN range from ꢃ2191 to
2196 cmꢀ1, which is within the range typically observed for other
metal–cyanide complexes [30,34].2 Surprisingly, no CN vibrations
were observed in the experimental FT-IR spectra of complexes
5–8, despite the conformation of a coordinated CNꢀ ligand by
X-ray crystallography and 13C NMR spectroscopy. The simulated
IR spectra of these complexes were obtained, however, and that
To further understand the charge localization onto the Zn(II) ion
in the bare [LZn]+ complexes, the electron occupancies of the
Zn(4s) and Zn(4p) orbitals were investigated by natural population
2
Calculated using the B3LYP functional, the SDD basis set and effective core
potential for Zn, and the 6-31g(d,p) basis set for all other atoms. These vibrational
frequency values have been scaled by a factor of 0.961 as recommended for this
functional and basis set combination.
1
Note: Geometries of the [LZn]+ fragments were not optimized in the absence of
the bromide ion prior to natural population analysis.

N.G. Spiropulos et al. / Inorganica Chimica Acta 376 (2011) 562–573
571
of the representative complex 8 is shown in Fig. S2. Indeed, the CN
vibration is predicted to give rise to a very low intensity IR absorp-
tion that is consistent with the lack of an observable CN stretch in
the experimental spectrum of these complexes [34].
CNꢀ (29% on C; 71% on N), the charge distribution within the
CNꢀ anion is slightly more polarized towards the carbon atom in
compounds 6 and 8 (34% on C; 66% on N). While these differences
are small, consistent shifts in the 13C NMR resonance for the bound
cyanide ligand in these complexes (d ꢄ 134–135 ppm for 5 and 7;
d ꢄ 138 ppm for 8) do suggest the possibility of electronic differ-
ences in the zinc centers imposed by variations in the N2Sꢀ ligands.
Such differences and their potential use in probing the Lewis acid-
ity of zinc(II) complexes are currently being explored further.
To gain further insight into the similarities and differences im-
parted upon the [LZn]+ fragments by the different classes of N2Sꢀ
ligands, important orbital interactions between the [LZn]+ frag-
ments and the CNꢀ anion for dithioacetate and thioacetamide li-
Natural atomic orbital populations and related natural atomic
charges [22] were computed for the optimized geometries of 5–8
at the B3LYP level of theory [20] using the SDD basis set and effec-
tive core potential [21] for zinc and the TZVP basis set [23] for all
other atoms. As seen in Table 5, the calculated natural atomic
charges of the zinc centers in the LZnCN complexes range from
0.85–0.89, while the calculated charges of the zinc centers in the
related ‘‘bare’’ [LZn]+ fragments range from 1.11–1.16. These
charges on the zinc centers of the LZnCN complexes are the same
as those computed for the LZnBr complexes 1–4 (0.85–0.89), sug-
gesting a comparable amount of charge delocalization onto the
zinc centers. Indeed, the occupancy of the Zn(4s) orbital is
calculated to range from 0.52–0.54 electrons in both series of com-
plexes, despite the coordination of the different anions. Moreover,
the sum of the orbital occupancies of the Zn(4p) orbitals for all
complexes is calculated to be between 0.57–0.65 electrons. Thus,
the Zn(4s) and Zn(4p) orbitals accept similar amounts of electron
density from the combination of the Brꢀ, CNꢀ and N2Sꢀ ligands
in all complexes 1–8.
gands L1 and L3 were investigated using AOMIX
-CDA [24]. Fig. 7
gives the orbital interaction diagram between the two fragments
in the representative complex 5. The main orbitals involved in
bonding between the two fragments are the filled
r-donating
HOFO (highest occupied fragment orbital) of the CNꢀ anion, and
the empty LUFO (lowest unoccupied fragment orbital) of the
In addition to quantification of charge on the zinc center of the
[LZn]+ fragments as an indicator of Lewis acidity, analysis of the
charge polarization within the cyanide anion is also informative.
Natural atomic charge calculations for the free cyanide anion at
the B3LYP level of theory using the TZVP basis set predict the neg-
ative charge to be localized mainly on the nitrogen atom (ꢀ0.75)
and less on the carbon atom (ꢀ0.25), as expected based on the
greater electronegativity of the nitrogen atom. Using the bare
[Zn(CN)]+ cation as a baseline for comparison, the binding of the
cyanide anion to the Zn(II) cation results in the net delocalization
of ꢃ0.57 electrons to the Lewis acidic zinc ion. Moreover, the
remaining charge distributed within the bound cyanide anion is
now significantly polarized towards the carbon atom (ꢀ0.32) rela-
tive to the nitrogen atom (ꢀ0.11). Thus, binding to the zinc center
results in both significant transfer of electron density to the metal
and away from the bound cyanide anion as well as a redistribution
of charge density within the cyanide molecule itself.
Upon binding to the [LZn]+ fragments in 5–8, average charge
delocalization from the bound cyanide anion to the [LZn]+ frag-
ment is calculated to be less than that observed for the bare zinc(II)
cation (ꢃ0.40 electrons versus 0.57 electrons), likely due to the
influence of the bound N2Sꢀ ligands which also donate electron
density into the zinc acceptor orbitals. The polarization of the
remaining charge on the CNꢀ anion is significantly less pronounced
than observed with the bare zinc(II) cation, with the natural atomic
charge of the carbon atom in 5–8 ranging from ꢀ0.17 to ꢀ0.21, and
the nitrogen charge calculated to be approximately ꢀ0.4 (Table 5).
While the relative charge distribution within the CNꢀ anion in
complexes 5 and 7 are roughly the same as that observed in free
Fig. 7. Orbital interaction diagram for L1ZnCN (only selected interfragment
donor/acceptor orbital interactions are shown for clarity).
Table 5
Calculated natural atomic charges for selected atoms in LZnCN complexes 5–8,
L2ZnCN and related ‘‘bare’’ [LZn]+ fragments.a
5
6
7
8
5-bare 6-bare 7-bare 8-bare
Zn
N
N0
S
C
N
0.85
0.88
0.86
0.89
1.13
1.11
ꢀ0.43
ꢀ0.43
ꢀ0.08
N/A
1.16
ꢀ0.41
ꢀ0.41
ꢀ0.24
N/A
1.13
ꢀ0.42
ꢀ0.43
ꢀ0.25
N/A
ꢀ0.34
ꢀ0.34
ꢀ0.07
ꢀ0.17
ꢀ0.41
ꢀ0.36
ꢀ0.36
ꢀ0.09
ꢀ0.21
ꢀ0.40
N/A
ꢀ0.33
ꢀ0.33
ꢀ0.24
ꢀ0.17
ꢀ0.41
134.8
ꢀ0.35 ꢀ0.42
ꢀ0.35 ꢀ0.42
ꢀ0.26 ꢀ0.07
ꢀ0.20 N/A
ꢀ0.40 N/A
N/A
N/A
N/A
N/A
N/A
N/A
13C 133.9
137.7
N/A
a
Geometries of [LZn]+ fragments used were those obtained from the optimiza-
Fig. 8. LUFO of [L1Zn]+ and [L3Zn]+ fragments (isovalue = 0.035).
tion of the full LZnCN complexes.

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N.G. Spiropulos et al. / Inorganica Chimica Acta 376 (2011) 562–573
[LZn]+ fragment, which is predominantly composed of the Zn(4s)
and Zn(4p) orbitals. This interaction very closely resembles the
‘‘classic’’ Lewis acid/base interaction that is expected for the zin-
c(II) center. Other filled frontier orbitals on the [LZn]+ fragment
are predominantly ligand based and are not significantly involved
in bonding with the cyanide ligand. The filled [LZn]+ fragment orbi-
tals having predominantly Zn(3d) character are low energy
(E = ꢀ18.3 to ꢀ19.9 eV) when compared to the energy of the empty
p⁄-orbitals of the CNꢀ fragment (E = 7.94 eV), resulting in no obser-
vable Zn(3d) ? CN(p⁄) backbonding interaction.
Appendix A. Supplementary material
CCDC 818529, 818530, 818531, 818532, 818533 and 818534
contain the supplementary crystallographic data for complexes 1,
2, 3, 4, 8 and 10, respectively. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at
doi:10.1016/j.ica.2011.07.033.
Comparison of the Lewis acidic LUFO of the [L1Zn]+ and [L3Zn]+
fragments for the dithioacetate and thioacetamide ligands reveals
them to be quite similar, both in energy and in composition.
Fig. 8 shows a representation of these two orbitals. Indeed, both
orbitals are predicted to have similar energies (E = ꢀ7.02 eV for
[L1Zn]+; E = ꢀ6.85 eV for [L3Zn]+) Moreover, the LUFO for [L1Zn]+
has 78% Zn character (42% Zn(4s) and 36% Zn(4p) character), while
the LUFO for [L3Zn]+ has 79% Zn character (42% Zn(4s) and 37%
Zn(4p) character). The remaining composition of both LUFOs re-
sults mostly from contributions from the sulfur atoms of the li-
gands. Because the orbital that dominates the Lewis acidic
behavior of these [LZn]+ fragments is the LUFO, the similar energies
and compositions of this orbital within the series of compounds
likely helps to explain their very similar Lewis acidities, despite
apparent differences in the two classes of ligands.
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4. Conclusion
The present study has examined the coordination chemistry of
sterically bulky bis(pyrazol-1-yl)dithioacetate and bis(pyrazol-1-
yl)thioacetamide ligands (L1–L4) with Zn(II) and Cd(II). For the
LZnBr complexes 1–4, the solid state structures are mononuclear
and the overall geometries of the complexes are distorted tetrahe-
dral. DOSY NMR studies confirm the solution structures of 1–4 are
mononuclear and correspond to the solid state structures. In con-
trast, the LCdCl complex 10, was determined to be a chloride-
bridged five-coordinate dimer. Despite the dinuclear structure of
10 in the solid state, cadmium chloride complexes 9–11 were each
shown to be mononuclear in solution by DOSY NMR. In an effort to
determine the electronic impact of the bis(pyrazol-1-yl)dithioace-
tate and bis(pyrazol-1-yl)thioacetamide ligands, computational
studies were performed on LZnX complexes 1–4 (X = Brꢀ) and 5–
8 (X = CNꢀ). Based on natural population analysis, the amount of
charge localization onto the zinc(II) center from ligands L1–L4 are
roughly equal, indicating a similar electronic impact from each of
these ligands despite their structural and electronic differences.
Moreover, examination of the LUFO of zinc fragments [L1Zn]+ and
[L3Zn]+ revealed similar energies and compositions, consistent
with zinc(II) centers having similar Lewis acidic properties despite
differences in supporting ligand.
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Acknowledgements
This work was funded by a Mountain State Tumor Institute
Small Project Grant and startup funds from Boise State University.
J.T.Y. thanks Stetson University for partial financial support of this
project. N.G.S. and M.S. were supported by a summer fellowship by
NIH Grant #P20 RR0116454 from the INBRE Program of the Na-
tional Center for Research Resources. We thank Dr. Atta M. Arif
(University of Utah X-ray Crystallographic Facility) for assistance
with X-ray crystallography and Dr. Greg Khitrov (University of Cal-
ifornia, Los Angeles Molecular Instrumentation Center) for assis-
tance with mass spectrometry. NMR and FT-IR instrumentation
was purchased through funds from NSF (CRIF/MU #0639251 and
CCLI #0737128).

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