Zinc-67 NMR study of tetrahedral and octahedral zinc sites with symmetrical oxygen, nitrogen, and sulfur ligands

Article abstract of DOI:10.1139/v99-154

We report a ⁶⁷Zn nuclear magnetic resonance (NMR) study of compounds containing zinc ions coordinated by oxygen, nitrogen, and sulfur ligands. New information concerning ⁶⁷Zn nuclear quadrupole coupling constants (NQCC) and chemical shift was obtained from magic-angle spinning (MAS) spectra of solid compounds containing both natural abundance and enriched ⁶⁷Zn isotopes. Rapid ligand exchange processes of [Zn(thiourea)₄]²⁺ in aqueous solutions were also observed.

Full text of DOI:10.1139/v99-154

1
782
Zinc -6 7 NMR s tudy of te tra he dra l a nd
oc ta he dra l zinc s ite s with s ym m e tric a l oxyge n,
nitroge n, a nd s ulfur liga nds
Sim on Sh a m a n d Ga n g Wu
Abstract: We report a 67Zn nuclear magnetic resonance (NMR) study of compounds containing zinc ions coordinated
6
7
by oxygen, nitrogen, and sulfur ligands. New information concerning Zn nuclear quadrupole coupling constants
NQCC) and chemical shift was obtained from magic-angle spinning (MAS) spectra of solid compounds containing
(
6
7
2+
both natural abundance and enriched Zn isotopes. Rapid ligand exchange processes of [Zn(thiourea)4] in aqueous
solutions were also observed.
Key words: 67Zn NMR, nuclear quadrupolar coupling constant, ligand exchange.
Résumé : On a mesuré les spectres de résonance magnétique nucléaire (RMN) du 67Zn de composés contenant des
ions zinc coordinés à des ligands oxygénés, azotés et sulfurés. Grâce aux spectres obtenus par la technique de rotation
6
7
à l’angle technique (magique) de composés solides contenant du Zn en abondance naturelle ou enrichie, on a obtenu
de nouvelles informations relatives aux constantes de couplage quadripolaire nucléaire (CCQN) et les déplacements
chimiques du 67Zn. On a aussi observé des processus d’échange rapide de ligand pour le [Zn(thiourée) ] .
2+
4
Mots clés : RMN du 67Zn, constante de couplage quadripolaire nucléaire, échange de ligands.
[
Traduit par la Rédaction] Sham and Wu 1787
2
+
In trod u c tion
[Zn(H O) ] (3–6). Despite the biological relevance of zinc
2 6
ions, the aforementioned practical difficulties have made
Zn NMR a nearly forgotten area. In the past, NMR studies
of the metal binding sites in zinc-containing proteins have
essentially relied on the utility of a surrogate nuclear probe,
6
7
Zinc is among the most important metal elements in biol-
ogy and is an integral constituent of a large number of en-
zymes (1, 2). To study the catalytic or structural role of zinc
ions in these enzymes, it is important to have a means of
probing the nature of the zinc binding site. In the absence of
X-ray crystallographic data, direct detection of diamagnetic
1
13
Cd (7).
Solid-state NMR represents a useful approach to obtain-
ing high-resolution NMR spectra for quadrupolar nuclei. In
many aspects, it is more advantageous to study quadrupolar
nuclei in the solid state than in solution. Since molecular
motion is largely restricted in solids, quadrupole relaxation
is not as efficient as in solution. As a result, the intrinsic res-
olution of solid-state NMR spectra is not limited by the life-
time of the quadrupole energy levels. However, solid-state
1
0
2+
d
Zn ions in biological systems is difficult because of the
lack of a suitable spectroscopic probe. From an NMR per-
6
7
spective, Zn (the only NMR-active zinc isotope) is among
a number of potentially important but insensitive metal
4
3
25
67
nuclei such as Ca and Mg. The insensitivity of Zn
6
7
NMR arises from the fact that Zn is a quadrupolar nucleus
spin 5/2, Q = 0.15 × 10– m ) with low natural abundance
28
–2
67
67
(
(
Zn NMR studies are still rare. Two solid-state Zn NMR
7
4.11%) and a small magnetogyric ratio (γ = 1.6768 × 10
studies have appeared in the literature (8, 9); but the com-
pounds under study are of cubic symmetry, so the electric
field gradient (EFG) at the zinc site vanishes. The 67Zn
NMR study of Zn(CH COO) ·2H O by Oldfield and co-
workers (10) is the first report dealing with a noncubic zinc
compound in the solid state. Later, Bastow and Stuart (11)
obtained Zn NMR spectra of both single-crystal and pow-
dered ZnO samples. Recently, we reported some preliminary
results of a natural abundance Zn NMR study of zinc com-
pounds with both tetrahedrally and octahedrally coordinated
–
1
–1
67
rad T s ). Consequently, Zn NMR experiments are re-
markably difficult. In addition to the low intrinsic sensitivity,
solution state Zn NMR is further hampered by the fact that
6
7
3
2
2
6
7
molecular tumbling motion always induces efficient Zn
quadrupole relaxation, resulting in short life times of
Zeeman energy levels, which lead to broad NMR lines. To
6
7
6
7
date, Zn NMR of liquid samples has been limited to stud-
ies of either highly symmetric species or unsymmetrical
complexes undergoing rapid exchange with large excessive
6
7
Received March 22, 1999.
This paper is dedicated to Professor Ted Schaefer in recognition of his outstanding contributions to Canadian chemistry.
1
S. Sham and G. Wu. Department of Chemistry, Queen’s University, Kingston, ON K7L 3N6, Canada.
1Author to whom correspondence may be addressed. Telephone: (613) 533-2644. Fax: (613) 533-6669.
e-mail: gangwu@chem.queensu.ca
Can. J. Chem. 77: 1782–1787 (1999)
© 1999 NRC Canada

Sham and Wu
1783
oxygen ligands (12). Here we extend our solid-state 67Zn
NMR study to compounds where the zinc center is coordi-
nated by either nitrogen or sulfur ligands. To the best of our
Fig. 1. 67Zn MAS spectra of (a) natural abundance ZnSe
(number of scans, 360; recycle delay, 1 s; spinning rate,
6
7
5.0 kHz) and (b) Zn-enriched Zn(ClO4)2·6H2O (number of
6
7
knowledge, Zn nuclear quadrupole coupling constants
NQCC) have not been determined for any noncubic zinc
scans, 128; recycle delay, 10 s; spinning rate, 6.0 kHz). Spinning
sidebands are marked as “ssb.”
(
center coordinated by nitrogen and sulfur ligands.
Exp e rim e n ta l s e c tion
All solid-state NMR spectra were obtained on a Bruker
Avance-400 NMR spectrometer operating at 25.036 MHz for
6
7
Zn nuclei. Solution NMR spectra were obtained on Bruker
Avance-500 and Avance-400 spectrometers. All 67Zn chemi-
cal shifts were referenced to an external sample of 1.0 M
Zn(NO ) aqueous solution (δ = 0 ppm). Magic-angle spin-
ning (MAS) Zn NMR experiments were performed using a
probe purchased from Doty Scientific Inc. (Columbia, S.C.).
Powder samples were packed into zirconium oxide rotors
3
2
iso
6
7
(
7
5 mm o.d.). Typical sample spinning speeds were between
and 9 kHz. A pulse width of 5 µs was used, and the recy-
cle time was between 1 and 10 s. Samples of ZnSe and
Zn(CH COO) ·2H O were obtained from commercial
3
2
2
sources and used without further purification. Anhydrous
Zn(CH COO) was obtained by heating the corresponding
3
2
67
hydrate sample in oven for 2 days at 80°C. ZnO (88% Zn-
enriched) was purchased from Trace Science International
Toronto, Ont.). Crystals of Zn(ClO ) ·6H O were formed
by dissolving 25 mg of ZnO in 3.6 mL of 0.25 M HClO4.
A solid sample of tetrakis(imidazole)zinc(II) perchlorate,
Zn[ImH] (ClO ) (ImH = imidazole), was then obtained by
6
7
(
4 2 2
6
7
smaller than that observed for the standard reference sample,
4
4 2
Zn(NO ) (aq), which has a line width of 80 Hz. In an early
slow evaporation of
a
1:4 molar ratio mixture of
3 2
6
7
study, Haller et al. (8) reported a 50 Hz line width for the
Zn(ClO ) ·6H O and imidazole (81 mg) from 1 mL of
5% ethanol. An aqueous solution of Zn(NO ) was pre-
4
2
2
6
7
6
7
Zn NMR signal from a stationary ZnSe sample. Close in-
9
3 2
6
7
spection of the spectrum presented in Fig. 1(a) showed no
indication of any indirect spin–spin coupling (J) between
pared by dissolving 24 mg of ZnO in 1.1 mL of 0.5 M
HNO . To this solution, 93 mg of thiourea, SC(NH ) , in
3
2 2
6
7
77
Zn and Se (spin 1/2, natural abundance = 7.58%) nuclei,
1
mL of distilled H O was added. Crystals of tetrakis(thio-
2
1
67
77
6
7
suggesting that J( Zn, Se) must be smaller than the ob-
urea)zinc(II) nitrate, Zn[SC(NH ) ] (NO ) , were then formed
2
2 4
3 2
served line width.
by slow evaporation of the solvent. For concentration-
dependent experiments, the concentration of [Zn]2+ was kept
constant at 0.65 mM, and an appropriate amount of thiourea
The isotropic 67Zn chemical shift observed for solid
Zn(ClO4)2·6H2O is –3 ppm, which suggests a similar zinc
6
7
2
+
was added to the solution to form a specific [Zn] to
thiourea concentration ratio. Solution 67Zn NMR experi-
ments were carried out at room temperature (298.5 K), using
a pulse width of 55.0 µs and a recycle delay of 100 ms.
Typically, 2000–3000 scans were collected. Prior to Fourier
Transformation (FT), FIDs were left-shifted to remove data
points due to probe ring-down, followed by 200–250 Hz ex-
ponential line broadening. Simulations of solid-state NMR
spectra were performed on a Pentium 200 personal computer
using the SOLIDS program package kindly provided by Dr.
Klaus Eichele and Professor Rod Wasylishen (Dalhousie
University, Halifax, N.S.).
environment in both solid and solution state. The crystal
structure of Zn(ClO4)2·6H2O at room temperature (space
group P63mc) (14) reveals that the zinc ion is coordinated by
six water molecules with a nearly perfect Oh site symmetry.
The Zn—O distances are 2.123 and 2.152 Å. The O-Zn-O
angles are very close to those expected for a regular
octahedron, e.g., 86.3°, 179.0°, 94.4°, and 84.9°. From the
6
7
line width observed in the Zn MAS spectrum, it is esti-
2
mated that e qQ/h is less than 0.2 MHz.
Figure 2(a) shows the natural abundance 67Zn MAS spec-
trum of Zn(CH COO) . The spectrum exhibits a typical line
3
2
shape arising from the second-order quadrupolar interaction
6
7
(
15). The isotropic Zn chemical shift of Zn(CH COO) is
3 2
found to be 245 ppm, which is similar to that found for ZnO
11, 12) where the zinc atom is tetrahedrally bonded to four
Re s u lts a n d d is c u s s ion
(
Figure 1 shows 67Zn MAS spectra of solid ZnSe and
Zn(ClO ) ·6H O obtained at 9.4 T. The cubic structure of
67
oxygen atoms. No solution Zn NMR data are available in
the literature for zinc centers coordinated to four oxygen lig-
ands. The crystal structure of Zn(CH COO) (space group
4
2
2
6
7
ZnSe (13) yields a negligible Zn NQCC at the zinc center,
and therefore, the MAS spectrum of ZnSe shows a sharp iso-
tropic peak centered at 276 ppm with a full width at the half-
height (FWHH) of 16 Hz. Interestingly, this value is even
3
2
Fdd2) indicates that the zinc ion is in a slightly distorted tet-
rahedral environment (16). The Zn—O bond length varies
from 1.914 to 1.955 Å, and the O-Zn-O angle ranges from
©
1999 NRC Canada

1
784
Can. J. Chem. Vol. 77, 1999
Fig. 2. Experimental (solid lines) and calculated (broken lines)
nated by four nitrogen atoms from the imidazole molecules
(17). The two independent Zn—N(imidazole) bond lengths
are nearly identical, 1.997 and 2.001 Å. The N-Zn-N angles
6
7
Zn MAS spectra of (a) natural abundance Zn(CH COO)
3
2
(
1
1
number of scans, 32 225; spinning rate, 8.5 kHz; recycle delay,
s), (b) 67Zn-enriched Zn(imidazole) (ClO ) (number of scans,
67
4
4 2
range from 105.3° to 113.3°. Analysis of the Zn MAS line
6
7
2
6 774; recycle time, 1 s; spinning rate, 8.1 kHz), and (c) Zn-
shape shown in Fig. 2(b) also yields that e qQ/h = 2.80 MHz
enriched Zn(thiourea) (NO ) (number of scans, 19 188, recycle
and η = 0.4. To our knowledge, Zn(ImH) (ClO ) represents
4
3 2
4
4 2
6
7
time, 2 s; spinning rate, 9.1 kHz).
the only example where the Zn NQCC for a ZnN coordi-
4
nation environment is determined.
Figure 2(c) shows the 7Zn MAS spectrum of
6
6
7
67
Zn[SC(NH ) ] (NO ) . The isotropic Zn chemical shift is
2
2 4
3 2
3
59 ppm, which is in agreement with those reported for zinc
sites coordinated by four sulfur atoms (3). From analysis of
6
7
67
the Zn MAS spectrum, we obtained that the Zn NQCC is
.15 MHz with η 1. The crystal structure of
Zn[SC(NH ) ] (NO ) (space group Pnaa) indicates that the
3
=
2
2 4
3 2
zinc ions are coordinated by four sulfur atoms from the
thiourea ligands (18). The two Zn—S distances are 2.324
and 2.361 Å, which are considerably longer than the Zn—N
distances in the zinc–imidazole complex. The S-Zn-S angle
between the two longer Zn—S bonds is 100.0°, and the an-
gle between the two shorter Zn—S bonds is 121.5°, both of
which deviate considerably from the perfect tetrahedral an-
6
7
gle. It is also noted that the
Zn EFG tensors in
Zn(ImH) (ClO ) and Zn[SC(NH ) ] (NO ) exhibit very
4
4 2
2 2 4
3 2
different asymmetry parameters. This observation is some-
what surprising, since the two compounds have a similar tet-
rahedral arrangement at the zinc center.
Compared with the 67Zn MAS spectrum of natural abun-
dance Zn(CH COO) , as shown in Fig. 2(a), the overall sen-
3
2
6
7
sitivity enhancement in the Zn MAS spectra of the two
Zn-enriched compounds does not appear as dramatic as ex-
pected from the enrichment level, i.e., a factor of 20. Al-
6
7
6
7
67
though solution
Zn NMR did prove that the
Zn
enrichment procedure was successful, several other factors
may contribute to the apparent lack of full signal enhance-
6
7
67
ment in the
Zn spectra of
Zn(ImH) (ClO ) and
4 4 2
6
7
Zn[SC(NH ) ] (NO ) . First, the molecular size of
2
2 4
3 2
Zn(ImH) (ClO ) (FW = 536) and Zn[SC(NH ) ] (NO )
4
4 2
2 2 4
3 2
1
02.2° to 115.9°. Analysis of the MAS line shape shown in
(FW = 493) is larger than that of Zn(CH3OO)2 (FW = 183).
Second, in the case of Zn(ImH)4(ClO4)2, a small sample of
2
67
Fig. 2(a) yields that e qQ/h = 2.42 MHz and η = 0.1.
To explore the sensitivity enhancement by isotopic enrich-
the enriched compound (ca. 50 mg) was used, which is
much less than ca. 150 mg used for the natural abundance
6
7
ment, we prepared two
Zn-enriched compounds:
6
7
67
67
Zn[ImH] (ClO ) and Zn[SC(NH ) ] (NO ) . The enrich-
Zn(CH3COO)2 sample. Third, the Zn spin-lattice relax-
4
4 2
2 2 4
3 2
6
7
ment level is about 88% for both compounds. Figures 2(b) and
ation time (T1) is much longer in Zn(ImH)4(ClO4)2 and
6
7
67
67
(
c) show the Zn MAS spectra of the two Zn-labeled com-
Zn[SC(NH2)2]4(NO3)2 than in Zn(CH3COO)2. By perform-
6
7
67
67
pounds. For Zn(ImH) (ClO ) , the isotropic Zn chemical
ing Zn MAS experiments with different recycle delays, we
4
4 2
6
7
shift is 291 ppm, which is comparable to the value of
64 ppm observed in acetone solution. The 67Zn chemical-
shift value for zinc ions coordinated by four nitrogen ligands
were able to provide an estimate for
Zn T1 in
67
2
Zn[SC(NH2)2](NO3)2, T1 ≈ 2 s. In Zn(CH3COO)2, however,
the T1 was estimated to be less than 0.5 s (12). After consid-
ering all these factors, the sensitivity enhancement in
Figs. 2(b) and (c) seems reasonable. It is worth pointing
2
+
is also in agreement with that observed in [Zn(NH ) ] from
3
4
6
7
solution Zn NMR studies (3). However, both the line width
and peak position of the solution state Zn spectrum of
6
7
67
out that our attempts to record Zn MAS spectra for natu-
6
7
Zn(ImH) (ClO ) were found to be solvent dependent. For
ral abundance samples of Zn(ImH)4(ClO4)2 and
Zn[SC(NH2)2]4(NO3)2 were unsuccessful, presumably due to
the long T1 values.
4
4 2
6
7
example, the Zn NMR spectrum of Zn(ImH) (ClO ) in
4
4 2
methanol exhibits a peak at 280 ppm with a much larger line
width, 2500 Hz, than that found in acetone, 500 Hz. Further-
more, we did not observe any 67Zn NMR signal from an
aqueous solution of Zn(ImH) (ClO ) , presumably due to
Similar to the case of the zinc imidazole complex, we also
observed rapid ligand exchange in the aqueous solution of
Zn[SC(NH ) ] (NO ) . Figure 3 shows the result of the con-
4
4 2
2 2 4
3 2
rapid ligand exchange (vide infra). The crystal structure of
centration study to examine the rapid ligand exchange pro-
cess in Zn[SC(NH ) ] (NO ) . When the concentration of
thiourea is low, the Zn chemical shift is close to 0 ppm, in-
Zn(ImH) (ClO ) (space group C2/c) shows that the zinc ion
4
4 2
2 2 4
3 2
6
7
lies on a crystallographic C axis and is tetrahedrally coordi-
2
©
1999 NRC Canada

Sham and Wu
1785
Table 1. Solid-state 67Zn NMR data for zinc compounds.
Compound
Zinc coordination
ZnO4
δiso (ppm)
e2qQ/h (MHz)
η
Ref.
ZnO (hexagonal)
240.1
2.407
2.40
0
0
0
<0.5
<0.4
—
—
—
0.0
0.0
—
—
—
0
11
12
8
11
12
11
12
8
2
40
ZnS (cubic)
ZnS4
378
3
3
80.5
81.9
ZnS (hexagonal)
ZnSe (cubic)
ZnS4
365
3
64.5
0
ZnSe4
273
—
—
—
—
0.85
0.2
0.1
0.4
1.0
—
2
76
This work
8
9
10
12
This work
This work
This work
This work
ZnTe (cubic)
ZnTe4
ZnC4
ZnO6
ZnO6
ZnO4
ZnN4
ZnS4
ZnO6
85
291
0
K Zn(CN) (cubic)
0
2
4
Zn(CH COO) ·2H O
5.3
1.70
2.42
2.80
3.15
<0.2
3
2
2
ZnSO ·7H O
10
4
2
Zn(CH COO)2
245
291
359
–3.0
3
Zn(imidazole) (ClO )
Zn(thiourea) (NO )
4
4 2
4
3 2
Zn(ClO ) ·6H O
4
2
2
Fig. 3. 67Zn NMR spectra of aqueous solutions of Zn(NO ) and
thiourea at different thiourea conc./[Zn ] ratios. The pH value of
the solution varies from 4.20 (1:1) to 3.78 (1:9).
is practically identical to that observed in the solid state,
359 ppm.
3
2
2
+
To examine possible trends in solid-state 67Zn NMR pa-
rameters, we summarize in Table 1 the presently available
data in the literature for zinc compounds. It is noted that the
6
7
2
Zn NQCC has also been determined in zinc metal, e qQ/h
=
11.983 MHz (19–21); however, it is reasonable not to
compare this value with those obtained for zinc compounds.
With this exception, the largest Zn NQCC found in zinc
compounds is that in Zn(CH COO) ·2H O, e qQ/h =
6
7
2
3
2
2
5
.3 MHz (10). The crystal structure of Zn(CH COO) ·2H O
3 2 2
(
22) indicates that the zinc ion is coordinated by six oxygen
atoms in a very distorted octahedral environment. Four of
the six oxygen atoms are from the acetate groups. The other
two oxygen atoms are those from the water molecules.
Whereas, the Zn—O bond length varies slightly from 2.14 to
2
.18 Å, a significant distortion from perfect O symmetry
h
exists in the O-Zn-O bond angles. For example, large distor-
tions are found when the two mutually trans oxygen atoms
are due to the acetate and water, respectively, i.e., O(Ac)-Zn-
O(W) = 152°, or when the two cis oxygen atoms are from
the same acetate group, O(Ac)-Zn-O(Ac) = 61°. The distor-
tion is, however, smaller when the two cis oxygen atoms
arise from the acetate and water, respectively, i.e., O(Ac)-
Zn-O(W) = 108°. To study the possible correlation between
6
7
Zn NQCC and molecular structure, we have used a param-
eter called the mean deviation of bond angles as a measure
of the degree of deviation of the zinc site of interest from a
perfect symmetry, T or O . The mean deviation of bond an-
d
h
gles can be defined for a tetrahedral site as
1
[
1]
∆(T ) = ∑ θ −109.5°
d
i
4
i
where θ is the ith bond angle across the zinc center. In a
i
2
+
dicating the existence of [Zn(H O) ] species. As the con-
similar fashion, one can define a mean deviation of bond an-
gles for an octahedral site using 90° and 180° as perfect an-
2
6
6
7
centration of the thiourea ligand is increased, the Zn NMR
signal is shifted toward higher frequencies. At the highest
metal–ligand ratio used in this study, thiourea conc./[Zn ] ≈ 9,
6
7
gles. In Fig. 4, Zn NQCC is plotted against the mean
deviation of bond angles as defined in eq. [1] for presently
2+
6
7
67
the Zn chemical shift of 355 ppm for the solution sample
available Zn NMR data. As seen in Fig. 4, the octahedral
©
1999 NRC Canada

1
786
Can. J. Chem. Vol. 77, 1999
Fig. 4. Plot of 67Zn NQCC versus mean bond angle deviation
from perfect T (᭿) and O (᭹) symmetry.
for the 67Zn NQCC in insulin by Shimizu and Hatano (25)
may be grossly underestimated.
d
h
Con c lu s ion s
In this study, we have focused on the solid-state 67Zn
NMR spectra of tetrahedral and octahedral zinc centers that
are symmetrically coordinated by water, acetate, imidazole,
and thiourea ligands. It is well known that the active zinc
sites in metalloproteins usually contain unsymmetrical lig-
ands. Therefore, it would be desirable, although technically
6
7
challenging, to extend these solid-state Zn NMR studies to
some model compounds where the zinc center resembles
those in zinc-containing proteins. Unfortunately, to this date,
6
7
our efforts in recording solid-state Zn NMR signals for un-
symmetrical zinc sites have been unsuccessful, presumably
due to the large quadrupole coupling constants associated
with those sites. It should be noted that the solid-state 67Zn
NMR results reported in this study were obtained at a rela-
tively low magnetic field, 9.4 T. Since the second-order
quadrupolar broadening is inversely proportional to the ap-
plied magnetic field strength, the availability of very high-
field instrument (18.8 T or greater) holds the promise of
making unsymmetrical coordination zinc sites accessible.2
compounds appear to show a correlation between the magni-
tude of the Zn NQCC and the degree of deviation from the
6
7
O symmetry, whereas the tetrahedral compounds are insen-
h
sitive to the mean deviation of bond angles, at least within
the presently available data. It is also somewhat surprising
Ac kn ow le d gm e n ts
that a very small distortion from the perfect T symmetry in
d
ZnO (O-Zn-O: 108.14° and 110.77°) (23) results in a rea-
This research was supported by research and equipment
grants from the Natural Sciences and Engineering Research
Council of Canada (NSERC).
6
7
sonably large
Zn NQCC, 2.40 MHz. However,
ZnSO ·7H O exhibits larger distortion (O-Zn-O: 80.6°,
4
2
6
7
9
1
7.5°, 172.3°) than ZnO but has a smaller Zn NQCC,
.7 MHz. It is clear that using the mean deviation of bond
Re fe re n c e s
angles alone to explain the results may be oversimplified
because the model does not consider the nature of the
Zn—ligand bond. For the octahedral compounds examined
in this study, the zinc ions are coordinated to six oxygen at-
oms with similar bond lengths, 2.09 ± 0.10 Å. Thus, it is
perhaps not surprising that a reasonable correlation is ob-
served among octahedral compounds. However, the
Zn—ligand distance for the tetrahedral compounds varies
from 1.914 to 2.000 to 2.361 Å for Zn—O (acetate), Zn—S
1
. S.J. Lippard and J.M. Berg. Principles of bioinorganic chemis-
try. University Science Books, Mill Valley, Calif. 1994.
. J.J.R. Frausto da Silva and R.J.P. Williams. The biological
chemistry of the elements. Oxford University Press, Oxford.
1991.
2
3. P. Granger. In Transition metal nuclear magnetic resonance.
Edited by. P.S. Pregosin. Elsevier Science Publishers B.V.,
Amsterdam, The Netherlands. 1991. pp. 285–346.
4. M. Kodaka, T. Shimizu, and M. Hatano. Inorg. Chim. Acta,
78, L55 (1983).
(
thiourea), and Zn—N (imidazole), respectively. This dis-
crepancy may be responsible for the lack of correlation be-
tween Zn NMR parameters and molecular structure among
tetrahedral compounds. Nevertheless, more investigations
6
7
5
6
7
. T. Shimizu and M. Hatano. Inorg. Chem. 24, 2003 (1985).
. A. Delville and C. Detellier. Can. J. Chem. 64, 1845 (1986).
. K. McAteer, A.S. Lipton, and P.D. Ellis. In Encyclopedia of
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are definitely required before one can draw any conclusion
about the correlation between 67Zn NMR parameters and
molecular structure.
8
9
Based on the presently available solid-state 67Zn NMR
data, it seems that the value of 5.3 MHz found in
Zn(CH COO) ·2H O may represent an upper limit of the
. G. Wu, S. Kroeker, and R.E. Wasylishen. Inorg. Chem. 34,
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Zn NQCC for zinc ions coordinated octahedrally by sym-
1
1
1
metrical ligands but a lower limit for octahedral compounds
with unsymmetrical ligands. The zinc binding geometry in
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coordinated by three imidazolyl nitrogen atoms and three
6
(
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6
7
water oxygen atoms (24). In light of the new solid-state Zn
NMR data, it appears that the value of 1.86 MHz suggested
2
After the submission of this manuscript, two solid-state 67Zn NMR studies have appeared in the literature (26, 27).
©
1999 NRC Canada

Sham and Wu
1787
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©
1999 NRC Canada