Zinc(II) complexes derived from potentially hexadentate (N4O2) acyclic ligands containing pyridinyl and phenolic groups

Article abstract of DOI:10.1039/a605706c

A group of zinc(II) complexes derived from potentially hexadentate (N₄O₂) acyclic ligands derived from Schiff bases and bearing pyridinyl and phenolic arms have been prepared and characterized. The crystal structures of two of them show that hydrolysis of the corresponding pro-ligand has occurred to give mononuclear zinc complexes. The co-ordination geometries around the zinc are approximately trigonal biopyramidal.

Full text of DOI:10.1039/a605706c

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Zinc(II) complexes derived from potentially hexadentate (N4O2)
acyclic ligands containing pyridinyl and phenolic groups
Cecilia O. Rodriguez de Barbarin, Neil A. Bailey, David E. Fenton* and Qing-Yu He
Department of Chemistry, Dainton Building, The University of Sheffield, Sheffield S3 7HF, UK
A group of zinc() complexes derived from potentially hexadentate (N4O2) acyclic ligands derived from Schiff
bases and bearing pyridinyl and phenolic arms have been prepared and characterized. The crystal structures of two
of them show that hydrolysis of the corresponding pro-ligand has occurred to give mononuclear zinc complexes.
The co-ordination geometries around the zinc are approximately trigonal biopyramidal.
Zinc() is present at the active sites of various enzymes in living
systems and plays a central role in catalysis and gene
expression.1–5 In enzymes with hydrolytic activity towards
phosphate esters5,6 the active zinc centre usually displays a
tetrahedral or distorted-tetrahedral co-ordination geometry in
which one site is occupied by a water molecule.5,7 During the
course of enzymatic turnover, the inner-sphere co-ordination
number of catalytic zinc may increase to five with accompany-
ing change of geometry to either trigonal bipyramidal or
‘octahedral-minus-one’. In (aqueous) solution, and in
small-molecule complexes, species containing zinc() with
co-ordination number six are common.8,9 It is the flexibility of
co-ordination number and co-ordination polyhedra that may
enable zinc() to reveal its essential catalytic function as dur-
ing a reaction this will enable a substrate to bind to the metal
without releasing another group from the co-ordination
sphere.8
In order to elucidate the mechanism of zinc() involvement
in hydrolytic enzymes, small-molecule zinc complexes have been
synthesized as models for the active site and their properties
investigated.10–15 In these models relatively rigid ligands provid-
ing three or four donor sites were used as such ligands leave
zinc() co-ordinatively unsaturated and so enable co-ordination
of the water that is believed to play an essential role in
hydrolysis. In addition, phosphate-bridged dinuclear zinc com-
plexes16 and a phenoxy-bridged homodinuclear zinc complex,17
related to the dinuclear component in some multizinc enzymes,
have been reported. Herein we report the synthesis of a series of
mononuclear zinc() complexes, with different co-ordination
numbers, derived from acyclic and potentially hexadentate
(N4O2) ligands containing pyridinyl and phenolic groups,
together with the crystal structures of two complexes which
result from hydrolytic cleavage of the precursor di-Schiff base
pro-ligand.
matrix used was 3-nitrobenzyl alcohol unless otherwise stated.
All solid samples prepared were dried in vacuum over silica gel
and MgSO4 overnight unless otherwise stated.
CAUTION: although no problems were encountered during
the preparation of the perchlorate salts and the use of per-
chloric acid in titration, suitable care and precautions should be
taken when handling such potentially hazardous compounds.
Preparation of pro-ligands
The pro-ligands are shown in scheme 1.
The compounds H2L1 and H2L2 were prepared as described
previously.19
H3L3. An ethanolic solution (10 cm3) containing diethyl-
enetriamine hydrochloride (5 mmol; 1.74 g) was added
dropwise to a tetrahydrofuran (thf) solution (60 cm3) contain-
ing 2-chloromethyl-4-nitrophenol (5 mmol, 0.94 g) while stir-
ring and then NEt3 (20 mmol, 2.02 g) was added. The mixture
was heated to reflux for 2 h. After cooling, the resulting suspen-
sion was filtered to remove solid NEt3ؒHCl. The filtrate was
evaporated to produce an oily syrup. This was dissolved in
CH2Cl2–MeOH (9:1) and chromatographed using a silica gel
1
column to yield H3L3 (1.30 g, 56.4%). NMR (CDCl3): H, δ
12.35 (s, 1 H), 8.30 (s, 2 H), 8.05 (d, 1 H), 7.95 (s, 1 H), 7.35–
6.70 (m, 9 H), 4.00 (s, 2 H), 3.75 (t, 4 H) and 3.00 (t, 4 H); 13C, δ
55.21, 57.15, 58.15, 115.20, 116.93, 118.53, 118.83, 121.66,
124.71, 125.60, 131.69, 132.56, 140.30, 160.76, 164.04 and
167.05. Mass spectrum: m/z 463 (M+, 100%).
H3L4. 3,3Ј-Iminobis(propylamine) (12.5 mmol, 1.64 g) and
salicylaldehyde (25 mmol, 3.05 g) were stirred together in abso-
lute ethanol at room temperature for 30 min and then 2-
chloromethyl-4-nitrophenol (12.5 mmol, 2.35 g; in 50 cm3
EtOH) and Na2CO3 (15 mmol, 1.6 g) were added to the yellow
solution. The mixture was heated to reflux overnight. On cool-
ing, the resulting suspension was filtered and the filtrate evap-
orated to remove solvent. Methanol (20 cm3) was added to the
syrupy residue and the mixture heated. The resulting solution
was allowed to stand at room temperature overnight and a yel-
low solid deposited. Recrystallization of the solid from MeOH
(65 cm3) gave H3L4 as a yellow microcrystalline powder (4.62 g,
75.4%) (Found: C, 65.8; H, 6.0; N, 11.3. Calc. for C27H30N4O5:
C, 66.1; H, 6.15; N, 11.4%). NMR (CDCl3): δ 8.30 (s, 2 H), 8.05
(d, 1 H), 7.90 (s, 1 H), 7.25 (m, 4 H), 6.85 (m, 5 H), 3.90 (s, 2 H),
3.60 (t, 4 H), 2.70 (t, 4 H) and 2.00 (m, 4 H); 13C, δ 27.32,
51.08, 56.99, 57.67, 116.50, 116.97, 118.57, 118.71, 121.59,
124.61, 125.40, 131.33, 132.41, 140.15, 160.94, 164.49 and
Experimental
Reagents and solvents used were of commercial reagent quality.
Purification of the pro-ligands was effected by using flash
chromatography18 with silica gel (40–63 µm). Their identity and
1
purity was judged by TLC, H NMR, and mass spectroscopy.
All elemental analyses were carried out by the University of
Sheffield Microanalytical Service. Infrared spectra were
recorded as KBr discs using a Perkin-Elmer 1710 Fourier-
transform spectrophotometer (4000–400 cmϪ1), electronic
absorption spectra using a Philips PU8720 UV/VIS scanning
spectrophotometer operating in the range 220–500 nm, 1H
NMR spectra at 220 MHz on a Perkin-Elmer R34 spectrometer
and 13C NMR spectra (62.9 MHz) using a Bruker AM-250
spectrometer. Positive-ion fast atom bombardment (FAB) mass
spectra were recorded on a Kratos Ms 80 spectrometer. The
165.72. IR (cmϪ1): 1636 (C᎐N). Mass spectrum: m/z 490 (M+,
᎐
100%).
J. Chem. Soc., Dalton Trans., 1997, Pages 161–166
161

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Scheme 1
H2L5 and H2L6. The Schiff-base precursor H2L1 (0.40 g) or
H2L2 (0.43 g) (1 mmol) was dissolved in 95% EtOH (75 cm3) to
which solution NaBH4 powder (10 mmol, 0.38 g) was added in
small portions. The mixture was stirred at room temperature for
2 h. 2 mol dmϪ3 Hydrochloric acid was added dropwise until
pH ≈ 8 and stirring was continued for 30 min. The resulting
suspension was filtered to remove any white solid and the fil-
trate evaporated to dryness. The residue was extracted with
CH2Cl2–water and the CH2Cl2 removed to leave a straw-yellow
oil (H2L5, H2L6). The oil H2L5 was dissolved in CHCl3 (5 cm3)
and allowed to stand at Ϫ20 ЊC for a few days, when clear
needle crystals were deposited; under the same conditions H2L6
1
remained as an oil. H2L5: H NMR (CDCl3) δ 2.62 (m, 8 H,
Et), 3.75 (s, 2 H, CH2), 3.86 (s, 4 H, CH2), 5.72 (br, 2 H, NH),
6.70–7.52 (m, 11 H, aryl) and 8.40 (d, 1 H, H6 of pyridyl); 13
C
NMR (CDCl3) δ 48.8, 52.8, 53.4, 60.0, 116.3, 118.9, 122.2,
122.3, 123.2, 128.6, 128.7, 136.7, 149.3, 157.6 and 158.4; mass
spectrum m/z = 407 (M + H+, 100%); accurate mass found
(required) 407.244 809 (407.244 702). H2L6: 1H NMR (CDCl3)
δ 1.40 (m, 4 H, CH2), 2.25 (t, 4 H, CH2), 2.45 (t, 4 H, CH2), 3.40
(s, 2 H, CH2), 3.70 (s, 4 H, CH2), 5.00 (br, 2 H, NH), 6.5–7.30
(m, 11 H, aryl), 8.00 (d, 1 H, H6 of pyridyl); 13C NMR (CDCl3)
δ 26.2, 47.3, 52.3, 52.6, 60.0, 116.4, 118.9, 122.2, 122.4, 123.2,
128.5, 128.7, 136.6, 149.1, 158.3 and 159.1; mass spectrum
m/z = 435 (M + H+, 100%); accurate mass found (required)
435.276 882 (435.276 002).
Scheme 2
Preparation of complexes
m/z = 469 (M+, 100%) [Found (required for C28H38ClN4O8.5Zn):
C, 50.35 (50.5); H, 5.75 (5.75); N, 8.5 (8.4%)].
The ligand frameworks for the complexes are depicted in
Scheme 2.
[Zn(HL5)]ClO4ؒMeCO2Etؒ0.5H2O 1. Pro-ligand H2L5 (1
mmol, 0.41 g) and NEt3 (2 mmol, 0.20 g) were dissolved in
MeOH (50 cm3) and Zn(ClO4)2ؒ6H2O (1.5 mmol, 0.55 g) in
MeOH (5 cm3) was added. The solution was heated to reflux for
3 h. On cooling, the resulting mixture was filtered and the fil-
trate evaporated to near dryness. Methanol (5 cm3) and then
ethyl acetate (40 cm3) were added to dissolve the residue, the
solution was warmed and then allowed to stand at room tem-
perature overnight, white small crystals deposited. Recrystal-
lization of the product from MeCN–MeOH gave crystals that
are unstable in the atmosphere. Yield = 0.38 g, 59%. IR (KBr
disc, ν/cmϪ1): 3422 (H2O), 3244 (OH), 2928 and 2879 (NH),
1727 (MeCO2Et), 1112 and 623 (ClO4). Mass spectrum:
[Zn(HL5)]BF4ؒMeCO2Et 2. This was prepared from
Zn(BF4)2ؒH2O (1.5 mmol, 0.36 g) using the same procedure as
for complex 1. The product is microcrystalline. Yield = 0.42 g,
64%. IR (KBr disc, ν/cmϪ1): 3286 (OH), 2916 and 2868 (NH),
1736 (MeCO2Et) and 1083 (BF4). Mass spectrum: m/z = 469
(M+, 100%) [Found (required for C28H37BF4N4O4Zn): C, 52.3
(52.15); H, 5.65 (5.8); N, 8.75 (8.7%)].
[Zn(HL6)]BF4 3. This was prepared from H2L6 (1 mmol, 0.44
g) and Zn(BF4)2ؒH2O using the same procedure. White needle-
like crystals were generated. Yield = 0.32 g, 46%. IR (KBr disc,
ν/cmϪ1): 3260 (OH), 3060, 2926 and 2879 (NH) and 1083 (BF4).
162
J. Chem. Soc., Dalton Trans., 1997, Pages 161–166

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overnight and yellow prismatic crystals suitable for X-ray
analysis were deposited (0.24 g, 53%) (Found: C, 50.5; H, 5.3; N,
12.0. Calc. for C19H24N4O5Zn: C, 50.3; H, 5.35; N, 12.35%). IR
(cmϪ1): 1637 (C᎐N) and 1294 (NO ). Mass spectrum: m/z 421
Table 1 Crystallographic data for complexes 6 and 8
6
8
᎐
2
(M+, 100%).
Formula
M
C41H41BN4OZn
681.96
C19H24N4O5Zn
453.80
[ZnL10] 9 and [ZnL11]ClO4ؒMeOH 10. A methanolic solu-
tion (55 cm3) containing ligand H3L4 (1 mmol, 0.49 g) and
Zn(ClO4)2ؒ6H2O (1.5 mmol, 0.56 g) was stirred and NEt3 (3
mmol, 0.30 g) added with immediate production of a yellow
precipitate. The suspension was heated to reflux for 2 h. On
cooling, the resulting mixture was filtered to obtain a yellow
powder that was recrystallized from MeCN–MeOH (1:1) to
give complex 9 (0.22 g, 48%) (Found: C, 53.1; H, 5.25; N, 12.15.
Calc. for C20H24N4O4Zn: C, 53.4; H, 5.4; N, 12.45%). IR (cmϪ1):
1628 (C᎐N) and 1293 (NO ). Mass spectrum: m/z 449 (M+,
89%). The filtrate was allowed to stand at room temperature for
a few days during which time complex 10 was deposited as light
yellow crystals (0.09 g, 21%) (Found: C, 35.2; H, 5.0; N, 12.15.
Calc. for C14H25ClN4O8Zn: C, 35.15; H, 5.25; N, 11.7%). IR
(cmϪ1): 1293 (NO2) and 1097 (ClO4Ϫ). Mass spectrum: m/z 345
(M+, 100%).
Crystal symmetry Monoclinic
Triclinic
P1
Space group
P21/n
a/Å
b/Å
c/Å
10.332(5)
23.057(6)
14.773(7)
10.285(6)
10.364(7)
10.619(6)
68.17(4)
82.69(4)
71.34(5)
995(1)
469.89
2
12.99
α/Њ
β/Њ
94.08(5)
γ/Њ
U/Å3
F(000)
Z
3510(3)
1432
4
µ(Mo-Kα)/cmϪ1
Crystal size/mm
Dc/g cmϪ3
λ(Mo-Kα)/Å
T/K
7.38
᎐
2
0.75 × 0.45 × 0.15 1.0 × 0.85 × 0.575
1.290
0.710 73
293(2)
5621
1.761
0.710 69
295
3531
2218
No. of reflections
Independent
reflections
Final R
4305
0.0405
0.0652
Crystallography
Mass spectrum: m/z = 497 (M+, 100%) [Found (required for
C26H33BF4N4O2Zn): C, 53.1 (53.4); H, 5.7 (5.7); N, 9.4 (9.55%)].
Complex 6.
A yellow crystal having dimensions of
0.75 × 0.45 × 0.15 mm was used to collect X-ray data at room
temperature in the range 3.5 < 2θ < 45.0Њ on a Siemens P4
diffractometer by the ω-scan method (h Ϫ1 to 11, k Ϫ1 to 24,
l Ϫ15 to 15). Of the 5621 reflections measured the 4305
independent reflections with |F |/σ(|F |) > 4.0 were corrected for
Lorentz-polarization effects and for absorption based upon
symmetry-equivalent reflections (maximum and minimum
transmission coefficients 0.397 and 0.302). The structure was
solved by direct methods and refined by full-matrix least
squares on F 2. Hydrogen atoms were included in calculated
positions and refined in riding mode with isotropic thermal
vibrational parameters related to those of the supporting
atoms. Refinement converged at a final R = 0.0405 (wR2 =
0.1193 for all 4299 unique data, 433 parameters, mean and
maximum δ/σ 0.000, 0.001), with allowance for the thermal
anisotropy of all non-hydrogen atoms. Minimum and
maximum final electron density Ϫ0.226 and 0.256 e ÅϪ3 close
[Zn(HL6)]ClO4ؒMeCO2Et 4. This was prepared from H2L6 (1
mmol, 0.44 g) using the same procedure. White microcrystals
were generated. Yield = 0.37 g, 53%. IR (KBr disc, ν/cmϪ1):
3222 (OH), 2934 and 2865 (NH), 1726 (MeCO2Et), 1108 and
623 (ClO4). Mass spectrum: m/z = 497 (M+, 100%) [Found
(required for C30H41ClN4O8Zn): C, 52.55 (52.6); H, 6.0 (6.05); N,
7.95 (8.2%)].
[Zn(HL6)]BPh4ؒH2O 5. This was prepared from equal molar
amounts of H2L6 (1 mmol, 0.44 g) and Zn(BF4)2ؒH2O (0.24 g)
in EtOH (30 cm3) with a similar procedure to that used for
complex 1. After treatment with sodium tetraphenylborate at
reflux temperature for 2 h the resulting yellow-white solid was
recrystallized from EtOH (50 cm3). Yield = 0.53 g, 63%. IR
(KBr disc, ν/cmϪ1): 3496 (OH), 2998, 2865 (NH), 766, 732 and
708 (BPh4). Mass spectrum: m/z = 497 (M+, 100%) [Found
(required for C50H55BN4O3Zn): C, 71.85 (71.8); H, 6.8 (6.65); N,
6.9 (6.7%)].
to the metal atom. A weighting scheme w = 1/[σ2(Fo ) +
2
(0.0736P)2 + 0.20P] where P = (Fo2 + 2Fc )/3 was used in the
2
latter stages of refinement. Complex scattering factors were
taken from the program package SHELXL 9320 as imple-
mented on a Viglen 486dx computer.
[ZnL7]BPh4 6. Equimolar amounts of H2L1 (1 mmol, 0.40 g)
and Zn(ClO4)2ؒ6H2O (0.37 g) in MeOH (50 cm3) were heated to
reflux in the presence of NEt3 (2 mmol, 0.20 g) for 1 h. Sodium
tetraphenylborate (1 mmol, 0.34 g in 5 cm3 MeOH) was added.
The clear solution was refluxed for half an hour, a white solid
emerging after a few minutes. The resulting suspension was fil-
tered and a yellow-white crystalline powder was recovered from
the filtrate on cooling (0.55 g, 82%). Recrystallization of the
powder from MeOH–MeCN (1:1) gave light yellow crystals
suitable for X-ray analysis (Found: C, 72.3; H, 6.3; N, 8.5. Calc.
for C41H41BN4OZn: C, 72.2; H, 6.05; N, 8.2%). IR (cmϪ1): 1632
Complex 8. Three-dimensional room-temperature X-ray
data were collected in the range 3.5 < 2θ < 50.0Њ on a Nicolet
R3 four-circle diffractometer by the ω-scan method. The 2218
independent reflections (of 3531 measured) for which |F |/
σ(|F |) > 5.0 were corrected for Lorentz-polarization effects,
and for absorption by analysis of eight azimuthal scans (min-
imum and maximum transmission coefficients 0.598 and
0.449). The structure was solved by standard Patterson and
Fourier techniques and refined by blocked-cascade least-
squares methods. Hydrogen atoms were included in calculated
positions with isotropic thermal parameters related to those of
the supporting atom and refined in riding model. Refinement
converged at a final R = 0.0652 for 262 parameters with allow-
ance for the thermal anisotropy of all non-hydrogen atoms
(mean and maximum δ/σ 0.001 and 0.010). Minimum and max-
imum final electron density Ϫ0.588 and 1.040 e ÅϪ3 located
around the zinc atom. Complex scattering factors were taken
from ref. 21 and from the program package SHELXTL 22 as
implemented on a Data General DG30 computer.
(C᎐N). Mass spectrum: m/z 361 (M+, 100%).
᎐
[ZnL8]BPh4 7. The complex was prepared similarly to 6
using H2L2 (1 mmol, 0.43 g). A white crystalline solid was col-
lected (0.5 g, 76%) (Found: C, 72.45; H, 6.5; N, 7.5. Calc. for
C43H45BN4OZn: C, 72.75; H, 6.4; N, 7.9%). IR (cmϪ1): 1619
(C᎐N). Mass spectrum: m/z 389 (M+, 100%).
᎐
[ZnL9]ؒMeOH 8. The compound H3L3 (1 mmol, 0.46 g) and
Zn(ClO4)2ؒ4dmso (dmso = dimethyl sulfoxide) (1.5 mmol, 0.52
g) in MeOH (50 cm3) were stirred and then NEt3 (3 mmol, 0.30
g) was added. The mixture was heated to reflux for 2 h. On
cooling, the resulting solution was filtered to remove residual
solids. The filtrate was allowed to stand at room temperature
Crystal data together with details of the X-ray diffraction
experiments for the two complexes are summarized in Table 1.
Atomic coordinates, thermal parameters, and bond lengths
J. Chem. Soc., Dalton Trans., 1997, Pages 161–166
163

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Table 2 Selected bond lengths (Å) and angles (Њ) for complex 6
Zn᎐O
Zn᎐N(3)
Zn᎐N(2)
1.961(3)
2.030(3)
2.313(3)
Zn᎐N(1)
Zn᎐N(4)
2.009(3)
2.042(3)
O᎐Zn᎐N(1)
N(1)᎐Zn᎐N(3) 118.55(13)
N(1)᎐Zn᎐N(4) 121.69(12)
93.15(12)
O᎐Zn᎐N(3)
O᎐Zn᎐N(4)
N(3)᎐Zn᎐N(4) 110.13(12)
109.18(12)
99.64(11)
O᎐Zn᎐N(2)
N(3)᎐Zn᎐N(2)
169.35(10)
81.34(11)
N(1)᎐Zn᎐N(2)
N(4)᎐Zn᎐N(2)
79.96(12)
77.53(11)
Table 3 Selected bond lengths (Å) and angles (Њ) for complex 8
Zn᎐O(1)
Zn᎐N(1)
Zn᎐N(3)
1.986(5)
2.045(6)
2.061(8)
Zn᎐O(2)
Zn᎐N(2)
1.979(7)
2.260(5)
O(1)᎐Zn᎐O(2) 100.3(2)
O(2)᎐Zn᎐N(1) 116.4(3)
O(1)᎐Zn᎐N(1)
91.1(2)
O(1)᎐Zn᎐N(2) 168.6(2)
N(1)᎐Zn᎐N(2) 80.5(2)
O(2)᎐Zn᎐N(3) 112.4(3)
N(2)᎐Zn᎐N(3) 81.3(2)
O(2)᎐Zn᎐N(2)
O(1)᎐Zn᎐N(3)
90.3(2)
98.0(2)
Fig. 1 Molecular geometry of the cation in complex 6
N(1)᎐Zn᎐N(3) 127.6(3)
that of the copper() analogue [Cu(HL6)]BF4.23 In this complex
one of the phenols in the pro-ligand does not deprotonate and
the metal is co-ordinated in a distorted trigonal-bipyramidal
manner by the three amine nitrogen atoms, one pyridine nitro-
gen atom and an oxygen atom from the phenolate; the phenolic
arm does not engage in co-ordination.
The IR spectra of these zinc() complexes are similar in pat-
tern to those of the pro-ligands. There are small shifts in the
characteristic bands with the most significant difference being a
decrease in intensity of the secondary amine bands at 3240–
2800 cmϪ1. There is a sharp band at ca. 3260 cmϪ1 assignable to
Fig. 2 Molecular geometry of complex 8
the single phenol OH. Solvent ethyl acetate in the complexes
Ϫ
gives an absorption at ca. 1730 cmϪ1 and the anions ClO4
,
,
BF4 and BPh4 have bands at 1110, 1083 and 732 cmϪ1
Ϫ
Ϫ
and angles, have been deposited at the Cambridge Crystallo-
graphic Data Centre (CCDC). See Instructions for Authors,
J. Chem. Soc., Dalton Trans., 1997, Issue 1. Any request to the
CCDC for this material should quote the full literature citation
and the reference number 186/310.
respectively. Positive-ion fast-atom bombardment (FAB) mass
spectra provide further evidence for complex formation. For
complexes 1, 2 and 3–5, the molecular peak can be identified as
[Zn(HL)]+ at m/z 469 and 497 respectively and whilst for com-
plexes 1 and 3–5 extremely small peaks are detected assignable
to species such as [Zn2(HL)2] and [Zn2(HL)2]X, probably arising
from aggregation processes in the matrix used; no intense peaks
corresponding to oligonuclear species are found.
Treatment of the Schiff-base imine pro-ligands with an
equimolar, or excess, amount of Zn(ClO4)2 in methanol in the
presence of triethylamine yielded the five-co-ordinate mono-
nuclear zinc() complexes 6–9. Hydrolytic cleavage of one
imine bond in the pro-ligand has occurred and although it is not
proven it is likely that under the conditions of the reaction an
activated nucleophile (OH) has been generated at the metal and
is responsible for the ensuing cleavage reaction. For H3L4 fur-
ther hydrolysis occurred during the reaction leading to the for-
mation of complex 10. This situation is very different from that
found in the reaction of the pro-ligands H2L1 and H2L2 with
Cu(ClO4)2 and triethylamine. No hydrolytic cleavage occurs and
trinuclear complexes are generated in which two ligand mol-
ecules share three copper() atoms.19
Results and Discussion
Pro-ligands H2L5 and H2L6 were prepared by reduction of the
corresponding Schiff-base ligand with NaBH4 in ethanol. The
absence of strong bands at 1635 cmϪ1 (characteristic of imine
C᎐N bond stretches) and the presence of medium bands at
᎐
1600–1580 cmϪ1 (attributable to N᎐H bending modes), in the
IR spectra, confirmed the successful in situ reduction. The mass
spectra gave clear molecular ion M + H+ peaks and accurate
mass peaks at 407.244 809 for H2L5 and 435.276 882 for H2L6
were determined and compared to the calculated values of
407.244 702 and 435.276 002; the isotopic patterns of these
peaks are consistent with the theoretical patterns.
The 1H NMR spectra for the pro-ligands showed the presence
of a singlet at δ ≈ 3.90 corresponding to the methylene protons
introduced by reduction of the imine bonds and no imine-related
proton singlet was observed. The 13C NMR data also confirmed
the successful reduction of imines as the carbon signals of imine
Structures of complexes 6 and 8
C᎐N bonds at δ ≈ 165 were replaced by methylene carbon signals
᎐
at δ ≈ 52 in the spectra of the reduced compounds.
The two complexes are mononuclear, five-co-ordinate species
that show similar approximately trigonal-bipyramidal (TBPY)
geometries with TBPY components τ = 0.79 (6) and 0.68 (8)
respectively. For perfect square-pyramidal and trigonal-
bipyramidal geometries the values of τ are zero and unity
respectively, τ being an index of the degree of trigonality within
the structural continuum between square-pyramidal and
trigonal-bipyramidal geometries.24 The molecular structures,
When the reduced amine phenol pro-ligands were employed
in complexation reactions with an excess amount of zinc() a
series of mononuclear zinc() complexes [Zn(HL)X] were
obtained. These are recovered as white-yellow microcrystals
after crystallization from appropriate solvents; unfortunately
crystals suitable for X-ray crystallography were not obtained.
The zinc complexes are proposed to have similar structures to
164
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with atom labelling, are illustrated in Figs. 1 and 2; relevant bond
lengths and angles with standard deviations in Tables 2 and 3.
Complex 6 consists of a complex cation and a tetraphenyl-
borate anion and the asymmetric unit of 8 comprises the neu-
tral zinc() species and a methanol solvent molecule. In each of
the two complexes one of the two imine bonds in the parent
ligand has hydrolysed on reaction with Zn(ClO4)2 and the
resultant primary amine nitrogen is co-ordinated to the metal
atom at an equatorial position of the distorted trigonal-
bipyramidal geometry adopted; the tertiary amino-nitrogen
atom occupies an axial position.
In complex 6 the second axial site is a deprotonated phenolic
oxygen [N(1), N(3), N(4) plane, with Zn Ϫ0.366, O Ϫ2.301
and N(2) 1.945 Å out of plane], with Zn᎐N distances 2.009,
2.030 and 2.042 Å in the equatorial plane, significantly short-
er than the Zn᎐N(2) amine distance of 2.313 Å at the axial
position; Zn᎐O 1.961. The phenyl and the pyridyl rings are
planar [root mean square (r.m.s.) deviations 0.003, 0.001 Å
respectively]; deviations of ring substituent atoms from
planarity are small [O 0.005, C(7) Ϫ0.056 and C(12) Ϫ0.024 Å].
The angle between the pyridine and phenyl rings is 55.7Њ.
Torsion angles of NCCN fragments are +45.1 and +55.6Њ
respectively. The primary amine does not engage in any
hydrogen bonding; this is very different from complex 3
where the appearance of p-nitrophenol results in the forma-
tion of hydrogen bonds.
In complex 8 two phenolate oxygens occupy different geo-
metric sites (one axial and one equatorial); N(1), N(3) and O(2)
make up the plane [Zn 0.221, O(1) 2.199 and N(2) Ϫ2.023 Å out
of plane], with two different Zn᎐N distances, two of 2.045,
2.061 Å in the equatorial plane and 2.26 Å in an axial position,
and two almost equal Zn᎐O distances of 1.979 and 1.986 Å. The
methanol solvent molecule is involved in hydrogen bonding to
the phenolate oxygen [O(5) ؒ ؒ ؒ O(2) 2.807, H(O5) ؒ ؒ ؒ O(2) 1.807
Å]. It is worth noting that the formation of the hydrogen bond
and the existence of the electron-withdrawing group (NO2) at
the para position does not weaken the Zn᎐O(2) bond. This bond
is marginally shorter than the second more normal zinc–oxygen
bond (1.986 Å) and is the shortest bond within the zinc co-
ordination sphere. The apical bonds average 2.125 Å; the mean
value for the basal ligation is 2.068 Å. The Zn᎐N(2) bond (2.260
Å) formed between the metal and tertiary nitrogen is the longest
and significantly longer than the average Zn᎐N distances. This
may be related to the induction of stress from the presence of
the two five-membered rings. The bond angles at the zinc atom
are further indicative of TBPY symmetry.
The phenyl rings in complex 8 are planar (r.m.s. deviations
0.019, 0.011 Å); deviations of oxygen and nitrogen substitu-
ent atoms from planarity are small (less than 0.053 Å), but
the imine and amine C are 0.157 and Ϫ0.141 Å out of
planarity. The nitro group is asymmetrically twisted by 8.4Њ
from the plane of the phenyl ring. The angle between the
phenyl rings is 35.3Њ. Torsion angles of both NCCN frag-
ments are +53Њ. The zinc OCCCN (imine) chelate ring is
approximately planar (r.m.s. deviation 0.024 Å) and inclined
at 5.7Њ to the phenyl ring.
for which the zinc complexes are co-ordinatively unsaturated
and/or sterically open for phosphate substrate approach can
catalyse the hydrolysis of phosphate esters.12 To test whether
flexible five-co-ordinate zinc complexes have the propensity for
inducing catalytic hydrolysis of phosphate esters, preliminary
kinetic studies for the two complexes 6 and 8, for which the
structures have been determined crystallographically, were con-
ducted using the UV/VIS spectroscopic method of Koike and
Kimura.11,25 The observed rate constant for hydrolysis of tris(4-
nitrophenyl) phosphate catalysed by complex 6 is 3.95 × 10Ϫ4
sϪ1 (based on a pseudo-first-order reaction) compared with
3.60 × 10Ϫ5 sϪ1 found in the absence of the complex [25 ЊC, NЈ-
(2-hydroxyethyl)piperazine-N-ethanesulfonic acid
(hepes)
buffer (10 mmol, 90% v/v EtOH and pH 7.37); I = 0.2 mol dmϪ3
(NaClO4)]. A small enhancement is also noted for hydrolysis of
bis(4-nitrophenyl) phosphate hydrolysis where the observed
rate constant is 2.69 × 10Ϫ7 sϪ1 compared with 6.73 × 10Ϫ8 sϪ1 in
the absence of the complex [35 ЊC; hepes buffer (10 mmol, 90%
v/v EtOH and pH 7.37); I = 0.2 mol dmϪ3 (NaClO4)]. Further
studies were hampered due to the emergence of a small precipi-
tate at higher pH. Potentiometric titrations on solutions (90%
v/v ethanol) of the pro-ligand (1.0 mmol dmϪ3) and an equi-
molar amount of zinc perchlorate showed that the complex has
essentially formed by pH 6.7, therefore at the concentrations
and pH values employed in this study it is likely that the com-
plex is fully formed and so the accelerations noted can be
attributed to its influence. For complex 8 no obviously
enhanced rates, even for tris(4-nitrophenyl) phosphate, were
observed under the present experimental conditions.
Acknowledgements
We thank the University of Sheffield for a Scholarship (to Q.-Y.
H.) and the SERC and Royal Society for funds towards the
purchase of the diffractometer.
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Received 14th August 1996; Paper 6/05706C
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J. Chem. Soc., Dalton Trans., 1997, Pages 161–166