Di- and poly-nuclear zinc(ii) Schiff base complexes: Synthesis, structural studies and reaction with an α-amino acid ester

Article abstract of DOI:10.1039/a907839h

In order to model peptide hydrolysis by dimetallic aminopeptidases, the dinuclear zinc(n) Schiff base complex [Zn₂L¹(CH₃CO₂)]ClO₄ la [HL¹ = 2,6-bis{Ar-[2-(dimethyIamino)ethyl]iminomethyl}-4-methylphenol] has been prepared and characterised by X-ray crystallography. The zinc ions are bridged by the deprotonated phenolic oxygen of L¹ and two acetate groups, the Zn ... Zn distance being 3.234(1) A. Complex la was shown to promote hydrolysis of glycine ethyl ester under mild conditions. Reaction of the hydrolysis product glycine with la led to the conversion of the ligand into L² (H₃L² = corresponding Schiff base derived from glycine). The X-ray analysis of the resulting zinc complex 2 revealed the novel pentanuclear complex [{Zn₂L²(CH₃CO₂)₂} ₂Zn(H₂O)₄]·4.5H₂O, and the crystal structure of polymeric {[ZnCHL²)]·3H₂O}∞, was determined. Complex 2 is built up of two dinuclear acetate-bridged Zn₂/ligand moieties that are linked through bridging carboxylate functions of the ligand to the fifth Zn atom. In the polymer one co-ordination site of the binucleating ligand is occupied by a Zn, while the second one is protonated. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a907839h

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Di- and poly-nuclear zinc(II) Schiff base complexes: synthesis,
structural studies and reaction with an ꢀ-amino acid ester†
Andrea Erxleben* and Jolante Hermann
Fachbereich Chemie, Universität Dortmund, 44221 Dortmund, Germany
Received 29th September 1999, Accepted 17th December 1999
In order to model peptide hydrolysis by dimetallic aminopeptidases, the dinuclear zinc() Schiff base complex
[Zn2L1(CH3CO2)2]ClO4 1a [HL1 = 2,6-bis{N-[2-(dimethylamino)ethyl]iminomethyl}-4-methylphenol] has been
prepared and characterised by X-ray crystallography. The zinc ions are bridged by the deprotonated phenolic oxygen
of L1 and two acetate groups, the Zn ؒ ؒ ؒ Zn distance being 3.234(1) Å. Complex 1a was shown to promote hydrolysis
of glycine ethyl ester under mild conditions. Reaction of the hydrolysis product glycine with 1a led to the conversion
of the ligand into L2 (H3L2 = corresponding Schiff base derived from glycine). The X-ray analysis of the resulting
zinc complex 2 revealed the novel pentanuclear complex [{Zn2L2(CH3CO2)2}2Zn(H2O)4]ؒ4.5H2O, and the crystal
structure of polymeric {[Zn(HL2)]ؒ3H2O}∞ was determined. Complex 2 is built up of two dinuclear acetate-bridged
Zn2/ligand moieties that are linked through bridging carboxylate functions of the ligand to the fifth Zn atom. In the
polymer one co-ordination site of the binucleating ligand is occupied by a Zn, while the second one is protonated.
2,6-bis{N-[2-(dimethylamino)ethyl]iminomethyl}-4-methyl-
Introduction
phenol (HL1) to mimic the dinickel site of urease as well as
Zinc-containing, carboxylate-bridged dimetallic centres are a
dimanganese sites.
widespread structural motif in hydrolytic metalloenzymes, such
Here we report on the synthesis and structural characteris-
as phosphatases and aminopeptidases.1 The catalytic role of Zn
comprises Lewis acid activation of the substrate, generation of
a reactive nucleophile (Zn–OH) and stabilisation of the leaving
group. Small-molecule metal complexes that can serve as struc-
tural and functional models for these enzymes receive continu-
ous interest. Over the years, numerous mononuclear model
ation of the dinuclear zinc() Schiff base complex [Zn2L1(CH3-
CO2)2]X 1 and its reaction with an α-amino acid ester. In the
course of our attempt to model peptide hydrolysis by a dimetal-
lic site we obtained the novel pentanuclear complex [{Zn2L2-
(CH3CO2)2}2Zn(H2O)4]ؒ4.5H2O 2. The crystal structure of 2 is
presented along with that of polymeric {[Zn(HL2)]ؒ3H2O}∞ 4.
complexes have been designed and studied in order to investi-
gate the mechanism of phosphate ester, carboxy ester and
amide hydrolysis and the relationship between structure and
reactivity.2 More recent studies on dinuclear model systems
have manifested the importance of polynuclear centres and of
co-operativity between the metal ions in phosphate diester
hydrolysis.3 In contrast, dinuclear aminopeptidase models are
rare.4 Aminopeptidases catalyse the hydrolysis of the amino-
terminal peptide bond in polypeptides. Bovine lens leucine
aminopeptidase (blLAP) and aminopeptidase from Aeromonas
proteolytica (AAP) which have been structurally characterised
contain µ-hydroxo-bis(µ-carboxylato)-dizinc() and µ-hydroxo-
µ-carboxylato-dizinc() cores.5,6 Two alternative mechanisms
have been proposed for blLAP, both including co-operativity
between the metal centres. On the basis of the crystal structure
of a transition-state analogue complex of blLAP, bidentate co-
ordination of the peptide through the terminal amino group to
Zn(1) and through the carbonyl oxygen to Zn(2) and nucleo-
philic attack by the bridging hydroxide at the carbonyl carbon
has been suggested by Sträter and Lipscomb.7 Alternatively,
monodentate co-ordination via the carbonyl oxygen and nucleo-
philic attack by terminally bound hydroxide has also been
discussed.5
One type of ligand often used to model dinuclear biosites
are phenol-based compartmental ligands of ‘end-off ’ type.8
Que and co-workers9 have studied diiron and dimanganese
active sites utilising 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-
4-methylphenol which was first described by Suzuki et al.10
Experimental
Syntheses
Zinc complexes of related ligands have been reported by
Krebs, Sakiyama and Wang.4,11 Okawa and co-workers12 used
2,6-Diformyl-4-methylphenol was prepared according to
ref. 13, HL1 by reaction of 2,6-diformyl-4-methylphenol and
N,N-dimethylethylenediamine in CHCl3 and subsequent evap-
† Electronic supplementary information (ESI) available: crystal pack-
ing diagrams. See http://www.rsc.org/suppdata/dt/a9/a907839h/
DOI: 10.1039/a907839h
J. Chem. Soc., Dalton Trans., 2000, 569–575
This journal is © The Royal Society of Chemistry 2000
569

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oration of the solvent and HL4 [2,6-bis{N-[2-(dimethylamino)-
ethyl]aminomethyl}-4-methylphenol] was prepared by reduc-
tion of HL1 by NaBH4 in methanol according to standard
literature procedures.14 Glycine and glycine ethyl ester
hydrochloride were obtained from Fluka. All chemicals and
solvents were reagent grade used without further purification.
CAUTION: perchlorate salts of metal complexes are poten-
tially explosive and should be handled with care.
piperazinepropanesulfonic acid] in D2O, pD 7.4). The reaction
was monitored by 1H NMR spectroscopy at 20 ЊC. The
hydrolysis was complete after ca. 2 d in the case of 1 and after
ca. 12 h in the case of 3. As a control experiment glycine ethyl
ester was treated with 2 equivalents Zn(CH3CO2)2 under the
same conditions (25 µl 0.2 M stock solution of glycine ethyl
ester in D2O, 50 mM EPPS in D2O, pD 7.4). 54% ester cleavage
was observed after 69 h. Treatment of glycine ethyl ester with
Zn(CH3CO2)2 (2 equivalents) in the presence of N,N,NЈ,NЈ-
tetramethylethylenediamine (2 equivalents) resulted in 66%
substrate conversion within 62 h.
[Zn2L1(CH3CO2)2]X (X ؍ ClO4 1a or PF6 1b). 2,6-Diformyl-
4-methylphenol (100 mg, 0.61 mmol) and Zn(CH3CO2)2ؒ2H2O
(335 mg, 1.53 mmol) were dissolved in ethanol (25 cm3). After
addition of N,N-dimethylethylenediamine (108 mg, 1.23 mmol)
dissolved in water (5 cm3) the reaction mixture was stirred over-
night at room temperature. Treatment with NaClO4ؒH2O (343
mg, 2.44 mmol) or KPF6 (336 mg, 1.83 mmol) gave complex 1a
as yellow cubes (303 mg, 76%) and 1b as a pale yellow powder
(312 mg, 73%). Found: C, 38.3; H, 5.3; N, 8.4. C21H33ClN4-
O9Zn2 requires C, 38.7; H, 5.1; N, 8.6. Found: C, 36.0; H, 4.6;
N, 8.0. C21H33F6N4O5PZn2 requires C, 36.2; H, 4.8; N, 8.0%. 1H
NMR (CD3OD): δ 8.54 (s, 2 H, Him), 7.39 (s, 2 H, Har), 3.81 (t, 4
H, CH2), 2.81 (t, 4 H, CH2), 2.47 (s, 12 H, NCH3), 2.29 (s, 3 H,
CH3) and 2.02 (s, 6 H, CH3CO2Ϫ). IR (cmϪ1): [Zn2L1(CH3-
CO2)2]ϩ 3005m, 2973m, 2926m, 2890m, 2872m, 2843m, 2795w,
1655s, 1647s, 1597s, 1549s, 1448s, 1374m, 1334m, 1276w,
1239m, 1196w, 1025m, 990m, 947m, 893m, 818m, 783m, 548m,
Instrumentation
The 1H NMR spectra were recorded on a Bruker AC200
spectrometer. Spectra were run in D2O, (CD3)2SO or CD3OD
solutions using sodium 3-trimethylsilylpropanesulfonate as
internal reference. The pD values of D2O solutions were
obtained by use of a glass electrode and addition of 0.4 to the
pH meter reading. Infrared spectra of KBr pellets were taken
on a Bruker IFS 28 FT-spectrometer.
Crystal structure analyses
Crystal data for compounds 1a, 2 and 4 were measured at room
temperature on an Enraf-Nonius-KappaCCD diffractometer15
using graphite-monochromated Mo-Kα radiation (λ = 0.71069
Å). For data reduction and cell refinement the programs
DENZO and SCALEPACK were applied.16 The structures
were solved by conventional Patterson (1a and 2) or direct (4)
methods and subsequent Fourier syntheses and refined by
full-matrix least squares on F 2 using the SHELXTL PLUS,
SHELXL 93 and SHELXL 97 programs.17 All non-hydrogen
atoms were refined anisotropically [with the following excep-
tions: C(11), C(16), C(17), O(3), O(11) in 1a and O(2W),
O(3W), O(4W), O(5W), O(6W), O(7W) and O(8W) in 4]. In 1a
and 2 hydrogen atoms except for those of the water molecules
were generated geometrically and given isotropic thermal
parameters equivalent to 1.2 times those of the atom to which
they were attached. In 4 the hydrogen atoms (except for those
of the water molecules) were located in the Fourier-difference
map and refined isotropically. The perchlorate oxygens in 1a
are disordered. The Fourier-difference syntheses revealed eight
oxygens to which occupancy factors of 0.6 [O(10), O(11), O(12)
and O(13)] and 0.4 [O(20), O(21), O(22) and O(23)] were
assigned. On the basis of the peak height in the Fourier map
site occupancy factors of 0.5 were assigned to O(5W) in 2 and
to O(4W) in 4 as well as of 0.25 to O(3W), O(5W), O(6W),
O(7W) and O(8W) in 4.
Ϫ
Ϫ
500w, 483w and 450m; ClO4 (1a) 1094s and 622m; PF6 (1b)
840s and 668m.
[{Zn2L2(CH3CO2)2}2Zn(H2O)4]ؒ4.5H2O 2 and {[Zn(HL2)]ؒ
3H2O}∞ 4. Glycine (304 mg, 4.05 mmol) was dissolved in water
(15 cm3) and added to a solution of 2,6-diformyl-4-methyl-
phenol (330 mg, 2.01 mmol) and Zn(CH3CO2)2ؒ2H2O (1.097
mg, 5.00 mmol) in ethanol (75 cm3). After stirring overnight
at room temperature a yellow precipitate was filtered off.
Recrystallisation from water afforded small yellow needles of
complex 2 in 52% yield. Found: C, 32.5; H, 3.8; N, 4.4.
1
C34H51N4O26.5Zn5 requires C, 32.2; H, 4.1; N, 4.4%. H NMR
(CD3OD): δ 8.45 (s, 2 H, Him), 7.35 (s, 2 H, Har), 4.16 (s, 4 H,
CH2), 2.28 (s, 3 H, CH3) and 1.96 (s, 6 H, CH3CO2Ϫ). IR (cmϪ1):
3399vs, br, 1645s, br, 1578s, br, 1448s, 1399s, 1333m, 1298m,
1239m, 1195w, 1079m, 1050m, 1007w, 935w, 877w, 822m,
762m, 699w, 669m, 620m, 575m and 488m. Slow evaporation
of the filtrate at room temperature gave yellow cubes of 4
(240 mg, 30%). Found: C, 38.9; H, 4.5; N, 6.5. C13H18N2O8Zn
1
requires C, 39.4; H, 4.6; N, 7.0%. H NMR (D2O, pD 6.9):
δ 8.40 (s, 1 H, Him), 8.33 (s, 1 H, Him), 7.59 (s, 1 H, Har), 7.44 (s,
1 H, Har), 4.38 (s, 2 H, CH2), 4.25 (s, 2 H, CH2) and 2.24 (s, 3 H,
CH3). IR (cmϪ1): 3426s, br, 1653s, 1631s, 1594s, 1541s, 1489w,
1456w, 1384s, 1305w, 1274m, 1223m, 1068m, 996m, 960w,
878w, 823w, 760w, 707w, 668w, 585m, 491w and 419w.
Crystallographic data and details of refinement are reported
in Table 1. Selected bond lengths and angles are listed in Tables
2–4.
CCDC reference number 186/1775.
See http://www.rsc.org/suppdata/dt/a9/a907839h/ for crystal-
lographic files in .cif format.
[Zn2L4Cl3]ؒ1.5H2O 3. Zinc chloride (552 mg, 4.05 mmol),
HL4 (500 mg, 1.62 mmol) and NaOCH3 (89 mg, 1.65 mmol)
were allowed to react in ethanol–water (5:1, 24 cm3) over-
night at room temperature. Slow evaporation of the solvent
yielded a white, microcrystalline precipitate of complex 3 (156
mg, 17%). Found: C, 35.5; H, 5.6; N, 9.8. C17H34Cl3N4O2.5Zn2
Results and discussion
Synthesis and characterisation of [Zn2L1(CH3CO2)2]X
1
requires C, 35.7; H, 6.0; N, 9.8%. H NMR (D2O, pD 7.2):
Schiff base complexes are generally easily accessible by con-
densation of an aldehyde with an amine in the presence of
the metal ion.18 Reaction of 2,6-diformyl-4-methylphenol with
N,N-dimethylethylenediamine in the presence of Zn(CH3-
CO2)2 leads to the dinuclear zinc complex [Zn2L1(CH3CO2)2]X
(X = ClO4 1a or PF6 1b). For 1a crystals suitable for
X-ray analysis were obtained, and the crystal structure was
determined.
A view of the cation of complex 1a is depicted in Fig. 1
and selected bond lengths and angles are presented in Table 2.
The Zn atoms, which are 3.234(1) Å apart, are bridged by the
deprotonated phenolic oxygen of L1 and two acetate groups.
δ 7.03 (s, 2 H, Har), 3.96 (s, br, 4 H, CarCH2), 3.02 (s, br, 4 H,
NCH2), 2.66 (s, br, 4 H, NCH2), 2.21 (s, 12 H, NCH3) and
2.10 (s, 3 H, CH3). IR (cmϪ1): 3503br, 3196m, 3001m, 2971m,
2916br, s, 1477s, 1312m, 1267m, 1165m, 1089m, 1030m, 964w,
937m, 886w, 860m, 803m, 780m, 668w, 592w, 556w, 501m,
462w and 401w.
Hydrolysis of glycine ethyl ester
A 25 µl 0.2 M stock solution of glycine ethyl ester in D2O and
25 µl 0.2 M stock solution of complex 1 or 3 in (CD3)2SO were
mixed in 0.5 cm3 buffer (50 mM EPPS [4-(2-hydroxyethyl)-1-
570
J. Chem. Soc., Dalton Trans., 2000, 569–575

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Table 1 Crystal data for [Zn2L1(CH3CO2)2]ClO4 1a, [{Zn2L2(CH3CO2)2}2Zn(H2O)4]ؒ4.5H2O 2 and {[Zn(HL2)]ؒ3H2O}∞ 4
1a
2
4
Empirical formula
Formula weight
C21H33ClN4O9Zn2
651.70
C34H51N4O26.5Zn5
1266.64
C13H18N2O8Zn
395.66
Crystal system
Space group
Triclinic
P1
Triclinic
P1
Monoclinic
C2/c
¯
¯
a/Å
b/Å
c/Å
α/Њ
8.717(1)
11.285(1)
15.057(1)
99.37(1)
94.00(3)
104.04(3)
1408.5(2)
2
8.192(2)
11.841(2)
14.520(3)
100.61(3)
91.98(3)
102.71(3)
1346.3(5)
1
20.290(4)
8.514(2)
19.488(4)
β/Њ
108.13(3)
γ/Њ
V/Å3
3199.4(12)
8
1.580
Z
µ(Mo-Kα)/mmϪ1
1.850
2.278
T/K
293
4573
4573
3164
0.048, 0.128
0.080, 0.158
293
4087
4064
2213
0.087, 0.206
0.149, 0.218
293
5372
2920
1864
0.043, 0.088
0.087, 0.119
No. measured reflections
No. independent reflections
No. observed reflections [I > 2σ(I)]
Final R1, wR2 indices [I > 2σ(I)]
(all data)
Table 2 Selected bond distances (Å) and angles (Њ) for [Zn2L1(CH3-
CO2)2]ClO4 1a
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–O(1)
Zn(1)–O(2)
Zn(1)–O(4)
N(1)–C(8)
O(2)–C(18)
O(4)–C(20)
2.013(3)
2.216(3)
2.093(2)
1.991(2)
1.966(2)
1.282(5)
1.247(4)
1.246(4)
Zn(2)–N(3)
Zn(2)–N(4)
Zn(2)–O(1)
Zn(2)–O(3)
Zn(2)–O(5)
N(3)–C(13)
O(3)–C(18)
O(5)–C(20)
2.031(3)
2.204(3)
2.101(2)
1.969(2)
1.976(2)
1.271(5)
1.266(4)
1.260(4)
N(1)–Zn(1)–N(2)
N(1)–Zn(1)–O(4)
O(1)–Zn(1)–O(4)
O(1)–Zn(2)–N(4)
O(3)–Zn(2)–O(5)
O(5)–Zn(2)–N(3)
C(8)–N(1)–Zn(1)
80.9(1)
139.1(1)
95.41(9)
163.9(1)
115.3(1)
110.0(1)
127.2(2)
N(1)–Zn(1)–O(2)
N(2)–Zn(1)–O(1)
O(2)–Zn(1)–O(4)
O(1)–Zn(2)–O(3)
O(3)–Zn(2)–N(3)
N(3)–Zn(2)–N(4)
C(10)–N(2)–Zn(1)
C(15)–N(4)–Zn(2)
C(1)–O(1)–Zn(2)
O(2)–C(18)–O(3)
108.5(1)
167.7(1)
111.4(1)
94.6(1)
134.0(1)
80.4(1)
101.8(2)
100.6(2)
129.2(2)
124.4(3)
C(13)–N(3)–Zn(2) 128.1(2)
C(1)–O(1)–Zn(1)
Zn(1)–O(1)–Zn(2)
O(4)–C(20)–O(5)
129.4(2)
100.92(9)
125.4(3)
Fig. 1 View of the cation of [Zn2L1(CH3CO2)2]ClO4 1a with the atom
numbering scheme.
Each metal centre is five-co-ordinate through the imine nitro-
gen, the amine nitrogen and the phenolate and acetate oxygens.
The co-ordination geometry of the Zn atoms is best described
as distorted trigonal bipyramidal with the phenolic oxygen and
amine nitrogen occupying the apical positions. The acetate
groups form an angle of 49.9(2)Њ with each other. The Zn–N
and Zn–O distances range from 1.966 to 2.216 Å, with the Zn–
N (imine) bonds [2.013(3) and 2.031(3) Å] being significantly
shorter than the Zn–N (amine) bonds [2.216(3) and 2.204(3) Å].
and HL1 in CD3OD shows two distinct sets of resonances
indicating that the complex is non-labile on the NMR time-
scale. The 0.14 ppm downfield shift of the acetate reson-
ance (δ 2.02) compared with that of ionic acetate (CH3CO2Na,
δ 1.88) in CD3OD suggests interaction of the acetate with the
metal centres in solution.
Despite the well known susceptibility of imine bonds to
hydrolysis, metal complexes of Schiff bases are generally resist-
ant towards hydrolysis.18 This is also the case for 1: no dissoci-
ation to aldehyde and amine could be detected in a solution of 1
in D2O (pD 7.4) after 1 week.
In the IR spectrum of complexes 1a and 1b the ν(C᎐N)
᎐
vibration is observed at 1647 cmϪ1. The asymmetric and sym-
metric stretching vibrations of the acetate groups appear at
1597 and 1448 cmϪ1, respectively.‡ The difference between
νasym(COO) and νsym(COO) (149 cmϪ1), which is smaller than
164 cmϪ1 observed in ionic acetate, reflects the bidentate
bridging co-ordination mode.19 The proton NMR spectrum
of 1 in CD3OD shows two singlets in the aromatic region
corresponding to the azomethine (δ 8.54) and phenolic ring
(δ 7.39) protons. Compared to the “free” ligand HL1 these
resonances are shifted to higher field by 0.04 and 0.16 ppm.
The methylene protons give two broad triplets at δ 3.81 and
2.81, and the signals of the N–CH3 and Ph–CH3 methyl
groups occur at δ 2.47 and 2.29. A mixture of complex 1
Hydrolysis of ꢀ-amino acid ester
Reaction of complex 1 with 1 equivalent glycine ethyl ester in
D2O at pD 7.4 and room temperature was monitored by NMR
1
spectroscopy (Fig. 2). The H NMR spectra clearly indicate
release of ethanol which is complete in about 2 d. In contrast, in
the absence of any metallocatalyst the half-life for the hydro-
lysis of carboxy esters at neutral pH is in the order of years.21
Besides NMR resonances corresponding to the hydrolysis
products ethanol and glycine, new signals appear in the aro-
matic region (δ 8.29, 7.73 and 7.36) (Fig. 2, right). By com-
parison with a genuine sample (see below) the singlets at δ 8.29
and 7.36 can be assigned to the zinc complex of the ligand H3L2
(2) that is formed by exchange of the ligand side arms with
glycine. Fig. 3 shows the NMR spectra of 1 in D2O after addi-
‡ The IR resonances of the acetate groups have been assigned by
comparison with the IR spectrum of the analogous complex
[Zn2L1Cl3], the X-ray analysis of which revealed one bridging and two
terminally bound chlorides.20
J. Chem. Soc., Dalton Trans., 2000, 569–575
571

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Table 3 Selected bond distances (Å), angles (Њ) and possible hydrogen-
bonding interactions for [{Zn2L2(CH3CO2)2}2Zn(H2O)4]ؒ4.5H2O 2
Zn(1)–N(1)
Zn(1)–O(1)
Zn(1)–O(3)
Zn(1)–O(6)
Zn(1)–O(8)
Zn(3)–O(5)
Zn(3)–O(11)
N(2)–C(11)
O(4)–C(13)
O(6)–C(14)
O(9)–C(16)
1.998(11)
2.088(8)
2.080(8)
1.965(8)
1.971(9)
2.17(1)
2.073(7)
1.30(2)
1.28(2)
Zn(2)–N(2)
Zn(2)–O(1)
Zn(2)–O(4)
Zn(2)–O(7)
Zn(2)–O(9)
Zn(3)–O(10)
N(1)–C(8)
O(3)–C(10)
O(5)–C(13)
O(7)–C(14)
C(10)–O(2)
2.009(10)
2.060(8)
2.092(8)
1.962(9)
1.966(9)
2.070(8)
1.29(2)
1.25(2)
1.22(1)
1.26(2)
1.24(2)
1.27(2)
1.25(1)
N(1)–Zn(1)–O(1)
N(1)–Zn(1)–O(6)
O(1)–Zn(1)–O(3)
O(6)–Zn(1)–O(8)
N(2)–Zn(2)–O(4)
O(1)–Zn(2)–O(4)
O(7)–Zn(2)–O(9)
O(5)–Zn(3)–O(10)
O(10)–Zn(3)–O(11)
C(8)–N(1)–Zn(1)
C(16)–O(9)–Zn(2)
O(2)–C(10)–O(3)
O(6)–C(14)–O(7)
89.9(4)
121.1(4)
170.0(3)
119.0(4)
79.6(4)
167.4(3)
113.6(4)
93.0(3)
88.6(3)
127.5(9)
126.6(9)
125(1)
N(1)–Zn(1)–O(3)
N(1)–Zn(1)–O(8)
O(3)–Zn(1)–O(6)
N(2)–Zn(2)–O(1)
N(2)–Zn(2)–O(9)
O(7)–Zn(2)–O(4)
O(7)–Zn(2)–N(2)
O(5)–Zn(3)–O(11)
Zn(2)–O(1)–Zn(1)
C(11)–N(2)–Zn(2)
C(13)–O(5)–Zn(3)
O(4)–C(13)–O(5)
O(8)–C(16)–O(9)
81.2(4)
119.1(4)
87.4(3)
88.2(3)
112.9(4)
87.0(4)
132.0(4)
86.9(3)
99.9(3)
127.6(8)
124.7(9)
126(1)
125(1)
126(1)
O(2) ؒ ؒ ؒ O(10)a
O(4) ؒ ؒ ؒ O(11)c
2.78(1)
2.68(1)
2.92(1)
2.84(1)
2.88(2)
2.76(1)
2.83(3)
2.84(4)
O(3) ؒ ؒ ؒ O(11)b
O(5) ؒ ؒ ؒ O(3W)d
O(6) ؒ ؒ ؒ O(3W)c
2.76(1)
2.88(1)
2.87(1)
Fig. 2 Hydrolysis of glycine ethyl ester by complex 1a at pD 7.4.
Proton NMR spectra were taken after (a) 1, (b) 5, (c) 11, (d) 24 and (e)
39 h at 20 ЊC. Resonances are assigned as follows: (᭿) CH3 of glycine
ethyl ester, (᭜) CH3 of EtOH, (᭹) Him and Har of 1a and (᭢) Him and
Har of 2.
O(5) ؒ ؒ ؒ O(11)
O(10) ؒ ؒ ؒ O(2W)e
O(1W) ؒ ؒ ؒ O(3W)f
O(2W) ؒ ؒ ؒ O(9)c
O(4W) ؒ ؒ ؒ O(4W)i
O(5W) ؒ ؒ ؒ O(5W)j
O(1W) ؒ ؒ ؒ O(2W)b 2.75(2)
O(1W) ؒ ؒ ؒ O(4W)g 2.76(2)
O(3W) ؒ ؒ ؒ O(5W)h 2.84(2)
O(4W) ؒ ؒ ؒ O(5W)
2.75(2)
Symmetry operations: a Ϫx, Ϫy ϩ 1, Ϫz; b x, y ϩ 1, z; c Ϫx, Ϫy, Ϫz;
d Ϫx, Ϫy Ϫ 1, Ϫz; e Ϫx ϩ 1, Ϫy, Ϫz; f x ϩ 1, y, z; g x, y Ϫ 1, z;
h Ϫx, Ϫy Ϫ 1, Ϫz Ϫ 1; i Ϫx ϩ 1, Ϫy, Ϫz Ϫ 1; j Ϫx, Ϫy, Ϫz Ϫ 1.
into 2 is almost complete. Possible pathways for the hydrolysis
reaction are (i) replacement of the ligand side arms by glycine
ethyl ester followed by hydrolysis of the ester group in Zn2L1
and (ii) 1 promoted ester cleavage and subsequent reaction of
the hydrolysis product glycine with 1. In order to distinguish
(i) and (ii), the imine ligand was reduced to the corresponding
amine HL4 that is incompatible with mechanism (i). The com-
plex [Zn2L4Cl3] 3‡ hydrolyses glycine ethyl ester, and the reac-
tion is complete within 12 h. The lower cleavage rate observed
with 1 is due to formation of 2 during the reaction. Generally,
the reactivity of a metal catalyst for hydrolysis reactions is
decreased by negatively charged donor atoms. The conversion
of 1 into 2 does not allow further clarification of the hydrolytic
mechanism, e.g. possible co-operativity between the two metal
ions. Detailed kinetic studies on 3 promoted amino acid ester
hydrolysis are in progress.
Zinc complexes of H3L2
Complex 2 was prepared on a preparative scale by treating
2,6-diformyl-4-methylphenol with 2 equivalents glycine in the
presence of Zn(CH3CO2)2. The complex precipitated from
ethanol–water and recrystallisation yielded small yellow
needles of composition [{Zn2L2(CH3CO2)2}2Zn(H2O)4]ؒ4.5H2O
2. Slow evaporation of the filtrate gave a second product of
composition [Zn(HL2)]ؒ3H2O 4. The structures of both com-
pounds were determined by X-ray analysis.
Fig. 3 Reaction of complex 1a with glycine in D2O (pD 7.3). Proton
NMR spectra were taken after addition of (a) 1, (b) 2 and (c) 5 equiv-
alents of glycine. Resonances (Him and Har) are assigned as follows:
(᭿) Zn2L1, (᭜) Zn2L2 and (᭹) Zn2L3.
Crystal structure of complex 2. Fig. 4 gives a view of complex
2, selected bond lengths, angles and possible hydrogen-bonding
interactions are listed in Table 3. The pentanuclear compound
is built up of two dinuclear acetate-bridged Zn2/ligand moieties
linked through bridging carboxylate functions of the ligand
side arm to the fifth Zn atom that is located on a crystallo-
tion of 1, 2 and 5 equivalents glycine. Treating 1 with 1 equiv-
alent glycine in D2O results in a mixture of 1 (δ 8.49 and 7.40),
[Zn2L2]ϩ 2 (δ 8.29 and 7.36) and [Zn2L3]2ϩ (δ 8.43, 8.23, 7.40 and
7.36; H2L3 = ligand containing one amine and one carboxylate
side arm). After addition of 5 equivalents glycine, conversion
572
J. Chem. Soc., Dalton Trans., 2000, 569–575

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Fig. 4 View of [{Zn2L2(CH3CO2)2}2Zn(H2O)4] 2 with the atom numbering scheme.
graphic inversion centre. The Zn atoms of the dinuclear unit
[Zn(1) and Zn(2)] are five-co-ordinate through imine nitrogen,
carboxylate, phenolate and two acetate oxygens. The inter-
metallic distance is 3.175(2) Å, slightly shorter than in 1a
[3.234(1) Å]. The co-ordination geometry of Zn(1) and Zn(2) is
trigonal bipyramidal. The carboxylate and bridging phenolic
oxygens occupy the apical positions, the equatorial planes are
formed by the acetate oxygens and the imine nitrogens. The
valence angles around Zn(1) are close to the ideal 90 and 120Њ
angles [ranges 119.0–121.1, 81.2–96.0Њ], while a more distorted
co-ordination sphere is observed for Zn(2) [ranges 112.9–132.0,
79.6–99.1Њ]. The greatest deviations from the ideal bond angles
are found for O(3)–Zn(1)–N(1) [81.2(4)Њ] and O(4)–Zn(2)–N(2)
[79.6(4)Њ] which may be caused by the narrow bite of the
chelating side arms of the ligand. The angle between the acetate
groups is 57.4(5)Њ. The Zn–O (acetate) bond distances
[1.962(9)–1.971(9) Å] are equal within experimental error and
significantly shorter than the Zn–O (carboxylate) bond dis-
tances [2.080(8)–2.17(1) Å]. In contrast to the symmetrically
bridging acetate, the carboxylate bridge between Zn(2) and
Zn(3) is asymmetric: the Zn(2)–O(4) bond [2.092(8) Å] is
significantly shorter than the Zn(3)–O(5) bond [2.17(1) Å]. The
central Zn atom is six-co-ordinate through two carboxylate
functions trans to each other and four water ligands. The
co-ordination geometry deviates only slightly from an ideal
octahedron, with the valence angles around Zn(3) ranging from
86.9 to 93.1Њ.
Fig. 5 Section of polymeric [Zn(HL2)]∞ 4. Hydrogen atoms were
located in the Fourier-difference map, but for clarity only hydrogen
atom H(1) is shown.
In the crystal packing the neutral molecules stack along the
crystallographic x axis. Adjacent stacks are connected via
hydrogen bonds involving water of crystallisation. Short con-
tacts are also found between the carboxylate oxygens O(2) and
O(3) and the water ligands bound to Zn(3) (Table 3).
deprotonated phenolic oxygen and one chelating side arm. The
other side arm binds via a carboxylate oxygen [O(2)] to the
second Zn of the dimer, while its imine nitrogen N(1) is
protonated with the proton forming a hydrogen bond to the
phenolate oxygen [N(1) ؒ ؒ ؒ O(1) 2.647(4) Å]. The correspond-
ing hydrogen atom H(1) could be located in the Fourier-
difference map. The five-co-ordination sphere of Zn is
completed by the carboxylate oxygen O(4) of the adjacent
dimer. As found for 1a and 2, the co-ordination geometry
around Zn is distorted trigonal bipyramidal with the phenolic
Crystal structure of complex 4. A section of the polymeric
compound 4 is shown in Fig. 5. Selected bond lengths, angles
and possible hydrogen-bonding interactions are reported in
Table 4. The polymeric, three-dimensional network is built up
of dimeric subunits {Zn(HL2)}2 interconnected through carb-
oxylate bridges. Each ligand co-ordinates one Zn through the
J. Chem. Soc., Dalton Trans., 2000, 569–575
573

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Table 4 Selected bond distances (Å), angles (Њ) and possible hydrogen-
bonding interactions for {[Zn(HL2)]ؒ3H2O}∞ 4
Acknowledgements
We thank Professor Bernhard Lippert for the generous
and continuous support of our work. Financial support by
the Ministerium für Wissenschaft und Forschung, NRW
(Lise-Meitner-Habilitationsstipendium to A. E.) is gratefully
acknowledged.
Zn(1)–N(2)
Zn(1)–O(2)a
Zn(1)–O(5)
O(2)–C(10)
O(4)–C(13)
2.036(3)
1.976(3)
2.219(3)
1.258(5)
1.257(4)
Zn(1)–O(1)
Zn(1)–O(4)b
N(1)–C(8)
O(3)–C(10)
O(5)–C(13)
2.040(3)
1.960(3)
1.284(5)
1.240(5)
1.239(4)
N(2)–Zn(1)–O(2)a
O(1)–Zn(1)–N(2)
O(1)–Zn(1)–O(5)
O(5)–Zn(1)–N(2)
C(11)–N(2)–C(12)
C(1)–O(1)–Zn(1)
133.2(1)
88.5(1)
161.1(1)
77.7(1)
N(2)–Zn(1)–O(4)b
O(1)–Zn(1)–O(4)b
O(2)a–Zn(1)–O(4)b
C(8)–N(1)–C(9)
C(11)–N(2)–Zn(1)
C(10)–O(2)–Zn(1)a
C(13)–O(5)–Zn(1)
O(4)–C(13)–O(5)
122.3(1)
106.4(1)
102.6(1)
123.0(4)
126.7(3)
122.8(3)
114.3(2)
127.2(4)
References
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C(13)–O(4)–Zn(1)c 143.5(3)
O(2)–C(10)–O(3)
126.5(4)
N(1) ؒ ؒ ؒ O(1)
2.647(4)
2.91(2)
2.62(3)
2.740(7)
2.68(3)
3.02 (2)
2.61(2)
2.91(2)
2.58(3)
2.81(2)
2.88(3)
O(3) ؒ ؒ ؒ O(3W)a
O(3) ؒ ؒ ؒ O(7W)a
O(5) ؒ ؒ ؒ O(1W)c
O(1W) ؒ ؒ ؒ O(6W)b
O(2W) ؒ ؒ ؒ O(3W)e
O(2W) ؒ ؒ ؒ O(7W)e
O(2W) ؒ ؒ ؒ O(8W)e
O(4W) ؒ ؒ ؒ O(7W)
O(4W) ؒ ؒ ؒ O(8W)
O(6W) ؒ ؒ ؒ O(7W)e
2.69(3)
2.73(3)
2.869(6)
2.61(2)
2.68(3)
2.61(2)
2.91(2)
2.58(3)
2.81(2)
2.77(3)
O(3) ؒ ؒ ؒ O(5W)a
O(3) ؒ ؒ ؒ O(8W)a
O(1W) ؒ ؒ ؒ O(2W)d
O(2W) ؒ ؒ ؒ O(3W)
O(2W) ؒ ؒ ؒ O(5W)
O(2W) ؒ ؒ ؒ O(7W)
O(2W) ؒ ؒ ؒ O(8W)
O(4W) ؒ ؒ ؒ O(7W)e
O(4W) ؒ ؒ ؒ O(8W)e
O(6W) ؒ ؒ ؒ O(8W)e
Symmetry operations: a Ϫx ϩ 1, Ϫy, Ϫz ϩ 1; b Ϫx ϩ ¹, y Ϫ ¹, Ϫz ϩ ¹;
¯
²
¯
²
¯
²
c Ϫx ϩ ¹, y ϩ ¹, Ϫz ϩ ¹; d x Ϫ ¹, y ϩ ¹, z; e Ϫx ϩ 1, y, Ϫz ϩ ¹.
¯
²
¯
²
¯
²
¯
²
¯
²
¯
²
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[O(1)] and carboxylate [O(2)] oxygens occupying the apical
positions. Owing to the narrow bite of the chelating side arm
the N(2)–Zn(1)–O(5) bond angle is reduced from the ideal 90Њ
angle to 77.7(1)Њ. The lengthening of the Zn(1)–O(5) bond to
2.219(3) Å [compared with 1.960(3) Å for Zn(1)–O(4) and
1.976(3) Å for Zn(1)–O(2)] may also be caused by steric effects
of the chelate ligand.
In the three-dimensional structure the dimeric subunits are
connected in such a way that long channels are produced
along the crystallographic y axis that are stabilised by water
of crystallisation. The disordered water molecules form
hydrogen bonds among each other and to the carboxylate O(3)
and O(5) oxygens (Table 4). The phenolic rings are stacked
along the y axis so that neighbouring rings overlap partly.
Spectroscopic characterisation of complexes 2 and 4. As
1
described above, the H NMR spectrum of complex 2 shows
two singlets in the aromatic region: in CD3OD the azo-
methine and phenolic ring protons are detected at δ 8.45
and 7.35. The singlets at δ 4.16 and 2.28 correspond to the
methylene and methyl protons. In the IR spectrum, the ν(C᎐N)
᎐
vibration of 2 appears at 1645 cmϪ1. The difference of 130
cmϪ1 between νasym(COO) [1578 cmϪ1] and νsym(COO) [1448
cmϪ1] is consistent with the bridging co-ordination mode of
the acetate groups. The IR spectrum of 4 shows two ν(C᎐N)
᎐
vibration modes at 1653 and 1631 cmϪ1. In the 1H NMR
spectrum (D2O, pD 6.9) the non-equivalent azomethine and
aromatic ring protons of the mono-co-ordinated compound
are detected at δ 8.40, 8.33 (azomethine), 7.59 and 7.44 (aro-
matic ring).
Conclusion
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The crystal structures of the pentanuclear complex [{Zn2L2-
(CH3CO2)2}2Zn(H2O)4]ؒ4.5H2O and of the polymeric complex
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575