Synthesis and structure of cyclen hydroxylamine ligands and their zinc(II) complexes

Article abstract of DOI:10.1039/b207571g

The synthesis of two new zinc(II) complexes with 1,4,7,10-tetraazacyclododecan-l-ol and 1,4,7,10-tetraazacyclododecan-1,7-diol as ligands is described. One of these complexes is characterized by X-ray structure analysis. By coordination to the metal catio

Full text of DOI:10.1039/b207571g

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Synthesis and structure of cyclen hydroxylamine ligands and their
zinc(II) complexes
Roland Reichenbach-Klinke, Manfred Zabel and Burkhard König*
Institut für Organische Chemie, Universität Regensburg, D-93040 Regensburg, Germany.
E-mail: burkhard.koenig@chemie.uni-regensburg.de
Received 2nd August 2002, Accepted 1st November 2002
First published as an Advance Article on the web 29th November 2002
The synthesis of two new zinc() complexes with 1,4,7,10-tetraazacyclododecan-1-ol and 1,4,7,10-tetraaza-
cyclododecan-1,7-diol as ligands is described. One of these complexes is characterized by X-ray structure analysis.
By coordination to the metal cation the acidity of the hydroxylamine groups of the ligands is significantly increased
as confirmed by potentiometric pH titrations.
Table 1 Selected bond distances (Å) and angles (Њ) for complex 3
Introduction
Metal complexes with substituted 1,4,7,10-tetraazacyclodo-
decane (cyclen) derivatives as ligands are widely used in molecu-
lar recognition and catalysis.1 Kimura et al.2,3 and others have
investigated the coordination of imides and phosphates to
Zn()-cyclen complexes in great detail. Hereby a deprotonated
imide moiety or phosphate anion binds reversibly to the Lewis-
acidic metal center in Zn()-cyclen. This binding motif, like
other reversible metal–ligand interactions, offers some advan-
tages over weaker non-covalent interactions such as hydrogen
bonds, salt bridges or hydrophobic interactions. It provides
sufficient binding strength to allow formation of reversible
aggregates in water under physiological conditions and at high
dilution. Moreover, the interaction is kinetically labile which is
important for applications in catalysis.
Br(1)–Zn(1)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–N(3)
Zn(1)–N(4)
O(1)–N(1)
2.3501(5)
2.192(2)
2.148(3)
2.122(3)
2.127(3)
1.447(4)
Br(1)–Zn(1)–N(1)
Br(1)–Zn(1)–N(2)
Br(1)–Zn(1)–N(3)
Br(1)–Zn(1)–N(4)
Zn(1)–N(1)–O(1)
O(1)–N(1)–C(1)
O(1)–N(1)–C(8)
108.73(7)
115.50(8)
115.76(7)
110.12(8)
115.50(17)
107.2(2)
106.8(2)
hydrobromic acid and after elution over a basic anion exchange
column the zinc() complexes were synthesized by reaction with
Zn(ClO4)2ؒ6H2O.
X-Ray structure of 3
Colourless crystals of complex 3 suitable for X-ray diffraction
were obtained by slow evaporation of a methanolic solution of
3. The structure of 3 in the crystal is shown in Fig. 1 and selected
bond distances and angles are listed in Table 1.
Hydroxylamine complexes of transition metals4 show some
interesting aspects regarding the acidity of the ligand. By
coordination of the hydroxylamine nitrogen atom to a metal
cation the pKa value of the hydroxy group is changed markedly.
While hydroxylamine itself is almost not acidic (pKa = 13.7 5),
the acidity increases significantly by coordination to transition
metals as outlined in Scheme 1.
The intention of our work presented here is to combine the
known excellent binding properties of Lewis-acidic Zn()-cyclen
with the interesting features of hydroxylamine ligands.
Results and discussion
Design and synthesis
In their synthesis of macrobicyclic Co() hydroxylamine com-
plexes Sargeson coworkers6 reacted the Co() azamacro-
bicyclic complex with H2O2 to obtain a mixture of complexes
with one to three hydroxylamine ligands. These complexes were
difficult to separate and purify. Therefore we chose a different
synthetic strategy starting with the selective preparation of the
free ligand followed by complexation with the metal.
Fig. 1 Structure of complex 3 and the perchlorate counter ion in the
crystal.
The synthesis starts with the selective protection of cyclen as
1,4,7-tris-tert-butoxycarbonyl-1,4,7,10-tetraazacyclododecane
1 7 and 1,7-bis(benzyloxycarbonyl)-1,4,7,10-tetraazacyclodo-
decane 4 (Scheme 2).8 These protected azamacrocycles were
then reacted with dimethyldioxirane9,10 to obtain the corre-
sponding hydroxylamines. The carbamates were cleaved with
In complex 3 the water molecule which is usually coordinated
to the axial position in aqueous solution is replaced by a brom-
ide anion. The unit cell contains a perchlorate anion as the
counter anion. The geometry of the complex is more distorted
if compared to Zn()-cyclen2 which is illustrated by the some-
what longer bond length of Zn(1)–N(1).
Scheme 1 pKa values of the hydroxylamine ligands in trans-[Pt(NH3)2(NH2OH)2]2ϩ
.
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 1 4 1 – 1 4 5
141

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Table 2 Stability constants of ligands L7H4, L8H4 and cyclen
The following equilibria were considered for
L7H4:
The following equilibria were considered for
L8H4:
The following equilibria for cyclen are given for
comparison:
1
6
β22: L7H3 ϩ Hϩ = L7H4
β27: L8H3 ϩ Hϩ = L8H4
β210: cyclen-H3 ϩ Hϩ = cyclen-H4
β211: cyclen-H2 ϩ Hϩ = cyclen-H3
β212: cyclen-H ϩ Hϩ = cyclen-H2
β23: L7H2 ϩ Hϩ = L7H3
β2 : L7H ϩ Hϩ = L7H2
β2 : L8H2 ϩ Hϩ = L8H3
2
K2 : Zn(ClO4)2 ϩ L8H2 = L8H2-
Zn(OH2)(ClO4)2
1
8
K2 : Zn(ClO4)2 ϩ L7H = L7H-
β2 : L8H-Zn(OH2)(ClO4Ϫ)2 ϩ Hϩ = L8H2-
β213: cyclen ϩ Hϩ = cyclen-H
Ϫ
Zn(OH2)(ClO4)2
Zn(OH2)(ClO4
)
2
4
9
3
β2 : L7H-Zn(OH)(ClO4Ϫ)2 ϩ Hϩ = L7H-
β2 : L8-Zn(OH2)ClO4Ϫ ϩ Hϩ = L8H-
K2 : Zn(ClO4)2 ϩ cyclen = cyclen-
Ϫ
Ϫ
Zn(OH2)(ClO4
)
Zn(OH2)ClO4
Zn(OH2)(ClO4)2
2
5
β2 : L7-Zn(OH)ClO4Ϫ ϩ Hϩ = L7H-
β214: cyclen-Zn(OH)(ClO4)2 ϩ Hϩ = cyclen-
Zn(OH2)(ClO4)2
Ϫ
Zn(OH)ClO4
a
a
L7H4
L8H4
Cyclenb
—
—
logβ210 < 2
logβ21 = 8.5 ( 0.05)c
logβ22 = 10.2 ( 0.03)
logβ23 = 11.6 ( 0.02)
logK21 = 14.4 ( 0.02)
logβ24 = 8.0 ( 0.04)
logβ25 = 8.4 ( 0.01)
logβ26 = 2.4 ( 0.04)
logβ27 = 8.9 ( 0.02)
—
logβ211 < 2
logβ212 = 9.9a 15
logβ213 = 11.0a 15
logK23 = 15.3a 15
logβ214 = 7.9 16
—
logK22 = 6.0 ( 0.1)
logβ28 = 7.8 ( 0.04)
logβ29 = 11.4 ( 0.04)
a All data were measured at I = 0.1 mol LϪ1 (tetraethylammonium perchlorate) and 25 0.5 ЊC. b Data were measured at I = 0.1 mol LϪ1 (NaClO4) and
25 ЊC. c Standard deviation.
Scheme 2 Synthesis of the complexes 3 and 6.
Potentiometric studies
(pKa = 12.88 in H2O at 25 ЊC)14 also does not support a de-
protonation of the hydroxy group.
The properties of complexes 3 and 6 and of the free ligands
were studied by potentiometric pH titration in aqueous
solution. All considered equilibria and the derived stability
constants are summarised in Table 2. The data for the parent
compound cyclen are added for comparison.
With a logK21 of 14.4 the zinc() cation is nearly as strongly
bound in the azamacrocycle as in Zn()-cyclen (logK23 = 15.3 in
H2O at 25 ЊC and I = 0.1 mol LϪ1).15
In the titration of complex 3 [L7H-Zn(OH2)] two equilibria
were observed (see Table 2 and Scheme 4). These values differ
only slightly making an exact discrimination impossible. So the
assignment in Scheme 4 is arbitrary. The Lewis acidity of the
metal center is preserved and it is in the range of the value
measured for Zn()-cyclen (logβ214 = 7.9 in H2O at 25 ЊC and I =
0.1 mol LϪ1).16 In addition, the acidity of the hydroxylamine is,
as expected, significantly enhanced by the coordination to the
zinc() cation. The species distribution plot calculated from the
potentiometric pH titration is shown in Fig. 2.
1
2
3
Three equilibrium constants (logβ2 , logβ2 , logβ2 ) were
derived from the pH profile for ligand L7H4 (see Table 2 and
Scheme 3). The values at 10.2 and 11.6 most likely correspond to
the protonation of secondary amines of the azamacrocycle.
Deprotection of 2 with hydrobromic acid to give L7H4 should
protonate the nitrogen of hydroxylamine, too, if its basicity
remains similar to reported values of N,N-diethylhydroxy-
lamine (pKa = 5.6 in H2O at 30 ЊC and I ≈ 0.02 mol LϪ1)11 or
N,N-dimethylhydroxylamine (pKa = 5.3 in H2O at 25 ЊC and I =
0.06 mol LϪ1).12 However, we could not determine a constant
corresponding to this value. Incorporation of the hydroxy-
lamine into the azamacrocycle therefore changes its nitrogen
basicity significantly.13 Deprotonation of the hydroxylamine
OH-group can be excluded because the elemental analysis of
L7H4 requires the trihydrobromide salt and only three equi-
librium constants were observed in the titration. Moreover,
comparison with literature data of N,N-diethylhydroxylamine
6
7
The two equilibrium constants (logβ2 , logβ2 ) observed in
the titration of ligand L8H4 were assigned to the protonation of
the secondary amines (see Table 2 and Scheme 5). The zinc()
cation is bound in a significantly weaker fashion by ligand L8H2
(logK22 = 6.0) if compared with L7H.
The titration of complex 6 [L8H2-Zn(OH2)] reveals two
acidic protons (Table 2). We assign the two equilibria to the
subsequent deprotonation of the hydroxylamine hydroxy
groups (see Table 2 and Scheme 6).17 The first logβ28 of 7.8 is in
D a l t o n T r a n s . , 2 0 0 3 , 1 4 1 – 1 4 5
142

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Scheme 3 Protonation and zinc coordination of ligand L7H4.
Scheme 4 Protonation equilibria of complex L7H-Zn(OH2) (complex 3).
Scheme
6
Protonation equilibria of complex L8H2-Zn(OH2)
(complex 6).
the range of the value measured for the hydroxylamine in com-
plex 3, while the second hydroxylamine is significantly less
acidic (for comparison see Scheme 1).
Conclusion
Fig. 2 Species distribution plot of complex 3. Stoichiometry of
equilibria: L7-Zn(OH)(ClO4)2 ϩ 2Hϩ = L7H-Zn(OH)(ClO4)2 ϩ Hϩ
=
We have prepared two new cyclen ligands bearing hydroxy-
lamino moieties. The ligands are able to bind zinc() ions
tightly. Moreover, by coordination of the metal cation the
acidity of the hydroxylamines is increased markedly. Hence
these complexes may find use as interesting binding sites for
molecular recognition.
L7H-Zn(OH2)(ClO4)2.
Experimental
General
Compounds 1 7 and 4 8 as well as dimethyldioxirane18 were
prepared by known methods. Melting points were taken on a
hot-plate microscope apparatus and are not corrected. UV/VIS
spectra: Varian Cary 50 Bio. IR spectra: Bio-Rad FTS 3000
1
FT-IR. H- and 13C-NMR spectra: Bruker AM 400 or Bruker
AC 250; chemical shifts relative to SiMe4; s = singlet, bs = broad
singlet, m = multiplet; the multiplicity of the 13C signals was
determined using the DEPT technique and quoted as (ϩ) for
CH3 or CH, (Ϫ) for CH2, and (Cquart) for quaternary carbons.
Scheme 5 Protonation and zinc() ion coordination of ligand L8H4.
D a l t o n T r a n s . , 2 0 0 3 , 1 4 1 – 1 4 5
143

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10-Hydroxy-1,4,7,10-tetraazacyclododecane-1,4,7-tricarboxylic
acid tri-tert-butyl ester (2)
1699, 2948, 3494, 3540. UV/VIS (CH3CN): λmax (logε) = 206 nm
(4.297). H NMR (400 MHz, CDCl3): δ 2.86–4.14 (m, 16 H,
1
CH2), 5.22 (m, 2 H, CH2), 5.27 (m, 2 H, CH2), 7.35 (m, 10 H,
CH). 13C NMR (100 MHz, CDCl3): δ 47.93 (Ϫ), 48.70 (Ϫ),
48.92 (Ϫ), 49.76 (Ϫ), 60.48 (Ϫ), 61.24 (Ϫ), 61.39 (Ϫ), 67.67 (Ϫ),
67.74 (Ϫ), 127.78 (ϩ), 127.90 (ϩ), 127.94 (ϩ), 128.14 (ϩ),
128.34 (ϩ), 128.40 (ϩ), 128.65 (ϩ), 128.69 (ϩ), 128.71 (ϩ),
128.76 (ϩ), 156.37 (Cquart), 156.47 (Cquart), 175.26 (2 C, Cquart).
MS (ESI, MeOH/CH2Cl2 ϩ 1% AcOH): m/z (%) = 473 (100)
1,4,7-Tris-tert-butoxycarbonyl-1,4,7,10-tetraazacyclododecane
1 (0.50 g, 1.06 mmol) was dissolved in 5 mL of degassed
acetone. Under cooling in an ice-bath 16 mL of dimethyl-
dioxirane in acetone (ca. 0.1 M) were added slowly. The reaction
mixture was then stirred for 4 h at room temperature. The
solvent was evaporated and the crude product was purified by
column chromatography on silica (eluent: ethyl acetate, Rf =
0.6). The hydroxylamine 2 (0.42 g, 0.86 mmol, 81%) was
ϩ
[MHϩ], 495 (2) [MNaϩ]; HRMS (C24H33N4O6 ): calc. 473.2400,
found. 473.2391 0.0006. Anal. calc. for C24H32N4O6ؒ2H2O: C,
56.68; H, 7.13; N, 11.02. Found: C, 56.83; H, 7.13; N, 11.02%.
—
obtained as a white solid. M.p. 84 ЊC. IR (KBr, cmϪ1): ν = 775,
1174, 1249, 1366, 1420, 1466, 1694, 2933, 2977, 3428. 1H NMR
(250 MHz, CDCl3): δ 1.45 (s, 18 H, CH3), 1.47 (s, 9 H, CH3),
2.91 (bs, 4 H, CH2), 3.31–3.95 (m, 12 H, CH2), 5.85 (bs, 1 H,
OH). 13C NMR (62 MHz, CDCl3): δ 28.47 (ϩ), 28.69 (ϩ), 45.27
(Ϫ), 46.14 (Ϫ), 47.81 (Ϫ), 48.15 (Ϫ), 49.57 (Ϫ), 50.14 (Ϫ), 59.07
(Ϫ), 59.69 (Ϫ), 79.41 (Cquart), 79.81 (Cquart), 79.94 (Cquart),
155.36 (Cquart), 156.07 (Cquart), 156.41 (Cquart). MS (ESI, MeOH/
CH2Cl2ϩ 1% AcOH): m/z (%) = 489 (100) [MHϩ]. Anal. calc.
for C23H44N4O7: C, 56.54; H, 9.08; N, 11.47. Found: C, 56.40;
H, 9.33; N, 11.12%.
1,4,7,10-Tetraazacyclododecane-1,7-diol di-hydrobromide
Compound 5 (0.28 g, 0.60 mmol) was mixed with 3.0 mL of
HBr in glacial acetic acid (33%, 4.1 M) and stirred for 30 min at
room temperature. Then 5 mL of diethyl ether were added to
the suspension. The precipitate was collected by suction
filtration and washed with diethyl ether. The hydrobromide
(0.20 g, 0.55 mmol, 93%) was obtained as a colourless powder.
—
M.p. 151 ЊC. IR (KBr, cmϪ1): ν = 1457, 1705, 2762, 2960, 3046,
1
3259, 3427. H NMR (250 MHz, D2O): δ 3.04–3.48 (m, CH2).
1,4,7,10-Tetraazacyclododecan-1-ol tri-hydrobromide
13C NMR (62 MHz, D2O): δ 42.91 (Ϫ), 45.02 (Ϫ), 53.08 (Ϫ).
MS (ESI, H2O): m/z (%) = 205 (100) [MHϩ]. Anal. calc. for
C8H22Br2N4O2ؒ0.5CH3CH2OCH2CH3: C, 29.79; H, 6.75; N,
13.90. Found: C, 30.23; H, 6.22; N, 14.18%.
Compound 2 (0.30 g, 0.62 mmol) was mixed with 2.0 mL of
HBr in glacial acetic acid (33%, 4.1 M) and stirred for 30 min at
room temperature. Then 5 mL of diethyl ether were added to
the suspension. The precipitate was collected by suction
filtration and washed with diethyl ether. The hydrobromide
(0.24 g, 0.56 mmol, 91%) was obtained as a colourless powder.
1,4,7,10-Tetraazacyclododecane-1,7-diol-zinc(II) di-perchlorate
(6)
—
Decomp. at 210 ЊC. IR (KBr, cmϪ1): ν = 1635, 2760, 2926, 3422.
Compound 6 was synthesized from the above hydrobromide
(0.20 g, 0.55 mmol) by a method similar to that of 3. The com-
plex 6 (0.15 g, 0.33 mmol, 60%) was obtained as a white solid.
1H NMR (250 MHz, D2O): δ 2.84–3.52 (m, CH2). 13C NMR
(100 MHz, D2O): δ 42.61 (Ϫ), 43.39 (Ϫ), 44.04 (Ϫ), 54.39 (Ϫ).
MS (ESI, H2O): m/z (%) = 189 (100) [MHϩ]. Anal. calc. for
C8H23Br3N4Oؒ0.5CH3COOH: C, 23.45; H, 5.47; N, 12.15.
Found: C, 23.03; H, 5.46; N, 12.53%.
—
M.p. 138–140 ЊC. IR (KBr, cmϪ1): ν = 628, 1090, 1389, 1450,
2941, 3167, 3224, 3422. 1H NMR (250 MHz, CD3CN): δ
2.65–2.75 (m, 4 H, CH2), 3.02–3.27 (m, 12 H, CH2), 3.71 (bs, 2
H, NH), 7.12 (bs, 1 H, OH), 7.39 (bs, 1 H, OH). 13C NMR
(62 MHz, CD3CN): δ 44.05 (Ϫ), 56.65 (Ϫ). MS (ESI, MeOH ϩ
10 mmol NH4Ac): m/z (%) = 327 (100) [(62ϩ ϩ CH3COOϪ)ϩ].
1,4,7,10-Tetraazacyclododecan-1-ol-zinc(II) di-perchlorate (3)
The above hydrobromide (0.22 g, 0.51 mmol) was dissolved in
2 mL of H2O and eluted over a basic anion exchange column.
The solution was evaporated to dryness and the solid residue
was dissolved in 2 mL of methanol. Then a solution of
Zn(ClO4)2ؒ6H2O (0.28 g, 0.75 mmol) in 2 mL of methanol was
added and the mixture was heated at 80 ЊC for 45 min. After
removal of most of the solvent, the reaction mixture was cooled
in an ice-bath and the precipitate was collected by suction
filtration. Complex 3 (0.15 g, 0.33 mmol, 65%) was isolated as a
Potentiometric titrations
Potentiometric pH titrations were performed with a computer-
controlled pH-meter (pH 3000, WTW; glass pH electrode
Metrohm) and automatic titration apparatus (dosimat 665,
Metrohm). For all titrations 0.1 M perchloric acid (Merck, p.a.)
and 0.1 M tetraethylammonium hydroxide (TEAOH) (Merck,
p.a.) in water containing tetraethylammonium perchlorate to
maintain an ionic strength of I = 0.1 mol LϪ1 were used. The
water used is of ultrapure HPLC grade deionized with the
Milli-Q185 apparatus form Millipore. Tetraethylammonium
perchlorate (Fluka, purum) was recrystallized twice from
acetonitrile. TEAOH solutions were calibrated with mono
sodium phthalate (Merck, p.a.). A titration of perchloric acid
with TEAOH solution was used for calibration and to deter-
mine log Kw and AI by the method of Gran.19 Titrations were
measured over a pH range from 2.7 to 12.6 with a minimum of
80 data points recorded. Titrations were performed in duplicate
or triplicate. The stability constants given and their standard
deviations are average values. All measurements were performed
—
white solid. Decomp. at 235 ЊC. IR (KBr, cmϪ1): ν = 628, 1089,
1459, 2933, 3163, 3254, 3410. 1H NMR (250 MHz, CD3CN): δ
2.60–2.79 (m, 4 H, CH2), 2.84–3.20 (m, 12 H, CH2), 3.36 (bs, 3
H, NH), 6.73 (bs, 1 H, OH). 13C NMR (62 MHz, CD3CN): δ
43.77 (Ϫ), 44.97 (Ϫ), 45.15 (Ϫ), 56.23 (Ϫ). MS (ESI, H2O ϩ 1%
AcOH): m/z (%) = 311 (100) [(32ϩ ϩ CH3COOϪ)ϩ], 351 (35)
[(32ϩ ϩ ClO4 )ϩ]. Anal. calc. for C8H20Cl2N4O9Znؒ0.5CD3CN:
Ϫ
C, 22.78; H, 4.88; N, 13.28. Found: C, 22.92; H, 4.98; N,
13.18%.
4,10-Dihydroxy-1,4,7,10-tetraazacyclododecane-1,7-dicarb-
oxylic acid dibenzyl ester (5)
at 25
0.5 ЊC. Data and error analysis was done with the
1,7-Bis(benzyloxycarbonyl)-1,4,7,10-tetraazacyclododecane
4
computer program Hyperquad 2000.20
(0.70 g, 1.60 mmol) was dissolved in 10 mL of degassed acet-
one. Under cooling in an ice-bath 41 mL of dimethyldioxirane
in acetone (ca. 0.1 M) were added. The reaction mixture was
then stirred for 30 min in the ice-bath and for a further 40 min
at room temperature. The solvent was evaporated and the crude
product was purified by column chromatography on silica
(eluent: CH2Cl2–MeOH = 10:1, Rf = 0.8). The hydroxylamine 5
(0.34 g, 0.73 mmol, 46%) was isolated as a white solid. M.p. 54
Crystal structure determination
Suitable crystals for X-ray experiments of complex 3 were
obtained by slow evaporation of a methanolic solution.
Formula: C8H20BrN4OZnؒClO4, M = 433.02, crystal system:
orthorhombic, space group P212121, a = 8.9049(5), b =
12.2325(8), c = 14.0969(11) Å, α = β = γ = 90Њ, V = 1535.56(18)
Å3, Z = 4, Dc = 1.873 g cmϪ3, F(000) = 872, µ = 4.398 mmϪ1,
—
ЊC. IR (KBr, cmϪ1): ν = 772, 1020, 1167, 1260, 1361, 1412, 1479,
D a l t o n T r a n s . , 2 0 0 3 , 1 4 1 – 1 4 5
144

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3 E. Kimura, T. Ikeda and M. Shionoya, Pure Appl. Chem., 1997, 69,
crystal size: 0.525 × 0.454 × 0.375 mm, colourless, rod-like
crystals.
2187.
4 K. Wieghardt, Adv. Inorg. Bioinorg. Mech., 1984, 3, 213.
5 A. F. Hollemann, E. Wiberg, N. Wiberg, Lehrbuch der
Anorganischen Chemie, de Gruyter, Berlin, 1985, p. 591.
6 D. J. Bull, I. I. Creaser, A. M. Sargeson, B. W. Skelton and
A. H. White, Inorg. Chem., 1987, 26, 3040.
7 S. Brandes, C. Gros, F. Denat, P. Pullumbi and R. Guilard, Bull. Soc.
Chim. Fr., 1996, 133, 65.
8 Z. Kovacs and A. D. Sherry, Synthesis, 1997, 759.
9 R. W. Murray and R. Jeyaraman, J. Org. Chem., 1985, 50, 2847.
10 R. W. Murray and M. Singh, US Pat., 5001233, 1991.
11 S. Ball and T. C. Bruice, J. Am. Chem. Soc., 1980, 102, 6498.
12 T. C. Bissot, R. W. Parry and D. H. Campbell, J. Am. Chem. Soc.,
1957, 79, 796.
13 On the basis of the titration data no final assignment of the pKa
values to the protonation of a specific nitrogen atom is possible.
14 M. J. Colthurst, A. J. S. S. Kanagasooriam, M. S. O. Wong,
C. Contini and A. Williams, Can. J. Chem., 1998, 76, 678.
15 T. Koike, S. Kajitani, I. Nakamura, E. Kimura and M. Shiro, J. Am.
Chem. Soc., 1995, 117, 1210.
Data collection was carried out at 173 K on a STOE-IPDS
diffractometer with graphite-monochromated Mo-Kα radiation
(λ = 0.71073 Å); 11144 reflections (2904 unique reflections, Rint
= 0.0396) were measured from 2.71 to 25.77Њ.
The structure was solved by direct methods (SIR97)21 and
refined by full-matrix least squares on F 2 for all non-H-atoms.22
H-atoms were included in calculated positions. ωR(F 2)
(all data) = 0.0843; R(F) = 0.0308, S = 1.073, max. ∆ρ = 1.248 e
ÅϪ3.
CCDC reference number 187581.
See http://www.rsc.org/suppdata/dt/b2/b207571g/ for crystal-
lographic data in CIF or other electronic format.
Acknowledgements
16 E. Kimura, T. Shiota, T. Koike, M. Shiro and M. Kodama, J. Am.
Chem. Soc., 1990, 112, 5805.
We thank the Volkswagen foundation (priority research area
electron transfer) and the DFG (Graduate College Sensory
Photoreceptors) for support of this work. We thank the
Schering AG, Berlin, and DSM, Regensburg, for their generous
gifts of cyclen. We thank Mrs H. Leffler-Schuster for her
help with potentiometric titrations.
17 Additional evidence for this interpretation comes from proton
NMR investigations: the proton NMR of compound L8H2-
Zn(OH2) in DMSO-d6 shows two hydroxy signals, at δ 4.82 [bs, 2H,
Zn(OH2)] and δ 8.58 (bs, 2H, N–OH). Upon addition of potassium
carbonate as base the signal at δ 8.58 disappears, which supports
preferred deprotonation of coordinated hydroxylamine moieties.
18 W. Adam, Y. Chan, D. Cremer, J. Gauss, D. Scheutzow and
M. Schindler, J. Org. Chem., 1987, 52, 2800.
19 G. Gran, Analyst, 1952, 77, 661.
References and notes
20 P. Gans, A. Sabatini and A. Vacca, Talanta, 1996, 43, 1739.
21 A. Altomare, G. Cascarano, C. Giacovazzo and A. Guagliardi,
J. Appl. Crystallogr., 1993, 26, 343.
1 R. Reichenbach-Klinke and B. König, J. Chem. Soc., Dalton Trans.,
2002, 121.
2 M. Shionoya, E. Kimura and M. Shiro, J. Am. Chem. Soc., 1993,
115, 6730.
22 G. M. Sheldrick, SHELXL97, Program for Refinement of Crystal
Structures, University of Göttingen, Germany, 1997.
D a l t o n T r a n s . , 2 0 0 3 , 1 4 1 – 1 4 5
145