Nickel(II) and copper(II) complexes with pyridine-containing macrocycles bearing an aminopropyl pendant arm: Synthesis, characterization, and modifications of the pendant amino group

Article abstract of DOI:10.1039/b211489e

The synthesis of three five-coordinate nickel(II) complexes with pendant arm-containing macrocycles has been achieved by the reduction of C=N bonds in the Schiff base precursors derived from diacetyl- or diformyl-pyridine and a tripodal tetramine. Demetallation of the nickel(II) macrocycles yielded stable pentadentate ligands that were used for the preparation of the copper(II) complexes. The structures of three nickel(II) complexes and two copper(II) complexes were determined by X-ray crystallography. Protonation of the pendant arm (pK_a = 6.3-6.6 for the nickel complexes, and 6.5-7.3 for the copper complexes) produced four-coordinate macrocycles, one of which was structurally characterized. The primary amino group of the pendant arm coordinated to the nickel(II) reacted with acetic anhydride or benzoyl chloride. The resulting mono-functionalized nickel(II) complexes and their copper(II) counterparts obtained by transmetallation displayed square-planar geometry in the solid state, as determined by X-ray crystallography, and remained four-coordinate in solutions below pH 11. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b211489e

View Article Online / Journal Homepage / Table of Contents for this issue
Nickel(II) and copper(II) complexes with pyridine-containing
macrocycles bearing an aminopropyl pendant arm: synthesis,
characterization, and modifications of the pendant amino group†
Aida M. Herrera,a Richard J. Staples,b Sergey V. Kryatov,a Alexander Y. Nazarenko*c and
Elena V. Rybak-Akimova*a
a Department of Chemistry, Tufts University, Medford, Massachusetts, 02155, USA
b Department of Chemistry, Chemical Biology, Harvard University, Cambridge, Massachusetts,
02138, USA
c Department of Chemistry, State University of New York, College at Buffalo, Buffalo,
New York, 14222-1095, USA
Received 20th November 2002, Accepted 6th January 2003
First published as an Advance Article on the web 3rd February 2003
The synthesis of three five-coordinate nickel() complexes with pendant arm-containing macrocycles has been
achieved by the reduction of C᎐N bonds in the Schiff base precursors derived from diacetyl- or diformyl-pyridine
᎐
and a tripodal tetramine. Demetallation of the nickel() macrocycles yielded stable pentadentate ligands that were
used for the preparation of the copper() complexes. The structures of three nickel() complexes and two copper()
complexes were determined by X-ray crystallography. Protonation of the pendant arm (pKa = 6.3–6.6 for the nickel
complexes, and 6.5–7.3 for the copper complexes) produced four-coordinate macrocycles, one of which was
structurally characterized. The primary amino group of the pendant arm coordinated to the nickel() reacted with
acetic anhydride or benzoyl chloride. The resulting mono-functionalized nickel() complexes and their copper()
counterparts obtained by transmetallation displayed square-planar geometry in the solid state, as determined by
X-ray crystallography, and remained four-coordinate in solutions below pH 11.
metals (nickel() or copper()).11 As metal-free Schiff base
Introduction
macrocycles derived from diacetyl- or diformyl-pyridine are
notoriously unstable, transmetallation is hardly possible for
these ligands. It is therefore desirable to modify the Schiff base
complexes in such a way that stable free ligands would be
accessible for future encapsulation of a variety of metal ions.
Stable free ligands are also necessary for bioimaging and target-
ing with radioisotopes (e.g., 67Cu), where “hot” isotopes are
added to the “cold kit” immediately before use.15–17 Reduction
of the azomethine bonds in pyridine macrocycles bearing a
pendant arm was reported to yield stable compounds suitable
for transmetallation.18–23 The reactivity of the pendant arm,
however, was not explored, in part because of the choice of
pendant functional groups (such as tertiary amine, pyridine, or
pyrazole) in the majority of known complexes.
This paper reports on the reduction of nickel() Schiff
base macrocycles with a primary amino group in the “long”
3-aminopropyl pendant arm, and on their transmetallation
with copper() (Scheme 1). Crystal structures of nine metal
complexes are discussed. Two aspects of the chemistry of the
pendant amino group are addressed: (1) “arm on”–“arm off”
equilibrium upon protonation/deprotonation; (2) selective
covalent modification of a coordinated amino group that yields
a mono-acylated product.
Interest in macrocycles with pendant arms is growing on
account of their unique coordination and structural properties,
their utility in enzyme mimicking studies and catalysis, and
their rapidly growing applications as radiopharmaceuticals,
magnetic resonance imaging reagents, and fluorescent probes.1–6
Biomedical applications of metallomacrocycles are often based
on targeted delivery of the metal to specific cells or tissues,
which requires covalent attachment of the macrocycle to a
recognition moiety (usually, peptide or antibody). Immobiliz-
ation of the macrocycles on solid supports is desirable in
catalysis. Unambiguous single-point attachment is best accom-
plished via reactions with a functional group in the pendant
arm.5 The preparation of monofunctionalized macrocycles,
however, is usually more elaborate than the synthesis of poly-
functionalized ligands (e.g., through extensive alkylation of the
secondary amino groups in azamacrocycles).3,6 The develop-
ment of simple methods for the preparation of mono-
functionalized macrocyclic ligands that can be metallated with
various transition metal ions is needed.
Pyridine-containing macrocycles are readily accessible and
versatile platforms that give rise to a variety of structural
motifs, including mono- and di-compartmental cycles, three-
dimensional cages, sterically enforced macrocycles, and ligands
with additional functional groups attached to the macro-
cycle.7–10 The non-symmetric structure of these ligands allows for
selective functionalization at non-equivalent amino group(s).
We 11 and others12–14 have previously reported a one-step syn-
thesis of Schiff base macrocyclic complexes bearing a primary
amino group in the pendant arm via a template condensation
between diacetylpyridine and tripodal aliphatic tetramines.
Howeverconvenient,thesereactionsarefarfrombeinguniversal,
as their outcome depends on the nature of the metal template,
and yields macrocyclic products with a limited selection of the
Results and discussion
Synthesis and characterization
Template condensation of diacetylpyridine and a tripodal
tetramine trpn in the presence of Ni() template in refluxing
ethanol cleanly yields a target macrocyclic Schiff base complex
[Ni(L1a)]2ϩ (Scheme 1).11 Although this reaction was previously
run at high dilution in order to avoid formation of acyclic
oligomers, we now found that the macrocycle remains a
predominant product in more concentrated solutions (up to
0.03 M), so that the cyclization could be conveniently scaled up.
Diformylpyridine undergoes similar template condensation at
room temperature (the reaction is complete in 4 days), while
† Electronic supplementary information (ESI) available: colour
versions of Figs. 4, 5 and 7. See http://www.rsc.org/suppdata/dt/b2/
b211489e/
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
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
846

View Article Online
Scheme 1
heating the reaction mixture results in rapid irreversible
degradation of the macrocycle (dark oligomeric products
down-up-up, with the last two structures being a pair of
enantiomers). In practice, two major isomers of the nickel()
complex were isolated from the reduced mixture by fractional
crystallization (combined crystallization yield 94%). The iso-
mers [Ni(L2a)]2ϩ (blue) and [Ni(L2b)]2ϩ (purple) give distinct
electronic spectra and distinct NMR of the free ligands
obtained by demetallation of the nickel() complexes (see
Experimental). X-Ray crystallography revealed that the
[Ni(L2a)]2ϩ isomer has the up-up-up stereochemistry, and the
[Ni(L2b)]2ϩ isomer is a racemic mixture of the up-up-down and
the down-up-up enantiomers (Scheme 1).
were not identified). The labile C᎐N bond(s) in macrocycles
᎐
derived from diformylpyridine undergo rapid and reversible
hydration, so that the crystallographically characterized mono-
hydrate [Ni(L1c)]2ϩ is the predominant component of the solid
product.24
Reduction of [Ni(L1a)]2ϩ and [Ni(L1c)]2ϩ with an excess of
sodium borohydride yields the corresponding aminomacro-
cycles (Scheme 1). Reduction proceeded smoothly when NaBH4
is added all at once; the black precipitate that forms redissolves
upon stirring. When NaBH4 was added gradually, in small por-
tions, incomplete reduction accompanied by contamination
with non-cyclic products was observed. The reduced reaction
mixture should not be left in solution for extended periods of
time, as it gradually decomposes.
Isolated and purified purple or blue nickel() complexes with
L2a–L2c were indefinitely stable in aqueous solutions. The
identity and purity of the reduced complexes was confirmed by
the IR (the absence of a C᎐N stretch at ca. 1640 cmϪ1), mass
᎐
spectrometry, elemental analysis, UV-Vis spectrophotometry
(reversible spectral changes upon protonation/deprotonation of
the pendant arm were indicative of the macrocyclic structures),
and, in most cases, X-ray crystallography.
Nickel() complexes with L2a–L2c, when treated with an
excess of NaCN, liberated macrocyclic ligands that could be
isolated by extraction in chloroform or methylene chloride
followed by solvent evaporation. Unlike their Schiff base pre-
cursors, these ligands were stable towards hydrolysis and could
be used to prepare complexes of other metals. The free ligands
were isolated as oily semi-solid materials, characterized by
NMR and mass-spectrometry (see Experimental), and used
without further purification.
Reduction of the diformylpyridine derivative [Ni(L1c)]2ϩ
gave only one product that was identified as [Ni(L2c)]2ϩ. There-
fore, facile and reversible hydration–dehydration of the un-
substituted Schiff base derived from diformylpyridine does
not cause complications in the preparation of the reduced
aminomacrocycle [Ni(L2c)]2ϩ. In the case of diacetylpyridine
derivative [Ni(L1a)]2ϩ, hydration of the Schiff base was not
observed under ambient conditions, but a different compli-
cation was expected. Reduction of [Ni(L1a)]2ϩ generates two
new stereocenters at the carbon atoms adjacent to the pyridine
rings. Two different orientations of each of the methyl substitu-
ents with respect to the macrocyclic plane become possible, giv-
ing rise to meso- and rac-diastereoisomers of the free ligand. In
metal complexes, two different orientations of the pendant arm
also need to be considered, since the inversion of the coordin-
ated tertiary nitrogen is expected to be very slow. Four stereo-
isomers of the reduced complex may exist that differ in relative
orientation of the alkyl substituents (methyl groups or amino-
propyl pendant arm) with respect to the macrocyclic plane
(CH3–CH3–C3H6NH2: up-up-up, up-up-down, up-down-up,
Preparation of copper() complexes was straightforward:
mixing the equivalent amounts of the ligand and copper()
perchlorate in acetonitrile solution resulted in rapid and almost
quantitative formation of the complexes [Cu(L2a)]2ϩ
–
[Cu(L2c)]2ϩ that were isolated as perchlorate salts and crystal-
lographically characterized.
Addition of one equivalent of a strong acid to copper()
or nickel() complexes with L2a–L2c resulted in selective
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
847

View Article Online
Scheme 2
protonation of the amino group in the pendant arm, as dis-
cussed in detail below (Scheme 2). A four-coordinate copper()
complex [Cu(HL2a)]3ϩ(ClO4)3 was isolated and structurally
characterized, confirming the dissociation of the pendant arm
from the metal in the singly protonated complex.
Fig. 3 ORTEP plot of the complex [Ni(L2c)](ClO4)2. Perchlorate ions
and hydrogen atoms are omitted for clarity.
Crystal structures
Three nickel() complexes and two copper() complexes with
neutral ligands L2a–L2c were crystallographically character-
ized (Figs. 1–5, Tables 1 and 2). Although crystals of two
of these materials ([Ni(L2c)(ClO4)2] and [Cu(L2a)](ClO4)2ؒ
CH3CN) were disordered, thus yielding somewhat less accurate
metric parameters, the geometry of all complexes could be
accurately determined. In all studied complexes with unmodi-
fied neutral ligands, the five nitrogen atoms are coordinated to
the metal, resulting in a square-pyramidal geometry (four
nitrogen donors of the macrocycle, N(1)–N(4), occupying the
equatorial positions, and the nitrogen N(5) from the pendant
arm being axial). No coordination of either the solvent mole-
cules or anions was observed. A similar coordination geometry
around the metal center was previously reported for analogous
Schiff base11,13 and reduced macrocyclic21 copper() and
nickel() complexes bearing a coordinating pendant arm
attached at the N(3) nitrogen. Severe distortion of the amino-
pyridine macrocycle (folding along N(2)–N(4) axis), however,
Fig.
4
Overlay of X-ray structures of [Ni(L2b)]2ϩ (solid) and
[Cu(L2b)]2ϩ (dotted). The atom numbering scheme (identical for both
complexes) is the same as that shown in Fig. 2.
Fig. 1 ORTEP67 plot of the complex [Ni(L2a)](ClO4)2. Perchlorate
ions and hydrogen atoms are omitted for clarity.
Fig.
5
Overlay of X-ray structures of [Ni(L2a)]2ϩ (solid) and
[Cu(L2a)]2ϩ (dotted). The atom numbering scheme (identical for both
complexes) is the same as that shown in Fig. 1.
occurred upon coordination to copper() of a pendant group
attached at the N(2) position.25
Nickel–nitrogen distances in the five-coordinate complexes
with L2a–L2c (1.94–2.17 Å, Table 1) are typical of high-spin
Ni().26 The shortest distance is between Ni() and the pyridine
nitrogen N(1), followed by Ni–N(3) (tertiary amine trans to
pyridine) and Ni–N(5) (primary amine in the pendant arm).
The two remaining bonds to the secondary nitrogens N(2) and
N(4) are slightly (in [Ni(L2b)]2ϩ) or significantly (in [Ni(L2a)]2ϩ
and [Ni(L2c)2ϩ) elongated, indicating trigonal distortion of the
coordination polyhedron. As Ni() is shifted towards the pyrid-
ine ring, the N(1)–Ni–N(2) and N(1)–Ni–N(4) angles are less
than 90Њ, and N(2)–Ni–N(3) and N(3)–Ni–N(4) angles are
greater than 90Њ. Nickel is displaced from the macrocyclic plane
Fig. 2 ORTEP plot of the complex [Ni(L2b)](ClO4)2. Perchlorate ions
and hydrogen atoms are omitted for clarity.
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
848

View Article Online
Table 1 Selected bond lengths (Å) and angles (Њ) in Ni() complexes
[Ni(L2c)](ClO4)2
[Ni(L2a)](ClO4)2
[Ni(L2b)](ClO4)2
[Ni(L3c)](ClO4)2ؒCH3NO2
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–N(3)
Ni(1)–N(4)
Ni(1)–N(5)
Ni(1)–Other
1.998(10)
2.166(11)
1.999(9)
2.039(9)
2.034(8)
—
2.002(5)
2.115(5)
2.057(5)
2.133(5)
2.019(5)
—
1.990(6)
2.124(6)
2.042(6)
2.101(6)
2.018(6)
—
1.835(5)
1.937(5)
1.953(5)
1.941(5)
—
2.799a
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–N(3)
N(1)–Ni(1)–N(4)
N(1)–Ni(1)–N(5)
N(2)–Ni(1)–N(3)
N(2)–Ni(1)–N(4)
N(2)–Ni(1)–N(5)
N(3)–Ni(1)–N(4)
N(3)–Ni(1)–N(5)
N(4)–Ni(1)–N(5)
Metal displacement from
N(1)N(2)N(3)N(4) mean plane
76.6(5)
164.30(3)
82.6(4)
99.1(4)
95.0(4)
146.6(4)
102.2(4)
98.4(4)
95.6(4)
106.8(4)
0.3855
79.57(19)
151.12(19)
79.97(19)
110.9(2)
94.20(19)
153.48(19)
105.62(19)
95.61(19)
97.9(2)
79.5(2)
162.9(2)
78.5(3)
102.7(2)
95.4(2)
145.4(3)
108.2(2)
97.9(2)
94.4(2)
102.5(3)
0.4167
83.1(2)
177.0(2)
83.3(2)
—
95.3(2)
162.6(2)
—
97.8(2)
—
97.4(2)
0.4503
—
0.1154
a Bonding to O of a ClO4Ϫ counterion of a different molecule.
Table 2 Selected bond lengths (Å) and angles (Њ) in Cu() complexes
[Cu(HL2a)](ClO4)3ؒ
CH3CN
[Cu(L2a)](ClO4)2ؒ
CH3CN
[Cu(L2b)](ClO4)2ؒ
2C3H6O
[Cu(L3c)](ClO4)2
[CuL4c)](ClO4)2
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(4)
Cu(1)–N(5)
Cu(1)–Other
1.943(5)
2.039(4)
2.003(4)
2.035(5)
—
1.975(3)
2.078(4)
2.032(3)
2.090(3)
2.163(4)
—
1.964(6)
2.080(7)
2.022(7)
2.077(6)
2.167(7)
—
1.936(5)
2.027(5)
2.010(5)
2.015(6)
—
1.952(8)
2.026(8)
2.025(8)
2.033(8)
—
2.235a
2.485–2.596b
2.535c
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–N(4)
N(1)–Cu(1)–N(5)
N(2)–Cu(1)–N(3)
N(2)–Cu(1)–N(4)
N(2)–Cu(1)–N(5)
N(2)–Cu(1)–N(6)
N(3)–Cu(1)–N(4)
N(3)–Cu(1)–N(5)
N(4)–Cu(1)–N(5)
Metal displacement from
N(1)N(2)N(3)N(3) mean plane
81.2(2)
166.80(18)
81.5(2)
—
96.5(2)
155.3(2)
—
98.5(2)
96.2(2)
—
78.78(14)
160.40(14)
80.19(13)
106.63(15)
96.12(13)
143.43(15)
106.79(15)
—
93.79(13)
92.97(14)
107.74(16)
0.4627
78.7(3)
162.9(3)
80.4(3)
103.9(3)
97.2(3)
143.7(3)
104.3(3)
—
94.1(3)
93.3(3)
109.4(3)
0.4377
82.6(2)
179.4(2)
82.2(2)
—
96.8(2)
161.2(2)
—
—
98.3(2)
—
82.7(4)
177.6(4)
81.6(4)
—
97.4(3)
159.7(4)
—
—
97.7(3)
—
—
—
—
0.2940
0.1052
0.1355
Ϫ
Ϫ
a Bonding to the N of a CH3CN solvent molecule. b Bonding to O of a ClO4 counterion. c Bonding to O of a ClO4 counterion of a different
molecule.
toward the pendant arm (separation from the mean N(1)N(2)-
N(3)N(4) plane ranges from 0.39 to 0.45 Å), resulting in a
decrease of the across-the-macrocycle bond angles (N(2)–Ni–
N(4) and N(1)–Ni–N(3) are less than 180Њ). The values of the
angular parameter of trigonal distortion, τ (which ranges from
0 for a perfect square pyramid to 1 for a perfect trigonal
bipyramid)27 are close to 0.3 for the majority of nickel
complexes described in this paper.
The in-plane copper()–nitrogen distances (1.94–2.08 Å) are,
on average, shorter than the corresponding distances in the
nickel() complexes (Tables 1 and 2), which is consistent with
the smaller radius of Cu2ϩ. The axial Cu–N(5) bond is signifi-
cantly elongated (to 2.163 Å in the complex with L2a, and 2.167
Å in the complex with L2b) due to Jahn–Teller distortion.
Copper() ion is also shifted toward the pyridine ring and
displaced out of the macrocyclic plane (Table 2).
(Ni(1)N(3)C(6)C(7)C(8)N(4)) is forced to adopt a skewed con-
formation (Fig. 1). In the remaining four complexes, one of the
NH vectors (at the N(2) atom) coincides with the direction of
the pendant arm, while the other NH vector (at the N(4) atom)
has the opposite direction (this hydrogen atom is located on
the opposite face of the macrocyclic plane). All saturated six-
membered chelate rings (two in the macrocyclic plane, and
one formed by the pendant arm) adopt chair conformations.
The similarity of the ligand conformations in copper()
and nickel() complexes with L2b can be seen in the almost
perfect overlay of the two structures (Fig. 4, root mean square
deviations 0.0347 Å). In contrast, the copper() and nickel()
complexes with L2a have a different configuration of the N(4)
center, different conformations of the MN(3)C(6)C(7)C(8)N(4)
chelate rings, and a somewhat different geometry of the
pendant arm (Fig. 5, root mean square deviations 0.0506 Å).
Protonation of the coordinated pendant arm in the copper
complex [Cu(L2a)]2ϩ yields a complex [Cu(HL2a)](ClO4)2ؒ
CH3CN, in which the four in-plane nitrogen atoms of the
macrocycle are coordinated to the copper(), while the proton-
ated pendant amino group is no longer bound to the metal
The conformations of the macrocycles L2a–L2c were very
similar in all investigated complexes, except for [Ni(L2a)]2ϩ. In
the latter complex, both hydrogen atoms at secondary amino
groups (N(2) and N(4)) are pointed in the same direction as the
pendant arm, and one of the six-membered chelate rings
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
849

View Article Online
Table 3 Pendant arm protonation constants and parameters of the electronic spectra of Ni() and Cu() complexes
c
Absorption bands, λmaxnm (ε/MϪ1 cmϪ1
)
Solid state
Complex
pKa
optical spectrad
pH 3.0
pH 10.0
Ni(L1a)a
Ni(L2a)
Ni(L2b)
Ni(L1c)
Ni(L2c)
Ni(L3c)
Cu(L1a)a
Cu(isoL1a)a
Cu(L2a)
Cu(L2b)
Cu(L2c)
Cu(L3c)
6.46 0.03
6.75 0.04
6.33 0.03
6.4 0.1
6.38 0.03
—
8.24 0.06
7.34 0.04
6.5 0.1
6.7 0.1
7.3 0.2
—
378, 580, 800
361, 560, 822
—
335 sh, 553, 836, 884
360 sh, 461
472 (85)
465 (102)
368 (143), 571 (53) 812 (20), 885 (13)
362 (147), 562 (52) 829 (15), 885 (12)
534 (59), 801 (19) 920 (17)
460 sh (86), 570 sh (29), 800 sh (11)
464 (89)
352 sh (315), 559 (48), 823 (24), 885 (12)
358 sh (233), 457 (110)b
650, 926
640, 911
—
579 (200)
575 (187)
562 (187)
556 (189)b
628 (224), 888 (60)
628 (229), 888 (67)
613 (229), 887 (67)
—
a Ref. 11. b The spectra were measured at pH 7. c Determined in aqueous solutions at 25 ЊC. d Determined in Nujol mulls in absorbance mode.
UV-Vis spectra of the Ni() complexes in solid state and solu-
tion (Table 3) strongly suggests that the five-coordinate geom-
etry found for the [NiL1]2ϩ complexes in the solid state by X-ray
crystallography is retained upon dissolving in water. It should
be noted that high-spin five-coordinate complexes of Ni() are
relatively rare.11,23,26,32,33,35,36 Nickel() complexes with penta-
dentate ligands often pick up a counter-ion or a donor solvent
molecule and become six-coordinate.6,21,37,38
“Arm-on” five-coordinate copper() complexes retain their
square pyramidal geometry in solutions, as can be seen from the
presence of two absorption bands in d–d spectra (at ca. 600 and
800–850 nm).39 When the protonated pendant arm dissociates,
square-planar complexes are formed, as evidenced by one
strong-field transition in the electronic spectra. Both four-
coordinate (protonated) and five-coordinate (non-protonated)
complexes in frozen solutions give well-resolved axial EPR
2
Ϫ
y2
spectra with g|| > g⊥ (see Experimental), indicating a dx
ground state.39 The spectral parameters reflect the usual
increase of the equatorial ligand field in four-coordinate com-
plexes vs. their five-coordinate counterparts.39,40 The values of
g|| decrease from 2.207–2.210 in five-coordinate species to
2.187–2.192 in protonated, square-planar complexes, and the
values of A|| increase from (166–170) × 10Ϫ4 cmϪ1 in five-
coordinate complexes to (193–202) × 10Ϫ4 cmϪ1 in protonated
complexes upon dissociation of the pendant arm from the
copper() center. Very similar UV-Vis and EPR spectral
changes were observed for the Schiff base complexes with
L1a,11 and for other macrocycles bearing pendant arms.18,41–43
Quantitative data on the pH-dependent protonation of the
pendant arm were obtained by spectrophotometrically moni-
tored pH-titrations (Fig. 7). All reduced nickel() complexes
Fig. 6 ORTEP plot of the complex [Cu(HL2a)](ClO4)3. Perchlorate
ions and hydrogen atoms are omitted for clarity.
(Fig. 6). The crystal structure of this complex is similar to the
structures of copper() complexes with tetradentate amino-
pyridine macrocycles.28–31 The Cu–N distances are now smaller
than the Cu–N distances in the five-coordinate (non-proton-
ated) precursor, as is the copper displacement out of the N4
plane (Table 2). The copper() ion is located on the opposite
face of the macrocycle with respect to the un-coordinated
pendant arm. As a result, both NH vectors (at N(2) and N(4))
coincide with the pendant arm without disrupting chair con-
formations of the six-membered chelate rings of the macro-
cyclic complex. The macrocycle is substantially “flattened” as
compared to the parent five-coordinate complex [Cu(L2a)]2ϩ
.
Interestingly, the fifth position is now occupied by the coordin-
ated acetonitrile molecule (which binds opposite to the pendant
arm). This situation is different from complex [Cu(L2a)]-
(ClO4)2ؒCH3CN, where all five nitrogens of the macrocycle were
coordinated, and a non-coordinated acetonitrile filled the voids
in the lattice.
Pendant arm protonation equilibria
The “long” aminopropyl pendant arm is coordinated to the
metal (copper() or nickel()) in neutral or slightly alkaline
solutions, but dissociates upon protonation in slightly acidic
media. The aqueous solutions of [NiL1]2ϩ become yellow upon
adjusting pH to 3, and their electronic spectra are typical of low
spin, square planar macrocyclic Ni() complexes, while purple
or blue solutions of the complexes at pH 10 contain several low-
intensity d–d transition bands with λmax (ca. 370, 570, 800, and
900 nm) characteristic of the high-spin square-pyramidal
NiIIN5 chromophore (Table 3).11,32–34 Comparison of the
Fig.
7
UV-Visible spectrum of the arm on/off behavior of
Concentration of initial [Ni(L2a)](ClO4)2 solution
[Ni(L2a)]2ϩ
.
0.006 M. The initial 2 mL of solution was titrated with 12 additions of
0.5 µL of 1.100 M HClO4. The final acid concentration was 0.003 M.
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
850

View Article Online
showed completely reversible behavior over a broad pH range
(3–11), and clear isosbestic points were present in UV-Vis
spectra (Fig. 7). Equilibrium constants (Scheme 2) were calcu-
lated from the titration curves with high accuracy. The results
for [Ni(L1a)]2ϩ (pKa = 6.47 0.03) agree perfectly with the pKa
value obtained previously by pH-potentiometric titrations
(6.46 0.03).11 The Schiff base complex [Ni(L1c)]2ϩ was less
stable and partially decomposed upon prolonged titrations
(especially in alkaline media). Copper() complexes with
reduced macrocycles reacted with acid or base reversibly over a
pH range from 3 to 8, but at higher pH, another process was
observed (presumably, deprotonation of a coordinated water
molecule) which was only partially reversible. This behavior
somewhat limited the accuracy of pKa determination for
[Cu(L2c)]2ϩ, because deprotonation of the pendant arm in this
complex occurred at relatively high pH, and no clear saturation
of the titration curve was reached. The pKa data are summar-
ized in Table 3.
The protonation constants for pendant amino groups
coordinated to metal ions in [M(L2a)]2ϩ–[M(L2c)]2ϩ are rel-
atively high, falling in the same pKa range (6–8) as the proton-
ation constants for aminopropyl pendant arm attached to
isocyclam copper() and nickel() complexes.42 Shorter side
arms, which form five-membered chelate rings with the central
metal ion, are usually less susceptible to protonation and dis-
sociate from the metal in acidic media (pH ≈ 2).18,21,23,42,43
The pH range for the proton-dependent dissociation of the
coordinated aminopropyl arm is very convenient as it allows
for switching between two different ligand binding modes under
mild conditions. Weak coordination is also favorable for
covalent modification of the primary amino group in the
macrocyclic complexes, as discussed below.
investigation. The experimental data show that the pendant
aminopropyl groups of the diacetylpyridine derivatives L2a
and L2b have higher affinity for copper() than their
diformylpyridine counterpart L2c (Table 3). The relatively high
pKa of [Cu(L2c)]2ϩ resembles the high pKa values previously
found for Schiff base copper() complexes.11
Covalent modification of the coordinated amino group
Protonation studies of [ML1]2ϩ and [ML2]2ϩ demonstrated that
two distinct metal binding sites with very different affinities for
the metal ion are present in the pentadentate ligands. The
aminopropyl pendant arm coordinates weakly and dissociates
upon protonation at pH 6–8, while the macrocycle remains
strongly bound to the metal even under acidic conditions. This
observation suggests that the coordinated amino groups may
display distinctly different reactivity with respect to covalent
modification. Selective modification of the pendant amine
would be beneficial for conjugation of the metallocomplexes to
biomolecules,44–46 or for fluorescent labeling of the complex.47–49
Because coordination to the metal ion decreases nucleophilicity
of the amino group, the modification of the free ligand followed
by metal insertion into the macrocycle remains the most
popular strategy for pendant arm functionalization.44,45,48,49
Recently, direct functionalization of the metal complexes was
reported,45–47 where either non-coordinated45,46 or weakly
coordinated47 amino group appended to the macrocycle or cage
reacted with an electrophilic agent (dansyl chloride, chloro-
acetic acid, etc.). A particularly attractive feature of these reac-
tions is the protecting role of the metal ion: metal coordination
prevented extensive modification of the secondary amino
groups of the macrocycles. Inspired by recent successes in deriv-
atizing metallomacrocycles bearing an amino group, we
decided to investigate the acylation of [Ni(L2c)]2ϩ. This strategy
for selective amide bond formation would be useful for bio-
conjugation. Numerous possible applications of metal-assisted
amide bond formation within the coordination sphere were
previously explored in peptide synthesis,50–55 ranging from
protecting the amino group,50,52 to activating the carbonyl
component,51,53,54 to templating the preparation of cyclic
tetrapeptides.55 Consequently, amide formation from coordin-
ated peptide precursors is possible, even though the amino
group is deactivated in metallocomplexes.
The structure of the macrocycle (double vs. single C–N
bonds and methyl substituents) exerts little effect on the pKas of
nickel() complexes (Table 3). The protonation constants of the
pendant arms cluster around pKa = 6.4 0.0.1 for all complexes
except for [Ni(L2a)]2ϩ that has somewhat higher pKa value of
6.75
0.04. The lower affinity of Ni() for pendant arm
coordination in [Ni(L2a)]2ϩ can be attributed to steric hin-
drance between the arm and two axial methyl groups facing the
same side of the macrocycle. Such steric interactions are absent
in the rest of our nickel() complexes: diformylpyridine deriv-
atives L1c and L2c lack methyl groups, Schiff base macrocycle
L1a contains methyl groups that are coplanar with the ring
and would not interfere with the arm coordination, and com-
plex [Ni(L2b)]2ϩ has only one methyl substituent in the same
configuration as the metal-bound aminopropyl arm.
In reactions of the complex [Ni(L2c)]2ϩ with acetic anhydride
or benzoyl chloride, the amide was cleanly formed in the pen-
dant arm, but no modification of the macrocyclic backbone
was observed, as evidenced by mass spectrometry and NMR
(Scheme 1). The acetylated product [Ni(L3c)](ClO4)2 was
isolated and fully characterized as a low-spin, square-planar
nickel() complex. The amide group in the long pendant arm
did not coordinate to the metal ion up to pH 11, in agreement
with the expected decrease in binding affinity of the pendant
arm due to an increase in the length of a flexible linker.
Kimura et al. found that an amide in a “short” pendant arm
(–CH2C(O)NH–) coordinates to the central zinc() ion, while a
longer arm separated from the macrocycle by a C2-linker did
not bind.56 Transmetallation of the acylated macrocyclic com-
plex proceeded uneventfully (NaCN followed by Cu2ϩ, Scheme
1, Experimental) and yielded the corresponding copper()
complexes [Cu(L3c)]2ϩ and [Cu(L4c)]2ϩ that were crystallo-
graphically characterized.
The coordination environments of copper() in the acetyl-
ated complex [Cu(L3c)](ClO4)2 (Fig. 8) and in the benzoylated
complex [Cu(L4c)](ClO4)2 (Fig. 9) are very similar to that found
in the singly protonated complex [Cu(HL2a)](ClO4)3ؒCH3CN
(Fig. 6). Again, short in-plane Cu–N distances (from 1.936 to
2.027 Å, Table 2) are observed, with the axial position being
occupied by a weak ligand (an oxygen of a perchlorate anion).
The macrocycle is essentially undistorted, having both six-
membered chelate rings in a chair conformation. The copper()
The protonation constants of the copper() complexes
depend on several factors and do not show one straightforward
trend. Unlike protonated nickel() complexes that are square-
planar, the analogous copper() “arm-off” macrocycles tend to
bind solvent molecules in the axial positions. A coordinated
acetonitrile molecule was found in the X-ray structure of
[Cu(HL2a)](ClO4)3ؒCH3CN (Fig. 6). Notably, this solvent mole-
cule is located at the opposite face of the ring with respect to
the methyl and aminopropyl substituents. It appears that the
axial methyl groups may screen the central copper() ion from
the solvent coordination, thus facilitating pendant arm binding.
Another important factor is the steric strain of the macrocyclic
ligands in square-pyramidal five coordinate copper() com-
plexes that could be partially relieved upon arm dissociation.
This factor appears to be of particular importance for the rigid
Schiff base complexes with the mismatch between the size of
the ring and the radius of the copper() ion.11 Although the
flexible reduced ligands L2a–L2c can better accommodate
copper(), the L2a ligand changes conformation (and configur-
ation of one of the macrocyclic NH-groups) upon proton-
induced dissociation of the pendant arm (Figs. 4 and 6). It is
difficult to quantify this effect in the absence of structural data
for all “arm on” and “arm off” copper() complexes under
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
851

View Article Online
complexes. Successful transmetallation allowed us to prepare
copper() complexes bearing an acetylated aminopropyl
substituent that remained detached from the metal under
ambient conditions.
Conclusions
A series of square pyramidal nickel() complexes with pyridine-
containing macrocycles bearing an aminopropyl pendant arm
were prepared and characterized. The free macrocycles were
stable, allowing for the performance of transmetallation reac-
tions that yielded copper() complexes. The coordinated
primary amino group on the long linker could be reversibly
detached from the metal (copper() or nickel()) by proton-
ation under mild conditions (pKa ≈ 6–8). Furthermore, the
weakly coordinated amino group was selectively acylated, yield-
ing four-coordinate square-planar nickel() and copper()
complexes. Covalent modification of the five-coordinate com-
plexes may be useful for attaching metallomacrocycles to
biomolecules.
Fig. 8 ORTEP plot of the complex [Cu(L3c)](ClO4)2. Perchlorate ions
and hydrogen atoms are omitted for clarity.
Experimental
General
All chemicals were of reagent grade and used as supplied with-
out further purification. MALDI-TOF mass spectrometry was
performed on a Bruker Biflex Instrument operating in positive
reflectron mode using a dithranol matrix. Electrospray mass
spectrometry was performed on a HP 5989B (Hewlett-Packard)
Instrument in a 1 : 1 mixture of methanol and water with acetic
acid. EPR spectra were obtained with a Bruker EMX spectro-
meter at 120 K in frozen propionitrile/2-methyltetrahydrofuran
solutions (1 : 2), using DPPH as a standard. NMR spectra were
recorded on a Bruker AM-300 spectrometer, IR spectra on a
Mattson 1000 FTIR Spectrometer. UV-Vis spectra were
obtained on a Hitachi U-2000 spectrometer (300–1100 nm).
The UV-Vis spectra of solids were determined in Nujol mulls
put on filter paper in absorbance mode as described else-
where.58 Elemental analyses were performed by Qualitative
Technologies Inc. (Whitehouse, NJ).
Fig. 9 ORTEP plot of the complex [Cu(L4c)](ClO4)2. Perchlorate ions
and hydrogen atoms are omitted for clarity.
Syntheses
Reduction of [Ni(L1a)](ClO4)2. 0.684 g (0.001 mol) of
[Ni(L1a)](ClO4)2 synthesized by a previously reported pro-
cedure was dissolved in 300 mL of water at ∼50 ЊC, through
Fig. 10 X-Ray structure of [Ni(L3c)](ClO4)2. Perchlorate ions and
hydrogen atoms are omitted for clarity.
11
the use of vigorous stirring, yielding a brick-red solution. Once
cooled to room temperature the solution was then filtered to
remove trace impurities. To this solution 0.874 g of NaBH4
(0.023 mol) was added over a period of 10–15 min. The reaction
mixture was seen to become black and precipitation of a gray
solid was observed. The solution was left vigorously stirring
overnight, during this time the solid re-dissolved, yielding a
violet solution. The pH was then adjusted to between 8.5–9
through the addition of concentrated HClO4, and the solution
was rotary evaporated to dryness (water bath temperature
∼30–40 ЊC). The complex was allowed to dry overnight under
vacuum. Impurities resulting from the use of NaBH4 were sub-
sequently removed by dissolving the desired product in nitro-
methane and filtering off insoluble material. Yield of a mixture
of [Ni(L2a)](ClO4)2 and [Ni(L2b)](ClO4)2: 0.318 g (46%).
Separation of [Ni(L2a)](ClO4)2 and [Ni(L2b)](ClO4)2.
Ethanol (70 mL) was added to 0.318 g (0.6 mmol) of the isomer
mixture. The suspension was stirred and brought to the boil.
The mixture was then allowed to cool to room temperature and
filtered, yielding a pink solution and blue solid. The solid was
placed back into a flask and this process was repeated three
more times. The periwinkle solid material [Ni(L2a)](ClO4)2 was
then collected and washed with ether. The complex was allowed
to dry overnight under vacuum. Yield of [Ni(L2a)](ClO4)2:
ion is displaced down the N4 mean plane by only 0.105 Å, and
the NH groups are parallel to the uncoordinated pendant arm.
The corresponding nickel() complex, [Ni(L3c)](ClO4)2, con-
tains a low-spin, four-coordinate metal in a square-planar
environment (Fig. 10) with a relatively long separation from the
Ϫ
oxygen atom of ClO4 (2.799 Å). All Ni–N distances are sub-
stantially shorter than those observed in the high-spin, five-
coordinate complexes, and range from 1.835 to 1.953 Å. These
distances are also shorter than the corresponding Cu–N dis-
tances in the complex cation [Cu(L3c)]2ϩ (Tables 1 and 2). Due
to uniform in-plane Ni–N bond lengths (only the Ni–N(1) bond
to a pyridine nitrogen is shorter by 0.1 Å), and small Ni() out-
of-plane displacement (0.11 Å), the bond angles approach their
ideal values (90 or 180Њ). The conformation of the macrocycle
in [Ni(L3c)]2ϩ is identical to that of its copper counterpart.
As expected, the structures of modified copper() and
nickel() complexes with a non-coordinated, N-acylated pen-
dant arm closely resemble the structures of the Cu() and Ni()
complexes with relevant tetradentate pyridine-containing
macrocycles.20,22,28–31,57
Structural data confirm that the pendant arm’s primary
amino group with its weak coordination to the nickel() ion
undergoes selective acylation, yielding four-coordinate
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
852

View Article Online
0.153 g (48%). Anal. found: C, 37.4; H, 5.63; N, 11.91. Calc. for
C16H33Cl2N5NiO8: C, 37.5; H, 5.76; N, 12.13%. ESMS: m/z 189
(NiLϩ). Dissolution of the material in an ethanol–nitromethane
mixture followed by ether diffusion yielded X-ray quality
crystals.
All ethanol fractions were consolidated and rotary evapor-
ated to a volume of 180 mL. This solution was placed into
the refrigerator and then frozen. After 1–2 days a crystalline
magenta precipitate of [Ni(L2b)](ClO4)2 formed and was
collected by suction filtration. The mother liquor was concen-
trated and the process repeated. The complex was allowed to
dry overnight under vacuum. Yield of [Ni(L2b)](ClO4)2: 0.147 g
(46% yield). Anal. found: C, 37.0; H, 5.63; N, 11. 69. Calc. for
C16H33Cl2N5NiO8: C, 37.5; H, 5.76; N, 12.13%. ESMS: m/z 189
(NiLϩ).
pyridine (synthesized by a previously reported procedure)59 in
200 mL of ethanol was subsequently added to the mixture over
a period of 20–25 minutes (the color changed from the original
sky blue to a deep red) and stirring at room temperature con-
tinued for four days. The solution was then rotary evaporated to
dryness, using a water bath of ∼25 ЊC and condenser with
chilled water (heating the reaction mixture resulted in decom-
position of the product), and the dark red solid containing
small amounts of a violet solid (determined by X-ray crystallo-
graphy to be the same complex with no acetonitrile in the
crystal lattice) was allowed to dry overnight under vacuum.
Yield: 5.26 g (96%). Crystallization occurred by dissolving the
residue in the minimum volume of acetonitrile followed by
ether diffusion into the solution. The complex was allowed to
dry overnight under vacuum. Found: C, 33.74; H, 5.02; N,
12.45. Calc. for C16H27Cl2N5NiO9: C, 34.13; H, 4.83; N, 12.44%.
IR (KBr): 1636w (νC᎐N), 1596m, 1542m, (pyridine and NH),
1100 (br, ClO4Ϫ) cmϪ1. MALDI-TOF MS: m/z 346 (NiLϩ,
100%).
Dissolution of the material in acetonitrile followed by ether
diffusion yielded X-ray quality crystals.
L2a. 40.4 mg (0.07 mmol) of [Ni(L2a)](ClO4)2 were dissolved
in ∼8.5 mL of warm water. The solution was then allowed to
cool to room temperature. To this solution six equivalents
(0.0206 g, 0.42 mmol) of NaCN were added and the solution
instantly turned from blue to yellow. The ligand was then
extracted with methylene chloride (3 × 5 mL). All methylene
chloride portions were consolidated and dried over sodium
sulfate overnight. The solution was concentrated to dryness
yielding yellow oil. The ligand was allowed to dry overnight
under vacuum. Yield: 0.018 g (80%). 1H NMR (CDCl3): δ 1.372
(2, 6H), 1.531–1.806 (m, 8H), 2.165–2.505 (m, 12H), 2.653 (t,
2H), 3.667 (quartet, 2H), 6.937 (d, 2H), 7.516 (t, 1H). 13C NMR
(CDCl3): δ 24.14, 27.57, 30.53, 40.84, 45.55, 51.01, 51.70, 59.61,
120.60, 136.45, 164.46 (11 signals). MS (DIP EI): m/z 320 (Lϩ,
100%)
[Ni(L2c)](ClO4)2. 1.47 g (2.61 mmol) of [Ni(L1c)](ClO4)2 was
dissolved in 500 mL of warm water and stirred. The brick-red
solution was then filtered. To the stirred solution 7.11 g
(0.19 mol) of NaBH4 was added. During this period, the form-
ation of a black precipitate was observed. The solution was left
to stir vigorously for two days during which time the precipitate
re-dissolved. To the resulting purple solution 1 g (0.027 mol) of
NaBH4 was added and left to sit an additional two days in order
to ensure complete reduction. The pH of the solution was then
adjusted to 9 with HClO4 (70%). The solution was filtered and
rotary evaporated to dryness. During the concentration of the
solution, a fluffy brown byproduct precipitated and was period-
ically filtered off. The complex was allowed to dry overnight
under vacuum. Nitromethane was added to the dried complex
and the solution then filtered. To the concentrated solution
small portions of ethanol (∼5 mL) were added. The addition
caused the precipitation of further impurities that were filtered
off and discarded. Additional purification of the material was
performed by adding diethyl ether to the nitromethane–ethanol
solution. The solution was then rotary evaporated to dryness
making sure the water bath temperature never exceeded 40 ЊC.
The complex was allowed to dry overnight under vacuum.
Yield: 0.62 g (45.3%). IR (KBr): ν(NH) 1612m, ν(pyridine)
1584m, ν(ClO4Ϫ) 1100 (br) cmϪ1. MALDI-TOF MS: m/z
350 (NiLϩ, 100%). Single crystals for X-ray diffraction analysis
were obtained by ether diffusion into an acetonitrile–
nitromethane (1 : 1) solution of [Ni(L2c)](ClO4)2.
L2b. The procedure described above for L2a was applied to a
magenta solution of 40.3 mg (0.07 mmol) of [Ni(L2a)](ClO4)2
1
in 5.5 mL of warm water, yielding 0.020 g (90%) of L2b. H
NMR (CDCl3): δ 1.372 (2, 6H), 1.531–1.806 (m, 8H), 2.165–
2.505 (m, 12H), 2.653 (t, 2H), 3.667 (quartet, 2H), 6.937
(d, 2H), 7.516 (t, 1H). 13C NMR (CDCl3): δ 24.14, 27.57,
30.53, 40.84, 45.55, 51.01, 51.70, 59.61, 120.60, 136.45, 164.46
(11 signals). MS (DIP EI): m/z 320 (Lϩ, 100%).
[Cu(L2a)](ClO4)2. 0.018 g (0.056 mmol) of L2a were dis-
solved in ∼2 mL of acetonitrile and to this solution 0.9 equiv-
alents of Cu(ClO4)2ؒ6H2O (0.0187 g, 0.050 mmol), dissolved in
1 mL of acetonitrile, was added. The acetonitrile solution
instantaneously turned blue, and the solid product precipitated
upon addition of ether. Yield: 0.030 g (90%). MALDI-TOF
MS: 383 (CuLϩ, 100%). EPR (EtCN–MeTHF): g|| = 2.210, A|| =
168 × 10Ϫ4 cmϪ1, g⊥ = 2.06. EPR (EtCN–MeTHF ϩ triflic acid):
g|| = 2.187, A|| = 196 × 10Ϫ4 cmϪ1, g⊥ = 2.046.
L2c. The ligand was isolated in 63% yield from 39.9 mg
(0.073 mmol) of [Ni(L2c)](ClO4)2 dissolved in ∼5.5 mL of warm
water using the procedure described above for the preparation
1
of L2a. H NMR (CDCl3): δ 1.550 (m, 2H), 1.710 (m, 4H),
Slow ether diffusion into the acetonitrile solution of
[Cu(L2a)](ClO4)2 yielded two types of crystals suitable for
X-ray analysis: needle like blue crystals of [Cu(L2a)](ClO4)2,
and purple crystals of [Cu(HL2a)](ClO4)3ؒCH3CN.
2.391 (m, 6H), 2.506 (t, 4H), 2.630 (t, 2H), 3.865 (s, 4H), 7.017
(d, 2H), 7.549 (t, 1H). 13C NMR (CDCl3): δ 27.42, 29.59, 40.45,
46.51, 50.38, 52.16, 54.22, 120.57, 136.51, 159.23 (10 signals).
MS (DIP EI): m/z 292 (Lϩ, 100%)
[Cu(L2b)](ClO4)2. The above procedure was applied to 0.020
g (0.063 mmol) of L2b, yielding 0.033 g (90%) of crystalline
complex. MALDI-TOF MS: m/z 383 (CuLϩ, 100%). EPR
(EtCN–MeTHF): g|| = 2.208, A|| = 170 × 10Ϫ4 cmϪ1, g⊥ = 2.06.
EPR (EtCN–MeTHF ϩ triflic acid): g|| = 2.192, A|| = 193 × 10Ϫ4
cmϪ1, g⊥ = 2.046.
[Cu(L2c)](ClO4)2. This complex was prepared in 90% yield
from 0.013 g (0.044 mmol) of the L2c ligand dissolved in ∼2 mL
of acetonitrile, using the procedure described above for the
preparation of [Cu(L2a)](ClO4)2. MALDI-TOF MS: m/z 354
(CuLϩ, 92%). EPR (EtCN–MeTHF): g|| = 2.207, A|| = 166 ×
10Ϫ4 cmϪ1, g⊥ = 2.06. EPR (EtCN–MeTHF ϩ triflic acid): g|| =
2.191, A|| = 202 × 10Ϫ4 cmϪ1, g⊥ = 2.044.
[Ni(L1c)](ClO4)2. 3.65 g (10 mmol) of Ni(ClO4)2ؒ6H2O was
dissolved in 168 mL of water. This solution was added to a
stirred solution of 1.883 g (10 mmol) of tris(3-amino-
propyl)amine in 650 mL of water. Upon addition of the nickel
salt the solution instantly changed from clear to sky blue (pH
between 8–9). A solution of 1.351 g (10 mmol) diformyl-
[Ni(L3c)](ClO4)2. 0.104 g (0.19 mmol) of [Ni(L2c)](ClO4)2
were dissolved in 5 mL of nitromethane to which 0.5 mL of 2,6-
lutidine was added followed by 0.25 mL of acetic anhydride.
Upon the addition of the last reagent the solution underwent a
color change from violet to orange–brown (∼30 seconds). The
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
853

View Article Online
mixture was then rotary evaporated to dryness and the solid
residue was repeatedly washed with warm toluene under vigor-
ous stirring until the solvent remained clear. The complex was
allowed to dry overnight under vacuum. Yield: 0.075 g (63%).
Found: C, 37.21; H, 5.10; N, 12.15. Calc. for C18H31Cl2N5NiO9:
C, 36.67; H, 5.30; N, 11.89%. Dissolution of the material
in acetonitrile–nitromethane (1 : 1) followed by slow ether
diffusion resulted in single crystals.
pH-meter equipped with a glass pH/ATC electrode. The values
of pKas were easily determined from the linear graphs
log{(ε[ML] Ϫ εi)/(εi Ϫ ε[M(HL)])} vs. pH, where ε[M(L)], ε[M(HL)], and
εi are molar optical absorbancies of [M(L)]2ϩ, [M(HL)]3ϩ, and
their mixutures during the titrations, respectively. The values of
ε were corrected for dilution during the titrations. Different
wavelengths were used for the calculation of pKas and gave
almost identical results. All measurements were done at 25.0
0.1 ЊC and the ionic strength of 0.100 M (KNO3). In a typical
experiment, 3 µmol of a solid complex [ML](ClO4)2 was dis-
solved in 3.00 mL of 0.100 M aqueous KNO3 solution. Nitric
acid (0.200 M) was added in 10 µL portions. UV-Vis spectrum
and pH of the solution were recorded after each addition.
When UV-Vis spectra no longer changed (usually at pH below
3), the titration was reversed with the additions of 10 µL
portions of 0.200 M aqueous KOH until the pH reached 11.
The calibration of the pH-meter was periodically checked
during the titration using standard buffer solutions. This pro-
cedure was tested on the [Ni(L1a)]2ϩ complex and gave the
value of pKa = 6.47(3), which agrees perfectly with the value of
6.46(3) obtained form a previous independent pH-potentio-
metric determination.11 The ε–pH titration curves showed a
L3c. 0.0389 g (0.066 mmol) of [Ni(L3c)](ClO4)2 dissolved in
10 mL of warm water. To the orange–brown solution 8 equiv-
alents (0.026 g, 0.53 mmol) of NaCN were added. The solution
turned yellow and the ligand was extracted into 15 mL of
chloroform (3 × 5 mL). The extract was then dried overnight
over Na2SO4 and rotary evaporated to dryness. The solid ligand
was allowed to dry overnight under vacuum. Yield: 0.0146 g
(65%). 1H NMR (CDCl3): δ 1.596 (m, 2H), 1.727 (m, 4H), 1.824
(s, 3H), 2.335 (t, 6H), 2.598 (t, 4H), 3.067 (m, 2H), 3.865 (s, 4H),
6.999 (d, 2H), 7.550 (t, 1H). 13C NMR (CDCl3): δ 24.92, 25.72,
27.80, 37.65, 47.90, 51.15, 53.36, 54.44, 120.91, 137.22, 159.37,
170.49 (12 signals). MS (DIP CI): m/z 334 (Lϩ, 100%).
single reversible process for complexes [Ni(L2a)]2ϩ, [Ni(L2b)]2ϩ
,
[Cu(L3c)](ClO4)2. 0.0146 g (0.044 mmol) of the amine L3c
was dissolved in 3 mL of ethanol. One equivalent of Cu(ClO4)2ؒ
6H2O (0.016 g) was dissolved in 1 mL of ethanol. Upon addi-
tion of the copper solution to that of the amine a purple pre-
cipitate formed. This precipitate was collected on a filter frit
and was then washed three times with 1 mL of cold ether. Addi-
tional amounts of product were collected from the mother
liqor: the ethanol purple filtrate was concentrated and placed in
the freezer (Ϫ10 ЊC) for a few hours. The precipitate was then
collected and washed with ether. The complex was allowed to
dry overnight under vacuum. Combined yield: 0.0249 g (98%).
X-Ray quality crystals were obtained by slow diffusion of ether
into acetonitrile solution of the complex.
and [Ni(L2c)]2ϩ and one quasi-reversible process for [Ni-
(L1c)]2ϩ, with an inflection point around pH 6–7. Complexes
[Cu(L2a)]2ϩ, [Cu(L2b)]2ϩ, and [Cu(L2c)]2ϩ showed a reversible
process around pH 6–7, followed by a quasi-reversible process
at pH above 8, which made the determination of the pKas for
the Cu complexes less accurate. Acylated complexes [M(L3c)]2ϩ
and [M(L4c)]2ϩ (M = Ni, Cu) showed pH-independent UV-Vis
spectra up to pH 11. Above pH 11, the nickel() complexes with
L3c and L4c were found to react with oxygen, and further
titrations were carried out under nitrogen. For both nickel()
and copper() complexes with acylated macrocycles, addition
of base after pH 11 under nitrogen caused quasi-reversible
spectral changes without saturation accompanied by the pre-
cipitation of solids. It is not clear if the spectral changes are due
to the deprotonation and coordination of the pendant amide
group in the complexes, or due to a decomposition process
(e.g., the formation of metal hydroxides).
[Ni(L4c)](ClO4)2. 0.110 g (0.20 mmol) of [Ni(L2c)](ClO4)2
were dissolved in 5 mL of nitromethane to which 0.5 mL of 2,6-
lutidine was added followed by 0.30 mL of benzoyl chloride.
Upon addition of the last reagent the solution underwent a
color change from violet to orange–brown (∼30 seconds). The
mixture was then rotary evaporated to dryness, and the solid
residue was washed with several 5 mL portions of warm tolu-
ene upon vigorous stirring. The orange solid was then dissolved
in nitromethane and precipitated out with ether yielding a tan
solid. The complex was dried overnight under vacuum. Yield:
0.06 g (52%).
X-Ray diffraction studies
Crystal data are summarized in Tables 4 and 5.
Data for [Ni(L2a)](ClO4)2, [Cu(L2a)](ClO4)2ؒCH3CN,
[Cu(HL2a)](ClO4)3ؒCH3CN, [Cu(L2b)](ClO4)2ؒ2C3H6O, and
[Cu(L3c)](ClO4)2 were collected using a Bruker SMART CCD
(charge coupled device) based diffractometer equipped with an
LT-3 low-temperature apparatus operating at 213 K. A suitable
crystal was chosen and mounted on a glass fiber using paratone
oil. Data were measured using omega scans of 0.3Њ per frame
for 30 seconds, such that a hemisphere was collected. A total of
1271 frames were collected with a maximum resolution of
0.75 Å. Data for [Ni(L3c)](ClO4)2ؒCH3NO2 and [Cu(L4c)]-
(ClO4)2 were collected at 298 K; the crystals were mounted on
a glass fiber using glue. A total of 1651 frames were collected
with a maximum resolution of 0.75 Å for [Ni(L3c)](ClO4)2ؒ
CH3NO2 and 0.80 Å for [Cu(L4c)](ClO4)2. The first 50 frames
were recollected at the end of data collection to monitor for
decay. Cell parameters were retrieved using SMART60 software
and refined using SAINT on all observed reflections. Data
reduction was performed using the SAINT61 software which
corrects for Lorentzian polarization and decay.
L4c. Free ligand L4c was isolated from 0.0430 g (0.066
mmol) of [Ni(L4c)](ClO4)2 in 65% yield, following the pro-
1
cedure for the preparation of L3c. H NMR (CDCl3): δ 1.721
(m, 4H), 2.375 (m, 6H), 2.679 (t, 4H), 3.199 (quartet, 2H), 3.844
(s, 4H), 6.980 (d, 2H), 7.456 (m, 3H), 7.539 (t, 1H), 7.912 (m,
2H). 13C NMR (CDCl3): δ 25.64, 27.36, 36.71, 48.45, 49.66,
53.84, 54.15, 120.67, 127.24, 128.76, 131.15, 135.08, 137.10,
158.92, 167.45 (15 signals). MS (DIP CI): m/z 396 (Lϩ, 48%).
[Cu(L4c)](ClO4)2. Was synthesized similarly to [Cu(L3c)]-
(ClO4)2, starting from 0.0169 g (0.043 mmol) of the amine L4c.
Crystallization from ether diffusion into a concentrated solu-
tion of the complex in acetone–acetonitrile (∼19 : 1) resulted in
rectangular crystals. Yield: 0.0183 g (95%). MALDI-TOF MS:
m/z 359 (CuL2ϩ, 36%).
Single crystal intensity measurements for [Ni(L2b)](ClO4)2
and [Ni(L2c)](ClO4)2 were collected at room temperature with a
Rigaku AFC5 diffractometer, using Mo-Kα radiation, graphite
monochromator and ω/2θ-scans. Lattice parameters were
obtained using least squares refinement of the angles of 24
reflections with 22 < 2θ < 26Њ. Psi-scan absorption correction
was applied to [Ni(L2b)](ClO4)2.
Determination of the protonation constants
The protonation constant of each complex’s side arm was
determined by combined spectrophotometric and pH-poten-
tionmetric titrations. UV-Vis spectra were recorded with a
Hithachi U-2000 spectrophotometer. The pH measurements
were carried out with a Denver Instrument Model 250
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
854

View Article Online
Table 4 Crystallographic parameters for Ni() complexes
[Ni(L2c)](ClO4)2
[Ni(L2a)](ClO4)2
[Ni(L2b)](ClO4)2
[Ni(L3c)](ClO4)2ؒCH3NO2
Formula
M
C16H29N5NiCl2O8
549.05
C18H33N5NiCl2O8
577.10
C18H33N5NiCl2O8
577.10
C19H34N6NiCl2O11
652.13
T /K
Crystal system
293(2)
213(2)
293(2)
293(2)
Monoclinic
P21/m (no. 11)
1164.5(4)
10.275(2)
9.314(2)
12.170(2)
90
91.03(3)
90
2
Monoclinic
P21/c (no. 14)
4948.2(8)
18.5147(17)
16.5059(13)
16.3212(15)
90
97.227(2)
90
8
Triclinic
Monoclinic
P21/n (no. 14)
2813.2(9)
12.144(2)
14.008(2)
16.681(3)
90
97.526(4)
90
4
¯
Space group
P1 (no. 2)
V/Å3
1303.9(6)
9.745(3)
16.107(3)
8.952(3)
105.17(2)
105.66(2)
88.86(2)
2
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
Z
µcalc/mmϪ1
1.114
1.053
0.999
0.944
N
Nind
Nobs
2329
33552
3755
12559
2202 (Rint 0.0166)
1396 [I > 2σ(I )]
0.0605
10739 (Rint 0.0856)
5244[I > 2σ(I )]
0.0767
3409 (Rint 0.0529)
2199 [I > 2σ(I )]
0.0593
3890 (Rint 0.0688)
2367 [I > 2σ(I )]
0.0581
Final R1 [I > 2σ(I )]
wR2 (all data)
0.2038
0.2505
0.1848
0.1845
Table 5 Crystallographic parameters for Cu() complexes
[Cu(HL2a)](ClO4)3ؒ
CH3CN
[Cu(L2a)](ClO4)2ؒ
CH3CN
[Cu(L2b)](ClO4)2ؒ
2C3H6O
[Cu(L3c)](ClO4)2
[Cu(L4c)](ClO4)2
Formula
M
C20H37N6CuCl3O12
723.45
C20H36N6CuCl2O8
622.99
C24H42N5CuCl2O10
695.07
C18H31N5CuCl2O9
595.92
C23H33N5CuCl2O9
657.989
T /K
Crystal system
293(2)
213(2)
213(2)
193(2)
273(2)
Triclinic
Monoclinic
P21/n (no. 14)
2779(3)
11.431(7)
8.861(5)
28.031(16)
90
101.766(7)
90
4
Monoclinic
P21/c (no. 14)
3063.3(8)
16.122(2)
13.595(2)
14.548(2)
90
106.128(3)
90
4
Monoclinic
P21/n (no. 14)
2615.4(7)
10.2060(17)
19.104(3)
13.750(2)
90
102.698(3)
90
4
Triclinic
¯
¯
Space group
P1 (no. 2)
P1 (no. 2)
V/Å3
1498.0(4)
9.0975(13)
11.9637(17)
13.886(2)
91.287(3)
94.926(3)
95.545(4)
2
1417.0(16)
8.769(6)
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
11.496(8)
15.502(11)
69.236(14)
80.586(18)
76.862(19)
2
Z
µcalc/mmϪ1
1.065
1.031
0.948
1.094
1.018
N
Nind
Nobs
8549
14389
15935
19047
6315
5249 (Rint 0.0371)
3983 [I > 2σ(I )]
0.0640
4906 (Rint 0.0514)
4663 [I > 2σ(I )]
0.0438
5392 (Rint 0.0551)
3780 [I > 2σ(I )]
0.0939
6539 (Rint 0.0548)
3661 [I > 2σ(I )]
0.0865
3756 (Rint 0.1078)
1540 [I > 2σ(I )]
0.0756
Final R1 [I > 2σ(I )]
wR2 (all data)
0.1849
0.1105
0.2685
0.2751
0.2166
The structures were solved by the direct method using the
SHELXS-9762 program and refined by least squares methods
on F 2, SHELXL-97,63 incorporated in SHELXTL V5.10.64
For [Ni(L2a)](ClO4)2, [Cu(L3c)](ClO4)2, and [Cu(L2a)](ClO4)2ؒ
CH3CN, absorption corrections were applied using SAD-
ABS65,66 supplied by George Sheldrick. All non-hydrogen
atoms are refined anisotropically. Hydrogens were calculated by
geometrical methods and refined as a riding model. For
[Cu(HL2a)](ClO4)3ؒCH3CN, one perchlorate anion was mod-
eled with disorder. For [Cu(L2b)](ClO4)2ؒ2C3H6O, the carbon
C17 was found to have two orientations; the perchlorate anions
were also found to be disordered.
The structure of [Cu(L2a)](ClO4)2ؒCH3CN was solved in the
space group P21/n (no. 14) that was identified by analysis of
systematic absences. Refinement in this space group resulted in
only an R1 of 0.22 and therefore further analysis was per-
formed. The data suggested that the crystal was twinned and
resembled space group C2221. Analysis as a pseudo-merohedral
twin of the monoclinic crystal, twin law Ϫ1 0 0 0 Ϫ1 0 1 0 1,
resulted in an initial R1 of 0.0526. Final refinement with
all hydrogens and the solvent molecule included gave a refined
percentage of the twin to be 33.089%.
consists of two ligand moieties with the occupancy of ½ sur-
rounding the central Ni atom that is located on the mirror
plane. This rather unusual type of disorder resulted in reason-
able bond lengths and angles. An attempt to solve the structure
in acentric P21 resulted in a similar Fourier map showing the
pseudo-mirror plane inside the complex moiety. Another,
independent data set was acquired on a different crystal of the
complex [Ni(L2c)](ClO4)2 using a Bruker SMART diffract-
ometer with a CCD detector and yielded identical cell
parameters and identical structural solution.
The crystals used for the diffraction study showed no decom-
position during data collection. All drawings are done at 50%
ellipsoids.
CCDC reference numbers 198022–198030.
See http://www.rsc.org/suppdata/dt/b2/b211489e/ for crystal-
lographic data in CIF or other electronic format.
Acknowledgements
This research was supported by Tufts University (Faculty
Research Award), the NSF (CHE 0111202), and the Research
Corporation (RI0223). The EPR facility at Tufts was supported
by the NSF (CHE 9816557), the X-ray facility at Tufts was
supported by AirForce DURIP grant F49620–01–1–0242, the
CCD-based X-ray diffractometer at Harvard University
was purchased through NIH grant (1S10RR11937–01). The
The structure of [Ni(L2c)](ClO4)2 was assigned to the space
group P21/m (no. 11) by analysis of systematic absences. It was
solved using heavy atom methods with subsequent analysis
of the resulting Fourier maps. The structure of the complex
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
855

View Article Online
34 M. Ciampolini, Inorg. Chem., 1966, 5, 35.
35 B. F. Hoskins and F. D. Whillans, J. Chem. Soc., Dalton Trans., 1975,
657.
authors thank Prof. T. E. Haas for helpful discussions and help
with X-ray diffraction studies at Tufts.
36 R. Morassi, I. Bertini and L. Sacconi, Coord. Chem. Rev., 1973, 11,
343.
References
37 J. R. Hartman, R. W. Vachet and J. H. Callahan, Inorg. Chim. Acta,
2000, 297, 79.
38 J. G. Gilbert, A. W. Addison and R. J. Butcher, Inorg. Chim. Acta,
2000, 308, 22.
39 B. J. Hathaway, Copper, in Comprehensive Coordination Chemistry,
G. Wilkinson, ed., Pergamon Press, Oxford, 1987, vol. 5, pp. 533–
774.
40 E. V. Rybak-Akimova, A. Y. Nazarenko, L. Chen, P. W. Krieger,
A. M. Herrera, V. V. Tarasov and P. D. Robinson, Inorg. Chim. Acta,
2001, 324, 1.
41 T. J. Lotz and T. A. Kaden, Helv. Chim. Acta, 1978, 61, 1376.
42 A. Schiegg and T. A. Kaden, Helv. Chim. Acta, 1990, 73, 716.
43 P. S. Pallavacini, A. Perotti, A. Poggi, B. Seghi and L. Fabbrizzi,
J. Am. Chem. Soc., 1987, 109, 5139.
44 P. V. Bernhardt and P. C. Sharpe, Inorg. Chem., 2000, 39, 4123.
45 P. S. Donnelly, J. M. Harrowfield, B. W. Skelton and A. H. White,
Inorg. Chem., 2000, 39, 5817.
46 P. S. Donnelly and J. M. Harrowfield, J. Chem. Soc., Dalton Trans.,
2002, 906.
47 T. Koike, T. Watanabe, S. Aoki, E. Kimura and M. Shiro, J. Am.
Chem. Soc., 1996, 118, 12696.
1 P. V. Bernhardt and G. A. Lawrence, Coord. Chem. Rev., 1990, 104,
297.
2 T. A. Kaden, Pure Appl. Chem., 1993, 65, 1477.
3 K. P. Wainwright, Coord. Chem. Rev., 1997, 166, 35.
4 J. Costamagna, G. Ferraudi, B. Matsuhiro, M. Campos-Valette,
J. Canales, M. Villagran, J. Vargas and M. J. Aguirre, Coord. Chem.
Rev., 2000, 196, 125.
5 K. P. Wainwright, Applications of polyaza macrocycles with
nitrogen-attached pendant arms, in Advances in Inorganic Chemistry,
A. G. Sykes, ed., Academic Press, San Diego, 2001, vol. 52, pp. 293–
334.
6 R. I. Haines, Rev. Inorg. Chem., 2002, 21, 165.
7 S. M. Nelson, Pure Appl. Chem., 1980, 52, 2461.
8 G. R. Newcome and V. K. Gupta, Macrocyclic pyridines, in Pyridine
and its derivatives, Wiley, New York, 1984, pp. 447–633.
9 D. E. Fenton and P. A. Vigato, Chem. Soc. Rev., 1988, 17, 69.
10 S. R. Collinson and D. E. Fenton, Coord. Chem. Rev., 1996, 148, 19.
11 E. V. Rybak-Akimova, A. Y. Nazarenko and S. S. Silchenko,
Inorg. Chem., 1999, 38, 2974.
12 H. Keypour and D. A. Stotter, Inorg. Chim. Acta, 1979, 33, 149.
13 H. Keypour, S. Salehzadeh, R. G. Pritchard and R. V. Parish,
Inorg. Chem., 2000, 39, 5787.
48 M. Aoki, H. Kawatani, T. Goto, E. Kimura and M. Shiro, J. Am.
Chem. Soc., 2001, 123, 1123.
14 H. Keypour and S. Salehzadeh, Transition Met. Chem., 2000, 25,
205.
15 D. Parker, Chem. Soc. Rev., 1990, 19, 271.
16 C. J. Anderson and M. J. Welch, Chem. Rev., 1999, 99, 2219.
17 W. A. Volkert and T. J. Hoffman, Chem. Rev., 1999, 99, 2269.
18 S. J. Grant, P. Moore, H. A. A. Omar and N. W. Alcock, J. Chem.
Soc., Dalton Trans., 1994, 485.
49 L. Fabbrizzi, F. Foti, M. Licchelli and A. Poggi, Inorg. Chem., 2002,
41, 4612.
50 R. J. Browne, D. A. Buckingham, C. R. Clark and P. A. Sutton,
Adv. Inorg. Chem., 2000, 49, 307.
51 N. Mensi and S. S. Isied, Inorg. Chem., 1986, 25, 147.
52 S. S. Isied, A. Vassilian and J. M. Lyon, J. Am. Chem. Soc., 1982,
104, 3910.
19 N. W. Alcock, K. P. Balakrishnan, A. Berry, P. Moore and C. J.
Reader, J. Chem. Soc., Dalton Trans., 1988, 1089.
20 N. W. Alcock, P. Moore and H. A. A. Omar, J. Chem. Soc.,
Dalton Trans., 1986, 985.
21 N. W. Alcock, K. P. Balakrishnan, P. Moore and H. A. A. Omar,
J. Chem. Soc., Dalton Trans., 1987, 545.
53 W. Beck and R. Kramer, Angew. Chem., Int. Ed. Engl., 1991, 30,
1467.
54 R. Kramer, M. Maurus, K. Polborn, K. Sunkel, C. Robl and
W. Beck, Chem. Eur. J., 1996, 2, 1518.
55 K. Haas, W. Ponikwar, H. Noth and W. Beck, Angew. Chem.,
Int. Ed., 1998, 37, 1086.
22 N. W. Alcock, P. Moore and H. A. A. Omar, J. Chem. Soc.,
Dalton Trans., 1987, 1107.
23 N. W. Alcock, R. G. Kingston, P. Moore and C. Pierpoint, J. Chem.
Soc., Dalton Trans., 1984, 1937.
56 E. Kimura, T. Gotoh and M. Shiro, Inorg. Chem., 2002, 41,
3239.
57 M. G. B. Drew and S. Hollis, Acta Crystallogr., Sect. B, 1980, 36,
718.
24 A. M. Herrera, A. V. Kalayda, J. S. Disch, A. Y. Nazarenko, R. J.
Staples and E. V. Rybak-Akimova, manuscript in preparation.
25 J. Costa, R. Delgado, M. G. B. Drew and V. Felix, J. Chem. Soc.,
Dalton Trans., 1999, 4331.
26 L. Sacconi, F. Mani and A. Bencini, in Comprehensive Coordination
Chemistry, G. Wilkinson, R. D. Gillard and J. A. McCleverty, eds.,
Pergamon Press, New York, 1987, vol. 5, p. 86.
27 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and G. C.
Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349.
28 M. Di Casa, L. Fabbrizzi, M. Licchelli, A. Poggi, D. Sacchi and
M. Zema, J. Chem. Soc., Dalton Trans., 2001, 1671.
29 L. F. Lindoy, T. Rambusch, B. W. Skelton and A. H. White, J. Chem.
Soc., Dalton Trans., 2001, 1857.
58 R. H. Lee, E. Griswold and J. Kleinberg, Inorg. Chem., 1964, 3,
1278.
59 A. L. Vance, N. W. Alcock, D. H. Busch and J. A. Heppert,
Inorg. Chem., 1997, 36, 5132.
60 SMART V 5.054 (NT), Software for the CCD Detector System,
Bruker Analytical X-Ray Systems, Madison, WI, 1998.
61 SAINT V 6.02 (NT), Software for the CCD Detector System, Bruker
Analytical X-Ray Systems, Madison, WI, 2000.
62 G. M. Sheldrick, SHELXS-97, Program for the Solution of Crystal
Structures, University of Göttingen, Germany, 1997.
63 G. M. Sheldrick, SHELXL-97, Program for the Refinement
of Crystal Structures, University of Göttingen, Germany,
1997.
30 T. H. Lu, S. F. Tung, T. Y. Chi and C. S. Chung, Acta Crystallogr.,
Sect. C, 1998, 54, 1069.
31 V. Felix, M. J. Calhorda, J. Costa, R. Delgado, C. Brito, M. T.
Duarte, T. Arcos and M. G. B. Drew, J. Chem. Soc., Dalton Trans.,
1996, 4543.
64 SHELXTL 5.10 (PC-Version), Program library for Structure
Solution and Molecular Graphics, Bruker Analytical X-Ray Systems,
Madison, WI, 1998.
65 SADABS, Program for absorption corrections using Siemens CCD
based on the methods of Robert Blessing (see ref. 66).
66 R. H. Blessing, Acta Crystallogr., Sect. A, 1995, 51, 33.
67 M. N. Burnett and C. K. Johnson, ORTEP-III: Oak Ridge Thermal
Ellipsoid Plot Program for Crystal Structure Illustrations, Report
ORNL-6895, Oak Ridge National Laboratory, Oak Ridge, TN,
USA, 1996.
32 A. T. Phillip, W. Mazurek and A. T. Casey, Aust. J. Chem., 1971, 24,
501.
33 S. V. Kryatov, B. S. Mohanraj, V. V. Tarasov, O. P. Kryatova,
E. V. Rybak-Akimova, B. Nuthakki, J. F. Rusling, R. J. Staples and
A. N. Nazarenko, Inorg. Chem., 2002, 41, 923.
D a l t o n T r a n s . , 2 0 0 3 , 8 4 6 – 8 5 6
856