Synthesis and Substitution Kinetics of Tricarbonylrhenium(I) Dendritic Complexes

Article abstract of DOI:10.1002/ejic.201700661

Tricarbonylrhenium(I) metallodendrimers functionalized with N,N-2-picolylamino chelates have been synthesized by the [2+1] approach. Methanol substitution reactions of the first- and second-generation poly(propylene amine) tetra- and octanuclear rhenium metallodendrimers by a range of monodentate nucleophiles (i.e. 4-dimethylaminopyridine, pyridine, and bromide ions) were investigated and compared to the substitution reactions of an analogous monomeric complex, fac-[Re(CO)₃(N,N-bidentate)(CH₃OH)]⁺. These detailed kinetic studies reveal that a greater activation is achieved using the metallodendrimers with no direct interactions between the metal centers.

Full text of DOI:10.1002/ejic.201700661

DOI: 10.1002/ejic.201700661
Full Paper
Metallodendrimers
Synthesis and Substitution Kinetics of Tricarbonylrhenium(I)
Dendritic Complexes
Siphelele Malaza,[a] Preshendren Govender,[a] Marietjie Schutte-Smith,[b]
Hendrik G. Visser,*[b] and Gregory S. Smith*[a]
Abstract: Tricarbonylrhenium(I) metallodendrimers functional- and bromide ions) were investigated and compared to the sub-
ized with N,N-2-picolylamino chelates have been synthesized stitution reactions of an analogous monomeric complex, fac-
by the [2+1] approach. Methanol substitution reactions of the [Re(CO)3(N,N-bidentate)(CH3OH)]+. These detailed kinetic
first- and second-generation poly(propylene amine) tetra- and studies reveal that a greater activation is achieved using the
octanuclear rhenium metallodendrimers by a range of mono- metallodendrimers with no direct interactions between the
dentate nucleophiles (i.e. 4-dimethylaminopyridine, pyridine, metal centers.
Introduction
organic molecules, and maintains its structural integrity under
harsh conditions.[11–16] In addition to its kinetic stability, its
kinetic behavior is an aspect worth considering, as it can influ-
ence the uptake and clearance of radiopharmaceutical agents.
Macromolecules such as dendrimers act as “vehicles” that offer
enhanced targeting efficiency via the “enhanced permeability
and retention” (EPR) effect.[1–5] The EPR effect is a phenomenon
in which macromolecules accumulate at the tumor site because
of an increase in blood vessel permeability within diseased tis-
sues compared to healthy tissues.[4] The multivalent nature of
dendrimers allows for the inclusion of a diverse range of metals
for chemotherapy or metallic radionuclides to deliver radiother-
apy and diagnostic imaging for follow-up treatment, surgical
biopsy guidance and automated pathologic analysis of cancer
biopsies and surgical samples.[5] Technetium-99m (γ-emitting
radionuclide) labelling of higher generation dendrimers is an
effective approach in evaluating organ distribution studies of
drug carriers for therapy and diagnosis.[6–10] However, explora-
tory chemical synthesis of technetium-99m complexes in devel-
opmental chemistry is limited by the lack of nonradioactive iso-
topes of Tc. Thus, complexes of nonradioactive rhenium, which
is a technetium congener, are often used as 99mTc models.[11]
Rhenium and technetium have similar ionic radii, and their
fac-[M(CO)3X]+ (where M = 99mTcI or ReI; X = halide) containing
complexes are often isostructural and relatively similar in chem-
ical reactivity (although they require different reaction condi-
tions).[11,12] It is also possible to use ꢀ-emitting rhenium iso-
topes (186Re and 188Re) for radiotherapy applications.[13] The
low-spin d6 fac-[M(CO)3]+ (where M = MnI, 99mTcI, and ReI) frag-
ment is lipophilic in nature, chemically robust compared to pure
The fac-[Re(CO)3(OH2)3]+ synthon is an attractive synthon
that has great stability in water and the ability to exchange
labile solvent ligands. Various synthetic strategies have
emerged to substitute the labile aqua ligands of fac-
[Re(CO)3(OH2)3]+, from the use of tridentate ligands to a combi-
nation of mono- and bidentate ligands, also known as the [2+1]
mixed-ligand approach.[17,18] As described by Mundwiler et al.,
the [2+1] approach comprises a bidentate ligand (displacing
two of the aqua ligands of the fac-[Re(CO)3(OH2)3]+ entity)
which can influence the overall lipophilicity of the tricarbonyl
complex, whilst the third [+1] aqua site is occupied by a
monodentate ligand (acting as a linker to a biomolecule).[18]
The advantage of a [2+1] mixed-ligand complexation is that
the apical [+1] site can be changed to modulate the physical
characteristics of a compound, such as solubility, lipophilicity,
and permeability. The integrity of mononuclear complexes
achieved via the [2+1] approach has been investigated with fac-
[Re(CO)3(N,N-bidentate)(OH2)]+ functionalized model com-
plexes.[19–25] It was shown in these studies, that the choice of
bidentate ligand has a huge effect on the rate of substitution,
with an increase of up to 20 000 times being achieved when
moving from an N,N-donor like bipyridine to an O,O-donor like
3-hydroxyflavone. In vivo, monodentate nucleophiles of varying
basicity are ubiquitous and can readily compete for the “labile”
aqua coordination site in potential fac-[99mTc(N,N-bidentate)-
(CO)3(OH2)]+ functionalized radiopharmaceuticals. The substitu-
tion kinetics of molecules containing more than one metal cen-
ter depends on the symmetry of the molecule and the distance
between the metal centers, as was reported by van Eldik and
others in dinuclear platinum(II) complexes.[26,27] However, the
substitution kinetics of dendrimers have not been examined in
great detail.
[a] Department of Chemistry, University of Cape Town,
Rondebosch, 7701 Cape Town, South Africa
E-mail: gregory.smith@uct.ac.za
www.gregsmith-research.co.za
[b] Department of Chemistry, University of the Free State,
P. O. Box 339, Bloemfontein 9300, South Africa
E-mail: visserhg@ufs.ac.za
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201700661.
Eur. J. Inorg. Chem. 2017, 3919–3927
3919
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Herein, we report a series of first- and second-generation tra for L1 (Figure S1), L2 (Figure S2), and L3, and is assigned to
poly(propylene amine) metallodendrimers of the type fac- the enantiotopic methyl protons adjacent to the pyridyl ring(s).
[Re(CO)3(N,N-bidentate)X]n (where X = Br (n = 0) or OH2 The 1H NMR spectra for L1 and L2 is characterized by broad
(n = +1)). Furthermore, to the best of our knowledge, this is the
first kinetic substitution studies involving metallodendrimers,
and more specifically, the first- and second-generation fac-
[Re(CO)3(N,N-bidentate)(OH2)]+ functionalized metallodendri-
mers, focusing on the rate of substitution of the apical [+1]
aqua ligand by a series of neutral and charged monodentate
ligands, with the aim to systematically study the influence on
the reactivity of the metal center towards substitution.
overlapping multiplets between 0.8–3.0 ppm, assigned to the
aliphatic protons of the dendrimer framework (i.e. protons of
the dendritic core and arms). The 13C{1H} NMR spectroscopic
data reveals a singlet in the range of 52–55 ppm and is assigned
to the sp3 carbon atom of the methyl group adjacent to the
pyridyl ring, further confirming successful reduction of the pre-
formed imine bond. In addition to the 1H and 13C{1H} NMR
spectra, strong absorption band(s) at about 1590 cm–1 were
observed in the IR spectra of L1–L3 and are assigned to the C=
N stretching vibration of the pyridyl ring.
Tricarbonylrhenium(I) complexes are commonly obtained
either from fac-(Et4N)2[Re(CO)3X3] (X = halogen), or a fac-
[Re(CO)3]+ precursor, as reported by Alberto et al.,[26] or from
fac-[Re(CO)3(OH2)3]+ under acidic conditions. The fac-
[Re(CO)3Br] functionalized tetranuclear (1), octanuclear (2), and
mononuclear (3) complexes were synthesized by reacting the
ligands L1–L3 with stoichiometric amounts of fac-
(Et4N)2[Re(CO)3Br3] in dry methanol (Scheme 1 and Scheme 2).
Metallodendrimers 1 and 2 were isolated as brown solids, and
Results and Discussion
Synthesis of Ligands (L1 and L2) and Rhenium(I)–Bromide
Complexes (1–3)
The synthesis of suitable chelating N,N-donor periphery-modi-
fied dendrimers (L1 and L2) for coordination to Re was
achieved via a reductive amination reaction of 2-pyridinecarb-
oxaldehyde with either the first-generation dendritic scaffold
DAB–G1–PPI-(NH2)4 (for L1) or the second-generation dendritic were purified by washing with water. When compared to the
1
scaffold DAB–G2–PPI-(NH2)8 (for L2) (Scheme 1). The water- 1H NMR spectra of the “metal-free” ligands L1–L3, the H NMR
soluble dendritic ligands L1 and L2 were isolated as brown oils spectra of 1 (Figure S3), 2 (Figure S4), and 3 (Figure S5) display
and further purified by reverse-phase column chromatography.
The monomeric ligand L3 and its corresponding mononuclear
complex 3 (Scheme 2) were prepared as models of the larger
dendritic ligands and metallodendrimers.
broadened and a general downfield shift in signals, and further
suggests that complexation has occurred. Moreover, the singlet
resonance previously assigned to the enantiotopic methylene
protons of the dendritic ligands L1 and L2, becomes diastereo-
Evidence of the reductive amination reaction is confirmed by topic following complexation and splits into two broad multi-
a singlet resonance peak at about 3.8 ppm in the 1H NMR spec- plets between 4.7 to 5.2 ppm. Three carbonyl carbon (C≡ O)
Scheme 1. Synthesis of dendritic ligands L1 and L2, and fac-[N,N-Re(CO)3Br] functionalized metallodendrimers 1 and 2.
Scheme 2. Synthesis of monomeric ligand L3 and fac-[N,N-Re(CO)3Br] functionalized mononuclear complex 3.
Eur. J. Inorg. Chem. 2017, 3919–3927 www.eurjic.org © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3920

Full Paper
resonances at approximately 191, 196, and 198 ppm are ob- refinement parameters, and Table S2 lists selected bond lengths
served in the 13C{1H} NMR spectra of 1 (Figure S6), 2 (Figure S7), and angles of 3.
and 3, and, in addition to the IR data, further confirm the pres-
Mononuclear complex 3 crystallizes in the monoclinic space
group, P21/c, with four formula units per unit cell (Z = 4), with
each asymmetric unit containing one independent molecule.
ence and geometric orientation of the fac-[Re(CO)3]+ entity.
1
The H NMR spectrum of 3 (Figure S5) supports the presence
X-ray Crystallography
of H1N (around 5 ppm). Furthermore, the crystal structure of 3
confirms the facial arrangement of the three CO ligands to-
gether with the pseudo octahedral geometry around the metal
center.
The molecular structure of the model mononuclear complex 3
was elucidated by single-crystal X-ray diffraction and is illus-
trated in Figure 1. Table S1 summarizes the crystal data and
The molecular structure of complex 3 shows that the central
rhenium atom is coordinated via the two nitrogen atoms of L3
to form a five-membered ring. The trans angles assigned to
the rhenium–CO [i.e. C(10)–Re(1)–N(2), C(11)–Re(1)–Br(1), C(12)–
Re(1)–N(1)] range between 171.56(18)–179.24(14)°, resulting in
a slight deviation from the anticipated octahedral limit of 180°.
Instead of the expected angle of 90°, the N(2)–Re(1)–N(1)
chelate angle of 75.17(16)°, delineates the formation of a geo-
metrically strained five-membered ring which contributes sig-
nificantly to the angular distortion of the octahedral complex
3. The structure is stabilized by an intermolecular C–H···O
hydrogen bond which is formed between one of the hydrogen
atoms of the pyridine moiety and the carbonyl oxygen of a
neighboring molecule (O12).
Synthesis of Rhenium(I)–Aqua Complexes 4–6
The organometallic precursor fac-[Re(CO)3(OH2)3]NO3, was pre-
pared by stirring fac-(Et4N)2[Re(CO)3Br3] in an aqueous solution
of HNO3 (pH 2.2) in the presence of silver nitrate (Scheme 3).
The substitution reaction produces Et4N+ as a side product,
which shows a similar polarity to the desired complexes and
renders the purification of these systems difficult. The acidic
Figure 1. Molecular structure of mononuclear complex 3, with thermal ellip-
soids at the 50 % probability level.
Scheme 3. Synthesis of aqua-derived metallodendrimers 4 and 5, and mononuclear complex 6.
Eur. J. Inorg. Chem. 2017, 3919–3927 www.eurjic.org
3921
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
reaction conditions serve to minimize dimerization of the orga-
nometallic precursor fac-[Re(CO)3(OH2)3]+, and its formation is
triggered under basic conditions that result in deprotonation of
the aqua ligands to form bridging hydroxide ligands (Re–OH–
Re).[28,29] The reaction between fac-[Re(CO)3(OH2)3]NO3 and
stoichiometric amounts of L1 – L3 in water, afforded the aqua-
derived complexes 4–6 (Scheme 3) as either a colorless or
brown oil. Complexes 4–6 were lyophilized to minimize the wa-
ter content, required for suitable characterization.
Table 1. IR data for the CO-stretching vibrations of 4 and 5 in MeOH and
DMAP.
νCO [cm–1
]
νCO [cm–1
]
Complex
(in MeOH)
(in DMAP)
4
5
2022, 2000, 1870
2022, 1999, 1872
2022, 1898
2021, 1898
a weaker σ-donor and/or π-acceptor ligand than the CO ligand.
In addition, the IR spectra of the complexes support the integ-
rity of the chelate around the ReI metal center as the dissocia-
tion of the bidentate ligand [+2] could not be observed, as
confirmed by the pyridyl C=N absorption band at about
1641 cm–1 in the IR spectra of both DMAP-derived complexes 4
and 5. As the ligand strength is enhanced, the π-backdonation
becomes stronger and the CO bond order is reduced, which
results in a decrease in the CO stretching frequencies, explain-
ing the increase of wavenumbers in charged complexes (4–6)
relative to those in neutral complexes (1–3).[23]
1H NMR spectra similar to that of the rhenium(I)–bromide
complexes 1–3 were obtained for the aqua-derived complexes
4–6. However, there is significant peak broadening and coalesc-
ing of signals observed in the 1H NMR spectra for the aqua-
derived metallodendrimers 4 and 5. This is due in part to the
multivalent nature of the metallodendrimers, and the line
broadening is a consequence of the enhanced relaxation time,
which affords the observation of instantaneous NMR signals
from all possible orientations of the molecule in solution, in-
stead of the averaged signals normally observed for protracted
relaxation events.[30–32] Compared to the IR spectra of the
“metal-free” dendritic ligands L1 and L2, which show the pyr-
idyl C=N absorption band at about 1590 cm–1, the IR spectra
of the aqua-derived metallodendrimers 4 and 5 reveal the pyr-
idyl C=N absorption peaks at higher wavenumbers of about
1620 cm–1. This feature was similarly observed in the IR spec-
trum for the mononuclear aqua-derived complex 6. Further-
more, the IR spectral analysis revealed two absorption bands at
about 2020 and 1860 cm–1 attributed to the symmetric A1 and
the asymmetric E stretching vibrations of the carbonyl ligands.
The positively charged complexes display higher frequencies
than their corresponding neutral complexes, where the carbon-
yls act as π-acceptors.[33] Jansen et al. have previously reported
on the capability of dendritic architectures to encapsulate small
molecules by forming a dendritic box through free rotation and
folding of the dendritic arms.[34] Thus, the elemental analysis
data of the metallodendrimers support inclusion of solvent
molecules.
Substitution Kinetics
The substitution kinetics in complexes 4, 5, and 6 of methanol
by a range of monodentate ligands using pyridine, DMAP, and
Br– ions was then studied in detail. Firstly, it has been shown in
the previous section that water is displaced by methanol upon
dissolution. Also, our preliminary experiments confirmed that
the reactions of all the metal complexes with monodentate li-
gands involve only one reaction (Figure 2). Figure 2 shows an
example of a typical time-resolved absorbance change scan,
which was observed for all substitution reactions in this study.
This may then be fitted to single exponentials (inset in Figure 2),
confirming first-order behavior. All subsequent plots of kobs vs.
ligand concentration yielded straight lines. The stability of all
the complexes in methanol was established by monitoring solu-
tions over several days using a UV/Vis spectrophotometer. The
concentration dependence of the pseudo-first-order rate con-
Substitution Experiments with Metallodendrimers 4 and 5
Investigations were performed to confirm methanol substitu-
tion on the metallodendrimers. The dendritic aqua-derived
metallodendrimers 4 and 5 were dissolved in methanol, to af-
ford MeOH-derived metallodendrimers by displacing the aqua
ligand. Such observations have been reported with structurally
similar tricarbonylrhenium(I) complexes,[19–24] achieved via the
[2+1] approach. The MeOH-derived metallodendrimers 4 and 5
were further reacted with a neutral monodentate ligand DMAP,
displacing the MeOH ligand. The IR absorption bands corre-
sponding to the carbonyl ligands of 4 and 5 in methanol and
in the presence of DMAP are listed in Table 1.
The IR data of the complexes 4 and 5 (Table 1) attest to the
lability of the [+1] aqua ligand. The displacement of the aqua
ligand by MeOH affords three CO absorption bands, whereas in
the presence of DMAP, two CO absorption bands are revealed
in the IR spectra of the DMAP-derived complexes 4 and 5. The
IR spectroscopy results illustrate that the N,N-picolyl chelate is
Figure 2. Typical UV/Vis spectral change for the coordinated methanol substi-
tution reaction of fac-[Re(CO)3(2,4-dPicoH)(MeOH)] with DMAP; [DMAP] =
0.05
M, 25.0 °C. The inset shows fit of absorbance vs. time data to first-order
exponential at 330 nm.
Eur. J. Inorg. Chem. 2017, 3919–3927
www.eurjic.org
3922
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
stant (kobs) for the substitution processes of the methanol li- ure 4 represents a fit of kobs vs. ligand concentration for all three
gand in fac-[ReN-{(pyridine-2-yl)methyl}propan-1-amine)(CO)3 the reactions at 25.0 °C.
(MeOH)]+ complexes by monodentate entering ligands (de-
noted by X) is given by Equation (1).
The reactions of 6, after dissolving it in methanol to form
the corresponding methanolato complex, with pyridine and
dimethylaminopyridine were followed at four different temper-
atures with the entering ligand in large excess in each case.
The reactions with Br– ions were only followed at 25.0 °C for
comparison. Figure 3 illustrates a typical plot of kobs vs. pyridine
concentration at different temperatures after fitting absorbance
vs. time plots to Equation (1).
Figure 4. Plot of kobs vs. [ligand] for the reaction between 6 and py, DMAP,
(1)
and Br– at 25.0 °C. [6] = 5.00 × 10–4
M, [ligand] = 0.005–0.05 M.
At 25.0 °C, the second-order rate constant, k1, for the reac-
tions of 6 with DMAP and Br– is almost a factor of 3 faster
than the comparative reactions with pyridine [k1(Br–) = 34.8(4);
k1(DMAP) = 28.7(1) and k1(py) = 10.2(1)
M
–1 s–1 respectively, at
25.0 °C]. This could be attributed to the favorable charge–
charge interactions between the Br– ion and the corresponding
cationic complex and the increased basicity of the entering li-
gand DMAP (pKa = 9.80) as opposed to pyridine (pKa = 5.23).
The stability constants, K1, for the formation of fac-[ReN-
{(pyridine-2-yl)methyl}propan-1-amine)(CO)3 (Br)] is in general
higher than that obtained for the corresponding pyridine and
DMAP complexes and is similar to what was found in previous
reports.[19–24]
This data, together with the high negative values obtained
for ΔS‡, suggests that these reactions proceed most likely
through an Ia type mechanism and corresponds to previous
results obtained for similar reactions.[19–24]
Figure 3. Plot of kobs vs. [py] at 5.0, 15.0, 25.0, and 35.0 °C. [6]
5.00 × 10–4
and [py] = 0.005–0.05 , methanol, λ = 317 nm.
=
M
M
The rate and equilibrium constants for the reactions of 6
with pyridine, DMAP, and Br– are reported in Table 2, together
with the activation parameters which were obtained from the
Eyring equation for the reactions with pyridine and DMAP (see
Supporting Information for Eyring plots, Figures S8 and S9). Fig-
Preliminary stability and kinetic studies of fac-[Re4(N,N′-
G1picolyl)(CO)12(OH2)4]4+ (4) and fac-[Re8(N,N′-G2picolyl)(CO)24-
(OH2)8]8+ (5) with py, DMAP, and Br–, after obtaining the corre-
sponding methanol complexes, were performed in dry meth-
anol. All the reactions were found to be too fast for conven-
tional UV/Vis spectroscopic studies and could only be followed
using a stopped-flow apparatus. Under pseudo-first-order con-
ditions, only one reaction was observed for all the reactions
studied, and the rates of these reactions increased systemati-
cally with the increase in ligand concentration.
Table 2. Rate and equilibrium data for the reactions between 6 and py, DMAP,
and Br– and activation parameters for the reactions between 6 and py and
DMAP.
k1
k–1
K1
ΔS‡
ΔH‡
–1
[M
–1 s–1
]
[s–1
]
[
M
]
[J K–1 mol–1 [kJ mol–1
]
If one considers that the rhenium metal centers are removed
from each other by at least ten atoms in these structures, and
considering the symmetry of the dendrimers, it can be rea-
soned that the substitution processes will occur simultaneously
at all the metal centers. This has also been observed in the
substitution reactions involving dinuclear platinum(II) com-
plexes.[26,27] The time-dependent spectra observed for all the
complexes match perfectly to a single exponential fit, and the
resulting kobs vs. ligand concentration data could thus be fitted
to Equation (1) to yield the second-order rate constants. The
data is presented in Table 3. A representative plot of kobs vs.
pyridine concentration for the monomer, 4, and the metallo-
dendrimers, 5 and 6, is presented in Figure 5.
Pyridine
1.80(1)
15.0 °C 3.68(3)
25.0 °C 10.2(1)
35.0 °C 20.4(5)
5.0 °C
0.025(7)
0.0382(1)
0.062(4)
0.24(3)
72(1)
96.3(6)
164(7)
85(3)
–34.1(10)
–79.1(9)
57(3)
40(2)
DMAP
5.0 °C
8.57(7)
0.202(8)
0.287(4)
0.301(3)
0.322(9)
42.4(4)
48.0(9)
95.3(6)
161(9)
15.0 °C 13.81(9)
25.0 °C 28.7(1)
35.0 °C 51.9(4)
Br–
25.0 °C 34.8(4)
0.165(1)
211(2)
Eur. J. Inorg. Chem. 2017, 3919–3927
www.eurjic.org
3923
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Table 3. Rate and equilibrium data for the reactions between 4 and 5 and
and 0.042(7) × 10–3
phen and bipy complexes, indicating a factor of 3000 increase
in rate. This is to be expected considering that aliphatic amines
are strong electron donors.
M
–1 s–1 were obtained for the respective
py, DMAP, and Br– at 25.0 °C.
–1
kn
[M
–1 s–1
]
k–n [s–1
]
Kn
[M
]
Complex 4
Py
19.9(8)
46.0(2)
60.8(2)
0.0749(2)
0.205(2)
0.274(4)
265(9)
224(5)
222(3)
DMAP
Br–
Conclusions
Complex 5
In summary, first- and second-generation poly(N,N-2-picolyl-
amino) DAB-PPI dendritic ligands were synthesized via reduc-
tive amination reactions and fully characterized. Their corre-
sponding rhenium(I) complexes were synthesized from two rhe-
nium precursors to afford tetra- and octanuclear rhenium(I)
metallodendrimers with fac-[Re(CO)3X]n+ (X = Br and n = 0; X =
OH2 and n = 1) peripheral entities. In addition, mononuclear
complexes were prepared as models for comparison to the
larger metallodendrimers. To demonstrate their potential,
kinetic studies showed that the second-order rate constants for
the reactions of the monomeric complex, 3, with various enter-
ing ligands are almost 2 times lower than what was obtained
for similar reactions with its metallodendrimer counterparts.
The reasons for this were ascribed to the fact that the monomer
has a more flexible substituent which might hinder the substi-
tution process and/or that the dendritic architectures are
slightly better electron donors than the corresponding mono-
mer. The preliminary investigation into the activation parame-
ters suggests an Ia type mechanism. The greater significance of
the kinetic results is that the dendritic ligands seem to have a
stronger labilizing effect than the monomer from which it was
derived. This might give an indication of what one can expect
for other macromolecules like proteins when coordinated to
more than one metal center. However, it is also clear that this
work has to be expanded to include more types of ligands and
of course, a deeper investigation of the intimate mechanism
that drives these substitution processes.
Py
22.7(4)
50.7(6)
60.7(2)
0.105(5)
0.255(3)
0.346(7)
216(4)
199(1)
175(6)
DMAP
Br–
Figure 5. Plot of kobs vs. [Br–] for the reaction of 4, 5 and 6 with Br– at 25.0 °C.
From Figure 5 and Table 3, it can be seen that the second-
order constant, k1, increases in each case when moving from
the monomer, 6, to the metallodendrimers, 4 and 5, for each
entering ligand used. Furthermore, the k1 values for 4 and 5
are comparable, with just a slight increase observed for the
higher order metallodendrimer in each case. For example, the
forward rate constants for the pyridine reactions at 25.0 °C in-
crease from 10.2(1) to 19.9(8) and 22.7(4)
M
–1 s–1, respectively,
when comparing the rates of 6 to 4 and 5, respectively. This
may be attributed to the increased inductive effect, by virtue
Experimental Section
of the dendritic arms, which influences the electron density General: All reagents were purchased from Sigma–Aldrich and used
around the ReI metal center. Another reason might be that the
without further purification. 1,4-Diaminobutane poly(propylene
amine) octaamine [DAB-PPI-(NH2)8] was purchased from SyMO-
metallodendrimers are more rigid than the monomer which has
a propyl substituent which could be sterically more hindering
as compared to the dendrimers.
Chem. fac-(Et4N)2[Re(CO)3Br3],[35,36] and ligand L3,[37,38] were pre-
pared using literature procedures. All reactions were performed at
room temperature using standard Schlenk-line techniques, and the
The values obtained for the stability constants, K1, for the
reductive amination reactions were performed under nitrogen gas.
newly formed dendrimer products are in general very similar,
but slightly higher than the monomeric complexes as illustrated
by K1 for the formation products of the reactions of 4 [224(5)
Dichloromethane (CH2Cl2) was dried using the molecular sieves
(Sigma Aldrich, 3 Å beads, 4–8 mesh) and methanol (MeOH) was
dried in the Innovative Technology Swagelok® (PS-Micro, PSM-13–
564) or over calcium hydride. Nuclear magnetic resonance (NMR)
spectra were acquired on a Bruker Ultrashield 400 Plus spectrome-
ter [1H: 399.95 MHz; 13C(1H): 100.60 MHz] at ambient temperature.
All chemical shifts are reported in the standard δ notation of parts
per million using tetramethylsilane (TMS) as an internal standard
and were referenced relative to the signal of corresponding deuter-
ated solvents. Infrared (IR) spectra were determined, either in the
solid state or as Nujol mulls between NaCl discs, with a Perkin–
Elmer Spectrum 100 FTIR spectrometer equipped with a SMART
iTR ATR unit. Melting points were determined on a Reichert-Jung
Thermovar hot-stage microscope and are uncorrected. Elemental
M
–1] and 5 [199(1)
M
–1] with DMAP compared to that of 6
[95.3(6) M
–1]. All these compare well to values obtained for sev-
eral ranges of fac-[Re(CO)3(Bid)X]n type complexes in literature
(Bid = N,N-, N,O- and O,O-bidentate ligands and X = Br–, DMAP,
py).[19–24]
The labilizing effect of the bidentate N,N-[(pyridine-2-
yl)methyl]propan-1-amine ligand in 6 is best illustrated when
comparing the value for the second-order rate constant of
0.165(1)
to that of fac-[Re(CO)3(Bid)(MeOH)] [Bid = 1,10-phenanthroline
(phen)/2,2′-bipydine (bipy)], where values of 0.050(3)
M
–1 s–1, obtained here for the reaction with Br– ions
M
–1 s–1 analyses were performed using Thermo Flash EA 112 Series com-
Eur. J. Inorg. Chem. 2017, 3919–3927
www.eurjic.org
3924 © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
bustion analyzer. For selected dendrimers, the analyses include sol-
vent molecules, which are ascribed to the propensity of the
dendrimers to encapsulate solvent molecules which are detected
1.12 mmol for 3) in MeOH (5.00 mL). The reagents were stirred at
room temp. for 3 h and the product was collected as a precipitate.
1 and 2 were collected as brown precipitates without further purifi-
after the comprehensive washing steps. Electrospray ionization cation. 3 was collected as a white fluffy solid and recrystallized from
(ESI+) mass spectra were recorded on a Waters API Quattro Micro
triple quadrupole mass spectrometer. Electron impact mass spec-
trometry (EI-MS) was carried out on a JEOL GCmateII mass spec-
trometer.
EtOH to afford colorless needles.
fac-[Re(CO)3(L1)Br] (1): Brown powder, yield: 0.169 g, 19.5 %. IR-
ATR: ν = 2014 (carbonyl, C≡ O), 1863 (br., carbonyl, C≡ O), 1609 (s,
˜
1
pyridyl, C=N) cm–1. H NMR [(CD3)2SO]: δ = 1.43–1.99 (br. m, 12 H,
Synthesis of Dendritic Ligands (L1 and L2): A solution of DAB–
G1–PPI-(NH2)4 (0.306 mL, 0.930 mmol for L1) or DAB–G2–PPI-(NH2)8
(0.360 mL, 0.466 mmol for L2) in CH2Cl2 (20.0 mL) was added drop-
wise to a stirred solution of 2-pyridinecarboxyaldehyde (0.355 mL,
3.73 mmol for L1; 0.400 mL, 3.73 mmol for L2) in dichloromethane
(40.0 mL), in the presence of MgSO4 (ca. 20.0 mg). The reaction
mixture was stirred for 24 h at room temperature. The reaction
mixture was filtered by gravity and the solvent of the filtrate re-
moved under vacuum to afford a brown crude oil. The brown oil
was dissolved in methanol (60.0 mL) and stirred for 30 min, whilst
the reaction flask was purged with nitrogen. The crude solution was
subsequently reacted with NaBH4 (0.353 g, 9.33 mmol for L1;
0.656 g, 17.3 mmol for L2) under nitrogen for 24 h. The excess
hydride was quenched with ice-cooled water (20.0 mL) and the
H2O/MeOH mixture was evaporated by rotary evaporation to afford
a yellow suspension. The crude product was extracted with CH2Cl2
(2 × 30.0 mL) and the organic fractions combined. The solvent was
removed under reduced pressure to once again afford a brown oil,
and was further purified using reverse-phase column chromatogra-
phy (H2O/MeOH, 100:0–50:50 %, 0:100 %). The desired organic frac-
tions were combined and dried with anhydrous MgSO4, filtered by
gravity, and the solvent of the filtrate removed under reduced pres-
sure to afford a brown oil.
NCH2 CH2 c o r e , NCH2 CH2 b r a n c h ), 2. 89–3. 23 (br. m, 16 H,
NCH2CH2CH2Nbranch, NCH2CH2CH2core), 4.15, 4.46–5.15 (br. m, 12 H,
Arpyr-CH2N, Arpyr-CH2N, NH), 7.54 (m, 4 H, Arpyr), 7.23 (m, 4 H, Arpyr),
8.09 (m, 4 H, Arpyr), 8.76 (br. dd, 4 H, Arpyr) ppm. 13C{1H} NMR
[(CD3)2SO]: δ = 24.5, 26.1, 50.9–57.2 (NCH2CH2core, 1st branch), 60.2
(Arpyr-CH2N), 123.1, 125.7, 140.2, 153.0 (CHpyr), 160.8 (Cpyr), 192.4,
196.8, 198.2 (C≡ O) ppm. C52H60Br4N10O12Re4 (2081.93): calcd. C
30.01, H 2.91, N 6.73; found C 30.18, H 3.33, N 6.65; MS (HR-ESI-TOF)
m/z calcd. for C52H60Br4N10O12Re4: 1041.8850, found 1041.9767 [M
+ 2H]2+. MP: 148–151 °C.
fac-[Re(CO)3(L2)Br] (2): Brown powder, yield: 0.0248 g, 18.0 %. IR-
ATR: ν = 2014 (carbonyl, C≡ O), 1865 (br., carbonyl, C≡ O), 1610 (s,
˜
pyridyl, C=N) cm–1. 1H NMR [(CD3)2SO]: δ = 1.38–1.86, 3.05–3.39 (br.
signals, 40 H, NCH2CH2core, NCH2CH2CH21st and 2nd branch), 4.07 (br. m,
8 H, NH), 4.42 (m, 8 H, Arpyr-CH2N), 5.08, (br. d, 8 H, Arpyr-CH2N), 7.50
(br. t, 8 H, Arpyr), 7.66 (br. s, 8 H, Arpyr), 8.02 (m, 8 H, Arpyr), 8.71
(m, 8 H, Arpyr) ppm. 13C{1H} NMR [(CD3)2SO]: δ = 25.6, 49.1, 50.8
(NCH2CH2core, 1st and 2nd branch), 60.3 (Arpyr-CH2N), 123.1, 125.7, 140.3,
152.9 (CHpyr), 160.7 (Cpyr), 192.3, 196.7, 198.3 (C≡ O) ppm.
C112H136Br8N22O24Re8 (4303.34): calcd. C 31.26, H 3.19, N 7.16; found
C 31.35, H 3.77, N 7.30; MS (HR-ESI-TOF) m/z calcd. for
C112H136Br8N22O24Re8: 1056.6145, found 1056.5388 [M + 3H – Br]4+
MP: 140–143 °C.
.
L1: Brown oil, yield: 0.190 g, 30.0 %. IR (Nujol between NaCl plates):
ν = 1593 (sharp, pyridyl, C=N), 1377 (sharp, 2° amine, C–N) cm–1. 1H
fac-[Re(CO)3(L3)Br] (3): Colorless needles, yield: 0.321 g, 57.0 %. IR-
˜
ATR: ν = 2012 (carbonyl, C≡ O), 1912 (s, carbonyl, C≡ O), 1875 (s,
˜
NMR [(CD3)2SO]: δ = 1.42 (m, 4 H NCH2CH2CH2 core), 1.70 (br. qn, 8
H, NCH2CH2CH2Nbranch), 2.42–2.52 (overlapping m, 12 H,
N C H 2 C H 2 c o r e , N C H 2 C H 2 C H 2 N b r a n c h ) , 2 . 6 9 ( b r. t , 8 H,
NCH2CH2CH2Nbranch), 2.81 (br. s, 4 H, NH), 3.91 (s, 8 H, Arpyr-CH2NH),
7.17 (m, 4 H, CHpyr), 7.33 (br. d, 3J = 7.80 Hz, 4 H, CHpyr), 7.66 (br. td,
4 H, CHpyr), 8.56 (br. d, 4 H, CHpyr) ppm. 13C{1H} NMR [(CD3)2SO]: δ =
24.4, 26.7, 47.2, 51.7, 53.4 (CH2 core,branch), 54.8 (Arpyr-CH2N), 121.6,
122.7, 136.2, 148.6 (CHpyr), 159.9 (Cpyr) ppm. C40H60N10·2H2O
(717.0200): calcd. C 67.01, H 9.00, N 19.54; found C 66.57, H 8.92, N
19.60; MS (HR-ESI-TOF) m/z calculated for C40H60N10: 341.2580,
found 341.2581 [M + 2H]2+. S25 °C = 0.01 mg/μL in water.
carbonyl, C≡ O), 1672 (s, pyridyl, C=N) cm–1. 1H NMR [(CD3)2SO]: δ =
0.93 (t, 3J = 7.41 Hz, 3 H, CH3CH2CH2N), 1.77 (m, 2 H, CH3CH2CH2N),
2
3
3.01 (m, 2 H, CH3CH2CH2N), 4.13 (dd, J = 15.68, J = 8.72 Hz, 1 H,
Arpyr-CH2N), 4.78 (dd, 2J = 15.72, 3J = 5.08 Hz, 1 H, Arpyr-CH2N), 5.03
(br. m, 1 H, NH), 7.51 (t, 3J = 6.06 Hz, 1 H, Arpyr), 7.71 (d, 3J = 7.84 Hz,
1 H, Arpyr), 8.02 (td, 3J = 7.75, 1.53 Hz, 1 H, Arpyr), 8.74 (d, 3J =
5.52 Hz, 1 H, Arpyr) ppm. 13C{1H} NMR [(CD3)2SO]: δ = 11.0
(CH3CH2CH2N), 21.6 (CH3CH2CH2N), 59.7 (CH3CH2CH2N, Arpyr-CH2N),
122.6, 125.1, 139.6, 152.4 (CHpyr), 160.4 (Cpyr), 191.9, 196.2, 197.7
(C≡ O) ppm. C12H14BrN2O3Re (500.37): calcd. C 28.81, H 2.82, N 5.60;
found C 28.92, H 2.77, N 5.42. MS (HR-ESI-TOF) m/z calcd. for
L2: Dark brown oil, yield: 0.89 g, 44.0 %. IR (Nujol between NaCl
C
12H14BrN2O3Re: 421.0562, found 421.0549 [M – Br]+. MP: 211–
plates): ν = 1592 (sharp, pyridyl, C=N), 1377 (sharp, 2° amine, C–N)
˜
213 °C.
cm–1
.
1H NMR [(CD3)2SO]: δ = 1.28, 1.50, 1.76 (br. m, 36 H,
Synthesis of fac-[Re(CO)3(OH2)]+-Functionalized Complexes (4–
6): fac-(Et4N)2[Re(CO)3Br3] (0.100 g, 0.130 mmol) was dissolved in
water (6.00 mL) and the solution was adjusted to pH 2.2 using
HNO3. AgNO3 (0.066 g, 0.389 mmol) was added to the solution and
the suspension was stirred for 24 h to afford fac-[Re(CO)3(OH2)3]NO3
(0.130 mmol) in solution, this was followed by the removal of AgBr
by filtration. Compound L1 (0.0340 g, 0.0325 mmol for 4) or L2
(0.0363 g, 0.0163 mmol for 5) or L3 (0.0195 g, 0.130 mmol for 6)
was added to the filtrate and the solution stirred for 10 h at room
temp. The product remained in solution and the water was reduced
using a freeze drier to afford a colorless or brown suspension.
NCH2CH2core, NCH2CH21st branch, NCH2CH2CH2NH2nd branch), 2.25, 2.34,
2.63 (m, 53 H, NCH2CH22nd branch, NCH2CH2core, NCH2CH2CH2N1st
branch, NH, CH2CH2N2nd branch), 3.74 (s, 16 H, Arpyr-CH2N), 7.22 (m, 8
H, Arpyr), 7.37 (m, 8 H, Arpyr), 7.67 (m, 8 H, Arpyr), 8.48 (m, 8 H, Arpyr
)
ppm. 13C{1H} NMR [(CD3)2SO]: δ = 20.4, 24.2, 27.5, 47.8, 51.5, 51.7
(NCH2CH2core,1st and 2nd branch), 51.9 (Arpyr-CH2N), 121.9, 122.8, 134.4,
148.6 (CHpyr), 160.1 (Cpyr) ppm. C88H136N22·CH2Cl2 (1587.1370): calcd.
C 67.35, H 8.76, N 19.42; found C 66.91, H 8.76, N 19.18. MS (HR-
ESI-TOF) m/z calculated for C88H136N22: 376.2908, found 376.2898
[M + 4H]4+. S25 °C = 0.01 mg/μL in water.
Synthesis of fac-[Re(CO)3Br]-Functionalized Complexes (1–3):
fac-(Et4N)2[Re(CO)3Br3] (0.765 g, 0.993 mmol for 1; 0.0980 g,
0.128 mmol for 2; 0.862 g, 1.12 mmol for 3) was dissolved in MeOH
(5.00 mL) and added to a stirring solution of either L1 (0.169 g,
fac-[Re(CO)3(L1)(OH2)]+ (4): Light-brown suspension, yield: 0.107 g.
IR-ATR: ν = 2020 (carbonyl, C≡ O), 1881 (s, carbonyl, C≡ O), 1644 (s,
˜
pyridyl, C=N) cm–1 1H NMR [(CD3)2SO]: δ = 1.71 (br. signal, 4 H,
.
0.248 mmol for 1) or L2 (0.0480 g, 0.0320 mmol for 2) or L3 (0.168 g, NCH2CH2core), 2.09 (br. signal, 8 H, NCH2CH2branch), 2.83–3.13 (br. m,
Eur. J. Inorg. Chem. 2017, 3919–3927
www.eurjic.org
3925 © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
16 H, NCH2CH2branch, CH2CH2NHbranch), 4.40, 4.94–4.99 (br. signal, 16 individual points, denoted by selected symbols. Activation parame-
H, NCH2CH2core, Arpyr-CH2N, NH), 7.45–7.53 (m, 8 H, 5-H, 3-H), 7.93
(m, 4 H, 4-H), 8.67 (m, 4 H, 6-H) ppm. C52H68N10O16Re4·5HNO3
(2149.06): calcd. C 29.06, H 3.42 N 9.78; found C 29.39, H 3.13, N
9.60; MS (HR-ESI-TOF) m/z calcd. for C52H60N10O14Re4: 898.6299,
found 898.7490 [M + 2H]2+. UHPLC (CH3CN/H2O, gradient, 20:80 %
– 70:30 %, flow rate 1.1 mL/min): tR = 6.23 min.
ters were determined from Eyring plots.
Acknowledgments
The authors would like to thank the Nuclear Technologies in
Medicine and the Biosciences Initiative (NTeMBI), a national
technology platform developed and managed by the South Af-
rican Nuclear Energy Corporation (NECSA) and funded by the
Department of Science and Technology, South Africa. Financial
support from the University of Cape Town (UCT), the University
of the Free State, the National Research Foundation (NRF) of
South Africa, and South African Nuclear Energy Corporation
(NECSA) is gratefully acknowledged and thanked for several
short term exchange visits between the various laboratories.
fac-[Re(CO)3(L2)(OH2)]+ (5): Brown suspension. IR-ATR: ν =
˜
2015 (carbonyl, C≡ O), 1863 (s, carbonyl, C≡ O), 1621 (s, pyridyl, C=
N) cm–1. C112H152N22O32Re8·15HNO3·Et4N (4883.66): calcd. C 29.90,
H 3.89, N 10.75; found C 29.54, H 3.43, N 10.54. MS (HR-ESI-TOF)
m/z calcd. for C112H152N22O32Re8: 460.9781, found 461.0756 [M +
30H2O + 2Et4N]10+
.
fac-[Re(CO)3(L3)(OH2)]+ (6): Colorless suspension, yield: 0.674 g. IR-
ATR: ν = 2022 (carbonyl, C≡ O), 1891 (s, carbonyl, C≡ O), 1624 (s,
˜
1
3
pyridyl, C=N) cm–1. H NMR [(CD3)2SO]: δ = 0.83 (t, J = 7.46 Hz, 3
3
H, CH3CH2CH2N), 1.59 (m, 2 H, CH3CH2CH2N), 2.94 (t, J = 7.71 Hz,
Keywords: Rhenium · Dendrimers · Kinetics · Ligand effects
2 H, CH3CH2CH2N), 4.26 (br. m, 2 H, Arpyr-CH2N), 7.47–7.55 (m, 2 H,
H5, Arpyr), 7.96 (td, 3J = 7.77, 1.73 Hz, 1 H, Arpyr), 8.52 (d, 3J = 5.13 Hz,
1 H, Arpyr) ppm. C12H16N2O4Re (438.48): calcd. C 32.87, H 3.86, N
6.39; Found C, 32.23; H, 3.45; N, 7.00. MS (HR-ESI-TOF) m/z calcd. for
C12H16N2O4Re: 421.0562, found 421.0560 [M – H2O]+.
[1] P. Govender, B. Therrien, G. S. Smith, Eur. J. Inorg. Chem. 2012, 2853–
2862.
[2] S. Langereis, A. Dirksen, T. M. Hackeng, M. H. P. van Genderen, E. W.
Meijer, New J. Chem. 2007, 31, 1152–1160.
[3] Z.-X. Liu, Y. Feng, Z.-Y. Zhao, Z.-C. Yan, Y.-M. He, X.-J. Luo, C.-Y. Liu, Q.-H.
Fan, Chem. Eur. J. 2014, 20, 533–541.
X-ray Crystallography
Diffraction data was collected on a Bruker ApexII 4 K CCD diffrac-
tometer using Mo-Kα (0.71073 Å) and ω-scans at 100 K. All reflec-
tions were merged and integrated with SAINT-PLUS[39] and cor-
rected for Lorentz, polarization, and absorption effects using SAD-
ABS.[40] Both structures were solved by the heavy atom method and
refined through full-matrix least-squares cycles using SHELX-97[41]
as part of the WinGX[42] package with Σ(||Fo| – |Fc||)2 being mini-
mized. All non-H atoms were refined with anisotropic displacement
parameters, while hydrogen atoms were constrained to parent atom
sites using a riding model {aromatic C–H = 0.95 Å [Uiso(H) = 1.2Ueq];
aliphatic C–H = 0.98 Å [Uiso(H) = 1.5Ueq]}. The graphics were ob-
tained with the visual crystal structure information system software
DIAMOND.
[4] D. F. Baban, L. W. Seymour, Adv. Drug Delivery Rev. 1998, 34, 109–119.
[5] D. Astruc, E. Boisselier, C. Ornelas, Chem. Rev. 2010, 110, 1857–1959.
[6] C. Ghobril, G. Lamanna, M. Kueny-Stotz, A. Garofalo, C. Billotey, D. Felder-
Flesch, New J. Chem. 2012, 36, 310–323.
[7] H. B. Agashe, A. K. Babbar, S. Jain, R. K. Sharma, A. K. Mishra, A. Asthana,
M. Garg, T. Dutta, N. K. Jain, Nanomed.: Nanotechnol. Biol. Med. 2007, 3,
120–127.
[8] M. C. Parrott, S. R. Benhabbour, C. Saab, J. A. Lemon, S. Parker, J. F. Valli-
ant, A. Adronov, J. Am. Chem. Soc. 2009, 131, 2906–2916.
[9] S. Hecht, J. M. J. Frechet, Angew. Chem. Int. Ed. 2001, 40, 74–91; Angew.
Chem. 2001, 113, 76.
[10] Y. Zhang, X. Xu, Y. Sun, Y. M. Shen, Chin. J. Chem. 2010, 28, 2447–2450.
[11] A. J. North, D. J. Hayne, C. Schieber, K. Price, A. R. White, P. J. Crouch, A.
Rigopoulos, G. J. O'Keefe, H. Tochon-Danguy, A. M. Scott, J. M. White, U.
Ackermann, P. S. Donnelly, Inorg. Chem. 2015, 54, 9594–9610.
[12] P. J. Blower, Dalton Trans. 2015, 44, 4819–4844.
[13] R. Alberto, R. Schibli, W. R. U. Abram, A. P. Schubiger, Coord. Chem. Rev.
1999, 190–192, 901–919.
CCDC 1472296 (for 3) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre.
[14] R. Alberto, J. Organomet. Chem. 2007, 692, 1179–1186.
[15] R. Alberto, H. Braband, H. W. P. N′Dongo, Curr. Radiopharm. 2009, 2, 254–
267.
Kinetic Studies
The kinetic measurements were initially performed on a Varian Cary
50 Conc UV/Vis spectrophotometer equipped with a Julabo F12-
mV temperature cell regulator (accurate within 0.1 °C). The kinetic
reactions of coordinated methanol substitution were observed to
occur rapidly under UV/Vis. Thus, rapid kinetic reactions (reaction
half-lives shorter than 20 seconds) were monitored at four different
temperatures 5.0, 15.0, 25.0, and 35.0 °C (accurate within 0.1 °C)
on Kinet AsystTM Hi-Tech Scientific M300 SHU-615X2 Stopped Flow
Spectrophotometer attached to a Julabo MPV thermostat water
bath (accurate within 0.1 °C). The third generation stopped-flow
system has a thermostatic sample handling unit and can be oper-
ated in the diode-array mode with a dead time < 5.00 microsec-
onds, yielding 400 nm spectral width scans at < 5.00 microseconds
per complete scan.
[16] R. Alberto, R. Schibli, A. Egli, P. A. Schubiger, U. Abram, T. A. Kaden, J.
Am. Chem. Soc. 1998, 120, 7987–7988.
[17] T. R. Hayes, B. B. Kasten, C. L. Barnes, P. D. Benny, Dalton Trans. 2014, 43,
6998–7001.
[18] S. Mundwiler, M. Kundig, K. Ortner, R. Alberto, Dalton Trans. 2004, 1320–
1328.
[19] A. Brink, H. G. Visser, A. Roodt, J. Coord. Chem. 2011, 64, 122–133.
[20] A. Brink, H. G. Visser, A. Roodt, Inorg. Chem. 2013, 52, 8950–8961.
[21] A. Brink, H. G. Visser, A. Roodt, Polyhedron 2013, 52, 416–423.
[22] A. Brink, H. G. Visser, A. Roodt, Inorg. Chem. 2014, 53, 12480–12488.
[23] M. Schutte, G. Kemp, H. G. Visser, A. Roodt, Inorg. Chem. 2011, 50, 12486–
12498.
[24] M. Schutte, A. Roodt, H. G. Visser, Inorg. Chem. 2012, 51, 11996–12006.
[25] R. Alberto, R. Schibli, A. Egli, P. A. Schubiger, W. A. Herrmann, G. Artus,
U. Abram, T. A. Kaden, J. Organomet. Chem. 1995, 493, 119–127.
[26] S. Hochreuther, R. Puchta, R. van Eldik, Inorg. Chem. 2011, 50, 12747–
12761.
[27] M. S. Davies, J. W. Cox, S. J. Berners-Price, W. Barklage, Y. Qu, N. Farrel,
Inorg. Chem. 2000, 39, 1710–1715.
[28] A. Egli, K. Hegetschweiler, R. Alberto, U. Abram, R. Schibli, R. Hedinger, V.
Gramlich, R. Kissner, P. A. Schubiger, Organometallics 1997, 16, 1833–
1840.
All the kinetic runs were performed under pseudo-first-order condi-
tions with the ligand in large excess in each case. Least-squares
analyses were performed on the absorbance vs. time data obtained
from the kinetics runs to appropriate functions using MicroMath
Scientist. The solid lines in the figures represent computer least-
squares fits of data, while experimental values are represented as
Eur. J. Inorg. Chem. 2017, 3919–3927
www.eurjic.org
3926
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[29] N. Lazarova, S. James, J. Babich, J. Zubieta, Inorg. Chem. Commun. 2004,
7, 1023–1026.
[36] R. Alberto, R. Schibli, P. A. Schubiger, U. Abram, T. A. Kaden, Polyhedron
1996, 15, 1079–1089.
[30] M. Cohn, T. R. J. Hughes, J. Biol. Chem. 1962, 237, 176–181.
[31] H. M. MacConnell, J. Chem. Phys. 1958, 28, 430.
[32] M. Prudencio, J. Rohovec, J. A. Peters, E. Tocheva, M. J. Boulanger, M. E. P.
Murphy, H.-J. Hupkes, W. Kosters, A. Impagliazzo, M. Ubbink, Chem. Eur.
J. 2004, 10, 3252–3260.
[33] P. V. Grundler, L. Helm, R. Alberto, A. E. Merbach, Inorg. Chem. 2006, 45,
10378–10390.
[34] J. F. G. A. Jansen, E. M. E. de Brabander-van den Berg, E. W. Meijer, Science
1994, 266, 1226–1269.
[37] D. Ma, F. Lu, T. Overstreet, D. E. Milenic, M. W. Brechbiel, Nucl. Med. Biol.
2002, 29, 91–105.
[38] P. Bandyopadhyay, S. Jha, S. K. I. Ali, J. Agric. Food Chem. 2009, 57, 9780–
9786.
[39] G. M. Sheldrick in SAINT, Bruker AXS Inc., Madison, WI, USA, 2006.
[40] G. M. Sheldrick in SADABS, University of Göttingen, Germany, 1997.
[41] G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
[42] L. J. Farrugia, J. Appl. Crystallogr. 1997, 30, 565.
[35] R. Alberto, A. Egli, U. Abram, K. Hegetschweiler, V. Gramlich, P. A. Schubi-
ger, J. Chem. Soc., Dalton Trans. 1994, 2815–2820.
Received: June 5, 2017
Eur. J. Inorg. Chem. 2017, 3919–3927
www.eurjic.org
3927
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim