Metallocorrole dendrimers: Sensitive corrole-chromium(V)-nitride spin probes for studying the solution structure of dendrimers

Article abstract of DOI:10.1021/ic200739y

The corrole-chromium(V)-nitrido moiety is introduced as a uniquely sensitive EPR spin probe. We describe a series of corrole-centered poly(amidoamine) (PAMAM) dendrimers and the selective incorporation of the chromium(V)-nitrido moiety. The chromium-corro

Full text of DOI:10.1021/ic200739y

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Metallocorrole Dendrimers: Sensitive CorroleꢀChromium(V)ꢀNitride
Spin Probes for Studying the Solution Structure of Dendrimers
Michael Pittelkow, Theis Brock-Nannestad, Jesper Bendix,* and Jørn B. Christensen*
Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark
S Supporting Information
b
Scheme 1. Synthesis of Corrole-Centered PAMAM Dendrimers
and Metal Insertion Protocol, Followed by Nitrogen Atom
Transfer To Give the Chromium(V)ꢀNitrido Complexes
ABSTRACT: The corroleꢀchromium(V)ꢀnitrido moiety
is introduced as a uniquely sensitive EPR spin probe. We
describe a series of corrole-centered poly(amidoamine)
(PAMAM) dendrimers and the selective incorporation of
the chromium(V)ꢀnitrido moiety. The chromiumꢀcorrole
cores are reactive toward both neutral and charged reagents,
and the accessibility of the dendrimer cores enables easy
manipulation of the spin probe. The spin probe reveals a
pronounced solvent dependence of the solution-phase
structure of the dendrimers.
he three-dimensional structure of dendrimers makes them
T
promising models for biological macromolecules such as
enzymes and proteins.1 The use of dendrimers as models for
biological systems encourages investigations into their structure
in solution,4 and the design, synthesis, and characterization of
new dendrimers incorporating functionalities capable of report-
ing subtle changes in the environment present an important
challenge.2ꢀ6 Electron paramagnetic resonance (EPR) spin
probes have been used extensively in the study of biomacromo-
lecule structure and dynamics.7 EPR spin probes have been
applied to investigate the structures of dendrimers using various
organic radicals, but the incorporation of a sensitive spin probe at
the center of dendrimers would enable a direct study of their
solution phase properties.8
acid in CH2Cl2) and amide bond formation via coupling with
PyBOP gave the dendrimers 4ꢀ6 and the model tripropylamide 3.
The resulting dendrimers 4ꢀ6 possess a corrole core, a PAMAM-
type interior, and benzyl esters at the periphery (Figure 1).
The corroles were converted into the corresponding
chromium(III) complexes by reaction with labile [CrCl3-
(THF)3] (THF = tetrahydrofuran) in N,N-dimethylformamide
(DMF) in a microwave reactor. It should be noted that failure to
strictly observe anaerobic conditions led to the formation of
chromium(V)ꢀoxo species, clearly demonstrating the prefer-
ence for high oxidation states in corrole chemistry, as shown in
the work of Gross et al.17 The chromium(III) complexes were
transformed into the corresponding chromium(V)ꢀnitrido
complexes by treatment with MnV(N)salen in DMF/hexafluor-
oisopropyl alcohol.18 This type of nitrogen atom transfer has
been investigated mechanistically by Groves and Takahashi,19a
Neely and Bottemley,19b Woo and Goll,19c and Gray et al.19d
Notably, the reaction has been shown unambiguously to be
inner-sphere,19b requiring a nitride-bridged intermediate, and to
be reversible.19d
In recent work, we showed that the chromium(V)ꢀnitrido
unit can function as a uniquely sensitive spectroscopic spin probe
for reactivity toward electrophiles.9 Here we demonstrate how
corroles can be used as a highly selective site for the preparation
of chromium(V) nitrides. We incorporate chromium(V) nitrides
inside large poly(amidoamine) (PAMAM) dendrimers and show
how this can be used to probe the solution-phase behavior of
the dendrimer. Metallocorroles10 have received attention as
catalysts for epoxidation,11 atom transfer,12 water oxidation,13 and
aziridination.14
A series of corrole-centered PAMAM dendrimers was synthe-
sized following a convergent synthesis approach similar to our
previously reported syntheses of PAMAM dendrimers (Scheme 1);
a triphenylcorrole functionalized with three carboxylic acids was
utilized as the core of the dendrimer.15 The corrole trimethylester
(1) was conveniently prepared according to the procedure of
Paolesse and co-workers, with some modifications in the purifica-
tion procedure, enabling routine isolation of 2 g batches. The triacid
(2) was obtained by means of basic hydrolysis of the triester.16 tert-
Butyloxycarbonyl deprotection of the dendrons (25% trifluoroacetic
Received: April 11, 2011
Published: June 02, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic200739y Inorg. Chem. 2011, 50, 5867–5869
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Inorganic Chemistry
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Figure 1. Structure of the PAMAM dendron arms exemplified by
dendron-8 of compound 6.
The resulting chromium(V)ꢀnitrido complexes and dendri-
mers (7 and 8ꢀ10, respectively) were isolated using preparative-
scale size-exclusion chromatography in CH2Cl2. All corrole-
centered dendrimers were analyzed by UV/vis spectroscopy
and matrix-assisted laser desorption ionization time-of-flight
(MALDI-TOF) mass spectrometry. In addition, the metal-free
systems were characterized by NMR spectroscopy and the
metalated systems by EPR spectroscopy.
The chromium(V)ꢀnitrido-functionalized dendrimers (8ꢀ10)
show insignificant variation in the Soret band energies with the
dendrimer size. However, contrary to the uncomplexed corrole
dendrimers, the bandwidths are constant across the three genera-
tions (Supporting Information). It is noteworthy that all of the
corrole amides exhibit significant photostability in solution, as
demonstrated by following the change in absorbance of the Soret
band over time upon leaving solutions of the corroles under
ambient laboratory light and ambient atmosphere. We note that
the addition of only one carboxy substituent per meso-phenyl
substituent is sufficient to achieve this photostability.20
Figure 2. Top: Nitride functionalization with trityl. Bottom: EPR
spectra of (a) corrole triester 1 coordinating CrV(N) and (b) CrV(N-trt)
[trt = C(C6H5)3]. The top trace is the derivative of trace (a) which
facilitates the counting of the 11 superhyperfine components.
the increase in complexity and splitting of the spectrum over that
of the simple nitride precursor (Figure 2a). The value of A-
(14Naxial) approximately doubles upon functionalization. Similar
increases in the nitrogen superhyperfine coupling were found for
nitrido-bridged complexes formed by reaction with [Pt(C2H4)Cl3]ꢀ
or [Rh(COD)Cl]2; the tripropylamide-substituted corrole (3)
behaved analogously (Supporting Information).
In the EPR spectra of the chromium(V)ꢀnitrido dendrimers,
on the other hand, only very poorly structured bands around g =
2 without resolved superhyperfine couplings were observed
(Figure 3aꢀc). The lacking resolution of the superhyperfine
couplings for the dendrimers can be attributed to a combination
of additional spinꢀspin interactions from the hydrogen atoms of
the dendron arms and incomplete averaging of the anisotropic
couplings due to intermediate tumbling rates on the EPR time
scale. Impþortantly, also for the dendrimers, the reaction with
C(C6H5)3 resulted in a large increase in the superhyperfine
coupling and a concomitant marked increase in the resolution of
the dendrimer EPR spectra, as illustrated in Figure 3dꢀf. The effect is
similar for all three generations of dendrimers. Attempts to react the
dendrimers with [Pt(C2H4)Cl3]ꢀ or [Rh(COD)Cl]2 in excess did
not have an effect on the EPR spectra on the time scale of hours. In
the functionalizations with C(C6H5)3þ, the reactions were complete
upon mixing for the corroles but slower and requiring minutes for
completion (at ca. 10ꢀ4ꢀ10ꢀ5 M solutions) for the higher genera-
tions of dendrimers.
As d1 systems, chromium(V)ꢀnitrido complexes generally
yield fairly narrow solution-phase EPR spectra with g ≈ 2.21
Furthermore, superhyperfine coupling of the electron spin to the
nuclear spins of the ligating nitrogen donors is expected to yield a
splitting of the signal. If the superhyperfine coupling is near
identical for all of the ligating nitrogen atoms, as is frequently
found experimentally,19 the splitting will result in 2Itot þ 1 = 11
equidistant lines for five equivalently coupled nitrogen donors
[I(14N) = 1], as in the chromium(V)ꢀnitrido-functionalized
corroles. Weak satellites from coupling to 53Cr (9.5%, I = 3/2) are
detectable for small molecules.22 Solution-phase EPR spectra of
the simple chromium(V)ꢀnitrido-functionalized corrole ester
(1) and amide (3) as well as of the chromium(V)ꢀoxo corrole
ester (11), which forms by air oxidation of the chromium(III)
complex, exhibit a resonance with g ≈ 1.98 split into the expected
number of lines (11, 11, and 9, respectively) by superhyperfine
couplings to ligating nitrogen atoms (Supporting Information).
The coupling constants fall within 10% of those observed for
related chromium(V) complexes.3,22
The chromium(V)ꢀnitridoꢀcorrole dendrimers were stu-
died in pure CH3NO2 and in MeNO2/DMF mixtures. In pure
CH3NO2, the spectra were similar (slightly broader; Figure 3aꢀc)
to the spectra obtained for complexes of simple corroles
(Figure 2a).
In recent work, we found that functionalization of chromium-
(V)ꢀnitrido complexes, either by conversion to imides or by
nitrido bridge formation to low-valent platinum metal ions,
results in dramatic, counterintuitive, increases in superhyperfine
couplings.9 This result also applies to the chromium(V)ꢀ
nitrido-complexed corrole ester (1), when compared to the
tritylated species CrV(N-trt), as shown in Figure 2b, illustrating
The sharp EPR signals obtained for all generations of den-
drimers in CH3NO2 indicate a fast tumbling of the dendrimers
on the EPR time scale.23 Addition of a small amount of DMF
caused a significant change in the EPR spectra. This is illustrated
in the upper part of Figure 3, where EPR spectra of the middle-
sized chromium(V)ꢀnitridoꢀcorrole dendrimer (9) is shown in
mixtures of the two solvents. Significant freezing of the spectra
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dx.doi.org/10.1021/ic200739y |Inorg. Chem. 2011, 50, 5867–5869

Inorganic Chemistry
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: bendix@kiku.dk (J.B.), jbc@kiku.dk (J.B.C.).
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’ ASSOCIATED CONTENT
S
Supporting Information. General experimental proce-
b
dures, UV/vis spectra, and EPR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
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