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Inorganic Chemistry
ARTICLE
Scheme 2. Covalent Attachment of the [Mn(CO)3
(tpm-L1)]þ Complex to the Surface of the (3-Azidoproply)-
Functionalized Silicium Dioxide Nanoparticles via the
Copper-Catalyzed AzideꢀAlkyne 1,3-Dipolar Cycloaddition
(CuAAC)
Figure 2. Overlay of the IR spectra of unfunctionalized (black) and
3-azidopropyl- (gray) and [Mn(CO)3(tpm-L1)]þ-functionalized (light
gray) silicium dioxide nanoparticles. (Inset) Region between 1800 and
2200 cmꢀ1 with the characteristic bands of the azido (2099 cmꢀ1) and
Mn(CO)3 (1958 and 2050 cmꢀ1) groups.
This is evident from the IR spectrum of the modified
nanoparticles which in addition to the two CtO vibrational
bands characteristic of the Mn(CO)3 moiety of the CORM at
1958 and 2050 cmꢀ1 still shows the azido band at 2099 cmꢀ1
(Figure 2), although of reduced intensity. In the 13C CP-MAS
NMR spectra, no additional signals could be detected compared
to the 3-azidopropyl-functionalized particles, probably due to
low loading and insufficient sensitivity of this method. However,
the CP-MAS 1H NMR spectrum shows a sharp signal at δ = 1.84
ppm and a broad peak at around 6ꢀ9 ppm due to the methylene
group and the three pyrazol protons, respectively. The latter,
however, could not be resolved to individual peaks (Figure S5,
Supporting Information). A TEM micrograph of the “clicked”
nanoparticles is shown in Figure 1C. Compared to the 3-azido-
propyl-functionalized nanoparticles, the CORM-functionalized
particles have the same shape and size distribution, demonstrat-
ing that the particles are stable under the reaction conditions of
the CuAAC (Figure 3A). The EDX spectrum (Figure 1D)
displays an additional peak at 5.9 keV corresponding to the
manganese of the [Mn(CO)3(tpm-L1)]þ moiety coupled to the
surface via a triazole linkage. In addition, the metal-functionalized
nanoparticles retain the yellow color of the parent complex even
after extended washing, while the nonmodified particles are
white (Figure 3B). The CHN and manganese content of the
functionalized nanoparticles was determined with an elemental
analyzer or atomic absorption spectroscopy (AAS), respectively.
Samples were found to contain 0.50% (w/w) of manganese and
2.47% (w/w) of nitrogen. This converts to a content of 0.09 mmol
of manganese per1 g of nanoparticle and 1.76mmol/g of nitrogen.
If one assumes that all manganese is exclusively present in the form
of [Mn(CO)3(tpm)]þ, then 0.54 mmol/g of the total nitrogen
should be due to the tpm ligands, leaving 1.22 mmol/g of nitrogen.
If one assumes that this is all in the form of azide or triazole, there
should be 0.41 mmol/g of these functional groups on the
nanoparticle. Thus, the degree of surface functionalization with
the [Mn(CO)3(tpm)]þ moiety can be estimated asapproximately
1C). The EDX spectrum of the 3-azidopropyl-functionalized
nanoparticles reflects the expected composition of the nanopar-
ticles and shows the characteristic peaks for silicium and oxygen
at 1.78 and 0.55 keV, respectively (Figure 1B).
The IR spectrum of the 3-azidopropyl-functionalized nano-
particles shows an additional band at 2099 cmꢀ1 compared to the
unfunctionalized particles (Figure 2), which indicates the pre-
sence of azido groups at the surface.
1
In the CP-MAS H NMR, a reference sample of silicium
dioxide nanoparticles capped with hexamethyldisilazane but
without the 3-azidopropyl functionalization shows two signals
at ꢀ0.11 and 3.65 ppm due to the SiCH3 and SiOCH3 groups,
respectively (Figure S1, Supporting Information). The corre-
sponding CP-MAS 13C NMR shows two bands at ꢀ6.79 and
46.18 ppm. These are also due to the SiCH3 and SiOCH3 groups
(Figure S2, Supporting Information). The 3-azidopropyl-func-
tionalized nanoparticles, in the CP-MAS 1H NMR, additionally
show small shoulders at about 2.26 and 3.42 ppm corresponding
to two of the three methylene groups of the 3-azidopropyl group
in addition to the main SiCH3 and SiOCH3 peaks (Figure S3,
Supporting Information). The third CH2 signal could not be
detected and is probably covered by the intense broad signal
at ꢀ0.31 ppm of the SiCH3 peak. Their CP-MAS 13C NMR
spectra, in addition to the signals at ꢀ6.60 and 46.26 ppm of
SiCH3 and SiOCH3 moieties also visible in the spectra of the
unmodified nanoparticles, display extra signals of the three
methylene groups from the propyl linker at 1.85, 7.89, and
19.79 ppm (Figure S4, Supporting Information).32
[Mn(CO)3(tpm-L1)]PF6-Functionalized Nanoparticles. To
couple the PhotoCORM [Mn(CO)3(tpm)]PF6 to the surface of
the azido-functionalized nanoparticles, a derivative of the parent
complex with an alkyne-substituted tpm ligand was utilized.19
The copper-catalyzed azideꢀalkyne 1,3-dipolar cycloaddition
(CuAAC) was performed in a 1:1:1 ethanol/N,N-dimethylform-
amide/water mixture using an excess of the metal carbonyl
complex (Scheme 2). Even at rather long reaction times (96 h),
the CuAAC reaction at the particle surface could not be brought to
completeness, probably due to the inaccessibility of some surface
site on the nanoparticles to the catalyst and/or CORM.
22% (0.09/0.41 100). Although there are other sources of nitro-
3
gen around during the synthesis which might remain in the sample
inunknownquantities, thisresult isinsurprisinglygood agreement
with the decrease of the intensity of the azide bandat2099 cmꢀ1 in
the IR spectra of the azide vs the “clicked” nanoparticles (inset in
Figure 2).
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dx.doi.org/10.1021/ic1024197 |Inorg. Chem. 2011, 50, 4362–4367