Nitrene-functionalized ruthenium nanoparticles

Article abstract of DOI:10.1039/c2jm33783e

Ruthenium nanoparticles protected by ruthenium-nitrene π bonds were prepared by refluxing bare ruthenium colloids (2.12 ± 0.72 nm in diameter) and 4-dodecylbenezenesulfonyl azide in sec-butylbenzene. Thermogravimetric analysis (TGA) of the resulting nanoparticles showed that on average there were about 84.1 ligands on the nanoparticle surface. XPS studies showed a 1:1 atomic ratio between nitrogen and sulfur, consistent with the formation of nitrene fragments by the thermal decomposition of the azide precursors. In addition, the binding energies of Ru3d and N1s electrons suggested a covalent nature of the RuN interfacial linkage which appeared in FTIR measurements with a vibrational band at 1246 cm^-1. Because of such conjugated bonding interactions, extensive intraparticle charge delocalization occurred, and the nanoparticle-bound nitrene moieties behaved analogously to azo derivatives, as manifested in UV-vis and fluorescence measurements. Further testimony of the formation of RuN interfacial linkages was highlighted in the unique reactivity of the nanoparticles with alkenes by imido transfer, which was evidenced in spectroscopic and electrochemical studies. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2jm33783e

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PAPER
Nitrene-functionalized ruthenium nanoparticles†
Xiongwu Kang, Yang Song and Shaowei Chen*
Received 12th June 2012, Accepted 27th July 2012
DOI: 10.1039/c2jm33783e
Ruthenium nanoparticles protected by ruthenium–nitrene p bonds were prepared by refluxing ‘‘bare’’
ruthenium colloids (2.12 ꢀ 0.72 nm in diameter) and 4-dodecylbenezenesulfonyl azide in sec-
butylbenzene. Thermogravimetric analysis (TGA) of the resulting nanoparticles showed that on average
there were about 84.1 ligands on the nanoparticle surface. XPS studies showed a 1 : 1 atomic ratio
between nitrogen and sulfur, consistent with the formation of nitrene fragments by the thermal
decomposition of the azide precursors. In addition, the binding energies of Ru3d and N1s electrons
suggested a covalent nature of the Ru]N interfacial linkage which appeared in FTIR measurements with
ꢁ
1
a vibrational band at 1246 cm . Because of such conjugated bonding interactions, extensive intraparticle
charge delocalization occurred, and the nanoparticle-bound nitrene moieties behaved analogously to azo
derivatives, as manifested in UV-vis and fluorescence measurements. Further testimony of the formation
of Ru]N interfacial linkages was highlighted in the unique reactivity of the nanoparticles with alkenes by
imido transfer, which was evidenced in spectroscopic and electrochemical studies.
Introduction
dimeric counterparts with a conjugated spacer, thanks to the
conducting metal cores.
Organically capped metal nanoparticles have been attracting
intensive research interest from both fundamental and techno-
logical perspectives, largely because these nanomaterials show
unique optical and electronic properties that can be readily
regulated by the chemical nature of the core metals, the organic
ligands as well as the metal–ligand interfacial bonding interac-
Within this experimental context, one may ask, is it possible to
extend the metal–ligand interfacial bonding interactions to other
functional moieties? One intriguing candidate is nitrene deriva-
tives. Note that metal–nitrene (M]N) bonds have been
proposed and/or recognized in a variety of organometallic
complexes that are involved in bond-activation reactions (e.g.,
C–H bond activation, nitrene-exchange or -transfer reactions,
1
tions. Among these, the last factor has been the central subject
of a series of recent studies, with nanoparticles passivated by
etc.), where the unique reactivity is attributed to the high
9–11
polarizability of the M]N bonds.
produced effectively by thermal or photo-decomposition of the
2–8
metal–carbon covalent linkages. Note that whereas mercapto
derivatives have been used as the ligands of choice for surface
passivation of metal nanoparticles because of the strong affinity
of thiol groups to transition metals, recent studies have shown
that other metal–ligand bonding chemistry may be exploited for
further and more deliberate functionalization of the nano-
particles, leading to the emergence of unprecedented optical and
The nitrene moieties can be
12–15
corresponding azide compounds.
For instance, Leinonen
15
et al. reported that p-toluenesulfonyl, methylsulfonyl and tri-
methylsilyl nitrene might be derived by microwave thermal
activation from the corresponding azides and covalently grafted
onto the surface of single-walled carbon nanotubes. Further-
more, it was found that whereas an appreciable amount of sulfur
dioxide might be produced in the decomposition of aliphatic
sulfonyl azides, very little is observed with aryl sulfonyl azides
which typically decompose in a clean, first-order reaction. This
suggests that the stability of the nitrene radicals might be
2–8
electronic properties. For instance, intraparticle charge delo-
calization has been observed with functional groups bound onto
ruthenium nanoparticle surfaces by ruthenium–carbene (Ru]C)
p bonds, ruthenium–acetylide (Ru–C^), or ruthenium–vinyli-
3,7,8
dene (Ru]C]CH–) linkages,
where the electrochemical and
spectroscopic characteristics are analogous to those of their
12
markedly enhanced by using aryl precursors. In metal–azido
complexes, thermal- or light-induced decomposition has also
been exploited to produce metal–nitrene bonds. For instance,
16
Meyer and co-workers reported that controlled decomposition
0
Department of Chemistry and Biochemistry, University of California, 1156
High Street, Santa Cruz, California 95064, USA. E-mail: shaowei@ucsc.
edu; Fax: +1 (831) 459-2935; Tel: +1 (831) 459-5841
of ruthenium(III) azido(bis-2,2 -bipyridine) complexes led to the
formation of the corresponding ruthenium–nitrene compounds.
It is generally believed that the nitrene group is stabilized by the
dp (metal) / pp (nitrene) back-bonding interactions that
†
Electronic supplementary information (ESI) available: Representative
TEM micrograph and core size histogram of Ru]N nanoparticles,
NMR spectra of the Ru]N and Ru]N(Fc) nanoparticles and
monomeric ligands. See DOI: 10.1039/c2jm33783e
16
involve both s and p components.
1
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Thus, in this study, using an aryl sulfonyl azide ligand as the
precursor, we demonstrated that upon thermal decomposition,
the resulting nitrene radicals could be readily bound onto ‘‘bare’’
ruthenium colloid surfaces forming ruthenium-nitrene p bonds.
The conjugated interfacial bonding interactions led to apparent
intraparticle charge delocalization, and the nanoparticle-bound
nitrene moieties behaved analogously to azo derivatives, as
manifested in a variety of spectroscopic measurements. Inter-
estingly, the resulting nanoparticles also exhibited apparent
activity in imido transfer with alkene derivatives, as highlighted
in the cyclic addition of vinyl ferrocene to the Ru]N interfacial
linkages by spectroscopic and electrochemical measurements. To
the best of our knowledge, this is the first report ever on the
functionalization of transition-metal nanoparticles with metal–
nitrene p bonds.
solution. After vigorous mixing for 1 h, the ruthenium nano-
particles were extracted into the sec-butylbenzene phase, as
manifested by the appearance of a dark brown colour and a
colourless propanediol phase. The sec-butylbenzene portion was
then collected and underwent a second thermal refluxing at
ꢂ
165 C for 24 h, where nitrene radicals were produced from the
thermolysis of the azide compounds and bound onto the Ru
12
nanoparticle surface. The solution was allowed to cool down
naturally to room temperature and the solvents were removed by
rotary evaporation. The solids were then rinsed with a copious
amount of methanol to remove excessive ligands. The resulting
nanoparticles were denoted as Ru]N, which were stable under
ambient conditions and readily dispersible in various apolar
solvents such as methylene chloride, chloroform and toluene, but
not in polar solvents (e.g., alcohols, acetone, etc.). The nano-
particle core size remained practically unchanged, as estimated
by TEM measurements, 2.06 ꢀ 0.31 nm (Fig. S1 in the ESI†).
Ferrocenyl functionalization of the Ru]N nanoparticles was
carried out by mixing the Ru]N nanoparticles with a calculated
Experimental section
Chemicals
2 2
amount of vinyl ferrocene in CH Cl under magnetic stirring for
Ruthenium chloride (RuCl
3
, 99+%, ACROS), 1,2-propanediol
three days (Scheme 1). The solution was then dried by rotary
evaporation and the remaining solids were rinsed extensively by
methanol to remove excessive ligands, affording Ru]N nano-
particles with multiple copies of ferrocenyl moieties. The result-
ing nanoparticles were denoted as Ru]N(Fc).
(
ACROS), sodium acetate trihydrate (NaOAc$3H O, MC&B),
2
vinyl ferrocene (97%, Sigma-Aldrich), tetra-n-butylammonium
nitrate (TBANO , $99%, Sigma-Aldrich), and 4-dodecylbenze-
3
nesulfonyl azide (Aldrich) were used as received. All solvents
were obtained from typical commercial sources and used without
further treatment. Water was supplied by a Barnstead Nanopure
water system (18.3 MU cm).
As a control experiment, carbene-stabilized ruthenium nano-
particles were prepared by following a similar procedure, as
17
described previously. Briefly, after the ‘‘bare’’ Ru colloids were
prepared thermolytically, a toluene solution with a calculated
amount of octyl diazoacetate (ODA) was added to the colloid
solution, where carbene fragments were self-assembled onto the
Ru nanoparticle surface forming Ru–carbene (Ru]C) p bonds
(and concurrently releasing nitrogen). The purified nanoparticles
were referred to as Ru]C8.
Ruthenium nanoparticles
The synthetic procedure of organically capped ruthenium
nanoparticles involved two major steps (Scheme 1). The first was
the preparation of ‘‘bare’’ ruthenium colloids by thermolysis of
17,18
RuCl
3
in 1,2-propanediol, as detailed previously.
Briefly,
0
1
1
.14 mmol of RuCl and 80 mg of NaOAc were co-dissolved in
3
00 mL of 1,2-propanediol and the mixed solution was heated to
ꢂ
Characterization
65 C and thermally refluxed for 30 min under vigorous stirring.
1
13
H and C NMR spectroscopic measurements were carried out
The solution displayed a dark brown colour, signifying the
formation of ruthenium colloids, which exhibited an average
core diameter of 2.12 ꢀ 0.72 nm, as determined by transmission
by using concentrated solutions of the nanoparticles in CDCl or
3
CD Cl with a Varian Unity 500 MHz NMR spectrometer. UV-
2
2
17,18
electron microscopic (TEM) measurements.
These ‘‘bare’’
vis spectroscopic studies were performed with an ATI Unicam
UV4 spectrometer using a 1 cm quartz cuvette at a resolution of 2
nm. Photoluminescence characteristics were examined with a
PTI fluorospectrometer. FTIR measurements were carried out
with a Perkin–Elmer FTIR spectrometer (Spectrum One,
colloids were then subject to nitrene functionalization in the
second step. Experimentally, when the ruthenium colloid solu-
ꢂ
tion was cooled down to 60 C, 0.52 mmol of 4-dodecylbenze-
nesulfonyl azide in 20 mL of sec-butylbenzene was added into the
Scheme 1
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ꢁ
1
spectral resolution 4 cm ); the samples were prepared by casting
the particle solutions onto a NaCl disk. Thermogravimetric
analysis (TGA) was performed with a Perkin-Elmer Pyris 1
(Fig. S2†), possibly because of the highly delocalized Ru]N
interfacial bonding linkages where extended spilling of core
3,8
electrons into the ligand shell might occur. Significantly, based
on the integrated peak areas, the atomic ratio of the S and N
elements was estimated to be 1.01 : 1, in sharp contrast to the
1 : 3 ratio that was observed with the azide precursors (Fig. S2†).
Taken together, the XPS data presented above strongly
suggest that indeed the azide precursors were thermally decom-
ꢂ
ꢁ1
instrument at a heating rate of 10 C min under a nitrogen
atmosphere. X-ray photoelectron spectra (XPS) were recorded
with a PHI 5400/XPS instrument equipped with an Al Ka source
ꢁ9
operated at 350 W and at 10 Torr. The spectra were charge-
referenced to the Au4f7/2 peak (83.8 eV) of sputtered gold.
3
1–33
posed into nitrene fragments,
bound onto the ruthenium nanoparticle surface forming Ru]N
which were then covalently
Electrochemistry
1
p bonds (Scheme 1). This was further confirmed in H and
13
C
Voltammetric measurements were carried out with a CHI 440
electrochemical workstation. A polycrystalline gold disk elec-
NMR measurements (Fig. S3†), where the spectral features of
the nanoparticles were all markedly broadened, as compared
with those of the monomeric ligands, indicating that the ligands
were indeed bound onto the nanoparticle surface and there were
no excess free ligands, consistent with earlier studies of organi-
2
trode with a surface area of 0.43 mm (sealed in glass tubing) was
used as the working electrode. A Ag/AgCl wire and a Pt coil were
used as the (quasi)reference and counter electrodes, respectively.
The gold electrode was first polished with alumina slurries of
34
cally capped nanoparticles.
0
.05 mm and then cleansed by sonication in 0.1 M HNO
and Nanopure water successively. The nanoparticles were
3
, H
2
SO
4
,
The molecular structures of the nitrene fragments on the
nanoparticle surface were further investigated by FTIR
measurements. Fig. 2 displays the FTIR spectra of the mono-
meric 4-dodecylbenzenesulfonyl azide ligands (black curve) and
the Ru]N nanoparticles (red curve). It can be seen that the
ꢁ
1
dispersed in CH
M TBANO
2 2
Cl at a concentration of 4.2 mg mL with 0.1
3
.
ꢁ
1
phenyl ]C–H stretching vibrations at ca. 3063 and 3028 cm as
ꢁ
Results and discussion
1
well as the ring skeleton vibrations between 1400 cm and
ꢁ1
The bonding interactions between the nitrene moieties and the
ruthenium nanoparticles were first examined by XPS measure-
ments. Fig. 1 (A) depicts the survey spectrum of Ru]N nano-
particles where the key elements of the nanoparticles can be
clearly identified, Ru, N, S, O and C. In addition, the high-reso-
lution scans of the Ru3d, N1s and S2p electrons are included in
panels (B) to (D), respectively. From panel (B), one can see that
the Ru 3d5/2 and 3d3/2 electrons of the Ru]N nanoparticles
exhibited a binding energy of 280.3 eV and 284.4 eV (note that the
1600 cm can be clearly identified in both the azide monomers
and nanoparticles. In addition, three vibrational bands can be
ꢁ
1
seen with the azide monomers at 1923 cm , 1801 cm and
ꢁ1
ꢁ
1
1667 cm . These may be assigned to the overtone and/or
combination of the out-of-plane ring C–H deformations which
ꢁ1
for disubstituted benzene typically appear between 800 cm and
ꢁ
1
35
;
1000 cm
in contrast, only two peaks are observed with the
ꢁ1
ꢁ1
Ru]N nanoparticles at 1979 cm
because the ligands are tightly packed on the nanoparticle
surface that sterically restricts the ring C–H deformations. In
and 1719 cm , likely
19–23
latter also overlaps with the C1s electrons
), respectively, only
ꢁ
+
ꢃ0.1 eV more positive than that observed with metallic ruthenium
fact, the antisymmetric (d ) and symmetric (d ) stretches of the
ligand methylene (CH ) groups of the dodecyl component
exhibited a red-shift from 2929 cm and 2858 cm for the
(
which typically shows two peaks at 280.2 eV and 284.3 eV for the
2
24,25
ꢁ1
ꢁ1
3
d
5/2 and 3d3/2 electrons, respectively
). This suggests little
ꢁ
1
ꢁ1
charge transfer between the ruthenium metal cores and the nitrene
moieties, consistent with the binding energy of the N1s electrons,
as depicted in panel (C). It can be seen that deconvolution of the
N1s spectrum yields only one peak (398.8 eV), as highlighted by
the good agreement between the experimental data (black curve)
and the curve fit (red curve), indicating that the nitrogen elements
are involved in only one chemical state. This is in sharp contrast to
the azide moieties (–N]N+]N–) of the monomeric ligands
where the nitrogen 1s electrons exhibit two distinctly different
binding energies at ca. 402.1 eV and 405.9 eV at an atomic ratio of
monomeric azide to 2925 cm and 2856 cm for the Ru]N
nanoparticles, indicating enhanced ordering of the ligands when
bound on the Ru particle surface as compared with the mono-
7
meric counterparts.
The tight packing of the nitrene fragments on the nanoparticle
surface may also account for the discrepancy of other vibrational
characteristics. For instance, the ring breathing (dring) at ca.
ꢁ
1
752 cm
particles than with the monomeric azide. The symmetrical and
asymmetrical stretches of the sulfonyl (–SO –) moieties can be
was substantially weaker with the Ru]N nano-
2
ꢁ1
ꢁ1
2
: 1, with the smaller peak corresponding to the relatively elec-
identified at 1171 cm and 1309 cm , respectively, for the
36
monomeric azide. Yet, for the nanoparticle sample, their
26
tron-deficient middle nitrogen atom (Fig. S2†). Furthermore,
such a binding energy of 398.8 eV for the N1s electrons in Ru]N
nanoparticles is consistent with those observed with pyridine-like
intensities diminished drastically, again likely due to the steric
restriction at the core–ligand interface.
27,28
nitrogen embedded within a graphene matrix (398.2 eV)
2
well as sp nitrogen in metal-coordinated porphyrin complexes
as
The most striking difference between the azide monomers and
the Ru]N nanoparticles, however, lies in the stretching vibra-
29
(
398.2–398.5 eV), signifying the covalent and conjugated nature
tions of the azide (N
metrical vibrations of N
ꢁ1 36
prominent peaks at 2127 cm and 2384 cm . Yet both of
3
) moieties. For the monomers, the asym-
of the Ru]N bonding interactions.
The S2p electrons can be identified with a peak at 167.8 eV, as
3
can be clearly identified by two
ꢁ
1
manifested in panel (D). This is 1.0 eV lower than that reported
these vanished completely in the nanoparticle samples, consistent
with the efficient decomposition of the azide ligands forming
nitrene radicals. Note that the azide symmetrical vibration,
30
in the literature for sulfonamide moieties (–SO
2
N–, 168.8 eV),
and 1.7 eV lower than that observed with the azide monomers
1
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Fig. 1 (A) Full XPS survey spectrum of Ru]N nanoparticles where all key elements of the nanoparticles are identified, and high-resolution scans of (B)
Ru3d, (C) N1s, and (D) S2p electrons. In panels (C) and (D), black curves are the experimental data, red curves are the fit, and green curves are the
background.
ꢁ
1
which most likely arises at 1383 cm , overlaps with the skeleton
with tpp ¼ dianion of 5,10,15,20-tetraphenylporphyrin, oep ¼
dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin, and Ts ¼
36
stretching of the phenyl ring. Thus, for the monomeric ligands,
the peak is very well-defined. Yet, for the Ru]N nanoparticles,
one can see a significant diminishment of the peak intensity
because of the loss of the azide moiety (Fig. 2).
VI
tosyl, where the asymmetric vibrational band of Ru ¼ NTs was
ꢁ
1
observed at 914 cm and 900 cm , respectively. In light of
ꢁ1
40
these earlier results, it is most likely that the new band observed
ꢁ1
Then one major question remains. Can the Ru]N bond be
resolved in infrared measurements? It should be noted that it
remains a challenge to unambiguously identify the spectroscopic
signatures of metal–nitrene bonds, although the syntheses of
metal–nitrene complexes have been reported in the literature,
largely because of the sensitive dependence of the vibrational
frequency on the charge state of the metal centers as well as on
the substituent groups on the nitrogen atom where coupling with
at 1246 cm with the nanoparticles (red curve) is due to the
Ru]N bonds formed at the metal–ligand interface. As
mentioned earlier, the Ru]N bond is stabilized by backbonding
interactions of the metal d electrons to nitrogen, and from the
XPS studies presented above (Fig. 1), the charge state of ruthe-
nium of the nanoparticles is close to that of neutral metal.
Therefore, it is reasonable to conclude that the Ru]N bonding
order and hence vibrational frequency of the nanoparticles
37–39
VI
other vibrations within the molecules is possible.
the ruthenium–nitrene (Ru]N) p bonds have been shown to
1
appear at frequencies ranging from 900 cm to 1300 cm in
Of these,
would be higher than those of the Ru ]N complexes.
The coverage of nitrene fragments on the nanoparticle surface
was then quantified by TGA measurements. Fig. 3 depicts the
decrease of the nanoparticle sample weight with temperature
(black curve), along with the corresponding first-order derivative
(green curve). One can see that the weight loss of the nanoparticle
ꢁ
ꢁ1
10,40,41
various organometallic complexes.
For instance, Che and
co-workers reported the preparation of stable tert-butylimido
complexes of ruthenium(VI) porphyrins and assigned the IR band
VI
at 1232 cm to the Ru ]NBu bonding stretch. In another
ꢁ1
t
42
ꢂ
organic components commenced at around 250 C and ended at
ꢂ
study, they prepared two bis(tosyl)imidoruthenium(VI)
VI
porphyrin complexes, Ru (tpp)(NTs)
around 500 C, with a total weight loss of 44.2%. Based on the
VI
and Ru (oep)(NTs)
2
2
,
Ru core diameter of 2.06 nm, the number of nitrene fragments on
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Fig. 2 FTIR spectra of 4-dodecylbenzenesulfonyl azide monomers (black curve) and Ru]N nanoparticles (red curve).
stronger than the ruthenium–carbon p bonds. In fact, the
ꢁ
1
vibrational frequency of the latter was estimated to be 1030 cm
44
by Ashkenazi et al. in the study of a metallaquinone complex.
This is markedly lower than that observed above in the present
study for the Ru]N bonds (Fig. 2).
Further studies revealed that the Ru]N nanoparticles
exhibited interesting optical properties. Fig. 4 depicts the UV-
vis absorption spectra of the azide monomers (black curve),
Ru]N nanoparticles (red curve), and Ru]C8 nanoparticles
(
green curve). Note that the spectra of Ru]N and Ru]C8
nanoparticles were both normalized to their respective absor-
bance at 300 nm, and their difference is shown as the blue
curve. It can be seen that the azide monomers exhibited three
absorption peaks at 238 nm, 267 nm and 278 nm. The first peak
may be ascribed to the p–p* transition of the phenyl p elec-
trons, whereas the last two peaks are most likely due to the
Fig. 3 TGA spectrum (black curve) of Ru]N nanoparticles at a heating
ꢁ1
ꢂ
rate of 10 C min . The corresponding first-order derivative is also
included as the green curve.
26
azide moiety. For the Ru]N nanoparticles, in addition to the
exponential decay profile that is characteristic of nanosized
45
the nanoparticle surface can be estimated to be ca. 84.1, corre-
metal nanoparticles (the so-called Mie scattering), the phenyl
absorption peak remained well-defined with a blue-shift to
230 nm, whereas the azide absorption features disappeared,
consistent with the decomposition of the azide groups forming
nitrene derivatives (vide ante). More interestingly, in compar-
ison with the absorption spectrum of the same ruthenium
nanoparticles stabilized by carbene fragments (Ru]C8), the
Ru]N nanoparticles displayed a new broad absorption band
within the range of 340 nm to 650 nm (centered at ca. 390 nm),
as manifested in the difference spectrum (blue curve). Such
ꢀ
2
sponding to an average footprint of 15.8 A for one nitrene
ligand. This is somewhat greater than that observed with alkyne
43
or alkynide ligands on ruthenium nanoparticles. Furthermore,
from the first-order derivative of the TG curve in Fig. 3, it can be
seen that the transition temperature of the organic weight loss is
ꢂ
ꢂ
around 414 C. This is more than 100 C higher than those
observed with ruthenium nanoparticles capped by alkyne mole-
ꢂ
7
cules through ruthenium–vinylidene bonds (<300 C), suggest-
ing that the ruthenium–nitrene p bonds are substantially
1
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at 375 nm, consistent with the UV-vis spectroscopic measurements
(
Fig. 4), and the emission peak at 467 nm, in agreement with those
48,49
of azoalkanes.
It should be noted that no fluorescence emission
was detected with the azide monomers. Thus, the apparent fluo-
rescence of the Ru]N nanoparticles further confirms the effective
intraparticle charge delocalization between the particle-bound
nitrene moieties that was facilitated by the conjugated Ru]N p
bonds, akin to that observed with ruthenium–carbene, –acetylide,
2–8
or –vinylidene interfacial linkages.
The formation of ruthenium–nitrene p bonds at the metal–
ligand interface was also manifested by the unique reactivity of
the Ru]N nanoparticles with vinyl derivatives, most probably
following the imido transfer mechanism that results in the cyclic
addition of the vinyl moiety to the Ru]N linkage, as depicted in
Scheme 1. Note that imido transfer has been observed with
ruthenium–imido complexes for the aziridination of varied are-
Fig. 4 UV-vis absorption spectra of 4-dodecylbenzenesulfonyl azide
monomers (black curve), Ru]N nanoparticles (red curve), and Ru]C8
10
nes such as styrene, norbornene, indene, etc. In the present
2 2
nanoparticles (green curve) in CH Cl . The spectra of Ru]N and Ru]
study, we used vinyl ferrocene as the probe molecule to highlight
the aziridination properties of the Ru]N nanoparticles. The
successful attachment of the ferrocenyl moieties onto the nano-
particle surface was confirmed by varied spectroscopic
C8 nanoparticles were normalized to their respective absorbance at 300
nm and their difference is shown as the blue curve (right axis).
emergence of new electronic energy states with the Ru]N
nanoparticles is most likely attributed to intraparticle charge
delocalization between the nanoparticle-bound nitrene moieties
because of the conjugated Ru]N interfacial bonding interac-
tions, such that the nitrene moieties behave analogously to azo
1
measurements. Fig. S4† depicts the H NMR spectrum of the
Ru]N(Fc) nanoparticles. First, one can see that, again, the
spectroscopic features of the Ru]N(Fc) nanoparticles were
markedly broadened, in comparison with those of the corre-
sponding monomeric ligands. This observation is similar to that
of the as-prepared Ru]N nanoparticles (Fig. S3†), where the
absence of sharp NMR signals indicates that nanoparticles were
free of excessive monomeric ligands and the responses were all
(–N]N–) derivatives. In fact, one may note that 1,2-dia-
lkyldiazenes (RN]NR) have been found to display a prom-
inent absorption band where the peak varied from 340 nm to
3
80 nm with the chemical structure of the aliphatic substitu-
3
,4,8,17
46,47
due to particle-bound molecules.
Ru]N nanoparticles, the Ru]N(Fc) sample exhibited a new
and broad) peak within the range of 3.8 and 4.5 ppm, which is
Second, in contrast to the
ents.
Such a behaviour has been observed previously with
other fluorophores (e.g., anthracene, pyrene, etc.) bound onto
the nanoparticle surface by ruthenium–carbene (Ru]C) p
(
7,8
6
assigned to the particle-bound ferrocenyl protons. In addition,
in XPS measurements (Fig. S5†), the Ru]N(Fc) nanoparticles
also exhibited two peaks at 708.6 eV and 720.7 eV, which are
bonds.
In fact, the Ru]N nanoparticles exhibited fluorescence char-
acteristics that are similar to those of azoalkanes. The black curves
of Fig. 5 show the excitation and emission spectra of the Ru]N
nanoparticles, where the excitation peak can be identified
50
ascribed to the Fe2p electrons.
The particle-bound ferrocenyl moieties were further man-
ifested in electrochemical measurements. Fig. 6 (A) shows the
cyclic voltammograms of the Ru]N(Fc) nanoparticles in
CH
ꢁ
2
Cl
2
with 0.1 M TBANO
3
at a potential sweep rate of 200 mV
1
s
. It can be seen that within the potential range of +0.2 to +0.6
V (vs. Ag/AgCl), a pair of prominent peaks appeared with a
ꢂ0
formal potential (E ) of +0.40 V, in contrast to the featureless
profile of the Ru]N nanoparticles (panel (B)) or the Ru]N
Fig. 5 Excitation and emission spectra of the Ru]N (black curve)
and Ru]N(Fc) (red curve) nanoparticles in CH
2
Cl
2
with the
Fig. 6 Cyclic voltammograms of (A) Ru]N(Fc) and (B) Ru]N
ꢁ1
emission (lem) and excitation (lex) wavelength set at 467 nm and 375 nm,
respectively. Inset shows the corresponding (unnormalized) UV-vis
absorption spectra.
nanoparticles both at a concentration of ca. 4.2 mg mL in CH
0.1 M TBANO . The working electrode was a gold electrode with a
surface area of 0.43 mm . The potential sweep rate was 200 mV s
2 2
Cl with
3
2
ꢁ1
.
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nanoparticles after mixing with ethyl ferrocene for 7 d (not
shown). This is ascribed to the 1e reduction–oxidation of nano-
In summary, the results presented above demonstrate, for the
first time ever, that nitrene chemistry may be readily utilized for
the surface functionalization of transition-metal nanoparticles.
More importantly, the unique chemical reactivity of the metal–
nitrene p bonds may be exploited as an unprecedented platform
for further and more complicated manipulation of the optical
and electronic properties of the nanoparticle materials. This will
be the focus of on-going work.
+
particle-bound ferrocenyl moieties, Fc 4 Fc + e. The small
ꢁ1
p
peak splitting (DE z 40 mV at 200 mV s ) is consistent with
the facile electron-transfer kinetics of ferrocenyl derivatives,
although in the Ru]N(Fc) nanoparticles, the ferrocenyl moie-
ties were buried within the hydrophobic ligand shell and the
accessibility to counter ions was likely impeded.
Importantly, the fact that only one pair of voltammetric peaks
was observed is consistent with the cyclic addition of the vinyl
moieties with the Ru]N linkage by imido transfer (Scheme 1)
where the ferrocenyl moieties were bound onto the nanoparticle
surface by saturated bonds. This is in sharp contrast with our
previous studies where olefin metathesis reactions of vinyl ferro-
cene with carbene-functionalized nanoparticles (e.g., Ru]C8) led
to the covalent attachment of the ferrocenyl moieties onto the
Acknowledgements
This work was supported, in part, by the National Science
Foundation (CHE – 1012258 and DMR – 0804049) and the ACS
Petroleum Research Fund (49137 – ND10). XPS and TEM
studies were carried out at the Molecular Foundry and National
Center for Electron Microscopy, Lawrence Berkeley National
Laboratory as part of a user project.
8
particle surface by Ru]C p bonds. Consequently, apparent
intraparticle charge delocalization occurred, where intervalence
transfer between the particle-bound ferrocenyl moieties was
manifested in electrochemical studies with two pairs of voltam-
metric peaks and in near-infrared measurements with a peak
emerging at mixed valence, a behaviour consistent with those of
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