Solution structure of cadmium carboxylate and its implications for the synthesis of cadmium chalcogenide nanocrystals

Article abstract of DOI:10.1039/c3cc44103b

Diffusion-ordered spectroscopy (DOSY) was used to investigate the solution structure of cadmium carboxylate. The molecular weights of cadmium complexes highly depend on the solvent; the complexes are polymeric in toluene but break up in the presence of polar solvents or coordinating ligands.

Full text of DOI:10.1039/c3cc44103b

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Solution structure of cadmium carboxylate and its
implications for the synthesis of cadmium
chalcogenide nanocrystals†
Cite this: Chem. Commun., 2013,
49, 7857
Received 31st May 2013,
Accepted 3rd July 2013
Rau´l Garc´ıa-Rodr´ıguez and Haitao Liu*
DOI: 10.1039/c3cc44103b
www.rsc.org/chemcomm
Diffusion-ordered spectroscopy (DOSY) was used to investigate the
A major challenge in studying the solution structure of
solution structure of cadmium carboxylate. The molecular weights cadmium carboxylate is the lack of suitable characterization
of cadmium complexes highly depend on the solvent; the complexes techniques. Cd2+ ions can have a wide range of coordination
are polymeric in toluene but break up in the presence of polar numbers and diverse geometries. In the solid state, cadmium
solvents or coordinating ligands.
carboxylates tend to form coordination polymers11,23,24 and break
down to smaller complexes in the presence of a coordinating
CdE (E = S, Se, Te) nanocrystals have made a great impact in many ligand.25,26 Although it is reasonable to assume that the solution
technological applications.1–4 Over the last two decades, a large species should have a similar structure to that found in the
number of synthetic methods have been developed to prepare solid state, it is equally conceivable that the polymeric structure
these materials,5–8 and significant efforts have been devoted to could be disrupted by solvent or ligand binding. Unfortunately,
understand the mechanism of nanocrystal formation in order to conventional characterization tools (e.g., mass spectrometry and
improve the control over its size, shape, and composition.9–11 The FTIR) could not provide quantitative information about the
fate of the chalcogenide sources was the subject of several recent nature of the metal coordination and degree of polymerization
studies. Both phosphine chalcogenide12,13 and dissolved S and Se in solution. For example, mass spectrometry is known to
in octadecene14 or amines15,16 have received significant attention. underestimate the concentration of large molecular weight
In contrast, not much is known about the solution structure and species and often requires polar solvents (e.g., for electrospray)
reactivity of cadmium precursors, not to mention their role in and/or additives that could affect the structure of cadmium
nanocrystal synthesis.
The solution structure of the cadmium precursor could affect of cadmium carboxylate has not been established.
the precursor-conversion kinetics and consequently the nucleation Herein we report a study of the solution structure of cadmium
complexes. To the best of our knowledge, the solution structure
and growth kinetics of nanocrystals. The Lewis acidity of the carboxylate using 113Cd and 1H DOSY. DOSY is a pseudo-2D NMR
cadmium precursor is known to significantly affect the rate of technique that measures diffusion coefficients, which in turn can
monomer production in phosphine-based syntheses.17–19 This fact be used to estimate the hydrodynamic radii and formula weight of
raises the question of whether other structural variations, such as the molecule.27,28 1H-DOSY has been employed to estimate the
degree of polymerization and coordination of Cd2+, could also particle size of II–VI and IV–VI nanocrystals and to study the
affect the reactivity and if so, to what degree. In addition, the presence of ligands and the interaction of ligands with the nano-
cadmium precursor may also directly impact the growth of crystal surface.29,30
nanocrystals by acting as a template. For example, a lamellar
A major goal of this study is to address two long-standing
mesophase of Cd(OAc)2 or CdCl2 layers separated by n-octylamine questions that are fundamental to the reaction mechanism
bi-layers was proposed to be responsible for the formation of of CdE nanocrystals: (1) what is the molecular weight of
planar CdSe nanostructures by Hyeon and Buhro groups.20–22 cadmium carboxylate in solution? (2) What is the coordination
A clear understanding of the solution structure of cadmium environment of the Cd center? In particular, are the carboxylate
carboxylate could clarify their roles in the nanocrystal synthesis ligands labile when dissolved in solution? Our result shows
and may lead to new synthetic methodologies.
that cadmium carboxylate exists as a polymer in toluene
and the formula weight (FW) decreases in the presence of
a coordinating ligand or polar solvent. However, even in the
latter case, 1H and 113Cd DOSY experiments revealed similar
diffusion coefficients suggesting that there is no dissociation of
carboxylate from Cd2+ (vide infra).31
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA.
E-mail: hliu@pitt.edu
† Electronic supplementary information (ESI) available: Experimental details and
additional figures and tables. See DOI: 10.1039/c3cc44103b
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To study the structure of cadmium carboxylate in a non-
coordinating solvent, we chose cadmium oleate, Cd(OA)2 for its
wide application in nanocrystal synthesis.5,18,36 Our Cd(OA)2 sample
was prepared by reacting CdO and oleic acid (HOA). The crude
reaction mixture was precipitated by adding acetone in order to
remove excess of free HOA (see the ESI,† for experimental details
and Fig. S7 and S8). When a sample of Cd(OA)2 was dissolved in
toluene a viscous solution was obtained, and the 1H NMR line
widths of the oleate ligand are considerably broader than the
residual toluene solvent signals (Fig. S8–S10, ESI†). This observation
is in contrast to the sharp NMR resonance observed for Cd(OAc)2 in
octylamine. In addition, we also observed strong, negative NOEs in
the 1H–1H NOESY spectrum (Fig. S11, ESI†), characteristic of slowly
tumbling molecules.30 These two facts suggest a polymeric structure
for Cd(OA)2 when dissolved in toluene.
In this case, 113Cd DOSY was challenging because of the broad
line width (Fig. S10, ESI†). However, given that toluene is neither
coordinating nor polar and the fact that Cd(OAc)2 does not
dissociate in octylamine we expect the dissociation of the oleate
ligand to be minimal in toluene (vide infra). Fig. 2 shows the
1H DOSY spectrum of this sample collected with three internal
standards (Table S2, ESI†). We estimated that the FW of this
Cd(OA)2 sample was 20 650 (see the ESI,† Fig. S12 and Table S3, for
details regarding the calculation of the FW and also Table S4 for
control experiments using polystyrene standards), which translates
to 30.5 units of Cd(OA)2. We note that there are other important
factors that can impact the exact structure of the metal carboxylate
in solution, such as concentration, temperature, and presence of
Fig. 1 (above) 1H DOSY and natural abundance 113Cd DOSY of Cd(OAc)2Á2H2O
dissolved in octylamine. Note that the acetate resonance of Cd(OAc)2 in the
1H DOSY diffuses with the same diffusion coefficient (1.78 Â 10À10 m2
s
À1) as
the one observed in the 113Cd DOSY (1.79 Â 10À10 m2 sÀ1). (below) 1H DOSY of
Cd(OAc)2Á2H2O dissolved in octylamine in the presence of polystyrene (Mw = 2400)
and di-tert-butylbiphenyl (Mw = 266.42) added as two inert internal references.
Our first model system is cadmium acetate, Cd(OAc)2, a adventitious water. For example, we found that the FW obtained
commonly used cadmium precursor for the synthesis of CdSe for Cd(OA)2 in toluene was highly dependent on the preparation of
nanostructures and clusters.21,32–35 Cd(OAc)2 is known to form the sample, and could fluctuate between 13 000 and 40 000.
coordination polymers in the solid state23 and is not soluble in Further studies are being carried out to probe the dependence of
non-polar organic solvents. It is soluble in coordinating solvents the FW on the synthesis and purification conditions.
such as methanol and octylamine. We studied the behavior of
Many nanocrystal syntheses are conducted in a non-coordinating
Cd(OAc)2Á2H2O dissolved in octylamine under similar conditions to solvent but in the presence of reagents or ligands that can coordinate
those employed in the synthesis of planar CdSe nanostructures.21,22 to Cd. Therefore, it is important to understand how these molecules
The solution in octylamine exhibited a single sharp signal in the affect the structure of cadmium carboxylate. To this end, we have
113Cd NMR spectrum at 154.6 ppm (see Fig. S1 (ESI†), referenced measured the diffusion coefficients of Cd(OA)2 in toluene in the
externally to 0.1 M Cd(ClO4)2 in D2O). No reaction between presence of three model ligands: SePMe3, OPMe3, and octyl-
octylamine and acetate was observed (see the ESI,† Fig. S2–S4). amine.16,21,33 We observed that the addition of ligand sharpened
The 1H DOSY spectrum (Fig. 1 and Fig. S5, ESI†) showed two
markedly different diffusion coefficients for octylamine (D = 6.00 Â
10À10 m2 sÀ1) and acetate (1.78 Â 10À10 m2 sÀ1). The diffusion
coefficient (1.79 Â 10À10 m2 sÀ1) extracted from 113Cd DOSY (Fig. 1
and Fig. S5, ESI†) matched perfectly with the one for acetate
extracted from 1H DOSY. Although this result strongly suggests
that acetate groups are tightly bound to the Cd2+ centers, the
formation of ion pairs cannot be ruled out since it would also
result in the observation of the same diffusion coefficient in the 1H
and 113Cd DOSY experiments. We note that exchange of carboxylate
from Cd2+ in aqueous solution is known to be fast on the NMR
time scale.31 The FW of the Cd complex was estimated to be
799 (see the ESI,† Table S1 and Fig. S6, for the calculation of the
FW), consistent with a small complex. Given that monomeric
complexes of the type Cd(OAc)2(octylamine)x have a FW = 488.99
and 747.47 for x = 2 and 4 respectively, the formation of species
larger than a dimer seems less likely.
Fig. 2 1H DOSY spectrum of Cd(OA)2 dissolved in toluene at 300 K in the presence
of three internal references: polystyrene (Mw
= 13 200), polydimethylsiloxane
(DMS, Mw = 28 500) and Poly(methyl methacrylate) (MMA, Mw = 68 500). Shape
of the signal at 2.09 ppm is due to overlapping toluene resonance. See the ESI†
(Table S3) for diffusion coefficients for each of the species.
c
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Notes and references
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Cd(OA)2 (Fig. S13–S15, ESI†), suggesting that the degree of oligo-
merization of Cd(OA)2 was reduced. As shown in Fig. 3, the addition
of just 1.5 equivalents of octylamine decreased the FW of Cd(OA)2 in
toluene from 23 500 to 2499. Subsequent additions of octylamine
slightly and progressively reduce its FW to 1985 after the addition of
9 equivalents. Similar observations were made with SePMe3 and
OPMe3 (see Fig. 3 and the ESI,† Tables S5 and S6 and Fig. S16).
In the presence of excess ligands, the FW calculated approaches
a monomeric or dimeric type [Cd(OA)2(L)x]. Additional support
for the formation of such complexes comes from 2D-NOESY and
2D-ROESY spectra of a mixture of Cd(OA)2 and 2 equivalents of
octylamine, both of which showed that the a-CH2 protons of oleate
and octylamine are close to each other in space (see the ESI,†
Fig. S17–S21 for details). This result indicates that octylamine and
oleate ligands are simultaneously coordinated to cadmium. Finally,
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cadmium oleate suggests that the nucleophilic attack of carboxylate
on SePMe3 must occur in a small cadmium complex, close to a
monomer or dimer, rather than in a polymeric one.9,13 In a similar
vein, the supramolecular structures observed by Buhro and
coworkers20–22 are likely constructed from and in equilibrium with
fast diffusing small complexes (see the ESI,† Scheme S1 for details).
1
In conclusion, we have used H and 113Cd DOSY to study the
solution structure of two cadmium carboxylate complexes under
conditions similar to those used in a wide range of low temperature
nanocrystal syntheses.16,21,22,37–39 The structure was found to be
polymeric in pure toluene and monomeric or dimeric in the
presence of excess ligands such as octylamine, SePMe3 or OPMe3.
Our result indicated that the dominant cadmium species during
nanocrystal synthesis are small complexes and not polymeric
ones.9,13 While the effect of ligands on nanocrystal growth and
shape control has been well documented, our results showed that
ligands also affect the nature of the cadmium precursor. With a
detailed understanding of the cadmium precursor, we hope future
work will develop more realistic kinetic analysis and theoretical
modelling of the effect of ligands on nanocrystal synthesis.
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We thank ONR (grant N000141310575) for partial support of
´
´
this work. RG-R acknowledges Fundacion Ramon Areces for a
postdoctoral fellowship. We thank Dr Damodaran Krishnan
and Sage Bowser for assistance in NMR experiments.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 7857--7859 7859