Azamacrocycles and tertiary amines can be used to form size tuneable hollow structures or monodisperse oxide nanoparticles depending on the 'M' source

Article abstract of DOI:10.1039/c9dt02080b

We show that the azamacrocycle 'cyclam' (1,4,8,11-tetraazacyclodecane) in conjunction with a silicon catecholate ion generates novel hollow tetragonal tube-like crystalline materials [(C₆H₄O₂)₃Si][C₁₀H₂₆N₄]·H₂O, whose dimensions can be tuned according to the pH of the reaction medium. The synthesis approach was successful for both silicon and germanium and we hypothesise that a range of other catecholate precursors of elements such as iron could be used to generate a large array of inorganic materials with interesting morphologies. The synthesis approach can be extended to tertiary diamines with functional group spacing playing an important role in the efficacy of complexation. Of the molecules explored to date, a C2 spacing (N,N,N′,N′-tetramethylethylenediamine (4MEDAE)), leads to the most efficient structure control with hollow hexagonal tube-like structures being formed. In addition, we show that azamacrocycles, in the presence of unbuffered tetramethoxysilane (TMOS) solutions can be used to manipulate silica formation and provide a fast (ca. 10 minutes) synthesis route to particles whose diameter can be tuned from ca. 20 nm to several hundreds of nm under reaction conditions (no extremes of pH) that make the sols suitable for direct use in biotechnological applications.

Full text of DOI:10.1039/c9dt02080b

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Azamacrocycles and tertiary amines can be used
to form size tuneable hollow structures or
monodisperse oxide nanoparticles depending on
the ‘M’ source†‡
Cite this: DOI: 10.1039/c9dt02080b
a
Graham E. Tilburey,a Toby J. Blundell, b Stephen P. Argentc and Carole C. Perry
*
We show that the azamacrocycle ‘cyclam’ (1,4,8,11-tetraazacyclodecane) in conjunction with a silicon
catecholate ion generates novel hollow tetragonal tube-like crystalline materials [(C6H4O2)3Si][C10H26N4]·H2O,
whose dimensions can be tuned according to the pH of the reaction medium. The synthesis approach
was successful for both silicon and germanium and we hypothesise that a range of other catecholate pre-
cursors of elements such as iron could be used to generate a large array of inorganic materials with inter-
esting morphologies. The synthesis approach can be extended to tertiary diamines with functional group
spacing playing an important role in the efficacy of complexation. Of the molecules explored to date, a
C2 spacing (N,N,N’,N’-tetramethylethylenediamine (4MEDAE)), leads to the most efficient structure
control with hollow hexagonal tube-like structures being formed. In addition, we show that azamacro-
cycles, in the presence of unbuffered tetramethoxysilane (TMOS) solutions can be used to manipulate
silica formation and provide a fast (ca. 10 minutes) synthesis route to particles whose diameter can be
tuned from ca. 20 nm to several hundreds of nm under reaction conditions (no extremes of pH) that
make the sols suitable for direct use in biotechnological applications.
Received 17th May 2019,
Accepted 24th June 2019
DOI: 10.1039/c9dt02080b
rsc.li/dalton
microporous aluminophosphates7 and magnesioaluminopho-
sphates.8 Interestingly, azamacrocycles have been found to
Introduction
The ability of azamacrocycles to bind to transition metal and interact with the crystallising inorganic framework in two
lanthanide ions forming thermodynamically stable complexes1–3 different ways.8–11 The first is found in gallium phosphate
has led to the development of a new generation of abiotic host hybrid materials where cyclam was found to be covalently
molecules that contain a signalling or responsive functional bonded to the zeolite-like gallium phosphate framework.9 This
group enabling the development of molecular sensors.1,2 The structure permits metal cation exchange with other ions
chelation of Gd(III) by azamacrocycles gives molecular species forming stronger complexes with the macrocycle, such as
that can be used as contrast agents in magnetic resonance Cu2+.9 This type of interaction was found in metalloalumino-
imaging (MRI), where strong binding of the metal and occu- phosphates where the azamacrocyclic molecule, 1,4,8,11 tetra-
pation of most of the available coordination sites is vital.4 methyl 1,4,8,11-tetraazacyclodecane (4MC) was found to act as
Cyclam, (1,4,8,11-tetraazacyclotetradecane) and related com- a structure directing agent around which the inorganic frame-
pounds have been used in the preparation of new inorganic– work crystallises forming STA-6, 7 and 8 structures depending
organic hybrid materials such as gallium phosphates,5,6 novel on the metal cation present.8,10,11 A further example is where
the azamacrocyclic molecule is present in the crystal structure
of the material, but does not coordinate a metal ion. Gallium
in the presence of 1,4,7,10,13,16-hexaazacyclooctadecane is
one such example where the gallium ion is too small to be
aBiomolecular and Materials Interface Research Group,
Interdisciplinary Biomedical Research Centre, Nottingham Trent University,
Clifton Lane, Nottingham NG11 8NS, UK. E-mail: carole.perry@ntu.ac.uk
bDepartment of Chemistry and Forensics, Nottingham Trent University, Clifton Lane,
Nottingham NG11 8NS, UK
effectively coordinated to the azamacrocyclic molecule result-
ing in the azamacrocyclic molecule being present in pores
within the structure and not bound to the framework.12
Molecular self-assembly has become one of the fastest
growing areas of research in materials chemistry over the past
few decades, leading to the production of biologically inspired
nanomaterials.13 One such area of research is the formation of
cSchool of Chemistry, University of Nottingham, University Park,
Nottingham NG7 2RD, UK
†In honour of the 60th birthday celebrations of Professor Annie Powell.
‡Electronic supplementary information (ESI) available. CCDC 1916071. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c9dt02080b
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Fig. 1 SEM images of (a) tetragonal tubes, prepared from potassium silicon catecholate and cyclam at a Si : N ratio of 10 : 1 (b) tetragonal tubes
from (a) after heat treatment to 900 °C, and (c) structures generated at a Si : N ratio of 2 : 1 showing multiple nucleation sites.
bioinspired nano-patterned silica materials where inspiration minimum of 14 ring atoms can be used to tune the sizes of
has been taken from silica accumulating organisms such as silica particles from ca. 20–800 nm (TMOS precursor).
diatoms,14 sponges and higher plants.15 Extensive studies of
silica formation have been performed using biomolecule
isolates (proteins and long chain polyamines) as well as
Results and discussion
simpler chemical mimics such as amino acids, peptides and
amines.16,17 Importantly, in the context of the present contri-
A range of aza- and oxo-compounds having C2 or C3 spacing
bution, self-assembly of the organic moiety has proved impor-
between the functional groups were tested for their ability to
tant in the generation of structurally organised amorphous
interact with an aqueous solution containing potassium
silicas.18 Molecular architecture has a significant effect on the
catalytic ability and morphological control exerted by (bio)
molecules on the formation of silica in vitro.19–21 Additives
ranging from polyelectrolytes to small molecules having a sig-
nificant effect on silica formation in vitro either catalytically or
through molecular templating usually contain some degree of
amine functionality.16,22 Complex structures which have been
generated include: hollow spheres formed in the presence of
long chain polyamines,23 amorphous silica hexagonal plates
formed in the presence of alpha-helical arrays of poly-
lysine,24,25 hollow nano-fibres,26 silica tubes27 and hybrid
organic–inorganic amorphous tubes.28
In this contribution we investigate the effect of amine mole-
cular architecture on the formation of novel silica mor-
phologies. Further, the unique ability of azamacrocyclic mole-
cules to interact with chemical species to generate crystalline
solids lead to the hypothesis that they might also interact with
a forming amorphous inorganic material such as silica. We
present the results of interaction studies with two different
silicic acid precursors, namely tetramethoxysilane (TMOS),
that is commonly used in biomimetic studies of silica for-
mation and potassium silicon catecholate (KSicat) where the
silicon is octahedrally coordinated and relatively ‘high’ concen-
trations of monomeric silicon species can be maintained in
solution. Specifically we show that according to the precursor
used cyclam can be used to induce the formation of a novel
layered crystalline structure in the form of hollow tetragonal Fig. 2 NMR and FTIR characterisation of hollow tubes formed from the
reaction between potassium silicon catecholate at a Si : N ratio of 1 : 1.
(a) 29-Si solid state nmr, CP-MAS, (b) 13-C solid state nmr, CPMAS, and
(c) FTIR spectra of tetragonal prism, KSiCat and cyclam, insert shows
close up of 1750–450 cm−1 where the Si–O peak at 1100 cm−1 is
needles (KSicat as precursor) or tune the sizes of silica sols at
the tens of nm level (TMOS precursor). Further, tertiary
amines with a range of functional group spacings can also be
used to generate macroscopic ordered silicon catecholate :
retained from KSiCat in the tetragonal prisms. Upon heat treatment the
diamine complexes and a range of azamacrocycles with a Si–O stretch is broad showing the presence of amorphous silica.
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silicon catecholate (see Materials section with images shown Hollow rod-like structures (needles) formed almost immedi-
in ESI1‡). Novel macroscopic structures (needles) formed in ately on mixing at a molar range of 10 : 1 for Si : N with the
the presence of 1,4,8,11-tetraazacyclodecane (cyclam) only. purest samples being formed by around 60 minutes after
mixing, Fig. 1a. Typical yields were ca. 90%+ based on the
amount of cyclam present, ESI2.‡ If samples were left con-
siderably longer than this then samples containing both
needles and other less ordered material formed, ESI3.‡ EDXA
analysis of the different material types showed that the
needles contained Si, C, N and O, ESI4.‡ No K was detected in
the rod-like structures, ESI4.‡ Treatment of structurally mixed
samples with acid (2 M HCl) destroyed the tube-like structures
but not the unordered materials giving further evidence that
the two were different in chemical composition and structure.
Material isolated at ca. 60 minutes after mixing yielded
material that was principally rod-like, Fig. 1a and was used
for further analysis by solid state NMR, FTIR, TGA and X-ray
diffraction (single crystal and powder). The temperature profile
of the TGA analysis of the rod-like structures was also used to
generate structures retaining the rod-like shape and consisting
of material that was resistant to heat treatment at 900 °C,
Fig. 1b and ESI6.‡
Fig. 3 Asymmetric unit of [(C6H4O2)3Si][C10H26N4]·H2O with labelling
scheme. Thermal ellipsoids set at 50% probability; some hydrogen
1
2
atoms omitted for clarity. Symmetry operations: −1 + x, y, z, 2 − x,
1 − y, 2 − z.
Fig. 4 a. Crystal packing of [(C6H4O2)3Si][C10H26N4]·H2O showing hydrogen bond network (dashed lines), top. C – black, O – Red, N – Blue,
Si – Orange, H – White. b. Crystal packing viewed down the crystallographic a axis showing layers of the two chiral enantiomers of [(C6H4O2)3Si]2−
Λ – Red, Δ – Green. Cyclam white and yellow for the respective layers. Thermal ellipsoids set at 50% probability; some hydrogen atoms omitted
for clarity.
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Solid state NMR of ‘pure’ samples of the tetragonal prisms
A network of hydrogen bonds exists between the oxygens on
was used to confirm the presence of silicon in an octahedrally the catechol ligands and the nitrogen atoms of the cyclam
coordinated environment (single peak at 142 ppm) and the (Fig. 4a, Table 1). Due to the centrosymmetric nature of the
presence of both catechol and cyclam in the structure, Fig. 2a unit cell two chiral enantiomers (delta and lambda) of the sili-
and b. 13C NMR shows characteristic peaks of aromatic carbon cate are present in the structure (Fig. 4b).
atoms in the same positions as the starting precursor dipotas-
The Si–O bond lengths (range 1.7775(11) Å for Si(1)–O(3) to
sium tris(1,2-benzenediolato-O,O′) silicate at 111, 150 and 1.7983(11) Å for Si(1)–O(2)) and angles (O(1)–Si(1)–O(2) 88.21(5)°,
180 ppm, and the peaks at 24, 42 and 46 ppm can be assigned O(3)–Si(1)–O(4) 88.89(5)°, and O(6)–Si(1)–O(5) 88.23(5)°) are
to cyclam which have been shifted to slightly lower ppm values similar to that of the analogous amine containing silicates
suggesting the involvement of hydrogen bonding in the tetra- [C3H7NH3]2[Si(C6H4O2)3]·0.5(C6H14N2),29 and [(C6H4O2)3Si]
gonal prisms. Thermogravimetric analysis coupled with CHN [{((CH3)2CH}2NH2]2·2(CH3CN)(H2O)30 (Si–O1 – 1.791(3) Å, Si–
analysis was used to confirm the composition of the needle- O(2) – 1.808(3) Å, Si–O(3) – 1.766(3) Å, Si–O(4) – 1.791(3) Å,
like phase, where the needles were suggested to contain a ca.
1 : 1 ratio of the silicon catecholate and cyclam, see ESI5‡ for
data and calculation. The lack of difference between the ther-
mogram collected on samples isolated at 5 minutes or
60 minutes after mixing further suggests that the needles once
Table 1 Table of relevant hydrogen bond lengths and angles.
Symmetry operation * = 1 + x, y, z
formed do not interact further with any residual silicon con-
taining species in solution, ESI6.‡
N–O distance
N–O length (esd) Å N–H distance Å (angle °)
N(1)–O(5)
N(2)–O(1)
O(6)–N(4)*
O(4)–N(3)*
2.7551(17)
2.9843(17)
3.0043(16)
2.7680(16)
1.946 (169.76)
2.029 (172.40)
2.095 (171.19)
1.778 (162.21)
FTIR analysis of the material as prepared, contained signa-
tures of both the cyclam and the Sicat complex, Fig. 2c. The
FTIR spectra contain two distinct regions; region 1 from
4000–2250 cm−1 and region 2; 1750–400 cm−1. Region 1 for
the rod-like structures included a secondary N–H stretching
band centred at ca. 3200 cm−1 and a loss of the O–H stretch at
ca. 3500 cm−1 suggesting that water in the native KSiCat salt is
largely replaced by cyclam in the structures formed. Region 2
showed only slight differences with the peaks shifted to
slightly higher wavenumbers in the tetragonal prisms. The
characteristic peaks at ca. 1500, 1250 and 1100 cm−1 attributed
to substituted aromatic molecules, the aryl vC–O and the
Si–O stretch respectively, all appear in both spectra indicating
the presence of tris(1,2-benzenediolato-O,O′)silicate (SiCat).
The Si–O band appears narrow in both the tetragonal prisms
and in KSiCat indicating that the Si–O bonds are in similar
environments and since the peak has not shifted in the
tetragonal prisms when compared to KSiCat we suggest that
the silicon atoms remain octahedrally coordinated. After
heat treatment of the tetragonal prisms (Fig. 2c insert) the
Si–O band at 1100 cm−1 appears broad, characteristic of the
Si–O–Si antisymmetric stretching vibrations of amorphous
silica as well as a peak at 805 cm−1 of symmetric Si–O–Si
stretching. Small differences in the fingerprint region
suggest that the Sicat complex remained intact within the
structure.
O(2)–O(7) [O(2)–H(7B)] 2.712(2) [2.0278]
The data obtained by FTIR, 29Si NMR and TGA provide evi-
dence that the tetragonal prisms are a complex formed
between undissociated SiCat and cyclam, where the tetragonal
prism morphology is assumed to form by molecular self-
assembly. Evidence for the assembly was gained from
single crystal X-ray analysis of the needle-like crystals.
[(C6H4O2)3Si][C10H26N4]·H2O crystallises in the triclinic crystal
Fig. 5 (a) Speciation diagram for cyclam calculated used SPARC.32 It is
known that cyclam exhibits an unusual speciation where protonation of
the fourth amine requires less energy than the third33 and as such the
calculated speciation diagram from SPARC was only used as a guide. (b)
pH range over which needles were formed and their sizes. A minimum
ˉ
system in the centrosymmetric space group P1. The asym-
metric unit consists of one slightly distorted octahedral
silicon(IV) atom coordinated by three catechol ligands as well as
one doubly protonated cyclam and one water molecule (Fig. 3). of 13 needles were measured for each sample.
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Fig. 6 Representative SEM images of organised structures formed in the presence of (a) 4MEDAE, (b) 4MEDAB and (c) 4MEDAH together with spe-
ciation diagrams generated using SPARC pKa calculator.32 The pH range over which organised structures form is indicated for each system.
Si–O(5) – 1.783(3) Å, Si–O(6) – 1.780(3) Å, O(1)–Si–O(2) – single enantiomer of the chiral silicate with the layers alternat-
87.53(13)°, O(3)–Si–O(4)
–
88.58(14)° and O(5)–Si–O(6)
–
ing between Λ and Δ (Fig. 4b).
88.41(13)°) and the bis-ethylaminediamine copper containing
A hydrogen bond network exists along the crystallographic
[(C6H4O2)3Si] [Cu(C2H8N2)2]31 (Si–O – 1.778–1.809 Å, O–Si–O – a axis between the cyclam N–H and catechol oxygen atoms as
88.3–89.3°). The silicon atom is in a slightly distorted octa- follows: the hydrogen atoms on N(1) and N(2) form hydrogen
hedral arrangement where the angles of the O–Si–O of each bonds with O(5) and O(1) respectively of the catecholate. O(4)
catechol deviate slightly from the idealised 90°. Each chelating and O(6) are hydrogen bond acceptors for the other two cyclam
catechol ligand forms two bonds to silicon via the oxygen nitrogen atoms (N(3) and N(4) respectively) completing the one-
atoms (Fig. 3). A single water molecule is present forming a dimensional hydrogen bond network that permeates through
hydrogen bond with the oxygen atom of one of the catechols the solid-state structure (Fig. 4a, bond lengths and hydrogen
(O(2)–O(7) – 2.712(2) Å, O(2)–H(7a) 2.0278 Å).
bond angles in Table 1). That similar structures are not formed
The structure consists of distinct layers or sheets of silicate between 1,8-dimethyl 1,4,8,11-tetraazacyclodecane (2MC) and
and cyclam molecules along the crystallographic b/c axes, with the silicon catecholate even though the doubly charged protona-
a hydrogen bond network between the cyclam and silicate tion state for the amine is possible suggests that it is principally
molecules of individual layers. Each layer contains only a sterics that preclude the formation of such materials.
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The coordination pattern for the azamacrocycle cyclam is (4MEDAE) showed a high dependency on the pH of the solu-
unlike that previously reported for azamacrocycle containing tion. Needle-like hollow hexagonal rods formed from pH 6.85
compounds where either covalent bonds were formed between and for experiments performed at pH 8.5, hexagonal rods were
species9 or the macrocycle was situated in pores12 and thus the formed. Experiments conducted at intermediate pH values (i.e.
structure described above represents a new coordination 8.21 showed a capping of the hollow rods. Example SEM
pattern.
Needles could be formed at a range of pH values, the
images of the structures formed can be found in ESI9.‡
success of which could be explained by considering the charge
state of both the catecholate and the azamacrocyclic with
success being achieved provided the catecholate salt was stable
(pH > 6.5) and the azamacrocyclic contained at least one, and
preferably two positive charges, Fig. 5.
Both pH and the Si : N ratio affected the structure of the
macroscopic materials formed. Tetragonal prisms, though
differing in aspect ratio could be formed over the pH range
(6.54–11.00), Fig. 5b that correlated with ability of the macro-
cycle to carry one or two positive charges, Fig. 5a as well as
being above the pH at which the silicon catecholate salt was
stable. Decreasing the Si : N ratio from 10 : 1 through to 2 : 1
did not improve the yield of rod-like materials with stars and
other aggregated structures being formed, Fig. 1c.
The formation and characterisation of tetragonal prisms using
dipotassium tris(1,2-benzenediolato-O,O′)germanate
We tested the generality of the approach using dipotassium
tris(1,2-benzenediolato-O,O′)germanate (KGeCat). In this case
base was added to the KGeCat solution to provide a range of
pH values compatible with the different protonation states of
cyclam. Hollow tetragonal prisms (ca. 30 × 3 microns-smaller
than for Si) were formed over a wide pH range (6.7–11.4)
whose X-ray powder diffraction pattern was very similar to that
generated for the Si based materials, ESI8 and SI10.‡
The precipitation of tris(1,2-benzenediolato-O,O′)silicate
complexes using tertiary amines
Although only one of the azamacrocylic compounds formed a
crystalline complex with silicon catecholate ions we tested a
range of diamines containing primary, secondary and tertiary
amine functionality at a Si : N ratio of 1 : 1 at pH 6.8 0.2 as
per the compounds listed in the materials and methods
section. Addition of tertiary amines led to the formation of
complex structures comprising the amine and the silicon cate-
cholate. The role of functional group spacing on the structures
formed was investigated using the tertiary diaminoalkanes
with 2, 4 and 6 carbon atoms between the amine groups
namely; N,N,N′,N′-tetramethylethylenediamine (4MEDAE), N,N,
N′,N′-tetramethyl-1,4-butanediamine (4MEDAB) and N,N,N′,N′-
tetramethyl-1,6-hexanediamine (4MEDAH). Images of typical
structures formed, the pH range and the speciation of the di-
amines as estimated using SPARC is presented in Fig. 6. The
Fig. 7 Reaction of hydrolysed TMOS in the presence of a range of aza-
macrocycles at pH 6.8. (a) Raw data from a molybdenum blue assay
pH range over which organised structures formed decreased as Si : N = 10.1 showing amount of orthosilicic acid left in solution after
reaction times in seconds, (b) kinetic rate constants for the early stages
of silica formation in the presence of azamacrocycles at a Si : N ratio of
10 : 1, (c) exemplar DLS data showing particle size versus time. The data
show a rapid stabilisation of structures at a range of sizes from TMOS
the number of carbon atoms between the amine groups was
increased corresponding to the reduced pH range over which
charge was carried by the amine.
Unlike the experiments conducted with cyclam, structures
with Si : N ratio of 1 : 1. Details on the specific azamacrocycles used is
formed in the presence of N,N,N′,N′-tetramethylethylenediamine given in ESI1.‡
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Characterisation of the materials formed showed that simi- Using azamacrocyles to generate monodisperse silica sols
larly to the cyclam silicon catecholate complexes, EDX con-
firmed the absence of metal cations, FTIR confirmed the pres- Although only cyclam was able to form a structural complex
ence of both the amine and the silicon catecholate salt, and with the silicon/germanium catecholate salt a number of aza-
powder XRD traces were similar to those obtained for the macrocycles impacted silica formation, both in the early stages
cyclam-silicon catecholate material, see ESI9.‡ TGA of the hex- of condensation and in the stabilization of mono-disperse sols
agonal rods formed at pH 8.51 showed that 88% of the when the silicon precursor was changed to unbuffered TMOS
materials mass is lost upon heating to 900 °C of which 37% at an Si : N ratio of 1 : 1, we found that azamacrocycles were
can be attributed to organic material lost between 400 and generally very good at catalysing silica formation, Fig. 7a.
650 °C. Silica only replicates of the initial structure could be There was an optimum ring size (14 atoms) for acceleration of
generated but it was not possible to obtain single crystal X-ray the initial stages of condensation Fig. 7b. Dynamic light scat-
diffraction data to confirm the local environment of the two tering was used to investigate the rates of aggregation of silica
ionic components. Based on the pH range over which com- with a range of stable sols being formed in the presence of
plexation occurred and assuming the mechanism for complex azamacrocycles, Fig. 7c. The materials that were isolated when
formation is similar to the that exhibited by cyclam then it either cyclam, 2MC or 4MC were the additives were essentially
would seem likely that the [4MEDAE]+ species is involved. If silica with low levels of entrapped organic material (ca. 2%),
the coordination pattern is similar to that encountered between data not shown. The material isolated after additions of 6NC
cyclam and the silicon catecholate ion then we would expect two was somewhat different as it contained ca. 10% entrapped
amine molecules to complex with one silicon catecholate ion.
organic material, data not shown.
Addition of a further two or four –CH2 groups between the
The effect of pH on silica particle size using the azamacro-
amine groups 4MEDAB and 6MEDAH led to the formation of cyclic molecules was investigated using the unbuffered TMOS
hexagonal plates (4MEDAB) or samples that contained both system, Fig. 8. Initial results show that the stabilised particle
disordered material and particulate sheets (6MEDAH), Fig. 6 size is dependent on the pH of the solution and the azamacro-
that although containing both the silicon catecholate ion and cyclic molecule used. According to the azamacrocyclic used a
the amine (evidence from FTIR ESI9‡) were not stable to exten- range of stable sols can be formed from 65–115 nm (cyclam);
sive washing. Further, the pH range over which such structures 105–300 nm (2MC), 30–210 nm (4MC) and 250–750 nm (6NC).
formed decreased in line with the amine speciation changes The mechanism of stabilisation is currently unknown but a
predicted by SPARC. We hypothesise that the structures suggested pathway may involve phase separation and droplet
observed contain both the amine and the silicon catecholate formation of the amine containing molecule as previously
ion though it is clear that as the spacing of the two amines encountered for silica formation in the presence of
increases the ability to form stable structures decreases mark- polyamines,23,33 alternatively, the azamacrocyles could act to
edly, perhaps due to steric constraints.
cap the silica materials once formed though this is unlikely
Fig. 8 The effect of azamacrocycle identity on silica sol formation. Si : N ratio of 1 : 1.
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due to the low level of retention of the molecules with the (18C6)),
amines:
(N,N′-bis(3-aminopropyl)-1,3-propanedi-
silica phase formed.
amine, N,N′-dimethyl-1,6-hexanediamine, tris(dimethylamino)
In contrast to the two common routes to monodisperse methane, 1,1,4,7,10,10-hexamethyltriethylenetetramine, 3,3′-
silicas which differ in their use of acetic acid (Karmaker)34 or iminobis(N,N-dimethylpropylamine), 1,3-diamino-2-hydroxy-
ammonia (Stöber)35 to catalyse the formation of silica, and propane-N,N,N′,N′-tetraacetic acid, 3-(dimethylamino)-1-propy-
take many hours to complete, our approach is significantly lamine, N,N,N′,N′-tetramethyldiaminomethane, N,N,N′,N′-tetra-
quicker. Materials prepared in the presence of several of the methylethylenediamine (4MEDAE), N,N,N′,N′-tetramethyl-1,4-
azamacrocyles tested retain very little organic material butanediamine (4METAB), N,N,N′,N′-tetramethyl-1,6-hexane-
meaning that the remaining, unbound additive could be diamine
(4MEDAH),
N,N,N′,N″,N″-pentamethyldiethyl-
recycled using a recently patented approach.36 Further detailed enetriamine, N,N,N′-trimethyl-1,3-propanediamine, N,N,N′-tri-
studies are required to understand the limitations of the methylethylenediamine. The silicon catecholate complex was
approach and the mechanism of particle formation. These are purified to >99% by recrystallising from methanol with purity
beyond the scope of the current contribution and may take assessed by 1H NMR (Jeol ECX-400 spectrometer) and a molyb-
considerable time to unpick as shown by the vast literature on denum blue assay.38 The charge speciation diagrams for
the Stöber process, the first major paper being reported in important cyclic compounds and amines was calculated using
1968 and only in 2017 was an article published ‘unravelling SPARC.32 The starting concentration of silica/germania precur-
the growth mechanism of silica particles in the Stöber sors was 30 mM throughout unless otherwise stated.
method’.37
Synthesis of crystalline inorganic–organic hybrid materials
Materials were prepared by mixing the silicon catecholate with
either azamacrocyclic compounds or diamines at Si : N ratios
Conclusion
from 10 : 1 through to 1 : 1 and a range of pH (adjusted using
We have shown that the azamacrocycle ‘cyclam’ can be used to acid or base but always above the pH where decomposition of
generate novel crystalline materials that exhibit a hollow tetra- the catecholate complex to orthosilicic acid occurs at around
gonal tube-like macrostructure, whose dimensions can be ca. pH 6.5) with the azamacrocyclic maintained at the same
tuned according to the pH of the synthesis medium. The syn- pH.
thesis approach was successful for both silicon and germa-
An example synthesis follows: KSiCat (0.14 g (30 mM)) was
nium and we hypothesise that a range of other catecholate pre- dissolved in 7.76 mL of distilled and deionised ice-cold water
cursors of elements such as iron could be used to generate a (dd water) to give a 30 mM solution on completion of addition
large array of inorganic materials with interesting mor- of all components. A solution of cyclam (2 mL) was prepared
phologies. The synthesis approach can be extended to tertiary in ice cold dd water at a Si : N ratio of 10 : 1 (0.0015 g, 7.5 µM).
diamines with functional group spacing playing an important Acid, 2 M HCl (240 µL) to generate the desired pH was added
role in the efficacy of complexation. Of the molecules explored to the cyclam solution. The two solutions were mixed and kept
to date, a C2 spacing leads to the most efficient structure at room temperature or 5 °C, the pH was taken after
control. In addition, we show that azamacrocycles can be used 15 minutes and the precipitate isolated by centrifugation after
to manipulate silica formation and provide a fast synthesis 60 minutes, unless otherwise stated. The precipitate was
route to particle sizes whose diameter can be tuned from ca. washed with dd water (40 mL) followed by centrifugation
20 nm to several hundred nm under mildly basic pH, con- 3 times and the material lyophilised (Virtis freeze dryer). For
ditions that will be suitable for their direct use in biotechnolo- materials prepared at a range of pH values, acid was added to
gical applications.
both solutions to give the desired pH (after 15 minutes) and
the solutions then mixed. The same procedure was used to
make materials using dipotassium tris(1,2-benzenediolato-
O,O′)germanate (KGeCat) as a starting material and for materials
prepared with 4MEDAE, 4MEDAB and 4MEDAH.
Materials and methods
Materials
Materials characterisation
All chemicals were purchased from Sigma Aldrich and used
without further purification unless otherwise stated. These Samples were investigated using Thermal Gravimetric Analysis
included silicon and germanium containing precursors (di- (TGA) using a PerkinElmer Pyris 6 TGA. The samples were
potassium tris(1,2-benzenediolato-O,O′)silicate (KSiCat), tetra- heated at 10 °C min−1 from 30 to 900 °C in air to ensure com-
methoxysilane) (TMOS) and dipotassium tris(1,2-benzenedio- plete combustion of organic material retained within the
lato-O,O′)germanate 97% (KGeCat), cyclic compounds contain- material. FTIR was performed using Nicolet Magna IR-750
ing heteroatoms (1,4,7-triazacyclononane (3NC), 1,4,7,10-tetra- spectrometer fitted with a Golden Gate attenuated total reflec-
azacyclododecane (4N2), 1,4,8,11-tetraazacyclodecane (cyclam), tion (ATR) accessory (Thermo Nicolet). Spectra were recorded
1,8-dimethyl 1,4,8,11-tetraazacyclodecane (2MC), 1,4,8,11-tetra at 4 cm−1 resolution, averaging 64 scans. Powder X-ray diffrac-
methyl 1,4,8,11-tetraazacyclodecane (4MC), 1,4,7,10,13,16- tion was performed using a Hilton Brooks modified Philips
hexaazacyclooctadecane (6NC). 12-Crown-4 (12C4), 18-crown-6 PW1050 powder diffractometer operating at 42.5 kV/18 mA
Dalton Trans.
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Paper
with a copper source Cu Kα = 1.540562 Å. Scans were per- for the pH ranges over which monodisperse silica sols were
formed in step mode with 0.02θ step size and 0.2 s dwell time. formed.
Data were recorded over the 2 range 3–80θ at room tempera-
ture. Single crystal X-ray diffraction measurements were per-
formed in Experimental Hutch 1 (EH1) of Beamline I19, at
Conflicts of interest
Diamond Light Source.39 The data were collected at a wave-
length of 0.6889 Å on a Fluid Film Devices 3-circle fixed-chi There are no conflicts to declare.
diffractometer using a Dectris Pilatus 2M detector. The crystal
was mounted on a MiTeGen micromount using a perfluoro-
polyether oil and cooled for data collection by a Cryostream
Acknowledgements
nitrogen-gas stream.40 The collected frames were integrated
using XIA2 41–45 software and the data were corrected for NMR spectra were recorded at the solid-state NMR Service in
absorption effects using AIMLESS,46 an empirical method.
Durham. We acknowledge Diamond Light Source for time on
All non-H atoms were located using direct methods and beamline I19 and beamline scientist Sarah Barnett under pro-
difference Fourier syntheses. Hydrogen atoms were placed and posal CY21755. We also thank Alex Slawin of Edinburgh
refined using a geometric riding model. All fully occupied University for preliminary X-ray data and colleagues at NTU
non-H atoms were refined with anisotropic displacement para- who attempted to generate single crystal X-ray data.
meters, unless otherwise specified. Crystal structures were
Funding is acknowledged from AFOSR FA9550-06-1-0154
solved and refined using the Olex2 software package47 with and FA9550-16-1-0213.
SHELXL48 (structure refinement) and SHELXT49 (structure
solution). CIF files were checked using checkCIF,50 CCDC
1916071‡
[(C6H4O2)3Si][C10H26N4]·H2O.
Solid state NMR (13C and 29Si) was carried out at Durham
University. solid-state silicon-29 NMR spectrum was
contains
the
supplementary
data
for
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