Two-Dimensional Frameworks Based on Ag(I)-N Bond Formation: Single Crystal to Single Molecular Sheet Transformation

Article abstract of DOI:10.1021/acs.inorgchem.6b01365

A series of new two-dimensional coordination framework materials, based on Ag(I)-N bond formation, has been synthesized and structurally characterized by single crystal methods. Reactions between the poly-monodentate bridging ligand N,N′-((1r,4r)-cyclohexane-1,4-diyl)bis(1-(pyridin-3-yl)methanimine), L1, and silver salts yield compounds {[Ag(L1)(MeCN)](CF₃SO₃^-)}_n, 1, {[Ag(L1)(PF₂O₂^-)]·H₂O}_n, 2, and {Ag₂(L1)(tosylate)₂}_n, 3. The frameworks of these materials exhibit two distinct net topologies: 3⁶.4⁶.5³ (1 and 2) and 4⁴.6² (3). In all cases, L1 ligands are found to be fully saturated, in terms of metal ion binding, with both sets of pyridyl and imino N atoms involved, though in 1 and 2, crystallographically independent L1 moieties also display pyridyl-only binding. Either solvent (1) or the anion (2 and 3) acts as a terminal ligand to support interlayer interactions in the solid state. For 2 and 3 the molecular sheet orientation lies in the plane of the largest crystal face, indicating that crystal growth is preferentially driven by coordinate bond formation. Despite the relatively labile nature, typical of such Ag(I)-N bonds, solvent-based exfoliation of crystals of 3 was shown to provide dispersions of large, μm², flakes which readily deposit on oxide surfaces as single-molecule sheets, as revealed by atomic force microscopy.

Full text of DOI:10.1021/acs.inorgchem.6b01365

Article
pubs.acs.org/IC
Two-Dimensional Frameworks Based on Ag(I)−N Bond Formation:
Single Crystal to Single Molecular Sheet Transformation
Glenn Lamming,† Osama El-Zubir,† James Kolokotroni,† Christopher McGurk,† Paul G. Waddell,‡
Michael R. Probert,‡ and Andrew Houlton*,†
†Chemical Nanoscience Lab and ‡Crystallography Lab, School of Chemistry, Newcastle University, Newcastle upon Tyne, NE1 7RU
United Kingdom
S
* Supporting Information
ABSTRACT: A series of new two-dimensional coordination framework
materials, based on Ag(I)−N bond formation, has been synthesized and
structurally characterized by single crystal methods. Reactions between the
poly-monodentate bridging ligand N,N′-((1r,4r)-cyclohexane-1,4-diyl)bis-
(1-(pyridin-3-yl)methanimine), L1, and silver salts yield compounds
−
−
{[Ag(L1)(MeCN)](CF3SO3 )}n, 1, {[Ag(L1)(PF2O2 )]·H2O}n, 2, and
{Ag2(L1)(tosylate)2}n, 3. The frameworks of these materials exhibit two
distinct net topologies: 36.46.53 (1 and 2) and 44.62 (3). In all cases, L1
ligands are found to be fully saturated, in terms of metal ion binding, with
both sets of pyridyl and imino N atoms involved, though in 1 and 2,
crystallographically independent L1 moieties also display pyridyl-only
binding. Either solvent (1) or the anion (2 and 3) acts as a terminal ligand
to support interlayer interactions in the solid state. For 2 and 3 the
molecular sheet orientation lies in the plane of the largest crystal face, indicating that crystal growth is preferentially driven by
coordinate bond formation. Despite the relatively labile nature, typical of such Ag(I)−N bonds, solvent-based exfoliation of
crystals of 3 was shown to provide dispersions of large, μm2, flakes which readily deposit on oxide surfaces as single-molecule
sheets, as revealed by atomic force microscopy.
polymers represent a somewhat smaller fraction, though many
examples are known, exhibiting a variety of topologies.22,31,32
While these materials have been shown to be capable of
catalysis,33 reversible anion binding33 and luminescence34
reports of Ag-based frameworks being prepared as single
molecular sheets are lacking.
With these factors in mind, we have begun to explore the
coordination chemistry of Ag(I) with poly-monodentate
ligands based on bis(1-pyridin-3-yl)methanimine. The labile
nature of the Ag−N bond21,22,35−39 means that high-quality
crystalline materials can be prepared at or below ambient
temperature, as well as by the hydrothermal methods40−43
typically used for metal−organic framework (MOF) materi-
als.44 However, this lability may have consequences for any
subsequent top-down exfoliation processing designed to isolate
single layer molecular sheets.
INTRODUCTION
■
In this “post-graphene” era there is an enormous interest in
two-dimensional or layered materials due to their potential for
applications in electronic, display, and sensor technologies,
etc.1−3 Metal−organic, or coordination polymer, frameworks4
provide a useful means to realize this type of structure.
Moreover, in contrast to graphene and binary inorganics, e.g.,
boron nitride (BN) and molybdenum disulfide (MoS2), such a
molecular-based approach offers an extensive level of
functionalization and diversity of architecture due to the
building-block nature of coordination chemistry.5
Notably, it has been demonstrated that two-dimensional
(2D) coordination framework materials comprising a few layers
or even a single molecular sheet can be obtained. Recent efforts
in this area include both “bottom-up” synthesis6−10 and “top-
down” exfoliation approaches.11−16 The successful and
reproducible preparation of these materials is an important
processing step for developments such as thin-film applica-
tions17,18 and membrane technologies.19
Here we report the synthesis and structural characterization
of a series of 2D coordination framework materials, {[Ag(L1)-
−
−
(MeCN)](CF3SO3 )}n, 1, {[Ag(L1)(PF2O2 )]·H2O}n, 2, and
{Ag2(L1)(tosylate)2}n, 3, (Scheme 1; L1 = N,N′-((1r,4r)-
cyclohexane-1,4-diyl)bis(1-(pyridin-3-yl)methanimine)). These
illustrate different ligand binding modes, metal:ligand stoi-
chiometry, and network topology. In addition, we demonstrate,
Ag(I)−N bond formation is very well established as a basis
for the synthesis of coordination polymers.20−28 The d10
electronic configuration of Ag(I) ensures a diversity of
coordination number and geometries though there is a general
tendency toward low-coordination-number, chainlike, systems
with many studies specifically aimed at producing one-
dimensional (1D) materials.21,29,30 Two-dimensional, sheet,
Received: June 6, 2016
© XXXX American Chemical Society
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Preparation of {[Ag(L1)(MeCN)](OTf)}n (1). Silver trifluorometha-
nesulfonate (0.1 mmol, 25.7 mg) and L1 (0.2 mmol, 58.5 mg) were
reacted using previously the described method and colorless single
crystals suitable for X-ray analysis were obtained upon standing. MPt:
decomposed to a dark brown material, 206−208 °C. FTIR: 1642 cm−1
(imine CN). ESI-MS (ESI, H2O): calcd for AgC4H6N2, 188.9582
([Ag + (MeCN)2]+); found, 188.9581 ([Ag + (MeCN)2]+, 38.3).
Calcd for C18H21N4, 293.1766 ([L1 + H]+); found, 293.1766 ([L1 +
H]+, 100.0). Calcd for AgC18H20N4, 399.0739 ([Ag + L1]+); found,
399.0738 ([Ag + L1]+, 1.5). Calcd for AgC20H23N5, 440.1004 ([Ag +
L1 + MeCN]+); found, 440.1007 ([Ag + L1 + MeCN]+, 7.7). Calcd
for AgC19F3H21N4O3S, 549.0337 ([Ag + L1 + OTf + H]+); found,
549.0340 ([Ag + L1 + OTf + H]+, 0.9). Calcd for AgC21F3H24N5O3S,
590.0603 ([Ag + L1 + MeCN + OTf + H]+); found, 590.0604 ([Ag +
L1 + MeCN + OTf + H]+, 0.9). Calcd for AgC36H40N8, 691.2427 ([Ag
+ (L1)2]+); found, 691.2426 ([Ag + (L1)2]+, 4.7). Elemental anal.
calcd for AgC19H20N4O3SF3: C, 41.54; H, 3.68; N, 10.20; found C,
41.65; H, 3.51; N, 10.23. Data indicates loss of solvent. Crystals of 1
were also obtained from reactions performed at L1:Ag(I) ratios of 1:1
and 3:1.
Scheme 1. Metallo-Ligand Frameworks All Prepared with
(1E,1′E)-N,N′-((1r,4r)-Cyclohexane-1,4-diyl)bis(1-(pyridin-
3-yl)methanimine), L1
for 3, that despite any relative lability of individual Ag−N
bonds, the material is amenable to solvent-based exfoliation and
provides dispersions of flakes of individual molecular sheets up
to several μm2 in size.
EXPERIMENTAL SECTION
■
General Methods and Reagents. All reagents were purchased
from Sigma-Aldrich or Alfa Aesar. Fourier transform infrared (FTIR)
spectra were recorded in air on a Shimadzu IRAffinity-1S or Varian
800 Scimitar Series. 1H NMR spectra were recorded on a Bruker
AC300 spectrometer at 300 MHz in CDCl3 (Sigma-Aldrich) unless
reported otherwise and referenced to residual non-deuterated solvent
(δ 7.26). High-resolution electrospray ionization mass spectrometry
(ESI-MS) spectra were recorded on a Waters Micromass LCT Premier
TOF mass spectrometer. Microanalyses were carried out by the
Elemental Analysis Service in the School of Human Sciences, London
Metropolitan University. Powder X-ray diffraction (XRD) data were
collected using a Panalytical X’pert MPD equipped with an
X’Celerator detector. Samples were mounted onto an oblique-cut Si
wafer as the substrate.
Preparation of {[Ag(L1)(PF2O2)]·H2O}n (2). Silver difluorophosphi-
nato (0.12 mmol, 25.3 mg) and L1 (0.2 mmol, 58.5 mg) were reacted
as described above and gave colorless single crystals suitable for X-ray
analysis upon standing. [The anion, initially identified through X-ray
47
−
crystallography, is a hydrolysis product of PF6 . ] Crystals of 2 were
also obtained from reactions with ∼1:3 ratio alongside free ligand. No
crystalline material was obtained from ratios of ∼1:1. 2 MPt:
decomposed to a dark brown material, 190−192 °C. FTIR: 1635
cm−1 (imine CN). ESI-MS (ESI, H2O): calcd for AgC4H6N2,
188.9582 ([Ag + (MeCN)2]+); found, 188.9581 ([Ag + (MeCN)2]+,
100.0). Calcd for C18H21N4, 293.1766 ([L1 + H]+); found, 293.1767
([L1 + H]+, 38.8). Calcd for AgC18H20N4, 399.0739 ([Ag + L1]+);
found, 399.0743 ([Ag + L1]+, 1.2). Calcd for AgC20H23N5, 440.1004
([Ag + L1 + MeCN]+); found, 440.1001 ([Ag + L1 + MeCN]+, 14.2).
Calcd for AgC36H40N8, 691.2427 ([Ag + (L1)2]+); found, 691.2425
([Ag + (L1)2]+, 0.8). Satisfactory elemental analysis could not be
obtained. The structure was confirmed by single crystal X-ray
crystallography.
Atomic force microscopy (AFM) images were acquired in air using
a MultiMode 8 atomic force microscope with a NanoScope V
controller (Bruker). The microscope was controlled by Nanoscope
software, version 9.1. The AFM was operated in ScanAsyst in air mode
as a peak force tapping mode at ultralow forces to minimize damage to
crystal surfaces. An isolation table (TMC) was used to reduce
vibrational noise. The probes used for AFM were silicon tips on silicon
nitride cantilevers (ScanAsyst, Bruker) with a nominal tip radius of
approximately 2 nm, resonant frequency 150 kHz, and spring constant
k = 0.7 N m−1. The AFM data was processed with NanoScope Analysis
1.50 software (Bruker). For imaging, P-type silicon wafers ⟨100⟩
oriented 1−10 Ω cm B-doped were used as substrates that were
cleaned by plasma ashing for 15 min at 8 kV followed by sonication in
ethanol for 15 min.
Synthesis of L1. (1E,1′E)-N,N′-((1r,4r)-Cyclohexane-1,4-diyl)bis-
(1-(pyridin-3-yl)methanimine), L1, was prepared based on literature
methods.45,46 trans-1,4-Diaminocyclohexane (25 mmol, 2.85 g) was
dissolved in ethanol (50 mL) and dichloromethane (50 mL). While
stirring, nicotinaldehyde (50 mmol, 5.36 g) was added to the solution
followed by two drops of formic acid. The solution was stirred at room
temperature overnight. The sample was evaporated under reduced
pressure until precipitation was observed. The solution was left to
stand, and vacuum assisted filtration followed by washing with ethanol
(10 mL) and hexane (10 mL) gave a white crystalline powder, L1 (5.4
g, 72%). mp: 160−162 °C. FTIR: 2920 m (CH), 2860 s (CH), and
1641 cm−1 vs (imine CN). 1H NMR (CDCl3): δ 1.79−1.88 (8H, m),
3.28−3.42 (2H, m), 7.34 (2H, dd, J 7.9, 4.8), 8.12 (2H, dt, J 7.9, 1.9),
8.41 (2H, s), 8.64 (2H, dd, J 4.8, 1.7), 8.86 (2H, d, J 1.6).
Preparation of {[Ag2(L1)(Tos)2]}n (3). Silver tosylate (0.1 mmol,
27.9 mg) and L1 (0.1 mmol, 29.2 mg) were reacted as described above
and gave colorless single crystals suitable for X-ray analysis upon
standing. 3 MPt: decomposed to a light brown material, 215−217 °C.
FTIR: 1641 cm−1 vs (imine CN). ESI-MS (ESI, H2O): calcd for
AgC4H6N2, 188.9582 ([Ag + (MeCN)2]+); found, 188.9582 ([Ag +
(MeCN)2]+, 99.4). Calcd for C18H21N4, 293.1766 ([L1 + H]+); found,
293.1767 ([L1 + H]+, 13.0). Calcd for AgC20H23N5, 440.1004 ([Ag +
L1 + MeCN]+); found, 440.1006 ([Ag + L1 + MeCN]+, 2.3). Calcd
for AgC25H28N4SO3, 571.0933 ([Ag + Tos + L1 + H]+); found,
571.0927 ([Ag + Tos + L1 + H]+, 0.7). Calcd for AgC36H40N8,
691.2427 ([Ag + (L1)2]+); found, 691.22426 ([Ag + (L1)2]+, 0.3).
Elemental anal. calcd for Ag2C32H34N4O6S2: C, 45.19; H, 4.03; N,
6.59; found C, 45.28; H, 4.16; N, 6.59. Crystals of 3 were also obtained
from reactions with a 1:3 ratio.
Single-Crystal X-ray Diffraction. Crystal structure data for 1, 2,
and 3 were collected at 150 K on an Xcalibur, Atlas, Gemini ultra
diffractometer equipped with a sealed X-ray tube (λCu Kα = 1.541 84 Å)
and an Oxford Cryosystems CryostreamPlus open-flow N2 cooling
device. Cell refinement, data collection, and data reduction were
undertaken via the software CrysAlisPro.48 Intensities were corrected
for absorption analytically using a multifaceted crystal model created
by indexing the faces of the crystal for which data were collected.49
Crystal structure data for L1 were collected at 120 K on a Bruker
D8 Venture diffractometer equipped with a Photonic CMOS detector,
microfocus sealed X-ray tube (λMo Kα = 0.710 73 Å) and an Oxford
Cryosystems CryostreamPlus open-flow N2 cooling device. An
empirical absorption correction using spherical harmonics was applied
to the intensities using the program SADABS.
Silver Complexes 1−3. The general method used to obtain
crystalline samples was a liquid/liquid diffusion technique. Briefly, L1
was dissolved in dichloromethane (10 mL) and the corresponding
silver salt was dissolved in acetonitrile (10 mL). Reactions were
typically performed for 1:1, 1:2, and 1:3 metal:ligand stoichiometric
ratios, and those yielding single crystals were examined further. All
solutions were cooled in dry ice during and after the layering of the
relevant solutions. The crystallization vessels were stored in the freezer
(−20 °C), in a dark container, for 3 days before transfer to a
refrigerator for a further week.
All structures were solved using XT50 and refined by XL51 using the
Olex252 interface. Hydrogen atoms were positioned with idealized
geometry, with the exception of those bound to heteroatoms which
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were located from peaks in the Fourier difference map. The thermal
parameters of the hydrogen atoms were constrained using the riding
model with U(H) set at 1.2Ueq for the parent atom.
molecule (Ag−NMeCN 2.400(1) Å). Details of the coordination
sphere are illustrated in Figure 1.
While the structure of L1 was collected at a different temperature
from those of 1, 2, and 3, the experimental collection conditions are
not significantly different and hence comparisons between the bond
geometries of the free ligand and the coordination complexes are valid.
Preparation of Single Molecular Sheet by Solvent-Based
Exfoliation.16 A crystalline sample of 3 (ca. 50 mg) was transferred to
filter paper. This was washed with hexane (2 × 10 mL) and ethanol (2
× 10 mL) and allowed to dry in air for 5 min. The sample was added
to toluene (1.5 mL) and the mixture was sonicated at 40 kHz using a
Langford Sonomatic 475H. Sonication times varied between 30 s and
5 min, with increased sonication time generally producing smaller
flakes. The sonicated material was centrifuged for 5 min at 8000 rpm,
and the supernatant was pipetted onto silicon wafer, allowed to dry,
and analyzed by AFM.
RESULTS AND DISCUSSION
■
Ligand L1 (1E,1′E)-N,N′-((1r,4r)-cyclohexane-1,4-diyl)bis(1-
(pyridin-3-yl)methanimine) (Scheme 1) was prepared accord-
ing to previously described methods.45,46 This molecule offers
two pairs of distinct, monodentate N-donor types: the pyridyl
and imino functionalities. Combined with the 3-substitution, a
range of metal binding site distances and metal−ligand bond
vectors can be brought about by simple rotation of the Py−C
bond. Single crystal X-ray diffraction studies reveal that L1 has
an E-arrangement with anti-anti-anti conformation (as shown in
Scheme 1) in the solid state at both 223 K53 and 120 K as
measured in this work.
Figure 1. Structural details of 1 with (upper) the coordination sphere
about the Ag ion and (lower) the different metal-binding patterns of
the crystallographically independent L1 groups.
The twist angles between the mean planes of the pyridyl
rings and cyclohexyl groups differ significantly between ligand−
N1 and ligand−N3 at 62.8(1) and 36.8(1)°, respectively. The
twist angle of ligand−N1 more closely matches that of the
crystal structure of free ligand L1 (58.2(1)°), suggesting that
Ag(I)−NIm coordination causes the deviation in ligand−N3. As
with the free ligand, both independent L1 ligands have the
same anti-anti-anti conformation with both pairs of imine and
pyridyl ring N-donor atoms (N1/N2) and (N3/N4) oriented
in opposite directions.
Of the two crystallographically independent L1 ligands of 1,
one coordinates through all four nitrogen donors, while the
other coordinates via the pyridyl nitrogen donor atoms only
(Figure 1). The CN imino bond is essentially unchanged by
metal binding (N4C16 1.2695(1); N2C7 1.2669(1); C
N bond free ligand L1 1.2662(1) Å). Similarly, the FTIR
spectrum also shows little evident shift of the νCN observed
compared to the free ligand (1641 cm−1 in 1, cf. 1640 cm−1),
though a slight shoulder is evident at higher wavenumber for 1
possibly due to the different imino groups (see Supporting
Information, Figure S1).
The observed metal−ligand connectivity gives rise to a 2D
36.46.53 net topology and forms two different sized metal-
locyclic rings: a 12-membered disilver ring and a larger 52-
membered tetrasilver ring. In the smaller metallocycle, the
pyridyl rings show parallel, partial, stacking with an interplanar
separation of 3.34(1) Å as measured from a calculated ring
centroid to ring plane (Figure 2). Also notable here is that the
Ag(I) ion lies out-of-plane of the N3-containing pyridyl group
(cf. ∠Py(N3)Cent−N3−Ag = 154.5(1)°), ∠Py(N1)Cent−N1−
Ag = 173.8(1)°).
Metal Complex Formation. As our primary interest was to
establish unambiguous structural detail of the compounds
through X-ray diffraction analysis, a liquid/liquid diffusion
method for preparation of complexes 1−3 as single crystals was
used. Solutions of dichloromethane containing L1 and
acetonitrile containing the corresponding silver salt, Ag(OTf),
Ag(Tos), or Ag(PF2O2) (where OTf = trifluoromethanesulfo-
nate and Tos = p-toluenesulfonic acid), were prepared, cooled
in dry ice, and then layered using a pipet. Reactions were
typically performed for 1:1, 1:2, and 1:3 metal:ligand ratios,
though a lack of stoichiometric control, quite common to the
coordination chemistry of Ag(I) ions,54,55 was apparent (vide
infra). The reaction mixtures were stored in a freezer (−20 °C)
for 3 days before transfer to a refrigerator. This method
typically provided samples suitable for single crystal X-ray
diffraction after 4−7 days. ESI-MS analysis of the solid material
collected from the respective reactions indicated metal ion
binding by the presence of species corresponding to [AgL1]+,
[AgL1(MeCN)]+, [AgL1(anion) + H]+, and [AgL12]+. No
degree of higher oligomerization was observed in solution,
however, consistent with a rather labile nature of the Ag−N
bond.
{[Ag(L1)(MeCN)](OTf)}n (1). Reactions of L1 with Ag(OTf)
at 1:1, 1:2, and 1:3 metal:ligand ratios yielded colorless
crystalline material; representative single crystals isolated from
each solution were identified by X-ray diffraction as the 1:1
complex {[Ag(L1)(MeCN)](OTf)}n 1. Compound 1 has a 2D
coordination polymer structure based on distorted tetrahedral
metal ion geometry about a single unique Ag(I) ion. The Ag(I)
is bound by the pyridyl groups of two crystallographically
independent L1 moieties (Ag−NPyr 2.258(1) and 2.330(1) Å
for N1 and N3, respectively), an imine nitrogen atom (Ag−NIm
2.318(1) Å), and the nitrogen atom of an MeCN solvent
The larger 52-membered metallocyle generates essentially
parallelogram-shaped pores with sides of ∼1.8 nm × 1.2 nm, as
measured by the respective Ag···Ag distances, in the single
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Figure 2. Parallel stacking of pyridyl rings that make up the smaller 12-
membered metallocycle.
sheet arrangement (Figure 3). These pores are occupied by
OTf− anions. The dipyridyl moieties, L1(N1/N2) and L1(N3/
Figure 4. Structural details of 2 showing (upper) the tetrahedral
coordination sphere about the Ag ion and (lower) the different metal-
binding patterns of crystallographically independent L1 groups.
result, the intradonor atom NPy···NPy distance for N1Py···N1Py is
shorter than N3Py···N3Py (13.9(1) and 15.0(1) Å, respectively)
with the equivalent distances in 1 being close to the latter value,
at 14.9(1) and 15.1(1) Å.
Figure 3. (upper) Parallelogram-shaped pores of the 2D coordination
polymer sheet of 1 as viewed essentially perpendicular to the plane.
(lower) View through the (110) plane showing individual sheets in
profile with a single sheet highlighted (red).
As in 1, the crystallograhically independent ligands
coordinate with either all four N-sites (N3/N4) or via only
the pyridyl, N1, sites. The imino bond lengths show little
change upon coordination (2: N4C16 1.2714(1) Å; N2C7
1.2694(1) Å; L1: NC 1.2660(7) Å) and there is little shift of
the FTIR νCN stretch (1636 cm−1) though a shoulder on the
main band is again observed (see Supporting Information,
Figure S1).
The Ag−N bonding generates the same framework topology
of 36.46.53 as in 1, but features 22-membered Ag2L12 and 42-
membered Ag4L14 metallomacrocycle rings. Figure 5 illustrates
the structure of an individual molecular sheet viewed onto the
plane and the packing of these sheets in the crystal structure
along with dimensions for layer thickness. There is notable
corrugation of the individual sheets, and interdigitation is more
extensive than is the case for 1.
{[Ag2(L1)(Tos)2]}n (3). Single crystals isolated from reactions
of L1 and Ag(Tos) at metal:ligand 1:1 stoichiometry were
found to be the 2:1 complex, {[Ag2(L1)(Tos)2]}n 3 (Figure 6).
We note that this same compound was also isolated from
reactions containing a metal:ligand 1:3 stoichiometry, alongside
at least one other product. Compound 3 contains a unique
Ag(I) ion 3-coordinated by donor atoms from pyridyl (Ag−N1
2.1892(1) Å) and imine nitrogen atoms (Ag−N2 2.1886(1) Å)
from different L1 moieties generating a 2D coordination
framework. The tosylate counterion acts as a weak terminal
ligand (Ag−O1 at 2.522(1) Å). The bond angles around the
metal ion indicate a distorted T-shape geometry (∠N1−Ag1−
N2 154.64(1)°; ∠O−Ag1−N1 99.24(1)°; ∠O−Ag1−N2
N4), make up the longer and shorter sides of the pore,
respectively. Two-dimensional sheets of 1 grow in the
crystallographic (101) plane and stack parallel to this (Figure
3). There is slight interdigitation of sheets involving the
terminal acetonitrile molecules. Individual sheet thickness, as
measured by the distance between planes calculated through
the methyl groups (C21) of the coordinated MeCN, is
approximately 8 Å.
{[Ag(L1)(PF2O2)]·H2O}n (2). Single crystals isolated from
reactions of L1 and Ag(PF2O2) at both 1:2 and 1:3
metal:ligand stoichiometries were analyzed by single crystal
X-ray diffraction and found to be the 1:1 complex {[Ag(L1)-
(PF2O2)]·H2O}n 2 (Figure 4). Compound 2 is essentially
similar to 1, adopting a 2D framework structure based on a
distorted tetrahedral coordination geometry about Ag(I). The
Ag(I) ion is similarly coordinated to two pyridyl nitrogen atoms
from crystallographically independent ligands (Ag−N1
2.357(1); Ag−N3 2.337(1) Å) and an imine nitrogen at a
slightly shorter distance (Ag−N4Im 2.259(1) Å). In this case the
−
fourth site is occupied by an oxygen from the terminal PF2O2
counterion (Ag−O = 2.5166(1) Å), resulting in an electroni-
cally neutral layer structure.
Compared to 1, the two independent ligands are more
similar with regard to the twist angles between the mean planes
of the pyridyl rings and cyclohexyl groups (ligand−N1 =
50.9(1)°, ligand−N3 = 71.2(1)°; cf. 62.8(1) and 36.8(1)°
respectively for 1). However, they adopt syn-anti-syn (N1/N2)
and anti-anti-anti (N3/N4) conformations, respectively. As a
D
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Considering the rotations about the Py−C bond in structures 1
and 2, L1 appears to exhibit little conformational preference to
the orientation of the N-donor atoms. The ligand L1 lies on an
inversion center, and all four N-donors are involved in metal
ion binding. X-ray and FTIR data indicate little change to the
CN function as a result of coordination (viz., 1: CN
1.2685(1) Å, νCN 1641 cm−1; L1: CN 1.2660(7) Å, νCN
1640 cm−1).
The resulting coordination mode forms a single type of 34-
membered metallocycle containing four Ag ions and four
contributing ligand moieties. These generate a 2D framework
with 44.62 topology where the centroid is a 4-connected ligand
node and the silver atoms are 2-connecting linkers. Within
individual layers the Ag(I) ions lie in a plane (Figure 7) and the
Figure 5. (upper) Single sheet of 2 viewed approximately through the
(121) plane. (lower) View approximately through (100) showing
sheets in profile. A single sheet is highlighted (red) and illustrates the
interdigitating of layers and the corrugation of the sheets.
Figure 7. (upper) Details of the crystal structure of 3 illustrating the
wavelike 34-membered metallocycles of individual MOF sheets viewed
onto the (100) plane. (lower) Packing of sheets in the crystal as
viewed onto the (010) plane with the (100) face uppermost.
layer thickness is ∼14.16(1) Å including the coordinated
tosylate anions. This is larger than the 11.5000(5) Å unit cell a-
axis, which is the approximate sheet stacking direction and the
measured interlayer spacing, due to interdigitation of the tosyl
groups of adjacent sheets.
Crystal Habit/Molecular Sheet Alignment and Single
Sheet Production. In order to identify the method and
compound most suited to produce single molecular sheets, we
initially established the relationship between bulk crystal habit
and the internal molecular orientation. From face-indexing of
single crystals of the compounds, it was noted that the layers
generally lie in the plane of the largest crystal face (see the
Supporting Information for a note on 1). For 2 the principal
Figure 6. Structural details of 3 with (upper) the trigonal coordination
sphere about the Ag ion and (lower) metal-binding pattern of L1.
Hydrogen atoms omitted for clarity.
103.07(1)°; ∑ ≅ 357.0°) with the Ag ion lying slightly
(0.19(1) Å) out of the mean coordination plane
The ligand adopts a syn-anti-syn conformation through
rotation about the Py−C bond which directs the imine and
pyridyl ring N-donor atoms (N1/N2) in the same direction.
face of the crystal was (011), while in 3 it was (100). Figure 8
̅
shows the superposition of the molecular structure in relation
to the individual crystal habit for 2 and 3. In the case of 3 this
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Figure 8. Superposition of the crystal habit and the relationship of the internal molecular structure for (upper) 2 showing largest crystal face as (011)
̅
with the corrugated sheet lying in this plane. (lower) Hexagonal crystal habit of 3 with (100) face as the large face as confirmed by AFM.
Figure 9. AFM height images (a and b) of the (100) face of a crystal of 3 and (c) corresponding line section showing the height of steps between
adjacent terraces in 1(b).
corresponds exactly to the crystallographic (011) plane and
stacking of sheets is then along the crystallographic [100]
direction, given the P21/c space group. The observed
relationship between crystal habit and the molecular orientation
can be rationalized from the view that the stronger, Ag−N,
bond formation drives crystal growth in the crystallographic
(011) plane while growth in the crystallographic [100]
direction is instead extended by weaker van der Waals
interactions of tosylate anions. Similar arguments for crystal
growth can be made for 2.
Due to issues of size and quality, as well as the smaller extent
to which individual layers interdigitate, crystals of 3 were
selected for further study and imaged by AFM. Scanning the
dominant (100) face revealed the expected step−terrace
features, with large, smooth, μm2 terrace regions (Figure 9).
The step height was persistently 1.1 nm corresponding to the
intersheet separations along the crystallographic [100]
direction length, viz. 11.45 Å (see Figure 7).
involved sonication in toluene for several minutes. Initial
indication that the procedure was successful was obtained by
observing the Tyndall effect,13 confirming dispersion in solvent.
Powder XRD analysis further established that the exfoliation
occurs with highly preferred orientation corresponding to the
(100) sheet in the crystal structure of 3 with peaks
corresponding to (100), (200), (300), and (400) planes in
the diffraction pattern observed (Figure 10).
AFM of the reaction mixture after deposition on SiO2
substrates revealed flakes ranging in size up to ∼30 μm2
(Figure 11). Analysis of individual flakes shows them to very
closely match the expected height of 1.4 nm for single tosylate-
capped molecular sheets. The difference between the heights
measured for these single sheets and the step heights on the
(100) crystal face is informative. As illustrated in Figure 7, the
larger of these two distances correspond to the thickness of an
individual sheet with tosylate anions binding above/below the
sheet plane. In the crystal, packing of these sheets involves
interdigitation of the tosylate anions on adjacent sheets which
has the effect of reducing the step height by ∼3 Å. This AFM
data indicates that after exfoliation the Ag(I)−L1 framework
deposits as electrically neutral sheets with the “weak” tosylate
anions bound.
Despite the optimal alignment of the sheets to the crystal
shape, little success was achieved with mechanical, adhesive
tape, exfoliation as a means for producing flakes of single
molecular sheets. However, samples of single molecular sheets
were prepared by solvent-based exfoliation.16 This procedure
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coordination framework compounds as precursors for single-
molecular layer material applications, and our efforts in this area
are ongoing.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01365.
X-ray crystallographic information (CIF)
FTIR, ESI-MS, summary crystallographic data, and
optical and AFM images (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: andrew.houlton@ncl.ac.uk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by the School of Chemistry,
Newcastle University, and EPSRC (EP/K018051/1). Dr.
Rantej Singh Kler is thanked for assistance with powder XRD
measurements.
Figure 10. Powder X-ray diffraction data for 3: (a) predicted bulk; (b)
experimental bulk; (c) predicted pattern for a preferred 100
orientation with a March−Dollase parameter56 of 0.25; (d)
experimental data for exfoliated flakes.
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