Pre-organisation of ligand donor sets modulates the supramolecular structure of bis(pyridyl-imine) silver(i) chelates

Article abstract of DOI:10.1039/d0ce01610a

Three bis(pyridyl-imine) ligands 1,2-bis(2-pyridyliminomethyl)benzene (L1), 1,3-bis(2-pyridyliminomethyl)benzene (L2) and 1,4-bis(2-pyridyliminomethyl)benzene (L3) were synthesised and complexed to silver(i) salts forming four novel complexes; 1,2-bis(2-p

Full text of DOI:10.1039/d0ce01610a

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Pre-organisation of ligand donor sets modulates
the supramolecular structure of bis(pyridyl–imine)
silver(I) chelates†
Cite this: CrystEngComm, 2021, 23,
1294
*
Chané Venter and Matthew P. Akerman
Three bis(pyridyl–imine) ligands 1,2-bis(2-pyridyliminomethyl)benzene (L1), 1,3-bis(2-pyridyliminomethyl)
benzene (L2) and 1,4-bis(2-pyridyliminomethyl)benzene (L3) were synthesised and complexed to silver(I)
salts forming four novel complexes; 1,2-bis(2-pyridyliminomethyl)benzene-silver(I) hexafluoroantimonate(V)
[Ag(L1)](SbF6), bis(1,3-bis(2-pyridyliminomethyl)benzene-di-silver(I)) bis(hexafluoroantimonate(V)) [Ag2-μ-
(L2)2](SbF6)2, catena-1,4-bis(2-pyridyliminomethyl)benzene-silver(I) hexafluoroantimonate(V) [Ag-μ-(L3)]
(SbF6) and catena-1,4-bis(2-pyridyliminomethyl)benzene-silver(I) tetraphenylborate(III) [Ag-μ-(L3)](BPh4). All
four complexes were characterized by 1H and 13C NMR, IR, mass spectrometry and UV/visible spectroscopy
as well as X-ray crystallography. The ligands were designed to preorganise the atom donor sets to control
the structures of the metal chelates. The pre-organised ligands control the supramolecular structure of the
bis(pyridyl–imine) silver(I) chelates, which results in the formation of a monomeric structure for chelate
[Ag(L1)](SbF6), a dimeric structure for chelate [Ag2-μ-(L2)2](SbF6)2, and polymeric structures for chelates
[Ag-μ-(L3)](SbF6) and [Ag-μ-(L3)](BPh4). The d10 electronic configuration of silver allows the metal ion to
adopt both tetrahedral and square planar coordination geometries in the above compounds, assisting the
formation of the various compounds. Additionally, the stabilising effects of the intermolecular interactions
were studied using DFT methods.
Received 6th November 2020,
Accepted 8th January 2021
DOI: 10.1039/d0ce01610a
rsc.li/crystengcomm
crystal engineering and coordination chemistry.17–20 A study
done by Deng et al. illustrates silver(I) being coordinated to
Introduction
Crystal engineering and the design of porous materials for
small molecule storage has been an area of intense research
in recent years. To this end, understanding the influence
ligand donor set geometry has on the supramolecular
structure of coordination compounds is highly relevant.1–4
Schiff base ligands are commonly used to stabilise metal ions
in various oxidation states through coordination.2,5 Their
application spans a range of fields including: catalysis,
metallodrugs, metal extraction and, significantly, crystal
engineering.2,5,6–9 The coordination of silver(I) to neutral,
tetradentate N-donor Schiff base ligands is relatively
uncommon; with metals ions such as Cu(II), Zn(II), Mn(II),
Co(II), Ni(II), and Au(III), among others, being more frequently
coordinated to these Schiff base ligands.8–16
flexible bis(pyridyl–amine) ligands with various spacers,
forming bimetallic structures.19 Since silver(I) has a d10
electronic configuration and hence zero crystal field
stabilisation energy (CFSE), a range of coordination geometries
are possible. This makes it a suitable candidate for testing the
impact of ligand geometry on the resultant structure of
supramolecular compounds. The range of coordination
geometries that are possible with little or no energy cost has
led to some interesting compounds such as helical silver(I)
coordination polymers from flexible bis(pyridyl) ligands.17 The
flexible coordination geometry of silver(I) is potentially ideal for
crystal engineering as small changes in the structures of
ligands and/or counter ions can lead to noticeable changes in
the three-dimensional crystal lattice.
The study of silver(I) coordination complexes with rigid or
flexible bis(pyridyl) ligands is gaining interest in the fields of
The various regioisomers of the bis(pyridyl–imine) ligands
with rigid aromatic di(azomethine) linkages have found
widespread
application.
1,2-Bis(2-pyridyliminomethyl)
benzene has been used to stabilise trivalent lanthanide
metals for applications as biological markers, photodynamic
therapeutics, NMR contrast reagents and phosphoryl transfer
catalysts.21 This regioisomer of the ligand has also been
coordinated to Ni(II), Cu(II) and Fe(II) which led to
mononuclear complexes in all cases.22,23 The ligand 1,4-bis(2-
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01,
Scottsville, Pietermaritzburg, 3209, South Africa. E-mail: akermanm@ukzn.ac.za
† Electronic supplementary information (ESI) available. CCDC 2033490–2033493,
contains the crystallographic data for compounds [Ag(L1)](SbF6), [Ag2-μ-(L2)2]
(SbF6)2, [Ag-μ-(L3)](SbF6) and [Ag-μ-(L3)](BPh4), respectively. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/d0ce01610a
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pyridyliminomethyl)benzene has been complexed to Re(III)
and Ir(III) and the crystallographic and luminescent
using a Puresolv™ MD 7 purification system from Innovative
Technologies. NMR spectra were recorded with a Bruker
Avance III 400 spectrometer equipped with a Bruker magnet
(9.395 T) using a 5 mm TBIZ probe at the following
properties of the complexes studied.24,25
A series of
phenylene-bridged dimer species complexed to Re(III) have
been synthesised and studied crystographically. The dimeric
meta- and para-phenylenediamine-bridged metal compounds
were investigated for photophysical applications using UV-
visible spectroscopy and cyclic voltammetry.26 Similar ligands
ortho-, meta- and para-bis(imino-pyrrole) benzene forming
three semi-rigid isomeric molecules have also been studied.
These examples show the significance of pre-organisation of
atom donor sets of free ligands for control of the
supramolecular structure in nanomaterials.27 The ligands are
also highly conjugated π-systems, capable of forming myriad
non-covalent interactions such as π⋯π and Ag⋯π
interactions, which have been previously shown to be
significant in the field of crystal engineering.28–33
Herein, we report the synthesis, solid-state structures and
UV/visible spectra of four silver(I) chelates (Scheme 1). The
chelates differ by the structure of the phenylenediamine
bridging unit. Through pre-organisation of the ligand donor
sets it is possible to control the supramolecular structure of
the metal chelates. The stabilising effects of some of the
intermolecular interactions are studied using DFT methods.
1
frequencies: H = 400 MHz and 13C = 100 MHz. The spectra
were recorded at 30 °C. All proton and carbon chemical shifts
are expressed in parts per million relative to DMSO-d6: 1H,
2.50 ppm and 13C, 39.5 ppm. The abbreviations in the script
are as follows; im: imine and py: pyridyl. FTIR spectra were
recorded using a Bruker Alpha FTIR spectrometer equipped
with an ATR platinum Diamond 1 reflectance accessory. The
instrument acquired the information in 32 scans with a
spectral resolution of 1.0 cm−1. The abbreviations used in the
script are as follows; s, strong; m, medium and w, weak
signals. Electronic spectra were recorded using a Perkin
Elmer Lambda 25 double-beam spectrophotometer (1.0 cm
path length cuvette). Spectra were recorded from 700 to 200
nm in acetonitrile. High resolution mass spectra were
determined with a Waters Acquity-LCT Premier coupled high
performance liquid chromatograph-mass spectrometer (time-
of-flight) using electrospray ionisation in positive mode.
The X-ray data were recorded on a Bruker Apex Duo
diffractometer equipped with an Oxford Instruments Cryojet
operating at 100(2) K and an Incoatec microsource operating
at 30 W power. Crystal and structure refinement data are
given in Table 1. In all four cases the data were collected with
Mo Kα (λ = 0.71073 Å) radiation at a crystal-to-detector
distance of 50 mm. The data collections were performed
using omega and phi scans with exposures taken at 30 W
X-ray power and 0.50° frame widths using APEX2.34 The data
were reduced with the programme SAINT34 using outlier
rejection, scan speed scaling, as well as standard Lorentz and
polarisation correction factors. A SADABS semi-empirical
multi-scan absorption correction34 was applied to the data.
Direct methods, SHELXL-2014 (ref. 35) and WinGX36 were
used to solve all four structures. All non-hydrogen atoms
were located in the difference density map and refined
anisotropically with SHELXL-2014.29 All hydrogen atoms were
included as idealised contributors in the least squares
process. Their positions were calculated using a standard
Experimental
Materials and methods
All reactions were carried out under inert conditions using a
double manifold vacuum line, cannula techniques and
Schlenkware. All reagents used in the syntheses were
purchased from Sigma-Aldrich and used as received. Organic
solvents were of analytical reagent grade and purchased from
Merck (South Africa). All solvents were dried prior to use,
riding model with C–Haromatic distances of 0.95 Å and Uiso
=
1.2 Ueq. Platon SQUEEZE37 was used to remove disordered
solvent from the lattice of complex [Ag-μ-(L3)](SbF6). The
process left solvent accessible voids of 884 Å3, which
accounts for 18.3% of the unit cell volume.
General procedure for the synthesis of [Ag(L1)](SbF6),
[Ag2-μ-(L2)2](SbF6)2, [Ag-μ-(L3)](SbF6) and [Ag-μ-(L3)](BPh4)
To a dry round-bottomed flask, two molar equivalents of
pyridine-2-carboxaldehyde (312 mg, 2.91 mmol), one molar
equivalent of the respective phenylendiamine regioisomers
(157 mg, 1.45 mmol) and molecular sieves were added. Dry
tetrahydrofuran (30 mL) was then added by cannula transfer
and the solution heated to reflux. A yellow solution was
observed after 30 minutes which was then transferred via
Scheme 1 Structure and naming scheme of the silver(I) complexes
studied in this work.
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Table 1 Summary of crystal data and structure refinement details for [Ag(L1)](SbF6), [Ag2-μ-(L2)2](SbF6)2, [Ag-μ-(L3)](SbF6) and [Ag-μ-(L3)](BPh4)
Crystal data
[Ag(L1)](SbF6)
[Ag2-μ-(L2)2](SbF6)2
[Ag-μ-(L3)](SbF6)
[Ag-μ-(L3)](BPh4)
Chemical formula
Molar mass g
mol−1
C18H14AgF6N4Sb
629.95
C36H28Ag2F12N8Sb2
1259.92
C18H14AgF6N4Sb
629.95
C42H34AgBN4
713.41
Crystal system,
space group
a, b, c/Å
Monoclinic, P21/n
Monoclinic, C2/c
Monoclinic, P21/c
Monoclinic, P21/c
7.7112(3), 12.8858(6),
19.6573(8)
β = 91.982(2)
100(2)
16.2711(10), 15.9716(11),
15.6817(11)
β = 102.447(3)
100(2)
13.6966(13), 13.1588(12),
26.895(3)
β = 96.078(4)
100(2)
16.2241(15), 9.2420(9),
23.889(2)
β = 107.447(2)
100(2)
β°
Temperature/K
Z
4
4
4
4
V/Å3
1952.08(14)
1208.0
2.455
3979.5(5)
2416.0
2.409
4820.1(5)
2416
1.99
3417.2(6)
1464.0
0.626
F (000)
μ/mm−1
Crystal Dim./mm
Data collection
0.09 × 0.06 × 0.05
0.11 × 0.07 × 0.04
0.010 × 0.080 × 0.210
0.15 × 0.06 × 0.04
Total, unique data 5696, 5072
5303, 4950
0.0264
11 340, 9294
0.0402
6707, 5621
0.0250
Rint
0.0239
Refinement
Final
Indices R1 = 0.024, wR2 = 0.055
R1 = 0.022, wR2 = 0.049
R1 = 0.091, wR2 = 0.185
R1 = 0.029, wR2 = 0.070
R[I > 2σ(I)]
cannula to a second dry round-bottomed flask containing
one molar equivalent silver(I) hexafluoroantimonate(V) (500
mg, 1.45 mmol) and the resulting solution stirred for 18
hours. The desired product precipitated from the solution as
a pale green powder. The isolation, crystallisation and
characterisation data for each complex are as follows.
NimCCHCH), 7.78 (m, 4H, NpyCHCH), 8.11 (d, 4H, NpyCCH),
8.20 (t of d, 4H, NpyCCHCH), 8.86 (d, 4H, NpyCH), 8.91 (s, 4H,
NimCH). 13C NMR (100 MHz, DMSO-d6, 303 K) [δ, ppm]: 121.1
(NimCCH), 128.2 (NimCCHCNim, NimCCHCH), 130.8
(NpyCHCH, NpyCCH), 139.4 (NpyCCHCH) 150.9 (NpyCH), 151.6
(NimCH). IR (cm−1): 652.69 (s, Sb–F), 766.79 (m, C–H out-of-
plane bend), 1480.5, 1593.0 (m, CC), 1630.0 (m, CN),
3061.0 (w, C–H). UV/vis (CH3CN) [λmax, nm; ε, mol−1 dm3 cm−1]:
251; 4.70 × 104, 281; 5.14 × 104. ES+ 393.03 m/z (M+).
1,2-Bis(2-pyridyliminomethyl)benzene-silver(I)
hexafluoroantimonate(V): [Ag(L1)](SbF6)
The product was separated from the reaction mixture by
centrifugation and washed with THF, yielding a green powder
(178 mg, 19.4% yield). Single crystals suitable for X-ray
diffraction were grown by slow liquid diffusion of diethylether
into a 2-methoxyethanol solution of the metal chelate. 1H NMR
(400 MHz, DMSO-d6, 303 K) [δ, ppm]: 7.58 (m, 2H, NimCCH),
7.75 (m, 2H, NimCCHCH), 7.79 (m, 2H, NpyCHCH), 8.04 (d,
2H, NpyCCH), 8.21 (t of d, 2H, NpyCCHCH), 8.95 (d, 2H,
NpyCH), 9.07 (s, 2H, NimCH). 13C NMR (100 MHz, DMSO-d6,
303 K) [δ, ppm]: 119.2 (NimCCH, NimCCHCH), 128.0
(NpyCHCH), 129.8 (NpyCCH), 139.7 (NpyCCHCH), 150.5
(NpyCH), 152.6 (NimCH). IR (cm−1): 649.60 (s, Sb–F), 771.86 (m,
C–H out-of-plane bend), 1484.1, 1592.8 (m, CC), 1629.3 (m,
CN). UV/vis (CH3CN) [λmax, nm; ε, mol−1 dm3 cm−1]: 244; 2.10
× 104, 300; 2.36 × 104. ES+ 393.03 m/z (M+).
catena-(μ-(1,4-Bis(2-pyridyliminomethyl)benzene)-silver(I)
hexafluoroantimonate(V)) [Ag-μ-(L3)](SbF6)
The precipitate was separated from solution by centrifugation
and washed with THF, giving a yellow-green powder (904 mg,
98.6% yield). Single crystals suitable for X-ray diffraction were
grown by slow liquid diffusion of diethylether into an
1
acetonitrile solution of the metal chelate. H NMR (400 MHz,
DMSO-d6, 303 K) [δ, ppm]: 7.42 (s, 4H, NimCCH), 7.78 (m,
2H, NpyCHCH), 8.12 (d, 2H, NpyCCH), 8.19 (t of d, 2H,
NpyCCHCH), 8.91 (d and s, 4H, NpyCH, NimCH). 13C NMR
(100 MHz, DMSO-d6, 303 K) [δ, ppm]: 123.4 (NimCCH), 128.0
(NpyCHCH, NpyCCH), 139.3 (NpyCCHCH), 151.1 (NpyCH),
151.6 (NimCH). IR (cm−1): 653.10 (s, Sb–F), 837.36 (m, C–H
out-of-plane bend), 1493.3, 1591.5 (s, CC), 1620.6 (m,
CN), 3080.1 (w, C–H). UV/vis (CH3CN) [λmax, nm; ε, mol−1
dm3 cm−1]: 236; 1.68 × 104, 284; 1.79 × 104, 354; 2.21 × 104.
ES+ 393.03 m/z (M+).
μ-di(1,3-Bis(2-pyridyliminomethyl)benzene)-di-silver(I)
hexafluoroantimonate(V): [Ag2-μ-(L2)2](SbF6)2
The product was separated from the reaction mixture by
centrifugation and washed with THF, resulting in a yellow-
green powder (858 mg, 93.6% yield). Single crystals suitable for
X-ray diffraction were grown by slow liquid diffusion of
diethylether into an acetonitrile solution of the metal chelate.
1H NMR (400 MHz, DMSO-d6, 303 K) [δ, ppm]: 7.34 (d, 4H,
NimCCH), 7.38 (s, 2H, NimCCHCNim), 7.46 (m, 2H,
catena-(μ-(1,4-Bis(2-pyridyliminomethyl)benzene)-silver(I)
tetraphenylborate(III)) [Ag-μ-(L3)](BPh4)
Ammonium tetraphenylborate(V) was added to [Ag-μ-(L3)]
(SbF6) dissolved in acetonitrile (30 mL), and stirred for 72
hours.
A
light green precipitate was harvested by
centrifugation, washed with a cold solution of acetonitrile
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and dried, yielding in a yellow-green powder. Single crystals
suitable for X-ray diffraction were grown by slow liquid
diffusion of diethylether into an acetonitrile solution of the
after a five-minute incubation period at room temperature. A
second aliquot comprising 0.5 molar equivalents was then
added to give a 1 : 1 metal–ligand ratio and the 1H NMR
spectrum again recorded after a five minute incubation
1
metal chelate. H NMR (400 MHz, DMSO-d6, 298 K) [δ, ppm]:
1
6.80 (t, 4H, BCHCHCH), 6.93 (m, 8H, BCHCH), 7.19 (t, 8H,
BCH), 7.41 (s, 4H, NimCCH), 7.79 (m, 2H, NpyCHCH), 8.12
(d, 2H, NpyCCH), 8.20 (t of d, 2H, NpyCCHCH), 8.91 (d and s,
4H, NpyCH, NimCH). 13C NMR (100 MHz, DMSO-d6, 303 K)
[δ, ppm]: 122.0 (BCHCHCH), 123.3 (NimCCH), 125.7
(BCHCH), 127.9 (NpyCHCH, NpyCCH), 136.0 (BCH), 139.2
(NpyCCHCH), 148.8 (NpyCH), 151.4 (NimCH). IR (cm−1):
701.21 (s, B–C stretch), 733.46 (s, C–H in plane bend), 838.21
(m, C–H out-of-plane bend), 1476.8, 1590.6 (m, CC), 1621.5
period. The change in H NMR chemical shifts of the ligand
with varying ratios of AgSbF6 in DMSO-d6 are shown in Fig.
S11† (the free ligand spectrum is shown in Fig. S10†). The
titration is interesting, indicating that the imine C–H and
α-pyridyl C–H switch positions (though maintain their
multiplicity) upon chelation to the metal ion. Although the
chemical shifts of both move downfield, the imine C–H
moves further downfield, past the chemical shift for the
α-pyridyl C–H. Importantly, the chemical shits at the end of
the titration match those observed when a crystalline sample
of [Ag-μ-(L3)](SbF6) is dissolved in DMSO-d6. These spectra
show that metal ion chelation occurs in solution, even in
coordinating solvent. It is therefore likely, the species
observed in the solid-state is also found in the solution state.
Due to the electron-withdrawing effects of the metal ion, the
chemical shifts of all hydrogen atoms move downfield; de-
shielded by the electron-withdrawing silver(I). As the
coordination polymer formed between L3 and silver(I) is
likely to be the least stable species in solution, it is probable
that the solid-state structures [Ag(L1)](SbF6) and [Ag2-μ-(L2)2]
(SbF6)2 are similarly stable in solution.
(m, CN), 2983.4, 3049.0 (m, C–H). UV/vis (CH3CN) [λmax
,
nm; ε, mol−1 dm3 cm−1]: 236; 3.18 × 104, 276; 1.94 × 104, 354;
2.13 × 104. ES+ 393.03 m/z (M+).
Results and discussion
NMR spectroscopy
The high symmetry of the compounds synthesised in this
work lead to uncomplicated NMR spectra. All four
compounds have the same chemical shift for the pyridyl
hydrogen atoms of the bis(pyridyl–imine) ligands due to all
pyridyl moieties having similar coordination modes. There
are some differences with respect to the bridging units as a
consequence of their varying degrees of symmetry. The imine
C–H has a chemical shift of 9.07 ppm in [Ag(L1)](SbF6),
which is slightly further downfield compared to the imine
C–H chemical shifts of [Ag2-μ-(L2)2](SbF6)2 and [Ag-μ-(L3)]X,
Fully assigned 1H and 13C NMR spectra are available in
the electronic ESI.†
UV-visible spectroscopy
−
−
(where X = BPh4 or SbF6 ) all of which have chemical shifts
of 8.91 pm. This is attributed to the minor variations in the
coordination geometries of the compounds: [Ag(L1)](SbF6) is
square planar while the other compounds exhibit nominally
tetrahedral coordination geometries. In the bridging
moieties, compound [Ag(L1)](SbF6) has two chemical shifts,
7.58 ppm and 7.75 ppm, whereas compound [Ag2-μ-(L2)2]
(SbF6)2 has three chemical shifts, 7.37, 7.38 and 7.49 ppm;
and compound [Ag-μ-(L3)]X has only one chemical shift, 7.72
ppm. In the latter, the high symmetry of the compound leads
to an equivalent chemical environment for the four phenyl
C–H groups of the bridging unit. The 13C NMR data show
similar patterns to those described above.
The electronic transitions of all three complexes are
dominated purely by ligand-based π → π* and n → π*
transitions as the silver(I) ion with
a
d10 electron
configuration precludes participation of the metal ion from
electronic transitions. Hence the absence of lower-energy
metal-to-ligand charge transfer bands, typically located
between 400–450 nm. All spectra were recorded in
acetonitrile giving visibly colourless solutions; this being
justified by the absorption bands outside of the visible region
of the electromagnetic spectrum. Despite the lack of
participation by the metal ion, each complex has unique
absorption bands attributed to the variable structures of the
bis(pyridyl–imine) ligands and their corresponding metal
complexes. The spectra show increased resolution of the
absorption bands as well as a bathochromic shift as a
function of positional substitution (Fig. 1).
Silver(I) is known to be a labile metal ion and the
structures elucidated in the solid-state may not be equivalent
to those of the solution state. This is particularly true for L3,
which forms a coordination polymer in the solid-state (vide
infra). In an effort to confirm the identity of the species in
solution, a simple titration experiment was performed as
follows: 4.0 mg of L3 was dissolved in DMSO-d6 (0.5 mL) and
Solid-state structures of [Ag(L1)](SbF6), [Ag2-μ-(L2)2](SbF6)2,
[Ag-μ-(L3)](SbF6) and [Ag-μ-(L3)](BPh4)
1
the H NMR spectrum recorded and referenced according to
All four complexes were studied by single crystal X-ray
diffraction. Each ligand was coordinated to silver(I) using
silver(I) hexafluoroantimonate(V) as the source of metal ions
in a 1 : 1 metal/ligand ratio. In addition, the counter anion of
the residual protonated DMSO at 2.50 ppm. A second stock
solution of AgSbF6 was prepared by dissolving 4.8 mg of the
salt (1 molar equivalent) in DMSO-d6 (0.5 mL). A 0.5 molar
equivalent of AgSbF6 (0.25 mL of the stock solution) was
chelate [Ag-μ-(L3)](SbF6) was exchanged for
a bulkier
1
added to the free ligand, and the H NMR spectrum recorded
tetraphenylborate(V) counter anion during synthesis to reduce
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Table 2. The rigid two-carbon di(azomethine) bridging unit
renders a constrained ligand bite and hence the N2–Ag–N3
bond angle is significantly acute measuring 64.71(6)°.
Consequently, the N1–Ag–N4 bond angle is obtuse,
measuring 154.40(6)°. The Npyridyl–Ag–Nimine bond angles are
approximately equal with a mean value of 70.58(8)°. The
strained coordination geometry leads to significant
differences in the Npyridyl–Ag and Nimine–Ag bond lengths,
which average 2.243(3) and 2.461(3) Å, respectively.
The compound shows evidence of
a
range of
intermolecular interactions including Ag⋯Ag, Ag⋯π and
π⋯π contacts. The compound packs as inversion dimers
stabilised by Ag⋯Ag and π⋯π interactions (Fig. 3). It would
seem that these interactions are moderately strong as the
interplanar spacing of the molecules is 3.236 Å. This distance
is significantly shorter than the interplanar spacing in the
π-system of graphite: 3.35 Å. The inversion dimers are then
linked by additional, similar interactions leading to one-
dimensional π-stacked columns. The Ag⋯Ag separation of
these close-packed columns measures 3.8497(3) Å.
Fig. 1 Electronic spectra of [Ag(L1)](SbF6), [Ag2-μ-(L2)2](SbF6)2, [Ag-μ-
(L3)](SbF6) showing improved resolution and bathochromic shift in the
order ortho < meta < para in terms of increased resolution and extent
of bathochromic shift.
the solvent accessible volume and hence yield a more stable
crystal lattice.
The silver chelate [Ag(L1)](SbF6) crystallised in the
monoclinic space group P21/n. The asymmetric unit (Fig. 2)
Ligand L2 is an interesting case. The arrangement of the
ligand donor set precludes the formation of a monomeric
species due to the position of C10–H10 unit, a multi-nuclear
structure is therefore anticipated. When the ligand L2 and
metal are reacted in a 1 : 1 ratio, the compound [Ag2-μ-(L2)2]
(SbF6)2 forms, crystallising in the monoclinic space group C2/c.
The structure is an unusual dimetallic chelate with two silver(I)
ions bridged by two ligands. The asymmetric unit (Fig. 4)
comprises a single ligand and metal ion with an associated
hexafluoroantimonate(V) anion. The asymmetric unit is the
substructure of the dimetallic, ligand-bridged species which
has inversion symmetry, thus Z = 4. The rigid aromatic
bis(imine) ligand bridging unit with 1,3-diamino substitution
is seemingly pre-organised for formation of a dinuclear
consists
one
independent
silver(I)
chelate
and
hexafluoroantimonate(V) counter anion, and Z = 4. The
chelate is monomeric with one metal ion coordinated to the
tetradentate bis(pyridyl–imine) ligand through the two
cis-pyridyl and two cis-imine nitrogen atoms of the complex.
The monomeric chelate shows a moderately planar structure,
this is the expected geometry of the ligand. Any significant
deviation from planarity would impede the π-conjugation of
the ligand making it less stable. The ligand donor set is thus
primed for square planar coordination. Correspondingly, the
metal ion has adopted
a
nominally square planar
coordination geometry with the silver(I) ion very slightly
displaced, 0.043 Å, from the four-atom mean plane defined
by the coordinating atoms. The bond lengths and bond
angles describing the coordination sphere are summarised in
complex. The silver(I) ions have
a distorted tetrahedral
coordination geometry. This distorted coordination geometry
has allowed the adjacent bridging phenyl rings to be co-planar.
This co-planar arrangement supports π⋯π interactions which
further stabilise the metal complex by ca. 11 kJ mol−1 (vide
infra). The mean plane separation of the two π-systems is 3.334
Å. This is comparable to the mean plane separation of graphite
(3.35 Å) and the interaction is therefore likely to be moderately
strong. Structural parameters for all four complexes are found
in Table 2.
It is noteworthy that the two silver(I) atoms in the
dimetallic structure are separated by a distance of 8.5247(6)
Å, which is significantly different to the distance reported in
the work done by Deng et al.: 13.0170(7), which is similarly a
meta-substituted, ligand-bridged dinuclear silver(I) species.19
Changing the bridging unit, from a flexible bis(pyridyl–
amine) to a semi-rigid bis(pyridyl–imine), alters the crystal
structure significantly. The less flexible ligand presented
ligand herein should be a more predictable entity for use in
crystal engineering. The Ag⋯Ag separation is, however,
longer than that reported by Pandey et al.: 7.905(2) Å.38 The
inductive effects of the trimethyl substitution on the phenyl
Fig. 2 Thermal ellipsoid plot (50% probability level) of [Ag(L1)](SbF6),
showing the approximately square planar coordination geometry of
the metal ion and the atom numbering scheme of the asymmetric unit.
Hydrogen atoms are rendered as spheres of arbitrary radius.
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Table 2 Bond lengths (Å) and bond angles (°) for compounds [Ag(L1)](SbF6), [Ag2-μ-(L2)2](SbF6)2, [Ag-μ-(L3)](SbF6), and [Ag-μ-(L3)](BPh4). Standard
uncertainties are given in parentheses
Bond lengths (Å)
Bond
Compound
[Ag(L1)](SbF6)
[Ag2-μ-(L2)2](SbF6)2
[Ag-μ-(L3)](SbF6)a
[Ag-μ-(L3)](BPh4)
N1–Ag1
N2–Ag1
N3–Ag1
N4–Ag1
2.240(2)
2.457(2)
2.464(2)
2.245(2)
2.221(2)
2.458(2)
2.434(2)
2.239(2)
2.322(9)
2.313(9)
2.320(9)
2.330(8)
2.239(2)
2.471(2)
2.413(2)
2.297(2)
Bond angles (°)
Compound
Bond
[Ag(L1)](SbF6)
[Ag2-μ-(L2)2](SbF6)2
[Ag-μ-(L3)](SbF6)a
[Ag-μ-(L3)](BPh4)
N1–Ag1–N2
N2–Ag1–N3
N3–Ag1–N4
N1–Ag1–N4
70.39(6)
64.71(6)
70.76(6)
154.4(6)
72.11(6)
84.69(6)
72.12(6)
151.74(7)
72.3(3)
107.3(3)
72.7(3)
72.01(6)
105.41(6)
72.27(6)
132.9(3)
139.01(7)
a
Mean values for the two independent molecules in the asymmetric unit.
bridge reported by Pandey et al. may assist with the
formation of the shorter bond distance (Fig. 5).
pyridyl nitrogen atom. The linear structure of the ligand
promotes the formation of coordination polymer
a
Two pseudopolymorphs of [Ag-μ-(L3)] (x) were synthesised
(Fig. 6 inset). The two molecules in the asymmetric unit are
−
−
and crystallised, where x = either SbF6 or BPh4 . (Fig. 6 and
−
7). When crystallised with an SbF6 anion, the crystal lattice
contains significant solvent accessible voids occupied by
numerous disordered acetonitrile solvent molecules. The
solvent molecules were removed in silico using Platon
SQUEEZE.37 The void contact surface was calculated using a
probe of 1.2 Å radius, indicating [Ag-μ-(L3)](SbF6) has solvent
accessible volumes of 884 Å3, i.e. 18.4% of the unit cell
volume. The chelate [Ag-μ-(L3)](SbF6) crystallised in the
monoclinic space group P21/c. The asymmetric unit (Fig. 6)
comprises two symmetry-independent ligands each
coordinated to a silver(I) ion through one imine and one
Fig. 3 Dimeric structures related through crystallographic inversion
symmetry, stabilised by Ag⋯Ag and π⋯π interactions, are linked
together through additional π⋯π and Ag⋯π interactions to form one-
dimensional columns. The Ag⋯Ag separation of the dimers measures
3.8497(3) Å and is indicated on the diagram as a dashed, black line.
Symmetry code: 1 − x, 1 − y, 1 − z.
Fig. 4 [Main] Thermal ellipsoid plot (50% probability level) of the
asymmetric unit of [Ag2-μ-(L2)2](SbF6)2, showing the atom numbering
scheme. [Inset] The symmetry-completed dimetallic structure showing
the co-planar bridging phenyl rings and approximately tetrahedral
coordination geometry of the metal ion. Hydrogen atoms are rendered
as spheres of arbitrary radius. Symmetry code: (i) −x, −y, −z.
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Fig.
5 One-dimensional chain (viewed down [a]) of [Ag2-μ-(L2)2]
(SbF6)2; the chain is co-linear with the c-axis supported by π⋯π
interactions. The intermolecular and intramolecular π⋯π interactions
are shown in green and red, respectively. Anions have been omitted
for clarity, and all atoms are shown as spheres of arbitrary radius. This
illustrates the influence of non-covalent interactions in crystal
engineering as described in previous works.28–33
Fig. 7 [Main] The symmetry-completed coordination polymer of [Ag-
μ-(L3)](BPh4) viewed down the a-axis, the counterions have been
omitted for clarity. The coordination polymer is col-linear with the
b-axis. All atoms are shown as spheres of arbitrary radius. [Inset]
Thermal ellipsoid plot (50% probability level) of the asymmetric unit of
[Ag-μ-(L3)](SbF6), showing the atom numbering scheme. Hydrogen
atoms are rendered as spheres of arbitrary radius. Symmetry code: (i) 1
1
− x, þ y, 1.5 − z.
2
similar in geometry and each propagates a coordination
polymer. The coordination geometry of the silver(I) ion is a
distorted tetrahedron; this leads to a herringbone pattern for
the coordination polymer. The relative rotation of the ligands
forming the coordination polymer is best described by the
N1–Ag1–N3–C10 torsion angle which measures 37.3(8)° and
N7–Ag2–N5–C23 which measures 35.7(6)°. The two nine-atom
mean planes each comprising the pyridyl–imine moiety and
silver ion form angles of approximately 63° for each
independent polymer.
Despite the extended π-conjugation of the ligands, the
ligands are not planar as may be expected. The two pyridyl
rings of each ligand are co-planar, but show a relative
rotation with respect to the bridging phenyl ring. This non-
planarity is illustrated by the angles subtended by the four
eight-atom mean planes each comprising the pyridyl–imine
moiety and the two six-atom mean planes of the bridging
phenyl rings. The mean of the four angles is 35(4)°. These
angles are the biggest difference between the two molecules
in the asymmetric unit. In molecule 1, the two pyridyl
imine groups form a mean angle of 31.3(2) and in molecule
two, the equivalent angle subtended by these groups
measures 39.3(6)°. The Ag⋯Ag separations are 7.925(1) and
7.912(1) Å for the Ag1⋯Ag1 and Ag2⋯Ag2 coordination
polymers, respectively.
Fig. 6 [Main] Thermal ellipsoid plot (50% probability level) of the
asymmetric unit of [Ag-μ-(L3)](SbF6), showing the atom numbering
scheme. Hydrogen atoms are rendered as spheres of arbitrary radius.
[Inset] The symmetry-completed coordination polymer. Note that the
two molecules in the asymmetric unit are stabilised by π⋯π
interactions (shown in red), but each propagates an independent chain
of molecules (coordination polymers). Symmetry code: (i) x, y, z.
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As described above, the crystal lattice of [Ag-μ-(L3)](SbF6)
has large solvent-accessible channels occupied by disordered
acetonitrile. The relatively volatile solvent would exit the
channels during crystal mounting and data collection,
lowering the quality of the data as sample crystallinity was
concomitantly lost. To stabilise the lattice and potentially
reduce the size of the channels in the lattice, the counter
−
anion was switched from SbF6− to BPh4 (the asymmetric unit
of [Ag-μ-(L3)](BPh4) is shown in Fig. 7). This still allowed for
formation of the coordination polymer (Fig. 7). However, the
anionic molecules now occupied the channels in the lattice
(Fig. 8) and reduced the solvent accessible surface from 884
Å3, 18.4% of the unit cell volume, (calculated using a 1.2 Å
radius probe) to 33.7 Å3, i.e. 1.0% of the unit cell volume
when calculated using the same probe radius. A comparison
of the solvent-accessible voids is shown in Fig. 9. This slightly
modified chelate [Ag-μ-(L3)](BPh4) still crystallised in the
same space group, P21/c, however, the asymmetric unit
(Fig. 7) now consists of
a
single chelate and
tetraphenylborate(III) counter anion.
Fig. 7 and the data in Table 2 show the silver(I) ion again
adopts a distorted tetrahedral coordination geometry in [Ag-μ-
(L3)](BPh4) with the coordination sphere comprising four
nitrogen atoms (two pyridyls and two imines) from two
independent ligands. Thus, each silver ion bridges two ligands
and each ligand bridges two silver ions. This leads to a one-
dimensional coordination polymer. In this case, the ligands of
Fig. 9 A comparison of solvent-accessible voids for chelates [top]
[Ag-μ-(L3)](SbF6) and [bottom] [Ag-μ-(L3)](BPh4). The volume and
structure of the voids are changed through exchanging the
counterions. The latter structure is thus also more stable.
the coordination polymer more closely approach
a
perpendicular orientation as illustrated by the N1–Ag1–N3–C10
torsion angle, which measures 74.6(2)°. The ligand again
deviates from planarity, the consistent deviation from planarity
despite the extended π-conjugation suggests that the
destabilising effect caused by the ligand twisting is perhaps
offset by the energy gains of more favourable metal ion
chelation and the formation of supramolecular structures. The
deviation from planarity in the case of [Ag-μ-(L3)](BPh4) is
illustrated by the relative rotations of the terminal pyridyl rings
with respect to the bridging phenyl ring. The six-atom mean
planes comprising N1, C1–C5, and N4, C14–C18, subtend
angles relative to the six-atom mean plane of the bridging
phenyl ring, C7–C12, of 24.9 and 15.7°, respectively.
Upon exchanging the counter anion of chelate [Ag-μ-(L3)]
(SbF6) from hexafluoroantimonate(V) to tetraphenylborate(V),
a
polymeric structure was once again formed. This is
significant as it shows that the formation of coordination
polymers is favoured under different reaction conditions. By
replacing SbF6, which has a relatively small molecular volume
−
when compared with BPh4 , the result is alternating layers of
coordination polymers and anions as shown in Fig. 8.
Although significantly smaller, this polymeric chelate once
again contains voids in the lattice, with a contact surface of
33.71 Å3 (Fig. 9). This is likely attributed to the larger
tetraphenylborate(III) counter ions occupying more space;
driving the solvent void pockets to be smaller.
The most notable feature of all four compounds is the
variation in bond angles as a consequence of the restrained
ligand bite and changes in coordination geometry. The angle
between the imine nitrogen atoms, N2–Ag–N3, varies
significantly as a function of the aromatic bridging unit
structure ortho- < meta- < para-. This change in bond angle also
reflects the change form an approximately square planar
Fig.
8
Packing of [Ag-μ-(L3)](BPh4) viewed down the c-axis. The
−
anionic BPh4 ions are rendered using their van der Waals radii and are
coloured blue. The coordination polymers are shown as spheres of an
arbitrary radius in yellow. This highlights the alternating layers of
polymer chains and anions which leads to a stable crystal lattice.
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coordination geometry in the case of ortho-substitution to
nominally tetrahedral in the case of para-substitution.
Correspondingly, the angle subtended by the pyridyl N-donor
atoms, N1–Ag1–N4, decrease significantly: ortho- > meta- >
para-. The trend in bond lengths is the same for all compounds
in this series with the Ag–Npyridyl bond lengths being
considerably shorter than the Ag–Nimine bond lengths. The
mean bond lengths for the four Ag–Npyridyl and Ag–Nimine bonds
are 2.27(4) and 2.42(6) Å, respectively. These bond length and
bond angles are comparable to those of related structures.38–45
The compound [Ag2-μ-(L2)2](SbF6)2 is similarly rigid, but
does show more structural deviation between the simulated
and experimental geometries (Fig. 10). The Ag⋯Ag distance
in the complex [Ag2-μ-(L2)2](SbF6)2 is 8.091
Å for the
geometry-optimised structure. This is significantly shorter
than the experimental structure: 8.5247(6) Å. Conversely, the
interplanar spacing of the phenyl bridging units is increased
in the optimised form with a distance of 3.866 Å between the
two six-atom mean planes. These geometric parameters show
that the compound has, in a sense been compressed, bringing
the silver(I) ion closer and forcing the π-systems further apart.
Through comparison of the geometry-optimised monomeric
and dimeric structures, it is evident the π⋯π interaction of
the bridging unit stabilises the structure by ca. 10.7 kJ mol−1.
Despite the differences described above, there is relatively
good agreement between the geometry-optimised and
experimental data with a least-squares-fit of all atoms yielding
a root-mean-square-deviation (RMSD) of 0.3211 Å (Fig. 10).
The polymeric nature of [Ag-μ-(L3)] (x) complicates the
geometry-optimisation process. A trimeric sub-structure of
the coordination polymer was used as a representative
example in the geometry optimisation (Fig. S23†). The least
squares fit of the bridging ligand shows that in the absence
of steric restraints imposed by the crystal lattice, the
geometry-optimised structure has the ligands subtending an
angle of ca. 90°. This is in contrast to the experimental
structures which subtend angles of approximately 65–75°.
Molecular simulations
Density functional theory (DFT) simulations were used to
gain a deeper understanding of the solid-state structures and
associated energies of the silver(I) chelates. The simulations
were performed using Gaussian 09W46 using the PBE hybrid
functional.47,48
A split basis set was applied in the
simulations, this split basis set specified the 6-311G(dp) level
of theory49,50 for the C, H, and N atoms and the LanL2DZ
(which makes use of effective core potentials) basis set for
the silver(I) ion.51–54 The X-ray coordinates of the metal
chelates were used for input structures. Normal geometry
convergence criteria were applied and no symmetry
constraints imposed. The input files were prepared using
GaussView 5.0;55 the same program was used to analyse the
output files. Structural overlays were performed using
Mercury 2020, v. 2.0.56
The rigid structure of L1 and the correspondingly
predictable structure of [Ag(L1)](SbF6) could be accurately
simulated. The root-mean-square deviation (RMSD) for the
least-squares fit of all atoms in the structure is 0.1814 Å
showing good correlation between the structures. The rigidity
of the ligand and few degrees of freedom allow for accurate
structure simulation in vacuo. This also shows the
intermolecular interactions in the solid state have little
influence on the molecular geometry. The least squares fit is
shown in Fig. S22.†
Conclusions
A series of ortho-, meta- and para-substituted phenylene-bridged
silver(I) bis(pyridyl–imine) complexes have been successfully
synthesised in facile
structurally distinct complexes as a consequence of ligand
donor atom pre-organisation. Due to the pre-organised
architecture of the di(azomethine) linkage in each ligand,
monomeric, dimeric and polymeric structures were synthesised.
a two-step process yielding three
Ligand 1,2-bis(2-pyridyliminomethyl)benzene (L1) forms
a
monomeric complex, [Ag(L1)](SbF6), ligand 1,3-bis(2-
pyridyliminomethyl)benzene (L2) forms a dimeric complex [Ag2-
μ-(L2)2](SbF6)2, and ligand 1,4-bis(2-pyridyliminomethyl)
benzene (L3) forms a polymeric complex [Ag-μ-(L3)](SbF6). The
latter coordination polymer was found to be robust with respect
to the counter anion; both [Ag-μ-(L3)](SbF6) and [Ag-μ-(L3)]
(BPh4) form polymeric structures in the solid state. The d10
electronic configuration of silver(I) and lack of CFSE (and
therefore no strong preference for coordination geometry)
allowed for both tetrahedral and square planar coordination
environments. This was necessary to form the different
monomeric and supramolecular structures. These examples
show the potential of ligand donor atom pre-organisation for
application in crystal engineering.
Fig. 10 Least-squares-fit of the geometry-optimised (yellow) and
experimental (purple) structures of [Ag2-μ-(L2)2](SbF6)2 showing the
shorter Ag⋯Ag separation and larger spacing between the bridging Conflicts of interest
phenyl rings. The π⋯π interaction between the bridging phenyl rings
stabilises the molecule by ca. 11 kJ mol−1
.
There are no conflicts to declare.
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Acknowledgements
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The authors gratefully acknowledge the University of Kwa-
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well as the National Research Foundation (South Africa) for
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Mr Craig Grimmer for the recording of the NMR spectra.
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