Binding of Mono- and Dianions within Silver Thiazolylurea Tweezers and Capsules

Article abstract of DOI:10.1021/acs.inorgchem.7b02028

A silver thiazolylurea complex, [Ag(TUTh)₂]⁺, has been used as a host species for geometrically differently shaped mono- and dianions: trigonal planar (NO₃^-), tetrahedral (SO₄^2-), and octah

Full text of DOI:10.1021/acs.inorgchem.7b02028

Article
pubs.acs.org/IC
Binding of Mono- and Dianions within Silver Thiazolylurea Tweezers
and Capsules
†
†
,‡
,†
Novia T. X. Lee, Jamie Hicks, Karl J. Wallace,* and David R. Turner*
†
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
‡
The Department of Chemistry and Biochemistry, The University of Southern Mississippi, Hattiesburg, Mississippi 39406, United
States
*
S Supporting Information
+
ABSTRACT: A silver thiazolylurea complex, [Ag(TUTh) ] ,
2
has been used as a host species for geometrically differently
−
shaped mono- and dianions: trigonal planar (NO ),
3
2
−
2−
tetrahedral (SO4 ), and octahedral (SiF6 ). In the presence
of nitrate a 1:1 near-planar tweezer host−guest species is
formed, with poor binding in solution despite excellent
geometric complementarity being found between the host
2
−
and the anion in the solid state. In the presence of either SO
4
or SiF62 a 2:1 host−guest species is formed, whereby the
guest is held in a capsulelike arrangement stabilized by an array
of eight NH hydrogen-bond donors, as confirmed by X-ray
−
crystallographic studies. Solution studies in DMSO-d support the host−guest stoichiometry seen in the solid state. The binding
6
2
4
−
+
−2
constant between SO and [Ag(TUTh) ] was calculated to be K = 2511 M and was shown to be the dominant species in
2
21
2−
solution, in excellent agreement with the solid-state studies. However, upon the addition of SiF6 ions different speciation is
observed: H ·G (capsule), H·G (tweezer), and H·G during the course of the study.
2
2
INTRODUCTION
along the metal−ligand bonds. For example, complexes of the
2+ + + 2+
10 11 12
■
type [PdL ] , [CuL ] , [AgL ] , and [FeL ] have been
4
2
2
3
Host−guest chemistry has been one of the main driving forces
behind the field of supramolecular chemistry. The challenges
and motivations associated with designing synthetic receptors
for important analytes, typically those that are environmentally
or biologically relevant, remain largely unchanged, although
there has been significant progress in enhancing our under-
13
1
reported.
Host−guest complexes in which the guest is completely
surrounded by the host can offer particularly high affinity and
14
selectivity. Host species can be templated by anions: for
15
example, M4L6 cages that contain tetrahedral dianions.
2
Molecular receptors containing the tris(2-aminoethyl)amine
standing and application of these systems. Some of the most
−
motif have been shown to either bind H PO ions via a
2
4
challenging analytes to selectively target are anionic species,
whereby particular difficulties are faced due to nonspherical
geometries, higher energies of solvation, and changes in
speciation due to pH.
Perhaps the most utilized molecular motifs for the binding of
anionic guests are those that can form two parallel N−H
hydrogen bonds with the guest. Classic examples of this motif
are seen in guanidinium and (thio)urea moieties. Another
group is the squaramide functionality, which is now being
widely incorporated into receptor design. In particular, this
arrangement of hydrogen bond donors lends itself to the
recognition of simple oxoanions due to the geometric
2
−
tripodal−guest interaction or completely encapsulate SO
ions in a 2:1 cage complex with an array of six urea groups
around the saturated anion. Other examples include 4:4 cages
4
16
−
17
that have been shown to encapsulate H PO or H PO ions.
3
4
2
4
There have recently been a number of examples whereby
flexible receptors form capsules or pseudocapsules containing
two or more hosts that self-assemble around the anionic
3
18
guest(s). Other common scaffolds used are trisubstituted
4
benzene derivatives, which have shown similarly complicated
19
anion encapsulation behavior. Organic bis-urea tweezers have
been shown to form complex assemblies with multiple anionic
and neutral guests inside a self-assembled cage, to form 2:1
complexes around tetrahedral anions or even to form anion-
complementarity between the H···H separation in the dono5r
and the O···O distance in the guest (ca. 2.0−2.4 Å).
20
templated helicates.
Preorganization of these donor groups into organic frameworks
or coordination complexes can give rise to very selective
+
Herein we report a bis-urea complex, [Ag(TUTh) ] , that
2
acts as a host to NO3− by a simple tweezer receptor and as a
6
7
binding: for example, [3]polynorbornane scaffolds, tris(2-
8
9
aminoethyl)amine, and trisubstituted benzene. Effective urea-
based receptors can also be constructed around metal ions,
which can allow for a significant amount of rotational freedom
Received: August 8, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b02028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
host to SiF 2 and SO42− through a “scissoring” motif involving
−
3
CH
), 7.11−7.13 (m, ArH and SCH), 7.35 (d, JHH = 8.3 Hz, 4H,
3
6
3
ArH), 7.39 (d, J = 3.5 Hz, 2H, NCH), 8.90 (s, 1H, TolylNH),
0.50 (br. s, 1H, ThiazoleNH). C{ H} NMR (75 MHz, DMSO-d ):
δ 20.4 (CH ), 112.7, 118.9, 129.4, 131.9, 135.9, 137.1 (ArC), 151.5
two complexes to form a cage that encapsulates the dianionic
guest (Figure 1) and the role that subtle structural differences
play in host−guest behavior.
HH
13
1
1
6
3
19
(
CO), 160.8 (NCS). F NMR (282 MHz, DMSO-d ): δ −70.1 (d,
6
1
31
J
= 710.4 Hz, 6F, PF ). P NMR (122 MHz, DMSO-d , 298 K): δ
FP
6
6
1
−1
−
3
(
1
144.2 (sept, J = 710.4 Hz, 1P, PF ). FTIR (ν/cm ): 3587 (w),
PF 6
404 (w), 1716 (m), 1692 (m), 1604 (m), 1534 (s), 1512 (s), 1458
m), 1406 (m), 1335 (w), 1314 (m), 1281 (m), 1237 (m), 1198 (m),
169 (m), 1131 (w), 1042 (w), 889 (w), 835 (s), 804 (s), 732 (s), 699
s). Anal. Calcd for C H AgF N O S : C, 36.73; H, 3.08; N, 11.68.
(
22
22
6
6
2 2
Found: C, 36.60; H, 3.14; N, 11.59. PXRD could not be obtained, as
the material quickly turned amorphous when it was removed from the
mother liquor.
[
Ag(TUTh) ]NO . Method A. To a solution of silver nitrate (7.2 mg,
2 3
4
3 μmol) in acetonitrile (2 mL) was added a solution of TUTh (20
mg, 86 μmol) in methanol (2 mL). The reaction mixture was stirred
briefly before being left to stand undisturbed in the dark for 18 h to
give [Ag(TUTh) ]NO as colorless crystals. Yield: 20 mg (73%).
2
3
Method B. A solution of [Ag(TUTh) ]PF (20 mg, 28 μmol) in
2
6
acetonitrile (2 mL) was added to a solution of tetrabutylammonium
nitrate (8.5 mg, 14 μmol) in methanol (2 mL) at room temperature.
The reaction mixture was stirred briefly before being left to stand
undisturbed in the dark for 18 h to produce [Ag(TUTh) ]NO as
Figure 1. Ligand TUTh used in this study (with hydrogen atom
labeling used in descriptions of binding behavior) and its ML complex
2
formed with AgPF that acts as a tweezer-shaped receptor.
6
2
3
1
colorless crystals. Yield: 15 mg (84%). Mp: 215−220 °C. H NMR
3
(
400 MHz, DMSO-d , 298 K): δ 2.24 (s, 6H, CH ), 7.12 (d, J = 8.2
6 3 HH
3
3
EXPERIMENTAL DETAILS
Hz, 4H, ArH), 7.16 (d, J = 3.6 Hz, 2H, SCH), 7.37 (d, J = 8.2
■
HH HH
3
General Details. All H, C, 19F, and 31P NMR spectra were
recorded on Bruker DPX300, Bruker Avance III 400, and Bruker
Avance III 600 spectrometers and were referenced to the resonances
of the solvents used or external CFCl or H PO . Mass spectra were
1
13
Hz, 4H, ArH), 7.45 (d, J = 3.6 Hz, 2H, NCH), 9.07 (s, 1H,
TolylNH), 10.65 (br s, 1H, ThiazoleNH). C{ H} NMR (101 MHz,
DMSO-d ): δ 20.4 (CH ), 112.6, 118.9, 129.4, 131.9, 136.0, 137.2
ArC), 151.5 (CO), 160.8 (NCS). FTIR (ν/cm ): 3282 (w), 3203
w), 3083 (w), 2920 (w), 1702 (s), 1611 (s), 1542 (s), 1508 (s), 1388
m), 1314 (s), 1278 (m), 1243 (s), 1200 (s), 1161 (s), 1122 (m),
HH
13
1
6
3
−1
(
(
(
3
3
4
recorded on an Agilent Technologies 5975D inert MSD instrument
with a solid-state probe. ATR-FTIR spectra were collected using an
Agilent Cary 630 spectrometer. Melting points were determined in
glass capillaries and are uncorrected. Microanalyses were carried out at
the Science Centre, London Metropolitan University. Tetrabutylam-
1037 (m), 890 (w), 862 (w), 809 (m), 704 (m). Anal. Calcd for
: C, 41.52; H, 3.48; N, 15.41. Found: C, 41.65; H,
C H22AgN O S
22 7 5 2
3.35; N, 15.37. Phase purity was confirmed by X-ray powder
diffraction (see Figure S8 in the Supporting Information).
−
2−
monium salts (NO3 and SO4 ) and other reagents were purchased
from standard commercial suppliers and used as supplied, except
tetrabutylammonium hexafluorosilicate, which was synthesized imme-
diately before use by titrating a dilute aqueous solution of
tetrabutylammonium hydroxide against an aqueous solution of
hexafluorosilicic acid until neutral. See Figures S1−S4 in the
Supporting Information for NMR spectra.
[Ag(TUTh) ] SiF . A solution of [Ag(TUTh) ]PF (20 mg, 28
2 2 6 2 6
μmol) in methanol (4 mL) was added to a solution of
tetrabutylammonium hexafluorosilicate (8.7 mg, 14 μmol) in methanol
(1 mL) at room temperature. The reaction mixture was stirred briefly
before being left to stand undisturbed (excluding light) for 18 h to give
[Ag(TUTh)
]
SiF
as colorless crystals. Yield: 15 mg (83%). Mp:
243−247 °C. H NMR (400 MHz, d -DMSO, 298 K): δ 2.25 (s, 12H,
CH ), 7.08−7.13 (m, 12H, ArH), 7.35−7.42 (m, 12H, ArH), 8.95 (s,
1H, TolylNH), 10.56 (br s, 1H, ThiazoleNH). C{ H} NMR (101
MHz, DMSO-d ): δ 20.4 (CH ), 112.4, 118.6, 129.3, 131.7, 136.1,
137.3 (ArC), 151.5 (CO), 160.2 (NCS). F NMR (282 MHz,
2
2
6
1
Synthetic Details. N-(Thiazol-2-yl)-N′-(p-tolyl)urea (TUTh). To a
6
solution of 2-aminothiazole (752 mg, 7.51 mmol) in dichloromethane
3
13
1
(
25 mL) was added p-tolyl isocyanate (1.00 g, 7.51 mmol) dropwise at
room temperature. The reaction mixture was heated to reflux and
stirred for a further 2 h, forming a white precipitate during this time
period. After the reaction mixture was cooled to room temperature,
the precipitate was isolated via filtration, washed with dichloromethane
6
3
19
−1
DMSO-d ): δ −134.5 (br, 4F), −120.7 (br, 2F). FTIR (ν/cm ): 3344
6
(m), 3305 (m), 1699 (s), 1608 (m), 1539 (s), 1509 (s), 1453 (m),
1403 (m), 1312 (s), 1280 (s), 1240 (s), 1199 (s), 1165 (s), 1120 (m),
1070 (m), 1038 (m), 865 (w), 803 (m), 756 (m), 738 (s), 703 (s).
(
3 × 5 mL), and dried in vacuo to give TUTh as a white solid (1.51 g,
1
8
2
8
3
6%). Mp: 203−207 °C. H NMR (600 MHz, DMSO-d , 298 K): δ
6
3
3
.25 (s, 3H, CH ), 7.09 (d, J = 3.5 Hz, 1H, SCH), 7.11 (d, J
.1 Hz, 2H, ArH), 7.35 (d, J = 8.1 Hz, 2H, ArH), 7.36 (d, J
=
=
Anal. Calcd for C44H44Ag F N O S Si: C, 40.94; H, 3.44; N, 13.02.
2 6 12 4 4
3
HH
HH
3
3
Found: C, 41.07; H, 3.32; N, 13.18. Phase purity was confirmed by X-
ray powder diffraction (see Figure S9 in the Supporting Information).
HH
HH
.5 Hz, 1H, NCH), 8.89 (s, 1H, TolylNH), 10.49 (br s, 1H,
13
1
ThiazoleNH). C{ H} NMR (101 MHz, DMSO-d ): δ 20.5 (CH ),
Preparation of [Ag(TUTh) ] SO . To a solution of [Ag(TUTh) ]-
2 2 4 2
6
3
1
12.5, 118.8, 129.5, 131.9, 136.1, 137.4 (ArC), 151.8 (CO), 159.8
PF (20 mg, 28 μmol) in methanol (4 mL) was added a solution of
6
−1
(
NCS). FTIR (ν/cm ): 3289 (w), 3027 (w), 1719 (m), 1619 (m),
tetrabutylammonium sulfate (8.1 mg, 14 μmol) in methanol (1 mL) at
room temperature. The reaction mixture was stirred briefly before
being left to stand undisturbed (excluding light) for 18 h to give
546 (m), 1509 (s), 1497 (s), 1314 (s), 1278 (m), 1249 (s), 1191 (s),
+
+
[Ag(TUTh)
]
SO
as colorless crystals. Yield: 17 mg (77%). Mp:
2
2
2
4
1
Anal. Calcd for C H N OS: C, 56.63; H, 4.75; N, 18.01. Found: C,
240−242 °C. H NMR (300 MHz, DMSO-d , 298 K): δ 2.25 (s, 12H,
6
11
11
3
3
3
5
6.54; H, 4.81; N, 17.96.
CH ), 7.07 (d, JHH = 8.4 Hz, 8H, ArH), 7.12 (d, JHH = 3.7 Hz 4H,
3
3
3
[
Ag(TUTh) ]PF . To a solution of silver hexafluorophosphate (109
SCH), 7.39 (d, JHH = 8.4 Hz, 8H, ArH), 7.51 (d, JHH = 3.7 Hz, 4H,
NCH), 9.62 (s, 1H, TolylNH), 11.35 (br s, 1H, ThiazoleNH).
2
6
mg, 430 μmol) in methanol (1 mL) was quickly added a warm
solution (ca. 60 °C) of TUTh (200 mg, 860 μmol) in methanol (1
mL) at room temperature. The reaction mixture was briefly stirred
before being left to stand in the dark undisturbed for 18 h to give
13
1
C{ H} NMR (101 MHz, DMSO-d ): δ 20.4 (CH ), 112.3, 118.6,
6 3
129.2, 131.5, 136.3, 137.3 (ArC), 151.5 (CO), 161.2 (NCS). FTIR
−1
(ν/cm ): 3291 (w), 3124 (w), 2921 (w), 1713 (m), 1689 (m), 1614
(m), 1545 (s), 1512 (s), 1460 (m), 1406 (m), 1313 (m), 1283 (m),
1245 (s), 1198 (s), 1170 (m), 1093 (m), 1063 (s), 1039 (s), 889 (w),
[
2
Ag(TUTh) ]PF as colorless crystals. Yield: 255 mg (83%). Mp:
2 6
1
26−229 °C. H NMR (600 MHz, DMSO-d , 298 K): δ 2.25 (s, 6H,
6
B
DOI: 10.1021/acs.inorgchem.7b02028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Crystallographic Structural and Refinement Parameters for All Structures
[
Ag(TUTh) ]NO
[Ag(TUTh) ] SO ·2MeCN
[Ag(TUTh) ] SiF ·MeOH
[Ag(TUTh) ]PF ·3MeOH
2
3
2
2
4
2
2
6
2
6
empirical formula
C H AgN O S
C H Ag N O S
C H Ag F N O S Si
C H AgF N O PS
25 34 6 6 5 2
2
2
22
7
5
2
48 50
2
14
8
5
45 48
2
6
12
5
4
formula wt
636.46
1327.06
1323.02
815.54
cryst syst
monoclinic
triclinic
triclinic
triclinic
space group
P2 /c
1
P1
̅
P1
̅
P1
̅
Z
8
2
2
2
a/Å
18.276(4)
13.899(3)
20.440(4)
90
13.8336(6)
14.9288(6)
15.3275(6)
88.020(2)
65.384(2)
69.584(2)
2674.1(2)
123
13.966(3)
14.348(3)
15.770(3)
75.46(3)
67.31(3)
65.21(3)
2631.9(13)
100
8.4917(5)
b/Å
14.2434(10)
15.9581(12)
113.608(2)
101.284(2)
97.412(2)
1687.6(2)
123
c/Å
α/deg
β/deg
106.09(3)
90
γ/deg
V/Å3
4988.8(17)
100
temp/K
no. of rflns measd
no. of indep rflns
no. of rflns (I ≥ 2σ(I))
Rint
35654
9739
57844
45631
29195
16327
12491
8350
8297
7400
11616
5386
0.0393
0.1192
0.0235
0.0779
R1 (I ≥ 2σ(I)/all data)
wR2(F ) (I ≥ 2σ(I)/all data)
GOF
0.0447/0.0514
0.1232/0.1268
1.053
0.0592/0.1619
0.1089/0.1475
0.960
0.0371/0.0396
0.0923/0.0937
1.042
0.0499/0.0898
0.1254/0.1065
1.003
2
residual min, max/e
CCDC no.
1.454, −0.983
1554115
1.196, −0.906
1554116
1.599, −1.408
1554117
0.820, −0.882
1554118
8
64 (w), 814 (m), 789 (m), 705 (m). Anal. Calcd for
by the ligands in a two-coordinate silver complex should differ
and will therefore alter the geometry of the potential tweezer
complex and hence affect the anion binding properties (vide
infra).
C H Ag N O S : C, 42.45; H, 3.56; N, 13.50. Found: C, 42.57;
H, 3.64; N, 13.40. Phase purity was confirmed by X-ray powder
diffraction (see Figure S10 in the Supporting Information).
Single-Crystal X-ray Diffraction. Single crystals were mounted
on nylon loops using viscous hydrocarbon oil. Data were collected
using the MX1 beamline at the Australian Synchrotron ([Ag-
44
44
2
12
8 5
The organic species 9N-(thiazol-2-yl)-N′-(p-tolyl)urea
TUTh) is synthesized in excellent yield (86%) from the
(
reaction of 2-aminothiazole and p-tolyl isocyanate (see
Experimental Details). Reaction of TUTh with AgPF
AgNO , Ag SO , and AgPF /(NBu ) SiF (prepared fresh
(TUTh) ]NO and [Ag(TUTh) ] SiF ) or using a Bruker Apex II
2 3 2 2 6
diffractometer ([Ag(TUTh) ] SO ·2MeCN and [Ag(TUTh) ]PF ·
,
6
2
2
4
2
6
3
MeOH).
Data from the MX1 beamline were collected using an energy of 17.4
keV (λ = 0.7109 Å), with the sample temperature maintained at 100 K
3
2
4
6
4 2
6
prior to use) yielded crystalline solids which show structures
+
that suggest the ability of the [Ag(TUTh) ] cation to act as an
2
2
1
by an open-flow N cryostream, using the BluICE control program.
Data indexing and integration were conducted using the XDS program
2
effective host species. As is common for silver(I) complexes,
−
addition of halides (X ) led to rapid precipitation of AgX.
22
suite. Data from the Bruker Apex II diffractometer were collected
using a graphite-monochromated Mo Kα source (λ = 0.71073 Å) with
the sample temperature maintained at 123 K by an open-flow N2
cryostream. Data were collected and processed using the SAINT
software suite.
Structures were solved by direct methods using SHELXS-2014 or
Solid-State Studies. Crystallization of a 2:1 mixture of
TUTh and AgPF in methanol yields the compound [Ag-
6
(
TUTh) ]PF ·3MeOH. The metal complex contains a near
2 6
23
linearly coordinated silver (N−Ag−N = 171°) with the two
+
ligands adopting a trans geometry around the Ag ion; as a
24
the dual-space method using SHELXT. Structures were refined by
conventional least-squares methods against F using SHELXL-2014.
consequence the two urea groups point in opposite directions
from the plane of the metal (Figure 2, and Figure S5 in the
Supporting Information). The PF6− anion is a notoriously poor
hydrogen bond acceptor. As a consequence, a single N−H···F
2
25
2
6
The program X-Seed was used as a graphical interface. All non-
hydrogen atoms were refined using an anisotropic model, except
where indicated below. All hydrogen CH and NH atoms were refined
using fixed, idealized X-ray distances and geometries using a riding
model with their displacement parameters 1.2 or 1.5 times that of the
Uiso value of the atom to which they are bound. Additional refinement
details are supplied in the Supporting Information.
Crystallographic structural and refinement parameters for all
structures are presented in Table 1, and detailed hydrogen bonding
tables are provided in the Supporting Information. All structural data
have been deposited with, and are available for free from, the
Cambridge Crystallographic Data Centre (www.ccdc.cam.ac.uk).
RESULTS AND DISCUSSION
■
[
Previous reports of a 3-pyridyl-substituted urea ligand in an
Figure 2. Part of the structure of [Ag(TUTh) ]PF ·3MeOH showing
2
6
+
AgL ] complex showed the formation of a molecular tweezer
2
the trans conformation of the complex and hydrogen bonding
interactions. CH hydrogen atoms and disorder of a tolyl group are
omitted for clarity.
−
12
that was selective for NO3 ions. When the Lewis basic group
is switched to a 2-substituted thiazole ring, the angle subtended
C
DOI: 10.1021/acs.inorgchem.7b02028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
+
Figure 3. Structures of host−guest complexes between [Ag(TUTh) ] and anionic guests as determined by X-ray crystallography: (a)
2
[
Ag(TUTh) ]NO ; (b) [Ag(TUTh) ] SO ; (c) [Ag(TUTh) ] SiF . CH hydrogen atoms are omitted for clarity (ellipsoid plots in the Supporting
2 3 2 2 4 2 2 6
Information).
1
Figure 4. Stack plots of H NMR spectra of TUTh and [Ag(TUTh) ]PF by themselves and upon addition of 1 equiv of anionic guests as their
2
6
tetrabutylammonium salts (labeling of hydrogen atoms as in Figure 1).
hydrogen bond interaction exists between one urea group of
the complex and the counterion. The other urea moiety forms a
bifurcated hydrogen-bonding interaction with a single methanol
molecule.
complexes form a near-perpendicular dual-tweezer motif. The
structure of [Ag(TUTh) ] SO ·2MeCN contains one unique
2
2
4
2:1 host−guest species in the asymmetric unit. The host−guest
complex is not symmetrical, with the two tweezers oriented at
85° with respect to each other (on the basis of the N−Ag−N
vectors). The tweezers themselves are nonplanar, with the
Structures containing NO , SO42−, and SiF62− as counter-
−
3
ions are significantly different, all containing a cis arrangement
of the ligands around the metal and good affinity toward the
anion. The structure of [Ag(TUTh) ]NO shows the
planes of the thiazole rings being offset by 47 and 38° in the
+
two unique [Ag(TUTh) ] species. As a consequence, the
2
3
2
SO42 anion does not sit symmetrically within the array of urea
groups that face the interior of the cavity. Hydrogen-bonding
N···O distances lie in the range 2.757(5)−3.030(6) Å with two
−
anticipated 1:1 host−guest species, with the asymmetric unit
containing two near-identical coordination compounds. The
host adopts the anticipated tweezer geometry, with the NO3−
ion residing within the triangular space that is presented by the
significantly longer, tentative interactions at ca. 3.2 Å (Figure S6
−
2
cation (Figure 3a). The NO3 ion is bound by the flanking urea
in the Supporting Information). Two nonplanar R (8) motifs
2
1
2
groups with N···O distances in the range 2.764(5)−3.218(5) Å;
and two R (6) motifs, both of which are commonly associated
longer hydrogen bonds are associated with the oxygen atom
with urea groups, are present in the host−guest complex (one
+
−
+
3
closest to the Ag ion. The NO guest lies close to the Ag ion
of each interaction is associated with each tweezer). There are
3
close contacts between the SO42 guest and the Ag ions, with
Ag···O distances of 2.794(4) and 2.900(3) Å being the closest
for the two unique metals.
−
+
with Ag−O distances of approximately 2.7 Å. The previously
reported 3-pyridyl-based system did not have close Ag−O
contacts, due to the cavity of the tweezer having a concave
nature rather than the convex form (i.e., silver protruding into
the cleft) that is brought about by the geometry enforced by the
thiazole rings in contrast to that of meta-substituted pyridyl
The SiF62 complex [Ag(TUTh) ] SiF is similar to that
−
2 2
6
obtained with SO42 . The tweezers are oriented at ca. 83° with
respect to each other and are nonplanar (with angles of 44 and
48° between the planes of the thiazolyl rings). The two
−
12
rings (Figure 4). This observation is borne out for the
dianions as well (vide infra) and demonstrates the significant
difference that can be brought about by a subtle change in the
geometry of the tweezer.
2−
tweezers are further separated in the SiF species than in the
6
SO42 complex to account for the larger guest (Ag···Ag = 8.4 Å
−
for SiF62 , in comparison to 7.4 Å for SO4 ). There are 11
hydrogen bonds between the host and guest within the N···F
range 2.767(3)−2.956(3) Å, alongside some longer “inter-
actions” (3.0−3.3 Å; see Table S2 and Figure S7 in the
−
2−
The adducts of TUTh with Ag SO and Ag SiF both form
2
4
2
6
+
cages comprising of two [Ag(TUTh) ] units enveloping the
2
central anion (Figure 3b,c). In both instances the two metal
D
DOI: 10.1021/acs.inorgchem.7b02028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Supporting Information). As with the SO complex, there are
Article
2
4
−
typical binding isotherm seen for 1:1 host−guest species
(Figures S20 and S21 in the Supporting Information).
However, attempts to calculate a binding constant in DMSO-
d6 proved to be difficult; therefore, no thermodynamic data
could be obtained. This is presumably due to the relative lack of
preorganization and the competitive nature of the DMSO
solvent system competing with the anion in equilibrium.
The solution behaviors of [Ag(TUTh) ]PF in the presence
2
1
both R (8) motifs and R (6) motifs between the tweezers and
2
2
2
−
the SiF6 guest, alongside bifurcated NH donor groups.
Perhaps the most revealing detail in the structure is the close
contact between the anion and the metals, with Ag−F distances
of 2.643(2) and 2.597(2) Å. These distances are comparable to
+
those observed in dimeric Ag cages containing tripodal ligands
and can reasonably be expected to significantly enhance the
2
6
2
7
2−
2−
−
stability of the host−guest complex.
of SO4 and SiF6 are strikingly different from that of NO .
3
2
−
Solution Studies. The solid-state structure of [Ag-
TUTh) ]PF shows that the two ligands are in an anti
Upon the addition of SO4 , a broad band is seen at 4.2 ppm
and the NH(tolyl) moiety disappears upon the first addition of
the anion (0.1 equiv). This can be explained by deprotonation,
and it is reasonable to assume that the species which is being
bound in solution is HSO ; however, there is no evidence of
this crystallographically. Nevertheless, this anionic species is
still interacting with the silver tweezer complex. The binding
isotherms for the other proton chemical environments are
significantly different (Figures S22 and S23 in the Supporting
Information), and it is clear that the binding isotherms are not
representative of a 1:1 binding ratio. Surprisingly, when the
binding constant was calculated by least-squares nonlinear
(
2
6
orientation with respect to the urea groups, whereas in the
−
+
presence of the NO3 ions the Ag coordination compound
−
unambiguously forms a tweezer-shaped host−guest complex.
4
Our attention turned to solution-based studies to investigate
+
the anion binding ability of [Ag(TUTh) ] . We have previously
2
2
+
2+
2+
shown that transition-metal ions (Ru , Pd , and Pt ) can be
used as inorganic backbones that have two pyridinium ligands
attached to the metal center, which can bind anions via
28
hydrogen-bonding and electrostatic interactions. Therefore,
we anticipated that the solution studies of [Ag(TUTh) ]PF6
2
2
9
will complement the solid-state results described above. The
fitting, using HYPNMR (see Table 2), an excellent fit was
anion binding behavior of [Ag(TUTh) ]PF was assessed by
2
6
1
H NMR titration with tetrabutylammonium (TBA) salts of
NO , SO , and SiF6 in DMSO-d due to the very limited
Table 2. Binding Constants Obtained for TUTh and
−
2−
2−
a
[Ag(TUTh) ]PF
3
4
6
2
6
solubility of the complexes in other solvents (evidenced by their
rapid crystallization). The hexafluorophosphate anion was
chosen due to its well-known low charge density, causing it
to be rapidly displaced by anions that are able to participate in
stronger hydrogen bonding interactions (requiring a consid-
erable excess of PF6 to bring about the reverse trans-
formation). Upon the addition of aliquots of the TBA salts, the
chemical environments for the hydrogen atoms in [Ag-
NO3−
SO42−
0.98
SiF62−
2.47
TUTh
log K11
b
[
Ag(TUTh) ]PF6
2
−
log K21
log K11
d
2.29
1.38
3.39
0.30
c
c
0.71
d
negligible
negligible
log K12
−1
a
−2
(
TUTh) ]PF experience chemical change upon the binding
K
11 (M ) and K21 (M ), and K12 refer to the formation of 1:1, 2:1,
and 1:2 host−guest complexes, respectively. Anions were added as
their tetrabutylammonium salts. Conditions: DMSO-d , 298 K. Not
determined, The speciation was included in the binding model to fit
data. The speciation was not needed to fit the data.
2
6
of the anion (Figures 4 and 5). The hydrogen atom that
b
6
c
d
2
−
complex (K21 = 2511 M−2)
obtained for a 2:1 host−SO
4
(
Table 2). The model did have to include 1:1 and 1:2 host−
anion ratios, but these species are negligible in solution with the
predominant species being the 2:1 host−anion complex (Figure
S24 in the Supporting Information), in excellent agreement
with the solid-state structure, in which the anion is encapsulated
between two Ag tweezers (Figure 3b).
+
2−
The binding behavior between [Ag(TUTh) ] and SiF in
2
6
2−
solution is drastically different from that of the SO4 anion. It
is evident that different species exist in solution (Figures S25
and S26 in the Supporting Information), unlike the case in the
1
Figure 5. H NMR titration curves (NH(tolyl), DMSO-d , 298 K):
2−
6
SO4 anion study, where only one major species was
a) TUTh plus SiF62 ; (b) [Ag(TUTh) ]PF plus SiF6 ; (c)
Ag(TUTh) ]PF6 plus SO4 ; (d) TUTh plus SO4 ; (e) [Ag-
TUTh) ]PF plus NO ; (f) TUTh plus NO .
−
2−
(
[
2
6
2
−
2−
determined to exist in solution. However, the nonlinear
regression calculation required all three species (2:1, 1:1, and
2
−
−
(
2
6
3
3
1
:2 host−guest complexes) as part of the model to fit the
−2
experimental data. At low concentrations (<2.3 × 10 mol
2
−
−3
2−
showed the greatest chemical shift upon the addition of SO
dm ) of SiF6 ions, it is reasonable to assume that the
pseudocapsule is the predominant species in solution, which is
supported by the solid-state structure. As the concentration of
the guest increases over the course of the titration, the capsule
collapses, forming a 1:1 complex, and at high concentrations
4
and SiF62 was the urea NH closest to the tolyl group (Figure
, hydrogen B). However, the H NMR spectrum of
Ag(TUTh) ]PF in the presence of NO showed very little
−
1
1
−
[
2 6 3
change (Figure S19 in the Supporting Information), with only a
modest change in chemical shift observed for the NH(tolyl)
proton (see Figure 1) (Δ = 0.08 ppm). The binding isotherm
drawn by plotting chemical shifts vs concentration does show a
(>4.2 × 10− mol dm ) a 1:2 host−anion complex is formed,
2
−3
whereby one anion is presumably associated with each arm of
+
the [Ag(TUTh) ] complex. This is supported by the sigmoidal
2
E
DOI: 10.1021/acs.inorgchem.7b02028
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
behavior of the binding isotherm for the NH(tolyl) functional
group (Figure 5), which is indicative of cooperative binding.
anions bind. The combination of eight hydrogen-bonding
interactions between the two tweezers and the guest inside the
capsule are seemingly strong enough in concert to overcome
competition from the highly polar solvent.
Furthermore, both the H :G and H:G species can be observed
2
during the course of the titration (Figure S27 in the Supporting
Information), whereby distinct signals for each species are seen.
The cooperativity is also supported by investigating the
model system (TUTh), whereby only a 1:1 binding isotherm
ASSOCIATED CONTENT
Supporting Information
■
*
S
−
2−
was obtained between TUTh plus the anion (NO , SO , and
3
4
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02028.
2−
SiF6 ). The calculated binding constants are significantly lower
than those of the coordination compound [Ag(TUTh) ] .
+
2
Interestingly the binding constants for the model compound
NMR spectra, PXRD traces, additional crystallographic
information, and detailed titration information and plots
2
−
(
TUTh) with SO4 (Figures S14 and S15 in the Supporting
2
−
Information) and SiF6 (Figures S17 and S18 in the
Supporting Information) could be determined precisely using
nonlinear regression, due to a significant amount of the
NH(tolyl) group (Figure 5a,d), whereas the NO3− binding
constant could not be determined due to the competition with
(PDF)
Accession Codes
CCDC 1554115−1554118 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
2
4
−
the solvent. However, both the tetrahedral SO
and the
octahedral SiF62 ion can form R (6) and R (8) motifs in the
−
1
2
2
2
solid state, respectively; therefore, we believe that both of these
complementary ring systems can stabilize the host−guest
interactions in solution (see Figure S28 in the Supporting
Information), therefore allowing us to determine the binding
AUTHOR INFORMATION
Corresponding Authors
*E-mail for K.J.W.: karl.wallace@usm.edu.
E-mail for D.R.T.: david.turner@monash.edu.
■
2
constant (Table 2). Interestingly, the R (8) motif formed
2
between an SiF62 anion and the TuTh model compound
−
appears to hinder the rotation of the thiazole group, thereby
*
“
locking” the molecule. As a consequence, a distinctive splitting
ORCID
1
of the aromatic signals is observed in the H NMR titration, in
contrast to the other anions studied, for which there is little
movement of these signals (Figure 4a). This is a consequence
David R. Turner: 0000-0003-1603-7994
Funding
The Australian Research Council is acknowledged for funding
FT120100300).
2
−
of the size and shape of the SiF6 ion preventing the rotation
of the thiazole group. This is not seen for the NO3− or SO4
ions, consistent with the small binding constant obtained for
2−
(
Notes
SO42 and the lack of data for NO3 (Table 2). Additionally,
this is consistent with the significant chemical shift of the NH
protons observed during the course of the titration (Figure 5a).
While crystallographic information reveals the solid-state
structure, this does not always translate to solution-based
−
−
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Part of this work was conducted using the MX1 beamline at the
Australian Synchrotron, Victoria, Australia.
■
30
1
host−guest behavior. However, in this instance the H NMR
titration data support the structural studies.
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