Heteroleptic metallosupramolecular racks, rectangles, and trigonal prisms: Stoichiometry-controlled reversible interconversion

Article abstract of DOI:10.1021/ic400328d

A simple approach toward preparation of heteroleptic two-dimensional (2D) rectangles and three-dimensional (3D) triangular prisms is described utilizing the HETPYP (HETeroleptic PYridyl and Phenanthroline metal complexes) concept. By mixing metal-loaded linear bisphenanthrolines of varying lengths with diverse (multi)pyridine (py) ligands in a proper ratio, six different self-assembled architectures arise cleanly and spontaneously in the absence of any template. They are characterized by ¹H and DOSY NMR, ESI-FT-ICR mass spectrometry as well as by Job plots and UV-vis titrations. Density functional theory (DFT) computations provide information about each structure. A stoichiometry-controlled supramolecule-to-supramolecule interconversion based on the relative amounts of metal bisphenanthroline and bipyridine forces the rectangular assembly to reorganize to a rack architecture and back to the rectangle, as clearly supported by variable temperature and DOSY NMR as well as dynamic light scattering data. The highly dynamic nature of the assemblies represents a promising starting point for constitutional dynamic materials.

Full text of DOI:10.1021/ic400328d

Article
pubs.acs.org/IC
Heteroleptic Metallosupramolecular Racks, Rectangles, and Trigonal
Prisms: Stoichiometry-Controlled Reversible Interconversion
Subhadip Neogi,*,† Yvonne Lorenz,‡ Marianne Engeser,‡ Debabrata Samanta,† and Michael Schmittel*,†
†Center of Micro and Nanochemistry and Engineering, Organische Chemie I, Universitat Siegen, Adolf-Reichwein-Strasse 2, D-57068
̈
Siegen, Germany
‡Kekule-
́
Institut fur Organische Chemie und Biochemie, Universitat Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A simple approach toward preparation of
heteroleptic two-dimensional (2D) rectangles and three-
dimensional (3D) triangular prisms is described utilizing the
HETPYP (HETeroleptic PYridyl and Phenanthroline metal
complexes) concept. By mixing metal-loaded linear bi-
sphenanthrolines of varying lengths with diverse (multi)-
pyridine (py) ligands in a proper ratio, six different self-
assembled architectures arise cleanly and spontaneously in the
absence of any template. They are characterized by 1H and DOSY NMR, ESI-FT-ICR mass spectrometry as well as by Job plots
and UV−vis titrations. Density functional theory (DFT) computations provide information about each structure. A
stoichiometry-controlled supramolecule-to-supramolecule interconversion based on the relative amounts of metal bisphenanthro-
line and bipyridine forces the rectangular assembly to reorganize to a rack architecture and back to the rectangle, as clearly
supported by variable temperature and DOSY NMR as well as dynamic light scattering data. The highly dynamic nature of the
assemblies represents a promising starting point for constitutional dynamic materials.
squares. Fortunately, there are various protocols to solve the
problem, with complexity rising significantly upon increasing
the number of components. By attaching a platinum-based
molecular clip with two parallel donor sites to linear ditopic
ligands, Stang and others self-assembled molecular rectangle-
s.1a,6e Using orthogonal binding motifs, that is, by connecting a
bis-chelating dianion to the equatorial sites of two fac-(CO)3Re
cores and a ditopic nitrogen-donor ligand, Lu and co-workers
prepared rectangles in high yield.1j Likewise, prisms may be
prepared. The simplest way of designing a trigonal prism, is to
use a two-component assembly. Based on the directional
bonding approach, Stang, and co-workers have designed
trigonal prisms using a 0° Pt-based molecular clip and planar
tripodal linkers by a 3 + 2 self-assembly reaction.2a More
control is required in the three-component protocol, as
reported by Fujita and co-workers. A combination of an end-
capped acceptor, a tritopic planar donor, and a bidentate linear
donor in a 6:2:3 molar ratio furnished the desired prism.2c
Recently, Lusby even reported on a kinetically controlled four-
component synthetic approach to trigonal prisms.2e
INTRODUCTION
■
With a large variety of successful strategies being readily
available to prepare two-dimensional (2D)1 and three-dimen-
sional (3D)2 supramolecular architectures, attention has
recently turned to functional dynamic structures that are able
to modulate their properties by directed compositional,
constitutional, and conformational changes. Detailed mecha-
nistic studies are thus important for understanding responsive
self-assemblies and their use in potential applications.3 So far,
the majority of work has focused on hydrogen-bond driven4
assemblies and constitutionally dynamic5 assemblies, but rarely
on coordination-driven6 self-assembly. As a notable example for
the latter category, Stang and co-workers recently described the
transformation of fused metallacyclic polygons through
stoichiometric control of multicomponent mixtures and the
supramolecular fusion of two 2-component homoleptic
architectures into a 3-component heteroleptic system by virtue
of charge separation.7 Lately, Schmittel and co-workers even
fabricated a 5-component scalene triangle by the 2-methylpyr-
idine-catalyzed supramolecular fusion of a 3-component
rectangle and a 2-component equilateral triangle.8
As already stated above, structural transformations and
interconversions9 of discrete self-assembled metallosupramole-
cules in solution have rarely been studied. Such protocols
essentially require, in contrast to traditional design principles,
to instate additionally an unique bidirectional sensitivity toward
external inputs. Herein, we will describe a reversible supra-
Herein, we will present a three-component self-assembly
toward the template-free formation of 2D and 3D supra-
molecules, such as fully dynamic racks, rectangles, and prisms,
and their responsiveness to a variation of the components’ ratio.
Despite the topological simplicity of our target structures, their
multicomponent preparation requires perfect heteroleptic
control. For example, synthesis of supramolecular rectangles
is often hampered by competitive formation of homoleptic
Received: February 7, 2013
© XXXX American Chemical Society
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Scheme 1. Schematic Illustration of Known Heteroleptic Complexation Motifs
Scheme 2. Ligands Used in the Present Study (with Atom Numbering Scheme)
molecule-to-supramolecule structural alteration upon varying
the ratio of components. The self-assembly will be based on
mixed phenanthroline pyridine copper(I) complexes.
encoded in individual subunits and their stoichiometric ratio
opens up a general platform to construct either trigonal
(HETPYP-I) or tetragonal (HETPYP-II) coordination scenar-
ios13 at the metal center (Scheme 1).
The sterically shielded 2,9-diarylphenanthroline (phenAr2)
has proven itself as a versatile capping unit for the quantitative
formation of a variety of nanosized heteroleptic supramolecular
assemblies either via the HETPHEN (HETeroleptic bis-
PHENanthroline complexes)10 or the HETTAP (HETeroleptic
Terpyridine And Phenanthroline complexes)11 approach
(Scheme 1). However, exchange of one or several bi- or
tridentate ligands at dynamically binding metal ions operates at
the cost of slow kinetics. HETPHEN and HETTAP
coordination scenario are thus not ideal for establishing facile
and possibly rapid interconversion of supramolecules. As a
promising alternative, we recently developed the HETPYP
(HETeroleptic PYridine and Phenanthroline metal com-
plexes)12 concept (Scheme 1) and prepared a series of discrete
three-component architectures in solution and the solid
state.12c The principal structural and bonding features are as
follows: (i) the combination of the sterically encumbered
phenanthroline ligand L and a metal ion Mn+ will generate a
capped [M(L)]n+ unit, in which the metal ion is still accessible
for the “slim” pyridine ligand; (ii) the π−π interaction between
the 2,9-diaryl substituents of L and the pyridine ligand stabilizes
the heteroleptic combination; (iii) the geometric arrangement
Using the fact that each capped copper(I) center can
effectively accommodate one pyridine ligand in the HETPYP-I
complexation scenario, we report herein the synthesis of
various discrete heteroleptic two- and three-dimensional
architectures by mixing preorganized linear metalated bis-
phenanthroline building blocks with oligopyridine ligands
(Scheme 2) in a proper ratio. Furthermore, we demonstrate a
reversible structural changeover between discrete rectangle and
rack assemblies, simply by tuning the pyridine/[Cu2(L)]2+ ratio
(L = 3, 4) (Scheme 2). Such receptiveness should open the
option to switch emergent properties of multicomponent
systems in response to the deliberate addition and removal of
components, via shuffling. So far, manifestations of rapid
additive-triggered supramolecule-to-supramolecule (inter)-
conversions in metallosupramolecular chemistry are rare.7,8,9a−c
RESULTS AND DISCUSSION
■
Pyridylethynyl units were incorporated into 1 and 2, via routine
Sonogashira coupling,14 to provide conformational flexibility for
coordination to the capped copper(I) center. The bisphenan-
throlines 3 and 4 controlling the HETPYP-I coordination were
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synthesized according to published procedures15 while ligand 5
was readily accessible via a Sonogashira cross-coupling reaction
of 2-(4-bromo-2,3,5,6-tetramethylphenyl)-3-(4-iodophenyl-
ethynyl)-9-mesityl-[1,10]-phenanthroline and 4,4′-diethynylbi-
phenyl (Scheme S1, Supporting Information).
X-ray Structure of Ligand 4. In addition to the solution
state characterization,15 it was also possible to establish the
structure of 4 (Scheme 2) by single crystal X-ray analysis.
Figure 1 shows that the structure is centrosymmetric with an
implies a tricoordination at each copper(I) center.12c Peak
assignments are further substantiated by COSY experiments. In
1
accordance with H NMR results, analysis of the 13C NMR
spectrum of the reaction mixture reveals only 25 aromatic, 3
acetylenic, and 4 aliphatic peaks (Figure S5, Supporting
Information), corroborating a time-averaged D2h symmetric
structure.
ESI-FT-ICR mass spectra obtained using extremely soft ESI
conditions16 furnished added support for R1 because signals are
seen for the doubly and triply charged aggregate with correct
isotope patterns and exact masses (Figure 3). The most
abundant fragment, formed during the ESI process from R1 in
acetone, contains a [Cu2(3)]2+ core with one or two
additionally coordinated molecules 1. Thus, the gas-phase
data fully confirm HETPYP-I complexation in R1. While the
strongly bound [Cu2(3)]2+-entities mostly stay intact in the gas
phase, the rather weakly binding pyridine ligands easily
dissociate in presence of excess energy. The latter property is
imperative for the use of this complex motif in the construction
of a highly dynamic supramolecular structure.12 Further
evidence toward the formation of R1 was obtained from the
method of continuous variation (Job plot) in UV−vis titrations,
indicating (Figure 3a) a perfect 1:1 stoichiometry of [Cu2(3)]2+
and ligand 1 at 25 °C.
More information on the structure of the metallosupramo-
lecular rectangle was obtained from 1H NMR DOSY (diffusion-
ordered spectroscopy) and DLS (dynamic light scattering)
experiments, carried out at 25 °C in dichloromethane. A single
diffusion coefficient in the DOSY NMR (Figure 3b) points
toward sole formation of R1 with D = 3.8 × 10−10 m2 s−1. The
experimental hydrodynamic diameter derived therefrom17
matches well with the value obtained from the DLS
measurement and the energy minimized structure18 (Table 2
and Table S2, Supporting Information), indicating that the
solution assembly of R1 is in good agreement with its
theoretically predicted structure.
Figure 1. Solid state structure of 4 with displacement ellipsoids at the
50% probability level.
inversion center at the midpoint of the biphenyl spacer unit.
Thus, the molecule displays a transoid conformation. The
crystal packing shows a number of weak intermolecular π−π
and C−H···π interactions.
Synthesis of Supramolecular Rectangles R1−R3. Clean
formation of R1, as concluded from NMR, VT-NMR (vide
infra), DOSY, MS and DLS data, is observed upon mixing the
copper bisphenanthroline [Cu2(3)]2+ (Cu+:3 = 2:1) and ligand
1
1 in a 1:1 ratio in dichloromethane. The H NMR spectrum
reveals only one set of sharp signals, where coordination of the
positively charged metal center results in typical downfield
shifts for all protons at the phenanthroline core (Table 1). In
contrast, the chemical shifts of the pyridine α-H are drastically
shifted upfield from 8.63 to 6.82 ppm while pyridine β-H
experience a shift from 7.42 to 7.22 ppm (Figure 2). The
upfield shifts suggest that each pyridine ring of 1 is located
within the cavity of the [Cu2(3)]+2 unit so that protons α-H
experience strong shielding effects from the mesityl groups of 3.
This observation is in line with our previous findings and
Since suitable single crystals for X-ray diffraction measure-
ments could not be obtained, DFT computations were used to
gain further insight into the structural characteristics of R1. The
energy-minimized structure is calculated with Gaussian0319
using the B3LYP/6-31G(d) exchange correlation functional20
Table 1. Characteristic 1H NMR (400 MHz, 25 °C) with Selected Chemical Shifts (in ppm) of the Ligands (see Scheme 2) and
Complexes
phenanthroline protons
pyridine protons
a
c
c
ligands/Complexes
4
7
5,6
8
Hα
Δδ (Hα)
Hβ
Δδ (Hβ)
1
2
3
4
8.63
8.62
7.42
7.42
8.49
8.51
8.47
8.83
8.83
8.85
8.82
8.83
8.84
8.84
8.85
8.33
8.33
8.29
8.71
8.71
8.73
8.70
8.71
8.72
8.71
8.72
7.86−7.93
7.87−7.93
7.85−7.90
8.16−8.20
8.16−8.20
8.18−8.22
8.16−8.20
8.16−8.20
8.16−8.20
8.16−8.20
8.17−8.21
7.57
7.57
7.58
7.94
7.94
7.97
7.94
7.94
7.96
7.94
7.94
b
5
R1
6.82
6.84
7.03
6.88
6.86
6.93
7.66
7.66
1.81
1.79
1.60
1.74
1.76
1.69
0.97
0.97
7.22
7.22
7.27
7.20
7.20
7.23
7.31
7.31
0.20
0.20
0.15
0.22
0.22
0.19
0.11
0.11
R2
R3
P1
P2
P3
RA1
RA2
a
b
c
Recorded in CD2Cl2. Recorded in CDCl3. 1H NMR shift difference for Hα or Hβ between the uncoordinated pyridine ligand and its resultant
complexes.
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1
Figure 2. H NMR spectra (partial, 400 MHz, 25 °C) of 1, 3 and complex R1 (pyridine α and β protons are shown in red and blue color,
respectively).
Figure 3. Data on R1: (a) Job plot from a UV−vis titration showing 1:1 stoichiometry for 1/[Cu2(3)]2+, (b) 1H DOSY spectrum, and (c) ESI-FT-
ICR mass spectrum with isotope distribution patterns for the doubly and triply charged aggregates.
in conjunction with LANL2DZ effective core potentials21 for
copper, nicely illustrating the rectangular arrangement of R1
(Figure 4).
R3 (Figure 4), equally DFT computations provided energy
minimized structures that are in agreement with DLS and
DOSY results. The energy minimized structures in R1−R3
show planes of bisphenanthroline and 1 to be perpendicular
thus increasing the π-stacking interactions between mesityl and
pyridine rings that appear to stabilize the overall structure.
Synthesis of Supramolecular Triangular Prisms P1−
P3. The successful formation of discrete trigonal prismatic
cages P1−P3 from the tritopic connector 2 (Scheme 2) and
various capped copper complexes [Cu2(L)]2+ (L = 3, 4, 5)
proceeded without template assistance. As a representative
example for all the cases studied so far, we report in detail on
the formation of P1 in dichloromethane. Because oligopyr-
To further extend the scope of the HETPYP-I approach and
evaluate the size limit of the self-assembled rectangles
attainable, we employed ligands 4 and 5 with 31 Å and 47 Å
length, respectively, in combination with 1 and copper(I) ion.
Analogously as for R1, the reaction of the individually capped
copper complexes [Cu2(L)]2+ (L = 4, 5) with ligand 1 (1:1
ratio) yielded cleanly the supramolecular rectangles R2 and R3.
The spectroscopic characterization and evaluation of purity was
performed as for R1 and shall not be presented in details here
but in the Supporting Information. For the other rectangles R2,
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S27, Supporting Information) suggesting the formation of a
time averaged D3h symmetric structure. The Job plot shows an
ideal 2:3 stoichiometry (Figure 6a) of ligand 2 and [Cu2(3)]2+.
ESI FT-ICR mass spectrometry of the yellow solution of P1
shows (Figure 6c) signals for the molecular species
[Cu6(3)3(2)2(PF6)3]3+, [Cu6(3)3(2)2(PF6)2]4+, and
[Cu6(3)3(2)2(PF6)]5+ with correct isotope patterns and exact
mass in addition to the expected fragments, in which again the
[Cu2(3)]2+ building blocks remain intact.
A battery of experimental evidence thus asserts that the rare
trigonal coordination motif of copper(I) can be realized in
presence of phenanthroline 3 and the weakly binding pyridine
2, leading to exclusive formation of P1 without a template or
additional directing factors. The latter bonding, as extensively
described in our earlier work,12c is further characterized by
strong π−π stacking between the pyridine ring and mesityl
groups of bisphenanthroline 3.
Table 2. Diffusion Coefficients, Hydrodynamic Diameters,
and Modeled Diameters of All the Complexes
d
D [10−10
dH (DOSY)
dH (DLS)
dav (calcd) [nm]
a
,
b
a
,
c
a
Complex
m2s−1
]
[nm]
[nm]
(Model)
R1
3.8
4.5
3.4
4.0
2.8
3.6
3.2
2.7
2.72
2.30
3.04
2.58
3.68
2.86
3.22
3.82
2.81
2.48
3.13
2.71
3.77
2.99
3.38
3.98
2.82
3.09
RA1
R2
RA2
R3
3.91
3.00
3.30
P1
P2
P3
4.06
a
b
All measurements done at 25 °C in dichloromethane. Diffusion
coefficient obtained from the 2D DOSY NMR experiments.
c
Hydrodynamic radii calculated via the Stokes−Einstein equation D
= kBT/6πηrH (kB, Boltzmann constant; T, absolute temperature; η,
d
viscosity of CH2Cl2 at 298 K). Average diameter dav = (dv + dh + dd)/
3 (see Table S2, Supporting Information).
Analogously, following a 3:2 stoichiometric ratio of the
respective capped pillar complex [Cu2(L)]2+ (L = 4, 5) and
ligand 2, the heteroleptic prisms P2 and P3 were obtained
exclusively, as evident from spectroscopic data (see full
spectroscopic characterization in Supporting Information) and
elemental analysis.
idines typically form oligomeric complexes in the presence of
many metal ions22 it is advisable to avoid formation of any
unwanted kinetically locked complex right at the onset by
combining all components sequentially. Accordingly, 3 is first
fed with 2 equiv of Cu+ in dichloromethane-d2 to form the
capped and coordinatively unsaturated complex [Cu2(3)]2+ as a
clear pale yellow solution. After mixing ligand 2 with
[Cu2(3)]2+ in a 2:3 stoichiometric ratio, the color of the
sonicated solution intensifies to deep yellow. A combination of
1H NMR, 1H-1H COSY, 13C NMR, and DOSY NMR
spectroscopy verifies the clean formation of P1. The 1H
NMR spectrum of the mixture reveals only one set of sharp
signals, with all phenanthroline protons being shifted upfield
because of metal complexation. The most diagnostic upfield
shift is observed for the pyridine α-H (Δδ = 1.74 ppm), while
pyridine β-H (Δδ = 0.22 ppm) are less shifted to upfield (Table
1). This observation is in line with the attachment of one
pyridine ring to each coordinatively unsaturated copper(I)
center of the [Cu2(3)]2+ unit. The proton corresponding to the
central benzene ring of 2 experiences a slight downfield shift
(Figure 5) upon complexation (Δδ = 0.12 ppm).
Suitable X-ray-quality crystals were not obtained for any of
the prisms P1−P3. Theoretical computation using Gaussian03
at the B3LYP/6-31G (d) level (for P1 and P2) were thus
performed (Table S3, Supporting Information) to obtain some
information about their structure and differently sized voids,
because the supramolecular prisms may become useful in host−
guest chemistry and as microreactors. Taking the Cu−Cu
distance as a measure, the model structure of P1 features a well-
defined trigonal prism with respective height, width, and
diagonal of 1.8, 1.7, and 2.5 nm (Figure 7). The roof is covered
by one tris-pyridine ligand 2 while each corner is defined by a
three-coordinate Cu+ ion. The computation of P2 shows a
similar trigonal structure with larger dimensions than in P1.
The ball-and-stick model for P3, computed using the
semiempirical method PM6 divulges the largest dimensions
among the three prism assemblies: ∼3.5 nm (height), ∼1.7 nm
(width), and ∼4.0 nm (Cu−Cu diagonal). For all the
nanoprisms the pyridine rings of ligand 2 adopt a parallel
orientation with respect to the mesityl’s aromatic plane of L
(L= 3, 4, 5) to maximize π−π stacking, but as a result the
overlap with the central benzene spacer is reduced. The
1
The complex is further studied by H DOSY spectroscopy
(Figure S25, Supporting Information), indicating the formation
of a single product. The 13C NMR spectrum of the solution
reveals 27 aromatic, 4 acetylenic, and 4 aliphatic peaks (Figure
Figure 4. DFT calculated structures of R1, R2, and R3 (from left to right) in ball-and-stick representations. Cu−Cu distances (solid lines) and
measures between two furthest atoms (dotted lines; vertical, horizontal, and diagonal) are depicted (in nm). Hydrogen atoms are omitted for clarity.
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Figure 5. Partial 1H NMR spectra (400 MHz, 25 °C, CD2Cl2) of 2 and each prism assembly. Pyridine α and β protons are shown in red and blue
color, respectively.
Figure 6. Data for supramolecular prism P1: (a) Job plot, (b) dynamic light scattering data, and (c) ESI FT-ICR mass spectrum including the
experimental and calculated isotopic distributions of 3+, 4+, and 5+ charged species.
augmented geometric strain for P1−P3, as a consequence to
maximize π−π stacking, is also reflected by a pronounced
bending in bisphenanthroline pillar ligands in the energy
minimized structures. The hydrodynamic diameter of each
prism assembly (Table 2), as determined from DOSY NMR,17
shows an excellent agreement with the data obtained from
dynamic light scattering experiments and the average size
(dav)18c as derived from computed vertical, horizontal, and
diagonal distances between the two furthest separated atoms in
the respective prisms (Figure 7 and Supporting Information,
Table S2).
spatial arrangement of the pyridine ring in the respective
assembly as compared to the situation in the free ligand. The
numbers indicate that for both individual series of rectangles or
prisms, the Δδ values are remarkably constant.
Stoichiometry-Induced Interconversion from Rectan-
gle to Rack Assembly. An instant reorganization of R1 was
1
detected in a H NMR titration experiment (Figure 8), in
which a diagnostic downfield shift of the α pyridine protons
from 6.82 to 6.96 ppm is witnessed at little excess of 1 (total 1.2
equiv.), while the pyridine β protons experience a slight shift
from 7.22 to 7.27 ppm. Further addition of 1 (total 1.5 equiv.)
to the solution promotes an extra downfield shift of pyridine α
and β protons, attesting a dynamic behavior of the assembly. At
2.0 equiv of 1 (1/[Cu2(3)]2+= 2: 1), the pyridine α and β
The structural closeness of all assemblies is best recognized
from the NMR shift differences Δδ(Hα) and Δδ(Hβ) at the
pyridine ring (Table 1) that witness sensibly the different
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Figure 7. Structural features of P1, P2, and P3 in ball and stick representations, derived from the respective energy minimized structures. Cu−Cu
distances (in solid lines) and measures between two furthest atoms (dotted lines; vertical, horizontal, and diagonal) are considered (in nm).
Hydrogen atoms are omitted for clarity.
immediate precipitation consistent with oligomer formation,
precluding supplementary NMR measurement.
Although the 1H NMR DOSY of the newly formed assembly
at 1/[Cu2(3)]2+ = 2:1 advocates a single trace (Figure S45,
Supporting Information) with no peaks found for free pyridine
protons, its diffusion coefficient was found to be larger than
that of R1 (Table 2). Furthermore, a DLS study on rectangle
R1 and the new assembly reveals a different monomodal size
distribution for both assemblies (Figure 9) with hydrodynamic
diameters d = 2.48 nm (for the unknown assembly) and 2.81
nm (for R1). Both diameters match with the size determination
from DOSY NMR18 (Table 2) thus suggesting that the new
HETPYP assembly at a ratio of [Cu2(3)]2+/ 1 = 1: 2 is smaller
than R1.
Because ESI-FT-ICR mass spectra, even under very soft
Figure 8. 1H NMR titration (partial spectrum, 400 MHz, RT) leading
ionization condition, do not allow us to establish unambigu-
ously the structure of the unknown assembly, we have to argue
solely on the basis of solution data. In principle, the R1
assembly with its coordinatively unsaturated Cu+ ion can
expand its coordination number from three to four (HETPYP-
I→HETPYP-II)12c in presence of excess of pyridine ligand (1/
[Cu2(3)]2+ = 2:1), leading to either an oligomeric assembly or a
discrete prism. However, based on the size data collected from
DLS and DOSY experiments, the possibility of a larger
architecture can be excluded. Formation of oligomeric
assembies has already been ruled out in the preceding section.
to stepwise developmentof R1 and RA1, starting from [Cu2(3)]2+ with
varying ratios of 1, plus back-titration of RA1 by addition of
[Cu2(3)]2+ to reform R1.
protons appear as two separate broad singlets at 7.66 and 7.31
ppm, respectively (Table 1), suggesting the formation of a
different species. Clearly, both α and β pyridine protons now
experience less shielding by the mesityl group of 3. Absence of
any precipitation additionally confirms that no homoleptic
pyridine/oligomer complexes form in between the trans-
formations. However, further addition of 1 (3.0 equiv.) causes
Figure 9. Dynamic light scattering data showing difference in monomodal size distribution curves and hydrodynamic diameters for R1 and RA1.
Various colored traces represent individual measurements.
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Figure 10. Top: Variable temperature 1H NMR spectra (400 MHz, in CD2Cl2) of 1/[Cu2(3)]2+ = 1:1 (R1) and 2:1 (RA1). Color code: R1, green;
RA1, pink; pyα, red; pyβ, blue. Bottom: Low temperature (−75 °C) partial 1H NMR spectra of (a) R1, (b) the products arising from a mixture of 1/
[Cu2(3)]2+ = 3:2 and (c) RA1.
Scheme 3. Cartoon Representation for the Construction of R1 and the Additive Triggered Dynamic Interconversion to Rack
Assembly
Based on thermochemical reasoning (vide infra), the simplest
structure formed from 1/[Cu2(3)]2+ = 2:1 is the molecular rack
RA1, where one [Cu2(3)]2+ unit is attached to two ligands 1 at
one of their pyridine terminals, while the second pyridine end is
left uncoordinated. Such rationale is further substantiated by
probing the mixture 1/[Cu2(3)]2+ at various ratios, that is, 1:1
H experience strong deshielding by the mesityl groups (Scheme
3).
In contrast, the dynamic nature of the rack assembly RA1 is
much more temperature dependent and more complex. At
room temperature, both pyridine terminals exhibit time-
averaged12c proton signals because of rapid exchange of “free”
and “bound” pyridine ends (intermolecular) with an NMR shift
placed between those of the free ligand 1 and R1 (Table 1). At
−50 °C, the swapping motion starts to freeze and the single
resonance for α-H separates into two resonances (Figure 10,
top right). Finally, the [Cu2(3)]2+ units and the pyridine
terminals display NMR shifts that increasingly indicate
formation of a rack architecture (Scheme 3), mainly based on
the splitting of the pyridine α-H (as stated above). At −75 °C,
even the dynamics about the single Cupy coordination bond
(rotational) freezes, as deduced from the splitting of the mesityl
protons. Obviously, intra- and intermolecular dynamics has
now come to a halt. With any of the intermolecular exchange
being stopped, we can see at −75 °C from the NMR that R1 is
not contaminated with RA1 and reciprocally RA1 is not
contaminated with R1 (Figure 10)
1
(= R1) and 2:1 (= RA1), and different temperatures in H
NMR (Figure 10, top). The signals of the pyridine α-H of 1 are
of particular help, as upon decreasing the temperature, they
experience a gradual shift. In contrast, pyridine β-H do not
experience any striking shift upon cooling because of their
distant position from the mesityl groups.
In R1, the pyridine α-H experience a slight downfield shift
with decreasing temperature that is obviously the result of a
tighter binding due to the positive (−T·ΔS) term. A somewhat
larger downfield shift is seen below −50 °C when protons x-H
and y-H split because of freezing of the rotation about the
mesityl-phenanthroline axis. The above NMR results (Figure
10, top left) thus suggest that assembly R1 is dynamic at room
temperature mostly because of its rotational freedom about the
long axis of 1 (intramolecular). Once the temperature reaches
−75 °C, a more rigid conformation is attained, in which the α-
H
dx.doi.org/10.1021/ic400328d | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Another crucial piece of information arises from the VT-
sterically encumbered bisphenanthroline ligands to set up a cap,
from which the rare trigonal [Cu(phenAr2)(py)]+ coordination
motif can be elaborated, using appropriate bi- and tris-pyridyl
ligands. An interesting feature of the present study is the
stoichiometry-controlled supramolecule-to-supramolecule
transformation, based on the relative amounts of oligopyridines.
Excess of bipyridine, for example, 1, forces the rectangle
assembly to alter into a rack architecture in solution. Vice versa,
the rectangle can again be reinstalled by titrating the rack
architecture [Cu2(1)2(L)]2+ with [Cu2(L)]2+ (L = 3, 4). The
observed rapid interconversion between the two structures, well
NMR of 1/[Cu2(3)]2+ = 3:2 which should correspond to a 1:1
1
mixture of RA1 and R1. Comparing the low temperature H
NMR with that of clean RA1 or R1, one realizes that at −75 °C
two sets of signals emerge that indeed reflect a mixture of R1:
RA1 = 1:1 (Figure 10, bottom). Above that temperature, at
about −50 °C, R1 and RA1 interconvert already as can be
derived from a single set of averaged NMR signals (Figure S51,
Supporting Information). The VT-NMR data thus support
existence of clean RA1 or R1 at the corresponding ratios 1/
[Cu2(3)]2+ of 2:1 or 1:1.
1
Finally, the stoichiometry-induced conversion of RA1→ R1
supported by variable temperature H NMR, arises from the
1
is demonstrated in a H NMR titration experiment at room
intrinsic dynamic nature of these assemblies owing to the labile
d10 Cu(I)−N(Py) bond. Although, the formation of rack
assemblies violates the maximum site occupancy rule, its
formation and subsequent equilibration behavior is of no
surprise in light of thermochemical reasoning. Given the great
interest in reversible structure alterations in supramolecular
assemblies, our study provides a new systematic to modify
constitution by exchanging and reshuffling ligand components.
temperature. The results as summarized in Figure 8 disclose
that increasing addition of complex [Cu(3)]2+ to the solution
of RA1 causes both the α and the β pyridine protons to shift
upfield in a steady manner. Finally, at 1.0 equiv of [Cu(3)]2+,
the resulting 1H NMR spectrum precisely matches with that for
R1. This experiment furthermore demonstrates that the
transformation is completely reversible. The process of
structural alteration from the rectangular assembly to a rack,
by changing the stoichiometric ratio of [Cu(3)]2+/ 1, can be
viewed as a supramolecular structural transformation, as a
function of the component stoichiometry.
The existence of RA1 with noncoordinating pyridine
terminals is unexpected because it violates the maximum site
occupancy rule. However, the exclusive formation of RA1 and
its subsequent equilibration behavior is of no surprise in light of
thermochemical reasoning, as the construction of a discrete
rack is always entropically favored over poly/oligomeric
assemblies. Although, formation of a heteroleptic rack has
been documented previously using the HETPHEN concept,10h
to the best of our knowledge, there are no examples in the
literature where the interconversion of a heteroleptic rack and
rectangle can be controlled by the relative stoichiometry of
individual components. Because of the weak Cu(I)−N(Py)
binding energy, the aforesaid changeovers in the supra-
molecular arrangement are facile.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data for 4 in CIF format. Experimental
details of synthesis, characterization for ligand 5 and all the
complexes, NMR spectra, DLS, FT-IR, and mass spectrometric
data, views of the single crystal X-ray structure, coordinates
from the computation. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: neogi@chemie.uni-siegen.de (S.N.), schmittel@
chemie.uni-siegen.de (M.S.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Based on the above protocol, a fully analogous stoichiometry
reliant reversible interconversion of R2 = [Cu4(4)2(1)2]4+
→
We are indebted to the DFG for continued support and to the
Alexander von Humboldt Foundation for a stipend to S.N. We
thank Dr. J. W. Bats (Frankfurt) for measuring the X-ray data of
4 and M. L. Saha for help with the FT-IR measurements.
RA2 = [Cu21(4)(1)2]2+ is convincingly established by room
temperature H NMR (Table 1 and Figure S52, Supporting
Information), low temperature 1H NMR (Figure S53,
Supporting Information), and further supported by DOSY
and DLS measurements (Table 2). Hence, the present study
demonstrates a reliable approach to dynamic HETPYP-I
complexation as a useful design principle for stoichiometry-
controlled supramolecular transformations. Such dynamic
modulation offers immense importance in the field of
functional dynamic materials to introduce stimuli-dependent
properties, for example through modifying constitution by
exchanging and reshuffling components.9 Unfortunately, no
analogous transformation could be studied for either R3 or the
prism assemblies as excess of pyridine ligands led to
precipitation of oligomeric complexes.
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