Synthesis and properties of metal-ligand complexes with endohedral amine functionality

Article abstract of DOI:10.1021/ic201092s

A series of tetracationic M₂L₄ palladium-pyridyl complexes with endohedral amine functionality have been synthesized. The complexes were analyzed by NMR techniques (including Diffusion NMR and 2D NOESY), electrospray ionization (ESI) mass spectrometry, and X-ray crystallography. The solid state analysis shows a large change in crystal morphology upon introduction of the endohedral amine groups, caused by deleterious interactions between the amines and the triflate counterions from the coordination process. Combination of different ligands allows analysis of ligand exchange rates via NMR analysis, with half-lives on the order of 3 h, independent of the donor properties of the ligand. Self-sorting behavior is observed, with more electron-rich ligands being favored. The amine-containing and extended complexes are strongly fluorescent, giving quantum yields of up to 83%.

Full text of DOI:10.1021/ic201092s

ARTICLE
pubs.acs.org/IC
Synthesis and Properties of MetalꢀLigand Complexes with
Endohedral Amine Functionality
Amber M. Johnson, Orly Moshe, Ana S. Gamboa, Brian W. Langloss, John F. K. Limtiaco, Cynthia K. Larive,
and Richard J. Hooley*
Department of Chemistry, University of California, Riverside, California 92521, United States
S Supporting Information
b
ABSTRACT: A series of tetracationic M2L4 palladium-pyridyl
complexes with endohedral amine functionality have been
synthesized. The complexes were analyzed by NMR techniques
(including Diffusion NMR and 2D NOESY), electrospray
ionization (ESI) mass spectrometry, and X-ray crystallography.
The solid state analysis shows a large change in crystal mor-
phology upon introduction of the endohedral amine groups,
caused by deleterious interactions between the amines and the
triflate counterions from the coordination process. Combina-
tion of different ligands allows analysis of ligand exchange rates via NMR analysis, with half-lives on the order of 3 h, independent of
the donor properties of the ligand. Self-sorting behavior is observed, with more electron-rich ligands being favored. The amine-
containing and extended complexes are strongly fluorescent, giving quantum yields of up to 83%.
’ INTRODUCTION
pointing a functional group perpendicular to the ligand is most
difficult. Even if this is achieved, the self-assembly process re -
quires donor ligands to coordinate to the metal vertices, and so
discriminating between “structural” lone pairs (to construct the
host) and “active” lone pairs (to display to the interior cavity) is
of vital importance. This can be achieved by postsynthetic modi-
fication of metalꢀorganic frameworks in the solid state,18 but the
reversibility of self-assembled solution-phase complexes renders
this strategy impractical. The attachment of functional groups
to self-assembled structures is extremely desirable, as it would
enable the variation of ligand electronics and provide methods
for analysis of the self-assembly and ligand exchange process, not
to mention promoting reactivity, catalysis, and sensing. In this
report, we discuss the synthesis and properties of a series of self-
assembled M2L4 complexes with variable amine groups, and the
effect of these functionalities on the self-assembly process and
optical properties of the system.19
The construction of self-assembled complexes with interior
cavities of defined size and shape is a rapidly advancing field of
research. Flat organic ligands can be coordinated by metal ions,1,2
forming polyhedra of various geometries3 with the metals at the
vertices4 and the ligands forming the faces.5 A variety of metalꢀ
ligand contacts are known, from octahedral iron and gallium
complexes with β-diketones6 and catecholates,7 to pyridylimine
complexes coordinated by iron and nickel,8 among others. In
addition, complexes of pyridine or acetylene ligands with square
planar palladium9,10 and platinum11 species give rise to a signifi-
cant number of self-assembled polyhedra. These polyhedra contain
interior cavities that are capable of noncovalently binding suitably
sized guests and so can act as excellent mimics of the behavior of
enzymes. As well as host:guest chemistry,6ꢀ9,12 biomimetic
reactions13 and catalysis14 are possible. Where the analogy with
enzymes falters is in the presence (or lack thereof) of appended
functionality. Few assemblies are known that display groups of
any kind on the interior of the complex.15 Most notably, Fujita
has displayed alkylammonium cations, fluorocarbons, and aromatic
groups on the interior of a self-assembled M12L24 nanocomplex.
Metalꢀligand complexes that introvert catalytically active groups
such as amines or carboxylic acids are unknown, although there
are examples of organic hoststhatcan mimic thebehavior ofamino
acid side chains in enzyme mechanisms in this way.16 In fact,
ligand derivatization in any form is still underutilized, although
examples of externally functionalized complexes have been
recently reported.17
’ RESULTS AND DISCUSSION
(1). Ligand Synthesis. The targeted ligand scaffold is shown
in Figures 1 and 2, and has three components; a central
substituted aniline core, acetylene spacers to provide complex
size and 3-pyridyl groups at the termini for metal coordination.20
Three types of ligands were synthesized to access complexes of
varying interior cavity size and substitution. To incorporate
functionality at different positions on the ligand (either endohe-
dral, exohedral, or terminal), two different strategies were used
The reasons for this are many; the efficient self-assembly of
metallocomplexes comes from the rigidity of the ligands that is often
conferred by flat aromatic or acetylenic groups. Correspondingly,
Received: May 23, 2011
Published: September 08, 2011
r
2011 American Chemical Society
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display amines on the interior of these ligands have so far been
unsuccessful.
(2). Self-Assembly and Complex Properties. Upon treat-
ment with 0.5 mol-eq. Pd(NO3)2, or PdOTf2, ligands 3,6, 8, and
11 all formed symmetrical self-assembled structures in DMSO
1
solution, as determined by H NMR spectroscopy. Molecular
modeling suggests an M2L4 “paddle-wheel” complex, as shown in
Figure 3.24 Self-assembly of the ligands was studied by five
methods: NMR, UVꢀvis absorption and fluorescence spectros-
copy, mass spectrometry, and X-ray crystallography.
NMR Analysis. All species showed representative downfield
chemical shifts due to coordination with the cationic Pd centers.
The complexes are fully symmetric, and only one structure is
observed in solution, irrespective of the ligand functionality.
Figure 3 shows the 1H NMR spectra for ligands 6a, 6b, and 3d,
and complexes [(6a)4 Pd2](NO3)4, [(6b)4 Pd2](NO3)4, and
3
3
[(3d)4 Pd2](NO3)4 (see Supporting Information for all other
3
spectra). Formation of the complex is extremely rapid, and
occurs in less than 10 s at ambient temperature and millimolar
concentration. Titration of Pd(NO3)2 into a dimethylsulfoxide
(DMSO) solution of ligand shows that complete complex
formation occurs upon addition of 0.5 mol equiv, indicating a
1:2 ratio of Pd: ligand in the self-assembled system (see
Supporting Information). Introduction of functional groups
has no effect on the coordination; neither the introverted amines
(6aꢀd), carboxylic acids nor other pyridine nitrogens are
competitive ligands for Pd, and are left uncoordinated in the
final structure. As can be seen in Figures 3b and 3d, the NH2
Figure 1. Synthesis of various ligands for complex formation.
(Figure 1). The unsubstituted ligand 3a was synthesized by
Sonogashira coupling6f,21 of 1,3-diethynylbenzene 1 and 3-bro-
mopyridine in good yield. Coupling with 5-substituted pyridines
2b and 2c allowed access to ligands with either electron rich or
electron poor pyridines at the termini. Ligand 3c was not suitable
for complex formation because of insolubility, so the diester was
hydrolyzed to diacid 3d for further study.
Commercially available 2,6-dihaloarenes 4aꢀd were used to
access amine substitution at the central core. The challenge in
creating endohedral amine substitution lies in metal-catalyzed
derivatization of these aniline reactants. Fujita has shown that
unreactive or protected species can be introverted in large
M12L24 palladium-pyridine complexes,15aꢀd but amine function-
alities are not innocent bystanders in metal-catalyzed coupling
reactions: if they are not reactants (e.g., in Buchwald-Hartwig22
coupling), they can react further with acetylene spacers to form
indoles or other unwanted side products.23 Double Sonogashira
coupling of 3-ethynylpyridine 5 with unsubstituted 2,6-dihaloar-
enes is well-precedented, but the presence of an amino sub-
stituent in the 1 position requires careful control of conditions for
optimized yields. The coupling conditions are remarkably similar
to the conditions needed for indole synthesis from aminophe-
nylacetylenes, and this was an especially problematic side reac-
tion in the synthesis of 6a. Protection of the amine 4a as a
carbamate or sulfonamide was ineffective, but use of Pd(dppf)Cl2
as catalyst and careful control of temperature and the proportions
of CuI and Pd allowed synthesis of 6a in adequate yield. The
presence of electron withdrawing groups on the central ring
(4bꢀd) allowed coupling under milder conditions, and so
competitive side reactions were minimized for the synthesis of
6b and 6c. The internal pyridine ligand 6d was synthesized in
moderate yield using the optimized conditions.
protons are visible for amine complexes [(6a)4 Pd2](NO3)4
3
and [(6b)4 Pd2](NO3)4 and have chemical shifts that are only
3
slightly changed from those of the ligand. The most notable
change in H NMR is from protons Ha and Hb which show a
1
downfield shift of >1 ppm. The other pyridine protons Hc and Hd
are also shifted significantly downfield, but the protons on the
central rings are only slightly changed because of their distance
from the cationic Pd centers.
Even though the Pd centers are tetracationic, the complexes
are stable to air, water, and light in both solution and the solid
state for months. They are sensitive to competitive ligands for the
Pd, however. Addition of stoichiometric amounts of salts such as
hydroxide, carbonate, chloride, bromide, or iodide caused dis-
ruption of the complexes, forming ligand and PdX2. Noncoordi-
nating anions (SO42-, BF4ꢀ, PF6ꢀ, SbF6ꢀ, NO3ꢀ or TfOꢀ) had
no disruptive effect on the complexes. Slight changes in chemical
shift were observed in complexes [(6a)4 Pd2](NO3)4 or
3
[(3a)4 Pd2](NO3)4 upon addition of solutions of NaSbF6 and
3
NaPF6, but these changes were minor, and no significant anion
binding occurred in solution. The octahedral geometry of these
anions should force four of the fluorides into coordination with
the introverted amines in [(6a)4 Pd2](NO3)4, but molecular
3
modeling suggests that the ions are too large for strong binding.
The counterion had a considerable effect on the solubility of the
complexes. The nitrate complexes were, without exception, only
soluble in the presence of DMSO. They are insoluble in water,
but an 80:20 mixture of water/DMSO is tolerated. No solubility
is observed in less polar solvents, and the complexes can be
precipitated from DMSO solution with addition of acetone. If
Pd(OTf)2 is used for the self-assembly, a greater range of solvents
More conjugated, larger ligands were synthesized via a similar
route, as shown in Figure 2. 1,3-Diethynylbenzene 1 was
selectively coupled with 1,4-bromoiodobenzene 7 under Sono-
gashira conditions, and the resulting dibromide subjected to
double Suzuki coupling with 3-pyridineboronic acid 9 to give the
extended ligand 8, albeit in poor yield. Para-bromophenylethy-
nylpyridine 10, accessed from 1,4-bromoiodobenzene and
3-ethynylpyridine (see Experimental Section) could be coupled
to 1 to access the fully conjugated, longer ligand 11. Attempts to
is possible. With the exception of [(3d)4 Pd2](OTf)4, all com-
3
plexes were soluble in either nitromethane or acetonitrile, and
these solutions tolerated small amounts of less polar solvents
such as toluene or chloroform. Interestingly, the complexes were
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Figure 2. Synthesis of larger ligands for complex formation.
Figure 4. DOSY spectrum of a 1:1 mixture of 6b and [(6b)4 Pd2]-
3
(NO3)4 (600 MHz, DMSO-d6, 298 K, relaxation delay 2.5 s, diffusion
delay 200 ms, gradient pulse length 2 ms). * = 6b, • = [(6b)4 Pd2]-
3
(NO3)4.27.
[(6b)4 Pd2](NO3)4 was subjected to DOSY analysis using the
3
stimulated echo pulse sequence.26 As expected, the larger com-
plex diffused more slowly than the ligand: the diffusion constant
for 6b was 2.60 ( 0.05 ꢁ 10ꢀ10 m2 sꢀ1, whereas the diffusion
Figure 3. 1H NMR spectra of representative ligands and complexes: (a)
6a, (b) [(6a)4 Pd2](NO3)4, (c) 6b, (d) [(6b)4 Pd2](NO3)4, (e) 3d,
3
3
(f) [(3d)4 Pd2](NO3)4 (400 MHz, DMSO-d6, 298K). Structure shown
constant for [(6b)4 Pd2](NO3)4 was 1.35 ( 0.01 ꢁ 10ꢀ10 m2 sꢀ1
3
3
is a molecular model of [(3a)4 Pd2](NO3)4 (SPARTAN, AM1
3
(Figure 4). Diffusion constants were determined by curve fitting
forcefield).
for the peaks at δ 8.92, 8.61 (6b) and δ 9.68, 9.39 ([(6b)4 Pd2]-
3
(NO3)4). The other peaks for the complex and ligand gave rise to
similar diffusion constants (see Supporting Information).
To confirm the M2L4 structure of the complexes, mass spectral
analysis was used. Because the nitrate complexes are only soluble
in DMSO, the triflate complexes were used for MS analysis.28
Either acetonitrile or nitromethane solutions of the complexes
were subjected to electrospray mass spectrometry. The compounds
were not completely stable to standard MS conditions, and low
stable to acid; addition of 2% HNO3 did not cause complex dis-
sociation, even for amine complex [(6b)4 Pd2](NO3)4. Other
3
metals were tested for their coordination ability. Pt formed similar
M2L4 complexes,19 but no coordination was observed upon
addition of various salts of CuI, CuII, NiII, ZnII, CdII, AgI, or
AuI with noncoordinating counterions (BF4ꢀ, PF6ꢀ, NO3ꢀ, etc).
The 3-pyridyl motif has free rotation, so could either be oriented
“out”, in a W-shaped conformation,25 or “down” as shown in the
Figures. No evidence of any other coordination patterns was
observed for any of the ligands, indicating the stability of the
M2L4 complex.
skimmer voltages were used to ensure that the [M2L4 OTf3]+
3
could be seen. All of the complexes formed from addition of
Pd(OTf)2 to ligands 3aꢀb, 6aꢀd, and 8 could be analyzed by
electrospray ionization mass spectrometry (ESI-MS). In each
case signals for the singly positive complexes (having lost one
triflate counterion) could be observed. The isotope patterns
The complexes could be analyzed by diffusion NMR, as
shown in Figure 4. A 1:1 mixture of ligand 6b and complex
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Figure 5. Observed and predicted regions of the M+ region for complexes [(6b)4 Pd2](OTf)4 and [(6d)4 Pd2](OTf)4.
3
3
Figure 6. Comparison between the structures of (a) unfunctionalized
[(3a)4 Pd2](OTf)4 (ref 19) and (b) amine complex [(6a)4 Pd2](OTf)4.
3
3
match the expected ratio for species with two Pd atoms present
(see Experimental Section and Figure 5). Complexes [(3d)4 Pd2]-
(OTf)4 were not sufficiently soluble in MeCN or MeNO2 for
3
Figure 7. Comparison between the unit cells of (a) unfunctionalized
[(3a)4 Pd2](OTf)4 (ref 19) and (b) amine complex [(6a)4 Pd2](OTf)4.
3
3
analysis, and [114 Pd2](OTf)4 was too unstable for MS analysis.
3
Only signals for [M2L4 OTf2]2+ were observed for [84 Pd2](OTf)4.
3
3
(3). X-ray Crystallographic Analysis. Crystallography of the
with disordered solvent molecules and triflate counterions. The
distance between Pd atoms on adjacent complexes is 15.01 Å,
and the aromatic rings of the ligand are completely offset, with no
πꢀπ stacking or CH-π interactions observed.
endohedral complexes proved challenging. X-ray quality crystals
of the unfunctionalized complex [(3a)4 Pd2](OTf)4 could be
3
obtained from slow evaporation of an acetonitrile solution of the
complex.19 X-ray diffraction analysis of this complex showed the
formation of the predicted paddle-wheel form, with the interior
cavity occupied by one disordered triflate molecule. Figures 6
and 7 show the structure; the complex is symmetrical and the
ligands form a perfect “cross” at the Pd centers, with no bending
or distortion. The space group was Cm, and as can be seen in
Figure 7a, there are large cavities between the complexes, filled
Crystallization of the endohedrally substituted complexes was
far less successful, and only the endohedral amine [(6a)4 Pd2]-
3
(OTf)4 was amenable to X-ray analysis. Crystals were grown by
slow evaporation of an acetonitrile-toluene solution of [(6a)4 Pd2]-
3
(OTf)4, forming the structure seen in Figures 6b and 7b. The cationic
complexes were well-resolved in the structure, but a signifi-
cant amount of disorder was present in all the counterions and
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Figure 8. 1H NMR spectra of the titration of Pd(NO3)2 into a 1:1 mixture of ligands 3a and 6a (7 mM, DMSO-d6, 500 MHz, 298K). Molar equivalents
Pd(NO3)2 (0.5 equiv of 3a, 0.5 equiv of 6a): (a) 0; (b) 0.16; (c) 0.32; (d) 0.50.
amine groups and the internally bound triflate anion have an
effect on the structure.
In solution, solvent and anions exchange rapidly from the
interior, but in the solid state one disordered triflate ion is forced
to occupy the cavity interior. It is likely that this triflate anion has
unfavorable interactions between the guest anion and the elec-
tron rich amines (interactions that are not present in [(3a)4 Pd2]-
3
(OTf)4). To allow its incorporation, the ligands twist out of plane,
changing the host structure and crystal morphology, from mono-
clinic crystals in the Cm space group for [(3a)4 Pd2](OTf)4 to
3
triclinic crystals in the P1 space group for [(6a)4 Pd2](OTf)4.
3
There are numerous CH-π contacts between the individual
[(6a)4 Pd2](OTf)4 complexes, notably between Hd from the
3
terminal pyridines (see Figure 3) and the central electron rich
aniline ring. In addition, there are πꢀπ stacks between the central
ring and the acetylene groups that are not observed in the
[(3a)4 Pd2](OTf)4 structure because of its offset nature.
3
(4). Ligand Dynamics. The synthesis of differentially func-
tionalized ligands with similar geometries allows construction
and analysis of unsymmetrical heterocomplexes. Analysis of self-
assembled complexes with differentially functionalized ligands
Figure 9. 1H NMR spectra of the incorporation of ligand 6a into of
complex [(3a)4 Pd2](NO3)4 (400 MHz, DMSO-d6, 14 mM, 300K).
3
has to date been hindered by unsuitable NMR spectra; most
published studies of ligand exchange behavior have relied upon
mass spectrometric analysis alone as the NMR spectra are too
complex or broad to allow accurate exchange analysis.30
solvent molecules; these are omitted from Figures 6 and 7 for
clarity, but occupy the empty spaces in the structure. Interest-
ingly, even though the only difference is the presence of internal
All ligands 3ꢀ6 have pyridine groups for metal binding, and
the coordination angle is identical for each. This allows analysis of
the effect of different electronics on the self-assembly process,
without significant change in metalꢀligand geometry or steric
interactions.29 The well-defined NMR spectra in this system
allow measurement of ligand exchange rate and heterocomplex
amine groups, the solid state structure of [(6a)4 Pd2](OTf)4
3
was markedly different from that of [(3a)4 Pd2](OTf)4. This
3
complex crystallizes in the P1 space group, and shows signifi-
cantly smaller cavities between complex molecules: the distance
between Pd atoms on adjacent complexes is 11.70 Å. As in
1
[(3a)4 Pd2](OTf)4, there is a disordered triflate molecule
3
composition by both H NMR and ESI-MS. If two ligands are
present on the interior of the tetracationic complex. While the
used, there are six products that can result - A4B0, A3B1, A2B2,
(two isomers - cis and trans), A1B3 and A0B4. To test the
selectivity, Pd(NO3)2 was titrated into a 1:1 solution of 3a and
6a (Figure 8). A complex (but well-defined) 1H NMR spectrum
was observed that did not change over time. Interestingly, the
incorporation of ligand into the complex with increasing [Pd]
remained the same; both ligands 3a and 6a form complexes at the
same rate, and the proportion of the heterocomplex mix is un-
changed over time. In the case of 3a/6a, the heterocomplex
mixture is both the thermodynamic and kinetic product. From
NMR and MS analysis (see Supporting Information), all six
heterocomplexes can be observed in an approximately statistical
unfunctionalized complex [(3a)4 Pd2](OTf)4 was cross-shaped
3
and symmetric, the arms in [(6a)4 Pd2](OTf)4 are twisted in
3
the solid state structure. Solution-phase studies (vide infra) show
that the formation of the tetramine complex [(6a)4 Pd2](OTf)4
3
is almost as favorable as forming the unsubstituted complex
[(3a)4 Pd2](OTf)4.29 Molecular modeling shows that the four
3
endohedral amines can be accommodated in the interior cavity
without unfavorable steric interactions between each other. While
the exact cause of the altered crystal packing in [(6a)4 Pd2](OTf)4
3
is not clear, the lack of interaction between the internal amine
groups themselves suggests that interactions between these
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ratio. There is a slight bias toward incorporation of 3a over 6a in
this case, but nothing substantial. The most useful region of the
1H NMR spectrum for analysis is that corresponding to pyridine
protons Ha and Hb (δ 9.0ꢀ10.0) as it is the least cluttered.
The sharp, distinguishable peaks for ligands and complexes in
the same sample suggested that NMR techniques could be used
to investigate the rate of ligand exchange. Unfortunately, the
ligand exchange rate is too slow for measurement by 2D EXSY
analysis of a mixture of ligand and corresponding homocomplex.
The propensity for definable heterocomplex formation, however,
allowed monitoring of the incorporation of a different ligand to a
preformed complex over time by simple dynamic NMR measure-
ments, as shown in Figure 9. Not all ligand combinations were
amenable to this analysis, as significant overlap was present in the
NMR spectra, making accurate integrations difficult. An equi-
molar amount of ligand 6a was added to a preformed solution of
complex [(3a)4 Pd2](NO3)4 and the decrease in [6a] was
3
measured over time. The observed initial rates were calculated
for ligand exchange, assuming that the rate of exchange of 6a with
3a is identical for each different heterocomplex ([(3a)4 Pd2]-
3
(NO3)4, [(3a)3 6a Pd2](NO3)4, [(3a)2 (6a)2 Pd2](NO3)4,
3
3
3
3
[3a (6a)3 Pd2](NO3)4). The fact that the different complexes
3
3
are formed at the same rate lends credence to this assumption, and
by measuring initial rates, the effect of different complex populations
is minimized. Very little difference was observed between the
exchange rate for incorporation of an electron rich ligand (ani-
line 6a) than that for an electron-poor ligand (pyridine 6d): t1/2
(6a:3a) = 3.25 h, whereas t1/2 (6d:3a) = 3.53 h. The variance in
t1/2 indicates that the ligand exchange process is not strongly
dependent on external ligand concentration and type, but is con-
sistent for the different ligands. Unfortunately, the complex NMR
spectra preclude a more detailed mechanistic analysis. Diffusion
NMR analysis of the [(3a)x (6a)y Pd2](NO3)4 heterocomplex
Figure 10. Analysis of the 6a(x) 6d(y) Pd2 heterocomplex system. (a)
3
3
ESI-MS spectrum of [6a(x) (6d)(y) Pd2](OTf)4; (b) Downfield region
3
3
of the 2D NOESY spectrum of [6a(x) 6d(y) Pd2](NO3)4, indicating
3
3
crosspeaks between Ha resonances on adjacent ligands (500 MHz,
DMSO-d6, 298 K).
3
3
mix was unsuccessful in distinguishing the individual complexes
to identify ligand combinations that would show self-sorting
behavior. Unfortunately, while all the ligands tested proved
suitable for heterocomplex formation, NMR spectra showed
statistical mixtures of products with only slight preference for
one complex over the others. The greatest bias was observed for
the [6a(x) 6d(y) Pd2](NO3)4 system shown above. The pre-
ferential incorporation of amine ligand 6a over pyridine ligand 6d
suggested that electron-rich ligands were more effective for
complex formation, but this phenomenon was not observed for
terminal methoxy ligand 3b, and no obvious pattern could be
observed in the self-assembly. This is not surprising, as the differ-
ence in donor ability for the ligands 3aꢀd and 6aꢀd is small. If
the elongated ligands 8 and 11 were mixed with the smaller
ligands 3/6, no mixing was observed, and only homocomplexes
were observed. This is consistent with the fact that no other coor-
dination geometries than the paddle-wheel are observed: the
M2L4 structure is the most favorable, and the longer ligands do
not mix with their smaller counterparts, a process that would require
a less favorable complex geometry.
(see Supporting Information).
Heterocomplex formation also allows NOE analysis of the
complexes. Obviously no NOE contacts are visible between
protons that are close in space for the symmetrical homocom-
plexes such as [(3a)4 Pd2](NO3)4, but once the ligands are
3
desymmetrized, NOESY spectra can be used to analyze the self-
assembled product. The most useful heterocomplex mix is
[6a(x) 6d(y) Pd2](NO3)2, as the different signals for Ha are
3
3
3
3
distinguished in the 1H NMR spectrum (see Figure 10). Varying
the proportions of ligands 6a and 6d ligand concentrations
allowed assignment of the different resonances. When a 1:1 ratio
of ligands is used, NOE contacts can be observed between pro-
tons Ha on adjacent ligands in the complex. The Ha peaks from
[(6a)3 6d Pd2](NO3)4 (δ 9.97, 9.70, and 9.58) show NOE
3
3
crosspeaks between each other, as do the Ha peaks from
[(6a)2 (6d)2 Pd2](NO3)4 (δ 9.85 and 9.63), but no crosspeaks
3
3
can be seen from [(6a)4 Pd2](NO3)4 (δ 9.66) to anything else.
3
1
No peaks are visible in the H spectrum for the [(6d)4 Pd2]
3
(NO3)4 complex, and any peaks from [(6a) (6d)3 Pd2](NO3)4
3
3
The lack of any strong preference between the small ligands
provides an interesting counterpart to the solid state structure of
are small and overlap with the other resonances. No other
interligand NOE signals are visible (although the different Hb
peaks are poorly differentiated in the 1H NMR spectrum). The
mass spectrum of the system is consistent with this preferential
incorporation; only four heterocomplexes are observed, [(6d)4 Pd2-
(OTf)4 is not seen at all and the signal for [6a (6d)3 Pd2](OTf)4
[(6a)4 Pd2](OTf)4. The complex with endohedral amines was
3
far less stable in the solid state than the unfunctionalized 3a,
presumably because of repulsive interactions between the amines
and the triflate ion in the cavity. These interactions forced the
complex from the cross-shaped ideal geometry into a twisted
form. In solution, that appears not to be the case: while the
3
3
3
is weak.
The formation of heterocomplexes between the other synthe-
sized ligands was investigated by 1H NMR spectra in an attempt
exact geometry of the [(6a)4 Pd2](OTf)4 complex is unknown,
3
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Table 1. UV-Vis Absorbance and Fluorescence Emission Characteristics of the Pd2L4 Complexes
a
entry
1
name
ε (cmꢀ1 Mꢀ1
)
λmax(abs) (nm)
excited @
λmax(emis) (nm)
Φ (%)
3
CH complex [3a4 Pd2](NO3)4
1.20 ꢁ 105
1.11 ꢁ 105
4.57 ꢁ 104
4.40 ꢁ 104
4.15 ꢁ 104
4.06 ꢁ 104
286, 305
287, 305
288
305
305
288
288
313
313
359, 378
359, 376
429
<0.2
<0.2
<0.2
<0.2
0.41
0.44
3
ligand 3a
2
3
diacid complex [3d4 Pd2](NO3)4
3
ligand 3d
288
437
dimethoxy complex [3b4 Pd2](NO3)2
287, 313
287, 313
356
3
ligand 3b
355
4
5
6
7
aniline complex [6a4 Pd2](NO3)4
1.43 ꢁ 105
1.32 ꢁ 105
3.64 ꢁ 104
3.33 ꢁ 104
4.80 ꢁ 104
4.44 ꢁ 104
5.64 ꢁ 104
5.17 ꢁ 104
280, 328, 374
281, 331, 374
267, 387
374
374
387
387
380
380
319
319
441
441
445
449
426
426
349
349
44
3
ligand 6a
36
nitroaniline complex [6b4 Pd2](NO3)4
0.27
<0.2
83
3
ligand 6b
266, 388
Acid-aniline complex [6c4 Pd2](NO3)4
281, 380
3
ligand 6c
281, 381
87
pyridine complex [6d4 Pd2](NO3)4
271, 319
1.1
1.1
3
ligand 6d
269, 324
8
9
elongated complex [84 Pd2](NO3)4
6.23 ꢁ 104
6.00 ꢁ 104
7.17 ꢁ 103
3.69 ꢁ 103
309
309
309
311
325
344, 361
21
23
10
38
3
ligand 8
309
344, 361
elongated complex [114 Pd2](NO3)4
311,475
325
354, 371, 381
354, 371
3
ligand 11
a εmax is quoted for the absorption at lowest wavelength.
Figure 11. Absorption spectra in DMSO solution for ligands 6c and 8, and complexes [(6c)4 Pd2](NO3)4 and [84 Pd2](NO3)4, showing weak high
3
3
energy MLCT peaks for the complexes.
formation of the complex is no less favorable than for any other
ligand with no endohedral substituents. This (along with the lack
of NMR evidence for anion binding) suggests that the triflate or
nitrate counterions are not present in the complex cavity in solu-
tion, rather that the DMSO solvent occupies the cavity interior,
giving rise to different complex stabilities in the solution phase
and the solid state.
(5). Optical Properties. The highly conjugated nature of the
ligands and complexes allows analysis of their optical properties.
All of the complexes described show strong UVꢀvis absorption
spectra, as shown in Table 1. The absorption spectra are domi-
nated by ligand-based πꢀπ* transitions, indicating that the
complexes are mainly Werner-type complexes with little electron
density residing on the metal centers, as would be expected from
the high charge. To eliminate any possibility of ligand dissociation
at the low concentrations used, the UVꢀvis absorption spectrum
of the Pd complexes were recorded at varying concentrations up
to 0.35 ꢁ 10ꢀ7 M, and their spectral features were found to be
identical to those recorded in more concentrated solution (2.2 ꢁ
10ꢀ5 M). We can therefore conclude that the Pd complexes did
not undergo dissociation at very low concentrations in DMSO.
The greatest variance in absorbance between ligand and
complex was an increased extinction coefficient for the complex,
along with small hypsochromic shifts from (1ꢀ14 nm). Only the
pyridine ligand 6d exhibited a bathochromic shift of 2 nm upon
complexation. These differences are consistent with other PdꢀN
complexes in the literature.31 For example, a 1.1-fold increase in
extinction coefficient and a hypsochromic shift of 1 nm was ob-
served between aniline ligand 6a and the corresponding complex
for the absorbance maximum at 281 nm. At the 331 nm absorbance
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Inorganic Chemistry
ARTICLE
Figure 12. Absorption/emission spectra for DMSO solutions of amine-containing complexes [(6a)4 Pd2](NO3)4, [(6b)4 Pd2](NO3)4, and
3
3
[(6c)4 Pd2](NO3)4.
3
Figure 13. Absorption/emission spectra for DMSO solutions of complex [(3a)4 Pd2](NO3)4 and elongated complexes [(6a)4 Pd2](NO3)4,
3
3
[84 Pd2](NO3)4, and [114 Pd2](NO3)4.
3
3
maximum, the opposite was observed; a 1.5-fold reduction in
extinction coefficient was recorded and a 3 nm hypsochromic
shift. Metalꢀligand charge transfer transitions were generally
weak, but they were observed for a number of the complexes. The
transitions had low ε values (∼ 102ꢀ103 cmꢀ1 Mꢀ1) and were
were observed in the presence of extroverted electron withdrawing
groups. The moderately electron withdrawing group in acid-aniline
complex [(6c)4 Pd2](NO3)4 resulted in a 3-fold decrease
3
in absorption with respect to [(6a)4 Pd2](NO3)4 as well as a
3
hypsochromic shift of 6 nm for the absorbance at longest wave-
observed most notably for extended complex [84 Pd2](NO3)4
length. Stronger acceptors such as nitroaniline [(6c)4 Pd2](NO3)4
3
3
and amino acid complex [(6c)4 Pd2](NO3)4 (Figure 11).
resulted in 4-fold decrease in absorption and a bathochromic shift
3
Fluorescence emission spectra were also recorded, and the
quantum yields for the fluorescent complexes established. The
quantum yields varied significantly for the various complexes,
of 13 nm.
All three complexes gave emission spectra, as shown in
Figure 12b. The Stokes’ shift of these complexes varied strongly
from very low values (<0.2% for diacid complex [(3d)4 Pd2]-
with substitution, with aniline complex [(6a)4 Pd2](NO3)4 giving
a 113 nm shift. The presence of electron withdrawing groups on
the ring reduced the shift, with a 58 nm Stokes’ shift observed for
3
3
(NO3)4) to high yields (83% for amino acid complex [(6c)4
3
Pd2](NO3)4). These variations will be discussed below. Stokes’
shifts also varied; the average shift was on the order of 70 nm, but
the strongly withdrawn nitroaniline complex [(6b)4 Pd2](NO3)4.
These complexes were strongly fluorescent, with quantum yields
varying between 44 and 83%. The presence of the nitro group in
3
this varied from 30 nm for pyridine complex [(6d)4 Pd2](NO3)4
3
to 141 nm for diacid complex [(3d)4 Pd2](NO3)4. The pre-
3
sence of heavy metals did not cause significant quenching of the
fluorescence, and an increase in Φ was observed for aniline
[(6b)4 Pd2](NO3)4 caused significant fluorescence quenching,
3
with Φ < 0.2%.
complex [(6a)4 Pd2](NO3)4.
(a). Amine-Functionalized Complexes. The absorption spec-
(b). Terminally Functionalized Complexes. The amine func-
tionality was essential for strong fluorescence in the smaller
complexes. The parent CH complex was only weakly fluorescent,
with Φ < 0.2%. Incorporation of electron donors and acceptors
to the terminal pyridine groups had little effect, although the
3
trum for the aniline complexes [(6a)4 Pd2](NO3)4ꢀ[(6c)4 Pd2]-
3
3
3
(NO3)4 is shown in Figure 12. The aniline complex [(6a)4 Pd2]-
(NO3)4 was strongly absorbing, with a molar extinction coefficient of
1.43 ꢁ 105 Mꢀ1 cmꢀ1. Three absorption peaks were observed at
280, 328, and 374 nm. Several variations in the absorption spectra
quantum yield for dimethoxy complex [(3b)4 Pd2](NO3)4 was
3
more than double that of [(3a)4 Pd2](NO3)4, with Φ = 0.41%.
3
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Inorganic Chemistry
ARTICLE
The emission spectrum for [(3a)4 Pd2](NO3)4 is broad, with
two maxima at 359 and 378 nm, whereas the emission spectra of
supplied with the Varian operating system. DOSY spectra were recorded
on a Bruker Avance 600 spectrometer. Proton (1H) chemical shifts are
reported in parts per million (δ) with respect to tetramethylsilane
(Si(CH3)4, δ = 0), and referenced internally with respect to the protio
solvent impurity. Carbon (13C) chemical shifts are reported in parts per
million (δ) with respect to tetramethylsilane (Si(CH3)4, δ = 0), and
referenced internally with respect to the solvent 13C signal (either
CDCl3 or DMSO-d6). Deuterated NMR solvents were obtained from
Cambridge Isotope Laboratories, Inc., Andover, MA, and used without
further purification. Mass spectra were recorded on an Agilent 6210 LC
TOF mass spectrometer using electrospray ionization and processed
with an Agilent MassHunter Operating System. The complexes were
diluted to concentrations of ∼50 μM. Gentle source conditions were
employed to keep the metalꢀligand complexes intact. This was achieved
by lowering voltages in the capillary/skimmer interface region. UVꢀvis
absorption spectra were recorded on a Varian Cary 50 UVꢀvisible
spectrophotometer. The absorption spectra of the free ligands and their
corresponding metal complexes were recorded in solution at room tem-
perature. Steady-state fluorescence measurements were obtained using a
Spex Fluorolog-Tau3 fluorescence spectrophotometer and recorded at
1 nm increments and 1 s integration time. The fluorescence emission
and excitation spectra of the free ligands and their corresponding metal
complexes were recorded in diluted solutions. Optical data was recorded
in standard 10 mm path length Spectrosil Far UV quartz cuvettes. Mole-
cules and reagents for ligand/complex synthesis were obtained from
Aldrich Chemical Co., St. Louis, MO, and were used as received. Solvents
were dried through a commercial solvent purification system (SG Water,
Inc.). Pd(OTf)2 was synthesized according to literature procedures.32
3
[(3b)4 Pd2](NO3)4 and [(3d)4 Pd2](NO3)4 are less struc-
3
3
tured (see Supporting Information). A hypsochromic shift is
noticed for the dimethoxy emission spectrum with λmax = 356 nm.
On the other hand, there is a noticeable bathochromic shift
observed for the extroverted acid complex corresponding to the
peak at λmax = 429 nm.
(c). Elongated Complexes. Increasing the conjugation of the
ligands caused slight changes in the absorption spectra; a bath-
ochromic shift of 4 nm was observed between the lowest energy
absorptions of complexes [(3a)4 Pd2](NO3)4 and [84 Pd2]-
3
3
(NO3)4 (Figure 13). A bathochromic shift of 6 nm was observed
between [(3a)4 Pd2](NO3)4 and the largest complex [114 Pd2]-
3
3
(NO3)4. Elongation reduces the absorption efficiency, though, as
the molar extinction coefficient of [(3a)4 Pd2](NO3)4 was 1.2 ꢁ
3
105 Mꢀ1 cmꢀ1, whereas[84 Pd2](NO3)4 showed a 2-fold decrease
3
in ε. A 17-fold decrease in ε was observed for the πꢀπ* intraligand
transition of [114 Pd2](NO3)4, but this system did show a metal-
3
to-ligand charge-transfer transition at 475 nm. The emission spectra
of [84 Pd2](NO3)4, and [114 Pd2](NO3)4 are more highly
3
3
structured than the absorption spectra, which is devoid of structure.
The more conjugated, longer ligands also show more efficient
fluorescence than their shorter counterparts, with Φ = 21% and
10% for [84 Pd2](NO3)4, and [114 Pd2](NO3)4, respectively.
3
3
’ CONCLUSIONS
The synthesis of ligand 3a and complex [(3a)4 Pd2](NO3)4, including
3
In conclusion, a series of tetracationic M2L4 palladium-pyridyl
paddle-wheel complexes have been synthesized with different
appended functionalities including endohedral amines and exo-
hedral carboxylic acids. The metalꢀligand coordination occurs
solely at the terminal pyridines, even in the presence of amine or
other pyridine groups, and the M2L4 structure is favored, with no
polymeric aggregates observed. Structural analysis is possible
using NMR techniques (including Diffusion NMR and 2D NOESY),
mass spectrometry, and X-ray crystallography. The solid state
analysis shows a large change in crystal morphology upon intro-
duction of the endohedral amine groups, because of a twisting of
the ligands in the paddle-wheel complex with respect to the un-
functionalized system. This twisting leads to a far more closely
packed structure that takes advantage of πꢀπ stacking and CH-
π interactions, something not observed in the unfunctionalized
system, which shows large cavities and highly offset crystal packing.
Combination of different ligands allows analysis of the self-
assembly properties of the system; both electron rich and elec-
tron poor ligands exchange with a half-life of ∼3 h. The different
donor properties of the functionalized ligands give rise to biased
heterocomplex formation, with favorability observed for self-
assembly of endohedral amines over pyridyl substrates. The most
efficient fluorescence belongs to complexes with endohedral
amines, giving quantum yields of up to 83%. The construction
of larger ligands is possible, and these more conjugated complexes
also show good fluorescence properties. Further research on the
properties of endohedrally functionalized complexes is underway
in our laboratory.
crystallographic analysis are described in ref 19. Molecular modeling
(semiempirical calculations) was performed using the AM1 force field
using SPARTAN.33
’ EXPERIMENTAL PROCEDURES
General Procedure for Complex Formation. The mixture of
ligand (0.1 mmol) and Pd(NO3)2 xH2O (0.05 mmol) in DMSO
3
(2 mL) was stirred at room temperature for 30 min, then precipitated
by addition of acetone and hexane. The precipitate was washed carefully
with acetone (6 mL) and dried under vacuum to yield the solid product.
1,3-Bis((5-methoxypyridin-3-yl)ethynyl)benzene 3b. To a 10 mL
round-bottomed flask equipped with stir bar and reflux condenser was
added 1,3-diethynylbenzene (48 mg, 0.376 mmol), 3-bromo-5-methox-
ypyridine (178 mg, 0.946 mmol), 1,10 bis(diphenylphosphino)ferrocene
palladium(II) dichloride (15.3 mg, 0.019 mmol), and copper(I) iodide
(4 mg, 0.022 mmol). The mixture was placed under nitrogen, and
degassed diisopropylamine (1 mL) was added. The reaction was heated
at 80 °C for 22 h. Ethyl acetate (10 mL) was added, the reaction filtered,
and the filtrate evaporated under reduced pressure. The crude product
was recrystallized from ethyl acetate to yield a brown solid (87 mg, 68%).
1H NMR (400 MHz, DMSO-d6) δ 8.37 (s, 2H); 8.33 (d, J = 2.0 Hz,
2H); 7.81 (s, 1H); 7.66 (dd, J = 1.2, 8.4 Hz, 2H); 7.60 (dd, J = 2.0, 2.8
Hz; 2H); 7.54 (t, J = 7.7 Hz, 1H); 3.87 (s, 6H); 13C NMR (100 MHz,
DMSO-d6) δ 154.9; 143.8; 138.15; 134.3; 132.0; 129.6; 122.4; 122.1;
119.4; 90.9; 87.0; 55.7; HRMS (ESI) m/z: calcd for C22H17N2O2
(M+H)+ 341.1285; found 341.1295.
Complex [(3b)4 Pd2](NO3)4. 1H NMR (400 MHz, DMSO-d6) δ 9.36
3
(s, 2H); 9.13 (d, J = 2.4 Hz, 2H); 7.92 (s, 1H); 7.91 (s, 2H); 7.70 (d,
J = 8.4 Hz, 2H); 7.59 (t, J = 7.6 Hz, 1H); 3.97 (s, 6H); 13C NMR
(100 MHz, DMSO-d6) δ 156.7; 144.4; 139.3; 134.2; 133.3; 130.0;
126.7; 122.3; 121.7; 93.5; 85.0; 56.8; HRMS (ESI) m/z: calcd for
C91H64N12O9F9S3106Pd108Pd (M ꢀ OTfꢀ)+ 2021.1476; found
2021.1514.
’ EXPERIMENTAL SECTION
General Information. 1H and 13C NMR spectra were recorded on
either a Varian Inova 400 or 500 spectrometer. 2D NOESY spectra were
recorded on a Varian Inova 500 spectrometer, using the pulse sequence
9438
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Inorganic Chemistry
ARTICLE
Dimethyl 5,50-(1,3-phenylenebis(ethyne-2,1-diyl))dinicotinate 3c.
To a 10 mL round-bottomed flask equipped with stir bar and reflux
condenser was added 1,3-diethynylbenzene (25 mg, 0.198 mmol), methyl
5-bromonicotinate (86.6 mg, 0.401 mmol), and 1,10 bis(diphenylpho-
sphino)ferrocene palladium(II) dichloride (8.4 mg, 0.010 mmol. The
mixture was placed under nitrogen and anhydrous N,N0-dimethylfor-
mamide (1 mL), and degassed triethylamine (0.5 mL) was added. The
reaction was heated at 80 °C for 16 h. Water (10 mL) was added, and the
resultant precipitate filtered. The crude product was purified by flash
column chromatography on silica gel (CH2Cl2) to yield a tan solid
(44 mg, 56%). 1H NMR (400 MHz, CDCl3) δ 9.15 (d, J = 1.6 Hz, 2H);
8.90 (d, J = 1.6 Hz, 2H); 8.41 (t, J = 2.0 Hz, 2H); 7.76 (s, 1H); 7.56 (dd,
J = 1.6, 7.6 Hz, 2H); 7.40 (t, J = 7.6 Hz, 1H); 3.98 (s, 6H); 13C NMR
(100 MHz, CDCl3) δ 165.22 155.6; 149.7; 139.5; 135.0; 132.3; 128.9;
125.8; 122.9; 120.3; 92.6; 85.9; 52.8; HRMS (ESI) m/z: calcd for
397.1183; found 397.1154.
bis(triphenylphosphine)-palladium(II) dichloride (20 mg), and triphe-
nylphosphine (20 mg). The mixture was placed under nitrogen and
diethylamine (5 mL) degassed was added by syringe. The reaction was
heated to 70 °C for 18 h. Ethyl Acetate (50 mL) was added to the reac-
tion mixture, and the mixture filtered. The filtration was subject to
aqueous workup and extracted with ethyl acetate. The combined organic
layer was dried over anhydrous sodium sulfate and evaporated under
reduced pressure. The residue was purified by flash column chroma-
tography on silica gel (CHCl3, CHCl3/methanol = 100:2, CHCl3/
methanol = 100:4) to yield a yellow solid (220 mg, 65%). 1H NMR (400
MHz, DMSO-d6) δ 8.92 (d, J = 1.9 Hz, 2H), 8.61 (dd, J = 1.6, 4.8 Hz,
2H), 8.24 (s, 2H), 8.14 (dt, J = 1.8, 7.9 Hz, 2H), 7.50 (dd, J = 5.0, 8.0 Hz,
2H), 7.28 (s, 2H). 13C NMR (100 MHz, DMSO-d6) δ 154.6, 152.1,
149.2, 138.9, 135.9, 129.1, 123.5, 119.2, 105.5, 92.6, 86.8. HRMS (ESI)
m/z: calcd for C20H12N4O2 (M+H)+ 341.1033; found 341.1009.
Complex [(6b)4 Pd2](NO3)4. 1H NMR (400 MHz, DMSO-d6) δ 9.68
3
5,50-(1,3-Phenylenebis(ethyne-2,1-diyl))dinicotinic acid 3d. To a
10 mL round-bottom flask equipped with stir bar and reflux condenser
was added dimethyl 5,50-(1,3-phenylenebis(ethyne-2,1-diyl))dinicotinate
(64 mg, 0.162 mmol), sodium hydroxide (53 mg, 1.32 mmol), tetra-
hydrofuran (0.8 mL), and water (0.2 mL). The reaction was heated at
65 °C for 16 h. The reaction was allowed to cool, and 10% aq HCl was
added until acidic.The resultant precipitate was filtered to yield a tan
solid (53 mg, 89%). 1H NMR (400 MHz, DMSO-d6) δ 13.68 (br s, 2H);
9.06 (d, J = 2.0 Hz, 2H); 8.99 (d, J = 1.6 Hz, 2H); 8.40 (t, J = 2.0 Hz, 2H);
7.91 (s, 1H); 7.71 (dd, J = 1.2, 7.6 Hz, 2H); 7.56 (t, J = 7.6 Hz, 1H); 13C
NMR (100 MHz, DMSO-d6) δ 165.5; 154.9; 149.5; 139.1; 134.6; 132.3;
129.6; 126.5; 122.2; 119.2; 92.0; 86.1; HRMS (ESI) m/z: calcd for
369.0870; found 369.0861.
(s, 2H); 9.40 (d, J = 4.0 Hz, 2H); 8.31 (d, J = 8.0 Hz, 2H); 8.29 (s, 2H);
7.85 (dd, J = 6.0, 8.0 Hz, 2H); 7.34 (s, 2H); 13C NMR (125 MHz,
DMSO-d6) δ 154.5; 152.8; 150.5; 142.5; 136.5; 130.2; 127.3; 122.3; 105.1;
90.5; 89.9. HRMS (ESI) m/z: calcd for C83H48N16O17F9S3106Pd108Pd
(M ꢀ OTfꢀ)+ 2021.0470; found 2021.0531.
4-Amino-3,5-bis(pyridin-3-ylethynyl)benzoic Acid 6c. To an oven-
dried 25 mL round-bottomed flask equipped with stir bar and reflux
condenser was added 4-amino-3,5-diiodobenzoic acid (195 mg, 0.501
mmol), 3-ethynylpyridine (130 mg, 1.261 mmol), bis(triphenylpho-
sphine)palladium(II) dichloride (15 mg, 0.021 mmol), and triphenyl-
phosphine (9 mg, 0.034 mmol). The mixture was placed under nitrogen,
and dry, degassed THF (4 mL), and degassed triethylamine (4 mL) were
added. The reaction was heated at 50 °C for 16 h. Tetrahydrofuran was
added (20 mL), and the reaction filtered. The filtrate was evaporated
under reduced pressure, and the residue purified by flash column chromato-
graphy on silica gel (CHCl3/MeOH 100:4; CHCl3/MeOH 100:10) to
Complex [(3d)4 Pd2](NO3)4. 1H NMR (400 MHz, DMSO-d6) δ 9.98
3
(d, J = 1.2 Hz, 2H); 9.87 (d, J = 1.6 Hz, 2H); 8.561 (s, 2H); 7.942
(s, 1H); 7.80 (dd, J = 1.2, 8.0 Hz, 2H); 7.60 (t, J = 8.0 Hz, 1H); 13C NMR
(100 MHz, DMSO-d6) δ 163.8; 155.8; 151.3; 142.5; 134.1; 133.7;
130.1; 128.5; 122.4; 121.7; 94.5; 84.9. HRMS data could not be obtained
because of solubility issues.
1
yield a yellow solid (65 mg, 38%). H NMR (400 MHz, DMSO-d6)
δ 12.64 (br s, 1H); 8.89 (d, J = 1.2 Hz, 2H); 8.59 (dd, J = 1.6, 4.8 Hz, 2H);
8.11 (td, J = 2.0, 8.0 Hz, 2H); 7.91 (s, 2H); 7.48 (dd, J = 4.8, 8.0 Hz, 2H);
6.64 (s, 2H); 13C NMR (100 MHz, DMSO- d6) δ 165.9; 152.9; 151.9;
148.9; 138.7; 134.7; 123.4; 119.6; 118.1; 105.4; 91.7; 88.0; HRMS (ESI)
m/z: calcd for C21H13N 3O2 (M + H)+ 340.1081; found 340.1073.
2,6-Bis(pyridin-3-ylethynyl)aniline 6a. To a 10 mL round-bottomed
flask equipped with stir bar and reflux condenser was added 2,6-dibro-
moaniline (129.8 mg, 0.517 mmol), 3-ethynylpyridine (129.2 mg, 1.253
mmol), 1,10 bis(diphenylphosphino)ferrocene palladium(II) dichloride
(21.6 mg, 0.026 mmol), and copper(I) iodide (5.6 mg, 0.029 mmol).
The mixture was placed under nitrogen and dry, degassed tetrahy-
drofuran (THF, 2 mL) and degassed diisopropylamine (2 mL) were
added. The reaction was heated at 80 °C for 48 h. Ethyl acetate (10 mL)
was added, the reaction filtered, and the filtrate washed with water
(3 ꢁ 20 mL) and brine (20 mL). The organic layer was dried over
magnesium sulfate and evaporated under reduced pressure. Boiling ethyl
acetate (2 mL) was added, and the mixture allowed to cool to room tem-
perature, then in an ice bath. The precipitate was washed with cold ethyl
Complex [(6c)4 Pd2](NO3)4. 1H NMR (400 MHz, DMSO-d6) δ 9.68
3
(s, 2H); 9.37 (d, J = 5.2 Hz, 2H); 8.29 (d, J = 8.0 Hz, 2H); 7.96 (s, 2H);
7.81 (dd, J = 6.0, 7.6 Hz, 2H); 6.83 (s, 2H); HRMS (ESI) m/z: calcd
for C87H52N12O17F9S3106Pd108Pd (M ꢀ OTfꢀ)+ 2017.0660; found
2017.0727. Note: this complex was too insoluble to obtain a 13C NMR
spectrum in any reasonable amount of time.
2,6-Bis(pyridin-3-ylethynyl)pyridine 6d. To an oven-dried 25 mL
round-bottom flask with a magnetic stirring bar was added 2,6-dibro-
mopyridine (471 mg, 2 mmol), 3-ethynylpyridine (500 mg, 4.8 mmol),
bis(triphenylphosphine)-palladium(II) dichloride (40 mg), triphenyl-
phosphine (30 mg), and copper iodide (15 mg). The mixture was placed
under nitrogen, and diethylamine (12 mL) degassed were added by
syringe. The reaction was heated to 60 °C under nitrogen for 18 h. Ethyl
acetate (50 mL) was added to the reaction mixture, and the mixture
filtered. The filtration was subject to aqueous workup and extracted with
ethyl acetate. The combined organic layer was dried over anhydrous sodium
sulfate and evaporated under reduced pressure. The residue was purified
by flash column chromatography on silica gel (CHCl3/methanol =
1
acetate to yield a brown solid (74.3 mg, 49%). H NMR (400 MHz,
DMSO-d6) δ 8.84 (s, 2H); 8.58 (d, J = 4.0 Hz, 2H); 8.06 (td, J = 4.0,
8.0 Hz, 2H); 7.47 (dd, J = 8.0, 8.0 Hz, 2H); 7.39 (d, J = 8.0 Hz, 2H); 6.64
(t, J = 8.0 Hz, 1H); 5.91 (s, 1H); 13C (100 MHz, DMSO-d 6) δ 152.8;
150.1; 142.5; 133.9; 133.3; 129.2; 127.3; 122.3; 121.2; 93.7; 85.1. HRMS
(ESI) m/z: calcd for C20H13N 3 (M + H+) 296.1182; found 296.1118.
Complex [(6a)4 Pd2](NO3)4. 1H NMR (400 MHz, DMSO-d6) δ 9.66
3
(s, 2H); 9.36 (d, J = 8.0 Hz, 2H); 8.21 (d, J = 8.0 Hz, 2H); 7.79 (dd, J =
8.0, 8.0 Hz, 2H); 7.43 (d, J = 8.0 Hz, 2H); 6.67 (t, J = 8.0 Hz, 1H); 6.24
(s, 2H); 13C (100 MHz, DMSO-d6) δ 152.6, 150.5, 149.9, 142.0, 134.8,
127.2, 122.9, 116.9, 105.1, 92.4, 89.6. HRMS (ESI) (OTfꢀ salt) m/z:
calcd for C83H52N12O9F9S3106Pd108Pd (M ꢀ OTfꢀ)+ 1841.1067; found
1841.1054.
1
100:2) to yield a yellow solid (640 mg 50%). H NMR (400 MHz,
DMSO-d6) δ 8.85 (s, 2H), 8.66 (d, J = 3.6 Hz, 2H), 8.08 (dt, J = 1.8,
7.9 Hz, 2H), 7.98 (t, J = 7.8 Hz, 2H), 7.74 (d, J = 7.8 Hz, 2H), 7.52 (dd,
J = 4.9, 7.9 Hz, 2H). 13C NMR (100 MHz, DMSO-d6) δ 152.0; 149.8;
142.3; 139.1; 137.9; 127.4; 123.8; 118.3; 90.9; 85.8. HRMS (ESI) m/z:
calcd for C19H11N3 (M + H)+ 282.1026; found 282.1002.
2,6-Bis(pyridylethynyl)-4-nitroaniline 6b. To an oven-dried 25 mL
round-bottom flask with a magnetic stirring bar was added 2,6-diiodoni-
troaniline (390 mg, 1 mmol), 3-ethynylpyridine (250 mg, 2.4 mmol),
Complex [(6d)4 Pd2](NO3)4. 1H NMR (400 MHz, DMSO-d6) δ 9.88
3
(s, 2H), 9.41 (d, J= 5.3 Hz, 2H), 8.32 (d, J= 8.0 Hz, 2H), 7.97 (t, J=7.8Hz,
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ARTICLE
2H), 7.84 (dd, J = 5.9, 7.9 Hz, 2H), 7.75 (d, J = 7.8 Hz, 2H); 13C NMR δ
153.5; 151.0; 143.3; 141.9; 138.0; 128.6; 127.3; 121.6; 93.3; 83.3. HRMS
(ESI) m/z: calcd for C79H44N12O9F9S3106Pd108Pd (M ꢀ OTfꢀ)+
1785.0441; found 1785.0473.
138.5; 133.2; 131.8; 123.2; 121.5; 120.3; 91.7; 87.0. HRMS (ESI)
m/z: calcd for C13H8BrN (M+H+) 256.9835; found at 256.9836.
1,3-Bis((4-(pyridin-3-ylethynyl)phenyl)ethynyl)benzene 11. To an
oven-dried, 25 mL round-bottom flask equipped with a magnetic stirrer
was added 1.3 diethynyl benzene (40 mg, 0.32 mmol), 3-((4-bromophenyl)-
ethynyl)pyridine 10 (166 mg, 0.64 mmol), bis(triphenylphosphine)-
palladium(II) dichloride (2.2 mg), and CuI (3 mg). The mixture was
placed under nitrogen, and anhydrous diisopropylamine (3 mL) was
added by syringe. The reaction was stirred at room temperature for 24 h.
The solvent was removed by rotary evaporation, and the remaining solid
was dissolved in methylene chloride (10 mL). The mixture was then
filtered through Celite, and the solvent from the filtrate was removed by
rotary evaporation to yield 1,3-bis((4-(pyridin-3-ylethynyl)phenyl)ethynyl)-
benzene 11 (42 mg, 27%). 1H NMR (400 MHz, CDCl3): δ 8.78 (s, 2H),
8.58 (d, J = 4.8 Hz, 2H), 7.83 (d, J = 7.9 Hz, 2H), 7.72 (s, 1H), 7.53
(s, 8H), 7.51 (d, J = 7.8 Hz, 2H), 7.382 (t, J = 7.1 Hz, 1H), 7.32 (m, J =
7.8 Hz, 2H). LIFDI-MS m/z: calcd for C36H20N2 (M+H+) found at 480.
1,3-Bis((4-(pyridin-3-yl)phenyl)ethynyl)benzene 8. To an oven-
dried, 50 mL round-bottom flask equipped with a magnetic stirrer was
added 1,3-diethynylbenzene (300 mg, 0.24 mmol), 1-bromo-4-iodo-
benzene (1.365 g, 0.48 mmol), bis(triphenylphosphine)palladium(II)
dichloride (168 mg), and CuI (23 mg). The mixture was placed under
nitrogen and anhydrous diisopropylamine (20 mL) was added by syringe.
The reaction was left to stir at room temperature for 30 h. The solvent
was removed by rotary evaporation, and the remaining solid was dis-
solved in methylene chloride (10 mL). The mixture was then filtered
through Celite, and the solvent from the filtrate was removed by rotary
evaporation. Recrystallization from methylene chloride gave 1,3-bis((4-
bromophenyl)-ethynyl)benzene (880 mg, 85%) as an off white powder.
1H NMR (300 MHz, CDCl3) δ 7.69 (br s, 1H), 7.51ꢀ7.45 (m, J =
8.67 Hz, 4 H), 7.40 (d, J = 8.23 Hz, 4 H), 7.34 (d, J = 7.68 Hz, 1 H).
13C NMR (400 MHz, CDCl3) δ 135.3; 134.6; 133.2; 131.7; 131.6; 123.5;
122.8; 122.0; 89.6. GCMS (EI) m/z: calcd for C22H12Br2 (M+H+)
433.9300; found 433.9283.
Complex [114 Pd2](NO3)4. 1H NMR (400 MHz, DMSO-d6): δ 9.60
3
(s, 8H), 9.29 (d, J = 8.52, 8H), 7.82ꢀ7.43 (m, 60H). This complex was
poorly stable, and HRMS/13C data could not be obtained.
’ X-RAY STRUCTURE DETERMINATION
To an oven-dried, 25 mL round-bottom flask equipped with a
magnetic stirrer was added 1,3-bis((4-bromophenyl)ethynyl)benzene
(100 mg, 0.23 mmol) and 3-pyridine boronic acid 9 (59 mg, 0.48 mmol),
bis(triphenylphosphine)palladium(II) dichloride (16 mg), and Cs2CO3
(450 mg, 1.4 mmol). The mixture was placed under nitrogen and
degassed ethanol (4 mL) and anhydrous toluene (2 mL) were added by
syringe. The reaction was stirred at 73 °C for 24 h. The solvent was
removed by rotary evaporation, and the remaining solid was dissolved in
methylene chloride (10 mL). The mixture was then filtered through
Celite, and the solvent from the filtrate was removed by rotary evapora-
tion. The crude product was purified by flash column chromatography
on silica gel (CH2Cl2/methanol = 100:1) to yield compound 8 (30 mg,
10%) as a light brown solid. 1H NMR (400 MHz, DMSO-d6): δ 8.97
(s, 2H), 8.61 (d, J = 3.8 Hz, 2H), 8.16 (d, J = 8.6 Hz, 2H), 7.85 (d, J =
8.4 Hz, 4H), 7.81 (s, 1H), 7.73 (d, J = 8.5 Hz, 4H), 7.65 (d, J = 7.4 Hz,
2H), 7.55ꢀ7.51 (m, J = 6.6 Hz, 3H). 13C NMR (100 MHz, CDCl3):
δ 148.7; 148.1; 137.8; 134.8; 134.5; 132.5; 132.1; 131.6; 129.8; 128.7;
127.2; 123.6; 123.1; 89.9; 89.7. HRMS (ESI) m/z: calcd for C32H20N2
(M + H+) 432.1621; found at 432.1614.
Crystal Data. A light yellow fragment of a prism (0.43 ꢁ 0.27
ꢁ 0.18 mm3) was used for the single crystal X-ray diffraction
study of [C80H52N12Pd2]4+[CF3SO3]4ꢀ. The crystal was coated
with paratone oil and mounted onto a cryo-loop glass fiber. X-ray
intensity data were collected at 100(2) K on a Bruker APEX2
platform-CCD X-ray diffractometer system (fine focus Mo-
radiation, λ = 0.71073 Å, 50KV/30 mA power). The CCD
detector was placed at a distance of 4.9890 cm from the crystal.
[C80H52N12Pd2]4+[CF3SO3]ꢀ4[partial solvents] M = 2111.77
[including partially occupied solvents], CCDC submission no.
828749, triclinic space group P1 (no. 2), a = 12.6089(15) Å,
b = 19.950(2) Å, c = 21.233(3) Å, α = 63.3356(16)°, β =
73.1549(17)°, γ = 75.7105(17)°, V = 4525.4(9) Å3, Z = 2,
calculated density Dc = 1.550 g/cm3, light yellow prism fragment
(0.43 ꢁ 0.27 ꢁ 0.18 mm3) coated with paratone oil, T = 100(2)
K, 36788 reflections measured (0.73 Å resolution), 25380 unique
(Rint = 0.0461, completeness = 99.6%), final R1 = 0.0685, wR2 =
0.1647 with intensity I > 2σ(I) (see Supporting Information for
full experimental details).
Complex [84 Pd2](NO3)4. 1H NMR (100 MHz, DMSO-d6): δ 9.98
3
(s, 8H), 9.42 (d, J = 5.9 Hz, 8H), 8.48 (d, J = 8.3 Hz, 8H), 7.94 (d, J =
8.4 Hz, 16H), 7.88 (m, J = 6.9 Hz, 8H), 7.81 (d, J = 8.2 Hz, 18H), 7.74
(s, 4H), 7.62 (d, J = 1.1 Hz, 8H), 7.51 (t, J = 7.2 Hz, 4H). 13C NMR (125
MHz, DMSO-d6): δ 149.9; 148.6; 138.4; 137.6; 134.4; 133.8; 131.9;
129.6; 127.4; 127.3; 123.2; 122.6; 90.1; 89.5. HRMS (ESI) m/z: calcd
for C130H81N8O6F6S2106Pd108Pd (M ꢀ 2OTfꢀ)2+ 1120.6844; found at
1120.6888. Note: in this case the monocation was not observed, and the
MS data for the dication is shown.
Procedure for Kinetics Measurements. Addition of Ligand
6d to Complex [(3a)4 Pd2](NO3)4. Ligand 3a was dissolved in
3
DMSO-d6 to make a 14 mM solution, and a 152 mM solution of
Pd(NO3)2 was titrated in until complex formation was almost
complete. A portion of this solution was added to an equal
volume 28 mM solution of ligand 6d. After mixing, spectra were
periodically acquired (3 s delay, 32 scans). Concentrations of
ligands and complexes were measured by integrating the peaks at
8.78, 8.85, and 9.94ꢀ9.61.
3-((4-Bromophenyl)ethynyl)pyridine 10. To an oven-dried, 50 mL
round-bottom flask equipped with a magnetic stirrer was added 3-ethynyl
pyridine (180 mg, 1.7 mmol), 1-bromo-4-iodobenzene (406 mg, 1.4 mmol),
bis(triphenylphosphine)palladium(II) dichloride (52 mg), and CuI
(8 mg). The mixture was placed under nitrogen and anhydrous diiso-
propylamine (3 mL) were added by syringe. The reaction was left to stir
at room temperature for 24 h. The solvent was removed by rotary eva-
poration, and the remaining solid was dissolved in methylene chloride
(10 mL). The mixture was then filtered through Celite, and the solvent
from the filtrate was removed by rotary evaporation to yield 3-((4-
bromophenyl)ethynyl)pyridine 10 (277 mg, 75%). 1H NMR (400
MHz, CDCl3): δ 8.77 (s, J = 1.04, 1H), 8.57 (d, J = 4.9 Hz, 1H), 7.8
(d, J = 7.8 Hz, 1H), 7.53 (d, J = 8.7 Hz, 1H), 7.42 (d, J = 8.7 Hz, 1H),
7.32ꢀ7.27 (m, 1H). 13C NMR (100 MHz, CDCl3): δ 152.2; 148.8;
Addition of Ligand 6a to Complex [(3a)4 Pd2](NO3)4.
3
Ligand 3a was dissolved in 500 μL of DMSO-d6 to make a
14 mM solution, and a 152 mM solution of Pd(NO3)2 was
titrated in until complex formation was complete. A portion of
this solution was added to an equal volume 28 mM solution of
ligand 6a. After mixing, spectra were periodically acquired (3 s delay,
32 scans). Concentrations of ligands and complex were measured by
integrating the peaks at 8.00, 8.06, and 9.59ꢀ9.80 ppm.
Procedure for Diffusion Experiments. Diffusion experi-
ments were performed using a Bruker Avance 600 MHz NMR
spectrometer equipped with a broadband inverse probe with
x-, y-, and z-gradients. Chemical shifts were referenced to the
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Inorganic Chemistry
ARTICLE
dimethyl sulfoxide-d5 resonance (2.504) ppm. Diffusion data
sets were acquired using the stimulated echo experiment with
bipolar gradients (stebpgp1s) included with the Topspin release
version 1.3. Gradient amplitudes were incremented as a square
dependence from 5% to 95% into 22 gradient increments. A spoil
gradient pulse of length 1 ms and amplitude of ꢀ17.13% was
used to effectively remove transverse magnetization following
the encode period of the pulse program. Spectra were acquired
with 128 transients coadded and 28,672 data points per transient
for each of the 22 increments. Diffusion (Δ) and gradient pulse
times (δ) were optimized using a one-dimensional version of the
stimulated echo pulse sequence, stebpgp1s1d to give values of 2.0
and 200 ms, respectively.
support by a 2010 GRMP graduate fellowship. The authors thank
Prof. Christopher Bardeen for stimulating discussions.
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’ ASSOCIATED CONTENT
S
Supporting Information. NMR Spectra, fitted diffusion
b
analysis, tabulated rate data, and X-ray Crystallographic analysis.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: richard.hooley@ucr.edu.
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’ ACKNOWLEDGMENT
We are grateful to the American Chemical Society Petroleum
Research Fund, UC Riverside, and the NSF-funded ACIF (NSF
CHE-9974924) for financial support. C.K.L. gratefully acknowl-
edges financial support from the National Science Foundation
Grant CHE 0848976. J.F.K.L. acknowledges support by a 2008-
2010 U.S. Pharmacopeia graduate fellowship. A.S.G. acknowledges
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