Double-Decker Coordination Cages

Article abstract of DOI:10.1002/ejic.201600259

Bis(pyridin-3-ylmethyl) pyridine-3,5-dicarboxylate (L) possessing one internal and two terminal pyridine moieties displayed differential coordination ability when combined with suitable Pd^IIcomponents. The compound L acted as a bidentate chelating ligand to form mononuclear complexes when combined with cis-[Pd(tmeda)(NO₃)₂] or Pd(NO₃)₂in calculated ratios. The combination of Pd(NO₃)₂with L in a ratio of 3:4, however, afforded the trinuclear “double-decker” cage [(NO₃)₂?Pd₃(L)₄](NO₃)₄, in which L acts as a nonchelating tridentate ligand and the counter anion (i.e., NO₃^–) acts as template. The encapsulated NO₃^–can be replaced by F^–, Cl^–, or Br^–but not by I^–. The F^–-encapsulated cage could not be isolated due to its reactivity, whereas the Cl^–or Br^–encapsulated cages could be isolated. Although anionic guests such as NO₃^–, Cl^–, or Br^–stabilized the cages, the presence of excess Cl^–or Br^–(not NO₃^–) facilitated decomplexation reactions releasing the ligand. The complexation of Pd(Y)₂(Y = BF₄^–, PF₆^–, CF₃SO₃^–, or ClO₄^–) with L afforded the corresponding mononuclear complexes under appropriate conditions. However, these counter anions could not act as templates for the construction of double-decker cages.

Full text of DOI:10.1002/ejic.201600259

DOI: 10.1002/ejic.201600259
Full Paper
Cage Compounds
Double-Decker Coordination Cages
Sreenivasulu Bandi,[a] Sagarika Samantray,[a] Rajan Deepan Chakravarthy,[a] Amlan K. Pal,[b]
Garry S. Hanan,*[b] and Dillip Kumar Chand*[a]
Abstract: Bis(pyridin-3-ylmethyl) pyridine-3,5-dicarboxylate (L) but not by I–. The F–-encapsulated cage could not be isolated
possessing one internal and two terminal pyridine moieties dis- due to its reactivity, whereas the Cl– or Br– encapsulated cages
played differential coordination ability when combined with could be isolated. Although anionic guests such as NO3 , Cl–, or
–
suitable PdII components. The compound L acted as a bidentate Br– stabilized the cages, the presence of excess Cl– or Br– (not
–
chelating ligand to form mononuclear complexes when com- NO3 ) facilitated decomplexation reactions releasing the ligand.
–
–
–
–
bined with cis-[Pd(tmeda)(NO3)2] or Pd(NO3)2 in calculated ra- The complexation of Pd(Y)2 (Y = BF4 , PF6 , CF3SO3 , or ClO4 )
tios. The combination of Pd(NO3)2 with L in a ratio of 3:4, how- with L afforded the corresponding mononuclear complexes un-
ever, afforded the trinuclear “double-decker” cage der appropriate conditions. However, these counter anions
[(NO3)2⊂Pd3(L)4](NO3)4, in which L acts as a nonchelating tri- could not act as templates for the construction of double-
–
dentate ligand and the counter anion (i.e., NO3 ) acts as tem- decker cages.
–
plate. The encapsulated NO3 can be replaced by F–, Cl–, or Br–
Introduction
The first designed and synthesized Pd2L4-type coordination
cage molecule was reported by McMorran and Steel[7] in 1998.
Subsequently, a large number of Pd2L4-type cages have been
prepared, studied, and reviewed.[8,9] The architecture of a Pd3L4-
type double-decker coordination cage was also originally con-
ceived by McMorran and Steel[10] in 2002, however, they could
not realize their design, probably due to limitations in ligand
design. In 2014 we successfully demonstrated the construction
of this long-awaited architecture by using a newly designed
ligand and named the self-assembled Pd3L4-type coordination
complex a “double-decker” cage (Figure 1).[6] In addition, Clever
and co-workers utilized a rigid tridentate ligand for the con-
The concept of self-assembly governs a variety of phenomena
in objects ranging from nanosized assemblies to the universe.[1]
We are interested in the synthesis and reactivity of PdII-based
self-assembled discrete coordination complexes.[2] The design
and synthesis of coordination cage molecules using a variety of
metal and ligand components is an alternative and convenient
approach to the preparation of large-sized supramolecular
hosts.[3,4] Understanding the emergent behaviors of certain self-
assembled coordination complexes[5] has been a very recent
trend in supramolecular chemistry.
In a preliminary communication we recently reported on struction of an elegant Pd3L4-type double-decker in 2015.[11]
stoichiometrically controlled and revocable self-assembled
“spiro” and quadruple-stranded “double-decker”-type coordina-
tion cages.[6] The molecular architectures of the “spiro”- and
“double-decker”-type assemblies could be classified, on the ba-
sis of their compositions, into Pd1L2 and Pd3L4 structures, re-
spectively. In this paper we disclose a detailed investigation of
the said double-decker cages and a few related compounds.
The nature and stoichiometry of the PdII components and the
role of the counter anion have been found to be crucial for the
formation of the cages.
Figure 1. Cartoon representation of the guest-induced “double-decker” coor-
[a] Department of Chemistry, Indian Institute of Technology Madras,
dination cage [(G)2⊂Pd3L4]4+. In this case G represents NO3
.
–
Chennai 600036, India
E-mail: dillip@iitm.ac.in
Complexation reactions of the ligand L were carried out with
the cis-protected PdII complex, that is, cis-[Pd(tmeda)(NO3)2] and
a simple PdII salt, that is, Pd(NO3)2. The complexation behaviors
displayed by both types of metal components were found to
be comparable as well as contrasting depending upon the
stoichiometry of the metal and ligand. Stoichiometrically con-
http://chem.iitm.ac.in/faculty/dillipkumarchand/
[b] Department of Chemistry, University of Montreal,
Montreal, Canada
E-mail: garry.hanan@umontreal.ca
http://en.chimie.umontreal.ca/department-directory/vue/hanan-garry/
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201600259.
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trolled complexation behavior in supramolecular coordination semi-flexible) ligand are expected to be randomly oriented, but
chemistry has not been extensively explored,[6,12] which we they could also be unidirectional and parallel to each other in
have sought to address in this work. The coordination chemistry one of the conformations of L. This would happen only if the
and anion-binding properties of a double-decker-type cage coordination vectors are reoriented, particularly during the
with the composition Pd3L4 is the major focus of this paper. process of complexation with suitable metal ions. Such a proc-
The anion-binding properties and nature of the Pd3L4 cage mol- ess could be driven by the thermodynamic stability of the ensu-
ecules have been found to be synergic in nature.
ing self-assembled complex. Thus, a given strand of the ligand
should coordinate with three different metal centers through
the three pyridine nitrogen centers. Both of the terminal coordi-
nation vectors are also capable of converging towards a single
metal center, due to the semi-flexible nature of the spacer, to
create a large chelate ring (Scheme 1). It is more appropriate to
term such a ring as a metallo-macrocycle. The ligand showed
differential coordination ability[12,13] under appropriate condi-
tions and acted either as a bi- or tridentate ligand in a stoichio-
metrically controlled manner upon complexation with a calcu-
lated amount of Pd(NO3)2 (see Scheme 2).
Results and Discussion
Design and Synthesis of Ligand L
The tridentate nonchelating ligand bis(pyridin-3-ylmethyl) pyr-
idine-3,5-dicarboxylate (L, Scheme 1) was prepared by the con-
densation of freshly prepared pyridine-3,5-dicarbonyl chloride
hydrochloride with 3-hydroxymethylpyridine in dry dichloro-
methane in the presence of triethylamine. The reaction mixture
was stirred at room temperature for 24 h followed by aqueous
workup and column chromatographic purification to afford li-
gand L as a white solid that was well characterized (see Fig-
ures S1–S5 in the Supporting Information).
The design and possible coordination modes of the ligand L
are deliberated here. Ligand L has three pyridine moieties as
donor sites crafted in the ligand strand. Two of these pyridines
are terminal and symmetrically disposed with respect to the
central/internal pyridine. The internal pyridine ring is substi-
tuted by two electron-withdrawing groups located at the 3- and
5-positions of the ring. The nature of the spacer units, that is,
-C(O)OCH2-, is such that the ligand is neither completely rigid
Complexation of cis-[Pd(tmeda)(NO3)2] with the Ligand L
A family of [Pdx(tmeda)xLy]-type self-assembled compounds
have been prepared by the complexation of cis-[Pd(tmeda)-
(NO3)2] with selected mono/polydentate ligands under suitable
reaction conditions.[4a] We have studied the complexation be-
havior of cis-[Pd(tmeda)(NO3)2] with the tridentate ligand L
(Scheme 1). In view of the possibility of differential coordination
of the ligand, the complexation reactions were performed at
two different ratios of metal component to ligand.
The reaction of cis-[Pd(tmeda)(NO3)2] with L in a ratio of 1:1
nor flexible. The three coordination vectors of the semi-rigid (or is a suitable stoichiometry for the preparation of a mononuclear
Scheme 1. Complexation of cis-[Pd(tmeda)(NO3)2] with the ligand L in ratios of (a) 1:1 and (b) 3:2. The mononuclear complex [Pd(tmeda)(L)](NO3)2 (1a)
was formed under both conditions as the exclusive self-assembled product. (c,d) Trinuclear [Pd3(tmeda)3(L)2](NO3)6 (2a) was not formed even at elevated
temperatures.
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self-assembled complex[14] with a large chelate ring. In the Figures S9 in the Supporting Information). The reaction condi-
mononuclear complex, the terminal pyridine nitrogen lone tions were further varied by changing the temperature and sol-
pairs are expected to participate in metal–ligand bond forma- vent; however, our efforts to prepare the trinuclear complex
tion. The ligand could potentially display tridentate nonchelat- were not successful. The reaction did not proceed beyond the
ing behavior when the ratio of cis-[Pd(tmeda)(NO3)2] to L is 3:2, mononuclear architecture when stirred at elevated temperature
to yield the target “double-saddle”-type[3] trinuclear assembly such as 90 °C in [D6]DMSO for a period as long as 3 days
–
of composition Pd3(tmeda)3L2.
(Scheme 1 c, d). The reaction of cis-[Pd(tmeda)(Y)2] (Y = NO3 ,
–
–
–
–
BF4 , PF6 , CF3SO3 , or ClO4 ) with L produced comparable re-
sults (see Scheme S1a).
The reaction of cis-[Pd(tmeda)(NO3)2] and L in a ratio of 1:1
resulted in the mononuclear complex [Pd(tmeda)(L)](NO3)2 (1a;
Scheme 1, a). The isolated complex 1a was characterized by
NMR spectroscopy (Figure 2 and Figures S6 and S7 in the Sup-
porting Information), MS (Figure S8), and single-crystal XRD
The reaction to yield the trinuclear complex 2a was studied
in the gaseous state by using the Gaussian 09 software pack-
age.[15] The geometries of the reactant and product molecules
were optimized and their frequencies were calculated at the
B3LYP/6-31G* level of theory. The overall Gibbs free energy (ΔG)
and enthalpy (ΔH) for the formation of the trinuclear complex
[Pd3(tmeda)3(L)2]6+ by the reaction of 1 equiv. of [Pd(tmeda)]2+
and 2 equiv. of [Pd(tmeda)(L)]2+ were found to be infeasible
(+240.339 kcal mol–1) and endothermic (+210.693 kcal mol–1),
respectively (see Figure S61 and Table S3).
1
(Figure 3). The H NMR spectrum of 1a recorded in [D6]DMSO
shows a single set of peaks supporting the exclusive formation
of a discrete complex. The signals of the terminal pyridine α-H
atoms are significantly shifted downfield (Δδ = 0.87 and
0.73 ppm for Ha and Hb, respectively) compared with the free
ligand L, thereby confirming the formation of a metal–ligand
bond (Figure 2). Furthermore, the insignificant change (Δδ =
0.04 ppm) in the position of the signal due to Hf indicates the
nonparticipation of the internal pyridine in the complex. This
observation supports the bidentate nature of the ligand in its
bonded form in complex 1a. Incidentally, the methylene pro-
tons (i.e., He) appear as a pair of doublets and hence the two
protons of a given methylene group are diastereotopic in the
bound form of the ligand, which indicates some restriction in
the rotation of the pyridine rings.
Crystal Structure of [Pd(tmeda)(L)](NO3)2 (1a)
Single crystals of 1a suitable for XRD data collection were
grown by the slow diffusion of ethyl acetate into a DMSO solu-
tion of 1a. The crystal structure (Figure 3, a and Table S4 in the
Supporting Information) shows one ligand L coordinated to
two sites of the cis-protected PdII ion forming a molecular loop-
like arrangement. The calculated Pd–N bond lengths in the
complex are in the range 2.03(2)–2.07(3) Å and the cis-N–Pd–N
bond angles span the range 86.44(13)–92.71(11)°.
Figure 2. 1H NMR (400 MHz) spectra of (i) ligand
L
and
(ii) [Pd(tmeda)(L)](NO3)2 (1a) in [D6]DMSO.
The composition of 1a was supported by MS (see Figure S8);
the mass spectrum shows a prominent isotopic peak pattern at
m/z = 285.52 for the dication [1a – 2NO3]2+, which corresponds
–
to the loss of two NO3 ions from the title mononuclear com-
pound. The isotopic pattern confirms the presence of one palla-
dium in the dication. The experimental pattern is similar to that
calculated.
Figure 3. Crystal structure of [Pd(tmeda)(L)](NO3)2 (1a). The counter anions
and solvent molecules have been omitted for clarity.
However, for a 3:2 ratio of cis-[Pd(tmeda)(NO3)2] to L
(Scheme 1, b), the probable trinuclear complex [Pd3(tmeda)3-
(L)2](NO3)6 (2a) was not formed. Instead, the mononuclear com-
plex 1a was isolated from the reaction mixture. The 1H NMR
spectrum of the isolated product is comparable to that of com-
plex 1a prepared with a 1:1 ratio of the reactants. The reaction
with a 3:2 ratio was therefore followed in situ by performing
the reaction in [D6]DMSO; the signals in the 1H NMR spectra
The packing arrangement of the cation of 1a is shown in
Figure S63. It displays a one-dimensional molecular-pillar-like
arrangement formed through weak supramolecular interac-
tions.[3,16]
Complexation of Pd(NO3)2 with the Ligand L
confirmed the formation of 1a as the exclusive product and the A family of PdmLn-type self-assembled complexes have been
presence of unreacted cis-[Pd(tmeda)(NO3)2] (Scheme 1, b and prepared by the complexation of PdII with selected mono/poly-
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–
dentate ligands under suitable reaction conditions.[4a] In this NO3 counter anions, as clarified in later sections. In particular,
–
work, the stoichiometry of metal to ligand was varied to study the choice of counter anion (NO3 or other anions) and the
the differential coordination ability of the ligand L. Thus, the method of preparation of the PdII salt immensely influenced
reaction of Pd(NO3)2 with the ligand L was performed in ratios the coordination chemistry of the system under investigation.
1
of 1:2 and 3:4 (Scheme 2). It is crucial to mention here that the
sample of Pd(NO3)2 used for the complexation reactions was
acquired from a commercial source (relevant to the discussion
in later sections).
The H NMR spectrum of the isolated self-assembled mono-
nuclear complex [Pd(L)2](NO3)2 (3a) was recorded in [D6]DMSO
and shows a single set of peaks supporting the exclusive forma-
tion of a discrete complex. The signals of terminal pyridine α-H
atoms are significantly shifted downfield (Δδ = 0.94 and
0.81 ppm for Ha and Hb, respectively) as compared with the
free ligand L, which confirms the formation of the metal–ligand
bond (Figure 4 and Figure S10 in the Supporting Information).
Furthermore, an insignificant change in the position of the sig-
nal due to Hf is observed (Δδ = 0.05 ppm), which indicates the
nonparticipation of the internal pyridine in the complexation
reaction. This observation supports the bidentate nature of the
ligand in its bound form in complex 3a. The signal of the meth-
ylene protons (i.e., He) in 3a appears as a singlet in contrast to
the doublet of doublets observed for 1a. This indicates faster
rotation of the pyridine rings and Pd–L exchange in 3a. The
1H NMR spectrum of the isolated complex 4a (Figure 4 and
Figure S13), which is best represented as [(NO3)2⊂Pd3-
(L)4](NO3)4, shows evidence for a tridentate binding mode of
the bound ligand. Although signals of Ha and Hb are shifted
downfield (Δδ = 1.16, and 0.65 ppm for Ha and Hb, respectively)
compared with in the free ligand, the signal of Hf is also consid-
erably shifted downfield (Δδ = 1.30 ppm), which confirms the
participation of the internal pyridine in the complexation reac-
tion.
Scheme 2. Complexation of Pd(NO3)2 with the ligand L at specific metal to
ligand ratios and temperatures showing the exclusive formation of the
“spiro”-type mononuclear complex [Pd(L)2](NO3)2 (3a) and “double-decker”-
type trinuclear complex [(NO3)2⊂Pd3(L)4](NO3)4 (4a). Complexes 3a and 4a
are interconvertible under appropriate conditions. Note: In step (f), 2 equiv.
of L are added to 1 equiv. of 4a to afford 3 equiv. of 3a.
All the reactions were carried out in either DMSO or
[D6]DMSO for isolation and monitoring purposes, respectively.
The precipitation method was used to isolate the desired prod-
ucts, through the addition of excess EtOAc. Although the
“spiro”-type[14] mononuclear product [Pd(L)2](NO3)2 (3a) was
isolated from the reaction carried out with a 1:2 ratio of metal
to ligand (Scheme 2, a), the “double-decker”-type trinuclear
product [Pd3(L)4](NO3)6 (4a) was formed with a 3:4 ratio of the
reagents (Scheme 2, b). Thus, in this latter case the internal
pyridine participates in the reaction of L with Pd(NO3)2 to give
the very symmetrical double-decker cage-like architecture 4a
(Scheme 2). A detailed investigation disclosed in the present
work satisfactorily accounts for the synergic role of NO3– as well
Figure 4. 1H NMR (400 MHz) spectra in [D6]DMSO of (i) ligand L, (ii) “spiro”-
type [Pd(L)2](NO3)2 (3a), and (iii) “double-decker”-type [(NO3)2⊂Pd3(L)4](NO3)4
(4a).
The “spiro”-type mononuclear complex [Pd(L)2](NO3)2 (3a)
as the ensuing three-dimensional cavity that facilitates the for- was spontaneously assembled at room temperature within
mation of “double-decker” 4a. This result is in sharp contrast 10 min when commercially available Pd(NO3)2 was combined
to the complexation behavior of cis-[Pd(tmeda)(NO3)2] with the with L in a ratio of either 1:2 (Scheme 2, a, Figure 4, and Fig-
ligand L, when the geometrically probable complex 2a could ure S10) or 3:4 (Scheme 2, c and Figure S16) in [D6]DMSO. In
not be formed (Scheme 1, b–d). The internal pyridine nitrogen the former ratio (1:2), only the mononuclear complex 3a was
of L is probably situated in a disadvantageous position for coor- formed (Scheme 2, a). However, in the latter case (3:4), pre-
dination with
[Pd(tmeda)(NO3)2] with L proved unsuitable for the synthesis of with 3a (Scheme 2, c).
2a (Scheme 1) and the internal pyridine is not utilized.
a metal center. The complexation of cis- sumably unreacted Pd(NO3)2 remained in the solution along
Subsequent addition of the required amount of Pd(NO3)2 to
We have acquired enough evidence to propose the chemical the former solution of 3a followed by heating at 90 °C
formula of the trinuclear complex 4a as the inclusion complex (Scheme 2, e and Figure S17) facilitated the formation of the
[(NO3)2⊂Pd3(L)4](NO3)4 instead of [Pd3(L)4](NO3)6. This formula- thermodynamically stable trinuclear “double-decker”-type
tion emphasizes the encapsulation/templating role of the two product 4a. In the latter case, the required amount of Pd(NO3)2
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was already present in solution, which favored the spontaneous ratio of 3:4, complex 4a was found, surprisingly, to be only a
reorganization of complex 3a upon heating at 90 °C to form minor product (see Scheme S22 and Figure S53).
complex 4a (Scheme 2, d and Figure S16). Therefore, compound
3a could be considered as an intermediate complex. Because
complex 3a could be detected in the preparation of 4a, it is
proposed that complex 3a is a kinetically controlled product in
the presence of excess Pd(NO3)2. However, the isolated complex
3a was found to be stable in the absence of additional
Pd(NO3)2; it could be stored both in the solid and solution states
without further reorganization. A solution of 3a in [D6]DMSO
was heated at 90 °C and no changes were observed (see Fig-
ures S19 and S44).
It was suspected, and later confirmed, that the presence of
small amounts of residual AgCl in solution was responsible for
the formation of the mixture of products. Thus, the solution of
Pd(NO3)2 was centrifuged to effectively settle the precipitated
AgCl and the solution was separated by using a syringe. This
process of centrifugation and separation of the Pd(NO3)2 solu-
tion was repeated until no AgCl sedimentation was visible to
the naked eye (four cycles). The purified Pd(NO3)2 was then
used for complexation reactions with the ligand L. To our satis-
faction, the targeted complex 3a was found to be an almost
exclusive product for a 1:2 ratio of metal to ligand (see
Scheme S21 and Figure S52), whereas the targeted complex
4a was found to be the major product (see Scheme S23 and
Figure S54) although not exclusive for the 3:4 ratio of metal to
ligand.
To determine whether AgI or Cl– or both the ions are respon-
sible for the formation of the mixture of products, the following
experiments were performed. The complexation of “commer-
cially available” Pd(NO3)2 with the ligand L was carried out in
the presence of deliberately added AgCl, AgNO3, TBACl, or
TBANO3 (TBA = tetra-n-butylammonium). Although the pres-
ence of AgNO3 or TBANO3 did not alter the nature of the com-
plexation, the presence of AgCl or TBACl produced a mixture
of products in line with the influence of residual AgCl on the
complexation reactions. Thus, it was concluded that AgI is an
innocent spectator whereas Cl– has a definite influence on the
course of the complexation reactions studied here.
As discussed above, the formation of complexes 3a and 4a
was found to be controlled by the stoichiometry of the metal
and ligand components. Both of these stoichiometrically con-
trolled complexes were also found to be revocable, through
dynamic processes, upon subjecting them to appropriate condi-
tions (Scheme 2, e, f). The reaction of 2 equiv. of 3a with
1 equiv. of Pd(NO3)2 could produce complex 4a quantitatively
(see Figure S17). Similarly, the reaction of 4a with 2 equiv. of
ligand L produced complex 3a (see Figure S18). The revocable
nature of the complexes was studied by DFT calculations using
the Gaussian 09 software package.[15] The results are presented
in Table S3 and a brief discussion is also provided there.
The compositions of 3a and 4a were further evidenced by
MS (ESI) and the structures of [Pd(L)2](NO3)2 (3a) and
[Pd(L)2](BF4)2 (3b) were confirmed by single-crystal X-ray dif-
fraction (see Figure 6). The MS (ESI) of compound 3a (see Fig-
ure S12) shows a prominent peak at m/z = 402.05 correspond-
ing to the dication [3a – 2NO3]2+ formed by the loss of two
–
NO3 ions. The isotopic pattern confirms the presence of one
unit of palladium and is in good agreement with the calculated
data. The MS (ESI) of compound 4a (see Figure S15) shows
peaks at m/z = 982.73, 634.35, and 460.23, which correspond
to the cations [4a – 2NO3]2+, [4a – 3NO3]3+, and [4a – 4NO3]4+
formed by the loss of two, three, and four NO3 ions, respec-
tively. The isotopic patterns of fragments containing three palla-
dium units are in good agreement with the calculated data.
Halide-Encapsulated Double-Decker Cages
–
The NO3 -encapsulated double-decker-type coordination cage
–
[(NO3)2⊂Pd3(L)4](NO3)4 (4a) was considered suitable for the
binding of appropriate anions in both of the cavities. In addi-
tion to the positive charges of the metal centers, the inner walls
of the cavities are delineated with several electron-deficient
pyridine α-H atoms, making the cavities suitable for binding
electron-rich guests. It was envisaged that spherical halide ions
could be encapsulated as guest molecules by combining 4a
with TBAX (X = F–, Cl–, Br–, or I–). In a typical experiment, a
solution of TBAX in [D6]DMSO was added portionwise to a solu-
Complexation of Pd(NO3)2 (Prepared from PdCl2 and
AgNO3) with the Ligand L
Stock solutions of Pd(NO3)2 were prepared separately in DMSO tion of 4a in [D6]DMSO to monitor the progress of the reaction.
and [D6]DMSO by treating PdCl2 with AgNO3 and then allowing The double-decker cage 4a could exchange the encapsulated
–
the solution to stand to allow the precipitated AgCl to settle. NO3 ions with two units of the incoming halide ions, one in
Complexation reactions of the as-prepared Pd(NO3)2 (in each of the cavities (Scheme 3, Figure 5, and Figures S20–S40).
[D6]DMSO) with the ligand L were carried out in ratios of 1:2 Although F–, Cl–, and Br– ions are accommodated, I– ions could
and 3:4 of metal to ligand, targeting the complexes not enter the cavities. After a thorough investigation by 1H NMR
[Pd(L)2](NO3)2 (3a) and [(NO3)2⊂Pd3(L)4](NO3)4 (4a), respectively. spectroscopy, it was understood that complexes of general for-
1
However, as confirmed by the H NMR spectra of the samples, mula [(X)(NO3)⊂Pd3(L)4](NO3)4 (X = F–, Cl–, or Br–, corresponding
a mixture of products, including the targeted compounds, was to 5a, 7a, and 9a) were initially formed along with other related
formed for each ratio. For the 1:2 ratio of metal to ligand, al- cages (see Figures S20, S25, and S31). Subsequently, complexes
though the expected complex 3a was found to be the major of general formula [(X)2⊂Pd3(L)4](NO3)4 (X = F–, Cl–, or Br–, corre-
1
product (see Scheme S20 and Figure S51 in the Supporting sponding to 6a, 8a, and 10a) were formed exclusively. The H
Information), it was not the exclusive product, whereas, for a NMR spectra of 6a, 8a, and 10a were recorded and compared
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with that of 4a (Figure 5). These complexes could also be pre-
Let us consider the case of Cl– encapsulation. The portion-
pared by alternative routes due to their dynamic behavior, as wise addition of TBACl to a solution of 4a resulted in the follow-
shown in Scheme 3, b, c (see also Figures S42 and S43). The ing situation. Initially, a mixture of [(NO3)2⊂Pd3(L)4](NO3)4 (4a),
binding of Cl– in the cavity of a binuclear Pd2L4 cage has previ- [(Cl)(NO3)⊂Pd3(L)4](NO3)4 (7a), and [(Cl)2⊂Pd3(L)4](NO3)4 (8a)
ously been reported by Puddephatt and co-workers.[17]
was observed that changes to a mixture of 7a and 8a and then
exclusively to 8a (see Figure S26). The addition of TBAF or
TBABr to a solution of 4a resulted in similar behavior, however,
the addition of TBAI showed no changes in 4a indicating no
entry of I– into the cavities (see Figures S21, S32, and S40).
The F– encapsulated complex 6a could neither be isolated in
the pure state from its solution nor was stable in solution. A
solution of 6a underwent decomplexation, releasing the ligand
within 1 day (see Figure S22). In contrast, complexes 8a and
10a could be isolated and were found to be stable enough.
However, in the presence of an additional amount of TBACl
(soluble Cl–), complex 8a was found to undergo decomplexa-
tion within 1 day, releasing the ligand L. Complex 10a behaved
similarly in the presence of an additional amount of TBABr (sol-
uble Br–). The strong interaction between PdII and F– ions is
probably responsible for the rapid decomposition of 6a.
The intermediates formed during the halide encapsulation5
reactions are formulated as [(X)(NO3)⊂Pd3(L)4](NO3)4 (X = F–, 5a;
Cl–, 7a; Br–, 9a). These intermediates are proposed on the basis
of their 1H NMR data. When the encapsulated guest moieties in
the two cavities of the double-decker are the same, the ligand
moieties should give one set of signals in the corresponding 1H
NMR spectrum. However, if the guests are different, the symme-
try of the bound ligand is lost and two sets of signals are ex-
pected. The complexes [(NO3)2⊂Pd3(L)4](NO3)4 (4a) and
[(Cl)2⊂Pd3(L)4](NO3)4 (8a) each show one set of signals, whereas
[(Cl)(NO3)⊂Pd3(L)4](NO3) (7a) exhibits two set of signals; the
chemical shifts of one of the two sets of signals of 7a were
found to be close to those of 4a, and the signals of the other
set were found to be close to those of 8a. The Hf atom clearly
shows four signals for a mixture of 4a, 7a, and 8a (see Fig-
ure S26). The chemical shifts of the signals of 5a–10a (see Fig-
ures S21, S26, and S32) were compared with those of 4a and
found to be in line with expectation. The separation of peaks
was noticed best for the encapsulation of Cl–, as shown in Fig-
ure S26.
Scheme 3. Preparation of [(X)2⊂Pd3(L)4](NO3)4 (6a, 8a, and 10a; X = F–, Cl–,
or Br–) by (a) encapsulation of two halide ions in the cage
[(NO3)2⊂Pd3(L)4](NO3)4 (4a) by anion exchange or (b) the complexation of
Pd(NO3)2 with [Pd(L)2](NO3)2 (3a) in a 1:2 ratio in the presence of TBAX, or
(c) the complexation of Pd(NO3)2 with L in a 3:4 ratio in the presence of
TBAX. (TBAX = tetra-n-butylammonium halide, nBu4NX.).
The formation of the complexes [(X)2⊂Pd3(L)4](NO3)4
(X = Cl–, 8a; Br–, 10a) was confirmed from their MS (ESI) data.
The single-crystal X-ray structures of [(Cl)2⊂Pd3(L)4](BF4)4 (8b)
and [(Cl)2⊂Pd3(L)4](PF6)4 (8c) (Figure 7) further supported the
claim. The MS (ESI) spectrum of compound 8a (see Figure S30)
shows peaks at m/z = 955.67, 616.13, and 446.58 corresponding
to the cations [8a – 2NO3]2+, [8a – 3NO3]3+, and [8a – 4NO3]4+
–
formed by the loss of two, three, and four NO3 ions, respec-
tively. The MS (ESI) spectrum of 8c (see Figure S39) shows peaks
at m/z = 1039.00, 644.34, and 446.76 corresponding to the cati-
ons [8c – 2PF6]2+, [8c – 3PF6]3+, and [8c – 4PF6]4+ formed by
the loss of two, three, and four hexafluorophosphate ions, re-
spectively. As expected on the basis of theoretical calculations,
the two encapsulated Cl– ions are retained in the formula of all
these fragments. Similarly, the MS spectrum of compound 10a
(see Figure S36) displays peaks at m/z = 645.99 and 468.98 cor-
responding to the cations [10a – 3NO3]3+ and [10a – 4NO3]4+
Figure 5. 1H NMR (400 MHz) spectra in [D6]DMSO for
(i)
[(NO3)2⊂Pd3(L)4](NO3)4
(4a),
(ii)
[(F)2⊂Pd3(L)4](NO3)4
(6a),
(iii) [(Cl)2⊂Pd3(L)4](NO3)4 (8a), and (iv) [(Br)2⊂Pd3(L)4](NO3)4 (10a).
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–
formed by the loss of three and four NO3 ions, respectively. which indicates that AgCl dissolves to an extent required for
Here also, the two encapsulated Br– ions are accounted for in encapsulation only. Thus, 4a could snatch Cl– from the AgI ion
the formula of the fragments. The calculated and observed iso- very easily to form the more stable complex 8a. The solubiliza-
topic distribution patterns of the peaks corresponding to the tion of AgCl in the cavities of an anion-driven interlocked cage
above-mentioned fragments are found to be comparable.
The exchange of the encapsulated NO3– with Cl– was studied
by DFT calculations using the Gaussian 09 software package[15]
and the results are presented in Table S3. The calculated free
energy and enthalpy for the formation of [(Cl)2⊂Pd3(L)4]4+ and
has been reported by Clever and co-workers.[18]
Nitrate Versus Chloride: Which is a Better Template for the
Double-Decker?
–
2 equiv. of NO3 from [(NO3)2⊂Pd3(L)4]4+ and 2 equiv. of Cl–
were found to be –66.198 and –40.922 kcal mol–1, respectively,
which indicates the feasibility and exothermic nature of the an-
ion-exchange reaction. The calculated global entropy (ΔS) is
0.084 kcal mol–1 K–1, which indicates the spontaneous nature of
the reaction.
The complex [Pd(L)2](NO3)2 (3a) reacted with Pd(NO3)2 to form
[(NO3)2⊂Pd3(L)4](NO3)4 (4a; Scheme 2). The reaction of 3a with
a calculated amount of Pd(NO3)2 as well as TBACl resulted in
the formation of [(Cl)2⊂Pd3(L)4](NO3)4 (8a; Scheme 3). Complex
3a remained unchanged upon addition of only TBANO3 (see
Figure S45 in the Supporting Information), whereas it under-
went reorganization upon addition of only TBACl, that is, even
in the absence of additional Pd(NO3)2 (see Scheme S15 and
Figure S46). The products formed from the reaction of 3 equiv.
Revisiting the Complexation of Pd(NO3)2 (Prepared from
PdCl2 and AgNO3) with the Ligand L
So far as the nature of Pd(NO3)2 is concerned, the commercially
available sample of Pd(NO3)2 provided stoichiometrically con-
trolled revocable single discrete compounds 3a and 4a, but the
presence of residual AgCl in the sample of Pd(NO3)2 prepared
in the laboratory resulted in a mixture of products. The nature
of the complexes present in the mixture could be deciphered
successfully by analyzing the 1H NMR spectra of the halide-
encapsulated double-decker cages at various stages of the en-
capsulation process. This was possible by using the results de-
scribed above for halide-encapsulated double-decker cages.
The compositions of the mixtures are summarized in Table S1
in the Supporting Information.
of 3a with TBACl were found to be
1
equiv. of
[(Cl)2⊂Pd3(L)4](NO3)4 (8a) and 2 equiv. of the free ligand L. Ironi-
cally, the ligand is released by the force of the templating Cl–.
Complex 4a can be converted into 8a upon addition of TBACl.
This indicates that Cl– is more suited as a template for the con-
–
struction of the double-decker cage than NO3 . The presence
–
of additional soluble NO3 was found to have no effect on the
–
NO3 -templated double-decker cage 4a or the Cl–-templated
double-decker cage 8a in solution. However, soluble Cl– was
found to be harmful to 4a and 8a. Thus, NO3 and Cl– are suit-
–
able for the creation of double-decker cages, with Cl– found to
–
be more powerful than NO3 . The stabilization of these cages,
We next considered a sample of Pd(NO3)2 prepared by the
reaction of PdCl2 and AgNO3 in [D6]DMSO with the precipitated
AgCl separated by allowing the solution to stand followed by
separation of the solution by using a syringe. The solution of
Pd(NO3)2 thus prepared is contaminated with AgCl. For a 1:2
ratio of metal to ligand, the otherwise expected complex 3a
was found to be the major product along with good amount
of 8a and uncomplexed ligand L (see Scheme S20 and Fig-
ure S51). This is due to the presence of Cl– originating from
AgCl. This proposal is supported by the fact that the isolated
pure complex 3a reacts with TBACl to form 8a and free ligand
L (see path c of Scheme S15 and Figure S46). For a 3:4 ratio
of metal to ligand, the otherwise expected complex 4a was
surprisingly found to be only a minor product, formed along
with major amounts of 7a and 8a (see Scheme S22 and Fig-
ure S53). This has also been attributed to the presence of Cl–
originating from AgCl. This is supported by the fact that the
isolated pure complex 4a reacts with TBACl to form a mixture
of 4a, 7a, and 8a when a small proportion of TBACl is added
and that upon increasing the amount of TBACl only 8a results
(see Scheme S8 and Figure S26). Complex 8a is, however, stable
enough if isolated; it undergoes decomplexation in the pres-
ence of excess soluble Cl– to give the free ligand.
however, requires a suitable environment.
Decomplexation of the Double-Decker Cages
The isolated cage 8a remained stable when redissolved in
[D6]DMSO, but it underwent decomplexation in the presence of
an additional amount of soluble Cl– (TBACl) to release the free
ligand (Scheme 4 and Figure S47 in the Supporting Informa-
tion). Thus, the decomplexation of 8a occurs upon the addition
of soluble Cl–. Like Cl–, soluble Br– in excess is also harmful
to the double-decker cages. The Hb protons of 8a are shifted
downfield upon addition of TBACl, whereas the other signals of
8a are unchanged. Thus, the bowl-like exohedral space is pro-
posed to accommodate the Cl– ion initially and the ligand is
released subsequently over a period of 12 h. In this process, the
replacement of ligands by halide ions and the existence of re-
lated equilibria cannot be ruled out. However, no evidence for
the intermediates formed during such exchanges could be
found. The addition of TBABr to 10a also exhibited similar be-
havior (see Scheme S17 and Figure S48). On addition of TBABr
to 8a and TBACl to 10a, the signals of Hb are shifted downfield
The deliberate addition of AgCl to 4a also resulted in 8a. with eventual decomplexation of the cage molecules (see Fig-
Interestingly, the architecture of 8a is retained even in the pres- ures S49 and S50). It is not very clear whether Cl– or Br– is the
ence of 50 equiv. of AgCl (see Scheme S12 and Figure S41), more powerful in promoting the decomplexation of the cages.
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Scheme 4. Addition of 2 equiv. of TBACl to isolated [(Cl)2⊂Pd3(L)4](NO3)4 (8a).
–
Complexation of Pd(Y)2 (Prepared from PdI2 and AgY)
with the Ligand L
[(NO3)2⊂Pd3(L)4](NO3)4 (4a). The binding of a unit of NO3 in
the cavity of a Pd2L4-type cage is a known phenomenon.[8a]
So far as the halide-encapsulated double-decker cages
–
[(X)2⊂Pd3(L)4](NO3)4 are concerned, NO3 , F–, Cl–, and Br–proved
suitable as guests ions, whereas I– is unsuitable. So far as the
nature of Pd(NO3)2 is concerned, the “commercially available”
salt provided stoichiometrically controlled, revocable, single dis-
crete compounds 3a and 4a, whereas the presence of residual
Cl– in the solution prepared in the laboratory resulted in a mix-
ture of products. We therefore prepared Pd(Y)2 from PdI2 and
–
–
–
–
–
AgY (Y = NO3 , BF4 , PF6 , CF3SO3 , or ClO4 ). It was confidently
anticipated that the presence of residual AgI, if any, should not
hamper the complexation reactions. However, the role of coun-
ter anion, Y, as a guest/template or otherwise should be re-
flected in this study.
The role of NO3– as a template for the synthesis of the Pd3L4
architecture was confirmed above. A sample of Pd(NO3)2 pre-
pared by reacting PdI2 with AgNO3 behaved very much like the
commercially acquired Pd(NO3)2 (see Figures S55 and S56). The
–
–
–
–
reaction of Pd(Y)2 (Y = BF4 , PF6 , CF3SO3 , or ClO4 ) with the
ligand L in a 1:2 ratio resulted in [Pd(L)2](Y)2 (3b–3e, respec-
tively; see Scheme S28a and Figure S59). The reaction of these
Pd(Y)2 salts with the ligand in a 3:4 ratio also resulted in the
mononuclear complexes 3b–3e along with unreacted PdII salts
(Scheme 5 and Figure S59). The mononuclear complexes did
not reorganize to form the corresponding trinuclear complexes,
–
Scheme 5. Reaction of Pd(Y)2 (prepared from PdI2 and AgY, Y = BF4–, PF6
,
CF3SO3–, or ClO4–) and ligand L in a 3:4 ratio to give the mononuclear com-
plexes [Pd(L)2](Y)2 (3b–3e). Upon addition of TBACl or TBANO3 the trinuclear
complexes [(Cl)2⊂Pd3(L)4](Y)4 (8b–8e) or [(NO3)2⊂Pd3(L)4](Y)4 (4b–4e) are
formed, respectively.
–
–
even at higher temperatures, which indicates that the BF4 , PF6
–
–
, CF3SO3 , and ClO4 anions are not suitable templates for the
construction of double-decker cages. However, the addition of
TBANO3 or TBACl to 3b–3e led to smooth reorganization to
Samples of Pd(Y)2 prepared from PdCl2 (instead of PdI2) and
the corresponding NO3 - and Cl–-encapsulated double-decker AgY will be contaminated with the Cl– ion. The reaction of such
cages [(NO3)2⊂Pd3(L)4](Y)4 (4b–4e) and [(Cl)2⊂Pd3(L)4](Y)4 (8b– samples of Pd(Y)2 with the ligand L is bound to furnish a mix-
8e), respectively (see Figure S60). It is easy to conclude that the ture of products due to the templating nature of the residual
required amount of PdII ion is not a sufficient criterion for the Cl–. The reaction of PdCl2 and AgPF6 resulted in Pd(PF6)2, which
synthesis of the double-decker cages. A suitable anion that is also contaminated with AgCl. The reaction of Pd(PF6)2 thus
could act as template is essential for the construction. Also, the prepared with the ligand L resulted in a mixture of products
–
–
–
double-decker formed in the presence of NO3 must encapsu- (see Figures S57 and S58), with PF6 an innocent spectator and
late two units of the anion, as the cavities are separated, and only the residual Cl– influencing the complexation by favoring
hence the formula of 4a would be best represented by the formation of the double-decker structure.
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Crystal Structures of [Pd(L)2](NO3)2·2CHCl3 (3a·2CHCl3) and
[Pd(L)2](BF4)2, (3b)
ture of 8c demonstrates a double-decker arrangement elabo-
rated with two cavities, each of which accommodates one Cl–
–
ion; the PF6 ions are located outside of the cavities. The Pd–N
Single crystals of 3a suitable for X-ray diffraction were grown
by layering a solution of commercially acquired Pd(NO3)2 in
acetonitrile over a solution of ligand L in chloroform. The mo-
lecular formula of the crystal structure [Pd(L)2](NO3)2·2CHCl3
(3a·2CHCl3) includes two CHCl3 molecules as the solvent of
crystallization. The coordination environment of the square-pla-
nar PdII center is occupied by two units of the ligand L. The
ligand adopts a “U-shape” and metallo-macrocycle loops are
formed to create the “spiro-type” complex Pd1L2, as shown in
Figure 6 (a). The two strands of each ligand are coordinated in
the cis positions of the PdII center with one ligand loop oriented
above and the other below the coordination square plane. The
Pd–N bond lengths in the complex are in the range 2.027(18)–
2.038(14) Å and the cis-N–Pd–N bond angles span the range
87.33(10)–92.67(7)°.
bond lengths involving the terminal and internal pyridine nitro-
gen atoms span the ranges 2.013(5)–2.030(4) and 2.066(3)–
2.067(4) Å, respectively. The cis-N–Pd–N bond angles of the two
outer PdII atoms and the single inner PdII atom span the ranges
88.83(20)–91.06(19) and 88.36(15)–91.64(14)°, respectively. The
Pd···Pd nonbonding distances across a given cavity and be-
tween two terminal PdII centers are 6.970(5) and 13.940(10) Å,
respectively. The encapsulated Cl– ions are in close contact with
the inwardly pointing H atoms of the ligand strands (i.e., Ha and
Hf, as denoted in Scheme 1 and Scheme 2) of the double-
decker cage. The calculated H(py)···Cl distances span the range
2.731(10)–2.807(9) Å,[17] and are indicative of hydrogen-bond-
ing interactions. The elaborated packing diagrams of 8b and 8c
in three-dimensions are shown Figures S66 and S67.
Figure 6. Crystal structures of (a) [Pd(L)2](NO3)2·2CHCl3 (3a·2CHCl3) and
(b) [Pd(L)2](BF4)2 (3b). The counter anions and solvent have been omitted for
clarity.
Figure 7. Crystal structures of (a) [(Cl)2⊂Pd3(L)4](BF4)4·2DMF (8b·2DMF) and
(b) [(Cl)2⊂Pd3(L)4](PF6)4 (8c) depicting the encapsulation of chloride ions in
the cavities. The counter anions outside of the cavity have been omitted for
clarity.
A variety of solvents were diffused into samples of 3b in
DMSO in separate experiments. The diffusion of tert-butyl alco-
hol into the solution of 3b afforded single crystals suitable for
X-ray diffraction. The crystal structure confirmed the molecular
structure of 3b as shown in Figure 6 (b). The Pd–N bond lengths
in the complex are in the range 2.018(3)–2.027(3) Å and the cis-
N–Pd–N bond angles span the range 88.40(13)–91.60(12)°.
Although the basic frameworks of the complexed cations in
the crystal structures of 3a·2CHCl3 and 3b are comparable,
close comparison of their structures showed a small difference
in the orientation of the internal pyridine rings and an appreci-
able difference in their packing, as shown in Figures S64 and
S65.
Conclusions
We have studied the differential coordinating abilities of a tri-
dentate ligand by using cis-protected or simple PdII moieties as
the metal components. A subtle change in the counter anion
of the metal components hugely influences the self-assembly
phenomena. It is now necessary to prepare more examples of
“double-decker” cages and study their nature to recognize new
and emergent behavior in supramolecular coordination chemis-
try with particular reference to discrete, self-assembled coordi-
nation cage molecules.
Crystal Structures of [(Cl)2⊂Pd3(L)4](BF4)4·2DMF (8b·2DMF)
and [(Cl)2⊂Pd3(L)4](PF6)4 (8c)
The isolated complex 8b was redissolved in DMF and EtOAc was
diffused slowly through this solution. Single crystals suitable for
X-ray diffraction were obtained over a period of 2 weeks. Single
crystals of 8c suitable for X-ray diffraction were obtained by the
slow diffusion of toluene into a solution of 8c in DMF.
Experimental Section
Ligand precursors (i.e., pyridine-3,5-dicarboxylic acid and 3-hydroxy-
methylpyridine), palladium salts [i.e., PdCl2, PdI2, and Pd(NO3)2],
tetra-n-butylammonium salts (i.e., TBAF, TBACl, TBABr TBAI, and
TBANO3), and silver salts (i.e., AgNO3, AgBF4, AgPF6, AgCF3SO3, and
The crystal structures of 8b and 8c are comparable (Figure 7)
and therefore we will describe only the latter here. The struc- AgClO4) were obtained from Sigma–Aldrich. Common reagents,
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such as tmeda, NH4PF6, and solvents were obtained from Spectro
Chem, India. The reagents were used as received unless specified
H, -CH3) ppm. 13C NMR (100 MHz, [D6]DMSO, 300 K): δ = 157.97,
151.85, 150.94, 140.24, 137.13, 131.04, 127.50, 112.92, 104.33, 65.56,
otherwise. cis-[Pd(tmeda)Cl2] and cis-[Pd(tmeda)(NO3)2] were pre- 62.65, 50.96, 50.73 ppm. MS (ESI): m/z = 284, [1a-2NO3]2+
.
pared according to literature procedures.[19] The cis-protected PdII
[Pd(tmeda)(L)](BF4)2 (1b): The ligand L (0.0175 g, 0.05 mmol) was
added to a solution of [Pd(tmeda)](BF4)2 (0.0198 g, 0.05 mmol) in
acetonitrile (5 mL). The reaction mixture was stirred at room tem-
perature for 1 h to obtain a clear yellow solution. The resulting
solution was poured into a watch-glass and subsequently evapo-
rated at room temperature over a period of 6 h to obtain a yellow
solid. The solid was washed with acetone (2 × 2 mL) and dried un-
der vacuum to afford complex 1b (0.0290 g, isolated yield 78 %).
–
components, that is, cis-[Pd(tmeda)(Y)2] (Y
=
NO3
,
BF4–, PF6–, CF3SO3–, and ClO4–) were prepared by the reaction of
cis-[Pd(tmeda)Cl2] with AgY (see Supporting Information). The deu-
teriated solvents CDCl3 and [D6]DMSO were obtained from Sigma–
Aldrich. NMR spectra were recorded in CDCl3 and [D6]DMSO at
room temperature with Bruker AV400 and AV500 spectrometers
(400 and 500 MHz for 1H NMR; 100 and 125 MHz for 13C NMR).
Chemical shifts are reported in ppm relative to residual solvent pro-
1
The H NMR spectrum of compound 1b is comparable with that of
1
tons (δ = 7.26 ppm for CDCl3 in H NMR and δ = 77.16 ppm in 13C
compound 1a.
NMR; δ = 2.50 ppm for [D6]DMSO in 1H NMR and δ = 39.43 ppm in
13C NMR). MS (ESI) spectra were recorded with Agilent Q-TOF and
Micromass Q-TOF spectrometers. Single-crystal X-ray diffraction
analysis was carried out by using a Bruker APEX II-CCD XRD diffrac-
tometer. CHN analysis was performed with a Perkin–Elmer 2400
series CHNS/O Analyzer. Melting points were determined using a
CINTEX apparatus.
[Pd(tmeda)(L)](Y)2 (1c–1e): Complexes [Pd(tmeda)(L)](Y)2 (1c–1e;
Y = PF6–, CF3SO3–, and ClO4–, respectively) were prepared in a simi-
lar manner as described for 1b using the appropriate cis-protected
PdII component [Pd(tmeda)](Y)2 (see the Supporting Information).
[Pd(L)2](NO3)2 (3a): The ligand L (0.0209 g, 0.06 mmol) was added
to a solution of Pd(NO3)2 (0.0069 g, 0.03 mmol) in DMSO (3 mL).
The reaction mixture was stirred at room temperature for 10 min.
Subsequently, the addition of ethyl acetate (10 mL) to the resulting
solution led to a white solid precipitate, which was separated by
centrifugation. The solid was washed with acetonitrile (2 × 2 mL)
and dried under vacuum to afford complex 3a (0.0222 g, isolated
yield 80 %), m.p. 232 °C (decomp.). 1H NMR (500 MHz, [D6]DMSO,
300 K): δ = 9.66 (s, 4 H, Ha), 9.39 (d, J = 1.0 Hz, 4 H, Hb), 9.38 (s, 4
H, Hf), 8.86 (s, 2 H, Hg), 8.09 (d, J = 8.0 Hz, 4 H, Hd), 7.73 (dd, J =
5.0, 5.0 Hz, 4 H, Hc), 5.43 (s, 8 H, He) ppm. 13C NMR (125 MHz,
[D6]DMSO, 300 K): δ = 162.96, 154.27, 151.20, 150.91, 140.84, 139.77,
136.16, 127.46, 125.45, 64.77 ppm. MS (ESI): m/z = 402.15 [3a –
Synthesis of the Ligand (L): A 100 mL round-bottomed flask was
charged with pyridine-3,5-dicarboxylic acid (0.618 g, 3.7 mmol) and
SOCl2 (20 mL) and the mixture was heated at reflux under nitrogen
for 48 h. The excess SOCl2 was removed by distillation under re-
duced pressure to give a pale-yellow solid. This solid was sus-
pended in dry dichloromethane (20 mL) and 3-hydroxymethylpyr-
idine (0.807 g, 7.4 mmol) was added followed by the dropwise addi-
tion of triethylamine (1 mL). The suspension was then heated at
reflux for 4 h, after which the suspension was cooled to room tem-
perature. A saturated aqueous solution of sodium hydrogen carb-
onate (20 mL) was then added. The organic layer was separated
and then evaporated to dryness under reduced pressure. The crude
product was purified by silica gel column chromatography prepared
by using hexane. The column was eluted with EtOAc/hexane (1:1)
to remove less polar impurities. Subsequently, elution with DCM/
acetone (3:7) afforded the product as a white solid (0.954 g, isolated
yield 74 %) after evaporation of the solvent and drying under vac-
uum, m.p. 121 °C. 1H NMR (500 MHz, CDCl3): δ = 9.38 (s, 2 H, Hf),
8.86 (s, 1 H, Hg), 8.79 (s, 2 H, Ha), 8.63 (d, J = 2.0 Hz, 2 H, Hb), 7.81
(d, J = 2.0 Hz, 2 H, Hd), 7.36–7.34 (dd, J = 0.5, 0.5 Hz, 2 H, Hc), 5.43
(s, 4 H, He) ppm. 13C NMR (125 MHz, CDCl3, 300 K): δ = 164.20,
154.65, 150.15, 149.98, 138.35, 136.58, 131.00, 125.86, 123.82,
65.12 ppm. 1H NMR (400 MHz, [D6]DMSO): δ = 9.33 (s, 2 H, Hf), 8.72
(s, 2 H, Ha), 8.68 (s, 1 H, Hg), 8.58 (d, J = 3.2 Hz, 2 H, Hb), 7.94 (d, J =
7.6 Hz, 2 H, Hd), 7.46–7.43 (dd, J = 4.8, 4.8 Hz, 2 H, Hc), 5.46 (s, 4 H,
He) ppm. 13C NMR(100 MHz, [D6]DMSO, 300 K): δ = 163.75, 153.79,
149.50, 149.41, 137.28, 136.16, 131.27, 125.59, 123.66, 64.70 ppm.
MS (ESI): m/z = 350.11 [M + H]+. C19H15N3O4 (349.3): calcd. C 65.32,
H 4.33, N 12.03; found C 64.37, H 4.38, N 11.77.
2NO3]2+
.
[Pd(L)2](BF4)2 (3b): A sample of Pd(BF4)2 was prepared in DMSO
(3 mL) by stirring a mixture of PdI2 (0.0108 g, 0.03 mmol) and AgBF4
(0.0116 g, 0.06 mmol) at 90 °C for 30 min. The precipitated AgI was
separated by centrifugation and the supernatant was transferred by
using a syringe. The ligand L (0.0209 g, 0.06 mmol) was added to
the solution of Pd(BF4)2 (0.0084 g, 0.03 mmol) prepared in DMSO
(3 mL), and the reaction mixture was stirred at room temperature
for 10 min. Subsequently, the addition of ethyl acetate (10 mL) to
the resulting solution led to a white solid precipitate, which was
separated by centrifugation. The solid was washed with acetonitrile
(2 × 10 mL) and dried under vacuum to afford complex 3b
(0.0241 g, isolated yield 82 %). The 1H NMR spectrum of compound
3b is similar to that of compound 3a.
–
[Pd(L)2](Y)2 (3c–3e): Complexes [Pd(L)2](Y)2 (3c–3e; Y = PF6
,
CF3SO3–, and ClO4–, respectively) were prepared in a similar manner
as described for 3b using the appropriate silver salt (see the Sup-
porting Information).
Synthesis of the Complexes
[Pd(tmeda)(L)](NO3)2 (1a): The ligand L (0.0175 g, 0.05 mmol) was
added to a solution of cis-[Pd(tmeda)(NO3)2] (0.0173 g, 0.05 mmol)
in acetonitrile (5 mL). The reaction mixture was stirred at room tem-
perature for 1 h to obtain a clear yellow solution. The resulting
[(NO3)2⊂Pd3(L)4](NO3)4 (4a): The ligand L (0.0139 g, 0.04 mmol)
was added to a solution of Pd(NO3)2 (0.0069 g, 0.03 mmol) in DMSO
(3 mL). The reaction mixture was stirred at 90 °C for 2 h. Subse-
quently, the addition of ethyl acetate (10 mL) to the resulting solu-
solution was poured into a watch-glass and then evaporated at tion led to a white solid precipitate, which was separated by centrif-
room temperature by allowing to stand for 6 h to obtain a yellow ugation. The solid was washed with acetonitrile (2 × 2 mL) and
solid. The solid was washed with acetone (2 × 2 mL) and dried un-
dried under vacuum to afford complex 4a (0.0462 g, isolated yield
74 %), m.p. 300 °C (decomp.). H NMR (500 MHz, [D6]DMSO, 300 K):
δ = 10.63 (s, 8 H, Hf), 9.88 (s, 8 H, Ha), 9.23 (d, J = 6.0 Hz, 8 H, Hb),
8.85 (s, 4 H, Hg), 8.12 (d, J = 8.0 Hz, 8 H, Hd), 7.79–7.76 (dd, J = 6.0,
6.0 Hz, 8 H, Hc), 5.51 (s, 16 H, He) ppm. 13C NMR (125 MHz,
[D6]DMSO, 300 K): δ = 162.47, 157.07, 151.13, 149.43, 142.97, 139.62,
1
der vacuum to afford complex 1a (0.0295 g, isolated yield 85 %),
1
m.p. 274 °C. H NMR (400 MHz, [D6]DMSO, 300 K): δ = 9.59 (s, 2 H,
Ha), 9.37 (s, 2 H, Hf), 9.31 (d, J = 5.2 Hz, 2 H, Hb), 8.83 (s, 1 H, Hg),8.17
(d, J = 8.0 Hz, 2 H, Hd), 7.79–7.76 (m, J = 6.0, 5.6 Hz, 2 H, Hc), 5.65–
5.27 (dd, J = 12.4, 12.8 Hz, 4 H, He), 2.96 (s, 4 H, -CH2-), 2.44 (s, 12
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135.12, 129.49, 126.98, 65.48 ppm. MS (ESI): m/z = 982.73 [4a – [(Cl)2⊂Pd3(L)4](BF4)4 (8b): A sample of Pd(BF4)2 in DMSO (3 mL)
2NO3]2+, 634.35 [4a – 3NO3]3+, 460.23 [4a – 4NO3]4+
.
was prepared in the same manner as described above for the syn-
thesis of 3b. The ligand L (0.0139 g, 0.04 mmol) was added to
Pd(BF4)2 (0.0084 g, 0.03 mmol) in DMSO (3 mL) and the solution
was stirred at room temperature for 10 min. Then TBACl (0.0111 g,
0.04 mmol) was added, the solution was heated to 90 °C, and the
reaction mixture stirred for 2 h. Subsequently, the addition of ethyl
acetate (10 mL) to the resulting solution led to an off-white solid
precipitate, which was separated by centrifugation. The solid was
washed with acetonitrile (2 × 10 mL) and dried under vacuum to
afford complex 8b (0.0499 g, isolated yield 78 %). The 1H NMR spec-
trum of compound 8b is similar to that of compound 8a.
[(NO3)2⊂Pd3(L)4](BF4)4 (4b): A sample of Pd(BF4)2 in DMSO (3 mL)
was prepared in same manner as described above for the synthesis
of 3b. The ligand L (0.0139 g, 0.04 mmol) was added to a solution
of Pd(BF4)2 (0.0084 g, 0.03 mmol) in DMSO (3 mL) followed by the
addition of TBANO3 (0.0122 g, 0.04 mmol) at room temperature.
The reaction mixture was then heated to 90 °C and stirred for 2 h.
Subsequently, the addition of ethyl acetate (10 mL) to the resulting
solution led to an off-white solid precipitate, which was separated
by centrifugation. The solid was washed with acetonitrile
(2 × 10 mL) and dried under vacuum to afford complex 4b
(0.0538 g, isolated yield 82 %). The 1H NMR spectrum of complex
4b is similar to that of compound 4a.
[(Cl)2⊂Pd3(L)4](Y)4 (8c–8e): Complexes [(Cl)2⊂Pd3(L)4](Y)4 (8c–8e;
Y = PF6–, CF3SO3–, and ClO4–, respectively) were prepared in a simi-
lar manner as described for 8b using the appropriate silver salt (see
the Supporting Information).
[(NO3)2⊂Pd3(L)4](Y)4 (4c–4e): Complexes [(NO3)2⊂Pd3(L)4](Y)4 (4c–
4e; Y = PF6–, CF3SO3–, and ClO4–, respectively) were prepared in a
similar manner as described for 4b using the appropriate silver salt
(see the Supporting Information).
We previously prepared complex 8c in a different manner, as de-
scribed below.
[(F)2⊂Pd3(L)4](NO3)4 (6a): A solution of tetra-n-butylammonium
fluoride (0.0313 g, 0.12 mmol) in [D6]DMSO (0.2 mL) was added
portionwise (4 × 0.05 mL) to a clear solution of compound 4a
(0.0626 g, 0.03 mmol of complex) in [D6]DMSO (0.4 mL, in situ) at
room temperature. The solution was monitored by 1H NMR spectro-
scopy at room temperature after the addition of each portion of
the guest. The final spectrum showed a single set of peaks and a
downfield shift of the pyridine α- and ꢀ-H atoms (Hf: Δδ =
0.512 ppm; Ha: Δδ = 0.378 ppm; Hb: Δδ = 0.313 ppm) as compared
with compound 4a, which indicates the quantitative formation of
[(Cl)2⊂Pd3(L)4](PF6)4 (8c): A sample of NH4PF6 (0.0489 g, 0.3 mmol)
was added to a clear solution of complex 8a (0.0610 g, 0.03 mmol)
in CH3CN/H2O (1:1, 5 mL). The mixture was stirred at room tempera-
ture for 12 h. The clear solution thus obtained was evaporated at
room temperature by allowing to stand. The pale-green solid ob-
tained was washed with water (2 × 2 mL) and dried under vacuum
to afford complex 8c evident from the absence of an N–O stretching
band due to NO3 ion at 1384 cm–1 [0.0624 g, isolated yield 88 %;
–
IR (KBr pellet): ν = 847 (P–F stretch) cm–1]. The 1H NMR spectrum
˜
shows a single set of peaks, comparable to the data of compound
8a, m.p. 263 °C (decomp.). 1H NMR (500 MHz, [D6]DMSO, 300 K):
δ = 11.48 (s, 8 H, Hf), 10.48 (s, 8 H, Ha), 9.56 (d, J = 6.0 Hz, 8 H, Hb),
8.92 (s, 4 H, Hg), 8.05 (d, J = 7.0 Hz, 8 H, Hd), 7.83–7.80 (dd, J = 6.0,
6.0 Hz, 8 H, Hc), 5.54 (s, 16 H, He) ppm. 13C NMR (125 MHz,
[D6]DMSO, 300 K): δ = 162.71, 156.92, 151.06, 148.47, 143.43, 139.14,
135.43, 129.40, 127.32, 65.26 ppm. MS (ESI): m/z = 1039.00 [8c –
–
compound 6a as a single product. The NO3 ions present in 4a
were exchanged with two F– ions, as is evident from the 1H NMR
spectroscopic data. The complex was not very stable and could not
be isolated in the solid state. In solution, additional signals started
to appear in the 1H NMR spectrum after 1 h and finally only signals
from the ligand were seen, indicating decomplexation. 1H NMR
(400 MHz, [D6]DMSO, 300 K): δ = 11.13 (s, 8 H, Hf), 10.25 (s, 8 H,
Ha), 9.54 (d, J = 5.6 Hz, 8 H, Hb), 8.92 (s, 4 H, Hg), 8.09 (d, J = 8.0 Hz,
8 H, Hd), 7.85–7.82 (dd, J = 6.4, 6.4 Hz, 8 H, Hc), 5.54 (s, 16 H,
He) ppm. 13C NMR (100 MHz, [D6]DMSO, 300 K): δ = 162.01, 155.98,
150.37, 147.70, 142.74, 139.64, 134.98, 129.01, 126.60, 64.59 ppm.
2PF6]2+, 644.34 [8c – 3PF6]3+, 446.77 [8c – 4PF6]4+
.
[(Br)2⊂Pd3(L)4](NO3)4 (10a): Complex 10a was prepared in a simi-
lar manner as described for 6a and 8a but by using a solution of
tetra-n-butylammonium bromide (0.0290 g, 0.09 mmol) in
1
[D6]DMSO (0.2 mL). The final H NMR spectrum shows a single set
[(Cl)2⊂Pd3(L)4](NO3)4 (8a): Complex 8a was prepared in a similar
manner as described for 6a, but by using a solution of tetra-n-
butylammonium chloride (0.0333 g, 0.12 mmol) in [D6]DMSO
(0.2 mL). The final spectrum showed a single set of peaks and a
downfield shift of pyridine α- and ꢀ-H atoms (Hf: Δδ = 0.872 ppm;
Ha: Δδ = 0.595 ppm; Hb: Δδ = 0.373 ppm) as compared with com-
pound 4a, which indicates the quantitative formation of 8a as a
of peaks and a downfield shift of the pyridine α- and ꢀ-H atoms
(Hf: Δδ = 1.051 ppm; Ha: Δδ = 0.766 ppm; Hb: Δδ = 0.394 ppm)
as compared with compound 4a, which indicates the quantitative
–
formation of compound 10a as the only product. The NO3 ions
present in 4a were exchanged with two Br– ions, as is evident from
1
the H NMR and MS (ESI) data.
–
Isolation of 10a: Complex 10a, prepared as described above in
DMSO, was isolated by the precipitation method by adding ethyl
acetate (10 mL) to the reaction mixture to afford a white precipitate.
The precipitate was separated by centrifugation. The solid was
washed with water (2 × 2 mL) and dried under vacuum to afford
complex 10a (0.0560 g, isolated yield 88 %), m.p. 300 °C (decomp.).
1H NMR (400 MHz, [D6]DMSO, 300 K): δ = 11.68 (s, 8 H, Hf), 10.65
(s, 8 H, Ha), 9.59 (d, J = 4.8 Hz, 8 H, Hb), 8.91 (s, 4 H, Hg), 8.09 (d, J =
7.2 Hz, 8 H, Hd), 7.84–7.83 (dd, J = 6.8, 6.8 Hz, 8 H, Hc), 5.57 (s, 16
H, He) ppm. 13C NMR (100 MHz, [D6]DMSO, 300 K): δ = 162.03,
156.76, 150.94, 148.43, 142.50, 138.41, 134.32, 128.52, 126.36,
64.88 ppm. MS (ESI): m/z = 645.99 [10a – 3NO3]3+, 468.98 [10a –
single product The NO3 ions present in 4a were exchanged with
1
two Cl– ions as is evident from the H NMR and MS (ESI) data.
Isolation of 8a: Complex 8a, as described above in DMSO, was
isolated by the precipitation method by adding ethyl acetate
(10 mL) to the reaction mixture to afford a white precipitate. The
precipitate was separated by centrifugation. The solid was washed
with water (2 × 2 mL) and dried under vacuum to afford complex
8a (0.0549 g, isolated yield 90 %), m.p. 289 °C (decomp.). 1H NMR
(400 MHz, [D6]DMSO, 300 K): δ = 11.50 (s, 8 H, Hf), 10.49 (s, 8 H,
Ha), 9.62 (d, J = 5.6 Hz, 8 H, Hb), 8.91 (s, 4 H, Hg), 8.07 (d, J = 8.4 Hz,
8 H, Hd), 7.86–7-83 (m, J = 5.6, 6.0 Hz, 8 H, Hc), 5.56 (s, 16 H, He) ppm.
13C NMR (100 MHz, [D6]DMSO, 300 K): δ = 161.96, 156.18, 150.36,
147.69, 142.58, 138.43, 134.69, 128.65, 126.59, 64.57 ppm. MS (ESI):
m/z = 955.67 [8a – 2NO3]2+, 616.42 [8a – 3NO3]3+, 446.58 [8a –
4NO3]4+
.
CCDC 1446609 (for 1a), 997082 (for 3a), 1446610 (for 3b), 1446611
(for 8b), and 997083 (for 8c) contain the supplementary crystallo-
4NO3]4+
.
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Acknowledgments
D. K. C. thanks the Science and Engineering Research Board,
India (SERB), Department of Science and Technology, Govern-
ment of India (project number SB/S1/IC-05/2014) for financial
support. We are grateful to Mr. V. Ramkumar for collecting sin-
gle-crystal X-ray diffraction data. S. B. and S. S. thank the Univer-
sity Grants Commission (UGC), New Delhi and the Council of
Scientific and Industrial Research (CSIR), New Delhi, respectively,
for research fellowships. G. S. H thanks Department of Foreign
Affairs and International Trade (DFAIT), Canada, Natural Scien-
ces and Engineering Research Council of Canada (NSERC) and
the University of Montreal, Canada for financial support.
Keywords: Template synthesis · Self-assembly · Palladium ·
Anions
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Received: March 8, 2016
Published Online: April 24, 2016
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim