Reactivity of M(en)Cl2 (M = PdII/PtII, en = 1,2-diaminoethane) with N,N′-bis(salicylidene)-p-phenylenediamine: Binding with hexafluorobenzene

Article abstract of DOI:10.1016/j.poly.2007.09.017

Schiff base N,N′-bis(salicylidene)-p-phenylenediamine (LH₂) complexed with Pt(en)Cl₂ and Pd(en)Cl₂ provided [Pt(en)L]₂ · 4PF₆ (1) and Pd(Salen) (2) (Salen = N,N′-bis(salicylidene)-ethylenediamine), respectively, which were characterized by their elemental analysis, spectroscopic data and X-ray data. A solid complex obtained by the reaction of hexafluorobenzene (hfb) with the representative complex 1 has been isolated and characterized as 3 (1 · hfb) using UV-Vis, NMR (¹H, ¹³C and ¹⁹F) data. A solid complex of hfb with a reported Zn-cyclophane 4 has also been prepared and characterized 5 (4 · hfb) for comparison with complex 3. The association of hfb with 1 and 4 has also been monitored using UV-Vis and luminescence data.

Full text of DOI:10.1016/j.poly.2007.09.017

Available online at www.sciencedirect.com
Polyhedron 27 (2008) 241–248
www.elsevier.com/locate/poly
Reactivity of M(en)Cl2 (M = PdII/PtII, en = 1,2-diaminoethane)
with N,N0-bis(salicylidene)-p-phenylenediamine: Binding
with hexafluorobenzene
*
Niraj Kumari, Rishikesh Prajapati, Lallan Mishra
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India
Received 2 August 2007; accepted 10 September 2007
Available online 29 October 2007
Abstract
Schiff base N,N0-bis(salicylidene)-p-phenylenediamine (LH2) complexed with Pt(en)Cl2 and Pd(en)Cl2 provided [Pt(en)L]2 Æ 4PF6 (1)
and Pd(Salen) (2) (Salen = N,N0-bis(salicylidene)-ethylenediamine), respectively, which were characterized by their elemental analysis,
spectroscopic data and X-ray data. A solid complex obtained by the reaction of hexafluorobenzene (hfb) with the representative complex
1 has been isolated and characterized as 3 (1 Æ hfb) using UV–Vis, NMR (1H, 13C and 19F) data. A solid complex of hfb with a reported
Zn-cyclophane 4 has also been prepared and characterized 5 (4 Æ hfb) for comparison with complex 3. The association of hfb with 1 and 4
has also been monitored using UV–Vis and luminescence data.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Keywords: N,N0-Bis(salicylidene)-p-phenylenediamine; PtII based cyclophane; X-ray study; NMR spectra; Association constant
1. Introduction
ing organic fluorine have also been a field of current inter-
est [12]. Several reports in the literature subscribe to the
conclusion that organic fluorine hardly accepts hydrogen
bonds and does not contribute to crystal packing [13,14].
We, therefore, thought to interact some fluorine containing
moieties, especially a strong electron acceptor molecule like
hexafluorobenzene, with newly synthesized metallocyclo-
phanes and to evaluate their association behaviour. In this
context, the tetradentate Schiff base N,N0-bis(salicylidene)-
p-phenylenediamine was chosen for its condensation with
well protected PdII and PtII metal centres. The association
of hfb with a representative complex of PtII-based cyclo-
phane 1 has been monitored using spectroscopic tech-
niques. Its association behaviour has also been compared
with the earlier reported ZnII cyclophane [15].
Supramolecular chemistry involving self assembly,
inclusion phenomena, molecular recognition and non-
covalent interactions are at the forefront of modern chem-
istry. These phenomena are commonly accessed by various
macrocycles and related assemblies. Diverse macrocycles
such as crown ethers, cyclophanes, cyclodextrins, calixare-
nes, cryptands, spherands, clathrates, cryptophanes and
molecular clefts as receptors are well known. The majority
of these macrocycles are reasonably flexible organic mole-
cules that preferentially interact with cationic guests
[1–10]. Conformationally, more rigid macrocycles incorpo-
rating metal centres have emerged as interesting members
of supramolecular assemblies [11]. Molecules containing
larger cavities bearing several functional groups open the
possibility of the encapsulation of various guest molecules.
However, among intermolecular interactions, those involv-
2. Experimental
2.1. Materials and methods
*
PdCl2 and K2PtCl4 purchased from Sigma–Aldrich were
converted into Pd(en)Cl2 and Pt(en)Cl2 using the reported
Corresponding author. Tel.: +91 542 2307321; fax: +91 542 2368174.
E-mail address: lmishrabhu@yahoo.co.in (L. Mishra).
0277-5387/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.09.017

242
N. Kumari et al. / Polyhedron 27 (2008) 241–248
procedures [16,17]. Elemental analysis and mass measure-
ments were carried on a Carbo-Erba elemental analyzer
1108 and a JEOL SX-102 mass spectrometer, respectively.
IR spectra were recorded as KBr pellets on a JASCO FT-
IR 5300 spectrometer and 1H NMR spectra were recorded
on a JEOL AL 300 MHZ spectrometer using CDCl3 and
DMSO-d6 as solvents and TMS as an internal reference.
19F NMR spectra are recorded on a JEOL AL 300 MHz
spectrometer using DMSO-d6 as the solvent and trifluoro-
acetic acid as an external reference at 282.5 MHz. UV–Vis
and luminescence measurements were made in the range
200–700 nm on a Shimadzu UV-1601 spectrometer and
Perkin–Elmer LS-45 Luminescence spectrometer, respec-
tively, and TGA-DTA measurements have been performed
on a Perkin–Elmer Pyris diamond TG-DTA instrument at
a heating rate of 20 ꢁC/min in the temperature range 50–
400 ꢁC.
solution of Pd(en)Cl2 (0.472 g, 2.0 mmol) in DMF (10 ml)
to a solution of LH2 (0.630 g, 2.0 mmol) in DMF contain-
ing Et3N (ꢀ5.0 mmol) while stirring. The product, after
heating on a sand bath for 4 h with stirring, was isolated
after the addition of H2O to the reaction mixture. It was
then filtered and washed with distilled water and metha-
nol followed by diethyl ether. The product was then
recrystallized in dichloromethane and characterized.
Yield: 40%. m.p.: >230 ꢁC. Elemental Anal. Calc. for
C16H14N2O2Pd: C, 51.56; H, 3.60; N, 8.56. Found: C,
50.56; H, 3.62; N, 8.03%. IR (KBr pellet, cmꢁ1): 1622
(mHC=N), 2934 (mC–H phenyl). 1H NMR (DMSO-d6,
300 MHz, ppm): d 8.5 (s, 2H, HC@N), 7.4 (m, 8H, aro-
matic), 6.5–6.8 (m, 8H, aromatic), 4.0 (s, 4H, N–CH2).
FAB-MS: m/z: 372 [M]+. UV–Vis (DMSO, 10ꢁ4 M):
kmax/(nm) (emax 104 Mꢁ1 cmꢁ1) 403 (1.94). Emission: non-
emissive. Conductivity: Km (DMSO, 10ꢁ4 M) 11.4 Xꢁ1
cm2 molꢁ1
.
2.2. Preparation of N,N0-bis(salicylidene)-
p-phenylenediamine (LH2)
This indicates that the PdII compound did not follow the
same reaction pattern as is followed by the PtII compound.
Instead it turned out to be a neutral PdII(Salen) [Salen =
N,N0-bis(salicylidene)-ethylenediamine] complex.
N,N0-Bis(salicylidene)-p-phenylenediamine (LH2) was
prepared and characterized using a reported procedure
[18].
2.5. Preparation of complex 3 (1 Æ hfb)
2.3. Preparation of PtII complex 1
The adduct of complex 1 with hexafluorobenzene was
prepared by the dropwise addition of a methanolic solution
of hfb to the solution of complex 1 in minimum amount of
DMSO. The resulting solution was stirred for 4 h and
cooled in a refrigerator for 24 h. After the addition of an
aqueous solution of NH4PF6, the solid thus obtained was
filtered and washed with methanol followed by diethyl
ether. It was further purified by dissolving it in a minimum
amount of DMSO followed by the addition of excess meth-
anol. Finally, the crystallized solid was dried in vacuo and
characterized as [Pt(en)LH2]2 Æ hfb 3. Yield 30%; m.p.:
>230 ꢁC. Elemental Anal. Calc. for C50H48N8O4P4F30Pt2:
C, 31.44; H, 2.51; N, 5.87. Found: C, 31.78; H, 2.55; N,
5.63%. IR (KBr pellet, cmꢁ1): 1622 (mCH@N), 1254 (mOH),
844 (mPF6). 1H NMR (DMSO-d6, 300 MHz, ppm): d
13.04 (s, OH), 9.02 (s, HC@N), 7.6–7.4 (m, aromatic), 6.9
(m, aromatic + NH2), 6.61 (m, aromatic), 3.6 (s, N–CH2),
3.5 (s, 4H, N–CH2), 19F NMR (DMSO-d6, 282.5 MHz,
ppm): d ꢁ71 (s; PF6), 162.35 (s; Ar–F). FAB-MS: m/z:
1908 [M]+, 1722 [Mꢁhfb]+, 1619 [Mꢁ2PF6+1]+, 1142
[Mꢁhfbꢁ4PF6]+, 1022 [Mꢁhfbꢁ4PF6ꢁ2en]+. UV/Vis
A solution of Pt(en)Cl2 (0.672 g, 2.0 mmol) in DMF
(10 ml) was added to a solution of LH2 (0.630 g,
2.0 mmol) in DMF followed by the addition of Et3N
(ꢀ5.0 mmol) while stirring. The resulting mixture was stir-
red whilst heating on a water bath for 5 h. The solid
product obtained after the addition of an aqueous solu-
tion of NH4PF6 was filtered, washed with distilled water
and methanol followed by diethyl ether and recrystallised
with a DMSO/MeOH mixture and finally dried in vacuo
as [Pt(en)LH2]2 Æ 4PF6 (1). However, crystals suitable for
single crystal X-ray crystallography could not be
obtained. Yield: 35%. m.p.: >230 ꢁC (dec). Elemental
Anal. Calc. for C44H48N8O4P4F24Pt2: C, 30.66; H, 2.78;
N, 6.50. Found: C, 30.78; H, 3.14; N, 6.72%. IR (KBr
pellet, cmꢁ1): 1633 (mHC = N), 1264 (mO–H phenolic), 2924
1
(mC–H phenyl), 846 (mPF6). H NMR (DMSO-d6, 300 MHz,
ppm): d 13.1 (s, 4H, OH), 9.0 (s, 4H, HC@N), 7.5 (m,
20H, aromatic), 7.0 (m, 8H, NH2), 6.5 (m, 4H, aromatic),
3.6 (s, 4H, N–CH2), 3.5 (s, 4H, N–CH2). FAB-MS: m/z:
1722 [M]+, 1577 [MꢁPF6], 1142 [Mꢁ4PF6]. UV–Vis
(DMSO, 10ꢁ4 M): kmax/(nm) (emax 104 Mꢁ1 cmꢁ1) 339
(1.69), 354 (1.68), 439 (0.43); Emission excited at k
340 nm (DMSO, 10ꢁ4 M): kmax (nm) (intensity in a.u.)
493 (459). Conductivity: Km (DMSO, 10ꢁ4 M)
(DMSO, 10ꢁ4 M): kmax/nm (emax 104 Mꢁ1 cmꢁ1
) 343
(1.41), 358 (1.49), 258 (4.27). Emission excited at k
340 nm (DMSO, 10ꢁ4 M): kmax (nm) (intensity in
a.u.) = 503.39 (362). Conductivity: Km (DMSO 10ꢁ4 M)
370 Xꢁ1 cm2 molꢁ1
.
360 Xꢁ1 cm2 molꢁ1
.
2.6. Preparation of ZnII complex 4
2.4. Preparation of PdII complex 2
The metallocyclophane [Zn(bpy)L]2 (bpy = 2,20-bipyri-
dine) 4 was prepared and characterized using the reported
procedure [15].
PdII complex 2 has also been prepared using a method
similar to that adopted for complex 1 by the addition of a

N. Kumari et al. / Polyhedron 27 (2008) 241–248
243
1
2.7. Preparation of complex 5 (4 Æ hfb)
ported by its H NMR spectrum, which shows a peak at
d 13.1 ppm assigned to OH protons. Additionally, in the
IR spectrum of adduct 3, peaks due to mHC@N are
observed at 1622 cmꢁ1 as compared to 1633 cmꢁ1 in com-
plex 1, showing that the HC@N groups are involved in the
association with hexafluorobenzene.
Additionally, the peaks of the phenyl ring protons in the
1H NMR spectrum of adduct 3 are broadened as compared
to the peaks in the spectrum of complex 1, in which they
are sharp (Fig. 1). The broadening of aromatic protons
could occur due to its restricted rotation on the NMR time-
scale upon the inclusion of hexafluorobenzene in the cavity
of complex 1. This behaviour is consistent with an earlier
report [20] on the inclusion complexes.
The UV/Vis spectra recorded in DMSO solution
(10ꢁ4 M) showed that the single peak observed at kmax
261 nm in the spectrum of free hfb is enhanced to twice
the intensity in the spectrum of its adduct 3 at kmax
260 nm. Additional peaks observed at kmax 340, 356 and
378 nm in the spectrum of complex 1 are assigned to
intra-ligand transitions, whereas the peak at kmax 439 nm
arises from a MLCT transition. In the adduct of 1 with
hfb, most of the peaks, including that of the MLCT transi-
tion, are blue shifted. This indicates that hfb is bound with
complex 1 internally.
The adduct of complex 4 with hfb was also prepared
using a similar method as followed for 3 by the addition
of a methanolic solution of excess hfb to the solution of
complex 4 in minimum amount of DMSO. The resulting
solution was stirred for 4 h then cooled in a refrigerator
for 24 h. The solid thus obtained was filtered and washed
with methanol followed by diethyl ether. It was further puri-
fied by dissolving in a minimum amount of DMSO followed
by the addition of excess methanol. Finally, the crystallized
solid was dried in vacuo and characterized as [4 Æ hfb]. Yield:
36%. m.p.: >230 ꢁC. Elemental Anal. Calc. (%) for
C66H44N8O4Zn2F6: C, 63.1; H, 3.5; N, 8.9. Found: C,
63.4; H, 3.4; N, 8.6%. IR (KBr pellet, cmꢁ1): 1601 (mCH@N),
1
1531 (mphenyl), 1149 (mhfb). H NMR (DMSO-d6, 300 MHz,
ppm): d 8.9 (s, 4H, HC@N), 6.4–6.7 (m, 12H, bpy), 7.3–
7.5 (m, 12H, phenyl), 6.9–7.1 (s, 4H, phenyl). 19F NMR
(DMSO d6, 282.5 MHz, ppm): d 162.47 (s, 6F, Ar–F).
FAB-MS: m/z: 1256 [M]+, 1071 [Mꢁhfb+1]+, 1036
[Mꢁhfbꢁ2OH]+, 1018 [Mꢁhfbꢁ3OH+1]+. UV/Vis
(DMSO, 10ꢁ4 M): kmax/nm (emax 104 Mꢁ1 cmꢁ1) 396 (1.59),
379 (2.40), 328 (1.41). Emission excited at k 396 nm (DMSO,
10ꢁ4 M): kmax (nm) (intensity in a.u.) 532 (150). Conductivity:
K
m (DMSO, 10ꢁ4 M) 5 Xꢁ1 cm2 molꢁ1
.
The composition of complex 2, assigned as Pd(Salen) on
the basis of its elemental analysis and mass measurements,
is further substantiated by its H NMR spectrum which
3. Results and discussion
1
The complexes are soluble in DMF and DMSO, com-
plex 1 and its adduct 3 show electrolytic behaviour (1:4)
in DMSO (10ꢁ3 M), whereas complexes 2,4 and 5 are
found to be non-electrolytic in nature [19].
shows the presence of N–CH2–CH2-N and HC@N protons
at d = 4.0 (s, 4H) and 8.5 (s, 2H) ppm, respectively. How-
ever, the disappearance of the signal of the phenolic OH
proton of the free ligand shows that the deprotonated
OH groups had coordinated with the PdII ion. To get addi-
tional support for its structure, it has been characterized by
single crystal X-ray crystallography.
Suitable X-ray quality crystals of complex 2 were
grown from a DMSO/MeOH solvent mixture at room
temperature, and X-ray crystallographic data are recorded
by mounting a single-crystal of sample 2 (0.26 · 0.18 ·
In the infrared spectra, the peak observed at 1610 cmꢁ1
(free ligand), assigned to m(CH@N), shifted to 1633 cmꢁ1
(complex 1). This indicates that the HC@N groups had
coordinated during complex formation. The peak observed
at 1282 cmꢁ1, assigned to mOH, (free ligand) [18] remains
constant in the spectrum of its complex and shows the pres-
ence of free OH groups. This observation is further sup-
Fig. 1. Comparison of 1H NMR spectra of complex 1 (a), and complex 3 (b) in the aromatic region.

244
N. Kumari et al. / Polyhedron 27 (2008) 241–248
0.16) mm3 on a glass fiber. An Oxford diffraction XCAL-
Table 1
Crystal data and structure refinement for complex 2
IBUR-S CCD area detector diffractometer equipped with
an LN-2 low-temperature attachment was used for the
cell determination and intensity data collection. Appropri-
ate empirical absorption corrections using the programs
multi-scan were applied. Monochromated Mo Ka radia-
Empirical formula
C16H14N2O2Pd
Formula weight
Temperature (K)
Wavelength (A)
Crystal system, space group
Unit cell dimensions
372.69
293(2)
0.71073
monoclinic, P21/c
˚
˚
tion (k = 0.71073 A) was used for the measurements.
Important crystallographic data and the structure
refinement details are listed in Table 1, selected bond
lengths and bond angles are summarized in Table 2, and
its ORTEP view is shown in Fig. 2. Crystallographic data
shows that the Pd–N distance, observed to be about
˚
a (A)
13.795(2)
7.252(10)
13.982(2
90
105.55(2)
90
1347.77(3)
4, 1.837
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
˚
˚
3
˚
1.95 A, is shorter than the reported [20] value of 2.1 A.
The N–Pd–N bond angle of 84.2ꢁ is a bit smaller than that
of a square (90ꢁ). Thus, the geometry around the PdII cen-
tre is a little deviated from the normal square planar. In
other words, although, the molecule’s shape is indeed that
of a square, it is not planar. The deviation from planarity is
Volume (A )
Z, Dcalc (Mg/m3)
Absorption coefficient (mmꢁ1
)
1.382
21921/4494 [0.0268]
4494/0/191
Reflections collected/unique [Rint
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
]
0.876
R1 = 0.0312, wR2 = 0.0909
R1 = 0.0414, wR2 = 0.1107
˚
about 3 A. It is important to note that N1–C8–C9–N2 is
largely deviated by 34.5ꢁ, which is quite logical as it con-
tains a flexible alkyl chain. The deviation from planarity
is also observed in the chelate ring but to a smaller extent
˚
(5 A). It shows that this deviation is again caused by the
Table 2
flexible alkyl chain.
˚
Selected bond lengths (A) and bond angles (ꢁ) for complex 2
Thus, the formation of Pd(Salen) shows that upon the
addition of Pd(en)Cl2 to a solution of LH2, the ligand rear-
ranges itself into the formation of an imine linkage between
ethylenediamine appended with PdII and salicylaldehyde
(most likely formed due to hydrolysis of the imine groups
followed by oxidation) as depicted in Scheme 1. The contri-
bution of ethylenediamine in this type of condensation is
further supported by the condensation of [Pd(bpy)Cl2] with
the Schiff base, which like the PtII complex, led to a (2 + 2)
condensation instead of the formation of Pd(Salen).
As mentioned above, upon the complexation of
Pt(en)Cl2 with LH2, a (2 + 2) condensation between the
two components occurred and it resulted in the formation
of a metallocyclophane, as reported earlier for the Zn(bpy)
system [15].
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–O(1)
Pd(1)–O(2)
O(1)–C(1)
O(2)–C(16)
N(1)–C(7)
N(1)–C(8)
N(2)–C(10)
N(2)–C(9)
1.952(2)
1.957(2)
1.9981(16)
2.0087(16)
1.311(3)
1.309(3)
1.286(3)
1.481(3)
1.285(3)
1.474(3)
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–O(1)
N(2)–Pd(1)–O(1)
N(1)–Pd(1)–O(2)
N(2)–Pd(1)–O(2)
O(1)–Pd(1)–O(2)
84.23(9)
93.91(8)
177.87(8)
177.72(8)
93.53(8)
88.33(7)
Fig. 2. ORTEP diagram of complex 2. Thermal ellipsoids are at 50% probability.

N. Kumari et al. / Polyhedron 27 (2008) 241–248
245
OH
Pt–N@C bond as compared to the Pd–N@C bond [21].
Theoretical calculations also support that the relativistic
bond stabilization is stronger for third-row transition metal
complexes (Pt) than for second-row transition metal com-
plexes (Pd) [22].
HO
N
N
N
N
N
OH
HO
H H
H
H
Pd(en)Cl2
N
N
H
O
Pd
Pd
N
H
N
H
The crystal packing pattern of complex 2 in the solid
state is shown in Fig. 4. The squares are stacked along
the ‘b’-axis, resulting in long channel like cavities of
H
H
NH2
NH2
OH
N
Hydrolysis in DMF
Pd(en)Cl2 in solution
˚
OH
ꢀ25 A diameter as shown by the centroid–centroid perpen-
HO
˚
˚
dicular distance ꢀ3.3 A and parallel distance of 8.2 A. It is
worth mentioning that p–p stacking between the phenyl
rings produces a near contact between the two Pd centres,
Scheme 1a
OH
H
˚
being separated by 3.206 A, though the presence of a Pd–
O
H2N
O
Pd bond between the metal centres has been ruled out in
view of the literature report [23,24]. Furthermore, it is quite
interesting to note that a cavity as desired from Scheme 1a
could be looked upon from this stacking pattern. Thus, the
stacking pattern of the complex as shown in Fig. 3
prompted us to make binding studies of phenol similar to
that reported earlier with a ZnII containing cyclophane.
Since this structure is obtained by p–p stacking interac-
tions, the association of phenol with this compound is con-
sidered to be weaker and in fact its binding constant [15]
gave the value K = 1.6 · 104, which is lower than that
reported for the ZnII analogue (K = 7.5 · 104) [15].
Cl
Cl
N
N
Pd
Pd
H2N
O
O
H
OH
Scheme 1.
Although the reason for this difference in reactivity of
PdII and PtII with the Schiff base cannot be understood
accurately, it may be considered in view of the stronger
Fig. 3. Stacking pattern in complex 2.

246
N. Kumari et al. / Polyhedron 27 (2008) 241–248
plex (5 · 10ꢁ5 M) monitored at k 340 nm is shown in Fig. 4.
The formation constant as calculated using the Benesi-
Hilde brand (BH) equation [15] is found to be 3.17 ·
104 Mꢁ1
.
To support the inclusion of hexafluorobenzene inside
the cavity of the complex, the 19F NMR spectrum of com-
plex 3 was recorded. A 19F signal observed at d
ꢁ162.35 ppm is found to be deshielded as compared to
the signal observed at d ꢁ161.54 for free hexafluoroben-
zene, as depicted in Fig. 5. It is worth mentioning that
the observed difference in 19F chemical shifts between the
included hexafluorobenzene and the free hexafluoroben-
zene, being 0.8 ppm, is almost two orders higher than the
digital resolution 0.01 ppm (acquisition time = 0.328 s).
Thus, the observed difference in chemical shifts is definitely
significant. It is further clear that upon heating, the 19F sig-
nal shifts towards high field (tending towards the 19F signal
Fig. 4. UV/Vis absorption spectrum of 1 titrated against hexafluoro-
benzene.
Since the formation constant Kc is an important param-
eter that measures the extent of inclusion of a guest in a
host complex, hexafluorobenzene being electron deficient
in nature was considered as a novel guest. The selectivity
of an electron acceptor molecule like hfb has been made
in view of the possibility of the electron rich nature of
the cavity of the macrocyclic frame due to the attachment
of electron donor groups like 1,2-diaminoethanes to the
metal centres as well as the presence of phenyl rings as elec-
tron sinks in the skeleton of the metallocyclophane. Thus,
the binding of hexafluorobenzene with the representative
complex 1 has been monitored by UV–Vis spectral data
(Fig. 4). The variation of spectral pattern obtained by the
addition of increasing concentrations of hexafluorobenzene
(2 · 10ꢁ5–1 · 10ꢁ4 M) to a fixed concentration of the com-
of free hfb). Additionally, upon heating, the number of 13
C
peaks of the phenyl rings also get enhanced. This could
happen only when hbf is included in the cavity since upon
heating hfb would tend to come out of the cavity and get
closer to the phenyl ring of ligand skeleton. A variable tem-
perature 1H NMR spectrum of adduct 3 was also recorded,
and it shows extensive splitting of the phenyl protons in the
temperature range 130–140 ꢁC. For further support, 19F
VT NMR of adduct 3 was also recorded from room tem-
perature 27.7 to 70 ꢁC. The variable temperature 19F
NMR spectrum shows a substantial decrease in the change
of chemical shift as compared to the position of the 19F sig-
Fig. 5. Comparison of 19F NMR spectra of free hexafluorobenzene (a); complex 3 (b) and complex 5 (c). The signal labelled with an asterisk represents
trifluoroacetic acid (external reference).

N. Kumari et al. / Polyhedron 27 (2008) 241–248
247
Fig. 6. Change in chemical shift in 19F VT NMR spectra of complex 3
(blue), trendline (red). (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
Fig. 8. TGA thermogram of complex 3.
ther supported by its DTA thermogram which shows an
endothermic peak for this temperature range, indicating
that a phase transition occurs most likely due to the
removal of hexafluorobenzene (guest) as reported for the
metal organic frameworks [25]. The second weight loss of
30.39% between 200 and 240 ꢁC is due to the removal of
ꢁ
four counter PF6 anions (30.54%) in view of the decom-
position temperature of complex 1, i.e. >230 ꢁC. The
appearance of an exotherm at 230 ꢁC in the DTA thermo-
gram also strengthens this finding. The second weight loss
in the TGA thermogram profile is followed by a third
weight loss in the temperature range 250–400 ꢁC. After
400 ꢁC about 12% mass remains, which may be due to
metal oxide.
Fig. 7. Ball and stick model showing the fitting of one hfb molecule within
the cavity of complex 1. All H atoms have been omitted for clarity.
4. Conclusion
nal of free hfb with an increase in temperature, as depicted
in Fig. 6.
The present study shows the construction of a PtII-based
metallocyclophane on the skeleton of a simple tetradentate
Schiff base which provides an electron rich cavity. The
space provided by the cavity is found to be suitable for
comparatively stronger binding of an electron acceptor
molecule like hexafluorobenzene. Additionally, the pres-
ently used Schiff base provides an alternate route to pre-
pare the Pd(Salen) complex in the form of beautiful
coloured crystals suitable for X-ray studies. This study also
allows the difference in the reactivity of PdII and PtII com-
plexes with a tetradentate Schiff base to be understood.
1
Thus, the H, 13C and 19F NMR spectral pattern of the
adduct show that the guest is included in the cavity gener-
ated by the metallocyclophane 1, consistent with the earlier
report. Since suitable crystals could not be grown, a model
of such host–guest binding was tried and is depicted in
Fig. 7, which shows that hfb is accommodated by the cav-
ity formed by metallocyclophane.
In order to compare the association of hfb with metallo-
cyclophane 1, hfb has also been allowed to interact with the
earlier reported metallocyclophane 4 and the association
constant thus evaluated was found to be 2.52 · 104 Mꢁ1
.
An 19F NMR spectrum of its adduct 5 along with the spec-
trum of free hfb is also shown in Fig. 5.
Acknowledgements
Thus, the results indicate that PtII–cyclophane derived
from the Schiff base (LH2) binds hexafluorobenzene more
strongly compared to the earlier reported ZnII–cyclophane.
To get evidence of guest mobility and framework stabil-
ity, thermal gravimetric analysis of adduct 3 has been made
and the corresponding thermogram is shown in Fig. 8.
Crystalline sample 3 (2.128 mg) was heated at a constant
rate of 20 ꢁC minꢁ1 in a N2 atmosphere with 180 ml/min
flow rate from 50 to 400 ꢁC. Three weight-loss steps were
observed: the first, corresponding to 9.39%, occurred
between 50 and 130 ꢁC, which could be attributed to the
loss of the guest molecules (calculated 9.74%). This is fur-
N.K. and R.P. thank UGC, New Delhi, India for finan-
cial assistance. The authors wish to thank CDRI, Luc-
know, India for providing analytical data and IITB,
Mumbai, India for X-ray data collection. We also wish
to thank Dr. R.K. Mandal, Department of Metallurgical
Engineering, BHU, India for recording TGA-DTA data.
Appendix A. Supplementary material
CCDC 647916 contains the supplementary crystallo-
graphic data for 2. These data can be obtained free of

248
N. Kumari et al. / Polyhedron 27 (2008) 241–248
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