Syn-anti conformational switching: Synthesis and X-ray structures of tweezer and anti form in a zinc porphyrin dimer induced by axial ligands

Article abstract of DOI:10.1016/j.ica.2011.01.101

Stepwise addition of 1,2-diaminobenzene to a Zn-1,2-bis(meso- octaethylporphyrinyl)ethane produces both tweezer and anti-form of the complex depending on the concentration of the axial ligand which exhibit two major equilibrium steps (with two step-wise binding constants): first, guest ligation leading to the formation of 1:1 host-guest tweezer structure (K₁) and, second, guest molecule ligation (K₂) forming 1:2 host-guest anti species and the corresponding binding constants are 1.82 × 10³ M^-1 and 1.34 × 10² M^-1, respectively. However, when guest like 1,4-diaminobenzene and 4-CN-pyridine are used, the ligand geometry prevents its entry into interporphyrin cavity to form a tweezer structure, thus producing only the 1:2 anti complex. Single crystal X-ray structures of both tweezer and anti form produced in a single Zn-bisporphyrin are reported here for the first time. The nonbonding Zn··· Zn distance within a molecule is 5.55 and 10.01 in tweezer and anti form, respectively. Although the average Zn-N (por) distances are comparable for both the forms, the Zn-N (1,2-diaminobenzene/4-CN-pyridine) distances and the displacement of Zn from the mean porphyrin planes are larger in tweezer compared to anti conformation.

Full text of DOI:10.1016/j.ica.2011.01.101

Inorganica Chimica Acta 372 (2011) 62–70
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Inorganica Chimica Acta
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Syn–anti conformational switching: Synthesis and X-ray structures of tweezer and
anti form in a zinc porphyrin dimer induced by axial ligands
⇑
Sanfaori Brahma, Sk. Asif Ikbal, Sankar Prasad Rath
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 4 February 2011
Stepwise addition of 1,2-diaminobenzene to a Zn–1,2-bis(meso-octaethylporphyrinyl)ethane produces
both tweezer and anti-form of the complex depending on the concentration of the axial ligand which
exhibit two major equilibrium steps (with two step-wise binding constants): first, guest ligation leading
to the formation of 1:1 host–guest tweezer structure (K1) and, second, guest molecule ligation (K2) form-
ing 1:2 host–guest anti species and the corresponding binding constants are 1.82 ꢀ 103 Mꢁ1 and
1.34 ꢀ 102 Mꢁ1, respectively. However, when guest like 1,4-diaminobenzene and 4-CN-pyridine are used,
the ligand geometry prevents its entry into interporphyrin cavity to form a tweezer structure, thus pro-
ducing only the 1:2 anti complex. Single crystal X-ray structures of both tweezer and anti form produced
Dedicated to Prof. S.S. Krishnamurthy on the
occasion of his 70th birthday
Keywords:
Zn-bisporphyrin
Molecular switching
Tweezer complex
Binding constant
Structure elucidation
in a single Zn-bisporphyrin are reported here for the first time. The nonbonding Zn
Zn distance within
ꢂꢂꢂ
a molecule is 5.55 and 10.01 Å in tweezer and anti form, respectively. Although the average Zn–N (por)
distances are comparable for both the forms, the Zn–N (1,2-diaminobenzene/4-CN-pyridine) distances
and the displacement of Zn from the mean porphyrin planes are larger in tweezer compared to anti
conformation.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
ecule between two binding sites of a molecular host that leads to a
fascinating spatial architecture resembles a pincer holding an ob-
Porphyrin dimers have been attracting considerable attention
for their functions such as light harvesting, catalysis, chirality sens-
ing, host–guest interactions, and molecular recognition [1–6]. Two
porphyrins linked in a face-to-face arrangement by a rigid/flexible
linker produce molecular clefts for the binding and activation of a
variety of substrates [1–34]. However, efficient molecular recogni-
tion of the bis-porphyrin host for guest molecules or ions requires
high structural and binding-site complementarity between the
host and the guest. Ligand selectivity and recognition are driven
by simultaneous cooperation of various non-covalent interactions
and coordinate bonding of axial ligands to a metal center. The con-
formational variations of a bisporphyrin host are responsible for a
wide range of physical and chemical properties with guest mole-
cules [1–6]. In the present work, we have used the Zn-1,2-bis(me-
so-octaethylporphyrinyl)ethane as a host in which the two meso
positions are connected through a highly flexible ethane spacer.
The spacer gives the compound enough flexibility for adjusting
the distance between two porphyrin units. Single crystal X-ray
structures of both tweezer and anti form in a single Zn–bisporphy-
rin induced by the axial ligands are reported here, for the first time.
A supramolecular tweezer arising from the insertion of a guest mol-
ject. Although bis-porphyrin derivatives are well suited to serve
as host compounds for the tweezers [18,19,24–30], only a few com-
plexes have been structurally characterized so far [18,19,27].
2. Experimental
2.1. Materials
1,2-Bis(meso-octaethylporphyrinyl)ethane 1, and its Zn-com-
plex 2, are prepared by the literature methods [35,36]. Reagents
and solvents are purchased from commercial sources and purified
by standard procedures before use.
2.1.1. Synthesis of tweezer-2 ODAB
ꢂ
Zn-bisporphyrin 2 (50 mg, 0.041 mmol) was dissolved in 2.5 mL
of distilled dichloromethane. 1,2-diaminobenzene (ODAB) (5.3 mg,
0.049 mmol) was added to it and stirred for 5–6 min. The solution
thus obtained was then filtered off to remove any solid residue and
carefully layered with cyclohexane in air at room temperature. On
standing for 6–7 days, a dark crystalline solid precipitated out
which was then isolated by filtration, washed well with cyclohex-
ane and dried in vacuum. Yield: 37 mg (68%). Anal. Calc. C, 72.36;
H, 7.44; N, 10.55. Found: C, 72.30; H, 7.52; N, 10.48%. UV–Vis
⇑
Corresponding author.
E-mail address: sprath@iitk.ac.in (S.P. Rath).
(chloroform) [kmax
,
nm
(e,
Mꢁ1 cmꢁ1)]: 410(2.60 ꢀ 105),
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.01.101

S. Brahma et al. / Inorganica Chimica Acta 372 (2011) 62–70
63
420sh(2.08 ꢀ 105),
441sh(7.11 ꢀ 104),
557(3.52 ꢀ 104),
Table 1
601(2.54 ꢀ 104). ESI-MS: m/z 1326.6558. 1H NMR (CDCl3, 295 K):
d 9.92, 8.16 (s,4H, 10,20-meso-H); 9.15 (s, 2H, 15-meso-H); 6.56,
6.01 (m, 4H, –CH2(b)), 4.87 (m, 4H, –CH, ODAB); 4.13–3.19 (m,
32H, –CH2CH3); 2.16–0.80 (m, 48H, –CH3); ꢁ6.38 (br, 4H, –NH2).
Crystal data and data collection parameters for the complexes.
tweezer-
ODAB C6H12
anti-
2 (CNPY)2 CNPY
2
ꢂ ꢂ
ꢂ
ꢂ
T (K)
100(2)
100(2)
Formula
Formula weight
Color
Crystal system
Space group
a (Å)
C86H110N10Zn2
1414.58
red
monoclinic
C2/c
19.635(2)
21.666(3)
38.468(5)
90
96.165(2)
90
C98H106N16Zn2
1638.73
red
2.1.2. Synthesis of anti-2 (PDAB)2
ꢂ
Zn-bisporphyrin 2 (50 mg, 0.041 mmol) was dissolved in 3 mL
of distilled dichloromethane. 1,4-diaminobenzene (PDAB)
(11.0 mg, 0.102 mmol) was then added to it and stirred for
10 min. The resulting solution was then filtered and carefully lay-
ered with cyclohexane. On standing for 6–7 days, dark solid were
precipitated out which was then isolated by filtration, washed well
with cyclohexane and dried in vacuum. (Yield 36 mg, 61%) Anal.
Calc. C, 71.93; H, 7.44; N, 11.71. Found: C, 71.85; H, 7.55; N,
triclinic
ꢀ
P1
11.866(2)
12.379(2)
15.353(3)
97.132(3)
105.597(3)
99.992(3)
2104.2(6)
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
16270(3)
Mꢁ1 cmꢁ1)]:
Radiation (k, Å)
Mo K
8
a
(0.71073)
Mo K
1
a (0.71073)
11.82%. UV–Vis (chloroform) [kmax
,
nm
(e
,
425(1.31 ꢀ 105), 438sh (9.55 ꢀ 104), 558(2.72 ꢀ 104), 600(1.58 ꢀ
104). 1H NMR (CDCl3, 295 K): d 9.72 (s, 2H, 15-meso-H); 8.42 (s,
4H, 10,20-meso-H); 5.76 (br, 8H, –CH, PDAB); 5.19 (s, 4H, –
CH2(b)); 4.13–4.08 (m, 4H, –CH2CH3); 4.00–3.95 (m, 4H, –CH2CH3);
3.80–3.74 (m, 4H, -CH2CH3); 3.69–3.64 (m, 4H, –CH2CH3); 3.38–
3.32 (m, 4H, -CH2CH3); 3.18–3.13 (m, 4H, –CH2CH3); 2.83–2.77
(m, 4H, –CH2CH3); 2.61–2.56 (m, 4H, –CH2CH3); 2.36(br, 4H, –
NH2); 1.86 (t, 12H, –CH3); 1.62(t, 12H, –CH3); 1.49(br, 4H, –
NH2);1.36 (t, 12H, –CH3); 1.14 (t,12H, –CH3).
Z
Dcalc (g cmꢁ3
)
1.155
0.638
6048
14308
30
1.293
0.629
866
7462
0
l
(mmꢁ1
F(0 0 0)
)
Number of unique data
Number of restraints
Number of parameters
refined
954
531
Goodness-of-fit (GOF) on F2 1.098
1.044
a
R1 [I > 2
r
(I)]
0.0936
0.1477
0.2303
0.0628
0.0852
0.1692
a
R1 (all data)
b
wR2 (all data)
P
jjFo jꢁjFc
j
a
P
2.1.3. Synthesis of anti-2 (CNPY)2
R1
¼
.
ꢂ
jFo
j
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
P
Zn-bisporphyrin 2 (50 mg, 0.041 mmol) was dissolved in 3 mL
of distilled dichloromethane. 4-Cyanopyridine (CNPY) (10.7 mg,
0.102 mmol) was then added to it and stirred for 10 min. After fil-
tration to remove the solid residue, the solution was carefully lay-
ered in air with cyclohexane at room temperature. Dark crystalline
solid of the complex were precipitated out after 6–8 days time
which was then isolated by filtration, washed well with cyclohex-
ane and dried in vacuum. (Yield 34 mg, 58%) Anal. Calc. C, 72.33; H,
6.92; N, 11.77. Found: C, 72.27, H, 6.98; N, 11.70%. UV–Vis (chloro-
2
½wðF2o ꢁF2c
Þ
ꢃ
ꢃ
b
P
wR2
¼
.
2
½wðF2o
Þ
package [39]. Non-hydrogen atoms were refined anisotropically.
In the refinement, hydrogens were treated as riding atoms using
SHELXL default parameters. Crystal data and data collection parame-
ters for the complexes are given in Table 1.
form) [kmax, nm (
e
, Mꢁ1 cmꢁ1)]: 424(2.10 ꢀ 105), 436sh(1.73 ꢀ 105),
557(2.6 ꢀ 104), 597(1.06 ꢀ 104). ESI-MS: m/z 1426.6626. 1H NMR
(CDCl3, 295 K): d 9.88 (s, 2H, 15-meso-H); 8.96 (s, 4H, 10,20-
meso-H); 6.93 (br, 4H, –CH, CNPY); 6.78(br, 4H, –CH, CNPY); 5.03
(s, 4H, –CH2(b)); 4.24–3.08 (m, 32H, –CH2CH3); 1.95 (t, 12H, –
CH3); 1.78(t, 12H, –CH3); 1.42 (t, 12H, –CH3); 1.23 (t, 12H, –CH3).
3. Results and discussion
The free ligand 1,2-bis(meso-octaethylporphyrinyl)ethane 1,
and Zn–1,2-bis(meso-octaethylporphyrinyl)ethane 2, are prepared
using the procedures reported earlier [35,36]. Two porphyrin
planes in 2 are found to be in a face-to-face conformation in non-
polar and non-coordinating solvent [20–23]. This spatial arrange-
ment is extremely stable with no observable changes in the
conformation even upon heating up to 110 °C. This high stability
arises due to strong p–p interaction between two planar chromo-
phoric moieties, which allows very effective interporphyrin
interactions.
The stepwise addition of an aromatic diamine such as 1,2-
diaminobenzene (ODAB), to 2 produces dramatic transformations
in the UV–Vis spectra. Fig. 1 demonstrates the spectral changes
of 2 in chloroform upon gradual addition of ODAB. As can be seen,
the intensity of the Soret band at 398 nm decreases while a new
peak at 410 nm progressively increases with increasing ODAB con-
centration that varies from host: guest molar ratio of 1:0.1–1:20.
Also, the porphyrin Soret band is bathochromically shifted in com-
parison to that of the nonligated 2 and exhibits three well-resolved
transitions: the main absorption peak at 410 nm with absorption
shoulders at 420 nm and 441 nm. However, the changes in the por-
phyrin Q-band region (500–700 nm) are less pronounced. The
spectral pattern is associated with 1:1 host–guest tweezer-
2.2. Instrumentation
UV–Vis spectra were recorded on a Perkin–Elmer UV/Vis spec-
trometer. Elemental (C, H, and N) analyses were performed on
CE-440 elemental analyzer. ESI-MS spectra were recorded on a
waters Micromass Quattro Micro triple quadrupole mass spec-
trometer. Isotopic pattern simulations are done by the software
‘‘ISOTOPE PATTERN CALCULATOR v4.0’’ developed by Junhua Yan 2001.9.
1H NMR spectra were recorded on a JEOL 500 MHz instrument.
The residual 1H resonances of the solvents were used as a second-
ary reference.
2.3. X-ray structure solution and refinement
Crystals were coated with light hydrocarbon oil and mounted in
the 100 K dinitrogen stream of Bruker SMART APEX CCD diffrac-
tometer equipped with CRYO Industries low-temperature
apparatus and intensity data were collected using graphite-
monochromated Mo K
a radiation (k = 0.71073 Å). The data inte-
gration and reduction were processed with SAINT software [37].
An absorption correction was applied [38]. Structures were solved
by the direct method using SHELXS-97 and were refined on F2 by
full-matrix least-squares technique using the SHELXL-97 program
2 ODAB by comparing with the similar spectrum reported previ-
ꢂ
ously for tweezer-2 DACH (DACH: 1,2-diaminocyclohexane)
ꢂ
[18,20–23] and was confirmed by the ESI mass spectral analysis
that reveals a signal at m/z 1326.6558 assigned for tweezer-

64
S. Brahma et al. / Inorganica Chimica Acta 372 (2011) 62–70
Fig. 1. UV–Vis spectral changes of 2 in chloroform upon addition of 1,2-diamino-
benzene (ODAB) as the host: guest molar ratio changes from 1:0.1 to 1:20 at 295 K.
Fig. 3. ESI-MS of 2 in presence of excess ODAB.
Fig. 2. UV–Vis spectral changes of 2 in chloroform upon addition of 1,2-diamino-
benzene (ODAB) as the host: guest molar ratio changes from 1:90 to 1:2200 at
295 K.
[2 ODAB]+. The isotopic distribution pattern of experimental mass
ꢂ
is also exactly matching with the theoretical pattern (Fig. S1). The
molecule was then isolated in solid and also structurally character-
ized (vide infra).
Fig. 4. UV–Vis spectral changes of 2 in chloroform upon addition of 4-cyanopyr-
idine (CNPY) as the host: guest molar ratio changes from 1:0.1 to 1:31000 at 295 K.
Upon further increase of 1,2-diaminobenzene of host: guest
molar ratio from 1:90 to 1:2200, the spectral pattern again changes
which includes splitting of the Soret band into two well-resolved
transitions at 422 and 441 nm as demonstrated in Fig. 2. These
observations are in a good agreement with Kasha’s exciton cou-
pling theory [40] and associated with the anti form of the complex.
The ESI-MS spectrum reveals a signal at m/z 1434.7247 assigned
pyridine and the molecule was then isolated in the solid and
structurally characterized as anti-2 (CNPY)2. Similar spectral
ꢂ
observation also obtained in case of PDAB and thus assigned as
anti-2 (PDAB)2. Fig. S3 (see Supporting Information) compares
ꢂ
the UV–Vis spectra of both anti-2 (PDAB)2 and anti-2 (CNPY)2.
ꢂ
ꢂ
Scheme 1 shows the synthetic outline of all the complexes re-
ported here along with their abbreviations while Fig. 5 shows the
UV–Vis spectra of all the complexes.
for the anti-[2 (ODAB)2]+ and the isotopic distribution pattern of
ꢂ
experimental mass is exactly matching with the theoretical pattern
(Fig. S2). Fig. 3 shows the ESI-MS spectra of 2 in presence of excess
ODAB.
3.1. Crystallographic characterization of tweezer-2 ODAB
ꢂ
The complexation behavior of 2 with 1,2-diaminobenzene
(ODAB) thus revealed that besides the tweezer-2 ODAB,
Dark brown crystals of the molecule are grown via slow diffu-
sion of cyclohexane into a dichloromethane solution of the com-
plex at room temperature. The complex crystallizes in the
monoclinic crystal system with C2/c space group. A perspective
view of the molecule is shown in Fig. 6 and the selected bond dis-
tances and angles are given in Table 2. The complex has two Zn–
centers each in a five coordinate square-pyramidal geometry in
which the Zn– atoms are displaced by 0.30 and 0.26 Å from the
mean porphyrin planes (C20N4 porphyrinato core) of core-I and
core-II, respectively. The average Zn–Np distances are found to
be 2.062(5) and 2.056(5) Å for core-I and core-II, respectively while
Zn–N(L) distances are 2.256(5) and 2.258(5) Å. The porphyrin rings
are nearly planar as can be seen from Fig. 6. However, the bridging
ꢂ
anti-2 (ODAB)2 structure was also formed. There are two distinct
concentration regions found in the process corresponding to the
ꢂ
formation of the tweezer-2 ODAB and anti-2 (ODAB)2 species.
ꢂ
ꢂ
Although bis-porphyrin derivatives are well suited to serve as host
compounds for the tweezer structures, only a few complexes of bis-
porphyrin based tweezers have been reported and structurally
characterized so far [18,19,27].
However, 2 does not form tweezer complex with 1,4-diamino-
benzene (PDAB) and 4-cyano pyridine (CNPY) and Fig. 4 demon-
strates the changes in the UV–Vis region of 2 on progressive
additions of 4-cyanopyridine. As can be seen, two well-resolved
transitions at 424 and 436 nm are observed in the case of 4-CN-

S. Brahma et al. / Inorganica Chimica Acta 372 (2011) 62–70
65
Scheme 1.
packing diagram of tweezer-2 ODAB which also shows the strong
ꢂ
p
-
p
interactions (with 3.7 Å separations) between the benzene ring
of ODAB and the pyrrole rings of the two adjacent rings. In the
crystal lattice one cyclohexane molecule is also present as a solvent
of crystallization.
The X-ray structure of tweezer-2 DACH (DACH: 1,2-diaminocy-
ꢂ
clohexane) is reported before [18] which, however, enable us to
compare the structural parameters with the complex reported
here. In the structure, the Zn
Zn nonbonding distance was
ꢂꢂꢂ
observed as 5.60 Å compared to 5.55 Å distance reported for
tweezer-2 ODAB. The average Zn–N(p) distances are longer while
Zn–N(L) distances are shorter in tweezer-2 DACH compared to
the similar distances found in tweezer-2 ODAB which suggest
the aliphatic diamine (DACH) binds the bisporphyrin guest 2 much
more strongly (vide infra). The reduced basicity of the aromatic
diamine (ODAB) compared to the aliphatic diamine (DACH) must
have contributed significantly towards the reduced stability of
ꢂ
ꢂ
Fig. 5. UV–Vis spectra in chloroform for 2 (orange line), tweezer-2 ODAB (green
ꢂ
line), anti-2 (ODAB)2 (red line) and anti-2 (CNPY)2 (blue line) at 295 K.
ꢂ
ꢂ
ꢂ
meso carbons, namely C20 and C120, are considerably out of the
mean porphyrin plane. This is presumably to minimize the non-
tweezer-2 ODAB reported here. Another reported tweezer struc-
ꢂ
bonded contacts between the two rings while Zn
Zn separation
ture of interest is Zn2(DPD)(2-aminopyrimidine), (DPD: diporphy-
rin dibenzofuran), in which 2-aminopyrimidine encapsulate
inside the cavity of bisporphyrin [19]. In the complex, the pyrimi-
dine guest is locked in a coplanar arrangement with the dibenzofu-
ran spacer. For the complex, the five-coordinate Zn(II) ions adopt a
square pyramidal geometry with an average Zn–N(p) bond length
ꢂꢂꢂ
is found to be 5.55 Å. The closest intramolecular non bonding
C distance is between two bridging meso carbon with a sep-
C
ꢂꢂꢂ
aration of 3.114(8) Å which is well below the van der Waals radius
of the atom. Fig. S4 highlights the encapsulations of a 1,2-diamino-
benzene within two porphyrin rings while Fig. S5 shows the

66
S. Brahma et al. / Inorganica Chimica Acta 372 (2011) 62–70
Fig. 6. A perspective view of tweezer-2 ODAB showing 50% thermal contours for all non-hydrogen atoms at 100 K (H-atoms and solvent cyclohexane have been omitted for
ꢂ
clarity).
Table 2
distance is significantly shorter compared to tweezer-2 ODAB.
ꢂ
Selected bond distances (Å) and angles (°).
Fig. S6 represents the packing diagrams of anti-2 (CNPY)2 which
ꢂ
also shows the uncoordinated 4-CN-pyridine that is present in
the crystal lattice. The ligand 4-CN-pyridine could, in principle,
bind to Zn either through the CN group or the pyridine nitrogen;
however, it binds through the pyridine nitrogen only due to more
basic nature.
tweezer-2 ODAB C6H12
anti-2 (CNPY)2 CNPY
ꢂ ꢂ
ꢂ
ꢂ
Core-1
Core-IIa
Zn1–N1
Zn1–N2
Zn1–N3
Zn1–N4
2.063(5)
2.078(5)
2.040(5)
2.069(5)
2.256(5)
90.1(2)
2.053(5)
2.051(5)
2.063(5)
2.059(5)
2.258(5)
90.9(2)
2.066(3)
2.075(3)
2.055(3)
2.065(3)
X-ray structure of 2 is reported previously in which the two
porphyrin rings are in syn conformation [23]. Table 3 compares
all the key structural parameters of the molecules reported here
along with structurally characterized Zn complexes with the same
bisporphyrin ligand, 1, known in the literature. As can be seen, the
average Zn–N(p) distances are shortest in 2 in the series while Zn
atom lies mostly on the plane of the porphyrin in the molecule. Zn–
Zn1–N5/N6
2.173(3)
N(1)–Zn1–N(2)
N(1)–Zn1–N(3)
N(1)–Zn1–N(4)
N(2)–Zn1–N(3)
N(2)–Zn1–N(4)
N(3)–Zn1–N(4)
N(1)–Zn1–N(5)/N6
N(2)–Zn1–N(5)/N6
N(3)–Zn1–N(5)/N6
N(4)–Zn1–N(5)/N6
90.14(11)
167.62(11)
88.14(11)
88.42(12)
166.03(11)
90.29(12)
96.35(11)
96.93(11)
96.03(11)
97.04(11)
165.8(2)
88.0(2)
169.51(19)
88.0(2)
88.0(2)
88.4(2)
169.24(19)
91.3(2)
167.36(19)
90.4(2)
97.0(2)
92.47(19)
97.1(2)
98.46(19)
96.83(19)
92.01(19)
95.79(19)
N(L) distance, however, is found longest in tweezer-2 ODAB in the
ꢂ
series. The average Zn–N(p) distances are comparable for both
tweezer and anti form reported here. Also, the displacement of Zn
from the mean porphyrin planes (24 atoms) are much less in anti
compared to the tweezer form. Two porphyrin rings are almost par-
98.27(19)
a
Actual numbering scheme of nitrogen starts from 101.
allel in 2 (without axial ligand) and also in the anti-2 (CNPY)2,
ꢂ
however, rings are highly angular with the angles of 39.5° and
of 2.076 Å while Zn–N(2-aminopyrimidine) bond distances are
2.257 and 2.346 Å. The zinc atoms are also displaced toward the
guest by 0.334 and 0.236 Å, from the N4 macrocyclic plane of the
cores.
34.7° in tweezer-2 DACH (DACH: 1,2-diamino cyclohexane) and
ꢂ
tweezer-2 ODAB respectively. The Zn
Zn nonbonding distances
ꢂ
ꢂꢂꢂ
are 5.55 and 10.01 Å, respectively in tweezer-2 ODAB and anti-
Zn nonbonding distance was ob-
served to be 5.91Å in 2 in the absence of axial ligand [23]. Thus,
Zn nonbonding distance decreases
in tweezer complex while the distance increases in anti-form of
ꢂ
2 (CNPY)2. However, Zn
ꢂ
ꢂꢂꢂ
3.2. Crystallographic characterization of anti-2 (CNPY)2
ꢂ
upon axial ligation, the Zn
ꢂꢂꢂ
Dark brown crystals of the molecule are grown via slow diffu-
sion of cyclohexane into a chloroform solution of the complex at
room temperature. The complex crystallizes in the triclinic crystal
the complex.
ꢀ
system with P1 space group; a perspective view is shown in Fig. 7
3.3. 1H NMR spectral studies
while the selected bond distance and angles are given in Table 2.
The complex has a five coordinate square-pyramidal geometry in
which Zn is displaced by 0.22 Å from the least-squares plane of
the C20N4 porphyrinato core. The average Zn–Np and Zn–N(L) dis-
tances are found to be 2.065(3)Å and 2.173(3) Å, respectively. Thus
the observed Zn–Np distance is relatively longer while Zn–N(L)
The 1H NMR titration experiments were carried out at 295 K in
CDCl3. Fig. 8 shows the relevant spectra coming from the reaction
between 2 and 1,2-diaminobenzene (ODAB). Trace A shows the
well-resolved spectrum of 2 in CDCl3 alone while trace B shows
the 1H NMR spectra after the addition of 1.0 M equivalent of ODAB

S. Brahma et al. / Inorganica Chimica Acta 372 (2011) 62–70
67
Fig. 7. A perspective view of anti-2 (CNPY)2 showing 50% thermal contours for all non-hydrogen atoms at 100 K (H-atoms have been omitted for clarity). The molecule also
ꢂ
contains 4-CN-pyridine in the crystal lattice which is not shown.
Table 3
Selected structural parameters.
Compound
Zn–Np
Zn–Nax
Zn
Znc
Ud
D24
,
References
[23]
b
b
Zn e
ꢂꢂꢂ
2a
Core-I
Core-II
Core-I
Core-II
Core-I
Core-II
2.037(3)
2.038(3)
2.079
–
–
5.91
8.5
0.03
0.01
0.47
0.40
0.30
0.26
0.22
tweezer-2 DACH
2.170
2.186
5.60
5.55
39.5
34.7
0.0
[18]
ꢂ
2.073
tweezer-2 ODAB
2.062(5)
2.056(5)
2.065(3)
2.256(5)
2.258(5)
2.173(3)
this work
this work
ꢂ
anti-2 (CNPY)2
10.01
ꢂ
a
b
c
Two rings are in face-to-face.
Average value in Å.
Nonbonding distance in Å.
Angle between two least-square plane of the C20N4 porphyrinato core.
Displacement (in Å) of Zn from mean plane of four pyrrole nitrogen.
d
e
which corresponds to tweezer-2 ODAB. Trace C, however, presents
ꢂ
the 1H NMR of free ODAB. The 1H NMR spectral feature of tweezer
are, upfield shift of –CH– (from 6.64 ppm to 4.87 ppm) and –NH2
protons (from 3.23 ppm to ꢁ6.38 ppm) of the 1,2-diaminobenzene
(Fig. 8), because of ligand’s close proximity to the two porphyrin
subunits and hence strong ring current effect. Similar shifts of
the guest’s protons clamped between the porphyrin subunits have
been also observed in tweezer-2 DACH reported earlier [22]. On
ꢂ
the other hand, protons of the porphyrin subunits (–CH2(b) and
10,20-meso-H) are significantly downfield shifted due to confor-
mational change upon tweezer formation, which result in moving
the two porphyrin rings further apart from each other (compared
to 2) and subsequently decreasing the shielding effect of the neigh-
boring porphyrin. Furthermore these protons become non-equiva-
lent as they experience different exposure to the ring current
effect. The largest split of ꢄ1.76 ppm is observed in 10,20-meso
proton, which are mostly affected by the anisotropy of the neigh-
boring porphyrin while the –CH2 (b) protons show a moderate split
by 0.55 ppm. The –CH2CH3 protons are transformed into several
broad multiplets showing a downfield shift in the center of their
signals. Similar trends in 1H NMR shifts of the porphyrin subunits
Fig. 8. 1H NMR spectra in CDCl3 at 295 K of (A) 2, (B) after addition of 1.0 equivalent
are also reported for the tweezer-2 DACH [22].
ꢂ
of 1,2-diaminobenzene (ODAB) showing the formations of tweezer-2 ODAB and (C)
Fig. 9 shows the relevant spectra coming from the reaction
ꢂ
free ODAB ligand.
between
2
and 4-CN-pyridine (CNPY). Trace
A
shows the

68
S. Brahma et al. / Inorganica Chimica Acta 372 (2011) 62–70
Fig. 9. 1H NMR spectra in CDCl3 at 295 K of (A) 2, (B) anti-2 (CNPY)2 and (C) free
ꢂ
CNPY ligand.
well-resolved spectrum of 2 in CDCl3 alone. Trace B shows the 1H
Fig. 10. (A) Normal plot (B) linear-log plot for the determination of stability
constant of binding 1,2-diaminobenzene (ODAB) in the cleft of host 2 at 295 K. The
solid line represents the best fit.
NMR spectra of polycrystalline sample of anti-2 (CNPY)2 in CDCl3
ꢂ
while trace C represents the 1H NMR of free CNPY ligand. The
10,20-meso protons show a downfield shift of 0.79 ppm, while
the 15-meso proton essentially does not show any noticeable
change upon interaction with axial ligand. This is because the
10,20-meso protons are strongly shielded by the ring current effect
of the two face-to-face porphyrin subunits in 2 and hence they are
mostly affected by the deshielding effect as the two porphyrin core
move apart during syn to anti conformational switching. There is
also an upfield shift of 0.16 ppm for the bridging –CH2 proton sig-
nals. The –CH protons of free CNPY ligand are noticeably upfield
of 2, tweezer-2 ODAB and anti-2 (ODAB)2, respectively, and [L1]
ꢂ
ꢂ
is the concentration of guest added.
ꢀ
ꢁ.ꢀ
ꢁ
2
2
A ¼ A0 þ K1½L1ꢃA1 þ K1K2½L1ꢃ A
1 þ K1½L1ꢃ þ K1K2½L1ꢃ
ð1Þ
1
When absorbance (A) is plotted against concentration of [L1]
(M), K1 and K2 are obtained by nonlinear curve-fitting as shown
in Fig. 10 and the binding constant values are found to be
1.82 ꢀ 103Mꢁ1 and 1.34 ꢀ 102 Mꢁ1, respectively.
shifted (Dd: 1.86 and 0.73 ppm) and this is because they are af-
fected by the ring current of the porphyrin rings. The eight well
resolved signals of –CH2CH3 protons (4.33–2.36 ppm) of 2 also
show a downfield shift (4.24–3.08 ppm).
The binding constant of 1,4-diaminobenzene (PDAB) and
4-CN-pyridine (CNPY) with 2, that generates anti-2 (PDAB)2 and
ꢂ
anti-2 (CNPY)2, respectively, were also determined by UV–Vis
ꢂ
spectroscopic titration method. Binding constants (K) were deter-
mined (Fig. 11) by nonlinear curve fitting of absorbance at
424 nm for single step 1:2 complexation by using Benesi–Hilde-
brand equation [42] and the values are 48 Mꢁ2 and 41 Mꢁ2, respec-
tively for PDAB and CNPY ligands.
3.4. Binding constant determination
The experimental points at low and high ligand (ODAB) concen-
tration regions are well-fitted to the set of theoretical equations for
the two-step binding model [41]. This model contains two major
steps for the total equilibrium shown in Scheme 1, the first ligation
of ODAB (with binding constant K1) resulting in incorporation of
As one can see, the K1 value in tweezer-2 ODAB is nearly thir-
ꢂ
teen-times larger than that of K2 reflecting the remarkable stability
of the tweezer-2 ODAB structure over the anti-2 (ODAB)2 form
ꢂ
ꢂ
mostly due to the chelate effect in the former. It is important to
note that besides binding affinity, the ligand preorganization also
plays an important role in the complexation process, which can
be illustrated with the difference in binding properties between
R,R-DACH [DACH: 1,2-diaminocyclohexane] and S,S-DACH on one
hand and cis-DACH on the other hand [22]. Two oppositely orien-
tated amino groups in R,R-DACH and S,S-DACH result in exclusive
formation of the tweezer structure with K1 = 1.25 ꢀ 107 Mꢁ1, while
location of the amino groups in cis-DACH on the same side of cyclo-
hexyl ring considerably reduces the K1 value by 1000 times and al-
lows the second equilibrium step to the anti structure formation to
take place [22]. In the present tweezer complex, however, the K1
value is remarkably less by nearly 104 times when aromatic dia-
mine such as 1,2-diaminobenzene is used to form 1:1 tweezer
the ligand between two porphyrin rings to yield tweezer-2 ODAB
ꢂ
and the subsequent second ligation (with binding constant K2)
inducing conformational switching to give anti-2 (ODAB)2. The
ꢂ
binding constants between Zn-bisporphyrin 2, and ODAB are
determined by UV–Vis spectroscopic titration method. The addi-
tion of ODAB (10ꢁ6 to 10ꢁ2 M) to chloroform solution of 2
(10ꢁ5 M) at room temperature primarily results red shift of Soret
band (398 to 410 nm) and Q-band (553–557 nm) and then clear
splitting of Soret band (411 and 422 nm) was observed at higher
ligand concentration. The nonlinear least-square curve-fitting of
the absorption spectral data at 424 nm for 1:1 and 1:2 complexa-
tion at low and high concentration of axial ligand was obtained
by applying Eq. (1), [41] in which A0, A1 and A are absorbances
1

S. Brahma et al. / Inorganica Chimica Acta 372 (2011) 62–70
69
entering the molecule into interporphyrin cavity to form the twee-
zer structures, thus producing only the 1:2 anti-2 (PDAB)2 and
ꢂ
anti-2 (CNPY)2. Single crystal X-ray structures of both tweezer-
ꢂ
2 ODAB and anti-2 (CNPY)2 form stabilized in the same bispor-
ꢂ
ꢂ
phyrin ligand 1 are reported here, for the first time. The Zn
Zn
ꢂꢂꢂ
non bonding distances are 5.55 and 10.01 Å, respectively in tweezer
and anti form. Although the average Zn–N(p) distances are compa-
rable for both the form, Zn–N(L) distances are longer in tweezer
compared to anti conformation. Also, the displacement of Zn from
the mean porphyrin planes (24 atoms) are much less in anti com-
pared to the tweezer form.
Acknowledgments
We are thankful to the Department of Science and Technology,
Government of India (and CSIR, New Delhi) for financial support.
S.B. and S.A.I. thank CSIR, India for their fellowships. We also thank
Prof. P. Sen for some valuable advice in binding constant
determinations.
Appendix A. Supplementary material
ESI-MS spectra of tweezer-2 ODAB (Fig. S1) and anti-
ꢂ
2 (ODAB)2 (Fig. S2), UV–Vis spectra of anti-2 (CNPY)2 and anti-
ꢂ
ꢂ
2 (PDAB)2 (Fig. S3), a view of tweezer-2 ODAB highlighting 1,2-
ꢂ
ꢂ
diaminobenzene (Fig. S4), packing diagrams of tweezer-2 ODAB
ꢂ
(Fig. S5) and anti-2 (CNPY)2 (Fig. S6). CCDC 764422 and 764423
contain the supplementary crystallographic data for complexes
tweezer-2 ODAB and anti-2 (CNPY)2, respectively. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.01.101.
ꢂ
Fig. 11. (A) Normal plot, (B) linear-log plot for the determination of stability
constant of binding 1,4-diaminobenzene (PDAB) with 2 at 295 K. The solid lines are
the best fit.
ꢂ
ꢂ
complex. As observed in the X-ray structure, Zn–N(L) distances are
longer by ꢄ0.09 Å in tweezer-2 ODAB compared to tweezer-
ꢂ
References
2 DACH which also suggestive of relatively weaker binding of
ꢂ
ODAB. However, attractive
the guest ligand and host porphyrin also contributes towards the
p
–
p
and CH–
p
interactions between
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