Synthesis and structure of a dinuclear zinc(II) triple helix of an N,N-bis-bidentate Schiff base: New building blocks for the construction of helical structures

Article abstract of DOI:10.1039/a701669g

The formation of a novel helical zinc(II) complex of an N,N-bis-bidentate Schiff base is reported; the edge-to-face and face-to-face π...π interactions play an important role in the metal-assisted self-assembling process in solution.

Full text of DOI:10.1039/a701669g

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Synthesis and structure of a dinuclear zinc(ii) triple helix of an
N,N-bis-bidentate Schiff base: new building blocks for the construction of
helical structures
Noboru Yoshida* and Kazuhiko Ichikawa
Laboratory of Molecular Functional Chemistry, Division of Material Science, Graduate School of Environmental Earth Science,
Hokkaido University, Sapporo 060, Japan
The formation of a novel helical zinc(II) complex of an N,N-
bis-bidentate Schiff base is reported; the edge-to-face and
face-to-face p···p interactions play an important role in the
metal-assisted self-assembling process in solution.
Here we report the facile synthesis of helical zinc(ii)
complexes utilizing weak interactions in which CH···p and p···p
interactions of aromatic rings between two or three bridging
groups (X = –C6H4CH2C6H4– or –C6H4SO2C6H4–) having
high conformational freedom may be involved in the self-
assembling process.
Weak non-covalent interactions play an important role in the
kinetics of the molecular recognition by a-cyclodextrin in
aqueous solution.1 Contrary to our expectation, enthalpic rather
than entropic factors are the principal driving force to form a
regioselective host–guest complex1c–e in which weak inter-
actions such as London dispersion forces, dipole–dipole and
CH···p interactions2 operate. Recently, great interest has been
focused on metal-assisted self-assembly of helical3 and box-
like4 supramolecular complexes in which the stereochemical
preference of a labile metal centre is exploited.3
Another very important aspect of self-assembly investiga-
tions has been the development of the design at ligands which
induces cooperative behaviour via the weak interactions. From
this point of view, a new type of bis-bidentate ligand has been
developed (Scheme 1).5,6
Recently, we have focused on some semi-N,O-bidentate and
bis-N,O-bidentate Schiff-base ligands which can be electronic-
ally and configurationally controlled, leading to a systematic
study of the self-assembly process in solution.7 Thus, bis-N,N-
and bis-N,O-bidentate Schiff-base ligands, L11, L16 and L17,
have been designed (Scheme 2) to bind two separate metal ions
C(2)
C(3)
C(27)
C(4)
C(26)
C(1)
C(52)
C(28)
C(53)
C(54)
C(51)
N(1)
N(5)
C(5)
C(6)
C(29)
C(8)
N(9)
N(10)
C(55)
C(56)
C(30)
C(31)
Zn(1)
N(2)
C(62)
C(61)
N(6)
C(57)
C(7)
C(32)
C(9)
C(12)
C(58)
C(37)
C(60)
C(36)
C(33)
C(10)
C(59)
C(11)
C(35)
C(63)
C(64)
C(38)
C(40)
C(41)
C(42)
C(34)
C(69)
C(13)
C(14)
C(65)
C(39)
C(66)
C(68)
C(19)
C(15)
owing
to
the
fact
that
the
bridging
group
C(67)
C(18)
C(70)
(X = –C6H4CH2C6H4– or –C6H4SO2C6H4–) prevents the metal
ions from forming 1:1 four coordinate complexes.
C(44)
C(43)
C(16)
C(17)
N(11)
C(45)
N(3)
C(20)
N(7)
C(71)
C(46)
Zn(2)
C(47)
C(72)
X
N(8)
N(12)
N(4)
C(48)
C(21)
C(49)
C(73)
C(50)
Scheme 1 X = –CONHC*H(R)CH2C*H(R)NHCO– (ref. 5) or –(CH2)n–
(ref. 6a); Ë = strong donor atom, 2 = weak donor atom
C(75)
C(22)
C(25)
C(24)
C(74)
C(23)
N
HO
N
HO
N
Fig. 1 Molecular structure of the complex cation of 1. Hydrogen atoms have
been omitted for clarity. Selected bond lengths (Å) and angles (°): Zn(1)–
N(1) 2.158(6), Zn(1)–N(2) 2.195(5), Zn(1)–N(5) 2.186(7), Zn(1)–N(6)
2.193(6), Zn(1)–N(9) 2.154(6), Zn(1)–N(10) 2.176(6), Zn(2)–N(3)
2.174(6), Zn(2)–N(4) 2.132(7), Zn(2)–N(7) 2.247(6), Zn(2)–N(8) 2.167(6),
Zn(2)–N(11) 2.200(6), Zn(2)–N(12) 2.187(6); N(1)–Zn(1)–N(2) 76.7(2),
N(1)–Zn(1)–N(5) 91.3(3), N(1)–Zn(1)–N(6) 164.7(2), N(1)–Zn(1)–N(9)
98.4(2), N(1)–Zn(1)–N(10) 94.5(2), N(2)–Zn(1)–N(5) 87.6(2),
N(2)–Zn(1)–N(6) 93.5(2), N(2)–Zn(1)–N(9) 173.6(2), N(2)–Zn(1)–N(10)
99.9(2), N(5)–Zn(1)–N(6) 76.5(2), N(5)–Zn(1)–N(9) 96.7(2), N(5)–Zn(1)–
N(10) 171.4(2), N(6)–Zn(1)–N(9) 92.1(2), N(6)–Zn(1)–N(10) 98.7(2),
N(9)–Zn(1)–N(10) 76.2(2), N(3)–Zn(2)–N(4) 77.3(3), N(3)–Zn(2)–N(7)
96.0(2), N(3)–Zn(2)–N(8) 170.1(2), N(3)–Zn(2)–N(11) 100.6(2), N(3)–
Zn(2)–N(12) 97.9(2), N(4)–Zn(2)–N(7) 93.2(2), N(4)–Zn(2)–N(8) 97.1(3),
N(4)–Zn(2)–N(11) 173.3(2), N(4)–Zn(2)–N(12) 98.4(2), N(7)–Zn(2)–N(8)
76.1(2), N(7)–Zn(2)–N(11) 93.4(2), N(7)–Zn(2)–N(12) 163.6(2),
N(8)–Zn(2)–N(11) 85.8(2), N(8)–Zn(2)–N(12) 90.9(2), N(11)–Zn(2)–
N(12) 75.4(2).
N
CH
CH
CH
4
4
4
H
H
H
H
O
O
S
4′
4′
4′
N
CH
N
N
CH
N
CH
HO
HO
L17
L16
Scheme 2
L11
Chem. Commun., 1997
1091

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Reactions of L16 and L11 with Zn(MeCO2)2·2H2O in 1:1
molar ratio in hot ethanol afforded a strongly fluorescent yellow
powder with many long fibres at the wall of the reaction vessel.
The melting point of the formed zinc(ii) complexes were
> 300 °C. They were insoluble in all organic solvents.
Elemental analysis indicated formation of 1:1 polymeric
complexes. Ligand L17 was synthesized by Schiff-base conden-
sation8 of bis(4-aminophenyl)methane and pyridine-2-aldehyde
and reaction with Zn(ClO4)2·6H2O in methanol at room temp.
gave a pale yellow crystalline powder (yield ca. 60%).† This
was recrystallized from dmf–MeCN (1:2)–diethyl ether to give
crystals of 1·2MeCN·dmf; MeCN was indispensable for
crystallization. Elemental analysis and FAB mass spectroscopy
were consistent with the empirical formula of a 2:3 complex
[Zn2L173][ClO4]4·dmf·2MeCN.
The structure of 1 shown in Fig. 1‡ confirms the existence of
a dinuclear triple helix. The complex contains two ZnII ions and
three L17 ligands. The building units, particularly the bridging
groups (–C6H4CH2C6H4–), are stacked face-to-face (p···p) and
edge-to-face (CH···p) to each other. Formation of a helix arises
from a twisting of these bridging groups. Each ZnII ion is six-
coordinate with Zn–N(pyridyl) distances in the range
2.132(7)–2.187(6) Å and Zn–N(–CHNN) 2.174(6)–2.247(6) Å.
There are three sets of bond angles N–Zn–N in the range
75.4(2)–77.3(3)°, 85.8(2)–100.6(2), and 163.6(2)–173.6(2)°,
respectively. A pseudo-octahedral array of nitrogen atoms
provided by three pyridylazomethine (C5H4NCHNN–) moieties
are in fac configuration. The pyridyl nitrogen atom as a stronger
donor of one ligand is situated trans to the weaker azomethine
(–CHNN–) nitrogen atom of another ligand (Fig. 1 and
Scheme 1).8
RAXIS IV imaging plate area detector with graphite monochromated Mo-
Ka radiation.
Crystal data: 1·dmf·2MeCN: C82H73Cl4N15O17Zn2; approximate dimen-
sions 0.30 3 0.10 3 0.05 mm, M
= 1813.14, pale yellow crystal,
monoclinic, space group C2/c (no. 15), a = 54.89(2), b = 13.822(2),
c = 22.01(1) Å, b = 95.40(3)°, U = 16639.9004 Å3, Z = 8, Dc = 1.447
g cm23, m(Cu-Ka) = 7.82 cm21, F(000) = 7472.00. Data were collected
on a Rigaku RAXISII diffractometer, 10268 reflections, 6756 observed [I >
3s(I)]. The structure was solved by direct methods, and expanded using
Fourier techniques. The final cycle of full-matrix least-squares refinements
was based on 65756 observed reflections [I > 3s(I)] and 1081 variable
parameters and converged at R = 0.061, Rw = 0.084, Maximum, minimum
residual electron density: +0.80, 20.61 e Å23. All calculations were
performed using the teXsan crystallographic software package of the
Molecular Structure coorporation. Atomic coordinates, bond lengths and
angles, and thermal parameters have been deposited at the Cambridge
Crystallograpic Data Centre (CCDC). See Information for Authors, Issue
No. 1. Any request to the CCDC for this material should quote the full
literature citation and the reference number 182/452.
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The new ligand system in this study is notable with regard to
the synthetic advantage that the use of the CNN bond9 allows a
wide range of different supramolecular structures. This provides
a basis for modelling many structural aspects and the utilization
of the weak interactions in metallosupramolecular chemistry.
Footnotes
* E-mail: nyoshida@high.hokudai.ac.jp
† CAUTION. Perchlorate salts of metal complexes with organic ligands are
potentially explosive. Only small amounts of material should be prepared,
and handled with care.
Received in Cambridge, UK, 10th March 1997; Com.
7/01669G
‡ Since the crystals were sensitive to moisture in the air, all measurements
were conducted with the sample sealed in a glass capilliary using a Rigaku
1092
Chem. Commun., 1997