First 'back-to-back' shaped compartmental ligand; structural characterization of a tetranuclear zinc(II) complex in a highly flattened form

Article abstract of DOI:10.1246/cl.2000.678

Schiff-base condensation of 3,3′,5,5′-tetraformyl-4,4′-biphenyldiol, which was converted from 4,4′-biphenyldiol by a modified Duff reaction, and N,N-diethylethylenediamine in the presence of Zn(CH₃CO₂)₂ followed by the addition of excess NH₄PF₆ gave a tetrazinc complex [Zn₄(L)(μ-CH₃CO₂)₄](PF₆)₂ (1), where L^2- is a new ligand bearing four Schiff-base and two phenoxo-bridging moieties; structure of 1 was fully characterized by X-ray crystallography.

Full text of DOI:10.1246/cl.2000.678

678
Chemistry Letters 2000
First 'Back-to-back' Shaped Compartmental Ligand; Structural Characterization of a
Tetranuclear Zinc(II) Complex in a Highly Flattened Form
Eiji Asato,* Mitsuyoshi Chinen, Atsushi Yoshino, Yoshiteru Sakata,† and Ken-ichi Sugiura†
Department of Chemistry, Biology and Marine Science, College of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213
†The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047
(Received March 8, 2000; CL-000230)
Schiff-base condensation of 3,3',5,5'-tetraformyl-4,4'-
biphenyldiol, which was converted from 4,4'-biphenyldiol by a
modified Duff reaction, and N,N-diethylethylenediamine in the
presence of Zn(CH3CO2)2 followed by the addition of excess
NH4PF6 gave a tetrazinc complex [Zn4(L)(µ-CH3CO2)4](PF6)2
(1), where L2- is a new ligand bearing four Schiff-base and two
phenoxo-bridging moieties; structure of 1 was fully character-
ized by X-ray crystallography.
Phenol-based ligands possessing functional coordinating
groups at the 2- and 6- positions, namely compartmental lig-
ands, are some of the most powerful tools for inorganic
chemists to create dinuclear metal cores. In addition to many
examples of these kinds of dinuclear metal complexes, recent
elaborated ligand synthesis has generated some multi-phenol-
based ligands such as large macrocycles1-4 into which several
metal ions (n > 3) can be incorporated. Similar chemical modi-
fication of coordinating groups onto the 3,3',5,5'-positions of
4,4'-biphenyldiol seems to be of great value, because 1) such
sists of a discrete dicationic complex [Zn4(L)(µ-CH3CO2)4]2+
phenol derivatives should have a high potential as an efficient
(Figure 1) and two PF6- counter anions. The asymmetric unit of
the cation is a dinuclear metal core formed by the phenoxo- and
two carboxylato bridges which has a distance of 3.2441(9) Å
for the Zn(1)-Zn(2) pair, and related to the other unit by a crys-
tallographically imposed center of symmetry located on the
middle point of the C(4)-C(4') bond. Thus, the tetra Schiff-base
ligand L2- has a very high planarity in its structure, character-
ized by the twisted angle of 0˚ between the two rings. The two
zinc ions in each dinuclear core have essentially the same coor-
dination geometry in slightly distorted trigonal-bipyramids
formed by the two N and three O donors where the O(1)-N(2)
and O(1)-N(4) atoms occupy the apical sites of Zn(1) and
Zn(2), respectively. Concerning the bond distances, the Zn-N
interaction (av. 2.096 Å) and the Zn-O bonds (av. 2.010 Å) fall
well within the range of distances observed for typical Zn(II)-
nitrogen and -oxygen coordination compounds with a trigonal-
bipyramidal configuration.4
Its charge balance consideration clearly indicates that the
potentially redox-active ligand is in a reduced form, i.e., depro-
tonated hydroquinone, because the metal ion is redox inactive
Zn(II) rejecting the possibility of electron transfer from the lig-
and to the metal ions. Thus, the highly flattened form causing a
significant π-conjugated character over the ligand should be
due to a crystallographical requirement, not to the contribution
by the quinoid structure. Although the cyclic voltammogram of
1 in acetonitrile showed no clear oxidation process up to 2.0 V,
the hydroquinone form is strongly supported by the following
four conclusive observations. First, the C(4)-C(4') distance of
1.512(4) Å is a typical value for a C-C single bond, which is
tetranucleating ligand providing two phenoxo-bridged dinuclear
metal cores at the two termini; and 2) the π-conjugated and
potentially redox-active property of the biphenyldiol unit make
it possible to study the electrochemical and magnetic interac-
tions between the terminal metal pairs. However, to the best of
our knowledge, no example of such tetranucleating ligands has
been reported thus far, being in contrast with the rapidly
increasing number of reports on dinuclear metal complexes
where deprotonated biphenyl- or oligophenyldiol units bridge
two metal ions in their termini.5,6 In this communication, we
wish to report the first example of the biphenyldiol-based
tetranucleating ligand categorized into a new type of compart-
mental ligand.
A recently reported modified Duff reaction7 was used for
the synthesis of the key compound in this study, i.e., 3,3',5,5'-
tetraformyl-4,4'-biphenyldiol (TFBD).8 Further attempts to
convert TFBD into tetra Schiff-base compounds were carried
out to prove its high potential as an effective precursor to a
variety of tetranucleating ligands. As a representative case, the
Schiff-base condensation of TFBD and N,N-diethylethylenedi-
amine (Et2en) in the presence of Zn(CH3CO2)2·2H2O with the
mole ratio of 1 : 4 : 4 in ethanol gave a dark brown solution.
The addition of excess NH4PF6 into the solution immediately
gave a yellow crystalline precipitate. Recrystallization of the
product from acetonitrile finally gave plate-like yellow crystals
9
formulated as [Zn4(L)(CH3CO2)4](PF6)2 (1) (ca. 80% yield
before recrystallization), where L2- is the tetra Schiff-base lig-
and shown in Scheme 1.
The X-Ray structural analysis10 of 1 revealed that it con-
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
679
bination and the expansion of the biphenyl unit to the longer π-
conjugated systems such as the terphenyl and tetraphenyl units.
This work was supported in part by a Grant-in-Aid for
Scientific Research on Priority Area (#11136242 "Metal-assem-
bled Complexes" to E.A.) from the Ministry of Education,
Science, Sports and Culture, Japan. We appreciate the analyti-
cal assistance provided by the Material Analysis Center of
ISIR, Osaka University.
References and Notes
1
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Gatteschi, J. C. Jeffery, J. A. McClererty, F. Totti, and M. S.
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2
3
4
5
6
more than 0.1 Å longer than that of the corresponding C=C
double bond for a biphenoquinone.12 Second, the C(1)-O(1) dis-
tance [1.296(7) Å] is quite normal for that of the metal-bridged
phenoxo moiety,1-4 but significantly longer (ca. 0.07 Å) than
that of the C=O double bond in a biphenoquinone.11 Third, the
1H-NMR chemical shift for the ring protons (7.79 ppm) showed
a downfield shift by ca. 0.5 ppm, compared with those for
biphenoquinones.12 Fourth, the UV-vis spectrum of 1 in ace-
tonitrile showed the absorption at λmax = 265 nm (π-π* of the
rings) and λmax = 394 nm (π-π * of the imines), the former of
which is close to that of biphenyldiol derivatives,12 but signifi-
cantly different from that of biphenoquinones.12
7
8
L. F. Lindon, G. V. Meehan, and N. Svenstrup, Synthesis, 1998, 1029.
4,4'-Biphenyldiol reacted with 10 equiv of hexamethylenete-
tramine (HMT) in absolute trifluoroacetic acid under Ar, and
refluxed for 7 days. The mixture was poured into 4 M HCl, and
precipitated yellow product was filtered, and then recrystalized
from hot DMSO, to give yellow micro-crystals in 65% yield
1
(Scheme 1). Selected data for TFBD: EI-mass, M+ = 296, H
NMR (DMSO-d6), δ10.31 (s, 4H, CHO), 8.24 (s, 4H, Ar-H), phe-
nol protons were not observed because of the exchanging with
H2O in the solvent used. Anal. Found: C, 63.35; H, 3.52%. Calcd
for C16H10O6·0.3H2O: C, 63.27; H, 3.52%. This compound was
previously prepared by a different multi-steps synthesis started
from 4,4'-biphenyldiol. (S. Taniguchi, The 37th Symposium of
Thermosetting Plastics, Tokyo, October 1987, Abstr., pp. 93–96).
Anal. Found: C, 39.13; H, 5.21; N, 7.58 %. Calcd for
C48H76N8O10Zn4P2F12: C, 39.04; H, 5.19; N, 7.59%. Selected
solution data: UV-Vis [MeCN; λmax (ε/dm3 mol-1 cm-1)]
The molar conductance (263 S cm2 mol-1) and the strongest
peak in the electrospray mass spectrum (M/z = 593) of 1 in ace-
tonitrile indicated that it behaves as a 1: 2 electrolyte and that
the dicationic structure seen in the solid is essentially main-
tained even in solutions. However, it is readily expected that the
π-conjugated character arising from the planarity of L2- should
be reduced to some extend by twisting between the two rings.
In fact, the imine protons were less influenced by the ring cur-
9
1
265(71,100), 394 (11,100 ). H NMR (CD3CN, 25 ˚C): δ 8.57 (s,
4H, imine-H), 7.79 (s, 4H, aryl), 3.78 (t, 8H, C=N-CH2CH2-NEt2),
2.89 (t, 8H, -C=N-CH2CH2-NEt2), 2.7–3.1 (br, 16H, NCH2CH3),
2.01 (s, 12H, CH3CO2), 1.01 (t, 24H, NCH2CH3).
10 Crystal data for C48H76N8O10Zn4P2F12 1: M = 1476.62, monoclin-
ic, a = 12.968(2), b = 15.156(1), c = 15.883(3) Å, b = 101.98(1)˚,
U = 3053.5(6) Å3, T = 190 K, space group P21/n (# 14), Z = 2,
µ(Mo-Kα) =17.01 cm-1, Of the 7224 which were collected, 6999
reflections were unique (Rint = 0.055). The structure was solved
by direct methods and refined using full-matrix least-squares pro-
cedures. All hydrogen atoms were located on the calculated posi-
tions. Refinement converged with R1 = 0.057 for 4046 data with I
> 2σ(I) , and R1 = 0.150 for all the data.
1
rent from the adjacent unit and appeared at 8.57 ppm in its H
NMR spectrum, which is close to 8.43 ppm of the correspon-
ding peak reported for a closely related dinuclear zinc(II) core
with a non-π-conjugated phenoxo-bridged system.4
In conclusion, the tetraformylation of 4,4'-biphenyldiol and
the subsequent Schiff-base condensation were proven to be an
effective way to obtain a new class of wire-like tetranuclear
metal complexes. Current efforts in our laboratory are directed
to variation in the chelation moieties, the ligand(s) metal com-
11 M. A. Khan, A. Osman, and D. G. Tuck, Acta Crystallogr., Sect.
C, 42, 1399(1986).
12 a) Y. Morita, A. Kashiwagi, and K. Nakasuji, J. Org. Chem., 62,
7464 (1997). b) W. J. Detroit and H. Hart, J. Am. Chem. Soc., 74,
5215 (1952).