Self-assembly of a helical dicopper(I) metallophane

Article abstract of DOI:10.1039/a802709i

X-Ray crystallographic and ¹H NMR spectroscopic studies show that, in the presence of copper(i), an oligopyridyl pyrazine derivative spontaneously forms a single chiral, cyclophane-like dimeric complex and that this is the sole species present in solution and the solid state.

Full text of DOI:10.1039/a802709i

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Self-assembly of a helical dicopper(i) metallophane
Thomas Bark,a Thomas Weyhermu¨llerb and Fenton Heirtzler*a†
a Departement Chemie der Universita¨t, Spitalstrasse 51, CH-4056, Basel Switzerland
b Max-Planck-Institut fu¨r Strahlenchemie, Stiftstrasse 34-36, D-45470 Mu¨lheim, Germany
1
X-Ray crystallographic and H NMR spectroscopic studies
show that, in the presence of copper(I), an oligopyridyl
pyrazine derivative spontaneously forms a single chiral,
cyclophane-like dimeric complex and that this is the sole
species present in solution and the solid state.
potassium cyanide afforded a mixture of the enediol 4 and
1,2-bis(2A-pyridyl)-1,2-dihydroxyethene. This mixture was di-
rectly oxidized with iodine to the corresponding a-diketones,
from which the desired product 5 was purified in 14% overall
yield. Condensation of 5 with 1,2-diaminoethane, then chlor-
anil-oxidation, afforded 2 in 64% yield.‡
The self-assembly1,2 of appropriate ligands and labile metal
centers to result in double-stranded helicates1a,c–e and cyclo-
phane-like structures2 is well documented. Either linear1c–e or
knotted 1a shapes are typical for the former, and a distinguishing
characteristic of the latter is frequently the coplanar orientation
of aromatic spacer units. In some of these ‘metallophanes’, this
results in a cavity, whilst in others, the non-bonding distances
between the spacers are within those values considered to be
crucial for p-stacking interactions3 (cf. [2.2]paracyclophane4).
In several of the latter cases, the spacer directly participates in
metal binding.2a,b
Treatment of 2 with 1 equiv. of [Cu(MeCN)4][BF4] in
methanol under reflux and addition of an excess of [NH4][BF4]
resulted in the precipitation of a dark red complex which could
be recrystallized from nitromethane–diethyl ether. This sub-
stance analysed as {[Cu2][BF4]}n‡ and in its FABMS spectrum
(noba matrix) prominent signals centered at m/z = 750 and 837
+
were observed, corresponding to [Cu2]2 and [Cu2]2[BF4]+,
respectively, and thus we assume a dimeric structure [Cu2]2-
[BF4]2. In its electronic spectrum in acetonitrile, metal–ligand
charge-transfer bands centered around 460 (1400) and 570 nm
(700 dm3 mol21 cm21) were visible, and suggest an
N4-environment for CuI.10 The cyclovoltammogram in MeCN
indicated a single, reversible CuI–CuII redox process at 20.16 V
vs. Fc–Fc+. In analogy to literature precedent,2b,10 we ascribe
this to a dimetallic complex containing two identical non-
interactive CuI centers.
We were, however uncertain of its exact structure, as both
meso-L,D and L, L/D,D-configured diasteromeric pairs were
reasonable structures. The 1H NMR spectrum of [Cu2]2[BF4]2
(400 MHz, CD3CN), recorded at 21 °C, displayed a single set of
broad resonances, integrating to 13 protons [Fig. 1(a)]. This
suggested either the ready interconversion of the three diastero-
mers, like in other dimeric oligopyridine complexes2d,f,g,6,11 or
some type of exchange process with the coordinating solvent, as
has been observed for other dicopper(i) bis-N4 systems2g,10
were occurring. Upon cooling to 240 °C [Fig. 1(b)], these
absorption sharpened to result in the profile of a single
compound, while at 40 °C, a broadening of the same shifts is
apparent. As well, the 13C NMR spectrum of the complex,
recorded at 240 °C, exhibited 13 C–H correlated resonances
displaying NOE enhancement. At no temperature could a de-
coalescence of signals in the 1H NMR spectra be observed, and
the chemical shifts were essentially temperature independent.
Since others have already demonstrated that low-temperature
While helicates are, by definition chiral,5 this is not so for
metallophanes. The majority of such complexes are achiral
(meso)2a–d,f although studies describing a chiral, dizinc(ii)
metallophane which is stable in dilute solution have also
recently appeared.2e Chiral metallophanes,whose formation is
templated by the inclusion of an aromatic guest molecule, have
also been recently described,2d,e as have equilibrating mixtures
of similar chiral and meso complexes.2d,g,6
We are interested in the preparation7 and supramolecular
complexation chemistry2a of 2,3-bis(2,2A-oligopyridyl)pyra-
zines. We anticipate that the influences of internitrogen
pyrazine base strength within the plane of that ring8a and
stacking effects perpendicular to it8b should together determine
the behaviour of this ligand class. Simpler pyrazine-containing
ligand systems are known to form cyclic trimeric or tetrameric
supramolecular complexes.8a,9
Along these lines, we have already shown that the symmet-
rical 2,3-bis(2,2A-bipyridyl)pyrazine 1 and CoII self-assemble to
1"
N
2'
N
N
2"
N
N
1"'
1'
2"'
2
N
N
1
N
N
OH
N
N
1
2
N
N
CHO
i
N
N
N
give a dimetallic meso-metallophane in which two roughly
orthogonal binding domains having different dendicity are
generated to geometrically satisfy the metal coordination
requirements.2a We wished to further test these premises on a
2,3-bis(2,2A-oligopyridyl)pyrazine derivative possessing oligo-
pyridyl groups of explicitly unlike dendicity. The simplest such
ligand is the previously unknown 2-(2,2A-bipyridyl)-3-(2-pyr-
idyl)pyrazine 2. Either tridentate/monodentate or bidentate/
bidentate binding modes are conceivable for 2, and thus we
anticipated that it should bind cooperatively with tetrahedral
CuI.
OH
3
4
ii
O
iii, iv
N
2
N
N
O
5
Scheme 1 Preparation of ligand 2. Reagents and conditions: i, 15 equiv.
pyridine-2-carbaldehyde, KCN, EtOH–H2O, reflux, 2 h; ii, 1 equiv. I2,
CH2Cl2, 25 °C, 15 h; iii, 1 equiv. 1,2-diaminoethane, EtOH, reflux, 2 h; iv,
1 equiv. chloranil, xylenes, reflux, 16 h.
A crude, but reasonable synthesis of 2 is described in Scheme
1. Thus, condensation of 2,2A-bipyridine-6-carbaldehyde 37
with an excess of pyridine-2-carbaldehyde in the presence of
Chem. Commun., 1998
1475

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We are currently investigating the extent which stacking
interactions control self-assembly in related substances.
We thank the companies Novartis Ag, Biosynth AG and Shell
Oil for chemical donations, the Treubel Fond for a stipend
(F. H.) and Professors E. C. Constable and K. Wieghardt for
encouraging this work.
(a)
Notes and References
† E-mail: heirtzler@ubaclu.unibas.ch
‡ Correct spectral and analytical data (C, H, N) were obtained for 2 and 5.
1
Spectral data for [Cu2]2[BF4]2: H NMR (400 MHz, CD3CN, 240 °C), d
(b)
8.72 (dd, J 1.0, 8.2 Hz, 1 H, H-3A/5A), 8.63 (d, J 2.7 Hz, 1 H, H-5/6), 8.57 (t,
J 7.8 Hz, 1 H, H-4A), 8.56 (d, J 8.2 Hz, 1 H, H-3B), 8.31 (dd, J 0.7, 7.6 Hz,
1 H,H-5A/3A), 8.14–8.19 (m, 2 H, H-4B, H-5B/6B), 8.09 (d, J 2.4 Hz, 1 H,
H-6/5), 7.53–7.58 (m, 3 H, H-6B/5B, H-4Ú,), 7.48 (dt, J 1.2, 4.8 Hz, 1 H,
H-6Ú), 7.21 (ddd, J 1.0, 5.3, 8.2 Hz, 1 H, H-5Ú), 7.15 (d, J 8.2 Hz, 1 H,
H-3Ú); 13C NMR (100 MHz, CD3CN 240 °C), d 149.35 (2C), 146.46,
141.96 (2C), 139.63, 137.79, 129.29, 127.68 (2C), 125.74, 124.18, 123.42.
Anal. Calc. for C38H26B2Cu2F8N10: C, 48.8; H, 2.97; N, 15.3. Found: C,
49.4; H, 2.84; N, 15.2%.
§ Crystallographic data for [Cu2]2[BF4]2, C38H26B2Cu2F8N10, Mr
=
923.39, dark red blocks, 0,43 3 0.28 3 0.25 mm, monoclinic, space group
C2/c, a = 13.271(2), b = 11.368(2), c = 24.200(4) Å, b = 95.72(2)°, U =
3632.7(10) Å3, Z = 4, Dc = 1.688 Mg m23, m = 1.259 mm21, F(000) =
1856, graphite monochromated radiation with l(Mo-Ka) = 0.710 73 Å, T
= 100(2) K, 17 678 reflections measured (1.69 < q < 30.00°) of which
5262 were independent (Rint = 0.0241), collected on a Siemens SMART
diffractometer with CCD detector taking frames at 0.3° in w. Data corrected
for Lorentz and polarization effects, absorption correction using SA-
DABS11 (min., max. transmission factors: 0,572, 0.832) and structure
solution and refinement on F2 using Siemens ShelXTL-V5. All non-
hydrogen atoms refined anisotropically, hydrogen atoms placed at calcu-
lated positions and refined isotropically. R1 = 0.0426, wR2 = 0.1041,
9.0
8.5
8.0
d
7.5
7.0
Fig. 1 1H NMR spectra of [Cu2]2[BF4]2 at (a) 21 and (b) 240 °C
1H NMR spectroscopy can distinguish equilibrating mixtures of
diastereomeric metallophanes2g it is evident that [Cu2]2[BF4]2
undergoes no such phenomenon.
In order to determine the stereochemistry of the dinuclear
complex, its crystal structure was determined.§ Complex
[Cu2]2[BF4]2 crystallizes in a centrosymmetric space group.
Consequently, the dication occurs as a racemic mixture of L,L-
and D,D-configured enantiomers, whereby the equivalent Cu2
fragments are inter-related by the C2 axis which runs parallel to
the pyridyl pyrazine surfaces and between the bipyridyl flanks.¶
The P-helical enantiomer is displayed in Fig. 2. The pyridyl
pyrazine copper(i) ‘decks’ of the metallophane are arranged in
a head-to-head fashion. Interdeck non-bonding distances be-
tween closest pairs of atoms are 3.47–3.57 Å for the pyrazine
rings and 3.42–3.62 Å for the monosubstituted pyridine rings;
the pairs of pyrazine and pyridine rings are parallel to within
2.10 and 2.03°, respectively. The bipyridyl and pyridylpyrazine
binding domains are twisted by 72.3° with respect to one
another, giving the observed rectangular molecular geometry.
The intermetallic distance is 5.08 Å. All metal–ligand bonding
parameters are within expected values.
goodness-of-fit: 1.049 for 4353 reflections with I
> 2s(I) and 271
parameters. Residual positive, negative electron density: +1.34, 20.49 Å23
.
CCDC 182/883.
¶ The hypothetical meso-dicopper(i) metallophane is characterized by a
‘head-to-tail’ orientation of the pyridylpyrazine decks and an inversion axis
(S2) in roughly the same location as for the chiral form.
1 (a) J.-C. Chambron, C. Dietrich-Buchecker and J.-P. Sauvage, Top.
Curr. Chem., 1993, 165, 131; (b) P. J. Stang and B. Olenyuk, Acc. Chem.
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Tetrahedron, 1992, 48, 10 013.
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K. Y. Wild, Angew. Chem., Int. Ed. Engl., 1998, 37, 114; (d) A. Bilyk,
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Harding, J. Chem. Soc., Dalton Trans., 1994, 77.
Ligand 2 diastereoselectively self-assembles to form a chiral
metallophane, which is also stable in solution. That this
phenomenon is influenced by stacking of metal-binding pyridyl
pyrazine fragments is suggested by molecular models of the
L,L/D,D-and meso-L,D-disastereomers, which indicate more
efficient overlap for the former compound.
3 C. A. Hunter, Chem. Soc. Rev., 1994, 23, 101.
4 H. Hope, Acta Crystallogr., Sect. B, 1972, 28, 1733.
5 See, however, M. Albrecht and C. Riether, Chem. Ber., 1996, 129, 829
and references therein.
6 C. O. Dietrich-Buchecker, J.-F. Nierengarten, J.-P. Sauvage, N.
Armaroli, V. Balzani and L. De Cola, J. Am. Chem. Soc., 1993, 115,
11 237.
C(6′″)
N(1′″)
C(2)
N(1)
C(2′)
7 F. R. Heirtzler, M. Neuburger, M. Zehnder and E. C. Constable, Liebigs
Ann./Recueil, 1997, 297.
N(1′)
C(2″)
8 (a) R.-D. Schnebeck, L. Randaccio, E. Zangrando and B. Lippert,
Angew. Chem., Int. Ed. Engl., 1998, 37, 119; (b) L. Carlucci, G. Ciani,
D. M. Proserpio and A. Sironi, J. Am. Chem. Soc., 1995, 117, 4562.
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1993, 32, 1607; (b) R. V. Stone, J. P. Hupp, C. L. Stern and T. E.
Albrecht-Schmitt, Inorg. Chem., 1996, 35, 4096.
Cu
N(1″)
10 E. C. Constable, M. J. Hannon, A. J. Edwards and P. R. Raithby,
J. Chem. Soc., Dalton Trans., 1994, 2669.
11 G. Sheldrick, University of Go¨ttingen, 1994.
Fig. 2 Crystal structure of the [Cu2]2-dication. Selected bond angles (°) and
lengths (Å): N(4)–Cu–N(1B) 138.46(7), N(1B)–Cu–N(1A) 82.17(7), N(1B)–
Cu–N(1BA) 114.39, N(4)–Cu–N(1A) 120.80(7), N(4)–Cu–N(1BA) 80.91(7),
N(1A)–Cu–N(1BA) 126.72(7); Cu–N(4) 1.991(2), Cu–N(1A) 2.033(2), Cu–
N(1B) 2.005(2), Cu–N(1BA) 2.038(2).
Received in Basel, Switzerland, 9th April 1998; 8/02709I
1476
Chem. Commun., 1998