Discrete Ag6L6 coordination nanotubular structures based on a T-shaped pyridyl diphosphine

Article abstract of DOI:10.1039/c0cc05235c

Ag₆L₆-type coordination nanotubular structures have been assembled from 6 Ag(i) ions and 6 T-shaped ligands, 4-(3,5- bis(diphenylphosphino)phenyl)pyridine; the nanotubes represent a discrete molecular architecture of a number of poly

Full text of DOI:10.1039/c0cc05235c

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Discrete Ag6L6 coordination nanotubular structures based on a T-shaped
pyridyl diphosphinew
Xiaobing Wang,a Jing Huang,a Shenglin Xiang,a Yu Liu,a Jianyong Zhang,*a
b
b
c
a
¨
Andreas Eichhofer, Dieter Fenske, Shi Bai and Cheng-Yong Su*
Received 28th November 2010, Accepted 28th January 2011
DOI: 10.1039/c0cc05235c
Ag6L6-type coordination nanotubular structures have been
assembled from Ag(I) ions and T-shaped ligands,
revealing some degree of coordination of the ligand to silver
in solution and some dynamic behaviour. As the L : AgBF4
ratio approaches 1 : 1, the 31P NMR spectrum of the mixture
reveals a sharp doublet of doublets centred at 9.84 ppm in
CDCl3–MeNO2 (v : v = 3 : 1), revealing the coupling of the
separate isotopes of 107Ag and 109Ag with 1J(109Ag–31P) = 311 Hz.
The upfield shift of ca. 16.2 ppm of the phosphorus peak in the
product from the free ligand, L, indicated clearly the formation
of a Ag–P coordination bond and the formation of a single
isomeric product. Discrete Ag6L6 architectures with other
anions such as SbF6ꢀ, PF6ꢀ and ClO4ꢀ could also be obtained.
These analogous complexes gave essentially similar 31P NMR
spectra consisting of doublet of doublets. That of Ag6L6–SbF6
is centred at 14.84 ppm with 1J(109Ag–31P) = 333 Hz in
CD2Cl2–CD3NO2 (v: v = 3 : 1) (Fig. 1), that of Ag6L6–PF6
centred at 9.33 ppm with 1J(109Ag–31P) = 408 Hz in
CDCl3–CD3CN (v : v = 2 : 1) and that of Ag6L6–ClO4 centred
at 15.68 ppm with 1J(109Ag–31P) = 408 Hz in CDCl3–CD3CN
(v : v = 2 : 1), which all have comparable coupling constants
with Ag6L6–BF4. 1H NMR study of Ag6L6–SbF6 indicates the
coordinative interaction between Ag(I) and the pyridyl N
atoms of L. Upon addition of Ag(I) salt, the signals of pyridyl
protons (Ha and Hb) are shifted upfield to 7.98 and 6.49 ppm,
respectively. The 1H NMR spectrum is also indicative of the
highly symmetrical arrangement of the ligands in solution and
in the complex, all six ligands are magnetically equivalent. The
6
6
4-(3,5-bis(diphenylphosphino)phenyl)pyridine; the nanotubes
represent a discrete molecular architecture of a number of
polymeric structures assembled from dimeric building blocks.
Inorganic nanotubes and organic nanotubular molecular
structures have been well-developed due to their promising
applications as one-dimensional (1D) molecular containers,
such as chemical sensors, gas absorption, molecular separation,
and catalysis.1,2 In contrast, discrete coordination tubular
structures remain rather uncommon even if self-assembly of
other discrete coordination aggregates such as cycles,3 cages,4
bowls,5 cubes,6 and prisms,7 have been explored in recent
years. Among a handful of examples of discrete coordination
tubular structures, pyridine8 or imidazole9-based molecular
strands have been employed to endow the structures with
interesting properties, e.g. tunable size.10 Herein we report
self-assembly of the first phosphine-based11 discrete M6L6
coordination tube with Ag(I) ions (L = 4-(3,5-bis(diphenyl-
phosphino)phenyl)pyridine) (Scheme 1).
The T-shaped pyridyl diphosphine, L was designed and
synthesised from 4-(3,5-difluorophenyl)-pyridine, by reaction
with two equivalents of KPPh2 in THF. The 31P NMR spectra
showed a single resonance at ꢀ6.46 ppm. Reactions examined
with different ratios of Ag/L from 2 : 1 to 0.5 : 1 showed the
Ag/L ratio to play a key role in the tubular structure formation
(see supporting informationw). With an L/AgBF4 ratio of
1.5 : 1 or 0.5 : 1, RT 31P NMR spectra showed a doublet of
broad peaks, owing to the silver–phosphorus coupling,
1
assignment of the signals was achieved on the basis of H–1H
COSY and NOESY experiments (see supporting informa-
tionw). Interestingly, Ag6L6–SbF6 has two geometrically
different sets of PPh2 phenyl rings: twelve axial phenyl rings
(Phax) lying up and down the central C3 axis of the tube and
twelve equatorial phenyl rings (Pheq) lying vertical to the
a KLGHEI of Environmental and Energy Chemistry, MOE
Laboratory of Bioinorganic and Synthetic Chemistry, State Key
Laboratory of Optoelectronic Materials and Technologies, School of
Chemistry and Chemical Engineering, Sun Yat-Sen University,
Guangzhou 510275, China. E-mail: zhjyong@mail.sysu.edu.cn,
cesscy@mail.sysu.edu.cn
0
0
0
central axis. The signals of Pheq protons (He , Hf and Hg )
are shifted downfield to 7.67–7.50 ppm compared with
those of Phax ones (He, Hf and Hg) at 7.50–7.23 ppm. These
b Forschungszentrum Karlsruhe, Institut fur Nanotechnologie,
¨
Postfach 3640, 76021 Karlsruhe, Germany
c Department of Chemistry and Biochemistry, University of Delaware,
Newark, DE 19716-2502, USA
w Electronic supplementary information (ESI) available: crystallo-
graphic data and figures, experimental details, NMR, ESI-TOF MS
and IR spectra. CCDC 802912–802914. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0cc05235c
Scheme 1 Self-assembly of the L6Ag6 tubular structures.
Chem. Commun., 2011, 47, 3849–3851 3849
c
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absorption band at 660 cmꢀ1 and the L/AgSbF6 ratio of 1 : 1
was confirmed by microanalysis. In spite of easy efflorescence
of the crystals and severe disorder of solvent molecules, X-ray
structural analysis enabled us to obtain an acceptable struc-
tural model, a discrete Ag6L6 tubular structure (Fig. 2).zz The
discrete tubular architecture consists of six Ag+ and six
ligands. The ligands are connected by Ag atoms in a head-to-
tail arrangement. Each Ag+ is tricoordinate and bound by
two phosphorus donors and one pyridyl nitrogen atom of
three separate ligands. There exists six uniform AgP2N centres.
Two phenyl rings of each PPh2 group are lying in the axial and
equatorial position, respectively, consistent with the NMR
ꢀ
results. Two of six SbF6 counteranions are located in the
tubular cavity. The tubular structure is ca. 2.1 nm long.
To clarify the formation mechanism of the present tubular
structure, its ring-opening polymerisation (ROP) relationship
with other polymeric structures is studied. The ROP relationship,
established between metallacycles/cages and 1D chain or 2D
polymeric structures,15 not only has mechanistic significance,
but also may guide synthesis of new polymeric or discrete
structures. Potential ROP relationships exist between discrete
Ag6L6 tubular architectures and a number of T-shaped ligand-
based polymeric structures, such as 1D ladder, 2D (4,82) and
3D (4,122) networks (Fig. 3).16,17 All these structures contain
dimeric L2M2 subunits as a 4-connecting node which can
be considered to be secondary building blocks. This point
is verified experimentally by a signal of [L2Ag2(SbF6)]+
(m/z 1499.0) in the ESI-MS spectra of Ag6L6–SbF6. It suggests
that new structures may be synthesised based on the L2M2
subunits.18
Fig. 1 (a) 1H NMR of L, (b) 1H NMR and (c) 31P{1H} NMR of
Ag6L6–SbF6 in CD2Cl2–CD3NO2.
NMR results clearly indicate that only one isomer was quan-
titatively constructed in solution when L and Ag+ are mixed
in a ratio of 1 : 1.
ESI-TOF mass spectrometry for Ag6L6–SbF6 showed a
signal at m/z 2366.01, which is assigned to the doubly charged
2+ 12
species L6Ag6(SbF6)4
,
giving further evidence of the
formation of a L6Ag6 tubular structure in solution as no ion
of higher molecular mass was obtained.
Thus discrete architectures have been obtained in solution
ꢀ
whenꢀthe ratio of Ag : L is 1 : 1 with BF4ꢀ, ClO4ꢀ, PF6 or
SbF6 as counterions indicated by combined NMR and
ESI-MS results. Precluding the formation of a common dinuclear,
cyclic structural motif [Ag2(diphosphine)2]2+ (Scheme 1, X),13
possible candidate structures to meet the criteria of these
results are tubular structures with D3d symmetry (Ag6L6),
D4d (Ag8L8) or D5d (Ag10L10), etc. Since such tubular struc-
tures are highly positively-charged, anions are reasonably
residing in the cavity.4 An Ag6L6 structure with D3d symmetry
may be expected to form, because its cavity size is suitable for
these counterions. To prove the effect of anion size, the 31P
NMR spectrum of a solution of AgBPh4/L = 1 : 1 in
CDCl3–MeCN–MeOH was tested, which shows a broad peak
at ca. 8.5 ppm, suggesting the tubular structure is not formed
for bigger BPh4ꢀ. Thus, anions with suitable size are important
for the tube formation.
To achieve new L2Ag2-based structures, coordinating an-
ions (TFAꢀ or OTsꢀ) were introduced. 31P NMR spectra of
the CDCl3–MeCN solutions of L : AgCF3CO2(AgTFA) or
L : AgCH3PhSO3(AgOTs) = 1 : 1 indicate that the P–Ag bond
is labile, with the RT resonance showing no sign of Ag
coupling. Diffusion of Et2O vapour into the solution mixtures
of L and AgTFA or AgOTs yielded polymeric structures of
[Ag2L(CF3CO2)2(H2O)]n * (Et2O)n (Ag–TFA * Et2O) and
19F NMR and 31P NMR spectra of Ag6L6–BF4 and
Ag6L6–PF6 indicate that there is no chemical shift difference
between the free and encapsulated counteranions in solution.
19F NMR of the former shows the signals of BF4 at ca.
ꢀ
153 ppm, corresponding to 10B/11B–19F coupling from RT to
200 K. 31P NMR of the latter shows the characterised multi-
plet of PF6ꢀ at ꢀ13.57 ppm with 1J(31P–19F) = 955 Hz. These
behaviours may be well explained by the Ag6L6 tubular
structure. In contrast to cage structures,14 the tubes are open
at both ends, resulting in fast exchange between the free and
capsulated anions on the NMR timescales without structural
dissociation.
Crystals of Ag6L6–SbF6 were grown by layering of a MeOH
solution of AgSbF6 into a CHCl3 solution of L. The presence
of SbF6ꢀ in Ag6L6–SbF6 was confirmed by its characteristic IR
Fig. 2 X-ray structure of the nanotubular complex Ag6L6–SbF6 with
anions and hydrogen atoms omitted for clarity, (a,b) side views, (c) top
ꢀ
view, and (d) two SbF6 anions located in the cavity.
c
3850 Chem. Commun., 2011, 47, 3849–3851
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We acknowledge the FRFCU and NSFC (20821001,
20731005, U0934003 and 20903121) for financial support.
Notes and references
z Ag6L6–SbF6: C210H162Ag6F36N6P12Sb6, monoclinic, P21/n, a =
18.4879(6), b = 34.8299(11), c = 20.2362(10) A, b = 102.860(4)1,
V = 12703.9(9) A3, Z = 2, dc = 1.360 g cmꢀ3, R1 = 0.1116, wR2 =
0.2544 (obs. data) Ag–TFA: C43H39Ag2F6NO6P2, monoclinic, P21/c,
a = 11.3131(6), b = 21.5140(9), c = 18.1697(8) A, b = 90.950(5)1
V = 4421.7(4) A3, Z = 4, dc = 1.588 g cmꢀ3, R1 = 0.0766, wR2
=
0.2011 (obs. data) Ag–OTs: C50H42Ag2Cl3NO6P2S2, monoclinic,
P21/n, a = 10.6348(5), b = 24.7251(11), c = 19.4535(10) A, b =
=
Fig.
3 Potential 1D–3D ring-opening polymers of the tubular
103.775(5)1, V = 4968.1(4) A3, Z = 4, dc = 1.606 g cmꢀ3, R1
0.0550, wR2 = 0.1651 (obs. data)
architecture.
1 V. G. Organoab and D. M. Rudkevich, Chem. Commun., 2007,
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[Ag2L(OTs)2]n * (CHCl3)n (Ag–OTs * CHCl3), respectively.
X-ray single-crystal analyses were performed to reveal unam-
biguously that both Ag–TFA and Ag–OTs have 2D (4,82)-type
networks consisting of dimeric L2Ag4 secondary building
blocks (Fig. 4, see also supporting informationw).z In these
structures, the nodes are bridged dimeric Ag2 species. In other
words, the L2Ag4 subunits are doubly bridged by TFAꢀ and
water in Ag–TFA, while they are doubly bridged by OTsꢀ
anions in Ag–OTs. In all the tubular architecture and the
polymeric structures of Ag–TFA and Ag–OTs, a divergent
conformation is adopted among possible orientations of the
lone pairs for meta-diphosphine groups (Fig. 4).13
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form 2D polymeric structures under specific conditions. The
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In summary, an unprecedented phosphine-based M6L6
coordination tubular architecture has been quantitatively
assembled based on a T-shaped pyridyl diphosphine ligand
and tricoordinate Ag(I) ions. The tubular structures represent
a discrete molecular architecture of a number of polymeric
structures assembled from dimeric building blocks, which may
guide future syntheses of novel discrete and polymeric structures.
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Fig. 4 Formation of the tubular architectures and 2D networks, and
a divergent dimeric synthon formed by L and Ag+ according to the
orientations of the lone pairs for meta-diphosphine groups.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 3849–3851 3851