Homologous size-extension of hybrid vanadate capsules - Solid state structures, solution stability and surface deposition

Article abstract of DOI:10.1039/c3cc48541b

The dimensions and cavity sizes of the molecular capsules with the general formula [V₁₀O₁₈L₄]^10- can be controlled modularly through the nature of the bifunctional, rigid organophosphonate ligands L¹ and L² (L¹ = bis(4-phosphonatophenyl)ethyne and L² = bis(4-phosphonatophenyl) butadiyne); the solution stability of the molecular entities as demonstrated by ESI-MS studies permits their assembly on the Au(111) surface on a sub-monolayer scale giving rise to a 2D supramolecular structure that is comparable to the packing arrangements of the capsules in the crystal structures. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3cc48541b

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Homologous size-extension of hybrid vanadate
capsules – solid state structures, solution stability
and surface deposition†
Mariyatra B. Mahimaidoss,a Sergey A. Krasnikov,b Lukas Reck,a Camelia I. Onet,a
John M. Breen,a Nianyong Zhu,a Bartosz Marzec,a Igor V. Shvetsb and
Wolfgang Schmitt*a
Cite this:DOI: 10.1039/c3cc48541b
Received 8th November 2013,
Accepted 11th December 2013
DOI: 10.1039/c3cc48541b
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The dimensions and cavity sizes of the molecular capsules with the bifunctional, rigid organophosphonate ligands L1 and L2 (Fig. 1). We
general formula [V10O18L4]10ꢀ can be controlled modularly through the highlight the solution stability of this class of compounds and demon-
nature of the bifunctional, rigid organophosphonate ligands L1 and L2 strate that individual cluster entities can be deposited on the Au(111)
(L1 = bis(4-phosphonatophenyl)ethyne and L2 = bis(4-phosphonato- surface and imaged using a scanning tunneling microscope (STM).
phenyl)butadiyne); the solution stability of the molecular entities as
Polyoxometalates constitute an interesting class of compounds for
demonstrated by ESI-MS studies permits their assembly on the Au(111) the preparation of cages.6 The chemistry of polyoxovanadates in
surface on a sub-monolayer scale giving rise to a 2D supramolecular aqueous reaction systems as developed by Mu¨ller, Zubieta and others
structure that is comparable to the packing arrangements of the capsules is characterised by condensation reactions whereby VIV and to a certain
in the crystal structures.
extent VV species in predominantly square pyramidal coordination
environments aggregate to form oligonuclear and polynuclear clusters.7
The geometrical restraints associated with the square pyramidal
The synthesis and characterisation of metallo-supramolecular cages
and molecular capsules have received significant scientific attention.1,2 {OQVO4} building unit often lead to convex oligonuclear species
In the last few decades preparative efforts were directed towards the
synthesis of new cage topologies or homologous species with con-
trollable inner cavities whose chemical, geometrical and electronic
attributes may give rise to unique properties that can be exploited in
homogeneous catalytic processes, size- and shape-restrictive chemical
transformations, separations, drug delivery and sensing devices.2,3
Importantly, for the majority of these and other advanced applica-
tions, the stability of the assembled molecular species in solution is a
key requirement. The development of homologous hollow molecular
entities with controllable outer and inner dimensions and chemical
attributes relies on the identification of geometrically suitable,
kinetically-stable building units with ligand-accessible coordination
sites allowing the assembly of the cage entities.4 Previously we
reported tetranuclear and pentanuclear organophosphonate and
-arsonate stabilised vanadate species whose assembly into molecular
capsules and cages can be directed through the use of suitable organic
ligands or anion-templates.5 Here we demonstrate that the dimen-
sions of the molecular capsules with the general formula [V10O18L4]10ꢀ
can modularly be controlled through the extent and the nature of the
a School of Chemistry & CRANN, University of Dublin, Trinity College, Dublin 2,
10ꢀ
Ireland. E-mail: schmittw@tcd.ie; Tel: +353-1-896-3495
Fig. 1 (a) Crystal structure of [V10O18L1
]
4
(L1
=
1,10-(1,2-ethynyl)-
10ꢀ
b School of Physics & CRANN, University of Dublin, Trinity College, Dublin 2, Ireland
† Electronic supplementary information (ESI) available: Synthetic details, addi-
tional figures, ESI-MS analysis, crystallographic table and data in CIF format.
CCDC 970548 and 970549. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3cc48541b
diphenyl-4,40-bisphosphonate); (b) crystal structure of [V10O18L2
]
4
(L2 = 1,10-(1,2,3,4-diethynyl)diphenyl-4,40-bisphosphonate); (c) structure
of the mixed-valent {V5O9} building unit that is stabilised by four fully
deprotonated organophosphonate moieties; (d) space filling models of 1
and 2 [VV light green, VIV darker green, P pink, C grey, O red].
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Fig. 2 (I–V) Size comparison of [V10O18L4]10ꢀ complexes containing benzenediphosphonate (I), naphthalene diphosphonate (II), biphenyl diphosphonate (III),
10ꢀ
highlighting that the acetylene-based ligands give rise to small openings.
L1 (IV) and L2 (V) as stabilising ligands. Right: space filling model for [V10O18L2
]
4
that further condense to form hollow metallo-supramolecular assem- edge dimensions of 13.202(2) ꢂ 5.568(1) Å2 and 15.776(6) ꢂ
blies, in which anions and other nucleophiles may act as templating 5.544(1) Å2. For both cages, the acetylene moieties bend significantly
agents giving rise to distinct topologies.8 It has been shown that outwards whereby the average inter-planar distances between parallel
these assembly principles apply to pure inorganic vanadates as well aligned, opposite located phenyl rings are ca. 8.5 Å. At their widest part
as hybrid systems that contain organophosphonate and -arsonate both capsular entities have a radial diameter of ca. 8.8 Å. In contrast to
ligands.7,9 Of particular interest is a mixed-valent {V5O9} unit that the smaller homologues, the incorporation of acetylene moieties now
forms upon partial reduction of VV salts in the presence of suitable gives rise to openings at the organic side-walls of the capsules (Fig. 2).
organic ligands. In this unit a central VV ion is linked via four m3-O2ꢀ The openings in 2 have dimensions of 6.7 ꢂ 6.1 Å2 thus demonstrating
ligands to four VIV centres to form a convex square unit in which the that it is feasible to design entities that selectively incorporate or release
central square pyramid connects to each of the peripheral pyramids small guests. In their pristine form, 1 and 2 encapsulate two stabilising
through a shared basal edge. The peripheral pyramids provide azide ions that may act as templates during the formation of the
ligand-accessible coordination sites for four phosphonate or convex capping units (closest V–N distances 3.02 and 3.06 Å for 1 and
-arsonate functionalities that adopt O,O0-syn,syn-bidentate bridging 2, respectively).5b,8 The capsules assemble in the crystal structures to
modes. The convex nature of the subunit allows the formation of give dense lamellar packing motifs in which hydrophilic inorganic and
capsular entities when rigid bifunctional organophosphonate hydrophobic organic areas are segregated. Partially hydrated sodium
ligands are employed in the reaction system.5a,d The stability of counterions bind to the terminal and bridging oxo ligands of the
the {V5O9} unit in aqueous systems not only allows the incorporation {V5O9} units and to O-donors of the stabilising phosphonate ligands.
of conventional benzene-, biphenyl- or naphthyl-bisphosphonates, These counterions connect the hydrophilic, inorganic ends of the
but it also permits the incorporation of significantly elongated molecular entities within the (001) plane; organophosphonate ligands
ligands such as bis(4-phosphonophenyl)ethyne (H4L1) and 1,10-bis- pillar between these planes.
(4-phosphonophenyl)butadiyne (H4L2), to form [V10O18L1 ]10ꢀ (1)
The results demonstrate that the capsular entities can
4
and [V10O18L2 ]10ꢀ (2), which crystallise phase-pure and in good systematically be extended (Fig. 2) and it appears plausible that
4
yields from the corresponding reaction systems. The two acetylene- even longer homologues of the rigid organophosphonate
based organophosphonate ligands were synthesised through a one- ligands can be incorporated. Limitations may arise from the
pot double Sonogashira coupling reaction (H4L1) and a combination poor solubility of extended ligands in the polar reaction mix-
of Sonogashira and Eglinton coupling reactions (H4L2) involving tures that are required to prepare the {V5O9} building unit.
diethyl-4-bromophenylphosphonate and ethynyltrimethylsilane.10
1
The stability of the capsular entities 1 and 2 in H2O was demon-
and 2 form in aqueous solution upon partial reduction of NaVO3 in strated by electrospray ionisation mass spectrometry (ESI-MS, Fig. 3).
the presence of the organophosphonates and N2H4. Despite small A set of ꢀ4 charged species corresponding to the decavanadate cages
crystal sizes and weak diffraction patterns the single crystal X-ray 1 and 2 could be identified in the m/z = 500–600 region of the spectra.
diffraction analysis provided a good structural understanding of the For 1, the signal observed at m/z = 544.3 corresponds to a
compounds to give the constitutional assignments of H6Na6- Na2H2[VV VIV6O18(O3PC14H8PO3)4]4ꢀ species. Additional signals at
4
[(N3)2*1]ꢁ42H2O and H6Na6[(N3)2*2]ꢁ40H2O.
m/z = 534.3, 538.6, 549.8 and 555.3 can be assigned to H8[VIV10O18-
The compounds crystallise in the tetragonal space group P4/mnc (O3PC14H8PO3)4]4ꢀ, NaH2[VV VIV5O18(O3PC14H8PO3)4]4ꢀ, Na3H[VV -
5
4
and their asymmetric units contain 1/8 of the capsular entities with VIV6O18(O3PC14H8PO3)4]4ꢀ and Na4[VV VIV6O18(O3PC14H8PO3)4]4ꢀ
4
the remainder generated through symmetry operations.
species, respectively. Similarly, in the mass spectrum of 2 (Fig. 3),
573.6 originates from Na3[VV VIV5O18-
The lengths of the capsular entities 1 and 2 as defined by the OꢁꢁꢁO the signal at m/z
=
5
distance of the terminal oxo ligands residing in the apical positions are (O3PC16H8PO3)4]4ꢀ species. Additional signals, comparable to those
22.371(2) Å and 24.836(9) Å for 1 and 2, respectively. Their inner cavities reported for 1, were also observed and assigned. The experimental
are characterised by square cuboid arrangements of the 8 P atoms with and modelled isotopic distributions of these anionic species are
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facilitate the uptake and release of small guest molecules giving rise to
catalytic activities or molecular recognition effects.2c The latter aspects
are the subject of our current investigations. The capsular entities are
stable in solution and adopt dense packing arrangements when
deposited on the Au(111) surface.
The authors thank the Science Foundation Ireland (SFI; 06/
RFP/CHE173 and 08/IN.1/I2047) for financial support.
Notes and references
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