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Self-Assembly in Inorganic Chemistry Guest Editors Paul Kruger and Thorri Gunnlaugsson

Published in issue 45, 2011 of Dalton Transactions

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PERSPECTIVE: Metal ion directed self-assembly of sensors for ions, molecules and biomolecules Jim A. Thomas Dalton Trans., 2011, DOI: 10.1039/C1DT10876J ARTICLES: Self-assembly between dicarboxylate ions and a binuclear europium complex: formation of stable adducts and heterometallic lanthanide complexes James A. Tilney, Thomas Just Sørensen, Benjamin P. Burton-Pye and Stephen Faulkner Dalton Trans., 2011, DOI: 10.1039/C1DT11103E Structural and metallo selectivity in the assembly of [2 × 2] grid-type metallosupramolecular species: Mechanisms and kinetic control Artur R. Stefankiewicz, Jack Harrowfield, Augustin Madalan, Kari Rissanen, Alexandre N. Sobolev and Jean-Marie Lehn Dalton Trans., 2011, DOI: 10.1039/C1DT11226K Visit the Dalton Transactions website for more cutting-edge inorganic and organometallic research

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Self-assembled Co(II) molecular squares incorporating the bridging ligand4,7-phenanthrolino-5,6:5¢,6¢-pyrazine†

Feng Li,a Jack K. Clegg,a,b Deanna M. D’Alessandro,a Laurence Goux-Capes,a Natasha F. Sciortino,a

Tony D. Keenea and Cameron J. Kepert*a

Received 1st July 2011, Accepted 11th October 2011DOI: 10.1039/c1dt11254f

Three high-spin tetranuclear cobalt(II) complexes have been prepared with the bridging ligand4,7-phenanthrolino-5,6:5¢,6¢-pyrazine (ppz) through metal-ion directed self-assembly. The complexesdiffer by the incorporation of three different coordinating anions: chloride, thiocyanide andselenocyanide. The physical properties of these complexes have been investigated in detail.

Introduction

Currently, considerable attention is being paid to supramolec-ular architectures formed by metal-ion directed self-assemblydue to their novel structures, which include helicates,1,2 cages3,4

and interlocked structures.2,5 These supramolecular systems havepromising applications in host–guest chemistry4,6 and often dis-play interesting magnetic properties.7,8 Of particular appeal aremetallocycles, grids and squares as they represent comparativelysimple systems in which the physico-chemical properties canbe tuned. Although some such systems displaying interestingmagnetic properties – particularly those exhibiting multiple spinstates – have been developed,8,9 their design and synthesis remainsa significant challenge.10 For example, in a recent study wereported the self-assembly of a Fe4-molecular square with a hybrid-grid/metallocyclic architecture that contained both high- andlow-spin metal centres.8 Herein, we elaborate upon that studyto investigate the magnetic and electronic properties of threeCo4-molecular squares incorporating the bridging ligand 4,7-phenanthrolino-5,6:5¢,6¢-pyrazine (ppz) (Scheme 1).

4,7-phenanthrolino-5,6:5¢,6¢-pyrazine (ppz) is an aromatic sys-tem incorporating both pyrazine and pyridyl rings which is bothrigid and bisbidentate, and is closely related to the 1,4,5,8,9,12-hexaazatriphenylene (HAT) ligand. Both ppz and HAT have beenemployed in a number of investigations with transition metals.11,12

Experimental

Materials and procedures

The ligand ppz and all other chemicals and solvents used were ofcommercial grade and used without further purification. All oper-

aSchool of Chemistry, University of Sydney, 2006, Australia.E-mail: c.kepert@chem.usyd.edu.aubDepartment of Chemistry, University of Cambridge, Lensfield Road,Cambridge, CB2 1EW, United Kingdom† CCDC reference numbers 846868–846870. For crystallographic data inCIF or other electronic format see DOI: 10.1039/c1dt11254f

Scheme 1 Cartoon representation of the self-assembly of a tetranuclearCo(II) molecular square.

ations with the Co(II)-containing solutions were performed undera nitrogen atmosphere. UV-vis-NIR spectroscopy was performedon a Cary 5E Spectrophotometer equipped with a Harrick Omni-Diff probe. Spectra are reported as the Kubelka–Munk transform,F(R), as a function of wavelength (cm-1). IR spectra were recordedon a Varian 800 Fourier transform infrared spectrometer (asKBr pellets). Magnetic susceptibility data were collected using aQuantum Design Physical Property Measurement System PPMS5 instrument equipped with a Vibrating Sample Magnetometer(VSM). All measurements were performed on uniform crystallinesamples of ~10 mg under an applied field of 0.5 T.

Solid state electrochemical measurements were performed un-der argon using a Bioanalytical Systems BAS 100A Electrochemi-cal Analyser. Cyclic voltammograms (CVs) were recorded in 0.1 M[(n-C4H9)4N]PF6 dissolved in anhydrous CH3CN at 25 ◦C using aglassy carbon working electrode (1.5 mm diameter), a platinumwire auxiliary electrode and an Ag/Ag+ wire quasi referenceelectrode. Solid state samples were mounted on the glassy carbonworking electrode by dipping the electrode in the powder sampledispersed in an acetone slurry. Ferrocene was added as an internal

12388 | Dalton Trans., 2011, 40, 12388–12393 This journal is © The Royal Society of Chemistry 2011

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standard on completion of each experiment and all potentials arequoted in mV versus Fc+/Fc0. To confirm that no dissolution ofsolid samples was occurring, the material was removed from theelectrode surface on completion of each scan and a CV of the bareelectrode was performed. In each case, the absence of any redoxprocesses confirmed that no dissolution had occurred.

Syntheses

[Co4(ppz)4Cl8]·4H2O (1). A mixture of ppz (30 mg,0.129 mmol) and CoCl2·6H2O (31 mg, 0.129 mmol) in 10 mLwater–EtOH (3 : 2 = v/v) was stirred at 70 ◦C for 30 min.The solution was subsequently left overnight to produce X-rayquality orange needle-like crystals. The product was collectedby filtration and washed with EtOH, and then allowed to standin air prior to analysis. Yield: ª91%. UV-vis-NIR (solid state):9150 cm-1; IR (cm-1, KBr): 3355(br), 3082(m), 1622(w), 1498(m),1422(m), 1392(s), 810(m), 766(m); elemental analysis (%) calcd.for C56H32Cl8Co4N16·15H2O: C 39.14, H 3.64, N 13.04; Found: C38.97, H 3.61, N 12.92.

[Co4(ppz)4(SCN)7(H2O)]·ClO4·4.5H2O·4MeCN (2). A solu-tion of a mixture of ppz (25 mg, 0.108 mmol) and Co(ClO4)2·6H2O(39.4 mg, 0.108 mmol) in 10 mL water–CH3CN (1 : 5 = v/v) waslayered onto an aqueous solution (15 mL) of KSCN (21 mg,0.216 mmol). The solution was left for two weeks at roomtemperature to yield X-ray quality dark red plate-like crystals.After filtration, the product was washed with CH3CN and thenallowed to stand in air prior to analysis. Yield: ª77%. UV-vis-NIR(solid state): 9395 cm-1; IR (cm-1, KBr): 3452(br), 3082(w), 2080(s),1620(w), 1497(m), 1421(m), 1392(s), 1089(m), 802(m), 766(m);elemental analysis (%) calcd. for C63H32ClCo4N23O4S7·2H2O: C44.33, H 2.13, N 18.88; Found: C 44.26, H 2.04, N 18.88.

[Co4(ppz)4(SeCN)7(H2O)]·ClO4·5H2O·4MeCN (3). A solu-tion of Co(ClO4)2·6H2O (39.4 mg, 0.108 mmol) in 10 mL water–CH3CN (1 : 1 = v/v) was layered onto an aqueous solution (15 mL)of ppz (25 mg, 0.108 mmol) and KSeCN (31 mg, 0.216 mmol). Thesolution was left for three weeks at room temperature to produceX-ray quality dark red needle-like crystals. After filtration, theproduct was washed with CH3CN and then allowed to standin air prior to analysis. Yield: ª64%. UV-vis-NIR (solid state):10310 cm-1; IR (cm-1, KBr): 3403(br), 3088(m), 2068(s), 1622(m),1587(m), 1487(m), 1392(s), 1092(s), 802(s), 765(m); elementalanalysis (%) calcd. for C63H32ClCo4N23O4Se7·8H2O: C 35.01, H2.33, N 14.91; Found: C 35.18, H 1.92, N 14.89.

X-ray data collection and structural determinations

Structural data were collected on a Bruker-Nonius APEXII-X8-FR591 diffractometer employing graphite-monochromated Mo-Ka radiation generated from a rotating anode (0.71073 A) withw and y scans.13 All data were collected to approximately 56◦

2q. Data integration and reduction were undertaken with SAINTand XPREP 13,14 and subsequent computations were carried outusing the WinGX-32 graphical user interface.15 Structures weresolved by direct methods using SIR97.16 Multi-scan empiricalabsorption corrections were applied to data sets using the programSADABS.17 Data were refined and extended with SHELXL-97.18

In general, non-hydrogen atoms with occupancies greater than 0.5were refined anisotropically. Carbon-bound hydrogen atoms were

included in idealised positions and refined using a riding model.Specific refinement details are reported below.

1 ([Co4(ppz)4Cl8]·4H2O). Formula C56H40Cl8Co4N16O4, M1520.36, tetragonal, space group P42/nnm (#134), a 17.7258(12),b 17.7258(12), c 12.3159(14) A, V 3869.7(6) A3, Dc 1.305 g cm-3,Z 2, crystal size 0.228 ¥ 0.07 ¥ 0.04 mm, colour orange, habitneedle, temperature 150(2) K, l(Mo-Ka) 0.71073 A, m(Mo-Ka)1.167 mm-1, T(SADABS)min,max 0.6199, 0.7446, 2qmax 41.56, hklrange -17 17, -17 13, -12 11, N 15 049, N ind 1073 (Rmerge 0.0571),Nobs 845(I > 2s(I)), Nvar 96, residuals R1(F) 0.0776, wR2(F 2)0.2292, GoF(all) 1.096, Drmin,max -0.572, 0.450 e- A-3. SQUEEZEapplied.

Specific details. The crystals employed in this study rapidlysuffered solvent loss and the quality of diffraction data recordedwas less than ideal despite rapid handling times and a lowtemperature collection (few reflections were observed at greaterthan 1 A resolution). In addition, there is a large region ofsmeared electron density present in the lattice. Despite attemptsto model this as a number of disordered water molecules withpartial occupancies, no satisfactory model could be found. TheSQUEEZE function of Platon19 was therefore employed to removethe contribution from this electron density. The molecular weightand density calculations were therefore based upon four watermolecules per molecular square.

2 ([Co4(ppz)4(SCN)7(H2O)]·ClO4·4.5H2O·4MeCN).Formula C71H55ClCo4N27O9.5S7, M 1934.01, monoclinic,space group P21/n (#14), a 17.445(2), b 25.389(3), c 18.285(2) A,b 90.097(7), V 8098.7(18) A3, Dc 1.586 g cm-3, Z 4, crystal size0.338 ¥ 0.323 ¥ 0.057 mm, colour red, habit plate, temperature100(2) K, l(Mo-Ka) 0.71073 A, m(Mo-Ka) 1.093 mm-1,T(SADABS)min,max 0.6451, 0.7458, 2qmax 58.38, hkl range -2323, -34 34, -25 25, N 201 303, N ind 21 719 (Rmerge 0.0471), Nobs

16 146(I > 2s(I)), Nvar 1117, residuals R1(F) 0.0553, wR2(F 2)0.1697, GoF(all) 1.050, Drmin,max -1.213, 1.552 e- A-3.

Specific details. The perchlorate anion is disordered over threepositions and the N(26)-containing thiocyanate was modelled overtwo positions. Disordered atoms in these molecules were includedwith identical thermal parameters. The acetonitrile and watermolecules are also disordered and were modelled over a numberof positions with total occupancies of 4 and 4.5, respectively. Thehydrogen atoms of the water molecules could not be located in thedifference Fourier map and were not included in the model.

3 ([Co4(ppz)4(SeCN)7(H2O)]·ClO4·5H2O·4MeCN).Formula C71H56ClCo4N27O10Se7, M 2271.32, monoclinic, spacegroup P21/n (#14), a 17.5332(8), b 25.5771(12), c 18.5360(9) A, b90.479(2), V 8312.2(7) A3, Dc 1.815 g cm-3, Z 4, crystal size 0.394 ¥0.108 ¥ 0.097 mm, colour red, habit needle, temperature 100(2) K,l(Mo-Ka) 0.71073 A, m(Mo-Ka) 3.955 mm-1, T(SADABS)min,max

0.5629, 0.7456, 2qmax 55.84, hkl range -19 23, -21 33, -24 24, N60 891, N ind 19 428 (Rmerge 0.0393), Nobs 13 433(I > 2s(I)), Nvar

1088, residuals R1(F) 0.0594, wR2(F 2) 0.1734, GoF(all) 1.047,Drmin,max -4.572, 3.136 e- A-3.

Specific details. The perchlorate anion is disordered over twopositions and two of the SeCN anions are disordered, one overtwo positions and the other over three. Disordered atoms in thesemolecules were included with identical thermal parameters. Inan effort to avoid over-parameterisation of the structure, many

This journal is © The Royal Society of Chemistry 2011 Dalton Trans., 2011, 40, 12388–12393 | 12389

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of the solvent molecules were refined anisotropically with fulloccupancy rather than being split over multiple positions. Thehydrogen atoms of the water molecules could not be located in thedifference Fourier map and were not included in the model.

Results and discussion

Syntheses and X-ray studies

The reaction of cobalt(II) chloride and ppz in a mixture ofwater and EtOH resulted in a clear orange solution thatyielded orange needle-shaped crystals of [Co4(ppz)4Cl8]·4H2O,1, in 91% yield. If instead a water/acetonitrile solutionof ppz and cobalt perchlorate was layered onto an aque-ous solution of potassium thiocyanate, red plate-like crystalsof [Co4(ppz)4(SCN)7(OH2)]·ClO4·4.5H2O·4MeCN, 2, resulted.Substitution of potassium selenocyanate for potassium thio-cyanate yielded red needle-shaped crystals of [Co4(ppz)4(SeCN)7

(H2O)]·ClO4·5H2O·4MeCN, 3.The X-ray structure of [Co4(ppz)4Cl8]·4H2O is given in Fig. 1.

The complex crystallises with tetragonal symmetry such that thereis one-eighths of the molecule in the asymmetric unit; centringis about the Wyckoff a sites (symmetry 42m) within P42/nnm.The four high-spin cobalt(II) centres within the molecular squareeach adopt a cis-orientation and are coordinated to two ppzligands and two chloride ligands. The bond lengths and anglesare typical for octahedral cobalt(II) as is evidenced by the Co–Nlengths of 2.141(5) and 2.166(6) A and the Co–Cl lengths of2.386(3) A. The metal centres form a square planar array witha 7.1 A separation between adjacent sites. Each of the ppz ligandsalternates pointing up and down in an almost grid-like fashion,resulting in a substantial void space within the cavity of themolecular square. Platon19 estimates that 37% of the unit cellrepresents a solvent accessible void.

Fig. 1 Schematic representation of the X-ray structure of[Co4(ppz)4Cl8]·4H2O.

The crystal packing (Fig. 2) is very similar to that reportedin [Co4(HAT)4Cl8]4·27H2O;20 molecules pack directly above eachother forming infinite 1D channels that extend along the crystallo-graphic c-axis. The cavities are filled with disordered water solventmolecules. Adjacent molecules interact via strong offset face-to-face p–p stacking (3.69 A; 83◦) and weak CHphenylene–Cl hydrogen

Fig. 2 Space filling representation showing part of the crystal packing of[Co4(ppz)4Cl8]·4H2O viewed down the c-axis.

bonding (2.65 A) forming an infinite 3D-network, suggesting thatthis complex could be a promising target for host–guest or gas-sorption investigations.

The crystal structure of 2 (Fig. 3) reveals a Co4(ppz)4 unitthat is very similar to that of 1. The complex crystallises in themonoclinic P21/n space group with one molecule per asymmetricunit, which is of lower symmetry than that found in 1. Thehigh-spin octahedral cobalt(II) centres are again arranged in aclose to planar array, although the Co–Co distances now rangefrom 7.1–7.2 A. In contrast to 1, the coordination geometryof each ‘corner’ is not identical. While three ‘corners’ are eachbound to two ppz ligands and two SCN- ligands, one anionon the fourth centre has been replaced with a solvent watermolecule such that the supramolecular square is best describedas [Co4(ppz)4(SCN)7(OH2)]+. Once again, the bond lengths aretypical for octahedral cobalt(II) with the Co–Naromatic lengths

Fig. 3 Schematic representation of the crystal structure of[ClO4ÃCo4(ppz)4(SCN)7(OH2)]·4.5H2O·4MeCN. Solvent molecules areremoved for clarity. The perchlorate anion is shown in a space fillingrepresentation.

12390 | Dalton Trans., 2011, 40, 12388–12393 This journal is © The Royal Society of Chemistry 2011

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ranging from 2.14 A to 2.21 A and Co–Nthiocyanate bond lengthsfrom 2.04 A to 2.14 A. The Co–O bond length is also typical at2.049(3) A.

The charge of the molecular square is accounted for by thepresence of a perchlorate anion, which is located within the cavityof the [Co4(ppz)4(SCN)7(OH2)]+ unit (therefore best described as[ClO4ÃCo4(ppz)4(SCN)7(OH2)]) and displaced from the Co4 planeby 3.12 A (measured to the Cl atom). Also lying partially within thecavity is an acetonitrile molecule, which sits directly above/belowthe perchlorate anion. The crystal packing is very similar to that in1, with strong p–p and weak CHphenylene–anion interactions resultingin the formation of a 3D network housing 1D channels that extendalong the crystallographic b-axis. These channels are filled with theanion and the residual water and acetonitrile solvent molecules.

The crystal structure of 3 (Fig. 4) is isomorphous with thatof 2, with the replacement of SCN- ligands by SeCN- resultingin a formula of [Co4(ppz)4(SeCN)7(OH2)]·ClO4·5H2O·4MeCN.Once again, a perchlorate anion occupies the central cavity ofthe molecular square. The crystal packing is dominated by p–pstacking and weak hydrogen bonding resulting in the formationof channels that are filled with anions, water and acetonitrilesolvent molecules. The bond lengths and Co–Co separations arevery similar to those given for 2 above.

Fig. 4 Schematic representation of the crystal structure of[ClO4ÃCo4(ppz)4(SeCN)7(OH2)] 5H2O·4MeCN. Solvent molecules areremoved for clarity. The perchlorate anion is shown in a space fillingrepresentation.

Vibrational and electronic spectroscopy

The solid state IR spectra of complexes 2 and 3 at 298 K exhibitintense n(CN) bands at 2080 and 2068 cm-1. The solid stateUV-vis-NIR spectra of the molecular squares over the region5000–28 000 cm-1 (Fig. 5) revealed a low intensity forbidden d–dtransition which was attributed to the high spin d7 Co2+ centres at9150, 9395 and 10 310 cm-1 for 1, 2 and 3, respectively. The increasein the energy of this feature upon variation of the coordinatedanion from Cl- (1) to SCN- (2) and SeCN- (3) is consistent withthe transition from a weak field ligand such as Cl- to increasingly

Fig. 5 Solid state UV-Vis-NIR spectra of 1 (orange), 2 (red) and 3(brown). The inset shows the forbidden d–d transitions of the high spin CoII

centres. The colours of the curves reflect those of the complexes themselves.

stronger field ligands. A series of broad, high intensity metal-to-ligand (MLCT) charge transfer bands were also observed in thevisible region of the spectra, with the energy of the band edgeincreasing in the order 3 < 2 < 1. The latter reflects the increasingstabilisation of the complexes as the ligand field of the coordinatedanion decreases from SeCN- in 3, to SCN- in 2 and Cl- in 1.

Magnetism

The magnetic susceptibilities, c, for compounds 1–3 were mea-sured over the range 3–290 K (Fig. 6). The three compoundsexhibit similar magnetic behaviour with room temperature cT (290 K)

values of 11.2, 10.5 and 8.77 cm3 K mol-1, respectively, for 1, 2 and3. This is consistent with four Co(II) ions with S = 3/2 (high-spin)in an octahedral environment with g values of 2.44, 2.37 and 2.16for 1, 2 and 3, respectively.

Fig. 6 Magnetic susceptibility data (c and cT) for compounds 1–3 as afunction of temperature.

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Below 125 K, the magnetic susceptibility of compounds 1–3starts to decrease, reaching 7.22 cm3 K mol-1 for 1, 6.30 cm3 Kmol-1 for 2 and 3.83 cm3 K mol-1 for 3 at 3 K. In octahedralCo(II) compounds the effects of spin orbit coupling and crystallinedistortions combine to give a spin-doublet ground state.21 Spinorbit coupling contributes to both the anisotropy of the g factorand to zero field splitting effects.22 Significantly, anisotropic spinmoments have been reported in the literature for Co compoundsin octahedral environments.21,23 A larger anisotropy of the gvalue is expected as the crystal field becomes more distorted.As was evident from the UV-Vis-NIR spectra, the ligand fieldin compounds 1–3 increases in the order Cl- < SCN- < SeCN-,which is consistent with the increase in g values for 1–3. Below 125K, the magnetic behaviour of the three compounds is mainly due tozero field splitting effects; however, the possibility of magnetic in-teractions cannot be excluded. Given that the Co ◊ ◊ ◊ Co separationdistance along the ppz bridging ligand is equal to or larger than7.1 A in the three compounds, the magnetic interactions, if any,should be small and cannot be distinguished from the overall zerofield splitting effect. Indeed, in Co3(HAT)[N(CN)2]6(OH2)2 wheresimilar Co ◊ ◊ ◊ Co distances along the HAT ligand were reported,the magnetic interactions were found to be negligible.12

Electrochemistry

Solid state cyclic voltammetry of the complexes in 0.1 M [(n-C4H9)4N]PF6/CH3CN electrolyte revealed a series of broad over-lapping redox processes in the region 500–2000 mV (versus theferrocene/ferrocenium couple) on the forward potential sweepfrom 0–2000 mV. Fig. 7 shows a representative CV for 2 whichdisplays the characteristic irreversible redox process correspond-ing to Co2+/3+ oxidation. Interestingly, at least two peaks at ~1145and 1250 mV can be discerned for oxidation in the first cycle;however, no resolution of these processes could be achieved.Square wave and differential pulse methods were also investigatedin an attempt to separate the individual redox couples. The almostsimultaneous oxidation of the four Co2+ centres suggests that onlyweak electronic coupling exists between the metal centres via the

Fig. 7 Solid state CV data (over three consecutive cycles) for 2 in0.1 M [(n-C4H9)4N]PF6/CH3CN electrolyte. The asterisk denotes theCo2+/3+ oxidation process.

bridging ppz ligands. Complexes 1 and 3 exhibit similar irreversibleCo2+/3+ redox processes at 926 and 1475 mV. Notably, the anodicshift in the metal centred potential (i.e., 1 < 2 < 3) mirrors thetrend in the ligand field observed from UV-Vis-NIR and magneticdata, i.e., Cl- is a weak field ligand compared with SCN- andSeCN-.

Conclusion

We have prepared three new tetranuclear cobalt(II) molecularsquares based on the bridging ligand 4,7-phenanthrolino-5,6:5¢,6¢-pyrazine (ppz) with different coordinated counter ions. Thesestructures have been characterised by X-ray crystallography.Magnetic measurements indicate similar behaviour for the threecompounds with each consisting of four high spin Co(II) ions inan octahedral environment. A larger anisotropy of the g value isfound as the ligand field effect increases for compounds 1–3 withthe change in the coordinated anion from Cl- to SCN- to SeCN-.The electrochemical data as well as the energies of the forbidden d–d transitions for Co(II) in the UV-Vis-NIR spectra of the complexesconfirmed the trend towards increasingly stronger field ligands (Cl-

< SCN- < SeCN-). At low temperatures, the magnetic behaviour isdominated by zero field splitting effects. Further investigations ofthe chemical and physical properties of the ppz ligand with othermetal ions are ongoing, with particular interest in the interplaybetween the magnetic, optical and electronic properties of thesupramolecular systems.

Acknowledgements

We thank the Australian Research Council and acknowledgethe Marie Curie IIF scheme of the 7th EU Framework Program(J.K.C.) for support.

References

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12392 | Dalton Trans., 2011, 40, 12388–12393 This journal is © The Royal Society of Chemistry 2011

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