A hexagon based Mn( II ) rod metal–organic framework – structure, SF 6 gas sorption, magnetism and electrochemistry

A manganese( II ) metal–organic framework based on the hexatopic hexakis(4-carboxyphenyl)benzene, cpb 6 (cid:2) : [Mn 3 (cpb)(dmf) 3 ], was solvothermally prepared showing a Langmuir area of 438 m 2 g (cid:2) 1 , rapid uptake OF sulfur hexafluoride (SF 6 ) as well as electrochemical and magnetic properties, while single crystal diffraction reveals an unusual rod-MOF topology.

Metal-Organic Frameworks, colloquially known as MOFs, 1 are coming of age with advanced 2 and basic 3 textbooks published, and gas storage devices on the market with other applications close to commercialization. 4 An emerging application is the capture of SF 6 , a greenhouse gas some 22 000 times more potent than CO 2 , used in industrial settings because of its dielectric properties, its non-toxicity, and thermal stabilty. 5,6anganese(II) is an attractive choice for such MOF construction as it is ubiquitous, non-toxic and present in relatively high amounts in most living systems.It is also a less common MOF metal ion, compared to i.e.Zn(II). 7In contrast to zinc(II) it is also magnetically and electrochemically active, properties that have recently been investigated. 8Higher stability may be induced in such MOFs by having the metal secondary building units (SBUs) forming an infinite rod. 9,10ere we explore Mn(II) with the hexatopic linker hexakis(4carboxyphenyl)benzene, cpb 6À , Fig. 1.This hexagon shaped linker, 11 has earlier led us to investigate unique topological properties, 12 and recently we implicated the concerted movement of the six carboxyphenyl groups in gate opening CO 2 gas sorption dynamics in a rod-MOF. 13We noted that the size of the rhombic channels formed by pairwise stacks of cpb linkers would probably conform to the 7 Å diameter suggested as optimal for SF 6 capture, 14 and therefore targeted this greenhouse gas in the present gas sorption study.
Solvothermal synthesis in dimethylformamide (dmf) at 120 1C gave the MOF [Mn 3 (cpb)(dmf) 3 ], CTH-18, in good yield.Contrary to some other M(II) cpb MOFs that form 2D kgd-nets with counter ions partly blocking the pores 12 the single crystal structure of CTH-18 shows an infinite rod metal SBU pairwise connecting the six-connected cpb linkers creating two types of channels running parallel with the rods having approximate minimal dimensions 4.3 Â 6.0 Å and 3.4 Â 4.7 Å when the van der Waals radii have been excluded.Thus, every Mn(II) only connects to four cpb linkers thereby maintaining a neutral framework, see Fig. 2 and Fig. S1 (ESI †).
CTH-18 is stable in organic solvents including ethanol, unstable in 1 M NaOH(aq) and transforms to other crystalline phases in 1 M HCl and deionized water.Thermogravimetric studies in air indicated gradual solvent loss from the framework below 300 1C, and rapid breakdown of the MOF at 425 1C.Taking the TGA results into account, the material was activated in dynamic vacuum at 250 1C prior to gas sorption analysis.N 2 sorption isotherms recorded at 77 K demonstrated the microporous nature of the material, shown by the steep N 2 adsorption isotherm shape at the very low-pressure regime, Fig. 3 top.However, we noted a very slow N 2 adsorption kinetics on CTH-18, especially at low relative pressures.In fact, repeated N 2 sorption experiments yielded different equilibrium uptakes due to this kinetic effect (and the equilibrium N 2 uptake points at low pressures would take several days to reach).The apparent hysteresis shown by the N 2 desorption isotherm was likely to be an artefact of this slow N 2 adsorption rather than N 2 entrapment, i.e.N 2 adsorption had not reached equilibrium when recording the adsorption isotherm.
The slow diffusion of N 2 is most likely related to the dimensions of the pore channels being very close to the kinetic diameter of N 2 (3.6 Å).We therefore carried out CO 2 sorption experiments at À78 1C as this molecule has a smaller kinetic diameter (3.3 Å) and thus faster diffusion within these narrow pore channels.Indeed, we did not observe slow CO 2 adsorption kinetics on CTH-18 and neither hysteresis upon desorption.The specific BET and Langmuir surface area of CTH-18 estimated using the CO 2 adsorption isotherm were 354 and 438 m 2 g À1 , respectively (values estimated using the N 2 isotherm were 289 and 356 m 2 g À1 ).
The pore size distributions (PSD) estimated using the CO 2 adsorption isotherm by Density Functional Theory (DFT), with the slit pore model.The DFT PSD of CTH-18 are plotted in Fig. S2 (ESI †) and showed two distinct types of pores with estimated diameters of B4.3 and B5.1 Å.The estimated pore diameters should not be taken as accurate numbers but they were in a comparable range to the crystallographic pore sizes of 4.3 Â 6.0 Å and 3.4 Â 4.7 Å.
The SF 6 adsorption isotherm showed very sharp increase at low pressures.This sharp increase also indicated that the pore size was close to the kinetic diameter of SF 6 (5.5 Å).As the pore size of CTH-18 was similar to the size of the kinetic diameter of the adsorbate gas (SF 6 ), enhanced van der Waals interaction between the pore surface and the adsorbate gas was expected. 6,14,15The enhanced interaction was reflected by the high isosteric heat of SF 6 adsorption (B40-50 kJ mol À1 ).
Water adsorption/desorption isotherms are shown in Fig. S3 (ESI †) indicating that CTH-18 has low water uptake at low relative pressures (p/p 0 ).Two steps in the water uptake were observed at p/p 0 between 0.1 and 0.3.The shape of the water sorption isotherms are comparable to some MOFs that have been investigated for water harvesting, 19 CTH-18 may thus be interesting for water harvesting applications, although this is beyond the scope of the present study.
Magnetic measurements on Mn(II) MOFs more often than not reveals antiferromagnetic couplings with a few notable exceptions like [Mn 12 O 12 (O 2 CR) 16 (H 2 O) 4 ] (R = Me, Et, etc.) and other so-called single molecule magnets. 20There is no reason to anticipate strong coupling through the cpb linker, and in the chain one would predict antiferromagnetic coupling by the relative close proximity of the d 5 Mn(II) ions (ca. 4 Å), either directly or through spin polarization of the carboxylate linkers. 21his is also what we find for CTH-18, the temperature dependence of the magnetisation shows a Ne ´el-like transition at around T N E 6 K, Fig. 5.The field-dependent magnetisation curves show the expected linear paramagnetic response in a wide temperature interval, as can be found in Fig. S4 (ESI †).
Calculations based on density functional theory (DFT) reveals an intra-chain antiferromagnetic coupling (5.7 meV Mn À1 lower than the ferromagnetic state) between the Mn moments along the x-axis whereas the magnetic coupling between the chains is negligibly small (tens of meV).The calculated Mn moments were around 4.5m B , establishing the oxidation state of Mn as 2+.
3][24] We note that an easily accessible Mn(IV) state may be beneficial for water oxidation, 25 but we found no evidence of this in the cyclic voltammetry of CTH-18 sweeping up to potentials of 1.5 V vs. Ag/AgCl (Fig. S7, ESI †).
The sweep rate dependence in a restricted potential region is shown in Fig. 6.
The redox couple is related to the oxidation of Mn(II) to Mn(III) in the MOF.To simulate the voltammetric response a mechanism separating the electrochemical oxidation and reduction steps was necessary.This is reasonable since the oxidation of Mn(II) to Mn(III) will require that an anion enter the structure. 20The resulting Mn(III)-anion site is reduced on the negative going scan and the anion leaves.Simulated voltammograms are shown in Fig. S6 (ESI †).
The detailed structure of CTH-18 shows no unusual features as far as Mn(II) coordination and cpb conformations are concerned,   but in contrast to [La 2 (cpb)], CTH-17 13 the cpb linkers are stacked with a longer spacing (5.7 Å vs. 5.5 Å) and linkers with opposite conformational chirality alter in the stacks, see Fig. S8 (ESI †).
The network topology on the other hand, merits a few words.CTH-18 is an example of a rod-MOF, [26][27][28] because the metal secondary building unit cannot be reduced to a 0D point forming a dot-MOF, instead it extends in one dimension.However, rod-MOFs pose a descriptive problem as they are commonly analyzed in a different way from dot-MOFs. 26,29,30he straight rod, STR, approach is the simplest but even that in this case gives an unusual topology.Each cpb connects pairwise to the same points on the rod giving a three nodal 4and 6-connected net with point symbol {5 2 .6 3 .7}{4.5 2 .6 2 .7}{4.5 4 .66 .7 4 } (Fig. S9, ESI †).The more elaborate points-ofextension method that also considers the metal SBU gives a four-nodal six-connected net.Details of the topology assignment can be found in the ESI.† In conclusion, CTH-18 shows high SF 6 uptake at 0.1 bar, a pressure practically relevant for SF 6 sorbents, combined with relatively fast kinetics.The SF 6 selectivity compared to N 2 is also higher than for CO 2 and CH 4 , and the thermal stability, judged from TGA, is up to 425 1C.It therefore shows some promise for practical applications, shown to be needed by recent reports of industrial safety issues. 31

Fig. 2 X
Fig. 2 X-Ray single crystal structure of [Mn 3 (cpb)(dmf) 3 ], CTH-18 with symmetry independent Mn atoms drawn in different shades of mauve.Gas sorption indicate the severely disordered dmf molecules can be removed by dynamic vacuum, and these have therefore been excluded for clarity.Left: View of the channels using van der Waals radii.Structure and gas sorption indicate apertures of around 5-7 Å. Right: Thermal ellipsoid drawing of the rod metal-SBU showing how the linkers connect pairwise to form the rod.

Fig. 5
Fig.5Temperature dependence of the magnetization for CTH-18.An applied field of 1 kOe was used.Below the Ne ´el transition at 6 K we can see the magnetic moment decrease as a result of the antiferromagnetic coupling and above 6 K we see paramagnetic behavior.