The heating of DMF:H2O mixtures of H384 and d-metals in a sealed vial was carried out in
the formation of the MOFs described herein. This solvent mixture was used as in-situ
hydrolysis of the DMF solvent facilitates the generation of dimethylamine342 which can
deprotonate the carboxylic acid groups of H384 resulting in the hard lewis base carboxylates suitable for metal ion binding. In this way, green blade-shaped crystals of 100 were obtained upon reaction of H384 and copper(II) nitrate hemipentahydrate in a 1 mL mixture of 95:5 DMF:H2O after heating at 100 °C for 2.5 hours. Phase purity was confirmed using
PXRD, see Figure 4.5 and the experimental elemental analysis revealed that the air-dried sample contained 12 H2O molecules per formula unit. The diffraction data were solved and
the structure model refined in the tetragonal I41cd space group with half of the molecular
unit [Cu3842], shown in Figure 4.6, in the asymmetric unit i.e. one fully deprotonated ligand
molecule of 84 coordinating 1.5 Cu(II) atoms and also containing two DMF molecules
modelled at 0.75 and 0.9 occupancy, respectively (Appendix E.6). The result is a two dimensional coordination polymer extending along the a and b-axes through metal-ligand coordination with packing in the c direction through π-π interactions. There are two Cu(II) coordination environments in the repeat structure: two btp ligands can be seen to be coordinating Cu1via the pyridyl nitrogen atom N4 and the two proximal triazolyl nitrogen atoms N3 and N5 (through a syn-syn orientation) giving an overall pseudo-octahedral N6
coordination environment. The bite angles ∠ (N3-Cu1-N4) and ∠ (N5-Cu1-N4) are essentially the same at 78.3(4)° and 78.7(5)°, respectively. The pyridyl-Cu(II) bond length
|N4-Cu1| is 1.978(8) Å and the corresponding triazolyl bonds lengths measure to be 2.207(12) Å and 2.184(12) Å for |N3-Cu1| and |N5-Cu1|, respectively. The second Cu(II) environment Cu2 displays five coordinate geometry that is slightly distorted from square
Figure 4.6 X-ray crystal structure of Cu(II) coordination polymer 100 displaying the two unique Cu(II) coordination environments with DMF molecules and hydrogen atoms omitted for clarity. Symmetry code used to generate only completed ligand equivalent atoms (-x, 1-y, +z).
pyramidal (τ5 = 0.18)343 within a di-copper paddlewheel cluster through coordination to the
aryl “arm” carboxylate groups. Each paddlewheel unit is comprised of only one crystallographically unique copper atom and three unique carboxylate groups. The aryl “arm” carboxylate oxygen atoms O1, O2, O5 and O6 are involved in linking two symmetry equivalent Cu2 ions in the bridging mode,μ2-κO:κO′,344 at the basal vertices of the square
pyramids. The respective bond lengths for |O1-Cu2’|, |O2-Cu2|, |O5-Cu2’| and |O6-Cu2| measure to be 1.973(9) Å, 1.941(9) Å, 2.003(9) Å and 1.942(9) Å. The apical site is occupied by the 4-pyridyl carboxylate oxygen atom O4 through a monodentate binding mode,345 the
bond length being significantly longer (|O4-Cu2| is 2.145(6) Å). The non-binding O3 oxygen atom is involved in a non-classical hydrogen bonding interaction with a carbonyl hydrogen atom of a partially occupied DMF molecule (|D···A| is 3.41(2) Å, ∠ (C-H···O) is ca. 172.6°).
The topology is best described as a (4,4) net with both the octahedral Cu1:btp centres and the Cu2 paddlewheel clusters acting as 4-connecting secondary building units (SBUs), see Figure 4.7(A). The di-copper [Cu2(μ2-COO)4(κ1-COO)2] moiety can be simplified in this
way from an alternative description as a binodal motif. This topology gives rise to the larger solvent channels existing parallel to the c-axis, see Figure 4.7(B) with narrower channels remaining in the two orthogonal directions along the a- and b-axes: the three in total accounting for approximately 12% of the total unit cell volume. The observed two dimensional array is the result of the trans conformation adopted by the aryl “arms” about
Figure 4.6 (A) Perspective view of the solvent channels that exist along the c-axis in the structure of
100, hydrogen atoms and solvent molecules omitted for clarity. (B) Perspective view of the solvent accessible surface (blue) highlighting the channels that exist in a unit cell with the b-axis orientated vertically.
the methylene linker; each one of them facing in essentially directly opposite direction to each other (the dihedral angle between the mean planes defined by the atoms comprising the benzene rings on opposite aryl “arms” of 84 is essentially zero 0.0(6)°). Packing parallel to
the c-axis is as a result of weak van der Waals and π-π stacking interactions between the benzoate arms and btp aromatic rings on adjacent two dimensional layers. These solvent channels visible from the single crystal X-ray diffraction analysis of coordination polymer system 100 indicate promising candidature for permanent porosity, however, there were
difficulties in optimizing the synthetic conditions required to reproduce crystalline 100. The PXRD of the failed attempts showed broad amorphous bands stretching the 2θ range (Appendix E.18). The reason for this remained unclear but one of the proposed causes of this inability to reproduce the synthesis may have been a result of varying degree of H2O content in the DMF:H2O solvent mixture, despite measuring out careful ratios of the solvent mixture.342 Many ratios of DMF:H2O were used (1:1, 2:1, 4:1, 9:1, 95:5, 98:2, 100:0) but to little avail, following the initial successful synthesis. Various concentrations of reactants were probed by altering the volume of the solvent mixture added (1 mL, 2 mL, 4 mL) and various stoichiometric ratios of metal to ligand were investigated. Different solvents such as DMSO and solvent mixtures including dimethylacetemide:H2O were also used in the attempts to reform 100. As a result, a sufficient quantity of material could not be synthesised such that gas adsorption studies could not be carried out in order to determine if 100 retained its structural integrity upon evacuation of the channels or in order to measure its gas uptake.
Nevertheless, the as-synthesised sample with which satisfactory phase purity was obtained was analysed by thermal analysis alongside the CH3CN exchanged sample.