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. 2020 Mar 31;13(7):1595.
doi: 10.3390/ma13071595.

Use of Pyrazole Hydrogen Bonding in Tripodal Complexes to Form Self Assembled Homochiral Dimers

Greg Brewer  1 , Raymond J Butcher  2 , Peter Zavalij  3
Affiliations

Use of Pyrazole Hydrogen Bonding in Tripodal Complexes to Form Self Assembled Homochiral Dimers

Greg Brewer et al. Materials (Basel). .

Abstract

The 3:1 condensation of 5-methyl-1H-pyrazole-3-carboxaldehyde (MepyrzH) with tris(2-aminoethyl)amine (tren) gives the tripodal ligand tren(MePyrzH)3. Aerial oxidation of a solution of cobalt(II) with this ligand in the presence of base results in the isolation of the insoluble Co(tren)(MePyrz)3. This complex reacts with acids, HCl/NaClO4, NH4ClO4, NH4BF4, and NH4I to give the crystalline compounds Co(tren)(MePyrzH)3(ClO4)3, {[Co(tren)(MePyrzH0.5)3](ClO4)1.5}2 {[Co(tren)(MePyrzH0.5)3](BF4)1.5}2 and [Co(tren)(MePyrzH)3][Co(tren)(MePyrzH)3]I2. The latter three complexes are dimeric, held together by three Npyrazole -H…Npyrazolate hydrogen bonds. The structures and symmetries of these homochiral dimers or pseudodimers are discussed in terms of their space group. Possible applications of these complexes by incorporation into new materials are mentioned.

Keywords: cobalt; crystal structure; dimer; hydrogen bonding; pyrazole; supramolecular.

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Conflict of interest statement

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Figures

Figure 1
Figure 1
Line drawings of tripodal azole ligands.
Figure 2
Figure 2
Partially deprotonated complexes have been observed for [Fetren(2-Im)2(2-ImH)]+ (linear, top) and [Cotren(2-ImH)2(2-Im)]2+ (zig-zag, bottom) that give 1D hydrogen bound chains. Please see [27] from preceding paragraph.
Figure 3
Figure 3
Face on upper and sideway lower views of extended 2D sheet structures for trigonal Fetren(4-MeImH0.5)3+1.5 (top) and hexagonal [Fetren(4-MeImH0.5)3]+1.5 (bottom). Please see [31] from preceding paragraph.
Figure 4
Figure 4
Structure of the [Co(tren(MepyrzH)3]3+ cation. Note that the apical nitrogen atom, labelled N above, caps the face of the three imine nitrogen atoms, N1A, N1B and N1C.
Figure 5
Figure 5
Structure of the [Cotren(MepyrzH)3][Cotren(Mepyrz)3]2+ cation. All the hydrogen atoms except for the bridging hydrogen atoms, shown in red, have been deleted. Note the three.Npyrazole–H…Npyrazolate hydrogen bonds, shown as dashed blue red bonds, holding the two homochiral halves of cation together and the alignment of the three pairs of pyrazole rings to promote π-π stacking. The apical nitrogen atoms of the tren units are in light blue at the bottom left and upper right of figure. The two octahedral cobalt atoms are in dark blue, the cobalt(III) on left and cobalt(II) on right.
Figure 6
Figure 6
Structure of [Cotren(4MeImH0.5)3]4(ClO4)6 (average level of protonation per Co) showing the tetrahedral array of cobalt complexes and the six hydrogen bonds folding the cluster together. All hydrogen atoms, except for the hydrogen bound bridging hydrogen atoms, shown in red, and the perchlorate anions have been omitted for clarity. Note that there are twelve hydrogen bound imidazole atoms which require six hydrogen atoms to link all the imidazoles together. Each cobalt complex forms three hydrogen bonds (through its three arms) to each of the other three cobalt complexes.
Figure 7
Figure 7
The unit cell (space group R32) of [Cotren(MepyrzH0.5)3]23+ is depicted above with one of the dimers (all H atoms and counterions omitted for clarity) that it contains. The red line is a three fold rotation axis and also an axis of the cell. Note that the three fold axis (red) runs through the apical tren nitrogen and cobalt atoms which result in a single tren arm in the asymmetric unit. R32 (D3) requires a three fold rotation axis (red) and three perpendicular two fold rotation axis (blue). The two fold rotation (blue) bisects the non bonded cobalt-cobalt axis and is perpendicular to a Npyrazole–H…Npyrazolate hydrogen bond. This results in a single cobalt atom in the asymmetric unit by a two fold rotation. The effect of both of these symmetry elements is that the asymmetric unit contains a single cobalt atom (not two) and a single tren arm (not three).
Figure 8
Figure 8
Structure of the pyrazole dimer in P43212. The unit cell axes are not depicted here. The hydrogen atoms and counterions have been omitted for clarity. Note the three hydrogen bonds (dashed blue) connecting the tree pairs of pyrazole rimgs and the alignment of rings for π-π stacking. The red line is a C2 proper rotation axis that bisects one of the three hydrogen bonds and the non bonded cobalt-cobalt axis. This two fold rotation axis (the only symmetry element in the molecule) means that there is a single cobalt atom and three tren arms in the asymmetric unit. This contrasts with the previous molecule in R32 in that it lacks a three fold rotation axis on the molecule. The four fold rotation axis in this space group (D4 symmetry) is not a rotation axis of the molecule itself but a screw axis of the cell. Clerly a tripodal ligand could not have a fold fold proper rotation axis.
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