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The persistence of dimeric R22(6) hydrogen bond motif in cadmium(II) acac-based complexes

The self-assembly process with cobalt(II)/nickel(II) cations and dibenzoylmethanato (dbm)/hexafluoracetylacetonato (hfac) anions and multifunctional pyridine-oxime ligands (4-pyridinealdoxime, 4-Hoxpy ; methyl 4- pyridyl ketoxime, 4-Meoxpy ; 3-pyridinealdoxime, 3-Hoxpy ; methyl 3-pyridyl ketoxime, 3-Meoxpy) was studied in order to target the desired supramolecular pattern - infinite 1-D chains based on O–H···N(oxime) hydrogen bonds (dimeric R22(6) motif). The self-assembly process produced the desired 1-D chains only in the case of [Co(dbm)2(3-Meoxpy)2], [Ni(dbm)2(3-Meoxpy)2] and all the hfac complexes prepared. The desired products were prepared by adjusting the reactants in an iterative manner to shift the magnitude of the electrostatic potential surfaces of competing hydrogen bond acceptor sites. Therefore, a robust synthetic protocol for the reproducible synthesis of the correct supramolecular products was obtained. We further wanted to examine if the same trend of persistence of the dimeric R22(6) hydrogen bond motif could be observed in the analogous cadmium(II) acac-based complexes. Indeed, our preliminary results showed the presence of R22(6) motif in the crystal structure of [Cd(hfac)2(4-Meoxpy)2] (1) and its absence in the crystal structure of [Cd(dbm)2(4-Meoxpy)2] (2) (R22(20) motif based on O–H···O(acac) hydrogen bonds was formed instead). In order to make the oxime N atom as good proton acceptor as possible and enable formation of the dimeric oxime motifs, it is not enough only to introduce an electron- donating methyl group in the vicinity of the oxime nitrogen atom, as it leads to the formation of R22(20) motif in 2. The hydrogen-bond accepting power of the oxygen atom can be reduced by introducing electron-withdrawing substituents (fluorine atoms) at the acac-based ligand. This approach resulted with the desired R22(6) motif in 1. The substitution of cobalt(II) or nickel(II) cations with larger cadmium(II) cation in the studied acac-based complexes results with the same general trend of R22(6) motif appearance, as established previously.

cadmium(II), acac-based complex, hydrogen bond motif

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