Exploring the Spatial Arrangement of Simple 18-Membered Hexaazatetraamine Macrocyclic Ligands in Their Metal Complexes

Abstract

Hexaazamacrocyclic Schiff bases have been extensively combined with lanthanoid (Ln) ions to obtain complexes with a highly axial geometry. However, the use of flexible hexaazatetraamine macrocycles containing two pyridines and acyclic spacers is rather uncommon. Accordingly, we obtained [DyL(OAc)₂]OAc·7H₂O·EtOH and [DyL^Me2(Cl)₂]Cl·2H₂O, where L and L^Me2 are the 18-membered macrocycles 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane and 3,10-dimethyl-3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane, respectively, which contain ethylene and methylethylene spacers between their N₃ moieties. [DyL(OAc)₂]OAc·7H₂O·EtOH represents the first crystallographically characterized lanthanoid complex of L, while [DyL^Me2(Cl)₂]Cl·2H₂O contributes to increasing the scarce number of Ln^III compounds containing L^Me2. Furthermore, the crystal structure of L·12H₂O was solved, and it was compared with those of other related macrocycles previously published. Likewise, the crystal structures of the Dy^III complexes were compared with those of the lanthanoid and d-metal complexes of other 18-membered N₆ donor macrocycles. This comparison showed some effect of the spacers employed, as well as the influence of the size of the ancillary ligands and the metal ion. Additionally, the distinct folding behaviors of these macrocycles influenced their coordination geometries. Moreover, the luminescent properties of [DyL(OAc)₂]OAc·7H₂O·EtOH and [DyL^Me2(Cl)₂]Cl·2H₂O were also investigated, showing that both complexes are fluorescent, with the emission being sensitized by the ligands.

Keywords: N6 macrocycle; amine ligands; conformation; dysprosium; fluorescence; pyridine; twisting.

Scheme 1

The 18-membered hexaazamacrocyclic ligands used in this work (L and L^Me2) as well as some other related ligands mentioned. All of them (L^x) are depicted as neutral species, and obviating any chirality of the asymmetric C atoms is marked with *.

Figure 1

Ellipsoid diagrams (50% probability) with the labeling scheme of L in L·12H₂O, only for atoms of the asymmetric unit and showing the opposite torsions of both ethylene spacers (left), as well as those of each branch (C_Py-CH₂-NH-CH₂) of the acetyl pyridine residue. In the right panel, another view is presented of the chirality of the amine N donor atoms shown in the crystal, with similar chirality for the two N atoms of each diacetylpyridine residue, an opposite for each ethylenediamine one.

Figure 2

(Left) The ellipsoid diagram for the [DyL(OAc)₂]⁺ cation (ellipsoids at 35% probability) showing the labeling scheme; (Right) the stick view from a different perspective to show the folding of the macrocycle.

Figure 3

Ellipsoids and sticks diagram for the [DyL^N6MeCl₂]⁺ cation. After coordination, the configurations of the chiral N atoms are N1: R, N3: S, N4: R, and N6: S.

Figure 4

Representations of the ionic radii R (for coordination number = 6) of some divalent d-block metal ions vs. deviations of continuous shape measures (CShMs) from a D6h hexagon (left) and Oh octahedron (center). Spacefill representation of the M helical spatial arrangement of L^Me4a in [NiL^Me4a]²⁺ for CSD_KUBCUG [12] (Right).

Figure 5

Spacefilling representations of the spatial arrangement of (from left to right): L, L^cyh2, and L^Me4a in [DyL^Me2Cl₂]⁺, CSD_RIVRAR [7] and CSD_SICCOX [8], respectively. They try to illustrate the terms used in this work to describe the macrocycle disposition “butterfly”, “saddle” [36], and “wave” [36], respectively.

Figure 6

Representations of the deviations of continuous shape measures (CShMs) observed from a D_6h hexagon versus the ionic radii of lanthanoid metal ions for deca- and undecacoordinate complexes; (a) all the complexes with L^x and (b) only for complexes containing L^cyh2 with coordination number > 9.

Figure 7

Excitation (dotted lines) and emission spectra (continuous lines) for [DyL(OAc)₂]OAc·7H₂O (a) and [DyLMe₂(Cl)₂]Cl·2H₂O (b), recorded with λ_em = 576 nm and λ_ex = 280 nm, respectively. Decay curves recorded monitoring the emission at 576 nm (⁴F_9/2 → ⁶H_13/2) under 280 nm excitation (circles) and its fit to a single-exponential decay (black line) at room temperature for [DyL(OAc)₂]OAc·7H₂O (c) and [DyLMe₂Cl₂]Cl·2H₂O (d).