Abstract
A QTAIM and IQA investigation on model compounds of two isostructural AgI and CuI coordination polymers (CPs) based on cyclic triimidazole (L), i.e. the [MIL] n 1D double-stranded stair chain and the [MClL] n 3D network (M = Cu, Ag), has allowed light to be shed on the different emissive behaviour associated with the two metal ions. According to a previously reported investigation [Malpicci et al. (2021). Inorg. Chem. Front. 8, 1312–1323], AgI CPs showed both fluorescence and multiple ligand-centred room-temperature phosphorescences, whereas CuI CPs displayed non-thermally equilibrated halogen and metal-to-ligand charge transfer and two ligand-centred phosphorescences, the latter observed only by their selective activation. Analysis of both local and integral QTAIM descriptors, including delocalization indices and source function, of the Ag—N and Cu—N bonds reveals a higher covalent and local character for the latter, explaining the greater metal–ligand electronic communication observed for the Cu compounds. Moreover, IQA investigation shows that the Cu—N bond is characterized by higher interaction energy, due to both higher electrostatic and exchange-correlation contributions. Analysis on the M—X (M = Ag, Cu; X = I, Cl) bonds, also present in these structures, highlights a much higher covalent and local character with respect to the M—N bonds.
Highlights
The development of new materials with selected properties and improved performance strongly relies on the understanding of the nature of chemical interactions governing their structure
The values of the Quantum Theory of Atoms In Molecules (QTAIM) properties for the M—N bond are in line with those reported in literature for the few studies on metal– nitrogen bonds (Cukrowski et al, 2014; Thomsen et al, 2015)
It should be noted that, compared with results obtained with the M06-2X wavefunction, the HBCP/bond critical points (BCPs), |VBCP|/GBCP and delocalization index (DI) values previously determined with the !B97X wavefunction, reported in Table 1 are systematically higher, with even significant differences regarding in particular the delocalization indices
Summary
The development of new materials with selected properties and improved performance strongly relies on the understanding of the nature of chemical interactions governing their structure. While the small dimensions of the [MIL]n and [MClL]n crystals precluded the collection of the high-resolution data required for experimental charge density studies, preliminary results of QTAIM topological analysis on the wavefunction of [MIL]4 model compounds (see Fig. 1, left) were reported by Malpicci et al (2021). According to this analysis, greater covalent character was obtained for the Cu—N bond with respect to the Ag—N one, as indicated, in particular, by a higher value of delocalization index (DI), i.e. the average number of electrons shared between M and N (Daudel et al, 1974; Bader & Stephens, 1975). QTAIM and IQA analyses on other relevant bonds and intramolecular interactions present in these structures are included
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