Redox Modification of EMACs Through the Tuning of Ligands: Heptametal(II) Complexes of Pyrazine-Modulated Oligo-α-pyridylamido Ligands

Theoretical Studies on the Structures of Metal String Complexes Crn(L)4Cl2 (n = 3, 5, 7; L = Oligo-α-Pyridylamide) under the Effect of an Electric Field

To study the electronic structures and properties of [Cr_n(L)₄Cl₂] (n = 3, L = dpa: di(2-pyridyl)amido; n = 5, L = tpda: tripyridyldiamido; n = 7, L = teptra: tetrapyridyltriamine) metal string complexes, the BP86 method was used by considering the influence of the electric field (EF) applied parallel to the metal axis. As the EF increases, the migration of more positively charged Cr_odd is more significant than that of Cr_even, which results in alternating long-short Cr-Cr bonds. This happens because of the natural charges on the Cr_odd of 1-3, which are more electropositive than those on Cr_even. The electrons are pulled to the Cr and Cl(r) atoms at the high-potential side from Cl(l) at the low-potential side by the EF, which leads to asymmetrical FMOs. After the critical electric field (E_c), the configuration turns into a remarkably asymmetric one with alternating Cr-Cr quadruple bonds and weak interactions. The electrons are transferred from equatorial ligands (L) to metal chains. In the meantime, the asymmetry of the FMOs increases and the delocalization is further reduced, which affects the conductivity. Especially for [Cr₇(teptra)₄Cl₂], the delocalized electrons of HOMO are completely transformed into a localized model after the critical electric field. It is observed that this supports the electric switching phenomenon ascribed to the conformations of delocalized and localized electrons. In addition, the longer the length of the metal chain, the smaller the E_c and the easier is for the complexes to be polarized by the EF.

The Origin of Magnetic Anisotropy and Single-Molecule Magnet Behavior in Chromium(II)-Based Extended Metal Atom Chains

Chromium(II)-based extended metal atom chains have been the focus of considerable discussion regarding their symmetric versus unsymmetric structure and magnetism. We have now investigated four complexes of this class, namely, [Cr3(dpa)4X2] and [Cr5(tpda)4X2] with X = Cl- and SCN- [Hdpa = dipyridin-2-yl-amine; H2tpda = N2,N6-di(pyridin-2-yl)pyridine-2,6-diamine]. By dc/ac magnetic techniques and EPR spectroscopy, we found that all these complexes have easy-axis anisotropies of comparable magnitude in their S = 2 ground state (|D| = 1.5-1.8 cm-1) and behave as single-molecule magnets at low T. Ligand-field and DFT/CASSCF calculations were used to explain the similar magnetic properties of tri- versus pentachromium(II) strings, in spite of their different geometrical preferences and electronic structure. For both X ligands, the ground structure is unsymmetric in the pentachromium(II) species (i.e., with an alternation of long and short Cr-Cr distances) but is symmetric in their shorter congeners. Analysis of the electronic structure using quasi-restricted molecular orbitals (QROs) showed that the four unpaired electrons in Cr5 species are largely localized in four 3d-like QROs centered on the terminal, "isolated" Cr2+ ion. In Cr3 complexes, they occupy four nonbonding combinations of 3d-like orbitals centered only on the two terminal metals. In both cases, then, QRO eigenvalues closely mirror the 3d-level pattern of the terminal ions, whose coordination environment remains quite similar irrespective of chain length. We conclude that the extent of unpaired-electron delocalization has little impact on the magnetic anisotropy of these wire-like molecular species.

Band Structures of Quasi-One-Dimensional Incommensurate Helical Systems: A Case Study of Infinite Chromium Extended Metal Atom Chain

Extended metal atom chains (EMACs) are promising candidates for molecular wires but their band structures remain to be explored. As a quasi-one-dimensional (Q1D) system, the incommensurate helical nature of EMACs hinders such calculations. In this work, we resolved this issue via explicit implementation of helical symmetry. Moreover, the pattern of metal d bands was rationalized by a systematic investigation on a series of related Q1D helical systems. Two critical factors, helical ligand field and chemically asymmetric ligand field, are proposed and identified. We found that the symmetry and ligand fields of the system dominate the pattern of the metal d bands, instead of specific chemical composition of ligands. The presented method and rationale are applicable to not only EMACs but also related Q1D helical systems.

Solution structure of a pentachromium(ii) single molecule magnet from DFT calculations, isotopic labelling and multinuclear NMR spectroscopy

Solution NMR spectroscopy on isotopically-labelled samples of [Cr₅(tpda)₄Cl₂] unveils a D₄ symmetric molecule, implying fast shuttling between the two unsymmetric ground configurations over NMR timescale.