Palladium complexes of unsymmetrical 1,3-bis(aryliminio)isoindoline (BAI) ligands: A novel class of complexes exhibiting unusual structural features

Luminescent Pt(II) Complexes Using Unsymmetrical Bis(2-pyridylimino)isoindolate Analogues

A series of ligands based upon a 1,3-diimino-isoindoline framework have been synthesized and investigated as pincer-type (N∧N∧N) chelates for Pt(II). The synthetic route allows different combinations of heterocyclic moieties (including pyridyl, thiazole, and isoquinoline) to yield new unsymmetrical ligands. Pt(L1-6)Cl complexes were obtained and characterized using a range of spectroscopic and analytical techniques: 1H and 13C NMR, IR, UV-vis and luminescence spectroscopies, elemental analyses, high-resolution mass spectrometry, electrochemistry, and one example via X-ray crystallography which showed a distorted square planar environment at Pt(II). Cyclic voltammetry on the complexes showed one irreversible oxidation between +0.75 and +1 V (attributed to Pt2+/3+ couple) and a number of ligand-based reductions; in four complexes, two fully reversible reductions were noted between -1.4 and -1.9 V. Photophysical studies showed that Pt(L1-6)Cl absorbs efficiently in the visible region through a combination of ligand-based bands and metal-to-ligand charge-transfer features at 400-550 nm, with assignments supported by DFT calculations. Excitation at 500 nm led to luminescence (studied in both solutions and solid state) in all cases with different combinations of the heterocyclic donors providing tuning of the emission wavelength around 550-678 nm.

Binding a Meridional Ligand in a Facial Geometry: A Square Peg in a Round Hole

The bis(pyridylimino)isoindoline (BPI) ligand is a tridentate chelate that binds to metals via a meridional coordination mode. However, when this ligand forms a complex with Re(CO)₃, an almost exclusively facial moiety, the BPI ligand deforms to coordinate in a facial mode. We have in-vestigated this deformation via structural and theoretical means, and the non-planar binding mode of the ligand bathochromically shifts the metal to ligand charge transfer (MLCT) transition.

Re(CO)3-Templated Synthesis of α-Amidinoazadi(benzopyrro)methenes

α-Amidinoazadi(benzopyrro)methenes were synthesized using the Re(CO)₃ unit as a templating agent. The products of these template reactions are six-coordinate rhenium complexes, with a facial arrangement of carbonyls, a noncoordinating anion, and a tridentate α-amidinoazadi(benzopyrro)methene ligand. The tridentate ligand shows the conversion of one diiminoisoindoline sp² carbon to a sp³ carbon, which has been seen in the "helmet" and bicyclic phthalocyanines. The bidentate diiminoisoindoline fragment tilts out of the plane of coordination. Five examples of α-amidinoazadi(benzopyrro)methenes produced from these reactions using different nitrile solvents, including the nitrile activation of acetonitrile, propionitrile, butyronitrile, cyclohexanecarbonitrile, and benzonitrile.

Co(iii) complexes of (1,3-selenazol-2-yl)hydrazones and their sulphur analogues

The first Co(iii) complexes with (1,3-selenazol-2-yl)hydrazones as an unexplored class of ligands were prepared and characterized by NMR spectroscopy and X-ray diffraction analysis. The novel ligands act as NNN tridentate chelators forming octahedral Co(iii) complexes. The impact of structural changes on ligands' periphery as well as that of isosteric replacement of sulphur with selenium on the electrochemical and electronic absorption features of complexes are explored. To support the experimental data, density functional theory (DFT) calculations were also conducted. Theoretical NMR chemical shifts, the relative energies and natural bond orbital (NBO) analysis are calculated within the DFT approach, while the singlet excited state energies and HOMO-LUMO energy gap were calculated with time-dependent density functional theory (TD-DFT). The electrophilic f^- and nucleophilic f⁺ Fukui functions are well adapted to find the electrophile and nucleophile centres in the molecules. Both (1,3-selenazol-2-yl)- and (1,3-thiazol-2-yl)hydrazone Co(iii) complexes showed potent antimicrobial and antioxidant activity. A significant difference among them was a smaller cytotoxicity of selenium compounds.

Isoindoline-derived ligands and applications

During the past decade isoindoline-based ligands became the subject of growing interest due to their modular set-up.

Adsorption behavior of asymmetric Pd pincer complexes on a Cu(111) surface

We address the adsorption of asymmetric Pd pincer complexes on a Cu(111) surface by scanning tunneling microscopy. The structural asymmetry is manifested in the observation of two chiral enantiomers. To enable an unambiguous identification of individual constituents, three closely related complexes with small modifications are investigated in parallel. Thereby, methyl substituents determine attractive molecule-molecule interaction. Depending on their distribution, dimerization and tetramerization can be observed.