Photophysical Studies of a Zr(IV) Complex with Two Pyrrolide-Based Tetradentate Schiff Base Ligands

The reaction of two equivalents of N,N'-bis(2-pyrrolylmethylidene)-1,2-phenylenediamine (H2bppda) with tetrabenzylzirconium provided the air- and moisture-stable eight-coordinate complex Zr(bppda)2. Temperature-dependent steady-state and time-resolved emission spectroscopy established weak photoluminescence (ΦPL = 0.4% at 293 K) by a combination of prompt fluorescence and thermally activated delayed fluorescence (TADF) upon visible light excitation at and around room temperature. TADF emission is strongly quenched by 3O2 and shows highly temperature-sensitive emission lifetimes of hundreds of microseconds. The lifetime of the lowest energy singlet excited state, S1, was established by transient absorption spectroscopy and shows rapid deactivation (τ = 142 ps) by prompt fluorescence and intersystem crossing to the triplet state, T1. Time-dependent density functional theory (TD-DFT) calculations predict moderate ligand-to-metal charge transfer (LMCT) contributions of 25-30% for the S1 and T1 states. A comparison of Zr(bppda)2 to related zirconium pyridine dipyrrolide complexes, Zr(PDP)2, revealed important electronic structure changes due to the eight-coordinate ligand environment in Zr(bppda)2, which were correlated to differences in the photophysical properties between the two compound classes.

Ligands with Two Monoanionic N,N-Binding Sites: Synthesis and Coordination Chemistry

Polytopic ligands have become ubiquitous in coordination chemistry because they grant access to a variety of mono- and polynuclear complexes of transition metals as well as rare-earth and main-group elements. Nitrogen-based ditopic ligands, in which two monoanionic N,N-binding sites are framed within one molecule, are of particular importance and are therefore the primary focus of this review. In detail, bis(amidine)s, bis(guanidine)s, bis(β-diimine)s, bis(aminotroponimine)s, bis(pyrrolimine)s, and miscellaneous bis(N,N-chelating) ligands are reviewed. In addition to the general synthetic protocols, the application of these ligands is discussed along with their coordination chemistry, the multifarious binding modes, and the ability of these ligands to bridge two (or more) metal(loids).

Titanium Amido- and Imido-Complexes Supported by a Tridentate Pyrrolyl Ligand: Syntheses, Characterisation and Catalytic Activities

The complexes [Ti(NMe2)2(pmpmi)] [H2pmpmi = (2-pyrrolylmethene)-(2-pyrrolylmethyl)imine], [Ti(NtBu)(pmpmi)(py)2], [Ti(NtBu)(pmpmi)(dpy)] and [Ti(NPh)(pmpmi)(py)2] have been prepared, characterised and shown to be pre-catalysts for the hydroamination of phenylacetylene with aniline and p-chloroaniline. The X-ray structures of [Ti(NMe2)2(pmpmi)], [Ti(NtBu)(pmpmi)(dpy)] and [Ti(NPh)(pmpmi)(py)2] have been determined.

Bis[N,N'-bis-[(1H-pyrrol-2-yl)methyl-ene]cyclo-hexane-1,2-diamine]titanium(IV) tetra-hydro-furan solvate

In the title compound, [Ti(C₁₆H₁₈N₄)₂]·C₄H₈O, the Ti^IV ion is chelated by two Schiff base dianions with a TiN₈ distorted square-anti­prismatic coordination geometry. The two cyclo­hexane rings assume the typical chair conformation. No hydrogen bonding is observed in the crystal structure.