Pd and Pt Complexes of Benzo-Fused Dipyrrins: Synthesis, Structure, Electrochemical, and Optical Properties

Benzo-fused dipyrrins are π-extended analogs of conventional dipyrrins, which exhibit bathochromically shifted absorption and possess the synthetic capability to bind various metal ions. We aimed to investigate the synthetic potential of benzo-fused dipyrrins in the complexation with transition metals. Two new complexes with Pd2+ and Pt2+ were synthesized and characterized. X-ray crystallography reveals that both complexes exhibit a zigzag geometry with square planar coordination of the central metal. The Pd2+ complex possesses a very weak fluorescence at 665 nm, while the Pt2+ complex is completely nonemissive. Transient absorption spectroscopy confirmed triplet excited state formation for both complexes; however, they are short-lived and no phosphorescence was observed even at 77K. DFT calculations support the experimental observation, revealing the existence of the low-lying ligand-metal charge-transfer (LMCT) triplet state acting as an energy sink.

Luminescent 1D heterometallic (Ir,Cd) coordination polymers based on bis-cyclometalated Ir(III) metallatectons and trinuclear Cd(II) dianionic nodes

Seven isostructural heterometallic luminescent (Ir,Cd) coordination polymers were prepared upon the combination of tris-chelate cationic Ir(III) complexes behaving as metalloligands with Cd(II) salts. Three octahedral Ir(III) complexes have been considered in the present report. They consist of a bipyridine unit functionalised with 3-pyridyl moieties as peripheral coordinating sites and two 2-phenylpyridyl cyclometalating derivatives. Three cadmium halide salts CdX₂ (X = Cl, Br, I) were used and rearranged themselves during the self-assembly process with the metallatectons to afford a dianonic trinuclear Cd node [Cd₃X₈]^2-. Seven out of the nine possible metallotecton-metal salt combinations could be characterised in the crystalline phase by X-ray diffraction on single crystals proving the isostructurality of the seven extended architectures studied. All of the CPs are luminescent with small shifts observed in the emission wavelength compared to the discrete complexes. Depending on the degree of fluorination of the two cyclometalating units, tuning of the emission wavelength of the discrete complexes as well as of the resulting coordination polymers is achieved.

Heteroleptic palladium(II) complexes of dipyrrin-1,9-dione supported by intramolecular hydrogen bonding

The dipyrrin-1,9-dione framework, which is characteristic of the propentdyopent pigments deriving from heme metabolism, coordinates metal ions as monoanionic bidentate donors. The resulting analogs of dipyrrinato complexes undergo reversible ligand-based reductions, thus showcasing the ability of the dipyrrindione scaffold to act as an electron reservoir. Herein we report the synthesis and characterization of three heteroleptic palladium complexes of the redox-active dipyrrindione ligand. Primary amines were chosen as additional ligands so as to assemble complexes of planar geometries with complementary interligand hydrogen-bonding. Full chemical characterization confirms the hydrogen bonding interactions between the primary amine ligands and the acceptor carbonyl groups on the dipyrrolic ligand. The resulting heteroleptic compounds display reversible one-electron reduction events that are centered on the dipyrrindione ligand as revealed by voltammetry and spectroelectrochemistry data. Within these planar Pd(II) complexes, the propentdyopent motif therefore combines reversible ligand-based redox chemistry with interligand hydrogen bonding in the primary coordination sphere of the metal center.

Quantifying Triplet State Formation in Zinc Dipyrrin Complexes

Photocatalysis is a promising method to harness solar energy and use it to form fuels and other high-value chemicals, but most sensitizers used in photocatalytic reactions are complexes of rare and expensive metals such as ruthenium and iridium. Zinc dipyrromethene complexes have potential to be a more earth-abundant alternative, but their photophysical properties are largely unexplored. In this study, triplet state formation was quantified in two zinc dipyrromethene complexes, with and without heavy atoms, by transient absorption spectroscopy. Without heavy atoms, the triplet quantum yield was 16% in toluene and 27% in THF. With the addition of heavy I atoms, the triplet quantum yield increased to 62-63% and was insensitive to solvent polarity. The fact that in the absence of heavy atoms the triplet yield is affected by solvent polarity and in the presence of heavy atoms it is not suggests that triplet formation occurs through different pathways in the two complexes. These triplet yields meet or exceed those of successful organic photosensitizers, illustrating the potential for zinc dipyrromethene complexes as photosensitizers.

Functional Supramolecular Architectures of Dipyrrin Complexes

Dynamic formation of self-assemblies from molecular components is a useful and efficient way to produce molecular and supramolecular architectures with sophisticated functions. The labile coordination bond and dynamic covalent bond as a reversible bond have often been used to create a well-organized supramolecular self-assembly. In order to realize sophisticated novel functions of the supramolecular self-assemblies, dipyrrin complexes have recently been employed as a functional unit and incorporated into the supramolecular architectures because of their outstanding properties and functions such as a high photostability and strong light absorption/emission. This review article summarizes recent development in functional supramolecular architectures of the dipyrrin complexes produced by coordination to a metal ion and dynamic covalent bond formation. We first describe the synthesis and unique functions of a series of discrete supramolecular architectures: helicates, macrocycles, and cages. The polymeric supramolecular self-assemblies with 1D, 2D, and 3D structures are then introduced as a functional infinite supramolecular architecture.

Solvent and anion effects on the organization of a luminescent [2 + 2] BODIPY/Ag(i) metallamacrocycle in the crystalline state

The assembly of a novel BODIPY bearing peripheral vinyl–benzonitrile groups with AgX salts (X⁻ = AsF₆⁻, BF₄⁻, ClO₄⁻) leads to the recurrent formation of a [2 + 2] [Ag₂(BODIPY)₂]²⁺ type metallamacrocycle featuring emission in the crystalline state.

Molecular tectonics: heterometallic (Ir,Cu) grid-type coordination networks based on cyclometallated Ir(III) chiral metallatectons

A chiral-at-metal Ir(III) organometallic metallatecton was synthesised as a racemic mixture and as enantiopure complexes and combined with Cu(II) to afford a heterobimetallic (Ir,Cu) grid-type 2D coordination network.

Assembly, disassembly, and reassembly: conversion of homometallic coordination networks into mixed metal-organic frameworks

A strategy for the conversion of homometallic coordination networks into mixed metal-organic frameworks (MM'MOFs) is proposed. Ni(II) complexes of dipyrrin (dpm) ligands bearing peripheral pyridyl or imidazolyl units have been shown to self-assemble into coordination polymers with the metal cation in an octahedral environment coordinated to two bis-pyrrolic chelates and two neutral monodentate coordinating units such as pyridyl or imidazolyl moieties. Taking advantage of the chelate effect, the two monodentate units may be replaced by a diimine ligand leading to the disassembly of the networks by the formation of discrete soluble complexes. The latter can be employed as metallatectons for the construction of heterometallic architectures upon reaction with a secondary metal salt. This approach was applied using either 1,10-phenanthroline (phen) or 2,2'-bipyrimidine (bpm) as chelates leading to a series of mono- and binuclear metallatectons of the (phen)Ni(dpm)2 and (bpm)[Ni(dpm)2]2 type. Subsequent assembly with CdCl2 afforded either interpenetrated 2D grid-type architectures or 3D MM'MOFs.