Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

Synthesis of Ethano-Bridged Diazapolycenes as Potential Precursors for Diazapolycenes and Their Properties

A series of ethanodiazapolycenes were prepared in 87%-89% yields by Friedländer reactions of three o-aminoarenecarbaldehydes with bicyclo[2.2.2]octane-2,5-dione and their spectral, thermal, and structural properties were studied. Subsequent attempts to convert them to diazapolycenes have proved unsuccessful.

Synthesis and properties of annulated 2-(Azaar-2-yl)- and 2,2'-Di(azaar-2-yl)-9,9'-spirobifluorenes

A series of 9,9'-spirobifluorene-derived N-heterocycles were prepared by the reactions of 8,9-dihydrospiro(benzo[b]fluorene-11,9'-fluoren)-6(7H)-one and 8,8',9,9'-tetrahydro-11,11'-spirobi(benzo[b]fluorene)-6,6'(7H,7'H)-dione with a series of 2-amino-arenecarbaldehydes such as 2-aminobenzaldehyde, 2-aminonicotinealdehyde, 1-amino-2-naphthaldehyde, and 8-aminoquinoline-7-carbaldehyde. In addition to the absorption maxima based on the parent 9,9'-spirobifluorene skeleton in the 225–234, 239–280, 296–298, and 308–328 nm regions, the absorptions due to the π-π* transitions of the heterocycles were observed in the 351–375 nm region in the UV absorption spectra. All the compounds showed strong photoluminescences in the 390–430 nm region.

Synthesis of benzo-annulated tryptanthrins and their biological properties

A series of benzo-annulated derivatives of tryptanthrin were prepared and their optical and redox properties were studied. Tryptanthrin and its benzo-annulated derivatives showed selective inhibitory activity on topo I with an increase of activity on topo II by benzo-annulation on quinazolin-4(3H)-one moiety. Although the benzo-annulation on quinazolin-4(3H)-one ring did not affect significantly on the inhibitory activities against topo I and II, the benzoannulation on indolin-3-one ring affected the inhibitory activity very much especially by linear annulation. Cytotoxicities were not significantly changed upon benzoannulation, which were not directly related either to the inhibitory activities against topo I and II or to the reduction potentials.