The coming of age: Understanding and tuning the electronic structure of pentalenides

Ever since its first report over 60 years ago the 10π dianionic hydrocarbon pentalenide and its intriguing organometallic chemistry has remained a rare curiosity due to the exceptional difficulty of precursor synthesis and associated limitations in ligand design. Here we report the first example of systematic tuning of the electronic properties of dianionic pentalenides through a straightforward design protocol which allows the controlled variation of substituents in the 1,3,4,6-positions to produce nine new species, representing the largest pentalenide study to date. Both electron-withdrawing as well as electron-donating aromatics have been incorporated to achieve different polarisations of the bicyclic 10π aromatic core, as indicated by characteristic 1H and 13C NMR shifts and evaluated by DFT calculations including nucleus-independent chemical shift (NICS) scans and anisotropy of the induced current density (ACID) calculations as well as natural bond orbital (NBO) charge distribution analysis. The introduction of methyl substituents to the pentalenide core required positional control in the dihydropentalene precursor to avoid exocyclic deprotonation during the metalation. Frontier orbital analyses showed arylated pentalenides to be slightly weaker donors but much better acceptor ligands than the unsubstituted pentalenide.