Amide-Linked Heterobi- and Heterotermetallocenes with Very Low HOMO–LUMO Gaps

The novel heterobi- and heterotermetallocenes [Fc-NHCO-Cc]<sup>+</sup> (<b>1</b>) and [Fc-NHCO-Cc-CONH-Fc]<sup>+</sup> (<b>2</b>) have been prepared by amide coupling of aminoferrocene and cobaltocenium carboxylic acid or 1,1′-cobaltocenium dicarboxylic acid. Very small differences of the redox potentials, Δ<i>G</i><sup>0</sup> ≈ 1 V, were observed due to the electron-donating and electron-withdrawing nature of the substituents at the ferrocene and cobaltocenium units, respectively. In accordance with this small zero-level energy difference, Δ<i>G</i><sup>0</sup> is the low-energy MM’CT absorption that extends into the near-infrared spectral region. Large solvatochromic shifts were observed for <b>1</b> and <b>2</b> due to the pronounced difference in ground-state and excited-state dipole moments. The low-energy optical transitions were assigned as MM’CT transitions by DFT-PCM and TDDFT-PCM approaches, and the experimental negative solvatochromic effect has been reproduced by the TDDFT-PCM calculations.