Ferrocene–Dithiolene Hybrids: Control of Strong Donor–Acceptor Electronic Communication to Reverse the Charge Transfer Direction

We prepared a novel class of ferrocene–dithiolene hybrid molecules, FcS<sub>4</sub>dt­(Me)<sub>2</sub> and FcS<sub>4</sub>dt­[Pt­(<sup><i>t</i></sup>Bu<sub>2</sub>bpy)] (where FcS<sub>4</sub>dt indicates 2-(1,3-dithia[3]­ferrocenophane-2-ylidene)-1,3-dithiole-4,5-dithiolate and <sup><i>t</i></sup>Bu<sub>2</sub>bpy indicates 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine), in which the ferrocene moiety was bound to the planar conjugated dithiolene skeleton via two sulfur atoms such that the cyclopentadienyl rings were perpendicular to the dithiolene backbone. The physical properties and electronic structures of the complexes and their oxidized species [FcS<sub>4</sub>dt­(Me)<sub>2</sub>]<sup>•+</sup> and [FcS<sub>4</sub>dt­[Pt­(<sup><i>t</i></sup>Bu<sub>2</sub>bpy)]]<sup>•+</sup> were investigated by means of single-crystal X-ray diffraction (XRD) analysis, cyclic voltammetry, electron paramagnetic resonance (EPR), and UV–vis near infrared (UV–vis–NIR) spectroscopy. The electron density distributions of the highest occupied molecular orbitals (HOMOs) of FcS<sub>4</sub>dt­(Me)<sub>2</sub> and FcS<sub>4</sub>dt­[Pt­(<sup><i>t</i></sup>Bu<sub>2</sub>bpy)] differed remarkably in that the HOMO of the former was ferrocene-based whereas that of the latter was dithiolene-based. The differences in the HOMO distributions originated from the energy level of the dithiolene-based π-orbital in each of the complexes, which was controlled by changing R in FcS<sub>4</sub>dt­(R)<sub>2</sub> (R = Me for FcS<sub>4</sub>dt­(Me)<sub>2</sub>; 2R = Pt­(<sup><i>t</i></sup>Bu<sub>2</sub>bpy) for FcS<sub>4</sub>dt­[Pt­(<sup><i>t</i></sup>Bu<sub>2</sub>bpy)]). We succeeded in analyzing the crystal structure of [FcS<sub>4</sub>dt­[Pt­(<sup><i>t</i></sup>Bu<sub>2</sub>bpy)]]­(F<sub>4</sub>TCNQ)·C<sub>6</sub>H<sub>14</sub>·CH<sub>2</sub>Cl<sub>2</sub> (where F<sub>4</sub>TCNQ indicates 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), which provided a rare example of the crystal structure of a [Pt­(diimine)­(dithiolate)]<sup>•+</sup> ion-based complex. A comparison of the bond lengths in FcS<sub>4</sub>dt­[Pt­(<sup><i>t</i></sup>Bu<sub>2</sub>bpy)] and [FcS<sub>4</sub>dt­[Pt­(<sup><i>t</i></sup>Bu<sub>2</sub>bpy)]]<sup>•+</sup> suggested that the latter complex displayed a conjugated dithiolene-based π-radical character. These considerations agreed well with the electronic structures calculated using density functional theory (DFT) and time-dependent­(TD)-DFT methods. Significant electronic communication between the ferrocene and dithiolene moieties was detected for both [FcS<sub>4</sub>dt­(Me)<sub>2</sub>]<sup>•+</sup> and [FcS<sub>4</sub>dt­[Pt­(<sup><i>t</i></sup>Bu<sub>2</sub>bpy)]]<sup>•+</sup> in the appearance of an intramolecular charge transfer band, which was hardly observed for previously reported ferrocene–dithiolene hybrid molecules. The charge transfer direction was reversed between the two cations. The electron coupling parameter <i>H</i><sub>AB</sub> and the potential energy curves of the oxidized complexes were estimated based on the classical two-state Marcus–Hush theory. These results suggest that FcS<sub>4</sub>dt-based metalladithiolenes can exhibit controllable electronic structures expressed as double-minimum potential energy curves.