A Redox-Switchable Germylene and its Ligating Properties in Selected Transition Metal Complexes.

The synthesis, structure, and full characterization of a redox-switchable germylene based on a [3]ferrocenophane ligand arrangement, [Fc(NMes)2 Ge] (4), is presented. The mesityl (Mes)-substituted title compound is readily available from Fc(NHMes)2 (2) and Ge{N(SiMe3 )2 }2 , or from the dilithiated, highly air- and moisture-sensitive compound Fc(NLiMes)2 ⋅3 Et2 O (3) and GeCl2 . Cyclic voltammetry studies are provided for 4, confirming the above-mentioned view of a redox-switchable germylene metalloligand. Although several 1:1 RhI and IrI complexes of 4 (5-7) are cleanly formed in solution, all attempts to isolate them in pure form failed due to stability problems. However, crystalline solids of [Mo(κ1 Ge-4)2 (CO)4 ] (8) and [W(κ1 Ge-4)2 (CO)4 ] (9) were isolated and fully characterized by common spectroscopic techniques (8 by X-ray diffraction). DFT calculations were performed on a series of model compounds to elucidate a conceivable interplay between the metal atoms in neutral and cationic bimetallic complexes of the type [Rh(κ1 E-qE)(CO)2 Cl]0/+ (qE=[Fc(NPh)2 E] with E=C, Si, Ge). The bonding characteristics of the coordinated Fc-based metalloligands (qE/qE+ ) are strongly affected upon in silico oxidation of the calculated complexes. The calculated Tolman electronic parameter (TEP) significantly increases by approximately 20 cm-1 (E=C) to 25 cm-1 (E=Si, Ge) upon oxidation. The change in the ligand-donating abilities upon oxidation can mainly be attributed to Coulombic effects, whereas an orbital-based interaction appears to have only a minor influence.