Comparative Study of Aqueous Acid–Base Properties of Tungstocene and Molybdocene Complexes

The aqueous chemistry of molybdenum and tungsten complexes is relevant due to their occurrence in metalloenzymes. However, water-stable and -soluble model complexes in biologically relevant higher oxidation states are rare. Metallocenes of the type [Cp₂M-(OH₂)]²⁺ (M = Mo, W) exhibit such properties despite the nonbiomimetic cyclopentadienyl (Cp) ligand. Therefore, the aqueous acid-base properties of these bis-aqua tungstocenes and molybdocenes were investigated, as their +IV oxidation states and coordinated H₂O render them ideal candidates. The precursors [Cp₂MoCl₂] (<b>1a</b>), [Cp₂Mo-(μ-OH)₂MoCp₂]-(<i>p</i>TsO)₂ (<b>1b</b>), [Cp₂Mo-(<i>p</i>TsO)₂] (<b>3</b>) and their tungsten analogues [Cp₂WCl₂] (<b>2a</b>), [Cp₂W-(μ-OH)₂WCp₂]-(<i>p</i>TsO)₂ (<b>2b</b>), and [Cp₂W-(<i>p</i>TsO)₂] (<b>4</b>) were studied via aqueous potentiometric titrations. Molybdocene acted as a triprotic acid, with deprotonation occurring from both mono- and dimeric species, while tungstocene reacted as a diprotic acid exclusively in its monomeric form. Tungsten complexes exhibit higher acidity with 1-1.5 lower p<i>K</i> _a values than molybdenum, which is of general interest for the understanding of tungstoenzymes. The substitution of chlorides by tosylates allowed the isolation of <b>3</b> and <b>4</b>, which proved to be less suitable precursors for aqueous chemistry, as they were more difficult to hydrolyze, leading to partial degradation upon hydrolysis. This indicates that simply the presence of a Cp₂M motif is not sufficient for the formation of the aqueous species.