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

## TL;DR

This study compares the acid-base behavior of water-soluble molybdenum and tungsten complexes, revealing differences in their acidity and stability in aqueous solutions.

## Contribution

The paper provides new insights into the aqueous acid-base properties of tungstocene and molybdocene complexes in biologically relevant oxidation states.

## Key findings

- Molybdocene acts as a triprotic acid, while tungstocene is diprotic and monomeric.
- Tungsten complexes show higher acidity than molybdenum complexes with 1–1.5 lower pKa values.
- Tosylate-substituted complexes are less suitable for aqueous chemistry due to poor hydrolysis and degradation.

## Abstract

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] (1a), [Cp2Mo­(μ-OH)2MoCp2]­(pTsO)2 (1b), [Cp2Mo­(pTsO)2]
(3) and their tungsten analogues [Cp2WCl2] (2a), [Cp2W­(μ-OH)2WCp2]­(pTsO)2 (2b), and [Cp2W­(pTsO)2] (4) 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 pK
a values than molybdenum, which is of general interest for the understanding
of tungstoenzymes. The substitution of chlorides by tosylates allowed
the isolation of 3 and 4, 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.