Unravelling 100-Year-Old Mystery of Rosenheim's Transition-Metal Paramolybdate Complexes

In 1916, Rosenheim reported the synthesis of transition-metal paramolybdate complexes of Co²⁺, Mn²⁺, and Cu²⁺ formulated as {M^II(MoO₄)₆}. However, later reports questioned the synthesis and structure of these compounds. Herein, we revisited this 100-year-old chemistry and clarified the nature of Rosenheim's complexes. We found that Rosenheim's synthesis method for {M^II(MoO₄)₆} (M = Co or Cu) indeed produced an Anderson-type [M^IIH₆Mo₆O₂₄]^4- cluster as the major product. Prolonged crystallization of the concentrated reaction mixture gave a Co²⁺-capped heptamolybdate or a Cu²⁺-linked octamolybdate, whereas crystallization of a dilute Cu²⁺/[Mo₇O₂₄]^6- mixture formed a new {Cu^II₆(OH)₆}-linked heptamolybdate. Rosenheim's synthesis method for {Mn^II(MoO₄)₆} does not produce the Anderson anion but a Mn²⁺-linked heptamolybdate. Moreover, adjusting the reactants' concentration and crystallization temperature gave three kinds of Mn²⁺-linked octamolybdates.