The dimolybdenum pentapivalate anion: X-ray crystal structure and dynamic solution behaviour and comments on the substitutional lability of containing compounds

The reaction between Mo2(O2CBut)4 and [Bun4N]+[ButCO2]− in toluene yields the yellow toluene-soluble salt [Bun4N]+[Mo2(O2CBut)5]− as large yellow plates upon crystallization. The molecular struture of the anion contains the now common paddle-wheel or lantern M2(O2C)4 core, = 2.1035(7) and MoO = 2.12(2) A (av), with a monodentate pivalate ligand coordinated to one metal atom, MoO = 2.298(5) A in the axial position and MoMoO (ηl-pivalate) = 176.6(1)°. In CD2Cl2 and toluene-d8 solutions the anion shows fluxional behaviour such that exchange between all pivalates is rapid on the NMR time-scale at room temperature. At −80°C this fluxional process becomes sufficiently slow to lead to the expected 4:1 ratio for the But protons. In pyridine-d5 the exchange of pivalates is slower but still sufficiently fast to lead to coalescence at elevated temperatures. In pyridine solution the predominant species are Mo2(O2CBut)4(py)2 and [Bun4N]+[ButCO2]−. These findings complement earlier work wherein [Et4N]+[Mo2(O2CCF3)5]− was prepared (Garner and Senior, J. Chem. Soc., Dalton Trans. 1975, 1171) and the exchange of CF3CO−2 groups in the reaction between Mo2(O2CCF3)4 and NaO2CCF3 in CH3CN was studied (Sasaki et al., Bull. Chem. Soc. Japan 1979, 52, 446). The facile intramolecular exchange of pivalate ligands in the Mo2(O2CBut)−5 anion emphasizes that Mo4+2 containing compounds are kinetically labile to associative nucleophilic substitution reactions. Possible reaction pathways leading to exchange of ButCO−2 are discussed.