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

Abstract The reaction between Mo 2 (O 2 CBu t ) 4 and [Bu n 4 N] + [Bu t CO 2 ] − in toluene yields the yellow toluene-soluble salt [Bu n 4 N] + [Mo 2 (O 2 CBu t ) 5 ] − as large yellow plates upon crystallization. The molecular struture of the anion contains the now common paddle-wheel or lantern M 2 (O 2 C) 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 (η l -pivalate) = 176.6(1)°. In CD 2 Cl 2 and toluene-d 8 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 Bu t protons. In pyridine-d 5 the exchange of pivalates is slower but still sufficiently fast to lead to coalescence at elevated temperatures. In pyridine solution the predominant species are Mo 2 (O 2 CBu t ) 4 (py) 2 and [Bu n 4 N] + [Bu t CO 2 ] − . These findings complement earlier work wherein [Et 4 N] + [Mo 2 (O 2 CCF 3 ) 5 ] − was prepared (Garner and Senior, J. Chem. Soc., Dalton Trans. 1975, 1171) and the exchange of CF 3 CO − 2 groups in the reaction between Mo 2 (O 2 CCF 3 ) 4 and NaO 2 CCF 3 in CH 3 CN was studied (Sasaki et al., Bull. Chem. Soc. Japan 1979, 52 , 446). The facile intramolecular exchange of pivalate ligands in the Mo 2 (O 2 CBu t ) − 5 anion emphasizes that Mo 4+ 2 containing compounds are kinetically labile to associative nucleophilic substitution reactions. Possible reaction pathways leading to exchange of Bu t CO − 2 are discussed.