Synthesis and reactivity of oxygen ligated molybdenum(II) carbonyl complexes

Abstract

Bis(acetylacetonate) and bis(tropolonate) complexes of MoII were synthesized by combining [NEt4][Mo(CO)4(I)3] and two equivalents of the ligand, followed by trapping with either an alkyne or phosphine. Alkyne complexes (trop)2Mo(CO)(PhCCH) (1b) and (acac)2Mo(CO)(PhCCR) (R=Me (2a), H (2b)) were characterized crystallographically. The alkyne ligands undergo rapid rotation at room temperature on the NMR time scale and the barrier to rotation for the butyne complexes (LX)2Mo(CO)(MeCCMe) (LX=trop (1c), acac (2c)) were measured by variable temperature NMR. The rotational barrier of the tropolonate derivative 1c is 1.6kcal/mol lower than that of the acetylacetonate derivative 2c despite the fact that trop is both smaller and less basic than acac. The coordinated alkynes will readily undergo exchange with free alkyne in solution and, on a slower time scale, the CO ligand can be displaced to form the bis(alkyne) species (LX)2Mo(RCCR)2 (4). The CO ligand of (LX)2Mo(CO)(PhCCH) can also be readily displaced by P(OMe)3 to form (LX)2Mo(P(OMe)3)(PhCCH), but larger phosphorus donors do not react. Alkyne complexes 1 and 2 will oxidize in air to the d2 species (LX)2Mo(O)(RCCR) in which the alkyne has been rotated perpendicular to the Mo–O bond and locked in position. The synthesis of the (LX)2Mo(CO) fragment reported here provides access to oxygen ligated low-valent Mo species which have thus far remained elusive.