A Second Metal Center Enhances the Reactivity of an Organomagnesate: Comparison of the Gas-Phase Reactions of Water with [RCCMgCl2] and [RCCMg2Cl4] (R = H, Ph)

The gas-phase formation and hydrolysis reactions of the mononuclear and binuclear organomagnesate ions [RCCMgnCl2n] (where R = H, Ph and n = 1, 2) were studied using a combination of ion trap mass spectrometry based experiments and DFT calculations. The organomagnesates were formed via decarboxylation of the carboxylates [RCCCO2MgnCl2n]. The binuclear organomagnesates [RCCMg2Cl4] were found to be at least 5 times more reactive than the mononuclear organomagnesates [RCCMgCl2] (where R = H, Ph). The DFT calculations highlight the role that a second magnesium center can have on both the formation and reaction of an organomagnesate. Thus, DFT calculations reveal that while decarboxylation of the mononuclear carboxylate [HCCCO2MgCl2] proceeds via only one pathway involving a four-centered transition state, there are two different decarboxylation pathways possible for the isomeric binuclear carboxylates [HCCCO2Mg2Cl4]: (i) a related four-centered transition state involving a single Mg, resulting in the organomagnesate isomer with a terminal acetylide [HCCMg(μ-Cl3)MgCl] and (ii) a six-centered transition state involving both Mg centers, which gives rise to the organomagnesate isomer with the bridging acetylide [ClMg(μ-CCH)(μ-Cl2)MgCl]. Decarboxylation via the six-centered transition state is slightly preferred on both thermodynamic and kinetic grounds. DFT calculations suggest that the source of the difference in the hydrolysis is the direct involvement of the second Mg center. Thus, while hydrolysis of the mononuclear organomagnesate can only proceed via a four-centered transition state, hydrolysis of the binuclear organomagnesate can proceed via a six-centered transition state in which the water binds to one Mg, while the departing acetylide coordinates to the other Mg.