Magnesium(I) Dimers Bearing Tripodal Diimine–Enolate Ligands: Proficient Reagents for the Controlled Reductive Activation of CO2 and SO2

The first examples of magnesium(I) dimers bearing tripodal ligands, [(Mg{κ3-N,N′,O-(ArNCMe)2(OCCPh2)CH})2] [Ar=2,6-iPr2C6H3 (Dip) 7, 2,6-Et2C6H3 (Dep) 8, or mesityl (Mes) 9] have been prepared by post-synthetic modification of the β-diketiminato ligands of previously reported magnesium(I) systems, using diphenylketene, OCCPh2. In contrast, related reactions between β-diketiminato magnesium(I) dimers and the isoelectronic ketenimine, MesNCCPh2, resulted in reductive insertion of the substrate into the MgMg bond of the magnesium(I) reactant, and formation of [{(Nacnac)Mg}2{μ-κ2-N,C-(Mes)NCCPh2}] (Nacnac=[(ArNCMe)2CH]−; Ar=Dep 10 or Mes 11). Reactions of the four-coordinate magnesium(I) dimer 8 with excess CO2 are readily controlled, and cleanly give carbonate [(LMg)2(μ-κ2:κ2-CO3)] 12 (L=[κ3-N,N′,O-(DepNCMe)2(OCCPh2)CH]−; thermodynamic product), or oxalate [(LMg)2(μ-κ2:κ2-C2O4)] 13 (kinetic product), depending on the reaction temperature. Compound 12 and CO are formed by reductive disproportionation of CO2, whereas 13 results from reductive coupling of two molecules of the gas. Treatment of 8 with an excess of N2O cleanly gives the μ-oxo complex [(LMg)2(μ-O)] 14, which reacts facilely with CO2 to give 12. This result presents the possibility that 14 is an intermediate in the formation of 12 from the reaction of 8 and CO2. In contrast to its reactions with CO2, 8 reacts with SO2 over a wide temperature range to give only one product; the first example of a magnesium dithionite complex, [(LMg)2(μ-κ2:κ2-S2O4)] 16, which is formed by reductive coupling of two molecules of SO2, and is closely related to f-block metal dithionite complexes derived from similar SO2 reductive coupling processes. On the whole, this study strengthens previously proposed analogies between the reactivities of magnesium(I) systems and low-valent f-block metal complexes, especially with respect to small molecule activations.