Accessing Stable Magnesium Acyl Compounds: Reductive Cleavage of Esters by Magnesium(I) Dimers.

The first examples of magnesium acyls, [(Nacnac)Mg{µ-C(Ph)O}(µ-OR)Mg(Nacnac)] (R=Me, tBu or Ph; Nacnac=[HC(MeCNAr)2 ]- ; Ar=C6 H2 Me3 -2,4,6 (Mes Nacnac), C6 H3 Et2 -2,6 (Dep Nacnac), C6 H3 iPr2 -2,6 (Dip Nacnac)), have been prepared by reductive cleavage of a series of esters using dimeric magnesium(I) reducing agents, [{(Nacnac)Mg}2 ]. Crystallographic studies reveal the complexes to be dimeric, being bridged by both phenyl-acyl and alkoxide/aryloxide fragments. The crystal structures, combined with results of spectroscopic and computational studies suggest that the nature of the acyl ligands within these complexes should be viewed as lying somewhere between anionic umpolung acyl and oxo-carbene. However, reactions of the acyl complexes with a variety of organic electrophiles did not provide evidence of umpolung acyl reactivity. A number of attempts to prepare alkoxide free magnesium acyls were carried out, and while these were unsuccessful, they did lead to unusual products, the crystallographic and spectroscopic details of which are discussed.

Synthesis and Reactivity of a Scandium Terminal Hydride: H2 Activation by a Scandium Terminal Imido Complex.

Dihydrogen is easily activated by a scandium terminal imido complex containing the weakly coordinated THF. The reaction proceeds through a 1,2-addition mechanism, which is distinct from the σ-bond metathesis mechanism reported to date for rare-earth metal-mediated H2 activation. This reaction yields a scandium terminal hydride, which is structurally well-characterized, being the first one to date. The reactivity of this hydride is reported with unsaturated substrates, further shedding light on the existence of the terminal hydride complex. Interestingly, the H2 activation can be reversible. DFT investigations further eludciate the mechanistic aspects of the reactivity of the scandium anilido-terminal hydride complex with PhNCS but also on the reversible H2 activation process.

Activation of SO2 by [Zn(Cp*)2] and [(Cp*)ZnI-ZnI(Cp*)].

Interesting reactivity was observed in reactions of SO2 with [Zn(Cp*)2] and [(Cp*)ZnI-ZnI(Cp*)]. These reactions proceeded with insertion of SO2 into the Zn-C bonds. Spectacularly, the lability of the C-S bond in the O2SCp* ligands led to the thermal decomposition of [Zn(O2SCp*)2(tmeda)] to afford [Zn2(µ-SO3)(µ-S2O4)(tmeda)2].

Uranium(iv) terminal hydrosulfido and sulfido complexes: insights into the nature of the uranium-sulfur bond.

Herein, we report the synthesis and characterization of a series of terminal uranium(iv) hydrosulfido and sulfido complexes, supported by the hexadentate, tacn-based ligand framework (Ad,MeArO)3tacn3- (= trianion of 1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane). The hydrosulfido complex [((Ad,MeArO)3tacn)U-SH] (2) is obtained from the reaction of H2S with the uranium(iii) starting material [((Ad,MeArO)3tacn)U] (1) in THF. Subsequent deprotonation with potassium bis(trimethylsilyl)amide yields the mononuclear uranium(iv) sulfido species in good yields. With the aid of dibenzo-18-crown-6 and 2.2.2-cryptand, it was possible to isolate a terminal sulfido species, capped by the potassium counter ion, and a "free" terminal sulfido species with a well separated cation/anion pair. Spectroscopic and computational analyses provided insights into the nature of the uranium-sulfur bond in these complexes.