Acyclic 1,2-dimagnesioethanes/-ethene derived from magnesium(i) compounds: multipurpose reagents for organometallic synthesis.

Open Access Article. Published on 04 February 2019. Downloaded on 3/21/2019 1:48:43 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Chemical Science View Article Online EDGE ARTICLE Cite this: Chem. Sci., 2019, 10, 3208 All publication charges for this article have been paid for by the Royal Society of Chemistry View Journal | View Issue Acyclic 1,2-dimagnesioethanes/-ethene derived from magnesium(I) compounds: multipurpose reagents for organometallic synthesis† Deepak Dange,a Andrew R. Gair,ab Dafydd D. L. Jones, Simon Aldridge b and Cameron Jones *a a Martin Juckel,a Reactions of three magnesium(I) dimers, [{(ArNacnac)Mg–}2] (ArNacnac ¼ [(ArNCMe)2CH]; Ar ¼ xylyl (Xyl), mesityl (Mes) or 2,6-diethylphenyl (Dep)), with either 1,1-diphenylethylene (DPE), a-methylstyrene (MS), trans-stilbene (TS) or diphenylacetylene (DPA) led to the 1,2-addition of the Mg–Mg bond across the substrate, giving rise to the 1,2-dimagnesioethanes, [{(XylNacnac)Mg}2(m-DPE)], [{(DepNacnac)Mg}2(m-MS)], [{(ArNacnac)Mg}2(m-TS)] (Ar ¼ Mes or Dep); and a 1,2-dimagnesioethene, [{(MesNacnac)Mg}2(m-DPA)]. The reactions involving the 1,1-substituted alkenes are shown to be readily redox reversible, in that the reaction products are in equilibrium with a significant proportion of the starting materials at room temperature. Variable temperature NMR spectroscopy and a van't Hoff analysis point to low kinetic barriers to these weakly exergonic reactions. [{(MesNacnac)Mg}2(m-DPE)] and [{(MesNacnac)Mg}2(m-DPA)] behave as 1,2-di-Grignard reagents in their reactions with very bulky amido-zinc bromides, yielding the first examples of a 1,2-dizincioethane, [(L*Zn)2(m-DPE)] (L* ¼ –N(Ar*)(SiPri3); Ar* ¼ C6H2Me{C(H)Ph2}24,2,6), and a 1,2-dizincioethene, [(TBoLZn)2(m-DPA)] (TBoL ¼ –N(SiMe3){B(DipNCH)2}, Dip ¼ 2,6diisopropylphenyl), respectively. Divergent reactivity is shown for [{(MesNacnac)Mg}2(m-DPE)], which behaves as a two-electron reducing agent when treated with amido-cadmium and amido-magnesium halide precursors, yielding the cadmium(I) and magnesium(I) dimers, [PhBoLCdCdPhBoL] (PhBoL ¼ –N(SiPh3) {B(DipNCH)2}) and [L†MgMgL†] (L† ¼ –N(Ar†)(SiMe3); Ar† ¼ C6H2Pri{C(H)Ph2}2-4,2,6), respectively. A Received 14th January 2019 Accepted 2nd February 2019 further class of reactivity for [{(MesNacnac)Mg}2(m-DPE)] derives from its reaction with the bulky amidogermanium chloride, L*GeCl, which gives a magnesio-germane, presumably via intramolecular C–H DOI: 10.1039/c9sc00200f activation of a highly reactive magnesiogermylene intermediate, [:Ge(L*){Mg(MesNacnac)}]. [{(MesNacnac) Mg}2(m-DPE)] can be considered as acting as a two-electron reducing, magnesium transfer reagent in rsc.li/chemical-science this reaction. Introduction Over the last 15 years or so, interest in low oxidation state pblock systems has rapidly grown, largely as a result of the fact that many such compounds have been shown capable of activating a variety of small molecules (e.g. H2, CO, CO2, alkenes etc.) under mild conditions.1 These activations are oen closely related to fundamental steps in important synthetic transformations that are typically catalysed by late transition metal complexes. Although the involvement of low oxidation state pa School of Chemistry, Monash University, PO Box 23, VIC, 3800, Australia b Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK. E-mail: ; Web: http://www.monash.edu/science/research-groups/chemistry/jonesgroup † Electronic supplementary information (ESI) available: Experimental procedures and characterisation data for all new compounds. CCDC 1890169–1890179. Crystal data, details of data collections and renements. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc00200f 3208 | Chem. Sci., 2019, 10, 3208–3216 block complexes in analogous catalytic transformations is generally prevented by the irreversibility of their oxidative small molecule activations, reports of reversible redox processes, and associated catalysis, at p-block element centres are becoming increasingly common.2 Given the more electropositive nature of s-block metals, it is perhaps not surprising that reversible redox processes involving those metals at room temperature were unknown until recently. Indeed, for such processes to occur, low oxidation state s-block metal compounds would need to be accessible. The only stable, well developed examples of compounds in this category are magnesium(I) dimers, LMg–MgL (L ¼ anionic ligand), rst described by us in 2007.3,4 Since that time, approximately 20 examples of magnesium(I) compounds have come forward, and their utility as widely applicable, selective, soluble reducing agents in many areas of inorganic and organic synthesis has been extensively exploited.5 This work has included the reductive activation of a wide array of small molecules and This journal is © The Royal Society of Chemistry 2019 View Article Online Open Access Article. Published on 04 February 2019. Downloaded on 3/21/2019 1:48:43 PM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Edge Article unsaturated substrates, none of which are reversible in the absence of an external reducing agent. This situation changed in 2017 when we reported that reactions between two magnesium(I) dimers and the activated alkene, 1,1-diphenylethylene (DPE), led to 1,2-addition of the Mg–Mg bonds across the alkene, and the formation of the 1,2dimagnesioethane compounds, 1 and 2 (Scheme 1).6 Remarkably, these reactions were shown to readily and rapidly reversible at room temperature, leading to equilibrium mixtures of products and reactants. As such, they represented the rst examples of reversible redox processes for s-block metal complexes. Also of interest is the fact that 1 and 2 can be considered as analogues of acyclic 1,2-di-Grignard reagents,7 which are unknown despite attempts to prepare them since the time of Grignard himself.8 Preliminary reactivity studies of 1 and 2 revealed that to some extent they do behave as Grignard reagents, but the high charge density on their doubly reduced C–C cores leads to them behaving as more highly activated magnesium alkyls. This was demonstrated through their facile reactions with H2, CO and ethylene which lead to hydrogenolysis of an Mg–C bond, C–C coupling of two CO molecules with the DPE dianion, and insertion of ethylene into an Mg–C bond, respectively. The high reactivity of 1 and 2 is evidenced by the fact that magnesium alkyls have previously proved unreactive towards these three gaseous substrates in the absence of catalysts. The activated nature of 1 and 2 is comparable to that of a trans-stilbene dianion bridged bis(amidinato-calcium) complex 3 (ref. 9 and 10) and a benzene dianion bridged bis(b-diketiminato-magnesium) complex 4,11 which Harder and co-workers have recently (...truncated)