Reduction of a 1,4-diazabutadiene and 2,2’-bipyridine using magnesium(I) compounds

β-diketiminate coordinated magnesium(I) compounds, [{(Nacnac)Mg}2] (ArNacnac = [(ArNCMe)2CH] (Ar = 2,6-diisopropylphenyl (Dip) or mesityl (Mes)), have been utilized as reducing agents in reactions with the redox active 1,4-diazabutadiene, (BuNCH)2 (ButDAB), and 2,2’-bipyrine (bipy). These reactions led to one-electron reductions of the unsaturated substrate, and formation of the highly colored radical complexes, [(ArNacnac) Mg(ButDAB∙)] and [(MesNacnac)Mg(bipy∙)(bipy)]. The X-ray crystal structures of the compounds reveal each to possess one singly reduced ButDAB or bipy ligand.

reduction; radical s-Block metals have been widely utilized as reagents for the reduction of organic substrates, yielding synthetically useful organometallic and inorganic products, for well over a century (Yamamoto and Oshima, 2004). Such reactions are often difficult to control because they are typically two-phase processes, and/or because the s-block metal involved is highly reducing. In 2007 we developed the first stable magnesium(I) compounds, e.g. LMg-MgL (L = β-diketiminate), and have since shown these to be widely applicable as soluble, stoichiometric, and safe reducing agents for the controlled reduction of numerous inorganic and unsaturated organic substrates (Green et al., 2007;Jones, 2017).
Two organic compound types we have not attempted to reduce using magnesium(I) compounds are redox noninnocent 1,4-diazabutadienes and 2,2'-bipyridines. While the one and two-electron reductions of such compounds have previously been achieved by a number of methods (e.g. reaction with activated elemental magnesium, and alkali metal reductions of magnesium halides in the presence of the unsaturate), product selectivity can be a problem (Freeman et al., 2019;Gao et al., 2011;Gardiner et al., 1994;Liu et al., 2009;Ren et al., 2017). We believed that the unique combination of properties that magnesium(I) reagents hold could lend them to the selective oneelectron reduction of 1,4-diazabutadienes (DAB) and 2,2'-bipyridine (bipy). Indeed, the radical products of these reductions could themselves be synthetically useful, as has previously shown to been the case for related radical systems (Ren et al., 2017). Here, we describe the synthesis and structural characterization of three such magnesium complexes of DAB and bipy radicals.
Compounds 1-3 are thermally stable in solution and the solid state, but all are extremely air sensitive. No signals were observed in their solution state NMR spectra, presumably due to their paramagnetic nature. Moreover, the extreme air sensitivity of the compounds hindered the acquisition of meaningful EPR spectroscopic data for them. However, all compounds were crystallographically characterized, and their molecular structures are shown in Figures 1 and 2 (see also Table 1). The magnesium centers  (7), N(4)-C(29) 1.324 (7), C(28)-C(29) 1.397 (8)  of 1 and 2 possess distorted tetrahedral coordination geometries, being chelated by both Ar Nacnac and radical But DAB ligands. The N-C and C-C bond lengths within the backbone of the But DAB ligands are strongly suggestive of electronic delocalization over those backbones, and are comparable to the N-C and C-C distances in other Mg complexes bearing singly reduced But DAB ligands, e.g. 1.32(1) Å and 1.40(1) Å respectively in [Mg( But DAB•) 2 ] (Gardiner et al., 1994).
The molecular structure of 3 reveals it to have a distorted octahedral geometry, with the magnesium atom being chelated by one Mes Nacnac and two bipy ligands. The N-C and C-C distances over the NCCN fragments of the two bipy ligands differ significantly, and suggest that the ligand incorporating the N(1) and N(2) centers is not reduced, while the ligand containing the N(3) and N(4) atoms is a singly reduced radical anion. This becomes apparent when those distances are compared to those in unreduced and singly reduced bipy ligands in related complexes, e.g. C-C 1.487(3) Å and N-C 1.349 Å (mean) in [( Dip Nacnac)Mg(SePh)(bipy)]; and C-C 1.425(3) Å and 1.392 Å (mean) in [( Dip Nacnac)Mg(bipy•)] (Ren et al., 2017).
In summary, reductions of either a 1,4-diazabutadiene or 2,2'-bipyrine with magnesium(I) compounds lead to single electron transfers to the redox active substrate, and formation of three magnesium radical complexes. The crystal structures of these highly air sensitive compounds show them all to possess one singly reduced radical ligand. We are currently exploring the use of these compounds as one electron reducing agents in both organic and inorganic synthesis.

Experimental General
All manipulations were carried out using standard Schlenk and glove box techniques under an atmosphere of high purity dinitrogen. Toluene and hexane were distilled over molten potassium, while pentane was distilled over 1:1 Na/K alloy. Mass spectra were collected using an Agilent Technologies 5975D inert MSD with a solid-state probe. FTIR spectra were collected as Nujol mulls on an Agilent Cary 630 attenuated total reflectance (ATR) spectrometer. Microanalyses were carried out at the Science Centre, London Metropolitan University. Melting points were determined in sealed glass capillaries under dinitrogen and are uncorrected. The starting materials But DAB (Haaf et al., 1998) and [{( Ar Nacnac)Mg} 2 ] (Ar = Dip or Mes) (Bonyhady et al., 2010;Hicks et al., 2018), were prepared by literature procedures. All other reagents were used as received.

Preparation of [( Mes Nacnac)Mg( But DAB•)] 2
[{( Mes Nacnac)Mg} 2 ] (150 mg, 0.210 mmol) and But DAB (71 mg, 0.420 mmol) were dissolved in 6 mL of toluene, yielding an orange-red solution. The reaction mixture was stirred for 2 h at room temperature. All volatiles were then removed in vacuo and the residue was extracted with n-pentane (10 mL). The extract was concentrated to ca. 2 mL and placed at -30°C for 5 days, after which time red-orange needles of 2 had deposited (

X-ray crystallography
Crystals of 1-3 suitable for X-ray structural determination were mounted in silicone oil. Crystallographic measurements were carried out at 123(2) K, and were made using an Rigaku Synergy diffractometer using a graphite monochromator with Mo Kα radiation (λ = 0.71073 Å) or Cu Kα radiation (λ = 1.54184 Å). The structures were solved by direct methods and refined on F 2 by full matrix least squares (SHELX16) (Sheldrick, 2016) using all unique data. All non-hydrogen atoms are anisotropic with hydrogen atoms included in calculated positions (riding model). The absolute structure parameter for the chiral crystal structure of 1 refined to 0.02(10). Crystal data, details of the data collection and refinement are given in Table 1. Crystallographic data for the structures have been deposited with the Cambridge Crystallographic Data Centre (CCDC no. 2023078-2023080). Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: +44-1223-336033; email: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).