Lawesson's Reagent: Providing a New Approach to the Forgotten 6-Thioverdazyl Radical.

A new method for the preparation of the underrepresented 1,5-dimethyl-6-thioverdazyl radicals has been developed employing Lawesson's reagent (LR). The synthetic route involves the direct thionation of the carbonyl group of the corresponding dialkylbishydrazone followed by cyclization to give the tetrazinanthione verdazyl precursor on a gram scale. Subsequent oxidation yields the 6-thioverdazyl radical. It was determined that thionation of substrates containing electron-withdrawing groups in the ortho- or para-positions was high yielding. In contrast, for the parent phenyl group or substrates bearing weakly electron-donating substituents, thionation efficiency was significantly reduced. This could be overcome by utilizing partial in situ cyclization, which occurs during work up, to generate the tetrazinanthione directly via a one-pot synthesis. Density functional theory suggests that the LR fragment interacts with the carbonyl prior to cycloaddition and subsequent to cycloreversion, leading to the thiocarbonyl. The electronic nature of the radical is characterized with electron paramagnetic resonance as well as the first report of 6-thioverdazyl redox properties.

A one-pot reduction route to bimetallic manganese 1,8-naphthyridine complexes.

Reaction of the dinucleating ligand 2,7-bis(6-methyl-2-pyridyl)-1,8-naphthyridine (MeL) with the MnI and MnII precursors MnBr(CO)5 and MnCl2 resulted in the formation of the monometallic complexes [MnBr(CO)3(MeL)] (1) and [MnCl2(MeL)] (3). In both cases, formation of bimetallic manganese complexes could be achieved by reduction with KC8, yielding the carbonyl-bridged complex [Mn2(CO)6(MeL)] (2) and the helicate complex [Mn2(MeL)2] (4), respectively. EPR results demonstrate that 4 represents a novel, weakly antiferromagnetically coupled homovalent dimer (J = -0.85 cm-1). The two formally Mn0 ions are both high spin (S = 3/2) and exhibit a zero-field splitting of ≈1 cm-1, suggesting reduction of the complex is substantially ligand centered, and may be better described as a MnII complex coupled to two open shell singlet ligands [MnII2(MeL2-)2]. X-ray crystallography, UV-Vis spectroscopy and DFT analysis support this finding.

One- and two-electron reductions of a bulky BODIPY compound.

The redox reaction between a bulky BODIPY and a magnesium(I) reducing agent leads to the formal one-electron reduction of the BODIPY, initially generating a dipyrromethene-centred radical compound that dimerises via C-C bond formation. In contrast, reduction with magnesium anthracene leads to the formal two-electron reduction of the BODIPY, resulting in the formation of the corresponding anion.

Ketyl Radicals Generated from Magnesium(I) Compounds: Useful Reagents for C-C Bond Forming Reactions.

The aryl ketones, 9-fluorenone (fluor), 9-xanthenone (xanth) and 9,10-anthraquinone (anth), were reacted with ß-diketiminato dimagnesium(I) compounds, [{(Ar Nacnac)Mg}2 ] (Ar Nacnac=[HC(MeCNAr)2 ]- , Ar=mesityl (Mes) or 2,6-diisopropylphenyl (Dip)). This gave stable magnesium ketyl complexes which are monomeric, [(Ar Nacnac)(DMAP)Mg(fluor⋅)] (Ar=Mes or Dip, DMAP=4-dimethylaminopyridine) and [(Mes Nacnac)Mg(xanth⋅)(xanth)]; dimeric, [{(Mes Nacnac)Mg(µ-fluor⋅)}2 ], or tetrameric, [{(Dip Nacnac)Mg(µ-anth⋅)}4 ]. In contrast, di-2-pyridylketone (OCPy2 ) is doubly reduced with [{(Xyl Nacnac)Mg}2 ] (Xyl=xylyl) to give a diamagnetic alkoxy/amido complex, [{(Xyl Nacnac)Mg}2 (µ-OCPy2 )]. These complexes have been characterized by X-ray crystallography, and in three cases, EPR spectroscopy. Regioselective C-C hetero-coupling reactions between magnesium ketyls and phenanthroline (phen) have yielded the alkoxy compounds, [(Mes Nacnac)Mg(OCR2 -2-phen)] (OCR2 =xanth, OCPh2 or OC(Ph)(2-Me-Ph)). In addition, homo- C-C couplings of the enones, chalcone (chalc) and dibenziylideneactetone (DBA), using magnesium(I) reducing agents, have afforded dimagnesium enolates, [{(Ar Nacnac)(THF)Mg}2 (µ-chalc2 )] (Ar=Mes or Xyl) and [{(Dip Nacnac)(THF)Mg}2 (µ-DBA2 )]. A pinacol coupling reaction between [{(Dip Nacnac)Mg}2 ] and 2-adamantanone (OAd) yielded [{(Dip Nacnac)(OAd)Mg}2 (µ-OAd2 )], presumably via a ketyl intermediate. This study further highlights the utility magnesium(I) compounds have as selective reducing agents in organic transformations.

Crystalline Germanium(I) and Tin(I) Centered Radical Anions.

An isostructural series of heavy Group 14 E(I) radical anions (Ge, Sn, Pb), stabilized by a bulky xanthene-based diamido ligand are reported. The radical anions were synthesised by the one-electron reduction of their corresponding E(II) precursor complexes with sodium naphthalenide in THF, yielding the radical anions as charge-separated sodium salts. The series of main group radicals have been comprehensively characterized by EPR spectroscopy, X-ray crystallography and DFT analysis, which reveal that in all cases, the spin density of the unpaired electron almost exclusively resides in a p-orbital of π symmetry located on the Group 14 center.