Structural and metal–halogen exchange reactivity studies of sodium magnesiate biphenolate complexes

Abstract

Bimetallic sodium magnesiates have been employed in metal–halogen exchange for the first time. Utilising the racemic phenoxide ligand 5,5′,6,6′-tetramethyl-3,3′-di-tert-butyl-1,1′-biphenyl-2,2′-diol [(rac)-BIPHEN-H₂], the dialkyl sodium magnesiates [(rac)-BIPHEN]Na₂MgBu₂(TMEDA)₂3 and [(rac)-BIPHEN]Na₂MgBu₂(PMDETA)₂4 have been synthesised. Both 3 and 4 can be easily prepared through co-complexation of di-n-butylmagnesium with the sodiated (rac)-BIPHEN precursor which can be prepared in situ in hydrocarbon solvent. Prior to the main investigation, synthesis of the sodiated precursor [BIPHEN]₂Na₄(THF)₄1 was explored in order to better understand the formation of sodium magnesiates utilising the dianionic (rac)-BIPHEN ligand as the parent ligand. In addition, a BIPHEN-rich sodium magnesiate [BIPHEN]₂Na₂Mg(THF)₄2 was prepared and characterised, and its formation was rationalised. Complex 1 and 4 have also been fully characterised in both solid and solution state. In terms of onward reactivity, 3 and 4 have been tested as potential exchange reagents with aryl and heteroaryl iodides to produce aryl and heteroaryl magnesium phenoxides utilising toluene as a non-polar hydrocarbon solvent. Complex 3 reacted smoothly to give a range of aryl and heteroaryl magnesium phenoxides, whilst 4′s reactivity is more sluggish.