Syntheses and Structures of Magnesium and Zinc Boraamidinates: EPR and DFT Investigations of Li, Mg, Zn, B, and In Complexes of the [PhB(NtBu)2]•- Anion Radical

Abstract

The first magnesium and zinc boraamidinate (bam) complexes have been synthesized via metathetical reactions between dilithio bams and Grignard reagents or MCl2 (M = Mg, Zn). The following new classes of bam complexes have been structurally characterized: heterobimetallic spirocycles {(L)μ-Li[PhB(μ-NtBu)2]}2M (6a,b, M = Mg, L = Et2O, THF; 6c, M = Zn, L = Et2O); bis(organomagnesium) complexes {[PhB(μ3-NtBu)2](MgtBu)2(μ3-Cl)Li(OEt2)3} (8) and {[PhB(μ3-NtBu)2](MgR)2(THF)2} (9a, R = iPr; 9b, R = Ph); mononuclear complex {[PhB(μ-NDipp)2]Mg(OEt2)2} (10). Oxidation of 6a or 6c with iodine produces persistent pink (16a, M = Mg) or purple (16b, M = Zn) neutral radicals {Lx-μ-Li[PhB(μ-NtBu)2]2M}• (L = solvent molecule), which are shown by EPR spectra supported by DFT calculations to be Cs-symmetric species with spin density localized on one of the bam ligands. In contrast, characterization of the intensely colored neutral radicals {[PhB(μ-NtBu)2]2M}• (5c, M = In, dark green; 5d, M = B, dark purple) reveals that the spin density is equally delocalized over all four nitrogen atoms in these D2d-symmetric spirocyclic systems. Oxidation of the dimeric dilithio complex {Li2[PhB(μ4-NtBu)2]}2 with iodine produces the monomeric neutral radical {[PhB(μ-NtBu)2]Li(OEt2)x}• (17), characterized by EPR spectra and DFT calculations. These findings establish that the bam anionic radical [PhB(NtBu)2]•- can be stabilized by coordination to a variety of early main-group metal centers to give neutral radicals whose relative stabilities are compared and discussed.