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

• Published in: Inorganic chemistry 2006-03, Vol.45 (5), p.2119-2131
• Main Authors: Chivers, Tristram, Eisler, Dana J, Fedorchuk, Chantall, Schatte, Gabriele, Tuononen, Heikki M, Boeré, René T
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0520014

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 {L x -μ-Li[PhB(μ-NtBu)2]2M}• (L = solvent molecule), which are shown by EPR spectra supported by DFT calculations to be C s -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 D 2 d -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.