Light group 13 chloride diazadiene complexes: consequences of varying substituent bulk

Abstract

BCl₃ cyclizes diazadiene (2,6-Prⁱ₂C₆H₃NCH)₂1 through a dichloroborated intermediate [(2,6-Prⁱ₂C₆H₃NCHCl)₂BCl] 3 to give, in polar aprotic solvents, a spontaneously dehyrochlorinated C-chloro diazaborole 4. In contrast, reaction of AlCl₃ with 1 forms only acyclic mono- or di-adducts 5a/b and 6. Alkali metal reductions of 3 gave mixtures of 4 and diazaborole [(2,6-Prⁱ₂C₆H₃NCH)₂BCl] 7. Pd(0) reduction cleanly gave diazaborole 7. Reduction of 6 gave a low yield of the closed shell C–C coupled dimer 8 of the putative diazadiene radical anion 1·AlCl₂ complex monomer. An alternative synthesis for diazadiene (2,6-Prⁱ₂C₆H₄NCPh)₂, 2, is reported. Reduction of 2/BCl₃, in which additional phenyl groups on the diazadiene C-2 and C-3 atoms hinder the radical coupling observed in 8, gave predominantly diazaborole 2·BCl, (9a) contaminated with 2·BCl₂, (9b) the first such stable radical diazadiene complex of boron. All compounds 2–9 were characterized by X-ray diffraction and NMR spectroscopy. Stable radical 9b was additionally characterized by EPR spectroscopy and density functional computation.