Reduction of Boron Heterocycles Supported by a Binucleating NacNac/NCN Pincer Janus Ligand
Chia Yun Chang1, Laurie J Donnelly1, Alexander Harrison1
1Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada.
Abstract:
The reduction chemistry of a difluoroboron β-diketiminato complex incorporating ortho-pyridyl groups as the substituent on the iminyl carbons, LBF2, has been explored using KC8 as the reductant. Compound LBF2 undergoes smooth, sequential reversible reductions with one or two electrons to yield fluoride ion-stabilized reduced boron heterocycles. These reaction outcomes are influenced by the binucleating capabilities of the Janus ligand employed, where the unligated NCN pincer bay sequesters the potassium ion generated in the reduction through the pendant pyridyl groups, preventing loss of KF and leaving the fluoride ion to act as a Lewis base stabilizer for the boron centers. Experimental spectroscopic (EPR, UV-vis) and structural (X-ray diffraction) studies, and computational (DFT) investigations, show that the orbital that receives the electrons is associated with a π* orbital of the central C3N2B ring that is primarily iminyl C-N antibonding in nature. Although there is little spin or orbital density on the boron center, reactivity that involves reoxidation of the reduced heterocycles is borocentric and indicative of formal B(II) and B(I) species for the mono and direduced heterocycles, respectively. Thus, use of binucleating ligands may be a fruitful strategy for generating sources of reactive low valent main group derivatives.
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