Experimental and computational exploration of the dynamic behavior of (PNP)BF2, a boron compound supported by an amido/bis(phosphine) pincer ligand

Abstract

The diarylamido/bis(phosphine) PNP pincer ligand (2-ⁱPr₂P-4-MeC₆H₃)₂N has been evaluated as a scaffold for supporting a BF₂ fragment. Compound (PNP)BF₂ (6) was prepared by simple metathesis of (PNP)Li (5) with Me₂SBF₃. NMR spectra of 6 in solution are of apparent C₂ symmetry, suggestive of a symmetric environment about boron. However, a combination of X-ray structural studies, low-temperature NMR investigations, and DFT calculations consistently establish that the ground state of this molecule contains a classical four-coordinate boron with a PNBF₂ coordination environment, with one phosphine donor in PNP remaining “free”. Fortuitous formation of a single crystal of (PNP)BF₂·HBF₄ (7), in which the “free” phosphine is protonated, furnished another structure containing the same PNBF₂ environment about boron for comparison and the two PNBF₂ environments in 6 and 7 are virtually identical. DFT studies on several other diarylamido/bis(phosphine) pincer (PNP)BF₂ systems were carried out and all displayed a similar four coordinate PNBF₂ environment in the ground state structures. The symmetric appearance of the room-temperature NMR spectra is explained by the rapid interconversion between two degenerate four-coordinate, C₁-symmetric ground-state forms. Lineshape analysis of the ¹H and ¹⁹F NMR spectra over a temperature range of 180–243 K yielded the activation parameters ΔH^‡ = 8.1(3) kcal mol⁻¹ and ΔS^‡ = −6.0(15) eu, which are broadly consistent with the calculated values. Calculations indicate that the exchange of phosphine donors at the boron center proceeds by an intrinsically dissociative mechanism.