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

The diarylamido/bis(phosphine) PNP pincer ligand (2-(Pr2P)-Pr-i-4-MeC6H3)(2)N 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 C-2 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 center dot 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, C-1-symmetric ground-state forms. Lineshape analysis of the H-1 and F-19 NMR spectra over a temperature range of 180-243 K yielded the activation parameters Delta H-double dagger = 8.1(3) kcal mol(-1) and Delta S-double dagger = -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.