Fluoride reactions with hydrotris(pyrazolyl)borate rhenium oxo complexes: Re-F vs B-F bond formation

Rhenium-oxo-fluoride complexes are readily prepared using hydrotris(3,5-dimethyl-1-pyrazolyl)borate [HB(3,5-Me(2)pz)(3)(-), Tp*] as a supporting ligand. For instance, treatment of Tp*Re(O)(OH)Cl with concentrated aqueous HF cleanly forms the chloro-fluoride complex Tp*Re(O)(F)Cl. Related fluoro-iodide, fluoro-triflate, and difluoride complexes are also described. In contrast, a variety of reactions of analogous compounds with the unsubstituted hydrotris(1-pyrazolyl)borate ligand [HB(pz)(3)(-), Tp] have failed to produce fluoro complexes. Extended refluxing of TpRe(O)I-2 with an excess of NaF in acetonitrile in the air gives a modest yield of an unusual rhenium mu-pyrazolyl mu-oxo dimer, {[kappa(2)-H(F)Bpz(2)]Re(O)}(2)(mu-pz)(2)(mu-O) (5) (pz = pyrazolyl). The X-ray crystal structure of 5 shows that one pyrazolyl of each Tp ligand has been substituted for fluoride. The different reactivity of the Tp and Tp* complexes is apparently due to the greater steric protection of the boron afforded by the Tp* ligand. Crystallographic data for 5: monoclinic; space group Cc; a = 19.103(4) Angstrom, b = 10.482(2) Angstrom, c = 14.240(3) Angstrom, beta = 114.82(3)degrees; Z = 4; R = 3.73%, R-w (observed data) = 4.73%; GOF = 1.20.