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 TpRe(O)(OH)Cl with concentrated aqueous HF cleanly forms the chloro-fluoride complex TpRe(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 &mgr;-pyrazolyl &mgr;-oxo dimer, {[kappa(2)-H(F)Bpz(2)]Re(O)}(2)(&mgr;-pz)(2)(&mgr;-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) Å, b = 10.482(2) Å, c = 14.240(3) Å, beta = 114.82(3) degrees; Z = 4; R = 3.73%, R(w) (observed data) = 4.73%; GOF = 1.20.