Tp* Rhenium(V) Oxo−Halide, −Hydride, −Alkyl, −Phenyl, and −Alkoxide Complexes: Syntheses and Oxidations

Rhenium−oxo complexes with the hydridotris(3,5-dimethyl-1-pyrazolyl)borate (Tp*) ligand
are described. Halide complexes Tp*Re(O)X(Cl) (X = Cl, I) are prepared by treatment of
Tp*Re(O)(OH)Cl with HX. Reaction of Tp*Re(O)Cl2 (3) with 1 or 2 equiv of LiPh/ZnCl2 or
Et2Zn produces the corresponding oxo−aryl and oxo−alkyl complexes Tp*Re(O)(Ph)Cl,
Tp*Re(O)(Ph)2, and Tp*Re(O)(Et)Cl. Alkoxide complexes Tp*Re(O)(OR)Cl and Tp*Re(O)(OR)2 are prepared from 3 and ROH, PhOH, or catechol. Triflate complexes Tp*Re(O)X(OTf) (X = halide, H, Et, Ph, OEt, OPh) have been prepared by halide metathesis with AgOTf
or by alkoxide metathesis with Me3SiOTf. Hydride complexes Tp*Re(O)(H)X (X = Cl, H)
are generated from the corresponding alkoxide complexes with BH3·THF. Oxidation of Tp*Re(O)Ph(OTf) with Me2SO gives a phenoxide complex, and oxidation of Tp*Re(O)Et(OTf) with
pyridine N-oxide gives acetaldehyde. Both reactions are similar to oxidations of related Tp
compounds. Tp*Re(O)H(OTf) is oxidized by either reagent to Tp*Re(O)3, with liberation of
H+; such a hydride complex has not been accessible in the Tp system. The Tp* ligand imparts
added stability to the rhenium(V) derivatives, making preparations easier. Steric constraints
of the Tp* ligand are illustrated by the lack of phenyl and pyridine ligand rotation on the
NMR time scale. The stability and crowding of the Tp* compounds inhibits reactions with
oxygen atom donors so that heating is often required, and the resulting oxidations are more
complex than for analogous Tp compounds.