Silver(i) complexes of tris(pyrazolyl)borate ligands bearing six trifluoromethyl and three additional electron-withdrawing substituents

Abstract

Sodium salts of two new tris(pyrazolyl)borates [HB(4-Cl-3,5-(CF₃)₂Pz)₃]⁻ and [HB(4-(NO₂)-3,5-(CF₃)₂Pz)₃]⁻, which are not only highly fluorinated, but also loaded with additional electron-withdrawing substituents, have been synthesized by reacting 4-Cl-3,5-(CF₃)₂PzH or 4-(NO₂)-3,5-(CF₃)₂PzH with NaBH₄ under nitrogen in a solventless process, and isolated after a work up in tetrahydrofuran (THF) or diethyl ether (Et₂O), as their THF or Et₂O adducts. Metathesis of these sodium salts with AgOTf in THF leads to [HB(4-Cl-3,5-(CF₃)₂Pz)₃]Ag(THF) and [HB(4-(NO₂)-3,5-(CF₃)₂Pz)₃]Ag(THF). The corresponding cis-cyclooctene (c-COE) complexes [HB(4-Cl-3,5-(CF₃)₂Pz)₃]Ag(c-COE) and [HB(4-(NO₂)-3,5-(CF₃)₂Pz)₃]Ag(c-COE) were obtained by displacing THF with cis-cyclooctene. The related [HB(3,5-(CF₃)₂Pz)₃]Ag(c-COE) can also be obtained via a similar process. X-ray crystal structures show that [HB(4-(R)-3,5-(CF₃)₂Pz)₃]Ag(c-COE) (R = H, Cl, NO₂) feature pseudo-tetrahedral silver sites supported by κ³-bound tris(pyrazolyl)borate ligands. [HB(4-(NO₂)-3,5-(CF₃)₂Pz)₃]Ag(c-COE) displays the smallest upfield shift of the alkene carbon peak (versus free alkene) followed by [HB(4-Cl-3,5-(CF₃)₂Pz)₃]Ag(c-COE) and [HB(3,5-(CF₃)₂Pz)₃]Ag(c-COE). Experimental and computational data indicate very electron poor silver(I) sites in all three [HB(4-(R)-3,5-(CF₃)₂Pz)₃]Ag(c-COE) adducts with minimal Ag → (cis-cyclooctene) backbonding. Although the impact of R (H, Cl, NO₂) on the alkene carbon chemical shift of these adducts and the olefin π/π* populations is small, the ¹³C NMR chemical shifts and NBO analysis suggest that [HB(4-(NO₂)-3,5-(CF₃)₂Pz)₃]Ag(c-COE) possesses the most electrophilic metal site, which correlates with the pK_a values of the free pyrazoles. [HB(4-(R)-3,5-(CF₃)₂Pz)₃]Ag(c-COE) adducts effectively catalyze the insertion of the carbene moiety of ethyl diazoacetate into C–H bonds of 2,3-dimethylbutane. The [HB(4-(NO₂)-3,5-(CF₃)₂Pz)₃]Ag(c-COE) catalyst shows a higher selectivity for primary C–H bonds compared to the reactions catalyzed by [HB(3,5-(CF₃)₂Pz)₃]Ag(c-COE).