Electrophilic substitution of metal-coordinated pyrazole: nitration, sulfonation and bromination of [Co(NH 3) 5(pyzH)] 3+ (pyzH = pyrazole)

Abstract

The nitration (HNO 3/H 2SO 4), sulfonation (oleum) and bromination (Br 2(aq.), HOBr) of the pyrazole ring of [Co(NH 3) 5pyzH] 3+ (pyzH = pyrazole) are reported. Nitration, sulfonation and bromination with Br 2(aq.) give the 4-substituted products, while bromination with HOBr gives both di- and tribrominated products. In all cases electrophilic substitution of the coordinated pyrazole appears to be faciliated in comparison with the analogous reaction of free pyrazole, and this is ascribed to both the electronic properties of the Co(NH 3) 5 3+ moiety, and the inability of the pyrazole complex to form the deactivated pyrazolium cation by protonation. The crystal structures of the unsubstituted complex [Co(NH 3) 5pyzH] (NO 3)Br 2 (C 3H 19Br 2CoN 8O 3, orthorhombic, Pbca, a= 13.855(3), b= 13.147(3), c= 15.383(3) A ̊ , α = β = γ = 90°, Z = 8) and the sulfonated complex [Co(NH 3) 54-SO 3pyzH](ClO 4) 2·H 2O (C 3H 18Cl 2CoN 7O 12S, orthorhombic, Pna2 1, a = 18.213(4), b = 8.199(2), c = 11.323(2), A ̊ , α = β = γ = 90°, Z = 4) are reported. The geometry of the pyrazole ring remains largely unaffected by coordination to Co(NH 3) 5 3+ or sulfonation at C-4. In both complexes the pyrazole ring is oriented in a staggered conformation with respect to the cis ammine ligands. Rate data (25.0°C, [Br − = 0.10 M, I = 1.00 M) for the bromination of [Co(NH 3) 5pyzH] 3+ over the [H +] range 0.18–0.90 M are reported. Data are consistent with bromination of the conjugate base [Co(NH 3) 5pyz] 3+ with the rate constant for this process being 1.32 × 10 9M −1 s −1. This value is some 10 4-fold greater than the reaction of Br 2 with pyrazole.