Second-sphere interaction of anions with a weakly binding metal complex host: probing the effect of counteranions

Abstract

The reaction of [Re(OTf)(CO)₅] with N-methylimidazole (MeIm) afforded [Re(CO)₃(MeIm)₃]OTf (1). The reactions of 1 with KPF₆, NaBPh₄ and NaBAr′₄ (Ar′ = 3,5-bis(trifluoromethyl)phenyl) afforded [Re(CO)₃(MeIm)₃]PF₆ (2) [Re(CO)₃(MeIm)₃]BPh₄ (3) and [Re(CO)₃(MeIm)₃]BAr′₄ (4) respectively. An analogous reaction using N-phenylimidazole (PhIm) yielded [Re(CO)₃(PhIm)₃]BAr′₄ (7). These new compounds were characterized by IR and NMR, and the structures of 1 and 2 were determined by X-ray diffraction. Compounds [Re(CO)₃(MeIm)₃]₂[PtCl₆] (5), [Re(CO)₃(MeIm)₃][HSO₄] (6), [Re(CO)₃(PhIm)₃][Br] (8) and [Re(CO)₃(PhIm)₃][NO₃] (9) were crystallized from equimolar mixtures of either 4 or 7 and the tetrabutylammonium salt of the corresponding anion, and their structures were determined by X-ray diffraction. The solution behavior of 1–4, 7 toward several anions was studied spectroscopically, including the quantitative determination of binding constants by ¹H NMR. The cationic tris(imidazole)complexes are stable against imidazole-by-anion substitution, and the main hydrogen bonding interactions involve the imidazole NC(H)N groups. The binding constants for compounds 1–4 with several external anions follow the order 1 < 2 < 3 < 4, indicating that the strength of the cationic complex–counteranion interaction follows the order OTf⁻ > PF₆⁻ > BPh₄⁻ > BAr′₄⁻.