Preparation and Protonation Studies of trans-Dioxorhenium(V) Complexes with Imidazoles

Reacting ReO 2 I(PPh 3 ) 2 with imidazole (ImH) and its methylated derivatives 1-MeIm, 1,2-Me 2 Im, 2-MeImH, and 4(5)-MeImH yielded salts of the trans -dioxo cation [ReO 2 L 4 ] + . Pure stable compounds were isolated for X - = I - and L = ImH, 1,2-Me 2 Im, and 1-MeIm and X - = B(C 6 H 5 ) 4 - and L = 1-MeIm, 2-MeImH and 5-MeImH. The ν as (O Re O) IR bands were observed between 765 and 794 cm - 1, whereas the ν s Raman band appeared in the 900−925 cm - 1 range. The compounds were also characterized by 1 H and 13 C NMR and UV−vis spectroscopies. Crystal structures were determined for three compounds: [ReO 2 (2-MeImH) 4 ][B(C 6 H 5 ) 4 ]·3CH 3 OH, triclinic, P 1̄, a = 10.696(3) Å, b = 15.128(4) Å, c = 15.497(4) Å, α = 113.57(2)°, β = 97.25(2)°, γ = 95.94(2)°, Z = 2, R = 0.030; [ReO 2 (1-MeIm) 4 ][B(C 6 H 5 ) 4 ]·H 2 O·0.5CH 3 OH, monoclinic, P 2 1 / n, a = 15.619(2) Å, b = 9.486(2) Å, c = 27.387(4) Å, β = 97.09(1)°, Z = 4, R = 0.057; [ReO 2 (5-MeImH) 4 ][B(C 6 H 5 ) 4 ], orthorhombic, P 2 1 2 1 2 1, a = 10.199(2) Å, b = 13.441(5) Å, c = 28.798(10) Å, Z = 4, R = 0.043. The complexes adopt the trans -dioxo geometry, and the octahedra are remarkably regular. Protonation of [ReO 2 L 4 ]I was studied in methanol and in water. Acid dissociation constants were determined in aqueous solution for the [ReO(OH)L 4 ] 2+ ( K a 1 ) and [ReO(OH 2 )L 4 ] 3+ ( K a 2 ) species of 1-MeIm (p K a 1 = 2.0, p K a 2 = ∼−4.0) and 1,2-Me 2 Im (p K a 1 = ∼3.8, p K a 2 = ∼−4.1). Iodide salts of [ReO(OH)L 4 ] 2+ were not stable enough to be isolated, but the B(C 6 H 5 ) 4 - salt of [ReO(OH)(1,2-Me 2 Im) 4 ] 2+ was obtained in pure form. The crystal structure of [ReO(OH)(1,2-Me 2 Im) 4 ](I 3 ) 0.5 (ReO 4 ) 1.5 (monoclinic, I 2/ a, a = 23.290(8) Å, b = 10.863(7) Å, c = 25.709(8) Å, β = 94.36(3)°, Z = 8, R = 0.060) showed the presence of octahedral dications in which the oxo and hydroxo groups are trans to one another and the Re−N bonds are displaced to the Re−OH side by the steric effect of the multiple Re−oxo bond. The σ-donor ability of imidazole, greater than that of pyridine, leads to more stable [ReO 2 L 4 ] + compounds in which the Re=O double bond character is lower and the oxo groups are better nucleophiles.