HYDROLYSIS OF THE ORGANOMETALLIC AQUA ION FAC-TRIAQUATRICARBONYLRHENIUM(I) - MECHANISM, PK(A), AND FORMATION-CONSTANTS OF THE POLYNUCLEAR HYDROLYSIS PRODUCTS

Hydrolytic pathways of the organometallic aqua ion [Re(CO)(3)(H2O)(3)] (+) (2) in aqueous media were investigated by means of potentiometric titration experiments. The aqua complex 2 was obtained quantitatively by dissolving (NEt4)(2)[Re(CO)(3)Br-3] (1) in water. Conventional alka limetric titrations (0.1 M KNO3, 25 degrees C) allowed the determinati on of the formation constants of [Re-3(CO)(9)(mu(2)-OH)(3)(mu(3)-OH)]( -) (3) and [Re-2(CO)(6)(mu(2)-OH)(3)](-) (4). The neutral dinuclear [R e-2(CO)(6)(mu(2)-OH)(2)(H2O)(2)] (5) was observed as a minor species. A fast titration technique was used to investigate a rapid preequilibr ium, consisting of the formation of the mononuclear deprotonation prod ucts [Re(CO)(3)(OH)(H2O)(2)] (6) and [Re(CO)(3)(OH)(2)(H2O)](-) (7). T he corresponding pK(a) values are 7.5(2) and 9.3(3). The immediate ext raction of an aqueous solution of 2 with diethyl ether after base addi tion (1 equiv) led to the quantitative isolation of the well-known cub ane cluster [Re(CO)(3)(OH)](4) (8), which was obtained as a DMF (8b) o r OPPh3 adduct (8c). Attempts to isolate the aqua complex 2 as a cryst alline material by precipitating the bromo ligands of 1 with AgCF3COO resulted in the formation of (NEt4)(2)[Re(CO)(3)(CF3COO)(3)] (9). The structures of 8b, 8c, and 9 were elucidated by single-crystal X-ray an alysis.