Hydrolysis of the Organometallic Aqua Ion fac-Triaquatricarbonylrhenium(I). Mechanism, pKa, and Formation Constants of the Polynuclear Hydrolysis Products

Hydrolytic pathways of the organometallic aqua ion [Re(CO) 3 (H 2 O) 3 ] + ( 2 ) in aqueous media were investigated by means of potentiometric titration experiments. The aqua complex 2 was obtained quantitatively by dissolving (NEt 4 ) 2 [Re(CO) 3 Br 3 ] ( 1 ) in water. Conventional alkalimetric titrations (0.1 M KNO 3, 25 °C) allowed the determination of the formation constants of [Re 3 (CO) 9 (μ 2 -OH) 3 (μ 3 -OH)] - ( 3 ) and [Re 2 (CO) 6 (μ 2 -OH) 3 ] - ( 4 ). The neutral dinuclear [Re 2 (CO) 6 (μ 2 -OH) 2 (H 2 O) 2 ] ( 5 ) was observed as a minor species. A fast titration technique was used to investigate a rapid preequilibrium, consisting of the formation of the mononuclear deprotonation products [Re(CO) 3 (OH)(H 2 O) 2 ] ( 6 ) and [Re(CO) 3 (OH) 2 (H 2 O)] - ( 7 ). The corresponding p K a values are 7.5(2) and 9.3(3). The immediate extraction of an aqueous solution of 2 with diethyl ether after base addition (1 equiv) led to the quantitative isolation of the well-known cubane cluster [Re(CO) 3 (OH)] 4 ( 8 ), which was obtained as a DMF ( 8b ) or OPPh 3 adduct ( 8c ). Attempts to isolate the aqua complex 2 as a crystalline material by precipitating the bromo ligands of 1 with AgCF 3 COO resulted in the formation of (NEt 4 ) 2 [Re(CO) 3 (CF 3 COO) 3 ] ( 9 ). The structures of 8b, 8c, and 9 were elucidated by single-crystal X-ray analysis.