Water-Exchange Study Revealed Unexpected Substitution Behavior of [(CO)2(NO)Re(H2O)3]2+ in Aqueous Media

The pH-dependent water-exchange rates of [(CO)2(NO)Re(H2Ocis)2(H2Otrans)]2+ (1) in aq. media were investigated by means of 17O NMR spectroscopy at 298 K. Because of the low pKa value found for 1 (pKa = 1.4 ± 0.3), the water-exchange rate const. kobsH2Otrans/cis was analyzed with a two-pathway model in which kReH2Otrans/cis and kReOHH2Otrans/cis denote the water-exchange rate consts. in trans or cis position to the nitrosyl ligand on 1 and on the monohydroxo species [(CO)2(NO)Re(H2O)2(OH)]+ (2), resp. Whereas the rate consts. kReOHH2Otrans and kReOHH2Ocis were detd. as (4.2 ± 2) ´ 10-3 s-1 and (5.8 ± 2) ´ 10-4 s-1, resp., kReH2Otrans and kReH2Ocis were too small to be detd. in the presence of the much more reactive species 2. Apart from the water exchange, an unexpectedly fast Cº16O ® CºO exchange was also obsd. via NMR and IR spectroscopy. It was found to proceed through 1 and 2, with rate consts. kReCO and kReOHCO of (19 ± 4) ´ 10-3 s-1 and (4 ± 3) ´ 10-3 s-1, resp. On the other hand, Nº16O ® NºO exchange was not obsd.