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

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Title: Water-Exchange Study Revealed Unexpected Substitution Behavior of [(CO)2(NO)Re(H2O)3]2+ in Aqueous Media.
Authors: Lehaire, Marie-Line1,2, Grundler, Pascal V.2,3, Steinhauser, Stefan4, Marti, Niklaus1, Helm, Lothar3, Hegetschweiler, Kaspar4, Schibii, Roger roger.schibli@psi.ch, Merbach, André E.3 andre.merbach@epfl.ch
Source: Inorganic Chemistry. 5/15/2006, Vol. 45 Issue 10, p4199-4204. 6p. 1 Chart, 5 Graphs.
Subjects: Nuclear magnetic resonance, Hydrogen-ion concentration, Acidity function, Spectrum analysis, Physical & theoretical chemistry
Abstract: The pH-dependent water-exchange rates of [(CO)2(NO)Re(H2Ocis)2(H2Otrans)]2+ (1) in aqueous media were investigated by means of 170 NMR spectroscopy at 298 K. Because of the low pKa value found for 1 (pKa = 1.4 ± 0.3), the water-exchange rate constant kobsH2Otrans/cis was analyzed with a two-pathway model in which kReH2Otrans/cis and kReOHH2Otrans/cis denote the water-exchange rate constants in trans or cis position to the nitrosyl ligand on 1 and on the monohydroxo species [(CO)2(NO)Re(H2O)2(OH)]+ (2), respectively. Whereas the rate constants kReOHH2Otrans and kReOHH2Ocis were determined as (4.2 ± 2) × 10-3 s-1 and (5.8 ± 2) × 10-4 s-1, respectively, kReH2Otrans and kReH2Ocis were too small to be determined in the presence of the much more reactive species 2. Apart from the water exchange, an unexpectedly fast C16O → C*O exchange was also observed via NMR and IR spectroscopy. It was found to proceed through 1 and 2, with rate constants kReCO and kReOHCO of (19 ± 4) × 10-3 s-1 and (4 ± 3) × 10-3 s-1, respectively. On the other hand, N16O → N*O exchange was not observed. [ABSTRACT FROM AUTHOR]
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Abstract:The pH-dependent water-exchange rates of [(CO)2(NO)Re(H2Ocis)2(H2Otrans)]2+ (1) in aqueous media were investigated by means of 170 NMR spectroscopy at 298 K. Because of the low pKa value found for 1 (pKa = 1.4 ± 0.3), the water-exchange rate constant kobsH2Otrans/cis was analyzed with a two-pathway model in which kReH2Otrans/cis and kReOHH2Otrans/cis denote the water-exchange rate constants in trans or cis position to the nitrosyl ligand on 1 and on the monohydroxo species [(CO)2(NO)Re(H2O)2(OH)]+ (2), respectively. Whereas the rate constants kReOHH2Otrans and kReOHH2Ocis were determined as (4.2 ± 2) × 10-3 s-1 and (5.8 ± 2) × 10-4 s-1, respectively, kReH2Otrans and kReH2Ocis were too small to be determined in the presence of the much more reactive species 2. Apart from the water exchange, an unexpectedly fast C16O → C*O exchange was also observed via NMR and IR spectroscopy. It was found to proceed through 1 and 2, with rate constants kReCO and kReOHCO of (19 ± 4) × 10-3 s-1 and (4 ± 3) × 10-3 s-1, respectively. On the other hand, N16O → N*O exchange was not observed. [ABSTRACT FROM AUTHOR]
ISSN:00201669
DOI:10.1021/ic060010m