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Rates of Water Exchange for Two Cobalt(II) Heteropolyoxotungstate Compounds in Aqueous Solution
Author(s) -
Ohlin C. André,
Harley Stephen J.,
McAlpin J. Gregory,
Hocking Rosalie K.,
Mercado Brandon Q.,
Johnson Rene L.,
Villa Eric M.,
Fidler Mary Kate,
Olmstead Marilyn M.,
Spiccia Leone,
Britt R. David,
Casey William H.
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201003550
Subject(s) - polyoxometalate , chemistry , aqueous solution , tungstate , cobalt , dissociation (chemistry) , molecule , electron paramagnetic resonance , extended x ray absorption fine structure , crystallography , catalysis , ion , self ionization of water , inorganic chemistry , absorption spectroscopy , biochemistry , physics , organic chemistry , nuclear magnetic resonance , quantum mechanics
Polyoxometalate ions are used as ligands in water‐oxidation processes related to solar energy production. An important step in these reactions is the association and dissociation of water from the catalytic sites, the rates of which are unknown. Here we report the exchange rates of water ligated to Co II atoms in two polyoxotungstate sandwich molecules using the 17 O‐NMR‐based Swift–Connick method. The compounds were the [Co 4 (H 2 O) 2 (B‐α‐PW 9 O 34 ) 2 ] 10− and the larger αββα‐[Co 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 16− ions, each with two water molecules bound trans to one another in a Co II sandwich between the tungstate ligands. The clusters, in both solid and solution state, were characterized by a range of methods, including NMR, EPR, FT‐IR, UV‐Vis, and EXAFS spectroscopy, ESI‐MS, single‐crystal X‐ray crystallography, and potentiometry. For [Co 4 (H 2 O) 2 (B‐α‐PW 9 O 34 ) 2 ] 10− at pH 5.4, we estimate: k 298 =1.5(5)±0.3×10 6 s −1 , Δ H ≠ =39.8±0.4 kJ mol −1 , Δ S ≠ =+7.1±1.2 J mol −1 K −1 and Δ V ≠ =5.6 ±1.6 cm 3 mol −1 . For the Wells–Dawson sandwich cluster (αββα‐[Co 4 (H 2 O) 2 (P 2 W 15 O 56 ) 2 ] 16− ) at pH 5.54, we find: k 298 =1.6(2)±0.3×10 6 s −1 , Δ H ≠ =27.6±0.4 kJ mol −1 Δ S ≠ =−33±1.3 J mol −1 K −1 and Δ V ≠ =2.2±1.4 cm 3 mol −1 at pH 5.2. The molecules are clearly stable and monospecific in slightly acidic solutions, but dissociate in strongly acidic solutions. This dissociation is detectable by EPR spectroscopy as S =3/2 Co II species (such as the [Co(H 2 O) 6 ] 2+ monomer ion) and by the significant reduction of the Co–Co vector in the XAS spectra.