Proton exchange kinetics from the bound waters on the oxo-centered rhodium(iii) trimer [Rh3(µ3-O)(µ-O2CCH3)6(OH2)3]+: a variable pH and temperature 1H NMR study

Abstract

Proton exchange from the bound to the bulk waters on the oxo-centered rhodium(III) trimer, [Rh(3)(micro(3)-O)(micro-O(2)CCH(3))(6)(OH(2))(3)](+)(abbreviated as Rh(3)(+)), was investigated over the temperature range of 219.1-313.9 K using a (1)H NMR line-broadening technique. By solving the modified Bloch equations for a two-site chemical exchange, lifetimes (tau) for proton transfer at pH = 2.7, 3.6, and 7.0 ([Rh(3)(+)]= 26 mM, T= 298 K) were determined to be 0.3 (+/-.08) ms, 2 (+/-0.3) ms, and 0.2 (+/-0.2) ms, respectively. From the temperature dependence of the rate, the activation parameters were determined to be DeltaH(++)= 16.2 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 123 (+/-2) J mol(-1) K(-1), DeltaH(++)= 14.9 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 141 (+/-2) J mol(-1) K(-1), and DeltaH(++)= 45 (+/-2) kJ mol(-1) and DeltaS(++)=- 22 (+/-5) J mol(-1) K(-1) for pH = 2.7, 3.6 and 7.0, respectively. All results are reported for a mixed solvent system [acetone : 250 mM NaClO(4)(aq)(3:1)], which was necessary to depress the freezing point of the solution so that the (1)H NMR signal due to bound water could be observed. The pK(a) of Rh(3)(+) was measured to be 8.9 (+/-0.2) in the mixed solvent, which is near the pK(a) for an aqueous solution (8.3 (+/-0.2)). Surprisingly, the lifetimes for protons on Rh(3)(+) are close to those observed for the Rh(OH(2))(6)(3+) ion, in spite of the considerable difference in structure, Brønsted acidity of the bound waters and average charge on the metal ion.

An overview of selected current approaches to the characterization of aqueous inorganic clusters

“…The NMR methods used to follow the exchange of isotopes or movement of mass into, and out of, the cluster are dominated by linebroadening approaches, [54][55][56][57][58] although there is no reason that selective-excitation methods cannot also be used. The NMR methods used to follow the exchange of isotopes or movement of mass into, and out of, the cluster are dominated by linebroadening approaches, [54][55][56][57][58] although there is no reason that selective-excitation methods cannot also be used.…”

“…The timescale for reaction dynamics in many large clusters fall into the 10 −9 -10 −2 s range, which makes the detection amenable to NMR if there exists a suitable nucleus. The NMR methods used to follow the exchange of isotopes or movement of mass into, and out of, the cluster are dominated by linebroadening approaches, [54][55][56][57][58] although there is no reason that selective-excitation methods cannot also be used. Application simply awaits a polyoxometalate system with appropriately spaced resonances and suitable kinetics of exchange.…”

Mechanism of Pyridine−Ligand Exchanges at the Different Labile Sites of 3d Heterometallic and Mixed Valence μ3-oxo Trinuclear Clusters

“…However, solution kinetic studies have been performed only on a few of them, mostly chosen for their inertness, as for example: [M 3 (µ 3 -O)(µ-O 2 CR) 6 L 3 ] + for M ) Cr III , Ru III , Rh III and L ) H 2 O, Py. [46][47][48][49][50][51][52][53] In these trinuclear monocataionic species the kinetic behavior dramatically differs from that observed in the corresponding trivalent hexaaqua ions: faster water exchange rates, and D exchange mechanisms instead of I a were observed. This was assigned to a trans effect of the central oxide ion and to additional cis effects of the bridging carboxylates.…”

Mechanisms of reactions in solution

“…The factors affecting the exchange rate of water from the trimeric species [Rh 3 (m 3 -O)(m-O 2 CCH 3 ) 6 (H 2 O) 3 ] + have been explored using high pressure 17 O NMR. 151,152 At 298 K, the rate of exchange is 5 Â 10 À3 s À1 , about 10 6 times faster than that for [Rh(H 2 O) 6 ] 3+ . The two factors suggested to explain the difference are trans-labilisation by the oxide ion and lower total charge on the Rh.…”