Water Exchange from the Oxo-Centered Rhodium(III) Trimer [Rh₃(μ₃-O)(μ-O₂CCH₃)₆(OH₂)₃]⁺:  A High-Pressure ¹⁷O NMR Study

Abstract

Water exchange from the oxo-centered rhodium(III) trimer, [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+, was investigated using variable-temperature (272.8-281.6 K) and variable-pressure (0.1-200 MPa) 17O NMR spectroscopy. The exchange reaction was also monitored at three different acidities (pH = 1.8, 2.9, and 5.7) in which the molecule is in the fully protonated form (pKa = 8.3 (+/-0.2), I = 0.1 M, T = 298 K). The temperature dependence of the pseudo-first-order rate coefficient for water exchange yields the following kinetic parameters: k(ex)298 = 5 x 10(-3) s(-1), deltaH(double dagger) = 99 (+/-3) kJ mol(-1), and deltaS(double dagger) = 43 (+/-10) J K(-1) mol(-1). The enhanced reactivity of the terminal waters, some 6 orders of magnitude faster than water exchange from Rh(H2O)6(3+), is likely due to trans-labilization from the central oxide ion. Also, another contributing factor is the low average charge on the metal ions (+0.33/Rh). Variation of reaction rate with pressure results in a deltaV(double dagger) = +5.3 (+/-0.4) cm3 mol(-1), indicative of an interchange-dissociative (I(d)) pathway. These results are consistent with those published by Sasaki et al. who proposed that water substitution from rhodium(III) and ruthenium(III) oxo-centered trimers follows a dissociative mechanism based on highly positive activation parameters (Sasaki, Y.; Nagasawa, A.; Tokiwa-Yamanoto, A.; Ito, T. Inorg. Chim. Acta 1993, 212, 175-182).

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.…”

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

“…28 Thus, proton transfer from bound water is much faster than from the bulk. No scalar coupling between the 103 Rh nuclei (spin 1 2 , 100% natural abundance) and water-molecule protons was observed, even at low temperatures, indicating that the rate of proton exchange is much greater than the two-bond coupling constant ( 2 J k).…”

“…An increase in the 1 Variation of the 1 H NMR linewidth of the bound-water resonance with temperature for a 26 mM solution of Rh 3 + at pH = 2.7, 3.6, and 7.0 is shown in Fig. 5.…”

“…10 −3 s −1 ),1 proton exchange by loss of an innersphere water can be ignored.Analogous to Cr(OH 2 ) 6 3+ and Rh(OH 2 ) 6 3+ , as well as [Gd(DTMA)(H 2 O)] 3+ , we see clear evidence that proton exchange rates increase at low pH for Rh 3 + . 3,4,11 At 250.7 K, a decrease in approximately one pH unit (from pH = 3.6 to pH = 2.7) results in a factor of six increase in exchange rate.…”

Observation of Solid-State 103Rh NMR by Cross-Polarization

“…15 The chemical shifts for these three sites, δ iso ) 9401, 9359, and 9354 ppm, are remarkably close to the measured value δ iso ) 9390 ppm for 2 in aqueous solution. 16 Analysis of the spinning sidebands for the resonance at δ iso ) 9401 ppm indicates a CSA characterized by Ω ) 1720 ppm and κ ) -0.64. The resonances at δ iso ) 9359 and 9354 ppm could not be analyzed separately, but treating their integrated intensities as a single resonance gave similar values of Ω ) 1620 ppm and κ ) -0.58.…”