Mechanism of Water Exchange for the Di- and Trivalent Metal Hexaaqua Ions of the First Transition Series

Abstract

The mechanism for the water-exchange reaction with the transition metal aqua ions from Sc III through Zn II has been investigated. The exchange mechanisms were analyzed on the previously reported model (Rotzinger, F. P. J. Am. Chem. Soc. 1996, 118, 6760) that involves the metal ion with six or seven water molecules. The structures of the reactants/products, transition states, and penta-or heptacoordinated intermediates have been computed with Hartree-Fock or CAS-SCF methods. Each type of mechanism, associative, concerted or dissociative, proceeds via a characteristic transition state. The calculated activation energies agree with the experimental ∆G q 298 or ∆H q 298 values, and the computed structural changes indicate whether an expansion or compression takes place during the transformation of the reactant into the transition state. These changes are in perfect agreement with the changes deduced from the experimental volumes of activation (∆V q 298 ). The motions of the ligands involved in the exchange reaction are described by the imaginary vibrational mode. All these computed quantities allow the attribution of the water-exchange reactions to the A, I a , or D mechanisms with use of the terminology of Merbach (Merbach, A. E. Pure Appl. Chem. 1982Chem. , 54, 1479. Within the present model, no transition state has been found for the I d mechanism. It remains to be verified, using an improved model, whether it really does not exist. The dissociative mechanism is always feasible, but it is the only possible pathway for high-spin d 8 , d 9 , and d 10 systems. In contrast, the associative mechanism requires that the transition metal ion does not have more than seven 3d electrons. Thus, Sc III , Ti III , and V III react via the A, Ni II , Cu II , and Zn II via the D (or I d ) mechanism, whereas for the elements in the middle of the periodic table, the high-spin 3d 3 -3d 7 systems, both associative (I a /A) and dissociative (D) pathways are feasible. The present results suggest that for Sc III hexa-and heptacoordinated species could coexist in aqueous solution.

QM/MM computational studies of substrate water binding to the oxygen-evolving centre of photosystem II

“…Mn (4) Mn (3) Mn (2) bond is the primary step in water exchange and presumably rate determining in this case (Rotzinger 1997;Lundberg et al 2003;Rotzinger 2005). Figure 3 shows the resulting energy profiles for the MEPs, while progressively detaching substrate water molecules from Ca 2C and the dangling manganese for the OEC in the S 1 and S 2 states Mn 4 (IV,IV,III,III ) and Mn 4 ( IV,IV,IV,III ), respectively.…”

Fenton chemistry at aqueous interfaces

“…Recent X-ray reflectivity studies confirmed the existence of nonmonotonic cation density profiles within ∼1-nm interfacial layers of aqueous electrolyte solutions (84). We cannot rule out the possibility that impinging gases are hydrated before colliding with the liquid surface (85, 86), but we deem it inconsequential because O 3 -(H 2 O) n and 6 2+ has a distorted octahedral geometry, on account of the broken symmetry, that lets O 3 approach the Fe 2+ center via low-energy associative interchange pathways (88). Against this backdrop, our findings reveal that the dynamics and thermodynamics of ion hydration at aqueous interfaces are quite different from those in bulk water (75,89).…”

Computational insights into the O2-evolving complex of photosystem II

“…These observation have been recently addressed by calculations of transition state energy barriers for water exchange in structural models of the OEC in the S 1 and S 2 states while progressively detaching substrate water molecules from Ca 2+ and the dangling Mn(4) (Sproviero et al 2007f). The resulting structural rearrangements provided insight on the water exchange mechanisms and the relative binding strengths, considering that elongation of the metal-oxygen bond is likely the primary step in water exchange and rate-determining (Lundberg et al 2003;Rotzinger 1997;Rotzinger 2005). These calculations complemented earlier studies of water exchange in transition metal complexes (Cady et al 2006;Helm & Merbach 1999;Houston et al 2006;Rotzinger 1997;Rotzinger 2005;Tagore et al 2006;Tagore et al 2007), including theoretical studies of manganese complexes, based on Hartree-Fock and complete active-space self-consistent field theories (Lundberg et al 2003;Rotzinger 1997;Rotzinger 2005;Tsutsui et al 1999) as well as DFT studies of water exchange in other transition metal complexes (Deeth & Elding 1996;Hartmann et al 1999;Hartmann et al 1997;Lundberg et al 2003;Vallet et al 2001).…”