Ligand Effects on the Rate and Mechanism of Solvent Exchange at Rhodium(III) and Iridium(III)

The rate constants and activation parameters for solvent exchange for a series of RhIII and IrIII complexes of the form [M(H2O)6]3+ (M = Rh or Ir) and [Cp*M(S)3]2+ (M = Rh or Ir, and S = H2O, MeCN, or Me2SO) have been determined by NMR spectroscopy as a function of free-solvent concentration, temperature, and pressure, and the mechanisms for the respective exchange processes have been inferred

High-Pressure Stopped-Flow Study of Inclusion Reactions with α-Cyclodextrin: Dynamic Aspects in Host-Guest Interactions

The full volume and entropy profiles of the inclusion reaction between α-cyclodextrin and the guest molecules, ethylorange (1) and mordant yellow 7 (2), have been constructed from variable-pressure and -temperature stopped-flow kinetic experiments

Theoretical study of the hydrated Gd3+ ion: Structure, dynamics, and charge transfer

The dynamical processes taking place in the first coordination shells of the gadolinium (III) ion are important for improving the contrast agent efficiency in magnetic-resonance imaging. An extensive study of the gadolinium (III) ion solvated by a water cluster is reported, based on molecular dynamics simulations. The AMOEBA force field [P. Y. Ren and J. W. Ponder, J. Phys. Chem. B 107, 5933 (2003)] that includes many-body polarization effects is used to describe the interactions among water molecules, and is extended here to treat the interactions between them and the gadolinium ion. In this purpose accurate ab initio calculations have been performed on Gd3+-H2O for extracting the relevant parameters. Structural data of the first two coordination shells and some dynamical properties such as the water exchange rate between the first and second coordination shells are compared to available experimental results. We also investigate the charge transfer processes between the ion and its solvent, using a fluctuating charges model fitted to reproduce electronic structure calculations on [Gd(H2O)n]3+ complexes, with n ranging from 1 to 8. Charge transfer is seen to be significant (about one electron) and correlated with the instantaneous coordination of the ion

Molecular dynamics simulations of the internal mobility of Gd3+-based MRI contrast agents: consequences for water proton relaxivity

The increasing use of contrast agents in magnetic resonance imaging (MRI) for medical diagnosis is due to the ability, called relaxivity, of these paramagnetic compounds to accelerate the relaxation of the surrounding water proton spins. A new classical force field for molecular dynamics simulations of Gd3+ polyaminocarboxylates has recently been published, which allows the study of the chelate internal mobility. We present two selected examples where such motions can affect relaxivity. Knowing the relationship between the bound water proton and oxygen mobility is important for the combined analysis of multinuclear NMR studies, and we show that they differ significantly. Next, we observe symmetry changes over time in the Gd3+ coordination polyhedron of the acyclic complexes. We propose that such rearrangements can play a role in the electron spin relaxation of Gd3+ chelates, an important result considering the uncertainty still attached to this particular factor

Transient versus static electron spin relaxation in Mn2+ complexes relevant as MRI contrast agents

[Abstract] The zero-field splitting (ZFS) parameters of the [Mn(EDTA)(H2O)]2–·2H2O and [Mn(MeNO2A)(H2O)]·2H2O systems were estimated by using DFT and ab initio CASSCF/NEVPT2 calculations (EDTA = 2,2′,2″,2‴-(ethane-1,2-diylbis(azanetriyl))tetraacetate; MeNO2A = 2,2′-(7-methyl-1,4,7-triazonane-1,4-diyl)diacetate). Subsequent molecular dynamics calculations performed within the atom-centered density matrix propagation (ADMP) approach provided access to the transient and static ZFS parameters, as well as to the correlation time of the transient ZFS. The calculated ZFS parameters present a reasonable agreement with the experimental values obtained from the analysis of 1H relaxation data. The correlation times calculated for the two systems investigated turned out to be very short (τc ∼ 0.02–0.05 ps), which shows that the transient ZFS is modulated by molecular vibrations. On the contrary, the static ZFS is modulated by the rotation of the complexes in solution, which for the small complexes investigated here is characterized by rotational correlation times of τR ∼ 35–60 ps. As a result, electron spin relaxation in small Mn2+complexes is dominated by the static ZFS.España. Ministerio de Economía y Competitividad; CTQ2013-43243-PEspaña. Ministerio de Economía y Competitividad; CTQ2015-71211-RED

EPR on aqueous Gd3+ complexes and a new analysis method considering both line widths and shifts

We performed variable temperature (0–100°C), concentration and frequency (9.425, 75, 150 and 225 GHz) continuous wave electron paramagnetic resonance (EPR) measurements on three different Gd(III) compounds: [Gd(H2O)8]3+, [Gd(DOTA)(H2O)]− (DOTA: 1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane) and [Gd(DTPA-BMA)(H2O)] (DTPA-BMA: 1,5-[bis(N-methylcarbamoyl)methyl]-1,3,5-tris(carboxymethyl)-1,5-diamino-3-azapentane) in aqueous solution. A simultaneous analysis of peak-to-peak widths and dynamic frequency shifts provides access to the transverse electronic relaxation, which is described using a transient zero field splitting (ZFS) mechanism with a spin rotation contribution. Our simultaneous analysis procedure involves numerical calculations using the full relaxation matrix and yields results in acceptable agreement with experimental data for reasonable values of the ZFS parameters (trace of the square of the ZFS Hamiltonian Δ2=1019–1020 s−2 depending on the complex, correlation time of the fluctuations τv298=10−11–10−10 s). We also discuss the relationship between our approach and recent developments found in the literature

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

The syntheses and single crystal X-ray structural analysis of five novel hetero- and homometallic μ3-oxo trinuclear cluster with the formula [FeIII2MII(μ3-O)(μ-O2CCH3)6(4-Rpy)3] · x(4-Rpy) · y(CH3CN) where R ) Ph for 1(Fe2Mn),2(Fe2Fe), 3(Fe2Co), 4(Fe2Ni) and R ) CF3 for 5(Fe2Co), are reported. The persistence of the structure for compounds 2-5 in dichloromethane solution in the temperature range 190-320 K is demonstrated by 1H and 19F NMR spectroscopy. Even at the lowest temperature, the electron exchange in the homometallic mixed-valence compound 2(Fe2Fe) is in the fast regime at the NMR time scale. Variable temperature and pressure NMR line broadening allowed quantifying the fast coordinated/free 4-Rpy exchanges at the two labile metal centers in these clusters: 2: FeIII(k298/103 s-1 ) 16.6; ΔH‡ ) 60.32 kJ mol-1; ΔS‡ ) + 34.8 J K-1 mol-1; ΔV‡ ) + 12.5 cm3 mol-1); 3: Fe(11.9; 58.92; +30.7; +10.6) and Co (2.8; 68.24; +49.8; +13.9); 4: Fe(12.2; 67.91; +61.0; -) and Ni (0.37;78.62; +67.8; +12.3); 5: Fe (46; 58.21; +39.3; +14.2) and Co (4.7; 55.37; +11.2; +10.9). A limiting D mechanism is assigned to these exchange reactions. This assignment is based on a first-order rate law, the detection of intermediates, the positive and large entropies and volumes of activation. The order of reactivity kCo > kNi is expected for a D mechanism at these metal centers: their low exchange rates are due to their strong binding with the 4-Rpy donor. Surrounded by oxygen donors the d5 iron(III) usually reacts associatively; however, here due to low affinity of this ion for nitrogen the mechanism is D and the rate of exchange is very fast, even faster than on the divalent ions. There is no significant effect of the divalent ion in cluster 2, 3, and 5 on the exchange rates of 4-Phpy at the iron center, which seems to indicate that the specific electronic interactions between the three ions making the clusters do not influence the FeIII-N bond strength