The Periodic Table and Kinetics?

Mendeleev in his first publication ordered the chemical elements following an apparent periodicity of properties such as atomic volume and valence. The reactivity of the elements was only studied systematically many years later. To illustrate the systematic variation of kinetics across the periodic table we compare water residence times for monoatomic ions in aqueous solution. A tremendous variation of τH2O by over 20 orders of magnitude is found, ranging from ~10 ps to about 200 years. Apart from some small +2 and +3 cations, all main group elements have very short residence times <10 ns. Transition metal cations of the d-block have water residence times that depend on the electronic configuration. τH2O of lanthanide ions are surprisingly short with values of 10 ns and shorter. This is due to an equilibrium between 8 and 9 coordinated ions leading to a low energy of the transition state for the water exchange reaction.

NMR study of ligand exchange and electron self-exchange between oxo-centered trinuclear clusters [Fe3(µ3-O)(µ-O2CR)6(4-R'py)3](+/0).

The syntheses, single crystal X-ray structures, and magnetic properties of the homometallic µ3-oxo trinuclear clusters [Fe3(µ3-O)(µ-O2CCH3)6(4-Phpy)3](ClO4) (1) and [Fe3(µ3-O)(µ-O2CAd)6(4-Mepy)3](NO3) (2) are reported (Ad = adamantane). The persistence of the trinuclear structure within 1 and 2 in CD2Cl2 and C2D2Cl4 solutions in the temperature range 190-390 K is demonstrated by ¹H NMR. An equilibrium between the mixed pyridine clusters [Fe3(µ3-O)(µ-O2CAd)6(4-Mepy)(3-x)(4-Phpy)(x)](NO3) (x = 0, 1, 2, 3) with a close to statistical distribution of these species is observed in CD2Cl2 solutions. Variable-temperature NMR line-broadening made it possible to quantify the coordinated/free 4-Rpy exchanges at the iron centers of 1 and 2: k(ex)²98 = 6.5 ± 1.3 × 10⁻¹ s⁻¹, ΔH(‡) = 89.47 ± 2 kJ mol⁻¹, and ΔS(‡) = +51.8 ± 6 J K⁻¹ mol⁻¹ for 1 and k(ex)²98 = 3.4 ± 0.5 × 10⁻¹ s⁻¹, ΔH(‡) = 91.13 ± 2 kJ mol⁻¹, and ΔS(‡) = +51.9 ± 5 J K⁻¹ mol⁻¹ for 2. A limiting D mechanism is assigned for these ligand exchange reactions on the basis of first-order rate laws and positive and large entropies of activation. The exchange rates are 4 orders of magnitude slower than those observed for the ligand exchange on the reduced heterovalent cluster [Fe(III)2Fe(II)(µ3-O)(µ-O2CCH3)6(4-Phpy)3] (3). In 3, the intramolecular Fe(III)/Fe(II) electron exchange is too fast to be observed. At low temperatures, the 1/3 intermolecular second-order electron self-exchange reaction is faster than the 4-Phpy ligand exchange reactions on these two clusters, suggesting an outer-sphere mechanism: k2²98 = 72.4 ± 1.0 × 103 M⁻¹ s⁻¹, ΔH(‡) = 18.18 ± 0.3 kJ mol⁻¹, and ΔS(‡) = -90.88 ± 1.0 J K⁻¹ mol⁻¹. The [Fe3(µ3-O)(µ-O2CCH3)6(4-Phpy)3](+/0) electron self-exchange reaction is compared with the more than 3 orders of magnitude faster [Ru3(µ3-O)(µ-O2CCH3)6(py)3](+/0) self-exchange reaction (ΔΔG(exptl)(‡298) = 18.2 kJ mol⁻¹). The theoretical estimated self-exchange rate constants for both processes compare reasonably well with the experimental values. The equilibrium constant for the formation of the precursor to the electron-transfer and the free energy of activation contribution for the solvent reorganization to reach the electron transfer step are taken to be the same for both redox couples. The larger ΔG(exptl)(‡298) for the 1/3 iron self-exchange is attributed to the larger (11.1 kJ mol⁻¹) inner-sphere reorganization energy of the 1 and 3 iron clusters in addition to a supplementary energy (6.1 kJ mol⁻¹) which arises as a result of the fact that each encounter is not electron-transfer spin-allowed for the iron redox couple.

Isomerization mechanisms of stereolabile tris- and bis-bidentate octahedral cobalt(II) complexes: X-ray structure and variable temperature and pressure NMR kinetic investigations.

The isomerization dynamics of five labile octahedral Co(II) compounds have been investigated by variable temperature and pressure (1)H and (19)F NMR spectroscopy in dichloromethane solution. The X-ray crystal structure of the two tris-chelates, [Co(HFA)(2)bpic] (1) and [Co(TTFA)(2)bpy] (2), show a distorted octahedral arrangement of the 4 oxygen and 2 nitrogen donor atoms, with bidentate ligand bite angles smaller than 90 degrees. On the other hand, in the three bis-chelates, trans(N)-[Co(HFA)(2)(CH(3)py)(2)](3), cis(N)-cis(CF(3))-trans(S)-[Co(TTFA)(2)(CH(3)py)(2)](4), and trans(N)-trans(CF(3))-[Co(TTFA)(2)(CF(3)py)(2)](5), the replacement of the bidentate nitrogen donor ligands by two monodentate Rpy ligands leads to relaxed structures with almost regular octahedral arrangements of the donor atoms (HFA = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato anion; TTFA = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionato anion; bpy = 2,2'-bipyridine; bpic = 4,4'-dimethyl-2,2'-bipyridine). In solution the five complexes are stereolabile and all possible isomers are formed: from one for 1 up to five for 4 and 5. All cis-N isomers form pairs of enantiomers, whereas the trans-N isomers are achiral. A solid state structure/isomerization mechanism/rate correlation has been established for the isomerization dynamics of these Co(II) tris- and bis-chelates. The two tris-chelate complexes 1 and 2, with a distorted octahedral solid state structure, show one and three isomers in solution and isomerize/tautomerize very rapidly according to Bailar twist mechanisms. The three bis-chelate complexes 3, 4, and 5, with a close to octahedral symmetry in the solid state, show two, five, and five isomers, respectively. They isomerize/tautomerize 3 orders of magnitude slower as the tris-chelates, by an intramolecular dissociative mechanism involving a ring-opening of an arm of a bidentate ligand to form a TBP intermediate with a dangling bidentate ligand. The results of this first systematic investigation of the isomerization mechanisms of highly labile Co(II) complexes are supported by the NMR observed exchange paths (up to five for complexes for 4 and 5), the variable temperature (185 to 312 K) and pressure (up to 200 MPa) activation parameters, and a detailed analysis of the solid state structures.

Mechanism of pyridine-ligand exchanges at the different labile sites of 3d heterometallic and mixed valence mu3-oxo trinuclear clusters.

The syntheses and single crystal X-ray structural analysis of five novel hetero- and homometallic mu 3-oxo trinuclear cluster with the formula [Fe (III) 2M (II)(mu 3-O)(mu-O 2CCH 3) 6(4-Rpy) 3]. x(4-Rpy). y(CH 3CN) where R = Ph for 1(Fe 2Mn), 2(Fe 2Fe), 3(Fe 2Co), 4(Fe 2Ni) and R = CF 3 for 5(Fe 2Co), are reported. The persistence of the structure for compounds 2- 5 in dichloromethane solution in the temperature range 190-320 K is demonstrated by (1)H and (19)F NMR spectroscopy. Even at the lowest temperature, the electron exchange in the homometallic mixed-valence compound 2(Fe 2Fe) 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: Fe (III)( k (298)/10 (3) s (-1) = 16.6; Delta H (++) = 60.32 kJ mol (-1); Delta S (++) = + 34.8 J K (-1) mol (-1); Delta V (++) = + 12.5 cm (3) 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 k (Co) > k (Ni) 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 d (5) 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 Fe (III)-N bond strength.

Design of Gd(III)-based magnetic resonance imaging contrast agents: static and transient zero-field splitting contributions to the electronic relaxation and their impact on relaxivity.

A multiple-frequency (9.4-325 GHz) and variable-temperature (276-320 K) electron paramagnetic resonance (EPR) study on low molecular weight gadolinium(III) complexes for potential use as magnetic resonance imaging (MRI) contrast agents has been performed. Peak-to-peak linewidths Delta Hpp and central magnetic fields have been analyzed within the Redfield approximation taking into account the static zero-field splitting (ZFS) up to the sixth order and the transient ZFS up to the second order. Longitudinal electronic relaxation is dominated by the static ZFS contribution at low magnetic fields (B < 0.3 T) and by the transient ZFS at high magnetic fields (B > 1.5 T). Whereas the static ZFS clearly depends on the nature of the chelating ligand, the transient ZFS does not. For the relatively fast rotating molecules studied water proton relaxivity is mainly limited by the fast rotation and electronic relaxation has only a marked influence at frequencies below 30 MHz. From our EPR results we can conclude that electronic relaxation will have no influence on the efficiency of Gd(III)-based MRI contrast agents designed for studies at very high magnetic fields (B > 3T).

Water contributes actively to the rapid crossing of a protein unfolding barrier.

The cold-shock protein CspB folds rapidly in a N <= => U two-state reaction via a transition state that is about 90% native in its interactions with denaturants and water. This suggested that the energy barrier to unfolding is overcome by processes occurring in the protein itself, rather than in the solvent. Nevertheless, CspB unfolding depends on the solvent viscosity. We determined the activation volumes of unfolding and refolding by pressure-jump and high-pressure stopped-flow techniques in the presence of various denaturants. The results obtained by these methods agree well. The activation volume of unfolding is positive (Delta V(++)(NU)=16(+/-4) ml/mol) and virtually independent of the nature and the concentration of the denaturant. We suggest that in the transition state the protein is expanded and water molecules start to invade the hydrophobic core. They have, however, not yet established favorable interactions to compensate for the loss of intra-protein interactions. The activation volume of refolding is positive as well (Delta V(++)(NU)=53(+/-6) ml/mol) and, above 3 M urea, independent of the concentration of the denaturant. At low concentrations of urea or guanidinium thiocyanate, Delta V(++)(UN) decreases significantly, suggesting that compact unfolded forms become populated under these conditions.

Destruction of perfluoroalkyl surfactant aggregates by beta-cyclodextrin.

A water 1H NMRD and 19F NMR spectroscopy study has proved, for the first time, that perfluoroalkyl surfactant micelles can be completely destroyed upon addition of beta-cyclodextrin to form successively 1:1 and 2:1 (beta-CD:R(F)) inclusion complexes.

HPLC separation of diastereomers of Ln(III)-ethylenepropylene-triamine-pentaacetate complexes. Direct assessment of their water exchange rate.

The diastereomers of two Ln(III)-EPTPA derivatives have been separated by reversed-phase HPLC, and the water exchange rate on their Gd(III) complexes has been directly determined by 17O NMR (H5EPTPA = ethylenepropylene-triamine-pentaacetic acid).

Novel heteroditopic chelate for self-assembled gadolinium(III) complex with high relaxivity.

[Fe(tpy-DTTA)(2)Gd(2)] is a self-assembled trinuclear complex based on a novel ligand in which a terpyridine and a poly(amino carboxylate) moiety are connected; it has a well-defined topology with favourable features to attain high relaxivities, i.e. a rigid Fe(II)(tpy)(2) core, reduced flexibility at the periphery thanks to a short linker, and efficient separation of the two Gd(III) centres.

Accelerating water exchange for GdIII chelates by steric compression around the water binding site.

The water exchange process was accelerated for nine-coordinate, monohydrated macrocyclic GdIII complexes by inducing steric compression around the water binding site; the increased steric crowding was achieved by replacing an ethylene bridge of DOTA4- by a propylene bridge; in addition to the optimal water exchange rate, the stability of [Gd(TRITA)(H2O)]- is sufficiently high to ensure safe medical use which makes it a potential synthon for the development of high relaxivity, macromolecular MRI contrast agents.

13C and (15)N NMR Mechanistic Study of Cyanide Exchange on Oxotetracyanometalate Complexes of Re(V), Tc(V), W(IV), Mo(IV), and Os(VI).

A range of complexes with general formula [MO(X)(CN)(4)](n)()(-) of W(IV), Mo(IV), Re(V), Tc(V), and Os(VI) were prepared and characterized by (13)C, (15)N, (17)O, and (99)Tc NMR, utilizing (13)C- and (15)N-enriched cyano complexes. A correlation between M-O and M-CN bond strength from X-ray crystallographic data and |(1)J((183)W-(13)C)| coupling is reported. The cyanide (HCN/CN(-)) exchange kinetics on the trans-dioxotetracyanometalate complexes and protonated/substituted ([MO(X)(CN)(4)](n)()(-)) forms thereof were studied in aqueous medium. The dioxotetracyano complexes show a trend of reactivity M(IV) > M(V) >M(VI), which is in agreement with the increase in M-L bond strength (L = O(2)(-) or CN(-)) and a dissociative activation for the cyanide and the oxygen exchange in these complexes. Rate constants (X, k(Xc)) in s(-)(1) at 298 K for the [MO(X)(CN)(4)](n)()(-) complexes are as follows for Mo(IV): O(2)(-), >0.4; OH(-), (1.7 +/- 0.1) x 10(-)(2); H(2)O, (1.5 +/- 0.1) x 10(-)(2); CN(-), (9.6 +/- 0.8) x 10(-)(3). For W(IV): O(2)(-), (4.4 +/- 0.4) x 10(-)(3); OH(-), (9.6 +/- 0.9) x 10(-)(5); H(2)O, (1.1 +/- 0.1) x 10(-)(4); CN(-), (1.1 +/- 0.1) x 10(-)(2); N(3)(-), (3.1 +/- 0.2) x 10(-)(4); F(-), (4.8 +/- 0.1) x 10(-)(5). For Tc(V): O(2)(-), (4.8 +/- 0.4) x 10(-)(3); H(2)O, <4 x 10(-)(5); NCS(-), <4 x 10(-)(5). For Re(V): O(2)(-), (3.6 +/- 0.3) x 10(-)(6) and (1.2 +/- 0.1) x 10(-)(4) for CN(-) and HCN, respectively; OH(2), <4 x 10(-)(8). For Os(VI): O(2)(-), <4 x 10(-)(9) and (1.2 +/- 0.1) x 10(-)(4) for CN(-) and HCN, respectively. The cyanide exchange kinetics were correlated with previously determined proton and oxygen exchange, spanning a kinetic domain of more than 12 orders of magnitude for the five metal centers studied.

Water Exchange and Rotational Dynamics of the Dimeric Gadolinium(III) Complex [BO{Gd(DO3A)(H(2)O)}(2)]: A Variable-Temperature and -Pressure (17)O NMR Study(1).

Rapid water exchange and slow rotation are essential for high relaxivity MRI contrast agents. A variable-temperature and -pressure (17)O NMR study at 14.1, 9.4, and 1.4 T has been performed on the dimeric BO(DO3A)(2), 2,11-dihydroxy-4,9-dioxa-1,12-bis[1,4,7,10-tetraaza-4,7,10-tris(carboxymethyl)cyclododecyl]dodecane, complex of Gd(III). This complex is of relevance to MRI as an attempt to gain higher (1)H relaxivity by slowing down the rotation of the molecule compared to monomeric Gd(III) complexes used as contrast agents. From the (17)O NMR longitudinal and transverse relaxation rates and chemical shifts we determined the parameters characterizing water exchange kinetics and the rotational motion of the complex, both of which influence (1)H relaxivity. The rate constant and the activation enthalpy for the water exchange, k(ex) and DeltaH(), are (1.0 +/- 0.1) x 10(6) s(-)(1)and (30.0 +/- 0.2) kJ mol(-)(1), respectively, and the activation volume, DeltaV(), of the process is (+0.5 +/- 0.2) cm(3) mol(-)(1), indicating an interchange mechanism. The rotational correlation time becomes about three times longer compared to monomeric Gd(III) polyamino-polyacetate complexes studied so far: tau(R) = (250 +/- 5) ps, which results in an enhanced proton relaxivity by raising the correlation time for the paramagnetic interaction.

Rhodium(III) and Iridium(III) Solvates of the Form [(eta(5)-C(5)Me(5))M(S)(3)](2+) as Models for the Reactivity of Half-Sandwich Compounds(1).

Solvent exchange on the half-sandwich organic solvates [(eta(5)-C(5)Me(5))M(S)(3)](2+) (M = Rh, S = MeCN (1) or Me(2)SO (3); and M = Ir, S = MeCN (2) or Me(2)SO (4)) has been investigated as a function of temperature, pressure, and concentration of free solvent by (1)H NMR line-broadening techniques in CD(3)CN and/or CD(3)NO(2). The exchange rates span several orders of magnitude, from k(ex)(298) = 8.8 x 10(-)(2) s(-)(1) for 2 to 3.6 x 10(3) s(-)(1) for 3, as a result of changes in the electronic and steric properties of the ligands. Nevertheless, the volume of activation remains consistently positive for compounds 1-4 with values ranging from +0.8 to +3.3 cm(3) mol(-)(1). In combination with the positive activation entropies obtained and the first-order rate law established for these systems, it was concluded that regardless of the nature of the ligand the solvent exchange process on 1-4 proceeds via a dissociative D mechanism. Of note, the intermolecular exchange with free Me(2)SO on 4 takes place exclusively from a conformational isomer of 4 (structure 4.2), which is itself in equilibrium with a second, more compact conformer (structure 4.1).

Trans- and Cis-Water Reactivities in d(6) Octahedral Ruthenium(II) Pentaaqua Complexes: Experimental and Density Functional Theory Studies(1)(,)(2).

The hexaaqua complex of ruthenium(II) represents an ideal starting material for the synthesis of isostructural compounds with a [Ru(H(2)O-ax)(H(2)O-eq)(4)L](2+) general formula. We have studied a series of complexes, where L = H(2)O, MeCN, Me(2)SO, H(2)C=CH(2), CO, and F(2)C=CH(2). We have evaluated the effect of L on the cyclic voltammetric response, on the rate and mechanism of exchange reaction of the water molecules, and on the structures calculated with the density functional theory (DFT). As expected, the formal redox potential, E degrees '(+2/+3), increases with the pi-accepting capabilities of the ligands. For L = N(2), the oxidation to Ru(III) is followed by a fast substitution of dinitrogen by a solvent molecule, revealing the poor stability of the Ru(III)-N(2) bond. The water exchange reactions have been followed by (17)O NMR spectroscopy. The variable-pressure and variable-temperature kinetic studies made on selected examples are all in accordance with a dissociative activation mode for exchange. The positive activation volumes obtained for the axial and equatorial water exchange reactions on [Ru(H(2)O)(5)(H(2)C=CH(2))](2+) (DeltaV(ax)() and DeltaV(eq)() = +6.5 +/- 0.5 and +6.1 +/- 0.2 cm(3) mol(-)(1)) are the strongest evidence of this conclusion. The increasing cis-effect series was established according to the lability of the equatorial water molecules and is as follows: F(2)C=CH(2) congruent with CO < Me(2)SO < N(2) < H(2)C=CH(2) < MeCN < H(2)O. The increase of the lability is accompanied by a decrease of the E degrees ' values, but no change was found in the calculated Ru-H(2)O(eq) bond lengths. The increasing trans-effect series, established from the lability of the axial water molecule, is the following: N(2) << MeCN < H(2)O < CO < Me(2)SO < H(2)C=CH(2) < F(2)C=CH(2). A variation of the Ru-H(2)O(ax) bond lengths is observed in the calculated structures. However, the best correlation is found between the lability and the calculated Ru-H(2)O(ax) bond energies. It appears, also, that a decrease of the electronic density along the Ru-O(ax) bond and the increase of the lability can be related to an increase of the pi-accepting capability of the ligand. For L = N(2), the calculations have shown that the Ru(II)-N(2) bond is weak. Consequently, the water exchange reaction proceeds through a different mechanism, where first the N(2) ligand is substituted by one water molecule to produce the hexaaqua complex of Ru(II). The water exchange takes place on this compound before re-formation of the [Ru(H(2)O)(5)N(2)](2+) complex.

Structure and Dynamics of a Trinuclear Gadolinium(III) Complex: The Effect of Intramolecular Electron Spin Relaxation on Its Proton Relaxivity(1).

The trinuclear [Gd(3)(H(-)(3)taci)(2)(H(2)O)(6)](3+) complex has been characterized in aqueous solution as a model compound from the point of view of MRI: the parameters that affect proton relaxivity have been determined in a combined variable temperature, pressure, and multiple-field (17)O NMR, EPR, and NMRD study. The solution structure of the complex was found to be the same as in solid state: the total coordination number of the lanthanide(III) ion is 8 with two inner-sphere water molecules. EPR measurements proved a strong intramolecular dipole-dipole interaction between Gd(III) electron spins. This mechanism dominates electron spin relaxation at high magnetic fields (B > 5 T). Its proportion to the overall relaxation decreases with decreasing magnetic field and becomes a minor term at fields used in MRI. Consequently, it cannot increase the electronic relaxation rates to such an extent that they limit proton relaxivity. [Gd(3)(H(-)(3)taci)(2)(H(2)O)(6)](3+) undergoes a relatively slow water exchange (k(ex)(298) = (1.1 +/- 0.2) x 10(7) s(-1)) compared to the Gd(III) aqua ion, while the mechanism is much more associatively activated as shown by the activation volume (DeltaV () = (-12.7 +/- 1.5) cm(3) mol(-)(1)). The lower exchange rate, as compared to [Gd(H(2)O)(8)](3+) and [Gd(PDTA)(H(2)O)(2)](-), can be explained with the higher rigidity of the [Gd(3)(H(-)(3)taci)(2)(H(2)O)(6)](3+) which considerably slows down the transition from the eight-coordinate reactant to the nine-coordinate transition state. The unexpectedly low rotational correlation time of the complex is interpreted in terms of a spherical structure with a large hydrophobic surface avoiding the formation of a substantial hydration sphere around [Gd(3)(H(-)(3)taci)(2)(H(2)O)(6)](3+).

Conformational and Coordination Equilibria on DOTA Complexes of Lanthanide Metal Ions in Aqueous Solution Studied by (1)H-NMR Spectroscopy.

A variable-temperature, -pressure, and -ionic strength (1)H NMR study of the DOTA complexes of different trivalent cations (Sc, Y, La, Ce --> Lu) (DOTA = 1,4,7,10-tetraaza-1,4,7,10-tetrakis(carboxymethyl)cyclododecane) yielded data that are in contradiction with the hitherto used model of only two enantiomeric pairs of diastereoisomers that differ in the ligand conformations. A two-isomer equilibrium cannot explain the newly observed apparent reversal of the isomer ratio at the end of the series. As both conformers may lose their inner sphere water molecule, a coordination equilibrium may be superimposed on this conformational equilibrium, as shown by large positive reaction volumes for the isomerization of [Ln(DOTA)(H(2)O)(x)()](-) (Ln = Yb, Lu; x = 1, 0). The isomerization of [Nd(DOTA)(H(2)O)](-) and [Eu(DOTA)(H(2)O)](-) is purely conformational, as shown by near-zero reaction volumes. The measured isomerization enthalpies and entropies agree with this model. The shift of the isomerization equilibria by a variety of non-coordinative salts depends on the ligand conformation rather than the presence or absence of the inner sphere water molecule. This results from weak ion binding and water solvent stabilization of one ligand conformation, rather than the decrease of the activity of the bulk water in the solution, as shown by UV-vis measurements of the coordination number sensitive transition (5)F(0) --> (7)D(0) of Eu(III) as a function of ionic strength. Fluoride ions replace a water molecule in the inner coordination sphere, preferentially for one of the conformational isomers, as proven by (19)F-NMR shifts and the appearance of a third set of resonances corresponding to [Eu(DOTA)F](2)(-) in the (1)H-NMR spectrum of [Eu(DOTA)(H(2)O)](-).

Trinuclear Lanthanoid Complexes of 1,3,5-Triamino-1,3,5-trideoxy-cis-inositol with a Unique, Sandwich-Type Cage Structure(1).

A variety of trinuclear complexes [M(3)(H(-)(3)L)(2)](3+) [M = Y, La, Eu, Gd, Dy; L = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci) and 1,3,5-trideoxy-1,3,5-tris(dimethylamino)-cis-inositol (tdci)] was prepared as solid materials of the composition M(3)(H(-)(3)L)(2)X(3).pH(2)O.qEtOH (X = Cl, NO(3); 2.5

A benzene-core trinuclear GdIII complex: towards the optimization of relaxivity for MRI contrast agent applications at high magnetic field.

A novel ligand, H(12)L, based on a trimethylbenzene core bearing three methylenediethylenetriamine-N,N,N'',N''-tetraacetate moieties (-CH(2)DTTA(4-)) for Gd(3+) chelation has been synthesized, and its trinuclear Gd(3+) complex [Gd(3)L(H(2)O)(6)](3-) investigated with respect to MRI contrast agent applications. A multiple-field, variable-temperature (17)O NMR and proton relaxivity study on [Gd(3)L(H(2)O)(6)](3-) yielded the parameters characterizing water exchange and rotational dynamics. On the basis of the (17)O chemical shifts, bishydration of Gd(3+) could be evidenced. The water exchange rate, k(ex)(298)=9.0+/-3.0 s(-1) is around twice as high as k(ex)(298) of the commercial [Gd(DTPA)(H(2)O)](2-) and comparable to those on analogous Gd(3+)-DTTA chelates. Despite the relatively small size of the complex, the rotational dynamics had to be described with the Lipari-Szabo approach, by separating global and local motions. The difference between the local and global rotational correlation times, tau(lO)(298)=170+/-10 ps and tau(gO)(298)=540+/-100 ps respectively, shows that [Gd(3)L(H(2)O)(6)](3-) is not fully rigid; its flexibility originates from the CH(2) linker between the benzene core and the poly(amino carboxylate) moiety. As a consequence of the two inner-sphere water molecules per Gd(3+), their close to optimal exchange rate and the appropriate size and limited flexibility of the molecule, [Gd(3)L(H(2)O)(6)](3-) has remarkable proton relaxivities when compared with commercial contrast agents, particularly at high magnetic fields (r(1)=21.6, 17.0 and 10.7 mM(-1)s(-1) at 60, 200 and 400 MHz respectively, at 25 degrees C; r(1) is the paramagnetic enhancement of the longitudinal water proton relaxation rate, referred to 1 mM concentration of Gd(3+)).