Formation and in situ Characterisation of the First Dihydrogen Aquacomplex: [Ru(H2O)(5)(H-2)](2+)

The product of the reaction between [Ru(H2O)(6)](2+) and pressurized H-2 in water is [Ru(H2O)(5)(H-2)](2+) whose nature was unambiguously demonstrated by H-1 and O-17 NMR and which is the first characterized dihydrogen aqua complex

Complete Carbonylation of fac-

The rate constant of ligand exchange on the complex fac-[(99)Tc(H(2)O)(3)(CO)(3)](+) was determined by means of (13)C, (17)O, and (99)Tc NMR spectroscopy under pressurized conditions in aqueous media. After keeping the sample under CO pressure for an extended period, the formation of [(99)Tc(CO)(6)](+) could unambiguously be detected in the (13)C and (99)Tc NMR spectra

Towards rational design of fast water-exchanging Gd(dota-like) contrast agents? Importance of the M/m ratio

H-1 NMR line-shape analysis and magnetisation-transfer experiments at variable temperature and pressure have been used to elucidate the solution dynamics of both M and m isomers of three [Eu(dota-tetraamide)(H2O)](3+) complexes. The direct H-1 NMR observation of the bound water signal allows the water- exchange rates on each isomer to be measured individually. They are definitely independent of the ligand for both M and m isomers (M: k(ex)(298) = 0.2 x 10(3)s(-1) for [Eu(dotam)(H2O)](3+), 8.2 +/- 02 x 10(3) s(-1) for [Eu(dtma)(H2O)](3+) and 11.2 +/- 1.4 x 10(3) s(-1) for [Eu(dotmam)(H2O)](3+), m: k(ex)(298) = 474 +/- 130 x 10(3) s(- 1) for [Eu(dotam)(H2O)](3+), 357 +/- 92 x 10(3) s(-1) for [Eu(dtma)(H2O)](3+)), and proceed through a dissociative mechanism (M isomers: DeltaV(double dagger) = + 4.9 cm(3) mol(- 1) for [Eu(dotam)(H2O)](3+) and + 6.9 cm(3) mol(-1) for [Eu(dtma)(H2O)](3+)). The overall water exchange only depends on the M/m isomeric ratio. The m isomer, which exchanges more quickly, is favoured by a-substitution of the ring nitrogen. Therefore the synthesis of DOTA-like ligands, which predominantly form complexes in the m form, should be a sufficient condition to ensure faster water exchange on potential Gd-III-based contrast agents. Furthermore the activation parameters for the water-exchange and isomerisation processes are both compatible with a nonhydrated complex as intermediate

High Pressure Cyanide Exchange Study on Square-planar Tetracyanometalate Complex of Pt(II)

Previous studies of cyanide exchange on square planar tetracyanoplatinate complex [Pt(CN)4]2- have been undertaken only at a high pH. For a more complete fundamental understanding of this system we extended the investigations of these exchanges over a large pH range. NMR kinetics methods (magnetisation transfer, isotopic exchange) proved to be very useful for obtaining quantitative rate data of the cyanide exchange on this complex. In fact it is quite significant that the reactivity of this metal center spans a ca. 9-order of magnitude range as a function of pH. Variable temperature and variable pressure studies were undertaken in aqueous solutions and the following activation parameters obtained: DeltaHDagger = (25.1 ± 0.4) kJmol-1 and activation entropy DeltaSDagger = -(142±2)JK-1mol-1 and activation volume DeltaVDagger = -(27±2)cm3mol-1

Solution and solid-state characterization of Eu(II) chelates: a possible route towards redox responsive MRI contrast agents.

We report the first solid state X-ray crystal structure for a Eu(II) chelate, [C(NH2)3]3[Eu(II)(DTPA)(H2O)].8H2O, in comparison with those for the corresponding Sr analogue, [C(NH2)3]3[Sr(DTPA)(H2O).8H2O and for [Sr(ODDA)].8H2O (DTPA5 = diethylenetriamine-N,N,N',N",N"-pentaacetate, ODDA2- =1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diacetate ). The two DTPA complexes are isostructural due to the similar ionic size and charge of Sr(2+) and Eu(2+). The redox stability of [Eu(II)(ODDA)(H2O)] and [Eu(II)(ODDM)]2- complexes has been investigated by cyclovoltammetry and UV/Vis spectrophotometry (ODDM4- =1,4,10,13-tetraoxa-7,16-diaza-cyclooctadecane-7,16-++ +dimalonate). The macrocyclic complexes are much more stable against oxidation than [Eu(II)(DTPA)(H2O)]3- (the redox potentials are E1/2 =-0.82 V, -0.92 V, and -1.35 V versus Ag/AgCl electrode for [Eu(III/II)(ODDA)(H2O)],[Eu(III/II)(ODDM)], and [Eu(III/II)(DTPA)(H2O)], respectively, compared with -0.63 V for Eu(III/II) aqua). The thermodynamic stability constants of [Eu(II)(ODDA)(H2O)], [Eu(II)(ODDM)]2-, [Sr(ODDA)(H2O)], and [Sr(ODDM)]2- were also determined by pH potentiometry. They are slightly higher for the EuII complexes than those for the corresponding Sr analogues (logK(ML)=9.85, 13.07, 8.66, and 11.34 for [Eu(II)(ODDA)(H2O)], [Eu(II)(ODDM)]2-, [Sr(ODDA)(H2O)], and [Sr(ODDM)]2-, respectively, 0.1M (CH3)4NCl). The increased thermodynamic and redox stability of the Eu(II) complex formed with ODDA as compared with the traditional ligand DTPA can be of importance when biomedical application is concerned. A variable-temperature 17O-NMR and 1H-nuclear magnetic relaxation dispersion (NMRD) study has been performed on [Eu(II)(ODDA)(H2O)] and [Eu(II)(ODDM)]2- in aqueous solution. [Eu(II)(ODDM)]2- has no inner-sphere water molecule which allowed us to use it as an outer-sphere model for [Eu(II)(ODDA)(H2O)]. The water exchange rate (k298(ex)= 0.43 x 10(9)s(-1)) is one third of that obtained for [Eu(II)(DTPA)(H2O)]3-. The variable pressure 17O-NMR study yielded a negative activation volume, deltaV (not=) = -3.9cm3mol(-1); this indicates associatively activated water exchange. This water exchange rate is in the optimal range to attain maximum proton relaxivities, which are, however, strongly limited by the fast rotation of the small molecular weight complex