Diffusion dynamics metals/metal complexes

To describe the dynamics of metals at biological interphases in the presence of various ligands, the kinetics of dissociation of the complexes have to be taken into account in relation to the diffusion and to the uptake kinetics ([14] and Chapters 3 and 10 in this volume). Based on kinetic criteria, labile and inert complexes can be distinguished as limiting cases with regard to biological uptake ([14] and Chapter 3, this volume). 

Solid-state nuclear magnetic resonance (NMR) has been extensively used to assess structural properties, electronic parameters and diffusion behavior of the hydride phases of numerous metals and alloys using mostly transient NMR techniques or low-resolution spectroscopy [3]. The NMR relaxation times are extremely useful to assess various diffusion processes over very wide ranges of hydrogen mobility in crystalline and amorphous phases [3]. In addition, several borohydrides [4-6] and alanates [7-11] have also been characterized by these conventional solid-state NMR methods over the years where most attention was on rotation dynamics of the BHT, A1H4, and AlHe anions detection of order-disorder phase transitions or thermal decomposition. There has been little indication of fast long-range diffusion behavior in any complex hydride studied by NMR to date [4-11]. 

However it turned out that the structural, chemical and dynamical details are essential for complex descriptions of long-range proton transport. These parameters appear to be distinctly different for different families of compounds, preventing proton conduction processes from being described by a single model or concept as is the case for electron transfer reactions in solutions (described within Marcus theory [23]) or hydrogen diffusion in metals (incoherent phonon assisted tunneling [24]). 

The recent extension of these thermodynamic models to include the kinetics and mechanisms of organo-metallic interactions has made it possible (1) to quantify the electrochemical availability of these metal complexes to voltammetric systems (Whitfield and Turner, 1980) (2) to examine diffusion and dissociation models for the tremsport of chelated iron to biological cells (Jackson and Morgan, 1978) and (3) to estimate the significance of adsorptive and convective removal processes on the equilibrium specia-tion of metals in natural waters (Lehrman and Childs, 1973). Thus both equOibrium and dynamic models have become an indispensable tool in the identification of the important chemical forms and critical reaction pathways of interactive elements in aquatic environments. 

The use by Avnir and co-workers (21) of excited-state dynamics of pyrene-doped sol-gel materials to probe the interior of the sol-gel environment has helped pave the way for similar smdies with luminescent probes such as [Ru(bpy)3] (14). From an applications standpoint, the attributes of [Ru(bpy)3] (Section lll.D.2.b) make this metal complex a particularly good O2 sensor. The matrix can have an enormous effect on O2 diffusibility to the doped sensor depending on the conditions of synthesis. The sensing ability of [Ru(bpy)3] " is probably best suited for gas-phase O2 as the molecule has been shown to leach from sol-gels under solution conditions (146). A typical optically based O2 sensing device using such a system is shown in Fig. 23 (143). 

Fortunately, this limitation in measuring protein dynamics can be circumvented by using donors with longer decay times. If the decay times are increased from 5 ns (Figure 14.22) to S is, the measurable diffusion coefficients decrease by a factor of 10, so that the time-resolved RET becomes sensitive to diffusion coefficients of 0 -10 cm /s, instead of I0" - 0 cm /s (Figure 14.22). Donors with longer decay times such as the lanthanides europium and terbium display 0.2- to 0.4-ms lifetimes, and the transition-metal-Ugand complexes display lifetimes from 100 ns to 10 These complexes are particularly prom-... 

The same information about coordination numbers and complexation obtained from molecular dynamics simulations is also obtained from Monte Carlo simulations. Molecular dynamics simulations, however, also allow us to investigate time dependant phenomena such as diffusion and the lifetime of metal complexes. 

This behaviour is explained in terms of a dynamic process in which inter-molecular electron transfer between (UO2 ) and metal complexes occurs. Other pertinent results are i) The quenching is highly efficient (approaching the diffusion controlled rate) with metallocenes or metal carbonyls which do not possess optically detectable electronic excited states... 

A different type of switch is displayed in Fig. 2.40. Here, a Ru-bipy complex is appended to a calixarene in which two of the phenoxyl groups have been oxidized to quinones, complex 90 [233, 234]. The calixarene also possesses a pendant bipyridine ligand. Upon illumination into the MLCT band localized on the metal complex, light-induced electron transfer takes place from the triplet state of Ru-bipy to one of the quinones. This process involves diffusive encounter between the reactants and it should be noted that NMR and molecular dynamics simulations indicate that the calixarene walls are highly mobile. Cations, such as Ba +, bind to the lower rim of the calixarene and are held in place by the additional bipyridine ligand. This has the effect of forcing the pendant Ru-bipy away from the calixarene in order to minimize electrostatic repulsion. The net effect is to curtail light-induced electron transfer. Thus, whereas 90 is nonluminescent the various cation-... 

In the context of Scheme 11-1 we are also interested to know whether the variation of K observed with 18-, 21-, and 24-membered crown ethers is due to changes in the complexation rate (k ), the decomplexation rate (k- ), or both. Krane and Skjetne (1980) carried out dynamic 13C NMR studies of complexes of the 4-toluenediazo-nium ion with 18-crown-6, 21-crown-7, and 24-crown-8 in dichlorofluoromethane. They determined the decomplexation rate (k- ) and the free energy of activation for decomplexation (AG i). From the values of k i obtained by Krane and Skjetne and the equilibrium constants K of Nakazumi et al. (1983), k can be calculated. The results show that the complexation rate (kx) does not change much with the size of the macrocycle, that it is most likely diffusion-controlled, and that the large equilibrium constant K of 21-crown-7 is due to the decomplexation rate constant k i being lower than those for the 18- and 24-membered crown ethers. Izatt et al. (1991) published a comprehensive review of K, k, and k data for crown ethers and related hosts with metal cations, ammonium ions, diazonium ions, and related guest compounds. 

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