Rotational dynamics, metals/metal complexes

Molecular mechanics and dynamics studies of metal-nucleotide and metal-DNA interactions to date have been limited almost exclusively to modeling the interactions involving platinum-based anticancer drugs. As with metal-amino-acid complexes, there have been surprisingly few molecular mechanics studies of simple metal-nucleotide complexes that provide a means of deriving reliable force field parameters. A study of bis(purine)diamine-platinum(II) complexes successfully reproduced the structures of such complexes and demonstrated how steric factors influenced the barriers to rotation about the Pt(II)-N(purine) coordinate bonds and interconversion of the head-to-head (HTH) to head-to-tail (HTT) isomers (Fig. 12.4)[2011. In the process, force field parameters for the Pt(II)/nucleotide interactions were developed. A promising new approach involving the use of ab-initio calculations to calculate force constants has been applied to the interaction between Pt(II) and adenine[202]. 

The first applications of NMR to the study of dynamic systems of the platinum group metals appear to have been studies on the rotation about the metal-olefin bond of coordinated olefins, and this process has been investigated by many workers. There are two reasonable orientations of an olefin with respect to the rest of a square planar complex, XXIV and XXV. 

Allylalkali metal compounds exhibit dynamic isomer-izations and bond rotations in solntion. In THF solntion, allylalkali metal complexes undergo C bond rotations with activation energies normally in the range of 45-75kJmoL. 1 - AUcylallylalkali metal compounds usually exist as a dynamic mixture of (E) and (Z) isomers (15), with a thermodynamic preference for the (Z) configuration. The rates of isomerization of frani -neopentylallyl metal complexes in THF solution decreases in the sequence Li > Na > K. Numerous studies of the structures and dynamic properties of allyllithium compounds have been reported. " ... 

The use of prochiral alkenes such as propene, 2-butene, tran -dideutereoethylene, dialkyl fumarates, and trani-dimethoxyethylenes, have allowed detailed structural and mechanistic studies of alkene complexes. The diastereomers produced upon binding of prochiral alkenes to CpM(CO)X centers provided the key complexes to prove that interconversions occurred by rotations about the metal-(C=C) axis. Thus observations that neither the chirality at the metal nor at the alkene is changed in the rearrangement of a trans-substituted alkene provided proof that the nature of the dynamic process was a propeller rotation. Note that in equation (9) the equilibrium between (20) and (21) averages enviromnents b and d separately from a and c. A key feature is that olefin rotation does not alter the chirahty at the olefin because of the olefin-metal bond. 

Pyridine complexes can show analogous hindered rotation to that observed with metal-aryl complexes. This has been observed for over SOyears, but recent interest in supramolecular chemistry, particularly by Fujita and Stang, wherein bipyridyls have been used as linkers has spurred renewed interest in the phenomenon. Several articles on dynamics provide leading references for applications of EXSY and other DNMR methods in these systems. BINAP complexes with picolines and methyl-substituted isoquinolines, which show syn and anti isomerism, have been investigated, for example, (48), (49), and (50). ... 

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]. 

Redfield limit, and the values for the CH2 protons of his- N,N-diethyldithiocarbamato)iron(iii) iodide, Fe(dtc)2l, a compound for which Te r- When z, rotational reorientation dominates the nuclear relaxation and the Redfield theory can account for the experimental results. When Te Ti values do not increase with Bq as current theory predicts, and non-Redfield relaxation theory (33) has to be employed. By assuming that the spacings of the electron-nuclear spin energy levels are not dominated by Bq but depend on the value of the zero-field splitting parameter, the frequency dependence of the Tj values can be explained. Doddrell et al. (35) have examined the variable temperature and variable field nuclear spin-lattice relaxation times for the protons in Cu(acac)2 and Ru(acac)3. These complexes were chosen since, in the former complex, rotational reorientation appears to be the dominant time-dependent process (36) whereas in the latter complex other time-dependent effects, possibly dynamic Jahn-Teller effects, may be operative. Again current theory will account for the observed Ty values when rotational reorientation dominates the electron and nuclear spin relaxation processes but is inadequate in other situations. More recent studies (37) on the temperature dependence of Ty values of protons of metal acetylacetonate complexes have led to somewhat different conclusions. If rotational reorientation dominates the nuclear and/or electron spin relaxation processes, then a plot of ln( Ty ) against T should be linear with slope Er/R, where r is the activation energy for rotational reorientation. This was found to be the case for Cu, Cr, and Fe complexes with Er 9-2kJ mol" However, for V, Mn, and... 

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