A Proton NMR Study of the Dynamics of Ligand Exchange on Tetrakis(1,1,3,3‐Tetramethylurea)Beryllium(II) and its Dimethyl Sulphoxide Analogue

Abstract

St. F. Lincoln and M. N. Tkaczuk: A Proton NMR Study of the Dynamics of Ligand Exchange etc. 433 proceeds with a quantum efficiency higher than 0.46. On the other hand, the quantum efficiency of RhB triplet formation is 0.006 in water (291. Hence, if process (9) is operative in cosensitization by Br-and C1-, these ions should enhance the RhB intersystem crossing by a factor larger than 80. This seems, by all means, improbable.Thus the origin for cosensitization by Br-and CI-ions should be sought in promotion of RhB adsorption t o the CdS surface (92). This type of mechanism has been put forth by Bode and Hauffe [9] for SCN-ion but not for halide ions. Although a quantitative agreement is not seen between the cosensitization ( Fig. 1) and adsorption promotion ( Fig. 3), this may again reflect the different surface conditions of CdS single crystal and powder.The above discussion could be rationalized with the redox level diagram illustrated in Fig. 4. Strong reducing agents such as HQ, I -, and Fe2+, with redox potentials negative of the RhB/RhBf couple, can regenerate RhB through process (8). Quenching of RhB fluorescence by all the additives employed is explained on account of the energetic correlation between the redox levels of these additives and that of the RhB*/RhB; couple. However, in the presence of a very fast process (7), a direct photoreduction of RhB is not possible at least by Br-and CI-ions. Figs. 1 and 2 show that H Q is more effective than I -, though the redox potential of the former lies at a somewhat positive value than the latter. This may have resulted from a kinetic factor in process (8), or from a difference in the reorganization energy [ 5 ] between H Q and I -.A 'H (90 MHz) NMR study of 1,1,3,3-tetramethylurea exchange on [Be(O= C(NMe2)2)4]2+ in the non-co-ordinating diluents CD3N02 and CD3CN shows that the exchange rate is independent of the free 1,1,3,3-tetramethylurea concentration. Typically k, (298.2 K) = 1.39 * 0.09 s-', AH' = 77.1 f 1.5 kJ mol-I, and AS' = 16.4 * 4.3 J K -' mol-' for a solution in which [Be(O=C(NMe2)2):+], [O=C(NMe2)2]free, and [CD3NOz] = 0.00743, 0.0334, and 18.3 mol dm-' respectively. The exchange parameters determined in CD3CN differ slightly from those determined in CD3N02 but in both cases the data are considered to typify a dissociative (D) solvent exchange mechanism. Dimethyl sulphoxide exchange on [Be(O=SMeZ),j2+ in CD3N02 diluent is characterised by an exchange rate = k2 [Be(O = SMez):'JIO = SMe2Jf,, for which k2(298.2 K) = 143 f 10 dm' m o l -'~-~, AH' = 51.1 rt 0.6 kJ mol-', and AS' = -32.3 * 2.2 JK-' mol-I, and which is considered to typify an associative (A) solvent exchange mechanism. Mechanistic comparisons are made with the data from other studies of solvent exchange on beryllium(I1). Fluoreszenz / Intersystem Crossing / Temperaturabhangigkeit / SubstituenteneinfluJThe energies of the excited states which are involved in the deactivation, the quantum yields of fluorescence and intersystem crossing, and the fluorescence decay times were determined fo...

Evidence for Interchange Ligand‐Exchange Processes on Solvated Beryllium Cations

“…This has been realized for a long time [7], and detailed studies on solvent-exchange reactions on Be 2 were performed by means of NMR techniques. Earlier mechanistic assignments were based on correlations between both stereochemistry and electron-donor ability of the solvent and its tendency to exchange with free solvent through a first-order and/or a secondorder pathway in non-coordinating polar diluents [8].…”

A Personal Account on Inorganic Reaction Mechanisms

“…Already before we became interested in Be 2 + , detailed studies on solvent-exchange reactions were performed by means of NMR techniques. [182,183] Due to the biological relevance, the most important mechanism is the exchange of coordinated water. To clarify available temperature-dependent data, Merbach et al [184] applied highpressure NMR techniques to study solvent-exchange reactions on Be 2 + , from which activation-volume data of such processes could be obtained.…”

Many-Body Potentials for Aqueous Be2+ Derived from ab Initio Calculations

“…6,15,18−25 The reactivity of Be 2+ in solution is generally controlled by the lability of the coordinated solvent molecules. 26 NMR studies have been used to elucidate the rate and mechanism of solvent exchange around Be 2+ . 16,27 In particular, Merbach applied highpressure NMR techniques to study the exchange of various solvents around Be 2+ and deduced an associative mechanism for water exchange around hydrated Be 2+ by measuring the pressure dependence of the exchange rate and determining the activation volume.…”

“…The study of aqueous Be 2+ in particular is of academic, industrial, and medical interest , as it has the highest surface charge density of any divalent ion. The behavior of aqueous Be 2+ has been the subject of numerous experimental − and theoretical studies. ,,− The reactivity of Be 2+ in solution is generally controlled by the lability of the coordinated solvent molecules . NMR studies have been used to elucidate the rate and mechanism of solvent exchange around Be 2+ . , In particular, Merbach applied high-pressure NMR techniques to study the exchange of various solvents around Be 2+ and deduced an associative mechanism for water exchange around hydrated Be 2+ by measuring the pressure dependence of the exchange rate and determining the activation volume. , …”