engleski

Hydride Affinities of Borane Derivatives: Novel Approach in Determining the Origin of Lewis Acidity Based on Triadic Formula

The problem of intrinsic Lewis acidities of simple boron compounds (BH3-mXm, m = 0-3, X = F, Cl, Br, CH3 and OH) is assessed by their gas-phase hydride affinities (HA). A simple and intuitively appealing picture of the interaction process including ionization of the hydride ion H- by ejecting an electron, attachment of the pruned electron to the investigated Lewis acid (LA) and subsequent formation of the homolytic chemical bond between two newly created radicals is proposed. It enables transparent and straightforward dissection of the initial and final state effects, which taken together determine the trend of changes in the hydride affinities. The former effects are reflected in the electron affinities of the neutral Lewis acids given within Koopmans' approximation, whilst the latter encompass properties of the formed Lewis acid-base adduct mirrored in the bond dissociation energy of the formed [LA-H]- chemical bond. It is demonstrated that unexpectedly low Lewis acidity of fluoroboranes relative to the corresponding chlorine and bromine derivatives can be traced down to the unfavorable Koopmans' electron affinities. Hence, it is a consequence of the initial state effect. In contrast, chloroboranes are more potent Lewis acids than fluoroboranes, because the relaxation and final state effects decisively influence their Lewis acidity. Finally, bromine-substituted borane compounds provide the most powerful studied Lewis acids. Their hydride affinities are result of a synergic synaction of the initial, intermediate relaxation and final state effects. It is shown that methyl- and hydroxy-derivatives of BH3 are weaker Lewis acids than the parent BH3. Pearson's global hardness indices defined within his hard and soft acid and base (HSAB) principle fail to adequately predict and interpret the calculated hydride affinities.

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