Composition of complex aluminum hydrides and borohydrides, as inferred from conductance, molecular association, and spectroscopic studies

Abstract

The equivalent conductance of THF solutions of LiAlH4, NaAlH4, Bu4NA1H4, and LiBH4 were obtained over a wide concentration range, i.e., 0.5 to 1 X 10~7 M. The data are consistent with the presence of free ions and ion pairs in dilute solution and the formation of triple ions as the concentration is increased. The application of the Fuoss treatment to the data, where only free ions and ion pairs are encountered, permits the evaluation of the infinite dilution equivalent conductance ( 0) and the ion-pair dissociation constant (Aa) for each hydride. Utilizing a sphere in a continuum model, the center-to-center distances between the ions in the ion pair calculated from the experimental Ka values indicate that the ion pairs of Bu4NA1H4 and LiBH4 are in intimate contact, whereas the LiAlH4 and NaAlH4 species are solvent-separated ion pairs. Thermodynamic parameters also suggest that LiAlH4 ion pairs are predominantly solvent separated, whereas a substantial fraction of the NaAlH4 ion pairs are are in intimate contact at 25°. The 0 values obtained for these hydrides also support these conclusions. In the region where ion pairs and triple ions are encountered, the triple ion dissociation constants (Kt) have been evaluated from the data using the Fuoss method. Combining the X» and Kt values, the fraction of each species present at any concentration below 0.1 M has been calculated. When these data are used to calculate association i values, good agreement is found between the observed ebullioscopic i values and those calculated from the conductance results. Nmr studies indicate the formation of a 4:1 THF solvate with LiAlH4, Lil, and LiC104 and the formation of a 2:1 THF solvent with LiBH4. Similar studies show no interaction for Bu4NA1H4. The appearance of infrared bands near 420 cm-1 (attributed to the vibration of the lithium ion in a solvent cage) are found for LiAlH4, LiClOi, Lil, and LiBr, but well-defined bands are absent for LiBH4 in THF.

Hydrogen electrode reaction of lithium and sodium aluminum hydrides

“…1. Our results are in good agreement with those reported by Ashby et al [4] and Alpatova et al [6]. Molar conductivity of LiAlH 4 2THF solution shows a minimum at around 0.02 M and a maximum at around 1.0 M. In case of NaAlH 4 2THF solution the molar conductivity shows a maximum similar to the case of LiAlH 4 although the minimum is not clear.…”

“…Molar conductivity of LiAlH 4 2THF solution shows a minimum at around 0.02 M and a maximum at around 1.0 M. In case of NaAlH 4 2THF solution the molar conductivity shows a maximum similar to the case of LiAlH 4 although the minimum is not clear. As mentioned in Section 1, Ashby et al [4] have provided a detailed information concerning the present species of LiAlH 4 2THF and NaAlH 4 2THF solutions over a wide concentration ð10 À7 20:1 MÞ. They found that at 25 C in dilute solution the predominant species are solvated lithium ion and ½AlH 4 À anion (I); at 0.01 M they are solvent-separated ion pairs (II); and at higher concentrations triple ions (III) are formed by the association of LiAlH 4 through hydrogen bridge…”

“…In order to deposit electrochemically metallic aluminum a hydride bath that consists of AlCl 3 and LiAlH 4 (or LiH) in anhydrous ether was developed in 1952 [3]. As for electrochemical properties, Ashby et al [4] have determined the molar conductivity of LiAlH 4 and NaAlH 4 in tetrahydrofuran (THF) to clarify the dissociation of them as ions in THF. For the electrode processes, anodic polarizations of LiAlH 4 were determined [5].…”

Hydrogen Dynamics in Lightweight Tetrahydroborates

“…[83]), in LiBH 4 dissolved in organic solvents (stretching vibrations measured by IR spectroscopy, Ref. [84]) and in solid LiBH 4 [62]. An important question related to the effect of additives is wether the additives interact with the hydrides.…”

Solutions of Lithium Tetrahydrido Aluminates in Ethers: a 7Li and 27Al NMR Study

“…This width is about four times as large as for monoglyme. Since X H broad band decoupling has no influence on the 7 Li line width it seems that the interaction of the Li-cations and AIH4 anions in the ion pair and triple ions present in these solutions [13] is less efficient in these solvents than in the diglyme allowing a more rapid quadrupol relaxation.…”