Fluoride affinities of fluorinated alanes

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Abstract

The fluoride affinities of fluorinated alanes, AlHmF3−m (m = 1–3) were measured using energy-resolved collision-induced dissociation of fluorinated aluminate anions. The AlHmF4−m anions were formed by reaction of dimethylethylamine-alane with fluoride ion and F2. From the measured bond dissociation energies, the fluoride affinities of fluorinated alanes are determined to be 93.2 ± 3.1, 97.5 ± 4.0, and 108.6 ± 3.7 kcal/mol for m = 3, 2, and 1, respectively. The fluoride affinities are in good agreement with the theoretical calculations at the CCSD(T)/CBS and B3LYP/6-31 + G* levels of theory. The increased Lewis acidity of more fluorinated alanes is attributed to increased positive charge density on the aluminum.

Research highlights

▶ Fluorinated aluminate anions (AlHmF4−m) can be formed in the gas phase by ionization of diethylmethylamine-alane with molecular fluorine. ▶ Energy-resolved collision-induced dissociation is used to measure fluoride affinities of the alanes. ▶ The measured fluoride affinities agree with predictions obtained from coupled-cluster calculations with very large basis sets. ▶ Fluoride affinities reflect the extent of positive charge character on the aluminum in the alanes.

Introduction

Thermochemical properties provide fundamental insight into the nature of molecular structure and bonding [1]. There are a wide variety of commonly measured thermochemical properties such as proton affinities, gas-phase acidities, and electron and hydride affinities. Fluoride affinity (FA) is also an important thermochemical property, serving as a measure of the Lewis acidity. The measurements of FAs have been determined experimentally using a variety of techniques [2], [3], [4]. Haartz and McDaniel used ion cyclotron resonance spectroscopy to determine the relative order of the fluoride affinity to be SF4, SF5 < SO2, HCl, AsF3 < SiF4 < BF3 < PF5 < BCl3 < ASF5 [2]. Larson and McMahon related fluoride affinities to the hydrogen bond strengths of various chemical species [3], which enabled them to create a wide-range fluoride affinity scale. The scale was fixed at the low end with the fluoride affinity of H2O (23.3 kcal/mol) and at the high end with the fluoride affinity of HCO2H (45.3 kcal/mol). Energy-resolved collision-induced dissociation (CID) has led to revisions of the fluoride affinity scale allowing the addition of new values [4], [5], [6].

Despite the advancement in FA measurements, very few aluminum containing systems have been studied. In fact, few thermochemical properties are known for any aluminum containing molecules, alanes in particular. Aluminates such as aluminum hydride are important because they are commonly used as reducing agents and have been proposed as a means for hydrogen storage [7]. In this study, we determined the fluoride affinities of fluorinated alanes using a flowing afterglow-triple quadrupole instrument.

Section snippets

Instrumental description and data analysis

The fluoride affinities of fluorinated alanes were determined using a flowing afterglow-triple quadrupole mass spectrometer that has been previously described elsewhere [8]. Fluoride ions were generated by 70 eV electron ionization of F2 (5% in helium). Helium buffer gas (P = 400 mTorr) was used to carry the ions through the flow tube at a flow rate of ca. 190 STP cm−3/s. Ions were then allowed to undergo ion-molecule reactions with the neutral reagent (dimethylethylamine-alane, DMEAA) added through

Results

Fluorinated aluminate ions were generated by chemical ionization (CI) of the Lewis acid–base complex dimethylethylamine-alane, DMEAA, with F generated from F2 as the CI reagent Eq. (4). Among the products observed are AlHmF4−m, where m = 0–3. By changing the source conditions, more of the higher masses can be formed (m = 1, 2). The fourth ion, AlF4 could not be formed in sufficient abundance for CID studies. AlH3F is likely generated by substitution of the dimethylethylamine in DMEAA. The more

Discussion

The experimentally measured FAs are in good agreement with the theoretical predicted values, with only the B3LYP value for AlHF2 at the edge of the assigned error limit. As noted above, the FA values are larger for alanes with more fluorine substituents, reflecting the greater Lewis acidity for these substrates. The increased Lewis acidity results from increased positive charge density on the aluminum in the alane when replacing a hydrogen atom with a more electronegative fluorine. Charge

Conclusions

Fluorination of alanes increases the fluoride affinity, and hence the Lewis acidity, by increasing the extent of positive charge character on the aluminum atom. The increase in fluoride affinity is not linear because the energies of fluorinated alanes are not linear with increased number of fluorines. The measured fluoride affinities agree well with theoretically predicted values.

Acknowledgements

This work was supported by the National Science Foundation (CHE04-54874 and CHE08-08964). Thanks also to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support. Calculations were carried out using the resources of the Center for Computational Studies of Open-Shell and Electronically Excited Species (iopenshell.usc.edu), supported by the National Science Foundation through the CRIF:CRF program.

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