The Lewis superacid Al[N(C6F5)2]3 and its higher homolog Ga[N(C6F5)2]3 - structural features, theoretical investigation and reactions of a metal amide with higher fluoride ion affinity than SbF5.

Herein we present the synthesis of the two Lewis acids Al[N(C6F5)2]3 (ALTA) and Ga[N(C6F5)2]3 (GATA) via salt elimination reactions. The metal complexes were characterized by NMR-spectroscopic methods and X-ray diffraction analysis revealing the stabilization of the highly Lewis acidic metal centers by secondary metal-fluorine contacts. The Lewis acidic properties of Al[N(C6F5)2]3 and Ga[N(C6F5)2]3 are demonstrated by reactions with Lewis bases resulting in the formation of metallates accompanied by crucial structural changes. The two metallates [Cs(Tol)3]+[FAl(N(C6F5)2)3]- and [AsPh4]+[ClGa(N(C6F5)2)3]- contain interesting weakly coordinating anions. The reaction of Al[N(C6F5)2]3 with trityl fluoride yielded [CPh3]+[FAl(N(C6F5)2)3]- which could find application in the activation of metallocene polymerization catalysts. The qualitative Lewis acidity of Al[N(C6F5)2]3 and Ga[N(C6F5)2]3 was investigated by means of competition experiments for chloride ions in solution. DFT calculations yielded fluoride ion affinities in the gas phase (FIA) of 555 kJ mol-1 for Al[N(C6F5)2]3 and 472 kJ mol-1 for Ga[N(C6F5)2]3. Thus, Al[N(C6F5)2]3 can be considered a Lewis superacid with a fluoride affinity higher than SbF5 (493 kJ mol-1) whereas the FIA of the corresponding gallium complex is slightly below the threshold to Lewis superacidity.

Superbasic alkyl-substituted bisphosphazene proton sponges: a new class of deprotonating matrices for negative ion matrix-assisted ionization/laser desorption mass spectrometry of low molecular weight hardly ionizable analytes.

RATIONALE: Here hardly ionizable and low molecular weight compounds are detected in negative ion mode by using novel superbasic proton sponges based on 1,8-bisphosphazenylnaphthalene (PN) as MALDI matrices. Among the selected proton sponges, 1,8-bis(trispyrrolidinophosphazenyl)naphthalene (TPPN) has shown the best behaviour as matrix since it allows the direct detection of intact cholesterol without derivatization also in real challenging samples. METHODS: Very weakly acidic compounds such as sterols, steroids, fatty alcohols and saccharides were detected in reflectron negative ion mode by a MALDI TOF/TOF system equipped with a neodymium-doped yttrium lithium fluoride (Nd:YLF) laser (345 nm) with typical mass accuracy of 10 ppm. MS/MS experiments were performed by using ambient air as the collision gas. RESULTS: Contrary to traditional MALDI matrices, superbasic proton sponges allowed the easy deprotonation of an alcohol functional group without a previous chemical derivatization step. Experimental evidence indicates that analyte deprotonation is achieved in the condensed phase, i.e. PN superbasic proton sponges operate according to a recently proposed model named matrix assisted ionization/laser desorption (MAILD). A detection limit of 3 pmol/spot of cholesterol (model compound) with a signal-to-noise ratio ≥ 10 was typically obtained. CONCLUSIONS: For the first time, the usefulness of novel superbasic proton sponges is demonstrated for MALDI detection of hardly ionizable compounds such as sterols, steroids, fatty alcohols and saccharides. The leading candidate TPPN has been successfully applied for negative ion MAILD-MS analysis of cholesterol, fatty acids and phospholipids in egg yolk and brain tissue extracts. Copyright © 2016 John Wiley & Sons, Ltd.