Salts of electrophilic cations and Lewis acid-base adducts with weakly coordinating anions

During the last decades, weakly coordinating anions (WCAs) have become a field of great interest both in basic and applied chemistry, as – if compared to "normal" classical anions like Cl- or SO42- – they have a lot of unique and unusual properties, such as high solubility in low dielectric media (like CH2Cl2 or toluene), "pseudo gas phase conditions" in condensed phases and the stabilization of strong electrophilic cations as well as of weakly bound and low charged complexes. Within the present thesis, several contributions to the chemistry of WCAs were made: Based on quantum chemical calculations, the stability of the most widely used WCAs have been investigated, using parameters such as the fluoride ion affinity (FIA), partial charges or frontier orbital energies. One class of the WCAs, the poly- and perfluorinated alkoxyaluminates of the type [Al(ORF)4]- and [(RFO)3Al-F-Al(ORF)3]-, has been used to investigate weakly bound Ag(P4S3) Lewis acid-base adducts and electrophilic cations: When the silver salts of fluorinated alkoxyaluminates are reacted with P4S3, different structure types are found for the adducts, depending on the anion used. The dynamics of these complexes has been explored by different spectroscopic methods (IR, Raman, MAS-NMR, NMR in solution) and X-ray diffraction. The first structurally characterized trihalocarbenium ions CI3+ as salts of different aluminates have been prepared and fully characterized using various spectroscopic techniques, e.g. NMR and vibrational spectroscopy, as well as single-crystal diffraction. The Lewis acidities of the halocarbenium ions have been investigated both on experimental and theoretical grounds and they have been compared to those of the isoelectronic haloboranes. The NO2+ salt of the [Al(OC(CF3)3)4]- anion has been synthesized by a metathesis reaction. It has been reacted with P4 in order to obtain naked phosphorus cations Px+. Another project was the synthesis and characterization of the tetraalkylammonium salts of the [Al(ORF)4]- anions, a promising class of substances for the use in electrochemical applications in non-polar solvents. All subjects treated experimentally within this thesis have been accompanied by quantum chemical calculations.

Stabilizing Unusual Oxidation States of Lanthanides in Molecular Complexes: Synthesis, Properties, and Reactivity

Aurélien René Willauer

The chemistry of divalent lanthanides has generated increasing interest in the past years due to their redox properties and unique reaction pathways. Notably, molecular complexes of low-valent lanthanides have been shown to be suitable one-electron reductants that are able to activate inert small molecules such as heteroallenes and dinitrogen in mild conditions. Lanthanide(II) complexes usually undergo mono-electron transfer, but multi-electron transfer can be achieved through multiple one-electron processes in polymetallic complexes. The first part of this thesis describes attempts to expand the redox chemistry of lanthanide complexes and lays the basis for a wider understanding of the chemical and reductive properties of low-valent polynuclear lanthanide complexes. Thus, Chapter 2, 3, and 4, reports the synthesis and characterization of different polynuclear lanthanides(II) complexes of various siloxide ligands. In Chapter 2, the outcome of carbon dioxide reduction (oxalate versus carbonate) by dinuclear Ln(II) tris(tert-butoxy)siloxide complexes, depending on the solvent polarity and on the nature of the Ln ions, was investigated. Furthermore, unprecedented reduction products could be isolated from the reaction with CS2, providing the first example of acetylenedithiolate ligand formation from metal reduction of carbon disulphide. In all cases, it has been shown that cooperative binding of the substrate plays an important role in the outcome of the reaction. Chapter 3 describes a rational route to the first metallasilsesquioxane of a dinuclear divalent lanthanide which can selectively effect the reductive disproportionation of CO2 and the two-electron reduction of azobenzene. In an attempt to confer increased reductive reactivity to Eu(II) ions, Chapter 4 describes the use of the tetraphenyldisiloxanediolate ligand to form polynuclear complexes of Eu(II) that can initiate for the first time carbon dioxide activation by Eu(II) ions in an isolated complex.Differently, until 2019, the +IV oxidation state of lanthanide in molecular complexes remained limited to a single lanthanide ion, cerium. It is only recently that the terbium ion was shown to be able to access the +IV oxidation state in molecular complexes. However, given the broad applications of cerium(IV), the discovery of other molecular complexes of Ln(IV) is of great interest for chemists. Chapter 5 describes the synthesis of the third example of a Tb(IV) complex displaying an open coordination sphere and the very first report of a molecular complex of praseodymium in the +IV oxidation state. The novel Ln(IV) complexes were characterized by UV-Visible, NMR, EPR and IR spectroscopy, magnetometry, single-crystal X-ray diffraction analysis, cyclic voltammetry, and computational studies. In particular, this provided an opportunity to measure and study the luminescent spectra of Pr(IV) for the first time, which upon binding to the siloxide ligand results in a large shift of the ligand triplet state. Furthermore, these complexes have been shown to be appropriate starting materials for the synthesis of Ln(IV) phosphine oxide adducts. Preliminary studies directed towards the isolation of Nd(IV) molecular complexes are discussed.

EPFL2022

Silver Complexes of Cyclic Hexachlorotriphosphazene

The first solid-state structures of complexed P3N3X6 (X=halogen) are reported for X=Cl. The compounds were obtained from P3N3Cl6 and Ag[Al(OR)4] salts in CH2Cl2/CS2 solution. The very weakly coordinating anion with R=C(CF3)3 led to the salt Ag(P3N3Cl6)2+[Al(OR)4]- (1), but the more strongly coordinating anion with R=C(CH3)(CF3)2 gave the molecular adduct (P3N3Cl6)AgAl(OR)4 (3). Crystals of [Ag(CH2Cl2)(P3N3Cl6)2]+[Al(OR)4]- (2), in which Ag+ is coordinated by two phosphazene and one CH2Cl2 ligands, were isolated from CH2Cl2 solution. The three compounds were characterized by their X-ray structures, and 1 and 3 also by NMR and vibrational spectroscopy. Solution and solid-state 31P NMR investigations in combination with quantum chemically calculated chemical shifts show that the 31P NMR shifts of free and silver-coordinated P3N3Cl6 differ by less than 3 ppm and indicate a very weakly bound P3N3Cl6 ligand in 1. The experimental silver ion affinity (SIA) of the phosphazene ligand was derived from the solid-state structure of 3. The SIA shows that (PNCl2)3 is only a slightly stronger Lewis base than P4 and CH2Cl2, while other ligands such as S8, P4S3, toluene, and 1,2-Cl2C2H4 are far stronger ligands towards the silver cation. The energetics of the complexes were assessed with inclusion of entropic, thermal, and solvation contributions (MP2/TZVPP, COSMO). The formation of the cations in 1, 2, and 3 was calculated to be exergonic by DeltarG°(CH2Cl2)=-97, -107, and -27 kJ mol-1, respectively. All prepared complexes are thermally stable; formation of P3N3Cl5+ and AgCl was not observed, even at 60 °C in an ultrasonic bath. Therefore, the formation of P3N3Cl5+ was investigated by quantum chemical calculations. Other possible reaction pathways that could lead to the successful preparation of P3N3X5+ salts were defined.

2006

PX4+, P2X5+, and P5X2+ (X = Br, I) salts of the superweak Al(OR)4- anion [R = C(CF3)3]

Lutz Müller, Ines Raabe

PX4+[Al(OR)4]- (X = I: 1a, X = Br: 1b) was prepd. from X2, PX3, and Ag[Al(OR)4] [R = C(CF3)3] in CH2Cl2 at -30 Deg in 69-86% yield. P2X5+ salts were prepd. from 2PX3 and Ag[Al(OR)4] in CH2Cl2 at -30 Deg yielding almost quant. P2X5+[Al(OR)4]- (X = I: 3a, X = Br: 3b). The P-rich P5X2+ salts arose from the reaction of cold (-78 Deg) mixts. of PX3, P4, and Ag[Al(OR)4] giving P5X2+[Al(OR)4]- (X = I: 4a, X = Br: 4b) with a C2v-sym. P5 cage. Silver salt metathesis presumably generated unstable PX2+ cations from PX3 and Ag[Al(OR)4] (X = Br, I) that acted as electrophilic carbene analogs and inserted into the X-X (P-X/P-P) bond of X2(PX3/P4) leading to the highly electrophilic and CH2Cl2-sol. PX4+ (P2X5+/P5X2+) salts. Reactions that aimed to synthesize P2I3+ from P2I4 and Ag[Al(OR)4] instead led to anion decompn. and the formation of P2I5(CS2)+[(RO)3Al-F-Al(OR)3]- (5). All salts were characterized by variable-temp. soln. NMR studies (3b also by 31P MAS NMR), Raman and/or IR spectroscopy as well as x-ray crystallog. (with the exception of 4a). The thermochem. vols. of the P-X cations are 121 (PBr4+), 161 (PI4+), 194 (P2Br5+), 271 (P2I5+), and 180 .ANG.3 (P5Br2+). The obsd. reactions were fully accounted for by thermochem. calcns. based on (RI-)MP2/TZVPP ab initio results and COSMO solvation enthalpy calcns. (CH2Cl2 soln.). The enthalpies of formation of the gaseous P-X cations were derived as +764 (PI4+), +617 (PBr4+), +749 (P2I5+), +579 (P2Br5+), +762 (P5I2+), and +705 kJ mol-1 (P5Br2+). The insertion of the intermediately prepd. carbene analog PX2+ cations into the resp. bonds were calcd., at the (RI-)MP2/TZVPP level, to be exergonic at 298 K in CH2Cl2 by DrG(CH2Cl2) = -133.5 (PI4+), -183.9 (PBr4+), -106.5 (P2I5+), -81.5 (P2Br5+), -113.2 (P5I2+), and -114.5 kJ mol-1 (P5Br2+). [on SciFinder (R)]

2002

Stabilizing Unusual Oxidation States of Lanthanides in Molecular Complexes: Synthesis, Properties, and Reactivity

Aurélien René Willauer

EPFL2022