Highly reactive carbenes as ligands for main group element fluorides. Syntheses and applications

Silylimidazolium Hexafluorophosphate Salts as Synthetic Precursors to N-Heterocyclic Carbene Pentafluorophosphorus Adducts

N-Heterocyclic carbene pentafluorophosphorus (NHC-PF5) adducts are six-coordinate phosphorus(v) compounds with emerging applications but poor synthetic accessibility. We have developed a simple and high yielding protocol for synthesizing imidazolylidene NHC-PF5 adducts from silylimidazolium hexafluorophosphate salts. Using this methodology, we have prepared a series of NHC-PF5 adducts in high yields, including new NHC-PF5 building blocks amenable to subsequent synthetic diversification. We also demonstrate that a similar approach enables access to analogous, synthetically elusive NHC-BF3 and NHC-SbF5 adducts.

Concatenating Structural Constraint Effects at Tin for the Sequential Generation, Stabilization, and Transfer of Acyclic Aminocarbenes

Structural constraint approaches have been employed toward different ends in recent years, from augmenting the nucleophilicity in pyramidalized low-valent p-block compounds to enhancing the Lewis acidities at planarized tetravalent p-block elements. While previous studies exploited these effects separately, this work introduces a strategy to concatenate structural constraint approaches at individual stages of a reaction sequence in a row to unlock a synthetic path unattainable by conventional methodologies. The boosted nucleophilicity resulting from the constrained tetracoordinated calix[4]pyrrolato stannate(II) dianion enables the reductive formation of sterically unprotected acyclic aminocarbenes. These amino carbenes are stabilized at the concomitantly formed square-planar stannane(IV) as air-stable adducts. Transfer of the carbenes onto copper(I) by cooperativity of the calix[4]pyrrole ligand finalizes this protocol to hitherto unreported yet prototypical carbene complexes. Detailed spectroscopic and quantum theoretical analyses establish the synergy of structural constraints and element-ligand cooperation as the linchpin to this reaction path and its selectivity.

Tertiary Phosphine and Arsine Complexes of Phosphorus Pentafluoride: Synthesis, Properties, and Electronic Structures

The reaction of PMe₃ or PPh₃ with PF₅ in anhydrous CH₂Cl₂ or hexane forms the white, moisture-sensitive complexes [PF₅(PR₃)] (R = Me, Ph). Similar reactions involving the diphosphines o-C₆H₄(PR₂)₂ afford the complexes [PF₄{o-C₆H₄(PR₂)₂}][PF₆]. The X-ray structures of [PF₅(PR₃)] and [PF₄{o-C₆H₄(PMe₂)₂}][PF₆] show pseudo-octahedral fluorophosphorus centers. Multinuclear NMR spectra (¹H, ¹⁹F{¹H}, ³¹P{¹H}) show that in solution in CH₂Cl₂/CD₂Cl₂ the structures determined crystallographically are the only species present for [PF₅(PMe₃)] and [PF₄{o-C₆H₄(PMe₂)₂}][PF₆] but that [PF₅(PPh₃)] and [PF₄{o-C₆H₄(PPh₂)₂}][PF₆] exhibit reversible dissociation of the phosphine at ambient temperatures, although exchange slows at low temperatures. The complex ¹⁹F{¹H} and ³¹P{¹H} NMR spectra have been analyzed, including those of the cation [PF₄{o-C₆H₄(PMe₂)₂}]⁺, which is a second-order AA'XX'B₂M spin system. The unstable [PF₅(AsMe₃)], which decomposes in a few hours at ambient temperatures, has also been isolated and spectroscopically characterized; neither AsPh₃ nor SbEt₃ forms similar complexes. The electronic structures of the PF₅ complexes have been explored by DFT calculations. The DFT optimized geometries for [PF₅(PMe₃)], [PF₅(PPh₃)], and [PF₄{o-C₆H₄(PMe₂)₂}]⁺ are in good agreement with their respective crystal structure geometries. DFT calculations on the PF₅-L complexes reveal the P-L bond strength falls with L in the order PMe₃ > PPh₃ > AsMe₃, consistent with the experimentally observed stabilities, and in the PF₅-L complexes, electron transfer from L to PF₅ on forming these complexes also follows the order PMe₃ > PPh₃ ≈ AsMe₃.

Thermal stability of dialkylimidazolium tetrafluoroborate and hexafluorophosphate ionic liquids: ex situ bulk heating to complement in situ mass spectrometry

Thermal decomposition (TD) products of the ionic liquids (ILs) [CnC1Im][BF4] and [CnC1Im][PF6] ([CnC1Im]+ = 1-alkyl-3-methylimidazolium, [BF4]- = tetrafluoroborate, and [PF6]- = hexafluorophosphate) were prepared, ex situ, by bulk heating experiments in a bespoke setup. The respective products, CnC1(C3N2H2)BF3 and CnC1(C3N2H2)PF5 (1-alkyl-3-methylimidazolium-2-trifluoroborate and 1-alkyl-3-methylimidazolium-2-pentafluorophosphate), were then vaporized and analyzed by direct insertion mass spectrometry (DIMS) in order to identify their characteristic MS signals. During IL DIMS experiments we were subsequently able, in situ, to identify and monitor signals due to both IL vaporization and IL thermal decomposition. These decomposition products have not been observed in situ during previous analytical vaporization studies of similar ILs. The ex situ preparation of TD products is therefore perfectly complimentary to in situ thermal stability measurements. Experimental parameters such as sample surface area to volume ratios are consequently very important for ILs that show competitive vaporization and thermal decomposition. We have explained these experimental factors in terms of Langmuir evaporation and Knudsen effusion-like conditions, allowing us to draw together observations from previous studies to make sense of the literature on IL thermal stability. Hence, the design of experimental setups are crucial and previously overlooked experimental factors.

Oxidation of a germanium(ii) dication to access cationic germanium(iv) fluorides

Herein we describe the synthesis and characterization of the tris(1-ethyl-benzoimidazol-2-ylmethyl)amine (BIMEt₃ = L) complex of Ge(ii)²⁺ and its oxidation with XeF₂ to access the cationic germanium fluorides [LGeF₂][OTf]₂ and [LGeF(OTf)][OTf]₂.