Preparation of stable AsBr₄⁺and I₂As–PI₃⁺salts. Why didn't we succeed to prepare AsI₄⁺and As₂X₅⁺? A combined experimental and theoretical study

Abstract

In analogy to our successful "PX2+" insertion reactions, an "AsX2+" insertion route was explored to obtain new arsenic halogen cations. Two new salts were prepared: AsBr4+[Al(OR)4]-, starting from AsBr3, Br2 and Ag[Al(OR)4], and I2As-PI3+[Al(OR)]4 from AsI3, PI3 and Ag[Al(OR)4](R=C(CF3)3). The first cation is formally a product of an "AsBr2+" insertion into the Br2 molecule and the latter clearly a "PI2+" insertion into the As-I bond of the AsI3 molecule. Both compounds were characterized by IR and NMR spectroscopy, the first also by its X-ray structure. Reactions of Ag[Al(OR)4] with AsI3 do not lead to ionization and AgI formation but rather lead to a marginally stable Ag(AsI3)2+[Al(OR)]4 salt. Despite many attempts we failed to prepare other PX-cation analogues such as AsI4+, As2X5+ and P4AsX2+(X=Br, I). To explain these negative results the thermodynamics of the formation of EX2+, EX4+ and E2X5+(E = As, P; X = Br, I) was carefully analyzed with MP2/TZVPP calculations and inclusion of entropy and solvation effects. We show that As2Br5+ is in very rapid equilibrium with AsBr2+ and AsBr3(DeltaGo((CH2Cl2))=+30 kJ mol(-1)). The extremely reactive AsBr2+ cation available in the equilibrium accounts for the observed decomposition of the [Al(OR)4]- anion. By contrast, the stability of AsI3 against Ag[Al(OR)4] appears to be kinetic and, if prepared by a suitable route, As2I5+ would be expected to have a stability intermediate between the known P2I5+ and P2Br5+.

Tetraalkylammonium Salts of Weakly Coordinating Aluminates: Ionic Liquids, Materials for Electrochemical Applications and Useful Compounds for Anion Investigation

“…Moreover we have shown that even very reactive cations like simple carbenium ions (CI 3 + ) [21,22] or reactive cations like PX 4 + , [23,24] AsBr 4 + , [25] [27] or P 7 S 6 I 2 + [28] (X = halogen) are compatible (i.e., could be stabilized) as salts of these aluminates. Especially the P À X cations are incompatible with all other anion types.…”

A Spirocyclic PS Cage Cation: Synthesis and Formation of P7S6I2+

“…The solution was filtered through the frit-plate, highly concentrated and crystallized at À28 8C. The first crystals obtained were P 5 850 mmol) were placed into a two-bulbed vessel connected through a frit plate and closed with two Young valves (with a glass stem). CH 2 Cl 2 (10 mL) was condensed onto the mixture at À78 8C.…”

“…Since the fluorinated [Al(OR F ) 4 ] À alkoxyaluminates allowed the synthesis of stable salts of phosphorus-rich cations such as P 3 I 6 + and P 5 X 2 + , it seemed likely that these anions are also suitable for stabilizing P x E y + or P x E y X z + . Ag-[Al(OR F 4 ] (R F = C(CF 3 ) 3 ), [4] tested in many halide abstraction reactions, [5][6][7][8][9][10][11] is a likely starting material to achieve this. Thus by reaction of Ag + with PX 3 and P 4 S 3 , the first [6] ternary P 5 S 3 X 2 + (1 ab), P 5 S 2 X 2 + (2 ab), and P 4 S 4 X + (3 ab) ions were recently obtained (X = Br, I).…”

“…This is in agreement with earlier observations. [5,6,8,11] IR spectroscopy: We recorded IR spectra of crystalline 4. To assign the vibrations of the cations, frequency calcula- …”

Aluminum Alkoxide Fluorides Involved in the Sol–Gel Synthesis of Nanoscopic AlF3

“…In 2004, we presented the first example of an isolated mixed aluminum alkoxide fluoride, Al 3 F(OiPr) 8 ·dmso, [5] and since then, only one more example was published: {(F)Al-[OC(CF 3 ) 3 ] 2 (thf)} 2 with the four-membered-ring unit [(σ 5 Al) 2 (µF) 2 ]. [6] Both structures are stabilized by donor molecules (dmso or thf). However, in performing the Al-(OiPr) 3 /aHF reactions in these solvents no further crystalline compounds could be synthesized by varying the stoichiometric ratios of Al/F.…”