From Ion‐Like Ethylzinc Aluminates to [EtZn(arene)₂]⁺[Al(OR^F)₄]⁻ Salts

Abstract

Attempts to prepare previously unknown simple and very Lewis acidic [RZn] [Al(OR ) ] salts from ZnR , AlR , and HO-R delivered the ion-like RZn(Al(OR ) ) (R=Me, Et; R =C(CF ) ) with a coordinated counterion, but never the ionic compound. Increasing the steric bulk in RZn to R=CH CMe , CH SiMe , or Cp*, thus attempting to induce ionization, failed and led only to reaction mixtures including anion decomposition. However, ionization of the ion-like EtZn(Al(OR ) ) compound with arenes yielded the [EtZn(arene) ] [Al(OR ) ] salts with arene=toluene, mesitylene, or o-difluorobenzene (o-DFB)/toluene. In contrast to the ion-like EtZn(η -C H )(CHB Cl ), which co-crystallizes with one benzene molecule, the less coordinating nature of the [Al(OR ) ] anion allowed the ionization and preparation of the purely organometallic [EtZn(arene) ] cation. These stable materials have further applications as, for example, initiators of isobutene polymerization. DFT calculations to compare the Lewis acidities of the zinc cations to those of a large number of organometallic cations were performed on the basis of fluoride ion affinity. The complexation energetics of EtZn with arenes and THF was assessed and related to the experiments.

Accessing Two‐Coordinate ZnII Organocations by NHC Coordination: Synthesis, Structure, and Use as π‐Lewis Acids in Alkene, Alkyne, and CO2 Hydrosilylation

“…[1.938(3) vs. 1.961(3) Å in 2 + ], indicative of a more electrophilic Zn(II) center. DOSY 1 H and19 F NMR data for[6][B(C6F5)4] agree with separated ions in solution (CD2Cl2, RT; FiguresS17-20). From PhBr, cation 6 + crystallizes as adduct (IDipp)Zn-C6F5 +.…”

“…Discrete [EtZn(arene)2] + Zn(II) cations, with arene π-coordination to the Zn(II) center, were also recently characterized. [6] Structurally characterized and isolable two-coordinate Zn(II) organocations of the type [(L)Zn-R] + (L monodentate) are unknown thus far, though (DMF)Zn-alkyl + cations have been observed in the gas phase. [7] In principle, in addition to their fundamental interest, such electrophilic entities may represent a better reactivity/stability balance when compared to, on the one hand, more robust but less Lewis acidic three-/four-coordinate cationic analogues and, on the other hand, elusive monocoordinate [RZn] + cations.…”

“…Cations 1 + and 2 + may be further derivatized for enhanced Lewis acidity at Zn(II). Thus, 1 + cleanly reacts with 1 equiv B(C6F5)3 (RT, PhF) via a Me/C6F5 exchange reaction to quantitatively afford two-coordinate cation (IDipp)Zn-C6F5 + (6 + ), isolated as [6][B(C6F5)4] (Scheme 1). [12] From CH2Cl2, it crystallizes as discrete (IDipp)Zn-C6F5 + and B(C6F5)4ions as determined from XRD studies (Figure 2a).…”

Robust Two-Coordinate Zn(II) Organocations Supported by Bulky-Yet-Flexible IPr* Carbene: Synthesis, Structure, and Distinct Reactivity in Hydrosilylation Catalysis

“…In addition to steric protection, a ligand such as IPr* may also provide additional electronic stabilization to electrophilic Zn II cations through arene π-stabilization (with the N–CH Ph 2 groups of the NHC ligand). As recently observed, low-coordinate organozinc cations may form labile M-arene complexes. − …”

Silver Salts of the Weakly Coordinating Anion [Me3NB12Cl11]–

“…In agreement with Pearsons acid‐base concept softer chlorine atoms form stronger interactions to silver than harder fluorine atoms. Similarly, using fluorinated aluminate [Al(OR F ) 4 ] – anions [EtZn(arene) 2 ] + complexes could be isolated, while [EtZn] + is coordinated to the chlorinated carborate . The differences in the basicities of halogenated borates and carborates has been discussed before in condensed and gas phase , …”