Synthesis and Characterization of an Almost Linear, Quasi-Two-Coordinate, Cationic Diorganoaluminum Compound

Abstract

The preparation of low-coordinate cationic organoaluminum compounds as reactive alternatives to transitionmetal catalysts for the polymerization of olefins or cyclic ethers has been reported by several groups in recent years. [1][2][3][4] Stable weakly coordinating anions such as [B(C 6 F 5 ) 4 ]were essential for the synthesis of these compounds. The stabilization of the more reactive alkylsubstituted cationic species required the employment of the more robust carborane ligands, as has been shown by Reed and co-workers for [Et 2 Al] + [CB 11 H 6 X 6 ] -(X ) Cl, Br). 5 The Lewis acidity of the cationic aluminum center is sufficiently high, even in these compounds, that there were relatively close Al‚‚‚X contacts leading to an essentially four-coordinate aluminum center and C-Al-C angles of 136.6 and 130.0°. These close contacts significantly decreased the reactivity of these species. As an alternative strategy to prepare and stabilize twocoordinate cationic aluminum compounds and at the same time minimize cation‚‚‚anion contacts, we have

Synthesis of 2,6‐Diarylphenyldimethylsilyl Cations: Polar‐π Distribution of Cation Character

“…[5] The premise was that the 2,6-diarylphenyl scaffold would offer steric protection of the Si center, as well as a thermodynamic stabilization of the entire cation by p-p interactions. [6,7] The flanking rings, which have restricted rotation about the biaryl bonds in the 2-and 6-positions, should prevent anion and solvent molecules from interacting with the positively charged cavity. Donation of p(aryl) electron density into the empty 3p(Si) orbital was expected to lead to a reduced amount of positive charge on the silicon center and a decreased silyl Lewis acidity.…”

m‐Terphenylaluminum and ‐gallium Compounds: Synthesis and Conversion into Low‐Coordinate Organogallium Cations

“…+ (M = Ga, [6] Al; [7] Mes = 2,4,6-Me 3 C 6 H 2 ), monosubstituted group 14 cations [Cp*Si] + [8] and [(2,6-Trip 2 C 6 H 3 )Pb] + , [9] or the interesting phosphorus-based cations [P 2 X 5 ] + and [P 5 X 2 ] + . [10] On the more applied side, there have been several studies of the potential of low-coordinate cationic organoaluminum compounds as catalysts for olefin oligomerization and polymerization.…”

“…The compound [14] + [CHB 11 Cl 11 ] -is stable at 70°C for at least 23 h. Addition of 1-octene to a solution of [14] + [CHB 11 Cl 11 ] -in C 6 cationic aluminum center. [7] In an effort to increase the reactivity of the cationic species, we have begun to investigate methods to achieve two-coordination that do not rely on excessive steric protection, which tends to decrease and change reactivity patterns. Based on the observed close secondary Al···C contacts in [(2,6-Mes 2 C 6 H 3 ) 2 Al] + involving the ipso-and ortho-carbon atoms of the flanking mesityl groups, [7] we speculate that a single m-terphenyl substituent in conjunction with the least-coordinating anions should be sufficient for the stabilization of a low-coordinate cationic center.…”

“…[7] In an effort to increase the reactivity of the cationic species, we have begun to investigate methods to achieve two-coordination that do not rely on excessive steric protection, which tends to decrease and change reactivity patterns. Based on the observed close secondary Al···C contacts in [(2,6-Mes 2 C 6 H 3 ) 2 Al] + involving the ipso-and ortho-carbon atoms of the flanking mesityl groups, [7] we speculate that a single m-terphenyl substituent in conjunction with the least-coordinating anions should be sufficient for the stabilization of a low-coordinate cationic center. The bulky substituents are expected to prevent close anion···cation contacts such as those observed in [Et 2 Al]-[CB 11 H 6 X 6 ] (X = Cl, Br) [13] and to stabilize the electronpoor center through secondary M···C contacts.…”

“…Its crystal structure (Figure 2) is isostructural with that of the gallium congener, [24] but features a significantly shorter Al-Cl bond with a value of 2.1098 (12) [26] [Equation (2)] at 70°C for 30 min afforded 7 in moderate to good yields in the form of well-shaped colorless crystals. [7] (2)…”