Main Group Lewis Acids in Frustrated Lewis Pair Chemistry: Beyond Electrophilic Boranes

Abstract

The advent of frustrated Lewis pair (FLP) chemistry almost a decade ago was based on the reactivity of sterically encumbered combinations of electrophilic boranes and phosphines with hydrogen. Since that time the chemistry has broadened dramatically in terms of reactivity and applications. Nonetheless, the majority of the work has continued to exploit the borane B(C6F5)3. In this review, we describe FLP chemistry that has developed by employing alternatives to this Lewis acid. Lewis acids derived from group 13, 14, and 15 based systems are described, thus demonstrating a growing area in the main group reactivity based on FLPs.

Mono- and Polynuclear Boron Dications

“…11 B NMR of [10] 2+ detected at ¹25.4 ppm is markedly upfield shifted compared to that observed for B resonance at 80 and 2530 ppm, respectively. In fact, the observed 11 B chemical shift is also more upfield shifted in comparison with the pyridine stabilized tetra-coordinated boron dications, re-emphasizing the strong σ-and π-donating ability of the carbodicarbene ligand.…”

“…(Figure 3). Structural characterization of [10][BH 4 ] 2 confirms the dicationic nature of [10] 2+ and the trigonal planar geometry at the boron atom with a CBC bond angle of 127.5°. The BC Ligand bond distances of 1.534 and 1.523 ¡ are noticeably shorter than that of NHC-stabilized borenium cations (1.58 ¡), indicating the presence of partial π-bonding character of the boroncarbodicarbene interaction.…”

“…NBO analysis reveals the multiple bonding characters of the BC Ligand bonds with significant polarization towards the carbon atom of carbodicarbene in both the σ-and π-bonding orbitals. The participation of two pairs of electrons in each of the BC Ligand bonds explains the observed upfield-shifted 11 B NMR of [10] 2+ . Natural population analysis (NPA) shows that the positive charges are delocalized through resonance conjugation to the four benzimidazolyl fragments and only a small amount of positive charges are left at the central [BH] 2+ unit.…”

“…The BC Ligand bond distances of 1.534 and 1.523 ¡ are noticeably shorter than that of NHC-stabilized borenium cations (1.58 ¡), indicating the presence of partial π-bonding character of the boroncarbodicarbene interaction. π-donation from ligand 9 also results in an abnormally shielded 11 B nuclei in [10] 2+ .…”

“…The molecular electronics of [10] 2+ has also been investigated computationally. NBO analysis reveals the multiple bonding characters of the BC Ligand bonds with significant polarization towards the carbon atom of carbodicarbene in both the σ-and π-bonding orbitals.…”