Superseding β‐Diketiminato Ligands: An Amido Imidazoline‐2‐Imine Ligand Stabilizes the Exhaustive Series of B=X Boranes (X=O, S, Se, Te)

Abstract

Boron reluctantly forms B=X (X=O, S, Se, Te) moieties, which has stimulated the quest for such species in the past few years. Based on the N,N′‐chelating β‐diketiminato ligand (HNacNac), a new amido imidazoline‐2‐imine ligand system (HAmIm) is presented, giving rise to the isolation of an exhaustive series of Lewis acid free, monomeric chalcogen B=X boranes with documented π‐bond character between boron and the chalcogen. The chalcogenoboranes are isoelectronic and isolobal to the respective ketones. The chemical behavior of the oxoborane (B=O) strongly resembles the classical carbonyl reactivity in C=O bonds. The improved stability provided by HAmIm arises from the formation of more‐stable five‐membered boron chelates versus the six‐membered NacNac analogues and from the imidazoline‐2‐imine moiety providing enhanced σ‐ and π‐donation. The HAmIm ligand class may supersede the widely employed NacNac system in certain applications.

Structural and Electronic Properties of Boranes Containing Boron‐Chalcogen Multiple Bonds and Stabilized by Amido Imidazoline‐2‐imine Ligands

“…In this regard, Frank et al. reported in 2021 a series of B=X compounds supported by an amido imidazoline‐2‐imine ligand system [28] . These species were characterized by structural information obtained from crystallographic X‐ray analysis and DFT‐computed Wiberg bond indices (WBI).…”

Synthesis, Structure, and Reactivity of Acid‐Free Neutral Oxoborane

“…[4][5][6][7] Very recently, acid free anionic oxoborane of the type [R 2 B = O] À stabilized by dianionic bulky N-heterocyclic ligands (VI, VII) were disclosed. [8,9] In addition, transition metal fragments were also used to stabilize the BO multiple-bonded compounds by Braunschweig and Yamashita. [10] It has been documented that there are two main strategies to stabilize reactive main group species: kinetic stabilization by sterically highly bulky ligands, and thermodynamic stabilization by Lewis acids/bases.…”

“…The exo boron atom is bonded to one oxygen, one cage carbon and one carbene-carbon in a trigonal planar geometry (Figure 1). It is noted that the B-O distance of 1.256(3) in 3 is the shortest one known so far among all reported acid stabilized oxoboranes (1.287(4)-1.329( 6) ), [3][4][5][6][7] and acid-free anionic oxoboranes (1.273(8) and 1.287-( 2) ), [8,9] suggesting a very strong interaction between the boron and oxygen atoms. The electronic structure and bonding properties of 3 were further confirmed by density functional theory (DFT) calculations at the B3LYP level of theory (see the SI for detail).…”

Synthesis, Structure, and Reactivity of Acid‐Free Neutral Oxoborane

“…The B=O boron in 3 was observed at δ =24.3 ppm as a broad singlet in its 11 B NMR spectrum, which falls in the range 18.7–32.3 ppm normally found in oxoboranes [3–5, 6c, 7–9] . This measured value is largely downfield shifted in comparison with that of −9.0 ppm in 4 and −13.5 ppm in 2 .…”

“…Subsequently, several Lewis/Brønsted acid stabilized oxoboranes ( II – V ) were reported by different groups (Scheme 1 a). [4–7] Very recently, acid free anionic oxoborane of the type [R 2 B=O] − stabilized by dianionic bulky N ‐heterocyclic ligands ( VI , VII ) were disclosed [8, 9] . In addition, transition metal fragments were also used to stabilize the BO multiple‐bonded compounds by Braunschweig and Yamashita [10]…”