Dual Reactivity of a Geometrically Constrained Phosphenium Cation

Abstract

A geometrically constrained phosphenium cation in bis(pyrrolyl)pyridine based NNN pincer type ligand (1⁺ ) was synthesized, isolated and its preliminary reactivity was studied with small molecules. 1⁺ reacts with MeOH and Et₂ NH, activating the O-H and N-H bonds via a P-center/ligand assisted path. The reaction of 1⁺ with one equiv. of H₃ NBH₃ leads to its dehydrogenation producing 5. Interestingly, reaction of 1⁺ with an excess H₃ NBH₃ leads to phosphinidene (P^I ) species coordinating to two BH₃ molecules (6). In contrast, [1⁺ ][OTf] reacts with Et₃ SiH by hydride abstraction yielding 1-H and Et₃ SiOTf, while [1⁺ ][B(C₆ F₅ )₄ ] reacts with Et₃ SiH via an oxidative addition type reaction of Si-H bond to P-center, affording a new P^V compound (8). However, 8 is not stable over time and degrades to a complex mixture of compounds in matter of minutes. Despite this, the ability of [1⁺ ][B(C₆ F₅ )₄ ] to activate Si-H bond could still be tested in catalytic hydrosilylation of benzaldehyde, where 1⁺ closely mimics transition metal behaviour.

Keywords: Phosphenium Cation; Phosphinidene; Phosphorus; Silane; Small Molecule Activation.

Scheme 1

a) Synthesis of 1‐Cl; b) RSs of 1′‐Cl obtained from NRT analysis.

Figure 1

a) POV‐ray depiction of 1‐Cl; b) POV‐ray depiction of 1⁺ . Thermal ellipsoids at 30 % probability, hydrogen atoms and TfO⁻ were omitted for clarity.

Scheme 2

Synthesis of [1 ⁺][OTf].

Scheme 3

Activation of MeO−H and Et₂N−H bonds by [1⁺ ][OTf] producing 3 _anti , 3 _syn and 4 _anti , 4 _syn , respectively.

Figure 2

a) POV‐ray depiction of 3 _anti ; b) POV‐ray depiction of 4 _syn . Thermal ellipsoids at the 30 % probability level, non‐relevant hydrogen atoms and TfO⁻ were omitted for clarity.

Figure 3

a) DFT calculated potential energy surface (PES) for the activation of MeO−H bond by 1⁺ . Free Gibbs energies (enthalpies) are given relative to the starting materials; b) DFT optimized structure of 1⁺ ‐O(H)Me and bond lengths around P atom.

Scheme 4

Reaction of [1⁺ ][OTf] with one equiv. of H₃BNH₃ producing 5 _anti and 5 _syn and with an excess of H₃BNH₃ producing phosphinidene 6 via stable intermediate 7.

Figure 4

a) POV‐ray depiction of 5 _syn ; b) POV‐ray depiction of 6. Thermal ellipsoids at the 30 % probability level, non‐relevant hydrogen atoms and TfO⁻ were omitted for clarity.

Scheme 5

Reaction of [1⁺ ][OTf] with Et₃SiH producing 1‐H and Et₃SiOTf reaction, and independent synthesis of 1‐H by reduction of 1‐Cl with DIBAL−H.

Figure 5

POV‐ray depiction of 1‐H. Thermal ellipsoids at the 30 % probability level, non‐relevant hydrogens were omitted for clarity.

Scheme 6

Oxidative addition type reaction of Et₃Si−H bond to an ambiphilic P‐center in [1 ⁺][B(C₆F₅)₄] giving intermediate 8, and independently formed 8 by reaction of 1‐H with [Et₃Si][B(C₆F₅)₄].

Figure 6

a) DFT calculated PES for the activation of Et₃SiH by 1⁺ . Free Gibbs energies (enthalpies) are given relative to the starting materials; b) DFT optimized structure of 1‐HSi and bond lengths around P atom.

Scheme 7

Proposed catalytic cycle of hydrosilylation of benzaldehyde by [1⁺ ][B(C₆F₅)₄].