Adaptable ligand donor strength: tracking transannular bond interactions in tris(2-pyridylmethyl)-azaphosphatrane (TPAP)

The Backbone of Success of P,N-Hybrid Ligands: Some Recent Developments

Organophosphorus ligands are an invaluable family of compounds that continue to underpin important roles in disciplines such as coordination chemistry and catalysis. Their success can routinely be traced back to facile tuneability thus enabling a high degree of control over, for example, electronic and steric properties. Diphosphines, phosphorus compounds bearing two separated P^III donor atoms, are also highly valued and impart their own unique features, for example excellent chelating properties upon metal complexation. In many classical ligands of this type, the backbone connectivity has been based on all carbon spacers only but there is growing interest in embedding other donor atoms such as additional nitrogen (–NH–, –NR–) sites. This review will collate some important examples of ligands in this field, illustrate their role as ligands in coordination chemistry and highlight some of their reactivities and applications. It will be shown that incorporation of a nitrogen-based group can impart unusual reactivities and important catalytic applications.

Synthesis, Characterizations and Applications of Fluoroazaphosphatranes

Haloazaphosphatranes are the halogenated parents of proazaphosphatranes, also known as Verkade's superbase. While the synthesis of iodo-, bromo- and chloroazaphosphatranes was reported more than thirty years ago by J. G. Verkade, the first synthesis of fluoroazaphosphatranes was only described in 2018 by Stephan et al. Currently, no common and versatile procedure exists to access fluoroazaphosphatranes platform with different structural characteristics. In this report, a new and simple synthesis of this class of compounds was developed based on the nucleophilic attack of the fluoride anion on chloroazaphosphatrane derivatives with good to high isolated yields for the corresponding fluoroazaphosphatranes (70-92%). The scope of the reaction was widened to fluoroazaphosphatranes bearing various substituents and X-ray molecular structures of two of them are reported. The stability of fluoroazaphosphatranes toward nucleophilic solvents like water has been investigated. As they revealed much more robust cations than their chloroazaphosphatrane parents, their chloride salts were tested as organocatalysts for the formation of cyclic carbonates from epoxides and CO₂ . Fluoroazaphosphatranes proved to be both efficient and stable catalytic systems for CO₂ conversion with catalytic activities similar to those of azaphosphatranes, and no decomposition of the cation was observed at the end of reaction.

Electric Fields Influence Intramolecular Vibrational Energy Relaxation and Line Widths

Intramolecular vibrational energy relaxation (IVR) is fundamentally important to chemical dynamics. We show that externally applied electric fields affect IVR and vibrational line widths by changing the anharmonic couplings and frequency detunings between modes. We demonstrate this effect in benzonitrile for which prior experimental results show a decrease in vibrational line width as a function of applied electric field. We identify three major channels for IVR that depend on electric field. In the dominant channel, the electric field affects the frequency detuning, while in the other two channels, variation of anharmonic couplings as a function of field is the underlying mechanism. Consistent with experimental results, we show that the combination of all channels gives rise to reduced line widths with increasing electric field in benzonitrile. Our results are relevant for controlling IVR with external or internal fields and for gaining a more complete interpretation of line widths of vibrational Stark probes.

The Chloroazaphosphatrane Motif for Halogen Bonding in Solution

Chloroazaphosphatranes, the corresponding halogenophosphonium cations of the Verkade superbases, were evaluated as a new motif for halogen bonding (XB). Their modulable synthesis allowed for synthetizing chloroazaphosphatranes with various substituents on the nitrogen atoms. The binding constants determined from NMR titration experiments for Cl^-, Br^-, I^-, AcO^-, and CN^- anions are comparable to those obtained with conventional iodine-based monodentate XB receptors. Remarkably, the protonated azaphosphatrane counterparts display no affinity for anions under the same conditions. The strength of the XB interaction is, to some extent, related to the basicity of the corresponding Verkade superbase. The halogen bonding abilities of this new class of halogen donor motif were also revealed by the Δδ(³¹P) NMR shift observed in CD₂Cl₂ solution in the presence of triethylphosphine oxide (TEPO). Thus, chloroazaphosphatranes constitute a new class of halogen bond donors, expanding the repertory of XB motifs mainly based on C_Ar-I bonds.

Rhodium(i) complexes derived from tris(isopropyl)-azaphosphatrane—controlling the metal–ligand interplay

Proazaphosphatranes are intriguing ligand architectures comprising a bicyclic cage of flexible nature. They can undergo structural deformations due to transannulation while displaying modular electronic and steric properties. Herein, we report the synthesis and coordination chemistry of rhodium(i) complexes bearing a tris(isopropyl)-azaphosphatrane (TⁱPrAP) ligand. The molecular structure of the primary complex (1) revealed the insertion of the metal center into a P–N bond of the ligand. The addition of a Lewis acid, i.e., lithium chloride, promoted the dynamic behavior of the complex in the solution, which was studied by state-of-the-art NMR spectroscopy. Substituting the cyclooctadiene ligand at the metal center with triphenylphosphine or 2-pyridyldiphenylphosphine unveiled the adaptive nature of the TⁱPrAP backbone capable of switching its axial nitrogen from interacting with the phosphorus atom to coordinate the rhodium center. This led the entire ligand edifice to change its binding to rhodium from a bidentate to tridentate coordination. Altogether, our study shows that introducing a TⁱPrAP ligand allows for unique molecular control of the immediate environment of the metal center, opening perspectives in controlled bond activation and catalysis.

The synthesis and coordination chemistry of Rh(i) complexes bearing a tris(isopropyl)-azaphosphatrane (TⁱPrAP) ligand are reported. The adaptive nature of TⁱPrAP ligands allows for molecular control of the immediate environment of the metal center.

Encapsulation of Azaphosphatranes and Proazaphosphatranes in Confined Spaces

Proazaphosphatranes (also named Verkade's superbases) and their azaphosphatrane conjugated acids have been recently been shown to be confined in either covalent or self-assembled molecular cages, or immobilized in nanopores of hybrid materials. The encapsulation of these phosphorus moieties turns out to strongly affect both their acid-base, catalytic, and recognition properties. The thermodynamics and kinetics of the proton transfer as well as the selectivity and catalytic activities of Verkade's superbases were strongly changed upon their confinement in a hemicryptophane cavity. Moreover, self-assembled cages, including azaphosphatrane moieties, were found to display remarkable anion recognition properties in water. In this Minireview, these new aspects of the chemistry of aza- and proaza-phosphatranes are presented, in order to highlight the great potential of such an approach.

On Transannulation in Azaphosphatranes: Synthesis and Theoretical Analysis

A combined synthetic-theoretical study has been undertaken to determine the factors that influence transannulation in azaphosphatranes. The commonly used proazaphosphatrane P(i-BuNCH₂CH₂)₃N and several of its oxidized congeners are used as model systems. The haloazaphosphatranes of P(i-BuNCH₂CH₂)₃N were synthesized, including a rare fluoroazaphopshatrane, and used as references for computational investigations. Comparisons of the experimental and theoretical observations highlight the flexibility observed in transannulated atranes and the potential for multiple local energetic minima depending on the identity of the equatorial substituents for a given azaphosphatrane. Theoretical calculations also identify the role of the ethylene linker in azaphosphatrane bonding, the influence of transannulation on P-electrophile interactions, and the contribution of electrostatic interactions to transannulation.

Crystal structure of NiFe(CO)5[tris(pyridyl-meth-yl)aza-phosphatrane]: a synthetic mimic of the NiFe hydrogenase active site incorporating a pendant pyridine base

The synthesis and the crystal structure of a NiFe complex containing an aza­phosphatrane ligand is discussed. The NiFe(TPAP)(CO)₅ complex contains a pendant pyridine base in close proximity to the NiFe metal center.

The reaction of Ni(TPAP)(COD) {where TPAP = [(NC₅H₄)CH₂]₃P(NC₂H₄)₃N} with Fe(CO)₅ resulted in the isolation of the title heterobimetallic NiFe(TPAP)(CO)₅ complex di-μ-carbonyl-tricarbon­yl[2,8,9-tris­(pyridin-2-yl­meth­yl)-2,5,8,9-tetra­aza-1-phosphabi­cyclo­[3.3.3]undeca­ne]ironnickel, [FeNi(C₂₄H₃₀N₇P)(CO)₅]. Characterization of the complex by ¹H and ³¹P NMR as well as IR spectroscopy are presented. The structure of NiFe(TPAP)(CO)₅ reveals three terminally bound CO mol­ecules on Fe⁰, two bridging CO mol­ecules between Ni⁰ and Fe⁰, and TPAP coordinated to Ni⁰. The Ni—Fe bond length is 2.4828 (4) Å, similar to that of the reduced form of the active site of NiFe hydrogenase (∼2.5 Å). Additionally, a proximal pendant base from one of the non-coordinating pyridine groups of TPAP is also present. Although involvement of a pendant base has been cited in the mechanism of NiFe hydrogenase, this moiety has yet to be incorporated in a structurally characterized synthetic mimic with key structural motifs (terminally bound CO or CN ligands on Fe). Thus, the title complex NiFe(TPAP)(CO)₅ is an unique synthetic model for NiFe hydrogenase. In the crystal, the complex mol­ecules are linked by C—H⋯O hydrogen bonds, forming undulating layers parallel to (100). Within the layers, there are offset π–π [inter­centroid distance = 3.2739 (5) Å] and C—H⋯π inter­actions present. The layers are linked by further C—H⋯π inter­actions, forming a supra­molecular framework.