Unravelling carbene moiety role in the N-heterocyclic carbene-phosphinidenes coordination chemistry: bonding and reactivity in gold chemistry

The study systematically explored the bonding features and reactivity of 13 N-heterocyclic carbene-phosphinidene (NHCP) gold hydride complexes, [NHCPAuH], bearing different classes of NHCs (CAACs, unconjugated DACs, and conjugated DACs), using an unbiased computational protocol.

Key findings include:

The analyses reveal that, although the π acceptor ability trend of NHCPs qualitatively parallels that of NHCs, the σ donor ability trend reverses, with more σ-donating NHC moieties generating less σ-donating NHCP ligands.

Concurrently, the nature of the NHC moiety impacts the coordination geometry at the P center, which ranges from nearly “planar” (CAACs), to pyramidal (unconjugated DACs) and PH3-like (conjugated DACs), with the CNHC-P-Au bond angle appearing as a structural descriptor to monitor and/or design NHCP ligands with desired σ-donor properties.

The NHCPs stronger σ-donor and weaker π-acceptor abilities compared to NHCs and their unique structural flexibility and electronic adaptability have been showcased to be directly controlled by the carbene moiety via modulation of the HOMO lone pair energy and its atomic phosphorous 3p character.

The reactivity of [NHCPAuH] complexes with CO2 demonstrates qualitatively similar mechanisms, with remarkably different activation barriers, which directly correlate with the NHCPs σ donor ability.

The investigation utilized Energy Decomposition Analysis (EDA), Natural Orbitals for Chemical Valence (NOCV), and Charge Displacement (CD) analysis to quantitatively assess the NHCP-AuH bond nature. The results showed that:

NHCP ligands are stronger σ-donor than NHCs, with a σ-donor trend opposite to that of NHCs, i.e. σ–donation increases from CAACs to conjugated DACs in the NHCP series.

NHCPs are much less π-acceptor and tunable compared to NHCs.

The coordination geometry at the P center is accordingly very flexible, shifting from “nearly planar” (in CAACs-based NHCP) to PH3-like (in conjugated DACs-based NHCP), driven by the lone pair P 3p percentage and energy under NHC moiety control.

The study concludes that the rate of CO2 insertion into the Au-H bond can be significantly changed by modifying the NHC/NHCP ligand, demonstrating a one-to-one mapping between bonding and reactivity: increasing electron density on the metal fragment by incorporating a stronger σ-donor NHC/NHCP results in faster CO2 insertion.

"In summary, from this analysis NHCPs emerge as a class of ligands possessing highly distinct coordination properties: a strong σ-donor ability and a unique structural flexibility and electronic adaptability, directly controlled by the carbene moiety, through modulation of the HOMO lone pair energy and P 3p contribution."

"In particular, introducing strong σ-donor CAACs moieties, NHCPs with a reduced σ-donor ability and a nearly “planar” coordination around P are generated, while inserting weaker σ-donor conjugated DACs moieties, NHCPs with an increased σ-donor ability and a PH3-like coordination around P are formed."

This work is devoted to understanding the NHCP highly tunable steric and electronic peculiar features as coordination ligands, as well as exploring their potential applications in small molecule activation and catalysis.

Computational details involved using the Amsterdam Density Functional (ADF) code combined with QUILD program for geometry optimizations and frequency calculations, employing functionals like BP86, basis sets like TZ2P, ZORA Hamiltonian, and Grimme’s D3-BJ dispersion correction. EDA was used to decompose interaction energy into Pauli repulsion, electrostatic interaction, dispersion contribution, and orbital interaction. CD-NOCV analysis was used to quantify charge transfer (CT) associated with different components of the NHCP-AuH bond.

The activation energy barriers ΔG‡ span a range from 29.1 to 21.2 kcal/mol and the ΔGs for the product formation show an endergonic reaction, with values from 6.4 to 1.0 kcal/mol.

Remarkably, all reactions in Figure 5 are endergonic, with similar ΔG values.

The NHCP complexes under study showed remarkable quantitative differences when reacting with CO2, with kinetic activation barriers nicely correlating with the tunable NHCP σ donor ability.

The computational protocol used includes Energy Decomposition Analysis (EDA) to decompose the interaction energy into contributions associated with Pauli repulsion, electrostatic interaction, dispersion contribution, and orbital interaction. The CD-NOCV scheme is used to partition the deformation density rearrangement into NOCV components to quantify charge transfer (CT). The analysis reveals that the NHCP-AuH bond is mainly described by σ-donation (with energies in a narrow range from-34.4 to-37.7 kcal/mol), with much smaller and comparable π backdonation contributions.

"The main driving force is the charge transfer from the Au-H σ bonding HOMO, also including NHC/NHCP-AuH contributions, towards the empty π

LUMO of CO2, thus suggesting a role of the ligand in the reactivity of these complexes. (This is inferred from Figure 5 analysis.)

The authors used a series of carbene moieties to investigate the impact on bonding and reactivity with AuH fragments and CO2 probes. The study provides insights into the design principle for NHCP ligands, highlighting their potential for applications in small molecule activation and catalysis. The computational setup was consistent with previous works used to study ligand effects on monomeric gold hydrides for CO2 reduction.

"In summary, this investigation suggests that the rate of CO2 insertion into the Au-H bond can be significantly changed by modifying the NHC/NHCP ligand, demonstrating a one-to-one mapping between the bonding and reactivity: increasing the electron density on the metal fragment by incorporating a stronger σ-donor NHC/NHCP, i.e. a CAAC for NHCs and a conjugated DAC-based for NHCPs, would result in faster CO2 insertion."

The study is devoted to understanding the NHCP highly tunable steric and electronic peculiar features as coordination ligands, as well as exploring their potential applications in small molecule activation and catalysis. The computational details involved using the Amsterdam Density Functional (ADF) code combined with QUILD program for geometry optimizations and frequency calculations, employing functionals like BP86, basis sets like TZ2P, ZORA Hamiltonian, and Grimme’s D3-BJ dispersion correction. Solvent effects were modeled employing the Conductor-like Screening Model (COSMO) with the default parameters for toluene as implemented in the ADF code. The same computational setup has also been used for the EDA and CD-NOCV calculations. The CD method has been validated against experimental values in previous works by Belpassi et al., demonstrating that NMR spectroscopy can separately probe σ donation (via 13C chemical shifts) and π back donation (via C–N bond rotational barriers).

"The coordination geometry at the P center is accordingly very flexible, shifting from “nearly planar” (in CAACs-based NHCP) to PH3-like (in conjugated DACs-based NHCP), driven by the lone pair P 3p percentage and energy under NHC moiety control, with the CNHC-P-Au bond angle that can be regarded as a structural descriptor to monitor and/or design NHCP ligands with desired σ-donor properties."

A second lone pair on P, available for additional coordination, bears strict isolobal analogies with the CNHC lone pair in NHCs, and as such can be more directly modulated by both the NHC and R substituent structures.

"The NHCP ligands are more σ-donor than NHCs (CTσ don: 0.30-0.35e for NHCPs vs. 0.29-0.31e for NHCs) and, remarkably, the σ –donor trend is opposite to that in NHCs, i.e. σ–donation increases from CAACs to conjugated DACs in the NHCP series."

NHCPs are much less π-acceptor and tunable compared to NHCs (CTπ total back: -0.09/-0.06 e for NHCPs vs. -0.14/-0.10 e for NHCs).

"The coordination geometry at the P center is accordingly very flexible, shifting from “nearly planar” (in CAACs-based NHCP) to PH3-like (in conjugated DACs-based NHCP), driven by the lone pair P 3p percentage and energy under NHC moiety control."

The computational details involved using the Amsterdam Density Functional (ADF) code combined with QUILD program for geometry optimizations and frequency calculations, employing functionals like BP86, basis sets like TZ2P, ZORA Hamiltonian, and Grimme’s D3-BJ dispersion correction. Solvent effects were modeled employing the Conductor-like Screening Model (COSMO) with the default parameters for toluene as implemented in the ADF code. The same computational setup has also been used for the EDA and CD-NOCV calculations. The CD method has been validated against experimental values in previous works by Belpassi et al., demonstrating that NMR spectroscopy can separately probe σ donation (via 13C chemical shifts) and π back donation (via C–N bond rotational barriers).

The NHCP-AuH bond is mainly described by σ-donation (with energies in a range from-34.4 to-37.7 kcal/mol), with much smaller and comparable π backdonation contributions.

"For the back donation components, the major contribution is given by π┴ back (energies range from-8.1 to-11.4 kcal/mol, larger for 6: -13.4 kcal/mol), with smaller and comparable πǁ back (-4.8/-6.1 kcal/mol) and σ back (-4.4/-5.5 kcal/mol) contributions."

The CTnet values vary substantially along the series (from 0.02 to 0.13 e), suggesting in all complexes a NHCPδ+ - AuHδ- bond polarity, higher for conjugated DACs-based complexes 8, 9, 12 and 13.

The orbital energy contributions associated to each bond component (Table 2) also show that the NHCP-AuH bond is mainly described by σ-donation (with energies in a narrow range from-34.4 to-37.7 kcal/mol).

NHCP ligands are much less π-acceptor and tunable compared to NHCs (CTπ total back: -0.09/-0.06 e for NHCPs vs. -0.14/-0.10 e for NHCs).

**"The orbital energy contributions associated to each bond component (Table 2) also show that the NHCP-AuH bond is mainly described by σ-donation (with energies in a narrow range from-34.4 to-37.7 kcal/mol).

1. AI systems can be trained to predict NHCP ligand properties, such as σ-donor ability and coordination geometry at P, based on a systematic mapping between carbene moiety classes (CAACs, unconjugated DACs, conjugated DACs) and resulting structural descriptors like CNHC-P-Au bond angle.

2. AI systems can be improved to quantitatively assess the NHCP AuH bond nature by integrating Energy Decomposition Analysis (EDA), Natural Orbitals for Chemical Valence (NOCV), and Charge Displacement (CD) analysis, allowing them to predict the specific charge transfer components such as CTsigma don and CTpi total back for any given ligand series.

3. AI systems can be developed to correlate the kinetic activation barriers of CO2 insertion reactions with the calculated NHCP sigma donor ability (CTsigma don values), enabling them to predict which NHCP complexes will exhibit faster or slower CO2 activation kinetics.

4. AI systems can be used for lead optimization in catalysis by predicting the thermodynamic stability of reaction products, such as formate PC species, based on their zwitterionic character and the nature of the ligand moiety to identify conditions favoring reversible hydride transfer reactions.

The coordination properties of the emerging class of N-heterocyclic carbene-phosphinidene (NHCP) ligands have been reported to be strongly affected by the carbene (NHC) moiety, but a clear understanding remains strikingly limited. In this work the bonding features and reactivity of 13 NHCP gold hydride complexes, [NHCPAuH], bearing different classes of NHCs have been systematically explored and compared to carbene analogues, using a consistent computational protocol. The analyses reveal that, although the π-acceptor ability trend of NHCPs qualitatively parallels that of NHCs, the σ-donor ability trend reverses, with more σ-donating NHC moieties generating less σ-donating NHCP ligands. Concurrently, the nature of the NHC moiety impacts the coordination geometry at the P center, with the CNHC-P-Au bond angle appearing as a structural descriptor to monitor and/or design NHCP ligands with desired σ-donor properties. The NHCPs stronger σ-donor and weaker π-acceptor abilities compared to NHCs and their unique structural flexibility and electronic adaptability have been showcased to be directly controlled by the carbene moiety via modulation of the HOMO lone pair energy and its atomic phosphorous 3p character. The reactivity of [NHCPAuH] complexes with CO2, taken as a probe for potential NHCP applications in small molecule activation processes, demonstrates qualitatively similar mechanisms, with remarkably different activation barriers, which directly correlate with the NHCPs σ-donor ability, thus reflecting the high tunability of their bonding properties. This work provides insights and perspectives on the design principle of NHCP ligands, with a spotlight on the pivotal role of NHC moiety in modulating their electronic and steric properties, offering opportunities for burgeoning applications across diverse fields.