Donor–Acceptor Properties of Bidentate Phosphines. DFT Study of Nickel Carbonyls and Molecular Dihydrogen Complexes

Closed Synthetic Cycle for Nickel-Based Dihydrogen Formation

Dihydrogen evolution was observed in a two-step protonation reaction starting from a Ni0 precursor with a tripodal N-heterocyclic carbene (NHC) ligand. Upon the first protonation, a NiII monohydride complex was formed, which was isolated and fully characterized. Subsequent protonation yields H2 via a transient intermediate (INT) and an isolable NiII acetonitrile complex. The latter can be reduced to regenerate its Ni0 precursor. The mechanism of H2 formation was investigated by using a deuterated acid and scrutinized by 1 H NMR spectroscopy and gas chromatography. Remarkably, the second protonation forms a rare nickel dihydrogen complex, which was detected and identified in solution and characterized by 1 H NMR spectroscopy. DFT-based computational analyses were employed to propose a reaction profile and a molecular structure of the Ni-H2 complex. Thus, a dihydrogen-evolving, closed-synthetic cycle is reported with a rare Ni-H2 species as a key intermediate.

Room-Temperature Amination of Chloroheteroarenes in Water by a Recyclable Copper(II)-Phosphaadamantanium Sulfonate System

Buchwald-Hartwig amination of chloroheteroarenes has been a challenging synthetic process, with very few protocols promoting this important transformation at ambient temperature. The current report discusses about an efficient copper-based catalytic system (Cu/PTABS) for the amination of chloroheteroarenes at ambient temperature in water as the sole reaction solvent, a combination that is first to be reported. A wide variety of chloroheteroarenes could be coupled efficiently with primary and secondary amines as well as selected amino acid esters under mild reaction conditions. Catalytic efficiency of the developed protocol also promotes late-stage functionalization of active pharmaceutical ingredients (APIs) such as antibiotics (floxacins) and anticancer drugs. The catalytic system also performs efficiently at a very low concentration of 0.0001 mol % (TON = 980,000) and can be recycled 12 times without any appreciable loss in activity. Theoretical calculations reveal that the π-acceptor ability of the ligand PTABS is the main reason for the appreciably high reactivity of the catalytic system. Preliminary characterization of the catalytic species in the reaction was carried out using UV-VIS and ESR spectroscopy, providing evidence for the Cu(II) oxidation state.

Chiral Catalysts for Pd0 -Catalyzed Enantioselective C-H Activation

In the past few decades, processes that involve transition-metal catalysis have represented a major part of the synthetic chemist's toolbox. Recently, the interest has shifted from the well-established cross-coupling reactions to C-H bond functionalization, thus making it a current frontier of transition-metal-catalyzed reactions. Constant progress in this field has led to the discovery of enantioselective methods to generate and control various types of stereogenic elements, thereby demonstrating its high value to generate scalemic chiral molecules. The present review is dedicated to enantioselective Pd⁰ -catalyzed C-H activation, which may be considered as an evolution of Pd⁰ -catalyzed cross-couplings, with a focus on the different chiral ligands and catalysts that enable these transformations.

Unexpected Nickel Complex Speciation Unlocks Alternative Pathways for the Reactions of Alkyl Halides with dppf-Nickel(0)

The mechanism of the reactions between dppf-Ni0 complexes and alkyl halides has been investigated using kinetic and mechanistic experiments and DFT calculations. The active species is [Ni(κ2-dppf)(κ1-dppf)], which undergoes a halide abstraction reaction with alkyl halides and rapidly captures the alkyl radical that is formed. The rates of the reactions of [Ni(COD)(dppf)] with alkyl halides and the yields of prototypical nickel-catalyzed Kumada cross-coupling reactions of alkyl halides are shown to be significantly improved by the addition of free dppf ligand.

Thermodynamic targeting of electrocatalytic CO2 reduction: advantages, limitations, and insights for catalyst design

Herein is reported the electrocatalytic reduction of CO₂ with the complex [Ni(bis-NHC)(dmpe)]²⁺ (1) (bis-NHC = 1,l':3,3'-bis(1,3-propanediyl)dibenzimidazolin-2,2'-diylidene; dmpe = 1,2-bis(dimethylphosphino)ethane). The hydricity of 1 was previously benchmarked to be , equating to a driving force of a minimum of ∼3.4 kcal mol^-1 for hydride transfer to CO₂. While hydride transfer to CO₂ is thermodynamically favorable, electrocatalytic and infrared spectroelectrochemical (IR-SEC) experiments reveal that hydride transfer is blocked by direct reactivity with CO₂ in the reduced, Ni(0) state of the catalyst, yielding CO via reductive disproportionation (2CO₂ + 2e^- = CO + CO₃^2-) and concomitant catalyst degradation. Although thermodynamic scaling relationships provide guidance in catalyst targeting, the findings herein illustrate the fundamental kinetic challenges in balancing substrate reactivity and selectivity in the design of CO₂ reduction electrocatalysts. Advantages and limitations of this scaling relationship as well as approaches by which divergence from it may be achieved are discussed, which provides insight on important parameters for future catalyst design.

Computational Ligand Descriptors for Catalyst Design

Ligands, especially phosphines and carbenes, can play a key role in modifying and controlling homogeneous organometallic catalysts, and they often provide a convenient approach to fine-tuning the performance of known catalysts. The measurable outcomes of such catalyst modifications (yields, rates, selectivity) can be set into context by establishing their relationship to steric and electronic descriptors of ligand properties, and such models can guide the discovery, optimization, and design of catalysts. In this review we present a survey of calculated ligand descriptors, with a particular focus on homogeneous organometallic catalysis. A range of different approaches to calculating steric and electronic parameters are set out and compared, and we have collected descriptors for a range of representative ligand sets, including 30 monodentate phosphorus(III) donor ligands, 23 bidentate P,P-donor ligands, and 30 carbenes, with a view to providing a useful resource for analysis to practitioners. In addition, several case studies of applications of such descriptors, covering both maps and models, have been reviewed, illustrating how descriptor-led studies of catalysis can inform experiments and highlighting good practice for model comparison and evaluation.

Hydrogen elimination reactivity of ruthenium pincer hydride complexes: a DFT study

The pincer effect is explained for various pincer hydride complexes, differing in the donor atoms, using activation barriers, and MESP parameters.

Syntheses, Structures, and Characterization of Nickel(II) Stibines: Steric and Electronic Rationale for Metal Deposition

Reactions of the homoleptic and heteroleptic antimony ligands Sb ⁱPr₃, Sb ⁱPr₂Ph, SbMe₂Ph, and SbMePh₂ with NiI₂ generate rare Ni^II stibine complexes in either square planar or trigonal bipyramidal (TBP) geometries, depending on the steric size of the ligands. Tolman electronic parameters were calculated (DFT) for each antimony ligand to provide a tabulated resource for the relative strengths of simple antimony ligands. The electronic absorbance spectra of the square planar complexes exhibit characteristic bands [λ_max ≈ 560 nm (17 900 cm^-1), ε ≈ 4330 M^-1 cm^-1] at lower energies compared to the reported phosphine complexes, indicating the weak donor strength of the stibine ligands and resultant low-energy ligand field d→ d transitions. The square planar complex Ni(I)₂(Sb ⁱPr₃)₂ reacts with CO to form the TBP complex Ni(I)₂(Sb ⁱPr₃)₂(CO). Lastly, the complexes were investigated for nickel metal deposition on Si|Cu(100 nm) substrates. The complexes with the strongest donating ligand, Sb ⁱPr₃, deposited the purest layer of NiCu alloy according to the balanced reaction Ni(I)₂(Sb^{III i}Pr₃)₂ → Ni⁰ + Sb^V( ⁱPr₃)I₂; the iodinated Sb^V byproduct was unambiguously detected in the supernatant by ¹H NMR and mass spectrometry. Complexes with weaker ligands (poor I₂ acceptors/scavengers) resulted undesired deposition of iodine and CuI on the surface. This work thus serves as a guide for the design and synthesis of 3 d metal complexes with neutral, heavy main-group donors that are useful for metal deposition applications.

Multimetallic Ni- and Pd-Catalyzed Cross-Electrophile Coupling To Form Highly Substituted 1,3-Dienes

The synthesis of highly substituted 1,3-dienes from the coupling of vinyl bromides with vinyl triflates is reported for the first time. The coupling is catalyzed by a combination of (5,5'-bis(trifluoromethyl)-2,2'-bipyridine)NiBr2 and (1,3-bis(diphenylphosphino)propane)PdCl2 in the presence of a zinc reductant. This method affords tetra- and penta-substituted 1,3-dienes that would otherwise be difficult to access and tolerates electron-rich and -poor substituents, heterocycles, an aryl bromide, and a pinacol boronate ester. Mechanistically, the reaction appears to proceed by an unusual zinc-mediated transfer of a vinyl group between the nickel and palladium centers.

Synthesis, characterization, and catalytic evaluation of ruthenium–diphosphine complexes bearing xanthate ligands

Nine ruthenium chelates with the generic formula [Ru(S₂COEt)₂(diphos)] were synthesized and fully characterized. Their catalytic activity was probed in three distinct reactions.

On the relation between carbonyl stretching frequencies and the donor power of chelating diphosphines in nickel dicarbonyl complexes

The relation between spectroscopic observables and the detailed metal-ligand bonding features in chelation complexes is addressed using both experimental and state-of-the-art theoretical and computational methods. We synthesized and characterized a set of six nickel dicarbonyl complexes of general formula [Ni(CO)2(PP)], where PP is an atropoisomeric chelating diphosphine ligand. The analysis of the obtained experimental data and the basicity and oxidative potentials of the free ligands suggests a close relation between the donor ability of the chelating ligand and the carbonyl stretching frequencies observed in the complexes. We then use theory to unravel the detailed mechanisms of chelation-bond formation in terms of partial charge flows between the molecular orbitals of the fragments. By extending the promising, recently published natural orbitals for chemical valence/charge displacement (NOCV/CD) analysis scheme we provide a thorough, quantitative description of the several charge fluxes following the metal-ligand bond formation and demonstrate that the carbonyl stretching frequencies in the considered complexes selectively respond to the σ-donation charge flow from the phosphorus lone pairs of the ligands, with the frequency shift being in quantitative correlation with the extent of the ligand-to-metal charge transfer.

Theoretical investigation on donor–acceptor interaction between a carbonyl compound and an N,N′-dioxide–Sc(iii) complex

The counterion and substituent on amide of the N,N′-dioxide ligand could affect electrostatic energy (ΔV_elstat) as well as orbital energy (ΔE_orb) between CH₂O and Sc(iii)-based catalyst, adjusting the Lewis acidity of the metal centre.

Ligand K-edge XAS, DFT, and TDDFT analysis of pincer linker variations in Rh(i) PNP complexes: reactivity insights from electronic structure

Ligand K-edge XAS and DFT studies of ligand variations in Rh(i) pincer complexes and correlations to small molecule reactivity.

Impact of Coordination Geometry, Bite Angle, and Trans Influence on Metal-Ligand Covalency in Phenyl-Substituted Phosphine Complexes of Ni and Pd

Despite the long-standing use of phosphine and diphosphine ligands in coordination chemistry and catalysis, questions remain as to their effects on metal-ligand bonding in transition metal complexes. Here we report ligand K-edge XAS, DFT, and TDDFT studies aimed at quantifying the impact of coordination geometry, diphosphine bite angle, and phosphine trans influence on covalency in M-P and M-Cl bonds. A series of four-coordinate NiCl2 and PdCl2 complexes containing PPh3 or Ph2P(CH2)nPPh2, where n = 1 (dppm), 2 (dppe), 3 (dppp), and 4 (dppb), was analyzed. The XAS data revealed that changing the coordination geometry from tetrahedral in Ni(PPh3)2Cl2 (1) to square planar in Ni(dppe)Cl2 (2) more than doubles the intensity of pre-edge features assigned to Ni-P and Ni-Cl 1s → σ* transitions. By way of comparison, varying the diphosphine in Pd(dppm)Cl2 (4), Pd(dppp)Cl2 (6), and Pd(dppb)Cl2 (7) yielded Pd-P 1s → σ* transitions with identical intensities, but a 10% increase was observed in the P K-edge XAS spectrum of Pd(dppe)Cl2 (5). A similar observation was made when comparing Ni(dppe)Cl2 (2) to Ni(dppp)Cl2 (3), and DFT and TDDFT calculations corroborated XAS results obtained for both series. Comparison of the spectroscopic and theoretical results to the diphosphine structures revealed that changes in M-P covalency were not correlated to changes in bite angles or coordination geometry. As a final measure, P and Cl K-edge XAS data were collected on trans-Pd(PPh3)2Cl2 (8) for comparison to the cis diphosphine complex Pd(dppe)Cl2 (5). Consistent with phosphine's stronger trans influence compared to chloride, a 35% decrease in the intensity of the Pd-P 1s → σ* pre-edge feature and a complementary 34% increase in Pd-Cl 1s → σ* feature was observed for 8 (trans) compared to 5 (cis). Overall, the results reveal how coordination geometry, ligand arrangement, and diphosphine structure affect covalent metal-phosphorus and metal-chloride bonding in these late transition metal complexes.

Synthesis, Structure, and Conformational Dynamics of Rhodium and Iridium Complexes of Dimethylbis(2-pyridyl)borate

Rhodium(I) and Iridium(I) borate complexes of the structure [Me₂B(2-py)₂]ML₂ (L₂ = (tBuNC)₂, (CO)₂, (C₂H₄)₂, cod, dppe) were prepared and structurally characterized (cod = 1,5-cyclooctadiene; dppe = 1,2-diphenylphosphinoethane). Each contains a boat-configured chelate ring that participates in a boat-to-boat ring flip. Computational evidence shows that the ring flip proceeds through a transition state that is near planarity about the chelate ring. We observe an empirical, quantitative correlation between the barrier of this ring flip and the π acceptor ability of the ancillary ligand groups on the metal. The ring flip barrier correlates weakly to the Tolman and Lever ligand parameterization schemes, apparently because these combine both σ and π effects while we propose that the ring flip barrier is dominated by π bonding. This observation is consistent with metal-ligand π interactions becoming temporarily available only in the near-planar transition state of the chelate ring flip and not the boat-configured ground state. Thus, this is a first-of-class observation of metal-ligand π bonding governing conformational dynamics.

Wave Function and Density Functional Theory Studies of Dihydrogen Complexes

We performed a benchmark study on a series of dihydrogen bond complexes and constructed a set of reference bond distances and interaction energies. The test set was employed to assess the performance of several wave function correlated and density functional theory methods. We found that second-order correlation methods describe relatively well the dihydrogen complexes. However, for high accuracy inclusion of triple contributions is important. On the other hand, none of the considered density functional methods can simultaneously yield accurate bond lengths and interaction energies. However, we found that improved results can be obtained by the inclusion of nonlocal exchange contributions.

Mechanisms and origins of switchable chemoselectivity of Ni-catalyzed C(aryl)-O and C(acyl)-O activation of aryl esters with phosphine ligands

Many experiments have shown that nickel with monodentate phosphine ligands favors the C(aryl)-O activation over the C(acyl)-O activation for aryl esters. However, Itami and co-workers recently discovered that nickel with bidentate phosphine ligands can selectively activate the C(acyl)-O bond of aryl esters of aromatic carboxylic acids. The chemoselectivity with bidentate phosphine ligands can be switched back to C(aryl)-O activation when aryl pivalates are employed. To understand the mechanisms and origins of this switchable chemoselectivity, density functional theory (DFT) calculations have been conducted. For aryl esters, nickel with bidentate phosphine ligands cleaves C(acyl)-O and C(aryl)-O bonds via three-centered transition states. The C(acyl)-O activation is more favorable due to the lower bond dissociation energy (BDE) of C(acyl)-O bond, which translates into a lower transition-state distortion energy. However, when monodentate phosphine ligands are used, a vacant coordination site on nickel creates an extra Ni-O bond in the five-centered C(aryl)-O cleavage transition state. The additional interaction energy between the catalyst and substrate makes C(aryl)-O activation favorable. In the case of aryl pivalates, nickel with bidentate phosphine ligands still favors the C(acyl)-O activation over the C(aryl)-O activation at the cleavage step. However, the subsequent decarbonylation generates a very unstable tBu-Ni(II) intermediate, and this unfavorable step greatly increases the overall barrier for generating the C(acyl)-O activation products. Instead, the subsequent C-H activation of azoles and C-C coupling in the C(aryl)-O activation pathway are much easier, leading to the observed C(aryl)-O activation products.