Ligand Electronic Effect on Reductive Elimination of Biphenyl from cis-[Pt(Ph)2(diphosphine)] Complexes Bearing Electron-Poor Diphosphine: Correlation Study between Experimental and Theoretical Results

Mechanistic study on the reductive elimination of (aryl)(fluoroaryl)palladium complexes: a key step in regiospecific dehydrogenative cross-coupling

Cross-dehydrogenative coupling (CDC) reactions have attracted attention as short-step synthetic methods for C-C bond formation. Recently, we have developed CDC reactions between naphthalene and fluorobenzene. Rather than exhibiting general regioselectivity, this reaction proceeds selectively at the β-position of naphthalene. In this study, investigation using model complexes as reaction intermediates revealed that the origin of the unique selectivity is the exclusive occurrence of reductive elimination at the β-position. Detailed studies on the reductive elimination showed that the steric hindrance of the naphthyl group and the electron-withdrawing properties of fluorobenzene determine the position at which the reductive elimination reaction proceeds. These results show that the selectivity of the C-H functionalisation of polycyclic aromatic hydrocarbons (PAHs) is determined not by the C-H cleavage step, but by the subsequent reductive elimination step. The regioselective CDC reaction was adaptable to various PAHs but was less selective for pyrene with extended π-conjugation. In fluorobenzene substrates, the F atoms at the two ortho positions of the C-H moiety are necessary for high selectivity. The substrate ranges are in good agreement with the proposed mechanism, in which the reductive elimination step determines the regioselectivity.

Diazaphospholene-Catalyzed Hydrodefluorination of Polyfluoroarenes with Phenylsilane via Concerted Nucleophilic Aromatic Substitution

The metal-free catalytic C-F bond activation of polyfluoroarenes was achieved with diazaphospholene as the catalyst and phenylsilane as the terminal reductant. Density functional theory calculations suggested a concerted nucleophilic aromatic substitution mechanism.

Force-modulated reductive elimination from platinum(ii) diaryl complexes

Coupled mechanical forces are known to drive a range of covalent chemical reactions, but the effect of mechanical force applied to a spectator ligand on transition metal reactivity is relatively unexplored. Here we quantify the rate of C(sp2)-C(sp2) reductive elimination from platinum(ii) diaryl complexes containing macrocyclic bis(phosphine) ligands as a function of mechanical force applied to these ligands. DFT computations reveal complex dependence of mechanochemical kinetics on the structure of the force-transducing ligand. We validated experimentally the computational finding for the most sensitive of the ligand designs, based on MeOBiphep, by coupling it to a macrocyclic force probe ligand. Consistent with the computations, compressive forces decreased the rate of reductive elimination whereas extension forces increased the rate relative to the strain-free MeOBiphep complex with a 3.4-fold change in rate over a ∼290 pN range of restoring forces. The calculated natural bite angle of the free macrocyclic ligand changes with force, but 31P NMR analysis and calculations strongly suggest no significant force-induced perturbation of ground state geometry within the first coordination sphere of the (P-P)PtAr2 complexes. Rather, the force/rate behavior observed across this range of forces is attributed to the coupling of force to the elongation of the O⋯O distance in the transition state for reductive elimination. The results suggest opportunities to experimentally map geometry changes associated with reactions in transition metal complexes and potential strategies for force-modulated catalysis.

Catalytic Hydrodefluorination via Oxidative Addition, Ligand Metathesis, and Reductive Elimination at Bi(I)/Bi(III) Centers

Herein, we report a hydrodefluorination reaction of polyfluoroarenes catalyzed by bismuthinidenes, Phebox-Bi(I) and OMe-Phebox-Bi(I). Mechanistic studies on the elementary steps support a Bi(I)/Bi(III) redox cycle that comprises C(sp²)-F oxidative addition, F/H ligand metathesis, and C(sp²)-H reductive elimination. Isolation and characterization of a cationic Phebox-Bi(III)(4-tetrafluoropyridyl) triflate manifests the feasible oxidative addition of Phebox-Bi(I) into the C(sp²)-F bond. Spectroscopic evidence was provided for the formation of a transient Phebox-Bi(III)(4-tetrafluoropyridyl) hydride during catalysis, which decomposes at low temperature to afford the corresponding C(sp²)-H bond while regenerating the propagating Phebox-Bi(I). This protocol represents a distinct catalytic example where a main-group center performs three elementary organometallic steps in a low-valent redox manifold.

Palladium-Catalyzed Homocoupling of Highly Fluorinated Arylboronates: Studies of the Influence of Strongly vs Weakly Coordinating Solvents on the Reductive Elimination Process

C-C reductive elimination from [PdL₂(C₆F₅)₂] to form polyfluorinated biaryls has been a challenge for over 50 years. Thus, palladium-catalyzed homocoupling of arylboronates (Ar_F-Bpin) containing two ortho-fluorine substituents is very difficult, as the reaction typically stops at the [PdL₂(Ar_F)₂] stage after two transmetalation steps. The transmetalated complexes cis-[Pd(MeCN)₂(C₆F₅)₂] (3a), cis-[Pd(MeCN)₂(2,4,6-C₆F₃H₂)₂] (3b), and cis-[Pd(MeCN)₂(2,6-C₆F₂H₃)₂] (3e) have been isolated from the reaction of Ar_F-Bpin with Pd(OAc)₂ in acetonitrile solvent, with no homocoupling observed. However, catalytic homocoupling proceeds smoothly in a "weakly coordinating" arene solvent as long as no ancillary ligands or coordinating solvents are present. DFT computations reveal that the active catalyst formed by arene solvent coordination leads to an overall reduced barrier for the reductive elimination step compared to the formation of stable [PdL₂(Ar_F)₂] complexes in the presence of a donor ligand or solvent L.

Platinum complexes of a boron-rich diphosphine ligand

Herein, we describe the preparation, characterization, and reactivity of two Pt^II bis-hydrocarbyl complexes containing the 1,2-bis(di(3-dicyclohexylboraneyl)propylphosphino)ethane (P₂B^Cy₄) ligand.

Syntheses and Structures of d10 Coinage Metal Complexes of Electron-Accepting Phosphine Ligands Featuring a 3,3,4,4,5,5-Hexafluorocyclopentene Framework

Cu(I), Ag(I), and Au(I) complexes of monophosphine or bisphosphine ligands based on the 3,3,4,4,5,5-hexafluorocyclopentene skeleton were synthesized and structurally characterized by X-ray crystallographic analysis. The electron-withdrawing nature of these polyfluorinated phosphines was experimentally revealed via UV/vis absorption studies and crystal structure analysis. Successful catalytic application of the Au(I) complex for alkyne hydration reactions was investigated.

Synthesis of Stable Diarylpalladium(II) Complexes: Detailed Study of the Aryl-Aryl Bond-Forming Reductive Elimination

The synthesis of diarylpalladium(II) complexes by twofold aryl C-H bond activation was developed. These intermediates of oxidative cyclization reactions are stabilized by chelation with acetyl groups while still maintaining sufficient reactivity to study their reductive elimination. Four distinct triggers were found for the reductive elimination of these complexes to dibenzofurans and carbazoles. Thermal elimination occurs at very high temperatures, whereas ligand-promoted and oxidatively induced reductive eliminations proceed readily at room temperature. Under these conditions, no isomerization occurs. In contrast, weak Brønsted acids, such as acetic acid, lead to a sequence of proto-demetalation, isomerization to a κ(3) -diarylpalladium(II) complex, and reductive elimination to non-symmetrical cyclization products.

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.

Mechanistic studies on C–C reductive coupling of five-coordinate Rh(iii) complexes

C–C reductive coupling of rare five-coordinate rhodium(iii) complexes has been studied in detail.

Highly electron-poor Buchwald-type ligand: application for Pd-catalysed direct arylation of thiophene derivatives and theoretical consideration of the secondary Pd0–arene interaction

Highly electron-poor SPhos ligands stabilised the Pd complex by secondary Pd⁰–arene interaction.

Long-lived and oxygen-responsive photoluminescence in the solid state of copper(i) complexes bearing fluorinated diphosphine and bipyridine ligands

Luminescence properties of a family of newly synthesized copper(i) complexes were investigated. The luminescence in the solid state is highly oxygen responsive.

A remote Lewis acid trigger dramatically accelerates biaryl reductive elimination from a platinum complex

A strategy for the control of electron density at a metal center is reported, which uses a remote chemical switch involving second-sphere Lewis acid binding that modulates electron density in the first coordination sphere. Binding of the Lewis acid B(C6F5)3 at remote nitrogen positions of a bipyrazine-diarylplatinum(II) complex accelerates biaryl reductive elimination by a factor of 64,000.

On the Role of Disproportionation Energy in Kumada Catalyst-Transfer Polycondensation

Kumada catalyst-transfer polycondensation (KCTP) is an effective method for the controlled polymerization of conjugated polymers. Nevertheless, side reactions leading to early termination and unwanted chain coupling cause deviations from the target molecular weight, along with increasing polydispersity and end group variation. The departure from the KCTP cycle stems from a disproportionation reaction that leads to experimentally observed side products. The disproportionation energies for a series of nickel-based initiators containing bidentate phosphino attendant ligands were computed using density functional theory at the B3LYP/DZP level. The initiator was found to be less favorable toward disproportionation by 0.5 kcal mol^-1 when ligated by 1,3-bis(diphenylphosphino)propane (dppp) rather than 1,2-bis(diphenylphosphino)ethane (dppe). Trends in disproportionation energy (E_disp) with a variety of bidentate phosphine ligands match experimental observations of decreased polymerization control. Theoretical E_disp values can thus be used to predict the likelihood of disproportionation in cross-coupling reactions and, therefore, aid in catalyst design.