Synthesis, Structures, and Ethylene Dimerization Reactivity of Palladium Alkyl Complexes That Contain a Chelating Phosphine–Trifluoroborate Ligand

Exploring the Reactivity of B‐Connected Carboranylphosphines in Frustrated Lewis Pair Chemistry: A New Frame for a Classic System

The primary phosphines MesPH₂ and tBuPH₂ react with 9‐iodo‐m‐carborane yielding B9‐connected secondary carboranylphosphines 1,7‐H₂C₂B₁₀H₉‐9‐PHR (R=2,4,6‐Me₃C₆H₂ (Mes; 1  a), tBu (1  b)). Addition of tris(pentafluorophenyl)borane (BCF) to 1  a, b resulted in the zwitterionic compounds 1,7‐H₂C₂B₁₀H₉‐9‐PHR(p‐C₆F₄)BF(C₆F₅)₂ (2  a, b) through nucleophilic para substitution of a C₆F₅ ring followed by fluoride transfer to boron. Further reaction with Me₂SiHCl prompted a H−F exchange yielding the zwitterionic compounds 1,7‐H₂C₂B₁₀H₉‐9‐PHR(p‐C₆F₄)BH(C₆F₅)₂ (3  a, b). The reaction of 2  a, b with one equivalent of R'MgBr (R’=Me, Ph) gave the extremely water‐sensitive frustrated Lewis pairs 1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄)B(C₆F₅)₂ (4  a, b). Hydrolysis of the B−C₆F₄ bond in 4  a, b gave the first tertiary B‐carboranyl phosphines with three distinct substituents, 1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄H) (5  a, b). Deprotonation of the zwitterionic compounds 2  a, b and 3  a, b formed anionic phosphines [1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄)BX(C₆F₅)₂]⁻[DMSOH]⁺ (R=Mes, X=F (6  a), R=tBu, X=F (6  b); R=Mes, X=H (7  a), R=tBu, X=H (7  b)). Reaction of 2  a, b with an excess of Grignard reagents resulted in the addition of R’ at the boron atom yielding the anions [1,7‐H₂C₂B₁₀H₉‐9‐PR(p‐C₆F₄)BR’(C₆F₅)₂]⁻ (R=Mes, R’=Me (8  a), R=tBu, R’=Me (8  b); R=Mes, R’=Ph (9  a), R=tBu, R’=Ph (9  b)) with [MgBr(Et₂O)_n]⁺ as counterion. The ability of the zwitterionic compounds 3  a, b to hydrogenate imines as well as the Brønsted acidity of 3  a were investigated.

Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity

Enhanced rates and selectivity in enzymes are enabled in part by precisely tuned electric fields within active sites. Analogously, the use of charged groups to leverage electrostatics in molecular systems is a promising strategy to tune reactivity. However, separation of the through space and through bond effects of charged functional groups is a long standing challenge that limits the rational application of electric fields in molecular systems. To address this challenge we developed a method using the phosphorus selenium coupling value (J _P-Se) of anionic phosphine selenides to quantify the electrostatic contribution of the borate moiety to donor strength. In this analysis we report the synthesis of a novel anionic phosphine, PPh₂CH₂BF₃K, the corresponding tetraphenyl phosphonium and tetraethyl ammonium selenides [PPh₄][SePPh₂CH₂BF₃] and [TEA][SePPh₂CH₂BF₃], and the Rh carbonyl complex [PPh₄][Rh(acac)(CO)(PPh₂(CH₂BF₃))]. Solvent-dependent changes in J _P-Se were fit using Coulomb's law and support up to an 80% electrostatic contribution to the increase in donor strength of [PPh₄][SePPh₂CH₂BF₃] relative to SePPh₂Et, while controls with [TEA][SePPh₂CH₂BF₃] exclude convoluting ion pairing effects. Calculations using explicit solvation or point charges effectively replicate the experimental data. This J _P-Se method was extended to [PPh₄][SePPh₂(2-BF₃Ph)] and likewise estimates up to a 70% electrostatic contribution to the increase in donor strength relative to SePPh₃. The use of PPh₂CH₂BF₃K also accelerates C-F oxidative addition reactivity with Ni(COD)₂ by an order of magnitude in comparison to the comparatively donating neutral phosphines PEt₃ and PCy₃. This enhanced reactivity prompted the investigation of catalytic fluoroarene C-F borylation, with improved yields observed for less fluorinated arenes. These results demonstrate that covalently bound charged functionalities can exert a significant electrostatic influence under common solution phase reaction conditions and experimentally validate theoretical predictions regarding electrostatic effects in reactivity.

Catalytic applications of zwitterionic transition metal compounds

This frontiers article highlights recent developments on the application of transition metal-based zwitterionic complexes in catalysis. Recent applications of selected zwitterionic catalysts in polymerization reactions, including the carbonylative polymerization of cyclic ethers, carbon-carbon coupling reactions, the asymmetric hydrogenation of unfunctionalized olefins, and the hydrofunctionalization of alkenes are reviewed. In addition, advances in the field of hydrogenation/dehydrogenation reactions related to energy applications, including the hydrogenation of CO₂ and the dehydrogenation of formic acid and N-heterocycles, the functionalization of CO₂ with amines and hydrosilanes, and the valorization of polyfunctional bio-based feedstocks, such as the dehygrogenation of glycerol to lactic acid or the reduction of levulinic acid into γ-valerolactone, are also described.

Imidazolyl-phenyl (IMP) anions: a modular structure for tuning solubility and coordinating ability

The effect of counteranion upon a cation's solution-phase reactivity depends on a subtle interplay of weak interactions.

Access to a pair of ambiphilic phosphine-borane regioisomers by rhodium-catalyzed hydroboration

Lewis basic substrates, such as vinylphosphines and enamines, can be problematic for transition-metal catalysed hydrofunctionalization reactions due to their propensity to ligate and deactivate transition-metal catalysts as well as form direct Lewis adducts with reaction partners. While exploring rhodium-catalyzed hydroboration of diphenylvinylphosphine with pinacolborane, we found that a high degree of regiocontrol could be achieved without the need to diminish the Lewis basicity of the phosphine by oxidation or prior-protection. At slightly elevated temperature, a high yield of the previously unreported branched regioisomer, 1-pinacolatoborono-1-diphenylphosphinoethane, was achieved with regioselectivity greater than 10 : 1 using [Rh(COD)Cl]2 as the catalyst and AgOTf as a catalytic additive. Inversion of regioselectivity occurred at low temperature and high yield of the linear regioisomer was observed. Subsequent functionalization of the new branched phosphine-boronic ester and its coordination to rhodium were also investigated.

Methylene-Bridged Bisphosphine Monoxide Ligands for Palladium-Catalyzed Copolymerization of Ethylene and Polar Monomers

A series of palladium complexes bearing a bisphosphine monoxide with a methylene linker, that is, [κ²-P,O-(R¹₂P)CH₂P(O)R²₂]PdMe(2,6-lutidine)][BAr^F₄] (Pd/BPMO), were synthesized and evaluated as catalysts for the homopolymerization of ethylene and the copolymerization of ethylene and polar monomers. X-ray crystallographic analyses revealed that these Pd/BPMO complexes exhibit significantly narrower bite angles and longer Pd-O bonds than Pd/BPMO complexes bearing a phenylene linker, while maintaining almost constant Pd-P bond lengths. Among the complexes synthesized, menthyl-substituted complex 3f (R¹ = (1R,2S,5R)-2-isopropyl-5-methylcyclohexan-1-yl; R² = Me) showed the best catalytic performance in the homo- and copolymerization in terms of molecular weight and polymerization activity. Meanwhile, complex 3e (R¹ = t-Bu; R² = Me) exhibited a markedly higher incorporation of comonomers in the copolymerization of ethylene and allyl acetate (≤12.0 mol %) or methyl methacrylate (≤0.6 mol %). The catalytic system represents one of the first examples of late-transition-metal complexes bearing an alkylene-bridged bidentate ligand that afford high-molecular-weight copolymers from the copolymerization of ethylene and polar monomers.

Copolymerisation of ethylene with polar monomers by using palladium catalysts bearing an N-heterocyclic carbene–phosphine oxide bidentate ligand

We report the synthesis and characterisation of palladium complexes bearing an N-heterocyclic carbene–phosphine oxide bidentate ligand and their use as catalysts for ethylene polymerisation and ethylene/polar monomer copolymerisation.

Insertion Copolymerization of Difunctional Polar Vinyl Monomers with Ethylene

A single-step synthesis, structural characterization and application of a neutral, acetonitrile ligated, palladium-phosphinesulfonate complex [{P^∧O}PdMe(L)] (P^∧O = κ²-P,O-Ar₂PC₆H₄SO₂O with Ar = 2-MeOC₆H₄; L = CH₃CN) (3) in coordination/insertion copolymerization of ethylene with difunctional olefin is investigated. In a significant development, complex 3 was found to catalyze insertion copolymerization of industrially relevant 1,1-disubstituted difunctional vinyl monomers for the first time. Thus, insertion copolymerization of ethyl-2-cyanoacrylate (ECA or super glue) and trifluoromethyl acrylic acid (TFMAA) with ethylene produced the corresponding copolymers with 6.5% ECA and 3% TFMAA incorporation. Increasing the concentration of difunctional olefins led to higher incorporation but at the expense of lower activities. These observations indicate that complex 3 tolerates difunctional vinyl monomers and provides direct access to difunctional polyolefins that have not been attempted before.