Pendant Phosphine Research Articles

Catalyst Editing via Post-Synthetic Functionalization by Phosphonium Generation and Anion Exchange for Nickel-Catalyzed Ethylene/Acrylate Copolymerization.

Rapid, efficient development of homogeneous catalysts featuring desired performance is critical to numerous catalytic transformations but remains a key challenge. Typically, this task relies heavily on ligand design that is often based on trial and error. Herein, we demonstrate a "catalyst editing" strategy in Ni-catalyzed ethylene/acrylate copolymerization. Specifically, alkylation of a pendant phosphine followed by anion exchange provides a high yield strategy for a large number of cationic Ni phosphonium catalysts with varying electronic and steric profiles. These catalysts are highly active in ethylene/acrylate copolymerization, and their behaviors are correlated with the electrophile and the anion used in late-stage functionalization.

Tuning a phosphine-substituted diimine ligand to afford an iron monocarbonyl complex

A series of low-valent iron complexes that feature a phosphine-substituted α-diimine (DI) ligand have been synthesized. Reduction of (Ph2PPrDI)FeBr2 with an excess of Na/Hg in the presence of carbon monoxide afforded the corresponding dicarbonyl complex, (Ph2PPrDI)Fe(CO)2. Through multinuclear NMR and single crystal X-ray diffraction analysis, this complex was found to possess a 3-coordinate DI ligand. Upon heating for 10 days at 110 °C while applying intermittent vacuum, (Ph2PPrDI)Fe(CO)2 was successfully converted to the corresponding monocarbonyl complex, (Ph2PPrDI)Fe(CO), which was found to feature a tetradentate chelate. Similar reactivity was explored using the analogous bis(tert-butyl)phosphine-substituted ligand, tBu2PPrDI. Addition of this chelate to FeBr2 afforded (tBu2PPrDI)FeBr2, and subsequent reduction yielded (tBu2PPrDI)FeBr, which was found to possess a tridentate DI ligand by single crystal X-ray diffraction. Performing the reduction of (tBu2PPrDI)FeBr2 in the presence of CO afforded the corresponding dicarbonyl complex, (tBu2PPrDI)Fe(CO)2. Like aryl-substituted (Ph2PPrDI)Fe(CO)2, alkyl-substituted (tBu2PPrDI)Fe(CO)2 was found to feature a pendant phosphine arm. However, heating (tBu2PPrDI)Fe(CO)2 under vacuum did not allow for phosphine substitution and conversion to the corresponding monocarbonyl complex, highlighting the importance of phosphine π-acidity for substitution and the stabilization of low-valent iron.

Synthesis and Reactivity of a Zinc Diazoalkyl Complex: [3+2] Cycloaddition Reaction with Carbon Monoxide

AbstractThis work reports the synthesis, characterization, and reactivity of the first example of a well‐defined zinc α‐diazoalkyl complex. Treatment of zinc(I)‐zinc(I) bonded compound L2Zn2 [L=CH3C(2,6‐iPr2C6H3N)CHC(CH3)(NCH2CH2PPh2)] or zinc(II) hydride LZnH with trimethylsilyldiazomethane affords zinc diazoalkyl complex LZnC(N2)SiMe3. This complex liberates N2 in the presence of a nickel catalyst to form an α‐zincated phosphorus ylide by reacting with the pendant phosphine. It selectively undergoes formal [3+2] cycloaddition with CO2 or CO to form the corresponding product with a five‐membered heterocyclic core. Notably, the use of CO in such a [3+2] cycloaddition reaction is unprecedented, reflecting a novel CO reaction mode.

Synthesis and Reactivity of a Zinc Diazoalkyl Complex: [3+2] Cycloaddition Reaction with Carbon Monoxide.

This work reports the synthesis, characterization, and reactivity of the first example of a well-defined zinc α-diazoalkyl complex. Treatment of zinc(I)-zinc(I) bonded compound L2 Zn2 [L=CH3 C(2,6-i Pr2 C6 H3 N)CHC(CH3 )(NCH2 CH2 PPh2 )] or zinc(II) hydride LZnH with trimethylsilyldiazomethane affords zinc diazoalkyl complex LZnC(N2 )SiMe3 . This complex liberates N2 in the presence of a nickel catalyst to form an α-zincated phosphorus ylide by reacting with the pendant phosphine. It selectively undergoes formal [3+2] cycloaddition with CO2 or CO to form the corresponding product with a five-membered heterocyclic core. Notably, the use of CO in such a [3+2] cycloaddition reaction is unprecedented, reflecting a novel CO reaction mode.

Kinetic and computational investigation on the reaction of Os5C(CO)15 with (Z)-Ph2PCH=CHPPh2 (dppen): Stepwise conversion to nido Os5C(CO)13(κ2-Pa,Pb-dppen)

The reaction of the pentaosmium cluster Os5C(CO)15 (1) with the diphosphine (Z)-Ph2PCHCHPPh2 (dppen) has been studied over the temperature range 328–358 K. Diphosphine addition to Os5C(CO)15 yields the arachno cluster Os5C(CO)15(κ1-Peq-dppen) (2) as the initially observed product. Cluster 2 transforms to the arachno cluster Os5C(CO)14(κ2-Peq,Peq-dppen) (3) upon CO loss and chelation of the pendent phosphine moiety. The loss of a second CO in cluster 3 is accompanied by polyhedral contraction to give the square pyramidal nido cluster Os5C(CO)13(κ2-Pa,Pb-dppen) (4). Clusters 2–4 were characterized in solution by IR and NMR spectroscopies, and the molecular structure of each new cluster was established by X-ray crystallography. The kinetics for the formation of 2 from 1 and dppen support a bimolecular process involving dppen addition to 1 analogous to the reported ligand additions to the family of M5C(CO)15 clusters (where M = Fe, Ru). Cluster 2 is labile and serves as the precursor to 3 + CO, which undergoes conversion to 4 + CO. The kinetics for these reactions have been explored, and these data, in conjunction with DFT calculations, have provided mechanistic insight into the stepwise conversion of 1 and dppen to 4 + 2CO.

Reductive reactivity of a Zn(I) − Zn(I)-bonded compound toward white phosphorus and beyond

This work reports the selective reduction of a number of small molecules by a Zn(I) − Zn(I)-bonded compound. Reaction of dizinc compound [L2Zn2] [L = CH3C(2,6-iPr2C6H3N)CHC(CH3)(NCH2CH2PPh2)] with white phosphorus generated a trimetallic cluster [(LZn)3(μ, κ2: κ2: κ1-P7)], which contained a zinc-coordinated Zintl ion [P7]3−. The low valent Zn(I) compound [L2Zn2] could also reduce 1-azidoadamantane and diphenyl disulfide to selectively afford Zn(II) hexazene complex [L2Zn2(μ-1,6-Ad2-N6)] and thiolate derivative [LZnSPh], respectively. In contrast, oxidation reaction selectively occurred at the pendant phosphine of ligand backbone when reacting [L2Zn2] with elemental sulfur, whereas Zn(I) − Zn(I) bond remained intact.

Base metal complexes featuring a new pyrazole-derived PCP pincer ligand.

A new pyrazole-derived PCP pincer ligand featuring a 1-methylpyrazole backbone tethered to two di(isopropyl)phosphine moieties via phenylene spacers (P(CH)P-iPr) was prepared. When reacting the ligand with group six carbonyl complexes [M(CO)6] (M = Cr, Mo, W) at 130 °C, complexes of the type [M(κ2PN-PCP-iPr)(CO)4] were obtained featuring a κ2P,N-bound ligand with a pendant phosphine arm. Upon an increase of the reaction temperature to 150 °C, in the case of molybdenum, the formation of the complex [Mo(κ3PCP-PCP-iPr)(CO)3] was observed featuring a weak Mo-C bond. DFT calculations reveal that there is no agostic η2-C-H interaction. Treatment of [Mn2(CO)10], [Fe2(CO)9], [Co2(CO)8] and [Ni(COD)2] afforded complexes [Mn(κ3PCP-PCP-iPr)(CO)3], [Fe(κ3PCP-PCP-iPr)(H)(CO)2], [Co(κ3PCP-PCP-iPr)(CO)2] and [Ni(κ3PCP-PCP-iPr)(H)], respectively, where the PCP ligand is coordinated in the typical meridional κ3-fashion. Postfunctionalization of the anionic PCP pincer ligand was possible via N-methylation of the second nitrogen atom of the pyrazole unit with the oxonium salt [Me3O]BF4. Treatment of [Mn(κ3PCP-PCP-iPr)(CO)3] and [Fe(κ3PCP-PCP-iPr)(H)(CO)2] with [Me3O]BF4 resulted in the formation of the cationic complexes [Mn(κ3PCP-PCPMe-iPr)(CO)3]+ and [Fe(κ3PCP-PCPMe-iPr)(Cl)(CO)2]+. In the case of the latter, the chloride ligand seems to originate from the solvent CH2Cl2 undergoing a hydride chloride exchange. All complexes were characterized by means of 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, IR spectroscopy and HR-MS. In addition, the structures of representative complexes were determined by X-ray crystallography.

Comparing the Electronic Structure of Iron, Cobalt, and Nickel Compounds That Feature a Phosphine-Substituted Bis(imino)pyridine Chelate

It was recently discovered that (Ph2PPrPDI)Mn (PDI = pyridine diimine) exists as a superposition of low-spin Mn(II) that is supported by a PDI dianion and intermediate-spin Mn(II) that is antiferromagnetically coupled to a triplet PDI dianion, a finding that encouraged the synthesis and electronic structure evaluation of late first row metal variants that feature the same chelate. The addition of Ph2PPrPDI to FeBr2 resulted in bromide dissociation and the formation of [(Ph2PPrPDI)FeBr][Br]. Reduction of this precursor using excess sodium amalgam afforded (Ph2PPrPDI)Fe, which possesses an Fe(II) center that is supported by a dianionic PDI ligand. Similarly, reduction of a premixed solution of Ph2PPrPDI and CoCl2 yielded the cobalt analog, (Ph2PPrPDI)Co. EPR spectroscopy and density functional theory calculations revealed that this compound features a high-spin Co(I) center that is antiferromagnetically coupled to a PDI radical anion. The addition of Ph2PPrPDI to Ni(COD)2 resulted in ligand displacement and the formation of (Ph2PPrPDI)Ni, which was found to possess a pendent phosphine group. Single-crystal X-ray diffraction, CASSCF calculations, and EPR spectroscopy indicate that (Ph2PPrPDI)Ni is best described as having a Ni(II)-PDI2- configuration. The electronic differences between these compounds are highlighted, and a computational analysis of Ph2PPrPDI denticity has revealed the thermodynamic penalties associated with phosphine dissociation from 5-coordinate (Ph2PPrPDI)Mn, (Ph2PPrPDI)Fe, and (Ph2PPrPDI)Co.

Accessing the main-group metal formyl scaffold through CO-activation in beryllium hydride complexes

Carbon monoxide (CO) is an indispensable C1 building block. For decades this abundant gas has been employed in hydroformylation and Pausen-Khand catalysis, amongst many related chemistries, where a single, non-coupled CO fragment is delivered to an organic molecule. Despite this, organometallic species which react with CO to yield C1 products remain rare, and are elusive for main group metal complexes. Here, we describe a range of amido-beryllium hydride complexes, and demonstrate their reactivity towards CO, in its mono-insertion into the Be-H bonds of these species. The small radius of the Be2+ ion in conjunction with the non-innocent pendant phosphine moiety of the developed ligands leads to a unique beryllium formyl complex with an ylidic P-COC fragment, whereby the carbon centre, remarkably, datively binds Be. This, alongside reactivity toward carbon dioxide, sheds light on the insertion chemistry of the Be-H bond, complimenting the long-known chemistry of the heavier Alkaline Earth hydrides.

Thermally stable zinc hydride catalyst for hydrosilylation of CO2 to silyl formate at atmospheric pressure.

Neutral zinc complexes supported by H[PNNO], a diaminophenolate ligand bearing a pendant phosphine group, were synthesized and characterized. The phosphine arm adopts two different configurations in solution and prevents aggregation. The monomeric zinc hydride complex is stable at elevated temperatures up to 125 °C and reacts readily with CO2 to afford a zinc formate complex. The zinc hydride is active for CO2 hydrosilylation at atmospheric CO2 pressure and is selective for CO2 reduction to the silyl-formate product.

Indium-Catalyzed CO2/Epoxide Copolymerization: Enhancing Reactivity with a Hemilabile Phosphine Donor.

Group 13 metal complexes have emerged as powerful catalysts for transforming CO2 into added-value products. However, direct comparisons of reactivity between Al, Ga, and In catalysts are rare. We report aluminum (1), gallium (2), and indium (3) complexes supported by a half-salen H[PNNO] ligand with a pendent phosphine donor and investigate their activity as catalysts for the copolymerization of CO2 and cyclohexene oxide. In solution, the P-donor is dissociated for the Al and Ga complexes while for the In complex it exhibits hemilabile behavior. The indium complex shows higher conversion and selectivity than the Al or Ga analogues. The mechanism of the reaction was studied by NMR and FTIR spectroscopy experiments as well as structural characterization of off-cycle catalytic intermediate indium trichloride complex [(PNNO)InCl3][TBA] (4). This study highlights the impact of a hemilabile phosphine group on group 13 metals and provides a detailed analysis of the initiation step in CO2/epoxide copolymerization reactions.

Nickel(0)-Induced β-H Elimination of Magnesium Alkyls: Formation and Reactivity of Heterometallic Hydrides.

We report the synthesis and reactivity of heterometallic Mg-Ni complexes with bridging hydrides. Treatment of magnesium monoalkyl complexes, which are supported by a tridentate β-diketiminato ligand bearing a pendent phosphine group, with nickel(0) reagent Ni(COD)2 (COD: 1,5-cyclooctadiene) at a molar ratio of 2:1 resulted in the formation of a heterotrimetallic hydride-bridged [Mg-Ni-Mg] complex via facile elimination of the corresponding alkenes. A heterobimetallic hydride-bridged [Mg-Ni] complex served as an intermediate species for the formation of the [Mg-Ni-Mg] complex. Computational studies revealed that the reaction was initiated by coordination of nickel to magnesium followed by an alkyl group transfer. β-H elimination at the nickel center subsequently occurred to give the heterometallic hydride-bridged complex. Density functional theory analysis also highlighted a three-center two-electron interaction for the Mg-H-Ni unit. The hydride-bridged [Mg-Ni-Mg] complex showed diverse reactivity toward unsaturated small molecules. For instance, reactions with isocyanides provided heterometallic species by coordination of isocyanides to the nickel center, with no subsequent reduction detected. Isocyanides could also be dissociated at 80 °C. In contrast, hydromagnesiation occurred upon treatment of the heterotrimetallic hydride with carbodiimides, affording C3-symmetric complexes with three heteroleptic magnesium mixed β-diketiminate/amidinate moieties. The hydride-bridged heterotrimetallic complex underwent dehydrogenation reaction with phenyl acetylene to produce an acetylide-bridged [Mg-Ni-Mg] complex.

Lemon twist catalysis

Organic Chemistry The limonene in citrus peels is a readily available and environmentally friendly chiral compound. Nonetheless, it lacks the functionality for direct use in most types of asymmetric catalysis. Nagata et al. report that when used as a solvent, limonene can bias the twist sense of an otherwise achiral polymer through subtle collective interactions. This polymer can then act as an asymmetric ligand, binding palladium at pendant phosphines to catalyze formation of binaphthyl compounds via Suzuki coupling in 98% enantiomeric excess. Asymmetric hydrosilylation was also demonstrated. ACS Cent. Sci. 10.1021/acscentsci.9b00330 (2019).

Dual Catalysis of the Selective Polymerization of Biosourced Myrcene and Methyl Methacrylate Promoted by Salicylaldiminato Cobalt(II) Complexes with a Pendant Donor

Cobalt complexes (Co1–Co6) bis-chelated by phenoxy (4,6-di-tert-butyl-2-phenol, naphthalen-2-ol)-imine ligands containing pendant hard oxygen (Co1 and Co4), soft sulfur (Co2 and Co5), or phosphine donors (Co3 and Co6) are reported for dual catalysis of selective polymerization for biosourced myrcene and methyl methacrylate (MMA). X-ray analysis reveals that the complexes features a distorted octahedral coordination of the cobalt center binding with phenolate oxygen, imine nitrogen, and pendant donor (P for Co3 and S for Co5). Activated by MMAO or AliBu3, all precursors are able to not only convert myrcene to cis-1,4-polymyrence but also polymerize MMA resulting in poly(methyl methacrylate) (PMMA) with moderate syndiotacticity. The introduction of pendant donor concurrently improves activity and stability particularly at 60–80 °C for polymerizations of both monomers. The enhanced activity and selectivity observed in myrcene cis-1,4 polymerization promoted by the sulfur and oxygen containing catalysts may relate to the weak Co–X (X = O, S) interaction, facilitating myrcene coordination. However, the donor effect on MMA polymerization activity is reversed, with phosphine donated catalyst demonstrating remarkably enhanced activity. This study represents the first example of cobalt catalyzed polymerization of both polar and nonpolar functions via possible mechanisms differentiated by pendant donor and also may provide valuable insights into their essential role in mediating the polymerization process.

Three Isomers in Equilibrium: Phosphine Exchange of Coordinated and Pendant Phosphines in a Unique Complex, (OC)5WL (L = 1,2-Bis(diphenylphosphino)-1-di-p-tolylphosphinoethane), and Implications for Understanding the k2 Term in the Rate Law for Phosphine Substitution in Group 6 Metal Carbonyl Complexes

The linkage isomers [(OC)5W{κ1-PPh2CH2CH(PPh2)[P(p-tol)2]}] 3a, [(OC)5W{κ1-P(p-tol)2CH(PPh2)CH2PPh2}] 3b, and [(OC)5W{κ1-PPh2CH[P(p-tol)2]CH2PPh2}] 3c isomerize at ambient temperatures to give a mixture of the three isomers in equilibrium. Rates of isomerization and equilibrium constants have been measured at temperatures from 10 to 55 °C, and from these, enthalpies and entropies have been determined. The isomerization reactions are initiated by a nucleophilic attack of a pendant phosphine on a bound carbonyl group followed by dissociation of a coordinated phosphine. These studies lead us to suggest that the k2 term in the rate law for CO replacement by phosphines is based on a similar mechanism and offers a plausible clarification of the currently accepted understanding. The crystal structure of 3b, the most stable of the three isomers, has been determined.

Synthesis of Heteroleptic Phosphorus(I) Cations by P+ Transfer.

Reported are general synthetic approaches for the syntheses of asymmetrically substituted phosphorus(I) cations by P+ transfer from [dppeP]+ (dppe = 1,2-bis(diphenylphosphino)ethane). The first method grants access to acyclic derivatives and is accomplished by the sequential substitution of dppe using first a sterically encumbered ligand which cannot form a stable homoleptic complex, followed by a second equivalent of a less sterically demanding ligand. The second method grants access to cyclic derivatives and utilizes asymmetric hybrid phosphine/N-heterocyclic carbene ligands. Interplay between the different ligand types and their stoichiometries relative to those of [dppeP]+ also allows for the isolation of symmetrical derivatives with pendant phosphines.

Dual reactivity pattern of Mn(I) carbyne complexes Cp(CO)2Mn+≡C–R (R = Ar, Alk) vs. dppm: Subtle balance between double intramolecular nucleophilic addition and nucleophilic addition followed by migratory CO insertion

The Mn(I) cationic carbyne complexes [Cp(CO)2Mn+≡C–R]BX4 (R = Ph, Tol, Me, Bn; X = F, Cl, Ph) react with bis(diphenylphosphino)methane to afford selectively either cyclic λ5-1,3-diphosphetium semi-ylides [RC–P+Ph2CH2P+Ph2)]BPh4 ([R4]BPh4; R = Ph, Tol), or Mn(I) complexes [Cp(CO)Mn(η3-(P,C,C)-Ph2PCH2P+Ph2C(R)=C=O)]BX4 ([R5]BX4; R = Me, X = Ph; R = Bn, X = F) featuring unprecedented phosphonioketene ligands depending on the nature of the carbyne substituent, aryl or alkyl, respectively. The selective formation of these products is tentatively rationalized through the key formation of a transient cationic Mn(I) phosphoniocarbene complexes [Cp(CO)2Mn=C(R)PPh2+CH2PPh2]BX4 bearing a pendent phosphine moiety. The molecular structures of semi-ylide [Ph4]BPh4 and complex [Bn5]BF4 were elucidated by single-crystal X-ray diffraction.

Nickel-Catalyzed Suzuki Cross Couplings with Unprotected Allylic Alcohols Enabled by Bidentate N-Heterocyclic Carbene (NHC)/Phosphine Ligands

Cross couplings between simple allylic alcohols and aryl and vinyl boronic acids are efficiently catalyzed by nickel(0) catalysts and bidentate N-heterocyclic carbene/phosphine ligands. The bidentate nature of the ligand is shown to extend catalyst lifetime and enable high yields of the corresponding cross coupling products. X-ray crystallography confirms the bidentate nature of the ligand scaffold. Multistep cross coupling-alkene/alkyne insertions reactions are also conducted and the bidentate nature of the substrate makes the pendant phosphine of the ligand unnecessary.

Phosphine functionalized polyphosphazenes: soluble and re-usable polymeric reagents for highly efficient halogenations under Appel conditions

In this paper we present the preparation and application of a novel soluble phosphine functionalized polyphosphazene (poly[3-(diphenylphosphino)propylamino]phosphazene) and investigate its application as a polymeric reagent. Upon chlorination of the pendant phosphine groups, the polymer was found to facilitate the rapid and efficient transformation of alcohols to the corresponding chlorides and bromides under Appel-type conditions. Reaction times followed by 31P NMR spectroscopy are shown to be rapid (several minutes) and the yields for the transformation of alcohols to the corresponding halides are in the range 80–99 %. The facile recovery of the oxidized polymeric agent by precipitation is also described, offering a significant advantage over notoriously difficult to remove small molecule phosphine oxide by-products. Furthermore the regeneration of the reactive phosphine chloride pendant groups is demonstrated, which could be efficiently re-used in a further chlorination reaction.

Modulation of Proton-Coupled Electron Transfer through Molybdenum-Quinonoid Interactions.

An expanded series of π-bound molybdenum-quinonoid complexes supported by pendant phosphines has been synthesized. These compounds formally span three protonation-oxidation states of the quinonoid fragment (catechol, semiquinone, quinone) and two different oxidation states of the metal (Mo(0), Mo(II)), notably demonstrating a total of two protons and four electrons accessible in the system. Previously, the reduced Mo(0)-catechol complex 1 and its reaction with dioxygen to yield the two-proton/two-electron oxidized Mo(0)-quinone compound 4 was explored, while, herein, the expansion of the series to include the two-electron oxidized Mo(II)-catechol complex 2, the one-proton/two-electron oxidized Mo-semiquinone complex 3, and the two-proton/four-electron oxidized Mo(II)-quinone complexes 5 and 6 is reported. Transfer of multiple equivalents of protons and electrons from the Mo(0) and Mo(II) catechol complexes, 1 and 2, to H atom acceptor TEMPO suggests the presence of weak O-H bonds. Although thermochemical analyses are hindered by the irreversibility of the electrochemistry of the present compounds, the reactivity observed suggests weaker O-H bonds compared to the free catechol, indicating that proton-coupled electron transfer can be facilitated significantly by the π-bound metal center.