PNP Pincer Complexes Research Articles

Benyzl Alcohol Dehydrogenative Coupling Catalyzed by Defined Mn and Re PNP Pincer Complexes – A Computational Mechanistic Study

Density functional theory computations have been carried out to investigate the base free acceptor‐less dehydrogenative coupling of benzyl alcohol to benzyl benzoate catalyzed by defined Mn and Re PNP pincer amido M(CO)2(PNP) and amino HM(CO)2(PNHP) complexes [M = Mn, Re; PNP = N{CH2CH2P(isopropyl)2}2]. Benchmark calculations show that B3PW91 have the best and closest agreement with the experiments than DLPNO‐CCSD(T). Within all proposed elementary steps, the non‐innocent outer‐sphere mechanism incorporating the amido complex and the N–H bond is more kinetically favored than the innocent outer‐sphere mechanism without the amido complex as well as the inner‐sphere mechanism via the de‐coordination of one phosphine ligand to create a vacant site for β‐hydride elimination. The dehydrogenation of hemiacetal to ester represents the rate‐determining step. Both Mn and Re‐based catalysts have close free energy barriers and similar catalytic activity. However, the computed apparent barrier is overestimated on the basis of the experimentally determined TOF for the Mn‐based complexes.

Synthesis and characterization of bis- and tris-carbonyl Mn(I) and Re(I) PNP pincer complexes

A series of neutral bis- and cationic tris-carbonyl complexes of the types cis-[M(κ3P,N,P-PNP)(CO)2Y] and [M(κ3P,N,P-PNP)(CO)3]+ was prepared by reacting [M(CO)5Y] (M = Mn, Re; Y = Cl or Br) with PNP pincer ligands derived from the 2,6-diaminopyridine, 2,6-dihydroxypyridine, and 2,6-lutidine scaffolds. With the most bulky ligand PNPNH-tBu, the cationic square-pyramidal 16e bis-carbonyl complex [Mn(PNPNH-tBu)(CO)2]+ was obtained. In contrast, in the case of rhenium, the 18e complex [Re(PNPNH-tBu)(CO)3]+ was formed. The dissociation of CO was studied by means of DFT calculation revealing in agreement with experimental findings that CO release from [M(κ3P,N,P-PNP)(CO)3]+ is in general endergonic, while for [Mn(κ3P,N,P-PNPNH-tBu)(CO)3]+, this process is thermodynamically favored. X-ray structures of representative complexes are provided.Graphical abstract

Chemoselective transfer hydrogenation of aldehydes in aqueous media catalyzed by a well-defined iron(II) hydride complex

An iron(II) hydride PNP pincer complex is applied as catalyst for the chemoselective transfer hydrogenation of aldehydes using an aqueous solution of sodium formate as hydrogen source. A variety of aromatic, heteroaromatic, and aliphatic aldehydes could be reduced to the corresponding alcohols in good to excellent yields with a catalyst loading of 1.0 mol% at 80 °C and 1 h reaction time. If present, C–C double bonds remained unaffected in course of the reaction, even when they are conjugated to the carbonyl group of the aldehyde. The catalyst’s lifetime and activity could be improved when the reactions were conducted in an ionic liquid-based micro emulsion.Graphical abstract

Selective Hydrogenation of Aldehydes Using a Well-Defined Fe(II) PNP Pincer Complex in Biphasic Medium.

A biphasic process for the hydrogenation of aldehydes was developed using a well‐defined iron (II) PNP pincer complex as model system to investigate the performance of various ionic liquids. A number of suitable hydrophobic ionic liquids based on the N(Tf)2 − anion were identified, allowing to immobilize the iron (II) catalyst in the ionic liquid layer and to facilitate the separation of the desired alcohols. Further studies showed that targeted Brønsted basic ionic liquids can eliminate the need of an external base to activate the catalyst.

Roles of Hydrogen Bonding in Proton Transfer to κP,κN,κP-N(CH2CH2P i Pr2)2-Ligated Nickel Pincer Complexes.

The nickel PNP pincer complex (iPrPNP)NiPh (iPrPNP = κP,κN,κP-N(CH2CH2PiPr2)2) was prepared by reacting (iPrPNP)NiBr with PhMgCl or deprotonating [(iPrPNHP)NiPh]Y (iPrPNHP = κP,κN,κP-HN(CH2CH2PiPr2)2; Y = Br, PF6) with KOtBu. The byproducts of the PhMgCl reaction were identified as [(iPrPNHP)NiPh]Br and (iPrPNP′)NiPh (iPrPNP′ = κP,κN,κP-N(CH=CHPiPr2)(CH2CH2PiPr2)). The methyl analog (iPrPNP)NiMe was synthesized from the reaction of (iPrPNP)NiBr with MeLi, although it was contaminated with (iPrPNP′)NiMe due to ligand oxidation. Protonation of (iPrPNP)NiX (X = Br, Ph, Me) with various acids, such as HCl, water, and MeOH, was studied in C6D6. Nitrogen protonation was shown to be the most favorable process, producing a cationic species [(iPrPNHP)NiX]+ with the NH moiety hydrogen-bonded to the conjugate base (i.e., Cl–, HO–, or MeO–). Protonation of the Ni–C bond was observed at room temperature with (iPrPNP)NiMe, whereas at 70 °C with (iPrPNP)NiPh, both resulting in [(iPrPNHP)NiCl]Cl as the final product. Protonation of (iPrPNP)NiBr was complicated by site exchange between Br– and the conjugate base and by the degradation of the pincer complexes. Indene, which lacks hydrogen-bonding capability, was unable to protonate (iPrPNP)NiPh and (iPrPNP)NiMe, despite being more acidic than water and MeOH. Neutral and cationic nickel pincer complexes involved in this study, including (iPrPNP′)NiBr, (iPrPNP)NiPh, (iPrPNP′)NiPh, (iPrPNP)NiMe, [(iPrPNHP)NiPh]Y (Y = Br, PF6, BPh4), [(iPrPNHP)NiPh]2[NiCl4], [(iPrPNHP)NiMe]Y (Y = Cl, Br, BPh4), [(iPrPNHP)NiBr]Br, and [(iPrPNHP)NiCl]Cl, were characterized by X-ray crystallography.

Chemoselective Hydrogenation of Aldehydes under Mild, Base-Free Conditions: Manganese Outperforms Rhenium.

Several hydride Mn(I) and Re(I) PNP pincer complexes were applied as catalysts for the homogeneous chemoselective hydrogenation of aldehydes. Among these, [Mn(PNP-iPr)(CO)2(H)] was found to be one of the most efficient base metal catalysts for this process and represents a rare example which permits the selective hydrogenation of aldehydes in the presence of ketones and other reducible functionalities, such as C=C double bonds, esters, or nitriles. The reaction proceeds at room temperature under base-free conditions with catalyst loadings between 0.1 and 0.05 mol% and a hydrogen pressure of 50 bar (reaching TONs of up to 2000). A mechanism which involves an outer-sphere hydride transfer and reversible PNP ligand deprotonation/protonation is proposed. Analogous isoelectronic and isostructural Re(I) complexes were only poorly active.

Synthesis and Characterization of Stable Gold(III) PNP Pincer Complexes

The first mononuclear AuIII PNP pincer complexes [PNP = bis(2‐diisopropylphosphinophenyl)amide] are reported. The chloro complex [(PNP)Au(Cl)][OAcF] (OAcF = OCOCF3) was synthesized by microwave irradiation of a tetrachloroaurate salt and the neutral PNHP ligand. Dehalogenation with AgOAcF afforded the trifluoroacetate‐bound complex [(PNP)Au(OAcF)][OAcF]. Both complexes were characterized by NMR spectroscopy and X‐ray crystallography. Electronic absorption spectroscopy and TD‐DFT studies assigned the electronic transition that imbues the complexes with a deep royal blue color. The AuIII trifluoroacetate complex is surprisingly stable, and no alkene insertion was observed under ethylene (C2H4), even under high pressures and at high temperatures. DFT calculations suggest that the lack of reactivity is due to the high energy of the putative dicationic C2H4‐bound intermediate invoked in a formal insertion reaction.

A DFT study on the mechanisms of hydrogenation and hydrosilylation of nitrous oxide catalyzed by a ruthenium PNP pincer complex

Abstract The mechanisms of hydrogenation and hydrosilylation of nitrous oxide catalyzed by a ruthenium PNP pincer complex l have been studied through quantum chemistry calculations at the M06 level of theory. It was found that the catalytic cycle of hydrogenation of nitrous oxide contains three steps, hydrogenation of the dearomatized PNP pincer complex 1, mono oxygen transfer and water release. In addition, the catalytic cycle of hydrosilylation of nitrous oxide also includes three steps, hydrosilylation of the dearomatized PNP pincer complex 1, mono oxygen transfer, dehydrogenative coupling of silanol and silane/direct Si O bond coupling. The calculations suggested that the water molecule play an important role to lower the barrier for N2 liberation. Frontier molecular orbital theory and NBO analysis indicated that nitrous oxide is activated by hydrido ligand via nucleophilical attacking. Present calculations are in good agreement with the experimental observations and can help to understand the activation of N2O.

Formation of Mono Oxo Molybdenum(IV) PNP Pincer Complexes: Interplay between Water and Molecular Oxygen.

The synthesis of cationic mono oxo MoIV PNP pincer complexes of the type [Mo(PNPMe‐iPr)(O)X]+ (X = I, Br) from [Mo(PNPMe‐iPr)(CO)X2] is described. These compounds are coordinatively unsaturated and feature a strong Mo≡O triple bond. The formation of these complexes proceeds via cationic 14e intermediates [Mo(PNPMe‐iPr)(CO)X]+ and requires both molecular oxygen and water. ESI MS measurements with 18O labeled water (H2 18O) and molecular oxygen (18O2) indicates that water plays a crucial role in the formation of the Mo≡O bond. A plausible mechanism based on DFT calculations is provided. The X‐ray structure of [Mo(PNPMe‐iPr)(O)I]SbF6 is presented.

Chemoselective Supported Ionic-Liquid-Phase (SILP) Aldehyde Hydrogenation Catalyzed by an Fe(II) PNP Pincer Complex

A base-tolerant supported ionic-liquid-phase (SILP) system containing a well-defined hydride Fe(II) PNP pincer complex has been prepared, structurally characterized, and used as catalyst in the hydrogenation of aldehydes to alcohols. The new SILP catalyst, with the optimum pore filling, was highly active exhibiting TONs and TOFs of up to 1000 and 4000 h–1, respectively, under mild conditions (25 °C, 10–50 bar H2 pressure) without significant leaching of both the complex and the IL.

Carbon-based SILP catalysis for the selective hydrogenation of aldehydes using a well-defined Fe(ii) PNP complex

In this work, the supported ionic liquid phase (SILP) method was applied for the immobilization of a newly developed, well-defined hydride Fe(ii) PNP pincer complex dissolved an in ionic liquid (IL) onto polymer-based spherical activated carbon.

Exploring the activities of vanadium, niobium, and tantalum PNP pincer complexes in the hydrogenation of phenyl-substituted CN, CN, CC, CC, and CO functional groups

The structures and stability of the designed PNP pincer amido M(NO) 2 (PNP) and amino H M(NO) 2 (PN H P) complexes [M = V, Nb, and Ta, PNP = N(CH 2 CH 2 P(isopropyl) 2 ) 2 , PN H P = HN(CH 2 CH 2 P(isopropyl) 2 ) 2 ] and their hydrogenation mechanisms for phenyl-substituted unsaturated functional groups have been explored at the B3PW91 level of density functional theory. Under H 2 environment, these conjugated complexes can form equilibrium and fulfill the criteria of metal–ligand cooperated bifunctional hydrogenation catalysts. For the hydrogenation of Ph-C N, Ph-CH NH, Ph-CH NH-Ph, Ph-CH N CH 2 Ph, Ph-C CH, Ph-CH CH 2 , Ph-CHO, and Ph-CO CH 3 , the reaction prefers either a two-step or one-step mechanism for the hydridic M H and protonic N H transfer. These results clearly show that the V, Nb, and Ta complexes are promising catalysts for the hydrogenation reactions, and these provide experimental challenges.

Cobalt‐Catalyzed Regioselective Hydroboration of Terminal Alkenes

Cobalt(II) catalysts based on flexible PNP or NNN ligands were explored for the regioselective hydroboration of alkenes. A known CoII–PNP pincer complex was found to efficiently catalyze alkene hydroboration with excellent anti‐Markovnikov selectivity, whereas a newly synthesized dinuclear CoII–NNN complex was found to catalyze the hydroboration of a range of aromatic terminal alkenes with good Markovnikov selectivities (up to 98:2, b/l). This represents a rare example of Markovnikov selectivity in the hydroboration of alkenes using an inexpensive, flexible‐ligand‐supported dinuclear cobalt catalyst.

Retracted: Catalytic activity of Fe(II) and Cu(II) PNP pincer complexes for Suzuki coupling reaction

Iron and copper PNP pincer complexes of the type [Fe(L)SO4] and [Cu(L)OCOCH3] are reported and represented as C‐1 and C‐2 catalyst. Both the complexes were synthesized using bis(diphenylphosphino)pyridine‐2,6‐diamine [L], and salts of ‘Fe’ and ‘Cu’ by direct coordination method. The as synthesized complexes were characterized using FTIR, UV–Vis, mass analysis and TGA. The effect of reaction time, catalyst load, solvent and base on the reaction between phenylboronic acid and para substituted bromobenzenes in the presence of the catalysts were investigated for evaluating the catalytic efficiency of the complexes. The results obtained highlight the enhanced C‐C coupling reactions with the use of 0.4 mol% of the catalyst C‐1 in 14 h and 0.6 mol% of C‐2 in 16 h respectively with Cs2CO3 base and ACN as solvent media. Of the two complexes reported, C‐1 with iron as catalytically active metal is more stable and active towards coupling which is reflected in its better coupling yields in lesser reaction time compared to copper bearing C‐2 complex.

Visible light-induced cis/trans isomerization of dicarbonyl Fe(II) PNP pincer complexes

The synthesis and characterization of dicarbonyl Fe(II) PNP pincer complexes of the type cis-[Fe(PNP-iPr)(CO)2(X)]+ (X=Br, Cl) is described. These complexes are slowly formed when solutions of complexes trans-[Fe(PNP-iPr)(CO)2(X)]+ are kept in the dark for 9h (X=Br) and 3days (X=Cl). Upon exposure to visible light these complexes isomerize to the respective trans-dicarbonyl complexes within a few hours. The visible-light reaction seems to involve reversible CO dissociation. The isomerization can be repeated serval times. A mechanistic rationale for this isomerization process is established by means of DFT calculations.

Manganese(I)-Catalyzed Enantioselective Hydrogenation of Ketones Using a Defined Chiral PNP Pincer Ligand.

A new chiral manganese PNP pincer complex is described. The asymmetric hydrogenation of several prochiral ketones with molecular hydrogen in the presence of this complex proceeds under mild conditions (30-40 °C, 4 h, 30 bar H2 ). Besides high catalytic activity for aromatic substrates, aliphatic ketones are hydrogenated with remarkable selectivity (e.r. up to 92:8). DFT calculations support an outer sphere hydrogenation mechanism as well as the experimentally determined stereochemistry.

Manganese(I)‐Catalyzed Enantioselective Hydrogenation of Ketones Using a Defined Chiral PNP Pincer Ligand

AbstractA new chiral manganese PNP pincer complex is described. The asymmetric hydrogenation of several prochiral ketones with molecular hydrogen in the presence of this complex proceeds under mild conditions (30–40 °C, 4 h, 30 bar H2). Besides high catalytic activity for aromatic substrates, aliphatic ketones are hydrogenated with remarkable selectivity (e.r. up to 92:8). DFT calculations support an outer sphere hydrogenation mechanism as well as the experimentally determined stereochemistry.

Manganese-Catalyzed Aminomethylation of Aromatic Compounds with Methanol as a Sustainable C1 Building Block.

This study represents the first example of a manganese-catalyzed environmentally benign, practical three-component aminomethylation of activated aromatic compounds including naphtols, phenols, pyridines, indoles, carbazoles, and thiophenes in combination with amines and MeOH as a C1 source. These reactions proceed with high atom efficiency via a sequence of dehydrogenation and condensation steps which give rise to selective C-C and C-N bond formations, thereby releasing hydrogen and water. A well-defined hydride Mn(I) PNP pincer complex, recently developed in our laboratory, catalyzes this process in a very efficient way, and a total of 28 different aminomethylated products were synthesized and isolated yields of up to 91%. In a preliminary study, a related Fe(II) PNP pincer complex was shown to catalyze the methylation of 2-naphtol rather than its aminomethylation displaying again the divergent behavior of isoelectronic Mn(I) and Fe(II) PNP pincer systems.

Manganese-Catalyzed Direct Deoxygenation of Primary Alcohols

Deoxygenation of alcohols is an important tool in the repertoire of defunctionalization methods in modern synthetic chemistry. We report the base-metal-catalyzed direct deoxygenation of benzylic and aliphatic primary alcohols via oxidative dehydrogenation/Wolff–Kishner reduction. The reaction is catalyzed by a well-defined PNP pincer complex of Earth-abundant manganese, evolving H2, N2, and water as the only byproducts.

Iridium–PNP Pincer Complexes for Methanol Dehydrogenation at Low Base Concentration

AbstractA catalytic system based on an iridium–PNP complex was developed for the aqueous‐phase reforming of methanol. Investigations revealed higher activity at low base concentrations and higher stability at high base concentrations as an opposing trend relative to that of known Ru and Fe PNP‐pincer‐containing systems for methanol dehydrogenation. During mechanistic investigations it was possible to identify a CO containing derivate of the deactivation species.