PNP Pincer Complexes Research Articles

Computational Insight into the Mechanism of Selective Imine Formation from Alcohol and Amine Catalyzed by the Ruthenium(II)‐PNP Pincer Complex

AbstractWe have used density functional theory computations to investigate the mechanism of the reaction of an amine with a primary alcohol catalyzed by the ruthenium(II)‐PNP pincer complex [PNP = 2,6‐bis(di‐tert‐butylphosphanylmethyl)pyridine]; the reaction produces an imine as the major product. The catalytic cycle includes four stages: (stage I) alcohol dehydrogenation to aldehyde, (stage II) coupling of aldehyde with amine to form hemiaminal, (stage III) hemiaminal dehydration to give imine, and (stage IV) catalyst regeneration by means of H2 elimination of the trans ruthenium dihydride complex produced in stage I. The mechanism is similar to that for amide formation from amine and alcohol that was catalyzed by the RuII‐PNN pincer complex [PNN = 2‐(di‐tert‐butylphosphanylmethyl)‐6‐(diethylaminomethyl)pyridine], the only difference being in stage III. Alcohol dehydrogenation (stage I) occurs by a bifunctional double hydrogen transfer mechanism and alcohol can facilitate stages II and III. The selectivity of imine over ester is governed by stage II: the formation of hemiaminal by means of aldehydeamine coupling is kinetically much more favorable than the alternative aldehydealcohol coupling reaction that yields hemiacetal. Furthermore, the hemiaminal dehydration to give an imine is also kinetically more favorable than the hemiacetal dehydrogenation to give an ester. The selectivity of imine over amide is determined by stage III: the hemiaminal dehydration to give an imine is kinetically much more favorable than the hemiaminal dehydrogenation to give an amide. The essential difference between the RuII‐PNP‐catalyzed imine synthesis and the RuII‐PNN‐catalyzed amide formation is that the former prefers hemiaminal dehydration whereas the latter prefers hemiaminal dehydrogenation. In addition, water produced during hemiaminal dehydration can catalyze stages II and III more effectively than alcohol can. By contrast, the water‐catalyzed hemiaminal formation does not happen in the RuII‐PNN‐catalyzed synthesis of an amide because no water is produced in any stage of the reaction.

Bimetallic Ruthenium PNP Pincer Complex As a Platform to Model Proposed Intermediates in Dinitrogen Reduction to Ammonia

A series of ruthenium complexes was isolated and characterized in the course of reactions aimed at studying the reduction of hydrazine to ammonia in bimetallic systems. The diruthenium complex {[HPNPRu(N(2))](2)(μ-Cl)(2)}(BF(4))(2) (2) (HPNP = HN(CH(2)CH(2)P(i)Pr(2))(2)) reacted with 1 equiv of hydrazine to generate [(HPNPRu)(2)(μ(2)-H(2)NNH(2))(μ-Cl)(2)](BF(4))(2) (3) and with an excess of the reagent to form [HPNPRu(NH(3))(κ(2)-N(2)H(4))](BF(4))Cl (5). When phenylhydrazine was added to 2, the diazene species [(HPNPRu)(2)(μ(2)-HNNPh)(μ-Cl)(2)](BF(4))(2) (4) was obtained. Treatment of 2 with H(2) or CO yielded {[HPNPRu(H(2))](2)(μ-Cl)(2)}(BF(4))(2) (7) and [HPNPRuCl(CO)(2)]BF(4) (8), respectively. Abstraction of chloride using AgOSO(2)CF(3) or AgBPh(4) afforded the species [(HPNPRu)(2)(μ(2)-OSO(2)CF(3))(μ-Cl)(2)]OSO(2)CF(3) (9) and [(HPNPRu)(2)(μ-Cl)(3)]BPh(4) (10), respectively. Complex 3 reacted with HCl/H(2)O or HCl/Et(2)O to produce ammonia stoichiometrically; the complex catalytically disproportionates hydrazine to generate ammonia.

A New Mode of Activation of CO2 by Metal–Ligand Cooperation with Reversible CC and MO Bond Formation at Ambient Temperature

Team work: Although CO(2) binding to metal centers usually involves π coordination to a C=O group or σ bonds to the carbon or oxygen atom of the CO(2) molecule, a new mode of metal-ligand cooperative activation of CO(2) to a ruthenium PNP pincer complex involving aromatization/dearomatization steps is presented in experimental and theoretical studies (see scheme).

Unexpected Direct Reduction Mechanism for Hydrogenation of Ketones Catalyzed by Iron PNP Pincer Complexes

The hydrogenation of ketones catalyzed by 2,6-bis(diisopropylphosphinomethyl)pyridine (PNP)-ligated iron pincer complexes was studied using the range-separated and dispersion-corrected ωB97X-D functional in conjunction with the all-electron 6-31++G(d,p) basis set. A validated structural model in which the experimental isopropyl groups were replaced with methyl groups was employed for the computational study. Using this simplified model, the calculated total free energy barrier of a previously postulated mechanism with the insertion of ketone into the Fe-H bond is far too high to account for the observed catalytic reaction. Calculation results reveal that the solvent alcohol is not only a stabilizer of the dearomatized intermediate but also more importantly an assistant catalyst for the formation of trans-(PNP)Fe(H)(2)(CO), the actual catalyst for hydrogenation of ketones. A direct reduction mechanism, which features the solvent-assisted formation of a trans dihydride complex trans-(PNP)Fe(H)(2)(CO), direct transfer of hydride to acetophenone from trans-(PNP)Fe(H)(2)(CO) for the formation of a hydrido alkoxo complex, and direct H(2) cleavage by hydrido alkoxo without the participation of the pincer ligand for the regeneration of trans-(PNP)Fe(H)(2)(CO), was predicted.

Cationic iridium complexes of the Xantphos ligand. Flexible coordination modes and the isolation of the hydride insertion product with an alkene

Reaction of the Ir(I)–Xantphos complex [Ir(κ 2–Xantphos)(COD)][BAr F 4] (Xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, Ar F = C 6H 3(CF 3) 2) with H 2 in acetone or CH 2Cl 2/MeCN affords the Ir(III)–hydrido complexes [Ir(κ 3–Xantphos)(H) 2(L)][BAr F 4], L = acetone or MeCN, whereas in non-coordinating CH 2Cl 2 solvent dimeric [Ir(κ 3–Xantphos)(H)(μ-H)] 2[BAr F 4] 2 is formed. A common intermediate in these reactions that invokes a (σ, η 2-C 8H 13) ligand is reported. Addition of excess tert-butylethene (tbe) to [Ir(κ 3–Xantphos)(H) 2(MeCN)][BAr F 4] results in insertion of a hydride into the alkene to form [Ir(κ 3–Xantphos)(MeCN)(CH 2CH 2C(CH 3) 3)(H)][BAr F 4], an Ir(III) alkyl–hydrido-Xantphos complex. This reaction is reversible, and heating (80 °C) results in the reformation of [Ir(κ 3–Xantphos)(H) 2(MeCN)][BAr F 4] and tbe. These complexes have been characterised by NMR spectroscopy, ESI-MS and single-crystal X-ray diffraction. They show variable coordination modes of the Xantphos ligand: cis-κ 2– P, P, fac–κ 3– P, O, P and mer-κ 3– P, O, P with the later coordination mode like that found in related PNP–pincer complexes.

Discovery of Environmentally Benign Catalytic Reactions of Alcohols Catalyzed by Pyridine-Based Pincer Ru Complexes, Based on Metal–Ligand Cooperation

We have developed a new mode of metal–ligand cooperation, based on aromatization–dearomatization of pyridine-based pincer-type ligands, and have demonstrated it in the activation of H2 and C–H bonds by PNP Ir complexes. This type of metal–ligand cooperation plays a key role in the recently discovered environmentally benign reactions of alcohols, catalyzed by PNP and PNN pincer complexes of ruthenium, including (a) dehydrogenation of secondary alcohols to ketones (b) dehydrogenative coupling of primary alcohols to form esters and H2 (c) hydrogenation of esters to alcohols under mild conditions (d) unprecedented amide synthesis: catalytic coupling of amines with alcohols, with liberation of H2. Ligand modification in the latter reaction has led to unprecedented synthesis of imines with H2 liberation. These reactions are very efficient, proceed under neutral conditions and produce no waste.

Kinetically Controlled Formation of Octahedral trans-Dicarbonyl Iron(II) PNP Pincer Complexes: The Decisive Role of Spin-State Changes

Treatment of either cis-[Fe(PNP)(X2)(CO)], trans-[Fe(PNP)(X2)(CO)], or [Fe(PNP)X2] (X = Cl, Br; PNP are tridentate pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine) with 1 equiv of AgBF4 in the presence of CO afforded selectively octahedral iron(II) complexes of the type trans-[Fe(PNP)(CO)2X]+. The same reaction carried out with trans-[Fe(PNP-iPr)(Cl)2(CO)] in the absence of CO affords also trans-[Fe(PNP-iPr)(CO)2Cl]+ together with unidentified paramagnetic species. This reaction involves an intermolecular CO transfer between coordinately unsaturated [Fe(PNP-iPr)(CO)(Cl)]+ intermediates. In all reactions studied, there was no evidence for the formation of cis dicarbonyl complexes. X-ray structures of representative complexes are presented. A detailed mechanism, based on DFT/B3LYP calculations, is presented, suggesting that upon irreversible removal of X− transient cationic intermediates [Fe(PNP)(CO)(X)]+ of two conformations, one with the CO in the apical and the halide in the ba...

Direct Synthesis of Imines from Alcohols and Amines with Liberation of H2

A clean sweep: Aryl and aliphatic imines can be synthesized directly and efficiently from alcohols and amines under mild, neutral conditions with the liberation of only molecular hydrogen and water (see scheme; R=isopropyl, tert-butyl). This general, environmentally benign reaction is catalyzed by a de-aromatized ruthenium PNP pincer complex (0.2 mol %), and can proceed in toluene under an inert atmosphere or under air.

Striking Differences between the Solution and Solid-State Reactivity of Iron PNP Pincer Complexes with Carbon Monoxide

Several new iron(II) complexes of the types [Fe(PNP)X2] (X = Cl, Br) containing tridentate PNP pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine have been prepared. They all exhibit intermolecular Fe−X···H−N hydrogen bonds, forming supramolecular networks in the solid state. In the case of X = Cl these compounds react readily with gaseous CO both in the solid state and in solution to give selectively the octahedral complexes cis- and trans-[Fe(PNP)(CO)(Cl)2], respectively, whereas with X = Br mixtures of cis and trans isomers are obtained. These transformations are accompanied by color and spin-state changes. CO binding is fully reversible in all cases, and heating solid samples of either cis- or trans-[Fe(PNP)(CO)(X)2] leads to complete regeneration of analytically pure [Fe(PNP)(X)2]. Mössbauer spectroscopy confirmed the high-spin nature of the parent five-coordinate Fe(II) complex (δ = 0.80(1) mm s−1) and the shift to two different low-spin octahedral species after reaction with CO in the solid (δ = 0.13(1) mm s−1) or in solution (δ = 0.15(1) mm s−1). Magnetization studies led to a magnetic moment close to 4.9 μB, reflecting the expected four unpaired d-electrons in [Fe(PNP)Cl2], which were confirmed by DFT calculations. The DFT study of the reaction pathway for CO capture led to low energy barriers (≤3.4 kcal mol−1). The cis−trans isomerization reaction was found to take place with a low energy barrier (10.8 kcal mol−1), after initial loss of chloride, and involves also spin-state changes

Facile Double C−H Activation of Tetrahydrofuran by an Iridium PNP Pincer Complex

The reaction of [IrCl(COE)2]2 (COE = cyclooctene) with bulky PNP amino pincer ligand HN(CH2CH2PtBu2)2, (HPNP)tBu, results in high-yield formation of iridium(III) complex [IrH(C8H13)Cl(HPNP)tBu] after oxidative addition of a vinylic COE C−H bond. Upon dissolving the product in polar solvents or chloride abstraction with TlPF6 the cationic iridium(I) complex [Ir(COE)(HPNP)tBu]+ is obtained, which is thermally unstable. This observation shows that the olefin isomer represents the thermodynamic minimum for the cationic complex, while the cyclooctenyl hydride isomer is trapped in nonpolar solvents by anion coordination. Owing to the lability of the olefin ligand, [Ir(COE)(HPNP)tBu]+ undergoes facile intermolecular C−H activation. Fischer-carbene complex [Ir(H)2(═CO(CH2)3)(HPNP)tBu]BPh4 was isolated in good yield upon α,α-dehydrogenation of THF. Mechanistic examinations suggest olefin dissociation to be rate-determining.

Palladium N(CH2CH2PiPr2)2-Dialkylamides: Synthesis, Structural Characterization, and Reactivity

Palladium(II) aminodiphosphine PNP pincer complexes [PdR(PNP(H))]PF(6) (1(R); R = Cl Me, Ph; PNP(H) = HN(CH(2)CH(2)P(i)Pr(2))(2)) were prepared. Deprotonation with KO(t)Bu affords dialkylamides [PdR(PNP)] (2(R); R = Cl Me, Ph; PNP = (NCH(2)CH(2)P(i)Pr(2))(2)) in high yield which are stable toward beta-H elimination. While AgPF(6) oxidizes the amides, cationic amido complexes [PdL(PNP)]PF(6) (3(L); L = CN(t)Bu, PMe(3)) were obtained upon chloride abstraction from 1(Cl) with TlPF(6). The reaction of amide 2(Cl) with MeOTf results in N-methylation yielding [PdCl(PNP(Me))]OTf (5) quantitatively. N-H acidities of the amino complexes 1(Me) (pK(a) = 24.2(1)) and 1(Ph) (pK(a) = 23.2(1)) were determined in dmso. Complexes 1(Cl), 1(Me), 2(Cl) 2(Me), 3(CNtBu), and 5 were structurally characterized by single crystal X-ray diffraction. The amido complexes feature pyramidal nitrogen atoms in the solid state. The molecular structures, high N-basicity, and reactivity of the amido complexes can be explained with Pd-N(amido) bonding that is characterized by strong N-->Pd sigma-donation and repulsive d(pi)-p(pi) pi-interactions. This interpretation was confirmed by density functional theory (DFT) calculations of 2(Cl).

Stereospecific and Reversible CO Binding at Iron Pincer Complexes

An opening for CO: An iron PNP pincer complex self-assembles in the solid state through intermolecular FeCl⋅⋅⋅HN hydrogen bonds to form a 3D supramolecular network. This compound reacts readily with gaseous CO in the solid state and in solution to stereospecifically give the octahedral complexes cis- and trans-[Fe(PNP-iPr)(CO)(Cl)2], respectively. In the solid-state process the supramolecular connectivities between individual molecules are maintained without loss of crystallinity.

Unusual Structural and Spectroscopic Features of Some PNP-Pincer Complexes of Iron

We report a structural, spectroscopic, and computational study of two tBuPNP (2,6-bis(di-tert-butyl-phosphinomethyl)pyridine) complexes of iron, (tBuPNP)FeCl2 (1) and (tBuPNP)Fe(CO)2 (3). Complex 1, (tBuPNP)FeCl2, also independently synthesized by Milstein, has unusually long iron−ligand bond distances. DFT calculations show that these are clearly attributable to its high-spin electronic structure, and in particular to occupancy of the strongly antibonding dx2−y2 orbital. The crystal structure of 3 reveals two unusual aspects. (1) The geometry around the iron atom in 3 is much closer to square pyramidal (SQP; apical CO) than to trigonal bipyramidal (TBP), although five-coordinate Fe(0) complexes are generally expected to be TBP; moreover, Chirik et al. have reported that (iPrPNP)Fe(CO)2 has essentially a perfect TBP structure (iPrPNP = 2,6-bis(di-isopropyl-phosphinomethyl)pyridine). (2) The apical carbonyl ligand in 3 deviates significantly from linearity (Fe−C−O = 171.9°). Additionally, complex 3 is inte...

ChemInform Abstract: Modularly Designed Transition Metal PNP and PCP Pincer Complexes Based on Aminophosphines: Synthesis and Catalytic Applications

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Modularly Designed Transition Metal PNP and PCP Pincer Complexes based on Aminophosphines: Synthesis and Catalytic Applications

Transition metal complexes are indispensable tools for any synthetic chemist. Ideally, any metal-mediated process should be fast, clean, efficient, and selective and take place in a catalytic manner. These criteria are especially important considering that many of the transition metals employed in catalysis are rare and expensive. One of the ways of modifying and controlling the properties of transition metal complexes is the use of appropriate ligand systems, such as pincer ligands. Usually consisting of a central aromatic backbone tethered to two two-electron donor groups by different spacers, this class of tridentate ligands have found numerous applications in various areas of chemistry, including catalysis, due to their combination of stability, activity, and variability. As we focused on pincer ligands featuring phosphines as donor groups, the lack of a general method for the preparation of both neutral (PNP) and anionic (PCP) pincer ligands using similar precursor compounds as well as the difficulty of introducing chirality into the structure of pincer ligands prompted us to investigate the use of amines as spacers between the aromatic ring and the phosphines. By introduction of aminophosphine and phosphoramidite moieties into their structure, the synthesis of both PNP and PCP ligands can be achieved via condensation reactions between aromatic diamines and electrophilic chlorophosphines (or chlorophosphites). Moreover, chiral pincer complexes can be easily obtained by using building blocks obtained from the chiral pool. Thus, we have developed a modular synthetic strategy with which the steric, electronic, and stereochemical properties of the ligands can be varied systematically. With the ligands in hand, we studied their reactivity towards different transition metal precursors, such as molybdenum, ruthenium, iron, nickel, palladium, and platinum. This has resulted in the preparation of a range of new pincer complexes, including various iron complexes, as well as the first heptacoordinated molybdenum pincer complexes and several pentacoordinated nickel complexes by using a controlled ligand decomposition pathway. In addition, we have investigated the use of some of the complexes as catalysts in different C-C coupling reactions: for example, the palladium PNP and PCP pincer complexes can be employed as catalysts in the well known Suzuki-Miyaura coupling, while the iron PNP complexes catalyze the coupling of aromatic aldehydes with ethyl diazoacetate under very mild reaction conditions to give selectively 3-hydroxyacrylates, which are otherwise difficult to prepare. While this Account presents an overview of current research on the chemistry of P-N bond containing pincer ligands and complexes, we believe that further investigations will give deeper insights into the reactivity and applicability of aminophosphine-based pincer complexes.

Selective Phosphoramidite Cleavage as a Route to Novel Chiral and Achiral Pentacoordinated Nickel(II) PNP Pincer Complexes

AbstractTreatment of NiBr2(DME) (DME = 1,2‐dimethoxyethane) or anhydrous NiBr2 with 2 equiv. of PNP pincer ligands featuring phosphoramidites in CH2Cl2 yields the novel neutral pentacoordinate complexes [Ni(PNP){κ1(P)‐R2P=O}Br]. Over the course of this reaction the P–N bonds of the phosphoramidite units of one PNP ligand are selectively cleaved due to hydrolysis affording an anionic κ1‐(P)‐coordinated phosphinite ligand [R2P=O]–, while a second PNP ligand remains intact and is coordinated in a κ3‐(P,N,P) fashion. The X‐ray structure of one [Ni(PNP){κ1(P)‐PR2=O}Br] complex has been determined.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Retardation of β-hydrogen elimination in PNP Pincer complexes of Pd

A series of new alkyl and alkoxide ( FPNP)Pd complexes have been synthesized. The alkyls and alkoxides containing β-hydrogens display remarkable thermal stability. Thermal decomposition of ( FPNP)PdOEt is very slow in pure C 6D 6 but is accelerated by the addition of EtOH co-solvent. It is proposed that the β-hydrogen elimination from ( FPNP)Pd–OCH 2R occurs via dissociation of the alkoxide anion.