PNP Complexes Research Articles

The Mechanism of Markovnikov-Selective Epoxide Hydrogenolysis Catalyzed by Ruthenium PNN and PNP Pincer Complexes.

The homogeneous catalysis of epoxide hydrogenolysis to give alcohols has recently received significant attention. Catalyst systems have been developed for the selective formation of either the Markovnikov (branched) or anti-Markovnikov (linear) alcohol product. Thus far, the reported catalysts exhibiting Markovnikov selectivity all feature the potential for Noyori/Shvo-type bifunctional catalysis, with either a RuH/NH or FeH/OH core structure. The proposed mechanisms of epoxide ring-opening have involved cooperative C-O bond hydrogenolysis involving the metal hydride and the acidic pendant group on the ligand, in analogy to the well-documented mechanism of polar double-bond hydrogenation exhibited by catalysts of this type. In this work, we present a combined computational/experimental study of the mechanism of epoxide hydrogenolysis catalyzed by Noyori-type PNP and PNN complexes of ruthenium. We find that, at least for these ruthenium systems, the previously proposed bifunctional pathway for epoxide ring-opening is energetically inaccessible; instead, the ring-opening proceeds through opposite-side nucleophilic attack of the ruthenium hydride on the epoxide carbon, without the involvement of the ligand N-H group. For both catalyst systems, the rate law and overall barrier predicted by density functional theory (DFT) are consistent with the results from kinetic studies.

Base-free, acceptorless dehydrogenative coupling of ethanol to ethyl acetate with PNP complexes.

A base-free, acceptorless dehydrogenative coupling of ethanol to ethyl acetate is presented. By using the pincer complex (PhPNP)RuH(BH4)(CO) (0.25 mol%) in the presence of m-xylene as the co-solvent, the catalysis proceeds with up to >99% conversion and >99% yield after 24 h at 120 °C. A scaled-up reaction is also effective under similar conditions.

Direct synthesis of cyanate anion from dinitrogen catalysed by molybdenum complexes bearing pincer-type ligand

Dinitrogen is an abundant and promising material for valuable organonitrogen compounds containing carbon–nitrogen bonds. Direct synthetic methods for preparing organonitrogen compounds from dinitrogen as a starting reagent under mild reaction conditions give insight into the sustainable production of valuable organonitrogen compounds with reduced fossil fuel consumption. Here we report the catalytic reaction for the formation of cyanate anion (NCO−) from dinitrogen under ambient reaction conditions. A molybdenum–carbamate complex bearing a pyridine-based 2,6-bis(di-tert-butylphosphinomethyl)pyridine (PNP)-pincer ligand is synthesized from the reaction of a molybdenum–nitride complex with phenyl chloroformate. The conversion between the molybdenum–carbamate complex and the molybdenum–nitride complex under ambient reaction conditions is achieved. The use of samarium diiodide (SmI2) as a reductant promotes the formation of NCO− from the molybdenum–carbamate complex as a key step. As a result, we demonstrate a synthetic cycle for NCO− from dinitrogen mediated by the molybdenum–PNP complexes in two steps. Based on this synthetic cycle, we achieve the catalytic synthesis of NCO− from dinitrogen under ambient reaction conditions.

New Ru PNP complexes as in situ Ru-oxo precursors in styrene and octane oxidation

The ruthenium complexes, [Ph2PN(R)PPh2]2RuCl2, where R = C6H11 (a); C3H7 (b); C5H11 (c), have been synthesized and characterized. These complexes were studied for n-octane and styrene oxidation with different hybridizations of CH bonds. Ru-oxo species were identified as active species in both reactions but showed very different catalytic behavior in the chosen catalytic models. These complexes were good candidates to catalyze C1 partial oxidation and result in high selectivity to octanal and 1-octanol, which have high industrial demand as precursors to α-octene (a good elasticizer for the polymer industry). Slight modification of the ligand backbone influenced both the rate of Ru-oxo formation and their nature. This resulted in different selectivity and conversion in both the studied oxidative reactions. These studies provide insights regarding the role and the mode of action of metal-oxo species during oxidation reactions, which is of interest to researchers in the field of homogenous catalysis.

Low-Valent Molybdenum PNP Pincer Complexes as Catalysts for the Semihydrogenation of Alkynes.

Low-valent molybdenum PNP pincer complexes were studied as catalysts for the semihydrogenation of alkynes. For that purpose, tBu-substituted PNP complexes PNPtBuMo(CO)2 (6a) and PNPtBuMo(CO)3 (6c) and the NNP complex NNPiPrMo(CO)2(PPh3) ((rac)-7) were synthesized and characterized. By utilizing the cyclohexyl-substituted complex PNPCyMo(CO)2(CH3CN) (5a), several diphenylacetylene derivatives are transformed to the corresponding (Z)-alkenes with good to very good diastereoselectivities (up to 91:9). Mechanistic experiments indicate an outer-sphere mechanism including metal–ligand cooperativity.

Comparative Coordination Chemistry of PNP and SNS Pincer Ruthenium Complexes

Ruthenium carbonyl complexes supported by PNP pincer ligands are prominent catalysts for a range of hydrogenation and dehydrogenation reactions. Recently, Ru complexes with cheaper, more air stable SNS pincer ligands have emerged as attractive alternatives for the development of improved catalysts. However, there is currently a paucity of information on how the replacement of the phosphine donors in PNP ligands with the sulfur donors in SNS ligands influences the synthesis, structure, and electronic properties of the resulting metal complexes. Herein, the coordination chemistry of a series of Ru carbonyl complexes with SNS pincer ligands has been systematically compared with related PNP-ligated species. Three different SNS pincer ligands were explored including a pyridyl based NC5H3{CH2(StBu)}2 ligand and two aliphatic ligands, HN{CH2CH2(StBu)}2 and NCH3{CH2CH2(StBu)}2, along with different combinations of monodentate ancillary ligands. The geometric structures of the SNS and PNP Ru complexes were studied using NMR spectroscopy and X-ray crystallography. Additionally, the redox properties and electronic structures of these complexes were probed through a combination of cyclic voltammetry and DFT calculations. Overall, differences between SNS and PNP complexes extend well beyond simply modulating inductive donation to the metal and include changes in synthetic outcomes, as well as variations in geometry that impact redox behavior. Our study reveals fundamental information about the coordination chemistry of the SNS ligand, which may aid in interpreting catalytic results.

Bulky PNP Ligands Blocking Metal-Ligand Cooperation Allow for Isolation of Ru(0), and Lead to Catalytically Active Ru Complexes in Acceptorless Alcohol Dehydrogenation.

We synthesized two 4Me-PNP ligands which block metal-ligand cooperation (MLC) with the Ru center and compared their Ru complex chemistry to their two traditional analogues used in acceptorless alcohol dehydrogenation catalysis. The corresponding 4Me-PNP complexes, which do not undergo dearomatization upon addition of base, allowed us to obtain rare, albeit unstable, 16 electron mono-CO Ru(0) complexes. Reactivity with CO and H2 allows for stabilization and extensive characterization of bis-CO Ru(0) 18 electron and Ru(II) cis and trans dihydride species that were also shown to be capable of C(sp2 ) -H activation. Reactivity and catalysis are contrasted to non-methylated Ru(II) species, showing that an MLC pathway is not necessary, with dramatic differences in outcomes during catalysis between i Pr and t Bu PNP complexes within each of the 4Me and non-methylated backbone PNP series being observed. Unusual intermediates are characterized in one of the new and one of the traditional complexes, and a common catalysis deactivation pathway was identified.

Isonitrile ruthenium and iron PNP complexes: synthesis, characterization and catalytic assessment for base-free dehydrogenative coupling of alcohols

Neutral and ionic ruthenium and iron aliphatic PNHP-type pincer complexes (PNHP = NH(CH2CH2PiPr2)2) bearing benzyl, n-butyl or tert-butyl isocyanide ancillary ligands have been prepared and characterized. Reaction of [RuCl2(PNHP)]2 with one equivalent CN-R per ruthenium center affords complexes [RuCl2(PNHP)(CNR)] (R = benzyl, 1a, R = n-butyl, 1b, R = t-butyl, 1c), with cationic [RuCl(PNHP)(CNR)2]Cl 2a-c as side-products. Dichloride species 1a-c react with excess NaBH4 to afford [RuH(PNHP)(BH4)(CN-R)] 3a-c, analogues to benchmark Takasago catalyst [RuH(PNHP)(BH4)(CO)]. Reaction of 1a-c with a single equivalent of NaBH4 results in formation of [RuHCl(PNHP) (CN-R)] (4a-c), from which 3a-c can be prepared upon reaction with excess NaBH4. Use of one equivalent of NaHBEt3 with 4a and 4c affords bishydrides [Ru(H)2(PNHP)(CN-R)] 5a and 5c. Deprotonation of 4c by KOtBu generates amido derivative [RuH(PNP)(CN-t-Bu)] (6, PNP = -N(CH2CH2PiPr2)2), unstable in solution. Addition of excess benzylisonitrile to 4a provides cationic hydride [RuH(PNHP) (CN-CH2Ph)2]Cl (7). Concerning iron chemistry, [Fe(PNHP)Br2] reacts with one equivalent of benzylisonitrile to afford [FeBr(PNHP)(CNCH2Ph)2]Br (8). The outer-sphere bromide anion can be exchanged by salt metathesis with NaBPh4 to generate [FeBr(PNHP) (CNCH2Ph)2](BPh4) (9). Cationic hydride species [FeH(PNHP) (CN-t-Bu)2](BH4) (10) is prepared from consecutive addition of excess CN-t-Bu and NaBH4 on [Fe(PNPH)Br2]. Ruthenium complexes 3a-c are active in acceptorless alcohol dehydrogenative coupling into ester under base-free conditions. From kinetic follow-up, the trend in initial activity is 3a ≈ 3b > [RuH(PNHP)(BH4)(CO)] ≫ 3c; for robustness, [RuH(BH4)(CO)(PNHP)] > 3a > 3b ≫ 3c. Hypotheses are given to account for the observed deactivation. Complexes 3b, 3c, 4a, 4c, 5c, 7, cis-8 and 9 were characterized by X-ray crystallography.

HCOOH disproportionation to MeOH promoted by molybdenum PNP complexes.

Molybdenum(0) complexes with aliphatic aminophosphine pincer ligands have been prepared which are competent for the disproportionation of formic acid, thus representing the first example so far reported of non-noble metal species to catalytically promote such transformation. In general, formic acid disproportionation allows for an alternative access to methyl formate and methanol from renewable resources. MeOH selectivity up to 30% with a TON of 57 could be achieved while operating at atmospheric pressure. Selectivity (37%) and catalyst performance (TON = 69) could be further enhanced when the reaction was performed under hydrogen pressure (60 bars). A plausible mechanism based on experimental evidence is proposed.

Combined Experimental and Computational Studies of the Mechanism of Dehydrogenative Borylation of Terminal Alkynes Catalyzed by PNP Complexes of Iridium

Iridium complexes supported by four different diarylamido/bis(phosphine) PNP ligands have been studied as catalysts for the dehydrogenative borylation of terminal alkynes (DHBTA). Experimental mechanistic investigations on a representative ligand system established that the reaction rate is zeroth order in the concentration of the borylating agent HBpin, first order in [alkyne] and in [Ir], and inverse first order in [H2]. These findings are understood as arising from the formation of the saturated, 18-electron resting state (PNP)IrH3Bpin (5a), which is required to dissociate H2 in order to enter the C–H oxidative addition reaction with the alkyne. However, the overall highest transition state corresponds to the B–H rotation following C–H oxidative addition. This realization follows from the density functional theory (DFT) calculations and is consistent with the modest experimentally determined kinetic isotope effect of 1.5(1) for C–H/C–D. DFT-calculated ΔG298⧧ = 17.6 kcal/mol and ΔH⧧ = 13.9 kcal/mol match the experimentally determined values (ΔG298⧧ = 20(2) kcal/mol and ΔH⧧ = 16(2) kcal/mol) reasonably well. DFT calculations were used to demonstrate that the boryl migration from the amido N to Ir is too high in energy to play a part in the DHBTA mechanism in the (PNP)Ir system. This is in surprising contrast to the seemingly closely related (SiNN)Ir system studied previously. DFT calculations further provide insights into the greater kinetic barrier for activating a C–H bond of a terminal alkyne trans to a boryl ligand versus trans to a hydride, explaining why diboryl Ir compounds are not catalytically competent in this system.

Synthesis of Molybdenum Pincer Complexes and Their Application in the Catalytic Hydrogenation of Nitriles

AbstractA series of molybdenum(0), (I) and (II) complexes ligated by different PNP and NNN pincer ligands were synthesized and structurally characterized. Along with previously described Mo−PNP complexes Mo‐1 and Mo‐2, all prepared compounds were tested in the catalytic hydrogenation of aromatic nitriles to primary amines. Among the applied catalysts, Mo‐1 is particularly well suited for the hydrogenation of electron‐rich benzonitriles. Additionally, two aliphatic nitriles were transformed into the desired products in 80 and 86 %, respectively. Moreover, catalytic intermediate Mo‐1a was isolated and its role in the catalytic cycle was subsequently demonstrated.

Efficient and selective catalytic hydrogenation of furanic aldehydes using well defined Ru and Ir pincer complexes

Homogeneous catalyzed hydrogenation of furanic aldehydes to their corresponding alcohols using PNP complexes.

Amido PNP complexes of iridium: Synthesis and catalytic olefin and alkyne hydrogenation

AbstractIn situ lithiation of HN(o‐C6H4PPh2)2 (H[1a]) or HN(o‐C6H4PiPr2)2 (H[1b]) with nBuLi in THF at −35°C followed by addition of [Ir(μ‐Cl)(COD)]2 (COD = 1,5‐cyclooctadiene) in toluene at −35°C generates 5‐coordinate [1a]Ir(η4‐COD) (2a) or 4‐coordinate [1b]Ir(η2‐COD) (2b), respectively. Oxidative addition of N‐H in H[1b] to [Ir(μ‐Cl)(COD)]2 affords square pyramidal [1b]Ir(H)(Cl) (3b). Metathetical reaction of 3b with LiBHEt3 in the presence of 1 atm of H2 in toluene produces [1b]Ir(H)2 (4b). Both 2a and 4b are active for catalytic hydrogenation of olefins and alkynes under extremely mild conditions.

Synthesis and Structural Dynamics of Five-Coordinate Rh(III) and Ir(III) PNP and PONOP Pincer Complexes

The synthesis and characterization of a homologous series of five-coordinate rhodium(III) and iridium(III) complexes of PNP (2,6-(tBu2PCH2)2C5H3N) and PONOP (2,6-(tBu2PO)2C5H3N) pincer ligands are described: [M(PNP)(biph)][BArF4] (M = Rh, 1a; Ir, 1b; biph = 2,2′-biphenyl; ArF = 3,5-(CF3)2C6H3) and [M(PONOP)(biph)][BArF4] (M = Rh, 2a; Ir, 2b). These complexes are structurally dynamic in solution, exhibiting pseudorotation of the biph ligand on the 1H NMR time scale (ΔG⧧ ca. 60 kJ mol–1) and, in the case of the flexible PNP complexes, undergoing interconversion between helical and puckered pincer ligand conformations (ΔG⧧ ca. 10 kJ mol–1). Remarkably, the latter is sufficiently facile that it persists in the solid state, leading to temperature-dependent disorder in the associated X-ray crystal structures. Reaction of 1 and 2 with CO occurs for the iridium congeners 1b and 2b, leading to the formation of sterically congested carbonyl derivatives.

Five-Coordinate Low-Spin {FeNO}7 PNP Pincer Complexes.

The synthesis and characterization of air-stable cationic mono nitrosonium Fe(I) PNP pincer complexes of the type [Fe(PNP)(NO)Cl]+ are described. These complexes are obtained via direct nitroslyation of [Fe(PNP)Cl2] with nitric oxide at ambient pressure. On the basis of magnetic and EPR measurements as well as DFT calculations, these compounds were found to adopt a low-spin d7 configuration and feature a nearly linear bound NO ligand suggesting FeINO+ rather than FeIINO• character. X-ray structures of all nitrosonium Fe(I) PNP complexes are presented. Preliminary investigations reveal that [Fe(PNPNH- iPr)(NO)(Cl)]+ efficiently catalyzes the conversion of primary alcohols and aromatic and benzylic amines to yield mono N-alkylated amines in good isolated yields.

Diastereoselective diazenyl formation: the key for manganese-catalysed alcohol conversion into (E)-alkenes.

The proposed reaction mechanism for the unprecedented direct transformation of primary alcohols into alkenes catalysed by Mn(i)-PNP complexes consists of two cycles. First, the acceptorless dehydrogenation of the alcohol into aldehyde is produced via a concerted mechanism. Secondly, in an excess of hydrazine, hydrazone is formed and reacts with the aldehyde to produce olefins. This process, taking place in base-free conditions, is characterised by the diastereoselective formation of diazenyl intermediates. Based on DFT data, the generation of the (SN,S,S) diastereoisomer is favoured over the rest, leading in its decomposition to the preferential formation of an (E)-alkene and liberating N2 and H2O as the only by-products.

A Carbon-Neutral CO2 Capture, Conversion, and Utilization Cycle with Low-Temperature Regeneration of Sodium Hydroxide.

A highly efficient recyclable system for capture and subsequent conversion of CO2 to formate salts is reported that utilizes aqueous inorganic hydroxide solutions for CO2 capture along with homogeneous pincer catalysts for hydrogenation. The produced aqueous solutions of formate salts are directly utilized, without any purification, in a direct formate fuel cell to produce electricity and regenerate the hydroxide base, achieving an overall carbon-neutral cycle. The catalysts and organic solvent are recycled by employing a biphasic solvent system (2-MTHF/H2O) with no significant decrease in turnover frequency (TOF) over five cycles. Among different hydroxides, NaOH and KOH performed best in tandem CO2 capture and conversion due to their rapid rate of capture, high formate conversion yield, and high catalytic TOF to their corresponding formate salts. Among various catalysts, Ru- and Fe-based PNP complexes were the most active for hydrogenation. The extremely low vapor pressure, nontoxic nature, easy regenerability, and high reactivity of NaOH/KOH toward CO2 make them ideal for scrubbing CO2 even from low-concentration sources-such as ambient air-and converting it to value-added products.

Crystal structure of Escherichia coli purine nucleoside phosphorylase in complex with 7-deazahypoxanthine.

Purine nucleoside phosphorylases (EC 2.4.2.1; PNPs) reversibly catalyze the phosphorolytic cleavage of glycosidic bonds in purine nucleosides to generate ribose 1-phosphate and a free purine base, and are key enzymes in the salvage pathway of purine biosynthesis. They also catalyze the transfer of pentosyl groups between purine bases (the transglycosylation reaction) and are widely used for the synthesis of biologically important analogues of natural nucleosides, including a number of anticancer and antiviral drugs. Potent inhibitors of PNPs are used in chemotherapeutic applications. The detailed study of the binding of purine bases and their derivatives in the active site of PNPs is of particular interest in order to understand the mechanism of enzyme action and for the development of new enzyme inhibitors. Here, it is shown that 7-deazahypoxanthine (7DHX) is a noncompetitive inhibitor of the phosphorolysis of inosine by recombinant Escherichia coli PNP (EcPNP) with an inhibition constant Ki of 0.13 mM. A crystal of EcPNP in complex with 7DHX was obtained in microgravity by the counter-diffusion technique and the three-dimensional structure of the EcPNP-7DHX complex was solved by molecular replacement at 2.51 Å resolution using an X-ray data set collected at the SPring-8 synchrotron-radiation facility, Japan. The crystals belonged to space group P6122, with unit-cell parameters a = b = 120.370, c = 238.971 Å, and contained three subunits of the hexameric enzyme molecule in the asymmetric unit. The 7DHX molecule was located with full occupancy in the active site of each of the three crystallographically independent enzyme subunits. The position of 7DHX overlapped with the positions occupied by purine bases in similar PNP complexes. However, the orientation of the 7DHX molecule differs from those of other bases: it is rotated by ∼180° relative to other bases. The peculiarities of the arrangement of 7DHX in the EcPNP active site are discussed.

Structure, Electronics and Reactivity of Ce(PNP) Complexes.

Synthetic methods for the coordination of the monoanionic bis[2-(diisopropylphosphino)-4-methylphenyl]amido (PNP) ligand framework to the cerium(III) cation have been developed and applied for the isolation of a series of {(PNP)Ce} and {(PNP)2 Ce} type complexes. The structures of the complexes were studied by X-ray diffraction and multinuclear NMR spectroscopy. We found that the cerium(III) ion can induce the elimination of one of the iPr groups at phosphorus to yield a new dianionic PNP tridentate framework (PNP-iPr ) featuring a phosphido-donor functionality, which is bound to the cerium ion with the shortest known Ce-P bond of 2.7884(14) Å for molecular compounds. The reaction of the complex [(PNP)Ce{N(SiMe3 )2 }2 ] (1) with Ph2 CO gave the Ce-bound product of C-C coupling, - N(SiMe3 )SiMe2 CH2 -CPh2 O- , through the C-H bond activation of a SiMe3 group. 31 P NMR spectroscopy was used to estimate the presence of a vacant coordination position at the cerium ion in the CeIII -PNP complexes by the examination of the δ(31 P) shift recorded both in non-polar (C6 D6 ) and polar ([D8 ]THF) solvents. Moreover, 31 P NMR spectroscopy was also found to be a useful tool for the estimation of the Ce-P bond distances in {(PNP)CeIII } and {(PNP)2 CeIII } systems. Electrochemical and computational studies for 1 and its lanthanum analogue containing a redox-innocent metal center revealed the stabilization of the CeIII oxidation state by the PNP ligand.

Efficient and selective catalysis for hydrogenation and hydrosilation of alkenes and alkynes with PNP complexes of scandium and yttrium

Scandium and yttrium congeneric complexes, supported by a monoanionic PNP ligand, were studied as catalysts for alkene hydrogenation and hydrosilation, and alkyne semihydrogenation and semihydrosilation. The yttrium congener was found to be much more active in all cases, but this greater activity is accompanied by more rapid catalyst decomposition and therefore higher total yields for some of the reactions with the scandium catalyst. Calculations indicate that the reactions may proceed via σ-bond metathesis of the alkyl complexes to form metal hydride intermediates into which alkenes/alkynes insert.