PNP Pincer Ligands Research Articles

Approaches to Sigma Complexes via Displacement of Agostic Interactions: An Experimental and Theoretical Investigation

A series of coordinatively unsaturated, 16-electron, cationic ruthenium complexes bearing PNP pincer ligands of the type [RuH(L)(PNRP)]+ (L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5); PNRP = RN(CH2CH2PPh2)2) has been prepared and characterized. These complexes exhibit agostic interaction in the sixth coordination site. The binding of various X–H (X = H, Si, B, and C) bonds in small molecules such as H2, silanes, tetracoordinate boranes, and CH4 to the ruthenium center via displacement of the relatively weak agostic interaction in these complexes has been studied. Structures of [RuH(L)(PNRP)]+ (L = PPh3, R = PhCH2 (3), Ph (4); L = CO, R = PhCH2 (5); PNRP = RN(CH2CH2PPh2)2) complexes and the sigma-borane complex trans-[RuH(CO)(η1-H-BH2·NMe3)(PNRP)]+ (R = PhCH2 (15)) have been established by X-ray crystallography. The relative binding strengths of the X–H bonds to ruthenium center in these complexes has been studied using computational methods.

Theoretical studies on the thermodynamics and kinetics of one-pot synthesis of aromatic PCP and PNP pincer ligands

In the present study, the kinetics and thermodynamics of the successful one-pot synthesis of arene-based PCP/PNP pincer ligands by less nucleophilic secondary halophosphines (mono/bis(phosphinomethyl)benzene/pyridine) via SN2 reactions, which have been recently developed by Shih and Ozerov (Organometallics 34:4591, 35), have been studied. By utilizing different substituents on the target molecules, the impact of such modifications has been investigated both in gas phase and in solution. Performed calculations showed clear dependence of the activation energy and reaction rates on the presence of lithium in the structure. Additionally, the nature of the leaving group was observed to influence the favorability of the reaction. Similarly, the type of the halogen on both substrate and nucleophile had impact on the reaction rates, with the use of iodide leading to the observation of the fastest reaction rates. The solvent used was also noted to exert considerable influence on the thermodynamics and kinetics of the reactions. However, the R group in the nucleophile (XPR2) showed no significant inductive effect on the activation energy, but a slight steric effect could be observed.

Iron pyrrole-based PNP pincer ligand complexes as catalyst precursors.

The structure of a pincer ligand consists of a backbone and two `arms' which typically contain a P or N atom. They are tridentate ligands that coordinate to a metal center in a meridional configuration. A series of three iron complexes containing the pyrrole-based PNP pincer ligand 2,5-bis[(diisopropylphosphanyl)methyl]pyrrolide (PNpyrP) has been synthesized. These complexes are possible precursors to new iron catalysts. {2,5-Bis[(diisopropylphosphanyl)methyl]pyrrolido-κ3P,N,P'}carbonylchlorido(trimethylphosphane-κP)iron(II), [Fe(C18H34NP2)Cl(C3H9P)(CO)] or [Fe(PNpyrP)Cl(PMe3)(CO)], (I), has a slightly distorted octahedral geometry, with the Cl and CO ligands occupying the apical positions. {2,5-Bis[(diisopropylphosphanyl)methyl]pyrrolido-κ3P,N,P'}chlorido(pyridine-κN)iron(II), [Fe(C18H34NP2)Cl(C5H5N)] or [Fe(PNpyrP)Cl(py)] (py is pyridine), (II), is a five-coordinate square-pyramidal complex, with the pyridine ligand in the apical position. {2,5-Bis[(diisopropylphosphanyl)methyl]pyrrolido-κ3P,N,P'}dicarbonylchloridoiron(II), [Fe(C18H34NP2)Cl(CO)2] or [Fe(PNpyrP)Cl(CO)2], (III), is structurally similar to (I), but with the PMe3 ligand replaced by a second carbonyl ligand from the reaction of (II) with CO. The two carbonyl ligands are in a cis configuration, and there is positional disorder of the chloride and trans carbonyl ligands.

Oxidative Transformations of Biosourced Alcohols Catalyzed by Earth‐Abundant Transition Metals

AbstractThe catalytic acceptorless dehydrogenative oxidation of biosourced alcohols into carboxylic acid salts was achieved using earth‐abundant Fe and Mn complexes that feature aliphatic PNP pincer ligands in good to excellent yields. The Fe derivatives were characterized by using 57Fe NMR spectroscopy. Mn pincer catalysts are catalytically more efficient than their Fe counterparts thanks to their robustness under basic conditions. Attempts to generate aldehydes from alcohols were not successful using the Fe and Mn species, but a commercially available Ru analogue achieves this transformation selectively under very mild conditions in the presence of a large excess of acetone as a hydrogen acceptor.

N-Atom transfer via thermal or photolytic activation of a Co-azido complex with a PNP pincer ligand.

Thermal or photolytic activation of well-defined mononuclear [Co(N3)(PNP)] (PNP = 2,2'-bis(diisopropylphosphino)-4,4'-ditolylamido) results in the structurally characterized dinuclear species [Co(μ-N;κ3-P,N,N-PNPN)]2 (3), with two N-bridging phosphiniminato bridgeheads. Density Functional Theory (DFT) calculations indicate the intermediacy of a mononuclear cobalt-nitrido complex, followed by N-migratory insertion into a Co-P bond. Reaction of 3 with two equiv. HCl leads to rupture of the dimer with formation of mononuclear [CoCl(PNPNH)] (4) by protonation of the N-bridges.

Hydrogen autotransfer and related dehydrogenative coupling reactions using a rhenium(i) pincer catalyst.

A novel rhenium complex bearing a non-innocent PNP pincer ligand was prepared. This novel catalyst is active in hydrogen autotransfer reactions to form new C-C and C-N bonds. More specifically, valuable alkylations of ketones and sulfonamides with primary alcohols are herein presented. In addition, the first examples of rhenium-catalysed synthesis of pyrroles are described by dehydrogenative coupling of diols, amines and ketones.

Air Stable Iron(II) PNP Pincer Complexes as Efficient Catalysts for the Selective Alkylation of Amines with Alcohols

AbstractA series of well‐defined iron(II) complexes of the types [Fe(PNP)Br2] and [Fe(PNP)(CO)Br2] with PNP pincer ligands based on triazine and pyridine backbones were prepared and fully characterized. These complexes were tested as catalysts for the alkylation of amines by alcohols. The high‐spin complexes [Fe(PNP)Br2] are catalytically inactive. The low‐spin complexes [Fe(PNP)(CO)Br2] bearing a carbonyl co‐ligand efficiently and selectively convert primary alcohols and aromatic and benzylic amines selectively into mono‐N‐alkylated amines in good to excellent isolated yields. A mechanistic proposal is given.magnified image

Computational Design of Cobalt Catalysts for Hydrogenation of Carbon Dioxide and Dehydrogenation of Formic Acid.

A series of cobalt complexes with acylmethylpyridinol and aliphatic PNP pincer ligands are proposed based on the active site structure of [Fe]-hydrogenase. Density functional theory calculations indicate that the total free energy barriers of the hydrogenation of CO2 and dehydrogenation of formic acid catalyzed by these Co complexes are as low as 23.1 kcal/mol in water. The acylmethylpyridinol ligand plays a significant role in the cleavage of H2 by forming a strong Co-Hδ-···Hδ+-O dihydrogen bond in a fashion of frustrated Lewis pairs.

Manganese‐Catalyzed Hydrogen‐Autotransfer C−C Bond Formation: α‐Alkylation of Ketones with Primary Alcohols

AbstractA novel catalytic hydrogen‐autotransfer protocol for the atom‐efficient α‐alkylation of ketones with readily available alcohols is presented. The use of manganese complexes bearing non‐innocent PNP pincer ligands enabled the functionalization of a broad range of valuable ketones, including 2‐oxindole, estrone 3‐methyl ether, and testosterone. Mechanistic investigations suggest the participation of an intramolecular amidate‐assisted alcohol‐dehydrogenation process.

Manganese-Catalyzed Hydrogen-Autotransfer C-C Bond Formation: α-Alkylation of Ketones with Primary Alcohols.

A novel catalytic hydrogen-autotransfer protocol for the atom-efficient α-alkylation of ketones with readily available alcohols is presented. The use of manganese complexes bearing non-innocent PNP pincer ligands enabled the functionalization of a broad range of valuable ketones, including 2-oxindole, estrone 3-methyl ether, and testosterone. Mechanistic investigations suggest the participation of an intramolecular amidate-assisted alcohol-dehydrogenation process.

Selective Formic Acid Dehydrogenation Catalyzed by Fe-PNP Pincer Complexes Based on the 2,6-Diaminopyridine Scaffold

Fe(II) hydrido carbonyl complexes supported by PNP pincer ligands based on the 2,6-diaminopyridine scaffold were studied as homogeneous, non-precious-metal-based catalysts for selective formic acid dehydrogenation to hydrogen and carbon dioxide, reaching quantitative yields and high TONs under mild reaction conditions.

Direct Transformation of Molecular Dinitrogen into Ammonia Catalyzed by Cobalt Dinitrogen Complexes Bearing Anionic PNP Pincer Ligands

AbstractThe direct formation of ammonia from molecular dinitrogen under mild reaction conditions was achieved by using new cobalt dinitrogen complexes bearing an anionic PNP‐type pincer ligand. Up to 15.9 equivalents of ammonia were produced based on the amount of catalyst together with 1.0 equivalent of hydrazine (17.9 equiv of fixed nitrogen atoms).

Direct Transformation of Molecular Dinitrogen into Ammonia Catalyzed by Cobalt Dinitrogen Complexes Bearing Anionic PNP Pincer Ligands.

The direct formation of ammonia from molecular dinitrogen under mild reaction conditions was achieved by using new cobalt dinitrogen complexes bearing an anionic PNP-type pincer ligand. Up to 15.9 equivalents of ammonia were produced based on the amount of catalyst together with 1.0 equivalent of hydrazine (17.9 equiv of fixed nitrogen atoms).

Catalytic transformation of dinitrogen into ammonia and hydrazine by iron-dinitrogen complexes bearing pincer ligand.

Synthesis and reactivity of iron-dinitrogen complexes have been extensively studied, because the iron atom plays an important role in the industrial and biological nitrogen fixation. As a result, iron-catalyzed reduction of molecular dinitrogen into ammonia has recently been achieved. Here we show that an iron-dinitrogen complex bearing an anionic PNP-pincer ligand works as an effective catalyst towards the catalytic nitrogen fixation, where a mixture of ammonia and hydrazine is produced. In the present reaction system, molecular dinitrogen is catalytically and directly converted into hydrazine by using transition metal-dinitrogen complexes as catalysts. Because hydrazine is considered as a key intermediate in the nitrogen fixation in nitrogenase, the findings described in this paper provide an opportunity to elucidate the reaction mechanism in nitrogenase.

Catalytic Conversion of Alcohols into Carboxylic Acid Salts in Water: Scope, Recycling, and Mechanistic Insights.

The catalytic conversion of alcohols into carboxylic acid salts in water was performed in the presence of ruthenium complexes supported by aliphatic PNP pincer ligands preformed or formed in situ. High activity toward a wide substrate scope was achieved with turnover number values of up to 4000. The air-stable catalytic system can be recycled by using toluene as a catalyst-immobilizing phase; the activity is maintained after five consecutive runs. Finally, mechanistic studies allowed some fundamental aspects related to water activation to be unveiled and to the mechanism postulated.

The synthesis of PNP-supported low-spin nitro manganese(I) carbonyl complexes

The coordination chemistry of Mn(CO)5Br was investigated with a series of PNP-pincer ligands. The ligands iPrPONOP (iPrPONOP=2,6-bis(diisopropylphosphinito)pyridine) and iPrPNHP (iPrPNHP=HN{CH2CH2(PiPr2)}2) gave the desired organometallic manganese complexes (iPrPONOP)Mn(CO)2Br and (iPrPNHP)Mn(CO)2Br, respectively, upon chelation to Mn(CO)5Br. The reactivity of iPrPNNNP (iPrPNNNP=N,N′-bis(diisopropylphosphino)-2,6-diaminopyridine) with Mn(CO)5Br yielded a pair of products, [(iPrPNNNP)Mn(CO)3][Br] and (iPrPNNNCO)Mn(CO)3. The formation of the asymmetric chelate arises from a formal loss of iPr2PBr and C–N bond formation from a carbonyl ligand and NH, yielding a Mn(I) amide core. The nitration reactions of (iPrPONOP)Mn(CO)2Br and (iPrPNHP)Mn(CO)2Br were carried out using silver nitrite, yielding the nitro compounds (iPrPONOP)Mn(CO)2(NO2) and (iPrPNHP)Mn(CO)2(NO2), respectively. The analogous iron complex (iPrPONOP)Fe(CO)Cl2 was nitrated under the same conditions to yield the salt pair [(iPrPONOP)Fe(CO)2][FeCl3NO]. This reactivity underlines the difference between iso-valent iron and manganese centers. The manganese complexes (iPrPONOP)Mn(CO)2(NO2) and (iPrPNHP)Mn(CO)2(NO2) were ineffective as oxygen atom transfer reagents for a variety of substrates.

Efficient and Mild Carbon Dioxide Hydrogenation to Formate Catalyzed by Fe(II) Hydrido Carbonyl Complexes Bearing 2,6-(Diaminopyridyl)diphosphine Pincer Ligands

Fe(II) hydrido carbonyl complexes supported by PNP pincer ligands based on the 2,6-diaminopyridine scaffold were found to promote the catalytic hydrogenation of CO2 and NaHCO3 to formate in protic solvents in the presence of bases, reaching quantitative yields and high TONs under mild reaction conditions, with pressures as low as 8.5 bar and temperatures as low as 25 °C. NMR and DFT studies highlighted the role of dihydrido and hydrido formate complexes in catalysis.

ChemInform Abstract: 2,6‐Diamination of Substituted Pyridines via Heterogeneous Chichibabin Reaction.

Abstract A series of ring substituted pyridines were selected for the sodium amide initiated heterogeneous Chichibabin amination to obtain 2,6-diaminopyridine derivatives which are important synthons for the preparation of PNP pincer ligands. The substrates were treated with an excess of sodium amide in neat mineral oil as solvent under an argon atmosphere. The reaction required temperatures of up to 215 °C under vigorous stirring with an overall reaction time of 3–5 h. In the case of methyl, tert -butyl, phenyl, pyridinyl, and hydroxyl substituted pyridines the desired products were obtained in good to excellent yields (63–96%). Thus, the Chichibabin reaction provides an inexpensive and economic alternative to methodologies starting from halopyridines or pyridine N -oxides provided that the substituents are inert under the harsh reaction conditions.

A Convenient Solvothermal Synthesis of Group 6 PNP Pincer Tricarbonyl Complexes

The solvothermal synthesis of a series of zerovalent Cr, Mo, and W complexes of the type [M(PNP)(CO)3] featuring PNP pincer ligands based on 2,6-diaminopyridine is described. We demonstrate that the solvothermal synthesis technique presented provides a powerful, simple, and practical synthetic method resulting in high isolated yields in a short time. In particular, Cr and W complexes are not readily accessible via conventional methods. Moreover, this study allows a direct comparison of steric and electronic properties of group 6 metal PNP pincer tricarbonyl complexes.

ChemInform Abstract: A Practical Synthesis of Substituted 2,6‐Diaminopyridines via Microwave‐Assisted Copper‐Catalyzed Amination of Halopyridines.

Abstract A microwave assisted copper-catalyzed amination protocol is reported utilizing a series of 2,6-dihalo- and 2-amino-6-halo pyridine precursors. Using this procedure, selective substitution of one or two halogens by aryl or alkylamines was achieved within 2–6 h with temperatures between 80 and 225 °C affording 2,6-diaminopyridines in good to excellent isolated yields. The reaction allows easy variation between educts and different N -substitutions. The target compounds are valuable precursors for the synthesis of bis-phosphorylated 2,6-diaminopyridines which are used as PNP pincer ligands in transition metal complexes.