NNN Pincer Research Articles

Guerbet-type β-alkylation of secondary alcohols catalyzed by chromium chloride and its corresponding NNN pincer complex

β-Alkylation of alcohols has been efficiently accomplished using readily available 3d metal Cr under microwave conditions in air. Well-defined molecular Cr is involved with a KIE of 7.33 and insertion of α-alkylated ketone into Cr–H bond as the RDS.

An unusual mixed-valence cobalt dimer as a catalyst for the anti-Markovnikov hydrophosphination of alkynes

A Co dimer with a 3e-reduced NNN pincer ligand [(PMe3)2CoII(L13−)CoI(PMe3)3] is an efficient catalyst for the hydrophosphination reaction of both internal and terminal alkynes including α-hydroxy-functionalized alkyne substrates with excellent stereo- and regioselectivity.

Regioselective Hydroalkylation of Vinylarenes by Cooperative Cu and Ni Catalysis

AbstractDisclosed here is a dual copper and nickel catalytic system with a silyl hydride source for promoting the linear selective hydroalkylation of vinylarenes. This carbon–carbon bond‐forming protocol is applied to couple a variety of functionalized vinylarenes with alkyl halides applying a nickel(II) NNN pincer complex in the presence of an NHC‐ligated copper catalyst. This combination allows for a 1 mol % loading of the nickel catalyst leading to turnover numbers of up to 72. Over 40 examples are presented, including applications for pharmaceutical diversification. Labeling experiments demonstrated the regioselectivity of the reaction and revealed that the copper catalyst plays a crucial role in enhancing the rate for formation of the reactive linear alkyl nickel complex. Overall, the presented work provides a complimentary approach for hydroalkylation reactions, whilst providing a preliminary mechanistic understanding of the cooperativity between the copper and nickel complexes.

Regioselective Hydroalkylation of Vinylarenes by Cooperative Cu and Ni Catalysis.

Disclosed here is a dual copper and nickel catalytic system with a silyl hydride source for promoting the linear selective hydroalkylation of vinylarenes. This carbon-carbon bond-forming protocol is applied to couple a variety of functionalized vinylarenes with alkyl halides applying a nickel(II) NNN pincer complex in the presence of an NHC-ligated copper catalyst. This combination allows for a 1 mol % loading of the nickel catalyst leading to turnover numbers of up to 72. Over 40 examples are presented, including applications for pharmaceutical diversification. Labeling experiments demonstrated the regioselectivity of the reaction and revealed that the copper catalyst plays a crucial role in enhancing the rate for formation of the reactive linear alkyl nickel complex. Overall, the presented work provides a complimentary approach for hydroalkylation reactions, whilst providing a preliminary mechanistic understanding of the cooperativity between the copper and nickel complexes.

Unraveling the Effect of Aromatic Groups in Mn(I)NNN Pincer Complexes on Carbon Dioxide Activation Using Density Functional Study.

Carbon dioxide utilization is necessary to reduce carbon footprint and also to synthesize value-added chemicals. The transition metal pincer complexes are attractive catalysts for the hydrogenation of carbon dioxide to formic acid. There is a need to understand the factors affecting the catalytic performance of these pincer complexes through a structure–activity relationship study using computational methods. It is a well-established fact that aromatic functionalities offer stability and selectivity to transition metal catalysts. However, their impact on the performance of the catalysts is lesser known in the case of metal pincer complexes. Hence, it is necessary to investigate the catalytic performance of Mn(I)NNN pincer complexes with variably activated aromatic functionalities. In this context, 15 catalysts are designed by placing different types of aromatic rings at the pincer carbons and two terminal nitrogen of Mn(I)NNN pincer complexes. A benzene moiety, placed at C2–C3 carbons of Mn(I)NNN pincer complex with identical aromatic groups at the terminal nitrogen, is found to be most efficient toward CO2 hydrogenation than the rest of the catalysts. On the other hand, when N,N-dimethyl aniline is placed at C2–C3 carbons of Mn(I)NNN pincer complexes, then the catalytic performance is significantly decreased. Thus, the present study unravels the impact of aromatic groups in Mn(I)NNN pincer complexes toward the catalytic hydrogenation of carbon dioxide.

Hydroxyl bond activation of formic acid by Metal-ligand cooperation of new designed aluminum ligated pincer fullerenes

• New designed aluminum ligated NNN pincer fullerene. • Aluminum-ligand cooperation ►►OH bond cleavage HCOOH. • H 2 elimination ►►protonation of imine carbon atom. • The aluminum complex with three HCOO►► active catalyst. • Catalytic cycle►► β-hydride elimination to dissociate CO 2 . The two parts of the formic acid dehydrogenation reaction including catalyst activation and catalytic cycle with a new designed aluminum ligated NNN pincer fullerene as catalyst in which Al(III)–bis(imino)pyridyl pincer are located on the (6, 6) ring fusion bond of C 60 fullerene was investigated with density functional calculations. Aluminum-ligand cooperation enables O H bond cleavage of formic acid in the HCOOH-assisted H 2 elimination pathway with protonation of imine carbon atom. The aluminum complex with three HCOO – units (complex 8 ) forms the active catalyst of HCOOH dehydrogenation. The existences of hydrogen bonds in the studied molecules were confirmed by NBO and AIM analyses. In the catalytic cycle, β-hydride elimination from 8 to dissociate CO 2 and formation of Al–H complex promote HCOOH dehydrogenation to generate H 2 . The β-hydride elimination is the rate-determining step with an overall barrier of +17.8 kcalmol −1 . The effect of fullerene cage on the catalytic cycle of HCOOH dehydrogenation is explored by elimination of fullerene cage from the Al(III)– bis(imino)pyridyl pincer fullerene.

Synthesis and characterization of trigonal bipyramidal FeIII complexes and their solution behavior

A series of air-stable trigonal bipyramidal FeIII complexes supported by a redox non-innocent NNN pincer ligand, CztBu(PyrR)2− (R = iPr, Me, or H), were synthesized, fully characterized, and utilized for the investigation of the interaction between acetone and the FeIII center. The magnetic moments determined from the paramagnetic 1H NMR spectra in conjunction with EPR and Mössbauer spectroscopy indicate the presence of a high-spin ferric center. Cyclic voltammetry studies feature two quasi-reversible events corresponding to a metal-centered FeIII/II reduction around −0.40 V (vs. Fc) and a ligand-centered CztBu(PyrR)2/CztBu(PyrR)2+ oxidation at potentials near +0.70 V (vs. Fc). UV–Visible spectroscopy in CH2Cl2 showcases ligand–metal charge transfer (LMCT) bands, as well as coordination of acetone to CztBu(PyrH)2FeCl2. In situ IR spectroscopy and solution conductivity (̺κ) measurements of CztBu(PyrR)2FeCl2 with varied equivalents of acetone reveal that acetone is weakly associated with the iron center.

Hydrogenation of Carbon Dioxide into Formic Acid by Aluminum Ligated NNN Pincer Fullerene Through Metal–Ligand H2O-Assisted Pathway: A Computational Study

Hydrogenation of Carbon Dioxide into Formic Acid by Aluminum Ligated NNN Pincer Fullerene Through Metal–Ligand H2O-Assisted Pathway: A Computational Study

Iron(II) Complexes Featuring a Redox-Active Dihydrazonopyrrole Ligand.

Nature uses control of the secondary coordination sphere to facilitate an astounding variety of transformations. Similarly, synthetic chemists have found metal-ligand cooperativity to be a powerful strategy for designing complexes that can mediate challenging reactivity. In particular, this strategy has been used to facilitate two electron reactions with first row transition metals that more typically engage in one electron redox processes. While NNN pincer ligands feature prominently in this area, examples which can potentially engage in both proton and electron transfer are less common. Dihydrazonopyrrole (DHP) ligands have been isolated in a variety of redox and protonation states when complexed to Ni. However, the redox-state of this ligand scaffold is less obvious when complexed to metal centers with more accessible redox couples. Here, we synthesize a new series of Fe-DHP complexes in two distinct oxidation states. Detailed characterization supports that the redox-chemistry in this set is still primarily ligand based. Finally, these complexes exist as 5-coordinate species with an open coordination site offering the possibility of enhanced reactivity.

CO2 hydrogenation into formate and methyl formate using Ru molecular catalysts supported on NNN pincer porous organic polymers

The Ru molecular catalyst immobilized on the porous organic polymer containing the NNN-pincer motif efficiently promoted the hydrogenation of CO2 into formate derivatives.

Reactivity of a T-shaped cobalt(I) pincer-complex.

The reactivity of a paramagnetic T-shaped cobalt(i) complex, [(iPrboxmi)Co], stabilised by a monoanionic bis(oxazolinylmethylidene)-isoindolate (boxmi) NNN pincer ligand is described. The exposure to carbon monoxide as an additional neutral ligand resulted in the square-planar species [(iPrboxmi)Co(CO)], accompanied by a change in the electronic spin state from S = 1 to S = 0. In contrast, upon treatment with trimethylphosphine the formation of the distorted tetrahedral complex [(iPrboxmi)Co(PMe3)] was observed (S = 1). Reacting [(iPrboxmi)Co] with iodine (I2), organic peroxides (tBu2O2, (SiMe3)2O2) and diphenyldisulphide (Ph2S2) yielded the tetracoordinated complexes [(iPrboxmi)CoI], [(iPrboxmi)Co(OtBu)], [(iPrboxmi)Co(OSiMe3)] and [(iPrboxmi)Co(SPh)], respectively, demonstrating the capability of the boxmi-supported cobalt(i) complex to homolytically cleave bonds and thus its distinct one-electron reactivity. Furthermore, a square-planar cobalt(ii) alkynyl complex [(iPrboxmi)Co(CCArF)] was identified as the main product in the reaction between [(iPrboxmi)Co] and a terminal alkyne, 4-fluoro-1-ethynylbenzene. Putting such species in the context of the previously investigated hydroboration catalysis, its stoichiometric reaction with pinacolborane revealed its potential conversion into a cobalt(ii) hydride complex, thus confirming its original attribution as off-cycle species.

NNN Pincer‐functionalized Porous Organic Polymer Supported Ru(III) as a Heterogeneous Catalyst for Levulinic Acid Hydrogenation to γ‐Valerolactone

AbstractA novel porous organic polymer that incorporates NNN pincer type of ligand, i. e., 2,6‐Bis(pyrazol‐3‐yl)pyridine, (3‐bpp‐POP) is prepared via solvent‐knitting method. Through the characterizations, 3‐bpp‐POP is demonstrated to possess abundant porous structure, high surface area, and excellent thermal durability. A ruthenium molecular catalyst immobilized on the 3‐bpp‐POP support exhibits substantial catalytic performance for the hydrogenation of LA to GVL with an initial turnover frequency (TOF) of 1680 h−1 and a maximum turnover number (TON) of 14762 over 24 h for LA conversion with selectivities of 91.1 % for GVL and 8.9 % for HPA. The catalyst demonstrated excellent durability during successive recycles without leaching Ru3+, which ascribe to the strong binding of pincer ligand to metal centers.

Magnetically coupled iron azide chains

Three well-defined iron(II) compounds CztBu(PyriPr)2FeCl (1), CztBu(PyriPr)2FeCl(THF) (2), and CztBu(PyriPr)2FeN3 (3), supported by the NNN pincer ligand CztBu(PyriPr)2− were synthesized and characterized.Compound 3 features a one-dimensional chain structure built by CztBu(PyriPr)2Fe units and bridged by end-to-end azido ligands. This chain material exhibits magnetic hysteresis at low temperatures and non-superimposable zero-field cooled and field cooled magnetization. These properties are consistent with single-chain magnet behavior, likely arising from superexchange coupling of Fe(II) centers via the azide bridges.

Coordination diversity in transition metal complexes with 4-aminoantipyrine tethered bis(imino)pyridine ligand: structures, superoxide dismutase and anticancer properties

In this article, we report Mn(II), Fe(II) and Cu(I) complexes (BIP-Mn, BIP-Fe and BIP-Cu) bearing a non-innocent, NNN pincer ligand, 4-aminoantipyrine tethered 2,6-bis(iminoantipyrine)pyridine (BIP) and structural diversity. The complexes were characterized by different spectroscopic and single-crystal X-ray analysis. Superoxide dismutase (SOD) activity has been evaluated for BIP and complexes BIP-Mn, BIP-Fe and BIP-Cu. It was observed that the seven-coordinate BIP-Mn complex showed preeminent SOD activity on comparing to BIP-Fe and BIP-Cu complexes. Besides, preliminary in vitro anticancer properties of BIP-Mn, BIP-Fe and BIP-Cu have been studied. Again, BIP-Mn complex showed stronger anticancer activity compared to ligand, BIP-Fe and BIP-Cu complexes.

Pd(II)‐NNN Pincer Complexes for Catalyzing Transfer Hydrogenation of Ketones

AbstractChiral and achiral Pd(II)‐NNN pincer complexes (1–3) containing acetonitrile as an ancillary ligand were synthesized and characterized by spectroscopic (UV‐visible, FT‐IR, NMR and mass) techniques. Polarimetric experiments were employed to measure the specific rotation values of the chiral complexes {[α]D27= +135° (2) and −136° (3)}. The structures resolved through single crystal X‐ray diffraction technique disclosed the distorted square planar geometry of the complexes. Transfer hydrogenation (TH) reactions of ketones were carried out in order to appraise the catalytic efficiency of the complexes. Remarkably, the TH results were promising for all the complexes, however, the chiral complexes (2 and 3) failed to induce chirality. Further probe led us to believe that the pincer complexes turned into Pd particles in situ and the later acted as an active catalyst. The scope of TH was extended to various substituted aromatic and heterocyclic ketones (Conversions=75–99%).

Reaction Pathways and Redox States in α‐Selective Cobalt‐Catalyzed Hydroborations of Alkynes

AbstractCobalt(II) alkyl complexes supported by a monoanionic NNN pincer ligand are pre‐catalysts for the regioselective hydroboration of terminal alkynes, yielding the Markovnikov products with α:β‐(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II) vinyl complex appear to determine the main reaction pathway. In a background reaction the highly reactive hydrido species specifically converts to a coordinatively unsaturated cobalt(I) complex which was found to re‐enter the main catalytic cycle.

Reaction Pathways and Redox States in α-Selective Cobalt-Catalyzed Hydroborations of Alkynes.

Cobalt(II) alkyl complexes supported by a monoanionic NNN pincer ligand are pre‐catalysts for the regioselective hydroboration of terminal alkynes, yielding the Markovnikov products with α:β‐(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II) vinyl complex appear to determine the main reaction pathway. In a background reaction the highly reactive hydrido species specifically converts to a coordinatively unsaturated cobalt(I) complex which was found to re‐enter the main catalytic cycle.

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.

Hydrophosphination of Activated Alkenes by a Cobalt(I) Pincer Complex

AbstractHerein we report the synthesis of three heteroleptic first‐row transition metal(II) complexes containing carbazolido NNN pincer ligands and conversion to the corresponding metal(I)‐carbonyl complexes via a reductive carbonylation route. These complexes are precatalysts for the hydrophosphination of activated alkenes, affording a cobalt‐catalysed hydrophosphination process that solely and selectively yields the β addition (anti‐Markovnikov) product. The scope of this transformation has been investigated using a variety of activated alkenes. Isolation and characterisation of substrate‐coordinated intermediates reveal available coordination sites, which provide insight into the proposed catalytic cycle.magnified image

Transfer hydrogenation via generation of hydride intermediate and base‐free alcohol oxidation activity studies on designed ruthenium complexes derived from NNN pincer type ligands

AbstractRuthenium complexes(1–3) have been synthesized using pincer‐type ligands L1 = (E)‐2‐((2‐phenyl‐2‐(pyridin‐2‐yl)hydrazono)methyl)pyridine, L2 = (E)‐2‐(1‐phenyl‐2‐(1‐(pyridin‐2‐yl)ethylidene)hydrazinyl)pyridine, L3 = (E)‐2‐(phenyl(2‐phenyl‐2‐(pyridin‐2‐yl)hydrazono) methyl)pyridine. The molecular structures of all the complexes 1, 2 and 3 were determined by using single crystal X‐ray diffraction. These complexes showed excellent catalytic activities such as transfer hydrogenation and alcohol oxidation. Theoretical calculations have been performed to understand the electronic properties of all the complexes using B3LYP as a function and LANL2DZ as a basis set.