NCN Pincer Complexes Research Articles

Symmetrical and unsymmetrical pincer complexes with group 10 metals: synthesis via aryl C–H activation and some catalytic applications

Aryl-based pincer metal complexes with anionic terdentate ligands have been widely applied in organic synthesis, organometallic catalysis and other related areas. Synthetically, the most simple and convenient method for the construction of these complexes is the direct metal-induced C(aryl)-H bond activation, which can be fulfilled by choosing the appropriate functional donor groups in the two side arms of the aryl-based pincer preligands. In this perspective, we wish to summarize some results achieved by our group in this context. Successful examples include symmetrical chiral bis(imidazoline) NCN pincer complexes with Ni(II), Pd(II) and Pt(II), bis(phosphinite) and bis(phosphoramidite) PCP pincer Pd(II) complexes, unsymmetrical (pyrazolyl)phosphinite, (amino)phosphinite and (imino)phosphinite PCN pincer Pd(II) complexes, chiral (imidazolinyl)phosphinite and (imidazolinyl)phosphoramidite PCN pincer complexes with Ni(II) and Pd(II) as well as unsymmetrical (oxazolinyl)amine and (oxazolinyl)pyrazole NCN' pincer Pd(II) complexes. Among them, the P-donor containing complexes are efficiently synthesized by the "one-pot phosphorylation/metalation" method. The obtained symmetrical and unsymmetrical pincer complexes have been used as catalysts in Suzuki-Miyaura reaction (Pd), asymmetric Friedel-Crafts alkylation of indole with trans-β-nitrostyrene (Pt) as well as in asymmetric allylation of aldehyde and sulfonimine (Pd). In the Suzuki couplings conducted at 40-50 °C, some unsymmetrical Pd complexes exhibit much higher activity than the related symmetrical ones which can be attributed to their faster release of active Pd(0) species resulting from the hemilabile coordination of the ligands. Literature results on the synthesis of some related pincer complexes as well as their activities in the above catalytic reactions are also presented.

Palladium NCN and CNC pincer complexes as exceptionally active catalysts for aerobic oxidation in sustainable media

The oxidation of secondary benzyl alcohols is catalyzed by two palladacycles at atmospheric pressure in PEG-400, a sustainable reaction media. Recycling of the active catalytic species is performed up to the 5th run, and catalyst loadings decreased down to 10−8 mol%, thus achieving unprecedented TON and TOF values. In addition, the same conditions proved to be effective for the aerobic oxidation of benzyl methylene compounds, a scarcely explored process by palladium catalysts.

Novel bis(azole) pincer palladium complexes: synthesis, structures and applications in Mizoroki–Heck reactions

Two novel NCN-pincer complex precursors bearing frameworks of 2,6-bis(oxazol-4-yl)benzene (A) and 2-(thiazol-4-yl)-6-(oxazol-4-yl)benzene (B) were synthesized. Palladations of A and B afforded two new bis(azole) pincer complexes, [(A-κ(3)NCN)PdBr] (1) and [(B-κ(3)NCN)PdBr] (2). Both complexes were fully characterized by NMR, MS, DSC-TGA and single-crystal X-ray diffraction analysis. Complex 1 crystallizes in a noncentrosymmetric orthorhombic space group Cmc2(1) (No. 36, Z=4). Complex 2 crystallizes in a centrosymmetric monoclinic space group P2(1)/n (No. 14, Z=4). Despite the similarity in their chemical formulas, the structures of the two complexes are subtly different: they are built up of two-dimensional supramolecular layers with identical topology, but stacked in different sequences, i.e., the layers in complex 1 are stacked in an AAAA-type fashion, while those in complex 2 are stacked in an alternating AA(-1)AA(-1) sequence (A denotes a layer; A(-1) stands for A's inversion symmetry equivalent). In addition, the complexes showed good catalytic activity toward Mizoroki-Heck reactions.

Manganese(II), Nickel(II), and Palladium(II) Complexes of a Terpyridine-Like Ligand Containing a Sulfur Linkage, and an Analogous NCN Pincer Palladium(II) Complex: Synthesis, Characterization, and Pd-Catalyzed Reactions

Abstract Coordination properties of new polypyridines containing a single sulfur linkage, 6-(2′′-pyridylthio)-2,2′-bipyridine (L1) and 1-(2′-pyridyl)-3-(2′′-pyridylthio)benzene (HL2), are described. The manganese(II) and nickel(II) complexes of the terpyridine-like ligand L1, [MnCl2(L1)] (1), [NiCl(μ-Cl)(L1)]2 (2), and [Ni(L1)2](SbF6)2 (3), were prepared and characterized by UV–vis spectroscopy and X-ray crystallography. The palladium(II) complex [PdCl(L1)]Cl (4) was synthesized from L1 and Li2[PdCl4] at room temperature, while the corresponding NCN pincer complex of L2, [PdCl(L2)] (5), was synthesized in DMSO at 150 °C by cyclopalladation of the ligand precursor HL2. The crystal structures of 4 and 5 revealed similar coordination geometries around Pd except for the longer Pd–Cl distance in 5, which is induced by the strong trans influence of the Pd–C bonding. The NMR spectra of 4 showed partial dissociation of L1 in DMSO-d6, which was not observed for L2 in 5. Catalytic activities of 4 and 5 for the Mizoroki–Heck reaction were investigated, and the lower activity of 5 can be related to the strong σ-donating character of L2.

Rhodium(III) NCN pincer complexes catalyzed transfer hydrogenation of ketones

Air-stable monomeric rhodium(III) NCN pincer complexes were synthesized via direct C–H bond activation of 1,3-bis(2-pyridyloxy)benzene, 3,5-bis(2-pyridyloxy)toluene and 3,5-bis(2-pyridyloxy)anisole with RhCl 3·3H 2O in ethanol under reflux. The synthesized complexes were characterized by elemental analysis and 1H NMR. One of the complexes was structurally characterized by X-ray analysis. An investigation into the catalytic activity of the complex 1a as catalyst for transfer hydrogenation of ketones to alcohols at 82 °C in the presence of iPrOH/KOH was undertaken with the conversions up to 99%.

First structurally characterized mixed-halogen nickel(III) NCN-pincer complex

A square-pyramidal mixed-halogen nickel(III) NCN-pincer complex (PipeNCN)NiClBr (where PipeNCN = 2,6-bis(piperidinomethyl)phenyl) was structurally characterized. Bromine occupies apical position; pincer ligand and chlorine atom are in the basal plane. EPR detects that complex in solution exists as a mixture of two structural isomers with bromine or chlorine atoms in the top of pyramid.

Chiral Nickel(II) and Palladium(II) NCN-Pincer Complexes Based on Substituted Benzene: Synthesis, Structure, and Lewis Acidity

Air- and moisture-stable nickel(II) and palladium(II) complexes from phebox pincer ligands 1−5 have been synthesized, and their Lewis activity was investigated. 2,6-Bis(oxazolinyl)phenylnickel halide complexes [NiX(phebox)] and 2,6-bis(oxazolinyl)phenylpalladium halide complexes [PdX(phebox)] were synthesized via oxidative addition with [Ni(COD)2] or Pd2dba3, respectively, followed by halide abstraction using silver(I) salts to form complexes such as [Ni(phebox)][ClO4] (41). Herein, X-ray crystal structures are reported for several pincer complexes having the expected square-planar geometries with the terdentate NCN pincer system. Complexes with general structure [M(phebox)]X where X is a halogen showed no relative Lewis acidity. On the other hand, complexes where X was exchanged for a less coordinating counterion showed increased Lewis acidity. The relative Lewis acidity varies depending on the substituents on the benzene core of the pincer ligands, due to the electronic effects of the ligand on the metal center.

Synthesis and reaction chemistry of 4-nitrile-substituted NCN-pincer palladium(II) and platinum(II) complexes (NCN = [N [tbnd]C-4-C 6H 2(CH 2NMe 2) 2-2,6] −)

Nitrile-functionalized NCN-pincer complexes of type [MBr(N C-4-C 6H 2(CH 2NMe 2) 2-2,6)] ( 6a, M = Pd; 6b, M = Pt) (NCN = [C 6H 2(CH 2NMe 2) 2-2,6] −) are accessible by the reaction of Br-1-N C-4-C 6H 2(CH 2NMe 2) 2-2,6 ( 2b) with [Pd 2(dba) 3 · CHCl 3] ( 5a) (dba = dibenzylidene acetone) and [Pt(tol-4) 2(SEt 2)] 2 ( 5b) (tol = tolyl), respectively. Complex 6b could successfully be converted to the linear coordination polymer {[Pt(N C-4-C 6H 2(CH 2NMe 2) 2-2,6)](ClO 4)} n ( 8) upon its reaction with the organometallic heterobimetallic π-tweezer compound {[Ti](μ-σ,π-C CSiMe 3) 2}AgOClO 3 ( 7) ([Ti] = (η 5-C 5H 4SiMe 3) 2Ti). The structures of 6a (M = Pd) and 6b (M = Pt) in the solid state are reported. In both complexes the d 8-configurated transition metal ions palladium(II) and platinum(II) possess a somewhat distorted square-planar coordination sphere. Coordination number 4 at the group-10 metal atoms M is reached by the coordination of two ortho-substituents Me 2NCH 2, the NCN ipso-carbon atom and the bromide ligand. The N C group is para-positioned with respect to M.

Palladium Pincer Complex‐Catalyzed Condensation of Sulfonimines and Isocyanoacetate to Imidazoline Derivatives. Dependence of the Stereoselectivity on the Ligand Effects

AbstractPalladium‐catalyzed condensation reactions of sulfonimines with isocyanoacetate were performed using various PCP, SCS, SeCSe and NCN pincer complexes as catalysts. The reactions proceeded rapidly (2 h) at room temperature using only 1 mol % pincer complex catalyst without any additives. The electron‐deficient and relatively bulky PCP complex provides imidazoline derivatives with a very high syn diastereoselectivity. The applied PCP catalyst proved to be very robust under the applied reaction conditions, as it could be recovered without any decomposition after the completed catalytic process. The stereoselectivity of the condensation reactions is reversed by employing the electron‐rich SeCSe type of complexes. Simple palladium salts, such as palladium acetate, Pd(OAc)2, catalyze the reaction with a poor stereoselectivity. The stereoselectivity of the PCP complex‐catalyzed process does not depend significantly on the steric bulk of the sulfonimine component. Mechanistic studies revealed that the PCP complex‐catalyzed reaction proceeds via an η1‐coordinated palladium isocyanoacetate pincer intermediate. This intermediate could also be isolated and its structure was determined by X‐ray diffraction. The X‐ray structure of this reaction intermediate indicates a surprisingly strong carbon‐metal bond between the palladium atom and the coordinated isocyanoacetate molecule. Our mechanistic studies show that the pincer complex catalyst does not undergo redox reactions and, thus it retains a +2 oxidation state under the catalytic process.

Synthesis and crystal structures of the first C2-symmetric bis-aldimine NCN–pincer complexes of platinum and palladium

( S, S)-2,6-bis[( N-α-methylbenzyl)imino]phenylpalladium bromide was synthesised by oxidative addition of palladium(0) to ( S, S)-1-bromo-2,6-bis[( N-α-methylbenzyl)imino]benzene. In contrast, ( S, S)-2,6-bis[( N-α-methylbenzyl)imino]phenylplatinum chloride was synthesised by direct C–H activation from the reaction of potassium tetrachloroplatinate with ( S, S)-1,3-bis[( N-α-methylbenzyl)imino]benzene. The X-ray crystal structures of both pincer complexes were obtained. Treatment of both complexes with silver hexafluoroanimonate gave effective but not stereoselective catalysts for a Michael reaction between methyl vinyl ketone and methyl 2-cyanopropanoate.

Heterobimetallic Fe–Pd and Fe–Pt NCN pincer complexes (NCN = [C 6H 2(CH 2NMe 2) 2-2,6] −)

The meta-diaminoaryl ferrocenes Fc–NCN–H ( 3) and Fc–C C–NCN–H ( 5) (Fc = (η 5-C 5H 5)(η 5-C 5H 4)Fe, NCN–H = C 6H 3(CH 2NMe 2) 2-3,5) can be used as precursors in the preparation of heterobimetallic transition metal complexes of structural type Fc–NCN–MX (NCN = [C 6H 2(CH 2NMe 2) 2-2,6] −; MX = PdCl ( 7), PtCl ( 8), PtI ( 9)) and Fc–C C–NCN–MX (MX = PdCl ( 11), PdI ( 12), PtCl ( 13)), respectively. They are accessible by applying different synthesis procedures, including oxidative addition and metallation– transmetallation processes. Cyclovoltammetric studies show that the ferrocene moieties in 3, 5, 7– 9 and 11- 13 can reversibly be oxidised. The potential of the Fe(II)/Fe(III) redox couple decreases with increasing electron density at the NCN pincer unit. The use of 8 as a possible (electro)chemical sensor in the detection of SO 2 is discussed as well. The solid-state structures of 8 and 13 are reported. The crystals of 8 contain two molecules of 8 in the asymmetric unit. The plane of the C 6H 2 moiety is with 27.2(3)° and 38.2(3)° tilted towards the C 5H 4 entity, while in 13 an angle of 45.9(3)° can be found. The d 8-electron configured platinum atoms possess a somewhat distorted square-planar surrounding, setup by two Me 2NCH 2 ortho-substituents, the NCN C ipso carbon atom and the chloride ligand.

NCN Pincer Palladium Complexes: Their Preparation via a Ligand Introduction Route and Their Catalytic Properties.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Catalytic hydroxylation of 1-propanol by platinum NCN and PCP pincer complexes using CuCl 2 as oxidant

The novel PCP-pincer Pt(II) complex, PtCl { C 6 H 3 - 2 , 6 - ( OPPr 2 i ) 2 } ( 1 ) has been prepared and characterized by 1H, 31P, and 13C NMR spectroscopy. The molecular structure of 1 has been determined through a single-crystal X-ray diffraction study. The pincer-ligated platinum complexes 1 and PtCl{C 6H 3-2,6-(CH 2NEt 2) 2} ( 2) have been explored as catalysts for the hydroxylation of 1-propanol to 1,3-propanediol under mild conditions. Product ratios and turnover numbers achieved with both complexes compare favorably to those obtained with [PtCl 4] 2−. Moreover, the pincer complexes catalyze this transformation even upon replacement of PtCl 4 by the more economical CuCl 2 as the requisite stoichiometric oxidant. Analysis of the reaction mixture by 31P NMR spectroscopy following the hydroxylation of 1-propanol by 1 in the presence of CuCl 2 revealed that 1 is partially converted to the ring substituted complex, PtCl { 3 - Cl - C 6 H 2 - 2 , 6 - ( OPPr 2 i ) 2 } ( 3 ) . The molecular structure of 3 has been determined through a single-crystal X-ray diffraction study.

NCN Pincer Palladium Complexes: Their Preparation via a Ligand Introduction Route and Their Catalytic Properties

A wide range of NCN pincer palladium complexes, [4-tert-butyl-2,6-bis(N-alkylimino)phenyl]chloropalladium (alkyl = n-butyl, benzyl, cyclohexyl, tert-butyl, adamantyl, phenyl, 4-methoxyphenyl), were readily prepared from trans-(4-tert-butyl-2,6-diformylphenyl)chlorobis(triphenylphosphine)palladium via dehydrative introduction of the corresponding alkylimino ligand groups (ligand introduction route) in excellent yields (71-98%). NMR studies on this route for forming pincer complexes revealed the intermediacy of [4-tert-butyl-2,6-bis(N-alkylimino)phenyl]chlorobis(triphenylphosphine)palladium which is in equilibrium with the corresponding NCN pincer complexes via coordination/dissociation of the intramolecular imino groups and triphenylphosphine ligands. A series of chiral NCN pincer complexes bearing pyrroloimidazolone units as the trans-chelating donor groups, [4-tert-butyl-2,6-bis{(3R,7aS)-2-phenylhexahydro-1H-pyrrolo[1,2-c]imidazol-1-on-3-yl}phenyl]chloropalladium, were also prepared from the same precursor via condensation with proline anilides in high yields. The catalytic properties of the NCN imino and the NCN pyrroloimidazolone pincer palladium complexes were examined in the Heck reaction and the asymmetric Michael reaction to demonstrate their high catalytic activity and high enantioselectivity.

Synthesis of 2,6‐Bis(2‐oxazolinyl)phenylplatinum(II) NCN Pincer Complexes by Direct Cyclometalation. Catalysts for Carbon—Carbon Bond Formation

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis and supramolecular association of immobilized NCN-pincer platinum(II) complexes on hyperbranched polyglycerol supports.

Pertosylation of hyperbranched polyglycerol (M(n)=2000; M(w)/M(n)=1.3) followed by partial displacement of the tosyl groups with carboxylic acid functionalized NCN-pincer platinum(II) complexes [PtI-2,6-(NMe(2)CH(2))(2)C(6)H(2)-4-COOH], resulted in covalent attachment of the NCN-pincer complexes to the polyglycerol. These functionalized hyperbranched macromolecules have been characterized by (1)H, (13)C, and (195)Pt NMR, UV-visible, and IR spectroscopy. The presence of Pt and I atoms renders them directly visible by transmission electron microscopy (TEM) without staining procedures, which offers images of associated hyperbranched macromolecules. TEM micrographs show disk-shaped structures with a small size-distribution (15-20 nm), and characteristic core-shell ring structures. The thickness of the corona observed in TEM could be correlated with the substitution degree with pincer platinum moieties.

Optically Active Hyperbranched Polyglycerol as Scaffold for Covalent and Noncovalent Immobilization of Platinum(II) NCN-Pincer Complexes. Catalytic Application and Recovery

New optically active hyperbranched polymers have been used as microenvironments as well as scaffolds for noncovalent and covalent immobilization of pincer platinum(II) complexes, respectively. The catalytic activity/selectivity of the incorporated platinum(II) complexes in these polymeric chiral supports was investigated. Chiral amphiphilic hyperbranched polyglycerols with core−shell “nanocapsules” structure, namely (−)-P(G40C160.5) (1) and (+)-P(G73C160.5) (2), have been prepared in two straightforward steps by ring-opening multibranching polymerization (ROMBP) of either (−)- or (+)-glycidol, resulting in (−)-PG40 (Mn = 3000, Mw/Mn = 1.3) or (+)-PG73 (Mn = 5500, Mw/Mn = 1.6), respectively. Polymerization was followed by partial esterification of a certain fraction of OH groups with palmitoyl chloride. Multifunctional hydrophilic NCN-pincer platinum(II) complexes with sulfonate groups, [PtCl(NCN-SO3H)] (3; NCN = 2,6-bis[(dimethylamino)methyl]phenyl anion), were encapsulated in 1 and 2. The NCN-pincer platinum(II) species encapsulated in the chiral nanocapsules 1 + 3 and 2 + 3 show only slight CD activity with intensity bands of opposite sign, +0.4 mdeg and −0.5 mdeg, respectively, at 320 nm. Pertosylation of the chiral polyglycerols (−)-PG40 and (+)-PG73 afforded the fully tosylated hyperbranched polymers (−)-PG40Tos1.0 (4) and (+)-PG73Tos1.0 (5). Partial displacement of the tosyl groups of 4 with potassium carboxylate functionalized NCN-pincer platinum(II) complexes [PtI(NCN-COOK)] (7) resulted in covalent attachment of the NCN-pincer complexes to the polyglycerol (8). CD spectra of the tosylated compounds 4 and 5 were recorded, showing opposite signals at 256 nm of −1.8 and +2.0 mdeg, respectively, which confirm retention of the chirality during the pertosylation. The decrease in CD intensity of 8 compared to 4 might be attributed to either partial inversion of chirality or racemization during functionalization. The noncovalent encapsulated Pt(II) complexes 1 + 3 and 2 + 3 as well as the covalently attached polymers 8 showed catalytic activity in Michael additions between methyl vinyl ketone and ethyl α-cyanopropionate, although no enantiomeric excess was observed. Quantitative recuperation of these catalysts from catalytic reactions was achieved by dialysis. The combination of the nanosize of the hyperbranched polymer with the catalytic activity of pincer platinum complexes renders these materials promising with respect to application in membrane reactors.

Synthesis of 2,6-Bis(2-oxazolinyl)phenylplatinum(II) NCN Pincer Complexes by Direct Cyclometalation. Catalysts for Carbon−Carbon Bond Formation

1,3-Bis(4‘,4‘-dimethyl-2‘-oxazolinyl)benzene (5a) and 5-nitro-1,3-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)benzene (5b) were heated in dry acetic acid with K2PtCl4 to give the corresponding 2,6-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)phenylplatinum(II) chloride complexes 6a and 6b in 49 and 11% yield, respectively. The X-ray structure of 6b is reported. The main side product observed in the platination of 5a was identified as di(2-methyl-2-N-acetyl)propyl isophthalate. In contrast, use of Pd(OAc)2 with 5a in this protocol, followed by addition of LiBr, gave 2,6-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)phenylpalladium(II) bromide in only 3% yield. Treatment of 6a with AgOTf and AgSbF6 in acetone gave quantitatively the corresponding cationic 2,6-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)phenyl(aquo)platinum(II) complexes 15a and 15b. Similarly treatment of 6b with AgOTf in acetone gave 4-nitro-2,6-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)phenyl(aquo)platinum(II) trifluoromethanesulfonate (15c) (72%). Complexes 15a−c, together with 2,6-bis(4‘,4‘-dimethyl-2‘-oxazolinyl)phenylaquopalladium(II) triflate (15d), were applied as catalysts for the Michael reaction between methyl vinyl ketone and ethyl cyanoacetate and the Diels−Alder reaction between acrylonitrile and cyclopentadiene. In both cases platinum complex 15a was found to be the most active, with the 4-nitro group of 15c resulting in decreased catalytic activity.

Catalysis of aldehyde and imine silylcyanation by platinum and palladium NCN-pincer complexes

The room temperature addition of trimethylsilylcyanide to aromatic and aliphatic aldehydes to give the corresponding cyanohydrins is efficiently catalysed by 1 mol% of ((2,6-bis( N-cyclohexyl)imino)phenyl)aquoplatinum(II) trifluoromethanesulfonate 1a . This methodology is also applicable to the addition of trimethylsilylcyanide to Schiff bases resulting in the formation of α-amino nitriles.

Synthesis of Novel Pd−NCN Pincer Complexes Having Additional Nitrogen Coordination Sites and Their Application as Catalysts for the Heck Reaction

A novel Pd−NCN pincer complex, 7, bearing two additional nitrogen atoms has been synthesized and characterized. The dynamic NMR study of 7 shows the two-site exchange reaction in the NCN pincer ligand with a relatively low ΔG⧧ value, suggesting easy interconversion in the pincer complex. The use of 7 as a catalyst in the Heck reaction was studied. Catalyst 7 is quite effective for all aryl iodides. A turnover number (TON) value of 4.3 × 106 was observed with 4-tolyl iodide. However, 7 is not an effective catalyst for nonactivated bromobenzene and chlorobenzene substrates. According to the study of the Heck reaction, the facile interconversion of the pincer ligand positively affects the catalytic activity.