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Pincer ligand complexes, which appeared nearly five decades ago, have provided a valuable platform for the study of fundamental chemical processes and the development of efficient catalysts for many chemical transformations. These complexes have usually contained transition metal atoms or ions, and their respective pincer ligands have often featured phosphine donor groups, which strongly coordinate to the metal center and allow its steric and electronic properties to be fine‐tuned. The increasing need to develop cost‐effective and sustainable catalytic processes has driven the search for main‐group metals as alternatives to commonly‐used transition metals. In this review, we show that despite the inherent mismatch between phosphines, which are soft Lewis bases, and main‐group metal ions, which are hard Lewis acids, a series of well‐defined phosphine‐based pincer complexes containing Li(I), Na(I), K(I), Mg(II), Ca(II), Zn(II) and Al(III) have been reported, which have proven to be active catalysts for industrially‐relevant transformations. [ABSTRACT FROM AUTHOR]

Herein we report the selective hydrogenation of cyclic imides to diols and amines, homogeneously catalyzed for the first time by a complex of an earth-abundant metal, a manganese pincer complex. A plausible catalytic cycle is proposed based on informative mechanistic experiments. [ABSTRACT FROM AUTHOR]

The first example of oxidative addition of a C–H bond to a square planar d8-Iridium complex, without any external additive, such as an acid, is described. Our mechanistic investigations show that metal–ligand cooperation through aromatization–dearomatization of the lutidine backbone is involved in this process, and that the actual C–H activation step occurs through an Ir(iii) intermediate. [ABSTRACT FROM AUTHOR]

The reactivity of the {RhNO}9 complex [Rh(PCPtBu)(NO)]Ṗ (1Ṗ) with NOṖ was studied. A disproportionation reaction takes place in which N2O is released quantitatively, while the complex Rh(PCPtBu)(NO)(NO2) (2), with coordinated nitrite, is formed. The new complex 2 was fully characterized by multinuclear NMR techniques, IR and X-ray diffraction. The X-ray structure reveals a square pyramidal geometry with an N-bound nitro ligand trans to the Cipso of the PCP ligand and a bent nitrosyl ligand in the apical position. IR measurement of released N2O confirms that one equivalent forms for each molecule of 1Ṗ. Infrared spectroscopic experiments with 1Ṗ-15NO and 14NOṖ suggest that the reaction occurs through the intermediacy of a dinitrosyl complex. In addition, DFT calculations were performed to provide more evidence on the structure of the intermediates and to support the observed reactivity. [ABSTRACT FROM AUTHOR]

The first example of base-metal-catalysed synthesis of amides from the coupling of primary amines with either alcohols or esters is reported. The reactions are catalysed by a new manganese pincer complex and generate hydrogen gas as the sole byproduct, thus making the overall process atom-economical and sustainable. [ABSTRACT FROM AUTHOR]

Homogeneous catalytic hydrogenation of esters to alcohols is an industrially important, environmentally benign reaction. While precious metal-based catalysts for this reaction are now well known, only very few catalysts based on first-row metal complexes were reported. Here we present the hydrogenation of esters catalyzed by a complex of earth-abundant manganese. The reaction proceeds under mild conditions and insight into the mechanism is provided based on an NMR study and the synthesis of novel Mn complexes postulated as intermediates. [ABSTRACT FROM AUTHOR]

Metal-ligand cooperation (MLC) has become an important concept in catalysis by transition metal complexes both in synthetic and biological systems. MLC implies that both the metal and the ligand are directly involved in bond activation processes, by contrast to 'classical' transition metal catalysis where the ligand (e.g. phosphine) acts as a spectator, while all key transformations occur at the metal center. In this Review, we will discuss examples of MLC in which 1) both the metal and the ligand are chemically modified during bond activation and 2) bond activation results in immediate changes in the 1st coordination sphere involving the cooperating ligand, even if the reactive center at the ligand is not directly bound to the metal (e.g. via tautomerization). The role of MLC in enabling effective catalysis as well as in catalyst deactivation reactions will be discussed. [ABSTRACT FROM AUTHOR]

Review: 411 refs. [ABSTRACT FROM AUTHOR]

The electrochemistry of the {RhNO}8complexes [Rh(PCPtBu)(NO)][BF4] (1), [Rh(PCPtBuCH2)(NO)][BF4] (2), and Rh(PCPtBu)(NO)Cl (3) was studied. Both four-coordinate complexes 1橷 2笘扮a reversible reduction within the CH2Cl2solventwindow. Nevertheless, the chemical or electrochemical reduction of 1橷 2 CH2Cl2led to the formation of thefive-coordinate {RhNO}8complexes 3and Rh(PCPtBuCH2)(NO)Cl (4),respectively, through chloride abstraction from CH2Cl2by the one-electron-reduced {RhNO}9species [Rh(PCPtBu)(NO)]•(1•) and [Rh(PCPtBuCH2)(NO)]•(2•), as has been observed for many other17-electron paramagnetic complexes. The new complex 4was fully characterized by multinuclear NMR techniques, IR, X-raydiffraction, CV, UV–vis, and elemental analysis. On the otherhand, the five-coordinate complexes 3and 4show only one irreversible oxidation in CH2Cl2and two irreversible reductions in THF. The {RhNO}9complex 1•could be obtained quantitativelyby one-electron reduction of 1숫 cobaltocene in nonchlorinated solvents and was characterizedby IR, EPR, and 1H NMR in solution. Activation of carbon–halogenbonds by complex 1•wasobserved by studying the reactivity of 1•with some aryl halides, giving in all cases the {RhNO}8Rh(PCPtBu)(NO)X (X = Cl–, 3, or X = I–, 6) asthe only rhodium complex, while a complex with coordination of thearyl moiety was not observed as a stable final product in any case.The fate of the aryl organic radicals could be determined in somecases. In addition, DFT calculations were performed to elucidate theelectronic structure of 1•and to support the observed reactivity. [ABSTRACT FROM AUTHOR]

Due to its significant contribution to stratospheric ozone depletion and its potent greenhouse effect, nitrous oxide has stimulated much research interest regarding its reactivity modes and its transformations, which can lead to its abatement. We report the homogeneously catalyzed reaction of nitrous oxide (N2O) with H2. The reaction is catalyzed by a PNP pincer ruthenium complex, generating efficiently only dinitrogen and water, under mild conditions, thus providing a green, mild methodology for removal of nitrous oxide. The reaction proceeds through a sequence of dihydrogen activation, "O"-atom transfer, and dehydration, in which metal-ligand cooperation plays a central role. This approach was further developed to catalytic O-transfer from N2O to Si-H bonds. [ABSTRACT FROM AUTHOR]

Aromaticity is a central concept in chemistry. Reaction pathways involving reversible ligand dearomatization sequences emerged as a powerful tool for bond activation by metal complexes. Exploring this concept with a metal-free system, we have synthesized a pyridine-coordinated aminoborane which undergoes a temperature-induced formal dearomatization of the pyridine ring. NMR studies and DFT calculations revealed that this formal dearomatization sequence led to an aromaticity switch and the formation of a six-π-electron boron-containing heteroaromatic system. Disrupting this aromatic system by coordination of an amine or a carboxylic acid to the boron center enabled N-H activation and O-H cleavage, leading to an unprecedented reversal aromaticity switch. [ABSTRACT FROM AUTHOR]

The use of carbon dioxide for synthetic applications presents a major goal in modern homogeneous catalysis. Rhodiumhydride PNP pincer complex 1 is shown to add CO2 in two disparate pathways: one is the expected insertion of CO2 into the metal--hydride bond, and the other leads to reductive cleavage of CO2, involving metalligand cooperation. The resultant rhodium--carbonyl complex was found to be photoactive, enabling the activation of benzene and formation of a new benzoyl complex. Organometallic intermediate species were observed and characterized by NMR spectroscopy and X-ray crystallography. Based on the series of individual transformations, a sequence for the photocarbonylation of benzene using CO2 as the feedstock was constructed and demonstrated for the production of benzaldehyde from benzene. [ABSTRACT FROM AUTHOR]

Herein we present a series of new α-iminopyridine-based iron-PNN pincer complexes [FeBr2LPNN] (1), [Fe(CO)2LPNN] (2), [Fe(CO)2LPNN](BF4) (3), [Fe(F)(CO)2LPNN](BF4) (4), and [Fe(H)(CO)2LPNN](BF4) (5) with formal oxidation states ranging from Fe(0) to Fe(II) (LPNN = 2-[(di-tert-butylphosphino)methyl]-6-[1-(2,4,6-mesitylimino)ethyl]pyridine). The complexes were characterized by a variety of methods including ¹H, 13C, 15N, and 31P NMR, IR, Mössbauer, and X-ray photoelectron spectroscopy (XPS) as well as electron paramagnetic resonance (EPR) and magnetic circular dichroism (MCD) spectroscopy, SQUID magnetometry, and X-ray crystallography, focusing on the assignment of the metal oxidation states. Ligand structural features suggest that the α-iminopyridine ligand behaves as a redox non-innocent ligand in some of these complexes, particularly in [Fe(CO)2LPNN] (2), in which it appears to adopt the monoanionic form. In addition, the NMR spectroscopic features (13C, 15N) indicate the accumulation of charge density on parts of the ligand for 2. However, a combination of spectroscopic measurements that more directly probe the iron oxidation state (e.g., XPS), density functional theory (DFT) calculations, and electronic absorption studies combined with time-dependent DFT calculations support the description of the metal atom in 2 as Fe(0). We conclude from our studies that ligand structural features, while useful in many assignments of ligand redox non-innocence, may not always accurately reflect the ligand charge state and, hence, the metal oxidation state. For complex 2, the ligand structural changes are interpreted in terms of strong back-donation from the metal center to the ligand as opposed to electron transfer. [ABSTRACT FROM AUTHOR]

A novel pincer ligand based on the pyrazine backbone (PNzP) has been synthesized, (2,6-bis(di(tert-butyl)phosphinomethyl)pyrazine), tBu-PNzP. It reacts with FeBr2 to yield [Fe(Br)2(tBu-PNzP)], 1. Treatment of 1 with NaBH4 in MeCN/MeOH gives the hydride complex [Fe(H)(MeCN)2(tBu-PNzP)][X] (X = Br, BH4), 2·X. Counterion exchange and exposure to CO atmosphere yields the complex cis-[Fe(H)(CO)(MeCN)(tBu-PNzP)][BPh4] 4·BPh4, which upon addition of Bu4NCl forms [Fe(H)(Cl)(CO)(tBu-PNzP)] 5. Complex 5, under basic conditions, catalyzes the hydrogenation of CO2 to formate salts at low H2 pressure. Treatment of complex 5 with a base leads to aggregates, presumably of dearomatized species B, stabilized by bridging to another metal center by coordination of the nitrogen at the backbone of the pyrazine pincer ligand. Upon dissolution of compound B in EtOH the crystallographically characterized complex 7 is formed, comprised of six iron units forming a 6-membered ring. The dearomatized species can activate CO2 and H2 by metal-ligand cooperation (MLC), leading to complex 8, trans-[Fe(PNzPtBu-COO)(H)(CO)], and complex 9, trans-[Fe(H)2(CO)(tBu-PNzP)], respectively. Our results point at a very likely mechanism for CO2 hydrogenation involving MLC. [ABSTRACT FROM AUTHOR]

Trifluoroacetic esters are reduced by hydrogen in the presence of an iron pincer complex to the corresponding alcohols and trifluoroethanol, for the first time. [ABSTRACT FROM AUTHOR]

Despite considerable interest in ruthenium carbonyl pincer complexes and their substantial catalytic activity, there has been relatively little study of the isoelectronic ruthenium nitrosyl complexes. Here we describe the synthesis and reactivity of several complexes of this type as well as the catalytic activity of complex 6. Reaction of the PNP ligand (PNP = 2,6-bis(tBu2PCH2)pyridine) with RuCl3(NO)(PPh3)2 yielded the Ru(II) complex 3. Chloride displacement by BArF- (BArF- = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) gave the crystallographicaly characterized, linear NO Ru(II) complex 4, which upon treatment with NaBEt3H yielded the Ru(0) complexes 5. The crystallographically characterized Ru(0) square planar complex 5·BF4 bears a linear NO ligand located trans to the pyridilic nitrogen. Further treatment of 5·BF4 with excess LiOH gave the crystallographicaly characterized Ru(0) square planar, linear NO complex 6. Complex 6 catalyzes the dehydrogenative coupling of alcohols to esters, reaching full conversion under air or under argon. Reaction of the PNN ligand (PNN = 2-(tBu2PCH2)-6-(Et2NCH2)pyridine) with RuCl3(NO)(H2O)2 in ethanol gave an equilibrium mixture of isomers 7a and 7b. Further treatment of 7a + 7b with 2 equivalent of sodium isopropoxide gave the crystallographicaly characterized, bent-nitrosyl, square pyramidal Ru(II) complex 8. Complex 8 was also synthesized by reaction of PNN with RuCl3(NO)(H2O)2 and Et3N in ethanol. Reaction of the "long arm" PN²N ligand (PN²N = 2-(tBu2PCH2-)-6-(Et2NCH2CH2)pyridine) with RuCl3(NO)(H2O)2 in ethanol gave complex 9, which upon treatment with 2 equiv of sodium isopropoxide gave complex 10. Complex 10 was also synthesized directly by reaction of PN²N with RuCl3(NO)(H2O)2 and a base in ethanol. A noteworthy aspect of these nitrosyl complexes is their preference for the Ru(0) oxidization state over Ru(II). This preference is observed with both aromatized and dearomatized pincer ligands, in contrast to the Ru(II) oxidation state which is preferred by the analogous carbonyl complexes. [ABSTRACT FROM AUTHOR]

Benzylcations are highly reactive compounds involved as intermediatesin various chemical and biochemical processes. In this work metalcoordination was utilized to stabilize different coordinated modesof benzyl cations, including methylene arenium (MA), π-benzyl,and σ-benzyl complexes. Two bidentate ligand frameworks, diphenylphosphinoethane(dppe) and di-tert-butylphosphinopropane (dtpp),were studied. η2-Coordination to Pd(II) allows forthe characterization and studies of the reactivity of the otherwiseunobserved methylene arenium species under ambient conditions. Therelative stability and electronic structure of the three forms ofthe coordinated benzyl molecule, η2-MA, η1-σ-benzylic, and η3- π-benzylic,were investigated experimentally and computationally. The MA and π-benzylicstructures are preferred in the absence of counteranions, while thedtpp bulky ligand contributes to stabilization of the methylene areniumform. Counteranions have a significant influence on the relative stability.The triflate anion stabilizes the σ-benzylic form upon coordinationto the metal center or the methylene arenium form as a result of compensationof positive charge on the MA ring. Use of the noncoordinating BArFcounteranion promotes conversion to the π-benzylic form. [ABSTRACT FROM AUTHOR]

The synthesis and characterization of new iron pincer complexes bearing bipyridine-based PNN ligands is reported. Three phosphine-substituted pincer ligands, namely, the known (t)Bu-PNN (6-((di-tert-butylphosphino)methyl)-2,2'-bipyridine) and the two new (i)Pr-PNN (6-((di-iso-propylphosphino)methyl)-2,2'-bipyridine) and Ph-PNN (6-((diphenylphosphino)methyl)-2,2'-bipyridine) ligands were synthesized and studied in ligation reactions with iron(II) chloride and bromide. These reactions lead to the formation of two types of complexes: mono-chelated neutral complexes of the type [(R-PNN)Fe(X)2] and bis-chelated dicationic complexes of the type [(R-PNN)2Fe](2+). The complexes [(R-PNN)Fe(X)2] (1: R = (t)Bu, X = Cl, 2: R = (t)Bu, X = Br, 3: R = (i)Pr, X = Cl, and 4: R = (i)Pr, X = Br) are readily prepared from reactions of FeX2 with the free R-PNN ligand in a 1:1 ratio. Magnetic susceptibility measurements show that these complexes have a high-spin ground state (S = 2) at room temperature. Employing a 2-fold or higher excess of (i)Pr-PNN, diamagnetic hexacoordinated dicationic complexes of the type [((i)Pr-PNN)2Fe](X)2 (5: X = Cl, and 6: X = Br) are formed. The reactions of Ph-PNN with FeX2 in a 1:1 ratio lead to similar complexes of the type [(Ph-PNN)2Fe](FeX4) (7: X = Cl, and 8: X = Br). Single crystal X-ray studies of 1, 2, 4, 6, and 8 do not indicate electron transfer from the Fe(II) centers to the neutral bipyridine unit based on the determined bond lengths. Density functional theory (DFT) calculations were performed to compare the relative energies of the mono- and bis-chelated complexes. The doubly deprotonated complexes [(R-PNN*)2Fe] (9: R = (i)Pr, and 10: R = Ph) were synthesized by reactions of the dicationic complexes 6 and 8 with KO(t)Bu. The dearomatized nature of the central pyridine of the pincer ligand was established by X-ray diffraction analysis of single crystals of 10. Reactivity studies show that 9 and 10 have a slightly different behavior in protonation reactions. [ABSTRACT FROM AUTHOR]

The synthesis of novel PNN rutheniumpincer complexes based on2,2′-dipyridinemethane phosphine derivatives, as well as on2,2′-oxobispyridine phosphine ligands, and their reactivitytoward dearomatization and cyclometalation are described. The dearomatizedcompounds 7a,bundergo cyclometalation toyield complexes 8a,b. In order for cyclometalationto proceed, the coordination sphere around the Ru center has to rearrange,and this depends on the flexibility of the system, showing that thecyclometalation is qualitatively faster in the case of the dimethylderivative 7athan in the case of the spyrocyclopentylderivative 7b. The cyclometalation occurs diastereoselectivelyand leads to only one diastereomer of the cyclometalated compounds.In the case of the 2,2′-oxobispyridine complex 6c, the dearomatized complex was too unstable to be isolated; howeverit was possible to isolate and characterize a stable dicarbonyl-dearomatizedruthenium(II) complex, 9c, when the deprotonation wasperformed under a CO atmosphere. Dearomatization of 6aunder CO also led to dicarbonyl-dearomatized ruthenium(II) complex 9a, which slowly rearranged into the dicarbonyl-aromatizedruthenium(0) complex 10a. These complexes were testedin catalytic alcohol–amine coupling, esterification of primaryalcohols, and hydrogenation of secondary amides. Moderate activitywas observed in hydrogenation of amides to alcohols and amines andlow activity in the other transformations, owing mainly to the formationof stable cyclometalated compounds. [ABSTRACT FROM AUTHOR]

The PNS pincer-type ligand 1and the novelRu(PNS)complexes 2–8were synthesized andcharacterized. The (PNS)RuH(Cl)CO complex 2was preparedby reaction of ligand 1with RuH(Cl)CO(PPh3)3. 2reacted with KHMDS (potassium bis(trimethylsilyl)amide)to form the symmetrical dimeric complex 4via the intermediacyof the dearomatized complex (PNS*)Ru(H)CO 3, in whichdeprotonation of the benzylic-S “arm” took place. Reactionof 2with excess NaH gave the dimeric 4,by a formal intermolecular attack of the benzylic “arm”on a second ruthenium center. Complex 4underwent spontaneoustransformation in solution to the dinuclear complex 5via C–S bond cleavage, resulting in the loss of a S-bound tBu group. Treatment of 2with KHMDS in the presenceof PEt3resulted in the trapping of intermediate 3in the form of the dearomatized complex 8.Reaction of 2with LiHBEt3gave the trans-dihydride complex 6, which reacted withCO2to give the formato complex 7, in whichthe formato ligand is located transto the hydride.Complexes 2, 4, and 5werealso investigated as catalysts for the dehydrogenative coupling ofalcohols with amines. [ABSTRACT FROM AUTHOR]

Abstract: Treatment of 7,8-benzo[h]quinoline (bhq-H, 1) and 10-methyl benzo[h]quinoline (bhq-Me, 3) with [Rh(C2H4)2(THF)2][BF4] resulted in double C–H activation of aliphatic and aromatic C–H bonds, yielding the Rh(III) complexes 4 and 5, respectively. The structures of 4 and 5 were revealed by X-ray diffraction. The reaction of 1 with two other slightly different rhodium precursors, [Rh(olefin) n (THF)2][BF4] (COE (n =2), COD (n =1)), led to completely different products, a dinuclear complex 7 and a trinuclear complex 6, respectively, which were characterized by X-ray diffraction. Complex 6 exhibits a rare linear Rh–Rh–Rh structure. Utilizing excess of 1 with [Rh(COD)(THF)2][BF4] led to the formation of a new product 8 with no C–H bond activation taking place. Additional C–H activation products of 1, cationic and neutral, in the presence of P i Pr3 (9a, 9b and 10) are also presented. [Copyright &y& Elsevier]

Discovery of an efficient artificial catalyst for the sunlight-driven splitting of water into dioxygen and dihydrogen is a major goal of renewable energy research. We describe a solution-phase reaction scheme that leads to the stoichiometric liberation of dihydrogen and dioxygen in consecutive thermal- and light-driven steps mediated by mononuclear, well-defined ruthenium complexes. The initial reaction of water at 25°C with a dearomatized ruthenium (11) [Ru(II)] pincer complex yields a monomeric aromatic Ru(ll) hydrido-hydroxo complex that, on further reaction with water at 100°C, releases H2 and forms a cis dihydroxo complex. Irradiation of this complex in the 320-to-420-nanometer range liberates oxygen and regenerates the starting hydrido-hydroxo Ru(II) complex, probably by elimination of hydrogen peroxide, which rapidly disproportionates. Isotopic labeling experiments with H217O and H218O show unequivocally that the process of oxygen-oxygen bond formation is intramolecular, establishing a previously elusive fundamental step toward dioxygen-generating homogeneous catalysis. [ABSTRACT FROM AUTHOR]

The synthesis of new ligand systems based on the bipyridine unit for bi- and trimetallic complexes, including a rare example of a chiral bimetallic complex, is presented. Ligands BBPX (bis-bipyridine-xylene, 3) and TBPBX (tris- bipyridine-bis-xylene, 4) were prepared in one step by reacting α,α'-dibromo-o-xylene (2) with 2 equiv of the monolithiated derivative of 4,4'-dimethyl-2,2'-bipyridine. Dilithium (S)-binaphtholate (5) reacted with 2 equiv of 4-bromomethyl-4'-methyl-2,2'-bipyridine (6), affording ligand (S)-BBPBINAP (bis-bipyridine-binaphtholate, 7). These ligands reacted cleanly with 1, 1.5, and 1 equiv of the rhodium dimer [Rh2Cl2(HD)2] (HD = 1,5-hexadiene), respectively. Chloride abstraction led to the isolation of the cationic complexes BBPX[Rh(HD)BF4]2 (8), TBPBX[Rh(HB)BF4]3 (10), and (S)-BBPBINAP[Rh(HD)BF4]2 (12). When BBPX (3), TBPBX (4), and (S)-BBPBINAP (7) were added to 2, 3, and 2 equiv of [Rh(NBD)2]BF4 or [Rh(NBD)(CH3CN)2]BF4 (NBD = norbornadiene), respectively, clean formation of BBPX[Rh(NBD)BF4]2 (9), TBPBX[Rh(NBD)BF4]3 (11), and (S)-BBPBINAP[Rh(NBD)BF4]2 (13) was observed. The neutral iridium complex (S)-BBPBNAP[IrCl(COD)]2 (14) was obtained by reaction of (S)-BBPBINAP (7) with 1 equiv of [Ir2Cl2(COD)2] (COD = cyclooctadiene). The complexes were fully characterized including X-ray structural studies of 8, 9, and 13, and preliminary studies on their catalytic activity were performed. [ABSTRACT FROM AUTHOR]

Presents research which prepared catalytically active Langmuir-Blodgett films of a rhodium complex to determine the possible effect of the molecular order of metal complexes on catalytic activity. Model reaction; Traits of complex in solution; Reactions and the substrate; Molecular order in catalysis.

The article discusses the study on the nitrogen (N)- hydrogen (H) activation of amines and ammonia by ruthenium (Ru) thru metal-ligand cooperation. It notes that activation of N-H bond of amines and ammonia by a transition-metal center may play a role in developing new catalytic processes based on these compounds. It notes that the study results, which involve aromatization/dearomatization of the ligand, represent a novel approach to N-H activation by transition-metal center