Pincer Complexes as Catalysts in Organic Chemistry. In: van Koten G, Milstein D, editors. Organometallic Pincer Chemistry. Berlin: Springer Berlin Heidelberg; 2013. pp. 203-41. [DOI: 10.1007/978-3-642-31081-2_7]

Nickel Pincer Complexes Catalyzed Sustainable Synthesis of 3,4-Dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxides via Acceptorless Dehydrogenative Coupling of Primary Alcohols

We report the atom-economic and sustainable synthesis of biologically important 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxide (DHBD) derivatives from readily available aromatic primary alcohols and 2-aminobenzenesulfonamide catalyzed by nickel(II)-N∧N∧S pincer-type complexes. The synthesized nickel complexes have been well-studied by elemental and spectroscopic (FT-IR, NMR, and HRMS) analyses. The solid-state molecular structure of complex 2 has been authenticated by a single-crystal X-ray diffraction study. Furthermore, a series of 3,4-dihydro-2H-1,2,4-benzothiadiazine-1,1-dioxide derivatives have been synthesized (24 examples) utilizing a 3 mol % Ni(II) catalyst through acceptorless dehydrogenative coupling of benzyl alcohols with benzenesulfonamide. Gratifyingly, the catalytic protocol is highly selective with the yield up to 93% and produces eco-friendly water/hydrogen gas as byproducts. The control experiments and plausible mechanistic investigations indicate that the coupling of the in situ generated aldehyde with benzenesulfonamide leads to the desired product. In addition, a large-scale synthesis of one of the thiadiazine derivatives unveils the synthetic usefulness of the current methodology.

Unexplored Facet of Pincer Ligands: Super-Reductant Behavior Applied to Transition-Metal-Free Catalysis

Pincer ligands are well-established supporting ancillaries to afford robust coordination to metals across the periodic table. Despite their widespread use in developing homogeneous catalysts, the redox noninnocence of the ligand backbone is less utilized in steering catalytic transformations. This report showcases a trianionic, symmetric NNN-pincer to drive C-C cross-coupling reactions and heterocycle formation via C-H functionalization, without any coordination to transition metals. The starting substrates are aryl chlorides that can tease the limit of a catalyst's ability to promote a reductive cleavage at a much demanding potential of -2.90 V vs SCE. The reducing power of the simple trianionic ligand backbone has been tremendously amplified by shining visible light on it. The catalyst's success relies on its easy access to the one-electron oxidized iminosemiquinonate form that has been thoroughly characterized by X-band electron paramagnetic resonance spectroscopy through spectroelectrochemical experiments. The moderately long-lived excited-state lifetime (10.2 ns) and such a super-reductive ability dependent on the one-electron redox shuttle between the bisamido and iminosemiquinonato forms make this catalysis effective.

Platinum thiolate complexes supported by PBP and POCOP pincer ligands as efficient catalysts for the hydrosilylation of carbonyl compounds

Diphosphino-boryl-based PBP pincer platinum thiolate complexes, [Pt(SR){B(NCH₂P^tBu₂)₂-1,2-C₆H₄}] (R = H, 1a; Ph, 1b), and benzene-based bisphosphinite POCOP pincer platinum thiolate complexes, [Pt(SR)(^tBu₂PO)₂-1,3-C₆H₃] (R = H, 2a; Ph, 2b), were prepared and fully characterized by multinuclear NMR, X-ray crystallography, HRMS and elemental analyses. The application of these complexes in the catalytic hydrosilylation of aldehydes and ketones was investigated. It was found that these platinum thiolate complexes are efficient catalysts for the hydrosilylation of aldehydes and ketones at 65-75 °C. Comparatively, the PBP complexes are more active than the corresponding POCOP complexes. Both phenylsilane and polymethylhydrosiloxane (PMHS) can be used as silyl reagents. The expected alcohols were obtained in good to excellent yields after the basic hydrolysis of the hydrosilylation products and many functional groups were not affected. With turnover frequencies (TOFs) of up to 67 000 h^-1, the present catalytic system represents the most effective platinum catalytic system for the hydrosilylation of carbonyl compounds. The reactions were likely catalysed by the in situ generated platinum hydride species.

Bimetallic complexes that merge metallocene and pincer-metal building blocks: synthesis, stereochemistry and catalytic reactivity

This perspective is to illustrate the synthesis and applications of bimetallic complexes by merging a metallocene and a (cyclopentadienyl/aryl) pincer metal complex. Four possible ways to merge metallocene and pincer-metal motifs are reported and representative examples are discussed in more detail. These bimetallic complexes have been employed in some important catalytic reactions such as cross-coupling, transfer hydrogenation or synthesis of ammonia. The metallocene fragment may tune the electronic properties of the pincer ligand, due to its redox reversible properties. Also, the presence of two metals in a single complex allows their electronic communication, which proved beneficial for, e.g., the catalytic activity of some species. The presence of the metallocene fragment provides an excellent opportunity to develop chiral catalysts, because the metallocene merger generally renders the two faces of the pincer-metal catalytic site diastereotopic. Besides, an extra chiral functionality may be added to the bimetallic species by using pincer motifs that are planar chiral, e.g. by using the different substituents of pincer ligand "arms" or non-symmetrical arene groupings. Post-functionalization of pre-formed pincer-metal complexes, via η⁶-coordination with an areneophile such as [CpRu]⁺ and [Cp*Ru]⁺ presents a striking strategy to obtain diastereomeric metallocene-pincer type derivatives, that actually involve half-sandwich metallocenes. This approach offers the possibility to create diastereomerically pure derivatives by using the chiral TRISPHAT anion. The authors hope that this report of the synthetic, physico-chemical properties and remarkable catalytic activities of metallocene-based pincer-metal complexes will inspire other researchers to continue exploring this realm.

The Stability of Diphosphino-Boryl PBP Pincer Backbone: PBP to POP Ligand Hydrolysis

Since moisture may frequently be present in many solvents, it is important to know the reactivity of a catalyst against water for catalytic reactions. In order to explore the stability and understand the transformation process of diphosphino-boryl-based PBP pincer platform, [PdCl{B(NCH₂ P^t Bu₂ )₂ -o-C₆ H₄ }] (1) was treated with PdCl₂ , HB(NCH₂ PPh₂ )₂ -o-C₆ H₄ was reacted with [PdCl₂ (cod)] (cod=cyclo-octa-1,5-diene) and [Pd₂ (dba)₃ ] (dba=dibenzylideneacetone), respectively, in the presence of water. Some novel palladium POP complexes, [Pd₂ Cl₂ (μ-Cl){μ-κ³ -P,O,P-OB(NCH₂ P^t Bu₂ )₂ -o-C₆ H₄ }] (2 a), [Pd₄ (μ-Cl)₂ (μ-O)₂ {μ-κ³ -P,O,P-OB(NCH₂ PPh₂ )₂ -o-C₆ H₄ }₂ ] (2 b), [Pd₂ {μ-κ⁴ -P,P,P,P-O(B(NCH₂ PPh₂ )₂ -o-C₆ H₄ )₂ }{μ-κ² -P,P-(NHCH₂ PPh₂ )₂ -o-C₆ H₄ }] (3), were obtained. It was found that the PBP pincer backbone can easily be converted into a POP backbone in the presence of water. From the crystal structures of the resultant palladium complexes, possible pincer backbone transformation pathways were discussed.

Palladium(II) Pincer Complexes of Functionalized Amides with S-Modified Cysteine and Homocysteine Residues: Cytotoxic Activity and Different Aspects of Their Biological Effect on Living Cells

In the search for potential new metal-based antitumor agents, two series of nonclassical palladium(II) pincer complexes based on functionalized amides with S-modified cysteine and homocysteine residues have been prepared and fully characterized by 1D and 2D NMR (¹H, ¹³C, COSY, HMQC or HSQC, ¹H-¹³C, and ¹H-¹⁵N HMBC) and IR spectroscopy and, in some cases, X-ray diffraction. Most of the resulting complexes exhibit a high level of cytotoxic activity against several human cancer cell lines, including colon (HCT116), breast (MCF7), and prostate (PC3) cancers. Some of the compounds under consideration are also efficient in both native and doxorubicin-resistant transformed breast cells HBL100, suggesting the prospects for the creation of therapeutic agents based on the related compounds that would be able to overcome drug resistance. An analysis of different aspects of their biological effects on living cells has revealed a remarkable ability of the S-modified derivatives to induce cell apoptosis and efficient cellular uptake of their fluorescein-conjugated counterpart, confirming the high anticancer potential of Pd(II) pincer complexes derived from functionalized amides with S-donor amino acid pendant arms.

The Reactivity of Mercapto Groups against Boron Hydrides in Pincer Ligated Nickel Mercapto Complexes

Several pincer ligated nickel mercapto complexes, [2,6-(R₂ PCH₂ )₂ C₆ H₃ ]NiSH (R=tBu, 1 a; iPr, 1 b), [2,6-(R₂ PO)₂ C₆ H₃ ]NiSH (R=tBu, 2 a; iPr, 2 b) and [4-MeOCO-2,6-(tBu₂ PO)₂ C₆ H₂ ]NiSH (3 a), were synthesized and fully characterized. The reactivity of the mercapto groups against boron hydrides and organic bases was investigated. It was found that the mercapto groups are difficult to be deprotonated by boron hydrides or organic bases. The treatment of complex 2 a or 2 b with an excess amount of catecholborane (HBcat) afforded the corresponding pincer ligated nickel borohydride complexes and the HBcat degradation product. The treatment of complex 1 a, 2 a or 2 b with an excess amount of BH₃ ⋅THF produced the corresponding nickel borohydride species and the S-bridged triborane species THF⋅BH₂ -μ₂ -S(B₂ H₅ ) (5). No reactions between these complexes and organic bases were observed. DFT calculations were carried out to understand this reactivity and get mechanistic insights into the reactions.

Backbone Dehydrogenation in Pyrrole-Based Pincer Ligands

Treatment of both [CoCl( ^tBuPNP)] and [NiCl( ^tBuPNP)] ( ^tBuPNP = anion of 2,5-bis((di- tert-butylphosphino)methyl)pyrrole) with one equivalent of benzoquinone affords the corresponding chloride complexes containing a dehydrogenated PNP ligand, ^tBudPNP ( ^tBudPNP = anion of 2,5-bis((di- tert-butylphosphino)methylene)-2,5-dihydropyrrole). Dehydrogenation of PNP to dPNP results in minimal change to steric profile of the ligand but has important consequences for the resulting redox potentials of the metal complexes, resulting in the ability to isolate both [CoH( ^tBudPNP)] and [CoEt( ^tBudPNP)], which are more challenging (hydride) or not possible (ethyl) to prepare with the parent PNP ligand. Electrochemical measurements with both the Co and Ni dPNP species demonstrate a substantial shift in redox potentials for both the M(II/III) and M(II/I) couples. In the case of the former, oxidation to trivalent Co was found to be reversible, and subsequent reaction with AgSbF₆ afforded a rare example of a square-planar Co(III) species. Corresponding reduction of [CoCl( ^tBudPNP)] with KC₈ produced the diamagnetic Co(I) species, [Co(N₂)( ^tBudPNP)]. Further reduction of the Co(I) complex was found to generate a pincer-based π-radical anion that demonstrated well-resolved EPR features to the four hydrogen atoms and lone nitrogen atom of the ligand with minor contributions from cobalt and coordinated N₂. Changes in the electronic character of the PNP ligand upon dehydrogenation are proposed to result from loss of aromaticity in the pyrrole ligand, resulting in a more reducing central amido donor. DFT calculations on the Co(II) complexes were performed to shed further insight into the electronic structure of the pincer complexes.

Synthesis of quinolinyl-based pincer copper(ii) complexes: an efficient catalyst system for Kumada coupling of alkyl chlorides and bromides with alkyl Grignard reagents

Well-defined quinolinamide-based pincer copper complexes have been developed and demonstrated in the Kumada coupling reaction of nonactivated alkyl chlorides and bromides with alkyl magnesium chloride.

A reaction of [2,6-(tBu2PO)2C6H3]NiSCH2Ph with BH3·THF: borane mediated C–S bond cleavage

C–S bond cleavage of the thiolato ligand of [2,6-(^tBu₂PO)₂C₆H₃]NiSCH₂Ph mediated by BH₃ as a suitable model for the transition metal catalyzed C–S bond activation of mercaptans.

Development of (quinolinyl)amido-based pincer palladium complexes: a robust and phosphine-free catalyst system for C–H arylation of benzothiazoles

Well-defined (quinolinyl)amido-pincer palladium complexes are developed and employed for the catalytic C–H bond arylation of benzothiazoles with aryl iodides, which can be recycled and reused for several cycles.

Computational and carbon-13 NMR studies of Pt-C bonds in P-C-P pincer complexes

A (13)C{(1)H} NMR based investigation was conducted to examine the electronic properties of C(aryl)-M bonds and their trans influence in P-C(aryl)-P pincer complexes. A series of structurally related platinum pincer complexes were rationally designed and their corresponding (13)C-(195)Pt coupling constants were systematically examined. By methodical substitution of the ligand trans to the organometallic C(aryl)-Pt bond, this study revealed the significant influence of the ligands on the nature of the C(aryl)-M bonds. The single crystal X-ray analysis of the complexes and computational studies further confirmed the observations that the C-M bond exhibits significant π-character.

Efficient and stereoselective synthesis of monomeric and bimetallic pincer complexes containing Pd-bonded stereogenic carbons

Optically-pure aliphatic N–C(sp³)–E Pd pincer complexes with C-stereogenic centers were synthesized via stepwise (1) enantioselective catalytic hydrophosphination, (2) P(iii) → P(v) oxidation and (3) diastereoselective C(sp³)–H activation.

The synthesis and efficient one-pot catalytic “self-breeding” of asymmetrical NC(sp3)E-hybridised pincer complexes

Asymmetric NC(sp³)E (E = S, O) pincer complexes were examined as catalysts in asymmetric hydrophosphination.

Bimetallic Complexes Supported by a Redox-Active Ligand with Fused Pincer-Type Coordination Sites

The remarkable chemistry of mononuclear complexes featuring tridentate, meridionally chelating "pincer" ligands has stimulated the development of ligand frameworks containing multiple pincer sites. Here, the coordination chemistry of a novel pentadentate ligand (L(N3O2)) that provides two closely spaced NNO pincer-type compartments fused together at a central diarylamido unit is described. The trianionic L(N3O2) chelate supports homobimetallic structures in which each M(II) ion (M = Co, Cu, Zn) is bound in a meridional fashion by the bridging diarylamido N atom and O,N-donors of the salicyaldimine arms. The metal centers are also coordinated by a mono- or bidentate auxiliary ligand (L(aux)), resulting in complexes with the general form [M2(L(N3O2))(L(aux))2](+) (where L(aux) = 1-methyl-benzimidazole (1MeBI), 2,2'-bipyridine (bpy), 4,4'-dibromo-2,2'-bipyridine (bpy(Br2)), or (S)-2-(4-isopropyl-4,5-dihydrooxazolyl)pyridine (S-(iPr)OxPy)). The fused nature of the NNO pincer sites results in short metal-metal distances ranging from 2.70 Å for [Co2(L(N3O2)) (bpy)2](+) to 3.28 Å for [Zn2(L(N3O2)) (bpy)2](+), as revealed by X-ray crystallography. The complexes possess C2 symmetry due to the twisting of the aryl rings of the μ-NAr2 core; spectroscopic studies indicate that chiral L(aux) ligands, such as S-(iPr)OxPy, are capable of controlling the helical sense of the L(N3O2) scaffold. Since the four- or five-coordinate M(II) centers are linked solely by the amido moiety, each features an open coordination site in the intermetallic region, allowing for the possibility of metal-metal cooperativity in small-molecule activation. Indeed, the dicobalt(II) complex [Co2(L(N3O2)) (bpy(Br2))2](+) reacts with O2 to yield a dicobalt(III) species with a μ-1,2-peroxo ligand. The bpy-containing complexes exhibit rich electrochemical properties due to multiple metal- and ligand-based redox events across a wide (3.0 V) potential window. Using electron paramagnetic resonance (EPR) spectroscopy and density functional theory (DFT), it was determined that one-electron oxidation of [Co2(L(N3O2)) (bpy)2](+) results in formation of a S = 1/2 species with a L(N3O2)-based radical coupled to low-spin Co(II) centers.

Design and development of POCN-pincer palladium catalysts for C-H bond arylation of azoles with aryl iodides

Well-defined and efficient POCN-ligated palladium complexes have been developed for the direct C-H bond arylation of azoles with aryl iodides. The phosphinite-amine pincer ligands 1-(R2PO)-C6H4-3-(CH2N(i)Pr2) [(R2)POCN(iPr2)-H; R = (i)Pr (), R = (t)Bu ()] and corresponding palladium complexes {2-(R2PO)-C6H3-6-(CH2N(i)Pr2)}PdCl [((R2)POCN(iPr2))PdCl; R = (i)Pr (), R = (t)Bu ()] were synthesized in good yields. Treatment of palladium complex with KI and AgOAc afforded the complexes ((iPr2)POCN(iPr2))PdI () and ((iPr2)POCN(iPr2))Pd(OAc) (), respectively. Similarly, the reaction of with benzothiazolyl-lithium produces the ((iPr2)POCN(iPr2))Pd(benzothiazolyl) () complex in a quantitative yield. The pincer palladium complex efficiently catalyzes the C-H bond arylation of benzothiazole, substituted-benzoxazoles and 5-aryl oxazoles with diverse aryl iodides in the presence of CuI as a co-catalyst under mild reaction conditions. This represents the first example of a pincer palladium complex being applied for the direct C-H bond arylation of any heterocycle with low catalyst loading. A preliminary mechanistic investigation reveals that palladium nanoparticles are presumably not the catalytically active form of and supports the direct involvement of the catalyst , with complex being a probable key intermediate in the catalytic reaction.

Recent advances in the use of chiral metal complexes with achiral ligands for application in asymmetric catalysis

Recent progress in asymmetric catalysis using chiral only-at-metal complexes is reviewed, with emphasis on Ir(iii) and Rh(iii) systems.

Mono- and binuclear palladacycles via regioselective C–H bond activation: syntheses, mechanistic insights and catalytic activity in direct arylation of azoles

Hybrid “POCN”-ligated mono- and binuclear palladacycles have been synthesized via the base-assisted regioselective C–H bond activation, and their mechanistic aspects and catalytic application for the arylation of azoles have been described.

Pincer versus pseudopincer: isomerism in palladium(ii) complexes bearing κ3C,S,C ligands

In NHC pincer complexes incorporating a hemilabile donor site, there exists an equilibrium between the true pincer form and a pseudopincer coordination isomer.