NHC Core Pincer Ligands Exhibiting Two Anionic Coordinating Extremities

The chemistry of NHCcore pincer ligands of LX₂ type bearing two pending arms, identical or not, whose coordinating center is anionic in nature, is here reviewed. In this family, the negative charge of the coordinating atoms can be brought either by a carbon atom via a phosphonium ylide (R₃P⁺-CR₂^-) or by a heteroatom through amide (R₂N^-), oxide (RO^-), or thio(seleno)oxide (RS^-, RSe^-) donor functionalities. Through selected examples, the synthetic methods, coordination properties, and applications of such tridentate systems are described. Particular emphasis is placed on the role of the donor ends in the chemical behavior of these species.

Steric and Electronic Effect of Cp-Substituents on the Structure of the Ruthenocene Based Pincer Palladium Borohydrides

Ruthenocene-based PCP^tBu pincer ligands were used to synthesize novel pincer palladium chloride Rc^F[PCP^tBu]PdCl (2a) and two novel palladium tetrahydroborates Rc^F[PCP^tBu]Pd(BH₄) (3a) and Rc^*[PCP^tBu]Pd(BH₄) (3b), where Rc^F[PCP^tBu] = κ³-{2,5-(tBu₂PCH₂)_2-C₅H₂}Ru(Cp^F) (Cp^F = C₅Me₄CF₃), and Rc*[PCP^tBu] = κ³-{2,5-(tBu₂PCH₂)₂C₅H₂}Ru(Cp*) (Cp* = C₅Me₅). These coordination compounds were characterized by X-ray, NMR and FTIR techniques. Analysis of the X-ray data shows that an increase of the steric bulk of non-metalated cyclopentadienyl ring in 3a and 3b relative to non-substituted Rc[PCP^tBu]Pd(BH₄) analogue (3c; where Rc[PCP^tBu] = κ³-{2,5-(tBu₂PCH₂)₂C₅H₂}Ru(Cp), Cp = C₅H₅) pushes palladium atom from the middle plane of the metalated Cp ring in the direction opposite to the ruthenium atom. This displacement increases in the order 3c < 3b < 3a following the order of the Cp-ring steric volume increase. The analysis of both X-ray and IR data suggests that BH₄ ligand in both palladium tetrahydroborates 3a and 3b has the mixed coordination mode η^1,2. The strength of the BH₄ bond with palladium atom increases in the order Rc[PCP^tBu]Pd(BH₄) < Rc*[PCP^tBu]Pd(BH₄) < Rc^F[PCP^tBu]Pd(BH₄) that appears to be affected by both steric and electronic properties of the ruthenocene moiety.

Cyclometalated Ruthenium Pincer Complexes as Catalysts for the α-Alkylation of Ketones with Alcohols

Ruthenium PNP pincer complexes bearing supplementary cyclometalated C,N‐bound ligands have been prepared and fully characterized for the first time. By replacing CO and H⁻ as ancillary ligands in such complexes, additional electronic and steric modifications of this topical class of catalysts are possible. The advantages of the new catalysts are demonstrated in the general α‐alkylation of ketones with alcohols following a hydrogen autotransfer protocol. Herein, various aliphatic and benzylic alcohols were applied as green alkylating agents for ketones bearing aromatic, heteroaromatic or aliphatic substituents as well as cyclic ones. Mechanistic investigations revealed that during catalysis, Ru carboxylate complexes are predominantly formed whereas neither the PNP nor the CN ligand are released from the catalyst in significant amounts.

A shape changing tandem Rh(CNC) catalyst: preparation of bicyclo[4.2.0]octa-1,5,7-trienes from terminal aryl alkynes

The preparation of a range of tetraaryl-substituted bicyclo[4.2.0]octa-1,5,7-trienes using a one-pot procedure starting from terminal aryl alkynes and catalysed by a rhodium(i) complex is reported. This synthesis proceeds by a reaction sequence involving head-to-tail homocoupling of the terminal alkyne and zipper annulation of the resulting gem-enyne. The rhodium catalyst employed is notable for the incorporation of a flexible NHC-based pincer ligand, which is suggested to interconvert between mer- and fac-coordination modes to fulfil the orthogonal mechanistic demands of the two transformations. Evidence for this interesting auto-tandem action of the catalyst is provided by reactions of the precatalyst with model substrates, corroborating proposed intermediates in both component cycles, and norbornadiene, which reversibly captures the change in pincer ligand coordination mode, along with a DFT-based computational analysis.

Electronic influence of substitution on the pyridine ring within NNN pincer-type molecules

Pincer molecules are of increasing interest due to the modular nature of modification and range of reactivity observed when coordinated to metal ions. A subset within the family of pincer molecules use a pyridine group to bridge the outer two arms as well as provide a N-donor atom for metal binding. While the arm appendages have been studied extensively, little research has been conducted on the electronic effects of the central, substituted pyridine systems. Therefore, a series of NNN pincer-type ligands with substitution on the 4-position of the pyridine ring with -OH, -OBn, -H, -Cl, and -NO2 functional groups were synthesized and characterized through NMR spectroscopy and ESI-HRMS. Each pincer was metalated with Cu(ii) salts and evaluated through X-ray diffraction analysis, cyclic voltammetry, and density functional theory analysis. The results indicate that the relatively unstudied -OBn group demonstrates both electron-withdrawing (XRD bond lengths) and electron-donating (NMR spectroscopy) properties. The -NO2 pincer ligand shows a redox event within experimental windows evaluated, in contrast to the other congeners studied. In addition, electron-donating groups increase the electron density around the Cu(ii) center based on DFT studies and cyclic voltammetry. These findings can be applied to other pyridine-based pincer systems when considering ligand design and warrants future characterization of 4-position substituted pyridines.

Synthesis and group 9 complexes of macrocyclic PCP and POCOP pincer ligands

The synthesis of macrocyclic variants of commonly employed phosphine-based pincer (pro)ligands derived from meta-xylene (PCP-14) and resorcinol (POCOP-14) is described, where the P-donors are trans-substituted with a tetradecamethylene linker. The former was accomplished using a seven-step asymmetric procedure involving (-)-cis-1-amino-2-indanol as a chiral auxiliary and ring-closing olefin metathesis. A related, but non-diastereoselective route was employed for the latter, which consequently necessitated chromatographic separation from the cis-substituted by-product. The proligands are readily metalated and homologous series of M^I(CO) and M^IIICl₂(CO) derivatives (M = Rh, Ir) have been isolated and fully characterised in solution and the solid state. Metal hydride complexes are generated during the synthesis of the former and have been characterised in situ using NMR spectroscopy.

Pyridine Wingtip in [Pd(Py-tzNHC)2]2+ Complex Is a Proton Shuttle in the Catalytic Hydroamination of Alkynes

The cationic palladium(II) complex 1 of pyridyl-mesoionic carbene ligand catalyzes Markovnikov-selective intermolecular hydroamination between anilines and terminal alkynes into the corresponding imines. The reaction proceeds at room temperature, in the absence of additives, with exquisite selectivity and diverse functional group tolerance. The key intrinsic feature of the catalyst is the pyridine wingtip confined to the proximity of the alkynophilic metal active site, which mimics the function of enzyme-like architectures by assisting entropically favored proton transfers.

Synthesis and rhodium complexes of macrocyclic PNP and PONOP pincer ligands

The synthesis of macrocyclic variants of commonly employed phosphine-based pincer ligands derived from lutidine (PNP-14) and 2,6-dihydroxypyridine (PONOP-14) is described, where the P-donors are trans-substituted with a tetradecamethylene linker. This was accomplished using an eight-step procedure involving borane protection, ring-closing olefin metathesis, chromatographic separation from the cis-substituted diastereomers, and borane deprotection. The rhodium coordination chemistry of these ligands has been explored, aided by the facile synthesis of 2,2'-biphenyl (biph) adducts [Rh(PNP-14)(biph)][BAr^F₄] and [Rh(PONOP-14)(biph)][BAr^F₄] (Ar^F = 3,5-(CF₃)₂C₆H₃). Subsequent hydrogenolysis enabled generation of dihydrogen, ethylene and carbonyl derivatives; notably the ν(CO) bands of the carbonyl complexes provide a means to compare the donor properties of the new pincer ligands with established acyclic congeners.

Conformation controlled stepwise hydride shuffling from the metal to the ligand backbone

The protonation of the benzo annulated cycloheptatriene PCP pincer ruthenium hydrido dicarbonyl complex using the superacid [H(Et₂O)₂][BArF₂₄] launches a cascade of hydride shifts which lead to the hydrogenation of the ligand backbone.

A Z-type PGeP pincer germylene ligand in a T-shaped palladium(0) complex

An unusual T-shaped palladium(0) complex having a σ-acceptor (Z-type) germylene scaffold has been prepared from a dipyrromethane-based PGeP germylene.

Reactions of Rh(PNP) pincer complexes with terminal alkynes: homocoupling through a ring or not at all

“Switching on” a metal's capacity to promote terminal alkyne coupling reactions using a macrocyclic pincer ligand.

Synthetic access to a phosphorescent non-palindromic pincer complex of palladium by a double oxidative addition – comproportionation sequence

A highly phosphorescent non-palindromic (C^C^N) palladium complex may be prepared by means of a double oxidative addition – comproportionation sequence, which is a new approach for the synthesis of non-palindromic pincer complexes.

New cobalt(ii) coordination designs and the influence of varying chelate characters, ligand charges and incorporated group I metal ions on enzyme-like oxidative coupling activity

In transition-metal-mediated catalysis, design of new, well defined coordination architectures and subjecting them to catalysis testing under the same reaction conditions is a necessity tool for improved understanding of desirable active site geometries and characteristics.

Strategies and mechanisms of metal–ligand cooperativity in first-row transition metal complex catalysts

The use of metal–ligand cooperation (MLC) by transition metal bifunctional catalysts has emerged at the forefront of homogeneous catalysis science.

Isomeric Tetradentate Ligand-Containing Cyclometalated Gold(III) Complexes

A simple one-pot two bond-forming reaction for the rapid construction of cyclometalated gold(III) complexes with fully π-conjugated tetradentate ligand is reported. The coupling of the bifunctional gold(III) precursor with the bifunctional aromatic compound has led to the formation of two regioisomers with either C- or N-coordination. Through monitoring by high-throughput high performance liquid chromatography, the regioselectivity of the reaction has been effectively tuned toward the formation of a single isomer, allowing easy separation of the metal complexes. The structures of the complexes have been determined by X-ray crystallography, and the photophysical, electrochemical, and electroluminescence (EL) studies have been carried out. Computational study has been performed to provide insights into the nature of the excited states. Isomeric effect has been shown to have a significant influence on the EL behavior of the organic light-emitting devices.

Preparation of a Series of Supported Nonsymmetrical PNP-Pincer Ligands and the Application in Ester Hydrogenation

In contrast to their symmetrical analogues, nonsymmetrical PNP-type ligand motifs have been less investigated despite the modular pincer structure. However, the introduction of mixed phosphorus donor moieties provides access to a larger variety of PNP ligands. Herein, a facile solid-phase synthesis approach towards a diverse PNP-pincer ligand library of 14 members is reported. Contrary to often challenging workup procedures in solution-phase, only simple workup steps are required. The corresponding supported ruthenium-PNP catalysts are screened in ester hydrogenation. Usually, industrially applied heterogeneous catalysts require harsh conditions in this reaction (250-350 °C at 100-200 bar) often leading to reduced selectivities. Heterogenized reusable Ru-PNP catalysts are capable of reducing esters and lactones selectively under mild conditions.

Employing Aryl-Linked Bis-mesoionic Carbenes as a Pincer-Type Platform to Access Ambient-Stable Palladium(IV) Complexes

The study of palladium(IV) species has great implications for Pd^II /Pd^IV -mediated catalysis. However, most of the Pd^IV complexes rapidly decompose under ambient conditions, which makes the isolation, characterization and further reactivity study very challenging. The reported ancillary ligand platforms to stabilize Pd^IV species are dominated by chelating N-donors such as bipyridines. In this work, we present two Pd^IV complexes with scarcely used C-donors as the supporting platform. The anionic aryl donor and MIC (MIC=mesoionic carbene) are combined in a [CC'C]-type pincer framework to access a series of ambient-stable Pd^IV tris(halido) complexes. Their synthesis, solid-state structures, stability, and reactivity are presented. To the best of our knowledge, the work presented herein reports the first isolated Pd^IV -MIC as well as the first Pd^IV carbene-based aryl pincer.

Synthesis, characterization, and crystal structures of organonickel(II) complexes coordinated to novel 1-bromo-2,6-bis{[(λ5-phosphanylidene)imino]methyl}benzene NCN-pincer ligands

A novel family of four 1-bromo-2,6-bis{[(λ⁵-phosphanylidene)imino]methyl}benzene ligands has been synthesized and characterized. The phosphiniminomethyl substituents are decorated with either three phenyl groups, two phenyl and one cyclohexyl group, one phenyl and two cyclohexyl groups, or three cyclohexyl groups. Each ligand was metallated using zero-valent nickel through an oxidative addition to form a family of organonickel(II) complexes, namely (2,6-bis{[(triphenyl-λ⁵-phosphanylidene)imino]methyl}phenyl-κ³N,C¹,N')bromidonickel(II) dichloromethane hemisolvate, [NiBr(C₄₄H₃₇N₂P₂)]·0.5CH₂Cl₂, (2,6-bis{[(cyclohexyldiphenyl-λ⁵-phosphanylidene)imino]methyl}phenyl-κ³N,C¹,N')bromidonickel(II) diethyl ether hemisolvate, [NiBr(C₄₄H₄₉N₂P₂)]·0.5C₄H₁₀O, (2,6-bis{[(dicyclohexylphenyl-λ⁵-phosphanylidene)imino]methyl}phenyl-κ³N,C¹,N')bromidonickel(II), [NiBr(C₄₄H₆₁N₂P₂)], and (2,6-bis{[(tricyclohexyl-λ⁵-phosphanylidene)imino]methyl}phenyl-κ³N,C¹,N')bromidonickel(II), [NiBr(C₄₄H₇₃N₂P₂)]. This family of complexes represents a useful opportunity to investigate the impact of incrementally changing the steric characteristics of a complex on its structure and reactivity.

Probing the Donor Properties of Pincer Ligands Using Rhodium Carbonyl Fragments: An Experimental and Computational Case Study

Metal carbonyls are commonly employed probes for quantifying the donor properties of monodentate ligands. With a view to extending this methodology to mer-tridentate "pincer" ligands, the spectroscopic properties [ν(CO), δ 13C, 1 J RhC] of rhodium(I) and rhodium(III) carbonyl complexes of the form [Rh(pincer)(CO)][BArF 4] and [Rh(pincer)Cl2(CO)][BArF 4] have been critically analysed for four pyridyl-based pincer ligands, with two flanking oxazoline (NNN), phosphine (PNP), or N-heterocyclic carbene (CNC) donors. Our investigations indicate that the carbonyl bands of the rhodium(I) complexes are the most diagnostic, with frequencies discernibly decreasing in the order NNN > PNP > CNC. To gain deeper insight, a DFT-based energy decomposition analysis was performed and identified important bonding differences associated with the conformation of the pincer backbone, which clouds straightforward interpretation of the experimental IR data. A correlation between the difference in carbonyl stretching frequencies Δν(CO) and calculated thermodynamics of the RhI/RhIII redox pairs was identified and could prove to be a useful mechanistic tool.

Emergence and Applications of Base Metals (Fe, Co, and Ni) in Hydroboration and Hydrosilylation

Base metal catalysis offers an alternative to reactions, which were once dominated by precious metals in hydrofunctionalization reactions. This review article details the development of some base metals (Fe, Co, and Ni) in the hydroboration and hydrosilylation reactions concomitant with a brief overview of recent advances in the field. Applications of both commercially available metal salts and well-defined metal complexes in catalysis and opportunities to further advance the field is discussed as well.

Rhodapentalenes: Pincer Complexes with Internal Aromaticity

Pincer complexes are a remarkably versatile family benefited from their stability, diversity, and tunability. Many of them contain aromatic organic rings at the periphery, and aromaticity plays an important role in their stability and properties, whereas their metallacyclic cores are not aromatic. Herein, we report rhodapentalenes, which can be viewed as pincer complexes in which the metallacyclic cores exhibit considerable aromatic character. Rhodapentalenes show good thermal stability, although the rhodium-carbon bonds in such compounds are fragile. Experimental and computational studies suggest that the stabilization of rigid CCC pincer architectures together with an intrinsic aromaticity is vital for these metallacyclic rhodium species. Dearomatization-aromatization reactions, corresponding to metal-ligand cooperation of classical aromatic pincer complexes, were observed in this system. These findings suggest a new concept for pincer chemistry, the internal aromaticity involving metal d-orbitals, which would be useful for exploiting the nature of construction motif and inspire further applications.

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A concept for pincer chemistry: internal aromaticity involving metal d-orbitals

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Stable cyclic rhodium complexes with obvious rhodium carbene character

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Dual stabilization of rigid CCC pincer architectures and intrinsic aromaticity

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Rhodium-containing aromatics with a single metal entity