A Dearomatized Anionic PNP Pincer Rhodium Complex: C–H and H–H Bond Activation by Metal–Ligand Cooperation and Inhibition by Dinitrogen

Metal-Ligand-Anion Cooperation in C-H Bond Formation at Platinum(II)

Thermolysis of [^H(BPI)Pt(CH₃)][OTf] (BPI = 1,3-bis(2-(4-tert-butyl)pyridylimino)isoindole) to release methane and form (BPI)Pt(OTf) is reported. Kinetic, mechanistic, and computational studies point to an unusual anion-assisted pathway that obviates the need for a higher oxidation state intermediate to couple the metal-bound methyl group with the ligand-bound hydrogen. Leveraging this insight, a triflimide derivative of the (BPI)Pt complex was shown to activate benzene, highlighting the role of the counteranion in controlling the activity of these complexes.

Heterolytic Oxidative Addition of sp2 and sp3 C-H Bonds by Metal-Ligand Cooperation with an Electron-Deficient Cyclopentadienone Iridium Complex

Oxidative addition reactions of C-H bonds that generate metal-carbon-bond-containing reactive intermediates have played essential roles in the field of organometallic chemistry. Herein, we prepared a cyclopentadienone iridium(I) complex 1 designed for oxidative C-H bond additions. The complex cleaves the various sp² and sp³ C-H bonds including those in hexane and methane as inferred from their H/D exchange reactions. The hydroxycyclopentadienyl(nitromethyl)iridium(III) complex 2 was formed when the complex was treated with nitromethane, which highlights this elementary metal-ligand cooperative C-H bond oxidative addition reaction. Mechanistic investigations suggested the C-H bond cleavage is mediated by polar functional groups in substrates or another iridium complex. We found that ligands that are more electron-deficient lead to more favorable reactions, in sharp contrast to classical metal-centered oxidative additions. This trend is in good agreement with the proposed mechanism, in which C-H bond cleavage is accompanied by two-electron transfer from the metal center to the cyclopentadienone ligand. The complex was further applied to catalytic transfer-dehydrogenation of tetrahydrofuran (THF).

Reactions of POP-pincer rhodium(I)-aryl complexes with small molecules: coordination flexibility of the ether diphosphine

Reactions of the aryl complexes Rh(aryl){κ3-P,O,P-[xant(PiPr2)2]} (1; aryl = 3,5-Me2C6H3 (a), C6H5 (b), 3,5-Cl2C6H3 (c), 3-FC6H4 (d); xant(PiPr2)2 = 9,9-dimethyl-4,5-bis-(diisopropylphosphino)xanthene) with O2, CO, and MeO2CC≡CCO2Me have been performed. Under 1 atm of O2, the pentane solutions of complexes 1 afford the dinuclear peroxide derivatives [Rh(aryl){κ2-P,P-xant(PiPr2)2}]2(μ-O2)2 (2a–2d) as yellow solids. In solution, these species are unstable. In dichloromethane, at room temperature, they are transformed into the dioxygen adducts Rh(aryl)(η2-O2){κ3-P,O,P-[xant(PiPr2)2]} (3a–3d), as a result of the rupture of the double peroxide bridge and the reduction of the metal center. Complex 3b decomposes in benzene, at 50 °C, to give diphosphine oxide, phenol, and biphenyl. Complexes 1 react with CO to give the square-planar mono carbonyl derivatives Rh(aryl)(CO){κ2-P,P-[xant(PiPr2)2]} (4a–4d), which under carbon monoxide atmosphere evolve to benzoyl species Rh{C(O)aryl}(CO){κ2-P,P-[xant(PiPr2)2]} (5a–5d), resulting from the migratory insertion of CO into the Rh-aryl bond and the coordination of a second CO molecule. The transformation is reversible; under vacuum, complexes 5 regenerate the precursors 4. The addition of the activated alkyne to complexes 1b and 1d initially leads to the π-alkyne intermediates Rh(aryl){η2-C(CO2Me)≡C(CO2Me)}{κ3-P,O,P-[xant(PiPr2)2]} (6b, 6d), which evolve to the alkenyl derivatives Rh{(E)-C(CO2Me)=C(CO2Me)aryl}{κ3-P,O,P-[xant(PiPr2)2]} (7b, 7d). The diphosphine adapts its coordination mode to the stability requirements of the different complexes, coordinating cis-κ2-P,P in complexes 2, fac-κ3-P,O,P in compounds 3, trans-κ2-P,P in the mono carbonyl derivatives 4 and 5, and mer-κ3-P,O,P in products 6 and 7.

Forays into rhodium macrocyclic chemistry stabilized by a P2N2 donor set. Activation of dihydrogen and benzene

The reaction of the dilithium diamido-diphosphine macrocycle, Li₂[N(SiMe₂CH₂P(Ph)CH₂SiMe₂)₂N] (Li₂[P₂N₂]) with [Rh(COD)Cl]₂ generates the dirhodium macrocyclic compound, [P₂N₂][Rh(COD)]₂ (where COD = η⁴-1,5-cyclooctadiene), wherein both rhodium-COD units are syn to each other and have square planar geometries. While this dirhodium derivative does react with H₂, no clean products could be isolated. Upon reaction of Li₂[P₂N₂] with [Rh(COE)₂Cl]₂ (where COE is η²-cyclooctene), the dilithium-dihodium derivative ([Rh(COE)][P₂N₂]Li)₂(dioxane) forms, which was characterized by single-crystal X-ray analysis and NMR spectroscopy. The cyclooctene derivative reacts with dihydrogen in benzene to generate the dilithium-dirhodium-dihydride complex ([Rh(H)₂][P₂N₂]Li)₂(dioxane); also formed is the dilithium-dirhodium-phenylhydride complex ([Rh(C₆H₅)H][P₂N₂]Li)₂(dioxane) via oxidative addition of a C-H bond of the solvent. The phenyl-hydride is eventually converted to the dihydride derivative via further reaction with H₂. This process is complicated by adventitious H₂O, which leads to the isolation of the amine-dihydride, Rh[P₂N₂H](H)₂; drying of the H₂ eliminates this side product. Nevertheless, careful addition of H₂O to ([Rh(COE)][P₂N₂]Li)₂(dioxane) results in protonation of one of the amido units and the formation of the rhodium-amine cyclooctene derivative, Rh[P₂N₂H](COE), which upon reaction with H₂ generates the aforementioned amine-dihydride, Rh[P₂N₂H](H)₂. The mechanism by which dihydrogen and C-H bonds of benzene are activated likely involves initial dissociation of cyclooctene from the 18-electron centers in ([Rh(COE)][P₂N₂]Li)₂(dioxane), followed by H-H and C-H bond activation. The ability of one of the amido units of the P₂N₂ macrocycle to be protonated is a potentially useful proton storage mechanism and is of interest in other bond activation processes.

Dehydropolymerisation of methylamine borane using highly active rhodium(iii) bis(thiophosphinite) pincer complexes: catalytic and mechanistic insights

The bis(thiophosphinite) pincer complexes [(^RPSCSP^R)Rh(py)(H)(Cl)] (^RPSCSP^R = C₆H₄–2,6-(SPR₂)₂ with R = ⁱPr, 2a and R = Ph, 2b) are highly active precatalysts for the dehydropolymerisation of methylamine borane.

Co(I) complexes with a tetradentate phenanthroline-based PNNP ligand as a potent new metal-ligand cooperation platform

A series of low spin cobalt(i) complexes bearing a tetradentate phenanthroline-based PNNP ligand (2,9-bis((diphenylphosphanyl)methyl)-1,10-phenanthroline), [CoCl(PNNP)] (1), [CoMe(PNNP)] (2) and [Co(CH2SiMe3)(PNNP)] (3), were synthesized and structurally identified. Complex 3 underwent a structural rearrangement of the PNNP skeleton upon heating to form [Co(PNNP')] (4), which is supported by an asymmetrical PNNP' ligand with a dearomatized phenanthroline backbone. Mechanistic studies supported that the transformation from 3 to 4 was initiated by the homolysis of either a Co-CH2SiMe3 bond or a benzylic C-H bond. Complex 4 achieved H-H bond cleavage of H2 (1 atm) at ambient temperature, to form [Co(PNNP'')] (6), in which two H atoms were incorporated into the endocyclic double bond of the PNNP'' ligand backbone.

Cleavage of Si-H, B-H, and C-H Bonds by Metal-Ligand Cooperation

Metal-ligand cooperation, in which metal and ligand participate in bond cleavage and formation, is gathering great attention in recent years. In contrast to the classical bond cleavage by active metal centers with spectator ligands, metal-ligand cooperation has enabled unprecedented reactivities. Especially, metal-ligand cooperative H-H bond cleavage has been extensively studied and applied to various catalysts. On the other hand, there are substantial efforts to expand the scope of the bond to be cleaved other than the H-H bond. This review summarizes the recent progress in the metal-ligand cooperative cleavages of Si-H, B-H, and C-H bonds and their catalytic applications.

Enabling Two-Electron Pathways with Iron and Cobalt: From Ligand Design to Catalytic Applications

Homogeneous catalysis with Earth-abundant, first-row transition metals, including iron and cobalt, has gained considerable recent attention as a potentially cost-effective and sustainable alternative to more commonly and historically used precious metals. Because fundamental organometallic transformations, such as oxidative addition and reductive elimination, are two-electron processes and essential steps in many important catalytic cycles, controlling redox chemistry-in particular overcoming one-electron chemistry-has been as a central challenge with Earth-abundant metals. This Perspective focuses on approaches to impart sufficiently strong ligand fields to generate electron-rich metal complexes able to promote oxidative addition reactions where the redox changes are exclusively metal-based. Emphasis is placed on how ligand design and exploration of fundamental organometallic chemistry coupled with mechanistic understanding have been used to discover iron catalysts for the hydrogen isotope exchange in pharmaceuticals and cobalt catalysts for C(sp²)-H borylation reactions. A pervasive theme is that first-row metal complexes often promote unique chemistry from their precious-metal counterparts, demonstrating that these elements offer a host of new opportunities for reaction discovery and for more sustainable catalysis.

Metal-Ligand Cooperation as Key in Formation of Dearomatized NiII-H Pincer Complexes and in Their Reactivity toward CO and CO2

The unique synthesis and reactivity of [(^RPNP*)NiH] complexes (1a,b), based on metal–ligand cooperation (MLC), are presented (^RPNP* = deprotonated PNP ligand, R = ⁱPr, ^tBu). Unexpectedly, the dearomatized complexes 1a,b were obtained by reduction of the dicationic complexes [(^RPNP)Ni(MeCN)](BF₄)₂ with sodium amalgam or by reaction of the free ligand with Ni⁰(COD)₂. Complex 1b reacts with CO via MLC, to give a rare case of a distorted-octahedral PNP-based pincer complex, the Ni(0) complex 3b. Complexes 1a,b also react with CO₂ via MLC to form a rare example of η¹ binding of CO₂ to nickel, complexes 4a,b. An unusual CO₂ cleavage process by complex 4b, involving C–O and C–P cleavage and C–C bond formation, led to the Ni–CO complex 3b and to the new complex [(PⁱPr₂NC₂O₂)Ni(P(O)ⁱPr₂)] (5b). All complexes have been fully characterized by NMR and X-ray crystallography.

Ligand K-edge XAS, DFT, and TDDFT analysis of pincer linker variations in Rh(i) PNP complexes: reactivity insights from electronic structure

Ligand K-edge XAS and DFT studies of ligand variations in Rh(i) pincer complexes and correlations to small molecule reactivity.

Synthesis and reactivity of coordinatively unsaturated halocarbonyl molybdenum PNP pincer complexes

In the present study a series of six-coordinate neutral 16e halocarbonyl Mo(ii) complexes of the type [Mo(PNP(Me)-iPr)(CO)X2] (X = I, Br, Cl), featuring the PNP pincer ligand N,N'-bis(diisopropylphosphino)-N,N'-dimethyl-2,6-diaminopyridine (PNP(Me)-iPr), were prepared and fully characterized. The synthesis of these complexes was accomplished by different methodologies depending on the halide ligands. For X = I and Br, [Mo(PNP(Me)-iPr)(CO)I2] and [Mo(PNP(Me)-iPr)(CO)Br2] were obtained by reacting [Mo(PNP(Me)-iPr)(CO)3] with stoichiometric amounts of I2 and Br2, respectively. In the case of X = Cl, [Mo(PNP(Me)-iPr)(CO)Cl2] was afforded by the reaction of [Mo(CO)4(μ-Cl)Cl]2 with 1 equiv. of PNP(Me)-iPr. The equivalent procedure also worked for X = Br. The modification of the 2,6-diaminopyridine scaffold by introducing NMe instead of NH spacers between the aromatic pyridine ring and the phosphine moieties changed the steric properties of the PNP-iPr ligand significantly. While in the present case exclusively neutral six-coordinate complexes of the type [Mo(PNP(Me)-iPr)(CO)X2] were obtained, with the parent PNP-iPr ligand, i.e. featuring NH spacers, cationic seven-coordinate complexes of the type [Mo(PNP-iPr)(CO)3X]X were afforded. Upon treatment of [Mo(PNP(Me)-iPr)(CO)X2] (X = Br, Cl) with Ag(+) in CH3CN, the cationic complexes [Mo(PNP(Me)-iPr)(CO)(CH3CN)X](+) were formed. Halide abstraction from [Mo(PNP(Me)-iPr)(CO)Cl2] in THF-CH2Cl2 afforded [Mo(PNP(Me)-iPr)(CO)(THF)Cl](+). In keeping with the facile synthesis of monocationic complexes preliminary ESI-MS and DFT/B3LYP studies revealed that one halide ligand in complexes [Mo(PNP(Me)-iPr)(CO)X2] is labile forming cationic fragments [Mo(PNP(Me)-iPr)(CO)X](+) which react with molecular oxygen in parallel pathways to yield mono and dioxo Mo(iv) and Mo(vi) species. Structures of representative complexes were determined by X-ray single crystal analyses.

An iron(II) complex featuring κ(3) and labile κ(2)-bound PNP pincer ligands - striking differences between CH2 and NH spacers

Treatment of anhydrous FeCl2 with 2 equiv. of the pincer ligand PNP-Ph afforded the diamagnetic cationic octahedral complex [Fe(κ(3)-P,N,P-PNP)(κ(2)-P,N-PNP)Cl](+) featuring a κ(2)-P,N-bound PNP ligand. Preliminary reactivity studies revealed that the κ(2)-P,N-bound PNP ligand is labile reacting with CO to afford trans-[Fe(PNP-Ph)(CO)2Cl](+).

Metal-ligand cooperation by aromatization-dearomatization as a tool in single bond activation

Metal-ligand cooperation (MLC) plays an important role in bond activation processes, enabling many chemical and biological catalytic reactions. A recent new mode of activation of chemical bonds involves ligand aromatization-dearomatization processes in pyridine-based pincer complexes in which chemical bonds are broken reversibly across the metal centre and the pincer-ligand arm, leading to new bond-making and -breaking processes, and new catalysis. In this short review, such processes are briefly exemplified in the activation of C-H, H-H, O-H, N-H and B-H bonds, and mechanistic insight is provided. This new bond activation mode has led to the development of various catalytic reactions, mainly based on alcohols and amines, and to a stepwise approach to thermal H2 and light-induced O2 liberation from water.

C–H and H–H bond activation via ligand dearomatization/rearomatization of a PN3P-rhodium(i) complex

A neutral complex PN³P-Rh(i)Cl (2) was prepared from a reaction of the PN³P pincer ligand (1) with [Rh(COD)Cl]₂ (COD = 1,5-cyclooctadiene).

Ligand assisted carbon dioxide activation and hydrogenation using molybdenum and tungsten amides

Molybdenum and tungsten amides M(NO)(CO)(PNP) {M = Mo, 3a; W, 3b; PNP = (iPr₂PCH₂CH₂)₂N} can activate CO₂ at room temperature forming carbamate species M(NO)(CO)(PNP)(OCO) (M = Mo, 4a(trans); W = 4b(trans). Employing 3a,b stoichiometric hydrogenation of carbon dioxide could be demonstrated.

A phosphomide based PNP ligand, 2,6-{Ph2PC(O)}2(C5H3N), showing PP, PNP and PNO coordination modes

Coordination chemistry of a versatile phosphomide ligand, 2,6-{Ph₂PC(O)}₂(C₅H₃N), is described.

Synthesis and reactivity of NHC-based rhodium macrocycles

Using a general synthetic procedure employing readily accessed terminal alkene-functionalized pro-ligands and macrocyclization by ring-closing olefin metathesis, rhodium carbonyl complexes have been prepared that contain lutidine (1a; n = 1) and pyridine (1b; n = 0) derived tridentate CNC macrocycles with dodecamethylene spacers. In solution, 1a shows temperature-invariant time-averaged C2 symmetry by (1)H NMR spectroscopy (CD2Cl2, 500 MHz), whereas in the solid-state, two polymorphs can be obtained showing different conformations of the alkyl spacer about the metal-carbonyl bond (asymmetric and symmetric). In contrast, time-averaged motion of alkyl spacer in 1b can be halted by cooling below 225 K (CD2Cl2, 500 MHz), and the complex crystallizes as a dimer with an interesting unsupported Rh···Rh bonding interaction (3.2758(6) Å). Oxidative addition reactions of 1a and 1b, using MeI and PhICl2, have been studied in situ by (1)H NMR spectroscopy, although pure Rh(III) adducts can be ultimately isolated only with the pyridine-based macrocyclic ligand. The lutidine backbone of 1a can be deprotonated by addition of K[N(SiMe3)2], and the resulting neutral dearomatized complex (5) has been fully characterized in solution, by variable-temperature (1)H NMR spectroscopy, and in the solid state, by X-ray diffraction.

Reversible CO2 binding triggered by metal–ligand cooperation in a rhenium(i) PNP pincer-type complex and the reaction with dihydrogen

Metal–ligand cooperation in a rhenium PNP pincer complex gives rise to the reversible activation of CO₂ and H₂ and the efficient catalytic decomposition of formic acid under base-free conditions.

Activation of nitriles by metal ligand cooperation. Reversible formation of ketimido- and enamido-rhenium PNP pincer complexes and relevance to catalytic design

The dearomatized complex cis-[Re(PNP(tBu)*)(CO)2] (4) undergoes cooperative activation of C≡N triple bonds of nitriles via [1,3]-addition. Reversible C-C and Re-N bond formation in 4 was investigated in a combined experimental and computational study. The reversible formation of the ketimido complexes (5-7) was observed. When nitriles bearing an alpha methylene group are used, reversible formation of the enamido complexes (8 and 9) takes place. The reversibility of the activation of the nitriles in the resulting ketimido compounds was demonstrated by the displacement of p-CF3-benzonitrile from cis-[Re(PNP(tBu)-N═CPh(pCF3))(CO)2] (6) upon addition of an excess of benzonitrile and by the temperature-dependent [1,3]-addition of pivalonitrile to complex 4. The reversible binding of the nitrile in the enamido compound cis-[Re(PNP(tBu)-HNC═CHPh)(CO)2] (9) was demonstrated via the displacement of benzyl cyanide from 9 by CO. Computational studies suggest a stepwise activation of the nitriles by 4, with remarkably low activation barriers, involving precoordination of the nitrile group to the Re(I) center. The enamido complex 9 reacts via β-carbon methylation to give the primary imino complex cis-[Re(PNP(tBu)-HN═CC(Me)Ph)(CO)2]OTf 11. Upon deprotonation of 11 and subsequent addition of benzyl cyanide, complex 9 is regenerated and the monomethylation product 2-phenylpropanenitrile is released. Complexes 4 and 9 were found to catalyze the Michael addition of benzyl cyanide derivatives to α,β-unsaturated esters and carbonyls.

Heterolytic Cleavage of Dihydrogen by an Iron(II) PNP Pincer Complex via Metal-Ligand Cooperation

The bis-carbonyl Fe(II) complex trans-[Fe(PNP-iPr)(CO)2Cl]+ reacts with Zn as reducing agent under a dihydrogen atmosphere to give the Fe(II) hydride complex cis-[Fe(PNP-iPr)(CO)2H]+ in 97% isolated yield. A crucial step in this reaction seems to be the reduction of the acidic NH protons of the PNP-iPr ligand to afford H2 and the coordinatively unsaturated intermediate [Fe(PNPH-iPr)(CO)2]+ bearing a dearomatized pyridine moiety. This species is able to bind and heterolytically cleave H2 to give cis-[Fe(PNP-iPr)(CO)2H]+. The mechanism of this reaction has been studied by DFT calculations. The proposed mechanism was supported by deuterium labeling experiments using D2 and the N-deuterated isotopologue of trans-[Fe(PNP-iPr)(CO)2Cl]+. While in the first case deuterium was partially incorporated into both N and Fe sites, in the latter case no reaction took place. In addition, the N-methylated complex trans-[Fe(PNPMe-iPr)(CO)2Cl]+ was prepared, showing no reactions with Zn and H2 under the same reaction conditions. An alternative synthesis of cis-[Fe(PNP-iPr)(CO)2H]+ was developed utilizing the Fe(0) complex [Fe(PNP-iPr)(CO)2]. This compound is obtained in high yield by treatment of either trans-[Fe(PNP-iPr)(CO)2Cl]+ or [Fe(PNP-iPr)Cl2] with an excess of NaHg or a stoichiometric amount of KC8 in the presence of carbon monoxide. Protonation of [Fe(PNP-iPr)(CO)2] with HBF4 gave the hydride complex cis-[Fe(PNP-iPr)(CO)2H]+. X-ray structures of both cis-[Fe(PNP-iPr)(CO)2H]+ and [Fe(PNP-iPr)(CO)2] are presented.

Facile N-H bond cleavage of ammonia by an iridium complex bearing a noninnocent PNP-pincer type phosphaalkene ligand

A novel PNP-pincer type phosphaalkene complex of iridium bearing a dearomatized pyridine unit (3) has been prepared. Complex 3 rapidly reacts with ammonia at room temperature to afford a parent amido complex in high yield. DFT calculations indicate that the phosphaalkene unit with a strong π-accepting property effectively facilitates the N-H bond cleavage of ammonia via metal-ligand cooperation.

Xantphos-type complexes of group 9: rhodium versus iridium

Treatment of the dimer [Rh(μ-Cl)(C8H14)2]2 (1a) with 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene [xant(P(i)Pr2)2] leads to the d(8) square-planar complex RhCl{xant(P(i)Pr2)2} (2), whereas reaction of the iridium counterpart [Ir(μ-Cl)(C8H14)2]2 (1b) gives the d(6) octahedral compound IrHCl{xant(P(i)Pr2)[(i)PrPCH(Me)CH2]} (3) as a result of the intramolecular C-H bond activation of one of the isopropyl substituents of the phosphine. Stirring 2 and 3 in 0.5 N KOH solutions of 2-propanol gives rise to the formation of hydrides RhH{xant(P(i)Pr2)2} (4) and IrH3{xant(P(i)Pr2)2} (5), respectively. In n-octane at 60 °C, complex 2 is stable. However, compound 3 activates the alkane to give the cis-dihydride IrH2Cl{xant(P(i)Pr2)2} (6) and a mixture of 3- and 4-octene. Complex 6 can be also obtained by the reaction of 3 with H2. Under the same conditions, 2 affords the rhodium analogue RhH2Cl{xant(P(i)Pr2)2} (7). Compounds 2-4 react with triflic acid (HOTf) to give RhHCl(OTf){xant(P(i)Pr2)2} (8), IrHCl(OTf){xant(P(i)Pr2)2} (9), and RhH2(OTf){xant(P(i)Pr2)2} (10), respectively. The related iridium derivative IrH2(OTf){xant(P(i)Pr2)2} (11) has also been prepared by the reaction of 6 with Tl(OTf). Complexes 2, 6, and 9 have been characterized by X-ray diffraction analysis. The {xant(P(i)Pr2)2}M skeleton is T-shaped with the metal center situated in the common vertex.

Aldehyde binding through reversible C-C coupling with the pincer ligand upon alcohol dehydrogenation by a PNP-ruthenium catalyst

Primary alcohol dehydrogenation by a PNP-Ru(II) catalyst was probed by low-temperature NMR experiments. Facile dehydrogenation occurred at -30 °C, but the resulting aldehydes were not found in solution, as they were trapped by the catalyst through a new mode of metal-ligand cooperation involving Ru-O coordination and an unusual, highly reversible C-C coupling with the PNP pincer ligand.