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

Carbene-Decorated Geometrically Constrained Borylenes for Bond Activations

While metal-ligand cooperativity is well-known, studies on element-ligand cooperativity involving main group species are comparatively much less explored. In this study, we computationally designed a few geometrically constrained borylenes supported by different carbenes. Our density functional theory studies indicate that they possess enhanced nucleophilicity as well as electrophilicity, thus rendering them promising candidates for exhibiting borylene-ligand cooperativity. The cooperation between the boron and adjacent carbene centers facilitates different bond activation processes, including the cycloaddition of acetylene across the boron-carbene bond as well as B-H/Si-H bond activation reactions, which have been analyzed in detail. To the best of our knowledge, the borylenes proposed in this study represent the first examples of theoretically proposed geometrically constrained bis(carbene)-stabilized borylenes capable of cooperative activation of enthalpically strong bonds.

Cooperativity between metal centers in homobimetallic PdII-NHC complexes: catalytic potential towards hydrodefluorination

Among the several unsymmetrical bis-NHC derived distinct homo-bimetallic and mono-NHC supported PdII complexes studied here (1-5), the bimetallic complex 1 was noted to be the most effective catalyst for the challenging hydrodefluorination. The electron richness of the metal centers and the synergistic cooperation between the PdII centers (cooperativity index, ɑ = 8.67) have been recognized to be the deciding factor for its better activity.

Bisgermylene-Stabilized Stannylone: Catalytic Reduction of Nitrous Oxide and Nitro Compounds via Element-Ligand Cooperativity

This study describes the synthesis, structural characterization, and catalytic application of a bis(germylene)-stabilized stannylone (2). The reduction of digermylated stannylene (1) with 2.2 equiv of potassium graphite (KC8) leads to the formation of stannylone 2 as a green solid in 78% yield. Computational studies showed that stannylone 2 possesses a formal Sn(0) center and a delocalized 3-c-2-e π-bond in the Ge2Sn core, which arises from back-donation of the p-type lone pair electrons on the Sn atom to the vacant orbitals of the Ge atoms. Stannylone 2 can serve as an efficient precatalyst for the selective reduction of nitrous oxide (N2O) and nitroarenes (ArNO2) with the formation of dinitrogen (N2) and hydrazines (ArNH-NHAr), respectively. Exposure of 2 with N2O (1 atm) resulted in the insertion of two oxygen atoms into the Ge-Ge and Ge-Sn bonds, yielding the germyl(oxyl)stannylene (3). Moreover, the stoichiometric reaction of 2 with 1-chloro-4-nitrobenzene afforded an amido(oxyl)stannylene (4) through the complete scission of the N-O bonds of the nitroarene. Stannylenes 3 and 4 serve as catalytically active species for the catalytic reduction of nitrous oxide and nitroarenes, respectively. Mechanistic studies reveal that the cooperation of the low-valent Ge and Sn centers allows for multiple electron transfers to cleave the N-O bonds of N2O and ArNO2. This approach presents a new strategy for catalyzing the deoxygenation of N2O and ArNO2 using a zerovalent tin compound.

Counterintuitive chemoselectivity in the reduction of carbonyl compounds

The reactivity of carbonyl functional groups largely depends on the substituents on the carbon atom. Reversal of the commonly accepted order of reactivity of different carbonyl compounds requires novel synthetic approaches. Achieving selective reduction will enable the transformation of carbon resources such as plastic waste, carbon dioxide and biomass into valuable chemicals. In this Review, we explore the reduction of less reactive carbonyl groups in the presence of those typically considered more reactive. We discuss reductions, including the controlled reduction of ureas, amides and esters to aldehydes, as well as chemoselective reductions of carbonyl groups, including the reduction of ureas over carbamates, amides and esters; the reduction of amides over esters, ketones and aldehydes; and the reduction of ketones over aldehydes.

P(V)-bis(amidophenolate) ligand cooperation: stoichiometric CO-bond cleavage in aldehydes and ketones

The cooperation between a geometrically constrained, highly electrophilic phosphorus(V) center, and an electronically rich tetradentate bis(amidophenolate) ligand enables the cleavage of the CO bond from typical aldehydes and ketones delivering iminio phosphoramidate species. The amphiphilic nature of these products, which is demonstrated through their reaction with typical Lewis acids and bases, enables their use as a mild source of silylium cations from silanes, allowing the selective reductive coupling of aldehydes to ethers under catalytic conditions.

Syntheses and exploration of the catalytic activities of organotin(IV) compounds

Six organotin(IV) compounds (1-6) have been synthesized by reaction of the polydentate pro-ligands H3L and H2L, respectively, with the corresponding diorganotin chlorides. All of the compounds were characterized by FT-IR spectroscopy, 1H, 13C{1H}, and 119Sn (1H) NMR spectroscopy, HRMS spectrometry, and single-crystal X-ray diffraction. The solid-state structures show that all of the compounds are monomeric (except compound 3) and contain a penta-coordinated tin atom. Compound 3 is a dimer with two hexa-coordinated tin atoms. Compounds 1-3 contain a non-coordinated hydroxymethyl group. All of the compounds have been screened for their catalytic efficacy in the synthesis of 1,2 disubstituted benzimidazoles using o-phenylenediamine and aldehyde derivatives. It has been observed that both the Lewis acidic Sn(IV) centre and the hydroxymethyl group (hydrogen bond donor) catalyse the reactions with a product yield of up to 92%.

Understanding Cooperativity in Homo- and Heterometallic Complexes: From Basic Concepts to Design

Cooperative effects have attracted considerable attention in recent years. These effects are ubiquitous in chemistry and biology and can govern interactions of proteins with other biomolecules, mechanisms of supramolecular recognition and polymerization, catalysis, assembly of compounds on surfaces, and physical properties such as magnetic, electronic or optical properties, e. g. Consequently, the understanding of cooperative effects can lead to a structure-property relation that can pave the way to future applications in various research areas; however, with regard to cooperative effects in homo- and heterometallic complexes, we still are at the beginning of understanding. Nevertheless, concepts to describe cooperativity of metal centers as well as methodologies to investigate and model these effects have emerged over the last years. This concept article gives an overview of these existing concepts, approaches, and strategies to understand cooperative effects in homo- and heterometallic complexes. Special emphasis is put on concepts to define cooperative effects, their quantification, as well as methods to investigate cooperative effects.

Calcium-Ligand Cooperation Promoted Activation of N2O, Amine, and H2 as well as Catalytic Hydrogenation of Imines, Quinoline, and Alkenes

Bond activation and catalysis using s-block metals are of great significance. Herein, a series of calcium pincer complexes with deprotonated side arms have been prepared using pyridine-based PNP and PNN ligands. The complexes were characterized by NMR and X-ray crystal diffraction. Utilizing the obtained calcium complexes, unprecedented N2O activation by metal-ligand cooperation (MLC) involving dearomatization-aromatization of the pyridine ligand was achieved, generating aromatized calcium diazotate complexes as products. Additionally, the dearomatized calcium complexes were able to activate the N-H bond as well as reversibly activate H2, offering an opportunity for the catalytic hydrogenation of various unsaturated molecules. DFT calculations were applied to analyze the electronic structures of the synthesized complexes and explore possible reaction mechanisms. This study is an important complement to the area of MLC and main-group metal chemistry.

Protic- or anionic-NHCs with a classical-NHC in a single [Ru(CNC)(PPh3)2Cl]Cl pincer complex: direct comparison of structure & electronic properties and heterolytic H2 splitting

Herein, we report the first set of pincer complexes 1 and 2 with the general formula [Ru(CNC)(PPh3)2Cl]Cl having a protic- and classical-NHC in the same molecule and nearly identical environments. Deprotonation of the protic-NHC complex 2 with one equivalent of base leads to the formation of anionic-NHC complex 2'. These complexes allow a direct comparison of protic- and anionic-NHCs with the classical-NHC ligand. A comparison of the molecular structure indicated that the metal carbene bond length trend is anionic-NHC > protic-NHC > classical-NHC. The electrochemical investigation revealed the electron donation tendency is classical-NHC > protic-NHC and anionic-NHC > protic-NHC. Cooperation between the metal and the ligand is indicated by the reaction of 2' with H2 gas at 1 atm pressure and 110 °C to give the Ru-hydride complex 3.

Unlocking the potential of metal ligand cooperation for enantioselective transformations

Metal-ligand cooperation, in which both the metal and the ligand of a transition metal complex actively participate in chemical transformations leading to enhanced reactivity or selectivity in chemical reactions, has emerged as a powerful and versatile concept in catalysis. This Viewpoint discusses the development trajectory of transition metal-based complexes as catalysts in (de)hydrogenative processes, in particular those cases where metal-ligand cooperation has been invoked to rationalise the observed high reactivities and excellent selectivities. The historical context, mechanistic aspects and current applications are discussed with the suggestion to explore the potential of the MLC mode of action of such catalysts in enantioselective transformations beyond (de)hydrogenative processes.

Neutral Chromium Complex with a Cr≡Si Triple Bond: Synthesis and Photoinduced H-H and Benzene C-H Bond Activation

A neutral silylyne complex with a Cr≡Si triple bond was prepared by dehydrogenation of a chromium silylene complex with Cr-H and Si-H bonds, and was isolated as monomeric crystals, unlike dimeric forms of its tungsten and molybdenum congeners. The strong Cr(δ-)-Si(δ+) bond polarity was revealed by the reaction with MeOH and DFT calculations. The chromium silylyne complex reacted with H2 under LED (365 nm) irradiation to reproduce the precursor silylene complex with a (H)Cr=Si(H) moiety, as a result of 1,2-H-H addition across the Cr≡Si triple bond. Similarly, the chromium silylyne complex reacted with benzene under irradiation to afford an 1,2-addition product with a (H)Cr=Si(Ph) moiety, via benzene C-H bond activation accompanied by Si-C bond forming.

Rhodium-Catalyzed Asymmetric Hydroboration/Cyclization of 1,6-Enynes Enabled by Spirosiladiphosphine Ligands: Constructing Chiral Five-Membered Rings with a Boron Handle

A rhodium-catalyzed enantioselective hydroboration/cyclization reaction of 1,6-enynes is achieved by employing a spirosiladiphosphine ligand. The process allows the synthesis of five-membered hetero- and carbocycles bearing a boron handle with high levels of activity and selectivity. Various enynes and organoboranes (HBdan, HBpin, HBmp, and HBamm) have been accommodated, and enynes containing terminal alkynes have been integrated into the process for the first time. The high yields and selectivities of the transformation highlight the synthetic utility of these novel spirosiladiphosphine ligands.

Ru(II) Complexes with Protic- and Anionic-Naked-NHC Ligands for Cooperative Activation of Small Molecules

A set of ruthenium(II)-protic-N-heterocyclic carbene complexes, [Ru(NNCH )(PPh3 )2 (X)]Cl (1, X=Cl and 2, X=H) and their deprotonated forms [Ru(NNC)(PPh3 )2 (X)] (1', X=Cl and 2', X=H), in which NNC is a new unsymmetrical pincer ligand, are reported. The four complexes are interconvertible by simple acid-base chemistry. The combined theoretical and spectroscopic investigations indicate charge segregation in anionic-NHC complexes (1' and 2') and can be described from a Lewis pair perspective. The chemical reactivity of deprotonated complex 1' shows cooperative small molecule activation. Complex 1' activates H-H bond of hydrogen, C(sp3 )-I bond of iodomethane, and C(sp)-H bond of phenylacetylene. The activation of CO2 using anionic NHC complex 1' at moderate temperature and ambient pressure and subsequent conversion to formate is also described. All the new compounds have been characterized using ESI-MS, 1 H, 13 C, and 31 P NMR spectroscopy. Molecular structures of 1, 2, and 2' have also been determined with single-crystal X-ray diffraction. The cooperative small molecule activation perspective broadens the scope of potential applications of anionic-NHC complexes in small molecule activation, including the conversion of carbon dioxide to formate, a much sought after reaction in the renewable energy and sustainable development domains.

Reactivity of a Strictly T-Shaped Phosphine Ligated by an Acridane Derived NNN Pincer Ligand

The steric tuning of a tridentate acridane-derived NNN pincer ligand allows for the isolation of a strictly T-shaped phosphine that exhibits ambiphilic reactivity. Well-defined phosphorus-centered reactivity towards nucleophiles and electrophiles is reported, contrasting with prior reports on this class of compounds. Reactions towards oxidants are also described. The latter result in the two-electron oxidation of the phosphorus atom from +III to +V and are accompanied by a strong geometric distortion of the NNN pincer ligand. By contrast, cooperative activation of E-H (HCl, HBcat, HOMe) bonds proceeds with retention of the phosphorus redox state. When using H2 O as a substrate, the reaction results in the full disassembly of H2 O to its constituent atoms, highlighting the potential of this platform for small molecule activation reactions.

Chemoselectivity change in catalytic hydrogenolysis enabling urea-reduction to formamide/amine over more reactive carbonyl compounds

The selective transformation of a less reactive carbonyl moiety in the presence of more reactive ones can realize straightforward and environmentally benign chemical processes. However, such a transformation is highly challenging because the reactivity of carbonyl compounds, one of the most important functionalities in organic chemistry, depends on the substituents on the carbon atom. Herein, we report an Ir catalyst for the selective hydrogenolysis of urea derivatives, which are the least reactive carbonyl compounds, affording formamides and amines. Although formamide, as well as ester, amide, and carbamate substituents, are considered to be more reactive than urea, the proposed Ir catalyst tolerated these carbonyl groups and reacted with urea in a highly chemoselective manner. The proposed chemo- and regioselective hydrogenolysis allows the development of a strategy for the chemical recycling of polyurea resins.

Understanding the mechanism and origins of stereoconvergence in nickel-catalyzed hydroarylation of 1,3-dienes with aryl boronates

Nickel-catalyzed stereoselective hydroarylation is one of the most efficient methods to access functionalized arenes. Herein, computational studies have been applied to reveal the mechanism and origins of ligand-controlled enantioselectivity of Ni-catalyzed hydroarylation of 1,3-dienes using ethanol as the hydrogen source. DFT calculations show that the hydroarylation of (E)-diene takes place via concerted hydronickelation aided by boronate leading to an alkylnickel(II) intermediate, which further undergoes transmetallation and C-C reductive elimination to deliver the final chiral alkylarene. The hydronickelation is found to be the rate-determining step and is irreversible. The enantioselectivity is dominated by the transmetallation step, in which the ligand-substrate interactions are analyzed to unveil the source of stereocontrol. Besides, mechanistic studies demonstrate that the (Z)-diene initially reacts to offer a (S)-Z-alkyl-Ni(II) species, which preferably undergoes facile isomerization via σ-π-σ-π-σ interconversion to the (R)-E-alkyl-Ni(II) complex rather than the transmetallation step, thus ultimately generating the same (R)-alkylarene product as (E)-diene. Overall, the mechanistic understanding will be useful for the further advancement of asymmetric hydroarylation of dienes.

Cooperative Bond Activation and Catalytic CO2 Functionalization with a Geometrically Constrained Bis(silylene)-Stabilized Borylene

Metal-ligand cooperativity has emerged as an important strategy to tune the reactivity of transition-metal complexes for the catalysis and activation of small molecules. Studies of main-group compounds, however, are scarce. Here, we report the synthesis, structural characterization, and reactivity of a geometrically constrained bis(silylene)-stabilized borylene. The one-pot reaction of [(SiNSi)Li(OEt₂)] (SiNSi = 4,5-bis(silylene)-2,7,9,9-tetramethyl-9H-acridin-10-ide) with 1 equiv of [BBr₃(SMe₂)] in toluene at room temperature followed by reduction with 2 equiv of potassium graphite (KC₈) leads to borylene [(SiNSi)B] (1), isolated as blue crystals in 45% yield. X-ray crystallography shows that borylene (1) has a tricoordinate boron center with a distorted T-shaped geometry. Computational studies reveal that the HOMO of 1 represents the lone pair orbital on the boron center and is delocalized over the Si-B-Si unit, while the geometric perturbation significantly increases its energy. Borylene (1) shows single electron transfer reactivity toward tris(pentafluorophenyl)borane (B(C₆F₅)₃), forming a frustrated radical pair [(SiNSi)B]^•+[B(C₆F₅)₃]^•-, which can be trapped by its reaction with PhSSPh, affording an ion pair [(SiNSi)BSPh][PhSB(C₆F₅)₃] (3). Remarkably, the cooperation between borylene and silylene allows the facile cleavage of the N-H bond of aniline, the P-P bond in white phosphorus, and the C═O bond in ketones and carbon dioxide, thus representing a new type of main-group element-ligand cooperativity for the activation of small molecules. In addition, 1 is a strikingly effective catalyst for carbon dioxide reduction. Computational studies reveal that the cooperation between borylene and silylene plays a key role in the catalytic chemical bond activation process.

Diverse Cooperative Reactivity at a Square Planar Aluminium Complex and Catalytic Reduction of CO2

The use of a sterically demanding pincer ligand to prepare an unusual square planar aluminium complex is reported. Due to the constrained geometry imposed by the ligand scaffold, this four-coordinate aluminium centre remains Lewis acidic and reacts via differing metal-ligand cooperative pathways for activating ketones and CO₂ . It is also a rare example of a single-component aluminium system for the catalytic reduction of CO₂ to a methanol equivalent at room temperature.

Reactivity of triphosphinoboranes towards H3B·SMe2: access to derivatives of boraphosphacycloalkanes with diverse substituents

Triphosphinoboranes activated the B-H bond in the BH₃ molecule without any catalysts at room temperature. Hydroboration reactions led to boraphosphacyloalkanes with diverse structures. The outcomes of reactions depend on the size of the phosphanyl substituent on the boron atom of the parent triphosphinoborane, where derivatives of boraphosphacyclobutane and boraphosphacyclohexane were obtained. Furthermore, the precursor of triphosphinoboranes, namely bromodiphosphinoborane, also exhibited high reactivity towards H₃B·SMe₂, yielding bromo-substituted boraphosphacyclobutane. The obtained products were characterized by heteronuclear NMR spectroscopy, single crystal X-ray diffraction, and elemental analysis.

Comprehensive Theoretical Study of Cp*IrIII-Catalyzed Intermolecular Enantioselective Allylic C-H Amidation: Reaction Mechanism, Electronic Processes, and Regioselectivity

Density functional theory was used to elucidate the reaction mechanism of Cp*Ir^III-catalyzed intermolecular regioselective C(sp³)-H amidation of alkenes with methyl dioxazolones. All substrates, intermediates, and transition states were fully optimized at the ωB97XD/6-31G(d,p) level (LANL2DZ(f) for Ir). The computational results revealed that this amidation occurred through the Ir^III/Ir^V catalytic cycle, involving four important elementary steps: C-H bond activation, oxidative addition of methyl dioxazolone, reductive elimination, and proto-demetalation, and the first was the rate-determining step. The C-H bond activation showed good α- and branch-regioselectivity, decided by the distortion energy of 2-pentene and the interaction energy of the transition state, respectively. The oxidative addition of dioxazolone occurred in one elementary step with CO₂ disassociation. The reductive elimination showed good branch-regioselectivity determined by the distorted energy of the allyl group. In the proto-demetalation, hydrogen directly transferred from the oxygen atom to the nitrogen atom. Moreover, to clarify the effect of the substituted groups, selected 12 substrates were also discussed in this text.

Partner effect in accelerating pincer-co catalyzed nitrile hydroboration reactions

The pincer-Co catalyzed nitrile hydroboration of nitrile has been presented as an elegant strategy to afford amine synthesis; however, ligand engineering is required. We show here a strategy to tune the catalytic behavior of the organometallic catalyst, as an alternative approach to ligand engineering, by means of computational investigations to understand the effect of partners such as (18-crown-6)K⁺, W(CO)₃ and W(PMe₃)₃ on the reactivity of the pincer-Co catalyzed nitrile hydroboration reaction through π-coordination to the ligand aromatic ring. The extra additives bind the central phenyl ring of the ligand by either dispersion or chemical bonding. The electron-richness of the cobalt center is tuned by the partner, and follows the order (18-crown-6)K⁺ > W(PMe₃)₃ > no partner > W(CO)₃. While the influence of the covalent W-containing partners parallels the electron-richness of the W, the non-covalent partner, (18-crown-6)K⁺, surprisingly increases the donor ability of the pincer ligand through the polarization effect. All the elementary steps involved in the nitrile hydroboration reaction are influenced by the partner, and the overall barrier is lowered by a surprisingly large amount of 4.9 kcal mol^-1 in the presence of (18-crown-6)K⁺, suggesting a notable partner effect to be explored by experimentalists so that the reactivity of a catalyst can be tuned without ligand modification.

Si-H Bond Activation and Dehydrogenative Coupling of Silanes across the Iron-Amide Bond of a Bis(amido)bis(phosphine) Iron(II) Complex

Despite the utility of Si-Si bonds, there are relatively few examples of Si-Si bond formation by base metals. In this work, a four-coordinate iron complex, (PNNP)Fe^II, is shown to strongly activate the Si-H bonds in primary silanes across the Fe-amide bonds in a metal-ligand cooperative fashion. Upon treatment with excess silane, Si-Si dehydrogenative homocoupling is shown to occur across the Fe-N_amide bond without concomitant oxidation and spin state changes at the Fe center.

Density Functional Theory Study on the H2-Acceptorless Dehydrogenative Boration of Alkenes Catalyzed by a Zirconium Complex

For the synthesis of vinyl boronate esters, the direct catalytic H₂-acceptorless dehydrogenative boration of alkenes is one of the promising strategies. In this paper, the density functional theory method was employed to investigate the reaction mechanism of dehydrogenative boration and transfer boration of alkenes catalyzed by a zirconium complex (Cp₂ZrH₂). There are two possible pathways for this reaction: the alkene insertion followed by the dehydrogenative boration (path A) and the alkene insertion after the dehydrogenative boration (path B). The calculated results showed that path A is more favorable than path B, and that the rate-determining step is the C-B coupling step with an energy barrier of 18.7 kcal/mol. The reaction modes of the C-B coupling assisted dehydrogenative boration and the alkene insertion were also discussed. These analyses reveal a novel hydrogen release behavior in dehydrogenative boration and the alkene insertion modes and sequences were proposed to be of importance in the chemoselectivity of this reaction. In addition, the X ligand effect (X = H, Cl) on the catalytic activity of the zirconium complex was explored, indicating that the H ligand could enhance the catalytic activity of the complex for styrene dehydrogenative boration.

E-selective Semi-hydrogenation of Alkynes under Mild Conditions by a Diruthenium Hydride Complex

The synthesis, characterization and catalytic activity of a new class of diruthenium hydrido carbonyl complexes bound to the ^tBu PNNP expanded pincer ligand is described. Reacting ^tBu PNNP with two equiv of RuHCl(PPh₃ )₃ (CO) at 140 °C produces an insoluble air-stable complex, which was structurally characterized as [Ru₂ (^tBu PNNP)H(μ-H)Cl(μ-Cl)(CO)₂ ] (1) using solid-state NMR, IR and X-ray absorption spectroscopies and follow-up reactivity. A reaction with KOtBu results in deprotonation of a methylene linker to produce [Ru₂ (^tBu PNNP^* )H(μ-H)(μ-OtBu)(CO)₂ ] (3) featuring a partially dearomatized naphthyridine core. This enables metal-ligand cooperative activation of H₂ analogous to the mononuclear analogue, [Ru(^tBu PNP*)H(CO)]. In contrast to the mononuclear system, the bimetallic analogue 3 catalyzes the E-selective semi-hydrogenation of alkynes at ambient temperature and atmospheric H₂ pressure with good functional group tolerance. Monitoring the semi-hydrogenation of diphenylacetylene by ¹ H NMR spectroscopy shows the intermediacy of Z-stilbene, which is subsequently isomerized to the E-isomer. Initial findings into the mode of action of this system are provided, including the spectroscopic characterization of a polyhydride intermediate and the isolation of a deactivated species with a partially hydrogenated naphthyridine backbone.

Activation of H-H, HO-H, C(sp2)-H, C(sp3)-H, and RO-H Bonds by Transition-Metal Frustrated Lewis Pairs Based on M/N (M = Rh, Ir) Couples

Reaction of the dimers [(Cp*MCl)₂(μ-Cl)₂] (Cp* = η⁵-C₅Me₅) with Ph₂PCH₂CH₂NC(NH(p-Tolyl))₂ (H₂L) in the presence of NaSbF₆ affords the chlorido complexes [Cp*MCl(κ²N,P-H₂L)][SbF₆] (M = Rh, 1; Ir, 2). Upon treatment with aqueous NaOH, solutions of 1 and 2 yield the corresponding complexes [Cp*M(κ³N,N′,P-HL)][SbF₆] (M = Rh, 3; Ir, 4) in which the ligand HL presents a fac κ³N,N′,P coordination mode. Treatment of THF solutions of complexes 3 and 4 with hydrogen gas, at room temperature, results in the formation of the metal hydrido-complexes [Cp*MH(κ²N,P-H₂L)][SbF₆] (M = Rh, 5; Ir, 6) in which the N(p-Tolyl) group has been protonated. Complexes 3 and 4 react with deuterated water in a reversible fashion resulting in the gradual deuteration of the Cp* group. Heating at 383 K THF/H₂O solutions of the complexes 3 and 4 affords the orthometalated complexes [Cp*M(κ³C,N,P-H₂L_-H)][SbF₆] [M = Rh, 7; Ir, 8, H₂L_-H = Ph₂PCH₂CH₂NC(NH(p-Tolyl))(NH(4-C₆H₃Me))], respectively. At 333 K, complexes 3 and 4 react in THF with methanol, primary alcohols, or 2-propanol giving the metal-hydrido complexes 5 and 6, respectively. The reaction involves the acceptorless dehydrogenation of the alcohols at a relatively low temperature, without the assistance of an external base. The new complexes have been characterized by the usual analytical and spectroscopic methods including the X-ray diffraction determination of the crystal structures of complexes 1–5, 7, and 8. Notably, the chlorido complexes 1 and 2 crystallize both as enantiopure conglomerates and as racemates. Reaction mechanisms are proposed based on stoichiometric reactions, nuclear magnetic resonance studies, and X-ray crystallography as well as density functional theory calculations.

In solution, masked transition-metal frustrated Lewis pairs (TMFLPs) give rise to the corresponding TMFLP species which activate dihydrogen, water, and alcohols following FLP reaction pathways. When D₂O or alcohols with deuterated OH groups were employed, H/D exchange at the Cp* ligand (involving C(sp³)−H activation) was observed. C(sp²)−H bond activation involving orthometalation of the p-Tolyl ring was also observed.

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.

Theoretical Study of N-H σ-Bond Activation by Nickel(0) Complex: Reaction Mechanism, Electronic Processes, and Prediction of Better Ligand

N-H σ-bond activation of alkylamine by Ni(PCy₃) was investigated using density functional theory (DFT) calculations. When simple alkylamine NHMe₂ is a reactant, both concerted oxidative addition in Ni(PCy₃)(NHMe₂) and ligand-to-ligand H transfer reaction in Ni(PCy₃)(C₂H₄)(NHMe₂) are endergonic and need a high activation energy. When NH(Me)(Bs) (Bs = SO₂Ph, a model of tosyl group used in experiments) is a reactant, both reactions are exergonic and occur easily with a much smaller activation energy. The much larger reactivity of NH(Me)(Bs) than that of NHMe₂ results from the stronger Ni-N(Me)(Bs) bond than the Ni-NMe₂ bond and the presence of the Ni-O bonding interaction between the Bs group and the Ni atom in the product. N-Heterocyclic carbene, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), is computationally predicted to be better than PCy₃ because the Ni-NMe₂ and Ni-N(Me)(Bs) bonds in the IPr complex are stronger, respectively, than those of the PCy₃ complex. The introduction of the electron-withdrawing Bs group to the N atom of amine and the use of IPr as a ligand are recommended for the N-H σ-bond activation. The C-H σ-bond activations of benzene via the oxidative addition and the ligand-to-ligand H transfer reaction were also investigated here for comparison with the N-H σ-bond activation. The differences between the C-H σ-bond activation of benzene and the N-H σ-bond activation of these amines are discussed in terms of the N-H, C-H, Ni-Ph, and Ni-NMe₂, and Ni-N(Me)(Bs) bond energies and back-donation to benzene from the Ni atom.

Solvent-Assisted Ketone Reduction by a Homogeneous Mn Catalyst

The choice of a solvent and the reaction conditions often defines the overall behavior of a homogeneous catalytic system by affecting the preferred reaction mechanism and thus the activity and selectivity of the catalytic process. Here, we explore the role of solvation in the mechanism of ketone reduction using a model representative of a bifunctional Mn-diamine catalyst through density functional theory calculations in a microsolvated environment by considering explicit solvent and fully solvated ab initio molecular dynamics simulations for the key elementary steps. Our computational analysis reveals the possibility of a Meerwein–Ponndorf–Verley (MPV) type mechanism in this system, which does not involve the participation of the N–H moiety and the formation of a transition-metal hydride species in ketone conversion. This path was not previously considered for Mn-based metal–ligand cooperative transfer hydrogenation homogeneous catalysis. The MPV mechanism is strongly facilitated by the solvent molecules present in the reaction environment and can potentially contribute to the catalytic performance of other related catalyst systems. Calculations indicate that, despite proceeding effectively in the second coordination sphere of the transition-metal center, the MPV reaction path retains the enantioselectivity preference induced by the presence of the small chiral N,N′-dimethyl-1,2-cyclohexanediamine ligand within the catalytic Mn(I) complex.

SNS ligand-assisted catalyst activation in Zn-catalysed carbonyl hydroboration

Ligands that include Lewis acid/base functionality have extensive applications in bifunctional catalysis using first row metals. In this work, zinc bis(amido), bis(thiolate) and amido-thiolate SNS complexes were prepared and compared as precatalysts for carbonyl hydroboration using pinacolborane. Mechanistic studies revealed two different ligand-assisted precatalyst activation pathways, both leading to an active and robust zinc alkoxide catalyst. This work furthers our understanding of metal-ligand cooperation in first-row metal catalysis.

Cycloaddition of isoselenocyanates to sodium and magnesium metallacycles

Heterocumulenes SeCNR (R = C₆H₄OMe-2, C₆H₄Me-2) undergo facile cycloaddition to [(H-dpp-bian)Na(Et₂O)₂] (1) (H-dpp-bian = N-protonated 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) resulting in cycloadducts [(H-dpp-bian)Na(SeCNR)(DME)] (2, 3), which are the first cycloadducts derived from a sodium metallacycle reported so far. A comparative reaction of [(dpp-bian)Mg(THF)₃] (10) with SeCNR gives magnesium cycloadducts [(dpp-bian)Mg(SeCNR)(Solv)₂] (11, 12), which undergo fast decomposition at room temperature. New compounds are characterized by NMR, EPR, and IR spectroscopy, and elemental and X-ray diffraction analysis. Their electronic structures and reaction pathways were probed using DFT calculations.

Insights into the Activation Mode of α-Carbonyl Sulfoxonium Ylides in Rhodium-Catalyzed C-H Activation: A Theoretical Study

A computational study has been performed to investigate the mechanism of Rh^III‐catalyzed C−H bond activation using sulfoxonium ylides as a carbene precursor. The stepwise and concerted activation modes for sulfoxonium ylides were investigated. Detailed theoretical results showed that the favored stepwise pathway involves C−H bond activation, carbonization, carbene insertion, and protonation. The free energy profiles for dialkylation of 2‐phenylpyridine were also calculated to account for the low yield of this reaction. Furthermore, the substituent effect was elucidated by comparing the energy barriers for the protonation of meta‐ and para‐substituted sulfoxonium ylides calculated by density functional theory.

Combining metal-metal cooperativity, metal-ligand cooperativity and chemical non-innocence in diiron carbonyl complexes

Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand's naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp3)-H bonds by ∼25 kcal mol-1. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = ) complex containing a mixed-valent iron(0)-iron(i) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity.

Homogeneous catalysis with polyhydride complexes

This review analyzes the role of transition metal polyhydrides as homogeneous catalysts for organic reactions. Discussed reactions involve nearly every main organic functional group.

Spin-state crossover in photo-catalyzed nitrile dihydroboration via Mn-thiolate cooperation

The role of a phosphine-free SNS-pincer ligand in metal–ligand cooperative hydroboration catalysis was investigated. The bifunctional thiolate donor and spin-state change to high-spin Mn are crucial to accessing low-energy activation barriers.

Activation of Si–H and B–H bonds by Lewis acidic transition metals and p-block elements: same, but different

In this Perspective we discuss the ability of transition metal complexes to activate and cleave the Si–H and B–H bonds of hydrosilanes and hydroboranes (tri- and tetra-coordinated) in an electrophilic manner, avoiding the need for the metal centre to undergo two-electron processes (oxidative addition/reductive elimination). A formal polarization of E–H bonds (E = Si, B) upon their coordination to the metal centre to form σ-EH complexes (with coordination modes η¹ or η²) favors this type of bond activation that can lead to reactivities involving the formation of transient silylium and borenium/boronium cations similar to those proposed in silylation and borylation processes catalysed by boron and aluminium Lewis acids. We compare the reactivity of transition metal complexes and boron/aluminium Lewis acids through a series of catalytic reactions in which pieces of evidence suggest mechanisms involving electrophilic reaction pathways.

In this Perspective we compare the ability of transition metals and p-block Lewis acids to activate electrophilically hydrosilanes and hydroboranes. The mechanistic similarities and dissimilarities in different catalytic transformations are analyzed.

Cooperative B-H bond activation: Dual sites borane activation by redox active κ2-N,S-chelated complexes

Cooperative dual site activation of boranes by redox-active 1,3-N,S-chelated ruthenium species, mer-[PR₃{κ²-N,S-(L)}₂Ru{κ¹-S-(L)}], (mer-2a: R = Cy, mer-2b: R = Ph; L = NC₇H₄S₂), generated from the aerial oxidation of borate complexes, [PR₃{κ²-N,S-(L)}Ru{κ³-H,S,S′-BH₂(L)₂}] (trans–mer-1a: R = Cy, trans–mer-1b: R = Ph; L = NC₇H₄S₂), has been investigated. Utilizing the rich electronic behaviour of these 1,3-N,S-chelated ruthenium species, we have established that a combination of redox-active ligands and metal–ligand cooperativity has a big influence on the multisite borane activation. For example, treatment of mer-2a–b with BH₃·THF led to the isolation of fac-[PR₃Ru{κ³-H,S,S′-(NH₂BSBH₂N)(S₂C₇H₄)₂}] (fac-3a: R = Cy and fac-3b: R = Ph) that captured boranes at both sites of the κ²-N,S-chelated ruthenacycles. The core structure of fac-3a and fac-3b consists of two five-membered ruthenacycles [RuBNCS] which are fused by one butterfly moiety [RuB₂S]. Analogous fac-3c, [PPh₃Ru{κ³-H,S,S′-(NH₂BSBH₂N)(SC₅H₄)₂}], can also be synthesized from the reaction of BH₃·THF with [PPh₃{κ²-N,S-(SNC₅H₄)}{κ³-H,S,S′-BH₂(SNH₄C₅)₂}Ru], cis–fac-1c. In stark contrast, when mer-2b was treated with BH₂Mes (Mes = 2,4,6-trimethyl phenyl) it led to the formation of trans- and cis-bis(dihydroborate) complexes [{κ³-S,H,H-(NH₂BMes)Ru(S₂C₇H₄)}₂], (trans-4 and cis-4). Both the complexes have two five-membered [Ru–(H)₂–B–NCS] ruthenacycles with κ²-H–H coordination modes. Density functional theory (DFT) calculations suggest that the activation of boranes across the dual Ru–N site is more facile than the Ru–S one.

Redox-active ruthenium complexes supported by hemilabile κ²-N,S-chelated ruthenacycles undergo unusual dual site B–H bond activation through metal–ligand cooperation with free and bulky boranes.

Insights into the mechanism and regioselectivity in Ni-catalysed redox-relay migratory hydroarylation of alkenes with arylborons

DFT studies reveal that Ni-catalysed redox-relay hydroarylation of alkenes occurs via concerted hydronickelation, iterative β-H elimination/migratory insertion and reductive elimination to yield the α-substituted product. The driving force for the redox-relay migratory hydroarylation arises from the stability of the LArNi(II)CHPhPr intermediate, which only allows its C-C elimination pathway to be opened up.

Alternative Conceptual Approach to the Design of Bifunctional Catalysts: An Osmium Germylene System for the Dehydrogenation of Formic Acid

The reaction of the hexahydride OsH₆(PⁱPr₃)₂ with a P,Ge,P-germylene-diphosphine affords an osmium tetrahydride derivative bearing a Ge,P-chelate, which arises from the hydrogenolysis of a P–C(sp³) bond. This Os(IV)–Ge(II) compound is a pioneering example of a bifunctional catalyst based on the coordination of a σ-donor acid, which is active in the dehydrogenation of formic acid to H₂ and CO₂. The kinetics of the dehydrogenation, the characterization of the resting state of the catalysis, and DFT calculations point out that the hydrogen formation (the fast stage) exclusively occurs on the coordination sphere of the basic metal center, whereas both the metal center and the σ-donor Lewis acid cooperatively participate in the CO₂ release (the rate-determining step). During the process, the formate group pivots around the germanium to approach its hydrogen atom to the osmium center, which allows its transfer to the metal and the CO₂ release.

An alternative class of bifunctional catalysts can be assembled by coordination of σ-donor Lewis acids to platinum-group-metal basic fragments. In contrast to what happens with the previously reported bifunctional catalysts, this design allows enhancing the basicity of the base and the acidity of the acid. According to this, a bifunctional catalyst for the dehydrogenation of formic acid, based on an osmium(IV)-germylene cooperative system, has been prepared and the mechanism of the catalysis established.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Same ligand, three first-row metals: comparing M-amido bifunctional reactivity (Mn, Fe, Co)

The bifunctional reactivity of three metal SNS (bis)amido complexes was computationally assessed by comparing the nucleophilicity of the M-N_amido donor (Mn, Fe, Co). Hirshfeld charges identified the Mn-N_amido donor as most nucleophilic and Fe as most electrophilic metal. Reaction energy profiles of a model bifunctional H₂ activation showed Mn with the lowest reaction barrier (17 kcal mol^-1), followed by Fe and Co (21 and 29 kcal mol^-1).

Cooperativity in Transition Metal Tetrylene Complexes

Cooperative reactivity between transition metals and ligands, or between two metals, has created significant opportunities for the development of new transformations that would be difficult to carry out with a single metal. Here we explore cooperativity between transition metals and divalent heavier group 14 elements (tetrylenes), a less-explored facet of the field of cooperativity. Tetrylenes combine their strong σ-donor properties with an empty p-orbital that can accept electron density. This ambiphilicity has allowed them to form metal tetrylene and metallotetrylene complexes that place a reactive site adjacent to the metal. We have selected examples to demonstrate what has been achieved so far regarding cooperative reactivity, as this already spans metal-, tetrylene- or multi-site-centred bond cleavage, cycloaddition, migration, metathesis, and insertion. We also highlight some challenges that need to be overcome for this cooperativity to make it to catalysis.

Using Postsynthetic X-Type Ligand Exchange to Enhance CO2 Adsorption in Metal-Organic Frameworks with Kuratowski-Type Building Units

Postsynthetic modification methods have emerged as indispensable tools for tuning the properties and reactivity of metal-organic frameworks (MOFs). In particular, postsynthetic X-type ligand exchange (PXLE) at metal building units has gained increasing attention as a means of immobilizing guest species, modulating the reactivity of framework metal ions, and introducing new functional groups. The reaction of a Zn-OH functionalized analogue of CFA-1 (1-OH, Zn(ZnOH)₄(bibta)₃, where bibta^2- = 5,5'-bibenzotriazolate) with organic substrates containing mildly acidic E-H groups (E = C, O, N) results in the formation of Zn-E species and water as a byproduct. This Brønsted acid-base PXLE reaction is compatible with substrates with pK_a(DMSO) values as high as 30 and offers a rapid and convenient means of introducing new functional groups at Kuratwoski-type metal nodes. Gas adsorption and diffuse reflectance infrared Fourier transform spectroscopy experiments reveal that the anilide-exchanged MOFs 1-NHPh_0.9 and 1-NHPh_2.5 exhibit enhanced low-pressure CO₂ adsorption compared to 1-OH as a result of a Zn-NHPh + CO₂ ⇌ Zn-O₂CNHPh chemisorption mechanism. The MFU-4l analogue 2-NHPh ([Zn₅(OH)_2.1(NHPh)_1.9(btdd)₃], where btdd^2- = bis(1,2,3-triazolo)dibenzodioxin), shows a similar improvement in CO₂ adsorption in comparison to the parent MOF containing only Zn-OH groups.