DFT Struggles to Predict the Energy Landscape for Iron Pyridine Diimine-Catalyzed [2 + 2] Cycloaddition of Alkenes: Insights into the Problem and Alternative Solutions

In recent years, noninnocent pyridine diimine (PDI) complexes featuring first-row transition metals have emerged as prominent catalysts, demonstrating efficacy in a diverse range of vital organometallic transformations. However, the inherent complexity of the fundamental reactivity paradigm in these systems arises from the presence of a noninnocent ligand and the multispin feasibility of 3d metals. While density functional theory (DFT) has been widely used to unravel mechanistic insights, its limitations as a single-reference method can potentially misrepresent spin-state energetics, compromising our understanding of these intricate systems. In this study, we employ extensive high-level ab initio state averaged-complete active space self-consistent field/N-electron valence state perturbation theory (SA-CASSCF/NEVPT2) calculations in combination with DFT to investigate an iron-PDI-catalyzed [2 + 2] cycloaddition reaction of alkenes. The transformation proceeds through two major steps: oxidative cyclization and reductive elimination. Contrary to the predictions of DFT calculations, which suggest two-state reactivity in the reaction and identify reductive elimination as the turnover-limiting step, SA-CASSCF/NEVPT2-corrected results unequivocally establish a single-state reactivity scenario with oxidative cyclization as the turnover-limiting step. SA-CASSCF/NEVPT2-based insights into electronic ground states and electron distribution elucidate the intriguing interactions between the PDI ligand and the iron center, revealing the highly multiconfigurational nature of these species and providing a precise depiction of metal-ligand cooperativity throughout the transformation. A comparative assessment of several widely recognized DFT functionals against SA-CASSCF/NEVPT2-corrected data indicates that single-point energy calculations using the modern density functional MN15 on TPSSh geometries offer the most reliable density functional methodology, in scenarios where SA-CASSCF/NEVPT2 computational cost is a consideration.

From Carbene-Dithiolene Zwitterion Mediated B-H Bond Activation to BH3·SMe2-Assisted Boron-Boron Bond Formation

The 1:1 reaction of the carbene-stabilized dithiolene zwitterion 1 with BH3·SMe2 gave the dithiolene-based hydroborane 2 and the doubly hydrogen-capped CAAC species 3 via hydride-coupled reverse electron transfer processes. The mechanism of this transformation was probed computationally using density functional theory. The subsequent 2:1 reaction of 2 with 1 resulted in 4 and 3, suggesting that 1 can mediate the B-H bond activation not only for BH3 but also for monohydroboranes. In the presence of BH3·SMe2, 2 was unexpectedly converted to the corresponding diborane(4) complex 5 through a dehydrocoupling reaction at an elevated temperature.

Cooperation towards nobility: equipping first-row transition metals with an aluminium sword

The exploration for noble metals substitutes in catalysis has become a highly active area of research, driven by the pursuit of sustainable chemical processes. Although the utilization of base metals holds great potential as an alternative, their successful implementation in predictable catalytic processes necessitates the development of appropriate ligands. Such ligands must be capable of controlling their intricate redox chemistry and promote two-electron events, thus mimicking well-established organometallic processes in noble metal catalysis. While numerous approaches for infusing nobility to base metals have been explored, metal-ligand cooperation has garnered significant attention in recent years. Within this context, aluminium-based ligands offer interesting features to fine-tune the activity of metal centres, but their application in base metal catalysis remains largely unexplored. This perspective seeks to highlight the most recent breakthroughs in the reactivity of heterobimetallic aluminium-base-metal complexes, while also showcasing their potential to develop novel and predictable catalytic transformations. By turning the spotlight on such heterobimetallic species, we aim to inspire chemists to explore aluminium-base-metal species and expand the range of their applications as catalysts.

Carbonylative Formal Cycloaddition between Alkylarenes and Aldimines Enabled by Palladium-Catalyzed Double C-H Bond Activation

Double C-H bond activation can enable an expeditious reaction pathway to cyclic compounds, offering an efficient tool to synthesize valuable molecules. However, cyclization reaction enabled by double C-H bond activation at one carbon atom is nearly unknown. Herein, we report a carbonylative formal cycloaddition of alkylarenes with imines via double benzylic C-H bond activation at one carbon atom, allowing a straightforward synthesis of β-lactams from readily accessible alkylarenes and imines, which paves the way for developing an annulation reaction through double C-H bond activation at one carbon atom.

Experimental and Theoretical Study of Ni II ‐ and Pd II ‐Promoted Double Geminal C(sp 3 )−H Bond Activation Providing Facile Access to NHC Pincer Complexes: Isolated Intermediates and Mechanism

We report the first examples of metal‐promoted double geminal activation of C(sp³)−H bonds of the N−CH₂−N moiety in an imidazole‐type heterocycle, leading to nickel and palladium N‐heterocyclic carbene complexes under mild conditions. Reaction of the new electron‐rich diphosphine 1,3‐bis((di‐tert‐butylphosphaneyl)methyl)‐2,3‐dihydro‐1H‐benzo[d]imidazole (1) with [PdCl₂(cod)] occurred in a stepwise fashion, first by single C−H bond activation yielding the alkyl pincer complex [PdCl(PCsp3^HP)] (3) with two trans phosphane donors and a covalent Pd−Csp3 bond. Activation of the C−H bond of the resulting α‐methine Csp3 H−M group occurred subsequently when 3 was treated with HCl to yield the NHC pincer complex [PdCl(PC^NHCP)]Cl (2). Treatment of 1 with [NiBr₂(dme)] also afforded a NHC pincer complex, [NiBr(PC^NHCP)]Br (6), but the reactions leading to the double geminal C−H bond activation of the N−CH₂−N group were too fast to allow identification or isolation of an intermediate analogous to 3. The determination of six crystal structures, the isolation of reaction intermediates and DFT calculations provided the basis for suggesting the mechanism of the stepwise transformation of a N−CH₂−N moiety in the N−C^NHC−N unit of NHC pincer complexes and explain the key differences observed between the Pd and Ni chemistries.

A Silylene Stabilized by a σ‐Donating Nickel(0) Fragment

A donor‐stabilized silylene 4 featuring a Ni⁰‐based donating ligand was synthesized. Complex 4 exhibits a pyramidalized and nucleophilic Si^II center and shows a peculiar behavior due to the cooperative reactivity of Si and Ni centers. Calculations indicate that the orientation of Ni‐ligands with respect to the silylene moiety is crucial in determining the role of the Ni‐fragment (Lewis acid or Lewis base) towards silylene. Indeed, a simple 90° rotation of the Si−Ni bond, reverses the role of Ni, and transforms a classical silylene→Ni⁰ complex into an unprecedented Ni⁰→silylene complex.

Nucleophilic Reactivity at a ═CH Arm of a Lutidine-Based CNC/Rh System: Unusual Alkyne and CO2 Activation

Reaction of an amido pincer complex [(CNC)*Rh(CO)] (1) (CNC* is the deprotonated form of CNC) with carbon dioxide gave a neutral complex [(CNC-CO₂)^Mes*Rh(CO)] (2), which is the result of a C-C bond-forming reaction between the deprotonated arm of the CNC* ligand and CO₂. The molecular structure of 2 showed a zwitterionic complex, where the CO₂ moiety is covalently connected to the former ═CH arm of the CNC* pincer ligand. The unusual structure of 1 allowed us to explore the reactivity of the CO₂ moiety with selected primary amines RNH₂ (benzylamine and ammonia), which afforded cationic complexes [(CNC)^MesRh(CO)][HRNC(O)O] (R = Bz (3), H (4)). Compounds 3 and 4 are the result of a C-N coupling between the incoming amine and the CO₂ fragment covalently connected to the pincer ligand in 2, a process that involves protonation of the "CH-CO₂" fragment in 2 from the respective amines. Once revealed the nucleophilic character of the ═CH fragment in 1, we explored its reactivity with alkynes, a study that enlightened a novel reactivity trend in alkyne activation. Reaction of 1 with terminal alkynes RC≡CH (R = Ph, 2-py, 4-C₆H₄-CF₃) yielded neutral complexes [(CNC-CH═CHR)^Mes*Rh(CO)] (R = Ph (5), 2-py (6), 4-C₆H₄-CF₃ (7)) in good yields. Deuterium labeling experiments with PhC≡CD confirmed that complex 5 is the product of a formal insertion of the alkyne into the C(sp²)-H bond of the deprotonated arm in 1. This structural proposal was further confirmed by the X-ray molecular structure of phenyl complex 5, which showed the alkyne covalently linked to the pincer ligand. Besides, this novel transformation was analyzed by DFT methods and showed a metal-ligand cooperative mechanism, based on the initial electrophilic attack of the alkyne to the ═CH arm of the CNC^Mes* ligand (making a new C-C bond) followed by the action of a protic base (HN(SiMe₃)₂), which is able to perform a proton rearrangement that leads to the final product 5.

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.

Cooperative approaches in catalytic hydrogenation and dehydrogenation

Metal-ligand cooperativity (MLC) is an established strategy for developing effective hydrogenation and dehydrogenation catalysts. Metal-metal cooperativity (MMC) in bimetallic complexes is not as well understood, and to date has had limited implementation in (de)hydrogenation. Herein we use (de)hydrogenation processes as a platform to examine modes of cooperativity, with a particular focus on catalytic mechanisms. We investigate how lessons learnt from the extensive development of metal-ligand cooperative catalysts can aid the ongoing development of metal-metal cooperative catalysts.

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.

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.

Tuning Ruthenium Carbene Complexes for Selective P-H Activation through Metal-Ligand Cooperation

The use of iminophosphoryl‐tethered ruthenium carbene complexes to activate secondary phosphine P−H bonds is reported. Complexes of type [(p‐cymene)‐RuC(SO₂Ph)(PPh₂NR)] (with R = SiMe₃ or 4‐C₆H₄−NO₂) were found to exhibit different reactivities depending on the electronics of the applied phosphine and the substituent at the iminophosphoryl moiety. Hence, the electron‐rich silyl‐substituted complex undergoes cyclometallation or shift of the imine moiety after cooperative activation of the P−H bond across the M=C linkage, depending on the electronics of the applied phosphine. Deuteration experiments and computational studies proved that cyclometallation is initiated by the activation process at the M=C bond and triggered by the high electron density at the metal in the phosphido intermediates. Consistently, replacement of the trimethylsilyl (TMS) group by the electron‐withdrawing 4‐nitrophenyl substituent allowed the selective cooperative P−H activation to form stable activation products.

Extensive Redox Non-Innocence in Iron Bipyridine-Diimine Complexes: a Combined Spectroscopic and Computational Study

Metal-ligand cooperation is an important aspect in earth-abundant metal catalysis. Utilizing ligands as electron reservoirs to supplement the redox chemistry of the metal has resulted in many new exciting discoveries. Here, we demonstrate that iron bipyridine-diimine (BDI) complexes exhibit an extensive electron-transfer series that spans a total of five oxidation states, ranging from the trication [Fe(BDI)]3+ to the monoanion [Fe(BDI]-1. Structural characterization by X-ray crystallography revealed the multifaceted redox noninnocence of the BDI ligand, while spectroscopic (e.g., 57Fe Mössbauer and EPR spectroscopy) and computational studies were employed to elucidate the electronic structure of the isolated complexes, which are further discussed in this report.

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.

Carbocyclic pincer carbene complexes of ruthenium: syntheses and reversible hydrogenation

The ruthenium carbene pincer complex 2 was synthesized treating the benzo annulated cycloheptatriene bisphosphine 1 with RuCl₃. Addition of three equivalents of hydrogen to the carbocyclic carbene complex 2 was achieved in reaction of 2 with hydrogen at elevated temperatures. Hydrogenated complex 4, exhibiting a rigid chair conformation in solution, was dehydrogenated by heating a toluene solution of complex 4 to reflux for 5-7 d. In reaction with ethylene, complex 4 transfers one equivalent of hydrogen, forming ethane and alkyl complex 5.

Recent Advances in Theoretical Studies on Transition-Metal-Catalyzed Carbene Transformations

Metal carbene plays a vital role in modern organic synthesis. The neutral divalent carbon of metal carbene renders it an active intermediate throughout a range of reactions. In experiments, diverse metal carbene-related transformation reactions have been established, including transition-metal-catalyzed cross-coupling reactions using N-heterocyclic carbenes as ligands, metal carbene insertion into σ bonds, cyclopropanations, ylide formation, and so forth. The remarkable progress achieved in synthetic chemistry, in turn, has increased the demand for mechanistic studies of carbene chemistry. A thorough understanding of reaction mechanisms can extend the application scope of metal carbene compounds and inspire the rational design of new carbene transformation reactions.Density functional theory (DFT) calculations have been performed in our group to gain more mechanistic insights into metal carbene-related reactions. This account focuses on computational studies of transition-metal-catalyzed carbene transformation reactions with nucleophiles. The generation of metal carbene or metal-ligated free carbene and subsequent carbene transformation pathways is discussed. According to our mechanistic studies of carbene transformation with nucleophiles, three generalized reaction models are summarized, including the intramolecular migratory insertion of metal carbene, intermolecular nucleophilic addition toward metal carbene, and outer-sphere nucleophilic addition to the metal-ligated free carbene.In general, the intermolecular nucleophilic addition mechanism is commonly proposed since metal carbene has an electrophilic carbene carbon. From a mechanistic point of view, the intramolecular migratory insertion mechanism is also widely used because metal carbene insertion into σ bonds formally occurs through this mechanism. An outer-sphere nucleophilic addition mechanism is proposed for reactions that form a metal-ligated free carbene complex instead of the commonly proposed metal carbene. The metal-ligated free carbene complex contains a naked carbene carbon that is not coordinated with the metal center. In this case, a transition-metal catalyst is used only as a Lewis acid, and nucleophilic addition occurs directly at the free carbene carbon. Our computational results suggested that outer-sphere nucleophilic addition is a facile step because metal ligation could stabilize the transition state as well as the generated intermediate. The intramolecular migratory insertion mechanism also has a low energy barrier due to the lack of an entropy penalty. Carbene formation from carbene precursors is usually the rate-determining step, except in intermolecular nucleophilic addition, and the reactivity of nucleophiles has a significant influence on the overall reaction rate. We can also envision that the weak nucleophilicity of nucleophiles would suppress outer-sphere nucleophilic addition. These computational studies showcase the characteristics of three carbene transformation models, and we hope that it will spur the development of mechanistic studies of carbene chemistry.

Access to and Reactivity of Fe0 , Fe-I , FeI , and FeII PCcarbene P Pincer Complexes

Despite their promising metal-ligand cooperative reactivity, PC_carbene P pincer ligands are rarely reported for first-row transition-metal centres. Using a dehydration methodology, we report access to an Fe⁰ PC_carbene P pincer complex (1) that proceeds via an isolated α-hydroxylalkyl hydrido complex (3). Reversible carbonyl migration to the carbene position in 1 is found to allow coordination chemistry and E-H bond addition (E=H, B, Cl) across the iron-carbene linkage, representing a unique mechanism for metal-ligand cooperativity. The PC_carbene P pincer ligand is also found to stabilize formal Fe^II , Fe^I , and Fe^-I oxidation states, as demonstrated with synthesis and characterization of the complexes [11-X][BAr^F ₂₀ ] (X=Br, I), 12, and K[13]. Compound K[13] is found to be highly reactive, and abstracts hydrogen from a range of aliphatic C-H sources. Computational analysis by DFT suggests that the formal Fe^I and Fe^-I complexes contain significant carbene radical character. The ability of the PC_carbene P ligand scaffold to partake in metal-ligand cooperativity and to support a range of iron oxidation states renders it as potentially useful in many catalytic applications.

Charge frustration in ligand design and functional group transfer

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e^- chemistry or switching.

Cooperative B-H and Si-H Bond Activations by κ2-N,S-Chelated Ruthenium Borate Complexes

Cooperative E-H (E = B, Si) bond activations employing κ²-N,S-chelated ruthenium borate species, [PPh₃{κ²-N,S-(NS₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(NC₇H₄S₂)₂}], (1) are established. Treatment of 1 with BH₃·SMe₂ yielded the six-membered ruthenaheterocycle [PPh₃{κ²-S,H-(BH₃NS₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (2) formed by a hemilabile ring opening of a Ru-N bond and capturing of a BH₃ unit coordinated in an "end-on" fashion. On the other hand, the bulky borane H₂BMes shows different reactivity with 1 that led to the formation of the two dihydroborate complexes [{κ³-S,H,H-(NBH₂Mes)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (3) and [PPh₃{κ³-S,H,H-(NBH₂Mes)(S₂C₇H₄)}Ru(κ²-N,S-C₇H₄NS₂)] (4), in which H₂BMes has been inserted into the Ru-N bond of the initial κ²-N,S-chelated ligand. In an attempt to directly activate hydrosilanes by 1, reactions were carried out with H₂SiPh₂ that yielded two isomeric five-membered ruthenium silyl complexes, namely [PPh₃{κ²-S,Si-(NSiPh₂)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (5a,b), and the hydridotrisilyl complex [Ru(H){κ²-S,Si-(SiPh₂NC₇H₄S₂}₃] (6). These complexes were generated by Si-H bond activation with the release of H₂ and the formation of N-Si and Ru-Si bonds. When the reaction of 1 was carried out in the presence of PhSiH_3, the reaction only produced the analogous complexes [PPh₃{κ²-S,Si-(NSiPhH)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (5a',b'). Density functional theory (DFT) calculations have been used to probe the bonding modes of boranes/silane with the ruthenium center.

N-Heterocyclic silylenes as ambiphilic activators and ligands

Recent developments of the use of N-heterocyclic silylenes (NHSis), higher homologues of Arduengo-carbenes, as ambiphilic activators and ligands are highlighted and a comparison of NHSi ligands with NHC and phosphine ligands is provided.

Enthalpy-Controlled Insertion of a "Nonspectator" Tricoordinate Phosphorus Ligand into Group 10 Transition Metal-Carbon Bonds

Insertion of a tricoordinate phosphorus ligand into late metal-carbon bonds is reported. Metalation of a P^P-chelating ligand (L1), composed of a nontrigonal phosphorous (i.e., P(III)) triamide moiety, P(N(o-N(Ar)C6H4)2, tethered by a phenylene linker to a -PiPr2 anchor, with group 10 complexes L2M(Me)Cl (M = Ni, Pd) results in insertion of the nontrigonal phosphorus site into the metal-methyl bond. The stable methylmetallophosphorane compounds thus formed are characterized spectroscopically and crystallographically. Metalation of L1 with (cod)PtII(Me)(Cl) does not lead to a metallophosphorane but rather to the standard bisphosphine chelate (κ2-L1)Pt(Me)(Cl). These divergent reactivities within group 10 are rationalized by reference to periodic variation in M-C bond enthalpies.

Reappraising Schmidpeter's bis(iminophosphoranyl)phosphides: coordination to transition metals and bonding analysis

The synthesis and characterization of a range of bis(iminophosphoranyl)phosphide (BIPP) group 4 and coinage metals complexes is reported. BIPP ligands bind group 4 metals in a pseudo fac-fashion, and the central phosphorus atom enables the formation of d⁰–d¹⁰ heterobimetallic complexes. Various DFT computational tools (including AIM, ELF and NCI) show that the phosphorus–metal interaction is either electrostatic (Ti) or dative (Au, Cu). A bridged homobimetallic Cu–Cu complex was also prepared and its spectroscopic properties were investigated. The theoretical analysis of the P–P bond in BIPP complexes reveals that (i) BIPP are closely related to ambiphilic triphosphenium (TP) cations; (ii) the P–P bonds are normal covalent (i.e. not dative) in both BIPP and TP.

The synthesis, characterization and computational analysis of a range of bis(iminophosphoranyl)phosphide (BIPP) group 4 and coinage metals complexes is reported. White phosphorus was used to install the central phosphorus atom.

Influence of Multisite Metal-Ligand Cooperativity on the Redox Activity of Noninnocent N2S2 Ligands

Metal-ligand cooperativity (MLC) relies on chemically reactive ligands to assist metals with small-molecule binding and activation, and it has facilitated unprecedented examples of catalysis with metal complexes. Despite growing interest in combining ligand-centered chemical and redox reactions for chemical transformations, there are few studies demonstrating how chemically engaging redox active ligands in MLC affects their electrochemical properties when bound to metals. Here we report stepwise changes in the redox activity of model Ru complexes as zero, one, and two BH₃ molecules undergo MLC binding with a triaryl noninnocent N₂S₂ ligand derived from o-phenylenediamine (L1). A similar series of Ru complexes with a diaryl N₂S₂ ligand with ethylene substituted in place of phenylene (L2) is also described to evaluate the influence of the o-phenylenediamine subunit on redox activity and MLC. Cyclic voltammetry (CV) studies and density functional theory (DFT) calculations show that MLC attenuates ligand-centered redox activity in both series of complexes, but electron transfer is still achieved when only one of the two redox-active sites on the ligands is chemically engaged. The results demonstrate how incorporating more than one multifunctional reactive site could be an effective strategy for maintaining redox noninnocence in ligands that are also chemically reactive and competent for MLC.

Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization

N-heterocyclic carbenes (NHCs) are ubiquitous ancillary ligands employed in metal-catalyzed homogeneous reactions and polymerization reactions. Of significance is the use of NHCs as the supporting ligand in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization. While the applications of transition metal catalysts ligated with NHCs in polymerization chemistry are well-documented, the use of analogous rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) catalysts in this area remains under-developed, despite the unique role of rare earth elements in regio- and stereo-specific (co)polymerization reactions. By using hetero-atom-tethered chelating NHCs and, more recently, the employment of other structurally related NHCs, NHC-ligated Ln complexes have proven to be promising and fruitful catalysts for selective polymerization reactions. This review summarizes the recent developments in the coordination chemistry of Ln complexes containing NHCs and their catalytic performance in polymerization.

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.

Metal-ligand cooperative C-O bond cleavage of propargylic alcohol with protic pyrazole complexes of ruthenium

The C-O bond cleavage of a propargylic alcohol, 1,1-dimethyl-3-phenylprop-2-yn-1-ol (3), by metal-ligand cooperation was investigated. The reactions of a naphthalene complex [CpRu(η6-C10H8)]PF6 (Cp = η5-C5H5) with 5-R-3-(pyrid-2-yl)pyrazoles (R-LH; R = Ph, CF3) in acetonitrile afforded the starting metal-ligand bifunctional ruthenium complexes [CpRu(R-LH)(MeCN)]PF6 (1a, R = Ph; 1b, R = CF3) featuring an N-N chelate protic pyrazole. The treatment of the CF3-substituted pyrazole complex 1b with 3 in 1,2-dichloroethane at 50 °C resulted in the formation of the η3-butadienyl complex 5. Meanwhile, the corresponding reaction of the phenylpyrazole complex 1a in 1,4-dioxane at 90 °C gave the N-allenylmethylpyrazole complex 6. The C-O and C-H bond cleavage of 3 in these reactions can be ascribed to the metal-ligand cooperation: initial formation of an η3-propargyl complex assisted by NHO hydrogen bonding and following C-H deprotonation by the neighboring pyrazolato ligand. On the other hand, in boiling methanol, the protic pyrazole complex 1a catalyzed the C-O bond cleavage of 3, which resulted in catalytic isomerization of 3 to an α,β-unsaturated enone, 3-methyl-1-phenylbut-2-en-1-one (8). The control experiments using non-protic and isocyanide ruthenium complexes indicated that both a labile nitrile ligand on the metal and the protic cooperating ligand are crucial for the catalysis.

Polymer-Anchored Bifunctional Pincer Catalysts for Chemoselective Transfer Hydrogenation and Related Reactions

A series of polymer-supported cooperative PC(sp³ )P pincer catalysts was synthesized and characterized. Their catalytic activity in the acceptorless dehydrogenative coupling of alcohols and the transfer hydrogenation of aldehydes with formic acid as a hydrogen source was investigated. This comparative study, examining homogeneous and polymer-tethered species, proved that carefully designing a link between the support and the catalytic moiety, which takes into consideration the mechanism underlying the target transformation, might lead to superior heterogeneous catalysis.