Cooperative Catalysis with First‐Row Late Transition Metals

Ligand Noninnocence in β-Diketiminate and β-Diketimine Copper Complexes

Metal-ligand cooperative systems have a long precedent in catalysis, with the classification depending on the site of substrate bond cleavage and formation and on redox state changes. Recently, our group reported the participation of a β-diketiminate ligand in chemical bonding to heterocumulenes such as CO2 and CS2 by tricopper complexes, leading to cooperative catalysis. Herein, we report the reactivity of these copper clusters, [Cu3EL]- (E = S, Se; L = tris(β-diketiminate) cyclophane ligand), toward other electrophiles, viz. alkyl halides and Brønsted acids. We identified a family of ligand-functionalized complexes, Cu3EL (R) (R = primary alkyls), and a series of disubstituted products, Cu3EL (R)2, through single-crystal X-ray diffraction, mass spectrometry, and infrared and UV-visible spectroscopy. As part of mechanistic studies on these alkylation reactions, we evaluated the acid-base reactivity of these complexes and the influence of the backbone substitution on the reduction potential. Implications of these findings for ligand noninnocence and the relevance of the metal core as a cofactor for the ligand's reactivity are discussed.

Ligand redox controlled amine dehydrogenation and imine hemilability in singlet diradical azo-aromatic Ni(II) complexes: characterization of the electron transfer series of azo-imine complexes of Ni(II)

Herein, using azo-amine (H2L) and azo-imine (L1-3) ligands, singlet diradical Ni(II) complexes [1] and [2] were synthesized from Ni(0)(COD)2 in THF. In separate reactions, homoleptic NiII complexes, [3a]2+-[3c]2+, were synthesized from [NiII(H2O)6](ClO4)2 and L1-3, respectively. All these complexes were characterized thoroughly. The X-ray structures of [1] and [2] showed that the amine side arm in [1] and the imine side arm in [2] are de-coordinated. The dN-N lengths in these two complexes were found to be ∼1.32 Å, which corresponds to the one-electron reduced azo-bond length. These complexes, [1] and [2], showed 1H NMR signals characteristic of diamagnetic compounds. These studies, along with DFT calculations, indicated that the unpaired spins on ligands coupled antiferromagnetically with the two unpaired spins on NiII to result in s = 0 ground states. Complex [1] showed ligand-based redox-induced dehydrogenation of the distal amine side arm to result in L1. Complexes [3a]2+-[3c]2+ have dN-N lengths of ∼1.27 Å and dC-N lengths of ∼1.28 Å. In cyclic voltammetry, complex [3a]2+ showed four well-resolved reversible reductive waves at 0.5 V to -1.6 V in dichloromethane. The first two waves became irreversible when they were measured in acetonitrile solution. The electron transfer series of [3a]2+ was further characterized by spectro-electrochemistry, EPR, and DFT calculations. These showed that all the reductions were associated with the ligand. It was further probed by redox events in complexes [3b]2+ and [3c]2+. While the electron donor -OMe group on the phenyl ring of the azo moiety in [3b]2+ showed a prominent cathodic shift of the potentials, the -F substitution on the phenyl group on the imine side arm of [3c]2+ has almost no effect. It has to be noted here that the oxidation of [2] by two electrons returns it back to complex [3a]2+. Reduction of [3a]2+ by two electrons also resulted in complex [2], indicating reversible ligand redox-induced hemilability of the imine moiety between [3a]2+ and [2]. Moreover, characterization of the electron transfer series of [3a]2+ and [2] has shown superior redox non-innocent behaviour and coordination ability of the azo-pyridine moiety in nickel(II) complexes over the imino-pyridine moiety of the ligand.

Nickel-Catalyzed α-Alkylation of Arylacetonitriles with Challenging Secondary Alcohols

Nickel(II) complex 1 was utilized as a sustainable catalyst for α-alkylation of arylacetonitriles with challenging secondary alcohols. Arylacetonitriles with a wide range of functional groups were tolerated, and various cyclic and acyclic secondary alcohols were utilized to yield a large number of α-alkylated products. The plausible mechanism involves the base-promoted activation of precatalyst 1 to an active catalyst 2 (dehydrochlorinated product) which activates the O-H and C-H bonds of the secondary alcohol in a dehydrogenative pathway.

One-pot C-N/C-C bond formation and oxidation of donor-acceptor cyclopropanes with tetrahydroisoquinolines: access to benzo-fused indolizines

One-pot C-N/C-C bond formation of donor-acceptor cyclopropanes (DACs) with tetrahydroisoquinolines (THIQs) has been achieved to furnish benzo-fused indolizines. These reactions involve a MgI2-catalyzed ring opening of DACs and oxidative annulation using Mn(OAc)3·2H2O. The substrate scope and functional group diversity are the important practical features.

Bimetallic H2 Addition and Intramolecular Caryl-H Activation Mediated by an Iron-Zinc Hydride

Heteronuclear Fe(μ-H)Zn hydride Cp*Fe(1,2-Cy2PC6H4)HZnEt (3) undergoes reversible intramolecular Caryl-H reductive elimination through coupling of the cyclometalated phosphinoaryl ligand and the hydride, giving rise to a formal Fe(0)-Zn(II) species. Addition of CO intercepts this equilibrium, affording Cp*(Cy2PPh)(CO)Fe-ZnEt that features a dative Fe-Zn bond. Significantly, this system achieves bimetallic H2 addition, as demonstrated by the transformation of the monohydride Fe(μ-H)Zn to a deuterated dihydride Fe-(μ-D)2-Zn upon reaction with D2.

An artificial metalloenzyme that can oxidize water photocatalytically: design, synthesis, and characterization

In nature, light-driven water oxidation (WO) catalysis is performed by photosystem II via the delicate interplay of different cofactors positioned in its protein scaffold. Artificial systems for homogeneous photocatalytic WO are based on small molecules that often have limited solubility in aqueous solutions. In this work, we alleviated this issue and present a cobalt-based WO-catalyst containing artificial metalloenzyme (ArM) that is active in light-driven, homogeneous WO catalysis in neutral-pH aqueous solutions. A haem-containing electron transfer protein, cytochrome B5 (CB5), served to host a first-row transition-metal-based WO catalyst, CoSalen (CoIISalen, where H2Salen = N,N'-bis(salicylidene)ethylenediamine), thus producing an ArM capable of driving photocatalytic WO. The CoSalen ArM formed a water-soluble pre-catalyst in the presence of [Ru(bpy)3](ClO4)2 as photosensitizer and Na2S2O8 as the sacrificial electron acceptor, with photocatalytic activity similar to that of free CoSalen. During photocatalysis, the CoSalen-protein interactions were destabilized, and the protein partially unfolded. Rather than forming tens of nanometer sized CoOx nanoparticles as free CoSalen does under photocatalytic WO conditions, the CB5 : CoSalen ArM showed limited protein cross-linking and remained soluble. We conclude that a weak, dynamic interaction between a soluble cobalt species and apoCB5 was formed, which generated a catalytically active adduct during photocatalysis. A detailed analysis was performed on protein stability and decomposition processes during the harsh oxidizing reaction conditions of WO, which will serve for the future design of WO ArMs with improved activity and stability.

Cooperative H2 activation at a nickel(0)-olefin centre

Catalytic olefin hydrogenation is ubiquitous in organic synthesis. In most proposed homogeneous catalytic cycles, reactive M-H bonds are generated either by oxidative addition of H2 to a metal centre or by deprotonation of a non-classical metal dihydrogen (M-H2) intermediate. Here we provide evidence for an alternative H2-activation mechanism that instead involves direct ligand-to-ligand hydrogen transfer (LLHT) from a metal-bound H2 molecule to a metal-coordinated olefin. An unusual pincer ligand that features two phosphine ligands and a central olefin supports the formation of a non-classical Ni-H2 complex and the Ni(alkyl)(hydrido) product of LLHT, in rapid equilibrium with dissolved H2. The usefulness of this cooperative H2-activation mechanism for catalysis is demonstrated in the semihydrogenation of diphenylacetylene. Experimental and computational mechanistic investigations support the central role of LLHT for H2 activation and catalytic semihydrogenation. The product distribution obtained is largely determined by the competition between (E)-(Z) isomerization and catalyst degradation by self-hydrogenation.

PhenTAA: A Redox-Active N4-Macrocyclic Ligand Featuring Donor and Acceptor Moieties

Here, we present the development and characterization of the novel PhenTAA macrocycle as well as a series of [Ni(R2PhenTAA)]n complexes featuring two sites for ligand-centered redox-activity. These differ in the substituent R (R = H, Me, or Ph) and overall charge of the complex n (n = -2, -1, 0, +1, or +2). Electrochemical and spectroscopic techniques (CV, UV/vis-SEC, X-band EPR) reveal that all redox events of the [Ni(R2PhenTAA)] complexes are ligand-based, with accessible ligand charges of -2, -1, 0, +1, and +2. The o-phenylenediamide (OPD) group functions as the electron donor, while the imine moieties act as electron acceptors. The flanking o-aminobenzaldimine groups delocalize spin density in both the oxidized and reduced ligand states. The reduced complexes have different stabilities depending on the substituent R. For R = H, dimerization occurs upon reduction, whereas for R = Me/Ph, the reduced imine groups are stabilized. This also gives electrochemical access to a [Ni(R2PhenTAA)]2- species. DFT and TD-DFT calculations corroborate these findings and further illustrate the unique donor-acceptor properties of the respective OPD and imine moieties. The novel [Ni(R2PhenTAA)] complexes exhibit up to five different ligand-based oxidation states and are electrochemically stable in a range from -2.4 to +1.8 V for the Me/Ph complexes (vs Fc/Fc+).

Metal-Tuned Ligand Reactivity Enables CX2 (X = O, S) Homocoupling with Spectator Cu Centers

Ligand non-innocence is ubiquitous in catalysis with ligands in synthetic complexes contributing as electron reservoirs or co-sites for substrate activation. The latter chemical non-innocence is manifested in H+ storage or relay at sites beyond the metal primary coordination sphere. Reaction of a competent CO2-to-oxalate reduction catalyst, namely, [K(THF)3](Cu3SL), where L3- is a tris(β-diketiminate) cyclophane, with CS2 affords tetrathiooxalate at long reaction times or at high CS2 concentrations, where otherwise an equilibrium is established between the starting species and a complex-CS2 adduct in which the CS2 is bound to the C atom on the ligand backbone. X-ray diffraction analysis of this adduct reveals no apparent metal participation, suggesting an entirely ligand-based reaction controlled by the charge state of the cluster. Thermodynamic parameters for the formation of the aforementioned Cligand-CS2 bond were experimentally determined, and trends with cation Lewis acidity were studied, where more acidic cations shift the equilibrium toward the adduct. Relevance of such an adduct in the reduction of CO2 to oxalate by this complex is supported by DFT studies, similar effects of countercation Lewis acidity on product formation, and the homocoupled heterocumulene product speciation as determined by isotopic labeling studies. Taken together, this system extends chemical non-innocence beyond H+ to effect catalytic transformations involving C-C bond formation and represents the rarest example of metal-ligand cooperativity, that is, spectator metal ion(s) and the ligand as the reaction center.

Hexaguanidino-Triptycenes and Triphenylenes: Electronic Coupling in Molecules Containing Three Redox-Active o-Diguanidinobenzene Units Connected either Directly or Interacting Through Homoconjugation

Novel redox-active hexaguanidine molecules with multiple redox states were synthesized by connecting three o-diguanidinobenzene units. In 2,3,6,7,14,15-hexaguanidino-triptycenes, the three redox-active o-diguanidinobenzene units are connected through C-C bonds to the sp3 -hybridized bridgehead C atoms, and in 2,3,6,7,10,11-hexaguanidino-triphenylenes they are directly connected. The connectivity difference leads to different electronic coupling between the three redox-active o-diguanidinobenzene units, with homoconjugation being present in the triptycene, but not in the triphenylene compounds. Motivated by the appearance of an intense low-energy electronic transition, we especially analysed the effect of homoconjugation on the electronic structure and charge delocalization in the dicationic redox state of the triptycene derivatives. Then, several trinuclear high-spin cobalt (and copper) complexes were synthesized with the triphenylene and triptycene ligands, and the magnetic coupling and redox properties analysed. By choice of the coligands (hexafluoroacetylacetonate, trifluoroacetylacetonate and acetylacetonate), oxidation could be switched between metal- and ligand-centered redox events, leading to drastic changes in the magnetic or optical properties, especially as a consequence of homoconjugation in the triptycene derivatives.

A Cooperative Cobalt-Driven System for One-Carbon Extension in the Synthesis of (Z)-Silyl Enol Ethers from Aldehydes: Unlocking Regio- and Stereoselectivity

The research presented herein explores a cobalt-based catalytic system, distinctively featuring a cooperative boron-centric element within its intricate ligand architecture. This system is strategically engineered to enable the integration of a singular carbon atom into aldehydes, a process culminating in the production of (Z)-silyl enol ethers. Beyond offering an efficient one-pot synthesis route, this method adeptly overcomes challenges inherent to conventional techniques, such as the need for large amounts of additives, restrictive functional group tolerance, and extreme reaction temperatures. Initial mechanistic studies suggest the potential role of a cobalt-carbene complex as a catalytically significant species and underscore the importance of the borane segment. Collectively, these observations highlight the potential of this system in advancing complex bond activation pursuits.

Reactions of Nickel(0)-Olefin Pincer Complexes with Terminal Alkynes: Cooperative C-H Bond Activation and Alkyne Coupling

Metal-ligand cooperation can facilitate the activation of chemical bonds, opening reaction pathways of interest for catalyst development. In this context, olefins occupying the central position of a diphosphine pincer ligand (PC=CP) are emerging as reversible H atom acceptors, e.g., for H2 activation. Here, we report on the reactivity of nickel complexes of PC=CP ligands with a terminal alkyne, for which two competing pathways are observed. First, cooperative and reversible C-H bond activation generates a Ni(II) alkyl/alkynyl complex as the kinetic product. Second, in the absence of a bulky substituent on the olefin, two alkyne molecules are incorporated in the ligand structure to form a conjugated triene bound to Ni(0). The mechanisms of these processes are studied by density functional theory calculations supported by experimental observations.

Advances in Group VI Metal-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions

Transition metal-catalyzed homogeneous hydrogenation and dehydrogenation reactions for attaining plethora of organic scaffolds have evolved as a key domain of research in academia and industry. These protocols are atom-economic, greener, in line with the goal of sustainability, eventually pave the way for numerous novel environmentally benign methodologies. Appealing progress has been achieved in the realm of homogeneous catalysis utilizing noble metals. Owing to their high cost, less abundance along with toxicity issues led the scientific community to search for sustainable alternatives. In this context, earth- abundant base metals have gained substantial attention culminating enormous progress in recent years, predominantly with pincer-type complexes of nickel, cobalt, iron, and manganese. In this regard, group VI chromium, molybdenum and tungsten complexes have been overlooked and remain underdeveloped despite their earth-abundance and bio-compatibility. This review delineates a comprehensive overview in the arena of homogeneously catalysed (de)hydrogenation reactions using group VI base metals chromium, molybdenum, and tungsten till date. Various reactions have been described; hydrogenation, transfer hydrogenation, dehydrogenation, acceptorless dehydrogenative coupling, hydrogen auto transfer, along with their scope and brief mechanistic insights.

A Combined Infrared and Computational Study of Gas-Phase Mixed-Ligand Rhodium Complexes: Rh(CO)n(N2O)m+ (n = 1-5, m = 1-4)

In this study, mixed carbonyl and nitrous oxide complexes with Rh+ were studied by mass-selective infrared photodissociation spectroscopy in a molecular beam. The infrared spectra, recorded in the region of the CO and N2O N═N stretches, were assigned and interpreted with the aid of simulated spectra of low-energy structural isomers. Clear evidence of an inner coordination shell of four ligands is observed. The observed vibrational structure can be understood on the basis of local mode vibrations in the two ligands. However, there is also evidence of multiple low-lying isomers and cooperative binding effects between the two ligands. In particular, σ donation from directly coordinated nitrous oxide ligands drives more classical carbonyl bonding than has been observed in pure carbonyl complexes. The observed fragmentation branching ratios following resonant infrared absorption are explained by simple statistical and energetic arguments, providing a contrast with those of equivalent Au+ complexes.

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.

Cleavage of Carbon Dioxide C=O Bond Promoted by Nickel-Boron Cooperativity in a PBP-Ni Complex

The synthesis and characterization of (tBu PBP)Ni(OAc) (5) by insertion of carbon dioxide into the Ni-C bond of (tBu PBP)NiMe (1) is presented. An unexpected CO2 cleavage process involving the formation of new B-O and Ni-CO bonds leads to the generation of a butterfly-structured tetra-nickel cluster (tBu PBOP)2 Ni4 (μ-CO)2 (6). Mechanistic investigation of this reaction indicates a reductive scission of CO2 by O-atom transfer to the boron atom via a cooperative nickel-boron mechanism. The CO2 activation reaction produces a three-coordinate (tBu P2 BO)Ni-acyl intermediate (A) that leads to a (tBu P2 BO)-NiI complex (B) via a likely radical pathway. The NiI species is trapped by treatment with the radical trap (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to give (tBu P2 BO)NiII (η2 -TEMPO) (7). Additionally, 13 C and 1 H NMR spectroscopy analysis using 13 C-enriched CO2 provides information about the species involved in the CO2 activation process.

A dinuclear Rh(-i)/Rh(i) complex bridged by biphilic phosphinine ligands

Bimetallic complexes have enabled precise control of catalysis by accumulating two discrete metal centres. In these complexes, bridging ligands are essential to combine multiple metals into one molecule. Among some bridging modes, an unsymmetric bridging mode will differentiate the electronic structures of the two metal centres. In this study, a dinuclear Rh(-i)/Rh(i) complex bridged by tridentate phosphine-phosphinine-phosphine ligands was prepared by reduction of the corresponding Rh(i) complex. Single-crystal X-ray analysis and DFT calculations suggest that the phosphinine ligands adopt an unsymmetric bridging mode wherein phosphinine accepts d-electrons from one Rh centre and, at the same time, donates lone pairs to the other Rh centre.

Asymmetrically Coordinated Heterodimetallic Ir-Ru System: Synthesis, Computational, and Anticancer Aspects

Herein, we present an unprecedented formation of a heterodinuclear complex [{(ppy)₂Ir^III}(μ-phpy){Ru^II(tpy)}](ClO₄)₂ {[1](ClO₄)₂} using terpyridyl/phenylpyridine as ancillary ligands and asymmetric phpy as a bridging ligand. The asymmetric binding mode (N^∧N-∩-N^∧N^∧C^-) of the phpy ligand in {[1](ClO₄)₂} is confirmed by ¹H, ¹³C, ¹H-¹H correlated spectroscopy (COSY), high-resolution mass spectrum (HRMS), single-crystal X-ray crystallography techniques, and solution conductivity measurements. Theoretical investigation suggests that the highest occupied molecular orbital (HOMO) and the least unoccupied molecular orbital (LUMO) of [1]²⁺ are located on iridium/ppy and phpy, respectively. The complex displays a broad low energy charge transfer (CT) band within 450-575 nm. The time-dependent density functional theory (TDDFT) analysis suggests this as a mixture of metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT), where both ruthenium, iridium, and ligands are involved. Complex {[1](ClO₄)₂} exhibits Ru^IIIr^III/Ru^IIIIr^III- and Ru^IIIIr^III/Ru^IIIIr^IV-based oxidative couples at 0.83 and 1.39 V, respectively. The complex shows anticancer activity and selectivity toward human breast cancer cells (IC₅₀; MCF-7: 9.3 ± 1.2 μM, and MDA-MB-231: 8.6 ± 1.2 μM) over normal breast cells (MCF 10A: IC₅₀ ≈ 21 ± 1.3 μM). The Western blot analysis and fluorescence microscopy images suggest that combined apoptosis and autophagy are responsible for cancer cell death.

Low-Coordinate Mixed Ligand NacNac Complexes of Rare Earth Metals

We report the synthesis and characterization of two types of new mixed-ligand rare earth complexes: tetracoordinate (NacNac^Mes)Ln(BIAN^dipp) (Ln = Dy (1), Er (2) and Y (3)) and pentacoordinate (NacNac^Mes)Ln(AP^dipp)(THF) (Ln = Dy (4), Er (5) and Y (6)). The first three compounds were prepared by the reaction of [(BIAN^Dipp)LnI] with potassium β-diketiminate. The salt metathesis of β-diketiminato-supported rare earth dichlorides (NacNac^Mes)LnCl₂(THF)₂ with sodium o-amidophenolate results in compounds 4-6. The crystal structures of complexes 1-6 were determined by single-crystal analysis. The combination of bulky monoanionic N-mesityl-substituted β-diketiminates with sterically hindered redox-active ligands led to the very low coordination numbers of rare earths and strong distortion of the chelate ligands.

Cooperativity-Driven Reactivity of a Dinuclear Copper Dimethylglyoxime Complex

In this report, we present the dinuclear copper(II) dimethylglyoxime (H₂ dmg) complex [Cu₂ (H₂ dmg)(Hdmg)(dmg)]⁺ (1), which, in contrast to its mononuclear analogue [Cu(Hdmg)₂ ] (2), is subject to a cooperativity-driven hydrolysis. The combined Lewis acidity of both copper centers increases the electrophilicity of the carbon atom in the bridging μ₂ -O-N=C-group of H₂ dmg and thus, facilitates the nucleophilic attack of H₂ O. This hydrolysis yields butane-2,3-dione monoxime (3) and NH₂ OH that, depending on the solvent, is then either oxidized or reduced. In ethanol, NH₂ OH is reduced to NH₄ ⁺ , yielding acetaldehyde as the oxidation product. In contrast, in CH₃ CN, NH₂ OH is oxidized by Cu^II to form N₂ O and [Cu(CH₃ CN)₄ ]⁺ . Herein are presented the combined synthetic, theoretical, spectroscopic and spectrometric methods that indicate and establish the reaction pathway of this solvent-dependent reaction.

Metal-Ligand Cooperativity in MnI -Catalysed N-Formylation of Secondary Amides and Lactams Using CO2 at Room Temperature

A Mn^I catalyst featuring redox-active tridentate phenalenyl (PLY) ligand has been used for catalytic N-formylation of secondary amides and lactams under 1 atm CO₂ as a C₁ source at room temperature for the first time. The protocol is applicable to a wide range of secondary amides including heterocycles, bio-active cinnamide derivatives and the diversification of therapeutic molecules. In-depth mechanistic investigations based on experimental outcomes and DFT calculations suggested an unconventional metal-ligand cooperation, where a ligand-centred radical plays a crucial role in initiating the reaction process.

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.

Efficient Hydrogenation of N-Heterocycles Catalyzed by NNP-Manganese(I) Pincer Complexes at Ambient Temperature

Manganese-catalyzed hydrogenation reactions have aroused widespread interest in recent years. Among the catalytic systems described, especially PNP- and NNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, ketones, nitriles, aldimines and esters. Furthermore, NNP-Mn pincer compounds are efficient catalysts for the hydrogenolysis of less reactive amides, ureas, carbonates, and carbamates. Herein, the synthesis and application of specific imidazolylaminophosphine ligands and the corresponding Mn pincer complexes are described. These new catalysts have been characterized and studied by a combination of experimental and theoretical investigations, and their catalytic activities have been tested in several hydrogenation reactions with good to excellent performance. Especially, the reduction of N-heterocycles can be performed under very mild conditions.

Interplay and Competition Between Two Different Types of Redox-Active Ligands in Cobalt Complexes: How to Allocate the Electrons?

The field of molecular transition metal complexes with redox-active ligands is dominated by compounds with one or two units of the same redox-active ligand; complexes in which different redox-active ligands are bound to the same metal are uncommon. This work reports the first molecular coordination compounds in which redox-active bisguanidine or urea azine (biguanidine) ligands as well as oxolene ligands are bound to the same cobalt atom. The combination of two different redox-active ligands leads to mono- as well as unprecedented dinuclear cobalt complexes, being multiple (four or six) center redox systems with intriguing electronic structures, all exhibiting radical ligands. By changing the redox potential of the ligands through derivatisation, the electronic structure of the complexes could be altered in a rational way.

Ni(II) complexes of a new tetradentate NN'N''O picolinoyl-1,2-phenylenediamide-phenolate redox-active ligand at different redox levels

Three square planar nickel(II) complexes of a new asymmetric tetradentate redox-active ligand H₃L² in its deprotonated form, at three redox levels, open-shell semiquinonate(1-) π radical, quinone(0) and closed-shell dianion of its 2-aminophenolate part, have been synthesized. The coordinated ligand provides N (pyridine) and N' and N'' (carboxamide and 1,2-phenylenediamide, respectively) and O (phenolate) donor sites. Cyclic voltammetry on the parent complex [Ni(L²)] 1 in CH₂Cl₂ established a three-membered electron-transfer series (oxidative response at E_1/2 = 0.57 V and reductive response at -0.32 V vs. SCE) consisting of neutral, monocationic and monoanionic [Ni(L²)]^z (z = 0, 1+ and 1-). Oxidation of 1 with AgSbF₆ affords [Ni(L²)](SbF₆) (2) and reduction of 1 with cobaltocene yields [Co(η⁵-C₅H₅)₂][Ni(L²)] (3). The molecular structures of 1·CH₃CN, 2·0.5CH₂Cl₂ and 3·C₆H₆ have been determined by X-ray crystallography at 100 K. Characterization by ¹H NMR, X-band EPR (g_av = 2.006 (solid); 2.008 (CH₂Cl₂-C₆H₅CH₃ glass); 80 K) and UV-VIS-NIR spectral properties established that 1, 2 and 3 have [Ni^II{(L²)˙^2-}], [Ni^II{(L²)^-}]⁺/1⁺ and [Ni^II{(L²)^3-}]^-/1^- electronic states, respectively. Thus, the redox processes are ligand-centred. While 1 possesses paramagnetic S_t (total spin) = 1/2, 2 and 3 possess diamagnetic ground-state S_t = 0. Interestingly, the variable-temperature (2-300 K) magnetic measurement reveals that 1 with the S_t = 1/2 ground state attains the antiferromagnetic S_t = 0 state at a very low temperature, due to weak noncovalent interactions via π-π stacking. Density functional theory (DFT) electronic structural calculations at the B3LYP level of theory rationalized the experimental results. In the UV-VIS-NIR spectra, broad absorptions are recorded for 1 and 2 in the range of 800-1600 nm; however, such an absorption is absent for 3. Time-dependent (TD)-DFT calculations provide a very good fit with the experimental spectra and allow us to identify the observed electronic transitions.

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.

Iron(III) Complexes of a Hexadentate Thioether-Appended 2-Aminophenol Ligand: Redox-Driven Spin State Switchover

A green complex [Fe(L³)] (1), supported by the deprotonated form of a hexadentate noninnocent redox-active thioether-appended 2-aminophenolate ligand (H₄L³ = N,N'-bis(2-hydroxy-3,5-di-tert-butylphenyl)-2,2'-diamino(diphenyldithio)ethane), has been synthesized and structurally characterized at 100(2) K and 298(2) K. In CH₂Cl₂, 1 displays two oxidative and a reductive one-electron redox processes at E_1/2 values of -0.52 and 0.20 V, and -0.85 V versus the Fc⁺/Fc redox couple, respectively. The one-electron oxidized 1⁺ and one-electron reduced 1^- forms, isolated as a blackish-blue solid 1(PF₆)·CH₂Cl₂ (2) and a gray solid [Co(η⁵-C₅H₅)₂]1·DMF (3), have been structurally characterized at 100(2) K. Structural parameters at 100 K of the ligand backbone and metrical oxidation state values unambiguously establish the electronic states as [Fe^III{(L^AP_O,N)^2-}{(L^ISQ_O,N)^•-}{(L_S,S)⁰}] (1) (two tridentate halves are electronically asymmetric-ligand mixed-valency), [Fe^III{(L^ISQ_O,N)^•-}{(L^ISQ_O,N)^•-}{(L_S,S)⁰}]⁺ (1⁺), and [Fe^III{(L^AP_O,N)^2-}{(L^AP_O,N)^2-}{(L_S,S)⁰}]^- (1^-) [dianionic 2-amidophenolate(2-) (L^AP_O,N)^2- and monoanionic 2-iminobenzosemiquinonate(1-) π-radical (S_rad = 1/2) (L^ISQ)^•- redox level]. Mössbauer spectral data of 1 at 295, 200, and 80 K reveal that it has a major low-spin (ls)-Fe(III) and a minor ls-Fe(II) component (redox isomers), and at 7 K, the major component exists exclusively. Thus, in 1, the occurrence of a thermally driven valence-tautomeric (VT) equilibrium (asymmetric) [Fe^III{(L^AP_O,N)^2-}{(L^ISQ_O,N)^•-}{(L_S,S)⁰}] ⇌ (symmetric) [Fe^II{(L^ISQ_O,N)^•-}{(L^ISQ_O,N)^•-}{(L_S,S)⁰}] (80-295 K) is implicated. Mössbauer spectral parameters unequivocally establish that 1⁺ is a ls-Fe(III) complex. In contrast, the monoanion 1^- contains a high-spin (hs)-Fe(III) center (S_Fe = 5/2), as is deduced from its Mössbauer and EPR spectra. Complexes 1-3 possess total spin ground states S_t = 0, 1/2, and 5/2, respectively, based on ¹H NMR and EPR spectra, the variable-temperature (2-300 K) magnetic behavior of 2, and the μ_eff value of 3 at 300 K. Broken-symmetry density functional theory (DFT) calculations at the B3LYP-level of theory reveal that the unpaired electron in 1⁺/2 is due to the (L^ISQ)^•- redox level [ls-Fe(III) (S_Fe = 1/2) is strongly antiferromagnetically coupled to one of the (L^ISQ)^•- radicals (S_rad = 1/2)], and 1^-/3 is a hs-Fe(III) complex, supported by (L³)^4- with two-halves in the (L^AP)^2- redox level. Complex 1 can have either a symmetric or asymmetric electronic state. As per DFT calculation, the former state is stabilized by -3.9 kcal/mol over the latter (DFT usually stabilizes electronically symmetric structure). Time-dependent (TD)-DFT calculations shed light on the origin of observed UV-vis-NIR spectral absorptions for 1-3 and corroborate the results of spectroelectrochemical experiments (300-1100 nm) on 1 (CH₂Cl₂; 298 K). Variable-temperature (218-298 K; CH₂Cl₂) absorption spectral (400-1000 nm) studies on 1 justify the presence of VT equilibrium in the solution-state.

Hydrogenation of CO2 or CO2 Derivatives to Methanol under Molecular Catalysis: A Review

The atmospheric CO2 concentration has been continuously increasing due to fossil fuel combustion. The transformations of CO2 and CO2 derivatives into high value-added chemicals such as alcohols are ideal routes to mitigate greenhouse gas emissions. Among alcohol products, methanol is very promising as it fulfills the carbon neutral cycle and can be used for direct methanol fuel cells. Herein, we summarize the recent progress in the hydrogenation of CO2 or CO2 derivatives to methanol, and focus on those systems with homogeneous catalysts and molecular hydrogen as the reductant. Discussions on the catalytic systems, efficiencies, and future outlooks will be given.

Multinuclear mesoionic 1,2,3-triazolylidene complexes: design, synthesis, and applications

The chemistry of multinuclear metal complexes bearing by N-heterocyclic carbene (NHC) ligands, is an area of fast growing interest in modern organometallic chemistry. In particular, complexes supported by mesoionic (MIC) 1,2,3-triazolylidenes are attracting a great deal attention due to their postulated superior donor capacity compared to classical NHC ligands. Despite the readily available synthetic routes to MIC-based ligand platforms featuring several substitution levels, most of the coordination chemistry of triazolylidenes is still dominated by mononuclear complexes. In this short review article, recent progress on the design and synthesis of multinuclear triazolylidene complexes (ranging from di- to tetranuclear species) is discussed. Special emphasis is placed on their structural features, electronic properties and catalytic applications.

Computational Investigation of the Metal and Ligand Substitution Effects on the Structure and Electronic States of the Phosphoranimide Tetramer Complexes of Cu(I), Ni(I), Co(I), and Fe(I)

The structurally unique saddle-shaped paramagnetic tetrametallic clusters of Co(I) and Ni(I) with phosphoranimide ligands have been synthesized and proposed as catalyst precursors. The analogous Cu(I) nanocluster is planar and diamagnetic. These notable variations in geometry and ground electronic states indicate that the effect of metal and ligand substituents on the structure and electronic properties of these complexes requires investigation. We present a computational study of a series of these novel homoleptic complexes containing Co(I), Ni(I), and Cu(I) as well as Fe(I) coordinated to phosphoranimides with electron-donating and withdrawing substituents, conducted at the relativistic density functional theory level using ZORA-PBE/TZP. The optimized structures of the saddle-shaped Co(I) and Ni(I) and planar Cu(I) tetramers with linear N-M-N coordination are validated with respect to X-ray diffraction determinations. The ground-state analysis indicates that Cu(I) complexes are diamagnetic, whereas Ni(I) and Co(I) complexes are in high-spin states, in agreement with magnetic susceptibility measurements. The computational results show that Fe(I) complexes are saddle shaped and high spin. The Co(I) complex is stabilized by a metal macrocycle distortion from square to diamond, as elucidated from its Walsh diagram. The effects of metals and ligand substituents on the ground electronic state, metal center coordination environment, and energy of the complexes are investigated. The bulky tertiary butyl substituent causes the largest saddle-shape distortion of the tetramer marcocycle, which partially offsets its electron-donating effect. Macrocycle distortions with N-M-N site angles ranging from obtuse to alternating obtuse reflex are correlated with the increasing number of unpaired electrons. The phenyl-substituted complexes are expected to have the highest reactivity toward electrophiles. Understanding the interplay between structural and electronic parameters is intended to guide the development of synthetic cooperative systems for multielectron redox reactions, models of biological systems, and molecular magnets.

Metal-ligand cooperative approaches in homogeneous catalysis using transition metal complex catalysts of redox noninnocent ligands

Catalysis offers a straightforward route to prepare various value-added molecules starting from readily available raw materials. The catalytic reactions mostly involve multi-electron transformations. Hence, compared to the inexpensive and readily available 3d-metals, the 4d and 5d-transition metals get an extra advantage for performing multi-electron catalytic reactions as the heavier transition metals prefer two-electron redox events. However, for sustainable development, these expensive and scarce heavy metal-based catalysts need to be replaced by inexpensive, environmentally benign, and economically affordable 3d-metal catalysts. In this regard, a metal-ligand cooperative approach involving transition metal complexes of redox noninnocent ligands offers an attractive alternative. The synergistic participation of redox-active ligands during electron transfer events allows multi-electron transformations using 3d-metal catalysts and allows interesting chemical transformations using 4d and 5d-metals as well. Herein we summarize an up-to-date literature report on the metal-ligand cooperative approaches using transition metal complexes of redox noninnocent ligands as catalysts for a few selected types of catalytic reactions.

Organometallic 3d transition metal NHC complexes in oxidation catalysis

The development of processes for the selective oxidation of hydrocarbons is a major focus in catalysis research. Making this process simultaneously environmentally friendly is still challenging. 3d transition metals are...

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.

Synthesis and reactivity of titanium ‘POCOP’ pincer complexes

Titanium ‘POCOP’ complexes have been made, and their ability to support further reactivity investigated, giving a rare isolable titanium chlorohydride.

High phenoxazinone synthase activity of two mononuclear cis-dichloro cobalt(ii) complexes with a rigid pyridyl scaffold

Two mononuclear cis-dichloro cobalt(II) complexes with bidentate pyridyl ligands have been successfully synthesized and employed as active o-aminophenol oxidation catalysts resulting in high turnover numbers under aerobic conditions.

Unsymmetric Dinuclear RhI2 and RhIRhIII Complexes Supported by Tetraphosphine Ligands and Their Reactivity of Oxidative Protonation and Reductive Dechlorination

Two linear tetradentate phosphine ligands, meso-Ph₂PCH₂P(Ph)CH₂XCH₂P(Ph)CH₂PPh₂ (X = CH₂ (meso-dpmppp), NBn (meso-dpmppmNBn; Bn = benzyl)) were utilized to synthesize unsymmetrical dinuclear Rh^I complexes, [Rh₂Cl₂(meso-dpmppp)(L)] (L = XylNC (1a), CO (1b)) and [Rh₂Cl₂(meso-dpmppmNBn)(L)] (L = XylNC (1c), CO (1d)), where electron-deficient Rh^I → Rh^I centers with 30 valence electrons are supported by a tetraphosphine in an unusual cis-/trans-P,P coordination mode. The Rh^I dimers of 1a-d were treated with HCl under air to afford the Rh^I → Rh^III dimers with 32 e^-, [Rh₂Cl₄(meso-dpmppp)(L)] (L = XylNC (4a), CO (4b)) and [Rh₂Cl₄(meso-dpmppmNBn)(L)] (L = XylNC (4c), CO (4d)), via intermediate hydride complexes, [{RhCl₂(μ-H)RhCl(L)}(meso-dpmppp)] (L = XylNC (2a), CO (2b)) and [{RhCl₂(μ-H)RhCl(L)}(meso-dpmppmNBn)] (L = XylNC (2c), CO (2d)), and [{Rh(H)Cl₂(μ-Cl)Rh(L)}(meso-dpmppp)] (L = XylNC (3a), CO (3b)) and [{Rh(H)Cl₂(μ-Cl)Rh(L)}(meso-dpmppmNBn)] (L = XylNC (3c), CO (3d)). The hydride intermediates 2 and 3 were monitored under nitrogen by ¹H{³¹P} and ³¹P{¹H} NMR techniques to reveal two reaction pathways depending on the terminal auxiliary ligand L. Further, the reductive dechlorination converting Rh^IRh^III (4b,d) to Rh^I₂ (1b,d) was accomplished with a CO terminal ligand by reacting with various amines that acted as one-electron reducing agents through an inner-sphere electron transfer mechanism. DFT calculations were performed to elucidate the electronic structures of 1a-d and 4a-d and to estimate the structures of the hydride intermediate complexes 2 and 3.

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).