Metal-ligand Cooperation Research Articles

Examining the Generality of Metal-Ligand Cooperativity Across a Series of First-Row Transition Metals: Capture, Bond Activation, and Stabilization.

We outline the generality and requirements for cooperative N2H4 capture, N-N bond scission, and amido stabilization across a series of first-row transition metal complexes bearing a pyridine(dipyrazole) ligand. This ligand contains a pair of flexibly tethered trialkylborane Lewis acids that enable hydrazine capture and M-NH2 stabilization. While the Lewis acids are required to bind N2H4, the identity of the metal dictates whether N-N bond scission can occur. The redox properties of the M(II) bis(amidoborane) series of complexes were investigated and reveal that ligand-based events prevail; oxidation results in the generation of a transiently formed aminyl radical, while reduction occurs at the redox-active pyridine(dipyrazole) ligand.

Recent Advances in Homogeneous Catalysis via Metal–Ligand Cooperation Involving Aromatization and Dearomatization

Recently, an increasing number of metal complex catalysts have been developed to achieve the activation or transformation of substrates based on cooperation between the metal atom and its ligands. In such “cooperative catalysis,” the ligand not only is bound to the metal, where it exerts steric and electronic effects, but also functionally varies its structure during the elementary processes of the catalytic reaction. In this review article, we focus on metal–ligand cooperation involving aromatization and dearomatization of the ligand, thus introducing the newest developments and examples of homogeneous catalytic reactions.

Generation and Oxidative Reactivity of a Ni(II) Superoxo Complex via Ligand-Based Redox Non-Innocence.

Metal ligand cooperativity is a powerful strategy in transition metal chemistry. This type of mechanism for the activation of O2 is best exemplified by heme centers in biological systems. While aerobic oxidations with Fe and Cu are well precedented, Ni-based oxidations are frequently less common due to less-accessible metal-based redox couples. Some Ni enzymes utilize special ligand environments for tuning the Ni(II)/(III) redox couple such as strongly donating thiolates in Ni superoxide dismutase. A recently characterized example of a Ni-containing protein, however, suggests an alternative strategy for mediating redox chemistry with Ni by utilizing ligand-based reducing equivalents to enable oxygen binding. While this mechanism has little synthetic precedent, we show here that Ni complexes of the redox-active ligand tBu,TolDHP (tBu,TolDHP = 2,5-bis((2-t-butylhydrazono)(p-tolyl)methyl)-pyrrole) activate O2 to generate a Ni(II) superoxo complex via ligand-based electron transfer. This superoxo complex is competent for stoichiometric oxidation chemistry with alcohols and hydrocarbons. This work demonstrates that coupling ligand-based redox chemistry with functionally redox-inactive Ni centers enables oxidative transformations more commonly mediated by metals such as Fe and Cu.

Metal–Ligand Cooperativity in Titanium-Catalyzed Anti-Markovnikov Hydroamination

Density function theory (DFT) calculations were performed to investigate a known diamido bis(amidate) Ti pre-catalyst that is excellent for anti-Markovnikov selective alkyne hydroamination. The coo...

Selective Room-Temperature Hydrogenation of Amides to Amines and Alcohols Catalyzed by a Ruthenium Pincer Complex and Mechanistic Insight.

We report a room-temperature protocol for the hydrogenation of various amides to produce amines and alcohols. Compared with most previous reports for this transformation, which use high temperatures (typically, 100–200 °C) and H2 pressures (10–100 bar), this system proceeds under extremely mild conditions (RT, 5–10 bar of H2). The hydrogenation is catalyzed by well-defined ruthenium-PNNH pincer complexes (0.5 mol %) with potential dual modes of metal–ligand cooperation. An unusual Ru-amidate complex was formed and crystallographically characterized. Mechanistic investigations indicate that the room-temperature hydrogenation proceeds predominantly via the Ru–N amido/amine metal–ligand cooperation.

Iridium-Catalyzed Selective Cross-Coupling of Ethylene Glycol and Methanol to Lactic Acid.

An atom-economic approach that has an unprecedented high selectivity for the synthesis of lactic acid (LA) based on a catalytic dehydrogenative cross-coupling by using inexpensive bulk ethylene glycol and methanol is described. This method relies on the synthesis and utilization of a novel iridium catalyst bearing three N-heterocyclic carbenes derived from 1,3-dimethylbenzimidazolium salts, and exhibits outstanding activity in the production of LA [turnover frequency (TOF) up to 3660 h-1 ] owing to an elegant metal-ligand cooperation.

Iridium‐Catalyzed Selective Cross‐Coupling of Ethylene Glycol and Methanol to Lactic Acid

AbstractAn atom‐economic approach that has an unprecedented high selectivity for the synthesis of lactic acid (LA) based on a catalytic dehydrogenative cross‐coupling by using inexpensive bulk ethylene glycol and methanol is described. This method relies on the synthesis and utilization of a novel iridium catalyst bearing three N‐heterocyclic carbenes derived from 1,3‐dimethylbenzimidazolium salts, and exhibits outstanding activity in the production of LA [turnover frequency (TOF) up to 3660 h−1] owing to an elegant metal–ligand cooperation.

Exploiting Metal-Ligand Cooperativity to Sequester, Activate, and Reduce Atmospheric Carbon Dioxide with a Neutral Zinc Complex.

As atmospheric levels of carbon dioxide (CO2) continue to increase, there is an immediate need to balance the carbon cycle. Current approaches require multiple processes to fix CO2 from the atmosphere or flue gas and then reduce it to value-added products. The zinc(II) catalyst Zn(DMTH) (DMTH = diacetyl-2-(4-methyl-3-thiosemicarbazonate)-3-(2-pyridinehydrazonato)) reduces CO2 from air to formate with a faradaic efficiency of 15.1% based on total charge. The catalyst utilizes metal-ligand cooperativity and redox-active ligands to fix, activate, and reduce CO2. This approach provides a new strategy that incorporates sustainable earth-abundant metals that are oxygen and water tolerant.

Ru0 or RuII: A Study on Stabilizing the "Activated" Form of Ru-PNP Complexes with Additional Phosphine Ligands in Alcohol Dehydrogenation and Ester Hydrogenation.

The complex Ru-MACHO has been previously shown to undergo uncontrolled degradation subsequent to base-induced dehydrochlorination in the absence of a substrate. In this study, we report that stabilization of the dehydrochlorinated Ru-MACHO with phosphines furnishes complexes whose structures depend on the phosphines employed: while PMe3 led to the expected octahedral RuII complex, PPh3 provided access to a trigonal-bipyramidal Ru0 complex. Because both complexes proved to be active in base-free (de)hydrogenation reactions, thorough quantum-chemical calculations were employed to understand the reaction mechanism. The calculations show that both complexes lead to the same mechanistic scenario after phosphine dissociation and, therefore, only differ energetically in this step. According to the calculations, the typically proposed metal-ligand cooperation mechanism is not the most viable pathway. Instead, a metal-ligand-assisted pathway is preferred. Finally, experiments show that phosphine addition enhances the catalyst's performance in comparison to the PR3-free "activated" Ru-MACHO.

Cobalt-Catalyzed Acceptorless Dehydrogenation of Alcohols to Carboxylate Salts and Hydrogen

The facile oxidation of alcohols to carboxylate salts and H2 is achieved using a simple and readily accessible cobalt pincer catalyst (NNNHtBuCoBr2). The reaction follows an acceptorless dehydrogenation pathway and displays good functional group tolerance. The amine-amide metal-ligand cooperation in cobalt catalyst is suggested to facilitate this transformation. The mechanistic studies indicate that in-situ-formed aldehydes react with a base through a Cannizzaro-type pathway, resulting in potassium hemiacetolate, which further undergoes catalytic dehydrogenation to provide the carboxylate salts and H2.

Highly Active and Robust Ruthenium Complexes Based on Hemilability of Hybrid Ligands for C-H Oxidation.

Evaluation of the hemilability of hybrid ligands provides a key to understand the metal-ligand cooperation in transition metal catalysis. Here, we design and synthesize a type of RuII complexes based on the hemilability of N-heterocyclic carbenes (NHCs), pyridine, and pyrazole, to compare their activity with other reported Ru catalysts in benzylic C-H oxidation. The RuII catalysts showed ultrastrong catalytic activity in water at room temperature and achieved a turnover frequency (TOF) of 114 h-1, which is the highest TOF value ever reported for Ru-catalyzed benzylic C-H oxidation. The addition of tridentate hybrid ligands in the Ru central position has two beneficial effects: NHCs with a stronger donor ability stabilize the Ru center; however, nitrogen ligands with a relatively weaker donor ability release from the Ru center, so that they induce a reaction. UV-vis, high-resolution electrospray ionization mass spectrometry (ESI-MS), electron paramagnetic resonance (EPR) spectrometry, the trapping of radicals, and the density functional theory calculations (DFT) suggested that a cation catalyst L-RuII-tBuO2H is formed via the reaction between starting RuII catalysts and tert-butyl hydroperoxide, which further undergoes a cleavage of the O-O bond to generate a radical and a cation L-RuIII-OH active intermediate.

Bidentate Triazolate-Based Ligand System: Synthesis, Coordination Modes, and Cooperative Bond Activation

The 1,2,3-triazole ring represents a highly attractive and useful platform for ligand synthesis. In this work, we reveal the design, preparation, and properties of a novel bidentate triazolate-based ligand and its potential in cooperative metal–ligand bond activation. Our triazolate system exhibits a multiphase mode of coordination to transition metals, supporting both dimeric and monomeric complexes. The complexes were carefully characterized both in solution and in the solid state. The metal–ligand cooperative potential of the basic triazolate pattern was demonstrated in HCl and H–H bond activation.

Metal–ligand cooperativity across two sites of a square planar iron(ii) complex ligated by a tetradentate PNNP ligand

An iron(ii) complex with a square planar geometry and S = 1 ground state was synthesized using a rigid tetradentate bisphosphine-bisamide [PNNP]2- ligand. This complex was shown to bind an equivalent of PMe3 to produce a five-coordinate low spin complex, and to react with two equivalents of borane reagents via addition of two B-H bonds across the metal-amide bonds in a metal-ligand cooperative fashion.

Metal-ligand cooperativity in the soluble hydrogenase-1 from Pyrococcus furiosus.

Metal–ligand cooperativity is an essential feature of bioinorganic catalysis. The design principles of such cooperativity in metalloenzymes are underexplored, but are critical to understand for developing efficient catalysts designed with earth abundant metals for small molecule activation. The simple substrate requirements of reversible proton reduction by the [NiFe]-hydrogenases make them a model bioinorganic system. A highly conserved arginine residue (R355) directly above the exogenous ligand binding position of the [NiFe]-catalytic core is known to be essential for optimal function because mutation to a lysine results in lower catalytic rates. To expand on our studies of soluble hydrogenase-1 from Pyrococcus furiosus (Pf SH1), we investigated the role of R355 by site-directed-mutagenesis to a lysine (R355K) using infrared and electron paramagnetic resonance spectroscopic probes sensitive to active site redox and protonation events. It was found the mutation resulted in an altered ligand binding environment at the [NiFe] centre. A key observation was destabilization of the Nia3+–C state, which contains a bridging hydride. Instead, the tautomeric Nia+–L states were observed. Overall, the results provided insight into complex metal–ligand cooperativity between the active site and protein scaffold that modulates the bridging hydride stability and the proton inventory, which should prove valuable to design principles for efficient bioinspired catalysts.

Reversible OH-bond activation and amphoterism by metal-ligand cooperativity of calix[4]pyrrolato aluminate.

Most p-block metal amides irreversibly react with metal alkoxides when subjected to alcohols, making reversible transformations with OH-substrates a challenging task. Herein, we describe how the combination of a Lewis acidic square-planar-coordinated aluminum(iii) center with metal–ligand cooperativity leverages unconventional reactivity toward protic substrates. Calix[4]pyrrolato aluminate performs OH-bond activation of primary, secondary, and tertiary aliphatic and aromatic alcohols, which can be fully reversed under reduced pressure. The products exhibit a new form of metal–ligand cooperative amphoterism and undergo counterintuitive substitution reactions of a polar covalent Al–O bond by a dative Al–N bond. A comprehensive mechanistic picture of all processes is buttressed by isolation of intermediates, spectroscopy, and computation. This study delineates how structural constraints can invert thermodynamics for seemingly simple addition reactions and invert common trends in bond energies.

The origin of different driving forces between O–H/N–H functional groups in metal ligand cooperation: mechanistic insight into Mn(i) catalysed transfer hydrogenation

The origin of different catalytic activity between two structurally similar Lewis basic bifunctional catalysts.

Copper(II)-hydroxide facilitated C-C bond formation: the carboxamido pyridine system versus the methylimino pyridine system.

A copper(ii)-hydroxide-induced carbon-carbon bond formation reaction is explored with the synthesis of an asymmetric carboxamido-methylimino pyridine Cu(i) complex of [CuI(py(N-C[double bond, length as m-dash]O)(N[double bond, length as m-dash]C-C)ph2Me2)2]- (12). Two imine-methyl groups are coupled to form a bridged C-C bond (N[double bond, length as m-dash]C-C-C-C[double bond, length as m-dash]N) at the methyl positions with the reduction of two Cu2+ center ions to Cu+. The reaction is checked with three dicarboxamido pyridine [CuII-OH] complexes, with which dinuclear Cu(i) complexes of [Cu2(py(N-C[double bond, length as m-dash]O)2ph2R2)2]2- (R = methyl (3), methyl and allyl (6)) and trinuclear [CuII-CuI-CuII] complex of [Cu3(⊂20-py(N-C[double bond, length as m-dash]O)2ph2dienMe3)2]+ (9) are obtained. The reactivities of the [CuII-L] (L = DMF, OH-) complexes in dicarboxamido pyridine, carboxamido-methylimino pyridine and dimethylimino pyridine systems are discussed in terms of the electron delocalization properties of ligands. A cooperative metal-ligand (Cu2+ and enamide ligand) interaction is proposed based on the characterization of ligated Cu(ii) intermediates with the techniques of X-ray crystallography, UV-vis spectroscopy, cyclic voltammogram, EPR spectroscopy, and DFT calculations.

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. Specially designed ligands can serve a Lewis base or Lewis acid function, as an aromatization/dearomatization shuttle, or as an electron reservoir with reversible redox activity. This review encapsulates advances that have been made in this field over the last ten years, focusing exclusively on first-row transition metals, and highlighting significant contributions to mechanistic understanding.

Bis[diphenylphosphino]methane and its bridge-substituted analogues as chemically non-innocent ligands for H2 activation.

Deprotonation of fac-[(κ2P,P-Ph2PCH(R)PPh2)Mn(CO)3Br] (R = H, Me, Ph) produces the corresponding diphosphinomethanide derivatives fac-[(κ3P,C,P-Ph2PC(R)PPh2)Mn(CO)3], which are prone to activate H2 to form the hydride complexes fac-[(κ2P,P-Ph2PCH(R)PPh2)Mn(CO)3H]. The substitution of the dppm bridge improves dramatically the reaction efficiency and this was rationalized by DFT calculations.

Advances for Ruthenium Catalysts with Metal-Ligand Cooperation for Hydrogenation of Oxalates into Ethylene Glycol

Advances for Ruthenium Catalysts with Metal-Ligand Cooperation for Hydrogenation of Oxalates into Ethylene Glycol