Tuning metal-metal interactions for cooperative small molecule activation

C(sp)-H, S-H, and Sn-H Bond Activation with a Cobalt(I) Pincer Complex

This study focuses on the stoichiometric reactions of {2,6-(iPr2PO)2C6H3}Co(PMe3)2 with terminal alkynes, thiols, and tin hydrides as part of an effort to develop catalytic, two-electron processes with cobalt. This specific Co(I) pincer complex proves to be effective for cleaving the C(sp)-H, S-H, and Sn-H bonds to give oxidative addition products with the general formula {2,6-(iPr2PO)2C6H3}CoHX(PMe3) (X = alkynyl, thiolate, and stannyl groups) along with the free PMe3. These reactions typically reach completion when the substituents on acetylene, sulfur, and tin are electron-withdrawing groups (e.g., phenyl, pyridyl, and alkenyl groups). In contrast, alkyl-substituted acetylenes, 1-pentanethiol, and tributyltin hydride are partially converted due to the equilibria with the corresponding oxidative addition products. The Co(I) pincer complex is not a hydrothiolation catalyst but capable of catalyzing the hydrostannation of terminal alkynes with Ph3SnH to produce β-(Z)-alkenylstannanes selectively.

Heterobimetallic 3d-4f complexes supported by a Schiff-base tripodal ligand

Complexes featuring multiple metal centres are of growing interest regarding metal-metal cooperation and its tuneability. Here the synthesis and characterisation of heterobimetallic complexes of a 3d metal (4: Mn, 5: Co) and lanthanum supported by a (1,1,1-tris[(3-methoxysalicylideneamino)methyl]ethane) ligand is reported, as well as discussion of their electronic structure via electron paramagnetic resonance (EPR) spectroscopy, electrochemical experiments and computational studies. Competitive binding experiments of the ligand and various metal salts unequivocally demonstrate that in these heterobimetallic complexes the 3d metal (Mn, Co) selectively occupies the κ6-N3O3 binding site of the ligand, whilst La occupies the κ6-O6 metal binding site in line with their relative oxophilicities. EPR spectroscopy supported by density functional theory analysis indicates that the 3d metal is high spin in both cases (S = 5/2 (Mn), 3/2 (Co)). Cyclic voltammetry studies on the Mn/La and Co/La bimetallic complexes revealed a quasi-reversible Mn2+/3+ redox process and poorly-defined irreversible oxidation events respectively.

Conversion of Methane at Room Temperature Mediated by the Ta-Ta σ-Bond

Metal-metal bonds constitute an important type of reactive centers for chemical transformation; however, the availability of active metal-metal bonds being capable of converting methane under mild conditions, the holy grail in catalysis, remains a serious challenge. Herein, benefiting from the systematic investigation of 36 metal clusters of tantalum by using mass spectrometric experiments complemented with quantum chemical calculations, the dehydrogenation of methane at room temperature was successfully achieved by 18 cluster species featuring σ-bonding electrons localized in single naked Ta-Ta centers. In sharp contrast, the other 18 remaining clusters, either without naked Ta-Ta σ-bond or with σ-bonding electrons delocalized over multiple Ta-Ta centers only exhibit molecular CH4-adsorption reactivity or inertness. Mechanistic studies revealed that changing cluster geometric configurations and tuning the number of simple inorganic ligands (e.g., oxygen) could flexibly manipulate the presence or absence of such a reactive Ta-Ta σ-bond. The discovery of Ta-Ta σ-type bond being able to exhibit outstanding activity toward methane conversion not only overturns the traditional recognition that only the metal-metal π- or δ-bonds of early transition metals could participate in bond activation but also opens up a new access to design of promising metal catalysts with dual-atom as reactive sites for chemical transformations.

A scandium metalloligand supported Ni(0) complex with a heterobimetallocycle: versatile reactivity with unsaturated bonds

A N2-bridged tetranuclear Sc(III)-Ni(0) complex featuring a Ni → Sc interaction and a 4-membered [Sc-N-C-Ni] ring was synthesized and characterized. Bimetallic reactivity was demonstrated via reactions with a series of unsaturated compounds containing NC, CN, CC, CO and NN bonds.

Valence Tautomerism Induced Proton Coupled Electron Transfer:X-H Bond Oxidation with a Dinuclear Au(II) Hydroxide Complex

We report the preparation and characterization of the dinuclear AuII hydroxide complex AuII 2(L)2(OH)2 (L=N,N'-bis (2,6-dimethyl) phenylformamidinate) and study its reactivity towards weak X-H bonds. Through the interplay of kinetic analysis and computational studies, we demonstrate that the oxidation of cyclohexadiene follows a concerted proton-coupled electron transfer (cPCET) mechanism, a rare type of reactivity for Au complexes. We find that the Au-Au σ-bond undergoes polarization in the PCET event leading to an adjustment of oxidation levels for both Au centers prior to C(sp3)-H bond cleavage. We thus describe the oxidation event as a valence tautomerism-induced PCET where the basicity of one reduced Au-OH unit provides a proton acceptor and the second more oxidized Au center serves as an electron acceptor. The coordination of these events allows for unprecedented radical-type reactivity by a closed shell AuII complex.

Small molecules (CO2 , iPrNCO, and iPrNCNiPr) activation by the metallomimetics (μ-Hydrido) diborane anion: A DFT investigation on mechanism and chemoselectivity controlling

Main-group metallomimetics provide a new way to replace transition metal complexes to activate inert small molecules under mild conditions. In this work, the activation mechanisms of CO2 , iPrNCO, and iPrNCNiPr by (μ-Hydrido) diborane anion ([1H]- ) have been investigated by density functional theory (DFT) calculations. Two different activation sites, BB versus BH bond of [1H]- , are investigated and compared. The results show that these inert molecules can be activated by [1H]- through cycloadditions under mild conditions. The reactions with iPrNCO and iPrNCNiPr are dynamic and thermodynamic controlling, the obtained products are related not only to the energy barrier but also to the stability of the products. Moreover, the competition for BB/BH bond site activation is directly related to the steric effect of small molecules. CO2 , which is without steric hindrance, can only be activated by the BB bond, whereas iPrNCNiPr can only be activated by the BH bond due to the large steric effect. The medium iPrNCO can be activated not only by the BB bond but also by the BH bond. Our study provides theoretical explanations for the reaction activity and chemoselectivity controlling of the title reaction, and displays the potential applications for compounds containing boron-boron bonds and inert small molecule activation.

A one-pot reduction route to bimetallic manganese 1,8-naphthyridine complexes

Reaction of the dinucleating ligand 2,7-bis(6-methyl-2-pyridyl)-1,8-naphthyridine (MeL) with the MnI and MnII precursors MnBr(CO)5 and MnCl2 resulted in the formation of the monometallic complexes [MnBr(CO)3(MeL)] (1) and [MnCl2(MeL)] (3). In both cases, formation of bimetallic manganese complexes could be achieved by reduction with KC8, yielding the carbonyl-bridged complex [Mn2(CO)6(MeL)] (2) and the helicate complex [Mn2(MeL)2] (4), respectively. EPR results demonstrate that 4 represents a novel, weakly antiferromagnetically coupled homovalent dimer (J = -0.85 cm-1). The two formally Mn0 ions are both high spin (S = 3/2) and exhibit a zero-field splitting of ≈1 cm-1, suggesting reduction of the complex is substantially ligand centered, and may be better described as a MnII complex coupled to two open shell singlet ligands [MnII2(MeL2-)2]. X-ray crystallography, UV-Vis spectroscopy and DFT analysis support this finding.

Unsymmetrical Imidazopyrimidine-Based Ligand and Bimetallic Complexes

With bimetallic catalysts becoming increasingly important, the development of electronically and structurally diverse binucleating ligands is desired. This work describes the synthesis of unsymmetric ligand 2,7-di(pyridin-2-yl)imidazo[1,2-a]pyrimidine (dpip) that is achieved in four steps on a multigram scale in an overall 54% yield. The ability of dpip to act as a scaffold for the formation of bimetallic complexes is demonstrated with the one-step syntheses of the dicopper complex [Cu2(dpip)(μ-OH)(CF3COO)3] (4), the dipalladium complex [Pd2(dpip)(μ-OH)(CF3COO)2](CF3COO)·CF3COOH (5), and the dimeric dinickel complex [Ni4(dpip)2(μ-Cl)4Cl2MeOH6][2Cl] (6) in good yields (79-92%). All bimetallic complexes were characterized by spectroscopic methods and X-ray crystallography, which revealed metal-metal distances between 3.4821(9) and 4.106(2) Å. Additionally, quantum chemical calculations were conducted on complex 4 and an analogous 1,8-naphthyridine-based dicopper complex to investigate the differences between the imidazopyrimidine motif reported here and the widely used 1,8-naphthyridine motif. Natural bonding orbital (NBO) and Mayer bond order (MBO) analyses validated the ability of dpip to coordinate metals more strongly. Finally, NBO calculations quantified the differences in the binding energy between the two pockets of the unsymmetrical dpip ligand.

Utilising a Proton-Responsive 1,8-Naphthyridine Ligand for the Synthesis of Bimetallic Palladium and Platinum Complexes

We present four proton-responsive palladium and platinum complexes, [MCl2 (R PONNHO)] (M=Pd, Pt; R=i Pr, t Bu) synthesised by complexation of PdCl2 or PtCl2 (COD) with the 1,8-naphthyridine ligand R PONNHO. Deprotonation of [MCl2 (tBu PONNHO)] switches ligand coordination from mono- to dinucleating, offering a synthetic pathway to bimetallic PdII and PtII complexes [M2 Cl2 (tBu PONNO)2 ]. Two-electron reduction gives planar MI -MI complexes [M2 (tBu PONNO)2 ] (M=Pd, Pt) containing a metal-metal bond. In contrast to the related nickel system that forms a metallophosphorane [Ni2 (tBu PONNOPONNO)], an unusual phosphinite binding mode is observed in [M2 (tBu PONNO)2 ] containing close phosphinite-naphthyridinone P⋅⋅⋅O interactions, which is investigated spectroscopically, crystallographically and computationally. The presented proton-responsive and structurally-responsive R PONNHO and bimetallic R PONNO complexes offer a novel platform for future explorations of metal-ligand and metal-metal cooperativity with palladium and platinum.

Influence of Metal Identity and Complex Nuclearity in Kumada Cross-Coupling Polymerizations with a Pyridine Diimine-Based Ligand Scaffold

Cross-coupling polymerizations have fundamentally changed the field of conjugated polymers (CPs) by expanding the scope of accessible materials. Despite the prevalence of cross-coupling in CP synthesis, almost all polymerizations rely on mononuclear Ni or Pd catalysts. Here, we report a systematic exploration of mono- and dinuclear Fe and Ni precatalysts with a pyridine diimine ligand scaffold for Kumada cross-coupling polymerization of a donor thiophene and an acceptor benzotriazole monomers. We observe that variation of the metal identity from Ni to Fe produces contrasting polymerization mechanisms, while complex nuclearity has a minimal impact on reactivity. Specifically, Fe complexes appear to catalyze step-growth Kumada polymerizations and can readily access both Csp2-Csp3 and Csp2-Csp2 cross-couplings, while Ni complexes catalyze chain-growth polymerizations and predominantly Csp2-Csp2 cross-couplings. Thus, our work sheds light on important design parameters for transition metal complexes used in cross-coupling polymerizations, demonstrates the viability of iron catalysis in Kumada polymerization, and opens the door to novel polymer compositions.

Copper(I) Complexes of Amide Functionalized Bisphosphine: Proximity Enhanced Metal-Ligand Cooperativity and Its Catalytic Advantage in C(sp3)-H Bond Activation of Unactivated Cycloalkanes in Dehydrogenative Carboxylation Reactions

The reactions of amide functionalized bisphosphine, o-Ph2PC6H4C-(O)N(H)C6H4PPh2-o (1) (BalaHariPhos), with copper salts is described. Treatment of 1 with CuX in a 1:1 molar ratio yielded chelate complexes of the type [CuX{(o-Ph2PC6H4C(O)N(H)C6H4PPh2-o)}-κ2-P,P] (X = Cl, 2; Br, 3; and I, 4), which on subsequent treatment with KOtBu resulted in a dimeric complex [Cu(o-Ph2PC6H4C(O)(N)C6H4PPh2-o)]2 (5). Interestingly, complexes 2-4 showed weak N-H···Cu interactions. These weak H-bonding interactions are studied in detail both experimentally and computationally. Also, CuI complexes 2-5 were employed in the oxidative dehydrogenative carboxylation (ODC) of unactivated cycloalkanes in the presence of carboxylic acids to form the corresponding allylic esters. Among complexes 2-5, halide-free dimeric CuI complex 5 showed excellent metal-ligand cooperativity in the oxidative dehydrogenative carboxylation (ODC) in the presence of carboxylic acids to form the corresponding allylic esters through C(sp3)-H bond activation of unactivated cycloalkanes. Mechanistic details of the catalytic process were established by isolating the precatalyst [Cu{(o-Ph2PC6H4C(O)(NH)C6H4PPh2-o)-κ2-P,P}(OOCPh)] (6) and fully characterized by mass spectrometry, NMR data, and single-crystal X-ray analysis. Density functional theory-based calculations further provided a quantitative understanding of the energetics of a series of H atom transfer steps occurring in the catalytic cycle.

Cooperative dihydrogen activation with unsupported uranium-metal bonds and characterization of a terminal U(iv) hydride

Cooperative chemistry between two or more metal centres can show enhanced reactivity compared to the monometallic fragments. Given the paucity of actinide-metal bonds, especially those with group 13, we targeted uranium(iii)-aluminum(i) and -gallium(i) complexes as we envisioned the low-valent oxidation state of both metals would lead to novel, cooperative reactivity. Herein, we report the molecular structure of [(C5Me5)2(MesO)U-E(C5Me5)], E = Al, Ga, Mes = 2,4,6-Me3C6H2, and their reactivity with dihydrogen. The reaction of H2 with the U(iii)-Al(i) complex affords a trihydroaluminate complex, [(C5Me5)2(MesO)U(μ2-(H)3)-Al(C5Me5)] through a formal three-electron metal-based reduction, with concomitant formation of a terminal U(iv) hydride, [(C5Me5)2(MesO)U(H)]. Noteworthy is that neither U(iii) complexes nor [(C5Me5)Al]4 are capable of reducing dihydrogen on their own. To make the terminal hydride in higher yields, the reaction of [(C5Me5)2(MesO)U(THF)] with half an equivalent of diethylzinc generates [(C5Me5)2(MesO)U(CH2CH3)] or treatment of [(C5Me5)2U(i)(Me)] with KOMes forms [(C5Me5)2(MesO)U(CH3)], which followed by hydrogenation with either complex cleanly affords [(C5Me5)2(MesO)U(H)]. All complexes have been characterized by spectroscopic and structural methods and are rare examples of cooperative chemistry in f element chemistry, dihydrogen activation, and stable, terminal ethyl and hydride compounds with an f element.

Switch in Selectivities by Dinuclear Nickel Catalysis: 1,4-Hydroarylation of 1,3-Dienes to Z-Olefins

One of the most challenging tasks in organic synthesis is to control selectivities, especially switching the well-known selectivity to obtain new isomers that were previously inaccessible. Inspired by biological catalysis involving multiple metal centers, catalysis enabled by binuclear metal complexes offers the potential to induce reactivity and selectivity that might not be available to mononuclear catalysts. Herein, we describe that using a macrocyclic bis pyridyl diimine dinickel complex as the catalyst, the commonly observed 4,3-regioselectivity of hydroarylation of 1,3-dienes is switched to 1,4-hydroarylation with thermodynamically less stable Z-stereoselectivity, offering challenging synthetic target Z-olefins. DFT calculations show that the activation of 1,3-diene proceeds through dinuclear Ni-diolefin coordination, and the synergistic effects of two Ni nuclei enable reactivity and selectivity of this binuclear catalysis substantially different from those of mononuclear nickel complexes in the current reaction.

Phenylalanine-Based Co2+ and Cd2+ 1D Coordination Polymers: Structural Properties and Catalytic Application for Solvent-Free Aerobic Oxidation of Cycloalkene

Two 1D coordination polymers (CPs) with general formula [M(L)(H2O)(AcO)]n, (M = Co (1) or Cd (2), AcO = acetate anion and L denotes l-phenylalanine based ligand), were synthesized and fully characterized by various spectroscopies (UV-vis, FTIR, and NMR), thermal techniques, magnetic measurements (for 1), and single-crystal and powder X-ray diffraction studies. They can be described as "ribbon-like" 1D polymers constructed through a zigzag arrangement. The polymeric structure is developed due to the coordination mode adopted by the amino acid ligand, classified as μ3-N1O1:O1:O2, which simultaneously links three metal centers. This moiety also plays an important role as a magnetic coupler between metal centers in the cobalt system, which shows a weak antiferromagnetic interaction. Both CPs have also been used in the catalytic oxidation of cyclohexene with molecular oxygen (O2) as an oxidant. Under mild conditions, both compounds demonstrated remarkable catalytic activity, with the cobalt system being more efficient than the cadmium analogue (conversion: 73 and 58% and selectivity for the major product, 2-cyclohexanone: 63 and 55%, for 1 and 2, respectively). Leaching experiments and the results obtained using a radical quencher are consistent with a radical-mediated mechanism for the Co compound. The presence of the superoxide radical was also confirmed using EPR spectroscopy and DMPO as a spin trap, which was further validated by DFT calculations. The activity observed for the Cd analogue is attributed to the organic scaffold assisted by the templating effect of the metal ion.

Binuclear Complexes Supported by a Tetrapyridyl Ligand with a Bending Anthraquinodimethane Linker

A tetrapyridyl ligand with a bending anthraquinodimethane linker has been synthesized, and its complexation with coinage metals has been examined. The treatment of the ligand with Ag(I) and Au(I) cations afforded binuclear complexes, wherein the two metal centers were in close proximity to the inside space of the ligand. X-ray analyses corroborated with theoretical calculations indicated that the ligand has reasonable flexibility toward a bending deformation of the linker moiety to provide a ligand pocket suitable for the proximal binuclear complexes, even though such deformations accompany a non-negligible amount of energetic cost. On the other hand, treatment with 2 equiv of Cu(I) salt afforded a binuclear complex, in which both copper atoms were coordinated at the periphery of the ligand.

Mono and Dinuclear Palladium Pincer Complexes of NNSe Ligand as a Catalyst for Decarboxylative Direct C-H Heteroarylation of (Hetero)arenes

This report describes the synthesis of a new NNSe pincer ligand and its mono- and dinuclear palladium(II) pincer complexes. In the absence of a base, a dinuclear palladium pincer complex (C1) was isolated, while in the presence of Et3 N base a mononuclear palladium pincer complex (C2) was obtained. The new ligand and complexes were characterized using techniques like 1 H, 13 C{1 H} nuclear magnetic resonance (NMR), fourier transform infrared (FTIR), high-resolution mass spectrometry (HRMS), ultraviolet-visible (UV-Visible), and cyclic voltammetry. Both the complexes showed pincer coordination mode with a distorted square planar geometry. The complex C1 has two pincer ligands attached through a Pd-Pd bond in a dinuclear pincer fashion. The air and moisture-insensitive, thermally robust palladium pincer complexes were used as the catalyst for decarboxylative direct C-H heteroarylation of (hetero)arenes. Among the complexes, dinuclear pincer complex C1 showed better catalytic activity. A variety of (hetero)arenes were successfully activated (43-87 % yield) using only 2.5 mol % of catalyst loading under mild reaction conditions. The PPh3 and Hg poisoning experiments suggested a homogeneous nature of catalysis. A plausible reaction pathway was proposed for the dinuclear palladium pincer complex catalyzed decarboxylative C-H bond activation reaction of (hetero)arenes.

Added Complexity!-Mechanistic Aspects of Heterobimetallic Complexes for Application in Homogeneous Catalysis

Inspired by multimetallic assemblies and their role in enzyme catalysis, chemists have developed a plethora of heterobimetallic complexes for application in homogeneous catalysis. Starting with small heterobimetallic complexes with σ-donating and π-accepting ligands, such as N-heterocyclic carbene and carbonyl ligands, more and more complex systems have been developed over the past two decades. These systems can show a significant increase in catalytic activity compared with their monometallic counterparts. This increase can be attributed to new reaction pathways enabled by the presence of a second metal center in the active catalyst. This review focuses on mechanistic aspects of heterobimetallic complexes in homogeneous catalysis. Depending on the type of interaction of the second metal with the substrates, heterobimetallic complexes can be subdivided into four classes. Each of these classes is illustrated with multiple examples, showcasing the versatility of both, the types of interactions possible, and the reactions accessible.

Homotropic Cooperativity in Iron-Catalyzed Alkyne Cyclotrimerizations

Enhancing catalytic activity through synergic effects is a current challenge in homogeneous catalysis. In addition to the well-established metal-metal and metal-ligand cooperation, we showcase here an example of self-activation by the substrate in controlling the catalytic activity of the two-coordinate iron complex [Fe(2,6-Xyl₂C₆H₃)₂] (1, Xyl = 2,6-Me₂C₆H₃). This behavior was observed for aryl acetylenes in their regioselective cyclotrimerization to 1,2,4-(aryl)-benzenes. Two kinetically distinct regimes are observed dependent upon the substrate-to-catalyst ratio ([RC≡CH]₀/[1]₀), referred to as the low ([RC≡CH]₀/[1]₀ < 40) and high ([RC≡CH]₀/[1]₀ > 40) regimes. Both showed sigmoidal kinetic response, with positive Hill indices of 1.85 and 3.62, respectively, and nonlinear Lineweaver-Burk replots with an upward curvature, which supports positive substrate cooperativity. Moreover, two alkyne molecules participate in the low regime, whereas up to four are involved in the high regime. The second-order rate dependence on 1 indicates that binuclear complexes are the catalytically competent species in both regimes, with that in the high one being 6 times faster than that involved in the low one. Moreover, Eyring plot analyses revealed two different catalytic cycles, with a rate-determining step more endergonic in the low regime than in the high one, but with a more ordered transition state in the high regime than in the low one.

On the Dynamic Behavior of Pacman Phosphanes─A Case of Cooperativity and Redox Isomerism

In solution, the Pacman chlorophosphane (2Cl) shows fast exchange of the endo/exo-orientation of the two P-Cl bonds in the molecule featuring cooperativity. Experimental and quantum mechanical investigations of the inversion on the phosphorus(III) centers reveal a crucial role of chloride ions in the dynamic process. To confirm the results, the homologous Pacman halogen-phosphanes 2X were prepared by halogen exchange reactions (X = F, Br, and I). Besides accelerated dynamic behavior for the heavier analogues, significant differences in the molecular structure are caused by the halogen exchange reactions, including the formation of an endo-endo substituted Pacman fluorophosphane as well as dicationic species by phosphorus halogen bond dissociation. The latter process can be regarded as redox isomerism since two P^III atoms in 2X become P^V centers in the dications.

Bimetallic Nickel Complexes Supported by a Planar Macrocyclic Diphosphoranide Ligand

Metal-metal cooperativity is emerging as an important strategy in catalysis. This requires appropriate ligand scaffolds that can support two metals in close proximity. Here we report nickel-promoted formation of a dinucleating planar macrocyclic ligand that can support bimetallic dinickel(II) and dinickel(I) complexes. Reaction outcomes can be tuned by variation of the substituents and reaction conditions to favour dinucleating macrocyclic, mononucleating macrocyclic or conventional pincer architectures.

Mg(I)-Fe(-II) and Mg(0)-Mg(I) covalent bonding in the MgnFe(CO)4- (n = 1, 2) anion complexes: an infrared photodissociation spectroscopic and theoretical study

Heteronuclear magnesium-iron carbonyl anion complexes MgFe(CO)₄^- and Mg₂Fe(CO)₄^- are produced in the gas phase and are detected by mass-selected infrared photodissociation spectroscopy in the carbonyl stretching frequency region. The geometric structures and the metal-metal bonding are discussed with the aid of quantum chemical calculations. Both complexes are characterized to have a doublet electronic ground state with C_3v symmetry containing a Mg-Fe bond or a Mg-Mg-Fe bonding unit. Bonding analyses indicate that each complex involves an electron-sharing Mg(I)-Fe(-II) σ bond. The Mg₂Fe(CO)₄^- complex involves a relatively weak covalent Mg(0)-Mg(I) σ bond.

Quantification of the Steric Properties of 1,8-Naphthyridine-Based Ligands in Dinuclear Complexes

Steric properties of ligands are an important parameter for tuning the reactivity of the corresponding complexes. For various ligands used in mononuclear complexes, methods have been developed to quantify their steric bulk. In this work, we present an expansion of the buried volume and the G-parameter to quantify the steric properties of 1,8-napthyridine-based dinuclear complexes. Using this methodology, we explored the tunability of the steric properties associated with these ligands and complexes.

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

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

Macrocycle-Induced Modulation of Internuclear Interactions in Homobimetallic Complexes

A synthetic route has been developed for a series of 3d homobimetallic complexes of Mn, Fe, Co, Ni, and Cu using three different pyridyldiimine and pyridyldialdimine macrocyclic ligands with ring sizes of 18, 20, and 22 atoms. Crystallographic analyses indicate that while the distances between the metals can be modulated by the size of the macrocycle pocket, the flexibility in the alkyl linkers used to construct the macrocycles enables the ligand to adjust the orientation of the PD(A)I fragments in response to the geometry of the [M₂(μ-Cl)₂]²⁺ core, particularly with respect to Jahn-Teller distortions. Analyses by UV-vis spectroscopy and SQUID magnetometry revealed deviations in the properties [M₂(μ-Cl)₂]²⁺-containing complexes bound by standard mononucleating ligands, highlighting the ability of macrocycles to use ring size to control the magnetic interactions of pseudo-octahedral, high-spin metal centers.

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.

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

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

Phosphinoamido Ligand Supported Heterobimetallic Rare-Earth Metal-Palladium Complexes: Versatile Structures and Redox Reactivities

Heterobimetallic Ln(III)-Pd(0) complexes (Ln = Y, Sm, Gd, Yb) featuring tetranuclear structures with COD as bridges were obtained via the metallation of tris(phosphinoamido) rare-earth metal complexes [Ph2PNAd]3Ln (Ad = admantyl)...

Heteropolymetallic Architectures as Snapshots of Transmetallation Processes at Different Degrees of Transfer

Novel heteropolymetallic architectures have been built by integrating Pd, Au and Ag systems. The dinuclear [(CNC)(PPh₃)Pd‐^G11M(PPh₃)](ClO₄) (^G11M=Au (3), Ag (4); CNC=2,6‐diphenylpyridinate) and trinuclear [{(CNC)(PPh₃)Pd}₂^G11M](ClO₄) (^G11M=Au (6), Ag (5)) complexes have been accessed or isolated. Structural and DFT characterization unveil striking interactions of one of the aryl groups of the CNC ligand(s) with the ^G11M center, suggesting these complexes constitute models of transmetallation processes. Further analyses allow to qualitatively order the degree of transfer, proving that Au promotes the highest one and also that Pd systems favor higher degrees than Pt. Consistently, Energy Decomposition Analysis calculations show that the interaction energies follow the order Pd−Au > Pt−Au > Pd−Ag > Pt−Ag. All these results offer potentially useful ideas for the design of bimetallic catalytic systems.

Main-group metal complexes of α-diimine ligands: structure, bonding and reactivity

α-Diimine ligands, in particular 1,4-diazabutadiene (dad) and bis(iminoacenaphthene) (bian) derivatives, have been widely used for coordination with various metals, including main-group, transition, and lanthanide and actinide metals. In addition to their tunable steric and electronic properties, the dad and bian ligands are redox-active and can readily accept one or two electrons, converting into the radical-anionic (L˙^-) or dianionic (enediamido, L^2-) form, respectively. This non-innocence brings about rich electronic structures and properties of the ligands and complexes thereof. For example, the dad ligands in their three redox levels can effectively stabilize a series of metal centers in different oxidation states, including low-valent metals. Moreover, these ligands can serve as electron reservoirs and can participate in reactions toward other molecules with or without metals. Therefore, such ligands are extremely useful in the areas of low-valent complexes and small molecule activation. Herein, we will discuss the use of dad (and bian) ligands in the stabilization of metal-metal-bonded compounds, in particular those of main-group metals, as well as small molecule activation by these (low-valent) metal coordination species where the non-innocence of the ligands plays a key role.

Dinickel Active Sites Supported by Redox-Active Ligands

Redox reactions that take place in enzymes and on the surfaces of heterogeneous catalysts often require active sites that contain multiple metals. By contrast, there are very few homogeneous catalysts with multinuclear active sites, and the field of organometallic chemistry continues to be dominated by the study of single metal systems. Multinuclear catalysts have the potential to display unique properties owing to their ability to cooperatively engage substrates. Furthermore, direct metal-to-metal covalent bonding can give rise to new electronic configurations that dramatically impact substrate binding and reactivity. In order to effectively capitalize on these features, it is necessary to consider strategies to avoid the dissociation of fragile metal-metal bonds in the course of a catalytic cycle. This Account describes one approach to accomplishing this goal using binucleating redox-active ligands.In 2006, Chirik showed that pyridine-diimines (PDI) have sufficiently low-lying π* levels that they can be redox-noninnocent in low-valent iron complexes. Extending this concept, we investigated a series of dinickel complexes supported by naphthyridine-diimine (NDI) ligands. These complexes can promote a broad range of two-electron redox processes in which the NDI ligand manages electron equivalents while the metals remain in a Ni(I)-Ni(I) state.Using (NDI)Ni₂ catalysts, we have uncovered cases where having two metals in the active site addresses a problem in catalysis that had not been adequately solved using single-metal systems. For example, mononickel complexes are capable of stoichiometrically dimerizing aryl azides to form azoarenes but do not turn over due to strong product inhibition. By contrast, dinickel complexes are effective catalysts for this reaction and avoid this thermodynamic sink by binding to azoarenes in their higher-energy cis form.Dinickel complexes can also activate strong bonds through the cooperative action of both metals. Norbornadiene has a ring-strain energy that is similar to that of cyclopropane but is not prone to undergoing C-C oxidative addition with monometallic complexes. Using an (NDI)Ni₂ complex, norbornadiene undergoes rapid ring opening by the oxidative addition of the vinyl and bridgehead carbons. An inspection of the resulting metallacycle reveals that it is stabilized through a network of secondary Ni-π interactions. This reactivity enabled the development of a catalytic carbonylative rearrangement to form fused bicyclic dienones.These vignettes and others described in this Account highlight some of the implications of metal-metal bonding in promoting a challenging step in a catalytic cycle or adjusting the thermodynamic landscape of key intermediates. Given that our studies have focused nearly exclusively on the (NDI)Ni₂ system, we anticipate that many more such cases are left to be discovered as other transition-metal combinations and ligand classes are explored.

Monometallic and Multimetallic Zinc Complexes of 2,7-Bis(2-pyridyl)-1,8-naphthyridine

A series of zinc complexes of 2,7-bis(2-pyridyl)-1,8-naphthyridine (BPNP) has been synthesised and characterised by single crystal X-ray diffraction and solution state NMR spectroscopic studies. Reactions of BPNP with zinc acetate and zinc chloride were found to give mononuclear complexes of the form [Zn(BPNP)X2] (X = OAc, Cl), whereas reactions with zinc triflate led to a mixture of products. Several of these were identified crystallographically as [Zn(BPNP-H)(H2O)4](OTf)3 and [Zn(BPNP-H)(NCMe)(OTf)2]OTf, in which protonation of one pyridyl group occurred, and the dimeric species [Zn2(BPNP)4(μ-H2O)2](OTf)4. A trimetallic complex [Zn3(μ2-BPNP)(μ2-OAc)3(OAc)2(μ3-OH)] was also isolated from reactions involving zinc acetate, and demonstrates the ability of BPNP to coordinate two zinc atoms in the adjacent binding pockets.

Synthesis of an expanded pincer ligand and its bimetallic coinage metal complexes

An expanded pincer ligand tBu-PONNOP (2,7-bis(di-tert-butylphosphinito)-1,8-naphthyridine) has been synthesised and its coordination to coinage metals has been studied. Bimetallic complexes were produced with metal halide salts of the type [M₂X₂(tBu-PONNOP)] (X = Cl, M = Au, Ag, Cu; X = I, M = Cu) with a varying degree of interaction with the naphthyridyl backbone in the order Au < Ag < Cu. The salts [Ag₂(tBu-PONNOP)₂][BAr^F₄]₂ (Ar^F = 3,5-C₆H₃(CF₃)₂) and [Ag₂(NCMe)₂(tBu-PONNOP)]X₂ (X = BAr^F₄, PF₆) were prepared, which may serve as a source of tBu-PONNOP via transmetallation.