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.

Investigating Metal-Metal Bond Polarization in a Heteroleptic Tris-Ylide Diiron System

This article describes the synthesis, characterization, and S-atom transfer reactivity of a series of C3v-symmetric diiron complexes. The iron centers in each complex are coordinated in distinct ligand environments, with one (FeN) bound in a pseudo-trigonal bipyramidal geometry by three phosphinimine nitrogens in the equatorial plane, a tertiary amine, and the second metal center (FeC). FeC is coordinated, in turn, by FeN, three ylidic carbons in a trigonal plane, and, in certain cases, by an axial oxygen donor. The three alkyl donors at FeC form through the reduction of the appended N═PMe3 arms of the monometallic parent complex. The complexes were studied crystallographically, spectroscopically (NMR, UV-vis, and Mössbauer), and computationally (DFT, CASSCF) and found to be high-spin throughout, with short Fe-Fe distances that belie weak orbital overlap between the two metals. Further, the redox nature of this series allowed for the determination that oxidation is localized to the FeC. S-atom transfer chemistry resulted in the formal insertion of a S atom into the Fe-Fe bond of the reduced diiron complex to form a mixture of Fe4S and Fe4S2 products.

C-C σ-Bond Oxidative Addition and Hydrofunctionalization by a Macrocycle-Supported Diiron Complex

This report describes the first examples of unassisted C(sp)-C(sp2) and C(sp)-C(sp3) bond oxidative addition reactions to give thermodynamically favorable products. Treatment of a diiron complex supported by a geometrically and electronically flexible macrocyclic ligand, (3PDI2)Fe2(μ-N2)(PPh3)2 ([Fe2N2]0), with stoichiometric amounts of various 4,4'-disubstituted diphenylacetylenes (ArX-C≡C-ArX; X = OMe, H, F, CF3) yielded C(sp)-C(sp2) bond oxidative addition products. When Ph-C≡C-R substrates were used as substrates (R = Me, Et, iPr, tBu), products of either C(sp)-C(sp2) or C(sp)-C(sp3) bond activation were obtained, with the less sterically encumbering alkynes exclusively undergoing C(sp)-C(sp3) bond activation. Treatment of the C-C activation species with either H2 or HBpin was found to form products of C-C σ-bond hydrofunctionalization. In both the hydrogenation and hydroboration schemes, the diiron species was observed to return to [Fe2N2]0, thereby completing synthetic cycles for C-C σ-bond functionalization.

Oriented internal electrostatic fields: an emerging design element in coordination chemistry and catalysis

The power of oriented electrostatic fields (ESFs) to influence chemical bonding and reactivity is a phenomenon of rapidly growing interest. The presence of strong ESFs has recently been implicated as one of the most significant contributors to the activity of select enzymes, wherein alignment of a substrate's changing dipole moment with a strong, local electrostatic field has been shown to be responsible for the majority of the enzymatic rate enhancement. Outside of enzymology, researchers have studied the impacts of "internal" electrostatic fields via the addition of ionic salts to reactions and the incorporation of charged functional groups into organic molecules (both experimentally and computationally), and "externally" via the implementation of bulk fields between electrode plates. Incorporation of charged moieties into homogeneous inorganic complexes to generate internal ESFs represents an area of high potential for novel catalyst design. This field has only begun to materialize within the past 10 years but could be an area of significant impact moving forward, since it provides a means for tuning the properties of molecular complexes via a method that is orthogonal to traditional strategies, thereby providing possibilities for improved catalytic conditions and novel reactivity. In this perspective, we highlight recent developments in this area and offer insights, obtained from our own research, on the challenges and future directions of this emerging field of research.

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.

Probing Ligand Effects on the Ultrafast Dynamics of Copper Complexes via Midinfrared Pump-Probe and 2DIR Spectroscopies

The effects of ligand structural variation on the ultrafast dynamics of a series of copper coordination complexes were investigated using polarization-dependent mid-IR pump-probe spectroscopy and two-dimensional infrared (2DIR) spectroscopy. The series consists of three copper complexes [(^R3P₃tren)Cu^IIN₃]BAr₄^F (1^PR3, ^R3P₃tren = tris[2-(phosphiniminato)ethyl]amine, BAr₄^F = tetrakis(pentafluorophenyl)borate) where the number of methyl and phenyl groups in the PR₃ ligand are systematically varied across the series (PR₃ = PMe₃, PMe₂Ph, PMePh₂). The asymmetric stretching mode of azide in the 1^PR3 series is used as a vibrational probe of the small-molecule binding site. The results of the pump-probe measurements indicate that the vibrational energy of azide dissipates through intramolecular pathways and that the bulkier phenyl groups lead to an increase in the spatial restriction of the diffusive reorientation of bound azide. From 2DIR experiments, we characterize the spectral diffusion of the azide group and find that an increase in the number of phenyl groups maps to a broader inhomogeneous frequency distribution (Δ₂). This indicates that an increase in the steric bulk of the secondary coordination sphere acts to create more distinct configurations in the local environment that are accessible to the azide group. This work demonstrates how ligand structural variation affects the ultrafast dynamics of a small molecular group bound to the metal center, which could provide insight into the structure-function relationship of the copper coordination complexes and transition-metal coordination complexes in general.

Mapping the Reactivity of Dicobalt Bridging Nitrides in Constrained Geometries

Low-nuclearity nitrides of the late transition metals are rare and reactive molecular species, with little experimental precedent. The first putative examples of dicobalt bridging nitrides, [(nPDI2)Co2(μ-N)(PMe3)2][OTf]3 (n[Co2N]3+; PDI = pyridyldiimine; n = 2 or 3, representing the length of the aliphatic chain linking PDI imino groups), were reported recently and shown to undergo a range of intramolecular reaction pathways, including N-H bond formation, C-H bond insertion, and P═N bond formation at the bridging nitride. The specific mode of reactivity changed with the phase of the reaction and the size of the macrocycle used to support the transient species. The present contribution offers a computational investigation into both the geometric and electronic structures of these nitrides as well as the factors governing their reaction selectivity. The compounds n[Co2N]3+ exhibit μ-N-based lowest unoccupied molecular orbitals (LUMOs) that are consistent with subvalent, electrophilic nitrides. The specific orientations of the LUMOs induce ring-size-dependent stereoelectronic effects, thereby causing the product selectivity observed experimentally. Notably, the nitrides also exhibit a degree of nucleophilicity at μ-N by way of a high-energy, μ-N-based lone pair. This ambiphilic character appears to be a direct result of the constrained environment imposed by the folded-ligand geometries of n[Co2N]3+. When combined with the experimental findings, these data led to the conclusion that the folded-ligand isomers are the reactive species and that the constrained geometry imposed by the macrocyclic ligand plays an important role in controlling the reaction outcome.

Tuning metal-metal interactions for cooperative small molecule activation

Cluster complexes have attracted interest for decades due to their promise of drawing analogies to metallic surfaces and metalloenzyme active sites, but only recently have chemists started to develop ligand scaffolds that are specifically designed to support multinuclear transition metal cores. Such ligands not only hold multiple metal centers in close proximity but also allow for fine-tuning of their electronic structures and surrounding steric environments. This Feature Article highlights ligand designs that allow for cooperative small molecule activation at cluster complexes, with a particular focus on complexes that contain metal-metal bonds. Two useful ligand-design elements have emerged from this work: a degree of geometric flexibility, which allows for novel small molecule activation modes, and the use of redox-active ligands to provide electronic flexibility to the cluster core. The authors have incorporated these factors into a unique class of dinucleating macrocycles (nPDI2). Redox-active fragments in nPDI2 mimic the weak-overlap covalent bonding that is characteristic of M-M interactions, and aliphatic linkers in the ligand backbone provide geometric flexibility, allowing for interconversion between a range of geometries as the dinuclear core responds to the requirements of various small molecule substrates. The union of these design elements appears to be a powerful combination for analogizing critical aspects of heterogeneous and metalloenzyme catalysts.

Pyridyldiimine macrocyclic ligands: Influences of template ion, linker length and imine substitution on ligand synthesis, structure and redox properties

A series of 2,6-diiminopyridine-derived macrocyclic ligands have been synthesized via [2+2] condensation around alkaline earth metal triflate salts. The inclusion of a tert-butyl group at the 4-position of the pyridine ring of the macrocyclic synthons results in macrocyclic complexes that are soluble in common organic solvents, thereby enabling a systematic comparison of the physical properties of the complexes by NMR spectroscopy, mass spectrometry, solution-phase UV-Vis spectroscopy, cyclic voltammetry and single-crystal X-ray crystallography. Solid-state structures determined crystallographically demonstrate increased twisting in the ligand, concurrent with either a decrease in ion size or an increase in macrocycle ring size (18, 20, or 22 membered rings). The degree of folding and twisting within the macrocycle can be quantified using parameters derived from the N_pyr-M-N_pyr bond angle and the relative orientation of the pyridinediimine (PDI) and pyridinedialdimine (PDAI) fragments to each other within the solid state structures. Cyclic voltammetry and UV-Vis spectroscopy were used to compare the relative energies of the imine π* orbital of the redox active PDI and PDAI components in the macrocycle when coordinated to redox inactive metals. Both methods indicate the change from a methyl to hydrogen substitution on the imine carbon lowers the energy of the ligand π* system.

Using internal electrostatic fields to manipulate the valence manifolds of copper complexes

A series of tetradentate tris(phosphinimine) ligands (^R3P₃tren) was developed and bound to Cu^I to form the trigonal pyramidal, C_3v-symmetric cuprous complexes [^R3P₃tren-Cu][BAr^F₄] (1PR3) (PR₃ = PMe₃, PMe₂Ph, PMePh₂, PPh₃, PMe₂(NEt₂), BAr^F₄ = B(C₆F₅)₄). Electrochemical studies on the Cu^I complexes were undertaken, and the permethylated analog, 1PMe3, was found to display an unprecedentedly cathodic Cu^I/Cu^II redox potential (−780 mV vs. Fc/Fc⁺ in isobutyronitrile). Elucidation of the electronic structures of 1PR3via density functional theory (DFT) studies revealed atypical valence manifold configurations, resulting from strongly σ-donating phosphinimine moieties in the xy-plane that destabilize 2e (d_xy/d_x²−y²) orbital sets and uniquely stabilized a₁ (d_z²) orbitals. Support is provided that the a₁ stabilizations result from intramolecular electrostatic fields (ESFs) generated from cationic character on the phosphinimine moieties in ^R3P₃tren. This view is corroborated via 1-dimensional electrostatic potential maps along the z-axes of 1PR3 and their isostructural analogues. Experimental validation of this computational model is provided upon oxidation of 1PMe3 to the cupric complex [^Me3P₃tren-Cu][OTf]₂ (2PMe3), which displays a characteristic Jahn–Teller distortion in the form of a see-saw, pseudo-C_s-symmetric geometry. A systematic anodic shift in the potential of the Cu^I/Cu^II redox couple as the steric bulk in the secondary coordination sphere increases is explained through the complexes' diminishing ability to access the ideal C_s-symmetric geometry upon oxidation. The observations and calculations discussed in this work support the presence of internal electrostatic fields within the copper complexes, which subsequently influence the complexes' properties via a method orthogonal to classic ligand field tuning.

Secondary coordination sphere electrostatic effects tune the valence manifolds of copper centers, impacting molecular geometries, photophysical properties, and redox potentials.

Unusual cyanide and methyl binding modes at a dicobalt macrocycle following acetonitrile C-C bond activation

This communication describes the C-C bond activation of acetonitrile and the trapping of the methyl and cyanide fragments by macrocyclic, dicobalt complexes. Both products display unique structural features as a result of the constraints imposed by the macrocycle. The bridged species [(3PDI2)Co2(μ-CN)(PMe3)2][OTf] ([Co2CN]+) exhibits atypical Co-CN-Co binding, and upon either phosphine dissociation or oxidation, the flexible ligand framework is able to switch between different binding modes of μ-cyanide. Further, the bridging methyl species [(3PDI2)Co2(μ-CH3)(PMe3)][OTf] ([Co2CH3]+) is the first structurally characterized dicobalt complex with a bridging methyl group.

N-H Bond Formation at a Diiron Bridging Nitride

Despite their connection to ammonia synthesis, little is known about the ability of iron-bound, bridging nitrides to form N-H bonds. Herein we report a linear diiron bridging nitride complex supported by a redox-active macrocycle. The unique ability of the ligand scaffold to adapt to the geometric preference of the bridging species was found to facilitate the formation of N-H bonds via proton-coupled electron transfer to generate a μ-amide product. The structurally analogous μ-silyl- and μ-borylamide complexes were shown to form from the net insertion of the nitride into the E-H bonds (E=B, Si). Protonation of the parent bridging amide produced ammonia in high yield, and treatment of the nitride with PhSH was found to liberate NH3 in high yield through a reaction that engages the redox-activity of the ligand during PCET.

Aryl Fluoride Activation Through Palladium-Magnesium Bimetallic Cooperation: A Mechanistic and Computational Study

Herein is described a mechanistic study of a palladium-catalyzed cross-coupling of aryl Grignard reagents to fluoroarenes that proceeds via a low-energy heterobimetallic oxidative addition pathway. Traditional oxidative additions of aryl chlorides to Pd complexes are known to be orders of magnitude faster than with aryl fluorides, and many palladium catalysts do not activate aryl fluorides at all. The experimental and computational studies outlined herein, however, support the view that at elevated Grignard : ArX ratios (i.e. 2.5 : 1) a Pd-Mg heterobimetallic mechanism predominates, leading to a remarkable decrease in the energy required for Ar-F bond activation. The heterobimetallic transition state for C-X bond cleavage is proposed to involve simultaneous Pd backbonding to the arene and Lewis acid activation of the halide by Mg to create a low-energy transition state for oxidative addition. The insights gained from this computational study led to the development of a phosphine ligand that was shown to be similarly competent for Ar-F bond activation.

Reversible nickel-metallacycle formation with a phosphinimine-based pincer ligand

Pincer ligands have a remarkable ability to impart control over small molecule activation chemistry and catalytic activity; therefore, the design of new pincer ligands and the exploration of their reactivity profiles continues to be a frontier in synthetic inorganic chemistry. In this work, a novel, monoanionic NNN pincer ligand containing two phosphinimine donors was used to create a series of mononuclear Ni complexes. Ligand metallation in the presence of NaOPh yielded a nickel phenoxide complex that was used to form a mononuclear hydride complex on treatment with pinacolborane. Attempts at ligand metallation with NaN(SiMe3)2 resulted in the activation of both phosphinimine methyl groups to yield an anionic, cis-dialkyl product, in which dissociation of one phosphinimine nitrogen leads to retention of a square planar coordination environment about Ni. Protonolysis of this dialkyl species generated a monoalkyl product that retained the 4-membered metallacycle. The insertion of 2,6-dimethylphenyl isocyanide (xylNC) into this nickel metallacycle, followed by proton transfer, generated a new five-membered nickel metallacycle. Kinetic studies suggested rate-limiting proton transfer (KIE ≥ 3.9 ± 0.5) from the α-methylene unit of the putative iminoacyl intermediate.

Mimicking the Constrained Geometry of a Nitrogen-Fixation Intermediate

Both biological and industrial nitrogen reduction catalysts activate N₂ at multinuclear binding sites with constrained Fe-Fe distances. This contrasts with molecular diiron systems, which routinely form linear N₂ bridges to minimize steric interactions. Model compounds that capture the salient geometric features of N₂ binding by the nitrogenase enzymes and Mittasch catalysts would contribute to understanding their high N₂-reduction activity. It is shown in the present study that use of a geometrically flexible, dinucleating macrocycle allows for the formation of a bridging N₂ ligand with an unusual Fe-Ct_N2-Fe angle of 150° (Ct_N2 = centroid of N₂), a geometry that approximates the α-N₂ binding mode on Fe(111) surfaces that precedes N₂ bond cleavage. The cavity size of the macrocycle prevents the formation of a linear Fe-N₂-Fe unit and leads to orbital interactions that are distinct from those available to the linear configuration.

Interdependent Metal-Metal Bonding and Ligand Redox-Activity in a Series of Dinuclear Macrocyclic Complexes of Iron, Cobalt, and Nickel

This report describes an isostructural series of dinuclear iron, cobalt, and nickel complexes bound by a redox-active macrocyclic ligand. The series spans five redox levels (34-38 e-/cluster core), allowing for a detailed investigation into both the degree of metal-metal interaction and the extent of ligand-based redox-activity. Magnetometry, electrochemistry, UV-vis-NIR absorption spectroscopy, and crystallography were used in conjunction with DFT computational analyses to extract the electronic structures of the six homodinuclear complexes. The isoelectronic, 34 e- species [(3PDI2)Fe2(PMe3)2(μ-Cl)](OTf) and [(3PDI2)Co2(PMe3)2(μ-Cl)](OTf)3 exhibit metal-metal single bonds, with varying amounts of electron density delocalization into the ligand as a function of the effective nuclear charge of the metal ions. One- and two-electron reductions of [(3PDI2)Co2(PMe3)2(μ-Cl)](OTf)3 lead to isolable products, which show successive increases in both the Co-Co distances and the extent of reduction of the ligand manifold. This trend results from reduction of a Co-Co σ* orbital, which was found to be heavily mixed with the redox-active manifold of the 3PDI2 ligand. A similar trend was observed in the 37 and 38 e- dinickel complexes [(3PDI2)Ni2(PMe3)2(μ-Cl)](OTf)2 and [(3PDI2)Ni2(PMe3)2(μ-Cl)](OTf); however, their higher electron counts lead to high-spin ground states that result from occupation of a high-lying δ/δ* manifold with significant Ni-NPDI σ* character. This change in ground state configuration reforms a M-M bonding interaction in the 37 e- complex, but formation of the 38 e- species again disrupts the M-M bond alongside the transfer of electron density to the ligand.

Tuning Metal-Metal Interactions through Reversible Ligand Folding in a Series of Dinuclear Iron Complexes

A dinucleating macrocyclic ligand with two redox-active, pyridyldiimine components was shown to undergo reversible ligand folding to accommodate various substitution patterns, metal ion spin states, and degrees of Fe-Fe bonding within the cluster. An unfolded-ligand geometry with a rectangular Fe₂(μ-Cl)₂ core and an Fe-Fe distance of 3.3262(5) Å served as a direct precursor to two different folded-ligand complexes. Chemical reduction in the presence of PPh₃ resulted in a diamagnetic, folded ligand complex with an Fe-Fe bonding interaction (d_Fe-Fe = 2.7096(17) Å) between two intermediate spin (S_Fe = 1) Fe(II) centers. Ligand folding was also induced through anion exchange on the unfolded-ligand species, producing a complex with three PhS^- ligands and a temperature-dependent Fe-Fe distance. In this latter example, the weak ligand field of the thiolate ligands led to a product with weakly coupled, high-spin Fe(II) ions (S_Fe = 2; J = -50.1 cm^-1) that form a bonding interaction in the ground state and a nonbonding interaction in the excited state(s), as determined by SQUID magnetometry and variable temperature crystallography. Finally, both folded-ligand complexes were shown to reform an unfolded-ligand geometry through convergent syntheses of a complex with an Fe-Fe bonded Fe₂(μ-SPh)₂ core (d_Fe-Fe = 2.7320(11) Å). Experimentally validated DFT calculations were used to investigate the electronic structures of all species as a way to understand the origin of Fe-Fe bonding interactions, the extent of ligand reduction, and the nature of the spin systems that result from multiple, weakly interacting spin centers.

Oxidation of uranium(iv) mixed imido-amido complexes with PhEEPh and to generate uranium(vi) bis(imido) dichalcogenolates, U(NR)2(EPh)2(L)2

This work provides new routes for the conversion of U(iv) into U(vi) bis(imido) complexes and offers new information on the manner in which the U(vi) compounds form. Many compounds from the series described by the general formula U(NR)₂(EPh)₂(L)₂ (R = 2,6-diisopropylphenyl, tert-butyl; E = S, Se, Te; L = py, EPh) were synthesized via oxidation of an in situ generated U(iv) amido-imido species with Ph₂E₂. This synthetic sequence provides a general route into bis(imido) U(vi) chalcogenolate complexes, circumventing the need to perform problematic salt metathesis reactions on U(vi) iodides. Investigation into the speciation of the U(iv) complexes that form prior to oxidation found a significant dependence on the identity of the ancillary ligands, with ^tBu₂bpy forming the isolable imido-(bis)amido complex, U(NDipp)(NHDipp)₂(^tBu₂bpy)₂. Together, these data are consistent with the view that the bis(imido) U(vi) motif - much like the uranyl ion, UO₂²⁺- is a thermodynamic sink into which simple ligand frameworks are unable to prevent uranium from falling when in the presence of a suitable retinue of imido proligands.

Ligand-Directed Reactivity in Dioxygen and Water Binding to cis-[Pd(NHC)2(η2-O2)]

Reaction of [Pd(IPr)₂] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and O₂ leads to the surprising discovery that at low temperature the initial reaction product is a highly labile peroxide complex cis-[Pd(IPr)₂(η²-O₂)]. At temperatures ≳ -40 °C, cis-[Pd(IPr)₂(η²-O₂)] adds a second O₂ to form trans-[Pd(IPr)₂(η¹-O₂)₂]. Squid magnetometry and EPR studies yield data that are consistent with a singlet diradical ground state with a thermally accessible triplet state for this unique bis-superoxide complex. In addition to reaction with O₂, cis-[Pd(IPr)₂(η²-O₂)] reacts at low temperature with H₂O in methanol/ether solution to form trans-[Pd(IPr)₂(OH)(OOH)]. The crystal structure of trans-[Pd(IPr)₂(OOH)(OH)] is reported. Neither reaction with O₂ nor reaction with H₂O occurs under comparable conditions for cis-[Pd(IMes)₂(η²-O₂)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene). The increased reactivity of cis-[Pd(IPr)₂(η²-O₂)] is attributed to the enthalpy of binding of O₂ to [Pd(IPr)₂] (-14.5 ± 1.0 kcal/mol) that is approximately one-half that of [Pd(IMes)₂] (-27.9 ± 1.5 kcal/mol). Computational studies identify the cause as interligand repulsion forcing a wider C-Pd-C angle and tilting of the NHC plane in cis-[Pd(IPr)₂(η²-O₂)]. Arene-arene interactions are more favorable and serve to further stabilize cis-[Pd(IMes)₂(η²-O₂)]. Inclusion of dispersion effects in DFT calculations leads to improved agreement between experimental and computational enthalpies of O₂ binding. A complete reaction diagram is constructed for formation of trans-[Pd(IPr)₂(η¹-O₂)₂] and leads to the conclusion that kinetic factors inhibit formation of trans-[Pd(IMes)₂(η¹-O₂)₂] at the low temperatures at which it is thermodynamically favored. Failure to detect the predicted T-shaped intermediate trans-[Pd(NHC)₂(η¹-O₂)] for either NHC = IMes or IPr is attributed to dynamic effects. A partial potential energy diagram for initial binding of O₂ is constructed. A range of low-energy pathways at different angles of approach are present and blur the distinction between pure "side-on" or "end-on" trajectories for oxygen binding.

Ring-Size-Modulated Reactivity of Putative Dicobalt-Bridging Nitrides: C-H Activation versus Phosphinimide Formation

Dicobalt complexes supported by flexible macrocyclic ligands were used to target the generation of the bridging nitrido species [(ⁿ PDI₂ )Co₂ (μ-N)(PMe₃ )₂ ]³⁺ (PDI=2,6-pyridyldiimine; n=2, 3, corresponding to the number of catenated methylene units between imino nitrogen atoms). Depending on the size of the macrocycle and the reaction conditions (solution versus solid-state), the thermolysis of azide precursors yielded bridging phosphinimido [(² PDI₂ )Co₂ (μ-NPMe₃ )(PMe₃ )₂ ]³⁺ , amido [(ⁿ PDI₂ )Co₂ (μ-NH₂ )(PMe₃ )₂ ]³⁺ (n=2, 3), and C-H amination [(³ PDI₂ *-μ-NH)Co₂ (PMe₃ )₂ ]³⁺ products. All results are consistent with the initial formation of [(ⁿ PDI₂ )Co₂ (μ-N)(PMe₃ )₂ ]³⁺ , followed by 1) PMe₃ attack on the nitride, 2) net hydrogen-atom transfer to form N-H bonds, or 3) C-H amination of the alkyl linker of the ⁿ PDI₂ ligand.

Positional Selectivity in C-H Functionalizations of 2-Benzylfurans with Bimetallic Catalysts

Metal-catalyzed carbon-carbon bond-forming reactions are a mainstay in the synthesis of pharmaceutical agents. A long-standing problem plaguing the field of transition metal catalyzed C-H functionalization chemistry is control of selectivity among inequivalent C-H bonds in organic reactants. Herein we advance an approach to direct site selectivity in the arylation of 2-benzylfurans founded on the idea that modulation of cooperativity in bimetallic catalysts can enable navigation of selectivity. The bimetallic catalysts introduced herein exert a high degree of control, leading to divergent site-selective arylation reactions of both sp(2) and sp(3) C-H bonds of 2-benzylfurans. It is proposed that the selectivity is governed by cation-π interactions, which can be modulated by choice of base and accompanying additives [MN(SiMe3)2, M = K or Li·12-crown-4].

Voltage clustering in redox-active ligand complexes: mitigating electronic communication through choice of metal ion

Homoleptic bis(imino)acenaphthene complexes of aluminum, chromium, and gallium were synthesized, characterized and modeled to compare the orbital contributions of main group elements and transition metals in ligand-based redox events toward increasing energy density of battery electrolytes.

Crystal structure of 2-azido-1H-imidazole-4,5-di-carbonitrile

In the title compound, C5HN7, the nitrile and azido substituents are close to being coplanar with the central ring. Mol-ecules in the crystal are linked via an N-H⋯N hydrogen bond to a nitrile acceptor, forming a chain extending along the c-axis direction.

Crystal structure of 4,5-di-nitro-1H-imidazole

The title compound, C3H2N4O4, forms crystals with two mol-ecules in the asymmetric unit which are conformationally similar. With the exception of the O atoms of the nitro groups, the mol-ecules are essentially planar. In the crystal, adjacent mol-ecules are associated by N-H⋯N hydrogen bonds involving the imidazole N-H donors and N-atom acceptors of the unsaturated nitro-gen of neighboring rings, forming layers parallel to (010).

Re-evaluating the Cu K pre-edge XAS transition in complexes with covalent metal-ligand interactions

Three [Me2NN]Cu(η2-L2) complexes (Me2NN = HC[C(Me)NAr]2; L2 = PhNO (2), (3), PhCH[double bond, length as m-dash]CH2 (4); Ar = 2,6-Me2-C6H3; ArF = 3,5-(CF3)2-C6H3) have been studied by Cu K-edge X-ray absorption spectroscopy, as well as single- and multi-reference computational methods (DFT, TD-DFT, CASSCF, MRCI, and OVB). The study was extended to a range of both known and theoretical compounds bearing 2p-element donors as a means of deriving a consistent view of how the pre-edge transition energy responds in systems with significant ground state covalency. The ground state electronic structures of many of the compounds under investigation were found to be strongly influenced by correlation effects, resulting in ground state descriptions with majority contributions from a configuration comprised of a Cu(ii) metal center anti-ferromagentically coupled to radical anion O2, PhNO, and ligands. In contrast, the styrene complex 4, which displays a Cu K pre-edge transition despite its formal d10 electron configuration, exhibits what can best be described as a Cu(i):(styrene)0 ground state with strong π-backbonding. The Cu K pre-edge features for these complexes increase in energy from 1 to 4, a trend that was tracked to the percent Cu(ii)-character in the ground state. The unexpected shift to higher pre-edge transition energies with decreasing charge on copper (Q Cu) contributed to an assignment of the pre-edge features for these species as arising from metal-to-ligand charge transfer instead of the traditional Cu1s → Cu3d designation.

A step beyond the Feltham-Enemark notation: spectroscopic and correlated ab initio computational support for an antiferromagnetically coupled M(II)-(NO)- description of Tp*M(NO) (M = Co, Ni)

Multiple spectroscopic and computational methods were used to characterize the ground-state electronic structure of the novel {CoNO}(9) species Tp*Co(NO) (Tp* = hydro-tris(3,5-Me(2)-pyrazolyl)borate). The metric parameters about the metal center and the pre-edge region of the Co K-edge X-ray absorption spectrum were reproduced by density functional theory (DFT), providing a qualitative description of the Co-NO bonding interaction as a Co(II) (S(Co) = 3/2) metal center, antiferromagnetically coupled to a triplet NO(-) anion (S(NO) = 1), an interpretation of the electronic structure that was validated by ab initio multireference methods (CASSCF/MRCI). Electron paramagnetic resonance (EPR) spectroscopy revealed significant g-anisotropy in the S = ½ ground state, but the linear-response DFT performed poorly at calculating the g-values. Instead, CASSCF/MRCI computational studies in conjunction with quasi-degenerate perturbation theory with respect to spin-orbit coupling were required for obtaining accurate modeling of the molecular g-tensor. The computational portion of this work was extended to the diamagnetic Ni analogue of the Co complex, Tp*Ni(NO), which was found to consist of a Ni(II) (S(Ni) = 1) metal center antiferromagnetically coupled to an S(NO) = 1 NO(-). The similarity between the Co and Ni complexes contrasts with the previously studied Cu analogues, for which a Cu(I) bound to NO(0) formulation has been described. This discrepancy will be discussed along with a comparison of the DFT and ab initio computational methods for their ability to predict various spectroscopic and molecular features.

Z-selective, catalytic internal alkyne semihydrogenation under H2/CO mixtures by a niobium(III) imido complex

The discovery of a Nb(III)-mediated catalytic hydrogenation of internal alkynes to (Z)-alkenes that proceeds through an unprecedented mechanism is reported. The mechanistic proposal involves initial reduction of the alkyne by the Nb(III) complex (BDI)Nb(N(t)Bu)(CO)(2) to provide a Nb(V) metallacyclopropene, itself capable of σ-bond metathesis reactivity with H(2). The resulting alkenyl hydride species then undergoes reductive elimination to provide the (Z)-alkene product and regenerate a metal complex in the Nb(III) oxidation state. Support for the proposed mechanism is derived from (i) the dependence of the product selectivity on the relative concentrations of CO and H(2), (ii) the isolation of complexes closely related to those proposed to be part of the catalytic cycle, (iii) H/D crossover experiments, and (iv) DFT studies of multiple possible reaction pathways.

Synthesis, Characterization, and Reactions of Isolable (β-Diketiminato)Nb(III) Imido Complexes

We have investigated both the chemical reduction of (BDI)Nb(V) imido complexes (BDI = HC[C(Me)NAr](2); Ar = 2,6-(i)Pr(2)-C(6)H(3)) to the formal Nb(III) oxidation state and the ability of these Nb(III) complexes to behave as two-electron reductants. The reduction of the Nb(V) species was found to depend heavily on the nature of available supporting ligands, but the chemistry of the reduced compounds proceeded cleanly with a number of unsaturated organic reagents. Accordingly, the novel Nb(V) bis(imido) complexes supported by the monoazabutadiene (mad) ligand (mad)Nb(N(t)Bu)(NAr)(L') (L' = py, thf) were formed by either KC(8) reduction of (BDI)Nb(N(t)Bu)Cl(2)(py) in the absence of strong π-acids or by H(2) reduction of the Nb(V) dimethyl complex (BDI)Nb(N(t)Bu)Me(2) in THF. These products are likely formed though an intramolecular, 2 e(-) reductive C-N bond cleavage, as has been observed previously for related Group 4 systems, suggesting that transient Nb(III) intermediates were present in both cases. In the presence of 1,2-bis(dimethylphosphino)ethane (dmpe), KC(8) reduction of (BDI)Nb(N(t)Bu)Cl(2)(py) was arrested at the Nb(IV) oxidation state to give (BDI)Nb(N(t)Bu)Cl(dmpe), which was characterized by solution-state EPR spectroscopy as a Nb-centered paramagnet with strong coupling to the two equivalent phosphorus nuclei (A(iso){(93)Nb} = 120.5×10(-4) cm(-1), A(iso){(31)P} = 31.0×10(-4) cm(-1), g(iso) = 1.9815). When strong π-acids were used to intercept the thermally unstable Nb(III) complex (BDI)Nb(N(t)Bu)(py) prior to reductive cleavage of the ligand C-N bond, the thermally stable Nb(III) species (BDI)Nb(N(t)Bu)(CX)(2)(L″) (X = O, L″ = py; X = NXyl, L″ = CNXyl; Xyl = 2,6-Me(2)-C(6)H(3)) were obtained in good yields. The Nb(III) complexes (BDI)Nb(N(t)Bu)py, (BDI)Nb(N(t)Bu)(CO)(2)(py) and (BDI)Nb(N(t)Bu)(CO)(2) were subsequently investigated for their ability to serve as two-electron reducing reagents for both metal-ligand multiple bond formation and for the reduction of organic π-systems. The reduction of mesityl azide by (BDI)Nb(N(t)Bu)(py) and diphenylsulfoxide by (BDI)Nb(N(t)Bu)(CO)(2) led to the monomeric bis(imido) and dimeric oxo complexes (BDI)Nb(N(t)Bu)(NMes)(py) and [(BDI)Nb(N(t)Bu)](2)(μ2-O)(2), respectively. MeLi addition to (BDI)Nb(N(t)Bu)(CO)(2)(py) resulted in the formation of a Nb-acylate via methide addition to one of the carbonyl carbons. The acylate product was revealed to have a short Nb-C(acylate) bond distance (2.059(4) Å), consistent with multiple Nb-C bond character resulting from Nb(III) back-bonding into the acylate carbon. The interaction of (BDI)Nb(N(t)Bu)(CO)(2) with two equivalents of 4,4'-dichlorobenzophenone resulted in the clean, quantitative formation of the corresponding pinacol coupling product, but introduction of the ketone in 1: 1 molar ratios resulted in mixtures of the pinacol product and the starting material, suggesting that ketone coordination to the Nb(III) complex may be reversible. Relatedly, addition of 1-phenyl-1-propyne to (BDI)Nb(N(t)Bu)(CO)(2) formed a thermally unstable 1: 1 Nb/alkyne complex, as characterized by NMR and IR spectroscopies; reaction of this species with HCl/MeOH yielded a 2: 1 mixture of 1-phenyl-1-propene and the free alkyne, suggesting a high degree of covalency in the Nb-C bonds.

Halo, Alkyl, Aryl, and Bis(imido) Complexes of Niobium Supported by the beta-Diketiminato Ligand

Synthesis of the complex (BDI)Nb(N(t)Bu)Cl(2)py (BDI = HC[C(Me)N(2,6-iPr(2)-C(6)H(3))](2)) was achieved in high yield following the treatment of Nb(N(t)Bu)Cl(3)py(2) with Li(BDI)(OEt(2)). Substitution of the chlorides for fluorides was effected by introducing 2.0 equiv of Me(3)SnF in toluene, providing the pyridine-coordinated difluoride complex (BDI)Nb(N(t)Bu)F(2)py in modest yield. The pyridine ligands from both halide compounds were removed by treatment of the pyridine adducts with B(C(6)F(5))(3), affording the Lewis base-free complexes (BDI)Nb(NtBu)X(2) (X = Cl, F). Additionally, the Lewis base-free dichlorides of the (t)Bu-imido and Ar-imido (Ar = 2,6-(i)Pr(2)-C(6)H(3)) complexes were obtained following treatment of Nb(NR)Cl(3)(dme) (R = tBu, Ar) with Li(BDI)(OEt(2)). The pyridine-coordinated dichloride was alkylated and arylated to form the dimethyl complex (BDI)Nb(N(t)Bu)Me(2) (described previously, see text) and the mono(p-tolyl) complex (BDI)Nb(N(t)Bu)Cl(p-tol), the latter of which was methylated with MeMgBr to yield the mixed alkyl/aryl complex (BDI)Nb(N(t)Bu)Me(p-tol) in good yield. A rare example of a Group 5 bis((t)Bu-imido) species was synthesized in good yield via treatment of (BDI)Nb(N(t)Bu)Cl(2)py with 2.0 equiv of LiNHtBu to form (BDI)Nb(NtBu)(2)py. Exchange of the coordinated pyridine ligand for either pyridine-d(5) or dmap (p-(dimethylamino)pyridine) was shown to occur through a dissociative mechanism, allowing for removal of the coordinated Lewis base by treatment with B(C(6)F(5))(3). The resulting average C(2v)-symmetric tetracoordinate bis(imido) complex (BDI)Nb(N(t)Bu)(2) was characterized in solution by NMR spectroscopy and observed to undergo clean thermal decomposition to yield (BDI(#))Nb(N(t)Bu)(NH(t)Bu) (BDI(#) = H(2)C=C(NAr)CH=C(NAr)Me) over several hours at room temperature. Treatment of the four-coordinate bis(imido) with (t)BuNCO resulted in clean [2 + 2] cycloaddition to yield an oxaazametallacyclobutane complex, which was further observed to extrude (t)BuN=C=N(t)Bu over 12 h at room temperature. The molecular structures of (BDI)Nb(N(t)Bu)Cl(2)py, (BDI)Nb(NAr)Cl(2), (BDI)Nb(N(t)Bu)Me(2), (BDI)Nb(N(t)Bu)Cl(p-tol), (BDI)Nb(N(t)Bu)(2)py, and (BDI)Nb(NtBu)(2)(dmap) were determined crystallographically. Finally, DFT (BP86) geometry optimization calculations on a model complex of the thermally unstable four-coordinate bis(imido) species allowed for identification of the orbital interactions leading to activation of the imido groups through mixing with the BDI frontier orbitals.

An unusally diverse array of products formed upon carbonylation of a dialkylniobium complex

The reaction of carbon monoxide with a beta-diketiminato dimethyl niobium complex (BDI)Me2Nb(NtBu) is shown to lead to a variety of products whose distribution displays a remarkable dependence on the reaction conditions. Among these, the products of metal reduction, enediolate formation, and intramolecular C-H activation have been fully characterized. An investigation into the individual steps leading to these products points to a transient initial monoacyl complex, whose fate may be perturbed via reaction conditions to allow for control over the product distribution. Furthermore, the reaction of (BDI)Me2Nb(NCMe3) with XylNC (Xyl = 2,6-Me2C6H3) yields the eta2-ketimine complex (BDI)(Me2C=NXyl)Nb(NCMe3), whose characterization and reactivity enhance our understanding of the sequences involving CO.