Controlling the Size of Molecular Copper Clusters Supported by a Multinucleating Macrocycle.

The use of a nonrigid, pyridyldialdimine-derived macrocyclic ligand (PDAI) enabled the synthesis of well-defined mono-, di-, tri-, and tetra-nuclear Cu(I) complexes in good yields through rational synthetic means. Starting from mono- and diargentous PDAI complexes, transmetalation to Cu(I) proceeded smoothly with formation of AgX (X = Cl, I) salts to generate mono-, di-, and trinuclear copper complexes. Monodentate supporting ligands (MeCN, xylNC, PMe, PPh) were found to either transmetallate with or bind various di- and trinuclear clusters.

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 -symmetric diiron complexes. The iron centers in each complex are coordinated in distinct ligand environments, with one (Fe) bound in a pseudo-trigonal bipyramidal geometry by three phosphinimine nitrogens in the equatorial plane, a tertiary amine, and the second metal center (Fe). Fe is coordinated, in turn, by Fe, three ylidic carbons in a trigonal plane, and, in certain cases, by an axial oxygen donor.

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(sp) and C(sp)-C(sp) bond oxidative addition reactions to give thermodynamically favorable products. Treatment of a diiron complex supported by a geometrically and electronically flexible macrocyclic ligand, (PDI)Fe(μ-N)(PPh) (), with stoichiometric amounts of various 4,4'-disubstituted diphenylacetylenes (Ar-C≡C-Ar; X = OMe, H, F, CF) yielded C(sp)-C(sp) bond oxidative addition products. When Ph-C≡C-R substrates were used as substrates (R = Me, Et, Pr, Bu), products of either C(sp)-C(sp) or C(sp)-C(sp) bond activation were obtained, with the less sterically encumbering alkynes exclusively undergoing C(sp)-C(sp) bond activation.

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 the addition of ionic salts to reactions and the incorporation of charged functional groups into organic molecules (both experimentally and computationally), and "externally" the implementation of bulk fields between electrode plates.

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 [M(μ-Cl)] core, particularly with respect to Jahn-Teller distortions. Analyses by UV-vis spectroscopy and SQUID magnetometry revealed deviations in the properties [M(μ-Cl)]-containing complexes bound by standard mononucleating ligands, highlighting the ability of macrocycles to use ring size to control the magnetic interactions of -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 [(Ptren)CuN]BAr (, Ptren = tris[2-(phosphiniminato)ethyl]amine, BAr = tetrakis(pentafluorophenyl)borate) where the number of methyl and phenyl groups in the PR ligand are systematically varied across the series (PR = PMe, PMePh, PMePh). The asymmetric stretching mode of azide in the series is used as a vibrational probe of the small-molecule binding site.

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.

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

A series of tetradentate tris(phosphinimine) ligands (Ptren) was developed and bound to Cu to form the trigonal pyramidal, -symmetric cuprous complexes [Ptren-Cu][BAr ] () (PR = PMe, PMePh, PMePh, PPh, PMe(NEt), BAr = B(CF)). Electrochemical studies on the Cu complexes were undertaken, and the permethylated analog, , was found to display an unprecedentedly cathodic Cu/Cu redox potential (-780 mV Fc/Fc in isobutyronitrile). Elucidation of the electronic structures of density functional theory (DFT) studies revealed atypical valence manifold configurations, resulting from strongly σ-donating phosphinimine moieties in the -plane that destabilize 2 (d /d ) orbital sets and uniquely stabilized (d ) orbitals.

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 -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).

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, [(PDI)Co(μ-N)(PMe)][OTf] (; PDI = pyridyldiimine; = 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.

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.

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 [(PDI)Co(μ-CN)(PMe)][OTf] ([CoCN]) 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.

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.

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.

Mimicking the Constrained Geometry of a Nitrogen-Fixation Intermediate.

Both biological and industrial nitrogen reduction catalysts activate N at multinuclear binding sites with constrained Fe-Fe distances. This contrasts with molecular diiron systems, which routinely form linear N bridges to minimize steric interactions. Model compounds that capture the salient geometric features of N binding by the nitrogenase enzymes and Mittasch catalysts would contribute to understanding their high N-reduction activity.

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.

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 Fe(μ-Cl) core and an Fe-Fe distance of 3.3262(5) Å served as a direct precursor to two different folded-ligand complexes.

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

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)(EPh)(L) (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 PhE. 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.

Ligand-Directed Reactivity in Dioxygen and Water Binding to cis-[Pd(NHC)(η-O)].

Reaction of [Pd(IPr)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and O leads to the surprising discovery that at low temperature the initial reaction product is a highly labile peroxide complex cis-[Pd(IPr)(η-O)]. At temperatures ≳ -40 °C, cis-[Pd(IPr)(η-O)] adds a second O to form trans-[Pd(IPr)(η-O)]. 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.

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 [( PDI )Co (μ-N)(PMe ) ] (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 [( PDI )Co (μ-NPMe )(PMe ) ] , amido [( PDI )Co (μ-NH )(PMe ) ] (n=2, 3), and C-H amination [( PDI *-μ-NH)Co (PMe ) ] products. All results are consistent with the initial formation of [( PDI )Co (μ-N)(PMe ) ] , followed by 1) PMe 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 PDI ligand.

Palladium-Catalyzed Enantioselective Arylation of Aryl Sulfenate Anions: A Combined Experimental and Computational Study.

A novel approach to produce chiral diaryl sulfoxides from aryl benzyl sulfoxides and aryl bromides via an enantioselective arylation of aryl sulfenate anions is reported. A (JosiPhos)Pd-based catalyst successfully promotes the asymmetric arylation reaction with good functional group compatibility. A wide range of enantioenriched diaryl, aryl heteroaryl, and even diheteroaryl sulfoxides were generated.

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

The redox-active bis(imino)acenapthene (BIAN) ligand was used to synthesize homoleptic aluminum, chromium, and gallium complexes of the general formula (BIAN)3M. The resulting compounds were characterized using X-ray crystallography, NMR, EPR, magnetic susceptibility and cyclic voltammetry measurements and modeled using both DFT and ab initio wavefunction calculations to compare the orbital contributions of main group elements and transition metals in ligand-based redox events. Complexes of this type have the potential to improve the energy density and electrolyte stability of grid-scale energy storage technologies, such as redox flow batteries, through thermodynamically-clustered redox events.

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.