Composition Determination of Heterometallic Trinuclear Clusters via Anomalous X-ray and Neutron Diffraction.

Anomalous X-ray diffraction (AXD) and neutron diffraction can be used to crystallographically distinguish between metals of similar electron density. Despite the use of AXD for structural characterization in mixed metal clusters, there are no benchmark studies evaluating the accuracy of AXD toward assessing elemental occupancy in molecules with comparisons with what is determined via neutron diffraction. We collected resonant diffraction data on several homo and heterometallic clusters and refined their anomalous scattering components to determine metal site occupancies.

C-H Insertion from Isolable Copper Benzylidenes.

Despite the utility of copper catalysts for the insertion of carbene moieties into C-H bonds, the copper carbene intermediate often invoked in these transformations has not been isolated. Herein, we describe the synthesis and structural characterization of a series of copper benzylidenes utilizing the sterically encumbered dipyrrin ligand (L)H. These isolated copper carbenes demonstrate intramolecular insertion into the primary C(sp)-H bond of the ligand (L)H and intermolecular insertion into ethereal and allylic C-H bonds.

High-Spin [Fe] Cluster Capable of Pnictogen Atom Capture.

Using a new hexanucleating anildophosphine ligand LH (1,3,5-CH(NHCH-5-F-2-P(Bu))), the all-monovalent [Fe] compound (L)Fe () was isolated and characterized by X-ray diffraction analysis, SQUID magnetometry, Fe Mössbauer spectroscopy, and cyclic voltammetry. The molecular structure of reveals very close Fe-Fe distances of 2.3825(7), 2.

Cluster dynamics of heterometallic trinuclear clusters during ligand substitution, redox chemistry, and group transfer processes.

Stepwise metalation of the hexadentate ligand LH (LH = 1,3,5-CH(NHCH--NHSiMeBu)) affords bimetallic trinuclear clusters (L)FeZn(thf) and (L)FeZn(py). Reactivity studies were pursued to understand metal atom lability as the clusters undergo ligand substitution, redox chemistry, and group transfer processes. Chloride addition to (L)FeZn(thf) resulted in a mixture of species including both all-zinc and all-iron products.

Lewis Acid Supported Nickel Nitrenoids.

Metalation of the polynucleating ligand LH (1,3,5-C H (NC H -4-F-2-NSiMe Bu) ) with two equivalents of Zn(N(SiMe ) ) affords the dinuclear product ( LH )Zn (1), which can be further deprotonated to yield ( L)Zn Li (OEt ) (2). Transmetalation of 2 with NiCl (py) yields the heterometallic, trinuclear cluster ( L)Zn Ni(py) (3). Reduction of 3 with KC affords [KC ][( L)Zn Ni] (4) which features a monovalent Ni centre.

Nitrene transfer from a sterically confined copper nitrenoid dipyrrin complex.

Despite the myriad Cu-catalyzed nitrene transfer methodologies to form new C-N bonds (, amination, aziridination), the critical reaction intermediates have largely eluded direct characterization due to their inherent reactivity. Herein, we report the synthesis of dipyrrin-supported Cu nitrenoid adducts, investigate their spectroscopic features, and probe their nitrene transfer chemistry through detailed mechanistic analyses. Treatment of the dipyrrin Cu complexes with substituted organoazides affords terminally ligated organoazide adducts with minimal activation of the azide unit as evidenced by vibrational spectroscopy and single crystal X-ray diffraction.

High-Spin Superatom Stabilized by Dual Subshell Filling.

Quantum confinement in small symmetric clusters leads to the bunching of electronic states into closely packed shells, enabling the classification of clusters with well-defined valences as superatoms. Like atoms, superatomic clusters with filled shells exhibit enhanced electronic stability. Here, we show that octahedral transition-metal chalcogenide clusters can achieve filled shell electronic configurations when they have 100 valence electrons in 50 orbitals or 114 valence electrons in 57 orbitals.

Reversible C-C Bond Cleavage of a Cobalt Diketimide into an Elusive Cobalt Aryl Nitrenoid Complex.

The reactivity of ( L)Co ( L=5-mesityl-1,9-(trityl)dipyrrin) toward various aryl azides was examined to elucidate the electronic structure and reactivity of dipyrrinato cobalt aryl nitrenoid complexes. Herein, we demonstrate the synthesis of a Co diketimide complex [( L)Co(NC F )] and its reversible C-C bond cleavage to yield a monomeric Co nitrenoid complex ( L)Co(NC F ). Exposure of [( L)Co(NC F )] to an excess amount of an H-atom donor cleanly affords the Co anilide complex ( L)Co(NHC F ).

Revealing redox isomerism in trichromium imides by anomalous diffraction.

In polynuclear biological active sites, multiple electrons are needed for turnover, and the distribution of these electrons among the metal sites is affected by the structure of the active site. However, the study of the interplay between structure and redox distribution is difficult not only in biological systems but also in synthetic polynuclear clusters since most redox changes produce only one thermodynamically stable product. Here, the unusual chemistry of a sterically hindered trichromium complex allowed us to probe the relationship between structural and redox isomerism.

Reversible Scavenging of Dioxygen from Air by a Copper Complex.

We report that exposing the dipyrrin complex (L)Cu(N) to air affords rapid, quantitative uptake of O in either solution or the solid-state to yield (L)Cu(O). The air and thermal stability of (L)Cu(O) is unparalleled in molecular copper-dioxygen coordination chemistry, attributable to the ligand flanking groups which preclude the [Cu(O)] core from degradation. Despite the apparent stability of (L)Cu(O), dioxygen binding is reversible over multiple cycles with competitive solvent exchange, thermal cycling, and redox manipulations.

O-Heterocycle Synthesis via Intramolecular C-H Alkoxylation Catalyzed by Iron Acetylacetonate.

Intramolecular alkoxylation of C-H bonds can rapidly introduce structural and functional group complexities into seemingly simple or inert precursors. The transformation is particularly important due to the ubiquitous presence of tetrahydrofuran (THF) motifs as fundamental building blocks in a wide range of pharmaceuticals, agrochemicals, and natural products. Despite the various synthetic methodologies known for generating functionalized THFs, most show limited functional group tolerance and lack demonstration for the preparation of spiro or fused bi- and tricyclic ether units prevalent in molecules for pharmacological purposes.

Luminescence from open-shell, first-row transition metal dipyrrin complexes.

Several first-row transition metal complexes of the 1,9-bis(2',4',6'-triphenylphenyl)-5-mesityl dipyrrinato ligand and its tetrahalogenated analogues have been synthesized and their luminescence spectra obtained. The protonated ligands, as well as the Li(i), Mn(ii), Cu(i), Cu(ii), and Zn(ii) chelates show appreciable luminescence, despite the paramagnetism of the Mn(ii) and Cu(ii) ions. Fluorescence quantum yields (ΦF) as high as 0.

Spectroscopic Investigation of a Metal-Metal-Bonded Fe Single-Molecule Magnet with an Isolated = / Giant-Spin Ground State.

The metal-metal-bonded molecule [BuN][(L)Fe(dmf)] (Fe) was previously shown to possess a thermally isolated spin = / ground state and found to exhibit slow magnetization relaxation below a blocking temperature of ∼5 K [ , , 13949-13956]. Here, we present a comprehensive spectroscopic investigation of this unique single-molecule magnet (SMM), combining ultrawideband field-swept high-field electron paramagnetic resonance (EPR) with frequency-domain Fourier-transform terahertz EPR to accurately quantify the spin Hamiltonian parameters of Fe. Of particular importance is the near absence of a 4th-order axial zero-field splitting term, which is known to arise because of quantum mechanical mixing of spin states on account of the relatively weak spin-spin (superexchange) interactions in traditional polynuclear SMMs such as the celebrated Mn-acetate.

Syntheses and solid-state structures of two cofacial (bis)dipyrrin dichromium complexes in different charge states.

The dichromium Pacman complex (dmx)CrCl·CHO (1) [(dmx)H is a dimethylxanthene-bridged cofacial (bis)dipyrrin, CHNO] was synthesized by salt metathesis using anhydrous CrCl and previously reported (dmx)K. Treatment of 1 with two equivalents of the reductant potassium graphite afforded K(dmx)CrCl(thf)·0.5CHO·0.

Enantioselective C-H Amination Catalyzed by Nickel Iminyl Complexes Supported by Anionic Bisoxazoline (BOX) Ligands.

The trityl-substituted bisoxazoline (BOX) was prepared as a chiral analogue to a previously reported nickel dipyrrin system capable of ring-closing amination catalysis. Ligand metalation with divalent NiI(py) followed by potassium graphite reduction afforded the monovalent (BOX)Ni(py) (). Slow addition of 1.

Efficient C-H Amination Catalysis Using Nickel-Dipyrrin Complexes.

A dipyrrin-supported nickel catalyst (L)Ni(py) (L: 1,9-di(1-adamantyl)-5-perfluorophenyldipyrrin; py: pyridine) displays productive intramolecular C-H bond amination to afford N-heterocyclic products using aliphatic azide substrates. The catalytic amination conditions are mild, requiring 0.1-2 mol% catalyst loading and operational at room temperature.

C-H Amination Mediated by Cobalt Organoazide Adducts and the Corresponding Cobalt Nitrenoid Intermediates.

Treatment of (L)CoBr (L = 5-mesityl-1,9-(2,4,6-PhCH)dipyrrin) with a stoichiometric amount of 1-azido-4-(-butyl)benzene N(CH--Bu) furnished the corresponding four-coordinate organoazide-bound complex (L)CoBr(N(CH--Bu)). Spectroscopic and structural characterization of the complex indicated redox innocent ligation of the organoazide. Slow expulsion of dinitrogen (N) was observed at room temperature to afford a ligand functionalized product via a [3 + 2] annulation, which can be mediated by a high-valent nitrene intermediate such as a Co iminyl (L)CoBr(N(CH--Bu)) or Co imido (L)CoBr(N(CH--Bu)) complex.

Synthesis and electronic structure studies of a Cr-imido redox series.

The effect of metal identity, d-electron count, and coordination geometry on the electronic structure of a metal-ligand multiple bond (MLMB) is an area of active exploration. Although high oxidation state Cr imidos have been extensively studied, very few reports on low-valent Cr imidos or the interconversion of redox isomers exist. Herein, we report the synthesis and characterization of a family of dipyrrinato Cr imido complexes in oxidation states ranging from CrIII to CrV, showcasing the influence of the weak-field dipyrromethene scaffold on the electronic structure and coordination geometries of these Cr imides.

Electronic Structures and Reactivity Profiles of Aryl Nitrenoid-Bridged Dicopper Complexes.

Dicopper complexes templated by dinucleating, pacman dipyrrin ligand scaffolds (dmx, dmx: dimethylxanthine-bridged, cofacial bis-dipyrrin) were synthesized by deprotonation/metalation with mesitylcopper (CuMes; Mes: mesityl) or by transmetalation with cuprous precursors from the corresponding deprotonated ligand. Neutral imide complexes (dmx)Cu(μ-NAr) (R: Mes, Bu; Ar: 4-MeOCH, 3,5-(FC)CH) were synthesized by treatment of the corresponding dicuprous complexes with aryl azides. While one-electron reduction of (dmx)Cu(μ-N(CHOMe)) with potassium graphite initiates an intramolecular, benzylic C-H amination at room temperature, chemical reduction of (dmx)Cu(μ-NAr) leads to isolable [(dmx)Cu(μ-NAr)] product salts.

Synthesis, characterization and C-H amination reactivity of nickel iminyl complexes.

Metalation of the deprotonated dipyrrin (L)Li with NiCl(py) afforded the divalent Ni product (L)NiCl(py) () (L: 1,9-di(1-adamantyl)-5-perfluorophenyldipyrrin; py: pyridine). To generate a reactive synthon on which to explore oxidative group transfer, we used potassium graphite to reduce , affording the monovalent Ni synthon (L)Ni(py) () and concomitant production of a stoichiometric equivalent of KCl and pyridine. Slow addition of mesityl- or 1-adamantylazide in benzene to afforded the oxidized Ni complexes (L)Ni(NMes) () and (L)Ni(NAd) (), respectively.

The Myth of d Copper(III).

Seventeen Cu complexes with formal oxidation states ranging from Cu to Cu are investigated through the use of multiedge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations. Analysis reveals that the metal-ligand bonding in high-valent, formally Cu species is extremely covalent, resulting in Cu K-edge and L-edge spectra whose features have energies that complicate physical oxidation state assignment. Covalency analysis of the Cu L-edge data reveals that all formally Cu species have significantly diminished Cu d-character in their lowest unoccupied molecular orbitals (LUMOs).

Direct Manipulation of Metal Imido Geometry: Key Principles to Enhance C-H Amination Efficacy.

We report the catalytic C-H amination mediated by an isolable Co imido complex (L)Co(NR) supported by a sterically demanding dipyrromethene ligand (L = 5-mesityl-1,9-(trityl)dipyrrin). Metalation of (L)Li with CoCl in THF afforded a high-spin ( = 3/2) three-coordinate complex (L)CoCl. Chemical reduction of (L)CoCl with potassium graphite yielded the high-spin ( = 1) Co synthon (L)Co which is stabilized through an intramolecular η-arene interaction.

Synthesis of a copper-supported triplet nitrene complex pertinent to copper-catalyzed amination.

Terminal copper-nitrenoid complexes have inspired interest in their fundamental bonding structures as well as their putative intermediacy in catalytic nitrene-transfer reactions. Here, we report that aryl azides react with a copper(I) dinitrogen complex bearing a sterically encumbered dipyrrin ligand to produce terminal copper nitrene complexes with near-linear, short copper-nitrenoid bonds [1.745(2) to 1.

Diiron oxo reactivity in a weak-field environment.

Concomitant deprotonation and metalation of a dinucleating cofacial Pacman dipyrrin ligand platform dmxH with Fe(Mes) results in formation of a diiron complex ( dmx)Fe(Mes). Treatment of ( dmx)Fe(Mes) with one equivalent of water yields the diiron μ-oxo complex ( dmx)Fe(μ-O) and free mesitylene. A two-electron oxidation of ( dmx)Fe(μ-O) gives rise to the diferric complex ( dmx)Fe(μ-O)Cl, and one-electron reduction from this FeFe state allows for isolation of a mixed-valent species [CpCo][( dmx)Fe(μ-O)Cl].

Exposing the inadequacy of redox formalisms by resolving redox inequivalence within isovalent clusters.

In this report we examine a family of trinuclear iron complexes by multiple-wavelength, anomalous diffraction (MAD) to explore the redox load distribution within cluster materials by the free refinement of atomic scattering factors. Several effects were explored that can impact atomic scattering factors within clusters, including 1) metal atom primary coordination sphere, 2) M-M bonding, and 3) redox delocalization in formally mixed-valent species. Complexes were investigated which vary from highly symmetric to fully asymmetric by Fe Mössbauer and X-ray diffraction to explore the relationship between MAD-derived data and the data available from these widely used characterization techniques.