Christopher C. Scarborough

Synthesis and isolation of a stable, axially-chiral seven-membered N-heterocyclic carbene

A chiral seven-membered N-heterocyclic carbene (NHC) has been synthesized from its phenol adduct (NHC-HOPh) by a novel base-induced alpha-elimination method, and its donor strength has been determined from the IR stretching frequencies of the NHC-Rh(CO)(2)Cl complex.

Characterization of the Fe-H bond in a three-coordinate terminal hydride complex of iron(I).

Hydride complexes of transition metals play a central role in organometallic chemistry.[1, 2] They are also implicated in biological inorganic chemistry, where hydrides are known or thought to be present in key intermediates in H2 utilization by hydrogenases[3, 4] and in N2 reduction by iron-molybdenum nitrogenases.[5, 6] In both cases, trapped intermediates exhibit large 1H hyperfine couplings from hydrides bonded to paramagnetic iron ion(s).[7-11] As these biological hydrides arise in catalytic intermediates that are not amenable to crystallographic characterization, it is essential to identify the spectroscopic signatures of crystallographically characterized transition-metal hydride complexes.[12]

Isolation and characterization of stable iron(I) sulfide complexes.

Iron-sulfur clusters are widespread in metalloproteins, where they most often function to transfer electrons, but also can act as sites for catalysis.[1] In known iron sulfide clusters, the iron ions are in the +2 and +3 oxidation states.[2,3] Even in synthetic chemistry, with a much broader range of supporting ligands, the iron ions in iron sulfide complexes are always Fe2+ or Fe3+. Synthetic all-Fe2+ clusters using cyanide or N-heterocyclic carbene ligands are a recent advance.[4] However, there are no reports of iron-sulfide compounds in which iron ions are reduced to the Fe1+ level.[5] In this contribution, we describe the first examples of isolable iron(I)-sulfide compounds, which establishes that iron(I) is a feasible oxidation state in iron sulfide chemistry.

Hydrogen Peroxide Complex of Zinc

Metal(H2O2) complexes have been implicated in kinetic and computational studies but have never been observed. Accordingly, H2O2 has been described as a very weak ligand. We report the first metal(H2O2) adduct, which is made possible by incorporating intramolecular hydrogen-bonding interactions with bound H2O2. This Zn(II)(H2O2) complex decays in solution by a second-order process that is slow enough to enable characterization of this species by X-ray crystallography. This report speaks to the intermediacy of metal(H2O2) adducts in chemistry and biology and opens the door to exploration of these species in oxidation catalysis.

Synthesis and Catalytic Reactivity of a Dicopper(II) μ-η2:η2-Peroxo Species Supported by 1,4,7-Tri-tert-butyl-1,4,7-triazacyclononane

O2-derived Cu(n)O2 adducts are attractive targets for aerobic oxidation catalysis because of their remarkable reactivity, but oxidation of the supporting ligand limits catalytic turnover. We report that (t)Bu3tacn (1,4,7-tri-tert-butyl-1,4,7-triazacyclononane) supports a dicopper(II) μ-η(2):η(2)-peroxo species with the highest solution stability outside of an enzyme. Decomposition of this species proceeds without oxidation of the (t)Bu3tacn ligand. Additive-free catalytic aerobic oxidation reactions at or above room temperature are described, highlighting the potential of oxidatively robust ligands in aerobic copper catalysis.

1,4,7-Triazacyclononane ligands bearing tertiary alkyl nitrogen substituents.

The first synthesis of 1,4,7-tri-tert-butyl-1,4,7-triazacyclononane ((t)Bu3tacn) and its adamantyl analog Ad3tacn are described. Cr(II), Mn(II), Fe(II), Co(II), Ni(II), and Cu(I) compounds of (t)Bu3tacn are reported: the steric properties of this ligand enforce four-coordinate geometries except in the case of five-coordinate Cr(II), enabling design of pseudotetrahedral compounds bearing this tridentate redox-inert ancillary ligand.

Hydrogen Peroxide Coordination to Cobalt(II) Facilitated by Second-Sphere Hydrogen Bonding

M(H2 O2 ) adducts have been postulated as intermediates in biological and industrial processes; however, only one observable M(H2 O2 ) adduct has been reported, where M is redox-inactive zinc. Herein, direct solution-phase detection of an M(H2 O2 ) adduct with a redox-active metal, cobalt(II), is described. This Co(II) (H2 O2 ) compound is made observable by incorporating second-sphere hydrogen-bonding interactions between bound H2 O2 and the supporting ligand, a trianionic trisulfonamido ligand. Thermodynamics of H2 O2 binding and decay kinetics of the Co(II) (H2 O2 ) species are described, as well as the reaction of this Co(II) (H2 O2 ) species with Group 2 cations.

Heterotrimetallic sandwich complexes supported by sulfonamido ligands

CoII complexes bearing sulfonamido ligands derived from tris(2-aminoethyl)amine (H6tren) assemble into complex architectures in the presence of Group II ions through interactions between the Group II ion and the sulfonyl oxygens. Novel ligands where the sulfonyl groups bear tert-butyl substituents afford well-defined isostructural heterotrimetallic sandwich complexes where two anionic [LCoII]− fragments are bridged by a Group II metal ion (Mg2+, Ca2+, Sr2+, or Ba2+). Solution studies on these novel sandwich complexes by cyclic voltammetry and electronic absorption spectroscopy indicate that these structures are not dramatically altered in solution, opening the door to the exploration of reactivity at CoII that is modified by Group II ions.

An investigation of the electron density of a Jahn-Teller-distorted Cr(II) cation: the crystal structure and charge density of hexakis(acetonitrile-κN)chromium(II) bis(tetraphenylborate) acetonitrile disolvate.

In the crystal structure of the title homoleptic Cr(II) complex, [Cr(CH3CN)6](C24H20B)2·CH3CN, the [Cr(CH3CN)6](2+) cation is a high-spin d(4) complex with strong static, rather than dynamic, Jahn-Teller distortion. The electron density of the cation was determined by single-crystal X-ray refinements using aspherical structure factors from wavefunction calculations. The detailed picture of the electronic density allowed us to assess the extent and directionality of the Jahn-Teller distortion of the Cr(II) cation away from idealized octahedral symmetry. The topological analysis of the aspherical d-electron density about the Cr(II) cation showed that there are significant valence charge concentrations along the axial Cr-N axes. Likewise, there were significant valence charge depletions about the Cr(II) cation along the equatorial Cr-N bonds. These charge concentrations are in accordance with a Jahn-Teller-distorted six-coordinate complex.

Coordination of Hydrogen Peroxide with Late-Transition-Metal Sulfonamido Complexes

Adducts of hydrogen peroxide and transition metals have been implicated as intermediates in biological and industrial processes but have only recently been observed. Therefore, knowledge of how hydrogen peroxide interacts with transition metals is extremely limited. Herein, we report the synthesis of H2O2 complexes of cobalt, nickel, and copper supported by sulfonamido ligands with second-sphere hydrogen bonding. Binding constant and decay kinetics are reported for four new M(H2O2) adducts, providing a foundation for future studies in H2O2 coordination and oxidation catalysis.