John F. Berry

An octahedral coordination complex of iron(VI)

The hexavalent state, considered to be the highest oxidation level accessible for iron, has previously been found only in the tetrahedral ferrate dianion, FeO42–. We report the photochemical synthesis of another Fe(VI) compound, an octahedrally coordinated dication bearing a terminal nitrido ligand. Mössbauer and x-ray absorption spectra, supported by density functional theory, are consistent with the octahedral structure having an FeN triple bond of 1.57 angstroms and a singlet \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{d}_{xy}^{2}\) \end{document} ground electronic configuration. The compound is stable at 77 kelvin and yields a high-spin Fe(III) species upon warming.

TERMINAL NITRIDO AND IMIDO COMPLEXES OF THE LATE TRANSITION METALS

Compounds that contain a late transition metal-nitrogen multiple bond represent important reactive intermediate species in many useful organic and inorganic transformations. In order to understand the role that these intermediates play in reactions, it is important to have a full understanding of their electronic structure. This paper reviews the known structural motifs that occur in late transition metal nitrido and imido compounds, and provides a correlation between geometric structure and electronic structure. Also, intermediate species that have been postulated but not yet isolated are discussed, as these compounds represent exciting targets for further efforts in synthetic inorganic chemistry.

Direct Spectroscopic Characterization of a Transitory Dirhodium Donor-Acceptor Carbene Complex

Catching a Carbene >> Divalent carbon fragments, or carbenes, vary widely in their stability, depending on their substituents. Some, such as N-heterocyclic carbenes, are independently isolatable. Others can be isolated in coordination complexes with metals. Kornecki et al. (p. 351, published online 12 September) synthesized a carbene coordinated to a rhodium dimer representative of an elusive class of short-lived intermediates long postulated to underlie a series of cyclopropanation and C-H insertion reactions. A long-postulated reactive chemical intermediate has been observed and characterized. A multitude of organic transformations catalyzed by dirhodium(II) (Rh2) complexes are thought to proceed via the intermediacy of highly reactive, electrophilic carbenoid intermediates that have eluded direct observation. Herein, we report the generation of a metastable Rh2-carbenoid intermediate supported by a donor-acceptor carbene fragment. This intermediate is stable for a period of ~20 hours in chloroform solution at 0°C, allowing for an exploration of its physical and chemical properties. The Rh=C bond, characterized by vibrational and nuclear magnetic resonance spectroscopy, extended x-ray absorption fine structure analysis, and quantum-chemical calculations, has weak σ and π components. This intermediate performs stoichiometric cyclopropanation and C–H functionalization reactions to give products that are identical to those obtained from analogous Rh2 catalysis.

Delocalized Metal–Metal and Metal–Ligand Multiple Bonding in a Linear RuRuN Unit: Elongation of a Traditionally Short RuN Bond

unmatched ability to catalyze reactions that directly functionalize C H bonds. These metal–metal bonded catalysts function by assisting the transfer of a carbene or nitrene group (CR2 or NR, respectively) to an organic substrate. [1] The key intermediates in these C H activation reactions are proposed to have structures such as B (Scheme 1) that feature both a metal–metal bond and a metal–ligand multiple bond. Despite many years of mechanistic study on these reactions, no multiply bonded species, such as B, has, to our knowledge, ever been isolated and characterized. In order to synthesize an M M=E metal–metal/metal–ligand multiply bonded system, we chose to target a Ru Ru N nitrido complex (Scheme 1, C) for which no structural precedents exist. This species could be synthesized from thermal or photolytic decomposition of the appropriate Ru Ru N3 azido precursor. This experimental strategy is attractive because synthetically useful [Ru2(L)4X] compounds (L = ligand) are well known and also because Ru is known to stabilize mononuclear nitrido Ru complexes that form a useful comparison to C. We used the previously reported azido compound [Ru2(dPhf)4N3] (2, dPhf = N,N’-diphenylformamidinate) [6] as a precursor for the photoreaction (Scheme 2), since it has

Metal–Metal Bonds in Chains of Three or More Metal Atoms: From Homometallic to Heterometallic Chains

Extended metal atom chains (EMACs) have attracted attention for their unique structural and bonding features. They allow for a systematic study of metal–metal bonding in discrete, oligomeric, polymetallic one-dimensional molecules. Because of their shape and bonding patterns, these complexes are often considered potential molecular wires for molecular electronic applications. As such, the electronic structure of the simplest EMACs, i.e., those that consist of three metals linked together, the ligand systems that have been used to support EMACs, and preliminary work on the conductance of EMACs at the molecular level are discussed. New heterometallic EMACs have also been recently synthesized and are discussed here. While these molecules may be of interest in molecular electronic applications, they also serve as a testing ground for studying the nature of heterometallic electronic effects.

Making connections with molecular wires: extending tri-nickel chains with axial cyanide, dicyanamide, and phenylacetylide ligands

New chemistry needed to facilitate the replacement of axial ligands (X) in Ni3(dpa)4X2 complexes (dpa = the anion of dipyridylamine) has been explored and used to replace X = Cl by X = CN, NCNCN, and CCPh. The resulting compounds, 3, 4, and 5, respectively, have been characterized by X-ray crystallography and cyclic voltammetry, inter alia. It is found that both the mean Ni⋯Ni separations (D), and the magnitude of the antiferromagnetic coupling (J) between the terminal, high spin (S = 1) Ni(II) atoms vary in a correlated way, with |J| decreasing with increasing D. The relationship is nearly linear over the available ranges of parameters, suggesting that the coupling may proceed mainly through the central, diamagnetic Ni(II) ion.

Oxygen Activation by Co(II) and a Redox Non-Innocent Ligand: Spectroscopic Characterization of a Radical-Co(II)-Superoxide Complex with Divergent Catalytic Reactivity.

Bimetallic (Et4N)2[Co2(L)2], (Et4N)2[1] (where (L)(3-) = (N(o-PhNC(O)(i)Pr)2)(3-)) reacts with 2 equiv of O2 to form the monometallic species (Et4N)[Co(L)O2], (Et4N)[3]. A crystallographically characterized analog (Et4N)2[Co(L)CN], (Et4N)2[2], gives insight into the structure of [3](1-). Magnetic measurements indicate [2](2-) to be an unusual high-spin Co(II)-cyano species (S = 3/2), while IR, EXAFS, and EPR spectroscopies indicate [3](1-) to be an end-on superoxide complex with an S = 1/2 ground state. By X-ray spectroscopy and calculations, [3](1-) features a high-spin Co(II) center; the net S = 1/2 spin state arises after the Co electrons couple to both the O2(•-) and the aminyl radical on redox non-innocent (L(•))(2-). Dianion [1](2-) shows both nucleophilic and electrophilic catalytic reactivity upon activation of O2 due to the presence of both a high-energy, filled O2(-) π* orbital and an empty low-lying O2(-) π* orbital in [3](1-).

Remarkable regioselectivity in the preparation of the first heterotrimetallic Mo[quadruple bond]W...Cr chain.

Addition of CrCl(2) to the dinuclear synthon MoW(dpa)(4) yields a regioselectively formed heterotrimetallic Mo[quadruple bond]W...Cr chain; computational studies suggest that the polarization of the Mo[quadruple bond]W quadruple bond partially accounts for this unexpected selectivity.

Extended Metal Atom Chains

15.1 Overview The previous chapters of this book have shown that the chemistry of dinuclear compounds with metal–metal bonds is extensive. But why should this chemistry be limited to bonds between only two metal atoms? As will be seen in this chapter, it is not. By using expanded bridging ligands as in 15.1, it is possible to synthesize extended metal atom chain (EMAC) compounds. Such EMACs with polypyridylamido or related ligands will be the main subject of this chapter. Brief reviews on this subject have appeared.

Electrophilic aryl C–H amination by dimetal nitrides: correlating electronic structure with reactivity

Using density functional methods we have modelled the intramolecular electrophilic aryl C–H amination for 15 dimetal nitrides, both homo- and heteronuclear, along with 2 mononuclear nitrides, in the pursuit of understanding the reactivity of the dimetal nitrido Ru2(DPhF)4N (DPhF = N,N′-diphenyl formamidinate) molecule, for which this amination reaction was experimentally observed and characterized. It was found that the 3-center bonding manifold (MMN) that arises between the metal–metal bond and axial nitrido moiety has a dominant influence in the electronic structure and consequently the reactivity at each step in the reaction. It was found that transition state energetics correlate strongly with product stabilization and that these quantities depend on the number of electrons available to occupy the MMN manifold. As the reaction proceeds the number of orbitals in the manifold decreases by one and the point at which this happens determines which of two transition states is rate limiting. The dimetallic nitrides are shown to be inherently more reactive than the mononuclear complexes and so the MMN manifold that is only active in the dimetallic complexes comes through as an important factor in facilitating this amination reaction. Overall, a strong correlation between electronic structure and reactivity is established for C–H amination and new synthetic targets are proposed to develop new facets of this reactivity.