Klaus H. Theopold

Variable character of O—O and M—O bonding in side-on (η2) 1:1 metal complexes of O2

The structures and the O—O and M—O bonding characters of a series of reported side-on (η2) 1:1 metal complexes of O2 are analyzed by using density functional theory calculations. Comparison of the calculated and experimental systems with respect to O—O bond distance, O—O stretching frequency, and O—O and M—O bond orders provides new insights into subtle influences relevant to O2 activation processes in biology and catalysis. The degree of charge transfer from the generally electron-rich metals to the dioxygen fragment is found to be variable, such that there are species well described as superoxides, others well described as peroxides, and several cases having intermediate character. Increased charge transfer to dioxygen takes place via overlap of the metal dxy orbital with the in-plane π* orbital of O2 and results in increased M—O bond orders and decreased O—O bond orders. Comparison of theory and experiment over the full range of compounds studied suggests that reevaluation of the O—O bond lengths determined from certain x-ray crystal structures is warranted; in one instance, an x-ray crystal structure redetermination was performed at low temperature, confirming the theoretical prediction. Librational motion of the coordinated O2 is identified as a basis for significant underestimation of the O—O distance at high temperature.

Homogeneous Chromium Catalysts for Olefin Polymerization

Chromium-based heterogeneous catalysts are used extensively for the polymerization of olefins. There are few homogeneous model systems that allow investigation of the reaction mechanisms and unique characteristics of chromium catalysts. Coordinatively unsaturated paramagnetic chromium alkyls containing a Cp*CrIIIR fragment (Cp* = η5-C5Me5, pentamethylcyclopentadienyl) catalyze the polymerization of ethylene and α-olefins. Correlations between molecular structure and catalytic activity are reviewed.

Design of a Monomeric Arsinogallane and Chemical Conversion to Gallium Arsenide

A monomeric arsinogallane containing a covalent gallium-arsenic bond has been prepared, and its molecular structure has been determined by x-ray crystallography. The compound reacted with tert-butanol at ambient temperature to yield the III-V semiconductor gallium arsenide as a finely divided amorphous solid. During the initial stages of the reaction small clusters of gallium arsenide were apparently present in solution. The band gaps of these particles, as observed by their absorption spectra, were larger than that of the bulk material. This work is a step toward the development of new molecular precursors for technologically important materials and the study of quantum size effects in small semiconductor particles.

A Family of Four‐Coordinate Iron(II) Complexes Bearing the Sterically Hindered Tris(pyrazolyl)borato Ligand TptBu,Me

A new family of 14-electron, four-coordinate iron(II) complexes of the general formula [Tp(tBu,Me)FeX] (Tp(tBu,Me) is the sterically hindered hydrotris(3-tert-butyl-5-methyl-pyrazolyl) borate ligand and X=Cl (1), Br, I) were synthesized by salt metathesis of FeX(2) with Tp(tBu,Me)K. The related fluoride complex was prepared by reaction of 1 with AgBF(4). Chloride 1 proved to be a good precursor for ligand substitution reactions, generating a series of four-coordinate iron(II) complexes with carbon, oxygen, and sulphur ligands. All of these complexes were fully characterized by conventional spectroscopic methods and most were characterized by single-crystal X-ray crystallographic analysis. Magnetic measurements for all complexes agreed with a high-spin (d(6), S=2) electronic configuration. The halide series enabled the estimation of the covalent radius of iron in these complexes as 1.24 Å.

Chromium Mediated Reductive Coupling of Isonitrile Forms Unusual Heterocycles

The quintuply bonded α-diimine chromium dimer [(H)L(iPr)Cr]2 reductively couples cyclohexyl isocyanide to produce various novel nitrogen heterocycles. Tetramerization yielded, inter alia, the aromatic squaramidinate, i.e. [C4(NCy)4](2-), whereas hexamerization produces a substituted 1,4-diaza-bicyclo[3.3.0]octadiene dianion. These unprecedented transformations complement the coupling reactions of isoelectronic CO, and they may prove synthetically useful.

A bimetallic N-heterocyclic carbene complex featuring a short Cr–Cr distance

The alkylation of a chelating bis(carbene) complex of CrCl(2) yields an unusual bimetallic complex featuring a short Cr-Cr distance, novel ligand coordination, and CH(3) ligand exchange.

The coordinative properties of bifunctional arsenic ligands in carbonylvanadium compounds

The complexes cis-[η5-CpV(CO)2LL] (LL = Ph2AsCH2CH2AsPh2 (dpase), Ph2AsCH2CH2PPh2 (arphos), and o-phenylenebis(dimethylarsine) (diars)) were isolated from the photo-induced reaction between η5-CpV(CO)4 and the corresponding ligand. With [Et4N][V(CO)6], the monosubstitution products [Et4N]2-[{V(CO)5}2(μ-LL)] (LL = dpase, arphos) are initially obtained. Further irradiation results in the formation of cis-[Et4N][V(CO)4(arphos)] or a mixture of cis-[Et4]N][V(CO)4(dpase)] and cis-[Et4N]2[{V(CO)4}2(μ-dpase)2], respectively. The complexes have been investigated by IR and 51V NMR spectroscopy. Mass spectral data are presented for the neutral complexes.

The direct oxidative addition of O2 to a mononuclear Cr(I) complex is spin forbidden.

Mononuclear chromium(I) alkyne complex (i-Pr2Ph)2nacnacCr(η(2)-C2(SiMe3)2) (1) reacts rapidly with dioxygen to yield chromium(V) dioxo species (i-Pr2Ph)2nacnacCr(O)2 (2). The mechanism of this oxygen cleavage has been studied experimentally and computationally. Isotope labeling studies rule out a direct four-electron oxidative addition of O2 to one chromium atom, which involves a spin-forbidden transformation. Instead, the reaction likely proceeds via an unsymmetric binuclear chromium bis(μ-oxo) complex. The latter has been independently prepared and structurally characterized. Its reactivity with O2 is consistent with the proposed mechanism.

Zeolite MCM-22 Supported Heterogeneous Chromium Catalyst for Ethylene Polymerization

The chromium alkyl complex Cp*Cr(py)Me2 (Cp* = η5-pentamethylcyclopentadienyl, py = pyridine, Me = methyl) has been affixed to the surface of zeolite MCM-22 via methane elimination. The resulting composite material has been characterized by a variety of physical methods. The results are consistent with the presence of isolated organometallic fragments linked to the solid support by a covalent chromium–oxygen bond. Activation of this material with methylaluminoxane (MAO) yielded a highly active catalyst for the polymerization of ethylene. The polyethylene thus formed featured high molecular weights (Mw ∼ 4 × 106) and narrow molecular weight distributions (Mw/Mn ∼ 3.5).

Synthesis and structure determination of [Cp*4Cr4(μ-F)5Cl2]PF6. A tetranuclear chromium complex formed by fluoride abstraction from hexafluorophosphate

ion of chloride from the binuclear chromium alkyl [Cp*Cr(Me)Cl]2 yields the substitutionally labile complex [Cp*Cr(THF)2Me]+ PF6−. This compound catalyzes the polymerization of ethylene, but it slowly decomposes by abstraction of fluoride from hexafluorophosphate and chloride from CH2Cl2. The crystal structure of the tetranuclear chromium complex [Cp*4Cr4(μ-F)5Cl2]+PF6− was determined. Fast atom bombardment mass spectrometry of the decomposition product revealed it to be a mixture of fluoride containing compounds. Magnetic susceptibility measurements indicated strong antiferromagnetic coupling of the S = 3/2 ions via the fluoride bridges.