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J P Collman

Publications and source records attributed to J P Collman.

At least 19 recordsLinked to original sources
PubMed2001

Mechanism of dihydrogen cleavage by high-valent metal oxo compounds: experimental and computational studies.

The oxidation of dihydrogen by metal tetraoxo compounds was investigated. Kinetic measurements of the oxidations of H(2) by MnO(4)(-) and RuO(4), performed by UV-vis spectroscopy, showed these reactions to be quite rapid at 25 degrees C (k(1) approximately (3-6) x 10(-2) M(-1) s(-1)). Rates measured for H(2) oxidation by MnO(4)(-) in aqueous solution (using KMnO(4)) and in chlorobenzene (using (n)Bu(4)NMnO(4)) revealed only a minor solvent effect on the reaction rate. Substantial kinetic isotope effects [(k(H)2/k(D)2 = 3.8(2) (MnO(4)(-), aq), 4.5(5) (MnO(4)(-), C(6)H(5)Cl soln), and 1.8(6) (RuO(4), CCl(4) soln)] indicated that H-H bond cleavage is rate determining and that the mechanism of dihydrogen cleavage is likely similar in aqueous and organic solutions. Third-row transition-metal oxo compounds, such as OsO(4), ReO(4)(-), and MeReO(3), were found to be completely unreactive toward H(2). Experiments were performed to probe for a catalytic hydrogen/deuterium exchange between D(2) and H(2)O as possible evidence of dihydrogen sigma-complex intermediates, but no H/D exchange was observed in the presence of various metal oxo compounds at various pH values. In addition, no inhibition of RuO(4)-catalyzed hydrocarbon oxidation by H(2) was observed. On the basis of the available evidence, a concerted mechanism for the cleavage of H(2) by metal tetraoxo compounds is proposed. Theoretical models were developed for pertinent MnO(4)(-) + H(2) transition states using density functional theory in order to differentiate between concerted [2 + 2] and [3 + 2] scissions of H(2). The density functional theory calculations strongly favor the [3 + 2] mechanism and show that the H(2) cleavage shares some mechanistic features with related hydrocarbon oxidation reactions. The calculated activation energy for the [3 + 2] pathway (DeltaH(++) = 15.4 kcal mol(-1)) is within 2 kcal mol(-1) of the experimental value.

Journal Article↗
PubMed2001

Superimposed saddle and ruffled distortions of the porphyrin in iodo(pyridine-N)(5,10,15,20-tetraphenylporphyrinato-kappa(4)N)rhodium(III) toluene solvate.

In the title compound, [RhI(C(44)H(28)N(4))(C(5)H(5)N)].C(7)H(8), the porphyrin ring experiences significant distortion from planarity (a saddle conformation with a superimposed ruffling), as a result of steric interactions with the 2,6-H atoms of the axial pyridine ligand. This also leads to a slight lengthening of the Rh-pyridine bond [Rh-N 2.102 (7) A] relative to those seen in other pyridine adducts of six-coordinate Rh(III). The metric parameters of the porphyrin core are comparable with those of related metalloporphyrin derivatives. No significant intermolecular interactions are observed between the metalloporphyrin and disordered solvate species.

Journal Article↗
PubMed2000

Formation of charge-transfer complexes from neutral bis(porphyrin) sandwiches.

Lanthanide(III) bis(porphyrin) sandwich complexes of octaethyltetraazaporphyrin (OETAP) were synthesized and characterized by UV-vis, IR, and NMR spectroscopies. Cyclic voltammetry results indicate that these neutral sandwich complexes are very easily reduced. Charge-transfer reactions were performed in solution with Ln-(OETAP)2 sandwiches and zirconium(IV) bis(porphyrin) sandwiches. The lanthanide sandwiches partially oxidize the zirconium sandwiches in solution, and a solvent dependence of the charge-transfer reaction was observed. The solid-state properties of these charge-transfer materials were also studied. Magnetic susceptibility results suggest weak intermolecular interactions between the sandwiches. The conductivities of the charge-transfer species are greatly improved relative to those of the insulating undoped sandwiches, but the conductivities are in the lower semiconducting region. The low conductivity values are thought to be due to poor intermolecular overlap.

Journal Article↗
PubMed1999

Efficient synthesis of a porphyrin-N-tripod conjugate with covalently linked proximal ligand: toward new-generation active-site models of cytochrome c oxidase.

[formula: see text] A new-generation cytochrome c oxidase active-site model compound (4) featuring both a trisimidazolyl moiety and a proximal base has been designed and efficiently synthesized. During this study, a facile method based on the chemistry of a 4-magnesioimidazole derivative to synthesize 4-imidazolyl-containing tripodal ligands (7) has been developed.

Binding Sites↗
PubMed1999

A general route to 4-imidazolyl-containing multidentate ligands for biomimetic studies.

[formula: see text] 4-iodo-1-tritylimidazole undergoes magnesium-iodine exchange with a Grignard reagent to give selectively the 4-magnesioimidazole derivative, which reacts with esters to form a variety of poly-4-imidazolyl carbinol compounds in 40-79% yields. A wide range of bi-, tri-, and pentadentate ligands featuring 4-substituted imidazole units have been efficiently synthesized.

Imidazoles↗
PubMed1997

A functional model related to cytochrome c oxidase and its electrocatalytic four-electron reduction of O2.

A cytochrome c oxidase model that consists of a cobalt(II) porphyrin with a copper(I) triazacyclononane macrocycle fastened on the distal face and an imidazole covalently attached to the proximal face has been synthesized and characterized. Redox titrations with molecular oxygen (O2) and cobaltocene were carried out, and O2 was found to bind irreversibly in a 1:1 ratio to the model compound. This O2 adduct (a bridged peroxide) can be fully reduced to the deoxygenated form with four equivalents of cobaltocene. The model compound was adsorbed on an edge-plane graphite electrode, and rotating ring-disk voltammetry was used to monitor the electrocatalytic reduction of O2. Four-electron reduction of O2 was observed at physiological pH.

Binding Sites↗
PubMed1996

Coupling H2 to electron transfer.

A catalytic transformation of dihydrogen into two protons and two electrons has been discovered with a ruthenium/iron complex. The chemical reactions of complexes between transition metals and dihydrogen give insights into the function of biological hydrogenases.

Crystallography, X-Ray↗
PubMed1993

Regioselective and enantioselective epoxidation catalyzed by metalloporphyrins.

Recent progress in regioselective and enantioselective epoxidations catalyzed by metalloporphyrins is discussed here, with an explanation of the biomimetic antecedents of this area and its relevance to synthetic applications. Classification of the catalysts that have been studied allows useful conclusions to be drawn about the development of this field. In particular, both the most promising biomimetic and practical catalysts have arisen from systems that can be systematically modified by convenient synthetic methodology.

Alkenes↗
PubMed1988

Gap-modulation infrared spectroscopy of high transition temperature superconductors.

Conventional methods of determining the coupling factor alpha(2)(omega)F(omega) for the newly discovered high transition temperature (T(c)) cuprate superconductors by using tunneling and infrared measurements have thus far failed to show the cause of the very high T(c) of these compounds. This is due in part to difficulties in sample preparation for tunneling studies and to difficulties in obtaining good data at relatively high tunneling voltages. Also, in IR (infrared) measurements, small differences in absorptivity between the normal and superconducting state can be masked by changes in the phonon occupation at high and low temperatures. Here we propose a technique for determing the coupling constant, which should be less dependent on the surface quality of the sample than with tunneling and should allow measurements at higher energies with greater precision than do tunneling or simple IR observations. This should make possible a definitive determination of any possible exciton contribution to this coupling term, which would appear at energies well above the range where conventional IR or tunneling measurements are effective.

Journal Article↗
PubMed1986

Conductive polymers derived from iron, ruthenium, and osmium metalloporphyrins: The shish-kebab approach.

The synthesis and characterization of pyrazine-bridged polymers of iron(II/III), ruthenium(II/III), and osmium(II/III) octaethylporphyrin (dubbed "shish-kebab" polymers) are presented. Optical and dc conductivity measurements reveal that the ruthenium and osmium polymers, when partially oxidized, are highly conductive. Electrochemical and ESR results are presented that indicate the existence of an interesting metal-centered conduction pathway. Unlike most of the previously reported porphyrinic molecular metals in which the conduction electrons are macrocyclic-based, electron transport in these materials proceeds exclusively along the metal-pyrazine backbone.

Journal Article↗
PubMed1984

Epoxidation of olefins by cytochrome P-450 model compounds: mechanism of oxygen atom transfer.

The mechanism of the Mn(III) porphyrin-catalyzed epoxidation of olefins by lithium hypochlorite is examined. The active oxidant is thought to be a high-valent manganese-oxo complex. It is shown that a relatively stable intermediate is reversibly formed upon interaction of the olefin and the oxo complex. The decomposition of this intermediate to Mn(III) porphyrin and epoxide is the rate-determining step of the catalytic cycle. Some analogies to the biochemical epoxidation of olefins catalyzed by cytochrome P-450 are discussed.

Alkenes↗
PubMed1984

Rotational barrier of a molybdenum-molybdenum quadruple bond.

The synthesis and characterization of molybdenum(II) porphyrin dimers containing unbridged metal-metal quadruple bonds are presented. Variable temperature 300 MHz (1)H NMR studies of meso-substituted derivatives provide novel solution evidence for both the existence of quadruple bonds and for a barrier to rotation about the metal-metal bonds in these complexes. The activation energy for this rotational process is 10.1 +/- 0.5 kcal.mol(-1).

Journal Article↗
PubMed1983

Oxygenation of hydrocarbons by cytochrome P-450 model compounds: modification of reactivity by axial ligands.

The rate of olefin oxygenation catalyzed by synthetic metalloporphyrins is examined, employing sodium hypochlorite as the oxygen atom source. The rate of epoxidation and the stability of the catalyst are shown to be dependent on the nature of the axial ligand employed. A rationale for this effect is presented and analogy is made to the role of the thiolate ligand in cytochrome P-450.

Cytochrome P-450 Enzyme System↗