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Peter C Ford

Publications and source records attributed to Peter C Ford.

At least 19 recordsLinked to original sources
PubMed2006

Electronic transitions involved in the absorption spectrum and dual luminescence of tetranuclear cubane [Cu4I4(pyridine)4] cluster: a density functional theory/time-dependent density functional theory investigation.

We present a combined density functional theory (DFT)/time-dependent density functional theory (TDDFT) study of the geometry, electronic structure, and absorption and emission properties of the tetranuclear "cubane" Cu4I4py4 (py = pyridine) system. The geometry of the singlet ground state and of the two lowest triplet states of the title complex were optimized, followed by TDDFT excited-state calculations. This procedure allowed us to characterize the nature of the excited states involved in the absorption spectrum and those responsible for the dual emission bands observed for this complex. In agreement with earlier experimental proposals, we find that while in absorption the halide-to-pyridine charge-transfer excited state (XLCT*) has a lower energy than the cluster-centered excited state (CC*), a strong geometrical relaxation on the triplet cluster-centered state surface leads to a reverse order of the excited states in emission.

Journal Article↗
PubMed2006

Henry Taube: inorganic chemist extraordinaire.

The numerous innovative contributions of Henry Taube to modern inorganic chemistry are briefly reviewed. Highlights include the determination of solvation numbers and lability, elucidation of substitution mechanisms, discovery and documentation of inner-sphere electron transfer, and discovery of the remarkable coordination chemistry of ruthenium and osmium ammine complexes with unsaturated ligands and mixed-valence complexes and their fundamental relationship to intramolecular electron transfer.

Journal Article↗
PubMed2006

Alkane bromination revisited: "reproportionation" in gas-phase methane bromination leads to higher selectivity for CH3Br at moderate temperatures.

The reaction of methane and bromine is a mildly exothermic and exergonic example of free radical alkane activation. We show here that the reaction of methane and bromine (CH4:Br2 > or = 1) may yield either a kinetically or a thermodynamically determined bromomethane product distribution and proceeds in two main phases between 450 and 550 degrees C under ambient pressure on the laboratory time scale. This is in contrast to the highly exothermic methane fluorination or chlorination reactions, which give kinetic product distributions, and to the endergonic iodination of methane, which yields an equilibrium distribution of iodomethanes. The first phase of reaction between methane and bromine is a relatively rapid consumption of bromine to yield a kinetic methane bromination product distribution characterized by low methane conversion, low methyl bromide selectivity, and higher polybromomethane selectivity. In the second slower phase CHxBr(4-x) reproportionation leads to significantly higher methane conversion and higher methyl bromide selectivity. For methane bromination at 525 degrees C, CH4 conversion and CH3Br selectivity reach 73.5% and 69.5%, respectively, after ample (60 s) time for reproportionation. The high selectivity and simple configuration make this pathway an attractive candidate for scale-up in halogen-mediated methane partial oxidation processes.

Journal Article↗
PubMed2006

In situ FT-IR and UV-vis spectroscopy of the low-temperature NO disproportionation mediated by solid state manganese(II) porphyrinates.

The heterogeneous reaction between NO gas and sublimed layers of manganese(II) porphyrinato complexes Mn(Por) (Por = TPP (tetraphenylporphyrinato dianion), TMP (tetramesitylporphyrinato dianion), or TPP(d20) (perdeuterated tetraphenylporphyrinato dianion)) has been monitored by IR and optical spectroscopy over the temperature range of 77 K to room temperature. These manganese porphyrins promote NO disproportionation to NO2 species and N2O, and the reaction proceeds via several distinct stages. At 90 K, the principal species observed spectrally are the nitric oxide dimer, cis-ONNO, two manganese nitrosyls, the simple NO adduct Mn(Por)(NO), and another intermediate (1) that is apparently critical to the disproportionation mechanism. This key intermediate is formed prior to N2O evolution, and proposals regarding its likely structure are offered. When the system is warmed to 130 K, the disproportionation products, N2O and the O-coordinated nitrito complex Mn(Por)(NO)(ONO) (2), are formed. IR spectral changes show that, upon further warming to 200 K, 2 isomerizes into the N-bonded nitro linkage isomer Mn(Por)(NO)(NO2) (3). After it is warmed to room temperature, the latter species loses NO and converts to the known 5-coordinate nitrito complex Mn(Por)(ONO) (4).

Catalysis↗
PubMed2006

A two-photon antenna for photochemical delivery of nitric oxide from a water-soluble, dye-derivatized iron nitrosyl complex using NIR light.

The experiments described here demonstrate the use of two-photon excitation (TPE) to sensitize nitric oxide (NO) release from a dye-derivatized iron/sulfur/nitrosyl cluster Fe2(mu-RS)2(NO)4 (Fluor-RSE, RS = 2-thioethyl ester of fluorescein) with near-infrared (NIR) light in the form of femtosecond pulses from a Ti:sapphire laser. TPE at 800 nm leads both to weak fluorescence from the organic chromophore at lambda(max) = 532 nm and to NO labilization from the cluster. Since the emission from the reference compound Fluor-Et (the ethyl ester of fluorescein) under identical conditions (50/50 CH3CN/phosphate buffer (1 mM) at pH 7.4) is considerably more intense, the weaker emission from Fluor-RSE and the NO generation indicate that the fluorescein excited states initially formed by TPE are largely quenched by energy transfer to the cluster core. The two-photon absorption (TPA) cross section of Fluor-RSE at 800 nm was determined to be delta = 63 +/- 7 GM via the TPA photoluminescence technique. This can be compared to the TPA cross section of 36 GM reported for fluorescein dye in pH 11 aqueous solution and of 32 +/- 3 GM for Fluor-Et measured under conditions comparable to those used for Fluor-RSE. Pulse intensity dependence studies showed that the quantity of NO released from the latter as the result of NIR photoexcitation follows a quadratic relationship to excitation intensity, consistent with the expectation for a TPE process. These studies demonstrate the potential utility of a two-photon antenna for sensitization of the photochemical release of an active agent (in this case, NO) from a photoactive pro-drug.

Fluoresceins↗
PubMed2006

Substituent effects on nitrosyl iron corrole complexes Fe(Ar3C)(NO).

A series of nitrosyl tris(5,10,15-aryl)corrolate complexes of iron(III) Fe(Ar3C)(NO) with different substituents on the aryl groups have been prepared, and certain spectroscopic and reaction properties were compared. The cyclic voltammetric analysis of the various Fe(Ar3C)(NO) complexes demonstrated that both the one-electron oxidation and one-electron reduction potentials respond in systematic and nearly identical trends relative to the electron-donor properties of the substituents. A similar pattern was seen in the nitrosyl stretching frequency, nu(NO), which modestly decreased with the stronger donor substituents. Flash photolysis of Fe(Ar3C)(NO) solutions in toluene leads to NO dissociation followed by rapid [NO]-dependent decay of the transients formed (presumably Fe(Ar3C)) to regenerate the original spectra. As was seen in an earlier flash photolysis study of Fe(TNPC)(NO) (TNPC3- = 5,10,15-tris(4-nitro-phenyl)corrolate; Joseph, C.; Ford, P. C. J. Am. Chem. Soc. 2005, 127, 6737-6743), the second-order rate constants, k(NO), are all much faster ((1-9) x 10(8) M(-1) s(-1) at 298 K) than those for analogous iron(III) complexes of porphyrins. However, on a more microscopic level there is no obvious pattern in these rates with respect to the donor properties of the aryl ring substituents. The high reactivity of the ferric triarylcorrolates with NO data is interpreted in terms of the strongly electron-donating character of the Ar3C3- ligand and the quartet electronic configuration of the Fe(Ar3C) intermediate.

Electrochemistry↗
PubMed2006

Photochemical and time resolved spectroscopic studies of intermediates relevant to iridium-catalyzed methanol carbonylation: photoinduced CO migratory insertion.

Photoreaction, time-resolved infrared (TRIR), and DFT studies were utilized to probe transformations between iridium complexes with possible relevance to the mechanisms of the iridium/iodide-catalyzed methanol carbonylation to acetic acid. Solution-phase continuous and laser flash photolysis of the tetraphenylarsonium salt of the fac-[CH3Ir(CO)2I3]- anion (1a) under excess carbon monoxide resulted in migratory insertion to give the acyl complex ion mer,trans-[Ir(C(O)CH3)(CO)2I3]- (2a). The latter was isolated as its AsPh4+ salt, and its X-ray crystal structure was determined. TRIR spectra indicate that several transients are generated upon flash photolysis of 1a. The principal photoreaction is CO dissociation, and this is proposed to generate the isomeric complexes fac-[CH3Ir(CO)(Sol)I3]- (I(CO)(fac), Sol = solvent) and mer,trans-[CH3Ir(CO)(Sol)I3]- (I(CO)(mer)). I(CO)(fac) reacts with CO to regenerate 1a with a second-order rate constant (k(CO)) approximately 2.5 x 10(7) M(-1) s(-1) in ambient dichloroethane, while I(CO)(mer) is the apparent precursor to 2a. Kinetics studies indicate the photoinduced formation of a third intermediate (I(M)), hypothesized to be the anionic acyl complex fac-[Ir(C(O)CH3)(CO)(Sol)I3]-. In the absence of added CO, these intermediates undergo dimerization to form a mixture of isomers with the apparent formula [Ir(C(O)CH3)(CO)I3]2(2-). One of these dimers was isolated as the AsPh4+ salt, and the crystal structure was determined. Addition of excess pyridine to a solution of the dimers gave the neutral complex mer,trans-[Ir(C(O)CH3)(CO)(py)2I2], which was characterized by FTIR, NMR, and X-ray crystallography. These transformations, especially the unprecedented photoinduced CO insertion reaction, are discussed and interpreted in terms of the factors favoring migratory insertion dynamics.

Journal Article↗
PubMed2006

Toward development of water soluble dye derivatized nitrosyl compounds for photochemical delivery of NO.

This report describes the synthesis, spectroscopy, and photochemistry of a new fluorescein-derivatized iron sulfur nitrosyl compound, the Roussin's red salt ester bis-((mu-S,mu-S')-fluorescein-2-thioethyl-ester)-tetranitrosyldiiron (Fluor-RSE). Under continuous photolysis Fluor-RSE decomposes with moderate quantum yields (0.0036 +/- 0.0005 at lambda(irr) = 436 nm) with the corresponding release of most of the NO carried by the Fe2S2NO4 cluster. Large changes in the optical absorptivity occur upon photolysis of the Fluor-RSE, and these changes have been attributed to the different protic forms available to the fluorescein chromophore as it is separated from the cluster. Steady-state luminescence experiments have shown that the fluorescence of Fluor-RSE is about 85% quenched relative to the model compound ethyl fluorescein (Fluor-Et). Thus, it is clear that excitation of the fluorescein chromophore antennae is followed by energy transfer to the Fe/S/NO cluster at a rate at least comparable to fluorescence. However, the effect of the iron-sulfur core on the fluorescent lifetimes from fluorescein chromophore is much smaller. A single-exponential decay (tau = 3.3 ns) was seen for Fluor-RSE that is only modestly shorter than that for Fluor-Et (tau = 4.5 ns), and this is the effect of the smaller radiative rate constant (k(r)) for the former. These systems further demonstrate that attachment of a pendant dye chromophore as an antenna significantly improves the effective rate for photochemical NO generation from the Roussin's red salt esters at longer excitation wavelengths.

Fluoresceins↗
PubMed2005

Probing shapes of bichromophoric metal-organic complexes using ion mobility mass spectrometry.

Ion mobility mass spectrometry (IM-MS) was used to probe the structures of several metal complexes carrying pendant chromophores. The three complexes investigated were the copper(II) complex Cu(DAC)2+ (DAC = 1,8-bis(9-methylanthracyl)cyclam, cyclam = 1,4,8,11-tetraazacyclotetradecane), the N-nitrosylated ligand DAC-NO, and the Roussin's red salt ester (mu-S,mu-S')-protoporphyrin-IX-bis(2-thioethyl ester)tetranitrosyldiiron (PPIX-RSE). From the IM-MS data coupled with theoretical calculations, it was found that [Cu(II)(DAC - H)]+ exists as a single conformer, with one anthracenyl group above the cyclam and the other below, similar to the crystal structure of Cu(II)(DAC)2+. The metal-free N-nitrosylated ligand (DAC-NO + H)+ has two conformations: one family of structures has one anthracenyl group above the cyclam and one below, while the other has both anthracenyl groups on the same side of the cyclam. These observations are consistent with 1H NMR data for the neutral DAC-NO complex that indicate the presence of two geometric isomers in solution. The third species, PPIX-RSE, has a porphyrin chromophore covalently linked to an Fe2S2(NO)4 cluster for use as a precursor for the photochemical delivery of nitric oxide in single- and two-photon excitation processes. Ion mobility indicates the presence of two (PPIX-RSE + H)+ conformations, consistent with the previous interpretation of the bimodal fluorescence lifetime decay seen for PPIX-RSE. DFT structures, in good agreement with the IM-MS cross sections, indicate two "bent" conformations with the planes of the porphyrin and Fe2S2 rings at different angles with respect to each other.

Journal Article↗
PubMed2005

Chromium(III) complexes for photochemical nitric oxide generation from coordinated nitrite: synthesis and photochemistry of macrocyclic complexes with pendant chromophores, trans-[Cr(L)(ONO)(2)]BF(4).

Several new dinitritochromium(III) complexes of the type trans-[Cr(L)(ONO)(2)]BF(4), where L is a derivative of the macrocyclic ligand cyclam having pendant aromatic chromophores attached (L = 5,7-dimethyl-6-(substituted)-1,4,8,11-tetraazacyclotetradecane), have been prepared and characterized. Photoexcitation of aqueous solutions containing these complexes at wavelengths corresponding to the pendant chromophore absorption bands led to the generation of NO as detected by an electrochemical sensor. Photophysical data show that the expected fluorescence of the pendant chromophores is largely quenched when the macrocyclic ligand is coordinated to these Cr(III) centers, and this is interpreted in terms of fast energy transfer processes from the ligand-centered pipi states to the Cr(III)-centered ligand field states leading to subsequent cleavage of the Cr(III)-coordinated nitrito ligand. Thus, the chromophores tethered to the coordinated cyclam serve as light-gathering antennae for the intramolecular sensitization of the NO-generating photoreactions at the metal center.

Chromium↗
PubMed2005

Synthesis and luminescence properties of Cr(III) complexes with cyclam-type ligands having pendant chromophores, trans-[Cr(L)Cl(2)]Cl.

The synthesis and spectroscopic properties of new cyclam-type ligands 5,7-dimethyl-6-R-1,4,8,11-tetraazacyclotetradecane (L), where R is a pendant chromophore such as an anthracene derivative, are reported. These ligands were prepared according to a nickel(II) template procedure, and the X-ray crystal structures of several Ni(II) intermediates are described. Reaction of the free base ligands L with CrCl(3)x3THF resulted in facile formation of trans-[Cr(L)Cl(2)]Cl complexes, and the structures and spectroscopic characterizations of these complexes are also described. Examination of the photophysical properties of trans-[Cr(L)Cl(2)]Cl solutions at 77 K demonstrated the emission spectra to be dominated by phosphorescence from the ligand field doublet of the chromium(III) center. This also applies to the Cr(III) complex trans-[Cr(mac)Cl(2)]Cl, where mac is the anthracene derivative 5,7-dimethyl-6-anthracenylcyclam. Excitation into the pi-pi(*) states of the anthracene leads to marked quenching of the fluorescence from this chromophore and sensitized phosphorescence from the metal-centered doublet state.

Chromium↗
PubMed2005

New structural motifs, unusual quenching of the emission, and second harmonic generation of copper(I) iodide polymeric or oligomeric adducts with para-substituted pyridines or trans-stilbazoles.

The structural, emissive, and nonlinear optical properties of new CuI adducts with para-substituted trans-stilbazolic and pyridinic ligands are reported. Single-crystal X-ray diffraction results indicate that the para-substituent on the organic ligand greatly influences the structural motif by its steric (tert-butyl), electronic/steric (dimethylamino), or bridging-donor (cyano) properties so that two absolutely new structural motifs, polymeric and oligomeric, are found when trans-stilbazole and pyridine carry a dimethylamino group in the para-position. In addition, a surprising photoemission behavior is observed, being the solid-state emission of [CuI(trans-4-stilbazole)]n, [CuI(trans-4'-(dimethylamino)-4-stilbazole)]n, and [CuI(4'-tert-butyl-4-stilbazole)]n totally quenched. In the case of the noncentrosymmetric CuI adduct of trans-4'-(dimethylamino)stilbazole a discrete second harmonic generation (SHG) occurs.

Journal Article↗
PubMed2005

The remarkable axial lability of iron(III) corrole complexes.

Flash photolysis of nitrosyl tris(aryl)corrolate complexes of iron(III), Fe(Ar(3)C)(NO) (Ar(3)C(3-) = 5,10,15-tris(4-nitro-phenyl)corrolate (TNPC(3-)), 5,10,15-tris(phenyl)corrolate (TPC(3-)) or 5,10,15-tris(4-tolyl)corrolate (H(3)TTC(3-))) leads to NO labilization. This is followed by the rapid reaction of NO with Fe(III)(C) to regenerate the starting complex. The second-order rate constants for the back reactions (k(NO)) were determined to be many orders of magnitude faster than the corresponding reactions of ferric porphyrin complexes and indeed are reminiscent of the very large values seen for those of the corresponding ferrous porphyrin analogues. These data are interpreted in terms of the strongly electron-donating character of the trianionic corrolate ligand and the likely triplet electronic configuration of the iron(III) complex. These reduce the affinity of the metal centers to Lewis bases to the extent that axial ligands bind very weakly or not at all. This property is illustrated by the nearly identical k(NO) values ( approximately 10(9) M(-1) s(-1) at 295 K) recorded for the back reaction of Fe(III)(TNPC) with NO after flash photolysis of Fe(TNPC)(NO) in toluene solution and in THF solution. Softer Lewis bases have a somewhat greater effect; for example, studies in 1:9 (v:v) acetonitrile:toluene and 1:9 pyridine:toluene gave k(NO) values decreased approximately 33% and approximately 85%, respectively, but these both remain >10(8) M(-1) s(-1). The potential roles of Lewis bases in controlling the dynamics of NO addition to Fe(TNPC) in toluene was investigated in greater detail by determining the rates as a function of pyridine concentration over a wide range (10(-4) to 2.5 M). These data suggest that, while a monopyridine complex, presumably Fe(TNPC)(py), is readily formed (K approximately 10(4) M), this species is about one-sixth as reactive as Fe(TNPC) itself. It appears that a much less reactive bis(pyridine) complex also is formed at high [py] but the equilibrium constant is quite small (<1 M(-1)).

Journal Article↗
PubMed2005

Reactions of nitrogen oxides with heme models. Spectral and kinetic study of nitric oxide reactions with solid and solute Fe(III)(TPP)(NO3).

The reaction(s) of nitric oxide (nitrogen monoxide) gas with sublimed layers containing the nitrato iron(III) complex Fe(III)(TPP)(eta(2)-O(2)NO) (1, TPP = meso-tetraphenyl porphyrinate(2)(-)) leads to formation of several iron porphyrin species that are ligated by various nitrogen oxides. The eventual products of these low-temperature solid-state reactions are the nitrosyl complex Fe(TPP)(NO), the nitro-nitrosyl complex Fe(TPP)(NO(2))(NO), and 1 itself, and the relative final quantities of these were functions of the NO partial pressure. It is particularly notable that isotope labeling experiments show that the nitrato product is not simply unreacted 1 but is the result of a series of transformations taking place in the layered material. Thus, the nitrato complex formed from solid Fe(TPP)(eta(2)-O(2)NO) maintained under a (15)NO atmosphere was found to be the labeled analogue Fe(TPP)(eta(2)-O(2)(15)NO). The reactivities of the layered solids are compared to the behaviors of the same species in ambient temperature solutions. To interpret the reactions of the labeled nitrogen oxides, the potential exchange reactions between N(2)O(3) and (15)NO were examined, and complete isotope scrambling was observed between these species under the reaction conditions (T = 140 K). Overall it was concluded from isotope labeling experiments that the sequence of reactions is initiated by reaction of 1 with NO to give the nitrato nitrosyl complex Fe(TPP)(eta(1)-ONO(2))(NO) (2) as an intermediate. This is followed by a reaction in the presence of excess NO that is equivalent to the loss of the nitrate radical NO(3)(*)( )()to give Fe(TPP)(NO) as another transient species. A plausible pathway involving NO attack on the coordinated nitrate of 2 resulting in the release of N(2)O(4) concerted with electron transfer to the metal center is proposed.

Biomimetic Materials↗
PubMed2005

NO and NOx interactions with group 8 metalloporphyrins.

There has been an ongoing interest in the reactions of nitric oxide (NO) with heme model compounds, with the goal of interpreting related reactions occurring in biology. With recent evidence that higher oxides (NO2-, *NO2, N2O3, etc.) may also be formed under bioregulatory conditions, there is a need to understand the reactivities of these compounds with such models. This review discusses the mechanistic studies of the reactions of iron, ruthenium, and osmium metalloporphyrin complexes with NO and the higher nitrogen oxides.

Animals↗
PubMed2005

Further evidence supporting an inner sphere mechanism in the NO reduction of the copper(II) complex Cu(dmp)2(2+) (dmp=2,9-dimethyl-1,10-phenanthroline).

Described are further studies directed towards elucidating the mechanism of the nitric oxide reduction of the copper(II) model system, Cu(dmp)2(2+) (I, dmp=2,9-dimethyl-1,10-phenanthroline). The reaction of I with NO in methanol results in the formation of Cu(dmp)2+ (II) and methyl nitrite (CH3ONO), with a second order rate constant kNO=38.1 M-1 s-1 (298K). The activation parameters for this reaction in buffered aqueous medium were measured to be DeltaH(double dagger)=41.6 kJ/mol and DeltaS(double dagger)=-82.7 kJ/mol deg. The addition of azide ion (N3-) as a competing nucleophile results in a marked acceleration in the rate of the copper(II) reduction. Analysis of the kinetics for the NO reduction of the bulkier Cu(dpp)(2)2+ (IV, dpp=2,9-diphenyl-1,10-phenanthroline) and the stronger oxidant, Cu(NO2-dmp)2(2+) (V, NO2-dmp=5-nitro-2,9-dimethyl-1,10-phenanthroline), gave the second order rate constants kNO=21.2 and 29.3 M-1 s-1, respectively. These results argue against an outer sphere electron transfer pathway and support a mechanism where the first step involves the formation of a copper-nitrosyl (Cu(II)-NO or Cu(I)-NO+) adduct. This would be followed by the nucleophilic attack on the bound NO and the labilization of RONO to form the nitrite products and the cuprous complex.

Azides↗