Jennifer C. Green

Photoelectron spectra of metallocenes

Abstract The He(II) spectra of the unsubstituted metallocenes {M(η-C 5 H 5 ) 2 }, M  V, Cr, Mn, Fe, Co, Ni and Ru, and of {Mn(η-C 5 H 4 Me) 2 } are reported; both He(I) and He(II) spectra of some decamethylmetallocenes {M(η-C 5 Me 5 ) 2 }, where M  Mg, V, Cr, Mn, Fe, Co and Ni, are also given. Intensity changes between the He(I) and He(II) spectra are shown to provide a reliable guide to band assignment. A ligand field treatment of the decamethylmanganocene cation, including limited configuration interaction, gives values for Δ 2 , B and C ; these values are also in good agreement with the photoelectron spectra of {M(η-C 5 Me 5 ) 2 } where M  V, Cr and Fe. Overlap between the ligand and metal “ d ” band structures prevents complete assignment in the cases of Co and Ni.

Bent metallocenes revisited

The orbital structure of bent metallocenes and how their geometry depends on the number of d electrons are described. Bonding by a metallocene unit is exemplified by reference to the known hydrides. The reactivity of metallocene derivatives is illustrated with particular emphasis on the differences between ansa-bridged and unbridged compounds; the reactions include ring opening polymerisation of ferrocenophanes, elimination from and addition to Group 6 metallocene derivatives and Ziegler–Natta polymerisation of olefins by Group 4 metallocenes.

Probing the mechanism of hypoxia selectivity of copper bis(thiosemicarbazonato) complexes: DFT calculation of redox potentials and absolute acidities in solution

Density functional theory (DFT) calculations have been performed using the uB3LYP/6-31++G(d,p) model to calculate the solution phase one-electron reduction potentials (E(calc)) and absolute pKa values of a series of copper bis(thiosemicarbazonato) complexes. The effects of solvation in water and dimethylsulfoxide (DMSO) are incorporated as a self-consistent reaction field (SCRF) using the integral equation formalism polarisable continuum model (IEFPCM) and are found to be essential for quantitative agreement with an average error in E(calc) of -0.02 V compared to experiment. The bonding and spin densities are examined through the use of Natural Bond Order analysis and the results used to rationalise the calculated and observed reduction potentials. Calculated estimates of pKa values of several copper(II) species are presented and their implications for the mechanisms of transport and trapping within hypoxic cells are considered. Reduction is found to be a prerequisite for protonation of the complexes which suggests their transport in the blood stream as neutral species, and the mechanistic sequence is identified as a sequential electrochemical-chemical (EC) process. The complex equilibria of protonation, reoxidation and dissociation are discussed and the copper(I) diprotonated, cationic complex of diacetyl bis(4-methyl-3-thiosemicarbazonato)copper(II), Cu(I)ATSMH2(+), is identified as a possible candidate for the initial species trapped in hypoxic cells.

A study of the bonding in the group iv tetrahalides by photoelectron spectroscopy

The photoelectron spectra excited by helium 21.2 eV radiation in the group IV halides, AB4, where A = C, Si, Ge, Sn, Ti and B = C1, Br, have been determined. The ionization potential data are discussed in terms of a simple molecular orbital description of the electronic structures.

Synthesis and structural characterisation of an yttrium–alkyl–alkylidene

The first structurally authenticated yttrium-alkyl-alkylidene is reported; structural, spectroscopic, and theoretical analyses show that whilst the yttrium-alkylidene bond is short, it possesses a bond order less than one and is comparable to the Y-C(alkyl) single bond within the same molecule.

COMPLEXES OF GOLD(I) AND PLATINUM(II) WITH POLYAROMATIC PHOSPHINE LIGANDS

Abstract Gold and platinum(II) complexes of the phosphine ligands PArnPh3−n (Ar=naphthyl, anthracenyl, ferrocenyl and other polyaromatic groups) have been synthesised. The electron donating abilities of naphthyl and anthracenyl phosphine ligands has been explored using gas phase photoelectron spectral data on the parent phosphines and their relative complexing ability to platinum precursor molecules has been assessed by means of synthetic studies and NMR experiments. Their steric parameters have been estimated by the Tolman cone angle methodology using X-ray crystallographic data. The molecular structures of the gold complexes [AuCl(PAn2Ph)]·CHCl3 and [Au(PFc2Ph)2]·CHCl3 have been determined.

Noninnocent Behavior of Ancillary Ligands: Apparent Trans Coupling of a Saturated N-Heterocyclic Carbene Unit with an Ethyl Ligand Mediated by Nickel

Oxidative addition of the tridentate N-heterocyclic carbene (NHC) diphosphine ligand precursor ([PCP]H)PF(6) (1) {[PCP] = o-(i)Pr(2)PC(6)H(4)(NC(3)H(4)N)o-C(6)H(4)P(i)Pr(2)} to Ni(COD)(2) results in the formation of the nickel(II) hydride complex ([PCP]NiH)PF(6) (2). This hydride undergoes a rapid reaction with ethylene to generate a nickel(0) complex in which an ethyl group has been transferred to the carbene carbon of the original NHC-diphosphine ligand. If the first intermediate is the anticipated square-planar nickel(II) ethyl species, then the formation of the product would require a process that involves a trans C-C coupling of the NHC carbon and a presumed Ni-ethyl intermediate. Deuterium-labeling studies provide evidence for migratory insertion of the added ethylene into the Ni-H bond rather than into the Ni-carbene linkage; this is based on the observed deuterium scrambling, which requires reversible beta-elimination, alkene rotation, and hydride readdition. However, density functional theory studies suggest that a key intermediate is an agostic ethyl species that has the Ni-C bond cis to the NHC unit. A possible transition state containing two cis-disposed carbon moieties was also identified. Such a process represents a new pathway for catalyst deactivation involving NHC-based metal complexes.

Photoelectron spectra of substituted benzenes

Photoelectron spectra of several substituted dimethylanilines, nitrobenzenes, acetophenones and nitrosobenzenes have been studied with a view to examine the electronic effects of substituents on the various φ and n levels. The results are discussed in the light of molecular orbital calculations and electronic absorption spectra. Correlation of substituent effects on the IEs with π-electron densities and Hammett substituent constants has enabled rationalization of all available data on mono- and disubstituted benzenes. The IEs generally increase with the electron-withdrawing power of the substituents.

Spectroelectrochemical and computational studies on the mechanism of hypoxia selectivity of copper radiopharmaceuticals

Detailed chemical, spectroelectrochemical and computational studies have been used to investigate the mechanism of hypoxia selectivity of a range of copper radiopharmaceuticals. A revised mechanism involving a delicate balance between cellular uptake, intracellular reduction, reoxidation, protonation and ligand dissociation is proposed. This mechanism accounts for observed differences in the reported cellular uptake and washout of related copper bis(thiosemicarbazonato) complexes. Three copper and zinc complexes have been characterised by X-ray crystallography and the redox chemistry of a series of copper complexes has been investigated by using electronic absorption and EPR spectroelectrochemistry. Time-dependent density functional theory (TD-DFT) calculations have also been used to probe the electronic structures of intermediate species and assign the electronic absorption spectra. DFT calculations also show that one-electron oxidation is ligand-based, leading to the formation of cationic triplet species. In the absence of protons, metal-centred one-electron reduction gives the reduced anionic copper(I) species, [CuIATSM](-), and for the first time it is shown that molecular oxygen can reoxidise this anion to give the neutral, lipophilic parent complexes, which can wash out of cells. The electrochemistry is pH dependent and in the presence of stronger acids both chemical and electrochemical reduction leads to quantitative and rapid dissociation of copper(I) ions from the mono- or diprotonated complexes, [CuIATSMH] and [Cu(I)ATSMH2]+. In addition, a range of protonated intermediate species have been identified at lower acid concentrations. The one-electron reduction potential, rate of reoxidation of the copper(I) anionic species and ease of protonation are dependent on the structure of the ligand, which also governs their observed behaviour in vivo.

Relative partial photoionization cross-sections and photoelectron branching ratios of the valence-bands of the group VIII metallocenes of iron, ruthenium and osmium

Relative partial photoionization cross-sections have been measured for the valence-bands of bis-η5-cyclopentadienyl iron, ruthenium and osmium over the photon energy range 16–115 eV. In each case the a′1 -1 and the e′2 -1 bands show cross-section features attributable to nd resonant photoemission in the vicinity of np → nd giant resonant absorption. For Ru(η-C5H5)2 a large intensity increase is observed, while Fe(η-C5H5)2 and Os(η-C5H5)2 show cross-section minima due to interference effects, followed by much weaker maxima. In the case of Ru(η-C5H5)2 all the ligand ionization bands exhibit resonance features very similar to the metal based orbitals, with those observed for e″1 -1 ionization being the most intense. Even the e″1 -1 orbital, which by symmetry has no metal d character, exhibits such a resonance in its cross-section. For ferrocene and osmocene the e″1 -1 bands also exhibit the greatest degree of cross-section correlation with the d bands. Significant differences are found between the a′1 -1 and...