Thermochemistry of small cationic iron–sulfur clusters

Fe-V sulfur clusters studied through photoelectron spectroscopy and density functional theory

Iron-vanadium sulfur cluster anions are studied by photoelectron spectroscopy (PES) at 3.492 eV (355 nm) and 4.661 eV (266 nm) photon energies, and by density functional theory (DFT) calculations. The structural properties, relative energies of different structural isomers, and the calculated first vertical detachment energies (VDEs) of different structural isomers for cluster anions FeVS1-3- and FemVnSm+n- (m + n = 3, 4; m > 0, n > 0) are investigated at a BPW91/TZVP theory level. The experimental first VDEs for these Fe-V sulfur clusters are reported. The most probable ground state structures and spin multiplicities for these clusters are tentatively assigned by comparing their theoretical and experiment first VDE values. For FeVS1-3- clusters, their first VDEs are generally observed to increase with the number of sulfur atoms from 1.45 eV to 2.86 eV. The NBO/HOMOs of the ground state of FeVS1-3- clusters are localized in a p orbital on a S atom; the partial charge distribution on the NBO/HOMO localized site of each cluster anion is responsible for the trend of their first VDEs. A less negative localized charge distribution is correlated with a higher first VDE. Structure and steric effect differences for FemVnSm+n- (m + n = 3, m > 0, n > 0) clusters are suggested to be responsible for their different first VDEs and properties. Two types of structural isomers are identified for FemVnSm+n- (m + n = 4, m > 0, n > 0) clusters: a tower structure isomer and a cubic structure isomer. The first VDEs for tower like isomers are generally higher than those for cubic like isomers of FemVnSm+n- (m + n = 4, m > 0, n > 0) clusters. Their first VDEs are can be understood through: (1) NBO/HOMO distributions, (2) structures (steric effects), and (3) partial charge numbers on the NBO/HOMO's localized sites. EBEs for excited state transitions for all Fe-V sulfur clusters are calculated employing OVGF and TDDFT approaches at the TZVP level. The OVGF approach for these Fe/V/S cluster anions is better for the higher transition energies than the TDDTF approach. The experimental and theoretical results for these Fe/V/S cluster anions are compared with their related pure iron sulfur cluster anions. Properties of the NBO/HOMO are essential for understanding and estimating the different first VDEs for Fe/V/S, and comparing them to those of the pure Fe/S cluster anions.

Thermal stability of iron-sulfur clusters

The thermal decomposition of free cationic iron-sulfur clusters Fe_xS_y⁺ (x = 0-7, y = 0-9) is investigated by collisional post-heating in the temperature range between 300 and 1000 K. With increasing temperature the preferential formation of stoichiometric Fe_xS_y⁺ (y = x) or near stoichiometric Fe_xS_y⁺ (y = x ± 1) clusters is observed. In particular, Fe₄S₄⁺ represents the most abundant product up to 600 K, Fe₃S₃⁺ and Fe₃S₂⁺ are preferably formed between 600 K and 800 K, and Fe₂S₂⁺ clearly dominates the cluster distribution above 800 K. These temperature dependent fragment distributions suggest a sequential fragmentation mechanism, which involves the loss of sulfur and iron atoms as well as FeS units, and indicate the particular stability of Fe₂S₂⁺. The potential fragmentation pathways are discussed based on first principles calculations and a mechanism involving the isomerization of the cluster prior to fragmentation is proposed. The fragmentation behavior of the iron-sulfur clusters is in marked contrast to the previously reported thermal dissociation of analogous iron-oxide clusters, which resulted in the release of O₂ molecules only, without loss of metal atoms and without any tendency to form particular prominent and stable Fe_xO_y⁺ clusters at high temperatures.

Photoelectron spectroscopy and density functional theory studies of (FeS)mH- (m = 2-4) cluster anions: effects of the single hydrogen

Single hydrogen containing iron hydrosulfide cluster anions (FeS)_mH^- (m = 2-4) are studied by photoelectron spectroscopy (PES) at 3.492 eV (355 nm) and 4.661 eV (266 nm) photon energies, and by Density Functional Theory (DFT) calculations. The structural properties, relative energies of different spin states and isomers, and the first calculated vertical detachment energies (VDEs) of different spin states for these (FeS)_mH^- (m = 2-4) cluster anions are investigated at various reasonable theory levels. Two types of structural isomers are found for these (FeS)_mH^- (m = 2-4) clusters: (1) the single hydrogen atom bonds to a sulfur site (SH-type); and (2) the single hydrogen atom bonds to an iron site (FeH-type). Experimental and theoretical results suggest such available different SH- and FeH-type structural isomers should be considered when evaluating the properties and behavior of these single hydrogen containing iron sulfide clusters in real chemical and biological systems. Compared to their related, respective pure iron sulfur (FeS)_m^- clusters, the first VDE trend of the diverse type (FeS)_mH_0,1^- (m = 1-4) clusters can be understood through (1) the different electron distribution properties of their highest singly occupied molecular orbital employing natural bond orbital analysis (NBO/HSOMO), and (2) the partial charge distribution on the NBO/HSOMO localized sites of each cluster anion. Generally, the properties of the NBO/HSOMOs play the principal role with regard to the physical and chemical properties of all the anions. The change of cluster VDE from low to high is associated with the change in nature of their NBO/HSOMO from a dipole bound and valence electron mixed character, to a valence p orbital on S, to a valence d orbital on Fe, and to a valence p orbital on Fe or an Fe-Fe delocalized valence bonding orbital. For clusters having the same properties for NBO/HSOMOs, the partial charge distributions at the NBO/HSOMO localized sites additionally affect their VDEs: a more negative or less positive localized charge distribution is correlated with a lower first VDE. The single hydrogen in these (FeS)_mH^- (m = 2-4) cluster anions is suggested to affect their first VDEs through the different structure types (SH- or FeH-), the nature of the NBO/HSOMOs at the local site, and the value of partial charge number at the local site of the NBO/HSOMO.

Tuning the oxidative power of free iron-sulfur clusters

The gas-phase reactions between a series of di-iron sulfur clusters Fe₂S_x⁺ (x = 1-3) and the small alkenes C₂H₄, C₃H₆, and C₄H₈ have been investigated by means of Fourier-transform ion-cyclotron resonance mass spectrometry. For all studied alkenes, the reaction efficiency is found to increase in the order Fe₂S⁺ < Fe₂S₂⁺ < Fe₂S₃⁺. In particular, Fe₂S⁺ and Fe₂S₂⁺ only form simple association products, whereas the sulfur-rich Fe₂S₃⁺ is able to dehydrogenate propene and 2-butene via desulfurization of the cluster and formation of H₂S. This indicates an increased propensity to induce oxidation reactions, i.e. oxidative power, of Fe₂S₃⁺ that is attributed to an increased formal oxidation state of the iron atoms. Furthermore, the ability of Fe₂S₃⁺ to activate and dissociate the C-H bonds of the alkenes is observed to increase with increasing size of the alkene and thus correlates with the alkene ionization energy.

Electronic, magnetic structure and water splitting reactivity of the iron-sulfur dimers and their hexacarbonyl complexes: A density functional study

The iron sulfide dimers (FeS)2 and their persulfide isomers with S-S bonds are studied with the B3LYP density functional as bare clusters and as hexacarbonyls. The disulfides are more stable than the persulfides as bare clusters and the persulfide ground state lies at 3.2 eV above the global minimum, while in the hexacarbonyl complexes this order is reversed: persulfides are more stable, but the energy gap between disulfides and persulfides becomes much smaller and the activation barrier for the transition persulfide → disulfide is 1.11 eV. Carbonylation also favors a non-planar Fe2S2 ring for both the disulfides and the persulfides and high electron density in the Fe2S2 core is induced. The diamagnetic ordering is preferred in the hexacarbonyls, unlike the bare clusters. The hexacarbonyls possess low-lying triplet excited states. In the persulfide, the lowest singlet-to-triplet state excitation occurs by electron transition from the iron centers to an orbital located predominantly at S2 via metal-to-ligand charge transfer. In the disulfide this excitation corresponds to ligand-to-metal charge transfer from the sulfur atoms to an orbital located at the iron centers and the Fe-Fe bond. Water splitting occurs on the hexacarbonyls, but not on the bare clusters. The singlet and triplet state reaction paths were examined and activation barriers were determined: 50 kJ mol(-1) for HO-H bond dissociation and 210 kJ mol(-1) for hydrogen evolution from the intermediate sulfoxyl-hydroxyl complexes Fe2S(OH)(SH)(CO)6 formed. The lowest singlet-singlet excitations in the hexacarbonyls, the water adsorption complexes and in the reaction intermediates, formed prior to dihydrogen release, fall in the visible light region. The energy barrier of 210 kJ mol(-1) for the release of one hydrogen molecule corresponds to one visible photon of 570 nm. The dissociation of a second water molecule, followed by H2 and O2 release via hydro-peroxide intermediate is a two-step process, with activation barriers of 218 and 233 kJ mol(-1), which also fall in the visible light region. A comparison of the full reaction path with that on diiron dioxide hexacarbonyls Fe2O2(CO)6 is traced.

Aqueous Fe2S2 cluster: structure, magnetic coupling, and hydration behaviour from Hubbard U density functional theory

We present a DFT + U investigation of the all-ferrous Fe2S2 cluster in aqueous solution. We determine the value of U by tuning the geometry of the cluster in the gas-phase to that obtained by the highly accurate CCSD(T) method. When the optimised value of U is employed for the aqueous Fe2S2 cluster (Fe2S2(aq)), the resulting geometry agrees well with the X-ray diffraction structure, while the magnetic coupling is in line with the estimate from Mössbauer data. Molecular dynamics trajectories predict Fe2S2(aq) to be stable in water, regardless of the introduction of U. However, significant differences arise in the geometry, hydration, and exchange constant of the solvated clusters.

Gas-phase ion chemistry in organometallic systems

This review essentially deals with positive ion/molecule reactions occurring in gas-phase organometallic systems, and encompasses a period of time of approximately 7 years, going from 1997 to early 2004. Following the example of the excellent review by Eller & Schwarz (1991; Chem Rev 91:1121-1177), in the first part, results of reaction of naked ions are presented by grouping them according to the neutral substrate, while in the second part, ligated ions are grouped according to the different ligands. Whenever possible, comparison among similar studies is attempted, and general trends of reactivities are evidenced.

Gas-Phase Reactivity of the Ti8C12+ Met-car with Triatomic Sulfur-Containing Molecules: CS2, SCO, and SO2

Gas-phase Ti(x)C(y)+ clusters (x/y = 3/5, 4/7, 5/9, 6/9, 7/12, 8/12, 9/12) including the magic Ti8C12+ (met-car) have been produced by reactive sputtering with a magnetron cluster source. The gas-phase reactivity of the met-car with SCO, CS2, and SO2 was investigated in a hexapole collision cell by way of tandem mass spectrometry. Results indicate an increase in activity as the oxygen-to-sulfur ratio increases (SO2 > SCO > CS2) with products ranging from association to break down of the met-car cluster. Trends in the mass spectra also indicate SCO and CS2 may bond to the met-car in a unique way not observed in previous reactivity studies on Ti8C12+. To investigate this, several possible single molecule-cluster bonding configurations were calculated with density functional theory. The results indicate that bridge bonding of the intact molecules is energetically preferred. In addition, the energy barriers and transition states leading to dissociation products were calculated and the trends are found to be in qualitative agreement with experiment. The effects of the different types of bonding and number of adsorbed species on the reactivity of the met-car along with proposed reaction mechanisms for product formation are also discussed.

Methane activation by nickel cluster cations, Ni[sub n][sup +] (n=2–16): Reaction mechanisms and thermochemistry of cluster-CH[sub x] (x=0–3) complexes

The kinetic energy dependences of the reactions of Ni+(n) (n=2-16) with CD(4) are studied in a guided ion beam tandem mass spectrometer over the energy range of 0-10 eV. The main products are hydride formation Ni(n)D+, dehydrogenation to form Ni(n)CD+(2), and double dehydrogenation yielding Ni(n)C+. These primary products decompose at higher energies to form Ni(n)CD+, Ni(n-1)D+, Ni(n-1)C+, Ni(n-1)CD+, and Ni(n-1)CD+(2). Ni(n)CD(2) (+) (n=5-9) and Ni(n-1)CD(2) (+) (n > or =4) are not observed. In general, the efficiencies of the single and double dehydrogenation processes increase with cluster size. All reactions exhibit thresholds, and cross sections for the various primary and secondary reactions are analyzed to yield reaction thresholds from which bond energies for nickel cluster cations to C, CD, CD(2), and CD(3) are determined. The relative magnitudes of these bond energies are consistent with simple bond order considerations. Bond energies for larger clusters rapidly reach relatively constant values, which are used to estimate the chemisorption energies of the C, CD, CD(2), and CD(3) molecular fragments to nickel surfaces.

Electronic States of Fe2S-/0/+

The relative energies of a multitude of low-lying electronic states of Fe2S-/0/+ are determined by complete active space self-consistent field (CASSCF) calculations. The numerous states obtained are assigned to spin ladders. For selected states, dynamic correlation has been included by multireference configuration interaction (MRCI) and the structures of some high-spin states have been optimized by CASSCF/MRCI. Comparison is made with structures obtained by density-functional theoretical calculations. The ground states of Fe2S-/0/+ are 10B2, 1A1 and 8A2, respectively, and the total splittings of the lowest-energy spin ladders are about 0.18, 0.07 and 0.13 eV, respectively. The spin ladders of Fe2S qualitatively reflect the picture of Heisenberg spin coupling. While both Fe2S- and Fe2S+ show an Fe-Fe distance of about 270 pm, that of Fe2S is about 100 pm longer. The calculated adiabatic electron affinity of Fe2S is 1.2 eV and the ionization energy 6.6 eV. An interpretation of the observed photoelectron spectrum of Fe2S- is given.