From the Iron Pentacarbonyl Cation to Heteroleptic η6-arene Carbonyls and bis-η6-arene Cations

Partial ligand substitution at the iron pentacarbonyl radical cation generates novel half-sandwich complexes of the type [Fe(η6-arene)(CO)2]⋅+ (arene=1,3,5-tri-tert-butylbenzene, 1,3,5-trimethylbenzene, benzene and fluorobenzene). Of those, the bulkier 1,3,5-tri-tert-butylbenzene (mes*) derivative [Fe(mes*)(CO)2]⋅+ was fully characterized by XRD analysis, IR, NMR, cw-EPR, Mössbauer spectroscopy and cyclic voltammetry as the [Al(ORF)4]- (RF=C(CF3)3) salt. Chemical electronation, i. e., the single electron reduction, with decamethylferrocene generates neutral [Fe(mes*)(CO)2], whereas further deelectronation under CO-pressure leads to a dicationic three-legged [Fe(mes*)(CO)3]2+ salt with [Al(ORF)4]- counterion. The full substitution of the carbonyl ligands in [Fe(CO)5]⋅+[Al(ORF)4]- mainly resulted in disproportionation reactions, giving solid Fe(0) and the dicationic bis-arene salts [Fe(η6-arene)2]2+([Al(ORF)4]-)2 (arene=1,3,5-trimethylbenzene, benzene and fluorobenzene). Only by employing the very large fluoride bridged anion [F-{Al(ORF)3}2]-, it was possible to isolate an open shell bis-arene cation salt [Fe(C6H6)2]⋅+[F-{Al(ORF)3}2]-. The highly reactive cation was characterized by XRD analysis, cw-EPR, Mössbauer spectroscopy and cyclic voltammetry. The disproportionation of [Fe(C6H6)2]⋅+ salts to give solid Fe(0) and [Fe(C6H6)2]2+ salts was analyzed by a suitable cycle, revealing that the thermodynamic driving force for the disproportionation is a function of the size of the anion used and the polarity of the solvent.

Theoretical study of novel complexed structures for methoxy derivatives of scytonemin: potential biomarkers in iron-rich stressed environments

Scytonemin is a cyanobacterial sheath pigment with potent UV (UVA, UVB, and UVC) absorbing properties. Di- and tetramethoxy derivatives of scytonemin have also been found and described in the literature. The importance of these biomolecules is their photoprotective function, which is one of the major survival strategies adopted by extremophiles in environmentally stressed conditions. Also, iron compounds [particularly iron(III) oxides] offer an additional UV-protecting facility to subsurface endolithic biological colonization; hence, banded iron formations (accompanied by zones of depletion of iron) in rock matrices have attracted attention with special interest in the method of transportation of iron compounds through the rock. Di- and tetramethoxyscytonemin and their iron(III) complexes have been modeled and studied computationally by using density functional theory calculations at the level of B3LYP/6-31G** methodology. We propose new structures that could feature in survival strategy and facilitate the movement of iron through the rock especially for iron-rich stressed terrestrial environments exemplified by the Río Tinto system with the added potential of subsurface Mars exploration. This study represents a continuation of our previous work on scytonemin. The calculated Raman spectra of the proposed iron complexes are compared with those of their parent compounds and discussed in relation to structural changes effected in the parent ligand upon complexation. This information leads to new insights to be gained by experimental Raman spectroscopists and the characterization of spectroscopic biosignatures for the database being compiled for the remote Raman analytical interrogation of the martian surface and subsurface being proposed for the ESA ExoMars mission planned for launch in 2018.

Iron-scytonemin complexes: DFT calculations on new UV protectants for terrestrial cyanobacteria and astrobiological implications

Cyanobacterial colonies produce the radiation-protectant biomolecule scytonemin as part of their response strategy for survival in environmentally stressed conditions in hot and cold deserts. These colonies frequently use sandstone rocks as host matrices for subsurface colonization, which is accompanied by a zone of depletion of iron and transportation of iron compounds to the mineral surface. It is suggested that an iron-scytonemin complex could feature in this survival strategy and facilitate the movement of iron through the rock. Calculations were carried out on several hypothetical iron-scytonemin complexes to evaluate the most stable structure energetically and examine the effect of the complexation of the biomolecule upon the electronic absorption characteristics of the radiation-protectant species. The implications for extraterrestrial planetary detection and analytical monitoring of an iron-scytonemin complex are assessed.

Ultraviolet and infrared photodissociation of Si(+)(C6H6)n and Si(+)(C6H6)(n)Ar clusters

Ion-molecule complexes of the form Si(+)(C6H6)n and Si(+)(C6H6)(n)Ar are produced by laser vaporization in a pulsed nozzle cluster source. These clusters are mass-selected and studied with ultraviolet (355 nm) photodissociation and resonance-enhanced infrared photodissociation spectroscopy in the C-H stretch region of benzene. In the UV, Si(+)(C6H6)n clusters (n = 1-5) fragment to produce the Si(+)(C6H6)n mono-ligand species, suggesting that this ion has enhanced relative stability. IR photodissociation of Si(+)(C6H6)n complexes occurs by the elimination of benzene, while Si(+)(C6H6)(n)Ar complexes lose Ar. Resonances reveal C-H vibrational bands in the 2900-3300 cm(-1) region characteristic of the benzene ligand with shifts caused by the silicon cation bonding. The IR spectra confirm that the major component of the Si(+)(C6H6)n ions studied have the pi-complex structure rather than the isomeric insertion products suggested previously.

What is required to stabilize Al3+? A gas-phase perspective

With a combination of experiment and theory (ab initio and DFT), we demonstrate that the Al(3+) cation can be stabilized in the gas phase using ligands, which have the ability to act as powerful sigma electron donors and electron acceptors. The latter property, which implies that electron density from the aluminum cation moves into ligand antibonding orbitals, has not previously been considered significant when accounting for the behavior of Al(3+). Of the three ligands identified as falling into the above category, acetonitrile appears to form the most stable complexes in the gas phase, which is in accord with the long established fact that solid-state complexes with Al(3+) are readily isolated. From the results, it is suggested that chain or ring compounds containing the -C triple bond N group might act as successful sequestering agents for Al(3+) from aqueous solutions.

Vibrational Spectroscopy of Ni+(benzene)n Complexes in the Gas Phase

Ni+ (benzene)n (n = 1-6) and Ni+ (benzene)n Ar(1,2) (n = 1,2) are produced by laser vaporization in a pulsed nozzle cluster source. The clusters are mass selected and studied by infrared laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer. The excitation laser is an OPO/OPA system that produces tunable IR in the C-H stretching region of benzene. Photodissociation of Ni+ (benzene)n complexes occurs by the elimination of intact neutral benzene molecules, while Ni+ (benzene)n Ar(1,2) complexes lose Ar. This process is enhanced on resonances, and the vibrational spectrum is obtained by monitoring the fragment yield versus the infrared wavelength. Vibrational bands in the 2700-3300 cm(-1) region are characteristic of the benzene molecular moiety with systematic shifts caused by the metal bonding. A dramatic change in the IR spectrum is seen at n = 3 and is attributed to the presence of external benzene molecules acting as solvent molecules in the cluster. The results of previous theoretical calculations are employed to investigate the structures, energetics, and vibrational frequencies of these complexes. The mono-benzene complex is found to have a C2v structure, with benzene distorted by the metal pi-bonding. The di-benzene complex is found to have a D2h structure, with both benzenes distorted. The comparison between experiment and theory provides intriguing new insight into the bonding in these prototypical pi-bonded organometallic complexes.

Revision of the second ionization energy of toluene

Charge stripping (CS) of the molecular ion of toluene, C(7)H(8) (+)-->C(7)H(8) (2+)+e, is often used as a reference for the determination of second ionization energies in energy-resolved CS experiments. For calibration of the kinetic energy scale, a value of IE(C(7)H(8) (+))=(15.7+/-0.2) eV derived from the appearance energy of the toluene dication upon electron ionization has been accepted generally. Triggered by some recent discrepancies between CS measurements on the one hand and different experimental methods as well as theoretical predictions on the other, we have reinvestigated the photon-induced double ionization of toluene using synchrotron radiation. These photoionization measurements yield phenomenological appearance energies of AE(C(7)H(8) (+))=(8.81+/-0.03) eV for the monocation and AE(C(7)H(8) (2+))=(23.81+/-0.06) eV for the dication. The former is in good agreement with a much more precise spectroscopic value, IE(C(7)H(8))=(8.8276+/-0.0006) eV. Explicit consideration of the Franck-Condon envelopes associated with photoionization to the dication in conjunction with the application of the Wannier law leads to an adiabatic ionization energy IE(a)(C(7)H(8) (+))=(14.8+/-0.1) eV, which is as much as 0.9 eV lower than the previous value derived from electron ionization. Because in many previous CS measurements the transition C(7)H(8) (+)-->C(7)H(8) (2+)+e was used as a reference, the energetics of several gaseous dications might need some readjustment.