Iron complexes, mass spectra

The pentanuclear carbido species Ms(CO)lsC (M = Fe, Ru, Os) have been prepared. The iron compound has been known for some considerable time (209), but the ruthenium and osmium complexes were prepared recently by pyrolysis reactions (210). The ruthenium adduct was only isolated in low yield (—1%), while the osmium complex was obtained in higher yield (—40%). The infrared spectrum and mass spectral breakdown pattern indicate a common structure to these compounds. The molecular structure of the iron complex is shown in Fig. 46. 

The mass spectrum of Fe2(CO)9, long assumed to be completely involatile, shows a parent ion, but the base peak is Fe2(CO) J, with a structure retaining the three bridging carbonyl groups [Fe(CO)3Fe]+ (70). Similar bridged ions containing iron have been postulated in the spectra of some phosphine and sulfide complexes (Section VI). 

Usually, atomic mass spectra are considerably simpler and easier to interpret than optical emission spectra. This property is particularly important for samples that contain rare earth elements and other heavy metals. such as iron, that yield complex emission spectra. Figure 11-15b illustrates this advantage. This spectral simplicity is further illustrated in Figure 11-16. which is the atontic mass spectrum for a mixture of fourteen rare earth elements that range in atomic mass number from 1.39 to 175. T he optical emission spectrum for such a mixture would be so complex that interpretation would be tedious, time-consuming, and perhaps impo.ssible. 

The olefin complex (22 X,Y = F P,Q = Br) has also been prepared in low yield by a photochemical reaction of CFi CBra with pentacarbonyliron, together with a small amount of material formulated from its mass spectrum and an i.r. band at 1675 cm" as the fenacyclopentene (23). A Russian patent claims that bis(halogenoperfluoroalkene)iron derivatives can be prepared by heating Fe(CO)s with CF2 CFX (X = Br, CF3, or CFiCFj) in an autoclave at 125—280° for 23—38 h. ... 

Excess iron nonacarbonylFe2(CO)9 was stirred with 3-chloro-(2-chloromethyl) propene in ether at room temperature for 12 hr. The pale yellow complex, mp 28-29°, was isolated from the reaction mixture. Propose a plausible structure from the following data Analysis C, 43.29 and H, 2.90 mass spectrum m/e 194, 166, 138, and 110 IR 1998 and 2064 cm NMR 8.00t(s) 7r-cyclobutadiene iron tricarbonyl 6.09t 7r-allyl-7r-cyclopentadienyl iron monocarbonyl 7.33t (syn), and 9.32t (anti). 

Very recently, the first example of hydroperoxoiron(III) complex was reported [77]. The treatment of iron(II) complex, [Fe(II)(N4py)(MeCN)](C104)2, with excess H2O2 affords a metastable purple species which can be assigned as [Fe(III)(N4py)(00H)](C104)2 based on the electronspray mass spectrum [77]. The schematic representation is shown at eq. (4). 

Figure 9 Anion photoelectron spectroscopy. Its unique features are (I) Intrinsic mass selectivity and (ii) neutrals as final states. Here, as an example the results for compounds of iron, carbon and hydrogen are shown which exist in catalytic processes, high-temperature terrestrial or low-temperature astrophysical chemistry. Bottom spectrum a primary anion mass spectrum containing anions of the complexes of interest. Top spectra anion photoelectron spectra obtained by electron kinetic energy analysis after laser-induced photodetachment. They reveal the change of molecular structure and electronic energies for increasing numbers of hydrogen atoms in the complex.

Hunt s group (50, 51) have pioneered the application of the Cl source to organometallics such as the iron tricarbonyl complex of heptafulvene, whose electron impact spectrum shows (M—CO)+ as the heaviest ion, in contrast to the methane Cl spectrum with the ion as base peak. Boron hydrides (52) and borazine (53) have also been studied. The methane Cl spectrum of arenechromium and -molybdenum (54) show protonation at the metal giving a protonated parent or molecular ion. Risby et al. have studied the isobutane Cl mass spectra of lanthanide 2,2,6,6-tetramethylheptane-3,5-dionates[Ln(thd)3] (55) and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-oetanedione [H(fod)] lanthanide complexes (56). These latter complexes have been suggested as a means of analysis for the lanthanide elements. 

Other aspects of the report (42) on [Fe3S2(NO)5] are surprising. Elemental analysis of the ammonium salt was reported to distinguish between iron(II) and iron(III) in [Fe3S2(NO)5] , but to find these two types of iron present in equal numbers is most unusual for a triiron complex. Second, the molecular weight of the potassium salt was measured as 420 by mass spectrometry. This value is close to the M/Z of 421 calculated for the most abundant isotopic form of the ion-pair cation [KFe3S2(NO)5] +. Finally, the ESR spectrum reported is that of a dini-trosyliron species, which bears a remarkable resemblance to that reported (22) for a complex formed from Fe(II) and nitric oxide in aqueous alkaline solution. 

However, spectroscopic studies of activated BLM indicate that it is not an Fev=0 species. It exhibits an S - 1/2 EPR spectrum with g values at 2.26, 2.17, and 1.94 [15], which is typical of a low-spin Fe111 center. This low-spin Fem designation is corroborated by Mossbauer and x-ray absorption spectroscopy [16,19], Furthermore, EXAFS studies on activated BLM show no evidence for a short Fe—0 distance, which would be expected for an iron-oxo moiety [19], These spectroscopic results suggest that activated BLM is a low-spin iron(III) peroxide complex, so the two oxidizing equivalents needed for the oxidation chemistry would be localized on the dioxygen moiety, instead of on the metal center. This Fe(III)BLM—OOH formulation has been recently confirmed by electrospray ionization mass spectrometry [20] and is supported by the characterization of related synthetic low-spin iron(III) peroxide species, e.g., [Fe(pma)02]+ [21] and [Fe(N4py)OOH]2+ [22], The question then arises whether the peroxide intermediate is itself the oxidant in these reactions or the precursor to a short-lived iron-oxo species that effects the cytochrome P-450-like transformations. This remains an open question and the subject of continuing interest. 

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