Gaseous iron(II) and iron(III) complexes with BINOLate ligands

Abstract

Electrospray ionization (ESI) of dilute solutions of 1,1=-bi-2-naphthol (BINOL) and iron(II) or iron(III) sulfate in methanol/water allows the generation of monocationic complexes of iron and deprotonated BINOL ligands with additional methanol molecules in the coordination sphere, and the types of complexes formed can be controlled by the valence of the iron precursors used in ESI. INOL (1,1=-bi-2-naphthol) is among the most important chiral ligands used in metal-mediated asymmetric synthesis [1]. Despite a huge base of knowledge, however, gas-phase models that would permit a detailed understanding of enantioselective reactions at a molecular level are still relatively scarce [2][3][4]. In the course of ongoing studies of possible systems that allow probing of chiral reactions with mass spectrometric means [3,5,6], we here report on the generation of BINOL complexes of iron(II) and iron(III) by electrospray ionization (ESI) and on the fragmentation behavior of the ions formed to provide a foundation for future studies about possible enantioselective effects in the ion/molecule reactions of these gaseous complexes. ExperimentalThe experiments were performed using a VG BIO-Q mass spectrometer, which consists of an ESI source combined with a tandem mass spectrometer of QHQ configuration (Q stands for quadrupole and H for hexapole) as described in detail elsewhere [7]. In the present experiments, millimolar solutions of BINOL and iron(II) or iron(III) sulfate, respectively, in methanol/water (1:1) were introduced through a fused-silica capillary to the ESI source by a syringe pump (ϳ5 L/min). Nitrogen was used as nebulizing and drying gas at a source temperature of about 100°C. Maximal yields of the desired ions (see following text) were achieved by adjusting the cone voltage to between 30 and 90 V, respectively. For collision-induced dissociation (CID) at low collision energies, the ions of interest were mass-selected using Q1, interacted with xenon as a collision gas in the hexapole H at variable collision energies (E lab ϭ 0 -20 eV), while scanning Q2 to monitor the ionic products. In most cases, this pressure of xenon (typically 2 ϫ 10 Ϫ4 mbar) corresponds to single-collision conditions, but for some of the more weakly bound methanol complexes, the CID cross sections are very large, such that a considerable number of multiple collisions take place [8]. Given that the primary aim of the CID experiments reported here is the illustration of trends in relative binding energies, this aspect is not pursued any further. The same holds true for the internal energy content of the ions, which somewhat depends on size of the complexes as well as the cone voltage used, but poses no particular obstacles for the purpose of this work.As pointed out previously, the VG Bio-Q does not allow one to directly extract quantitative threshold information from CID experiments due to several limitations of the commercial instrument [7]. For weakly

Studies of gas‐phase reactions of cationic iron complexes of 2‐pyrimidinyloxy‐N‐arylbenzylamines by electrospray ionization tandem mass spectrometry

“…The theoretical computations provided more detailed information for such a transient process, which showed that the N-atom on the neighboring pyrimidine might associate with the proton dissociation step from benzylamine, especially from the step A1 to A3. Totally, the step from A1 to B and propanol is an endoergic process with energy 9.7 kcal/mol (listed in Table [52] Such results also suggested the different gas-phase chemistries initiated by Fe(II) and Fe(III). The relative stabilities of the Fe(II) and Fe(III) complexes could also be explained by HSAB (hard and soft acids and bases).…”

“…Our ESI‐MS experimental results presented the gas‐phase reactivities of Fe(III) and Fe(II) complexes of pyrimidinyloxy‐ N ‐arylbenzylamines 1 – 6 in the solvent‐free condition, showing the distinct intrinsic catalytic effects of Fe(III) and Fe(II) cations: Fe(II) could initiate a train of fascinating gas‐phase reactions including skeletal rearrangement, retro‐ Michael reaction and some others while there is no evidence to show that Fe(III) complexes also occurred in such gas‐phase Smiles rearrangement reactions but give the interesting SET products 1 •+ – 6 •+ , as well as some fragment ions. Previously, Schröder and Schwarz studied the gaseous Fe(II) and Fe(III) complexes with BINOLate ligands, and the results showed that Fe(II) complexes and Fe(III) complexes have different gas‐phase chemistries: [(BINOLdiate)Fe] + , a Fe(III) species, undergoes decarbonylation as a typical fragmentation of ionized phenols and phenolates, while the [(BINOLate)Fe] + cation, a Fe(II) species, on the other hand, preferentially loses neutral FeOH to afford an organic C 20 H 12 O +• cation radical 52. Such results also suggested the different gas‐phase chemistries initiated by Fe(II) and Fe(III).…”

Thiol‐disulfide redox equilibria of glutathione metaboloma compounds investigated by tandem mass spectrometry

“…An accurate estimate of this amount of energy entails a fine control on all experimental variables and can only be accomplished with guided ion beam instruments purposely designed for energy‐resolved tandem mass spectrometry 45. However, experimentally simpler approaches can take advantage of the ability of the analytically oriented triple quadrupole tandem mass spectrometer to record fragment spectra at several defined values of center‐of‐mass collision energy and to examine them to obtain at least semi‐quantitative estimates of the involved energies 37–40, 46…”

“…45 However, experimentally simpler approaches can take advantage of the ability of the analytically oriented triple quadrupole tandem mass spectrometer to record fragment spectra at several defined values of center-of-mass collision energy and to examine them to obtain at least semi-quantitative estimates of the involved energies. [37][38][39][40]46 When this process is applied to the set of fragment spectra obtained from protonated disulfides such as glutathione disulfides and other analogues, the onset potential of the fragment appearance curves obtained for the same fragment (e.g., the 'sulfenium' fragment) from different precursor ions (such as from several glutathione heterodisulfides) appear very close to one another, as can be found in the left column of Fig. 2.…”

Enantioselective Dehydrogenation of Alkan‐2‐ols by Gaseous (BINOLate)Ni+

“…For alkan-2-ol ligands, RCHOHCH 3 , CID of the mass-selected [(BINOLate)Ni(RCHOHCH 3 )] þ ions leads to two primary channels, Reactions 1 and 2; at elevated collision energies, consecutive fragmentations of the BINOL residue are observed (e.g., decarbonylation) which are not pursued any further here [8] [12].…”