Article pubs.acs.org/JPCA
Formation of a Spin-Forbidden Product, 1[MnO4]−, from Gas-Phase Decomposition of 6[Mn(NO3)3]− Johnny Lightcap, Thomas H. Hester, Daniel Patterson, Joseph T. Butler, and Daniel J. Goebbert* Department of Chemistry, The University of Alabama, Box 870336, Tuscaloosa, Alabama 35487, United States ABSTRACT: The manganese nitrate complex, [Mn(NO3)3]−, was generated via electrospray ionization and studied by tandem quadrupole mass spectrometry. The complex is assumed to decompose into [MnO(NO3)2]− by elimination of NO2•. The [MnO(NO3)2]− product undergoes elimination of NO2• to yield [MnO2(NO3)]−, or elimination of NO • to yield [MnO 3 (NO 3 )] − . Both [MnO 2 (NO 3 )] − and [MnO3(NO3)]− yield [MnO4]− via the transfer of oxygen atoms from the remaining nitrate ligand. The mechanism of permanganate formation is interesting because it can be generated through two competing pathways, and because the singlet ground state is spin-forbidden from the high-spin sextet [Mn(NO3)3]− precursor. Theory and experiment suggest [MnO2(NO3)]− is the major intermediate leading to formation of [MnO4]−. Theoretical studies show crossing from the high-spin to low-spin surface upon neutral oxygen atom transfer from the nitrate ligand in [MnO2(NO3)]− allows formation of 1[MnO4]−. Relative energy differences for the formation of 1[MnO4]− and 1[MnO3]− predicted by theory agree with experiment.
1. INTRODUCTION Metal oxides are common heterogeneous catalysts used in a wide variety of industrial processes. In particular, manganese oxides are an important class of compounds used in the oxidative coupling of methane1−3 and the reduction of nitric oxide into N2 and H2O.4−6 The reactivity of manganese oxides depends on the surface active site geometry and its electronic structure. However, conclusive identification of reactive surface sites is challenging, and properties of local active sites are difficult to probe in bulk materials.7−9 The lack of fundamental information pertaining to these critical locations limits understanding of catalytic mechanisms and the development or design of new catalytic materials. Experimental studies on small model complexes provide valuable information on fundamental properties at the molecular level, such as the nature of metal−oxygen bonding and the role of electronic structure on reactivity. Mass spectrometry has proven to be a powerful technique for investigating small model systems,10−15 because compounds with known composition can be isolated and studied under carefully controlled conditions. Gas-phase experiments reveal intrinsic properties of compounds in the absence of complicating factors such as extended materials, interfaces, and solvent. Small model systems can also be studied with relatively high levels of theory, 16−33 whereas accurate theoretical treatments on extended bulk systems remain challenging. The synergy between experiment and theory has made mass spectrometry an indispensible tool for studying reactivity and properties of metal clusters. Gas-phase ions of metal oxides and metal oxide complexes have been generated by several techniques. One of the most comment methods is laser-desorption-ionization,11−15,34−39 whereas less frequently used methods include discharges, sputtering10,40−43 and electrospray ionization, ESI.44−46 Gen© XXXX American Chemical Society
eration of ions by ESI is attractive because these ion sources are readily available on many instruments and have excellent stability and reproducibility. The decomposition of metal nitrate anion complexes generated by ESI, including [Mn(NO3)3]−, was originally investigated by Li et al., using a tandem quadrupole mass spectrometer.44 They reported three products from [Mn(NO3)3]− corresponding to NO3−, or consecutive elimination of NO2• to form [MnO(NO3)2]− and [MnO2(NO3)]−. A more recent fragmentation study by Frański and co-workers reported a number of smaller fragment ions from decomposition of [Mn(NO 3 ) 3 ] − , including [MnO3(NO3)]−, [MnO4]−, and [MnO3]−.45 These studies demonstrated that bare metal oxide anions, and metal oxide complexes decorated by ligands can be generated by dissociation of metal nitrate anion complexes generated from ESI. The product corresponding to [MnO4]− was the most intense peak in the recently reported fragmentation spectrum of [Mn(NO3)3]−.45 Observation of [MnO4]− as the major product from [Mn(NO3)3]− is suggestive of metal oxidation upon oxygen abstraction from the nitrate ligands.45 The permanganate ion is a well-known, stable, species, where manganese is in the highest oxidation state, +7, with a singlet electronic ground state. However, the electronic ground state of the [Mn(NO3)3]− precursor does not allow formation of singlet [MnO4]−. The five 3d electrons on Mn2+ yield a highspin 6[Mn(NO3)3]− precursor because the nitrate ligands are weak-field ligands. The formation of [MnO4]− from [Mn(NO3)3]− requires the net elimination of 2NO2• and NO•, all doublets, from the three nitrate ligands. The elimination Received: July 12, 2016 Revised: August 19, 2016
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DOI: 10.1021/acs.jpca.6b06978 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A products account for pairing of three of the five unpaired metal electrons; therefore, the spin-allowed product corresponds to 3 [MnO4]−. The energetically favored product, 1[MnO4]−, is spin-forbidden. Important questions remain unanswered about the decomposition of [Mn(NO3)3]−. The goal of this work is to investigate decomposition reactions, energies, metal oxidation states, and product spin states using a combination of experiment and theory. The major focus of this study is the mechanism of [MnO4]− formation, and the properties of simple manganese oxides. Intersystem crossing would allow formation of 1[MnO4]−, but there must be compelling experimental and theoretical evidence to support this claim. The concept of two-state reactivity has been well-established for transition gas-phase metals and metal oxides.23 Previous studies on reactions involving Mn+ and MnO+ have invoked surface crossing to explain reactivity and product distributions,47−49 and a similar process is expected in the formation of 1 [MnO4]− from 6[Mn(NO 3)3]−. Oxygen atom transfer reactions involving MnV−oxo−porphyrin complexes may undergo similar spin-forbidden two-state reactivity.50
ion kinetic energy distribution is relatively broad, and a small fraction of ions undergo either multiple collisions or collisions outside the collision cell, resulting in a broad low intensity signal in the threshold region. Pressure-dependent fragmentation studies would minimize many of these complicating factors but were not possible due to drift in the pressure gauge. However, the relative appearance energies measured in this work remain useful quantities for comparison with theory. We have successfully used this approach to study the fragmentation of several ions and continue the practice in this study.54−59 2.3. Theoretical Calculations. All calculations were performed using Gaussian 0960 at the density functional level of theory, (U)B3LYP, with the 6-311+G(3df) basis set. Geometries were optimized at various spin states to find the lowest energy structures for all detected ions. Vibrational frequencies were calculated to ensure the optimized structures converged to a minimum with no imaginary frequencies. Transition states were characterized by a single imaginary frequency corresponding to motion along the reaction coordinate. All reported energies include zero-point corrections. Orbital occupancies were calculated from a natural population analysis, NPA,61−63 on the optimized structures.
2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Manganese(II) nitrate hydrate (99.98%, Alfa Aesar) was purchased from VWR and used without further purification. HPLC grade acetonitrile (50% vol) and HPLC grade methanol (50% vol) were used to prepare 1 mM solutions of manganese nitrate. 2.2. Mass Spectrometry. A tandem quadrupole mass spectrometer (TSQ-7000, Finnigan MAT, San Jose, CA) equipped with an electrospray ionization source was used for all experiments. Solutions were injected by a syringe pump (8 μL/min) through a metal needle maintained at −4 kV with a current of about 0.2 μA. Gaseous ions were transferred through a heated capillary (200 °C) into a low pressure region (about 800 mTorr), and focused by a tube lens through a skimmer into a second differentially pumped region (1 mTorr). The tube lens voltage is generally minimized, but can be increased to induce fragmentation in the high pressure region. Ions were collimated by an ion guide and focused by a pair of electrostatic lenses into the high vacuum chamber for mass analysis. The tandem mass spectrometer used in this experiment has a quadrupole−-octopole−quadrupole configuration. Ions were initially studied by collision induced dissociation, CID. Major fragment ions were studied by energy-resolved mass spectrometry, ERMS, where each product was monitored as a function of the collision energy offset voltage applied to the collision cell. In ERMS experiments, the collision cell pressure was reduced to ≤0.20 mTorr in order to minimize the probability of multiple collision conditions. The lab-frame collision energy was calibrated by a retarding potential analysis used to locate the stopping potential of the incident ion beam. The lab-frame collision energies were converted to center-of-mass collision energies using the equation: Ecm = Elab[M/(m + M)], where Ecm is the center-of-mass energy, Elab is the lab-frame collision offset voltage after correction for zero kinetic energy, M is the mass of the argon target gas, and m is the ion mass. Approximate appearance energies were determined by extrapolation of the linearly increasing portion of the curves to the energy axis intercept. Threshold energies were not modeled51−53 because the precursor ion internal energy from the electrospray ionization source is not well characterized. The
3. RESULTS 3.1. Collision Induced Dissociation Mass Spectra. A typical CID mass spectrum for [Mn(NO3)3]− is shown in Figure 1a. The spectrum was recorded at 40 eVlab collision energy with a collision gas pressure of 0.50 mTorr. The spectrum in Figure 1a is in good agreement with the recently
Figure 1. Representative collision induced dissociation mass spectra of manganese anion complexes. Manganese oxide complexes were generated by in-source fragmentation of [Mn(NO3)3]−. All spectra were recoded using identical conditions, 0.50 mTorr Ar collision gas and a collision energy of 40 eVlab. B
DOI: 10.1021/acs.jpca.6b06978 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
Figure 2. Representative energy resolved fragmentation spectra of manganese anion complexes. All spectra were recorded using an Ar collision gas pressure of
